Conversion of estrone to 2- and 4-hydroxyestrone by hamster kidney and liver microsomes: Implications for the mechanism of estrogen-induced carcinogenesis

As part of an ongoing investigation of the role of metabolic activation of estrogens in the genesis of cancers such as estrogen-induced renal tumors in hamsters, we have 1) determined steroid-17β-oxidoreductase activity of microsomes and cytosol prepared from hamster kidney and liver; 2) compared the rates of 2-, 4-, and 16α-hydroxylations of estrone by microsomes from hamster kidney and liver; and 3) determined the rates of inactivation of 2- and 4-hydroxyestrone by catechol-O-methyltransferase from hamster kidney and by purified enzyme. Microsomal steroid-17β-oxidoreductase activity in hamster kidney and liver was low and favored the conversion of estrone to estradiol. Cytosolic steroid-17β-oxidoreductase activity was only barely detectable in both liver and kidney. Using hepatic microsomes, the rate of 2- hydroxylation of estrone was comparable to that found previously using estradiol as substrate, whereas 4-hydroxylation of estrone was double that of estradiol. Using renal microsomes, the rates of 2- and 4-hydroxylation of estrone were 10- to 20-fold higher than those with estradiol as substrate, and the ratio of 2- to 4-hydroxylation was about 2:1. Fadrozole hydrochloride was an equally good inhibitor of rates of 2- and 4-hydroxylation of estrone (20 μM) by hepatic microsomes (IC₅₀, ~25 μM). Corresponding IC₅₀ values with renal microsomes were less than 2 μM, and 2 hydroxylation of estrone was inhibited by Fadrozole hydrochloride up to 15% more than 4- hydroxylation. Treatment of hamsters with estradiol for 2 months decreased rates of 2- and 4-hydroxylation of estrone by renal microsomes by approximately 95%. The rate of conversion of estrone to 16α-hydroxyestrone by hepatic microsomes was 10-20% that of 2-hydroxylation. Renal microsomes catalyzed 16α-hydroxylation of estrone at an even lower rate (~5% of that of 2-hydroxylation). Rates of O-methylation of 2- and 4-hydroxyestrone by hamster kidney cytosol were comparable to those of 2- and 4-hydroxyestradiol. In conclusion, conversion of estrone to its catechol metabolites by microsomes of hamster kidney, a target organ of estrogen-induced carcinogenesis, is quantitatively more important than the conversion to 16α- hydroxyestrone. The findings are consistent with the postulated role of catechol estrogens generated in situ in estrone-induced carcinogenesis.