Newly synthesized RNA: simultaneous measurement in intact cells of transcription rates and RNA stability of insulin-like growth factor I, actin, and albumin in growth hormone-stimulated hepatocytes

A selective and sensitive detection system for 4-thiouridine modification in RNA

4-Thiouridine (s4U) is a modified nucleoside, found at positions 8 and 9 in tRNA from eubacteria and archaea. Studies of the biosynthetic pathway and physiological role of s4U in tRNA are ongoing in the tRNA modification field. s4U has also recently been utilized as a biotechnological tool for analysis of RNAs. Therefore, a selective and sensitive system for the detection of s4U is essential for progress in the fields of RNA technologies and tRNA modification. Here, we report the use of biotin-coupled 2-aminoethyl-methanethiosulfonate (MTSEA biotin-XX) for labeling of s4U and demonstrate that the system is sensitive and quantitative. This technique can be used without denaturation; however, addition of a denaturation step improves the limit of detection. Thermus thermophilus tRNAs, which abundantly contain 5-methyl-2-thiouridine, were tested to investigate the selectivity of the MTSEA biotin-XX s4U detection system. The system did not react with 5-methyl-2-thiouridine in tRNAs from a T. thermophilus tRNA 4-thiouridine synthetase (thiI) gene deletion strain. Thus, the most useful advantage of the MTSEA biotin-XX s4U detection system is that MTSEA biotin-XX reacts only with s4U and not with other sulfur-containing modified nucleosides such as s2U derivatives in tRNAs. Furthermore, the MTSEA biotin-XX s4U detection system can analyze multiple samples in a short time span. The MTSEA biotin-XX s4U detection system can also be used for the analysis of s4U formation in tRNA. Finally, we demonstrate that the MTSEA biotin-XX system can be used to visualize newly transcribed tRNAs in S. cerevisiae cells.

Determining mRNA Stability by Metabolic RNA Labeling and Chemical Nucleoside Conversion

The varying rates at which mRNAs decay are tightly coordinated with transcriptional changes to shape gene expression during development and disease. But currently available RNA sequencing approaches lack the temporal information to determine the relative contribution of RNA biogenesis, processing and turnover to the establishment of steady-state gene expression profiles.Here, we describe a protocol that combines metabolic RNA labeling with chemical nucleoside conversion by thiol-linked alkylation of 4-thiouridine to determine RNA stability in cultured cells (SLAMseq). When coupled to cost-effective mRNA 3' end sequencing approaches, SLAMseq determines the half-life of polyadenylated transcripts in a global and transcript-specific manner using untargeted or targeted cDNA library preparation protocols.We provide a step-by-step instruction for time-resolved mRNA 3' end sequencing, which augments traditional RNA-seq approaches to acquire the temporal resolution necessary to study the molecular principles that control gene expression.

Critical appraisal of the role of serum albumin in cardiovascular disease

Concentration of serum albumin (SA), a multifunctional circulatory protein, is influenced by several factors, including its synthesis rate, catabolism rate, extravascular distribution, and exogenous loss. Moreover, both nutritional status and systemic inflammation affect the synthesis of SA. Determining SA concentration aids in risk prediction in various clinical settings. It is of interest to understand the prognostic value of SA in the full spectrum of cardiovascular disease (CVD) in the era of newly developed pharmacological and interventional treatments. Proper interpretation of SA in addition to established risk factors potentially provides a better risk discrimination and thereby presents an option to modify therapeutic strategies accordingly. In this narrative review, we summarize the basic features of SA and its associated physiological functions contributing to its prognostic impacts on CVD. Finally, we discuss the prognostic role of SA in CVDs based on existing evidence.

Metabolic labeling and recovery of nascent RNA to accurately quantify mRNA stability

Changes in the rate of mRNA decay are closely coordinated with transcriptional changes and together these events have profound effects on gene expression during development and disease. Traditional approaches to assess mRNA decay have relied on inhibition of transcription, which can alter mRNA decay rates and confound interpretation. More recently, metabolic labeling combined with chemical modification and fractionation of labeled RNAs has allowed the isolation of nascent transcripts and the subsequent calculation of mRNA decay rates. This approach has been widely adopted for measuring mRNA half-lives on a global scale, but has proven challenging to use for analysis of single genes. We present a series of normalization and quality assurance steps to be used in combination with 4-thiouridine pulse labeling of cultured eukaryotic cells. Importantly, we demonstrate how the relative amount of 4sU-labeled nascent RNA influences accurate quantification. The approach described facilitates reproducible measurement of individual mRNA half-lives using 4-thiouridine and could be adapted for use with other nucleoside analogs.

Temporally and spatially restricted gene expression profiling

Identifying gene function in specific cells is critical for understanding the processes that make cells unique. Several different methods are available to isolate actively transcribed RNA or actively translated RNA in specific cells at a chosen time point. Cell-specific mRNA isolation can be accomplished by the expression of transgenes in cells of interest, either directly from a specific promoter or using a modular system such as Gal4/UAS or Cre/lox. All of the methods described in this review, namely thiol-labeling of RNA (TU-tagging or RABT), TRAP (translating ribosome affinity purification) and INTACT (isolation of nuclei tagged in specific cell types), allow next generation sequencing, permitting the identification of enriched gene transcripts within the specific cell-type. We describe here the general concept of each method, include examples, evaluate possible problems related to each technique, and suggest the types of questions for which each method is best suited.

Genome-wide survey of interindividual differences of RNA stability in human lymphoblastoid cell lines

The extent to which RNA stability differs between individuals and its contribution to the interindividual expression variation remain unknown. We conducted a genome-wide analysis of RNA stability in seven human HapMap lymphoblastoid cell lines (LCLs) and analyzed the effect of DNA sequence variation on RNA half-life differences. Twenty-six percent of the expressed genes exhibited RNA half-life differences between LCLs at a false discovery rate (FDR) < 0.05, which accounted for ~ 37% of the gene expression differences between individuals. Nonsense polymorphisms were associated with reduced RNA half-lives. In genes presenting interindividual RNA half-life differences, higher coding GC3 contents (G and C percentages at the third-codon positions) were correlated with increased RNA half-life. Consistently, G and C alleles of single nucleotide polymorphisms (SNPs) in protein coding sequences were associated with enhanced RNA stability. These results suggest widespread interindividual differences in RNA stability related to DNA sequence and composition variation.

Eukaryotic mRNA decay: methodologies, pathways, and links to other stages of gene expression

mRNA concentration depends on the balance between transcription and degradation rates. On both sides of the equilibrium, synthesis and degradation show, however, interesting differences that have conditioned the evolution of gene regulatory mechanisms. Here, we discuss recent genome-wide methods for determining mRNA half-lives in eukaryotes. We also review pre- and posttranscriptional regulons that coordinate the fate of functionally related mRNAs by using protein- or RNA-based trans factors. Some of these factors can regulate both transcription and decay rates, thereby maintaining proper mRNA homeostasis during eukaryotic cell life.

Long-term persistance of the pathophysiologic response to severe burn injury

Background

Main contributors to adverse outcomes in severely burned pediatric patients are profound and complex metabolic changes in response to the initial injury. It is currently unknown how long these conditions persist beyond the acute phase post-injury. The aim of the present study was to examine the persistence of abnormalities of various clinical parameters commonly utilized to assess the degree hypermetabolic and inflammatory alterations in severely burned children for up to three years post-burn to identify patient specific therapeutic needs and interventions.

Methodology/Principal Findings

Patients: Nine-hundred seventy-seven severely burned pediatric patients with burns over 30% of the total body surface admitted to our institution between 1998 and 2008 were enrolled in this study and compared to a cohort non-burned, non-injured children. Demographics and clinical outcomes, hypermetabolism, body composition, organ function, inflammatory and acute phase responses were determined at admission and subsequent regular intervals for up to 36 months post-burn. Statistical analysis was performed using One-way ANOVA, Student's t-test with Bonferroni correction where appropriate with significance accepted at p<0.05. Resting energy expenditure, body composition, metabolic markers, cardiac and organ function clearly demonstrated that burn caused profound alterations for up to three years post-burn demonstrating marked and prolonged hypermetabolism, p<0.05. Along with increased hypermetabolism, significant elevation of cortisol, catecholamines, cytokines, and acute phase proteins indicate that burn patients are in a hyperinflammatory state for up to three years post-burn p<0.05.

Conclusions

Severe burn injury leads to a much more profound and prolonged hypermetabolic and hyperinflammatory response than previously shown. Given the tremendous adverse events associated with the hypermetabolic and hyperinflamamtory responses, we now identified treatment needs for severely burned patients for a much more prolonged time.

Mammal cells double their total RNAs against diabetes, ischemia reperfusion and malaria-induced oxidative stress

Total cellular RNA level is stable usually, although it may increase gradually during growth or decrease gradually under certain stressors. However, we found that mammal cell RNAs could be doubled within 24 h in response to free heme accumulation (ischemia reperfusion and malaria infection) or a high level of glucose treatment (diabetes). Clinical investigations in rats showed that pretreatment with heme (24 h for doubling total RNAs) alleviated oxidative damages caused by diabetes, and pretreatment with glucose (24 h for trebling total RNAs) alleviated oxidative damages caused by ischemia reperfusion or malaria infection. Therefore, this rapid RNA amplification may play an important role in mammal adaptation to diabetes, ischemia reperfusion and malaria infection-derived oxidative stress. This rapid RNA amplification is derived from glucose and heme, but not from their accompanying reactive oxygen species. Hexokinases endure glucose-derived reactive oxygen species accumulation but are not related glucose-derived RNA amplification. In contrast, the TATA box-binding protein (TBP) mediates all glucose- and heme-induced RNA amplification in mammal cells.

Endo-siRNAs depend on a new isoform of loquacious and target artificially introduced, high-copy sequences

Colonization of genomes by a new selfish genetic element is detrimental to the host species and must lead to an efficient, repressive response. In vertebrates as well as in Drosophila, piRNAs repress transposons in the germ line, whereas endogenous siRNAs take on this role in somatic cells. We show that their biogenesis depends on a new isoform of the Drosophila TRBP homologue loquacious, which arises by alternative polyadenylation and is distinct from the one that functions during the biogenesis of miRNAs. For endo-siRNAs and piRNAs, it is unclear how an efficient response can be initiated de novo. Our experiments establish that the endo-siRNA pathway will target artificially introduced sequences without the need for a pre-existing template in the genome. This response is also triggered in transiently transfected cells, thus genomic integration is not essential. Deep sequencing showed that corresponding endo-siRNAs are generated throughout the sequence, but preferentially from transcribed regions. One strand of the dsRNA precursor can come from spliced mRNA, whereas the opposite strand derives from independent transcripts in antisense orientation.

Pathophysiologic response to severe burn injury

OBJECTIVE

To improve clinical outcome and to determine new treatment options, we studied the pathophysiologic response postburn in a large prospective, single center, clinical trial.

SUMMARY BACKGROUND DATA

A severe burn injury leads to marked hypermetabolism and catabolism, which are associated with morbidity and mortality. The underlying pathophysiology and the correlations between humoral changes and organ function have not been well delineated.

METHODS

Two hundred forty-two severely burned pediatric patients [>30% total body surface area (TBSA)], who received no anabolic drugs, were enrolled in this study. Demographics, clinical data, serum hormones, serum cytokine expression profile, organ function, hypermetabolism, muscle protein synthesis, incidence of wound infection sepsis, and body composition were obtained throughout acute hospital course.

RESULTS

Average age was 8 +/- 0.2 years, and average burn size was 56 +/- 1% TBSA with 43 +/- 1% third-degree TBSA. All patients were markedly hypermetabolic throughout acute hospital stay and had significant muscle protein loss as demonstrated by a negative muscle protein net balance (-0.05% +/- 0.007 nmol/100 mL leg/min) and loss of lean body mass (LBM) (-4.1% +/- 1.9%); P < 0.05. Patients lost 3% +/- 1% of their bone mineral content (BMC) and 2 +/- 1% of their bone mineral density (BMD). Serum proteome analysis demonstrated profound alterations immediately postburn, which remained abnormal throughout acute hospital stay; P < 0.05. Cardiac function was compromised immediately after burn and remained abnormal up to discharge; P < 0.05. Insulin resistance appeared during the first week postburn and persisted until discharge. Patients were hyperinflammatory with marked changes in IL-8, MCP-1, and IL-6, which were associated with 2.5 +/- 0.2 infections and 17% sepsis.

CONCLUSIONS

In this large prospective clinical trial, we delineated the complexity of the postburn pathophysiologic response and conclude that the postburn response is profound, occurring in a timely manner, with derangements that are greater and more protracted than previously thought.

Stability of alternatively spliced IGF-I mRNA in growth plate chondrocytes

Insulin-like growth factor-I (IGF-I) enhances the proliferation and hypertrophy of growth plate chondrocytes leading to increased growth plate width thus promoting bone elongation. Differing quantities of the multiple IGF-I transcripts within the growth plate suggest differential regulation of IGF-I. To assess this, the relative stabilities of the 1Ea, 1Eb, and 2Ea IGF-I mRNA classes in 2-6 week old rat growth plates were evaluated. The mean estimated half-life of class 1Ea was 3.7+/-0.05 h, while classes 1Eb and 2Ea decayed gradually over the course of the experiment. Estimated half-lives for each IGF-I mRNA species were unchanged at all ages examined. Incubation with Act D enhanced the transcription of class 1Ea, 1Eb, and 2Ea mRNAs to varying degrees. This implies that the differential stability of alternative IGF-I mRNA classes may be an inherent regulatory component that is not influenced by an animal's developmental state, and the differential abundance of alternative IGF-I mRNA species in the growth plate throughout development is due to transcriptional differences.

Recombinant human growth hormone treatment in pediatric burn patients and its role during the hepatic acute phase response

OBJECTIVE

Recombinant human growth hormone (rHGH) has been shown to increase mortality in adult trauma patients; however, little has been reported on its side effects in children. The acute phase response has been suggested to be a contributing factor to trauma mortality. Therefore, the purpose of this study was to examine the effects of exogenous rHGH on the acute phase response in pediatric bum patients.

DESIGN

Prospective, randomized, double-blind study.

SETTING

Shriners Hospital for Children.

PATIENTS

Thermally injured pediatric patients, ranging in age from 0.1 to 16 yrs.

INTERVENTIONS

Twenty-eight thermally injured children received either 0.2 mg/kg/day of rHGH or saline (placebo) within 3 days of admission and for at least 25 days.

MEASUREMENTS AND MAIN RESULTS

Measurements were patient demographics, incidence of sepsis, inhalation injury, mortality, serum constitutive proteins, acute phase proteins, proinflammatory cytokines and insulin-like growth factor-I (IGF-I), insulin-like growth factor binding protein (IGFBP)-1, and IGFBP-3. No differences could be demonstrated in age, gender, burn size, incidence in sepsis (20% vs. 26%), inhalation injury (46% vs. 27%), or mortality (8% vs. 7%) between those receiving rHGH or placebo. Serum IGF-I and IGFBP-3 increased with rHGH treatment, whereas serum IGFBP-1 decreased compared with placebo (p < .05). Burned children treated with rHGH required significantly less albumin substitution to maintain normal levels compared with placebo (p < .05). Those receiving rHGH demonstrated a decrease in serum C-reactive protein and serum amyloid-A and an increase in serum retinol-binding protein compared with placebo (p < .05). rHGH decreased serum tumor necrosis factor-alpha and interleukin (IL)-1beta, whereas no changes were found for serum IL-1alpha, IL-6, and IL-10 compared with placebo (p < .05). Free fatty acids were elevated in burned children who received rHGH (p < .05).

CONCLUSION

Data indicate that rHGH does not increase mortality. rHGH decreased acute phase proteins, tumor necrosis factor-alpha, and IL-1beta, which is associated with increases in constitutive hepatic proteins and IGF-I.

Hepatocyte growth factor modulates the hepatic acute-phase response in thermally injured rats

OBJECTIVE

Hepatocyte growth factor (HGF) has been shown to modulate the acute-phase response in vitro. The specific in vivo role of HGF in this multifactorial response, however, remains unknown. This study examines the effects of exogenous HGF on the acute-phase response in thermally injured rats.

DESIGN

Prospective, randomized, laboratory study.

SETTINGS

Shriners Hospital for Children and University of Texas Medical Branch laboratories.

SUBJECTS

Fifty-six male Sprague-Dawley rats (weight range, 300-325 g).

INTERVENTION

Animals received a 60% total body surface area third-degree scald burn and were randomly divided to receive either 400 microg/kg/day i.v. HGF or saline (control).

MEASUREMENTS AND MAIN RESULTS

Serum acute-phase proteins, cytokines, and insulin-like growth factor (IGF)-I concentrations, as well as liver weight, protein and triglyceride content, IGF-I concentrations, and cytokine gene expression were measured 1, 2, 5, or 7 days after burn. Serum albumin was increased on days 2, 5, and 7 after burn, and transferrin was increased on day 7 after burn in HGF-treated rats compared with controls (p<.05). HGF increased alpha2-macroglobulin concentrations on postburn days 2, 5, and 7 compared with controls (p<.05). Serum interleukin-6 and tumor necrosis factor-alpha were significantly higher within 2 days of burn in rats treated with HGF (p<.05). HGF increased the hepatic gene expression of tumor necrosis factor-alpha compared with controls (p<.05). Serum IGF-I decreased in rats receiving HGF 1, 2, and 5 days after burn, whereas liver IGF-I concentrations were higher on days 1 and 7 after burn compared with controls (p<.05). Hepatic protein concentrations were higher in the HGF group compared with controls on postburn days 1, 2, and 7, with a concomitant increase in total liver weight (p<.05). HGF exerted a strong mitogenic effect on hepatocytes 1 and 2 days after thermal injury compared with controls (p<.05).

CONCLUSIONS

These findings suggest that HGF modulates the acute-phase response in vivo after burn and causes changes in liver morphology.

Hypoxia increases glyceraldehyde-3-phosphate dehydrogenase transcription in rat alveolar epithelial cells

Alveolar epithelial type II (ATII) cells are particularly hypoxia-tolerant in vitro. As one of the mechanisms of hypoxia tolerance is the induction of certain proteins, one of which is glyceraldehyde-3-phosphate dehydrogenase (GAPDH), we investigated whether hypoxia modified GAPDH expression in ATII cells. Hypoxia induced a time- and O(2) concentration-dependent accumulation of GAPDH mRNA in cultured rat ATII cells (2- to 3-fold the normoxic value after 18 h in 0% O(2)), an effect completely reversed by reoxygenation. GAPDH mRNA induction was accounted for by an increase in GAPDH gene transcription during hypoxia with no change in mRNA stability. GAPDH protein synthesis increased 3- to 4-fold after 18 h of 0% O(2), while the GAPDH protein steady-state level rose by 75%. GAPDH enzymatic activity in hypoxic cell homogenates increased by 45%. These results indicate that hypoxia induces GAPDH expression in ATII cells through an increase in transcription.

Cyclic AMP regulates G(omicronalpha) protein and mRNA levels by modulating the transcriptional rate of G(omicronalpha) gene

In rat astroglial cells, four G(omicronalpha) transcripts were found: G(omicron2alpha) mRNA (5.7 kb) and three G(omicron1alpha) mRNAs (4.0, 3.0 and 2.3 kb). However, G(omicron2alpha) but little G(omicronalpha1) proteins were present in membrane-enriched fractions. Culturing astroglial cells with forskolin (10 microM) or isoproterenol (10 microM) a beta-adrenergic agonist increased transiently in a time-dependent manner the levels of G(omicronalpha) proteins. The degradation rate of G(omicronalpha) proteins was slightly decreased by the cAMP treatment. In parallel, forskolin (10 microM) treatment increased transiently the amounts of both G(omicron1alpha) and G(omicron2alpha) mRNAs. The relative transcription rate of G(omicronalpha) gene was increased by 1.7-fold in forskolin-treated cells whereas the half-lives of G(omicron1alpha) and G(omicron2alpha) mRNAs were not significantly changed. These results suggest that cAMP regulates the transcription rate of G(omicronalpha) gene and this is compatible with the existence of a cAMP responsive element in the promoter of the G(omicronalpha) gene.

Increased expression of insulin-like growth factor-I in serum and liver after recombinant human growth hormone administration in thermally injured rats

BACKGROUND

Recombinant human growth hormone (rhGH) has been shown to modulate the hypermetabolic response and the hepatic acute-phase response after thermal injury. In vitro studies, however, demonstrated that rhGH activates insulin-like growth factor-I (IGF-I) gene transcription and production, suggesting that rhGH may exert some of its effects indirectly through IGF-I stimulation. The purpose of this study was to determine the effects of rhGH on serum and hepatic IGF-I in thermally injured rats.

METHODS

Sprague-Dawley rats (56 males) receiving a 60% TBSA third-degree scald burn were randomly divided to receive either rhGH (2.5 mg/kg/day im) or saline (control). Rats were sacrificed on postburn days 1, 2, 5, and 7 and serum IGF-I, hepatic IGF-I mRNA, and IGF-I protein concentration were measured. The physiologic response to changes in IGF-I levels was evaluated by measuring hepatocyte proliferation, total liver protein concentration, and muscle dry/wet weights.

RESULTS

Serum IGF-I was increased from postburn day 1 through day 7 in rats receiving rhGH compared to controls (P < 0.05). Hepatic IGF-I mRNA and IGF-I protein expression were increased from day 1 to 7 after burn in animals receiving rhGH when compared to controls (P < 0.05). Recombinant hGH increased hepatocyte proliferation at 5 days and total liver protein concentration at 5 and 7 days postburn compared to controls (P < 0.05). Muscle dry/wet weights increased in rats receiving rhGH at 7 days after burn compared to controls (P < 0.05).

SUMMARY

Liver and serum IGF-I levels decreased after a thermal injury. Recombinant hGH attenuated this decrease by stimulating hepatic IGF-I expression. Increases in IGF-I were associated with increases in hepatocyte proliferation and protein concentration in liver and muscle.

CONCLUSION

We suggest that rhGH modulates the hypermetabolic response through IGF-I stimulation in the hepatic parenchyma.

Both transcriptional and posttranscriptional mechanisms regulate human telomerase template RNA levels

The human telomerase RNA component (hTR) is present in normal somatic cells at lower levels than in cancer-derived cell lines. To understand the mechanisms regulating hTR levels in different cell types, we have compared the steady-state hTR levels in three groups of cells: (i) normal telomerase-negative human diploid cells; (ii) normal cells transfected with the human telomerase catalytic subunit, hTERT; and (iii) cells immortalized in vitro and cancer cells expressing their own endogenous hTERT. To account for the differences in steady-state hTR levels observed in these cell types, we compared the transcription rate and half-life of hTR in a subset of these cells. The half-life of hTR in telomerase-negative cells is about 5 days and is increased 1.6-fold in the presence of hTERT. The transcription rate of hTR is essentially unchanged in cells expressing exogenous hTERT, and the increased steady-state hTR level appears to be due to the increased half-life. However, the transcription rate of hTR is greatly increased in cells expressing endogenous hTERT, suggesting some overlap in transcriptional regulatory control. We conclude that the higher hTR level in cells expressing an endogenous telomerase can be a result of both increased transcription and a longer half-life and that the longer half-life might be partially a result of protection or stabilization by the telomerase catalytic subunit. The 4-week half-life of hTR in H1299 tumor cells is the longest half-life yet reported for any RNA.

Recombinant human growth hormone alters acute phase reactant proteins, cytokine expression, and liver morphology in burned rats

BACKGROUND

The effects of exogenous recombinant human growth hormone (rhGH) on hepatic acute phase reactant proteins, cytokine expression, and liver morphology were studied in thermally injured rats to define whether rhGH alters the acute phase response.

MATERIALS AND METHODS

Sprague-Dawley rats (56 males) receiving a 60% TBSA third-degree scald burn were randomly divided into two groups to receive either 2.5 mg/kg/day sc rhGH or saline. Rats were sacrificed on Postburn Days 1, 2, 5, and 7. Serum acute phase reactant proteins and cytokines TNF-alpha, IL-1alpha, IL-1beta, and IL-6 were measured. Hepatocyte proliferation, hepatic cytokine gene expression, and liver protein concentrations were determined.

RESULTS

Recombinant hGH increased serum albumin on Days 5 and 7 after burn (P < 0.05). Serum haptoglobin and alpha1-acid glycoprotein levels decreased at 2, 5, and 7 days after burn compared to saline (P < 0.05). In rats treated with rhGH, serum IL-1beta decreased 1 day postburn, while serum TNF-alpha increased 5 days after burn compared to saline (P < 0.05). Serum IL-6 and IL-1alpha did not change. Hepatic RNA levels for TNF-alpha were significantly elevated on Day 1 postburn compared to saline (P < 0. 05). Hepatic protein content increased on Days 2, 5, and 7 postburn compared to saline (P < 0.05). Hepatocyte proliferation in rhGH-treated rats increased on Day 5 after burn (P < 0.05).

CONCLUSION

Data indicate that rhGH alters the hepatic acute phase response by decreasing type I acute phase proteins and modulating IL-1-like cytokine expression. These changes are associated with increased hepatocyte mitosis and serum and total liver protein concentrations.

Growth hormone inhibits rat liver alpha-1-acid glycoprotein gene expression in vivo and in vitro

The gene encoding alpha-1-acid glycoprotein (AGP), one of the major acute-phase proteins, is positively controlled at the transcriptional level by cytokines (interleukin-1 [IL-1], IL-6, and tumor necrosis factor alpha) and glucocorticoids. Here, we show that growth hormone (GH) treatment of isolated rat hepatocytes in vitro reduces AGP messenger RNA (mRNA) expression. AGP gene expression remained inducible by IL-1, IL-6, and phenobarbital (PB) in GH-treated hepatocytes. Interestingly, the repressive effect of GH on AGP gene expression was also observed in vivo: liver AGP mRNA content was strongly increased in hypophysectomized rats, and GH treatment of these animals led to a decrease in mRNA to levels lower than those in untreated control animals. Moreover, the inhibitory effect of GH mainly occurs at the transcriptional level and can be observed as little as 0.5 hours after GH adding in vitro to isolated hepatocytes. These results show negative regulation of AGP gene expression and strongly suggest that GH is a major endogenous regulator of constitutive AGP gene expression. Moreover, transfection assays showed that the region of the AGP promoter located at position -147 to -123 is involved in AGP gene regulation by GH. Furthermore, GH deeply modifies the pattern of nuclear protein binding to this region. GH treatment of hypophysectomized rats led to the release of proteins of 42 to 45 and 80 kd and to the binding of proteins of 48 to 50 and 90 kd.

Albumin synthesis and bone collagen formation in human immunodeficiency virus-positive subjects: differential effects of growth hormone administration

Loss of lean tissue often accompanies human immunodeficiency virus (HIV) infection. Exogenous human recombinant GH (hrGH) has been shown to be beneficial in reversing this wasting. However, catabolic effects of hrGH on muscle protein metabolism have also been reported. Therefore, the responsiveness of other GH-sensitive tissues, including bone formation and albumin synthesis, has been examined. Anabolic activity in bone, from serum levels of carboxy-terminal propeptide of type I collagen, was stimulated by 2 weeks of hrGH in controls (56 +/- 15%, P = 0.002), patients with asymptomatic HIV (24 +/- 10%, not significant), patients with AIDS (47 +/- 7%, P < 0.001), and patients with AIDS and > 10% weight loss (21 +/- 12%, P = 0.02). Albumin synthesis, determined from the incorporation of L-[2H5]phenylalanine, was increased in response to hrGH in controls (23 +/- 7%, P < 0.05), HIV+ subjects (39 +/- 16%, P < 0.05), and patients with AIDS (25 +/- 7%, P < 0.01). Patients with AIDS and weight loss, however, did not increase albumin synthesis (-0.6 +/- 12%) in response to hrGH. The results indicate variable anabolic responses to hrGH. Bone collagen synthesis remained sensitive to hrGH, whereas, the anabolic action of hrGH on the synthesis of albumin diminished with severity of disease. However unlike muscle protein synthesis, albumin synthesis was not depressed below basal levels by hrGH.

Induction of oxytocin receptor gene expression in rabbit amnion cells

Oxytocin (OT)-stimulated PGE2 release by rabbit amnion is enhanced by the up-regulation of oxytocin receptors (OTR), which increase about 200-fold at the end of pregnancy. As recent studies have shown that PGs are essential for parturition, the rise in amnion OTR and associated PGE2 synthesis are probably essential for labor initiation. The present work was directed toward understanding the mechanisms of OTR up-regulation. Levels of agents that stimulate adenylyl cyclase activity and cortisol are increased in amniotic fluid at the end of pregnancy. Addition of either forskolin or cortisol to cultured amnion cells caused an increase in OTR ligand-binding sites and steady state OTR messenger RNA (mRNA) levels. Forskolin treatment elevated OTR mRNA levels rapidly, but transiently, whereas cortisol's effects were slower and sustained. Actinomycin or cycloheximide, added 3 h after forskolin, led to a sustained elevation in OTR mRNA levels, suggesting that forskolin increases the activities of OTR mRNA-destabilizing factors along with increasing OTR mRNA concentration. Cortisol did not appear to affect OTR mRNA stability. Measurement of OTR mRNA transcription rates showed that forskolin's effects were maximal within 1 h of treatment. In contrast, cortisol-induced transcription was not apparent until 8 h. The effects of forskolin and cortisol on OTR gene transcription were synergistic. Thus, the increase in OTR mRNA levels occurring after either forskolin or cortisol treatments is the result of induction of OTR gene expression, but the effects of the two agents appear to occur at separate sites.

Overexpression of growth hormone affects alternatively spliced IGF-I mRNA expression in oMt1a-oGH transgenic mice

Restorative growth hormone (GH) treatment of hypophysectomized rats differentially enhances the transcription of alternative IGF-I mRNA classes in liver. The goal of the present study was to determine the effects of GH overexpression on various classes of hepatic IGF-I mRNA in GH transgenic mice. Unstimulated oMt1a-oGH transgenic mice had low levels of transgene expression, and therefore were used to determine the effects of long-term, slightly elevated GH levels on the abundance on each alternative IGF-I mRNA class. The acute effects of high GH levels on the expression of alternative IGF-I mRNA were studied by gavaging transgenic mice with 25 mM zinc sulfate to activate oMt1a-oGH transgene expression. Long-term, low levels of oGH transgene expression in unstimulated transgenic mice resulted in a 73% down regulation of IGF-I 2Ea mRNA but not 1Ea and 2Eb mRNA. Acute stimulation of transgene expression triggered a rapid, 240% increase in 1Ea mRNA levels within 4 hours of transgene expression while 2Ea mRNA was down regulated to nearly non-detectable levels by 6 hours. IGF-I 2Eb mRNA was not affected by the short-term GH elevation. Our results showed that IGF-I 1Ea and 2Ea mRNA were differentially regulated by chronic low or acute high levels of GH. These results suggest that the regulation of IGF-I 1Ea and 2Ea mRNA transcription involve different postreceptor molecules and/or feedback mechanisms.

Hormonal regulation of liver fatty acid-binding protein in vivo and in vitro: effects of growth hormone and insulin

Liver fatty acid-binding protein (LFABP) is an abundant protein in hepatocytes that binds most of the long chain fatty acids present in the cytosol. It is suggested to be of importance for fatty acid uptake and utilization in the hepatocyte. In the present study, the effects of bovine GH (bGH) and other hormones on the expression of LFABP and its messenger RNA (mRNA) were studied in hypophysectomized rats and in vitro using primary cultures of rat hepatocytes. One injection of bGH increased LFABP mRNA levels about 5-fold after 6 h, but there was no effect of this treatment on LFABP levels. However, 7 days of bGH treatment increased both LFABP mRNA and LFABP protein levels 2- to 5-fold. Female rats had higher levels of LFABP than male rats. Hypophysectomy of female rats, but not that of male rats, decreased LFABP levels markedly. Treatment of hypophysectomized rats with bGH for 7 days as two daily injections or as a continuous infusion increased LFABP levels to a similar degree. This finding indicates that the sex difference in the expression of LFABP is not regulated by the sexually dimorphic secretory pattern of GH. Neither insulin nor insulin-like growth factor I treatment of hypophysectomized rats for 6-7 days had any effect on LFABP mRNA or LFABP levels. In vitro, bGH dose-dependently increased the expression of LFABP mRNA, but only in the presence of insulin. Insulin alone had a marked dose-dependent effect on LFABP mRNA levels and was of importance for maintaining the expression of LFABP mRNA during the culture. Incubation with bGH increased LFABP mRNA levels within 3 h. GH had no effect on LFABP mRNA levels in the presence of actinomycin D, indicating a transcriptional effect of GH. Incubation with glucagon in vitro decreased LFABP mRNA levels markedly, indicating that glucagon, in contrast to GH, has an effect opposite that of insulin on LFABP mRNA expression. It is concluded that GH is an important regulator of LFABP in vivo and in vitro. In contrast to the effect of GH on insulin-like growth factor I mRNA, the presence of insulin was a prerequisite for the effect of GH on LFABP mRNA expression in vitro. The results emphasize the role of GH in the regulation of hepatic fatty acid metabolism.

Recombinant human growth hormone (rhGH) downregulates hepatocyte growth factor (HGF) in burns

INTRODUCTION

Hepatocyte growth factor (HGF) and recombinant human growth hormone (rhGH) have both been shown to increase albumin serum concentrations after major injury. However, the effect of rhGH on HGF production after injury is unknown. We postulated that rhGH effects constitutive protein concentrations by inducing HGF expression.

METHODS

In order to determine a dose response, 20 male Sprague-Dawley rats received three different concentrations of rhGH 1, 2.5, and 5 mg/kg and a saline treatment. Rats were sacrificed 10 days after burn and serum albumin and HGF plasma concentrations were measured. Eighty male Sprague-Dawley rats received a 60% TBSA third-degree scald burn injury and were randomly divided into three groups, control, burn plus saline treatment, or burn plus rhGH treatment (2.5 mg/kg SQ qD). Rats were sacrificed 2, 5, 7, and 14 days after burn, and serum albumin, plasma, and hepatic tissue HGF concentrations were determined (ELISA, Institute of Immunology, Tokyo, Japan).

RESULTS

At 10 days postburn serum albumin levels were significantly increased with rhGH 2.5 mg/kg treatment (P < 0.05); however, HGF plasma concentrations were significantly decreased with a dose of 5 mg/kg of rhGH compared to control and rhGH 1.0 mg/kg (P < 0.05). Serum albumin concentrations decreased immediately after burn and remained low until at least the 14th day after injury. RhGH-treated animals had higher levels of albumin on Day 7 after burn (P < 0.05). Plasma HGF levels decreased immediately after burn, but increased after the second day postburn. Beginning on the 5th day after injury, HGF levels in non-rhGH-treated rats were significantly higher compared to those in rhGH-treated rats (P < 0.05). Hepatic tissue HGF concentrations were higher in non-rhGH-treated rats compared to rhGH treated animals 7 days after burn (P < 0.01).

CONCLUSION

Although rhGH treatment improves constitutive protein synthesis, rhGH decreases HGF concentration in a dose-dependent manner. The improvements in constitutive protein concentrations do not occur via a HGF dependent pathway.

Increased transcription and modified growth state-dependent expression of the plasminogen activator inhibitor type-1 gene characterize the senescent phenotype in human diploid fibroblasts

The type-1 inhibitor of plasminogen activator (PAI-1) is a major physiologic regulator of pericellular proteolytic activity and, as such, influences matrix integrity, cell-to-substrate adhesion, and cellular proliferation. Excessive accumulation of both PAI-1 mRNA and protein correlates with the progressive acquisition of morphological and growth traits characteristic of the senescent phenotype (Mu and Higgins, 1995, J. Cell. Physiol., 165:647-657). Compared to early-passage IMR-90 human diploid fibroblasts, a late-passage senescence-associated 11-fold elevation in steady-state PAI-1 mRNA content reflected a 15-fold increase in constitutive PAI-1 gene transcription. Differential mRNA stability was not a factor in age-associated PAI-1 overexpression in IMR-90 cells. Upon removal of serum, early-passage human fibroblasts enter into a state of growth arrest with marked down-regulation of PAI-1 synthesis. Rapid induction of both the 3.0- and 2.2-kb PAI-1 mRNA species was evident upon serum-induced "activation" of quiescent early-passage fibroblasts; induced PAI-1 transcripts were maximal at 2 hr post-serum stimulation and declined in late G1 prior to entry into S phase. In contrast, late-passage (p32) fibroblasts maintained a significant level of PAI-1 expression under serum-free culture conditions. Although the PAI-1 gene was further responsive to serum in senescent cells, transcript abundance remained elevated and actually increased over the 12 to 16 hr post-serum addition period (a time when early-passage fibroblasts down-regulate PAI-1 mRNA content). Development of the senescent phenotype in human fibroblasts is associated, therefore, with significant changes in PAI-1 gene regulation. Such reprogramming involves predominantly transcriptional events and results in a marked increase in steady-state PAI-1 transcript abundance involving both the 3.0- and 2.2-kb mRNA species.

Messenger RNA deadenylylation precedes decapping in mammalian cells

In yeast, the major mRNA degradation pathway is initiated by poly(A) tail shortening that triggers mRNA decapping. The mRNA is then degraded by 5'-to-3' exonucleolysis. In mammalian cells, even though poly(A) tail shortening also precedes mRNA degradation, the degradation pathway has not been elucidated. We have used a reverse transcription-PCR approach that relies on mRNA circularization to measure the poly(A) tail length of four mammalian mRNAs. This approach allows for the simultaneous analysis of the 5' and 3' ends of the same mRNA molecule. For all four mRNAs analyzed, this strategy permitted us to demonstrate the existence of small amounts of decapped mRNA species which have a shorter poly(A) tail than their capped counterparts. Kinetic analysis of one of these mRNAs indicates that the decapped species with a short poly(A) tail are mRNA degradation products. Therefore, our results indicate that decapping is preceded by a shortening of the poly(A) tail in mammalian cells, as it is in yeast, suggesting that this mRNA degradation pathway is conserved throughout eukaryotic evolution.

Growth hormone down-regulates growth hormone receptor mRNA in chickens but developmental increases in growth hormone receptor mRNA occur independently of growth hormone action

The purpose of this study was to determine the role of growth hormone (GH) in regulating expression of the chicken GH receptor (cGHR) gene by comparing the levels of cGHR mRNA in livers of normal chickens with that of GHR-deficient dwarf chickens. Since the sex-linked dwarf chicken lacks a functional cGHR, there are no genes activated as a result of GH action. Examination of the early developmental profile of hepatic cGHR mRNA in normal and dwarf chickens should yield information on the relative contribution of developmental and hormonal factors to the regulation of cGHR gene expression. Using a sensitive RNase protection assay, we found that the abundance of the major cGHR transcripts (4.3, 3.2 and 0.8 kb) in normal chickens increases about 2-fold between 1 and 7 weeks of age. Due to a splice site mutation in the dwarf chicken, the two larger transcripts encoding the full-length cGHR are not expressed. However, the expression of the truncated cGHR transcript (0.8 kb) in dwarf chickens increases about 5-fold between 1 and 7 weeks of age which suggests that the cGHR gene is overexpressed when not down-regulated by GH. Furthermore, a single promoter, appears to control expression of cGHR transcripts in liver since primer extension analysis revealed the same 5'-end in both full-length and 0.8 kb transcripts. These observations suggest that even though developmental increases in cGHR gene expression occur independently of GH action, GH, either directly or indirectly, down-regulates expression of the cGHR gene in normal chickens.

Angiotensin II receptor subtypes AT1 and AT2 are down-regulated by angiotensin II through AT1 receptor by different mechanisms

The regulatory effects of angiotensin II (AngII) on its receptor subtypes, AT1 and AT2, were studied using cultured bovine adrenal cells (BAC), which express both receptor subtypes, and PC12W and R3T3 cells, which express only AT2 receptors. In BAC, AngII caused a decrease in AT1- and AT2-binding sites and their corresponding messenger RNAs (mRNAs), but with different kinetics. AT1-binding sites decreased by more than 50% within the first 3 h, whereas AT1 mRNA started to decline after a lag period of 3 h. Both AT2-binding sites and mRNA remained stable within the first 6 h of AngII treatment. Then, AT2 mRNA decreased rapidly with an apparent half-life of 2-3 h, whereas AT2-binding sites declined with an apparent half-life of about 16 h. Measurement of transcription rate and mRNA half-life by the [3H]uridine-thiouridine method revealed that AngII reduced by 90% the rate of AT1 transcription, but had no effect on AT1 mRNA half-life, whereas it slightly reduced AT2 transcription, but markedly reduced AT2 mRNA stability. All of the effects of AngII on both AT1 and AT2 receptors were blocked by losartan, indicating that they were mediated exclusively through the AT1 receptor. In PC12W cells, AngII was unable to modify AT2-binding sites or mRNA. Moreover, in BAC, [125I]AngII was internalized through the AT1 receptor, whereas occupancy of AT2 receptors in either BAC or PC12W did not produce internalization of the hormone. These results indicate that AngII, through the AT1 receptor, down-regulates both AT1 and AT2, but by different mechanisms; AT1 receptor is regulated through internalization-degradation of the occupied receptor and inhibition of transcription, whereas AT2 receptor is regulated mainly by decreasing the stability of its mRNA. Moreover, the phorbol ester phorbol 12-myristate 13-acetate mimicked most of the effects of AngII in BAC and decreased both AT2-binding sites and mRNA on PC12W cells, indicating that the hormonal regulation of both AT1 and AT2 receptors is mediated through protein kinase C activation.

Growth hormone therapy in elderly people: an age-delaying drug?

The aims of this review are to present a brief overview of growth hormone (GH) physiology and to summarize the studies of GH treatment in adults. Special attention has been paid to randomized controlled trials. Studies have revealed a partial deficiency of GH secretion in the elderly. GH secretion on the average declines by 14% with each decade in normal adults after 20 years of age. Aging has a central effect on the GH secretion and peripheric effect on insulin-like growth factor 1 (IGF-1) through changes in the body composition. GH administration may attenuate several important decrements in body composition and in function associated with aging. GH may also have very potent anabolic effects in surgical situations. Short-term side-effects of GH therapy include edema, carpal tunnel syndrome and arthralgia. A number of agents such as oral GH-releasing peptides (GHRPs) increase GH secretion; they may be an alternative to GH treatment in the future. Further studies of GH replacement are needed, examining issues such as dosage, tolerance and efficacy before the widespread use of GH in the elderly is advocated.

Effects of diabetes mellitus on hepatocyte nuclear factor 1 decrease albumin gene transcription

We have previously reported that albumin gene transcription is reduced in diabetes mellitus (DM). The present study explored the mechanism by which albumin gene transcription is down-regulated in DM. Deletional studies and displacement of factors binding to site B of the albumin promoter indicated that the repressive effects of DM are mediated by nuclear factors binding to this site. Since hepatocyte nuclear factor 1 (HNF1) activates albumin promoter activity and is the predominant factor binding to site B, we examined HNF1. The abundance and binding activity of HNF1 were reduced in hepatonuclear extracts from diabetic compared to control rats. However, HNF1 mRNA levels were unchanged, suggesting that the effect of DM on HNF1 is at the post-transcriptional level. Extracts from diabetic animals also contained another protein, distinct from HNF1 and vHNF1, which bound to site B in gel retardation studies. In summary, our studies demonstrate that the reduced abundance and binding activity of HNF1 correlates with decreased albumin gene transcription in DM.

mRNA stability in mammalian cells

This review concerns how cytoplasmic mRNA half-lives are regulated and how mRNA decay rates influence gene expression. mRNA stability influences gene expression in virtually all organisms, from bacteria to mammals, and the abundance of a particular mRNA can fluctuate manyfold following a change in the mRNA half-life, without any change in transcription. The processes that regulate mRNA half-lives can, in turn, affect how cells grow, differentiate, and respond to their environment. Three major questions are addressed. Which sequences in mRNAs determine their half-lives? Which enzymes degrade mRNAs? Which (trans-acting) factors regulate mRNA stability, and how do they function? The following specific topics are discussed: techniques for measuring eukaryotic mRNA stability and for calculating decay constants, mRNA decay pathways, mRNases, proteins that bind to sequences shared among many mRNAs [like poly(A)- and AU-rich-binding proteins] and proteins that bind to specific mRNAs (like the c-myc coding-region determinant-binding protein), how environmental factors like hormones and growth factors affect mRNA stability, and how translation and mRNA stability are linked. Some perspectives and predictions for future research directions are summarized at the end.

mRNA stability in mammalian cells

This review concerns how cytoplasmic mRNA half-lives are regulated and how mRNA decay rates influence gene expression. mRNA stability influences gene expression in virtually all organisms, from bacteria to mammals, and the abundance of a particular mRNA can fluctuate manyfold following a change in the mRNA half-life, without any change in transcription. The processes that regulate mRNA half-lives can, in turn, affect how cells grow, differentiate, and respond to their environment. Three major questions are addressed. Which sequences in mRNAs determine their half-lives? Which enzymes degrade mRNAs? Which (trans-acting) factors regulate mRNA stability, and how do they function? The following specific topics are discussed: techniques for measuring eukaryotic mRNA stability and for calculating decay constants, mRNA decay pathways, mRNases, proteins that bind to sequences shared among many mRNAs [like poly(A)- and AU-rich-binding proteins] and proteins that bind to specific mRNAs (like the c-myc coding-region determinant-binding protein), how environmental factors like hormones and growth factors affect mRNA stability, and how translation and mRNA stability are linked. Some perspectives and predictions for future research directions are summarized at the end.

Transcriptional up-regulation of the mouse gene for the muscle-specific subunit of enolase during terminal differentiation of myogenic cells

The glycolytic enzyme enolase (EC 4.2.1.11) exists as dimers formed from three structurally related subunits alpha, beta, and gamma, encoded by separate genes. The gene encoding the beta-subunit is expressed only in striated muscles. We have previously shown that the beta-enolase gene belongs to a small subset of muscle-specific genes showing transcriptional activity in cultured myoblasts, prior to withdrawal from the cell cycle. An increase in the level of beta-enolase mRNA occurs during terminal differentiation of myoblasts. To investigate the mechanisms underlying this increase, we have simultaneously estimated, under steady state conditions, the rate of synthesis and the stability of beta-enolase mRNA in proliferating C2.7 myoblasts as well as in differentiating myotubes. The method used is based on the isolation of newly synthesized RNA from the total RNA pool, following pulse-labeling of intact cells in the presence of 4-thiouridine. The results described here demonstrate a coordinate increase in newly synthesized and total beta-enolase mRNA, while the mRNA half-life, about 4 hr, remains unchanged in the course of terminal differentiation. The expression of the gene for insulin-like growth factor-II (IGF-II), a major positive regulator of myogenesis, was analyzed using the same approach. It is concluded that the up-regulation of beta-enolase as well as IGF-II gene expression in differentiating muscle cells reflects an increased rate of entry of newly synthesized mRNAs into the general pool of transcripts without changes in their respective half-lives.

Vitamin B6 modulates expression of albumin gene by inactivating tissue-specific DNA-binding protein in rat liver

The level of albumin mRNA in the liver of vitamin B6-deficient rats was found to be 7-fold higher than that of control rats. Since the transcriptional activity of the albumin gene, as measured by a nuclear run-on assay, was increased 5-fold in vitamin B6 deficiency, the higher concentration of albumin mRNA in the liver of vitamin-deficient rats could be attributed to the enhanced rate of transcription. The promoter proximal sequences of the albumin gene interact with a number of tissue-specific transcription factors including HNF-1 and C/EBP. We determined the binding activities of liver nuclear extracts to the HNF-1- and C/EBP-binding sites by gel mobility-shift assay and found that the activities of the extract prepared from liver of vitamin B6-deficient rats were greater than those of controls. As the concentrations of C/EBP in nuclear extracts from control and vitamin-deficient rats, estimated by Western-blot analysis, were essentially the same, the lower binding activity of the extract from control liver is probably due to inactivation of tissue-specific factors by pyridoxal phosphate and/or its analogues. We therefore examined the effect of pyridoxal phosphate and its analogues on the binding activity of nuclear extract in vitro and found that only pyridoxal phosphate effectively inhibited the binding. These observations indicate that vitamin B6 modulates albumin gene expression through a novel mechanism that involves inactivation of tissue-specific transcription factors by direct interaction with pyridoxal phosphate.

Regulation of human ornithine decarboxylase expression following prolonged quiescence: role for the c-Myc/Max protein complex

WI-38 cells can remain quiescent for long periods of time and still be induced to reenter the cell cycle by the addition of fresh serum. However, the longer these cells remain growth arrested, the more time they require to enter S phase. This prolongation of the prereplicative phase has been localized to a point early in G1, after the induction of "immediate early" G1 genes such as c-fos and c-jun but before maximal expression of "early" G1 genes such as ornithine decarboxylase (ODC). Understanding the molecular basis for ODC mRNA induction can therefore provide information about the molecular events which regulate the progression of cells out of long-term quiescence into G1 and subsequently into DNA synthesis. Studies utilizing electrophoretic mobility shift assays (EMSA) of nuclear extracts from short- and long-term quiescent WI-38 cells identified a region of the human ODC promoter at -491 bp to -474 bp which exhibited a protein binding pattern that correlated with the temporal pattern of ODC mRNA expression. The presence of a CACGTG element within this fragment, studies with antibodies against c-Myc and Max, the use of purified recombinant c-Myc protein in the mobility shift assay, and antisense studies suggest that these proteins can specifically bind this portion of the human ODC promoter in a manner consistent with growth-associated modulation of the expression of ODC and other early G1 genes following prolonged quiescence. These studies suggest a role for the c-Myc/Max protein complex in regulating events involved in the progression of cells out of long-term quiescence into G1 and subsequently into S.

Growth hormone administration in older adults: effects on albumin synthesis

Evidence suggests that the albumin gene contains a growth hormone (GH) responsive element. Our purpose was to determine if GH administration to older men increases the rate of albumin synthesis and whether this is related to the increase in nitrogen retention observed during short-term recombinant human GH (rhGH) administration. Five older men (60-75 yr) received daily injections (40 micrograms/kg) of rhGH for 2 wk, whereas four others received daily injections (10 micrograms/kg) for 4 wk. In both the 2- and 4-wk recipients, rhGH administration increased (P < 0.05) fasting plasma insulin-like growth factor I levels and reduced (P < 0.05) 24-h urinary nitrogen excretion. However, during an overnight fast, the fractional rate of albumin synthesis determined by the in vivo rate of incorporation of intravenously infused L-[1-13C]leucine into plasma albumin was unchanged after 2 or 4 wk of treatment. The average plasma albumin fractional synthetic rate was 8.6 +/- 0.6%/day before and 9.4 +/- 0.7%/day after rhGH treatment (P = 0.12). We conclude that short-term rhGH administration and the subsequent increase in urinary nitrogen retention does not result in an increase in the rate of plasma albumin synthesis in older men.

Suppression of albumin enhancer activity by H-ras and AP-1 in hepatocyte cell lines

We demonstrated, using a transient transfection assay, that the albumin enhancer increased the expression of the albumin promoter in a highly differentiated, simian virus 40 (SV40)-immortalized hepatocyte cell line, CWSV1, but was not functional in two ras-transformed cell lines (NR3 and NR4) derived from CWSV1 by stable transfection with the T24ras oncogene. A transient cotransfection assay showed that T24ras and normal c-Ha-ras were each able to inhibit the activity of the albumin enhancer in an immortal hepatocyte cell line. DNase I footprinting and gel mobility shift assays demonstrated that the DNA binding activities specific to the albumin enhancer were not decreased in the ras-transformed cells. ras also did not diminish the expression of HNF1 alpha, C/EBP alpha, HNF3 alpha, HNF3 beta, or HNF3 gamma but did significantly increase AP-1 binding activity. Three AP-1 binding sites were identified within the albumin enhancer, and DNA binding activities specific to these AP-1 sites were induced in the ras-transformed hepatocytes. Subsequent functional assays showed that overexpression of c-jun and c-fos inhibited the activity of the albumin enhancer. Site-directed mutagenesis of the AP-1 binding sites in the albumin enhancer partially abrogated the suppressing effect of ras and c-jun/c-fos on the enhancer. These functional studies therefore supported the results of the structural studies with AP-1. We conclude that the activity of the albumin enhancer is subject to regulation by ras signaling pathways and that the effect of ras on the albumin enhancer activity may be mediated by AP-1.

Suppression of albumin enhancer activity by H-ras and AP-1 in hepatocyte cell lines

We demonstrated, using a transient transfection assay, that the albumin enhancer increased the expression of the albumin promoter in a highly differentiated, simian virus 40 (SV40)-immortalized hepatocyte cell line, CWSV1, but was not functional in two ras-transformed cell lines (NR3 and NR4) derived from CWSV1 by stable transfection with the T24ras oncogene. A transient cotransfection assay showed that T24ras and normal c-Ha-ras were each able to inhibit the activity of the albumin enhancer in an immortal hepatocyte cell line. DNase I footprinting and gel mobility shift assays demonstrated that the DNA binding activities specific to the albumin enhancer were not decreased in the ras-transformed cells. ras also did not diminish the expression of HNF1 alpha, C/EBP alpha, HNF3 alpha, HNF3 beta, or HNF3 gamma but did significantly increase AP-1 binding activity. Three AP-1 binding sites were identified within the albumin enhancer, and DNA binding activities specific to these AP-1 sites were induced in the ras-transformed hepatocytes. Subsequent functional assays showed that overexpression of c-jun and c-fos inhibited the activity of the albumin enhancer. Site-directed mutagenesis of the AP-1 binding sites in the albumin enhancer partially abrogated the suppressing effect of ras and c-jun/c-fos on the enhancer. These functional studies therefore supported the results of the structural studies with AP-1. We conclude that the activity of the albumin enhancer is subject to regulation by ras signaling pathways and that the effect of ras on the albumin enhancer activity may be mediated by AP-1.

Differential regulation of multiple gap junction transcripts and proteins during rat liver regeneration

The mRNA and protein expression of alpha 1 (connexin 43), beta 1 (connexin 32), and beta 2 (connexin 26) gap junction genes were examined in the regenerating rat liver after 70% partial hepatectomy (PH). Expression of beta 1 and beta 2 steady-state mRNA levels changed minimally until 12 h after PH when both transcripts decreased to approximately 15% of baseline values. A similar decrease in assembled connexin levels was detected by immunoblot and indirect immunofluorescence at 18 h after PH. Both transcripts simultaneously increased between 24 and 42 h and again rapidly decreased by 48 h post-PH. beta 1 and beta 2 assembled gap junction protein expression increased at 48 h post-PH and rapidly decreased by 56 h. By 72 to 84 h post-PH, beta 1 and beta 2 mRNA and assembled protein expression returned to near baseline levels and were maintained. Interestingly, inhibition of protein synthesis with cycloheximide completely inhibited disappearance of the beta 2 transcript, in contrast to beta 1 mRNA which was unaffected. Nuclear run-on assays showed no change in transcriptional rates for either gene during the regenerative period. However, both beta 1 and beta 2 transcripts exhibited significantly decreased mRNA half-lives at 12 h post-PH (3.8 and 3.7 h, respectively) relative to those at 0 h (10.9 and 6.1 h, respectively). Surprisingly, although the transcriptional rate for alpha 1 was similar to that observed for beta 2, no alpha 1 transcripts were detectable by northern or RNase protection analysis. The results suggest that in the regenerating rat liver, beta 1 and beta 2 gap junction genes are not regulated at the transcriptional level. Rather, the cyclical modulation of their steady-state transcripts is regulated primarily by posttranscriptional events of which mRNA stability is at least one critical factor in the control process.

Effects of actinomycin D and cycloheximide on transcript levels of IGF-I, actin, and albumin in hepatocyte primary cultures treated with growth hormone and insulin

The stability of several RNA transcripts in cultured hepatocytes is known to increase when serum is omitted from the culture medium. In order to investigate possible mechanisms for this phenomenon, we examined the effects of actinomycin D and cycloheximide on the levels of actin, albumin, and insulin-like growth factor I transcripts in primary cultures incubated in serum-free medium. The levels of IGF-I and albumin transcripts per culture increased for the first 4 hours following addition of actinomycin D and then declined. The levels of actin transcripts and total RNA per culture declined immediately following actinomycin D addition in a manner consistent with exponential decay. IGF-I and albumin transcript levels were relatively unaffected by cycloheximide, while actin transcript levels increased 7-fold over 7 hours. The half-lives of actin transcripts and total RNA were calculated to be 4.6 to 7.7 hours and 11 to 19 hours, respectively, with no statistically significant correlation with hormone treatment. The data suggest that the stability of albumin and IGF-I transcripts, but not actin transcripts, is controlled in part by an actinomycin D-sensitive process.