Oncogenic kinase signalling

E-cadherin-mediated adhesion inhibits ligand-dependent activation of diverse receptor tyrosine kinases

E-cadherin is an essential adhesion protein as well as a tumor suppressor that is silenced in many cancers. Its adhesion-dependent regulation of signaling has not been elucidated. We report that E-cadherin can negatively regulate, in an adhesion-dependent manner, the ligand-dependent activation of divergent classes of receptor tyrosine kinases (RTKs), by inhibiting their ligand-dependent activation in association with decreases in receptor mobility and in ligand-binding affinity. E-cadherin did not regulate a constitutively active mutant RTK (Neu*) or the ligand-dependent activation of LPA receptors or muscarinic receptors, which are two classes of G protein-coupled receptors. EGFR regulation by E-cadherin was associated with complex formation between EGFR and E-cadherin that depended on the extracellular domain of E-cadherin but was independent of beta-catenin binding or p120-catenin binding. Transfection of E-cadherin conferred negative RTK regulation to human melanoma and breast cancer lines with downregulated endogenous E-cadherin. Abrogation of E-cadherin regulation may contribute to the frequent ligand-dependent activation of RTK in tumors.

Development and Validation of a Fluorescence Technology for both Primary and Secondary Screening of Kinases That Facilitates Compound Selectivity and Site-Specific Inhibitor Determination

The IQ Technology has been developed to serve as a homogeneous, universal detection platform for HTS of kinases and phosphatases. The technology is a direct, noncompetitive assay format that does not require antibodies or radioactive reagents to measure phosphorylation state. Fluorophore-labeled peptides are used as enzyme substrates, and kinase or phosphatase activity is quantitated by direct measurement of the phosphorylation state of the substrate. Phosphorylation is measured by the change in fluorescence intensity that occurs when a proprietary iron-containing compound binds specifically to phosphoryl groups on peptides. This change in observed fluorescence is proportional to the extent of phosphorylation of the fluorophore-labeled peptide. The technology provides a universal method that can be used with any peptide sequence and is insensitive to high concentrations of ATP. Inhibition at the ATP-binding site versus the phosphorylation site can be differentiated and compound selectivity identified using the same detection method as in the primary screen. The technology has been tested against a large number of detergents, organics, and other reagents found in reaction mixtures, and the detection method eliminates common issues associated with fluorescent and chromogenic compounds. The technology has been formatted for 96-, 384-, and 1,536-well microplate formats, and a representative Z' value of 0.7 was obtained. IC(50) values generated using this platform correlate with previously reported values, and screening of a small compound library was performed to evaluate the assay further.

Phosphatidylinositol triphosphate kinase-dependent and c-jun NH2-terminal kinase-dependent induction of telomerase by calcium requires Pyk2

Calcium signaling has been linked to activation of Pyk2, a calcium-dependent, focal adhesion kinase-related, non-receptor tyrosine kinase. Signaling via Pyk2 can activate c-jun NH(2)-terminal kinase (JNK). Calcium has also been shown to activate phosphatidylinositol triphosphate kinase and/or JNK. Here, we show that calcium signaling in ovarian surface epithelial cells not only induces telomerase activity via JNK but also activates Pyk2. Moreover, telomerase activation by Pyk2 requires JNK activation. In contrast, a kinase-deficient Pyk2 construct failed to activate either JNK or telomerase. Finally, we demonstrate that Pyk2 is capable of driving the human telomerase reverse transcriptase promoter, resulting in telomerase activation. These data suggest a novel role of Pyk2 for telomerase regulation.

Pinning down cell signaling, cancer and Alzheimer's disease

Protein phosphorylation on certain serine or threonine residues preceding proline (Ser/Thr-Pro) is a pivitol signaling mechanism in diverse cellular processes and its deregulation can lead to human disease. However, little is known about how these phosphorylation events actually control cell signaling. Pin1 is a highly conserved enzyme that isomerizes only the phosphorylated Ser/Thr-Pro bonds in certain proteins, thereby inducing conformational changes. Recent results indicate that such conformational changes following phosphorylation are a novel signaling mechanism pivotal in regulating many cellular functions. This mechanism also offers new insights into the pathogenesis and treatment of human disease, most notably cancer and Alzheimer's disease. Thus, Pin1 plays a key role in linking signal transduction to the pathogenesis of cancer and Alzheimer's disease - two major age-related diseases.

Expression of the receptor tyrosine kinase Axl promotes ocular melanoma cell survival

Metastatic tumor cells originating from cancers of a variety of tissues such as breast, skin, and prostate may remain dormant for long periods of time. In the case of uveal melanoma, the principal malignancy of the eye, complete removal of the primary tumor by enucleation can nonetheless be followed by metastatic tumor growth in distant organs months, years, or even decades later. This suggests that tumor cells have already spread to secondary sites at the time of treatment and remain dormant as micrometastases. Identifying factors that govern long-lived survival of metastatic tumor cells is therefore key to decreasing mortality associated with this and other diseases. While investigating factors differentially expressed in melanoma cells and normal melanocytes, we identified the receptor tyrosine kinase Axl and found up-regulation of Axl in uveal melanomas and melanoma cell lines by RNase protection, Western analysis, and immunohistochemistry. Axl has been shown to mediate cell growth and survival through its ligand Gas6 in non-transformed cells. To test whether stimulation of Axl can enhance survival of uveal melanoma cells, we assessed the degree of mitogenesis and cell survival by bromodeoxyuridine incorporation and trypan blue exclusion, respectively, upon stimulation of Mel 290 uveal melanoma cells with Gas6 in vitro. We show that Gas6 mediates mitogenesis and cell survival in Mel 290 cells. We further demonstrate that these effects occur specifically through the Axl receptor by modulating the expression of Axl with an antisense construct. cDNA microarray analysis of 12,687 genes then revealed that Gas6 stimulation of Axl in Mel 290 cells results primarily in the down-regulation of Cyr61, a member of the CCN protein family involved in tumor progression. These data show that the Axl pathway mediates increased survival of uveal melanoma cells, potentially advantageous during cancer dormancy, and that Axl may function in part through regulation of Cyr61.

Forkhead box M1B transcriptional activity requires binding of Cdk-cyclin complexes for phosphorylation-dependent recruitment of p300/CBP coactivators

Previous liver regeneration studies demonstrated that the mouse forkhead box M1B (FoxM1B) transcription factor regulates hepatocyte proliferation through expression of cell cycle genes that stimulate cyclin-dependent kinase 2 (Cdk2) and Cdk1 activity. In this study, we demonstrated that disruption of the FoxM1B Cdk1/2 phosphorylation site at Thr residue 596 significantly reduced both FoxM1B transcriptional activity and Cdk phosphorylation of the FoxM1B T596A mutant protein in vivo. Retention of this FoxM1B 596 Cdk phosphorylation site was found to be essential for recruiting the histone acetyltransferase CREB binding protein (CBP) to the FoxM1B transcriptional activation domain. Consistent with these findings, dominant negative Cdk1 protein significantly reduced FoxM1B transcriptional activity and inhibited FoxM1B recruitment of the CBP coactivator protein. Likewise, Cdc25B-mediated stimulation of Cdk activity together with elevated levels of the CBP coactivator protein provided a 6.2-fold synergistic increase in FoxM1B transcriptional activity. Furthermore, mutation of the FoxM1B Leu 641 residue within an LXL motif (residues 639 to 641) inhibited recruitment of Cdk-cyclin complexes and caused significant reduction in both FoxM1B transcriptional activity and in vivo Cdk phosphorylation of the FoxM1B Thr 596 residue. We demonstrated that FoxM1B transcriptional activity requires binding of either S-phase or M-phase Cdk-cyclin complexes to mediate efficient Cdk phosphorylation of the FoxM1B Thr 596 residue, which is essential for recruitment of p300/CBP coactivator proteins.

Development by Self-Digestion

Autophagy is the major cellular pathway for the degradation of long-lived proteins and cytoplasmic organelles. It involves the rearrangement of subcellular membranes to sequester cargo for delivery to the lysosome where the sequestered material is degraded and recycled. For many decades, it has been known that autophagy occurs in a wide range of eukaryotic organisms and in multiple different cell types during starvation, cellular and tissue remodeling, and cell death. However, until recently, the functions of autophagy in normal development were largely unknown. The identification of a set of evolutionarily conserved genes that are essential for autophagy has opened up new frontiers for deciphering the role of autophagy in diverse biological processes. In this review, we summarize our current knowledge about the molecular machinery of autophagy and the role of the autophagic machinery in eukaryotic development.

Signal transduction and oncogenesis by ErbB/HER receptors

Growth factors enable cells to escape irradiation-induced death (apoptosis). One important family of growth factors share an epidermal growth factor motif, and all bind to ErbB transmembrane receptors. In response to growth factor ligands, ErbB receptor tyrosine kinases induce a variety of cellular responses, including proliferation, differentiation and motility. Signal transduction pathways are initiated upon ligand-induced receptor homo- or heterodimerization and activation of tyrosine kinase activity. The complement of induced signaling pathways, as well as their magnitude and duration, determines the biological outcome of signaling, and in turn, is regulated by the identity of the ligand and the receptor composition. Recent insights into the structural basis for receptor dimerization, as provided by crystallographic analysis, are described, as is the differential activation of signaling pathways and downregulatory mechanisms. Further, dysregulation of the ErbB network is implicated in a variety of human cancers, and the nature of aberrant signaling through ErbB proteins, as well as current therapeutic approaches, are discussed, highlighting the role of the highly oncogenic ErbB-2 molecule.

Cancer metastasis therapeutic targets and drug discovery: emerging small-molecule protein kinase inhibitors

Cancer metastasis is a significant problem and a tremendous challenge to drug discovery relative to identifying key therapeutic targets as well as developing breakthrough medicines. Recent progress in unravelling the complex molecular circuitry of cancer metastasis, including receptors, intracellular proteins and genes, is highlighted. Furthermore, recent advances in drug discovery to provide novel proof-of-concept ligands, in vivo effective lead compounds and promising clinical candidates, are summarised. Such drug discovery efforts illustrate the integration of functional genomics, cell biology, structural biology, drug design, molecular/cellular screening and chemical diversity (e.g., small molecules, peptides/peptidomimetics, natural products, antisense, vaccines and antibodies). Promising therapeutic targets for cancer metastasis have been identified, including Src, focal adhesion kinase, the integrin receptor, the vascular endothelial growth factor receptor, the epidermal growth factor receptor, Her-2/neu, c-Met, Ras/Rac GTPases, Raf kinase, farnesyl diphosphate synthase (i.e., amino-bisphosphonate therapeutic target) and matrix metalloproteases within the context of their implicated functional roles in cancer growth, invasion, angiogenesis and survival at secondary sites. Clinical and preclinical drug discovery is described and emerging small-molecule inhibitors of protein kinases are highlighted.

Role of protein ubiquitylation in regulating endocytosis of receptor tyrosine kinases

Growth factors and their transmembrane receptor tyrosine kinases play pivotal roles in morphogenesis, cell fate determination and pathogenesis, including multiple stages of cancer. The amplitude and kinetics of signaling by growth factor receptors are determined by an endocytic process, which sorts activated, autophosphorylated receptors to degradation in lysosomes. Recent studies uncovered the role of protein ubiquitylation in vesicular trafficking of growth factor receptors. Decoration of ligand-activated receptors by multiple monomeric ubiquitins distinguishes this degradative route from the proteasome-mediated pathway, which involves polymeric chains of ubiquitin. Although receptor ubiquitylation occurs at the cell surface, its major role is to sort internalized receptors to the lumen of the multivesicular body, en route to the lysosome. The ubiquitin ligases that control this late sorting event belong to the Cbl family of RING finger adaptors, which bind specific phosphotyrosine residues in the receptors upon activation by ligand. Another group of E3 ubiquitin ligases, the Nedd4 family, regulates the initial sorting event, which targets receptors to clathrin-coated regions of the plasma membrane. This step entails ubiquitin-dependent assembly of a clathrin-binding complex of adaptors such as epsins, which share ubiquitin-interacting motifs. The concerted action of both ubiquitin-binding adaptors of membrane coats and E3 ligases, as well as their regulation by protein phosphorylation and ubiquitylation, ensure robust endocytosis of growth factor receptors. Genetic defects and virus-mediated manipulations of the endocytic pathway divert receptors to a default recycling pathway, thereby enabling unrestrained signaling characteristic to transformed cells.

Identification of Compounds with Nanomolar Binding Affinity for Checkpoint Kinase-1 Using Knowledge-Based Virtual Screening

A virtual screen of a subsection of the AstraZeneca compound collection was performed for checkpoint kinase-1 (Chk-1 kinase) using a knowledge-based strategy. This involved initial filtering of the compound collection by application of generic physical properties followed by removal of compounds with undesirable chemical functionality. Subsequently, a 3-D pharmacophore screen for compounds with kinase binding motifs was applied. A database of approximately 200K compounds remained for docking into the active site of Chk-1 kinase, using the FlexX-Pharm program. For each compound that docked successfully into the binding site, up to 100 poses were saved. These poses were then postfiltered using a customized consensus scoring scheme for a kinase, followed by visual inspection of a selection of the docked compounds. This resulted in 103 compounds being ordered for testing in the project assay, and 36 of these (corresponding to four chemical classes) were found to inhibit the enzyme in a dose-response fashion with IC(50) values ranging from 110 nM to 68 microM.

Activation of tyrosine kinases in cancer

Receptor and nonreceptor tyrosine kinases (TKs) have emerged as clinically useful drug target molecules for treating certain types of cancer. Epidermal growth factor receptor (EGFR)-TK is a transmembrane receptor TK that is overexpressed or aberrantly activated in the most common solid tumors, including non-small cell lung cancer and cancers of the breast, prostate, and colon. Activation of the EGFR-TK enzyme results in autophosphorylation, which drives signal transduction pathways leading to tumor growth and malignant progression. Randomized clinical trials of the EGFR-TK inhibitor gefitinib have demonstrated clinical benefits in patients with advanced non-small cell lung cancer whose disease had previously progressed on platinum- and docetaxel-based chemotherapy regimens. Bcr-Abl is a constitutively activated nonreceptor TK enzyme found in the cytoplasm of Philadelphia chromosome-positive leukemia cells. STI571 (imatinib mesylate) inhibits the Bcr-Abl TK, blocks the growth of these leukemia cells, and induces apoptosis. STI571 also inhibits other TKs, including the receptor TK c-kit, which is expressed in gastrointestinal stromal tumors. As TK inhibitors become available for clinical use, new challenges include predicting which patients are most likely to respond to these targeted TK inhibitors. Additional clinical trials are needed to develop the full potential of receptor and nonreceptor TK inhibitors for cancer treatment.

Structure-Based Optimization of Novel Azepane Derivatives as PKB Inhibitors

Novel azepane derivatives were prepared and evaluated for protein kinase B (PKB-alpha) and protein kinase A (PKA) inhibition. The original (-)-balanol-derived lead structure (4R)-4-(2-fluoro-6-hydroxy-3-methoxy-benzoyl)-benzoic acid (3R)-3-[(pyridine-4-carbonyl)amino]-azepan-4-yl ester (1) (IC(50) (PKB-alpha) = 5 nM) which contains an ester moiety was found to be plasma unstable and therefore unsuitable as a drug. Based upon molecular modeling studies using the crystal structure of the complex between PKA and 1, the five compounds N-[(3R,4R)-4-[4-(2-fluoro-6-hydroxy-3-methoxy-benzoyl)-benzoylamino]-azepan-3-yl]-isonicotinamide (4), (3R,4R)-N-[4-[4-(2-fluoro-6-hydroxy-3-methoxy-benzoyl)-benzyloxy]-azepan-3-yl]-isonicotinamide (5), N-[(3R,4S)-4-[4-(2-fluoro-6-hydroxy-3-methoxy-benzoyl)-phenylamino]-methyl]-azepan-3-yl)-isonicotinamide (6), N-[(3R,4R)-4-[4-(2-fluoro-6-hydroxy-3-methoxy-benzoyl)-benzylamino]-azepan-3-yl]-isonicotinamide (7), and N-[(3R,4S)-4-(4-[trans-2-[4-(2-fluoro-6-hydroxy-3-methoxy-benzoyl)-phenyl]-vinyl]-azepan-3-yl)-isonicotinamide (8) with linkers isosteric to the ester were designed, synthesized, and tested for in vitro inhibitory activity against PKA and PKB-alpha and for plasma stability in mouse plasma.(1) Compound 4 was found to be plasma stable and highly active (IC(50) (PKB-alpha) = 4 nM). Cocrystals with PKA were obtained for 4, 5, and 8 and analyzed for binding interactions and conformational changes in the ligands and protein in order to rationalize the different activities of the molecules.

Differential regulation of signal transduction pathways in wild type and mutated p53 breast cancer epithelial cells by copper and zinc

Previous studies have suggested that cells may differ in their response to metal stress. This study was undertaken to investigate the role of PI3K/Akt signaling pathway in metal resistance in human breast cancer epithelial cells with different p53 and estrogen receptor status. Exposure to copper and zinc increased Akt phosphorylation with its nuclear localization only in MDA-MB-231 cells with no estrogen receptor and mutated p53. Cyclin D1 expression and cell-cycle progression followed the metal-induced Akt phosphorylation. Treatment with LY294002 abrogated these effects, suggesting the essential role of PI3-kinase. In contrast, in MCF-7 cells with wild type p53 and estrogen receptor, there was no change in Akt activation, while suppression of p53 activity by pifithrin-alpha increased phosphorylation of Akt after the treatment with copper. In MCF-7 cells, the metal treatment increased the phosphorylation of p53 at serine 15, up-regulated p21 expression, and resulted in cell-cycle arrest in G1 phase with apoptosis. These results demonstrate that copper-induced apoptosis in MCF-7 cells is p53 dependent, whereas the metal resistance in MDA-MB-231 cells may be due to activation of Akt in the absence of a functional p53.

Design of hypoxia-targeting protein tyrosine kinase inhibitor using an innovative pharmacophore 2-methylene-4-cyclopentene-1,3-dione

We review in this report our strategy and tactics for the design of 2-hydroxyarylidene-4-cyclopentene-1,3-diones as protein tyrosine kinase (PTK) inhibitors having low mitochondrial toxicities and/or hypoxia-targeting function. We based our synthetic design on an innovative pharmacophore, 2-methylene-4-cyclopentene-1,3-dione. We first showed the effectiveness of this pharmacophore in the development of 2-methylene-4-cyclopentene-1,3-dione as PTK inhibitor that have lower mitochondrial toxicity than the potent PTK inhibitor tyrphostin AG17. Our results show that the cyclopentenedione-derived TX-1123 is a more potent antitumor tyrphostin and also shows lower mitochondrial toxicity than the malononitrile-derived AG17. The O-methylation product of TX-1123 (TX-1925) retained its tyrphostin-like properties, including mitochondrial toxicity and antitumor activities. However, the methylation product of AG17 (TX-1927) retained its tyrphostin-like antitumor activities, but lost its mitochondrial toxicity. Our comprehensive evaluation of these agents with respect to PTK inhibition, mitochondrial inhibition, antitumor activity, and hepatotoxicity demonstrates that PTK inhibitors TX-1123 and TX-1925 are more promising candidates for antitumor agents than tyrphostin AG17. Secondly, as a further investigation of the promising power of this 4-cyclopentene-1,3-dione as an innovative pharmacophore, we discuss our strategy of development of hypoxia-targeting PTK inhibitor TX-1123 analogues, 2-nitroimidazole-aminomethylenecyclopentenediones, such as TX-2036, for cancer treatment, especially for pancreatic cancers, which have a high level of hypoxia.

TEL-Syk fusion constitutively activates PI3-K/Akt, MAPK and JAK2-independent STAT5 signal pathways

We previously reported the fusion of the TEL gene to the Syk gene in myelodysplastic syndrome with t(9;12)(q22;p12). TEL-Syk fusion transformed interleukin-3 (IL-3)-dependent murine hematopoietic cell line BaF3 to growth factor independence. Here, we investigate the intracellular signal transduction of the stable transfectants. TEL-Syk fusion protein was associated with the p85 subunit of phosphatidyl inositol 3 kinase (PI3-K) followed by the activation of Akt in the absence of IL-3. Vav, phospholipase C-gamma2 and mitogen-activated protein kinase (MAPK) were also constitutively activated. TEL-Syk also activated the signal transducer and activator of transcription 5 (STAT5) in the absence of Janus kinase 2 activation. None of these kinases were phosphorylated in the BaF3 cells transfected with TELDeltaPNT-Syk in which the oligomerization domain of TEL was deleted. Inhibitor analysis showed that the MAPK pathway was important in TEL-Syk-mediated cell proliferation. The immunofluorescence technique revealed that the TEL-Syk fusion protein was located in the cytoplasm. These data suggest that TEL-Syk fusion protein in the cytoplasm leads to the constitutive activation of PI3-K/Akt, MAPK and STAT5 signal pathways, which are closely involved in IL-3-independent cell proliferation of BaF3 cells.

KIT mutations are common in testicular seminomas

Expression of KIT tyrosine kinase is critical for normal germ cell development and is observed in the majority of seminomas. Activating mutations in KIT are common in gastrointestinal stromal tumors and mastocytosis. In this study we examined the frequency and spectrum of KIT mutations in 54 testicular seminomas, 1 ovarian dysgerminoma and 37 non-seminomatous germ cell tumors (NSGCT). Fourteen seminomas (25.9%) contained exon 17 point mutations including D816V (6 cases), D816H (3 cases), Y823D (2 cases), and single examples of Y823C, N822K, and T801I. No KIT mutations were found in the ovarian dysgerminoma or the NSGCTs. In transient transfection assays, mutant isoforms D816V, D816H, Y823D, and N822K were constitutively phosphorylated in the absence of the natural ligand for KIT, stem cell factor (SCF). In contrast, activation of T801I and wild-type KIT required SCF. Mutants N822K and Y823D were inhibited by imatinib mesylate (Gleevec, previously STI571) whereas D816V and D816H were both resistant to imatinib mesylate. Biochemical evidence of KIT activation, as assessed by KIT phosphorylation and KIT association with phosphatidylinositol (PI) 3-kinase in tumor cell lysates, was largely confined to seminomas with a genomic KIT mutation. These findings suggest that activating KIT mutations may contribute to tumorigenesis in a subset of seminomas, but are not involved in NSGCT.

A highly conserved NTRK3 C-terminal sequence in the ETV6-NTRK3 oncoprotein binds the phosphotyrosine binding domain of insulin receptor substrate-1: an essential interaction for transformation

Receptor tyrosine kinases are integral components of cellular signaling pathways and are frequently deregulated in malignancies. The NTRK family of neurotrophin receptors mediate neuronal cell survival and differentiation, but altered NTRK signaling has also been implicated in oncogenesis. The ETV6-NTRK3 (EN) gene fusion occurs in human pediatric spindle cell sarcomas and secretory breast carcinoma, and encodes the oligomerization domain of the ETV6 transcription factor fused to the protein-tyrosine kinase domain of NTRK3. The EN protein functions as a constitutively active protein-tyrosine kinase with potent transforming activity in multiple cell lineages, and EN constitutively activates both the Ras-MAPK and phosphatidylinositol 3-kinase-Akt pathways. EN transformation is associated with constitutive tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1). Further, IRS-1 functions as the adaptor protein linking EN to downstream signaling pathways. However, the exact nature of the EN-IRS-1 interaction remains unknown. We now demonstrate that EN specifically binds the phosphotyrosine binding domain of IRS-1 via an interaction at the C terminus of EN. An EN mutant lacking the C-terminal 19 amino acids does not bind IRS-1 and lacks transforming ability. Moreover, expression of an IRS-1 polypeptide containing the phosphotyrosine binding domain acts in a dominant negative manner to inhibit EN transformation, and overexpression of IRS-1 potentiates EN transforming activity. These findings indicate that EN.IRS-1 complex formation through the NTRK3 C terminus is essential for EN transformation.

New agents in acute myeloid leukemia and other myeloid disorders

Over the past several decades, improvements in chemotherapeutic agents and supportive care have resulted in significant progress in treating patients with acute myeloid leukemia (AML). More recently, advances in understanding the biology of AML have resulted in the identification of new therapeutic targets. The success of all-trans-retinoic acid in acute promyelocytic leukemia and of imatinib mesylate in chronic myeloid leukemia have demonstrated that targeted therapy may be more effective and less toxic when well defined targets are available. At the same time, understanding mechanisms of drug resistance and means to overcome them has led to modification of some of the existing cytotoxic agents. Rational design and conduct of clinical trials is necessary to ensure that the full potential of these new agents is realized.

Overexpression of ErbB2 receptor inhibits IGF-I-induced Shc-MAPK signaling pathway in breast cancer cells

Overexpression of the ErbB2 receptor in one-third of human breast cancers contributes to the transformation of epithelial cells and predicts poor prognosis for breast cancer patients. We report that the overexpression of ErbB2 inhibits IGF-I-induced MAPK signaling. IGF-I-induced MAPK phosphorylation and MAPK kinase activity are reduced in ErbB2 overexpressing MCF-7/HER2-18 cells relative to control MCF-7/neo cells. In SKBR3/IGF-IR cells, reduction of ErbB2 by antisense methodology restores the IGF-I-induced MAPK activation. The inhibition of IGF-I-induced MAP kinase activation in ErbB2 overexpressing breast cancer cells is correlated with decreased IGF-I-induced Shc tyrosine-phosphorylation, leading to a decreased association of Grb2 with Shc and decreased Raf phosphorylation. However, IGF-I-induced tyrosine-phosphorylation of IGF-I receptor and IRS-I and AKT phosphorylation were unaffected by ErbB2 overexpression. Consistent with these results, we observed that the proportion of IGF-I-stimulated proliferation blocked by the MAPK inhibitor PD98059 fell from 82.6% in MCF-7/neo cells to 41.2% in MCF-7/HER2-18 cells. These data provide evidence for interplay between the IGF-IR and ErbB2 signaling pathways. They are consistent with the view that the IGF-IR mediated attenuation of trastuzumab-induced growth inhibition we recently described is dependent on IGF-I-induced PI3K signaling rather than IGF-I-induced MAPK signaling.

Enhanced cell cycle progression and down regulation of p21(Cip1/Waf1) by PRL tyrosine phosphatases

Human PRL-1, PRL-2, and PRL-3 tyrosine phosphatases induce the malignant transformation of epithelial cells. We tested the hypothesis that the oncogenic effects of PRL occur by increasing cellular proliferation. Cells stably transfected with PRL-1 or PRL-2 exhibited 2.7-3.3-fold increases over control cells in the rate of DNA synthesis and the proportion of cells in S-phase, and they progressed more rapidly from G1 into S. In addition, cells overexpressing either PRL-1 or PRL-2 exhibited enhanced cyclin-dependent kinase 2 (CDK2) activity and significantly lower p21(Cip1/Waf1) protein levels, and PRL-1 overexpressing cells had higher cyclin A protein levels than control cells. We conclude that PRL phosphatases increase cell proliferation by stimulating progression from G1 into S phase, and this process may be dependent on the down regulation of the cyclin dependent kinase inhibitor p21(Cip1/Waf1).

Helicobacter pylori CagA induces Ras-independent morphogenetic response through SHP-2 recruitment and activation

The CagA protein of Helicobacter pylori, which is injected from the bacteria into bacteria-attached gastric epithelial cells, is associated with gastric carcinoma. CagA is tyrosine-phosphorylated by Src family kinases, binds the SH2 domain-containing SHP-2 phosphatase in a tyrosine phosphorylation-dependent manner, and deregulates its enzymatic activity. We established AGS human gastric epithelial cells that inducibly express wild-type or a phosphorylation-resistant CagA, in which tyrosine residues constituting the EPIYA motifs were substituted with alanines. Upon induction, wild-type CagA, but not the mutant CagA, elicited strong elongation of cell shape, termed the "hummingbird" phenotype. Time-lapse video microscopic analysis revealed that the CagA-expressing cells exhibited a marked increase in cell motility with successive rounds of elongation-contraction processes. Inhibition of CagA phosphorylation by an Src kinase inhibitor, PP2, or knockdown of SHP-2 expression by small interference RNA (siRNA) abolished the CagA-mediated hummingbird phenotype. The morphogenetic activity of CagA also required Erk MAPK but was independent of Ras or Grb2. In AGS cells, CagA prolonged duration of Erk activation in response to serum stimulation. Conversely, inhibition of SHP-2 expression by siRNA abolished the sustained Erk activation. Thus, SHP-2 acts as a positive regulator of Erk activity in AGS cells. These results indicate that SHP-2 is involved in the Ras-independent modification of Erk signals that is necessary for the morphogenetic activity of CagA. Our work therefore suggests a key role of SHP-2 in the pathological activity of H. pylori virulence factor CagA.

Expression of Ligand-Activated KIT and Platelet-Derived Growth Factor Receptor β Tyrosine Kinase Receptors in Synovial Sarcoma

PURPOSE

The use of tyrosine kinase receptor inhibitors is increasingly becoming a valuable therapeutic alternative in tumors carrying activated tyrosine kinase receptors. In a previous study, we described a coexpression of KIT and stem cell factor (SCF) mRNA in Synovial sarcomas, (SS) and in a limited number of cases, we demonstrated the presence of an activated receptor. Here, in a wider number of cases, we investigated the expression level and phosphorylation status of two structurally related tyrosine kinase receptors, KIT and platelet-derived growth factor receptor beta (PDGFRbeta), at the light of their role as possible targets of tyrosine kinase receptors inhibitor molecules.

EXPERIMENTAL DESIGN

Forty-three SS cases were analyzed for KIT and PDGFRbeta expression/activation by immunoprecipitation/Western blotting experiments. The cognate ligands, SCF and PDGFB, were detected by reverse transcription-PCR.

RESULTS

KIT was observed in 48 and 41% (45% total) whereas PDGFRbeta in 54 and 33% (45% total) of monophasic and biphasic SS cases, respectively. With respect to the fusion transcript type SYTSSX1 and SYTSSX2, KIT was more expressed in SYTSSX1 carrying cases (48 versus 38%), whereas PDGFRbeta resulted more frequently expressed in SYTSSX2 ones (54 versus 37%). When expressed, the receptors were phosphorylated. Their ligands were detected in all of the activated cases.

CONCLUSIONS

About 70% of the cases express one of the two activated tyrosine kinase receptors with a mutually exclusive expression trend. Coexpression is not frequent and seems to be restricted to monophasic subtype. These data indicate that a consistent fraction of this tumor type could represent a good candidate for kinase inhibitor molecules effective on KIT and PDGFRbeta where their activation is sustained by an autocrine loop.

A genomic perspective on protein tyrosine phosphatases: gene structure, pseudogenes, and genetic disease linkage

The protein tyrosine phosphatases (PTPs) are now recognized as critical regulators of signal transduction under normal and pathophysiological conditions. In this analysis we have explored the sequence of the human genome to define the composition of the PTP family. Using public and proprietary sequence databases, we discovered one novel human PTP gene and defined chromosomal loci and exon structure of the additional 37 genes encoding known PTP transcripts. Direct orthologs were present in the mouse genome for all 38 human PTP genes. In addition, we identified 12 PTP pseudogenes unique to humans that have probably contaminated previous bioinformatics analysis of this gene family. PCR amplification and transcript sequencing indicate that some PTP pseudogenes are expressed, but their function (if any) is unknown. Furthermore, we analyzed the enhanced diversity generated by alternative splicing and provide predicted amino acid sequences for four human PTPs that are currently defined by fragments only. Finally, we correlated each PTP locus with genetic disease markers and identified 4 PTPs that map to known susceptibility loci for type 2 diabetes and 19 PTPs that map to regions frequently deleted in human cancers. We have made our analysis available at http://ptp.cshl.edu or http://science.novonordisk.com/ptp and we hope this resource will facilitate the functional characterization of these key enzymes.

The role of protein tyrosine phosphatase 1B in Ras signaling

Protein tyrosine phosphatase (PTP) 1B has been implicated as a negative regulator of multiple signaling pathways downstream of receptor tyrosine kinases. Inhibition of this enzyme was initially thought to potentially lead to increased oncogenic signaling and tumorigenesis. Surprisingly, we show that platelet-derived growth factor-stimulated extracellular-regulated kinase signaling in PTP1B-deficient cells is not significantly hyperactivated. Moreover, these cells exhibit decreased Ras activity and reduced proliferation by way of previously uncharacterized pathways. On immortalization, PTP1B-deficient fibroblasts display increased expression of Ras GTPase-activating protein (p120RasGAP). Furthermore, we demonstrate that p62Dok (downstream of tyrosine kinase) is a putative substrate of PTP1B and that tyrosine phosphorylation of p62Dok is indeed increased in PTP1B-deficient cells. Consistent with the decreased Ras activity in cells lacking PTP1B, introduction of constitutively activated Ras restored extracellular-regulated kinase signaling and their proliferative potential to those of WT cells. These results indicate that loss of PTP1B can lead to decreased Ras signaling, despite enhanced signaling of other pathways. This finding may in part explain the absence of increased tumor incidence in PTP1B-deficient mice. Thus, PTP1B can positively regulate Ras activity by acting on pathways distal to those of receptor tyrosine kinases.

Vascular cell senescence and vascular aging

Vascular cells have a finite lifespan when cultured in vitro and eventually enter an irreversible growth arrest called "cellular senescence". A number of genetic animal models carrying targeted disruption of the genes that confer the protection against senescence in vitro have been reported to exhibit the phenotypes of premature aging. Similar mutations have been found in the patients with premature aging syndromes. Many of the changes in senescent vascular cell behavior are consistent with the changes seen in age-related vascular diseases. We have demonstrated the presence of senescent vascular cells in human atherosclerotic lesions but not in non-atherosclerotic lesions. Moreover, these cells express increased levels of pro-inflammatory molecules and decreased levels of endothelial nitric oxide synthase, suggesting that cellular senescence in vivo contributes to the pathogenesis of human atherosclerosis. One widely discussed hypothesis of senescence is the telomere hypothesis. An increasing body of evidence has established the critical role of the telomere in vascular cell senescence. Another line of evidence suggests that telomere-independent mechanisms are also involved in vascular cell senescence. Activation of Ras, an important signaling molecule involved in atherogenic stimuli, induces vascular cell senescence and thereby promotes vascular inflammation in vitro and in vivo. It is possible that mitogenic-signaling pathways induce telomere-dependent and telomere-independent senescence, which results in vascular dysfunction. Further understanding of the mechanism underlying cellular senescence will provide insights into the potential of antisenescence therapy for vascular aging.

Integration of IGF, FGF, and anti-BMP signals via Smad1 phosphorylation in neural induction

How do very diverse signaling pathways induce neural differentiation in Xenopus? Anti-BMP (Chordin), FGF8, and IGF2 signals are integrated in the embryo via the regulation of Smad1 phosphorylation. Neural induction results from the combined inhibition of BMP receptor serine/threonine kinases and activation of receptor tyrosine kinases that signal through MAPK and phosphorylate Smad1 in the linker region, further inhibiting Smad1 transcriptional activity. This hard-wired molecular mechanism at the level of the Smad1 transcription factor may help explain the opposing activities of IGF, FGF, and BMP signals not only in neural induction, but also in other aspects of vertebrate development.

Convergent evidence for impaired AKT1-GSK3beta signaling in schizophrenia

AKT-GSK3beta signaling is a target of lithium and as such has been implicated in the pathogenesis of mood disorders. Here, we provide evidence that this signaling pathway also has a role in schizophrenia. Specifically, we present convergent evidence for a decrease in AKT1 protein levels and levels of phosphorylation of GSK3beta at Ser9 in the peripheral lymphocytes and brains of individuals with schizophrenia; a significant association between schizophrenia and an AKT1 haplotype associated with lower AKT1 protein levels; and a greater sensitivity to the sensorimotor gating-disruptive effect of amphetamine, conferred by AKT1 deficiency. Our findings support the proposal that alterations in AKT1-GSK3beta signaling contribute to schizophrenia pathogenesis and identify AKT1 as a potential schizophrenia susceptibility gene. Consistent with this proposal, we also show that haloperidol induces a stepwise increase in regulatory phosphorylation of AKT1 in the brains of treated mice that could compensate for an impaired function of this signaling pathway in schizophrenia.

Molecular mechanisms underlying IGF-I-induced attenuation of the growth-inhibitory activity of trastuzumab (Herceptin) on SKBR3 breast cancer cells

The clinical usefulness of trastuzumab (Herceptin; Genentech, San Francisco, CA) in breast cancer treatment is limited by the rapid development of resistance. We previously reported that IGF-I signaling confers resistance to the growth-inhibitory actions of trastuzumab in a model system, but the underlying molecular mechanism remains unknown. We used SKBR3/neo cells (expressing few IGF-I receptors) and SKBR3/IGF-IR cells (overexpressing IGF-I receptor) as our experimental model. IGF-I antagonized the trastuzumab-induced increase in the level of the Cdk inhibitor p27(Kip1). This resulted in decreased association of p27(Kip1) with Cdk2, restoration of Cdk2 activity and attenuation of cell-cycle arrest in G(1) phase, all of which had been induced by trastuzumab treatment in SKBR3/IGF-IR cells. We also found that the decrease in p27(Kip1) induced by IGF-I was accompanied by an increase in expression of Skp2, which is a ubiquitin ligase for p27(Kip1), and by increased Skp2 association with p27(Kip1). A specific proteasome inhibitor (LLnL) completely blocked the ability of IGF-I to reduce the p27(Kip1) protein level, while IGF-I increased p27(Kip1) ubiquitination. This suggests that the action of IGF-I in conferring resistance to trastuzumab involves targeting of p27(Kip1) to the ubiquitin/proteasome degradation machinery. Finally, specific inhibitors of MAPK and PI3K suggest that the IGF-I-mediated reduction in p27(Kip1) protein level by increased degradation predominantly involves the PI3K pathway. Our results provide an example of resistance to an antineoplastic therapy that targets one tyrosine kinase receptor by increased signal transduction through an alternative pathway in a complex regulatory network.

Imatinib mesylate (STI571) inhibits multiple myeloma cell proliferation and potentiates the effect of common antimyeloma agents

c-Kit has been shown to be mutated in several types of tumours, and its activity has been correlated with increased proliferation rates in a subset of multiple myeloma (MM) patients. We have investigated the effect of imatinib mesylate (STI571), an inhibitor of c-Kit, on MM cells. STI571 inhibited the proliferation of MM cells by arresting cell cycle progression. Western blotting of cell cycle proteins showed that STI571 increased the levels of p21 and p16. MM cells expressed abl, but its level of tyrosine phosphorylation was low and unaffected by treatment with STI571. c-Kit was also expressed in certain MM cell lines, and its phosphorylation was stimulated by stem cell factor. However, the failure to detect the receptor protein in other MM cell lines in which cell proliferation was inhibited by STI571 suggests that its effect on these c-Kit-negative MM cell lines might be caused by the action of the drug on yet unknown targets. STI571 inhibited the proliferation of MM cells resistant to dexamethasone or melphalan and had an additive effect when combined with dexamethasone. Efforts to understand the action of STI571 in MM cells may help to identify these potentially useful targets in the treatment of this and other disorders.

Natural history and functional divergence of protein tyrosine kinases

Cellular signaling is important for many biological processes including growth, differentiation, adhesion, motility and apoptosis. The protein tyrosine kinase (PTK) supergene family is the key mediator in cellular signaling in metazoans, directly associated with a variety of human diseases. All PTKs contain a highly conserved catalytic kinase domain, in spite of variable multi-domain structures. Within each PTK gene family, members exhibit functional divergence in substrate-specificity or temporal/tissue-specific expression, although their primary function is conserved. After conducting phylogenetic analysis on major PTK gene families, we found that the expanding of each PTK family was likely caused by gene or genome duplication event(s) that occurred before the emergence of teleosts but after the vertebrate-amphioxus split. We further investigated the evolutionary pattern of functional divergence after gene duplication in those gene families. Our results show that site-specific shifted evolutionary rate (altered functional constraint) is a common pattern in PTK gene family evolution.

Gastrointestinal stromal tumours and their response to treatment with the tyrosine kinase inhibitor imatinib

Gastrointestinal stromal tumours (GISTs), the most common mesenchymal tumours of the digestive tract, are largely resistant to chemo- and radiotherapy. They are currently defined by their overexpression of the KIT receptor tyrosine kinase (CD117), a member of the family of receptor tyrosine kinases (RTKs), and exhibit KIT mutations in more than 85% of cases. Additionally, in more than one-third of KIT wild-type GISTs, mutations of platelet-derived growth factor receptor alpha (PDGF-R alpha), which also belongs to the family of RTKs, were recently found. Since these data indicate that uncontrolled RTK signalling may be implicated in the pathogenesis of GISTs, RTKs and the activated downstream signalling cascades are attractive targets in the therapy of these tumours. Imatinib is a small-molecule inhibitor that selectively blocks the activity of the PDGF-R, ABL and KIT receptor tyrosine kinases by competitive binding to the adenosine triphosphate binding site of their catalytic domains. We herein review the molecular pathological, preclinical and clinical data that identify imatinib as a valuable new agent in the treatment of GISTs.

The tethered configuration of the EGF receptor extracellular domain exerts only a limited control of receptor function

Quantitative epidermal growth factor (EGF)-binding experiments have shown that the EGF-receptor (EGFR) is displayed on the surface of intact cells in two forms, a minority of high-affinity and a majority of low-affinity EGFRs. On the basis of the three-dimensional structure of the extracellular ligand binding domain of the EGFR, it was proposed that the intramolecularly tethered and autoinhibited configuration corresponds to the low-affinity receptor, whereas the extended configuration accounts for the high-affinity EGFRs on intact cells. Here we test this model by analyzing the properties of EGFRs mutated in the specific regions responsible for receptor autoinhibition and dimerization, respectively. Our results show that mutagenic disruption of the autoinhibitory tether in EGFR results in a decrease in the dissociation rate of EGF without a detectable change in EGFR activation and signaling through EGFR even in response to stimulation with low concentrations of EGF. Mutagenic disruption of the dimerization arm, on the other hand, increased the rate of EGF dissociation and impaired EGFR activation and signaling via the EGFR. This study demonstrates that the extended configuration of EGFR does not account for the apparent high-affinity EGF-binding to EGFR on intact cells. Furthermore, the autoinhibition conferred by the tethered configuration of the extracellular ligand-binding domain provides only a limited control of EGFR function.

A structure-based model for ligand binding and dimerization of EGF receptors

On the basis of the 3D structures of the extracellular ligand-binding domains of the epidermal growth factor (EGF) receptor (EGFR) and ErbB3, a mechanism has been proposed for how the extracellular region of the EGFR is maintained in an autoinhibited configuration and for how EGF binding induces EGFR dimerization and activation. We have attempted to derive a mathematical model for EGF binding to the EGFR and for ligand-induced receptor dimerization and activation that uses this structural information and can explain the characteristic concave-up curvilinear Scatchard plots seen when EGF binding to intact EGFR is studied in living cells. We show that these curvilinear plots cannot be accounted for by simply ascribing different affinities to the autoinhibited and extended (dimeric) configurations of the receptor seen in structural studies. Concave-up plots can only be obtained by including in the mathematical model an additional binding event in which occupied EGFR dimers bind to an "external site." The external site may represent receptor interactions with coated-pit regions in the cell membrane or with other cellular components involved in receptor endocytosis and turnover. We conclude in this study and in the accompanying article that the active extended EGFR configuration binds EGF 5- to 20-fold more strongly than the autoinhibited monomeric receptor configuration. However, these extended receptors do not correspond directly with the "high-affinity" EGF-binding sites seen in EGF-binding studies on intact cells.

G Proteins in Cancer: The Prostate Cancer Paradigm

Signal transduction research investigating mechanisms of androgen-independent prostate cancer cell proliferation has historically focused on the role of androgen and peptide growth factor receptors. More recent work has raised the idea that intracellular signaling mechanisms triggered by extracellular hormonal factors acting through heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) can also mediate and sustain this pathologic process. Prostate cancer patients with advanced disease express elevated levels of GPCRs and GPCR ligands, suggesting that the GPCR system is activated in the cancerous gland and may contribute to tumor growth. Importantly, inhibition of G protein signaling attenuates prostate cancer cell growth in animal models. The nature of intracellular signaling pathways mediating mitogenic effects of GPCRs in prostate cancer is poorly defined, although the G protein-dependent activation of the Ras-to-mitogen-activated protein kinase pathway has emerged as a critical regulatory event. Activated GPCRs may also exert their mitogenic effects in the prostate by activating the androgen receptor.

Inactivation of platelet-derived growth factor receptor by the tumor suppressor PTEN provides a novel mechanism of action of the phosphatase

PTEN, mutated in a variety of human cancers, is a dual specificity protein phosphatase and also possesses D3-phosphoinositide phosphatase activity on phosphatidylinositol 3,4,5-tris-phosphate (PIP(3)), a product of phosphatidylinositol 3-kinase. This PIP(3) phosphatase activity of PTEN contributes to its tumor suppressor function by inhibition of Akt kinase, a direct target of PIP(3). We have recently shown that Akt regulates PDGF-induced DNA synthesis in mesangial cells. In this study, we demonstrate that expression of PTEN in mesangial cells inhibits PDGF-induced Akt activation leading to reduction in PDGF-induced DNA synthesis. As a potential mechanism, we show that PTEN inhibits PDGF-induced protein tyrosine phosphorylation with concomitant dephosphorylation and inactivation of tyrosine phosphorylated and activated PDGF receptor. Recombinant as well as immunopurified PTEN dephosphorylates autophosphorylated PDGF receptor in vitro. Expression of phosphatase deficient mutant of PTEN does not dephosphorylate PDGF-induced tyrosine phosphorylated PDGF receptor. Rather its expression increases tyrosine phosphorylation of PDGF receptor. Furthermore, expression of PTEN attenuated PDGF-induced signal transduction including phosphatidylinositol 3-kinase and Erk1/2 MAPK activities. Our data provide the first evidence that PTEN is physically associated with platelet-derived growth factor (PDGF) receptor and that PDGF causes its dissociation from the receptor. Finally, we show that both the C2 and tail domains of PTEN contribute to binding to the PDGF receptor. These data demonstrate a novel aspect of PTEN function where it acts as an effector for the PDGF receptor function and negatively regulates PDGF receptor activation.

Akt negatively regulates the in vitro lifespan of human endothelial cells via a p53/p21-dependent pathway

The signaling pathway of insulin/insulin-like growth factor-1/phosphatidylinositol-3 kinase/Akt is known to regulate longevity as well as resistance to oxidative stress in the nematode Caenorhabditis elegans. This regulatory process involves the activity of DAF-16, a forkhead transcription factor. Although reduction-of-function mutations in components of this pathway have been shown to extend the lifespan in organisms ranging from yeast to mice, activation of Akt has been reported to promote proliferation and survival of mammalian cells. Here we show that Akt activity increases along with cellular senescence and that inhibition of Akt extends the lifespan of primary cultured human endothelial cells. Constitutive activation of Akt promotes senescence-like arrest of cell growth via a p53/p21-dependent pathway, and inhibition of forkhead transcription factor FOXO3a by Akt is essential for this growth arrest to occur. FOXO3a influences p53 activity by regulating the level of reactive oxygen species. These findings reveal a novel role of Akt in regulating the cellular lifespan and suggest that the mechanism of longevity is conserved in primary cultured human cells and that Akt-induced senescence may be involved in vascular pathophysiology.