Obesity is now the most common metabolic disorder in industrialized countries and represents one of the most prevalent risk factors for the development of type 2 diabetes. While the close relationship between obesity and diabetes has long been recognized, understanding how an expanded fat mass results in the progressive development of insulin resistance and type 2 diabetes has proven difficult. One possible mechanism linking obesity and insulin resistance is the excess mobilization of free fatty acids (FFAs) from adipocyte stores, which leads to derangements in insulin signaling either through direct actions of FFAs or through ectopic lipid deposition in non-adipose tissues. We recently developed a novel mouse model that lacks the expression of GX secretory phospholipase A2 (GX KO mice). Unexpectedly, these mice have significantly increased adiposity compared to wild-type littermates. The increase in adiposity is associated with adipocyte hypertrophy, which appears to be due at least in part to a defect in 2-adrenergic stimulated lipolysis. However, despite a marked increase in adiposity, GX KO mice show improved response to glucose challenge compared to wild- type mice, suggesting that insulin sensitivity may be protected in these mice. Based on our preliminary findings, we hypothesize that GX sPLA2 expressed by adipose tissue macrophages modulates the equilibrium between adipocyte lipid storage and lipid catabolism. As a consequence, GX sPLA2 regulates adipocyte hypertrophy, adipose tissue inflammation, FFA flux, and whole body insulin sensitivity. To test these hypotheses, we propose the following 3 specific aims: Aim 1. To test the hypothesis that reduced FFA flux in adipose tissue of GX sPLA2-deficient mice is associated with increased insulin sensitivity and reduced adipose tissue inflammation. This will be achieved by a) determining whether decreased FFA mobilization in adipose tissue of GX KO mice is associated with increased glucose uptake and insulin sensitivity in non-adipose tissue;b) determining whether GX sPLA2 deficiency protects against high fat diet-induced insulin resistance;and c) defining the effect of GX sPLA2 deficiency on adipose tissue inflammation in mice. Aim 2. To define the role of macrophage-expressed GX sPLA2 in adipose tissue. This will be achieved by performing a bone marrow transplant experiment in which wild-type and GX KO mice will be reconstituted with bone marrow cells derived from either wild-type or GX KO mice. Adipocyte hypertrophy, adipose tissue inflammation and insulin sensitivity will be assessed in each of the groups of transplanted mice. Aim 3. To test the hypothesis that GX sPLA2 positively regulates TAG mobilization. This will be achieved by a) determining whether GX sPLA2 alters the rate of TAG synthesis or hydrolysis in adipocytes;and b) determining whether GX sPLA2 promotes TAG mobilization by activating hormone sensitive lipase. Results from these studies have the potential to provide important insights into a novel mechanism regulating lipolysis, and may identify new targets for modulating adipocyte hypertrophy and FFA flux to reduce insulin resistance. PUBLIC HEALTH RELEVANCE: In the U.S., obesity is now the most common risk factor for the development of type-2 diabetes. Studies in humans and animal models suggest that inefficient storage of fat during long-term exposure to nutrient excess may trigger pathological inflammatory responses that disrupt the ability of insulin to function. The goal of this proposal is to test the hypothesis that enhancing the capacity of adipose tissue to store fat will have a beneficial effect on inflammation and insulin sensitivity. This proposal could provide new strategies to prevent or treat diabetes.