The role of calcium in apoptosis induced by 7β‐hydroxycholesterol and cholesterol‐5β,6β‐epoxide |
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Authors: | Sinéad Lordan Nora M. O'Brien John J. Mackrill |
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Affiliation: | 1. Department of Food and Nutritional Sciences, University College, Cork, Republic of Ireland;2. Department of Physiology, University College, Cork, Republic of Ireland |
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Abstract: | Oxysterols, such as 7β‐hydroxy‐cholesterol (7β‐OH) and cholesterol‐5β,6β‐epoxide (β‐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 (Ca2+) homeostasis to apoptotic cell death, the aim of the present study was to determine the influence of intracellular Ca2+ changes on apoptosis induced by 7β‐OH and β‐epoxide. Ca2+ 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 μM of 7β‐OH induced a slow but significant rise in fura‐2 ratio. The Ca2+ channel blocker nifedipine and the chelating agent EGTA blocked the increase in cytoplasmic Ca2+. Moreover, dihydropyridine (DHP) binding sites identified with BODIPY‐FLX‐DHP were blocked following pretreatment with nifedipine, indicating that the influx of Ca2+ occurred through L‐type channels. However, following long‐term incubation with 7β‐OH, elevated levels of cytoplasmic Ca2+ were not maintained and nifedipine did not provide protection against apoptotic cell death. Our results indicate that the increase in Ca2+ may be an initial trigger of 7β‐OH–induced apoptosis, but following chronic exposure to the oxysterol, the influence of Ca2+ on apoptotic cell death appears to be less significant. In contrast, Ca2+ did not appear to be involved in β‐epoxide–induced apoptosis. © 2009 Wiley Periodicals, Inc. J Biochem Mol Toxicol 23:324–332, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/jbt.20295 |
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Keywords: | Apoptosis Calcium Cholesterol‐5β ,6β ‐Epoxide 7β ‐Hydroxycholesterol U937 Cells |
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