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Estrogen and Tamoxifen Interplay with T₃ in Male Rats: Pharmacologically Distinct Classes of Estrogen Responses Affecting Growth, Bone, and Lipid Metabolism, and Their Relation to Serum GH and IGF-I

Department of Pharmacology (J.M.F., C.A.P.) and Department of Radiology (R.M.K.), New York Medical College, Valhalla, New York 10595

Address all correspondence and requests for reprints to: C. Andrew Powers, Ph.D., Department of Pharmacology, New York Medical College, Valhalla, New York 10595. E-mail: andrew_powers{at}nymc.edu

Estrogen (E) and T₃ regulate gene expression by receptor mechanisms that may enable hormonal interplay affecting growth and metabolism. Prior studies of E and tamoxifen (TM) interplay with T₃ in female rats identified a subset of E responses that required T₃ for expression and exhibited large agonist responses to TM. In contrast, TM acted more like an antagonist in most T₃-independent E responses. This study used male rats to further explore the role of T₃ in E effects on growth and metabolism, and the relation of such effects to changes in serum GH and IGF-I. Orchidectomized, hypothyroid rats were treated 6 wk with vehicle, E2 benzoate (E2B), or TM with or without T₃. The following parameters were measured: body weight change; tibia length and bone mineral density; heart and kidney weight; food intake and body temperature; serum levels of glucose, cholesterol, triglycerides, GH, and IGF-I; seminal vesicle weight; and anterior pituitary levels of GH, PRL, glandular kallikrein, and total protein. Interplay with T₃ contributed to multiple E effects on growth and metabolism, and some E responses involved both T₃-dependent and T₃-independent components. Both E2B and TM increased serum GH, but the increases were poorly coupled to IGF-I. Correlation/regression analysis of individual rat data sets suggested distinct roles for GH and IGF-I in specific E effects. E2B and TM effects on somatic growth exhibited positive correlations with IGF-I and negative correlations with GH; effects on bone mineral density and triglycerides exhibited positive correlations with GH and negative correlations with IGF-I. Three pharmacologically distinct classes of in vivo E responses were identified in this study, and TM displayed a profile of biological activity that may be useful for men undergoing androgen-deprivation therapy.

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