Coordinate actions of calcium and protein kinase-C in the expression of primary response genes in pituitary gonadotrophs

Abstract

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 junB. GnRH-induced stimulation peaked at 30 min and was dose related, with similar EC50 values (approximately 1 nM) for all three PRGs and higher maximum responses for junB than for c-jun and c-fos. The agonist-induced 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. Depletion of cellular protein kinase-C by prior treatment with PMA reduced GnRH- and PMA-induced expression of PRGs. The protein kinase-C inhibitor staurosporine also attenuated agonist- and phorbol ester-induced PRG expression. Activation of Ca2+ entry by the calcium channel agonist BayK 8644 or high K(+)-induced depolarization caused a concentration-dependent rise in intracellular Ca2+ ([Ca2+]i) and a concentration-dependent and transient expression of PRGs, albeit of smaller amplitudes than those elicited by GnRH and PMA. Ca(2+)-dependent PRG expression was 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 expression. At lower intracellular calcium concentrations, calcium acted additively with low concentrations of GnRH and PMA. However, high calcium 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 indicate that GnRH-induced PRG expression in gonadotrophs is mediated by protein kinase-C and calcium, and that protein kinase-C-dependent induction of PRGs is modulated both positively and negatively by physiological changes in [Ca2+]i. Such coordinate actions of the two signaling molecules provide a mechanism for the control of PRG expression by preferential integration of low strength, and attenuation of high strength, extracellular signals.