Site-specific mutagenesis of avian erythroblastosis virus: erb-B is required for oncogenicity

Relationship between retroviral DNA-integration-site selection and host cell transcription

Retroviral DNA integration occurs throughout the genome; however, local "hot spots" exist where a strong preference for certain sites over others are seen, and more global preferences associated with genes have been reported. Previous data from our laboratory suggested that there are fewer integration events into a DNA template when it is undergoing active transcription than when it is not. Because these data were generated by using a stably transfected foreign gene that was only weakly inducible, we have extended this observation by comparing integration events into a highly inducible endogenous gene under both induced and uninduced transcriptional states. To examine the influence of transcription on site selection directly, we analyzed the frequency and distribution of integration of avian retrovirus DNA into the metallothionein gene, before and after its induction to a highly sustained level of expression by addition of ZnSO4. We found a 6-fold reduction in integration events after 100-fold induction of transcription. This result implies that, despite an apparent preference for integration of retroviral DNA into transcribed regions of host DNA, high-level transcription can be inhibitory to the integration process. Several possible models for our observation are as follows. First, when a DNA template is undergoing active transcription, integration might be blocked by the RNA polymerase II complex because of steric hindrance. Alternatively, the integrase complex may require DNA to be in a double-stranded conformation, which would not be the case during active transcription. Last, transcription might lead to remodeling of chromatin into a structure that is less favorable for integration.

Relationship between retroviral DNA integration and gene expression

Although retroviruses can integrate their DNA into a large number of sites in the host genome, factors controlling the specificity of integration remain controversial and poorly understood. To assess the effects of transcriptional activity on integration in vivo, we created quail cell clones containing a construct with a minigene cassette, whose expression is controlled by the papilloma virus E2 protein. From these clones we derived transcriptionally active subclones expressing the wild-type E2 protein and transcriptionally silent subclones expressing a mutant E2 protein that binds its target DNA but is unable to activate transcription. By infecting both clones and subclones with avian leukosis virus and using a PCR-based assay to determine viral DNA integration patterns, we were able to assess the effects of both protein binding and transcriptional activity on retroviral DNA integration. Contrary to the hypothesis that transcriptional activity enhances integration, we found an overall decrease in integration into our gene cassette in subclones expressing the wild-type E2 protein. We also found a decrease in integration into our gene cassette in subclones expressing the mutant E2 protein, but only into the protein binding region. Based on these findings, we propose that transcriptionally active DNA is not a preferred target for retroviral integration and that transcriptional activity may in fact be correlated with a decrease in integration.

Detection of avian leukosis virus in albumen of chicken eggs using reverse transcription polymerase chain reaction

A reverse transcriptase polymerase chain (RT-PCR) assay was developed to detect avian leukosis retrovirus (ALV) in egg albumen. Eggs of Single Comb White Leghorns were from a commercial breeder (stock F) and from a pathogen-free flock (stock N). RT-PCR was undertaken on isolated RNA from 20 unfertilized egg samples using seven sets of primers that correspond to the ALV gp85 envelope glycoprotein which determines the ALV subgroup classification. An ELISA assay for ALV gs antigen of egg albumen was positive for all stock F birds tested and negative for all stock N birds. Virus isolation was undertaken by inoculating egg albumen, feather pulp, or blood from five stock F chickens onto cultures of chicken embryo fibroblasts (C/E). IFA analysis of the inoculated C/E cultures indicated that all stock F birds tested contained infectious ALV. For the virus-positive stock F chickens, RT-PCR analyses using primers designed to detect all ALV subgroups detected ALV in 15/15 (100%) egg albumen samples, while primers designed to detect subgroup A ALV were positive for 12/15 (80%) egg albumen samples. RT-PCR products were not detected from five egg albumen samples from five stock N chickens by any primer sets. Direct sequencing using primers specific for subgroup A ALV verified the viral subgroup in the RT-PCR amplification products. The combined use of RT-PCR and direct sequencing of the RT-PCR product provides a new approach for identifying ALV-infected poultry.

Selection of an avian retrovirus mutant with extended receptor usage

Receptor recognition by avian retroviruses is thought to involve the interaction of two regions of the SU protein, hr1 and hr2, with the host cell surface receptor. These regions exhibit considerable variation, concordant with differences in receptor usage among the many avian leukosis virus subgroups. We hypothesize that some retroviruses have altered receptor usage in response to selective pressures imposed by receptor polymorphisms in their hosts. To test this hypothesis, we passaged td-Pr-RSV-B on cocultured permissive chicken (C/E) and nonpermissive quail (QT6/BD) cells. A variant virus with an expanded host range was identified at passage 29 and ultimately shown to be identical in sequence to td-Pr-RSV-B, except for changes at codons 155 and 156 of SU amino acid corresponding to two amino acid changes within hr1. Superinfection resistance studies suggest that the variant virus recognizes the subgroup B receptor on chicken cells and the subgroup E receptor on quail cells. These findings indicate that altered receptor usage can be conferred by small changes in env and may point to a key region for receptor interaction. Further, they demonstrate the evolutionary potential of retroviral env genes to alter receptor usage in response to appropriate selective pressure.

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.

Increased expression of and sensitivity to transforming growth factor-alpha: a promotive role during rat liver carcinogenesis

The influence of the tumor promoter 2-acetylaminofluorene (2-AAF) on cell proliferation and on the epidermal growth factor receptor (EGFR) system was assessed in normal and nodular rat livers. DNA replication in vivo was inhibited below the detection level after 8d of dietary 2-AAF treatment of previously unexposed rats. The 2-AAF-induced growth inhibition was accompanied by downregulation of the number of epidermal growth factor (EGF)-binding sites and decreased levels of EGFR transcripts, whereas no changes in the transforming growth factor-alpha (TGF-alpha) mRNA levels were observed. The persistent liver nodules generated by intermittent 2-AAF-feeding had a 30- to 35-fold higher replicating cell fraction than normal liver. Treatment with 2-AAF in vivo reduced the replicating cell fraction to one third in nodules after 14 d of 2-AAF treatment. The initial EGFR mRNA levels and number of EGF binding sites in nodules before 2-AAF administration was about 605 that of control livers and was slightly reduced by 2-AAF feeding. The levels of EGFR mRNA after 14 d of 2-AAF feeding were thus similar in the nodules and in the 2-AAF-treated control livers, whereas the fraction of proliferating cells in nodules after the 2-AAF treatment was much larger than in normal liver. The TGF-alpha mRNA level in the nodules was found to be 1.4-fold and in malignant hepatomas 1.7-fold the level in normal liver. Primary hepatocytes isolated from control livers were four to five times more sensitive to replicative stimulation with EGF than with TGF-alpha, whereas nodular cells responded at lower concentrations than control cells and equally well to both EGF and TGF-alpha. We conclude that the decreased amounts of EGFR in the nodular cells with respect to proliferative stimulation could be more than compensated for by elevated synthesis of TGF-alpha combined with an increased TGF-alpha sensitivity. Collectively, these changes implicate TGF-alpha in sustaining cell proliferation during chemically induced rat liver carcinogenesis.

Influence of sequences in the long terminal repeat and flanking cell DNA on polyadenylation of retroviral transcripts

Readthrough transcripts are formed during retrovirus infection by polyadenylation of viral RNA in cellular sequences adjacent to the provirus. We have studied such transcripts in avian leukosis virus-infected cell clones containing a single provirus, either the wild type or one with an inactivating mutation in the poly(A) addition signal. All individual wild-type proviruses produced readthrough transcripts, implying that this property is not restricted to a few integration sites. The range of sizes of viral RNA in the mutant lacking a correct signal for poly(A) addition reflected both the occurrence of functional polyadenylation sites within flanking cell DNA and increased usage of cryptic sites within viral sequences.

Induction of renal adenocarcinoma by a nonmutated erbB oncogene

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

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

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

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

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

Mechanism of transduction by retroviruses

Retroviruses can capture cellular sequences and express them as oncogenes. Capture has been proposed to be a consequence of the inefficiency of polyadenylation of the viral genome that allows the packaging of cellular sequences flanking the integrated provirus in virions; after transfer into virions, these sequences could be incorporated into the viral genome by illegitimate recombination during reverse transcription. As a test for this hypothesis, a tissue culture system was developed that mimics the transduction process and allows the analysis and quantitation of capture events in a single step. In this model, transduction of sequences adjacent to a provirus depends on the formation of readthrough transcripts and their transmission in virions and leads to various recombinant structures whose formation is independent of sequence similarity at the crossover site. Thus, all events in the transduction process can be attributed to the action of reverse transcriptase on readthrough transcripts without involving deletions of cellular DNA.

Clonal cells from embryonic retinal cell lines express qualitative electrophysiological differences

Cells from the embryonic quail retina were immortalized with the v-mil oncogene and cloned by limiting dilution. Their phenotype was examined using the whole-cell patch clamp method. Three membrane currents, IK(IR), INa and IK, were found at different frequencies within a sample of 170 cells drawn from a large clone. Nearly all combinations of these three markers were found and the frequency of combinations showed that the markers assorted independently. Examination of clones of less than 10 cells showed that heterogeneity originates with a high probability within clones, arguing that chromosomal mutation, for example, is unlikely to account for phenotypic diversity. A possible explanation is that phenotypic differences between cells might reflect the local exchange of instructive signals. If so, then the genes for the three phenotypic markers are controlled independently.

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.

Ontogeny of the v-erbA oncoprotein from the thyroid hormone receptor: an alteration in the DNA binding domain plays a role crucial for v-erbA function

The avian erythroblastosis virus v-erbA oncogene is imprecisely derived from a cellular gene (c-erbA) encoding a thyroid hormone receptor: the v-erbA protein has sustained both small terminal deletions and internal amino acid sequence changes relative to c-erbA. We report here that one of these missense differences between v- and c-erbA proteins, located in a zinc finger DNA binding domain, has dramatic effects on the biological activities of the encoded protein. Back mutation of the viral coding sequence to resemble c-erbA at this site severely impairs erythroid transformation and produces subtle changes in DNA binding by the encoded protein, suggesting that differences in DNA binding by the viral and cellular proteins may be involved in the activation of v-erbA as an oncogene.

Phosphorylation of the v-erbA protein is required for its function as an oncogene

The v-erbA oncogene of avian erythroblastosis virus (AEV) encodes a ligand-independent mutated version of the chicken c-erbA alpha-encoded thyroid hormone receptor. The v-erbA gene product, a 75-kD gag/v-erbA fusion protein, is phosphorylated on Ser-16/17 of its v-erbA-encoded domain, and phosphorylation at this site is increased in vivo after activation of either the PKA or PKC signal transduction pathways. To test the hypothesis that phosphorylation of Ser-16/17 regulates gag/v-erbA protein function, mutant proteins in which Ser-16/17 had been changed to alanine or threonine residues were analyzed for their ability to inhibit erythroid differentiation of ts v-erbB or ts v-sea-transformed erythroblasts at nonpermissive temperature. Conversion of Ser-16/17 into alanine, although not affecting nuclear localization or DNA binding of the gag/erbA protein, prevented phosphorylation of the v-erbA-encoded domain of the protein both in unstimulated cells or after stimulation by PKA and PKC activators. The nonphosphorylatable AA-gag/v-erbA protein proved unable to inhibit temperature-induced differentiation of ts v-erbB and ts v-sea-transformed erythroblasts and to block expression of the erythrocyte-specific genes band 3 and carbonic anhydrase II. Back mutation of these alanine residues to serine resulted in the recovery of both normal phosphorylation levels and wild-type biological activity. In contrast, substitution of Ser-16/17 for threonine, which preserved phosphorylation in unstimulated cells but not PKA- and PKC-enhanced phosphorylation, resulted in a partially active gag/v-erbA protein. These results, together with the fact that the protein kinase inhibitor H7 resulted in both a dose-dependent inhibition of gag/v-erbA protein phosphorylation and the induction of terminal differentiation of AEV-transformed erythroblasts show that phosphorylation of gag/v-erbA protein is required for full biological activity. These results support the hypothesis that phosphorylation of the gag/v-erbA protein is important for transcriptional repression of at least some of its target genes in erythroid cells.

Delineation of functional determinants in the transforming protein of Fujinami sarcoma virus

We analyzed linker insertion mutations throughout the 3' region of the v-fps gene of Fujinami sarcoma virus to identify tyrosine kinase transforming protein (P130gag-fps) determinants that are important for catalysis and transforming activity and, in particular, to define residues that participate in substrate selection. Mutations that encode kinase-active, transformation-defective v-fps alleles were recovered, defining sites in the transforming protein that may normally facilitate kinase-substrate interaction. Additionally, one region within the catalytic domain of the transforming protein (amino acid residues 1012 to 1020) that tolerates peptide insertions without loss of transforming activity was discovered, although the insertion mutations in this region of v-fps exhibited qualitatively abnormal transforming function. Transformed rat cell lines that express these mutations displayed unusual phenotypes, including giant cells and cells with an extremely fusiform shape. Furthermore, the insertion mutations in this region were temperature sensitive, transformed cells assumed a flat morphology, cellular protein phosphotyrosine was reduced, and the kinase activity of the transforming protein was decreased when cells were incubated at 40.5 degrees C. Point mutations that specify the ancestral chicken c-fps sequence in the insertion-tolerant region were also introduced into v-fps. These back mutations led to a modest decrease in kinase activity, decreased tumorigenic potential in chickens, and an unexpected increase in transforming activity in rat cells. These results indicate that the insertion-tolerant region of P130gag-fps influences the biologic activity and thermostability of the kinase.

Transformation of chicken embryo fibroblasts by direct DNA transfection of single oncogenes: comparative analyses of src, erbB, myc, and ras

Chicken embryo fibroblasts (CEF) have been used extensively to study the transformation parameters of a number of avian sarcoma-leukemia viruses. Previously, oncogene transformation of CEF has been conducted almost exclusively with replicating viruses, because of perceived difficulties with direct DNA transfection. Here, we show that CEF can be efficiently and stably transfected by selection for the neomycin resistance gene (neo). Cotransfection of neo with various oncogenes resulted in CEF transformation in vitro and, in several instances, sarcoma formation in vivo. Transfection of src, myc, erbB, and ras, either singly or in combination, resulted in soft-agar colonies with unique morphologies. Transfection of a family of v-src, c-src, and v/c-src chimeric constructs demonstrated the ability of the assay to discriminate between transforming and nontransforming genes. Transfection of a number of erbB variants showed that internal mutations, primarily in the kinase domain, contribute significantly to the ability to transform fibroblasts. The tumorigenic potential detected by transfection of oncogenes faithfully reproduced those previously reported by using viral infections. Our studies establish the utility of CEF transformation by direct DNA transfection. This method should prove useful in analyzing oncogenes, (e.g., myc) that do not readily transform rodent cell lines and in studying host-range mutants of oncogenes, such as those recently identified for src and erbB.

EGF receptor down-regulation attenuates ligand-induced second messenger formation

Epidermal growth factor (EGF)-induced increases in cytosolic Ca2+ and inositol polyphosphate production were compared in a human hepatocellular carcinoma-derived cell line, PLC/PRF/5, and in an EGF receptor-overexpressing subline, NPLC/PRF/5. Formation of these second messengers was correlated to EGF receptor display at the cell surface by monitoring ligand-induced EGF receptor down-regulation. Both cell lines exhibited a strikingly similar cytosolic Ca2+ increase upon exposure to EGF. The initial inositol phosphate responses were also similar in the two cell lines; inositol 1,4,5-trisphosphate increased within 10-15 s and returned to prestimulatory values after 2 min in both cell lines, while inositol tetrakisphosphate and inositol 1,3,4-trisphosphate were elevated after a 2-min exposure to EGF. At later times the responses were markedly different; NPLC/PRF/5 cells exhibited prolonged production of inositol 1,3,4-trisphosphate and inositol tetrakisphosphate (maximum at 1-3 h) but PLC/PRF/5 cells showed decreased levels of these isomers after 10 min and a return to basal values by 1 h. Exposure of PLC/PRF/5 cells to EGF caused a progressive decrease in the amount of EGF receptor at the cell surface whereas such treatment did not change the surface receptor levels in NPLC/PRF/5 cells. Kinetic analysis of EGF receptor down-regulation showed that receptor internalization was rapid enough to account for the transient nature of the inositol phosphate response in PLC/PRF/5 cells. Thus, the divergent patterns of signaling exhibited by the two cell lines may reflect differences in the efficiency of EGF-induced down-regulation of surface receptors.

c-erbB-2/c-erbA co-amplification indicative of lymph node metastasis, and c-myc amplification of high tumour grade, in human breast carcinoma

A panel of 73 samples, including 52 primary breast carcinomas, 10 normal breast tissues and 11 axillary lymph nodes, has been analysed for the presence of amplifications and gross structural alterations, in the oncogenes c-erbB-2, c-erbA, c-myc, N-myc, c-mos and c-Ha-ras. The tumours were also classified, graded and staged histopathologically and their DNA ploidy (42 samples) was determined by flow cytometry. Three breast cancer cell lines (MCF7, ZR-75-1 and T47D) were also included in the study. Amplification of c-erbB-2 was detected in 28% of the tumours, of which 91% had an increased steady-state level of c-erbB-2 mRNA. Amplification of c-erbA was found in 23% of tumours and was always associated with the amplification of c-erbB-2. Ten out of 12 (83%) tumours which had c-erbB-2 and c-erbA co-amplification had metastasised to axillary lymph nodes (P less than 0.006). However, the human thymidine kinase gene, which is present at the same chromosomal location as these two oncogenes (17q21-22), was amplified in only tw tumours. Amplification of c-myc was detected in 21% of the tumours studied, of which 82% (P less than 0.005) were of histopathological grade 3 and none were of grade 1. Flow cytometry showed that 90% (P less than 0.01) of the analysed tumours with c-erbB-2 and c-erbA co-amplification, and 70% (P less than 0.1) of those with c-myc amplification were DNA aneuploid. This study demonstrates the potential value of c-myc amplification in the assessment of the tumour grade, rather than metastatic potential; and of the co-amplification of c-erbB-2 and c-erbA as a strong indicator of metastatic potential, rather than tumour grade.

Polyadenylation at correct sites in genome RNA is not required for retrovirus replication or genome encapsidation

RNA transcripts polyadenylated at sites derived from flanking cellular DNA (readthrough transcripts) make up about 15% of the viral RNA in cells infected with avian leukosis virus. To test the functionality of such transcripts, a virus was created by introducing two mutations into the AAUAAA polyadenylation signal of Rous-associated virus 1, converting it to AAGGAA. The replication of this virus was not greatly affected at any level. However, less than 1% of viral transcripts produced during mutant virus replication were cleaved and polyadenylated at the correct site within viral long terminal repeat-related sequence. These results imply that readthrough transcripts, which are produced during normal viral replication, are polyadenylated and packaged into virions as normal transcripts and can serve as RNA genomes in the next round of replication. These results show that polyadenylation within virus-related sequences is not a necessary requirement for virus replication and that readthrough transcripts have the necessary properties to be intermediates in the process of transduction of cellular sequences.

Retrovirally induced murine B-cell tumors rarely show proviral integration in sites common in T-cell tumors

The molecular etiology of retrovirally induced T-cell tumors has been shown in many cases to involve proviral integration near a cellular oncogene, c-myc, N-myc, Pim-1 and pvt-1 being frequent targets for insertional activation. Murine B-cell tumors induced by infection with murine leukemia virus have been studied for rearrangements in these and other loci. In contrast to the T-cell lymphomas, tumors of the B-cell lineage, either early B-cell tumors induced in nude mice or late B-cell tumors in immunocompetent mice, did not show disruption of N-myc or Pim-1 in any of the tumors studied, although those lymphomas had acquired many new proviruses. The loci c-abl, bcl-2, fis-1, c-erbB, c-myb, and neu were likewise not involved. Rearrangement of c-myc was seen in 1 out of 71 and rearrangement of the pvt-1 locus in 4 out of 73 (5%) of the B-cell tumors. Thus it appears that mechanistic differences exist in the development of T-cell tumors and B-cell tumors caused by the same etiological agent.

Avian erythroblastosis virus transforms a novel mast cell-basophil precursor target in the Japanese quail

Hematopoietic cells of the Japanese quail were transformed by avian erythroblastosis virus in vivo and in vitro. In both circumstances, the infected hematopoietic tissues exhibited a dual oncogenic response of erythroid and mast cell-basophil elements. The erythroid transformants escaped the avian erythroblastosis virus block in differentiation and progressed to hemoglobinization. Resulting basophilic cells were morphologically, biochemically, and ultrastructurally identical to mast cell-basophils observed in other species. None of the virally transformed cells actively produced reverse transcriptase activity. Nonproducer cell lines synthesized viral RNA and both v-erbA and v-erbB proteins. These results indicate that the Japanese quail has a viral target cell different from that of the chicken. The implications of a single bipotential transformation target yielding both erythroid and mast cell-basophil colonies is discussed.

Segregation, viral phenotype, and proviral structure of 23 avian leukosis virus inserts in the germ line of chickens

We have artificially introduced 23 avian leukosis virus (ALV) proviral inserts into the chicken germ line by injection of wild-type and recombinant subgroup A ALV near the blastoderm of fertile eggs just before incubation. Eight viremic males were identified as germline mosaics because they transmitted proviral DNA to their generation 1 (G-1) progeny at a low frequency. Eleven female and 9 male G-1 progeny carried 23 distinct proviruses that had typical major clonal proviral-host DNA junction fragments detectable after digestion of their DNA with SacI, Southern blotting and hybridization with a probe representing the complete ALV genome. These proviruses, identified by their typical proviral-host DNA junction fragments, were transmitted to approximately 50% of their G-2 progeny after mating the G-1 parents to a line of chickens lacking endogenous ALV proviral inserts. One G-1 female carried 2 proviruses and another 3. The proviruses appeared to be scattered throughout the genome. One of the 14 proviruses carried by females was on the sex (Z) chromosome. Two of the 3 proviruses carried by a single G-1 female were linked with a recombination frequency of about 0.20. Twenty-one of the proviruses coded for infectious ALV. Two proviruses coded for envelope glycoprotein, and cell cultures carrying them were relatively resistant to subgroup A sarcoma virus, but failed to produce infectious ALV. One of these proviruses coded for internal gag proteins, had a deletion in pol, but produced non-infectious virus particles. The other failed to code for gag proteins and had no detectable internal deletions nor did it produce virus particles. Thus, we have shown that replication-competent ALV can artificially infect germ-line cells and that spontaneous defects in the inherited proviruses occur at a rather low rate.

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.

Thyroid hormone action and the erbA oncogene family

In this review, we discuss the biological action and biochemical function of the v-erbA oncogene product, and the role of c-erbA proto-oncogene products as thyroid hormone receptors, as related to the molecular structure and function of the nuclear hormone receptors at large.

c-erbA encodes multiple proteins in chicken erythroid cells

To identify and characterize the proteins encoded by the erbA proto-oncogene, we expressed the C-terminal region of v-erbA in a bacterial trpE expression vector system and used the fusion protein to prepare antiserum. The anti-trp-erbA serum recognized the P75gag-erbA protein encoded by avian erythroblastosis virus and specifically precipitated six highly related proteins ranging in size from 27 to 46 kilodaltons from chicken embryonic erythroid cells. In vitro translation of a chicken erbA cDNA produced essentially the same pattern of proteins. Partial proteolytic maps and antigenicity and kinetic analyses of the in vivo and in vitro proteins indicated that they are related and that the multiple bands are likely to arise from internal initiations within c-erbA to generate a nested set of proteins. All of the c-erbA proteins are predominantly associated with chicken erythroblast nuclei. However, Nonidet P-40 treatment resulted in extraction of the three smaller proteins, whereas the larger proteins were retained. During differentiation of erythroid cells in chicken embryos, we found maximal levels of c-erbA protein synthesis at days 7 to 8 of embryogenesis. By contrast, c-erbA mRNA levels remained essentially constant from days 5 to 12. Together, our results indicate that posttranscriptional or translational mechanisms are involved in regulation of c-erbA expression and in the complexity of its protein products.

Genetic dissection of functional domains within the avian erythroblastosis virus v-erbA oncogene

The avian erythroblastosis virus v-erbA locus potentiates the oncogenic transformation of erythroid and fibroblast cells and is derived from a host cell gene encoding a thyroid hormone receptor. We report here the use of site-directed mutagenesis to identify and characterize functional domains within the v-erbA protein. Genetic lesions introduced into a putative hinge region or at the extreme C-terminus of the v-erbA coding domain had no significant effect on the biological activity of this polypeptide. In contrast, mutations introduced within the cysteine-lysine-arginine-rich center of the v-erbA coding region, a DNA-binding domain in the thyroid and steroid hormone receptors, abolished or severely compromised the ability of the viral protein to function. Our results suggest that the mechanism of action of the v-erbA protein in establishing the neoplastic phenotype is closely related to its ability to interact with DNA, presumably thereby altering expression of host target genes by either mimicking or interfering with the action of the normal c-erbA gene product.

Mega base map of the epidermal growth factor (EGF) receptor gene flanking regions and structure of the amplification units in EGF receptor-hyperproducing squamous carcinoma cells

We have established a mega base scale physical map of the 5'- and 3'-flanking regions of the epidermal growth factor (EGF) receptor gene using CpG-recognition rare-cutting restriction enzymes and pulsed-field gel electrophoresis. In this map, a methylation-free CpG island (HTF island) is located within an 8-kilobase pair (kb) EcoRI fragment which includes exon 1 of the EGF receptor gene. From this HTF island, a 390-kb NotI fragment was identified as the longest 5'-flanking region and a 540-kb MluI fragment as the longest 3'-flanking region. Utilizing this map information, we have analyzed the structure of the flanking regions of amplified EGF receptor genes which are found in various squamous carcinoma cells. Among seven cell lines tested, four cell lines carrying EGF receptor genes in amounts more than 20 times that of normal cells showed amplification together with large 5'- and 3'-flanking regions. The amplified 5'-flanking regions were rearranged in different forms but were distinct in each cell line. The amplified 3'-flanking regions were at least 540 kb in size and common to all the cell lines, except that A431 had rearrangement points within 540 kb downstream of the HTF island. Thus, the size of amplification units appears to be large and different in each cell line.

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.

Genetic dissection of functional domains within the avian erythroblastosis virus v-erbA oncogene

The avian erythroblastosis virus v-erbA locus potentiates the oncogenic transformation of erythroid and fibroblast cells and is derived from a host cell gene encoding a thyroid hormone receptor. We report here the use of site-directed mutagenesis to identify and characterize functional domains within the v-erbA protein. Genetic lesions introduced into a putative hinge region or at the extreme C-terminus of the v-erbA coding domain had no significant effect on the biological activity of this polypeptide. In contrast, mutations introduced within the cysteine-lysine-arginine-rich center of the v-erbA coding region, a DNA-binding domain in the thyroid and steroid hormone receptors, abolished or severely compromised the ability of the viral protein to function. Our results suggest that the mechanism of action of the v-erbA protein in establishing the neoplastic phenotype is closely related to its ability to interact with DNA, presumably thereby altering expression of host target genes by either mimicking or interfering with the action of the normal c-erbA gene product.

c-erbA encodes multiple proteins in chicken erythroid cells

To identify and characterize the proteins encoded by the erbA proto-oncogene, we expressed the C-terminal region of v-erbA in a bacterial trpE expression vector system and used the fusion protein to prepare antiserum. The anti-trp-erbA serum recognized the P75gag-erbA protein encoded by avian erythroblastosis virus and specifically precipitated six highly related proteins ranging in size from 27 to 46 kilodaltons from chicken embryonic erythroid cells. In vitro translation of a chicken erbA cDNA produced essentially the same pattern of proteins. Partial proteolytic maps and antigenicity and kinetic analyses of the in vivo and in vitro proteins indicated that they are related and that the multiple bands are likely to arise from internal initiations within c-erbA to generate a nested set of proteins. All of the c-erbA proteins are predominantly associated with chicken erythroblast nuclei. However, Nonidet P-40 treatment resulted in extraction of the three smaller proteins, whereas the larger proteins were retained. During differentiation of erythroid cells in chicken embryos, we found maximal levels of c-erbA protein synthesis at days 7 to 8 of embryogenesis. By contrast, c-erbA mRNA levels remained essentially constant from days 5 to 12. Together, our results indicate that posttranscriptional or translational mechanisms are involved in regulation of c-erbA expression and in the complexity of its protein products.

Mechanism of c-erbB transduction: newly released transducing viruses retain poly(A) tracts of erbB transcripts and encode C-terminally intact erbB proteins

We have previously shown that avian leukosis virus (ALV) induces erythroblastosis by insertional activation of the c-erbB gene. In 25% of the ALV-induced leukemic samples we have analyzed, acute retroviruses that have captured the activated erbB oncogene were released. The unusually high frequency at which erbB transduction occurs makes this an ideal system for studying the mechanism of oncogene transduction. In addition, these leukemic samples provide a rich source for the isolation of novel erbB-transducing viruses. We report here our characterization of several new erbB-transducing proviruses. The 5' recombination points of all these viruses mapped to the same intron in which proviral insertions cluster, supporting the hypothesis that transduction begins with proviral insertion near the oncogene. The 3' recombination points usually occurred within the 3' untranslated region downstream from the termination codon of the c-erbB gene. Three of the erbB-containing proviruses were molecularly cloned and analyzed in detail. Two of them were capable of releasing acute viruses, and interestingly, both retained poly(A) tracts of erbB messages in their genomes. A stretch of six adenosine residues in the ALV env gene appeared to mediate the 3' recombination events required for the generation of these viruses. These data provide further insight into the mechanism by which oncogenes are transduced into retroviral genomes.

Molecular characterization of three erbB transducing viruses generated during avian leukosis virus-induced erythroleukemia: extensive internal deletion near the kinase domain activates the fibrosarcoma- and hemangioma-inducing potentials of erbB

Three new erbB transducing viruses generated during avian leukosis virus-induced erythroblastosis have been cloned and sequenced, and their transforming abilities have been analyzed. Provirus 9134 E1 expresses an amino-terminally truncated erbB product that is analogous to the proviral insertionally activated c-erbB gag-erbB fusion product. This virus efficiently induces erythroblastosis, but does not transform fibroblasts in vitro or induce sarcomas in vivo. In contrast, virus 9134 S3 expresses an erbB product identical to the erbB product of 9134 E1, with the exception of a large internal deletion located between the kinase domain and the putative autophosphorylation site, P1. Interestingly, this virus is no longer capable of inducing erythroblastosis, but can induce both fibrosarcomas and hemangiomas in vivo. Provirus 9134 F3 has sustained an approximately 23-amino-acid carboxy-terminal truncation and is capable of inducing both erythroblastosis and sarcomagenesis. This virus expresses an erbB product with the shortest carboxy-terminal truncation sufficient to reveal the sarcomagenic potential of this protein. The distinct transforming properties of these viruses indicate that different structural domains of the erbB product confer distinct disease specificities.

Separation of simian virus 40 large-T-antigen-transforming and origin-binding functions from the ability to block differentiation

Wild-type simian virus 40 large T antigen is very effective at blocking adipocyte differentiation in 3T3-F442A cells as assayed by triglyceride accumulation, induction of glycerophosphate dehydrogenase activity, and expression of mRNAs for glycerophosphate dehydrogenase, the adipocyte serine protease adipsin, and the putative lipid-binding protein adipocyte P2. Point mutants defective for either origin-specific DNA binding or transformation blocked differentiation as completely as wild type.

Proviral-activated c-erbB is leukemogenic but not sarcomagenic: characterization of a replication-competent retrovirus containing the activated c-erbB

Avian leukosis virus (ALV) induces erythroblastosis in chickens by integrating its DNA into the host c-erbB locus and by activating expression of truncated c-erbB transcripts. Although there is a 100% correlation of c-erbB activation with ALV-induced erythroblastosis, direct evidence that the activated c-erbB is oncogenic has not been established. We have constructed a replication-competent retrovirus containing the activated c-erbB to investigate its transforming potential. The Rous c-erbB virus (REB-c) was constructed by inserting the activated c-erbB cDNA into a Rous sarcoma virus vector in place of src. When transfected into transformed quail fibroblasts (QT6), the REB-c construct stably integrates and expresses c-erbB-specific transcripts and produces infectious virus. The REB-c retrovirus produces short-latency polyclonal erythroblastosis in chickens. However, in contrast to avian erythroblastosis virus which contains v-erbB, the REB-c construct does not transform chicken embryo fibroblasts in vitro, nor does the REB-c virus produce sarcomas when injected into the wing web of chickens. Our results provide the first direct evidence that the activated c-erbB which lacks the amino-terminal extracellular domain but which retains the entire carboxy-terminal sequences is leukemogenic but not sarcomagenic.

Separation of simian virus 40 large-T-antigen-transforming and origin-binding functions from the ability to block differentiation

Wild-type simian virus 40 large T antigen is very effective at blocking adipocyte differentiation in 3T3-F442A cells as assayed by triglyceride accumulation, induction of glycerophosphate dehydrogenase activity, and expression of mRNAs for glycerophosphate dehydrogenase, the adipocyte serine protease adipsin, and the putative lipid-binding protein adipocyte P2. Point mutants defective for either origin-specific DNA binding or transformation blocked differentiation as completely as wild type.

The avian erythroblastosis virus erbA oncogene encodes a DNA-binding protein exhibiting distinct nuclear and cytoplasmic subcellular localizations

The protein product of the v-erbA oncogene of avian erythroblastosis virus was analyzed by use of site-specific antisera. The v-erbA protein was found to exist in distinct nuclear and cytoplasmic forms. Both nuclear and cytoplasmic species of the v-erbA protein were capable of binding to DNA, a property predicted based on the structural relatedness the v-erbA polypeptide shares with the thyroid and steroid hormone receptors. A mutation within the v-erbA coding region which inhibited DNA binding and nuclear localization also inhibited the ability of the v-erbA protein to potentiate erythroid transformation, consistent with a model of the v-erbA protein as a transcriptional regulator.

The avian retrovirus env gene family: molecular analysis of host range and antigenic variants

The nucleotide sequence of the env gp85-coding domain from two avian sarcoma and leukosis retrovirus isolates was determined to identify host range and antigenic determinants. The predicted amino acid sequence of gp85 from a subgroup D virus isolate of the Schmidt-Ruppin strain of Rous sarcoma virus was compared with the previously reported sequences of subgroup A, B, C, and E avian sarcoma and leukosis retroviruses. Subgroup D viruses are closely related to the subgroup B viruses but have an extended host range that includes the ability to penetrate certain mammalian cells. There are 27 amino acid differences shared between the subgroup D sequence and three subgroup B sequences. At 16 of these sites, the subgroup D sequence is identical to the sequence of one or more of the other subgroup viruses (A, C, and E). The remaining 11 sites are specific to subgroup D and show some clustering in the two large variable regions that are thought to be major determinants of host range. Biological analysis of recombinant viruses containing a dominant selectable marker confirmed the role of the gp85-coding domain in determining the host range of the subgroup D virus in the infection of mammalian cells. We also compared the sequence of the gp85-coding domain from two subgroup A viruses, Rous-associated virus type 1 and a subgroup A virus of the Schmidt-Ruppin strain of Rous sarcoma virus. The comparison revealed 24 nonconservative amino acid changes, of which 6 result in changes in potential glycosylation sites. The positions of 10 amino acid differences are coincident with the positions of 10 differences found between two subgroup B virus env gene sequences. These 10 sites identify seven domains in the sequence which may constitute determinants of type-specific antigenicity. Using a molecular recombinant, we demonstrated that type-specific neutralization of two subgroup A viruses was associated with the gp85-coding domain of the virus.

Efficient transformation by Prague A Rous sarcoma virus plasmid DNA requires the presence of cis-acting regions within the gag gene

A region in addition to and outside the long terminal repeats (LTRs) in the gag gene of the Prague A strain of Rous sarcoma virus was found to be essential in cis for efficient cell transformation by cloned viral DNA. Transformation in chicken embryo fibroblasts, which requires infectious virus production and reinfection, was facilitated in cis by sequences between nucleotides 630 and 1659. Efficient transformation of NIH 3T3 cells in which secondary spread of virus is not necessary (as it is in chicken embryo fibroblasts) required sequences between nucleotides 630 and 1149. A src cDNA clone which also lacks this region demonstrated low transformation efficiency, indicating that the role of the cis element cannot be attributed to interference with RNA splicing. The gag gene segment required in cis for transformation, between nucleotides 630 and 1149, could substitute for the simian virus 40 enhancer in either orientation, and cells transfected with Rous sarcoma virus LTR-driven plasmids containing the gag cis element had a two- to threefold increase in steady-state viral RNA levels compared with plasmids lacking this region. Thus, additional cis-acting regulatory elements located outside the viral LTRs may modulate viral gene expression and contribute to the efficiency of cell transformation.

The chicken c-erbA proto-oncogene is preferentially expressed in erythrocytic cells during late stages of differentiation

We analyzed the expression of the c-erbA proto-oncogene in different tissues of chicken embryos. c-erbA transcripts were found at low levels in the lung, kidney, liver, and heart and in high amounts in embryonic blood cells. Nuclease mapping assays proved that these transcripts were true c-erbA transcripts. In situ hybridization on fractionated embryonic blood cells showed that c-erbA transcripts were predominantly found in erythroblasts, particularly during the final step of differentiation. Life span analysis of c-erbA mRNAs revealed their relative instability, demonstrating that the high level of c-erbA transcripts in embryonic erythroblasts was not the result of passive accumulation. These results suggest that the c-erbA genes play some role in erythrocyte differentiation.

Amplification of the neu (c-erbB-2) oncogene in human mammmary tumors is relatively frequent and is often accompanied by amplification of the linked c-erbA oncogene

We investigated alterations in the structure and expression of oncogenes in mammary tumors and mammary tumor-derived cell lines. In 16 of 95 samples, we detected amplification of the human neu oncogene, also known as c-erB-2, accompanied by overexpression in the tumors from which intact RNA could be isolated. In 10 of these DNAs, the linked oncogene c-erbA was also amplified, whereas another gene on human chromosome 17, p53, was present in normal copy numbers. Overexpression of c-erbA could not be detected in the tumors analyzed. The relatively high frequency of neu amplification points to a functional role in human breast cancer. Coamplification of the c-erbA oncogene could contribute to this disease as well but is most likely fortuitous.

The chicken c-erbA proto-oncogene is preferentially expressed in erythrocytic cells during late stages of differentiation

We analyzed the expression of the c-erbA proto-oncogene in different tissues of chicken embryos. c-erbA transcripts were found at low levels in the lung, kidney, liver, and heart and in high amounts in embryonic blood cells. Nuclease mapping assays proved that these transcripts were true c-erbA transcripts. In situ hybridization on fractionated embryonic blood cells showed that c-erbA transcripts were predominantly found in erythroblasts, particularly during the final step of differentiation. Life span analysis of c-erbA mRNAs revealed their relative instability, demonstrating that the high level of c-erbA transcripts in embryonic erythroblasts was not the result of passive accumulation. These results suggest that the c-erbA genes play some role in erythrocyte differentiation.