The tyrosine phosphatase SHP-2 is required for sustained activation of extracellular signal-regulated kinase and epithelial morphogenesis downstream from the met receptor tyrosine kinase

c-Met as a Target for Personalized Therapy

MET and its ligand HGF are involved in many biological processes, both physiological and pathological, making this signaling pathway an attractive therapeutic target in oncology. Downstream signaling effects are transmitted via mitogen-activated protein kinase (MAPK), PI3K (phosphoinositide 3-kinase protein kinase B)/AKT, signal transducer and activator of transcription proteins (STAT), and nuclear factor-κB. The final output of the terminal effector components of these pathways is activation of cytoplasmic and nuclear processes leading to increases in cell proliferation, survival, mobilization and invasive capacity. In addition to its role as an oncogenic driver, increasing evidence implicates MET as a common mechanism of resistance to targeted therapies including EGFR and VEGFR inhibitors. In the present review, we summarize the current knowledge on the role of the HGF-MET signaling pathway in cancer and its therapeutic targeting (HGF activation inhibitors, HGF inhibitors, MET antagonists and selective/nonselective MET kinase inhibitors). Recent advances in understanding the role of this pathway in the resistance to current anticancer strategies used in lung, kidney and pancreatic cancer are discussed.

c-Met as a Target for Personalized Therapy

MET and its ligand HGF are involved in many biological processes, both physiological and pathological, making this signaling pathway an attractive therapeutic target in oncology. Downstream signaling effects are transmitted via mitogen-activated protein kinase (MAPK), PI3K (phosphoinositide 3-kinase protein kinase B)/AKT, signal transducer and activator of transcription proteins (STAT), and nuclear factor-κB. The final output of the terminal effector components of these pathways is activation of cytoplasmic and nuclear processes leading to increases in cell proliferation, survival, mobilization and invasive capacity. In addition to its role as an oncogenic driver, increasing evidence implicates MET as a common mechanism of resistance to targeted therapies including EGFR and VEGFR inhibitors. In the present review, we summarize the current knowledge on the role of the HGF-MET signaling pathway in cancer and its therapeutic targeting (HGF activation inhibitors, HGF inhibitors, MET antagonists and selective/nonselective MET kinase inhibitors). Recent advances in understanding the role of this pathway in the resistance to current anticancer strategies used in lung, kidney and pancreatic cancer are discussed.

Endothelial cell activation by hemodynamic shear stress derived from arteriovenous fistula for hemodialysis access

Intimal hyperplasia (IH) is the first cause of failure of an arteriovenous fistula (AVF). The aim of the present study was to investigate the effects on endothelial cells (ECs) of shear stress waveforms derived from AVF areas prone to develop IH. We used a cone-and-plate device to obtain real-time control of shear stress acting on EC cultures. We exposed human umbilical vein ECs for 48 h to different shear stimulations calculated in a side-to-end AVF model. Pulsatile unidirectional flow, representative of low-risk stenosis areas, induced alignment of ECs and actin fiber orientation with flow. Shear stress patterns of reciprocating flow, derived from high-risk stenosis areas, did not affect EC shape or cytoskeleton organization, which remained similar to static cultures. We also evaluated flow-induced EC expression of genes known to be involved in cytoskeletal remodeling and expression of cell adhesion molecules. Unidirectional flow induced a significant increase in Kruppel-like factor 2 mRNA expression, whereas it significantly reduced phospholipase D1, α4-integrin, and Ras p21 protein activator 1 mRNA expression. Reciprocating flow did not increase Kruppel-like factor 2 mRNA expression compared with static controls but significantly increased mRNA expression of phospholipase D1, α4-integrin, and Ras p21 protein activator 1. Reciprocating flow selectively increased monocyte chemoattractant protein-1 and IL-8 production. Furthermore, culture medium conditioned by ECs exposed to reciprocating flows selectively increased smooth muscle cell proliferation compared with unidirectional flow. Our results indicate that protective vascular effects induced in ECs by unidirectional pulsatile flow are not induced by reciprocating shear forces, suggesting a mechanism by which oscillating flow conditions may induce the development of IH in AVF and vascular access dysfunction.

AKT inactivation causes persistent drug tolerance to EGFR inhibitors

Drug resistance is a major obstacle to the success of EGFR-targeted therapy. We recently studied the mechanism by which a small subset of EGFR mutant lung cancer cells remains viable after EGFR inhibition. We found that this drug-tolerant subpopulation develops because EGFR inhibition prevents AKT activity and thus inactivates Ets-1 function. In this article, we discuss how changes in intrinsic cell signaling after EGFR inhibition open a new avenue to drug resistance in NSCLCs, and comment on combined TKI and MEK inhibitor treatment to reduce the probability of emergent resistance to EGFR TKIs.

Gab1 and Mapk Signaling Are Essential in the Hair Cycle and Hair Follicle Stem Cell Quiescence

Gab1 is a scaffold protein that acts downstream of receptor tyrosine kinases. Here, we produced conditional Gab1 mutant mice (by K14- and Krox20-cre) and show that Gab1 mediates crucial signals in the control of both the hair cycle and the self-renewal of hair follicle stem cells. Remarkably, mutant hair follicles do not enter catagen, the destructive phase of the hair cycle. Instead, hair follicle stem cells lose quiescence and become exhausted, and thus no stem cell niches are established in the bulges. Moreover, conditional sustained activation of Mapk signaling by expression of a gain-of-function Mek1(DD) allele (by Krox20-cre) rescues hair cycle deficits and restores quiescence of the stem cells. Our data thus demonstrate an essential role of Gab1 downstream of receptor tyrosine kinases and upstream of Shp2 and Mapk in the regulation of the hair cycle and the self-renewal of hair follicle stem cells.

SHP2 sails from physiology to pathology

Over the two past decades, mutations of the PTPN11 gene, encoding the ubiquitous protein tyrosine phosphatase SHP2 (SH2 domain-containing tyrosine phosphatase 2), have been identified as the causal factor of several developmental diseases (Noonan syndrome (NS), Noonan syndrome with multiple lentigines (NS-ML), and metachondromatosis), and malignancies (juvenile myelomonocytic leukemia). SHP2 plays essential physiological functions in organism development and homeostasis maintenance by regulating fundamental intracellular signaling pathways in response to a wide range of growth factors and hormones, notably the pleiotropic Ras/Mitogen-Activated Protein Kinase (MAPK) and the Phosphoinositide-3 Kinase (PI3K)/AKT cascades. Analysis of the biochemical impacts of PTPN11 mutations first identified both loss-of-function and gain-of-function mutations, as well as more subtle defects, highlighting the major pathophysiological consequences of SHP2 dysregulation. Then, functional genetic studies provided insights into the molecular dysregulations that link SHP2 mutants to the development of specific traits of the diseases, paving the way for the design of specific therapies for affected patients. In this review, we first provide an overview of SHP2's structure and regulation, then describe its molecular roles, notably its functions in modulating the Ras/MAPK and PI3K/AKT signaling pathways, and its physiological roles in organism development and homeostasis. In the second part, we describe the different PTPN11 mutation-associated pathologies and their clinical manifestations, with particular focus on the biochemical and signaling outcomes of NS and NS-ML-associated mutations, and on the recent advances regarding the pathophysiology of these diseases.

c-Met targeting in advanced gastric cancer: An open challenge

Despite significant improvements in systemic chemotherapy over the last two decades, the prognosis of patients with advanced gastric and gastroesophageal junction adenocarcinoma (GC) remains poor. Because of molecular heterogeneity, it is essential to classify tumors based on the underlying oncogenic pathways and to develop targeted therapies acting on individual tumors. High-quality research and advances in technology have contributed to the elucidation of molecular pathways underlying disease progression and have stimulated many clinical studies testing target therapies in an advanced disease setting. In particular, strong preclinical evidence for the aberrant activation of the HGF/c-Met signaling pathways in GC cancers exists. This review will cover the c-Met pathway, the mechanisms of c-Met activation and the different strategies of its inhibition. Next, we will focus on the current state of the art in the clinical evaluation of c-Met-targeted therapies and the description of ongoing randomized trials with the idea that in this disease, high quality translational research to identify and validate biomarkers is a priority task.

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.

EGFR inhibition evokes innate drug resistance in lung cancer cells by preventing Akt activity and thus inactivating Ets-1 function

Nonsmall cell lung cancer (NSCLC) is the leading cause of cancer death worldwide. About 14% of NSCLCs harbor mutations in epidermal growth factor receptor (EGFR). Despite remarkable progress in treatment with tyrosine kinase inhibitors (TKIs), only 5% of patients achieve tumor reduction >90%. The limited primary responses are attributed partly to drug resistance inherent in the tumor cells before therapy begins. Recent reports showed that activation of receptor tyrosine kinases (RTKs) is an important determinant of this innate drug resistance. In contrast, we demonstrate that EGFR inhibition promotes innate drug resistance despite blockade of RTK activity in NSCLC cells. EGFR TKIs decrease both the mitogen-activated protein kinase (MAPK) and Akt protein kinase pathways for a short time, after which the Ras/MAPK pathway becomes reactivated. Akt inhibition selectively blocks the transcriptional activation of Ets-1, which inhibits its target gene, dual specificity phosphatase 6 (DUSP6), a negative regulator specific for ERK1/2. As a result, ERK1/2 is activated. Furthermore, elevated c-Src stimulates Ras GTP-loading and activates Raf and MEK kinases. These observations suggest that not only ERK1/2 but also Akt activity is essential to maintain Ets-1 in an active state. Therefore, despite high levels of ERK1/2, Ets-1 target genes including DUSP6 and cyclins D1, D3, and E2 remain suppressed by Akt inhibition. Reduction of DUSP6 in combination with elevated c-Src renews activation of the Ras/MAPK pathway, which enhances cell survival by accelerating Bim protein turnover. Thus, EGFR TKIs evoke innate drug resistance by preventing Akt activity and inactivating Ets-1 function in NSCLC cells.

Functions of Shp2 in cancer

Diagnostics and therapies have shown evident advances. Tumour surgery, chemotherapy and radiotherapy are the main techniques in treat cancers. Targeted therapy and drug resistance are the main focus in cancer research, but many molecular intracellular mechanisms remain unknown. Src homology region 2-containing protein tyrosine phosphatase 2 (Shp2) is associated with breast cancer, leukaemia, lung cancer, liver cancer, gastric cancer, laryngeal cancer, oral cancer and other cancer types. Signalling pathways involving Shp2 have also been discovered. Shp2 is related to many diseases. Mutations in the ptpn11 gene cause Noonan syndrome, LEOPARD syndrome and childhood leukaemia. Shp2 is also involved in several cancer-related processes, including cancer cell invasion and metastasis, apoptosis, DNA damage, cell proliferation, cell cycle and drug resistance. Based on the structure and function of Shp2, scientists have investigated specific mechanisms involved in cancer. Shp2 may be a potential therapeutic target because this phosphatase is implicated in many aspects. Furthermore, Shp2 inhibitors have been used in experiments to develop treatment strategies. However, conflicting results related to Shp2 functions have been presented in the literature, and such results should be resolved in future studies.

Structure and function of Gab2 and its role in cancer (Review)

The docking proteins of the Grb-associated binder (Gab) family transduce cellular signals between receptors and intracellular downstream effectors, and provide a platform for protein-protein interactions. Gab2, a key member of the Gab family of proteins, is involved in the amplification and integration of signal transduction, evoked by a variety of extracellular stimuli, including growth factors, cytokines and antigen receptors. Gab2 protein lacks intrinsic catalytic activity; however, when phosphorylated by protein-tyrosine kinases (PTKs), Gab2 recruits several Src homology-2 (SH2) domain-containing proteins, including the SH2-containing protein tyrosine phosphatase 2 (SHP2), the p85 subunit of phosphoinositide-3 kinase (PI3K), phospholipase C-γ (PLCγ)1, Crk, and GC-GAP. Through these interactions, the Gab2 protein triggers various downstream signal effectors, including SHP2/rat sarcoma viral oncogene/RAF/mitogen-activated protein kinase kinase/extracellular signal-regulated kinase and PI3K/AKT, involved in cell growth, differentiation, migration and apoptosis. It has been previously reported that aberrant Gab2 and/or Gab2 signaling is closely associated with human tumorigenesis, particularly in breast cancer, leukemia and melanoma. The present review aimed to focus on the structure and effector function of Gab2, its role in cancer and its potential for use as an effective therapeutic target.

Dysregulation of the Met pathway in non-small cell lung cancer: implications for drug targeting and resistance

The receptor tyrosine kinase, Met, orchestrates a complex signalling network that physiologically drives a programme of 'invasive growth'. In cancer however, this process may be co-opted to promote proliferation, survival and metastasis of cancer cells. Met is thus a key therapeutic target, not least in non-small cell lung cancer (NSCLC) where it is one of the most commonly dysregulated driver oncogenes. Identifying robust biomarkers that allow the selection of patients most likely to respond to Met targeted therapies will however be essential to realising their potential. This has been underlined recently by the early termination of three pivotal phase III trials investigating Met targeted agents in NSCLC, all of which failed to show clinical benefit. In contrast to these trials, which were relatively unselective, a couple of early phase trials have recently been instigated that select patients on the basis of Met amplification. While still at an early stage, interim results are relatively encouraging and strengthen the rationale for using Met amplifaction as a biomarker. Here we will discuss this and other aberrations in Met signalling in relation to their significance in the therapeutic targeting of Met.

Adaptor protein CRK induces epithelial-mesenchymal transition and metastasis of bladder cancer cells through HGF/c-Met feedback loop

We have previously reported that an adaptor protein CRK, including CRK-I and CRK-II, plays essential roles in the malignant potential of various aggressive human cancers, suggesting the validity of targeting CRK in molecular targeted therapy of a wide range of cancers. Nevertheless, the role of CRK in human bladder cancer with marked invasion, characterized by distant metastasis and poor prognosis, remains obscure. In the present study, immunohistochemistry indicated a striking enhancement of CRK-I/-II, but not CRK-like, in human bladder cancer tissues compared to normal urothelium. We established CRK-knockdown bladder cancer cells using 5637 and UM-UC-3, which showed a significant decline in cell migration, invasion, and proliferation. It is noteworthy that an elimination of CRK conferred suppressed phosphorylation of c-Met and the downstream scaffold protein Gab1 in a hepatocyte growth factor-dependent and -independent manner. In epithelial–mesenchymal transition-related molecules, E-cadherin was upregulated by CRK elimination, whereas N-cadherin, vimentin, and Zeb1 were downregulated. A similar effect was observed following treatment with c-Met inhibitor SU11274. Depletion of CRK significantly decreased cell proliferation of 5637 and UM-UC-3, consistent with reduced activity of ERK. An orthotopic xenograft model with bioluminescent imaging revealed that CRK knockdown significantly attenuated not only tumor volume but also the number of circulating tumor cells, resulted in a complete abrogation of metastasis. Taken together, this evidence uncovered essential roles of CRK in invasive bladder cancer through the hepatocyte growth factor/c-Met/CRK feedback loop for epithelial–mesenchymal transition induction. Thus, CRK might be a potent molecular target in bladder cancer, particularly for preventing metastasis, leading to the resolution of clinically longstanding critical issues.

MET: a new promising biomarker in non-small-cell lung carcinoma

Non-small-cell lung cancer (NSCLC) leads cancer-related deaths worldwide. Mutations in the kinase domain of the EGFR gene provide sensitivity to tyrosine kinase inhibitors (TKI) drugs. TKI show initial response rates over 75% in mutant EGFR-NSCLC patients, although most of these patients acquire resistance to EGFR inhibitors after therapy. EGFR-TKI resistance mechanisms include amplification in MET and its ligand, and also MET mutations. MET signaling dysregulation has been involved in tumor cell growth, survival, migration and invasion, angiogenesis and activation of several pathways, therefore representing an attractive target for anticancer drug development. In this review, we will discuss MET-related mechanisms of EGFR-TKI resistance in NSCLC, as well as the main drugs targeted to inhibit MET pathway.

MET genetic lesions in non-small-cell lung cancer: pharmacological and clinical implications

Lung cancer is the leading cause of death for solid tumors worldwide with an annual mortality of over one million. Lung carcinoma includes a series of different diseases which are roughly divided into two groups based on clinical and histo-pathological features: non-small cell lung cancer (NSCLC), accounting for almost 80% of lung cancer diagnosis and small cell lung cancer (SCLC) responsible for the remaining 20%. The NSCLC molecular profile has been deeply investigated; alterations in several oncogenes, tumor suppressor genes and transcription factors have been detected, mainly in adenocarcinomas. Dissection of such a complex scenario represents a still open challenge for both researchers and clinicians. MET, the receptor for Hepatocyte Growth Factor (HGF), has been recently identified as a novel promising target in several human malignancies, including NSCLC. Deregulation of the HGF/MET signaling pathway can occur via different mechanisms, including HGF and/or MET overexpression, MET gene amplification, mutations or rearrangements. While the role of MET mutations in NSCLC is not yet fully understood, MET amplification emerged as a critical event in driving cell survival, with preclinical data suggesting that MET-amplified cell lines are exquisitely sensitive to MET inhibition. True MET amplification, which has been associated with poor prognosis in different retrospective series, is a relatively uncommon event in NSCLC, occurring in 1-7% of unselected cases. Nevertheless, in highly selected cohorts of patients, such as those harboring somatic mutations of EGFR with acquired resistance to EGFR tyrosine kinase inhibitors, MET amplification can be observed in up to 20% of cases. Preclinical data suggested that a treatment approach including a combination of EGFR and MET tyrosine kinases could be an effective strategy in this setting and led to the clinical investigation of multiple MET inhibitors in combination with anti-EGFR agents. Results from ongoing and future trials will clarify the role of anti-MET molecules for the treatment of NSCLC and will provide insights into the most appropriate timing for their use. The present review recapitulates the current knowledge on the role of MET signaling in NSCLC mainly focusing on its implications in molecular diagnostic approach and on the novel targeted inhibitors.

Predicting resistance by selection of signaling pathways

Epidermal growth factor receptor (EGFR) mutations occur in 17% of non-small-cell lung cancer (NSCLC) patients with notable response to single agent therapy but with low complete remission rate and, eventually, disease progression. Priming BIM, a pro-apoptotic signaling BH3-only protein, induces sensitivity to erlotinib in EGFR-mutant cell lines. Synthetic lethal approaches and preemptive therapies based on the initial expression of BIM may significantly improve the treatment outcome. EGFR mutations result in transient pro-death imbalance of survival and apoptotic signaling in response to EGFR inhibition. SHP2 is essential to the balance between ERK and the phosphoinositide-3-kinase (PI3K)/AKT and signal transducer activator of transcription (STAT) activity, while mTOR can be an additional marker for patients with high BIM expression. Furthermore, stromal hepatocyte growth factor (HGF) confers EGFR tyrosine kinase inhibitor (TKI) resistance and induces interreceptor crosstalk with integrin-b4, Eph2, CUB domain-containing protein-1 (CDCP1), AXL and JAK1. Only by understanding better, and in more depth, complex cancer molecular biology will we have the information that will help us to design strategies to augment efficacy of EGFR TKIs and offer our patients the best, most correct therapeutic option.

Regulation of EGFR signal transduction by analogue-to-digital conversion in endosomes

An outstanding question is how receptor tyrosine kinases (RTKs) determine different cell-fate decisions despite sharing the same signalling cascades. Here, we uncovered an unexpected mechanism of RTK trafficking in this process. By quantitative high-resolution FRET microscopy, we found that phosphorylated epidermal growth factor receptor (p-EGFR) is not randomly distributed but packaged at constant mean amounts in endosomes. Cells respond to higher EGF concentrations by increasing the number of endosomes but keeping the mean p-EGFR content per endosome almost constant. By mathematical modelling, we found that this mechanism confers both robustness and regulation to signalling output. Different growth factors caused specific changes in endosome number and size in various cell systems and changing the distribution of p-EGFR between endosomes was sufficient to reprogram cell-fate decision upon EGF stimulation. We propose that the packaging of p-RTKs in endosomes is a general mechanism to ensure the fidelity and specificity of the signalling response.

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.

Physiological Signaling and Structure of the HGF Receptor MET

The "hepatocyte growth factor" also known as "scatter factor", is a multifunctional cytokine with the peculiar ability of simultaneously triggering epithelial cell proliferation, movement and survival. The combination of those proprieties results in the induction of an epithelial to mesenchymal transition in target cells, fundamental for embryogenesis but also exploited by tumor cells during metastatization. The hepatocyte growth factor receptor, MET, is a proto-oncogene and a prototypical transmembrane tyrosine kinase receptor. Inhere we discuss the MET molecular structure and the hepatocyte growth factor driven physiological signaling which coordinates epithelial proliferation, motility and morphogenesis.

Systemic treatment in EGFR-ALK NSCLC patients: second line therapy and beyond

Lung cancer is the most frequently diagnosed cancer and a leading cause of cancer mortality worldwide, with adenocarcinoma being the most common histological subtype. Deeper understanding of the pathobiology of non-small cell lung cancer (NSCLC) has led to the development of small molecules that target genetic mutations known to play critical roles in progression to metastatic disease and to influence response to targeted therapies. The principle goal of precision medicine is to define those patient populations most likely to respond to targeted therapies. However, the cancer genome landscape is composed of relatively few "mountains" [representing the most commonly mutated genes like KRAS, epidermal growth factor (EGFR), and anaplastic lymphoma kinase (ALK)] and a vast number of "hills" (representing low frequency but potentially actionable mutations). Low-frequency lesions that affect a druggable gene product allow a relatively small population of cancer patients for targeted therapy to be selected.

The Hepatocyte Growth Factor (HGF)/Met Axis: A Neglected Target in the Treatment of Chronic Myeloproliferative Neoplasms?

Met is the receptor of hepatocyte growth factor (HGF), a cytoprotective cytokine. Disturbing the equilibrium between Met and its ligand may lead to inappropriate cell survival, accumulation of genetic abnormalities and eventually, malignancy. Abnormal activation of the HGF/Met axis is established in solid tumours and in chronic haematological malignancies, including myeloma, acute myeloid leukaemia, chronic myelogenous leukaemia (CML), and myeloproliferative neoplasms (MPNs). The molecular mechanisms potentially responsible for the abnormal activation of HGF/Met pathways are described and discussed. Importantly, inCML and in MPNs, the production of HGF is independent of Bcr-Abl and JAK2V617F, the main molecular markers of these diseases. In vitro studies showed that blocking HGF/Met function with neutralizing antibodies or Met inhibitors significantly impairs the growth of JAK2V617F-mutated cells. With personalised medicine and curative treatment in view, blocking activation of HGF/Met could be a useful addition in the treatment of CML and MPNs for those patients with high HGF/MET expression not controlled by current treatments (Bcr-Abl inhibitors in CML; phlebotomy, hydroxurea, JAK inhibitors in MPNs).

Cell density modulates SHC3 expression and survival of human glioblastoma cells through Fak activation

Shc3 protein levels are high in human glioblastoma but they decrease in vitro. We found that SHC3 mRNA and protein increased when glioblastoma cells grew as multicellular tumor spheroid (MTS). Shc3 expression was also induced in adherent cultures by increasing cell density. Among the Shc family members, only Shc2 and Shc3 increased with cell density. Shc3 and focal adhesion kinase (Fak) interact as shown by co-immunoprecipitation. Inhibition of Fak activation reduced Shc3 increase and MTS formation and changed Shc3 phosphorylation pattern. Our results suggest that in gliomas cell density modulates Shc3 protein levels and its activity, at least in part, through Fak activation.

Molecular basis of gain-of-function LEOPARD syndrome-associated SHP2 mutations

The Src homology 2 (SH2) domain-containing protein tyrosine phosphatase 2 (SHP2) is a critical signal transducer downstream of growth factors that promotes the activation of the RAS-ERK1/2 cascade. In its basal state, SHP2 exists in an autoinhibited closed conformation because of an intramolecular interaction between its N-SH2 and protein tyrosine phosphatase (PTP) domains. Binding to pTyr ligands present on growth factor receptors and adaptor proteins with its N-SH2 domain localizes SHP2 to its substrates and frees the active site from allosteric inhibition. Germline mutations in SHP2 are known to cause both Noonan syndrome (NS) and LEOPARD syndrome (LS), two clinically similar autosomal dominant developmental disorders. NS-associated SHP2 mutants display elevated phosphatase activity, while LS-associated SHP2 mutants exhibit reduced catalytic activity. A conundrum in how clinically similar diseases result from mutations to SHP2 that have opposite effects on this enzyme's catalytic functionality exists. Here we report a comprehensive investigation of the kinetic, structural, dynamic, and biochemical signaling properties of the wild type as well as all reported LS-associated SHP2 mutants. The results reveal that LS-causing mutations not only affect SHP2 phosphatase activity but also induce a weakening of the intramolecular interaction between the N-SH2 and PTP domains, leading to mutants that are more readily activated by competing pTyr ligands. Our data also indicate that the residual phosphatase activity associated with the LS SHP2 mutant is required for enhanced ERK1/2 activation. Consequently, catalytically impaired SHP2 mutants could display gain-of-function properties because of their ability to localize to the vicinity of substrates for longer periods of time, thereby affording the opportunity for prolonged substrate turnover and sustained RAS-ERK1/2 activation.

Dynamic reprogramming of signaling upon met inhibition reveals a mechanism of drug resistance in gastric cancer

The Met receptor tyrosine kinase is activated or genetically amplified in some gastric cancers, but resistance to small-molecule inhibitors of Met often emerges in patients. We found that Met abundance correlated with a proliferation marker in patient gastric tumor sections, and gastric cancer cell lines that have MET amplifications depended on Met for proliferation and anchorage-independent growth in culture. Inhibition of Met induced temporal changes in gene expression in the cell lines, initiated by a rapid decrease in the expression of genes encoding transcription factors, followed by those encoding proteins involved in epithelial-mesenchymal transition, and finally those encoding cell cycle-related proteins. In the gastric cancer cell lines, microarray and chromatin immunoprecipitation analysis revealed considerable overlap between genes regulated in response to Met stimulation and those regulated by signal transducer and activator of transcription 3 (STAT3). The activity of STAT3, extracellular signal-regulated kinase (ERK), and the kinase Akt was decreased by Met inhibition, but only inhibitors of STAT3 were as effective as the Met inhibitor in decreasing tumor cell proliferation in culture and in xenografts, suggesting that STAT3 mediates the pro-proliferative program induced by Met. However, the phosphorylation of ERK increased after prolonged Met inhibition in culture, correlating with decreased abundance of the phosphatases DUSP4 and DUSP6, which inhibit ERK. Combined inhibition of Met and the mitogen-activated protein kinase kinase (MEK)-ERK pathway induced greater cell death in cultured gastric cancer cells than did either inhibitor alone. These findings indicate combination therapies that may counteract resistance to Met inhibitors.

The Met receptor tyrosine kinase: a key player in oncogenesis and drug resistance

The Met receptor tyrosine kinase (RTK) is an attractive oncology therapeutic target. Met and its ligand, HGF, play a central role in signaling pathways that are exploited during the oncogenic process, including regulation of cell proliferation, invasion, angiogenesis, and cancer stem cell regulation. Elevated Met and HGF as well as numerous Met genetic alterations have been reported in human cancers and correlate with poor outcome. Alterations of pathways that regulate Met, such as the ubiquitin ligase c-Cbl are also likely to activate Met in the oncogenic setting. Moreover, interactive crosstalk between Met and other receptors such as EGFR, HER2 and VEGFR, underlies a key role for Met in resistance to other RTK-targeted therapies. A large body of preclinical and clinical data exists that supports the use of either antibodies or small molecule inhibitors that target Met or HGF as oncology therapeutics. The prognostic potential of Met expression has been suggested from studies in numerous cancers including lung, renal, liver, head and neck, stomach, and breast. Clinical trials using Met inhibitors indicate that the level of Met expression is a determinant of trial outcome, a finding that is actively under investigation in multiple clinical scenarios. Research in Met prognostics and predictors of drug response is now shifting toward more sophisticated methodologies suitable for development as validated and effective biomarkers that can be partnered with therapeutics to improve patient survival.

Molecular mechanisms of SH2- and PTB-domain-containing proteins in receptor tyrosine kinase signaling

Intracellular signaling is mediated by reversible posttranslational modifications (PTMs) that include phosphorylation, ubiquitination, and acetylation, among others. In response to extracellular stimuli such as growth factors, receptor tyrosine kinases (RTKs) typically dimerize and initiate signaling through phosphorylation of their cytoplasmic tails and downstream scaffolds. Signaling effectors are recruited to these phosphotyrosine (pTyr) sites primarily through Src homology 2 (SH2) domains and pTyr-binding (PTB) domains. This review describes how these conserved domains specifically recognize pTyr residues and play a major role in mediating precise downstream signaling events.

Frs2α and Shp2 signal independently of Gab to mediate FGF signaling in lens development

Fibroblast growth factor (FGF) signaling requires a plethora of adaptor proteins to elicit downstream responses, but the functional significances of these docking proteins remain controversial. In this study, we used lens development as a model to investigate Frs2α and its structurally related scaffolding proteins, Gab1 and Gab2, in FGF signaling. We show that genetic ablation of Frs2α alone has a modest effect, but additional deletion of tyrosine phosphatase Shp2 causes a complete arrest of lens vesicle development. Biochemical evidence suggests that this Frs2α-Shp2 synergy reflects their epistatic relationship in the FGF signaling cascade, as opposed to compensatory or parallel functions of these two proteins. Genetic interaction experiments further demonstrate that direct binding of Shp2 to Frs2α is necessary for activation of ERK signaling, whereas constitutive activation of either Shp2 or Kras signaling can compensate for the absence of Frs2α in lens development. By contrast, knockout of Gab1 and Gab2 failed to disrupt FGF signaling in vitro and lens development in vivo. These results establish the Frs2α-Shp2 complex as the key mediator of FGF signaling in lens development.

Targeting the yin and the yang: combined inhibition of the tyrosine kinase c-Src and the tyrosine phosphatase SHP-2 disrupts pancreatic cancer signaling and biology in vitro and tumor formation in vivo

OBJECTIVES

Although c-Src (Src) has emerged as a potential pancreatic cancer target in preclinical studies, Src inhibitors have not demonstrated a significant therapeutic benefit in clinical trials. The objective of these studies was to examine the effects of combining Src inhibition with inhibition of the protein tyrosine phosphatase SHP-2 in pancreatic cancer cells in vitro and in vivo.

METHODS

SHP-2 and Src functions were inhibited by siRNA or small molecule inhibitors. The effects of dual Src/SHP-2 functional inhibition were evaluated by Western blot analysis of downstream signaling pathways; cell biology assays to examine caspase activity, viability, adhesion, migration, and invasion in vitro; and an orthotopic nude mouse model to observe pancreatic tumor formation in vivo.

RESULTS

Dual targeting of Src and SHP-2 induces an additive or supra-additive loss of phosphorylation of Akt and ERK-1/2 and corresponding increases in expression of apoptotic markers, relative to targeting either protein individually. Combinatorial inhibition of Src and SHP-2 significantly reduces viability, adhesion, migration, and invasion of pancreatic cancer cells in vitro and tumor formation in vivo, relative to individual Src/SHP-2 inhibition.

CONCLUSIONS

These data suggest that the antitumor effects of Src inhibition in pancreatic cancer may be enhanced through simultaneous inhibition of SHP-2.

MET: a critical player in tumorigenesis and therapeutic target

Since its discovery more than 25 years ago, numerous studies have established that the MET receptor is unique among tyrosine kinases. Signaling through MET is necessary for normal development and for the progression of a wide range of human cancers. MET activation has been shown to drive numerous signaling pathways; however, it is not clear how MET signaling mediates diverse cellular responses such as motility, invasion, growth, and angiogenesis. Great strides have been made in understanding the pleotropic aspects of MET signaling using three-dimensional molecular structures, cell culture systems, human tumors, and animal models. These combined approaches have driven the development of MET-targeted therapeutics that have shown promising results in the clinic. Here we examine the unique features of MET and hepatocyte growth factor/scatter factor (HGF/SF) structure and signaling, mutational activation, genetic mouse models of MET and HGF/SF, and MET-targeted therapeutics.

The hemopexin domain of MMP3 is responsible for mammary epithelial invasion and morphogenesis through extracellular interaction with HSP90β

Matrix metalloproteinases (MMPs) are crucial mediators in sculpting tissue architecture and are required for many physiological and pathological processes. MMP3 has been shown to regulate branching morphogenesis in the mammary gland. Ectopic expression of proteolytically active MMP3 in mouse mammary epithelia triggers supernumerary lateral branching and, eventually, tumors. Using a three-dimensional collagen-I (Col-1) gel assay that simulates epithelial invasion and branching, we show that it is the hemopexin domain that directs these processes. Using three different engineered constructs containing a variation on MMP3 structural domains, we confirmed the importance of the hemopexin domain also in primary organoids of the mammary gland. A proteomic screen of MMP3-binding partners surprisingly revealed that the intracellular chaperone heat-shock protein 90 β (HSP90β) is present extracellularly, and its interaction with the hemopexin domain of MMP3 is critical for invasion. Blocking of HSP90β with inhibitory antibodies added to the medium abolished invasion and branching. These findings shift the focus from the proteolytic activity of MMP3 as the central player to its hemopexin domain and add a new dimension to HSP90β's functions by revealing a hitherto undescribed mechanism of MMP3 regulation. Our data also may shed light on the failure of strategies to use MMP inhibitors in cancer treatment and other related disorders.

Modulation of c-Met signaling and cellular sensitivity to radiation: potential implications for therapy

The c-Met/hepatocyte growth factor receptor and its family members are known to promote cancer cell migration and invasion. Signaling within and beyond this pathway contributes to the systemic spread of metastases through induction of the epithelial-mesenchymal transition, a process also implicated in mediating resistance to current anticancer therapies, including radiation. Induction of c-Met has also been observed after irradiation, suggesting that c-Met participates in radiation-induced disease progression through the epithelial-mesenchymal transition. Therefore, c-Met inhibition is an attractive target for potentially mitigating radiation resistance. This article summarizes key findings regarding crosstalk between radiotherapy and c-Met and discusses studies performed to date in which c-Met inhibition was used as a strategy to increase cellular radiosensitivity.

Gab docking proteins in cardiovascular disease, cancer, and inflammation

The docking proteins of the Grb2-associated binder (Gab) family have emerged as crucial signaling compartments in metazoans. In mammals, the Gab proteins, consisting of Gab1, Gab2, and Gab3, are involved in the amplification and integration of signal transduction evoked by a variety of extracellular stimuli, including growth factors, cytokines, antigens, and other molecules. Gab proteins lack the enzymatic activity themselves; however, when phosphorylated on tyrosine residues, they provide binding sites for multiple Src homology-2 (SH2) domain-containing proteins, such as SH2-containing protein tyrosine phosphatase 2 (SHP2), phosphatidylinositol 3-kinase regulatory subunit p85, phospholipase Cγ, Crk, and GC-GAP. Through these interactions, the Gab proteins transduce signals from activated receptors into pathways with distinct biological functions, thereby contributing to signal diversification. They are known to play crucial roles in numerous physiological processes through their associations with SHP2 and p85. In addition, abnormal Gab protein signaling has been linked to human diseases including cancer, cardiovascular disease, and inflammatory disorders. In this paper, we provide an overview of the structure, effector functions, and regulation of the Gab docking proteins, with a special focus on their associations with cardiovascular disease, cancer, and inflammation.

Novel role for SHP-2 in nutrient-responsive control of S6 kinase 1 signaling

Amino acids are required for the activation of the mammalian target of rapamycin complex 1 (mTORC1), which plays a critical role in cell growth, proliferation, and metabolism. The branched-chain amino acid leucine is an essential nutrient that stimulates mTORC1 to promote protein synthesis by activating p70 S6 kinase 1 (S6K1). Here we show that the protein tyrosine phosphatase SHP-2 is required for leucine-induced activation of S6K1 in skeletal myoblasts. In response to leucine, S6K1 activation is inhibited in myoblasts either lacking SHP-2 expression or overexpressing a catalytically inactive mutant of SHP-2. Activation of S6K1 by leucine requires the mobilization of intracellular calcium (Ca(2+)), which we show is mediated by SHP-2 in an inositol-1,4,5-trisphosphate-dependent manner. Ectopic Ca(2+) mobilization rescued the S6K1 activation defect in SHP-2-deficient myoblasts. SHP-2 was identified to act upstream of phospholipase C β4, linking it to the generation of nutrient-induced Ca(2+) release and S6K1 phosphorylation. Consistent with these results, SHP-2-deficient myoblasts exhibited impaired leucine sensing, leading to defective autophagy and reduced myoblast size. These data define a new role for SHP-2 as a nutrient-sensing regulator in skeletal myoblasts that is required for the activation of S6K1.

The signaling adaptor GAB1 regulates cell polarity by acting as a PAR protein scaffold

Cell polarity plays a key role in development and is disrupted in tumors, yet the molecules and mechanisms that regulate polarity remain poorly defined. We found that the scaffolding adaptor GAB1 interacts with two polarity proteins, PAR1 and PAR3. GAB1 binds PAR1 and enhances its kinase activity. GAB1 brings PAR1 and PAR3 into a transient complex, stimulating PAR3 phosphorylation by PAR1. GAB1 and PAR6 bind the PAR3 PDZ1 domain and thereby compete for PAR3 binding. Consequently, GAB1 depletion causes PAR3 hypophosphorylation and increases PAR3/PAR6 complex formation, resulting in accelerated and enhanced tight junction formation, increased transepithelial resistance, and lateral domain shortening. Conversely, GAB1 overexpression, in a PAR1/PAR3-dependent manner, disrupts epithelial apical-basal polarity, promotes multilumen cyst formation, and enhances growth factor-induced epithelial cell scattering. Our results identify GAB1 as a negative regulator of epithelial cell polarity that functions as a scaffold for modulating PAR protein complexes on the lateral membrane.

Detection of a rare BCR-ABL tyrosine kinase fusion protein in H929 multiple myeloma cells using immunoprecipitation (IP)-tandem mass spectrometry (MS/MS)

Hypothesis directed proteomics offers higher throughput over global analyses. We show that immunoprecipitation (IP)-tandem mass spectrometry (LC-MS/MS) in H929 multiple myeloma (MM) cancer cells led to the discovery of a rare and unexpected BCR-ABL fusion, informing a therapeutic intervention using imatinib (Gleevec). BCR-ABL is the driving mutation in chronic myeloid leukemia (CML) and is uncommon to other cancers. Three different IP-MS experiments central to cell signaling pathways were sufficient to discover a BCR-ABL fusion in H929 cells: phosphotyrosine (pY) peptide IP, p85 regulatory subunit of phosphoinositide-3-kinase (PI3K) IP, and the GRB2 adaptor IP. The pY peptides inform tyrosine kinase activity, p85 IP informs the activating adaptors and receptor tyrosine kinases (RTKs) involved in AKT activation and GRB2 IP identifies RTKs and adaptors leading to ERK activation. Integration of the bait-prey data from the three separate experiments identified the BCR-ABL protein complex, which was confirmed by biochemistry, cytogenetic methods, and DNA sequencing revealed the e14a2 fusion transcript. The tyrosine phosphatase SHP2 and the GAB2 adaptor protein, important for MAPK signaling, were common to all three IP-MS experiments. The comparative treatment of tyrosine kinase inhibitor (TKI) drugs revealed only imatinib, the standard of care in CML, was inhibitory to BCR-ABL leading to down-regulation of pERK and pS6K and inhibiting cell proliferation. These data suggest a model for directed proteomics from patient tumor samples for selecting the appropriate TKI drug(s) based on IP and LC-MS/MS. The data also suggest that MM patients, in addition to CML patients, may benefit from BCR-ABL diagnostic screening.

Diminished functional role and altered localization of SHP2 in non-small cell lung cancer cells with EGFR-activating mutations

Non-small cell lung cancer (NSCLC) cells harboring activating mutations of the epidermal growth factor receptor (EGFR) tend to display elevated activity of several survival signaling pathways. Surprisingly, these mutations also correlate with reduced phosphorylation of ERK and SHP2, a protein tyrosine phosphatase required for complete ERK activation downstream of most receptor tyrosine kinases. As ERK activity influences cellular response to EGFR inhibition, altered SHP2 function could have a role in the striking response to gefitinib witnessed with EGFR mutation. Here, we demonstrate that impaired SHP2 phosphorylation correlates with diminished SHP2 function in NSCLC cells expressing mutant, versus wild-type, EGFR. In NSCLC cells expressing wild-type EGFR, SHP2 knockdown decreased ERK phosphorylation, basally and in response to gefitinib, and increased cellular sensitivity to gefitinib. In cells expressing EGFR mutants, these effects of SHP2 knockdown were less substantial, but the expression of constitutively active SHP2 reduced cellular sensitivity to gefitinib. In cells expressing EGFR mutants, which do not undergo efficient ligand-mediated endocytosis, SHP2 was basally associated with GRB2-associated binder 1 (GAB1) and EGFR, and SHP2's presence in membrane fractions was dependent on EGFR activity. Whereas EGF promoted a more uniform intracellular distribution of initially centrally localized SHP2 in cells expressing wild-type EGFR, SHP2 was basally evenly distributed and did not redistribute in response to EGF in cells with EGFR mutation. Thus, EGFR mutation may promote association of a fraction of SHP2 at the plasma membrane with adapters that promote SHP2 activity. Consistent with this, SHP2 immunoprecipitated from cells with EGFR mutation was active, and EGF treatment did not change this activity. Overall, our data suggest that a fraction of SHP2 is sequestered at the plasma membrane in cells with EGFR mutation in a way that impedes SHP2's ability to promote ERK activity and identify SHP2 as a potential target for co-inhibition with EGFR in NSCLC.

Functional characterization of cancer-associated Gab1 mutations

Grb2-associated binder 1 (Gab1) is a docking protein that transduces signals from a variety of tyrosine kinases, including Met and the epidermal growth factor receptor (EGFR). Although the related protein Gab2 is strongly implicated in human cancer, a role for Gab1 has been less clear. However, a screen for gene mutations in breast cancer identified two somatic mutations in Gab1, Y83C and T387N. In this paper we describe the functional characterization of these Gab1 mutants. MCF-10A immortalized mammary epithelial cells overexpressing Gab1 Y83C and T387N exhibited a more elongated, fibroblastic phenotype compared with wild-type Gab1 controls. Expression of Gab1 or the mutants promoted epidermal growth factor (EGF)-independent proliferation in monolayer culture to a similar degree. However, in Matrigel culture, both mutants enhanced the formation of acini exhibiting an aberrant, branched morphology. In addition, expression of the mutants modestly increased Erk activation. The two mutants also enhanced branching morphogenesis in a different mammary epithelial cell line, HC11. To gain further insights into the mechanism of action of these mutations, we mapped Gab1 phosphorylation sites by mass spectrometry. This detected phosphorylation of T387 but ;not Y83. Cellular stimulation with EGF or hepatocyte growth factor (HGF) led to a transient, or sustained, induction of T387 phosphorylation, respectively. As T387 corresponds in position to Gab2 T391, which suppresses Gab2 signaling in a phosphorylation-dependent manner, these data support a model in which the T387N mutation abrogates negative-feedback regulation of Gab1. Interrogation of publically-available databases revealed additional cancer-associated mutations at, or in close proximity to, identified serine/threonine phosphorylation sites in other docking proteins. These data indicate that aberrant Gab1 signaling can directly contribute to breast cancer progression, and that negative feedback sites in docking proteins can be targeted by oncogenic mutations.

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.

Targeting MET in cancer: rationale and progress

Uncontrolled cell survival, growth, angiogenesis and metastasis are essential hallmarks of cancer. Genetic and biochemical data have demonstrated that the growth and motility factor hepatocyte growth factor/scatter factor (HGF/SF) and its receptor, the tyrosine kinase MET, have a causal role in all of these processes, thus providing a strong rationale for targeting these molecules in cancer. Parallel progress in understanding the structure and function of HGF/SF, MET and associated signalling components has led to the successful development of blocking antibodies and a large number of small-molecule MET kinase inhibitors. In this Review, we discuss these advances, as well as results from recent clinical studies that demonstrate that inhibiting MET signalling in several types of solid human tumours has major therapeutic value.

GGA3 functions as a switch to promote Met receptor recycling, essential for sustained ERK and cell migration

Cells are dependent on correct sorting of activated receptor tyrosine kinases (RTKs) for the outcome of growth factor signaling. Upon activation, RTKs are coupled through the endocytic machinery for degradation or recycled to the cell surface. However, the molecular mechanisms governing RTK recycling are poorly understood. Here, we show that Golgi-localized gamma ear-containing Arf-binding protein 3 (GGA3) interacts selectively with the Met/hepatocyte growth factor RTK when stimulated, to sort it for recycling in association with "gyrating" clathrin. GGA3 loss abrogates Met recycling from a Rab4 endosomal subdomain, resulting in pronounced trafficking of Met toward degradation. Decreased Met recycling attenuates ERK activation and cell migration. Met recycling, sustained ERK activation, and migration require interaction of GGA3 with Arf6 and an unexpected association with the Crk adaptor. The data show that GGA3 defines an active recycling pathway and support a broader role for GGA3-mediated cargo selection in targeting receptors destined for recycling.

SHP-1 expression accounts for resistance to imatinib treatment in Philadelphia chromosome-positive cells derived from patients with chronic myeloid leukemia

We prove that the SH2-containing tyrosine phosphatase 1 (SHP-1) plays a prominent role as resistance determinant of imatinib (IMA) treatment response in chronic myelogenous leukemia cell lines (sensitive/KCL22-S and resistant/KCL22-R). Indeed, SHP-1 expression is significantly lower in resistant than in sensitive cell line, in which coimmunoprecipitation analysis shows the interaction between SHP-1 and a second tyrosine phosphatase SHP-2, a positive regulator of RAS/MAPK pathway. In KCL22-R SHP-1 ectopic expression restores both SHP-1/SHP-2 interaction and IMA responsiveness; it also decreases SHP-2 activity after IMA treatment. Consistently, SHP-2 knocking-down in KCL22-R reduces either STAT3 activation or cell viability after IMA exposure. Therefore, our data suggest that SHP-1 plays an important role in BCR-ABL-independent IMA resistance modulating the activation signals that SHP-2 receives from both BCR/ABL and membrane receptor tyrosine kinases. The role of SHP-1 as a determinant of IMA sensitivity has been further confirmed in 60 consecutive untreated patients with chronic myelogenous leukemia, whose SHP-1 mRNA levels were significantly lower in case of IMA treatment failure (P < .0001). In conclusion, we suggest that SHP-1 could be a new biologic indicator at baseline of IMA sensitivity in patients with chronic myelogenous leukemia.

Distinct involvement of the Gab1 and Grb2 adaptor proteins in signal transduction by the related receptor tyrosine kinases RON and MET

Although the signal transduction mechanisms of the receptor tyrosine kinase MET are well defined, less is known about its close relative RON. MET initiates intracellular signaling by autophosphorylation on specific cytoplasmic tyrosines that form docking sites for the adaptor proteins Grb2 and Gab1. Grb2 binds directly and is essential for all of the biological activities of MET. Gab1 docks either directly or indirectly via Grb2 and controls only a subset of MET functions. Because MET and RON possess similar adaptor binding sites, it was anticipated that their adaptor interactions would be conserved. Here we show that in contrast to MET, RON relies primarily on Gab1 for signal transmission. Surprisingly, disruption of the Grb2 docking site of RON or Grb2 depletion augments activity, whereas enhancement of Grb2 binding attenuates Gab1 recruitment and signaling. Hence, RON and MET differ in their adaptor interactions; furthermore, Grb2 performs a novel antagonistic role in the context of RON signaling.

Docking Protein Gab1 Is an Essential Component of Postnatal Angiogenesis After Ischemia via HGF/c-Met Signaling

Rationale:

Grb2-associated binder (Gab) docking proteins, consisting of Gab1, Gab2, and Gab3, have crucial roles in growth factor–dependent signaling. Various proangiogenic growth factors regulate angiogenesis and endothelial function. However, the roles of Gab proteins in angiogenesis remain elusive.

Objective:

To elucidate the role of Gab proteins in postnatal angiogenesis.

Methods and Results:

Endothelium-specific Gab1 knockout (Gab1ECKO) mice were viable and showed no obvious defects in vascular development. Therefore, we analyzed a hindlimb ischemia (HLI) model of control, Gab1ECKO, or conventional Gab2 knockout (Gab2KO) mice. Intriguingly, impaired blood flow recovery and necrosis in the operated limb was observed in all of Gab1ECKO, but not in control or Gab2KO mice. Among several proangiogenic growth factors, hepatocyte growth factor (HGF) induced the most prominent tyrosine phosphorylation of Gab1 and subsequent complex formation of Gab1 with SHP2 (Src homology-2–containing protein tyrosine phosphatase 2) and phosphatidylinositol 3-kinase subunit p85 in human endothelial cells (ECs). Gab1-SHP2 complex was required for HGF-induced migration and proliferation of ECs via extracellular signal-regulated kinase (ERK)1/2 pathway and for HGF-induced stabilization of ECs via ERK5. In contrast, Gab1-p85 complex regulated activation of AKT and contributed partially to migration of ECs after HGF stimulation. Microarray analysis demonstrated that HGF upregulated angiogenesis-related genes such as KLF2 (Krüppel-like factor 2) and Egr1 (early growth response 1) via Gab1-SHP2 complex in human ECs. In Gab1ECKO mice, gene transfer of vascular endothelial growth factor, but not HGF, improved blood flow recovery and ameliorated limb necrosis after HLI.

Conclusion:

Gab1 is essential for postnatal angiogenesis after ischemia via HGF/c-Met signaling.

Gab1 mediates hepatocyte growth factor-stimulated mitogenicity and morphogenesis in multipotent myeloid cells

Hepatocyte growth factor (HGF)-stimulated mitogenesis, motogenesis and morphogenesis in various cell types begins with activation of the Met receptor tyrosine kinase and the recruitment of intracellular adaptors and kinase substrates. The adapter protein Gab1 is a critical effector and substrate of activated Met, mediating morphogenesis, among other activities, in epithelial cells. To define its role downstream of Met in hematopoietic cells, Gab1 was expressed in the HGF-responsive, Gab1-negative murine myeloid cell line 32D. Interestingly, the adhesion and motility of Gab1-expressing cells were significantly greater than parental cells, independent of growth factor treatment. Downstream of activated Met, Gab1 expression was specifically associated with rapid Shp-2 recruitment and activation, increased mitogenic potency, suppression of GATA-1 expression and concomitant upregulation of GATA-2 transcription. In addition to enhanced proliferation, continuous culture of Gab1-expressing 32D cells in HGF resulted in cell attachment, filopodia extension and phenotypic changes suggestive of monocytic differentiation. Our results suggest that in myeloid cells, Gab1 is likely to enhance HGF mitogenicity by coupling Met to Shp-2 and GATA-2 expression, thereby potentially contributing to normal myeloid differentiation as well as oncogenic transformation.

shRNA-mediated decreases in c-Met levels affect the differentiation potential of human mesenchymal stem cells and reduce their capacity for tissue repair

Mesenchymal stem cells/marrow stromal cells (MSC) are adult multipotent cells that can augment tissue repair. We previously demonstrated that culturing MSC in hypoxic conditions causes upregulation of the hepatocyte growth factor (HGF) receptor c-Met, allowing them to respond more robustly to HGF. MSC preconditioned in hypoxic environments contributed to restoration of blood flow after an ischemic injury more rapidly than MSC cultured in normoxic conditions. We now investigated the specific role of HGF/c-Met signaling in MSC function. An shRNA-mediated knockdown (KD) of c-Met in MSC did not alter their phenotypic profile, proliferation, or viability in vitro. However, we determined that while HGF/c-Met signaling does not play a role in the adipogenic differentiation of the cells, the disruption of this signaling pathway inhibited the ability of MSC to differentiate into the osteogenic and chondrogenic lineages. We next assessed the impact of c-Met KD on human MSC function in a xenogeneic hindlimb ischemia injury model. A 70% KD of c-Met in MSC resulted in a significant decrease in their capacity to regenerate blood flow to the ischemic limb, as compared to the MSC transduced with control shRNA. MSC with only a 60% KD of c-Met exhibited an intermediate capacity to restore blood flow, suggesting that MSC function is sensitive to the dosage of c-Met signaling. The current study highlights the significance of HGF/c-Met signaling in the capacity of MSC to restore blood flow after an ischemic injury and in their ability to differentiate into the osteogenic and chondrogenic lineages.