Expression of the Proenkephalin A Gene and [Met⁵]‐Enkephalin Secretion Induced by Arachidonic Acid in Bovine Adrenal Medullary Chromaffin Cells: Involvement of Second Messengers

Abstract

We have previously reported that arachidonic acid (AA) increases the long‐term secretion of [Met5]‐enkephalin (ME) and the expression of proenkephalin A (proENK) mRNA in bovine adrenal medullary chromaffin (BAMC) cells. To characterize the underlying signal transductional mechanisms for the AA‐induced responses, the interactions of AA with several second messenger systems were studied. Long‐term (24‐h) treatment with AA (100 µM) increased both the secretion of ME and the expression of proENK mRNA. Pretreatment of BAMC cells with nimodipine (1 µM), but not with ω‐conotoxin GVIA (1 µM), inhibited the secretion of ME and the expression of proENK mRNA induced by AA. Calmidazolium (1 µM), a calmodulin antagonist, also significantly inhibited AA‐induced responses. However, a protein kinase C (PKC) inhibitor, sphingosine (36 µM), was ineffective in blocking AA‐induced responses. In addition, the down‐regulation of PKC by phorbol 12‐myristate 13‐acetate (0.1 µM) for 48 h did not inhibit the AA‐induced responses. Forskolin (5 µM), an adenyl cyclase activator, alone increased the secretion of ME as well as proENK mRNA levels and, when coincubated with AA, showed an additive effect on the secretion of ME and the levels of proENK mRNA. The results suggest that the Ca2+/calmodulin pathway, but not the protein kinase A or PKC pathway, is partially involved in mediating the AA‐induced increases of the long‐term secretion of ME and the levels of proENK mRNA.

Differential activation of enkephalin, galanin, somatostatin, NPY, and VIP neuropeptide production by stimulators of protein kinases A and C in neuroendocrine chromaffin cells

“…It is, therefore, possible that PKA and PKC regulation of neuropeptide production may involve control of proneuropeptide gene expression. Indeed, the neuropeptides examined in this study – (Met)enkephalin, galanin, somatostatin, NPY, and VIP – utilize PKA- or PKC-mediated transcriptional regulators to control proneuropeptide gene expression (Tezapsidis et al, 1995; Suh et al, 1995; Rokaeus et al, 1990; Montminy et al, 1986; Magni et al, 1998; Williams et al, 1998; Hahm and Eiden, 1999). It will be fruitful in future studies to investigate the coordinate regulation of proneuropeptide gene expression and proneuropeptide processing to gain knowledge of how PKA and PKC may influence transcriptional and post-translational regulatory mechanisms in the control of neuropeptide biosynthesis.…”