Met-induced JNK activation is mediated by the adapter protein Crk and correlates with the Gab1 - Crk signaling complex formation

Modulating Growth Factor Receptor Signaling to Promote Corneal Epithelial Homeostasis

The corneal epithelium is the first anatomical barrier between the environment and the cornea; it is critical for proper light refraction onto the retina and prevents pathogens (e.g., bacteria, viruses) from entering the immune-privileged eye. Trauma to the highly innervated corneal epithelium is extremely painful and if not resolved quickly or properly, can lead to infection and ultimately blindness. The healthy eye produces its own growth factors and is continuously bathed in tear fluid that contains these proteins and other nutrients to maintain the rapid turnover and homeostasis of the ocular surface. In this article, we review the roles of growth factors in corneal epithelial homeostasis and regeneration and some of the limitations to their use therapeutically.

The Emerging Role of c-Met in Carcinogenesis and Clinical Implications as a Possible Therapeutic Target

Background

c-MET is a receptor tyrosine kinase receptor (RTK) for the hepatocyte growth factor (HGF). The binding of HGF to c-MET regulates several cellular functions: differentiation, proliferation, epithelial cell motility, angiogenesis, and epithelial-mesenchymal transition (EMT). Moreover, it is known to be involved in carcinogenesis. Comprehension of HGF-c-MET signaling pathway might have important clinical consequences allowing to predict prognosis, response to treatment, and survival rates based on its expression and dysregulation. Discussion. c-MET represents a useful molecular target for novel engineered drugs. Several clinical trials are underway for various solid tumors and the development of new specific monoclonal antibodies depends on the recent knowledge about the definite c-MET role in each different malignance. Recent clinical trials based on c-MET molecular targets result in good safety profile and represent a promising therapeutic strategy for solid cancers, in monotherapy or in combination with other target drugs.

Conclusion

The list of cell surface receptors crosslinking with the c-MET signaling is constantly growing, highlighting the importance of this pathway for personalized target therapy. Research on the combination of c-MET inhibitors with other drugs will hopefully lead to discovery of new effective treatment options.

KRAS and MET in non-small-cell lung cancer: two of the new kids on the 'drivers' block

Non-small-cell lung cancer (NSCLC) is a heterogeneous disease, and therapeutic management has advanced to identify various critical oncogenic mutations that promote lung cancer tumorigenesis. Subsequent studies have developed targeted therapies against these oncogenes in the hope of personalized treatment based on the tumor's molecular genomics. This review presents a comprehensive review of the biology, new therapeutic interventions, and resistance patterns of two well-defined subgroups, tumors with KRAS and MET alterations. We also discuss the status of molecular testing practices for these two key oncogenic drivers, considering the progressive introduction of next-generation sequencing (NGS) and RNA sequencing in regular clinical practice.

Dnmt3b catalytic activity is critical for its tumour suppressor function in lymphomagenesis and is associated with c-Met oncogenic signalling

BACKGROUND

DNA methylation regulates gene transcription in many physiological processes in mammals including development and haematopoiesis. It is catalysed by several DNA methyltransferases, including Dnmt3b that mediates both methylation-dependant and independent gene repression. Dnmt3b is critical for mouse embryogenesis and functions as a tumour suppressor in haematologic malignancies in mice. However, the extent to which Dnmt3b's catalytic activity (CA) is involved in development and cancer is unclear.

METHODS

We used a mouse model expressing catalytically inactive Dnmt3b (Dnmt3bCI) to study a role of Dnmt3b's CA in development and cancer. We utilized global approaches including Whole-genome Bisulfite sequencing and RNA-seq to analyse DNA methylation and gene expression to identify putative targets of Dnmt3b's CA. To analyse postnatal development and haematopoiesis, we used tissue staining, histological and FACS analysis. To determine potential involvement of selected genes in lymphomagenesis, we used overexpression and knock down approaches followed by in vitro growth assays.

FINDINGS

We show that mice expressing Dnmt3bCI only, survive postnatal development and develop ICF (the immunodeficiency-centromeric instability-facial anomalies) -like syndrome. The lack of Dnmt3b's CA promoted fibroblasts transformation in vitro, accelerated MLL-AF9 driven Acute Myeloid Leukaemia and MYC-induced T-cell lymphomagenesis in vivo. The elimination of Dnmt3b's CA resulted in decreased methylation of c-Met promoter and its upregulation, activated oncogenic Met signalling, Stat3 phosphorylation and up-regulation of Lin28b promoting lymphomagenesis.

INTERPRETATION

Our data demonstrates that Dnmt3b's CA is largely dispensable for mouse development but critical to prevent tumourigenesis by controlling events involved in cellular transformation.

FUNDING

This study was supported by Department of Anatomy and Cell Biology and Cancer Centre at the University of Florida start-up funds, NIH/NCI grant 1R01CA188561-01A1 (R.O.).

Overexpression of CrkL as a novel biomarker for poor prognosis in gastric cancer

BACKGROUND

The signaling adapter protein CrkL plays vital roles in multiple cancers. However, the expression pattern of CrkL protein and its clinical significance have not been well characterized in human gastric cancer (GC) so far.

OBJECTIVE

To investigate the association of tissue-based CrkL protein expression level with the clinicopathological characteristics and prognosis of GC patients.

METHODS

The expression level of CrkL protein in 380 GC patients was analyzed by immunohistochemistry. The associations of CrkL protein expression level with clinicopathologicalal characteristics and clinical outcome were evaluated.

RESULTS

Compared with the matched adjacent non-tumor tissues, CrkL protein expression level was significantly up-regulated in tumor tissues. In addition, there was a positive correlation between CrkL and Ki67 expression levels in GC patients. An elevated CrkL level statistically correlated with aggressive clinicopathologicalal characteristics, such as larger tumor size, deeper local invasion, more lymph node metastasis, advanced TNM stage, and poorer prognosis. Notably, multivariate analysis identified tissue-based CrkL level as an independent predictor for the unfavorable prognosis of GC.

CONCLUSIONS

These results indicate that CrkL protein may serve as a novel prognostic biomarker in GC.

Activated HGF-c-Met Axis in Head and Neck Cancer

Head and neck squamous cell carcinoma (HNSCC) is a highly morbid disease. Recent developments including Food and Drug Administration (FDA) approved molecular targeted agent’s pembrolizumab and cetuximab show promise but did not improve the five-year survival which is currently less than 40%. The hepatocyte growth factor receptor; also known as mesenchymal–epithelial transition factor (c-Met) and its ligand hepatocyte growth factor (HGF) are overexpressed in head and neck squamous cell carcinoma (HNSCC); and regulates tumor progression and response to therapy. The c-Met pathway has been shown to regulate many cellular processes such as cell proliferation, invasion, and angiogenesis. The c-Met pathway is involved in cross-talk, activation, and perpetuation of other signaling pathways, curbing the cogency of a blockade molecule on a single pathway. The receptor and its ligand act on several downstream effectors including phospholipase C gamma (PLCγ), cellular Src kinase (c-Src), phosphotidylinsitol-3-OH kinase (PI3K) alpha serine/threonine-protein kinase (Akt), mitogen activate protein kinase (MAPK), and wingless-related integration site (Wnt) pathways. They are also known to cross-talk with other receptors; namely epidermal growth factor receptor (EGFR) and vascular endothelial growth factor receptor (VEGFR) and specifically contribute to treatment resistance. Clinical trials targeting the c-Met axis in HNSCC have been undertaken because of significant preclinical work demonstrating a relationship between HGF/c-Met signaling and cancer cell survival. Here we focus on HGF/c-Met impact on cellular signaling in HNSCC to potentiate tumor growth and disrupt therapeutic efficacy. Herein we summarize the current understanding of HGF/c-Met signaling and its effects on HNSCC. The intertwining of c-Met signaling with other signaling pathways provides opportunities for more robust and specific therapies, leading to better clinical outcomes.

Analysis of progress and challenges for various patterns of c-MET-targeted molecular imaging: a systematic review

Background

Mesenchymal–epithelial transition factor also named c-MET is a receptor tyrosine kinase for the hepatocyte growth factor that plays a pivotal role in tumorigenesis. c-MET-targeted therapies have been tested in preclinical models and patients, with significant benefits for cancer treatment. In recent years, many studies have shown that the expression level and activation status of c-MET are closely correlated to c-MET-targeted therapy response and clinical prognosis, thus highlighting the importance of evaluating the c-MET status during and prior to targeted therapy. Molecular imaging allows the monitoring of abnormal alterations of c-MET in real time and in vivo.

Results

In this review, we initially summarize the recent advances in c-MET-targeted molecular imaging, with a special focus on the development of imaging agents ranging in size from monoclonal antibody to small molecule. The aim of this review is to report the preclinical results and clinical application of all molecular imaging studies completed until now for in vivo detection of c-MET in cancer, in order to be beneficial to development of molecular probe and the combination of molecular imaging technologies for in vivo evaluation of c-MET. Various molecular probe targeted to c-MET possesses distinctive advantages and disadvantages. For example, antibody-based probes have high binding affinity but with long metabolic cycle as well as remarkable immunogenicity.

Conclusions

Although studies for c-MET-targeted molecular imaging have made many important advances, most of imaging agents specifically target to extracellular area of c-MET receptor; however, it is difficult to reflect entirely activation of c-MET. Therefore, small molecule probes based on tyrosine kinase inhibitors, which could target to intracellular area of c-MET without any immunogenicity, should be paid more attention.

Targeting c-MET in gastrointestinal tumours: rationale, opportunities and challenges

Data from many preclinical studies, including those using cellular models of colorectal, gastric, gastro-oesophageal and gastro-oesophageal junction cancers, indicate that the hepatocyte growth factor (HGF)-hepatocyte growth factor receptor (c-MET) pathway is vital for the growth, survival and invasive potential of gastrointestinal cancers. Following the availability of data from these various studies, and data on c-MET expression as a biomarker that indicates a poor prognosis in patients with gastrointestinal cancer and increased c-MET expression, inhibitors targeting this pathway have entered the clinic in the past decade. However, the design of clinical trials that incorporate the use of HGF/c-MET inhibitors in their most appropriate genetic and molecular context remains crucial. Recognizing and responding to this challenge, the European Commission funded Framework 7 MErCuRIC programme is running a biomarker-enriched clinical trial investigating the efficacy of combined c-MET/MEK inhibition in patients with RAS-mutant or RAS-wild-type metastatic colorectal cancer with aberrant c-MET expression. The design of this trial enables the continued refinement of the predictive biomarker and co-development of companion diagnostics. In this Review, we focus on advances in our understanding of inhibition of the HGF/c-MET pathway in patients with gastro-intestinal cancers, the prominent challenges facing the clinical translation and implementation of agents targeting HGF/c-MET, and discuss the various efforts, and associated obstacles to the discovery and validation of biomarkers that will enable patient stratification in this context.

c-Met expression and activity in urogenital cancers - novel aspects of signal transduction and medical implications

C-Met is a receptor tyrosine kinase with multiple functions throughout embryonic development, organogenesis and wound healing and is expressed in various epithelia. The ligand of c-Met is Hepatocyte Growth Factor (HGF) which is secreted among others by mesenchymal stroma/stem (MSC) cells.

Physiological c-Met functions are centred around processes that underly cellular motility and invasive growth. Aberrant c-Met expression and activity is observed in numerous cancers and makes major contributions to cell malignancy. Importantly, HGF/c-Met signaling is crucial in the context of communication between cancer cells and the the tumor stroma.

Here, we review recent findings on roles of dysregulated c-Met in urogenital tumors such as cancers of the urinary bladder, prostate, and ovary. We put emphasis on novel aspects of cancer-associated c-Met expression regulation on both, HGF-dependent and HGF-independent non-canonical mechanisms. Moreover, this review focusses on c-Met-triggered signalling with potential relevance for urogenital oncogenesis, and on strategies to specifically inhibit c-Met activity.

JNK Pathway Activation Modulates Acquired Resistance to EGFR/HER2-Targeted Therapies

Resistance limits the effectiveness of receptor tyrosine kinase (RTK)-targeted therapies. Combination therapies targeting resistance mechanisms can considerably improve response, but will require an improved understanding of when particular combinations will be effective. One common form of resistance is bypass signaling, wherein RTKs not targeted by an inhibitor can direct reactivation of pathways essential for survival. Although this mechanism of resistance is well appreciated, it is unclear which downstream signaling events are responsible. Here, we apply a combined experimental- and statistical modeling-based approach to identify a set of pathway reactivation essential for RTK-mediated bypass resistance. Differences in the downstream pathway activation provided by particular RTKs lead to qualitative differences in the capacity of each receptor to drive therapeutic resistance. We identify and validate that the JNK pathway is activated during and strongly modulates bypass resistance. These results identify effective therapeutic combinations that block bypass-mediated resistance and provide a basic understanding of this network-level change in kinase dependence that will inform the design of prognostic assays for identifying effective therapeutic combinations in individual patients. Cancer Res; 76(18); 5219-28. ©2016 AACR.

JNK suppression of chemotherapeutic agents-induced ROS confers chemoresistance on pancreatic cancer stem cells

Chemoresistance associated with cancer stem cells (CSCs), which is now being held responsible for the pervasive therapy resistance of pancreatic cancer, poses a major challenge to the successful management of this devastating malignancy. However, the molecular mechanism underlying the marked chemoresistance of pancreatic CSCs remains largely unknown. Here we show that JNK, which is upregulated in pancreatic CSCs and contributes to their maintenance, is critically involved in the resistance of pancreatic CSCs to 5-fluorouracil (5-FU) and gemcitabine (GEM). We found that JNK inhibition effectively sensitizes otherwise chemoresistant pancreatic CSCs to 5-FU and GEM. Significantly, JNK inhibition promoted 5-FU- and GEM-induced increase in intracellular reactive oxygen species (ROS), and scavenging intracellular ROS by use of N-acetylcysteine impaired JNK inhibition-mediated promotion of the cytotoxicity of 5-FU and GEM. Our findings thus suggest that JNK may contribute to the chemoresistance of pancreatic CSCs through prevention of chemotherapeutic agents-induced increase in intracellular ROS. Our findings also suggest that JNK inhibition combined with 5-FU- and/or GEM-based regimens may be a rational therapeutic approach to effectively eliminate pancreatic CSCs.

Gab1 amplifies signaling in response to low-intensity stimulation by HGF

The receptor tyrosine kinases EGFR and Met induce phosphorylation of the docking protein Gab1, and there is evidence that Gab1 may have a role in the signaling from these receptors. Studying hepatocytes, we previously found that although Gab1 mechanistically interacted in different ways with EGFR and Met, it was involved in mitogenic signaling induced by both EGF and HGF. It has been reported that in EGFR, Gab1 is required particularly at a low dose of EGF. Whether this also applies to HGF/Met signaling has not been investigated. We have studied the role of Gab1 in activation of the Akt and ERK pathways at low- and high-intensity stimulation with EGF and HGF in cultured hepatocytes. In cells where Gab1 was depleted by a specific Gab1-directed siRNA, the EGF-induced phosphorylation of ERK was lowered and HGF-induced phosphorylation of both ERK and Akt was substantially reduced. These effects were more marked at low-dose HGF stimulation. The inhibitory consequence of Gab1 depletion was particularly pronounced for HGF-induced Akt phosphorylation. The results suggest that Gab1 is an important signal amplifier for low-intensity stimulation by HGF.

c-Met and Other Cell Surface Molecules: Interaction, Activation and Functional Consequences

The c-Met receptor, also known as the HGF receptor, is one of the most studied tyrosine kinase receptors, yet its biological functions and activation mechanisms are still not fully understood. c-Met has been implicated in embryonic development and organogenesis, in tissue remodelling homeostasis and repair and in cancer metastasis. These functions are indicative of the many cellular processes in which the receptor plays a role, including cell motility, scattering, survival and proliferation. In the context of malignancy, sustained activation of c-Met leads to a signalling cascade involving a multitude of kinases that initiate an invasive and metastatic program. Many proteins can affect the activation of c-Met, including a variety of other cell surface and membrane-spanning molecules or receptors. Some cell surface molecules share structural homology with the c-Met extracellular domain and can activate c-Met via clustering through this domain (e.g., plexins), whereas other receptor tyrosine kinases can enhance c-Met activation and signalling through intracellular signalling cascades (e.g., EGFR). In this review, we provide an overview of c-Met interactions and crosstalk with partner molecules and the functional consequences of these interactions on c-Met activation and downstream signalling, c-Met intracellular localization/recycling and c-Met degradation.

CRK SH3N Domain Diminishes Cell Invasiveness of Non-Small Cell Lung Cancer

CRK (c-Crk) as an adaptor protein is involved in several oncogenic signal transduction pathways, conveying oncogenic signals to its downstream effectors and thereby affecting multiple cellular processes including proliferation, differentiation, and migration. For example, we have observed that CRK expression and phosphorylation influence the invasiveness of non-small cell lung cancer (NSCLC) cells. To intervene in CRK signaling pathway, we examined whether CRK protein domains can be used as therapeutic tools to interrupt CRK signaling, thus influencing the biological behavior of NSCLC cells. For this purpose, Src Homology domains of CRK-I (i.e., SH2 and SH3N domains) were overexpressed in H157, Rh2, and A549 cells. CRK-SH3N domain expression induced epithelial morphology in H157 cells and enhanced epithelial morphology of A549 and Rh2 cells as compared to cells transfected with CRK-SH2 domain or empty vector. In addition, CRK-SH3N domain expression significantly decreased the motility and invasiveness of A549 and H157 cells. Furthermore, CRK-SH3N domain expression disrupted the interaction of CRK-II with DOCK180. In summary, these data provide evidence that the CRK-SH3N domain can be used to influence the malignant phenotype of NSCLC cells and also reduce the metastatic potential of these cells.

Hepatocyte growth factor is an attractive target for the treatment of pulmonary fibrosis

INTRODUCTION

Pulmonary fibrosis (PF) is a progressive fatal disorder and is characterized by alveolar epithelial injury, myofibroblast proliferation, and extracellular matrix remodeling, resulting in irreversible distortion of lung's architecture. Available therapies are associated with side effects and show restricted efficacy. Therefore, there is an urgent need to find a therapeutic solution to PF. Therapeutic strategies interfering myofibroblast expansion, apoptosis of epithelial and endothelial cells might be beneficial for treatment of PF. Hepatocyte growth factor (HGF), a pleiotropic growth factor, plays an important role in lung development, inflammation, repair, and regeneration. In animal model of PF, administration of recombinant HGF protein or ectopic HGF expression ameliorates fibrosis.

AREAS COVERED

The focus of this review is to highlight HGF as a promising therapeutic approach for the treatment of PF. The review discusses the currently available treatment option for PF as well as highlights the possible beneficial effect of HGF as a drug target.

EXPERT OPINION

HGF with its anti-fibrotic effect provides a promising new therapeutic approach by protecting lung from fibrotic remodeling and also promoting normal regeneration of lung. The development of HGF mimetics may provide a potential attractive therapy for treatment of this devastating and complex disease.

Protein tyrosine phosphatase-1B regulates the tyrosine phosphorylation of the adapter Grb2-associated binder 1 (Gab1) in the retina

BACKGROUND

Gab1 (Grb2-associated binder 1) is a key coordinator that belongs to the insulin receptor substrate-1 like family of adaptor molecules and is tyrosine phosphorylated in response to various growth factors, cytokines, and numerous other molecules. Tyrosine phosphorylated Gab1 is able to recruit a number of signaling effectors including PI3K, SHP2 and PLC-γ. In this study, we characterized the localization and regulation of tyrosine phosphorylation of Gab1 in the retina.

RESULTS

Our immuno localization studies suggest that Gab1 is expressed in rod photoreceptor inner segments. We found that hydrogen peroxide activates the tyrosine phosphorylation of Gab1 ex vivo and hydrogen peroxide has been shown to inhibit the protein tyrosine phosphatase PTP1B activity. We found a stable association between the D181A substrate trap mutant of PTP1B and Gab1. Our studies suggest that PTP1B interacts with Gab1 through Tyrosine 83 and this residue may be the major PTP1B target residue on Gab1. We also found that Gab1 undergoes a light-dependent tyrosine phosphorylation and PTP1B regulates the phosphorylation state of Gab1. Consistent with these observations, we found an enhanced Gab1 tyrosine phosphorylation in PTP1B deficient mice and also in retinas treated ex vivo with a PTP1B specific allosteric inhibitor.

CONCLUSIONS

Our laboratory has previously reported that retinas deficient of PTP1B are resistant to light damage compared to wild type mice. Since Gab1 is negatively regulated by PTP1B, a part of the retinal neuroprotective effect we have observed previously in PTP1B deficient mice could be contributed by Gab1 as well. In summary, our data suggest that PTP1B regulates the phosphorylation state of retinal Gab1 in vivo.

An overview of the c-MET signaling pathway

c-MET is a receptor tyrosine kinase that, after binding with its ligand, hepatocyte growth factor, activates a wide range of different cellular signaling pathways, including those involved in proliferation, motility, migration and invasion. Although c-MET is important in the control of tissue homeostasis under normal physiological conditions, it has also been found to be aberrantly activated in human cancers via mutation, amplification or protein overexpression. This paper provides an overview of the c-MET signaling pathway, including its role in the development of cancers, and provides a rationale for targeting the pathway as a possible treatment option.

Met receptor tyrosine kinase signals through a cortactin-Gab1 scaffold complex, to mediate invadopodia

Invasive carcinoma cells form actin-rich matrix-degrading protrusions called invadopodia. These structures resemble podosomes produced by some normal cells and play a crucial role in extracellular matrix remodeling. In cancer, formation of invadopodia is strongly associated with invasive potential. Although deregulated signals from the receptor tyrosine kinase Met (also known as hepatocyte growth factor are linked to cancer metastasis and poor prognosis, its role in invadopodia formation is not known. Here we show that stimulation of breast cancer cells with the ligand for Met, hepatocyte growth factor, promotes invadopodia formation, and in aggressive gastric tumor cells where Met is amplified, invadopodia formation is dependent on Met activity. Using both GRB2-associated-binding protein 1 (Gab1)-null fibroblasts and specific knockdown of Gab1 in tumor cells we show that Met-mediated invadopodia formation and cell invasion requires the scaffold protein Gab1. By a structure–function approach, we demonstrate that two proline-rich motifs (P4/5) within Gab1 are essential for invadopodia formation. We identify the actin regulatory protein, cortactin, as a direct interaction partner for Gab1 and show that a Gab1–cortactin interaction is dependent on the SH3 domain of cortactin and the integrity of the P4/5 region of Gab1. Both cortactin and Gab1 localize to invadopodia rosettes in Met-transformed cells and the specific uncoupling of cortactin from Gab1 abrogates invadopodia biogenesis and cell invasion downstream from the Met receptor tyrosine kinase. Met localizes to invadopodia along with cortactin and promotes phosphorylation of cortactin. These findings provide insights into the molecular mechanisms of invadopodia formation and identify Gab1 as a scaffold protein involved in this process.

Overexpression of CRKII increases migration and invasive potential in oral squamous cell carcinoma

CT10 regulator of kinase (CRK) was originally identified as an oncogene product of v-CRK in a CT10 chicken retrovirus system. Overexpression of CRKII has been reported in several human cancers. CRKII regulates cell migration, morphogenesis, invasion, phagocytosis, and survival; however, the underlying mechanisms are not well understood. In the present study, we evaluated the possibility of CRKII as an appropriate molecular target for cancer gene therapy. The expression of CRKII in 71 primary oral squamous cell carcinomas and 10 normal oral mucosal specimens was determined immunohistochemically, and the correlation of CRKII overexpression with clinicopathological factors was evaluated. Overexpression of CRKII was detected in 41 of 70 oral squamous cell carcinomas, the frequency being more significant than in normal oral mucosa. In addition, CRKII overexpression was more frequent in higher-grade cancers according to the T classification, N classification, and invasive pattern. Moreover, RNAi-mediated suppression of CRKII expression reduced the migration and invasion potential of an oral squamous cell carcinoma cell line, OSC20. Downregulation of CRKII expression also reduced the expression of Dock180, p130Cas, and Rac1, and the actin-associated scaffolding protein cortactin. These results indicate that the overexpression of CRKII is tightly associated with an aggressive phenotype of oral squamous cell carcinoma. Therefore, we propose that CRKII could be a potential molecular target of gene therapy by RNAi-targeting in oral squamous cell carcinoma.

MET signalling: principles and functions in development, organ regeneration and cancer

The MET tyrosine kinase receptor (also known as the HGF receptor) promotes tissue remodelling, which underlies developmental morphogenesis, wound repair, organ homeostasis and cancer metastasis, by integrating growth, survival and migration cues in response to environmental stimuli or cell-autonomous perturbations. The versatility of MET-mediated biological responses is sustained by qualitative and quantitative signal modulation. Qualitative mechanisms include the engagement of dedicated signal transducers and the subcellular compartmentalization of MET signalling pathways, whereas quantitative regulation involves MET partnering with adaptor amplifiers or being degraded through the shedding of its extracellular domain or through intracellular ubiquitylation. Controlled activation of MET signalling can be exploited in regenerative medicine, whereas MET inhibition might slow down tumour progression.

Docking proteins

Docking proteins comprise a distinct category of intracellular, noncatalytic signalling protein, that function downstream of a variety of receptor and receptor-associated tyrosine kinases and regulate diverse physiological and pathological processes. The growth factor receptor bound 2-associated binder/Daughter of Sevenless, insulin receptor substrate, fibroblast growth factor receptor substrate 2 and downstream of tyrosine kinases protein families fall into this category. This minireview focuses on the structure, function and regulation of these proteins.

Scaffolding adaptor protein Gab1 is required for TLR3/4- and RIG-I-mediated production of proinflammatory cytokines and type I IFN in macrophages

RIG-I-like helicases and TLRs are critical sensors in the induction of type I IFN and proinflammatory cytokines to initiate innate immunity against invading pathogens. However, the mechanisms for the full activation of TLR and RIG-I-triggered innate response remain to be fully investigated. Grb2-associated binder 1 (Gab1), a member of scaffolding/adaptor proteins, can mediate signal transduction from many receptors, however, whether and how Gab1 is required for TLR and RIG-I-triggered innate responses remain unknown. In this study, we demonstrated that Gab1 significantly enhances TLR4-, TLR3-, and RIG-I-triggered IL-6, IL-1beta, and IFN-alpha/beta production in macrophages. Gab1 knockdown in primary macrophages or Gab1 deficiency in mouse embryonic fibroblasts significantly suppresses TLR3/4- and RIG-I-triggered production of IL-6, IL-1beta, and IFN-alpha/beta. Consistently, Gab1 deficiency impairs vesicular stomatitis virus (VSV) infection-induced IFN-alpha/beta production. In addition to promoting both MyD88- and TLR/IL-1 receptor domain-containing adaptor protein inducing IFN-beta-dependent MAPKs and NF-kappaB activation, Gab1 enhances PI3K/Akt activation by directly binding p85 in TLR signaling and VSV infection. Accordingly, Gab1 inhibits VSV replication and VSV infection-induced cell damage by inducing type I IFNs and IFN-inducible gene expression via PI3K/Akt pathway. Therefore, Gab1 is needed for full activation of TLR3/4- and RIG-I-triggered innate responses by promoting activation of PI3K/Akt, MAPKs, and NF-kappaB pathways.

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 and CrkL adaptor proteins: networks for physiological and pathological signaling

The Crk adaptor proteins (Crk and CrkL) constitute an integral part of a network of essential signal transduction pathways in humans and other organisms that act as major convergence points in tyrosine kinase signaling. Crk proteins integrate signals from a wide variety of sources, including growth factors, extracellular matrix molecules, bacterial pathogens, and apoptotic cells. Mounting evidence indicates that dysregulation of Crk proteins is associated with human diseases, including cancer and susceptibility to pathogen infections. Recent structural work has identified new and unusual insights into the regulation of Crk proteins, providing a rationale for how Crk can sense diverse signals and produce a myriad of biological responses.

Transforming signals resulting from sustained activation of the PDGFbeta receptor in mortal human fibroblasts

The platelet-derived growth factor beta receptor (PDGFbetaR) plays an important role in proliferation and motility of fibroblasts. We have been investigating the effects of sustained PDGFbetaR activation in mortal human diploid fibroblasts (HDFs), which are typically difficult to transform. We have previously shown that the bovine papillomavirus E5 protein, through its ability to crosslink and constitutively activate the PDGFbetaR, induces morphological transformation, enhanced growth and loss of contact inhibition (focus formation) in HDFs. Here, we characterized two E5 mutants as being severely defective for focus formation but still competent for enhanced growth, suggesting that proliferation is insufficient for loss of contact inhibition. These E5 mutants were then used in a comparative study to distinguish the PDGFbetaR signaling intermediates required for the enhanced growth phenotype from those required for focus formation. Our data suggested that a PI 3-kinase (PI3K)-AKT-cyclin D3 pathway, a Grb2-Gab1-SHP2 complex and JNK played a role in the enhanced growth phenotype. However, a SHP2-p66Shc-p190BRhoGAP complex and ROCK were implicated exclusively in focus formation. We speculate that a SHP2-p66Shc-p190BRhoGAP signaling complex recruited to the activated PDGFbetaR promotes a distinct Rho-dependent process required for focus formation but not growth of HDFs.

Host-pathogen systems biology: logical modelling of hepatocyte growth factor and Helicobacter pylori induced c-Met signal transduction

Background

The hepatocyte growth factor (HGF) stimulates mitogenesis, motogenesis, and morphogenesis in a wide range of tissues, including epithelial cells, on binding to the receptor tyrosine kinase c-Met. Abnormal c-Met signalling contributes to tumour genesis, in particular to the development of invasive and metastatic phenotypes. The human microbial pathogen Helicobacter pylori can induce chronic gastritis, peptic ulceration and more rarely, gastric adenocarcinoma. The H. pylori effector protein cytotoxin associated gene A (CagA), which is translocated via a type IV secretion system (T4SS) into epithelial cells, intracellularly modulates the c-Met receptor and promotes cellular processes leading to cell scattering, which could contribute to the invasiveness of tumour cells. Using a logical modelling framework, the presented work aims at analysing the c-Met signal transduction network and how it is interfered by H. pylori infection, which might be of importance for tumour development.

Results

A logical model of HGF and H. pylori induced c-Met signal transduction is presented in this work. The formalism of logical interaction hypergraphs (LIH) was used to construct the network model. The molecular interactions included in the model were all assembled manually based on a careful meta-analysis of published experimental results. Our model reveals the differences and commonalities of the response of the network upon HGF and H. pylori induced c-Met signalling. As another important result, using the formalism of minimal intervention sets, phospholipase Cγ1 (PLCγ1) was identified as knockout target for repressing the activation of the extracellular signal regulated kinase 1/2 (ERK1/2), a signalling molecule directly linked to cell scattering in H. pylori infected cells. The model predicted only an effect on ERK1/2 for the H. pylori stimulus, but not for HGF treatment. This result could be confirmed experimentally in MDCK cells using a specific pharmacological inhibitor against PLCγ1. The in silico predictions for the knockout of two other network components were also verified experimentally.

Conclusion

This work represents one of the first approaches in the direction of host-pathogen systems biology aiming at deciphering signalling changes brought about by pathogenic bacteria. The suitability of our network model is demonstrated by an in silico prediction of a relevant target against pathogen infection.

Gab1 but not Grb2 mediates tumor progression in Met overexpressing colorectal cancer cells

Hepatocyte growth factor receptor (Met) plays an important role in the progression of multiple cancer types. The overexpression of Met in DLD-1 colon carcinoma cells with kirsten rat sarcoma oncogene homolog (KRAS) oncogene activation resulted in enhanced subcutaneous and orthotopic tumor growth rate and increased metastatic potential. To elucidate the mechanism of this effect, we stably expressed kinase-inactive Met(K1110A), Src homology 2 (SH2)-binding domain-inactive Met(Y1349/1356F), growth factor receptor-bound protein 2 (Grb2) non-binding Met(N1358H) and mutant receptors with ability to selectively recruit signaling proteins Grb2, src homology domain c-terminal adaptor homolog (Shc), phospholipase c-gamma (PLCgamma) and p85 phosphatidyl inositol 3 kinase. As subcutaneous implants, DLD-1 cells that expressed the majority of these receptor constructs failed to recapitulate the tumor growth-enhancing effect of the wild-type Met receptor. The Grb2- and Shc-recruiting Met mutants demonstrated slight but consistent tumor-suppressive activity, whereas the expression of N1358H mutant stimulated tumor growth rate comparable with the wild-type receptor. This suggests that direct Grb2/Shc binding does not contribute to the tumor progression activity of Met receptor. The tumors expressing Grb2- and Shc-recruiting Met receptors demonstrated a marked loss in Grb2-associated adaptor protein 1 (Gab1) protein levels, which was not observed in the cell lines, consistent with a post-translationally regulated process. Moreover, a moderate level of Gab1 overexpression stimulated tumor growth. The findings suggest a delicate balance for intact Y1349/1356 SH2-binding domain to mediate the tumor progression activity of the coactivated Met-rat sarcoma oncogene homolog (RAS) pathways. Selectivity for specific adaptor protein involvement may be the key that determines the tissue- and cell-type specificity of Met-mediated tumorigenicity in human cancers.

Gab2 requires membrane targeting and the met binding motif to promote lamellipodia, cell scatter, and epithelial morphogenesis downstream from the met receptor

Gab1 and Gab2 are conserved scaffolding proteins that amplify and integrate signals stimulated by many growth factor receptors including the Met receptor. Gab1 acts to diversify the signal downstream from Met through the recruitment of multiple signaling proteins, and is essential for epithelial morphogenesis. However, whereas Gab1 and Gab2 are both expressed in epithelial cells, Gab2 fails to support a morphogenic response. We demonstrate that Gab1 and Gab2 are divergent in their function whereby Gab1, but not Gab2, promotes lamellipodia formation, and is localized to the membrane of lamellipodia upon Met activation. We have identified activation of ERK1/2 as a requirement for lamellipodia formation. Moreover, activated ERK1/2 are localized to lamellipodia in Gab1 expressing cells but not in cells that overexpress Gab2. By structure-function studies, we identify that enhanced membrane localization conferred through the addition of a myristoylation signal, together with the addition of the direct Met binding motif (MBM) from Gab1, are required to promote lamellipodia and confer a morphogenic signaling response to Gab2. Moreover, the morphogenesis competent myristoylated Gab2MBM promotes localization of activated ERK1/2 to the leading edge of lamellipodia in a similar manner to Gab1. Hence, subcellular localization of the Gab scaffold, as well as the ability of Gab to interact directly with the Met receptor, are both essential components of the morphogenic signaling response which involves lamellipodia formation and the localization of ERK1/2 activation in membrane ruffles.

Adaptor molecule Crk is required for sustained phosphorylation of Grb2-associated binder 1 and hepatocyte growth factor-induced cell motility of human synovial sarcoma cell lines

Activation of the c-Met receptor tyrosine kinase through its ligand, hepatocyte growth factor (HGF), promotes mitogenic, motogenic, and morphogenic cellular responses. Aberrant HGF/c-Met signaling has been strongly implicated in tumor cell invasion and metastasis. Both HGF and its receptor c-Met have been shown to be overexpressed in human synovial sarcoma, which often metastasizes to the lung; however, little is known about HGF-mediated biological effects in this sarcoma. Here, we provide evidence that Crk adaptor protein is required for the sustained phosphorylation of c-Met-docking protein Grb2-associated binder 1 (Gab1) in response to HGF, leading to the enhanced cell motility of human synovial sarcoma cell lines SYO-1, HS-SY-II, and Fuji. HGF stimulation induced the sustained phosphorylation on Y307 of Gab1 where Crk was recruited. Crk knockdown by RNA interference disturbed this HGF-induced tyrosine phosphorylation of Gab1. By mutational analysis, we identified that Src homology 2 domain of Crk is indispensable for the induction of the phosphorylation on multiple Tyr-X-X-Pro motifs containing Y307 in Gab1. HGF remarkably stimulated cell motility and scattering of synovial sarcoma cell lines, consistent with the prominent activation of Rac1, extreme filopodia formation, and membrane ruffling. Importantly, the elimination of Crk in these cells induced the disorganization of actin cytoskeleton and complete abolishment of HGF-mediated Rac1 activation and cell motility. Time-lapse microscopic analysis revealed the significant attenuation in scattering of Crk knockdown cells following HGF treatment. Furthermore, the depletion of Crk remarkably inhibited the tumor formation and its invasive growth in vivo. These results suggest that the sustained phosphorylation of Gab1 through Crk in response to HGF contributes to the prominent activation of Rac1 leading to enhanced cell motility, scattering, and cell invasion, which may support the crucial role of Crk in the aggressiveness of human synovial sarcoma.

Lenalidomide inhibits proliferation of Namalwa CSN.70 cells and interferes with Gab1 phosphorylation and adaptor protein complex assembly

Lenalidomide (Revlimid, CC-5013) belongs to a line of compounds known as immunomodulatory drugs (IMiDs) that are under clinical investigation in hematopoietic and solid tumor cancers. Lenalidomide efficacy has been reported in clinical trials of multiple myeloma and myelodysplastic syndromes (MDS), particularly in MDS patients with a del 5q cytogenetic abnormality, with or without other cytogenetic abnormalities. Here we report that lenalidomide inhibits proliferation of chromosome 5 deleted hematopoietic tumor cell lines in vitro, whether from the B cell, T cell, or myeloid lineage. There was diversity in the responses of the various cell lines to lenalidomide, with one undergoing cell cycle arrest, and others undergoing apoptosis. In the most lenalidomide-sensitive chromosome 5 deleted cell line, Namalwa CSN.70, the compound induced G0/G1 cell cycle arrest, inhibited Akt and Gab1 phosphorylation, and inhibited the ability of Gab1 to associate with a receptor tyrosine kinase. Lenalidomide also enhanced AP-1 transcriptional activity in Namalwa, but not in the other cell lines tested. These studies provide evidence for the mechanism of action of lenalidomide in chromosome 5 deleted hematopoietic tumors in vitro, and may provide a better understanding of the drug's activity in clinical applications.

Gab1 is required for cell cycle transition, cell proliferation, and transformation induced by an oncogenic met receptor

We have shown previously that either Grb2- or Shc-mediated signaling from the oncogenic Met receptor Tpr-Met is sufficient to trigger cell cycle progression in Xenopus oocytes. However, direct binding of these adaptors to Tpr-Met is dispensable, implying that another Met binding partner mediates these responses. In this study, we show that overexpression of Grb2-associated binder 1 (Gab1) promotes cell cycle progression when Tpr-Met is expressed at suboptimal levels. This response requires that Gab1 possess an intact Met-binding motif, the pleckstrin homology domain, and the binding sites for phosphatidylinositol 3-kinase and tyrosine phosphatase SHP-2, but not the Grb2 and CrkII/phospholipase Cgamma binding sites. Importantly, we establish that Gab1-mediated signals are critical for cell cycle transition promoted by the oncogenic Met and fibroblast growth factor receptors, but not by progesterone, the natural inducer of cell cycle transition in Xenopus oocytes. Moreover, Gab1 is essential for Tpr-Met-mediated morphological transformation and proliferation of fibroblasts. This study provides the first evidence that Gab1 is a key binding partner of the Met receptor for induction of cell cycle progression, proliferation, and oncogenic morphological transformation. This study identifies Gab1 and its associated signaling partners as potential therapeutic targets to impair proliferation or transformation of cancer cells in human malignancies harboring a deregulated Met receptor.

Transcriptional targets of hepatocyte growth factor signaling and Ki-ras oncogene activation in colorectal cancer

Both Ki-ras mutation and hepatocyte growth factor (HGF) receptor Met overexpression occur at high frequency in colon cancer. This study investigates the transcriptional changes induced by Ki-ras oncogene and HGF/Met signaling activation in colon cancer cell lines in vitro and in vivo. The model system used in these studies included the DLD-1 colon cancer cell line with a mutated Ki-ras allele, and the DKO-4 cell line generated from DLD-1, with its mutant Ki-ras allele inactivated by targeted disruption. These cell lines were transduced with cDNAs of full-length Met receptor. Microarray transcriptional profiling was conducted on cell lines stimulated with HGF, as well as on tumor xenograft tissues. Overlapping genes between in vitro and in vivo microarray data sets were selected as a subset of HGF/Met and Ki-ras oncogene-regulated targets. Using the Online Predicted Human Interaction Database, novel HGF/Met and Ki-ras regulated proteins with putative functional linkage were identified. Novel proteins identified included histone acetyltransferase 1, phosphoribosyl pyrophosphate synthetase 2, chaperonin containing TCP1, subunit 8, CSE1 chromosome segregation 1-like (yeast)/cellular apoptosis susceptibility (mammals), CCR4-NOT transcription complex, subunit 8, and cyclin H. Transcript levels for these Met-signaling targets were correlated with Met expression levels, and were significantly elevated in both primary and metastatic human colorectal cancer samples compared to normal colorectal mucosa. These genes represent novel Met and/or Ki-ras transcriptionally coregulated genes with a high degree of validation in human colorectal cancers.

CrkI and CrkII function as key signaling integrators for migration and invasion of cancer cells

Crk adaptor proteins play an important role during cellular signaling by mediating the formation of protein complexes. Increased levels of Crk proteins are observed in several human cancers and overexpression of Crk in epithelial cell cultures promotes enhanced cell dispersal and invasion, implicating Crk as a regulator of invasive responses. To determine the requirement of Crk for invasive signals, we targeted the CRKI/II gene by RNA interference. Consistent knockdown of CrkI/II was observed with two small interfering RNA targeting sequences in all human cancer cell lines tested. CrkI/II knockdown resulted in a significant decrease in migration and invasion of multiple malignant breast and other human cancer cell lines (MDA-231, MDA-435s, H1299, KB, and HeLa). Moreover, CrkI/II knockdown decreased cell spreading on extracellular matrix and led to a decrease in actin stress fibers and the formation of mature focal adhesions. Using immunohistochemistry, we show elevated CrkI/II protein levels in patients with breast adenocarcinoma. Together, these studies identify Crk adaptor proteins as critical integrators of upstream signals for cell invasion and migration in human cancer cell lines and support a role for Crk in metastatic spread.

Protein kinase C modulates negatively the hepatocyte growth factor-induced migration, integrin expression and phosphatidylinositol 3-kinase activation

Previously, we reported that, in hepatocyte growth factor (HGF)-induced HepG2 cells, protein kinase C (PKC) decreased the duration of intensive Erk1/Erk2 MAP kinase activation. This study shows that the inhibition of PKC enhanced significantly the HGF-induced integrin expression. Beside the prolonged activation of Erk1/Erk2, the activity of phosphatidylinositol 3-kinase (PI 3K) was required for growth factor-induced integrin expression. PI 3-kinase was activated to a higher extent in response to HGF than to epidermal growth factor (EGF), though the activation was transient in both cases. In EGF-induced cells, PI 3K activation was terminated by the loss of phosphotyrosine docking sites for PI 3K. To the contrary, the decrease of PI 3K activation, which followed the HGF-induced increase was not accompanied by the loss of phosphotyrosine docking sites and was prevented by the inhibition of PKC. The negative modulator effects of PKC on integrin expression and PI 3-kinase activation correlated with its ability to limit the HGF-induced motogen response.

Role of the Grb2-associated binder 1/SHP-2 interaction in cell growth and transformation

Grb2-associated binder 1 (Gab1) is a docking protein that is tyrosine phosphorylated following the activation of multiple cytokine receptors and receptor tyrosine kinases. Its function then is to recruit and activate multiple signaling molecules. In our previous work, we showed that Gab1 enhances cell growth and induces the transformed phenotype in NIH3T3 cells downstream of the epidermal growth factor (EGF) receptor. In this report, we analyze how it produces these effects. Because SHP-2 is the major binding partner of Gab1, we mutated its binding site in the Gab1 cDNA (Gab1/DeltaSHP-2). This construct was stably overexpressed in NIH3T3 cells (3T3-Gab1/DeltaSHP-2) and in the wild-type Gab1 cDNA (3T3-Gab1) or an empty expression vector (3T3-CTR). Our findings show that after EGF stimulation, Gab1/DeltaSHP-2 has a higher level of tyrosine phosphorylation at early time points than Gab1. Gab1/DeltaSHP-2 recruits more phosphatidylinositol 3'-kinase than Gab1 after EGF triggering, which accounts for a higher and more sustained AKT activation in 3T3-Gab1/DeltaSHP-2 cells relative to 3T3-Gab1 fibroblasts. Moreover, 3T3-Gab1/DeltaSHP-2 cells demonstrate a higher level of extracellular-regulated kinase 1 activation at early time points of EGF stimulation. However, there was an unexpected decrease in c-fos promoter induction in 3T3-Gab1/DeltaSHP-2 cells when compared with 3T3-Gab1 cells. Additionally, the 3T3-Gab1/DeltaSHP-2 cells show a reversion of the transformed phenotype, including fewer morphologic changes, an increase in stress fiber cytoskeletal organization, and a decrease in cell proliferation and anchorage independent growth. These results reveal that the Gab1/SHP-2 interaction is essential for cell growth and transformation but that this must occur through a novel pathway that is independent of extracellular-regulated kinase or AKT. On the basis of its role in growth and transformation, the Gab1/SHP-2 interaction may become an attractive target for the pharmacologic intervention of malignant cell growth.

Role of Gab1 in UV-induced c-Jun NH2-terminal kinase activation and cell apoptosis

Exposure of mammalian cells to UV irradiation leads to activation of the c-Jun NH(2)-terminal protein kinase (JNK) pathway, which is associated with cell apoptosis. However, the molecular mechanism for JNK activation by UV exposure is not fully understood. We show here an essential role of a multisubstrate adapter, Gab1, in this signaling cascade. Gab1-deficient mouse fibroblast cells were defective in induction of JNK activity by UV exposure or heat shock, and this defect was rescued by reintroduction of Gab1 into Gab1(-/-) cells. Consistently, Gab1(-/-) cells displayed reduced caspase 3 induction and apoptotic cell death in response to UV irradiation. Gab1 was constitutively complexed with JNK and became tyrosine phosphorylated in UV-irradiated cells. Genetic and pharmaceutical analyses suggest the involvement of c-Met and the Src family tyrosine kinases in mediating UV-induced Gab1 phosphorylation as well as JNK activation. In aggregate, these observations identify a new function of Gab1 in the response of mammalian cells to UV light.

Matrix metalloproteinases (MMP), EMMPRIN (extracellular matrix metalloproteinase inducer) and mitogen-activated protein kinases (MAPK): co-expression in metastatic serous ovarian carcinoma

Activation or suppression of intracellular signaling via the mitogen-activated protein kinase (MAPK) family has been linked to expression of matrix metalloproteinases (MMP) in experimental models, but this association has not been demonstrated in clinical material. The objective of this study was to investigate the possible association between expression and activity of MMP, expression of the MMP inducer EMMPRIN, and the expression (level) and phosphorylation status (activity) of the extracellular-regulated kinase (ERK), c-Jun amino-terminal kinase (JNK) and high osmolarity glycerol response kinase (p38) in effusions from patients diagnosed with serous ovarian carcinoma. MAPK level and activity were studied in 55 effusions using immunoblotting. MMP-1, MMP-2, MMP-9 and EMMPRIN expression was studied using immunocytochemistry (ICC) and mRNA in situ hybridization (ISH). The gelatinolytic activity of MMP-2 and MMP-9 was measured by zymography. ERK and phospho-ERK (p-ERK) were detected in 54/55 (98%) and 50/55 (91%) specimens, respectively. JNK and p-JNK were detected in 53/55 (96%) and 38/55 (69%) specimens, respectively. p38 was expressed in 54/55 (98%) specimens, and its phosphorylated form was found in 51/55 (92%). MMP-2 mRNA expression (P = 0.048), protein expression (P = 0.046) and gelatinolytic activity (P = 0.039) correlated with ERK phosphorylative activity. MMP-2 activity also correlated with p38 activity (P = 0.017). MMP-9 protein expression correlated with phosphorylation of p38 (P = 0.046), but enzyme activity showed inverse relationship with both p-ERK (P = 0.05) and p-p38 (P = 0.033) expression. EMMPRIN expression correlated with MMP-1 (P < 0.001), MMP-2 (P = 0.042) and MMP-9 (P = 0.029) expression, as well as with ERK activity (P = 0.001). Our results present the first evidence of a possible link between MAPK signaling and MMP expression and activity in vivo. These data may expand our understanding regarding the mechanisms by which MMP synthesis is regulated in effusions and possibly affect treatment strategies for this form of malignancy.

Src phosphorylates Grb2-associated binder 1 upon hepatocyte growth factor stimulation

Grb2-associated binder 1 (Gab1) is known to play an important role in hepatocyte growth factor (HGF) signaling, which rapidly becomes tyrosine-phosphorylated upon HGF stimulation. In this study, we found that the tyrosine phosphorylation of Gab1 in the cells derived from Src/Yes/Fyn null mouse embryos was approximately 40% lower than that in their wild type counterparts upon HGF stimulation. Increased expression of wild-type Src enhanced HGF-induced phosphorylation of Gab1, and, in contrast, expression of the Src kinase-deficient mutant or treatment of the specific Src inhibitor PP1 suppressed it. Expression of a constitutively active Src mutant (Y527F) or oncogenic v-Src led to a prominent increase in Gab1 phosphorylation independent of HGF stimulation. Moreover, Src interacted with Gab1 via both its Src homology 2 and 3 domains and was capable of phosphorylating purified Gab1 in vitro. Finally, the increased phosphorylation of Gab1 by Src selectively potentiated HGF-induced activation of ERK and AKT. Taken together, our results establish a new role for Src in HGF-induced Gab1 phosphorylation.

Hepatocyte growth factor signaling regulates transactivation of genes belonging to the plasminogen activation system via hypoxia inducible factor-1

Hepatocyte growth factor (HGF) plays an important role in tumor growth and progression also by regulating invasive/metastatic phenotype and angiogenesis. Here we report that a molecular mechanism possibly contributing to these functions of HGF may be hypoxia inducible factor-1 (HIF-1)-dependent expression of genes of the plasminogen activation system. The following findings support this conclusion: (1) HGF enhanced the activity of a luciferase reporter construct under the control of multiple HIF-1 responsive elements (HRE) in HepG2 cells, and the cotransfection of the dominant negative for the beta-subunit (ARNT) prevented this increase; (2) HGF activated uPA and PAI-1 promoters through HIF-1 activity regulated by PI3K/JNK1 transducers, as demonstrated by cotransfection with the reporter gene promoters and the dominant negative for ARNT, p85 subunit of PI3K or JNK1; (3) hypoxia was additive to HGF in increasing reporter vector activities, but probably through different transduction pathways; (4) JNK1 wild-type expression vector increased HIF-1alpha protein expression probably in a phosphorylated state and, thus, functional for transactivating activity; and (5) c-Jun did not seem to be involved in the activation of the luciferase construct containing multiple HREs because it was not prevented by expression of TAM-67, which is the dominant negative mutant form for c-Jun.

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.

Gab1 is an integrator of cell death versus cell survival signals in oxidative stress

Upon the addition of different growth factors and cytokines, the Gab1 docking protein is tyrosine phosphorylated and in turn activates different signaling pathways. On the basis of the large body of evidence concerning cross talk between the signaling pathways activated by growth factors and oxidative stress, we decided to investigate the role of Gab1 in oxidative injury. We stimulated wild-type mouse embryo fibroblasts (MEF) or MEF with a homozygous deletion of the Gab1 gene (-/- MEF) with H(2)O(2). Our results show that Gab1 is phosphorylated in a dose- and time-dependent manner after H(2)O(2) triggering. Gab1 then recruits molecules such as SHP2, phosphatidylinositol 3-kinase (PI3K), and Shc. Gab1 phosphorylation is sensitive to the Src family kinase inhibitor PP2. Furthermore, we demonstrate that Gab1 is required for H(2)O(2)-induced c-Jun N-terminal kinase (JNK) activation but not for ERK2 or p38 activation. Reconstitution of Gab1 in -/- MEF rescues JNK activation, and we find that this is dependent on the SHP2 binding site in Gab1. Cell viability assays reveal that Gab1 has a dual role in cell survival: a positive one through its interaction with PI3K and a negative one through its interaction with SHP2. This is the first report identifying Gab1 as a component in oxidative stress signaling and one that is required for JNK activation.

Membrane targeting of Grb2-associated binder-1 (Gab1) scaffolding protein through Src myristoylation sequence substitutes for Gab1 pleckstrin homology domain and switches an epidermal growth factor response to an invasive morphogenic program

The hepatocyte growth factor receptor tyrosine kinase Met promotes cell dissociation and the inherent morphogenic program of epithelial cells. In a search for substrates downstream from Met, we have previously identified the Grb2-associated binder-1 (Gab1) as critical for the morphogenic program. Gab1 is a scaffold protein that acts to diversify the signal downstream from the Met receptor through its ability to couple with multiple signal transduction pathways. Gab1 contains a pleckstrin homology (PH) domain with specificity for phosphatidylinositol 3,4,5-trisphosphate. The phospholipid binding capacity of the Gab1 PH domain is required for the localization of Gab1 at sites of cell-cell contact in colonies of epithelial cells and for epithelial morphogenesis, suggesting that PH domain-dependent subcellular localization of Gab1 is a prerequisite for function. We have investigated the requirement for membrane localization of Gab1 for biological activity. We show that substitution of the Gab1 PH domain with the myristoylation signal from the c-Src protein is sufficient to replace the Gab1 PH domain for epithelial morphogenesis. The membrane targeting of Gab1 enhances Rac activity in the absence of stimulation and switches a nonmorphogenic noninvasive response to epidermal growth factor to a morphogenic invasive program. These results suggest that the subcellular localization of Gab1 is a critical determinant for epithelial morphogenesis and invasiveness.

Macrophage-stimulating protein cooperates with erythropoietin to induce colony formation and MAP kinase activation in primary erythroid progenitor cells

We have shown that Fv2, the Friend virus susceptibility 2 locus, encodes a naturally occurring amino-terminally truncated form of the STK receptor tyrosine kinase (Sf-Stk). Sf-Stk appears to interact with the viral glycoprotein gp55 and drive erythropoietin (Epo)-independent expansion of Friend virus-infected erythroblasts. Presumably, Sf-Stk provides signals that cooperate with EpoR signaling to induce the polyclonal expansion of infected cells. In this report, we show that macrophage-stimulating protein (MSP), the ligand for full-length STK, can also cooperate with Epo to enhance burst-forming units-erythroid (BFU-E) formation. To evaluate the signals induced by MSP/STK in primary erythroid progenitor cells, we adapted a method for the expansion of murine bone marrow mononuclear cells. The expanded progenitor cells express STK and respond to MSP in a colony assay. Furthermore, we demonstrate that low doses of MSP and Epo stimulation of the expanded cells cooperate to induce the phosphorylation of MAP kinase. Using the MEK inhibitor PD98059, we show that the activation of ERK is required for the enhanced BFU-E formation in response to MSP. These findings suggest that MSP has the ability to enhance erythroid colony formation in response to Epo, and that this response is dependent on the ability of MSP to induce the MAP kinase pathway.

Multiple regions of internalin B contribute to its ability to turn on the Ras-mitogen-activated protein kinase pathway

Internalin B (InlB) is a protein present on the surface of Listeria monocytogenes that mediates bacterial entry into mammalian cells. It is thought that InlB acts by binding directly to the hepatocyte growth factor (HGF) receptor, present on the surface of host cells. Binding of InlB to the HGF receptor results in mitogen-activated protein (MAP) kinase and phosphoinositide 3-kinase activation, followed by changes in the organization of the actin cytoskeleton. Here we have compared signaling by HGF and InlB. Whereas stimulation with equivalent concentrations of HGF and InlB elicits similar activation of the HGF receptor, we observed striking differences in downstream activation of MAP kinase. InlB leads to a greater activation of the Ras-MAP kinase pathway than does HGF. The leucine-rich repeat region, which was previously shown to be sufficient for binding and activation of the HGF receptor, lacks the ability to super-activate the Ras-MAP kinase pathway. Analysis of a series of deletion mutants suggests that it is the B repeat region between the leucine-rich repeat and GW domains that endows InlB with an increased ability to turn on the Ras-MAP kinase pathway. These unexpected observations suggest that HGF and InlB use alternative mechanisms to turn on cellular signaling pathways.

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.

A Crk-II/TC10 signaling pathway is required for osmotic shock-stimulated glucose transport

Osmotic shock stimulates the translocation of the glucose transporter Glut 4 to plasma membrane by a tyrosine kinase signaling pathway involving Gab-1 (the Grb2-associated binder-1 protein). We show here that, in response to osmotic shock, Gab-1 acts as a docking protein for phospholipase Cgamma1, the p85 subunit of the phosphoinositide 3-kinase and Crk-II. It has been shown that the adapter Crk-II is constitutively associated with C3G, a GDP to GTP exchange factor for several small GTP-binding proteins. We found that inhibition of the activity of phosphoinositide 3-kinase or phospholipase C did not prevent the stimulation of glucose transport by osmotic shock, whereas inactivation of Rho proteins by Clostridium difficile toxin B severely inhibited glucose uptake. Among the Rho family members, overexpression of dominant-interfering TC10/T31N mutant inhibited osmotic shock-mediated Glut 4 translocation suggesting that TC10 is required for this process. Further, disruption of cortical actin integrity by latrunculin B or jasplakinolide severely impaired osmotic shock-induced glucose transport. In contrast, osmotic shock increased the amount of cortical actin associated with caveolin-enriched plasma membrane domains. These data provide the first evidence that activation of TC10 and remodeling of cortical actin, which could occur through the TC10 signaling, are required for osmotic shock-mediated Glut 4 translocation and glucose uptake.