Reactive oxygen species (ROS) has been shown to mediate apoptosis signaling in many cell types. Even though selenium is a known anti-oxidant at a nutritional level of intake, we and others have shown that selenium compounds at levels intended for cancer chemoprevention and therapy induce apoptosis by caspase-dependent and independent pathways. We investigated here whether ROS played a role in apoptosis in LNCaP and DU145 prostate cancer cells induced by methylseleninic acid (MSeA) vs. sodium selenite. Methylseleninic acid is a synthetic precursor of the putative active anti-cancer metabolite methylselenol. Whereas MSeA induced caspase-mediated apoptosis in both cell lines, selenite induced apoptosis in the androgen-independent and p53-mutant DU145 cells in the absence of caspase-3 activity and cleavage of its canonical substrate poly (ADP ribose) polymerase. Furthermore, selenite induced rapid p53 serine-15 phosphorylation and caspase-activation in the androgen-responsive LNCaP cells. Flow cytometric detection of 2,7-dichlorofluorescein oxidation indicative of hydrogen peroxide and hydroxyl free radicals showed that selenite induced ROS generation in LNCaP cells within 3 hr, a time frame that coincided with p53 phosphorylation but preceded caspase activation. In DU145 cells, ROS was not increased within the first 3 hr of selenite treatment and was significantly higher at 12 hr, on pace with the onset of apoptosis. In both cell lines, selenite treatment increased the phosphorylation of c-Jun N-terminal kinase (JNK)-1/2 and a JNK inhibitor SP600125 attenuated the apoptosis responses. Contrary to selenite, MSeA exposure in both cell lines did not alter ROS status and did not increase the phosphorylation of JNK1/2. In summary, apoptosis signaling induced by MSeA in both prostate cancer cell lines did not appear to involve ROS and JNK activation. Selenite exposure increased ROS generation and JNK activation. The significance of ROS in relationship to JNK activation, p53 phosphorylation and caspase activation induced by selenium is currently being investigated, especially in the context of androgen signaling in prostate cancer cells. Grant supports: CA95642 from NCI and DAMD17-02-1-0007 from DOD.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]