COORDINATE ACTIONS OF CALCIUM AND PROTEIN-KINASE-C IN THE EXPRESSION OF PRIMARY RESPONSE GENES IN PITUITARY GONADOTROPHS

Activation of GnRH receptors in cultured pituitary cells and alpha T3- 1 gonadotrophs caused prominent, but transient, increases in messenger RNAs for primary response genes (PRGs) including c-fos, c-jun, and ju nB. GnRH-induced stimulation peaked at 30 min and was dose related, wi th similar EC(50) values (similar to 1 nM) for all three PRGs and high er maximum responses for junB than for c-jun, and c-fos. The agonist-i nduced expression of PRGs was mimicked by activation of protein kinase -C with the phorbol ester phorbol 12-myristate 13-acetate (PMA), which acted additively with GnRH at low concentrations of both stimuli. Dep letion of cellular protein kinase-C by prior treatment with PMA reduce d GnRH- and PMA-induced expression of PRGs. The protein kinase-C inhib itor staurosporine also attenuated agonist- and phorbol ester-induced PRG expression. Activation of Ca2+ entry by the calcium channel agonis t BayK 8644 or high Kc-induced depolarization caused a concentration-d ependent rise in intracellular Ca2+ ([Ca2+](i)) and a concentration-de pendent and transient expression of PRGs, albeit of smaller amplitudes than those elicited by GnRH and PMA. Ca2+-dependent PRG expression wa s abolished by the calmodulin inhibitor W-7. Parallel measurements of [Ca2+](i) and steady-state levels of PRG messenger RNAs indicated that intracellular Ca2+ exerted both additive and suppressive actions over its physiological concentration range on GnRH- and PMA-induced PRG ex pression. At lower intracellular calcium concentrations, calcium acted additively with low concentrations of GnRH and PMA. However, high cal cium concentrations suppressed high agonist- and phorbol ester-induced PRG expression. In contrast, omission of Ca2+ from the extracellular medium significantly enhanced induction of PRGs. These findings indica te that GnRH-induced PRG expression in gonadotrophs is mediated by pro tein kinase-C and calcium, and that protein kinase-C-dependent inducti on of PRGs is modulated both positively and negatively by physiologica l changes in [Ca2+](i). Such coordinate actions of the two signaling m olecules provide a mechanism for the control of PRG expression by pref erential integration of low strength, and attenuation of high strength , extracellular signals.