Lipoproteins containing apolipoprotein B (apoB-LP) are important causal risk factors for coronary artery disease (CAD). The most common cause of elevated apoB-LP is overproduction by the liver of VLDL, but the molecular processes regulating hepatic VLDL assembly and secretion remain incompletely understood. Human genetics has identified previously unsuspected genes that are strongly associated with plasma levels of apoB-LP as well as with CAD. Two of the most compelling new genes associated with apoB-LP and CAD are SORT1, encoding a protein called sortilin, and TRIB1, encoding a protein known as tribbles-1. We recently published studies of the SORT1 locus, demonstrating in mice that overexpression of sortilin in the liver reduces VLDL secretion and LDL-C levels whereas knockdown of sortilin increases VLDL secretion and LDL-C levels. In parallel, we investigated the TRIB1 locus, and found that overexpression of tribbles-1 reduced VLDL secretion and Trib1 knockout mice had increased VLDL secretion. Thus, both of these loci encode proteins that regulate the hepatic production of VLDL. However, the molecular mechanisms by which they influence this complex process remain unknown. The goal of this proposal is to perform a series of detailed experiments intended to elucidate the molecular and cellular mechanisms by which these proteins modulate hepatic VLDL assembly and secretion. With regard to sortilin, we will test the hypothesis that sortilin acts as a sorting protein to bind and sort nascent VLDL from Golgi to lysosome, diverting it from the secretory pathway. With regard to tribbles-1, we will test the hypotheses that tribbles-1 regulates MAP kinase activity in hepatocytes, and that tribbles-1 targets C/EBP1 and/or 2 for post-translational degradation in hepatocytes. We will perform experiments in hepatocyte cell culture systems, as well as in mice with gain and loss of function of sortilin and tribbles-1. In addition, we will test naturally-occurring mutations in SORT1 and TRIB1 for functional effects. Successful completion of the proposed aims promises to yield novel insights regarding new biological pathways regulating VLDL production and thus plasma lipids in humans, and could provide novel therapeutic targets for reducing VLDL production and cardiovascular risk.
Lowering blood levels of LDL (bad cholesterol) is among the most powerful ways to reduce risk of heart attack, and new treatments to reduce LDL are needed. New genes have been identified that powerfully control LDL levels, but are poorly understood. This proposal will establish a detailed understanding of these new genes and potentially pave the way for new treatments to reduce LDL levels and reduce heart disease.
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