Endoplasmic Reticulum Chaperone as a Regulator of Obesity and Diabetes

University Of Southern California, Los Angeles CA

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Abstract

DESCRIPTION (provided by applicant): The endoplasmic reticulum (ER) is a cellular organelle where secretory and membrane-associated proteins are synthesized and modified. It has been proposed that obesity promotes nutrient stress and chronic inflammation that involve increased demand on the synthetic machinery of the cells in many secretory organ systems, such as adipose tissue. Thus, obesity acts as a chronic stimulus for ER stress in peripheral tissues, triggering insulin resistance and type 2 diabetes. The multifunctional ER chaperone protein GRP78/BiP, is a master regulator of ER homeostasis due to its control of protein folding and the activation of trans-membrane ER stress sensors. Serendipitously, we discovered that the Grp78 mice in the C57BL/6 background exhibit resistance to high-fat diet (HFD)-induced obesity and improved insulin sensitivity. Our preliminary studies revealed that HFD-fed Grp78 mice showed increased energy expenditure without changes in food intake and lipid absorption. Preliminary euglycemic clamp studies showed a striking increase in insulin-stimulated glucose uptake most prominently in the white adipose tissue of the Grp78 mice (P<0.001). We further discovered that in adipose tissue, Grp78 heterozygosity leads to diet-induced upregulation of ER chaperones. In contrast, glucose metabolism was not altered in skeletal muscle and no chaperone upregulation was observed. Thus, Grp78 mice offer new opportunities to investigate the basic mechanisms linking ER integrity to energy balance, glucose homeostasis and adipocyte stress. We propose that during chronic stress induced by HFD, Grp78 heterozygosity in adipose tissue triggers compensatory and protective measures, such as upregulation of chaperones and increase in mitochondrial function, which lead to enhanced energy expenditure, attenuation of ER stress and inflammatory responses resulting in improved insulin sensitivity. Based on our preliminary data that the Grp78mice are more insulin sensitive than the wild-type littermates on chow diet with similar body weights, Aim 1 will identify the mechanism by which Grp78 improves insulin sensitivity. Aim 2 will determine how obesity affects the unfolded protein response signaling and how Grp78 heterozygosity alters energy expenditure and causes resistance to diet-induced obesity. Aim 3 will generate and characterize mouse models with white adipose tissue-specific overexpression or knockout of GRP78 to determine its role in energy balance and insulin sensitivity. Aim 4 will determine the functional contribution of other ER chaperones upregulated in adipose tissue of HFD-fed Grp78 mice in adipocyte metabolic function, utilizing primary adipocytes and MEFs from the Grp78 and mice, as well as the 3T3-L1 adipocyte culture system. The clinical relevance of our studies is that they may identify novel regulatory pathways for diet- induced obesity and insulin resistance which may represent new therapeutic targets for human metabolic diseases. PUBLIC HEALTH RELEVANCE: The dramatic increase in the incidence of obesity, insulin resistance and type 2 diabetes has become one of the most serious threats to human health. Hence, understanding the molecular mechanisms underlying these diseases is critical. This proposal is based on the serendipitous observation from a novel mouse model recently created in the laboratory that may provide clues to prevent high-fat diet-induced obesity and subsequent insulin resistance. This proposal will fully characterize the metabolic phenotypes of the mutant mice and investigate the underlying mechanisms for increased energy expenditure and improved insulin sensitivity of these mice under the chronic stress of high-fat diet. If validated, our findings may lead to new targets for therapy against obesity and type 2 diabetes in humans.