Dissecting the Circuitry of Protein Kinase A and cAMP Signaling in Cancer Genesis

Abstract

Expression of the RIα subunit of the cAMP‐dependent protein kinase type I (PKA‐I) is enhanced in human cancer cell lines, in primary tumors, in transformed cells, and in cells upon stimulation of growth. Signaling via the cAMP pathway may be complex, and the biological effects of the pathway in normal cells may depend upon the physiological state of the cells. However, results of different experimental approaches such as antisense exposure, 8‐Cl‐cAMP treatment, and gene overexpression have shown that the inhibition of RIα/PKA‐I exerts antitumor activity in a wide variety of tumor‐derived cell lines examined in vitro and in vivo. cDNA microarrays have further shown that in a sequence‐specific manner, RIα antisense induces alterations in the gene expression profile of cancer cells and tumors. The cluster of genes that define the “proliferation‐transformation” signature are down‐regulated, and those that define the “differentiation‐reverse transformation” signature are up‐regulated in antisense‐treated cancer cells and tumors, but not in host livers, exhibiting the molecular portrait of the reverted (flat) phenotype of tumor cells. These results reveal a remarkable cellular regulation, elicited by the antisense RIα, superimposed on the regulation arising from the Watson‐Crick base‐pairing mechanism of action. Importantly, the blockade of both the PKA and PKC signaling pathways achieved with the CRE‐transcription factor decoy inhibits tumor cell growth without harming normal cell growth. Thus, a complex circuitry of cAMP signaling comprises cAMP growth regulatory function, and deregulation of the effector molecule by this circuitry may underlie cancer genesis and tumor progression.