The avian transcription factor c-Rel is expressed in lymphocyte precursor cells and antigen-presenting cells during thymus development.

Transcription factors of the Rel/NF-kappaB family are widely involved in the immune system. In this study, we investigate the in vivo expression of the avian protein c-Rel in the T-cell lineage during thymus development. The majority of thymocytes do not express the c-Rel protein. However, lymphocyte precursor cells that colonize the thymus express the c-Rel protein shortly after their homing in the organ and before they begin to differentiate. c-Rel is also detected in different subsets of antigen-presenting cells such as epithelial cells, dendritic cells, and macrophages. In vitro studies have shown that Rel/NF-kappaB proteins are sequestered in an inactive form in the cytoplasm by interaction with the IkappaBalpha inhibitory protein. By immunocytochemistry, we show that in vivo c-Rel is localized in the cytoplasm of antigen-presenting cells but in both the cytoplasm and nucleus of lymphocyte precursor cells. The cytoplasmic localization of c-Rel in antigen-presenting cells correlates with a high expression of IkappaBalpha, whereas the nuclear localization of c-Rel in lymphocyte precursor cells correlates with a much lower expression of IkappaBalpha. These results suggest that c-Rel might be constitutively activated in lymphocyte precursor cells.

A chicken c-Rel-estrogen receptor chimeric protein shows conditional nuclear localization, DNA binding, transformation and transcriptional activation.

In this report, we characterize the biological and biochemical properties of a conditional protein containing chicken c-Rel fused to the hormone-binding domain of the human estrogen receptor. This chimeric c-RelER protein causes estrogen-dependent, but otherwise c-Rel-specific, transformation of avian fibroblasts in vitro. Our results demonstrate that c-RelER heterodimerizes with wild-type c-Rel and forms specific complexes with IkappaB-alpha. Estrogen causes translocation of c-RelER to the nucleus and stabilizes its binding to DNA. Hormone-activated c-RelER induces transcription of at least four cellular genes that are constitutively active in wild-type c-Rel-transformed fibroblasts. Two distinct cell populations were examined that differed with respect to their growth phenotypes. The growth of fibroblasts with moderate expression levels of c-RelER was stimulated by estrogen. In contrast, the addition of estrogen to cells with high cRelER expression levels resulted in inhibition of cytokinesis and the arrest of growth. The carboxy terminal transactivation domain of c-Rel was required for the induction of these effects since neither v-Rel nor c-Rel deletion mutants were able to induce similar changes. Taken together, our results demonstrate that high levels of c-Rel expression can affect cell cycle control and/or cytokinesis. Furthermore, they also indicate that the biological properties of c-Rel in cell growth and differentiation will potentially differ depending on the level of expression.

Characterization of changes in gene expression associated with malignant transformation by the NF-kappaB family member, v-Rel.

In this study, alterations in gene expression patterns have been examined in v-Rel-transformed avian bone marrow cells. Using a conditional v-Rel estrogen receptor chimera (v-RelER) which transforms cells in an estrogen-dependent manner, we constructed subtraction cDNA libraries from v-RelER-transformed bone marrow cells. Several different sequences were identified whose expression was altered upon hormone activation of v-RelER. These include two genes related to the MIP-1 chemokine family (mip-1beta and a tca3 homologue), a cell surface antigen sca-2 and the transcription factor nfkb1. The expression of each gene was assayed in a number of wild-type and mutant v-Rel-expressing fibroblast and hematopoietic cells. All v-Rel-transformed hematopoietic cells tested express high levels of nfkb1 and sca-2. In fibroblasts, wild-type v-Rel induced expression of mip-1beta and nfkb1, while nontransforming mutants of v-Rel failed to do so, suggesting a role for these two genes in v-Rel mediated transformation. Finally, these genes are expressed at high levels in cells overexpressing wild-type and truncated forms of c-Rel, implying that v-Rel transforms, in part, by induction of c-Rel target genes.

Analysis of the role of v-rel in transcriptional regulation of high mobility group 14.

The oncogene, v-rel is a member of the rel/NF-kappaB family of transcription factors. It causes a rapidly fatal lymphoma in young chicks and is capable of transforming both fibroblasts and primitive hematopoietic cells in culture. To understand the role of v-rel in transformation we constructed an inducible form of v-rel and used it to identify potential cellular target genes for v-rel regulation. In this paper we show that High Mobility Group Protein 14 (HMG 14) is expressed in a wide variety of v-rel transformed cell types. In addition we show that v-rel participates in the transcriptional regulation of HMG 14 and that extracts from v-relER cells interact with the HMG 14 promoter. These experiments suggest a role for v-rel in the regulation of a unique gene whose protein product may influence gene transcription in a global fashion.

Isolation of a cDNA encoding a novel chicken chemokine homologous to mammalian macrophage inflammatory protein-1 beta.

A cDNA encoding a novel chicken chemokine homologous to mammalian chemokine macrophage inflammatory protein 1 beta (MIP-1 beta) was isolated and characterised. The cDNA encodes a protein which is 75-80% homologous to human and mouse MIP-1 beta. All conserved amino acids characteristic of the mammalian chemokine family have been evolutionarily preserved in chicken MIP-1 beta, suggesting similar protein folding patterns and functional properties.

The role of the carboxy terminus of v-Rel in transformation and activation of endogenous gene expression.

Proteins within the Rel/NF-kappa B transcription factor family can be divided into two functional domains, a homologous amino terminal region, the Rel Homology Domain, and a divergent carboxy terminal domain. The amino terminal sequences specify DNA binding, nuclear localization, and interaction with the I kappa B family of inhibitory proteins. The carboxy terminus of each protein functions as a transcriptional activation domain, however, precise definition of sequence requirements has been difficult. To further define these sequences, small 100 bp deletions were constructed throughout the carboxy terminus of v-Rel. Each resulting mutant was assayed for DNA binding, localization, protein complex formation, activation of endogenous gene expression and ability to transform bone marrow cells and fibroblasts. Surprisingly, deletion within the carboxy terminus had marginal effects on transforming potential. However, three separate regions were required for full activation of gene expression. Taken together, these results suggest that the carboxy terminus of v-Rel contains multiple sequences that participate in the activation of gene expression.

Constitutive and inducible kappa B binding activities in the cytosol of v-Rel-transformed lymphoid cells.

Constitutive and inducible kapp B binding activities associated with v-Rel and c-Rel in the cytosol of v-Rel-transformed cells have been identified. These activities were resolved by electrophoretic mobility shift assay and chromatographic techniques into a high-molecular-weight protein-DNA complex designated complex I containing v- and c-Rel and lower-molecular-weight complexes II, III and IV which contained only v-Rel and which were stimulated by nucleotides, low pH, and detergent. These experiments suggest that interaction of v-Rel and c-Rel decreases the DNA-binding activity of each.

Dendritic cell progenitor is transformed by a conditional v-Rel estrogen receptor fusion protein v-RelER.

A conditional v-Rel estrogen receptor fusion protein, v-RelER, causes estrogen-dependent but otherwise unaltered v-rel-specific transformation of chicken bone marrow cells. Here, we demonstrate that such v-relER-transformed cells exhibit B lymphoid determinants in line with earlier studies on v-rel-transformed cells. However, following inactivation of v-RelER oncoprotein activity by administration of an estrogen antagonist, cells differentiate into antigen-presenting dendritic cells as judged by several morphological and functional criteria. Additionally, under yet different culture conditions, v-relER cells differentiate into cells resembling polymorphonuclear neutrophils. Our studies therefore suggest that the conditional v-RelER, and probably also the authentic v-Rel, transform a common progenitor for neutrophils and dendritic cells.

The Rel family of proteins in oncogenesis and differentiation.

The Rel family consists of a large set of related proteins containing a conserved protein region, termed the Rel Homology Domain. Although the prototype member of the Rel family, v-Rel, was originally discovered on the basis of its transforming ability, recent studies have indicated that other members are also important mediators of embryonic development and differentiation. In particular, the dorsal protein is an essential embryonic polarity gene required for the proper dorsal/ventral development of the Drosophila embryo, while the avian c-rel gene has been implicated in the control of cell death and tissue molding. Other members may play a part in the differentiation of dendritic antigen presenting cells and the control of general and lymphoid specific gene transcription.

High levels of c-rel expression are associated with programmed cell death in the developing avian embryo and in bone marrow cells in vitro.

To determine the physiological processes in which the transcription factor c-Rel may act, we have examined its pattern of expression in the avian embryo by in situ hybridization. These studies showed that c-rel is expressed ubiquitously at low levels and at high levels in isolated cells undergoing programmed cell death by apoptosis or autophagocytosis. To further establish a functional link between expression of c-rel and cell death, we examined the biological consequences of c-rel overexpression in vitro. In primary avian fibroblasts, overexpression of c-rel leads to transformation and dramatic life span extension. In contrast, bone marrow cells expressing high levels of c-rel undergo a process of programmed cell death displaying features of both apoptosis and autophagocytic cell death. Thus, these experiments suggest a critical role for c-rel not only in the control of cell proliferation, but also in the induction of cell death.

Hormone-regulated v-rel estrogen receptor fusion protein: reversible induction of cell transformation and cellular gene expression.

We describe the construction of a v-rel estrogen receptor fusion protein (v-relER) which allows the regulation of v-rel oncoprotein activity by hormone. In the presence of estrogen, v-relER readily transformed primary chicken fibroblasts and bone marrow cells in vitro. In both cell types, v-rel-specific transformation was critically dependent on the presence of estrogen or the estrogen agonist 4-hydroxytamoxifen (OHT). Withdrawal of estrogen or application of an estrogen antagonist, ICI164,384 (ICI) caused a reversal of the transformed phenotype. We also demonstrate that the v-relER protein binds to NF-kappa B sites in an estrogen-dependent manner, thereby showing that sequence-specific DNA binding of v-relER is critical for the activation of its transforming capacity. In transient transfection experiments, we failed to demonstrate a clear repressor or activator function of the v-rel moiety in v-relER. However, in v-relER-transformed bone marrow cells, estrogen and OHT induced elevated mRNA levels of two cellular genes whose expression is constitutive and high in v-rel-transformed cells. These results suggest that v-rel might exert part of its activity as an activator of rel-responsive genes.

Mutations in the rel-homology domain alter the biochemical properties of v-rel and render it transformation defective in chicken embryo fibroblasts.

Sequential deletions of approximately 100 base pairs were made in the rel-homology domain of the viral rel protein. Each deletion mutant was cloned into a replication-competent viral vector and assayed in chicken embryo fibroblasts (CEFs). The deleted v-rel proteins were analysed for localization, complex formation and ability to induce transformation. In vitro-translated mutant proteins were assayed for binding to a NF-kappa B consensus sequence. All the deletion mutants between nucleotides 37 and 798 in v-rel were transformation defective. Each of the mutants localized predominantly in the cytoplasm, whereas wild-type v-rel localizes predominantly in the nucleus of CEFs. Any disruption of the rel-homology domain reduced binding of the mutant v-rel proteins to the cellular protein, p36, while the requirements for binding to p68c-rel, p115 and p124 appeared to be more complicated. The binding of these three proteins to v-rel appeared to be linked and mediated through c-rel, suggesting that v-rel disrupts normal c-rel function. None of the deletion mutants in this region were able to bind to the NF-kappa B site. However, mutants which lie outside the rel-homology domain retained the ability to transform CEFs, localize to the nucleus, complex with p36, p115, p124 and p68c-rel and bind to the NK-kappa B site. These results suggest that transformation by v-rel requires an intact rel-homology domain and that the biochemical properties of v-rel are linked and dependent upon higher order protein structure for full function.

Expression of v-rel in a replication competent virus: transformation and biochemical characterization.

The avian reticuloendotheliosis virus strain T (REV-T) transforms bone marrow cells and may cause phenotypic changes in fibroblasts. Both events are thought to result from expression of the v-rel oncoprotein, a member of the NF-kappa B family of transcription factors. Most REV stocks contain a cytopathic and immunosuppressive helper virus (REV-A) unrelated to standard avian retroviruses, and thus the degree to which v-rel expression alone contributes to the transformed phenotype in bone marrow cells and fibroblasts is complicated by helper virus expression. To gain a more accurate picture of how v-rel contributes to transformation, we have cloned the v-rel gene into a replication-competent avian retrovirus vector (RCAS) and have expressed it in both chick embryo fibroblasts (CEF) and bone marrow cells. Transfection of RCAS-rel into CEF readily produced a partially transformed phenotype, demonstrating that expression of the v-rel protein is sufficient for fibroblast transformation. The RCAS-rel virus also transformed bone marrow cells in vitro, but required culture conditions different from those normally required for transformation by REV-T. The v-rel protein expressed in transformed CEF was biochemically indistinguishable from that expressed in transformed bone marrow cells, being localized to the cytoplasm and the nucleus, and forming a complex with cellular proteins. We also demonstrate that the RCAS-rel-transformed hematopoietic cells exhibited a distinct differentiation phenotype.

Cell transformation by the epidermal growth factor receptor and v-erbB.

It has become increasingly apparent that growth factor receptors can function as oncogenic proteins and play causal roles in cell transformation. A prime example is the epidermal growth factor receptor (EGFR), which belongs to the ligand-activated tyrosine kinase receptor family and is overexpressed in certain human tumors. The transforming gene of avian erythroblastosis virus, v-erbB, encodes a truncated form of the EGFR whose kinase domain is constitutively activated by deletion of the ligand binding domain. Recent comparative studies of the v-erbB gene in different viral isolates have revealed that subtle sequence changes (point mutations, small deletions) can alter both the pathogenic spectrum of the virus and the range of cell types susceptible to transformation in vitro. Therefore, the possibility exists that similar, as-yet-unidentified, mutations may exist in the EGFR gene; if so, the EGFR may be an oncogenic factor in tumors other than those with which it has been associated to date.

Interaction of the v-rel protein with an NF-kappa B DNA binding site.

The avian reticuloendotheliosis virus T contains within its genome the oncogene rel. The expression of this gene is responsible for the induction of lymphoid tumors in birds. Recently, the rel gene was shown to be related to the p50 DNA binding subunit of the transcription factor complex NF-kappa B. Binding sites for the NF-kappa B complex are found in the enhancer regions of a number of genes, including the immunoglobulin kappa gene and the human immunodeficiency virus long terminal repeat. In this communication we identify an activity from avian reticuloendotheliosis virus T-transformed avian lymphoid cells that binds in an electrophoretic-mobility-shift assay to an NF-kappa B binding site from the kappa enhancer. This activity contains proteins immunologically related to rel, as detected by polyclonal and monoclonal antibodies directed against v-rel. In a DNA affinity precipitation assay using the NF-kappa B site from the human immunodeficiency virus long terminal repeat, v-rel and several other proteins were identified. These data suggest that oncogenic transformation by v-rel is the result of an altered pattern of gene expression.

The rel gene and its role in oncogenesis.

The rel oncogene induces rapidly fatal lymphomas in birds. The mechanism by which this occurs is not clear, but investigations into the properties of rel have shown it to be an unusual protein. Unlike other 'nuclear' oncogenes described in this issue, v-rel can be found in both the cytoplasm and nucleus. In addition, it appears to transform avian lymphoid cells regardless of its localization. Interestingly, the rel gene is highly homologous to the Drosophila gene dorsal, which is involved in determination of dorsal-ventral polarity in the developing embryo. In this review, the biology of the virus containing the rel oncogene, REV-T, will be described. The structure of the gene and the properties of the rel protein will be examined. The rel protein will be compared to the Drosophila homolog, dorsal, both structurally and functionally.

Characterization of a novel promoter insertion in the c-rel locus.

Avian leukosis virus (ALV)-induced neoplasias are commonly found associated with integrations of proviral DNA in proximity to the myc gene. However, studies suggest that other genetic events are necessary for the complete neoplastic phenotype. A cell line (HP46) derived from an ALV-induced tumor has been analyzed and found to contain, in addition to an alteration in the myc gene, a promoter insertion in the c-rel locus. Both loci expressed large amounts of mRNA coding for their respective proteins. Several rel-related transcripts were expressed in the HP46 line, and four rel-related proteins of lower molecular weight than the wild-type p68c-rel product were detected. At least two of these transcripts contained U5 long terminal repeat sequences on the 5' end of the RNA. Structural data suggest that the messages may have evolved by an alternative splicing mechanism. This is the first example of a promoter insertion in the c-rel locus, a gene whose viral counterpart v-rel is responsible for the induction of lymphoid tumors.

Viral rel and cellular rel associate with cellular proteins in transformed and normal cells.

The rel oncogene from the avian reticuloendotheliosis virus strain T is a 59 kd phosphoprotein localized primarily to the cytoplasm of transformed cells. Recently, the v-rel protein was shown to associate with several cellular proteins with molecular weights of 124 kd, 115 kd, and 36 kd. We have analysed the subcellular distribution of v-rel protein complexes after biochemical fractionation of [35S]methionine and [32P]orthophosphate labeled cells. Our results demonstrate that the v-rel protein coprecipitates with a characteristic set of proteins, some of which are distinct to nuclear or cytoplasmic fractions. We also demonstrate that the normal cellular homolog of the viral rel protein, c-rel, coprecipitates with several cellular proteins from normal chick hematopoietic tissue. These cellular proteins have apparent molecular weights similar to those which are coprecipitated with v-rel from cytoplasmic fractions. Our results demonstrate that both v-rel and c-rel interact with a variety of cellular proteins and suggest that this association is important for the function or regulation of the rel protein.