Diverse regulation of full-length and truncated growth hormone receptor expression in 3T3-L1 adipocytes

Angiotensin-(1-7) suppresses oxidative stress and improves glucose uptake via Mas receptor in adipocytes

Although reactive oxygen species (ROS) contribute to glucose intolerance induced by the renin-angiotensin system (RAS) is well documented, the role of the newly discovered pathway of RAS, angiotensin (Ang)-(1-7)/Mas axis, in this process remains unknown. Here, we examined the effect of Ang-(1-7) on oxidative stress and glucose uptake in adipocytes. We used primary cultured epididymal adipocytes from C57 mice to study Ang-(1-7) effects on glucose uptake. We also treated fully differentiated 3T3-L1 adipocytes with exogenous Ang-(1-7) or overexpression of angiotensin-converting enzyme 2 (ACE2) to induce endogenous generation of Ang-(1-7) to clarify its effects on ROS production. Intracellular ROS was measured by flow cytometry, dihydroethidium (DHE), and nitroblue tetrazolium assay. Levels of NADPH oxidase and adiponectin mRNA were measured by real-time PCR. Ang-(1-7) improved glucose uptake both in basal and insulin-stimulated states. ROS production was slightly but significantly decreased in adipocytes treated with Ang-(1-7). Additionally, Mas receptor antagonist D-Ala7-Ang-(1-7) (A779) reversed the effect of Ang-(1-7) on glucose uptake and oxidative stress. Furthermore, treatment of adipocytes with Ang-(1-7) decreased NADPH oxidase mRNA levels. We also found that oxidative stress induced by glucose oxidase-suppressed expression of adiponectin, an insulin-sensitive protein. However, the suppression of oxidative stress by Ang-(1-7) restored adiponectin expression, while A779 agonists these changes induced by Ang-(1-7). In conclusion, Ang-(1-7) can protect against oxidative stress and improve glucose metabolism in adipocytes. These results show that Ang-(1-7) is a novel target for the improvement of glucose metabolism by preventing oxidative stress.

The insulin-like growth factors in adipogenesis and obesity

Adipose tissue has been recognized as a major target of growth hormone (GH) action. GH was shown to inhibit adipocyte differentiation but stimulated preadipocyte proliferation in vitro. GH acts directly via its receptor or via upregulating insulin-like growth factor (IGF)-I, which is a critical mediator of preadipocyte proliferation, differentiation, and survival. Results from clinical studies on GH treatment in patients with GH deficiency or GH insensitivity syndrome can be used to dissect GH and IGF as well as IGF-binding protein (IGFBP) actions in vivo. In this article, changes of the GH/IGF system during adipocyte differentiation in vitro as well as related signaling pathways and their impact on adipose tissue growth and function are discussed. Clinical considerations include the effects of GH and IGF-I on adipose tissue during treatment of GH deficiency, differences in the IGF system between visceral and subcutaneous adipose tissue depots as well as the recently emerging role for adipose tissue in the regulation of glucose homeostasis.

IGF-I stimulates reactive oxygen species (ROS) production and inhibits insulin-dependent glucose uptake via ROS in 3T3-L1 adipocytes

OBJECTIVE

IGF-I is known to enhance insulin sensitivity in whole body mainly via the IGF-I receptors in muscles. However, the effect of IGF-I on the regulation of insulin sensitivity in the adipose tissue is yet unclear. Insulin sensitivity was found to be higher in the IGF-I receptor-deficient adipocytes than that in wild-type adipocytes, suggesting that IGF-I signaling induces insulin resistance in adipocytes. However, the underlying mechanism has not yet been elucidated. In addition, the effect of superphysiological levels of IGF-I, as is observed in patients with acromegaly, on insulin sensitivity remains unclear.

DESIGN

To clarify the role of IGF-I on insulin sensitivity in adipocytes, we determined insulin-induced glucose uptake and IRS-1 status in 3T3-L1 adipocytes treated with IGF-I. Since reactive oxygen species (ROS) are causally related to insulin resistance, we investigated the effect of IGF-I on ROS production to elucidate the molecular mechanism underlying insulin resistance.

RESULTS

Preincubation of the adipocytes with IGF-I attenuated insulin-dependent glucose uptake. Interestingly, we found that IGF-I significantly stimulated ROS production. Furthermore, preincubation of adipocytes with an antioxidant, N-acetyl-cysteine (NAC) restored the IGF-I-induced attenuation of insulin-dependent glucose uptake; this indicates that IGF-I induces insulin resistance via ROS. Serine phosphorylation of IRS-1 was strongly induced and the insulin-dependent tyrosine phosphorylation of IRS-1 was suppressed by preincubating the adipocytes with IGF-I. Further, NAC restored these changes induced by IGF-I on both serine and tyrosine phosphorylation of IRS-1.

CONCLUSIONS

These data indicate that IGF-I inhibited insulin activity in the 3T3-L1 adipocytes via ROS production, which affects IRS-1 phosphorylation status.

Cloning of Atlantic halibut growth hormone receptor genes and quantitative gene expression during metamorphosis

To gain insight into the possible regulatory role of the growth hormone (GH)-insulin-like growth factor I (IGF-I) system in flatfish metamorphosis, body GHR gene expression as well as IGF-I protein content was quantified in larval Atlantic halibut throughout metamorphosis (developmental stages 5-10). The cDNA of the full-length GH receptor (hhGHR) was cloned from adult liver and characterized. The hhGHR shows common features of a GHR, including a (Y/F)GEFS motif in the extracellular domain, a single transmembrane region, and an intracellular domain containing a Box 1 and Box 2. Additionally, a truncated GHR (hhGHRtr), similar to turbot and Japanese flounder GHRtr, was cloned and sequenced. These sequences are highly similar to the full-length and truncated GHRs in turbot (89%/86%) and Japanese flounder (93%/91%) with lower identity with other fish type I GHR (81%) and type II GHRs (58%). A quantitative real-time RT-PCR assay was used to measure hhGHR and hhGHRtr mRNA content in normally and abnormally metamorphosed individuals at six developmental stages, from early pre-metamorphosis to post-metamorphosis, when the fish is considered a juvenile. The level of hhGHR gene expression was highest at pre-metamorphic stage 6 and at stage 8 at the onset of metamorphosis, and then decreased during metamorphic climax and post-metamorphosis. Expression of hhGHRtr reached highest levels at stage 6 and then decreased to post-metamorphosis. The ratio of expression between the full-length and the truncated GHR (hhGHR:hhGHRtr) varied among stages and was highest at the onset of metamorphosis and at metamorphic climax. A radioimmunoassay was used to measure halibut IGF-I body content throughout metamorphosis. IGF-I increases from early metamorphosis to the onset of metamorphosis and then decreases towards post-metamorphosis. In comparison between normally and abnormally metamorphosing larvae, IGF-I content, hhGHR and hhGHRtr mRNA levels were reduced in the abnormal fish. These data indicate that the GH-IGF-I system either has a regulatory role in metamorphosis, or is being affected as a consequence of the abnormal metamorphosis.

Differential regulation of membrane associated-growth hormone binding protein (MA-GHBP) and growth hormone receptor (GHR) expression by growth hormone (GH) in mouse liver

Growth hormone (GH) binding to GH receptor (GHR) is the initial step that leads to the physiological functions of the hormone. Proteolytical cleavage of the GHR in humans and rabbits and alternative processing of the GHR transcript in rodents generates circulating growth hormone binding protein (GHBP). Moreover, other GHR truncated forms that result from alternative processing of the GHR mRNA transcript have been described. These GHR short forms are inserted in the plasma membrane but they are unable to transduce the signal. In rodents, membrane associated-GHBP (MA-GHBP), which accounts for a significant proportion of liver GH binding capacity, represents the main GHR short form found in membranes, and may therefore function as a negative form of the receptor. In the present study, GHR and MA-GHBP content in liver were analyzed using mutant and transgenic mice expressing different concentrations of growth hormone to evaluate the correlation between GH levels, body weight (BW), GHR and MA-GHBP expression. It was found that GH deficiency was associated with diminished BW, GHR and MA-GHBP expression, while increased GH concentration led to increased BW, GHR and MA-GHBP expression, but MA-GHBP upregulation was more pronounced than the observed increase in GHR expression. Since GHR and MA-GHBP both contribute to liver GH binding capacity, GH-induced enrichment of the dominant negative form would represent a compensatory mechanism triggered by high levels of the hormone. This attempt to attenuate the effects of supraphysiological concentrations of GH may be critical to reduce or prevent their plausible damaging effects on the organism.

Growth hormone and prolactin receptors in adipogenesis: STAT-5 activation, suppressors of cytokine signaling, and regulation of insulin-like growth factor I

Growth hormone GH stimulates lipolysis in mature adipocytes and primary preadipocytes but promotes adipogenesis in preadipocyte cell lines. The lactogenic hormones (prolactin PRL and placental lactogen) also stimulate adipogenesis in preadipocyte cell lines but have variable lipolytic and lipogenic effects in mature adipose tissue. We hypothesized that differences in expression of GH receptors GHR and PRL receptors PRLR during adipocyte development might explain some of the differential effects of the somatogens and lactogens on fat metabolism. To that end, we compared: (a) the expression of GHR and PRLR mRNAs in 3T3-L1 preadipocytes during the course of adipocyte differentiation; (b) the induction of STAT-5 activity by GH and PRL during adipogenesis; and (c) the acute effects of GH and PRL on the suppressors of cytokine signaling (SOCS-1-3 and cytokine-inducible SH2-domain-containing protein CIS) and IGF-I. In confluent, undifferentiated 3T3-L1 cells, the levels of GHR mRNA were approximately 250-fold higher than the levels of PRLR mRNA. Following induction of adipocyte differentiation the levels of PRLR mRNA rose 90-fold but GHR mRNA increased only 0.8-fold. Expression of both full-length (long) and truncated (short) isoforms of the PRLR increased during differentiation but the long isoform predominated at all time points. Mouse GH mGH stimulated increases in STAT-5a and 5b activity in undifferentiated as well as differentiating 3T3-L1 cells; mouse PRL mPRL had little or no effect on STAT-5 activity in undifferentiated cells but stimulated increases in STAT-5a and 5b activity in differentiating cells. mGH stimulated increases in SOCS-2 and SOCS-3 mRNAs in undifferentiated cells and SOCS-1-3 and CIS mRNAs in differentiating cells; mPRL induced CIS in differentiating cells but had no effect on SOCS-1-3. mPRL and mGH stimulated increases in IGF-I mRNA in differentiating cells but not in undifferentiated cells; the potency of mGH (3-6-fold increase, p < 0.01) exceeded that of mPRL (40-90% increase, p < 0.05). Our findings reveal disparities in the expression of PRLR and GHR during adipocyte development and differential effects of the hormones on STAT-5, the SOCS proteins, CIS, and IGF-I. These observations suggest that somatogens and lactogens regulate adipocyte development and fat metabolism through distinct but overlapping cellular mechanisms.

Physiology and pathophysiology of growth hormone-binding protein: methodological and clinical aspects

Circulating GH is partly bound to a high-affinity binding protein (GHBP), which in humans is derived from cleavage of the extracellular domain of the GH receptor. The precise biological function GHBP is unknown, although a regulation of GH bioactivity appears plausible. GHBP levels are determined by GH secretory status, body composition, age, and sex hormones, but the cause-effect relationships remain unclarified. In addition to the possible in vivo significance of GHBP, the interaction between GH and GHBP has methodological implications for both GH and GHBP assays. The present review concentrates on methodological aspects of GHBP measurements, GHBP levels in certain clinical conditions with a special emphasis on disturbances in the GH-IGF axis, and discusses the possible relationship between plasma GHBP and GH receptor status in peripheral tissues.

Regulation of development and metabolism of adipose tissue by growth hormone and the insulin-like growth factor system

White adipose tissue plays a key role in the regulation of the energy balance of vertebrates. This tissue is also now recognized to secrete a variety of factors such as leptin, which is thought to be involved in the modulation of adipose mass. Unlike other tissues, adipose tissue mass has considerable capacity to expand. The review deals primarily on the regulation of development and metabolism of adipose tissue by growth hormone (GH) and the insulin-like growth factor (IGF) system, with a special focus on the pig. The anti-insulin effects of GH are well-documented in pigs as in other species. In vitro exposure of adipose precursor cells to GH leads to a decrease in differentiation of those cells in pigs, in contrast to data obtained in murine cell lines. In vivo treatment and prolonged in vitro incubation of adipose tissue or isolated adipocytes with GH result in a decrease in glucose transport and lipogenesis, especially at the level of the fatty acid synthase gene, resulting in a reduction of the lipid content and adipose tissue mass. The mechanism by which GH antagonizes insulin stimulation of lipogenesis is still unresolved, as it is not mediated by protein kinase A, protein kinase C and Janus kinase-2 at the signaling level, or upstream stimulatory factor 1 or sterol regulatory element binding protein-1 at the transcriptional level. GH is apparently the main regulator of IGF-I mRNA expression in adipose tissue, however, the effects of IGF-I on this tissue are rather unclear.

Regulation of full-length and truncated growth hormone (GH) receptor by GH in tissues of lit/lit or bovine GH transgenic mice

Two truncated isoforms of growth hormone (GH) receptor (GHR) were identified in mice and in humans. The proteins encoded by these isoforms lack most of the intracellular domain of the GHR and inhibit GH action in a dominant negative fashion. We have quantified the mRNAs encoding the GHR isoforms in mouse tissues by use of real-time RT-PCR and examined the effect of GH excess or deficiency on regulation of mRNA levels of the GHR isoforms in vivo. In the liver, the truncated GHR mRNAs (mGHR-282 and mGHR-280) were 0.5 and <0.1%, respectively, the level of full-length GHR (mGHR-fl). In skeletal muscle, the values were 2-3 and 0.1-0.5% of mGHR-fl, respectively, and in subcutaneous fat, the values were 3-5 and 0.1-0.5% of mGHR-fl, respectively. The bovine GH transgenic mice showed a significant increase of mGHR-fl in liver but a significant decrease in skeletal muscle, with no difference in subcutaneous fat when compared with control mice. The lit/lit mice showed a significant decrease of mGHR-fl in liver, no difference of mGHR-fl in muscle, and a significant increase of mGHR-fl in subcutaneous fat when compared with lit/+ mice. The mRNA of mGHR-282 was regulated in parallel with mGHR-fl in all tissues of all mice examined, whereas that of mGHR-280 was not changed in either GH-excess or GH-deficient states. In conclusion, two truncated isoforms of GHR mRNAs were detected in liver, skeletal muscle, and subcutaneous fat of mice. The ratio of GHR-tr to GHR-fl mRNA was tissue specific and not affected by chronic excess or deficiency of GH.

Chronic treatment with growth hormone stimulates adiponectin gene expression in 3T3-L1 adipocytes

Growth hormone (GH) is an important regulator of adiposity and systemic energy metabolism. Here, we have investigated the effects of GH on production of adiponectin, an anti-diabetic and anti-atherogenic hormone secreted exclusively from adipocytes. Analysis using real time quantitative PCR revealed that GH significantly increased adiponectin gene expression in a dose-dependent manner. Time course study showed that the expression of adiponectin gene started to increase only after 30 h of GH treatment (10(-8) M), suggesting it to be a chronic effect. GH-mediated induction of adiponectin gene expression was completely blocked by treatment with the Janus kinase2 (JAK2) inhibitor AG490 and the P38 mitogen activated protein (MAP) kinase inhibitor SB203580, while the specific inhibitors of phosphatidylinositol-3-kinase (LY294002) and p70S6 kinase (rapamycin) moderately enhanced GHs effect. Co-incubation of adipocytes with GH and the PPARgamma agonist rosiglitazone produced additive effects on induction of adiponectin gene expression. These results collectively suggest that GH increases adiponectin gene expression through the JAK2-P38 MAP kinase pathway, and that elevation of adiponectin production might represent a novel mechanism by which GH regulates systemic energy metabolism and insulin sensitivity.