The lipidation by hepatocytes of human apolipoprotein A-I occurs by both ABCA1-dependent and -independent pathways.
The pathways of hepatic intra- and peri-cellular lipidation of apolipoprotein A-I (apoA-I) were studied by infecting primary mouse hepatocytes from either apoA-I-deficient or ABCA1-deficient mice with a recombinant adenovirus expressing the human apoA-I (hapoA-I) cDNA (endo apoA-I) or incubating the hepatocytes with exogenously added hapoA-I (exo apoA-I) and examining the hapoA-I-containing lipoproteins formed. The cells, maintained in serum-free medium, were labeled with [(3)H]choline, and the cell medium was separated by fast protein liquid chromatography or immunoprecipitated to quantify labeled choline phospholipids specifically associated with hapoA-I. With the apoA-I-deficient hepatocytes, the high density lipoprotein fraction formed with endo apoA-I contained proportionally more phospholipids than that formed with exo apoA-I. However, the lipoprotein size and electrophoretic mobility and phospholipid profiles were similar for exo apoA-I and endo apoA-I. Taken together, these data demonstrate that a significant proportion of hapoA-I is secreted from hepatocytes in a phospholipidated state but that hapoA-I is also phospholipidated peri-cellularly. With primary hepatocytes from ABCA1-deficient mice, the expression and net secretion of adenoviral-generated endogenous apoA-I was unchanged compared with control mice, but (3)H-phospholipids associated with endo apoA-I and exo apoA-I decreased by 63 and 25%, respectively. The lipoprotein size and electrophoretic migration and their phospholipid profiles remained unchanged. In conclusion, we demonstrated that intracellular and peri-cellular lipidation of apoA-I represent distinct and additive pathways that may be regulated independently. Hepatocyte expression of ABCA1 is central to the lipidation of newly synthesized apoA-I but also contributes to the lipidation of exogenous apoA-I. However, a significant basal level of phospholipidation occurs in the absence of ABCA1.
Kiss
Robert S
RS
Lipoprotein and Atherosclerosis Research Group, Department of Pathology & Laboratory Medicine, University of Ottawa Heart Institute, Ontario K1Y 4W7, Canada.
McManus
Dan C
DC
Franklin
Vivian
V
Tan
Wei Ling
WL
McKenzie
Andrea
A
Chimini
Giovanna
G
Marcel
Yves L
YL
D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters / Q000502:physiology*; D000077556:Alitretinoin; D000818:Animals; D016632:Apolipoprotein A-I / Q000378:metabolism*; D002784:Cholesterol / Q000378:metabolism; D002853:Chromatography, Liquid; D022781:Hepatocytes / Q000378:metabolism*; D006801:Humans; D050356:Lipid Metabolism; D008075:Lipoproteins, HDL / Q000378:metabolism; D008077:Lipoproteins, LDL / Q000378:metabolism; D008079:Lipoproteins, VLDL / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D010743:Phospholipids / Q000378:metabolism; D018168:Receptors, Retinoic Acid / Q000502:physiology; D047488:Retinoid X Receptors; D014157:Transcription Factors / Q000502:physiology; D014212:Tretinoin / Q000494:pharmacology
C577836:ABCA1 protein, human; D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters; D016632:Apolipoprotein A-I; D008075:Lipoproteins, HDL; D008077:Lipoproteins, LDL; D008079:Lipoproteins, VLDL; D010743:Phospholipids; D018168:Receptors, Retinoic Acid; D047488:Retinoid X Receptors; D014157:Transcription Factors; D000077556:Alitretinoin; D014212:Tretinoin; D002784:Cholesterol