We have recently shown that uterine insulin-like growth factor I (IGF-I) gene expression is up-regulated by tamoxifen, a uterotrophic partial antagonist to the estrogen receptor, but down-regulated by the complete estrogen receptor antagonist ICI 182780, which causes uterine involution. This result is consistent with prior reports indicating that the uterotrophic effects of estradiol are mediated at least in part by estradiolstimulated uterine IGF-I gene expression. We demonstrate here that the uterotrophic agents estradiol and tamoxifen each suppress expression of the IGF binding protein 3 (IGFBP-3) gene in uterus to less than one-third of control values, while oophorectomy or administration of the complete estrogen receptor antagonist ICI 182780, both of which result in uterine involution, are associated with a greater than 3-fold stimulation of uterine IGFBP-3 gene expression. The data reveal a negative correlation between uterine weight and uterine IGFBP-3 gene expression as well as reciprocal regulation by estradiol of expression in uterus of the genes encoding IGF-I and IGFBP-3. In vitro, IGFBP-3 protein accumulation in media conditioned by primary uterine cultures was decreased by estradiol treatment and increased by ICI 182780 treatment. Together, these observations provide a novel mechanism by which estradiol and antiestrogens modulate uterine IGF-I physiology that is consistent with the view that the mitogenic activity of IGF-I is reduced in the presence of IGFBP-3. The uterotrophic toxicity of chronic estradiol or tamoxifen treatment may be causally related to both the inhibition of uterine IGFBP-3 expression and the stimulation of uterine IGF-I expression by these compounds.

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Supported by grants from the Riesman Family Foundation and the National Cancer Institute of Canada (to M. P.).

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