PT  - JOURNAL ARTICLE
AU  - Marina S. Feschenko
AU  - Elizabeth Stevenson
AU  - Angus C. Nairn
AU  - Kathleen J. Sweadner
TI  - A Novel cAMP-Stimulated Pathway in Protein Phosphatase 2A Activation
AID  - 10.1124/jpet.302.1.111
DP  - 2002 Jul 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 111--118
VI  - 302
IP  - 1
4099  - http://jpet.aspetjournals.org/content/302/1/111.short
4100  - http://jpet.aspetjournals.org/content/302/1/111.full
SO  - J Pharmacol Exp Ther2002 Jul 01; 302
AB  - Elevated cAMP in NRK-52E and L6 cells causes a marked reduction in the phosphorylation of numerous phosphoproteins, as detected initially with phosphoserine-specific antibodies. Here, we show that elevation of cAMP in NRK cells by forskolin/3-isobutyl-1-methylxanthine (IBMX) treatment decreased phosphorylation of substrates for different protein kinases, pointing to a common protein phosphatase as a target for cAMP-dependent regulation. Forskolin/IBMX treatment completely dephosphorylated a selective protein phosphatase 2A (PP2A) substrate, elongation factor-2 (EF-2), at its Ca2+ calmodulin-dependent kinase site, and decreased phosphorylation of substrates for cyclin-dependent kinases, including retinoblastoma (Rb) protein. As reported before, forskolin/IBMX also decreased phosphorylation of a protein kinase C substrate, the Na,K-ATPase. The cAMP-stimulated dephosphorylation was blocked by the protein phosphatases 1 (PP1) and PP2A inhibitor okadaic acid at concentrations selective for PP2A but was not blocked by tautomycin at concentrations selective for PP1. The data implicate PP2A as a cAMP-activated phosphatase. Contrary to expectation, we found evidence that cAMP-dependent activation of PP2A did not depend on protein kinase A (PKA). Pretreatment of cells with the PKA inhibitor H89 abolished PKA activity measured in cell extracts and significantly decreased cAMP-activated phosphorylation of a known PKA substrate, ARPP-19, in cells, but failed to block the cAMP-stimulated dephosphorylation of EF-2, Rb, and other proteins. This novel pathway of PP2A activation, acting on the time scale of minutes, represents yet another example of a cAMP-mediated, PKA-independent signaling mechanism. Because PP2A is active toward a variety of endogenous substrates, cAMP-stimulated dephosphorylation may have complicated the interpretation of many prior studies. The American Society for Pharmacology and Experimental Therapeutics