Rapid regulation of a cyclic AMP-specific phosphodiesterase (PDE IV) by forskolin and isoproterenol in LRM55 astroglial cells
Abstract Elevation of intracellular cyclic AMP (cAMP) levels by incubation of intact LRM55 astroglial cells with 0.1 mM forskolin or 0.1 μM isoproterenol (IPR) caused a rapid increase in soluble cAMP phospho-diesterase (PDE) activity. Activation did not require de novo protein synthesis and reached a maximum of ⩾100% increase over basal PDE activity after 15 min of treatment. The increase in activity was recovered in a single peak (peak 3) following DEAE chromatography; the other two peaks separated by this procedure showed no change. Peak 3 had all the characteristics of PDE IV: it was sensitive to rolipram, was insensitive to CI-930 and cyclic GMP (cGMP), had a high affinity for cAMP ( K m ∼- 4 μM), and had a very low affinity for cGMP ( K m > 100 μM). Forskolin treatment resulted in an increase of the V max of peak 3 without affecting its K m . In vitro treatment of peak 3 with the catalytic subunit of protein kinase A increased activity, whereas treatment with alkaline phosphatase decreased activity. The rapid activation of this specific PDE in response to forskolin and IPR represents a novel regulation of PDE IV by a mechanism that seems to involve its phosphorylation by a cAMP-dependent protein kinase.
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