Rac Signaling Drives Clear Cell Renal Carcinoma Tumor Growth by Priming the Tumor Microenvironment for an Angiogenic Switch.

Clear cell renal cell carcinoma (ccRCC) remains a common cause of cancer mortality. Better understanding of ccRCC molecular drivers resulted in the development of antiangiogenic therapies that block the blood vessels that supply tumors with nutrients for growth and metastasis. Unfortunately, most ccRCC patients eventually become resistant to initial treatments, creating a need for alternative treatment options. We investigated the role of the small GTPase Rac1 in ccRCC. Analysis of ccRCC clinical samples indicates that Rac signaling drives disease progression and predicts patients with poorer outcomes. Investigation of Rac1 identifies multiple roles for Rac1 in the pathogenesis of ccRCC. Rac1 is overexpressed in RCC cell lines and drives proliferation and migratory/metastatic potential. Rac1 is also critical for endothelial cells to grow and form endothelial tubular networks potentiated by angiogenic factors. Importantly, Rac1 controls paracrine signaling of angiogenic factors including VEGF from renal carcinoma cells to surrounding blood vessels. A novel Rac1 inhibitor impaired the growth and migratory potential of both renal carcinoma cells and endothelial cells and reduced VEGF production by RCC cells, thereby limiting paracrine signaling both in vitro and in vivo Lastly, Rac1 was shown to be downstream of VEGF receptor (VEGFR) signaling and required for activation of MAPK signaling. In combination with VEGFR2 inhibitors, Rac inhibition provides enhanced suppression of angiogenesis. Therefore, targeting Rac in ccRCC has the potential to block the growth of tumor cells, endothelial cell recruitment, and paracrine signaling from tumor cells to other cells in the tumor microenvironment. ©2020 American Association for Cancer Research.