On the involvement of cyclic AMP and extracellular Ca2+ in the regulation of hormone release from rat pituitary tumour (GH3) cells in culture

Involvement of mitogen-activated protein kinase in cyclic adenosine 3',5'-monophosphate-induced hormone gene expression in rat pituitary GH(3) cells

We examined whether mitogen-activated protein (MAP) kinase was activated by stimulation of the cAMP pathway and whether MAP kinase activation was involved in synthesis of PRL and GH in GH(3) cells. Treatment of the cells with a cAMP analog, 8-(4-chlorophenylthio)cAMP (CPT-cAMP), activated MAP kinase and increased PRL at both the protein and messenger RNA levels. The protein and messenger RNA of GH were decreased by the treatment. We constructed the luciferase reporter genes after the promoters of PRL and GH and found the activation of both promoters by the CPT-cAMP treatment. We confirmed that overexpression of the catalytic subunit of cAMP-dependent protein kinase had essentially the same effects on MAP kinase activation and synthesis of PRL and GH as the CPT-cAMP treatment. Furthermore, treatment of the cells with pituitary adenylate cyclase-activating polypeptide 27 activated MAP kinase. The activation of PRL promoter by CPT-cAMP and pituitary adenylate cyclase-activating polypeptide 27 was abolished by pretreatment with PD098059 and H89. Although the increase in PRL and GH secretion by CPT-cAMP was inhibited by H89, PD098059 had no effect on secretion. These results suggest that cAMP-induced MAP kinase activation is essential for PRL gene expression, but not for secretion of PRL and GH.

GH3 cells expressing constitutively active Gs alpha (Q227L) show enhanced hormone secretion and proliferation

cAMP levels in GH3gsp cells (Q227L mutation of Gs alpha), in comparison with uninfected GH3 and GH3vt (with vector alone) cells, were two to three fold (P< 0.01) higher (basal), and 10-20 fold (P<0.001) higher (in the presence of isobutyl methylxanthine, (IBMX)). Proliferation of GH3gsp cells after 7 days in culture, as determined by cell number and [(3)H]thymidine incorporation, were up to 25% (respectively P <0.001 and P < 0.02) higher. After chronic (4 days) but not acute (15 min) exposure to forskolin (10 microM) or dibutyryl cAMP (50 microM) all cell types showed a greater than 200% (P < 0.001) increase in [(3)H]thymidine incorporation. Secretion of prolactin and growth hormone by GH3gsp cells were two to four fold (P <0.001) higher than GH3 and GH3vt cells after 4 h and 10-12 fold (P <0.001) higher after 8 h. In conclusion GH3 cells possessing Q227L have a higher proliferation rate and secrete higher levels of prolactin and growth hormone which are associated with higher levels of cAMP.

C-type natriuretic peptide stimulates secretion of growth hormone from rat-pituitary-derived GH3 cells via a cyclic-GMP-mediated pathway

Although C-type natriuretic peptide (CNP) has been shown to exist at the highest concentration in the anterior pituitary in rat tissues, its physiological role(s) there is (are) not clear. In this study, we report a novel function of CNP examined with anterior pituitary-derived cell lines, GH3 and AtT20/D16v-F2. Both CNP and atrial natriuretic peptide (ANP) increased cellular cGMP levels in both cell lines in dose-dependent manners. CNP, but not ANP, stimulated growth hormone (GH) release from GH3 cells. In contrast, neither ANP nor CNP had any significant effect on the corticotropin release from AtT20/D16v-F2 cells. An activator for cGMP-dependent protein kinase (cGK), dibutyryl cGMP, mimicked the stimulation of GH release from GH3 cells by CNP. Constitutive GH release from GH3 cells was greatly diminished in the presence of inhibitors for cAMP-dependent protein kinase, while stimulative GH release by CNP was not affected. However, inhibitors which can block cGK almost completely diminished the stimulative effect of CNP. An inhibitor for protein kinase C did not show any effect on either constitutive or CNP-stimulative GH release. Our observations indicate that the stimulation of GH release from GH3 cells by CNP is mediated mainly by the cGK signal-transduction pathway, not by cAMP-dependent protein kinase or protein kinase C, through a CNP-specific receptor (possibly ANP-B receptor). Thus, CNP may act as a local modulator in the anterior pituitary.