ESTROGEN EFFECTS ON INSULIN-LIKE-GROWTH-FACTOR GENE-EXPRESSION IN A HUMAN OSTEOBLASTIC CELL-LINE WITH HIGH-LEVELS OF ESTROGEN-RECEPTOR

Insulin-like growth factors (IGF)-I and IGF-II are produced by osteobl asts and are important paracrine/ autocrine regulators of osteoblast p roliferation and differentiation. Estrogen has been reported to increa se gene expression of IGF-I in rodent osteoblasts, However, because sp ecies differences have been demonstrated in expression of various aspe cts of the IGF system in bone cells, it is nor known whether this acti on also occurs In human osteoblasts. Thus, we assessed the effects of estrogen treatment on IGF-I and IGF-II gene expression in vitro in a r ecently developed human fetal osteoblast cell line that has high level s of estrogen receptors, As assessed by a quantitative reverse transcr iptase-polymerase chain reaction method, treatment of kFOB/ER9 cells w ith 17 beta-estradiol (E-2) increased steady state levels of IGF-I mRN A in a time-and dose-dependent fashion with a maximal increase of 319% +/- 33% (P < 0.01) of control occurring after treatment with 10(-7) M E-2 for 48 hours, In contrast, E-2 did not alter steady state levels of IGF-II mRNA. The pure (type 2) antiestrogens ICI 182,780 (10(-7) M) and ICI 164,384 (10(-6) M) blocked the E-2-induced increase in IGF-I mRNA levels. Interestingly, 4-hydroxytamoxifen (10(-7) M), a documente d pure antiestrogen in reproductive tissues, also increased IGF-I mRNA to levels similar to those observed In E-2-treated cells. Since E-2 w as shown to mediate its effects on some target genes through a cAMP-de pendent pathway, we studied the interaction between E-2 and agents tha t are known to increase intracellular cAMP. Forskolin (10(-8) M) and d ibutyryl cAMP (10(-3) M) increased IGF-I mRNA levels sixfold, and cotr eatment with E-2 did not affect these changes, consistent with a possi ble mediation of the estrogen effect on IGF-I gene expression by cAMP. We conclude that in human osteoblastic cells, the IGF-I gene is a tar get Tor estrogen action, suggesting that IGF-I may mediate part of the effects of estrogen in human bone.