Evidence of Direct Hepatic Secretion of Low density Lipoprotein (LDL) in Mice

Abstract

<h3>Lead Author's Financial Disclosures</h3> Nothing to disclose. <h3>Study Funding</h3> National Lipid Association Junior Faculty Research Award 2020, NIH P01HL151328 and HL45095 (IJG). <h3>Background/Synopsis</h3> LDL is the major atherogenic lipoprotein causing atherosclerosis, leading to cardiovascular disease (CVD), the primary cause of morbidity and mortality in the United States. Lipoprotein lipase (LpL) is the rate limiting enzyme hydrolyzing triglyceride rich lipoproteins in the circulation. Current understanding is that LpL mediated hydrolysis of very low density lipoprotein (VLDL) generates cholesterol rich LDL particles in the circulation. However, evidence from in vitro studies as well as some human kinetic data suggest that the liver might directly secrete LDL into the circulation. <h3>Objective/Purpose</h3> In this study, we determine whether combined deficiency of lipoprotein lipase and hepatic LDL receptor (LDLR) would lead to decreased circulating LDL cholesterol (LDL-C) levels. <h3>Methods</h3> We used a mouse model of tamoxifen inducible global LpL deficiency (iLpl-/-). To knock down hepatic genes like LDLR and hepatic lipase (HL) we utilized anti sense oligonucleotides (ASOs). <h3>Results</h3> As expected, iLpl-/- mice display reduced plasma LDL-C levels compared to control mice (5 mg/dl vs 30 mg/dl). To determine whether LDL-C would increase with knock down of hepatic LDLRs, we used LDLR ASO in control and iLpl-/- mice. As anticipated, LDL-C levels increased in control mice to 150 mg/dl. Remarkably, plasma LDL-C also increased in iLpl-/- mice to levels similar to that of control mice. We then fed the mice a high cholesterol western diet for two weeks; this increased LDL-C levels equally in both mice to 300 mg/dl. When we overexpressed PCSK9 to inhibit LDLR in iLpl-/- mice, we again observed increases in LDL-C similar to control mice with PCSK9 overexpression. Further, to test if HL, another extracellular triglyceride lipase, is converting VLDL to LDL in iLpl-/- mice, we knocked down HL in control and iLpl-/- mice. HL knockdown did not alter LDL-C levels in control or iLpl-/- mice. <h3>Conclusions</h3> These data suggest the presence of direct LDL secretion from liver. We are investigating why this is apparent only with concomitant loss of LpL and LDLRs. We postulate that LDLRs have an additional gate-keeping function to halt secretion of newly synthesized LDL particles from the liver in addition to their well-established role in clearance of plasma LDL. When LDLRs are limiting or in patients with combined hyperlipidemia, direct LDL production may constitute a major source of circulating LDL. If so, this could uncover a novel approach to treat familial hypercholesterolemia.