While there is an increasing interest in
selenium chemoprevention against human colon
polyp recurrence and other
cancers, the mechanism(s) by which these agents inhibit
carcinogenesis are uncertain. Some of the proposed mechanisms include the inhibition of
cytosine methyltransferases,
carcinogen bioactivation, and inhibition of
cyclooxygenase (COX). More recently, it has been suggested that
selenium may exert growth inhibitory effects by activating p53. However, the molecular mechanisms of action of
selenomethionine, an organoselenium compound present in selenized yeast and currently being investigated in human clinical trials for colon
polyp prevention, are unclear. In the present study we tested the hypothesis that
selenomethionine might affect
colon cancer cell growth by p53 mediated apoptosis and/or cell cycle regulation. Four human
colon cancer cell lines including HCT116 and RKO (wild type p53), HCT116-p53KO (isogenic control of HCT116 cells with p53 knocked out) and Caco-2 (mutant p53) were treated with 0-100 microM of
selenomethionine for 24, 48 and 72 h. Cell viability rates were determined by the MTT assay. Cell cycle analysis was performed by flow cytometry and apoptosis measured by
Annexin V-Cy5 staining. Expression of p53
protein was determined by Western blotting and immunofluorescence assays. All cell lines showed concentration and time dependent growth inhibition with
selenomethionine, although HCT116 and RKO cells were the most sensitive to such treatments. Interestingly, although HCT116 and HCT116-p53KO are isogenic cell lines,
selenomethionine caused a G2/M cell cycle arrest in HCT116 and RKO cells, but not in HCT116-p53KO cells. Similarly, both HCT116 and RKO demonstrated a significant increase in apoptosis (100-170%; p < 0.01) with 50-100 microM
selenomethionine. Cell cycle arrest and apoptosis observed in HCT116 and RKO cell lines were accompanied by a marked increase in p53
protein expression following
selenium treatment. These results clearly suggest that
selenomethionine exerts p53 dependent growth inhibitory effects in
colon cancer cells by inducing G2/M cell cycle arrest as well as apoptosis.