Src tyrosine kinase is a novel direct effector of G proteins

A transforming Src mutant increases the bioavailability of EGFR ligands via stimulation of the cell-surface metalloproteinase ADAM17

ADAM17 (a disintegrin and metalloproteinase 17, also referred to as TNFα converting enzyme or TACE) is a cell-surface metalloproteinase that regulates signaling via the epidermal growth factor receptor (EGFR) and has important roles in diseases such as cancer and rheumatoid arthritis. ADAM17 can be activated by stimulation of several tyrosine kinase receptors, raising questions about whether oncogenic tyrosine kinases could also enhance EGFR signaling and activation of extracellular signal-regulated kinase (ERK) via stimulation of ADAM17. The main goal of this study was to evaluate the role of Src in activating ADAM17. We provide evidence that a constitutively active transforming form of Src, the E378G mutant, as well as v-Src enhance ADAM17-mediated shedding of the EGFR ligand TGFα. Moreover, we demonstrate that constitutive shedding of TGFα can be reduced by inhibition of Src in several cell lines, including COS7, MCF7 (the human breast cancer cell line), PAE (a pig aortic endothelial cell line) and HaCaT (the human keratinocyte cell line) cells. Src(E378G)-stimulated shedding of TGFα is abolished in Adam17(-/-) cells, but can be rescued by wild-type (wt) ADAM17 and a mutant ADAM17 lacking its cytoplasmic domain. These findings demonstrate that ADAM17 is the principal TGFα sheddase that is activated by Src in a manner that does not require the cytoplasmic domain of ADAM17. Finally, we show that stimulation of ADAM17 by Src(E378G) leads to enhanced paracrine signaling via release of EGFR ligands into the culture supernatant. These results raise the possibility that activation of ADAM17 by oncogenic forms of Src can aid in promoting tumorigenesis by enhancing signaling via the EGFR and ERK in an autocrine and paracrine manner. Enhanced autocrine signaling could further activate tumor cells expressing oncogenic mutants of Src, whereas paracrine signaling could stimulate EGFR and ERK signaling in surrounding non-transformed cells such as stromal cells, thereby contributing to crosstalk between tumor cells and stromal cells.

CXCR4-mediated bone marrow progenitor cell maintenance and mobilization are modulated by c-kit activity

RATIONALE

The mobilization of bone marrow (BM) progenitor cells (PCs) is largely governed by interactions between stromal cell-derived factor (SDF)-1 and CXC chemokine receptor (CXCR)4. Ischemic injury disrupts the SDF-1-CXCR4 interaction and releases BM PCs into the peripheral circulation, where the mobilized cells are recruited to the injured tissue and contribute to vessel growth. BM PCs can also be mobilized by the pharmacological CXCR4 antagonist AMD3100, but the other components of the SDF-1-CXCR4 signaling pathway are largely unknown. c-kit, a membrane-bound tyrosine kinase and the receptor for stem cell factor, has also been shown to play a critical role in BM PC mobilization and ischemic tissue repair.

OBJECTIVE

To investigate the functional interaction between SDF-1-CXCR4 signaling and c-kit activity in BM PC mobilization.

METHODS AND RESULTS

AMD3100 administration failed to mobilize BM PCs in mice defective in c-kit kinase activity or in mice transplanted with BM cells that expressed a constitutively active c-kit mutant. Furthermore, BM levels of phosphorylated (phospho)-c-kit declined after AMD3100 administration and after CXCR4 deletion. In cells adhering to culture plates coated with vascular cell adhesion molecule 1, SDF-1 and stem cell factor increased phospho-c-kit levels, and AMD3100 treatment suppressed SDF-1-induced, but not SCF-induced, c-kit phosphorylation. SDF-1-induced c-kit phosphorylation also required the activation of Src nonreceptor tyrosine kinase: pretreatment of cells with a selective Src inhibitor blocked both c-kit phosphorylation and the interaction between c-kit and phospho-Src.

CONCLUSIONS

These findings indicate that the regulation of BM PC trafficking by SDF-1 and CXCR4 is dependent on Src-mediated c-kit phosphorylation.

Src family kinases participate in the regulation of encephalomyocarditis virus-induced cyclooxygenase-2 expression by macrophages

Src family kinases (SFKs) are non-receptor tyrosine kinases that have been implicated as regulators of the inflammatory response. In this study, the role of SFK activation in the inflammatory response of macrophages to encephalomyocarditis virus (EMCV) infection was examined. Virus infection of macrophages stimulates the expression of cyclooxygenase-2 (COX-2), interleukin (IL)-1β and inducible nitric oxide synthase (iNOS). Inhibition of SFK attenuates EMCV-induced COX-2 expression and prostaglandin E₂ production, iNOS expression and subsequent nitric oxide production, and IL-1β expression. EMCV-induced COX-2 expression requires the activation of nuclear factor-κB and the mitogen-activated protein kinase p38. Consistent with these previous findings, inhibition of SFKs attenuated the phosphorylation of p38 in response to EMCV infection, suggesting that SFKs may act upstream of p38. These findings provide evidence that SFK activation plays an active role in the regulation of inflammatory gene expression by virus-infected macrophages.

Src-dependent TrkA transactivation is required for pituitary adenylate cyclase-activating polypeptide 38-mediated Rit activation and neuronal differentiation

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a potent neuropeptide that possesses both neurotrophic and neurodevelopmental effects. Recently, the Rit GTPase was found to be activated by a novel Galpha/cAMP/exchange protein activated by cyclic AMP (Epac)-dependent signaling pathway and required for PACAP-dependent cAMP response element-binding protein activation and neuronal differentiation. However, Epac did not function as a Rit guanine nucleotide exchange factor (GEF), and the nature of the PACAP regulatory cascade remained unclear. Here, we show that PACAP-mediated Rit activation involves Src family kinase-dependent TrkA receptor transactivation. PACAP receptor (PACR1) stimulation triggered both G(i)alpha and G(s)alpha/cAMP/Epac regulatory cascades resulting in Src kinase activity, which in turn induced TrkA kinase tyrosine phosphorylation. Importantly, Src inhibition, or the lack of functional Trk receptors, was found to inhibit PACAP-mediated Rit activation, whereas constitutively active Src alone was sufficient to stimulate Rit-guanosine triphosphate levels. A single tyrosine (Y(499)) phosphorylation event was identified as critical to both PACAP-mediated transactivation and TrkA-dependent Rit activation. Accordingly, PACAP stimulation resulted in TrkA-dependent phosphorylation of both the Shc adaptor and son of sevenless (SOS)1/2 GEFs, and Rit activation was inhibited by RNA interference silencing of SOS1/2, implicating a TrkA/Shc/SOS signaling complex in Rit regulation. Together, these observations expand upon the nature of PACR1-mediated transactivation and identify TrkA-Rit signaling as a key contributor to PACAP-dependent neuronal differentiation.

Multi-component signaling complexes of the delta-opioid receptor with STAT5B and G proteins

Besides mediating opioid responses in the nervous system and the peripheral tissues, opioid receptors are implicated in signaling mechanisms shared by cytokine receptors. Recent observations have shown that the Signal Transducer and Activator of Transcription 5A (STAT5A) interacts with the mu-opioid receptor (mu-OR) and is phosphorylated upon mu-OR stimulation (Mazarakou and Georgoussi, 2005). In the present study we demonstrate that another member of the STAT family, STAT5B, associates constitutively with the C-terminal tail of the delta-opioid receptor (delta-CT). [D-Ser(2), Leu(5), Thr(6)]-enkephalin-exposure of HEK293 cells, expressing stably the delta-opioid receptor (delta-OR), leads to receptor-dependent STAT5B tyrosine phosphorylation and transcriptional activation. This phosphorylation occurs in a G protein-dependent manner and is carried out by a c-Src kinase. Co-immunoprecipitation studies indicate that STAT5B forms pairs with selective Galpha and Gbetagamma subunits of G proteins and activated c-Src kinase in HEK293 cells. These interactions are formed either constitutively, or upon receptor stimulation. We also demonstrate that the delta-CT serves as a platform for the formation of a multi-component signaling complex (signalosome), consisting of STAT5B, c-Src and selective G protein members. We can thus conclude that STAT5B signaling can be modulated by its coupling with a specific subset of G protein subunits, revealing a novel signaling mechanism for the transcriptional regulation of STAT5B-dependent genes.

Beta-2 adrenergic receptor mediated ERK activation is regulated by interaction with MAGI-3

The beta-2 adrenergic receptor (beta2AR) has a carboxyl terminus motif that can interact with PSD-95/discs-large/ZO1 homology (PDZ) domain-containing proteins. In this paper, we identified membrane-associated guanylate kinase inverted-3 (MAGI-3) as a novel binding partner of beta2AR. The carboxyl terminus of beta2AR binds with high affinity to the fifth PDZ domain of MAGI-3, with the last four amino acids (D-S-L-L) of the receptor being the key determinants of the interaction. In cells, the association of full-length beta2AR with MAGI-3 occurs constitutively and is enhanced by agonist stimulation of the receptor. Our data also demonstrated that beta2AR-stimulated extracellular signal-regulated kinase-1/2 (ERK1/2) activation was substantially retarded by MAGI-3 expression. These data suggest that MAGI-3 regulates beta2AR-mediated ERK activation through the physical interaction between beta2AR and MAGI-3.

Ligand-directed signalling at beta-adrenoceptors

beta-Adrenoceptors (ARs) classically mediate responses to the endogenous ligands adrenaline and noradrenaline by coupling to Gsalpha and stimulating cAMP production; however, drugs designed as beta-AR agonists or antagonists can activate alternative cell signalling pathways, with the potential to influence clinical efficacy. Furthermore, drugs acting at beta-ARs have differential capacity for pathway activation, described as stimulus trafficking, biased agonism, functional selectivity or ligand-directed signalling. These terms refer to responses where drug A has higher efficacy than drug B for one signalling pathway, but a lower efficacy than drug B for a second pathway. The accepted explanation for such responses is that drugs A and B have the capacity to induce or stabilize distinct active conformations of the receptor that in turn display altered coupling efficiency to different effectors. This is consistent with biophysical studies showing that drugs can indeed promote distinct conformational states. Agonists acting at beta-ARs display ligand-directed signalling, but many drugs acting as cAMP antagonists are also able to activate signalling pathways central to cell survival and proliferation or cell death. The observed complexity of drug activity at beta-ARs, prototypical G protein-coupled receptors, necessitates rethinking of the approaches used for screening and characterization of novel therapeutic agents. Most studies of ligand-directed signalling employ recombinant cell systems with high receptor abundance. While such systems are valid for examining upstream signalling events, such as receptor conformational changes and G protein activation, they are less robust when comparing downstream signalling outputs as these are likely to be affected by complex pathway interactions.

Immunologic messenger molecules: cytokines, interferons, and chemokines

Cytokines and chemokines are secreted proteins involved in numerous aspects of cell growth, differentiation, and activation. A prominent feature of these molecules is their effect on the immune system with regard to cell trafficking and development of immune tissue and organs. The nature of an immune response determines which cytokines are produced and ultimately whether the response is cytotoxic, humoral, cell mediated, or allergic. For this chapter, cytokines are grouped according to those that are predominantly antigen-presenting cell or T lymphocyte derived; that mediate cytotoxic, humoral, cell mediated, and allergic immunity; or that are immunosuppressive. A discussion of chemokine function and their role in cell trafficking and disease follows.

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

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

Proteinase-activated receptor 2 expression in breast cancer and its role in breast cancer cell migration

Proteinase-activated receptor 2 (PAR2) is a G protein-coupled receptor that is activated by trypsin-like proteinases. PAR2 is detected in breast tumor specimens; however, it is not clear how PAR2 level in breast cancer cell/tissues compares with normal cell/tissues. Here, we show the elevation of PAR2 protein level in 76 of 105 breast tumor specimens but only 5 of 24 normal breast tissues. PAR2 level is also higher in breast cancer cell lines than that in normal breast cells and non-cancerous breast cell lines. To determine the role of PAR2 in breast carcinogenesis, we examined the effect of PAR2 agonists on cell proliferation and migration. Our studies show that PAR2 agonists (PAR2-activating peptide and trypsin) are neither potent growth enhancers nor chemoattractants to breast cancer cells. Instead, PAR2 agonists induce significant chemokinesis. PAR2-mediated chemokinesis is G(alphai)-dependent, and inhibiting Src kinase activity or silencing c-Src expression blocks PAR2-mediated chemokinesis. These results suggest that c-Src works downstream of G(alphai) to mediate this PAR2 agonist-induced event. To characterize c-Src effector, we reveal that PAR2 agonists activate JNKs in a Src-dependent manner and that JNK activity is essential for PAR2-mediated chemokinesis. Moreover, PAR2 agonist stimulation leads to paxillin Ser(178) phosphorylation and paxillin(S178A) mutant inhibits PAR2-mediated chemokinesis. In conclusion, our studies show that PAR2 agonists facilitate breast cancer cell chemokinesis through the G(alphai)-c-Src-JNK-paxillin signaling pathway.

NaKtide, a Na/K-ATPase-derived peptide Src inhibitor, antagonizes ouabain-activated signal transduction in cultured cells

We have previously shown that the Na/K-ATPase binds and inhibits Src. Here, we report the molecular mechanism of Na/K-ATPase-mediated Src regulation and the generation of a novel peptide Src inhibitor that targets the Na/K-ATPase/Src receptor complex and antagonizes ouabain-induced protein kinase cascades. First, the Na/K-ATPase inhibits Src kinase through the N terminus of the nucleotide-binding domain of the alpha1 subunit. Second, detailed mapping leads to the identification of a 20-amino acid peptide (NaKtide) that inhibits Src (IC50 = 70 nm) in an ATP concentration-independent manner. Moreover, NaKtide does not directly affect the ERK and protein kinase C family of kinases. It inhibits Lyn with a much lower potency (IC50 = 2.5 microm). Third, highly positively charged leader peptide conjugates including HIV-Tat-NaKtide (pNaKtide) readily enter cultured cells. Finally, the following functional studies of pNaKtide demonstrate that this conjugate can specifically target the Na/K-ATPase-interacting pool of Src and act as a potent ouabain antagonist in cultured cells: 1) pNaKtide, unlike PP2, resides in the membranes. Consistently, it affects the basal Src activity much less than that of PP2. 2) pNaKtide is effective in disrupting the formation of the Na/K-ATPase/Src receptor complex in a dose-dependent manner. Consequently, it blocks ouabain-induced activation of Src, ERK, and hypertrophic growth in cardiac myocytes. 3) Unlike PP2, pNaKtide does not affect IGF-induced ERK activation in cardiac myocytes. Taken together, we suggest that pNaKtide may be used as a novel antagonist of ouabain for probing the physiological and pathological significance of the newly appreciated signaling function of Na/K-ATPase and cardiotonic steroids.

Interplay between the heterotrimeric G-protein subunits Galphaq and Galphai2 sets the threshold for chemotaxis and TCR activation

Background

TCR and CXCR4-mediated signaling appears to be reciprocally regulated pathways. TCR activation dampens the chemotactic response towards the CXCR4 ligand CXCL12, while T cells exposed to CXCL12 are less prone to subsequent TCR-activation. The heterotrimeric G proteins G_αq and G_αi2 have been implicated in CXCR4-signaling and we have recently also reported the possible involvement of G_αq in TCR-dependent activation of Lck (Ngai et al., Eur. J. Immunol., 2008, 38: 32083218). Here we examined the role of G_αq in migration and TCR activation.

Results

Pre-treatment of T cells with CXCL12 led to significantly reduced Lck Y394 phosphorylation upon TCR triggering indicating heterologous desensitization. We show that knockdown of G_αq significantly enhanced basal migration in T cells and reduced CXCL12-induced SHP-1 phosphorylation whereas G_αi2 knockdown inhibited CXCL12-induced migration.

Conclusion

Our data suggest that G_αi2 confers migration signals in the presence of CXCL12 whereas G_αq exerts a tonic inhibition on both basal and stimulated migrational responses. This is compatible with the notion that the level of G_αq activation contributes to determining the commitment of the T cell either to migration or activation through the TCR.

Galpha(i1) and Galpha(i3) are required for epidermal growth factor-mediated activation of the Akt-mTORC1 pathway

The precise mechanism whereby epidermal growth factor (EGF) activates the serine-threonine kinase Akt and the mammalian target of rapamycin (mTOR) complex 1 (mTORC1) remains elusive. Here, we report that the alpha subunits of the heterotrimeric guanine nucleotide-binding proteins (G proteins) Galpha(i1) and Galpha(i3) are critical for this activation process. Both Galpha(i1) and Galpha(i3) formed complexes with growth factor receptor binding 2 (Grb2)-associated binding protein 1 (Gab1) and the EGF receptor (EGFR) and were required for the phosphorylation of Gab1 and its subsequent interaction with the p85 subunit of phosphatidylinositol 3-kinase in response to EGF. Loss of Galpha(i1) and Galpha(i3) severely impaired the activation of Akt and of p70 S6 kinase and 4E-BP1, downstream targets of mTORC1, in response to EGF, heparin-binding EGF-like growth factor, and transforming growth factor alpha, but not insulin, insulin-like growth factor, or platelet-derived growth factor. In addition, ablation of Galpha(i1) and Galpha(i3) largely inhibited EGF-induced cell growth, migration, and survival and the accumulation of cyclin D1. Overall, this study suggests that Galpha(i1) and Galpha(i3) lie downstream of EGFR, but upstream of Gab1-mediated activation of Akt and mTORC1, thus revealing a role for Galpha(i) proteins in mediating EGFR signaling.

Signaling pathways involved in the effect of follicle-stimulating hormone on chick embryo testis cell proliferation

The aim of the present study was to evaluate the signaling pathways involved in the follicle-stimulating hormone (FSH) regulated mitogenic activity. For this purpose, 18-d-old chick embryo testis cells were dissociated and cultured for 60 h on polycarbonate membranes. The culture medium was Dulbecco modified Eagle's medium with or without high pure human FSH (hFSH), human recombinant FSH, or different regulators of tyrosine kinase activity as herbimycin A and genistein, or serine/threonine kinases [cyclic adenosine monophosphate (cAMP)-dependent protein serine kinase and protein kinase C] as cAMP, phorbol myristate, and forskolin. In some experiments the regulators were added simultaneously with hFSH. The [3H]-thymidine incorporation was used as an indicator of DNA synthesis. In addition, fragments of chick embryo testis were cultured in the presence or absence of FSH or herbimycin A, and 5-bromo-2'-deoxy-uridine was added to identify the proliferating cell subpopulations. The effect of hFSH on [3H]-thymidine incorporation began at 24 h, and the increment was significant at 36 and 60 h of culture. The hFSH as well as human recombinant FSH significantly stimulated [3H]-thymidine incorporation to testicular cells. The 5-bromo-2'-deoxy-uridine technique showed a high signal in pericordonal and interstitial cells of the hFSH-treated groups, confirming the results obtained using [3H]-thymidine uptake. The treatment with the tyrosine kinase inhibitor herbimycin A increased the [3H]-thymidine uptake, but genistein did not. Regulators of PKA such as cAMP and forskolin, as well as PKC regulators and the phorbol ester phorbol myristate, did not influence cell proliferation. In summary, an inhibitor of tyrosine kinase, herbimycin A, induced per se an increment in chick embryo testis cell proliferation, a fact that strongly suggests that tyrosine kinase signaling pathway functions by inhibiting the proliferation of these cells. On the other hand, the cAMP-PKA pathway had no significant role during the embryonic stage of chick embryo testis. Our results also showed that the effect of FSH on chick embryo cell proliferation occurs mainly in pericordonal and interstitial testis cells.

Direct and specific inactivation of protein tyrosine kinases in the Src and FGFR families by reversible cysteine oxidation

Accumulating evidence suggests that protein tyrosine phosphorylation-based signaling pathways are under the regulation of reactive oxygen species. Although protein tyrosine phosphatases are directly regulated by reversible oxidation, it is not clear whether protein tyrosine kinases (PTKs) are also directly regulated by reduction/oxidation (redox). In this study we report a mechanism of direct oxidative inactivation specific for the PTKs in the Src and fibroblast growth factor receptor (FGFR) families, key enzymes in mammalian signal transduction. Src is fully active when reduced and retains 8-25% of the full activity toward various substrates when oxidized. This inactivation is caused by oxidation of a specific cysteine residue (Cys-277), which results in homodimerization of Src linked by a disulfide bridge. Cys-277 is located in the Gly loop in the catalytic domain. This cysteine residue is conserved only in 8 of the >90 PTKs in the human kinome, including 3 of the 10 Src family kinases and all 4 kinases of the FGFR family. FGFR1 is also reversibly regulated by redox because of this cysteine residue, whereas Csk, a PTK that lacks a cysteine residue at the corresponding position, is not similarly regulated. These results demonstrate a mechanism of direct redox regulation conserved in certain specific PTKs.

Identification of N-terminal lobe motifs that determine the kinase activity of the catalytic domains and regulatory strategies of Src and Csk protein tyrosine kinases

Csk and Src protein tyrosine kinases are structurally homologous but use opposite regulatory strategies. The isolated catalytic domain of Csk is intrinsically inactive and is activated by interactions with the regulatory Src homology 3 (SH3) and SH2 domains, while the isolated catalytic domain of Src is intrinsically active and is suppressed by interactions with the regulatory SH3 and SH2 domains. The structural basis for why one isolated catalytic domain is intrinsically active while the other is inactive is not clear. In this study, we identified structural elements in the N-terminal lobe of the catalytic domain that render the Src catalytic domain active. These structural elements include the alpha-helix C region, a beta turn between the beta4 and beta5 strands, and an Arg residue at the beginning of the catalytic domain. These three motifs interact with one another to activate the Src catalytic domain, but the equivalent motifs in Csk directly interact with the regulatory domains that are important for Csk activation. The Src motifs can be grafted to the Csk catalytic domain to obtain an active Csk catalytic domain. These results, together with available Src and Csk tertiary structures, reveal an important structural switch that determines the kinase activity of a catalytic domain and dictates the regulatory strategy of a kinase.

The heterotrimeric G-protein alpha-subunit Galphaq regulates TCR-mediated immune responses through an Lck-dependent pathway

Here, we examined the functional involvement of heterotrimeric G-proteins in TCR-induced immune responses. TCR/CD3 crosslinking resulted in activation of both Galphaq and Galphas, but not Galphai-2. Targeting of Galphas, Galphai-2 and Galphaq using siRNA demonstrated a specific role of Galphaq in TCR signaling. Jurkat TAg T cells with Galphaq knockdown displayed reduced activation of Lck and LAT phosphorylation, but paradoxically showed sustained ERK1/2 phosphorylation and increased NFAT-AP-1-reporter activity implicating Galphaq in the negative control of downstream signaling and IL-2-promoter activity. Primary T cells isolated from Galphaq-deficient mice had a similar TCR signaling response with reduced proximal LAT phosphorylation, sustained ERK1/2 phosphorylation and augmented immune responses including increased secretion of IL-2, IL-5, IL-12 and TNF-alpha. The effects on NFAT-AP-1-reporter activity were sensitive to the Src family kinase inhibitor PP2 and were reversed by transient expression of constitutively active Lck. Furthermore, expression of constitutively active Galphaq Q209L elevated Lck activity and Zap-70 phosphorylation. Together these data argue for a role of Galphaq in the fine-tuning of proximal TCR signals at the level of Lck and a negative regulatory role of Galphaq in transcriptional activation of cytokine responses.

beta-Arrestin 1 is required for PAC1 receptor-mediated potentiation of long-lasting ERK1/2 activation by glucose in pancreatic beta-cells

In pancreatic beta-cells, the pituitary adenylate cyclase-activating polypeptide (PACAP) exerts a potent insulin secretory effect via PAC(1) and VPAC receptors (Rs) through the Galpha(s)/cAMP/protein kinase A pathway. Here, we investigated the mechanisms linking PAC(1)R to ERK1/2 activation in INS-1E beta-cells and pancreatic islets. PACAP caused a transient (5 min) increase in ERK1/2 phosphorylation via PAC(1)Rs and promoted nuclear translocation of a fraction of cytosolic p-ERK1/2. Both protein kinase A- and Src-dependent pathways mediated this transient ERK1/2 activation. Moreover, PACAP potentiated glucose-induced long-lasting ERK1/2 activation. Blocking Ca(2+) influx abolished glucose-induced ERK1/2 activation and PACAP potentiating effect. Glucose stimulation during KCl depolarization showed that, in addition to the triggering signal (rise in cytosolic [Ca(2+)]), the amplifying pathway was also involved in glucose-induced sustained ERK1/2 activation and was required for PACAP potentiation. The finding that at 30 min glucose-induced p-ERK1/2 was detected in both cytosol and nucleus while the potentiating effect of PACAP was only observed in the cytosol, suggested the involvement of the scaffold protein beta-arrestin. Indeed, beta-arrestin 1 (beta-arr1) depletion (in beta-arr1 knockout mouse islets or in INS-1E cells by siRNA) completely abolished PACAP potentiation of long-lasting ERK1/2 activation by glucose. Finally, PACAP potentiated glucose-induced CREB transcriptional activity and IRS-2 mRNA expression mainly via the ERK1/2 signaling pathway, and likewise, beta-arr1 depletion reduced the PACAP potentiating effect on IRS-2 expression. These results establish for the first time that PACAP potentiates glucose-induced long-lasting ERK1/2 activation via a beta-arr1-dependent pathway and thus provide new insights concerning the mechanisms of PACAP and glucose actions in pancreatic beta-cells.

Beta-adrenergic receptor mediated protection against doxorubicin-induced apoptosis in cardiomyocytes: the impact of high ambient glucose

Recent studies have demonstrated that the beta2-adrenergic receptor (beta2AR)-Galphai signaling pathway exerts a cardiac antiapoptotic effect. The goals of this study were to determine the intracellular signaling factors involved in beta2AR-mediated protection against doxorubicin-induced apoptosis in H9c2 cardiomyocyte and explore the impact of high ambient glucose on the antiapoptotic effect. Under physiological glucose environment (100 mg/dl), beta2AR stimulation prevented doxorubicin-induced apoptosis, which was attenuated by cotreatment with wortmannin, a phosphoinositide 3-kinase (PI3K) inhibitor, or transfection of a dominant-negative Akt. Inhibition of Src kinase with 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d] pyrimidine or cSrc small interfering RNA 32 also attenuated the antiapoptotic effect. Inhibition of platelet-derived growth factor receptor (PDGFR) with AG1296 reversed the beta2AR-induced antiapoptotic effect. Transfection of an active Src cDNA (Y529F) alone was sufficient to render the cells resistant to apoptosis, and the resistance was blocked by wortmannin. Transfection of an active PI3K minigene (iSH2-p110) alone also induced resistance to apoptosis, and the resistance was reversed by an Akt-inhibitor but not by AG1296. High ambient glucose (450 mg/dl) caused two major effects: 1) it significantly reduced betaAR-induced PDGFR phosphorylation, Src kinase activity, and activation of PI3K signaling pathway; and 2) it partially attenuated beta2AR-induced antiapoptotic effect. These data provide in vitro evidence supporting a signaling cascade by which beta2AR exerts a protective effect against doxorubicin-induced apoptosis via sequential involvement of Galphai, Gbetagamma, Src, PDGFR, PI3K, and Akt. High ambient glucose significantly attenuates beta2AR-mediated cardioprotection by suppressing factors involved in this cascade including PDGFR, Src, and PI3K/Akt.

Src-protein tyrosine kinases are required for cocaine-induced increase in the expression and function of the NMDA receptor in the ventral tegmental area

Cocaine-induced long-term potentiation of glutamatergic synapses in the ventral tegmental area (VTA) has been proposed as a key process that contributes to the development of addictive behaviors. In particular, the activation of ionotrophic glutamate NMDA receptor (NMDAR) in the VTA is critical for the initiation of cocaine sensitization. Here we show that application of cocaine both in slices and in vivo induced an increase in tyrosine phosphorylation of the NR2A, but not the NR2B subunit of the NMDAR in juvenile rats. Cocaine induced an increase in the activity of both Fyn and Src kinases, and the Src-protein tyrosine kinase (Src-PTKs) inhibitor, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2), abolished both cocaine-induced increase in tyrosine phosphorylation of the NR2A subunit and the increase in the expression of NR1, NR2A, and NR2B in the VTA. Moreover, cocaine-induced enhancement in NMDAR-mediated excitatory post-synaptic currents was completely abolished by PP2. Taken together, these results suggest that acute cocaine induced an increase in the expression of NMDAR subunits and enhanced tyrosine phosphorylation of NR2A-containing NMDAR through members of the Src-PTKs. This in turn, increased NMDAR-mediated currents in VTA dopamine neurons. These results provide a potential cellular mechanism by which cocaine triggers NMDAR-dependent synaptic plasticity of VTA neurons that may underlie the development of behavioral sensitization.

Src phosphorylation of micro-receptor is responsible for the receptor switching from an inhibitory to a stimulatory signal

Recent studies have revealed that in G protein-coupled receptor signalings switching between G protein- and beta-arrestin (betaArr)-dependent pathways occurs. In the case of opioid receptors, the signal is switched from the initial inhibition of adenylyl cyclase (AC) to an increase in AC activity (AC activation) during prolonged agonist treatment. The mechanism of such AC activation has been suggested to involve the switching of G proteins activated by the receptor, phosphorylation of signaling molecules, or receptor-dependent recruitment of cellular proteins. Using protein kinase inhibitors, dominant negative mutant studies and mouse embryonic fibroblast cells isolated from Src kinase knock-out mice, we demonstrated that mu-opioid receptor (OPRM1)-mediated AC activation requires direct association and activation of Src kinase by lipid raft-located OPRM1. Such Src activation was independent of betaArr as indicated by the ability of OPRM1 to activate Src and AC after prolonged agonist treatment in mouse embryonic fibroblast cells lacking both betaArr-1 and -2. Instead the switching of OPRM1 signals was dependent on the heterotrimeric G protein, specifically Gi2 alpha-subunit. Among the Src kinase substrates, OPRM1 was phosphorylated at Tyr336 within NPXXY motif by Src during AC activation. Mutation of this Tyr residue, together with mutation of Tyr166 within the DRY motif to Phe, resulted in the complete blunting of AC activation. Thus, the recruitment and activation of Src kinase by OPRM1 during chronic agonist treatment, which eventually results in the receptor tyrosine phosphorylation, is the key for switching the opioid receptor signals from its initial AC inhibition to subsequent AC activation.

Cytokines and myelination in the central nervous system

Myelin abnormalities that reflect damage to developing and mature brains are often found in neurological diseases with evidence of inflammatory infiltration and microglial activation. Many cytokines are virtually undetectable in the uninflamed central nervous system (CNS), so that their rapid induction and sustained elevation in immune and glial cells contributes to dysregulation of the inflammatory response and neural cell homeostasis. This results in aberrant neural cell development, cytotoxicity, and loss of the primary myelin-producing cells of the CNS, the oligodendrocytes. This article provides an overview of cytokine and chemokine activity in the CNS with relevance to clinical conditions of neonatal and adult demyelinating disease, brain trauma, and mental disorders with observed white matter defects. Experimental models that mimic human disease have been developed in order to study pathogenic and therapeutic mechanisms, but have shown mixed success in clinical application. However, genetically altered animals, and models of CNS inflammation and demyelination, have offered great insight into the complexities of neuroimmune interactions that impact oligodendrocyte function. The intracellular signaling pathways of selected cytokines have also been highlighted to illustrate current knowledge of receptor-mediated events. By learning to interpret the actions of cytokines and by improving methods to target appropriate predictors of disease risk selectively, a more comprehensive understanding of altered immunoregulation will aid in the development of advanced treatment options for patients with inflammatory white matter disorders.

Microfluidic switching system for analyzing chemotaxis responses of wortmannin-inhibited HL-60 cells

The chemotaxis of phosphoinositide kinase-3 (PI3K)-inhibited differentiated HL-60 cells stably expressing CXCR2 was studied in a microfluidic switching gradient device that can generate stable and well-defined forward and reverse gradients. Wortmannin, a widely used PI3K inhibitor, was added during cell preparation and the experiment process. The studies quantify the chemotaxis gradient and the effects of a change in the direction of a CXCL-8 gradient on cell migration. PI3K-inhibited HL-60 cells migrated more efficiently toward the gradient before gradient switching than after, as measured by the effective chemotactic index. The inhibited HL-60 cells also showed that inadequate polarization, slower response time, and reduced cell populations can follow the gradient change. We observed that the role of PI3K in directing cellular response to gradient reversal was important in cell polarization and directional sensing associated with gradient switching.

Arrestin-3 is essential for the activation of Fyn by the luteinizing hormone receptor (LHR) in MA-10 cells

Recent studies showed that Fyn is a mediator of the LHR-induced activation of the ERK1/2 cascade in MA-10 cells. Since the LHR is a G protein-coupled receptor and the Src family of kinases can be activated by some Galpha subunits and by the non-visual arrestins we investigated the role of these signaling molecules in the LHR-provoked activation of Fyn. Small interfering RNAs (siRNAs) that target two Galpha subunits that participate in LHR signaling (Galpha(s) and Galpha(11)) and one that targets arrestin-3 were co-transfected with the hLHR in MA-10 cells. We then determined the effects of these siRNAs on the LHR-provoked activation of Fyn, the phosphorylation of FAK (a prominent Fyn substrate) and the release of EGF-like growth factors (a Fyn-mediated process). Expression of the siRNA against Galpha(s) decreased the level of Galpha(s) and LHR-stimulated cAMP production by approximately 50% but did not affect LHR-stimulated Fyn activation or FAK phosphorylation. Likewise, expression of the siRNA against Galpha(11) decreased the level of Galpha(11) and LHR-stimulated inositol phosphate production by approximately 50% but did not affect LHR-stimulated Fyn activation or FAK phosphorylation. Expression of the siRNA against arrestin-3 decreased the level of arrestin-3 and the rate of internalization of hCG by approximately 50% and it also inhibited the LHR-provoked stimulation of Fyn, the phosphorylation of FAK and the release of EGF-like growth factors. These results show that, in MA-10 cells, the hLHR activates Fyn through an arrestin-3-dependent pathway and that this pathway is a mediator of the hLHR-provoked release of EGF-like growth factors.

Parallel phosphatidylinositol 3-kinase (PI3K)-dependent and Src-dependent pathways lead to CXCL8-mediated Rac2 activation and chemotaxis

The requirement for phosphatidylinositol 3-kinase (PI3K) in the establishment of cell polarity and motility in a number of cell types has recently come into question. In this study, we demonstrate that inhibition of PI3K by wortmannin in neutrophil-like differentiated HL60 cells expressing CXCR2 resulted in reduced cell motility but normal chemotaxis in response to a gradient of CXCL8. However, wortmannin inhibition of PI3K did impair the ability of cells to re-orient their polarity and respond quickly to a change in the direction of the CXCL8 gradient. We hypothesized that Src-regulated ELMO-Dock2-Rac2 activation mediates chemotaxis in the absence of PI3K activity. Inhibition of Src with the small molecule inhibitor, PP2, or inhibition of Dock2 by shRNA knockdown confirmed the functional role of Src and Dock2 in regulating chemotaxis when PI3K was inhibited. Moreover, neutrophils isolated from bone marrow of hck(-/-)fgr(-/-)lyn(-/-) mice exhibited much more severe inhibition of chemotaxis when PI3K was blocked with wortmannin as compared with neutrophils isolated from bone marrow of wild-type mice. Thus, PI3K and Src-ELMO-Dock2 pathways work in parallel to activate Rac2 and modulate chemotaxis in response to a CXCL8 gradient in neutrophils.

PSGL-1 engagement by E-selectin signals through Src kinase Fgr and ITAM adapters DAP12 and FcR gamma to induce slow leukocyte rolling

E-selectin binding to P-selectin glycoprotein ligand-1 (PSGL-1) can activate the β₂ integrin lymphocyte function-associated antigen-1 by signaling through spleen tyrosine kinase (Syk). This signaling is independent of Gα_i-protein–coupled receptors, results in slow rolling, and promotes neutrophil recruitment to sites of inflammation. However, the signaling pathways linking E-selectin engagement of PSGL-1 to Syk activation are unknown. To test the role of Src family kinases and immunoreceptor tyrosine-based activating motif (ITAM)–containing adaptor proteins, we used different gene-deficient mice in flow chamber, intravital microscopy, and peritonitis studies. E-selectin–mediated phosphorylation of Syk and slow rolling was abolished in neutrophils from fgr^−/− or hck^−/− lyn^−/− fgr^−/− mice. Neutrophils from Tyrobp^−/− Fcrg^−/− mice lacking both DAP12 and FcRγ were incapable of sustaining slow neutrophil rolling on E-selectin and intercellular adhesion molecule-1 and were unable to phosphorylate Syk and p38 MAPK. This defect was confirmed in vivo by using mixed chimeric mice. Gα_i-independent neutrophil recruitment into the inflamed peritoneal cavity was sharply suppressed in Tyrobp^−/− Fcrg^−/− mice. Our data demonstrate that an ITAM-dependent pathway involving the Src-family kinase Fgr and the ITAM-containing adaptor proteins DAP12 and FcRγ is involved in the initial signaling events downstream of PSGL-1 that are required to initiate neutrophil slow rolling.

Alpha-1 adrenergic receptor transactivates signal transducer and activator of transcription-3 (Stat3) through activation of Src and epidermal growth factor receptor (EGFR) in hepatocytes

Hepatocytes express adrenergic receptors (ARs) that modulate several functions, including liver regeneration, hepatocyte proliferation, glycogenolysis, gluconeogenesis, synthesis of urea and fatty acid metabolism. Adrenergic hepatic function in adults is mainly under the control of alpha(1)-ARs; however, the mechanism through which they influence diverse processes remains incompletely understood. This study describes a novel alpha(1)-AR-mediated transactivation of signal transducer and activator of transcription-3 (Stat3) in primary and transformed hepatocytes. Treatment of primary rat hepatocytes with the alpha(1)-AR agonist, phenylephrine (PE), induced a rapid phosphorylation of Stat3. PE also increased Stat3 phosphorylation, DNA binding and transcription activity in transformed human hepatocellular carcinoma cells (Hep3B). The PE-induced Stat3 phosphorylation, DNA binding and reporter activity were completely blocked by the selective alpha(1)-AR antagonist, prazosin. In addition, transfection of Hep3B cells with human alpha(1B)-AR expression vector also enhanced Stat3 phosphorylation and reporter activity. Moreover, overexpression of RGS2, a protein inhibitor of G(q/11) signaling, blocked PE-induced Stat3 phosphorylation and reporter activity. The observations that PE induced the formation of c-Src-Stat3 binding complex and phosphorylation of epidermal growth factor receptor (EGFR) and that inhibiting Src and EGFR prevented PE-induced Stat3 activation indicate the involvement of Src and EGFR. Taken together, these observations demonstrate a novel alpha(1)-AR-mediated Stat3 activation that involves G(q/11), Src, and EGFR in hepatic cells.

c-Src tyrosine kinase, a critical component for 5-HT2A receptor-mediated contraction in rat aorta

Serotonin (5-hydroxytryptamine, 5-HT) receptors (5-HTRs) play critical roles in brain and cardiovascular functions. In the vasculature, 5-HT induces potent vasoconstrictions, which in aorta are mainly mediated by activation of the 5-HT(2A)R subtype. We previously proposed that one signalling mechanism of 5-HT-induced vasoconstriction could be c-Src, a member of the Src tyrosine kinase family. We now provide evidence for a central role of c-Src in 5-HT(2A)R-mediated contraction. Inhibition of Src kinase activity with 10 mum 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) prior to contraction resulted in approximately 90-99% inhibition of contractions induced by 5-HT or by alpha-methyl-5-HT (5-HT(2)R agonist). In contrast, PP2 pretreatment only partly inhibited contractions induced by angiotensin II and the thromboxane A(2) mimetic, U46619, and had no significant action on phenylephrine-induced contractions. 5-Hydroxytryptamine increased Src kinase activity and PP2-sensitive tyrosine-phosphorylated proteins. As expected for c-Src identity, PP2 pretreatment inhibited 5-HT-induced contraction with an IC(50) of approximately 1 mum. Ketanserin (10 nm), a 5-HT(2A) antagonist, but not antagonists of 5-HT(2B)R (100 nm SB204741) or 5-HT(2C)R (20 nm RS102221), prevented 5-HT-induced contractions, mimicking PP2 and implicating 5-HT(2A)R as the major receptor subtype coupled to c-Src. In HEK 293T cells, c-Src and 5-HT(2A)R were reciprocally co-immunoprecipitated and co-localized at the cell periphery. Finally, 5-HT-induced Src activity was unaffected by inhibition of Rho kinase, supporting a role of c-Src upstream of Rho kinase. Together, the results highlight c-Src activation as one of the early and pivotal mechanisms in 5-HT(2A)R contractile signalling in aorta.

Pituitary adenylate cyclase-activating polypeptide 38-mediated Rin activation requires Src and contributes to the regulation of HSP27 signaling during neuronal differentiation

Pituitary adenylate cyclase-activating polypeptide 38 (PACAP38) is a potent neuropeptide that acts through G-protein-coupled receptors. While it is well established that PACAP mediates both neurotrophic and neurodevelopmental effects, the signaling cascades that underlie these diverse actions remain incompletely characterized. Here we show that the Ras-related Rin GTP-binding protein, a GTPase that is expressed predominantly in neurons, is regulated by PACAP38 signaling, and loss-of-function analysis demonstrates that Rin makes an essential contribution to PACAP38-mediated pheochromocytoma cell differentiation. Rin is activated following stimulation of both Gsalpha and Gialpha cascades but does not rely upon cyclic AMP (cAMP)-, Ca(2+)-, or Epac-dependent signaling pathways. Instead, Rin is activated in a Src kinase-dependent manner. Surprisingly, Rin knockdown significantly inhibits PACAP38-mediated neurite outgrowth, without affecting mitogen-activated protein kinase signaling cascades. Instead, Rin loss attenuates PACAP38-mediated HSP27 activation by disrupting a cAMP-protein kinase A cascade. RNA interference-mediated HSP27 silencing suppresses both PACAP38- and Rin-mediated neurite outgrowth, while expression of a constitutively active Rin mutant increases both HSP27 protein and phospho-HSP27 levels, supporting a role for Rin-HSP27 signaling in neuronal differentiation. Together, these observations identify an unsuspected role for Rin in neuronal PACAP signaling and establish a novel Galpha-Src-Rin-HSP27 signal transduction pathway as a critical element in PACAP38-mediated neuronal differentiation signaling.

From individual Wnt pathways towards a Wnt signalling network

Wnt proteins play important roles during vertebrate and invertebrate development. They obviously have the ability to activate different intracellular signalling pathways. Based on the characteristic intracellular mediators used, these are commonly described as the Wnt/beta-catenin, the Wnt/calcium and the Wnt/Jun N-terminal kinase pathways (also called planar cell polarity pathway). In the past, these different signalling events were mainly described as individual and independent signalling branches. Here, we discuss the possibility that Wnt proteins activate a complex intracellular signalling network rather than individual pathways and suggest a graph representation of this network. Furthermore, we discuss different ways of how to predict the specific outcome of an activation of this network in a particular cell type, which will require the use of mathematical models. We point out that the use of deterministic approaches via the application of differential equations is suitable to model only small aspects of the whole network and that more qualitative approaches are possibly a suitable starting point for the prediction of the global behaviour of such large protein interaction networks.

Intracellular signal transduction by the extracellular calcium-sensing receptor of Xenopus melanotrope cells

The extracellular calcium-sensing receptor (CaR) is expressed in various types of endocrine pituitary cell, but the intracellular mechanism this G protein-coupled receptor uses in these cells is not known. In the present study we investigated possible intracellular signal transduction pathway(s) utilized by the CaR of the endocrine melanotrope cells in the intermediate pituitary lobe of the South African-clawed toad Xenopus laevis. For this purpose, the effects of various pharmacological agents on CaR-evoked secretion of radiolabeled secretory peptides from cultured melanotrope cells were assessed. CaR-evoked secretion, induced by the potent CaR agonist L-phenylalanine (L-Phe), could not be inhibited by cholera toxin, nor by NPC-15437 and PMA, indicating that neither G(s)/PKA nor G(q)/PKC pathways are involved. However, pertussis toxin (G(i/o) protein inhibitor), genistein (inhibitor of PTKs), wortmannin/LY-294002 (PI3-K inhibitor) and U-0126 (inhibitor of extracellular signal-regulated kinase, ERK) all substantially inhibited CaR-evoked secretion, indicating that the Xenopus melanotrope cell possesses a PI3-K/MAPK system that plays some role in CaR-signaling. Since no direct effect of L-Phe on ERK phosphorylation could be shown it is concluded that CaR must act primarily through another, still unknown, signaling pathway in Xenopus melanotropes. Our results indicate that the PI3-K/MAPK system has a facilitating effect on CaR-induced secretion, possibly by sensitizing the CaR.

Regulation of motor neuron specification by phosphorylation of neurogenin 2

The mechanisms by which proneural basic helix-loop-helix (bHLH) factors control neurogenesis have been characterized, but it is not known how they specify neuronal cell-type identity. Here, we provide evidence that two conserved serine residues on the bHLH factor neurogenin 2 (Ngn2), S231 and S234, are phosphorylated during motor neuron differentiation. In knockin mice in which S231 and S234 of Ngn2 were mutated to alanines, neurogenesis occurs normally, but motor neuron specification is impaired. The phosphorylation of Ngn2 at S231 and S234 facilitates the interaction of Ngn2 with LIM homeodomain transcription factors to specify motor neuron identity. The phosphorylation-dependent cooperativity between Ngn2 and homeodomain transcription factors may be a general mechanism by which the activities of bHLH and homeodomain proteins are temporally and spatially integrated to generate the wide diversity of cell types that are a hallmark of the nervous system.

Galpha12 regulates protein interactions within the MDCK cell tight junction and inhibits tight-junction assembly

The polarized functions of epithelia require an intact tight junction (TJ) to restrict paracellular movement and to separate membrane proteins into specific domains. TJs contain scaffolding, integral membrane and signaling proteins, but the mechanisms that regulate TJs and their assembly are not well defined. Galpha12 (GNA12) binds the TJ protein ZO-1 (TJP1), and Galpha12 activates Src to increase paracellular permeability via unknown mechanisms. Herein, we identify Src as a component of the TJ and find that recruitment of Hsp90 to activated Galpha12 is necessary for signaling. TJ integrity is disrupted by Galpha12-stimulated Src phosphorylation of ZO-1 and ZO-2 (TJP2); this phosphorylation leads to dissociation of occludin and claudin 1 from the ZO-1 protein complex. Inhibiting Hsp90 with geldanamycin blocks Galpha12-stimulated Src activation and phosphorylation, but does not affect protein levels or the Galpha12-ZO-1 interaction. Using the calcium-switch model of TJ assembly and GST-TPR (GST-fused TPR domain of PP5) pull-downs of activated Galpha12, we demonstrate that switching to normal calcium medium activates endogenous Galpha12 during TJ assembly. Thrombin increases permeability and delays TJ assembly by activating Galpha12, but not Galpha13, signaling pathways. These findings reveal an important role for Galpha12, Src and Hsp90 in regulating the TJ in established epithelia and during TJ assembly.

Synergistic activation of phospholipases Cgamma and Cbeta: a novel mechanism for PI3K-independent enhancement of FcepsilonRI-induced mast cell mediator release

Antigen/IgE-mediated mast cell activation via FcvarepsilonRI can be markedly enhanced by the activation of other receptors expressed on mast cells and these receptors may thus contribute to the allergic response in vivo. One such receptor family is the G protein-coupled receptors (GPCRs). Although the signaling cascade linking FcvarepsilonRI aggregation to mast cell activation has been extensively investigated, the mechanisms by which GPCRs amplify this response are relatively unknown. To investigate this, we utilized prostaglandin (PG)E2 based on initial studies demonstrating its greater ability to augment antigen-mediated degranulation in mouse mast cells than other GPCR agonists examined. This enhancement, and the ability of PGE2 to amplify antigen-induced calcium mobilization, was independent of phosphoinositide 3-kinase but was linked to a pertussis toxin-sensitive synergistic translocation to the membrane of phospholipase (PL)Cgamma and PLCbeta and to an enhancement of PLCgamma phosphorylation. This "trans-synergistic" activation of PLCbeta and gamma, in turn, enhanced production of inositol 1,4,5-trisphosphate, store-operated calcium entry, and activation of protein kinase C (PKC) (alpha and beta). These responses were critical for the promotion of degranulation. This is the first report of synergistic activation between PLCgamma and PLCbeta that permits reinforcement of signals for degranulation in mast cells.

Src tyrosine kinase alters gating of hyperpolarization-activated HCN4 pacemaker channel through Tyr531

We recently discovered that the constitutively active Src tyrosine kinase can enhance hyperpolarization-activated, cyclic nucleotide-gated (HCN) 4 channel activity by binding to the channel protein. To investigate the mechanism of modulation by Src of HCN channels, we studied the effects of a selective inhibitor of Src tyrosine kinase, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2), on HCN4 and its mutant channels expressed in HEK 293 cells by using a whole cell patch-clamp technique. We found that PP2 can inhibit HCN4 currents by negatively shifting the voltage dependence of channel activation, decreasing the whole cell channel conductance, and slowing activation and deactivation kinetics. Screening putative tyrosine residues subject to phosphorylation yielded two candidates: Tyr(531) and Tyr(554). Substituting HCN4-Tyr(531) with phenylalanine largely abolished the effects of PP2 on HCN4 channels. Replacing HCN4-Tyr(554) with phenylalanine did not abolish the effects of PP2 on voltage-dependent activation but did eliminate PP2-induced slowing of channel kinetics. The inhibitory effects of HCN channels associated with reduced Src tyrosine activity is confirmed in HL-1 cardiomyocytes. Finally, we found that PP2 can decrease the heart rate in a mouse model. These results demonstrate that Src tyrosine kinase enhances HCN4 currents by shifting their activation to more positive potentials and increasing the whole cell channel conductance as well as speeding the channel kinetics. The tyrosine residue that mediates most of Src's actions on HCN4 channels is Tyr(531).

Mitogenic signaling pathways induced by G protein-coupled receptors

G protein-coupled receptor (GPCR) agonists, including neurotransmitters, hormones, chemokines, and bioactive lipids, act as potent cellular growth factors and have been implicated in a variety of normal and abnormal processes, including development, inflammation, and malignant transformation. Typically, the binding of an agonistic ligand to its cognate GPCR triggers the activation of multiple signal transduction pathways that act in a synergistic and combinatorial fashion to relay the mitogenic signal to the nucleus and promote cell proliferation. A rapid increase in the activity of phospholipases C, D, and A2 leading to the synthesis of lipid-derived second messengers, Ca2+ fluxes and subsequent activation of protein phosphorylation cascades, including PKC/PKD, Raf/MEK/ERK, and Akt/mTOR/p70S6K is an important early response to mitogenic GPCR agonists. The EGF receptor (EGFR) tyrosine kinase has emerged as a transducer in the signaling by GPCRs, a process termed transactivation. GPCR signal transduction also induces striking morphological changes and rapid tyrosine phosphorylation of multiple cellular proteins, including the non-receptor tyrosine kinases Src, focal adhesion kinase (FAK), and the adaptor proteins CAS and paxillin. The pathways stimulated by GPCRs are extensively interconnected by synergistic and antagonistic crosstalks that play a critical role in signal transmission, integration, and dissemination. The purpose of this article is to review recent advances in defining the pathways that play a role in transducing mitogenic responses induced by GPCR agonists.

Interaction between src family kinases and rho-kinase in agonist-induced Ca2+-sensitization of rat pulmonary artery

Aims

We investigated the role of src family kinases (srcFK) in agonist-mediated Ca²⁺-sensitization in pulmonary artery and whether this involves interaction with the rho/rho-kinase pathway.

Methods and results

Intra-pulmonary arteries (IPAs) and cultured pulmonary artery smooth muscle cells (PASMC) were obtained from rat. Expression of srcFK was determined at the mRNA and protein levels. Ca²⁺-sensitization was induced by prostaglandin F_2α (PGF_2α) in α-toxin-permeabilized IPAs. Phosphorylation of the regulatory subunit of myosin phosphatase (MYPT-1) and of myosin light-chain-20 (MLC₂₀) and translocation of rho-kinase in response to PGF_2α were also determined. Nine srcFK were expressed at the mRNA level, including src, fyn, and yes, and PGF_2α enhanced phosphorylation of three srcFK proteins at tyr-416. In α-toxin-permeabilized IPAs, PGF_2α enhanced the Ca²⁺-induced contraction (pCa 6.9) approximately three-fold. This enhancement was inhibited by the srcFK blockers SU6656 and PP2 and by the rho-kinase inhibitor Y27632. Y27632, but not SU6656 or PP2, also inhibited the underlying pCa 6.9 contraction. PGF_2α enhanced phosphorylation of MYPT-1 at thr-697 and thr-855 and of MLC₂₀ at ser-19. This enhancement, but not the underlying basal phosphorylation, was inhibited by SU6656. Y27632 suppressed both basal and PGF_2α-mediated phosphorylation. The effects of SU6656 and Y27632, on both contraction and MYPT-1 and MLC₂₀ phosphorylation, were not additive. PGF_2α triggered translocation of rho-kinase in PASMC, and this was inhibited by SU6656.

Conclusions

srcFK are activated by PGF_2α in the rat pulmonary artery and may contribute to Ca²⁺-sensitization and contraction via rho-kinase translocation and phosphorylation of MYPT-1.

Formyl peptide-receptor like-1 requires lipid raft and extracellular signal-regulated protein kinase to activate inhibitor-κB kinase in human U87 astrocytoma cells

Formyl peptide-receptor like-1 (FPRL-1) may possess critical roles in Alzheimer's diseases, chemotaxis and release of neurotoxins, possibly through its regulation of nuclear factor-kappaB (NFkappaB). Here we illustrate that activation of FPRL-1 in human U87 astrocytoma or Chinese hamster ovary cells stably expressing the receptor resulted in the phosphorylations of inhibitor-kappaB kinase (IKK), an onset kinase for NFkappaB signaling cascade. FPRL-1 selective hexapeptide Trp-Lys-Tyr-Met-Val-Met (WKYMVM) promoted IKK phosphorylations in time- and dose-dependent manners while pre-treatment of pertussis toxin abrogated the Galpha(i/o)-dependent stimulations. The FPRL-1-mediated IKK phosphorylation required extracellular signal-regulated protein kinase (ERK), phosphatidylinositol 3-kinase and cellular Src (c-Src), but not c-Jun N-terminal kinase and p38 mitogen-activated protein kinase. Despite its ability to mobilize Ca(2+), WKYMVM did not require Ca(2+) for the modulation of IKK phosphorylation. Activation of FPRL-1 also induced NFkappaB-driven luciferase expression. Interestingly, cholesterol depletion from plasma membrane by methyl-beta-cyclodextrin abolished the FPRL-1-stimulated IKK phosphorylation, denoting the important role of lipid raft integrity in the FPRL-1 to IKK signaling. Furthermore, we demonstrated that in U87 cells, several signaling intermediates in the FPRL-1-IKK pathway including Galpha(i2), c-Src and ERK were constitutively localized at the raft microdomains. WKYMVM administration not only resulted in higher amount of ERK recruitment to the raft region, but also specifically stimulated raft-associated c-Src and ERK phosphorylations. Taken together, these results demonstrate that FPRL-1 is capable of activating NFkappaB signaling through IKK phosphorylation and this may serve as a useful therapeutical target for FPRL-1-related diseases.

When a G protein-coupled receptor does not couple to a G protein

Classically, G protein-coupled receptors (GPCRs) relay signals by directly activating heterotrimeric guanine nucleotide-binding proteins (G proteins). Increasing evidence indicates that GPCRs may also signal through G protein-independent pathways. JAK/STATs, Src-family tyrosine kinases, GRKs/beta-arrestins, and PDZ domain-containing proteins have been suggested to directly relay signals from GPCRs independent of G proteins. In addition, our laboratory recently reported that the beta(2) adrenergic receptor (beta(2)AR) could switch from G protein-coupled to G protein-independent ERK (extracellular signal-regulated kinase) activation in an agonist dosage-dependent manner. This finding provides a novel mechanism for G protein-independent GPCR signaling. This review focuses on recent progress in understanding the mechanisms by which G protein-independent GPCR signaling occurs.

Formyl peptide receptor like 1 differentially requires mitogen-activated protein kinases for the induction of glial fibrillary acidic protein and interleukin-1α in human U87 astrocytoma cells

Mitogen-activated protein kinases (MAPKs) are not only pivotal mediators of signal transduction but they also regulate diverse biological processes ranging from survival, proliferation and differentiation to apoptosis. By using human U87 astrocytoma and transfected FPRL1/CHO cells, we have demonstrated that activation of FPRL1 with WKYMVM effectively phosphorylated JNK and ERK. Interestingly, p38 MAPK activation was only seen with FPRL1/CHO cells. The MAPK phosphorylations in response to WKYMVM were blocked by WRW(4) (a selective FPRL1 antagonist), but not cyclosporine H (a well-known FPR antagonist). The key signaling intermediates in the MAPK pathways were also delineated. G(i)/G(o) proteins, Src family tyrosine kinases, but not phosphatidylinositol-3 kinase, protein kinase C and calmodulin-dependent kinase II, were required to transmit signals from FPRL1 toward JNK, ERK and p38 MAPK. Furthermore, phospholipase Cbeta was distinctively involved in the regulation of JNK but not the other MAPKs. Importantly, WKYMVM-stimulated U87 cells triggered noticeable increases in glial fibrillary acidic protein (GFAP) and interleukin-1alpha (IL-1alpha), which are correlated with reactive astrocytosis. In contrast, GFAP expression was not altered following stimulation with N-formyl-methionyl-leucyl-phenylalanine. Moreover, inhibitions of G(i)/G(o) proteins and JNK completely abolished both GFAP and IL-1alpha upregulations by FPRL1, while blockade of the MEK/ERK cascade exclusively suppressed the GFAP production. Consistently, overexpression of MEK1 and constitutively active JNKK in U87 cells led to ERK and JNK activation, respectively, which was accompanied with markedly increased GFAP production. We have thus identified a possible linkage among FPRL1, MAPKs, astrocytic activation and the inflammatory response.

Stress hormones regulate interleukin-6 expression by human ovarian carcinoma cells through a Src-dependent mechanism

Recent studies have demonstrated that chronic stress promotes tumor growth, angiogenesis, and metastasis. In ovarian cancer, levels of the pro-angiogenic cytokine, interleukin 6 (IL-6), are known to be elevated in individuals experiencing chronic stress, but the mechanism(s) by which this cytokine is regulated and its role in tumor growth remain under investigation. Here we show that stress hormones such as norepinephrine lead to increased expression of IL-6 mRNA and protein levels in ovarian carcinoma cells. Furthermore, we demonstrate that norepinephrine stimulation activates Src tyrosine kinase and this activation is required for increased IL-6 expression. These results demonstrate that stress hormones activate signaling pathways known to be critical in ovarian tumor progression.

Studies of the regulation and function of the Gs alpha gene Gnas using gene targeting technology

The heterotrimeric G protein alpha-subunit G(s)alpha is ubiquitously expressed and mediates receptor-stimulated intracellular cAMP generation. Its gene Gnas is a complex imprinted gene which uses alternative promoters and first exons to generate other gene products, including the G(s)alpha isoform XL alpha s and the chromogranin-like protein NESP55, which are specifically expressed from the paternal and maternal alleles, respectively. G(s)alpha itself is imprinted in a tissue-specific manner, being biallelically expressed in most tissues but paternally silenced in a few tissues. Gene targeting of specific Gnas transcripts demonstrates that heterozygous mutation of G(s)alpha on the maternal (but not the paternal) allele leads to early lethality, perinatal subcutaneous edema, severe obesity, and multihormone resistance, while the paternal mutation leads to only mild obesity and insulin resistance. These parent-of-origin differences are the consequence of tissue-specific G(s)alpha imprinting. XL alpha s deficiency leads to a perinatal suckling defect and a lean phenotype with increased insulin sensitivity. The opposite metabolic effects of G(s)alpha and XL alpha s deficiency are associated with decreased and increased sympathetic nervous system activity, respectively. NESP55 deficiency has no metabolic consequences. Other gene targeting experiments have shown Gnas to have 2 independent imprinting domains controlled by 2 different imprinting control regions. Tissue-specific G(s)alpha knockout models have identified important roles for G(s)alpha signaling pathways in skeletal development, renal function, and glucose and lipid metabolism. Our present knowledge gleaned from various Gnas gene targeting models are discussed in relation to the pathogenesis of human disorders with mutation or abnormal imprinting of the human orthologue GNAS.

A novel signaling pathway for β-adrenergic receptor-mediated activation of phosphoinositide 3-kinase in H9c2 cardiomyocytes

Stimulation of cardiac β-adrenergic receptors (β-AR) activates both the Gs- and Gi-coupled signaling cascades, including the phosphoinositide 3 kinase (PI3K) pathway, that have important physiological implications. Multiple isoforms of PI3K exist in the heart. The goals of this study were to examine the intracellular signaling pathways linking β-AR to PI3K and to identify the PI3K isoform mediating this transactivation in a cardiac context. Acute β-AR stimulation with isoproterenol resulted in increased tyrosine kinase-associated PI3K activity and phosphorylation of Akt and p70S6K in H9c2 cardiomyocytes. Cotreatment with ICI-118,551, but not CGP-20712, abolished the increase in PI3K activity, suggesting a β2-AR-mediated event. PI3K activation was also abrogated by cotreatment with pertussis toxin, 4-amino-5-(4-chlorophenyl)-7-( t-butyl)pyrazolol[3,4-d]pyrimidine (PP2, a selective Src-family tyrosine kinases inhibitor), or AG-1296 [selective platelet-derived growth factor receptor (PDGFR) inhibitor] but not with an inhibitor for protein kinase A, protein kinase C, Ras, adenylyl cyclase, epidermal growth factor receptor, or insulin-like growth factor-1 receptor. β-AR stimulation induced an increase in tyrosine phosphorylation of PDGFR, which was abolished by inhibition of Src either by PP2 or small interfering RNA. Moreover, H9c2 cardiomyocytes stably transfected with a vector expressing a Gβγ sequestrant peptide derived from the COOH-terminus of β-AR kinase-1 failed to activate PI3K after β-AR stimulation, suggesting Gβγ is required for the transactivation. Furthermore, acute β-AR stimulation in vivo resulted in increases in PDGFR-associated PI3K and PI3Kα isoform activities but not the activities of other isoforms (PI3Kβ, -δ, -γ) in adult mouse heart. Taken together, these data provide in vitro and in vivo evidence for a novel mechanism of β-AR-mediated transactivation of cardiac PI3Kα via sequential involvement of Gαi/Gβγ, Src, and PDGFR.