Gankyrin Plays An Essential Role In Activation Of Akt Through RhoA-ROCK-PTEN Pathway

Gankyrin was initially characterized as the p28 protein,a component of the 26S proteasome,and was recently identified as an oncoprotein with 7 repeated ankyrin domains.Although the protein levels of Gankyrin are high in hepatocellular carcinomas and Gankyrin functions as an oncoprotein,the exact mechanism of Gankyrin oncogenic effect in cellular transformation remains elusive.Gankyrin is found in other complexes that contain Rb(retinoblastoma protein),the ubiquitin protein ligase Mdm2(murine double minute 2),and NFkB.Gankyrin increases the hyperphosphorylation of Rb and therefore activates E2F-dependent transcription of DNA synthesis genes.Additionally, gankyrin increases the ubiquitylation and degradation of p53 and prevents apoptosis by binding to Mdm2.Gankyrin also inhibit the transcription of NFkB by binding to it. Previously we reported that the protein levels of Gankyrin are elevated in Rastransformed cells.Here,we report that Gankyrin is a key regulator of PI3K/Akt signaling pathway through inhibiting RhoA/ROCK activity.We performed yeast two-hybrid screening with Gankyrin as the bait and identified Rho guanine nucleotide dissociation inhibitor 1 (RhoGDI1).RhoA is a member of Rho-GTPase family,and stimulates multiple signaling events which contribute to cell growth and transformation.GDIs are pivotal regulators of RhoA function,which control the access of RhoA to GEFs and GAPs,to effector targets and to membranes where such effectors reside.We demonstrated that Gankyrin is critical for RhoGDI to interact with RhoA,a critical step to inhibit RhoA and ROCK activity.We also show that RhoA/ROCK inhibits Akt activation through PTEN,and the inhibition of RhoA/ROCK activity by the elevated level of Gankyrin results in a prolonged Akt activation,which is critical for activated Ras-induced transformation and tumorigenesis. Therefore,our findings suggest that Gankyrin is a critical mediator in the activation of Akt and that Gankyrin is a potential therapeutic target in tumors.