Oncogenic kinase signalling

Structure of a c-kit product complex reveals the basis for kinase transactivation

The c-Kit proto-oncogene is a receptor protein-tyrosine kinase associated with several highly malignant human cancers. Upon binding its ligand, stem cell factor (SCF), c-Kit forms an active dimer that autophosphorylates itself and activates a signaling cascade that induces cell growth. Disease-causing human mutations that activate SCF-independent constitutive expression of c-Kit are found in acute myelogenous leukemia, human mast cell disease, and gastrointestinal stromal tumors. We report on the phosphorylation state and crystal structure of a c-Kit product complex. The c-Kit structure is in a fully active form, with ordered kinase activation and phosphate-binding loops. These results provide key insights into the molecular basis for c-Kit kinase transactivation to assist in the design of new competitive inhibitors targeting activated mutant forms of c-Kit that are resistant to current chemotherapy regimes.

Expression of c-ABL, c-KIT, and platelet-derived growth factor receptor-beta in ovarian serous carcinoma and normal ovarian surface epithelium

BACKGROUND

Tyrosine kinases, such as c-KIT, c-ABL, and platelet-derived growth factor-beta (PDGFR-beta), are important regulators of cell growth. Highly potent and selective inhibitors of tyrosine kinases are being investigated as alternatives to standard chemotherapy. One such inhibitor, imatinib mesylate, is being used to treat gastrointestinal stromal tumors and chronic myelogenous leukemia. Ovarian carcinomas frequently develop resistance to conventional chemotherapeutic agents. Immunohistochemical expression of c-ABL, PDGFR-beta, and c-KIT was evaluated in ovarian carcinomas to determine whether treatment with imatinib mesylate might be feasible.

METHODS

The expression of c-ABL, c-KIT, and PDGFR-beta in tumors was evaluated by immunohistochemical analysis of 52 ovarian serous carcinomas, including 21 low-grade (well differentiated) and 31 high-grade (poorly differentiated) tumors. Fourteen normal ovaries were also evaluated.

RESULTS

In normal ovarian surface epithelium, c-ABL was expressed universally. PDGFR-beta was expressed in the majority (93%) of samples of normal ovarian epithelium, whereas the c-KIT protein was undetectable in normal ovarian surface epithelium. Overall, c-ABL was expressed in 71% of serous carcinomas. c-ABL was expressed more frequently in the low-grade serous carcinomas (81%) compared with the high-grade serous carcinomas (65%). PDGFR-beta expression was observed in 81% of serous carcinomas overall and was observed more frequently in higher-grade tumors. c-KIT immunohistochemical staining was absent in low-grade tumors but was present in 26% of high-grade serous carcinomas.

CONCLUSIONS

The majority of ovarian serous carcinomas express one or more of the kinases targeted by the tyrosine kinase inhibitor, imatinib mesylate, suggesting the potential usefulness of this drug in the treatment of ovarian carcinoma.

Application of comparative genomics in the identification and analysis of novel families of membrane-associated receptors in bacteria

BACKGROUND

A great diversity of multi-pass membrane receptors, typically with 7 transmembrane (TM) helices, is observed in the eukaryote crown group. So far, they are relatively rare in the prokaryotes, and are restricted to the well-characterized sensory rhodopsins of various phototropic prokaryotes.

RESULTS

Utilizing the currently available wealth of prokaryotic genomic sequences, we set up a computational screen to identify putative 7 (TM) and other multi-pass membrane receptors in prokaryotes. As a result of this procedure we were able to recover two widespread families of 7 TM receptors in bacteria that are distantly related to the eukaryotic 7 TM receptors and prokaryotic rhodopsins. Using sequence profile analysis, we were able to establish that the first members of these receptor families contain one of two distinct N-terminal extracellular globular domains, which are predicted to bind ligands such as carbohydrates. In their intracellular portions they contain fusions to a variety of signaling domains, which suggest that they are likely to transduce signals via cyclic AMP, cyclic diguanylate, histidine phosphorylation, dephosphorylation, and through direct interactions with DNA. The second family of bacterial 7 TM receptors possesses an alpha-helical extracellular domain, and is predicted to transduce a signal via an intracellular HD hydrolase domain. Based on comparative analysis of gene neighborhoods, this receptor is predicted to function as a regulator of the diacylglycerol-kinase-dependent glycerolipid pathway. Additionally, our procedure also recovered other types of putative prokaryotic multi-pass membrane associated receptor domains. Of these, we characterized two widespread, evolutionarily mobile multi-TM domains that are fused to a variety of C-terminal intracellular signaling domains. One of these typified by the Gram-positive LytS protein is predicted to be a potential sensor of murein derivatives, whereas the other one typified by the Escherichia coli UhpB protein is predicted to function as sensor of conformational changes occurring in associated membrane proteins

CONCLUSIONS

We present evidence for considerable variety in the types of uncharacterized surface receptors in bacteria, and reconstruct the evolutionary processes that model their diversity. The identification of novel receptor families in prokaryotes is likely to aid in the experimental analysis of signal transduction and environmental responses of several bacteria, including pathogens such as Leptospira, Treponema, Corynebacterium, Coxiella, Bacillus anthracis and Cytophaga.

Tyrosine phosphate in melanoma progression

BACKGROUND

Cellular tyrosine phosphorylation is regulated by two large families of enzymes. Protein tyrosine kinases (PTK) mediate addition, and protein tyrosine phosphatases (PTP), removal of phosphate from protein substrates. PTKs are oncogenes and PTPs have been hypothesized to function as tumour suppressor genes.

OBJECTIVES

To determine changes in tyrosine phosphate and PTP activity that occur during melanoma progression.

METHODS

Immunohistochemistry was used to study phosphotyrosine in melanocytic lesions. In addition, PTP activity of normal melanocytes and melanoma cell lines was measured using an enzyme-linked immunosorbent assay-based system.

RESULTS

Melanocytes in normal skin and most (67%) benign naevi were not immunostained. Neither were early malignant lesions (80% of malignant melanoma in situ and radial growth phase melanomas) stained. However, most advanced melanomas (100% of vertical growth phase, and 90% of metastatic melanomas) were immunoreactive. When total PTP enzyme activity was assayed in normal melanocytes and malignant melanoma cell lines, there was a significant increase in activity associated with melanoma progression.

CONCLUSIONS

Taken together, the data suggest increased phosphotyrosine signalling occurs during melanoma progression at the stage when cells first become competent for metastasis.

New perspectives in arsenic-induced cell signal transduction

Although the carcinogenicity of arsenic has been well established, the underlying molecular mechanisms have not yet been fully identified. Accumulating evidence indicates that the alteration of cellular signal transduction is directly related to the carcinogenesis of arsenic. This review focuses on recent advances in arsenic-induced signal transduction, including reactive oxygen species (ROS) production, tyrosine phosphorylation, MAPK signaling, NF-kappaB activation, cell cycle arrest, and apoptosis.

Immunization in tumor prevention

Recent experimental data suggest that immunity can be activated to prevent tumors. The rationale for prevention is strong because the setting is one endowed with an immune system that is neither impaired by tumor and treatment-induced suppression, nor tolerant to tumor-associated antigens. Oncogenic growth factor receptors are tumor antigens and rational targets for specific immunoprevention. Successful prevention of mammary carcinomas in Her-2/neu transgenic mice is cited as an evidence of the validity of this approach. The specific properties of the Her-2/neu gene product as an antigen and the nature of the immune responses induced by effective preventive treatments are summarized.

Activation of the p70 S6 kinase and phosphorylation of the 4E-BP1 repressor of mRNA translation by type I interferons

The Type I IFN receptor-generated signals required for initiation of mRNA translation and, ultimately, induction of protein products that mediate IFN responses, remain unknown. We have previously shown that IFNalpha and IFNbeta induce phosphorylation of insulin receptor substrate proteins and downstream engagement of the phosphatidylinositol (PI) 3'-kinase pathway. In the present study we provide evidence for the existence of a Type I IFN-dependent signaling cascade activated downstream of PI 3'-kinase, involving p70 S6 kinase. Our data demonstrate that p70 S6K is rapidly phosphorylated on threonine 421 and serine 424 and is activated during treatment of cells with IFNalpha or IFNbeta. Such activation of p70 S6K is blocked by pharmacological inhibitors of the PI 3'-kinase or the FKBP 12-rapamycin-associated protein/mammalian target of rapamycin (FRAP/mTOR). Consistent with this, the Type I IFN-dependent phosphorylation/activation of p70 S6K is defective in embryonic fibroblasts from mice with targeted disruption of the p85alpha and p85beta subunits of the PI 3'-kinase (p85alpha-/-beta-/-). Treatment of sensitive cell lines with IFNalpha or IFNbeta also results in phosphorylation/inactivation of the 4E-BP-1 repressor of mRNA translation. Such 4E-BP1 phosphorylation is also PI3'-kinase-dependent and rapamycin-sensitive, indicating that the Type I IFN-inducible activation of PI3'-kinase and FRAP/mTOR results in dissociation of 4E-BP1 from the eukaryotic initiation factor-4E (eIF4E) complex. Altogether, our data establish that the Type I IFN receptor-activated PI 3'-kinase pathway mediates activation of the p70 S6 kinase and inactivation of 4E-BP1, to regulate mRNA translation and induction of Type I IFN responses.

Inactivation of NF-kappaB by genistein is mediated via Akt signaling pathway in breast cancer cells

Genistein, a natural isoflavonoid found in soybean products, has been proposed to be associated with a lower rate of breast cancer in Asian women. Studies from our laboratory and others have shown that genistein can induce apoptosis by regulating the expression of apoptosis-related genes in breast cancer cells. However, the precise molecular mechanism(s) by which genistein induces apoptotic cell death is not clear. In order to investigate such mechanism, we tested the role of Akt and NF-kappaB in genistein-treated MDA-MB-231 breast cancer cells. We found that inhibition of cell growth and induction of apoptosis by genistein are partly mediated through the downregulation of Akt and NF-kappaB pathways. Gel shift assay showed that NF-kappaB DNA-binding activity in MDA-MB-231 cells transfected with Akt cDNA was induced, suggesting that there is a cross-talk between NF-kappaB and Akt signaling pathway. Moreover, we found that genistein could abrogate EGF and Akt induced NF-kappaB activation. From these results, we conclude that the inactivation of NF-kappaB by genistein in MDA-MB-231 breast cancer cells is partly mediated via Akt pathway, which could be useful for rational design of strategies for the prevention and/or treatment of breast cancer.

Protein kinase CK2: a challenge to canons

CK2 is an extremely conserved pleiotropic protein kinase with a growing list of more than 300 substrates, the majority of which are proteins implicated in signal transduction, gene expression and other nuclear functions. The CK2 phosphoacceptor sites are specified by multiple acidic residues, with the one at position +3 relative to the target residue being of crucial relevance. The CK2 holoenzyme is composed of two catalytic subunits (alphaalpha, alpha'alpha' or alphaalpha'), which are essential for cell viability, and a dimer of two non-catalytic beta subunits, whose precise function is still poorly understood. Although the beta subunits deeply affect many properties of CK2, both the isolated catalytic subunits and the holoenzyme are constitutively active, which is probably responsible for the oncogenic potential of CK2. Given the structure of the holoenzyme, the beta subunits could undergo reversible dissociation under physiological conditions and play a role as anchoring elements and/or as a docking platform for protein substrates and effectors. These unusual features are likely to be instrumental in the involvement of CK2 in a number of key biological functions, notably RNA synthesis, Wnt signaling, ubiquitination and cell survival.

Differing roles of Akt and serum- and glucocorticoid-regulated kinase in glucose metabolism, DNA synthesis, and oncogenic activity

Serum- and glucocorticoid-regulated kinase (SGK) is a serine kinase that has a catalytic domain homologous to that of Akt, but lacks the pleckstrin homology domain present in Akt. Akt reportedly plays a key role in various cellular actions, including glucose transport, glycogen synthesis, DNA synthesis, anti-apoptotic activity, and cell proliferation. In this study, we attempted to reveal the different roles of SGK and Akt by overexpressing active mutants of Akt and SGK. We found that adenovirus-mediated overexpression of myristoylated (myr-) forms of Akt resulted in high glucose transport activity in 3T3-L1 adipocytes, phosphorylated glycogen synthase kinase-3 (GSK3) and enhanced glycogen synthase activity in hepatocytes, and the promotion of DNA synthesis in interleukin-3-dependent 32D cells. In addition, stable transfection of myr-Akt in NIH3T3 cells induced an oncogenic transformation in soft agar assays. The active mutant of SGK (D-SGK, substitution of Ser422 with Asp) and myr-SGK were shown to phosphorylate GSK3 and to enhance glycogen synthase activity in hepatocytes in a manner very similar to that observed for myr-Akt. However, despite the comparable degree of GSK3 phosphorylation between myr-Akt and d-SGK or myr-SGK, d-SGK and myr-SGK failed to enhance glucose transport activity in 3T3-L1 cells, DNA synthesis in 32D cells, and oncogenic transformation in NIH3T3 cells. Therefore, the different roles of SGK and Akt cannot be attributed to ability or inability to translocate to the membrane thorough the pleckstrin homology domain, but rather must be attributable to differences in the relatively narrow substrate specificities of these kinases. In addition, our observations strongly suggest that phosphorylation of GSK3 is either not involved in or not sufficient for GLUT4 translocation, DNA synthesis, or oncogenic transformation. Thus, the identification of substrates selectively phosphorylated by Akt, but by not SGK, may provide clues to clarifying the pathway leading from Akt activation to these cellular activities.

Mass spectrometry and site-directed mutagenesis identify several autophosphorylated residues required for the activity of PrkC, a Ser/Thr kinase from Bacillus subtilis

We have shown recently that PrkC, which is involved in developmental processes in Bacillus subtilis, is a Ser/Thr kinase with features of the receptor kinase family of eukaryotic Hanks kinases. In this study, we expressed and purified from Escherichia coli the cytoplasmic domain of PrkC containing the kinase and a short juxtamembrane region. This fragment, which we designate PrkCc, undergoes autophosphorylation in E.coli. PrkCc is further autophosphorylated in vitro, apparently through a trans-kinase, intermolecular reaction. PrkC also displays kinase activity with myelin basic protein. Using high mass accuracy electrospray tandem mass spectrometry (LC-MS/MS) and nanoelectrospray tandem mass spectrometry, we identified seven phosphorylated threonine and one serine residue in PrkCc. All the corresponding residues were replaced by systematic site-directed mutagenesis and the purified mutant proteins were tested for in vitro kinase activity. Single and multiple replacement of four threonine residues, clustered between residues 162 and 167 in a putative activation loop, substantially reduced kinase activity and the effect was clearly additive. Replacement of the other three threonine residues, clustered between residues 290 and 320, had relatively little effect on activity. In contrast, substitution of Ser214, which is conserved in closely related receptor kinase-like bacterial proteins, independently affected activity and may represent a novel regulatory mechanism. When projected onto a 3D structure of PrkC modelled on the structure of known Hanks kinases, the first cluster of phospho-threonine residues falls precisely in the activation loop, controlling the access of substrate and ATP to the catalytic site of many eukaryotic receptor kinases, whereas the second cluster is located in the juxtamembrane region. These results indicate that regulation of PrkC kinase activity (and presumably autophosphorylation) includes a conserved activation loop mechanism. The juxtamembrane phospho-threonine residues may be essential, for example for the recruitment of other proteins necessary for a PrkC signalling cascade or for coupling to other signalling pathways. This is the first structure-function analysis of a bacterial receptor-like kinase of the Hanks family.

Profiling, comparison and validation of gene expression in gastric carcinoma and normal stomach

Gastric carcinoma is the fourth most common cause of cancer death worldwide but its molecular biology is poorly understood. We catalogued the genes expressed in two gastric adenocarcinomas and normal stomach, using serial analysis of gene expression (SAGE), and compared the profiles on-line with other glandular epithelia. Candidates were validated by Northern blotting and immunohistochemistry. A total of 29 480 transcripts, derived from 10 866 genes, were identified. In all, 1% of the genes were differentially expressed (>/=fivefold difference plus P-value </=0.01) between cancers and normal stomach. The most abundant transcripts included ribosomal and mitochondrial proteins, of which most were upregulated in the tumours, as were other widely expressed genes including transcription factors, signalling molecules (serine/threonine protein kinases), thymosin beta 10 and collagenase I. Transcripts abundant in normal stomach were functionally important, including gastrin, immunoglobulin alpha, lysozyme, MUC5, pS2 and pepsinogens, which were among 55 gastric-specific genes. Many transcripts were minimally characterized or new, some cancer-associated genes reflected their intestinal morphology, and some normal gastric genes had previously been considered as pancreatic carcinoma markers. The gastric carcinoma profiles resembled other tumours', supporting the existence of common cancer-associated targets. These data provide a catalogue from which to develop markers for better diagnosis and therapy of gastric carcinoma.

Altered localization and activity of the intracellular tyrosine kinase BRK/Sik in prostate tumor cells

Breast tumor kinase (BRK) is an intracellular tyrosine kinase expressed in differentiating epithelial cells of the gastrointestinal tract and skin, and in several epithelial cancers including carcinomas of the breast and colon. We examined expression of BRK and its mouse ortholog Src-related intestinal kinase (Sik) in prostate tissues and detected it in the nuclei of normal luminal prostate epithelial cells. BRK localization was then examined in 58 human prostate biopsy samples representing various grades of prostate cancer. While nuclear localization of BRK was present in well-differentiated tumors, it was absent in poorly differentiated tumors. However localization of Sam68, a nuclear substrate of BRK/Sik, was unaltered in all prostate tumors examined. Consistent with these results, nuclear BRK was detected in the more differentiated androgen-responsive LNCaP human prostate cancer cell line that is poorly tumorigenic in host animals, but it was primarily cytoplasmic in the undifferentiated androgen-unresponsive PC3 prostate cancer cell line that forms aggressive tumors. While PC3 cells expressed higher levels of endogenous BRK than LNCaP cells, BRK was less active in these cells. Our data suggest that BRK plays a role in differentiation of prostate epithelial cells. Altered BRK localization and/or activity may provide a prognostic indicator for prostate tumor progression and be a potential target for therapeutic intervention.

Putative phosphatidylinositol 3-kinase (PI3K) binding motifs in ovine betaretrovirus Env proteins are not essential for rodent fibroblast transformation and PI3K/Akt activation

Jaagsiekte sheep retrovirus (JSRV) and enzootic nasal tumor virus (ENTV) are simple betaretroviruses that cause epithelial cell tumors in the lower and upper airways of sheep and goats. The envelope (Env) glycoproteins of both viruses can transform rodent and chicken fibroblasts, indicating that they play an essential role in oncogenesis. Previous studies found that a YXXM motif in the Env cytoplasmic tail, a putative docking site for phosphatidylinositol 3-kinase (PI3K) after tyrosine phosphorylation, was necessary for rodent cell transformation but was not required for transformation of DF-1 chicken fibroblasts. Here we show that JSRV and ENTV Env proteins with tyrosine or methionine mutations in the YXXM motif can still transform rodent fibroblasts, albeit with reduced efficiency. Akt was activated in cells transformed by JSRV or ENTV Env proteins and in cells transformed by the proteins with tyrosine mutations. Furthermore, the PI3K-specific inhibitor LY294002 could inhibit Akt activation and cell transformation in all cases, indicating that Akt activation and transformation is PI3K dependent. However, we could not detect tyrosine phosphorylation of JSRV or ENTV Env proteins or an interaction between the Env proteins and PI3K in the transformed cells. We found no evidence for mitogen-activated protein kinase activation in cells that were transformed by the JSRV or ENTV Env proteins. We conclude that ovine betaretrovirus Env proteins transform the rodent fibroblasts by indirectly activating the PI3K/Akt pathway.

Clinical applications of proteomics

Proteomics, the systematic evaluation of changes in the protein constituency of a cell, is more than just the generation of lists of proteins that increase or decrease in expression as a cause or consequence of disease. The ultimate goal is to characterize the information flow through protein pathways that interconnect the extracellular microenvironment with the control of gene transcription. The nature of this information can be a cause or a consequence of disease processes. Clinical applications of proteomics involve the use of proteomic technologies at the bedside. The analysis of human cancer as a model for how proteomics can have an impact at the bedside is now employing several new proteomic technologies that are being developed for early detection, therapeutic targeting and finally, patient-tailored therapy.

Inhibition of zebrafish epidermal growth factor receptor activity results in cardiovascular defects

The physiological role of any of the epidermal growth factor (EGF) receptor tyrosine kinases has yet to be determined in zebrafish. We isolated a zebrafish homologue of EGFR (egfr) that shows a 63% amino acid overall identity to human EGFR but with 90% amino acid identity in the kinase domain. Whole mount in situ hybridization showed ubiquitous distribution of egfr transcripts during gastrulation, somitogenesis and later stages. When expressed in Chinese hamster ovary cells, zebrafish Egfr was a functional receptor that responded to EGF by receptor tyrosine phosphorylation and activation of MAP kinase. The function of zebrafish Egfr in vivo was determined by inhibiting its activity using EGFR kinase inhibitors and antisense morpholinos (MO), which inhibited Egfr kinase activity and translation of egfr messenger RNA into protein, respectively. The zebrafish is a particularly excellent model for studying cardiovascular development because zebrafish are transparent allowing direct visualization of the heart and circulation in the blood vessels. Inhibition of zebrafish Egfr activity in vivo impeded blood flow via the outflow tract into the aorta and impeded circulation in the axial and intersegmental vessels by 80 h post-fertilization. Analysis of the heart showed that the heart chambers and pericardial sacs were dilated and the outflow tracts were narrowed. Together these results suggested that zebrafish Egfr has a cardiovascular function in the developing zebrafish that is required for normal circulation.

Liquid chromatography-Fourier transform ion cyclotron resonance mass spectrometric characterization of protein kinase C phosphorylation

A vented column, capillary liquid chromatography (LC) microelectrospray ionization (ESI) Fourier transform ion cyclotron resonance (FT-ICR (9.4 T)) mass spectrometry (MS) approach to phosphopeptide identification is described. A dual-ESI source capable of rapid (approximately 200 ms) switching between two independently controlled ESI emitters was constructed. The dual-ESI source, combined with external ion accumulation in a linear octopole ion trap, allowed for internal calibration of every mass spectrum during LC. LC ESI FT-ICR positive-ion MS of protein kinase C (PKC) revealed four previously unidentified phosphorylated peptides (one within PKC(alpha), one within PKC(delta), and two within PKC(zeta)). Internal calibration improved the mass accuracy for LC MS spectra from an absolute mean (47 peptide ions) of 11.5 ppm to 1.5 ppm. Five additional (out of eight known) activating sites of PKC phosphorylation, not detected in positive-ion experiments, were observed by subsequent negative-ion direct infusion nanoelectrospray. Extension of the method to enable infrared multiphoton dissociation of all ions in the ICR cell prior to every other mass measurement revealed the diagnostic neutral loss of H3PO4 from phosphorylated peptide ions. The combination of accurate-mass MS and MS/MS offers a powerful new tool for identifying the presence and site(s) of phosphorylation in peptides, without the need for additional wet chemical derivatization.

Novel approaches with targeted therapies in bladder cancer. Therapy of bladder cancer by blockade of the epidermal growth factor receptor family

The improved understanding of the molecular biology of urothelial malignancies is helping to define the role of new targets and prognostic indices that can direct the most appropriate choice of treatment for advanced disease. Many human tumors express high levels of growth factors and their receptors that can be used as potential therapeutical targets. Tyrosine-kinase receptors, including many growth factor receptors such the receptors for epidermal growth factor (EGF), vascular endothelial growth factor (VEGF), and Her2/neu, have been found overexpressed in urothelial tumors. For many of these growth factor receptors, the degree of expression has been associated with the progression of cancer and a poor prognosis. Among the best studied growth factor receptors are the two members of EGF receptor familiy EGFr (ErbB-1), and Her2/neu (ErbB-2). Several preclinical studies in bladder cancer models, have confirmed that systemic administration of growth factor inhibitors inhibits the growth and metastasis of human transitional cell carcinoma established in the bladder wall of athymic nude mice. Additional studies indicate that therapy with EGFR inhibitors enhances the activity of conventional cytoreductive chemotherapeutic agents, in part by inhibiting tumor cell proliferation, angiogenesis, and inducing apoptosis. Novel targeted therapy hold promise to improve the current results of bladder cancer treatment. Based on the success seen with anti-HER2 monoclonal antibodies (Herceptin) and the promising results with EGFR targeted agents (IMC-C225 Cetuximab, ZD1389 Iressa, OSI-774 Tarceva, GW 57016) in other tumor types, and based on the results obtained in preclinical models, there is a great interest in assessing these agents in patients with bladder cancer. Several trials are now ongoing testing these new agents alone or in combination with chemotherapy in bladder cancer patients. The integration of these newer biologic agents, probably to supplement rather than to supplant chemotherapeutic drugs, should be a primary direction of research with the objective to interfere with multiple aspects of bladder cancer progression. However, the value of integration of biologically targeted agents into combined modality treatment for patients with bladder cancer has still to be proven.

TSG101 interaction with HRS mediates endosomal trafficking and receptor down-regulation

Down-regulation of mitogenic signaling in mammalian cells relies in part on endosomal trafficking of activated receptors into lysosomes, where the receptors are degraded. These events are mediated by ubiquitination of the endosomal cargo and its consequent sorting into multivesicular bodies that form at the surfaces of late endosomes. Tumor susceptibility gene 101 (tsg101) recently was found to be centrally involved in this process. Here we report that TSG101 interacts with hepatocyte growth factor-regulated tyrosine kinase substrate (HRS), an early endosomal protein, and that disruption of this interaction impedes endosomal trafficking and endocytosis-mediated degradation of mitogenic receptors. TSG101/HRS interaction occurs between a ubiquitin-binding domain of TSG101 and two distinct proline-rich regions of HRS, and is modulated by a C-terminal TSG101 sequence that resembles a motif targeted in HRS. Mutational perturbation of TSG101/HRS interaction prevented delivery of epidermal growth factor receptor (EGFR) to late endosomes, resulted in the cellular accumulation of ubiquitinated EGFR in early endosomes, and inhibited ligand-induced down-regulation of EGFR. Our results reveal the TSG101 interaction with HRS as a crucial step in endocytic down-regulation of mitogenic signaling and suggest a role for this interaction in linking the functions of early and late endosomes.

Systematic discovery of multicomponent therapeutics

Multicomponent therapies, originating through deliberate mixing of drugs in a clinical setting, through happenstance, and through rational design, have a successful history in a number of areas of medicine, including cancer, infectious diseases, and CNS disorders. We have developed a high-throughput screening method for identifying effective combinations of therapeutic compounds. We report here that systematic screening of combinations of small molecules reveals unexpected interactions between compounds, presumably due to interactions between the pathways on which they act. Through systematic screening of approximately 120,000 different two-component combinations of reference-listed drugs, we identified potential multicomponent therapeutics, including (i) fungistatic and analgesic agents that together generate fungicidal activity in drug-resistant Candida albicans, yet do not significantly affect human cells, (ii) glucocorticoid and antiplatelet agents that together suppress the production of tumor necrosis factor-alpha in human primary peripheral blood mononu-clear cells, and (iii) antipsychotic and antiprotozoal agents that do not exhibit significant antitumor activity alone, yet together prevent the growth of tumors in mice. Systematic combination screening may ultimately be useful for exploring the connectivity of biological pathways and, when performed with reference-listed drugs, may result in the discovery of new combination drug regimens.

Escape from Cbl-mediated downregulation: a recurrent theme for oncogenic deregulation of receptor tyrosine kinases

Deregulation of growth factor receptor tyrosine kinases (RTKs) is linked to a large number of malignancies. This occurs through a variety of mechanisms that result in enhanced activity of the receptor. Considerable evidence now supports the idea that loss of negative regulation plays an important role in receptor deregulation. RTKs are removed from the cell surface via endocytosis and many are subsequently degraded in the lysosome. Lysosomal targeting has recently been linked with receptor ubiquitination. We review here molecular alterations that uncouple RTKs from ubiquitination and implicate loss of ubiquitination as a process that plays a significant role in the pathogenesis of cancer.

The raison d'être of constitutively active protein kinases: the lesson of CK2

Protein kinases generally are tightly controlled signaling molecules that are switched on only in response to specific stimuli. Exceptionally few protein kinases are constitutively active, the most striking example being provided by CK2 (formerly "casein kinase 2"). Owing to unique structural features, the catalytic activity of CK2 is constantly on, although its targeting can be deeply influenced by the association of its two catalytic subunits (alpha and/or alpha') with a dimeric non catalytic beta subunit. Constitutive activity of CK2 reflects its extraordinary pleiotropy documented by its growing list of >300 protein substrates and is consistent with emerging evidence that CK2 plays an essential role in the cell by counteracting premature and/or unscheduled apoptosis, thus ensuring cell survival under stress conditions.

Analysis of protein tyrosine phosphorylation by nanoelectrospray ionization high-resolution tandem mass spectrometry and tyrosine-targeted product ion scanning

A novel highly sensitive strategy is introduced for analysis of tyrosine phosphorylation in previously identified proteins channelling for this aim all analytical and sequence information available. Nanoelectrospray high-resolution MS/MS analysis is targeted to precalculated m/z values corresponding to phosphotyrosine-containing tryptic peptides. Identification of these peptides is supported by the occurrence of the phosphotyrosine immonium ion at m/z 216, neutral loss of 79.97/z (= loss of HPO3), and similarity of the fragmentation patterns of phosphotyrosine-containing peptides with their nonphosphorylated analogues. This tyrosine-targeted tandem mass spectrometry strategy is demonstrated for epidermal growth factor receptor showing that phosphotyrosine-containing tryptic peptides invisible in the survey spectrum can be safely identified.

An extracellular matrix-specific microarray allowed the identification of target genes downstream of discoidin domain receptors

The two discoidin domain receptors, DDR1 and DDR2, are tyrosine kinases that are activated by collagen and are essential regulators of cell-matrix communication. However, the target genes downstream of activated DDRs and their physiological significance are largely unknown. Here, we describe a novel method to dissect signaling pathways induced by extracellular matrix (ECM) receptors. Using the doxycycline-inducible repression system (tet-off), we generated human fibrosarcoma and mouse fibroblast cell lines over-expressing DDR1 or DDR2. These cell lines were employed for gene expression analysis using microarrays specific for human and mouse genes coding for ECM proteins or ECM-interacting factors. We found that approximately 10% of the genes studied were up- or down-regulated more than twofold in response to signals generated by over-expressing DDRs. A common event downstream of DDR1 and DDR2 in human and mouse cells was the up-regulation of P-selectin glycoprotein ligand. Key target genes repressed upon DDR activation were agrin, syndecan-1 and alpha3 integrin. ECM-specific microarrays were found a valuable tool to dissect gene expression changes induced by collagen-receptor signaling pathways.

Determinants of substrate recognition in nonreceptor tyrosine kinases

Cytoplasmic tyrosine kinases do not occur as isolated catalytic domains. Instead, each kinase family possesses a characteristic array of additional domains that are appended to the catalytic domain. The combination and the arrangement of these modular domains are important in kinase regulation and function. This Account describes how the noncatalytic regions of Src family tyrosine kinases are involved in enzyme regulation, substrate selection, and multisite phosphorylation.

Phosphotyrosyl mimetics in the development of signal transduction inhibitors

Phosphotyrosyl (pTyr) residues play important roles in cellular signal transduction by facilitating recognition and binding necessary for critical protein-protein interactions, and for this reason pTyr motifs represent attractive starting points in the development of signaling antagonists. Although the pTyr phosphoryl moiety is central in these phenomena, its incorporation into signaling inhibitors is contraindicated due to enzymatic lability and limited bioavailability associated with phosphate esters. To address these limitations, an entire field of study has arisen devoted to the design and utilization of pTyr mimetics. This Account provides a perspective on the roles of pTyr residues in signal transduction and approaches to pTyr mimetic development.

Modern proteomic strategies in the study of complex neuropsychiatric disorders

An insight into protein mechanisms involved in disease is critical to the discovery and design of new therapeutic tools. Direct protein analysis provides a method for studying the proteome of a tissue irrespective of an in-depth knowledge of its transcriptome. The development of a human central nervous system (CNS) proteome database ultimately will serve to accelerate the development of specific diagnostic and prognostic markers, neuropsychiatric disease markers, and the corresponding therapeutic tools. It may also reduce the uncertainties in in silico gene predictions by direct open reading frame verification and the ambiguities that experimental models of disease may provide. Advances in gel independent proteomic analyses by solid phase isotope tagging provide greater scope for the characterization of previously elusive membrane proteins; approximately half of all drug targets are key CNS membrane proteins. These advances hold great promise for improvements in the understanding, diagnosis, and therapy of central nervous system disorders.

Tyrosine phosphatase-epsilon activates Src and supports the transformed phenotype of Neu-induced mammary tumor cells

Few tyrosine phosphatases support, rather than inhibit, survival of tumor cells. We present genetic evidence that receptor-type protein-tyrosine phosphatase (RPTP)-epsilon performs such a function, as cells from mammary epithelial tumors induced by activated Neu in mice genetically lacking RPTPepsilon appeared morphologically less transformed and exhibited reduced proliferation. We show that at the molecular level, RPTPepsilon activates Src, a known collaborator of Neu in mammary tumorigenesis. Lack of RPTPepsilon reduced Src activity and altered Src phosphorylation in tumor cells; RPTPepsilon dephosphorylated and activated Src; and Src bound a substrate-trapping mutant of RPTPepsilon. The altered morphology of tumor cells lacking RPTPepsilon was corrected by exogenous Src and exogenous RPTPepsilon or RPTPalpha; exogenous activated Src corrected also the growth rate phenotype. Together, these results suggest that the altered morphology of RPTPepsilon-deficient tumor cells is caused by reduced Src activity, caused, in turn, by lack of RPTPepsilon. Unexpectedly, the phenotype of RPTPepsilon-deficient tumor cells occurs despite expression of the related RPTPalpha, indicating that endogenous RPTPalpha does not compensate for the absence of RPTPepsilon in this case. We conclude that RPTPepsilon is a physiological activator of Src in Neu-induced mammary tumors and suggest that pharmacological inhibition of phosphatases that activate Src may be useful to augment direct pharmacological inhibition of Src.

Phosphoinositide 3-kinase-mediated reduction of insulin receptor substrate-1/2 protein expression via different mechanisms contributes to the insulin-induced desensitization of its signaling pathways in L6 muscle cells

Impaired glucose tolerance precedes type 2 diabetes and is characterized by hyperinsulinemia, which develops to balance peripheral insulin resistance. To gain insight into the deleterious effects of hyperinsulinemia on skeletal muscle, we studied the consequences of prolonged insulin treatment of L6 myoblasts on insulin-dependent signaling pathways. A 24-h long insulin treatment desensitized the phosphoinositide 3-kinase (PI3K)/protein kinase B (PKB) and p42/p44 MAPK pathways toward a second stimulation with insulin or insulin-like growth factor-1 and led to decreased insulin-induced glucose uptake. Desensitization was correlated to a reduction in insulin receptor substrate (IRS)-1 and IRS-2 protein levels, which was reversed by the PI3K inhibitor LY294002. Co-treatment of cells with insulin and LY294002, while reducing total IRS-1 phosphorylation, increased its phosphotyrosine content, enhancing IRS-1/PI3K association. PDK1, mTOR, and MAPK inhibitors did not block insulin-induced reduction of IRS-1, suggesting that the PI3K serine-kinase activity causes IRS-1 serine phosphorylation and its commitment to proteasomal degradation. Contrarily, insulin-induced IRS-2 down-regulation occurred via a PI3K/mTOR pathway. Suppression of IRS-1/2 down-regulation by LY294002 rescued the responsiveness of PKB and MAPK toward acute insulin stimulation. Conversely, adenoviral-driven expression of constitutively active PI3K induced an insulin-independent reduction in IRS-1/2 protein levels. IRS-2 appears to be the chief molecule responsible for MAPK and PKB activation by insulin, as knockdown of IRS-2 (but not IRS-1) by RNA interference severely impaired activation of both kinases. In summary, (i) PI3K mediates insulin-induced reduction of IRS-1 by phosphorylating it while a PI3K/mTOR pathway controls insulin-induced reduction of IRS-2, (ii) in L6 cells, IRS-2 is the major adapter molecule linking the insulin receptor to activation of PKB and MAPK, (iii) the mechanism of IRS-1/2 down-regulation is different in L6 cells compared with 3T3-L1 adipocytes. In conclusion, the reduction in IRS proteins via different PI3K-mediated mechanisms contributes to the development of an insulin-resistant state in L6 myoblasts.

Fusion of FIG to the receptor tyrosine kinase ROS in a glioblastoma with an interstitial del(6)(q21q21)

The transmembrane proto-oncogene receptor tyrosine kinase (RTK) ROS is an orphan receptor that is aberrantly expressed in neoplasms of the central nervous system. Here, we report the fusion of its carboxy-terminal kinase domain to the amino-terminal portion of a protein called FIG (Fused in Glioblastoma) in a human glioblastoma multiforme (GBM). By characterizing both FIG and ROS genes in normal and in U118MG GBM cells, we determined that an intra-chromosomal homozygous deletion of 240 kilobases on 6q21 is responsible for the formation of the FIG-ROS locus. The FIG-ROS transcript is encoded by 7 FIG exons and 9 ROS-derived exons. We also demonstrate that the FIG-ROS locus encodes for an in-frame fusion protein with a constitutively active kinase activity, suggesting that FIG-ROS may act as an oncogene. This is the first example of a fusion RTK protein that results from an intra-chromosomal deletion, and it represents the first fusion RTK protein isolated from a human astrocytoma.

Chemometrical classification of ephrin ligands and Eph kinases using GRID/CPCA approach

Eph receptor tyrosine kinases are divided on two subfamilies based on their affinity for ephrin ligands and play a crucial role in the intercellular processes such as angiogenesis, neurogenesis, and carcinogenesis. As such, Eph kinases represent potential targets for drug design, which requires the knowledge of structural features responsible for their specific interactions. To overcome the existing gap between available sequence and structure information we have built 3D models of eight ephrins and 13 Eph kinase ligand-binding domains using homology modeling techniques. The interaction energies for several molecular probes with binding sites of these models were calculated using GRID and subjected to chemometrical classification based on consensus principal component analysis (CPCA). Despite inherent limitations of the homology models, CPCA was able to successfully distinguish between ephrins and Eph kinases, between Eph kinase subfamilies, and between ephrin subfamilies. As a result we have identified several amino acids that may account for selectivity in ephrin-Eph kinase interactions. In general, although the difference in charge between ephrin and Eph kinase binding domains creates an attractive long-range electrostatic force, the hydrophobic and steric interactions are highly important for the short-range interactions between two proteins. The chemometrical analysis also provides the pharmacophore model, which could be used for virtual screening and de novo ligand design.

Detection of FLT3 internal tandem duplication and D835 mutations by a multiplex polymerase chain reaction and capillary electrophoresis assay

FLT3 is a receptor tyrosine kinase that is expressed on early hematopoietic progenitor cells and plays an important role in stem cell survival and differentiation. Two different types of functionally important FLT3 mutations have been identified. Internal tandem duplication mutations arise from duplications of the juxtamembrane portion of the gene and result in constitutive activation of the FLT3 protein. This alteration has been identified in approximately 20% to 30% of patients with acute myelogenous leukemia and appears to be associated with a worse prognosis. The second type of FLT3 mutation, missense mutations at aspartic acid residue 835, occurs in approximately 7.0% of acute myelogenous leukemia cases. These mutations also appear to be activating and to portend a worse prognosis. Identification of FLT3 mutations is important because it provides prognostic information and may play a pivotal role in determining appropriate treatment options. We have developed an assay to identify both internal tandem duplication and D835 FLT3 mutations in a single multiplex polymerase chain reaction. After amplification, the polymerase chain reaction products are analyzed by capillary electrophoresis for length mutations and resistance to EcoRV digestion. Here we describe the performance characteristics of the assay, assay validation, and our clinical experience using this assay to analyze 147 clinical specimens.

Tyrosine kinase inhibitor STI-571/Gleevec down-regulates the beta-catenin signaling activity

Beta-Catenin is a critical transducer of the Wnt signal pathway and plays an important role in many developmental and cellular processes. Deregulation of beta-catenin signaling has been observed in a broad range of human tumors. In this report, we investigated whether tyrosine kinase inhibitor STI-571 could inhibit the beta-catenin signaling activity and hence suppress cell proliferation. Our results demonstrated that STI-571 effectively inhibited the constitutive activity of beta-catenin signaling in human colon cancer cells as well as the Wnt1-induced activation of beta-catenin signaling in HOS, HTB-94, and HEK 293 cells. Furthermore, STI-571 was shown to effectively suppress the proliferation of human colon cancer cells. Finally, we demonstrated that the Wnt1-mediated activation of a GAL4-beta-catenin heterologous transcription system was effectively inhibited by STI-571. Thus, our findings suggest that tyrosine phosphorylation may play an important role in regulating beta-catenin signaling activity, and inhibition of this signaling pathway by STI-571 may be further explored as an important target for alternative/adjuvant treatments for a broader range of human cancer.

Phosphorylation of Tyr-176 of the yeast MAPK Hog1/p38 is not vital for Hog1 biological activity

Mitogen-activated protein kinases are crucial components in the life of eukaryotic cells. The current dogma for MAPK activation is that dual phosphorylation of neighboring Thr and Tyr residues at the phosphorylation lip is an absolute requirement for their catalytic and biological activity. In this study we addressed the role of Tyr and Thr phosphorylation in the yeast MAPK Hog1/p38. Taking advantage of the recently isolated hyperactive mutants, whose intrinsic basal activity is independent of upstream regulation, we demonstrate that Tyr-176 is not required for basal catalytic and biological activity but is essential for the salt-induced amplification of Hog1 catalysis. We show that intact Thr-174 is absolutely essential for biology and catalysis of the mutants but is mainly required for structural reasons and not as a phosphoacceptor. The roles of Thr-174 and Tyr-176 in wild type Hog1 molecules were also tested. Unexpectedly we found that Hog1(Y176F) is biologically active, capable of induction of Hog1 target genes and of rescuing hog1Delta cells from osmotic stress. Hog1(Y176F) was not able, however, to mediate growth arrest induced by constitutively active MAPK kinase/Pbs2. We propose that Thr-174 is essential for stabilizing the basal active conformation, whereas Tyr-176 is not. Tyr-176 serves as a regulatory element required for stimuli-induced amplification of kinase activity.

Ataxia telangiectasia mutated proteins, MAPKs, and RSK2 are involved in the phosphorylation of STAT3

Phosphorylation at Ser(727) is known to be required for complete activation of STAT3 by diverse stimuli including UV irradiation, but the kinase(s) responsible for phosphorylating STAT3 (Ser(727)) is still not well discerned. In the present study, we observed that activation of ATM is required for a UVA-stimulated increase in Ser(727) phosphorylation of STAT3 as well as in activation and phosphorylation of p90 ribosomal protein S6 kinases (RSKs). Moreover, UVA-stimulated activation of upstream kinases, such as c-Jun N-terminal kinases (JNKs) and ERKs, involved in mediating phosphorylation of RSKs and STAT3 was defective or delayed in ATM-deficient cells. Furthermore, we provide evidence that RSK2-deficient cells were defective for UV-induced Ser(727) phosphorylation of STAT3, and the defect was restored after ectopic expression of transfected full-length RSK2. In vitro experiments showed that active RSK2 and JNK1 induce the phosphorylation of STAT3 precipitates from immunoprecipitation but not from glutathione S-transferase (GST) pull-down. Interestingly, the GST fusion STAT3 proteins mixed together with STAT3 immunoprecipitates can be phosphorylated by JNK. However, the in vitro phosphorylation of STAT3 was reduced by the GST-STAT3 beta protein, a dominant negative form of STAT3. Taken together, our results demonstrate that the STAT3 phosphorylation at Ser(727) is triggered by active RSK2 or JNK1 in the presence of a downstream kinase or a cofactor, and thereby the intracellular phosphorylation process is stimulated through a signaling pathway involving ATM, MAPKs, RSK2, and an as yet unidentified kinase or cofactor. Additionally, RSK2-mediated phosphorylation of STAT3 (Ser(727)) was further determined to be required for basal and UVA-stimulated STAT3 transcriptional activities.

Caspase-dependent cleavage of c-Abl contributes to apoptosis

The nonreceptor tyrosine kinase c-Abl may contribute to the regulation of apoptosis. c-Abl activity is induced in the nucleus upon DNA damage, and its activation is required for execution of the apoptotic program. Recently, activation of nuclear c-Abl during death receptor-induced apoptosis has been reported; however, the mechanism remains largely obscure. Here we show that c-Abl is cleaved by caspases during tumor necrosis factor- and Fas receptor-induced apoptosis. Cleavage at the very C-terminal region of c-Abl occurs mainly in the cytoplasmic compartment and generates a 120-kDa fragment that lacks the nuclear export signal and the actin-binding region but retains the intact kinase domain, the three nuclear localization signals, and the DNA-binding domain. Upon caspase cleavage, the 120-kDa fragment accumulates in the nucleus. Transient-transfection experiments show that cleavage of c-Abl may affect the efficiency of Fas-induced cell death. These data reveal a novel mechanism by which caspases can recruit c-Abl to the nuclear compartment and to the mammalian apoptotic program.

Clinical, cellular, and molecular aspects of cancer invasion

Invasion causes cancer malignancy. We review recent data about cellular and molecular mechanisms of invasion, focusing on cross-talk between the invaders and the host. Cancer disturbs these cellular activities that maintain multicellular organisms, namely, growth, differentiation, apoptosis, and tissue integrity. Multiple alterations in the genome of cancer cells underlie tumor development. These genetic alterations occur in varying orders; many of them concomitantly influence invasion as well as the other cancer-related cellular activities. Examples discussed are genes encoding elements of the cadherin/catenin complex, the nonreceptor tyrosine kinase Src, the receptor tyrosine kinases c-Met and FGFR, the small GTPase Ras, and the dual phosphatase PTEN. In microorganisms, invasion genes belong to the class of virulence genes. There are numerous clinical and experimental observations showing that invasion results from the cross-talk between cancer cells and host cells, comprising myofibroblasts, endothelial cells, and leukocytes, all of which are themselves invasive. In bone metastases, host osteoclasts serve as targets for therapy. The molecular analysis of invasion-associated cellular activities, namely, homotypic and heterotypic cell-cell adhesion, cell-matrix interactions and ectopic survival, migration, and proteolysis, reveal branching signal transduction pathways with extensive networks between individual pathways. Cellular responses to invasion-stimulatory molecules such as scatter factor, chemokines, leptin, trefoil factors, and bile acids or inhibitory factors such as platelet activating factor and thrombin depend on activation of trimeric G proteins, phosphoinositide 3-kinase, and the Rac and Rho family of small GTPases. The role of proteolysis in invasion is not limited to breakdown of extracellular matrix but also causes cleavage of proinvasive fragments from cell surface glycoproteins.

Protein microarrays: meeting analytical challenges for clinical applications

Protein microarrays, one emerging class of proteomic technologies, have broad applications for discovery and quantitative analysis. A rapidly expanding use of this technology is the acquisition of information about the posttranslational modifications of proteins reflecting the activity state of signal pathways and networks, and is now employed for the analysis of biopsy samples in clinical trial research.

Genomic dissection for characterization of cancerous oral epithelium tissues using transcription profiling

Genome-wide and high-throughput functional genomic tools offer the potential of identifying disease-associated genes and dissecting disease regulatory patterns. There is a need for a set of systematic bioinformatic tools that handles efficiently a large number of variables for extracting biological meaning from experimental outputs. We present well-characterized statistical tools to discover genes that are differentially expressed between malignant oral epithelial and normal tissues in microarray experiments and to construct a robust classifier using the identified discriminatory genes. Those tools include Wilks' lambda score, error rate estimated from leave-one out cross-validation (LOOCV) and Fisher Discriminant Analysis (FDA). High Density DNA microarrays and Real Time Quantitative PCR were employed for the generation and validation of the transcription profile of the oral cancer and normal samples. We identified 45 genes that are strongly correlated with malignancy. Of the 45 genes identified, six have been previously implicated in the disease, and two are uncharacterized clones.

Issues and progress with protein kinase inhibitors for cancer treatment

Identification of the key roles of protein kinases in cancer has led to extensive efforts to develop kinase inhibitors for the treatment of a wide range of cancers, and more than 30 such agents are now in clinical trials. Here, we consider the crucial issues in the development of kinase inhibitors for cancer, and discuss strategies to address the challenges raised by these issues in the light of preclinical and clinical experiences so far.

Nuclear receptors: integration of multiple signalling pathways through phosphorylation

Nuclear receptors (NRs) orchestrate the transcription of specific gene networks in response to binding of their cognate ligand. They also act as mediators in a variety of signalling pathways through integrating diverse phosphorylation events. NR phosphorylation concerns all three major domains, the N-terminal activation function (AF-1), the ligand-binding and the DNA binding domains. Often, phosphorylation of NRs by kinases that are associated with general transcription factors (e.g. cdk7 within TFIIH), or activated in response to a variety of signals (MAPKs, Akt, PKA, PKC), facilitates the recruitment of coactivators or of components of the transcription machinery and, therefore, cooperates with the ligand to enhance transcription activation. But phosphorylation can also contribute to the termination of the ligand response through inducing DNA dissociation or NR degradation or through decreasing ligand affinity. These different modes of regulation reveal an unexpected complexity of the dynamics of NR-mediated transcription. In addition, deregulation of NR phosphorylation may impact their action in certain diseases or cancers.

Overexpression and functional alterations of the EphA2 tyrosine kinase in cancer

Cancer is a disease of aberrant signal transduction. The expression and function of intracellular signaling pathways are frequently subverted as cells progress towards a metastatic phenotype. In particular, tyrosine kinases initiate powerful signals that govern many different aspects of cell behavior. In Recent studies have demonstrated that the EphA2 receptor tyrosine kinase is frequently overexpressed and functionally altered in aggressive tumor cells, and that these changes promote metastatic character. Herein, we provide an overview of our current understanding of EphA2, with emphasis upon the differential regulation of EphA2 expression and function. We also show that differential EphA2 expression and function may provide a unique opportunity for selective therapeutic targeting of EphA2 in metastatic disease.

Regulation of a transient receptor potential (TRP) channel by tyrosine phosphorylation. SRC family kinase-dependent tyrosine phosphorylation of TRPV4 on TYR-253 mediates its response to hypotonic stress

The recently identified transient receptor potential (TRP) channel family member, TRPV4 (formerly known as OTRPC4, VR-OAC, TRP12, and VRL-2) is activated by hypotonicity. It is highly expressed in the kidney as well as blood-brain barrier-deficient hypothalamic nuclei responsible for systemic osmosensing. Apart from its gating by hypotonicity, little is known about TRPV4 regulation. We observed that hypotonic stress resulted in rapid tyrosine phosphorylation of TRPV4 in a heterologous expression model and in native murine distal convoluted tubule cells in culture. This tyrosine phosphorylation was sensitive to the inhibitor of Src family tyrosine kinases, PP1, in a dose-dependent fashion. TRPV4 associated with Src family kinases by co-immunoprecipitation studies and confocal immunofluorescence microscopy, and this interaction required an intact Src family kinase SH2 domain. One of these kinases, Lyn, was activated by hypotonic stress and phosphorylated TRPV4 in an immune complex kinase assay and an in vitro kinase assay using recombinant Lyn and TRPV4. Transfection of wild-type Lyn dramatically potentiated hypotonicity-dependent TRPV4 tyrosine phosphorylation whereas dominant negative-acting Lyn modestly inhibited it. Through mutagenesis studies, the site of tonicity-dependent tyrosine phosphorylation was mapped to Tyr-253, which is conserved across all species from which TRPV4 has been cloned. Importantly, point mutation of Tyr-253 abolished hypotonicity-dependent channel activity. In aggregate, these data indicate that hypotonic stress results in Src family tyrosine kinase-dependent tyrosine phosphorylation of the tonicity sensor TRPV4 at residue Tyr-253 and that this residue is essential for channel function in this context. This is the first example of direct regulation of TRP channel function through tyrosine phosphorylation.

Tumoricidal activity of lauryl gallate towards chemically induced skin tumours in mice

Lauryl gallate (antioxidant food additive E-312) prevents the formation of dimethylbenzanthracene-induced skin tumours in mice, and kills, selectively, tumoral cells on established tumours. This results in total remission, after topical application of the compound on the tumoral mass, without affecting the surrounding tissue.

A myristoyl/phosphotyrosine switch regulates c-Abl

The c-Abl tyrosine kinase is inhibited by mechanisms that are poorly understood. Disruption of these mechanisms in the Bcr-Abl oncoprotein leads to several forms of human leukemia. We found that like Src kinases, c-Abl 1b is activated by phosphotyrosine ligands. Ligand-activated c-Abl is particularly sensitive to the anti-cancer drug STI-571/Gleevec/imatinib (STI-571). The SH2 domain-phosphorylated tail interaction in Src kinases is functionally replaced in c-Abl by an intramolecular engagement of the N-terminal myristoyl modification with the kinase domain. Functional studies coupled with structural analysis define a myristoyl/phosphotyrosine switch in c-Abl that regulates docking and accessibility of the SH2 domain. This mechanism offers an explanation for the observed cellular activation of c-Abl by tyrosine-phosphorylated proteins, the intracellular mobility of c-Abl, and it provides new insights into the mechanism of action of STI-571.