Role of VAV2 in EGFR-mediated Rac1 responses in human prostate cancer

The small G-protein Rac1, a member of the Rho family of GTPases, is a central player in cancer progression and metastatic dissemination. Rac1 has been established as a bona fide effector of receptor tyrosine kinases, acting as a signaling node for growth factor-mediated responses such as motility, invasiveness, mitogenesis and gene expression. Previous studies demonstrated that Rac1 is aberrantly hyperactivated in aggressive cellular models of prostate cancer. Here we show that CRISPR-mediated knockout of Rac1 in prostate cancer cells leads to impaired proliferation and migration. Rac1-null cells display major alterations in gene expression, most notably affecting transcriptional programs associated with cell adhesion and extracellular matrix (ECM) regulation, but not those related to epithelial-to-mesenchymal (EMT) transition. Expression analysis of GEFs, the guanine nucleotide exchange factors responsible for Rac1 activation, followed by an unbiased RNAi screening, identified VAV2 as the foremost mediator of epidermal growth factor (EGF)-induced GTP loading onto Rac1 in prostate cancer cells. VAV2-deficient prostate cancer cells display significant proliferative and migratory defects, without noticeable changes in the expression of Rac1-regulated genes such as CDH1 and CDH3, indicating that VAV2 controls a discrete subset of Rac1-dependent responses. Finally, immunohistochemical assessment of VAV2 expression in human prostate specimens revealed significant VAV2 up-regulation in tumoral vs. non-tumoral areas. Our studies revealed important contributions of VAV2 to Rac1 activation in prostate cancer cells and highlight its potential prognostic value as a marker of prostate cancer progression.