There are genetic and environmental causes of insulin resistance, and clearly these two inputs can be additive and interactive. A high fat intake is an important environmental factor which can cause, or exacerbate, insulin resistance and enhance the risk for the development of Type II diabetes. Our recent studies have shown that lipid/heparin infusions lead to insulin resistance in men, but not in pre-menopausal women. We also have preliminary data showing that post-menopausal women are fully susceptible to fat-induced insulin resistance and that estrogen replacement therapy re-establishes the protective state. In addition, we have conducted a series of studies in rats, demonstrating that estrogenization (endogenous or exogenous) will protect females from fat -induced insulin resistance. Based on these findings, we propose that men and non-replaced post-menopausal women will exhibit fat-induced insulin resistance, whereas, adequately estrogenized women will be protected. We will test these ideas, not only by employing the lipid/heparin infusion glucose clamp technique, but also by placing experimental subjects on control and high fat diets. It is also possible that adequate estrogen can ameliorate the effects of other physiologic causes of insulin resistance. Thus, we also will conduct studies to determine whether estrogenization can protect women from the insulin resistance induced by obesity and aging. Using muscle biopsy samples collected during the glucose clamp studies, we will conduct experiments aimed at identifying cellular mechanisms for these protective effects of estrogens. We also propose an extensive series of animal studies, in which we will explore in more detail the mechanisms of estrogen protection from fat-induced insulin resistance. We will conduct studies in normal male and female rats, ovariectomized rats, and old estrogen deficient female rats+/- treatment with estradiol, an estrogen antagonist, or estrogen receptor isoform specific agonists. Studies in mice with deletion of the alpha or beta forms of the estrogen receptor, as well as muscle specific estrogen receptor specific knockout animals are also proposed. We will also determine whether the fat cell secreted protein ACRP3O is modulated by estrogen status, and whether the insulin sensitizing effects of ACRP3O are responsible for the estrogen induced protection from insulin resistance. If the concepts contained in this application prove correct, then these findings could have significant implications concerning the mechanisms of insulin resistance as well as the treatment and possibly prevention of this disorder.