Dose-dependent effects of 4-hydroxytamoxifen, the active metabolite of tamoxifen, on estrogen receptor-α expression in the rat uterus

Tamoxifen, a selective estrogen receptor modulator, has agonist or antagonist activity, depending on the target tissue. The estrogen-like agonist effects of tamoxifen in the uterus are mediated primarily by 4-hydroxytamoxifen (4OH), the major active metabolite. Tamoxifen, 4OH and estradiol-17β (E2) all bind to estrogen receptors (ERα and ERβ), but with different affinities, suggesting that these ligands are capable of producing differential in vivo effects on the uterus. However, differences in short-term effects of tamoxifen, 4OH and E2 on the uterus have not been compared in the rat in vivo. Thus, we treated adult, ovariectomized rats (225–250 g) with vehicle (sesame oil), tamoxifen (1 mg/kg body weight), 4OH (0.01, 0.1 or 1.0 mg/kg body weight), E2 (40 μg/kg body weight), estradiol valerate (a long-lasting estrogen; 40 μg/kg body weight) or ICI 182,780 (a pure anti-estrogen; 1 mg/kg body weight). Animals were sacrificed at 0, 3, 6, 12 or 24 h post-injection, and protein and mRNA levels for ERα and two estrogen-regulated early response genes, c-fos and jun-B, were examined. Administration of E2 and 4OH (1 mg/kg body weight dose) resulted in down-regulation of uterine ERα protein in the uterine luminal and glandular epithelium by 6 h post-treatment. In contrast, no change in ERα level was observed after treatment with tamoxifen. Rapid (by 3 h) and transient increases in c-fos and jun-B mRNA levels were observed after E2 treatment; however, c-fos and jun-B induction by 4OH was highly dose dependent, and higher 4OH doses induced rapid but persistent proto-oncogene expression in vivo. Our results demonstrate that tamoxifen and its major metabolite have differential effects on uterine gene expression, and 4OH is highly estrogenic in the rat uterus.