Hepatic Rap1a in cholesterol homeostasis

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PROJECT SUMMARY Title: Hepatic Rap1a in cholesterol homeostasis Atherosclerotic cardiovascular disease is the leading cause of death worldwide and lowering of proatherogenic low-density lipoprotein (LDL) cholesterol (LDL-C) levels is the primary target of therapy. Recent work has identified proprotein convertase subtilisin/ kexin type 9 (PCSK9) as a promising therapeutic target that regulates serum LDL-C. Despite its critical role as a regulator of the LDL receptor (LDLR) and attractiveness as a pharmacological target, the mechanism(s) by which PCSK9 is regulated remain largely unknown. Interestingly, the most widely prescribed drugs for lowering plasma cholesterol, statins, dose-dependently increase PCSK9 levels, which largely negate further statin-induced increases in hepatic LDLR and attenuate their efficacy. However, our understanding of the underlying cellular and molecular mechanisms of this regulation remains incomplete. This proposal is based on a new finding from our laboratory suggesting that activation of the small G protein of the Ras superfamily, Rap1a, lowers PCSK9 and plasma LDL-C levels. Further, Rap1a geranylgeranyl prenylation and activity is inhibited in statin-treated hepatocytes and human liver, and restoring intracellular geranylgeranyl isoprenoid levels prevents statin-induced PCSK9 and further lowers LDL-C. We propose that hepatic Rap1a is a novel regulator of PCSK9 protein, and its inhibition via lower intracellular isoprenoid levels may provide an additional mechanism to explain how statins increase PCSK9. Based on these, we will define the mechanisms by which Rap1a regulates PCSK9 and LDL-C metabolism (Aim 1), elucidate it’s in role statin-mediated PCSK9 induction (Aim 2), and explore the effect of combined statin and Rap1a activation in the prevention of atherosclerotic lesion development. (Aim 3). We will test these new ideas in vivo and ex vivo, using a combination of mouse genetic, physiological, and biochemical approaches.