PHYSICAL PARTITIONING IS THE MAIN MECHANISM OF ALPHA-TOCOPHEROL AND CHOLESTEROL TRANSFER BETWEEN LIPOPROTEINS AND P388D(1) MACROPHAGE-LIKE CELLS

The regulation of cellular vitamin E concentration was studied in P388 D(1) macrophage-like cells, Cellular alpha-tocopherol levels increased more than 5000-fold over constitutive levels without reaching saturat ion when P388D(1) cells were cultured in vitamin-E-supplemented fetal calf serum. The uptake of alpha-tocopherol was accompanied by accumula tion of alpha-[H-3]tocopherol and [C-14]cholesterol in these cells. Hu man unmodified low-density lipoprotein (LDL) inhibited the uptake of a lpha-[H-3]tocopherol and [C-14]cholesterol in a dose-dependent manner and with very similar IC50. Acetylated, Cu2+-oxidized and aggregated h uman LDL and human very-low-density-lipoprotein (VLDL) were similarly potent, whereas human HDL was at least tenfold less effective than hum an LDL when inhibitory activity was correlated to lipoprotein protein levels. The rate of vitamin E uptake by P388D(1) cells, however, alway s correlated with the extracellular alpha-tocopherol/cholesterol ratio . Efflux of alpha-[H-3]tocopherol from labeled P388D(1) cells required extracellular accepters and was accompanied by the concomitant releas e of [C-14]cholesterol. Both human LDL and HDL could serve as accepter s, Changes in the cellular alpha-tocopherol level appear to be the dir ect consequence of changes in the extracellular alpha-tocopherol/chole sterol ratio due to a rapid exchange of lipids between P388D(1) cells and their extracellular environment. While the transfer of alpha-tocop herol from LDL, VLDL, and fetal calf serum into P388D(1) cells appears to occur mainly by diffusion, HDL-stimulated efflux of alpha-tocopher ol may underlie a different mechanism. The alpha-tocopherol/cholestero l ratio of the extracellular environment may be a critical factor in d etermining cellular vitamin E levels in vivo.