IN-VIVO AND IN-VITRO REGULATION OF PITUITARY TRANSCRIPTION FACTOR-I (PIT-1) BY CHANGES IN THE HORMONE ENVIRONMENT

Pituitary transcription factor-1 (Pit-1 or GHF-1) is a transcription f actor specific to the anterior pituitary and is involved in the expres sion and regulation of the growth hormone (GH), prolactin (PRL) and th yroid-stimulating hormone (TSH) beta-subunit genes. The expression of these three genes can be modulated by changes in the hormone environme nt and it is thought that some of these effects are mediated through P it-1, but little is known about the physiological regulation of this t ranscription factor. Therefore, we first asked whether Pit-1 gene expr ession is modified as a result of changes in the in vivo gonadal stero id environment and if this could be correlated with changes in GH and/ or PRL mRNA levels. Secondly, we sought to determine if sex steroids a ffect the mRNA levels of these three peptides by acting at the level o f the pituitary and whether these effects are androgen or estrogen med iated. Finally, how sex steroids modulate the response of these three genes to the hypothalamic neuropeptides growth hormone-releasing hormo ne (GHRH) and somatostatin (SS) was analyzed. To this end, we compared Pit-1, GH and PRL mRNA levels in the anterior pituitary of intact, ca strated, and castrated testosterone-replaced adult male rats. In addit ion, primary cultures of adult male pituitaries were used to study the direct effects of both androgens and estrogens on Pit-1, GH, and PRL mRNA levels. In situ hybridization histochemistry was used to compare relative levels of Pit-1, GH and PRL mRNA. Densitometric analysis of t he in vivo studies showed that castration resulted in a 57, 40 and 55% decline in Pit-1, CH and PRL mRNA signal levels, respectively. Furthe rmore, replacement with testosterone (T) at the time of castration com pletely prevented the decline in all three mRNA species (ANOVA: Pit-1 mRNA, p < 0.0001: GH mRNA, p < 0.0001; PRL mRNA, p < 0.0001). In vivo, both T (10(-7) M) and estradiol (10(-9) M) were capable of stimulatin g Pit-1 mRNA and PRL mRNA levels, while dihydrotestosterone (DHT; 10(- 7) M) had no effect. There was no effect of any of these steroid treat ments on GH mRNA levels in vitro. Addition of GHRH to the cultures inc reased GH mRNA levels, as well as those of Pit-1 and PRL, and SS had t he opposite effect on GH mRNA levels. Whereas the GH response to GHRH was not significantly modified by exposure to sex steroids, the effect of SS was. The presence of sex steroids was capable of modifying the Pit-1 and PRL responses to both GHRH and SS. These results clearly ind icate that changes in circulating levels of sex steroids modulate the expression of Pit-1 in the anterior pituitary and that these changes c all be correlated with commensurate modifications in GH and PRL mRNA l evels. Furthermore, the effect on both Pit-1 and PRL mRNA levels occur s, at least in part, at the level of the anterior pituitary and is an estrogen-receptor-mediated event. In contrast, the effects of gonadal steroids on GH mRNA levels are less direct and are most likely mediate d at the level of the hypothalamus, as well as through modulation of t he response of the somatotroph to hypothalamic factors. We conclude th at the transcription factor Pit-1 is actively regulated physiologicall y and may be involved in mediating some of the effects of sex steroids and hypothalamic factors on the synthesis of certain anterior pituita ry hormones.