Protein S (PS) is a unique protein because it functions non-enzymatically as an anticoagulant, and because it interacts with plasma components that function in both hemostasis and inflammation. PS deficiency is a major cause of hypercoagulability that manifests most prominently as deep vein thrombosis, stroke and myocardial infarction. Many factors contribute to acquired plasma PS deficiency. One factor is the concentration of sex hormones, especially estrogen. In women, an increase in estrogen from use of oral contraceptives (OCA) causes a decrease in PS. Importantly, this contraceptive-induced PS decrease enhances the risk of thrombosis by 3-fold. Decreased PS levels are also associated with obesity, which enhances the risk of thrombosis by 2-3 fold. Dramatically, the risk of thrombosis increases as much as 24- fold in obese women using OCA. Because 40% of the US female population is obese and 10% of females is morbidly obese, thrombotic complications represent a sizeable clinical side effect among premenopausal women using OCA. We will determine the biochemical and molecular basis for the dramatic increase in the incidence of thrombosis in obese women using OCA. Specifically, we will answer two important questions: (1) How do OCA and obesity, individually, cause changes in plasma PS level and (2) How do OCA and obesity synergistically elevate thrombotic risk. We have preliminary findings suggesting that two key transcription factors, HIF1α, which is active in obesity, and ERα, which is responsive to estrogen, individually repress expression of the PS gene, and, importantly, cooperate to still further, synergistically reduce PS gene expression. To assess the activities of HIF1α and ERα in various states, we will use in vitro assays, ex vivo assays, cell-based assays, and mouse models to measure PS expression in the presence of HIF1α and ERα. In addition, we will test several methods to augment PS levels in obese, OCA-treated mice to reduce the incidence of thrombosis. Our studies will provide the impetus to devise novel therapies for the significant risk of thrombosis in the female population.