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Translational Control of Metabolism and Cell Cycle Progression in Hepatocellular Carcinoma. Int J Mol Sci 2023; 24:ijms24054885. [PMID: 36902316 PMCID: PMC10002961 DOI: 10.3390/ijms24054885] [Citation(s) in RCA: 4] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2023] [Revised: 02/27/2023] [Accepted: 03/01/2023] [Indexed: 03/06/2023] Open
Abstract
The liver is a metabolic hub characterized by high levels of protein synthesis. Eukaryotic initiation factors, eIFs, control the first phase of translation, initiation. Initiation factors are essential for tumor progression and, since they regulate the translation of specific mRNAs downstream of oncogenic signaling cascades, may be druggable. In this review, we address the issue of whether the massive translational machinery of liver cells contributes to liver pathology and to the progression of hepatocellular carcinoma (HCC); it represents a valuable biomarker and druggable target. First, we observe that the common markers of HCC cells, such as phosphorylated ribosomal protein S6, belong to the ribosomal and translational apparatus. This fact is in agreement with observations that demonstrate a huge amplification of the ribosomal machinery during the progression to HCC. Some translation factors, such as eIF4E and eIF6, are then harnessed by oncogenic signaling. In particular, the action of eIF4E and eIF6 is particularly important in HCC when driven by fatty liver pathologies. Indeed, both eIF4E and eIF6 amplify at the translational level the production and accumulation of fatty acids. As it is evident that abnormal levels of these factors drive cancer, we discuss their therapeutic value.
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Effects of different dietary protein sources on expression of genes related to protein metabolism in growing rats. Br J Nutr 2010; 104:1421-8. [DOI: 10.1017/s000711451000231x] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
Protein metabolism is known to be affected by dietary proteins, but the fundamental mechanisms that underlie the changes in protein metabolism are unclear. The aim of the present study was to test the effects of feeding growing rats with balanced diets containing soya protein isolate, zein and casein as the sole protein source on the expression of genes related to protein metabolism responses in skeletal muscle. The results showed that feeding a zein protein diet to the growing rats induced changes in protein anabolic and catabolic metabolism in their gastrocnemius muscles when compared with those fed either the reference protein casein diet or the soya protein isolate diet. The zein protein diet increased not only the mRNA levels and phosphorylation of mammalian target of rapamycin (mTOR), but also the mRNA expression of muscle atrophy F-box (MAFbx)/atrogin-1 and muscle ring finger 1 (MuRF1), as well as the forkhead box-O (FoxO) transcription factors involved in the induction of the E3 ligases. The amino acid profile of proteins seems to control signalling pathways leading to changes in protein synthesis and proteolysis.
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Kantor L, Pinchasi D, Mintz M, Hathout Y, Vanderver A, Elroy-Stein O. A point mutation in translation initiation factor 2B leads to a continuous hyper stress state in oligodendroglial-derived cells. PLoS One 2008; 3:e3783. [PMID: 19023445 PMCID: PMC2583043 DOI: 10.1371/journal.pone.0003783] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/07/2008] [Accepted: 11/04/2008] [Indexed: 11/18/2022] Open
Abstract
Background Mutations in eukaryotic translation initiation factor 2B (eIF2B) cause Childhood Ataxia with CNS Hypomyelination (CACH), also known as Vanishing White Matter disease (VWM). The disease is manifested by loss of brain myelin upon physiological stress. In a previous study, we showed that fibroblasts isolated from CACH/VWM patients are hypersensitive to pharmacologically-induced endoplasmic reticulum (ER) stress. Since brain cells from affected individuals are not available for research, we wished to assess the effect of eIF2B mutation on oligodendroglial-derived cells. Methodology/Principal Findings A rat oligodendroglial-derived cell line was used for a stable knock-down of eIF2B5 followed by stable expression of mutated eIF2B5(R195H) cDNA. In response to a pharmacological ER-stress agent, eIF2B5(R195H) expressing cells exhibited heightened ER-stress response demonstrated by hyper induction of ATF4, GADD34, Bip, PDIA1, PDIA3, PDIA4 and PDIA6 proteins. Moreover, even in the absence of a pharmacological stress agent, eIF2B5(R195H)-expressing cells exhibited high basal levels of ATF4, GADD34 and ER-associated Bip, PDIA1 and PDIA3. Significance The data provide evidence that oligodendroglial-derived cells expressing a mutated eIF2B constantly use their stress response mechanism as an adaptation mean in order to survive. The current study is the first to demonstrate the effects of eIF2B5 mutation on ER homeostasis in oligodendroglial-derived cells.
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Affiliation(s)
- Liraz Kantor
- Department of Cell Research & Immunology, George S. Wise Faculty of Life Sciences, Tel-Aviv University, Tel-Aviv, Israel
| | - Dalia Pinchasi
- Department of Cell Research & Immunology, George S. Wise Faculty of Life Sciences, Tel-Aviv University, Tel-Aviv, Israel
| | - Michelle Mintz
- Center for Genetic Medicine, Children's National Medical Center, Washington D. C., United States of America
| | - Yetrib Hathout
- Center for Genetic Medicine, Children's National Medical Center, Washington D. C., United States of America
| | - Adeline Vanderver
- Center for Genetic Medicine, Children's National Medical Center, Washington D. C., United States of America
| | - Orna Elroy-Stein
- Department of Cell Research & Immunology, George S. Wise Faculty of Life Sciences, Tel-Aviv University, Tel-Aviv, Israel
- * E-mail:
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Wang H, Kubica N, Ellisen LW, Jefferson LS, Kimball SR. Dexamethasone represses signaling through the mammalian target of rapamycin in muscle cells by enhancing expression of REDD1. J Biol Chem 2006; 281:39128-34. [PMID: 17074751 DOI: 10.1074/jbc.m610023200] [Citation(s) in RCA: 196] [Impact Index Per Article: 10.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
The mammalian target of rapamycin (mTOR), a critical modulator of cell growth, acts to integrate signals from hormones, nutrients, and growth-promoting stimuli to downstream effector mechanisms involved in the regulation of protein synthesis. Dexamethasone, a synthetic glucocorticoid that represses protein synthesis, acts to inhibit mTOR signaling as assessed by reduced phosphorylation of the downstream targets S6K1 and 4E-BP1. Dexamethasone has also been shown in one study to up-regulate the expression of REDD1 (also referred to RTP801, a novel stress-induced gene linked to repression of mTOR signaling) in lymphoid, but not nonlymphoid, cells. In contrast to the findings of that study, here we demonstrate that REDD1, but not REDD2, mRNA expression is dramatically induced following acute dexamethasone treatment both in rat skeletal muscle in vivo and in L6 myoblasts in culture. In L6 myoblasts, the effect of the drug on mTOR signaling is efficiently blunted in the presence of REDD1 RNA interference oligonucleotides. Moreover, the dexamethasone-induced assembly of the mTOR regulatory complex Tuberin. Hamartin is disrupted in L6 myoblasts following small interfering RNA-mediated repression of REDD1 expression. Finally, overexpression of Rheb, a downstream target of Tuberin function and a positive upstream effector of mTOR, reverses the effect of dexamethasone on phosphorylation of mTOR substrates. Overall, the data support the conclusion that REDD1 functions upstream of Tuberin and Rheb to down-regulate mTOR signaling in response to dexamethasone.
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Affiliation(s)
- Hongmei Wang
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA
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Frank JW, Escobar J, Suryawan A, Nguyen HV, Kimball SR, Jefferson LS, Davis TA. Dietary protein and lactose increase translation initiation factor activation and tissue protein synthesis in neonatal pigs. Am J Physiol Endocrinol Metab 2006; 290:E225-33. [PMID: 16144813 DOI: 10.1152/ajpendo.00351.2005] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Protein synthesis and eukaryotic initiation factor (eIF) activation are increased in muscle and liver of pigs parenterally infused with amino acids and insulin. To examine the effects of enteral protein and carbohydrate on protein synthesis, pigs (n = 42, 1.7 kg body wt) were fed isocaloric milk diets containing three levels of protein (5, 15, and 25 g x kg body wt(-1) x day(-1)) and two levels of lactose (low = 11 and high = 23 g x kg body wt(-1) x day(-1)) from 1 to 6 days of age. On day 7, pigs were gavage fed after 4-h food deprivation, and tissue protein synthesis rates and biomarkers of mRNA translation were assessed. Piglet growth and protein synthesis rates in muscle and liver increased with dietary protein and plateaued at 15 g x kg body wt(-1) x day(-1) (P < 0.001). Growth tended to be greater in high-lactose-fed pigs (P = 0.07). Plasma insulin was lowest in pigs fed 5 g x kg body wt(-1) x day(-1) protein (P < 0.0001). Plasma branched-chain amino acids increased as protein intake increased (P < 0.0001). Muscle (P < 0.001) and liver (P < or = 0.001) ribosomal protein S6 kinase-1 and eIF4E-binding protein phosphorylation increased with protein intake and plateaued at 15 g x kg body wt(-1) x day(-1). The results indicate that growth and protein synthesis rates in neonatal pigs are influenced by dietary protein and lactose intake and might be mediated by plasma amino acids and insulin levels. However, feeding protein well above the piglet's requirement does not further stimulate the activation of translation initiation or protein synthesis in skeletal muscle and liver.
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Affiliation(s)
- Jason W Frank
- USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, 1100 Bates St., Houston, TX 77030, USA
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Frank JW, Escobar J, Suryawan A, Kimball SR, Nguyen HV, Jefferson LS, Davis TA. Protein synthesis and translation initiation factor activation in neonatal pigs fed increasing levels of dietary protein. J Nutr 2005; 135:1374-81. [PMID: 15930440 DOI: 10.1093/jn/135.6.1374] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Limited data suggest that the growth of low-birth-weight infants is enhanced by feeding a high-protein diet; however, the mechanisms involved in the effect have not been delineated. To identify these mechanisms, 34 pigs were fed from 2 to 7 d of age [60 g dry matter/(kg body weight . d)] isocaloric milk diets that contained levels of dietary protein that were marginal, adequate, and in excess of the piglets protein requirement (21, 33, and 45% of dry matter, respectively). Dietary protein replaced lactose and fat on an isocaloric basis. Fractional protein synthesis rates, various biomarkers of translational regulation, and plasma glucose and insulin levels were measured in overnight food-deprived and fed pigs. Mean daily weight gain of pigs fed the 33 and 45% protein diets was greater than that of pigs fed the 21% protein diet (P < 0.01). Plasma glucose (P = 0.07) and insulin (P < 0.01) levels decreased as dietary protein increased 60 min after feeding. Protein synthesis rates in longissimus dorsi, gastrocnemius, masseter, heart, liver, kidney, jejunum, and pancreas were greater in the fed than in the food-deprived state (P < 0.01). Protein synthesis in skeletal muscle did not change with protein intake in the fed state, but decreased quadratically (P < 0.01) with increasing dietary protein in the food-deprived state. Protein kinase B, ribosomal protein S6 kinase 1(S6K1), and eukaryotic initiation factor (eIF) 4E binding protein-1 (4E-BP1) were more phosphorylated, and assembly of the inactive eukaryotic initiation factor 4E . 4E-BP1 complex in muscle and liver was reduced in the fed state (P < 0.001) and were not consistently affected by dietary protein level. The results suggest that feeding stimulates protein synthesis, and this is modulated by the activation of initiation factors that regulate mRNA binding to the ribosomal complex. However, the provision of a high-protein diet that exceeds the protein requirement does not further enhance protein synthesis or translation initiation factor activation.
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Affiliation(s)
- Jason W Frank
- U.S. Department of Agriculture/Agriculture Research Service, Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, TX 77030, USA
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Reiter AK, Bolster DR, Crozier SJ, Kimball SR, Jefferson LS. Repression of protein synthesis and mTOR signaling in rat liver mediated by the AMPK activator aminoimidazole carboxamide ribonucleoside. Am J Physiol Endocrinol Metab 2005; 288:E980-8. [PMID: 15613684 DOI: 10.1152/ajpendo.00333.2004] [Citation(s) in RCA: 61] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The studies described herein were designed to investigate the effects of 5-aminoimidazole-4-carboxamide-1-beta-D-ribonucleoside (AICAR), an activator of the AMP-activated protein kinase (AMPK), on the translational control of protein synthesis and signaling through the mammalian target of rapamycin (mTOR) in rat liver. Effects of AICAR observed in vivo were compared with those obtained in an in situ perfused liver preparation to investigate activation of AMPK in the absence of accompanying changes in hormones and nutrients. AMPK became hyperphosphorylated, as assessed by a gel-shift analysis, in response to AICAR both in vivo and in situ; however, increased relative phosphorylation at the Thr172 site on the kinase was observed only in perfused liver. Phosphorylation of AMPK either in vivo or in situ was associated with a repression of protein synthesis as well as decreased phosphorylation of a number of targets of mTOR signaling including ribosomal protein S6 kinase 1, eukaryotic initiation factor (eIF)4G, and eIF4E-binding protein (4E-BP)1. The phosphorylation changes in eIF4G and 4E-BP1 were accompanied by a reduction in the amount of eIF4E present in the active eIF4E.eIF4G complex and an increase in the amount present in the inactive eIF4E.4E-BP1 complex. Reduced insulin signaling as well as differences in nutrient availability may have contributed to the effects observed in vivo as AICAR caused a fall in the serum insulin concentration. Overall, however, the results from both experimental models support a scenario in which AICAR directly represses protein synthesis and mTOR signaling in the liver through an AMPK-dependent mechanism.
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Affiliation(s)
- Ali K Reiter
- Department of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, P.O. Box 850, Hershey, PA 17033, USA
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O'Connor PMJ, Kimball SR, Suryawan A, Bush JA, Nguyen HV, Jefferson LS, Davis TA. Regulation of neonatal liver protein synthesis by insulin and amino acids in pigs. Am J Physiol Endocrinol Metab 2004; 286:E994-E1003. [PMID: 14761876 DOI: 10.1152/ajpendo.00391.2003] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The high efficiency of protein deposition during the neonatal period is driven by high rates of protein synthesis, which are maximally stimulated after feeding. Infusion of amino acids, but not insulin, reproduces the feeding-induced stimulation of liver protein synthesis. To determine whether amino acid-stimulated liver protein synthesis is independent of insulin in neonates, and to examine the role of amino acids and insulin in the regulation of translation initiation in neonatal liver, we performed pancreatic glucose-amino acid clamps in overnight-fasted 7-day-old pigs. Pigs (n = 9-12/group) were infused with insulin at 0, 10, 22, and 110 ng.kg(-0.66).min(-1) to achieve 0, 2, 6, and 30 microU/ml insulin, respectively. At each insulin dose, amino acids were maintained at fasting or fed levels or, in conjunction with the highest insulin dose, allowed to fall to below fasting levels. Insulin had no effect on the fractional rate of protein synthesis in liver. Amino acids increased fractional protein synthesis rates in liver at each dose of insulin, including the 0 microU/ml dose. There was a dose-response effect of amino acids on liver protein synthesis. Amino acids and insulin increased protein S6 kinase and 4E-binding protein 1 (4E-BP1) phosphorylation; however, only amino acids decreased formation of the inactive 4E-BPI.eukaryotic initiation factor-4E (eIF4E) complex. The results suggest that amino acids regulate liver protein synthesis in the neonate by modulating the availability of eIF4E for 48S ribosomal complex formation and that this response does not require insulin.
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Affiliation(s)
- Pamela M J O'Connor
- United States Department of Agriculture/Agricultural Research Service, Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, 1100 Bates St., Suite 9064, Houston, TX 77030, USA
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Kubica N, Kimball SR, Jefferson LS, Farrell PA. Alterations in the expression of mRNAs and proteins that code for species relevant to eIF2B activity after an acute bout of resistance exercise. J Appl Physiol (1985) 2004; 96:679-87. [PMID: 14565967 DOI: 10.1152/japplphysiol.00962.2003] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023] Open
Abstract
The focus of the study described herein was to examine the relative expression levels of mRNAs and proteins relevant to the regulation of translational initation, and hence protein synthesis, in the time course after an acute bout of resistance exercise in male Sprague-Dawley rats. Significant increases in the relative abundance of the mRNAs coding for the epsilon (33%) and gamma (26%) subunits of eukaryotic initiation factor (eIF) 2B were observed 48 h after the exercise bout. Furthermore, the mRNA coding for the delta subunit of eIF2B was also significantly increased, both 24 h (46%) and 48 h (44%) postexercise. There was a relative decrease in three eIF2Bϵ kinase mRNAs, namely sequences coding for glycogen synthase kinase 3β (49%), casein kinase I (48%), and casein kinase II (42%) 48 h into the recovery period. Additionally, there was a significant decrease in expression of the mRNAs coding for eIF2α (28% 24 h postexercise) and one of its regulatory kinases, double-stranded RNA-activated protein kinase (33% 48 h postexercise). Finally, an increase in eIF2B total protein (124%) was observed within 3 h postexercise. These results suggest that there may be rapid translational regulation of mRNAs coding for species relevant to translational initiation after an acute bout of resistance exercise. Furthermore, transcription of these mRNAs is altered further into the recovery period, and this might play a role in protein synthetic capacity on subsequent bouts of resistance exercise.
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Affiliation(s)
- Neil Kubica
- Noll Physiological Research Center, The Pennsylvania State University, University Park 16802, USA
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O'Connor PMJ, Kimball SR, Suryawan A, Bush JA, Nguyen HV, Jefferson LS, Davis TA. Regulation of translation initiation by insulin and amino acids in skeletal muscle of neonatal pigs. Am J Physiol Endocrinol Metab 2003; 285:E40-53. [PMID: 12637260 DOI: 10.1152/ajpendo.00563.2002] [Citation(s) in RCA: 97] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
Previous studies have shown that intravenous infusion of insulin and/or amino acids reproduces the feeding-induced stimulation of muscle protein synthesis in neonates and that insulin and amino acids act independently to produce this effect. The goal of the present study was to delineate the regulatory roles of insulin and amino acids on muscle protein synthesis in neonates by examining translational control mechanisms, specifically the eukaryotic translation initiation factors (eIFs), which enable coupling of initiator methionyl-tRNAi and mRNA to the 40S ribosomal subunit. Insulin secretion was blocked by somatostatin in fasted 7-day-old pigs (n = 8-12/group), insulin was infused to achieve plasma levels of approximately 0, 2, 6, and 30 microU/ml, and amino acids were clamped at fasting or fed levels or, at the high insulin dose, below fasting. Both insulin and amino acids increased the phosphorylation of ribosomal protein S6 kinase (S6K1) and the eIF4E-binding protein (4E-BP1), decreased the binding of 4E-BP1 to eIF4E, increased eIF4E binding to eIF4G, and increased fractional protein synthesis rates but did not affect eIF2B activity. In the absence of insulin, amino acids had no effect on these translation initiation factors but increased the protein synthesis rates. Raising insulin from below fasting to fasting levels generally did not alter translation initiation factor activity but raised protein synthesis rates. The phosphorylation of S6K1 and 4E-BP1 and the amount of 4E-BP1 bound to eIF4E and eIF4E bound to eIF4G were correlated with insulin level, amino acid level, and protein synthesis rate. Thus insulin and amino acids regulate muscle protein synthesis in skeletal muscle of neonates by modulating the availability of eIF4E for 48S ribosomal complex assembly, although other processes also must be involved.
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Affiliation(s)
- Pamela M J O'Connor
- United States Department of Agriculture/Agricultural Research Service, Children's Nutrition Research Center, Baylor College of Medicine, Houston, Texas 77030, USA
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Kimball SR, Horetsky RL, Ron D, Jefferson LS, Harding HP. Mammalian stress granules represent sites of accumulation of stalled translation initiation complexes. Am J Physiol Cell Physiol 2003; 284:C273-84. [PMID: 12388085 DOI: 10.1152/ajpcell.00314.2002] [Citation(s) in RCA: 208] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
In eukaryotic cells subjected to environmental stress, untranslated mRNA accumulates in discrete cytoplasmic foci that have been termed stress granules. Recent studies have shown that in addition to mRNA, stress granules also contain 40S ribosomal subunits and various translation initiation factors, including the mRNA binding proteins eIF4E and eIF4G. However, eIF2, the protein that transfers initiator methionyl-tRNA(i) (Met-tRNA(i)) to the 40S ribosomal subunit, has not been detected in stress granules. This result is surprising because the eIF2. GTP. Met-tRNA(i) complex is thought to bind to the 40S ribosomal subunit before the eIF4G. eIF4E. mRNA complex. In the present study, we show in both NIH-3T3 cells and mouse embryo fibroblasts that stress granules contain not only eIF2 but also the guanine nucleotide exchange factor for eIF2, eIF2B. Moreover, we show that phosphorylation of the alpha-subunit of eIF2 is necessary and sufficient for stress granule formation during the unfolded protein response. Finally, we also show that stress granules contain many, if not all, of the components of the 48S preinitiation complex, but not 60S ribosomal subunits, suggesting that they represent stalled translation initiation complexes.
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Affiliation(s)
- Scot R Kimball
- Department of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA.
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Bigot K, Taouis M, Tesseraud S. Refeeding and insulin regulate S6K1 activity in chicken skeletal muscles. J Nutr 2003; 133:369-73. [PMID: 12566469 DOI: 10.1093/jn/133.2.369] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Broiler chickens are characterized by fast muscle growth and high protein deposition, most likely subsequent to a high protein synthesis. However, the regulation of protein synthesis in chicken muscle is still unknown. In contrast, it has been clearly demonstrated in mammals that S6K1 is a key regulator of protein synthesis. In the present study, S6K1 was characterized in both pectoralis and gastrocnemius muscles in chickens. A 133-bp fragment of chicken S6K1 cDNA had 84% identity to mammalian S6K1. We investigated in vivo the effects of refeeding and insulin treatment after 16 h starvation. S6K1 enzyme activity was significantly increased in both pectoralis and gastrocnemius muscles by refeeding (two- to threefold greater than in food-deprived chickens, P < 0.05). Optimal activation occurred 30 min after refeeding following 16 h starvation. S6K1 activation was associated with its phosphorylation on serine and Thr 389 residues, which occurred within the first 5 min of refeeding. S6K1 was also significantly stimulated in both pectoralis and gastrocnemius muscles after a single insulin injection (nine- to 12-fold greater than in control chickens, P < 0.001). Our results indicate that S6K1 is expressed in chickens muscles and activated by refeeding and insulin treatment.
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Affiliation(s)
- Karine Bigot
- Régulation du métabolisme des oiseaux, Station de Recherches Avicoles, Institut National de la Recherche Agronomique, Centre de Tours-Nouzilly, Nouzilly, France
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Zhang P, McGrath BC, Reinert J, Olsen DS, Lei L, Gill S, Wek SA, Vattem KM, Wek RC, Kimball SR, Jefferson LS, Cavener DR. The GCN2 eIF2alpha kinase is required for adaptation to amino acid deprivation in mice. Mol Cell Biol 2002; 22:6681-8. [PMID: 12215525 PMCID: PMC134046 DOI: 10.1128/mcb.22.19.6681-6688.2002] [Citation(s) in RCA: 338] [Impact Index Per Article: 15.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022] Open
Abstract
The GCN2 eIF2alpha kinase is essential for activation of the general amino acid control pathway in yeast when one or more amino acids become limiting for growth. GCN2's function in mammals is unknown, but must differ, since mammals, unlike yeast, can synthesize only half of the standard 20 amino acids. To investigate the function of mammalian GCN2, we have generated a Gcn2(-/-) knockout strain of mice. Gcn2(-/-) mice are viable, fertile, and exhibit no phenotypic abnormalities under standard growth conditions. However, prenatal and neonatal mortalities are significantly increased in Gcn2(-/-) mice whose mothers were reared on leucine-, tryptophan-, or glycine-deficient diets during gestation. Leucine deprivation produced the most pronounced effect, with a 63% reduction in the expected number of viable neonatal mice. Cultured embryonic stem cells derived from Gcn2(-/-) mice failed to show the normal induction of eIF2alpha phosphorylation in cells deprived of leucine. To assess the biochemical effects of the loss of GCN2 in the whole animal, liver perfusion experiments were conducted. Histidine limitation in the presence of histidinol induced a twofold increase in the phosphorylation of eIF2alpha and a concomitant reduction in eIF2B activity in perfused livers from wild-type mice, but no changes in livers from Gcn2(-/-) mice.
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Affiliation(s)
- Peichuan Zhang
- Department of Biology, The Pennsylvania State University, University Park 16802, USA
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Shah OJ, Kimball SR, Jefferson LS. The Src-family tyrosine kinase inhibitor PP1 interferes with the activation of ribosomal protein S6 kinases. Biochem J 2002; 366:57-62. [PMID: 12014987 PMCID: PMC1222761 DOI: 10.1042/bj20020198] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2002] [Revised: 04/26/2002] [Accepted: 05/15/2002] [Indexed: 11/17/2022]
Abstract
Considerable biochemical and pharmacological evidence suggests that the activation of ribosomal protein S6 kinases (S6Ks) by activated receptor tyrosine kinases involves multiple co-ordinated input signals. However, the identities of many of these inputs remain poorly described, and their precise involvement in S6K activation has been the subject of great investigative effort. In the present study, we have shown that 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP1), a selective inhibitor of the Src family of non-receptor tyrosine kinases, interferes with the activation of 70 and 85 kDa S6K gene products (p70S6K1 and p85S6K1) by insulin, insulin-like growth factor 1, sodium orthovanadate and activated alleles of phosphoinositide 3-kinase and H-Ras. PP1 also impedes the activation of AKT/protein kinase B and the extracellular signal-regulated protein kinases 1 and 2 by these various stimuli. Insulin-like growth factor 1 was observed to induce a sustained increase in c-Src autophosphorylation as revealed using anti-phospho-Y416 antisera, but this effect was absent from the cells treated with PP1. To conclude, an activated allele of p70S6K1 is compared with the wild-type allele, resistant to inhibition by PP1 when co-expressed with phosphoinositide-dependent kinase 1 (PDK1), suggesting that PP1 affects p70S6K1 via a PDK1-independent pathway. Thus activation of Src may supply a necessary signal for the activation of p70S6K1 and possibly other S6Ks.
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Affiliation(s)
- O Jameel Shah
- Department of Cellular and Molecular Physiology, College of Medicine, The Pennsylvania State University, 500 University Drive, Hershey, PA 17033, U.S.A
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Kimball SR, Farrell PA, Nguyen HV, Jefferson LS, Davis TA. Developmental decline in components of signal transduction pathways regulating protein synthesis in pig muscle. Am J Physiol Endocrinol Metab 2002; 282:E585-92. [PMID: 11832361 DOI: 10.1152/ajpendo.00269.2001] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Our previous studies showed that the feeding-induced stimulation of protein synthesis in skeletal muscle of neonatal pigs is accompanied by enhanced phosphorylation of the eukaryotic initiation factor (eIF)4E-binding protein (4E-BP1) and the ribosomal protein S6 kinase (S6K1). These effects of feeding are substantially reduced with development. The goal of the present investigation was to delineate the basis for the reduced responsiveness to feeding observed in the older animals. In these studies, the content and activity of protein kinases located upstream of S6K1 and 4E-BP1 in signal transduction pathways activated by amino acids, insulin, and insulin-like growth factor I were examined in 7- and 26-day-old pigs that were either fasted overnight or fed porcine milk after an overnight fast. Feeding stimulated phosphatidylinositol (PI) 3-kinase activity to the same extent in muscle of 7- and 26-day-old pigs, suggesting that PI 3-kinase is not limiting in muscle of older animals. In contrast, protein kinase B (PKB) activity was significantly less in muscle from 26- vs. 7-day-old pigs, regardless of nutritional status, suggesting that its activity is regulated by mechanisms distinct from PI 3-kinase. In part, the reduced PKB responsiveness can be attributed to a developmental decline in PKB content. Likewise, muscle content of the protein kinase termed mammalian target of rapamycin (mTOR) in 26-day-old pigs was <25% of that in 7-day-old animals. Finally, in agreement with our earlier work showing that S6K1 phosphorylation is reduced in older animals, S6K1 activity was stimulated to a lesser extent in 26- compared with 7-day-old pigs. Overall, the results suggest that the blunted protein synthetic response observed in 26- vs. 7-day-old neonatal pigs is due in part to decreased content and/or activity of signaling components downstream of PI 3-kinase, e.g., PKB, mTOR, and S6K1.
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Affiliation(s)
- Scot R Kimball
- Department of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA.
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Clemens MJ. Initiation factor eIF2 alpha phosphorylation in stress responses and apoptosis. PROGRESS IN MOLECULAR AND SUBCELLULAR BIOLOGY 2002; 27:57-89. [PMID: 11575161 DOI: 10.1007/978-3-662-09889-9_3] [Citation(s) in RCA: 169] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The alpha subunit of polypeptide chain initiation factor eIF2 can be phosphorylated by a number of related protein kinases which are activated in response to cellular stresses. Physiological conditions which result in eIF2 alpha phosphorylation include virus infection, heat shock, iron deficiency, nutrient deprivation, changes in intracellular calcium, accumulation of unfolded or denatured proteins and the induction of apoptosis. Phosphorylated eIF2 acts as a dominant inhibitor of the guanine nucleotide exchange factor eIF2B and prevents the recycling of eIF2 between successive rounds of protein synthesis. Extensive phosphorylation of eIF2 alpha and strong inhibition of eIF2B activity can result in the downregulation of the overall rate of protein synthesis; less marked changes may lead to alterations in the selective translation of alternative open reading frames in polycistronic mRNAs, as demonstrated in yeast. These mechanisms can provide a signal transduction pathway linking eukaryotic cellular stress responses to alterations in the control of gene expression at the translational level.
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Affiliation(s)
- M J Clemens
- Department of Biochemistry and Immunology, St George's Hospital Medical School, University of London, Cranmer Terrace, London SW17 0RE, UK
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Sans MD, Kimball SR, Williams JA. Effect of CCK and intracellular calcium to regulate eIF2B and protein synthesis in rat pancreatic acinar cells. Am J Physiol Gastrointest Liver Physiol 2002; 282:G267-76. [PMID: 11804848 DOI: 10.1152/ajpgi.00274.2001] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
Pancreatic secretagogues enhance acinar protein synthesis at physiological concentrations and inhibit protein synthesis at high concentrations. We investigated the potential role in this process of the eukaryotic translation initiation factor (eIF)2B. Cholecystokinin (CCK) at 10-100 pM did not significantly affect eIF2B activity, which averaged 35.4 nmol guanosine 5'-diphosphate exchanged per minute per milligram protein under control conditions; higher CCK concentrations reduced eIF2B activity to 38.2% of control. Carbamylcholine chloride (Carbachol, CCh), A-23187, and thapsigargin also inhibited eIF2B and protein synthesis, whereas bombesin and the CCK analog JMV-180 were without effect. Previous studies have shown that eIF2B can be negatively regulated by glycogen synthase kinase-3 (GSK-3). However, GSK-3 activity, as assessed by phosphorylation state, was inhibited at high concentrations of CCK, an effect that should have stimulated, rather than repressed, eIF2B activity. An alternative mechanism for regulating eIF2B is through phosphorylation of the alpha-subunit of eIF2, which converts it into an inhibitor of eIF2B. CCK, CCh, A-23187, and thapsigargin all enhanced eIF2alpha phosphorylation, suggesting that eIF2B activity is regulated by eIF2alpha phosphorylation under these conditions. Removal of Ca(2+) from the medium enhanced the inhibitory action of CCK on both protein synthesis and eIF2B activity as well as further increasing eIF2alpha phosphorylation. Although it is likely that other mechanisms account for the stimulation of acinar protein synthesis, these results suggest that the inhibition of acinar protein synthesis by CCK occurs as a result of depletion of Ca(2+) from the endoplasmic reticulum lumen leading to phosphorylation of eIF2alpha and inhibition of eIF2B.
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Affiliation(s)
- Maria Dolors Sans
- Department of Physiology, University of Michigan, 1301 St. Catherine St., 7737 Med Sci II, Ann Arbor, MI 48109, USA.
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Shah OJ, Iniguez-Lluhi JA, Romanelli A, Kimball SR, Jefferson LS. The activated glucocorticoid receptor modulates presumptive autoregulation of ribosomal protein S6 protein kinase, p70 S6K. J Biol Chem 2002; 277:2525-33. [PMID: 11705993 DOI: 10.1074/jbc.m105935200] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023] Open
Abstract
Protein metabolism in eukaryotic organisms is defined by a synthesis-degradation equilibrium that is subject to regulation by hormonal and nutritional signals. In mammalian tissues such as skeletal muscle, glucocorticoid hormones specify a catabolic response that influences both protein synthetic and protein degradative pathways. With regard to the former, glucocorticoids attenuate mRNA translation at two levels: translational efficiency, i.e. translation initiation, and translational capacity, i.e. ribosome biogenesis. Glucocorticoids may impair translational capacity through the ribosomal S6 protein kinase (p70 S6K), a recognized glucocorticoid target and an effector of ribosomal protein synthesis. We demonstrate here that the reduction in growth factor-activated p70 S6K activity by glucocorticoids depends upon a functional glucocorticoid receptor (GR) and that the GR is both necessary and sufficient to render p70 S6K subject to glucocorticoid regulation. Furthermore, the DNA binding and transcriptional activation but not repression properties of the GR are indispensable for p70 S6K regulation. Finally, a mutational analysis of the p70 S6K carboxyl terminus indicates that this region confers glucocorticoid sensitivity, and thus glucocorticoids may facilitate autoinhibition of the enzyme ultimately reducing the efficiency with which T389 is phosphorylated.
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Affiliation(s)
- O Jameel Shah
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033-0850, USA
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Kimball SR, Clemens MJ, Tilleray VJ, Wek RC, Horetsky RL, Jefferson LS. The double-stranded RNA-activated protein kinase PKR is dispensable for regulation of translation initiation in response to either calcium mobilization from the endoplasmic reticulum or essential amino acid starvation. Biochem Biophys Res Commun 2001; 280:293-300. [PMID: 11162513 DOI: 10.1006/bbrc.2000.4103] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
Abstract
The alpha-subunit of eukaryotic initiation factor eIF2 is a preferred substrate for the double-stranded RNA-activated protein kinase, PKR. Phosphorylation of eIF2alpha converts the factor from a substrate into a competitive inhibitor of the guanine nucleotide exchange factor, eIF2B, leading to a decline in mRNA translation. Early studies provided evidence implicating PKR as the kinase that phosphorylates eIF2alpha under conditions of cell stress such as the accumulation of misfolded proteins in the lumen of the endoplasmic reticulum, i.e., the unfolded protein response (UPR). However, the recent identification of a trans-microsomal membrane eIF2alpha kinase, termed PEK or PERK, suggests that this kinase, and not PKR, might be the kinase that is activated by misfolded protein accumulation. Similarly, genetic studies in yeast provide compelling evidence that a kinase termed GCN2 phosphorylates eIF2alpha in response to amino acid deprivation. However, no direct evidence showing activation of the mammalian homologue of GCN2 by amino acid deprivation has been reported. In the present study, we find that in fibroblasts treated with agents that promote the UPR, protein synthesis is inhibited as a result of a decrease in eIF2B activity. Furthermore, the reduction in eIF2B activity is associated with enhanced phosphorylation of eIF2alpha. Importantly, the magnitude of the change in each parameter is identical in wildtype cells and in fibroblasts containing a chromosomal deletion in the PKR gene (PKR-KO cells). In a similar manner, we find that during amino acid deprivation the inhibition of protein synthesis and extent of increase in eIF2alpha phosphorylation are identical in wildtype and PKR-KO cells. Overall, the results show that PKR is not required for increased eIF2alpha phosphorylation or inhibition of protein synthesis under conditions promoting the UPR or in response to amino acid deprivation.
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Affiliation(s)
- S R Kimball
- Department of Cellular and Molecular Physiology, Pennsylvania State University, College of Medicine, Hershey, Pennsylvania 17033, USA
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Davis TA, Nguyen HV, Suryawan A, Bush JA, Jefferson LS, Kimball SR. Developmental changes in the feeding-induced stimulation of translation initiation in muscle of neonatal pigs. Am J Physiol Endocrinol Metab 2000; 279:E1226-34. [PMID: 11093908 DOI: 10.1152/ajpendo.2000.279.6.e1226] [Citation(s) in RCA: 74] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The rapid gain in skeletal muscle mass in the neonate is associated with a marked elevation in skeletal muscle protein synthesis in response to feeding. The feeding-induced response decreases with development. To determine whether the response to feeding is regulated at the level of translation initiation, the expression, phosphorylation, and function of a number of eukaryotic initiation factors (eIF) were examined. Pigs at 7 and 26 days of age were either fasted overnight or fed porcine milk after an overnight fast. In muscle of 7-day-old pigs, the hyperphosphorylated form of the eIF4E repressor protein, 4E-binding protein 1 (4E-BP1), was undetectable in the fasting state but rose to 60% of total 4E-BP1 after feeding; eIF4E phosphorylation was unaffected by feeding status. The amount of eIF4E in the inactive 4E-BP1. eIF4E complex was reduced by 80%, and the amount of eIF4E in the active eIF4E. eIF4G complex was increased 14-fold in muscle of 7-day-old pigs after feeding. The amount of 70-kDa ribosomal protein S6 (p70(S6)) kinase in the hyperphosphorylated form rose 2.5-fold in muscle of 7-day-old pigs after feeding. Each of these feeding-induced responses was blunted in muscle of 26-day-old pigs. eIF2B activity in muscle was unaffected by feeding status but decreased with development. Feeding produced similar changes in eIF characteristics in liver and muscle; however, the developmental changes in liver were not as apparent as in skeletal muscle. Thus the results demonstrate that the developmental change in the acute stimulation of skeletal muscle protein synthesis by feeding is regulated by the availability of eIF4E for 48S ribosomal complex formation. The results further suggest that the overall developmental decline in skeletal muscle protein synthesis involves regulation by eIF2B.
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Affiliation(s)
- T A Davis
- Department of Pediatrics, United States Department of Agriculture/Agricultural Research Service, Children's Nutrition Research Center, Baylor College of Medicine, Houston, Texas 77030, USA.
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Shah OJ, Anthony JC, Kimball SR, Jefferson LS. Glucocorticoids oppose translational control by leucine in skeletal muscle. Am J Physiol Endocrinol Metab 2000; 279:E1185-90. [PMID: 11052975 DOI: 10.1152/ajpendo.2000.279.5.e1185] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Glucocorticoids comprise an important class of hormonal mediators of fuel and protein homeostasis in normal and pathological scenarios. In skeletal muscle, exposure to glucocorticoids is characterized by a reduction in protein synthetic rate coincident with hampered translation initiation. However, it is unclear whether this involves attenuation of anabolic stimuli or is simply due to inhibition of the basally activated translational apparatus. Therefore, this inquiry was designed to determine whether leucine, administered orally, could rescue the translational inhibition induced by glucocorticoids. Dexamethasone, injected intraperitoneally, acutely diminished protein synthetic rates to 80% of control values in skeletal muscle from rat hindlimb. The eukaryotic initiation factor (eIF)4 regulatory element was simultaneously and negatively impacted via sequestration of eIF4E by the hypophosphorylated form of the translational suppressor, eIF4E binding protein 1 (4E-BP1). The 70-kDa ribosomal protein S6 kinase (S6K1) was also dephosphorylated, notably at T389, in response to glucocorticoids. Leucine, administered orally, effectively restored each aforementioned translational parameter to control levels. Inasmuch as leucine's potency in modulation of the translational machinery, and indeed of protein turnover in general, is widely appreciated, this amino acid may prove useful in normalizing the impairment of mRNA translation associated with various muscle-wasting pathologies, such as glucocorticoid excess.
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Affiliation(s)
- O J Shah
- Department of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA
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Kimball SR, Jefferson LS, Nguyen HV, Suryawan A, Bush JA, Davis TA. Feeding stimulates protein synthesis in muscle and liver of neonatal pigs through an mTOR-dependent process. Am J Physiol Endocrinol Metab 2000; 279:E1080-7. [PMID: 11052963 DOI: 10.1152/ajpendo.2000.279.5.e1080] [Citation(s) in RCA: 85] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Protein synthesis is repressed in both skeletal muscle and liver after a short-term fast and is rapidly stimulated in response to feeding. Previous studies in rats and pigs have shown that the feeding-induced stimulation of protein synthesis is associated with activation of the 70-kDa ribosomal protein S6 kinase (S6K1) as well as enhanced binding of eukaryotic initiation factor eIF4E to eIF4G to form the active eIF4F complex. In cells in culture, hormones and nutrients regulate both of these events through a protein kinase termed the mammalian target of rapamycin (mTOR). In the present study, the involvement of mTOR in the feeding-induced stimulation of protein synthesis in skeletal muscle and liver was examined. Pigs at 7 days of age were fasted for 18 h, and then one-half of the animals were fed. In addition, one-half of the animals in each group were administered rapamycin (0.75 mg/kg) 2 h before feeding. The results reveal that treating 18-h fasted pigs with rapamycin, a specific inhibitor of mTOR, before feeding prevented the activation of S6K1 and the changes in eIF4F complex formation observed in skeletal muscle and liver after feeding. Rapamycin also ablated the feeding-induced stimulation of protein synthesis in liver. In contrast, in skeletal muscle, rapamycin attenuated, but did not prevent, the stimulation of protein synthesis in response to feeding. The results suggest that feeding stimulates hepatic protein synthesis through an mTOR-dependent process involving enhanced eIF4F complex formation and activation of S6K1. However, in skeletal muscle, these two processes may account for only part of the stimulation of protein synthesis, and thus additional steps may be involved in the response.
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Affiliation(s)
- S R Kimball
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA.
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Shah OJ, Kimball SR, Jefferson LS. Glucocorticoids abate p70(S6k) and eIF4E function in L6 skeletal myoblasts. Am J Physiol Endocrinol Metab 2000; 279:E74-82. [PMID: 10893325 DOI: 10.1152/ajpendo.2000.279.1.e74] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
The catabolic properties of glucocorticoid hormones are largely attributable to dual regulation of protein degradation and synthesis. With regard to the latter, glucocorticoids modulate the translational machinery, namely that component functional in translation initiation. This investigation revealed that in L6 myoblasts, dexamethasone, a synthetic glucocorticoid, deactivated the ribosomal protein S6 kinase (p70(S6k)) within 4 h, as evidenced by diminished phosphorylation of its physiological substrate, the 40S ribosomal protein S6. This deactivation correlated with dephosphorylation of p70(S6k) at Thr(389), whereas phosphorylation of Ser(411) was unaffected. Furthermore, glucocorticoid administration induced dephosphorylation of the cap-dependent translational repressor, eukaryotic initiation factor 4E (eIF4E) binding protein 1 (4E-BP1), thereby facilitating conjunction of the inhibitor and eIF4E. The mechanism of action is reminiscent of classical transcriptional regulation by steroid hormone receptors in that these effects were preceded by a temporal lag and were sensitive to inhibitors of glucocorticoid receptor function as well as transcriptional and translational inhibition. Okadaic acid and calyculin A corrected the dexamethasone-induced dephosphorylation of p70(S6k) and 4E-BP1, implicating a PP1- and/or PP2A-like protein phosphatase(s) in the observed phenomena. Hence, glucocorticoids attenuate distal constituents of the phosphatidylinositol-3 kinase signaling pathway and thereby encumber the protein synthetic apparatus.
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Affiliation(s)
- O J Shah
- Department of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA
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Svanberg E, Ohlsson C, Kimball SR, Lundholm K. rhIGF-I/IGFBP-3 complex, but not free rhIGF-I, supports muscle protein biosynthesis in rats during semistarvation. Eur J Clin Invest 2000; 30:438-46. [PMID: 10809904 DOI: 10.1046/j.1365-2362.2000.00652.x] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
BACKGROUND The aim of this study was to evaluate the effect of insulin like growth factor-I (rhIGF-I) in complex with binding protein 3 (IGFBP 3) compared to the effect of free IGF-I on muscle protein biosynthesis in undernourished animals. METHODS Three groups of female Sprague-Dawley rats (200 g) were initially semi-starved for 3 days and then treated with saline (controls), rhIGF-I (1 microg g-1) or equimolar amounts of rhIGF-I/rhIGFBP-3 complex (5 microg g-1) i.v. twice daily for 3 days during continuous semistarvation. Protein metabolism in hind limb skeletal muscle was studied by incorporation of L-[14C-U]phenylalanine into proteins, western blot determination of translation initiation factors involved in the binding of the 40S ribosomal subunit to mRNA, and quantification of mRNA content for IGF-I, IGF-IR and GH-R. Plasma measurements of insulin, IGF-I and amino acids were also performed. RESULTS rhIGF-I/rhIGFBP-3, but not rhIGF-I alone, stimulated protein synthesis by 177 +/- 26% (P </= 0.05) in semi-starved rats. This stimulation was associated with dissociation of the 4E-BP1. eIF-4E complex, implicating increased binding of the 40S ribosomal subunit to mRNA, and hence increased initiation of protein synthesis in these animals. Muscle content of IGF-I mRNA was reduced in semi-starved animals, whereas IGF-I receptor mRNA was unaltered despite food restriction. Plasma concentration of IGF-I was 20% (P </= 0.05) higher in rhIGF-I/rhIGFBP-3 treated animals as compared to rats treated with saline or free IGF-I. Plasma concentrations of amino acids were increased in rhIGF-I/rhIGFBP-3 treated animals (P </= 0.05 vs. semi-starved controls). CONCLUSION rhIGF-I/rhIGFBP-3 (SomatoKine) was a significant stimulator of muscle protein synthesis in chronically semi-starved animals whereas IGF-I alone failed to increase protein synthesis during the same experimental conditions. This stimulation was because of increased initiation of translation, likely induced by more physiologic concentrations/kinetics of plasma IGF-I and amino acids following rhIGF-I/rhIGFBP-3 treatment, compared to IGF-I in its free form.
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Shah OJ, Kimball SR, Jefferson LS. Among translational effectors, p70S6k is uniquely sensitive to inhibition by glucocorticoids. Biochem J 2000; 347:389-97. [PMID: 10749668 PMCID: PMC1220971 DOI: 10.1042/0264-6021:3470389] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Fundamental cellular processes such as cell differentiation and growth, apoptosis and cellular metabolism are regulated differentially by glucocorticoid hormones in a cell-context-related fashion. However, these basic processes are not governed by isolated signals but are influenced by the integration of both synergistic and antagonistic extracellular and intracellular stimuli. Because glucocorticoids and insulin-like growth factor I (IGF-I) reciprocally modulate growth-regulated processes such as translation initiation, especially in skeletal muscle, a study was undertaken to address the nature of this counter-regulation. Quiescent L6 skeletal myoblasts pretreated for 4 h with the synthetic glucocorticoid dexamethasone exhibited a marked attenuation of IGF-I-induced activation of the ribosomal protein S6 kinase (p70(S6k)). The adverse effects of glucocorticoids on the activity of the endogenous enzyme were due to differential dephosphorylation at discrete residues, suggesting that, physiologically, some but not all phosphorylation sites are subject to mitogenic regulation. Furthermore, the translational repressor eIF4E-binding protein 1 (4E-BP1), which in many circumstances is co-ordinately regulated with p70(S6k), was dephosphorylated in response to glucocorticoids; however, hyperphosphorylation of the protein after stimulation with IGF-I was refractory to inhibition by glucocorticoids, as was its dissociation from its binding partner, eIF4E. Although both basal and IGF-I-stimulated rates of protein synthesis were modestly affected by glucocorticoids, the synthesis of EF1A, whose mRNA precursor is a prototype for the terminal oligopyrimidine ('TOP') transcript family and whose expression is controlled by the activity of p70(S6k), was markedly affected. Therefore in this cell system it seems that, despite the mutual control of p70(S6k) and 4E-BP1 that is often observed, p70(S6k) is more sensitive to down-regulation by glucocorticoids under growth-promoting conditions than is 4E-BP1.
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Affiliation(s)
- O J Shah
- Department of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, P.O. Box 850, Hershey, PA 17033, USA
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Farrell PA, Hernandez JM, Fedele MJ, Vary TC, Kimball SR, Jefferson LS. Eukaryotic initiation factors and protein synthesis after resistance exercise in rats. J Appl Physiol (1985) 2000; 88:1036-42. [PMID: 10710401 DOI: 10.1152/jappl.2000.88.3.1036] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Translational control of protein synthesis depends on numerous eukaryotic initiation factors (eIFs) and we have previously shown (Am. J. Physiol. Endocrinol. Metab. 276: E721-E727, 1999) that increases in one factor, eIF2B, are associated with increases in rates of protein synthesis after resistance exercise in rats. In the present study we investigated whether the eIF4E family of initiation factors is also involved with an anabolic response to exercise. Male Sprague-Dawley rats either remained sedentary (n = 6) or performed acute resistance exercise (n = 6), and rates of protein synthesis were assessed in vivo 16 h after the last session of resistance exercise. eIF4E complexed to eIF4G (eIF4E x eIF4G), eIF4E binding protein 1 (4E-BP1) complexed to eIF4E, and phosphorylation state of eIF4E and 4E-BP1 (gamma-form) were assessed in gastrocnemius. Rates of protein synthesis were higher in exercised rats compared with sedentary rats [205 +/- 8 (SE) vs. 164 +/- 5.5 nmol phenylalanine incorporated x g muscle(-1) x h(-1), respectively; P < 0.05]. Arterial plasma insulin concentrations were not different between the two groups. A trend (P = 0.09) for an increase in eIF4E x eIF4G with exercise was noted; however, no statistically significant differences were observed in any of the components of the eIF4E family in response to resistance exercise. These new data, along with our previous report on eIF2B, suggest that the regulation of peptide chain initiation after exercise is more dependent on eIF2B than on the eIF4E system.
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Affiliation(s)
- P A Farrell
- Noll Physiological Research Center and Graduate Program in Physiology, Pennsylvania State University, University Park 16802, USA.
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Lang CH, Frost RA, Kumar V, Wu D, Vary TC. Impaired Protein Synthesis Induced by Acute Alcohol Intoxication Is Associated With Changes in eIF4E in Muscle and eIF2B in Liver. Alcohol Clin Exp Res 2000. [DOI: 10.1111/j.1530-0277.2000.tb04614.x] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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Shah OJ, Kimball SR, Jefferson LS. Acute attenuation of translation initiation and protein synthesis by glucocorticoids in skeletal muscle. Am J Physiol Endocrinol Metab 2000; 278:E76-82. [PMID: 10644539 DOI: 10.1152/ajpendo.2000.278.1.e76] [Citation(s) in RCA: 90] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
Glucocorticoids are diabetogenic factors that not only antagonize the action of insulin in target tissues but also render these tissues catabolic. Therefore, in rats, we endeavored to characterize the effects in skeletal muscle of glucocorticoids on translation initiation, a regulated process that, in part, governs overall protein synthesis through the modulated activities of eukaryotic initiation factors (eIFs). Four hours after intraperitoneal administration of dexamethasone (100 microg/100 g body wt), protein synthesis in skeletal muscle was reduced to 59% of the value recorded in untreated control animals. Furthermore, translation initiation factor eIF4E preferred association with its endogenous inhibitor 4E-BP1 rather than eIF4G. Dexamethasone treatment resulted in dephosphorylation of both 4E-BP1 and the 40S ribosomal protein S6 kinase concomitant with enhanced phosphorylation of eIF4E. Moreover, the guanine nucleotide exchange activity of eIF2B was unaffected as was phosphorylation of the alpha-subunit of eIF2. Hence glucocorticoids negatively modulate the activation of a subset of the protein synthetic machinery, thereby contributing to the catabolic properties of this class of hormones in vivo.
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Affiliation(s)
- O J Shah
- Department of Cellular and Molecular Physiology, The Pennsylvania State University, College of Medicine, Hershey, Pennsylvania 17033, USA
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29
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Lang CH, Wu D, Frost RA, Jefferson LS, Vary TC, Kimball SR. Chronic alcohol feeding impairs hepatic translation initiation by modulating eIF2 and eIF4E. THE AMERICAN JOURNAL OF PHYSIOLOGY 1999; 277:E805-14. [PMID: 10567006 DOI: 10.1152/ajpendo.1999.277.5.e805] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The present study examined potential cellular mechanisms responsible for the inhibition of protein synthesis in liver after chronic alcohol consumption. Rats were maintained on an alcohol-containing diet for 14 wk; control animals were fed isocalorically. Hepatic ATP content was not different in alcohol-fed and control animals. No alcohol-induced reduction in total hepatic RNA content (an estimate of ribosomal RNA) was detected, suggesting that alcohol decreased translational efficiency. Alcohol feeding increased the proportion of 40S and 60S ribosomal subunits in the nonpolysome-associated fraction by 30%. To identify mechanisms responsible for the impairment in initiation, several eukaryotic initiation factors (eIF) were analyzed. Alcohol feeding decreased hepatic eIF2B activity by 36%. This reduction was associated with a 20% decrease in eIF2Bepsilon content and a 90% increase in eIF2alpha phosphorylation. Alcohol also dramatically influenced the distribution of eIF4E. Compared with pair-fed control values, alcohol feeding increased the amount of eIF4E present in the inactive 4E-binding protein 1 (4E-BP1). eIF4E complex by 80% and decreased binding of eIF4G to eIF4E by 70%. However, the phosphorylation status of 4E-BP1 and eIF4E was not altered by alcohol. Although the plasma concentrations of threonine, proline, and citrulline were mildly decreased, the circulating amount of total amino acids was not altered by alcohol feeding. In summary, these data suggest that chronic alcohol consumption impairs translation initiation in liver by altering eIF2B activity as well as eIF4F function via changes in eIF4E availability.
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Affiliation(s)
- C H Lang
- Department of Cellular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA.
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Karpa KD, Lidow MS, Pickering MT, Levenson R, Bergson C. N-linked glycosylation is required for plasma membrane localization of D5, but not D1, dopamine receptors in transfected mammalian cells. Mol Pharmacol 1999; 56:1071-8. [PMID: 10531415 DOI: 10.1124/mol.56.5.1071] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023] Open
Abstract
We have analyzed the role of N-linked glycosylation in functional cell surface expression of the D1 and D5 dopamine receptor subtypes. Treatment of transfected HEK 293 cells with tunicamycin, an inhibitor of N-linked oligosaccharide addition, was found to prevent localization of D5 receptors in the plasma membrane. In contrast, tunicamycin treatment had no effect on the plasma membrane localization of the D1 receptor. Polymerase chain reaction mutagenesis was used to generate a panel of D5 receptors containing mutations in the three predicted sites of N-linked glycosylation. Expression of mutant receptors indicated that glycosylation of residue N7 was the major determinant of D5 receptor plasma membrane localization. Mutation of a comparable site in the D1 receptor at position N5 had no effect on the delivery of the D1 receptor to the cell surface. Tunicamycin treatment during receptor biosynthesis, but not N-glycosidase F digestion of mature receptors, abrogated binding of the D5 receptor antagonist [(3)H]SCH23390, suggesting that while oligosaccharide moieties play a key role in the cell surface expression of D5 receptors, they do not appear to contribute to the receptor's ligand binding properties. Together, our data indicate a differential requirement for N-linked glycosylation in functional cell surface expression of D1 and D5 dopamine receptors.
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Affiliation(s)
- K D Karpa
- Department of Pharmacology, Penn State College of Medicine, Milton S. Hershey Medical Center, Hershey, Pennsylvania 17033, USA.
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Lang CH, Wu D, Frost RA, Jefferson LS, Kimball SR, Vary TC. Inhibition of muscle protein synthesis by alcohol is associated with modulation of eIF2B and eIF4E. THE AMERICAN JOURNAL OF PHYSIOLOGY 1999; 277:E268-76. [PMID: 10444422 DOI: 10.1152/ajpendo.1999.277.2.e268] [Citation(s) in RCA: 67] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The present study examined potential mechanisms for the inhibition of protein synthesis in skeletal muscle after chronic alcohol consumption. Rats were maintained on an alcohol-containing diet for 14 wk; control animals were pair fed. Alcohol-induced myopathy was confirmed by a reduction in lean body mass as well as a decrease in the weight of the gastrocnemius and psoas muscles normalized for tibial length. No alcohol-induced decrease in total RNA content (an estimate of ribosomal RNA) was detected in any muscle examined, suggesting that alcohol reduced translational efficiency but not the capacity for protein synthesis. To identify mechanisms responsible for regulating translational efficiency, we analyzed several eukaryotic initiation factors (eIF). There was no difference in the muscle content of either total eIF2alpha or the amount of eIF2alpha in the phosphorylated form between alcohol-fed and control rats. Similarly, the relative amount of eIF2Bepsilon in muscle was also not different. In contrast, alcohol decreased eIF2B activity in psoas (fast-twitch) but not in soleus or heart (slow-twitch) muscles. Alcohol feeding also dramatically influenced the distribution of eIF4E in the gastrocnemius (fast-twitch) muscle. Compared with control values, muscle from alcohol-fed rats demonstrated 1) an increased binding of the translational repressor 4E-binding protein 1 (4E-BP1) with eIF4E, 2) a decrease in the phosphorylated gamma-form of 4E-BP1, and 3) a decrease in eIF4G associated with eIF4E. In summary, these data suggest that chronic alcohol consumption impairs translation initiation in muscle by altering multiple regulatory sites, including eIF2B activity and eIF4E availability.
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Affiliation(s)
- C H Lang
- Departments of Cellular and Molecular Physiology and Surgery, Pennsylvania State College of Medicine, Hershey, Pennsylvania 17033, USA.
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Kimball SR, Shantz LM, Horetsky RL, Jefferson LS. Leucine regulates translation of specific mRNAs in L6 myoblasts through mTOR-mediated changes in availability of eIF4E and phosphorylation of ribosomal protein S6. J Biol Chem 1999; 274:11647-52. [PMID: 10206976 DOI: 10.1074/jbc.274.17.11647] [Citation(s) in RCA: 277] [Impact Index Per Article: 11.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Regulation of translation of mRNAs coding for specific proteins plays an important role in controlling cell growth, differentiation, and transformation. Two proteins have been implicated in the regulation of specific mRNA translation: eukaryotic initiation factor eIF4E and ribosomal protein S6. Increased phosphorylation of eIF4E as well as its overexpression are associated with stimulation of translation of mRNAs with highly structured 5'-untranslated regions. Similarly, phosphorylation of S6 results in preferential translation of mRNAs containing an oligopyrimidine tract at the 5'-end of the message. In the present study, leucine stimulated phosphorylation of the eIF4E-binding protein, 4E-BP1, in L6 myoblasts, resulting in dissociation of eIF4E from the inactive eIF4E.4E-BP1 complex. The increased availability of eIF4E was associated with a 1.6-fold elevation in ornithine decarboxylase relative to global protein synthesis. Leucine also stimulated phosphorylation of the ribosomal protein S6 kinase, p70(S6k), resulting in increased phosphorylation of S6. Hyperphosphorylation of S6 was associated with a 4-fold increase in synthesis of elongation factor eEF1A. Rapamycin, an inhibitor of the protein kinase mTOR, prevented all of the leucine-induced effects. Thus, leucine acting through an mTOR-dependent pathway stimulates the translation of specific mRNAs both by increasing the availability of eIF4E and by stimulating phosphorylation of S6.
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Affiliation(s)
- S R Kimball
- Department of Cellular and Molecular Physiology, The Pennsylvania State University, College of Medicine, Hershey, Pennsylvania 17033, USA.
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Cooney R, Kimball SR, Eckman R, Maish G, Shumate M, Vary TC. TNF-binding protein ameliorates inhibition of skeletal muscle protein synthesis during sepsis. THE AMERICAN JOURNAL OF PHYSIOLOGY 1999; 276:E611-9. [PMID: 10198295 DOI: 10.1152/ajpendo.1999.276.4.e611] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
We examined the effects of TNF-binding protein (TNFBP) on regulatory mechanisms of muscle protein synthesis during sepsis in four groups of rats: Control; Control+TNFBP; Septic; and Septic+TNFBP. Saline (1. 0 ml) or TNFBP (1 mg/kg, 1.0 ml) was injected daily starting 4 h before the induction of sepsis. The effect of TNFBP on gastrocnemius weight, protein content, and the rate of protein synthesis was examined 5 days later. Sepsis reduced the rate of protein synthesis by 35% relative to controls by depressing translational efficiency. Decreases in protein synthesis were accompanied by similar reductions in protein content and muscle weight. Treatment of septic animals with TNFBP for 5 days prevented the sepsis-induced inhibition of protein synthesis and restored translational efficiency to control values. TNFBP treatment of Control rats for 5 days was without effect on muscle protein content or protein synthesis. We also assessed potential mechanisms regulating translational efficiency. The phosphorylation state of p70(S6) kinase was not altered by sepsis. Sepsis reduced the gastrocnemius content of eukaryotic initiation factor 2Bepsilon (eIF2Bepsilon), but not eIF2alpha. The decrease in eIF2Bepsilon content was prevented by treatment of septic rats with TNFBP. TNFBP ameliorates the sepsis-induced changes in protein metabolism in gastrocnemius, indicating a role for TNF in the septic process. The data suggest that TNF may impair muscle protein synthesis by reducing expression of specific initiation factors during sepsis.
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Affiliation(s)
- R Cooney
- Department of Cellular and Molecular Physiology and Department of Surgery, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA
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Jefferson LS, Fabian JR, Kimball SR. Glycogen synthase kinase-3 is the predominant insulin-regulated eukaryotic initiation factor 2B kinase in skeletal muscle. Int J Biochem Cell Biol 1999; 31:191-200. [PMID: 10216953 DOI: 10.1016/s1357-2725(98)00141-1] [Citation(s) in RCA: 87] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Eukaryotic initiation factor eIF2B is a guanine nucleotide exchange protein involved in regulation of translation initiation. Phosphorylation of the epsilon-subunit is thought to be important in insulin-mediated changes in eIF2B activity. However, elucidation of insulin's action has proven elusive, primarily because eIF2B epsilon is a substrate in vitro for at least three different protein kinases. In the present study, we observed changes in eIF2B epsilon kinase activity only in those muscles previously shown to exhibit alterations in protein synthesis in response to insulin. Specifically, eIF2B epsilon kinase activity was increased in psoas muscle from diabetic rats compared to controls. Treating diabetic rats with insulin rapidly reduced eIF2B epsilon kinase activity below control values. Changes were not observed in heart. To identify the kinase(s) in psoas responsible for phosphorylating eIF2B epsilon, the wildtype and two variant forms of the epsilon-subunit were expressed in and purified from Sf9 insect cells, and were used as substrates in protein kinase assays. The first variant contained a point mutation in the eIF2B epsilon cDNA that converted the glycogen synthase kinase-3 (GSK-3) phosphorylation site, Ser535, to a nonphosphorylatable Ala residue. In the second variant, the putative GSK-3 'priming' site, Ser539, was converted to Asp. Based on the pattern of phosphorylation of the wildtype and two variant forms of eIF2B epsilon using casein kinase (CK)-I, CK-II, or GSK-3 as well as that observed with skeletal muscle extracts, we conclude that the predominant eIF2B epsilon kinase in psoas muscle is GSK-3. Thus, insulin-mediated changes in eIF2B activity are likely to involve GSK-3.
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Affiliation(s)
- L S Jefferson
- Department of Cellular and Molecular Physiology, Pennsylvania State University, College of Medicine, Hershey 17033, USA.
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Kimball SR, Horetsky RL, Jefferson LS. Implication of eIF2B rather than eIF4E in the regulation of global protein synthesis by amino acids in L6 myoblasts. J Biol Chem 1998; 273:30945-53. [PMID: 9812990 DOI: 10.1074/jbc.273.47.30945] [Citation(s) in RCA: 136] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The present study was designed to investigate the mechanism through which leucine and histidine regulate translation initiation in L6 myoblasts. The results show that both amino acids stimulate initiation and coordinately regulate the activity of eukaryotic initiation factor eIF2B. The changes in eIF2B activity could be explained in part by modulation of the phosphorylation state of the alpha-subunit of eIF2. The activity changes might also be a result of modulation of the phosphorylation state of the eIF2B epsilon-subunit, because deprivation of either amino acid caused a decrease in eIF2Bepsilon kinase activity. Leucine, but not histidine, additionally caused a redistribution of eIF4E from the inactive eIF4E.4E-BP1 complex to the active eIF4E.eIF4G complex. The redistribution was a result of increased phosphorylation of 4E-BP1. The changes in 4E-BP1 phosphorylation and eIF4E redistribution associated with leucine deprivation were not observed in the presence of insulin. However, the leucine- and histidine-induced alterations in global protein synthesis and eIF2B activity were maintained in the presence of the hormone. Overall, the results suggest that both leucine and histidine regulate global protein synthesis through modulation of eIF2B activity. Furthermore, under the conditions employed herein, alterations in eIF4E availability are not rate-controlling for global protein synthesis but might be necessary for regulation of translation of specific mRNAs.
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Affiliation(s)
- S R Kimball
- Department of Cellular and Molecular Physiology, The Pennsylvania State University, College of Medicine, Hershey, Pennsylvania 17033, USA.
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36
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Fabian JR, Kimball SR, Jefferson LS. Reconstitution and purification of eukaryotic initiation factor 2B (eIF2B) expressed in Sf21 insect cells. Protein Expr Purif 1998; 13:16-22. [PMID: 9631509 DOI: 10.1006/prep.1998.0860] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Eukaryotic initiation factor eIF2B plays a key role in the regulation of protein synthesis through its ability to catalyze the exchange of GDP bound to a second initiation factor, eIF2, for free GTP. In contrast to other GDP-GTP exchange factors (GEFs), which are often single subunit proteins, eIF2B consists of five dissimilar subunits. In the studies reported here the baculovirus expression vector system (BEVS) was used to express FLAG epitope tagged alleles for the alpha, beta, gamma, delta, and epsilon subunits of rat eIF2B in Sf21 cells. The eIF2B holoprotein was reconstituted in vivo by coexpression of all five subunits in Sf21 cells and was subsequently purified to greater than 98% homogeneity using a two-step procedure involving an anti-FLAG immunoaffinity column followed by gel filtration chromatography. The purified five-subunit eIF2B complex had high GEF activity as assayed by using [3H]GDP-bound to eIF2 as a substrate. Alternatively, eIF2B with high GEF activity was reconstituted in vitro by mixing crude cell lysates containing different eIF2B subunits. The latter results suggest that eIF2B activity in vivo could involve alterations in the concentration and/or the availability of individual subunits for holoprotein assembly. Overall, the results show the utility of the baculovirus-insect cell system for the expression, assembly, and purification of active recombinant multisubunit factors.
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Affiliation(s)
- J R Fabian
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey 17033, USA
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Kimball SR, Heinzinger NK, Horetsky RL, Jefferson LS. Identification of interprotein interactions between the subunits of eukaryotic initiation factors eIF2 and eIF2B. J Biol Chem 1998; 273:3039-44. [PMID: 9446619 DOI: 10.1074/jbc.273.5.3039] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Modulation of protein/protein interaction is an important mechanism involved in regulation of translation initiation. Specifically, regulation of the interaction of eIF2 with the guanine nucleotide exchange factor, eIF2B, is a key mechanism for controlling translation under a variety of conditions. Phosphorylation of the alpha-subunit of eIF2 converts the protein into a competitive inhibitor of eIF2B by causing an increase in the binding affinity of eIF2B for eIF2. Consequently, it has been assumed that the alpha-subunit of eIF2 is directly involved in binding to eIF2B. In the present study, eIF2 was found to bind only to the delta- and epsilon-subunits of eIF2B, and eIF2B was shown to bind only to the beta-subunit of eIF2 by far-Western blot analysis. The binding site on eIF2beta for either the eIF2B holoprotein, or the isolated delta- or epsilon-subunits of eIF2B was shown to be located within approximately 70 amino acids of the C terminus of the protein. Phosphorylation of the alpha-subunit of eIF2 did not promote binding of eIF2B to the isolated subunit. However, it did cause an increase in the affinity of eIF2B for eIF2. Finally, phosphorylation by protein kinase A of the beta-subunit of eIF2 in the C-terminal portion of the protein increased the guanine nucleotide exchange activity of eIF2B, whereas phosphorylation by casein kinase II or protein kinase C was without effect.
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Affiliation(s)
- S R Kimball
- Department of Cellular and Molecular Physiology, The Pennsylvania State University, College of Medicine, Hershey, Pennsylvania 17033, USA.
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Kimball SR, Horetsky RL, Jefferson LS. Signal transduction pathways involved in the regulation of protein synthesis by insulin in L6 myoblasts. THE AMERICAN JOURNAL OF PHYSIOLOGY 1998; 274:C221-8. [PMID: 9458731 DOI: 10.1152/ajpcell.1998.274.1.c221] [Citation(s) in RCA: 121] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
The phosphorylation states of three proteins implicated in the action of insulin on translation were investigated, i.e., 70-kDa ribosomal protein S6 kinase (p70S6k), eukaryotic initiation factor (eIF) 4E, and the eIF-4E binding protein 4E-BP1. Addition of insulin caused a stimulation of protein synthesis in L6 myoblasts in culture, an effect that was blocked by inhibitors of phosphatidylinositide-3-OH kinase (wortmannin), p70S6k (rapamycin), and mitogen-activated protein kinase (MAP kinase) kinase (PD-98059). The stimulation of protein synthesis was accompanied by increased phosphorylation of p70S6k, an effect that was blocked by rapamycin and wortmannin but not PD-98059. Insulin caused dephosphorylation of eIF-4E, an effect that appeared to be mediated by the p70S6k pathway. Insulin also stimulated phosphorylation of 4E-BP1 as well as dissociation of the 4E-BP1.eIF-4E complex. Both rapamycin and wortmannin completely blocked the insulin-induced changes in 4E-BP1 phosphorylation and association of 4E-BP1 and eIF-4E; PD-98059 had no effect on either parameter. Finally, insulin stimulated formation of the active eIF-4G.eIF-4E complex, an effect that was not prevented by any of the inhibitors. Overall, the results suggest that insulin stimulates protein synthesis in L6 myoblasts in part through utilization of both the p70S6k and MAP kinase signal transduction pathways.
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Affiliation(s)
- S R Kimball
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey 17033, USA
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Yoshizawa F, Kimball SR, Jefferson LS. Modulation of translation initiation in rat skeletal muscle and liver in response to food intake. Biochem Biophys Res Commun 1997; 240:825-31. [PMID: 9398653 DOI: 10.1006/bbrc.1997.7652] [Citation(s) in RCA: 66] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Protein synthesis is altered in both skeletal muscle and liver in response to nutritional status with food deprivation being associated with an inhibition of mRNA translation. In the present study, the effect of food-intake on the initiation of mRNA translation was examined in rats fasted for 18-h and then refed a complete diet. Fasting and refeeding caused alterations in translation initiation in both skeletal muscle and liver that were not associated with any detectable changes in the activity of eIF2B or in the phosphorylation state of eIF2 alpha. Instead, alterations in initiation were associated with changes in the phosphorylation state of eIF4E and/or the association of eIF4E with eIF4G as well as the eIF4E binding protein, 4E-BP1. In muscle from fasted rats, the amount of eIF4E present in an inactive complex with 4E-BP1 was increased 5-fold compared to freely fed control animals. One hour after refeeding a complete diet, the amount of 4E-BP1 bound to eIF4E was reduced to freely fed control values. Reduced association of the two proteins was the result of increased phosphorylation of 4E-BP1. Refeeding a complete diet also stimulated the binding of eIF4E to eIF4G to form the active eIF4F complex. In liver, the amount of eIF4E associated with eIF4G, but not the amount of eIF4E associated with 4E-BP1, was altered by fasting and refeeding. Furthermore, in liver, but not in skeletal muscle, fasting and refeeding resulted in modulation of the phosphorylation state of eIF4E. Overall, the results suggest that protein synthesis may be differentially regulated in muscle and liver in response to fasting and refeeding. In muscle, protein synthesis is regulated through modulation of the binding of eIF4E to eIF4G and in liver through modulation of both phosphorylation of eIF4E as well as binding of eIF4E to eIF4G.
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Affiliation(s)
- F Yoshizawa
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey 17033, USA
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40
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Brostrom CO, Brostrom MA. Regulation of translational initiation during cellular responses to stress. PROGRESS IN NUCLEIC ACID RESEARCH AND MOLECULAR BIOLOGY 1997; 58:79-125. [PMID: 9308364 DOI: 10.1016/s0079-6603(08)60034-3] [Citation(s) in RCA: 207] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Chemicals and conditions that damage proteins, promote protein misfolding, or inhibit protein processing trigger the onset of protective homeostatic mechanisms resulting in "stress responses" in mammalian cells. Included in these responses are an acute inhibition of mRNA translation at the initiation step, a subsequent induction of various protein chaperones, and the recovery of mRNA translation. Separate, but closely related, stress response systems exist for the endoplasmic reticulum (ER), relating to the induction of specific "glucose-regulated proteins" (GRPs), and for the cytoplasm, pertaining to the induction of the "heat shock proteins" (HSPs). Activators of the ER stress response system, including Ca(2+)-mobilizing and thiol-reducing agents, are discussed and compared to activators of the cytoplasmic stress system, such as arsenite, heavy metal cations, and oxidants. An emerging integrative literature is reviewed that relates protein chaperones associated with cellular stress response systems to the coordinate regulation of translational initiation and protein processing. Background information is presented describing the roles of protein chaperones in the ER and cytoplasmic stress response systems and the relationships of chaperones and protein processing to the regulation of mRNA translation. The role of chaperones in regulating eIF-2 alpha kinase activities, eIF-2 cycling, and ribosomal loading on mRNA is emphasized. The putative role of GRP78 in coupling rates of translation to processing is modeled, and functional relationships between the HSP and GRP chaperone systems are discussed.
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Affiliation(s)
- C O Brostrom
- Department of Pharmacology, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, Piscataway 08854, USA
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Atchuta Ramaiah KV, Chen JJ, Gallop PM, London IM. The effects of pyrroloquinoline quinone on heme-regulated eIF-2alpha kinase and eIF-2B activities in eukaryotic protein synthesis. Blood Cells Mol Dis 1997; 23:177-87. [PMID: 9236156 DOI: 10.1006/bcmd.1997.0135] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Pyrroloquinoline quinone (PQQ), a novel cofactor of biological redox processes, is ubiquitous in animal cells. We have examined the effects of PQQ on protein synthesis. PQQ inhibits protein synthesis in hemin-supplemented rabbit reticulocyte lysates. This inhibition is characterized by increased phosphorylation of eIF-2alpha and by diminished guanine nucleotide exchange activity of eIF-2B. The increased eIF-2alpha phosphorylation is the result of activation by PQQ of the heme-regulated eIF-2alpha kinase (HRI). The addition of 10 microM PQQ completely inhibits the increase in protein synthesis that occurs on the addition of hemin (20 microM) to heme-deficient lysates, whereas a lower concentration of PQQ (100 nM) causes a very slight stimulation of protein synthesis. The increased eIF-2alpha phosphorylation that occurs at high concentrations of PQQ inhibits eIF-2B activity, presumably due to formation of a 15S complex [eIF-2(alphaP).eIF-2B] in which eIF-2B becomes non-functional. Low concentrations of PQQ (0.1-1 microM) do not affect eIF-2alpha phosphorylation, but rather enhance the guanine nucleotide exchange activity of eIF-2B in reticulocyte lysates. In Chinese hamster ovary cell extract which is devoid of significant eIF-2alpha kinase activity, addition of both low and high concentrations of PQQ results in an increase in eIF-2B activity. The addition of PQQ to reticulocyte lysates activates HRI whereas addition of PQQ to purified HRI in vitro inhibits the autokinase and eIF-2alpha kinase activity of the HRI; the inhibition of purified HRI by PQQ is observed both in the presence and absence of hemin. These findings suggest that PQQ inhibits purified HRI by acting as an oxidant whereas in lysates in which PQQ is readily reduced, the PQQ acts as a reductant and increases the activities of both HRI and eIF-2B.
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Affiliation(s)
- K V Atchuta Ramaiah
- Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, 77 Massachusetts Avenue, E25-551, Cambridge, MA 02139, USA
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Fabian JR, Kimball SR, Heinzinger NK, Jefferson LS. Subunit assembly and guanine nucleotide exchange activity of eukaryotic initiation factor-2B expressed in Sf9 cells. J Biol Chem 1997; 272:12359-65. [PMID: 9139680 DOI: 10.1074/jbc.272.19.12359] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
Eukaryotic initiation factor-2B (eIF-2B) is a guanine nucleotide exchange factor (GEF) that plays a key role in the regulation of protein synthesis. In this study, we have used the baculovirus-infected Sf9 insect cell system to express and characterize the five dissimilar subunits of rat eIF-2B. GEF activity was detected in extracts of Sf9 cells expressing the epsilon-subunit alone and was greatly increased when all five subunits were coexpressed. In addition, high GEF activity was observed in extracts containing a four-subunit complex lacking the alpha-subunit. Assembly of an eIF-2B holoprotein was confirmed by coimmunoprecipitation of all five subunits. Gel filtration chromatography revealed that recombinant eIF-2B had the same molecular mass as eIF-2B purified from rat liver and that it did indeed possess GEF activity. Phosphorylation of the substrate eIF-2 inhibited the GEF activity of the five-subunit eIF-2B; this inhibition required the eIF-2B alpha-subunit. The results demonstrate that eIF-2Balpha functions as a regulatory subunit that is not required for GEF activity, but instead mediates the regulation of eIF-2B by substrate phosphorylation. Furthermore, eIF-2Bepsilon is necessary and is perhaps sufficient for GEF activity in vitro.
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Affiliation(s)
- J R Fabian
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA
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Svanberg E, Jefferson LS, Lundholm K, Kimball SR. Postprandial stimulation of muscle protein synthesis is independent of changes in insulin. THE AMERICAN JOURNAL OF PHYSIOLOGY 1997; 272:E841-7. [PMID: 9176184 DOI: 10.1152/ajpendo.1997.272.5.e841] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Protein synthesis in skeletal muscle is markedly stimulated (approximately 180% of control rate) within 3 h of oral feeding in mice subjected to an overnight fast (18 h). The stimulation of protein synthesis is the result of a faster rate of translation initiation; however, neither the mediators (i.e., hormones or nutrients) nor the mechanisms responsible for the effect of feeding are well understood. Results of the present study revealed that the amount of eukaryotic initiation factor 4E (eIF-4E) present in the phosphorylated form (i.e., 70%) was not changed after overnight starvation or a subsequent 3-h refeeding period compared with muscles from freely fed mice. In contrast, the phosphorylation state of the eIF-4E binding protein 1 (4E-BP1) was changed with nutritional state. Starvation increased the proportion of the unphosphorylated form of 4E-BP1, whereas feeding promoted a shift to the more highly phosphorylated forms of the protein. Moreover, starvation increased the amount of 4E-BP1 recovered by almost threefold, indicative of an increase in the eIF-4E.4E-BP1 complex. The increased association of 4E-BP1 with eIF-4E was completely reversed within 3 h of feeding. Starvation and refeeding also altered the amount of eIF-4G that coimmunoprecipitated with eIF-4E. However, in contrast to the results obtained for 4E-BP1, starvation decreased the amount of eIF-4G recovered in the eIF-4E immunoprecipitate, suggesting that starvation causes a decrease in the formation of the active eIF-4F complex. The alterations in 4E-BP1 phosphorylation and association of 4E-BP1 and eIF-4G with eIF-4E observed in control mice in response to starvation and refeeding were also observed in diabetic mice exhibiting characteristics of type I or type II diabetes subjected to the same conditions, suggesting that insulin alone does not mediate the observed changes. Thus the integrated feeding response represents an important area of investigation for understanding the regulation of translation initiation.
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Affiliation(s)
- E Svanberg
- Department of Surgery, Sahlgrenska University Hospital, University of Göteborg, Sweden
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Kimball SR, Jurasinski CV, Lawrence JC, Jefferson LS. Insulin stimulates protein synthesis in skeletal muscle by enhancing the association of eIF-4E and eIF-4G. THE AMERICAN JOURNAL OF PHYSIOLOGY 1997; 272:C754-9. [PMID: 9124320 DOI: 10.1152/ajpcell.1997.272.2.c754] [Citation(s) in RCA: 145] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Insulin stimulated protein synthesis in gastrocnemius muscle of perfused rat hindlimb preparations by approximately twofold. The stimulation of protein synthesis was associated with a 12-fold increase in the amount of eukaryotic initiation factor eIF-4G bound to the mRNA cap-binding protein eIF-4E. In part, the increased binding of eIF-4G to eIF-4E was a result of release of eIF-4E bound to the translational regulator, PHAS-I, through a mechanism involving enhanced phosphorylation of PHAS-I. However, the insulin-induced association of eIF-4E and eIF-4G was not due to increased net phosphorylation of eIF-4E because insulin decreased the amount present in the phosphorylated form from 86 to 59% of total eIF-4E. Overall, the results suggest that insulin stimulates protein synthesis in gastrocnemius muscle through a mechanism involving increased binding of eIF-4G to eIF-4E, which is in part due to phosphorylation of PHAS-I, resulting in a release of eIF-4E from the inactive PHAS-I x eIF-4E complex.
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Affiliation(s)
- S R Kimball
- Department of Cellular and Molecular Physiology, The Pennsylvania State University, College of Medicine, Hershey 17033, USA
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Price NT, Mellor H, Craddock BL, Flowers KM, Kimball SR, Wilmer T, Jefferson LS, Proud CG. eIF2B, the guanine nucleotide-exchange factor for eukaryotic initiation factor 2. Sequence conservation between the alpha, beta and delta subunits of eIF2B from mammals and yeast. Biochem J 1996; 318 ( Pt 2):637-43. [PMID: 8929216 PMCID: PMC1217679 DOI: 10.1042/bj3180637] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
The guanine nucleotide-exchange factor eIF2B mediates the exchange of GDP bound to translation initiation factor eIF2 for GTP. This exchange process is a key regulatory step for the control of translation initiation in eukaryotic organisms. To improve our understanding of the structure, function and regulation of eIF2B, we have obtained and sequenced cDNA species encoding all of its five subunits. Here we report the sequences of eIF2B beta and delta from rat. This paper focuses on sequence similarities between the alpha, beta and delta subunits of mammalian eIF2B. Earlier work showed that the amino acid sequences of the corresponding subunits of eIF2B in the yeast Saccharomyces cerevisiae (GCN3, GCD7 and GCD2) exhibit considerable similarity. We demonstrate that this is also true for the mammalian subunits. Moreover, alignment of the eIF2B alpha, beta and delta sequences from mammals and yeast, along with the sequence of the putative eIF2B alpha subunit from Caenorhabditis elegans and eIF2B delta from Schizosaccharomyces pombe shows that a large number of residues are identical or conserved between the C-terminal regions of all these sequences. This strong sequence conservation points to the likely functional importance of these residues. The implications of this are discussed in the light of results concerning the functions of the subunits of eIF2B in yeast and mammals. Our results also indicate that the large apparent differences in mobility on SDS/PAGE between eIF2B beta and delta subunits from rat and rabbit are not due to differences in their lengths but reflect differences in amino acid composition. We have also examined the relative expression of mRNA species encoding the alpha, beta, delta and epsilon subunits of eIF2B in a range of rat tissues by Northern blot analysis. As might be expected for mRNA species encoding subunits of a heterotrimeric protein, the ratios of expression levels of these subunits to one another did not vary between the different rat tissues examined (with the possible exception of liver). This represents the first analysis of the levels of expression of mRNA species encoding the different subunits of eIF2B.
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Affiliation(s)
- N T Price
- Department of Biochemistry, University of Bristol, U.K
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Vary TC, Voisin L, Cooney RN. Regulation of peptide-chain initiation in muscle during sepsis by interleukin-1 receptor antagonist. THE AMERICAN JOURNAL OF PHYSIOLOGY 1996; 271:E513-20. [PMID: 8843745 DOI: 10.1152/ajpendo.1996.271.3.e513] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
The mechanism by which interleukin-1 (IL-1) regulates protein synthesis in skeletal muscle during hypermetabolic sepsis in rats was investigated. Treatment of septic rats with a specific interleukin-1 receptor antagonist (IL-1ra) prevented the sepsis-induced inhibition of protein synthesis and translational efficiency in gastrocnemius. Analysis of ribosomal subunits revealed that the increase in free 40S and 60S ribosomal subunits observed in septic rats was prevented by infusion of IL-1ra, indicating peptide-chain initiation was maintained at control values. The failure of sepsis to inhibit peptide-chain initiation after infusion of IL-1ra correlated with a maintenance of the epsilon-subunit of eukaryotic initiation factor (eIF) 2B (eIF-2B epsilon) protein at control values. The alterations in the eIF-2B epsilon protein content in gastrocnemius of septic rats treated with or without IL-1ra were associated with corresponding changes in the abundance of eIF 2B epsilon mRNA. The results provide evidence that infusion of IL-1ra attenuates the sepsis-induced inhibition of protein synthesis by preventing the inhibition of peptide-chain initiation and downregulation of eIF-2B expression during sepsis.
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Affiliation(s)
- T C Vary
- Department of Cellular and Molecular Physiology, Pennsylvania State University, College of Medicine, Hershey 17033, USA
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Price NT, Kimball SR, Jefferson LS, Proud CG. Cloning of cDNA for the gamma-subunit of mammalian translation initiation factor 2B, the guanine nucleotide-exchange factor for eukaryotic initiation factor 2. Biochem J 1996; 318 ( Pt 2):631-6. [PMID: 8809057 PMCID: PMC1217667 DOI: 10.1042/bj3180631] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
Peptide sequence data were obtained from rabbit protein synthesis initiation factor subunit eIF2B gamma. Searching the database of expressed sequence tags (dbEST) revealed nucleotide sequences potentially encoding human eIF2B gamma that contained peptides corresponding to those from the rabbit subunit. PCR primers were derived from these sequences and used to generate a probe. This was used to screen a rat skeletal muscle cDNA library, and a clone encoding rat eIF2B gamma was isolated. This cDNA gave a product in coupled transcription/translation that co-migrated with the gamma-subunit of purified eIF2B under SDS/PAGE. The sequence of this rat eIF2B gamma cDNA is reported. The protein sequence shows homology with that of yeast eIF2B gamma (the GCD1 gene product). We have also identified an open reading frame from the Caenorhabditis elegans genome project that probably encodes the gamma-subunit of C. elegans eIF2B. All these sequences show similarity to nucleotidyl- and acyltransferases, as previously reported for GCD1 [Koonin (1995) Protein Sci. 4, 1608-1617], and contain conserved motifs potentially involved in nucleotide binding. They also contain "I-patch' motifs: isoleucine-rich hexamer repeats that have been associated with the binding of acyl groups in bacterial acyltransferases. The roles of these motifs are discussed in relation to the known properties of eIF2B.
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Affiliation(s)
- N T Price
- Department of Biochemistry, University of Bristol, U.K
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Asuru AI, Mellor H, Thomas NS, Yu L, Chen JJ, Crosby JS, Hartson SD, Kimball SR, Jefferson LS, Matts RL. Cloning and characterization of cDNAs encoding the epsilon-subunit of eukaryotic initiation factor-2B from rabbit and human. BIOCHIMICA ET BIOPHYSICA ACTA 1996; 1307:309-17. [PMID: 8688466 DOI: 10.1016/0167-4781(96)00054-1] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
A rabbit reticulocyte lysate cDNA library was screened with a polyclonal antiserum directed against eukaryotic initiation factor eIF-2B (eIF-2B). A 2508 base pair cDNA (pA1) was isolated and determined to encode the epsilon-subunit of eIF-2B based on the immunoreactivity of the fusion protein expressed from the cDNA in Escherichia coli and the presence of two peptide sequences obtained from two V8 fragments of purified nonrecombinant eIF-2B epsilon in the deduced amino acid sequence of the cDNA. The open reading frame of the cDNA began with the third nucleotide of the cDNA with the first AUG codon at nucleotide 522. Mutational analysis of pA1 indicated that the cDNA did not code for full-length eIF-2B epsilon. Seven missing codons of the reading-frame and the 71 nucleotide 5' non-coding region of the eIF-2B epsilon mRNA were obtained by 5' RACE. A human eIF-2B epsilon cDNA fragment, which corresponded to a similar 2.3 kb fragment generated by digestion of the rabbit pA1 cDNA with EcoRI, was isolated from a human histiocytic lymphoma (U-937) cell cDNA library constructed in lambda gt10. The nucleotide and amino acid sequences were highly conserved between the rabbit and human cDNAs, showing approx. 90% sequence identity within the open reading frame. Northern and Western blot analyses of reticulocyte lysate and other rabbit tissue extracts indicated that the eIF-2B epsilon polypeptide has a similar apparent molecular weight in all tissues examined, and is coded for by a single approximately 2.8 kilobase mRNA species which is ubiquitously expressed.
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Affiliation(s)
- A I Asuru
- Department of Biochemistry and Molecular Biology, Oklahoma State University, Stillwater 74078, USA
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Flowers KM, Mellor H, Matts RL, Kimball SR, Jefferson LS. Cloning and characterization of complementary and genomic DNAs encoding the epsilon-subunit of rat translation initiation factor-2B. BIOCHIMICA ET BIOPHYSICA ACTA 1996; 1307:318-24. [PMID: 8688467 DOI: 10.1016/0167-4781(96)00055-3] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Eukaryotic initiation factor-2B (eIF-2B) is a guanine nucleotide-exchange protein involved in the recycling of eIF-2 during peptide-chain initiation. Regulation of eIF-2B activity occurs under a wide range of conditions by diverse mechanisms. To better understand the regulation of eIF-2B activity as well as the coordinate expression of its five subunits, we have begun to clone and characterize the cDNAs and genes encoding these proteins. In the present study, complementary and genomic DNAs encoding the epsilon-subunit of rat eIF-2B were cloned and characterized. The cDNA is 2517 bp in length, including a 30 nt poly(A) tail, and recognizes both 2.7 and 3.5 kb mRNA species on Northern blots of rat RNA. The cDNA contains a 2151 bp open reading frame encoding 716 amino acids producing a protein with a predicted molecular mass of 80 kDa. The derived amino acid sequence contains regions identical to three peptides obtained from bovine liver eIF-2B epsilon and is 31% identical to Gcd6, the putative yeast eIF-2B epsilon. Examination of the derived amino acid sequence of rat eIF-2B epsilon reveals phosphorylation site motifs for several protein kinases which have been implicated in regulation of guanine nucleotide exchange activity. The mRNA for eIF-2B epsilon is expressed to a similar extent in most rat tissues examined with the exception of testis, where its expression is approx, three-fold greater. We have also isolated and sequenced the coding and 5'-flanking region of the rat eIF-2B epsilon gene. The 16 exons encoding rat eIF-2B epsilon are contained within 9.5 kb of genomic DNA. Examination of the promoter region of the gene reveals a consensus binding site for the alpha-Pal transcription factor as well as possible cytokine-response elements and binding sites for testis-specific transcription factors.
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Affiliation(s)
- K M Flowers
- Department of Cellular and Molecular Physiology, College of Medicine, Pennsylvania State University, Hershey, PA 17033, USA
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Voisin L, Gray K, Flowers KM, Kimball SR, Jefferson LS, Vary TC. Altered expression of eukaryotic initiation factor 2B in skeletal muscle during sepsis. THE AMERICAN JOURNAL OF PHYSIOLOGY 1996; 270:E43-50. [PMID: 8772472 DOI: 10.1152/ajpendo.1996.270.1.e43] [Citation(s) in RCA: 23] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
Sepsis causes an inhibition of protein synthesis in skeletal muscles composed of fast-twitch fibers, in part, as a result of a decreased activity of the eukaryotic initiation factor 2B (eIF-2B). In the present study, we investigated the expression of two subunits of eIF-2B, i.e., the beta- and epsilon-subunits during sepsis. The expression of both beta- and epsilon-subunits of eIF-2B in gastrocnemius was decreased approximately 50% from control values during the first 5 days after induction of sepsis. The decreased expression of eIF-2B epsilon during sepsis correlated with similar reductions in eIF-2B epsilon mRNA. Restoration of protein synthesis (10 days postsurgery) was associated with a return of eIF-2B epsilon expression to values observed in control rats. Expression of eIF-2B epsilon was not altered in heart during sepsis or in gastrocnemius from nonseptic abscess animals. Amrinone, which ameliorated the inhibition of protein synthesis during sepsis, also prevented the fall in eIF-2B epsilon protein after 5 days of infection. The data provide evidence that expression of eIF-2B epsilon is markedly influenced in gastrocnemius during the course of the septic episode and support the concept that this change is a mechanism responsible for the inhibition of protein synthesis observed under this condition.
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Affiliation(s)
- L Voisin
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey 17033, USA
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