Estrogen modulation of prolactin gene expression requires an intact mitogen-activated protein kinase signal transduction pathway in cultured rat pituitary cells

Expression of the PRL gene is regulated by many factors, including cAMP, es tradiol (E-2), phorbol esters, epidermal growth factor (EGF), and TRH. The promoter region of the rat PRL gene has been shown to contain DNA sequences that are thought to support the direct interaction of estrogen receptors ( ERs) with DNA. It is by this direct ER/DNA interaction that estrogen is tho ught to modulate expression of PRL. We report here that estrogen-induced PR L expression requires an intact mitogen-activated protein kinase (MAPK) sig nal transduction pathway in cultured rat pituitary cells (PR1 lactotroph an d GH3 somatolactotroph cell lines). Interfering with the MARK signaling cas cade by inhibiting the activity of MARK kinase (MEK) ablates the ability of estrogen to induce PRL mRNA and protein. In these cell lines, estrogen act ivates extracellular regulated protein kinases ERK-1 and ERK-2 enzyme activ ities maximally within 10 min of 1 nM E-2 treatment. This activity is block ed by pretreatment of the cells with the MEK inhibitors PD98059 and UO126. The mechanism by which ERKs-1 and -2 are activated by estrogen appears to b e independent of c-Src since the effects of estrogen on PRL gene expression are not affected by herbimycin A or PP1 administration. c-Raf-1 may be inv olved in the effects of E-2 because estrogen causes the rapid and transient tyrosine phosphorylation of c-Raf-1. The ER antagonist ICI 182,780 blocks both ERK-1 and ERK-2 activation in addition to PRL protein and mRNA, implyi ng a central role for the classical ER in the activation of the MARK pathwa y resulting in PRL gene expression.