In addition to their well-known stimulatory action, estrogens have an anti-proliferative effect. The present study was undertaken to investigate the mechanism by which 17β-estradiol (E2) inhibits insulin-like growth factor-1 (IGF-1)-induced proliferation in vitro in the rat pituitary lactotroph, a typical estrogen-responsive cell. E2 treatment of pituitary cells did not change levels of IGF-1-induced phosphorylation of proliferation-related protein kinases such as Erk1/2 and Akt. We performed global gene expression profiling by DNA microarray analysis and identified 177 genes regulated by E2 in the presence of IGF-1. These results were verified by quantitative real time PCR. The estrogen-regulated genes included several NFκB family related genes. As pharmacological inhibition of the NFκB pathway blocked IGF-1-induced lactotroph proliferation, we chose to investigate whether one NFκB pathway gene, Bcl3, was involved in the anti-proliferative action of E2. RNA interference-mediated knockdown of Bcl3 expression attenuated IGF-1-induced lactotroph proliferation. Even minimal induced overexpression of Bcl3 blocked the anti-proliferative action of E2. In contrast, Nfkb2, another E2-downregulated protein, required maximal overexpression to block the anti-proliferative action of E2. These results suggest that inhibition of Bcl3 expression is involved in the anti-proliferative action of estrogens in pituitary lactotrophs in culture.
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