Activated ERK2 interacts with and phosphorylates the docking protein GAB1

The Role of GAB1 in Cancer

GRB2-associated binder 1 (GAB1) is the inaugural member of the GAB/DOS family of pleckstrin homology (PH) domain-containing proteins. Upon receiving various stimuli, GAB1 transitions from the cytoplasm to the membrane where it is phosphorylated by a range of kinases. This event recruits SH2 domain-containing proteins like SHP2, PI3K's p85 subunit, CRK, and others, thereby activating distinct signaling pathways, including MAPK, PI3K/AKT, and JNK. GAB1-deficient embryos succumb in utero, presenting with developmental abnormalities in the heart, placenta, liver, skin, limb, and diaphragm myocytes. Oncogenic mutations have been identified in the context of cancer. GAB1 expression levels are disrupted in various tumors, and elevated levels in patients often portend a worse prognosis in multiple cancer types. This review focuses on GAB1's influence on cellular transformation particularly in proliferation, evasion of apoptosis, metastasis, and angiogenesis-each of these processes being a cancer hallmark. GAB1 also modulates the resistance/sensitivity to antitumor therapies, making it a promising target for future anticancer strategies.

Phosphorylation of threonine residues on Shc promotes ligand binding and mediates crosstalk between MAPK and Akt pathways in breast cancer cells

Scaffold proteins play important roles in regulating signalling network fidelity, the absence of which is often the basis for diseases such as cancer. In the present work, we show that the prototypical scaffold protein Shc is phosphorylated by the extracellular signal-regulated kinase, Erk. In addition, Shc threonine phosphorylation is specifically up-regulated in two selected triple-negative breast cancer (TNBC) cell lines. To explore how Erk-mediated threonine phosphorylation on Shc might play a role in the dysregulation of signalling events, we investigated how Shc affects pathways downstream of EGF receptor. Using an in vitro model and biophysical analysis, we show that Shc threonine phosphorylation is responsible for elevated Akt and Erk signalling, potentially through the recruitment of the 14-3-3 ζ and Pin-1 proteins.

Extracellular Signal-Regulated Kinases 1 and 2 Phosphorylate Gab2 To Promote a Negative-Feedback Loop That Attenuates Phosphoinositide 3-Kinase/Akt Signaling

The scaffolding adapter protein Gab2 (Grb2-associated binder) promotes cell proliferation, survival, and motility by engaging several signaling pathways downstream of growth factor and cytokine receptors. In particular, Gab2 plays essential roles in mast cells, as it is required for phosphoinositide 3-kinase (PI3K) activation in response to Kit and the high-affinity IgE receptor. While the positive role of Gab2 in PI3K signaling is well documented, very little is known about the mechanisms that attenuate its function. Here we show that Gab2 becomes phosphorylated on multiple proline-directed sites upon stimulation of the Ras/extracellular signal-regulated kinase (ERK) signaling pathway. We demonstrate that ERK1 and ERK2 interact with Gab2 via a novel docking motif, which is required for subsequent Gab2 phosphorylation in response to ERK1/2 activation. We identified four ERK1/2-dependent phosphorylation sites in Gab2 that prevent the recruitment of the p85 regulatory subunit of PI3K. Using bone marrow-derived mast cells to study Gab2-dependent signaling, we found that the inhibition of ERK1/2 activity promotes Akt signaling in response to Kit and the high-affinity IgE receptor. Together, our results indicate that ERK1/2 participates in a negative-feedback loop that attenuates PI3K/Akt signaling in response to various agonists.

Essential roles of Gab1 tyrosine phosphorylation in growth factor-mediated signaling and angiogenesis

Growth factors and their downstream receptor tyrosine kinases (RTKs) mediate a number of biological processes controlling cell function. Adaptor (docking) proteins, which consist exclusively of domains and motifs that mediate molecular interactions, link receptor activation to downstream effectors. Recent studies have revealed that Grb2-associated-binders (Gab) family members (including Gab1, Gab2, and Gab3), when phosphorylated on tyrosine residues, provide binding sites for multiple effector proteins, such as Src homology-2 (SH2)-containing protein tyrosine phosphatase 2 (SHP2) and phosphatidylinositol 3-kinase (PI3K) regulatory subunit p85, thereby playing important roles in transducing RTKs-mediated signals into pathways with diversified biological functions. Here, we provide an up-to-date overview on the domain structure and biological functions of Gab1, the most intensively studied Gab family protein, in growth factor signaling and biological functions, with a special focus on angiogenesis.

Receptor tyrosine kinase (RTK) signalling in the control of neural stem and progenitor cell (NSPC) development

Important developmental responses are elicited in neural stem and progenitor cells (NSPC) by activation of the receptor tyrosine kinases (RTK), including the fibroblast growth factor receptors, epidermal growth factor receptor, platelet-derived growth factor receptors and insulin-like growth factor receptor (IGF1R). Signalling through these RTK is necessary and sufficient for driving a number of developmental processes in the central nervous system. Within each of the four RTK families discussed here, receptors are activated by sets of ligands that do not cross-activate receptors of the other three families, and therefore, their activation can be independently regulated by ligand availability. These RTK pathways converge on a conserved core of signalling molecules, but differences between the receptors in utilisation of signalling molecules and molecular adaptors for intracellular signal propagation become increasingly apparent. Intracellular inhibitors of RTK signalling are widely involved in the regulation of developmental signalling in NSPC and often determine developmental outcomes of RTK activation. In addition, cellular responses of NSPC to the activation of a given RTK may be significantly modulated by signal strength. Cellular propensity to respond also plays a role in developmental outcomes of RTK signalling. In combination, these mechanisms regulate the balance between NSPC maintenance and differentiation during development and in adulthood. Attribution of particular developmental responses of NSPC to specific pathways of RTK signalling becomes increasingly elucidated. Co-activation of several RTK in developing NSPC is common, and analysis of co-operation between their signalling pathways may advance knowledge of RTK role in NSPC development.

MEK inhibition leads to PI3K/AKT activation by relieving a negative feedback on ERBB receptors

The phosphoinositide 3-kinase (PI3K)/AKT and RAF/MEK/ERK signaling pathways are activated in a wide range of human cancers. In many cases, concomitant inhibition of both pathways is necessary to block proliferation and induce cell death and tumor shrinkage. Several feedback systems have been described in which inhibition of one intracellular pathway leads to activation of a parallel signaling pathway, thereby decreasing the effectiveness of single-agent targeted therapies. In this study, we describe a feedback mechanism in which MEK inhibition leads to activation of PI3K/AKT signaling in EGFR and HER2-driven cancers. We found that MEK inhibitor-induced activation of PI3K/AKT resulted from hyperactivation of ERBB3 as a result of the loss of an inhibitory threonine phosphorylation in the conserved juxtamembrane domains of EGFR and HER2. Mutation of this amino acid led to increased ERBB receptor activation and upregulation of the ERBB3/PI3K/AKT signaling pathway, which was no longer responsive to MEK inhibition. Taken together, these results elucidate an important, dominant feedback network regulating central oncogenic pathways in human cancer.

Function, regulation and pathological roles of the Gab/DOS docking proteins

Since their discovery a little more than a decade ago, the docking proteins of the Gab/DOS family have emerged as important signalling elements in metazoans. Gab/DOS proteins integrate and amplify signals from a wide variety of sources including growth factor, cytokine and antigen receptors as well as cell adhesion molecules. They also contribute to signal diversification by channelling the information from activated receptors into signalling pathways with distinct biological functions. Recent approaches in protein biochemistry and systems biology have revealed that Gab proteins are subject to complex regulation by feed-forward and feedback phosphorylation events as well as protein-protein interactions. Thus, Gab/DOS docking proteins are at the centre of entire signalling subsystems and fulfil an important if not essential role in many physiological processes. Furthermore, aberrant signalling by Gab proteins has been increasingly linked to human diseases from various forms of neoplasia to Alzheimer's disease.

In this review, we provide a detailed overview of the structure, effector functions, regulation and evolution of the Gab/DOS family. We also summarize recent findings implicating Gab proteins, in particular the Gab2 isoform, in leukaemia, solid tumours and other human diseases.

Crk adaptor protein-induced phosphorylation of Gab1 on tyrosine 307 via Src is important for organization of focal adhesions and enhanced cell migration

Upon growth factor stimulation, the scaffold protein, Gab1, is tyrosine phosphorylated and subsequently the adaptor protein, Crk, transmits signals from Gab1. We have previously shown that Crk overexpression, which is detectable in various human cancers, induces tyrosine phosphorylation of Gab1 without extracellular stimuli. In the present study, the underlying mechanisms were further investigated. Mutational analyses of CrkII demonstrated that the SH2 domain, but not the SH3(N) or the regulatory Y221 residue of CrkII, is critical for the induction of Gab1-Y307 phosphorylation. SH2 mutation of CrkII also decreased the interaction with Gab1. In GST pull-down assay, Crk-SH2 bound to wild-type Gab1, whereas Crk-SH3(N) interacted with the Gab1 mutant, which lacks the clustered tyrosine region (residues 242-410). Tyrosine phosphorylation of Gab1 was induced by all Crk family proteins, but not other SH2-containing signalling adaptors. Src-family kinase inhibitor, PP2, abrogates Crk-induced tyrosine phosphorylations of Gab1. Y307 phosphorylation was undetectable in fibroblasts lacking Src, Yes, and Fyn, even upon overexpression of Crk, whereas cells lacking only Yes and Fyn still contained Gab1 with phosphorylated Y307. Furthermore, Crk induced the phosphorylation of Src-Y416; accordingly the interaction between Crk and Csk was increased. The Gab1-Y307F mutant failed to localize near the plasma membrane even upon HGF stimulation and decreased cell migration. Moreover, Gab1-Y307F disturbed the localization of Crk, FAK, and paxillin, which are the typical components of focal adhesions. Taken together, these results indicate that Crk facilitates tyrosine phosphorylation of Gab1-Y307 through Src, contributing to the organization of focal adhesions and enhanced cell migration, thereby possibly promoting human cancer development.

Identification of ERK2-binding domain of EBITEIN1, a novel ERK2-binding protein

We recently cloned a cDNA encoding a novel extracellular signal-regulated kinase 2 (ERK2) binding protein, EBITEIN1, by yeast two-hybrid screening. In this study, we further characterized EBITEIN1. Binding experiments using various deletion mutants identified a 40-amino acid minimal sequence for binding ERK2. Binding experiments using substitution mutants indicated the crucial role of arginine residues in this sequence. Based on empirical and bioinformatic analyses, we propose two domains in EBITEIN1. One is the minimal sequence for binding ERK2 (EB domain) and the other is the EBITEIN1 C-terminal domain (ECT domain). These results might pave the way for further empirical and bioinformatic analyses of EBITEIN1- and ERK2-mediated events.

Oscillatory dynamics arising from competitive inhibition and multisite phosphorylation

There have been a growing number of observations of oscillating protein levels (p53 and NFkB) in eukaryotic signalling pathways. This has resulted in a renewed interest in the mechanism by which such oscillations might occur. Recent computational work has shown that a multisite phosphorylation mechanism such as that found in the MAPK cascade can theoretically exhibit bistability. The bistable behavior was shown to arise from sequestration and saturation mechanisms for the enzymes that catalyse the multisite phosphorylation cycle. These effects generate the positive feedback necessary for bistability. In this paper we describe two kinds of oscillatory dynamics which can occur in a network by which, both use such bistable multisite phosphorylated cycles. In the first example, the fully phosphorylated form of the phosphorylated cycle represses the production of the kinase, which carries out the phosphorylation of the unphosphorylated states of the cycle. The dynamics of this system leads to a relaxation oscillator. In the second example, we consider a cascade of two cycles, in which the fully phosphorylated form of the kinase, in the first cycle, phosphorylates the unphosphorylated forms in the second cycle. A feedback loop, by which the fully phosphorylated form of the second cycle inhibits the kinase step in the first cycle is also present. In this case we obtain a ring oscillator. Both these networks illustrate the versatility of the multisite bistable network.

Scaffolding protein Grb2-associated binder 1 sustains epidermal growth factor-induced mitogenic and survival signaling by multiple positive feedback loops

Grb2-associated binder 1 (GAB1) is a scaffold protein involved in numerous interactions that propagate signaling by growth factor and cytokine receptors. Here we explore in silico and validate in vivo the role of GAB1 in the control of mitogenic (Ras/MAPK) and survival (phosphatidylinositol 3-kinase (PI3K)/Akt) signaling stimulated by epidermal growth factor (EGF). We built a comprehensive mechanistic model that allows for reliable predictions of temporal patterns of cellular responses to EGF under diverse perturbations, including different EGF doses, GAB1 suppression, expression of mutant proteins, and pharmacological inhibitors. We show that the temporal dynamics of GAB1 tyrosine phosphorylation is significantly controlled by positive GAB1-PI3K feedback and negative MAPK-GAB1 feedback. Our experimental and computational results demonstrate that the essential function of GAB1 is to enhance PI3K/Akt activation and extend the duration of Ras/MAPK signaling. By amplifying positive interactions between survival and mitogenic pathways, GAB1 plays the critical role in cell proliferation and tumorigenesis.

MET meet adaptors: functional and structural implications in downstream signalling mediated by the Met receptor

The tyrosin kinase Met receptor regulates multiple cellular events, ranging from cell motility and angiogenesis to morphological differentiation and tissue regeneration. To conduce these activities, the cytoplasmic C-terminal region of this receptor acts as a docking site for multiple protein substrates, including Grb 2, Gab 1, STAT 3, Shc, SHIP-1 and Src. These substrates are characterised by the presence of multiple domains, including the PH, PTB, SH 2 and SH 3 domains, which directly interact with the multisubstrate C-terminal region of Met. How this receptor recognises and binds a specific substrate in a space-temporal mode is a central question in cell signalling. The recently solved crystal structure of the tyrosine kinase domain of the Met receptor and that of domains of diverse Met substrates provides the molecular framework to understand Met substrate specificity. This structural information also gives new insights on the plasticity of Met signalling and the implications of Met deregulation in tumorigenic processes. In the light of these advances, the present work discusses the molecular basis of Met-substrate recognition and its functional implications in signalling events mediated by this pleiotropic receptor.

Phosphorylation of Grb2-Associated Binder 2 on Serine 623 by ERK MAPK Regulates Its Association with the Phosphatase SHP-2 and Decreases STAT5 Activation

IL-2 stimulation of T lymphocytes induces the tyrosine phosphorylation and adaptor function of the insulin receptor substrate/Grb2-associated binder (Gab) family member, Gab2. In addition, Gab2 undergoes a marked decrease in its mobility in SDS-PAGE, characteristic of migration shifts induced by serine/threonine phosphorylations in many proteins. This migration shift was strongly diminished by treating cells with the MEK inhibitor U0126, indicating a possible role for ERK in Gab2 phosphorylation. Indeed, ERK phosphorylated Gab2 on a consensus phosphorylation site at serine 623, a residue located between tyrosine 614 and tyrosine 643 that are responsible for Gab2/Src homology 2 domain-containing tyrosine phosphatase (SHP)-2 interaction. We report that pretreatment of Kit 225 cells with U0126 increased Gab2/SHP-2 association and tyrosine phosphorylation of SHP-2 in response to IL-2, suggesting that ERK phosphorylation of serine 623 regulates the interaction between Gab2 and SHP-2, and consequently the activity of SHP-2. This hypothesis was confirmed by biochemical analysis of cells expressing Gab2 WT, Gab2 serine 623A or Gab2 tyrosine 614F, a mutant that cannot interact with SHP-2 in response to IL-2. Activation of the ERK pathway was indeed blocked by Gab2 tyrosine 614F and slightly increased by Gab2 serine 623A. In contrast, STAT5 activation was strongly enhanced by Gab2 tyrosine 614F, slightly reduced by Gab2 WT and strongly inhibited by Gab2 serine 623A. Analysis of the rate of proliferation of cells expressing these mutants of Gab2 demonstrated that tyrosine 614F mutation enhanced proliferation whereas serine 623A diminished it. These results demonstrate that ERK-mediated phosphorylation of Gab2 serine 623 is involved in fine tuning the proliferative response of T lymphocytes to IL-2.

Extracellular signal-regulated kinase activated by epidermal growth factor and cell adhesion interacts with and phosphorylates vinexin

Extracellular signal-regulated kinase 1/2 (ERK1/2) is activated by various extracellular stimuli including growth factors and cytokines and plays a pivotal role in regulating cell proliferation and differentiation by phosphorylating nuclear transcription factors. Recently, it was reported that activated ERK1/2 also concentrates at adhesion sites and regulates cell spreading and migration. Vinexin is a focal adhesion protein regulating both cell spreading and growth factor signaling. We show here that vinexin was directly phosphorylated by ERK1/2 upon stimulation with growth factors. ERK1/2 phosphorylated the linker region of vinexin between the second and third SH3 domains. Site-directed mutagenesis revealed that ERK2 mainly phosphorylated the serine 189 residue of vinexin beta. Furthermore, vinexin beta interacted with ERK1/2 both in vitro and in vivo. Vinexin interacted with the active but not inactive form of ERK1/2. A putative DEF (docking for ERK FXFP) domain located in the linker region of vinexin was required for the interaction with ERK1/2 and efficient phosphorylation of vinexin beta by ERK2. Finally, we showed that cell adhesion to fibronectin also induced the association of vinexin beta with ERK2 and the phosphorylation of vinexin beta. Furthermore, vinexin and ERK were co-localized to the periphery of cells during cell spreading on fibronectin. Together, these results suggest that vinexin is a novel substrate of ERK2 and may play roles in ERK-dependent cell regulation during cell spreading as well as in growth factor-induced responses.

ERK1/2 associates with the c-Met-binding domain of growth factor receptor-bound protein 2 (Grb2)-associated binder-1 (Gab1): role in ERK1/2 and early growth response factor-1 (Egr-1) nuclear accumulation

Endothelial cell (EC) migration contributes to reendothelialization after angioplasty or rupture of atherosclerotic plaques. Extracellular signal-regulated kinase (ERK)1/2 translocates to the nucleus and activates transcription factors such as Ets-like transcription factor-1 and early growth response factor-1 (Egr-1) during reendothelialization. Because ERK1/2 does not possess a nuclear localization signal (NLS), its mechanism of translocation and accumulation in the nucleus remains unclear. Because Gab1 has a putative NLS in its N-terminal region, and Gab1 associates with phosphorylated ERK1/2, we hypothesized that Gab1 participates in ERK1/2 and Egr-1 nuclear accumulation. Using regenerating EC as a model system, we found that endogenous growth factor receptor-bound protein 2-associated binder-1 (Gab1) translocates into the nucleus in migrating EC. Wild-type red fluorescent protein-tagged Gab1 could be observed in both nucleus and cytoplasm, whereas the putative NLS deletion mutant (deltaNLS-Gab1) specifically localized in the cytoplasm. In addition, reduction of Gab1 expression by antisense Gab1 oligos or overexpression of deltaNLS-Gab1 inhibited serum-induced ERK1/2 and Egr-1 nuclear accumulation, suggesting a functional role for the NLS of Gab1 and a role for Gab1-ERK1/2 interactions in ERK1/2-Egr-1 nuclear accumulation. To investigate whether Gab1-ERK1/2 interaction is critical for ERK1/2 and Egr-1 nuclear accumulation, we created a dominant-negative Gab1 construct that consisted of the c-Met binding domain (amino acids 442-536) of Gab1. We found that overexpression of the c-Met binding domain of Gab1 disrupted serum-induced Gab1-ERK1 interaction and inhibited ERK1 and Egr-1 nuclear accumulation. These data suggest that Gab1-ERK1/2 binding and their nuclear translocation play a crucial role in Egr-1 nuclear accumulation.

Dual Phosphorylation of Phosphoinositide 3-Kinase Adaptor Grb2-Associated Binder 2 Is Responsible for Superoxide Formation Synergistically Stimulated by Fcγ and Formyl-Methionyl-Leucyl-Phenylalanine Receptors in Differentiated THP-1 Cells

The class Ia phosphoinositide (PI) 3-kinase consisting of p110 catalytic and p85 regulatory subunits is activated by Tyr kinase-linked membrane receptors such as FcgammaRII through the association of p85 with the phosphorylated receptors or adaptors. The heterodimeric PI 3-kinase is also activated by G protein-coupled chemotactic fMLP receptors, and activation of the lipid kinase plays an important role in various immune responses, including superoxide formation in neutrophils. Although fMLP-induced superoxide formation is markedly enhanced in FcgammaRII-primed neutrophils, the molecular mechanisms remain poorly characterized. In this study, we identified two Tyr-phosphorylated proteins, c-Cbl (Casitas B-lineage lymphoma) and Grb2-associated binder 2 (Gab2), as PI 3-kinase adaptors that are Tyr phosphorylated upon the stimulation of FcgammaRII in differentiated neutrophil-like THP-1 cells. Interestingly, Gab2 was, but c-Cbl was not, further Ser/Thr phosphorylated by fMLP. Thus, the adaptor Gab2 appeared to be dually phosphorylated at the Ser/Thr and Tyr residues through the two different types of membrane receptors. The Ser/Thr phosphorylation of Gab2 required the activation of extracellular signal-regulated kinase, and fMLP receptor stimulation indeed activated extracellular signal-regulated kinase in the cells. Enhanced superoxide formation in response to Fcgamma and fMLP was markedly attenuated when the Gab2 Ser/Thr phosphorylation was inhibited. These results show the importance of the dual phosphorylation of PI 3-kinase adaptor Gab2 for the enhanced superoxide formation in neutrophil-type cells.

GC-GAP, a Rho family GTPase-activating protein that interacts with signaling adapters Gab1 and Gab2

Gab1 and Gab2 are scaffolding proteins acting downstream of cell surface receptors and interact with a variety of cytoplasmic signaling proteins such as Grb2, Shp-2, phosphatidylinositol 3-kinase, Shc, and Crk. To identify new binding partners for GAB proteins and better understand their functions, we performed a yeast two-hybrid screening with hGab2-(120-587) as bait. This work led to identification of a novel GTPase-activating protein (GAP) for Rho family GTPases. The GAP domain shows high similarity to the recently cloned CdGAP and displays activity toward RhoA, Rac1, and Cdc42 in vitro. The protein was named GC-GAP for its ability to interact with GAB proteins and its activity toward Rac and Cdc42. GC-GAP is predominantly expressed in the brain with low levels detected in other tissues. Antibodies directed against GC-GAP recognized a protein of approximately 200 kDa. Expression of GC-GAP in 293T cells led to a reduction in active Rac1 and Cdc42 levels but not RhoA. Suppression of GC-GAP expression by siRNA inhibited proliferation of C6 astroglioma cells. In addition, GC-GAP contains several classic proline-rich motifs, and it interacts with the first SH3 domain of Crk and full-length Nck in vitro. We propose that Gab1 and Gab2 in cooperation with other adapter molecules might regulate the cellular localization of GC-GAP under specific stimuli, acting to regulate precisely Rac and Cdc42 activities. Given that GC-GAP is specifically expressed in the nervous system and that it is localized to the dendritic processes of cultured neurons, GC-GAP may play a role in dendritic morphogenesis and also possibly in neural/glial cell proliferation.

Grb2-independent recruitment of Gab1 requires the C-terminal lobe and structural integrity of the Met receptor kinase domain

The Gab1 docking protein forms a platform for the assembly of a multiprotein signaling complex downstream from receptor tyrosine kinases. In general, recruitment of Gab1 occurs indirectly, via the adapter protein Grb2. In addition, Gab1 interacts with the Met/hepatocyte growth factor receptor in a Grb2-independent manner. This interaction requires a Met binding domain (MBD) in Gab1 and is essential for Met-mediated epithelial morphogenesis. The Gab1 MBD has been proposed to act as a phosphotyrosine binding domain that binds Tyr-1349 in the Met receptor. We show that a 16-amino acid motif within the Gab1 MBD is sufficient for interaction with the Met receptor, suggesting that it is unlikely that the Gab1 MBD forms a structured domain. Alternatively, the structural integrity of the Met receptor, and residues upstream of Tyr-1349 located in the C-terminal lobe of the kinase domain, are required for Grb2-independent interaction with the Gab1 MBD. Moreover, the substitution of Tyr-1349 with an acidic residue allows for the recruitment of the Gab1 MBD and for phosphorylation of Gab1. We propose that Gab1 and the Met receptor interact in a novel manner, such that the activated kinase domain of Met and the negative charge of phosphotyrosine 1349 engage the Gab1 MBD as an extended peptide ligand.

EGFR and FGFR signaling through FRS2 is subject to negative feedback control by ERK1/2

Fibroblast growth factor (FGF) receptor substrate 2 (FRS2) is a membrane-anchored docking protein that has been shown to play an important role in linking FGF, nerve growth factor (NGF) and glial cell-derived neurotrophic factor (GDNF) receptors with the Ras/mitogen-activated protein (MAP) kinase signaling cascade. Here we provide evidence that FRS2 can also play a role in epidermal growth factor (EGF) signaling. Upon EGF stimulation, FRS2 mediates enhanced MAPK activity and undergoes phosphorylation on tyrosine as well as serine/threonine residues. This involves the direct interaction of the FRS2 PTB domain with the EGFR and results in a significantly altered mobility of FRS2 in SDS-PAGE which is also observed in FGF stimulated cells. This migration shift of FRS2 is completely abrogated by U0126, a specific MAPK kinase 1 (MEK1) inhibitor, suggesting that ERK1/2 acts as serine/threonine kinase upstream of FRS2. Indeed, we show that the central portion of FRS2 constitutively associates with ERK1/2, whereas the FRS2 carboxy-terminal region serves as substrate for ERK2 phosphorylation in response to EGF and FGF stimulation. Notably, tyrosine phosphorylation of FRS2 is enhanced when ERK1/2 activation is inhibited after both EGF and FGF stimulation. These results indicate a ligand-stimulated negative regulatory feedback loop in which activated ERK1/2 phosphorylates FRS2 on serine/threonine residues thereby down-regulating its tyrosine phosphorylation. Our findings support a broader role of FRS2 in EGFR-controlled signaling pathways in A-431 cells and provide insight into a molecular mechanism for ligand-stimulated feedback regulation with FRS2 as a central regulatory switch point.

The "Gab" in signal transduction

Tyrosine phosphorylation plays an important role in controlling cellular growth, differentiation and function. Abnormal regulation of tyrosine phosphorylation can result in human diseases such as cancer. A major challenge of signal transduction research is to determine how the initial activation of protein-tyrosine kinases (PTKs) by extracellular stimuli triggers multiple downstream signaling cascades, which ultimately elicit diverse cellular responses. Recent studies reveal that members of the Gab/Dos subfamily of scaffolding adaptor proteins (hereafter, "Gab proteins") play a crucial role in transmitting key signals that control cell growth, differentiation and function from multiple receptors. Here, we review the structure, mechanism of action and function of these interesting molecules in normal biology and disease.

Distinct recruitment and function of Gab1 and Gab2 in Met receptor-mediated epithelial morphogenesis

The Gab family of docking proteins (Gab1 and Gab2) are phosphorylated in response to various cytokines and growth factors. Gab1 acts to diversify the signal downstream from the Met receptor tyrosine kinase through the recruitment of multiple signaling proteins, and is essential for epithelial morphogenesis. To determine whether Gab1 and Gab2 are functionally redundant, we have examined the role of Gab2 in epithelial cells. Both Gab1 and Gab2 are expressed in epithelial cells and localize to cell-cell junctions. However, whereas overexpression of Gab1 promotes a morphogenic response, the overexpression of Gab2 fails to induce this response. We show that Gab2 recruitment to the Met receptor is dependent on the Grb2 adapter protein. In contrast, Gab1 recruitment to Met is both Grb2 dependent and Grb2 independent. The latter requires a novel amino acid sequence present in the Met-binding domain of Gab1 but not Gab2. Mutation of these residues in Gab1 impairs both association with the Met receptor and the ability of Gab1 to promote a morphogenic response, whereas their insertion into Gab2 increases Gab2 association with Met, but does not confer on Gab2 the ability to promote epithelial morphogenesis. We propose that the Grb2-independent recruitment of Gab proteins to Met is necessary but not sufficient to promote epithelial morphogenesis.

Insulin-like growth factor-1 enhances inflammatory responses in endothelial cells: role of Gab1 and MEKK3 in TNF-alpha-induced c-Jun and NF-kappaB activation and adhesion molecule expression

Insulin-like growth factor (IGF)-1 and the type I IGF-1 receptor are important regulators of vascular function that may contribute to cardiovascular disease. We hypothesized that IGF-1 causes endothelial cell dysfunction and expression of neutrophil and monocyte adhesion molecules by enhancing pro-inflammatory cytokine signal transduction. Long-term IGF-1 treatment of endothelial cells potentiated c-Jun and nuclear factor NF-kappaB activation by tumor necrosis factor (TNF)-alpha and enhanced TNF-alpha-mediated adhesion molecule expression. In response to IGF-1 treatment, the expression of kinases in the c-Jun/c-Jun NH(2)-terminal kinase signaling pathway (MEKK1, MEK4, and JNK1/2) was unchanged, but expressions of insulin receptor substrate-1 and Grb2-associated binder-1 (Gab1) were significantly decreased. Because Gab1 is involved in both c-Jun and NF-kappaB activation by TNF-alpha, we focused on Gab1-dependent signaling. Gab1 inhibited c-Jun and NF-kappaB transcriptional activation by TNF-alpha. Interestingly, Gab1 inhibited c-Jun transcriptional activity induced by MEKK3 but not MEKK1 and MEK4. Gab1 associated with MEKK3, and a catalytically inactive form of MEKK3 inhibited TNF-alpha-induced c-Jun and NF-kappaB transcriptional activation, suggesting a critical role for Gab1 and MEKK3 in TNF-alpha signaling. These data demonstrate that Gab1 and MEKK3 play important roles in endothelial cell inflammation via regulating the activation of c-Jun and NF-kappaB. Furthermore, the IGF-1-mediated downregulation of Gab1 expression represents a novel mechanism to promote vascular inflammation and atherosclerosis.

Distinct recruitment and function of Gab1 and Gab2 in Met receptor-mediated epithelial morphogenesis

The Gab family of docking proteins (Gab1 and Gab2) are phosphorylated in response to various cytokines and growth factors. Gab1 acts to diversify the signal downstream from the Met receptor tyrosine kinase through the recruitment of multiple signaling proteins, and is essential for epithelial morphogenesis. To determine whether Gab1 and Gab2 are functionally redundant, we have examined the role of Gab2 in epithelial cells. Both Gab1 and Gab2 are expressed in epithelial cells and localize to cell-cell junctions. However, whereas overexpression of Gab1 promotes a morphogenic response, the overexpression of Gab2 fails to induce this response. We show that Gab2 recruitment to the Met receptor is dependent on the Grb2 adapter protein. In contrast, Gab1 recruitment to Met is both Grb2 dependent and Grb2 independent. The latter requires a novel amino acid sequence present in the Met-binding domain of Gab1 but not Gab2. Mutation of these residues in Gab1 impairs both association with the Met receptor and the ability of Gab1 to promote a morphogenic response, whereas their insertion into Gab2 increases Gab2 association with Met, but does not confer on Gab2 the ability to promote epithelial morphogenesis. We propose that the Grb2-independent recruitment of Gab proteins to Met is necessary but not sufficient to promote epithelial morphogenesis.

ERK negatively regulates the epidermal growth factor-mediated interaction of Gab1 and the phosphatidylinositol 3-kinase

We have examined the ability of epidermal growth factor (EGF)-stimulated ERK activation to regulate Grb2-associated binder-1 (Gab1)/phosphatidylinositol 3-kinase (PI3K) interactions. Inhibiting ERK activation with the MEK inhibitor U0126 increased the EGF-stimulated association of Gab1 with either full-length glutathione S-transferase-p85 or the p85 C-terminal Src homology 2 (SH2) domain, a result reproduced by co-immunoprecipitation of the native proteins from intact cells. This increased association of Gab1 and the PI3K correlates with an increase in PI3K activity and greater phosphorylation of Akt. This result is in direct contrast to what we have previously reported following HGF stimulation where MEK inhibition decreased the HGF-stimulated association of Gab1 and p85. In support of this divergent effect of ERK on Gab1/PI3K association following HGF and EGF stimulation, U0126 decreased the HGF-stimulated association of p85 and the Gab1 c-Met binding domain but did not alter the EGF-stimulated association of p85 and the c-Met binding domain. An examination of the mechanism of this effect revealed that the treatment of cells with EGF + U0126 increased the tyrosine phosphorylation of Gab1 as well as its association with another SH2-containing protein, SHP2. Furthermore, overexpression of a catalytically inactive form of SHP2 or pretreatment with pervanadate markedly increased EGF-stimulated Gab1 tyrosine phosphorylation. These experiments demonstrate that EGF and HGF-mediated ERK activation result in divergent effects on Gab1/PI3K signaling. HGF-stimulated ERK activation increases the Gab1/PI3K association, whereas EGF-stimulated ERK activation results in a decrease in the tyrosine phosphorylation of Gab1 and a decreased association with the PI3K. SHP2 is shown to associate with and dephosphorylate Gab1, suggesting that EGF-stimulated ERK might act through the regulation of SHP2.

Hepatocyte growth factor induces ERK-dependent paxillin phosphorylation and regulates paxillin-focal adhesion kinase association

Hepatocyte growth factor (HGF) modulates cell adhesion, migration, and branching morphogenesis in cultured epithelial cells, events that require regulation of cell-matrix interactions. Using mIMCD-3 epithelial cells, we studied the effect of HGF on the focal adhesion proteins, focal adhesion kinase (FAK) and paxillin and their association. HGF was found to increase the tyrosine phosphorylation of paxillin and to a lesser degree FAK. In addition, HGF induced association of paxillin and activated ERK, correlating with a gel retardation of paxillin that was prevented with the ERK inhibitor U0126. The ability of activated ERK to phosphorylate and induce gel retardation of paxillin was confirmed in vitro in both full-length and amino-terminal paxillin. Several potential ERK phosphorylation sites in paxillin flank the paxillin-FAK association domains, so the ability of HGF to regulate paxillin-FAK association was examined. HGF induced an increase in paxillin-FAK association that was inhibited by pretreatment with U0126 and reproduced by in vitro phosphorylation of paxillin with ERK. The prevention of the FAK-paxillin association with U0126 correlated with an inhibition of the HGF-mediated FAK tyrosine phosphorylation and inhibition of HGF-dependent cell spreading and adhesion. An examination of cellular localization of FAK and paxillin demonstrated that HGF caused a condensation of focal adhesion complexes at the leading edges of cell processes and FAK-paxillin co-localization in these large complexes. Thus, these data suggest that HGF can induce serine/threonine phosphorylation of paxillin most probably mediated directly by ERK, resulting in the recruitment and activation of FAK and subsequent enhancement of cell spreading and adhesion.

PKB-mediated negative feedback tightly regulates mitogenic signalling via Gab2

Heregulin (HRG)-induced tyrosine phosphorylation of the Gab2 docking protein was enhanced by pretreatment with wortmannin, indicating negative regulation via a PI3-kinase-dependent pathway. This represents phosphorylation by the serine/threonine kinase protein kinase B (PKB), since PKB constitutively associates with Gab2, phosphorylates Gab2 on a consensus phosphorylation site, Ser159, in vitro and inhibits Gab2 tyrosine phosphorylation. However, expression of Gab2 mutated at this site (S159A Gab2) not only enhanced HRG-induced Gab2 tyrosine phosphorylation and association with Shc and ErbB2, but also markedly increased tyrosine phosphorylation of ErbB2 and other cellular proteins and amplified activation of the ERK and PKB pathways. The impact of this negative regulation was further emphasized by a potent transforming activity for S159A Gab2, but not wild-type Gab2, in fibroblasts. These studies establish Gab2 as a proto-oncogene, and a model in which receptor recruitment of Gab2 is tightly regulated via an intimate association with PKB. Release of this negative constraint enhances growth factor receptor signalling, possibly since Gab2 binding limits dephosphorylation and disassembly of receptor-associated signalling complexes.

ERK regulates the hepatocyte growth factor-mediated interaction of Gab1 and the phosphatidylinositol 3-kinase

Based on our previous observations that active ERK associates with and phosphorylates Gab1 in response to HGF, and the prediction that the ERK phosphorylation site is adjacent to one of the phosphatidylinositol 3-kinase (PI3K) SH2 binding motifs, we examined the possibility that ERK phosphorylation can regulate the Gab1/PI3K association. The HGF-mediated association of Gab1 with either full-length GST-p85 or its isolated N- or C-terminal SH2 domains was inhibited by approximately 50% in the setting of ERK inhibition, a result confirmed by co-immunoprecipitation of the native proteins. A 14-amino acid peptide encoding (472)YVPMTP(477) (one of the major p85 binding sites in Gab1 and the predicted ERK phosphorylation site) was synthesized with either phosphotyrosine alone (pY), or phosphotyrosine + phosphothreonine (pYT). In both pull-down assays and competition assays, pYT demonstrated a higher affinity for p85 than did pY alone. Finally, examination of the phosphorylation state of Akt after HGF stimulation revealed that ERK inhibition resulted in a decrease in Akt activation at both 5 and 10 min. These results suggest that activated ERK can phosphorylate Gab1 in response to HGF stimulation and thereby potentiate the Gab1/PI3K association and subsequent PI3K activation.

HGF promotes adhesion of ATP-depleted renal tubular epithelial cells in a MAPK-dependent manner

Hepatocyte growth factor (HGF) has been shown to enhance recovery from renal tubular ischemia. We investigated the possibility that HGF improves recovery by preventing ischemia-induced loss of cell adhesion. Murine inner medullary collecting duct-3 (mIMCD-3) cells subjected to 90% ATP depletion demonstrated a 55% decrease in adhesion, an effect that was completely reversed by the addition of HGF. Assays examining release of adherent cells revealed similar results with 30 min of ATP depletion causing loss of adhesion of 25% of mIMCD-3 cells and HGF completely reversing this effect. In contrast, HGF was unable to reverse the loss of adhesion of cells exposed to 99% ATP depletion. Examination of the mitogen-activated protein kinase (MAPK) signaling pathway revealed that HGF could induce extracellular signal-regulated kinase (ERK) phosphorylation in control and 90% ATP-depleted cells but not in 99% ATP-depleted cells. Inhibition of ERK activation with U0126 completely blocked the HGF-dependent reversal of ATP-depleted cell adhesion. Thus ATP-depleted cells demonstrate a marked decrease in cell adhesion that is reversible by the addition of HGF. This effect of HGF requires activation of the MAPK pathway.

Differential MAPK pathways utilized for HGF- and EGF-dependent renal epithelial morphogenesis

Cells derived from the inner medullary collecting duct undergo in vitro branching tubulogenesis to both the c-met receptor ligand hepatocyte growth factor (HGF) as well as epidermal growth factor (EGF) receptor ligands. In contrast, many other cultured renal epithelial cells respond in this manner only to HGF, suggesting that these two receptors may use independent signaling pathways during morphogenesis. We have therefore compared the signaling pathways for mIMCD-3 cell morphogenesis in response to EGF and HGF. Inhibition of the p42/44 mitogen-activated protein kinase (MAPK) pathway with the mitogen-activated protein kinase kinase (MKK1) inhibitor PD98059 (50 microm) markedly inhibits HGF-induced cell migration with only partial inhibition of EGF-induced cell motility. Similarly, HGF-dependent, but not EGF-dependent, branching morphogenesis was more greatly inhibited by the MKK1 inhibitor. Examination of EGF-stimulated cells demonstrated that extracellular-regulated kinase 5 (ERK5) was activated in response to EGF but not HGF, and that activation of ERK5 was only 60% inhibited by 50 microm PD98059. In contrast, the MKK inhibitor U0126 markedly inhibited both ERK1/2 and ERK5 activation and completely prevented HGF- and EGF-dependent migration and branching process formation. Expression of dominant negative ERK5 (dnBMK1) likewise inhibited EGF-dependent branching process formation, but did not affect HGF-dependent branching process formation. Our results indicate that activation of the ERK1/ERK2 signaling pathway is critical for HGF-induced cell motility/morphogenesis in mIMCD-3 cells, whereas ERK5 appears to be required for EGF-dependent morphogenesis.

Gab1 phosphorylation: a novel mechanism for negative regulation of HGF receptor signaling

Signal transduction by HGF receptor, the tyrosine kinase encoded by the MET oncogene, switches on a genetic program called 'invasive growth' inducing epithelial cell dissociation, migration, growth, and ultimately leading to differentiation into branched tubular structures. Sustained tyrosine phosphorylation of the downstream adaptor protein Gab1 is required for the HGF response. Here we show that serine/threonine phosphorylation of Gab1 provides a control mechanism for negative regulation. Treatment with okadaic acid, a potent inhibitor of the serine/threonine protein phosphatases PP1 and PP2A, was followed by activation of a number of serine/threonine kinases, hyper-phosphorylation in serine and threonine of Gab1 and severe inhibition of the HGF-induced biological responses. Under these conditions, Gab1 was found to be concomitantly hypo-phosphorylated in tyrosine, and thus endowed with reduced ability to recruit SH2 containing signal transducers such as PI3 kinase. Among the serine-threonine kinases activated by PP1 and PP2A inhibition, we found that PKC-alpha and PKC-beta1 are required for negative regulation of Gab1. These data provide a novel negative mechanism for the HGF receptor signaling pathways and highlight a potentially useful target for inhibitors of invasive growth.

Hepatocyte growth factor induces MAPK-dependent formin IV translocation in renal epithelial cells

Renal epithelial tubule formation in cultured cells occurs after the addition of tubulogenic growth factors such as the hepatocyte growth factor (HGF). HGF activates the tyrosine kinase receptor c-met, initiating a series of complex events that regulate cell morphology, cell-cell interactions, and cell-matrix interactions and eventually result in the formation of branching tubular structures. The discovery that disruption of the formin gene locus in mice causes agenesis of the kidneys secondary to failure of ureteric bud outgrowth and branching tubule formation suggested that this family of proteins may be critical to the development of renal epithelial tubules. In this study, we investigated whether formin is involved in the HGF/c-met signaling pathway of in vitro tubulogenesis in renal epithelial cells. mIMCD-3 cells were analyzed by reverse transcription-PCR and found to express formin IV mRNA. With the use of an antibody that recognizes the carboxy terminus of all known formin isoforms, it was observed a formin isoform of approximately 165 kD markedly increased in the detergent soluble cell lysate after 10 min of stimulation with HGF. An antibody that is specific for formin IV was then generated and confirmed that the formin isoform regulated by HGF was formin IV. Cell fractionation and confocal localization of formin IV revealed that formin IV is primarily found in a submembranous band that co-localizes with the actin cytoskeleton and in a perinuclear location in quiescent epithelial cells but undergoes a rapid relocalization after HGF stimulation with translocation into the cell cytosol and into the nucleus. Formin IV was found to be a phosphorylation substrate for activated extracellular signal-regulated kinase in vitro, and pretreatment of cells with the mitogen-activated protein kinase inhibitor U0126 prevented the translocation of formin IV and inhibited HGF-dependent phosphorylation of formin IV in intact cells. In conclusion, activation of the c-met receptor results in cellular relocalization of formin IV in a mitogen-activated protein kinase-dependent manner.

New role for Shc in activation of the phosphatidylinositol 3-kinase/Akt pathway

Most, if not all, cytokines activate phosphatidylinositol 3-kinase (PI-3K). Although many cytokine receptors have direct binding sites for the p85 subunit of PI-3K, others, such as the interleukin-3 (IL-3) receptor beta common chain (betac) and the IL-2 receptor beta chain (IL-2Rbeta), lack such sites, leaving the mechanism by which they activate PI-3K unclear. Here, we show that the protooncoprotein Shc, which promotes Ras activation by recruiting the Grb2-Sos complex in response to stimulation of cytokine stimulation, also signals to the PI-3K/Akt pathway. Analysis of Y-->F and "add-back" mutants of betac shows that Y577, the Shc binding site, is the major site required for Gab2 phosphorylation in response to cytokine stimulation. When fused directly to a mutant form of IL-2Rbeta that lacks other cytoplasmic tyrosines, Shc can promote Gab2 tyrosyl phosphorylation. Mutation of the three tyrosyl phosphorylation sites of Shc, which bind Grb2, blocks the ability of the Shc chimera to evoke Gab2 tyrosyl phosphorylation. Overexpression of mutants of Grb2 with inactive SH2 or SH3 domains also blocks cytokine-stimulated Gab2 phosphorylation. The majority of cytokine-stimulated PI-3K activity associates with Gab2, and inducible expression of a Gab2 mutant unable to bind PI-3K markedly impairs IL-3-induced Akt activation and cell growth. Experiments with the chimeric receptors indicate that Shc also signals to the PI-3K/Akt pathway in response to IL-2. Our results suggest that cytokine receptors lacking direct PI-3K binding sites activate Akt via a Shc/Grb2/Gab2/PI-3K pathway, thereby regulating cell survival and/or proliferation.