Transformation parameters of chicken embryo fibroblasts infected with the ts34 mutant of avian erythroblastosis virus

Early replication timing of the chicken alpha-globin gene domain correlates with its open chromatin state in cells of different lineages

The vertebrate alpha-globin gene domain is an open chromatin domain overlapping a neighboring house-keeping gene. The tissue-specific cluster of alpha-globin genes and the overlapping housekeeping gene share the same replication origin. We have studied the replication timing of chicken alpha-globin genes in cells of different lineages using the FISH-based approach and found that alpha-globin genes replicate early both in erythroid and in non-erythroid cells, i.e. regardless of their transcriptional activity. Early replication timing of chicken alpha-globin genes in cells of different lineages was in good correlation with the open chromatin configuration of the alpha-globin gene domain in both erythroid and non-erythroid cells. We propose that active transcription of the housekeeping gene overlapping the alpha-globin gene domain enables an access of Origin Recognition Complex (ORC) proteins to the replication origin resulting in early replication of alpha-globin genes even in non-erythroid cells.

Transformation of chicken fibroblasts by the v-fms oncogene

The v-fms oncogene of the McDonough strain of feline sarcoma virus (SM-FeSV) encodes a plasma-membrane-associated tyrosine kinase (gp140v-fms) which is closely related, both structurally and functionally, to the c-fms-specified receptor for the macrophage colony stimulating factor (CSF-1). In mammalian fibroblasts, the natural producers of CSF-1, expression of v-fms leads to cell transformation. To study the interaction between CSF-1 and gp140v-fms molecules in a cell system that does not produce endogenous cross-reactive CSF-1, we have expressed the entire v-fms gene as well as a nontransforming deletion mutant (SC2) in chicken embryo cells (CEC). For this purpose the avian retroviral vectors pDS3 and pREP, based on Rous sarcoma virus, were used to isolate recombinant virus particles. CEC infected with virus that carried the entire v-fms gene expressed high amounts of gp140v-fms, comparable to those in SM-FeSV transformed NRK cells. However, these CEC remained flat, retained their fibronectin network, and did not produce enhanced levels of plasminogen activator. The cells grew faster than control CEC for more than 8 weeks but failed to form colonies in soft agar. Within 2 days after addition of CSF-1 to the growth medium, a transformed cell phenotype was induced, as judged by loss of the fibronectin network, again with a growth rate fourfold faster than that of the parental cells and with colony formation in soft agar. Moreover, human CSF-1 caused a rapid tyrosine phosphorylation of v-fms molecules detectable within 5 min after addition of the growth factor. In contrast, CSF-1 had none of the above effects on cells that expressed the SC2 v-fms deletion mutant.

Temperature-sensitive v-sea transformed erythroblasts: a model system to study gene expression during erythroid differentiation

The isolation and characterization of a temperature-sensitive mutant (ts1 S13) of the avian erythroblastosis virus, S13, is described. The temperature-sensitive lesion in ts1 S13 was identified as affecting the tyrosine kinase activity but not the plasma membrane localization of the ts1 S13 v-sea gene product. Erythroblasts transformed by ts1 S13 can be induced to synchronously differentiate into erythrocytes in an erythropoietin (EPO)-dependent fashion. Analysis of erythrocyte-specific gene expression in ts1 S13 erythroblasts reveals that the transformed, self-renewing erythroblasts obtained at permissive temperature already express all erythrocyte genes tested for, although at a low level. Upon differentiation induction, expression of erythrocyte-specific genes is not coordinately regulated but rather involves complex regulatory mechanisms that appear to be specific for the individual genes.

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.

A new avian erythroblastosis virus, AEV-H, carries erbB gene responsible for the induction of both erythroblastosis and sarcomas

The genome structure of a newly isolated avian erythroblastosis virus, AEV-H, was analyzed. Using DNA probes specific for the LTR sequence of SR-RSV-A, and for the erbA gene and the erbB gene of the ES4 strain of AEV, we have shown that the genome of AEV-H is 35S in size and carries the erbB gene but not the erbA gene. Comparison of the restriction sites of molecularly cloned AEV-H DNA with that of cloned DNA of the associated virus revealed that the env gene of the associated virus was replaced with the erbB gene to generate AEV-H. The genome structure of AEV-H is, therefore, determined to be 5'-gag-pol-erbB-3'. Moreover, we have isolated a mutant of AEV-H, td-130, that can induce sarcomas but not erythroblastosis in chickens. The restriction analysis of proviral DNA of the td-130 showed that it carries a deletion of about 150 to 200 nucleotides in the erbB gene. These data indicate that the erbB protein is responsible for both erythroblastosis and sarcomas.

Identification and characterization of the avian erythroblastosis virus erbB gene product as a membrane glycoprotein

Avian erythroblastosis virus causes erythroid leukemia and sarcomas in chickens. The viral oncogene responsible for these diseases, erb, is divided into two regions known as erbA and erbB, and recent evidence suggests that it is the erbB gene that is responsible for the transforming activity. From rats bearing avian erythroblastosis virus-induced sarcomas, we have obtained antisera which are specific for the erb gene products. Using such antisera, we have been able to characterize the erbB gene product as a 68,000 molecular weight protein. Pulse-chase and cell-free in vitro translation experiments show that the initial product is a 62,500 dalton protein which is initially modified to a 66,000 dalton protein, and then further modified to a 68,000 dalton form. These modifications could be shown to be associated with glycosylation and phosphorylation. Cell fractionation experiments revealed that the 66,000 and 68,000 dalton proteins were located in cell membrane fractions, and immunofluorescence results showed the erbB gene product to be expressed on the cell surface.

Transforming capacities of avian erythroblastosis virus mutants deleted in the erbA or erbB oncogenes

Mutants of avian erythroblastosis virus (AEV) were constructed by deleting large nucleotide segments in each of the viral oncogenes termed v-erbA and v-erbB. Mutants in erbA (erbA -B +) retained the ability to transform fibroblasts in vitro, and these cells exhibited most of the transformation characteristics that typify wild-type AEV-transformed fibroblasts. In addition, the mutants induced small erythroid colonies upon infection of bone marrow cells in culture. Chickens inoculated with erbA -B + virus or with erbA -B +-transformed cells developed sarcomas or atypical erythroid leukemias. The erythroid cells transformed in vivo or in vitro by the erbA -B + viruses appeared not to be as tightly blocked in differentiation as wild-type transformed cells. In contrast, fibroblasts infected with the erbA +B - mutant resembled normal cells in all transformation parameters tested, and no bone marrow cell transformation was observed with the mutant. The results indicate that the main transforming properties of AEV are encoded in erbB and that its effects are enhanced by erbA.

Nuclear location of the putative transforming protein of avian myelocytomatosis virus

The putative transforming protein of avian myelocytomatosis virus MC29 is a 110,000 dalton (P110gag-myc) polyprotein comprised of sequences derived from both the gag region and the MC29-specific myc region. Two approaches have been taken to determine the location of the MC29 gag-related proteins in transformed cells: subcellular fractionation and immunofluorescence. Analysis of subcellular fractions of MC29-transformed cells by immunoprecipitation indicates that the majority of the gag-myc polyprotein is found in the nuclear fractions of Q8 cells (a nonproducer line of MC29-transformed quail embryo fibroblasts) and nonproducer cells derived from a liver tumor of MC20-infected quail. This is in contrast to the distribution of gag-related helper virus proteins lacking myc, which are found only in nonnuclear fractions of superinfected Q8 cells. The purity of unlabeled nuclei was assessed by electron microscopy and enzyme assays, revealing little contaminating material from other subcellular fractions. Immunofluorescence experiments using monospecific anti-gag serum showed specific, intense immunofluorescence in the nuclei of fixed Q8 cells. In contrast, the majority of P75gag-erb, a candidate transforming protein produced by avian erythroblastosis virus (AEV), is absent from the nuclei of nonproducer AEV-transformed chick embryo fibroblasts. The nuclear association of the MC29 transforming protein may be related to some of the unique properties of MC29-transformed cells.

Retrovirus-encoded transformation-specific polyproteins: expression coordinated with malignant phenotype in cells from different germ layers

A transformation-associated polyprotein designated "gag-x" was previously shown to be induced by the feline sarcoma virus (FeSV) after the nonproductive transformation of rat or mink cells. We found that this protein was also expressed in cells derived from the native species (cat) with or without the production of feline leukemia helper virus (FeLV) and that cats could mount a humoral antibody response to the transformation-specific (x) portion of the molecule. Such antisera also reacted with the feline oncornavirus-associated cell membrane antigen (FOCMA) by membrane immunofluorescence. Expression of the gag-x protein was coordinated with malignant phenotype in that both transformed cat fibroblasts and cultured cells from a FeSV-induced melanoma expressed antigenically indistinguishable proteins of the same size. These cells are derived from different embryonic germ layers, suggesting that such transformation-related proteins may function in a pleiotropic manner when introduced by a virus.

Avian erythroblastosis virus produces two mRNA's

We analyzed the viral mRNA's present in fibroblast nonproducer clones transformed by avian erythroblastosis virus. Two size classes of mRNA (28 to 30S and 22 to 24S) were identified by solution hybridization with both complementary DNA strong stop and complementary DNA made against the unique sequences of avian erythroblastosis virus. Based upon the kinetics of hybridization with complementary DNA made against the unique sequences of avian erythroblastosis virus, we estimated that there were 400 to 500 copies of the 28 to 30S RNA per cell and 200 to 250 copies of the 22 to 24S RNA per cell. Both RNA species were packaged in the virion. In vitro translation of the 28 to 30S virion RNA yielded a 75,000-dalton protein which was the 75,000-dalton gag-related polyprotein found in avian erythroblastosis virus-transformed cells. In vitro translation of the 22 to 24S virion RNA yielded two proteins (46,000 and 48,000 daltons). This indicates that there may be two genes in avian erythroblastosis virus, one coding for the 75,000-dalton gag-related polyprotein and the second coding for the 46,000- or 48,000-dalton protein or both.

Mutant of avian erythroblastosis virus defective for erythroblast transformation: deletion in the erb portion of p75 suggests function of the protein in leukemogenesis

Previous studies have shown that td359 AEV, a mutant of avian erythroblastosis virus (AEV), is unable to transform erythroblasts in vitro or in vivo but is capable of transforming fibroblasts in vitro and of causing sarcomas in chicks. In this paper we show that the mutant synthesizes a gag-gene related protein (delta p75) which is about 1000 daltons smaller than the protein, p75, induced by wild-type AEV. The mutant protein lacks 3 of the approximately 53 lysine-arginine tryptic peptides resolved in p75 and also contains an additional peptide. By cleavage of delta p75 with p15 protease and analysis of the fragments for size and peptide composition, the deletion in delta p75 could be located in the non-gag region of the molecule. In contrast, with p40 AEV, a second AEV-specific protein synthesized in in vitro translation experiments, there is no change in size of translation products obtained from td359 AEV RNA. Our data provide direct evidence that p75 is required for erythroblast transformation.