Published OnlineFirst August 14, 2017; DOI: 10.1158/0008-5472.CAN-17-1456 Cancer Review Research The Rac GTPase in Cancer: From Old Concepts to New Paradigms Marcelo G. Kazanietz1 and Maria J. Caloca2 Abstract Rho family GTPases are critical regulators of cellular func- mislocalization of Rac signaling components. The unexpected tions that play important roles in cancer progression. Aberrant pro-oncogenic functions of Rac GTPase-activating proteins also activity of Rho small G-proteins, particularly Rac1 and their challenged the dogma that these negative Rac regulators solely regulators, is a hallmark of cancer and contributes to the act as tumor suppressors. The potential contribution of Rac tumorigenic and metastatic phenotypes of cancer cells. This hyperactivation to resistance to anticancer agents, including review examines the multiple mechanisms leading to Rac1 targeted therapies, as well as to the suppression of antitumor hyperactivation, particularly focusing on emerging paradigms immune response, highlights the critical need to develop ther- that involve gain-of-function mutations in Rac and guanine apeutic strategies to target the Rac pathway in a clinical setting. nucleotide exchange factors, defects in Rac1 degradation, and Cancer Res; 77(20); 5445–51. Ó2017 AACR. Introduction directed toward targeting Rho-regulated pathways for battling cancer. Exactly 25 years ago, two seminal papers by Alan Hall and Nearly all Rho GTPases act as molecular switches that cycle colleagues illuminated us with one of the most influential dis- between GDP-bound (inactive) and GTP-bound (active) forms. coveries in cancer signaling: the association of Ras-related small Activation is promoted by guanine nucleotide exchange factors GTPases of the Rho family with actin cytoskeleton reorganization (GEF) responsible for GDP dissociation, a process that normally (1, 2). Those findings set the mechanistic basis for the control of occurs very slowly, thereby facilitating the exchange for GTP that is cell motility, invasiveness, and metastasis in response to extra- present at much higher cytosolic concentrations. On the other cellular receptors. The Rho small (21 kDa) G-protein family hand, GTPase-activating proteins (GAP) inactivate Rho proteins comprises 20 members categorized into Rac (Rac1, Rac2, Rac3, by accelerating their intrinsic rate of GTP hydrolysis. Once in the and RhoG), Rho (RhoA, RhoB, and RhoC), and Cdc42 (Cdc42, inactive conformation, Rho GTPases associate with guanine TC10, Chip, TCL, and Wrch-1) subfamilies, and other less studied nucleotide dissociation inhibitors (GDI), a step that contributes GTPases that include RhoD, RhoE, and RhoH (3). Rho GTPases to their stabilization and precludes them from getting activated control a variety of cellular functions through regulation of actin (3, 4). In cancer, changes in abundance of Rho GTPases and their contractility and peripheral actin structures, including cell mor- regulators, or excessive input signals leading to their activation, phology, locomotion, and polarity. Accordingly, they are key have been associated with disease progression (3–5). More recent- players in physiologic processes such as embryonic development, ly, point mutations and deregulated stability or localization of neuronal plasticity, phagocytosis, and stem cell formation. Dereg- these proteins have been identified as mechanisms that contribute ulation of Rho GTPase function in cancer is associated with to tumorigenesis and metastasis, which will be discussed in this fundamental hallmarks of progression, including changes in gene review particularly for the GTPase Rac (summarized in Fig. 1 expression, cell survival, oncogenic transformation, tumor metab- and Table 1). olism, and invasiveness (3–5). Deciphering the key effectors and regulators of Rho family members became a crucial undertaking in cancer cell biology, and significant research efforts have been Distinctive Roles for Rho GTPases in Cancer Initiation and Progression The initial evidence that Rho GTPases are positively involved in cancer cell growth arised from studies showing transforming 1Department of Systems Pharmacology and Translational Therapeutics, Perel- man School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania. activity by active forms and inhibition by dominant-negative 2Instituto de Biología y Genetica Molecular, Consejo Superior de Investigaciones forms (see for example ref. 6). Elevated expression of Rac1, the Científicas, Universidad de Valladolid, Valladolid, Spain. active splice variant Rac1b, and other related GTPases has been Corresponding Authors: Marcelo G. Kazanietz, Perelman School of Medicine, frequently observed in human cancer, which in some cases University of Pennsylvania, 1256 Biomedical Research Building II/III, 421 Curie correlated with aggressiveness and poor prognosis (3). Consis- Blvd., Philadelphia, PA 19104-6160. Phone: 215-898-0253; Fax: 215-746-8941; tently, studies in mouse models supported the requirement of Rac E-mail: [email protected]; and Maria J. Caloca, Instituto de Biología y for tumor growth. For example, genetic deletion of the Rac1 gene Genetica Molecular, Consejo Superior de Investigaciones Científicas, Universi- in mice impairs the development of mutant KRas-driven cancer in dad de Valladolid, 47003 Valladolid, Spain. E-mail: [email protected] skin, lung, and pancreas (7–9). Loss of Rac2, but not Rac1, delays doi: 10.1158/0008-5472.CAN-17-1456 the initiation of acute myeloid leukemia, although the survival of Ó2017 American Association for Cancer Research. fully transformed leukemia cells is dependent on Rac1 and Rac2 www.aacrjournals.org 5445 Downloaded from cancerres.aacrjournals.org on October 2, 2021. © 2017 American Association for Cancer Research. Published OnlineFirst August 14, 2017; DOI: 10.1158/0008-5472.CAN-17-1456 Kazanietz and Caloca Activating Active Rac1 Defective Rac Rac-GEF Rac-GAP mutations splice variant degradation hyperactivation downregulation GTP GTP GTP GDP GTP GDP Rac Rac1b Rac Rac Rac Rac GEF GAPs GEF HACE1 GEF GTP GEF Rac GDI Ub Protumorigenic Ub Ub Rac-GAPs Ub GDP GAP Rac GAP Proteasome GAP GAP GAP-independent Altered functions Rac/Rho balance GTP GTP Nucleolus Rac Rac GDP Ect2 GTP Rac Rac Rac GTP mislocalization Rho rDNA transcription Actin polymerization Nuclear plasticity Actomyosin contractility © 2017 American Association for Cancer Research Figure 1. Mechanisms of Rac deregulation in cancer. Activating mutations in Rac1 have been recently found in various cancers, including Rac1P29S, identified as a driver mutation in melanomas. Another hyperactive form is the splice variant Rac1b present in a number of cancers. Degradation of Rac1 by the proteasome contributes to the control of Rac protein expression levels, and it could be impaired in tumors as a consequence of missense mutations in the ubiquitin ligase HACE1. Hyperactivation of Rac-GEFs as a consequence of overexpression or mutation is also a prominent cause of Rac1 deregulation in cancer. Rac-GAPs have a complex role in cancer, as their reduced expression contributes to Rac1 hyperactivation in some tumors; however, overexpression of specific GAPs has been also linked to aggressiveness, most likely through GAP-independent functions. Abnormal nuclear Rac1 localization is also common in cancer, an effect that has been associated with improper nucleocytoplasmic shuttling. Deregulated GEF activity in nuclear compartments, such as the nucleolus, may lead to localized Rac1 activation and enhanced rRNA synthesis. (10). Similarly, deletion of Rac3 resulted in increased survival of a ablation of the RhoB gene in mice (a gene deleted in many human À À transgenic mouse model of acute lymphoblastic leukemia (11). cancers) enhances carcinogen-induced skin cancer (15). Rhob / The involvement of Rac1 in cancer progression has also been mice are less prone to form squamous cell carcinomas following strengthened by means of genetic deletion or silencing of Rac- chronic exposure to UVB, arguing that RhoB favors early stages of specific GEFs such as P-Rex1 and Tiam1 in mouse models, oncogenesis; however, RhoB could also limit the progression to resulting in impaired tumorigenic and metastatic phenotypes highly aggressive tumors (16). Deletion of the RhoC gene in the (12–14). It became increasingly evident, however, that unlike MMTV-PyVT mouse model of breast cancer significantly reduces Rac, Rho could exert either pro- or antioncogenic actions in vivo lung metastasis of mammary tumors (17). However, deletion of in different contexts. For example, early studies revealed that RhoC or RhoA genes failed to suppress KRas-mediated lung 5446 Cancer Res; 77(20) October 15, 2017 Cancer Research Downloaded from cancerres.aacrjournals.org on October 2, 2021. © 2017 American Association for Cancer Research. Published OnlineFirst August 14, 2017; DOI: 10.1158/0008-5472.CAN-17-1456 Rac GTPases in Cancer Table 1. Common cancer-associated alterations in Rac1, Rac-GEFs, and mutants in KRas) and Rac1Q61R (paralogous to Q61 mutants in Rac-GAPs KRas) have been also identified in head and neck, and prostate Alteration Examples cancer, respectively (22). It should not be assumed that the same Rac1 Upregulation Lung and breast cancer scenario is true for other Rho GTPases, because RhoA mutations Point mutations Rac1Q61R (prostate cancer) Rac1P29S (melanoma) found in gastric adenocarcinomas, head and neck cancer, and Rac1G12V/G12R/P34R/Q61R/Q61K (germ cell lymphomas could be either gain-of-function (for example, Y42C G17V testicular