Effects of TGF-beta1 on prolactin synthesis and secretion: an in-vitro study.
The hypothalamus exerts a predominantly inhibitory influence on prolactin secretion through dopamine. In addition, the expression of anterior pituitary hormone-gene products are regulated by intrapituitary growth factors. In particular, TGF-beta1 produced in the pituitary regulates lactotroph cell proliferation and prolactin gene-expression. This study characterized the regulation of in-vitro prolactin synthesis and secretion by TGF-beta1 using rat anterior pituitary cells in monolayer culture. Furthermore, we studied the interaction of TGF-beta1 with other signals involved in the neuroregulation of prolactin secretion, such as dopamine and TRH, as well as the importance of different signal transduction pathways in this response. TGF-beta1 inhibited prolactin secretion in a time- and concentration-dependent manner, with half-maximal inhibition occurring at the range of 15-30 pM. The inhibitory effect was observed after 4 h, being maximal after 4 days of exposure of the cells to the peptide. This inhibitory effect was mimicked by TGF-beta2 but not by inhibin, and was not influenced by oestrogens, being similar in male, normal female or oestradiol-treated rats. Prolonged pretreatment of the cells with TGF-beta1(4 days) did not modify GH or TSH secretion nor dopamine-induced inhibition of prolactin secretion, and blunted prolactin responses to TRH, Forskolin, But2-cAMP and to the calcium ionophore A23187. The effect observed after long-term treatment (24 h to 4 days) is essentially caused by a decrease in prolactin synthesis, since TGF-beta1 inhibited prolactin mRNA levels and de novo prolactin protein synthesis. However, in the short term (up to 12 h) TGF-beta1 inhibition of prolactin secretion was associated with an increase in intracellular prolactin content, dissecting a dual mechanism of action of TGF-beta1. The short-term TGF-beta1 effect did not modify Erk-2 phosphorylation, basal or TRH-induced increase in intracellular calcium concentration, but blunted basal and forskolin stimulated cAMP levels. But2-cAMP replacement did not revert the inhibition of prolactin secretion. However, pertussis toxin was able to recover a large percentage of TGF-beta1-induced inhibition of prolactin secretion. This study indicates that TGF-beta1 plays a crucial role as a modulator of lactotroph function, inhibiting prolactin biosynthesis after long-term treatment, as well as, after short-term exposure prolactin secretion at the level of the secretory process, through a mechanism pertussis toxin sensitive but independent of Erk-2 phosphorylation, calcium concentrations or intracellular cAMP.
D000818:Animals; D000001:Calcimycin / Q000037:antagonists & inhibitors / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D005576:Colforsin / Q000037:antagonists & inhibitors / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D004298:Dopamine / Q000494:pharmacology; D004967:Estrogens / Q000494:pharmacology; D005260:Female; D013006:Growth Hormone / Q000378:metabolism; D007265:Inhibins / Q000494:pharmacology; D008297:Male; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism; D037342:Pertussis Toxin; D010766:Phosphorylation / Q000187:drug effects; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D011388:Prolactin / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013972:Thyrotropin / Q000378:metabolism; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology; D016212:Transforming Growth Factor beta / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D010566:Virulence Factors, Bordetella / Q000494:pharmacology