Increased secretion and higher levels of apoB-containing lipoproteins (BLp) are a major cause of the dyslipidemia seen in familial hyperlipidemia, obesity, and diabetes. Plasma phospholipid transfer protein (PLTP) is known to mediate transfer of phospholipids between BLp and HDL during their intravascular metabolism, To address a possible role of PLTP in dyslipidemia and atherogenesis, PLTP gene knock-out (PLTPO) mice were bred with differing hyperlipidemic strains. In the apoB transgenic (apoBTg) and apoE knock-out (apoE0) backgrounds, PLTP deficiency resulted in reduced production and levels of BLp, and markedly decreased atherosclerosis. BLp secretion was diminished in hepatocytes from PLTP-deficient mice, an effect that was canceled when PLTP was reintroduced in the adenovirus. Preliminary studies reveal a major and quite unexpected role for PLTP in regulating the secretion of BLp. The biosynthesis of BLp is a two-step process: initial lipidation of apoB occurs in the endoplasmic reticulum, and requires the activity of the microsomal triglyceride transfer protein. A poorly understood, slower second step probably involves the addition of further lipid to the nascent very low-density lipoprotein (VLDL) particle. Our working hypothesis is that PLTP may be involved in the second lipidation step of nascent BLp. Preliminary studies also show that apoB production is not affected in LDL receptor gene knock-out (LDLrO)/PLTPO mice compared with LDLrO mice, but that atherosclerotic lesions are significantly reduced, suggesting that: 1) the influence of PLTP on the production of BLp may require the presence of functional LDL receptors, at least in the liver; and 2) there are some unknown factors, other than lowering BLp, involved in the reduction of atherosclerosis in PLTPO mice. The goal of this project is to investigate further the role of PLTP in BLp metabolism and atherosclerosis.
Specific aims are: 1) to evaluate the hypothesis that PLTP may play a role in BLp assembly and secretion in nonlipoprotein-producing model systems (cotransfected by PLTP, apoB, and MTP), primary hepatocyte and hepatoma cells (liver-like cells); 2) to evaluate the hypothesis that PLTP plays a role in BLp assembly and secretion in intestinal cells; and 3) to determine antiatherogenic mechanisms in PLTPO mice other than lowering BLp. This project will provide new information on the relationship between PLTP activity and BLp production, between PLTP activity and atherosclerosis, and will further evaluate PLTP as a therapeutic target.
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