Role of Reduced Hepatocyte-Derived tPA Activity in Obesity-Associated Atherothrombosis

In the US, 40 percent of the adult population is considered obese. Obesity leads to many comorbidities, including atherosclerosis and thrombosis. Atherosclerosis is initiated and promoted by the arterial accumulation of cholesterol-rich apoB-lipoproteins (apoB LP-Chol), which activate a chronic inflammatory response. Atherosclerotic lesions may rupture or erode to trigger occlusive platelet/fibrin clots ('atherothrombosis') and organ infarction. However, how obesity affects these processes is not fully understood. The overarching objective of this proposal is to explore how hepatocyte expression of the key fibrinolytic protein tissue plasminogen activator (tPA) dysregulates fibrinolysis and, surprisingly, plasma cholesterol in obesity. The function of tPA in fibrinolysis is balanced by plasminogen activator inhibitor-1 (PAI1). The premise is based on strong preliminary data showing that hepatocytes contribute substantially to basal plasma tPA level, thereby altering systemic fibrinolysis. I have shown that the transcriptional co-repressor DACH1 downregulates tPA and upregulates PAI1 in hepatocytes. In human liver biopsy samples, DACH1 expression correlates positively with BMI and negatively with plasma tPA activity. Finally, I discovered that silencing hepatic tPA in obese mice increases the secretion of apoB LPs and the development of atherosclerotic lesions. I propose the following two aims: Aim 1 will explore the mechanisms by which obesity reduces hepatocyte-derived plasma tPA fibrinolytic activity. By lowering hepatic tPA expression to a comparable level as lean mice, I will first test the hypothesis that the moderate increase of hepatic tPA in obesity is a partially protective mechanism that limits the extent of impaired fibrinolysis. I will then explore the mechanism of the increase in hepatic PAI1 and tPA expression in obesity. For this, I will explore hepatic gene expression and systemic fibrinolysis in obese mice lacking either CREB or DACH1 in hepatocytes. Aim 2 will explore the mechanism through which hepatocyte tPA leads to a decrease in atherogenic plasma cholesterol, atherosclerosis, and thrombosis on atherosclerotic lesions. I will explore the mechanism by which steps in apoB production are suppressed by hepatic tPA activity. I will next test the hypothesis that improving hepatocyte-derived tPA activity with genetic manipulations of hepatic tPA and PAI1. (AHA Program: Career Development Award)