SNX9 promotes metastasis by enhancing cancer cell invasion via differential regulation of RhoGTPases

Despite current advances in cancer research, metastasis remains the leading factor in cancer-related deaths. Here we identify sorting nexin 9 (SNX9) as a new regulator of breast cancer metastasis. We detect an increase in SNX9 expression in human breast cancer metastases compared with primary tumors and demonstrate that SNX9 expression in MDA-MB-231 breast cancer cells is necessary to maintain their ability to metastasize in a chick embryo model. Conversely, SNX9 knockdown impairs this process. In vitro studies using several cancer cell lines derived from a variety of human tumors reveal a role for SNX9 in cell invasion and identify mechanisms responsible for this novel function. We show that SNX9 controls the activation of RhoA and Cdc42 GTPases and also regulates cell motility via the modulation of well-known molecules involved in metastasis, namely RhoA-ROCK and N-WASP. In addition, we find that SNX9 is required for RhoGTPase-dependent, clathrin-independent endocytosis, and in this capacity can functionally substitute to the bona fide Rho GAP, GTPase regulator associated with focal adhesion kinase (GRAF1). Taken together, our data establish novel roles for SNX9 as a multifunctional protein scaffold that regulates, and potentially coordinates, several cellular processes that together can enhance cancer cell metastasis.