High-density lipoprotein (HDL3)-associated alpha-tocopherol is taken up by HepG2 cells via the selective uptake pathway and resecreted with endogenously synthesized apo-lipoprotein B-rich lipoprotein particles.
alpha-Tocopherol (alphaTocH) is transported in association with lipoproteins in the aqueous milieu of the plasma. Although up to 50% of circulating alphaTocH is transported by high-density lipoproteins (HDLs), little is known about the mechanisms of uptake of HDL-associated alphaTocH. During the current study, human apolipoprotein (apo)E-free HDL subclass 3 (HDL3) labelled with [14C]alphaTocH was used to investigate uptake mechanisms of HDL3-associated alphaTocH by a permanent hepatoblastoma cell line (HepG2). HDL3-associated alphaTocH was taken up independently of HDL3 holoparticles in excess of apoA-I comparable with the non-endocytotic delivery of cholesteryl esters to cells termed the 'selective' cholesteryl ester uptake pathway. Experiments with unlabelled HDL3 demonstrated net mass transfer of alphaTocH to HepG2 cells. Time-dependent studies with [14C]alphaTocH-labelled HDL3 revealed tracer uptake in 80-fold excess of apoA-I and in 4-fold excess of cholesteryl linoleate. In addition to HLDs, low-density lipoprotein (LDL)-associated alphaTocH was also taken up in excess of holoparticles, although to a lesser extent. These findings were confirmed with unlabelled lipoprotein preparations, in which HDL3 displayed a 2- to 3-fold higher alphaTocH donor efficiency than LDLs (lipoproteins adjusted for equal amounts of alphaTocH). An important factor affecting particle-independent uptake of alphaTocH was the cellular cholesterol content (a 2-fold increase in cellular cholesterol levels resulted in a 2.3-fold decrease in uptake). Pulse-chase studies demonstrated that some of the HDL3-associated alphaTocH taken up independently of holoparticle uptake was resecreted along with a newly synthesized apoB-containing lipoprotein fraction.
Goti
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Department of Medical Biochemistry, University of Graz, Harrachgasse 21, 8010 Graz, Austria.
Reicher
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Malle
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Kostner
G M
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Panzenboeck
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U
Sattler
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D020106:Acrylamide; D000178:Acrylamides / Q000494:pharmacology; D000251:Adenosine Triphosphatases / Q000037:antagonists & inhibitors; D001055:Apolipoproteins B / Q000493:pharmacokinetics*; D006528:Carcinoma, Hepatocellular / Q000378:metabolism*; D002784:Cholesterol / Q000378:metabolism; D003078:Colchicine / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D008075:Lipoproteins, HDL / Q000493:pharmacokinetics*; D008077:Lipoproteins, LDL / Q000493:pharmacokinetics; D008985:Monensin / Q000494:pharmacology; D014407:Tumor Cells, Cultured; D014810:Vitamin E / Q000493:pharmacokinetics*
D000178:Acrylamides; D001055:Apolipoproteins B; D004791:Enzyme Inhibitors; D008075:Lipoproteins, HDL; D008077:Lipoproteins, LDL; D014810:Vitamin E; D020106:Acrylamide; D008985:Monensin; D002784:Cholesterol; D000251:Adenosine Triphosphatases; D003078:Colchicine