A novel Epac-Rap-PP2A signaling module controls cAMP-dependent Akt regulation.
Rap1b has been implicated in the transduction of the cAMP mitogenic signal. It is phosphorylated and activated by cAMP, and its expression in models where cAMP is mitogenic leads to proliferation and tumorigenesis. Akt is a likely downstream effector of cAMP-Rap1 action. cAMP elevation induced a rapid and transient Akt inhibition that required activated and phosphorylated Rap1b. However, the mechanism(s) by which cAMP-Rap regulates Akt remains unclear. Here we show that (i) upstream regulators, PIK and PDK1, are not the target(s) of the cAMP inhibitory action; (ii) constitutively active Akt and calyculin A-stimulated Akt are resistant to cAMP inhibition, suggesting the action of a phosphatase; (iii) cAMP increases the rate of Akt dephosphorylation, directly implicating an Akt-phosphatase; (iv) Epac- and protein kinase A (PKA)-specific analogs synergistically inhibit Akt, indicating the involvement of both cAMP-dependent effector pathways; (v) H89 and dominant negative Epac 279E block cAMP-inhibitory action; (vi) Epac associates in a complex with Akt and PP2A, and the associated-phosphatase activity is positively modulated by cAMP in a PKA- and Rap1-dependent manner; (vii) like its action on Akt inhibition, PKA- and Epac-specific analogs synergistically activate Epac-associated PP2A; and (viii) dominant negative PP2A blocks cAMP-inhibitory action. Thus, we uncovered a novel cAMP-Epac/PKA-Rap1b-PP2A signaling module involved in Akt regulation that may represent a physiological event in the process of cAMP stimulation of thyroid mitogenesis.
Department of Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA.
Kyoungja
K
Hong
Liguang
L
Lou
Sandhya
S
Gupta
Fernando
F
Ribeiro-Neto
Daniel L
DL
Altschuler
D000818:Animals; D002460:Cell Line; D000242:Cyclic AMP / Q000378:metabolism; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015971:Gene Expression Regulation, Enzymologic; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism*; D020128:Inhibitory Concentration 50; D007546:Isoquinolines / Q000494:pharmacology; D054648:Protein Phosphatase 2 / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D051381:Rats; D015398:Signal Transduction; D013449:Sulfonamides / Q000494:pharmacology; D013961:Thyroid Gland / Q000378:metabolism; D013997:Time Factors; D020827:rap1 GTP-Binding Proteins / Q000378:metabolism*
D004791:Enzyme Inhibitors; D020662:Guanine Nucleotide Exchange Factors; D007546:Isoquinolines; C526973:Rapgef3 protein, rat; D013449:Sulfonamides; D000242:Cyclic AMP; D051057:Proto-Oncogene Proteins c-akt; D017868:Cyclic AMP-Dependent Protein Kinases; C514014:Ppp2ca protein, rat; D054648:Protein Phosphatase 2; D020827:rap1 GTP-Binding Proteins; C063509:N-(2-(4-bromocinnamylamino)ethyl)-5-isoquinolinesulfonamide