The role of calcium in apoptosis induced by 7beta-hydroxycholesterol and cholesterol-5beta,6beta-epoxide

Oxysterols, such as 7beta-hydroxy-cholesterol (7beta-OH) and cholesterol-5beta,6beta-epoxide (beta-epoxide), may have a central role in promoting atherogenesis. This is thought to be predominantly due to their ability to induce apoptosis in cells of the vascular wall and in monocytes/macrophages. Although there has been extensive research regarding the mechanisms through which oxysterols induce apoptosis, much remains to be clarified. Given that experimental evidence has long associated alterations of calcium (Ca(2+)) homeostasis to apoptotic cell death, the aim of the present study was to determine the influence of intracellular Ca(2+) changes on apoptosis induced by 7beta-OH and beta-epoxide. Ca(2+) responses in differentiated U937 cells were assessed by epifluorescence video microscopy, using the ratiometric dye fura-2. Over 15-min exposure of differentiated U937 cells to 30 muM of 7beta-OH induced a slow but significant rise in fura-2 ratio. The Ca(2+) channel blocker nifedipine and the chelating agent EGTA blocked the increase in cytoplasmic Ca(2+). Moreover, dihydropyridine (DHP) binding sites identified with BODIPY-FLX-DHP were blocked following pretreatment with nifedipine, indicating that the influx of Ca(2+) occurred through L-type channels. However, following long-term incubation with 7beta-OH, elevated levels of cytoplasmic Ca(2+) were not maintained and nifedipine did not provide protection against apoptotic cell death. Our results indicate that the increase in Ca(2+) may be an initial trigger of 7beta-OH-induced apoptosis, but following chronic exposure to the oxysterol, the influence of Ca(2+) on apoptotic cell death appears to be less significant. In contrast, Ca(2+) did not appear to be involved in beta-epoxide-induced apoptosis.