Transformation by v-Myb

The v-myb oncogene of the avian myeloblastosis virus (AMV) is unique among known oncogenes in that it causes only acute leukemia in animals and transforms only hematopoietic cells in culture. AMV was discovered in the 1930s as a virus that caused a disease in chickens that is similar to acute myelogenous leukemia in humans (Hall et al., 1941). This avian retrovirus played an important role in the history of cancer research for two reasons. First, AMV was used to demonstrate that all oncogenic viruses did not contain a single cancer-causing principle. In particular, although both Rous sarcoma virus (RSV) and AMV could replicate in cultures of either embryonic fibroblasts or hematopoietic cells, RSV could transform only fibroblasts whereas AMV could transform only hematopoietic cells (Baluda, 1963; Durban and Boettiger, 1981a). Second, chickens infected with AMV develop remarkably high white counts and therefore their peripheral blood contains remarkably large quantities of viral particles (Beard, 1963). For this reason AMV was often used as a prototypic retrovirus in order to study viral assembly and later to produce large amounts of reverse transcriptase for both research and commercial purposes. Following the discovery of the v-src oncogene of RSV and the demonstration that it arose from the normal c-src proto-oncogene, a number of acute leukemia viruses were analysed by similar techniques and found to also contain viral oncogenes of cellular origin (Roussel et al., 1979). In the case of AMV, it was shown that almost the entire retroviral env gene had been replaced by a sequence of cellular origin (initially called mab or amv, but later renamed v-myb) (Duesberg et al., 1980; Souza et al., 1980). Remarkably, sequences contained in this myb oncogene were shared between AMV and the avian E26 leukemia virus, but were not contained in any other acutely transforming retroviruses. In addition, the E26 virus contained a second sequence of cellular origin (ets) that was unique. The E26 leukemia virus was first described in the 1960s and causes an acute erythroblastosis in chickens, more reminiscent of the disease caused by avian erythroblastosis virus (AEV) than by AMV (Ivanov et al., 1962).

Differential regulation of c-Myb-induced transcription activation by a phosphorylation site in the negative regulatory domain

The c-myb protooncogene encodes a highly conserved 75-89-kDa transcription factor that contains three functional domains, an amino-terminal DNA binding domain (DBD), a central acidic transactivation domain, and a carboxyl-terminal negative regulatory domain (NRD). Two acute transforming retroviruses, avian myeloblastosis virus and the E26 leukemia virus, transduced portions of c-myb and encode Myb proteins that are truncated in both the DBD and the NRD. Several conserved potential sites for phosphorylation by proline-directed serine/threonine protein kinases reside in or near the NRD, suggesting that phosphorylation might play a role in regulating c-Myb. We have previously demonstrated that serine 528, located in the NRD, is a target for p42(mapk) in vitro. Serine 528 is phosphorylated in vivo in several cell lines, and substitution of serine 528 to alanine (S528A) resulted in an increased ability of Myb to transactivate a synthetic promoter containing five copies of the mim-1A Myb-responsive element and a minimal herpes tk promoter. We have tested the ability of S528A Myb to transactivate a series of cellular target promoters and report that the serine to alanine substitution increased the ability of Myb to activate transcription from the CD34 promoter but not the c-myc or mim-1 promoters. This suggests that phosphorylation of serine 528 may differentially regulate c-Myb activity at different promoters. The DNA binding and multimerization activities of c-Myb appear to be unaffected by the S528A substitution, suggesting that phosphorylation of serine 528 may mediate its effect on the transcription transactivating activity of c-Myb by regulating interactions with other proteins.

Avian myeloblastic cell lines transformed by two nuclear oncoproteins, P135gag-myb-ets and p61/63myc: a model of retinoic acid-induced differentiation not abrogated by v-erbA

We previously demonstrated that the retroviral construct MHE226 transducing both the P135gag-myb-ets and p61/63myc nuclear proteins induces solid hemopoietic tumors in early chicken embryos. In the present paper, we report the characterization of two MHE226-transformed cell lines established from such hemopoietic tumors retrieved from the heart of a 13-day embryo. Cytological analysis indicated a myeloblastic phenotype. These MHE226 cell lines were positive for the MEP17 monoclonal antibody but were negative for the myeloblast-specific 51/2 monoclonal antibody. MHE226 cell lines displayed a doubling time of about 20-24 h and were maintained for at least 1 year. Contrary to E26 myeloblastic cell lines, MHE226 cell lines were independent of chicken myelomonocytic growth factor and could be maintained in serum-free medium. MHE226 cell lines could be induced to differentiate toward the monocytic lineage by retinoic acid. Retinoic acid inhibited proliferation of MHE226 cell lines as early as day 1. After 3 days, MHE226 cells displayed cytological, enzymatic (alpha-naphthyl acetate esterase and chloroacetate esterase), and functional (phagocytosis) characteristics of monocytic cells. The retinoic acid-induced differentiation of MHE226 cells could not be inhibited by v-erbA. Thus, MHE226-transformed cell lines represent a novel model of cell transformation by two nuclear oncoproteins. Furthermore, they provide a model to study molecular mechanisms implicated in the monocytic differentiation program.

Synergistic activation of the chicken mim-1 gene by v-myb and C/EBP transcription factors

The retroviral oncogene v-myb encodes a transcriptional activator which is responsible for the activation of the mim-1 gene in myelomonocytic cells transformed by v-myb. The mim-1 promoter contains several myb consensus binding sites and has previously been shown to be regulated directly by v-myb. Here we report that the mim-1 gene is activated synergistically by v-myb and different C/EBP transcription factors. We have cloned a chicken C/EBP-related gene that is highly expressed in myeloid cells and identified it as the chicken homolog of C/EBP beta. A dominant-negative variant of chicken C/EBP beta interferes with the v-myb induced activation of the mim-1 gene in these cells, suggesting that C/EBP beta or another C/EBP transcription factor is required for the activation of mim-1 by v-myb. We found that C/EBP beta and other C/EBP transcription factors confer to fibroblasts the ability to induce the mim-1 gene in the presence of v-myb. Finally we show that, in contrast to v-myb, c-myb synergizes with C/EBP transcription factors only at low concentrations of c-myb protein. Our results suggest a role for C/EBP beta, and possibly for other C/EBP transcription factors, in v-myb function and in myeloid-specific gene activation.

Identification of genes differentially expressed in two types of v-myb-transformed avian myelomonocytic cells

In an earlier study we found that different forms of the v-myb oncogene transform myeloid cells which resemble either monoblasts [when v-myb of avian myeloblastosis virus (AMV) was used] or promyelocytes [when a point mutant in v-myb of AMV was used; Introna, M., Golay, J., Frampton J., Nakano, T., Ness, S.A. & Graf, T. (1990). Cell, 63, 1287-1297]. In the present study we have searched for genes expressed in AMV mutant-transformed promyelocytes that are not expressed in AMV-transformed monoblasts using a differential screening approach. Eight different genes were identified among more than 500 differentially expressed clones. The most abundant of these was the previously identified myb-regulated mim-1 gene. The others were found to encode a small calcium-binding (MRP-like) protein; the p20K protein; goose-type lysozyme; a ribonuclease A/angiogenin-related protein; and three non-identified proteins. Although these genes appear to be rather lineage restricted, their expression varied in different subtypes of transformed myelomonocytic cells, and only two of them (goose lysozyme and ribonuclease) showed a similar expression pattern in normal promyelocytes and macrophages, suggesting an aberrant gene regulation in the transformed cells. Co-transfection experiments of a reporter construct containing the promoter of the ribonuclease A-related gene indicated that this promoter is regulated by the v-Myb oncoprotein without the involvement of Myb-specific binding sequences.

Reduced in vivo nonspecific resistance to Listeria monocytogenes infection during avian retrovirus-induced immunosuppression

Ten-day-old chickens infected with an avian osteopetrosis virus [MAV-2(O)] were more susceptible to challenge with Listeria monocytogenes than virus-free chickens, as demonstrated by reduced bacterial clearance from their spleens. Reduced clearance of L. monocytogenes was observed throughout a 26-day period after MAV-2(O) infection.

Sequential study of visceral lesions caused by isolates of an avian osteopetrosis virus (myeloblastosis-associated virus)

Ten-day-old chicken embryos were inoculated with isolates of myeloblastosis-associated virus that induced osteopetrosis of slow or rapid onset. Bursa of Fabricius, thymus, spleen, bone marrow, kidney, liver, and lung were examined at 15, 17, and 19 days in ovo and at 7 and 25 days after hatching by histologic and immunoperoxidase techniques. Tissues from 19-day-old in ovo embryos also were examined by electron microscopy. The lymphoid organs of embryos inoculated with all isolates manifested changes suggesting inhibited development. Virus was most often associated with macrophages, heterophils, and nonlymphoid stromal cells in these organs. Viral particles and antigen were abundant in tissues from embryos inoculated with slow-onset isolates, but cell necrosis was infrequent. The kidney and bursa had especially abundant viral particles and antigen. Conversely, viral particles and antigen were minimal in tissues from embryos inoculated with the rapid-onset isolate, yet intravascular cellular thrombi, substantial cell necrosis, and increased heterophils and hemocytoblasts were found.

Proto-oncogene expression in chicken leukemic cells induced by avian myeloblastosis virus

Sixteen proto-oncogenes which have generated retroviral oncogenes were tested for their expression in chicken leukemic cells induced by avian myeloblastosis virus (AMV) and five were found to be expressed (c-ets, c-fps, c-mht, c-myc, and c-rel). The size of the c-fps transcript (4.0 kb) was not in good agreement with the size (approximately 3.0 kb) previously reported but was uniform in the leukemic cells from 10 different chickens. The size of the other proto-oncogene transcripts appeared normal. The five expressed proto-oncogenes represent cellular genes involved in hematopoiesis. Interestingly the c-myb gene was not expressed in any of the leukemic cells despite its expression in the immature myeloid cells which are targets for AMV transformation. This could represent down regulation of c-myb by v-myb or a differentiation-related arrest of c-myb expression. The leukemic phenotype induced by v-myb may therefore become expressed at a stage of myeloid differentiation when c-myb expression is repressed.

Subnuclear associations of the v-myb oncogene product and actin are dependent on ionic strength during nuclear isolation

The method used to isolate nuclei has a direct effect on the subnuclear association of the v-myb product, p48v-myb, and nuclear actin. Analysis of nuclei subjected to various isolation procedures showed that disruption of native nuclear structure during hypotonic treatment resulted in dissociation of p48v-myb from the nuclear matrix.

[Tumors of the brain vascular plexuses caused by the avian myeloblastosis virus]

A series of experiments on the induction of tumours in the central nervous system with the help of avian myeloblastosis virus was carried out. 142 highly sensitive and inbred chickens, 1 or 2 days of age, were used. The intracerebral virus dose was 0.01-0.02 ml. The titer was 10(9)--10(12) PFU/ml. 25 tumours of choroid plexus were induced during the experiments, 3 tumours developed in the control after intravenous inoculation. Out of 33 chickens highly sensitive to AMV tumours of choroid plexus were found in 6 (15.1%), and out of 75 inbred chickens tumours were found in 16 (21.3%). Three main stages of experimental tumours development in the brain choroid plexus were identified and their pathomorphological description was given. A relatively big amount of choroid plexus tumours have developed upon the brain inoculation with avian myeloblastosis virus. This might be attributed to blastomogenic virus action.

Antiretroviral activity in a marine red alga: reverse transcriptase inhibition by an aqueous extract of Schizymenia pacifica

An aqueous extract from the marine red alga, Schizymenia pacifica has been tested in a cell free system for its effect on reverse transcriptase from avian retrovirus (avian myeloblastosis virus), and mammalian retrovirus (Rauscher murine leukemia virus). The extract inhibited reverse transcriptase from both these retroviruses but showed almost no effect, if any, on the activity of cellular DNA polymerase alpha and RNA polymerase II in vitro. Consequently it is unlikely to have an adverse effect on the growth of cultured cell. The inhibitory activity of the extract was stable over a relatively wide pH range (pH 1-11) and was not lost after pronase digestion. Inhibitory activity of the extract was lost after boiling at 100 degrees C in 0.67 N HCl, and after treatment with 100 mM NaIO4. The active principle in the extract has an apparent molecular weight in excess of 100,000 daltons. This new reverse transcriptase inhibitor is probably a polysaccharide.

A sequential study of bone lesions caused by isolates of an avian osteopetrosis virus, MAV-2(0)

The pathogenesis of avian osteopetrosis caused by rapid and slow-onset isolates of myeloblastosis associated virus, MAV-2(0), was studied by inoculation of 10-day-old chick embryos with virus. Femur and calvarium were examined at 15, 17 and 19 days in ovo and 7 and 25 days after hatching by histologic and immunoperoxidase techniques. Femur and calvarium were also examined by electron microscopy at 17 and 19 days in ovo and at 7 days after hatching. Avian osteopetrotic bone lesions were characterized by exuberant periosteal proliferation; the time of onset varied with different virus isolates. In the femur virus was first associated with osteoprogenitor cells, then with osteoblasts and finally with osteocytes as the cells progressed through normal sequences of differentiation. The amount of virus produced by these cells did not correlate with onset of periosteal proliferation. Slow onset isolates provoked early virus production, but proliferative lesions did not develop until later. Conversely, the rapid onset isolate induced little early virus production, although lesions were present. Periosteal proliferation was associated with and preceded by perivascular edema and perivascular cell necrosis within the bone cortex following infection by all isolates. However, the rapid onset isolate caused more severe lesions than other isolates. These lesions included vascular thrombosis, capillary necrosis and focal bone necrosis. The relationship between early vascular lesions and late periosteal proliferation seen with the slow onset isolates is not as clear as with the rapid onset isolate. Calvarial bone, a representative flat bone, was found to have virus present, but at a level less than the femur. Vascular lesions were rarely seen in the calvarium and bone proliferation did not occur at this site.

Identification of c-myb (chicken), c-myb (mouse) and v-myb (AMV) protein products by immunoprecipitation with antibodies directed against a synthetic peptide

A synthetic nonadecapeptide (IL 19) derived from a sequence of v-myb was covalently bound to haemocyanin and used for immunization. Anti-IL 19 serum immunoprecipitated a 75 kDa protein in the lysate of metabolically labelled chicken and murine thymus cells. Presaturation of the serum with IL 19 abolished this immunoprecipitation, thus indicating that the product of c-myb in both chicken and murine thymuses is the 75 kDa protein (p75c-myb). Anti IL 19 serum also precipitated p48v-myb in the lysate of nonproducer myeloblasts.

Sequence-specific DNA-binding proteins which interact with (G + C)-rich sequences flanking the chicken c-myc gene

The interaction of nuclear sequence-specific DNA-binding proteins from definitive chicken erythrocytes, thymus and proliferating transformed erythroid precursor (HD3) cells with the 700-base-pair (700-bp) DNA 5'-flanking region of the chicken c-myc gene was investigated by in vitro footprint analysis. The major HD3 protein-binding activity binds to a site (site V) 200 bp upstream from the 'cap' site but, after further fractionation, a second distinct binding activity is detected to a site (site VIII) which contains both the 'CAAT' and 'SP1-binding' consensus sequences. Protein from thymus and erythrocyte cells which express c-myc at lower levels, bind to seven and eight sites respectively. In common with HD3 cell protein, they both bind to site VIII and, although binding to the sequence at site V is also detected, the footprint protection pattern is sufficiently different (site V') to suggest the involvement of different proteins in terminally differentiated and proliferating cells. The DNA-binding activities were partially fractionated by high-performance liquid chromatography gel filtration and include an erythrocyte-specific protein which binds to a c-myc gene poly(dG) homopolymer sequence similar to that found upstream of the chicken beta A-globin gene.

Structural organization and nucleotide sequence of mouse c-myb oncogene: activation in ABPL tumors is due to viral integration in an intron which results in the deletion of the 5' coding sequences

Bacteriophage libraries of mouse DNA were screened for sequences homologous to the v-myb oncogene and two overlapping clones containing the v-myb related region were isolated. Restriction enzyme mapping, heteroduplex analysis and nucleotide sequence analysis revealed the presence of nine exons. Six of these exons are homologous to the v-myb region while the other three exons are derived from the 5' region which is deleted in the viral oncogene. The sequences downstream to the sixth v-myb exon are not included in the 17 kbp of DNA sequences analyzed in this study. Comparison of the structure of the normal c-myb clone with its rearranged couterpart present in plasmacytoid lymphosarcomas revealed that the rearrangements occur in this locus as a result of viral integration. Present studies demonstrate that such a viral insertion interrupts the c-myb coding region at a region identical to that observed in the generation of the v-myb gene of avian myeloblastosis virus and results in the synthesis of mRNAs that lack the same 5' coding region.

Antibodies to the evolutionarily conserved amino-terminal region of the v-myb-encoded protein detect the c-myb protein in widely divergent metazoan species

Antibodies directed against a bacterial fusion protein that contains the domain encoded by the highly evolutionarily conserved 5' one-third of the v-myb oncogene of avian myeloblastosis virus (AMV) detect the protein products of various members of the myb gene family. Immunoprecipitation or immunoblot analyses with these antibodies yielded the following information. First, the products of the v-myb oncogenes of AMV (p48v-myb) and of E26 virus (p135gag-myb-ets) contain this highly conserved amino acid sequence, as previously hypothesized. Second, p75c-myb, the product of the chicken c-myb protooncogene, also contains this protein domain. Third, these antibodies have identified the products of the human, murine, and Drosophila c-myb genes, which were all found to be nuclear proteins of Mr 75,000-80,000. The human c-myb protein product is present in immature cells of the erythroid, myeloid, and lymphoid lineages.

Enhanced viral inhibition of lymphocyte mitogenesis in patients with advanced breast cancer

Virus particles are frequently able to non-specifically inhibit the capacity of human peripheral blood lymphocytes to respond to mitogenic or antigenic stimuli. In the case of breast cancer patients with advanced disease, the quantity of virus required to abrogate responsiveness to phytohaemagglutinin (PHA) was approximately four-fold less than that found when cells from healthy donors were employed. The results show that such virus co-incubated cultures are deficient with regard to their ability to synthesize detectable quantities of T cell growth factor (TCGF) activity, and that the extent of responsiveness to PHA in each case corresponds roughly to the amount of TCGF activity that is present in the cultures. While the addition of exogenous purified TCGF to cultures containing virus, normal cells and stimulus caused a reversal of the usual inhibitory effect, this finding was generally not obtained in the case of lymphocytes obtained from patients with advanced breast cancer. These data suggest that one mechanism of explaining diminished cellular immune responsiveness in breast cancer patients may be a relative inability of appropriate cells or subsets of cells to respond effectively to TCGF.

Effect of avian osteopetrosis virus infection on cells and their collagen synthesis in vitro

Primary avian tendon fibroblasts and calvarial osteoblasts were infected with the avian osteopetrosis virus MAV.2-O, in vitro. The infected tendon cells could be cloned in soft agar and kept in culture for at least 25 passages, a number not reached by uncloned infected cells. In contrast to many other virus-transformed fibroblasts, these cells continued making collagen and fibronectin, and there were no gross morphological changes as observed in the light microscope. Changes were seen in their cytoskeletal structure, however, as observed by immunofluorescence. The cloned cells were not tumorigenic in nude mice, nor had they an altered pattern of protein phosphorylation. MAV.2-O-infected fibroblasts and the cloned cells synthesized 2-3 times more collagen type I, the main product of their biosynthetic machinery, than control cells. The proportion of the total cellular RNA consisting of specific mRNAs for the precursor of collagen, procollagen pro-alpha 1 and pro-alpha 2 chains, was higher in the infected cells than in normal fibroblasts. Southern blotting experiments indicated that there was no rearrangement of the collagen genes after infection with this virus. Furthermore, large viral DNA fragments were not integrated into the immediate vicinity of the 5' end of the alpha 2-collagen gene.

Induction of colony-stimulating factor from quiescent fibroblasts by avian macrophages and monocytic leukemic cells

Confluent, quiescent chick embryo fibroblasts maintained in protein-free medium released CSF(s) active on granulocyte and monocyte progenitors and on monocytic leukemia cells induced by avian 'myeloblastosis' virus (AMV) when exposed to formaldehyde-fixed AMV leukemic cells. The CSF was assayed under serum-free conditions. Of several normal cell types tested including populations enriched for blast cells, only macrophages exhibited this capacity.

Inhibition of the initiation of leukemic transcription by N-trifluoroacetyladriamycin-14-O-hemiadipate in vitro. Impaired formation of RNA polymerase-DNA complex

The effect of N-trifluoroacetyladriamycin-14-O-hemiadipate (AD 143), a new derivative of adriamycin, on various steps of the enzymic reaction catalyzed by chicken myeloblastosis RNA polymerase II was studied. AD 143 inhibition of RNA synthesis, which was evident at the beginning of the reaction, could not be reversed by increasing the concentrations of any one of the four nucleoside triphosphate substrates of the reaction. Furthermore, the RNA synthesis inhibition was not affected by varying the concentrations of template DNA. The AD 143-induced inhibition caused a reduction of the frequency of RNA chain initiation, whereas the average chain length of RNA synthesized at the end of the reaction remained unaltered. The susceptible step in the initiation process was found to be the formation of stable complexes between RNA polymerase and the DNA template. While AD 143 causes no inhibition of Escherichia coli RNA polymerase activity, it was found not to affect the E. coli RNA polymerase-template DNA complex formation.

Morphological transformation, autonomous proliferation and colony formation by chicken heart mesenchymal cells infected with avian sarcoma, erythroblastosis and myelocytomatosis viruses

Normal chicken heart mesenchymal cells at low density in monolayer culture in plasma-containing medium have a polygonal shape and are proliferatively quiescent. The combination of epidermal growth factor and insulin at hyperphysiological concentration, an insulin-like growth factor surrogate, causes these cells to assume a fusiform shape and to increase 40-fold in number during four days of incubation. These mitogenic hormones do not, however, induce normal chicken heart mesenchymal cells to form colonies in agarose suspension culture. Chicken heart mesenchymal cells infected with the Schmidt-Ruppin or Prague-A strains of Rous sarcoma virus or with the Fujinami or Y73 avian sarcoma viruses assume spindle and round shapes, increase 50-100 fold in number during four days of monolayer culture in the absence of mitogenic hormones and form macroscopic colonies during 3-4 days of agarose suspension culture. The autonomous (mitogenic hormone-independent) proliferation, in monolayer culture, of cells infected with temperature-sensitive transformation mutants of Rous sarcoma virus (tsNY68, tsNY72, tsLA24, tsLA29) is temperature-sensitive. Chicken heart mesenchymal cells infected with avian erythroblastosis virus assume spindle shapes and proliferate in monolayer culture at a rate comparable to that of sarcoma virus-infected cells but do not, however, form colonies in agarose suspension culture. Cells infected with the myelocytomatosis virus MC29 assume stellate shapes and increase 18-fold in number during four days of monolayer culture. Cells infected with the myelocytomatosis virus MH2 assume fusiform shapes and increase fourfold in number during four days of monolayer culture. Neither MC29 nor MH2 renders chicken heart mesenchymal cells capable of colony formation in agarose suspension culture. Infection with avian leukosis viruses (RAV-1, RAV-2, RPL-42) or with transformation-defective mutants of Rous sarcoma virus (tdNY105, 107, 109) does not affect the morphology or proliferative behavior of chicken heart mesenchymal cells. Monolayer culture of chicken heart mesenchymal cells in plasma-containing medium appears, therefore, to define the ability of onc genes of acute transforming avian retroviruses to induce autonomous (mitogenic hormone-independent) cell proliferation, the essential characteristic of neoplasia. The differences in transformed morphology and rates of autonomous proliferation between cells infected with different acute transforming retroviruses probably reflects differences in the modes of action of the transforming proteins encoded by the onc genes of the respective viruses.(ABSTRACT TRUNCATED AT 400 WORDS)

Catalase depression in malignant liver from chickens with myeloblastosis and Marek's disease

In rapidly frozen livers from chickens affected with myeloblastosis and Marek's disease and from unaffected control birds there exists a strong correlation between catalase activity and catalase Electron Paramagnetic Resonance (EPR) signal intensities. The diseased chickens had activities and signals reduced to as little as 10% of control values. There were no changes in the EPR parameters in diseased liver and the data support the hypothesis that the lowering in activity is due to lowered catalase levels rather than to catalase inhibition. The rate of transformation of catalase to catalase-formate in liver was studied by freeze-clamping liver in anaesthetised chickens, then warming to 37 degrees for 1 or 2 minutes anaerobiosis, and then refreezing. The only difference of significance in this transformation between diseased and normal livers was the greater percentage of total catalase present as catalase-formate (approximately + 15%) in aerobic diseased liver, which may indicate a lowered production of hydrogen peroxide, relative to formate, in these livers. The rate of transformation was far faster in chickens (t1/2 less than 1 min) than in the rat (t1/2 = 7.7 min).

Target cells for avian myeloblastosis virus in embryonic yolk sac and relationship of cell differentiation to cell transformation

The yolk sac of the 12-day chicken embryo retains the blast stage progenitors to cells of the myeloid lineages with a very low level of contamination by more mature myeloid cells which have begun to express the characteristic myeloid cell markers. Both in vivo and in vitro experiments have supported the hypothesis that target cells for the BAI-A strain of avian myeloblastosis virus are contained within the myeloid lineages. An assay system for avian myeloblastosis virus was developed which utilizes this yolk sac cell system and which appears to be more sensitive than previous published assays. In addition, the kinetics of a liquid culture transformation system is presented in which at least 4% of the yolk sac cell population was transformed in a relatively synchronous fashion at 2 days after infection. The morphological transformation preceded an increased rate of cell proliferation. Cell separation procedures provided a 10- to 20-fold enrichment of target cells and demonstrated that the target cell population copurifies with macrophage colony-forming cells which are the committed progenitors to the macrophage lineage. In combination with earlier work, this work demonstrated that cells committed to the macrophage lineage at all stages of differentiation may serve as target cells for infection by avian myeloblastosis virus.

Inhibition of concanavalin A response during osteopetrosis virus infection

Infection of animals with oncogenic viruses frequently leads to an immunosuppressed state. We have examined immunosuppression induced by an avian osteopetrosis virus, myeloblastosis-associated virus of subgroup B inducing osteopetrosis [MAV-2(O)], and our results suggest that this virus induces immunosuppression by a novel mechanism. Lymphoid cells from osteopetrotic chickens did not respond to a wide dose range of concanavalin A (Con A) over a wide cell density range. Failure to undergo blastogenesis was not due to a lack of Con A-binding sites, since 125I-labeled Con A bound to lymphocytes from infected and uninfected chickens. Infected lymphocytes failed to respond to sodium metaperiodate stimulation, indicating that failure of blastogenesis was not due to a blockage of Con A receptor sites. MAV-2(O) infection of chicks 8 days of age resulted in a transient immunosuppression which appeared 1 to 2 weeks after infection. Cell-mixing experiments showed that MAV-2(O)-induced immunosuppression was not attributable to suppressor cells. In contrast, adherent cells from normal lymphoid preparations restored mitogenicity to lymphocytes from MAV-2(O)-infected animals. Adherent cells were present in the spleen and peripheral blood lymphocytes of MAV-2(O)-infected chickens in numbers comparable to those of the uninfected animal, and both sets of cells contained Fc-dependent phagocytic activity and nonspecific esterase. Peritoneal exudate cells were elicited from osteopetrotic and normal chickens in similar numbers. We conclude that MAV-2(O) induces immunosuppression by interfering with an accessory function of macrophage-like adherent cells.

DNA polymerase and thymidine kinase activities in MC-29 virus-induced transplantable hepatoma and the effect of cytostatic treatment of these activities

The activity of DNA polymerases and thymidine kinase was compared in the MC-29 leukosis virus-induced transplantable hepatoma and in the livers of rats treated with cyclophosphamide (CP), cytosine-arabinoside (ara-C) and 5-fluoro-uracil (5-FU). The specific activity of DNA polymerase was twenty times greater in the MC-29 leukosis virus-induced hepatoma, while thymidine kinase was only 3-5 times greater than in liver. All three enzymes showed Michaelis-Menten kinetics in their substrate and template saturation curves. The template utilization of DNA polymerases from hepatoma and from liver was compared. Both had higher activities on a poly(dA) . poly(dT) template at pH 8.0, than on DNA at pH 7.5. After chromatography on a phosphocellulose column two polymerases were separated. The first peak eluted by 0.15 m KCl preferred DNA as template (polymerase alpha). The second eluted by 0.5 M KCl worked better on poly(dA) . poly(dT) (polymerase beta). Thymidine kinase was eluted by 0.25 m KCl. Inhibition by N-ethylmaleimide (NEM) showed the polymerase alpha to be sensitive and the polymerase beta to be resistant to the sulfhydryl blocking agent; similar to the respective enzymes of other eukaryotic cells. The specific activity of DNA polymerase decreased after CP treatment at 6 h and 72 h and after ara-C treatment at 72 h. The specific activities of thymidine kinase were not changed significantly in response to the drug administrations.

Avian nephroblastoma virus MAV-2(N) and avian osteopetrosis virus MAV-2(O) are genetically distinct

Avian myeloblastosis virus consists of a mixture of a defective leukaemia virus and several non-defective associated avian leukosis viruses. The genomes of two of the associated avian leukosis viruses were examined in this study and were chosen because one of them, MAV-2(N), induces predominantly nephroblastoma, while the other, MAV-2(O), induces predominantly osteopetrosis. Competitive hybridization studies employing labelled virion RNA and DNA from normal and malignant tissue failed to demonstrate a difference the genomes. However, examination of ribonuclease T1-resistant oligonucleotide maps revealed that MAV-2(N) RNA had five oligonucleotide fragments which were not present in the MAV-2(O) genome. Poly(A) selection of the oligonucleotides at the 3' end of the genome showed that the fragments unique to MAV-2(N) were not present at this end of the genome. These results suggest that two viruses differing in oncogenic manifestation also differ in genome composition.

Levels of colony-stimulating and inhibiting activities in chicks with myeloblastic leukemia are related to disease progression

Chicks with myeloblastic leukemia induced by avian myeloblastosis virus (AMV) exhibited increased levels of plasma granulocyte/monocyte (GM) colony-stimulating activity (CSA). Chromatography of plasma from AMV-infected chicks revealed that this CSA eluted in the first protein peak from a column of Sephadex G-200. A second protein peak from the same column contained an inhibitor of GM colony-formation. The presence of the inhibitor and the increased CSA was studied during the development of myeloblastic leukemia. The disease course was separated into two distinct phases: 1) an early phase characterized by the onset of viremia, the induction of leukemic colony-forming cells (CFC) in the marrow, and moderately increased marrow cellularity, 2) a late phase characterized by marked increases in the early parameters, dramatic increases in the number of myeloblasts and decreases in the number of erythrocytic cells. Induction of the GM-CFC-inhibitor occurred during the initial stages of the first phase. Increased GM-CSA occurred later in this phase. We conclude that induction of the inhibitor and increased CSA were associated with disease progression rather than ancillary characteristics of frank disease.

A monoclonal antibody with specificity for leukemic cells transformed by defective avian leukemia viruses

Mouse anti-chicken monoclonal antibodies were raised against an avian myeloblastosis virus (AMV)-transformed myeloblastic leukemic cell line. One monoclonal antibody, S1-37 (IgG2a), reacted with producer and nonproducer myeloblastic leukemia cell lines transformed by AMV and by E-26 virus, but it did not react with chicken fibroblasts infected with RAV-2, MAV-2, MAV-1, or RAV-7. S1-37 also did not react with normal chicken hemopoietic cells, except for yolk sack macrophages and a small population of embryonal and adult bone marrow cells that morphologically resembled macrophages. Cytotoxicity studies of GM-CFU, the normal stem cell population of the granulocytic macrophage lineage, indicated that these cells lack the surface antigen recognized by S1-37. Immunoprecipitation studies of 125I surface-labeled myeloblastic leukemic cells indicated that S1-37 binds a 42,000 Mr polypeptide. The possible role of this polypeptide in the process of transformation and differentiation of chicken myeloid cells is discussed.

Characterization of an inhibitor of granulocyte/monocyte colony formation in leukemic chicken plasma

The plasma of chicks with myeloblastic leukemia (induced by avian myeloblastosis virus, AMV) contained an inhibitor which blocked colony formation in vitro by marrow cells. It eluted in the second protein peak obtained by Sephadex G-200 gel filtration and was found in the diafiltrate following defiltration through a UM 10 membrane under acidic conditions. It was not extractable with chloroform, was heat-stable (65 degrees C, 30 min), pronase-sensitive and had a molecular weight less than or equal to 5000 daltons as determined by high performance liquid chromatography. Thus, it appears to be a small, acid-soluble, heat-stable peptide. It had no interferon activity. It was not present or present only at very low levels in normal plasma. Furthermore, it was not elevated in chicks infected only with the nonleukemogenic helper virus of AMV, and was, therefore, associated with leukemia rather than virus-replication. Leukemic myeloblasts, purified by passage in suspension culture, released the inhibitor. It acted directly on the colony-forming cell and inhibited normal cells much more than leukemic cells. In normal marrow, macrophage and granulocyte progenitors were affected. A similar inhibitor in normal plasma inhibited only granulocyte progenitors.

Collagen and noncollagen protein synthesis in chick limb bud cells infected with a virus that causes osteoblastoma

Infection of cultured stage 24 (day 4.5) chick limb cells (spot cultures) with myeloblastosis-associated virus 2-(osteo) (MAV-2(0)) produces cell density-dependent alterations of collagen and noncollagen protein synthesis over the course of 8 days in culture. Collagen synthesis in infected, low density cultures (100 k cells/spot) was maximal on day 4 and increased 12 fold compared with maximal values on days 5-6 for uninfected counterparts. The patterns of collagen synthesis were similar in uninfected or MAV-2(0)-infected cultures seeded at 200 (medium density) or 400 k cells (high density) per spot with peaks on days 5-6. Noncollagen protein synthesis (NCP) in infected, low density cultures was maximal on day 4 and increased 15.7 fold compared with maximal values on days 5-6 for uninfected counterparts. The patterns for NCP synthesis were similar for uninfected and MAV-2(0)-infected cultures seeded at medium density (peak on day 5). However, in MAV-2(0)-infected , high density cultures, NCP synthesis was maximal on day 4 and increased 2.9 fold compared with maximal values on days 5-6 for uninfected counterparts. Although kinetics of cell division and DNA synthesis were similar in uninfected and MAV-2(0)-infected limb bud cells, the virus may have diverse affects on different cell populations at the three cell densities employed in these experiments.

[DNA polymerase in the microsomal fraction of the myeloblasts of chickens infected with avian myeloblastosis virus]

Only one DNA polymerase is present in the microsomal fraction of the cells producing AMV. Chromatographically purified enzyme shows the properties of revertase, that is it transcribes in DNA the information encoded in natural RNA. The enzyme possesses identical chromatographic characteristics and the same template specificity as the enzyme isolated from pure AMV virus. Thus the virus enzyme and the cellular DNA polymerase from the microsomal fraction cannot be differentiated on the basis of certain properties.

Hybridization studies of poly A-RNA from 5'-bromodeoxyuridine treated neuroblastoma cells

Hybridization studies were carried out to measure sequence complexity and relative complexity of poly A-RNA populations from M1 neuroblastoma cells cultivated under proliferating conditions and after BrdU treatment. BrdU treatment is known to induce morphological differentiation. Hybridization kinetics were performed with [3H] labelled complementary DNA synthetized by reverse transcriptase action. The total complexities and the complexities of three classes of sequences measured for the two developmental states differed significantly. In particular, the total complexities as well as the complexity of the rare sequences class were higher in the poly A-RNA population of morphological differentiated M1 cells. Heterologous hybridization between poly A-RNA of proliferating cells with cDNA of differentiated M1 cells was very close to the homologous hybridization of poly A-RNA and cDNA from differentiated cells, nevertheless significant differences, were found in the intermediate and in the rare sequences classes. On the other hand the inverse heterologous hybridization (poly A-RNA of differentiated state X cDNA of proliferating cells) showed a lower hybridization in the region of Rot higher than 1. The plateau reached only 87 per cent compared to that of the homologous hybridization, suggesting that certain sequences expressed in the differentiated state. Nevertheless the number of different poly A-RNA species present per cell (seen by homologous hybridization experiments) was higher in differentiated state indicating that selective transcription took place beside repression with morphological differentiation.

Purification and characterization of the RNA-directed DNA polymerase of a primate type-D retrovirus: Mason-Pfizer virus

The RNA-directed DNA polymerase of the primate type-D retrovirus Mason-Pfizer virus was purified using ion-exchange and affinity chromatography, and molecular sieving. The enzyme was shown to have a molecular weight of approx. 80 000 as determined by sedimentation analysis, molecular sieving and sodium dodecyl sulfate polyacrylamide gel electrophoresis. The purified RNA-directed DNA polymerase retained its ability to use a heteropolymeric RNA as a template. The Mason-Pfizer virus RNA-directed DNA polymerase was also characterized as to its divalent cation preference for several synthetic primertemplates and for heteropolymeric RNA. Mg2+ was preferred as its divalent cation for all primer-templates except oligo(dG).poly(rC)m for which it prefers Mn2+. The Mason-Pfizer virus enzyme was also shown to have a pH optimum of 8-8.5 and a temperature optimum of 37-40 degrees C. The stability of the Mason-Pfizer virus RNA-directed DNA polymerase was shown to differ when measured using different primer-templates.

Stimulation of sugar uptake and glycolysis in chicken embryo fibroblasts by the major glycoprotein from avian myeloblastosis virus

Addition of purified major glycoprotein from avian myeloblastosis virus to growing or quiescent chicken embryo fibroblasts rapidly stimulates the rate of hexose transport and increases the lactic acid production. These stimulatory effects are dependent on the time of exposure and the dose of viral glycoprotein. In contrast, the glycoprotein only marginally affects hexose transport in chicken cells transformed by Rous sarcoma virus. Some effects of the glycoprotein on serum-starved quiescent cells were similar to those observed upon re-addition of serum; however, the viral glycoprotein did not stimulate DNA synthesis. Quiescent cells stimulated by saturating levels of serum showed little further stimulation of hexose uptake upon exposure to viral glycoprotein for 3 hr. This behavior suggests that the glycoprotein may be acting on a system that is also a target for serum action.

Alfalfa mosaic virus RNA. Determination of the sequence homology between the four RNA species and a comparison with the four RNA species of cucumber mosaic virus

The method of Taylor et al. [Taylor, J. M., Illmensee, R & Summers, J. (1976) Biochim. Biophys. Acta, 442, 324--330 and Gould and Symons (1977) Nucleic Acids Res. 4, 3787--3802] has been used to transcribe complementary DNA probes from the four major RNAs of alfalfa mosaic virus (AMV). Analysis of the kinetics of hybridization of these probes in homologous and heterologous complementary DNA . RNA hybridization reactions has shown that the sequence of the smallest RNA (RNA 4), which contains the coat protein gene, is present within RNA 3 and located at the 3' end of this RNA species. RNAs 1 and 2 are unique RNA molecules with little or no sequence homology between them or RNAs 3 and 4. This latter observation contrasts with the situation that occurs in cucumber mosaic virus (CMV) as CMV RNAs 1--4 were shown to have a common nucleotide stretch of 200 bases at their 3' termini; the location of RNA 4 within RNA 3 of CMV was also shown to be at the 3' end of this RNA species.

Particles from mouse melanoma B16 containing reverse transcriptase and 70S RNA related to human melanoma cytoplasmic RNA

The mouse melanoma B16 contains particles encapsulating high molecular weight RNA of 60--70S size associated with a reverse transcriptase. The [3H]DNA synthesized by these particles possesses homology with RNA isolated from a hamster melanoma and from three human malignant melanomas.

Effect of metal activators during synthesis with avian myeloblastosis virus DNA polymerase

The effect of metal activators on the fidelity of DNA synthesis has been examined. Using the DNA polymerase from avian myeloblastosis virus, the accuracy of Co2+-, M2+-, and Ni2+-activated DNA synthesis was determined with different polynucleotide templates. With poly[d(A-T)] as the template, the error frequency for dCMP incorporation was 1:1400, 1:1100, and 1:600 for Mg2+, Co2+, and Mn2+, respectively, at maximally activating concentrations. The error frequency was invariant with respect to [Mg2+] but increased with greater than activating concentrations of Co2+ and Mn2+. This increase resulted from differential rates of complementary and noncomplementary nucleotide incorporation. The enhanced error frequency was nonspecific as it occurred with all polynucleotide templates and with all noncomplementary deoxy- and ribonucleotides which were tested. Nearest neighbor analyses of the reaction products indicated that the noncomplementary deoxynucleotides were incorporated as single base substitutions. The fidelity of Ni2+-activated DNA synthesis was invariant with respect to [Ni2+] and was similar to that obtained using Mg2+. During DNA synthesis with Mg2+, the addition of Co2+, Mn2+, or Ni2+ resulted in a decrease in the fidelity of DNA synthesis. The relationship between decreases in the fidelity of DNA synthesis and metal mutagenesis, or carcinogenesis, or both, is considered.

Biological characterization of avian osteopetrosis

Chicks infected as 12-day-old embryos with an end-point purified derivative of avian myeloblastosis virus developed a rapidly progressive osteopetrosis that manifested within 1 week of hatching. A detailed comparison of osteopetrotic chicks and normal hatchmates revealed the following. (i) Osteopetrotic chicks exhibited a stunting syndrome, growing at a mean rate that was 26% of the control rats. (ii) At autopsy, the mass of the lymphoid organs was reduced, whereas the mass of the heart, pancreas, kidneys, lungs, brain, liver, and bones of osteopetrotic chicks was increased. Edema was likely responsible for most of the increase in organ weight. (iii) Infected chicks exhibited a normochromic, normocytic anemia that was virus dose dependent and was not required for the development of osteopetrosis. (iv) Bone collagen content was normal. (v) Osteopetrotic bone was initially hypomineralized, but later became more fully mineralized. (vi) The concentrations of alpha, beta, and gamma globulins in the plasma were elevated in osteopetrotic chicks, whereas albumin concentration was decreased. (vii) The level of plasma alkaline phosphatase was elevated in osteopetrotic chicks, yet the level of acid phosphatase was unchanged. (viii) Body and bone temperatures were unchanged.

Search for virus specific DNA sequences and viral particles in mitochondria of avian leukemic myeloblasts

The intracellular localization of the avian myeloblastosis virus (AMV) genome was studied. Nuclear and mitochondrial DNAs from myeloblasts were examined by hybridization with 32P labeled AMV-RNA of high molecular weight for the presence of virus specific DNA sequences. Nuclear DNA (nDNA) from myeloblasts specifically hybridized with viral RNA, whereas purified closed circular mitochondrial DNA (mtDNA) did not hybridize with viral RNA. It was therefore concluded that viral genome was present in nuclear DNA and not in mitochondrial DNA. Likewise, in normal chick cells, nDNA but not mtDNA hybridized with viral RNA.