Regulation of neuropeptide-processing enzymes by nitric oxide in cultured astrocytes

Nitric oxide (NO), a recently discovered neurotransmitter, has been shown to have a cytostatic effect on cultured glia. A NO-generating agent, S-nitroso-N-acetyl-penicillamine (SNAP), was used to treat C6 glioma and primary cortical astrocytes. The levels of a monobasic peptide-processing enzyme activity and carboxypeptidase E activity were examined. The cellular levels of these two enzymes are specifically reduced in response to treatment with SNAP. A decrease of approximately 30-50% in these two enzyme activities was seen in both primary astrocytes and C6 glioma cells. This decrease in cellular enzyme activities is not due to increased secretion because the secreted activity is also reduced in response to SNAP treatment in both the glioma cells and the primary astrocytes. Removal of SNAP treatment causes the carboxypeptidase enzyme activity to return to control levels within 3 days. Northern and western blot analyses indicate that the reduced cellular level of carboxypeptidase E is not due to reduced expression of the messenger RNA or protein, suggesting that the SNAP treatment is affecting factors that influence carboxypeptidase E activity. Taken together, these results imply that NO is involved in the regulation of peptide biosynthetic enzymes and this could lead to the antimitogenic action of SNAP on glia.