Role of insulin-like growth factor-1 in regulating estrogen receptor-alphagene expression

The role of insulin-like growth factor-I (IGF-I) in regulating estrogen rec eptor-alpha (ER-alpha) gene expression and activity was investigated in the human breast cancer cell line MCF-7. Treatment of cells with 40 ng/ml IGF- I resulted in a 60% decrease in ER-alpha protein concentration by 3 h, and the amount of ER-alpha remained suppressed for 24 h. A multiple-dose ligand -binding assay demonstrated that the decrease in ER-alpha protein correspon ded to a similar decrease of 50% in estradiol-binding sites with no effect on the binding affinity of ER-alpha. The dissociation constant of the estra diol-ER-ce complex in the absence of ICF-I (K-d = 3 X 10(-10) +/- 0.5 X 10( -10) M) was Similar to the dissociation constant in the presence of ICF-I ( K-d = 6 X 10(-10) +/- 0.3 X 10(-10) M). The decrease in ER-alpha protein co ncentration was paralleled by an 80% decrease in the steady-state amount of ER-alpha mRNA by 3 h. The IGF-I induced decrease in ER-cr mRNA was due to the inhibition of ER-alpha gene transcription. When an 128-base pair ER-alp ha-promoter-CAT construct was transfected into MCF-7 cells, treatment with IGF-I resulted in a 40% decrease in CAT activity. In contrast to the effect s on ER-alpha, treatment with IGF-I induced two endogenous estrogen-regulat ed genes, progesterone receptor and pS2, by 4- and twofold, respectively. T he pure antiestrogen ICI-164,384 blocked this induction, suggesting that ER -alpha mediates the effects of IGF-Il. Transient co-transfections of wild-t ype ER-alpha and an estrogen response element-CAT reporter into COS-1 cells demonstrated that IGF-1 increased reporter gene activity. This effect was also blocked by ICI 164,384. Protein kinase A and phosphatidylinositol 3-ki nase inhibitors blocked the IGF-I effects on ER-alpha expression and activi ty, suggesting that these kinases may be involved in the cross-talk between the IGF-I and ER-alpha pathways. (C) 2000 Wiley-Liss, Inc.