Molecular mechanisms underlying the chemopreventive effect of selenium and vitamin E combination in human prostate cancer
The ongoing Sel enium and Vitamin E C ancer Prevention T rial (SELECT) is designed to evaluate the efficacy of these two agents, either individually or in combination, on prostate cancer morbidity. Little information, however, is available on the potential benefit of combining these two agents from either in vitro or in vivo studies. The present project was designed to address this gap of knowledge. α-Tocopheryl succinate (α-TOS) and methylseleninic acid (MSA) are the agents of choice in all the experiments with the PC-3 human prostate cancer cell line. A synergistic effect on cell growth suppression was observed with the combination. The synergy was accounted for primarily by an augmented apoptotic response. Caspase activation profiling studies suggested that the mitochondrial pathway and the ER stress pathway are the targets of apoptosis signaling by α-TOS and MSA, respectively. The next step was to elucidate the agent-specific mechanisms associated with mitochondrial or ER stress signal transduction. Microarray analysis showed that three upstream components of the stress-activated MAP kinase (JNK) pathway are novel targets of α-TOS. A systematic examination of the Bcl-2 family proteins revealed that the up-regulation of JNK resulted in increased phosphorylation of Bcl-2 and Bim in a manner consistent with enhanced mitochondrial translocation of Bax and Bim. Collectively, these molecular changes might contribute to the stimulation of mitochondrial death signaling by α-TOS. The effect of MSA on induction of ER stress response was also methodically investigated. All three ER stress transducer pathways, PERK/eIF2α, ATF6, and IRE1/XBP1, were activated very rapidly, leading to induction of ER stress target genes. GRP78, an ER-resident chaperone and the master regulator of ER stress response, was knocked down by RNA interference in order to intensify ER stress. Among the three transducer pathways, ATF6 responded most robustly to the depletion of GRP78 in the presence of MSA. The effect of MSA on cell growth arrest was magnified by GRP78 knockdown; this was accompanied by altered gene expression of key cell cycle regulatory molecules. The above findings suggest a central role of ER stress in mediating the anticancer activity of selenium.