Cyclic AMP Inhibits Akt Activity by Blocking the Membrane Localization of PDK1

Akt is a protein serine/threonine kinase that plays an important role in the mitogenic responses of cells to variable stimuli. Akt contains a pleckstrin homology (PH) domain and is activated by phosphorylation at threonine 308 and serine 473. Binding of 3′-OH phosphorylated phosphoinositides to the PH domain results in the translocation of Akt to the plasma membrane where it is activated by upstream kinases such as (phosphoinositide-dependent kinase-1 (PDK1). Over-expression of constitutively active forms of Akt promotes cell proliferation and survival, and also stimulates p70 S6 kinase (p70S6K). In many cells, an increase in levels of intracellular cyclic AMP (cAMP) diminishes cell growth and promotes differentiation, and in certain conditions cAMP is even antagonistic to the effect of growth factors. Here, we show that cAMP has inhibitory effects on the phosphatidylinositol 3-kinase/PDK/Akt signaling pathway. cAMP potently inhibits phosphorylation at threonine 308 and serine 473 of Akt, which is required for the protein kinase activities of Akt. cAMP also negatively regulates PDK1 by inhibiting its translocation to the plasma membrane, despite not affecting its protein kinase activities. Furthermore, when we co-expressed myristoylated Akt and PDK1 mutants which constitutively co-localize in the plasma membrane, Akt activity was no longer sensitive to raised intracellular cAMP concentrations. Finally, cAMP was also found to inhibit the lipid kinase activity of PI3K and to decrease the levels of phosphatidylinositol 3,4,5-triphosphate in vivo, which are required for the membrane localization of PDK1. Collectively, these data strongly support the theory that the cAMP-dependent signaling pathway inhibits Akt activity by blocking the coupling between Akt and its upstream regulators, PDK, in the plasma membrane. Akt is a protein serine/threonine kinase that plays an important role in the mitogenic responses of cells to variable stimuli. Akt contains a pleckstrin homology (PH) domain and is activated by phosphorylation at threonine 308 and serine 473. Binding of 3′-OH phosphorylated phosphoinositides to the PH domain results in the translocation of Akt to the plasma membrane where it is activated by upstream kinases such as (phosphoinositide-dependent kinase-1 (PDK1). Over-expression of constitutively active forms of Akt promotes cell proliferation and survival, and also stimulates p70 S6 kinase (p70S6K). In many cells, an increase in levels of intracellular cyclic AMP (cAMP) diminishes cell growth and promotes differentiation, and in certain conditions cAMP is even antagonistic to the effect of growth factors. Here, we show that cAMP has inhibitory effects on the phosphatidylinositol 3-kinase/PDK/Akt signaling pathway. cAMP potently inhibits phosphorylation at threonine 308 and serine 473 of Akt, which is required for the protein kinase activities of Akt. cAMP also negatively regulates PDK1 by inhibiting its translocation to the plasma membrane, despite not affecting its protein kinase activities. Furthermore, when we co-expressed myristoylated Akt and PDK1 mutants which constitutively co-localize in the plasma membrane, Akt activity was no longer sensitive to raised intracellular cAMP concentrations. Finally, cAMP was also found to inhibit the lipid kinase activity of PI3K and to decrease the levels of phosphatidylinositol 3,4,5-triphosphate in vivo, which are required for the membrane localization of PDK1. Collectively, these data strongly support the theory that the cAMP-dependent signaling pathway inhibits Akt activity by blocking the coupling between Akt and its upstream regulators, PDK, in the plasma membrane. The phosphatidylinositol 3-kinase (PI3K)1-dependent cell signaling pathway has emerged as a key regulatory pathway involved in a number of cellular events (1Rameh L.E. Cantley L.C. J. Biol. Chem. 1999; 274: 8347-8350Abstract Full Text Full Text PDF PubMed Scopus (848) Google Scholar). Upon activation of growth factor tyrosine kinase receptors, the p85 regulatory subunit of PI3K recruits the p110 catalytic subunit to the plasma membrane (2Vanhaesebroeck B. Leevers S.J. Panayotou G. Waterfield M.D. Trends Biochem. Sci. 1997; 22: 267-272Abstract Full Text PDF PubMed Scopus (827) Google Scholar). The p110 catalytic subunit increases the level of PtdIns-3,4,5-P3and phosphatidylinositol 3,4-bisphosphate (PtdIns-3,4-P2), which induce the membrane translocation of PDK1 and Akt (also called PKB or RAC-PK) by binding to the pleckstrin homology domain (3Anderson K.E. Coadwell J. Stephens L.R. Hawkins P.T. Curr. Biol. 1998; 8: 684-691Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar). In the membrane, PDK1 phosphorylates and activates Akt in a PtdIns-3,4,5-P3- or PtdIns-3,4-P2-dependent manner (4Alessi D.R. James S.R. Downes C.P. Holmes A.B. Gaffney P.R. Reese C.B. Cohen P. Curr. Biol. 1997; 7: 261-269Abstract Full Text Full Text PDF PubMed Google Scholar, 5Stephens L. Anderson K. Stokoe D. Erdjument-Bromage H. Painter G.F. Holmes A.B. Gaffney P.R. Reese C.B. McCormick F. Tempst P. Coadwell J. Hawkins P.T. Science. 1998; 279: 710-714Crossref PubMed Scopus (910) Google Scholar). By a mechanism that involves phospholipase C (6Ferguson K.M. Lemmon M.A. Sigler P.B. Schlessinger J. Nature Struct. Biol. 1995; 2: 715-718Crossref PubMed Scopus (59) Google Scholar), activated Akt is released from the membrane and phosphorylates various targets.This complex and unique signaling pathway has been implicated in a variety of cellular events such as cell proliferation and survival (1Rameh L.E. Cantley L.C. J. Biol. Chem. 1999; 274: 8347-8350Abstract Full Text Full Text PDF PubMed Scopus (848) Google Scholar,7Datta S.R. Brunet A. Greenberg M.E. Genes Dev. 1999; 13: 2905-2927Crossref PubMed Scopus (3707) Google Scholar). Previously, it has been shown that various survival factors, such as nerve growth factor, require the activation of PI3K to prevent various cell types from undergoing apoptosis (8Yao R.J. Cooper G.M. Science. 1995; 267: 2003-2006Crossref PubMed Scopus (1288) Google Scholar, 9Kulik G. Klippel A. Weber M.J. Mol. Cell. Biol. 1997; 17: 1595-1606Crossref PubMed Scopus (964) Google Scholar). The mechanism by which the PI3K pathway protects cells from programmed cell death has been studied intensively. Recently, it was shown that Akt can phosphorylate serine 136 of BAD, a member of the pro-apoptotic Bcl-2 family, forming a binding site for 14-3-3 (10Del Peso L. Gonzalez-Garcia M. Page C. G. Science. 1997; PubMed Scopus Google Scholar, S.R. H. H. Greenberg M.E. Cell. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar). it can no longer to Bcl-2 and to inhibit Akt also phosphorylates important cellular involved in such as Science. 1998; PubMed Scopus Google and A. A. M.J. P. Anderson M.J. J. Greenberg M.E. Cell. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar), which results in the of blocking the PI3K signaling pathway is involved in D.R. Cohen P. M. 1995; PubMed Scopus Google Scholar, Mol. Cell. Biol. 1997; 17: PubMed Scopus Google Scholar), B. L. L. J. Mol. Cell. Biol. PubMed Scopus Google Scholar), and protein G. Curr. Biol. 1997; PubMed Scopus Google Scholar). The activities of Akt are these important activities. is phosphorylated at serine and its activities are by Akt D.R. Cohen P. M. 1995; PubMed Scopus Google Scholar). of and which phosphorylate a factor, and a are as of Akt Sci. A. 1998; PubMed Scopus Google Scholar, 1995; PubMed Scopus Google Scholar, M.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, H. K. B. J. D. J. 1999; PubMed Scopus Google and cAMP-dependent protein kinase are of in many and are involved in many cellular cAMP is a key by a mechanism in which inhibits the activation of the factor D. Genes Dev. 1997; PubMed Scopus Google Scholar). In cAMP regulates and to as as the activity of and the binding and B. A. Science. 1997; PubMed Scopus Google Scholar, D. G. J. 1998; 17: PubMed Scopus Google cAMP and its are implicated in a variety of between intracellular signaling was that the from in required the activation of the pathway D. A. M.E. Science. PubMed Scopus Google Scholar). is also that between the and cAMP-dependent in the M.A. G. Sci. A. PubMed Scopus Google and the A. Sci. A. 1998; PubMed Scopus Google Scholar). it was that cAMP inhibits the pathway M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar), which is important in cAMP-dependent protein kinase inhibits by at a serine M. PubMed Scopus Google Scholar). In the protein kinase pathway is strongly by cAMP-dependent signaling in various J. P. A. Weber M.J. Science. PubMed Scopus Google Scholar, S.J. McCormick F. Science. PubMed Scopus Google Scholar, P. P. J. PubMed Scopus Google Scholar, K.E. Sci. A. PubMed Scopus Google that is also between the PI3K pathway and the cAMP-dependent pathway. an level of cAMP inhibits the activation of M. L. J. J. Mol. Cell. Biol. 1995; PubMed Scopus Google Scholar). In in which activities are by phosphorylation at serine is phosphorylated at the serine by and K. M. J. Biol. Chem. 1998; Full Text Full Text PDF PubMed Scopus Google we that an increase in the level of intracellular cAMP inhibits the activities of Akt, and PDK1 activity was not by cAMP its plasma membrane localization was these results support that the cAMP-dependent signaling pathway inhibits Akt of PI3K lipid kinase activity and the of PDK1 localization at the plasma that cAMP Akt activity by the membrane localization of PDK1 and inhibiting the lipid kinase activity of we that the kinase activity of myristoylated Akt was strongly by we that phosphorylate Akt and inhibit the activity of Akt, as Akt has a phosphorylation site in its catalytic the catalytic subunit of was to phosphorylate Akt in and an Akt that the phosphorylation site was strongly by in not Furthermore, the required for the activity of Akt by and results to the effects of cAMP on PDK1 and upstream of that PDK1 is constitutively active and that its activity is by membrane localization binding to (3Anderson K.E. Coadwell J. Stephens L.R. Hawkins P.T. Curr. Biol. 1998; 8: 684-691Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar, D.R. James S.R. Downes C.P. Holmes A.B. Gaffney P.R. Reese C.B. Cohen P. Curr. Biol. 1997; 7: 261-269Abstract Full Text Full Text PDF PubMed Google Scholar, H. Downes C.P. J. Biol. Chem. 1998; Full Text Full Text PDF PubMed Scopus Google Scholar). In support of a PDK1 an constitutively activates co-expressed Akt (3Anderson K.E. Coadwell J. Stephens L.R. Hawkins P.T. Curr. Biol. 1998; 8: 684-691Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar). results that the pleckstrin homology localization of PDK1 and Akt in the plasma membrane is for the coupling between the protein results strongly support the theory that cAMP signaling by inhibiting the lipid kinase activity of PI3K and the in of we not cAMP inhibits PI3K in the data at that a phosphorylation of the p110 and p85 of PI3K by is not a mechanism the of the PI3K and also the in tyrosine phosphorylation of the p85 subunit of PI3K by cAMP found that and tyrosine phosphorylation of the p85 protein was not by cAMP In we found that cAMP not the of we for the mechanism of PI3K by are to we an inhibitory role of cAMP for Akt in various cells and D. Stephens L.R. Gaffney P.R. Reese C.B. Painter G.F. Holmes A.B. McCormick F. Hawkins P.T. Mol. Biol. 1997; on the activation of Akt by a pathway. not was the activation of Akt by cAMP and cAMP phosphorylation at serine 473 of Akt D. Stephens L.R. Gaffney P.R. Reese C.B. Painter G.F. Holmes A.B. McCormick F. Hawkins P.T. Mol. Biol. 1997; Scholar). not by that phosphorylation at serine 473 of Akt is for its activity (4Alessi D.R. James S.R. Downes C.P. Holmes A.B. Gaffney P.R. Reese C.B. Cohen P. Curr. Biol. 1997; 7: 261-269Abstract Full Text Full Text PDF PubMed Google Scholar, D.R. M. B. P. Cohen P. J. PubMed Scopus Google Scholar, D. Stephens L.R. Gaffney P.R. Reese C.B. Painter G.F. Holmes A.B. McCormick F. Hawkins P.T. Science. 1997; PubMed Scopus Google Scholar). the we that the phosphorylation of Akt at threonine 308 and serine 473 was to levels by also the of PI3K activity the in of translocation of PDK1 to the plasma membrane and activity by these the of the inhibitory effect of cAMP on the PI3K as it for cAMP to Akt inhibiting its upstream Furthermore, in a variety of cell we also in that cAMP induce the of Akt at threonine 308 and serine 473 and Akt activity Sci. A. PubMed Scopus Google Scholar, M. M.J. Mol. Cell. Biol. PubMed Scopus Google Scholar). also that cAMP the activity of Akt Sci. A. PubMed Scopus Google Scholar). we that we a the of Akt and of the PI3K pathway by cAMP and is by results that cAMP inhibits a of Akt and in M. L. J. J. Mol. Cell. Biol. 1995; PubMed Scopus Google and that cAMP to Akt in J. Biol. Chem. 1997; PubMed Scopus Google plays important in cells from various shown that Akt its activities by important for apoptosis such as (10Del Peso L. Gonzalez-Garcia M. Page C. G. Science. 1997; PubMed Scopus Google Scholar, S.R. H. H. Greenberg M.E. Cell. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar), Science. 1998; PubMed Scopus Google Scholar), A. A. M.J. P. Anderson M.J. J. Greenberg M.E. Cell. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar), activation of the cAMP-dependent pathway can and P. P. M. 1997; PubMed Scopus Google shown that activation of the cAMP pathway apoptosis activation in a In the cAMP programmed cell death G. C. B. A. H. Mol. Cell. Biol. 1997; 17: PubMed Scopus (59) Google Scholar). In and cAMP in a to induce apoptosis activation 1999; PubMed Scopus Google Scholar). we that these to the cAMP-dependent pathway to the for we that of the Akt activity of the the of the we shown that cAMP inhibits the in of and the activities of Akt and which that of the PI3K pathway are also by the and cAMP-dependent are involved in various cellular events cell and important to the and its role in the The phosphatidylinositol 3-kinase (PI3K)1-dependent cell signaling pathway has emerged as a key regulatory pathway involved in a number of cellular events (1Rameh L.E. Cantley L.C. J. Biol. Chem. 1999; 274: 8347-8350Abstract Full Text Full Text PDF PubMed Scopus (848) Google Scholar). Upon activation of growth factor tyrosine kinase receptors, the p85 regulatory subunit of PI3K recruits the p110 catalytic subunit to the plasma membrane (2Vanhaesebroeck B. Leevers S.J. Panayotou G. Waterfield M.D. Trends Biochem. Sci. 1997; 22: 267-272Abstract Full Text PDF PubMed Scopus (827) Google Scholar). The p110 catalytic subunit increases the level of PtdIns-3,4,5-P3and phosphatidylinositol 3,4-bisphosphate (PtdIns-3,4-P2), which induce the membrane translocation of PDK1 and Akt (also called PKB or RAC-PK) by binding to the pleckstrin homology domain (3Anderson K.E. Coadwell J. Stephens L.R. Hawkins P.T. Curr. Biol. 1998; 8: 684-691Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar). In the membrane, PDK1 phosphorylates and activates Akt in a PtdIns-3,4,5-P3- or PtdIns-3,4-P2-dependent manner (4Alessi D.R. James S.R. Downes C.P. Holmes A.B. Gaffney P.R. Reese C.B. Cohen P. Curr. Biol. 1997; 7: 261-269Abstract Full Text Full Text PDF PubMed Google Scholar, 5Stephens L. Anderson K. Stokoe D. Erdjument-Bromage H. Painter G.F. Holmes A.B. Gaffney P.R. Reese C.B. McCormick F. Tempst P. Coadwell J. Hawkins P.T. Science. 1998; 279: 710-714Crossref PubMed Scopus (910) Google Scholar). By a mechanism that involves phospholipase C (6Ferguson K.M. Lemmon M.A. Sigler P.B. Schlessinger J. Nature Struct. Biol. 1995; 2: 715-718Crossref PubMed Scopus (59) Google Scholar), activated Akt is released from the membrane and phosphorylates various complex and unique signaling pathway has been implicated in a variety of cellular events such as cell proliferation and survival (1Rameh L.E. Cantley L.C. J. Biol. Chem. 1999; 274: 8347-8350Abstract Full Text Full Text PDF PubMed Scopus (848) Google Scholar,7Datta S.R. Brunet A. Greenberg M.E. Genes Dev. 1999; 13: 2905-2927Crossref PubMed Scopus (3707) Google Scholar). Previously, it has been shown that various survival factors, such as nerve growth factor, require the activation of PI3K to prevent various cell types from undergoing apoptosis (8Yao R.J. Cooper G.M. Science. 1995; 267: 2003-2006Crossref PubMed Scopus (1288) Google Scholar, 9Kulik G. Klippel A. Weber M.J. Mol. Cell. Biol. 1997; 17: 1595-1606Crossref PubMed Scopus (964) Google Scholar). The mechanism by which the PI3K pathway protects cells from programmed cell death has been studied intensively. Recently, it was shown that Akt can phosphorylate serine 136 of BAD, a member of the pro-apoptotic Bcl-2 family, forming a binding site for 14-3-3 (10Del Peso L. Gonzalez-Garcia M. Page C. G. Science. 1997; PubMed Scopus Google Scholar, S.R. H. H. Greenberg M.E. Cell. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar). it can no longer to Bcl-2 and to inhibit Akt also phosphorylates important cellular involved in such as Science. 1998; PubMed Scopus Google and A. A. M.J. P. Anderson M.J. J. Greenberg M.E. Cell. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar), which results in the of blocking the PI3K signaling pathway is involved in D.R. Cohen P. M. 1995; PubMed Scopus Google Scholar, Mol. Cell. Biol. 1997; 17: PubMed Scopus Google Scholar), B. L. L. J. Mol. Cell. Biol. PubMed Scopus Google Scholar), and protein G. Curr. Biol. 1997; PubMed Scopus Google Scholar). The activities of Akt are these important activities. is phosphorylated at serine and its activities are by Akt D.R. Cohen P. M. 1995; PubMed Scopus Google Scholar). of and which phosphorylate a factor, and a are as of Akt Sci. A. 1998; PubMed Scopus Google Scholar, 1995; PubMed Scopus Google Scholar, M.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, H. K. B. J. D. J. 1999; PubMed Scopus Google Scholar). cAMP and cAMP-dependent protein kinase are of in many and are involved in many cellular cAMP is a key by a mechanism in which inhibits the activation of the factor D. Genes Dev. 1997; PubMed Scopus Google Scholar). In cAMP regulates and to as as the activity of and the binding and B. A. Science. 1997; PubMed Scopus Google Scholar, D. G. J. 1998; 17: PubMed Scopus Google Scholar). cAMP and its are implicated in a variety of between intracellular signaling was that the from in required the activation of the pathway D. A. M.E. Science. PubMed Scopus Google Scholar). is also that between the and cAMP-dependent in the M.A. G. Sci. A. PubMed Scopus Google and the A. Sci. A. 1998; PubMed Scopus Google Scholar). it was that cAMP inhibits the pathway M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar), which is important in cAMP-dependent protein kinase inhibits by at a serine M. PubMed Scopus Google Scholar). In the protein kinase pathway is strongly by cAMP-dependent signaling in various J. P. A. Weber M.J. Science. PubMed Scopus Google Scholar, S.J. McCormick F. Science. PubMed Scopus Google Scholar, P. P. J. PubMed Scopus Google Scholar, K.E. Sci. A. PubMed Scopus Google Scholar). that is also between the PI3K pathway and the cAMP-dependent pathway. an level of cAMP inhibits the activation of M. L. J. J. Mol. Cell. Biol. 1995; PubMed Scopus Google Scholar). In in which activities are by phosphorylation at serine is phosphorylated at the serine by and K. M. J. Biol. Chem. 1998; Full Text Full Text PDF PubMed Scopus Google Scholar). Here, we that an increase in the level of intracellular cAMP inhibits the activities of Akt, and PDK1 activity was not by cAMP its plasma membrane localization was these results support that the cAMP-dependent signaling pathway inhibits Akt of PI3K lipid kinase activity and the of PDK1 localization at the plasma membrane. that cAMP Akt activity by the membrane localization of PDK1 and inhibiting the lipid kinase activity of we that the kinase activity of myristoylated Akt was strongly by we that phosphorylate Akt and inhibit the activity of Akt, as Akt has a phosphorylation site in its catalytic the catalytic subunit of was to phosphorylate Akt in and an Akt that the phosphorylation site was strongly by in not Furthermore, the required for the activity of Akt by and results to the effects of cAMP on PDK1 and upstream of that PDK1 is constitutively active and that its activity is by membrane localization binding to (3Anderson K.E. Coadwell J. Stephens L.R. Hawkins P.T. Curr. Biol. 1998; 8: 684-691Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar, D.R. James S.R. Downes C.P. Holmes A.B. Gaffney P.R. Reese C.B. Cohen P. Curr. Biol. 1997; 7: 261-269Abstract Full Text Full Text PDF PubMed Google Scholar, H. Downes C.P. J. Biol. Chem. 1998; Full Text Full Text PDF PubMed Scopus Google Scholar). In support of a PDK1 an constitutively activates co-expressed Akt (3Anderson K.E. Coadwell J. Stephens L.R. Hawkins P.T. Curr. Biol. 1998; 8: 684-691Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar). results that the pleckstrin homology localization of PDK1 and Akt in the plasma membrane is for the coupling between the protein results strongly support the theory that cAMP signaling by inhibiting the lipid kinase activity of PI3K and the in of we not cAMP inhibits PI3K in the data at that a phosphorylation of the p110 and p85 of PI3K by is not a mechanism the of the PI3K and also the in tyrosine phosphorylation of the p85 subunit of PI3K by cAMP found that and tyrosine phosphorylation of the p85 protein was not by cAMP In we found that cAMP not the of we for the mechanism of PI3K by are to we an inhibitory role of cAMP for Akt in various cells and D. Stephens L.R. Gaffney P.R. Reese C.B. Painter G.F. Holmes A.B. McCormick F. Hawkins P.T. Mol. Biol. 1997; on the activation of Akt by a pathway. not was the activation of Akt by cAMP and cAMP phosphorylation at serine 473 of Akt D. Stephens L.R. Gaffney P.R. Reese C.B. Painter G.F. Holmes A.B. McCormick F. Hawkins P.T. Mol. Biol. 1997; Scholar). not by that phosphorylation at serine 473 of Akt is for its activity (4Alessi D.R. James S.R. Downes C.P. Holmes A.B. Gaffney P.R. Reese C.B. Cohen P. Curr. Biol. 1997; 7: 261-269Abstract Full Text Full Text PDF PubMed Google Scholar, D.R. M. B. P. Cohen P. J. PubMed Scopus Google Scholar, D. Stephens L.R. Gaffney P.R. Reese C.B. Painter G.F. Holmes A.B. McCormick F. Hawkins P.T. Science. 1997; PubMed Scopus Google Scholar). the we that the phosphorylation of Akt at threonine 308 and serine 473 was to levels by also the of PI3K activity the in of translocation of PDK1 to the plasma membrane and activity by these the of the inhibitory effect of cAMP on the PI3K as it for cAMP to Akt inhibiting its upstream Furthermore, in a variety of cell we also in that cAMP induce the of Akt at threonine 308 and serine 473 and Akt activity Sci. A. PubMed Scopus Google Scholar, M. M.J. Mol. Cell. Biol. PubMed Scopus Google Scholar). also that cAMP the activity of Akt Sci. A. PubMed Scopus Google Scholar). we that we a the of Akt and of the PI3K pathway by cAMP and is by results that cAMP inhibits a of Akt and in M. L. J. J. Mol. Cell. Biol. 1995; PubMed Scopus Google and that cAMP to Akt in J. Biol. Chem. 1997; PubMed Scopus Google plays important in cells from various shown that Akt its activities by important for apoptosis such as (10Del Peso L. Gonzalez-Garcia M. Page C. G. Science. 1997; PubMed Scopus Google Scholar, S.R. H. H. Greenberg M.E. Cell. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar), Science. 1998; PubMed Scopus Google Scholar), A. A. M.J. P. Anderson M.J. J. Greenberg M.E. Cell. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar), activation of the cAMP-dependent pathway can and P. P. M. 1997; PubMed Scopus Google shown that activation of the cAMP pathway apoptosis activation in a In the cAMP programmed cell death G. C. B. A. H. Mol. Cell. Biol. 1997; 17: PubMed Scopus (59) Google Scholar). In and cAMP in a to induce apoptosis activation 1999; PubMed Scopus Google Scholar). we that these to the cAMP-dependent pathway to the for we that of the Akt activity of the the of the we shown that cAMP inhibits the in of and the activities of Akt and which that of the PI3K pathway are also by the and cAMP-dependent are involved in various cellular events cell and important to the and its role in the that cAMP Akt activity by the membrane localization of PDK1 and inhibiting the lipid kinase activity of we that the kinase activity of myristoylated Akt was strongly by we that phosphorylate Akt and inhibit the activity of Akt, as Akt has a phosphorylation site in its catalytic the catalytic subunit of was to phosphorylate Akt in and an Akt that the phosphorylation site was strongly by in not Furthermore, the required for the activity of Akt by and results to the effects of cAMP on PDK1 and upstream of Akt. that PDK1 is constitutively active and that its activity is by membrane localization binding to (3Anderson K.E. Coadwell J. Stephens L.R. Hawkins P.T. Curr. Biol. 1998; 8: 684-691Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar, D.R. James S.R. Downes C.P. Holmes A.B. Gaffney P.R. Reese C.B. Cohen P. Curr. Biol. 1997; 7: 261-269Abstract Full Text Full Text PDF PubMed Google Scholar, H. Downes C.P. J. Biol. Chem. 1998; Full Text Full Text PDF PubMed Scopus Google Scholar). In support of a PDK1 an constitutively activates co-expressed Akt (3Anderson K.E. Coadwell J. Stephens L.R. Hawkins P.T. Curr. Biol. 1998; 8: 684-691Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar). results that the pleckstrin homology localization of PDK1 and Akt in the plasma membrane is for the coupling between the protein results strongly support the theory that cAMP signaling by inhibiting the lipid kinase activity of PI3K and the in of we not cAMP inhibits PI3K in the data at that a phosphorylation of the p110 and p85 of PI3K by is not a mechanism the of the PI3K and also the in tyrosine phosphorylation of the p85 subunit of PI3K by cAMP found that and tyrosine phosphorylation of the p85 protein was not by cAMP In we found that cAMP not the of we for the mechanism of PI3K by are to we an inhibitory role of cAMP for Akt in various cells and D. Stephens L.R. Gaffney P.R. Reese C.B. Painter G.F. Holmes A.B. McCormick F. Hawkins P.T. Mol. Biol. 1997; on the activation of Akt by a pathway. not was the activation of Akt by cAMP and cAMP phosphorylation at serine 473 of Akt D. Stephens L.R. Gaffney P.R. Reese C.B. Painter G.F. Holmes A.B. McCormick F. Hawkins P.T. Mol. Biol. 1997; Scholar). not by that phosphorylation at serine 473 of Akt is for its activity (4Alessi D.R. James S.R. Downes C.P. Holmes A.B. Gaffney P.R. Reese C.B. Cohen P. Curr. Biol. 1997; 7: 261-269Abstract Full Text Full Text PDF PubMed Google Scholar, D.R. M. B. P. Cohen P. J. PubMed Scopus Google Scholar, D. Stephens L.R. Gaffney P.R. Reese C.B. Painter G.F. Holmes A.B. McCormick F. Hawkins P.T. Science. 1997; PubMed Scopus Google Scholar). the we that the phosphorylation of Akt at threonine 308 and serine 473 was to levels by also the of PI3K activity the in of translocation of PDK1 to the plasma membrane and activity by these the of the inhibitory effect of cAMP on the PI3K as it for cAMP to Akt inhibiting its upstream Furthermore, in a variety of cell we also in that cAMP induce the of Akt at threonine 308 and serine 473 and Akt activity Sci. A. PubMed Scopus Google Scholar, M. M.J. Mol. Cell. Biol. PubMed Scopus Google Scholar). also that cAMP the activity of Akt Sci. A. PubMed Scopus Google Scholar). we that we a the of Akt and of the PI3K pathway by cAMP and is by results that cAMP inhibits a of Akt and in M. L. J. J. Mol. Cell. Biol. 1995; PubMed Scopus Google and that cAMP to Akt in J. Biol. Chem. 1997; PubMed Scopus Google Scholar). Akt plays important in cells from various shown that Akt its activities by important for apoptosis such as (10Del Peso L. Gonzalez-Garcia M. Page C. G. Science. 1997; PubMed Scopus Google Scholar, S.R. H. H. Greenberg M.E. Cell. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar), Science. 1998; PubMed Scopus Google Scholar), A. A. M.J. P. Anderson M.J. J. Greenberg M.E. Cell. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar), activation of the cAMP-dependent pathway can and P. P. M. 1997; PubMed Scopus Google shown that activation of the cAMP pathway apoptosis activation in a In the cAMP programmed cell death G. C. B. A. H. Mol. Cell. Biol. 1997; 17: PubMed Scopus (59) Google Scholar). In and cAMP in a to induce apoptosis activation 1999; PubMed Scopus Google Scholar). we that these to the cAMP-dependent pathway to the for we that of the Akt activity of the the of In the we shown that cAMP inhibits the in of and the activities of Akt and which that of the PI3K pathway are also by the and cAMP-dependent are involved in various cellular events cell and important to the and its role in the are to D. J. J. and A. for phosphatidylinositol 3-kinase growth factor kinase cyclic AMP cAMP-dependent protein kinase myristoylated Akt p70 S6 kinase phosphatidylinositol phosphatidylinositol 3,4-bisphosphate and of