β-Sitosterol Induces Anti-proliferation and Apoptosis in Human Leukemic U937 Cells through Activation of Caspase-3 and Induction of Bax/Bcl-2 Ratio

抄録

β-Sitosterol is the main dietary phytosterol found in plants and has been shown to inhibit proliferation and induce apoptosis in human solid tumors such as colon and breast cancers. However, the mechanism by which β-sitosterol induces apoptosis is not completely understood in leukemic cells. This study investigated the mechanism of apoptosis induced by β-sitosterol in human leukemic U937 cells. β-Sitosterol induced cytotoxicity and apoptosis in U937 cells in a concentration dependent manner, as measured by hemocytometer counts, fluorescence microscopy, agarose gel electrophoresis, and flow cytometry analysis. The increase in apoptosis induced by β-sitosterol was associated with down-regulation of Bcl-2, degradation of poly-(ADP-ribose) polymerase (PARP) and phospholipase C (PLC)-γ1 protein, and activation of caspase-3. β-Sitosterol induced apoptosis was not associated with changes in the expression of Bcl-xL, Bax, or inhibitor of apoptosis proteins (IAPs). z-DEVD-fmk, a caspase-3 specific inhibitor, blocked caspase-3 activation and PARP degradation, and significantly attenuated β-sitosterol-induced apoptosis. This suggests that caspase-3 activation is partially essential for β-sitosterol-induced apoptosis. Bcl-2 overexpression also significantly blocked caspase-3 activation and the decrease in PARP cleavage by β-sitosterol, and effectively attenuated the apoptotic response to β-sitosterol. These results show that β-sitosterol potently induces apoptosis in U937 cells and that β-sitosterol-induced apoptosis is related to the selective activation of caspase-3 and induction of Bax/Bcl-2 ratio.