Growth Hormone-induced Insulin Resistance Research Articles

The Role of PTEN in Chronic Growth Hormone-Induced Hepatic Insulin Resistance

Chronic growth hormone (GH) therapy has been shown to cause insulin resistance, but the mechanism remains unknown. PTEN, a tumor suppressor gene, is a major negative regulator of insulin signaling. In this study, we explored the effect of chronic GH on insulin signaling in the context of PTEN function. Balb/c healthy mice were given recombinant human or bovine GH intraperitoneally for 3 weeks. We found that phosphorylation of Akt was significantly decreased in chronic GH group and the expression of PTEN was significantly increased. We further examined this effect in the streptozotocin-induced Type I diabetic mouse model, in which endogenous insulin secretion was disrupted. Insulin/PI3K/Akt signaling was impaired. However, different from the observation in healthy mice, the expression of PTEN did not increase. Similarly, PTEN expression did not significantly increase in chronic GH-treated mice with hypoinsulinemia induced by prolonged fasting. We conducted in-vitro experiments in HepG2 cells to validate our in-vivo findings. Long-term exposure to GH caused similar resistance of insulin/PI3K/Akt signaling in HepG2 cells; and over-expression of PTEN enhanced the impairment of insulin signaling. On the other hand, disabling the PTEN gene by transfecting the mutant PTEN construct C124S or siPTEN, disrupted the chronic GH induced insulin resistance. Our data demonstrate that PTEN plays an important role in chronic-GH-induced insulin resistance. These findings may have implication in other pathological insulin resistance.

OR05-5 Growth hormone-induced insulin resistance involves reduced pyruvat dehydrogenase activity in skeletal muscle

OR05-5 Growth hormone-induced insulin resistance involves reduced pyruvat dehydrogenase activity in skeletal muscle

Growth hormone-induced insulin resistance is associated with increased intramyocellular triglyceride content but unaltered VLDL-triglyceride kinetics

The ability of growth hormone (GH) to stimulate lipolysis and cause insulin resistance in skeletal muscle may be causally linked, but the mechanisms remain obscure. We investigated the impact of GH on the turnover of FFA and VLDL-TG, intramuscular triglyceride content (IMTG), and insulin sensitivity (euglycemic clamp) in nine healthy men in a randomized double-blind placebo-controlled crossover study after 8 days treatment with (A) Placebo+Placebo, (B) GH (2 mg daily)+Placebo, and (C) GH (2 mg daily)+Acipimox (250 mgx3 daily). In the basal state, GH (B) increased FFA levels (P<0.05), palmitate turnover (P<0.05), and lipid oxidation (P=0.05), but VLDL-TG kinetics were unaffected. Administration of acipimox (C) suppressed basal lipolysis but did not influence VLDL-TG kinetics. In the basal state, IMTG content increased after GH (B; P=0.03). Insulin resistance was induced by GH irrespective of concomitant acipimox (P<0.001). The turnover of FFA and VLDL-TG was suppressed by hyperinsulinemia during placebo and GH, whereas coadministration of acipimox induced a rebound increase FFA turnover and VLDL-TG clearance. We conclude that these results show that GH-induced insulin resistance is associated with increased IMTG and unaltered VLDL-TG kinetics; we hypothesize that fat oxidation in muscle tissue is an important primary effect of GH and that circulating FFA rather than VLDL-TG constitute the major source for this process; and the role of IMTG in the development of GH-induced insulin resistance merits future research.

Prolactin and growth hormone regulate adiponectin secretion and receptor expression in adipose tissue

Adiponectin is a hormone secreted from adipose tissue, and serum levels are decreased with obesity and insulin resistance. Because prolactin (PRL) and growth hormone (GH) can affect insulin sensitivity, we investigated the effects of these hormones on the regulation of adiponectin in human adipose tissue in vitro and in rodents in vivo. Adiponectin secretion was significantly suppressed by PRL and GH in in vitro cultured human adipose tissue. Furthermore, PRL increased adiponectin receptor 1 (AdipoR1) mRNA expression and GH decreased AdipoR2 expression in the cultured human adipose tissue. In transgenic mice expressing GH, and female mice expressing PRL, serum levels of adiponectin were decreased. In contrast, GH receptor deficient mice had elevated adiponectin levels, while PRL receptor deficient mice were unaffected. In conclusion, we demonstrate gene expression of AdipoR1 and AdipoR2 in human adipose tissue for the first time, and show that these are differentially regulated by PRL and GH. Both PRL and GH reduced adiponectin secretion in human adipose tissue in vitro and in mice in vivo. Decreased serum adiponectin levels have been associated with insulin resistance, and our data in human tissue and in transgenic mice suggest a role for adiponectin in PRL and GH induced insulin resistance.

Glucose flux is normalized by compensatory hyperinsulinaemia in growth hormone-induced insulin resistance in healthy subjects, while skeletal muscle protein synthesis remains unchanged

Glucose flux is normalized by compensatory hyperinsulinaemia in growth hormone-induced insulin resistance in healthy subjects, while skeletal muscle protein synthesis remains unchanged

Glucose flux is normalized by compensatory hyperinsulinaemia in growth hormone-induced insulin resistance in healthy subjects, while skeletal muscle protein synthesis remains unchanged.

The aim of this present investigation was to study the relationship between the reduction in insulin sensitivity accompanying 5 days of treatment with growth hormone (GH; 0.05 mg.24 h(-1).kg(-1)) and intracellular substrate oxidation rates in six healthy subjects, while maintaining glucose flux by a constant glucose infusion and adjusting insulin infusion rates to achieve normoglycaemia (feedback clamp). Protein synthesis rates in skeletal muscle (flooding dose of L-[(2)H(5)]phenylalanine) were determined under these conditions. We also compared changes in insulin sensitivity after GH treatment with simultaneous changes in energy requirements, protein synthesis rates, nitrogen balance, 3-methylhistidine excretion in urine, body composition and the hormonal milieu. After GH treatment, 70% more insulin was required to maintain normoglycaemia (P<0.01). The ratio between glucose infusion rate and serum insulin levels decreased by 34% at the two levels of glucose infusion tested (P<0.05). Basal levels of C-peptide, insulin-like growth factor (IGF)-I and IGF-binding protein-3 increased almost 2-fold, while levels of glucose, insulin, glucagon, GH and IGF-binding protein-1 remained unchanged. Non-esterified fatty acid levels decreased (P<0.05). In addition, 24 h urinary nitrogen excretion decreased by 26% (P<0.01) after GH treatment, while skeletal muscle protein synthesis and 3-methylhistidine excretion in urine remained unchanged. Energy expenditure increased by 5% (P<0.05) after treatment, whereas fat and carbohydrate oxidation were unaltered. In conclusion, when glucose flux was normalized by compensatory hyperinsulinaemia under conditions of GH-induced insulin resistance, intracellular rates of oxidation of glucose and fat remained unchanged. The nitrogen retention accompanying GH treatment seems to be due largely to improved nitrogen balance in non-muscle tissue.

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.

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.

Growth hormone-induced insulin resistance and its relationship to lipid availability in the rat.

The role of increased lipid availability in the generation of insulin resistance by growth hormone has not been established. We investigated this in rats infused with saline (controls) or human growth hormone (hGH) (500 micrograms x kg-1 x 24 h-1) for 5 h or 3 days. hGH infusion increased basal plasma insulin at 5 h (335 +/- 33 vs. 197 +/- 15 [controls]; P < 0.005) and at 3 days (396 +/- 34 vs. 218 +/- 14 pmol/l; P < 0.0001). Plasma nonesterified fatty acid (0.26 +/- 0.01 vs. 0.21 +/- 0.01 [controls] g/l) and liver long-chain acyl-CoA (21.2 +/- 1.9 vs. 14.1 +/- 1.1 [controls] nmol/g wet wt) were elevated at 5 h (P < 0.01 for both) but were below control levels after 3 days of hGH infusion (P < 0.01 for both); indirect calorimetry after 3 days demonstrated decreased lipid oxidation. Clamp studies showed similar degrees of peripheral insulin resistance at 5-h and 3-day hGH infusion (glucose disposal reduced by 25% versus controls). Insulin-stimulated glucose metabolic index (Rg') in red gastrocnemius muscle (red muscle) was reduced (P < 0.05 and P < 0.01) at 5 h and 3 days of hGH infusion, respectively (e.g., 5 h, 10.0 +/- 1.8 vs. 24.1 +/- 4.4 [controls] micromol x 100 g-1 x min-1), whereas insulin-mediated muscle glycogen synthesis was reduced (P < 0.03) only in rats infused with hGH for 3 days. We conclude that in the rat, hGH rapidly induces persistent peripheral insulin resistance and basal hyperinsulinemia. However, the transient nature of increased lipid mobilization suggests that it is not an important factor in the manifestation of muscle insulin resistance during prolonged hGH elevation. The persistent insulin resistance is not associated with increased lipid oxidation but is associated with hyperinsulinemia and reduced insulin-mediated muscle glycogen synthesis.

Growth hormone-induced insulin resistance and its relationship to lipid availability in the rat

Growth hormone-induced insulin resistance and its relationship to lipid availability in the rat

Growth hormone induces insulin resistance in Laron dwarf cells via lactogenic receptors

Growth hormone induces insulin resistance in Laron dwarf cells via lactogenic receptors

Growth hormone induces insulin resistance in Laron dwarf cells via lactogenic receptors.

GH is the hormone primarily responsible for regulating body size within the genetic program. While GH has pleiotropic actions on cellular growth and metabolism, most of its effects are believed to be mediated by a single GH receptor. This receptor is not functional in tissues from patients with Laron dwarfism. We used human T-cell leukemia virus-immortalized T-lymphoblast cell lines from Laron dwarfs and normal individuals to examine the mechanism of GH-induced insulin resistance at the cellular level. GH (5-500 micrograms/L) caused a profound decrease in the sensitivity of normal T-lymphoblasts in response to all insulin concentrations (P < 0.0001 vs. insulin alone); pretreatment with GH and GH receptor antibody significantly improved sensitivity to all concentrations of insulin (P = NS vs. insulin alone). Preincubation with GH and PRL receptor antibody was associated with partial improvement in insulin sensitivity (P = 0.004 vs. insulin alone). Thus, in normal T-cell lines, the major pathway of GH-induced insulin resistance appears to be directed by the GH receptor, with a smaller effect mediated through the PRL receptor. While T-cell lines from Laron dwarfs do not respond to GH in clonal proliferation assays, GH (50 and 100 micrograms/L) caused profound insulin resistance in these cells (P = 0.008 and P < 0.0001, respectively, vs. insulin alone). GH receptor antibody did not abrogate this effect at any insulin concentration (P = NS vs. insulin alone), but there was partial restoration of insulin sensitivity when GH and PRL receptor antibody were coincubated (P = 0.0069 vs. insulin alone). Thus, in Laron T-cell lines, PRL and perhaps other lactogenic receptors appear to mediate GH-induced insulin resistance. The kinetics of GH-induced insulin resistance in Laron T-cells were also distinct from the pattern seen in normal T-cells, and unlike in normal cells, GH had no effect on insulin-like growth factor-I-induced clonal expansion of Laron T-cell lines (P = NS vs. insulin-like growth factor-I alone). These results provide evidence for an alternative pathway of GH action revealed in cells lacking classical growth responses to GH.

Isolated adipocytes from growth hormone-treated obese (ob/ob) mice exhibit insulin resistance

The genetically obese (ob/ob) mouse is a useful model for the study of the diabetogenic action of growth hormone (GH), because treatment of these animals with GH results in decreased responsiveness of their adipose tissue to insulin in vitro. Studies of the mechanisms involved in GH-induced insulin resistance using isolated adipocytes of ob/ob mice have not been possible, however, because of their extreme fragility and the lack of an adequate system for the maintenance of these cells. This study describes a new method for the isolation of ob/ob mouse adipocytes. The isolated cells are stable, viable and metabolically responsive to insulin. In addition, these adipocytes have been maintained in primary culture, in serum-free medium, for up to 3 days. During culture, the cells exhibit large increases in 125I-hGH binding (10–20-fold) and porcine 125I-insulin binding (5–10-fold). The induction of insulin resistance by GH has also been demonstrated in these freshly isolated ob/ob mouse adipocytes. The studies to date indicate that the ob/ob mouse adipocyte system should provide a useful model for detailed studies of the cellular and molecular mechanisms of GH induced insulin resistance.

Effects of growth hormone on insulin action in man. Mechanisms of insulin resistance, impaired suppression of glucose production, and impaired stimulation of glucose utilization.

The present studies were undertaken to assess the mechanisms responsible for growth hormone-induced insulin resistance in man. The insulin dose-response characteristics for suppression of glucose production and stimulation of glucose utilization and their relationship to monocyte insulin binding were determined in six normal volunteers after 12-h infusion of growth hormone and 12-h infusion of saline. The infusion of growth hormone (2 micrograms . kg-1 . h-1) increased plasma growth hormone nearly threefold (to congruent to 9 ng/ml) within the range observed during sleep and exercise. This increased plasma insulin (14 +/- 1 versus 8 +/- 1 microunits/ml, P less than 0.005) concentrations without significantly altering plasma glucose concentrations or basal rates of glucose production and utilization. Insulin dose-response curves for both suppression of glucose production (half-maximal response at 37 +/- 3 versus 20 +/- 3 microunits/ml, P less than 0.01) and stimulation of glucose utilization (half-maximal response at 98 +/- 8 versus 52 +/- 8 microunits/ml, P less than 0.01) were shifted to the right with preservation of normal maximal responses to insulin. Monocyte insulin binding was unaffected. Thus, except at near maximal insulin receptor occupancy, the action of insulin on glucose production and utilization per number of monocyte insulin receptors occupied was decreased. These results indicate that increases in plasma growth hormone within the physiologic range can cause insulin resistance in man, which is due to decreases in both hepatic and extrahepatic effects of insulin. Assuming that insulin binding to monocytes reflects insulin binding in insulin sensitive tissues, this decrease in insulin action can be explained on the basis of a postreceptor defect.