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

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, ϳ50% of LDL-[ 3 H]oleate-PC was converted to triacylglycerol rather than degraded in lysosomes as occurs for LDLderived 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.

Lipoprotein profiles in SCID/uPA mice transplanted with human hepatocytes become human-like and correlate with HCV infection success

“…Mouse hepatocytes are capable of efficiently removing human LDL from the circulation (15), as is also supported by our own studies. We injected high amounts of human lipoproteins (VLDL and LDL) in mice, which resulted in a human-like lipoprotein profile shortly after injection (4 h; not shown).…”

Specificity and rate of human and mouse liver and plasma phosphatidylcholine synthesis analyzed in vivo

“…The fractional enrichment of methyl -D 9 -choline phosphate ( Fig. 6A ) in liver followed a nonlinear distribution, with a rapid initial decline to 3 h, followed by a slower decrease to 24 h. Although these data are limited, they suggest a biphasic response, with the latter period representing uptake of lipoprotein by the liver and recycling of labeled choline ( 25 ). Given that at 3 h the mean enrichment of methyl -D 9 -PC was 4.15% and that of methyl -D 9 -choline phosphate was 5.97% between 1.5 and 3 h, an estimate of the maximal rate of synthesis by this pathway was calculated as (4.15 × 100)/(5.97 × 1.5) = 46.3% of total liver PC/h.…”

“…As acyl remodeling apparently occurs over a period of 3-6 h ( 9, 11 ) and as our estimate of the combined fl ux rate through both pathways is approximately 50%/h for both mouse liver and plasma PC, PC acyl remodeling must be only one of a number of activities that determine liver PC molecular species composition. In this context, the relative contributions to the equilibrium composition of liver and plasma PC of LPCAT ( 37 ), lecithin-cholesterol acyltransferase ( 38 ), recycling of extrahepatic PC to the liver ( 25 ), and secretion of PC in bile ( 39 ) have yet to be fully evaluated.…”

Extrahepatic Diseases and NAFLD: The Triangular Relationship between NAFLD, Type 2-Diabetes and Dysbiosis

“…Choline is an essential nutrient and it is mostly obtained from food such as red meat and eggs. Choline is essential for neurotransmitter synthesis, methyl group metabolism, cell membrane structure and signaling, lipid metabolism and synthesis of very low-density lipoprotein in the liver [32] . Moreover, choline is metabolized in the liv- er to phosphatidylcholine, which is necessary for the packaging and export of triglycerides in very low-density lipoproteins.…”