A dominant negative mutation suppresses the function of normal epidermal growth factor receptors by heterodimerization

The new and recurrent FLT3 juxtamembrane deletion mutation shows a dominant negative effect on the wild-type FLT3 receptor

In acute myeloid leukemia (AML), the Fms-like tyrosine kinase 3 (FLT3) is one of the most frequently mutated genes. Recently, a new and recurrent juxtamembrane deletion mutation (p.Q569Vfs*2) resulting in a truncated receptor was identified. The mutated receptor is expressed on the cell surface and still binds its ligand but loses the ability to activate ERK signaling. FLT3 p.Q569fs-expressing Ba/F3 cells show no proliferation after ligand stimulation. Furthermore, coexpressed with the FLT3 wild-type (WT) receptor, the truncated receptor suppresses stimulation and activation of the WT receptor. Thus, FLT3 p.Q569Vfs*2, to our knowledge, is the first FLT3 mutation with a dominant negative effect on the WT receptor.

Actin binding GFP allows 4D in vivo imaging of myofilament dynamics in the zebrafish heart and the identification of Erbb2 signaling as a remodeling factor of myofibril architecture

RATIONALE

Dilated cardiomyopathy is a leading cause of congestive heart failure and a debilitating complication of antineoplastic therapies. Despite disparate causes for dilated cardiomyopathy, maladaptive cardiac remodeling and decreased systolic function are common clinical consequences, begging an investigation of in vivo contractile dynamics in development and disease, one that has been impossible to date.

OBJECTIVE

To image myocardial contractile filament dynamics in vivo and to assess potential causes of dilated cardiomyopathy in antineoplastic therapies targeting the epidermal growth factor receptor Erbb2.

METHODS AND RESULTS

We generated a transgenic zebrafish line expressing an actin-binding green fluorescent protein in cardiomyocytes, allowing an in vivo imaging of myofilaments. Analysis of this line revealed architectural differences in myofibrils of the distinct cardiomyocyte subtypes. We used this model to investigate the effects of Erbb2 signaling on myofibrillar organization because drugs targeting ERBB2 (HER2/NEU) signaling, a mainstay of breast cancer chemotherapy, cause dilated cardiomyopathy in many patients. High-resolution in vivo imaging revealed that Erbb2 signaling regulates a switch between a dense apical network of filamentous myofibrils and the assembly of basally localized myofibrils in ventricular cardiomyocytes.

CONCLUSIONS

Using this novel line, we compiled a reference for myofibrillar microarchitecture among myocardial subtypes in vivo and at different developmental stages, establishing this model as a tool to analyze in vivo cardiomyocyte contractility and remodeling for a broad range of cardiovascular questions. Furthermore, we applied this model to study Erbb2 signaling in cardiomyopathy. We show a direct link between Erbb2 activity and remodeling of myofibrils, revealing an unexpected mechanism with potentially important implications for prevention and treatment of cardiomyopathy.

Antagonism of the prostaglandin E2 EP1 receptor in MDCK cells increases growth through activation of Akt and the epidermal growth factor receptor

The actions of prostaglandin E2 (PGE2) in the kidney are mediated by G protein-coupled E-prostanoid (EP) receptors, which affect renal growth and function. This report examines the role of EP receptors in mediating the effects of PGE2 on Madin-Darby canine kidney (MDCK) cell growth. The results indicate that activation of Gs-coupled EP2 and EP4 by PGE2 results in increased growth, while EP1 activation is growth inhibitory. Indeed, two EP1 antagonists (ONO-8711 and SC51089) stimulate, rather than inhibit, MDCK cell growth, an effect that is lost following an EP1 knockdown. Similar observations were made with M1 collecting duct and rabbit kidney proximal tubule cells. ONO-8711 even stimulates growth in the absence of exogenous PGE2, an effect that is prevented by ibuprofen (indicating a dependence upon endogenous PGE2). The involvement of Akt was indicated by the observation that 1) ONO-8711 and SC51089 increase Akt phosphorylation, and 2) MK2206, an Akt inhibitor, prevents the increased growth caused by ONO-8711. The involvement of the EGF receptor (EGFR) was indicated by 1) the increased phosphorylation of the EGFR caused by SC51089 and 2) the loss of the growth-stimulatory effect of ONO-8711 and SC51089 caused by the EGFR kinase inhibitor AG1478. The growth-stimulatory effect of ONO-8711 was lost following an EGFR knockdown, and transduction of MDCK cells with a dominant negative EGFR. These results support the hypothesis that 1) signaling via the EP1 receptor involves Akt as well as the EGFR, and 2), EP1 receptor pharmacology may be employed to prevent the aberrant growth associated with a number of renal diseases.

Receptor tyrosine kinases: legacy of the first two decades

Receptor tyrosine kinases (RTKs) and their cellular signaling pathways play important roles in normal development and homeostasis. Aberrations in their activation or signaling leads to many pathologies, especially cancers, motivating the development of a variety of drugs that block RTK signaling that have been successfully applied for the treatment of many cancers. As the current field of RTKs and their signaling pathways are covered by a very large amount of literature, spread over half a century, I am focusing the scope of this review on seminal discoveries made before tyrosine phosphorylation was discovered, and on the early days of research into RTKs and their cellular signaling pathways. I review the history of the early days of research in the field of RTKs. I emphasize key early findings, which provided conceptual frameworks for addressing the questions of how RTKs are activated and how they regulate intracellular signaling pathways.

Both stromal cell and colonocyte epidermal growth factor receptors control HCT116 colon cancer cell growth in tumor xenografts

Colon cancer growth requires growth-promoting interactions between malignant colonocytes and stromal cells. Epidermal growth factor receptors (EGFR) are expressed on colonocytes and many stromal cells. Furthermore, EGFR is required for efficient tumorigenesis in experimental colon cancer models. To dissect the cell-specific role of EGFR, we manipulated receptor function on stromal cells and cancer cells. To assess the role of stromal EGFR, HCT116 human colon cancer cells were implanted into immunodeficient mice expressing dominant negative (DN) Egfr(Velvet/+) or Egfr(+/+). To assess the role of cancer cell EGFR, HCT116 transfectants expressing inducible DN-Egfr were implanted into immunodeficient mice. To dissect EGFR signals in vitro, we examined colon cancer cells in monoculture or in cocultures with fibroblasts for EGFR transactivation and prostaglandin synthase 2 (PTGS2) induction. EGFR signals were determined by blotting, immunostaining and real-time PCR. Tumor xenografts in Egfr(Velvet/+) mice were significantly smaller than tumors in Egfr(+/+) mice, with decreased proliferation (Ki67) and increased apoptosis (cleaved caspase-3) in cancer cells and decreased stromal blood vessels. Mouse stromal transforming growth factor alpha (TGFA), amphiregulin (AREG), PTGS2 and Il1b and interleukin-1 receptor 1 (Il1r1) transcripts and cancer cell beta catenin (CTNNB1) and cyclin D1 (CCND1) were significantly lower in tumors obtained from Egfr(Velvet/+) mice. DN-EGFR HCT116 transfectants also formed significantly smaller tumors with reduced mouse Areg, Ptgs2, Il1b and Il1r1 transcripts. Coculture increased Caco-2 phospho-active ERBB (pERBB2), whereas DN-EGFR in Caco-2 cells suppressed fibroblast PTGS2 and prostaglandin E2 (PGE2). In monoculture, interleukin 1 beta (IL1B) transactivated EGFR in HCT116 cells. Stromal cell and colonocyte EGFRs are required for robust EGFR signals and efficient tumor growth, which involve EGFR-interleukin-1 crosstalk.

Interactions between Type III receptor tyrosine phosphatases and growth factor receptor tyrosine kinases regulate tracheal tube formation in Drosophila

The respiratory (tracheal) system of the Drosophila melanogaster larva is an intricate branched network of air-filled tubes. Its developmental logic is similar in some ways to that of the vertebrate vascular system. We previously described a unique embryonic tracheal tubulogenesis phenotype caused by loss of both of the Type III receptor tyrosine phosphatases (RPTPs), Ptp4E and Ptp10D. In Ptp4E Ptp10D double mutants, the linear tubes in unicellular and terminal tracheal branches are converted into bubble-like cysts that incorporate apical cell surface markers. This tube geometry phenotype is modulated by changes in the activity or expression of the epidermal growth factor receptor (Egfr) tyrosine kinase (TK). Ptp10D physically interacts with Egfr. Here we demonstrate that the Ptp4E Ptp10D phenotype is the consequence of the loss of negative regulation by the RPTPs of three growth factor receptor TKs: Egfr, Breathless and Pvr. Reducing the activity of any of the three kinases by tracheal expression of dominant-negative mutants suppresses cyst formation. By competing dominant-negative and constitutively active kinase mutants against each other, we show that the three RTKs have partially interchangeable activities, so that increasing the activity of one kinase can compensate for the effects of reducing the activity of another. This implies that SH2-domain downstream effectors that are required for the phenotype are likely to be able to interact with phosphotyrosine sites on all three receptor TKs. We also show that the phenotype involves increases in signaling through the MAP kinase and Rho GTPase pathways.

Palmitoylation of the EGF receptor impairs signal transduction and abolishes high-affinity ligand binding

The intracellular juxtamembrane domain of the EGF receptor has been shown to be involved in the stimulation of the receptor's tyrosine kinase activity. To further explore the function of this portion of the EGF receptor, a consensus site for protein palmitoylation was inserted at the beginning of the juxtamembrane domain of the receptor. The altered EGF receptor incorporated [(3)H]palmitate, demonstrating that it was palmitoylated. Compared to the wild-type EGF receptor, the palmitoylated EGF receptor was significantly impaired in EGF-stimulated receptor autophosphorylation as well as ligand-induced receptor internalization. While both the wild-type and the palmitoylated EGF receptors exhibited a similar propensity to associate with lipid rafts, only the wild-type receptor exited lipid rafts in response to EGF. Binding of [(125)I]EGF to the wild-type EGF receptor showed a curvilinear Scatchard plot with both high- and low-affinity forms of the receptor. By contrast, the palmitoylated receptor exhibited only low-affinity EGF binding. These data suggest that the cytoplasmic juxtamembrane domain is involved not only in the transmission of the proliferative signal generated by ligand binding but also in facilitating the adoption of the high-affinity conformation by the extracellular ligand binding domain.

SplicerAV: a tool for mining microarray expression data for changes in RNA processing

Background

Over the past two decades more than fifty thousand unique clinical and biological samples have been assayed using the Affymetrix HG-U133 and HG-U95 GeneChip microarray platforms. This substantial repository has been used extensively to characterize changes in gene expression between biological samples, but has not been previously mined en masse for changes in mRNA processing. We explored the possibility of using HG-U133 microarray data to identify changes in alternative mRNA processing in several available archival datasets.

Results

Data from these and other gene expression microarrays can now be mined for changes in transcript isoform abundance using a program described here, SplicerAV. Using in vivo and in vitro breast cancer microarray datasets, SplicerAV was able to perform both gene and isoform specific expression profiling within the same microarray dataset. Our reanalysis of Affymetrix U133 plus 2.0 data generated by in vitro over-expression of HRAS, E2F3, beta-catenin (CTNNB1), SRC, and MYC identified several hundred oncogene-induced mRNA isoform changes, one of which recognized a previously unknown mechanism of EGFR family activation. Using clinical data, SplicerAV predicted 241 isoform changes between low and high grade breast tumors; with changes enriched among genes coding for guanyl-nucleotide exchange factors, metalloprotease inhibitors, and mRNA processing factors. Isoform changes in 15 genes were associated with aggressive cancer across the three breast cancer datasets.

Conclusions

Using SplicerAV, we identified several hundred previously uncharacterized isoform changes induced by in vitro oncogene over-expression and revealed a previously unknown mechanism of EGFR activation in human mammary epithelial cells. We analyzed Affymetrix GeneChip data from over 400 human breast tumors in three independent studies, making this the largest clinical dataset analyzed for en masse changes in alternative mRNA processing. The capacity to detect RNA isoform changes in archival microarray data using SplicerAV allowed us to carry out the first analysis of isoform specific mRNA changes directly associated with cancer survival.

Drosophila pico and its mammalian ortholog lamellipodin activate serum response factor and promote cell proliferation

MIG-10/RIAM/lamellipodin (MRL) proteins link activated Ras-GTPases with actin regulatory Ena/VASP proteins to induce local changes in cytoskeletal dynamics and cell motility. MRL proteins alter monomeric (G):filamentous (F) actin ratios, but the impact of these changes had not been fully appreciated. We report here that the Drosophila MRL ortholog, pico, is required for tissue and organismal growth. Reduction in pico levels resulted in reduced cell division rates, growth retardation, increased G:F actin ratios and lethality. Conversely, pico overexpression reduced G:F actin ratios and promoted tissue overgrowth in an epidermal growth factor (EGF) receptor (EGFR)-dependent manner. Consistently, in HeLa cells, lamellipodin was required for EGF-induced proliferation. We show that pico and lamellipodin share the ability to activate serum response factor (SRF), a transcription factor that responds to reduced G:F-actin ratios via its co-factor Mal. Genetics data indicate that mal/SRF levels are important for pico-mediated tissue growth. We propose that MRL proteins link EGFR activation to mitogenic SRF signaling via changes in actin dynamics.

Heterogeneity in EGF-binding affinities arises from negative cooperativity in an aggregating system

Scatchard analysis of the binding of EGF to its receptor yields concave up plots that indicate the presence of two classes of binding sites. However, how two independent classes of sites arise from the expression of a single EGF receptor protein has never been adequately explained. Using a new analytical approach involving the simultaneous fitting of binding isotherms from cells expressing increasing levels of EGF receptors, we show that (125)I-EGF-binding data can be completely explained by a model involving negative cooperativity in an aggregating system. This approach provides an experimentally determined value for the monomer-dimer equilibrium constant, which, for wild-type EGF receptors, corresponds to approximately 50,000 receptors per cell. Therefore, changes in receptor expression within the physiological range can modulate the outcome of a signaling stimulus. Analysis of the L680N-EGF receptor mutant, in which the formation of asymmetric kinase domain dimers is blocked, indicates that the kinase dimers make a substantial energetic contribution to the ligand-independent association of EGF receptor monomers, but are not necessary for negative cooperativity. The model accurately predicts the behavior of receptor mutants, such as the dimerization-defective Y246D-EGF receptor, which exhibit a single class of binding sites and provides a framework for understanding secondary dimer formation and lateral signaling in the EGF receptor family.

On the nature of low- and high-affinity EGF receptors on living cells

The small subpopulation of high-affinity EGF receptors (EGFRs) on living cells revealed by Scatchard analysis of (125)I-EGF binding results was discovered nearly three decades ago, yet not much is known about the underlying mechanism. After the determination of the structure of different forms of EGFR extracellular domain it was proposed that the monomeric tethered configuration corresponds to the majority of low-affinity receptors, whereas the extended dimeric configuration corresponds to the minority of the high-affinity class of EGFRs. Mathematical modeling of EGF-binding experiments to different conformational mutants of EGFR has shown that the high-affinity class of EGFR on living cells does not correspond to the extended configuration of EGFR and can only be accounted for by including in the mathematical model an additional binding event that is attributed to the dynamic nature of EGFR on living cells. To circumvent this problem we have performed similar experiments in the background of an EGFR mutant that does not form high-affinity sites. Quantitative analysis and mathematical modeling of these data show that release of the intramolecular tether causes a 2-fold increase in EGF-binding affinity, whereas elimination of the dimerization arm reduces EGF-binding affinity by approximately 6-fold. These experiments confirm the salient features of the structural model for EGFR regulation and argue further that the intramolecular tether provides only limited autoinhibitory control of EGFR activity and that the low-affinity class of EGF-binding sites on living cells reflects interconverting, tethered, and extended receptor configurations.

Suppression of ovarian cancer cell tumorigenicity and evasion of Cisplatin resistance using a truncated epidermal growth factor receptor in a rat model

The overexpression of the epidermal growth factor receptor (EGFR) is associated with a poor prognosis in ovarian cancer. The dominant-negative EGFR (EGFR-DNR) is a truncated receptor that lacks the tyrosine kinase domain and is devoid of signaling capability. This study tested the effects of a EGFR-DNR approach in ovarian cancer cells. NuTu-19, a rat ovarian cancer cell line was rendered resistant to cisplatin. Both NuTu-19 and resistant cells were infected with a retroviral vector containing the EGFR-DNR. NuTu-19 and NuTu-DNR (NuTu-19 cells expressing the EGFR-DNR) were injected into Fisher 344 immunocompetent rats. Western blot analyses were used to assess signal transduction pathways. All rats injected with NuTu-DNR cells remained healthy following tumor injection. In contrast, 100% of the rats injected with the NuTu-19 and NuTu-Sham (NuTu-19 cells expressing an empty vector) died of disease progression at the end of 15 weeks (P = 0.00009). On Western blot analysis, both NuTu-19 and NuTu-Sham cells showed a strong activation of mitogen-activated protein kinase (MAPK) after exposure to EGF. Cisplatin-resistant cell lines showed an enhanced EGF stimulatory effect via the MAPK pathway compared with parental cells. The EGFR-DNR significantly reduced the ability of EGF to induce cell signaling through the MAPK pathway. Lastly, the EGFR-DNR can partially reverse cisplatin resistance in drug-resistant cells. The EGFR-DNR approach suggests that EGFR confers a growth advantage to NuTu-19 cells in vivo. Thus, EGFR blockade may ultimately prove to be a useful therapeutic tool in the treatment of cisplatin-sensitive and cisplatin-resistant ovarian cancers.

Wound-healing studies in transgenic and knockout mice

Injury to the skin initiates a cascade of events including inflammation, new tissue formation, and tissue remodeling, that finally lead to at least partial reconstruction of the original tissue. Historically, animal models of repair have taught us much about how this repair process is orchestrated and, over recent years, the use of genetically modified mice has helped define the roles of many key molecules. Aside from conventional knockout technology, many ingenious approaches have been adopted, allowing researchers to circumvent such problems as embryonic lethality, or to affect gene function in a tissue- or temporal-specific manner. Together, these studies provide us with a growing source of information describing, to date, the in vivo function of nearly 100 proteins in the context of wound repair. This article focuses on the studies in which genetically modified mouse models have helped elucidate the roles that many soluble mediators play during wound repair, encompassing the fibroblast growth factor (FGF) and transforming growth factor-beta (TGF-beta) families and also data on cytokines and chemokines. Finally, we include a table summarizing all of the currently published data in this rapidly growing field. For a regularly updated web archive of studies, we have constructed a Compendium of Published Wound Healing Studies on Genetically Modified Mice which is avaialble at http://icbxs.ethz.ch/members/grose/woundtransgenic/home.html.

Adhesion-mediated squamous cell carcinoma survival through ligand-independent activation of epidermal growth factor receptor

The survival and growth of squamous epithelial cells require signals generated by integrin-matrix interactions. After conversion to squamous cell carcinoma, the cells remain sensitive to detachment-induced anoikis, yet in tumor cell aggregates, which are matrix-deficient, these cells are capable of suprabasal survival and proliferation. Their survival is enhanced through a process we call synoikis, whereby junctional adhesions between neighboring cells generate specific downstream survival signals. Here we show that in squamous cell carcinoma cells, E-cadherin-mediated cell-cell contacts specifically induce activation of epidermal growth factor receptor (EGFR). EGFR activation in turn triggers the ERK/MAPK signaling module, leading to elevation of anti-apoptotic Bcl-2. After intercellular adhesion, formation of adherens junctions triggers the formation of E-cadherin-EGFR complexes, correlating with EGFR transactivation. Analysis of the process with a dominant-negative EGFR mutant indicated that activation of EGFR is ligand-independent. Our data implicate cell-cell adhesion-induced activation of EGFR as a cooperative mechanism that generates compensatory survival signaling, protecting malignant cells from detachment-induced death.

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

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

ERBB receptor tyrosine kinases and cellular radiation responses

Ionizing radiation induces in autocrine growth-regulated carcinoma and malignant glioma cells powerful cytoprotective responses that confer relative resistance to consecutive radiation exposures. Understanding the mechanisms of these responses should provide new molecular targets for tumor radiosensitization. ERBB and other receptor Tyr kinases have been identified as immediate early response gene products that are activated by radiation within minutes, as by their physiological growth factor ligands, and induce secondary stimulation of cytoplasmic protein kinase cascades. The simultaneous activation of all receptor Tyr kinases and nonreceptor Tyr kinases leads to complex cytoprotective responses including increased cell proliferation, reduced apoptosis and enhanced DNA repair. Since these responses contribute to cellular radioresistance, ERBB1, the most extensively studied ERBB receptor, is examined as a target for tumor cell radiosensitization. The three methods of ERBB1 inhibition include blockade of growth factor binding by monoclonal antibody against the ligand-binding domain, inhibition of the receptor Tyr kinase-mediating receptor activation, and overexpression of a dominant-negative epidermal growth factor receptor-CD533 that lacks the COOH-terminal 533 amino acids and forms nonfunctional heterodimeric complexes with wild-type receptors. All the three approaches enhance radiation toxicity in vitro and in vivo. The different mechanisms of inhibition have contributed to the understanding of cellular responses to radiation, vary in relative effectiveness and pose different challenges for translation.

Mistargeting hippocampal axons by expression of a truncated Eph receptor

Topographic mapping of axon terminals is a general principle of neural architecture that underlies the interconnections among many neural structures. The Eph family tyrosine kinase receptors and their ligands, the ephrins, have been implicated in the formation of topographic projection maps. We show that multiple Eph receptors and ligands are expressed in the hippocampus and its major subcortical projection target, the lateral septum, and that expression of a truncated Eph receptor in the mouse brain results in a pronounced alteration of the hippocamposeptal topographic map. Our observations provide strong support for a critical role of Eph family guidance factors in regulating ontogeny of hippocampal projections.

A screen for dominant mutations applied to components in the Drosophila EGF-R pathway

The Drosophila epidermal growth factor receptor (EGF-R) controls many critical cell fate choices throughout development. Several proteins collaborate to promote localized EGF-R activation, such as Star and Rhomboid (Rho), which act sequentially to ensure the maturation and processing of inactive membrane-bound EGF ligands. To gain insights into the mechanisms underlying Rho and Star function, we developed a mutagenesis scheme to isolate novel overexpression activity (NOVA) alleles. In the case of rho, we isolated a dominant neomorphic allele, which interferes with Notch signaling, as well as a dominant-negative allele, which produces RNA interference-like flip-back transcripts that reduce endogenous rho expression. We also obtained dominant-negative and neomorphic Star mutations, which have phenotypes similar to those of rho NOVA alleles, as well as dominant-negative Egf-r alleles. The isolation of dominant alleles in several different genes suggests that NOVA mutagenesis should be widely applicable and emerge as an effective tool for generating dominant mutations in genes of unknown function.

Expression of herstatin, an autoinhibitor of HER-2/neu, inhibits transactivation of HER-3 by HER-2 and blocks EGF activation of the EGF receptor

The four members of the EGF receptor family are capable of homomeric as well as heteromeric interactions. HER-2/neu (erbB-2) dominates as the preferred coreceptor that amplifies mitogenic signaling. An alternative HER-2/neu product, herstatin, consists of a segment of the ectodomain of p185HER-2 and an intron-encoded C-terminus. Recombinant herstatin was found to bind with nM affinity and inhibit p185HER-2. To further examine the impact on receptor activity, herstatin was expressed with various receptor tyrosine kinases. In CHO cells that overexpressed HER-2, herstatin caused a sevenfold inhibition of colony formation that corresponded to a reduction in the tyrosine phosphorylation of p185HER-2. Herstatin also prevented HER-2 mediated transactivation of the kinase impaired HER-3 as reflected in transphosphorylation of HER-3 and heteromers between HER-2 and HER-3. In EGF receptor-overexpressing cells, EGF induction of receptor dimerization and tyrosine phosphorylation were reduced more than 90%, and receptor down-regulation as well as colony formation were also suppressed by coexpression with herstatin. Inhibition was selective for the EGF receptor family since herstatin expression did not reduce tyrosine phosphorylation mediated by the FGF receptor-2 or by insulin-like growth factor -1. Herstatin bound to the EGF receptor as well as to p185HER-2 in pull-down assays suggesting that complex formation may be involved in receptor inhibition. Our findings indicate that herstatin has the capability to negatively regulate combinations of interactions between group I receptor tyrosine kinases that confer synergistic growth signals.

Dominant-negative activity of an alpha(1B)-adrenergic receptor signal-inactivating point mutation

alpha(1)-adrenergic receptors (alpha(1)-ARs) are members of the G-protein-coupled receptor (GPCR) superfamily and activate inositol phosphate (IP) turnover. We show that glycine and asparagine mutations of Phe303 in transmembrane segment VI (TMVI) of the alpha(1B)-AR, a highly conserved residue in GPCRs, although increasing agonist affinity, abolish agonist-activated IP signalling. Co-expression of the Phe303 mutants also inhibited (-)epinephrine-stimulated IP signalling by wild-type alpha(1B)-AR and other G(q)-coupled receptors, as well as IP signalling mediated by AlF(4)(-) stimulation of both wild-type G(q alpha) and a constitutively active mutant. The inability of the Phe303 mutants to signal is due to induction of a receptor conformation that dissociates G-protein binding from activation. As a result, the Phe303 mutants sequester G(q alpha) and stoichiometrically inhibit Gq signalling in a dominant-negative manner. We further show that both the enhanced basal and agonist-stimulated IP-signalling activity of the constitutively active alpha(1B)-AR mutants, C128F and A293E, are inhibited in the double mutants, C128F/F303G and A293E/F303G. Phe303, therefore, appears to be critically involved in coupling TMVI alpha-helical movement, a key step in receptor activation, to activation of the cognate G-protein.

Targeted expression of a dominant-negative EGF-R in the kidney reduces tubulo-interstitial lesions after renal injury

The role of EGF in the evolution of renal lesions after injury is still controversial. To determine whether the EGF expression is beneficial or detrimental, we generated transgenic mice expressing a COOH-terminal-truncated EGF-R under the control of the kidney-specific type 1 gamma-glutamyl transpeptidase promoter. As expected, the transgene was expressed exclusively at the basolateral membrane of proximal tubular cells. Under basal conditions, transgenic mice showed normal renal morphology and function. Infusion of EGF to transgenic animals revealed that the mutant receptor behaved in a dominant-negative manner and prevented EGF-signaled EGF-R autophosphorylation. We next evaluated the impact of transgene expression on the development of renal lesions in two models of renal injury. After 75% reduction of renal mass, tubular dilations were less severe in transgenic mice than in wild-type animals. After prolonged renal ischemia, tubular atrophy and interstitial fibrosis were reduced in transgenic mice as compared with wild-type mice. The beneficial effect of the transgene included a reduction of tubular cell proliferation, interstitial collagen accumulation, and mononuclear cell infiltration. In conclusion, functional inactivation of the EGF-R in renal proximal tubular cells reduced tubulo-interstitial lesions after renal injury. These data suggest that blocking the EGF pathway may be a therapeutic strategy to reduce the progression of chronic renal failure.

Gene therapy for carcinoma of the breast: Genetic ablation strategies

The gene therapy strategy of mutation compensation is designed to rectify the molecular lesions that are etiologic for neoplastic transformation. For dominant oncogenes, such approaches involve the functional knockout of the dysregulated cellular control pathways provoked by the overexpressed oncoprotein. On this basis, molecular interventions may be targeted to the transcriptional level of expression, via antisense or ribozymes, or post-transcriptionally, via intracellular single chain antibodies (intrabodies). For carcinoma of the breast, these approaches have been applied in the context of the disease linked oncogenes erbB-2 and cyclin D1, as well as the estrogen receptor. Neoplastic revision accomplished in modal systems has rationalized human trials on this basis.

Dominant negative EGFR-CD533 and inhibition of MAPK modify JNK1 activation and enhance radiation toxicity of human mammary carcinoma cells

Exposure of MDA-MB-231 human mammary carcinoma cells to an ionizing radiation dose of 2 Gy results in immediate activation and Tyr phosphorylation of the epidermal growth factor receptor (EGFR). Doxycycline induced expression of a dominant negative EGFR-CD533 mutant, lacking the COOH-terminal 533 amino acids, in MDA-TR15-EGFR-CD533 cells was used to characterize intracellular signaling responses following irradiation. Within 10 min, radiation exposure caused an immediate, transient activation of mitogen activated protein kinase (MAPK) which was completely blocked by expression of EGFR-CD533. The same radiation treatment also induced an immediate activation of the c-Jun-NH2-terminal kinase 1 (JNK1) pathway that was followed by an extended rise in kinase activity after 30 min. Expression of EGFR-CD533 did not block the immediate JNK1 response but completely inhibited the later activation. Treatment of MDA-TR15-EGFR-CD533 cells with the MEK1/2 inhibitor, PD98059, resulted in approximately 70% inhibition of radiation-induced MAPK activity, and potentiated the radiation-induced increase of immediate JNK1 activation twofold. Inhibition of Ras farnesylation with a concomitant inhibition of Ras function completely blocked radiation-induced MAPK and JNK1 activation. Modulation of EGFR and MAPK functions also altered overall cellular responses of growth and apoptosis. Induction of EGFR-CD533 or treatment with PD98059 caused a 3-5-fold increase in radiation toxicity in a novel repeated radiation exposure growth assay by interfering with cell proliferation and potentiating apoptosis. In summary, this data demonstrates that both MAPK and JNK1 activation in response to radiation occur through EGFR-dependent and -independent mechanisms, and are mediated by signaling through Ras. Furthermore, we have demonstrated that radiation-induced activation of EGFR results in downstream activation of MAPK which may affect the radiosensitivity of carcinoma cells.

Fas gene mutation in the progression of adult T cell leukemia

Fas antigen (Apo-1/CD95) is an apoptosis-signaling cell surface receptor belonging to the tumor necrosis factor receptor superfamily. Adult T cell leukemia (ATL) cells express Fas antigen and show apoptosis after treatment with an anti-Fas monoclonal antibody. We established the ATL cell line KOB, which showed resistance to Fas-mediated apoptosis, and found that KOB expressed two forms of Fas mRNA, the normal form and a truncated form. The truncated transcript lacked 20 base pairs at exon 9, resulting in a frame shift and the generation of a premature stop codon at amino acid 239. The same mutation was detected in primary ascitic cells and peripheral blood cells. The mutation was not detected in lymph node cells, however, although all of the primary ATL cells were of the same clonal origin. A retroviral-mediated gene transfer of the truncated Fas to Jurkat cells rendered the cells resistant to Fas-mediated apoptosis, suggesting a dominant negative interference mechanism. These results indicate that an ATL subclone acquires a Fas mutation in the lymph nodes, enabling the subclone to escape from apoptosis mediated by the Fas/Fas ligand system and proliferate in the body. Mutation of the Fas gene may be one of the mechanisms underlying the progression of ATL.

Blockade of type beta transforming growth factor signaling prevents liver fibrosis and dysfunction in the rat

We eliminated type beta transforming growth factor (TGF-beta) signaling by adenovirus-mediated local expression of a dominant-negative type II TGF-beta receptor (AdCATbeta-TR) in the liver of rats treated with dimethylnitrosamine, a model of persistent liver fibrosis. In rats that received a single application of AdCATbeta-TR via the portal vein, liver fibrosis as assessed by histology and hydroxyproline content was markedly attenuated. All AdCATbeta-TR-treated rats remained alive, and their serum levels of hyaluronic acid and transaminases remained at low levels, whereas all the AdCATbeta-TR-untreated rats died of liver dysfunction. The results demonstrate that TGF-beta does play a central role in liver fibrogenesis and indicate clearly in a persistent fibrosis model that prevention of fibrosis by anti-TGF-beta intervention could be therapeutically useful.

Dominant-negative mutations in the G-protein-coupled alpha-factor receptor map to the extracellular ends of the transmembrane segments

G-protein-coupled receptors (GPCRs) transduce the signals for a wide range of hormonal and sensory stimuli by activating a heterotrimeric guanine nucleotide-binding protein (G protein). The analysis of loss-of-function and constitutively active receptor mutants has helped to reveal the functional properties of GPCRs and their role in human diseases. Here we describe the identification of a new class of mutants, dominant-negative mutants, for the yeast G-protein-coupled alpha-factor receptor (Ste2p). Sixteen dominant-negative receptor mutants were isolated based on their ability to inhibit the response to mating pheromone in cells that also express wild-type receptors. Detailed analysis of two of the strongest mutant receptors showed that, unlike other GPCR interfering mutants, they were properly localized at the plasma membrane and did not alter the stability or localization of wild-type receptors. Furthermore, their dominant-negative effect was inversely proportional to the relative amount of wild-type receptors and was reversed by overexpressing the G-protein subunits, suggesting that these mutants compete with the wild-type receptors for the G protein. Interestingly, the dominant-negative mutations are all located at the extracellular ends of the transmembrane segments, defining a novel region of the receptor that is important for receptor signaling. Altogether, our results identify residues of the alpha-factor receptor specifically involved in ligand binding and receptor activation and define a new mechanism by which GPCRs can be inactivated that has important implications for the evaluation of receptor mutations in other G-protein-coupled receptors.

Ligand-independent activation of platelet-derived growth factor receptor is a necessary intermediate in lysophosphatidic, acid-stimulated mitogenic activity in L cells

Growth factor-derived mitogenic signals from the cell surface are transmitted to the nucleus via receptor tyrosine kinases (RTKs), the adaptor proteins Shc and Grb2, and a Ras-dependent protein kinase cascade that activates the extracellular signal regulated kinase (ERK) subfamily of mitogen-activated protein kinases. ERKs also are activated by hormones that stimulate G protein-coupled receptors (GPCRs). We report here that, in agreement with previous data, the epidermal growth factor receptor (EGFR) is a signaling intermediate in ERK activation by GPCRs. Of import, we show that cross-talk between two classes of surface receptors, RTKs and GPCRs, is a general feature. Lysophosphatidic acid not only induces ligand-independent tyrosine autophosphorylation of EGFR but also of platelet-derived growth factor beta receptor (PDGF-beta-R) as shown by detection of tyrosine phosphorylation and by the use of specific inhibitors of RTKs. The cross-talk appears to be cell type-specific: In L cells that lack EGFR, lysophosphatidic acid-induced Shc and ERK activation is prevented completely by specific inhibition of PDGFR, whereas in COS-7 cells expressing only EGFR, the pathway via EGFR is chosen. In Rat-1 cells, however, that express both EGFR and PDGFR, the EGFR pathway dominates.

Disruption of cellular translational control by a viral truncated eukaryotic translation initiation factor 2alpha kinase homolog

Phosphorylation of eukaryotic translation initiation factor 2alpha (eIF2alpha) is a common cellular mechanism to limit protein synthesis in stress conditions. Baculovirus PK2, which resembles the C-terminal half of a protein kinase domain, was found to inhibit both human and yeast eIF2alpha kinases. Insect cells infected with wild-type, but not pk2-deleted, baculovirus exhibited reduced eIF2alpha phosphorylation and increased translational activity. The negative regulatory effect of human protein kinase RNA-regulated (PKR), an eIF2alpha kinase, on virus production was counteracted by PK2, indicating that baculoviruses have evolved a unique strategy for disrupting a host stress response. PK2 was found in complex with PKR and blocked kinase autophosphorylation in vivo, suggesting a mechanism of kinase inhibition mediated by interaction between truncated and intact kinase domains.

Signal characteristics of G protein-transactivated EGF receptor

The epidermal growth factor receptor (EGFR) tyrosine kinase recently was identified as providing a link to mitogen-activated protein kinase (MAPK) in response to G protein-coupled receptor (GPCR) agonists in Rat-1 fibroblasts. This cross-talk pathway is also established in other cell types such as HaCaT keratinocytes, primary mouse astrocytes and COS-7 cells. Transient expression of either Gq- or Gi-coupled receptors in COS-7 cells allowed GPCR agonist-induced EGFR transactivation, and lysophosphatidic acid (LPA)-generated signals involved the docking protein Gab1. The increase in SHC tyrosine phosphorylation and MAPK stimulation through both Gq- and Gi-coupled receptors was reduced strongly upon selective inhibition of EGFR function. Inhibition of phosphoinositide 3-kinase did not affect GPCR-induced stimulation of EGFR tyrosine phosphorylation, but inhibited MAPK stimulation, upon treatment with both GPCR agonists and low doses of EGF. Furthermore, the Src tyrosine kinase inhibitor PP1 strongly interfered with LPA- and EGF-induced tyrosine phosphorylation and MAPK activation downstream of EGFR. Our results demonstrate an essential role for EGFR function in signaling through both Gq- and Gi-coupled receptors and provide novel insights into signal transmission downstream of EGFR for efficient activation of the Ras/MAPK pathway.

A role for fibroblast growth factor signaling in the lobuloalveolar development of the mammary gland

The inappropriate expression of growth factors, or activating mutations of their receptors, have been implicated as causative factors in mouse and human mammary cancer. For example, it has been known for some time that three members of the fibroblast growth factor (FGF) family behave like oncogenes in virally induced mammary cancer of mice. In normal circumstances, signaling via FGF receptors is known to mediate growth, differentiation, and patterning, during embryogenesis and fetal development. A powerful approach to dissecting the roles for these signaling pathways is to determine the developmental consequences of abrogating their function in transgenic mice. In this review, we describe the use of dominant negative FGF receptors to evaluate the contribution of specific FGF signals in normal mammary gland development. These studies have revealed that normal lobuloalveolar development requires FGF signaling to the mammary epithelium, a function that is presumably usurped by MMTV in mouse mammary tumorigenesis.

Loss of functional cell surface transforming growth factor beta (TGF-beta) type 1 receptor correlates with insensitivity to TGF-beta in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is the most common form of adult leukemia in Western countries, and there is significant variability in survival within CLL clinical stages. Earlier studies showed that CLL cells produce and are usually growth inhibited by transforming growth factor beta type 1 (TGF-beta1), suggesting a mechanism for the clinically indolent course of most CLL. Here we studied the mechanism by which CLL cells from about one-third of the patients are insensitive to TGF-beta1. Of the 13 patients studied, CLL cells isolated from the peripheral blood of 8 patients were sensitive to growth inhibition by TGF-beta1, as determined by incorporation of tritiated thymidine, whereas those from 5 patients were completely resistant to TGF-beta1. As judged by binding of radiolabeled TGF-beta1 followed by cross-linking and immunoprecipitation with anti-receptor antisera, CLL cells sensitive to TGF-beta1 exhibited normal cell surface expression of both types 1 and 2 TGF-beta receptors. In contrast, all CLL cells resistant to TGF-beta1 exhibited no detectable surface type I receptors able to bind TGF-beta1, but normal expression of type II receptors. Both TGF-beta1-sensitive and TGF-beta1-resistant CLL cells contained normal amounts of both type 1 and type 2 receptor mRNAs. Specific loss of type 1 receptor expression represents a new mechanism by which cells acquire resistance to TGF-beta1-mediated growth inhibition in the development and progression of human lymphoproliferative malignancies.

Trans receptor inhibition of human glioblastoma cells by erbB family ectodomains

Our aim has been to understand the features of erbB receptor homo- and heterodimer assembly to develop approaches to disrupt receptor activation. We have developed a general approach to cause erbB receptor-specific trans inhibition of human neoplasia. The clonal progression of human astrocytomas to a more malignant phenotype often involves the amplification and overexpression of the epidermal growth factor receptor (EGFr) gene. We have selectively targeted the EGFr in human glioblastoma cells with kinase-deficient mutants of the erbB family derived from the ectodomain of the Neu oncogene that are able to form heterodimers with EGFr and inhibit EGFr-dependent phenotypes. In EGFr-positive U87MG human glioblastoma cells, expression of the Neu ectodomain inhibits EGF-, but not platelet-derived growth factor-, induced DNA synthesis; inhibits cell proliferation in the presence of EGF, but not platelet-derived growth factor; inhibits the ability of U87MG to form colonies in soft agar; and inhibits transforming efficiency in athymic mice. These studies establish that EGFr-mediated signal transduction is important in the maintenance of malignant glioma, and that trans receptor inhibition is a novel way to abrogate abnormal growth of these tumors. Neu ectodomains will be useful in determining the manner in which the EGFr contributes to glial tumorigenesis and in the design of pharmaceuticals that disable erbB family oncoproteins. In addition, these studies provide a rationale for the application of the Neu ectodomain in gene therapy approaches to human malignant glioma and, potentially, to other systemic epithelial malignancies expressing erbB family receptors.

Glycosylation is critical for natriuretic peptide receptor-B function

Co-transfection of a truncated natriuretic peptide receptor-B (NPR-B) with the full length receptor results in a decrease of 60-80% in wild-type receptor activity. This reduction correlates with a loss of glycosylation of the full length NPR-B. This effect is dose-dependent, and occurs with no change in the glycosylation of the truncated receptor. Co-transfection of the full length NPR-B with other receptors yields similar results. These data suggest that glycosylation may be crucial for NPR-B function. Cross-linking studies further demonstrate that only fully glycosylated NPR-B receptors are able to bind ligand. Our data therefore argue that carbohydrate modification may be critical for NPR-B receptor ligand binding.

A dominant inhibitory mutant of the type II transforming growth factor beta receptor in the malignant progression of a cutaneous T-cell lymphoma

In many cancers, inactivating mutations in both alleles of the transforming growth factor beta (TGF-beta) type 11 receptor (TbetaRII) gene occur and correlate with loss of sensitivity to TGF-beta. Here we describe a novel mechanism for loss of sensitivity to growth inhibition by TGF-beta in tumor development. Mac-1 cells, isolated from the blood of a patient with an indolent form of cutaneous T-cell lymphoma, express wild-type TbetaRII and are sensitive to TGF-beta. Mac-2A cells, clonally related to Mac-1 and isolated from a skin nodule of the same patient at a later, clinically aggressive stage of lymphoma, are resistant to TGF-beta. They express both the wild-type TbetaRII and a receptor with a single point mutation (Asp-404-Gly [D404G]) in the kinase domain (D404G-->TbetaRII); no TbetaRI or TbetaRII is found on the plasma membrane, suggesting that D404G-TbetaRII dominantly inhibits the function of the wild-type receptor by inhibiting its appearance on the plasma membrane. Indeed, inducible expression, under control of a tetracycline-regulated promoter, of D404G-TbetaRII in TGF-beta- sensitive Mac-1 cells as well as in Hep3B hepatoma cells results in resistance to TGF-beta and disappearance of cell surface TbetaRI and TbetaRII. Overexpression of wild-type TbetaRII in Mac-2A cells restores cell surface TbetaRI and TbetaRH and sensitivity to TGF-beta. The ability of the D404G-TbetaRH to dominantly inhibit function of wild-type TGF-beta receptors represents a new mechanism for loss of sensitivity to the growth-inhibitory functions of TGF-beta in tumor development.

Biological activity of the receptor for macrophage colony-stimulating factor in the human endometrial cancer cell line, Ishikawa

Previously we found that the Ishikawa endometrial cancer cell line expresses macrophage colony-stimulating factor (M-CSF) and c-fms transcripts and that its proliferation is enhanced by the addition of recombinant M-CSF. This suggested that Ishikawa cells are constitutively stimulated by M-CSF. In support of this we now show that Ishikawa cells secrete M-CSF and that known stimulators of M-CSF production increase the amount detected in Ishikawa cell conditioned medium. Using retroviral infections to introduce and express exogenous c-fms genes in Ishikawa cells we also demonstrate proliferation to be partially inhibited by a dominant negative, mutant c-fms gene, yet enhanced approximately 3-fold by a normal c-fms gene, under conditions in which the only source of M-CSF was that produced by the cells. The data provide evidence for the existence of an active M-CSF/receptor loop in these endometrial cancer cells and suggests the possibility of such activity in tumours of the endometrium and ovary that aberrantly express M-CSF and fms genes.

Stimulation of proliferation of a human osteosarcoma cell line by exogenous acidic fibroblast growth factor requires both activation of receptor tyrosine kinase and growth factor internalization

U2OS Dr1 cells, originating from a human osteosarcoma, are resistant to the intracellular action of diphtheria toxin but contain toxin receptors on their surfaces. These cells do not have detectable amounts of fibroblast growth factor receptors. When these cells were transfected with fibroblast growth factor receptor 4, the addition of acidic fibroblast growth factor to the medium induced tyrosine phosphorylation, DNA synthesis, and cell proliferation. A considerable fraction of the cell-associated growth factor was found in the nuclear fraction. When the growth factor was fused to the diphtheria toxin A fragment, it was still bound to the growth factor receptor and induced tyrosine phosphorylation but did not induce DNA synthesis or cell proliferation, nor was any fusion protein recovered in the nuclear fraction. On the other hand, when the fusion protein was associated with the diphtheria toxin B fragment to allow translocation to the cytosol by the toxin pathway, the fusion protein was targeted to the nucleus and stimulated both DNA synthesis and cell proliferation. In untransfected cells containing toxin receptors but not fibroblast growth factor receptors, the fusion protein was translocated to the cytosol and targeted to the nucleus, but in this case, it stimulated only DNA synthesis. These data indicate that the following two signals are required to stimulate cell proliferation in transfected U2OS Dr1 cells: the tyrosine kinase signal from the activated fibroblast growth factor receptor and translocation of the growth factor into the cell.

Modulation of tumor cell gene expression and phenotype by the organ-specific metastatic environment

The mechanistic basis of a metastatic cell's ability to proliferate in the parenchyma of certain organs and develop organ-specific metastases is under intense investigation. Signals from paracrine or autocrine pathways, alone or in combination, may regulate tumor cell proliferation with the eventual outcome dependent on the net balance of stimulatory and inhibitory factors. This article summarizes recent reports from our laboratory and others demonstrating that the organ microenvironment can profoundly influence the pattern of gene expression and the biological phenotype of metastatic tumor cells, including induction of melanocyte stimulating hormone receptor and production of melanin, regulation of terminal differentiation and apoptosis, resistance to chemotherapy, and regulation of growth at the organ-specific metastatic site. These recent data from both murine and human tumor models support the concept that the microenvironment of different organs can influence the pattern of gene expression and hence the phenotype of tumor cells at different steps of the metastatic process. These findings have obvious implications for the therapy of neoplasms in general and metastases in particular.

Heterodimerization and functional interaction between EGF receptor family members: a new signaling paradigm with implications for breast cancer research

The EGF receptor (EGFR) and HER2 are members of a growth factor receptor family. Overexpression of either protein in advanced breast cancer correlates with poor prognosis. EGF stimulates growth by binding to EGFR, activating the receptor's intracellular tyrosine kinase. The initial consequence is phosphorylation of specific tyrosine-containing sequences in the receptor's carboxyl terminus. These phosphotyrosines serves as high affinity recognition sites for proteins that, in turn, transmit the growth signal inside the cell. Mechanistic studies suggest that EGF binds to a single EGFR, triggering dimerization with another like receptor molecule. This dimerization is thought to initiate the tyrosine kinase activation. The EGF receptor family was recently expanded with the sequencing of HER3 and HER4. Each of the four family members was postulated to regulate a unique growth or differentiation signaling repertoire when activated by a receptor-specific ligand. However, new data from numerous laboratories suggest that EGFR family members may play a complex and ultimately more flexible role in signaling by forming heterodimers between family members, e.g. EGFR:HER2 or HER4:HER2. These heterodimers may form even when only one member of the pair binds its ligand. This review summarizes current work on heterodimerization and attempts to predict the consequences for downstream signaling. In brief, when compared to ligand-dependent receptor homodimers comprised of two proteins with the same internalization sequence and phosphorylated tyrosine residues, heterodimers are likely to: i) expand substrate selection and downstream signaling pathway activation; ii) promote interaction between sets of substrates in the mixed receptor complexes that would not ordinarily be physically juxtaposed; iii) alter the duration of receptor signaling by changing rates of receptor internalization, ligand loss, kinase inactivation, recycling, etc.; and iv) alter rates of receptor and substrate dephosphorylation. In addition to understanding interactions of heterodimers with the internalization machinery, identification of receptor-specific substrates and binding proteins for each EGFR family member will be necessary to explicate the role of heterodimers in growth and differentiation.

Cell transformation by fibroblast growth factors can be suppressed by truncated fibroblast growth factor receptors

Ligand-induced dimerization and transphosphorylation are thought to be important events by which receptor tyrosine kinases generate cellular signals. We have investigated the ability of signalling-defective, truncated fibroblast growth factor (FGF) receptors (FGFR-1 and FGFR-2) to block the FGF response in cells that express both types of endogenous FGF receptors. When these dominant negative receptors are expressed in NIH 3T3 cells transformed by the secreted FGF-4, the transformed properties of the cells can be reverted to various degrees, with better reversion phenotype correlating with higher levels of truncated receptor expression. Furthermore, truncated FGFR-2 is significantly more efficient at producing reversion than FGFR-1, indicating that FGF-4 preferentially utilizes the FGFR-2 signalling pathway. NIH 3T3 clones expressing these truncated receptors are more resistant to FGF-induced mitogenesis and also exhibit reduced tyrosine phosphorylation upon treatment with FGF. The block in FGF-signalling, however, can be overcome by the addition of excess growth factor. The truncated receptors have binding affinities that are four- to eightfold lower than those of wild-type receptors, as measured by Scatchard analysis. We also observed a partial specificity in the responses of truncated-receptor-expressing clones to FGF-2 or FGF-4. Our results suggest that the block to signal transduction produced by kinase-negative FGF receptors is achieved through a combination of dominant negative effects and competition for growth factor binding with functional receptors.

Cell transformation by fibroblast growth factors can be suppressed by truncated fibroblast growth factor receptors

Ligand-induced dimerization and transphosphorylation are thought to be important events by which receptor tyrosine kinases generate cellular signals. We have investigated the ability of signalling-defective, truncated fibroblast growth factor (FGF) receptors (FGFR-1 and FGFR-2) to block the FGF response in cells that express both types of endogenous FGF receptors. When these dominant negative receptors are expressed in NIH 3T3 cells transformed by the secreted FGF-4, the transformed properties of the cells can be reverted to various degrees, with better reversion phenotype correlating with higher levels of truncated receptor expression. Furthermore, truncated FGFR-2 is significantly more efficient at producing reversion than FGFR-1, indicating that FGF-4 preferentially utilizes the FGFR-2 signalling pathway. NIH 3T3 clones expressing these truncated receptors are more resistant to FGF-induced mitogenesis and also exhibit reduced tyrosine phosphorylation upon treatment with FGF. The block in FGF-signalling, however, can be overcome by the addition of excess growth factor. The truncated receptors have binding affinities that are four- to eightfold lower than those of wild-type receptors, as measured by Scatchard analysis. We also observed a partial specificity in the responses of truncated-receptor-expressing clones to FGF-2 or FGF-4. Our results suggest that the block to signal transduction produced by kinase-negative FGF receptors is achieved through a combination of dominant negative effects and competition for growth factor binding with functional receptors.

Activation and inhibition of erythropoietin receptor function: role of receptor dimerization

Members of the cytokine receptor superfamily have structurally similar extracellular ligand-binding domains yet diverse cytoplasmic regions lacking any obvious catalytic domains. Many of these receptors form ligand-induced oligomers which are likely to participate in transmembrane signaling. A constitutively active (factor-independent) mutant of the erythropoietin receptor (EPO-R), R129C in the exoplasmic domain, forms disulfide-linked homodimers, suggesting that the wild-type EPO-R is activated by ligand-induced homodimerization. Here, we have taken two approaches to probe the role EPO-R dimerization plays in signal transduction. First, on the basis of the crystal structure of the ligand-bound, homodimeric growth hormone receptor (GH-R) and sequence alignment between the GH-R and EPO-R, we identified residues of the EPO-R which may be involved in intersubunit contacts in an EPO-R homodimer. Residue 129 of the EPO-R corresponds to a residue localized to the GH-R dimer interface region. Alanine or cysteine substitutions were introduced at four other residues of the EPO-R predicted to be in the dimer interface region. Substitution of residue E-132 or E-133 with cysteine renders the EPO-R constitutively active. Like the arginine-to-cysteine mutation at position 129 in the exoplasmic domain (R129C), E132C and E133C form disulfide-linked homodimers, suggesting that constitutive activity is due to covalent dimerization. In the second approach, we have coexpressed the wild-type EPO-R with inactive mutants of the receptor missing all or part of the cytosolic domain. These truncated receptors have a dominant inhibitory effect on the proliferative action of the wild-type receptor. Taken together, these results strengthen the hypothesis that an initial step in EPO- and EPO-R-mediated signal transduction is ligand-induced receptor dimerization.

Activation and inhibition of erythropoietin receptor function: role of receptor dimerization

Members of the cytokine receptor superfamily have structurally similar extracellular ligand-binding domains yet diverse cytoplasmic regions lacking any obvious catalytic domains. Many of these receptors form ligand-induced oligomers which are likely to participate in transmembrane signaling. A constitutively active (factor-independent) mutant of the erythropoietin receptor (EPO-R), R129C in the exoplasmic domain, forms disulfide-linked homodimers, suggesting that the wild-type EPO-R is activated by ligand-induced homodimerization. Here, we have taken two approaches to probe the role EPO-R dimerization plays in signal transduction. First, on the basis of the crystal structure of the ligand-bound, homodimeric growth hormone receptor (GH-R) and sequence alignment between the GH-R and EPO-R, we identified residues of the EPO-R which may be involved in intersubunit contacts in an EPO-R homodimer. Residue 129 of the EPO-R corresponds to a residue localized to the GH-R dimer interface region. Alanine or cysteine substitutions were introduced at four other residues of the EPO-R predicted to be in the dimer interface region. Substitution of residue E-132 or E-133 with cysteine renders the EPO-R constitutively active. Like the arginine-to-cysteine mutation at position 129 in the exoplasmic domain (R129C), E132C and E133C form disulfide-linked homodimers, suggesting that constitutive activity is due to covalent dimerization. In the second approach, we have coexpressed the wild-type EPO-R with inactive mutants of the receptor missing all or part of the cytosolic domain. These truncated receptors have a dominant inhibitory effect on the proliferative action of the wild-type receptor. Taken together, these results strengthen the hypothesis that an initial step in EPO- and EPO-R-mediated signal transduction is ligand-induced receptor dimerization.

A dominant negative erythropoietin (EPO) receptor inhibits EPO-dependent growth and blocks F-gp55-dependent transformation

The erythropoietin receptor (EPO-R), a member of the cytokine receptor superfamily, can be activated to signal cell growth by binding either EPO or F-gp55, the Friend spleen focus-forming virus glycoprotein. Activation by F-gp55 results in constitutive EPO-R signalling and the first stage of Friend virus-induced erythroleukemia. We have generated a truncated form of the EPO-R polypeptide [EPO-R(T)] which lacks the critical cytoplasmic signal-transducing domain of the EPO-R required for EPO- or F-gp55-induced cell growth. EPO-R(T) specifically inhibited the EPO-dependent growth of EPO-R-expressing Ba/F3 cells without changing the interleukin-3-dependent growth of these cells. In addition, Ba/F3 cells that coexpressed wild-type EPO-R and EPO-R(T) were resistant to transformation by F-gp55 despite efficient expression of the F-gp55 transforming oncoprotein in infected cells. EPO-R(T) inhibited the EPO-dependent tyrosine phosphorylation of wild-type EPO-R, the tyrosine kinase (JAK2), and the SH2 adaptor protein (Shc). In conclusion, the EPO-R(T) polypeptide is a dominant negative polypeptide which specifically interferes with the early stages of EPO-R-mediated signal transduction and which prevents Friend virus transformation of erythroblasts.

A dominant negative erythropoietin (EPO) receptor inhibits EPO-dependent growth and blocks F-gp55-dependent transformation

The erythropoietin receptor (EPO-R), a member of the cytokine receptor superfamily, can be activated to signal cell growth by binding either EPO or F-gp55, the Friend spleen focus-forming virus glycoprotein. Activation by F-gp55 results in constitutive EPO-R signalling and the first stage of Friend virus-induced erythroleukemia. We have generated a truncated form of the EPO-R polypeptide [EPO-R(T)] which lacks the critical cytoplasmic signal-transducing domain of the EPO-R required for EPO- or F-gp55-induced cell growth. EPO-R(T) specifically inhibited the EPO-dependent growth of EPO-R-expressing Ba/F3 cells without changing the interleukin-3-dependent growth of these cells. In addition, Ba/F3 cells that coexpressed wild-type EPO-R and EPO-R(T) were resistant to transformation by F-gp55 despite efficient expression of the F-gp55 transforming oncoprotein in infected cells. EPO-R(T) inhibited the EPO-dependent tyrosine phosphorylation of wild-type EPO-R, the tyrosine kinase (JAK2), and the SH2 adaptor protein (Shc). In conclusion, the EPO-R(T) polypeptide is a dominant negative polypeptide which specifically interferes with the early stages of EPO-R-mediated signal transduction and which prevents Friend virus transformation of erythroblasts.

Ligand and p185c-neu density govern receptor interactions and tyrosine kinase activation

The neu protooncogene (also known as c-erbB2, NGL, and HER2) encodes a 185-kDa transmembrane glycoprotein with intrinsic tyrosine kinase activity that resembles the receptor for epidermal growth factor. The p185 gene and protein were originally identified in the brain and are thought to play a critical role in neurogenesis. Aberrant c-erbB2 protein overexpression also occurs in several human adenocarcinomas. A ligand for p185, neu-activating factor (NAF), specifically binds to neu receptor and increases the p185c-neu tyrosine phosphorylation in vitro and in vivo in a dose-dependent manner. We now show that NAF specifically binds to purified p185 expressed in baculovirus. Direct binding analysis showed that NAF binds with high affinity (Kd = 1.3 nM). We have investigated changes in the structure and association state of baculovirus-produced neu holoreceptor that are induced by ligand binding. In this study, we used sucrose gradients to show that purified p185c-neu exists mainly in the monomeric form at low concentrations, whereas at higher concentrations p185c-neu exists as dimers or multimers. At low concentrations, but in the presence of ligand, p185c-neu sediments as a dimeric or multimeric form. Monomer-oligomer interconversion is absolutely ligand dependent at low receptor concentrations. The high molecular weight form of the receptor is enzymatically more active, as a consequence of ligand-driven activation of the receptor kinase. Oncogenic p185neu receptors sediment predominantly as high molecular weight forms and have constitutively active kinases.

Heterodimerization of epidermal growth factor receptor and wild-type or kinase-deficient Neu: a mechanism of interreceptor kinase activation and transphosphorylation

We have shown that members of the erbB family undergo homodimer and heterodimer formation. The rat p185c-neu and the epidermal growth factor receptor (EGFR) can associate into an active heterodimeric tyrosine kinase. Overexpression of these two receptors also results in a transformed phenotype. We now show that mutant Neu proteins resulting from a point mutation at the ATP-binding site (N757) or cytoplasmic domain deletions (N691stop) are still able to undergo EGF-induced heterodimerization with EGFR. Analysis of heterodimer formation between EGFR and truncated Neu proteins revealed that heterodimerization is preferred over homodimerization of EGFR. N757 can be transphosphorylated by associated EGFR upon EGF stimulation. However, the heterodimer composed of EGFR and N691stop is kinase inactive. These results provided evidence that the Neu ectodomain is sufficient to associate with EGFR physically, and the cytoplasmic domain interaction is required for heterodimeric kinase activation, indicating that Neu/c-erbB2 is not just a simple substrate for EGFR but a transactivator as well.

A dominant negative mutant of 2-5A-dependent RNase suppresses antiproliferative and antiviral effects of interferon

2-5A-dependent RNase is the terminal factor in the interferon-regulated 2-5A system thought to function in both the molecular mechanism of interferon action and in the general control of RNA stability. However, direct evidence for specific functions of 2-5A-dependent RNase has been generally lacking. Therefore, we developed a strategy to block the 2-5A system using a truncated form of 2-5A-dependent RNase which retains 2-5A binding activity while lacking RNase activity. When the truncated RNase was stably expressed to high levels in murine cells, it prevented specific rRNA cleavage in response to 2-5A transfection and the cells were unresponsive to the antiviral activity of interferon alpha/beta for encephalomyocarditis virus. Remarkably, cells expressing the truncated RNase were also resistant to the antiproliferative activity of interferon. The truncated RNase is a dominant negative mutant that binds 2-5A and that may interfere with normal protein-protein interactions through nine ankyrin-like repeats.

Dominant negative mutations of the scavenger receptor. Native receptor inactivation by expression of truncated variants

The bovine scavenger receptor was truncated at amino acid 266 or 310 to delete either all or part, respectively, of the collagen-like domain. The truncated receptors were inactive in the binding and internalization of acetyl (Ac) low density lipoprotein (LDL). Coexpression of truncated receptor with the native receptor dramatically reduced the percentage of cells internalizing fluorescently labeled Ac LDL, compared with cells expressing the native receptor alone. The mutant truncated at amino acid 266 was most effective in receptor inactivation, resulting in a 42% or 80% decrease in the percentage of cells expressing active receptor when transfected in a 1:1 or 1:2 molar ratio (native:mutant), respectively, with native receptor. Degradation of 125I-Ac LDL was reduced up to 90% when the native and truncated mutant receptors were coexpressed. Scavenger receptor inhibition was specific because the activity of the LDL receptor was not altered. Transient transfection of the mouse macrophage cell line P388D1 with truncated scavenger receptor resulted in a 65% decrease in the uptake and degradation of Ac LDL but did not decrease the degradation of beta-migrating very low density lipoprotein, which is LDL receptor-mediated. These results demonstrate that expression of truncated bovine scavenger receptor inactivates both the native bovine and murine scavenger receptors, producing a dominant negative phenotype in vitro.

Control of fibroblast growth factor receptor kinase signal transduction by heterodimerization of combinatorial splice variants

A differentiated liver cell (HepG2), which exhibits a dose-dependent growth-stimulatory and growth-inhibitory response to heparin-binding fibroblast growth factor type 1 (FGF-1), displays high- and low-affinity receptor phenotypes and expresses specific combinatorial splice variants alpha 1, beta 1, and alpha 2 of the FGF receptor (FGF-R) gene (flg). The extracellular domains of the alpha and beta variants consist of three and two immunoglobulin loops, respectively, while the intracellular variants consist of a tyrosine kinase (type 1) isoform and a kinase-defective (type 2) isoform. The type 2 isoform is also devoid of the two major intracellular tyrosine autophosphorylation sites (Tyr-653 and Tyr-766) in the type 1 kinase. An analysis of ligand affinity, dimerization, autophosphorylation, and interaction with src homology region 2 (SH2) substrates of the recombinant alpha 1, beta 1, and alpha 2 isoforms was carried out to determine whether dimerization of the combinatorial splice variants might explain the dose-dependent opposite mitogenic effects of FGF. Scatchard analysis indicated that the alpha and beta isoforms exhibit low and high affinity for ligand, respectively. The three combinatorial splice variants dimerized in all combinations. FGF enhanced dimerization and kinase activity, as assessed by receptor autophosphorylation. Phosphopeptide analysis revealed that phosphorylation of Tyr-653 was reduced relative to phosphorylation of Tyr-766 in the type 1 kinase component of heterodimers of the type 1 and type 2 isoforms. The SH2 domain substrate, phospholipase C gamma 1 (PLC gamma 1), associated with the phosphorylated type 1-type 2 heterodimers but was phosphorylated only in preparations containing the type 1 kinase homodimer. The results suggest that phosphorylation of Tyr-653 within the kinase catalytic domain, but not Tyr-766 in the COOH-terminal domain, may be stringently dependent on a trans intermolecular mechanism within FGF-R kinase homodimers. Although phosphotyrosine 766 is sufficient for interaction of PLC gamma 1 and other SH2 substrates with the FGF-R kinase, phosphorylation and presumably activation of substrates require the kinase homodimer and phosphorylation of Tyr-653. We propose that complexes of phosphotyrosine 766 kinase monomers and SH2 domain signal transducers may constitute unactivated presignal complexes whose active or inactive fate depends on homodimerization with a kinase or heterodimerization with a kinase-defective monomer, respectively. The results suggest a mechanism for control of signal transduction by different concentrations of ligand through heterodimerization of combinatorial splice variants from the same receptor gene.