Aberrant expression and activity of G proteins and G protein coupled receptors (GPCRs) are frequently associated with tumorigenesis. Deep sequencing studies show that 4.2% of tumors carry activating mutations in GNAS (encoding Gαs), and that oncogenic activating mutants in genes encoding Gαq family members (GNAQ or GNA11) are present in ~66% and ~6% of melanomas arising in the eye and skin, respectively. Furthermore, nearly 20% of human tumors harbor mutations in GPCRs. Many human cancer-associated viruses also express constitutively active viral GPCRs. These studies indicate that G proteins, GPCRs and their linked signaling circuitry represent novel therapeutic targets for cancer prevention and treatment.

Plexins represent a novel family of transmembrane receptors that transduce attractive and repulsive signals mediated by the axon-guiding molecules semaphorins. Emerging evidence implicates Rho GTPases in these biological events. However, Plexins lack any known catalytic activity in their conserved cytoplasmic tails, and how they transduce signals from semaphorins to Rho is still unknown. Here we show that Plexin B2 associates directly with two members of a recently identified family of Dbl homology/pleckstrin homology containing guanine nucleotide exchange factors for Rho, PDZ-Rho-GEF, and Leukemia-associated Rho GEF (LARG). This physical interaction is mediated by their PDZ domains and a PDZ-binding motif found only in Plexins of the B family. In addition, we show that ligand-induced dimerization of Plexin B is sufficient to stimulate endogenous RhoA potently and to induce the reorganization of the cytoskeleton. Moreover, overexpression of the PDZ domain of PDZ-RhoGEF but not its regulator of G protein signaling domain prevents cell rounding and neurite retraction of differentiated PC12 cells induced by activation of endogenous Plexin B1 by semaphorin 4D. The association of Plexins with LARG and PDZ-RhoGEF thus provides a direct molecular mechanism by which semaphorins acting on Plexin B can control Rho, thereby regulating the actin-cytoskeleton during axonal guidance and cell migration.The Rho family of small GTP-binding proteins, which includes Rho, Rac, and Cdc42, participate in a number of key cellular events, such as the regulation of cell morphology, cell aggregation, tissue polarity, cytokinesis, cell motility, smooth muscle contraction, and cell growth control (reviewed in Refs. 1 and 2). In vivo, the activity of small GTP-binding proteins of the Rho family is tightly regulated. Guanine nucleotide exchange factors (GEFs) 1 act as activators by promoting the conversion of the inactive GDP-bound to the active GTP-bound species. All known GEFs for GTP-binding proteins have in common a 250-amino acid stretch of significant similarity with Dbl, called Dbl homology (DH) domain, that is critical for their ability to stimulate nucleotide exchange toward GTPases of the Rho family and is often adjacent to a pleckstrin-homology (PH) domain (reviewed in Ref.3). In turn, these GEFs link a variety of cell surface receptors to the activation of Rho proteins, thereby regulating the dynamic remodeling of actin-containing cytostructures and gene expression.Recently, a new family of DH-domain containing RhoGEFs, including PDZ-RhoGEF, leukemia-associated RhoGEF (LARG), and p115RhoGEF, has been identified (4 -6). In addition to their DH/PH domains, these GEFs exhibit a number of structural motifs suggestive of a role in signal transduction. Indeed, these GEFs associate with G␣ 12 and G␣ 13 through a regulators of G protein signaling (RGSs)-like domain (5, 7, 8), thus providing a direct link from these heterotrimeric G protein ␣-subunits and their coupled cell surface receptors to Rho. Of interest, PDZ-RhoGEF and LARG, but n...

Polarized cell migration results from the transduction of extracellular cues promoting the activation of Rho GTPases with the intervention of multidomain proteins, including guanine exchange factors. P-Rex1 and P-Rex2 are Rac GEFs connecting G␤␥ and phosphatidylinositol 3-kinase signaling to Rac activation. Their complex architecture suggests their regulation by protein-protein interactions. Novel mechanisms of activation of Rho GTPases are associated with mammalian target of rapamycin (mTOR), a serine/ threonine kinase known as a central regulator of cell growth and proliferation. Recently, two independent multiprotein complexes containing mTOR have been described. mTORC1 links to the classical rapamycin-sensitive pathways relevant for protein synthesis; mTORC2 links to the activation of Rho GTPases and cytoskeletal events via undefined mechanisms. Here we demonstrate that P-Rex1 and P-Rex2 establish, through their tandem DEP domains, interactions with mTOR, suggesting their potential as effectors in the signaling of mTOR to Rac activation and cell migration. This possibility was consistent with the effect of dominant-negative constructs and short hairpin RNA-mediated knockdown of P-Rex1, which decreased mTOR-dependent leucine-induced activation of Rac and cell migration. Rapamycin, a widely used inhibitor of mTOR signaling, did not inhibit Rac activity and cell migration induced by leucine, indicating that P-Rex1, which we found associated to both mTOR complexes, is only active when in the mTORC2 complex. mTORC2 has been described as the catalytic complex that phosphorylates AKT/PKB at Ser-473 and elicits activation of Rho GTPases and cytoskeletal reorganization. Thus, P-Rex1 links mTOR signaling to Rac activation and cell migration.P-Rex1 and P-Rex2 are Rac guanine exchange factors connecting G protein-coupled receptors, through G␤␥ and phosphatidylinositol 3-kinase, to Rac activation. In particular, P-Rex1 has been associated with the activation of Rac2, generating reactive oxygen species in neutrophils. P-Rex2 (showing two splice variants) is similarly regulated by G␤␥ and phosphatidylinositol 3-kinase. Northern blot assays revealed a differential distribution of the two members of the P-Rex 3 family, suggesting that they exert equivalent functions in different cellular populations (1-3, 7-11). The complex architecture of this family of proteins, constituted by a catalytic DH domain, followed by a phosphatidylinositol 3,4,5-trisphosphate-sensitive pleckstrin homology domain, two DEP and two PDZ domains in tandem, and a long carboxyl terminus (except for P-Rex2b, which is the short version, having a reduced carboxyl terminus), suggests that these Rac guanine exchange factors might be regulated by diverse protein-protein interactions modulating signal transduction pathways associated with the activation of Rac. In fact, in the developing brain, P-Rex1 is associated with neuronal migration in response to nerve growth factor (12, 13).The mammalian target of rapamycin, mTOR, a highly conserved serine-threonine k...

Heterotrimeric G proteins of the G i , G s , and G q family control a wide array of physiological functions primarily by regulating the activity of key intracellular second messenger-generating systems. ␣ subunits of the G 12 family, G␣ 12 and G␣ 13 , however, can promote cellular responses that are independent of conventional second messengers but that result from the activation of small GTP-binding proteins of the Rho family and their downstream targets. These findings led to the identification of a novel family of guanine-nucleotide exchange factors (GEFs) that provides a direct link between G␣ 12/13 and Rho stimulation. Recent observations suggest that many cellular responses elicited by G␣ q and its coupled receptors also require the functional activity of Rho. However, available evidence suggests that G␣ q may act on pathways downstream from Rho rather than by promoting Rho activation. These seemingly conflicting observations and the recent development of sensitive assays to assess the in vivo levels of active Rho prompted us to ask whether G␣ q and its coupled receptors can stimulate endogenous Rho. Here we show that the expression of activated forms of G␣ q and the stimulation of G qcoupled receptors or chimeric G␣ q molecules that respond to G i -linked receptors can promote a robust activation of endogenous Rho in HEK-293T cells. Interestingly, this response was not prevented by molecules interfering with the ability of G␣ 13 to stimulate its linked RhoGEFs, together suggesting the existence of a novel molecular mechanism by which G␣ q and the large family of G q -coupled receptors can regulate the activity of Rho and its downstream signaling pathways. G protein-coupled receptors (GPCRs)1 represent the largest family of cell surface molecules involved in signal transmission. They owe their name to their extensively studied interaction with heterotrimeric G proteins (␣, ␤, and ␥ subunits), which undergo conformational changes that lead to the exchange of GDP for GTP bound to the ␣ subunit upon receptor activation. Consequently, GTP-bound G␣ subunits of the G␣ i , G␣ s , G␣ q , and G␣ 12 family and free G ␤␥ subunits stimulate a variety of effector molecules thereby activating or inhibiting key second messenger-generating systems (1). However, recent evidence suggests that many cellular responses elicited by activation of heterotrimeric G proteins are not mediated by classical second messengers, but they involve not yet fully understood molecular mechanisms that result in the activation of small GTPbinding proteins of the Ras and Rho families. For example, it was observed that activated forms of G␣ 12 and G␣ 13 promote stress fiber formation, the assembly focal adhesions, the transcriptional activation of the serum responsive factor, and cellular transformation through Rho-dependent pathways without affecting conventional second messengers (2-4). These observations prompted the study of the underlying molecular mechanisms by which G␣ 12/13 activate Rho. In this regard, recent work has revealed that a nov...

The effect of endothelin-1 on the phosphorylation of ␣ 1b -adrenoreceptors, transfected into rat-1 fibroblasts, was studied. Basal ␣ 1b -adrenoreceptor phosphorylation was markedly increased by endothelin-1, norepinephrine, and phorbol esters. The effect of endothelin-1 was dose dependent (EC 50 Ϸ 1 nM), reached its maximum 5 min after stimulation, and was inhibited by BQ-123, an antagonist selective for ET A receptors. Endothelin-1-induced ␣ 1b -adrenoreceptor phosphorylation was attenuated by staurosporine or genistein and essentially abolished when both inhibitors were used together. The effect of norepinephrine was not modified by either staurosporine or genistein alone, and it was only partially inhibited when both were used together. These data suggest the participation of protein kinase C and tyrosine kinase(s) in endothelin-1-induced receptor phosphorylation. However, phosphoaminoacid analysis revealed the presence of phosphoserine and traces of phosphothreonine, but not of phosphotyrosine, suggesting that the putative tyrosine kinase(s), activated by endothelin, could act in a step previous to receptor phosphorylation. The effect of endothelin-1 on ␣ 1b -adrenoreceptor phosphorylation was not mediated through pertussis toxin-sensitive G proteins. Calcium mobilization induced by norepinephrine was diminished by endothelin-1. Norepinephrine and endothelin-1 increased [ 35 S]GTP␥S binding to control membranes. The effect of norepinephrine was abolished in membranes obtained from cells pretreated with endothelin-1. Interestingly, genistein plus staurosporine inhibited this effect of the endothelial peptide. Endothelin-1 did not induce ␣ 1b -adrenoreceptor internalization. Our data indicate that activation of ET A receptors by endothelin-1 induces ␣ 1b -adrenoreceptor phosphorylation and alters G protein coupling.

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Somatic gain-of-function mutations of GNAQ and GNA11, which encode α subunits of heterotrimeric Gαq/11 proteins, occur in about 85% of cases of uveal melanoma (UM), the most common cancer of the adult eye. Molecular therapies to directly target these oncoproteins are lacking, and current treatment options rely on radiation, surgery, or inhibition of effector molecules downstream of these G proteins. A hallmark feature of oncogenic Gαq/11 proteins is their reduced intrinsic rate of hydrolysis of guanosine triphosphate (GTP), which results in their accumulation in the GTP-bound, active state. Here, we report that the cyclic depsipeptide FR900359 (FR) directly interacted with GTPase-deficient Gαq/11 proteins and preferentially inhibited mitogenic ERK signaling rather than canonical phospholipase Cβ (PLCβ) signaling driven by these oncogenes. Thereby, FR suppressed the proliferation of melanoma cells in culture and inhibited the growth of Gαq-driven UM mouse xenografts in vivo. In contrast, FR did not affect tumor growth when xenografts carried mutated B-RafV600E as the oncogenic driver. Because FR enabled suppression of malignant traits in cancer cells that are driven by activating mutations at codon 209 in Gαq/11 proteins, we envision that similar approaches could be taken to blunt the signaling of non-Gαq/11 G proteins.

We have identified an activator of Rac, P-REX2, that is structurally related to the exchange factor PtdIns(3,4,5)-dependent Rac exchanger (P-REX1), but exhibits distinct tissuespecific expression. P-REX2 is spliced into two RNA species, 3.5 and 10 kb in size. The cDNA corresponding to the smaller transcript encodes a protein that exhibits strong similarity with P-REX1 within its N-terminal domains, but differs in the C-terminal region. P-REX2 promoted increased levels of GTP-bound Rac that could be further stimulated by enhancing PI-3K activity. Thus, P-REX2 may serve as a novel link between Rac activation and the PI-3 kinase pathway.