Galanin gene expression in chromaffin cells is controlled by calcium and protein kinase signaling pathways

The neuropeptide galanin (GAL) is widely distributed throughout the diffuse neuroendocrine system, and is coexpressed with acetylcholine, norepinephrine, prolactin, and a variety of other messenger substances in different cell types. Bovine chromaffin cells in primary culture synthesize and store GAL along with catecholamines, chromogranin A, and enkephalin peptides, as well as other neurosecretory products, and secrete all these molecules in response to nicotinic stimulation. The regulation of GAL biosynthesis and secretion were studied by measuring changes in messenger RNA (mRNA(GAL], and peptide immunoreactivity, 24-72 h after stimulation of secretion (40 mM potassium or 10 microM veratridine), or exposure to stimulators of protein kinase C (100 nM phorbol myristate acetate) and protein kinase A (25 microM forskolin). Depolarization-induced stimulation of GAL biosynthesis, like that of enkephalin and other neuropeptides, was calcium dependent, suggesting that calcium generally mediates both exocytotic release and new peptide synthesis thus coordinating maintenance of neuropeptide levels in chromaffin cells. GAL and mRNA(GAL) were also upregulated by stimulation of protein kinase A with forskolin. Treatment with PMA increased GAL and mRNA(GAL) to an even greater extent than depolarization. Thus GAL expression can be regulated by three distinct signal transduction systems in chromaffin cells: depolarization-stimulated calcium influx, activation of protein kinase C and activation of protein kinase A, which in addition differentially up- and down-regulate the expression of several other neurosecretory proteins and peptides resulting in different patterns of GAL/neuropeptide coexistence in bovine chromaffin cells. GAL coexistence with diverse neuroendocrine substances may reflect the relative activity of these three signalling systems in other neuroendocrine cell types as well.