Gα12 and endoplasmic reticulum stress-mediated pyroptosis in a single cycle of dextran sulfate-induced mouse colitis

Inflammatory bowel disease (IBD) pathogenesis involves complex inflammatory events and cell death. Although IBD involves mainly necrosis in the digestive tract, pyroptosis has also been recognized. Nonetheless, the underlying basis is elusive. Gα12/13 overexpression may affect endoplasmic reticulum (ER) stress. This study examined how Gα12/13 and ER stress affect pyroptosis using dextran sulfate sodium (DSS)-induced colitis models. Gα12/13 levels were increased in the distal and proximal colons of mice exposed to a single cycle of DSS, as accompanied by increases of IRE1α, ATF6, and p-PERK. Moreover, Il-6, Il-1β, Ym1, and Arg1 mRNA levels were increased with caspase-1 and IL-1β activation, supportive of pyroptosis. In the distal colon, RIPK1/3 levels were enhanced to a greater degree, confirming necroptosis. By contrast, the mice subjected to three cycles of DSS treatments showed decreases of Gα12/13, as accompanied by IRE1α and ATF6 suppression, but increases of RIPK1/3 and c-Cas3. AZ2 treatment, which inhibited Gα12, has an anti-pyroptotic effect against a single cycle of colitis. These results show that a single cycle of DSS-induced colitis may cause ER stress-induced pyroptosis as mediated by Gα12 overexpression in addition to necroptosis, but three cycles model induces only necroptosis, and that AZ2 may have an anti-pyroptotic effect.

Gα12 signaling regulates transcriptional and phenotypic responses that promote glioblastoma tumor invasion

In silico interrogation of glioblastoma (GBM) in The Cancer Genome Atlas (TCGA) revealed upregulation of GNA12 (Gα12), encoding the alpha subunit of the heterotrimeric G-protein G12, concomitant with overexpression of multiple G-protein coupled receptors (GPCRs) that signal through Gα12. Glioma stem cell lines from patient-derived xenografts also showed elevated levels of Gα12. Knockdown (KD) of Gα12 was carried out in two different human GBM stem cell (GSC) lines. Tumors generated in vivo by orthotopic injection of Gα12KD GSC cells showed reduced invasiveness, without apparent changes in tumor size or survival relative to control GSC tumor-bearing mice. Transcriptional profiling of GSC-23 cell tumors revealed significant differences between WT and Gα12KD tumors including reduced expression of genes associated with the extracellular matrix, as well as decreased expression of stem cell genes and increased expression of several proneural genes. Thrombospondin-1 (THBS1), one of the genes most repressed by Gα12 knockdown, was shown to be required for Gα12-mediated cell migration in vitro and for in vivo tumor invasion. Chemogenetic activation of GSC-23 cells harboring a Gα12-coupled DREADD also increased THBS1 expression and in vitro invasion. Collectively, our findings implicate Gα12 signaling in regulation of transcriptional reprogramming that promotes invasiveness, highlighting this as a potential signaling node for therapeutic intervention.

Ogt Deficiency Induces Abnormal Cerebellar Function and Behavioral Deficits of Adult Mice through Modulating RhoA/ROCK Signaling

Previous studies have shown the essential roles of O-GlcNAc transferase (Ogt) and O-GlcNAcylation in neuronal development, function and neurologic diseases. However, the function of Ogt and O-GlcNAcylation in the adult cerebellum has not been well elucidated. Here, we have found that cerebellum has the highest level of O-GlcNAcylation relative to cortex and hippocampus of adult male mice. Specific deletion of Ogt in granule neuron precursors (GNPs) induces abnormal morphology and decreased size of the cerebellum in adult male Ogt deficient [conditional knock-out (cKO)] mice. Adult male cKO mice show the reduced density and aberrant distribution of cerebellar granule cells (CGCs), the disrupted arrangement of Bergman glia (BG) and Purkinje cells. In addition, adult male cKO mice exhibit aberrant synaptic connection, impaired motor coordination, and learning and memory abilities. Mechanistically, we have identified G-protein subunit α12 (Gα12) is modified by Ogt-mediated O-GlcNAcylation. O-GlcNAcylation of Gα12 facilitates its binding to Rho guanine nucleotide exchange factor 12 (Arhgef12) and consequently activates RhoA/ROCK signaling. RhoA/ROCK pathway activator LPA can rescue the developmental deficits of Ogt deficient CGCs. Therefore, our study has revealed the critical function and related mechanisms of Ogt and O-GlcNAcylation in the cerebellum of adult male mice.SIGNIFICANCE STATEMENT Cerebellar function are regulated by diverse mechanisms. To unveil novel mechanisms is critical for understanding the cerebellar function and the clinical therapy of cerebellum-related diseases. In the present study, we have shown that O-GlcNAc transferase gene (Ogt) deletion induces abnormal cerebellar morphology, synaptic connection, and behavioral deficits of adult male mice. Mechanistically, Ogt catalyzes O-GlcNAcylation of Gα12, which promotes the binding to Arhgef12, and regulates RhoA/ROCK signaling pathway. Our study has uncovered the important roles of Ogt and O-GlcNAcylation in regulating cerebellar function and cerebellum-related behavior. Our results suggest that Ogt and O-GlcNAcylation could be potential targets for some cerebellum-related diseases.

Overexpressed Gα13 activates serum response factor through stoichiometric imbalance with Gβγ and mislocalization to the cytoplasm

Gα13, a heterotrimeric G protein α subunit of the G12/13 subfamily, is an oncogenic driver in multiple cancer types. Unlike other G protein subfamilies that contribute to cancer progression via amino acid substitutions that abolish their deactivating, intrinsic GTPase activity, Gα13 rarely harbors such mutations in tumors and instead appears to stimulate aberrant cell growth via overexpression as a wildtype form. It is not known why this effect is exclusive to the G12/13 subfamily, nor has a mechanism been elucidated for overexpressed Gα13 promoting tumor progression. Using a reporter gene assay for serum response factor (SRF)-mediated transcription in HEK293 cells, we found that transiently expressed, wildtype Gα13 generates a robust SRF signal, approximately half the amplitude observed for GTPase-defective Gα13. When epitope-tagged, wildtype Gα13 was titrated upward in cells, a sharp increase in SRF stimulation was observed coincident with a "spillover" of Gα13 from membrane-associated to a soluble fraction. Overexpressing G protein β and γ subunits caused both a decrease in this signal and a shift of wildtype Gα13 back to the membranous fraction, suggesting that stoichiometric imbalance in the αβγ heterotrimer results in aberrant subcellular localization and signalling by overexpressed Gα13. We also examined the acylation requirements of wildtype Gα13 for signalling to SRF. Similar to GTPase-defective Gα13, S-palmitoylation of the wildtype α subunit was necessary for SRF activation but could be replaced functionally by an engineered site for N-terminal myristoylation. However, a key difference was observed between wildtype and GTPase-defective Gα13: whereas the latter protein lacking palmitoylation sites was rescued in its SRF signalling by either an engineered polybasic sequence or a C-terminal isoprenylation site, these motifs failed to restore signalling by wildtype, non-palmitoylated Gα13. These findings illuminate several components of the mechanism in which overexpressed, wildtype Gα13 contributes to growth and tumorigenic signalling, and reveal greater stringency in its requirements for post-translational modification in comparison to GTPase-defective Gα13.

Neonatal Platelets: Lower G12/13 Expression Contributes to Reduced Secretion of Dense Granules

Despite fully functional primary hemostasis, platelets of healthy neonates exhibit hypoaggregability and secretion defects, which may be adaptations to specific requirements in this developmental stage. The etiologies for reduced signal transduction vary with the type of agonist. The discovered peculiarities are lower receptor densities, reduced calcium mobilization, and functional impairments of G proteins. Reduced secretion of dense granules has been attributed to lower numbers of granules. Signaling studies with adult platelets have shown a regulating effect of the G_12/13 signaling pathway on dense granule secretion via RhoA. We comparatively analyzed secretion profiles using flow cytometry and expression levels of G_q, G_i, and G_12/13 using Western blot analysis in platelets from cord blood and adults. Furthermore, we evaluated Rho activation after in vitro platelet stimulation with thrombin using a pulldown assay. We observed a markedly reduced expression of the dense granule marker CD63 on neonatal platelets after thrombin stimulation. G_α12/13 expression was significantly decreased in neonatal platelets and correlated with lower Rho activation after thrombin stimulation. We conclude that lower expression of G_12/13 in neonatal platelets results in attenuated activation of Rho and may contribute to reduced secretion of dense granules after exposure to thrombin.

Gα13 loss in Kras/Tp53 mouse model of pancreatic tumorigenesis promotes tumors susceptible to rapamycin

Gα13 transduces signals from G-protein-coupled receptors. While Gα13 functions as a tumor suppressor in lymphomas, it is not known whether Gα13 is pro-tumorigenic or tumor suppressive in genetically engineered mouse (GEM) models of epithelial cancers. Here, we show that loss of Gα13 in the Kras/Tp53 (KPC) GEM model promotes well-differentiated tumors and reduces survival. Mechanistically, tumors developing in KPC mice with Gα13 loss exhibit increased E-cadherin expression and mTOR signaling. Importantly, human pancreatic ductal adenocarcinoma (PDAC) tumors with low Gα13 expression also exhibit increased E-cadherin expression and mTOR signaling. Treatment with the mTOR inhibitor rapamycin decreases the growth of syngeneic KPC tumors with Gα13 loss by promoting cell death. This work establishes a tumor-suppressive role of Gα13 in pancreatic tumorigenesis in the KPC GEM model and suggests targeting mTOR in human PDAC tumors with Gα13 loss.

The adaptive immune system is a major driver of selection for tumor suppressor gene inactivation

During tumorigenesis, tumors must evolve to evade the immune system and do so by disrupting the genes involved in antigen processing and presentation or up-regulating inhibitory immune checkpoint genes. We performed in vivo CRISPR screens in syngeneic mouse tumor models to examine requirements for tumorigenesis both with and without adaptive immune selective pressure. In each tumor type tested, we found a marked enrichment for the loss of tumor suppressor genes (TSGs) in the presence of an adaptive immune system relative to immunocompromised mice. Nearly one-third of TSGs showed preferential enrichment, often in a cancer- and tissue-specific manner. These results suggest that clonal selection of recurrent mutations found in cancer is driven largely by the tumor’s requirement to avoid the adaptive immune system.

TSH Activates Macrophage Inflammation by G13- and G15-dependent Pathways

Thyroid-stimulating hormone (TSH) treatment activates inhibitor of NF-κB/nuclear factor κB (IκB/NFκB) and extracellular signal-regulated kinase (ERK)-P38 in macrophages, but how these pathways are activated, and how they contribute to the proinflammatory effect of TSH on macrophages remain unknown. The TSH receptor (TSHR) is coupled to 4 subfamilies of G proteins (Gs, Gi/o, Gq/11, and G12/13) for its downstream signaling. This study investigated the G protein subtypes responsible for the proinflammatory effect of TSH on macrophages. qPCR showed that Gi2, Gi3, Gas, Gq, G11, G12, G13, and G15 are abundantly expressed by macrophages. The contribution of different G protein pathways to the proinflammatory effect was studied by the corresponding inhibitors or siRNA interference. While TSH-induced IκB phosphorylation was not inhibited by Gs inhibitor NF449, Gi inhibitor pertussis toxin, or Gq or G11 siRNA, it was blocked by phospholipase C inhibitor U73122 or G15 siRNA interference. TSH-induced ERK and P38 phosphorylation was blocked by G13 but not G12 siRNA interference. Interference of either G13 or G15 could block the proinflammatory effect of TSH on macrophages. The present study demonstrate that TSH activates macrophage inflammation by the G13/ERK-P38/Rho GTPase and G15/phospholipase C (PLC)/protein kinases C (PKCs)/IκB pathways.

Targeting Gα13-integrin interaction ameliorates systemic inflammation

Systemic inflammation as manifested in sepsis is an excessive, life-threatening inflammatory response to severe bacterial or viral infection or extensive injury. It is also a thrombo-inflammatory condition associated with vascular leakage/hemorrhage and thrombosis that is not effectively treated by current anti-inflammatory or anti-thrombotic drugs. Here, we show that MB2mP6 peptide nanoparticles, targeting the Gα13-mediated integrin "outside-in" signaling in leukocytes and platelets, inhibited both inflammation and thrombosis without causing hemorrhage/vascular leakage. MB2mP6 improved mouse survival when infused immediately or hours after onset of severe sepsis. Furthermore, platelet Gα13 knockout inhibited septic thrombosis whereas leukocyte Gα13 knockout diminished septic inflammation, each moderately improving survival. Dual platelet/leukocyte Gα13 knockout inhibited septic thrombosis and inflammation, further improving survival similar to MB2mP6. These results demonstrate that inflammation and thrombosis independently contribute to poor outcomes and exacerbate each other in systemic inflammation, and reveal a concept of dual anti-inflammatory/anti-thrombotic therapy without exacerbating vascular leakage.

Identification of key transcriptome biomarkers based on a vital gene module associated with pathological changes in Alzheimer's disease

Dysregulation of transcriptome expression has been reported to play an increasingly significant role in AD. In this study, we firstly identified a vital gene module associated with the accumulation of β-amyloid (Aβ) and phosphorylated tau (p-tau) using the WGCNA method. The vital module, named target module, was then employed for the identification of key transcriptome biomarkers. For coding RNA, GNA13 and GJA1 were identified as key biomarkers based on ROC analysis. As for non-coding RNA, MEG3, miR-106a-3p, and miR-24-3p were determined as key biomarkers based on analysis of a ceRNA network and ROC analysis. Experimental analyses firstly confirmed that GNA13, GJA1, and ROCK2, a downstream effector of GNA13, were all increased in 5XFAD mice, compared to littermate mice. Moreover, their expression was increased with aging in 5XFAD mice, as Aβ and p-tau pathology developed. Besides, the expression of key ncRNA biomarkers was verified to be decreased in 5XFAD mice. GSEA results indicated that GNA13 and GJA1 were respectively involved in ribosome and spliceosome dysfunction. MEG3, miR-106a-3p, and miR-24-3p were identified to be involved in MAPK pathway and PI3K-Akt pathway based on enrichment analysis. In summary, we identified several key transcriptome biomarkers, which promoted the prediction and diagnosis of AD.

Shear and Integrin Outside-In Signaling Activate NADPH-Oxidase 2 to Promote Platelet Activation

[Figure: see text].

NLRP3 inflammasome priming and activation in cholestatic liver injury via the sphingosine 1-phosphate/S1P receptor 2/Gα(12/13)/MAPK signaling pathway

NLRP3 inflammasome-driven inflammation represents a key trigger for hepatic fibrogenesis during cholestatic liver injury. However, whether sphingosine 1-phosphate (S1P) plays a role in NLRP3 inflammasome priming and activation remains unknown. Here, we found that the expression of NLRP3 in macrophages and NLRP3 inflammasome activation were significantly elevated in the liver injured by bile duct ligation (BDL). In vitro, S1P promoted the NLRP3 inflammasome priming and activation via S1P receptor 2 (S1PR2) in bone marrow-derived monocyte/macrophages (BMMs). Focusing on BMMs, the gene silencing of Gα12 or Gα13 by specific siRNA suppressed NLRP3 inflammasome priming and pro-inflammatory cytokine (IL-1β and IL-18) secretion, whereas Gα(i/o) and Gαq were not involved in this process. The MAPK signaling pathways (P38, ERK, and JNK) mediated NLRP3 inflammasome priming and IL-1β and IL-18 secretion, whereas blockage of PI3K, ROCK, and Rho family had no such effect. Moreover, JTE-013 (S1PR2 inhibitor) treatment markedly reduced NLRP3 inflammasome priming and activation in BDL-injured liver. Collectively, S1P promotes NLRP3 inflammasome priming and pro-inflammatory cytokines (IL-1β and IL-18) secretion via the S1PR2/Gα(12/13)/MAPK pathway, which may represent an effective therapeutic strategy for liver disease. KEY MESSAGE: • Hepatic NLRP3 expression was significantly elevated in BMMs of BDL-injured mouse liver. • S1P promoted NLRP3 inflammasome priming and activation in BMMs, depending on the S1PR2/Gα(12/13)/MAPK pathway. • Blockade of S1PR2 by JTE-013 reduced NLRP3 inflammasome priming and activation inflammasome in vivo.

Gαs directly drives PDZ-RhoGEF signaling to Cdc42

Gα proteins promote dynamic adjustments of cell shape directed by actin-cytoskeleton reorganization via their respective RhoGEF effectors. For example, Gα13 binding to the RGS-homology (RH) domains of several RH-RhoGEFs allosterically activates these proteins, causing them to expose their catalytic Dbl-homology (DH)/pleckstrin-homology (PH) regions, which triggers downstream signals. However, whether additional Gα proteins might directly regulate the RH-RhoGEFs was not known. To explore this question, we first examined the morphological effects of expressing shortened RH-RhoGEF DH/PH constructs of p115RhoGEF/ARHGEF1, PDZ-RhoGEF (PRG)/ARHGEF11, and LARG/ARHGEF12. As expected, the three constructs promoted cell contraction and activated RhoA, known to be downstream of Gα13 Intriguingly, PRG DH/PH also induced filopodia-like cell protrusions and activated Cdc42. This pathway was stimulated by constitutively active Gαs (GαsQ227L), which enabled endogenous PRG to gain affinity for Cdc42. A chemogenetic approach revealed that signaling by Gs-coupled receptors, but not by those coupled to Gi or Gq, enabled PRG to bind Cdc42. This receptor-dependent effect, as well as CREB phosphorylation, was blocked by a construct derived from the PRG:Gαs-binding region, PRG-linker. Active Gαs interacted with isolated PRG DH and PH domains and their linker. In addition, this construct interfered with GαsQ227L's ability to guide PRG's interaction with Cdc42. Endogenous Gs-coupled prostaglandin receptors stimulated PRG binding to membrane fractions and activated signaling to PKA, and this canonical endogenous pathway was attenuated by PRG-linker. Altogether, our results demonstrate that active Gαs can recognize PRG as a novel effector directing its DH/PH catalytic module to gain affinity for Cdc42.

Naturally occurring hotspot cancer mutations in Gα13 promote oncogenic signaling

Heterotrimeric G-proteins are signaling switches broadly divided into four families based on the sequence and functional similarity of their Gα subunits: Gs, Gi/o, Gq/11, and G12/13 Artificial mutations that activate Gα subunits of each of these families have long been known to induce oncogenic transformation in experimental systems. With the advent of next-generation sequencing, activating hotspot mutations in Gs, Gi/o, or Gq/11 proteins have also been identified in patient tumor samples. In contrast, patient tumor-associated G12/13 mutations characterized to date lead to inactivation rather than activation. By using bioinformatic pathway analysis and signaling assays, here we identified cancer-associated hotspot mutations in Arg-200 of Gα13 (encoded by GNA13) as potent activators of oncogenic signaling. First, we found that components of a G12/13-dependent signaling cascade that culminates in activation of the Hippo pathway effectors YAP and TAZ is frequently altered in bladder cancer. Up-regulation of this signaling cascade correlates with increased YAP/TAZ activation transcriptional signatures in this cancer type. Among the G12/13 pathway alterations were mutations in Arg-200 of Gα13, which we validated to promote YAP/TAZ-dependent (TEAD) and MRTF-A/B-dependent (SRE.L) transcriptional activity. We further showed that this mechanism relies on the same RhoGEF-RhoGTPase cascade components that are up-regulated in bladder cancers. Moreover, Gα13 Arg-200 mutants induced oncogenic transformation in vitro as determined by focus formation assays. In summary, our findings on Gα13 mutants establish that naturally occurring hotspot mutations in Gα subunits of any of the four families of heterotrimeric G-proteins are putative cancer drivers.

G12/13 is activated by acute tethered agonist exposure in the adhesion GPCR ADGRL3

The adhesion G-protein-coupled receptor (GPCR) latrophilin 3 (ADGRL3) has been associated with increased risk of attention deficit hyperactivity disorder (ADHD) and substance use in human genetic studies. Knockdown in multiple species leads to hyperlocomotion and altered dopamine signaling. Thus, ADGRL3 is a potential target for treatment of neuropsychiatric disorders that involve dopamine dysfunction, but its basic signaling properties are poorly understood. Identification of adhesion GPCR signaling partners has been limited by a lack of tools to acutely activate these receptors in living cells. Here, we design a novel acute activation strategy to characterize ADGRL3 signaling by engineering a receptor construct in which we could trigger acute activation enzymatically. Using this assay, we found that ADGRL3 signals through G12/G13 and Gq, with G12/13 the most robustly activated. Gα12/13 is a new player in ADGRL3 biology, opening up unexplored roles for ADGRL3 in the brain. Our methodological advancements should be broadly useful in adhesion GPCR research.

The Vasopressin Receptor 2 Mutant R137L Linked to the Nephrogenic Syndrome of Inappropriate Antidiuresis (NSIAD) Signals through an Alternative Pathway that Increases AQP2 Membrane Targeting Independently of S256 Phosphorylation

NSIAD is a rare X-linked condition, caused by activating mutations in the AVPR2 gene coding for the vasopressin V2 receptor (V2R) associated with hyponatremia, despite undetectable plasma vasopressin levels. We have recently provided in vitro evidence that, compared to V2R-wt, expression of activating V2R mutations R137L, R137C and F229V cause a constitutive redistribution of the AQP2 water channel to the plasma membrane, higher basal water permeability and significantly higher basal levels of p256-AQP2 in the F229V mutant but not in R137L or R137C. In this study, V2R mutations were expressed in collecting duct principal cells and the associated signalling was dissected. V2R-R137L and R137C mutants had significantly higher basal pT269-AQP2 levels -independently of S256 and PKA-which were reduced to control by treatment with Rho kinase (ROCK) inhibitor. Interestingly, ROCK activity was found significantly higher in V2R-R137L along with activation of the Gα12/13–Rho–ROCK pathway. Of note, inhibition of ROCK reduced the basal elevated osmotic water permeability to control. To conclude, our data demonstrate for the first time that the gain-of-function mutation of the V2R, R137L causing NSIAD, signals through an alternative PKA-independent pathway that increases AQP2 membrane targeting through ROCK-induced phosphorylation at S/T269 independently of S256 of AQP2.

Divergent C-terminal motifs in Gα12 and Gα13 provide distinct mechanisms of effector binding and SRF activation

The G12/13 subfamily of heterotrimeric guanine nucleotide binding proteins comprises the α subunits Gα12 and Gα13, which transduce signals for cell growth, cytoskeletal rearrangements, and oncogenic transformation. In an increasing range of cancers, overexpressed Gα12 or Gα13 are implicated in aberrant cell proliferation and/or metastatic invasion. Although Gα12 and Gα13 bind non-redundant sets of effector proteins and participate in unique signalling pathways, the structural features responsible for functional differences between these α subunits are largely unknown. Invertebrates encode a single G12/13 homolog that participates in cytoskeletal changes yet appears to lack signalling to SRF (serum response factor), a transcriptional activator stimulated by mammalian Gα12 and Gα13 to promote growth and tumorigenesis. Our previous studies identified an evolutionarily divergent region in Gα12 for which replacement by homologous sequence from Drosophila melanogaster abolished SRF signalling, whereas the same invertebrate substitution was fully tolerated in Gα13 [Montgomery et al. (2014) Mol. Pharmacol. 85: 586]. These findings prompted our current approach of evolution-guided mutagenesis to identify fine structural features of Gα12 and Gα13 that underlie their respective SRF activation mechanisms. Our results identified two motifs flanking the α4 helix that play a key role in Gα12 signalling to SRF. We found the region encompassing these motifs to provide an interacting surface for multiple Gα12-specific target proteins that fail to bind Gα13. Adjacent to this divergent region, a highly-conserved domain was vital for SRF activation by both Gα12 and Gα13. However, dissection of this domain using invertebrate substitutions revealed different signalling mechanisms in these α subunits and identified Gα13-specific determinants of binding Rho-specific guanine nucleotide exchange factors. Furthermore, invertebrate substitutions in the C-terminal, α5 helical region were selectively disruptive to Gα12 signalling. Taken together, our results identify key structural features near the C-terminus that evolved after the divergence of Gα12 and Gα13, and should aid the development of agents to selectively manipulate signalling by individual α subunits of the G12/13 subfamily.

Auto-regulation of Secretory Flux by Sensing and Responding to the Folded Cargo Protein Load in the Endoplasmic Reticulum

Maintaining the optimal performance of cell processes and organelles is the task of auto-regulatory systems. Here we describe an auto-regulatory device that helps to maintain homeostasis of the endoplasmic reticulum (ER) by adjusting the secretory flux to the cargo load. The cargo-recruiting subunit of the coatomer protein II (COPII) coat, Sec24, doubles as a sensor of folded cargo and, upon cargo binding, acts as a guanine nucleotide exchange factor to activate the signaling protein Gα12 at the ER exit sites (ERESs). This step, in turn, activates a complex signaling network that activates and coordinates the ER export machinery and attenuates proteins synthesis, thus preventing large fluctuations of folded and potentially active cargo that could be harmful to the cell or the organism. We call this mechanism AREX (autoregulation of ER export) and expect that its identification will aid our understanding of human physiology and diseases that develop from secretory dysfunction.

G-protein Gα13 functions as a cytoskeletal and mitochondrial regulator to restrain osteoclast function

Excessive osteoclastic bone erosion disrupts normal bone remodeling and leads to bone loss in many skeletal diseases, including inflammatory arthritis, such as rheumatoid arthritis (RA) and psoriatic arthritis, periodontitis and peri-prosthetic loosening. Functional control of osteoclasts is critical for the maintenance of bone homeostasis. However, the mechanisms that restrain osteoclast resorptive function are not fully understood. In this study, we identify a previously unrecognized role for G-protein Gα13 in inhibition of osteoclast adhesion, fusion and bone resorptive function. Gα13 is highly expressed in mature multinucleated osteoclasts, but not during early differentiation. Deficiency of Gα13 in myeloid osteoclast lineage (Gα13ΔM/ΔM mice) leads to super spread morphology of multinucleated giant osteoclasts with elevated bone resorptive capacity, corroborated with an osteoporotic bone phenotype in the Gα13ΔM/ΔM mice. Mechanistically, Gα13 functions as a brake that restrains the c-Src, Pyk2, RhoA-Rock2 mediated signaling pathways and related gene expressions to control the ability of osteoclasts in fusion, adhesion, actin cytoskeletal remodeling and resorption. Genome wide analysis reveals cytoskeleton related genes that are suppressed by Gα13, identifying Gα13 as a critical cytoskeletal regulator in osteoclasts. We also identify a genome wide regulation of genes responsible for mitochondrial biogenesis and function by Gα13 in osteoclasts. Furthermore, the significant correlation between Gα13 expression levels, TNF activity and RA disease activity in RA patients suggests that the Gα13 mediated mechanisms represent attractive therapeutic targets for diseases associated with excessive bone resorption.

GNA13 promotes tumor growth and angiogenesis by upregulating CXC chemokines via the NF-κB signaling pathway in colorectal cancer cells

GNA13 has been found overexpressed in various types of cancer, which is related to tumor metastasis and progression. However, the biological functions of GNA13 in colorectal cancer (CRC) progression remain unclear. This study aimed to explore the role of GNA13 in CRC and investigate the mechanism of how GNA13 promotes tumor growth. Interestingly, our findings showed that GNA13 is commonly upregulated in CRC, where these events are associated with a worse histologic grade and poor survival. Increased expression levels of GNA13 promoted cell growth, migration, invasion, and epithelial-mesenchymal transition, whereas GNA13 silencing abrogated these malignant phenotypes. In addition, overexpressing GNA13 in cancer cells increased the levels of the chemokines CXCL1, CXCL2, and CXCL4, which contributed to CRC proliferation and colony formation. Moreover, our mechanistic investigations suggest that the NF-κB/p65 signaling pathway was activated by the increase in GNA13 levels. Inhibiting the NF-κB/p65 pathway with an inhibitor decreased GNA13-induced migration, invasion and CXCL chemokine level increases, indicating the critical role of NF-κB/p65 signaling in mediating the effects of GNA13 in CRC. Together, these results demonstrate a key role of GNA13 overexpression in CRC that contributes to malignant behavior in cancer cells, at least in part through stimulating angiogenesis and increasing the levels of the NF-κB-dependent chemokines CXCL1, CXCL2, and CXCL4.

Overlap in signaling between Smoothened and the α subunit of the heterotrimeric G protein G13

The Hedgehog family of morphogens has long been known to utilize, through the 7-transmembrane protein Smoothened (Smo), the heterotrimeric G protein Gi in both canonical and noncanonical forms of signaling. Other G proteins, while not specifically utilized by Smo, may nonetheless provide access to some of the events controlled by it. We reported several years ago that the G protein G13 activates one or more forms of the Gli family of transcription factors. While the Gli transcription factors are well known targets for Smo, the uncertain mechanism of activation by G13 and the identity of the targeted Gli(s) limited predictions as to the extent to which G13 might mimic Smo's actions. We evaluate here the potential for overlap in G13 and Smo signaling using C3H10T1/2 and 3T3-L1 cells as models of osteogenesis and adipogenesis, respectively. We find in C3H10T1/2 cells that a constitutively active form of Gα13 (Gα13QL) increases Gli1 mRNA, as does a constitutively active form of Smo (SmoA1). We find as well that Gα13QL induces alkaline phosphatase activity, a marker of osteogenesis, albeit the induction is far less substantial than that achieved by SmoA1. In 3T3-L1 cells both Gα13QL and SmoA1 markedly suppress adipogenic differentiation as determined by triglyceride accumulation. RNA sequencing reveals that Gα13QL and SmoA1 regulate many of the same genes but that quantitative and qualitative differences exist. Differences also exist, we find, between SmoA1 and purmorphamine, an agonist for Smo. Therefore, while comparisons of constitutively active proteins are informative, extrapolations to the setting of agonists require care.

A FRET-based biosensor for measuring Gα13 activation in single cells

Förster Resonance Energy Transfer (FRET) provides a way to directly observe the activation of heterotrimeric G-proteins by G-protein coupled receptors (GPCRs). To this end, FRET based biosensors are made, employing heterotrimeric G-protein subunits tagged with fluorescent proteins. These FRET based biosensors complement existing, indirect, ways to observe GPCR activation. Here we report on the insertion of mTurquoise2 at several sites in the human Gα13 subunit, aiming to develop a FRET-based Gα13 activation biosensor. Three fluorescently tagged Gα13 variants were found to be functional based on i) plasma membrane localization and ii) ability to recruit p115-RhoGEF upon activation of the LPA2 receptor. The tagged Gα13 subunits were used as FRET donor and combined with cp173Venus fused to the Gγ2 subunit, as the acceptor. We constructed Gα13 biosensors by generating a single plasmid that produces Gα13-mTurquoise2, Gβ1 and cp173Venus-Gγ2. The Gα13 activation biosensors showed a rapid and robust response when used in primary human endothelial cells that were exposed to thrombin, triggering endogenous protease activated receptors (PARs). This response was efficiently inhibited by the RGS domain of p115-RhoGEF and from the biosensor data we inferred that this is due to GAP activity. Finally, we demonstrated that the Gα13 sensor can be used to dissect heterotrimeric G-protein coupling efficiency in single living cells. We conclude that the Gα13 biosensor is a valuable tool for live-cell measurements that probe spatiotemporal aspects of Gα13 activation.

Gα12 overexpression induced by miR-16 dysregulation contributes to liver fibrosis by promoting autophagy in hepatic stellate cells

BACKGROUND & AIMS

Hepatic stellate cells (HSCs) have a role in liver fibrosis. Guanine nucleotide-binding α-subunit 12 (Gα₁₂) converges signals from G-protein-coupled receptors whose ligand levels are elevated in the environment during liver fibrosis; however, information is lacking on the effect of Gα₁₂ on HSC trans-differentiation. This study investigated the expression of Gα₁₂ in HSCs and the molecular basis of the effects of its expression on liver fibrosis.

METHODS

Gα₁₂ expression was assessed by immunostaining, and immunoblot analyses of mouse fibrotic liver tissues and primary HSCs. The role of Gα₁₂ in liver fibrosis was estimated using a toxicant injury mouse model with Gα₁₂ gene knockout and/or HSC-specific Gα₁₂ delivery using lentiviral vectors, in addition to primary HSCs and LX-2 cells using microRNA (miR) inhibitors, overexpression vectors, or adenoviruses. miR-16, Gα₁₂, and LC3 were also examined in samples from patients with fibrosis.

RESULTS

Gα₁₂ was overexpressed in activated HSCs and fibrotic liver, and was colocalised with desmin. In a carbon tetrachloride-induced fibrosis mouse model, Gα₁₂ ablation prevented increases in fibrosis and liver injury. This effect was attenuated by HSC-specific lentiviral delivery of Gα₁₂. Moreover, Gα₁₂ activation promoted autophagy accompanying c-Jun N-terminal kinase-dependent ATG12-5 conjugation. In addition, miR-16 was found to be a direct inhibitor of the de novo synthesis of Gα₁₂. Modulations of miR-16 altered autophagy in HSCs. In a fibrosis animal model or patients with severe fibrosis, miR-16 levels were lower than in their corresponding controls. Consistently, cirrhotic patient liver tissues showed Gα₁₂ and LC3 upregulation in desmin-positive areas.

CONCLUSIONS

miR-16 dysregulation in HSCs results in Gα₁₂ overexpression, which activates HSCs by facilitating autophagy through ATG12-5 formation. This suggests that Gα₁₂ and its regulatory molecules could serve as targets for the amelioration of liver fibrosis.

LAY SUMMARY

Guanine nucleotide-binding α-subunit 12 (Gα₁₂) is upregulated in activated hepatic stellate cells (HSCs) as a consequence of the dysregulation of a specific microRNA that is abundant in HSCs, facilitating the progression of liver fibrosis. This event is mediated by c-Jun N-terminal kinase-dependent ATG12-5 formation and the promotion of autophagy. We suggest that Gα₁₂ and its associated regulators could serve as new targets in HSCs for the treatment of liver fibrosis.

Gα12 facilitates shortening in human airway smooth muscle by modulating phosphoinositide 3-kinase-mediated activation in a RhoA-dependent manner

BACKGROUND AND PURPOSE

PI3K-dependent activation of Rho kinase (ROCK) is necessary for agonist-induced human airway smooth muscle cell (HASMC) contraction, and inhibition of PI3K promotes bronchodilation of human small airways. The mechanisms driving agonist-mediated PI3K/ROCK axis activation, however, remain unclear. Given that G12 family proteins activate ROCK pathways in other cell types, their role in M3 muscarinic acetylcholine receptor-stimulated PI3K/ROCK activation and contraction was examined.

EXPERIMENTAL APPROACH

Gα12 coupling was evaluated using co-immunoprecipitation and serum response element (SRE)-luciferase reporter assays. siRNA and pharmacological approaches, as well as overexpression of a regulator of G-protein signaling (RGS) proteins were applied in HASMCs. Phosphorylation levels of Akt, myosin phosphatase targeting subunit-1 (MYPT1), and myosin light chain-20 (MLC) were measured. Contraction and shortening were evaluated using magnetic twisting cytometry (MTC) and micro-pattern deformation, respectively. Human precision-cut lung slices (hPCLS) were utilized to evaluate bronchoconstriction.

KEY RESULTS

Knockdown of M3 receptors or Gα12 attenuated activation of Akt, MYPT1, and MLC phosphorylation. Gα12 coimmunoprecipitated with M3 receptors, and p115RhoGEF-RGS overexpression inhibited carbachol-mediated induction of SRE-luciferase reporter. p115RhoGEF-RGS overexpression inhibited carbachol-induced activation of Akt, HASMC contraction, and shortening. Moreover, inhibition of RhoA blunted activation of PI3K. Lastly, RhoA inhibitors induced dilation of hPCLS.

CONCLUSIONS AND IMPLICATIONS

Gα12 plays a crucial role in HASMC contraction via RhoA-dependent activation of the PI3K/ROCK axis. Inhibition of RhoA activation induces bronchodilation in hPCLS, and targeting Gα12 signaling may elucidate novel therapeutic targets in asthma. These findings provide alternative approaches to the clinical management of airway obstruction in asthma.

The balance between Gαi-Cdc42/Rac and Gα12/13-RhoA pathways determines endothelial barrier regulation by sphingosine-1-phosphate

The bioactive sphingosine-1-phosphatephosphate (S1P) is present in plasma, bound to carrier proteins, and involved in many physiological processes, including angiogenesis, inflammatory responses, and vascular stabilization. S1P can bind to several G-protein-coupled receptors (GPCRs) activating a number of different signaling networks. At present, the dynamics and relative importance of signaling events activated immediately downstream of GPCR activation are unclear. To examine these, we used a set of fluorescence resonance energy transfer-based biosensors for different RhoGTPases (Rac1, RhoA/B/C, and Cdc42) as well as for heterotrimeric G-proteins in a series of live-cell imaging experiments in primary human endothelial cells. These experiments were accompanied by biochemical GTPase activity assays and transendothelial resistance measurements. We show that S1P promotes cell spreading and endothelial barrier function through S1PR1-Gαi-Rac1 and S1PR1-Gαi-Cdc42 pathways. In parallel, a S1PR2-Gα12/13-RhoA pathway is activated that can induce cell contraction and loss of barrier function, but only if Gαi-mediated signaling is suppressed. Our results suggest that Gαq activity is not involved in S1P-mediated regulation of barrier integrity. Moreover, we show that early activation of RhoA by S1P inactivates Rac1 but not Cdc42, and vice versa. Together, our data show that the rapid S1P-induced increase in endothelial integrity is mediated by a S1PR1-Gαi-Cdc42 pathway.

PPARγ agonists negatively regulate αIIbβ3 integrin outside-in signaling and platelet function through up-regulation of protein kinase A activity

Essentials peroxisome proliferator-activated receptor γ (PPARγ) agonists inhibit platelet function. PPARγ agonists negatively regulate outside-in signaling via integrin αIIbβ3. PPARγ agonists disrupt the interaction of Gα13 with integrin β3. This is attributed to an upregulation of protein kinase A activity.

SUMMARY

Background Agonists for the peroxisome proliferator-activated receptor (PPARγ) have been shown to have inhibitory effects on platelet activity following stimulation by GPVI and GPCR agonists. Objectives Profound effects on thrombus formation led us to suspect a role for PPARγ agonists in the regulation of integrin αIIbβ3 mediated signaling. Both GPVI and GPCR signaling pathways lead to αIIbβ3 activation, and signaling through αIIbβ3 plays a critical role in platelet function and normal hemostasis. Methods The effects of PPARγ agonists on the regulation of αIIbβ3 outside-in signaling was determined by monitoring the ability of platelets to adhere and spread on fibrinogen and undergo clot retraction. Effects on signaling components downstream of αIIbβ3 activation were also determined following adhesion to fibrinogen by Western blotting. Results Treatment of platelets with PPARγ agonists inhibited platelet adhesion and spreading on fibrinogen and diminished clot retraction. A reduction in phosphorylation of several components of αIIbβ3 signaling, including the integrin β3 subunit, Syk, PLCγ2, focal adhesion kinase (FAK) and Akt, was also observed as a result of reduced interaction of the integrin β3 subunit with Gα13. Studies of VASP phosphorylation revealed that this was because of an increase in PKA activity following treatment with PPARγ receptor agonists. Conclusions This study provides further evidence for antiplatelet actions of PPARγ agonists, identifies a negative regulatory role for PPARγ agonists in the control of integrin αIIbβ3 outside-in signaling, and provides a molecular basis by which the PPARγ agonists negatively regulate platelet activation and thrombus formation.

Gα13 Switch Region 2 Binds to the Talin Head Domain and Activates αIIbβ3 Integrin in Human Platelets

Even though GPCR signaling in human platelets is directly involved in hemostasis and thrombus formation, the sequence of events by which G protein activation leads to αIIbβ3 integrin activation (inside-out signaling) is not clearly defined. We previously demonstrated that a conformationally sensitive domain of one G protein, i.e. Gα13 switch region 1 (Gα13SR1), can directly participate in the platelet inside-out signaling process. Interestingly however, the dependence on Gα13SR1 signaling was limited to PAR1 receptors, and did not involve signaling through other important platelet GPCRs. Based on the limited scope of this involvement, and the known importance of G13 in hemostasis and thrombosis, the present study examined whether signaling through another switch region of G13, i.e. Gα13 switch region 2 (Gα13SR2) may represent a more global mechanism of platelet activation. Using multiple experimental approaches, our results demonstrate that Gα13SR2 forms a bi-molecular complex with the head domain of talin and thereby promotes β3 integrin activation. Moreover, additional studies provided evidence that Gα13SR2 is not constitutively associated with talin in unactivated platelets, but becomes bound to talin in response to elevated intraplatelet calcium levels. Collectively, these findings provide evidence for a novel paradigm of inside-out signaling in platelets, whereby β3 integrin activation involves the direct binding of the talin head domain to the switch region 2 sequence of the Gα13 subunit.

Endoderm convergence controls subduction of the myocardial precursors during heart-tube formation

Coordination between the endoderm and adjacent cardiac mesoderm is crucial for heart development. We previously showed that myocardial migration is promoted by convergent movement of the endoderm, which itself is controlled by the S1pr2/Gα13 signaling pathway, but it remains unclear how the movements of the two tissues is coordinated. Here, we image live and fixed embryos to follow these movements, revealing previously unappreciated details of strikingly complex and dynamic associations between the endoderm and myocardial precursors. We found that during segmentation the endoderm underwent three distinct phases of movement relative to the midline: rapid convergence, little convergence and slight expansion. During these periods, the myocardial cells exhibited different stage-dependent migratory modes: co-migration with the endoderm, movement from the dorsal to the ventral side of the endoderm (subduction) and migration independent of endoderm convergence. We also found that defects in S1pr2/Gα13-mediated endodermal convergence affected all three modes of myocardial cell migration, probably due to the disruption of fibronectin assembly around the myocardial cells and consequent disorganization of the myocardial epithelium. Moreover, we found that additional cell types within the anterior lateral plate mesoderm (ALPM) also underwent subduction, and that this movement likewise depended on endoderm convergence. Our study delineates for the first time the details of the intricate interplay between the endoderm and ALPM during embryogenesis, highlighting why endoderm movement is essential for heart development, and thus potential underpinnings of congenital heart disease.

Cannabinoid-induced actomyosin contractility shapes neuronal morphology and growth

Endocannabinoids are recently recognized regulators of brain development, but molecular effectors downstream of type-1 cannabinoid receptor (CB1R)-activation remain incompletely understood. We report atypical coupling of neuronal CB1Rs, after activation by endo- or exocannabinoids such as the marijuana component ∆(9)-tetrahydrocannabinol, to heterotrimeric G12/G13 proteins that triggers rapid and reversible non-muscle myosin II (NM II) dependent contraction of the actomyosin cytoskeleton, through a Rho-GTPase and Rho-associated kinase (ROCK). This induces rapid neuronal remodeling, such as retraction of neurites and axonal growth cones, elevated neuronal rigidity, and reshaping of somatodendritic morphology. Chronic pharmacological inhibition of NM II prevents cannabinoid-induced reduction of dendritic development in vitro and leads, similarly to blockade of endocannabinoid action, to excessive growth of corticofugal axons into the sub-ventricular zone in vivo. Our results suggest that CB1R can rapidly transform the neuronal cytoskeleton through actomyosin contractility, resulting in cellular remodeling events ultimately able to affect the brain architecture and wiring.

Gα12 structural determinants of Hsp90 interaction are necessary for serum response element-mediated transcriptional activation

The G12/13 class of heterotrimeric G proteins, comprising the α-subunits Gα12 and Gα13, regulates multiple aspects of cellular behavior, including proliferation and cytoskeletal rearrangements. Although guanine nucleotide exchange factors for the monomeric G protein Rho (RhoGEFs) are well characterized as effectors of this G protein class, a variety of other downstream targets has been reported. To identify Gα12 determinants that mediate specific protein interactions, we used a structural and evolutionary comparison between the G12/13, Gs, Gi, and Gq classes to identify "class-distinctive" residues in Gα12 and Gα13. Mutation of these residues in Gα12 to their deduced ancestral forms revealed a subset necessary for activation of serum response element (SRE)-mediated transcription, a G12/13-stimulated pathway implicated in cell proliferative signaling. Unexpectedly, this subset of Gα12 mutants showed impaired binding to heat-shock protein 90 (Hsp90) while retaining binding to RhoGEFs. Corresponding mutants of Gα13 exhibited robust SRE activation, suggesting a Gα12-specific mechanism, and inhibition of Hsp90 by geldanamycin or small interfering RNA-mediated lowering of Hsp90 levels resulted in greater downregulation of Gα12 than Gα13 signaling in SRE activation experiments. Furthermore, the Drosophila G12/13 homolog Concertina was unable to signal to SRE in mammalian cells, and Gα12:Concertina chimeras revealed Gα12-specific determinants of SRE activation within the switch regions and a C-terminal region. These findings identify Gα12 determinants of SRE activation, implicate Gα12:Hsp90 interaction in this signaling mechanism, and illuminate structural features that arose during evolution of Gα12 and Gα13 to allow bifurcated mechanisms of signaling to a common cell proliferative pathway.

The sphingolipid receptor S1PR2 is a receptor for Nogo-a repressing synaptic plasticity

This study identifies a GPCR, S1PR2, as a receptor for the Nogo-A-Δ20 domain of the membrane protein Nogo-A, which inhibits neuronal growth and synaptic plasticity.

Nogo-A is a membrane protein of the central nervous system (CNS) restricting neurite growth and synaptic plasticity via two extracellular domains: Nogo-66 and Nogo-A-Δ20. Receptors transducing Nogo-A-Δ20 signaling remained elusive so far. Here we identify the G protein-coupled receptor (GPCR) sphingosine 1-phosphate receptor 2 (S1PR2) as a Nogo-A-Δ20-specific receptor. Nogo-A-Δ20 binds S1PR2 on sites distinct from the pocket of the sphingolipid sphingosine 1-phosphate (S1P) and signals via the G protein G₁₃, the Rho GEF LARG, and RhoA. Deleting or blocking S1PR2 counteracts Nogo-A-Δ20- and myelin-mediated inhibition of neurite outgrowth and cell spreading. Blockade of S1PR2 strongly enhances long-term potentiation (LTP) in the hippocampus of wild-type but not Nogo-A^−/− mice, indicating a repressor function of the Nogo-A/S1PR2 axis in synaptic plasticity. A similar increase in LTP was also observed in the motor cortex after S1PR2 blockade. We propose a novel signaling model in which a GPCR functions as a receptor for two structurally unrelated ligands, a membrane protein and a sphingolipid. Elucidating Nogo-A/S1PR2 signaling platforms will provide new insights into regulation of synaptic plasticity.

Recent studies have demonstrated an important role of Nogo-A signaling in the repression of structural and synaptic plasticity in mature neuronal networks of the central nervous system. These insights extended our understanding of Nogo-A's inhibitory function far beyond its well-studied role as axonal-growth inhibitor. Repression is mediated via two different Nogo-A extracellular domains: Nogo-66 and Nogo-A-Δ20. Here, we identify the G-protein coupled receptor S1PR2 as a high-affinity receptor for Nogo-A-Δ20 and demonstrate that S1PR2 binds this domain with sites different from the recently proposed S1P binding pocket. Interfering with S1PR2 activity, either pharmacologically or genetically, prevented Nogo-A-Δ20-mediated inhibitory effects. Similar results were obtained when we blocked G₁₃, LARG, and RhoA, components of the downstream signaling pathway. These findings revealed a strong increase in hippocampal and cortical synaptic plasticity when acutely interfering with Nogo-A/S1PR2 signaling, similar to previous results obtained by blocking Nogo-A. We thus provide a novel biological concept of multi-ligand GPCR signaling in which this sphingolipid-activated GPCR is also bound and activated by the high molecular weight membrane protein Nogo-A.

Leucine-rich repeat-containing G-protein coupled receptor 5/GPR49 activates G12/13-Rho GTPase pathway

Leucine-rich repeat-containing G-protein coupled receptor 5 (LGR5/GPR49) is highly expressed in adult stem cells of various tissues, such as intestine, hair follicles, and stomach. LGR5 is also overexpressed in some colon and ovarian tumors. Recent reports show that R-spondin (RSPO) family ligands bind to and activate LGR5, enhancing canonical Wnt signaling via the interaction with LRP5/6 and Frizzled. The identity of heterotrimeric G-proteins coupled to LGR5, however, remains unclear. Here, we show that Rho GTPase is a downstream target of LGR5. Overexpression of LGR5 induced SRF-RE luciferase activity, a reporter of Rho signaling. RSPOs, ligands for LGR4, LGR5, and LGR6, however, did not induce SRF-RE reporter activity in the presence of LGR5. Consistently, LGR5-induced activity of the SRF-RE reporter was inhibited by Rho inhibitor C3 transferase and RhoA N19 mutant, and knockdown of Gα12/13 genes blocked the reporter activity induced by LGR5. In addition, focal adhesion kinase, NF-κB and c-fos, targets of Rho GTPase, were shown to be regulated by LGR5. Here, we have demonstrated, for the first time, that LGR5 is coupled to the Rho pathway through G12/13 signaling.

Gα₁₂ drives invasion of oral squamous cell carcinoma through up-regulation of proinflammatory cytokines

Oral squamous cell carcinoma (OSCC) ranks among the top ten most prevalent cancers worldwide. Like most head and neck squamous cell carcinomas (HNSCCs), OSCC is highly inflammatory and aggressive. However, the signaling pathways triggering the activation of its inflammatory processes remain elusive. G protein-coupled receptor signaling regulates the inflammatory response and invasiveness of cancers, but it remains unclear whether Gα₁₂ is a critical player in the inflammatory cytokine pathway during the tumorigenesis of OSCC. This study was undertaken to determine the role of Gα₁₂ signaling in the regulation of proinflammatory cytokines in their mediation of OSCC invasion. We found that both the transcription and protein levels of Gα₁₂ are up-regulated in OSCC tumors. The elevated Gα₁₂ expressions in OSCC patients also correlated with extra-capsular spread, an indicator of tumor invasiveness in HNSCCs. This clinical finding was supported by the studies of overexpression and RNAi knockdown of Gα₁₂ in OSCC cells, which demonstrated that Gα₁₂ promoted tumor cell migration and invasion. To understand how Gα₁₂ modulates OSCC invasiveness, we analyzed key biological processes in microarray data upon depletion of Gα₁₂ and found that cytokine- and other immune-related pathways were severely impaired. Importantly, the mRNA levels of IL-6 and IL-8 proinflammatory cytokines in clinical samples were found to be significantly correlated with the increased Gα₁₂ levels, suggesting a potential role of Gα₁₂ in modulating the IL-6 and IL-8 expressions. Supporting this hypothesis, overexpression or RNAi knockdown of Gα₁₂ in OSCC cell lines both showed that Gα₁₂ positively regulated the mRNA and protein levels of IL-6 and IL-8. Finally, we demonstrated that the Gα₁₂ promotion of tumor cell invasiveness was suppressed by the neutralization of IL-6 and IL-8 in OSCC cells. Together, these findings suggest that Gα₁₂ drives OSCC invasion through the up-regulation of IL-6 and IL-8 cytokines.

Activation of p115-RhoGEF requires direct association of Gα13 and the Dbl homology domain

RGS-containing RhoGEFs (RGS-RhoGEFs) represent a direct link between the G(12) class of heterotrimeric G proteins and the monomeric GTPases. In addition to the canonical Dbl homology (DH) and pleckstrin homology domains that carry out the guanine nucleotide exchange factor (GEF) activity toward RhoA, these RhoGEFs also possess RGS homology (RH) domains that interact with activated α subunits of G(12) and G(13). Although the GEF activity of p115-RhoGEF (p115), an RGS-RhoGEF, can be stimulated by Gα(13), the exact mechanism of the stimulation has remained unclear. Using combined studies with small angle x-ray scattering, biochemistry, and mutagenesis, we identify an additional binding site for activated Gα(13) in the DH domain of p115. Small angle x-ray scattering reveals that the helical domain of Gα(13) docks onto the DH domain, opposite to the surface of DH that binds RhoA. Mutation of a single tryptophan residue in the α3b helix of DH reduces binding to activated Gα(13) and ablates the stimulation of p115 by Gα(13). Complementary mutations at the predicted DH-binding site in the αB-αC loop of the helical domain of Gα(13) also affect stimulation of p115 by Gα(13). Although the GAP activity of p115 is not required for stimulation by Gα(13), two hydrophobic motifs in RH outside of the consensus RGS box are critical for this process. Therefore, the binding of Gα(13) to the RH domain facilitates direct association of Gα(13) to the DH domain to regulate its exchange activity. This study provides new insight into the mechanism of regulation of the RGS-RhoGEF and broadens our understanding of G protein signaling.

The pseudokinase NIPI-4 is a novel regulator of antimicrobial peptide gene expression

Hosts have developed diverse mechanisms to counter the pathogens they face in their natural environment. Throughout the plant and animal kingdoms, the up-regulation of antimicrobial peptides is a common response to infection. In C. elegans, infection with the natural pathogen Drechmeria coniospora leads to rapid induction of antimicrobial peptide gene expression in the epidermis. Through a large genetic screen we have isolated many new mutants that are incapable of upregulating the antimicrobial peptide nlp-29 in response to infection (i.e. with a Nipi or 'no induction of peptide after infection' phenotype). More than half of the newly isolated Nipi mutants do not correspond to genes previously associated with the regulation of antimicrobial peptides. One of these, nipi-4, encodes a member of a nematode-specific kinase family. NIPI-4 is predicted to be catalytically inactive, thus to be a pseudokinase. It acts in the epidermis downstream of the PKC∂ TPA-1, as a positive regulator of nlp antimicrobial peptide gene expression after infection. It also controls the constitutive expression of antimicrobial peptide genes of the cnc family that are targets of TGFß regulation. Our results open the way for a more detailed understanding of how host defense pathways can be molded by environmental pathogens.

Angiotensin II modulates VEGF-driven angiogenesis by opposing effects of type 1 and type 2 receptor stimulation in the microvascular endothelium

Vascular endothelial growth factor (VEGF) is a main stimulator of pathological vessel formation. Nevertheless, increasing evidence suggests that Angiotensin II (Ang II) can play an augmentory role in this process. We thus analyzed the contribution of the two Ang II receptor types, AT(1)R and AT(2)R, in a mouse model of VEGF-driven angiogenesis, i.e. oxygen-induced proliferative retinopathy. Application of the AT(1)R antagonist telmisartan but not the AT(2)R antagonist PD123,319 largely attenuated the pathological response. A direct effect of Ang II on endothelial cells (EC) was analyzed by assessing angiogenic responses in primary bovine retinal and immortalized rat microvascular EC. Selective stimulation of the AT(1)R by Ang II in the presence of PD123,319 revealed a pro-angiogenic activity which further increased VEGF-driven EC sprouting and migration. In contrast, selective stimulation of the AT(2)R by either CGP42112A or Ang II in the presence of telmisartan inhibited the VEGF-driven angiogenic response. Using specific inhibitors (pertussis toxin, RGS proteins, kinase inhibitors) we identified G(12/13) and G(i) dependent signaling pathways as the mediators of the AT(1)R-induced angiogenesis and the AT(2)R-induced inhibition, respectively. As AT(1)R and AT(2)R stimulation displays opposing effects on the activity of the monomeric GTPase RhoA and pro-angiogenic responses to Ang II and VEGF requires activation of Rho-dependent kinase (ROCK), we conclude that the opposing effects of the Ang II receptors on VEGF-driven angiogenesis converge on the regulation of activity of RhoA-ROCK-dependent EC migration.

G12 signaling through c-Jun NH2-terminal kinase promotes breast cancer cell invasion

Signaling through the heterotrimeric G protein, G12, via Rho induces a striking increase in breast cancer cell invasion. In this study, evidence is provided that the c-Jun NH₂-terminal kinase (JNK) is a key downstream effector of G12 on this pathway. Expression of constitutively-active Gα12 or activation of G12 signaling by thrombin leads to increased JNK and c-Jun phosphorylation. Pharmacologic inhibition of JNK or knockdown of JNK expression by siRNA significantly decreases G12-induced JNK activation as well as the ability of breast cancer cells to invade a reconstituted basement membrane. Furthermore, expression of dominant-negative Rho or treatment of cells with an inhibitor of the Rho kinase, ROCK, reduces G12-induced JNK and c-Jun activation, and ROCK inhibitor treatment also inhibits G12-induced cellular invasion. JNK knockdown or ROCK inhibitor treatment has no effect on activation of Rho by G12. Taken together, our data indicate that JNK activation is required for G12-induced invasion of breast cancer cells and that JNK is downstream of Rho and ROCK on this pathway. This study implicates a G12-stimulated mitogen-activated protein kinase cascade in cancer cell invasion, and supports a role for JNK in cancer progression.

Freeze-thawing single human embryonic stem cells induce e-cadherin and actin filament network disruption via g13 signaling

Poor adhesion of single human embryonic stem (hES) cells after freeze-thawing causes death. To investigate mechanisms responsible for this, Rho-dependent protein kinase (ROCK) inhibitor Y-27632-treated and untreated single hES cells were analyzed for E-cadherin and F-actin distribution by immunostaining and phalloidin staining respectively and for G13 signaling pathway components by DNA microarray and quantitative polymerase chain reaction (PCR). Y-27632-treated cells clustered rapidly and maintained E-cadherin and F-actin distribution without losing Oct-3/4. Immediately after thawing, E-cadherin in untreated hES cells dotted along the membrane and then displayed eccentric cytoplasmic localization. Bleb formation and early Oct-3/4 loss occurred after F-actin network condensation in the cytoplasm. Microarray analyses and quantitative PCR indicated upregulation of two actin reorganization-associated components of the G13 signaling pathway, Arhgdib and Cdc42, in untreated cells. Considering these findings and that cell death was partly interrupted by Y-27632, E-cadherin and actin cytoskeleton network disruption through the G13 signaling pathway may cause hES cell death after freeze-thawing.

G-protein alpha-s and -12 subunits are involved in androgen-stimulated PI3K activation and androgen receptor transactivation in prostate cancer cells

BACKGROUND

The androgen receptor (AR) is a ligand-dependent transcription factor that mediates androgenic hormone action in cells. We recently demonstrated the involvement of phosphoinositide 3-OH kinase (PI3K) p110beta in AR transactivation and gene expression. In this study, we determined the upstream signals that lead to PI3K/p110beta activation and AR transactivation after androgen stimulation.

METHODS

Human prostate cancer LAPC-4 and 22Rv1 cell lines were used for the experiments. AR transactivation was assessed using an androgen responsive element-driven luciferase (ARE-LUC) assay. Cell proliferation was examined using BrdU incorporation and MTT assays. Target genes were silenced using small interfering RNA (siRNA) approach. Gene expression was evaluated at the mRNA level (real-time RT-PCR) and protein level (Western blot). PI3K kinase activities were measured using immunoprecipitation-based in vitro kinase assay. The AR-DNA-binding activity was determined using chromatin-immunoprecipitation (ChIP) assay.

RESULTS

First, at the cellular plasma membrane, disrupting the integrity of caveolae microdomain with methyl-β-cyclodextrin (M-β-CD) abolished androgen-induced AR transactivation and gene expression. Then, knocking down caveolae structural proteins caveolin-1 or -2 with the gene-specific siRNAs significantly reduced androgen-induced AR transactivation. Next, silencing Gα(s) and Gα(12) genes but not other G-proteins blocked androgen-induced AR transactivation and cell proliferation. Consistently, overexpression of Gα(s) or Gα(12) active mutants enhanced androgen-induced AR transactivation, of which Gα(s) active mutant sensitized the AR to castration-level of androgen (R1881). Most interestingly, knocking down Gα(s) but not Gα(12) subunit significantly suppressed androgen-stimulated PI3K p110beta activation. However, ChIP analysis revealed that both Gα(s) or Gα(12) subunits are involved in androgen-induced AR interaction with the AR target gene PSA promoter region.

CONCLUSION

These data suggest that caveolae-associated G-protein alpha subunits are involved in AR transactivation by modulating the activities of different PI3K isoforms.

Lysophosphatidylinositol causes neurite retraction via GPR55, G13 and RhoA in PC12 cells

GPR55 was recently identified as a putative receptor for certain cannabinoids, and lysophosphatidylinositol (LPI). Recently, the role of cannabinoids as GPR55 agonists has been disputed by a number of reports, in part, because studies investigating GPR55 often utilized overexpression systems, such as the GPR55-overexpressing HEK293 cells, which make it difficult to deduce the physiological role of endogenous GPR55. In the present study, we found that PC12 cells, a neural model cell line, express endogenous GPR55, and by using these cells, we were able to examine the role of endogenous GPR55. Although GPR55 mRNA and protein were expressed in PC12 cells, neither CB₁ nor CB₂ mRNA was expressed in these cells. GPR55 was predominantly localized on the plasma membrane in undifferentiated PC12 cells. However, GPR55 was also localized in the growth cones or the ruffled border in differentiated PC12 cells, suggesting a potential role for GPR55 in the regulation of neurite elongation. LPI increased intracellular Ca²⁺ concentration and RhoA activity, and induced ERK1/2 phosphorylation, whereas endogenous and synthetic cannabinoids did not, thereby suggesting that cannabinoids are not GPR55 agonists. LPI also caused neurite retraction in a time-dependent manner accompanied by the loss of neurofilament light chain and redistribution of actin in PC12 cells differentiated by NGF. This LPI-induced neurite retraction was found to be G_q-independent and G₁₃-dependent. Furthermore, inactivation of RhoA function via C3 toxin and GPR55 siRNA knockdown prevented LPI-induced neurite retraction. These results suggest that LPI, and not cannabinoids, causes neurite retraction in differentiated PC12 cells via a GPR55, G₁₃ and RhoA signaling pathway.

Identification of critical residues in G(alpha)13 for stimulation of p115RhoGEF activity and the structure of the G(alpha)13-p115RhoGEF regulator of G protein signaling homology (RH) domain complex

RH-RhoGEFs are a family of guanine nucleotide exchange factors that contain a regulator of G protein signaling homology (RH) domain. The heterotrimeric G protein Gα(13) stimulates the guanine nucleotide exchange factor (GEF) activity of RH-RhoGEFs, leading to activation of RhoA. The mechanism by which Gα(13) stimulates the GEF activity of RH-RhoGEFs, such as p115RhoGEF, has not yet been fully elucidated. Here, specific residues in Gα(13) that mediate activation of p115RhoGEF are identified. Mutation of these residues significantly impairs binding of Gα(13) to p115RhoGEF as well as stimulation of GEF activity. These data suggest that the exchange activity of p115RhoGEF is stimulated allosterically by Gα(13) and not through its interaction with a secondary binding site. A crystal structure of Gα(13) bound to the RH domain of p115RhoGEF is also presented, which differs from a previously crystallized complex with a Gα(13)-Gα(i1) chimera. Taken together, these data provide new insight into the mechanism by which p115RhoGEF is activated by Gα(13).

New insights into the mechanism of light modulated signaling by heterotrimeric G-proteins: ENVOY acts on gna1 and gna3 and adjusts cAMP levels in Trichoderma reesei (Hypocrea jecorina)

Sensing of environmental signals is often mediated by G-protein coupled receptors and their cognate heterotrimeric G-proteins. In Trichoderma reesei (Hypocrea jecorina) the signals transmitted via the G-protein alpha subunits GNA1 and GNA3 cause considerable modulation of cellulase transcript levels and the extent of this adjustment is dependent on the light status. We therefore intended to elucidate the underlying mechanism connecting light response and heterotrimeric G-protein signaling. Analysis of double mutant strains showed that constitutive activation of GNA1 or GNA3 in the absence of the PAS/LOV domain protein ENVOY (ENV1) leads to the phenotype of constitutive G-alpha activation in darkness. In light, however the deletion-phenotype of Δenv1 was observed with respect to growth, conidiation and cellulase gene transcription. Additionally deletion of env1 causes decreased intracellular cAMP accumulation, even upon constitutive activation of GNA1 or GNA3. While supplementation of cAMP caused an even more severe growth phenotype of all strains lacking env1 in light, addition of the phosphodiesterase inhibitor caffeine rescued the growth phenotype of these strains. ENV1 is consequently suggested to connect the light response pathway with nutrient signaling by the heterotrimeric G-protein cascade by adjusting transcript levels of gna1 and gna3 and action on cAMP levels - presumably through inhibition of a phosphodiesterase.

G(alpha)12/13 induction of CYR61 in association with arteriosclerotic intimal hyperplasia: effect of sphingosine-1-phosphate

OBJECTIVE

Gα(12/13) play a role in oncogenic transformation and tumor growth. Cysteine-rich protein 61 (CYR61) is a growth-factor-inducible angiogenic factor. In view of potential overlapping functions between Gα(12/13) and CYR61, this study investigated the role of these G proteins in CYR61 induction in association with hyperplastic vascular abnormality.

METHODS AND RESULTS

Overexpression of activated Gα(12) or Gα(13) induced CYR61 expression in vascular smooth muscle cells (VSMCs). Gene knockdown and knockout experiments revealed that sphingosine-1-phosphate (S1P) treatment induced CYR61 via Gα(12/13). JunD/activator protein-1 (AP-1) was identified as a transcription factor required for CYR61 transactivation by S1P. Deficiencies in Gα(12/13) abrogated AP-1 activation and AP-1-mediated CYR61 induction. c-Jun N-terminal kinase was responsible for CYR61 induction. Moreover, deficiencies of Gα(12/13) abolished c-Jun N-terminal kinase-dependent CYR61 induction by S1P. N-acetyl-l-cysteine or NADPH oxidase inhibitor treatment reversed CYR61 induction by S1P, indicating that reactive oxygen species are responsible for this process. The levels of Gα(12/13) were increased within thickened intimas and medias in wire-injured mouse femoral arteries, which was accompanied by simultaneous CYR61 induction. Moreover, Gα(12/13) and CYR61 were costained in the arteriosclerotic lesions immediately adjacent to human tumor tissues.

CONCLUSIONS

Gα(12/13) regulate AP-1-dependent CYR61 induction in VSMCs and promote VSMC migration, and they are upregulated with CYR61 in arteriosclerotic lesions.

Analysis of two novel midgut-specific promoters driving transgene expression in Anopheles stephensi mosquitoes

Background

Tissue-specific promoters controlling the expression of transgenes in Anopheles mosquitoes represent a valuable tool both for studying the interaction between these malaria vectors and the Plasmodium parasites they transmit and for novel malaria control strategies based on developing Plasmodium-refractory mosquitoes by expressing anti-parasitic genes. With this aim we have studied the promoter regions of two genes from the most important malaria vector, Anopheles gambiae, whose expression is strongly induced upon blood feeding.

Results

We analysed the A. gambiae Antryp1 and G12 genes, which we have shown to be midgut-specific and maximally expressed at 24 hours post-bloodmeal (PBM). Antryp1, required for bloodmeal digestion, encodes one member of a family of 7 trypsin genes. The G12 gene, of unknown function, was previously identified in our laboratory in a screen for genes induced in response to a bloodmeal. We fused 1.1 kb of the upstream regions containing the putative promoter of these genes to reporter genes and transformed these into the Indian malaria vector A. stephensi to see if we could recapitulate the expression pattern of the endogenous genes. Both the Antryp1 and G12 upstream regions were able to drive female-predominant, midgut-specific expression in transgenic mosquitoes. Expression of the Antryp1-driven reporter in transgenic A. stephensi lines was low, undetectable by northern blot analysis, and failed to fully match the induction kinetics of the endogenous Antryp1 gene in A. gambiae. This incomplete conservation of expression suggests either subtle differences in the transcriptional machinery between A. stephensi and A. gambiae or that the upstream region chosen lacked all the control elements. In contrast, the G12 upstream region was able to faithfully reproduce the expression profile of the endogenous A. gambiae gene, showing female midgut specificity in the adult mosquito and massive induction PBM, peaking at 24 hours.

Conclusions

Our studies on two putative blood-meal induced, midgut-specific promoters validate the use of G12 upstream regulatory regions to drive targeted transgene expression coinciding spatially and temporally with pre-sporogonic stages of Plasmodium parasites in the mosquito, offering the possibility of manipulating vector competence or performing functional studies on vector-parasite interactions.

Aldosterone stimulates elastogenesis in cardiac fibroblasts via mineralocorticoid receptor-independent action involving the consecutive activation of Galpha13, c-Src, the insulin-like growth factor-I receptor, and phosphatidylinositol 3-kinase/Akt

We previously demonstrated that aldosterone, which stimulates collagen production through the mineralocorticoid receptor (MR)-dependent pathway, also induces elastogenesis via a parallel MR-independent mechanism involving insulin-like growth factor-I receptor (IGF-IR) signaling. The present study provides a more detailed explanation of this signaling pathway. Our data demonstrate that small interfering RNA-driven elimination of MR in cardiac fibroblasts does not inhibit aldosterone-induced IGF-IR phosphorylation and subsequent increase in elastin production. These results exclude the involvement of the MR in aldosterone-induced increases in elastin production. Results of further experiments aimed at identifying the upstream signaling component(s) that might be activated by aldosterone also eliminate the putative involvement of pertussis toxin-sensitive Galphai proteins, which have previously been shown to be responsible for some MR-independent effects of aldosterone. Instead, we found that small interfering RNA-dependent elimination of another heterotrimeric G protein, Galpha13, eliminates aldosterone-induced elastogenesis. We further demonstrate that aldosterone first engages Galpha13 and then promotes its transient interaction with c-Src, which constitutes a prerequisite step for aldosterone-dependent activation of the IGF-IR and propagation of consecutive downstream elastogenic signaling involving phosphatidylinositol 3-kinase/Akt. In summary, the data we present reveal new details of an MR-independent cellular signaling pathway through which aldosterone stimulates elastogenesis in human cardiac fibroblasts.

Molecular mechanisms for activation of voltage-independent Ca2+ channels by endothelin-1/endothelin-A receptors

Endothelin-1 (ET-1) activates two types of Ca2+- permeable non-selective cation channels (designated NSCC-1 and NSCC-2) and a store-operated Ca2+ channel (SOCC) in Chinese hamster ovary cells expressing endothelin-A receptors (CHOETAR), which couple with Gq, Gs and G12. The purpose of this study was to identify the G proteins involved in the activation of these Ca channels, using mutated ETARs with coupling to either Gq or Gs/G12 (designated ETAR(Delta)385 and SerETAR, respectively) and a dominant negative mutant of G12 (G12G228A). ETAR(Delta)385 is truncated downstream of Cys385 in the C-terminal as palmitoylation sites, whereas SerET(A)R is unpalmitoylated because of substitution of all the cysteine residues to serine (CysCys --> SerSer). ET-1 activated SOCC in CHO-ET(A)R(Delta)385. In CHO-SerET(A)R or CHO-ET(A)R pretreated with U73122, an inhibitor of phospholipase C, ET-1 activated NSCC-1. ET-1 activated SOCC in CHO-ETAR microinjected with G12G228A. Moreover, ET-1 activated NSCC-1 in CHO-ETAR treated with LY 294002, the phosphoinositide 3-kinase inhibitor. These results indicate that NSCC-1 is activated via a G12-dependent pathway, NSCC-2 via Gq/phospholipase C-dependent and G12-dependent pathways, and SOCC via a Gq-phospholipase C-dependent pathway. In addition, NSCC-2 and SOCC are stimulated by ET-1 via a phosphoinositide 3-kinase-dependent cascade, whereas NSCC-1 is stimulated via a phosphoinositide 3-kinase-independent cascade.

Selective activation of human atrial Galpha12 and Galpha13 by Galphaq-coupled angiotensin and endothelin receptors

Galphaq-coupled receptors such as alpha1-adrenergic, angiotensin, and endothelin receptors, play key roles in cardiac physiology. These receptors have also been shown to couple to G proteins of the G12 family, including Galpha12 and Galpha13. In this report, we determined whether these G proteins interact with endothelin, angiotensin, and alpha1-adrenergic receptors in the human heart. We find that these receptors activate cardiac Galpha12 and Galpha13 differentially; endothelin receptors activate only Galpha12 (to 218 +/- 22% of unstimulated levels), angiotensin receptors activate only Galpha13 (to 236 +/- 49% of unstimulated levels), and alpha1-adrenergic receptors activate neither Galpha12 (123 +/- 18% of unstimulated levels) nor Galpha13 (113 +/- 12% of unstimulated levels). Consistent with these data, translocation of guanine nucleotide exchange factor p115RhoGEF, which responds to Galpha13, occurs only after stimulation of angiotensin receptors (shifting from 73 +/- 12% to 41 +/- 10% cytosolic). These differences in the activation of Galpha12 and Galpha13 by Galphaq-coupled receptors may underlie reported differences in the functions of these receptors.

The P2Y2 nucleotide receptor requires interaction with alpha v integrins to access and activate G12

The P2Y2 nucleotide receptor (P2Y2R) interacts with alpha v integrins to activate G(o) and induce chemotaxis in human 1321N1 astrocytoma cells. In this study, it was determined that the P2Y2R also requires interaction with alpha v integrins to activate G12 and associated signaling pathways that control chemotaxis in 1321N1 cells. Mutation of the Arg-Gly-Asp (RGD) integrin-binding sequence in the first extracellular loop of the human P2Y2R to Arg-Gly-Glu (RGE), which prevents integrin interaction, did not inhibit G(q) or ERK1/2 signaling by the P2Y2R agonist UTP but completely inhibited activation of G12 and G12-mediated events, including Rho activation, cofilin and myosin light chain-2 phosphorylation, stress fiber formation and chemotaxis towards UTP. The involvement of G12 in all these events was verified by using a dominant negative G alpha12 construct. G12 activation by the P2Y2R also was inhibited by anti-alpha v beta5 integrin antibodies and alpha v integrin antisense oligonucleotides, suggesting that alpha v integrin activity and expression are required for the P2Y2R to activate G12. Co-immunoprecipitation experiments confirmed that G alpha12 protein associates with the wild-type P2Y2R and with alpha v integrins but not with the RGE mutant P2Y2R or with alpha3 integrins. Collectively, these results suggest that alpha v integrin complexes provide the P2Y2R with access to G12, thereby allowing activation of this heterotrimeric G protein that controls actin cytoskeletal rearrangements required for chemotaxis.

The heterotrimeric G protein G(i3) regulates hepatic autophagy downstream of the insulin receptor

Compelling evidence suggests that the heterotrimeric G protein G(i3) specifically transmits the antiautophagic effects of insulin and amino acids in the liver. This points to a previously unrecognized cross talk between the insulin receptor tyrosine kinase and G(i3). Interestingly, G(i3) is localized not only to plasma membranes but also to membranes of the autophagosomal compartment. Furthermore, as part of insulin's or phenylalanine's actions to inhibit autophagy, G(i3) is redistributed away from autophagosomes. Therefore, endomembrane-associated rather than plasma membrane-localized G(i3) may serve as the target of insulin's endocrine and metabolic actions. We therefore propose that the function and regulation of organelle-associated heterotrimeric G proteins may be different from their roles at the plasma membrane where they act as signal transducers of seven-transmembrane receptors. Here, we discuss recent findings and propose a function for G(i3) in mTOR-dependent signaling pathways. We hypothesize that G(i) family members may have tissue-specific roles in the regulation of autophagy under different physiological and pathological conditions.

G alpha 12/13 basally regulates p53 through Mdm4 expression

G alpha(12/13), which belongs to the G alpha(12) family, participates in the regulation of diverse physiologic processes. In view of the control of G alpha(12/13) in cell proliferation, this study investigated the role of G alpha(12/13) in the regulation of p53 and mdm4. Immunoblotting and immunocytochemistry revealed that p53 was expressed in control embryonic fibroblasts and was largely localized in the nuclei. G alpha(12) deficiency decreased p53 levels and its DNA binding activity, accompanying p21 repression with Bcl(2) induction, whereas G alpha(13) deficiency exerted weak effects. G alpha(12) or G alpha(13) deficiency did not change p53 mRNA expression. ERK1/2 or Akt was not responsible for p53 repression due to G alpha(12) deficiency. Mdm4, a p53-stabilizing protein, was repressed by G alpha(12) deficiency and to a lesser extent by G alpha(13) deficiency, whereas mdm2, PTEN, beta-catenin, ATM, and Chk2 were unaffected. p53 accumulation by proteasomal inhibition during G alpha(12) deficiency suggested the role of G alpha(12) in p53 stabilization. Constitutively active G alpha(12) (G alpha(12)QL) or G alpha(13) (G alpha(13)QL) promoted p53 accumulation with mdm4 induction in MCF10A cells. p53 accumulation by mdm4 overexpression, but no mdm4 induction by p53 overexpression, and small interfering RNA knockdown verified the regulatory role of mdm4 for p53 downstream of G alpha(12/13). In control or G alpha(12)/G alpha(13)-deficient cells, genotoxic stress led to p53 accumulation. At concentrations increasing the flow cytometric pre-G(1) phase, doxorubicin or etoposide treatment caused serine phosphorylations in G alpha(12)-/- or G alpha(12/13)-/- cells, but did not induce mdm4. G alpha(12/13)QL transfection failed to phosphorylate p53 at serines. Our results indicate that G alpha(12/13) regulate basal p53 levels via mdm4, which constitutes a cell signaling pathway distinct from p53 phosphorylations elicited by genotoxic stress.