Sfrp5 increases glucose-stimulated insulin secretion in the rat pancreatic beta cell line INS-1E

Previous studies reported that secreted frizzled-related protein-5 (Sfrp5) decreases beta cell proliferation and increases fasting insulin levels, but studies on direct effects of Sfrp5 on insulin secretion and its underlying mechanisms are missing. This study examined effects of Sfrp5 on (i) beta cell viability and proliferation, (ii) basal and glucose-stimulated insulin secretion and (iii) canonical and non-canonical Wnt signalling pathways. We incubated rat INS-1E cells with 0.1, 1 or 5 μg/ml recombinant Sfrp5 for 24h. We measured basal and glucose-stimulated insulin secretion at glucose concentrations of 2.5 and 20 mmol/l. Phosphorylated and total protein content as well as mRNA levels of markers of cell proliferation, canonical and non-canonical Wnt signalling pathways were examined using Western blotting and real-time PCR. Differences between treatments were analysed by repeated measurement one-way ANOVA or Friedman’s test followed by correction for multiple testing using the Benjamini-Hochberg procedure. At 5 μg/ml, Sfrp5 reduced mRNA levels of cyclin-B1 by 25% (p<0.05). At 1 and 5 μg/ml, Sfrp5 increased glucose-stimulated insulin secretion by 24% and by 34% (both p<0.05), respectively, but had no impact on basal insulin secretion. Sfrp5 reduced the phosphorylation of the splicing forms p46 and p54 of JNK by 39% (p<0.01) and 49% (p<0.05), respectively. In conclusion, Sfrp5 reduced markers of cell proliferation, but increased in parallel dose-dependently glucose-stimulated insulin secretion in INS-1E cells. This effect is likely mediated by reduced JNK activity, an important component of the non-canonical Wnt signalling pathway.

Intestinal insulin/IGF1 signalling through FoxO1 regulates epithelial integrity and susceptibility to colon cancer

Obesity promotes the development of insulin resistance and increases the incidence of colitis-associated cancer (CAC), but whether a blunted insulin action specifically in intestinal epithelial cells (IECs) affects CAC is unknown. Here, we show that obesity impairs insulin sensitivity in IECs and that mice with IEC-specific inactivation of the insulin and IGF1 receptors exhibit enhanced CAC development as a consequence of impaired restoration of gut barrier function. Blunted insulin signalling retains the transcription factor FOXO1 in the nucleus to inhibit expression of Dsc3, thereby impairing desmosome formation and epithelial integrity. Both IEC-specific nuclear FoxO1ADA expression and IEC-specific Dsc3 inactivation recapitulate the impaired intestinal integrity and increased CAC burden. Spontaneous colonic tumour formation and compromised intestinal integrity are also observed upon IEC-specific coexpression of FoxO1ADA and a stable Myc variant, thus suggesting a molecular mechanism through which impaired insulin action and nuclear FOXO1 in IECs promotes CAC.

Distinct Roles for JNK and IKK Activation in Agouti-Related Peptide Neurons in the Development of Obesity and Insulin Resistance

Activation of c-Jun N-terminal kinase 1 (JNK1)- and inhibitor of nuclear factor kappa-B kinase 2 (IKK2)-dependent signaling plays a crucial role in the development of obesity-associated insulin and leptin resistance not only in peripheral tissues but also in the CNS. Here, we demonstrate that constitutive JNK activation in agouti-related peptide (AgRP)-expressing neurons of the hypothalamus is sufficient to induce weight gain and adiposity in mice as a consequence of hyperphagia. JNK activation increases spontaneous action potential firing of AgRP cells and causes both neuronal and systemic leptin resistance. Similarly, activation of IKK2 signaling in AgRP neurons also increases firing of these cells but fails to cause obesity and leptin resistance. In contrast to JNK activation, IKK2 activation blunts insulin signaling in AgRP neurons and impairs systemic glucose homeostasis. Collectively, these experiments reveal both overlapping and nonredundant effects of JNK- and IKK-dependent signaling in AgRP neurons, which cooperate in the manifestation of the metabolic syndrome.

Dwarfism in mice lacking collagen-binding integrins α2β1 and α11β1 is caused by severely diminished IGF-1 levels

Mice with a combined deficiency in the α2β1 and α11β1 integrins lack the major receptors for collagen I. These mutants are born with inconspicuous differences in size but develop dwarfism within the first 4 weeks of life. Dwarfism correlates with shorter, less mineralized and functionally weaker bones that do not result from growth plate abnormalities or osteoblast dysfunction. Besides skeletal dwarfism, internal organs are correspondingly smaller, indicating proportional dwarfism and suggesting a systemic cause for the overall size reduction. In accordance with a critical role of insulin-like growth factor (IGF)-1 in growth control and bone mineralization, circulating IGF-1 levels in the sera of mice lacking either α2β1 or α11β1 or both integrins were sharply reduced by 39%, 64%, or 81% of normal levels, respectively. Low hepatic IGF-1 production resulted from diminished growth hormone-releasing hormone expression in the hypothalamus and, subsequently, reduced growth hormone expression in the pituitary glands of these mice. These findings point out a novel role of collagen-binding integrin receptors in the control of growth hormone/IGF-1-dependent biological activities. Thus, coupling hormone secretion to extracellular matrix signaling via integrins represents a novel concept in the control of endocrine homeostasis.

High-fat feeding promotes obesity via insulin receptor/PI3K-dependent inhibition of SF-1 VMH neurons

SF-1-expressing neurons of the ventromedial hypothalamus (VMH) control energy homeostasis, but the role of insulin action in these cells remains undefined. We show that insulin activates PI3-kinase (PI3k) signaling in SF-1 neurons and reduces firing frequency in these cells via activation of K_ATP-channels. These effects are abrogated in mice with insulin receptor (IR) deficiency restricted to SF-1 neurons (SF-1^ΔIR-mice). While body weight and glucose homeostasis remain unaltered in SF-1^ΔIR-mice under normal chow diet, they exhibit protection from diet-induced leptin resistance, weight gain, adiposity and impaired glucose tolerance. High-fat feeding activates PI3k signaling in SF-1 neurons of control mice, and this response is attenuated in the VMH of SF-1^ΔIR-mice. Mimicking diet-induced overactivation of PI3k signaling by disruption of the PIP₃-phosphatase PTEN leads to increased body weight and hyperphagia under normal chow diet. Collectively, our experiments reveal a critical role for HFD-induced, insulin-dependent PI3k activation in VMH neurons to control energy homeostasis.

Mutant huntingtin causes metabolic imbalance by disruption of hypothalamic neurocircuits

In Huntington's disease (HD), the mutant huntingtin protein is ubiquitously expressed. The disease was considered to be limited to the basal ganglia, but recent studies have suggested a more widespread pathology involving hypothalamic dysfunction. Here we tested the hypothesis that expression of mutant huntingtin in the hypothalamus causes metabolic abnormalities. First, we showed that bacterial artificial chromosome-mediated transgenic HD (BACHD) mice developed impaired glucose metabolism and pronounced insulin and leptin resistance. Selective hypothalamic expression of a short fragment of mutant huntingtin using adeno-associated viral vectors was sufficient to recapitulate these metabolic disturbances. Finally, selective hypothalamic inactivation of the mutant gene prevented the development of the metabolic phenotype in BACHD mice. Our findings establish a causal link between mutant huntingtin expression in the hypothalamus and metabolic dysfunction, and indicate that metabolic parameters are powerful readouts to assess therapies aimed at correcting dysfunction in HD by silencing huntingtin expression in the brain.

Novel roles for JNK1 in metabolism

Activation of stress-kinase signaling has recently been recognized as an important pathophysiological mechanism in the development of diet-induced obesity, type 2 diabetes mellitus and other aging-related pathologies. Here, c-Jun N-terminal Kinase (JNK) 1 knockout mice have been shown to exhibit protection from diet-induced obesity, glucose intolerance, and insulin resistance. Nonetheless, the tissue-specific role of JNK1-activation in the development of the metabolic syndrome has been poorly defined so far. Recently, it was demonstrated that JNK1 signaling plays a crucial role in the central nervous system (CNS) and in the pituitary to control systemic glucose and lipid metabolism partially through regulation of hormones involved in growth and energy expenditure.

CNS leptin and insulin action in the control of energy homeostasis

The obesity and diabetes pandemics have made it an urgent necessity to define the central nervous system (CNS) pathways controlling body weight, energy expenditure, and fuel metabolism. The pancreatic hormone insulin and the adipose tissue-derived leptin are known to act on diverse neuronal circuits in the CNS to maintain body weight and metabolism in a variety of species, including humans. Because these homeostatic circuits are disrupted during the development of obesity, the pathomechanisms leading to CNS leptin and insulin resistance are a focal point of research. In this review, we summarize the recent findings concerning the mechanisms and novel neuronal mediators of both insulin and leptin action in the CNS.

Hepatic Bax inhibitor-1 inhibits IRE1alpha and protects from obesity-associated insulin resistance and glucose intolerance

The unfolded protein response (UPR) or endoplasmic reticulum (ER) stress response is a physiological process enabling cells to cope with altered protein synthesis demands. However, under conditions of obesity, prolonged activation of the UPR has been shown to have deteriorating effects on different metabolic pathways. Here we identify Bax inhibitor-1 (BI-1), an evolutionary conserved ER-membrane protein, as a novel modulator of the obesity-associated alteration of the UPR. BI-1 partially inhibits the UPR by interacting with IRE1alpha and inhibiting IRE1alpha endonuclease activity as seen on the splicing of the transcription factor Xbp-1. Because we observed a down-regulation of BI-1 expression in liver and muscle of genetically obese ob/ob and db/db mice as well as in mice with diet-induced obesity in vivo, we investigated the effect of restoring BI-1 expression on metabolic processes in these mice. Importantly, BI-1 overexpression by adenoviral gene transfer dramatically improved glucose metabolism in both standard diet-fed mice as well as in mice with diet-induced obesity and, critically, reversed hyperglycemia in db/db mice. This improvement in whole body glucose metabolism and insulin sensitivity was due to dramatically reduced gluconeogenesis as shown by reduction of glucose-6-phosphatase and phosphoenolpyruvate carboxykinase expression. Taken together, these results identify BI-1 as a critical regulator of ER stress responses in the development of obesity-associated insulin resistance and provide proof of concept evidence that gene transfer-mediated elevations in hepatic BI-1 may represent a promising approach for the treatment of type 2 diabetes.

MyD88 signaling in the CNS is required for development of fatty acid-induced leptin resistance and diet-induced obesity

Obesity-associated activation of inflammatory pathways represents a key step in the development of insulin resistance in peripheral organs, partially via activation of TLR4 signaling by fatty acids. Here, we demonstrate that palmitate acting in the central nervous system (CNS) inhibits leptin-induced anorexia and Stat3 activation. To determine the functional significance of TLR signaling in the CNS in the development of leptin resistance and diet-induced obesity in vivo, we have characterized mice deficient for the TLR adaptor molecule MyD88 in the CNS (MyD88(DeltaCNS)). Compared to control mice, MyD88(DeltaCNS) mice are protected from high-fat diet (HFD)-induced weight gain, from the development of HFD-induced leptin resistance, and from the induction of leptin resistance by acute central application of palmitate. Moreover, CNS-restricted MyD88 deletion protects from HFD- and icv palmitate-induced impairment of peripheral glucose metabolism. Thus, we define neuronal MyD88-dependent signaling as a key regulator of diet-induced leptin and insulin resistance in vivo.

Hormone and glucose signalling in POMC and AgRP neurons

In the wake of the obesity pandemic, increased research efforts are under way to define how peripheral hormones and metabolites regulate energy homeostasis. The melanocortin system, comprising anorexigenic proopiomelanocortin (POMC) expressing neurons and orexigenic agouti-related protein (AgRP)/neuropeptide Y (NPY) coexpressing neurons in the arcuate nucleus of the hypothalamus are crucial for normal energy homeostasis both in rodents and humans. They are regulated by peripheral hormones such as leptin and insulin, as well as nutrients such as glucose, amino acids and fatty acids. Although much progress has been made, recent reports continue to underline how restricted our understanding of POMC and AgRP/NPY neuron regulation by these signals is. Importantly, ATP-dependent potassium (K(ATP)) channels are regulated both by ATP (from glucose metabolism) and by leptin and insulin, and directly control electrical excitability of both POMC and AgRP neurons. Thus, this review attempts to offer an integrative overview about how peripheral signals, particularly leptin, insulin and glucose, converge on a molecular level in POMC and AgRP neurons of the arcuate nucleus of the hypothalamus to control energy homeostasis.

PDK1 deficiency in POMC-expressing cells reveals FOXO1-dependent and -independent pathways in control of energy homeostasis and stress response

Insulin- and leptin-stimulated phosphatidylinositol-3 kinase (PI3K) activation has been demonstrated to play a critical role in central control of energy homeostasis. To delineate the importance of pathways downstream of PI3K specifically in pro-opiomelanocortin (POMC) cell regulation, we have generated mice with selective inactivation of 3-phosphoinositide-dependent protein kinase 1 (PDK1) in POMC-expressing cells (PDK1(DeltaPOMC) mice). PDK1(DeltaPOMC) mice initially display hyperphagia, increased body weight, and impaired glucose metabolism caused by reduced hypothalamic POMC expression. On the other hand, PDK1(DeltaPOMC) mice exhibit progressive, severe hypocortisolism caused by loss of POMC-expressing corticotrophs in the pituitary. Expression of a dominant-negative mutant of FOXO1 specifically in POMC cells is sufficient to ameliorate positive energy balance in PDK1(DeltaPOMC) mice but cannot restore regular pituitary function. These results reveal important but differential roles for PDK1 signaling in hypothalamic and pituitary POMC cells in the control of energy homeostasis and stress response.

Central insulin action in energy and glucose homeostasis

Insulin has pleiotropic biological effects in virtually all tissues. However, the relevance of insulin signaling in peripheral tissues has been studied far more extensively than its role in the brain. An evolving body of evidence indicates that in the brain, insulin is involved in multiple regulatory mechanisms including neuronal survival, learning, and memory, as well as in regulation of energy homeostasis and reproductive endocrinology. Here we review insulin's role as a central homeostatic signal with regard to energy and glucose homeostasis and discuss the mechanisms by which insulin communicates information about the body's energy status to the brain. Particular emphasis is placed on the controversial current debate about the similarities and differences between hypothalamic insulin and leptin signaling at the molecular level.

Enhanced PIP3 signaling in POMC neurons causes KATP channel activation and leads to diet-sensitive obesity

Leptin and insulin have been identified as fuel sensors acting in part through their hypothalamic receptors to inhibit food intake and stimulate energy expenditure. As their intracellular signaling converges at the PI3K pathway, we directly addressed the role of phosphatidylinositol3,4,5-trisphosphate-mediated (PIP3-mediated) signals in hypothalamic proopiomelanocortin (POMC) neurons by inactivating the gene for the PIP3 phosphatase Pten specifically in this cell type. Here we show that POMC-specific disruption of Pten resulted in hyperphagia and sexually dimorphic diet-sensitive obesity. Although leptin potently stimulated Stat3 phosphorylation in POMC neurons of POMC cell-restricted Pten knockout (PPKO) mice, it failed to significantly inhibit food intake in vivo. POMC neurons of PPKO mice showed a marked hyperpolarization and a reduction in basal firing rate due to increased ATP-sensitive potassium (KATP) channel activity. Leptin was not able to elicit electrical activity in PPKO POMC neurons, but application of the PI3K inhibitor LY294002 and the KATP blocker tolbutamide restored electrical activity and leptin-evoked firing of POMC neurons in these mice. Moreover, icv administration of tolbutamide abolished hyperphagia in PPKO mice. These data indicate that PIP3-mediated signals are critical regulators of the melanocortin system via modulation of KATP channels.