Acute in vivo elevation of insulin-like growth factor (IGF) binding protein-1 decreases plasma free IGF-I and muscle protein synthesis

This study examined whether the acute elevation of IGF-binding protein-1 (IGFBP-1) decreases the plasma free IGF-I concentration and alters in vivo rates of muscle protein synthesis and glucose uptake. The plasma concentration of human IGFBP-1 was increased to approximately 95 ng/ml in conscious catheterized rats infused iv with human IGFBP-1 for 4 h. Infusion of IGFBP-1 also increased the concentration of endogenous (e.g. rat) IGFBP-1 in the blood, and this response was associated with a 2- to 3-fold elevation of IGFBP-1 mRNA in liver and kidney. IGFBP-1 did not significantly alter the plasma concentration of total IGF-I, but decreased circulating free IGF-I levels by about 50%. IGFBP-1 decreased protein synthesis in the predominantly fast-twitch gastrocnemius muscle (20%), and this change resulted from a decreased translational efficiency that was associated with a decreased phosphorylation of S6K1, but not 4E-BP1. Complementary studies demonstrated that IGFBP-1 also decreased the rates of protein synthesis under basal conditions and in response to stimulation by IGF-I when added in vitro to the fast-twitch epitrochlearis muscle. In contrast, IGFBP-1 did not alter in vivo-determined rates of protein synthesis in the slow-twitch soleus muscle, heart, liver, or kidney. The infusion of IGFBP-1 did not significantly alter the plasma glucose or lactate concentration or the whole body rate of glucose production or disposal. The above-mentioned changes were not mediated indirectly by changes in the plasma insulin or corticosterone concentrations, decreased high energy phosphate content in muscle, or hepatoxicity produced by the infused IGFBP-1. These results demonstrate that acute in vivo elevation in IGFBP-1, of the magnitude observed in various catabolic conditions, is capable of selectively decreasing protein synthesis in fast-twitch skeletal muscle and up-regulating the hepatic and renal syntheses of IGFBP-1 per se. Hence, elevations in circulating and tissue levels of IGFBP-1 may be an important mediator for the muscle catabolism observed in various stress conditions.

Sepsis-induced muscle growth hormone resistance occurs independently of STAT5 phosphorylation

Growth hormone (GH) stimulates insulin-like growth factor I (IGF-I) synthesis in both liver and muscle. During sepsis, proinflammatory cytokines inhibit GH action in liver, but it is unknown whether sepsis also produces GH resistance in muscle. Sepsis was induced by cecal ligation and puncture, and 18 h later the effect of GH on signal transducer and activator of transcription (STAT) phosphorylation and IGF-I mRNA content was assessed in rat gastrocnemius and liver. The relative abundance of phosphorylated (p)STAT5a, pSTAT5b, pSTAT3, and pSTAT1 was increased in liver from control rats after GH. Sepsis alone also increased hepatic pSTAT5a, pSTAT3, and pSTAT1. Sepsis dramatically impaired the ability of GH to stimulate the phosphorylation of STAT5a and -5b, as well as to increase IGF-I mRNA in liver. In muscle from control rats, GH increased pSTAT5a and -5b, whereas content of pSTAT3 and pSTAT1 was not affected. Sepsis increased basal content of pSTAT3 but not pSTAT5a, pSTAT5b, or pSTAT1 in muscle. The GH-induced increase of pSTAT5a and -5b in muscle from septic rats was not inhibited, suggesting that muscle was not GH resistant. In contrast to these changes in pSTAT5, the ability of GH to increase IGF-I mRNA was completely absent in muscle from septic rats. Because the suppressor of cytokine signaling (SOCS) proteins may function as negative regulators of GH signaling, we examined the content of these proteins. Sepsis produced small (30-50%), albeit statistically significant, increases in SOCS-1, -2, and -3 protein in muscle. In contrast to muscle, the SOCS proteins in the liver did not change under the various experimental conditions, suggesting that these proteins are not responsible for the impaired phosphorylation of STAT5 by GH. In conclusion, sepsis produces GH resistance in both muscle and liver, with the locus of this impairment in muscle differing from that in liver and being independent of a defect in STAT5 phosphorylation.

Capsaicin-sensitive nerves regulate the metabolic response to abdominal sepsis

BACKGROUND

Both the systemic release of inflammatory mediators and activation of the neuroendocrine axis by sensory afferent nerves (SANs) have been implicated as initiators of the metabolic response to infection. In this study, we investigate the role of SANs as mediators of protein catabolism and the insulin-like growth factor (IGF) axis during abdominal sepsis using capsaicin (Cap) to selectively destroy nociceptive sensory axons.

METHODS

Four groups of male Sprague-Dawley rats were studied: Control, Control+Cap, Sepsis, and Sepsis+Cap. Rats were injected with Cap (75 mg/kg) on day 1 and (50 mg/kg) on day 2 to destroy SANs. Time-matched control and septic rats were pair-fed and injected with vehicle on the same schedule. Controls underwent sham laparotomy, while septic rats had a fecal-agar pellet inoculated with Escherichia coli and Bacteroides fragilis implanted in the peritoneal cavity. Blood and tissues were harvested 5 days after the induction of sepsis. Plasma IGF-I, IGFBP-1, and -3 were measured by radioimmunoassay and Western blot analysis. IGF-I, acid-labile subunit (ALS), IGFBP-1 and -3 mRNA levels were determined by Northern blot analysis.

RESULTS

Mortality was 40% in septic rats vs 0% in the sepsis+Cap group. Capsaicin had no effect on muscle mass, protein content, or the IGF system in control rats. However, sepsis-induced reductions in gastrocnemius mass (25%) and protein content (35%) were ameliorated by capsaicin. The sepsis-induced decrease in hepatic IGF-I mRNA and circulating IGF-I (26%), as well as the 4-fold increase in plasma IGFBP-1 and hepatic IGFBP-1 mRNA were prevented by capsaicin.

CONCLUSIONS

Capsaicin-sensitive nerves mediate mortality, the catabolism of skeletal muscle, and selected elements of the IGF system during abdominal sepsis. The results suggest an important role for nociceptive SANs and the neuroendocrine system in mediating the host response to abdominal infection.

Endotoxin stimulates in vivo expression of inflammatory cytokines tumor necrosis factor alpha, interleukin-1beta, -6, and high-mobility-group protein-1 in skeletal muscle

The presence of increased levels of proinflammatory cytokines in the blood is associated with decreased muscle protein synthesis and the erosion of lean body mass in many catabolic conditions. However, little is known regarding the role of endogenous cytokine synthesis in muscle per se. The purpose of the present study was to characterize the cytokine expression profile of skeletal muscle in response to an in vivo injection of endotoxin (lipopolysaccharide, LPS). Intraperitoneal injection of a nonlethal dose of LPS (1,000 microg/kg Escherichia coli) into male rats increased the mRNA content of tumor necrosis factor-alpha (TNF-alpha) and interleukin (IL)-1beta in gastrocnemius muscle as early as 1 h; IL-6 mRNA was not increased until 2 h post-LPS. Expression of TNF-alpha and IL-1beta peaked at 2 h (10- and 80-fold, respectively), whereas the increased IL-6 mRNA content (150-fold) peaked later at 4 h. The abundance of all measured cytokine mRNAs in skeletal muscle declined thereafter. The LPS-induced increase in muscle mRNA content for TNF-alpha, IL-6, and IL-1beta was dose-dependent with elevations being seen with as little as 10 microg/kg of LPS (2.5-, 8-, and 9-fold, respectively). In general, pretreatment of rats with dexamethasone attenuated but did not completely prevent the LPS-induced increase in muscle cytokine mRNA. LPS increased muscle TNF-alpha protein content approximately 2-fold and this increase was prevented by pretreatment with dexamethasone. LPS-induced increases in muscle IL-1beta and IL-6 protein were not detected. LPS also produced a 2-fold increase in the mRNA content of the high-mobility-group protein-1, a late-phase cytokine, in muscle at 12-24 h. Finally, although skeletal muscle was found to contain both the toll-like receptor (TLR)-2 and TLR4, LPS did not alter the mRNA content of TLR4 and produced a small (50%) but significant increase in TLR2 mRNA. These changes in TLRs were less dramatic than those observed for liver, spleen or cardiac muscle. Collectively these data indicate that skeletal muscle possesses many of the components of the innate immune system, including increases in both early- and late-phase cytokines and the presence of toll-like receptors.

Tumor necrosis factor-alpha decreases insulin-like growth factor-I messenger ribonucleic acid expression in C2C12 myoblasts via a Jun N-terminal kinase pathway

IGF-I is a major anabolic hormone for skeletal muscle in vivo. Yet the mechanisms by which GH and cytokines regulate IGF-I expression remain obscure. Lipopolysaccharide (LPS) dramatically alters the circulating concentration of both TNF alpha and IGF-I, and TNF alpha in part mediates the cachectic activity of LPS. Little is known about the local synthesis of IGF-I and TNF alpha in skeletal muscle per se. The purpose of the present study was to determine whether LPS alters the expression of TNF alpha and IGF-I in mouse skeletal muscle and whether TNF alpha directly inhibits IGF-I mRNA expression in C2C12 myoblasts. Intraperitoneal injection of LPS in C3H/SnJ mice increased the expression of TNF alpha protein in plasma (16-fold) and TNF alpha mRNA in skeletal muscle (8-fold). LPS also decreased the plasma concentration of IGF-I (30%) and IGF-I mRNA in skeletal muscle (50%, between 6 and 18 h after LPS administration). Addition of LPS or TNF alpha directly to C2C12 myoblasts decreased IGF-I mRNA by 50-80%. The TNF alpha-induced decrease in IGF-I mRNA was both dose and time dependent and occurred in both myoblasts and differentiated myotubes. TNF alpha selectively decreased IGF-I but not IGF-II mRNA levels, and the effect of TNF alpha was blocked by a specific TNF-binding protein. TNF alpha did not alter IGF-I mRNA levels in the presence of the protein synthesis inhibitor cycloheximide. TNF alpha did not change the half-life of IGF-I mRNA. TNF alpha completely prevented GH-inducible IGF-I mRNA expression, but this GH resistance was not attributable to impairment in signal transducer and activator of transcription-3 or -5 phosphorylation. TNF alpha increased both nitric oxide synthase-II mRNA and protein, and the nitric oxide donor sodium nitroprusside decreased IGF-I mRNA levels in C2C12 cells. Yet inhibitor studies indicate that nitric oxide did not mediate the effect of TNF alpha on IGF-I mRNA expression. TNF alpha stimulated the phosphorylation of c-Jun and specific inhibition of the Jun N-terminal kinase pathway, but not other MAPK pathways, completely prevented the TNF alpha-induced drop in IGF-I mRNA. These data suggest that LPS stimulates TNF alpha expression in mouse skeletal muscle and autocrine-derived cytokines may contribute to the reduced expression of IGF-I in this tissue.

Regulation of insulin-like growth factor-I in skeletal muscle and muscle cells

Growth hormone (GH) and insulin-like growth factor-I (IGF-I) are potent regulators of muscle mass. Transgenic mice that over-express these proteins exhibit dramatically enlarged skeletal muscles. In contrast, malnutrition, critical illness, sepsis, and aging are all associated with a dramatic reduction in muscle mass and function. The circulating concentration of IGF-I and the expression of IGF-I in skeletal muscle are also reduced during catabolic states. Consequently, GH has been used clinically to increase lean body mass in patients with muscle wasting. Likewise, delivery of IGF-I specifically into muscle has been proposed as a genetic therapy for muscle disorders. A better understanding of the regulation of IGF-I expression in skeletal muscle and muscle cells is therefore of importance. Yet, our knowledge in this area has been limited by a lack of GH responsive muscle cells. In addition the IGF-I gene spans over 90 kb of genomic DNA and it exhibits a very complex regulatory pattern. This review will summarize our knowledge of the control of muscle mass by GH, IGF-I, anabolic steroids, exercise and other growth enhancing hormones. We will also highlight recent advances in the regulation of IGF-I and signal transducers and activators of transcription (Stats) by GH. A special emphasis will be placed on the interaction of IGF-I and proinflammatory cytokines in skeletal muscle and muscle cells.

IGF-I induced phosphorylation of S6K1 and 4E-BP1 in heart is impaired by acute alcohol intoxication

BACKGROUND

The purpose of the present study was to determine whether acute alcohol (EtOH) intoxication impairs the signal transduction pathway used to coordinate insulin-like growth factor (IGF)-I stimulation of myocardial protein synthesis.

METHODS

Rats were injected intraperitoneally with EtOH or saline. After 2.5 h, IGF-I or saline was injected intravenously and the heart was excised at 2 min or 20 min. Additional rats were pretreated with RU486 or tumor necrosis factor (TNF) binding protein (BP) to assess the importance of elevations in glucocorticoids or TNF, respectively, as endogenous modulators of IGF-I signal transduction.

RESULTS

EtOH did not alter the total amount or tyrosine phosphorylation of the IGF-I receptor, IRS-1 or PKB under basal or IGF-stimulated conditions. However, EtOH attenuated the ability of IGF-I to phosphorylate ribosomal S6 kinase (S6K)-1 on residues T389 ( approximately 62%) and T421/S424 ( approximately 40%), and also reduced ribosomal protein S6 phosphorylation. Under basal conditions, EtOH altered the distribution of eukaryotic initiation factor (eIF) 4E, as evidenced by a decreased amount of the active eIF4E.eIF4G complex (53%), an increased amount of inactive eIF4E.4E-BP1 complex ( approximately 3-fold), and decreased phosphorylation of 4E-BP1 (56%). EtOH also impaired the ability of IGF-I to reverse the above-mentioned changes in the eIF4E system. Pretreatment of rats with RU486 or TNFBP was unable to attenuate the EtOH-induced changes in either eIF4E distribution or the phosphorylation state of 4E-BP1, S6K1 or S6.

CONCLUSIONS

These data indicate that acute EtOH intoxication alters selected aspects of translational control in the heart under basal conditions. Furthermore, despite appropriate stimulation of IGF-I receptor, IRS-1 and PKB, EtOH impairs IGF-I signaling via S6K1 and 4E-BP1 pathways, and this defect is regulated in a glucocorticoid- and TNF-independent manner. This IGF-I resistance may represent a participating mechanism by which alcohol limits protein synthesis in heart.

Differential effects of alcohol consumption on eukaryotic elongation factors in heart, skeletal muscle, and liver

BACKGROUND

Acute and chronic alcohol administration diminishes rates of protein synthesis in a variety of tissues including skeletal muscle, heart, and liver, through a diminished translational efficiency rather than a reduction in the number of ribosomes.

METHODS

The purpose of the present study was to examine the effect of chronic alcohol exposure (8, 12, or 16 weeks) on elongation factors (eEF) as a potential mechanism for controlling mRNA translation in psoas, soleus, heart, and liver. The cellular content of eEF1A and eEF2 and the phosphorylation state of eEF2 in each tissue was measured using immunoblot techniques.

RESULTS

The protein content of eEF1A was reduced in psoas, heart, and liver (but not soleus) from rats fed a diet containing alcohol for 16 weeks, but not for 8 or 12 weeks, compared with time-matched pair-fed controls. eEF2 content was only reduced in myocardium after feeding rats an alcohol-containing diet for 16 weeks. In other tissues, no change in eEF2 content was observed. The decreases in eEF protein content were not associated with a concomitant reduction in the mRNA abundance for eEF1A or eEF2. The phosphorylation state of eEF2 was not affected by chronic alcohol consumption in the skeletal muscle or heart. In contrast, the level of eEF2 phosphorylation in the liver was reduced after 8, 12, and 16 weeks of feeding rats an alcohol-containing diet. In contrast, acute alcohol intoxication failed to modulate the content of eEF1A or eEF2 in any of the tissues examined. The phosphorylation state of eEF2 was reduced in psoas following acute alcohol intoxication.

CONCLUSIONS

A decreased eEF1A protein content could account, in part, for the inhibition of translational efficiency following chronic (16 weeks) alcohol feeding but not the response to acute alcohol intoxication.

Alcohol impairs insulin and IGF-I stimulation of S6K1 but not 4E-BP1 in skeletal muscle

The present study determined whether acute alcohol (ethanol; EtOH) intoxication in rats impaired components of the insulin- and IGF-I-signaling pathway in skeletal muscle. Rats were administered EtOH, and 2.5 h thereafter either insulin, IGF-I, or saline was injected and the gastrocnemius removed. EtOH did not alter the total amount or tyrosine phosphorylation of the insulin receptor, IGF-I receptor, insulin receptor substrate (IRS)-1, or protein kinase B (PKB)/Akt under basal or hormone-stimulated conditions. In contrast, the ability of insulin or IGF-I to phosphorylate T389 and T421/S424 on S6K-1 was markedly diminished by EtOH, and these changes were associated with a reduction in the phosphorylation of the ribosomal protein S6. Under basal conditions, EtOH altered the distribution of eukaryotic initiation factor (eIF)4E, as evidenced by a decreased amount of active eIF4E. eIF4G complex, an increased amount of inactive eIF4E. 4E-binding protein (BP)1 complex, and decreased 4E-BP1 phosphorylation. In contrast, EtOH did not impair the ability of either hormone to reverse the changes in eIF4E distribution or 4E-BP1 phosphorylation. Pretreatment with a glucocorticoid receptor antagonist was unable to attenuate either the basal EtOH-induced changes in eIF4E distribution or the impaired ability of IGF-I to stimulate S6K1 and S6 phosphorylation. Hence, acute alcohol intoxication alters selected aspects of translational control under both basal and anabolic hormone-stimulated conditions in skeletal muscle in a glucocorticoid-independent manner.

Alcohol-induced increases in insulin-like growth factor binding protein-1 are partially mediated by TNF

BACKGROUND

Alcohol (EtOH) alters the plasma and tissue content of insulin-like growth factor (IGF)-I, an important anabolic hormone. However, the bioavailability and bioactivity of IGF-I can also be modulated by changes in soluble proteins that bind IGF-I (IGFBPs). The purpose of the present study was to determine whether EtOH intoxication in rats alters the plasma concentration and tissue mRNA content of various IGFBPs. Based on initial results subsequent studies were performed to assess potential mechanisms by which EtOH increased IGFBP-1.

METHODS

Rats were administered EtOH (75 mmol/kg) and blood and tissues collected at various times thereafter. Separate groups of rats were also pretreated with 4-methylpyrazole (4-MP; alcohol dehydrogenase inhibitor), cyanamide (inhibitor of acetaldehyde metabolism), RU486 (glucocorticoid receptor antagonist) or the tumor necrosis factor (TNF) antagonist (TNF(BP)) prior to EtOH administration.

RESULTS

Acute EtOH intoxication did not alter the mRNA content of IGFBP-3, -4 or -5 in liver or kidney. However, EtOH increased IGFBP-1 in blood (5-fold), which was associated with an up-regulation of IGFBP-1 mRNA content in liver and kidney (2- to 3-fold). Likewise, the injection of the nonmetabolizable alcohol -butanol also increased IGFBP-1 in plasma, liver, and kidney. The increased IGFBP-1 in blood and tissues was not prevented by inhibiting alcohol metabolism with 4-MP. However, pretreatment with cyanamide markedly accentuated the EtOH-induced increase in IGFBP-1 in blood (20-fold), liver (3.5-fold), and kidney (12-fold), indicating that accumulation of acetaldehyde can enhance IGFBP-1 synthesis. A time course study indicated that EtOH increased plasma IGFBP-1 levels as early as 0.5-1 hr, and that this response was associated with elevated IGFBP-1 mRNA in liver but not kidney. Pretreatment with RU486 did not prevent or attenuate the EtOH-induced increase in IGFBP-1. However, the alcohol-induced increase in IGFBP-1 was attenuated by TNF(BP).

CONCLUSIONS

These data suggest that the acute alcohol-induced increase in IGFBP-1 is mediated, at least in part, by TNF and is independent of EtOH metabolism and increases in endogenous glucocorticoids.

Tumor necrosis factor mediates hepatic growth hormone resistance during sepsis

During sepsis, growth hormone (GH) resistance contributes to the catabolism of muscle protein. To determine the role of tumor necrosis factor (TNF) as a mediator of GH resistance, we examined the effects of a TNF antagonist [TNF-binding protein (TNFbp)] on the GH/insulin-like growth factor (IGF) I system during abdominal sepsis. To investigate potential mechanisms, the effects of TNF on the IGF-I response to GH and GH signaling were examined in cultured rat hepatocytes (CWSV-1). Three groups of rats were studied: Control, Sepsis, and Sepsis + TNFbp. Liver, gastrocnemius, and plasma were collected on day 5. In gastrocnemius, neither sepsis nor TNFbp altered the abundance of IGF-I mRNA. However, septic rats demonstrated an increase in circulating GH and a reduction in plasma IGF-I concentrations that was ameliorated by pretreatment with TNFbp. Liver from septic rats demonstrated a 50% reduction in GH receptor (GHR) and IGF-I mRNA on day 5 that was attenuated by TNFbp. However, the abundance of GHR protein was not different in liver from Control, Sepsis, or Sepsis + TNFbp rats. Consequently, a decreased amount of hepatic GHR does not explain the GH-resistant septic state. In CWSV-1 hepatocytes, TNF-alpha had no effect on GHR protein level but inhibited the induction of IGF-I mRNA by GH. Nuclear protein from TNF-treated hepatocytes demonstrated similar levels of phosphorylated signal transducer and activator of transcription-5 (STAT5) and DNA binding relative to controls 5 min after GH treatment. However, both of these parameters were decreased (vs. control) in TNF-treated cells 60 min after GH treatment. Collectively, these results suggest that TNF mediates hepatic GH resistance during sepsis by inhibiting the duration of signaling via the janus kinase-2/STAT5 pathway.

Lipopolysaccharide regulates proinflammatory cytokine expression in mouse myoblasts and skeletal muscle

The purpose of the present study was to examine the regulation of tumor necrosis factor (TNF)-alpha and interleukin (IL)-6 by lipopolysaccharide (LPS) in C2C12 myoblasts and mouse skeletal muscle. LPS produced dose- and time-dependent increases in TNF-alpha and IL-6 mRNA content in C2C12 myoblasts. The LPS-induced cytokine response could be mimicked by peptidoglycan from the cell wall of Staphylococcus aureus but not by zymosan A, a cell wall component from Saccharomyces cerevisiae. Ongoing protein synthesis was not necessary for the increase in the two cytokine mRNAs. The transcriptional inhibitor 5,6-dichloro-beta-D-ribofuranosyl-benzimidazole blocked LPS-stimulated IL-6 mRNA expression without changing its mRNA half-life. The anti-inflammatory glucocorticoid dexamethasone selectively blocked LPS-stimulated IL-6 mRNA accumulation but not TNF-alpha. In contrast, the proteasomal inhibitor MG-132 blocked TNF-alpha mRNA expression but not IL-6. Exposure of myoblasts to LPS was associated with a rapid decrease in the inhibitor of nuclear factor-kappaB (I kappaB, alpha, and epsilon), and this response was also blocked by MG-132. Treatment of myocytes with IL-1 or TNF-alpha also increased IL-6 mRNA content, but the increase in IL-6 mRNA due to LPS could not be prevented by pretreatment with antagonists to either IL-1 or TNF. Under in vivo conditions, LPS increased the plasma concentration of TNF-alpha and IL-6 and stimulated the accumulation of their mRNAs in multiple tissues including skeletal muscle from wild-type mice. In contrast, the ability of LPS to stimulate the same cytokines was markedly decreased in mice that harbor a mutation in the Toll-like receptor 4. Our data suggest that LPS stimulates cytokine expression not only in classical immune tissues but also in skeletal muscle.

Role of growth hormone, insulin-like growth factor-I, and insulin-like growth factor binding proteins in the catabolic response to injury and infection

The erosion of lean body mass resulting from protracted critical illness remains a significant risk factor for increased morbidity and mortality in this patient population. Previous studies have documented the well known impairment in nitrogen balance results from both an increase in muscle protein degradation as well as a decreased rate of both myofibrillar and sacroplasmic protein synthesis. This protein imbalance may be caused by an increased presence or activity of various catabolic agents, such as tumor necrosis factor-alpha, interleukin-1 beta, interleukin-6 or glucocorticoids, or may be mediated via a decreased concentration or responsiveness to various anabolic hormones, such as growth hormone or insulin-like growth factor-I. This review focuses on recent developments pertaining to the importance of alterations in the growth hormone-insulin-like growth factor-I axis as a mechanism for the observed defects in muscle protein balance.

Contribution of insulin to the translational control of protein synthesis in skeletal muscle by leucine

Enhanced protein synthesis in skeletal muscle after ingestion of a balanced meal in postabsorptive rats is mimicked by oral leucine administration. To assess the contribution of insulin to the protein synthetic response to leucine, food-deprived (18 h) male rats (approximately 200 g) were intravenously administered a primed-constant infusion of somatostatin (60 microg + 3 microg.kg(-1).h(-1)) or vehicle beginning 1 h before administration of leucine (1.35 g L-leucine/kg) or saline (control). Rats were killed 15, 30, 45, 60, or 120 min after leucine administration. Compared with controls, serum insulin concentrations were elevated between 15 and 45 min after leucine administration but returned to basal values by 60 min. Somatostatin maintained insulin concentrations at basal levels throughout the time course. Protein synthesis was increased between 30 and 60 min, and this effect was blocked by somatostatin. Enhanced assembly of the mRNA cap-binding complex (composed of eukaryotic initiation factors eIF4E and eIF4G) and hyperphosphorylation of the eIF4E-binding protein 1 (4E-BP1), the 70-kDa ribosomal protein S6 kinase (S6K1), and the ribosomal protein S6 (rp S6) were observed as early as 15 min and persisted for at least 60 min. Somatostatin attenuated the leucine-induced changes in 4E-BP1 and S6K1 phosphorylation and completely blocked the change in rp S6 phosphorylation but had no effect on eIF4G small middle dot eIF4E assembly. Overall, the results suggest that the leucine-induced enhancement of protein synthesis and the phosphorylation states of 4E-BP1 and S6K1 are facilitated by the transient increase in serum insulin. In contrast, assembly of the mRNA cap-binding complex occurs independently of increases in insulin and, by itself, is insufficient to stimulate rates of protein synthesis in skeletal muscle after leucine administration.

Orally administered leucine enhances protein synthesis in skeletal muscle of diabetic rats in the absence of increases in 4E-BP1 or S6K1 phosphorylation

In this study, food-deprived (18 h) control rats and rats with alloxan-induced diabetes were orally administered saline or the amino acid leucine to assess whether it regulates protein synthesis independently of a change in serum insulin concentrations. Immediately after leucine administration, diabetic rats were infused with insulin (0.0, 4.0, or 20 pmol small middle dot min(-1) small middle dot kg(-1)) for 1 h to examine the role of the hormone in the protein synthetic response to leucine. In control rats, leucine stimulated protein synthesis by 58% and increased phosphorylation of the translational repressor, eukaryotic initiation factor (eIF) 4E-binding protein (BP)-1, 4E-BP1, fivefold. Consequently, association of the mRNA cap-binding protein eukaryotic initiation factor (eIF)4E with 4E-BP1 was reduced to 50% of control values, and eIF4G*eIF4E complex assembly was increased 80%. Furthermore, leucine increased the phosphorylation of the 70-kDa ribosomal protein S6 (rp S6) and the ribosomal protein S6 kinase (S6K1). Diabetes attenuated protein synthesis compared with control rats. Nonetheless, in diabetic rats, leucine increased protein synthesis by 53% without concomitant changes in the phosphorylation of 4E-BP1 or S6K1. Skeletal muscle protein synthesis was stimulated in diabetic rats infused with insulin, but rates of synthesis remained less than values in nondiabetic controls that were administered leucine. Phosphorylation of 4E-BP1 and S6K1 was increased in diabetic rats infused with insulin in a dose-dependent manner, and the response was enhanced by leucine. The results suggest that leucine enhances protein synthesis in skeletal muscle through both insulin-dependent and -independent mechanisms. The insulin-dependent mechanism is associated with increased phosphorylation of 4E-BP1 and S6K1. In contrast, the insulin-independent effect on protein synthesis is mediated by an unknown mechanism.

Regulation of IGF-I mRNA and signal transducers and activators of transcription-3 and -5 (Stat-3 and -5) by GH in C2C12 myoblasts

GH and IGF-I are critical hormones for the regulation of longitudinal growth and the maintenance of lean body mass in humans. The regulation of IGF-I expression by GH in hepatocytes is well documented; however less is known about the regulation of IGF-I in peripheral tissues such as muscle. We have examined the regulation of IGF-I mRNA by GH and IGF-I in C2C12 myoblasts. GH stimulated the accumulation of IGF-I mRNA dose- and time-dependently. An elevation of IGF-I mRNA was observed with GH doses as low as 0.75 ng/ml and after exposure to GH for as little as 1 h, and the increase required ongoing transcription and translation. GH applied in a pulsatile fashion for 10 min followed by an 8-h interpulse interval increased IGF-I mRNA to a greater extent than continuous exposure. GH stimulated tyrosine phosphorylation of the GH receptor, signal transducer and activator of transcription-3 (Stat3), and Stat5. Stat5 was resistant to additional phosphorylation if cells were given a GH pulse within 2 h of a previous GH exposure. The refractory period lasted for 4 h, and cells could be maximally stimulated again after 6 h. Stat3 phosphorylation was also enhanced in cells that were allowed to recover from a previous application of GH. The tyrosine kinase inhibitors, genistein, PP1, and AG-490, and the MAPK kinase inhibitor, PD98059, did not block Stat3 or Stat5 phosphorylation. In contrast, WHI-P154, a Janus kinase-3 inhibitor, dose-dependently prevented Stat3, but not Stat5, phosphorylation. GH-inducible nuclear transport of Stat3 was likewise inhibited by WHI-P154. Most importantly, GH-dependent IGF-I mRNA expression was inhibited by WHI-P154. In contrast, IGF-I mRNA expression was inhibited by IGF-I peptide, and the effect of IGF-I was dominant over that of GH. IGF-I mRNA was regulated by both PI3K and MAPK signal transduction pathways, but IGF-I peptide signaled predominantly through a wortmannin-sensitive pathway to down-regulate its own mRNA. Our data suggest that Janus kinases (Jak2 or Jak3) and their downstream targets (Stat3 and Stat5) may play important roles in the expression of IGF-I mRNA and the myoblast response to GH. In addition, C2C12 cells appear to be a good model system to examine GH regulation of Janus kinase/Stat signaling and the regulation of IGF-I mRNA.

TNF-alpha impairs heart and skeletal muscle protein synthesis by altering translation initiation

This study examined potential mechanisms contributing to the inhibition of protein synthesis in skeletal muscle and heart after administration of tumor necrosis factor (TNF)-alpha. Rats had vascular catheters implanted, and TNF-alpha was infused continuously for 24 h. TNF-alpha decreased in vivo-determined rates of global protein synthesis in gastrocnemius (39%) and heart (25%). The TNF-alpha-induced decrease in protein synthesis in the gastrocnemius involved a reduction in the synthesis of both myofibrillar and sarcoplasmic proteins. To identify potential mechanisms responsible for regulating mRNA translation, we examined several eukaryotic initiation factors (eIFs) and elongation factors (eEFs). TNF-alpha decreased the activity of eIF-2B in muscle (39%) but not in heart. This diminished activity was not caused by a reduction in the content of eIF-2B epsilon or the content and phosphorylation state of eIF-2 alpha. Skeletal muscle and heart from TNF-alpha-treated rats demonstrated 1) an increased binding of the translation repressor 4E-binding protein-1 (4E-BP1) with eIF-4E, 2) a decreased amount of eIF-4E associated with eIF-4G, and 3) a decreased content of the hyperphosphorylated gamma-form of 4E-BP1. In contrast, the infusion of TNF-alpha did not alter the content of eEF-1 alpha or eEF-2, or the phosphorylation state of eEF-2. In summary, these data suggest that TNF-alpha impairs skeletal muscle and heart protein synthesis, at least in part, by decreasing mRNA translational efficiency resulting from an impairment in translation initiation associated with alterations in eIF-4E availability.

In vitro and in vivo inhibition of LPS-induced tumor necrosis factor-alpha production by dimeric gallotannin analogues

Designed dimeric gallotannin analogues featuring two tetragalloylglucopyranose cores connected by various hydrocarbon linkers inhibit tumor necrosis factor-alpha secretion from lipopolysaccharide-stimulated human peripheral blood mononuclear cells by up to 53% (5-24 microM concentration range) compared to control. Comparable suppression of tumor necrosis factor-alpha levels (approximately 50% vs control) was observed in the plasma of rats co-treated with lipopolysaccharide and specific tannin analogues selected for their lack of interleukin 1-beta stimulating activity.

Molecular pathology and clinical aspects of alcohol-induced tissue injury

Proceedings of a symposium at the 2001 RSA Meeting in Montreal, Canada; organized and co-chaired by Patricia E. Molina and Manuela Neuman. The presentations were (1) Mechanisms of alcohol-induced cell injury by Craig McClain; (2) Cytokines in alcoholic steatohepatitis and non-alcoholic steatohepatitis by Manuela Neuman; (3) Combination of alcohol and hepatitis C virus and liver injury by Dominique Valla; (4) Chronic ethanol exposure potentiates lipopolysaccharide liver injury, despite inhibiting Jun N-Terminal kinase and caspase 3 activation by Anna Mae Diehl; (5) Glutathione homeostasis in alcoholism: Role in alveolar epithelial barrier and lung injury by David M. Guidot; (6) Metabolic and inflammatory contribution of alcohol to trauma-induced tissue injury by Patricia E. Molina; (7) Growth factor and protein synthesis dysregulation: Role in alcoholic myopathy by Charles H. Lang.

Burn-induced changes in IGF-I and IGF-binding proteins are partially glucocorticoid dependent

The purpose of the present study was to determine whether burn-induced changes in various components of the insulin-like growth factor (IGF) system are mediated by the actions of endogenous glucocorticoids or tumor necrosis factor (TNF). To address this aim, a 30% total body surface area full-thickness scald burn was produced in anesthetized rats, and the animals were studied 24 h later. Separate groups of time-matched control and burned rats were pretreated with either an antagonist to glucocorticoids (RU-486) or to TNF (TNF-binding protein; TNFBP). Thermal injury decreased the plasma concentration of IGF-I (38%) as well as the IGF-I mRNA abundance in muscle and kidney (31 and 48%, respectively). While RU-486 prevented the burn-induced decrease in plasma IGF-I, it did not ameliorate the reduction in tissue IGF-I mRNA. Burn increased the plasma concentration of IGF-binding protein (IGFBP)-1 as well as the mRNA content of IGFBP-1 in liver and kidney (15- to 20-fold). These burn-induced increases were partially or largely prevented by RU-486. In contrast, burn decreased the plasma concentration of IGFBP-3 (30%). Burn concomitantly decreased hepatic IGFBP-3 mRNA abundance (42%) but increased IGFBP-3 mRNA in kidney and muscle (50% and 10-fold, respectively). RU-486 largely prevented the burn-induced changes in IGFBP-3 mRNA in kidney and muscle but failed to attenuate the decreases in plasma and liver. Finally, burn injury decreased hepatic acid-labile subunit (ALS) mRNA by 80% and increased the mRNA content of IGFBP-related protein-1 (mac25) in liver by twofold, and these changes were not modified by pretreatment with RU-486. The above-mentioned changes in the IGF system were associated with a burn-induced decrease in muscle protein content that was prevented by RU-486. TNFBP failed to completely ameliorate any of the burn-induced changes in the IGF system. These results demonstrate that glucocorticoids, but not TNF, mediate many but not all of the burn-induced changes in the IGF system.

Alcohol impairs protein synthesis and degradation in cultured skeletal muscle cells

BACKGROUND

Acute and chronic alcohol intoxication decreases skeletal muscle protein synthesis under in vivo conditions. We investigated whether ethanol (EtOH) and its major metabolites, acetaldehyde and acetate, can directly modulate protein balance under in vitro conditions.

METHODS

Human myocytes were incubated with different doses of EtOH for varying periods of time (i.e., 4-72 hr). Alternatively, cells were incubated with acetaldehyde, acetate, insulin, insulin-like growth factor-I (IGF-I), or with a combination of EtOH plus insulin or IGF-I. Rates of protein synthesis or degradation were determined by 35S-methionine/cysteine incorporation into or release from cellular protein.

RESULTS

A significant, 15% to 20%, decrease in basal protein synthesis was observed after 24 hr, but not at earlier time points, in response to 80 mM EtOH. Incubation of myocytes for 72 hr decreased synthesis in cells incubated with EtOH ranging between 60 and 120 mM. The ability of IGF-I or insulin to stimulate protein synthesis was impaired by 30% and 60%, respectively, in cells incubated with 80 mM EtOH for 72 hr. Exposure of cells to 200 microM acetaldehyde or 5 mM Na-acetate also decreased basal protein synthesis. In contrast, neither EtOH, acetaldehyde, nor acetate altered the basal rate of protein degradation. However, EtOH completely impaired the ability of insulin and IGF-I to inhibit proteolysis. Finally, EtOH did not impair IGF-I receptor autophosphorylation, but inhibited the ability of insulin to phosphorylate its own receptor. EtOH also did not alter the number of insulin or IGF-I receptors or the formation of insulin/IGF-I hybrid receptors.

CONCLUSIONS

We have demonstrated that EtOH can directly inhibit muscle protein synthesis under in vitro conditions. Neither EtOH nor its metabolites altered basal protein degradation, although EtOH did compromise the ability of both insulin and IGF-I to slow proteolysis. This impairment seems to be mediated by different defects in signal transduction.

Effects of chronic alcohol consumption on regulation of myocardial protein synthesis

Heart disease represents an important etiology of mortality in chronic alcoholics. The purpose of the present study was to examine potential mechanisms for the inhibitory effect of chronic alcohol exposure (16 wk) on the regulation of myocardial protein metabolism. Chronic alcohol feeding resulted in a lower heart weight and 25% loss of cardiac protein per heart compared with pair-fed controls. The loss of protein mass resulted in part from a diminished (30%) rate of protein synthesis. Ethanol exerted its inhibition of protein synthesis through diminished translational efficiency rather than lower RNA content. Chronic ethanol administration decreased the abundance of eukaryotic initiation factor (eIF)4G associated with eIF4E in the myocardium by 36% and increased the abundance of the translation response protein (4E-BP1) associated with eIF4E. In addition, chronic alcohol feeding significantly reduced the extent of p70S6 kinase (p70(S6K)) phosphorylation. The decreases in the phosphorylation of 4E-BP1 and p70(S6K) did not result from a reduced abundance of mammalian target of rapamycin (mTOR). These data suggest that a chronic alcohol-induced impairment in myocardial protein synthesis results in part from inhibition in peptide chain initiation secondary to marked changes in eIF4E availability and p70(S6K) phosphorylation.

Tissue-specific regulation of IGF-I and IGF-binding proteins in response to TNFalpha

The circulating concentration of insulin-like growth factor-I (IGF-I) is regulated by both its rate of synthesis and its ability to form stable complexes with IGF-binding proteins (IGFBPs). An equilibrium between IGF-I and IGFBPs is thought to help maintain muscle protein balance. In contrast, catabolic conditions disrupt the IGF system and result in the loss of skeletal muscle protein. We have examined the mechanisms by which tumour necrosis factor alpha (TNFalpha), a catabolic cytokine, alters the IGF system. Conscious rats were infused intravenously with recombinant human TNFalpha or vehicle for 24 h. TNFalpha decreased the concentration of both total and free IGF-I in the plasma (30-40%). This change was associated with a reduction in IGF-I mRNA expression in liver (39%), gastrocnemius (73%), soleus (46%) and heart (63%), but a 2.5-fold increase in the whole kidney. In contrast, TNFalpha did not alter IGF-II mRNA expression in skeletal muscle. TNFalpha also increased IGFBP-1 in the blood (4-fold) and this response was associated with an increase in IGFBP-1 mRNA expression in both liver (3-fold) and kidney (9-fold). In contrast, IGFBP-3 levels in the blood were reduced 38% in response to the infusion of TNFalpha. This change was accompanied by a 60-80% reduction of IGFBP-3 mRNA in liver and kidney but no significant change in muscle. Hepatic mRNA levels of the acid-labile subunit were also reduced by TNFalpha (46%). Finally, tissue expression of mac25 (also referred to IGFBP-related protein-1) mRNA was increased in gastrocnemius (50%) but remained unchanged in liver and kidney. These results more fully characterize the changes in various elements of the IGF system and, thereby, provide potential mechanisms for the alterations in the circulating IGF system as well as for changes in tissue metabolism observed during catabolic insults associated with increased TNFalpha expression.

In vitro and In vivo inhibition of LPS-stimulated tumor necrosis factor-alpha secretion by the gallotannin beta-D-pentagalloylglucose

The naturally occurring gallotannin beta-D-pentagalloylglucose (beta-PGG) decreases tumor necrosis factor-alpha (TNF-alpha) output from human peripheral blood mononucleocytes exposed to lipopolysaccharide (LPS) by as much as 90% (vs control) at approximately 5 microM concentration. A qualitatively similar but less pronounced effect ( approximately 50% decrease) was observed in the serum of rats dosed with both LPS and beta-PGG. These results may have relevance to therapies that target disease states characterized by an overproduction of TNF-alpha.

Immunization against IGF-I prevents increases in protein synthesis in diabetic rats after resistance exercise

These studies examined whether passive immunization against insulin-like growth factor I (IGF-I) would prevent increases in rates of protein synthesis in skeletal muscle of diabetic rats after resistance exercise. Male Sprague-Dawley rats were pancreatectomized and randomly assigned to either an exercise or a sedentary group. Animals in each of these groups received either an IGF-I antibody or a nonspecific IgG from a subcutaneous osmotic pump. Exercise did not change plasma or gastrocnemius IGF-I concentrations in nondiabetic rats. However, plasma and muscle IGF-I concentrations were higher in IgG-treated diabetic rats that exercised compared with respective sedentary groups (P < 0.05). Passively immunized diabetic rats did not exhibit the same exercise-induced increase in IGF-I concentrations. In nondiabetic rats, protein synthesis rates were higher after exercise in both control and immunized groups. In diabetic rats, exercise increased protein synthesis in the IgG-treated animals but not in those treated with IGF-I antibody. There was also a significant positive correlation between both plasma and gastrocnemius IGF-I concentrations and rates of protein synthesis in diabetic (P < 0.01), but not nondiabetic, rats. These results suggest that IGF-I is compensatory for insulin in hypoinsulinemic rats by facilitating an anabolic response after acute resistance exercise.

Alcohol myopathy: impairment of protein synthesis and translation initiation

Alcohol consumption leads to numerous morphological, biochemical and functional changes in skeletal and cardiac muscle. One such change observed in both tissues after either acute alcohol intoxication or chronic alcohol consumption is a characteristic decrease in the rate of protein synthesis. A decrease in translation efficiency appears to be responsible for at least part of the reduction. This review highlights advances in determining the molecular mechanisms by which alcohol impairs protein synthesis and places these observations in context of earlier studies on alcoholic myopathy. Both acute and chronic alcohol administration impairs translational control by modulating various aspects of peptide-chain initiation. Moreover, this alcohol-induced impairment in initiation is associated with a decreased availability of eukaryotic initiation factor (eIF) 4E in striated muscle, as evidenced by an increase in the amount of the inactive eIF4E.4E-BP1 complex and decrease in the active eIF4E.eIF4G complex. In contrast, alcohol does not produce consistent alterations in the control of translation initiation by the eIF2 system. The etiology of these changes remain unresolved. However, defects in the availability or effectiveness of various anabolic hormones, particularly insulin-like growth factor-I, are consistent with the alcohol-induced decrease in protein synthesis and translation initiation.

Impaired myocardial protein synthesis induced by acute alcohol intoxication is associated with changes in eIF4F

The purpose of the present study was to examine potential mechanisms for the known inhibitory effect of acute alcohol exposure on myocardial protein synthesis. Rats were injected intraperitoneally with either ethanol (75 mmol/kg) or saline, and protein synthesis was measured in vivo 2.5 h thereafter by use of the flooding-dose L-[(3)H]phenylalanine technique. Rates of myocardial protein synthesis and translational efficiency in alcohol-treated rats were decreased compared with control values. Free (nonpolysome bound) 40S and 60S ribosomal subunits were increased 50% after alcohol treatment, indicating an impaired peptide-chain initiation. To identify mechanisms responsible for this impairment, several eukaryotic initiation factors (eIF) were analyzed. Acute alcohol intoxication did not significantly alter the myocardial content of eIF2 alpha or eIF2B epsilon, the extent of eIF2 alpha phosphorylation, or the activity of eIF2B. Acute alcohol exposure increased the binding of 4E-binding protein 1 (4E-BP1) to eIF4E (55%), diminished the amount of eIF4E bound to eIF4G (70%), reduced the amount of 4E-BP1 in the phosphorylated gamma-form (40%), and decreased the phosphorylation of p70S6 kinase and the ribosomal protein S6. There was no significant difference in either the plasma insulin-like growth factor (IGF) I concentration (total or free) or expression of IGF-I or IGF-II mRNA in heart between the two groups. These data suggest that the acute alcohol-induced impairment in myocardial protein synthesis results, in part, from an inhibition in peptide-chain initiation, which is associated with marked changes in eIF4E availability and p70S6 kinase phosphorylation but is independent of changes in the eIF2/2B system and IGFs.

IGF-I/IGFBP-3 binary complex modulates sepsis-induced inhibition of protein synthesis in skeletal muscle

The present study evaluated the ability of insulin-like growth factor I (IGF-I) complexed with IGF binding protein-3 (IGFBP-3) to modulate the sepsis-induced inhibition of protein synthesis in gastrocnemius. Beginning 16 h after the induction of sepsis, either the binary complex or saline was injected twice daily via a tail vein, with measurements made 3 and 5 days later. By day 3, sepsis had reduced plasma IGF-I concentrations approximately 50% in saline-treated rats. Administration of the binary complex provided exogenous IGF-I to compensate for the sepsis-induced diminished plasma IGF-I. Sepsis decreased rates of protein synthesis in gastrocnemius relative to controls by limiting translational efficiency. Treatment of septic rats with the binary complex for 5 days attenuated the sepsis-induced inhibition of protein synthesis and restored translational efficiency to control values. Assessment of potential mechanisms regulating translational efficiency showed that neither the sepsis-induced change in gastrocnemius content of eukaryotic initiation factor 2B (eIF2B), the amount of eIF4E associated with 4E binding protein-1 (4E-BP1), nor the phosphorylation state of 4E-BP1 or eIF4E were altered by the binary complex. Overall, the results are consistent with the hypothesis that decreases in plasma IGF-I are partially responsible for enhanced muscle catabolism during sepsis.

Stimulation of insulin-like growth factor binding protein-1 synthesis by interleukin-1beta: requirement of the mitogen-activated protein kinase pathway

Insulin-like growth factor (IGF) binding protein-1 (IGFBP-1) is a 28-kDa plasma protein that binds to IGF-I and IGF-II with high affinity. IGFBP-1 is elevated in the blood as a result of sepsis, AIDS, excessive alcohol consumption, and diabetes and may, in part, be responsible for the wasting observed during these pathophysiological conditions. The liver is the principal site of IGFBP-1 synthesis, and we have previously shown that proinflammatory cytokines can directly stimulate IGFBP-1 secretion in a human hepatoma cell line (HepG2). The purpose of the present study was to investigate the role of the MAP kinase pathway in regulating IGFBP-1 synthesis by IL-1beta. We show that IL-1beta stimulates the phosphorylation of ERK-1 and -2 in a time- and dose-dependent manner. In addition, the MAP kinase-kinase MEK-1 and the ribosomal S6-kinase RSK-1 are also phosphorylated in response to IL-1beta. The transcription factor CREB, a potential substrate of both protein kinase A (PKA) and RSK-1, is phosphorylated in response to IL-1beta and cAMP in HepG2 cells. The ability of IL-1beta to stimulate the expression of IGFBP-1 and the phosphorylation of the above kinases was specifically inhibited by PD98059, a MEK-1 inhibitor. cAMP also stimulated IGFBP-1 synthesis, but PD98059 failed to block the cAMP effect. Conversely, a PKA inhibitor (H-89) inhibited the ability of cAMP, but not IL-1beta to stimulate IGFBP-1 synthesis. The effect of IL-1beta and cAMP on IGFBP-1 messenger RNA (mRNA) accumulation was additive. IL-1beta, cAMP, PD98059, and H-89 had similar effects on the accumulation of IGFBP-1 protein and mRNA. IL-1beta and cAMP did not change the half-life of IGFBP-1 mRNA, but PD98059 and SB202190, a p38 MAP kinase inhibitor, destabilized IGFBP-1 mRNA and blocked the phosphorylation of RSK-1 in response to IL-1beta. Our data demonstrate that the MAP kinase signal transduction pathway plays an important role in the regulation of IGFBP-1 synthesis by IL-1beta.

Effect of hyperthyroidism on insulin-like growth factor-I (IGF-I) and IGF-binding proteins in adolescent children

A study was performed on adolescent hyperthyroid patients to determine the effects of hyperthyroidism on insulin-like growth factor (IGF)-I and its binding proteins. Serum concentrations of immunoreactive total and free IGF-I, and IGF binding protein (IGFBP)-2 and IGFBP-3 were determined before and after correction of hyperthyroidism in eight patients with Grave's disease and compared to control patients matched for age, sex and pubertal stage. The concentration of serum total IGF-I was not significantly different in the hyperthyroid state and euthyroid state, and did not differ significantly from euthyroid controls. IGFBP-2 levels were elevated three-fold in hyperthyroid patients at the time of diagnosis of hyperthyroidism compared to control subjects, and fell significantly during treatment. There was also a significant positive correlation between serum IGFBP-2 concentrations and thyroxine (T4) concentrations in all subjects. Serum IGFBP-3 concentrations were also elevated in hyperthyroid subjects and normalized with correction of the hyperthyroidism. There was also a positive correlation between serum T4 and IGFBP-3 concentrations in all subjects. Despite the hyperthyroid-induced elevations in IGFBP-2 and -3, no significant difference in the serum concentration of free IGF-I before or after correction of the hyperthyroid condition was observed. We conclude that hyperthyroidism does not cause alterations in the serum concentrations of either free or total IGF-I. However, both serum IGFBP-2 and IGFBP-3 concentrations were elevated during hyperthyroidism and correlated with serum T4 levels. These abnormalities reversed with normalization of thyroid function.

Muscle damage impairs insulin stimulation of IRS-1, PI 3-kinase, and Akt-kinase in human skeletal muscle

Physiological stress associated with muscle damage results in systemic insulin resistance. However, the mechanisms responsible for the insulin resistance are not known; therefore, the present study was conducted to elucidate the molecular mechanisms associated with insulin resistance after muscle damage. Muscle biopsies were obtained before (base) and at 1 h during a hyperinsulinemic-euglycemic clamp (40 mU x kg(-1) x min(-1)) in eight young (age 24+/-1 yr) healthy sedentary (maximal O(2) consumption, 49.7+/-2.4 ml x kg(-1) x min(-1)) males before and 24 h after eccentric exercise (ECC)-induced muscle damage. To determine the role of cytokines in ECC-induced insulin resistance, venous blood samples were obtained before (control) and 24 h after ECC to evaluate ex vivo endotoxin-induced mononuclear cell secretion of tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, and IL-1beta. Glucose disposal was 19% lower after ECC (P<0.05). Insulin-stimulated insulin receptor substrate (IRS)-1 tyrosine phosphorylation was 45% lower after ECC (P<0.05). Insulin-stimulated phosphatidylinositol (PI) 3-kinase, Akt (protein kinase B) serine phosphorylation, and Akt activity were reduced 34, 65, and 20%, respectively, after ECC (P < 0.05). TNF-alpha, but not IL-6 or IL-1beta production, increased 2.4-fold 24 h after ECC (P<0.05). TNF-alpha production was positively correlated with reduced insulin action on PI 3-kinase (r = 0.77, P = 0.04). In summary, the physiological stress associated with muscle damage impairs insulin stimulation of IRS-1, PI 3-kinase, and Akt-kinase, presumably leading to decreased insulin-mediated glucose uptake. Although more research is needed on the potential role for TNF-alpha inhibition of insulin action, elevated TNF-alpha production after muscle damage may impair insulin signal transduction.

Endotoxin-induced decrease in muscle protein synthesis is associated with changes in eIF2B, eIF4E, and IGF-I

The present study examined potential mechanisms contributing to the inhibition of protein synthesis in skeletal muscle after administration of endotoxin (LPS). Rats implanted with vascular catheters were injected intravenously with a nonlethal dose of Escherichia coli LPS, and samples were collected at 4 and 24 h thereafter; pair-fed control animals were also included. The rate of muscle (gastrocnemius) protein synthesis in vivo was reduced at both time points after LPS administration. LPS did not alter tissue RNA content, but the translational efficiency was consistently reduced at both time points. To identify mechanisms responsible for regulating translation, we examined several eukaryotic initiation factors (eIFs). The content of eIF2alpha or the amount of eIF2alpha in the phosphorylated form did not change in response to LPS. eIF2B activity was decreased in muscle 4 h post-LPS but activity returned to control values by 24 h. A decrease in the relative amount of eIF2Balpha protein was not responsible for the LPS-induced reduction in eIF2B activity. LPS also markedly altered the distribution of eIF4E in muscle. Compared with control values, LPS-treated rats demonstrated 1) a transient increase in binding of the translation repressor 4E-binding protein-1 (4E-BP1) with eIF4E, 2) a transient decrease in the phosphorylated gamma-form of 4E-BP1, and 3) a sustained decrease in the amount of eIF4G associated with eIF4E. LPS also decreased insulin-like growth factor (IGF) I protein and mRNA expression in muscle at both times. A significant linear relationship existed between muscle IGF-I and the rate of protein synthesis or the amount of eIF4E bound to eIF4G. In summary, these data suggest that LPS impairs muscle protein synthesis, at least in part, by decreasing translational efficiency, resulting from an impairment in translation initiation associated with alterations in both eIF2B activity and eIF4E availability.

Acute response of IGF-I and IGF binding proteins induced by thermal injury

Previous studies demonstrate that thermal injury decreases circulating levels of insulin growth factor I (IGF-I) and alters the plasma concentration of several IGF binding proteins (IGFBP), but the mechanisms for these alterations have not been elucidated. In the current study, a 30% total body surface area full-thickness scald burn was produced in anesthetized rats, and animals were studied 24 h later. The plasma concentration of both total and free IGF-I was decreased (38 and 65%, respectively) in burn rats compared with values from time-matched control animals. Thermal injury decreased the IGF-I peptide content in liver approximately 40%, as well as in fast-twitch skeletal muscle (56-69%) and heart (28%). In contrast, IGF-I content in kidney was elevated by 36% in burn rats. Northern blot analysis of liver indicated that burn decreased the expression of small (1.7- and 0.9- to 1.2-kb) IGF-I mRNA transcripts but increased the expression of the 7.5-kb transcript. In contrast, there was a coordinate decrease in all IGF-I mRNA transcripts in muscle and kidney of approximately 30%. For liver, muscle, and kidney, there was no significant difference in the expression of growth hormone receptor mRNA between control and burn rats. Thermal injury increased plasma IGFBP-1 levels, and this change was associated with increased IGFBP-1 mRNA in both liver and kidney. IGFBP-3 levels in plasma were concomitantly decreased by burn injury. This change was associated with a reduction in IGFBP-3 mRNA in liver but an increased expression of IGFBP-3 in kidney and muscle. Thermal injury also decreased the concentration of the acid-labile subunit (ALS) in plasma and ALS mRNA expression in liver. Finally, hepatic expression of IGFBP-related peptide-1 was increased twofold in liver but was unchanged in kidney or muscle of burn rats. These results characterize burn-induced changes in various components of the IGF system in select tissues and thereby provide potential mechanisms for alterations in the circulating IGF system and for changes in tissue metabolism.

Alterations in tissue glucose uptake during the hyperglycemic and hypoglycemic phases of sepsis

The purpose of the present study was to characterize the alterations in tissue glucose uptake during the hyperglycemic, euglycemic, and hypoglycemic phases of peritonitis. Rats had vascular catheters implanted, and sepsis was induced by cecal ligation and puncture. Rates of whole-body glucose appearance (Ra), disappearance (Rd), and metabolic clearance (MCR) were determined by the constant infusion of 3H-glucose, and in vivo glucose uptake (Rg) by individual tissues was assessed by using 14C-deoxyglucose. During the hyperglycemic phase of sepsis (2 h), glucose Ra and Rd were increased, but glucose MCR was unaltered. In contrast, during the euglycemic phase (6 h), the sepsis-induced increase in glucose Ra and Rd was associated with an elevation in the MCR. Finally, during the hypoglycemic phase (24 h), sepsis decreased glucose Ra and Rd and the glucose MCR. The sepsis-induced changes in Rg for skeletal muscle and adipose tissue mimic those seen for the whole body at each time point. Rg for skin and intestine was elevated at 2 h and 6 h but was not different from control values at 24 h. In contrast, the Rg for liver, lung, and spleen was increased at all 3 time points. In a second study, there was no difference in Rg for any tissue between 2-h septic rats and control animals in which blood glucose and insulin levels were artificially elevated to the same degree. In a third study, the prevailing glucose and insulin levels in control animals were decreased, by injection of the gluconeogenic inhibitor 3-mercaptopicolinic acid, to levels seen in 24-h septic rats. There was no difference in the Rg for muscle and adipose tissue between 24-h septic rats and hypoglycemic insulinopenic control animals. However, the Rg for liver, lung, and spleen remained elevated in 24-h septic rats, compared with hypoglycemic insulinopenic control values. These data indicate that the increased tissue glucose uptake observed during the early phase of sepsis is a consequence of concomitant changes in plasma glucose and insulin. In contrast, during the euglycemic and hypoglycemic stages of sepsis, glucose uptake in macrophage-rich tissues remains elevated and is independent of changes in glucose and insulin.

Growth hormone does not attenuate the inhibitory effects of sepsis on wound healing

Chronic abdominal sepsis is associated with impaired tissue repair. Treatment of burn patients with growth hormone results in improved healing of skin graft donor sites. The goal of this study was to determine whether administration of growth hormone could attenuate the inhibitory effects of sepsis on cutaneous wound healing. Four groups of male Sprague Dawley rats were studied: control, control + growth hormone, sepsis, and sepsis + growth hormone. Sepsis was caused by implantation of a bacterial focus in the peritoneal cavity. Control animals underwent sham laparotomy, and polyvinyl alcohol sponge implants were placed in subdermal pockets in all animals. Saline or growth hormone (400 microg) was injected subcutaneously every 12 hours. On day 5, the incisional wounds and polyvinyl alcohol sponge implants were harvested. The breaking strength of abdominal incisions was measured. Granulation tissue penetration and quality were determined by scoring polyvinyl alcohol sponge implant histology from 1 to 4 in a blinded fashion. Collagen deposition in polyvinyl alcohol sponge implants was quantitated by hydroxyproline assay. Septic mortality was not altered by growth hormone administration. Septic animals showed a reduction in food consumption for 2 days after surgery (p < 0.05 vs. controls), which was not affected by growth hormone administration. The breaking strength of incisional wounds and hydroxyproline content of polyvinyl alcohol sponge implants was reduced in septic rats (p < 0.001 vs. controls) but administration of growth hormone for 5 days did not improve breaking strength or collagen deposition in either group. We conclude that the administration of growth hormone for 5 days did not improve collagen deposition or breaking strength in cutaneous wounds from control or septic animals. The results suggest that growth hormone treatment is unlikely to improve tissue repair in sepsis-induced catabolic illness.

Impaired protein synthesis induced by acute alcohol intoxication is associated with changes in eIF4E in muscle and eIF2B in liver

BACKGROUND

Acute alcohol intoxication in rats decreases protein synthesis in skeletal muscle and, to a lesser extent, in liver. The purpose of the present study was to examine potential mechanisms for the inhibitory effect of acute ethanol exposure.

METHODS

Rats were injected intraperitoneally with either ethanol (75 mmol/kg) or saline, and tissues were examined 2.5 hr later. Rates of protein synthesis in vivo were determined by [3H]phenylalanine incorporation into protein, and various eukaryotic initiation factors (eIFs) were quantitated by Western blot analysis to identify possible mechanisms for regulating translation.

RESULTS

Protein synthesis in gastrocnemius and liver was decreased (39% and 21%, respectively) after alcohol administration, compared with saline-injected control animals. Alcohol administration did not alter tissue RNA content but diminished translational efficiency in muscle (43%) and liver (24%). Hepatic eIF2B activity was decreased 24% in alcohol-treated rats, and this was associated with a 95% increase in eIF2alpha phosphorylation. However, alcohol did not alter the amount of 4E-binding protein 1 (4E-BP1) bound to eIF4E, cIF4E bound to eIF4G, or the phosphorylation state of either 4E-BP1 or eIF4E. In contrast to liver, neither eIF2B activity nor the phosphorylation of eIF2alpha was affected in muscle of alcohol-treated rats. However, acute alcohol intoxication increased binding of 4E-BP1 to eIF4E (113%), decreased the amount of cIF4E bound to cIF4G (81%), and decreased the amount of 4E-BP1 in the phosphorylated gamma-form (77%). The plasma concentrations of insulin and insulin-like growth factor-I were unchanged by alcohol, but muscle insulin-like growth factor-I messenger ribonucleic acid abundance was decreased 35%.

CONCLUSIONS

These data suggest that acute alcohol intoxication decreases translation initiation and protein synthesis in liver and muscle via different mechanisms. Changes in eIF2B appear to predominate in liver, whereas alterations in eIF4E availability appear more critical in skeletal muscle for controlling translation initiation.

Acute effects of growth hormone in alcohol-fed rats

The present study examined whether administration in vivo of a maximally stimulating dose of growth hormone (GH) was capable of modulating selected aspects of the GH-insulin-like growth factor (IGF) system to the same extent in alcohol-fed and control animals. Rats were maintained on an alcohol-containing diet for 14 weeks, while control animals were fed isocalorically. After surgical implantation of a catheter in the carotid artery, rats were starved overnight. The next morning, rats were injected with recombinant human GH (500 microg/kg, s.c.) or an equal volume of saline at time 0 and 12 h. Blood samples were collected prior to GH and at 6, 12 and 24 h thereafter; tissues were collected at the end of the study. Time-matched control and alcohol-fed rats not receiving GH were also included. Although the plasma concentrations of both total and free IGF-I were decreased 30-40% in alcohol-fed rats, the ability of GH to elevate circulating IGF-I was not diminished. GH was equally effective at increasing IGF-I peptide levels in both liver and skeletal muscle. GH also produced comparable increases in IGF-I mRNA in muscle in both groups. Hepatic GH receptor (GHR) peptide levels were not significantly altered by either alcohol or GH. Alcohol feeding decreased plasma levels of IGF binding protein (IGFBP)-3 and increased IGFBP-1, and GH did not significantly alter this profile. Hepatic expression of suppressor of cytokine signalling (SOCS-3) mRNA was not different between the groups. However, SOCS-3 mRNA was increased by approximately 50% in control animals in response to GH, but remained unchanged in alcohol-fed rats. These data indicate that the decrease in hepatic IGF-I synthesis and plasma IGF-I observed in alcohol-fed rats was independent of a change in GHR levels. In contrast, the ability of a maximally stimulating dose of GH to modulate selected biological responses in vivo was not impaired by chronic alcohol consumption and was associated with a lack of a GH-induced increase in SOCS-3 mRNA.

Severe diabetes prohibits elevations in muscle protein synthesis after acute resistance exercise in rats

This study determined whether rates of protein synthesis increase after acute resistance exercise in skeletal muscle from severely diabetic rats. Previous studies consistently show that postexercise rates of protein synthesis are elevated in nondiabetic and moderately diabetic rats. Severely diabetic rats performed acute resistance exercise (n = 8) or remained sedentary (n = 8). A group of nondiabetic age-matched rats served as controls (n = 9). Rates of protein synthesis were measured 16 h after exercise. Plasma glucose concentrations were >500 mg/dl in the diabetic rats. Rates of protein synthesis (nmol phenylalanine incorporated. g muscle(-1). h(-1), means +/- SE) were not different between exercised (117 +/- 7) and sedentary (106 +/- 9) diabetic rats but were significantly (P < 0.05) lower than in sedentary nondiabetic rats (162 +/- 9) and in exercised nondiabetic rats (197 +/- 7). Circulating insulin concentrations were 442 +/- 65 pM in nondiabetic rats and 53 +/- 11 and 72 +/- 19 pM in sedentary and exercised diabetic rats, respectively. Plasma insulin-like growth factor I concentrations were reduced by 33% in diabetic rats compared with nondiabetic rats, and there was no difference between exercised and sedentary diabetic rats. Muscle insulin-like growth factor I was not affected by resistance exercise in diabetic rats. The results show that there is a critical concentration of insulin below which rates of protein synthesis begin to decline in vivo. In contrast to previous studies using less diabetic rats, severely diabetic rats cannot increase rates of protein synthesis after acute resistance exercise.

Chronic alcohol feeding impairs hepatic translation initiation by modulating eIF2 and eIF4E

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.

Differential effects of insulin-like growth factor I (IGF-I) and IGF-binding protein-1 on protein metabolism in human skeletal muscle cells

Insulin-like growth factor-binding protein-1 (BP-1) is a multifunctional protein that binds IGF-I in solution and integrins on the cell surface. BP-1 is overexpressed during catabolic illnesses, and the protein accumulates in skeletal muscle. To define a potential physiological role for BP-1 in regulating muscle protein balance, we have examined the effect of IGF-I and BP-1 on protein synthesis and degradation in human skeletal muscle cells. IGF-I-stimulated protein synthesis by 20%, and this was completely inhibited by either phosphorylated or nonphosphorylated BP-1. Half-maximal inhibition of protein synthesis occurred at a molar ratio of BP-1 to IGF-I of 1.5:1. BP-1 failed to form a complex with a truncated form of IGF-I (desIGF-I), and consequently, BP-1 failed to inhibit the ability of desIGF-I to stimulate protein synthesis. IGF-I and BP-1 dose-dependently inhibited protein degradation individually, and both BP-1 phosphovariants failed to block the ability of IGF-I to do the same. Blocking integrin receptor occupancy with the integrin antagonist echistatin blunted the ability of BP-1 to inhibit protein degradation, but had no significant effect on IGF-I-mediated changes in protein synthesis or degradation. The extracellular matrix protein vitronectin also inhibited protein degradation, but vitronectin receptor antibodies failed to block BP-1 action. In contrast, antibodies to the beta1 integrin subunit blocked BP-1-mediated inhibition of protein degradation. Rapamycin inhibited IGF-I-dependent protein synthesis, but not the ability of IGF-I to inhibit proteolysis. In contrast, rapamycin completely blocked the ability of BP-1 to inhibit proteolysis. Our results demonstrate that BP-1 inhibits IGF-I-mediated protein synthesis by binding to IGF-I. BP-1, acting independently of IGF-I, inhibits protein degradation. The IGF-independent response occurs via beta1 integrin binding and stimulation of a rapamycin-sensitive signal transduction pathway.

Insulin-like growth factor system in patients with HIV infection: effect of exogenous growth hormone administration

The purpose of this study was to characterize changes in the levels of insulin-like growth factor-I (IGF-I) and IGF binding proteins (BP) 1, 2, and 3 in HIV-infected adults throughout the course of their disease, and to assess the responsiveness of the IGF system components to growth hormone (GH) administration (6 mg/day) for 2 weeks. Healthy control study subjects (n = 10) were compared with patients who were either HIV-positive (n = 9), had AIDS without weight loss (n = 13), or had AIDS with >10% weight loss (n = 6), all of whom had been free of acute illness for at least 3 months. Under basal conditions, fasting serum concentrations of epinephrine, norepinephrine, cortisol, glucagon, insulin, IGF-I, and IGFBP-3 were not significantly different among the four groups. The serum concentrations of IGFBP-1 and IGFBP-2 were significantly higher in AIDS patients with wasting than in the other three groups (p < .05). In addition, there was a statistically significant positive correlation between the levels of IGFBP- 1 (p = .004) and IGFBP-2 (p = .03) and the stage of disease. Following GH administration, the serum concentrations of insulin and IGF-I were increased in all groups (p < .05). In addition, the increases in insulin levels correlated with stage of disease (p = .004). The responses of the IGFBPs were more variable. GH administration significantly increased the levels of IGFBP-3 in all groups except the patients with AIDS wasting, whereas the levels of IGFBP-1 were significantly decreased in controls and AIDS patients. These results demonstrate that there is a continuum of both elevations in the IGFBPs and altered metabolic responsiveness in patients infected with HIV that increases with the severity of the disease. These data also demonstrate that AIDS patients, who are free from secondary infection, respond to administration of GH by significantly increasing hepatic IGF-I production.

Hypertrophy of skeletal muscle in diabetic rats in response to chronic resistance exercise

This study had the following objectives: 1) to determine whether diabetic rats could increase muscle mass due to a physiological manipulation (chronic resistance exercise), 2) to determine whether exercise training status modifies the effect of the last bout of exercise on elevations in rates of protein synthesis, and 3) to determine whether chronic resistance exercise alters basal glycemia. Groups consisted of diabetic or nondiabetic rats that performed progressive resistance exercise for 8 wk, performed acute resistance exercise, or remained sedentary. Arterial plasma insulin in diabetic groups was reduced by about one-half (P < 0.05) compared with nondiabetic groups. Soleus and gastrocnemius-plantaris complex muscle wet weights were lower because of diabetes, but in response to chronic exercise these muscles hypertrophied in diabetic (0.028 +/- 0.003 vs. 0.032 +/- 0.0015 g/cm for sedentary vs. exercised soleus and 0.42 +/- 0.068 vs. 0.53 +/- 0.041 g/cm for sedentary vs. exercised gastrocnemius-plantaris, both P < 0.05) but not in nondiabetic (0.041 +/- 0.0026 vs. 0.042 +/- 0.003 g/cm for sedentary vs. exercised soleus and 0.72 +/- 0.015 vs. 0.69 +/- 0.013 g/cm for sedentary vs. exercised gastrocnemius-plantaris) rats when muscle weight was expressed relative to tibial length or body weight (data not shown). Another group of diabetic rats that lifted heavier weights showed muscle hypertrophy. Rates of protein synthesis were higher in red gastrocnemius in chronically exercised than in sedentary rats: 155 +/- 11 and 170 +/- 7 nmol phenylalanine incorporated x g muscle(-1) x h(-1) in exercised diabetic and nondiabetic rats vs. 110 +/- 14 and 143 +/- 7 nmol phenylalanine incorporated x g muscle(-1) x h(-1) in sedentary diabetic and nondiabetic rats. These elevations, however, were lower than in acutely exercised (but untrained) rats: 176 +/- 15 and 193 +/- 8 nmol phenylalanine incorporated x g muscle(-1) x h(-1) in diabetic and nondiabetic rats. Finally, chronic exercise training in diabetic rats was associated with reductions in basal glycemia, and such reductions did not occur in sedentary diabetic groups. These data demonstrate that, despite lower circulating insulin concentrations, diabetic rats can increase muscle mass in response to a physiological stimulus.

Inhibition of muscle protein synthesis by alcohol is associated with modulation of eIF2B and eIF4E

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.