Conversion of Low Density Lipoprotein-associated Phosphatidylcholine to Triacylglycerol by Primary Hepatocytes

Carlos Minahk,Kyung-Wook Kim,Randy Nelson,Bernardo Trigatti,Richard Lehner,Dennis E Vance

doi:10.1074/jbc.m706995200

Abstract

We have studied the uptake and metabolism of phosphatidylcholine (PC), the major phospholipid of low density lipoproteins (LDL), by cultures of primary hepatocytes. Strikingly, in the absence of the LDL receptor, PC incorporation into hepatocytes was inhibited by only 30%, whereas cholesteryl ether uptake was inhibited by 60-70%. On the other hand, scavenger receptor class B, type I, the other important receptor for LDL in the liver, was found to be responsible for the uptake of the remaining 30-40% of LDL-cholesteryl ether. PC uptake was, however, only partially inhibited (30%) in scavenger receptor class B, type I, knock-out hepatocytes. Once LDL-PC was taken up by hepatocytes, approximately 50% of LDL-[(3)H]oleate-PC was converted to triacylglycerol rather than degraded in lysosomes as occurs for LDL-derived cholesteryl esters. The remainder of the LDL-derived PC was not significantly metabolized to other products. Triacylglycerol synthesis from LDL-PC requires a PC-phospholipase C activity as demonstrated by inhibition with the phospholipase C inhibitor D609 or activation with rattlesnake venom. Small interfering RNA-mediated suppression of acyl-CoA:diacylglycerol acyltransferase 2 (DGAT2), but not DGAT1, decreased the acylation of the LDL-derived diacylglycerol. These findings show that PC in LDL particles is taken up not only by the classical receptors but also by additional mechanism(s) followed by metabolism that is completely different from the cholesteryl esters or apoB100, the other main components of LDL.

Highlights

low density lipoproteins (LDL)-cholesteryl esters (LDL-CE) is associated with enhanced risk of atherosclerosis, leading to cardiovascular diseases, including heart attack and stroke [2, 3]
We showed that the production of TG from LDL-PC in LDLRϪ/Ϫ hepatocytes was inhibited by 30%, the same extent of inhibition as LDL-PC uptake (Fig. 2B)
The cell model chosen was primary hepatocytes because liver is the main organ involved in LDL clearance [36, 54]

Summary

Introduction

LDL-cholesteryl esters (LDL-CE) is associated with enhanced risk of atherosclerosis, leading to cardiovascular diseases, including heart attack and stroke [2, 3]. The pioneering work of Brown and Goldstein in the 1970s on the cellular basis of cholesterol homeostasis led to the detailed characterization of the uptake and metabolism of LDL following receptor-mediated endocytosis. In this pathway, LDL binds to the LDL receptor (LDLR) [4, 5], and the complex is internalized from clathrincoated pits [6]. High affinity binding of LDL was shown to involve SR-BI that mediated the selective import of the major lipoprotein-associated phospholipids [34]. A role for SR-BI in PC uptake from LDL and HDL by endothelial cells was demonstrated by Sattler and co-workers [35]

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Conclusion