Enkephalin biosynthesis is coupled to secretory activity via transcription of the proenkephalin A gene

Abstract

The molecular mechanisms regulating neuropeptide and secretory protein biosynthesis in neuroendocrine cells were examined using the prototype neuropeptide and secretory proteins enkephalin and chromogranin A (CGA). Treatment with the secretogogue nicotine results in the calcium-dependent secretion of enkephalin peptides from bovine chromaffin cells in primary culture and a concomitant increase in enkephalin peptide biosynthesis. Both secretion and biosynthesis are also stimulated by cell depolarization with elevated potassium. Elevation of intracellular cyclic AMP, on the other hand, results in stimulation of enkephalin biosynthesis and long-term, but not acute, secretion of enkephalin peptides. Coupling of enkephalin biosynthesis to calcium influx occurs at the level of transcription of the enkephalin gene. Thus, potassium depolarization causes a calcium-dependent elevation of enkephalin mRNA which is preceded by an increase in the rate of transcription of the enkephalin gene in the chromaffin cell. The accumulation of enkephalin message or peptide by potassium depolarization or treatment with nicotine is prevented by D600 or hexamethonium respectively, added 1 h after addition of nicotine or KCl and following acute release, suggesting that calcium acts as a continuous rather than triggering stimulus for enkephalin biosynthesis. Sequence analysis of the bovine enkephalin promoter identified sequence conservation of three enhancers previously reported in the human gene which are required for regulation of the gene by calcium, cAMP, and phorbol ester in vitro. In contrast to the regulation of the enkephalin system, no increase in either CGA or CGB mRNA or gene transcription attended depolarization-induced secretion from chromaffin cells. Since enkephalin and CGA are co-stored in and co-released from the same secretory vesicles in these cells, the results imply that a surplus of CGA is constitutively synthesized in chromaffin cells such that compensatory up-regulation during changes in the secretory state of the cell, such as occurs for enkephalin, is not required for the secretory proteins.