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

The role of growth hormone receptor in β cell function

Growth hormone (GH) exerts numerous effects on tissues through binding to its receptor, GHR, which resides on cell membranes in many different organs and tissues. Endocrine pancreatic β cells are the only source of insulin secretion in response to metabolic demand, thereby regulating blood glucose and maintaining metabolic homeostasis. β cell dysfunction is the main composition of diabetes mellitus. Numerous studies have provided strong evidence that GHR signaling plays an independent role in β cell function. In this review, we focus on the role of GHR signaling in β cell actions and the underlying molecular mechanisms.

The beneficial effect of growth hormone treatment on islet mass in streptozotocin-treated mice

BACKGROUND

Type 1 diabetes is an autoimmune disease, characterized by a loss of pancreatic β-cell mass and function, which results in dramatic reductions in insulin secretion and circulating insulin levels. Patients with type 1 diabetes are traditionally treated with insulin injections and insulin pumps ex vivo or undergo transplantation. Growth hormone (GH) has been shown to be involved in β-cell function and survival in culture.

METHODS

Twelve-week-old female C57BL/6 mice were treated with streptozotocin and monitored for their weight and blood glucose levels. Fourteen days post-initial injection, these mice were separated into two groups at random. One group was treated with GH while the other treated with vehicle for up to 3 weeks. These mice were compared with mice not treated with streptozotocin.

RESULTS

Under our experimental conditions, we observed that mice treated with GH had larger islets and higher serum insulin levels than streptozotocin-treated mice treated with saline (0.288 vs. 0.073 ng/mL, p < 0.01).

CONCLUSIONS

Our data demonstrate that GH may rescue islets and therefore may possess therapeutic potential in the treatment of type 1 diabetes, although consideration should be made regarding GH's effect on insulin resistance.

Betacellulin ameliorates hyperglycemia in obese diabetic db/db mice

We found that administration of a recombinant adenovirus (rAd) expressing betacellulin (BTC) into obese diabetic db/db mice ameliorated hyperglycemia. Exogenous glucose clearance was significantly improved, and serum insulin levels were significantly higher in rAd-BTC-treated mice than rAd-β-gal-treated control mice. rAd-BTC treatment increased insulin/bromodeoxyuridine double-positive cells in the islets, and islets from rAd-BTC-treated mice exhibited a significant increase in the level of G1-S phase-related cyclins as compared with control mice. In addition, BTC treatment increased messenger RNA (mRNA) and protein levels of these cyclins and cyclin-dependent kinases in MIN-6 cells. BTC treatment induced intracellular Ca(2+) levels through phospholipase C-γ1 activation, and upregulated calcineurin B (CnB1) levels as well as calcineurin activity. Upregulation of CnB1 by BTC treatment was observed in isolated islet cells from db/db mice. When treated with CnB1 small interfering RNA (siRNA) in MIN-6 cells and isolated islets, induction of cell cycle regulators by BTC treatment was blocked and consequently reduced BTC-induced cell viability. As well as BTC's effects on cell survival and insulin secretion, our findings demonstrate a novel pathway by which BTC controls beta-cell regeneration in the obese diabetic condition by regulating G1-S phase cell cycle expression through Ca(2+) signaling pathways.

KEY MESSAGES

Administration of BTC to db/db mice results in amelioration of hyperglycemia. BTC stimulates beta-cell proliferation in db/db mice. Ca(2+) signaling was involved in BTC-induced beta-cell proliferation. BTC has an anti-apoptotic effect and potentiates glucose-stimulated insulin secretion.

Glucagon-Like Peptide-1 Receptor Activation Does not Affect Re-Endothelialization but Reduces Intimal Hyperplasia via Direct Effects on Smooth Muscle Cells in a Nondiabetic Model of Arterial Injury

Diabetic patients have an increased risk of restenosis and late stent thrombosis after angioplasty, i.e. complications that are related to a defective re-endothelialization. Exendin-4, a stable glucagon-like peptide (GLP)-1 receptor agonist, has been suggested to influence the formation of intimal hyperplasia and to increase endothelial cell proliferation in vitro. Thus, the aim of this study was to investigate the mechanisms by which treatment with exendin-4 could influence re-endothelialization and intimal hyperplasia after vascular injury.

METHODS

Sprague-Dawley rats were subjected to balloon injury of the left common carotid artery and treated for 4 weeks with exendin-4 or vehicle. Intimal hyperplasia and vessel wall elasticity were monitored noninvasively by high-frequency ultrasound, and re-endothelialization was evaluated upon sacrifice using Evans blue dye.

RESULTS AND CONCLUSION

Exendin-4 selectively reduced the proliferation of smooth muscle cells (SMCs) and intimal hyperplasia in vivo without affecting the re-endothelialization process, but treatment with exendin-4 improved arterial wall elasticity. Our data also show that exendin-4 significantly decreased the proliferation and increased the apoptosis of SMCs in vitro, effects that appear to be mediated through cAMP signaling and endothelial nitric oxide synthase following GLP-1 receptor activation. Together, these effects of exendin-4 are highly desirable and may lead to an improved outcome for patients undergoing vascular interventions.

Effects of some anti-diabetic and cardioprotective agents on proliferation and apoptosis of human coronary artery endothelial cells

BACKGROUND

The leading cause of death for patients suffering from diabetes is macrovascular disease. Endothelial dysfunction is often observed in type 2 diabetic patients and it is considered to be an important early event in the pathogenesis of atherogenesis and cardiovascular disease. Many drugs are clinically applied to treat diabetic patients. However, little is known whether these agents directly interfere with endothelial cell proliferation and apoptosis. This study therefore aimed to investigate how anti-diabetic and cardioprotective agents affect human coronary artery endothelial cells (HCAECs).

METHODS

The effect of anti-diabetic and cardioprotective agents on HCAEC viability, proliferation and apoptosis was studied. Viability was assessed using Trypan blue exclusion; proliferation in 5 mM and 11 mM of glucose was analyzed using [3H]thymidine incorporation. Lipoapoptosis of the cells was investigated by determining caspase-3 activity and the subsequent DNA fragmentation after incubation with the free fatty acid palmitate, mimicking diabetic lipotoxicity.

RESULTS

Our data show that insulin, metformin, BLX-1002, and rosuvastatin improved HCAEC viability and they could also significantly increase cell proliferation in low glucose. The proliferative effect of insulin and BLX-1002 was also evident at 11 mM of glucose. In addition, insulin, metformin, BLX-1002, pioglitazone, and candesartan significantly decreased the caspase-3 activity and the subsequent DNA fragmentation evoked by palmitate, suggesting a protective effect of the drugs against lipoapoptosis.

CONCLUSION

Our results suggest that the anti-diabetic and cardioprotective agents mentioned above have direct and beneficial effects on endothelial cell viability, regeneration and apoptosis. This may add yet another valuable property to their therapeutic effect, increasing their clinical utility in type 2 diabetic patients in whom endothelial dysfunction is a prominent feature that adversely affect their survival.

Growth hormone receptor regulates β cell hyperplasia and glucose-stimulated insulin secretion in obese mice

Insulin, growth hormone (GH), and insulin-like growth factor-1 (IGF-1) play key roles in the regulation of β cell growth and function. Although β cells express the GH receptor, the direct effects of GH on β cells remain largely unknown. Here we have employed a rat insulin II promoter-driven (RIP-driven) Cre recombinase to disrupt the GH receptor in β cells (βGHRKO). βGHRKO mice fed a standard chow diet exhibited impaired glucose-stimulated insulin secretion but had no changes in β cell mass. When challenged with a high-fat diet, βGHRKO mice showed evidence of a β cell secretory defect, with further deterioration of glucose homeostasis indicated by their altered glucose tolerance and blunted glucose-stimulated insulin secretion. Interestingly, βGHRKO mice were impaired in β cell hyperplasia in response to a high-fat diet, with decreased β cell proliferation and overall reduced β cell mass. Therefore, GH receptor plays critical roles in glucose-stimulated insulin secretion and β cell compensation in response to a high-fat diet.

Growth hormone can improve insulin resistance and differentiation in pancreas of senescence accelerated prone male mice (SAMP8)

OBJECTIVE

The aim of the present study was to investigate the effect of aging on several parameters related to glucose metabolism, proliferation and differentiation in the pancreas and how GH administration to old SAMP8 mice could affect these parameters.

MATERIALS AND METHODS

Pancreas samples were obtained from two types of male mice models: senescence-accelerated prone (SAMP8) and senescence-accelerated-resistant (SAMR1) mice SAMP8 and SAMR1 mice and the influence of exogenous administration of GH (2mgs.c./kg/day) on SAMP8 mice. RNA was isolated from pancreas samples of male mice using the kit RNeasy total RNA kit Ref. 50974104 (Qiagen). Insulin was measured in plasma by RIA kit and glucose was measured in plasma by an assay kit.

RESULTS

Aging decreases the expression of differentiation in the pancreas of Pdx-1, FoxO 1 and FoxO 3A but not of Sirt 1 or of the expression of the proliferative genes PCNA and Sei1. The expression of glucagon and GLUT2 were increased with aging and no differences were observed in somatostatin and insulin expressions. Insulin levels in plasma were increased with aging in SAMP8 mice. IGF-1 expression was reduced with aging. The treatment with GH was able to increase the expression of Sirt 1, Pdx-1, FoxO 3A and IGF-1. On the other hand, the treatment decreased the expression of glucagon, GLUT2, somatostatin and insulin, furthermore GH was able to decrease the plasma levels of insulin in old male SAMP8 mice (p<0.0004).

CONCLUSION

The present study has shown that aging is associated with significant alterations in the relative expression of pancreatic genes involved in insulin secretion as well as in the differentiation and in the intra islet glucose metabolism. According to our results, GH administration to old SAMP8 mice was able to improve the pancreatic function of the old SAMP8 mice and to decrease insulin and glucagon expressions in the pancreas improving instead insulin levels and glucose metabolism.

Intrinsic regulators of pancreatic beta-cell proliferation

Once thought incapable of significant proliferation, the pancreatic beta-cell has recently been shown to harbor immense powers of self-renewal. Pancreatic beta-cells, the sole source of insulin in vertebrate animals, can grow facultatively to a degree unmatched by other organs in experimental animals. beta-cell growth matches changes in systemic insulin demand, which increase during common physiologic states such as aging, obesity, and pregnancy. Compensatory changes in beta-cell mass are controlled by beta-cell proliferation. Here we review recent advances in our understanding of the intrinsic factors and mechanisms that control beta-cell cycle progression. Dysregulation of beta-cell proliferation is emerging as a fundamental feature in the pathogenesis of human disease states such as cancer and diabetes mellitus. New experimental observations and studies of these diseases suggest that beta-cell fate and expansion are coordinately regulated. We speculate on how these advances may accelerate the discovery of new strategies for the treatment of diseases characterized by a deficiency or excess of beta-cells.

Pioglitazone acutely influences glucose-sensitive insulin secretion in normal and diabetic human islets

We have studied acute effects of the PPARgamma agonist pioglitazone in vitro on human islets from both non-diabetic and type 2 diabetic subjects. In 5 mM glucose, pioglitazone caused a transient increase in insulin secretion in non-diabetic, but not diabetic, islets. Continuous presence of the drug suppressed insulin release in both non-diabetic and diabetic islets. In islets from non-diabetic subjects, both high glucose and tolbutamide-stimulated insulin secretion was inhibited by pioglitazone. When islets were continuously perifused with 5 mM glucose, short-term pretreatment with pioglitazone caused approximately 2-fold increase in insulin secretion after drug withdrawal. Pioglitazone pretreatment of diabetic islets restored their glucose sensitivity. Examination of cytosolic free Ca(2+) concentration ([Ca(2+)](i)) in non-diabetic islets revealed slight Ca(2+) transient by pioglitazone at 3 mM glucose with no significant changes at high glucose. Our data suggest that short-term pretreatment with pioglitazone primes both healthy and diabetic human islets for enhanced glucose-sensitive insulin secretion.

Phospholipase Cgamma1 negatively regulates growth hormone signalling by forming a ternary complex with Jak2 and protein tyrosine phosphatase-1B

Growth hormone binds to its membrane receptor (GHR), whereby it regulates many cellular functions, including proliferation, differentiation and chemotaxis. However, although the activation of growth hormone-mediated signalling is well understood, the precise mechanism responsible for its regulation has not been elucidated. Here, we demonstrate that phospholipase Cgamma1 (PLCgamma1) modulates the action of growth hormone-mediated signalling by interacting with tyrosine kinase Jak2 (janus kinase 2) in a growth hormone-dependent manner. In the absence of PLCgamma1 (PLCgamma1(-/-)), growth hormone-induced JAK2 and STAT5 phosphorylation significantly increased in mouse embryonic fibroblasts (MEFs). Furthermore, the re-expression of PLCgamma1 reduced growth hormone-induced Jak2 activation. Growth hormone-induced Jak2 phosphorylation was enhanced by siRNA-specific knockdown of PLCgamma1. Interestingly, PLCgamma1 physically linked Jak2 and protein tyrosine phosphatase-1B (PTP-1B) by binding to both using different domains, and this process was implicated in the modulation of cytokine signalling through Jak2. In addition, in PLCgamma1(-/-) MEFs, growth hormone-dependent c-Fos activation was upregulated and growth hormone-induced proliferation was potentiated. These results suggest that PLCgamma1 has a key function in the regulation of growth hormone-mediated signalling by negatively regulating Jak2 activation.

Involvement of JAK2 and Src kinase tyrosine phosphorylation in human growth hormone-stimulated increases in cytosolic free Ca2+and insulin secretion

We previously reported that human growth hormone (hGH) increases cytoplasmic Ca2+concentration ([Ca2+]i) and proliferation in pancreatic β-cells (Sjöholm Å, Zhang Q, Welsh N, Hansson A, Larsson O, Tally M, and Berggren PO. J Biol Chem 275: 21033–21040, 2000) and that the hGH-induced rise in [Ca2+]iinvolves Ca2+-induced Ca2+release facilitated by tyrosine phosphorylation of ryanodine receptors (Zhang Q, Kohler M, Yang SN, Zhang F, Larsson O, and Berggren PO. Mol Endocrinol 18: 1658–1669, 2004). Here we investigated the tyrosine kinases that convey the hGH-induced rise in [Ca2+]iand insulin release in BRIN-BD11 β-cells. hGH caused tyrosine phosphorylation of Janus kinase (JAK)2 and c-Src, events inhibited by the JAK2 inhibitor AG490 or the Src kinase inhibitor PP2. Although hGH-stimulated rises in [Ca2+]iand insulin secretion were completely abolished by AG490 and JAK2 inhibitor II, the inhibitors had no effect on insulin secretion stimulated by a high K+concentration. Similarly, Src kinase inhibitor-1 and PP2, but not its inactive analog PP3, suppressed [Ca2+]ielevation and completely abolished insulin secretion stimulated by hGH but did not affect responses to K+. Ovine prolactin increased [Ca2+]iand insulin secretion to a similar extent as hGH, effects prevented by the JAK2 and Src kinase inhibitors. In contrast, bovine GH evoked a rise in [Ca2+]ibut did not stimulate insulin secretion. Neither JAK2 nor Src kinase inhibitors influenced the effect of bovine GH on [Ca2+]i. Our study indicates that hGH stimulates rise in [Ca2+]iand insulin secretion mainly through activation of the prolactin receptor and JAK2 and Src kinases in rat insulin-secreting cells.

Muscarinic M1 and M3 receptor binding alterations in pancreas during pancreatic regeneration of young rats

The importance of muscarinic receptors in proliferation of different cell types and in insulin secretion from pancreatic beta cells has been extensively studied. However, the role of pancreatic muscarinic receptors during pancreatic regeneration has not yet been studied. For the first time, the functional status of the muscarinic M1 and M3 receptors in regeneration of the pancreas is investigated here. It is observed that the number and affinity of high-affinity muscarinic M3 receptors increased at the time of regeneration. The low-affinity M3 receptors also showed a similar trend. In the case of muscarinic M1 receptors, the receptor number increased with a decrease in affinity. We also observed an increase in the circulating insulin levels at the time of active regeneration. The in vitro studies confirmed that muscarinic receptors are stimulatory to insulin secretion. Our results suggest that the increased muscarinic M1 and M3 receptor subtypes stimulate insulin secretion and islet cell proliferation during the regeneration of pancreas.

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.

The phospholipase C-InsP3 pathway is involved in calcium mobilization induced by growth hormone in hepatocytes

We investigated the effects of bovine GH (bGH) on Ca(2+) handling, phospholipase C (PLC) activation and inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] formation in hepatocytes. bGH generates oscillations in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) in single male rat hepatocytes microinjected with the photoprotein aequorin. In the absence of extracellular Ca(2+) these transients persisted for more than 10 min indicating a requirement for intracellular Ca(2+). Treatment of the hepatocyte with the phosphatidylinositol-specific phospholipase C (PI-PLC) inhibitor U-73122 removed the oscillations. These results suggest bGH-induced oscillations are due to PLC activation and generation of Ins(1,4,5)P(3). We measured the mass of Ins(1,4,5)P(3) in freshly isolated hepatocyte suspensions in response to bGH, and vasopressin as a control. Both agonists rapidly increased the levels of Ins(1,4,5)P(3). This is the first study to indicate that early events in the signal transduction pathways mediated by GH in hepatocytes involve intracellular Ca(2+) mobilization via activation of a PI-PLC and subsequent Ins(1,4,5)P(3) production.

Inositol hexakisphosphate and sulfonylureas regulate β-cell protein phosphatases

In human type 2 diabetes, loss of glucose-stimulated insulin exocytosis from the pancreatic beta-cell is an early pathogenetic event. Mechanisms controlling insulin exocytosis are, however, not fully understood. We show here that inositol hexakisphosphate (InsP(6)), whose concentration transiently increases upon glucose stimulation, dose-dependently and differentially inhibits enzyme activities of ser/thr protein phosphatases in physiologically relevant concentrations. None of the hypoglycemic sulfonylureas tested affected protein phosphatase-1 or -2A activity at clinically relevant concentrations in these cells. Thus, an increase in cellular phosphorylation state, through inhibition of protein dephosphorylation by InsP(6), may be a novel regulatory mechanism linking glucose-stimulated polyphosphoinositide formation to insulin exocytosis in insulin-secreting cells.

Role of protein kinase C in arginine vasopressin-stimulated ERK and p70S6 kinase phosphorylation

We previously showed in rat renal glomerular mesangial cells, that arginine vasopressin (AVP)-stimulated cell proliferation was mediated by epidermal growth factor receptor (EGF-R) transactivation, and activation (phosphorylation) of ERK1/2 and p70S6 kinase (Ghosh et al. [2001]: Am J Physiol Renal Physiol 280:F972-F979]. In this paper, we extend these observations and show that different protein kinase C (PKC) isoforms play different roles in mediating AVP-stimulated ERK1/2 and p70S6 kinase phosphorylation and cell proliferation. AVP treatment for 0-60 min stimulated the serine/threonine phosphorylation of PKC isoforms alpha, delta, epsilon, and zeta. The activation of PKC was dependent on EGF-R and phosphatidylinositol 3-kinase (PI3K) activation. In addition, inhibition of conventional and novel PKC isoforms by chronic (24 h) exposure to phorbol 12-myristate 13-acetate (PMA) inhibited AVP-induced activation of ERK and p70S6 kinase as well as EGF-R phosphorylation. Rottlerin, a specific inhibitor of PKCdelta, inhibited both ERK and p70S6 kinase phosphorylation and cell proliferation. In contrast, a PKCepsilon translocation inhibitor decreased ERK1/2 activation without affecting p70S6 kinase or cell proliferation, while a dominant negative PKCzeta (K281W) cDNA delayed p70S6 kinase activation without affecting ERK1/2. On the other hand, Gö6976, an inhibitor of conventional PKC isoforms, did not affect p70S6 kinase, but stimulated ERK1/2 phosphorylation without affecting cell proliferation. Our results indicate that PKCdelta plays an important role in AVP-stimulated ERK and p70S6 kinase activation and cell proliferation.

Glibenclamide inhibits islet carnitine palmitoyltransferase 1 activity, leading to PKC-dependent insulin exocytosis

Hypoglycemic sulfonylureas such as glibenclamide have been widely used to treat type 2 diabetic patients for 40 yr, but controversy remains about their mode of action. The widely held view is that they promote rapid insulin exocytosis by binding to and blocking pancreatic beta-cell ATP-dependent K+ (KATP) channels in the plasma membrane. This event stimulates Ca2+ influx and sets in motion the exocytotic release of insulin. However, recent reports show that >90% of glibenclamide-binding sites are localized intracellularly and that the drug can stimulate insulin release independently of changes in KATP channels and cytoplasmic free Ca2+. Also, glibenclamide specifically and progressively accumulates in islets in association with secretory granules and mitochondria and causes long-lasting insulin secretion. It has been proposed that nutrient insulin secretagogues stimulate insulin release by increasing formation of malonyl-CoA, which, by blocking carnitine palmitoyltransferase 1 (CPT-1), switches fatty acid (FA) catabolism to synthesis of PKC-activating lipids. We show that glibenclamide dose-dependently inhibits beta-cell CPT-1 activity, consequently suppressing FA oxidation to the same extent as glucose in cultured fetal rat islets. This is associated with enhanced diacylglycerol (DAG) formation, PKC activation, and KATP-independent glibenclamide-stimulated insulin exocytosis. The fat oxidation inhibitor etomoxir stimulated KATP-independent insulin secretion to the same extent as glibenclamide, and the action of both drugs was not additive. We propose a mechanism in which inhibition of CPT-1 activity by glibenclamide switches beta-cell FA metabolism to DAG synthesis and subsequent PKC-dependent and KATP-independent insulin exocytosis. We suggest that chronic CPT inhibition, through the progressive islet accumulation of glibenclamide, may explain the prolonged stimulation of insulin secretion in some diabetic patients even after drug removal that contributes to the sustained hypoglycemia of the sulfonylurea.

Signal transducer and activator of transcription 5 activation is sufficient to drive transcriptional induction of cyclin D2 gene and proliferation of rat pancreatic beta-cells

Signal transducer and activator of transcription 5 (STAT5) activation plays a central role in GH- and prolactin-mediated signal transduction in the pancreatic beta-cells. In previous experiments we demonstrated that STAT5 activation is necessary for human (h)GH-stimulated proliferation of INS-1 cells and hGH-induced increase of mRNA-levels of the cell cycle regulator cyclin D2. In this study we have further characterized the role of STAT5 in the regulation of cyclin D expression and beta-cell proliferation by hGH. Cyclin D2 mRNA and protein levels (but not cyclin D1 and D3) were induced in a time-dependent manner by hGH in INS-1 cells. Inhibition of protein synthesis by coincubation with cycloheximide did not affect the hGH-induced increase of cyclin D2 mRNA levels at 4 h. Expression of a dominant negative STAT5 mutant, STAT5aDelta749, partially inhibited cyclin D2 protein levels. INS-1 cells transiently transfected with a cyclin D2 promoter-reporter construct revealed a 3- to 5-fold increase of transcriptional activity in response to hGH stimulation. Furthermore, coexpression of a constitutive active STAT5 mutant (either CA-STAT5a or CA-STAT5b) was sufficient to drive transactivation of the promoter. CA-STAT5b was stably expressed in INS-1 cells under the control of a doxycycline-inducible promoter. Gel retardation experiments using a probe representing a putative STAT5 binding site in the cyclin D2 promoter revealed binding of the doxycycline-induced CA-STAT5b. Furthermore, induction of CA-STAT5b stimulated transcriptional activation of the cyclin D2 promoter and induced hGH-independent proliferation in these cells. In primary beta-cells, adenovirus-mediated expression of CA-STAT5b profoundly stimulated DNA-synthesis (5.3-fold over control) in the absence of hGH. Our studies indicate that STAT5 activation is sufficient to drive proliferation of the beta-cells and that cyclin D2 may be a critical target gene for STAT5 in this process.

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.

Growth hormone signal transduction

Multiple signaling pathways mediate the diverse effects of growth hormone (GH) on growth and metabolism. The interaction of GH with GH receptors (GHR) on target cells promotes the association of the cellular tyrosine kinase JAK2 with the GHR, initiating tyrosine phosphorylation of GHR and JAK2, and activation of multiple signaling cascades. GH-stimulated activation of signal transducers and activators of transcription (STATs), mitogen activated protein kinase (MAPK) and phosphatidylinositol 3' kinase (PI3K) cascades have been shown to regulate the transcription of GH-responsive genes. Cross-talk among these signaling cascades in regulating specific genes suggests that GH signaling to the nucleus involves a GH-regulated signaling network.

Signal transduction via the growth hormone receptor

Rapid progress has been made recently in the definition of growth hormone (GH) receptor signal transduction pathways. It is now apparent that many cytokines, including GH, share identical or similar signalling components to exert their cellular effects. This review provides a brief discourse on the signal transduction pathways, which have been demonstrated to be utilized by GH. The identification of such pathways provides a basis for understanding the pleiotropic actions of GH. The mechanisms by which the specific cellular effects of GH are achieved remain to be elucidated.

Casein phosphopeptides influence calcium uptake by cultured human intestinal HT-29 tumor cells

We investigated the direct effects of casein phosphopeptides (CPP), which are formed by the proteolytic degradation of alpha- and beta-caseins, on calcium uptake by human HT-29 intestinal tumor cells, which undergo an enterocytically oriented differentiation in culture. A commercial preparation containing a mixture of purified CPP and an individual CPP of 25 amino acids, both containing the characteristic Ca(2+) binding motif, ser(P)-ser(P)-ser(P)-glu-glu, were employed. The study was performed at the single-cell level and on a cell population and measured the changes in cytosolic calcium concentration before and after CPP addition. In the presence of 2 mmol/L extracellular calcium, both CPP preparations induced a transient rise of free intracellular calcium ions, which did not influence ATP-induced release of calcium from intracellular stores, and which disappeared completely in the absence of extracellular calcium. Pretreatment of these cells with thapsigargin, which completely empties the intracellular calcium stores, did not abolish the cell responses to CPP. Repetitive stimulation of HT-29 cells with CPP always elicited a transient calcium rise, suggesting a lack of desensitization. The CPP-stimulated cytosolic calcium rise was dependent on CPP dose, in a seemingly nonsaturating mode, and on cell numbers. All of this is consistent with the hypothesis that CPP do not influence membrane-bound receptors or ion channels, but may act as calcium ionophores or calcium carriers across the membrane. The reported findings provide a new basis on which to assess the possibility that CPP enhance calcium absorption and bioavailability in animals.

Arginine vasopressin stimulates mesangial cell proliferation by activating the epidermal growth factor receptor

The potent vasoconstrictor arginine vasopressin (AVP) is also a mitogen for mesangial cells. Treatment with AVP decreased transit time through the cell cycle. AVP-stimulated mesangial cell growth by activating both the Ras mitogen-activated protein kinase (MAPK) and the phosphatidylinositol 3-kinase (PI3K) cell signaling pathways. Both the selective PI3K inhibitor LY-294002 and the MAPK kinase (MEK) inhibitor PD-98059 inhibited AVP-stimulated mesangial cell proliferation. However, LY-294002 was more potent, indicating an important role for PI3K activation in AVP-stimulated mesangial cell proliferation. AVP appeared to exert its effect on MAPK and PI3K activation, as well as on cell proliferation, by activating the epidermal growth factor receptor (EGF-R). Pretreatment with the tyrphostin-derived EGF-R antagonist AG-1478 inhibited mesangial cell proliferation as well as the activation of extracellular signal-regulated kinase 1/2 (ERK1/2 or p42/p44MAPK), and p70S6 kinase, a downstream effector of PI3K, providing evidence that MAPK and PI3K activation, respectively, occurred downstream of EGF-R activation. Treatment with rapamycin, an inhibitor of the p70S6 kinase activator mTOR, also resulted in growth inhibition, further suggesting the importance of the PI3K signaling pathway in AVP-induced proliferation. AVP treatment appeared to transactivate EGF-R by inducing tyrosine phosphorylation of the Ca2+/protein kinase C (PKC)-dependent nonreceptor tyrosine kinase, Pyk2, leading to Pyk2/c-Src association and c-Src activation. This was followed by association of c-Src with EGF-R and EGF-R activation. These data suggested that AVP-stimulated Pyk2 tyrosine phosphorylation to activate c-Src, thereby leading to EGF-R transactivation.