cAMP stimulates protein kinase B in a Wortmannin‐insensitive manner

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Abstract
Activation of protein kinase B (PKB) by growth factors has been demonstrated to proceed via phosphatidylinositol 3‐kinase (PI3‐kinase). Here, we show that agents which raise intracellular cAMP can also stimulate PKB. However, this effect is not sensitive to wortmannin, indicating that it is PI3‐kinase independent. This activation does not appear to result from direct phosphorylation by protein kinase A (PKA) since GST–PKB is not an effective PKA substrate. In addition, the activation pathway of PKB by cAMP seems to be linked to that of growth factors, albeit downstream of PI3‐kinase. Evidence for this is that a constitutive active PKB, T308D, S473D, containing activating mutations in the serine and threonine residues which are phosphorylated subsequent to PI3‐kinase activation, cannot be further stimulated by cAMP elevations. Hence, these data suggest that, in addition to growth factors, cAMP can also lead to activation of PKB. This cAMP stimulatory action appears to require phosphorylation of T308 and S473, and hence would indicate that cAMP modulates the phosphorylation event of these PKB regulatory sites.