RETINOIC ACID INDUCES EXPRESSION OF THE TRANSCRIPTION FACTOR GHF-1/PIT-1 IN PITUITARY PROLACTIN-PRODUCING AND GROWTH-HORMONE-PRODUCING CELL-LINES

PRL and GH gene expression depend on the presence of the pituitary-specific transcription factor GHF-1/Pit-1. We have examined the effects of retinoic acid (RA) on the expression of the GHF-1/Pit-1 gene. RA induced a time- and dose-dependent increase in GHF-1/Pit-1 messenger RNA in the PRL-producing cell line 235-1. A maximal effect (a 2- to 3-fold increase) was obtained after 12-24 h of incubation with 1 mu M RA. The level of the transcription factor determined by both Western blotting and gel retardation analysis with a GHF-1/Pit-1-binding site was increased in RA-treated cells compared to that in control cells. Sequences located between -400 and -90 bp mediated a 2- to 3-fold activation of the GHF-1/Pit-1 promoter by RA. The retinoid also increased the response to cAMP and phorbol esters that is mediated by two cAMP-responsive elements (CREs) located in the same promoter fragment. Both CREs are required for RA induction, as deletion of either CRE abolished the response to the retinoid. RA also induced GHF-1/Pit-1 gene expression in GH(4)C(1) cells, which produce both PRL and GH. T-3 did not affect expression of the GHF-1/Pit-1 gene in 235-1 cells, but decreased basal GHF-1/Pit-1 messenger RNA and promoter activity in GH(4)C(1) cells and blocked the stimulatory effect of RA.