GABP Mediates Insulin-increased Prolactin Gene Transcription

The insulin-response element from the prolactin gene is identical to the Ets-binding site, and dominant-negative Ets protein inhibits insulin-increased prolactin gene expression. Immunoblotting identified the Ets-related transcription factor GABP in nuclear extracts from GH cells. Expression of GABPα and GABPβ1 squelches insulin-increased prolactin gene expression. GABPα and GABPβ1 bind the insulin-response element of the prolactin promoter, and anti-GABPα and anti-GABPβ1 antibodies supershift a species seen with nuclear extracts from GH cells. GABPα immunoprecipitated from insulin-treated, P-labeled GH cells was phosphorylated 3-fold more than GABPα from control cells. There was no increase in phosphorylation of GABPβ in response to insulin. Mitogen-activated protein (MAP) kinase activity is increased 10-fold in insulin-treated GH4 cells. MAP kinase immunoprecipitated from control cells does not phosphorylate GABPα while MAP kinase immunoprecipitated from insulin-treated cells shows substantial phosphorylation of GABPα. These studies suggest that GABP mediates insulin-increased transcription of the prolactin gene. GABP may be regulated by MAP kinase phosphorylation.