Unraveling the role of membrane progesterone receptor α (mPRα) in the lactotroph population

DÍAZ-TORGA, GRACIELA; FARAONI, ERIKA Y.; FERRARIS, JIMENA; THOMAS, PETER; CONVERSE, AUBREY; CAMILLETTI, M. ANDREA; PISERA, DANIEL; ABELEDO-MACHADO, ALEJANDRA

Chicago

Congreso; ENDO 2018; 2018

Endocrine Society

The membrane progesterone receptors (mPRα, -β, -γ, -δ, -ε) belong to the Progestin and AdipoQ receptor family (PAQR) and are known to mediate rapid non-genomic progesterone functions in different cell types. However, the functions of these receptors in the pituitary have not been reported to date. Previous research from our group demonstrated that the relative expression of mPRα mRNA was the highest among all subtypes in the anterior pituitary of female Sprague Dawley (SD) rats. Then, the role of mPRα in the pituitary was investigated. Immunostaining of mPRα was detected in rat pituitary sections by fluorescent immunohistochemistry in somatotrophs and gonadotrophs but lactotrophs showed the greatest expression. By flow cytometry, we demonstrated that 63.0 ± 4.9% of the mPRα-positive cells were also PRL-positive cells. With mPRα being primarily expressed in lactotrophs, we hypothesized the involvement of this receptor in controlling PRL secretion. To test this hypothesis, pituitaries from SD rats were incubated (1h) with progesterone (P4) or with mPRα specific agonist Org OD 02-0 (02) and PRL secretion was measured by RIA at the end of the treatment period. Interestingly, both P4 and 02 decreased PRL secretion in the collected medium and increased the PRL content in the pituitary. Accordingly, using the GH3 cell line (CCL-82.1?) we showed that P4 and 02 inhibited PRL release, whereas the nuclear PR agonist R5020 was ineffective. To corroborate the specificity of P4 and 02 binding sites in this cell line, a single-point competition assay in presence of [3H]-P4 was performed. P4 and 02 but not R5020 significantly displaced [3H]-P4 binding to the plasma membranes of GH3 cells. Finally, the cellular mechanisms behind mPRα action were investigated. We found that P4 and 02 decreased cAMP accumulation and increased ERK phosphorylation, while R5020 did not. In addition, 02-effect on PRL release was blocked by pretreatment with pertussis toxin, an inhibitor of G0/Gi proteins. Furthermore, since it has been shown that TGFβ1 acts as a potent inhibitor of PRL secretion in lactotrophs, we evaluated if TGFβ1 was activated by progesterone and whether this effect was mediated by mPRα. Our results showed that (1) P4 and 02, but not R5020, increased active TGFβ1 levels in GH3 cells after 15 min of treatment; and (2) the 02-induced effect was completely abolished when cells were transfected with mPRα-siRNA (ON-TARGET plus Rat Paqr7 siRNA). Taking together, these data provide the first evidence of mPRα function in the lactotroph population. This rapid progesterone action in the pituitary seems to be associated with decreased cAMP levels and increased ERK phosphorylation, suggesting a role of a mPR-coupled G0/Gi protein signaling pathway. Also, mPRα triggers the activation of TGFβ1 providing a possible mechanism by which progesterone can decrease PRL secretion.