GPCR signaling to Rho GTPases plays a critical role in the tumor microenvironment. Cancer progression relies on the communication among different cell populations recruited into the tumor stroma. These include fibroblasts and endothelial cells from surrounding tissues and bone marrow–derived cells (BMDCs), such as macrophages, leukocytes, monocytes, endothelial progenitor cells, and other myeloid precursors. Chemokines secreted by stromal and cancer cells are key mediators of inflammation, angiogenesis, and immunosuppression, contributing to tumor growth, invasion and metastasis. Among them, several GPCR agonists in the tumor microenvironment activate signaling pathways to Rho GTPases via Gα and Gβγ subunits and their RhoGEF effectors. The leading edge of migrating cells contributing to tumor progression is amplified in the inset to highlight the role of Gβγ as an activator of RhoGEFs that regulate actin cytoskeleton dynamics leading chemotactic cell migration.

GPCR-dependent chemotaxis in individual migrating cells. A general model of chemotactic cell migration is based on studies of fast-moving individual cells, such as neutrophils and Dictyostelium discoideum amoebas (Devreotes and Horwitz, 2015). Accordingly, a chemotactic gradient is sensed by GPCRs located at the proximity of incoming stimuli (A). Then, Gβγ is locally released, recruiting PI3K and RhoGEFs (B). These effectors of Gβγ promote a spatiotemporal restricted activation of Rho GTPases resulting in polarized morphologic changes based on the dynamics of the actin cytoskeleton (C). In this context, cell migration occurs as an integrated effect of protrusions, adhesion, and contraction at the advancing front, followed by rear-edge retraction. Maintained cell migration involves translocation of signaling proteins (D). The process is positively influenced by vesicle trafficking (Murphy et al., 2009). In this regard, Gβγ trafficking, mediated by its interaction with Rab11, activates a PI3K-AKT signaling axis at early endosomes (EE) (García-Regalado et al., 2008). Whether Gβγ-trafficking leads to endosomal activation of RhoGEFs is currently being investigated.

RhoGEFs identified as Gα or Gβγ interactors. GPCRs activate Rho GTPases via direct interactions between Gβγ, GTP-bound Gα_12/13, CaSRs), or Gα_q with the indicated RhoGEFs. These RhoGEF have a modular organization characterized by the presence of a catalytic DH (Dbl-homology) domain, followed by a Pleckstrin homology (PH) domain predicted with the SMART analysis tool (http://smart.embl-heidelberg.de/). The list of GPCR-regulated RhoGEFs is based on studies showing, at least by coimunoprecipitation or other equivalent assays, interaction between the indicated G proteins and RhoGEFs. Gα_12/13-regulated RhoGEFs: p115RhoGEF/ARHGEF1 (Hart et al., 1998), LARG/ARHGEF12 (Fukuhara et al., 2000), PDZ-RhoGEF/ARHGEF11 (Fukuhara et al., 1999), MCF2/Dbl/ARHGEF21 (Jin and Exton, 2000), ARHGEF2/GEF-H1 (Meiri et al., 2014), and ARHGEF28/RGNEF (Masià-Balagué et al., 2015). Gα_q-regulated RhoGEFs: p63RhoGEF/ARHGEF25 (Lutz et al., 2005, 2007), TRIO/ARHGEF23 (Rojas et al., 2007; Vaqué et al., 2013), and ARHGEF28/RGNEF (Masià-Balagué et al., 2015). Gβγ-regulated RhoGEFs: P-Rex1 (Welch et al., 2002) and P-Rex2 (Donald et al., 2004), PLEKHG2 (Ueda et al., 2008), p114RhoGEF/ARHGEF18 (Niu et al., 2003), ARHGEF5/TIM (Wang et al., 2009), and MCF2/Dbl/ARHGEF21(Nishida et al., 1999). As shown at the bottom, the cycle of Rho GTPases is regulated by GEFs, GTPase-activating proteins (GAPs) and guanine-nucleotide dissociation inhibitors (GDIs). G protein–regulated RhoGEFs activate Rho GTPases by promoting exchange of GDP for GTP. GAPs are negative regulators that help the GTPase to hydrolyze GTP to GDP. GDIs inhibit GDP dissociation, keeping the GTPase in an inactive state.