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

Transmembrane domain of the AEV erb B oncogene protein is not required for partial manifestation of the transformed phenotype

The transmembrane domain was deleted from within the v-erb B protein coding region of avian erythroblastosis virus. The mutant oncogene encoded a shortened, apparently soluble form of the normally membrane bound v-erb B protein. Despite this alteration in subcellular distribution, the mutant polypeptide retained the ability to induce fibroblast transformation by several parameters, including the ability to display anchorage-independent growth. It appears that the transmembrane domain, although important for full manifestation of the transformed phenotype, is not essential for v-erb B-mediated oncogenic transformation.

Expression of the v-erbA oncogene in chicken embryo fibroblasts stimulates their proliferation in vitro and enhances tumor growth in vivo

In contrast to uninfected chicken embryo fibroblasts (CEFs), CEFs infected with a retroviral vector that carries the v-erbA gene of avian erythroblastosis virus displayed new properties. These included limited anchorage-independent growth in soft agar, growth without latency in serum-supplemented medium, ability to overcome quiescence induced by serum deprivation, growth at low cell density, and an extended life span in vitro. Furthermore, when explanted in vivo onto the chorioallantoic membrane of chicken embryo, the transformed CEFs expressing v-erbA in addition to v-erbB exhibited a high proliferative rate, giving rise to fibrosarcoma tumors that were ten times larger than those developed from transformed CEFs expressing v-erbB alone. All these data show that CEFs expressing the v-erbA oncogene display activated growth and suggest that the v-erbA product interferes with the mechanisms regulating the growth and/or differentiation of primary CEFs.

Creation of a chimeric oncogene: analysis of the biochemical and biological properties of v-erbB/src fusion polypeptide

A novel gene was created that linked complementary portions of two different tyrosine kinase oncogenes: v-erB and v-src. The v-erbB/src chimera encoded a glycoprotein exhibiting the subcellular distribution of the v-erbB protein but containing the kinase catalytic domain of the v-src parent. Fibroblasts expressing the v-erbB/src gene product became transformed to an oncogenic state and closely resembled cells expressing the v-erbB parent oncogene. Our results indicated that v-erbB sequences can be functionally replaced by sequences derived from a different oncogene, v-src, and that important determinants of the transformed phenotype appear to be encoded in oncogene sequences distinct from those defining the kinase catalytic domain itself.

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.

Characterization of Rous sarcoma virus sequences essential for viral gene expression

Using the Escherichia coli lacZ gene product beta-galactosidase as an indicator of gene expression, we analyzed sequences that are required for expression of the Rous sarcoma virus (RSV) genome in avian cells. The RSV long terminal repeat (LTR) and leader region were sufficient to direct the synthesis of high levels of enzymatically active gag-lacZ fusion proteins. A portion of U3 greater than 140 nucleotides upstream from the cap site was essential for gene expression. This element functioned in either orientation, but its activity was attenuated when it was relocated further away from the cap site. The insertion of exogenous LTRs 3' of lacZ augmented the expression of that gene by increasing the level of stable gag-lacZ transcripts. Furthermore, 3' LTRs could partially compensate for certain defects within the 5' LTR. Insertion of various fragmentary LTRs allowed the identification of at least three synergistically acting domains within the 3' LTR that influence gene expression. Interestingly, the gag-lacZ expression was only stimulated by a 3' LTR when the exogenous 3'-untranslated region was adjacent. Our results imply that the two LTRs of a provirus interact in a complex manner to promote high levels of stable transcripts. It was also found that gag-lacZ expression was independent of viral gene products, suggesting that trans-activation is not a key mechanism regulating RSV expression in avian cells.

High-affinity interleukin 2 binding by an oncogenic hybrid interleukin 2-epidermal growth factor receptor molecule

Both interleukin 2 (IL-2) and epidermal growth factor (EGF) receptors exist in two forms that differ with respect to affinity for their ligand. Only the high-affinity receptors appear to be responsible for the proliferation signal delivered upon binding of the growth factor. Fibroblasts transfected with IL-2 receptor cDNA generate only low-affinity receptors for IL-2, but fusion of membranes from these fibroblasts with T-cell membranes converts some receptors to high affinity, indicating the involvement of a T cell-specific factor in the generation of high-affinity receptors. We have constructed a chimeric cDNA molecule containing the extracellular IL-2-binding domain of the IL-2 receptor cDNA and the transmembrane and intracellular tyrosine kinase domains of the EGF receptor cDNA. When transfected into fibroblasts, this IL-2-EGF receptor cDNA generated high-affinity receptors for IL-2. Moreover, fibroblasts transfected with the chimeric molecule were morphologically transformed and produced rapidly growing tumors in nude mice.

Transgenic chickens: insertion of retroviral genes into the chicken germ line

We infected early chicken embryos by injection of wild-type and recombinant avian leukosis viruses into the yolk of unincubated, fertile eggs. The viremic males (designated generation 0 (G-0] were tested for transmission of proviral DNA to their G-1 progeny. Nine of 37 G-0 viremic males were mosiac and proviral DNA was transmitted to their progeny at frequencies varying from 1 to 11%. All of the G-1 progeny examined by restriction enzyme analysis for clonality of proviral junction fragments had one to three simple but different fragments. The proviral DNA was transmitted from G-1 to the G-2 progeny in a Mendelian fashion thus proving that retroviral genes have been inserted into the chicken germ line. One of the viruses is a candidate vector for insertion of foreign genes into the chicken germ line.

Coordinate regulation of myelomonocytic phenotype by v-myb and v-myc

Both avian myeloblastosis virus (by the action of v-myb) and avian myelocytomatosis virus MC29 (by the action of v-myc) transform cells of the myelomonocytic lineage. Whereas avian myeloblastosis virus elicits a relatively immature phenotype, cells transformed by MC29 resemble mature macrophages. When cells previously transformed by v-myb were superinfected with MC29, their phenotype was rapidly altered to that of a more mature cell. These superinfected cells expressed both v-myb (at a level similar to that found before superinfection) and v-myc. It therefore appears that the expression of v-myc can elicit certain properties of a more differentiated phenotype. In addition, unlike cells transformed by v-myb alone, the cells expressing both v-myb and v-myc could not be induced by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate to differentiate to fully mature macrophages. Cells with a morphology similar to that of the superinfected cells were elicited by simultaneously infecting yolk sac macrophages with avian myeloblastosis virus and MC29. Such cells expressed both v-myb and v-myc. These results indicate that expression of v-myb and v-myc in infected cells coordinately regulates myelomonocytic phenotype and that the two viral oncogenes vary in their ability to interfere with tumor promoter-induced differentiation. Our findings also sustain previous suggestions that the oncogenes v-myb and v-myc may not transform target cells by simply blocking differentiation.

A single amino acid substitution in v-erbB confers a thermolabile phenotype to ts167 avian erythroblastosis virus-transformed erythroid cells

A library of recombinant bacteriophage was prepared from ts167 avian erythroblastosis virus-transformed erythroid precursor cells (HD6), and integrated proviruses from three distinct genomic loci were isolated. A subclone of one of these proviruses (pAEV1) was shown to confer temperature-sensitive release from transformation of erythroid precursor cells in vitro. The predicted amino acid sequence of the v-erbB polypeptide from the mutant had a single amino acid change when compared with the wild-type parental virus. When the wild-type amino acid was introduced into the temperature-sensitive avian erythroblastosis virus provirus in pAEV1, all erythroid clones produced in vitro were phenotypically wild type. The mutation is a change from a histidine to an aspartic acid in the temperature-sensitive v-erbB polypeptide. It is located in the center of the tyrosine-specific protein kinase domain and corresponds to amino acid position 826 of the human epidermal growth factor receptor sequence.

The c-erb-A gene encodes a thyroid hormone receptor

The cDNA sequence of human c-erb-A, the cellular counterpart of the viral oncogene v-erb-A, indicates that the protein encoded by the gene is related to the steroid hormone receptors. Binding studies with the protein show it to be a receptor for thyroid hormones.

Mammalian cell transformation by a murine retrovirus vector containing the avian erythroblastosis virus erbB gene

A recombinant murine retrovirus vector containing the v-erbB gene of avian erythroblastosis virus was constructed to investigate v-erbB as a transforming gene for mammalian cells. A restriction fragment containing the v-erbB sequences from a molecular clone of avian erythroblastosis virus was inserted into a Moloney murine leukemia virus vector. The construct, designated MuLV/erbB, transformed NIH 3T3 cells at a high efficiency in the DNA transfection assay. Individual MuLV/erbB transfectants grew in soft agar and were tumorigenic. The transfectants contained v-erbB DNA sequences, expressed v-erbB-specific transcripts, and synthesized v-erbB-related glycoproteins. The majority of transfectants produced two major v-erbB gene products of 58 and 66 kilodaltons. However, some transfectants produced much smaller v-erbB-specific proteins. Tunicamycin experiments revealed that the size heterogeneity observed between different transfectants was not due to variations in glycoprotein processing, implying that, in some cases, alterations in the MuLV/erbB genome occurred during the transfection process. These findings indicate that expression of the complete v-erbB gene product is not required for transformation of NIH 3T3 cells. A transmissible murine v-erbB (M-erbB) virus was generated by infection of nonproducer transfectants with amphotrophic murine leukemia virus. Transmission of the rescued M-erbB virus was confirmed by DNA, RNA, and protein analyses. The introduction of a transforming v-erbB gene into mammalian cells by virus infection provides a means of analyzing the mechanism by which this epidermal growth factor receptor-related gene alters the growth and differentiation of cells from various lineages.

Gene expression from both intronless and intron-containing Rous sarcoma virus clones is specifically inhibited by anti-sense RNA

To distinguish the inhibitory effect of anti-sense RNA on translation from the effect on splicing, a plasmid (pLC32) was constructed from a cDNA clone of the Rous sarcoma virus (RSV) envelope gene (env) mRNA. Transcription of this plasmid results in the synthesis of RNA identical to the RSV env gene mRNA which does not require splicing to be expressed. Plasmids derived from pLC32 were also constructed in which the env gene coding sequence and 5' noncoding leader sequences were inserted in the opposite orientation relative to the RSV long terminal repeats (LTRs). pLC32 DNA transfected by the calcium phosphate coprecipitation technique efficiently rescued infectious virus from quail cells infected with an RSV mutant deleted in the env gene [R(-)Q cells], indicating that the intron sequences are dispensable in env gene expression. When the inverted constructs were cotransfected with pLC32, significantly less infectious virus was produced. The extent of the inhibition depended upon the concentration ratio of the two plasmids. The maximum inhibition (80%) occurred when the ratio of inverted constructs to pLC32 was 12:1. The inhibition is specific for the inverted orientation since cotransfection of pLC32 with several other plasmids containing viral LTRs and defective src and env genes at similar concentrations did not inhibit the production of infectious virus. In addition, the inverted constructs did not interfere with the expression of an LTR-driven chloramphenicol acetyltransferase gene. When cotransfected with a wild-type Prague A RSV DNA plasmid (pJD100), the inverted constructs also greatly inhibited expression and replication of virus in R(-)Q quail cells. These data suggest that the specific inhibition is caused by hybridization of complementary RNA transcribed from the inverted constructs to the env mRNA, thereby blocking its expression. The fact that expression of both intron-containing and intronless clones are inhibited to the same extent suggest that inhibition by anti-sense RNA from the env exon regions does not act at the level of RNA splicing.

Mutagenesis of the avian erythroblastosis virus erbB coding region: an intact extracellular domain is not required for oncogenic transformation

Avian erythroblastosis virus (AEV) is an oncogenic retrovirus of birds. The AEV-encoded erbB polypeptide, a transmembrane glycoprotein bearing an N-terminal domain exposed on the surface of virally transformed cells, plays a crucial role in AEV-mediated oncogenesis. We report here a characterization of a mutated form of the AEV erbB protein which lacks over two-thirds of the extracellular region of this oncogenic protein. This mutant v-erbB protein, although lacking the three possible extracellular sites of N-linked protein glycosylation, appears unimpaired in the ability to transform cells to an oncogenic phenotype.

Gene insertion into the chicken germ line by retroviruses

We injected chick syncytial strain of reticuloendotheliosis virus (CS-REV) and wild type and recombinant avian leukosis virus (ALV) near the blastoderm of unincubated fertilized embryos and CS-REV intra-abdominally at day of hatch, and we progeny tested the surviving ALV viremic males and REV viremic males and females for transmitted viral genetic material. A number of positive progeny were identified and their deoxyribonucleic acid (DNA) analyzed for restriction enzyme fragments that hybridized with viral genetic material. Most of the progeny had simple restriction enzyme patterns unlike the viremic parents or congenitally infected progeny. This is suggestive evidence that retroviral genetic information has been inserted into the germ line of chickens.

Differences in sequences encoding the carboxyl-terminal domain of the epidermal growth factor receptor correlate with differences in the disease potential of viral erbB genes

Eleven recently isolated erbB-transducing viruses as well as avian erythroblastosis virus (AEV)-R (ES4) and AEV-H have been characterized for the type of disease they cause, their ability to transform fibroblasts in culture, their ability to cause disease in pedigrees of chicken that differ in susceptibility to erbB-induced erythroblastosis, and the structure of their erbB genes. Differences in each of the biological parameters correlated with differences in erbB sequences encoding the C-terminal domain of the epidermal growth factor receptor (EGFR). Seven viruses were strain restricted in their ability to induce erythroblastosis and did not transform fibroblasts. These seven viruses contained v-erbB genes encoding the complete C terminus of the EGFR. AEV-R and AEV-H were not pedigree restricted in their ability to induce erythroblastosis and could transform fibroblasts. These viruses contain v-erbB genes that lack codons for the immediate C terminus of the EGFR. Three viruses caused angiosarcoma and one caused fibrosarcoma. The angiosarcoma and fibrosarcoma-inducing viruses were not strain restricted and did not cause erythroblastosis. The v-erbB genes of each of these viruses contained extensive internal deletions or 3' truncations in sequences encoding the C-terminal domain of the EGFR.

The product of the human c-erbB-2 gene: a 185-kilodalton glycoprotein with tyrosine kinase activity

Antibodies were raised against a synthetic peptide corresponding to 14 amino acid residues at the COOH-terminus of a protein deduced from the human c-erbB-2 nucleotide sequence. These antibodies immunoprecipitated a 185-kilodalton glycoprotein from MKN-7 adenocarcinoma cells. Incubation of the immunoprecipitates with (gamma-32P)ATP resulted in the phosphorylation of this protein on tyrosine residues. These results indicate that the human c-erbB-2 gene product is the 185-kilodalton glycoprotein that is associated with tyrosine kinase activity. Although the c-erbB-2 protein was predicted to encode a protein very similar to epidermal growth factor (EGF) receptor, EGF did not stimulate this kinase activity either in vivo or in vitro.

Coordinate regulation of myelomonocytic phenotype by v-myb and v-myc

Both avian myeloblastosis virus (by the action of v-myb) and avian myelocytomatosis virus MC29 (by the action of v-myc) transform cells of the myelomonocytic lineage. Whereas avian myeloblastosis virus elicits a relatively immature phenotype, cells transformed by MC29 resemble mature macrophages. When cells previously transformed by v-myb were superinfected with MC29, their phenotype was rapidly altered to that of a more mature cell. These superinfected cells expressed both v-myb (at a level similar to that found before superinfection) and v-myc. It therefore appears that the expression of v-myc can elicit certain properties of a more differentiated phenotype. In addition, unlike cells transformed by v-myb alone, the cells expressing both v-myb and v-myc could not be induced by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate to differentiate to fully mature macrophages. Cells with a morphology similar to that of the superinfected cells were elicited by simultaneously infecting yolk sac macrophages with avian myeloblastosis virus and MC29. Such cells expressed both v-myb and v-myc. These results indicate that expression of v-myb and v-myc in infected cells coordinately regulates myelomonocytic phenotype and that the two viral oncogenes vary in their ability to interfere with tumor promoter-induced differentiation. Our findings also sustain previous suggestions that the oncogenes v-myb and v-myc may not transform target cells by simply blocking differentiation.

A single amino acid substitution in v-erbB confers a thermolabile phenotype to ts167 avian erythroblastosis virus-transformed erythroid cells

A library of recombinant bacteriophage was prepared from ts167 avian erythroblastosis virus-transformed erythroid precursor cells (HD6), and integrated proviruses from three distinct genomic loci were isolated. A subclone of one of these proviruses (pAEV1) was shown to confer temperature-sensitive release from transformation of erythroid precursor cells in vitro. The predicted amino acid sequence of the v-erbB polypeptide from the mutant had a single amino acid change when compared with the wild-type parental virus. When the wild-type amino acid was introduced into the temperature-sensitive avian erythroblastosis virus provirus in pAEV1, all erythroid clones produced in vitro were phenotypically wild type. The mutation is a change from a histidine to an aspartic acid in the temperature-sensitive v-erbB polypeptide. It is located in the center of the tyrosine-specific protein kinase domain and corresponds to amino acid position 826 of the human epidermal growth factor receptor sequence.

Structural domains of the avian erythroblastosis virus erbB protein required for fibroblast transformation: dissection by in-frame insertional mutagenesis

Avian erythroblastosis virus (AEV) induces erythroblastosis and fibrosarcomas. The viral erbB protein is required for AEV-mediated oncogenesis. To explore the structural aspects of the v-erbB polypeptide necessary for its oncogenic function, we created a series of small in-frame insertions in different domains of the v-erbB oncogene. AEV genomes bearing lesions within the v-erbB kinase domain demonstrated a drastically decreased ability to transform avian fibroblasts, establishing a functional role for this structurally conserved oncogene domain. In contrast, mutations in the extracellular domain, between the transmembrane region and the kinase domain, or at the extreme C terminus of the v-erbB protein had no effect on AEV-mediated fibroblast transformation. One lesion within the v-erbB kinase domain, a 10-amino acid insertion, produced a temperature-sensitive mutant capable of fibroblast transformation at 36 degrees C but not at 41 degrees C, suggesting that small in-frame insertions have general utility for the in vitro creation of conditional mutants.

Domain structure of human glucocorticoid receptor and its relationship to the v-erb-A oncogene product

The interaction of steroids with their nuclear receptors induces a cascade of regulatory events that results from the activation of specific sets of genes by the hormone/receptor complex. Steroids, either acting alone or possibly synergistically with other growth factors, can influence the DNA synthesis and proliferation of specific target cells, initiate developmental pathways and activate expression of the differentiated phenotype. Moreover, steroid hormones have been implicated in abnormal growth regulation both in tumours and tumour-derived cell lines. The identification of complementary DNAs encoding the human glucocorticoid receptor (hGR) predicts two protein forms (alpha and beta; 777 and 742 amino acids long, respectively) which differ at their carboxy termini. We report here that both forms of the receptor are related, with respect to their domain structure, to the v-erb-A oncogene product of avian erythroblastosis virus (AEV), which suggests that steroid receptor genes and the c-erb-A proto-oncogene are derived from a common primordial regulatory gene. Therefore, oncogenicity by AEV may result, in part, from the inappropriate activity of a truncated steroid receptor or a related regulatory molecule encoded by v-erb-A. This suggests a mechanism by which transacting factors may facilitate transformation. We also identify a short region of hGR that is homologous with the Drosophila homoeotic proteins encoded by Antennapedia and fushi tarazu.

A new pathway in the generation of defective retrovirus DNA

We used a retrovirus shuttle vector to make molecular clones of circular viral DNA from infected cells. One-third of the molecules examined had deletions that started within or near the U5 domain of the long terminal repeat (LTR) region and extended a variable distance toward the gag gene. We present evidence that some of these deletions arose by cleavage of a single LTR unit, in contrast to the cleavage of tandem LTR units associated with the integration reaction. These results suggest that in the formation of defective circular DNA, the U5 domain can be recognized and cleaved in the absence of an adjacent U3 domain. The cleavage of isolated U5 domains may represent an important mechanism responsible for the generation of certain forms of both defective circular DNA and defective integrated DNA.

Expression of transfected DNA in avian cells can be enhanced in trans by retroviral infection

Using a quantitative S1 nuclease protection assay, we demonstrated that acute or chronic infection of avian cells enhances expression of an exogenously introduced rat preproinsulin II gene by approximately equal to 50-fold. The degree of enhancement is shown to vary with the transfection technique used but is independent of the transcription control region of the transfected gene. We conclude that retroviral infection of avian cells enhances expression of transfected DNA in trans by facilitating the uptake of DNA rather than by activating the transfected promoter.

Gene expression from both intronless and intron-containing Rous sarcoma virus clones is specifically inhibited by anti-sense RNA

To distinguish the inhibitory effect of anti-sense RNA on translation from the effect on splicing, a plasmid (pLC32) was constructed from a cDNA clone of the Rous sarcoma virus (RSV) envelope gene (env) mRNA. Transcription of this plasmid results in the synthesis of RNA identical to the RSV env gene mRNA which does not require splicing to be expressed. Plasmids derived from pLC32 were also constructed in which the env gene coding sequence and 5' noncoding leader sequences were inserted in the opposite orientation relative to the RSV long terminal repeats (LTRs). pLC32 DNA transfected by the calcium phosphate coprecipitation technique efficiently rescued infectious virus from quail cells infected with an RSV mutant deleted in the env gene [R(-)Q cells], indicating that the intron sequences are dispensable in env gene expression. When the inverted constructs were cotransfected with pLC32, significantly less infectious virus was produced. The extent of the inhibition depended upon the concentration ratio of the two plasmids. The maximum inhibition (80%) occurred when the ratio of inverted constructs to pLC32 was 12:1. The inhibition is specific for the inverted orientation since cotransfection of pLC32 with several other plasmids containing viral LTRs and defective src and env genes at similar concentrations did not inhibit the production of infectious virus. In addition, the inverted constructs did not interfere with the expression of an LTR-driven chloramphenicol acetyltransferase gene. When cotransfected with a wild-type Prague A RSV DNA plasmid (pJD100), the inverted constructs also greatly inhibited expression and replication of virus in R(-)Q quail cells. These data suggest that the specific inhibition is caused by hybridization of complementary RNA transcribed from the inverted constructs to the env mRNA, thereby blocking its expression. The fact that expression of both intron-containing and intronless clones are inhibited to the same extent suggest that inhibition by anti-sense RNA from the env exon regions does not act at the level of RNA splicing.

Susceptibility to erbB-induced erythroblastosis is a dominant trait of 151 chickens

Rous-associated virus-1 (RAV-1)-induced erythroblastosis results from proviral insertions into or viral transductions of the c-erbB region of the epidermal growth factor gene. Most chickens develop low incidences (less than 5%) of RAV-1-induced erythroblastosis. However, an inbred line of chickens (151) suffers high incidences (approximately 80%) of RAV-1-induced erythroblastosis. Analysis of 151, K28, and (K28 X 151) X K28 chickens for susceptibility to RAV-1-induced erythroblastosis revealed that susceptibility to RAV-1-induced erythroblastosis is a dominant trait of line 151 chickens. Analysis of 151 X K28 and K28 chicks for susceptibility to the induction of erythroblastosis by two new c-erbB-transducing viruses (avian erythroblastosis virus strains AEV-5005 and AEV-5009) revealed that susceptibility to transformation by new c-erbB-transducing viruses is also a dominant trait of 151 chickens. We think it is likely that both of these dominant traits are encoded by the same gene or genes. Our hypothesis is that this gene (or genes) potentiates the ability of the transmembrane and cytoplasmic domains of the epidermal growth factor receptor to transform cells.

c-erbB activation in ALV-induced erythroblastosis: novel RNA processing and promoter insertion result in expression of an amino-truncated EGF receptor

ALV-induced erythroblastosis results from the specific interruption of the host oncogene, c-erbB, by the insertion of an intact provirus. Integrated proviruses are oriented in the same transcriptional direction as c-erbB, and expression of truncated c-erbB transcripts is observed. Evidence, including sequence analysis of cDNA clones, indicates that transcription of truncated c-erbB mRNA is initiated in the 5' LTR of the integrated provirus. This transcript is processed through a series of remarkable splicing reactions to yield viral gag and env sequences fused to erbB sequences. These results establish a novel pathway of promoter insertion oncogenesis that stands in contrast to the pathways used in the activation of c-myc in B lymphomas.

Expression of transfected DNA in avian cells can be enhanced in trans by retroviral infection

Using a quantitative S1 nuclease protection assay, we demonstrated that acute or chronic infection of avian cells enhances expression of an exogenously introduced rat preproinsulin II gene by approximately equal to 50-fold. The degree of enhancement is shown to vary with the transfection technique used but is independent of the transcription control region of the transfected gene. We conclude that retroviral infection of avian cells enhances expression of transfected DNA in trans by facilitating the uptake of DNA rather than by activating the transfected promoter.

Molecular cloning of the PRCII sarcoma viral genome and the chicken proto-oncogene c-fps

The class II avian sarcoma viruses comprise PRCII, PRCIIp, PRCIV, URI, 16L, and Fujinami. The members of this class are all replication-defective viruses containing various amounts of a transforming sequence called v-fps. PRCII contains the smallest amount of fps-specific sequences, transforms fibroblasts in tissue culture, but is only weakly tumorigenic. As a first step in understanding variations in pathogenicity among the class II avian sarcoma viruses and the mechanism by which the oncogene of these viruses was transduced from a single cellular locus, we have molecularly cloned the viral genome of PRCII, its related helper PRCII-AV, and the chicken proto-oncogene (c-fps) from which v-fps derived. The fps-specific region within the cloned PRCII genome was shown to be 0.8-1.0 kb smaller than that of the Fujinami fps-specific region, in agreement with previous studies. Transfection of the cloned DNAs into primary chicken cells demonstrated that both clones (PRCII and PRCII-AV) are biologically active. The cloned PRCII genome is helper dependent and produces a gag-fusion phosphoprotein (P105) which is phosphorylated on a tyrosine residue. The cloned PRCII-AV genome produces infectious virus and can function as a helper for the cloned PRCII genome in transfection assays. Three overlapping recombinant lambda clones homologous to v-fps from a chicken genomic library have been isolated. One of these, lambda-c-fps(2), contains all of the cellular sequences homologous to v-fps. In the aggregate, the three molecular clones may represent the entirety of c-fps.

Induction of angiosarcoma by a c-erbB transducing virus

Recently, 12 new transductions of c-erbB have been identified in a series of Rous-associated virus type 1-induced erythroleukemias. During the passage of these new transducing viruses it has become apparent that the erythroleukemia in chicken 5005 contained two different c-erbB transducing viruses. One induces erythroblastosis, whereas the second induces angiosarcoma. The angiosarcoma- and erythroblastosis-inducing viruses appear to have had a common ancestor, since tumors induced by each contain a novel, 4.3-kilobase c-erbB-related EcoRI fragment. The angiosarcoma-inducing virus has been named avian angiosarcoma virus and is designated for the chicken in which it originated.

Molecular and biological properties of MH2D12, a spontaneous mil deletion mutant of avian oncovirus MH2

Avian oncogenic retrovirus MH2 carries two cell-derived oncogenes, v-mil and v-myc. From an infectious stock of MH2 a spontaneous deletion mutant, MH2D12, that has lost most of the v-mil gene but has retained a complete and functional v-myc gene, has been isolated. Nonproducer quail embryo cells transformed by MH2D12 in the absence of helper virus contain two virus-specific proteins: a gag-related protein of 53,000 Da (p53gag), and a v-myc gene product of 59,000/61,000 Da (p59/61v-myc) indistinguishable from the v-myc protein encoded by MH2. MH2D12 viral RNA contains all T1-oligonucleotides specific for the MH2 v-myc gene but none of those characteristic for the v-mil gene. The genetic structure of molecularly cloned proviral DNA of MH2D12 was revealed by restriction mapping, blot hybridization, heteroduplex analysis, and nucleotide sequencing. The MH2D12 provirus is homologous to the MH2 genome but has suffered a deletion of 1271 nucleotides from the central region encompassing the 3' end of delta gag and all of v-mil except the very 3' 31 nucleotides directly adjacent to the v-myc gene. A nine-nucleotide overlap of homology to gag or mil at the delta gag/delta mil junction suggests that recombination between homologous sequence elements of the delta gag and v-mil domains of MH2 was involved in the genesis of MH2D12. The nucleotide sequence analysis predicts that the carboxyterminal 17 amino acids of p53gag are encoded by the residual v-mil sequences and by intron-derived v-myc sequences. Transformation of quail embryo cells by MH2D12 can be assayed by focus and colony formation of transformed cells. This indicates that the v-mil gene is not essential for these activities. However, size and morphology of foci and colonies, and cellular morphology of cultured MH2D12-transformed cell lines can easily be distinguished from those observed in cell transformation by MH2 and resemble more those seen in cell transformation by viruses containing the myc oncogene only.

Bacterial beta-galactosidase as a marker of Rous sarcoma virus gene expression and replication

We have developed a convenient and sensitive assay of eucaryotic gene expression which uses the Escherichia coli lacZ gene product, beta-galactosidase, as a nonselectable marker. This system has been applied to the analysis of Rous sarcoma virus replication and gene expression. Avian cells were transfected with plasmids encoding in-frame gene fusions of the N-terminal portion of the gag gene to a 'lacZ gene, which requires both transcriptional and translational initiation signals; these were supplied by the virus long terminal repeat and leader region. Readily detectable quantities of beta-galactosidase were synthesized in transfected cells; it was demonstrated that the levels of enzyme activity induced in such cultures increased linearly with the input DNA concentration and also correlated with mRNA levels. By using a Rous sarcoma virus-derived vector containing the src gene and a related virus as a helper, it was shown that lac sequences were compatible with all phases of the virus life cycle. gag-lacZ fusion proteins were immunoprecipitable from cultures which stably expressed lacZ as well as src. Virus rescued from stably transfected cultures resulted in continued lac and src expression in recipient cells. One particular construction was efficiently transmitted as virus, although it lacked sequences thought to be important for encapsidation of RNA into virions. The data presented here demonstrate the use of lacZ as a marker of retrovirus gene expression and replication.

Protein phosphorylation at tyrosine is induced by the v-erbB gene product in vivo and in vitro

The v-erbB gene product of avian erythroblastosis virus (AEV) has extensive homology with the receptor for epidermal growth factor (EGF). We report here that chicken embryo fibroblasts (CEF) transformed by AEV show enhanced tyrosine phosphorylation of a number of cellular polypeptides, including the 36 kd protein, which is phosphorylated in avian sarcoma virus-transformed fibroblasts, and the 42 kd protein, which is phosphorylated in mitogen-stimulated cells. CEF infected by AEV mutants with deletions in v-erbA showed enhanced tyrosine phosphorylation, whereas CEF infected by mutants with deletions in v-erbB did not. When membranes from AEV-transformed cells were incubated with gamma-32P-ATP, both the v-erbB gene product and the 36 kd cellular protein became phosphorylated at tyrosine. These results indicate that the v-erbB protein induces tyrosine phosphorylation in vivo and in vitro, and suggest that, like the EGF receptor, it possesses tyrosine-specific protein kinase activity.

Antibodies against a synthetic peptide as a probe for the kinase activity of the avian EGF receptor and v-erbB protein

The transforming protein v-erbB of avian erythroblastosis virus (AEV) displays extensive sequence homology with the presumptive protein-tyrosine kinase domain of the human EGF receptor and with the src protein-tyrosine kinase family of oncogenes. However, no kinase activity has previously been demonstrated for the v-erbB protein. Here antibodies generated against a synthetic peptide from the C terminus of human EGF receptor are shown to immunoprecipitate the EGF receptor from human and avian cells, as well as the v-erbB proteins from AEV-transformed cells that become phosphorylated on tyrosine residues upon the addition of gamma-32P-ATP. The immunoprecipitates are also able to phosphorylate exogenous tyrosine-containing substrates. Hence, it is likely that both avian EGF receptor and v-erbB proteins are protein tyrosine-specific protein kinases. Since the kinase activity of v-erbB protein cannot be regulated by EGF, it is proposed that the tyrosine protein kinase function of v-erbB may be constitutively activated.

Bacterial beta-galactosidase as a marker of Rous sarcoma virus gene expression and replication

We have developed a convenient and sensitive assay of eucaryotic gene expression which uses the Escherichia coli lacZ gene product, beta-galactosidase, as a nonselectable marker. This system has been applied to the analysis of Rous sarcoma virus replication and gene expression. Avian cells were transfected with plasmids encoding in-frame gene fusions of the N-terminal portion of the gag gene to a 'lacZ gene, which requires both transcriptional and translational initiation signals; these were supplied by the virus long terminal repeat and leader region. Readily detectable quantities of beta-galactosidase were synthesized in transfected cells; it was demonstrated that the levels of enzyme activity induced in such cultures increased linearly with the input DNA concentration and also correlated with mRNA levels. By using a Rous sarcoma virus-derived vector containing the src gene and a related virus as a helper, it was shown that lac sequences were compatible with all phases of the virus life cycle. gag-lacZ fusion proteins were immunoprecipitable from cultures which stably expressed lacZ as well as src. Virus rescued from stably transfected cultures resulted in continued lac and src expression in recipient cells. One particular construction was efficiently transmitted as virus, although it lacked sequences thought to be important for encapsidation of RNA into virions. The data presented here demonstrate the use of lacZ as a marker of retrovirus gene expression and replication.

Cellular oncogenes (c-erb-A and c-erb-B) located on different chicken chromosomes can be transduced into the same retroviral genome

Avian erythroblastosis virus has transduced two cellular genes, c-erb-A and c-erb-B. Using fractionated chicken chromosomes, we found that the two genes are located on different chromosomes in the chicken genome: c-erb-A is on a microchromosome, and c-erb-B is on a large chromosome. The locations of two other cellular oncogenes (c-fps and c-myb) were also determined: c-fps is on a microchromosome, and c-myb is on chromosome of an intermediate size. Our results suggest that avian erythroblastosis virus had transduced the two cellular genes independently, conforming to previous indications that cellular oncogenes are dispersed among multiple chromosomes in every species that has been examined.

src- and fps-containing avian sarcoma viruses transform chicken erythroid cells

We report here that several oncogene-transducing avian sarcoma virus strains, namely Rous sarcoma virus (src), Fujinami sarcoma virus (fps), and PRCII (fps), transform avian erythroid cells in vitro and in vivo. The src- and fps-transformed erythroblasts grow in vitro for 20-30 generations, require special growth conditions, and tend to differentiate spontaneously. In these properties, they resemble erythroid cells transformed with the erbB-containing H strain of avian erythroblastosis virus (AEV-H) but differ from those transformed with AEV-ES4 (erbA, erbB), which grow under standard culture conditions and rarely differentiate spontaneously. Erythroblasts transformed with viruses carrying temperature-sensitive mutations in the src or fps oncogene and then shifted to the nonpermissive temperature in the presence of anemic serum (as a source of an erythropoietin-like factor) differentiate terminally into erythrocytes. These results demonstrate that several members of the src gene family other than erbB have the capacity to transform erythroid cells.

Cellular oncogenes (c-erb-A and c-erb-B) located on different chicken chromosomes can be transduced into the same retroviral genome

Avian erythroblastosis virus has transduced two cellular genes, c-erb-A and c-erb-B. Using fractionated chicken chromosomes, we found that the two genes are located on different chromosomes in the chicken genome: c-erb-A is on a microchromosome, and c-erb-B is on a large chromosome. The locations of two other cellular oncogenes (c-fps and c-myb) were also determined: c-fps is on a microchromosome, and c-myb is on chromosome of an intermediate size. Our results suggest that avian erythroblastosis virus had transduced the two cellular genes independently, conforming to previous indications that cellular oncogenes are dispersed among multiple chromosomes in every species that has been examined.

Subcellular localization of the v-erb-B protein, the product of a transforming gene of avian erythroblastosis virus

Avian erythroblastosis virus (AEV) is an oncogenic retrovirus capable of transforming both fibroblasts and immature erythroid cells. The v-erb-B locus within the AEV genome encodes a glycosylated protein, expression of which is required for oncogenic transformation of either cell type. Subcellular localization of the v-erb-B glycoprotein in AEV-transformed cells is reported here. Results indicate that the v-erb-B protein is synthesized on dense membrane fractions and appears to possess the properties of an integral membrane protein. The bulk of the v-erb-B protein remains with dense membranes after synthesis, although a small quantity may slowly become associated with the plasma membrane. The biogenesis and subcellular location of the v-erb-B protein are thus quite different from those of the transforming proteins that display protein kinase activity. These differences are especially provocative because the amino acid sequences of the v-erb-B protein and the protein kinases are closely related to one another.

Identification of a form of the avian erythroblastosis virus erb-B gene product at the cell surface

Avian erythroblastosis virus (AEV) induces both erythroblastosis and fibrosarcoma in chickens. The viral oncogene responsible for these diseases, erb, is divided into two regions, erb-A and erb-B, although recent evidence suggests that it is primarily the erb-B gene product that is responsible for the transforming activity. The erb-B gene product has been reported previously to be a membrane glycoprotein of 68,000 molecular weight (MW), gp68erb -B. However, we show here that gp68erb -B is an intracellular precursor which is modified further to a 74,000 MW protein, gp74erb -B. By the criteria of resistance to digestion with endoglycosidase H, subcellular fractionation and inhibition of biosynthesis by the ionophore monensin, gp74erb -B appears to be located at the cell surface. Recently, a comparison of the erb-B sequence with that of the epidermal growth factor (EGF) receptor has shown that these two genes are highly homologous, and that erb-B appears to represent a truncated form of this growth factor. In light of these data the identification of gp74erb -B at the plasma membrane suggests that this may be the functionally important form of the erb-B gene product.

FBR murine osteosarcoma virus. II. Nucleotide sequence of the provirus reveals that the genome contains sequences acquired from two cellular genes

The complete nucleotide sequence of the FBR proviral DNA has been determined. The provirus of 3791 nucleotides (specifying a genome of 3284 bases) encodes a single gag- fos fusion product of 554 amino acids. The fos portion of the gene lacks the sequences which code for the first 24 and the last 98 amino acids of the 380-amino acid mouse c- fos gene product. In addition, the coding region has sustained three in-frame deletions, one in the p30gag portion, and two in the fos region, as compared to the sequences of AKR-MLV and the c- fos gene, respectively. The gene product terminates in sequences, termed v-fox, that are present in uninfected mouse DNA at loci unrelated to the c- fos gene. The c-fox gene(s) is expressed as an abundant class of polyadenylated RNA in normal mouse tissues.