Cyclic AMP-independent Activation of Protein Kinase A by Vasoactive Peptides

Protein kinase A (PKA) is an important effector enzyme commonly activated by cAMP. The present study focuses on our finding that the vasoactive peptide endothelin-1 (ET1), whose signaling is not coupled to cAMP production, stimulates PKA in two independent cellular models. Using an in vivo assay for PKA activity, we found that ET1 stimulated PKA in HeLa cells overexpressing ET1 receptors and in aortic smooth muscle cells expressing endogenous levels of ET1 receptors. In these cell models, ET1 did not stimulate cAMP production, indicating a novel mechanism for PKA activation. The ETl-induced activation of PKA was found to be dependent on the degradation of inhibitor of κB, which was previously reported to bind and inhibit PKA. ET1 potently stimulated the nuclear factor-κB pathway, and this effect was inhibited by overexpression of the inhibitor of κB dominant negative mutant (IκBαm) and by treatment with the proteasome inhibitor MG-132. Importantly, IκBαm and MG-132 had similar inhibitory effects on ET1-induced activation of PKA without affecting G_s-mediated activation of PKA or ET1-induced phosphorylation of mitogen-activated protein kinase. Finally, another vasoactive peptide, angiotensin II, also stimulated PKA in a cAMP-independent manner in aortic smooth muscle cells. These findings suggest that cAMP-independent activation of PKA might be a general response to vasoactive peptides.