Heterotrimeric G Protein Signaling: Roles in Immune Function and Fine-Tuning by RGS Proteins
1998; Cell Press; Volume: 8; Issue: 1 Linguagem: Inglês
10.1016/s1074-7613(00)80453-7
ISSN1097-4180
Autores Tópico(s)Cytokine Signaling Pathways and Interactions
ResumoImmunologists hope to understand how extracellular signals modulate and regulate the function of cells that participate in immune responses. Not surprisingly, most recent signal transduction studies examining immune cells have focused on Fc, antigen, and cytokine receptor signaling (reviewed by25Daeron M Fc receptor biology.Annu. Rev. Immunol. 1997; 15: 203-234Crossref PubMed Scopus (1038) Google Scholar, 85Qian D Weiss A T cell antigen receptor signal transduction.Curr. Opin. Cell. Biol. 1997; 9: 205-212Crossref PubMed Scopus (286) Google Scholar, 83O'Shea J.J Jaks, STATs, cytokine signal transduction, and immunoregulation are we there yet?.Immunity. 1997; 7: 1-11Abstract Full Text Full Text PDF PubMed Scopus (345) Google Scholar, respectively). In these pathways, tyrosine kinases and phosphatases play dominant roles as signal transducers. The discovery of the Jak/STAT (Janus kinase/signal transducer and activator of T cells) pathway as a major mediator of cytokine receptor signaling is an excellent example of their importance. However, the recognition of the critical importance of chemokine receptor signaling (reviewed by75Murphy P.M Chemokine receptors structure, function and role in microbial pathogenesis.Cytokine Growth Factor Rev. 1996; 7: 47-64Abstract Full Text PDF PubMed Scopus (282) Google Scholar) is leading to a resurgence of interest in heterotrimeric guanine nucleotide–binding proteins (G proteins) as signal transducers in immunocompetent cells. Also, the findings in many cellular systems of significant cross-talk between signaling pathways provide an impetus to examine how signaling through G protein–coupled receptors (GPCRs) might impinge on signaling through Fc, antigen, or cytokine receptors. This review focuses on the known and potential roles of heterotrimeric G protein signaling in immune cells and how a new class of proteins, termed regulators of G protein signaling (RGS), may function to fine-tune these responses. GPCRs are heptahelical receptors such as the chemokine receptors that link to downstream signaling pathways by activating heterotrimeric G proteins. In their inactive state, heterotrimeric G proteins are composed of three subunits, termed α, β, and γ (reviewed by77Neer E.J Heterotrimeric G proteins organizers of transmembrane signals.Cell. 1995; 80: 249-257Abstract Full Text PDF PubMed Scopus (1287) Google Scholar, 46Hamm H.E Gilchrist A Heterotrimeric G proteins.Curr. Opin. Cell Biol. 1996; 8: 189-196Crossref PubMed Scopus (204) Google Scholar, 79Offermanns S Simon M.I Organization of transmembrane signaling by heterotrimeric G proteins.Cancer Surv. 1996; 27: 177-198PubMed Google Scholar). There are 23 distinct α subunits encoded by 17 different genes that are divided into four subfamilies based on primary sequence homology and shared intracellular effector molecules: Giα (Giα1-3, Gzα, Goα1/2, Gtα, and Ggustα); Gqα (Gqα, G11α, G14α, and G15/16α); Gsα (Gsα and Golfα); and G12α (G12α and G13α). The β and γ subunits exist either as part of the heterotrimeric G protein or together as a dimer, but not independently. There are 5 different β subunits and 10 different γ subunits, which do not pair indiscriminately, since certain β subunits prefer certain γ subunits. By combining different α, β, and γ subunits, a large number of distinct heterotrimeric G proteins can be assembled within a cell. Upon ligand binding, GPCRs stimulate the α subunit of a heterotrimeric G protein to release guanosine diphosphate (GDP) and to bind guanosine triphosphate (GTP) in its place (Figure 1). In the GTP-bound form, Gα dissociates from Gβγ, each of which independently binds and activates downstream effectors. Signaling is terminated when Gα subunits hydrolyze GTP, returning to the GDP-bound state, which results in reassembly with Gβγ to form the inactive heterotrimers. Besides their intrinsic GTPase activity, several mechanisms suppress downstream signaling by Gα. Since Gβγ inhibits guanine nucleotide dissociation, assembly of the heterotrimer inhibits Gα nucleotide exchange. In addition, RGS proteins accelerate the slow intrinsic rate of the Gα GTPase reaction (see below). RGS proteins represent a new class of GTPase activators (GAPs) that function in vivo to modulate signal transduction through GPCRs. G proteins have been shown to interact directly with only a few effectors: namely, retinal cyclic guanosine monophosphate phosphodiesterase, several isoforms of adenylyl cyclase, phospholipase C (PLC) β, and an inward rectifying potassium channel. However, a variety of signaling pathways are activated by the different subfamilies of heterotrimeric G proteins (reviewed by79Offermanns S Simon M.I Organization of transmembrane signaling by heterotrimeric G proteins.Cancer Surv. 1996; 27: 177-198PubMed Google Scholar). The major cell signaling events and pathways activated by different Gα subunits and their associated Gβγ subunits are detailed below and summarized in Table 1. Of note, the majority of studies examining heterotrimeric G proteins signaling have been performed with nonlymphoid cells. Although there is relatively little information about heterotrimeric G protein expression in lymphoid cell subsets, a summary of what is known is provided below.Table 1Signaling by Heterotrimeric G ProteinsSubunit FamilySignalingGsα(1) Activation of adenylyl cyclasesGiα(1) Inhibition of adenylyl cyclases(2) Activation of PI3Kγ(3) Activation of the Ca2+ permeable cation channel CD20(4) Activation of a voltage-independent Ca2+ channel in erythroid precursorsGqα(1) Activation of PLCβ(2) Activation of BtkG12α(1) Increase Na+/H+ exchange(2) Increased stress-activated protein kinase activity (G12α via a Ras-dependent pathway and G13α via a Ras-independent pathway)(3) Induction of actin polymerization and focal adhesions (Rho dependent)(4) Activation of early response genes and mitogenesis(5) Induction of nitric oxide synthaseGβγ(1) Activation of G protein–coupled receptor kinases(2) Activation of PLCβ(3) Activation of Src family tyrosine kinases(4) Activation of ion channels (IKG and ICa)(5) Activation of PI3KSee the review by 79Offermanns S Simon M.I Organization of transmembrane signaling by heterotrimeric G proteins.Cancer Surv. 1996; 27: 177-198PubMed Google Scholar. For Gβγ activation of PI3K see 101Stephens L.R Eguinoa A Erdjument-Bromage H Lui M Cooke F Coadwell J Smrcka A.S Thelen M Cadwallader K Tempst P et al.The G beta gamma sensitivity of a PI3K is dependent upon a tightly associated adaptor, p101.Cell. 1997; 89: 105-114Abstract Full Text Full Text PDF PubMed Scopus (493) Google Scholar. Open table in a new tab See the review by 79Offermanns S Simon M.I Organization of transmembrane signaling by heterotrimeric G proteins.Cancer Surv. 1996; 27: 177-198PubMed Google Scholar. For Gβγ activation of PI3K see 101Stephens L.R Eguinoa A Erdjument-Bromage H Lui M Cooke F Coadwell J Smrcka A.S Thelen M Cadwallader K Tempst P et al.The G beta gamma sensitivity of a PI3K is dependent upon a tightly associated adaptor, p101.Cell. 1997; 89: 105-114Abstract Full Text Full Text PDF PubMed Scopus (493) Google Scholar. Gs subfamily members stimulate adenylyl cyclases, ubiquitous enzymes that catalyze the conversion of adenosine triphosphate to the intracellular messenger cyclic adenosine monophosphate (cAMP) (reviewed by43Gilman A.G G proteins and regulation of adenylyl cyclase.Biosci. Rep. 1995; 15: 65-97Crossref PubMed Scopus (175) Google Scholar). There are nine isoforms of adenylyl cyclase, all of which are stimulated by Gs. Gsα can be activated by cholera toxin, which adenosine diphosphate (ADP)–ribosylates an arginine residue, causing the irreversible binding of GTP to Gsα. Cholera toxin or Gs activators increase cAMP levels, which activates cAMP-dependent protein kinase A (PKA). In endocrine-derived cells, Gs-mediated signals enhance cell proliferation, and mutationally activated forms of Gsα have been detected in some endocrine tumors (reviewed by99Spiegel A.M The molecular basis of disorders caused by defects in G proteins.Hormone Res. 1997; 47: 89-96Crossref PubMed Scopus (69) Google Scholar). Conversely, activated forms of Gsα suppress the transformation of NIH3T3 cells by oncogenic Ras and inhibit mitogen-activated protein kinase (MAPK) activation (21Chen J Iyengar R Suppression of Ras-induced transformation of NIH 3T3 cells by activated Gαs.Science. 1994; 263: 1278-1281Crossref PubMed Scopus (107) Google Scholar). Thus, the effects of Gs-mediated signals may have profoundly different effects, depending on the cellular context. Gsα is well expressed in most B and T cells and cell lines, although an occasional cell line contains low levels. For example, the B cell lines Daudi and WEHI231 have nearly undetectable levels by immunoblotting (45Grant K.R Harnett W Millignan G Harnett M.M Differential G-protein expression during B- and T-cell development.Immunology. 1997; 90: 564-571Crossref PubMed Scopus (28) Google Scholar). Similar to its effects on NIH3T3 cells, Gs-mediated signals and subsequent PKA activation inhibit lymphocyte proliferation and impair activation of MAPK and the related stress-activated protein kinase (SAPK, also known as Jun kinase) pathways (102Tamir A Granot Y Isakov N Inhibition of T lymphocyte activation by cAMP is associated with down-regulation of two parallel mitogen-activated protein kinase pathways, the extracellular signal-related kinase and c-Jun N-terminal kinase.J. Immunol. 1996; 157: 1514-1522PubMed Google Scholar). The effects of β-adrenoceptor–mediated signals (Gs-coupled) on lymphocyte function depend on the developmental stage and differentiation status of the cell (reviewed by91Sanders V.M The role of adrenoceptor-mediated signals in the modulation of lymphocyte function.Adv. Neuroimmunol. 1995; 5: 283-298Abstract Full Text PDF PubMed Scopus (53) Google Scholar, 92Scherer L.J Diamond R.A Rothenberg E.V Developmental regulation of cAMP signaling pathways in thymocyte development.Thymus. 1995; 23: 231-257Google Scholar). Prostaglandin E2–mediated signals (also Gs-coupled) raise cAMP levels in T cells and reportedly shift the balance of a cellular immune response toward a T helper type 2 (Th2) response and away from a Th1 response (reviewed by38Fedyk E.R Adawi A Looney R.J Phipps R.P Regulation of IgE and cytokine production by cAMP implications for extrinsic asthma.Clin. Immunol. Immunopathol. 1996; 81: 101-113Crossref PubMed Scopus (41) Google Scholar). Prostaglandin E2 inhibits interleukin-2 (IL-2) gene transcription by interfering with a calcium-sensitive T cell signal transduction pathway (84Paliogianni F Kincaid R.L Boumpas D.T Prostaglandin E2 and other cyclic AMP elevating agents inhibit interleukin 2 gene transcription by counteracting calcineurin-dependent pathways.J. Exp. Med. 1993; 178: 1813-1817Crossref PubMed Scopus (84) Google Scholar). In B lymphocytes it blocks lipopolysaccharide- and IL-4–induced increases in B lymphocyte cell size and major histocompatibility complex class II expression but enhances IL-4–directed isotype switching to IgE (89Roper R.L Ludlow J.W Phipps R.P Prostaglandin E2 inhibits B lymphocyte activation by a cAMP-dependent mechanism PGE-inducible regulatory proteins.Cell. Immunol. 1994; 154: 296-308Crossref PubMed Scopus (52) Google Scholar, 37Fedyk E.R Phipps R.P Prostaglandin E2 receptors of the EP2 and EP4 subtypes regulate activation and differentiation of mouse B lymphocytes to IgE-secreting cells.Proc. Natl. Acad. Sci. USA. 1996; 93: 10978-10983Crossref PubMed Scopus (140) Google Scholar). Increased intracellular levels of cAMP are also implicated in the triggering of apoptosis in thymocytes and B lymphocytes. The latter is rescuable by either CD2 or CD40 cross-linking (4Baixeras E Garcia-Lozano E Martinez C Decrease in cAMP levels promoted by CD48-CD2 interaction correlates with inhibition of apoptosis in B cells.Scand. J. Immunol. 1996; 43: 406-412Crossref PubMed Scopus (11) Google Scholar). Gi subfamily members inhibit adenylyl cyclase types I, V, and VI and thereby lower cAMP levels (reviewed by43Gilman A.G G proteins and regulation of adenylyl cyclase.Biosci. Rep. 1995; 15: 65-97Crossref PubMed Scopus (175) Google Scholar). Giα1-3 and Goα are substrates for ADP ribosylation by pertussis toxin, which leads to their inactivation. Sensitivity of a cellular function to pertussis toxin suggests receptor coupling to one of these G proteins. Some GPCRs activate MAPK through a pertussis toxin–sensitive mechanism that is not mediated by Giα but rather by associated βγ subunits. In addition, Gi/o βγ subunits account for the pertussis toxin–dependent activation of PLC (reviewed by79Offermanns S Simon M.I Organization of transmembrane signaling by heterotrimeric G proteins.Cancer Surv. 1996; 27: 177-198PubMed Google Scholar). PLC catalyzes the hydrolysis of phosphatidyl-4,5-bisphosphate, generating the second messengers inositol-1,4,5-trisphosphate (IP3) and diacylglycerol. Among the different families of enzymes with PLC activity, the four isoforms of the PLCβ family are specifically regulated by heterotrimeric G proteins. Giα subunits also modulate certain potassium and calcium channels. Exposure of erythroid progenitors to erythropoietin increases cytosolic calcium, apparently through a Giα2-activated, voltage-independent calcium channel (74Miller B.A Bell L Hansen C.A Robishaw J.D Linder M.E Cheung J.Y G-protein alpha subunit Gi(alpha)2 mediates erythropoietin signal transduction in human erythroid precursors.J. Clin. Invest. 1996; 98: 1728-1736Crossref PubMed Scopus (25) Google Scholar). Giα subunits are present in all lymphoid cell lines and primary cells that have been examined (45Grant K.R Harnett W Millignan G Harnett M.M Differential G-protein expression during B- and T-cell development.Immunology. 1997; 90: 564-571Crossref PubMed Scopus (28) Google Scholar). Giα, predominantly Giα2, expression is highest in proliferating lymphoid cell lines, and large in vivo activated B cells have higher levels than do resting B cells. No Giα1 expression is found in immature B or T cells, although it is induced during B cell differentiation. Numerous studies implicate Gi-signaling in the regulation of lymphocyte function. Gi-coupled receptors clearly regulate lymphocyte migration. Treatment of lymphoid cells with pertussis toxin blocks their binding to high endothelial venules and their entrance into lymph nodes and splenic white pulp (97Spangrude G.J Braaten B.A Daynes R.A Molecular mechanisms of lymphocyte extravasation. I. Studies of two selective inhibitors of lymphocyte recirculation.J. Immunol. 1984; 132: 354-362PubMed Google Scholar, 6Bargatze R.F Butcher E.C Rapid G protein-regulated activated event involved in lymphocyte binding to high endothelial venules.J. Exp. Med. 1993; 178: 367-372Crossref PubMed Scopus (252) Google Scholar, 24Cyster J.G Goodnow C.C Pertussis toxin inhibits migration of B and T lymphocytes into splenic white pulp cords.J. Exp. Med. 1995; 182: 581-586Crossref PubMed Scopus (164) Google Scholar). A Gi-coupled receptor may mediate B cell differentiation because a B cell differentiation factor (446-BCDF)–triggered calcium flux and enhancement of immunoglobulin secretion are sensitive to pertussis toxin (52Huang R Cioffi J Berg K London R Cidon M Maayani S Mayer L B cell differentiation factor-induced B cell maturation regulation via reduction in cAMP.Cell. Immunol. 1995; 162: 49-55Crossref PubMed Scopus (8) Google Scholar). The calcium-permeable cation channel CD20, which is highly expressed by mature B cells, can be activated by Giα2, suggesting that a Giα2-linked GPCR may regulate calcium influx into B cells (56Kanzaki M Lindorfer M.A Garrison J.C Kojima I Activation of the calcium-permeable cation channel CD20 by alpha subunits of the Gi protein.J. Biol. Chem. 1997; 272: 14733-14739Crossref PubMed Scopus (36) Google Scholar). Gi-mediated signals may also affect the induction of lymphocyte apoptosis, since pertussis toxin treatment inhibits activation-induced apoptosis of monocytes, thymocytes, pre-B leukemia cells, T cells, and natural killer cells (88Ramirez R Carracedo J Zamzami N Castedo M Kroemer G Pertussis toxin inhibits activation-induced cell death of human thymocytes, pre-B leukemia cells and monocytes.J. Exp. Med. 1994; 180: 1147-1152Crossref PubMed Scopus (27) Google Scholar, 17Carracedo J Ramirez R Marchetti P Pintado O.C Baixeras E Martinez C Kroemer G Pertussis toxin-sensitive GTP-binding proteins regulate activation-induced apoptotic cell death of human natural killer cells.Eur. J. Immunol. 1995; 25: 3094-3099Crossref PubMed Scopus (17) Google Scholar). Also implicating Giα2-coupled receptors in the regulation of T cell function is the finding that Giα2-/- mice have increased numbers of single-positive thymic T cells with high-intensity CD3 staining and hyperresponsive T cells in their periphery (Table 2). Although of unknown significance, Giα2 and Giα3 have been found associated with the glycosylphosphatidylinositol-anchored proteins CD59, CD48, and Thy-1 (96Solomon K.R Rudd C.E Finberg R.W The association between glycosylphosphatidylinositol-anchored proteins and heterotrimeric G protein α subunits in lymphocytes.Proc. Natl. Acad. Sci. USA. 1996; 93: 6053-6058Crossref PubMed Scopus (100) Google Scholar).Table 2Results of Gene Targeting Experiments Relevant to G Protein Signaling in Lymphocytes and Inflammatory cellsMoleculeFunctionPhenotypeGiα2G α subunitIncreased numbers of single-positive thymic T cells; peripheral T cells that produce high levels of IL-2, interferon-γ, and tumor necrosis factor in response to CD3 cross-linking (51Hornquist C.E Lu X Rogers-Fani P.M Rudolph U Shappell S Birnbaumer L Harriman G.R Gαi2-deficient mice with colitis exhibit a local increase in memory CD4+ T cells and proinflammatory Th1-type cytokines.J. Immunol. 1997; 158: 1068-1077PubMed Google Scholar)SDF-1ChemokineMajor defect in fetal liver and bone marrow B lymphopoiesis (76Nagasawa T Hirota S Tachibana K Takakura N Nishikawa S Kitamura Y Yoshida N Kikutani H Kishimoto T Defects of B-cell lymphopoiesis and bone-marrow myelopoiesis in mice lacking the CXC chemokine PBSF/SDF-1.Nature. 1996; 382: 635-638Crossref PubMed Scopus (2009) Google Scholar)MIP-1αChemokineNo overt abnormality; impaired inflammatory response to influenza virus (23Cook D.N Beck M.A Coffman T.M Kirby S.L Sheridan J.F Pragnell I.B Smithies O Requirement of MIP-1 alpha for an inflammatory response to viral infection.Science. 1995; 269: 1583-1585Crossref PubMed Scopus (561) Google ScholarEotaxinChemokineImpaired eosinophil recruitment following antigen challenge; decreased peripheral eosinophils (90Rothenberg M.E MacLean J.A Pearlman E Luster A.D Leder P Targeted disruption of the chemokine eotaxin partially reduces antigen-induced tissue eosinophilia.J. Exp. Med. 1997; 185: 785-790Crossref PubMed Scopus (473) Google ScholarBLR1Putative chemokineLack of inguinal lymph nodes and germinal receptor centers; migration of lymphocytes into splenic follicles is abnormal (40Forster R Mattis A.E Kremmer E Wolf E Brem G Lipp M A putative chemokine receptor, BLR1, directs B cell migration of defined lymphoid organs and specific anatomic compartments of the spleen.Cell. 1996; 87: 1037-1047Abstract Full Text Full Text PDF PubMed Scopus (954) Google ScholarCCR1Chemokine receptorImpaired neutrophil chemotaxis; no obvious lymphocyte phenotype, although there may be an alteration in the balance between type 1 and type 2 cytokines (42Gao J.-L Wynn T.A Chang Y Lee E.J Broxmeyer H.E Cooper S Tiffany H.L Westphal H Kwon-Chung J Murphy P.M Impaired host defense, hematopoiesis, granulomatous inflammation and type 1-type 2 cytokine balance in mice lacking CC chemokine receptor 1.J. Exp. Med. 1997; 185: 1959-1968Crossref PubMed Scopus (386) Google ScholarCCR2Chemokine receptorImpaired monocyte trafficking (67Kuziel W.A Morgan S.J Dawson T.C Griffin S Smithies O Ley K Maeda N Severe reduction in leukocyte adhesion and monocyte extravasation in mice deficient in CC chemokine receptor 2.Proc. Natl. Acad. Sci. USA. 1997; 94: 12053-12058Crossref PubMed Scopus (564) Google Scholar)IL-8RChemokine receptorImpaired neutrophil migration; possibly, expansion of B cells (15Cacalano G Lee J Kikly K Ryan A.M Pitts-Meek S Hultgren B Wood W.I Moore M.W Neutrophil and B cell expansion in mice that lack the murine IL-8 receptor homolog.Science. 1995; 269: 1590-1591Crossref PubMed Scopus (33) Google Scholar) Open table in a new tab All Gq subfamily members can activate all four PLCβ isoforms, but in a pertussis toxin–independent manner and with a rank order different from that of Gi/o βγ subunits (reviewed by39Fields T.A Casey P.J Signaling functions and biochemical properties of pertussis toxin-resistant G-proteins.Biochem. J. 1997; 321: 561-571Crossref PubMed Scopus (248) Google Scholar). Although the molecular mechanisms remain obscure, some GPCRs activate MAPK via a Gq-stimulated protein kinase C–dependent pathway (47Hawes B.E van Biesen T Koch W.J Luttrell L.M Lefkowitz R.J Distinct pathways of Gi- and Gq-mediated mitogen-activated protein kinase activation.J. Biol. Chem. 1995; 270: 17148-17153Abstract Full Text Full Text PDF PubMed Scopus (413) Google Scholar). GTPase-deficient forms of Gq are oncogenic in NIH3T3, although with low potency. In an illustration of the importance of Gq-mediated signaling, platelets from Gqα-/- mice fail to aggregate, generate IP3, or mobilize calcium in response to physiological activators. Newborn Gqα-/- mice often die of intraabdominal bleeding (80Offermanns S Toombs C.F Hu Y.-H Simon M.I Defective platelet activation in Gαq-deficient mice.Nature. 1997; 389 (a): 183-186Crossref PubMed Scopus (495) Google Scholar). Gq and G11 α subunits are thought to be expressed ubiquitously (98Spicher K Kalkbrenner F Zobel A Harhammer R Nurnberg B Soling A Schultz G G12 and G13 α-subunits are immunochemically detectable in most membranes of various mammalian cells and tissues.Biochem. Biophys. Res. Commun. 1994; 198: 906-914Crossref PubMed Scopus (63) Google Scholar); however, expression levels vary markedly in lymphoid cell lines. Of 13 B and T cell lines tested as well as in thymocytes, tonsil B and T cells, and splenic B cells, only Jurkat cells and 2 B cell lines expressed Gqα. G11α was found in mature B cells and B cell lines and in Jurkat cells, but not in thymocytes or primary T cells (45Grant K.R Harnett W Millignan G Harnett M.M Differential G-protein expression during B- and T-cell development.Immunology. 1997; 90: 564-571Crossref PubMed Scopus (28) Google Scholar). Implicating Gqα in B cell and mast cell function, Gqα activates Bruton's tyrosine kinase (Btk) (7Bence K Ma W Kozasa T Huang X.-Y Direct stimulation of Bruton's tyrosine kinase by Gq-protein α-subunit.Nature. 1997; 389: 296-299Crossref PubMed Scopus (169) Google Scholar). Another Gqα subfamily member, G16α (G15α in the mouse), is expressed in most hematopoietic cells and found at high levels in progenitor B cells. A progressive down-regulation of G16α accompanies B cell differentiation (1Amatruda T.T Steele D.A Slepak V.Z Simon M.I Gα16, a G protein α subunit specifically expressed in hematopoietic cells.Proc. Natl. Acad. Sci. USA. 1991; 88: 5587-5591Crossref PubMed Scopus (240) Google Scholar, 72Mapara M.Y Bommert K Bargou R.C Leng C Beck C Ludwig W.D Gierschik P Dorken B G protein subunit Gα16 expression is restricted to progenitor B cells during human B-cell differentiation.Blood. 1995; 85: 1836-1842Crossref PubMed Google Scholar). Relatively few GPCRs are known to couple to G16α, although in the hemapoietic cell line HEL purine receptors do so (5Baltensperger K Porzig H The P2U purinoceptor obligatorily engages the heterotrimeric G protein G16 to mobilize intracellular Ca2+ in human erythroleukemia cells.J. Biol. Chem. 1997; 272: 10151-10159Crossref PubMed Scopus (58) Google Scholar) The downstream targets of the G12 subfamily members are largely undefined, although the expression of GTPase-deficient forms has provided some hints (reviewed by30Dhanasekaran N Dermott J.M Signaling by the G12 class of G proteins.Cell Signal. 1996; 8: 235-245Crossref PubMed Scopus (128) Google Scholar, 39Fields T.A Casey P.J Signaling functions and biochemical properties of pertussis toxin-resistant G-proteins.Biochem. J. 1997; 321: 561-571Crossref PubMed Scopus (248) Google Scholar). The activation of the small GTPases Ras and Rho, the SAPK pathway, Na+/H+ exchangers, and focal adhesion assemblies have been observed. 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The receptors that couple to G12 and G13 are largely unidentified, and their roles in chemokine signaling as well as lymphocyte function are unknown. Within cells certain key molecules integrate signals from parallel signaling pathways. A good example is the small GTPase Ras, which activates pathways that regulate cellular growth and differentiation (reviewed by57Katz M.E McCormick F Signal transduction from multiple Ras effectors.Curr. Opin. Genet. Dev. 1997; 7: 75-79Crossref PubMed Scopus (275) Google Scholar). A major consequence of Ras activation is stimulation of the MAPK pathway. Growth factor receptors, lymphocyte antigen receptors, and GPCRs, which couple to pertussis toxin–sensitive G proteins, utilize Ras for this purpose (Figure 2). Growth factor receptors that possess intrinsic tyrosine kinase activity undergo ligand-induced autophosphorylation, creating binding sites for docking proteins containing a Src homology 2 (SH2) domain, leading to the recruitment of Ras exchange factors and Ras activation. Although lymphocyte antigen receptors lack intrinsic tyrosine kinase activity, they utilize nonreceptor protein tyrosine kinases to assemble a signaling complex for Ras activation (reviewed by10Berridge M.J Lymphocyte activation in health and disease.Crit. Rev. Immunol. 1997; 17: 155-178Crossref PubMed Google Scholar). Similarly, GPCR-mediated Ras activation requires tyrosine kinases. Free Gi βγ subunits likely activate Src family kinases, leading to the phosphorylation of adapter proteins and members of the focal adhesion kinase family (reviewed by79Offermanns S Simon M.I Organization of transmembrane signaling by heterotrimeric G proteins.Cancer Surv. 1996; 27: 177-198PubMed Google Scholar, 62Kranenburg O Verlaan I Hordijk P.L Moolenaar W.H Gi-mediated activation of the Ras/MAP kinase pathway involves a 100 kDa tyrosine-phosphorylated Grb2 SH3 binding protein, but not Src nor Shc.EMBO J. 1997; 16: 3097-3105Crossref PubMed Scopus (89) Google Scholar). Several GPCR ligands trigger the tyrosine phosphorylation of Pyk2, a focal adhesion kinase family member, and its association with activated Src. Since dominant negative forms of Grb2, Sos, and Pyk2 block MAPK activation, Pyk2 and Src activation may link Gi- and Gq-coupled receptors to the MAPK pathway in certain cell types (31Dikic I Tokiwa G Lev S Courtneidge S.A Schlessinger J A role for Pyk2 and Src in linking G-protein-coupled receptors with MAP kinase activation.Nature. 1996; 383: 547-550Crossref PubMed Scopus (878) Google Scholar, 28Della Rocca G.J van Biesen T Daaka Y Luttrell D.K Luttrell L.M Lefkowitz R.J Ras-dependent mitogen-activated protein kinase activation by G protein-coupled receptors. Convergence of Gi- and Gq-mediated pathways on calcium/calmodulin, Pyk2, and Src kinase.J. Biol. Chem. 1997; 272: 19125-19132Crossref PubMed Scopus (413) Google Scholar). Because T cell receptor (TCR) cross-linking also induces Pyk2 activation in T cells, Pyk2 potentially serves as a convergence point for TCR and GPCR signaling (86Qian D Lev S vanOers N.S.C Dikic I Schlessinger J Weiss A Tyrosine phosphorylation of Pyk2 is selectively regulated by fyn during TCR signaling.J. Exp. Med. 1997; 185: 1253-1259Crossref PubMed Scopus (151) Google Scholar). In some cell types, phosphatidylinositol 3-kinase-γ (PI3Kγ) may be required f
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