Myung-Shik Lee

Saturated fatty acids inhibit induction of insulin gene transcription by JNK-mediated phosphorylation of insulin-receptor substrates

JNKs are attractive targets for treatment of obesity and type-2 diabetes. A sustained increase in JNK activity was observed in dietary and genetic models of obesity in mice, whereas JNK deficiency prevented obesity-induced insulin resistance. A similar insulin-sensitizing effect was seen upon treatment of obese mice with JNK inhibitors. We now demonstrate that treatment with the saturated fatty acid palmitic acid results in sustained JNK activation and insulin resistance in primary mouse hepatocytes and pancreatic β-cells. In the latter, palmitic acid treatment inhibits glucose-induced insulin gene transcription, in part, by interfering with autocrine insulin signaling through phosphorylation of insulin-receptor substrates 1 and 2 at sites that interfere with binding to activated insulin receptors. This mechanism may account for the induction of central insulin resistance by free fatty acids.

Systemic autophagy insufficiency compromises adaptation to metabolic stress and facilitates progression from obesity to diabetes.

Despite growing interest in the relationship between autophagy and systemic metabolism, how global changes in autophagy affect metabolism remains unclear. Here we show that mice with global haploinsufficiency of an essential autophagy gene (Atg7(+/-) mice) do not show metabolic abnormalities but develop diabetes when crossed with ob/ob mice. Atg7(+/-)-ob/ob mice show aggravated insulin resistance with increased lipid content and inflammatory changes, suggesting that autophagy haploinsufficiency impairs the adaptive response to metabolic stress. We further demonstrate that intracellular lipid content and insulin resistance after lipid loading are increased as a result of autophagy insufficiency, and provide evidence for increased inflammasome activation in Atg7(+/-)-ob/ob mice. Imatinib or trehalose improves metabolic parameters of Atg7(+/-)-ob/ob mice and enhances autophagic flux. These results suggest that systemic autophagy insufficiency could be a factor in the progression from obesity to diabetes, and autophagy modulators have therapeutic potential against diabetes associated with obesity and inflammation.

Gut Microbiota and Metabolic Disorders

Gut microbiota plays critical physiological roles in the energy extraction and in the control of local or systemic immunity. Gut microbiota and its disturbance also appear to be involved in the pathogenesis of diverse diseases including metabolic disorders, gastrointestinal diseases, cancer, etc. In the metabolic point of view, gut microbiota can modulate lipid accumulation, lipopolysaccharide content and the production of short-chain fatty acids that affect food intake, inflammatory tone, or insulin signaling. Several strategies have been developed to change gut microbiota such as prebiotics, probiotics, certain antidiabetic drugs or fecal microbiota transplantation, which have diverse effects on body metabolism and on the development of metabolic disorders.

Differentiation and Transplantation of Functional Pancreatic Beta Cells Generated from Induced Pluripotent Stem Cells Derived from a Type 1 Diabetes Mouse Model

The nonobese diabetic (NOD) mouse is a classical animal model for autoimmune type 1 diabetes (T1D), closely mimicking features of human T1D. Thus, the NOD mouse presents an opportunity to test the effectiveness of induced pluripotent stem cells (iPSCs) as a therapeutic modality for T1D. Here, we demonstrate a proof of concept for cellular therapy using NOD mouse-derived iPSCs (NOD-iPSCs). We generated iPSCs from NOD mouse embryonic fibroblasts or NOD mouse pancreas-derived epithelial cells (NPEs), and applied directed differentiation protocols to differentiate the NOD-iPSCs toward functional pancreatic beta cells. Finally, we investigated whether the NPE-iPSC-derived insulin-producing cells could normalize hyperglycemia in transplanted diabetic mice. The NOD-iPSCs showed typical embryonic stem cell-like characteristics such as expression of markers for pluripotency, in vitro differentiation, teratoma formation, and generation of chimeric mice. We developed a method for stepwise differentiation of NOD-iPSCs into insulin-producing cells, and found that NPE-iPSCs differentiate more readily into insulin-producing cells. The differentiated NPE-iPSCs expressed diverse pancreatic beta cell markers and released insulin in response to glucose and KCl stimulation. Transplantation of the differentiated NPE-iPSCs into diabetic mice resulted in kidney engraftment. The engrafted cells responded to glucose by secreting insulin, thereby normalizing blood glucose levels. We propose that NOD-iPSCs will provide a useful tool for investigating genetic susceptibility to autoimmune diseases and generating a cellular interaction model of T1D, paving the way for the potential application of patient-derived iPSCs in autologous beta cell transplantation for treating diabetes.

NF-κB Activation in Hypothalamic Pro-opiomelanocortin Neurons Is Essential in Illness- and Leptin-induced Anorexia

Anorexia and weight loss are prevalent in infectious diseases. To investigate the molecular mechanisms underlying these phenomena, we established animal models of infection-associated anorexia by administrating bacterial and viral products, lipopolysaccharide (LPS) and human immunodeficiency virus-1 transactivator protein (Tat). In these models, we found that the nuclear factor-κB (NF-κB), a pivotal transcription factor for inflammation-related proteins, was activated in the hypothalamus. In parallel, administration of LPS and Tat increased hypothalamic pro-inflammatory cytokine production, which was abrogated by inhibition of hypothalamic NF-κB. In vitro, NF-κB activation directly stimulated the transcriptional activity of pro-opiomelanocortin (POMC), a precursor of anorexigenic melanocortin, and mediated the stimulatory effects of LPS, Tat, and pro-inflammatory cytokines on POMC transcription, implying the involvement of NF-κB in controlling feeding behavior. Consistently, hypothalamic injection of LPS and Tat caused a significant reduction in food intake and body weight, which was prevented by blockade of NF-κB and melanocortin. Furthermore, disruption of IκB kinase-β, an upstream kinase of NF-κB, in POMC neurons attenuated LPS- and Tat-induced anorexia. These findings suggest that infection-associated anorexia and weight loss are mediated via NF-κB activation in hypothalamic POMC neurons. In addition, hypothalamic NF-κB was activated by leptin, an important anorexigenic hormone, and mediates leptin-stimulated POMC transcription, indicating that hypothalamic NF-κB also serves as a downstream signaling pathway of leptin.

p62/Sqstm1 promotes malignancy of HCV-positive hepatocellular carcinoma through Nrf2-dependent metabolic reprogramming

p62/Sqstm1 is a multifunctional protein involved in cell survival, growth and death, that is degraded by autophagy. Amplification of the p62/Sqstm1 gene, and aberrant accumulation and phosphorylation of p62/Sqstm1, have been implicated in tumour development. Herein, we reveal the molecular mechanism of p62/Sqstm1-dependent malignant progression, and suggest that molecular targeting of p62/Sqstm1 represents a potential chemotherapeutic approach against hepatocellular carcinoma (HCC). Phosphorylation of p62/Sqstm1 at Ser349 directs glucose to the glucuronate pathway, and glutamine towards glutathione synthesis through activation of the transcription factor Nrf2. These changes provide HCC cells with tolerance to anti-cancer drugs and proliferation potency. Phosphorylated p62/Sqstm1 accumulates in tumour regions positive for hepatitis C virus (HCV). An inhibitor of phosphorylated p62-dependent Nrf2 activation suppresses the proliferation and anticancer agent tolerance of HCC. Our data indicate that this Nrf2 inhibitor could be used to make cancer cells less resistant to anticancer drugs, especially in HCV-positive HCC patients.

Autophagy is a major regulator of beta cell insulin homeostasis

Aims/hypothesisWe studied the role of protein degradation pathways in the regulation of insulin production and secretion and hypothesised that autophagy regulates proinsulin degradation, thereby modulating beta cell function.MethodsProinsulin localisation in autophagosomes was demonstrated by confocal and electron microscopy. Autophagy was inhibited by knockdown of autophagy-related (ATG) proteins and using the H+-ATPase inhibitor bafilomycin-A1. Proinsulin and insulin content and secretion were assessed in static incubations by ELISA and RIA.ResultsConfocal and electron microscopy showed proinsulin localised in autophagosomes and lysosomes. Beta-Atg7−/− mice had proinsulin-containing sequestosome 1 (p62 [also known as SQSTM1])+ aggregates in beta cells, indicating proinsulin is regulated by autophagy in vivo. Short-term bafilomycin-A1 treatment and ATG5/7 knockdown increased steady-state proinsulin and hormone precursor chromogranin A content. ATG5/7 knockdown also increased glucose- and non-fuel-stimulated insulin secretion. Finally, mutated forms of proinsulin that are irreparably misfolded and trapped in the endoplasmic reticulum are more resistant to degradation by autophagy.Conclusions/interpretationIn the beta cell, transport-competent secretory peptide precursors, including proinsulin, are regulated by autophagy, whereas efficient clearance of transport-incompetent mutated forms of proinsulin by alternative degradative pathways may be necessary to avoid beta cell proteotoxicity. Reduction of autophagic degradation of proinsulin increases its residency in the secretory pathway, followed by enhanced secretion in response to stimuli.

Role of autophagy in the progression from obesity to diabetes and in the control of energy balance

Autophagy plays a crucial role in cellular homeostasis through the degradation and recycling of organelles such as mitochondria or endoplasmic reticulum (ER) that are closely related to the pathogenesis of diabetes. In pancreatic β-cells producing insulin, autophagy helps maintain β-cell mass, structure and function. In mice with β-cell-specific deletion of Atg7 (autophagy-related 7), a critical autophagy gene, reduction of β-cell mass and pancreatic insulin content were observed together with impaired insulin secretory function. Because of such structural and functional defects, β-cell-specific Atg7-null mice showed hypoinsulinemia and hyperglycemia. However, those mice never developed diabetes. Obesity and lipids are physiological ER stressors that can precipitate β-cell dysfunction and insulin resistance. Recent studies showed that β-cell-specific Atg7-null mice, when bred with ob/ob mice, developed severe diabetes, suggesting that autophagy-deficient β-cells can handle basal metabolic stress but have problems dealing with increased metabolic stress. Thus, autophagy deficiency in β-cells could be a factor in the progression from obesity to diabetes due to an inappropriate response to obesity-induced ER stress. Autophagy also appears to play a role in the hypothalamic control of energy expenditure, appetite and body weight. Thus, autophagy is important to body and nutrient metabolism in many ways, and its dysregulation could contribute to the pathogenesis of metabolic disorders and diabetes.

Autophagy as a crosstalk mediator of metabolic organs in regulation of energy metabolism

Autophagy plays an important role in the regulation of cellular homeostasis through elimination of aggregated proteins, damaged organelles, and intracellular pathogens. Autophagy also contributes to the maintenance of energy balance through degradation of energy reserves such as lipids, glycogen, and proteins in the setting of increased energy demand. Recent studies have suggested that autophagy, or its deficiency, is implicated in the pathogenesis of insulin resistance, obesity, and diabetes. These effects of autophagy or its deficiency in regulation of energy metabolism are mediated not only by cell-autonomous effects, such as direct autophagic degradation of energy stores or intracellular organelles (endoplasmic reticulum and mitochondria) but also by non-cell-autonomous effects, such as induction/suppression of secreted factors or changes of sympathetic tone. In the present review, we highlight a recent surge in the research on the autophagy in the regulation of energy homeostasis, with a focus on its role as a mediator for crosstalk between metabolic organs.

FGF21 as a mediator of adaptive responses to stress and metabolic benefits of anti-diabetic drugs.

Most hormones secreted from specific organs of the body in response to diverse stimuli contribute to the homeostasis of the whole organism. Fibroblast growth factor 21 (FGF21), a hormone induced by a variety of environmental or metabolic stimuli, plays a crucial role in the adaptive response to these stressful conditions. In addition to its role as a stress hormone, FGF21 appears to function as a mediator of the therapeutic effects of currently available drugs and those under development for treatment of metabolic diseases. In this review, we highlight molecular mechanisms and the functional importance of FGF21 induction in response to diverse stress conditions such as changes of nutritional status, cold exposure, and exercise. In addition, we describe recent findings regarding the role of FGF21 in the pathogenesis and treatment of diabetes associated with obesity, liver diseases, pancreatitis, muscle atrophy, atherosclerosis, cardiac hypertrophy, and diabetic nephropathy. Finally, we discuss the current understanding of the actions of FGF21 as a crucial regulator mediating beneficial metabolic effects of therapeutic agents such as metformin, glucagon/glucagon-like peptide 1 analogues, thiazolidinedione, sirtuin 1 activators, and lipoic acid.