Hematopoietic cell transformation by reticuloendotheliosis virus: characterization of the genetic defect

Integrative Analyses of Transcriptome Sequencing Identify Functional miRNAs in the Chicken Embryo Fibroblasts Cells Infected With Reticuloendotheliosis Virus

In this study, we found a much higher proportion of reticuloendotheliosis virus (REV) infected chicken embryo fibroblasts (CEF) were in active cell division phase than that of control cells which indicated that REV can affect the fate of CEF. So, we performed high-throughput sequencing and transcriptomic analysis to identify functional miRNAs, in order to figure out the possible mechanism in the interaction of REV with CEF. In total, 50 differentially expressed miRNAs (DEmiRNAs) were identified. Then target genes of DEmiRNAs were predicted and identified by transcriptome profile results. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment were conducted to analyze the identified target genes of miRNAs which showed that metabolism, cell cycle, and apoptosis were the most related pathways involved in infection of REV. We analyzed the genes related to cell cycle which indicated that CyclinD1-CDK6 complex played an important role in regulating the transition of the cell cycle from G1 phase to S phase during REV infection. Fluorescence microscope identification showed that REV inhibited the apoptosis of CEF which was in accordance with transcriptome results. A novel miRNA, named novel-72 was found, KEGG analysis was conducted to predict the biological function of its target genes which showed that those target genes were significantly enriched in mTOR signaling pathway and functioned to promote cell cycle and cell growth during the REV infection. In conclusion, REV could induce the up-regulation of cell metabolism, cell cycle and mTOR signaling pathway while inhibit apoptosis of the cell.

Analysis of the spleen proteome of chickens infected with reticuloendotheliosis virus

Infection with reticuloendotheliosis virus (REV), a gammaretrovirus in the family Retroviridae, can result in immunosuppression and subsequent increased susceptibility to secondary infections. In the present study, we identified differentially expressed proteins in the spleens of chickens infected with the REV-A HLJ07I strain, using two-dimensional gel electrophoresis on samples from time points coinciding with different phases of the REV life cycle. Differentially expressed proteins were identified using one-dimensional liquid chromatography electrospray ionization tandem mass spectrometry (1D LC ESI MS/MS). Comparative analysis of multiple gels revealed that the majority of changes occurred at early stages of infection. In total, 60 protein spots representing 28 host proteins were detected as either quantitatively (false discovery rate [FDR] ≤0.05 and fold change ≥2) or qualitatively differentially expressed at least once during different sampling points. The differentially expressed proteins identified in this study included antioxidants, molecular chaperones, cellular metabolism, formation of the cytoskeleton, signal transduction, cell proliferation and cellar aging. The present findings provide a basis for further studies to elucidate the role of these proteins in REV-host interactions. This could lead to a better understanding of REV infection mechanisms that cause immune suppression.

Methods for assessing the in vitro transforming activity of NF-κB transcription factor c-Rel and related proteins

Among NF-κB transcription factors, c-Rel and c-Rel-derived proteins, including v-Rel, are the only ones that have shown consistent and frank transforming activity in cell culture. In particular, viral, chicken, mouse, and human Rel proteins can rapidly transform primary chicken spleen and bone marrow cells. Overexpression of a human Rel protein missing a C-terminal transactivation domain can also enhance the transformed state of the human B-lymphoma cell line BJAB. As described in this chapter, these in vitro assays can be used to quantitatively assess the transforming activity of Rel proteins.

Effects of reticuloendotheliosis virus infection on cytokine production in SPF chickens

Infection with reticuloendotheliosis virus (REV), a gammaretrovirus in the Retroviridae family, can result in immunosuppression and subsequent increased susceptibility to secondary infections. The effects of REV infection on expression of mRNA for cytokine genes in chickens have not been completely elucidated. In this study, using multiplex branched DNA (bDNA) technology, we identified molecular mediators that participated in the regulation of the immune response during REV infection in chickens. Cytokine and chemokine mRNA expression levels were evaluated in the peripheral blood mononuclear cells (PBMCs). Expression levels of interleukin (IL)-4, IL-10, IL-13 and tumor necrosis factor (TNF)-α were significantly up-regulated while interferon (IFN)-α, IFN-β, IFN-γ, IL-1β, IL-2, IL-3, IL-15, IL-17F, IL-18 and colony-stimulating factor (CSF)-1 were markedly decreased in PBMCs at all stages of infection. Compared with controls, REV infected chickens showed greater expression levels of IL-8 in PBMCs 21 and 28 days post infection. In addition, REV regulates host immunity as a suppressor of T cell proliferative responses. The results in this study will help us to understand the host immune response to virus pathogens.

Identification of a conserved B-cell epitope on reticuloendotheliosis virus envelope protein by screening a phage-displayed random peptide library

Background

The gp90 protein of avian reticuloendotheliosis-associated virus (REV-A) is an important envelope glycoprotein, which is responsible for inducing protective antibody immune responses in animals. B-cell epitopes on the gp90 protein of REV have not been well studied and reported.

Methods and Results

This study describes the identification of a linear B-cell epitope on the gp90 protein by screening a phage-displayed 12-mer random peptide library with the neutralizing monoclonal antibody (mAb) A9E8 directed against the gp90. The mAb A9E8 recognized phages displaying peptides with the consensus motif SVQYHPL. Amino acid sequence of the motif exactly matched ²¹³SVQYHPL²¹⁹ of the gp90. Further identification of the displayed B cell epitope was conducted using a set of truncated peptides expressed as GST fusion proteins and the Western blot results indicated that ²¹³SVQYHPL²¹⁹ was the minimal determinant of the linear B cell epitope recognized by the mAb A9E8. Moreover, an eight amino acid peptide SVQYHPLA was proven to be the minimal unit of the epitope with the maximal binding activity to mAb A9E8. The REV-A-positive chicken serum reacted with the minimal linear epitopes in Western blot, revealing the importance of the eight amino acids of the epitope in antibody-epitope binding activity. Furthermore, we found that the epitope is a common motif shared among REV-A and other members of REV group.

Conclusions and Significance

We identified ²¹³SVQYHPL²¹⁹ as a gp90-specific linear B-cell epitope recognized by the neutralizing mAb A9E8. The results in this study may have potential applications in development of diagnostic techniques and epitope-based marker vaccines against REV-A and other viruses of the REV group.

Malignant transformation of primary chicken spleen cells by human transcription factor c-Rel

Rel/NF-kappaB transcription factors control a variety of cellular processes, such as cell growth and apoptosis, that are relevant to oncogenesis, and mutations in genes encoding Rel/NF-kappaB transcription factors have been found in several human lymphoid cell cancers. In this study, we have used a sensitive cell outgrowth assay to demonstrate that wild-type human c-Rel can malignantly transform primary chicken spleen cells, and that transformation by c-Rel is accelerated by co-expression of Bc1-2. Full-length mouse c-Rel can also transform chicken spleen cells. These results are the first demonstration of a lymphoid cell malignant transforming ability for mammalian Rel/NF-kappaB transcription factors, and implicate c-Rel as a molecular target for cancer therapeutics.

Multiple mutations contribute to the oncogenicity of the retroviral oncoprotein v-Rel

The avian Rev-T retrovirus encodes the v-Rel oncoprotein, which is a member of the Rel/NF-kappaB transcription factor family. v-Rel induces a rapidly fatal lymphoma/leukemia in young birds, and v-Rel can transform and immortalize a variety of avian cell types in vitro. Although Rel/NF-kappaB transcription factors have been associated with oncogenesis in mammals, v-Rel is the only member of this family that is frankly oncogenic in animal model systems. The potent oncogenicity of v-Rel is the consequence of a number of mutations that have altered its activity and regulation: for example, certain mutations decrease its ability to be regulated by IkappaBalpha, change its DNA-binding site specificity, and endow it with new transactivation properties. The study of v-Rel will continue to increase our knowledge of how cellular Rel proteins contribute to oncogenesis by affecting cell growth, altering cell-cycle regulation, and blocking apoptosis. This review will discuss biological and molecular activities of v-Rel, with particular attention to how these activities relate to structure - function aspects of the Rel/NF-kappaB transcription factors.

RRR-alpha-tocopheryl succinate induces apoptosis in avian retrovirus-transformed lymphoid cells

The RRR-alpha-tocopheryl succinate form of vitamin E [vitamin E succinate (VES)] inhibits the proliferation of avian reticuloendotheliosis virus-transformed RECC-UTC4-1 (C4-1) lymphoblastoid cells in a dose-dependent manner, blocks the cells in the G2/M cell cycle phase, and induces the cells to undergo apoptosis. Apoptosis was documented by demonstrating changes that are characteristic of this type of cell death, including morphological analyses of chromatin condensation by 4',6-diamidine-2'-phenylindole dihydrochloride (DAPI) staining using scanning confocal and traditional fluorescent microscopy; flow cytometry analyses of propidium iodide-labeled DNA showing fragmented DNA as a pre-G1 peak; two-color flow cytometry analyses of intact cells labeled first by the TUNEL procedure (terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick-end-labeled DNA stained with fluorescein isothiocyanate-labeled avidin) and then by propidium iodide demonstrating fragmented DNA; and electrophoresis of DNA showing a DNA ladder created by internucleosomal DNA fragmentation. The percentage of apoptotic cells was determined by DAPI staining and showed 11%, 27%, and 49% of cells to be apoptotic after treatment with 10 micrograms/ml VES for one, two, and three days, respectively. Analyses of mRNA levels of genes that have been implicated in the apoptotic process, namely, bcl-2, c-myc, and c-jun, revealed no change in bcl-2, decreases in c-myc mRNA levels after 36 hours of treatment, and increases in c-jun mRNA levels within four hours after treatment. Western immunoblotting analyses of protein levels for the transcription factors c-Myc and c-Jun showed normal levels of c-Myc at early time points and decreased levels at 24 and 48 hours after treatment. c-Jun increased as early as 6 hours after treatment and returned to lower (yet still elevated over control) levels by 48 hours. To determine possible functional consequences of increased c-Jun expression, gel electrophoretic mobility assays were conducted that showed increased AP-1 binding at 24 and 48 hours after treatment. These data show that VES induces apoptosis in reticuloendotheliosis virus-transformed lymphoid cells and suggest that decreases of c-Myc protein and increases of c-Jun protein and DNA binding capacity may be playing a role in VES-mediated events leading to apoptosis in this cell type.

v-rel Induces ectopic expression of an adhesion molecule, DM-GRASP, during B-lymphoma development

In an effort to identify aberrantly expressed genes in v-rel-induced tumors, monoclonal antibodies were developed that reacted selectively with avian B-cell tumors. One antibody, HY78, immunoprecipitated a 120-kDa glycoprotein (p120) from cells that express v-rel. N-terminal amino acid sequencing of p120 identified a 27-amino-acid sequence that is also present in DM-GRASP, an adhesion molecule belonging to the immunoglobulin superfamily. Evidence from tissue distribution, immunological cross-reaction, PCR amplification, cDNA cloning, and DNA sequence shows that p120 is indeed DM-GRASP. Northern (RNA) analysis using a probe from the DM-GRASP gene identified a 5.3-kb transcript in mRNA from bursa, thymus, and brain as well as from v-rel-induced B-cell lymphomas but not from bursal B cells. The induction of this protein by v-rel during the development of bursal B-cell lymphomas appears, therefore, to be ectopic in nature. Overexpression of v-rel or c-rel in chicken embryonic fibroblasts, B-cell lines, and spleen mononuclear cells induces the expression of DM-GRASP. The ratio of DM-GRASP to v-Rel was fivefold higher than that of DM-GRASP/c-Rel in a B-cell line, DT95. Interestingly, the presence of HY78 antibody inhibits the in vitro proliferation of v-rel-transformed cells but not cells that immortalized by myc. These data suggest that DM-GRASP is one of the genes induced during v-rel-mediated tumor development and that DM-GRASP may be involved in the growth of v-rel tumor cells.

A mutant v-rel with increased ability to transform B lymphocytes

We observed that two strains of REV-T differ in the ability to transform bursal cells in vitro. REV-TW, with v-rel derived from a well-characterized clone and considered the prototype of the wild type, fails to generate colonies in soft agar. In contrast, REV-S2A3, derived from the S2A3 cell line, readily transforms bursal cells. With PCR, a 1,591-bp fragment containing v-rel from the REV-S2A3 provirus was cloned into plasmid pREV-0. Except for the absence of v-rel, pREV-0 is identical to pREV-TW. Five clones of pREV-PCR, each produced by an independent amplification, were obtained. The REV-PCR viruses displayed the strong transforming phenotype of REV-S2A3. Two mutations were identified in the 5' region of v-rel from REV-PCR1 to REV-PCR5: a silent mutation and a G-to-T transversion, changing the alanine at position 40 to serine. To confirm the relevance of this amino acid substitution, a 478-bp fragment containing the mutations was exchanged between REV-TW and REV-PCR1. Only the mutant viruses were able to form large colonies of bursal cells in liquid culture and to generate bursal cell colonies in soft agar. When tested on splenocytes, the wild-type viruses induced predominantly non-B-cell colonies while the mutant viruses gave origin mainly to B-cell colonies. The above results indicate that the substitution of serine for alanine at position 40 of v-Rel enhances the ability of REV-T to transform B lymphocytes in vitro. This mutation is close to the DNA-binding region, and the variant v-Rel oncoprotein shows increased kappa B-binding activity, thus confirming the relevance of this property for transformation.

Temperature-sensitive transforming mutants of the v-rel oncogene

By making site-directed mutations in the avian retroviral oncogene v-rel, we created two temperature-sensitive (ts) transforming mutants; these changes were analogous to mutations previously shown to confer a ts function onto the Dorsal protein of Drosophila melanogaster. Chicken spleen cells infected with the ts v-rel mutants formed colonies in agar at 36.5 degrees C but not at 41.5 degrees C. In addition, spleen cells derived from the ts v-rel-transformed colonies could be propagated in liquid culture at 36.5 degrees C but rapidly senesced at 41.5 degrees C. Both mutant v-Rel proteins were also ts for DNA binding in vitro. These mutants may be valuable for identifying genes directly regulated by v-rel.

Biological actions of oncogenes

Cancer, in many cases, results from multistep genetic mutation. Certain genes can have a predisposed susceptibility to mutations that lead to cancer because of chromosome location or their importance in the control of cell cycles. Mutations that deregulate the expression or activity of enzymes involved in the biochemical pathways of growth and differentiation or that suppress the expression of negative cell cycle control factors result in activation of oncogenesis. The study of oncogenes and tumor suppressor genes has greatly influenced our understanding of the molecular origins of cancer. We focus here on the normal biological action of proto-oncogenes compared with the transforming activities of oncogenes and tumor suppressor genes, and we discuss possible mechanisms of oncogenic transformation.

A protein kinase-A recognition sequence is structurally linked to transformation by p59v-rel and cytoplasmic retention of p68c-rel

The Rel family of proteins includes a number of proteins involved in transcriptional control, such as the retroviral oncoprotein v-Rel, c-Rel, the Drosophila melanogaster developmental protein Dorsal, and subunits of the transcription factor NF-kappa B. These proteins are related through a highly conserved domain of approximately 300 amino acids, called the Rel homology domain, that contains dimerization, DNA binding, and nuclear targeting functions. Also within the Rel homology domain, there is a conserved consensus sequence (Arg-Arg-Pro-Ser) for phosphorylation by cyclic AMP-dependent protein kinase (PKA). We used linker insertion mutagenesis and site-directed mutagenesis to determine the importance of this sequence for the transformation of avian spleen cells by v-Rel and the subcellular localization of c-Rel in chicken embryo fibroblasts (CEF). The insertion of 2 amino acids (Pro-Trp) within this sequence completely abolished transformation and transcriptional repression by v-Rel and resulted in a shift in the localization of c-Rel from cytoplasmic to nuclear in CEF. When the conserved Ser within the PKA recognition sequence was replaced by Ala, there was no significant effect on transformation and transcriptional repression by v-Rel or on cytoplasmic retention of c-Rel. However, when this Ser was changed to Asp or Glu, transformation and transcriptional repression by v-Rel were significantly inhibited and c-Rel showed a diffuse nuclear and cytoplasmic localization in CEF. Although a peptide containing the recognition sequence from v-Rel can be phosphorylated by PKA in vitro, this site is not constitutively phosphorylated to a high degree in vivo in transformed spleen cells incubated with okadaic acid. Our results indicate that the transforming and transcriptional repressing activities of v-Rel and the cytoplasmic retention of c-Rel are dependent on the structure of the conserved PKA recognition motif. In addition, they suggest that phosphorylation at the conserved PKA site could have a negative effect on transformation and transcriptional repression by v-Rel and induce the nuclear localization of c-Rel.

A protein kinase-A recognition sequence is structurally linked to transformation by p59v-rel and cytoplasmic retention of p68c-rel

The Rel family of proteins includes a number of proteins involved in transcriptional control, such as the retroviral oncoprotein v-Rel, c-Rel, the Drosophila melanogaster developmental protein Dorsal, and subunits of the transcription factor NF-kappa B. These proteins are related through a highly conserved domain of approximately 300 amino acids, called the Rel homology domain, that contains dimerization, DNA binding, and nuclear targeting functions. Also within the Rel homology domain, there is a conserved consensus sequence (Arg-Arg-Pro-Ser) for phosphorylation by cyclic AMP-dependent protein kinase (PKA). We used linker insertion mutagenesis and site-directed mutagenesis to determine the importance of this sequence for the transformation of avian spleen cells by v-Rel and the subcellular localization of c-Rel in chicken embryo fibroblasts (CEF). The insertion of 2 amino acids (Pro-Trp) within this sequence completely abolished transformation and transcriptional repression by v-Rel and resulted in a shift in the localization of c-Rel from cytoplasmic to nuclear in CEF. When the conserved Ser within the PKA recognition sequence was replaced by Ala, there was no significant effect on transformation and transcriptional repression by v-Rel or on cytoplasmic retention of c-Rel. However, when this Ser was changed to Asp or Glu, transformation and transcriptional repression by v-Rel were significantly inhibited and c-Rel showed a diffuse nuclear and cytoplasmic localization in CEF. Although a peptide containing the recognition sequence from v-Rel can be phosphorylated by PKA in vitro, this site is not constitutively phosphorylated to a high degree in vivo in transformed spleen cells incubated with okadaic acid. Our results indicate that the transforming and transcriptional repressing activities of v-Rel and the cytoplasmic retention of c-Rel are dependent on the structure of the conserved PKA recognition motif. In addition, they suggest that phosphorylation at the conserved PKA site could have a negative effect on transformation and transcriptional repression by v-Rel and induce the nuclear localization of c-Rel.

Malignant transformation by mutant Rel proteins

A newly described family of transcriptional regulatory proteins, the Rel family, has recently been the subject of much interest. The Rel family includes proteins known to be important in Drosophila development, replication of HIV-1, oncogenesis and general transcriptional control. Nevertheless, there is still much to be learned about their precise mechanism of action, including the process by which the original member of this family, v-Rel, malignantly transforms cells.

The v-rel oncogene of avian reticuloendotheliosis virus transforms immature and mature lymphoid cells of the B cell lineage in vitro

Heavy chain gene rearrangements were analyzed in 67 independently derived reticuloendotheliosis virus (REV-T) transformed avian lymphoid cell lines. The status of the heavy chain genes in these REV-T transformed cell lines was determined, in part, by the age of the chicken whose spleen cells were transformed. Cell lines derived by the in vitro transformation of splenic lymphocytes obtained from embryos did not contain heavy chain gene Ig rearrangements. By contrast, splenic lymphocytes transformed by REV-T obtained from birds 1 week or older generally exhibited heavy chain gene rearrangements. The REV-T transformed lymphoid cell lines with heavy chain rearrangements also had light chain gene rearrangements. The Ig gene rearrangements in REV-T transformed cells were functional. The majority of the cells which had heavy chain rearrangements expressed a 2.2-kb mu transcript and synthesized and secreted IgM. An REV-T transformant was also identified which produced IgG, suggesting that v-rel can transform a terminally differentiated cell. Irrespective of their Ig chain gene status the REV-T transformed cell lines expressed variable amounts of some but not all normal B cell-specific markers and failed to express T cell markers. All the cell lines analyzed expressed the B-L (Ia-like) antigen as well as a common leukocyte antigen. Based on the expression of these surface molecules, the transformants with or without Ig gene rearrangements all appear to be committed to the B cell pathway.

Reticuloendotheliosis virus REV-T(REV-A)-induced neoplasia: development of tumors within the T-lymphoid and myeloid lineages

Infection of 1-day-old chicks with reticuloendotheliosis virus strain T induces a neoplastic disease that kills the chicks 7 to 14 days postinfection. In association with reticuloendotheliosis-associated virus (REV-A), reticuloendotheliosis virus T (REV-T) induces tumors that are predominantly immunoglobulin M (IgM) negative. We examined a variety of REV-T(REV-A)-induced tumors and tumor-derived cell lines and concluded that the principal IgM-negative tumors that develop in REV-T(REV-A)-infected chicks are neither pre-B or pre-B-pre-T but rather mature T lymphoid and myeloid. Without exception, the immunoglobulin heavy- and light-chain loci were in germ line configuration. Furthermore, the cell lines expressed neither sterile transcripts of the heavy- or light-chain immunoglobulin genes nor elevated levels of c-myb, two characteristics associated with murine pre-B lymphomas. Cell lines were also examined by using monoclonal antibodies for expression of a variety of cell surface markers expressed on B lymphocytes and/or T lymphocytes and/or myeloid cells. These reagents defined two types of IgM-negative tumor cell lines, one CIa+ CT-3+ (T lymphoid) and the other CIa+ CT-3-. By using the same approaches, tumor development was examined following REV-T(REV-A) infection at 1 and 3 weeks post-hatching of cyclophosphamide-treated chicks shown to be devoid of B-lymphoid cells. Again, the tumors that developed were either CIa+ CT-3+ (T lymphoid) or CIa+ CT-3-. Furthermore, the frequency and rate with which IgM-negative tumors developed in cyclophosphamide-treated chicks were not different from those observed in normal chicks. In 3-week-old cyclophosphamide-treated chicks, the presence of CIa+ CT-3- tumors bearing hematopoietic lineage markers, such as CLA-3 and 5M19, are most likely to have been derived from cells within the myeloid lineage.

The N-terminal env-derived amino acids of v-rel are required for full transforming activity

Expression of the v-rel oncogene of the reticuloendotheliosis virus, strain T (REV-T), can mediate the transformation of chicken spleen and bone marrow cells. Although the majority of the coding sequence of the v-rel oncogene is derived from the cellular rel sequence, the N- and C-terminal amino acids are coded for by remnants of the REV env gene. The resulting v-rel protein can be described as an env-rel-(out of frame env) fusion protein. Terminal deletion mutants were constructed to determine the role that env sequences play in the transforming activity of v-rel. Deletions were designed to remove only sequences of v-rel derived from former env sequence. Additional deletions removed more substantial amounts of coding sequence. Introduction of deleted genes into an REV-T based retroviral vector permitted the transforming activities to be determined. Deletion analysis indicated that the N-terminal region of pp59v-rel is required for the transforming activity, whereas as many as 100 C-terminal amino acids could be deleted without complete loss of the activity.

Oncogenic transformation by vrel requires an amino-terminal activation domain

The mechanism by which the products of the v-rel oncogene, the corresponding c-rel proto-oncogene, and the related dorsal gene of Drosophila melanogaster exert their effects is not clear. Here we show that the v-rel, chicken c-rel, and dorsal proteins activated gene expression when fused to LexA sequences and bound to DNA upstream of target genes in Saccharomyces cerevisiae. We have defined two distinct activation regions in the c-rel protein. Region I, located in the amino-terminal half of rel and dorsal proteins, contains no stretches of glutamines, prolines, or acidic amino acids and therefore may be a novel activation domain. Lesions in the v-rel protein that diminished or abolished oncogenic transformation of avian spleen cells correspondingly affected transcription activation by region I. Region II, located in the carboxy terminus of the c-rel protein, is highly acidic. Region II is not present in the v-rel protein or in a transforming mutant derivative of the c-rel protein. Our results show that the oncogenicity of Rel proteins requires activation region I and suggest that the biological function of rel and dorsal proteins depends on transcription activation by this region.

Oncogenic transformation by vrel requires an amino-terminal activation domain

The mechanism by which the products of the v-rel oncogene, the corresponding c-rel proto-oncogene, and the related dorsal gene of Drosophila melanogaster exert their effects is not clear. Here we show that the v-rel, chicken c-rel, and dorsal proteins activated gene expression when fused to LexA sequences and bound to DNA upstream of target genes in Saccharomyces cerevisiae. We have defined two distinct activation regions in the c-rel protein. Region I, located in the amino-terminal half of rel and dorsal proteins, contains no stretches of glutamines, prolines, or acidic amino acids and therefore may be a novel activation domain. Lesions in the v-rel protein that diminished or abolished oncogenic transformation of avian spleen cells correspondingly affected transcription activation by region I. Region II, located in the carboxy terminus of the c-rel protein, is highly acidic. Region II is not present in the v-rel protein or in a transforming mutant derivative of the c-rel protein. Our results show that the oncogenicity of Rel proteins requires activation region I and suggest that the biological function of rel and dorsal proteins depends on transcription activation by this region.

The v-rel oncogene product is complexed with cellular proteins including its proto-oncogene product and heat shock protein 70

The oncogene product, pp59v-rel, of avian reticuloendotheliosis virus (REV-T) is complexed in the cytosol of REV-T transformed lymphoid cells with cellular proteins. Monoclonal antibodies and antisera directed against different regions of pp59v-rel coimmunoprecipitate five cellular proteins (p124, p115, p75, p70, and p40) in addition to pp59v-rel. Cellular proteins with the same apparent molecular mass also copurify with pp59v-rel during sequential Sephacryl S200 and immunoaffinity chromatography. Antisera directed against the most abundant cellular protein in the complex, pp40, coimmunoprecipitate pp59v-rel and several cellular proteins with the same apparent molecular mass. The 75-kDa protein in the pp59v-rel complex is the product of c-rel proto-oncogene and is weakly phosphorylated. In MSB-1 cells this protein is not detectably phosphorylated or associated with cellular proteins. The 70-kDa protein in the pp59v-rel containing cytosolic complex is the constitutive form of avian heat shock protein 70 (HSC70). The p70 protein coimmunoprecipitates and copurifies with pp59v-rel using antisera directed against pp59v-rel and coimmunoprecipitates with antisera specific for pp40. The p70 isolated from immune complexes containing pp59v-rel shares V8 protease fragments with HSC70.

Rearrangement and diversification of immunoglobulin light-chain genes in lymphoid cells transformed by reticuloendotheliosis virus

Avian lymphoid cells transformed by reticuloendotheliosis virus (REV-T) serve as a model to analyze the mechanism by which B-cell differentiation and antibody diversification occur in birds. Immunoglobulin light-chain gene rearrangements, diversification, and expression were analyzed in 72 independently derived REV-T-transformed cell lines. Lymphoid cells transformed as the result of expression of the v-rel oncogene were divided into two distinct groups based on light-chain gene rearrangements. The status of the light-chain gene loci in these REV-T-transformed cell lines was determined in part by the ages of the chickens whose spleen cells were transformed. In embryonic spleen cell lines transformed by the v-rel oncogene, rearrangements were not detected, even after prolonged culture in vitro, indicating that these cells are arrested in B-cell differentiation. REV-T transformants derived from spleens obtained from chickens 2 weeks old or older, however, had at least one light-chain allele rearranged. All of the cell lines analyzed which exhibited rearranged light-chain genes contained light-chain transcripts, and most of the REV-T-transformed cells which displayed light-chain rearrangements expressed immunoglobulin protein. REV-T, therefore, transforms B-lymphoid cells at phenotypically different stages of development. Many REV-T-transformed cells undergo immunoglobulin chain gene rearrangements during prolonged propagation in vitro. Most of the cell lines which rearrange their light-chain alleles also undergo diversification during cultivation in vitro. Light-chain diversification occurs during or after the rearrangement event.

Growth-inhibitory effects of vitamin E succinate on retrovirus-transformed tumor cells in vitro

Vitamin E succinate inhibited proliferation of C4#1 cells, an established avian retrovirus [reticuloendotheliosis virus (REV)]-transformed immature lymphoid tumor cell line, in a dose-dependent manner. The cytostatic effects of vitamin E succinate were reversible in that treated cells regained their ability to divide after vitamin E succinate removal. Possible mechanism(s) for the antiproliferative actions of vitamin E succinate were investigated. Analyses of C4#1 cell surface membrane antigen profiles and morphology indicated that vitamin E succinate was not inducing differentiation of the tumor cells to a more mature, differentiated, nonproliferative state. Five antioxidants, including a synthetic analogue of vitamin E, Trolox, as well as the active vitamin form, DL-alpha-tocopherol, were incapable of inhibiting C4#1 tumor cell growth, indicating that a mechanism of action other than or in addition to functions as an antioxidant may be operating. Cell cycle analyses suggested that C4#1 tumor cells treated with vitamin E succinate were blocked in the G0G1/early S phases of the cell cycle. Tumor growth arrested by vitamin E succinate did not affect the expression of the REV-encoded oncogene, v-rel, at either the RNA or protein level. These studies demonstrated that vitamin E, in the form of vitamin E succinate, inhibited the growth of retrovirus-transformed tumor cells in vitro and suggested that the antiproliferative effects of vitamin E succinate did not involve antioxidant properties but rather, as yet, unidentified mechanisms leading to cell cycle blockage.

Transactivation of gene expression by nuclear and cytoplasmic rel proteins

Transcriptional activation of gene expression by oncogenic proteins can lead to cellular transformation. It has recently been demonstrated that the protein encoded by the v-rel oncogene from reticuloendotheliosis virus strain T can transactivate gene expression from certain promoters in a cell-specific manner. We have examined the cytological location, transforming properties, and transactivation properties of proteins encoded by chimeric turkey v-rel/chicken c-rel genes. We found that whereas the v-rel protein was nuclear in both chicken embryo and rat fibroblasts, the presence of the C terminus of the c-rel protein inhibited nuclear localization of the rel protein in these fibroblasts. Cytoplasmic rel proteins containing C-terminal c-rel sequences transactivated gene expression from the polyomavirus late promoter as efficiently as did similar rel proteins located in the nucleus. These results indicate that the cellular location of rel proteins is not important for transactivation of gene expression and suggest that transactivation by rel proteins is indirect, perhaps by affecting an intracellular signal transduction pathway that eventually results in the alteration of gene expression. The transforming properties of the rel protein were unaltered by the presence of the c-rel C terminus, but, as previously reported for turkey c-rel sequences, substitution of chicken c-rel sequences for internal v-rel sequences reduced the transforming activity of the rel protein and eliminated the immortalization ability. However, all of the chimeric v/c-rel proteins were able to transactivate gene expression, indicating that transactivation does not correlate with transformation. These results suggest that transactivation may be necessary but is not sufficient for transformation by rel proteins.

Rearrangement and diversification of immunoglobulin light-chain genes in lymphoid cells transformed by reticuloendotheliosis virus

Avian lymphoid cells transformed by reticuloendotheliosis virus (REV-T) serve as a model to analyze the mechanism by which B-cell differentiation and antibody diversification occur in birds. Immunoglobulin light-chain gene rearrangements, diversification, and expression were analyzed in 72 independently derived REV-T-transformed cell lines. Lymphoid cells transformed as the result of expression of the v-rel oncogene were divided into two distinct groups based on light-chain gene rearrangements. The status of the light-chain gene loci in these REV-T-transformed cell lines was determined in part by the ages of the chickens whose spleen cells were transformed. In embryonic spleen cell lines transformed by the v-rel oncogene, rearrangements were not detected, even after prolonged culture in vitro, indicating that these cells are arrested in B-cell differentiation. REV-T transformants derived from spleens obtained from chickens 2 weeks old or older, however, had at least one light-chain allele rearranged. All of the cell lines analyzed which exhibited rearranged light-chain genes contained light-chain transcripts, and most of the REV-T-transformed cells which displayed light-chain rearrangements expressed immunoglobulin protein. REV-T, therefore, transforms B-lymphoid cells at phenotypically different stages of development. Many REV-T-transformed cells undergo immunoglobulin chain gene rearrangements during prolonged propagation in vitro. Most of the cell lines which rearrange their light-chain alleles also undergo diversification during cultivation in vitro. Light-chain diversification occurs during or after the rearrangement event.

Transactivation of gene expression by nuclear and cytoplasmic rel proteins

Transcriptional activation of gene expression by oncogenic proteins can lead to cellular transformation. It has recently been demonstrated that the protein encoded by the v-rel oncogene from reticuloendotheliosis virus strain T can transactivate gene expression from certain promoters in a cell-specific manner. We have examined the cytological location, transforming properties, and transactivation properties of proteins encoded by chimeric turkey v-rel/chicken c-rel genes. We found that whereas the v-rel protein was nuclear in both chicken embryo and rat fibroblasts, the presence of the C terminus of the c-rel protein inhibited nuclear localization of the rel protein in these fibroblasts. Cytoplasmic rel proteins containing C-terminal c-rel sequences transactivated gene expression from the polyomavirus late promoter as efficiently as did similar rel proteins located in the nucleus. These results indicate that the cellular location of rel proteins is not important for transactivation of gene expression and suggest that transactivation by rel proteins is indirect, perhaps by affecting an intracellular signal transduction pathway that eventually results in the alteration of gene expression. The transforming properties of the rel protein were unaltered by the presence of the c-rel C terminus, but, as previously reported for turkey c-rel sequences, substitution of chicken c-rel sequences for internal v-rel sequences reduced the transforming activity of the rel protein and eliminated the immortalization ability. However, all of the chimeric v/c-rel proteins were able to transactivate gene expression, indicating that transactivation does not correlate with transformation. These results suggest that transactivation may be necessary but is not sufficient for transformation by rel proteins.

Cytotoxic T lymphocytes in reticuloendotheliosis virus-infected chickens

Cytotoxic T lymphocytes were functionally demonstrated in spleen cells from chickens 7 days post inoculation with reticuloendotheliosis virus using a Cr-release assay. Major histocompatibility complex (MHC)-restricted cytotoxicity was demonstrated using effector and target cells from two different strains of chickens of known avian MHC haplotype. Anti-viral specificity was shown and in vivo generation of MHC-restricted cytotoxicity was evaluated. Cytotoxic T cells were distinguished from macrophages and natural killer cells. Their cytotoxicity was not antibody dependent. Higher levels of cytolysis were found with cytotoxic T cells from embryonally bursectomized vs. intact chickens over a large range of effector to target cell ratios. Using monoclonal antibodies, cytotoxic T cells were further defined as Ia+ T cells by immunofluorescence, antibody plus complement-mediated lysis of effector cells and blocking of cytolysis in the Cr-release assay.

p59v-rel, the transforming protein of reticuloendotheliosis virus, is complexed with at least four other proteins in transformed chicken lymphoid cells

Previous studies have identified the protein product of v-rel, the oncogene carried by reticuloendotheliosis virus (REV), as a 59,000-dalton phosphoprotein located predominantly in the cytosol of transformed chicken lymphoid cells. In immune precipitates of p59v-rel, there is a closely associated protein kinase activity. In chicken lymphoid cells that do not contain REV, p68c-rel is found free in the cytosol not associated with other proteins and not detectably phosphorylated. In this study, we found that immune precipitates of 59v-rel from REV-transformed cells contain at least four other proteins, of approximate molecular weights 124, 115, 68, and 36 kilodaltons (kDa). The 124-, 115-, and 36-kDa proteins are apparently unrelated to p59v-rel in sequence, and their coprecipitation suggests that they are complexed with p59v-rel. The coprecipitating 68-kDa protein was found to be p68c-rel, which, like the other three proteins, precipitates by virtue of its association with p59v-rel. Glycerol gradient analysis suggested the presence of more than one type of complex: one containing p115, p68c-rel, p59v-rel, and p36, and another containing p124, p115, p59v-rel, and possibly p68c-rel. In vitro kinase activity was found in all size classes, coinciding with the distribution of p115 and p59v-rel. The complex(es) was stable under a variety of conditions, including a wide range of ionic strengths, chelators, and detergents, and through multiple cycles of immune precipitation and elution. This suggests a specific and functionally significant interaction among the members that may be of direct relevance to the mechanism of REV-induced transformation.

Site-directed cytotoxic antibody against the C-terminal segment of the surface glycoprotein gp90 of avian reticuloendotheliosis virus

The major mature env-gene products of avian reticuloendotheliosis-associated virus (REV-A) are the surface glycoprotein (gp90) and the transmembrane protein (gp20). We have previously reported that gp90 was detected in the REV-A virus by Western blot analysis as well as in the REV-A-infected cells by radioimmunoprecipitation with antibodies raised in rabbits against the gp90 C-terminal tridecapeptide which was predicted from the nucleotide sequence (Wilhemsen et al., J. Virol., 52, 172, 1984). We have now shown that this antibody detected antigens on the REV-A-infected cells by fluorescence-activated cell sorter (FACS) analysis, and conferred specific cytotoxic effects on the infected cells in the presence of rabbit complement using the chromium release assay. These results clearly indicate that the C-terminal epitope of gp90 is situated on the surface of the REV-A-infected cells and accessible to site-directed antibodies which cause cytotoxicity by activating the complement system. The possible in vivo roles of this antibody are discussed.

Autogenous growth factor production by reticuloendotheliosis virus-transformed hematopoietic cells

Reticuloendotheliosis virus strain T (REV-T)-transformed cells gave rise spontaneously to variants which secrete a factor that forms a distinct visible ring of precipitation (halo) surrounding colonies grown in soft agar. An Mr 15,000 protein was produced at higher levels by halo variants than by nonhalo-producing cells. An assay designed to detect the formation of precipitates enabled purification of an Mr 15,000 protein, p15, from serum-free medium conditioned by the growth of REV-T-transformed hematopoietic cells. Fractions enriched in p15 permitted the growth of REV-T-transformed cells under conditions where they normally failed to proliferate.

The v-rel oncogene product is complexed to a 40-kDa phosphoprotein in transformed lymphoid cells

The transforming protein encoded by the v-rel oncogene of avian reticuloendotheliosis virus (REV-T) is a very low copy number molecule in the cytosol of transformed cells. Analysis of cytosolic extracts from a REV-T-transformed lymphoid cell line by gel filtration on Sephacryl S-300 indicated that most of the v-rel oncogene product, pp59v-rel, eluted with an apparent molecular mass of 400 kDa. The size of this complex was confirmed by analysis on a fast-protein liquid chromatography gel filtration column. A 40-kDa cellular protein copurified with pp59v-rel on sequential gel filtration on Sephacryl S-200 and immunoaffinity chromatography with a monoclonal antibody directed against pp59v-rel. The 40-kDa cellular protein could also be immunoprecipitated together with pp59v-rel from cell extracts of [35S]methionine-labeled cells, suggesting that pp59v-rel is complexed with the 40-kDa protein in transformed lymphoid cells. Both the 59- and 40-kDa proteins were phosphorylated when the highly purified preparation containing pp59v-rel was incubated with [gamma-32P]ATP and 10 mM MgCl2 in vitro. The identity of the kinase in the highly purified preparation containing pp59v-rel, however, is unknown. Immune complexes recovered from extracts of REV-T-transformed lymphoid cells labeled with [32P]orthophosphate also contained the 59- and 40-kDa phosphoproteins. These observations suggest that pp59v-rel is complexed with a 40-kDa cellular phosphoprotein to form a 400-kDa heteropolymer in the cytoplasm of transformed lymphoid cells.

Transforming viruses spontaneously arise from nontransforming reticuloendotheliosis virus strain T-derived viruses as a result of increased accumulation of spliced viral RNA

The highly oncogenic avian retrovirus reticuloendotheliosis virus strain T (Rev-T) contains a substitution of the oncogene v-rel for much of env and a deletion of gag and pol relative to the helper virus Rev-A. Replacement of gag and pol sequences in Rev-T suppresses transformation by reducing the accumulation of spliced viral mRNA and v-rel protein in infected cells (C. K. Miller and H. M. Temin, J. Virol 58:75-80, 1986). After infection of spleen cells with viruses containing gag and pol sequences, revertant viruses that are strongly transforming were found. Approximately three-fourths of the revertant viruses appeared structurally the same as the parental virus, and approximately one-fourth of the revertant viruses had large deletions (similar in size and location to the deletion in Rev-T). Two revertant viruses that appeared structurally the same as the parental virus were molecularly cloned. The regions sufficient to change the parental virus to a strongly transforming virus were determined by construction of recombinant viruses. In one revertant virus, the region sufficient for transformation contained a 327-base-pair insertion 5' of the 3' splice site used by Rev-T. In the other revertant virus, the region sufficient for transformation contained a 1-base-pair transition and a deletion of one copy of a 9-base-pair direct repeat, both 3' of the 3' splice site used by Rev-T. These differences resulted in the accumulation of increased levels of subgenomic v-rel mRNA and protein, ultimately leading to transformation.

v-rel oncoproteins in the nucleus and in the cytoplasm transform chicken spleen cells

The transforming protein encoded by the v-rel oncogene of the highly oncogenic avian retrovirus reticuloendotheliosis virus strain T (Rev-T) is a 59,000-dalton protein, p59v-rel. The mechanism by which p59v-rel induces transformation of early lymphoid cells is unknown. As a step towards understanding the mechanism of v-rel-induced transformation, we sought to establish the subcellular site of action of p59v-rel. In this report, we show that p59v-rel contains sequences that are necessary for its efficient localization in the nucleus of infected chicken embryo fibroblasts. These v-rel sequences when added to the normally cytoplasmic protein, beta-galactosidase, directed that protein to the nucleus. A mutation in the v-rel nuclear-localizing sequence did not affect the transforming function, although it did alter the nuclear-localizing function. The addition of a supplemental nuclear-localizing sequence from simian virus 40 large T-antigen to v-rel resulted in the expression of a transforming rel protein which was located exclusively in the nucleus of transformed spleen cells, in contrast to wild-type p59v-rel, which was largely cytoplasmic in transformed spleen cells. Our results support the hypothesis that v-rel encodes a protein which can act either in the nucleus or in the cytoplasm to transform spleen cells.

Expression of the v-rel oncogene in reticuloendotheliosis virus-transformed fibroblasts

Reticuloendotheliosis virus (REV-T) induces a rapidly fatal lymphoma in chickens through the expression of its oncogene, v-rel, REV-T also morphologically transforms avian fibroblasts in vitro. These transformed cells displayed limited anchorage-independent growth and reached higher saturation density than uninfected or REV-A-infected fibroblasts. Morphologically transformed fibroblasts were tumorigenic when injected into the wing web of chickens. In transformed fibroblasts, the v-rel oncogene was expressed as a 57 kDa phosphoprotein with a half-life of 2 to 4 hr. A cellular phosphoprotein of about 40 kDa was also observed in immunoprecipitates of transformed fibroblasts. The subcellular location of the v-rel-encoded protein was determined using cell fractionation procedures and immunofluorescent staining. In acutely infected, nontransformed fibroblasts, pp57v-rel was associated with the nuclear region, but in morphologically transformed cells the v-rel protein was found in the cytoplasm. These observations suggest that the expression of the v-rel oncogene is insufficient for transformation and that the cellular localization of this transforming protein to the cytoplasm may be required for the progression to an altered cell phenotype in avian fibroblasts.

Rearrangements of chicken immunoglobulin genes in lymphoid cells transformed by the avian retroviral oncogene v-rel

The retroviral oncogene v-rel transforms poorly characterized lymphoid cells. We have explored the nature of these cells by analyzing the configuration and expression of immunoglobulin genes in chicken hemopoietic cells transformed by v-rel. None of the transformed cells expressed their immunoglobulin genes. The cells fell into three classes: class I cells have their immunoglobulin genes potentially in an embryonic configuration; class II and class III cells have lost one copy of the lambda light chain locus and have one copy of the heavy chain locus rearranged into a configuration that differs from what is found in mature B cells. In class II cells, the other heavy chain locus may be in embryonic configuration, whereas it is deleted in class III cells. The first of these classes may represent the earliest stage of the lymphoid lineage yet encountered among virus-transformed cells, whereas the second and third classes represent an apparently anomalous rearrangement whose origin remains unknown.

Transformation of avian lymphoid cells by reticuloendotheliosis virus

Avian reticuloendotheliosis virus (REV-T) is the most virulent of all retroviruses, inducing an invariably fatal leukemia in chickens with a latent period of 7-10 days. Unlike avian cells transformed by other acutely transforming viruses, lymphoid cells transformed by REV-T are immortalized. Furthermore, in vitro derived, REV-T transformed cells which do not produce virus are tumorigenic and induce lethal reticuloendotheliosis when injected into histocompatible birds. Thus REV-T transforms its target cell both in vitro and in vivo. In addition this transformation is independent of any helper virus functions. Like other acute leukemia viruses, REV-T is replication-defective and must co-replicate with a reticuloendotheliosis associated virus (REV-A). During evolution, a substantial portion of its genome has been deleted and replaced with a host-derived genetic sequence, designated v-rel. Presumably, the v-rel oncogene was transduced from a normal turkey DNA locus, c-rel. There are 9 regions of homology between c-rel and v-rel, however, several differences exist between these genes, suggesting that transformation by REV-T results from the production of an altered v-rel protein. The v-rel sequence is distinct from other known oncogenes and encodes a 57-kDa phosphoprotein. In REV-T transformed cells, this pp57v-rel protein is localized in the cytoplasm. The product of the v-rel oncogene is present at a low level, representing only about 0.003% of total methionine-labelled protein. In addition, pp57v-rel is relatively stable, having an estimated half-life of 4-10 h. The v-rel protein when purified close to homogeneity is complexed with a 40-kDa cellular phosphoprotein in transformed lymphoid cells and possesses serine kinase activity. This review discusses the molecular aspects of transformation by REV-T in the context of other oncogene-encoded proteins.

The transforming protein of avian reticuloendotheliosis virus is a soluble cytoplasmic protein which is associated with a protein kinase activity

We have identified the product (p57v-rel) of the transforming gene, v-rel, of avian reticuloendotheliosis virus (REV-T) using antisera generated against nonoverlapping sequences representing the middle and carboxy-terminal regions of the v-rel protein expressed in Escherichia coli (N.K. Herzog and H.R. Bose, Jr., 1986, Proc. Natl. Acad. Sci. USA 83, 812-816). The amino-terminal region of the v-rel protein was also expressed in E. coli and used to generate antisera. The immunoglobulin-enriched fractions of these antisera were used to determine the subcellular location of p57v-rel in REV-T transformed lymphoid cells. Cells were fractionated into nuclear, mitochondrial, microsomal, and cytoplasmic fractions. The majority of p57v-rel was found in the cytoplasm. Examination of REV-T transformed lymphoid cells labeled with 32Pi revealed that the majority of the phosphorylated form of the v-rel protein was also found in the cytoplasm. Indirect immunofluorescence of REV-T transformed cells gave a diffuse cytoplasmic pattern indicating that p57v-rel was not associated with any discrete cellular organelle. The distribution of p57v-rel was similar in REV-T transformed lymphoid cells labeled with [35S]methionine for short and long periods of time, suggesting that p57v-rel is a soluble cytoplasmic protein throughout its lifetime. The v-rel protein was phosphorylated when immune complexes precipitated from transformed cells with the immunoglobulin fractions obtained from antisera against the amino-terminal, middle, and carboxy-terminal regions of v-rel were incubated with [gamma-32P]ATP and Mn2+. The phosphorylation of p57v-rel in the in vitro immune complex kinase assay was inhibited when the immunoglobulin-enriched fraction of these antisera was preincubated with the homologous v-rel fusion proteins. Preincubation with heterologous proteins did not block the phosphorylation of p57v-rel. These observations suggest that p57v-rel is associated with a protein kinase activity. Most of the kinase activity was found in the soluble cytoplasmic fraction of transformed cells. The transforming protein encoded by v-rel is a relatively stable protein with a half-life of approximately 7 to 8 hr in transformed lymphoid cells.

Activation of oncogenicity of the c-rel proto-oncogene

Reticuloendotheliosis virus strain T (Rev-T) induces a lethal lymphoma in young birds and transforms avian lymphoid cells in vitro. The transforming gene of Rev-T, v-rel, was derived from the turkey proto-oncogene c-rel. Comparison of the nucleotide sequences of v-rel and c-rel indicates that in addition to several internal amino acid changes relative to c-rel, p59v-rel has amino acid sequences at both ends derived from the reticuloendotheliosis virus strain A-related virus env gene (K. C. Wilhelmsen, K. Eggleton, and H. M. Temin, J. Virol. 52:172-182, 1984). In this report, the v-rel sequences important for transformation were defined by constructing recombinant retroviruses in which c-rel sequences replaced the analogous v-rel sequences. These recombinant viruses expressing chimeric proteins were tested for their ability to transform spleen cells in vitro and to induce tumors in young chickens. Activation of the oncogenicity of c-rel in Rev-T required alteration of the amino terminus and the central region of the protein. Deletion of the noncoding sequences 3' to c-rel and of most of the helper virus-related env sequences was necessary for the formation of Rev-T.

Biosynthesis and chemical and immunological characterization of avian reticuloendotheliosis virus env gene-encoded proteins

Two glycosylated proteins designated gp90 and gp20 were purified from replication-competent avian reticuloendotheliosis associated virus (REV-A). The N-terminal sequences of gp90 and gp20 were determined and found to match the REV-A-env-gene sequence. The alignments of the determined amino acid sequences with the predicted sequence indicate that gp20 and gp90 are the REV-A-encoded viral transmembrane and surface glycoprotein, respectively, and predict a signal peptide of 36 residues on the 5' end of the env-gene. Furthermore, gp90 of REV-A was detected by Western blot analysis with antibodies to a tridecapeptide corresponding to an env-gene nucleotide segment immediately preceding gp20 and thus representing the C-terminal portion of gp90. The env-gene precursor polyprotein gPr75-79env and Pr22(E), the precursor to gp20 and p2(E) were identified in the infected cells by monospecific antibodies raised against purified gp20. Thus the organization of gPR75-79env is likely to be N-gp90-gp20-p2(E), resembling that of M-MuLV gp85env. Sequence comparisons showed that the env gene of REV-A is highly related to both baboon endogenous virus and Type D retroviruses. In Western blot analyses, antibodies to REV-A gp20 cross-reacted with a panel of mammalian Type C and Type D viruses. Evolutionary aspects of these findings are discussed.

Activation of oncogenicity of the c-rel proto-oncogene

Reticuloendotheliosis virus strain T (Rev-T) induces a lethal lymphoma in young birds and transforms avian lymphoid cells in vitro. The transforming gene of Rev-T, v-rel, was derived from the turkey proto-oncogene c-rel. Comparison of the nucleotide sequences of v-rel and c-rel indicates that in addition to several internal amino acid changes relative to c-rel, p59v-rel has amino acid sequences at both ends derived from the reticuloendotheliosis virus strain A-related virus env gene (K. C. Wilhelmsen, K. Eggleton, and H. M. Temin, J. Virol. 52:172-182, 1984). In this report, the v-rel sequences important for transformation were defined by constructing recombinant retroviruses in which c-rel sequences replaced the analogous v-rel sequences. These recombinant viruses expressing chimeric proteins were tested for their ability to transform spleen cells in vitro and to induce tumors in young chickens. Activation of the oncogenicity of c-rel in Rev-T required alteration of the amino terminus and the central region of the protein. Deletion of the noncoding sequences 3' to c-rel and of most of the helper virus-related env sequences was necessary for the formation of Rev-T.

Localization of the v-rel protein in reticuloendotheliosis virus strain T-transformed lymphoid cells

The protein (p59rel) encoded by the transforming gene of reticuloendotheliosis virus strain T (REV-T) has been identified in REV-T-transformed avian lymphoid cells by using antisera raised against synthetic peptides whose sequences were derived from three nonoverlapping regions of v-rel (N. R. Rice, T. D. Copeland, S. Simek, S. Oroszlan, and R. V. Gilden, Virology 149:217-229, 1986). To obtain polyclonal antibodies directed against a larger number of p59rel epitopes, a 262-amino acid segment was expressed in bacteria. Antisera raised against this fusion protein (v-delta-rel) precipitated p59rel from lysates of [35S]methionine-labeled REV-T-transformed cells, thus confirming previous results obtained with the peptide antisera. We used this new antiserum to localize p59rel in REV-T-transformed cells by subcellular fractionation using differential centrifugation and by indirect immune fluorescent staining. After fractionation and immune precipitation, the majority of p59rel was found in the cytosolic fraction. Indirect immunofluorescence experiments also gave results consistent with the cytoplasmic localization of the v-rel protein in transformed lymphoid cells. In previous studies (Rice et al., Virology 149:217-229, 1986) it was shown that immune precipitates formed with one of the three p59rel peptide antisera possessed in vitro protein kinase activity. Immune precipitates formed with the fusion protein antiserum also showed kinase activity in the in vitro assay. Most of this activity was found in the soluble cytoplasmic fraction, indicating that the kinase may be p59rel or a protein closely associated with it.

Insertion of several different DNAs in reticuloendotheliosis virus strain T suppresses transformation by reducing the amount of subgenomic mRNA

The highly oncogenic retrovirus reticuloendotheliosis virus (Rev) strain T (Rev-T) has, relative to its helper virus Rev strain A, a substitution of the oncogene v-rel for most of the env gene and a large deletion of gag and pol sequences. When the helper virus sequences that are deleted in Rev-T are replaced, the recombinant virus is nontransforming (I. S. Y. Chen and H. M. Temin, Cell 31: 111-120, 1982). We show that suppression of transformation occurs when several different DNA sequences are inserted in Rev-T and that suppression is correlated with a reduction in the amount of v-rel mRNA and v-rel protein in infected cells. The reduced amount of v-rel protein is insufficient for transformation.

Detection and characterization of the protein encoded by the v-rel oncogene

To identify the protein encoded by v-rel, the oncogene of reticuloendotheliosis virus (REV-T), antisera have been raised to three synthetic peptides derived from the translation of our previously published v-rel DNA sequence [R.M. Stephens, N.R. Rice, R.R. Hiebsch, H.R. Bose, Jr., and R.V. Gilden, Proc. Natl. Acad. Sci. USA 80, 6229-6233 (1983)]. Sera to all three peptides precipitate a 59,000 Da protein from REV-T-transformed chicken lymphoid cells. This protein is not detectable in uninfected chick embryo fibroblasts, and its observed size is in good agreement with the 56,000 Da predicted by the DNA sequence. We conclude that this protein is the v-rel product and designate it p59rel. To search for evidence of post-translational processing of this protein, cells were grown in the presence of glycosylation inhibitors. These resulted in no detectable difference in the size of p59rel. Nor was its size detectably altered during the course of a pulse-chase experiment. Growth of cells in the presence of [32P] orthophosphate, however, revealed that p59rel is a phosphoprotein. It is also closely associated with a protein kinase activity, for precipitation with one of the peptide antisera (but not the other two) resulted in strong kinase activity in the immune complex pellet. During this reaction, p59rel itself becomes phosphorylated. Kinase activity was retained in the immune complex following detergent and high salt washes, leaving open the possibility that p59rel is itself a kinase.