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

Overview of Retrovirology

In the 100 years since their discovery, retroviruses have played a special role in virology and in molecular biology. These agents have been at the center of cancer research and shaped our understanding of cell growth, differentiation and survival in ways that stretch far beyond investigations using these viruses. The discovery of retroviral oncogenes established the central paradigm that altered cellular genes can provide a dominant signal initiating cancer development. Their unique replication mechanism and their integration into cellular DNA allow these viruses to alter the properties of their hosts beyond the life span of the infected individual and contribute to the evolution of species. This same property has made retroviral vectors an important tool for gene therapy. Indeed, the impact of retrovirus research has been far-reaching and despite the amazing progress that has been made, retroviruses continue to reveal new insights into the host – pathogen interaction.

Genetic, biochemical and evolutionary facets of Xmrk-induced melanoma formation in the fish Xiphophorus

Certain interspecific hybrids of the fish Xiphophorus spontaneously develop melanoma induced by the derepression of the Xmrk oncogene. Xmrk is a recent duplicate of an orthologue of the mammalian epidermal growth factor receptor gene Egfr. In addition to a specific overexpression in melanoma, amino-acid substitutions in the extracellular domain leading to ligand-independent dimerisation and constitutive autophosphorylation are responsible for the tumorigenic potential of Xmrk. The Xmrk receptor induces several signal transduction pathways mediating cell proliferation and resistance to apoptosis and initiating dedifferentiation. Moreover, Xmrk upregulates the expression of the secreted protein osteopontin, inducing an autocrine loop possibly allowing invasion and survival in the dermis as a first step in malignancy. Hence, Xmrk is able to induce pathways essential for a transformed phenotype. Some of these events are equivalent to those found downstream of the mammalian Egfr, but others have clearly evolved differently or are specific for pigment cells. Xmrk is potentially hazardous, nonessential and located in a very unstable genomic region. Nevertheless, Xmrk has been maintained under purifying selection in divergent Xiphophorus species. Hence, Xmrk has probably a beneficial function under certain conditions. The analysis of this function is a major challenge for future research in the Xiphophorus model.

Membrane localization of v-ErbB is required but not sufficient for ligand-independent transformation

The v-ErbB retroviral oncogene is a transduced, mutated copy of the avian EGF receptor gene, and its expression is sufficient to induce tumor formation in vivo. The structural alterations that release the oncogenic potential of the v-ErbB oncogene are similar to EGFR gene mutations described in human tumors. Thus, the study of v-ErbB tumor biology offers a useful model through which we can gain insight into the mechanism of EGFR-induced malignancies. Despite years of study, however, questions remain regarding the domains of v-ErbB required for oncogenicity. We sought to clarify the role of the transmembrane domain of v-ErbB during transformation using S3-v-ErbB, an acutely transforming retroviral oncogene isolated from avian sarcomas. Infection of primary fibroblasts with a retroviral vector containing S3-v-ErbB results in the formation of a transformation-associated phosphoprotein signaling complex, soft agar colony formation, and the rapid induction of highly vascularized sarcomas in vivo. To address contribution of the transmembrane domain of S3-v-ErbB during these processes, we constructed a mutant version of this oncogene with a precise deletion in this domain. Specifically, the S3-v-ErbB-TM- mutant was created through an in-frame deletion of the entire transmembrane domain. Primary fibroblasts expressing this S3-v-ErbB-TM- mutant fail to form a characteristic transformation-associated phosphoprotein complex and do not grow in an anchorage-independent manner. In addition, day-old chicks injected with a helper-independent retrovirus expressing the S3-v-ErbB-TM- mutant exhibit only limited tumor formation in vivo. These results demonstrate that the transmembrane domain and, consequently membrane localization, are essential for S3-v-ErbB-mediated transformation.

Generation and characterization of polyclonal antibodies specific for human p110 sEGFR

The EGFR/ERBB family of receptor tyrosine kinases mediates intracellular signal transduction pathways important in the regulation of cell growth, differentiation, and transformation. We previously have reported the cloning and expression of a 3 kb alternative EGFR transcript which encodes a 110 kDa form of the receptor (p110 sEGFR). This receptor isoform is identical to the extracellular region of the full-length 170 kDa EGFR through amino acid 603; in addition, p110 sEGFR contains 78 unique carboxy-terminal amino acids. Here, we report the generation and characterization of polyclonal antisera specific for the unique carboxy-terminal sequence of p110 sEGFR. Polyclonal antisera were generated by immunizing rabbits with synthetic peptides corresponding to peptides contained within the unique carboxy-terminal sequence of p110 sEGFR. Immunoglobulin fractions from antisera which tested positive for immune reactivity to these peptides by ELISA were affinity-purified by protein G and peptide-based chromatography. This affinity-purified immunoglobulin fraction specifically recognizes p110 sEGFR by ELISA, immunoprecipitation, immunoblot analysis, and immunocytochemical methods. No cross-reactivity with full-length p170 EGFR is observed using any of these detection methods. These new polyclonal antibodies will be useful in determining the expression, localization, and function of p110 sEGFR, and importantly will allow us to distinguish between the expression of this receptor isoform and p170 EGFR.

Activation of Rho is required for ligand-independent oncogenic signaling by a mutant epidermal growth factor receptor

Mutations in the epidermal growth factor receptor have been identified in several human tumor types, including gliomas. These receptor mutants have deletions in their extracellular ligand-binding domains and are, therefore, no longer regulated by ligand, resulting in constitutive activation of the receptor kinase. These mutants have been proposed to transduce oncogenic signals via ligand-independent signaling pathways. Avian viral homologues of these oncogenic epidermal growth factor receptors exhibit structurally homologous deletions and form tumors in chickens. One such mutant, S3v-ErbB, transforms fibroblasts in vitro, and transformation has been correlated with the formation of a novel tyrosine phosphoprotein complex. V-ErbB-mediated complex formation and transformation have been shown to occur independently of Ras activation. The major aims of this study are to further characterize this ligand-independent v-ErbB oncogenic signaling pathway. Here we show that both v-ErbB-mediated phosphoprotein complex formation and transformation are inhibited by a dominant negative mutant of Rho. This inhibition is specific for dominant negative Rho; dominant negative mutants of Rac and Cdc42 have no effect on transformation or on tyrosine phosphorylation of the phosphoprotein complex. Based on these observations, we propose that S3v-ErbB stimulates a Rho-dependent tyrosine kinase, resulting in complex formation and ultimately oncogenic transformation.

Ras-independent oncogenic transformation by an EGF-receptor mutant

Mutations in the ligand-binding domain of the epidermal growth factor receptor have been identified in several types of human cancers, including malignant gliomas. These mutations render signaling by this receptor to be constitutively ligand-independent. In fibroblasts transformed with ligand-independent epidermal growth factor receptor mutants, there is a correlation between the formation of a unique phosphotyrosine protein complex and oncogenic transformation. This phosphoprotein complex includes Grb2, Shc, Sos, tyrosine-phosphorylated form of caldesmon, and two, as yet, unidentified proteins. The presence of Grb2, Shc, and Sos in this complex implicates Ras in ligand-independent signaling by these oncogenic epidermal growth factor receptor mutants. We, therefore, have used retroviral co-infections of cultured primary fibroblasts to determine if Ras activation is required for phosphoprotein complex formation, stress fiber loss, or transformation. As predicted, expression of a dominant-negative Ras mutant (N17Ras) completely abrogates ligand-stimulated soft agar colony growth of primary fibroblasts. In contrast, N17Ras expression has no effect on v-ErbB mediated stress fiber disassembly, soft agar colony growth, or phosphoprotein complex assembly. In addition, our data suggest that ligand-dependent Ras activation may be suppressed by oncogenic v-ErbB expression. Together these observations suggest that oncogenic signaling by v-ErbB does not require Ras activation, and implicate an alternative signal transduction pathway in ligand-independent epidermal growth factor receptor oncogenic signaling.

A sandwich type acridinium-linked immunosorbent assay (ALISA) detects soluble ErbB1 (sErbB1) in normal human sera

The epidermal growth factor receptor (ErbB1) is overexpressed in various human tumor-derived cell lines and neoplasms, where it is believed that receptor dysregulation plays a role in oncogenic transformation and tumor progression. In addition to the ErbB1 holoreceptor, numerous studies demonstrate that cells synthesize soluble or secreted forms of ErbB1, i.e., sErbB1. Overexpression of ErbB1 in a variety of tumors has led us to hypothesize that sErbB levels also may be altered during oncogenesis, tumor progression, and/or metastasis; and that these molecules may be useful tumor biomarkers. To address this hypothesis we have developed an acridinium-linked immunosorbent assay (ALISA) specific for the extracellular domain of ErbB1 that can be used to quantify the levels of sErbB1 molecules in body fluids and conditioned culture media. This assay can also detect full-length ErbB1 in cell and tissue extracts. Our ALISA is characterized by high sensitivity (intra-assay LLD < 1 fmol/ml), a broad linear range (approximately 1 to 4000 fmol/ml), and good reproducibility (CVs < 10%). Specificity experiments show that this ALISA detects p170 ErbB1 and soluble forms of ErbB1 that embody extracellular subdomains I through IV, but not forms of sErbB1 lacking subdomain IV. Our ALISA does not detect full-length ErbB2, ErbB3, or ErbB4; or p105 soluble ErbB2. We report that serum sErbB1 levels of healthy women (median = 3716 fmol/ml), ranging in age from 43 to 76 years, differ significantly from those of healthy men (median = 24,512 fmol/ml), ranging in age from 25 to 79 years. Additional analyses do not indicate that serum sErbB1 levels change with age in either healthy men or women. Immunoprecipitation experiments show that monoclonal antibodies specific for extracellular epitopes of ErbB1 completely neutralize the detection of sErbB1 in normal human sera by ALISA. Finally, we show by immunoprecipitation and Western immunoblot analyses with monoclonal antibodies specific for the extracellular domain of ErbB1 that normal human female and male sera contain a approximately 110-kDa protein. We conclude that our ALISA is measuring the relative levels of this p110 sErbB1 analog in normal human sera. Our ALISA, therefore, should be useful for measuring the levels of ErbB1 and sErbB1 molecules in tumor biopsy specimens and body fluids, respectively, and for determining whether sErbB1, like ErbB1, is a useful tumor biomarker.

Tissue- and transformation-specific phosphotyrosyl proteins in v-erbB-transformed cells

To understand the mechanism of tissue-specific and transformation-specific signaling by the v-ErbB oncoprotein, we have investigated signaling pathways downstream of this transmembrane tyrosine kinase. In this report, we describe tissue-specific patterns of phosphotyrosyl proteins in three distinct cell types transformed by the v-erbB oncogene: fibroblasts, erythroblasts, and endothelial cells. In addition, we describe transformation-specific tyrosine phosphorylation events and signal complex formation in v-erbB-transformed fibroblasts. Two patterns of phosphotyrosyl proteins have been detected in v-erbB-transformed cells. The first is a fibroblast-specific pattern which includes unique phosphotyrosyl proteins of 170 kDa (c-ErbB1), 158 kDa, and 120 kDa (the catenin-like protein p120cas). The second is an erythroblast/endothelial cell-specific pattern which includes a prominent unidentified phosphotyrosyl protein of 120 kDa. Evaluation of the phosphotyrosyl proteins p120cas and SHC in chicken embryo fibroblasts infected with transforming and nontransforming v-erbB mutants reveals transformation-specific patterns of tyrosine phosphorylation. One corollary of these phosphorylation events in v-erbB-transformed fibroblasts is the formation of a complex involving SHC, growth factor receptor-bound protein 2, and a novel 75-kDa phosphotyrosyl protein. The results of these studies suggest that the v-ErbB oncoprotein can couple to multiple signal transduction pathways, that these pathways are tissue specific, and that v-erbB-mediated transformation involves specific tyrosine phosphorylation events.

EGF-R as a hemopoietic growth factor receptor: the c-erbB product is present in chicken erythrocytic progenitors and controls their self-renewal

c-erbB, encoding the EGF receptor (EGF-R), was originally identified as the cellular homolog of a chicken leukemia oncogene. In humans, EGF-R is distributed widely except in hemopoietic tissues, and its amplification is associated with epidermal and glial malignancies. Here we show that c-erbB is present in normal chicken erythrocytic progenitors and transmits the mitogenic signal induced by TGF alpha. Cells that contain high affinity EGF-R are at approximately the BFU-E stage, and their long-term renewal can be induced by TGF alpha. Upon addition of insulin and erythropoietin, they can be induced to terminally differentiate into red cells. We previously demonstrated that v-erbA blocks differentiation of chicken erythrocytic progenitors but does not abrogate their growth factor dependence for proliferation. These data indicate that proliferation and differentiation are not necessarily coupled in these cells. They also demonstrate a direct role of c-erbB in the control of self-renewal of normal chicken erythrocytic progenitors and could account for the predominant leukemogenic potential of the chicken erbB gene.

Magnesium transport in Salmonella typhimurium: expression of cloned genes for three distinct Mg2+ transport systems

In Salmonella typhimurium, the corA, mgtA, and mgtB loci are involved in active transport of Mg2+ (S. P. Hmiel, M. D. Snavely, C. G. Miller, and M. E. Maguire, J. Bacteriol. 168:1444-1450, 1988; S. P. Hmiel, M. D. Snavely, J. B. Florer, M. E. Maguire, and C. G. Miller, J. Bacteriol. 171:4742-4751, 1989). In this study, the gene products coded for by the corA, mgtA, and mgtB genes were identified by using plasmid expression in Escherichia coli maxicells. Complementation was assessed by introducing plasmids into a Mg2+-dependent corA mgtA mgtB strain and determining the ability of the plasmid to restore growth on medium without a Mg2+ supplement. Complementing plasmids containing corA expressed a 42-kilodalton (kDa) protein. This protein was not expressed by plasmids containing insertions or deletions that eliminated complementation. A plasmid containing mgtA expressed 37- and 91-kDa gene products. Data obtained with subclones and insertions in this plasmid indicated that plasmids expressing only the 91-kDa polypeptide complemented; plasmids that did not express this protein did not complement regardless of whether they expressed the 37-kDa protein. Plasmids carrying mgtB expressed a single protein of 102 kDa whose presence or absence correlated with the ability of the plasmid to complement the Mg2+-dependent triple mutant. Fractionation of labeled maxicells demonstrated that the 42-kDa corA, the 91-kDa mgtA, and the 102-kDa mgtB gene products are all tightly associated with the membrane, a location consistent with involvement in a transport process. These data provide further support the for existence of three distinct systems for Mg2+ transport in S. typhimurium.