Age and sex differences in the incidence of colonic cancer, together with epidemiological data on patients taking hormone replacement therapy, suggest the involvement of estrogens. Analogous to the role of aromatase in breast cancer, we postulated that steroid metabolism within the colon itself may be a crucial mechanism in regulating tissue exposure to estrogens. We have characterized expression of aromatase (responsible for converting C19 androgens to C18 estrogens) and 17beta-hydroxysteroid dehydrogenase (17beta-HSD) [responsible for interconversion of active estradiol (E2) to less potent estrone (E1)] in normal and neoplastic human colon from 24 patients undergoing tumor resection. Aromatase activity was similar in homogenates from normal mucosa, tissue adjacent to tumors, and the tumors themselves. Analysis of 17beta-HSD activity indicated that the predominant activity was oxidative (E2 to E1), and this conversion was significantly lower in colonic tumors [444 (90-1735); median (95% confidence interval) pmol/mg protein x h], compared with normal mucosa [1709 (415-13828), P < 0.001]. Northern blot analyses indicated expression of messenger RNAs (mRNAs) for the type 2 and 4 isozymes of 17beta-HSD in normal colon; messenger RNA for 17beta-HSD 4 was significantly lower in tumor tissue [0.75 +/- 0.22 (mean +/- SD) arbitrary U vs. 0.43 +/- 0.17, P < 0.01]. Studies in vitro, using three colonic cancer cell lines, indicated that there was an inverse correlation between 17beta-HSD oxidative activity and the rate of cell proliferation. In addition, E1, but not E2, was shown to significantly decrease proliferation when added exogenously to the colonic epithelial cell line, SW620 cells. Colonic mucosa can regulate estrogen hormone action in an intracrine fashion. The loss of estrogen inactivation may be an important mechanism in the pathogenesis of colonic cancer.