Targeted deletion of the PRL receptor: effects on islet development, insulin production, and glucose tolerance

Targeted inactivation of hepatocyte growth factor receptor c-met in beta-cells leads to defective insulin secretion and GLUT-2 downregulation without alteration of beta-cell mass

Overexpression of hepatocyte growth factor (HGF) in the beta-cell of transgenic mice enhances beta-cell proliferation, survival, and function. In the current studies, we have used conditional ablation of the c-met gene to uncover the physiological role of HGF in beta-cell growth and function. Mice in which c-met is inactivated in the beta-cell (MetCKO mice) display normal body weight, blood glucose, and plasma insulin compared with control littermates. In contrast, MetCKO mice displayed significantly diminished glucose tolerance and reduced plasma insulin after a glucose challenge in vivo. This impaired glucose tolerance in MetCKO mice was not caused by insulin resistance because sensitivity to exogenous insulin was similar in both groups. Importantly, in vitro glucose-stimulated insulin secretion in MetCKO islets was decreased by approximately 50% at high glucose concentrations compared with control islets. Furthermore, whereas insulin and glucokinase expression in MetCKO islets were normal, GLUT-2 expression was decreased by approximately 50%. These changes in beta-cell function in MetCKO mice were not accompanied by changes in total beta-cell mass, islet morphology, islet cell composition, and beta-cell proliferation. Interestingly, however, MetCKO mice display an increased number of small islets, mainly single and doublet beta-cells. We conclude that HGF/c-met signaling in the beta-cell is not essential for beta-cell growth, but it is essential for normal glucose-dependent insulin secretion.

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

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

Roles of the lactogens and somatogens in perinatal and postnatal metabolism and growth: studies of a novel mouse model combining lactogen resistance and growth hormone deficiency

To delineate the roles of the lactogens and GH in the control of perinatal and postnatal growth, fat deposition, insulin production, and insulin action, we generated a novel mouse model that combines resistance to all lactogenic hormones with a severe deficiency of pituitary GH. The model was created by breeding PRL receptor (PRLR)-deficient (knockout) males with GH-deficient (little) females. In contrast to mice with isolated GH or PRLR deficiencies, double-mutant (lactogen-resistant and GH-deficient) mice on d 7 of life had growth failure and hypoglycemia. These findings suggest that lactogens and GH act in concert to facilitate weight gain and glucose homeostasis during the perinatal period. Plasma insulin and IGF-I and IGF-II concentrations were decreased in both GH-deficient and double-mutant neonates but were normal in PRLR-deficient mice. Body weights of the double mutants were reduced markedly during the first 3-4 months of age, and adults had striking reductions in femur length, plasma IGF-I and IGF binding protein-3 concentrations, and femoral bone mineral density. By age 6-12 months, however, the double-mutant mice developed obesity, hyperleptinemia, fasting hyperglycemia, relative hypoinsulinemia, insulin resistance, and glucose intolerance; males were affected to a greater degree than females. The combination of perinatal growth failure and late-onset obesity and insulin resistance suggests that the lactogen-resistant/GH-deficient mouse may serve as a model for the development of the metabolic syndrome.

Characterization of mice doubly transgenic for parathyroid hormone-related protein and murine placental lactogen: a novel role for placental lactogen in pancreatic beta-cell survival

Transgenic overexpression of either parathyroid hormone-related peptide (PTHrP) or mouse placental lactogen type 1 (mPL1) in pancreatic beta-cells, using the rat insulin II promoter (RIP), results in islet hyperplasia either through prolonged beta-cell survival or through increased beta-cell proliferation and hypertrophy, respectively. For determining whether the two proteins might exert complementary, additive, or synergistic effects on islet mass and function when simultaneously overexpressed in beta-cells in vivo, RIP-PTHrP and RIP-mPL1 mice were crossed to generate mice doubly transgenic for PTHrP and mPL1. These double-transgenic mice displayed marked islet hyperplasia (threefold), hypoglycemia, increased beta-cell proliferation (threefold), and resistance to the diabetogenic and cytotoxic effects of streptozotocin compared with their normal siblings. Although the phenotype of the double-transgenic mice was neither additive nor synergistic relative to their single-transgenic counterparts, it was indeed complementary, yielding the maximal salutary phenotypic features of both individual transgenes. Finally, mPL1, for the first time, was shown to exert a protective effect on the survival of beta-cells, placing it among the few proteins that can improve function and proliferation and prolong the survival of beta-cells. Placental lactogen 1 is an attractive target for future therapeutic strategies in diabetes.

Disruption of growth hormone receptor gene causes diminished pancreatic islet size and increased insulin sensitivity in mice

Growth hormone, acting through its receptor (GHR), plays an important role in carbohydrate metabolism and in promoting postnatal growth. GHR gene-deficient (GHR(-/-)) mice exhibit severe growth retardation and proportionate dwarfism. To assess the physiological relevance of growth hormone actions, GHR(-/-) mice were used to investigate their phenotype in glucose metabolism and pancreatic islet function. Adult GHR(-/-) mice exhibited significant reductions in the levels of blood glucose and insulin, as well as insulin mRNA accumulation. Immunohistochemical analysis of pancreatic sections revealed normal distribution of the islets despite a significantly smaller size. The average size of the islets found in GHR(-/-) mice was only one-third of that in wild-type littermates. Total beta-cell mass was reduced 4.5-fold in GHR(-/-) mice, significantly more than their body size reduction. This reduction in pancreatic islet mass appears to be related to decreases in proliferation and cell growth. GHR(-/-) mice were different from the human Laron syndrome in serum insulin level, insulin responsiveness, and obesity. We conclude that growth hormone signaling is essential for maintaining pancreatic islet size, stimulating islet hormone production, and maintaining normal insulin sensitivity and glucose homeostasis.

Distinctive roles for prolactin and growth hormone in the activation of signal transducer and activator of transcription 5 in pancreatic islets of langerhans

Although the beta-cells of the pancreatic islets of Langerhans express both prolactin (PRL) and GH receptors, we have observed that PRL is considerably more effective than GH in the up-regulation of islet function in vitro. This study examined whether differences in the activation of the Janus kinase 2/signal transducer and activator of transcription (STAT) 5 signaling pathway by these closely related receptors may be involved in this disparity. The activation of STAT5B by PRL was biphasic, with an initial peak within 30 min, a nadir between 1 and 3 h, and prolonged activation after 4 h. In contrast, the response to GH was transient for 1 h. The importance of the long-term activation of STAT5B by PRL was supported by the similar dose response curves for STAT5B activation and the PRL-induced increases in insulin secretion and islet cell proliferation. Because the pulsatile secretion of GH affects its actions in other target tissues, the ability of pretreatment with either hormone to affect subsequent stimulation was also examined. Surprisingly, the response to PRL was inhibited by prior exposure for less than 3 h to either PRL or GH and disappeared with a longer pretreatment with either hormone. Similar to other tissues, the response to GH was inhibited by any length of prior exposure to GH. However, pretreatment with PRL had no effect. These experiments are the first demonstration of the transient desensitization of the PRL receptor by either PRL or GH pretreatment in any tissue and the desensitization of GH stimulation in islet cells. These observations provide insight into the mechanisms that regulate the desensitization of these receptors and, more importantly, allow the long-term activation of STAT5B by the PRL receptor. These results may apply to other members of the cytokine superfamily of receptors. We also demonstrate that the increase in islet cell proliferation required continuous stimulation with PRL, whereas the smaller effect with GH occurred with either continuous or pulsatile stimulation. In summary, this study demonstrates that islets are sensitive to the temporal pattern of stimulation by these hormones and provides a new basis for understanding their physiological roles in the regulation of islet function.

Prolactin and the prolactin receptor: new targets of an old hormone

Prolactin (PRL) is one of a family of related hormones including growth hormone (GH) and placental lactogen (PL) that are hypothesized to have arisen from a common ancestral gene about 500 million years ago. Over 300 different functions of PRL have been reported, highlighting the importance of this pituitary hormone. PRL is also synthesized by a number of extra-pituitary tissues including the mammary gland and the uterus. Most of PRL's actions are mediated by the unmodified 23 kDa peptide, however, PRL may be modified post-translation, thereby altering its biological effects. PRL exerts these effects by binding to its receptor, a member of the class I cytokine receptor super-family. This activates a number of signaling pathways resulting in the transcription of genes necessary for the tissue specific changes induced by PRL. Mouse knockout models of the major forms of the PRL receptor have confirmed the importance of PRLs role in reproduction. Further knockout models have provided insight into the importance of PRL signaling intermediates and the advent of transcript profiling has allowed the elucidation of a number of PRL target genes.

The maternal endocrine environment in the low-protein model of intra-uterine growth restriction

Many adult diseases, including type 2 diabetes, hypertension and cardiovascular disease, are related to low birth weight. The mechanistic basis of this relationship is not known. To investigate the role of fetal undernutrition, we used a rat model of maternal protein restriction in which dams were fed a diet containing 80 g protein/kg (v. 200 g/kg in the control group) throughout gestation and lactation. Offspring were born smaller than controls and in adulthood developed diabetes, hyperinsulinaemia and tissue insulin resistance. To determine possible mechanisms of fetal programming, circulating levels of several hormones were measured in maternal plasma at gestational days 14, 17 and 21 and fetal plasma at gestational day 21. Several differences were noted at day 14, when glucose concentrations in maternal and feto-placental blood were raised significantly (P=0.04 and P=0.0001 respectively); insulin levels in the low-protein (LP) dams were raised (P=0.04), prolactin levels were raised (P=0.047) and progesterone levels were reduced (P=0.02). Circulating 17beta-oestradiol in the LP dams was raised by 35 % over those of the controls from day 17 to day 21 (P=0.008). A significant decrease in maternal leptin levels (P=0.004) was observed at gestation on day 21. Neither oestradiol nor leptin levels were altered in the fetal circulation at day 21. Maternal and fetal corticosterone levels were comparable with control levels, suggesting that they do not initiate the programming effects in this model. Our present results suggest that maternal protein restriction imposes changes in maternal levels of glucose, insulin, prolactin, progesterone, oestradiol and leptin; these changes could influence the programming of eventual adult disease in the developing fetus.

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.

Differential activation mechanisms of Erk-1/2 and p70(S6K) by glucose in pancreatic beta-cells

Glucose can activate the mitogen-activated kinases, Erk-1/2, and the ribosomal-S6 kinase, p70(S6K), in beta-cells, contributing to an increase in mitogenesis. However, the signaling mechanism by which glucose induces Erk-1/2 and p70(S6K) phosphorylation activation is undefined. Increased glucose metabolism increases [Ca(2+)](i) and [cAMP], and it was investigated if these secondary signals were linked to glucose-induced Erk-1/2 and p70(S6K) activation in pancreatic beta-cells. Blocking Ca(2+) influx with verapamil, or inhibiting protein kinase A (PKA) with H89, prevented glucose-induced Erk-1/2 phosphorylation. Increasing cAMP levels by GLP-1 potentiated glucose-induced Erk-1/2 phosphorylation via PKA activation. Elevation of [Ca(2+)](i) by glyburide potentiated Erk-1/2 phosphorylation, which was also inhibited by H89, suggesting increased [Ca(2+)](i) preceded PKA for glucose-induced Erk-1/2 activation. Adenoviral-mediated expression of dominant negative Ras in INS-1 cells decreased IGF-1-induced Erk-1/2 phosphorylation but had no effect on that by glucose. Collectively, our study indicates that a glucose-induced rise in [Ca(2+)](i) leads to cAMP-induced activation of PKA that acts downstream of Ras and upstream of the MAP/Erk kinase, MEK, to mediate Erk-1/2 phosphorylation via phosphorylation activation of Raf-1. In contrast, glucose-induced p70(S6K) activation, in the same beta-cells, was mediated by a distinct signaling pathway independent of Ca(2+)/cAMP, most likely via mTOR-kinase acting as an "ATP-sensor."