br Rho protein networks in

Rho protein networks in malignancy It has long been recognized that Rho activity and expression are frequently elevated in tumors (Fritz et al., 1999, Orgaz et al., 2014). In this chapter we will introduce the most prominent Rho proteins, their regulators and effectors and provide an overview of their involvement in cancer. Increased expression of RhoA occurs in a variety of carcinomas such as hepatocellular, lung, and colorectal (Vega & Ridley, 2008). Similar, expression of RhoC positively correlates with cancer metastasis and invasion. Indeed, its Boc-D-Asp(OtBu)-OH.DCHA impairs such processes (Bellovin et al., 2006, Clark et al., 2000, van Golen et al., 2002, Vega et al., 2011, Xing et al., 2015) while overexpression enhances cancer cell migration and metastasis (Dietrich et al., 2009, Ma et al., 2007). Conversely, expression of RhoB is downregulated in several malignancies (Huang & Prendergast, 2006) where its expression promotes apoptosis in cancer cell lines (Croft and Olson, 2011, Prendergast, 2001) and its depletion stimulates cancer cell migration (Vega, Colomba, Reymond, Thomas, & Ridley, 2012). Accordingly, miR-21, which is one of the most frequently upregulated miRNAs in cancer, suppresses RhoB expression and thereby increases migration, proliferation and reduces apoptosis (Connolly et al., 2010, Liu et al., 2011). Therefore, it is generally believed that RhoB acts as a tumor suppresso while RoA and RhoC acts as tumor promotors, although recent evidence is challenging this notion for RhoA for at least colorectal tumors (Rodrigues et al., 2014). To date, mutations in Rho proteins have only been sparsely described in cancer and are mainly limited to subsets of gastric carcinoma (Kakiuchi et al., 2014, Kumar et al., 2016). Nonetheless, activation levels of Rho proteins are often found altered. The activation or inactivation of Rho proteins is regulated by numerous signal transduction pathways, such as G-protein coupled receptors (GPCRs) and growth factor receptors. Of the different pathways that mediate GPCR-induced activation of Rho, signaling through Gα12/13 is best understood. Activated Gα12/13 interacts with the Rho exchange factors PDZ-RhoGEF, LARG, and p115RhoGEF (Aittaleb et al., 2010, Fukuhara et al., 2001, Rossman et al., 2005). Thereby, Gα12/13-coupled receptors activate RhoA and control cell migration (Dorsam & Gutkind, 2007). Therefore, it is important to recognize that, in addition to altered expression, deregulation of Rho proteins in cancer mainly occurs at the level of their activation. An example of such deregulation in cancer that has recently received considerable attention is the Rho GAP deleted in liver 1 (DLC-1), of which downregulation is often observed in carcinomas (Braun & Olayioye, 2015) where its appears to be involved in the regulation of cell motility and migration as expression of DLC-1 reduces migration of carcinoma cells (Goodison et al., 2005, Wong et al., 2005). Strikingly, restoring DLC-1 expression in human cancer cells lacking the endogenous protein results in lower capacity to form tumors in mouse xenograft models, suggesting that DLC-1 functions as a tumor suppressor (Durkin et al., 2007, Zhou et al., 2004), while in breast cancer cells, silencing of DLC-1 enhances cell motility (Holeiter et al., 2008). Similarly, the other DLC members, DLC-2 and DLC-3, have comparable roles in cancer progression (Durkin et al., 2007, Leung et al., 2005). Furthermore, in addition to altered expression, regulatory mechanisms exist that alter the function of DLC-1, such as phosphorylation by protein kinase A (PKA), a process enhancing the GAP activity of DLC-1 and thereby inhibiting motility of hepatocellular carcinoma in vitro and metastasis in vivo (Ko et al., 2013). For an overview over deregulated Rho GEFs, we refer to a recent publication by Cook and colleagues (Cook, Rossman, & Der, 2014). Rho proteins exert their effects through activation of their downstream effectors, which include protein kinases, actin nucleation promoting molecules, and adaptor proteins. With regard to cancer, one interesting class of downstream Rho protein effectors, are the serine-threonine kinases, which can effectively be targeted with small-molecule inhibitors, making them interesting targets for cancer therapy. Activation of the Rho-associated protein kinases (ROCKs) regulates actomyosin contractility by increasing the phosphorylation of myosin light chain (MLC). This in turn stimulates myosin to interact with and move along actin filaments (Riento & Ridley, 2003). On the one hand, ROCKs stimulate the activity of MLC kinase (MLCK), while on the other hand ROCKs phosphorylate myosin binding subunit (MBS), which results in the inactivation of MLC phosphatase (MLCP). Increased ROCK activity and gene expression have been demonstrated in various tumors, with increased expression or activation of ROCKs being correlated with metastasis (Kale et al., 2015, Loirand, 2015, Rodriguez-Hernandez et al., 2016).