Regulation of cAMP levels by protein kinase C in C6 rat glioma cells.

Cultures of rat C6 rat glioma cells exhibit a diminished response to isoproterenol and forskolin after being treated with phorbol 12,13-dibutyrate (PDbU). An IC50 for PDbU of 38 +/- 5 nM and 62 +/- 8 nM was observed in the isoproterenol and forskolin response, respectively. Similarly, C6 cultures exhibited a diminished response to isoproterenol and forskolin after an overnight incubation with phospholipase C. We previously demonstrated that this treatment will increase diacylglycerol levels in these cells (Bressler: J Neurochem 48:181-186, 1987). An IC50 for phospholipase C of 6.0 +/- 0.1 x 10(-1) and 7.0 +/- 0.1 x 10(-1) units/ml was observed for the isoproterenol and forskolin response, respectively. A kinetic analysis suggests that the site of PDbU-mediated inhibition to beta-adrenergic and forskolin stimulation was different. Degradation of cAMP was a contributory factor since elevated cAMP levels decreased faster in PDbU treated cells than in nontreated cells. In addition, PDbU treated cells exhibited a significantly higher level of phosphodiesterase activity. We conclude that activation of protein kinase C and subsequent stimulation of phosphodiesterase activity contributes to the inhibition of the beta-adrenergic and forskolin mediated increase in cAMP levels in intact C6 rat glioma cells. The consequences of lower cAMP levels in sustaining differentiated function in the C6 rat glioma cell line will be discussed.