Aim:To investigate whether subtoxic concentration of β-sitosterol (SITO) com-bined with TNF-related apoptosis-inducing ligand (TRAIL) induces apoptosis in TRAIL-resistant MDA-MB-231 breast cancer cells.Methods:Cell viability and growth were assessed by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphnyl-2H-tetrazolim bromide assays,chromatin condensation,release of lactate dehydrogenase (LDH),and Annexin V+ cells.The apoptosis-related proteins were detected by Weste blotting.Results:Treatment with TRAIL in combination with subtoxic concen-trations of SITO sensitized MDA-MB-231 breast cancer cells to TRAIL-mediated apoptosis.The synergistic treatment induced chromatin condensation,DNA fragmentation,the release of LDH,and Annexin V cells.The indicators of apoptosis are correlated to the induction of caspase activities,which results in the cleavage ofpoly(ADP-ribose)polymerase.Both the cytotoxic effects and apoptotic characteristics induced by the synergistic treatment were significantly inhibited by a pan-caspase inhibitor z-VAD-fmk,demonstrating the important role of caspases.These results indicate that caspases are crucial regulators of apoptosis induced by the combined treatment of SITO and TRAIL in MDA-MB-231 cells.Conclusion:The synergistic treatment of SITO and TRAIL induces apoptosis,which can serve as a potential preventive and therapeutic agent.