HIGH-DENSITY-LIPOPROTEIN (HDL3)-ASSOCIATED ALPHA-TOCOPHEROL IS TAKEN UP BY HEPG2 CELLS VIA THE SELECTIVE UPTAKE PATHWAY AND RESECRETED WITHENDOGENOUSLY SYNTHESIZED APOLIPOPROTEIN B-RICH LIPOPROTEIN PARTICLES

alpha-Tocopherol (alpha TocH) is transported in association with lipop roteins in the aqueous milieu of the plasma. Although up to 50% of cir culating alpha TocH is transported by high-density lipoproteins (HDLs) , little is known about the mechanisms of uptake of HDL-associated alp ha TocH. During the current study, human apolipoprotein (apo)E-free HD L subclass 3 (HDL3) labelled with [C-14]alpha TocH was used to investi gate uptake mechanisms of HDL3-associated alpha TocH by a permanent he patoblastoma cell line (HepG2). HDL3-associated alpha TocH was taken u p independently of HDL3 holoparticles in excess of apoA-I comparable w ith the non-endocytotic delivery of cholesteryl esters to cells termed the 'selective' cholesteryl ester uptake pathway. Experiments with un labelled HDL3 demonstrated net mass transfer of alpha TocH to KepG2 ce lls. Time-dependent studies with [C-14]alpha ToCH-labelled HDL3 reveal ed tracer uptake in 80-fold excess of apoA-I and in 4-fold excess of c holesteryl linoleate. In addition to HLDs, low-density lipoprotein (LD L)-associated alpha TocH was also taken up in excess of holoparticles, although to a lesser extent. These findings were confirmed with unlab elled lipoprotein preparations, in which HDL, displayed a 2- to 3-fold higher alpha TocH donor efficiency than LDLs (lipoproteins adjusted f or equal amounts of alpha TocH). An important factor affecting particl e-independent uptake of alpha TocH was the cellular cholesterol conten t (a 2-fold increase in cellular cholesterol levels resulted in a 2.3- fold decrease in uptake). Pulse-chase studies demonstrated that some o f the HDL3-associated alpha TocH taken up independently of holoparticl e uptake was resecreted along with a newly synthesized apoB-containing lipoprotein fraction.