PKG induces the expression of tumor suppressor genes in colon cancer cells

Abstract

The role of cyclic guanosine 3',5′‐monophosphate (cGMP)‐dependent protein kinase (PKG) as an anticancer messenger emerged recently, but there is little known about its action. In this study, we examined the role of PKG‐Iβ on the beta‐catenin pathway and the expression of tumor suppressor proteins in human colon cancer cells. Our results revealed that cells overexpressing PKG‐Iβ showed β‐catenin translocation to the nucleus compared to control cells. This translocation of β‐catenin was accompanied with an increase in T‐cell factor (TCF)‐luciferase activity and β‐catenin binding to TCF oligonucleotide as tested by elecromobility shift assays in both LoVo and SW480 cells. We analyzed the expression of the tumor suppressor retinoblastoma (Rb), p27KIP1, p53, and p21CIP1 at both the protein levels and promoter activities. Cells overexpressing PKG‐Iβ showed an increase in expression of the p21CIP1, p27KIP1, and Rb proteins. Both p21CIP1 and p27KIP1 promoters were activated in LoVo or SW480 cells overexpressing PKG as demonstrated by luciferase reporter assays. Specific PKG inhibitors (DT2/DT3) as well as phosphatase inhibitors (okadaic acid, Calyculin A) reduced β‐catenin accumulation in the nucleus as well as the promoter activities of p21 and Rb luciferase reporter constructs. These results suggest that PKG is stabilizing β‐catenin, probably through activation of a phosphatase. These effects of PKG activation on the expression of tumor suppressor genes may explain, at least in part, the anticancer effects of activation of PKG.