Constitutively active Galphaq and Galpha13 trigger apoptosis through different pathways

GQ262 Attenuates Pathological Cardiac Remodeling by Downregulating the Akt/mTOR Signaling Pathway

Cardiac remodeling, a critical process that can lead to heart failure, is primarily characterized by cardiac hypertrophy. Studies have shown that transgenic mice with Gαq receptor blockade exhibit reduced hypertrophy under induced pressure overload. GQ262, a novel Gαq/11 inhibitor, has demonstrated good biocompatibility and specific inhibitory effects on Gαq/11 compared to other inhibitors. However, its role in cardiac remodeling remains unclear. This study aims to explore the anti-cardiac remodeling effects and mechanisms of GQ262 both in vitro and in vivo, providing data and theoretical support for its potential use in treating cardiac remodeling diseases. Cardiac hypertrophy was induced in mice via transverse aortic constriction (TAC) for 4 weeks and in H9C2 cells through phenylephrine (PE) induction, confirmed with WGA and H&E staining. We found that GQ262 improved cardiac function, inhibited the protein and mRNA expression of hypertrophy markers, and reduced the levels of apoptosis and fibrosis. Furthermore, GQ262 inhibited the Akt/mTOR signaling pathway activation induced by TAC or PE, with its therapeutic effects disappearing upon the addition of the Akt inhibitor ARQ092. These findings reveal that GQ262 inhibits cardiomyocyte hypertrophy and apoptosis through the Akt/mTOR signaling pathway, thereby reducing fibrosis levels and mitigating cardiac remodeling.

The role of signaling pathways mediated by the GPCRs CysLTR1/2 in melanocyte proliferation and senescence

In contrast with sun exposure-induced melanoma, rarer melanocytic tumors and neoplasms with low mutational burden present opportunities to study isolated signaling mechanisms. These include uveal melanoma and blue nevi, which are often driven by mutations within the G protein-coupled signaling cascade downstream of cysteinyl leukotriene receptor 2. Here, we review how the same mutations within this pathway drive the growth of melanocytes in one tissue but can inhibit the growth of those in another, exemplifying the role of the tissue environment in the delicate balance between uncontrolled cell growth and senescence.

GPCR-Gα13 Involvement in Mitochondrial Function, Oxidative Stress, and Prostate Cancer

Gα13 and Gα12, encoded by the GNA13 and GNA12 genes, respectively, are members of the G12 family of Gα proteins that, along with their associated Gβγ subunits, mediate signaling from specific G protein-coupled receptors (GPCRs). Advanced prostate cancers have increased expression of GPCRs such as CXC Motif Chemokine Receptor 4 (CXCR4), lysophosphatidic acid receptor (LPAR), and protease activated receptor 1 (PAR-1). These GPCRs signal through either the G12 family, or through Gα13 exclusively, often in addition to other G proteins. The effect of Gα13 can be distinct from that of Gα12, and the role of Gα13 in prostate cancer initiation and progression is largely unexplored. The oncogenic effect of Gα13 on cell migration and invasion in prostate cancer has been characterized, but little is known about other biological processes such as mitochondrial function and oxidative stress. Current knowledge on the link between Gα13 and oxidative stress is based on animal studies in which GPCR-Gα13 signaling decreased superoxide levels, and the overexpression of constitutively active Gα13 promoted antioxidant gene activation. In human samples, mitochondrial superoxide dismutase 2 (SOD2) correlates with prostate cancer risk and prognostic Gleason grade. However, overexpression of SOD2 in prostate cancer cells yielded conflicting results on cell growth and survival under basal versus oxidative stress conditions. Hence, it is necessary to explore the effect of Gα13 on prostate cancer tumorigenesis, as well as the effect of Gα13 on SOD2 in prostate cancer cell growth under oxidative stress conditions.

The emerging roles of Gα12/13 proteins on the hallmarks of cancer in solid tumors

G12 proteins comprise a subfamily of G-alpha subunits of heterotrimeric GTP-binding proteins (G proteins) that link specific cell surface G protein-coupled receptors (GPCRs) to downstream signaling molecules and play important roles in human physiology. The G12 subfamily contains two family members: Gα12 and Gα13 (encoded by the GNA12 and GNA13 genes, respectively) and, as with all G proteins, their activity is regulated by their ability to bind to guanine nucleotides. Increased expression of both Gα12 and Gα13, and their enhanced signaling, has been associated with tumorigenesis and tumor progression of multiple cancer types over the past decade. Despite these strong associations, Gα12/13 proteins are underappreciated in the field of cancer. As our understanding of G protein involvement in oncogenic signaling has evolved, it has become clear that Gα12/13 signaling is pleotropic and activates specific downstream effectors in different tumor types. Further, the expression of Gα12/13 proteins is regulated through a series of transcriptional and post-transcriptional mechanisms, several of which are frequently deregulated in cancer. With the ever-increasing understanding of tumorigenic processes driven by Gα12/13 proteins, it is becoming clear that targeting Gα12/13 signaling in a context-specific manner could provide a new strategy to improve therapeutic outcomes in a number of solid tumors. In this review, we detail how Gα12/13 proteins, which were first discovered as proto-oncogenes, are now known to drive several "classical" hallmarks, and also play important roles in the "emerging" hallmarks, of cancer.

Heterotrimeric Gα12/13 proteins in kidney injury and disease

Gα₁₂ and Gα₁₃ are ubiquitous members of the heterotrimeric guanine nucleotide-binding protein (G protein) family that play central and integrative roles in the regulation of signal transduction cascades within various cell types in the kidney. Gα₁₂/Gα₁₃ proteins enable the kidney to adapt to an ever-changing environment by transducing stimuli from cell surface receptors and accessory proteins to effector systems. Therefore, perturbations in Gα₁₂/Gα₁₃ levels or their activity can contribute to the pathogenesis of various renal diseases, including renal cancer. This review will highlight and discuss the complex and expanding roles of Gα₁₂/Gα₁₃ proteins on distinct renal pathologies, with emphasis on more recently reported findings. Deciphering how the different Gα₁₂/Gα₁₃ interaction networks participate in the onset and development of renal diseases may lead to the discovery of new therapeutic strategies.

Ric-8A gene deletion or phorbol ester suppresses tumorigenesis in a mouse model of GNAQ(Q209L)-driven melanoma

The heterotrimeric G protein α subunit oncogenes GNAQ or GNA11 carry Q209X or R183X activating mutations and are present with ~90% frequency in human uveal melanomas. Forced expression of GNAQ/11^Q209L in melanocytes is sufficient to drive metastatic melanoma in immune-compromised mice. No known drugs directly target these oncogenic G proteins. Ric-8A is the molecular chaperone that selectively folds Gαq/i/13 subunits. Targeting Ric-8A serves as a rational, yet unexplored approach to reduce the functional abundance of oncogenic Gαq/11 in order to blunt cancer signaling. Here, using mouse melanocyte cell graft tumorigenesis models, we determined that Ric-8A genetic ablation attenuated the abundance and melanoma-driving potential of Gαq-Q209L. A new conditional Ric-8A^Flox/Flox; Rosa-CreER^+/− mouse strain was derived and used as a tissue source to culture an immortalized, tamoxifen-inducible Ric-8A knockout melanocyte cell line that required 12-O-tetradecanoylphorbol-13-acetate (TPA, phorbol ester) for growth. The cell line failed to grow tumors when grafted into immune-compromised mice regardless of Ric-8A expression. Stable expression of human GNAQ^Q209L, but not GNAQ^WT in the cell line promoted TPA-independent cell proliferation, and upon cell grafting in mice, the initiation and robust growth of darkly-pigmented melanoma tumors. Deletion of Ric-8A in GNAQ^Q209L cells restored TPA-dependent growth, reduced Gαq-Q209L below detectable levels and completely mitigated tumorigenesis from primary or secondary cell line grafts. Interestingly, TPA treatment of cultured GNAQ^Q209L cells or host animals grafted with GNAQ^Q209L cells also sharply reduced Gαq-Q209L abundance and tumorigenic capacity. Finally, tumorigenesis initiated from GNAQ^Q209L cell grafts, followed by host mouse systemic tamoxifen treatment to delete Ric-8A in the grafted cells completely abrogated GNAQ^Q209L-driven tumor progression unless a stable human RIC-8A transgene was used to rescue the floxed Ric-8A alleles. Our work defines two new rational targets that may be developed as potential uveal melanoma therapies through reduction of Gαq/11-Q209L oncoprotein abundance: (1) Ric-8A inhibition and (2) phorbol ester treatment.

Heterotrimeric G-protein alpha-12 (Gα12) subunit promotes oral cancer metastasis

Oral squamous cell carcinoma (OSCC) has a propensity to spread to the cervical lymph nodes (LN). The presence of cervical LN metastases severely impacts patient survival, whereby the two-year survival for oral cancer patients with involved LN is ~30% compared to over 80% in patients with non-involved LN. Elucidation of key molecular mechanisms underlying OSCC metastasis may afford an opportunity to target specific genes, to prevent the spread of OSCC and to improve patient survival. In this study, we demonstrated that expression of the heterotrimeric G-protein alpha-12 (Gα12) is highly up-regulated in primary tumors and LN of OSCC patients, as assessed by quantitative polymerase chain reaction (qPCR) and immunohistochemistry (IHC). We also found that exogenous expression of the constitutively activated-form of Gα12 promoted cell migration and invasion in OSCC cell lines. Correspondingly, inhibition of Gα12 expression by shRNA consistently inhibited OSCC cell migration and invasion in vitro. Further, the inhibition of G12 signaling by regulator of G-protein signaling (RGS) inhibited Gα12-mediated RhoA activation, which in turn resulted in reduced LN metastases in a tongue-orthotopic xenograft mouse model of oral cancer. This study provides a rationale for future development and evaluation of drug candidates targeting Gα12-related pathways for metastasis prevention.

Gq protein-induced apoptosis is mediated by AKT kinase inhibition that leads to protein kinase C-induced c-Jun N-terminal kinase activation

G(q) protein-coupled receptors (G(q)PCRs) regulate various cellular processes, including mainly proliferation and differentiation. In a previous study we found that in prostate cancer cells, the G(q)PCR of gonadotropin-releasing hormone (GnRH) induces apoptosis by reducing the PKC-dependent AKT activity and elevating JNK phosphorylation. Because it was thought that G(q)PCRs mainly induce activation of AKT, we first undertook to examine how general this phenomenon is. In a screen of 21 cell lines we found that PKC activation results in the reduction of AKT activity, which correlates nicely with JNK activation and in some cases with apoptosis. To understand further the signaling pathways involved in this stimulation, we studied in detail SVOG-4O and αT3-1 cells. We found that prostaglandin F2α and GnRH agonist (GnRH-a) indeed induce significant Gα(q)- and PKC-dependent apoptosis in these cells. This is mediated by two signaling branches downstream of PKC, which converge at the level of MLK3 upstream of JNK. One branch consists of c-Src activation of the JNK cascade, and the second involves reduction of AKT activity that alleviates its inhibitory effect on MLK3 to allow the flow of the c-Src signal to JNK. At the MAPKK level, we found that the signal is transmitted by MKK7 and not MKK4. Our results present a general mechanism that mediates a G(q)PCR-induced, death receptor-independent, apoptosis in physiological, as well as cancer-related systems.

Transformation by a nucleotide-activated P2Y receptor is mediated by activation of Galphai, Galphaq and Rho-dependent signaling pathways

Background

Nucleotide-actived P2Y receptors play critical roles in the growth of tumor cells by regulating cellular proliferation, differentiation and survival.

Results

Here we demonstrate that an avian P2Y purinoceptor (tP2YR) with unique pharmacological and signal transduction properties induces morphologic and growth transformation of rodent fibroblasts. tP2YR induced a transformed phenotype similar to the mas oncogene, a G protein-coupled receptor which causes transformation by activation of Rac-dependent pathways. tP2YR-transformed cells exhibited increased steady-state activation of Rac1 and RhoA. Like activated Rho GTPases, tP2YR cooperated with activated Raf and caused synergistic transformation of NIH3T3 cells. Our data indicate that the ability of tP2YR to cause transformation is due to its unique ability among purinergic receptors to simultaneously activate Gα_q and Gα_i. Co-expression of constitutively activated mutants of these two Gα subunits caused the same transformed phenotype as tP2YR and Mas. Furthermore, transformation by both tP2YR and Mas was blocked by pharmacological inhibition of Gα_I by pertussis toxin (PTX) indicating an essential role for Gα_i in transformation by these G-protein coupled receptors.

Conclusions

Our data suggest that coordinated activation of Gα_q and Gα_i may link the tP2YR and possibility the Mas oncogene with signaling pathways resulting in activation of Rho family proteins to promote cellular transformation.

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.

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.

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.

Focal adhesion kinase as a RhoA-activable signaling scaffold mediating Akt activation and cardiomyocyte protection

RhoA a small G-protein that has an established role in cell growth and in regulation of the actin cytoskeleton. Far less is known about whether RhoA can modulate cell fate. We previously reported that sustained RhoA activation induces cardiomyocyte apoptosis (Del Re, D. P., Miyamoto, S., and Brown, J. H. (2007) J. Biol. Chem. 282, 8069-8078). Here we demonstrate that less chronic RhoA activation affords a survival advantage, protecting cardiomyocytes from apoptotic insult induced by either hydrogen peroxide treatment or glucose deprivation. Under conditions where RhoA is protective, we observe Rho kinase-dependent cytoskeletal rearrangement and activation of focal adhesion kinase (FAK). Activation of endogenous cardiomyocyte FAK leads to its increased association with the p85 regulatory subunit of phosphatidylinositol-3-kinase (PI3K) and to concomitant activation of Akt. Treatment of isolated perfused hearts with sphingosine 1-phosphate recapitulates this response. The pathway by which RhoA mediates cardiomyocyte Akt activation is demonstrated to require Rho kinase, FAK and PI3K, but not Src, based on studies with pharmacological inhibitors (Y-27632, LY294002, PF271 and PP2) and inhibitory protein expression (FAK-related nonkinase). Inhibition of RhoA-mediated Akt activation at any of these steps, including inhibition of FAK, prevents RhoA from protecting cardiomyocytes against apoptotic insult. We further demonstrate that stretch of cardiomyocytes, which activates endogenous RhoA, induces the aforementioned signaling pathway, providing a physiologic context in which RhoA-mediated FAK phosphorylation can activate PI3K and Akt. We suggest that RhoA-mediated effects on the cardiomyocyte cytoskeleton provide a novel mechanism for protection from apoptosis.

G protein regulation of MAPK networks

G proteins provide signal-coupling mechanisms to heptahelical cell surface receptors and are critically involved in the regulation of different mitogen-activated protein kinase (MAPK) networks. The four classes of G proteins, defined by the G(s), G(i), G(q) and G(12) families, regulate ERK1/2, JNK, p38MAPK, ERK5 and ERK6 modules by different mechanisms. The alpha- as well as betagamma-subunits are involved in the regulation of these MAPK modules in a context-specific manner. While the alpha- and betagamma-subunits primarily regulate the MAPK pathways via their respective effector-mediated signaling pathways, recent studies have unraveled several novel signaling intermediates including receptor tyrosine kinases and small GTPases through which these G-protein subunits positively as well as negatively regulate specific MAPK modules. Multiple mechanisms together with specific scaffold proteins that can link G-protein-coupled receptors or G proteins to distinct MAPK modules contribute to the context-specific and spatio-temporal regulation of mitogen-activated protein signaling networks by G proteins.

Biologic functions of the G12 subfamily of heterotrimeric g proteins: growth, migration, and metastasis

The G12 subfamily of heterotrimeric G proteins has been the subject of intense scientific interest for more than 15 years. During this period, studies have revealed more than 20 potential G12-interacting proteins and numerous signaling axes emanating from the G12 proteins, Galpha12 and Galpha13. In addition, more recent studies have begun to illuminate the various and sundry functions that the G12 subfamily plays in biology. In this review, we summarize the diverse range of proteins that have been identified as Galpha12 and/or Galpha13 interactors and describe ongoing studies designed to dissect the biological roles of specific Galpha-effector protein interactions. Further, we describe and discuss the expanding role of G12 proteins in the biology of cells, focusing on the distinct properties of this subfamily in regulating cell proliferation, cell migration, and metastatic invasion.

RhoA/Rho kinase up-regulate Bax to activate a mitochondrial death pathway and induce cardiomyocyte apoptosis

The small G-protein RhoA regulates the actin cytoskeleton, and its involvement in cell proliferation has also been established. In contrast, little is known about whether RhoA participates in cell survival or apoptosis. In cardiomyocytes in vitro, RhoA induces hypertrophic cell growth and gene expression. In vivo, however, RhoA expression leads to development of heart failure (Sah, V. P., Minamisawa, S., Tam, S. P., Wu, T. H., Dorn, G. W., Ross, J. Jr., Chien, K. R., and Brown, J. H. (1999) J. Clin. Investig. 103, 1627-1634), a condition widely associated with cardiomyocyte apoptosis. We demonstrate here that adenoviral overexpression of activated RhoA in cardiomyocytes induces hypertrophy, which transitions over time to apoptosis, as evidenced by caspase activation and nucleosomal DNA fragmentation. The Rho kinase inhibitors Y-27632 and HA-1077 and expression of a dominant negative Rho kinase block these responses. Caspase-9, but not caspase-8, is activated, and its inhibition prevents DNA fragmentation, consistent with involvement of a mitochondrial death pathway. Interestingly, RhoA expression induces a 3-4-fold up-regulation of the proapoptotic Bcl-2 family protein Bax. RhoA also increases levels of activated Bax and the amount of Bax protein localized at mitochondria. Bax mRNA is increased by RhoA, indicating transcriptional regulation, and the ability of a dominant negative p53 mutant to block Bax up-regulation implicates p53 in this response. The involvement of Bax in RhoA-induced apoptosis was examined by treatment with a Bax-inhibitory peptide, which was found to significantly attenuate DNA fragmentation and caspase-9 and -3 activation. The dominant negative p53 also prevents RhoA-induced apoptosis. We conclude that RhoA/Rho kinase activation up-regulates Bax through p53 to induce a mitochondrial death pathway and cardiomyocyte apoptosis.

Endothelial nitric-oxide synthase reveals a new face in G protein signaling

In this issue of Molecular Pharmacology, Andreeva et al. (p. 975) report a novel functional link between the heterotrimeric G protein G alpha12 and endothelial nitric-oxide synthase (eNOS). Based on studies characterizing the interaction of G alpha12 and the molecular chaperone Hsp90 and the interaction of eNOS and Hsp90, the group proposed an interaction between G alpha12 and eNOS and sought to determine the regulatory mechanisms, including the inferred dependence on Hsp90. Their experiments using an overexpression model lead to the observation that the cotransfection of G alpha12 and eNOS expression vectors increased overall eNOS expression. Additional studies in the overexpression model and in human umbilical vein endothelial cells (HUVEC) provide evidence for a mechanism that involves G alpha12-dependent stabilization of eNOS protein and possibly mRNA. These data present yet another paradigm by which heterotrimeric G proteins, through stabilization of target proteins, can regulate the activity of downstream signaling pathways.

Novel mechanisms of G protein-dependent regulation of endothelial nitric-oxide synthase

Endothelial nitric-oxide synthase (eNOS) plays a crucial role in the regulation of a variety of cardiovascular and pulmonary functions in both normal and pathological conditions. Multiple signaling inputs, including calcium, caveolin-1, phosphorylation by several kinases, and binding to the 90-kDa heat shock protein (Hsp90), regulate eNOS activity. Here, we report a novel mechanism of G protein-dependent regulation of eNOS. We demonstrate that in mammalian cells, the alpha subunit of heterotrimeric G12 protein (G alpha12) can form a complex with eNOS in an activation- and Hsp90-independent manner. Our data show that G alpha12 does not affect eNOS-specific activity, but it strongly enhances total eNOS activity by increasing cellular levels of eNOS. Experiments using inhibition of protein or mRNA synthesis show that G alpha12 increases the expression of eNOS by increasing half-life of both eNOS protein and eNOS mRNA. Small interfering RNA-mediated depletion of endogenous G alpha12 decreases eNOS levels. A quantitative correlation can be detected between the extent of down-regulation of G alpha12 and eNOS in endothelial cells after prolonged treatment with thrombin. G protein-dependent increase of eNOS expression represents a novel mechanism by which heterotrimeric G proteins can regulate the activity of downstream signaling molecules.

Unbiased Examination of Changes in Plasma Membrane Proteins in Drug Resistant Cancer Cells

In this study, an unbiased examination is made of the abundance changes between proteins found in the basolateral plasma membranes of a drug susceptible parental MCF-7 breast cancer cell line and a cell line selected from the parent line for resistance to the anticancer drug mitoxantrone. Plasma membrane proteins were differentially labeled metabolically, enriched using the colloidal silica pellicle method, and characterized by tandem mass spectrometry. Fifteen proteins were identified with significant (>2) changes, including receptors, adhesion proteins, proteins involved in amino acid uptake, and proteins involved in glucose uptake. From 40 mug of membrane proteins, 3227 unique peptides and 540 proteins were identified.

Kisspeptin-10-Induced Signaling of GPR54 Negatively Regulates Chemotactic Responses Mediated by CXCR4: a Potential Mechanism for the Metastasis Suppressor Activity of Kisspeptins

The product of the KiSS-1 gene is absent or expressed at low level in metastatic melanoma and breast cancer compared with their nonmetastatic counterparts. A polypeptide derived from the KiSS-1 product, designated kisspeptin-10 (Kp-10), activates a receptor coupled to Galphaq subunits (GPR54 or KiSS-1R). To study the mechanism by which Kp-10 antagonizes metastatic spread, the effect on CXCR4-mediated signaling, which has been shown to direct organ-specific migration of tumor cells, was determined. Kp-10 blocked chemotaxis of tumor cells expressing CXCR4 in response to low and high concentrations of SDF-1/CXCL12 and inhibited mobilization of calcium ions induced by this ligand. Pretreatment with Kp-10 did not induce down-modulation of cell surface CXCR4 expression, reduce affinity for SDF-1/CXCL12, or alter Galphai subunit activation stimulated by this ligand. Although Kp-10 stimulated prolonged phosphorylation of extracellular signal-regulated kinase 1/2, it inhibited the phosphorylation of Akt induced by SDF-1. The ability of Kp-10 to inhibit signaling and chemotaxis induced by SDF-1 indicates that activation of GPR54 signaling may negatively regulate the role of CXCR4 in programming tumor metastasis.

The human cytomegalovirus-encoded chemokine receptor US28 induces caspase-dependent apoptosis

Viral subversion of apoptosis regulation plays an important role in the outcome of host/virus interactions. Although human cytomegalovirus (HCMV) encodes several immediate early (IE) antiapoptotic proteins (IE1, IE2, vMIA and vICA), no proapoptotic HCMV protein has yet been identified. Here we show that US28, a functional IE HCMV-encoded chemokine receptor, which may be involved in both viral dissemination and immune evasion, constitutively induces apoptosis in several cell types. In contrast, none of nine human cellular chemokine receptors, belonging to three different subfamilies, induced any significant level of apoptosis. US28-induced cell death involves caspase 10 and caspase 8 activation, but does not depend on the engagement of cell-surface death receptors of the tumour necrosis factor receptor/CD95 family. US28 cell-death induction is prevented by coexpression of C-FLIP, a protein that inhibits Fas-associated death domain protein (FADD)-mediated activation of caspase 10 and caspase 8, and by coexpression of the HCMV antiapoptotic protein IE1. The use of US28 mutants indicated that the DRY sequence of its third transmenbrane domain, required for constitutive G-protein signalling, and the US28 intracellular terminal domain required for constitutive US28 endocytosis, are each partially required for cell-death induction. Thus, in HCMV-infected cells, US28 may function either as a chemokine receptor, a phospholipase C activator, or a proapoptotic factor, depending on expression levels of HCMV and/or cellular antiapoptotic proteins.

Cell signalling diversity of the Gqalpha family of heterotrimeric G proteins

Many receptors for neurotransmitters and hormones rely upon members of the Gqalpha family of heterotrimeric G proteins to exert their actions on target cells. Galpha subunits of the Gq class of G proteins (Gqalpha, G11alpha, G14alpha and G15/16alpha) directly link receptors to activation of PLC-beta isoforms which, in turn, stimulate inositol lipid (i.e. calcium/PKC) signalling. Although Gqalpha family members share a capacity to activate PLC-beta, they also differ markedly in their biochemical properties and tissue distribution which predicts functional diversity. Nevertheless, established models suggest that Gqalpha family members are functionally redundant and that their cellular responses are a result of PLC-beta activation and downstream calcium/PKC signalling. Growing evidence, however, indicates that Gqalpha, G11alpha, G14alpha and G15/16alpha are functionally diverse and that many of their cellular actions are independent of inositol lipid signalling. Recent findings show that Gqalpha family members differ with regard to their linked receptors and downstream binding partners. Reported binding partners distinct from PLC-beta include novel candidate effector proteins, various regulatory proteins, and a growing list of scaffolding/adaptor proteins. Downstream of these signalling proteins, Gqalpha family members exhibit unexpected differences in the signalling pathways and the gene expression profiles they regulate. Finally, genetic studies using whole animal models demonstrate the importance of certain Gqalpha family members in cardiac, lung, brain and platelet functions among other physiological processes. Taken together, these findings demonstrate that Gqalpha, G11alpha, G14alpha and G15/16alpha regulate both overlapping and distinct signalling pathways, indicating that they are more functionally diverse than previously thought.

Two G12 family G proteins, G alpha12 and G alpha13, show different subcellular localization

The G alpha subunits of the G12 family of heterotrimeric G proteins, G alpha12 and G alpha13, are closely related in sequences and some effectors, but they often act through different pathways or bind to different proteins. We have examined subcellular distribution of these two G proteins and found that endogenous G alpha12 and G alpha13 localize in membrane and cytoplasmic fractions, respectively. Exogenously expressed G alpha12 and G alpha13 also localize in membrane and cytoplasmic fractions, respectively, in COS-7 cells. Stimulation of lysophosphatidic acid receptor coupled to G alpha13 markedly promotes the translocation of G alpha13 from cytoplasm to membrane. This different localization of G alpha12 and G alpha13 may explain some of the nonoverlapping actions of G alpha12 and G alpha13.

Functional consequences of G alpha 13 mutations that disrupt interaction with p115RhoGEF

The G-protein alpha subunit, alpha(13), regulates cell growth and differentiation through the monomeric Rho GTPase. Alpha(13) activates Rho through direct stimulation of the guanine nucleotide exchange factor p115RhoGEF, which contains a regulator of G-protein signaling homology domain (RH) in its N-terminus. Through its RH domain, p115RhoGEF also functions as a GAP for G alpha(13). The mechanism for the G alpha(13)/p115RhoGEF interaction is not well understood. Here, we determined specific alpha(13) residues important for its interaction with p115RhoGEF. GST-pulldowns and co-immunoprecipitation assays revealed that individually mutating alpha(13) residues Lys204, Glu229, or Arg232 to opposite charge residues disrupts the interaction of activated alpha(13) with the RH domain of p115RhoGEF or full-length p115RhoGEF. We further demonstrate that mutation of Glu229, and to a lesser extent Lys204 or Arg232, disrupts the ability of activated alpha(13) to induce the recruitment of p115RhoGEF to the plasma membrane (PM) and to activate Rho-mediated serum response element-luciferase gene transcription. Interestingly, an alpha(13) mutant where a conserved Gly was mutated to a Ser (G205S) retained its ability to bind to p115RhoGEF, induce p115RhoGEF recruitment to the PM, and activate Rho-dependent signaling, even though identical Gly to Ser mutations in other alpha disrupt their interaction with regulator of G-protein signaling (RGS) proteins. These results demonstrate that, whereas several features of a typical alpha/RGS interaction are preserved in the alpha(13)/p115RhoGEF interaction, there are also significant differences.

Differential Effects of Gqα, G14α, and G15α on Vascular Smooth Muscle Cell Survival and Gene Expression Profiles

Gqalpha family members (Gqalpha, G11alpha, G14alpha, and G15/16alpha) stimulate phospholipase Cbeta (PLCbeta) and inositol lipid signaling but differ markedly in amino acid sequence and tissue distribution predicting unappreciated functional diversity. To examine functional differences, we compared the signaling properties of Gqalpha, G14alpha, and G15alpha and their cellular responses in vascular smooth muscle cells (VSMC). Constitutively active forms of Gqalpha, G14alpha, or G15alpha elicit markedly different responses when introduced to VSMC. Whereas each Galpha stimulated PLCbeta to similar extents when expressed at equal protein levels, Gqalpha and G14alpha but not G15alpha initiated profound cell death within 48 h. This response was the result of activation of apoptotic pathways, because Gqalpha and G14alpha, but not G15alpha, stimulated caspase-3 activation and did not alter phospho-Akt, a regulator of cell survival pathways. Gqalpha and G14alpha stimulate nuclear factor of activated T cell (NFAT) activation in VSMC, but Galpha-induced cell death seems independent of PKC, InsP(3)/Ca(2+), and NFAT, in that pharmacological inhibitors of these pathways did not block cell death. Gene expression analysis indicates that Gqalpha, G14alpha, and G15alpha each elicit markedly different profiles of altered gene sets in VSMC after 24 h. Whereas all three Galpha stimulated changes (> or =2-fold) in 50 shared mRNA, Gqalpha and G14alpha (but not G15alpha) stimulated changes in 221 shared mRNA, many of which are reported to be pro-apoptotic and/or involved with TNF-alpha signaling. We were surprised to find that each Galpha also stimulated changes in nonoverlapping Galpha-specific gene sets. These findings demonstrate that Gqalpha family members activate both overlapping and distinct signaling pathways and are more functionally diverse than previously thought.

Galphaq/11 signaling induces apoptosis through two pathways involving reduction of Akt phosphorylation and activation of RhoA in HeLa cells

We have previously reported that expression of the constitutively active mutant of Galpha11 or stimulation of m1 muscarinic acetylcholine receptor induced proteolytic activation of Rho-associated kinase (ROCK-I) by caspase and apoptosis in HeLa cells. In this study, we investigate the molecular mechanisms of Galphaq/11-induced apoptosis in m1 muscarinic acetylcholine receptor-expressing HeLa cells. Overexpression of Bcl-2 inhibited carbachol-induced ROCK-I cleavage, indicating a mitochondrial apoptotic pathway. Overexpression of the constitutively active mutant of Akt that delivers an anti-apoptotic survival signal had a similar influence. Insulin, a major survival factor in many cells, strongly increased phosphorylation of Akt, which was completely blocked by carbachol. This latter effect was partially inhibited by treatment with the tyrosine phosphatase inhibitors, orthovanadate and pervanadate. In parallel with these observations, carbachol attenuated insulin-stimulated tyrosine phosphorylation of insulin receptor substrate-1, an effect eliminated by orthovanadate. On the other hand, carbachol induced rapid stimulation of endogenous RhoA, and expression of a constitutively active mutant of RhoA increased ROCK-I cleavage. Orthovanadate and the dominant negative mutant of RhoA partially, and their combination completely, inhibited carbachol-induced ROCK-I cleavage and apoptosis. These results demonstrate that Gq/11 signaling induces apoptosis by reducing insulin-stimulated Akt phosphorylation through tyrosine dephosphorylation and activating RhoA in HeLa cells.

Urotensin II Promotes Hypertrophy of Cardiac Myocytes via Mitogen-Activated Protein Kinases

Urotensin II and its receptor are coexpressed in the heart and up-regulated during cardiac dysfunction. In cultured neonatal cardiomyocytes, we mimicked this up-regulation using an adenovirus to increase expression of the urotensin receptor. In this model system, urotensin II promoted strong hypertrophic growth and phenotypic changes, including cell enlargement and sarcomere reorganization. Urotensin II potently activated the MAPKs, ERK1/2 and p38, and blocking these kinases with PD098059 and SB230580, respectively, significantly inhibited urotensin II-mediated hypertrophy. In contrast, urotensin II did not activate JNK. The activation of ERK1/2 and p38 as well as cellular hypertrophy was independent of protein kinase C, and calcium and phosphoinositide 3-kinase, yet dependent on the capacity of the urotensin receptor to trans-activate the epidermal growth factor receptor. Urotensin II promoted the tyrosine phosphorylation of epidermal growth factor receptors, which was inhibited by the selective epidermal growth factor receptor kinase inhibitor, AG1478. These data indicate that perturbations in cardiac homeostasis, which lead to up-regulation of urotensin II receptors, promote urotensin II-mediated cardiomyocyte hypertrophy via ERK1/2 and p38 signaling pathways in an epidermal growth factor receptor-dependent manner.

Vasodilator-stimulated phosphoprotein activation of serum-response element-dependent transcription occurs downstream of RhoA and is inhibited by cGMP-dependent protein kinase phosphorylation

Vasodilator-stimulated phosphoprotein (VASP) associates with cytoskeletal structures and promotes F-actin formation. RhoA, a member of the Ras superfamily of proteins, activates serum response element (SRE)-dependent transcription through changes in actin dynamics. We now show that the F-actin binding region of VASP is required for VASP stimulation of SRE-dependent transcription, and that VASP is downstream of RhoA in stimulating SRE-dependent transcription. The isolated carboxyl-terminal coiled-coil region of VASP mediates protein tetramerization and has been used as a dominant negative form of VASP; we found that it forms complexes with endogenous VASP in vivo and inhibits in a dose-dependent fashion serum-, RhoA-, and VASP-stimulated SRE-dependent transcription. Cyclic GMP-dependent protein kinase (G-kinase) inhibits RhoA activation of SRE-dependent transcription (Gudi, T., Chen, J. C., Casteel, D. E., Seasholtz, T. M., Boss, G. R., and Pilz, R. B. (2002) J. Biol. Chem. 277, 37382-37393). We now show that the G-kinase inhibition that occurs downstream of RhoA can be explained, at least in part, by G-kinase phosphorylation of VASP on Ser(239) at the carboxyl-terminal end of the G-actin binding site, with some contribution by phosphorylation of Ser(157), which is proximal to the profilin binding site. A phosphorylation-deficient VASP mutant can partly prevent cGMP/G-kinase inhibition of serum- and RhoA-induced SRE-dependent transcription. These studies show that VASP, an important component of the cellular microfilament system, plays a major role in regulating SRE-dependent transcription, and that G-kinase regulates VASP activity.

Phospholipase C-independent Activation of Glycogen Synthase Kinase-3β and C-terminal Src Kinase by Gαq

It is generally thought that activation of phospholipase Cbeta (PLCbeta) by Galphaq accounts for most of the effects of Gq-coupled receptors. Here we describe a novel effect of Galphaq that is independent of the PLCbeta pathway. Expression of the constitutively active Galphaq mutant Galphaq(Q209L) promoted an increase in glycogen synthase kinase-3beta (GSK-3beta) activity that was associated with increased phosphorylation of Tyr216 on GSK-3beta. Galphaq(Q209L)-AA, a mutant that cannot activate PLCbeta, also induced GSK-3beta activation and phosphorylation of Tyr216. We speculate that the protein-tyrosine kinase Csk (C-terminal Src kinase), which is also activated by Galphaq(Q209L) and Galphaq(Q209L)-AA, acts upstream of GSK-3beta. Expression of Csk accentuated the activation of GSK-3beta by Galphaq(Q209L), whereas catalytically inactive Csk blocked GSK-3beta activation by Galphaq(Q209L). Recombinant Csk phosphorylated and activated GSK-3beta in vitro, and GSK-3beta coprecipitated with Csk from cell lysates. These results suggest that activation of Csk and GSK-3beta by Galphaq may contribute to the physiological and pathological effects of Gq-coupled receptors.

G alpha 13-mediated transformation and apoptosis are permissively dependent on basal ERK activity

We previously reported that the alpha-subunit of heterotrimeric G13 protein induces either mitogenesis and neoplastic transformation or apoptosis in a cell-dependent manner. Here, we analyzed which signaling pathways are required for G alpha 13-induced mitogenesis or apoptosis using a novel mutant of G alpha 13. We have identified that in human cell line LoVo, the mutation encoding substitution of Arg260 to stop codon in mRNA of G alpha 13 subunit produced a mutant protein (G alpha 13-T) that lacks a COOH terminus and is endogenously expressed in LoVo cells as a polypeptide of 30 kDa. We found that G alpha 13-T lost its ability to promote proliferation and transformation but retained its ability to induce apoptosis. We found that full-length G alpha 13 could stimulate Elk1 transcription factor, whereas truncated G alpha 13 lost this ability. G alpha 13-dependent stimulation of Elk1 was inhibited by dominant-negative extracellular signal-regulated kinase (MEK) but not by dominant-negative MEKK1. Similarly, MEK inhibitor PD-98059 blocked G alpha 13-induced Elk1 stimulation, whereas JNK inhibitor SB-203580 was ineffective. In Rat-1 fibroblasts, G alpha 13-induced cell proliferation and foci formation were also inhibited by dominant-negative MEK and PD-98059 but not by dominant-negative MEKK1 and SB-203580. Whereas G alpha 13-T alone did not induce transformation, coexpression with constitutively active MEK partially restored its ability to transform Rat-1 cells. Importantly, full-length but not G alpha 13-T could stimulate Src kinase activity. Moreover, G alpha 13-dependent stimulation of Elk1, cell proliferation, and foci formation were inhibited by tyrosine kinase inhibitor, genistein, or by dominant-negative Src kinase, suggesting the involvement of a Src-dependent pathway in the G alpha 13-mediated cell proliferation and transformation. Importantly, truncated G alpha 13 retained its ability to stimulate apoptosis signal-regulated kinase ASK1 and c-Jun terminal kinase, JNK. Interestingly, the apoptosis induced by G alpha 13-T was inhibited by dominant-negative ASK1 or by SB-203580.

The proteinase-activated receptor-2 mediates phagocytosis in a Rho-dependent manner in human keratinocytes

Recent work shows that the G-protein-coupled receptor proteinase activated receptor-2 activates signals that stimulate melanosome uptake in keratinocytes in vivo and in vitro. The Rho family of GTP-binding proteins is involved in cytoskeletal remodeling during phagocytosis. We show that proteinase-activated receptor-2 mediated phagocytosis in human keratinocytes is Rho dependent and that proteinase-activated receptor-2 signals to activate Rho. In contrast, Rho activity did not affect either proteinase-activated receptor-2 activity or mRNA and protein levels. We explored the signaling mechanisms of proteinase-activated receptor-2 mediated Rho activation in human keratinocytes and show that activation of proteinase-activated receptor-2, either through specific proteinase-activated receptor-2 activating peptides or through trypsinization, elevates cAMP in keratinocytes. Proteinase-activated receptor-2 mediated Rho activation was pertussis toxin insensitive and independent of the protein kinase A signaling pathway. These data are the first to show that proteinase-activated receptor-2 mediated phagocytosis is Rho dependent and that proteinase-activated receptor-2 signals to Rho and cAMP in keratinocytes. Because phagocytosis of melanosomes is recognized as an important mechanism for melanosome transfer to keratinocytes, these results suggest that Rho is a critical signaling intermediate in melanosome uptake in keratinocytes.

Activated G alpha q inhibits p110 alpha phosphatidylinositol 3-kinase and Akt

Some Gq-coupled receptors have been shown to antagonize growth factor activation of phosphatidylinositol 3-kinase (PI3K) and its downstream effector, Akt. We used a constitutively active Galphaq(Q209L) mutant to explore the effects of Galphaq activation on signaling through the PI3K/Akt pathway. Transient expression of Galphaq(Q209L) in Rat-1 fibroblasts inhibited Akt activation induced by platelet-derived growth factor or insulin treatment. Expression of Galphaq(Q209L) also attenuated Akt activation promoted by coexpression of constitutively active PI3K in human embryonic kidney 293 cells. Galphaq(Q209L) had no effect on the activity of an Akt mutant in which the two regulatory phosphorylation sites were changed to acidic amino acids. Inducible expression of Galphaq(Q209L) in a stably transfected 293 cell line caused a decrease in PI3K activity in p110alpha (but not p110beta) immunoprecipitates. Receptor activation of Galphaq also selectively inhibited PI3K activity in p110alpha immunoprecipitates. Active Galphaq still inhibited PI3K/Akt in cells pretreated with the phospholipase C inhibitor U73122. Finally, Galphaq(Q209L) co-immunoprecipitated with the p110alpha-p85alpha PI3K heterodimer from lysates of COS-7 cells expressing these proteins, and incubation of immunoprecipitated Galphaq(Q209L) with purified recombinant p110alpha-p85alpha in vitro led to a decrease in PI3K activity. These results suggest that agonist binding to Gq-coupled receptors blocks Akt activation via the release of active Galphaq subunits that inhibit PI3K. The inhibitory mechanism seems to be independent of phospholipase C activation and might involve an inhibitory interaction between Galphaq and p110alpha PI3K.

The lysophospholipid receptor G2A activates a specific combination of G proteins and promotes apoptosis

G2A, a G protein-coupled receptor for which lysophosphatidylcholine (LPC) is a high affinity ligand, belongs to a newly defined lysophospholipid receptor subfamily. Expression of G2A is transcriptionally up-regulated by stress-inducing and cell-damaging agents, and ectopic expression of G2A leads to growth inhibition. However, the G proteins that functionally couple to G2A have not been elucidated in detail. We report here that G2A ligand independently stimulates the accumulation of both inositol phosphates and cAMP. LPC does not further enhance inositol phosphate accumulation but dose-dependently augments intracellular cAMP concentration. Expression of G alpha(q) and G alpha(13) with G2A potentiates G2A-mediated activation of a NF-kappa B-luciferase reporter. These results demonstrate that G2A differentially couples to multiple G proteins including G alpha(s), G alpha(q), and G alpha(13), depending on whether it is bound to ligand. G2A-transfected HeLa cells display apoptotic signs including membrane blebbing, nuclear condensation, and reduction of mitochondrial membrane potential. Furthermore, G2A-induced apoptosis can be rescued by the caspase inhibitors, z-vad-fmk and CrmA. Although apoptosis occurs without LPC stimulation, LPC further enhances G2A-mediated apoptosis and correlates with its ability to induce cAMP elevation in both HeLa cells and primary lymphocytes. Rescue from G2A-induced apoptosis was achieved by co-expression of a G alpha(12/13)-specific inhibitor, p115RGS (regulator of G protein signaling), in combination with 2',5'-dideoxyadenosine treatment. These results demonstrate the ability of G2A to activate a specific combination of G proteins, and that G2A/LPC-induced apoptosis involves both G alpha(13)- and G alpha(s)-mediated pathways.

Constitutive inositol phosphate formation in cytomegalovirus-infected human fibroblasts is due to expression of the chemokine receptor homologue pUS28

An open reading frame (ORF), US28, with homology to mammalian chemokine receptors has been identified in the genome of human cytomegalovirus (HCMV). Its protein product, pUS28, has been shown to bind several human CC chemokines, including RANTES, MCP-1, and MIP-1 alpha, and the CX(3)C chemokine fractalkine with high affinity. Addition of CC chemokines to cells expressing pUS28 was reported to cause a pertussis toxin-sensitive increase in the concentration of cytosolic free Ca(2+). Recently, pUS28 was shown to mediate constitutive, ligand-independent, and pertussis toxin-insensitive activation of phospholipase C via G(q/11)-dependent signaling pathways in transiently transfected COS-7 cells. Since these findings are not easily reconciled with the former observations, we analyzed the role of pUS28 in mediating CC chemokine activation of pertussis toxin-sensitive G proteins in cell membranes and phospholipase C in intact cells. The transmembrane signaling functions of pUS28 were studied in HCMV-infected cells rather than in cDNA-transfected cells. Since DNA sequence analysis of ORF US28 of different laboratory and clinical strains had revealed amino acid sequence differences in the amino-terminal portion of pUS28, we compared two laboratory HCMV strains, AD169 and Toledo, and one clinical strain, TB40/E. The results showed that infection of human fibroblasts with all three HCMV strains led to a vigorous, constitutively enhanced formation of inositol phosphates which was insensitive to pertussis toxin. This effect was critically dependent on the presence of the US28 ORF in the HCMV genome but was independent of the amino acid sequence divergence of the three HCMV strains investigated. The constitutive activity of pUS28 is not explained by expression of pUS28 at high density in HCMV-infected cells. The pUS28 ligands RANTES and MCP-1 failed to stimulate binding of guanosine 5'-O-(3-[(35)S]thiotriphosphate to membranes of HCMV-infected cells and did not enhance constitutive activation of phospholipase C in intact HCMV-infected cells. These findings raise the possibility that the effects of CC chemokines and pertussis toxin on G protein-mediated transmembrane signaling previously observed in HCMV-infected cells are either independent of or not directly mediated by the protein product of ORF US28.

Exogenous myristoylated-G(i2)alpha subunits of GTP-binding proteins are mitogens following their internalization by astrocytes in culture

Heterotrimeric GTP-binding proteins (G proteins) are involved in the coupling of a variety of cell surface receptors to different intracellular signalling pathways, some of which take part in the regulation of growth by affecting cell proliferation and/or differentiation. In cultured astrocytes, many receptors of neuropeptides and hormones are coupled to the heterotrimeric G(i) proteins which regulate the mitogen-activated protein kinase (MAPK/ERK) cascade through both the Galpha and Gbetagamma subunits. We have previously reported that functionally active recombinant myr-G(i2)alpha subunits added to such cultures are internalised and distributed within the plasma membrane and cytosol as well as in the nuclei of dividing astrocytes. Here we show that astrocytes proliferate dose-dependently in response to exogenous myr-G(i2)alpha subunits. Concentrations of 100 pM-30 nM myr-G(i2)alpha caused more than 2.5-fold increase of [3H]thymidine incorporation over basal levels. Other classes of myr-Galpha subunits, such as G(i3)alpha or G(o)alpha, induced a much lower proliferative effect. The addition of G(i1)alpha subunits to the cultures produced no change, indicating the selectivity of this effect. Even though myr-G(i2)alpha subunits are internalised by the cells regardless of their guanine nucleotide-bound state, much less [3H]thymidine incorporation was observed in the presence of GDPbetaS-myr-G(i2)alpha or GTPgammaS-myr-G(i2)alpha. Further, the fluorescent labelling was dissimilarly distributed, the signal being concentrated in the nucleus and perinuclear regions of the astrocytes. Selective disassembly of caveolae impaired both myr-G(i2)alpha internalisation and DNA induction. Together, these data reveal a proliferative effect of myr-G(i2)alpha subunits in astrocytes, and provide evidence for the incorporation of exogenous myr-G(i2)alpha subunits into the mitogen cascade activated by neurotransmitters or growth factors. The fact that Galpha proteins can enter cells is particularly interesting because options for delivering functional proteins into cells are limited. Thus, these proteins may have clinical applications for compensating deficits in the transduction mechanisms associated with several neurological diseases, or as a non-invasive membrane traversing carriers.

cGMP-dependent protein kinase inhibits serum-response element-dependent transcription by inhibiting rho activation and functions

RhoA, in its active GTP-bound form, stimulates transcription through activation of the serum-response factor (SRF). We found that cGMP inhibited serum-induced Rho.GTP loading and transcriptional activation of SRF-dependent reporter genes in smooth muscle and glial cells in a cGMP-dependent protein kinase (G-kinase)-dependent fashion. Serum stimulation of the SRF target gene vinculin was also blocked by cGMP/G-kinase. G-kinase activation inhibited SRF-dependent transcription induced by upstream RhoA activators including Galpha(13) and p115RhoGEF, with Galpha(13)-induced Rho.GTP loading inhibited by G-kinase. G-kinase had no effect on the high activation levels of RhoA(63L) or the double mutant RhoA(63L,188A) but inhibited transcriptional activation by these two RhoA mutants to a similar extent, suggesting an effect downstream of RhoA and independent of RhoA Ser(188) phosphorylation. Constitutively active forms of the Rho effectors Rho kinase (ROK), PKN, and PRK-2 induced SRF-dependent transcription in a cell type-specific fashion with ROK being the most efficient; G-kinase inhibited transcription induced by all three effectors without affecting ROK catalytic activity. G-kinase had no effect on RhoA(63L)-induced morphological changes in glial cells, suggesting distinct transcriptional and cytoskeletal effectors of RhoA. We conclude that G-kinase inhibits SRF-dependent transcription by interfering with RhoA signaling; G-kinase acts both upstream of RhoA, inhibiting serum- or Galpha(13)-induced Rho activation, and downstream of RhoA, inhibiting steps distal to the Rho targets ROK, PKN, and PRK-2.

Hsp90 interactions and acylation target the G protein Galpha 12 but not Galpha 13 to lipid rafts

The heterotrimeric G proteins, G(12) and G(13), are closely related in their sequences, signaling partners, and cellular effects such as oncogenic transformation and cytoskeletal reorganization. Yet G(12) and G(13) can act through different pathways, bind different proteins, and show opposing actions on some effectors. We investigated the compartmentalization of G(12) and G(13) at the membrane because other G proteins reside in lipid rafts, membrane microdomains enriched in cholesterol and sphingolipids. Lipid rafts were isolated after cold, nonionic detergent extraction of cells and gradient centrifugation. Galpha(12) was in the lipid raft fractions, whereas Galpha(13) was not associated with lipid rafts. Mutation of Cys-11 on Galpha(12), which prevents its palmitoylation, partially shifted Galpha(12) from the lipid rafts. Geldanamycin treatment, which specifically inhibits Hsp90, caused a partial loss of wild-type Galpha(12) and a complete loss of the Cys-11 mutant from the lipid rafts and the appearance of a higher molecular weight form of Galpha(12) in the soluble fractions. These results indicate that acylation and Hsp90 interactions localized Galpha(12) to lipid rafts. Hsp90 may act as both a scaffold and chaperone to maintain a functional Galpha(12) only in discrete membrane domains and thereby explain some of the nonoverlapping functions of G(12) and G(13) and control of these potent cell regulators.

Interaction of G alpha(12) with G alpha(13) and G alpha(q) signaling pathways

The G(12) subfamily of heterotrimeric G-proteins consists of two members, G(12) and G(13). Gene-targeting studies have revealed a role for G(13) in blood vessel development. Mice lacking the alpha subunit of G(13) die around embryonic day 10 as the result of an angiogenic defect. On the other hand, the physiological role of G(12) is still unclear. To address this issue, we generated G alpha(12)-deficient mice. In contrast to the G alpha(13)-deficient mice, G alpha(12)-deficient mice are viable, fertile, and do not show apparent abnormalities. However, G alpha(12) does not seem to be entirely redundant, because in the offspring generated from G alpha(12)+/- G alpha(13)+/- intercrosses, at least one intact G alpha(12) allele is required for the survival of animals with only one G alpha(13) allele. In addition, G alpha(12) and G alpha(13) showed a difference in mediating cell migratory response to lysophosphatidic acid in embryonic fibroblast cells. Furthermore, mice lacking both G alpha(12) and G alpha(q) die in utero at about embryonic day 13. These data indicate that the G alpha(12)-mediated signaling pathway functionally interacts not only with the G alpha(13)- but also with the G alpha(q/11)-mediated signaling systems.

Leukemia-associated Rho guanine nucleotide exchange factor promotes G alpha q-coupled activation of RhoA

Leukemia-associated Rho guanine-nucleotide exchange factor (LARG) belongs to the subfamily of Dbl homology RhoGEF proteins (including p115 RhoGEF and PDZ-RhoGEF) that possess amino-terminal regulator of G protein signaling (RGS) boxes also found within GTPase-accelerating proteins (GAPs) for heterotrimeric G protein alpha subunits. p115 RhoGEF stimulates the intrinsic GTP hydrolysis activity of G alpha 12/13 subunits and acts as an effector for G13-coupled receptors by linking receptor activation to RhoA activation. The presence of RGS box and Dbl homology domains within LARG suggests this protein may also function as a GAP toward specific G alpha subunits and couple G alpha activation to RhoA-mediating signaling pathways. Unlike the RGS box of p115 RhoGEF, the RGS box of LARG interacts not only with G alpha 12 and G alpha 13 but also with G alpha q. In cellular coimmunoprecipitation studies, the LARG RGS box formed stable complexes with the transition state mimetic forms of G alpha q, G alpha 12, and G alpha 13. Expression of the LARG RGS box diminished the transforming activity of oncogenic G protein-coupled receptors (Mas, G2A, and m1-muscarinic cholinergic) coupled to G alpha q and G alpha 13. Activated G alpha q, as well as G alpha 12 and G alpha 13, cooperated with LARG and caused synergistic activation of RhoA, suggesting that all three G alpha subunits stimulate LARG-mediated activation of RhoA. Our findings suggest that the RhoA exchange factor LARG, unlike the related p115 RhoGEF and PDZ-RhoGEF proteins, can serve as an effector for Gq-coupled receptors, mediating their functional linkage to RhoA-dependent signaling pathways.

Interaction between the G alpha subunit of heterotrimeric G(12) protein and Hsp90 is required for G alpha(12) signaling

The G alpha subunit of G(12) protein, one of the heterotrimeric G proteins, regulates diverse and complex cellular responses by transducing signals from the cell surface, presumably involving more than one downstream effector. Yeast two-hybrid screening of a human testis cDNA library identified a large fragment of Hsp90 as a protein that interacted with G alpha(12). The interaction between G alpha(12) and Hsp90 was further substantiated by a co-immunoprecipitation technique. We have determined that Hsp90 is not required for the interaction of G alpha(12) with its binding partners, p115(RhoGEF) and the G beta subunit. Importantly, Hsp90 is required for G alpha(12)-induced serum response element activation, cytoskeletal changes, and mitogenic response. Closely related to G alpha(12), the G alpha(13) subunit did not interact with Hsp90 and did not require functional Hsp90 for serum response element activation. Thus, our results identify a novel signaling module of G alpha(12) and Hsp90.

Galpha11 induces caspase-mediated proteolytic activation of Rho-associated kinase, ROCK-I, in HeLa cells

Expression of the constitutively active mutant of Galpha(11) (Galpha(11)QL) induces the formation of vinculin-containing focal adhesion-like structures in HeLa cells. This was found to be inhibited by Y-27632, a specific inhibitor of Rho-associated kinases (ROCK), but not by co-expression with a dominant negative mutant of RhoA, suggesting Rho-independent activation of ROCK by Galpha(11)QL. Investigation of trypan blue exclusion and immunocytochemistry with an antibody against cleaved caspase revealed the cellular phenotype of Galpha(11)QL-expressing cells to be identical to that displayed by cells undergoing apoptosis, and the caspase inhibitor zVAD-fmk blocked all morphological changes induced by Galpha(11)QL. Transfection of Galpha(11)QL induced cleavage of ROCK-I, and this proteolysis was also prevented by zVAD-fmk. ROCK-I C-terminally truncated at its authentic caspase sites also induced the formation of vinculin-containing focal adhesion-like structures. In addition, cleavage of ROCK-I was observed when cells overexpressing m1 muscarinic acetylcholine receptors were stimulated with carbachol. These results suggest that Galpha(11) induces proteolytic activation of ROCK-I by caspase and thereby regulates the actin cytoskeleton during apoptosis.

beta -Amyloid peptide-induced apoptosis regulated by a novel protein containing a g protein activation module

Degeneration of neurons in Alzheimer's disease is mediated by beta-amyloid peptide by diverse mechanisms, which include a putative apoptotic component stimulated by unidentified signaling events. This report describes a novel beta-amyloid peptide-binding protein (denoted BBP) containing a G protein-coupling module. BBP is one member of a family of three proteins containing this conserved structure. The BBP subtype bound human beta-amyloid peptide in vitro with high affinity and specificity. Expression of BBP in cell culture induced caspase-dependent vulnerability to beta-amyloid peptide toxicity. Expression of a signaling-deficient dominant negative BBP mutant suppressed sensitivity of human Ntera-2 neurons to beta-amyloid peptide mediated toxicity. These findings suggest that BBP is a target of neurotoxic beta-amyloid peptide and provide new insight into the molecular pathophysiology of Alzheimer's disease.

Involvement of c-Jun N-terminal kinase and p38 mitogen-activated protein kinase in alpha1B-adrenergic receptor/Galphaq-induced inhibition of cell proliferation

Certain G protein-coupled receptors (GPCRs) stimulate the activities of c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK), members of the MAPK family. We investigated the role of JNK and p38 MAPK activation induced by the alpha1B-adrenergic receptor in the proliferation of human embryonic kidney 293T cells. Activation of the alpha1B-adrenergic receptor resulted in inhibition of cell proliferation. This receptor-induced inhibition of proliferation was blocked by a kinase-deficient MKK4 and by the p38 MAPK inhibitor SB203580. Additionally, transfection of constitutively activated Galphaq into cells also led to inhibition of proliferation in a JNK- and p38 MAPK-dependent manner. These results demonstrate that the alpha1B-adrenergic receptor/Galphaq signaling inhibits cell proliferation through pathways involving JNK and p38 MAPK.

Differential expression of a gene homologous to a G-alpha protein gene in neonatal mouse skin during development of hair follicles

The development of mouse hair follicles depends on the proliferation, differentiation and migration of epithelial matrix cells in the follicle bulb. In particular, induction of the proliferation of epithelial cells is thought to be signalled by the dermal papilla at the base of the bulb. Neonatal mouse skin is useful for studying changes in gene expression during development of the follicles, as the mitotic activity of skin cells changes shortly after birth. Using RNA differential display, a 248-bp message has been identified, which is expressed in the skin, specifically on day 2 and day 3 but not on day 4 after birth. Confirmation of expression of this gene by ribonuclease protection assay showed that strong expression is seen on day 2 and day 3, but weak expression is also shown on day 1, day 4 and day 5. In situ hybridization data revealed that it is mainly localized in the dermal papilla. Analysis of its nucleotide sequence showed 99% identity between nucleotide 2 and 232 of the mouse uncoupled S49 cell mRNA for stimulatory GTP-binding protein (G(S)) alpha subunit, suggesting it is a segment of G(S)alpha. As the G(S)alpha subunit is involved in transducing extracellular signals across the cell, the finding of its expression in the papilla suggests it may be a molecular signal to the induction of epithelial proliferation in the follicle bulb. Evidence of strong expression on day 2, at the time when the mitotic activity of epithelial matrix cells starts to increase, also suggests that the G(S)alpha is a potential candidate for involvement in the initiation of follicle growth.

Dual regulation of Akt/protein kinase B by heterotrimeric G protein subunits

While positive regulation of c-Akt (also known as protein kinase B) by receptor tyrosine kinases is well documented, compounds acting through G protein-coupled receptors can also activate Akt and its downstream targets. We therefore explored the role of G protein subunits in the regulation of Akt in cultured mammalian cells. In HEK-293 and COS-7 cells transiently transfected with beta(2)-adrenergic or m2 muscarinic receptors, respectively, treatment with agonist-induced phosphorylation of Akt at serine 473 as evidenced by phosphoserine-specific immunoblots. This effect was blocked by the phosphatidylinositol-3-OH kinase inhibitor LY294002 and wild-type Galpha(i1), and was not duplicated by co-transfection of the constitutively active Galpha(s)-Q227L or Galpha(i)-Q204L mutant. Co-transfection of Gbeta(1), Gbeta(2) but not Gbeta(5) together with Ggamma(2) activated the kinase when assayed in vitro following immunoprecipitation of the epitope-tagged enzyme. In contrast, constitutively activated G protein subunits representing the four Galpha subfamilies were found unable to activate Akt in either cell line. The latter results are in disagreement with a report by Murga et al. (Murga, C., Laguinge, L., Wetzker, R., Cuadrado, A., and Gutkind, J. S. (1998) J. Biol. Chem. 273, 19080-19085) that described activation of Akt in response to mutationally activated Galpha(q) and Galpha(i) transfection in COS cells. To the contrary, in our experiments Galpha(q)-Q209L inhibited Akt activation resulting from betagamma or mutationally activated H-Ras co-transfection in these cells. In HEK-293 cells Galpha(q)-Q209L transfection inhibited insulin-like growth factor-1 activation of epitope-tagged Akt. In m1 muscarinic receptor transfected HEK-293 cells, carbachol inhibited insulin-like growth factor-1 stimulated phosphorylation at Ser(473) of endogenous Akt in an atropine-reversible fashion. We conclude that G proteins can regulate Akt by two distinct and potentially opposing mechanisms: activation by Gbetagamma heterodimers in a phosphatidylinositol-3-OH kinase-dependent fashion, and inhibition mediated by Galpha(q). This work identifies Akt as a novel point of convergence between disparate signaling pathways.

[D-Arg(1),D-Trp(5,7,9),Leu(11)]Substance P inhibits bombesin-induced mitogenic signal transduction mediated by both G(q) and G(12) in Swiss 3T3cells

Substance P (SP) analogues including [d-Arg(1),d-Trp(5,7,9), Leu(11)]SP are broad spectrum neuropeptide antagonists and potential anticancer agents, but their mechanism of action is not fully understood. Here, we examined the mechanism of action of [d-Arg(1), d-Trp(5,7,9),Leu(11)]SP as an inhibitor of G protein-coupled receptor (GPCR)-mediated signal transduction and cellular DNA synthesis in Swiss 3T3 cells. Addition of [d-Arg(1),d-Trp(5,7,9), Leu(11)]SP, at 10 micrometer, caused a striking rightward shift in the dose-response curves of DNA synthesis induced by bombesin, bradykinin, or vasopressin and markedly inhibited the activation of p42(mapk) (ERK-2) and p44(mapk) (ERK-1) induced by these GPCR agonists. In addition, this SP analogue also prevented the protein kinase C-dependent activation of protein kinase D induced by these agonists. [d-Arg(1),d-Trp(5,7,9),Leu(11)]SP, at a concentration (10 micrometer) that inhibited these G(q)-mediated events, also prevented GPCR agonist-induced responses mediated through the G proteins of the G(12) subfamily. These include bombesin-induced assembly of focal adhesions, formation of parallel arrays of actin stress fibers, increase in the tyrosine phosphorylation of focal adhesion kinase (FAK), p130(Cas), and paxillin, and formation of a complex between FAK and Src. We conclude that [d-Arg(1),d-Trp(5,7,9),Leu(11)]SP acts as a mitogenic antagonist of neuropeptide GPCRs blocking signal transduction via both G(q) and G(12).

Polarity exchange at the interface of regulators of G protein signaling with G protein alpha-subunits

RGS proteins are GTPase-activating proteins (GAPs) for G protein alpha-subunits. This GAP activity is mediated by the interaction of conserved residues on regulator of G protein signaling (RGS) proteins and Galpha-subunits. We mutated the important contact sites Glu-89, Asn-90, and Asn-130 in RGS16 to lysine, aspartate, and alanine, respectively. The interaction of RGS16 and its mutants with Galpha(t) and Galpha(i1) was studied. The GAP activities of RGS16N90D and RGS16N130A were strongly attenuated. RGS16E89K increased GTP hydrolysis of Galpha(i1) by a similar extent, but with an about 100-fold reduced affinity compared with non-mutated RGS16. As Glu-89 in RGS16 is interacting with Lys-210 in Galpha(i1), this lysine was changed to glutamate for compensation. Galpha(i1)K210E was insensitive to RGS16 but interacted with RGS16E89K. In rat uterine smooth muscle cells, wild type RGS16 abolished G(i)-mediated alpha(2)-adrenoreceptor signaling, whereas RGS16E89K was without effect. Both Galpha(i1) and Galpha(i1)K210E mimicked the effect of alpha(2)-adrenoreceptor stimulation. Galpha(i1)K210E was sensitive to RGS16E89K and 10-fold more potent than Galpha(i1). Analogous mutants of Galpha(q) (Galpha(q)K215E) and RGS4 (RGS4E87K) were created and studied in COS-7 cells. The activity of wild type Galpha(q) was counteracted by wild type RGS4 but not by RGS4E87K. The activity of Galpha(q)K215E was inhibited by RGS4E87K, whereas non-mutated RGS4 was ineffective. We conclude that mutation of a conserved lysine residue to glutamate in Galpha(i) and Galpha(q) family members renders these proteins insensitive to wild type RGS proteins. Nevertheless, they are sensitive to glutamate to lysine mutants of RGS proteins. Such mutant pairs will be helpful tools in analyzing Galpha-RGS specificities in living cells.