Minna Woo

Muscle-Specific Pten Deletion Protects against Insulin Resistance and Diabetes

ABSTRACT Pten (phosphatase with tensin homology), a dual-specificity phosphatase, is a negative regulator of the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway. Pten regulates a vast array of biological functions including growth, metabolism, and longevity. Although the PI3K/Akt pathway is a key determinant of the insulin-dependent increase in glucose uptake into muscle and adipose cells, the contribution of this pathway in muscle to whole-body glucose homeostasis is unclear. Here we show that muscle-specific deletion of Pten protected mice from insulin resistance and diabetes caused by high-fat feeding. Deletion of muscle Pten resulted in enhanced insulin-stimulated 2-deoxyglucose uptake and Akt phosphorylation in soleus but, surprisingly, not in extensor digitorum longus muscle compared to littermate controls upon high-fat feeding, and these mice were spared from developing hyperinsulinemia and islet hyperplasia. Muscle Pten may be a potential target for treatment or prevention of insulin resistance and diabetes.

Erythropoietin protects against diabetes through direct effects on pancreatic β cells

In mouse models of type 1 and type 2 diabetes, administration of human erythropoietin protects against disease by acting directly on pancreatic β cells.

Caspase-3-Dependent β-Cell Apoptosis in the Initiation of Autoimmune Diabetes Mellitus

ABSTRACT β-Cell apoptosis is a key event contributing to the pathogenesis of type 1 diabetes mellitus. In addition to apoptosis being the main mechanism by which β cells are destroyed, β-cell apoptosis has been implicated in the initiation of type 1 diabetes mellitus through antigen cross-presentation mechanisms that lead to β-cell-specific T-cell activation. Caspase-3 is the major effector caspase involved in apoptotic pathways. Despite evidence supporting the importance of β-cell apoptosis in the pathogenesis of type 1 diabetes, the specific role of caspase-3 in this process is unknown. Here, we show that Caspase-3 knockout (Casp3− /−) mice were protected from developing diabetes in a multiple-low-dose streptozotocin autoimmune diabetes model. Lymphocyte infiltration of the pancreatic islets was completely absent in Casp3 − /− mice. To determine the role of caspase-3-dependent apoptosis in disease initiation, a defined antigen-T-cell receptor transgenic system, RIP-GP/P14 double-transgenic mice with Casp3 null mutation, was examined. β-cell antigen-specific T-cell activation and proliferation were observed only in the pancreatic draining lymph node of RIP-GP/P14/Casp3 + /− mice, but not in mice lacking caspase-3. Together, our findings demonstrate that caspase-3-mediated β-cell apoptosis is a requisite step for T-cell priming, a key initiating event in type 1 diabetes.

Regulation of obesity-related insulin resistance with gut anti-inflammatory agents.

Obesity has reached epidemic proportions, but little is known about its influence on the intestinal immune system. Here we show that the gut immune system is altered during high-fat diet (HFD) feeding and is a functional regulator of obesity-related insulin resistance (IR) that can be exploited therapeutically. Obesity induces a chronic phenotypic pro-inflammatory shift in bowel lamina propria immune cell populations. Reduction of the gut immune system, using beta7 integrin-deficient mice (Beta7(null)), decreases HFD-induced IR. Treatment of wild-type HFD C57BL/6 mice with the local gut anti-inflammatory, 5-aminosalicyclic acid (5-ASA), reverses bowel inflammation and improves metabolic parameters. These beneficial effects are dependent on adaptive and gut immunity and are associated with reduced gut permeability and endotoxemia, decreased visceral adipose tissue inflammation, and improved antigen-specific tolerance to luminal antigens. Thus, the mucosal immune system affects multiple pathways associated with systemic IR and represents a novel therapeutic target in this disease.

Essential Role of Pten in Body Size Determination and Pancreatic β-Cell Homeostasis In Vivo

ABSTRACT PTEN (phosphatase with tensin homology) is a potent negative regulator of phosphoinositide 3-kinase (PI3K)/Akt signaling, an evolutionarily conserved pathway that signals downstream of growth factors, including insulin and insulin-like growth factor 1. In lower organisms, this pathway participates in fuel metabolism and body size regulation and insulin-like proteins are produced primarily by neuronal structures, whereas in mammals, the major source of insulin is the pancreatic β cells. Recently, rodent insulin transcription was also shown in the brain, particularly the hypothalamus. The specific regulatory elements of the PI3K pathway in these insulin-expressing tissues that contribute to growth and metabolism in higher organisms are unknown. Here, we report PTEN as a critical determinant of body size and glucose metabolism when targeting is driven by the rat insulin promoter in mice. The partial deletion of PTEN in the hypothalamus resulted in significant whole-body growth restriction and increased insulin sensitivity. Efficient PTEN deletion in β cells led to increased islet mass without compromise of β-cell function. Parallel enhancement in PI3K signaling was found in PTEN-deficient hypothalamus and β cells. Together, we have shown that PTEN in insulin-transcribing cells may play an integrative role in regulating growth and metabolism in vivo.

Insulin granule recruitment and exocytosis is dependent on p110γ in insulinoma and human β-cells

OBJECTIVE Phosphatidylinositol 3-OH kinase (PI3K) has a long-recognized role in β-cell mass regulation and gene transcription and is implicated in the modulation of insulin secretion. The role of nontyrosine kinase receptor–activated PI3K isoforms is largely unexplored. We therefore investigated the role of the G-protein–coupled PI3Kγ and its catalytic subunit p110γ in the regulation of insulin granule recruitment and exocytosis. RESEARCH DESIGN AND METHODS The expression of p110γ was knocked down by small-interfering RNA, and p110γ activity was selectively inhibited with AS605240 (40 nmol/l). Exocytosis and granule recruitment was monitored by islet perifusion, whole-cell capacitance, total internal reflection fluorescence microscopy, and electron microscopy in INS-1 and human β-cells. Cortical F-actin was examined in INS-1 cells and human islets and in mouse β-cells lacking the phosphatase and tensin homolog (PTEN). RESULTS Knockdown or inhibition of p110γ markedly blunted depolarization-induced insulin secretion and exocytosis and ablated the exocytotic response to direct Ca2+ infusion. This resulted from reduced granule localization to the plasma membrane and was associated with increased cortical F-actin. Inhibition of p110γ had no effect on F-actin in β-cells lacking PTEN. Finally, the effect of p110γ inhibition on granule localization and exocytosis could be rapidly reversed by agents that promote actin depolymerization. CONCLUSIONS The G-protein–coupled PI3Kγ is an important determinant of secretory granule trafficking to the plasma membrane, at least in part through the negative regulation of cortical F-actin. Thus, p110γ activity plays an important role in maintaining a membrane-docked, readily releasable pool of secretory granules in insulinoma and human β-cells.

Distinct In Vivo Roles of Caspase-8 in β-Cells in Physiological and Diabetes Models

Inadequate pancreatic β-cell mass resulting from excessive β-cell apoptosis is a key defect in type 1 and type 2 diabetes. Caspases are the major molecules involved in apoptosis; however, in vivo roles of specific caspases in diabetes are unclear. The purpose of this study is to examine the role of Caspase (Casp)8 in β-cells in vivo. Using the Cre-loxP system, mice lacking Casp8 in β-cells (RIPcre+Casp8fl/fl mice) were generated to address the role of Casp8 in β-cells in physiological and diabetes models. We show that islets isolated from RIPcre+Casp8fl/fl mice were protected from Fas ligand (FasL)–and ceramide-induced cell death. Furthermore, RIPcre+Casp8fl/fl mice were protected from in vivo models of type 1 and type 2 diabetes. In addition to being the central mediator of apoptosis in diabetes models, we show that Casp8 is critical for maintenance of β-cell mass under physiological conditions. With aging, RIPcre+Casp8fl/fl mice gradually develop hyperglycemia and a concomitant decline in β-cell mass. Their islets display decreased expression of molecules involved in insulin/IGF-I signaling and show decreased pancreatic duodenal homeobox-1 and cAMP response element binding protein expression. At the level of individual islets, we observed increased insulin secretory capacity associated with increased expression of exocytotic proteins. Our results show distinct context-specific roles of Casp8 in physiological and disease states; Casp8 is essential for β-cell apoptosis in type 1 and type 2 diabetes models and in regulating β-cell mass and insulin secretion under physiological conditions.

Deletion of Pten in Pancreatic β-Cells Protects Against Deficient β-Cell Mass and Function in Mouse Models of Type 2 Diabetes

OBJECTIVE Type 2 diabetes is characterized by diminished pancreatic β-cell mass and function. Insulin signaling within the β-cells has been shown to play a critical role in maintaining the essential function of the β-cells. Under basal conditions, enhanced insulin-PI3K signaling via deletion of phosphatase with tensin homology (PTEN), a negative regulator of this pathway, leads to increased β-cell mass and function. In this study, we investigated the effects of prolonged β-cell–specific PTEN deletion in models of type 2 diabetes. RESEARCH DESIGN AND METHODS Two models of type 2 diabetes were employed: a high-fat diet (HFD) model and a db/db model that harbors a global leptin-signaling defect. A Cre-loxP system driven by the rat insulin promoter (RIP) was employed to obtain mice with β-cell–specific PTEN deletion (RIPcre+ Ptenfl/fl). RESULTS PTEN expression in islets was upregulated in both models of type 2 diabetes. RIPcre+ Ptenfl/fl mice were completely protected against diabetes in both models of type 2 diabetes. The islets of RIPcre+ Ptenfl/fl mice already exhibited increased β-cell mass under basal conditions, and there was no further increase under diabetic conditions. Their β-cell function and islet PI3K signaling remained intact, in contrast to HFD-fed wild-type and db/db islets that exhibited diminished β-cell function and attenuated PI3K signaling. These protective effects in β-cells occurred in the absence of compromised response to DNA-damaging stimuli. CONCLUSIONS PTEN exerts a critical negative effect on both β-cell mass and function. Thus PTEN inhibition in β-cells can be a novel therapeutic intervention to prevent the decline of β-cell mass and function in type 2 diabetes.

Executionary pathway for apoptosis: lessons from mutant mice

ABSTRACTApoptosis or programmed cell death (PCD) is an evolutionarily conserved cellular process that is essential for normal development and homeostasis of multicellular organisms. Defects in the apoptosis signaling result in many diseases including autoimmune diseases and cancer. The apoptosis signaling pathway was first described genetically in the nematode Caenorhabditis elegans which serves as a framework for the more complex apoptotic pathways that exist in mammals. In this review, we will discuss the apoptotic pathways that are emerging in mammals as elucidated by studies of gene-targeted mutant mice.

Hepatocyte-specific Deletion of Janus Kinase 2 (JAK2) Protects against Diet-induced Steatohepatitis and Glucose Intolerance

Background: JAK2 mediates signaling by a number of cytokines in the liver. Results: Hepatic JAK2 KO mice developed spontaneous steatosis but were protected from high fat diet-induced steatohepaitits and insulin resistance. Conclusion: Hepatic JAK2 is required for the development of diet-induced steatohepatitis and glucose intolerance. Significance: Understanding the role of JAK2 in metabolism will provide insights into the pathogenesis of the metabolic syndrome. Non-alcoholic fatty liver disease (NAFLD) is becoming the leading cause of chronic liver disease and is now considered to be the hepatic manifestation of the metabolic syndrome. However, the role of steatosis per se and the precise factors required in the progression to steatohepatitis or insulin resistance remain elusive. The JAK-STAT pathway is critical in mediating signaling of a wide variety of cytokines and growth factors. Mice with hepatocyte-specific deletion of Janus kinase 2 (L-JAK2 KO mice) develop spontaneous steatosis as early as 2 weeks of age. In this study, we investigated the metabolic consequences of jak2 deletion in response to diet-induced metabolic stress. To our surprise, despite the profound hepatosteatosis, deletion of hepatic jak2 did not sensitize the liver to accelerated inflammatory injury on a prolonged high fat diet (HFD). This was accompanied by complete protection against HFD-induced whole-body insulin resistance and glucose intolerance. Improved glucose-stimulated insulin secretion and an increase in β-cell mass were also present in these mice. Moreover, L-JAK2 KO mice had progressively reduced adiposity in association with blunted hepatic growth hormone signaling. These mice also exhibited increased resting energy expenditure on both chow and high fat diet. In conclusion, our findings indicate a key role of hepatic JAK2 in metabolism such that its absence completely arrests steatohepatitis development and confers protection against diet-induced systemic insulin resistance and glucose intolerance.