Phase II enzyme expression in rat liver in response to the antiestrogen tamoxifen

The genotoxicity and carcinogenicity of tamoxifen have been attributed to metabolic activation of tamoxifen to an electrophile. Phase II enzymes are known to be involved in the metabolism of the drug and possibly in the formation or elimination of the active metabolite. To determine the effects of tamoxifen on phase II enzyme expression, the drug was administered to F344 rats, and hepatic glutathione S-transferase (GST), UDP-glucuronosyltransferase (UGT), and sulfotransferase (ST) expression was evaluated. Some of the tamoxifen-induced effects, including dramatic suppression of selected GST enzymes and activity, were observed at a dose in rats that is directly equivalent, on a mg/kg b.w. basis, to the doses used for breast cancer treatment. Most of the observed responses are not consistent with the previously described phenobarbital-like properties of tamoxifen and could be the result of the partial agonist activity of tamoxifen at the estrogen receptor. Northern blot analysis was performed with isozyme-specific oligonucleotide probes for rat GST, ST, and UGT. In addition, GST subunit protein levels were assayed by high-performance liquid chromatography. In females, tamoxifen treatment resulted in a 60% suppression of GST Ya1 mRNA and protein levels and a 40% suppression of GST Ya2 levels. In males, tamoxifen treatment suppressed GST Ya1 expression approximately 60%, and GST Ya2 expression was suppressed at low doses but induced above control at high doses. Male GST Yc1 was induced approximately 80% over control. The expression of all other major forms of rat hepatic GST subunit protein, including GST Yb1, Yb2, Yb3, Yp, and Yl, was unaffected by tamoxifen treatment. GST conjugation activity toward delta 5-androstene-3,17-dione, a GST Ya1- and Ya2-specific substrate, was suppressed approximately 40% in both sexes, consistent with our protein and mRNA data. Total GST activity, as measured by the rate of chlorodinitrobenzene conjugation, was not changed. Tamoxifen also produced a dose-dependent increase in UGT2B1 mRNA, a phenobarbital-inducible enzyme; mRNA levels reached 210 and 420% of control in females and males, respectively. In addition, mRNA levels for ST2A2, a female-specific ST gene, were suppressed 50% in females and induced 120% over control in males. mRNA expression for all other forms of rat liver UGT and ST isozymes that were tested was not significantly affected by tamoxifen treatment. Overall, these results demonstrate that tamoxifen has significant effects on hepatic phase II enzyme expression that may have implications for the carcinogenicity and/or therapeutic activity of the drug.