Regulation of Chromogranin A and Chromogranin B (Secretogranin I) Synthesis in Bovine Cultured Chromaffin Cells

Abstract

In the present study we investigated the regulation of Chromogranin A (CGA) and Chromogranin B (CGB) biosynthesis in bovine chromaffin cells maintained in primary culture. Cellular proteins were labelled with [(35)S]methionine and the incorporated radioactivity was used as an index of the synthesis rate. The radioactivity incorporated into CGA was determined by immunoprecipitation, and that into CGB was quantified by a dot immunobinding assay using specific antibodies. Incubation of cells with carbamylcholine, nigh K(+) or histamine, three potent stimulators of catecholamine secretion in chromaffin cells, increased the rate of CGA and CGB synthesis. On the other hand bradykinin, angiotensin II and prostaglandin E(2), which cause little secretion, also produced an increase in both CGA and CGB synthesis. These results suggest that in chromaffin cells, the biosynthesis of chromogranins is not closely linked to the secretory activity. Inhibition of protein kinase C by sphingosine or by long-term treatment with phorbol esters, completely abolished the synthesis of CGA and CGB induced by carbamylcholine, bradykinin and prostaglandin E(2) but decreased only partially the stimulating effect of histamine. Thus, protein kinase C may not be the sole effector involved in the secretagogue-induced modulation of Chromogranin synthesis. Forskolin, an activator of adenylate cyclase had no effect on CGA synthesis, but significantly enhanced the incorporation of radioactivity into CGB. The effect of forskolin was not modified by protein kinase C inhibitors and was additive to that induced by phorbol esters indicating that cyclic AMP did not stimulate CGB synthesis through a protein kinase C-dependent pathway. These observations suggest that the biosynthesis of CGA and CGB in chromaffin granules is independently regulated.