Physical Partitioning is the Main Mechanism of α‐Tocopherol and Cholesterol Transfer between Lipoproteins and P388D₁Macrophage‐Like Cells

The regulation of cellular vitamin E concentration was studied in P388D1 macrophage-like cells. Cellular alpha-tocopherol levels increased more than 5000-fold over constitutive levels without reaching saturation when P388D1 cells were cultured in vitamin-E-supplemented fetal calf serum. The uptake of alpha-tocopherol was accompanied by accumulation of alpha-[3H]tocopherol and [14C]cholesterol in these cells. Human unmodified low-density lipoprotein (LDL) inhibited the uptake of alpha-[3H]tocopherol and [14C]cholesterol in a dose-dependent manner and with very similar IC50. Acetylated, Cu2+-oxidized and aggregated human LDL and human very-low-density-lipoprotein (VLDL) were similarly potent, whereas human HDL was at least tenfold less effective than human LDL when inhibitory activity was correlated to lipoprotein protein levels. The rate of vitamin E uptake by P388D1 cells, however, always correlated with the extracellular alpha-tocopherol/cholesterol ratio. Efflux of alpha-[3H]tocopherol from labeled P388D1 cells required extracellular acceptors and was accompanied by the concomitant release of [14C]cholesterol. Both human LDL and HDL could serve as acceptors. Changes in the cellular alpha-tocopherol level appear to be the direct consequence of changes in the extracellular alpha-tocopherol/cholesterol ratio due to a rapid exchange of lipids between P388D1 cells and their extracellular environment. While the transfer of alpha-tocopherol from LDL, VLDL, and fetal calf serum into P388D1 cells appears to occur mainly by diffusion, HDL-stimulated efflux of alpha-tocopherol may underlie a different mechanism. The alpha-tocopherol/cholesterol ratio of the extracellular environment may be a critical factor in determining cellular vitamin E levels in vivo.