Insulin resistance is a characteristic feature of obesity and type 2 diabetes mellitus. The PPARgamma nuclear receptor is the target of Thiazolidinediones (TZD), a class of anti-diabetic drugs known to ameliorate insulin resistance. Despite the known action of TZDs to improve insulin sensitivity, many aspects of PPARgamma biology remain relatively unclear. In this application we plan to utilize gene knockout technology to elucidate the physiologic and cellular mechanisms of PPARgamma biology relating to insulin action and glucose homeostasis. Previous studies from our laboratory have shown that animals heterozygous for a PPAR gene deletion (PPARgamma+/-) demonstrate enhanced insulin sensitivity in muscle and liver. This led us to hypothesize that the PPARgamma receptor and its natural ligands normally serve to dampen insulin action. We believe that PPARgamma gene deletion releases this dampening effect leading to heightened insulin sensitivity. Since the PPARgamma+/-mice display enhanced insulin sensitivity, (1) we will test the hypothesis that when crossed into genetically insulin resistant animals, the insulin resistance phenotype will be rescued. While it is known that TZDs improve the ability of insulin to stimulate glucose disposal, the site of TZD action remains unknown. (2) To assess the differential role of PPARgamma receptor expression in fat vs. muscle, our laboratory has used the Cre-Lox system to create animals with a PPARgamma gene deletion in either skeletal muscle (MCK-Cre mice), or adipose tissue (aP2-Cre mice). The phenotype of these animals will be studied in vivo and in vitro, with or without perturbations including: a) TZD treatment and b) high fat feeding. In summary, in this application we propose to utilize the techniques of mouse genetics combined with a variety of in vivo and in vitro studies to elucidate the role of PPARgamma nuclear receptors in whole body, skeletal muscle, and adipose tissue metabolism. We feel that the findings from the proposed investigations will improve our general understanding of insulin resistance and offer potential insights into new therapeutic modalities.