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Am J Blood Res. 2011; 1(2): 130–134.
Published online Aug 8, 2011.
PMCID: PMC3232456
NIHMSID: NIHMS341308
Grb2-associated binding (Gab) proteins in hematopoietic and immune cell biology
Tamisha Y Vaughan, Sheetal Verma, and Kevin D Bunting
Department of Pediatrics, Aflac Cancer Center of Children's Healthcare of Atlanta and Emory University, School of Medicine, Atlanta, GA, USA
Address correspondence to: Kevin D. Bunting, Ph.D. Department of Pediatrics, Division of Hem/Onc/BMT & Aflac Cancer Center and Blood Disorders Service, Emory University School of Medicine, 2015 Upper-gate Dr. NE, ECC #444, Atlanta, GA 30322, Tel: 404-778-4039, E-mail: Kevin.bunting/at/emory.edu
Received June 30, 2011; Accepted July 25, 2011.
Grb2-associated binding (Gab) scaffolding/adapter proteins are a family of three members including mammalian Gab1, Gab2, and Gab3 that are highly conserved. Since the discovery of these proteins, there has been an extensive amount of work done to better understand Gab functional roles in multiple signaling pathways, typically acting as a downstream effectors of receptor-tyrosine kinase (RTK)-triggered signal transduction. In addition to their participation in hematopoiesis, Gabs play important roles in regulation of immune response and in also in cancer cell signaling. Gabs may play complex roles and thus a complete understanding of their interactions and how they modulate hematopoietic and immune cell biology remains to be determined. This review will cover the most recent findings including the involvement of Gabs in disease development and signaling which will be important for design of future therapeutic interventions.
Keywords: Adapter protein, cytokine signaling, Grb2-associated binding protein, Gab, receptor tyrosine kinase, cancer signaling
The first Gab molecule was originally identified as the mammalian homolog of the Daughter of Sevenless (DOS) Drosophila adapter protein while also displaying sequence similarity to Suppressor of Clear 1 (Soc1), which was identified by genetic screens in C. elegans [1, 2]. In mammals, there are three family members with high sequence identity, Gab1, Gab2, and Gab3. Since the discovery of these Gabs, it has been a priority to understand their similarities, differences and functions [3-12]. To date, they have been shown to be expressed in a variety of cells such as T cells, B cells, macrophages and mast cells [6, 13-16]. However, Gab3 has been found at only low levels in the hematopoietic system and lymphocytes during steady state [7]. Gab proteins are located in the cytosol until modifications occur such as phosphorylation which then causes migration to the plasma membrane where they function as adapter proteins. Although, all Gab proteins become tyrosine phosphorylated and contain PH domains with a recognition sequence PXXXR for Grb2, the protein-protein interaction is distinct [6,15,17-21]. Importantly, consensus binding sites for SH2 domains of SHP2, p85 regulatory subunit of PI3-K, PLCγ and Crk were identified in all 3 Gabs. The exact role of Gabs remains to be determined.
Gab1, the most widely studied Gab member, was originally isolated as a Grb2-binding protein from a human glial tumor expressing library and found to be tyrosine phosphorylated [22]. Gab1 has a vital role in signal transduction of multiple receptors controlling cell growth, differentiation and function. Interestingly, a major limitation in studying the function of Gab1 is embryonic lethality when knocked out [21, 23], occurring between E13.5 and E18.5 as a result of organ development failure and possibly cell differentiation. Conditional Gab1 KO mice have been generated [11]; however until there is a complete grasp on the redundancies of other Gab members it has not been determined whether Gab1 has in vivo functions in hematopoiesis. Gab2 on the other hand is involved in MAPK/ PI3K pathways and is a highly phosphorylated protein with 10 tyrosine, 18 serine and 5 threonine phosphorylation sites. Deletion of Gab2 in mice results in impaired fertility, lack of allergic responsiveness, and altered mast cell development [4]. Gab2-null bone marrow mast cells have severely impaired IgE-induced mast cell degranulation and largely absent activation of downstream PI3K/AKT signaling [4]. Gab3 was identified for its homology with the other members of the Dos/Gab family. There hasn't been a phenotypic defect associated with the deletion of Gab3, although a role in macrophage cell differentiation has been inconsistent [24-26](Table 1). In this review, we discuss the role of Gabs in signaling.
Table 1
Table 1
Description of Gabs phenotype and health relevance
Phosphorylation of Gab proteins plays several roles in downstream signal activation. All members of the Gab have binding sites for the SH2 domain of PI3-K p85 regulatory subunit. Phosphatidylinositol 3-kinase (PI3K) is a key component of multiple signaling pathways, where it typically promotes survival, proliferation, and/or adhesion [17]. Gab2 has been proven to be a principle activator of PI3K in response to Fc (epsilon)RI activation in vivo [4, 6]. Hepatocyte Growth Factor (HGF), a multipotent protein with several functions which include development functions has been shown to phosphorylate several sites of Gab1 upon activation of its receptor c-Met [27]. In addition, mutations to c-Met receptor show similar defects to that of Gab1 deficiencies [20]. Most recently, studies have been done investigating the protein-protein interaction of Gab1 to Grb2. They were able to eloquently show the cSH3 domain of Grb2 only recognizes two out of four distinct Gab1 RXXK motifs [18]. This signaling information is critical as it leads to better understanding of Gab involvement in health complications such as immune deficiencies and cancer therapeutics.
Cells of the immune system have tyrosine phosphorylated Gab1 and Gab2, occurring by a number of mechanisms including antigen receptors of B and T cells [6,13-15]. In contrast, Gab3 is phosphorylated mostly upon Macrophage-Colony Stimulating Factor (M-CSF) activation [25]. Most recently, Gab1 has been identified to play a role in immune response through macrophage differentiation and toll-like receptor (TLR) signaling [28]. TLRs are key receptors involved in the innate immune processes and play vital roles in host protection against invading pathogens. They function by utilizing downstream signaling molecules and adapter proteins (Figure 1). Macrophage, however, have been identified as a key player in innate immune response as well as playing a role in the initiation and progression of diverse inflammatory diseases.
Figure 1
Figure 1
Gabs involvement in immunity. Activation of TLR3/4 initiates a signaling cascade responsible for the production of proinflammatory cytokines and IFNs. In turn, Gab proteins have the potential to interact with several molecules and modulate the interferon (more ...)
Again, as the first identified family member, Gab1 has been most widely studied in this area. Studies show Gab1 promotes TLR-triggered pro-inflammatory cytokine production by enhancing MAPKs and NFκB activation in macrophage [28]. NFκB activation is not only a key player in inflammation but it plays a novel role in a number of complications including hematological disorders (Figure 1). In addition to triggering proinflammatory cytokines, Gab1 has also been linked to the production of type I interferon by directly binding p85 to activate PI3K/Akt PI3K/ Akt cascade [28]. Interestingly, it has been shown that SHP-2 deficiency increased TBK1-activated IFN-beta and TNF-alpha expression in which Gab members may be involved [29]. SHP-2 also inhibited poly (I: C)-induced activation of MAP kinase pathway which led to SHP-2 specifically negatively regulating TRIF-mediated gene expression in TLR signaling. Work by An et al., demonstrated that SHP-2 negatively regulated TLR4- and TLR3-activated IFN-beta production [29]. However, Gab involvement hasn't been extensively studied.
Interestingly, an investigation of Gab2 was done on the pharmacological effects of an antisense oligonucleotide targeted at Gab2 on the immune responses of rat basophilic leukemic (RBL)-2H3 cells. Not only did they find that Gab2 knockdown inhibits phosphorylation of Akt, p38 mitogen-activated protein kinase and PKCdelta, but knocking down Gab2 under these conditions causes suppression of mast cell functions [30]. These findings give further insight to the Gab family's involvement in immune response and their role as adapter proteins during signaling processes. However, there still remains a gap of knowledge about whether these proteins overlap or have distinct functions.
Gab1 and Gab2 have both been proven to play critical roles in colon, gastric, and breast cancer [10,19,31-33](Table 1). The Bcr-Abl oncoprotein promotes leukemia development by activating a number of signaling pathways regulating cell proliferation, transformation, and survival. Signaling pathways activated by Bcr-Abl include the PI3K-mTOR pathway, the RAS/RAF/MEK/ERK pathway, and the JAK-STAT pathway. However, Gab2 has been most widely characterized for its role in leukemia via its interaction with the BCR-ABL complex. Chronic myelogenous leukemia (CML) is a myeloproliferative disease (MPD) caused by hematopoietic stem cells that possess the Philadelphia chromosome, which encodes the Bcr-Abl oncoprotein. The Bcr-Abl complex has become a target for therapeutic interventions in cancer treatments [8, 34]. A commonly used inhibitor, Imatinib, inhibits kinase activity in Bcr-Abl and has been shown to prolong survival in CML patients [34-36]. However, Gab proteins may contribute to better therapeutics. Currently, Bcr-Abl negative MPD results from mutations in other receptors, most commonly JAK2 [29,30]. New findings show that Bcr -Abl signaling can be suppressed in CML by inhibiting or knocking down Jak2 [34]. Jak2 inhibition subsequently led to the decrease of Grb2-BCR-Abl binding while under normal conditions would lead to phosphorylation of Gab2 involved in binding the regulatory subunit of PI3 kinase [8, 35]. In addition, they also show in three CML lines such modifications of Jak2 not only compromise the signaling complex of Grb2 but reduce phosphorylation of Gab2 after just 30 minutes of inhibitor treatment [34].
In addition to CML, Gab2 plays important roles in Juvenile Myelomonocytic Leukemia (JMML) [34,35,37]. JMML is a myeloproliferative disease (MPD) of early childhood and studies have shown involvement of Gab2 by the Ras pathway. Interestingly, thirty-five percent of patients diagnosed with JMML have activating mutations in phosphatase PTPN11 (Shp2), a ubiquitous tyrosine phosphatase. Gab2 plays an important role in PTPN11 mutations which are associated with this condition [37]. Prior studies have shown how the disruption of Gab2 binding sites for SHP2 [8,19,38] and p85 effect cell growth, migration and activation of downstream cascades. But the Ptpn11 aberrant hematopoietic stem cell activities caused by the mutation were corrected by deleting Gab2 [37]. Although mechanisms aren't clearly understood, advances in understanding how Gabs promote such diseases will be needed for development of new therapeutic interventions.
1. Schutzman JL, Borland CZ, Newman JC, Robinson MK, Kokel M, Stern MJ. The Caenor-habditis elegans EGL-15 signaling pathway implicates a DOS-like multisubstrate adaptor protein in fibroblast growth factor signal trans-duction. Mol Cell Biol. 2001;21:8104–8116. [PMC free article] [PubMed]
2. Herbst R, Carroll PM, Allard JD, Schilling J, Raabe T, Simon MA. Daughter of sevenless is a substrate of the phosphotyrosine phosphatase Corkscrew and functions during sevenless signaling. Cell. 1996;85:899–909. [PubMed]
3. Eulenfeld R, Schaper F. A new mechanism for the regulation of Gab1 recruitment to the plasma membrane. J Cell Sci. 2009;122:55–64. [PubMed]
4. Gu H, Saito K, Klaman LD, Shen J, Fleming T, Wang Y, Pratt JC, Lin G, Lim B, Kinet JP, Neel BG. Essential role for Gab2 in the allergic response. Nature. 2001;412:186–190. [PubMed]
5. Lu Y, Xiong Y, Huo Y, Han J, Yang X, Zhang R, Zhu DS, Klein-Hessling S, Li J, Zhang X, Han X, Li Y, Shen B, He Y, Shibuya M, Feng GS, Luo J. Grb-2-associated binder 1 (Gab1) regulates postnatal ischemic and VEGF-induced angiogenesis through the protein kinase A-endothelial NOS pathway. Proc Natl Acad Sci U S A. 2011;108:2957–2962. [PubMed]
6. Neumann K, Oellerich T, Heine I, Urlaub H, Engelke M. Fc gamma receptor IIb modulates the molecular Grb2 interaction network in activated B cells. Cell Signal. 2011;23:893–900. [PubMed]
7. Nishida K, Hirano T. The role of Gab family scaffolding adapter proteins in the signal transduction of cytokine and growth factor receptors. Cancer Sci. 2003;94:1029–1033. [PubMed]
8. Sattler M, Mohi MG, Pride YB, Quinnan LR, Malouf NA, Podar K, Gesbert F, Iwasaki H, Li S, Van Etten RA, Gu H, Griffin JD, Neel BG. Critical role for Gab2 in transformation by BCR/ABL. Cancer Cell. 2002;1:479–492. [PubMed]
9. Schjeide BM, Hooli B, Parkinson M, Hogan MF, DiVito J, Mullin K, Blacker D, Tanzi RE, Bertram L. GAB2 as an Alzheimer disease susceptibility gene: follow-up of genomewide association results. Arch Neurol. 2009;66:250–254. [PMC free article] [PubMed]
10. Seiden-Long I, Navab R, Shih W, Li M, Chow J, Zhu CQ, Radulovich N, Saucier C, Tsao MS. Gab1 but not Grb2 mediates tumor progression in Met overexpressing colorectal cancer cells. Carcinogenesis. 2008;29:647–655. [PubMed]
11. Shioyama W, Nakaoka Y, Higuchi K, Minami T, Taniyama Y, Nishida K, Kidoya H, Sonobe T, Naito H, Arita Y, Hashimoto T, Kuroda T, Fujio Y, Shirai M, Takakura N, Morishita R, Yamauchi-Takihara K, Kodama T, Hirano T, Mochizuki N, Komuro I. Docking protein Gab1 is an essential component of postnatal angiogenesis after ischemia via HGF/c-met signaling. Circ Res. 2011;108:664–675. [PubMed]
12. Wohrle FU, Daly RJ, Brummer T. Function, regulation and pathological roles of the Gab/ DOS docking proteins. Cell Commun Signal. 2009;7:22. [PMC free article] [PubMed]
13. Angyal A, Medgyesi D, Sarmay G. Grb2-associated binder 1 (Gab1) adaptor/ scaffolding protein regulates Erk signal in human B cells. Ann N Y Acad Sci. 2006;1090:326–331. [PubMed]
14. Hibi M, Hirano T. Gab-family adapter molecules in signal transduction of cytokine and growth factor receptors, and T and B cell antigen receptors. Leuk Lymphoma. 2000;37:299–307. [PubMed]
15. Pratt JC, Igras VE, Maeda H, Baksh S, Gelfand EW, Burakoff SJ, Neel BG, Gu H. Cutting edge: gab2 mediates an inhibitory phosphati-dylinositol 3'-kinase pathway in T cell antigen receptor signaling. J Immunol. 2000;165:4158–4163. [PubMed]
16. Sarmay G, Angyal A, Kertesz A, Maus M, Medgyesi D. The multiple function of Grb2 associated binder (Gab) adaptor/scaffolding protein in immune cell signaling. Immunol Lett. 2006;104:76–82. [PubMed]
17. Gu H, Neel BG. The “Gab” in signal transduction. Trends Cell Biol. 2003;13:122–130. [PubMed]
18. McDonald CB, Seldeen KL, Deegan BJ, Bhat V, Farooq A. Binding of the cSH3 domain of Grb2 adaptor to two distinct RXXK motifs within Gab1 docker employs differential mechanisms. J Mol Recognit. 2011;24:585–596. [PMC free article] [PubMed]
19. Meng S, Chen Z, Munoz-Antonia T, Wu J. Participation of both Gab1 and Gab2 in the activation of the ERK/MAPK pathway by epidermal growth factor. Biochem J. 2005;391:143–151. [PubMed]
20. Sachs M, Brohmann H, Zechner D, Muller T, Hulsken J, Walther I, Schaeper U, Birchmeier C, Birchmeier W. Essential role of Gab1 for signaling by the c-Met receptor in vivo. J Cell Biol. 2000;150:1375–1384. [PMC free article] [PubMed]
21. Schaeper U, Gehring NH, Fuchs KP, Sachs M, Kempkes B, Birchmeier W. Coupling of Gab1 to c-Met, Grb2, and Shp2 mediates biological responses. J Cell Biol. 2000;149:1419–1432. [PMC free article] [PubMed]
22. Holgado-Madruga M, Emlet DR, Moscatello DK, Godwin AK, Wong AJ. A Grb2-associated docking protein in EGF- and insulin -receptor signalling. Nature. 1996;379:560–564. [PubMed]
23. Itoh M, Yoshida Y, Nishida K, Narimatsu M, Hibi M, Hirano T. Role of Gab1 in heart, placenta, and skin development and growth factor- and cytokine-induced extracellular signal-regulated kinase mitogen-activated protein kinase activation. Mol Cell Biol. 2000;20:3695–3704. [PMC free article] [PubMed]
24. Wolf I, Jenkins BJ, Liu Y, Seiffert M, Custodio JM, Young P, Rohrschneider LR. Gab3, a new DOS/Gab family member, facilitates macrophage differentiation. Mol Cell Biol. 2002;22:231–244. [PMC free article] [PubMed]
25. Bourgin C, Bourette RP, Arnaud S, Liu Y, Rohrschneider LR, Mouchiroud G. Induced expression and association of the Mona/Gads adapter and Gab3 scaffolding protein during monocyte/macrophage differentiation. Mol Cell Biol. 2002;22:3744–3756. [PMC free article] [PubMed]
26. Seiffert M, Custodio JM, Wolf I, Harkey M, Liu Y, Blattman JN, Greenberg PD, Rohrschneider LR. Gab3-deficient mice exhibit normal development and hematopoiesis and are immunocompetent. Mol Cell Biol. 2003;23:2415–2424. [PMC free article] [PubMed]
27. Sakkab D, Lewitzky M, Posern G, Schaeper U, Sachs M, Birchmeier W, Feller SM. Signaling of hepatocyte growth factor/scatter factor (HGF) to the small GTPase Rap1 via the large docking protein Gab1 and the adapter protein CRKL. J Biol Chem. 2000;275:10772–10778. [PubMed]
28. Zheng Y, An H, Yao M, Hou J, Yu Y, Feng G, Cao X. Scaffolding adaptor protein Gab1 is required for TLR3/4- and RIG-I-mediated production of proinflammatory cytokines and type I IFN in macrophages. J Immunol. 2010;184:6447–6456. [PubMed]
29. An H, Zhao W, Hou J, Zhang Y, Xie Y, Zheng Y, Xu H, Qian C, Zhou J, Yu Y, Liu S, Feng G, Cao X. SHP-2 phosphatase negatively regulates the TRIF adaptor protein-dependent type I interferon and proinflammatory cytokine production. Immunity. 2006;25:919–928. [PubMed]
30. Chan JH, Liao W, Lau HY, Wong WS. Gab2 antisense oligonucleotide blocks rat basophilic leukemic cell functions. Int Immunopharmacol. 2007;7:937–944. [PubMed]
31. Lee SH, Jeong EG, Nam SW, Lee JY, Yoo NJ. Increased expression of Gab2, a scaffolding adaptor of the tyrosine kinase signalling, in gastric carcinomas. Pathology. 2007;39:326–329. [PubMed]
32. Bocanegra M, Bergamaschi A, Kim YH, Miller MA, Rajput AB, Kao J, Langerod A, Han W, Noh DY, Jeffrey SS, Huntsman DG, Borresen-Dale AL, Pollack JR. Focal amplification and oncogene dependency of GAB2 in breast cancer. Oncogene. 2010;29:774–779. [PubMed]
33. Chernoff KA, Bordone L, Horst B, Simon K, Twadell W, Lee K, Cohen JA, Wang S, Silvers DN, Brunner G, Celebi JT. GAB2 amplifications refine molecular classification of melanoma. Clin Cancer Res. 2009;15:4288–4291. [PMC free article] [PubMed]
34. Samanta A, Perazzona B, Chakraborty S, Sun X, Modi H, Bhatia R, Priebe W, Arlinghaus R. Janus kinase 2 regulates Bcr-Abl signaling in chronic myeloid leukemia. Leukemia. 2011;25:463–472. [PMC free article] [PubMed]
35. Samanta AK, Lin H, Sun T, Kantarjian H, Arlinghaus RB. Janus kinase 2: a critical target in chronic myelogenous leukemia. Cancer Res. 2006;66:6468–6472. [PubMed]
36. Warsch W, Kollmann K, Eckelhart E, Fajmann S, Cerny-Reiterer S, Holbl A, Gleixner KV, Dworzak M, Mayerhofer M, Hoermann G, Herrmann H, Sillaber C, Egger G, Valent P, Moriggl R, Sexl V. High STAT5 levels mediate imatinib resistance and indicate disease progression in chronic myeloid leukemia. Blood. 2011;117:3409–3420. [PubMed]
37. Xu D, Wang S, Yu WM, Chan G, Araki T, Bunting KD, Neel BG, Qu CK. A germline gain-of-function mutation in Ptpn11 (Shp-2) phosphatase induces myeloproliferative disease by aberrant activation of hematopoietic stem cells. Blood. 2010;116:3611–3621. [PubMed]
38. Caron C, Spring K, Laramee M, Chabot C, Cloutier M, Gu H, Royal I. Non-redundant roles of the Gab1 and Gab2 scaffolding adapters in VEGF-mediated signalling, migration, and survival of endothelial cells. Cell Signal. 2009;21:943–953. [PubMed]
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