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

Aberrant MET Receptor Tyrosine Kinase Signaling in Glioblastoma: Targeted Therapy and Future Directions

Brain tumors represent a heterogeneous group of neoplasms characterized by a high degree of aggressiveness and a poor prognosis. Despite recent therapeutic advances, the treatment of brain tumors, including glioblastoma (GBM), an aggressive primary brain tumor associated with poor prognosis and resistance to therapy, remains a significant challenge. Receptor tyrosine kinases (RTKs) are critical during development and in adulthood. Dysregulation of RTKs through activating mutations and gene amplification contributes to many human cancers and provides attractive therapeutic targets for treatment. Under physiological conditions, the Met RTK, the hepatocyte growth factor/scatter factor (HGF/SF) receptor, promotes fundamental signaling cascades that modulate epithelial-to-mesenchymal transition (EMT) involved in tissue repair and embryogenesis. In cancer, increased Met activity promotes tumor growth and metastasis by providing signals for proliferation, survival, and migration/invasion. Recent clinical genomic studies have unveiled multiple mechanisms by which MET is genetically altered in GBM, including focal amplification, chromosomal rearrangements generating gene fusions, and a splicing variant mutation (exon 14 skipping, METex14del). Notably, MET overexpression contributes to chemotherapy resistance in GBM by promoting the survival of cancer stem-like cells. This is linked to distinctive Met-induced pathways, such as the upregulation of DNA repair mechanisms, which can protect tumor cells from the cytotoxic effects of chemotherapy. The development of MET-targeted therapies represents a major step forward in the treatment of brain tumours. Preclinical studies have shown that MET-targeted therapies (monoclonal antibodies or small molecule inhibitors) can suppress growth and invasion, enhancing the efficacy of conventional therapies. Early-phase clinical trials have demonstrated promising results with MET-targeted therapies in improving overall survival for patients with recurrent GBM. However, challenges remain, including the need for patient stratification, the optimization of treatment regimens, and the identification of mechanisms of resistance. This review aims to highlight the current understanding of mechanisms underlying MET dysregulation in GBM. In addition, it will focus on the ongoing preclinical and clinical assessment of therapies targeting MET dysregulation in GBM.

The Role of GAB1 in Cancer

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

High expression of GRB2 associated binding protein 3 mRNA predicts positive prognosis in melanoma

Malignant melanoma is the most aggressive form of skin cancer, and it is characterized by poor prognosis in patients with metastatic diseases. Accurate prediction of prognosis is crucial for therapeutic decisions. In this study, bioinformatics analysis was used to explore the prognostic value of growth factor receptor-bound protein 2-associated binding protein 3 (GAB3) mRNA. RNA transcriptome sequencing data and clinical data from The Cancer Genome Atlas and genotype-tissue expression (GTEx) were analyzed for differentially expressed genes in high and low GAB3 mRNA expression groups in melanoma. Performing gene enrichment analysis and constructing protein-protein interaction networks. High expression of GAB3 was significantly correlated with a lower T stage, melanoma Clark level, Breslow depth, and melanoma ulceration. And high GAB3 expression was also associated with better progression-free interval in T1 and T2 stages and N0 stage and longer overall survival in T1 and T2 stages, N0 stage, and N1 stage. GAB3 promoted high levels of infiltration of macrophages and activated natural killer cells in melanoma. High expression of GAB3 predicted a positive prognosis in early-stage melanoma that may be mediated by the anticancer immune response.

3-phosphoinositide-dependent protein kinase 1 (PDK1) mediates crosstalk between Src and Akt pathways in MET receptor signaling

The high-affinity tyrosine kinase receptor MET plays a pivotal role in several facets of cell regulation. Although its mitogenic effect is well documented, some aspects of connection patterns between signaling pathways involved in cell cycle progression remain to be deciphered. We have used a tractable heterologous expression system, the Xenopus oocyte, to detect connections between distinct MET signaling cascades involved in G2/M progression. Our results reveal that Src acts as an adapter via its SH2 domain to recruit 3-phosphoinositide-dependent protein kinase 1 (PDK1) to the MET signaling complex leading to Akt phosphorylation. These data define an original crosstalk between Src and Akt signaling pathways that contributes to MET-induced entry into the M phase, and deserves further investigation in pathologies harboring deregulation of this receptor.

GAB1 alleviates septic lung injury by inhibiting the TLR4/ NF-κB pathway

Sepsis, with its high morbidity and mortality, is a difficult problem in critical care medicine. The purpose of this study is to investigate the involvement of GRB2-associated binding protein 1 (GAB1) in septic lung injury. Lipopolysaccharide (LPS)-induced mouse model and A549 cell model were used to simulate septic lung injury. Haematoxylin and eosin (H&E) staining was used to observe the pathological changes. The terminal-deoxynucleotidyl transferase/(TdT)-mediated dUTP-biotin nick end labelling (TUNEL) staining and flow cytometry were used to detect apoptosis. The levels of inflammatory factors in the bronchoalveolar lavage fluid (BALF) were determined by enzyme-linked immunosorbent assay (ELISA). In LPS-induced sepsis mice, GAB1 expression was markedly reduced, and GAB1 overexpression significantly attenuated cell apoptosis and decreased levels of macrophages, neutrophils, and inflammatory factors in the BALF. Our results also demonstrated that GAB1 overexpression significantly reduced LPS-induced apoptosis and inflammation of A549 cells. More importantly, GAB1 overexpression significantly inhibited the Toll-like receptor/ NFkappaB (TLR4/NF-κB) pathway, while silencing GAB1 significantly activated the TLR4/NF-κB pathway and induced apoptosis and increased expression of inflammatory factors. However, the TLR4 inhibitor TAK-242 eliminated the effect of GAB1 silencing on A549. In conclusion, GAB1 is a key regulator of sepsis by inhibiting TLR4/NF-κB mediated apoptosis and inflammation.

PAK4, a target of miR-9-5p, promotes cell proliferation and inhibits apoptosis in colorectal cancer

Background

Colorectal cancer (CRC) is a leading cause of cancer-related death worldwide. P21-activated kinase 4 (PAK4) and miR-9-5p have emerged as attractive therapeutic targets in several tumor types, but in CRC, the regulation of their biological function and their target association remain unclear.

Methods

The expression of PAK4 in CRC tissues was determined using quantitative real-time PCR and immunohistochemistry analyses. The targeted regulation between miR-9-5p and PAK4 was predicted and confirmed with bioinformatics analysis and the dual-luciferase reporter assay. Functional experiments, including the MTT assay and flow cytometry, were performed to investigate the impact of PAK4 knockdown and miR-9-5p overexpression on cell proliferation and apoptosis in CRC cells.

Results

We found that the expression of PAK4 was upregulated in CRC tissues. PAK4 knockdown significantly suppressed cell proliferation and promoted apoptosis in cells of the CRC cell lines HCT116 and SW1116. We also found that miR-9-5p directly targeted the 3′-UTR of PAK4 mRNA and negatively regulated its expression. The degree of downregulation of miR-9-5p inversely correlated with PAK4 expression. Intriguingly, enforced expression of miR-9-5p suppressed cell proliferation and promoted apoptosis. This could be partially reversed by PAK4 overexpression.

Conclusion

These results suggest that miR-9-5p targeting of PAK4 could have therapeutic potential for CRC treatment.

Cholinergic Grb2-Associated-Binding Protein 1 Regulates Cognitive Function

Grb2-associated-binding protein 1 (Gab1) is a docking/scaffolding molecule known to play an important role in cell growth and survival. Here, we report that Gab1 is decreased in cholinergic neurons in Alzheimer's disease (AD) patients and in a mouse model of AD. In mice, selective ablation of Gab1 in cholinergic neurons in the medial septum impaired learning and memory and hippocampal long-term potentiation. Gab1 ablation also inhibited SK channels, leading to an increase in firing in septal cholinergic neurons. Gab1 overexpression, on the other hand, improved cognitive function and restored hippocampal CaMKII autorphosphorylation in AD mice. These results suggest that Gab1 plays an important role in the pathophysiology of AD and may represent a novel therapeutic target for diseases involving cholinergic dysfunction.

Hepatocyte growth control by SOCS1 and SOCS3

The extraordinary capacity of the liver to regenerate following injury is dependent on coordinated and regulated actions of cytokines and growth factors. Whereas hepatocyte growth factor (HGF) and epidermal growth factor (EGF) are direct mitogens to hepatocytes, inflammatory cytokines such as TNFα and IL-6 also play essential roles in the liver regeneration process. These cytokines and growth factors activate different signaling pathways in a sequential manner to elicit hepatocyte proliferation. The kinetics and magnitude of these hepatocyte-activating stimuli are tightly regulated to ensure restoration of a functional liver mass without causing uncontrolled cell proliferation. Hepatocyte proliferation can become deregulated under conditions of chronic inflammation, leading to accumulation of genetic aberrations and eventual neoplastic transformation. Among the control mechanisms that regulate hepatocyte proliferation, negative feedback inhibition by the 'suppressor of cytokine signaling (SOCS)' family proteins SOCS1 and SOCS3 play crucial roles in attenuating cytokine and growth factor signaling. Loss of SOCS1 or SOCS3 in the mouse liver increases the rate of liver regeneration and renders hepatocytes susceptible to neoplastic transformation. The frequent epigenetic repression of the SOCS1 and SOCS3 genes in hepatocellular carcinoma has stimulated research in understanding the growth regulatory mechanisms of SOCS1 and SOCS3 in hepatocytes. Whereas SOCS3 is implicated in regulating JAK-STAT signaling induced by IL-6 and attenuating EGFR signaling, SOCS1 is crucial for the regulation of HGF signaling. These two proteins also module the functions of certain key proteins that control the cell cycle. In this review, we discuss the current understanding of the functions of SOCS1 and SOCS3 in controlling hepatocyte proliferation, and its implications to liver health and disease.

Dissecting Oncogenic RTK Pathways in Colorectal Cancer Initiation and Progression

Colorectal cancer (CRC) is a progressive disorder associated with an accumulation of multiple heterogeneous genetic alterations in intestinal epithelial cells (IEC). However, when these cells undergo neoplastic transformation and become cancerous and metastatic, they invariably acquire hallmarks conferring them the ability to hyperproliferate, escape growth-inhibitory and death-inducing cues, and promote angiogenesis as well as epithelial-to-mesenchymal transformation (EMT), fostering their invasive dissemination from primary tumor into distant tissues. Compelling clinical and experimental evidence suggest that aberrant engagement of cell surface growth factor receptor tyrosine kinase (RTK) signaling, like that of the hepatocyte growth factor (HGF)/MET receptor, underlies CRC metastatic progression by promoting these cancer hallmarks. To date, though, the use of RTK-targeting agents has been viewed as a promising approach for the treatment of metastatic CRC, clinical success has been modest.Our vision is that the prospect of designing RTK-based, improved and innovative CRC therapies and prognostic markers likely rests on a comprehensive understanding of the biological processes and underlying regulatory molecular mechanisms by which deregulation of RTK signaling governs IEC's neoplastic transformation and their transition from noninvasive to metastatic and malignant cells. Herein, we describe our scheme for defining the full scope of oncogenic MET-driven cancer biological processes, in cellulo and in vivo, as well as the individual contribution of MET-binding effectors in a nontransformed IEC model, the IEC-6 cell line.

Novel miR-5582-5p functions as a tumor suppressor by inducing apoptosis and cell cycle arrest in cancer cells through direct targeting of GAB1, SHC1, and CDK2

MicroRNAs (miRNAs) play pivotal roles in tumorigenesis as either tumor suppressors or oncogenes. In the present study, we discovered and demonstrated the tumor suppressive function of a novel miRNA miR-5582-5p. miR-5582-5p induced apoptosis and cell cycle arrest in cancer cells, but not in normal cells. GAB1, SHC1, and CDK2 were identified as direct targets of miR-5582-5p. Knockdown of GAB1/SHC1 or CDK2 phenocopied the apoptotic or cell cycle arrest-inducing function of miR-5582-5p, respectively. The expression of miR-5582-5p was lower in tumor tissues than in adjacent normal tissues of colorectal cancer patients, while the expression of the target proteins exhibited patterns opposite to that of miR-5582-5p. Intratumoral injection of a miR-5582-5p mimic or induced expression of miR-5582-5p in tumor cells suppressed tumor growth in HCT116 xenografts. Collectively, our results suggest a novel tumor suppressive function for miR-5582-5p and its potential applicability for tumor control.

The Tyrosine Kinome Dictates Breast Cancer Heterogeneity and Therapeutic Responsiveness

Phospho-tyrosine signaling networks control numerous biological processes including cellular differentiation, cell growth and survival, motility, and invasion. Aberrant regulation of the tyrosine kinome is a hallmark of malignancy and influences all stages of breast cancer progression, from initiation to the development of metastatic disease. The success of specific tyrosine kinase inhibitors strongly validates the clinical relevance of tyrosine phosphorylation networks in breast cancer pathology. However, a significant degree of redundancy exists within the tyrosine kinome. Numerous receptor and cytoplasmic tyrosine kinases converge on a core set of signaling regulators, including adaptor proteins and tyrosine phosphatases, to amplify pro-tumorigenic signal transduction pathways. Mutational activation, amplification, or overexpression of one or more components of the tyrosine kinome represents key contributing events responsible for the tumor heterogeneity that is observed in breast cancers. It is this molecular heterogeneity that has become the most significant barrier to durable clinical responses due to the development of therapeutic resistance. This review focuses on recent literature that supports a prominent role for specific components of the tyrosine kinome in the emergence of unique breast cancer subtypes and in shaping breast cancer plasticity, sensitivity to targeted therapies, and the eventual emergence of acquired resistance. J. Cell. Biochem. 117: 1971-1990, 2016. © 2016 Wiley Periodicals, Inc.

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.

Host response during Yersinia pestis infection of human bronchial epithelial cells involves negative regulation of autophagy and suggests a modulation of survival-related and cellular growth pathways

Yersinia pestis (Yp) causes the re-emerging disease plague, and is classified by the CDC and NIAID as a highest priority (Category A) pathogen. Currently, there is no approved human vaccine available and advances in early diagnostics and effective therapeutics are urgently needed. A deep understanding of the mechanisms of host response to Yp infection can significantly advance these three areas. We employed the Reverse Phase Protein Microarray (RPMA) technology to reveal the dynamic states of either protein level changes or phosphorylation changes associated with kinase-driven signaling pathways during host cell response to Yp infection. RPMA allowed quantitative profiling of changes in the intracellular communication network of human lung epithelial cells at different times post infection and in response to different treatment conditions, which included infection with the virulent Yp strain CO92, infection with a derivative avirulent strain CO92 (Pgm-, Pst-), treatment with heat inactivated CO92, and treatment with LPS. Responses to a total of 111 validated antibodies were profiled, leading to discovery of 12 novel protein hits. The RPMA analysis also identified several protein hits previously reported in the context of Yp infection. Furthermore, the results validated several proteins previously reported in the context of infection with other Yersinia species or implicated for potential relevance through recombinant protein and cell transfection studies. The RPMA results point to strong modulation of survival/apoptosis and cell growth pathways during early host response and also suggest a model of negative regulation of the autophagy pathway. We find significant cytoplasmic localization of p53 and reduced LC3-I to LC3-II conversion in response to Yp infection, consistent with negative regulation of autophagy. These studies allow for a deeper understanding of the pathogenesis mechanisms and the discovery of innovative approaches for prevention, early diagnosis, and treatment of plague.

Met receptor-induced Grb2 or Shc signals both promote transformation of intestinal epithelial cells, albeit they are required for distinct oncogenic functions

Background

Deregulation of receptor tyrosine kinases (RTK) contributes to the initiation and progression of intestinal-derived epithelial cancers, including colorectal cancer (CRC). However, the roles of the proximal signaling molecules engaged by RTKs in different oncogenic functions of CRC remain unclear.

Methods

Herein, the functional impact of expressing variant forms of the oncogenic Met receptor (Tpr-Met) that selectively recruit the adaptor proteins Grb2 or Shc was investigated in a model derived from normal intestinal epithelial cells (IEC-6). An RNA interference (RNAi) approach was used to define the requirement of Grb2 or Shc in Tpr-Met-transformed IEC-6 cells. Since Grb2 and Shc couple RTKs to the activation of the Ras/MEK/Erk and PI3K/Akt pathways, Erk and Akt phosphorylation/activation states were monitored in transformed IEC-6 cells, and a pharmacological approach was employed to provide insights into the roles of these pathways in oncogenic processes evoked by activated Met, and downstream of Grb2 and Shc.

Results

We show, for the first time, that constitutive activation of either Grb2 or Shc signals in IEC-6 cells, promotes morphological transformation associated with down-regulation of E-cadherin, as well as increased cell growth, loss of growth contact inhibition, anchorage-independent growth, and resistance to serum deprivation and anoikis. Oncogenic activation of Met was revealed to induce morphological transformation, E-cadherin down-regulation, and protection against anoikis by mechanisms dependent on Grb2, while Shc was shown to be partly required for enhanced cell growth. The coupling of activated Met to the Ras/MEK/Erk and PI3K/Akt pathways, and the sustained engagement of Grb2 or Shc in IECs, was shown to trigger negative feedback, limiting the extent of activation of these pathways. Nonetheless, morphological alterations and E-cadherin down-regulation induced by the oncogenic Tpr-Met, and by Grb2 or Shc signals, were blocked by MEK, but not PI3K, inhibitors while the enhanced growth and resistance to anoikis induced by Tpr-Met were nearly abolished by co-treatment with both inhibitors.

Conclusion

Overall, these results identify Grb2 and Shc as central signaling effectors of Met-driven progression of intestinal epithelial-derived cancers. Notably, they suggest that Grb2 may represent a promising target for the design of novel CRC therapies.

The involvement of the docking protein Gab1 in mitogenic signalling induced by EGF and HGF in rat hepatocytes

Grb2-associated binder (Gab) family proteins are docking molecules that can interact with receptor tyrosine kinases (RTKs) and cytokine receptors and bind several downstream signalling proteins. Studies in several cell types have shown that Gab1 may have a role in signalling mediated by the two RTKs epidermal growth factor (EGF) receptor (EGFR) and Met, the receptor for hepatocyte growth factor (HGF), but the involvement of Gab1 in EGFR and Met signalling has not been directly compared in the same cell. We have studied mechanisms of activation and role in mitogenic signalling of Gab1 in response to EGF and HGF in cultured rat hepatocytes. Gab1, but not Gab2, was expressed in the hepatocytes and was phosphorylated upon stimulation with EGF or HGF. Depletion of Gab1, using siRNA, decreased the ERK and Akt activation, cyclin D1 expression, and DNA synthesis in response to both EGF and HGF. Studies of mechanisms of recruitment to the receptors showed that HGF induced co-precipitation of Gab1 and Met while EGF induced binding of Gab1 to Grb2 but not to EGFR. Gab1 activation in response to both EGF and HGF was dependent on PI3K. While EGF activated Gab1 and Shc equally, within the same concentration range, HGF very potently and almost exclusively activated Gab1, having only a minimal effect on Shc. Collectively, our results strongly suggest that although Gab1 interacts differently with EGFR and Met, it is involved in mitogenic signalling mediated by both these growth factor receptors in hepatocytes.

Acquired substrate preference for GAB1 protein bestows transforming activity to ERBB2 kinase lung cancer mutants

Activating mutations in the αC-β4 loop of the ERBB2 kinase domain, such as ERBB2(YVMA) and ERBB2(G776VC), have been identified in human lung cancers and found to drive tumor formation. Here we observe that the docking protein GAB1 is hyper-phosphorylated in carcinomas from transgenic mice and in cell lines expressing these ERBB2 cancer mutants. Using dominant negative GAB1 mutants lacking canonical tyrosine residues for SHP2 and PI3K interactions or lentiviral shRNA that targets GAB1, we demonstrate that GAB1 phosphorylation is required for ERBB2 mutant-induced cell signaling, cell transformation, and tumorigenesis. An enzyme kinetic analysis comparing ERBB2(YVMA) to wild type using physiologically relevant peptide substrates reveals that ERBB2(YVMA) kinase adopts a striking preference for GAB1 phosphorylation sites as evidenced by ∼150-fold increases in the specificity constants (kcat/Km) for several GAB1 peptides, and this change in substrate selectivity was predominantly attributed to the peptide binding affinities as reflected by the apparent Km values. Furthermore, we demonstrate that ERBB2(YVMA) phosphorylates GAB1 protein ∼70-fold faster than wild type ERBB2 in vitro. Notably, the mutation does not significantly alter the Km for ATP or sensitivity to lapatinib, suggesting that, unlike EGFR lung cancer mutants, the ATP binding cleft of the kinase is not significantly changed. Taken together, our results indicate that the acquired substrate preference for GAB1 is critical for the ERBB2 mutant-induced oncogenesis.

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.

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.

Profiling phospho-signaling networks in breast cancer using reverse-phase protein arrays

Measuring the states of cell signaling pathways in tumor samples promises to advance the understanding of oncogenesis and identify response biomarkers. Here, we describe the use of Reverse Phase Protein Arrays (RPPAs or RPLAs) to profile signaling proteins in 56 breast cancers and matched normal tissue. In RPPAs, hundreds to thousands of lysates are arrayed in dense regular grids and each grid is probed with a different antibody (100 in the current work, of which 71 yielded strong signals with breast tissue). Although RPPA technology is quite widely used, measuring changes in phosphorylation reflective of protein activation remains challenging. Using repeat deposition and well-validated antibodies, we show that diverse patterns of phosphorylation can be monitored in tumor samples and changes mapped onto signaling networks in a coherent fashion. The patterns are consistent with biomarker-based classification of breast cancers and known mechanisms of oncogenesis. We explore in detail one tumor-associated pattern that involves changes in the abundance of the Axl receptor tyrosine kinase (RTK) and phosphorylation of the cMet RTK. Both cMet and Axl have been implicated in breast cancer, or in resistance to anticancer drugs, but the two RTKs are not known to be linked functionally. Protein depletion and overexpression studies in a 'triple-negative' breast cell line reveal cross talk between Axl and cMet involving Axl-mediated modification of cMet, a requirement for cMet in efficient and timely signal transduction by the Axl ligand Gas6 and the potential for the two receptors to interact physically. These findings have potential therapeutic implications, as they imply that bi-specific receptor inhibitors (for example, ATP-competitive small-kinase inhibitors such as GSK1363089, BMS-777607 or MP470) may be more efficacious than the mono-specific therapeutic antibodies currently in development (for example, Onartuzumab).

Novel role of proline-rich nonreceptor tyrosine kinase 2 in vascular wall remodeling after balloon injury

OBJECTIVE

To investigate the role of Pyk2, a proline-rich nonreceptor tyrosine kinase, in G protein-coupled receptor agonist, thrombin-induced human aortic smooth muscle cell growth and migration, and injury-induced vascular wall remodeling.

METHODS AND RESULTS

Thrombin, a G protein-coupled receptor agonist, activated Pyk2 in a time-dependent manner and inhibition of its stimulation attenuated thrombin-induced human aortic smooth muscle cell migration and proliferation. Thrombin also activated Grb2-associated binder protein 1, p115 Rho guanine nucleotide exchange factor, Rac1, RhoA, and p21-activated kinase 1 (Pak1) and interference with stimulation of these molecules attenuated thrombin-induced human aortic smooth muscle cell migration and proliferation. In addition, adenovirus-mediated expression of dominant negative Pyk2 inhibited thrombin-induced Grb2-associated binder protein 1, p115 rho guanine nucleotide exchange factor, Rac1, RhoA and Pak1 stimulation. Balloon injury also caused activation of Pyk2, Grb2-associated binder protein 1, p115 rho guanine nucleotide exchange factor, Rac1, RhoA, and Pak1 in the carotid artery of rat, and these responses were sensitive to inhibition by the dominant negative Pyk2. Furthermore, inhibition of Pyk2 activation resulted in reduced recruitment of smooth muscle cells onto the luminal surface and their proliferation in the intimal region leading to suppression of neointima formation.

CONCLUSIONS

Together, these results demonstrate for the first time that Pyk2 plays a crucial role in G protein-coupled receptor agonist thrombin-induced human aortic smooth muscle cell growth and migration, as well as balloon injury-induced neointima formation.

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.

Gab adapter proteins as therapeutic targets for hematologic disease

The Grb-2 associated binder (Gab) family of scaffolding/adaptor/docking proteins is a group of three molecules with significant roles in cytokine receptor signaling. Gabs possess structural motifs for phosphorylation-dependent receptor recruitment, Grb2 binding, and activation of downstream signaling pathways through p85 and SHP-2. In addition, Gabs participate in hematopoiesis and regulation of immune response which can be aberrantly activated in cancer and inflammation. The multifunctionality of Gab adapters might suggest that they would be too difficult to consider as candidates for “targeted” therapy. However, the one drug/one target approach is giving way to the concept of one drug/multiple target approach since few cancers are addicted to a single signaling molecule for survival and combination drug therapies can be problematic. In this paper, we cover recent findings on Gab multi-functionality, binding partners, and their role in hematological malignancy and examine the concept of Gab-targeted therapy.

Order and disorder in large multi-site docking proteins of the Gab family--implications for signalling complex formation and inhibitor design strategies

Large multi-site docking (LMD) proteins of the Gab, IRS, FRS, DOK and Cas families consist of one or two folded N-terminal domains, followed by a predominantly disordered C-terminal extension. Their primary function is to provide a docking platform for signalling molecules (including PI3K, PLC, Grb2, Crk, RasGAP, SHP2) in intracellular signal transmission from activated cell-surface receptors, to which they become coupled. A detailed analysis of the structural nature and intrinsic disorder propensity of LMD proteins, with Gab proteins as specific examples, is presented. By primary sequence analysis and literature review the varying levels of disorder and hidden order are predicted, revealing properties and a physical architecture that help to explain their biological function and characteristics, common for network hub proteins. The virulence factor, CagA, from Helicobacter pylori is able to mimic Gab function once injected by this human pathogen into stomach epithelial cells. Its predicted differential structure is compared to Gab1 with respect to its functional mimicry. Lastly, we discuss how LMD proteins, in particular Gab1 and Gab2, and their protein partners, such as SH2 and SH3 domain-containing adaptors like Grb2, might qualify for future anti-cancer strategies in developing protein-protein interaction (PPI) inhibitors towards binary interactors consisting of an intrinsically disordered epitope and a structured domain surface.

Oncogenic engagement of the Met receptor is sufficient to evoke angiogenic, tumorigenic, and metastatic activities in rat intestinal epithelial cells

The deregulation of Met/hepatocyte growth factor (HGF) receptor tyrosine kinase signaling constitutes a common event in colorectal cancers. However, the physiopathological functions of such a deregulation remain poorly understood. In the present study, we investigated the role of the deregulation of Met receptor in the neoplastic transformation of intestinal epithelial cells. To do so, the normal, well-established and characterized rat intestinal epithelial IEC-6 cells were transduced with a retrovirus carrying the oncogenic constitutive active form of Met receptor, Tpr-Met. Herein, we show that compared with control IEC-6 cells, Tpr-Met-IEC-6 cells exhibit enhanced proliferation, loss of growth-contact inhibition, cell morphological alterations, actin cytoskeletal reorganization, loss of E-cadherin expression and anchorage-independent growth. Moreover, Tpr-Met-IEC-6 cells are conferred the capacity to produce the proangiogenic factor VEGF and to reduce the potent antiangiogenic factor thrombospondin-1. Of significance, Tpr-Met-IEC-6 cells are endowed with the ability to elicit angiogenic responses and to form tumors and metastases in vivo. Hence, our study demonstrates for the first time that the sole oncogenic engagement of Met receptor in normal intestinal epithelial cells is sufficient to induce a wide array of cancerous biological processes that are fundamental to the initiation and malignant progression of colorectal cancers.

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.

Distinct requirements for Gab1 in Met and EGF receptor signaling in vivo

Gab1 is a multiadaptor protein that has been shown to be required for multiple processes in embryonic development and oncogenic transformation. Gab1 functions by amplifying signal transduction downstream of various receptor tyrosine kinases through recruitment of multiple signaling effectors, including phosphatidylinositol 3-kinase and Shp2. Until now, the functional significance of individual interactions in vivo was not known. Here we have generated knockin mice that carry point mutations in either the P13K or Shp2 binding sites of Gab1. We show that different effector interactions with Gab1 play distinct biological roles downstream of Gab1 during the development of different organs. Recruitment of phosphatidylinositol 3-kinase by Gab1 is essential for EGF receptor-mediated embryonic eyelid closure and keratinocyte migration, and the Gab1-Shp2 interaction is crucial for Met receptor-directed placental development and muscle progenitor cell migration to the limbs. Furthermore, we investigate the dual association of Gab1 with the Met receptor. By analyzing knockin mice with mutations in the Grb2 or Met binding site of Gab1, we show that the requirements for Gab1 recruitment to Met varies in different biological contexts. Either the direct or the indirect interaction of Gab1 with Met is sufficient for Met-dependent muscle precursor cell migration, whereas both modes of interaction are required and neither is sufficient for placenta development, liver growth, and palatal shelf closure. These data demonstrate that Gab1 induces different biological responses through the recruitment of distinct effectors and that different modes of recruitment for Gab1 are required in different organs.

Haloperidol induces the nuclear translocation of phosphatidylinositol 3'-kinase to disrupt Akt phosphorylation in PC12 cells

OBJECTIVE

The antipsychotic drug haloperidol (HAL) has been linked to apoptosis and to inhibition of prosurvival Akt signalling in pheochromocytoma (PC12) and neuronal cell cultures. However, the mechanism involved is unclear.

METHODS

We used HAL to induce cytotoxicity in preneuronal PC12 cells. The expression and the subcellular localization of selected components of the PI3K-Akt survival cascade were monitored with standard biochemical approaches, such as subcellular fractionation, western blot analysis, gene transfer and fluorescence microscopy.

RESULTS

PC12 cell stimulation with the epidermal growth factor (used as a control) results in normal processing of phosphatidylinositol 3'-kinase (PI3K)-Akt signalling (e.g., localization of PI3K to the plasma membrane and phosphorylation of Akt (Ser473). Surprisingly, HAL induces PI3K-generated phosphoinositol [phosphatidylinositol-3,4,5-triphosphate (PIP3), which conflicts with its ability to inhibit Akt. In fact, the production of PIP3s is nuclear, as assessed by the localized concentration of a fluorophore-tagged PIP3-targeting pleckstrin homology protein and a fluorophore-tagged substrate-trapping mutant of the phosphoinositide phosphatase, phosphatase and tensin homologue deleted on chromosome 10 (PTEN). However, phosphoinositide-dependent protein kinase 1 (PDK1, the activating kinase of Akt) does not colocalize to the nucleus with the PI3K complex. This effectively inactivates both cytoplasmic and nuclear pools of Akt.

CONCLUSION

The differential compartmentalization of effectors of the PI3K-PDK1-Akt pathway is a unique means by which HAL disrupts Akt functioning in PC12 cells.