INCREASE IN PIT-1 MESSENGER-RNA IS NOT REQUIRED FOR THE ESTROGEN-INDUCED EXPRESSION OF PROLACTIN GENE AND LACTOTROPH PROLIFERATION

Estrogen has been shown to stimulate lactotroph proliferation and expr ession of the prolactin (PRL) gene. Recently it has been established t hat Pit-1, a pituitary-specific transcription factor, is required for lactotroph proliferation. Furthermore, in vitro studies showed that an increase in the PRL promoter activity caused by estrogen was dependen t of the amount of cotransfected Pit-1-expressing plasmid. These findi ngs led us to examine whether the induction of Pit-1 mRNA is required for the estrogen-increased PRL gene expression in the rat anterior pit uitary in vivo. Short term estrogen treatment was achieved by means of a single intramuscular injection of estradiol dipropionate. DNA synth esis, the levels of PRL and Pit-1 messenger RNAs in the anterior pitui tary were determined. Estradiol dipropionate resulted in a significant increase in DNA synthesis 24 h after administration and in PRL mRNA a fter 48 h. In contrast, the Pit-1 mRNA level was not altered. Since Pi t-1 is expressed not only in lactotroph but also in somatotroph and th yrotroph, and the lactotroph cell population has been reported to be l ess than 10% in the pituitary, the change in the Pit-1 mRNA level in l actotrophs was not seen following only short term estrogen treatment. An increase in the lactotroph cell population was therefore achieved b y chronic estrogen treatment (subcutaneous implantation of a silastic tube containing 17 beta-estradiol powder for 30 days). This treatment resulted in the marked proliferation of lactotrophs and a 3-fold incre ase in PRL mRNA. However, no alteration in Pit-1 mRNA was observed. Th ese results suggest that the increase in Pit-1 mRNA is not required fo r the estrogen-induced lactotroph proliferation or PRL gene expression .