Dissecting the activating mutations in v-erbB of avian erythroblastosis virus strain R

EGFRvIV: a previously uncharacterized oncogenic mutant reveals a kinase autoinhibitory mechanism

Tumor cells often subvert normal regulatory mechanisms of signal transduction. This study shows this principle by studying yet uncharacterized mutants of the epidermal growth factor receptor (EGFR) previously identified in glioblastoma multiforme, which is the most aggressive brain tumor in adults. Unlike the well-characterized EGFRvIII mutant form, which lacks a portion of the ligand-binding cleft within the extracellular domain, EGFRvIVa and EGFRvIVb lack internal segments distal to the intracellular tyrosine kinase domain. By constructing the mutants and by ectopic expression in naive cells, we show that both mutants confer an oncogenic potential in vitro, as well as tumorigenic growth in animals. The underlying mechanisms entail constitutive receptor dimerization and basal activation of the kinase domain, likely through a mechanism that relieves a restraining molecular fold, along with stabilization due to association with HSP90. Phosphoproteomic analyses delineated the signaling pathways preferentially engaged by EGFRvIVb-identified unique substrates. This information, along with remarkable sensitivities to tyrosine kinase blockers and to a chaperone inhibitor, proposes strategies for pharmacological interception in brain tumors harboring EGFRvIV mutations.

Malfunctions within the Cbl interactome uncouple receptor tyrosine kinases from destructive transport

Proteins of the Cbl family are adaptor molecules and ubiquitin ligases with major functions in the regulation, intracellular transport and degradation of receptor tyrosine kinases (RTKs). Due to this central role, mutations that cause malfunctions of Cbl or their associated proteins - termed the Cbl interactome - easily lead to the transformation of affected cells and eventually the development of cancer. This review intends to give an overview on the mechanisms of Cbl-mediated cell transformation in light of the dysregulated intracellular trafficking of RTKs.

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.

Signaling through monoubiquitination

Ubiquitination is a post-translational modification in which a small conserved peptide, ubiquitin, is appended to target proteins in the cell, through a series of complex enzymatic reactions. Recently, a particular form of ubiquitination, monoubiquitination, has emerged as a nonproteolytic reversible modification that controls protein function. In this review, we highlight recent findings on monoubiquitination as a signaling-induced modification, controlled, among others, by pathways originating from active receptor tyrosine kinases. Furthermore, we review the major cellular processes controlled by ubiquitin modification, including membrane trafficking, histone function, transcription regulation, DNA repair, and DNA replication.

A mutant EGF-receptor defective in ubiquitylation and endocytosis unveils a role for Grb2 in negative signaling

Ligand-induced desensitization of the epidermal growth factor receptor (EGFR) is controlled by c-Cbl, a ubiquitin ligase that binds multiple signaling proteins, including the Grb2 adaptor. Consistent with a negative role for c-Cbl, here we report that defective Tyr1045 of EGFR, an inducible c-Cbl docking site, enhances the mitogenic response to EGF. Signaling potentiation is due to accelerated recycling of the mutant receptor and a concomitant defect in ligand-induced ubiquitylation and endocytosis of EGFR. Kinetic as well as morphological analyses of the internalization-defective mutant receptor imply that c-Cbl-mediated ubiquitylation sorts EGFR to endocytosis and to subsequent degradation in lysosomes. Unexpectedly, however, the mutant receptor displayed significant residual ligand-induced ubiquitylation, especially in the presence of an overexpressed c-Cbl. The underlying mechanism seems to involve recruitment of a Grb2 c-Cbl complex to Grb2-specific docking sites of EGFR, and concurrent acceleration of receptor ubiquitylation and desensitization. Thus, in addition to its well-characterized role in mediating positive signals, Grb2 can terminate signal transduction by accelerating c-Cbl-dependent sorting of active tyrosine kinases to destruction.

Ca2+/calmodulin-dependent kinase II phosphorylates the epidermal growth factor receptor on multiple sites in the cytoplasmic tail and serine 744 within the kinase domain to regulate signal generation

Down-regulation of receptor tyrosine kinase activity plays an essential role in coordinating and controlling cellular growth/differentiation. Ca2+/calmodulin-dependent kinase II (CaM kinase II)-mediated phosphorylation of threonine 1172 in the cytoplasmic tail of HER2/c-erbB2 can modulate tyrosine kinase activity and consensus phosphorylation sites are also found at serines 1046/1047 in the structurally related epidermal growth factor receptor (EGFR). We show that serines 1046/1047 are sites for CaM kinase II phosphorylation, although there is a preference for serine 1047, which resides within the consensus -R-X-X-S-. In addition, we have identified major phosphorylation sites at serine 1142 and serine 1057, which lie within a novel -S-X-D- consensus. Mutation of serines 1046/1047 in full-length EGFR enhanced both fibroblast transformation and tyrosine autokinase activity that was significantly potentiated by additional mutation of serines 1057 and 1142. A single CaM kinase II site was also identified at serine 744 within sub-kinase domain III, and autokinase activity was significantly affected by mutation of this serine to an aspartic acid making this site appear constitutively phosphorylated. We have addressed the mechanism by which CaM kinase II phosphorylation of the EGFR might regulate receptor autokinase activity and show that this modification can hinder association of the cytoplasmic tail with the kinase domain to prevent an enzyme-substrate interaction. We postulate that the location and greater number of CaM kinase II phosphorylation sites in the EGFR compared with HER-2/c-erbB2, leading to differential regulation of autokinase activity, contributes to differences in the strength of downstream signaling events and may explain the higher relative transforming potential of HER-2/cerbB2.

Novel mutations of the MET proto-oncogene in papillary renal carcinomas

Hereditary papillary renal carcinoma (HPRC) is characterized by multiple, bilateral papillary renal carcinomas. Previously, we demonstrated missense mutations in the tyrosine kinase domain of the MET proto-oncogene in HPRC and a subset of sporadic papillary renal carcinomas. In this study, we screened a large panel of sporadic papillary renal carcinomas and various solid tumors for mutations in the MET proto-oncogene. Summarizing these and previous results, mutations of the MET proto-oncogene were detected in 17/129 sporadic papillary renal carcinomas but not in other solid tumors. We detected five novel missense mutations; three of five mutations were located in the ATP-binding region of the tyrosine kinase domain of MET. One novel mutation in MET, V1110I, was located at a codon homologous to an activating mutation in the c-erbB proto-oncogene. These mutations caused constitutive phosphorylation of MET when transfected into NIH3T3 cells. Molecular modeling studies suggest that these activating mutations interfere with the intrasteric mechanism of tyrosine kinase autoinhibition and facilitate transition to the active form of the MET kinase. The low frequency of MET mutations in noninherited papillary renal carcinomas (PRC) suggests that noninherited PRC may develop by a different mechanism than hereditary papillary renal carcinoma.

A patient with Werner's syndrome and erythroleukemia: just coincidence?

Werner's syndrome is a rare clinical entity and approximately 150 cases have been reported in the medical literature. Werner's syndrome, inherited by autosomal recessive transmission, is characterized primarily by a short stature, premature greying and balding, trophic ulceration of the legs, diabetes mellitus and hypogonadism. These features combine to present a picture of adult progeria. In this brief report we describe a 51-year-old Bedouin male with Werner's syndrome, diagnosed as erythroleukemia (AML-6), and presenting as acute pancytopenia. The patient died two months after diagnosis. This is a rare case of erythroleukemia in a patient with Werner's syndrome. We survey current knowledge of the cytogenetic pathogenesis of Werner's syndrome and erythroleukemia, and attempt to explain the possible link between these two rare syndromes.

The v-erbB oncogene confers enhanced cellular susceptibility to reovirus infection

We have previously demonstrated that two mouse cell lines that are poorly infectible by reovirus become highly susceptible upon transfection with the gene encoding the epidermal growth factor receptor (EGFR) (J. E. Strong, D. Tang, and P. W. K. Lee, Virology 197:405-411, 1993). This enhancement of infection efficiency requires a functional EGFR, since such an enhancement is not observed in cells expressing a mutated (kinase-inactive) EGFR. The additional finding that reovirus is capable of directly binding to the N-terminal ectodomain of the EGFR (D. Tang, J. E. Strong, and P. W. K. Lee, Virology 197:412-414, 1993) has led us to question whether this interaction is required for the activation of a signalling cascade that somehow augments the ensuing infection process. In the present study, we address this question, using cells transfected with the v-erbB oncogene, which encodes a protein structurally related to the EGFR but lacking a large portion of the N-terminal ligand-binding domain. The v-erbB protein also possesses ligand-independent, constitutive tyrosine kinase activity. Control NIH 3T3 cells, which are poorly infectible by reovirus (serotype 3, strain Dearing), and NIH 3T3 cells transfected with the v-erbB oncogene (THC-11) were assayed for their susceptibilities to reovirus infection. Infectivity was determined by immunofluorescent detection of viral proteins, sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of radiolabeled cells, and plaque titration. All three assays demonstrated a drastically higher degree of susceptibility to infection in the THC-11 cell line. This enhanced susceptibility was found to be abrogated by treatment of the cells with genistein, an inhibitor of tyrosine protein kinases, but only partially by treatment with daidzein, an inactive analog of genistein. We propose that the mechanism of enhancement of infection efficiency conferred by EGFR and v-erbB is through the opportunistic utilization by the virus of an already activated signal transduction pathway.

Disease specificity of kinase domains: the src-encoded catalytic domain converts erbB into a sarcoma oncogene

src and erbB are two tyrosine kinase-encoding oncogenes carried by retroviruses, which have distinct disease specificities. The former induces predominantly sarcomas, and the latter, leukemias. Src and ErbB have similar catalytic domains but have very different regulatory domains. A wealth of information exists concerning how different regulatory domains [Src homology 2 (SH2) and SH3 domains and autophosphorylation sites] control substrate and disease specificities. Whether the catalytic domain helps determine these specificities remains to be explored. Here we show that the Src catalytic domain is enzymatically active when substituted into the ErbB backbone and interacts with the ErbB regulatory domain. This ErbB/Src chimera displays autophosphorylation and substrate phosphorylation patterns different from those of both Src and ErbB. Neither SH2 and SH3 nor autophosphorylation sites are required for the Src catalytic domain to exert its fibroblast transforming ability. Most significantly, the catalytic domain can convert erbB from a leukemogenic oncogene into a sarcomagenic oncogene, suggesting that the leukemogenic determinants in part reside within the ErbB catalytic domain.

A minor tyrosine phosphorylation site located within the CAIN domain plays a critical role in regulating tissue-specific transformation by erbB kinase

Avian c-erbB encodes a protein that is homologous to the human epidermal growth factor receptor. Truncation of the amino-terminal, ligand-binding domain of this receptor results in an oncogene product which is a potent inducing agent for erythroleukemias but not fibrosarcomas in chickens. Here we show that mutation of a single tyrosine residue, p5, in the carboxyl terminus of the erbB oncogene product allows it to become sarcomagenic in vivo and to transform fibroblasts in vitro. Mutations of other autophosphorylation sites do not generate comparable effects. The increased transforming activity of the p5 mutant is accompanied by an elevated level of mitogen-activated protein kinase phosphorylation. By analogy to the human epidermal growth factor receptor, p5 is a minor autophosphorylation site and is located in a domain known to be involved in regulating calcium influx and receptor internalization (CAIN domain). This area of the erbB product has been found to be repeatedly deleted in various sarcomagenic avian erythroblastosis virus isolates. We precisely deleted the CAIN domain and also made point mutations of the acidic residues within the CAIN domain. In both cases, fibroblast-transforming potential is activated. We interpret these data to mean that p5 and its surrounding region negatively regulate fibroblast-transforming and sarcomagenic potential. To our knowledge, this represents the first point mutation of an autophosphorylation site that activates erbB oncogenicity.

Insights into erythroid differentiation obtained from studies on avian erythroblastosis virus

Analysis of the oncogenes v-erbB and v-erbA and their normal proto-oncogene counterparts has revealed several novel aspects of erythroid differentiation. A new erythroid progenitor capable of extended self-renewal has been described, tyrosine kinase receptors and steroid hormone receptors have been found to cooperate in controlling self-renewal, and dramatic alterations in the cell cycle have been found to accompany induction of terminal differentiation.

Modulation of erbB kinase activity and oncogenic potential by single point mutations in the glycine loop of the catalytic domain

Avian c-erbB is activated to a leukemia oncogene following truncation of its amino-terminal ligand-binding domain by retroviral insertion. The insertionally activated transcripts encode protein products which have constitutive tyrosine kinase activity and can induce erythroleukemia but not sarcomas. We have previously found that a valine-to-isoleucine point mutation at position 157 (V157I mutant) within the tyrosine kinase domain of this truncated erbB can dramatically activate the sarcomagenic potential of the oncogene and increase the kinase activity of this oncoprotein. This mutation lies at position 157 of the insertionally activated c-erbB product, affecting a highly conserved valine residue of the glycine loop involved in ATP binding and phosphate transfer. To investigate the functional importance of this residue in the catalytic activity of kinases, we have introduced at this position, by site-directed mutagenesis, codons representing the remaining 18 amino acid residues. Most of the mutants have diminished activity, with six of them completely devoid of kinase activity, indicating the sensitivity of this region to conformational changes. Some of these mutants displayed increased kinase activity and greater transforming potential in comparison with IA c-erbB, but none had levels as high as those of the V157I mutant. In general, the sarcomagenic potential of the various erbB mutants correlated with their autophosphorylation state and their ability to cause phosphorylation of MAP kinase. However, there are important exceptions such as the V157G mutant, which lacks enhanced autophosphorylation but is highly sarcomagenic. Studies of this and other autophosphorylation site mutants point to the existence of an autophosphorylation-independent pathway in sarcomagenesis. The requirement for leukemogenic potential is much less stringent and correlates with positivity of kinase activity. When the valine-to-isoleucine substitution was put in context of the full-length erbB protein, the mutation relaxed the ligand dependence and had a positive effect on the transforming potential of the full-length c-erbB.

Modulation of erbB kinase activity and oncogenic potential by single point mutations in the glycine loop of the catalytic domain

Avian c-erbB is activated to a leukemia oncogene following truncation of its amino-terminal ligand-binding domain by retroviral insertion. The insertionally activated transcripts encode protein products which have constitutive tyrosine kinase activity and can induce erythroleukemia but not sarcomas. We have previously found that a valine-to-isoleucine point mutation at position 157 (V157I mutant) within the tyrosine kinase domain of this truncated erbB can dramatically activate the sarcomagenic potential of the oncogene and increase the kinase activity of this oncoprotein. This mutation lies at position 157 of the insertionally activated c-erbB product, affecting a highly conserved valine residue of the glycine loop involved in ATP binding and phosphate transfer. To investigate the functional importance of this residue in the catalytic activity of kinases, we have introduced at this position, by site-directed mutagenesis, codons representing the remaining 18 amino acid residues. Most of the mutants have diminished activity, with six of them completely devoid of kinase activity, indicating the sensitivity of this region to conformational changes. Some of these mutants displayed increased kinase activity and greater transforming potential in comparison with IA c-erbB, but none had levels as high as those of the V157I mutant. In general, the sarcomagenic potential of the various erbB mutants correlated with their autophosphorylation state and their ability to cause phosphorylation of MAP kinase. However, there are important exceptions such as the V157G mutant, which lacks enhanced autophosphorylation but is highly sarcomagenic. Studies of this and other autophosphorylation site mutants point to the existence of an autophosphorylation-independent pathway in sarcomagenesis. The requirement for leukemogenic potential is much less stringent and correlates with positivity of kinase activity. When the valine-to-isoleucine substitution was put in context of the full-length erbB protein, the mutation relaxed the ligand dependence and had a positive effect on the transforming potential of the full-length c-erbB.

Expression of the erb B oncogene in the Morris hepatoma 7777

Altered expression of protooncogenes/oncogenes is believed to be involved in hepatocarcinogenesis of the chemically induced, transplantable Morris hepatoma 7777. We compared the mRNA expression of c-N-ras and v-erb B mRNA of normal rat liver with that of Morris hepatoma 7777 using Northern blot analysis and in situ hybridization. Northern blot analysis revealed a strong overexpression of the v-erb B related mRNA, while the c-N-ras mRNA was only slightly increased. In situ hybridization using a c-N-ras mRNA probe also showed only a slightly increased number of silver grains in the hepatoma cells compared with normal rat liver. On the other hand, the v-erb B related mRNA was strongly overexpressed in the hepatoma cells, while the connective-tissue capsule, the blood vessels, blood cells and the necrotic foci did not show an elevated v-erb B related gene mRNA expression. Similar results were obtained in liver metastases. The detectable v-erb B hybridization signal was lost by pretreatment with RNase A. We conclude that the c-N-ras gene is of minor importance in the chemically induced, transplantable Morris hepatoma 7777, while the increased expression of the v-erb B related mRNA is due to a selection of ligand-independent tyrosine kinase activity.

Oncogenes, Protein Tyrosine Kinases, and Signal Transduction

Many oncogenes encode protein tyrosine kinases (PTKs). Oncogenic mutations of these genes invariably result in constitutive activation of these PTKs. Autophosphorylation of the PTKs and tyrosine phosphorylation of their cellular substrates are essential events for transmission of the mitogenic signal into cells. The recent discovery of the characteristic amino acid sequences, of the src homology domains 2 and 3 (SH2 and SH3), and extensive studies on proteins containing the SH2 and SH3 domains have revealed that protein tyrosine-phosphorylation of PTKs provides phosphotyrosine sites for SH2 binding and allows extracellular signals to be relayed into the nucleus through a chain of protein-protein interactions mediated by the SH2 and SH3 domains. Studies on oncogenes, PTKs and SH2/SH3-containing proteins have made a tremendous contribution to our understanding of the mechanisms for the control of cell growth, oncogenesis, and signal transduction. This review is intended to provide an outline of the most recent progress in the study of signal transduction by PTKs. Copyright 1994 S. Karger AG, Basel

Oncogenic activation of tyrosine kinases

Tyrosine kinases comprise the largest group of oncoproteins, a fact that underscores the importance of reversible tyrosine phosphorylation in the regulation of essential cellular functions. Oncogenic activation of tyrosine kinases results in the constitutive activation of what is normally a conditionally regulated enzyme activity. Studies of tyrosine kinase oncoproteins, and a comparison with their corresponding proto-oncogene products, have identified important functional and regulatory domains within these proteins, positive and negative regulators of their enzyme activities and signalling cascades that control cell growth and differentiation.

Induction of renal adenocarcinoma by a nonmutated erbB oncogene

Oncogenicity tests have revealed that a nonmutated erbB oncogene induces renal adenocarcinoma in addition to erythroblastosis. The erbB oncogene is a truncated form of the chicken epidermal growth factor receptor that lacks the extracellular ligand-binding domain. Previously, the nonmutated erbB oncogene has been reported to cause only erythroblastosis. The expansion of the disease potential of erbB to additional neoplasms has been associated with mutations (truncations, deletions, and point mutations) within the erbB gene. Our results indicate that a nonmutated virally expressed erbB oncogene (REB-c) causes a 100% incidence of renal neoplasia.

Protein tyrosine kinase activities of the epidermal growth factor receptor and ErbB proteins: correlation of oncogenic activation with altered kinetics

We have compared the protein tyrosine kinase activities of the chicken epidermal growth factor receptor (chEGFR) and three ErbB proteins to learn whether cancer-activating mutations affect the kinetics of kinase activity. In immune complex assays performed in the presence of 15 mM Mn2+, ErbB proteins and the chEGFR exhibited highly reproducible tyrosine kinase activity. Under these conditions, the ErbB and chEGFR proteins had similar apparent Km [Km(app)] values for ATP. The ErbB proteins appeared to be activated, as they had at least 3-fold-higher relative Vmax(app) for autophosphorylation and approximately 2-fold higher relative Vmax(app) for the phosphorylation of the exogenous substrate TK6 (a bacterially expressed fusion protein containing the C-terminal domain of the human EGFR). The ErbB kinases had both higher Km(app) and higher Vmax(app) for the phosphorylation of the exogenous substrate TK6 than did the chEGFR. The ratios of the Vmax(app) to the Km(app) for TK6 phosphorylation suggested that the ErbB proteins had lower catalytic efficiencies for the exogenous substrate than did the chEGFR. The three tested ErbB proteins had cytoplasmic domain mutations that conferred distinctive disease potentials. These mutations did not affect the kinetics for the phosphorylation of the exogenous substrate TK6. Two of the ErbB proteins contained all of the sites used for autophosphorylation. In these, a mutation that broadened oncogenic potential to endothelial cells caused an additional increase in Vmax(app) for autophosphorylation. Thus, mutations that change the EGFR into an ErbB oncogene cause multiple changes in the kinetics of protein tyrosine kinase activity.

Protein tyrosine kinase activities of the epidermal growth factor receptor and ErbB proteins: correlation of oncogenic activation with altered kinetics

We have compared the protein tyrosine kinase activities of the chicken epidermal growth factor receptor (chEGFR) and three ErbB proteins to learn whether cancer-activating mutations affect the kinetics of kinase activity. In immune complex assays performed in the presence of 15 mM Mn2+, ErbB proteins and the chEGFR exhibited highly reproducible tyrosine kinase activity. Under these conditions, the ErbB and chEGFR proteins had similar apparent Km [Km(app)] values for ATP. The ErbB proteins appeared to be activated, as they had at least 3-fold-higher relative Vmax(app) for autophosphorylation and approximately 2-fold higher relative Vmax(app) for the phosphorylation of the exogenous substrate TK6 (a bacterially expressed fusion protein containing the C-terminal domain of the human EGFR). The ErbB kinases had both higher Km(app) and higher Vmax(app) for the phosphorylation of the exogenous substrate TK6 than did the chEGFR. The ratios of the Vmax(app) to the Km(app) for TK6 phosphorylation suggested that the ErbB proteins had lower catalytic efficiencies for the exogenous substrate than did the chEGFR. The three tested ErbB proteins had cytoplasmic domain mutations that conferred distinctive disease potentials. These mutations did not affect the kinetics for the phosphorylation of the exogenous substrate TK6. Two of the ErbB proteins contained all of the sites used for autophosphorylation. In these, a mutation that broadened oncogenic potential to endothelial cells caused an additional increase in Vmax(app) for autophosphorylation. Thus, mutations that change the EGFR into an ErbB oncogene cause multiple changes in the kinetics of protein tyrosine kinase activity.