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Differential Sensitivity of Rat Uterine Growth and Epithelium Hypertrophy to Estrogens and Antiestrogens

Abstract Triphenylethylene antiestrogens are considered weak estrogen agonists based on their limited ability to induce estrogen responses, in particular uterine growth. We compared the uterotrophic activity of naturally occurring and synthetic estrogens with that of antiestrogens by quantitating uterine wet weight and hypertrophy in the uterine luminal and glandular epithelium. Immature rats received five daily injections of either an estrogen (17β-estradiol [E2], diethylstilbestrol [DES], or ethynyl estradiol [EE]) or an antiestrogen (tamoxifen [TAM], monohydroxytamoxifen [OH-TAM], or clom-iphene citrate [CC]) (0.001-100 μg/rat/day) subcutaneously in sesame oil and were sacrificed approximately 2 hr after the last injection. Both DES and EE increased uterine weight at doses between 0.01-100 μg/rat/day; E2 was about 10-fold less potent. The antiestrogens increased uterine weight only slightly. DES, EE, and the three antiestrogens each increased luminal epithelium hypertrophy to over 3-fold above that in controls. While the potencies of these synthetic compounds differed (DES = EE > OH-TAM > TAM = CC), each hypertrophic response occurred over two log doses, and the response curves displayed identical slopes. E2, however, required a range of four log doses to achieve the same degree of luminal epithelium hypertrophy. The three antiestrogens elicited glandular epithelium hypertrophy up to 2-fold above controls at the same doses that induced luminal epithelium hypertrophy; the order of potency was OH-TAM > TAM = CC. However, the three estrogens increased glandular epithelium hypertrophy only marginally. Thus, under dosing conditions commonly used to assess uterotrophic activity, these “antiestrogens” are complete, albeit less potent, estrogen agonists in the luminal epithelium and, unlike estrogens, induce hypertrophy in the glandular epithelium.