The protective effect of estrogen against chemically induced murine colon carcinogenesis is associated with decreased CpG island methylation and increased mRNA and protein expression of the colonic vitamin D receptor

Epidemiological studies suggest that estrogen prevents neoplastic transform ation in the intestinal mucose. Estrogen was shown to increase the expressi on of vitamin D receptors (VDR) in a variety of tissues. 1,25-Dihydroxyvita min D [1,25-(OH)(2)D] and several of its analogues are known as potent anti neoplastic and prodifferentiative in many cell types, including colon-deriv ed cells. The present study was designed to examine the effect of estradiol (E-2) on dimethylhydrazine (DMH)-induced colon cancer in rats, and the pos sibility that E-2 may exert its protective effect on the colon through modu lation of the vitamin D-endocrine system. The in vivo effect of E-2 on DMH- induced colorectal cancer was studied in four groups of ovariectomized fema le rats: (I) untreated control, (II) E-2 treated, (III) DMH treated, and (I V) combined E-2 and DMH treated. Significantly higher uterine weights and h igher colonic estrogen receptor content confirmed the effectiveness of ovar iectomy and E-2 replacement. The number of malignant tumors in group IV was 2.3 +/- 1.1 (mean +/- SE) per rat, compared with 8.1 +/- 1.9 in group III (P < 0.001). Exposure to estrogen was associated with a marked increase in VDR mRNA content and VDR protein expression in the normal colonic mucosa. I n tumor extracts VDR protein expression was considerably lower compared wit h normal mucosa. Estrogen treatment did not affect serum levels of 25(OH)D, 1,25(OH)(2)D, and PTH. Significant CpG island methylation in the VDR gene was observed in colonic tissue DNA harvested from rats treated with DMH, bu t not in colonic mucosae from rats treated with DMH + E-2. The highest freq uency of CpG methylation in the VDR gene was detected in DNA extracted from cancer tissue rims. In summary, the protective effect of estrogen against chemically induced colonic carcinogenesis is associated with reduced methyl ation of the VDR gene and with upregulation of both VDR gene transcription and protein expression. We suggest that estrogen may interfere with the pro cess of CpG DNA methylation in the colonic mucosa to prevent silencing of t he VDR gene. Increased VDR activity could be one of the mechanisms by which estrogen protects against neoplastic transformation in the colon.