G-protein-coupled receptor signaling in cancer development and treatment

Division Of Basic Sciences - Nci

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Abstract

The PKA inhibitor (PKI) family members PKIalpha, PKIbeta, and PKIgamma, bind with high affinity to PKA and block its kinase activity, modulating the extent and duration of PKA mediated signaling events. While PKA is a well known regulator of physiological and oncogenic events, the role of PKI proteins in these pathways has remained elusive. I have demonstrated that overexpression of the PKA binding domain of PKIalpha (PKIalpha1 24) in the skin is sufficient to induce BCC formation. My results suggested that expression levels of endogenous PKIs could have a role in mediating tumor formation or suppression by modulating PKA activity. Indeed, analysis of PKI genomic alterations in the TCGA dataset indicated that amplification of the gene coding for PKIalpha (PKIA) is common in various cancers. The finding that PKIA amplifications are common in several tumor types suggested that PKI overexpression could have functional consequences in tumor growth pathways. Considering the known regulation of MAPK signaling by GPCRs and PKA and the importance of this pathway for oncogenesis, we characterized the effect of PKI on ERK activation downstream of cAMP in HEK293 cells. Our results demonstrated that PKI enhances ERK phosphorylation by driving GPCR Galphas cAMP signaling away from PKA, acting as a molecular switch. This effect was dependent on the stimulation of exchange proteins directly activated by cAMP, or EPACs. While PKIs and PKA do not directly regulate EPAC, PKI inhibition of PKA resulted in increased and sustained cAMP concentrations that, in turn, could intensify the activation of EPAC and its target RAP1. RAP1 may link PKI and MAPK activation, as RAP1 is known to increase MEK and ERK phosphorylation. We next focused on prostate cancer (PC), which exhibited the most significant PKIA amplification frequency. PC cell lines showed significantly increased PKI mRNA levels and recapitulated the effects of PKI and EPAC on MAPK activation. Depleting PKIA mRNA in DU145 castration resistant PC cells reduced metastatic phenotypes in vitro (reduced migration and increased sensitivity to anoikis) and reduced tumor growth in mouse xenograft experiments. While more research is needed to understand the main downstream events regulated by PKI proteins in cancer, our results establish that PKI expression levels can alter GPCR Galphas signaling and promote tumor growth.