Nitric oxide stimulates embryonic somatotroph differentiation and growth hormone mRNA and protein expression through a cyclic guanosine monophosphate-independent mechanism

Renewing an old interest: Pituitary folliculostellate cells

Anterior pituitary folliculostellate (FS) cells, first described almost 50 years ago, have a wide range of functions with respect to supporting and coordinating endocrine cell function, in particular through paracrine and gap junction-mediated signalling. Our previous studies identified the morphological organisation of FS cells, which mediates coordinated calcium activity throughout the homotypic FS network and allows signalling across the whole pituitary gland. It is also clear that FS cells can modify endocrine output and feedback on pituitary axes over a range of timescales. Recently, several studies have defined FS cells as a source of anterior pituitary endocrine cell renewal, which has resulted in a renaming of FS cells as "Sox2+ve stem cells". Here, we highlight the broader potential of the FS cell population in fine-tuning and coordinating pituitary axes function. In addition, we identify a need for: the definition of the possible subtypes of FS cell and their relationship with the stem cell population; the potential role of FS cells in pulsatile hormone secretion and coordination of heterotypic cell networks; and the roles that FS cells may play in both early-life programming of pituitary axes and in memory, or anticipation, of demand. Further studies of FS cells may demonstrate the fundamental importance of this cell type and its potential as a therapeutic target to correct pituitary gland dysfunction, one of which is stem cell therapy. Clearly, a thorough understanding of all of these interactions and relationships of FS and endocrine cells is required whatever therapeutic use is suggested by their various roles.

Nitric oxide inhibits ghrelin-induced cell proliferation and ERK1/2 activation in GH3 cells

Ghrelin stimulates growth hormone release and cell proliferation, which strongly supports a significant role for this peptide in the control of growth hormone-releasing adenomas function and growth. Nitric oxide can influence the stimulatory effects of ghrelin on growth hormone secretion in growth hormone-releasing adenomas. However, the effect of nitric oxide (NO) on ghrelin-induced cell proliferation and the mechanism of this effect in the adenoma were not clarified. In this study, we observed that ghrelin, at a concentration of 10⁻⁹ to 10⁻⁶ M, significantly increased BrdU incorporation into rat GH3 cells. A NO donor, S-nitroso-N-acetylpenicillamine (SNAP), blunted basal, and ghrelin-induced cell proliferation. A blocker of NO synthase, Nw-nitro-L-arginine methyl ester hydrochloride (NAME), had no influence on these actions. The activation of extracellular signal-regulated kinase (ERK) 1/2 was examined by western blotting. The results showed that SNAP reduced ghrelin-stimulated ERK1/2 activation but NAME had no influence on this activation. Together, this study indicates that NO inhibited ghrelin-induced cell proliferation by blocking ERK1/2 activation in GH3 cells.

Sustained gonadotropin-releasing hormone stimulation mobilizes the cAMP/PKA pathway to induce nitric oxide synthase type 1 expression in rat pituitary cells in vitro and in vivo at proestrus

Previous in vivo studies have established that pituitary nitric oxide synthase type 1 (NOS1) is regulated by gonadotropin-releasing hormone (GnRH). The aim of our study was to elucidate the mechanisms of NOS1 regulation by GnRH in rat pituitary cells. Using a perifused cell system, we demonstrated that NOS1 induction was sensitive to GnRH pulse frequency and was maximally induced under continuous GnRH stimulation. In primary cultures of rat pituitary cells, sustained stimulation with the GnRH agonist triptorelin (GnRHa) increased NOS1 protein levels, whereas NOS2 and NOS3 levels were unaffected. NOS1 up-regulation occurred in gonadotroph cells only, in a time-dependent and concentration-dependent manner (maximum increase, 2.5-fold; half-maximal concentration, 0.17 nM). GnRHa effect was mimicked by cAMP pathway activators and, most importantly, was blocked by disruption of the protein kinase A (PKA) pathway using pharmacological inhibitors such as Rp-cAMP or drug phosphatase technology-protein kinase inhibitor (DPT-PKI), a cell-permeant PKI peptide. In contrast, modulation of the PKC pathway and inhibition of the MAPK cascade were ineffective. Overall, these experiments demonstrated that GnRH-induced up-regulation of pituitary NOS1 is mediated notably by the cAMP/PKA pathway. Last, in vivo administration of a GnRH antagonist markedly inhibited the pituitary cAMP rise at proestrus in addition to suppressing NOS1 increase. Altogether, our data suggest that the cAMP/PKA signaling pathway is preferentially recruited under sustained GnRH stimulation in vivo during proestrus, allowing the expression of a specific set of PKA-regulated proteins, including NOS1, in gonadotroph cells.