Differential sensitivity of astrocyte primary cultures and derived spontaneous transformed cell lines to 70-hydroxycholesterol: effect on plasma membrane lipid composition and fluidity, and on cell surface protein expression

Abstract

The cytotoxicity of 7 beta-hydroxycholesterol (7 beta-OHC) was investigated on rat astrocyte primary cultures and spontaneously transformed cell lines derived from them. Confluent astrocyte primary cultures (normal cells) were unaffected by 20 microM 7 beta-OHC over a period of 72 h whereas 30 microM markedly affected the viability of the transformed cells within the first 72 h. Both cell types incorporated 18% of the total amount of 7 beta-OHC added to the cultures at concentrations of 20 microM or 30 microM. Cellular fractionation after incubation with 20 microM or 30 microM 7 beta-OHC indicated that the plasma membrane incorporated 2 or 6 fold more 7 beta-OHC than the intracellular one's respectively. Plasma membrane cholesterol (CH) and phospholipid (PL) analysis showed that 20 microM 7 beta-OHC did not affect CH/PL in normal cells; in contrast, plasma membranes of transformed cells displayed a significant CH/PL decrease, which was more pronounced with 30 microM 7 beta-OHC treatment. Fluorescence anisotropy measurements indicated that 20 microM 7 beta-OHC slightly fluidified the plasma membrane of normal cells whereas it has not effect on that of the transformed cells one; however, an increase in plasma membrane fluidity was observed when the transformed cells were treated with 30 microM 7 beta-OHC. Lactoperoxidase catalyzed radioiodination of cell surface proteins and subsequent autoradioelectrophoretic analysis demonstrated that the labelled protein pattern was unchanged when both cell types were incubated with 30 microM 7 beta-OHC. These findings demonstrate that 7 beta-OHC is lethal to highly proliferating cultured glial cells. The high accumulation of 7 beta-OHC in the plasma membrane and its decrease in fluidity, by themselves, do not seem to be involved in the processes leading to cellular death. However, increase of plasma membrane fragility associated with the decrease of CH/PL, which occurs exclusively in plasma membranes isolated from 7 beta-OHC treated transformed cells together with high 7 beta-OHC uptake, are probably implicated in 7 beta-OHC cytotoxicity. The possibility of an additional action mechanism is discussed.