Isolation of a somatic cell mutant resistant to the induction of apoptosisby oxidized low density lipoprotein

Oxidized low density Lipoprotein (oxLDL) induces apoptosis in macrophages, smooth muscle cells, and endothelial cells. To elucidate the molecular mech anism of oxLDL-induced cytotoxicity and determine its tissue specificity, w e have used Chinese hamster ovary (CHO)-K1 cells expressing human CD36 (CHO /CD36), Expression of CD36 rendered these cells susceptible to killing by o xLDL, This cytotoxicity was due to the induction of apoptosis, Therefore, C D36 expression is the only requirement for oxLDL-induced apoptosis. Oxyster ols apparently mediate the cytotoxicity of oxLDL in macrophage foam cells a nd endothelial cells. 25-Hydroxycholesterol at concentrations higher than 1 mu g/ml, killed CHO-K1 cells, by apoptosis, in medium supplemented with se rum as a source of cholesterol, These effects mere not seen in a 25-hydroxy cholesterol-resistant CHO/CD36 mutant (OXR), which was otherwise capable of undergoing apoptosis in response to staurosporine. This mutant was also re sistant to killing by oxLDL, suggesting that oxysterols are at least partia lly responsible for the toxic effects of oxLDL. Oxysterol-induced apoptosis did not involve regulation of sterol regulatory element-binding protein pr oteolysis or the cholesterol biosynthetic pathway. 25-Hydroxycholesterol st imulated calcium uptake by CHO-K1 cells within 2 min after addition. Treatm ent of CHO or THP-1 (macrophage) cells with the calcium channel blocker nif edipine prevented 25-hydroxycholesterol induction of apoptosis, OXR showed no enhanced calcium uptake in response to 25-hydroxycholesterol.