Mutations in the DNA-binding and dimerization domains of v-Rel are responsible for altered kappa B DNA-binding complexes in transformed cells

The activation of TC10, a Rho small GTPase, contributes to v-Rel-mediated transformation

v-Rel is the oncogenic member of the Rel/NF-kappaB family of transcription factors and transforms hematopoietic cells and fibroblasts. Differential display was employed to identify target genes that exhibit altered expression in v-Rel transformed cells. One of the cDNAs identified encodes the chicken ortholog of TC10, a member of the Rho small GTPase family. The expression of TC10 was increased in v-Rel-transformed chicken embryonic fibroblasts (CEFs) 3 to 6-fold relative to control cells at both the RNA and protein levels. An elevated level of active, GTP-bound TC10 was also detected in v-Rel-transformed cells relative to control cells. Expression of a dominant-negative TC10 mutant (TC10T32N) decreased the colony formation potential of v-Rel-transformed cells. Furthermore, overexpression of wild-type TC10 or a gain-of-function mutant (TC10Q76L) greatly enhanced the ability of v-Rel transformed CEFs to form colonies in soft agar. In addition to enhance the transformation potential of v-Rel, the overexpression of wild-type TC10 or the gain-of-function mutant alone enhanced the saturation density of CEFs and was sufficient for their anchorage-independent growth in vitro. These results indicate that elevated TC10 activity contributes to v-Rel-mediated transformation of CEFs and demonstrate for the first time that a Rho factor alone is capable of inducing the in vitro transformation of primary cells.

The suppression of SH3BGRL is important for v-Rel-mediated transformation

The v-rel oncogene is the most efficient transforming member of the Rel/NF-kappaB family of transcription factors. v-Rel induces avian and mammalian lymphoid cell tumors and transforms chicken embryo fibroblasts in culture by the aberrant regulation of genes under the control of Rel/NF-kappaB proteins. Here we report that the expression of SH3BGRL, a member of the SH3BGR (SH3 domain-binding glutamic acid-rich) family of proteins, is downregulated in v-Rel-expressing fibroblasts, lymphoid cells, and splenic tumor cells. Chromatin immunoprecipitation experiments demonstrated that v-Rel binds to the sh3bgrl promoter in transformed cells. Coexpression of SH3BGRL with v-Rel in primary splenic lymphocytes reduced the number of colonies formed by 76%. Mutations in the predicted SH3-binding domain of SH3BGRL abolished the suppressive effect on v-Rel transformation and resulted in colony numbers comparable to those formed by v-Rel alone. However, mutations in the predicted EVH1-binding domain of SH3BGRL only had a modest effect on suppression of v-Rel transformation. This study provides the first example of a gene that is downregulated in v-Rel-expressing cells that also plays a role in v-Rel transformation.

The c-Rel transcription factor and B-cell proliferation: a deal with the devil

Activation of the Rel/NF-kappaB signal transduction pathway has been associated with a variety of animal and human malignancies. However, among the Rel/NF-kappaB family members, only c-Rel has been consistently shown to be able to malignantly transform cells in culture. In addition, c-rel has been activated by a retroviral promoter insertion in an avian B-cell lymphoma, and amplifications of REL (human c-rel) are frequently seen in Hodgkin's lymphomas and diffuse large B-cell lymphomas, and in some follicular and mediastinal B-cell lymphomas. Phenotypic analysis of c-rel knockout mice demonstrates that c-Rel has a normal role in B-cell proliferation and survival; moreover, c-Rel nuclear activity is required for B-cell development. Few mammalian model systems are available to study the role of c-Rel in oncogenesis, and it is still not clear what features of c-Rel endow it with its unique oncogenic activity among the Rel/NF-kappaB family. In any event, REL may provide an appropriate therapeutic target for certain human lymphoid cell malignancies.

Differential regulation of the inhibitor of apoptosis ch-IAP1 by v-rel and the proto-oncogene c-rel

The v-rel oncogene encoded by reticuloendotheliosis virus is the acutely transforming member of the Rel/NF-kappaB family of transcription factors. v-Rel is a truncated and mutated form of c-Rel and transforms cells by inducing the aberrant expression of genes regulated by Rel/NF-kappaB proteins. The expression of ch-IAP1, a member of the inhibitor-of-apoptosis family, is highly elevated in cells expressing v-Rel and contributes to the immortalization of cells transformed by this oncoprotein. In this study we demonstrate that the elevated expression of ch-IAP1 in v-Rel-expressing cells is due to an increased rate of transcription. The ch-IAP1 promoter was isolated, and four Rel/NF-kappaB binding sites were identified upstream of the transcription start site. Two kappaB sites proximal to the transcription start site were required for v-Rel to activate the ch-IAP1 promoter. While c-Rel also utilized these sites, a third more-distal kappaB site was required for its full activation of the ch-IAP1 promoter. Differences in the transactivation domains of v-Rel and c-Rel are responsible for their different abilities to utilize these sites and account for their differential activation of the ch-IAP1 promoter. Although c-Rel was a more potent activator of the ch-IAP1 promoter than v-Rel in transient reporter assays, cells stably overexpressing c-Rel failed to maintain high levels of ch-IAP1 expression. The reduction of ch-IAP1 expression in these cells correlated with the efficient regulation of c-Rel by IkappaBalpha. The ability of v-Rel to escape IkappaBalpha regulation allows for the gradual and sustained elevation of ch-IAP1 expression directly contributing to the transforming properties of v-Rel.

Mutational analysis of the v-Rel dimerization interface reveals a critical role for v-Rel homodimers in transformation

The v-rel oncogene encoded by reticuloendotheliosis virus strain T is the acutely transforming member of the Rel/NF-kappaB family of transcription factors. In v-Rel-transformed cells, v-Rel exists as homodimers or heterodimers with the endogenous Rel/NF-kappaB proteins c-Rel, NF-kappaB1, NF-kappaB2, and RelA. To examine the contribution of these complexes to v-Rel-mediated transformation, mutations were introduced into the dimerization interface of v-Rel to generate v-Rel mutants with selective dimerization properties. Nine mutants are described in this study that are defective in homodimer and/or heterodimer formation with specific Rel/NF-kappaB family members. Viruses expressing mutants that failed to homodimerize but were able to form heterodimeric complexes were unable to transform splenic lymphocytes in vitro, indicating that the dimerization of v-Rel with endogenously expressed Rel/NF-kappaB proteins is not in itself sufficient for transformation. In addition, two partially transforming mutants were identified that exhibited an impaired ability to form homodimers. Sequence analysis of the proviral DNA from cells transformed by these mutants revealed the presence of multiple secondary mutations in sequences responsible for dimerization and DNA binding. Two of these mutations either enhanced or restored the ability of these proteins to bind DNA as a homodimer. Viruses expressing these proteins transformed cells at levels comparable to or slightly less than v-Rel, suggesting that a threshold level of DNA binding by v-Rel homodimers is required for transformation.

X-ray crystal structure of proto-oncogene product c-Rel bound to the CD28 response element of IL-2

BACKGROUND

The proto-oncogene product c-Rel is a Rel/NF-kappaB family transcription factor that plays a critical role in lymphoid cell development and mediates CD28-induced expression of interleukin 2 (IL-2). The CD28 response element (CD28RE) in the IL-2 enhancer is nonameric and similar to the kappaB DNA target sites recognized by p65 homodimers.

RESULTS

We have determined and refined the X-ray crystal structure of the c-Rel homodimer complexed to the CD28RE DNA site, 5'-AGAAATTCC-3', to 2.85 A resolution. The c-Rel homodimer binds CD28RE in a mode similar to that observed in the p65/IL-8 kappaB crystallographic complex. Binding studies reveal that the c-Rel homodimer recognizes the CD28RE with higher affinity as compared to other canonical kappaB sequences despite the nonconsensus A:T base pair at the 5' end of the CD28RE. Preferential recognition of the CD28RE by c-Rel results from the direct contacts between the protein and the DNA as well as intrasubunit interactions between the beta(f)-beta(g) loop in the dimerization domain and the DNA-contacting loop L1 of the N-terminal domain. Not only do these loops have different conformations in other Rel/DNA crystallographic complexes, but they also contain two of the five oncogenic point mutations found in v-Rel.

CONCLUSIONS

The current structure indicates that a non-DNA-contacting loop in the dimerization domain and the DNA-contacting loop L1 may play critical roles in defining affinity and specificity. Two amino acid changes in these segments may account for the differential DNA binding by v-Rel as compared to that of c-Rel.

Three mutations in v-Rel render it resistant to cleavage by cell-death protease caspase-3

The retroviral oncoprotein v-Rel is a transcriptional activator in the Rel/NF-kappa B family. v-Rel causes rapidly fatal lymphomas in young chickens, and transforms and immortalizes chicken lymphoid cells in vitro. Several mutations that have enhanced the oncogenicity of v-Rel have been selected during in vitro and in vivo passage of v-Rel-containing retroviruses. In this report, we show that the C-terminal deletion and two point mutations (Asp-->Gly at residue 91 and Asp-->Asn at residue 437) in v-Rel make it resistant to cleavage by the cell-death protease caspase-3. In contrast, c-Rel, which has Asp residues at these sites, can be cleaved by caspase-3 in vitro as well as in vivo in cells induced to undergo apoptosis. We have characterized activities of v-Rel mutants with recreated single caspase-3 cleavage sites, two cleavage sites, or an introduced artificial cleavage site. All of these mutant v-Rel proteins are sensitive to caspase-3 cleavage in vitro, and show wild-type activity in terms of nuclear localization in chicken fibroblasts and DNA binding in vitro. Moreover, all caspase-3-sensitive v-Rel mutants transform chicken spleen cells in vitro and induce fatal lymphoid tumors in vivo to approximately the same extent as wild-type v-Rel. As with v-Rel mutants, caspase-3-resistant c-Rel mutants behave similarly to caspase-3-sensitive wild-type c-Rel in terms of DNA binding, transcriptional activation, in vitro transformation, and tumorigenicity. Mammalian c-Rel proteins can also be cleaved by caspase-3 in vitro, and a c-Rel mutant from a human pre-T lymphoma cell line is less sensitive than wild-type human c-Rel to cleavage by caspase-3. Taken together, these results demonstrate that specific mutations render oncogenic forms of Rel proteins resistant to cleavage by a cell-death caspase; however, the biological relevance of this resistance remains unclear. Nevertheless, to our knowledge, this is the first demonstration of mutations in caspase-3 recognition sites occurring during the evolution of an oncogenic protein.

Regulation of DNA binding by Rel/NF-kappaB transcription factors: structural views

Rel/NF-kappaB transcription factors form homo- and heterodimers with different DNA binding site specificities and DNA binding affinities. Several intracellular pathways evoked by a wide range of biological factors and environmental conditions can lead to the activation of Rel/NF-kappaB dimers by signaling degradation of the inhibitory IkappaB protein. In the nucleus Rel/NF-kappaB dimers modulate the expression of a variety of genes including those encoding cytokines, growth factors, acute phase response proteins, immunoreceptors, other transcription factors, cell adhesion molecules, viral proteins and regulators of apoptosis. The primary focus of this review is on structural and functional aspects of Rel/NF-kappaB:DNA complexes and their formation. The salient features of the Rel/NF-kappaB dimer:DNA structure are described, as are modes of transcriptional regulation by phosphorylation, altered DNA binding properties, varying protein conformations, and interactions with IkappaB proteins.

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.

Degradation of proto-oncoprotein c-Rel by the ubiquitin-proteasome pathway

The c-rel proto-oncogene product, c-Rel, belongs to the Rel/NF-kappaB transcription factor family, which regulates a large variety of cellular functions. The activation of NF-kappaB involves the degradation of the inhibitor, IkappaB, through the ubiquitin-proteasome (Ub-Pr)-mediated pathway. Here we report that the turnover of c-Rel is also regulated by the Ub-Pr pathway, thus adding another level of complexity to the regulation of NF-kappaB. High molecular weight ubiquitinated c-Rel conjugates are detected in cells and accumulate in cells treated with proteasome inhibitors. In a cell-free in vitro degradation assay, c-Rel is degraded specifically through the Ub-Pr pathway. N-terminally truncated c-Rel is readily degraded, implying the dispensability of N-terminal sequence; in contrast, a series of deletion mutants missing C-terminal sequences display a reduced susceptibility to the degradation. Interestingly, the sequence between residues 118 and 171 of c-Rel, i.e. the region immediately following the c-Rel/v-Rel homology domain, appears to play an important role in mediating ubiquitin conjugation and the subsequent degradation. Together with our previous study showing an elevated tumorigenic potential for C-terminally truncated mutants, our data suggest that the C-terminal domain of c-Rel plays an important role in mediating c-Rel degradation and growth control.

Identification of v-Rel oncogene-induced inhibitor of apoptosis by differential display

The v-Rel oncoprotein is a member of the Rel/NF-kappaB family of transcription factors. v-Rel induces oncogenic transformation and inhibits apoptosis. To identify aberrantly expressed cellular genes in v-Rel transformed cells, gene expression patterns in normal and v-Rel transformed cells were compared by mRNA differential display. Northern blotting analysis with radiolabeled cDNAs from differential display confirmed the reproducible differential expression of 10 transcripts in v-Rel transformed cells. One of the identified genes, termed ch-IAP1, encodes a chicken homolog of the inhibitor-of-apoptosis protein (IAP) family. ch-IAP1 contains N-terminal baculovirus IAP repeats (BIR), the hallmark of IAPs, and has a C-terminal RING finger motif commonly present in the other IAPs. Like other IAPs, ch-IAP1 is expressed predominantly in the cytoplasm of cells. ch-IAP1 is highly expressed in v-Rel transformed fibroblasts, B- and T-cell lines, and spleen cell lines. In contrast, ch-IAP1 expression levels were low in chicken cell lines transformed by several other unrelated tumor viruses. Additionally, ch-IAP1 expression is downregulated in temperature-sensitive (ts) v-Rel transformed spleen cells at the nonpermissive temperature. Overexpression of the full-length ch-IAP1 suppresses mammalian cell apoptosis induced by the interleukin-1-converting enzyme (ICE), a member of the mammalian caspase family of cysteine proteases. Furthermore, expression of exogenous ch-IAP1 inhibits apoptosis of ts v-Rel transformed spleen cells at the nonpermissive temperature.

Loss of IkappaB alpha-mediated control over nuclear import and DNA binding enables oncogenic activation of c-Rel

The IkappaB alpha protein is able both to inhibit nuclear import of Rel/NF-kappaB proteins and to mediate the export of Rel/NF-kappaB proteins from the nucleus. We now demonstrate that the c-Rel-IkappaB alpha complex is stably retained in the cytoplasm in the presence of leptomycin B, a specific inhibitor of Crm1-mediated nuclear export. In contrast, leptomycin B treatment results in the rapid and complete relocalization of the v-Rel-IkappaB alpha complex from the cytoplasm to the nucleus. IkappaB alpha also mediates the rapid nuclear shuttling of v-Rel in an interspecies heterokaryon assay. Thus, continuous nuclear export is required for cytoplasmic retention of the v-Rel-IkappaB alpha complex. Furthermore, although IkappaB alpha is able to mask the c-Rel-derived nuclear localization sequence (NLS), IkappaB alpha is unable to mask the v-Rel-derived NLS in the context of the v-Rel-IkappaB alpha complex. Taken together, our results demonstrate that IkappaB alpha is unable to inhibit nuclear import of v-Rel. We have identified two amino acid differences between c-Rel and v-Rel (Y286S and L302P) which link the failure of IkappaB alpha to inhibit nuclear import and DNA binding of a mutant c-Rel protein to oncogenesis. Our results support a model in which loss of IkappaB alpha-mediated control over c-Rel leads to oncogenic activation of c-Rel.

ch-IAP1, a member of the inhibitor-of-apoptosis protein family, is a mediator of the antiapoptotic activity of the v-Rel oncoprotein

The oncoprotein v-Rel, a member of the Rel/NF-kappaB family of transcription factors, induces neoplasias and inhibits apoptosis. To identify differentially regulated cellular genes and to evaluate their relevance to transformation and apoptosis in v-Rel-transformed cells, mRNA differential display has been used. One of the recovered cDNAs corresponds to a gene that was highly expressed in v-Rel-transformed fibroblasts. Analysis of the isolated full-length cDNA of a chicken inhibitor-of-apoptosis protein (ch-IAP1) revealed that it encodes a 68-kDa protein that is highly homologous to members of the IAP family, such as human c-LAP1. Like other IAPs, ch-IAP1 contains the N-terminal baculovirus IAP repeats and C-terminal RING finger motifs. Northern blot analysis identified a 3.3-kb ch-IAP1 transcript expressed at relatively high levels in the spleen, thymus, bursa, intestine, and lungs. Expression of v-Rel in fibroblasts, a B-cell line, and spleen cells up-regulated the expression of ch-IAP1. In contrast, ch-IAP1 expression levels were low in chicken cell lines transformed by several other unrelated tumor viruses. ch-IAP1 was expressed predominantly in the cytoplasm of the v-Rel-transformed cells. ch-IAP1 suppressed mammalian cell apoptosis induced by the overexpression of the interleukin-1-converting enzyme. Expression of exogenous ch-IAP1 in temperature-sensitive v-Rel transformed spleen cells inhibited apoptosis of these cells at the nonpermissive temperature. Collectively, these results suggest that ch-IAP1 is induced during the v-Rel-mediated transformation process and functions as a suppressor of apoptosis in v-Rel-transformed cells.

Avian I kappa B alpha is transcriptionally induced by c-Rel and v-Rel with different kinetics

The Rel/NF-kappa B family of transcription factors participates in the regulation of genes involved in defense responses, inflammation, healing and regeneration processes, and embryogenesis. The control of the transcriptional activation potential of the Rel/NF-kappa B proteins is mediated, in part, by their association with inhibitory proteins of the I kappa B family. This association results in the cytoplasmic retention of these factors until the cell receives a proper stimulatory signal. The I kappa B alpha gene is a target for regulation by the Rel/NF-kappa B proteins and is in fact upregulated in response to Rel/NF-kappa B activation. A naturally occurring oncogenic variant of the Rel/NF-kappa B family, v-rel, transforms avian lymphocytes, bone marrow cells, monocytes, and fibroblasts. Avian I kappa B alpha expression is upregulated in cells transformed by v-Rel. Avian I kappa B alpha is also upregulated in fibroblasts overexpressing c-Rel and oncogenic variants of c-Rel. c-Rel, a carboxy-terminally truncated variant of c-Rel, and v-Rel are all able to directly transactivate the expression of the avian I kappa B alpha gene. However, c-Rel was the most potent activator of this gene, and the induction of I kappa B alpha expression showed faster kinetics in cells overexpressing c-Rel than in those overexpressing v-Rel. The regulation of I kappa B alpha induction by the Rel proteins was shown to be dependent on a 362-bp region of the I kappa B alpha promoter that contains two potential NF-kappa B binding sites and one AP-1-like binding site. Results of electrophoretic mobility shift assays using these NF-kappa B binding sites indicate that major changes in the profile of DNA binding complexes in fibroblasts overexpressing v-Rel correlated temporally with the kinetic changes in v-Rel's ability to activate the expression of the I kappa B alpha gene.