Abl interactor 1 binds to sos and inhibits epidermal growth factor- and v-Abl-induced activation of extracellular signal-regulated kinases

Adaptor protein Abelson interactor 1 in homeostasis and disease

Dysregulation of Abelson interactor 1 (ABI1) is associated with various states of disease including developmental defects, pathogen infections, and cancer. ABI1 is an adaptor protein predominantly known to regulate actin cytoskeleton organization processes such as those involved in cell adhesion, migration, and shape determination. Linked to cytoskeleton via vasodilator-stimulated phosphoprotein (VASP), Wiskott-Aldrich syndrome protein family (WAVE), and neural-Wiskott-Aldrich syndrome protein (N-WASP)-associated protein complexes, ABI1 coordinates regulation of various cytoplasmic protein signaling complexes dysregulated in disease states. The roles of ABI1 beyond actin cytoskeleton regulation are much less understood. This comprehensive, protein-centric review describes molecular roles of ABI1 as an adaptor molecule in the context of its dysregulation and associated disease outcomes to better understand disease state-specific protein signaling and affected interconnected biological processes.

CBLC promotes the development of colorectal cancer by promoting ABI1 degradation to activate the ERK signaling pathway

CBLC (CBL proto-oncogene C) is an E3 ubiquitin protein ligase that plays a key role in cancers. However, the function and mechanism of CBLC in colorectal cancer (CRC) has not been fully elucidated. The aim of this study was to investigate the function of CBLC in CRC and its underlying molecular mechanism. High CBLC levels were certified in tumor tissues of CRC patients, and its expression was positively associated with TNM stage. Next, we explored the role of CBLC in CRC using gain or loss of function. For biological function analysis, CCK-8 cell proliferation, colony formation, flow cytometry, scratch, and transwell assays collectively suggested that CBLC overexpression promoted cell proliferation, cell cycle progression, migration and invasion. As observed, CBLC knockdown exhibited exactly opposite effects, resulting in impaired tumorigenicity in vitro. Xenograft studies displayed that CBLC overexpression accelerated tumor growth and promoted tumor metastasis to the lung, while the inhibitory effects of CBLC knockdown on tumorigenicity and metastasis ability of CRC cells was also confirmed. Furthermore, the molecular mechanism of CBLC in CRC was explored. CBLC induced the activation of ERK signaling pathway, further leading to its pro-tumor role. Notably, CBLC decreased ABI1 (Abelson interactor protein-1, a candidate tumor suppressor) protein levels through its ubiquitin ligase activity, while ABI1 upregulation abolished the effects of CBLC on the tumorigenesis of CRC. Taken together, these results demonstrate that CBLC acts as a tumor promoter in CRC through triggering the ubiquitination and degradation of ABI1 and activating the ERK signaling pathway. CBLC may be a potential novel target for CRC.

Circulating miR-30e-3p induces disruption of neurite development in SH-SY5Y cells by targeting ABI1, a novel biomarker for schizophrenia

Schizophrenia (SCZ) represents a set of enduring mental illnesses whose underlying etiology remains elusive, posing a significant challenge to public health. Previous studies have shown that the neurodevelopmental process involving small molecules such as miRNA and mRNA is one of the etiological hypotheses of SCZ. We identified and verified that miR-30e-3p and ABI1 can be used as biomarkers in peripheral blood transcriptome sequencing data of patients with SCZ, and confirmed the regulatory relationship between them. To further explore their involvement, we employed retinoic acid (RA)-treated SH-SY5Y differentiated cells as a model system. Our findings indicate that in RA-induced SH-SY5Y cells, ABI1 expression is up-regulated, while miR-30e-3p expression is down-regulated. Functionally, both miR-30e-3p down-regulation and ABI1 up-regulation promote apoptosis and inhibit the proliferation of SH-SY5Y cells. Subsequently, the immunofluorescence assay detected the expression location and abundance of the neuron-specific protein β-tubulinIII. The expression levels of neuronal marker genes MAPT, TUBB3 and SYP were detected by RT-qPCR. We observed that these changes of miR-30e-3p and ABI1 inhibit the neurite growth of SH-SY5Y cells. Rescue experiments further support that ABI1 silencing can correct miR-30e-3p down-regulation-induced SH-SY5Y neurodevelopmental defects. Collectively, our results establish that miR-30e-3p's regulation of neurite development in SH-SY5Y cells is mediated through ABI1, highlighting a potential mechanism in SCZ pathogenesis.

Functional Classification and Interaction Selectivity Landscape of the Human SH3 Domain Superfamily

SRC homology 3 (SH3) domains are critical interaction modules that orchestrate the assembly of protein complexes involved in diverse biological processes. They facilitate transient protein-protein interactions by selectively interacting with proline-rich motifs (PRMs). A database search revealed 298 SH3 domains in 221 human proteins. Multiple sequence alignment of human SH3 domains is useful for phylogenetic analysis and determination of their selectivity towards PRM-containing peptides (PRPs). However, a more precise functional classification of SH3 domains is achieved by constructing a phylogenetic tree only from PRM-binding residues and using existing SH3 domain-PRP structures and biochemical data to determine the specificity within each of the 10 families for particular PRPs. In addition, the C-terminal proline-rich domain of the RAS activator SOS1 covers 13 of the 14 recognized proline-rich consensus sequence motifs, encompassing differential PRP pattern selectivity among all SH3 families. To evaluate the binding capabilities and affinities, we conducted fluorescence dot blot and polarization experiments using 25 representative SH3 domains and various PRPs derived from SOS1. Our analysis has identified 45 interacting pairs, with binding affinities ranging from 0.2 to 125 micromolar, out of 300 tested and potential new SH3 domain-SOS1 interactions. Furthermore, it establishes a framework to bridge the gap between SH3 and PRP interactions and provides predictive insights into the potential interactions of SH3 domains with PRMs based on sequence specifications. This novel framework has the potential to enhance the understanding of protein networks mediated by SH3 domain-PRM interactions and be utilized as a general approach for other domain-peptide interactions.

The Abl-interactor Abi suppresses the function of the BRAG2 GEF family member Schizo

Guanine nucleotide exchange factors (GEF) of the BRAG subfamily activate small Arf GTPases, which are pivotal regulators of intracellular membrane traffic and actin dynamics. Consequently, BRAG proteins have been implicated to regulate the surface levels of adhesive and signaling receptors. However, not much is known about the mechanism leading to the regulation of these surface proteins. In this study, we found that the Drosophila BRAG GEF Schizo interacts physically with the Abl-interactor (Abi). schizo mutants display severe defects in myoblast fusion during syncytial muscle formation and show increased amounts of the cell adhesion protein N-cadherin. We demonstrate that the schizo myoblast fusion phenotype can be rescued by the expression of the Schizo GEF (Sec7) and membrane-binding (pleckstrin homology) domain. Furthermore, the expression of the Sec7-PH domain in a wild-type background decreases the amounts of N-cadherin and impairs myoblast fusion. These findings support the notion that the Sec7-PH domain serves as a constitutive-active form of Schizo. Using a yeast-two hybrid assay, we show that the SH3 domain of Abi interacts with the N-terminal region of Schizo. This region is also able to bind to the cytodomain of the cell adhesion molecule N-cadherin. To shed light on the function of Schizo and Abi in N-cadherin removal, we employed epistasis experiments in different developmental contexts of Drosophila. These studies point towards a new model for the regulation of Schizo. We propose that the binding of Abi to the N-terminal part of Schizo antagonizes Schizo function to inhibit N-cadherin removal.

Establishment of a schizophrenia classifier based on peripheral blood signatures and investigation of pathogenic miRNA-mRNA regulation

To date, the diagnosis of schizophrenia (SCZ) mainly relies on patients' or guardians' self-reports and clinical observation, and the pathogenesis of SCZ remains elusive. In this study, we sought to develop a reliable classifier for diagnosing SCZ patients and provide clues to the etiology and pathogenesis of SCZ. Based on the high throughput sequencing analysis of peripheral blood miRNA expression profile and weighted gene co-expression network analysis (WGCNA) in our previous study, we selected eleven hub miRNAs for validation by qRT-PCR in 51 SCZ patients and 51 controls. miR-939-5p, miR-4732-3p let-7d-3p, and miR-142-3p were confirmed to be significantly up-regulated, and miR-30e-3p and miR-23a-3p were down-regulated in SCZ patients. miR-30e-3p with the most considerable fold change and statistically significance was selected for targeting validation. We first performed bioinformatics prediction followed by qRT-PCR and verified the up-regulation of potential target mRNAs (ABI1, NMT1, HMGB1) expression. Next, we found that the expression level of ABI1 was significantly up-regulated in SH-SY5Y cells transfected with miR-30e-3p mimics. Lastly, we conducted a luciferase assay in 293T cells confirming that miR-30e-3p could directly bind with the 3'untranslated region (3'-UTR) of ABI1, revealing that miR-30e-3p might play a role in the polymerization of neuronal actin and the reconstruction of the cytoskeleton via the downstream regulation of ABI1. In addition, we constructed a classifier by a series of bioinformatics algorithms and evaluated its diagnostic performance. It appears that the classifier consists of miRNAs and mRNAs possess a better discrimination performance than individual miRNA or mRNA in SCZ.

SOS2 Comes to the Fore: Differential Functionalities in Physiology and Pathology

The SOS family of Ras-GEFs encompasses two highly homologous and widely expressed members, SOS1 and SOS2. Despite their similar structures and expression patterns, early studies of constitutive KO mice showing that SOS1-KO mutants were embryonic lethal while SOS2-KO mice were viable led to initially viewing SOS1 as the main Ras-GEF linking external stimuli to downstream RAS signaling, while obviating the functional significance of SOS2. Subsequently, different genetic and/or pharmacological ablation tools defined more precisely the functional specificity/redundancy of the SOS1/2 GEFs. Interestingly, the defective phenotypes observed in concomitantly ablated SOS1/2-DKO contexts are frequently much stronger than in single SOS1-KO scenarios and undetectable in single SOS2-KO cells, demonstrating functional redundancy between them and suggesting an ancillary role of SOS2 in the absence of SOS1. Preferential SOS1 role was also demonstrated in different RASopathies and tumors. Conversely, specific SOS2 functions, including a critical role in regulation of the RAS-PI3K/AKT signaling axis in keratinocytes and KRAS-driven tumor lines or in control of epidermal stem cell homeostasis, were also reported. Specific SOS2 mutations were also identified in some RASopathies and cancer forms. The relevance/specificity of the newly uncovered functional roles suggests that SOS2 should join SOS1 for consideration as a relevant biomarker/therapy target.

RNA Splicing of the Abi1 Gene by MBNL1 contributes to macrophage-like phenotype modulation of vascular smooth muscle cell during atherogenesis

BACKGROUND

Vascular smooth muscle cells (VSMC) switch to macrophage-like cells after cholesterol loading, and this change may play an important role in atherogenesis. Muscleblind-like splicing regulator 1 (MBNL1) is a well-known splicing factor that has been implicated in many cellular processes. However, the role of MBNL1 in VSMC macrophage-like transdifferentiation is largely unknown. In this study, we aim to characterize the role of MBNL1-induced gene splicing during atherogenesis.

METHODS

The expression of MBNL1 and Abelson interactor 1 (Abi1) splice variants (Abi1-e10 and Abi1-Δe10) was compared between artery tissues from healthy donors and atherosclerosis patients. Regulatory mechanisms of MBNL1-induced Abi1 gene splicing were studied, and the signal pathways mediated by Abi1 splice variants were investigated in VSMC.

RESULTS

Loss of MBNL1 was found in the macrophage-like VSMC (VSMC-M) in artery wall from atherosclerosis patients. In vitro and in vivo evidence confirmed that Abi1 is one of the MBNL1 target genes. Loss of MBNL1 significantly induces the Abi1-Δe10 isoform expression. Compared to the known actin organization activities of the Abi1 gene, we discovered a novel action of Abi1-Δe10, whereby Abi1-Δe10 activates Rac1 independent of upstream stimulation and triggers the Rac1-NOX1-ROS pathway, which results in increased expression of transcription factor Kruppel-like factor 4 (KLF4). While Abi1-Δe10 inhibits contractile VSMC biomarkers expression and cell contraction, it stimulates VSMC proliferation, migration and macrophage-like transdifferentiation.

CONCLUSION

Loss-of-function of MBNL1 activates VSMC-M transdifferentiation to promote atherogenesis through regulating Abi1 RNA splicing.

Effects and mechanisms of Eps8 on the biological behaviour of malignant tumours (Review)

Epidermal growth factor receptor pathway substrate 8 (Eps8) was initially identified as the substrate for the kinase activity of EGFR, improving the responsiveness of EGF, which is involved in cell mitosis, differentiation and other physiological functions. Numerous studies over the last decade have demonstrated that Eps8 is overexpressed in most ubiquitous malignant tumours and subsequently binds with its receptor to activate multiple signalling pathways. Eps8 not only participates in the regulation of malignant phenotypes, such as tumour proliferation, invasion, metastasis and drug resistance, but is also related to the clinicopathological characteristics and prognosis of patients. Therefore, Eps8 is a potential tumour diagnosis and prognostic biomarker and even a therapeutic target. This review aimed to describe the structural characteristics, role and related molecular mechanism of Eps8 in malignant tumours. In addition, the prospect of Eps8 as a target for cancer therapy is examined.

The Role of the Human Cytomegalovirus UL133-UL138 Gene Locus in Latency and Reactivation

Human cytomegalovirus (HCMV) latency, the means by which the virus persists indefinitely in an infected individual, is a major frontier of current research efforts in the field. Towards developing a comprehensive understanding of HCMV latency and its reactivation from latency, viral determinants of latency and reactivation and their host interactions that govern the latent state and reactivation from latency have been identified. The polycistronic UL133-UL138 locus encodes determinants of both latency and reactivation. In this review, we survey the model systems used to investigate latency and new findings from these systems. Particular focus is given to the roles of the UL133, UL135, UL136 and UL138 proteins in regulating viral latency and how their known host interactions contribute to regulating host signaling pathways towards the establishment of or exit from latency. Understanding the mechanisms underlying viral latency and reactivation is important in developing strategies to block reactivation and prevent CMV disease in immunocompromised individuals, such as transplant patients.

CRISPR/CAS9-mediated knockout of Abi1 inhibits p185Bcr-Abl-induced leukemogenesis and signal transduction to ERK and PI3K/Akt pathways

Background

Abl interactor 1 (Abi1) is a downstream target of Abl tyrosine kinases and a component of the WAVE regulatory complex (WRC) that plays an important role in regulating actin cytoskeleton remodeling and membrane receptor signaling. While studies using short hairpin RNA (shRNA) have suggested that Abi1 plays a critical role in Bcr-Abl-induced leukemogenesis, the mechanism involved is not clear.

Methods

In this study, we knocked out Abi1 expression in p185^Bcr-Abl-transformed hematopoietic cells using CRISPR/Cas9-mediated gene editing technology. The effects of Abi1 deficiency on actin cytoskeleton remodeling, the Bcr-Abl signaling, IL-3 independent growth, and SDF-induced chemotaxis in these cells were examined by various in vitro assays. The leukemogenic activity of these cells was evaluated by a syngeneic mouse transplantation model.

Results

We show here that Abi1 deficiency reduced the IL3-independent growth and SDF-1α-mediated chemotaxis in p185^Bcr-Abl-transformed hematopoietic cells and inhibited Bcr-Abl-induced abnormal actin remodeling. Depletion of Abi1 also impaired the Bcr-Abl signaling to the ERK and PI3 kinase/Akt pathways. Remarkably, the p185^Bcr-Abl-transformed cells with Abi1 deficiency lost their ability to develop leukemia in syngeneic mice. Even though these cells developed drug tolerance in vitro after prolonged selection with imatinib as their parental cells, the imatinib-tolerant cells remain incapable of leukemogenesis in vivo.

Conclusions

Together, this study highlights an essential role of Abi1 in Bcr-Abl-induced leukemogenesis and provides a model system for dissecting the Abi1 signaling in Bcr-Abl-positive leukemia.

Inhibitory short peptides targeting EPS8/ABI1/SOS1 tri-complex suppress invasion and metastasis of ovarian cancer cells

Background

We aimed to develop inhibitory short peptides that can prevent protein interactions of SOS1/EPS8/ABI1 tri-complex, a key component essential for ovarian cancer metastasis.

Methods

Plasmids containing various regions of HA-tagged ABI1 were co-transfected into ovarian cancer cells with Flag-tagged SOS1 or Myc-tagged EPS8. Co-immunoprecipitation and GST-pulldown assay were used to identify the regions of ABI1 responsible for SOS1 and EPS8 binding. Inhibitory short peptides of these binding regions were synthesized and modified with HIV-TAT sequence. The blocking effects of the peptides on ABI1-SOS1 or ABI1-EPS8 interactions in vitro and in vivo were determined by GST-pulldown assay. The capability of these short peptides in inhibiting invasion and metastasis of ovarian cancer cell was tested by Matrigel invasion assay and peritoneal metastatic colonization assay.

Results

The formation of endogenous SOS1/EPS8/ABI1 tri-complex was detected in the event of LPA-induced ovarian cancer cell invasion. In the tri-complex, ABI1 acted as a scaffold protein holding together SOS1 and EPS8. The SH3 and poly-proline+PxxDY regions of ABI1 were responsible for SOS1 and EPS8 binding, respectively. Inhibitory short peptides p + p-8 (ppppppppvdyedee) and SH3–3 (ekvvaiydytkdkddelsfmegaii) could block ABI1-SOS1 and ABI1-EPS8 interaction in vitro. TAT-p + p-8 peptide could disrupt ABI1-EPS8 interaction and suppress the invasion and metastasis of ovarian cancer cells in vivo.

Conclusions

TAT-p + p-8 peptide could efficiently disrupt the ABI1-EPS8 interaction, tri-complex formation, and block the invasion and metastasis of ovarian cancer cells.

Human Cytomegalovirus UL135 Interacts with Host Adaptor Proteins To Regulate Epidermal Growth Factor Receptor and Reactivation from Latency

Human cytomegalovirus, HCMV, is a betaherpesvirus that establishes a lifelong latent infection in its host that is marked by recurrent episodes of reactivation. The molecular mechanisms by which the virus and host regulate entry into and exit from latency remain poorly understood. We have previously reported that UL135 is critical for reactivation, functioning in part by overcoming suppressive effects of the latency determinant UL138 We have demonstrated a role for UL135 in diminishing cell surface levels and targeting epidermal growth factor receptor (EGFR) for turnover. The attenuation of EGFR signaling promotes HCMV reactivation in combination with cellular differentiation. In this study, we sought to define the mechanisms by which UL135 functions in regulating EGFR turnover and viral reactivation. Screens to identify proteins interacting with pUL135 identified two host adaptor proteins, CIN85 and Abi-1, with overlapping activities in regulating EGFR levels in the cell. We mapped the amino acids in pUL135 necessary for interaction with Abi-1 and CIN85 and generated recombinant viruses expressing variants of pUL135 that do not interact with CIN85 or Abi-1. These recombinant viruses replicate in fibroblasts but are defective for reactivation in an experimental model for latency using primary CD34+ hematopoietic progenitor cells (HPCs). These UL135 variants have altered trafficking of EGFR and are defective in targeting EGFR for turnover. These studies demonstrate a requirement for pUL135 interactions with Abi-1 and CIN85 for regulation of EGFR and mechanistically link the regulation of EGFR to reactivation.IMPORTANCE Human cytomegalovirus (HCMV) establishes a lifelong latent infection in the human host. While the infection is typically asymptomatic in healthy individuals, HCMV infection poses life-threatening disease risk in immunocompromised individuals and is the leading cause of birth defects. Understanding how HCMV controls the lifelong latent infection and reactivation of replication from latency is critical to developing strategies to control HCMV disease. Here, we identify the host factors targeted by a viral protein that is required for reactivation. We define the importance of this virus-host interaction in reactivation from latency, providing new insights into the molecular underpinnings of HCMV latency and reactivation.

Analysis of K-Ras Interactions by Biotin Ligase Tagging

BACKGROUND

Mutations of the human K-Ras 4B (K-Ras) G protein are associated with a significant proportion of all human cancers. Despite this fact, a comprehensive analysis of K-Ras interactions is lacking. Our investigations focus on characterization of the K-Ras interaction network.

MATERIALS AND METHODS

We employed a biotin ligase-tagging approach, in which tagged K-Ras proteins biotinylate neighbor proteins in a proximity-dependent fashion, and proteins are identified via mass spectrometry (MS) sequencing.

RESULTS

In transfected cells, a total of 748 biotinylated proteins were identified from cells expressing biotin ligase-tagged K-Ras variants. Significant differences were observed between membrane-associated variants and a farnesylation-defective mutant. In pancreatic cancer cells, 56 K-Ras interaction partners were identified. Most of these were cytoskeletal or plasma membrane proteins, and many have been identified previously as potential cancer biomarkers.

CONCLUSION

Biotin ligase tagging offers a rapid and convenient approach to the characterization of K-Ras interaction networks.

The NCK and ABI adaptor genes in catfish and their involvement in ESC disease response

Adaptor proteins non-catalytic region of tyrosine kinase (NCK) and Abelson interactor (ABI) are crucial for disease response. NCK1 was identified to be a candidate gene for enteric septicemia of catfish (ESC) disease resistance, and was speculated to play similar roles during ESC and enteropathogenic Escherichia coli (EPEC) pathogenicity. ABI1 was reported as a positional candidate gene for bacterial cold water disease (BCWD) resistance in rainbow trout. In this study, three NCK genes and six ABI genes were identified in the channel catfish (Ictalurus punctatus) genome and blue catfish (I. furcatus) transcriptome, and annotated by domain structures, phylogenetic and syntenic analyses. Their expression patterns were examined in the intestine and liver of catfish after challenge with Edwardsiella ictaluri. In the intestine, NCK1, ABI2a, ABI2b, ABI3a were differentially expressed after E. ictaluri infection. In the liver, NCK2a, NCK2b, ABI1b, ABI2a, ABI2b were significantly upregulated in ESC susceptible fish. In general, the NCK and ABI genes, with exception of ABI3a gene and NCK1 gene, were expressed at higher levels in susceptible fish after infection than in control fish, but were expressed at lower levels in resistant fish than in the control fish. Taken together, these results support the notion that NCK and ABI genes are involved in disease processes facilitating pathogenesis of the E. ictaluri bacteria.

Reciprocal regulation of Abl kinase by Crk Y251 and Abi1 controls invasive phenotypes in glioblastoma

Crk is the prototypical member of a class of Src homology 2 (SH2) and Src homology 3 (SH3) domain-containing adaptor proteins that positively regulate cell motility via the activation of Rac1 and, in certain tumor types such as GBM, can promote cell invasion and metastasis by mechanisms that are not well understood. Here we demonstrate that Crk, via its phosphorylation at Tyr251, promotes invasive behavior of tumor cells, is a prominent feature in GBM, and correlating with aggressive glioma grade IV staging and overall poor survival outcomes. At the molecular level, Tyr251 phosphorylation of Crk is negatively regulated by Abi1, which competes for Crk binding to Abl and attenuates Abl transactivation. Together, these results show that Crk and Abi1 have reciprocal biological effects and act as a molecular rheostat to control Abl activation and cell invasion. Finally, these data suggest that Crk Tyr251 phosphorylation regulate invasive cell phenotypes and may serve as a biomarker for aggressive GBM.

Hepatitis C Virus Nonstructural 5A Protein Interacts with Abelson Interactor 1 and Modulates Epidermal Growth Factor-mediated MEK/ERK Signaling Pathway

The propagation of hepatitis C virus (HCV) is highly dependent on host cellular factors. To identify the cellular factors involved in HCV propagation, we have previously performed protein microarray assays using the HCV nonstructural 5A (NS5A) protein as a probe. Of ∼9,000 host proteins immobilized in a microarray, ∼90 cellular proteins were identified as HCV NS5A interacting partners. Of these candidates, we selected Abelson interactor 1 (Abi1) for further characterization. Binding of HCV NS5A to Abi1 was verified by both in vitro pulldown and coimmunoprecipitation assays. HCV NS5A interacted with Abi1 through regions I + II of Abi1 and domain I of NS5A. We further demonstrated that Abi1 colocalized with the HCV NS5A protein in the cytoplasm. We showed that NS5A inhibited epidermal growth factor-mediated ERK and Egr1 activations and this inhibitory activity of NS5A was nullified in Abi1-knockdown cells. Moreover, silencing of Abi1 expression impaired HCV replication, whereas overexpression of Abi1 promoted HCV propagation. Collectively, these data indicate that HCV exploits host Abi1 protein via NS5A to modulate MEK/ERK signaling pathway for its own propagation.

Opposing Regulation of the EGF Receptor: A Molecular Switch Controlling Cytomegalovirus Latency and Replication

Herpesviruses persist indefinitely in their host through complex and poorly defined interactions that mediate latent, chronic or productive states of infection. Human cytomegalovirus (CMV or HCMV), a ubiquitous β-herpesvirus, coordinates the expression of two viral genes, UL135 and UL138, which have opposing roles in regulating viral replication. UL135 promotes reactivation from latency and virus replication, in part, by overcoming replication-suppressive effects of UL138. The mechanism by which UL135 and UL138 oppose one another is not known. We identified viral and host proteins interacting with UL138 protein (pUL138) to begin to define the mechanisms by which pUL135 and pUL138 function. We show that pUL135 and pUL138 regulate the viral cycle by targeting that same receptor tyrosine kinase (RTK) epidermal growth factor receptor (EGFR). EGFR is a major homeostatic regulator involved in cellular proliferation, differentiation, and survival, making it an ideal target for viral manipulation during infection. pUL135 promotes internalization and turnover of EGFR from the cell surface, whereas pUL138 preserves surface expression and activation of EGFR. We show that activated EGFR is sequestered within the infection-induced, juxtanuclear viral assembly compartment and is unresponsive to stress. Intriguingly, these findings suggest that CMV insulates active EGFR in the cell and that pUL135 and pUL138 function to fine-tune EGFR levels at the cell surface to allow the infected cell to respond to extracellular cues. Consistent with the role of pUL135 in promoting replication, inhibition of EGFR or the downstream phosphoinositide 3-kinase (PI3K) favors reactivation from latency and replication. We propose a model whereby pUL135 and pUL138 together with EGFR comprise a molecular switch that regulates states of latency and replication in HCMV infection by regulating EGFR trafficking to fine tune EGFR signaling.

Cytomegalovirus, a herpesvirus, persists in its host through complex interactions that mediate latent, chronic or productive states of infection. Defining the mechanistic basis viral persistence is important for defining the costs and possible benefits of viral persistence and to mitigate pathologies associated with reactivation. We have identified two genes, UL135 and UL138, with opposing roles in regulating states of latency and replication. UL135 promotes replication and reactivation from latency, in part, by overcoming suppressive effects of UL138. Intriguingly, pUL135 and pUL138 regulate the viral cycle by targeting the same receptor tyrosine kinase, epidermal growth factor receptor (EGFR). EGFR is a major homeostatic regulator controlling cellular proliferation, differentiation, and survival, making it an ideal target for viruses to manipulate during infection. We show that CMV insulates and regulates EGFR levels and activity by modulating its trafficking. This work defines a molecular switch that regulates latent and replicative states of infection through the modulation of host trafficking and signaling pathways. The regulation of EGFR at the cell surface provides a novel means by which the virus may sense and respond to changes in the host environment to enter into or exit the latent state.

Scavenger receptor class A member 5 (SCARA5) and suprabasin (SBSN) are hub genes of coexpression network modules associated with peripheral vein graft patency

OBJECTIVE

Approximately 30% of autogenous vein grafts develop luminal narrowing and fail because of intimal hyperplasia or negative remodeling. We previously found that vein graft cells from patients who later develop stenosis proliferate more in vitro in response to growth factors than cells from patients who maintain patent grafts. To discover novel determinants of vein graft outcome, we have analyzed gene expression profiles of these cells using a systems biology approach to cluster the genes into modules by their coexpression patterns and to correlate the results with growth data from our prior study and with new studies of migration and matrix remodeling.

METHODS

RNA from 4-hour serum- or platelet-derived growth factor (PDGF)-BB-stimulated human saphenous vein cells obtained from the outer vein wall (20 cell lines) was used for microarray analysis of gene expression, followed by weighted gene coexpression network analysis. Cell migration in microchemotaxis chambers in response to PDGF-BB and cell-mediated collagen gel contraction in response to serum were also determined. Gene function was determined using short-interfering RNA to inhibit gene expression before subjecting cells to growth or collagen gel contraction assays. These cells were derived from samples of the vein grafts obtained at surgery, and the long-term fate of these bypass grafts was known.

RESULTS

Neither migration nor cell-mediated collagen gel contraction showed a correlation with graft outcome. Although 1188 and 1340 genes were differentially expressed in response to treatment with serum and PDGF, respectively, no single gene was differentially expressed in cells isolated from patients whose grafts stenosed compared with those that remained patent. Network analysis revealed four unique groups of genes, which we term modules, associated with PDGF responses, and 20 unique modules associated with serum responses. The "yellow" and "skyblue" modules, from PDGF and serum analyses, respectively, correlated with later graft stenosis (P = .005 and P = .02, respectively). In response to PDGF, yellow was also associated with increased cell growth. For serum, skyblue was also associated with inhibition of collagen gel contraction. The hub genes for yellow and skyblue (ie, the gene most connected to other genes in the module), scavenger receptor class A member 5 (SCARA5) and suprabasin (SBSN), respectively, were tested for effects on proliferation and collagen contraction. Knockdown of SCARA5 increased proliferation by 29.9% ± 7.8% (P < .01), whereas knockdown of SBSN had no effect. Knockdown of SBSN increased collagen gel contraction by 24.2% ± 8.6% (P < .05), whereas knockdown of SCARA5 had no effect.

CONCLUSIONS

Using weighted gene coexpression network analysis of cultured vein graft cell gene expression, we have discovered two small gene modules, which comprise 42 genes, that are associated with vein graft failure. Further experiments are needed to delineate the venous cells that express these genes in vivo and the roles these genes play in vein graft healing, starting with the module hub genes SCARA5 and SBSN, which have been shown to have modest effects on cell proliferation or collagen gel contraction.

Synergistic effects of proteasome inhibitor carfilzomib in combination with tyrosine kinase inhibitors in imatinib-sensitive and -resistant chronic myeloid leukemia models

The tyrosine kinase inhibitor (TKI) imatinib has transformed the treatment and outlook of chronic myeloid leukemia (CML); however, the development of drug resistance and the persistence of TKI-resistant stem cells remain obstacles to eradicating the disease. Inhibition of proteasome activity with bortezomib has been shown to effectively induce apoptosis in TKI-resistant cells. In this study, we show that exposure to the next generation proteasome inhibitor carfilzomib is associated with a decrease in ERK signaling and increased expression of Abelson interactor proteins 1 and 2 (ABI-1/2). We also investigate the effect of carfilzomib in models of imatinib-sensitive and -resistant CML and demonstrate a potent reduction in proliferation and induction of apoptosis in a variety of models of imatinib-resistant CML, including primitive CML stem cells. Carfilzomib acts synergistically with the TKIs imatinib and nilotinib, even in imatinib-resistant cell lines. In addition, we found that the presence of immunoproteasome subunits is associated with an increased sensitivity to carfilzomib. The present findings provide a rational basis to examine the potential of carfilzomib in combination with TKIs as a potential therapy for CML, particularly in imatinib-resistant disease.

The C11R gene, which encodes the vaccinia virus growth factor, is partially responsible for MVA-induced NF-κB and ERK2 activation

MVA is an attenuated strain of vaccinia virus (VACV) that is a popular vaccine vector. MVA infection activates NF-κB. For 293T cells, it is known that MVA early gene expression activates extracellular signal-regulated kinase 2 (ERK2), resulting in NF-κB activation. However, other viral and cellular mechanisms responsible for this event are ill defined. The data presented here show that the epidermal growth factor receptor (EGFR) is at least one apical trigger in this pathway: ERK2 and NF-κB activation was diminished when MVA infections occurred in cells devoid of the EGFR (CHO K1 cells) or in the presence of a drug that inhibits EGFR activation (AG1478) in 293T cells. The expression of dominant negative Ras or Raf proteins still permitted NF-κB activation, suggesting that a nonclassical EGFR-based signal transduction pathway triggered ERK2-NF-κB activation. C11R is an early gene present in MVA and other orthopoxviruses. It encodes the soluble, secreted vaccinia virus growth factor (VGF), a protein that binds to and stimulates the EGFR. Here it was observed that NF-κB was activated in 293T cells transfected with a plasmid encoding the C11R gene. Silencing by small interfering RNA (siRNA) or deletion of the C11R gene (MVAΔC11R) reduced both MVA-induced ERK2 and NF-κB activation in 293T cells or the keratinocyte line Hacat, suggesting that this mechanism of MVA-induced NF-κB activation may be common for several cell types.

Downregulation of ABI1 expression affects the progression and prognosis of human gastric carcinoma

Abelson interactor protein-1 (ABI1) is a promising candidate tumor suppressor, and plays critical roles both in the pathogenesis of BCR-Abl-induced leukemia and in the spread of several solid tumors. The expression of ABI1 and its role in cancer progression and prognosis are largely unknown in the majority of solid tumors, including gastric cancer. In this study, we analyzed the correlation between ABI1 expression and the clinicopathological characteristics, tumor progression, and prognosis of patients with gastric carcinoma. Tissue specimens were from 103 gastric cancer patients who underwent gastrectomy in our hospital between January 2000 and December 2007. Among them 59 tumor tissue samples were matched with normal tissue samples. The expression of ABI1 protein was measured using immunohistochemical staining of paraffin-embedded tissue specimens. Meanwhile, quantitative real-time RT-PCR and Western blotting were used to identify the expression of ABI1 in human gastric normal mucosal cell line (GES-1) and gastric cancer cell lines (N87, AGS). We performed a statistical analysis of the potential correlation between ABI1 expression and the patients' clinicopathological characteristics, 5-year survival, and median survival time. The immunohistochemical staining results of 59 patients showed that ABI1 was expressed in 28.8% (17/59) of gastric cancer tissues, compared to 91.5% (54/59) of normal samples. ABI1 expression in 103 patients was strongly correlated with tumor differentiation, clinical stage, and lymph node status (P < 0.01). The 5-year survival rate was 15.3% in the ABI1-negative group and 63.7% in the ABI1-positive group. Median survival time in the ABI1-negative and ABI1-positive groups was 25.0 months (95% CI: 19.7-30.3) and 74.0 months (95% CI: 54.6-93.3), respectively. There was a significant difference between the two groups (chi(2) = 10.888, P = 0.001). Furthermore, we found that ABI1 expressed lowly in poor differentiated AGS, whereas highly in GES-1 and well-differentiated N87. Downregulation of ABI1 expression in human gastric carcinoma may play a critical role in tumor progression and in determining patient prognosis. ABI1 may be a useful diagnostic or prognostic molecular biomarker, and might be a potential target for therapeutic intervention.

Generation of branched actin networks: assembly and regulation of the N-WASP and WAVE molecular machines

The Arp2/3 complex is a molecular machine that generates branched actin networks responsible for membrane remodeling during cell migration, endocytosis, and other morphogenetic events. This machine requires activators, which themselves are multiprotein complexes. This review focuses on recent advances concerning the assembly of stable complexes containing the most-studied activators, N-WASP and WAVE proteins, and the level of regulation that is provided by these complexes. N-WASP is the paradigmatic auto-inhibited protein, which is activated by a conformational opening. Even though this regulation has been successfully reconstituted in vitro with isolated N-WASP, the native dimeric complex with a WIP family protein has unique additional properties. WAVE proteins are part of a pentameric complex, whose basal state and activated state when bound to the Rac GTPase were recently clarified. Moreover, this review attempts to put together diverse observations concerning the WAVE complex in the conceptual frame of an in vivo assembly pathway that has gained support from the recent identification of a precursor.

Abl interactor 1 regulates Src-Id1-matrix metalloproteinase 9 axis and is required for invadopodia formation, extracellular matrix degradation and tumor growth of human breast cancer cells

Abl interactor 1 (Abi1) is a key regulator of actin polymerization/depolymerization. The involvement of Abi1 in the development of abnormal cytoskeletal functions of cancer cells has recently been reported. It remains unclear, however, how Abi1 exerts its effects in tumor cells and whether it contributes to tumor progression in vivo. We report here a novel function for Abi1 in the regulation of invadopodia formation and Src-inhibitor of differentiation protein 1 (Id1)-matrix metalloproteinase (MMP)-9 pathway in MDA-MB-231 human breast cancer cells. Abi1 is found in the invadopodia of MDA-MB-231 cells. Epigenetic silencing of the Abi1 gene by short hairpin RNA in MDA-MB-231 cells impaired the formation of invadopodia and resulted in downregulation of the Src activation and Id1/MMP-9 expression. The decreased invadopodia formation and MMP-9 expression correlate with a reduction in the ability of these cells to degrade extracellular matrix. Remarkably, the knockdown of Abi1 expression inhibited tumor cell proliferation and migration in vitro and slowed tumor growth in vivo. Taken together, these results indicate that the Abi1 signaling plays a critical role in breast cancer progression and suggest that this pathway may serve as a therapeutic target for the treatment of human breast cancer.

Abl kinase inhibits the engulfment of apoptotic [corrected] cells in Caenorhabditis elegans

The engulfment of apoptotic cells is required for normal metazoan development and tissue remodeling. In Caenorhabditis elegans, two parallel and partially redundant conserved pathways act in cell-corpse engulfment. One pathway includes the adaptor protein CED-2 CrkII and the small GTPase CED-10 Rac, and acts to rearrange the cytoskeleton of the engulfing cell. The other pathway includes the receptor tyrosine kinase CED-1 and might recruit membranes to extend the surface of the engulfing cell. Although many components required for engulfment have been identified, little is known about inhibition of engulfment. The tyrosine kinase Abl regulates the actin cytoskeleton in mammals and Drosophila in multiple ways. For example, Abl inhibits cell migration via phosphorylation of CrkII. We tested whether ABL-1, the C. elegans ortholog of Abl, inhibits the CED-2 CrkII-dependent engulfment of apoptotic cells. Our genetic studies indicate that ABL-1 inhibits apoptotic cell engulfment, but not through CED-2 CrkII, and instead acts in parallel to the two known engulfment pathways. The CED-10 Rac pathway is also required for proper migration of the distal tip cells (DTCs) during the development of the C. elegans gonad. The loss of ABL-1 function partially restores normal DTC migration in the CED-10 Rac pathway mutants. We found that ABI-1 the C. elegans homolog of mammalian Abi (Abl interactor) proteins, is required for engulfment of apoptotic cells and proper DTC migration. Like Abl, Abi proteins are cytoskeletal regulators. ABI-1 acts in parallel to the two known engulfment pathways, likely downstream of ABL-1. ABL-1 and ABI-1 interact physically in vitro. We propose that ABL-1 opposes the engulfment of apoptotic cells by inhibiting ABI-1 via a pathway that is distinct from the two known engulfment pathways.

Cell death or apoptosis is a normal part of animal development, as is the engulfment and removal of dead cells by other cells. In the nematode Caenorhabditis elegans, ten highly conserved proteins have been characterized previously for their roles in engulfment and in cell migration, both of which involve the formation of cellular extensions. Little is known, however, about how engulfment is inhibited. In mammals, the tyrosine kinase Abl, which regulates the actin cytoskeleton and which when misexpressed causes two types of leukemia, prevents the CrkII protein from facilitating cell migration. CrkII functions in engulfment in C. elegans and mammals. We tested whether the C. elegans homolog of Abl, ABL-1, could inhibit engulfment. We found that ABL-1 functions as an inhibitor of apoptotic cell engulfment and cell migration. However, our analysis further showed that ABL-1 does not function by inhibiting other known engulfment proteins, including C. elegans CrkII. Our data indicate that ABL-1 blocks ABI-1, the C. elegans homolog of the mammalian and Drosophila Abl-interactor (Abi) cytoskeletal-regulatory proteins. We propose that ABL-1 acts via ABI-1 to inhibit a newly identified pathway during cell corpse engulfment and cell migration.

We show thatC. elegans Abl (ABL-1) inhibits the engulfment of apoptotic cells via a newly defined pathway that includes theC. elegans homolog of the cytoskeletal regulator Abl-interactor.

Abi1 gene silencing by short hairpin RNA impairs Bcr-Abl-induced cell adhesion and migration in vitro and leukemogenesis in vivo

Abl interactor (Abi) 1 was first identified as the downstream target of Abl tyrosine kinases and was found to be dysregulated in leukemic cells expressing oncogenic Bcr-Abl and v-Abl. Although the accumulating evidence supports a role of Abi1 in actin cytoskeleton remodeling and growth factor/receptor signaling, it is not clear how it contributes to Bcr-Abl-induced leukemogenesis. We show here that Abi1 gene silencing by short hairpin RNA attenuated the Bcr-Abl-induced abnormal actin remodeling, membrane-type 1 metalloproteinase clustering and inhibited cell adhesion and migration on fibronectin-coated surfaces. Although the knock down of Abi1 expression did not affect growth factor-independent growth of Bcr-Abl-transformed Ba/F3 cells in vitro, it impeded competitive expansion of these cells in non obese diabetic (NOD)/ severe combined immuno-deficiency (SCID) mice. Remarkably, the knock down of Abi1 expression in Bcr-Abl-transformed Ba/F3 cells impaired the leukemogenic potential of these cells in NOD/SCID mice. Abi1 contributes to Bcr-Abl-induced leukemogenesis in part through Src family kinases, as the knock down of Abi1 expression attenuates Bcr-Abl-stimulated activation of Lyn. Together, these data provide for the first time the direct evidence that supports a critical role of Abi1 pathway in the pathogenesis of Bcr-Abl-induced leukemia.

A new interaction between Abi-1 and betaPIX involved in PDGF-activated actin cytoskeleton reorganisation

Members of the Rho family of GTPases are key regulators of the actin cytoskeleton. In particular, activated Rac1 stimulates membrane dorsal ruffle formation in response to platelet-derived growth factor (PDGF). Abl-interactor (Abi)-1 and betaPIX, a guanine nucleotide exchange factor for Rac1, localise at these Rac1-induced actin structures and play important roles in the induction of membrane dorsal ruffling in response to PDGF in fibroblasts. Here, we demonstrate a novel interaction between Abi-1 and betaPIX using the yeast two-hybrid system, in vitro pull-down assays, and in vivo co-immunoprecipitation experiments. In vitro, the C-terminal fragment of betaPIX interacted with Abi-1, while in vivo the N-terminal fragment of betaPIX interacted with Abi-1. The biological function of this interaction was investigated in mouse fibroblasts in response to PDGF stimulation. Abi-1 and betaPIX co-localised in the cytoplasm and to membrane dorsal ruffles after PDGF treatment. We show that the co-expression of Abi-1 and truncated forms of betaPIX in mouse fibroblasts blocked PDGF-induced membrane dorsal ruffles. Together, these results show that the interaction between Abi-1 and betaPIX is involved in the formation of growth factor-induced membrane dorsal ruffles.

Abelson interactor protein-1 positively regulates breast cancer cell proliferation, migration, and invasion

Abelson interactor protein-1 (ABI-1) is an adaptor protein involved in actin reorganization and lamellipodia formation. It forms a macromolecular complex containing Hspc300/WASP family verprolin-homologous proteins 2/ABI-1/nucleosome assembly protein 1/PIR121 or Abl/ABI-1/WASP family verprolin-homologous proteins 2 in response to Rho family-dependent stimuli. Due to its role in cell mobility, we hypothesized that ABI-1 has a role in invasion and metastasis. In the present study, we found that weakly invasive breast cancer cell lines (MCF-7, T47D, MDA-MB-468, SKBR3, and CAMA1) express lower levels of ABI-1 compared with highly invasive breast cancer cell lines (MDA-MB-231, MDA-MB-157, BT549, and Hs578T), which exhibit high ABI-1 levels. Using RNA interference, ABI-1 was stably down-regulated in MDA-MB-231, which resulted in decreased cell proliferation and anchorage-dependent colony formation and abrogation of lamellipodia formation on fibronectin. Down-regulation of ABI-1 decreased invasiveness and migration ability and decreased adhesion on collagen IV and actin polymerization in MDA-MB-231 cells. Additionally, compared with control parental cells, ABI-1 small interfering RNA-transfected cells showed decreased levels of phospho-PDK1, phospho-Raf, phospho-AKT, total AKT, and AKT1. These data suggest that ABI-1 plays an important role in the spread of breast cancer and that this role may be mediated via the phosphatidylinositol 3-kinase pathway.

Abl tyrosine kinase promotes dorsal ruffles but restrains lamellipodia extension during cell spreading on fibronectin

The nonreceptor Abl tyrosine kinase stimulates F-actin microspikes and membrane ruffles in response to adhesion and growth factor signals. We show here that induced dimerization of Abl-FKBP, but not the kinase-defective AblKD-FKBP, inhibits cell spreading on fibronectin. Conversely, knockdown of cellular Abl by shRNA stimulates cell spreading. The Abl kinase inhibitor, imatinib, also stimulates cell spreading and its effect is overridden by the imatinib-resistant AblT315I. Expression of Abl but not AbkKD in Abl/Arg-deficient cells again inhibits spreading. Furthermore, Abl inhibits spreading of cells that express the activated Rac, RacV12, correlating with RacV12 localization to dorsal membrane protrusions. Ectopic expression of CrkII, a Rac activator that is inactivated by Abl-mediated tyrosine phosphorylation, antagonizes Abl-mediated dorsal membrane localization of RacV12. Ectopic expression of a dynamin-2 mutant, previously shown to induce Rac-GTP localization to the dorsal membrane, abolishes the stimulatory effect of imatinib on cell spreading. These results suggest that Abl tyrosine kinase, through CrkII phosphorylation and in collaboration with dynamin-2 can regulate the partitioning of Rac-GTP to favor dorsal ruffles during cell spreading. The Abl-dependent dorsal membrane localization of activated Rac explains its positive role in ruffling and negative role in cell spreading and migration.

Purification of phosphoproteins by immobilized metal affinity chromatography and its application to phosphoproteome analysis

Prefractionation procedures facilitate the identification of lower-abundance proteins in proteome analysis. Here we have optimized the conditions for immobilized metal affinity chromatography (IMAC) to enrich for phosphoproteins. The metal ions, Ga(III), Fe(III), Zn(II), and Al(III), were compared for their abilities to trap phosphoproteins; Ga(III) was the best. Detailed analyses of the pH and ionic strength for IMAC enabled us to determine the optimal conditions (pH 5.5 and 0.5 m NaCl). When whole cell lysates were fractionated in this way, about one-tenth of the total protein was recovered in the eluate, and the recovery of phosphorylated extracellular signal-regulated kinase (ERK) was more than 90%. Phosphorylated forms of ribosomal S6 kinase (RSK) and Akt were also enriched efficiently under the same conditions. Our Ga(III) IMAC and a commercially available purification kit for phosphoproteins performed similarly, with a slight difference in the spectrum of phosphoproteins. When phosphoproteins enriched from NIH3T3 cells in which ERK was either activated or suppressed were analyzed by two-dimensional fluorescence difference gel electrophoresis, phosphorylated ERK was detected as discrete spots unique to ERK-activated cells, which overlapped with surrounding spots in the absence of prefractionation. We applied the same technique to search for Akt substrates and identified Abelson interactor 1 as a novel potential target. These results demonstrate the efficacy of phosphoprotein enrichment by IMAC and suggest that this procedure will be of general use in phosphoproteome research.

c-Abl-binding protein interacts with p21-activated kinase 2 (PAK-2) to regulate PDGF-induced membrane ruffles

p21-Activated kinases (PAKs) are serine/threonine kinases involved in multiple cellular functions including cytoskeleton regulation, proliferation and apoptosis. We performed a screen for proteins interacting with PAK-2, a ubiquitously expressed kinase involved in apoptotic signaling. Among the PAK-2 interacting proteins were different members of the Abl-binding protein family. Abl-binding proteins bound to a proline-rich region of PAK-2 located in the regulatory N terminus. Moreover, active PAK-2 phosphorylated Abl-binding proteins in vitro. Interestingly, we show that PAK-2 also interacted with c-Abl but via a different domain than with the Abl-binding proteins. PAK-2 and Abi-1 co-localized in the cytoplasm and to membrane dorsal ruffles induced by PDGF treatment. Expression of mutant PAK-2 deficient in binding to Abl-binding proteins or silencing of PAK-2 expression prevented the formation of membrane dorsal ruffles in response to PDGF. Our findings define a new class of PAK-interacting proteins, which play an important role in actin cytoskeletal reorganization.

Abi-1 forms an epidermal growth factor-inducible complex with Cbl: role in receptor endocytosis

The Abl-interactor (Abi) proteins are involved in the regulation of actin polymerization and have recently been shown to modulate epidermal growth factor receptor (EGFR) endocytosis. Here we describe the identification of a novel complex between Abi-1 and the Cbl ubiquitin ligase that is induced by stimulation with EGF. Notably, an Abi-1 mutant lacking the SH3 domain (DeltaSH3) fails to interact with Cbl and inhibits EGFR internalization. We show that expression of the Abi-1DeltaSH3 mutant inhibits Cbl accumulation at the plasma membrane after EGF treatment. We have previously shown that the oncogenic Abl tyrosine kinase inhibits EGFR internalization. Here we report that the oncogenic Abl kinase disrupts the EGF-inducible Abi-1/Cbl complex, highlighting the importance of Abl kinases and downstream effectors in the regulation of EGFR internalization. Thus, our work reveals a new role for oncogenic Abl tyrosine kinases in the regulation of the Abi-1/Cbl protein complex and uncovers a role for the Abi-1/Cbl complex in the regulation of EGFR endocytosis.

Abelson interacting protein 1 (Abi-1) is essential for dendrite morphogenesis and synapse formation

Synaptogenesis and synaptic plasticity depend crucially on the dynamic and locally specific regulation of the actin cytoskeleton. We identified an important component for controlled actin assembly, abelson interacting protein-1 (Abi-1), as a binding partner for the postsynaptic density (PSD) protein ProSAP2/Shank3. During early neuronal development, Abi-1 is localized in neurites and growth cones; at later stages, the protein is enriched in dendritic spines and PSDs, as are components of a trimeric complex consisting of Abi-1, Eps8 and Sos-1. Abi-1 translocates upon NMDA application from PSDs to nuclei. Nuclear entry depends on abelson kinase activity. Abi-1 co-immunoprecipitates with the transcription factor complex of Myc/Max proteins and enhances E-box-regulated gene transcription. Downregulation of Abi-1 by small interfering RNA results in excessive dendrite branching, immature spine and synapse morphology and a reduction of synapses, whereas overexpression of Abi-1 has the opposite effect. Data show that Abi-1 can act as a specific synapto-nuclear messenger and is essentially involved in dendrite and synapse formation.

The role of the lens actin cytoskeleton in fiber cell elongation and differentiation

The vertebrate ocular lens is a fascinating and unique transparent tissue that grows continuously throughout life. During the process of differentiation into fiber cells, lens epithelial cells undergo dramatic morphological changes, membrane remodeling, polarization, transcriptional activation and elimination of cellular organelles including nuclei, concomitant with migration towards the lens interior. Most of these events are presumed to be influenced in large part, by dynamic reorganization of the cellular actin cytoskeleton and by intercellular and cell: extracellular matrix interactions. In light of recent and unprecedented advancement in our understanding of the mechanistic bases underlying regulation of actin cytoskeletal dynamics and the role of the actin cytoskeleton in cell function, this review attempts to summarize current knowledge regarding the role of the cellular actin cytoskeleton, in lens fiber cell elongation and differentiation, and regulation of actin cytoskeletal organization in the lens.

Phosphoinositide 3-Kinase C2beta regulates cytoskeletal organization and cell migration via Rac-dependent mechanisms

Receptor-linked class I phosphoinositide 3-kinases (PI3Ks) induce assembly of signal transduction complexes through protein-protein and protein-lipid interactions that mediate cell proliferation, survival, and migration. Although class II PI3Ks have the potential to make the same phosphoinositides as class I PI3Ks, their precise cellular role is currently unclear. In this report, we demonstrate that class II phosphoinositide 3-kinase C2beta (PI3KC2beta) associates with the Eps8/Abi1/Sos1 complex and is recruited to the EGF receptor as part of a multiprotein signaling complex also involving Shc and Grb2. Increased expression of PI3KC2beta stimulated Rac activity in A-431 epidermoid carcinoma cells, resulting in enhanced membrane ruffling and migration speed of the cells. Conversely, expression of dominant negative PI3KC2beta reduced Rac activity, membrane ruffling, and cell migration. Moreover, PI3KC2beta-overexpressing cells were protected from anoikis and displayed enhanced proliferation, independently of Rac function. Taken together, these findings suggest that PI3KC2beta regulates the migration and survival of human tumor cells by distinct molecular mechanisms.

The expression of Abl interactor 2 in leiomyoma and myometrium and regulation by GnRH analogue and transforming growth factor-β

BACKGROUND

Abelson (Abl) interactor 2 (Abi-2) has been considered as a key regulator of cell/tissue structural organization and is differentially expressed in leiomyomas. The objective of this study was to evaluate the expression of Abi-2 in leiomyoma/myometrium during the menstrual cycle and following GnRH analogue (GnRHa) therapy, as well as regulation by transforming growth factor (TGF)-beta1 in leiomyoma and myometrial smooth muscle cells (LSMC and MSMC).

METHODS

We used real-time PCR, Western blotting and immunohistochemistry to determine the expression of Abi-2 in paired leiomyoma and myometrium (n = 27) from proliferative (n = 8) and secretory (n = 12) phases of the menstrual cycle and from patients who received GnRHa therapy (n = 7). Time-dependent action of TGF-beta1 (2.5 ng/ml) and GnRHa (0.1 microM) on Abi-2 expression was determined in LSMC and MSMC.

RESULTS

Leiomyomas express elevated levels of Abi-2 as compared with myometrium from the proliferative but not the secretory phase of the menstrual cycle, with a significant reduction following GnRHa therapy (P < 0.05). Western blotting showed a similar trend in Abi-2 protein expression in leiomyoma/myometrial tissue extracts, which was immunolocalized in LSMC and MSMC, connective tissue fibroblasts and arterial walls. The expression of Abi-2 in LSMC and MSMC was increased by TGF-beta1 (2.5 ng/ml) and was inhibited by GnRHa (0.1 microM) in a time- and cell-dependent manner, and pretreatment with Smad3 SiRNA and U0126, an MEK-1/2 inhibitor, respectively, reversed their actions.

CONCLUSION

Based on the menstrual cycle-dependent expression, the influence of GnRHa therapy, and regulation by TGF-beta in LSMC/MSMC, we conclude that Abi-2 may have a key regulatory function in leiomyomas cellular/tissue structural organization during growth and regression.

E3B1, a human homologue of the mouse gene product Abi-1, sensitizes activation of Rap1 in response to epidermal growth factor

E3B1, a human homologue of the mouse gene product Abi-1, has been implicated in growth-factor-mediated regulation of the small GTPases p21Ras and Rac. E3b1 is a regulator of Rac because it can form a complex with Sos-1 and eps8, and such a Sos-1-e3B1-eps8 complex serves as a guanine nucleotide exchange factor for Rac. In the present study, we found that overexpression of e3B1 in NIH3T3/EGFR cells sensitized EGF-induced activation of Rac1, whereas it had no impact on EGF-induced activation of p21Ras. Remarkably, we found that EGF-induced activation of the p21Ras-related GTPase Rap1 was also sensitized in NIH3T3/EGFR-e3B1 cells. Thus, in NIH3T3/EGFR-e3B1 cells, maximal EGF-induced activation of Rap1 occurs with a dose of EGF much lower than in NIH3T3/EGFR cells. We also report that overexpression of e3B1 in NIH3T3/EGFR cells renders EGF-induced activation of Rap1 completely dependent on Src tyrosine kinases but not on c-Abl. However, EGF-induced tyrosine phosphorylation of the Rap GEF C3G occurred regardless of whether e3B1 was overexpressed or not, and this did not involve Src tyrosine kinases. Accordingly, we propose that overexpression of e3B1 in NIH3T3/EGFR cells leads to mobilization of Src tyrosine kinases that participate in EGF-induced activation of Rap1 and inhibition of cell proliferation.

Abi enhances Abl-mediated CDC2 phosphorylation and inactivation

Abelson tyrosine kinase (Abl) is a non-receptor tyrosine kinase which is frequently coupled with adaptor proteins to interact with its substrates for the regulation of cytoskeleton rearrangement, cell growth and apoptosis in response to a variety of biological stimuli. The Abl interactor (Abi) family members were first identified as adaptor proteins of Abl for regulating Abl transforming and kinase activity. In the present study, we used a yeast two-hybrid screen to identify Cdc2 as a novel Abi-binding protein. This finding led us to investigate the role of Abi in linking Abl and Cdc2. These three proteins formed a trimeric complex in Drosophila and mammalian cells. The expression of Abi in cells greatly enhanced the formation of the Abl-Cdc2 complex, suggesting that Abi functions as an adaptor protein facilitating the binding between Abl and Cdc2. We show that Abi promotes Abl-mediated phosphorylation of Cdc2 at tyrosine 15 and inactivation of Cdc2 kinase activity. Furthermore, coexpression of Abl and Abi in Drosophila S2 cells led to suppression of cell growth. These data suggest that Abl signaling may be involved in the downregulation of Cdc2 kinase in cell cycle control.

ABI2-deficient mice exhibit defective cell migration, aberrant dendritic spine morphogenesis, and deficits in learning and memory

The Abl-interactor (Abi) family of adaptor proteins has been linked to signaling pathways involving the Abl tyrosine kinases and the Rac GTPase. Abi proteins localize to sites of actin polymerization in protrusive membrane structures and regulate actin dynamics in vitro. Here we demonstrate that Abi2 modulates cell morphogenesis and migration in vivo. Homozygous deletion of murine abi2 produced abnormal phenotypes in the eye and brain, the tissues with the highest Abi2 expression. In the absence of Abi2, secondary lens fiber orientation and migration were defective in the eye, without detectable defects in proliferation, differentiation, or apoptosis. These phenotypes were consistent with the localization of Abi2 at adherens junctions in the developing lens and at nascent epithelial cell adherens junctions in vitro. Downregulation of Abi expression by RNA interference impaired adherens junction formation and correlated with downregulation of the Wave actin-nucleation promoting factor. Loss of Abi2 also resulted in cell migration defects in the neocortex and hippocampus, abnormal dendritic spine morphology and density, and severe deficits in short- and long-term memory. These findings support a role for Abi2 in the regulation of cytoskeletal dynamics at adherens junctions and dendritic spines, which is critical for intercellular connectivity, cell morphogenesis, and cognitive functions.

Loss of c-abl facilitates anchorage-independent growth of p53- and RB- deficient primary mouse embryonic fibroblasts

The c-abl tyrosine kinase is the proto-oncogene of the v-abl oncogene of the Abelson murine leukemia virus. Although mutational variants of c-Abl can exhibit gain of function and can produce a transformed phenotype, the function of c-Abl in transformation remained unclear. Here, we report that the loss of c-abl facilitates transformation. c-abl-knockout mouse embryonic fibroblasts (MEFs) immortalized by SV40 T antigen acquired anchorage-independent growth, and by constructing mutational variants of T antigen we showed that binding of large T antigen to p53 and RB was necessary to induce anchorage-independent growth. Although c-abl/p53 double-knockout MEFs did not undergo anchorage-independent growth, those expressing human papilloma virus 16 E7, which mainly inactivates RB, did. Our results show that the loss of c-abl facilitates anchorage-independent growth in the context of p53 and RB deficiency, and suggest that loss of function of c-abl facilitates some types of transformation.

Pak regulates calpain-dependent degradation of E3b1

E3b1, a binding partner of Eps8, plays a critical role in receptor tyrosine kinase (RTK)-mediated Rac activation by facilitating the interaction of Eps8 with Sos-1 and the consequent activation of the Rac-specific guanine nucleotide exchange factor activity of Sos-1. Here we present evidence that E3b1 levels are regulated by the Ca(2+)-activated protease calpain, and also by Pak, a downstream target of Rac signaling. Serum starvation of Rat2 or COS7 cells resulted in rapid loss of E3b1 that was reversed by calpain inhibitors. Loss was also prevented by expressing the constitutively active Pak1 mutant, Pak1(H83,86L). Activation of endogenous Pak by platelet-derived growth factor or the constitutively active Rac1 mutant, Rac1(G12V), also inhibited degradation. In contrast, inhibition of endogenous Pak activity by expressing the Pak auto-inhibitory domain caused degradation of over-expressed E3b1 even in the presence of serum. Taken together, these findings indicate that E3b1 is down-regulated by calpain activation and stabilized by Pak activation. They also suggest that RTK-mediated Rac activation can be modulated by changes in the level of E3b1 in response to signals that affect the activity of calpain or Pak.

Abl interactor 1 (Abi-1) wave-binding and SNARE domains regulate its nucleocytoplasmic shuttling, lamellipodium localization, and wave-1 levels

The Abl interactor 1 (Abi-1) protein has been implicated in the regulation of actin dynamics and localizes to the tips of lamellipodia and filopodia. Here, we show that Abi-1 binds the actin nucleator protein Wave-1 through an amino-terminal Wave-binding (WAB) domain and that disruption of the Abi-1-Wave-1 interaction prevents Abi-1 from reaching the tip of the lamellipodium. Abi-1 binds to the Wave homology domain of Wave-1, a region that is required for translocation of Wave-1 to the lamellipodium. Mouse embryo fibroblasts that lack one allele of Abi-1 and are homozygous null for the related Abi-2 protein exhibit decreased Wave-1 protein levels. This phenotype is rescued by Abi-1 proteins that retain Wave-1 binding but not by Abi-1 mutants that cannot bind to Wave-1. Moreover, we uncovered an overlapping SNARE domain in the amino terminus of Abi-1 that interacts with Syntaxin-1, a SNARE family member. Further, we demonstrated that Abi-1 shuttles in and out of the nucleus in a leptomycin B (LMB)-dependent manner and that complete nuclear translocation of Abi-1 in the absence of LMB requires the combined inactivation of the SNARE, WAB, and SH3 domains of Abi-1. Thus, Abi-1 undergoes nucleocytoplasmic shuttling and functions at the leading edge to regulate Wave-1 localization and protein levels.

Identification and characterization of EBP, a novel EEN binding protein that inhibits Ras signaling and is recruited into the nucleus by the MLL-EEN fusion protein

The chimeric MLL-EEN fusion protein is created as a result of chromosomal translocation t(11;19)(q23;p13). EEN, an Src homology 3 (SH3) domain–containing protein in the endophilin family, has been implicated in endocytosis, although little is known about its role in leukemogenesis mediated by the MLL-EEN fusion protein. In this study, we have identified and characterized EBP, a novel EEN binding protein that interacts with the SH3 domain of EEN through a proline-rich motif PPERP. EBP is a ubiquitous protein that is normally expressed in the cytoplasm but is recruited to the nucleus by MLL-EEN with a punctate localization pattern characteristic of the MLL chimeric proteins. EBP interacts simultaneously with EEN and Sos, a guanine-nucleotide exchange factor for Ras. Coexpressoin of EBP with EEN leads to suppression of Ras-induced cellular transformation and Ras-mediated activation of Elk-1. Taken together, our findings suggest a new mechanism for MLL-EEN–mediated leukemogenesis in which MLL-EEN interferes with the Ras-suppressing activities of EBP through direct interaction.

Cytoplasmic c-Abl provides a molecular 'Rheostat' controlling carcinoma cell survival and invasion

Tumor cell metastasis involves the coordinated activation of migration and survival mechanisms necessary for cell invasion of foreign tissues. Here, we report that cytoplasmic c-Abl tyrosine kinase determines whether a cell invades the ECM or commits suicide. c-Abl phosphorylates the cytoskeleton-associated adaptor protein, Crk, at tyrosine 221, causing disassociation of Crk from the Crk-associated substrate (CAS) and disassembly of Crk/CAS complexes. c-Abl-induced disruption of Crk/CAS complexes inhibits cell migration and promotes apoptosis in normal cells, and is deregulated in highly invasive carcinoma cells. c-Abl-mediated disassembly of Crk/CAS complexes and induction of death occur via disruption of the cytoskeleton, which is distinct from nuclear c-Abl-induced apoptosis in response to DNA-damaging agents. Inhibition of c-Abl kinase activity or Crk binding to Abl's polyproline region prevents Crk phosphorylation and apoptosis, leading to increased cell survival and invasion of the extracellular matrix. Together, these data illustrate that c-Abl prevents aberrant motility and survival through Crk 221 phosphorylation and modulation of Crk/CAS complexes, and that deregulation of this pathway contributes to cell metastasis.

Induction of colonic epithelial cell apoptosis by p47-dependent oxidants

Exogenous oxidants appear capable of initiating both proliferative and death signals, but the role of endogenous oxidants in either tumorigenesis or tumor suppression is unclear. We found that expression of the NAD(P)H oxidase adapter p47(phox) was suppressed in human colon carcinoma specimens relative to adjacent normal colon. Overexpression of p47(phox) increased apoptosis in colon cancer cell lines independent of p53 and mismatch-repair competency. p47(phox) was found to interact with the c-Abl adapter Abl interactor-1 (ABI-1), and p47(phox) coprecipitated with both ABI-1 and c-Abl. Ectopic expression of p47(phox) in colon cancer cells increased oxidant production with phosphorylation and activation of nuclear c-Abl and consequent apoptosis. Colonic epithelial p47(phox) may be specifically targeted to a c-Abl-containing complex that serves a physiologic tumor suppressing function.

Phosphoinositide 3-kinase activates Rac by entering in a complex with Eps8, Abi1, and Sos-1

Class I phosphoinositide 3-kinases (PI3Ks) are implicated in many cellular responses controlled by receptor tyrosine kinases (RTKs), including actin cytoskeletal remodeling. Within this pathway, Rac is a key downstream target/effector of PI3K. However, how the signal is routed from PI3K to Rac is unclear. One possible candidate for this function is the Rac-activating complex Eps8-Abi1-Sos-1, which possesses Rac-specific guanine nucleotide exchange factor (GEF) activity. Here, we show that Abi1 (also known as E3b1) recruits PI3K, via p85, into a multimolecular signaling complex that includes Eps8 and Sos-1. The recruitment of p85 to the Eps8-Abi1-Sos-1 complex and phosphatidylinositol 3, 4, 5 phosphate (PIP3), the catalytic product of PI3K, concur to unmask its Rac-GEF activity in vitro. Moreover, they are indispensable for the activation of Rac and Rac-dependent actin remodeling in vivo. On growth factor stimulation, endogenous p85 and Abi1 consistently colocalize into membrane ruffles, and cells lacking p85 fail to support Abi1-dependent Rac activation. Our results define a mechanism whereby propagation of signals, originating from RTKs or Ras and leading to actin reorganization, is controlled by direct physical interaction between PI3K and a Rac-specific GEF complex.

Serine/threonine phosphorylation of ShcA. Regulation of protein-tyrosine phosphatase-pest binding and involvement in insulin signaling

Serine phosphorylation of the ShcA signaling molecule has been reported recently. In this work, we have identified 12-O-tetradecanoylphorbol-13-acetate (TPA)- and growth factor-induced serine/threonine phosphorylation sites in p52(Shc) and p66(Shc). Among them, Ser(29) in p52(Shc) (equivalent to Ser(138) in p66(Shc)) was phosphorylated only after TPA stimulation. Phosphorylation of this site together with the intact phosphotyrosine-binding domain was essential for ShcA binding to the protein-tyrosine phosphatase PTP-PEST. TPA-induced ShcA phosphorylation at this site (and hence, its association with PTP-PEST) was inhibited by a protein kinase C-specific inhibitor and was induced by overexpression of constitutively active mutants of protein kinase Calpha, -epsilon, and -delta isoforms. Insulin also induced ShcA/PTP-PEST association, although to a lesser extent than TPA. Overexpression of a PTP-PEST binding-defective mutant of p52(Shc) (S29A) enhanced insulin-induced ERK activation in insulin receptor-overexpressing HIRc-B cells. Consistent with this, p52(Shc) S29A was more tyrosine-phosphorylated than wild-type p52(Shc) after insulin stimulation. Thus, we have identified a new mechanism whereby serine phosphorylation of ShcA controls the ability of its phosphotyrosine-binding domain to bind PTP-PEST, which is responsible for the dephosphorylation and down-regulation of ShcA after insulin stimulation.

The two hats of SOS

Son of sevenless (SOS) is a guanine nucleotide exchange factor that activates Ras in response to growth factor stimulation. SOS also appears to serve as a guanine nucleotide exchanger for Rac and, thus, may be involved in cytoskeleton reorganization. Nimnual and Bar-Sagi discuss how these two activities of SOS can be regulated and how SOS may be recruited to different cellular locations through interactions with the adaptor proteins Grb2 and E3b1.

Functional involvement of Akt signaling downstream of Jak1 in v-Abl-induced activation of hematopoietic cells

Activation of intracellular signaling pathways is important for cellular transformation and tumorigenesis. The nonreceptor tyrosine kinases Jak1 and Jak3, which bind to the v-Abl oncoprotein, are constitutively activated in cells transformed with the Abelson murine leukemia virus. A mutant of p160 v-Abl lacking the Jak1-binding region (v-Abl Delta858-1080) has a significant defect in Jak/STAT (signal transducers and activators of transcription) activation, cytokine-independent cell growth/survival, and tumorigenesis. To identify the pathways downstream of Jak kinases in v-Abl-mediated signaling, we examined the activation of several signaling molecules by p160 v-Abl or the v-Abl Delta858-1080 mutant. We demonstrate that, in addition to the decreased Ras activation, signaling through phosphatidylinositol-3 kinase and Akt are impaired in cells expressing mutant v-Abl. The proliferative defect of v-Abl Delta858-1080 was rescued by activated v-Akt and was also moderately rescued by activated v-H-Ras. However, constitutive active phosphatidylinositol-3 kinase (p110CAAX) did not complement this effect. Cells expressing v-Abl Delta858-1080 demonstrated reduced tumor formation in nude mice. In contrast, cells coexpressing v-Akt with v-Abl Delta858-1080 demonstrated reduced latency and increased frequency of tumor formation in nude nice compared with cells expressing v-Abl Delta858-1080 alone, whereas v-H-Ras or p110CAAX had minimum effects on tumor formation. These results suggest that Jak1-dependent Akt activation is important in v-Abl-mediated transformation.