Diacylglycerol production induced by growth hormone in Ob1771 preadipocytes arises from phosphatidylcholine breakdown

The metabolic effects of growth hormone in adipose tissue

There is a general consensus that a reduction in growth hormone (GH) secretion results in obesity. However, the pathophysiologic role of GH in the metabolism of lipids is yet to be fully understood. The major somatic targets of GH are bones and muscles, but GH stimulates lipolysis and seems to regulate lipid deposition in adipose tissue. Patients with isolated GH deficiency (GHD) have enlarged fat depots due to higher fat cell volume, but their fat cell numbers are lower than those of matched controls. The treatment of patients with GH results in a relative loss of body fat and shifts both fat cell number and fat cell volume toward normal, indicating an adipogenic effect of GH. Adults with GHD are characterized by perturbations in body composition, lipid metabolism, cardiovascular risk profile, and bone mineral density. It is well established that GHD is usually accompanied by an increase in fat accumulation; GH replacement in GHD results in the reduction of fat mass, particularly abdominal fat mass. In addition, abdominal obesity results in a secondary reduction in GH secretion that is reversible with weight loss. However, whereas GH replacement in patients with GHD leads to specific depletion of intra-abdominal fat, administering GH to obese individuals does not seem to result in a consistent reduction or redistribution of body fat. Although administering GH to obese non-GHD subjects has only led to equivocal results, more recent studies indicate that GH still remains a plausible metabolic candidate.

Effects of growth hormone on insulin signal transduction in rat adipose tissue maintained in vitro

Growth hormone treatment (GH) decreases adipose tissue sensitivity to insulin. However, the exact molecular mechanism(s) involved remains unclear. In the present study, we have evaluated the chronic effects of GH on adipose tissue explants cultured in a defined media. The objective was to determine the effects of GH treatment for 24 and 48 hours on the early steps of the insulin signal transduction, including IRS-3. The 24-hour culture media contained no hormones or 100 ng/ml GH. The 48-hour culture media contained insulin and dexamethasone supplemented with or without 100 ng/ml of GH. Results demonstrated a reduction in the cellular concentration of IRS-1 by around 30% when adipose tissue was chronically treated with growth hormone for either 24 or 48 hours. IRS-3 protein levels were also decreased by 15% after the 24-hour treatment, and by 27% after culture with GH for 48 hours in the presence of insulin and dexamethasone. PI 3-kinase concentrations were also reduced by GH in both experiments by around 25%. At the end of the 24-hour culture with GH adipose explants were stimulated with insulin in a short-term incubation, after which phosphorylation and association of the IRSs with PI 3-kinase were evaluated. After the insulin stimulus, the association of PI 3-kinase with IRS-1 and IRS-3 were decreased in explants chronically cultured with GH by 44 and 28%, respectively. After this short-term insulin stimulus, the IRS-3 phosphorylation was also lowered in GH-treated explants. The results with chronic cultures of adipose presented here are consistent with similar changes in IRS-1 and IRS-2 concentration and phosphorylation observed for liver and muscle after long-term (3-5 days) in vivo treatment with GH. The data suggest that chronic GH treatment alters the early steps of the insulin signal transduction pathway, and may explain the changes in adipose tissue sensitivity to insulin.

Growth hormone-induced diacylglycerol and ceramide formation via Galpha i3 and Gbeta gamma in GH4 pituitary cells. Potentiation by dopamine-D2 receptor activation

Growth hormone (GH) secretion is regulated by indirect negative feedback mechanisms. To address whether GH has direct actions on pituitary cells, lipid signaling in GH(4)ZR(7) somatomammotroph cells was examined. GH (EC(50) = 5 nm) stimulated diacylglycerol (DAG) and ceramide formation in parallel by over 10-fold within 15 min and persisting for >3 h. GH-induced DAG/ceramide formation was blocked by pertussis toxin (PTX) implicating G(i)/G(o) proteins and was potentiated 1.5-fold by activation of G(i)/G(o)-coupled dopamine-D2S receptors, which had no effect alone. Following PTX pretreatment, only PTX-resistant Galpha(i)3, not Galpha(o) or Galpha(i)2, rescued GH-induced DAG/ceramide signaling. GH-induced DAG/ceramide formation was also blocked in cells expressing Gbetagamma blocker GRK-ct. In GH(4)ZR(7) cells, GH induced phosphorylation of JAK2 and STAT5, which was blocked by PTX and mimicked by ceramide analogue C2-ceramide or sphingomyelinase treatment to increase endogenous ceramide. We conclude that in GH(4) pituitary cells, GH induces formation of DAG/ceramide via a novel Galpha(i)3/Gbetagamma-dependent pathway. This novel pathway suggests a mechanism for autocrine feedback regulation by GH of pituitary function.

Rapid Ca2+ influx and diacylglycerol synthesis in growth hormone-mediated islet beta -cell mitogenesis

Growth hormone (GH) is an important mitogenic stimulus for the insulin-producing beta-cell. We investigated the effects of GH on Ca(2+) handling and diacylglycerol (DAG) and cAMP formation in the beta-cell. GH elicited a rapid increase in the cytoplasmic free [Ca(2+)], which required extracellular Ca(2+) and was also blocked by pertussis toxin or protein kinase C (PKC) inhibition. GH also elevated islet DAG content, which should lead to PKC activation. Pertussis toxin and PKC inhibitors obliterated the mitogenicity of GH, suggesting involvement of GTP-binding proteins. PKC activation stimulated beta-cell proliferation, and it also activated phospholipase D. Islet cAMP content was not elevated by GH. Addition of a specific protein kinase A antagonist failed to influence the mitogenicity of GH, whereas a stimulatory cAMP agonist stimulated beta-cell replication. We conclude that GH rapidly increases the beta-cell cytoplasmic free [Ca(2+)] and also evokes a similar increase in DAG content via a phosphatidylcholine-specific phospholipase C, but does not affect mitogen-activated protein kinases, phospholipase D, or the cAMP signaling pathway. This rise in DAG may be of importance in translation of the stimulatory signal of GH into a proliferative response by the beta-cell, which seems to occur through GTP-binding proteins and PKC-dependent mechanisms.

Transgenic models of growth hormone action

A growth-promoting principle of the pituitary gland was discovered in 1921, and bovine growth hormone (GH) was isolated in 1944. Since then, the structure of GH as it relates to its biological activities has been an exciting research topic. Equally fascinating is the relationship between GH structure and its metabolic activities. In attempts to define some of these activities, several investigators have used GH transgenic mice as models. In this review we summarize what is known about the molecular mechanisms of GH action. We then describe some of the GH transgenic models and point out potential targets for nutrition research.

Proliferation and differentiation of progeny of ovine unilocular fat cells (adipofibroblasts)

The responsiveness of progency of sheep-derived unilocular fat cells (adipofibroblasts) to dexamethasone, insulin, insulinlike growth factor I (IGF-I), growth hormone (GH), and basic fibroblst growth factor (FGF) was determined in a clonal culture system. Primary cultures of mature adipocytes were obtained from intermuscular adipose tissue (semimembranosus/semitendinosus seam depot) of sheep by ceiling culture techniques. Following degeneration of unilocular fat droplets and re-establishment of fibroblasticlike adipofibroblasts, all adipofibroblasts adhering to upper flask surfaces were collected and isolated away from fibroblasts (which had no multilocular vesicles) by Percoll gradient centrifugation. Progeny derived from a single adipofibroblast were isolated and tested for the ability to proliferate, differentiate, and accumulate lipids. Stock cultures of adipofibroblasts reached confluence in 5 d and were induced to differentiate from 7 to 9 d with dexamethasone-methyl isobutylxanthine-insulin (DMI). Incubation with insulin, IGF-I, GH, or FGF prior to confluence followed by induction with DMI produced no direct (priming) effect on subsequent differentiation. When substituted individually in place of DMI during the 2 d differentiation/induction period, all factors induced differentiation of cultured adipofibroblasts as determined by lipogenesis (P < .05) and lipoprotein lipase activity (P < .05). Thus, isolated adipofibroblasts from sheep muscle may be induced by hormones and growth factors to display mature adipocyte morphology in cell culture. Further definition of the adipofibroblast culture system may aid in the identification of mechanisms regulating adipocyte development in sheep skeletal muscle, as well as in the study of intercommunication between fat and muscle cells.

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.

Growth hormone increases calcium uptake in rat fat cells by a mechanism dependent on protein kinase C

Growth hormone (GH; 500 ng/ml) rapidly doubled cytosolic free Ca2+ concentration ([Ca2+]i) in rat adipocytes as determined with the Ca2+ indicator fura 2. No response was seen in Ca(2+)-free medium, suggesting that the increase in [Ca2+]i was due to Ca2+ influx. GH also doubled the influx of Mn2- as inferred from the rate of fluorescence quenching. Depolarization with 30 mMK+ also increased [Ca2+]i, and the increase in [Ca2+]i due to either GH or 30 mMK+ was blocked by 100 nM nimodipine, suggesting that GH increases [Ca2+]i by activating voltage-sensitive L-type Ca2+ channels. GH increased [Ca2+]i even when K+ channels were blocked, suggesting that activation of Ca2+ uptake was not secondary to closure of K+ channels and consequent depolarization. A diacylglycerol (PAG) analogue, 1,2-dioctanoyl-sn-glycerol (50 microM), duplicated, and the protein kinase C(PKC) inhibitors calphostin C (100 nM), chelerythrine (1 microM), and bis-indolylmaleimide (250 nM) inhibited the effects of GH on [Ca2+]i. Xanthogenate tricyclodecan-9-yl (D609), a specific inhibitor of phospholipase C(PLC), abolished the increase in [Ca2+]i due to GH but not to DAG. The results suggest that GH increases [Ca2+]i by activation of PLC, release of DAG, and activation of a Ca(2+)-independent isoform of PKC. PKC-catalyzed phosphorylation of either the Ca2+ channels or a protein that regulates them may account for the influx of Ca2+ produced by GH.

Bovine growth hormone induces oscillations in cytosolic free Ca2+ in single rat hepatocytes

Single rat hepatocytes microinjected with the photoprotein aequorin generate oscillations in the cytosolic free Ca2+ concentration ([Ca2+]i) when stimulated with agonists acting through the phosphoinositide signalling pathway. We show here that, in single rat hepatocytes, bovine growth hormone (bGH) is able to induce [Ca2+]i oscillations which display similarities with oscillations induced by phenylephrine. Thus the rate of rise of intracellular Ca2+ in each oscillation closely resembles that induced by Ins(1,4,5)P3-mediated agonists. However, the duration of bGH-induced oscillations increases with agonist concentration, in contrast to phenylephrine-induced oscillations, which undergo an increase in frequency as the agonist concentration is raised, without any increase in the duration of individual oscillations.

Postreceptor signaling mechanisms for Growth Hormone

Recent data have shed significant new light on the mechanisms involved in the transmission of a biologic signal by GH. Following ligand-induced dimerization of the GH receptor, multiple cascades are involved in GH signaling. These include activation of nonreceptor tyrosine kinases, in particular JAK2, which is a mechanism shared by the newly described cytokine receptor superfamily. Furthermore, several classic pathways (for example, guanine-nucleotide-binding proteins and protein kinase C), shared by numerous hormones, growth factors, and neurotransmitters, are also involved in many of the actions of GH. The interrelationships between the various signaling pathways for GH have not yet been fully defined. This review briefly summarizes the current state of knowledge with respect to the processes involved in the effects of GH in target cells.

Growth hormone (GH) induces tyrosine-phosphorylated proteins in mouse L cells that express recombinant GH receptors

Porcine and bovine GH receptor (GHR) cDNAs were stably expressed in mouse L cells, which normally do not possess detectable levels of mouse GHR. Expression of the GHR cDNAs resulted in specific binding of 125I-labeled GH by these cell lines. To study GHR-related signaling events in these cells, protein tyrosine phosphorylation was examined. In GH-treated cells, a tyrosine-phosphorylated protein with a molecular mass of approximately 95 kDa (pp95) was increased dramatically (approximately 100-fold) relative to non-GH-treated cells. The amount of pp95 within the cells after GH treatment was positively correlated with the number of GHRs on the cells. Tyrosine phosphorylation of pp95 could not be induced by prolactin, insulin, insulin-like growth factor I, interleukin 2, epidermal growth factor, platelet-derived growth factor, or fibroblast growth factor. Phosphorylation of pp95 was found to be a rapid event that could be observed 60 sec after GH treatment. Also, pp95 appears to exist as a complex of two proteins, i.e., pp95 and pp96. The GH-induced response by these cells may be of use in screening GH analogs for biological activity.

Growth hormone regulation of lipid metabolism in cells transfected with growth hormone receptor cDNA

The functional properties of the growth hormone (GH) receptor was studied using cellular transfection of GH receptor cDNA. GH treatment (1.5-2 h) of Chinese hamster ovary cells, stably transfected with GH receptor cDNA (CHO4), resulted in increased cellular lipid synthesis (240% of control). This effect was blocked by staurosporine, suggesting a dependence on cellular kinases. However, if GH treatment of CHO4 cells was prolonged (16 h), this instead stimulated lipolysis (128% of control). The GH receptor in CHO4 cells was also shown to be functional in terms of ligand internalization. A GH receptor mutant, in which 183 amino acids had been deleted in the carboxyterminal of the intracellular domain was functionally active, while a receptor without its intracellular domain was shown to be inactive. In conclusion, GH receptors expressed in CHO cells are functional and GH was also shown to have both an acute insulin-like effect, which was kinase dependent, and a long-term anti-insulin-like effect on the lipid metabolism. This suggests that an approach using GH receptor cDNA transfected cells can be of value in understanding the mechanism of GH action.

Inhibition of prostacyclin-induced Ca2+ mobilization by phorbol esters in Ob1771 preadipocytes

In addition to cAMP production, a transient elevation of intracellular free Ca2+ has been shown to take place in preadipose cells upon stimulation by carbaprostacyclin (cPGI2), both messengers acting in synergy to initiate adipose cell differentiation (Vassaux, G., Gaillard, D., Ailhaud, G., and Négrel, R. (1992) J. Biol. Chem.267, 11092-11097). Further studies reported herein show that this Ca2+ transient is i) elicited by the natural prostaglandin PGI2, ii) independent of the presence of extracellular Ca2+, suggesting a mobilization of Ca2+ from intracellular pools and ii) unaffected by cAMP elevating agents. Moreover, and in contrast to the InsP3-dependent Ca2+ signal evoked by PGF2 alpha, that induced by PGI2 is fully abolished by pretreatment with phorbol esters (EC50: 1-5 nM). Furthermore, experiments designed to empty the Ca2+ pools, using PGI2 or PGF2 alpha as Ca2+ mobilizing agents as well as pretreatments with drugs, allow to conclude that PGI2 mobilizes Ca2+ from an InsP3 sensitive, ryanodine insensitive intracellular pool. Altogether, these results strongly suggest that PGI2 mobilizes Ca2+ from an intracellular store common to that affected by InsP3, by means of a mechanism which remains to be elucidated.

Evidence that growth hormone stimulates protein kinase C activity in isolated rat hepatocytes

The mechanism of action of growth hormone (GH) is not known, although indirect evidence suggests that protein kinase C (PKC) might play an important role in the insulin-like actions of GH. In this investigation, we directly examined the effects of GH relative to those of insulin on PKC activity in isolated rat hepatocytes. Human GH (10(-7) mol/L) significantly increased the activity of PKC in both cytosolic and particulate fractions. The effect was maximal at 1 minute, disappeared at 5 minutes, and then increased again at 30 minutes in both fractions. At 1 minute, maximal and half-maximal stimulation of PKC activity occurred at hGH concentrations of 10(-7) and 5 x 10(-9) mol/L, respectively. Insulin (10(-7) mol/L) also induced a significant and transient increase in enzyme activity at 2 minutes in cytosolic and particulate fractions; at 30 minutes, PKC activity was decreased in the soluble fraction (-17%) and increased in the particulate fraction (+65%). Measurement of specific [3H]-phorbol dibutyrate (PDBu) binding suggested translocation of PKC from the cytosol to the membrane fraction after 30 minutes of incubation, only after insulin treatment. The early effects of GH and insulin on PKC activity were additive in both the particulate and cytosolic fractions. Although the later effects of GH and insulin on PKC were quite different, both hormones rapidly activated PKC in isolated hepatocytes, suggesting that PKC might be involved in triggering the insulin-like actions of GH.

The role and mechanism of growth hormone in the regulation of sexually dimorphic P450 enzymes in rat liver

The determination of sexually dimorphic hepatic steroid metabolism in rat liver has been shown to involve growth hormone. However, the mechanisms by which growth hormone controls the cytochrome P450 enzymes responsible for this dimorphic steroid metabolism is largely unknown. In this review we discuss different levels of growth hormone signal transduction, including receptor binding, signal transduction and activation of target genes by growth hormone.

Growth hormone activates mitogen-activated protein kinase and S6 kinase and promotes intracellular tyrosine phosphorylation in 3T3-F442A preadipocytes

Physiological concentrations of growth hormone induced a rapid and transient activation of mitogen-activated protein kinase (MAP kinase) and S6 kinase in 3T3-F442A preadipocytes. These effects were abrogated by staurosporine and in cells chronically pretreated with phorbol esters, suggesting that protein kinase C is involved in the mechanism of activation. In addition, three cytosolic proteins exhibited a growth-hormone-dependent increase in tyrosine phosphorylation.

Terminal differentiation of mouse preadipocyte cells: the mitogenic-adipogenic role of growth hormone is mediated by the protein kinase C signalling pathway

The role of growth hormone (GH) in the differentiation process of Ob1771 mouse preadipocyte cells has been studied under culture conditions that were serum-free and hormone-supplemented and which were previously shown to lead to terminal differentiation. In the absence of GH, a dramatic decrease in the adipogenic activity of the culture medium could be observed, as indicated 12 days after confluence by the low levels of glycerol-3-phosphate dehydrogenase activity and the sharp reduction of the number of triacylglycerol-containing cells. This decrease in adipogenic activity was accompanied by a parallel loss of the mitogenic potency of the culture medium. Determination of the half-maximal and maximal concentrations of GH required for the restoration of growth and differentiation were identical, 0.5 and 2 nM, respectively. Despite the presence of insulin-like growth factor-I (IGF-I) to substitute for supraphysiological concentrations of insulin and to saturate IGF-I receptor, GH was still required to induce terminal differentiation of a maximal number of cells. However, protein kinase C activators such as prostaglandin F2 alpha, phorbol esters and diacylglycerol were able to mimic GH in promoting a maximal mitogenic-adipogenic response, indicating that the ability of GH to induce diacylglycerol production (Doglio et al., 1989; Catalioto et al., 1990) plays a prominent role in this process. Furthermore, in agreement with the fact that the mitoses which precede terminal differentiation of Ob1771 preadipocytes are strictly controlled by cAMP and only modulated by protein kinase C, terminal differentiation of Ob1771 preadipocytes occurred in the absence of GH upon supplementation with high concentrations of carbaprostacyclin, added as a cAMP-elevating agent or with 8-Br-cAMP, added as a cAMP analogue. It is concluded that the control exerted by GH on terminal differentiation of mouse preadipocytes corresponds to a modulating mitogenic effect mediated through protein kinase C activation and leading to a potentiation of the cAMP and IGF-I mitogenic signalling pathways.

Growth hormone regulation of hepatic cytochrome P450 expression in the rat

GH by means of its sexually differentiated secretory pattern is the predominant regulator of the expression of cytochrome P450 enzymes responsible for a sexual dimorphism of hepatic steroid metabolism. Other hormones, such as gonadal, thyroid and glucocorticoid hormones, as well as insulin appear to modulate the sexually differentiated expression of these enzymes. The major constitutively expressed sex specific forms of P450, belonging to the P4502C-subfamily, have been shown to be regulated by GH at the level of transcription. However, the GH postreceptor events leading to increased or decreased transcriptional activity are essentially unknown. Neither is the functional role of the soluble GH binding protein yet resolved. On-going protein synthesis is a prerequisite for GH transcriptional activation of the female specific P4502C12 but not for all GH effects in the hepatocyte. With regard to signalling mechanisms PKC activity appears to be permissive for the GH induction of P4502C12 but some as yet unidentified factor/kinase(s) may also be activated. The transcriptional control exerted on the rat P4502C-gene subfamily by the pattern of GH secretion offers a versatile tool to elucidate the molecular mechanisms of GH regulation of cytochrome P450 expression.