| Popis: |
Epidemiological studies suggest that estrogen prevents neoplastic transformation in the intestinal mucosa. Estrogen was shown to increase the expression of vitamin D receptors (VDR) in a variety of tissues. 1,25-Dihydroxyvitamin D [1,25-(OH)2D] and several of its analogues are known as potent antineoplastic and prodifferentiative in many cell types, including colon-derived cells. The present study was designed to examine the effect of estradiol (E2) on dimethylhydrazine (DMH)-induced colon cancer in rats, and the possibility that E2 may exert its protective effect on the colon through modulation of the vitamin D-endocrine system. The in vivo effect of E2 on DMH-induced colorectal cancer was studied in four groups of ovariectomized female rats: (I) untreated control, (II) E2 treated, (III) DMH treated, and (IV) combined E2 and DMH treated. Significantly higher uterine weights and higher colonic estrogen receptor content confirmed the effectiveness of ovariectomy and E2 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 (P0.001). Exposure to estrogen was associated with a marked increase in VDR mRNA content and VDR protein expression in the normal colonic mucosa. In tumor extracts VDR protein expression was considerably lower compared with normal mucosa. Estrogen treatment did not affect serum levels of 25(OH)D, 1,25(OH)2D, and PTH. Significant CpG island methylation in the VDR gene was observed in colonic tissue DNA harvested from rats treated with DMH, but not in colonic mucosae from rats treated with DMH + E2. The highest frequency 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 methylation of the VDR gene and with upregulation of both VDR gene transcription and protein expression. We suggest that estrogen may interfere with the process of CpG DNA methylation in the colonic mucosa to prevent silencing of the VDR gene. Increased VDR activity could be one of the mechanisms by which estrogen protects against neoplastic transformation in the colon. |
