Effects of human growth hormone on insulin-stimulated glucose metabolism in 3T3-F442A adipocytes

Computational and functional analysis of growth hormone (GH)-regulated genes identifies the transcriptional repressor B-cell lymphoma 6 (Bc16) as a participant in GH-regulated transcription

For insight into transcriptional mechanisms mediating physiological responses to GH, data mining was performed on a profile of GH-regulated genes induced or inhibited at different times in highly responsive 3T3-F442A adipocytes. Gene set enrichment analysis indicated that GH-regulated genes are enriched in pathways including phosphoinositide and insulin signaling and suggested that suppressor of cytokine signaling 2 (SOCS2) and phosphoinositide 3' kinase regulatory subunit p85alpha (Pik3r1) are important targets. Model-based Chinese restaurant clustering identified a group of genes highly regulated by GH at times consistent with its key physiological actions. This cluster included IGF-I, phosphoinositide 3' kinase p85alpha, SOCS2, and cytokine-inducible SH2-containing protein. It also contains the most strongly repressed gene in the profile, B cell lymphoma 6 (Bcl6), a transcriptional repressor. Quantitative real-time PCR verified the strong decrease in Bcl6 mRNA after GH treatment and induction of the other genes in the cluster. Transcriptional network analysis of the genes implicated signal transducer and activator of transcription (Stat) 5 as hub regulating the most responsive genes, Igf1, Socs2, Cish, and Bcl6. Transcriptional activation analysis demonstrated that Bcl6 inhibits SOCS2-luciferase and blunts its stimulation by GH. Occupancy of endogenous Bcl6 on SOCS2 DNA decreased after GH treatment, whereas occupancy of Stat5 increased concomitantly. Thus, GH-mediated inhibition of Bcl6 expression may reverse the repression of SOCS2 and facilitate SOCS2 activation by GH. Together these analyses identify Bcl6 as a participant in GH-regulated gene expression and suggest an interplay between the repressor Bcl6 and the activator Stat5 in regulating genes, which contribute to GH responses.

Evidence for distinct effects of GH and IGF-I in the metabolic syndrome

AIMS

The metabolic syndrome is a cluster of cardiovascular risk factors which include central obesity, dyslipidaemia, glucose intolerance and hypertension. These risk factors are common in patients with growth hormone (GH) deficiency, suggesting a role for the somatotropic axis in the development of metabolic syndrome.

METHODS

We used factor analysis to investigate the relationships linking serum levels of GH and insulin-like growth factor I (IGF-I) to metabolic syndrome variables (high-density lipoprotein cholesterol, triglycerides, fasting glucose, blood pressure and waist circumference). We studied 359 men and 388 women from the Data from an Epidemiological Study on the Insulin Resistance syndrome (DESIR). Their age range was 30-64 years.

RESULTS

Three independent latent factors explained 61% of the total variance in women and four factors explained 73% in men. In both men and women, IGF-I showed a strong positive correlation with the lipid factor and a negative correlation with the obesity/glucose factor. In women, GH showed a strong negative correlation with the obesity/glucose factor but not the lipid factor. In men, GH was unrelated to the lipid and obesity/glucose factors. The blood pressure factor was not related to GH or IGF-I. In contrast with IGF-I, GH was significantly lower in women with metabolic syndrome (1575 +/- 449 pg/ml) than in the other women (2121 +/- 520 pg/ml, P = 0.002). No significant difference was observed in men for GH or IGF-I.

CONCLUSION

Our results support a link between the somatotropic axis and several features of the metabolic syndrome, and suggest distinct effects of GH and IGF-I on these parameters.

Metabolic and histochemical characteristics of fat and muscle tissues in homozygous or heterozygous pigs for the body composition QTL located on chromosome 7

Quantitative trait loci (QTL) influencing many traits including backfat thickness and carcass composition have been detected on porcine chromosome 7 (SSC7) in an F2 cross between Large White (LW) and Meishan (MS) pigs. However, the genes and controlled pathways underlying the QTL effects on body phenotype remain unknown. This study aimed at investigating the tissue characteristics at metabolic and cellular levels in pigs that were either homozygous or heterozygous for a body composition SSC7 QTL. A backcross pig (BC3) was first progeny tested to confirm its heterozygoty for the SSC7 QTL; results on all offspring (n = 80) confirmed the QTL effects on body fatness. This boar was then mated with three sows known to be heterozygous for this QTL. In the subset of pigs per genotype, we found that heterozygous LW(QTL7)/MS(QTL7) pigs had smaller adipocytes in backfat, together with a lower basal rate of glucose incorporation into lipids and lower activities of selected lipogenic enzymes in backfat isolated cells, compared with homozygous LW(QTL7)/LW(QTL7) pigs. A higher number of adipocytes was also estimated in backfat of LW(QTL7)/MS(QTL7) animals compared with LW(QTL7)/LW(QTL7) pigs. The SSC7 QTL did not influence oxidative and glycolytic metabolisms of longissimus and trapezius muscles, as estimated by the activities of specific energy metabolism enzymes, or the myofiber type properties. Altogether, this study provides new evidence for an altered adipocyte cellularity in backfat of pigs carrying at least one MS allele for the SSC7 QTL. Some candidate genes known for their functions on adipocyte growth and differentiation are suggested.

Lipid metabolism and secretory function of porcine intramuscular adipocytes compared with subcutaneous and perirenal adipocytes

The function of adipocytes interspersed between myofiber fasciculi in skeletal muscle physiology and physiopathology is poorly documented. Because regional differences in adipocyte features have been reported in various species, we hypothesized that lipid metabolism and secretory function of intramuscular (IM) adipocytes differ from that of nonmuscular adipocytes. In the present study, adipocytes isolated from trapezius muscle were compared with subcutaneous and perirenal adipocytes in growing pigs. Between 80 and 210 days of age, gene expressions and/or activities of enzymes involved in lipogenesis or lipolysis were much lower (P < 0.05) in adipocytes isolated from muscle than in those from other locations. Insulin-induced lipogenesis and lipolytic efficiency after catecholamine addition were also the lowest (P < 0.05) in IM adipocytes. In these cells, the age-related increase (+300%) in the ratio of mRNA levels of fatty acid synthase to hormone-sensitive lipase paralleled the enlargement of adipocyte diameters (+70%, P < 0.05) and the increase in lipid content in muscle (+135%, P < 0.05) during growth. Expressions of genes coding for leptin, adiponectin, and IGF-I, as well as for various hormonal receptors, were lower (P < 0.05) in IM adipocytes than in other adipocytes, whereas levels of TNF-alpha mRNA did not differ between sites. Interestingly, IGF-II mRNA levels were higher (P < 0.05) in IM adipocytes than in other adipocytes. These data support the view that IM fat is not just an ectopic extension of other fat locations but displays specific biological features during growth.

Profiles of Growth Hormone (GH)-regulated Genes Reveal Time-dependent Responses and Identify a Mechanism for Regulation of Activating Transcription Factor 3 By GH

In examination of mechanisms regulating metabolic responses to growth hormone (GH), microarray analysis identified 561 probe sets showing time-dependent patterns of expression in GH-treated 3T3-F442A adipocytes. Biological functions significantly over-represented among GH-regulated genes include regulators of transcription at early times, and lipid biosynthesis, cholesterol biosynthesis, and mediators of immune responses at later times (48 h). One novel GH-induced gene encodes activating transcription factor 3 (ATF3). Atf3 mRNA expression and promoter activity were stimulated by GH. Genes for ATF3 and growth arrest and DNA damage-inducible gene 45 gamma (GADD45gamma) showed similar time-dependent patterns of responses to GH, suggesting similar regulatory mechanisms. A conserved sequence in the promoters of the Atf3 and Gadd45gamma genes contains a CCAAT/enhancer-binding protein (C/EBP) site previously observed in the Gadd45gamma promoter, suggesting a novel corresponding C/EBP site in the Atf3 promoter. C/EBPbeta was found to bind to the predicted Atf3 C/EBP site, and C/EBPbeta enhanced the activation of the wild-type Atf3 promoter. Mutation of the predicted Atf3 C/EBP site disrupted Atf3 promoter activation not only by C/EBPbeta but also by GH. These findings suggest that GH regulates transcription of Atf3 through a mechanism utilizing factors, such as C/EBPbeta, which bind to a novel C/EBP site.

A simple and sensitive assay for GH activity based on 3T3-F442A cell differentiation

We describe a fast, sensitive, specific, and simple in vitro assay for GH biological activity, based on the differentiation of 3T3-F442A cells into adipocytes. The 3T3-F442A cells were directly plated at 1.5 x 10(4)cells/cm(2) in medium with or without various concentrations of human growth hormone (hGH). After 7 days, cells were lysed with buffer containing 0.5 % (v/v) Triton X-100, and adipose conversion was quantitated by the activity of the adipogenic enzyme glycerophosphate dehydrogenase. The assay is highly sensitive and specific for GH from different species. These culture conditions have shortened the time for the cells to undergo adipose differentiation, and they might also be useful to design and test drugs or agents that modify adipocyte differentiation or lipid metabolism, or for evaluation of cytotoxic and pharmacologic effects of drugs and other compounds.

Humoral regulation of resistin expression in 3T3-L1 and mouse adipose cells

Resistin is a hormone secreted by adipocytes that acts on skeletal muscle myocytes, hepatocytes, and adipocytes themselves, reducing their sensitivity to insulin. In the present study, we investigated how the expression of resistin is affected by glucose and by mediators known to affect insulin sensitivity, including insulin, dexamethasone, tumor necrosis factor-alpha (TNF-alpha), epinephrine, and somatropin. We found that resistin expression in 3T3-L1 adipocytes was significantly upregulated by high glucose concentrations and was suppressed by insulin. Dexamethasone increased expression of both resistin mRNA and protein 2.5- to 3.5-fold in 3T3-L1 adipocytes and by approximately 70% in white adipose tissue from mice. In contrast, treatment with troglitazone, a thiazolidinedione antihyperglycemic agent, or TNF-alpha suppressed resistin expression by approximately 80%. Epinephrine and somatropin were both moderately inhibitory, reducing expression of both the transcript and the protein by 30-50% in 3T3-L1 adipocytes. Taken together, these data make it clear that resistin expression is regulated by a variety of hormones and that cytokines are related to glucose metabolism. Furthermore, they suggest that these factors affect insulin sensitivity and fat tissue mass in part by altering the expression and eventual secretion of resistin from adipose cells.

Growth hormone-induced alterations in the insulin-signaling system

Growth hormone (GH) counteracts insulin action on lipid and glucose metabolism. However, the sequence of molecular events leading to these changes is poorly understood. Insulin action is initiated by binding of the hormone to its cell surface receptor (IR). This event activates the intrinsic tyrosine kinase activity residing in the beta-subunit of the IR and leads to autophosphorylation of the cytoplasmic portion of the beta-subunit and further activation of its tyrosine kinase towards several intermediate proteins, including the family of IR substrates (IRS) and the Shc proteins. When tyrosine phosphorylated, these cellular substrates connect the IR with several downstream signaling molecules. One of them is the enzyme phosphatidylinositol (PI) 3-kinase. The insulin antagonistic action of GH is not a consequence of a direct interaction with the IR. Instead, long-term exposure to GH is, in general, associated with hyperinsulinemia, which leads to a reduction of IR levels and an impairment of its tyrosine kinase activity. The signals of GH and insulin may converge at post-receptor levels. The signaling pathway leading to activation of PI 3-kinase appears to be an important site of convergence between the signals of these two hormones and seems to be mediated principally by IRS-1. Rodent models of chronic GH excess have been useful tools to investigate the mechanism by which GH induces insulin resistance. Decreased IR, IRS-1, and IRS-2 tyrosyl phosphorylation in response to insulin was found in skeletal muscle, whereas a chronic activation of the IRS-PI 3-kinase pathway was found in liver. The induction of the expression of proteins that inhibit IR signaling such as suppressors of cytokine signaling (SOCS)-1 and -6 may also be involved in this alteration. Interestingly, the modulation of insulin signaling and action observed in states of GH excess, deficiency, or resistance seems to be relevant to the changes in longevity associated with those states.

Diabetogenic activity of 20 kDa human growth hormone (20K-hGH) and 22K-hGH in rats

To compare the diabetogenic activity of 20 kDa human growth hormone (20K-hGH) with that of 22K-hGH, we evaluated insulin sensitivity with a euglycaemic clamp in rats. The glucose infusion rate (GIR) in euglycaemic clamp studies was measured as an indicator of insulin sensitivity. [(14)C]glucose and 2-[(3)H] deoxy- D -glucose injection were used to calculate the rate of glucose utilization (R(d)), the hepatic glucose output (HGO), and the glucose metabolic index (R(g)'). Both 20K- and 22K-hGH were infused at equivalent rates (1.0 (mg/kg)/day). A 24 h infusion of 20K-hGH had no significant effects on the GIR, R(d), HGO and R(g)(')except for in gastrocnemius muscle. In contrast, 22K-hGH significantly lowered the GIR compared with the control (P< 0.001) and 20K-hGH groups (P< 0.01). The infusion of 22K-hGH also reduced R(d)compared with the controls and the 20K-hGH rats by 46.6% (P< 0.001) and 39.6% (P< 0.05) respectively, while no differences were observed in the HGO. Moreover, 22K-hGH inhibited glucose uptake, which was estimated from the insulin-stimulated R(g)' in some tissues. These results suggest that 22K-hGH inhibits the uptake and use of glucose in various tissues, which then leads to insulin resistance. In conclusion, the diabetogenicity of 20K-hGH is much weaker than that of 22K-hGH, and the reduced insulin-antagonizing action of 20K-hGH could have important clinical benefits.

Somatotropin-dependent decrease in fatty acid synthase mRNA abundance in 3T3-F442A adipocytes is the result of a decrease in both gene transcription and mRNA stability

Somatotropin (ST) markedly decreases lipogenesis, fatty acid synthase (FAS) enzyme activity and mRNA abundance in pig adipocytes. The present study was conducted to determine whether the decrease in FAS mRNA in 3T3-F442A adipocytes was the result of a decrease in transcription of the FAS gene and/or a change in FAS mRNA stability. Insulin increased the abundance of FAS mRNA 2-13-fold and fatty acid synthesis 3-7-fold. Somatotropin decreased the stimulatory effect of insulin on the abundance of FAS mRNA and lipogenesis by 40-70% and 20-60% respectively. Subsequent run-on analyses demonstrated that the decrease observed in FAS mRNA in response to ST was associated with an 82% decrease in transcription; ST significantly shortened the half-life of FAS mRNA from 35 to 11 h. To corroborate the run-on analyses, cells were stably transfected with a pFAS-CAT5 (in which CAT stands for chloramphenicol acetyltransferase) reporter construct that contained 2195 bp of the 5' flanking region of the rat FAS gene. Insulin treatment increased FAS-CAT activity 4.7-fold. When ST was added to the insulin-containing medium there was an approx. 60% reduction in FAS-CAT activity. In summary, our results indicate that ST decreases FAS mRNA levels and that this is the result of a marked decrease in both transcription of the FAS gene and stability of the FAS mRNA.

Growth hormone-induced insulin resistance: role of the insulin receptor, IRS-1, GLUT-1, and GLUT-4

Treatment of rats with growth hormone (GH; 1 mg/kg sc) twice daily over 2.5 days did not alter fasting plasma glucose or glucose tolerance but increased fasting plasma insulin levels 65% and peak insulin response to a glucose load 35% over controls, indicating the development of insulin resistance. Studies on partially purified insulin receptors from soleus muscles showed that GH increased the abundance of insulin receptor beta-subunits by 48% as measured by immunoblotting. Despite this increase, GH abolished the increase in autophosphorylation of the insulin receptor beta-subunit in response to physiological hyperinsulinemia and diminished by 28% the response to supraphysiological hyperinsulinemia. Similarly, insulin-stimulated phosphorylation of insulin receptor substrate-1 (IRS-1) was decreased 25% by GH, but the abundance of IRS-1 was not affected. Studies on rats pretreated with streptozotocin suggested that the effects of GH are direct and not secondary to GH-induced hyperinsulinemia. GH decreased basal GLUT-1 abundance in the low-density microsome and plasma membrane fractions of epididymal adipocytes by 50 and 42%, respectively, but decreased basal GLUT-4 abundance only in the low-density microsome fraction by 24%. Despite these alterations, the abundance of both transporters in the plasma membrane fraction of adipocytes incubated with 0.1 U insulin/ml was not diminished by GH.

Effects of growth hormone on fuel utilization and muscle glycogen synthase activity in normal humans

To examine the insulin antagonistic effects of growth hormone (GH), seven healthy subjects underwent, in random order, two 5-h euglycemic clamp studies with moderate hyperinsulinemia. A GH infusion (45 ng.kg-1.min-1) was given throughout one of the studies. GH inhibited the insulin-stimulated glucose disposal by 27% from 4.4 +/- 0.7 to 3.3 +/- 0.4 mg.kg-1.min-1 (P less than 0.02) and raised the nonprotein energy expenditures (NPEE) from 18.7 +/- 0.5 to 20.5 +/- 0.3 kcal.kg-1.24 h-1 (P less than 0.03). Lipid oxidation contributed 71.7 +/- 5.6% of NPEE during the GH infusion as compared with 48.7 +/- 5.2% during the control clamp (P less than 0.02). In skeletal muscle biopsies, insulin binding to wheat germ agglutinin-purified insulin receptors and insulin receptor kinase activity were unaffected by GH infusion. Glycogen synthase activation by insulin was inhibited by 41% during the GH clamp (fractional velocity 14.1 +/- 2.5 vs. 8.3 +/- 1.4%, P less than 0.03). In conclusion, GH 1) increases energy expenditures and inhibits glucose oxidation in favor of an increased lipid oxidation, and 2) inhibits insulin-mediated activation of the glycogen synthase in skeletal muscle biopsies by a mechanism distal to insulin receptor binding and kinase activity.

Effects of 3 years of growth hormone therapy in short normal children

The effect of 3 years of growth hormone (GH) treatment on growth rate, predicted height, carbohydrate and metabolic status, and thyroid function was studied in 16 short prepubertal children growing with a normal pretreatment growth rate. The height velocity SDS increased from a pretreatment value of -0.44 +/- 0.33 (mean +/- SD) to a value of +2.20 +/- 1.03 during the first year of treatment. It was maintained at a value above zero over the subsequent 2 years. By the end of the third year of treatment, the predicted final height had increased by 6.8 cm in the boys and by 4.2 cm in the girls (p less than 0.001 and p less than 0.01, respectively). Increasing the dose of GH on a body surface area basis reduced the deceleration of growth observed during the second year of treatment, leading to an improvement in height prognosis over that year. Glucose homoeostasis was achieved initially at the expense of an elevation in fasting serum insulin concentration, but this had returned to pretreatment values by the end of the second year of therapy.