alpha-Hemolysin-induced dephosphorylation of EGF receptor of A431 cells is carried out by rPTPsigma

Understanding Glycogen Synthase Kinase-3: A Novel Avenue for Alzheimer's Disease

Alzheimer's Disease (AD) is the most prevalent form of age-related dementia. Even though a century has passed since the discovery of AD, the exact cause of the disease still remains unknown. As a result, this poses a major hindrance in developing effective therapies for treating AD. Glycogen synthase kinase-3 (GSK-3) is one of the kinases that has been investigated recently as a potential therapeutic target for the treatment of AD. It is also known as human tau protein kinase and is a proline-directed serine-threonine kinase. Since dysregulation of this kinase affects all the major characteristic features of the disease, such as tau phosphorylation, amyloid formation, memory, and synaptic function, it is thought to be a major player in the pathogenesis of AD. In this review, we present the most recent information on the role of this kinase in the onset and progression of AD, as well as significant findings that identify GSK-3 as one of the most important targets for AD therapy. We further discuss the potential of treating AD by targeting GSK-3 and give an overview of the ongoing studies aimed at developing GSK-3 inhibitors in preclinical and clinical investigations.

Mini-review: Role of the PI3K/Akt pathway and tyrosine phosphatases in Alzheimer's disease susceptibility

A variety of findings from in vitro experiments and animal models support the hypothesis that one contribution to pathogenesis in Alzheimer's disease (AD) is enhanced phosphorylation of tau protein, which may be triggered by amyloid β (Aβ) and mediated by impaired activity of the PI3K/Akt signaling pathway. A number of tyrosine phosphatases act to reduce PI3K/Akt activity, and inhibition of tyrosine phosphatases is protective against Aβ toxicity in cell cultures and whole animals. Results from analysis of exome sequenced late onset AD cases and controls similarly show that rare coding variants predicted to damage PI3K functioning increase AD risk, whereas those which are predicted to damage genes for tyrosine phosphatase genes are protective. Taken together, these results support the proposition that tyrosine phosphatase antagonists might be trialed as therapeutic agents to protect against the development of AD.

The Diacetyl-Exposed Human Airway Epithelial Secretome: New Insights into Flavoring-Induced Airways Disease

Bronchiolitis obliterans (BO) is an increasingly important lung disease characterized by fibroproliferative airway lesions and decrements in lung function. Occupational exposure to the artificial food flavoring ingredient diacetyl, commonly used to impart a buttery flavor to microwave popcorn, has been associated with BO development. In the occupational setting, diacetyl vapor is first encountered by the airway epithelium. To better understand the effects of diacetyl vapor on the airway epithelium, we used an unbiased proteomic approach to characterize both the apical and basolateral secretomes of air-liquid interface cultures of primary human airway epithelial cells from four unique donors after exposure to an occupationally relevant concentration (∼1,100 ppm) of diacetyl vapor or phosphate-buffered saline as a control on alternating days. Basolateral and apical supernatants collected 48 h after the third exposure were analyzed using one-dimensional liquid chromatography tandem mass spectrometry. Paired t tests adjusted for multiple comparisons were used to assess differential expression between diacetyl and phosphate-buffered saline exposure. Of the significantly differentially expressed proteins identified, 61 were unique to the apical secretome, 81 were unique to the basolateral secretome, and 11 were present in both. Pathway enrichment analysis using publicly available databases revealed that proteins associated with matrix remodeling, including degradation, assembly, and new matrix organization, were overrepresented in the data sets. Similarly, protein modifiers of epidermal growth factor receptor signaling were significantly altered. The ordered changes in protein expression suggest that the airway epithelial response to diacetyl may contribute to BO pathogenesis.

Unbiased identification of substrates of protein tyrosine phosphatase ptp-3 in C. elegans

The leukocyte antigen related (LAR) family of receptor-like protein tyrosine phosphatases has three members in humans - PTPRF, PTPRD and PTPRS - that have been implicated in diverse processes including embryonic development, inhibition of cell growth and axonal guidance. Mutations in the LAR family are associated with developmental defects such as cleft palate as well as various cancers including breast, neck, lung, colon and brain. Although this family of tyrosine phosphatases is important for many developmental processes, little is known of their substrates. This is partially due to functional redundancy within the LAR family, as deletion of a single gene in the LAR family does not have an appreciable phenotype, but a dual knockout is embryonically lethal in mouse models. To circumvent the inability to knockout multiple members of the LAR family in mouse models, we used a knockout of ptp-3, which is the only known ortholog of the LAR family in Caenorhabditis elegans and allows for the study of the LAR family at the organismal level. Using SILAC-based quantitative phosphoproteomics, we identified 255 putative substrates of ptp-3, which included four of the nine known annotated substrates of the LAR family. A motif analysis of the identified phosphopeptides allowed for the determination of sequences that appear to be preferentially dephosphorylated. Finally, we discovered that kinases were overrepresented in the list of identified putative substrates and tyrosine residues whose phosphorylation is known to increase kinase activity were dephosphorylated by ptp-3. These data are suggestive of ptp-3 as a potential negative regulator of several kinase families, such as the mitogen activated kinases (MAPKs), and multiple tyrosine kinases including FER, MET, and NTRK2.

Protein tyrosine phosphatase σ targets apical junction complex proteins in the intestine and regulates epithelial permeability

Protein tyrosine phosphatase (PTP)σ (PTPRS) was shown previously to be associated with susceptibility to inflammatory bowel disease (IBD). PTPσ(-/-) mice exhibit an IBD-like phenotype in the intestine and show increased susceptibility to acute models of murine colitis. However, the function of PTPσ in the intestine is uncharacterized. Here, we show an intestinal epithelial barrier defect in the PTPσ(-/-) mouse, demonstrated by a decrease in transepithelial resistance and a leaky intestinal epithelium that was determined by in vivo tracer analysis. Increased tyrosine phosphorylation was observed at the plasma membrane of epithelial cells lining the crypts of the small bowel and colon of the PTPσ(-/-) mouse, suggesting the presence of PTPσ substrates in these regions. Using mass spectrometry, we identified several putative PTPσ intestinal substrates that were hyper-tyrosine-phosphorylated in the PTPσ(-/-) mice relative to wild type. Among these were proteins that form or regulate the apical junction complex, including ezrin. We show that ezrin binds to and is dephosphorylated by PTPσ in vitro, suggesting it is a direct PTPσ substrate, and identified ezrin-Y353/Y145 as important sites targeted by PTPσ. Moreover, subcellular localization of the ezrin phosphomimetic Y353E or Y145 mutants were disrupted in colonic Caco-2 cells, similar to ezrin mislocalization in the colon of PTPσ(-/-) mice following induction of colitis. Our results suggest that PTPσ is a positive regulator of intestinal epithelial barrier, which mediates its effects by modulating epithelial cell adhesion through targeting of apical junction complex-associated proteins (including ezrin), a process impaired in IBD.

Anti-epidermal growth factor receptor therapy in head and neck squamous cell carcinoma: focus on potential molecular mechanisms of drug resistance

Targeted therapy against the epidermal growth factor receptor (EGFR) is one of the most promising molecular therapeutics for head and neck squamous cell carcinoma (HNSCC). EGFR is overexpressed in a wide range of malignancies, including HNSCC, and initiates important signal transduction pathways in HNSCC carcinogenesis. However, primary and acquired resistance are serious problems and are responsible for low single-agent response rate and tumor recurrence. Therefore, an improved understanding of the molecular mechanisms of resistance to EGFR inhibitors may provide valuable indications to identify biomarkers that can be used clinically to predict response to EGFR blockade and to establish new treatment options to overcome resistance. To date, no predictive biomarker for HNSCC is available in the clinic. Therapeutic resistance to anti-EGFR therapy may arise from mechanisms that can compensate for reduced EGFR signaling and/or mechanisms that can modulate EGFR-dependent signaling. In this review, we will summarize some of these molecular mechanisms and describe strategies to overcome that resistance.

Genomic dissection of the epidermal growth factor receptor (EGFR)/PI3K pathway reveals frequent deletion of the EGFR phosphatase PTPRS in head and neck cancers

Activation of the PI3K and epidermal growth factor receptor (EGFR) pathway is able to drive oncogenesis in multiple human cancers, including head and neck squamous cell carcinoma. Targeted agents such as cetuximab and erlotinib are currently used in patients with head and neck squamous cell carcinoma, but, in this disease, the genomic alterations that cause pathway activation and determine response to pharmacologic inhibition remain ill-defined. Here, we present a detailed dissection of the EGFR/PI3K pathway, composed of sequencing of the core pathway components, and high-resolution genomic copy number assessment. Mutations were found in PIK3CA (6%), but no point mutations were observed in other pathway genes such as PTEN and EGFR. In contrast, we observed frequent copy number alterations of genes in the pathway, including PIK3CA, EGFR, protein tyrosine phosphatase receptor S (PTPRS), and RICTOR. In total, activating genetic pathway alterations were identified in 74% of head and neck tumors. Importantly, intragenic microdeletions of the EGFR phosphatase PTPRS were frequent (26%), identifying this gene as a target of 19p13 loss. PTPRS loss promoted EGFR/PI3K pathway activation, modulated resistance to EGFR inhibition, and strongly determined survival in lung cancer patients with activating EGFR mutations. These findings have important implications for our understanding of head and neck cancer tumorigenesis and for the use of targeted agents for this malignancy.

Membrane bound monomer of Staphylococcal alpha-hemolysin induces caspase activation and apoptotic cell death despite initiation of membrane repair pathway

Background

Wild type Staphylococcal α-hemolysin (α-HL) assembly on target mammalian cells usually results in necrotic form of cell death; however, caspase activation also occurs. The pathways of caspase activation due to binding/partial assembly by α-HL are unknown till date.

Results

Cells treated with H35N (a mutant of α-HL that remains as membrane bound monomer), have been shown to accumulate hypodiploid nuclei, activate caspases and induce intrinsic mitochondrial apoptotic pathway. We have earlier shown that the binding and assembly of α-HL requires functional form of Caveolin-1 which is an integral part of caveolae. In this report, we show that the caveolae of mammalian cells, which undergo a continuous cycle of ‘kiss and run’ dynamics with the plasma membrane, have become immobile upon the binding of the monomer. The cells treated with H35N were unable to recover despite activation of membrane repair mechanism involving caspase-1 dependent activation of sterol regulatory element binding protein-1.

Conclusions

This is for the first time we show the range of cellular changes and responses that take place immediately after the binding of the monomeric form of staphylococcal α-hemolysin.

Vinexin β regulates the phosphorylation of epidermal growth factor receptor on the cell surface

Epidermal growth factor (EGF) regulates various cellular events, including proliferation, differentiation, migration and oncogenesis. In this study, we found that exogenous expression of vinexin beta enhanced the phosphorylation of 180-kDa proteins in an EGF-dependent manner in Cos-7 cells. Western blot analysis using phospho-specific antibodies against EGFR identified EGFR as a phosphorylated 180-kDa protein. Vinexin beta did not stimulate the phosphorylation of EGFR but suppressed the dephosphorylation, resulting in a sustained phosphorylation. Mutational analyses revealed that both the first and third SH3 domains were required for a sustained phosphorylation of EGFR. Small interfering RNA-mediated knockdown of vinexin beta reduced the phosphorylation of EGFR on the cell surface in HeLa cells. The sustained phosphorylation of EGFR induced by vinexin beta was completely abolished by adding the EGFR-specific inhibitor AG1478 even after EGF stimulation, suggesting that the kinase activity of EGFR is required for the sustained phosphorylation induced by vinexin beta. We also found that E3 ubiquitin ligase c-Cbl is a binding partner of vinexin beta through the third SH3 domain. Expression of wild-type vinexin beta but not a mutant containing a mutation in the third SH3 domain decreased the cytosolic pool of c-Cbl and increased the amount of membrane-associated c-Cbl. Furthermore, over-expression of c-Cbl suppressed the sustained phosphorylation of EGFR induced by vinexin beta. These results suggest that vinexin beta plays a role in maintaining the phosphorylation of EGFR on the plasma membrane through the regulation of c-Cbl.