Modulation by celecoxib and difluoromethylornithine of the methylation of DNA and the estrogen receptor-alpha gene in rat colon tumors.

Abstract

The ability of celecoxib and alpha-difluoromethylornithine (DFMO) to modulate the DNA hypomethylation and the hypermethylation of the estrogen receptor (ER)-alpha gene in colon tumors was evaluated as potential biomarkers for chemoprevention. Colon tumors were induced in rats by azoxymethane. Celecoxib (500 mg/kg), DFMO (100, 1000 and 3000 mg/kg) or celecoxib + 1000 mg/kg DFMO were administered in the diet for 7 or 28 days prior to death at week 37. Relative to the normal colon mucosa, colon tumors contained global hypomethylated DNA but simultaneous hypermethylation of the promoter plus exon-1 region of the ER-alpha gene. Limited treatment with celecoxib (500 p.p.m. in diet) or DFMO (1000 or 3000 p.p.m. in diet) reversed the DNA hypomethylation. Administering 1000 and 3000 p.p.m. DFMO for 7-days decreased the number of methylated CpG sites in the ER-alpha gene from 5.00 +/- 0.95 to 3.83 +/- 0.75 and 1.75 +/- 0.49 these levels were further reduced to 0.50 +/- 0.26 following administration of 1000 mg/kg for 28 days. Celecoxib administered for 7 and 28 days reduced the number of methylated sites to 4.25 +/- 0.48 and 1.5 +/- 0.50. The combination containing celecoxib and DFMO reduced the number of methylated sites to 0.20 +/- 0.20 at both 7 and 28 days. In parallel with the hypermethylation of the ER-alpha gene, the mRNA expression of the gene was decreased in colon tumors and was increased by celecoxib, DFMO or the combination. Celecoxib and DFMO reversed DNA hypomethylation and the hypermethylation of the ER-alpha gene in colon tumors supporting the hypothesis that modulation of methylation is a biomarker of chemoprevention.