The erbB gene of avian erythroblastosis virus is a member of the src gene family

Oncogenes and tumour-suppressor genes in squamous cell carcinoma of the head and neck

Cancer is now considered to be a multi-hit process which involves a number of aberrant genetic events culminating in malignant transformation. In squamous cell carcinoma (SCC) of the head and neck the action of both oncogenes and tumour-suppressor genes has been identified during the course of the disease. Cytogenetic analysis of these carcinomas has demonstrated chromosomal breakpoints, particularly in the regions of 1p22 and 11q13 together with frequent amplification of the proto-oncogenes in the 11q13 amplicon; int-2, hst-1 and bcl-1. Ras mutations have been infrequently identified in the Western World whereas ras over-expression has been a common finding and may be associated with the early development of head and neck cancer. C-myc over-expression appears to correlate with a poor prognosis for these patients. The tumour-suppressor gene p53 is also thought to be involved in the development of SCC in head and neck tumours and its aberrant expression is associated with a history of heavy smoking and heavy drinking. E-cadherin, a putative tumour-suppressor gene is down-regulated in poorly differentiated head and neck SCC and maybe important in nodal metastasis. A recent study has indicated that the Human Papilloma Virus (HPV 16 and 33) has a role in the aetiology of tonsillar carcinomas and HPV has been shown to produce transforming proteins which bind to and inactivate the p53 tumour suppressor gene. This evidence suggests that the possibility of a viral mechanism for the development of SCC in the head and neck should be considered. This paper proposes a series of genetic events to explain the development of SCC of the head and neck.

Large-scale purification and characterisation of a recombinant epidermal growth-factor receptor protein-tyrosine kinase. Modulation of activity by multiple factors

The human epidermal-growth-factor receptor (EGF-R) is a 170-kDa transmembrane glycoprotein that mediates the mitogenic response of cells to EGF and transforming growth factor alpha. Culture conditions have been developed for the large-scale expression of the cytoplasmic domain of the EGF-R in insect cells using a recombinant baculovirus. From 61 Sf9 cells, grown to high density using a bioreactor, 20 mg of the EGF-R kinase was purified to greater than 95% purity. Purification, which was carried out in the absence of detergents using classical purification methods, yielded an EGF-R protein that was not phosphorylated on tyrosine. This procedure has enabled us to produce high quality enzyme for both structural and biochemical studies on the EGF-R kinase. The in vitro activity of the cytoplasmic domain of the EGF-R kinase was modulated by multiple assay factors which include substrates, divalent cations and conformational modulators. Kinetic analysis in the presence of Mn2+ gave an apparent Vmax value of 20 nmol min-1 mg-1 and Km values of 4.5 microM for ATP and 1.43 mM for angiotensin II. This corresponds to a turnover number of 1.4 mol min-1 mol-1. Ammonium sulfate (1 M) resulted in an eightfold stimulation of kinase activity when assayed using angiotensin II as substrate. The specific activity of the intracellular domain of the EGF-R, when assayed at 20 degrees C in the presence of 1M ammonium sulfate, was 160 nmol min-1 mg-1. Activation of the EGF-R kinase by ammonium sulfate was found to be substrate-specific. No activation was found when assayed using polymeric substrates. Addition of Me(2+)-ATP to the purified enzyme resulted in autophosphorylation and was accompanied by retardation of SDS/PAGE migration. Kinetic constants and metal ion preferences of a number of co-polymers and peptide substrates have been compared. Dramatic differences in kinetic constants were found which were dependent on both the substrate and metal ion used. Activation of EGF-R autophosphorylation was found to be influenced by the use of charged polymers. The random polymer of Glu, Lys, Ala, Tyr (2:5:6:1), which was not phosphorylated by the EGF-R kinase, dramatically activates autophosphorylation of the EGF-R. Thus the intracellular domain of the EGF-R appears to be in a low-activity conformation which, under appropriate assay conditions, can be activated to a similar specific activity to that reported for the purified EGF-R holoenzyme.

Heterogeneity in epidermal growth factor responsiveness and tumor growth of human maxillary cancer cell lines

We have established three cell lines (IMC-2, IMC-3, and IMC-4) from a human maxillary tumor, which exhibited different sensitivities to epidermal growth factor (EGF). It was inhibitory to colony-forming abilities of IMC-3 and IMC-4 cells in culture, while it affected that of IMC-2 cells slightly if at all. The differential sensitivities to EGF among the three cell lines were reproducibly observed when several cell sublines were further established from tumors appearing in nude mice. Saturation-binding kinetics with 125I-EGF showed similar levels of EGF-binding activities among the three cell lines. However, IMC-2, IMC-3, and IMC-4 showed almost similar sensitivities to cisplatin. Autophosphorylation of EGF receptor in the presence of EGF proceeded at similar levels among the three cell lines. Tumor growth was followed in nude mice when IMC-2, IMC-3, and IMC-4 at 1 x 10(7) cells were inoculated. The IMC-2 tumors enlarged at much faster rates than the other two cell lines. The IMC-4 tumors showed very slow growth rates, and IMC-3 tumors enlarged at an intermediate rate. These data suggest that the maxillary tumor used comprised cell populations that differed in their growth behaviors in response to EGF.

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.

Identification and interpretation of epidermal growth factor and c-erbB-2 overexpression

Overexpression of normal cellular genes may be one mechanism by which malignant cells can acquire a selective growth advantage. The epidermal growth factor receptor and the c-erbB-2 protein are members of the erbB family and are good examples of genes that appear to act through this mechanism. Molecular and biochemical analyses of these two proteins also illustrate how studies of growth factors, growth factor receptors and oncogenic retroviruses may lead to new approaches to diagnosis and treatment. In particular, overexpression of these growth factor receptors has identified clinical subgroups that may respond differently to chemotherapy and provides the opportunity for antibody targeted therapy. Overexpression of these proteins can be identified using immunocytochemistry on both histological sections and fine-needle aspirates, thus enabling these parameters to be assessed preoperatively and to be monitored during therapy.

Signal transmission by epidermal growth factor receptor: coincidence of activation and dimerization

Dimerization of epidermal growth factor receptor dissolved in a solution of nonionic detergent was followed with a resolution of 1 min by quantitative cross-linking with glutaraldehyde. Upon addition of epidermal growth factor to the solution, the initially monomeric protein dimerized in a reaction that was second-order in the concentration of receptor. A second-order rate constant, on the basis of enzymatic activity as a measure of the concentration of functional receptor, was calculated from time courses of dimerization at various initial concentrations of receptor. The activation of the protein tyrosine kinase of the receptor was monitored directly under the same conditions with an exogenous substrate. The increase in tyrosine kinase activity displayed kinetics that were also second-order in the concentration of receptor. A second-order rate constant for the activation of the tyrosine kinase could be calculated from the time courses. The second-order rate constant for the activation of the tyrosine kinase by epidermal growth factor was indistinguishable from the second-order rate constant for the dimerization induced by epidermal growth factor. Therefore, dimerization of epidermal growth factor receptor and activation of its tyrosine kinase are coincident events, both initiated by the binding of epidermal growth factor.

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.

Molecular mechanisms for generation of neural diversity and specificity: roles of polypeptide factors in development of postmitotic neurons

Development of postmitotic neurons is influenced by two groups of polypeptide factors. Neurotrophic factors promote neuronal survival both in vivo and in vitro. Neuronal differentiation factors influence transmitter phenotypes without affecting neuronal survival. The list of neurotrophic factors is increasing partly because certain growth factors and cytokines have been shown to possess neurotrophic activities and also because new neurotrophic factors including new members of the nerve growth factor (NGF) family have been identified at the molecular level. In vitro assays using recombinant neurotrophic factors and distributions of their mRNAs and proteins have indicated that members of a neurotrophic gene family may play sequential and complementary roles during development and in the adult nervous system. Most of the receptors for neurotrophic factors contain tyrosine kinase domains, suggesting the importance of tyrosine phosphorylation and subsequent signal transduction for their effects. Molecules such as LIF (leukemia inhibitory factor) and CNTF (ciliary neurotrophic factor) have been identified as neuronal differentiation factors in vitro. At the moment, however, it remains to be determined whether or not the receptors for a group of neuronal differentiation factors constitute a gene family or contain domains of kinase or phosphatase activity. Synergetic combinations of neurotrophic and neuronal differentiation factors as well as their receptors may contribute to the generation of neural specificity and diversity.

Gene amplification and overexpression of epidermal growth factor receptor in squamous cell carcinoma of the head and neck

The degree of gene amplification for epidermal growth factor receptor (EGFR) and its expression levels were examined in 4 cases of tumor lesions and their cell lines of human squamous cell carcinoma (SCC) of the oral cavity. The amplification was detected in 1 case (ZA), but not significantly in 3 other cases (HOC605, HOC815, and HOC927) in which the amplification did not occur during the cell line establishment. In those 3 cases, levels of EGFR synthesis and human EGF (hEGF) binding capacity were varied: HOC605 and HOC815 had slightly increased levels of hEGF binding capacity and EGFR synthesis, respectively. While HOC927 had the lowest levels of both, the hEGF binding capacity was elevated in the tumor lesion when compared with the normal counterpart of the same patient. These results suggest that the increased capacity for EGF binding plays a more important role than does gene amplification on the tumorigenesis of SCC of the head and neck.

Expression and characterization of kinase-active v-erbB protein using a baculovirus vector system

The v-erbB gene is an oncogene of the avian erythroblastosis virus encoding a protein that is a truncated version of the epidermal growth factor receptor. The v-erbB protein was expressed alone or as polyhedrin-erbB fusion proteins using the Bombyx mori nuclear polyhedrosis virus vector. The expression level of the fusion protein whose polyhedrin portion consisted of only 8 amino-terminal amino acids was more than ten times higher than that of the non-fusion protein. Studies with tunicamycin showed that the recombinant v-erbB proteins were glycosylated. The recombinant protein autophosphorylated tyrosine residues, and phosphorylated a synthetic tyrosine-containing peptide and lipocortin I. These observations indicate that functional v-erbB protein can be expressed in silkworm-derived cells, and furthermore, that this system can be used for large-scale production.

The role of growth factors in the embryogenesis and differentiation of the eye

The vertebrate eye is composed of a variety of tissues that, embryonically, have their derivation from surface ectoderm, neural ectoderm, neural crest, and mesodermal mesenchyme. During development, these different types of cells are subjected to complex processes of induction and suppressive interactions that bring about their final differentiation and arrangement in the fully formed eye. With the changing concept of ocular development, we present a new perspective on the control of morphogenesis at the cellular and molecular levels by growth factors that include fibroblast growth factors, epidermal growth factor, nerve growth factor, platelet-derived growth factor, transforming growth factors, mesodermal growth factors, transferrin, tumor necrosis factor, neuronotrophic factors, angiogenic factors, and antiangiogenic factors. Growth factors, especially transforming growth factor-beta, have a crucial role in directing the migration and developmental patterns of the cranial neural-crest cells that contribute extensively to the structures of the eye. Some growth factors also exert an effect on the developing ocular tissues by influencing the synthesis and degradation of the extracellular matrix. The mRNAs for the growth factors that are involved in the earliest aspects of the growth and differentiation of the fertilized egg are supplied from maternal sources until embryonic tissues are able to synthesize them. Subsequently, the developing eye tissues are exposed to both endogenous and exogenous growth factors that are derived from nonocular tissues as well as from embryonic fluids and the systemic circulation. The early interaction between the surface head ectoderm and the underlying chordamesoderm confers a lens-forming bias on the ectoderm; later, the optic vesicle elicits the final phase of determination and enhances differentiation by the lens. After the blood-ocular barrier is established, the internal milieu of the eye is controlled by the interactions among the intraocular tissues; only those growth factors that selectively cross the barrier or that are synthesized by the ocular tissues can influence further development and differentiation of the cells. An understanding of the tissue interactions that are regulated by growth factors could clarify the precise mechanism of normal and abnormal ocular development.

Epidermal growth factor and transforming growth factor alpha

I have attempted here to outline the basic biochemical knowledge that we have now secured on the EGF family of proteins. In the future we will learn much more about the differential role of EGF versus TGF-alpha, about the physiological significance of amphiregulin, the newest member of this family, and about the roles of TGF-alpha and amphiregulin in cancer. Many questions remain. What is the importance of these factors in embryogenesis and fetal development? Is there an involvement of the EGF-like domains of extracellular proteins in cell-to-extracellular-matrix interactions? Do these extracellular matrix EGF-like entities function in a similar manner to fibroblast growth factor in cell growth and in mediating the relationship of cells to the extracellular matrix? What is the significance of cell membrane-bound forms of EGF and TGF-alpha as potential cell-to-cell contact regulators? What is the role of the viral EGF-like proteins in the viral infective and transforming process? These and other questions will be addressed in the next decade. The key question has already been well stated: 'what is the normal physiological role of EGF during development and homeostasis? The answers to these and a host of other questions must be found before we can fully comprehend this important regulatory system' (Cohen, 1987).

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

The v-erbB oncogene isolated from the R (or ES4) strain of avian erythroblastosis virus is capable of inducing erythroleukemia and fibrosarcomas. This oncogene differs from the proto-oncogene c-erbB, the avian homolog of the epidermal growth factor receptor, by its lack of an intact ligand-binding domain as well as additional alterations in its cytoplasmic coding sequences. By contrast, the insertionally activated c-erbB, a variant oncogene, which encodes a product that also lacks the ligand-binding domain but is otherwise unaltered in its cytoplasmic coding sequences, is capable of inducing leukemia but cannot induce sarcomas. In this report, we show that the critical changes for activating the sarcomagenic potential displayed by v-erbB R are two point mutations within the tyrosine kinase domain and an internal deletion of 21 amino acids in the carboxyl-terminal regulatory domain. The removal of the carboxyl-terminal autophosphorylation sites is not obligatory. These activating mutations (Arg-263 to His, Ile-384 to Ser, and the deletion of residues 494 to 514), when introduced singly into the insertionally activated c-erbB, all dramatically increase fibroblast-transforming potential. Arg-263 resides near the highly conserved HRD motif of the kinase domain, and its mutation to His increases the autophosphorylation activity. The other two mutations do not alter the intrinsic kinase activity and presumably affect other aspects of the receptor involved in growth signaling. Therefore, the high transforming potential of v-erbB R is a consequence of synergism among multiple activating mutations.

The amino-terminal 14 amino acids of v-src can functionally replace the extracellular and transmembrane domains of v-erbB

The retroviral oncogene v-erbB encodes a truncated form of the receptor for epidermal growth factor, an integral membrane protein-tyrosine kinase. By contrast, the oncogene v-src encodes a protein-tyrosine kinase that is a peripheral membrane protein. The morphologies and spectra of cells transformed by these two oncogenes differ. In an effort to identify the functional determinant(s) of these differences, we constructed and tested first deletion mutants of v-erbB and then chimeras between v-src and v-erbB. As reported previously, the absence of any membrane anchorage eliminated transformation by v-erbB. Anchorage of the cytoplasmic kinase domain of v-erbB to membranes with amino-terminal portions of the v-src protein permitted transformation. The phenotype and spectrum of transformation were those expected for v-erbB rather than for v-src. The transforming chimeras lost their biological activity if the signal for myristylation at the amino terminus of v-src was compromised by mutation. Biochemical fractionations revealed a correlation between transforming activity and the association of chimeric gene products with the membrane fraction of the cell. For reasons not yet apparent, the combined presence of membrane anchorage domains of v-src, and the transmembrane domain of v-erbB in the same chimera typically (but not inevitably) impeded transformation. Our results suggest that the specificity of transformation by v-erbB resides in the selection of substrates by the cytoplasmic domain of the gene product. The protein retains access to those substrates even when anchored to the membrane in the manner of a peripheral rather than a transmembrane protein.

The amino-terminal 14 amino acids of v-src can functionally replace the extracellular and transmembrane domains of v-erbB

The retroviral oncogene v-erbB encodes a truncated form of the receptor for epidermal growth factor, an integral membrane protein-tyrosine kinase. By contrast, the oncogene v-src encodes a protein-tyrosine kinase that is a peripheral membrane protein. The morphologies and spectra of cells transformed by these two oncogenes differ. In an effort to identify the functional determinant(s) of these differences, we constructed and tested first deletion mutants of v-erbB and then chimeras between v-src and v-erbB. As reported previously, the absence of any membrane anchorage eliminated transformation by v-erbB. Anchorage of the cytoplasmic kinase domain of v-erbB to membranes with amino-terminal portions of the v-src protein permitted transformation. The phenotype and spectrum of transformation were those expected for v-erbB rather than for v-src. The transforming chimeras lost their biological activity if the signal for myristylation at the amino terminus of v-src was compromised by mutation. Biochemical fractionations revealed a correlation between transforming activity and the association of chimeric gene products with the membrane fraction of the cell. For reasons not yet apparent, the combined presence of membrane anchorage domains of v-src, and the transmembrane domain of v-erbB in the same chimera typically (but not inevitably) impeded transformation. Our results suggest that the specificity of transformation by v-erbB resides in the selection of substrates by the cytoplasmic domain of the gene product. The protein retains access to those substrates even when anchored to the membrane in the manner of a peripheral rather than a transmembrane protein.

Oncogene expression in the liver tissue of patients with nonneoplastic liver disease

The expression of cellular oncogenes in nonneoplastic human liver tissue was examined to determine if there was a correlation between oncogene expression and physiologic regeneration in liver disease. Human liver tissue specimens from 70 patients with various histologic findings from almost normal to cirrhosis were examined (using northern blot analysis) for the expression of nine cellular oncogenes. With c-K-ras, four RNA bands (5.6-kilobase [kb], 2.1-kb, 1.5-kb, and 1.2-kb RNA species) were detected in all liver tissue examined. Expression of c-fos was also detected in a few samples examined when 50-micrograms samples of total RNA were applied. Other oncogenes such as H-ras, myc, erbB, raf, fms, fes, and myb were not detected. These results indicate that particular oncogene(s) may not be highly expressed during liver regeneration in human liver disease, or that populations of regenerating hepatocytes may be too small to show significant elevations of oncogene expression. The new finding of a constant expression of c-K-ras in human liver tissue suggests that it is linked to essential hepatocellular function rather than carcinogenesis.

Phospholipase C activity in microorganisms associated with reproductive tract infection

Phospholipase C (lecithinase or phosphatidylcholine phosphorylase) catalyzes the hydrolysis of lecithin into phosphorylcholine and 1,2-diglyceride. Bacterial production of phospholipase C may damage reproductive tract tissues by both direct and indirect mechanisms. Use of the synthetic substrate p-nitrophenylphosphorylcholine phospholipase C activity was determined in 204 isolates representative of those found in female genital tract. Multiple aerobic (28%) and anaerobic (28%) reproductive tract microorganisms showed phospholipase C activity. Phospholipase C-producing isolates included strains of Bacteroides fragilis, B. bivius, B. thetaiotaomicron, Gardnerella vaginalis, and group B streptococcus. Phospholipase C activity was heterogenous; not all isolates that belong to a particular species showed activity. Phospholipase C production may be a possible virulence factor produced by a number of microflora commonly implicated in various reproductive tract infections or conditions, as well as in some instances of preterm birth.

Extrachromosomal amplification of the epidermal growth factor receptor gene in a human colon carcinoma cell line

In situ hybridization, using a biotinylated cDNA probe for the epidermal growth factor receptor (EGFR) gene, indicates that the amplified EGFR genes in the colon tumor cell line, DiFi, are localized in many small double minute chromosomes (dmin) of varying size and visibility. Analysis of the electrophoretic mobility of gamma-irradiated DNA from DiFi by pulsed-field gel electrophoresis and Southern blot hybridization using EGFR probe, indicates that the amplified EGFR in DiFi exists in extrachromosomal, covalently closed circular episomes, presumably equivalent to dmin. Two major and one minor species were observed which had estimated sizes of 650, 1,300, and 2,000 kb, respectively. The DiFi cell line appears to represent a unique case of extrachromosomal EGFR gene amplification in human cells.

Tissue-specific transformation by epidermal growth factor receptor: a single point mutation within the ATP-binding pocket of the erbB product increases its intrinsic kinase activity and activates its sarcomagenic potential

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 that have constitutive tyrosine kinase activity and that can induce erythro-leukemia but not sarcomas. We have found that a single point mutation within the ATP-binding pocket of the tyrosine kinase domain in this truncated molecule can increase the ability of this oncogene to induce anchorage-independent growth of fibroblasts in vitro and fibrosarcoma formation in vivo. Associated with this increased transforming potential is a corresponding increase in the kinase activity of the mutant erbB protein product. The mutation, which converts a valine to isoleucine at position 157 of the insertionally activated c-erbB product, is at a residue that is highly conserved within the protein kinase family. To our knowledge, this is the first demonstration of a point mutation in the ATP-binding pocket that activates a tyrosine kinase.

Cooperation of v-jun and v-erbB oncogenes in embryo fibroblast transformation in vitro and in vivo

Retroviral vectors carrying either the v-jun and v-erbB sequences or the v-jun gene linked to the neomycin resistance gene were constructed on the basis of the structural genome organization of avian erythroblastosis virus (AEV). These viruses, called JB and JN, respectively, were rescued as Rous-associated virus-1 pseudotypes, and they were shown to successfully transform chicken embryo fibroblasts in vitro. However, in agar, colonies developed from JB-infected fibroblasts were three to five times larger than those obtained after infection with JN or with AEV Pst124 carrying only a functional v-erbB gene. In vivo, on chorioallantoic membrane (CAM) assays, JB produced fibrosarcomas that were more rapidly growing and much larger than those induced by JN or AEV Pst124. Moreover, in chickens infected in ovo with JB, multiple fibrosarcomas arose in different organs a few days after birth, whereas no tumor could be detected in parallel experiments in either JN- or AEV Pst124-infected animals. These results demonstrate that in embryo fibroblast cells, v-jun and v-erbB can act synergistically to enhance the transformation potential of either oncogene alone both in vitro and in vivo.

A deletion mutation within the ligand binding domain is responsible for activation of epidermal growth factor receptor gene in human brain tumors

Two transplantable cell lines of human glioblastoma multiforme GL-3 and GL-5 carried an amplification and overexpression of structurally altered epidermal growth factor (EGF) receptor gene: the 140 kilodalton EGF receptors in these cases exhibited a constitutively expressed tyrosine kinase activity without the ligand. Here, we isolated the abnormal EGF receptor cDNA from GL-5 cell line, and demonstrated that this cDNA bears a single large intramolecular deletion mutation 801 base pairs long within the ligand binding domain of EGF receptor. In other regions no amino acid substitution was observed. At the level of genomic DNA, this deletion appeared to start from the 1st intron and terminate in the 6th intron of the EGF receptor gene. However, in the two lines of glioblastoma, GL-3 and GL-5, the positions of the start or the end of the deletion mutation in these introns were not identical, suggesting an involvement of a unique recombination mechanism in the formation of deletion mutation. A weak but ligand-independent transforming activity was observed in the deletion-carrying EGF receptor cDNA.

Epidermal growth factor receptor expression in 72 meningiomas

The expression of epidermal growth factor receptor (EGF-R) was studied immunohistochemically in 72 meningiomas using two monoclonal antibodies with specificities to protein and carbohydrate components, respectively, of the external domain of the EGF-R. One third of the tumors had cytoplasmic and membrane positivity with the protein-specific antibody but in none were there positive tumor cells with the carbohydrate-specific antibody which recognizes the blood group A antigen. There was no difference in EGF-R expression between typical and aggressive meningiomas. No evidence was found to support previous reports of specific EGF-R immunoreactivity in the vascular endothelial cells of meningiomas. The authors believe this discrepancy to be due to detection of normal blood group A antigen attached to endothelial cells in patients of blood group A or AB. This occurs because many monoclonal anti-EGF-R antibodies are specific for A antigen which is found on the EGF-R of A431 cells but has not been reported on EGF-R elsewhere.

Epidermal growth factor receptor cytoplasmic domain mutations trigger ligand-independent transformation

The transforming gene product of avian erythroblastosis virus, v-erbB, is derived from the epidermal growth factor (EGF) receptor but has lost its extracellular ligand-binding domain and was mutated in its cytoplasmic portion, which is thought to be responsible for biological signal generation. We have repaired the deletion of extracellular EGF-binding sequences and investigated the functional consequences of cytoplasmic erbB mutations. Within the resulting EGF receptors, the autophosphorylation activities of the cytoplasmic domains of v-erbB-H and v-erbB-ES4 were fully ligand dependent in intact cells. However, the mitogenic and transforming signaling activities of an EGF receptor carrying v-erbB-ES4 (but not v-erbB-H) cytoplasmic sequences remained ligand independent, whereas those of a receptor with a v-erbB-H cytoplasmic domain were regulated by EGF or transforming growth factor alpha. Thus, structural alterations in the cytoplasmic domain of growth factor receptor tyrosine kinases may induce constitutive signaling activity without autophosphorylation. These findings provide new insight into the mechanism of receptor-mediated signal transduction and suggest a novel alternative for subversion of cellular control mechanisms and proto-oncogene activation.

A truncated, secreted form of the epidermal growth factor receptor is encoded by an alternatively spliced transcript in normal rat tissue

Two independent cDNA clones corresponding to a 2.7-kilobase (kb) epidermal growth factor receptor (EGF-R) mRNA were isolated from a rat liver cDNA library. Sequence analysis revealed 100% homology in the external domain when compared with the full-length rat EGF-R nucleotide sequence and 80 to 90% similarity relative to the human EGF-R. However, the 3'-terminal sequence of these clones did not match EGF-R or any other known sequence(s) and was distinct from the 3' end of the 2.8-kb mRNA, which encodes a truncated EGF-R in A431 cells. The deduced amino acid sequence revealed an open reading frame which is homologous to the external domain of the EGF-R but which terminates prior to the transmembrane region. Southern blot analysis of rat genomic DNA indicated that the 3'-terminal sequence of this transcript is derived from the EGF-R gene. Analysis of a genomic clone containing the 3' end of the 2.7-kb transcript revealed that this sequence is present as a discrete exon in the mid-region of the receptor gene in proximity to the exon encoding the transmembrane domain. Introduction of an expression vector containing the truncated EGF-R cDNA into Chinese hamster ovary (CHO) cells led to the expression of a 95-kilodalton protein which was detected in conditioned media, by using antisera directed against the EGF-R. A similarly sized protein was also detected in the media of WB cells, a continuous, nontransformed line of rat hepatic epithelial cells. Northern (RNA blot) analysis established that the truncated receptor is encoded by a 2.7-kb transcript found in normal rat liver. Furthermore, Northern analysis of rat poly(A)+ RNA showed that the 2.7-kb EGF-R transcript is expressed at differing levels in various fetal and adult tissues. These data indicate that alternative splicing of the EGF-R primary transcript yields a 2.7-kb mRNA which codes for a truncated form of the receptor. This receptor is secreted by rat hepatic epithelial cells in culture, which suggests that it may be secreted by normal rat cells or tissues and perhaps serve an as yet unknown physiological function.

Characterization of ROS1 cDNA from a human glioblastoma cell line

We have isolated and characterized a human ROS1 cDNA from the glioblastoma cell line SW-1088. The cDNA, 8.3 kilobases long, has the potential to encode a transmembrane tyrosine-specific protein kinase with a predicted molecular mass of 259 kDa. The putative extracellular domain of ROS1 is homologous to the extracellular domain of the sevenless gene product from Drosophila. No comparable similarities in the extracellular domains were found between ROS1 and other receptor-type tyrosine kinases. Together, ROS1 and sevenless gene products define a distinct subclass of transmembrane tyrosine kinases.

A truncated, secreted form of the epidermal growth factor receptor is encoded by an alternatively spliced transcript in normal rat tissue

Two independent cDNA clones corresponding to a 2.7-kilobase (kb) epidermal growth factor receptor (EGF-R) mRNA were isolated from a rat liver cDNA library. Sequence analysis revealed 100% homology in the external domain when compared with the full-length rat EGF-R nucleotide sequence and 80 to 90% similarity relative to the human EGF-R. However, the 3'-terminal sequence of these clones did not match EGF-R or any other known sequence(s) and was distinct from the 3' end of the 2.8-kb mRNA, which encodes a truncated EGF-R in A431 cells. The deduced amino acid sequence revealed an open reading frame which is homologous to the external domain of the EGF-R but which terminates prior to the transmembrane region. Southern blot analysis of rat genomic DNA indicated that the 3'-terminal sequence of this transcript is derived from the EGF-R gene. Analysis of a genomic clone containing the 3' end of the 2.7-kb transcript revealed that this sequence is present as a discrete exon in the mid-region of the receptor gene in proximity to the exon encoding the transmembrane domain. Introduction of an expression vector containing the truncated EGF-R cDNA into Chinese hamster ovary (CHO) cells led to the expression of a 95-kilodalton protein which was detected in conditioned media, by using antisera directed against the EGF-R. A similarly sized protein was also detected in the media of WB cells, a continuous, nontransformed line of rat hepatic epithelial cells. Northern (RNA blot) analysis established that the truncated receptor is encoded by a 2.7-kb transcript found in normal rat liver. Furthermore, Northern analysis of rat poly(A)+ RNA showed that the 2.7-kb EGF-R transcript is expressed at differing levels in various fetal and adult tissues. These data indicate that alternative splicing of the EGF-R primary transcript yields a 2.7-kb mRNA which codes for a truncated form of the receptor. This receptor is secreted by rat hepatic epithelial cells in culture, which suggests that it may be secreted by normal rat cells or tissues and perhaps serve an as yet unknown physiological function.

Epidermal growth factor receptor cytoplasmic domain mutations trigger ligand-independent transformation

The transforming gene product of avian erythroblastosis virus, v-erbB, is derived from the epidermal growth factor (EGF) receptor but has lost its extracellular ligand-binding domain and was mutated in its cytoplasmic portion, which is thought to be responsible for biological signal generation. We have repaired the deletion of extracellular EGF-binding sequences and investigated the functional consequences of cytoplasmic erbB mutations. Within the resulting EGF receptors, the autophosphorylation activities of the cytoplasmic domains of v-erbB-H and v-erbB-ES4 were fully ligand dependent in intact cells. However, the mitogenic and transforming signaling activities of an EGF receptor carrying v-erbB-ES4 (but not v-erbB-H) cytoplasmic sequences remained ligand independent, whereas those of a receptor with a v-erbB-H cytoplasmic domain were regulated by EGF or transforming growth factor alpha. Thus, structural alterations in the cytoplasmic domain of growth factor receptor tyrosine kinases may induce constitutive signaling activity without autophosphorylation. These findings provide new insight into the mechanism of receptor-mediated signal transduction and suggest a novel alternative for subversion of cellular control mechanisms and proto-oncogene activation.

Enhancement of tyrosine kinase activity of the Drosophila epidermal growth factor receptor homolog by alterations of the transmembrane domain

The Drosophila epidermal growth factor receptor homolog (DER) displays sequence similarity to both the epidermal growth factor (EGF) receptor and the neu vertebrate proteins. We have examined the possibility of deregulating the tyrosine kinase activity of DER by introducing structural changes which mimic the oncogenic alterations in the vertebrate counterparts. Substitution of valine by glutamic acid in the transmembrane domain, in a position analogous to the oncogenic mutation in the rat neu gene, elevated the in vivo kinase activity of DER in Drosophila Schneider cells sevenfold. A chimera containing the oncogenic neu extracellular and transmembrane domains and the DER kinase region, also showed a threefold elevated activity relative to a similar chimera with normal neu sequences. Double truncation of DER in the extracellular and cytoplasmic domains, mimicking the deletions in the v-erbB oncogene, did not however result in stimulation of in vivo kinase activity. The chimeric constructs were also expressed in monkey COS cells, and similar results were obtained. The ability to enhance the DER kinase activity by a specific structural modification of the transmembrane domain demonstrates the universality of this activation mechanism and strengthens the notion that this domain is intimately involved in signal transduction. These results also support the inclusion of DER within the tyrosine-kinase receptor family.

Soluble factors including proteinases released from damaged cells may trigger the wound healing process

The wound healing process is initiated as soon as tissue is injured. Herein, we demonstrate that c-fos and c-myc mRNA transcripts are promptly increased in the wounded tissue in vivo and in vitro. A buffer solution from scraped serum-starved quiescent fibroblasts, when added to resting fibroblasts, caused the increase of c-fos and c-myc mRNA among the indicator cells. Soluble factors contained in the wounding supernatant are responsible for these phenomena and we call them wounding factors. Addition of proteinase inhibitors to the culture medium drastically reduced the c-fos mRNA induction by the wounding factors. Exogenously added trypsin or thrombin mimicked the activity of wounding factors. These results suggest that wounding causes soluble factors including various proteinases to be released from the damaged cells, which trigger the adjacent cells to respond to the injury.

Tissue-specific expression of rat c-ros-1 gene and partial structural similarity of its predicted products with sev protein of Drosophila melanogaster

The expression and predicted products of rat c-ros-1 gene, the proto-oncogene of v-ros in UR2 sarcoma virus, were characterized. The c-ros-1 gene was found to be expressed in a tissue-specific manner, and the sizes of its transcripts were heterogeneous: 8.2 kilobases (kb) long in lung and kidney tissues, 6.9 kb in heart tissue, and 2.4 kb and 1.9 kb in testis tissue. The c-ros-1 cDNAs were isolated from lung and heart tissues. The predicted product of the c-ros-1 gene in lung tissue was a receptor-type tyrosine kinase 2,317 amino acids long (including a very large extracellular domain of approximately 1,800 amino acids) which showed a partial but significant structural homology with the sev gene product of Drosophila melanogaster. An alternatively sliced lung transcript was found to encode a protein with external and transmembrane domains but not a tyrosine kinase catalytic domain. The predicted product in heart tissue was essentially identical to that in lung tissue except for a shorter amino-terminal region and a 21-amino-acid insertion in the extracellular domain. On the basis of these results, the c-ros-1 gene appears to be active in the lungs and kidneys and probably in the hearts of rats.

EGF receptor and erbB-2 tyrosine kinase domains confer cell specificity for mitogenic signaling

The epidermal growth factor (EGF) receptor (EGFR) can efficiently couple with mitogenic signaling pathways when it is transfected into interleukin-3 (IL-3)-dependent 32D hematopoietic cells. When expression vectors for erbB-2, which is structurally related to EGFR, or its truncated counterpart, delta NerbB-2, were introduced into 32D cells, neither was capable of inducing proliferation. This was despite overexpression and constitutive tyrosine kinase activity of their products at levels associated with potent transformation of fibroblast target cells. Thus, EGFR and erbB-2 couple with distinct mitogenic signaling pathways. The region responsible for the specificity of intracellular signal transduction was localized to a 270-amino acid stretch encompassing their respective tyrosine kinase domains. Thus, tissue- or cell-specific regulation of growth factor receptor signaling can occur at a point after the initial interaction of growth factor with receptor. Such specificity in signal transduction may account for the selection of certain oncogenes in some malignancies.

Inhibition of epidermal growth factor receptor activity by retinoic acid in glioma cells

The growth inhibitory effects of exogenously added retinoic acid (RA) on various cultured human glioma cells was observed to be heterogenous, with an ID50 ranging from 10(-7) M to no response. The protein tyrosine kinase activity of epidermal growth factor receptor (EGF-receptor) appeared to parallel the cell's growth responsiveness to RA. Cells sensitive to RA-induced growth inhibition exhibited a dose-dependent decrease in EGF-receptor activity, whereas RA-resistant cells showed no alterations in EGF-receptor protein tyrosine kinase activity or expression. The modulation of EGF-receptor by RA was further examined with RA-sensitive (LG) and -resistant (NG-1) cell lines. Both cell lines were approximately equal in their ability to bind and internalize epidermal growth factor in the presence or absence of RA. Several independent assays suggested that the inhibition of EGF-receptor activity was independent of protein kinase C modulation as mediated by phorbol myristate acetate. However, alterations in associated glycoconjugates of EGF-receptor were observed among the sensitive cells but not the resistant cells. These results suggest RA-induced growth inhibition in sensitive cells may arise, at least in part, through alterations in EGF-receptor and structure.

Induction of high-grade anti-tumor immunity by use of a recombinant H-2Kb/avian erythroblastosis virus erbB gene transfectant

The recombinant H-2Kb-erbB gene, encoding for a part of the H-2 class I antigen and the kinase domain of the V-erbB peptide, was successfully introduced into murine mastocytoma P815 variant P1.HTR cells, which resulted in low but significant cell-surface expression of the hybrid gene product. When the chimeric gene transfectant was inoculated into the CDF1 mice, it soon grew but regressed thereafter. The tumorigenicity of this transfectant was lower than the H-2Kb gene transfectant that expressed the H-2Kb antigen at a comparable level. These CDF1 mice that had received the chimeric gene transfectant obtained a high-grade anti-tumor immunity against the challenge of a high dose of parental tumor. Corresponding to these observations, anti-tumor cytotoxic T lymphocytes, which lyse parental P1.HTR cells but not syngeneic L1210 or NS-1 tumor cells, were developed in the peritoneal cavity of mice that had been inoculated with the transfectant and parental tumor. Definite antibody activity binding to parental P1.HTR tumor cells was also demonstrated in the sera of these mice, precipitating 40-kDa, 74-kDa and 98-kDa molecules from the surface of the radiolabeled P1-HTR tumor cells. The results suggested that the chimeric H-2-erB gene transfectant efficiently triggers both cellular and humoral anti-tumor immune responses.

Elevated expression of secondary, but not early, responding genes to phorbol ester tumor promoters in papillomas and carcinomas of mouse skin

A single topical treatment of mouse skin with the potent tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) results in transient inductions of a variety of genes. Based on the time courses of their inductions, these genes can be classified into two main groups: "early" response genes whose mRNA expression reaches a maximum 0.5-2 h after TPA treatment and "secondary" response genes whose mRNA expression is maximal 4 h or more after treatment. The nuclear oncogenes c-fos, c-myc, and c-jun belong to the early response group, whereas the metallothionein, osteopontin, and urokinase genes belong to the secondary response group. The steady-state expressions of these early and secondary response genes are all very low in normal skin, except that of c-jun, which is relatively high. Steady-state levels of expression and inducibility of these genes by TPA were not altered in initiated skin or in apparently normal skin during tumor promotion. We examined the expressions of these genes in papillomas and carcinomas produced by two-stage (initiator-promoter) and three-stage (initiator-promoter-initiator) protocols in mouse skin. Steady-state expression of the early responding nuclear oncogenes in papillomas and carcinomas was found to remain at the same low level as in normal skin. However, all the secondary responding genes were found to be expressed constitutively at high levels in these tumors. Elevated expressions of the genes for transforming growth factor alpha and beta were also observed in papillomas and to varying extents in carcinomas. These observations suggest that the regulatory machinery for transcription by the protein kinase C-mediated pathway through nuclear oncogenes is altered during the processes of tumor promotion and progression. The genes whose expression is elevated may be associated directly or indirectly with tumor promotion and progression.

Autocrine growth of leukemic cells

Autocrine growth is a process whereby a cell both secretes and responds to a growth factor. This paper describes the stepwise malignant progression of leukemic cells which has been demonstrated in many experimental models of autocrine leukemic growth. In contrast, autocrine growth has not been proven as a major physiopathological mechanism for the growth of leukemic cells in vivo in human myeloid and lymphocytic leukemias. Growth-factor independency of human leukemic cell lines may be due to clonal selection.

Oncogenes, growth, and the cell cycle: an overview

In spite of the complexity of the network of regulatory factors which control the balance between the cell cycle and quiescence, a picture is emerging, if only in outline. Several dozens of protooncogenes participate in growth signal transduction and integration, and, when expressed inappropriately, generate growth signals that may override other cellular controls. Some of these controls are provided by the negatively regulating growth factors, and when these are lost (e.g. by chromosomal deletion), or inactivated (e.g. by binding to an inactive analogue or a DNA viral oncoprotein), cell cycle activity is favoured over quiescence. Embryonic tissues are rapidly growing, so their cells are actively cycling and expression of proto-oncogenes is usually observed (Schuuring et al., 1989). As embryonic and stem cells in adult tissues mature, expression of the active proto-oncogenes is generally lost, but other proto-oncogenes may now be expressed (e.g. Muller et al., 1982). These changes in proto-oncogene expression are not achieved by modulation of transcriptional rates alone; transcriptional attenuation, message processing and stability, and post-translational protein modifications are all known to be important for the regulation of proto-oncogene expression during the transition from growth to the differentiated state. When quiescent cells re-enter the cell cycle approximately 60 genes become up-regulated, including proto-oncogene c-fos, the jun family, and c-myc (Zipfel et al., 1989). Evidence is strong that fos and jun proteins are transcriptional regulators. Terminal differentiation, on the other hand, is sometimes accompanied by the up-regulation of the ras gene family, as well as of several other proto-oncogenes. Proto-oncogene function is essential to the cell cycle traverse, but the genes involved are different in various cell types, and the precise order of oncogene expression may not turn out to be important. This is because cell cycle traverse appears to be more dependent on a critical threshold of growth signals propagated by parallel pathways, rather than on a strict order of predetermined steps. The participation of proto-oncogenes in growth signal transduction offers opportunities for errors, and abnormal growth may result from aberrant oncogene products generating a persistent or excessive growth signal, which shifts the balance of input to the integrating genes from quiescence to an active cell cycle. Thus, cancer may result from an entirely normal processing of growth signals that are abnormal.(ABSTRACT TRUNCATED AT 400 WORDS)

The expression of epidermal growth factor receptors in the oral mucosa of patients with oral cancer

Representative tumor samples and mucosal samples were taken from three different groups of patients and were stained immunohistochemically for their expressions of epidermal growth factor receptors (EGFR). Patients in group 1 had oral squamous carcinoma, with specimens taken from the tumor as well as from the mucosa without tumor invasion. Patients in group 2 had no evidence of tumor but had heavy drinking and smoking habits. Tumor-free patients who do not drink or smoke served as the control group. The findings in the present study showed that the tumor and mucosal samples from groups 1 and 2 had increased EGFR expression while the control group showed significantly less EGFR. These results suggest that EGFR may play a role in the development of premalignant tissue changes, which are probably influenced by chronic toxic irritation.

Proteolytic generation of constitutive tyrosine kinase activity of the human insulin receptor

Structural modification induced by partial digestion with trypsin has been shown to stimulate the tyrosine kinase activity of the insulin receptor both in solution and in intact cells [Tamura, Fujita-Yamaguchi & Larner (1983) J. Biol. Chem. 258, 14749-14752; Goren, White & Kahn (1987) Biochemistry 26, 2374-2382; Leef & Larner (1987) J. Biol. Chem. 262, 14837-14842]. Furthermore, experiments involving deletion of sequences encoding the extracellular domain of the insulin receptor suggest that it may function as a protooncogene in fibroblasts [Wang et al., (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 5725-5729]. To further understand the structural requirements that generate this activity, the major activated fragments generated in solution following trypsin digestion have been characterized here, one of which is shown to have a similar amino acid sequence to a transforming protein. Furthermore, treatment with trypsin of intact Chinese hamster ovary cells that overexpress the human insulin receptor stimulates both autophosphorylation of the receptor and 2-deoxyglucose uptake into the cells, but does not enhance receptor internalization. Unlike digestion in solution, no proteolysis or loss of activity of the activated insulin receptor beta-subunit could be detected using intact cells, even at high trypsin concentrations, despite the existence of extracellular sites that are readily cleaved by trypsin in the solubilized receptor. These studies provide further detail of a mechanism used during trypsinization of cells in culture which mimics activation of the insulin receptor and contributes to stimulation of growth.

Epidermal growth factor receptor is down-regulated by a 10,400 MW protein encoded by the E3 region of adenovirus

Epidermal growth factor (EGF) binds to specific high affinity receptors (EGF-Rs) and induces endosome-specific internalization and degradation of ligand-receptor complexes in lysosomes. We report here that EGF-R is down-regulated in an analogous manner during early infection of a variety of cell types by group C human adenoviruses. This effect is not a function of viral entry, nor is it due to a nonspecific increase in turnover of membrane proteins. Using a series of virus deletion mutants, the gene responsible for EGF-R down-regulation was mapped to the E3 transcription unit. The E3 gene product, a protein of MW 10,400 (10.4K), induces internalization and degradation of EGF-R, but does not affect synthesis of the EGF-R precursor. The 10.4K protein is not an EGF-like autocrine growth factor, but is similar in sequence to a region in EGF-R at the cytoplasmic face of the transmembrane domain. This suggests that down-regulation of EGF-R during adenovirus infection may occur by a novel mechanism that involves the formation of hetero-oligomers composed of 10.4K and EGF-R.

Gene amplification and overexpression of EGF receptor in squamous cell carcinomas of the head and neck

Tumours of the head and neck were examined for gene amplification and expression of the epidermal growth factor (EGF) receptor by Southern blot and Western blot analyses. The EGF receptor gene was found to be amplified in four (19%) of 21 squamous cell carcinomas. The EGF receptor was overexpressed in eight (53%) of 15 squamous cell carcinomas examined, including all four tumours showing gene amplification. No amplification or overexpression of the EGF receptor gene was detected in any of nine malignant or eight benign tumours of other types of the head and neck. The tumours showing amplification and/or overexpression of the EGF receptor gene (8/15) were all identified histologically as well differentiated squamous cell carcinomas, whereas none of the histologically less differentiated squamous cell carcinomas (0/9) showed amplification and/or overexpression of the EGF receptor gene. Within our sample set, no correlation was evident between amplification and/or overexpression and the clinical stage or tumour site. Our results support the possible involvement of gene amplification and overexpression of the EGF receptor in a subclass of squamous cell carcinomas of the head and neck.

The Drosophila EGF receptor homolog (DER) gene is allelic to faint little ball, a locus essential for embryonic development

Recessive lethal mutations in the genetic locus of the Drosophila EGF receptor homolog (DER) were isolated. Identification of mutations in the gene is based on assays of DER protein autophosphorylation activity. Most DER alleles show little or no in vivo autophosphorylation. The ability to monitor these activities in vivo and in vitro offers a preliminary insight into the functional defects in the different mutant proteins. The identification of the DER locus was also confirmed by partial rescue of the mutant phenotype with a DER P-element construct. Homozygous DER mutants display a complex embryonic phenotype. Most notably, the anterior structures deteriorate, ventral denticle bands are missing, the germ band does not retract, and the central nervous system shows a collapse of commissure and midline pattern. Mutations in DER were shown to be allelic to the previously described locus faint little ball.

Proviral insertional activation of c-erbB: differential processing of the protein products arising from two alternate transcripts

Proviral insertional activation of c-erbB results in the expression of two alternate transcripts (ENV+ and ENV-). We used cDNA clones representing the two alternate transcripts to generate stably transformed quail fibroblast cell lines which express the products of these transcripts independently. Analysis of the co- and posttranslational processing of the insertionally activated c-erbB products expressed in these cell lines revealed that the protein products of the ENV+ and ENV- transcripts were processed differently. The ENV+ transcript produced a primary translation product which was rapidly cotranslationally cleaved near the amino terminus to form a 79,000-Mr product. This protein product was efficiently converted to a higher-molecular-weight form, of between 82,000 and 88,000 (gp82-88), which was terminally glycosylated and expressed on the cell surface. A small portion of the ENV+ primary translation product underwent a second proteolytic cleavage to generate an unglycosylated 53,000-Mr species. In contrast, the primary translation product of the ENV- transcript, p80, was not proteolytically processed; this precursor form was rapidly converted to two discrete glycosylation intermediates, gp82 and go84. Only a small portion (less than 10%) of the total ENV- insertionally activated c-erbB product was slowly converted to the terminally glycosylated cell surface form, gp85-88. The processing differences that distinguished the ENV+ and ENV- products were similar to processing differences that we observed in parallel studies on the viral erbB products of the avian erythroblastosis viruses AEV-H and AEV-R, respectively. Since all four erbB protein products shared the same number, position, and sequence context of potential N-linked glycosylation sites, yet differed in the extent of their carbohydrate maturation, these data suggest that the mechanisms used by these truncated receptor molecules to associate with cellular membranes may be distinct.