Sex differences in the pituitary transforming growth factor-ß1 system: studies in a model of resistant prolactinomas.

Dopamine and estradiol interact in the regulation of lactotroph cell proliferation and prolactin secretion. Ablation of the dopamine D2 receptor gene (Drd2(-/-)) in mice leads to a sexually dimorphic phenotype of hyperprolactinemia and pituitary hyperplasia, which is stronger in females. TGF-ß1 is a known inhibitor of lactotroph proliferation. TGF-ß1 is regulated by dopamine and estradiol, and it is usually down-regulated in prolactinoma experimental models. To understand the role of TGF-ß1 in the gender-specific development of prolactinomas in Drd2(-/-) mice, we compared the expression of different components of the pituitary TGF-ß1 system, including active cytokine content, latent TGF-ß-binding protein isoforms, and possible local TGF-ß1 activators, in males and females in this model. Furthermore, we evaluated the effects of dopamine and estradiol administration to elucidate their role in TGF-ß1 system regulation. The expression of active TGF-ß1, latent TGF-ß-binding protein isoforms, and several putative TGF-ß1 activators evaluated was higher in male than in female mouse pituitary glands. However, Drd2(-/-) female mice were more sensitive to the decrease in active TGF-ß1 content, as reflected by the down-regulation of TGF-ß1 target genes. Estrogen and dopamine caused differential regulation of several components of the TGF-ß1 system. In particular, we found sex- and genotype- dependent regulation of active TGF-ß1 content and a similar expression pattern for 2 of the putative TGF-ß1 activators, thrombospondin-1 and kallikrein-1, suggesting that these proteins could mediate TGF-ß1 activation elicited by dopamine and estradiol. Our results indicate that (1) the loss of dopaminergic tone affects the pituitary TGF-ß1 system more strongly in females than in males, (2) males express higher levels of pituitary TGF-ß1 system components including active cytokine, and (3) estradiol negatively controls most of the components of the system. Because TGF-ß1 inhibits lactotroph proliferation, we propose that the higher levels of the TGF-ß1 system in males could protect or delay the development of prolactinomas in Drd2(-/-) male mice.

Active and total transforming growth factor-ß1 are differentially regulated by dopamine and estradiol in the pituitary.

Dopamine, acting through the dopamine type 2 receptor (Drd2), is the main inhibitor of pituitary prolactin (PRL) secretion and lactotroph proliferation. TGF-ß1 is involved, at least in part, in mediating these actions. It was described that TGF-ß1 synthesis in rat pituitary lactotrophs is up-regulated by dopamine and down-regulated by estradiol. TGF-ß1 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-ß1 content, as well as TGF-ß1 mRNA, and the in vivo role of dopamine and estradiol on pituitary TGF-ß1 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-ß1 mRNA expression, in active and total cytokine content, and in TGF-ß type II receptor expression. Dopamine regulation of pituitary TGF-ß1 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-ß1. PRL secretion correlated with active, but not total cytokine. TGF-ß1 inhibitory action on lactotroph proliferation and PRL secretion was decreased in Drd2(-/-) pituitary cells, in correlation with decreased TGF-ß type II receptor. The study of the TGF-ß1 activation process and its regulation is essential to understand the cytokine activity. As an intermediary of dopamine inhibition of lactotroph function, TGF-ß1 and local activators may be important targets in the treatment of dopamine agonist-resistant prolactinomas.