Conversion of low density lipoprotein-associated phosphatidylcholine to triacylglycerol by primary hepatocytes.
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.
Minahk
Carlos
C
Department of Biochemistry and the Group on the Molecular and Cell Biology of Lipids, University of Alberta, Edmonton, Alberta, Canada.
Kim
Kyung-Wook
KW
Nelson
Randy
R
Trigatti
Bernardo
B
Lehner
Richard
R
Vance
Dennis E
DE
D000215:Acylation / Q000187:drug effects; D000818:Animals; D053299:Apolipoprotein B-100 / Q000378:metabolism; D001952:Bridged-Ring Compounds / Q000494:pharmacology; D002460:Cell Line; D002788:Cholesterol Esters / Q000378:metabolism; D008078:Cholesterol, LDL / Q000378:metabolism*; D003435:Crotalid Venoms / Q000494:pharmacology; D051048:Diacylglycerol O-Acyltransferase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D004789:Enzyme Activation / Q000187:drug effects; D022781:Hepatocytes / Q000166:cytology / Q000201:enzymology*; D008099:Liver / Q000166:cytology / Q000378:metabolism; D008247:Lysosomes / Q000378:metabolism; D051379:Mice; D009636:Norbornanes; D010713:Phosphatidylcholines / Q000378:metabolism*; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D011973:Receptors, LDL / Q000378:metabolism; D051122:Scavenger Receptors, Class B / Q000378:metabolism; D013859:Thiocarbamates; D013871:Thiones / Q000494:pharmacology; D014280:Triglycerides / Q000096:biosynthesis*; D010738:Type C Phospholipases / Q000037:antagonists & inhibitors / Q000378:metabolism*
D053299:Apolipoprotein B-100; D001952:Bridged-Ring Compounds; D002788:Cholesterol Esters; D008078:Cholesterol, LDL; D003435:Crotalid Venoms; D009636:Norbornanes; D010713:Phosphatidylcholines; D010726:Phosphodiesterase Inhibitors; D011973:Receptors, LDL; C454739:Scarb1 protein, mouse; D051122:Scavenger Receptors, Class B; D013859:Thiocarbamates; D013871:Thiones; D014280:Triglycerides; C046498:tricyclodecane-9-yl-xanthogenate; C516957:DGAT2 protein, mouse; C494882:Dgat1 protein, mouse; D051048:Diacylglycerol O-Acyltransferase; D010738:Type C Phospholipases; C105598:phosphatidylcholine-specific phospholipase C