Active and Total Transforming Growth Factor-β1 Are Differentially Regulated by Dopamine and Estradiol in the Pituitary

Abstract

Dopamine, acting through the dopamine type 2 receptor (Drd2), is the main inhibitor of pituitary prolactin (PRL) secretion and lactotroph proliferation. TGF-β1 is involved, at least in part, in mediating these actions. It was described that TGF-β1 synthesis in rat pituitary lactotrophs is up-regulated by dopamine and down-regulated by estradiol. TGF-β1 is secreted as a large latent complex. The local regulation of cytokine activation in the pituitary has not yet been explored. In this work, we studied pituitary active and total TGF-β1 content, as well as TGF-β1 mRNA, and the in vivo role of dopamine and estradiol on pituitary TGF-β1 levels. Adult female mice (wild type), and female mice with a null mutation in the Drd2 (Drd2(-/-)), were used. The loss of dopaminergic tone induced a decrease in TGF-β1 mRNA expression, in active and total cytokine content, and in TGF-β type II receptor expression. Dopamine regulation of pituitary TGF-β1 activation process was inferred by the inhibition of active cytokine by in vivo sulpiride treatment. Interestingly, in the absence of dopaminergic tone, estradiol induced a strong increase in active TGF-β1. PRL secretion correlated with active, but not total cytokine. TGF-β1 inhibitory action on lactotroph proliferation and PRL secretion was decreased in Drd2(-/-) pituitary cells, in correlation with decreased TGF-β type II receptor. The study of the TGF-β1 activation process and its regulation is essential to understand the cytokine activity. As an intermediary of dopamine inhibition of lactotroph function, TGF-β1 and local activators may be important targets in the treatment of dopamine agonist-resistant prolactinomas.