Dietary selenite and azadeoxycytidine treatments affect dimethylhydrazine-induced aberrant crypt formation in rat colon and DNA methylation in HT-29 cells

Several observations implicate a role for altered DNA methylation in cancer pathogenesis. The global level of DNA methylation is generally lower; however, DNA methyltransferase (Dnmt1) activity is usually higher in tumor cells than in normal cells. The purpose of this study was to investigate whether the Dnmt1 inhibitor, 5-aza-2'-deoxycytidine (aza-dC) would alter the effect of dietary selenium on the formation of aberrant crypts. Weanling rats (n = 60) were fed three concentrations of selenium (deficient, 0.1 and 2.0 mg/kg diet) in a Torula yeast-based diet. Half of the rats were injected weekly with aza-dC (1 mg/kg, subcutaneously) and half were injected with the vehicle control (PBS). After 3.5 wk of consuming the experimental diets, the rats were given two injections of dimethylhydrazine (DMH; 25 mg/kg, intraperitoneally). Rats fed the selenium-deficient diet and injected with PBS had significantly (P < 0.006) more aberrant crypts than rats fed 0.1 or 2.0 mg selenium/kg diet (244 +/- 21 vs. 165 +/- 9 and 132 +/- 14, respectively). In contrast, when rats were injected with aza-dC, there was a significant (P < 0.0001) reduction in aberrant crypt formation and dietary selenium had no effect (62 +/- 8 vs. 77 +/- 13 vs. 54 +/- 8, in rats fed 0, 0.1 and 2.0 mg selenium/kg diet, respectively). HT-29 cells cultured in the absence of selenium had significantly hypomethylated DNA but significantly more Dnmt1 protein expression than cells cultured in the presence of 1 or 2 mumol/L selenium. These results suggest that aza-dC treatment may protect selenium-deficient rats against carcinogen-induced aberrant crypt formation.