Autoregulatory mechanisms in protein-tyrosine kinases

Crystal structure of the EphA4 protein tyrosine kinase domain in the apo- and dasatinib-bound state

The Eph family of receptor tyrosine kinases regulates diverse cellular processes while the over-expression of a member of this family, EphA4, has been reported in a variety of malignant carcinomas. To gain insight into molecular mechanisms and to facilitate structure-based inhibitor design, we solved the crystal structure of the native EphA4 kinase domain in both the apo and dasatinib bound forms. Analysis of the two structures provides insight into structural features of inhibitor binding and revealed a hydrophobic back-pocket in the ATP- binding site of EphA4 which was previously unidentified. The structures suggest a route towards development of novel and specific inhibitors.

Biochemical characterization of a novel type-II VEGFR2 kinase inhibitor: comparison of binding to non-phosphorylated and phosphorylated VEGFR2

A pyrrolo[3,2-d]pyrimidine-based type-II vascular endothelial growth factor receptor 2 (VEGFR2) kinase inhibitor, compound 20d, displayed time-dependent inhibition of the non-phosphorylated catalytic domain of VEGFR2. In contrast, 20d did not show time-dependent inhibition of the phosphorylated enzyme. Dissociation of 20d from non-phosphorylated VEGFR2 was slow and the half-life of the complex was longer than 4h. In contrast, dissociation of 20d from the phosphorylated enzyme was very fast (half-life <5min). A fluorescent tracer based displacement assay and surface plasmon resonance (SPR) analysis confirmed the slow dissociation of 20d from only non-phosphorylated VEGFR2. Thus, activity based and binding kinetic analyses both supported slow dissociation of 20d from only non-phosphorylated VEGFR2. Additionally SPR analysis revealed that association rates were rapid and nearly identical for these two phosphorylation forms of VEGFR2. From these results, the preferential effect of 20d on non-phosphorylated VEGFR2 is dominated by its slow dissociation from the enzyme and this characteristically long residence time may increase its potency in vivo. The present findings may assist in the design of novel type-II kinase inhibitors.

A chimeric receptor of the insulin-like growth factor receptor type 1 (IGFR1) and a single chain antibody specific to myelin oligodendrocyte glycoprotein activates the IGF1R signalling cascade in CG4 oligodendrocyte progenitors

In order to generate neural stem cells with increased ability to survive after transplantation in brain parenchyma we developed a chimeric receptor (ChR) that binds to myelin oligodendrocyte glycoprotein (MOG) via its ectodomain and activates the insulin-like growth factor receptor type 1 ‎‎(IGF1R) signalling cascade. Activation of this pro-survival pathway in response to ligand broadly available in the brain might increase neuroregenerative potential of transplanted precursors. The ChR was produced by fusing a MOG-specific single ‎chain antibody with the extracellular boundary of the IGF1R transmembrane segment. The ChR is expressed on the cellular surface, predominantly as a monomer, and is not N-glycosylated. To show MOG-dependent functionality of the ChR, neuroblastoma cells B104 expressing this ChR were stimulated with monolayers of cells expressing recombinant MOG. The ChR undergoes MOG-dependent tyrosine phosphorylation and homodimerisation. It promotes insulin and IGF-independent growth of the oligodendrocyte progenitor cell line CG4. The proposed mode of the ChR activation is by MOG-induced dimerisation which promotes kinase domain transphosphorylation, by-passing the requirement of conformation changes known to be important for IGF1R activation. Another ChR, which contains a segment of the β-chain ectodomain, was produced in an attempt to recapitulate some of these conformational changes, but proved non-functional.

Oncogenic mutations of ALK in neuroblastoma

Neuroblastoma is one of the most common solid cancers among children. Prognosis of advanced neuroblastoma is still poor despite the recent advances in chemo/radiotherapies. In view of improving the clinical outcome of advanced neuroblastoma, it is important to identify the key molecules responsible for the pathogenesis of neuroblastoma and to develop effective drugs that target these molecules. Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase, initially identified through the analysis of a specific translocation associated with a rare subtype of non-Hodgkin's lymphoma. Recently it was demonstrated that ALK is frequently mutated in sporadic cases with advanced neuroblastoma. Moreover, germline mutations of ALK were shown to be responsible for the majority of hereditary neuroblastoma. ALK mutants found in neuroblastoma show constitutive active kinase activity and oncogenic potentials. Inhibition of ALK in neuroblastoma cell lines carrying amplified or mutated ALK alleles results in compromised downstream signaling and cell growth, indicating potential roles of small molecule ALK inhibitors in the therapeutics of neuroblastoma carrying mutated ALK kinases.

Biochemical characterization of TAK-593, a novel VEGFR/PDGFR inhibitor with a two-step slow binding mechanism

Inhibition of tumor angiogenesis leads to a lack of oxygen and nutrients in the tumor and therefore has become a standards of care for many solid tumor therapies. Dual inhibition of vascular endothelial growth factor receptor (VEGFR) and platelet-derived growth factor receptor (PDGFR) protein kinase activities is a popular strategy for targeting tumor angiogenesis. We discovered that TAK-593, a novel imidazo[1,2-b]pyridazine derivative, potently inhibits tyrosine kinases from the VEGFR and PDGFR families. TAK-593 was highly selective for these families, with an IC(50) >1 μM when tested against more than 200 protein and lipid kinases. TAK-593 displayed competitive inhibition versus ATP. In addition, TAK-593 inhibited VEGFR2 and PDGFRβ in a time-dependent manner, classifying it as a type II kinase inhibitor. Analysis of enzyme-inhibitor preincubation experiments revealed that the binding of TAK-593 to VEGFR2 and PDGFRβ occurs via a two-step slow binding mechanism. Dissociation of TAK-593 from VEGFR2 was extremely slow (t(1/2) >17 h), and the affinity of TAK-593 at equilibrium (K(i)*) was less than 25 pM. Ligand displacement analysis with a fluorescent tracer confirmed the slow dissociation of TAK-593. The dissociation rate constants were in good agreement between the activity and ligand displacement data, and both analyses supported slow dissociation of TAK-593. The long residence time of TAK-593 may achieve an extended pharmacodynamic effect on VEGFR2 and PDGFRβ kinases in vivo that differs substantially from its observed pharmacokinetic profile.

Genotoxicity and cytotoxicity of zinc oxide and titanium dioxide in HEp-2 cells

Aims: The rapidly growing industrial and medical use of nanomaterials, especially zinc oxide and titanium dioxide, has led to growing concerns about their toxicity. Accordingly, the intrinsic genotoxic and cytotoxic potential of these nanoparticles have been evaluated. Materials & methods: Using a HEp-2 cell line, cytotoxicity was tested along with mitochondrial activity and neutral red uptake assays. The genotoxic potential was determined using the Comet and the cytokinesis-blocked micronucleus assays. In addition, tyrosine phosphorylation events were investigated. Results & conclusion: We found concentration- and time-dependent cytotoxicity and an increase in DNA and cytogenetic damage with increasing nanoparticle concentrations. Mainly for zinc oxide, genotoxicity was clearly associated with an increase in tyrosine phosphorylation. Our results suggest that both types of nanoparticles can be genotoxic over a range of concentrations without being cytotoxic.

Regulation of phagocytosis by TAM receptors and their ligands

The TAM family of receptors is preferentially expressed by professional and non-professional phagocytes, including macrophages, dendritic cells and natural killer cells in the immune system, osteoclasts in bone, Sertoli cells in testis, and retinal pigmental epithelium cells in the retina. Mutations in the Mertk single gene or in different combinations of the double or triple gene mutations in the same cell cause complete or partial impairment in phagocytosis of their preys; and as a result, either the normal apoptotic cells cannot be efficiently removed or the tissue neighbor cells die by apoptosis. This scenario of TAM regulation represents a widely adapted model system used by phagocytes in all different tissues. The present review will summarize current known functional roles of TAM receptors and their ligands, Gas 6 and protein S, in the regulation of phagocytosis.

Reaction-diffusion systems in intracellular molecular transport and control

Chemical reactions make cells work only if the participating chemicals are delivered to desired locations in a timely and precise fashion. Most research to date has focused on active-transport mechanisms, although passive diffusion is often equally rapid and energetically less costly. Capitalizing on these advantages, cells have developed sophisticated reaction-diffusion (RD) systems that control a wide range of cellular functions-from chemotaxis and cell division, through signaling cascades and oscillations, to cell motility. These apparently diverse systems share many common features and are "wired" according to "generic" motifs such as nonlinear kinetics, autocatalysis, and feedback loops. Understanding the operation of these complex (bio)chemical systems requires the analysis of pertinent transport-kinetic equations or, at least on a qualitative level, of the characteristic times of the constituent subprocesses. Therefore, in reviewing the manifestations of cellular RD, we also describe basic theory of reaction-diffusion phenomena.

Perspectives for the use of structural information and chemical genetics to develop inhibitors of Janus kinases

Gain-of-function mutations in the genes encoding Janus kinases have been discovered in various haematologic diseases. Jaks are composed of a FERM domain, an SH2 domain, a pseudokinase domain and a kinase domain, and a complex interplay of the Jak domains is involved in regulation of catalytic activity and association to cytokine receptors. Most activating mutations are found in the pseudokinase domain. Here we present recently discovered mutations in the context of our structural models of the respective domains. We describe two structural hotspots in the pseudokinase domain of Jak2 that seem to be associated either to myeloproliferation or to lymphoblastic leukaemia, pointing at the involvement of distinct signalling complexes in these disease settings. The different domains of Jaks are discussed as potential drug targets. We present currently available inhibitors targeting Jaks and indicate structural differences in the kinase domains of the different Jaks that may be exploited in the development of specific inhibitors. Moreover, we discuss recent chemical genetic approaches which can be applied to Jaks to better understand the role of these kinases in their biological settings and as drug targets.

The strong dimerization of the transmembrane domain of the fibroblast growth factor receptor (FGFR) is modulated by C-terminal juxtamembrane residues

The fibroblast growth factor receptor 3 (FGFR3) is a member of the FGFR subfamily of the receptor tyrosine kinases (RTKs) involved in signaling across the plasma membrane. Generally, ligand binding leads to receptor dimerization and activation. Dimerization involves the transmembrane (TM) domain, where mutations can lead to constitutive activation in certain cancer types and also in skeletal malformations. Thus, it has been postulated that FGFR homodimerization must be inherently weak to allow regulation, a feature reminiscent of alpha and beta integrin TM interactions. However, we show herein that in FGFR3-TM, four C-terminal residues, CRLR, have a profound destabilizing effect in an otherwise strongly dimerizing TM peptide. In the absence of these four residues, the dimerizing propensity of FGFR3-TM is comparable to glycophorin, as shown using various detergents. In addition, the expected enhanced dimerization induced by the mutation associated to the Crouzon syndrome A391E, was observed only when these four C-terminal residues were present. In the absence of these four residues, A391E was dimer-destabilizing. Finally, using site specific infrared dichroism and convergence with evolutionary conservation data, we have determined the backbone model of the FGFR3-TM homodimer in model lipid bilayers. This model is consistent with, and correlates with the effects of, most known pathological mutations found in FGFR-TM.

RAG: a recombinase diversified

During B cell and T cell development, the lymphoid-specific proteins RAG-1 and RAG-2 act together to initiate the assembly of antigen receptor genes through a series of site-specific somatic DNA rearrangements that are collectively called variable-diversity-joining (V(D)J) recombination. In the past 20 years, a great deal has been learned about the enzymatic activities of the RAG-1-RAG-2 complex. Recent studies have identified several new and exciting regulatory functions of the RAG-1-RAG-2 complex. Here we discuss some of these functions and suggest that the RAG-1-RAG-2 complex nucleates a specialized subnuclear compartment that we call the 'V(D)J recombination factory'.

Synthetic pentapeptides inhibiting autophosphorylation of insulin receptor in a non-ATP-competitive mechanism

In an attempt to develop non-ATP-competitive inhibitors of the autophosphorylation of IR, the effects of the synthetic peptides, Ac-DIY(1158)ET-NH(2) and Ac-DY(1162)Y(1163)RK-NH(2), on the phosphorylation of IR were studied in vitro. The peptides were derived from the amino-acid sequence in the activation loop of IR. They inhibited the autophosphorylation of IR to 20.5 and 40.7%, respectively, at 4000 microM. The Asp/Asn- and Glu/Gln-substituted peptides, Ac-NIYQT-NH(2) and Ac-NYYRK-NH(2), more potently inhibited the autophosphorylation than did the corresponding parent peptides. The inhibitory potencies of the substituted peptides were decreased with increasing concentrations of ATP, indicating that these peptides employ an ATP-competitive mechanism in inhibiting the autophosphorylation of IR. In contrast, those of the parent peptides were not affected. Mass spectrometry showed that the parent peptides were phosphorylated by IR, suggesting that they interact with the catalytic loop. Moreover, docking simulations predicted that the substituted peptides would interact with the ATP-binding region of IR, whereas their parent peptides would interact with the catalytic loop of IR. Thus, Ac-DIYET-NH(2) and Ac-DYYRK-NH(2) are expected to be non-ATP-competitive inhibitors. These peptides could contribute to the development of a drug employing a novel mechanism.

The precise sequence of FGF receptor autophosphorylation is kinetically driven and is disrupted by oncogenic mutations

Autophosphorylation of the tyrosine kinase domain of fibroblast growth factor receptor 1 (FGFR1) is mediated by a sequential and precisely ordered three-stage autophosphorylation reaction. First-stage autophosphorylation of an activation loop tyrosine leads to 50- to 100-fold stimulation of kinase activity and is followed by second-stage phosphorylation of three additional tyrosine residues, which are binding sites for signaling molecules. Finally, third-stage phosphorylation of a second activation loop tyrosine leads to an additional 10-fold stimulation of FGFR1 catalytic activity. In this report, we show that sequential autophosphorylation of five tyrosines in the FGFR1 kinase domain is under kinetic control, mediated by both the amino acid sequence surrounding the tyrosines and their locations within the kinase structure, and, moreover, that phosphoryl transfer is the rate-limiting step. Furthermore, the strict order of autophosphorylation is disrupted by a glioblastoma-derived, oncogenic FGFR1 point mutation in the kinase domain. We propose that disrupted stepwise activation of tyrosine autophosphorylation caused by oncogenic and other activating FGFR mutations may lead to aberrant activation of and assembly of signaling molecules by the activated receptor.

Flexibility and charge asymmetry in the activation loop of Src tyrosine kinases

Regulated activity of Src kinases is critical for cell growth. Src kinases can be activated by trans-phosphorylation of a tyrosine located in the central activation loop of the catalytic domain. However, because the required exposure of this tyrosine is not observed in the down-regulated X-ray structures of Src kinases, transient partial opening of the activation loop appears to be necessary for such processes. Umbrella sampling molecular dynamics simulations are used to characterize the free energy landscape of opening of the hydrophilic part of the activation loop in the Src kinase Hck. The loop prefers a partially open conformation where Tyr416 has increased accessibility, but remains partly shielded. An asymmetric distribution of the charged residues in the sequence near Tyr416, which contributes to shielding, is found to be conserved in Src family members. A conformational equilibrium involving exchange of electrostatic interactions between the conserved residues Glu310 and Arg385 or Arg409 affects activation loop opening. A mechanism for access of unphosphorylated Tyr416 into an external catalytic site is suggested based on these observations.

Insights into the conformational variability and regulation of human Nek2 kinase

The Nek family of serine/threonine kinases regulates centrosome and cilia function; in addition, several of its members are potential targets for drug discovery. Nek2 is dimeric, is cell cycle regulated and functions in the separation of centrosomes at G2/M. Here, we report the crystal structures of wild-type human Nek2 kinase domain bound to ADP at 1.55-A resolution and T175A mutant in apo form as well as that bound to a non-hydrolyzable ATP analog. These show that regions of the Nek2 structure around the nucleotide-binding site can adopt several different but well-defined conformations. None of the conformations was the same as that observed for the previously reported inhibitor-bound structure, and the two nucleotides stabilized two conformations. The structures suggest mechanisms for the auto-inhibition of Nek2 that we have tested by mutagenesis. Comparison of the structures with Aurora-A and Cdk2 gives insight into the structural mechanism of Nek2 activation. The production of specific inhibitors that target individual kinases of the human genome is an urgent challenge in drug discovery, and Nek2 is especially promising as a cancer target. We not only identify potential challenges to the task of producing Nek2 inhibitors but also propose that the conformational variability provides an opportunity for the design of Nek2 selective inhibitors because one of the conformations may provide a unique target.

In vivo response of Mesocestoides vogae to human insulin

SUMMARY

Successful host invasion by parasitic helminths involves detection and appropriate response to a range of host-derived signals. Insulin signal response pathways are ancient and highly-conserved throughout the metazoans. However, very little is known about helminth insulin signalling and the potential role it may play in host-parasite interactions. The response of Mesocestoides vogae (Cestoda: Cyclophyllidea) larvae to human insulin was investigated, focusing on tyrosine-phosphorylation status, glucose content, survival and asexual reproduction rate. Parasite larvae were challenged with different levels of insulin for variable periods. The parameters tested were influenced by human insulin, and suggested a host-parasite molecular dialogue.

Insights into the regulation of 5-HT2A serotonin receptors by scaffolding proteins and kinases

5-HT(2A) serotonin receptors are essential molecular targets for the actions of LSD-like hallucinogens and atypical antipsychotic drugs. 5-HT(2A) serotonin receptors also mediate a variety of physiological processes in peripheral and central nervous systems including platelet aggregation, smooth muscle contraction, and the modulation of mood and perception. Scaffolding proteins have emerged as important regulators of 5-HT(2A) receptors and our recent studies suggest multiple scaffolds exist for 5-HT(2A) receptors including PSD95, arrestin, and caveolin. In addition, a novel interaction has emerged between p90 ribosomal S6 kinase and 5-HT(2A) receptors which attenuates receptor signaling. This article reviews our recent studies and emphasizes the role of scaffolding proteins and kinases in the regulation of 5-HT(2A) trafficking, targeting and signaling.

Biochemical basis for the functional switch that regulates hepatocyte growth factor receptor tyrosine kinase activation

Ligand-induced dimerization of receptor tyrosine kinases (RTKs) modulates a system of linked biochemical reactions, sharply switching the RTK from a quiescent state to an active state that becomes phosphorylated and triggers intracellular signaling pathways. To improve our understanding of this molecular switch, we developed a quantitative model for hepatocyte growth factor receptor (c-MET) activation using parameters derived in large part from c-MET kinetic and thermodynamic experiments. Our model accurately produces the qualitative and quantitative dynamic features of c-MET phosphorylation observed in cells following ligand binding, including a rapid transient buildup of phosphorylated c-MET at high ligand concentrations. In addition, our model predicts a slow buildup of phosphorylated c-MET under conditions of reduced phosphatase activity and no extracellular agonist. Significantly, this predicted response is observed in cells treated with phosphatase inhibitors, further validating our model. Parameter sensitivity studies clearly show that synergistic oligomerization-dependent changes in c-MET kinetic, thermodynamic, and dephosphorylation properties result in the selective activation of the dimeric receptor, confirming that this model can be used to accurately evaluate the relative importance of linked biochemical reactions important for c-MET activation. Our model suggests that the functional differences observed between c-MET monomers and dimers may have incrementally evolved to optimize cell surface signaling responses.

Activation of growth hormone receptors by growth hormone and growth hormone antagonist dimers: insights into receptor triggering

GH binds dimerized GH receptors (GHRs) to form a trimolecular complex and induces downstream signaling events. The mechanism by which GH binding converts the inactive predimerized GHR to its active signaling conformation is uncertain. GH has no axis of symmetry. Its interaction with GHR is mediated by two asymmetric binding sites on GH, each with distinct affinity. Site 1 is of high affinity and is thought to mediate the first binding step. Mutation of binding site 2 (as in the human GH mutant, G120R) disrupts the second binding but leaves site 1 binding intact. G120R is a GH antagonist; it binds only one GHR and thus fails to signal, and it prevents productive GHR binding by normal GH. We previously demonstrated that prolactin receptor signaling was achieved by a dimeric version of a prolactin antagonist. We now employ assays of cellular signaling and receptor conformational changes to examine whether GH molecules harboring two site 1 regions can trigger GHR activation. We used recombinantly produced GH-GH and G120R-G120R dimers in which monomers in tandem are connected by a short linker peptide. Rabbit GHR-expressing human fibrosarcoma cells (C14) were treated with GH, G120R, GH-GH, or G120R-G120R. As expected, GH and GH-GH, but not G120R, induced GHR disulfide linkage, as assessed by anti-GHR blotting of cell extracts resolved by SDS-PAGE under nonreducing conditions. Disulfide linkage of GHRs reflects attainment of the active signaling conformation. Likewise, GH and GH-GH, but not G120R, caused Janus kinase 2 (JAK2) and signal transducer and activator of transcription 5 (STAT5) activation. Notably, G120R-G120R, despite its lack of an intact site 2 in either dimer partner, also promoted GHR disulfide linkage and JAK2 and STAT5 activation, albeit less potently than either GH or GH-GH. Time-course responses of the three agonists were similar in terms of JAK2 and STAT5 activation. Pretreatment of cells with our conformation-sensitive inhibitory monoclonal antibody, anti-GHR ext-mAb, prevented ligand-induced receptor activation for all three agonists. GHR was also rendered less immunoprecipitable by anti-GHR ext-mAb after treatment with these agonists. These results are important in that they indicate that a ligand with two intact binding sites 1 causes GHR to adopt similar conformational changes as does GH and thus triggers activation of JAK2 and downstream signaling. Furthermore, we infer that there is substantial flexibility in the GHR extracellular domain, such that it productively accommodates GH dimers that are much larger than GH.

Crystal structure of the T315I mutant of AbI kinase

Imatinib (Gleevec) is currently the frontline therapy for chronic myeloid leukemia (CML), a disease characterized by the presence of a constitutively activated chimeric tyrosine kinase protein Bcr-AbI. However, drug resistance often occurs at later stages of the disease, principally because of the occurrence of mutations in the kinase domain. Second generation Bcr-AbI inhibitors, such as dasatinib and nilotinib are capable of inhibiting many imatinib-resistant forms of the kinase but not the form in which threonine is mutated to isoleucine at the gatekeeper position (T315I). In this study, we present the crystal structure of the kinase domain of the c-AbI T315I mutant, as well as the wild-type form, in complex with a pyrrolopyridine inhibitor, PPY-A. The side chain of Ile315 is accommodated in the AbI T315I mutant structure without large conformational changes proximal to the site of mutation. In contrast to other inhibitors, such as imatinib and dasatinib, PPY-A does not occupy the hydrophobic pocket behind the gatekeeper residue. This binding mode, coupled with augmented contacts with the glycine-rich loop, appears to be critical for its ability to override the T315I mutation. The data presented here may provide structural guidance for the design of clinically useful inhibitors of Bcr-AbI T315I.

Complex interactions between thyroid hormone and fibroblast growth factor signalling

PURPOSE OF REVIEW

Thyroid hormone and fibroblast growth factors are critically important for normal development. Recent evidence points to complex interactions between thyroid hormone and fibroblast growth factors that regulate cell proliferation and differentiation. We discuss mechanisms of thyroid hormone and fibroblast growth factor action, and identify downstream signalling responses that offer opportunities for regulatory crosstalk.

RECENT FINDINGS

Thyroid hormone action is mediated by nuclear receptors that regulate gene expression in response to thyroid hormone. Recent studies have shown thyroid hormone also acts at the cell membrane via the alpha(V)beta(3) integrin receptor and these actions also communicate with nuclear responses to thyroid hormone. Fibroblast growth factors act via receptor tyrosine kinases to stimulate second messenger pathways that also communicate with nuclear events. Several common pathways, including mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and signal transducer and activator of transcription signalling, are activated by thyroid hormone and fibroblast growth factor, and may act as points of convergence for interaction in tissues, such as bone, central nervous system and heart, as well as in the extra-cellular matrix and during angiogenesis.

SUMMARY

Although there is convincing evidence that thyroid hormone and fibroblast growth factors interact widely, little is known about molecular mechanisms that determine this interplay. Future research in this expanding field may result in identification of new pharmacological targets for manipulation of cell proliferation and differentiation.

Targeting receptor tyrosine kinase signaling in acute myeloid leukemia

Acute myeloid leukemia (AML) is a quickly progressing, heterogeneous clonal disorder of hematopoietic progenitor cells. Significant progress in understanding the pathogenesis of AML has been achieved in the past few years. Two major types of genetic events are thought to give rise to leukemic transformation: alterations in the activity of transcription factors controlling hematopoietic differentiation and activation of components of receptor tyrosine kinase (RTK) signaling pathways. This has led to the development of promising new therapeutic strategies for the disease. In this article, we will discuss recent developments in the field of molecularly targeted therapies for AML, which involve RTKs such as FMS-like tyrosine kinase 3 (Flt3), c-Kit and signal transduction via the phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) pathways. Initial results imply that targeting RTKs is a very promising approach for AML and that other receptors, such as the insulin-like growth factor receptor (IGF-IR), could also represent new targets in the future.

Pathophysiology of tumor neovascularization

Neovascularization is essential to the process of development and differentiation of tissues in the vertebrate embryo, and is also involved in a wide variety of physiological and pathological conditions in adults, including wound repair, metabolic diseases, inflammation, cardiovascular disorders, and tumor progression. Thanks to cumulative studies on vasculature, new therapeutic approaches have been opened for us to some life-threatening diseases by controlling angiogenesis in the affected organs. In cancer therapy, for example, modulation of factors responsible for tumor angiogenesis may be beneficial in inhibiting of tumor progression. Several antiangiogenic approaches are currently under preclinical trial. However, the mechanisms of neovascularization in tumors are complicated and each tumor shows unique features in its vasculature, depending on tissue specificity, angiogenic micromilieu, grades and stages, host immunity, and so on. For better understanding and effective therapeutic approaches, it is important to clarify both the general mechanism of angiogenic events and the disease-specific mechanism of neovascularization. This review discusses the general features of angiogenesis under physiological and pathological conditions, mainly in tumor progression. In addition, recent topics such as contribution of the endothelial progenitor cells, tumor vasculogenic mimicry, markers for tumor-derived endothelial cells and pericytes, and angiogenic/angiostatic chemokines are summarized.

A novel fusion of RBM6 to CSF1R in acute megakaryoblastic leukemia

Activated tyrosine kinases have been frequently implicated in the pathogenesis of cancer, including acute myeloid leukemia (AML), and are validated targets for therapeutic intervention with small-molecule kinase inhibitors. To identify novel activated tyrosine kinases in AML, we used a discovery platform consisting of immunoaffinity profiling coupled to mass spectrometry that identifies large numbers of tyrosine-phosphorylated proteins, including active kinases. This method revealed the presence of an activated colony-stimulating factor 1 receptor (CSF1R) kinase in the acute megakaryoblastic leukemia (AMKL) cell line MKPL-1. Further studies using siRNA and a small-molecule inhibitor showed that CSF1R is essential for the growth and survival of MKPL-1 cells. DNA sequence analysis of cDNA generated by 5'RACE from CSF1R coding sequences identified a novel fusion of the RNA binding motif 6 (RBM6) gene to CSF1R gene generated presumably by a t(3;5)(p21;q33) translocation. Expression of the RBM6-CSF1R fusion protein conferred interleukin-3 (IL-3)-independent growth in BaF3 cells, and induces a myeloid proliferative disease (MPD) with features of megakaryoblastic leukemia in a murine transplant model. These findings identify a novel potential therapeutic target in leukemogenesis, and demonstrate the utility of phosphoproteomic strategies for discovery of tyrosine kinase alleles.

Diagnostic relevance of overexpressed mRNA of novel oncogene with kinase-domain (NOK) in lung cancers

There have been no target molecules that have enabled us to diagnose lung cancer with high sensitivity and specificity even in its early clinical stages. A molecule termed novel oncogene with kinase-domain (NOK) was recently reported as a receptor protein tyrosine kinase that is expressed in some cancer cell lines and causes the transformation and progressive proliferation of normal cells. Therefore, NOK could be a possible candidate for a diagnostic marker for human cancers. We examined here, the degree of NOK mRNA expression in lung cancer tissues and compared it to that in non-cancerous tissues. More than 60% of non-cancerous samples (8/13) showed undetectable levels of mRNA. In contrast, NOK mRNA was detected in 97.6% (40/41) of lung cancer tissues, resulting in a sensitivity of 80.5% and a specificity of 92.3% that was estimated using the cutoff obtained from receiver operating characteristic curve analysis. Further, NOK mRNA expression was found to be elevated in 92.3% (12/13) of cancerous tissues when paired cancerous and non-cancerous tissues from identical patients were compared. There were no obvious correlations between clinicopathological factors and NOK mRNA expression; however, NOK mRNA was highly expressed even at the early clinical stages of the cancer. These results suggest that NOK mRNA might be a new tool to support the diagnosis of lung cancers, irrespective of the clinical stages.

Theanaphthoquinone inhibits fatty acid synthase expression in EGF-stimulated human breast cancer cells via the regulation of EGFR/ErbB-2 signaling

Fatty acid synthase (FAS) is a major lipogenic enzyme catalyzing the synthesis of long-chain saturated fatty acids. Most breast cancers require lipogenesis for growth. Here, we demonstrated the effects of theanaphthoquinone (TNQ), a member of the thearubigins generated by the oxidation of theaflavin (TF-1), on the expression of FAS in human breast cancer cells. TNQ was found to suppress the EGF-induced expression of FAS mRNA and FAS protein in MDA-MB-231 cells. Expression of FAS has previously been shown to be regulated by the SREBP family of transcription factors. In this study, we demonstrated that the EGF-induced nuclear translocation of SREBP-1 was blocked by TNQ. Moreover, TNQ also modulated EGF-induced ERK1/2 and Akt phosphorylation. Treatment of MDA-MB-231 cells with PI 3-kinase inhibitors, LY294002 and Wortmannin, inhibited the EGF-induced expression of FAS and nuclear translocation of SREBP-1. Treatment with TNQ inhibited EGF-induced EGFR/ErbB-2 phosphorylation and dimerization. Furthermore, treatment with kinase inhibitors of EGFR and ErbB-2 suggested that EGFR/ErbB-2 activation was involved in EGF-induced FAS expression. In constitutive FAS expression, TNQ inhibited FAS expression and Akt autophosphorylation in BT-474 cells. The PI 3-kinase inhibitors and tyrosine kinase inhibitors of EGFR and ErbB-2 also reduced constitutive FAS expression. In addition, pharmacological blockade of FAS by TNQ decreased cell viability and induced cell death in BT-474 cells. In summary, our findings suggest that TNQ modulates FAS expression by the regulation of EGFR/ErbB-2 pathways and induces cell death in breast cancer cells.

Signaling by estrogens

By regulating activities and expression levels of key signaling molecules, estrogens control mechanisms that are responsible for crucial cellular functions. Ligand binding to estrogen receptor (ER) leads to conformational changes that regulate the receptor activity, its interaction with other proteins and DNA. In the cytoplasm, receptor interactions with kinases and scaffolding molecules regulate cell signaling cascades (extranuclear/nongenomic action). In the nucleus, estrogens control a repertoire of coregulators and other auxiliary proteins that are associated with ER, which in turn determines the nature of regulated genes and level of their expression (genomic action). The combination of genomic and nongenomic actions of estrogens ultimately confers the cell-type and tissue-type selectivity. Recent studies have revealed some important new insights into the molecular mechanisms underlying ER action, which may help to explain the functional basis of existing selective ER modulators (SERMs) and provide evidence into how ER might be selectively targeted to achieve specific therapeutic goals. In this review, we will summarize some new molecular details that relate to estrogen signaling. We will also discuss some new strategies that may potentially lead to the development of functionally selective ER modulators that can separate between the beneficial, prodifferentiative effects in bone, the cardiovascular system and the CNS as well as the "detrimental," proliferative effects in reproductive tissues and organs.

Protein tyrosine phosphorylation and reversible oxidation: two cross-talking posttranslation modifications

In addition to protein phosphorylation, redox-dependent posttranslational modification of proteins is emerging as a key signaling system, conserved throughout evolution, and influencing many aspects of cellular homeostasis. Recent data have provided new insight about the interplay between phosphorylation- and redox-dependent signaling, and reactive oxygen species have been included among intracellular signal transducers of growth factor and extracellular matrix receptors. Both tyrosine phosphorylation and thiol oxidation are reversible and dynamic, and this review will particularly focus on the cross-talk between these posttranslational protein regulatory means. Although these modifications share their reversibility, their effects on enzymatic activity of protein tyrosine phosphatases (PTPs) and protein tyrosine kinases (PTKs) may be even opposite. Indeed, while tyrosine phosphorylation is frequently correlated to enzyme activation, thiol oxidation leads to inactivation of PTPs and to superactivation of PTKs. Several papers describe that both these modifications occur during the same input, (i.e., cell proliferation and motility induced by numerous growth factors and cytokines). The review will discuss several aspects of phosphorylation\oxidation interplay, describing both convergent and divergent features of the integrated and coordinated function of PTPs and PTKs during signaling.

Molecular mechanisms of RET receptor-mediated oncogenesis in multiple endocrine neoplasia 2B

Multiple endocrine neoplasia 2B (MEN 2B) is an inherited syndrome of early onset endocrine tumors and developmental anomalies. The disease is caused primarily by a methionine to threonine substitution of residue 918 in the kinase domain of the RET receptor (2B-RET); however, the molecular mechanisms that lead to the disease phenotype are unclear. In this study, we show that the M918T mutation causes a 10-fold increase in ATP binding affinity and leads to a more stable receptor-ATP complex, relative to the wild-type receptor. Further, the M918T mutation alters local protein conformation, correlating with a partial loss of RET kinase autoinhibition. Finally, we show that 2B-RET can dimerize and become autophosphorylated in the absence of ligand stimulation. Our data suggest that multiple distinct but complementary molecular mechanisms underlie the MEN 2B phenotype and provide potential targets for effective therapeutics for this disease.

Mechanisms of disease: understanding resistance to HER2-targeted therapy in human breast cancer

Trastuzumab is a monoclonal antibody targeted against the human epidermal growth factor receptor (HER) 2 tyrosine kinase receptor, which is overexpressed in approximately 25% of invasive breast cancers. The majority of patients with metastatic breast cancer who initially respond to trastuzumab, however, demonstrate disease progression within 1 year of treatment initiation. Preclinical studies have indicated several molecular mechanisms that could contribute to the development of trastuzumab resistance. Increased signaling via the phosphatidylinositol 3-kinase/Akt pathway could contribute to trastuzumab resistance because of activation of multiple receptor pathways that include HER2-related receptors or non-HER receptors such as the insulin-like growth factor 1 receptor, which appears to be involved in a cross-talk with HER2 in resistant cells. Additionally, loss of function of the tumor suppressor PTEN gene, the negative regulator of Akt, results in heightened Akt signaling that leads to decreased sensitivity to trastuzumab. Decreased interaction between trastuzumab and its target receptor HER2, which is due to steric hindrance of HER2 by cell surface proteins such as mucin-4 (MUC4), may block the inhibitory actions of trastuzumab. Novel therapies targeted against these aberrant molecular pathways offer hope that the effectiveness and duration of response to trastuzumab can be greatly improved.

Regulation of receptor tyrosine kinase signaling by GRKs and beta-arrestins

Receptor tyrosine kinases (RTKs) are a unique family of cell surface receptors, each containing a common intracellular domain that has tyrosine kinase activity. However, RTKs share many signaling molecules with another unique family of cell surface receptors, the seven-transmembrane receptors (7TMRs), and these receptor families can activate similar signaling cascades. In this review of RTK signaling, we describe the role of cross talk between RTKs and 7TMRs, focusing specifically on the role played in this process by beta-arrestins and by G proteins.

Molecular regulation of receptor tyrosine kinases in hematopoietic malignancies

Dysregulation of receptor tyrosine kinase (RTK) activity has been implicated in the progression of a variety of human leukemias. Most notably, mutations and chromosomal translocations affecting regulation of tyrosine kinase activity in the Kit receptor, the Flt3 receptor, and the PDGFbeta/FGF1 receptors have been demonstrated in mast cell leukemia, acute myeloid leukemia (AML), and chronic myelogenous leukemias (CML), respectively. In addition, critical but non-overlapping roles for the Ron and Kit receptor tyrosine kinases in the progression of animal models of erythroleukemia have been demonstrated [Persons, D., Paulson, R., Loyd, M., Herley, M., Bodner, S., Bernstein, A., Correll, P. and Ney, P., 1999. Fv2 encodes a truncated form of the Stk receptor tyrosine kinase. Nat. Gen. 23, 159-165.; Subramanian, A., Teal, H.E., Correll, P.H. and Paulson, R.F., 2005. Resistance to friend virus-induced erythroleukemia in W/Wv mice is caused by a spleen-specific defect which results in a severe reduction in target cells and a lack of Sf-Stk expression. J. Virol. 79 (23), 14586-14594.]. The various classes of RTKs implicated in the progression of leukemia have been recently reviewed [Reilly, J., 2003. Receptor tyrosine kinases in normal and malignant haematopoiesis. Blood Rev. 17 (4), 241-248.]. Here, we will discuss the mechanism by which alterations in these receptors result in transformation of hematopoietic cells, in the context of what is known about the molecular regulation of RTK activity, with a focus on our recent studies of the Ron receptor tyrosine kinase.

Vascular endothelial growth factor receptors: molecular mechanisms of activation and therapeutic potentials

Angiogenesis-associated eye diseases are among the most common cause of blindness in the United States and worldwide. Recent advances in the development of angiogenesis-based therapies for treatment of angiogenesis-associated diseases have provided new hope in a wide variety of human diseases ranging from eye diseases to cancer. One group of growth factor receptors critically implicated in angiogenesis is vascular endothelial growth factor receptors (VEGFR), a subfamily of receptor tyrosine kinases (RTKs). VEGFR-1 and VEGFR-2 are closely related receptor tyrosine kinases and have both common and specific ligands. VEGFR-1 is a kinase-impaired RTK and its kinase activity is suppressed by a single amino acid substitution in its kinase domain and by its carboxyl terminus. VEGFR-2 is highly active kinase, stimulates a variety of signaling pathways and broad biological responses in endothelial cells. The mechanisms that govern VEGFR-2 activation, its ability to recruit signaling proteins and to undergo downregulation are highly regulated by phosphorylation activation loop tyrosines and its carboxyl terminus. Despite their differential potentials to undergo tyrosine phosphorylation and kinase activation, both VEGFR-1 and VEGFR-2 are required for normal embryonic development and pathological angiogenesis. VEGFR-1 regulates angiogenesis by mechanisms that involve ligand trapping, receptor homodimerization and heterodimerization. This review highlights recent insights into the mechanism of activation of VEGFR-1 and VEGFR-2, and focuses on the signaling pathways employed by VEGFR-1 and VEGFR-2 that regulate angiogenesis and their therapeutic potentials in angiogenesis-associated diseases.

Inhibition of autophosphorylation of epidermal growth factor receptor by small peptides in vitro

1. Inhibition of uncontrolled epidermal growth factor receptor (EGFR) is one of the approaches for the treatment of breast and lung cancers. We designed oligopeptides consisting of amino-acid sequences of the major (Y1068, Y1148, and Y1173) and minor (Y992) autophosphorylation sites of EGFR. These peptides may be exogenous substrates or pseudosubstrates that interfere with the autophosphorylation of EGFR. The effects of the peptides on autophosphorylation of EGFR were studied. 2. Purified EGFR was phosphorylated in vitro with EGF in the presence of various synthetic peptides. The phosphorylation level of EGFR was then evaluated after SDS-PAGE separation, followed by Western blot analysis with antiphosphotyrosine antibody. 3. Ac-VPEYINQ-NH2 (Y1068) and Ac-DYQQD-NH2 (Y1148) showed the most potent inhibitory effects, followed by Ac-ENAEYLR-NH2 (Y1173). These peptides at 4 mM suppressed phosphorylation to 30-50%. 4. Combination of the three kinds of peptides much more strongly inhibited autophosphorylation. The 50% inhibitory concentration (IC50) value was 0.5 mM as a mixture and was comparable to that of AG1478 (IC50, 0.3 mM) at 0.2 mM ATP. 5. Neither Ac-DIYET-NH2 or Ac-KIYEK-NH2, designed previously based on the amino-acid sequence of an autophosphorylation site of insulin receptor, nor their related (Ac-KIFMK-NH2) or unrelated (Ac-LPFFD-NH2) peptides showed an inhibitory effect. These results suggest that the small peptides that originated from the autophosphorylation sites of EGFR interact solely with EGFR. 6. The peptides containing the sequences surrounding Y1068, Y1148, and Y1173 may be a promising seed for the development of therapeutic agents for breast and lung cancers.

The c-Src tyrosine kinase associates with the catalytic domain of ErbB-2: implications for ErbB-2 mediated signaling and transformation

c-Src associates with and is activated by the ErbB-2 receptor tyrosine kinase, but is unable to bind the EGFR. Although c-Src has been found to interact directly and specifically with the ErbB-2 receptor, the significance of this interaction is unclear. Using both chimeric receptor and site-directed mutagenesis approaches, the region of interaction of c-Src on ErbB-2 was identified. Significantly, EGFR could be converted into a receptor capable of binding c-Src by replacement of a catalytic domain of ErbB-2. We further demonstrated that MDCK cells that express mutant EGFR that are competent in c-Src recruitment lose epithelial polarity in organoid cultures, whereas cells overexpressing the wild-type EGFR retain a polarized phenotype. ErbB-2-dependent activation of c-Src results in disruption of epithelial cell-cell contacts leading to cell dispersal that correlates with the re-localization of phospho-MAPK to focal adhesions. Taken together, these observations suggest that recruitment of c-Src to these closely related EGFR family members plays a critical role in modulating cell polarity.

ErbB3 expression predicts tumor cell radiosensitization induced by Hsp90 inhibition

The ability to identify tumors that are susceptible to a given molecularly targeted radiosensitizer would be of clinical benefit. Towards this end, we have investigated the effects of a representative Hsp90 inhibitor, 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17DMAG), on the radiosensitivity of a panel of human tumor cell lines. 17DMAG was previously shown to enhance the radiosensitivity of a number of human cell lines, which correlated with the loss of ErbB2. We now report on cell lines in which 17DMAG induced the degradation of ErbB2, yet had no effect on radiosensitivity. In a comparison of ErbB family members, ErbB3 protein was only detectable in cells resistant to 17DMAG-induced radiosensitization. To determine whether ErbB3 plays a casual role in this resistance, short interfering RNA (siRNA) was used to knockdown ErbB3 in the resistant cell line AsPC1. Whereas individual treatments with siRNA to ErbB3 or 17DMAG had no effect on radiosensitivity, the combination, which reduced both ErbB2 and ErbB3, resulted in a significant enhancement in AsPC1 radiosensitivity. In contrast to siRNA to ErbB3 or 17DMAG treatments only, AsPC1 cell exposure to the combination also resulted in a decrease in ErbB1 kinase activity. These results indicate that ErbB3 expression predicts for tumor cell susceptibility to and suggests that the loss of ErbB1 signaling activity is necessary for 17DMAG-induced radiosensitization. However, for cell lines sensitized by 17DMAG, treatment with siRNA to ErbB2, which reduced ErbB1 activity, had no effect on radiosensitivity. These results suggest that, whereas the loss of ErbB1 signaling may be necessary for 17DMAG-induced radiosensitization, it is not sufficient.

STI 571 inhibition effect on KITAsn822Lys-mediated signal transduction cascade

OBJECTIVE

Alterations in growth factor signaling pathways may be a frequent collaborating event in AML1-ETO-mediated leukemogenesis. Gain-of-function KIT receptor mutations have been reported in adult AML patients, especially those with core binding factor leukemia (CBFL). We have previously reported a new gain-of-function KIT(Asn822Lys) mutation that is constitutively expressed in the Kasumi-1 CBFL cell line, and has recently been described in two childhood AML patients. To explore the molecular basis of the effects of this mutation in the appropriate context of hemopoietic dysregulation, we investigated KIT downstream signaling in the Kasumi-1 cell line by means of STI 571 (Imatinib, Gleevec) pharmacological inhibition.

MATERIALS AND METHODS

We investigated KIT(Asn822Lys) mutant-initiated signaling in Kasumi-1 cell line, and characterized the inhibitory effect of the STI 571 protein tyrosine kinase inhibitor on downstream signaling.

RESULTS

The use of STI 571-mediated inhibition impaired the tyrosine phosphorylation of KIT(Asn822Lys) and its association with the p85 subunit of phosphatidylinositol 3'-kinase (p85PI3K). The downstream constitutive phosphorylation of JNK1/2 and STAT3 was also significantly inhibited, but STI 571 had no effect on the constitutive activation of Akt, thus suggesting that it is due to other signaling in Kasumi-1 cells. STI 571 inhibited the KIT-mediated proliferation of Kasumi-1 cells in a dose-dependent manner.

CONCLUSIONS

These findings show the role of PI3K in KIT(Asn822Lys)-mediated constitutive activation through the Akt-independent downstream signaling pathway of JNK, and also demonstrate the mutant's susceptibility to STI 571, which may therefore have therapeutic potential in CBFL patients with susceptible KIT mutations.

Teaching resources. Protein domains that interact with receptor tyrosine kinases: structural aspects

This Teaching Resource provides lecture notes and slides for a class covering insights gained from structural analysis of the regulation of receptor tyrosine kinases and is part of the course "Cell Signaling Systems: A Course for Graduate Students." The lecture begins with an overview of the many protein domains thus far implicated in cell signaling and then describes in detail the phosphotyrosine-binding domain (PTB). The application of structural information to rational drug design by targeting protein interaction domains is also covered.

Two conserved cysteine residues are critical for the enzymic function of the human platelet-derived growth factor receptor-beta: evidence for different roles of Cys-822 and Cys-940 in the kinase activity

The platelet-derived growth factor receptor-beta (PDGFR-beta) has a number of conserved cysteine residues on its cytoplasmic domain. We have examined whether the cysteine residues play a role in the enzymic function of PDGFR-beta. We found that N-ethylmaleimide, which selectively alkylates free thiol groups of cysteine residues, completely inhibited the kinase activity of PDGFR-beta. We then identified, through site-directed mutagenesis, two conserved cysteine residues critical for the enzymic function of PDGFR-beta. Cys to Ser mutations for either Cys-822, positioned in the catalytic loop, or Cys-940, located in the C-terminal kinase subdomain, significantly reduced the activities of autophosphorylation and phosphorylation towards exogenous substrates. The non-reducing gel analysis indicated that neither of these cysteine residues contributes to the kinase activity by disulphide-bond formation. In addition, the individual mutation of Cys-822 and Cys-940 had no effect on protein stability or the binding of substrates or ATP, implying that these cysteine residues are involved in enzyme catalysis. Finally, proteolytic cleavage assays showed that the mutation of Cys-940, but not Cys-822, induced a protein conformational change. Taken together, these results suggest that Cys-940 contributes to the catalytic activity of PDGFR-beta by playing a structural role, whereas Cys-822 contributes through a different mechanism.

The SH2 domain: versatile signaling module and pharmaceutical target

The Src homology 2 (SH2) domain is the most prevalent protein binding module that recognizes phosphotyrosine. This approximately 100-amino-acid domain is highly conserved structurally despite being found in a wide variety proteins. Depending on the nature of neighboring protein module(s), such as catalytic domains and other protein binding domains, SH2-containing proteins play many different roles in cellular protein tyrosine kinase (PTK) signaling pathways. Accumulating evidence indicates SH2 domains are highly versatile and exhibit considerable flexibility in how they bind to their ligands. To illustrate this functional versatility, we present three specific examples: the SAP, Cbl and SOCS families of SH2-containing proteins, which play key roles in immune responses, termination of PTK signaling, and cytokine responses. In addition, we highlight current progress in the development of SH2 domain inhibitors designed to antagonize or modulate PTK signaling in human disease. Inhibitors of the Grb2 and Src SH2 domains have been extensively studied, with the aim of targeting the Ras pathway and osteoclastic bone resorption, respectively. Despite formidable difficulties in drug design due to the lability and poor cell permeability of negatively charged phosphorylated SH2 ligands, a variety of structure-based strategies have been used to reduce the size, charge and peptide character of such ligands, leading to the development of high-affinity lead compounds with potent cellular activities. These studies have also led to new insights into molecular recognition by the SH2 domain.

Developmental profile of ErbB receptors in murine central nervous system: Implications for functional interactions

The ErbB family, ErbB1 (also known as the epidermal growth factor receptor EGFR), ErbB2, ErbB3, and ErbB4 comprise a group of receptor tyrosine kinases that interact with ligands from the epidermal growth factor (EGF) superfamily, subsequently dimerize, catalytically activate each other by cross-phosphorylation, and then stimulate various signaling pathways. To gain a better understanding of in vivo functions of ErbB receptors in the central nervous system, the current study examined their mRNA expression throughout development in the mouse brain via in situ hybridization. EGFR, ErbB2, and ErbB4 exhibited distinct but sometimes overlapping distributions in multiple cell types within germinal zones, cortex, striatum, and hippocampus in prenatal and postnatal development. In addition, a subpopulation of cells positive for ErbB4 mRNA in postnatal cortex and striatum coexpressed mRNA for either EGFR or GAD67, a marker for gamma-aminobutyric acid (GABA)ergic interneurons, suggesting that both ErbB4 and EGFR are coexpressed in GABAergic interneurons. In contrast, ErbB3 mRNA was not detected within the brain during development and only appeared in white matter tracts in adulthood. Together, these findings suggest that ErbB receptors might mediate multiple functions in central nervous system development, some of which may be initiated by EGFR/ErbB4 heterodimers in vivo.

A unique structure for epidermal growth factor receptor bound to GW572016 (Lapatinib): relationships among protein conformation, inhibitor off-rate, and receptor activity in tumor cells

GW572016 (Lapatinib) is a tyrosine kinase inhibitor in clinical development for cancer that is a potent dual inhibitor of epidermal growth factor receptor (EGFR, ErbB-1) and ErbB-2. We determined the crystal structure of EGFR bound to GW572016. The compound is bound to an inactive-like conformation of EGFR that is very different from the active-like structure bound by the selective EGFR inhibitor OSI-774 (Tarceva) described previously. Surprisingly, we found that GW572016 has a very slow off-rate from the purified intracellular domains of EGFR and ErbB-2 compared with OSI-774 and another EGFR selective inhibitor, ZD-1839 (Iressa). Treatment of tumor cells with these inhibitors results in down-regulation of receptor tyrosine phosphorylation. We evaluated the duration of the drug effect after washing away free compound and found that the rate of recovery of receptor phosphorylation in the tumor cells reflected the inhibitor off-rate from the purified intracellular domain. The slow off-rate of GW572016 correlates with a prolonged down-regulation of receptor tyrosine phosphorylation in tumor cells. The differences in the off-rates of these drugs and the ability of GW572016 to inhibit ErbB-2 can be explained by the enzyme-inhibitor structures.

Structural basis of constitutive activity and a unique nucleotide binding mode of human Pim-1 kinase

Pim-1 kinase is a member of a distinct class of serine/threonine kinases consisting of Pim-1, Pim-2, and Pim-3. Pim kinases are highly homologous to one another and share a unique consensus hinge region sequence, ER-PXPX, with its two proline residues separated by a non-conserved residue, but they (Pim kinases) have <30% sequence identity with other kinases. Pim-1 has been implicated in both cytokine-induced signal transduction and the development of lymphoid malignancies. We have determined the crystal structures of apo Pim-1 kinase and its AMP-PNP (5'-adenylyl-beta,gamma-imidodiphosphate) complex to 2.1-angstroms resolutions. The structures reveal the following. 1) The kinase adopts a constitutively active conformation, and extensive hydrophobic and hydrogen bond interactions between the activation loop and the catalytic loop might be the structural basis for maintaining such a conformation. 2) The hinge region has a novel architecture and hydrogen-bonding pattern, which not only expand the ATP pocket but also serve to establish unambiguously the alignment of the Pim-1 hinge region with that of other kinases. 3) The binding mode of AMP-PNP to Pim-1 kinase is unique and does not involve a critical hinge region hydrogen bond interaction. Analysis of the reported Pim-1 kinase-domain structures leads to a hypothesis as to how Pim kinase activity might be regulated in vivo.

Velvet, a dominant Egfr mutation that causes wavy hair and defective eyelid development in mice

In the course of a large-scale program of ENU mutagenesis, we isolated a dominant mutation, called Velvet. The mutation was found to be uniformly lethal to homozygotes, which do not survive E13.5. Mice heterozygous for the Velvet mutation are born with eyelids open and demonstrate a wavy coat and curly vibrissae. The mutation was mapped to the proximal end of chromosome 11 by genome-wide linkage analysis. On 249 meioses, the locus was confined to a 2.7-Mb region, which included the epidermal growth factor receptor gene (Egfr). An A --> G transition in the Egfr coding region of Velvet mice was identified, causing the amino acid substitution D833G. This substitution alters an essential triad of amino acids (DFG --> GFG) that is normally required for coordination of the ATP substrate. As such, kinase activity is at least mostly abolished, but quaternary structure of the receptor is presumably maintained, accounting for the dominant effect. Velvet is the first known dominant representative of the Egfr allelic series that is fully viable, a fact that makes it particularly useful for developmental studies.

United States Food and Drug Administration Drug Approval summary: Gefitinib (ZD1839; Iressa) tablets

On May 5, 2003, gefitinib (Iressa; ZD1839) 250-mg tablets (AstraZeneca Inc.) received accelerated approval by the United States Food and Drug Administration as monotherapy for patients with locally advanced or metastatic non-small cell lung cancer after failure of both platinum-based and docetaxel chemotherapies. Information provided in this summary includes chemistry manufacturing and controls, clinical pharmacology, and clinical trial efficacy and safety results. Gefitinib is an anilinoquinazoline compound with the chemical name 4-quinazolinamine,N-(3-chloro-4-flurophenyl)-7-methoxy-6-[3-(4-morpholinyl)propoxy]. It has the molecular formula C(22)H(24)ClFN(4)O(3). Gefitinib is often referred to as a "specific" or "selective" inhibitor of epidermal growth factor receptor. Studies demonstrate, however, that gefitinib inhibits the activity of other intracellular transmembrane tyrosine-specific protein kinases at concentrations similar to those at which it inhibits the epidermal growth factor signal. Maximum plasma concentrations resulting from clinically relevant doses are 0.5-1 microM or more, well within the IC(50) values of several tyrosine kinases. No clinical studies have been performed that demonstrate a correlation between epidermal growth factor receptor expression and response to gefitinib. Gefitinib is 60% available after oral administration and is widely distributed throughout the body. Gefitinib is extensively metabolized in the liver by cytochrome P450 3A4 enzyme. Over a 10-day period, approximately 86% of an orally administered radioactive dose is recovered in the feces, with <4% of the dose in the urine. After daily oral administration, steady-state plasma levels are reached in 10 days and are 2-fold higher than those achieved after single doses. Gefitinib effectiveness was demonstrated in a randomized, double-blind, Phase II, multicenter trial comparing two oral doses of gefitinib (250 versus 500 mg/day). A total of 216 patients were enrolled. The 142 patients who were refractory to or intolerant of a platinum and docetaxel comprised the evaluable population for the efficacy analysis. A partial tumor response occurred in 14% (9 of 66) of patients receiving 250 mg/day gefitinib and in 8% (6 of 76) of patients receiving 500 mg/day gefitinib. The overall objective response rate (RR) for both doses combined was 10.6% (15 of 142 patients; 95% confidence interval, 6.0-16.8%). Responses were more frequent in females and in nonsmokers. The median duration of response was 7.0 months (range, 4.6-18.6+ months). Other submitted data included the results of two large trials conducted in chemotherapy-naive, stage III and IV non-small cell lung cancer patients. Patients were randomized to receive gefitinib (250 or 500 mg daily) or placebo, in combination with either gemcitabine plus cisplatin (n = 1093) or carboplatin plus paclitaxel (n = 1037). Results from this study showed no benefit (RR, time to progression, or survival) from adding gefitinib to chemotherapy. Consequently, gefinitib is only recommended for use as monotherapy. Common adverse events associated with gefitinib treatment included diarrhea, rash, acne, dry skin, nausea, and vomiting. Interstitial lung disease has been observed in patients receiving gefitinib. Worldwide, the incidence of interstitial lung disease was about 1% (2% in the Japanese post-marketing experience and about 0.3% in a United States expanded access program). Approximately one-third of the cases have been fatal. Gefitinib was approved under accelerated approval regulations on the basis of a surrogate end point, RR. No controlled gefitinib trials, to date, demonstrate a clinical benefit, such as improvement in disease-related symptoms or increased survival. Accelerated approval regulations require the sponsor to conduct additional studies to verify that gefitinib therapy produces such benefit.