Receptor binding properties and insulin-like effects of human growth hormone and its 20 kDa-variant in rat adipocytes

Growth hormone isoforms

Human growth hormone (GH) is a heterogeneous protein hormone consisting of several isoforms. The sources of this heterogeneity reside at the level of the genome, mRNA splicing, post-translational modification and metabolism. The GH gene cluster on chromosome 17q contains 2 GH genes (GH1 or GH-N and GH2 or GH-V) in addition to 2(-3) genes encoding the related chorionic somatomammotropin. Alternative mRNA splicing of the GH1 transcript yields two products: 22K-GH (the principal pituitary GH form) and 20K-GH. Post-translationally modified GH forms include N(alpha)-acylated, deamidated and glycosylated monomeric GH forms, as well as both non-covalent and disulfide-linked oligomers up to at least pentameric GH. GH fragments generated in the course of peripheral metabolism may be measured in immunoassays for GH. The GH-N gene is expressed in the pituitary, the GH-V gene in the placenta. Secretion of pituitary GH forms is pulsatile under control from the hypothalamus, whereas secretion of placental GH-V is tonic and rises progressively in maternal blood during the 2nd and 3rd trimester. Pituitary GH forms are co-secreted during a secretory pulse; no isoform-specific stimuli have been identified. There are minor differences in somatogenic and metabolic bioactivity among the GH isoforms, depending on species and assay system used. Both 20K-GH and GH-V have poor lactogenic activity. Oligomeric GH forms have variably diminished bioactivity compared to monomeric forms. GH isoforms cross-react in most immunoassays, but assays specific for 22K-GH, 20K-GH and GH-V have been developed. The metabolic clearance of 20K-GH and GH oligomers is delayed compared to that of 22K-GH. The heterogeneous mixture of GH isoforms in blood is further complicated by the presence of two GH-binding proteins, which form complexes with GH; isoform proportions also vary depending on the lag time from a secretory pulse because of different half-lives. GH forms excreted in the urine reflect monomeric GH isoforms in blood, but constitute only a minute fraction of the GH production rate. The heterogeneity of GH is one important reason for the notorious disparity among assay results. It also presents an opportunity for distinguishing endogenous from exogenous GH.

22- and 20 kDa-human growth hormones bind to different sites within certain cellular receptors

Human growth hormone 20 kDa (20 kDa-hGH) is a natural variant of the main hGH isoform (22 kDa-hGH). Since some 20- and 22 kDa-hGH biological activities are not identical, we decided to map the prolactin (PRLR) and growth hormone receptor (GHR) binding sites for both isoforms. Monoclonal antibody (MAb) R7B4, directed to both receptors, was employed to estimate the relative proximity between 20- and 22 kDa-hGH receptors binding sites. Results indicated that although both hGH isoforms share the same PRLR present in Nb2-cells and rat liver, MAb R7B4 differently affected hormone binding, suggesting that their receptor binding sites would be close in Nb2-cells and separate in rat liver membranes. Since labelled 20 kDa-hGH did not bind significantly to hGHR, we added to the incubating medium an allosteric MAb anti-hGH that improved 20 kDa-hGH affinity for receptors. Under these experimental conditions MAb R7B4 inhibited 20 kDa-hGH binding to human liver but not to placenta, whereas the Ab impaired 22 kDa-hGH binding to both receptors. Data thus suggested that both hGH isoforms share the same hGHR binding site in liver tissue but bind to different overlapped regions in placenta. Consequently, results presented in this paper indicate that PRLR and GHR binding sites for 22- and 20 kDa-hGH should not be always identical, a fact that could explain some of the isoforms different activities.

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.

The mechanism of effect of growth hormone on preadipocyte and adipocyte function

Growth hormone (GH) is not only the major regulator of postnatal somatic growth but also exerts profound effects on body composition through a combination of anabolic, lipolytic and antinatriuretic actions. GH enhancement of the lipolytic activity of adipose tissue in combination with a reduction of triglyceride accumulation via inhibition of lipoprotein lipase activity appears to be the major mechanism by which GH results in a reduction of the total fat mass. Recently, much progress has been made in understanding the molecular mechanism by which GH affects cellular function. This review provides a brief discourse and summary of the mechanism of effects of GH on preadipocyte/adipocyte function. It is intended to provide a functional understanding of the mechanism of action of GH as it relates to adipogenesis and adipocyte function.

GH induced lipolysis stimulation in 3T3-L1 adipocytes stably expressing hGHR: analysis on signaling pathway and activity of 20K hGH

We have constructed a cell line of 3T3-L1 which can efficiently express human GHR (3T3-L1-hGHR) after differentiation to adipocytes. The expressed hGHR was detected as two bands with approximate molecular sizes of 120K by Western analysis using hGHR specific monoclonal antibody. Maximum lipolytic activity induced by hGH in the 3T3-L1-hGHR was enhanced 10-fold as compared to that in 3T3-L1, suggesting that expressed hGHR is functionally active. Comparative analysis using bGH and hGH revealed that 70% of lipolysis stimulation by 1-10 ng/ml hGH could be attributed to hGHR-mediated response. Analyses on inhibition and phosphorylation of signaling molecules suggested that GH-induced lipolysis stimulation is dependent on gene expression and not mediated through PKA-, PKC-, PLA-, PLC-, nor MAPK-pathway but possibly through JAK-STATs pathway. Duration of STAT5 activation by hGH continued up to 48 h. We also revealed that 22 K hGH isoform, 20K hGH which has been reported as a weaker agonist for GH-induced lipolysis stimulation, possesses equipotent activity and shows stronger action in the presence of hGHBP as compared to 22 K hGH. Taken together we conclude that the hGH-induced lipolysis was not mediated through MAP-, PKA-, PKC-, nor PLA-pathway but might be mediated through STAT pathway and that 20K hGH might show higher lipolytic activity than 22 K hGH in adipose tissue that produces a large amount of GHBP.

Growth hormone stimulates the tyrosine kinase activity of JAK2 and induces tyrosine phosphorylation of insulin receptor substrates and Shc in rat tissues

GH stimulates the tyrosine phosphorylation of various cellular polypeptides, including the GH receptor itself, in an early part of the intracellular response. Some of these phosphorylations are catalyzed by a GH receptor-associated kinase identified as JAK2, a member of the Janus family of tyrosine kinases. In cultured cells, GH stimulates the tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1), IRS-2, and Shc. This study investigated whether GH could cause the tyrosine phosphorylation of IRSs and Shc proteins in fasted rat tissues in vivo. GH was administered to fasted Wistar rats via a portal vein, and extracts of different tissues were immunoprecipitated with specific antibodies. GH increased the tyrosine phosphorylation of IRS-1, IRS-2, JAK2, and Shc proteins in the liver, heart, kidney, muscle, and adipose tissue of rats. The roles of these substrates as signaling molecules for GH were further demonstrated by the finding that GH stimulated the association of IRS-1/2 with phosphatidylinositol 3-kinase, Grb2, and phosphotyrosine phosphatase and of Shc with Grb2. The correlation between JAK2 tyrosyl phosphorylation and IRS-1 tyrosyl phosphorylation in response to GH together with the results of the in vitro tyrosine kinase assay are consistent with the hypothesis that JAK2 may mediate GH-induced phosphorylation of IRS-1.

Serum levels of 20-kilodalton human growth hormone (GH) are parallel those of 22-kilodalton human GH in normal and short children

Twenty-kilodalton human GH (20K), which is one of the human GH (hGH) variants, is thought to be produced by alternative premessenger ribonucleic acid splicing. However, its physiological role is still unclear due to the lack of a specific assay. We have measured serum 20K and 22-kDa hGH (22K) by specific ELISAs to investigate the physiological role of 20K in children. The subjects were 162 normal children, aged 1 month to 20 yr; 12 patients with GH deficiency (GHD), aged 11 months to 13 yr; 57 children with non-GHD short stature, aged 2-17 yr; and 13 girls with Turner's syndrome, aged 5 months to 15 yr. Samples were collected at random from normal children and were collected after hGH provocative tests and 3-h nocturnal sleep from GHD, non-GHD short stature, and Turner's syndrome children. The mean basal serum concentrations of 22K and 20K were 2.4 +/- 2.8 ng/mL and 152.3 +/- 184.0 pg/mL in normal boys and 2.5 +/- 3.1 ng/mL and 130.6 +/- 171.5 pg/mL in normal girls, respectively. The percentages of 20K (%20K) were 5.8 +/- 2.1% and 6.0 +/- 3.2% in 83 normal boys and 79 normal girls, respectively. There was no significant difference in %20K either among ages or between the prepubertal stage and the pubertal stage in normal boys and girls. The mean %20K values in basal samples of provocative tests in 12 patients with GHD, non-GHD short stature, and Turner's syndrome were 6.5 +/- 2.4%, 6.5 +/- 3.8%, and 5.9 +/- 3.2%, respectively. There was no significant difference in %20K among normal children and these growth disorders, and there was no significant difference in %20K throughout the hGH provocative tests and 3-h nocturnal sleep in these growth disorders. There was also no significant correlation between the percentage of 20K and the height SD score or body mass index in either normal children or subjects with these growth disorders. In conclusion, the %20K is constant, regardless of age, sex, puberty, height SD score, body mass index, and GH secretion status. The regulation of serum 20K levels remains to be established.

Insulin produces a growth hormone-like increase in intracellular free calcium concentration in okadaic acid-treated adipocytes

In vivo, GH and insulin usually produce opposing effects on carbohydrate and lipid metabolism in adipocytes, even though their signal transduction pathways overlap. However, when added to rat adipocytes that have been made GH deficient, GH briefly produces responses that are qualitatively like those of insulin. Subsequently, GH induces refractoriness to this acute insulin-like response, in a sense restricting its effects to a unique subset of possible physiological actions. Okadaic acid is an inhibitor of type I and IIa phosphoprotein phosphatases and affects glucose metabolism in fat cells in a manner that is reminiscent of GH. Okadaic acid initially mimics the actions of insulin, and subsequently, even after it has been removed by thorough washing, blunts the ability of adipocytes to accelerate glucose metabolism in response to insulin or GH. Because refractoriness to the insulin-like effect of GH is associated with GH-induced increases in intracellular free calcium concentrations ([Ca2+]i), we examined the effects of insulin on [Ca2+]i in okadaic acid-treated adipocytes. Adipocytes were incubated with 0.25 microM okadaic acid for 1 h, washed, and reincubated without okadaic acid for 2 h before measurement of [Ca2+]i using fura-2 as a calcium indicator. Neither GH (500 ng/ml) nor insulin (100 microU/ml) affected [Ca2+]i in cells in which glucose metabolism was stimulated, but both hormones rapidly increased [Ca2+]i in adipocytes that were refractory to insulin-like stimulation. The characteristics of the increase in [Ca2+]i produced by insulin were identical to those previously reported for GH. The effect of insulin was mimicked by the dihydropyridine calcium channel activator BayK 5552 or depolarization of the cell membrane with 30 mM KCl and was blocked by the dihydropyridine calcium channel blocker, nimodipine (100 nM), implicating activation of voltage-sensitive L-type Ca2+ channels. The increase in [Ca2+]i was also mimicked by sn-1,2-dioctanoylglycerol and blocked by inhibitors of protein kinase C (staurosporine, chelerythrine chloride, and calphostin), and D609, an inhibitor of phospholipase C, as reported for GH. Acquisition of the ability to increase [Ca2+]i in response to insulin required a lag period of at least 2 h after removal of okadaic acid and was prevented by inhibitors of RNA and protein synthesis. Adipocytes that were incubated with inhibitors of protein kinase A (KT-5720), or protein kinase C (staurosporine) along with okadaic acid also failed to increase [Ca2+]i in response to insulin. Conversely, adipocytes that were incubated with dibutyryl cAMP, methylisobutyl xanthine, or phorbol ester instead of okadaic acid increased [Ca2+]i when treated with insulin 2 h later. These results suggest that phosphorylated substrates of protein kinases A and C may mediate the transcriptional event(s) that enable adipocytes to activate L-type Ca2+ channels in response to insulin. Blockade of protein kinases A or C or removal of calcium from the incubation medium did not restore the ability of okadaic acid-treated adipocytes to increase glucose metabolism in response to insulin, nor did pretreatment of adipocytes with dibutyryl cAMP or phorbol ester decrease insulin-induced stimulation of glucose metabolism. The failure of insulin to increase glucose metabolism in okadaic acid-treated adipocytes thus cannot be ascribed to the increase in [Ca2+]i. These findings indicate that just as GH can produce an insulin-like response, so too can insulin produce a GH-like response, and highlight the need to understand how specificity of hormone action is achieved in cells that respond to different hormones that share elements of their transduction pathways.

Secretion of authentic 20-kDa human growth hormone (20K hGH) in Escherichia coli and properties of the purified product

Using Bacillus amyloliquefaciens neutral protease gene (npr), we have constructed a secretion system of 20-kDA human growth hormone (20K hGH) in E. coli. The secretion-signal region from npr was modified inserting a fragment coding a 2Lys-5Leu cluster. In this system we found that co-expression of glutathione reductase remarkably increased accumulation level of 20K hGH in periplasm and confirmed that secreted 20K hGH was correctly processed. The recombinant 20K hGH was highly purified and subjected to analyses of physicochemical properties and biological activities which are still unclear and controversial due to difficulty in preparing the sample with authentic structure. The secreted recombinant product had authentic disulfide linkages and showed molecular weight of 20,270.5 +/- 3.7 (theoretical value, 20,269.9). The results suggest that the recombinant 20K hGH is a full agonist on rat growth promotion and lipolysis stimulation in isolated rat adipose tissues. In particular, the lipolysis-stimulating activity of 20K hGH was distinct as compared with that of 22K hGH under physiological concentration. Cell proliferation activity via prolactin-receptor in Nb-2 lymphoma was obviously low as compared with that of 22K hGH.

Magnesium deficiency and glucose metabolism in rat adipocytes

We examined the effect of reducing ambient and intracellular free Mg ion ([Mg]i) concentrations on insulin action in epididymal adipocytes from male Sprague-Dawley rats in terms of (1) cellular transport of nonmetabolizable 2-deoxyglucose, (2) [U-14C]glucose oxidation to CO2, and (3) D-[3H]glucose incorporation into triglycerides. There were no significant differences in basal or insulin-stimulated transport of 2-deoxyglucose between adipocytes cultured in physiologic (1.24 mmol) or low (0.16 mmol) Mg for up to 24 hours. In contrast, insulin-stimulated but not basal [U-14C]glucose oxidation to CO2 was significantly reduced in adipocytes cultured in low versus physiologic Mg (P < .05 to .01). Similarly, there were no differences in basal glucose incorporation into triglycerides between cells cultured in low or physiologic Mg media for up to 24 hours. However, long-term (24-hour) but not short-term (2-hour) exposure of cells to low Mg was associated with a significant 30% reduction in insulin-stimulated D-[3H]glucose incorporation into triglycerides. When adipocytes incubated in low Mg were reincubated in high Mg (1.24 or 5 mmol) for 30 minutes, normal insulin-stimulated D-[3H]glucose incorporation into triglycerides was restored. Incubation of adipocytes in low Mg (0.16 mmol) for 24 hours resulted in a significant decrease in [Mg]i (264 +/- 89 v 437 +/- 125 micromol/cell [mean +/- SEM]) as compared with cells incubated in physiologic Mg (1.24 mmol; P < .01). These data support a role for intracellular Mg deficiency in the development of insulin resistance and suggest that the effect occurs at a site(s) distal to glucose entry into the cell. The effect of Mg deficiency on insulin action appears to be reversible.

Androgen-independent effects of prolactin on the different lobes of the immature rat prostate

In this study, we have examined the respective roles of androgens and prolactin (Prl) on rat prostate development and function. Hypophysectomized immature rats, castrated or not after hypophysectomy and treated or not with a 5 alpha-reductase inhibitor, were used to study the different aspects of Prl action on the rat prostate and its synergy with androgens in vivo. Using Northern blot analysis and quantitation of prostatic mRNAs, we have shown that Prl significantly increases the steady-state levels of transcripts coding for several lobe-specific proteins: the C3 subunit of prostatein, probasin, and RWB. We have confirmed these observations in vitro, on explants of immature rat prostate treated with either saline, Prl, or testosterone. In addition, we have demonstrated by a nuclear run-on assay that Prl significantly enhances the transcription rate of the C3 gene in the rat prostate. We conclude that the effects of Prl concern all lobes of the organ and are, at least in part, androgen-independent. Moreover, Prl is able, via an androgen-independent pathway, to increase the rate of transcription of the C3 gene, one of the major products of the rat prostate.

Growth hormone and prolactin stimulate androgen receptor, insulin-like growth factor-I (IGF-I) and IGF-I receptor levels in the prostate of immature rats

In this study we investigated the involvement of several different pituitary hormones on rat prostate development. 22-day-old Wistar rats, hypophysectomized (hypox) at 19 days of age were supplemented with highly purified human prolactin (hPRL), human luteinizing hormone (hLH), porcine follicle-stimulating hormone (pFSH), and bovine growth hormone (bGH) or with saline. Quantitative analysis of RNAs shows that treatment with either PRL or GH increases significantly steady-state mRNAs levels of the following genes in the prostate: androgen receptor (AR) (respectively 3.5- and 4.8-fold above hypox controls), IGF-I (5- and 2.7-fold), and IGF-I receptor (2.9- and 2.3-fold). LH and FSH, by contrast, have negative effects on these parameters. To test whether the enhancing effect of PRL and GH on AR-mRNA abundance was followed by increased content in the protein itself, binding assays were performed with the androgen agonist [3H]R1881 (131 and 153 fmol/mg protein while hypox controls contained 110 fmol/mg protein). In addition to the well-documented presence of prolactin receptors in prostatic tissues, we have further demonstrated, by means of nuclease S1 protection assays plus dot- and Northern-blot analyses, that a GH receptor mRNA is produced in the immature rat prostate. Moreover, we observed not only strong lactogenic but also purely somatogenic binding to be occurring in the immature prostates. Finally, we have studied IGF-I mRNA content in separated epithelial/stromal cell fractions and have concluded that IGF-I expression is principally located in the prostatic stroma. Taken together, these results suggest that PRL and GH are involved in regulating AR synthesis, at least partially by direct action on the organ. In this context IGF-I appears as a paracrine factor playing a role in epithelium/stroma interactions during prostatic development.

Actions of bovine somatotropin on polymorphonuclear leukocytes and lymphocytes in cattle

Objectives were to determine 1) in vitro effects of bST on function of polymorphonuclear leukocytes and lymphocytes and 2) in vivo effects of bST on leukocyte function of heifers fed to maintain medium or high growth rates. When administered in vitro, bST did not affect function of polymorphonuclear leukocytes. [Methyl-3H]thymidine incorporation by resting lymphocytes was stimulated by 1000 ng/ml bST. When given in vitro, bST did not further enhance [methyl-3H]thymidine uptake by mitogen-stimulated lymphocytes cultured at 38.5 degrees C but reduced the depression of mitogen-stimulated proliferation caused by incubating cells at 42 degrees C. When bST was administered in vivo, phagocytosis and killing of Escherichia coli by polymorphonuclear leukocytes from bST-treated heifers were not different from cells of control heifers. As measured by [methyl-3H]thymidine uptake after stimulation with phytohemagglutinin, lymphocytes from bST-treated heifers responded similarly to those of control heifers when incubated at 38.5 degrees C, but the depression in [methyl-3H]thymidine uptake due to culture at 42 degrees C was less for lymphocytes obtained from bST-treated heifers. In conclusion, bST had little effect on function of polymorphonuclear leukocytes but could promote proliferation of lymphocytes in vitro and protect cells from effects of elevated temperature.

The 20,000 Da variant of human growth hormone does not bind to growth hormone receptors in human liver

The 20,000 Da variant of human growth hormone (hGH) (20K) exhibited no specific binding to hGH receptors in human liver plasma membranes. This contrasts with the 22,000 Da form of human growth hormone (22K), which bound with high affinity to the same hepatic receptor preparation. Since the liver is considered a major target organ for the somatogenic pathway of growth hormone action, this finding implies that in humans the 20K form plays little role in that pathway. The homologous hormone-receptor system examined here yielded results that differ from heterologous receptor binding experiments in animals. The differences are likely explained by the presence in non-primate mammals of more than one type of growth hormone receptor with varied specificities. In man, the 20K form of growth hormone may have a biological role distinct from that of the main 22K form of growth hormone.

Growth hormone binding proteins and various forms of growth hormone: implications for measurements

GH measurements are complicated by numerous physiologically occurring GH forms, by the lack of availability of completely specific reagents for the various GH isoforms, and by interference of the circulating BP in some assays. The discrepancies between assay results are partly due to these factors, with monoclonal immunoassays or RRAs being more affected than polyclonal immunoassays.

Growth hormone receptors

The receptor for rabbit liver growth hormone (GH) has recently been purified and cloned. Sequencing data demonstrates that this site represents a new class of transmembrane receptor proteins containing covalently linked ubiquitin and N-linked oligosaccharides. Homology with the human GH liver receptor is estimated to be 84%. GH receptors are also present on non-liver cells, however, their function is poorly understood, and it is not yet known if there is homology with the liver site. Direct stimulation of differentiation of preadipocytes to adipocytes by GH has been demonstrated. In bone, binding of GH to the resting cell zone, but not the proliferative layers has been reported. Taken together, these data support the hypothesis of Green and colleagues that GH has a dual mechanism of action which includes direct stimulation of differentiation of precursor cells and a secondary effect of enhancing the clonal proliferative response.

Role of kinase C in the insulin-like effects of human growth hormone in rat adipocytes

Insulin and to a smaller extent, human growth hormone (hGH), both stimulate lipogenesis in isolated rat adipocytes preincubated 4 hours in the absence of hormone. The non-additivity of maximal doses suggested that hGH may share a subset of the metabolic pathways stimulated by insulin. We explored whether kinase C may be involved in the common lipogenic effect of both hormones. The stimulation of lipogenesis by phorbol ester 12-myristate 13-acetate (PMA) (an activator of kinase C) was not additive to the stimulation by either insulin or hGH. Downregulation of kinase C resulted in a marked decrease of the maximal insulin effect (44 +/- 9%) and even more of the hGH effect (64 +/- 14%). These data suggest that kinase C either mediates part of, or modulates, the effect of insulin and hGH on lipogenesis.