JAK2/STAT5 Pathway Mediates Prolactin-Induced Apoptosis of Lactotropes

Prolactinomas are increasingly viewed as a “problem of signal transduction.” Consequently, the identification of factors and signaling pathways that control lactotrope cell turnover is needed in order to encourage new therapeutic developments. We have previously shown that prolactin (PRL) acts as a proapoptotic and antiproliferative factor on lactotropes, maintaining anterior pituitary cell homeostasis, which contrasts with the classical antiapoptotic and/or proliferative actions exerted by PRL in most other target tissues. We aimed to investigate the PRLR-triggered signaling pathways mediating these nonclassical effects of PRL in the pituitary. Our results suggest that (i) the PRLR/Jak2/STAT5 pathway is constitutively active in GH3 cells and contributes to PRL-induced apoptosis by increasing the Bax/Bcl-2 ratio, (ii) PRL inhibits ERK1/2 and Akt phosphorylation, thereby contributing to its proapoptotic effect, and (iii) the PI3K/Akt pathway participates in the PRL-mediated control of lactotrope proliferation. We hypothesize that the alteration of PRL actions in lactotrope homeostasis due to the dysregulation of any of the mechanisms of actions described above may contribute to the pathogenesis of prolactinomas.

The Pulsatility of ACTH Secretion in the Rat Anterior Pituitary Cell Perifusion System

Aims: This study aimed to examine the physiological mechanism whereby the corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) exert their influence on adrenocorticotropic hormone (ACTH) secretion in pituitary cells. Methods: Anterior pituitary cells were harvested from male rats and placed in the perifusion system. Cells were perifused with serum-free medium for 6 hours before fraction collection. After 30-minute of baseline collection, perifusion media was changed to expose the cells to CRH with or without AVP. ACTH concentration in each fraction was measured using enzyme immunoassay chemiluminescent kit. Results: The lowest physiological concentration of CRH (10 pM) or AVP (10 pM) was not able to induce a marked increase in ACTH secretion. Higher concentration of CRH (30 pM) or AVP (100 pM) in the physiological range caused sustained elevation of ACTH secretion (P < 0.001), while the secretion remained at similar levels for up to 1 hour with continuous stimulation. Perifusion with 10 pM AVP and 10 pM or 30 pM CRH caused a 2.38-fold and 2.99-fold increase in pulsatile ACTH secretion in pituitary cells, respectively. The duration of pulsatility caused by perifusion with 10 pM AVP and 30 pM CRH was close to that observed under physiological condition. Conclusions: By using the rat anterior pituitary cell perifusion system, we found that CRH and AVP potentiate the effect of each other on ACTH secretion, but AVP was a less potent agonist than CRH. The data suggest that CRH and AVP are essential for the pulsatility of ACTH, and AVP acts like a switch of the pulsatility.