The role of hepatocyte tPA in hepatic VLDL production.

Atherosclerosis is initiated and promoted by the arterial accumulation of apolipoprotein B (apoB)-containing lipoproteins which activate a chronic inflammatory response. The hepatocyte is the major source of apoB- lipoprotein particles via its ability to secrete very-low-density lipoprotein (VLDL), which is then hydrolyzed into intermediate-density lipoprotein (IDL) and then low-density lipoprotein (LDL) in the blood. Current cholesterol- lowering therapies primarily target LDL-cholesterol levels by enhancing LDL receptor (LDLR)-mediated LDL clearance. However, these LDL-lowering treatments (e.g., statins and PCSK9 inhibitors) have only modest effects on the remnant atherogenic apoB-containing lipoprotein-cholesterol constituents, including VLDL- and IDL-cholesterol. Despite reaching optimal levels of LDL-cholesterol with these LDL-lowering treatments, uncontrolled VLDL- and IDL-cholesterols still significantly contribute to the high residual atherosclerotic cardiovascular disease (CVD) risk in the population. Our group recently made the novel discovery that a key blood clot lysis protein, tissue-type plasminogen activator (tPA), in hepatocytes limits the production of apoB- lipoproteins in mice and cultured primary hepatocytes. The overarching objective of this proposal is to explore the underlying mechanisms by which hepatocyte tPA lowers plasma apoB-containing lipoprotein-cholesterol levels. Aim 1: Determine the molecular mechanisms by which hepatocyte tPA limits VLDL lipidation. Secretion of VLDL particles requires proper apoB lipidation, but major gaps remain in our understanding of the mechanisms by which this happens. Our preliminary data strongly support our hypothesis that silencing hepatocyte tPA increases plasma apoB lipoprotein-cholesterol by promoting hepatic VLDL lipidation. Completion of this aim will add novel insights to the understanding of the production of atherogenic apoB-lipoproteins. Aim 2: Determine whether hepatocyte tPA enhances apoB-VLDL intracellular degradation before secretion. Intracellular degradation of apoB-VLDL particles prior to their secretion is important to maintain the optimal plasma levels of atherogenic cholesterol and normal liver lipid levels, but the mechanism is poorly understood. We will increase hepatocyte tPA expression in mice to test hypothesis that hepatocyte tPA enhances VLDL intracellular degradation before secretion. Completion of this aim will provide a novel mechanism to maintain intrahepatic lipid homeostasis. Aim 3: Determine whether increasing hepatocyte tPA in dyslipidemia reduces atherosclerosis, without raising the risk of fatty liver disease. Lowering circulating remnant cholesterol (the cholesterol found in VLDL and IDL) while maintaining the homeostasis of intrahepatic lipid levels is a promising strategy to reduce the residual atherosclerotic risk. However, therapeutic gaps remain in lowering atherogenic cholesterol without increasing liver lipid accumulation. We hypothesize that increasing tPA expression in dyslipidemic mice reduces apoB lipoprotein-cholesterol, alleviates atherosclerosis, without accumulating lipids in the liver. Completion of this aim will provide novel therapeutic targets to reduce residual atherosclerotic CVD risk.