The G12 family of heterotrimeric G proteins promotes breast cancer invasion and metastasis

Site-specific metastasis formation: chemokines as regulators of tumor cell adhesion, motility and invasion

The metastatic spread of tumors is a well-coordinated process in which different types of cancers tend to form metastases in defined organs. The formation of site-specific metastases requires full compatibility between the intrinsic properties of the tumor cells and the tumor microenvironment. It was recently found that chemokines which are expressed in specific loci promote the adhesion, migration and invasion of tumor cells that express the corresponding receptor(s). Of the different members of the family, the CXCL12 chemokine and its cognate CXCR4 receptor are the prototypes of this process, although other members of the family (e.g. CCR7 and CCR10) also play a role in determination of the metastatic spread. This commentary addresses the fundamental roles of chemokines and their receptors in site-specific metastasis, with emphasis on CXCL12-CXCR4. The article also describes some of the efforts that were performed thus far in order to identify the intracellular components involved in this process. The focus is put on the roles played by proteins that regulate adhesion and migration of tumor cells in response to CXCL12, including mainly focal adhesion kinase (FAK), Pyk2/RAFTK and members of the Rho family of GTPases (RhoA, Rac, Cdc42). This is followed by discussion of open questions that need to be addressed in future research, and of the potential therapeutic implications of the findings that are available to date in this field.

G alpha 12 inhibits alpha2 beta1 integrin-mediated Madin-Darby canine kidney cell attachment and migration on collagen-I and blocks tubulogenesis

Regulation of epithelial cell attachment and migration are essential for normal development and maintenance of numerous tissues. G proteins and integrins are critical signaling proteins regulating these processes, yet in polarized cells little is known about the interaction of these pathways. Herein, we demonstrate that G alpha 12 inhibits interaction of MDCK cells with collagen-I, the major ligand for alpha2 beta1 integrin. Activating G alpha 12 (QL point mutation or stimulating endogenous G alpha 12 with thrombin) inhibited focal adhesions and lamellipodia formation and led to impaired cell migration. Consistent with G alpha 12-regulated attachment to collagen-I, G alpha 12-silenced MDCK cells revealed a more adherent phenotype. Inhibiting Rho kinase completely restored normal attachment in G alpha 12-activated cells, and there was partial recovery with inhibition of Src and protein phosphatase pathways. G alpha 12 activation led to decreased phosphorylation of focal adhesion kinase and paxillin with displacement of alpha2 integrin from the focal adhesion protein complex. Using the MDCK cell 3D-tubulogenesis assay, activated G alpha 12 inhibited tubulogenesis and led to the formation of cyst-like structures. Furthermore, G alpha 12-silenced MDCK cells were resistant to thrombin-stimulated cyst development. Taken together, these studies provide direct evidence for G alpha 12-integrin regulation of epithelial cell spreading and migration necessary for normal tubulogenesis.

Activation of choline kinase by extracellular Ca2+ is Ca(2+)-sensing receptor, Galpha12 and Rho-dependent in breast cancer cells

Breast cancer cell metastases to bone result in osteolysis and release of large quantities of Ca2+ into the bone microenviroment. Extracellular Ca2+ (Ca(o)2+) acting through the Ca(2+)-sensing receptor (CaR), a member of G protein-coupled receptor superfamily, plays an important role in the regulation of multiple signaling pathways. Here, we find that expression of the CaR and Galpha(12) is significantly up-regulated in breast cancer cells (MDA-MB-231 and MCF-7) compared with nonmalignant breast cells (Hs 578Bst and MCF-10A). Ca(o)2+ induces a significant increase in extracellular [(3)H]phosphocholine (P-cho) production in breast cancer cells. Using an anti-CaR antibody to block Ca(o)2+ binding to the CaR and small interfering RNA (siRNA) to silence CaR gene expression, our data demonstrate that [(3)H]P-cho production in response to Ca(o)(2+)-stimulation is CaR-dependent. By analyzing cellular lipid profiles and using siRNA to silence choline kinase (ChoK) expression, we determine that the production of [3H]P-cho is primarily related to CaR-induced ChoK activation, and not degradation of choline phospholipids. Finally, by pretreatment of the cells with either pertussis toxin or C3 exoenzyme, co-immunoprecipiation of Galpha(i), Galpha(q) or Galpha12 with the CaR, and RhoA translocation, we found that the enhancement of ChoK activation and P-cho production in breast cancer cells occurs via a CaR-Galpha12-Rho signaling pathway.

G(alpha)12-mediated pathway promotes invasiveness of nasopharyngeal carcinoma by modulating actin cytoskeleton reorganization

The molecular mechanisms behind the aggressiveness of nasopharyngeal carcinoma (NPC), a highly invasive and metastatic head and neck malignancy, have not been made clear. In this study investigating these mechanisms, guanine nucleotide-binding protein alpha(12) subunit (G(alpha)(12)) signaling was found by microarray analysis to be increased in primary NPC cells and NPC-derived cell lines. Using small interfering RNA to knock down G(alpha)(12) in NPC cells resulted in a reduction in cell migration and invasion as well as a reversal in fibroblastoid morphology. Using microarray analysis, we also found a reduction in expression of key actin dynamics regulators and several epithelial-to-mesenchymal transition-related genes in G(alpha)(12)-depleted NPC cells. Knocking down one of those genes, IQ motif containing GTPase activating protein 1, reduced the migration and formation of adherens junctions and reversed the fibroblastoid morphology of NPC cells, as knocking down G(alpha)(12) was found to do. Immunohistochemical analysis found NPC tumors to have significantly greater levels of G(alpha)(12) protein than the normal basal epithelial cells. Quantitative real-time PCR analysis revealed a significant correlation between G(alpha)(12) mRNA levels and NPC lymph node metastasis. Together, our findings support a model in which activation of G(alpha)(12) signaling promotes tumorigenesis and progression of NPC by modulating actin cytoskeleton reorganization and expression of epithelial-to-mesenchymal transition-related genes. =

Breast cancer migration and invasion depend on proteasome degradation of regulator of G-protein signaling 4

Aberrant signaling through G-protein coupled receptors promotes metastasis, the major cause of breast cancer death. We identified regulator of G-protein signaling 4 (RGS4) as a novel suppressor of breast cancer migration and invasion, important steps of metastatic cascades. By blocking signals initiated through G(i)-coupled receptors, such as protease-activated receptor 1 and CXC chemokine receptor 4, RGS4 disrupted Rac1-dependent lamellipodia formation, a key step involved in cancer migration and invasion. RGS4 has GTPase-activating protein (GAP) activity, which inhibits G-protein coupled receptor signaling by deactivating G-proteins. An RGS4 GAP-deficient mutant failed to inhibit migration and invasion of breast cancer cells in both in vitro assays and a mouse xenograft model. Interestingly, both established breast cancer cell lines and human breast cancer specimens showed that the highest levels of RGS4 protein were expressed in normal breast epithelia and that RGS4 down-regulation by proteasome degradation is an index of breast cancer invasiveness. Proteasome blockade increased endogenous RGS4 protein to levels that markedly inhibit breast cancer cell migration and invasion, which was reversed by an RGS4-targeted short hairpin RNA. Our findings point to the existence of a mechanism for posttranslational regulation of RGS4 function, which may have important implications for the acquisition of a metastatic phenotype by breast cancer cells. Preventing degradation of RGS4 protein should attenuate aberrant signal inputs from multiple G(i)-coupled receptors, thereby retarding the spread of breast cancer cells and making them targets for surgery, radiation, and immune treatment.

Heterotrimeric G proteins and apoptosis: intersecting signaling pathways leading to context dependent phenotypes

Apoptosis, a programmed cell death mechanism, is a fundamental process during the normal development and somatic maintenance of all multicellular organisms and thus is highly conserved and tightly regulated through numerous signaling pathways. Apoptosis is of particular clinical importance as its dysregulation contributes significantly to numerous human diseases, primarily through changes in the expression and activation of key apoptotic regulators. Each of the four families of heterotrimeric G proteins (G(s), G(i/o), G(q/11) and G(12/13)) has been implicated in numerous cellular signaling processes, including proliferation, transformation, migration, differentiation, and apoptosis. Heterotrimeric G protein signaling is an important but not widely studied mechanism regulating apoptosis. G protein Signaling and Apoptosis broadly cover two large bodies of literature and share numerous signaling pathways. Examination of the intersection between these two areas is the focus of this review. Several studies have implicated signaling through each of the four heterotrimeric G protein families to regulate apoptosis within numerous disease contexts, but the mechanism(s) are not well defined. Each G protein family has been shown to stimulate and/or inhibit apoptosis in a context-dependent fashion through regulating numerous downstream effectors including the Bcl-2 family, NF-kappaB, PI3 Kinase, MAP Kinases, and small GTPases. These cell-type specific and G protein coupled receptor dependent effects have led to a complex body of literature of G protein regulation of apoptosis. Here, we review the literature and summarize apoptotic signaling through each of the four heterotrimeric G protein families (and the relevant G protein coupled receptors), and discuss limitations and future directions for research on regulating apoptosis through G protein coupled mechanisms. Continued investigation in this field is essential for the identification of important targets for pharmacological intervention in numerous diseases.

Activation of Rap1 promotes prostate cancer metastasis

Elucidating the mechanisms of prostate cancer (CaP) survival and metastasis are critical to the discovery of novel therapeutic targets. The monomeric G protein Rap1 has been implicated in cancer tumorigenesis. Rap1 signals to pathways involved in cell adhesion, migration, and survival, suggesting Rap1 may promote several processes associated with cancer cell metastasis. Examination of CaP cell lines revealed cells with a high metastatic ability exhibited increased Rap1 activity and reduced expression of the negative regulator Rap1GAP. Rap1 can be further stimulated in these cells by stromal-derived factor (SDF-1), an agonist known to regulate tumor cell metastasis and tropism to bone. Activation of Rap1 increased CaP cell migration and invasion, and inhibition of Rap1A activity via RNAi-mediated knockdown or ectopic expression of Rap1GAP markedly impaired CaP cell migration and invasion. Additional studies implicate integrins alpha4, beta3, and alphavbeta3 in the mechanism of Rap1-mediated CaP migration and invasion. Extending the effect of Rap1 activity in CaP metastasis in vivo, introduction of activated Rap1 into CaP cells dramatically enhanced the rate and incidence of CaP metastasis in a xenograft mouse model. These studies provide compelling evidence to support a role for aberrant Rap1 activation in CaP progression, and suggest that targeting Rap1 signaling could provide a means to control metastatic progression of this cancer.

Role of G12 proteins in oncogenesis and metastasis

The G12 subfamily of heterotrimeric guanine nucleotide-binding proteins consists of two alpha subunits, G alpha12 and G alpha13. These proteins mediate signalling via G protein-coupled receptors and have been implicated in various physiological and pathophysiological processes. A number of direct and indirect effectors of G alpha12 and G alpha13 have been identified that mediate, or have been proposed to mediate, the diverse cellular responses accompanying activation of G12 proteins. This review describes the signalling pathways and cellular events stimulated by G12 proteins, with a particular emphasis on processes that are important in regulating cell migration and invasion, and could potentially be involved in the pathophysiology of cancer metastasis. Experimental findings directly implicating G12 proteins in the spread of metastatic disease are also summarized, indicating the importance of targeted inhibition of G12 signalling as a potential therapeutic option for locally advanced and metastatic disease.

Meta-analysis of genome-wide scans for human adult stature identifies novel Loci and associations with measures of skeletal frame size

Recent genome-wide (GW) scans have identified several independent loci affecting human stature, but their contribution through the different skeletal components of height is still poorly understood. We carried out a genome-wide scan in 12,611 participants, followed by replication in an additional 7,187 individuals, and identified 17 genomic regions with GW-significant association with height. Of these, two are entirely novel (rs11809207 in CATSPER4, combined P-value = 6.1×10⁻⁸ and rs910316 in TMED10, P-value = 1.4×10⁻⁷) and two had previously been described with weak statistical support (rs10472828 in NPR3, P-value = 3×10⁻⁷ and rs849141 in JAZF1, P-value = 3.2×10⁻¹¹). One locus (rs1182188 at GNA12) identifies the first height eQTL. We also assessed the contribution of height loci to the upper- (trunk) and lower-body (hip axis and femur) skeletal components of height. We find evidence for several loci associated with trunk length (including rs6570507 in GPR126, P-value = 4×10⁻⁵ and rs6817306 in LCORL, P-value = 4×10⁻⁴), hip axis length (including rs6830062 at LCORL, P-value = 4.8×10⁻⁴ and rs4911494 at UQCC, P-value = 1.9×10⁻⁴), and femur length (including rs710841 at PRKG2, P-value = 2.4×10⁻⁵ and rs10946808 at HIST1H1D, P-value = 6.4×10⁻⁶). Finally, we used conditional analyses to explore a possible differential contribution of the height loci to these different skeletal size measurements. In addition to validating four novel loci controlling adult stature, our study represents the first effort to assess the contribution of genetic loci to three skeletal components of height. Further statistical tests in larger numbers of individuals will be required to verify if the height loci affect height preferentially through these subcomponents of height.

The first genetic association studies of adult height have confirmed a role of many common variants in influencing human height, but to date, the genetic basis of differences between different skeletal components of height have not been addressed. Here, we take advantage of recent technical and methodological advances to examine the role of common genetic variants on both height and skeletal components of height. By examining nearly 20,000 individuals from the UK and the Netherlands, we provide statistically significant evidence that 17 genomic regions are associated with height, including four novel regions. We also examine, for the first time, the association of these 17 regions with skeletal size measurements of spine, femur, and hip axis length, a measurement of hip geometry known to influence the risk of osteoporotic fractures. We find that some height loci are also associated with these skeletal components, although further statistical tests will be required to verify if these genetic variants act differentially on the individual skeletal measurements. The knowledge generated by this and other studies will not only inform the genetics of human quantitative variation, but will also lead to the potential discovery of many medically important polymorphisms.

Regulation of RhoGEF proteins by G12/13-coupled receptors

G protein-coupled receptors (GPCRs) represent a large family of seven transmembrane receptors, which communicate extracellular signals into the cellular lumen. The human genome contains 720-800 GPCRs, and their diverse signal characteristics are determined by their specific tissue and subcellular expression profiles, as well as their coupling profile to the various G protein families (G(s), G(i), G(q), G(12)). The G protein coupling pattern links GPCR activation to the specific downstream effector pathways. G(12/13) signalling of GPCRs has been studied only recently in more detail, and involves activation of RhoGTPase nucleotide exchange factors (RhoGEFs). Four mammalian RhoGEFs regulated by G(12/13) proteins are known: p115-RhoGEF, PSD-95/Disc-large/ZO-1 homology-RhoGEF, leukemia-associated RhoGEF and lymphoid blast crisis-RhoGEF. These link GPCRs to activation of the small monomeric GTPase RhoA, and other downstream effectors. Misregulated G(12/13) signalling is involved in multiple pathophysiological conditions such as cancer, cardiovascular diseases, arterial and pulmonary hypertension, and bronchial asthma. Specific targeting of G(12/13) signalling-related diseases of GPCRs hence provides novel therapeutic approaches. Assays to quantitatively measure GPCR-mediated activation of G(12/13) are only emerging, and are required to understand the G(12/13)-linked pharmacology. The review gives an overview of G(12/13) signalling of GPCRs with a focus on RhoGEF proteins as the immediate mediators of G(12/13) activation.

Regulation and physiological functions of G12/13-mediated signaling pathways

Accumulating data indicate that G12 subfamily (Galpha12/13)-mediated signaling pathways play pivotal roles in a variety of physiological processes, while aberrant regulation of this pathway has been identified in various human diseases. It has been demonstrated that Galpha12/13-mediated signals form networks with other signaling proteins at various levels, from cell surface receptors to transcription factors, to regulate cellular responses. Galpha12/13 have slow rates of nucleotide exchange and GTP hydrolysis, and specifically target RhoGEFs containing an amino-terminal RGS homology domain (RH-RhoGEFs), which uniquely function both as a GAP and an effector for Galpha12/13. In this review, we will focus on the mechanisms regulating the Galpha12/13 signaling system, particularly the Galpha12/13-RH-RhoGEF-Rho pathway, which can regulate a wide variety of cellular functions from migration to transformation.

Gene expression in human oral squamous cell carcinoma is influenced by risk factor exposure

Oral squamous cell carcinoma (OSCC) is a world health problem and is associated with exposure to different risk factors. In the west, smoking and alcohol consumption are considered to be the main risk factors whilst in India and southeast Asia, betel quid (BQ) chewing is predominant. In this study, we compared the gene expression patterns of oral cancers associated with BQ chewing to those caused by smoking using Affymetrix microarrays. We found that 281 genes were differentially expressed between OSCC and normal oral mucosa regardless of aetiological factors including MMP1, PLAU, MAGE-D4, GNA12, IFITM3 and NMU. Further, we identified 168 genes that were differentially expressed between the BQ and smoking groups including CXCL-9, TMPRSS2, CA12 and RNF24. The expression of these genes was validated using qPCR using independent tissue samples. The results demonstrate that whilst common genes/pathways contribute to the development of oral cancer, there are also other gene expression changes that are specific to certain risk factors. The findings suggest that different carcinogens activate or inhibit specific pathways during cancer development and progression. These unique gene expression profiles should be taken into consideration when developing biomarkers for future use in prognostic or therapeutic applications.

Proteomic analysis of membrane rafts of melanoma cells identifies protein patterns characteristic of the tumor progression stage

The molecular mechanisms controlling the progression of melanoma from a localized tumor to an invasive and metastatic disease are poorly understood. In the attempt to start defining a functional protein profile of melanoma progression, we have analyzed by LC-MS/MS the proteins associated with detergent resistant membranes (DRMs), which are enriched in cholesterol/sphingolipids-containing membrane rafts, of melanoma cell lines derived from tumors at different stages of progression. Since membrane rafts are involved in several biological processes, including signal transduction and protein trafficking, we hypothesized that the association of proteins with rafts can be regulated during melanoma development and affect protein function and disease progression. We have identified a total of 177 proteins in the DRMs of the cell lines examined. Among these, we have found groups of proteins preferentially associated with DRMs of either less malignant radial growth phase/vertical growth phase (VGP) cells, or aggressive VGP and metastatic cells suggesting that melanoma cells with different degrees of malignancy have different DRM profiles. Moreover, some proteins were found in DRMs of only some cell lines despite being expressed at similar levels in all the cell lines examined, suggesting the existence of mechanisms controlling their association with DRMs. We expect that understanding the mechanisms regulating DRM targeting and the activity of the proteins differentially associated with DRMs in relation to cell malignancy will help identify new molecular determinants of melanoma progression.

Activation of leukemia-associated RhoGEF by Galpha13 with significant conformational rearrangements in the interface

The transient protein-protein interactions induced by guanine nucleotide-dependent conformational changes of G proteins play central roles in G protein-coupled receptor-mediated signaling systems. Leukemia-associated RhoGEF (LARG), a guanine nucleotide exchange factor for Rho, contains an RGS homology (RH) domain and Dbl homology/pleckstrin homology (DH/PH) domains and acts both as a GTPase-activating protein (GAP) and an effector for Galpha(13). However, the molecular mechanism of LARG activation upon Galpha(13) binding is not yet well understood. In this study, we analyzed the Galpha(13)-LARG interaction using cellular and biochemical methods, including a surface plasmon resonance (SPR) analysis. The results obtained using various LARG fragments demonstrated that active Galpha(13) interacts with LARG through the RH domain, DH/PH domains, and C-terminal region. However, an alanine substitution at the RH domain contact position in Galpha(13) resulted in a large decrease in affinity. Thermodynamic analysis revealed that binding of Galpha(13) proceeds with a large negative heat capacity change (DeltaCp degrees ), accompanied by a positive entropy change (DeltaS degrees ). These results likely indicate that the binding of Galpha(13) with the RH domain triggers conformational rearrangements between Galpha(13) and LARG burying an exposed hydrophobic surface to create a large complementary interface, which facilitates complex formation through both GAP and effector interfaces, and activates the RhoGEF. We propose that LARG activation is regulated by an induced-fit mechanism through the GAP interface of Galpha(13).

Activated G(alpha)13 impairs cell invasiveness through p190RhoGAP-mediated inhibition of RhoA activity

The GTPase RhoA is a downstream target of heterotrimeric G(13) proteins and plays key roles in cell migration and invasion. Here, we show that expression in human melanoma cells of a constitutively active, GTPase-deficient Galpha(13) form (G(alpha)(13)QL) or lysophosphatidylcholine (LPC)-promoted signaling through G(alpha)(13)-coupled receptors led to a blockade of chemokine-stimulated RhoA activation and cell invasion that was rescued by active RhoA. Melanoma cells expressing G(alpha)(13)QL or cells stimulated with LPC displayed an increase in p190RhoGAP activation, and defects in RhoA activation and invasion were recovered by knocking down p190RhoGAP expression, thus identifying this GTPase-activating protein (GAP) protein as a downstream G(alpha)(13) target that is responsible for these inhibitory responses. In addition, defective stress fiber assembly and reduced migration speed underlay inefficient invasion of G(alpha)(13)QL melanoma cells. Importantly, G(alpha)(13)QL expression in melanoma cells led to impairment in lung metastasis associated with prolonged survival in SCID mice. The data indicate that G(alpha)(13)-dependent downstream effects on RhoA activation and invasion tightly depend on cell type-specific GAP activities and that G(alpha)(13)-p190RhoGAP signaling might represent a potential target for intervention in melanoma metastasis.

RNA aptamer blockade of osteopontin inhibits growth and metastasis of MDA-MB231 breast cancer cells

Osteopontin (OPN) is a secreted phosphoprotein which mediates tumorigenesis, local growth, and metastasis in a variety of cancers. It is a potential therapeutic target for the regulation of cancer metastasis. RNA aptamer technology targeting OPN may represent a clinically viable therapy. In this study, we characterize the critical sequence of an RNA aptamer, termed OPN-R3, directed against human OPN. It has a K(d) of 18 nmol/l and binds specifically to human OPN as determined by RNA electrophoretic mobility assays. In MDA-MB231 human breast cancer cells examined under fluorescence microscopy, OPN-R3 ablates cell surface binding of OPN to its cell surface CD44 and alpha(v)beta(3) integrin receptors. Critical enzymatic components of the OPN signal transduction pathways, PI3K, JNK1/2, Src and Akt, and mediators of extracellular matrix degradation, matrix metalloproteinase 2 (MMP2) and uroplasminogen activator (uPA), are significantly decreased following exposure to OPN-R3. OPN-R3 inhibits MDA-MB231 in vitro adhesion, migration, and invasion characteristics by 60, 50, and 65%, respectively. In an in vivo xenograft model of breast cancer, OPN-R3 significantly decreases local progression and distant metastases. On the basis of this "proof-of-concept" study, we conclude that RNA aptamer targeting of OPN has biologically relevance for modifying tumor growth and metastasis.

PCR differential display-based identification of regulator of G protein signaling 10 as the target gene in human colon cancer cells induced by black tea polyphenol theaflavin monogallate

We have previously reported that black tea polyphenol theaflavin monogallate (TF-2) suppressed COX-2 and induced apoptosis in human colon cancer cells [Lu, J.B., Ho, C.-T., Ghai, G., Chen, K.Y., 2000. Differential effects of theaflavin monogallates on cell growth, apoptosis and Cox-2 gene expression in cancerous versus normal cells. Cancer Res. 60, 6465-6471.]. We now extended the study by using PCR-based differential display to search for genes that were selectively induced by TF-2. Here we report the identification of Regulator of G-binding protein signaling 10 (RGS10) as the target gene, which was induced as early as 4 h after the TF-2 treatment. We then examined the effect of TF-2 on several other RGS genes and found that, in addition to RGS10, TF-2 induced the expression of RGS14, but not RGS4. Other tea polyphenols, including theaflavin-3,3'-digallate (TF-3) and (-) epigallocatechin-3-gallate (EGCG), also induced the expression of RGS10 and RGS14, but not RGS4. However, theaflavin (TF-1), which does not contain the gallate moiety, was ineffective. These results showed for the first time that tea polyphenols can induce the expression of selective RGS genes and that the gallate moiety may be important in this induction. In view of the role of RGS in modulating G-protein mediated signal transduction pathways, our findings may be significant since dysregulation of G-signaling is prominently implicated in carcinogenesis.

Tre1 GPCR initiates germ cell transepithelial migration by regulating Drosophila melanogaster E-cadherin

Despite significant progress in identifying the guidance pathways that control cell migration, how a cell starts to move within an intact organism, acquires motility, and loses contact with its neighbors is poorly understood. We show that activation of the G protein–coupled receptor (GPCR) trapped in endoderm 1 (Tre1) directs the redistribution of the G protein Gβ as well as adherens junction proteins and Rho guanosine triphosphatase from the cell periphery to the lagging tail of germ cells at the onset of Drosophila melanogaster germ cell migration. Subsequently, Tre1 activity triggers germ cell dispersal and orients them toward the midgut for directed transepithelial migration. A transition toward invasive migration is also a prerequisite for metastasis formation, which often correlates with down-regulation of adhesion proteins. We show that uniform down-regulation of E-cadherin causes germ cell dispersal but is not sufficient for transepithelial migration in the absence of Tre1. Our findings therefore suggest a new mechanism for GPCR function that links cell polarity, modulation of cell adhesion, and invasion.

G(12)/G(13)-mediated signalling in mammalian physiology and disease

The human genome encodes hundreds of G-protein-coupled receptors. Their intracellular effects, however, are mediated by only four families of heterotrimeric G proteins: G(s), G(i)/G(o), G(q)/G(11) and G(12)/G(13). Progress in the knowledge about the G(12)/G(13) family has somewhat lagged behind because their downstream effectors remained unknown for several years, and tools to specifically interfere with G(12)/G(13)-mediated signalling were, therefore, missing. However, with the identification of G(12)/G(13)-regulated signalling pathways and the recent application of new techniques, such as conditional gene inactivation, RNA interference or expression of inhibitory proteins, new insights into the in vivo functions of this G-protein family have been gained. It has become clear that this pathway regulates cellular proliferation, movement and morphology in many different organs and that it is centrally involved in various diseases including cancer and cardiovascular disorders. Here, we focus on recent progress made in the analyses of the in vivo functions of mammalian G(12)/G(13)-mediated signalling.

Novel role of thromboxane receptors beta isoform in bladder cancer pathogenesis

These studies were undertaken to determine the potential role of thromboxane receptors (TP) in bladder cancer. The data reported herein show that expression of the TP-beta receptor protein is increased in tissue obtained from patients with bladder cancer and associated with a significantly poorer prognosis (P < 0.005). Bladder cancer cell lines express the TP-beta isoform, unlike immortalized nontransformed urothelial cells (SV-HUC) that express only the TP-alpha isoform. TP-beta receptor expression, but not TP-alpha, promoted cell proliferation, migration, and invasion in vitro, and also resulted in malignant transformation of SV-HUC cells in vivo. Agonist-mediated phosphorylation of extracellular signal-regulated kinase and FAK was dependent on the expression of TP-beta. Furthermore, TP-beta mediated multiple biological effects by signaling through either G-protein alpha subunit 12 or beta-arrestin 2. Treatment of mice with the TP receptor antagonist GR32191, alone or in combination with cisplatin, significantly delayed tumor onset and prolonged survival of mice transplanted with TCC-SUP bladder cancer cells compared with vehicle or cisplatin alone. These results support the model that the TP-beta receptor isoform plays a unique role in bladder cancer progression and its expression may have predictive value and provide a novel therapeutic target.

Extra-nuclear signaling of progesterone receptor to breast cancer cell movement and invasion through the actin cytoskeleton

Progesterone plays a role in breast cancer development and progression but the effects on breast cancer cell movement or invasion have not been fully explored. In this study, we investigate the actions of natural progesterone and of the synthetic progestin medroxyprogesterone acetate (MPA) on actin cytoskeleton remodeling and on breast cancer cell movement and invasion. In particular, we characterize the nongenomic signaling cascades implicated in these actions. T47-D breast cancer cells display enhanced horizontal migration and invasion of three-dimensional matrices in the presence of both progestins. Exposure to the hormones triggers a rapid remodeling of the actin cytoskeleton and the formation of membrane ruffles required for cell movement, which are dependent on the rapid phosphorylation of the actin-regulatory protein moesin. The extra-cellular small GTPase RhoA/Rho-associated kinase (ROCK-2) cascade plays central role in progesterone- and MPA-induced moesin activation, cell migration and invasion. In the presence of progesterone, progesterone receptor A (PRA) interacts with the G protein Gα₁₃, while MPA drives PR to interact with tyrosine kinase c-Src and to activate phosphatidylinositol-3 kinase, leading to the activation of RhoA/ROCK-2. In conclusion, our findings manifest that progesterone and MPA promote breast cancer cell movement via rapid actin cytoskeleton remodeling, which are mediated by moesin activation. These events are triggered by RhoA/ROCK-2 cascade through partially differing pathways by the two compounds. These results provide original mechanistic explanations for the effects of progestins on breast cancer progression and highlight potential targets to treat endocrine-sensitive breast cancers.

Persistent transactivation of EGFR and ErbB2/HER2 by protease-activated receptor-1 promotes breast carcinoma cell invasion

Hyperactivation of ErbB signaling is implicated in metastatic breast cancer. However, the mechanisms that cause dysregulated ErbB signaling and promote breast carcinoma cell invasion remain poorly understood. One pathway leading to ErbB activation that remains unexplored in breast carcinoma cell invasion involves transactivation by G-protein-coupled receptors (GPCRs). Protease-activated receptor-1 (PAR1), a GPCR activated by extracellular proteases, is overexpressed in invasive breast cancer. PAR1 is also proposed to function in breast cancer invasion and metastasis, but how PAR1 contributes to these processes is not known. In this study, we report that proteolytic activation of PAR1 by thrombin induces persistent transactivation of EGFR and ErbB2/HER2 in invasive breast carcinoma, but not in normal mammary epithelial cells. PAR1-stimulated EGFR and ErbB2 transactivation leads to prolonged extracellular signal-regulated kinase-1 and -2 signaling and promotes breast carcinoma cell invasion. We also show that PAR1 signaling through Galpha(i/o) and metalloprotease activity is critical for ErbB transactivation and cellular invasion. Finally, we demonstrate that PAR1 expression in invasive breast carcinoma is essential for tumor growth in vivo assessed by mammary fat pad xenografts. These studies reveal a critical role for PAR1, a receptor activated by tumor-generated proteases, in hyperactivation of ErbB signaling that promotes breast carcinoma cell invasion.

Thromboxane A2 receptors in prostate carcinoma: expression and its role in regulating cell motility via small GTPase Rho

Thromboxane A(2) (TxA(2)) is a prostanoid formed by thromboxane synthase using the cyclooxygenase product prostaglandin H(2) as the substrate. Previously, increased expression of thromboxane synthase was found in prostate tumors, and tumor cell motility was attenuated by inhibitors of thromboxane synthase. This study was undertaken to elucidate how tumor motility is regulated by TxA(2). Here, we report that human prostate cancer cells express functional receptors for TxA(2) (TP). Ligand binding assay found that PC-3 cells binded to SQ29548, a high-affinity TP antagonist, in a saturable manner with K(d) of 3.64 nmol/L and B(max) of 120.4 fmol per million cells. Treatment of PC-3 cells by U46619, a TP agonist, induced PC-3 cell contraction, which was blocked by pretreatment with the TP antagonist SQ29548 or pinane TxA(2). The migration of prostate cancer cells was significantly inhibited either by sustained activation of TP or by blockade of TP activation, suggesting that TP activation must be tightly controlled during cell migration. Further studies found that small GTPase RhoA was activated by TP activation, and pretreatment of PC-3 cells with Y27632, a Rho kinase (ROCK) inhibitor, blocked U46619-induced cell contraction. A dominant-negative mutant of RhoA also blocked U46619-induced cell contraction. Taken together, the data suggest that TPs are expressed in prostate cancer and activation of TPs regulates prostate cancer cell motility and cytoskeleton reorganization through activation of Rho.

Colonization of experimental murine breast tumours by Escherichia coli K-12 significantly alters the tumour microenvironment

The successful application of live bacteria in cancer therapy requires a more detailed understanding of bacterial interaction with the tumour microenvironment. Here, we analysed the effect of Escherichia coli K-12 colonization on the tumour microenvironment by immunohistochemistry and fluorescence microscopy in the murine 4T1 breast carcinoma model. We described the colonization of tumour-bearing mice, as well as the spatiotemporal distribution of E. coli K-12 in the 4T1 tumour tissue over a period of 14 days. The colonization resulted within 3 days in large avascular necrotic tissue, redistribution of hypoxic areas and an enhanced collagen IV deposition within the colonized tumour tissue, which changed the tumoral perfusion of systemically injected immunoglobulins. In addition, E. coli K-12 colonization led to the redistribution of tumour-associated macrophages, forming a granulation tissue around bacterial colonies, and also to an increase in TNFalpha and matrix metalloproteinase 9 expression. Colonization of 4T1 tumours by E. coli K-12 resulted in strong reduction of pulmonary metastatic events. These new insights will contribute to the general understanding of the tumour-microbe cross-talk and to the design of bacterial strains with enhanced anticancer efficiency.

LARG and mDia1 link Galpha12/13 to cell polarity and microtubule dynamics

Regulation of cell polarity is a process observed in all cells. During directed migration, cells orientate their microtubule cytoskeleton and the microtubule-organizing-center (MTOC), which involves integrins and downstream Cdc42 and glycogen synthase kinase-3beta activity. However, the contribution of G protein-coupled receptor signal transduction for MTOC polarity is less well understood. Here, we report that the heterotrimeric Galpha(12) and Galpha(13) proteins are necessary for MTOC polarity and microtubule dynamics based on studies using Galpha(12/13)-deficient mouse embryonic fibroblasts. Cell polarization involves the Galpha(12/13)-interacting leukemia-associated RhoGEF (LARG) and the actin-nucleating diaphanous formin mDia1. Interestingly, LARG associates with pericentrin and localizes to the MTOC and along microtubule tracks. We propose that Galpha(12/13) proteins exert essential functions linking extracellular signals to microtubule dynamics and cell polarity via RhoGEF and formin activity.

Galpha12 stimulates apoptosis in epithelial cells through JNK1-mediated Bcl-2 degradation and up-regulation of IkappaBalpha

Apoptosis is an essential mechanism for the maintenance of somatic tissues, and when dysregulated can lead to numerous pathological conditions. G proteins regulate apoptosis in addition to other cellular functions, but the roles of specific G proteins in apoptosis signaling are not well characterized. Galpha12 stimulates protein phosphatase 2A (PP2A), a serine/threonine phosphatase that modulates essential signaling pathways, including apoptosis. Herein, we examined whether Galpha12 regulates apoptosis in epithelial cells. Inducible expression of Galpha12 or constitutively active (QL)alpha12 in Madin-Darby canine kidney cells led to increased apoptosis with expression of QLalpha12, but not Galpha12. Inducing QLalpha12 led to degradation of the anti-apoptotic protein Bcl-2 (via the proteasome pathway), increased JNK activity, and up-regulated IkappaBalpha protein levels, a potent stimulator of apoptosis. Furthermore, the QLalpha12-stimulated activation of JNK was blocked by inhibiting PP2A. To characterize endogenous Galpha12 signaling pathways, non-transfected MDCK-II and HEK293 cells were stimulated with thrombin. Thrombin activated endogenous Galpha12 (confirmed by GST-tetratricopeptide repeat (TPR) pull-downs) and stimulated apoptosis in both cell types. The mechanisms of thrombin-stimulated apoptosis through endogenous Galpha12 were nearly identical to the mechanisms identified in QLalpha12-MDCK cells and included loss of Bcl-2, JNK activation, and up-regulation of IkappaBalpha. Knockdown of the PP2A catalytic subunit in HEK293 cells inhibited thrombin-stimulated apoptosis, prevented JNK activation, and blocked Bcl-2 degradation. In summary, Galpha12 has a major role in regulating epithelial cell apoptosis through PP2A and JNK activation leading to loss of Bcl-2 protein expression. Targeting these pathways in vivo may lead to new therapeutic strategies for a variety of disease processes.

Roles of G-protein-coupled receptor signaling in cancer biology and gene transcription

G-protein-coupled receptors (GPCRs) are ubiquitous mediators of signal transduction across mammalian cell membranes. Among other roles, GPCRs are known to regulate cellular motility, growth and differentiation, and gene transcription, three factors central to the biology of cancer. Because GPCRs are tractable drug targets, mechanisms by which receptors and their associated proteins impact cellular transformation and metastasis might lead to novel cancer therapies. Recent work has elucidated mechanisms explaining correlations between cancer progression and the expression of GPCRs, such as a protease-activated receptor (PAR1), and G-proteins, such as Galpha(12/13). Of special interest, the discovery of novel nuclear roles for heterotrimeric G-proteins expands the direct impact of G-protein signaling on processes fundamental to the pathology of cancer.

A Role for the G12 Family of Heterotrimeric G Proteins in Prostate Cancer Invasion

Many studies have suggested a role for the members of the G12 family of heterotrimeric G proteins (Galpha12 and Galpha13) in oncogenesis and tumor cell growth. However, few studies have examined G12 signaling in actual human cancers. In this study, we examined the role of G12 signaling in prostate cancer. We found that expression of the G12 proteins is significantly elevated in prostate cancer. Interestingly, expression of the activated forms of Galpha12 or Galpha13 in the PC3 and DU145 prostate cancer cell lines did not promote cancer cell growth. Instead, expression of the activated forms of Galpha12 or Galpha13 in these cell lines induced cell invasion through the activation of the RhoA family of G proteins. Furthermore, inhibition of G12 signaling by expression of the RGS domain of the p115-Rho-specific guanine nucleotide exchange factor (p115-RGS) in the PC3 and DU145 cell lines did not reduce cancer cell growth. However, inhibition of G12 signaling with p115-RGS in these cell lines blocked thrombin- and thromboxane A2-stimulated cell invasion. These observations identify the G12 family proteins as important regulators of prostate cancer invasion and suggest that these proteins may be targeted to limit invasion- and metastasis-induced prostate cancer patient mortality.

Transducing the signals: a G protein takes a new identity

The prevailing dogma is that heterotrimeric G proteins exclusively transduce signals from the seven-transmembrane motif-containing cell surface receptors, also known as G protein-coupled receptors (GPCRs). New evidence indicates that Galpha(13), the alpha subunit of the G protein G(13), breaks away from this traditional exclusive signaling alliance with GPCRs to transmit signals from receptor tyrosine kinases (RTKs), such as platelet-derived growth factor receptor (PDGFR), epidermal growth factor receptor (EGFR), and vascular endothelial growth factor receptor (VEGFR). Galpha(13) is involved in cell migration in response to GPCRs activated by lysophosphatidic acid (LPA) or thrombin. A new report indicates that Galpha(13) is also required for cell migration induced by the growth factors, such as PDGF, EGF, or VEGF. GPCR coupling is not required for such RTK-to-Galpha(13) signaling. This new identity for Galpha(13) as a signal transducer for both GPCRs and RTKs may be a forerunner for similar findings involving other Galpha subunits. This expanding role of G proteins in both GPCR signaling and RTK signaling is likely to have a great impact not only on our understanding of cell signaling in general, but also more specifically where the dysregulation of signaling by GPCRs, RTKs, and G proteins cause pathophysiological changes such as in the case of tumorigenesis, tumor progression and/or metastasis.