Downregulation of Ral GTPase-activating protein promotes tumor invasion and metastasis of bladder cancer

The small GTPase Ral is known to be highly activated in several human cancers, such as bladder, colon and pancreas cancers. It is reported that activated Ral is involved in cell proliferation, migration and metastasis of bladder cancer. This protein is activated by Ral guanine nucleotide exchange factors (RaIGEFs) and inactivated by Rat GTPase-activating proteins (RaIGAPs), the latter of which consist of heterodimers containing a catalytic alpha 1 or alpha 2 subunit and a common beta subunit. In Ras-driven cancers, such as pancreas and colon cancers, constitutively active Ras mutant activates Ral through interaction with RaIGEFs, which contain the Ras association domain. However, little is known with regard to the mechanism that governs aberrant activation of Ral in bladder cancer, in which Ras mutations are relatively infrequent. Here, we show that Ral was highly activated in invasive bladder cancer cells due to reduced expression of RaIGAP alpha 2, the dominant catalytic subunit in bladder, rather than increased expression of RaIGEFs. Exogenous expression of wild-type RaIGAP alpha 2 in KU7 bladder cancer cells with invasive phenotype, but not mutant RaIGAID alpha 2-N1742K lacking RaIGAP activity, resulted in attenuated cell migration in vitro and lung metastasis in vivo. Furthermore, genetic ablation of Ralgapa2 promoted tumor invasion in a chemically-induced murine bladder cancer model. Importantly, immunohistochennical analysis of human bladder cancer specimens revealed that lower expression of RaIGAP alpha 2 was associated with advanced clinical stage and poor survival of patients. Collectively, these results are highly indicative that attenuated expression of RaIGAP alpha 2 leads to disease progression of bladder cancer through enhancement of Ral activity. Oncogene (2013) 32, 894-902; doi:10.1038/onc.2012.101; published online 26 March 2012