DOPAMINE RECEPTOR ACTIVATION INHIBITS ESTROGEN-STIMULATED TRANSFORMING GROWTH FACTOR-ALPHA GENE-EXPRESSION AND GROWTH IN ANTERIOR-PITUITARY, BUT NOT IN UTERUS

Transforming growth factor-alpha (TGF-alpha) has been localized to the anterior pituitary, specifically to the lactotroph and somatotroph cell populations, by our previous studies. Since pituitary lactotrophs are known to undergo growth in response to estrogens, we have used an estradiol-induced pituitary hyperplasia/adenoma model. Estradiol treatment resulted in induction of TGF-alpha mRNA in anterior pituitary, evident by 48 h, preceding actual macroscopic growth, which attained a maximum greater than 500% by 12 weeks. This rapid effect of estradiol also enhanced PRL mRNA, but did not affect other species of mRNA encoding for proenkephalin, D2 receptor mRNA, or hexosaminidase-A. TGF-alpha mRNA remained elevated for the duration of rapid pituitary growth. D2 receptor activation by its agonist bromocriptine resulted in marked attenuation of TGF-alpha mRNA preceding regression of growth. Coadministration of bromocriptine with estradiol resulted in an involution of pituitary size, indicating the overriding influence of dopamine in spite of a continued estrogenic stimulus. Epidermal growth factor receptor mRNA was not affected by any of these manipulations, suggesting that the receptor was not coregulated in this tissue similarly to TGF-alpha. Estradiol also induced uterine TGF-alpha mRNA and marked growth of the organ, but TGF-alpha in this location was not regulated by dopamine. These results indicate that TGF-alpha in the anterior pituitary is rapidly induced by estrogen in a time course preceding the growth of the gland. Estrogen-induced TGF-alpha is rapidly attenuated by D2 dopamine receptor activation and is accompanied by a regression of pituitary growth. Interaction between these opposing hormonal/transmitter responses will determine the growth potential of the anterior pituitary.