vRel is an inactive member of the Rel family of transcriptional activating proteins

Molecular mimicry of NF-κB by vaccinia virus protein enables selective inhibition of antiviral responses

Infection of mammalian cells with viruses activates NF-κB to induce the expression of cytokines and chemokines and initiate an antiviral response. Here, we show that a vaccinia virus protein mimics the transactivation domain of the p65 subunit of NF-κB to inhibit selectively the expression of NF-κB-regulated genes. Using co-immunoprecipitation assays, we found that the vaccinia virus protein F14 associates with NF-κB co-activator CREB-binding protein (CBP) and disrupts the interaction between p65 and CBP. This abrogates CBP-mediated acetylation of p65, after which it reduces promoter recruitment of the transcriptional regulator BRD4 and diminishes stimulation of NF-κB-regulated genes CXCL10 and CCL2. Recruitment of BRD4 to the promoters of NFKBIA and CXCL8 remains unaffected by either F14 or JQ1 (a competitive inhibitor of BRD4 bromodomains), indicating that BRD4 recruitment is acetylation-independent. Unlike other viral proteins that are general antagonists of NF-κB, F14 is a selective inhibitor of NF-κB-dependent gene expression. An in vivo model of infection demonstrated that F14 promotes virulence. Molecular mimicry of NF-κB may be conserved because other orthopoxviruses, including variola, monkeypox and cowpox viruses, encode orthologues of F14.

LIM domain-containing protein trip6 can act as a coactivator for the v-Rel transcription factor

The retroviral oncoprotein v-Rel is a transcriptional activator in the Rel/NF-kappaB family of eukaryotic transcription factors. v-Rel malignantly transforms a variety of cell types in vitro and in vivo, and its transforming activity is dependent on the ability of v-Rel to bind to DNA and activate transcription. In this report, we used the yeast two-hybrid assay to identify proteins that interact with C-terminal sequences of v-Rel that are needed for transcriptional activation and transformation. One protein, Trip6, that we identified in this screen was previously identified as a thyroid hormone receptor-interacting protein. Trip6 is a member of a subfamily of LIM domain-containing proteins that are thought to transport intracellular signals from the cell surface to the nucleus. By several criteria, we show that sequences from Trip6, which include the LIM domains, behave as a coactivator for transcriptional activation by v-Rel. That is, a GAL4-Trip6 fusion protein can activate transcription in yeast and chicken cells, Trip6 can enable C-terminal sequences of v-Rel to activate transcription in yeast, and Trip6 can enhance activation by v-Rel from a kappaB site reporter plasmid in yeast. Although full-length Trip6 localizes to adhesion plaques, deletion of N-terminal sequences allows human Trip6 to enter the nucleus of chicken cells. Lastly, Northern blotting shows that Trip6 mRNA is expressed in many human tissues. Coexpression of Trip6 does not affect the transforming activity of v-Rel. Taken together, our results indicate that Trip6 may be a protein that is important for the ability of v-Rel to activate transcription and transform cells, and may represent a potential target for blocking Rel-mediated oncogenesis and transcriptional activation.

Evidence for an oncogenic modifier role for mutant histone acetyltransferases in diffuse large B-cell lymphoma

Mutations in histone acetyltransferases (HATs) are among the most common mutations in diffuse large B-cell lymphoma (DLBCL). We previously showed that two human DLBCL cell lines, RC-K8 and SUDHL2, express C-terminally truncated, HAT domain-deficient p300 proteins (p300ΔC) that are required for optimal cell proliferation. Microarray analysis of mRNA expression in RC-K8 cells following p300ΔC knockdown shows upregulation of NF-κB and p53 gene expression programs and downregulation of a MYC gene expression program. Experiments indicate that these gene expression changes are due to inhibitory effects of p300ΔC on NF-κB activity and on p53 protein levels and stimulatory effects on MYC protein levels, suggesting that p300ΔC mutants enhance the proliferation of DLBCL cells by adjusting the transcriptional output of cell-specific oncoproteins. We propose that p300/CBP gene truncation represents a new class of oncogenic mutation that optimizes the activity of context-specific oncogenic transcription factors. We propose 'oncogenic modifier' to describe such mutations.

Myb proteins inhibit fibroblast transformation by v-Rel

Genes that cause cancer have been divided into two general classes--oncogenes that act in a dominant fashion to transform normal cells into a malignant state, and tumor suppressor genes that act in a dominant fashion to prevent such transformation. In this report, we demonstrate that both the v-myb retroviral oncogene, which causes leukemic transformation of hematopoietic cells, and the c-myb proto-oncogene can also function as inhibitors of fibroblast transformation by the v-rel oncogene. These results imply that the myb genes can function either as oncogenes or as tumor suppressors in different cellular contexts.

Leukemia virus long terminal repeat activates NFkappaB pathway by a TLR3-dependent mechanism

The long terminal repeat (LTR) region of leukemia viruses plays a critical role in tissue tropism and pathogenic potential of the viruses. We have previously reported that U3-LTR from Moloney murine and feline leukemia viruses (Mo-MuLV and FeLV) upregulates specific cellular genes in trans in an integration-independent way. The U3-LTR region necessary for this action does not encode a protein but instead makes a specific RNA transcript. Because several cellular genes transactivated by the U3-LTR can also be activated by NFkappaB, and because the antiapoptotic and growth promoting activities of NFkappaB have been implicated in leukemogenesis, we investigated whether FeLV U3-LTR can activate NFkappaB signaling. Here, we demonstrate that FeLV U3-LTR indeed upregulates the NFkappaB signaling pathway via activation of Ras-Raf-IkappaB kinase (IKK) and degradation of IkappaB. LTR-mediated transcriptional activation of genes did not require new protein synthesis suggesting an active role of the LTR transcript in the process. Using Toll-like receptor (TLR) deficient HEK293 cells and PKR(-/-) mouse embryo fibroblasts, we further demonstrate that although dsRNA-activated protein kinase R (PKR) is not necessary, TLR3 is required for the activation of NFkappaB by the LTR. Our study thus demonstrates involvement of a TLR3-dependent but PKR-independent dsRNA-mediated signaling pathway for NFkappaB activation and thus provides a new mechanistic explanation of LTR-mediated cellular gene transactivation.

High levels of p105 (NFKB1) and p100 (NFKB2) proteins in HPV16-transformed keratinocytes: role of E6 and E7 oncoproteins

We have previously shown that functional components of the NF-kappaB signaling pathway are up-regulated and sequestered in the cytoplasm of human papillomavirus 16 (HPV16)-transformed cell lines leading to a reduced activity of NF-kappaB. In this study, we examined the expression of the NF-kappaB precursors p100 and p105 in keratinocytes transformed or not by HPV16. Western immunoblotting experiments demonstrated high levels of p100 and p105 proteins not only in HPV16+ cervical carcinoma-derived keratinocytes but also in keratinocytes stably transfected by HPV16 E6 or E7 oncogenes. Moreover, p100 and p105 proteins were predominantly cytoplasmic and nuclear in keratinocytes expressing E7 and E6, respectively. A predominantly cytoplasmic localization of E7 protein was also detected in all keratinocytes expressing E7. Our results suggest that HPV16 E6 and E7 proteins modulate the expression and the subcellular localization of p100 and p105 NF-kappaB precursors.

Divergent C-terminal transactivation domains of Rel/NF-κB proteins are critical determinants of their oncogenic potential in lymphocytes

rel/nf-kappaB genes are amplified, overexpressed, or constitutively activated in many human hematopoietic tumors; however, the molecular mechanisms by which they contribute to tumorigenesis remain to be determined. Here, we explored the oncogenic potential of cellular Rel/NF-kappaB proteins in vitro and in vivo. We show that overexpression of wild-type mouse and human c-rel genes suffices to malignantly transform primary spleen cells in stringent soft agar assays and produce fatal tumors in vivo. In contrast relA and a constitutively active form of IKKbeta did not. Importantly, a hybrid RelA protein with its C-terminal transactivation domain substituted by that of v-Rel was potently oncogenic in vitro and in vivo. The transactivation domain of v-Rel selectively conferred an oncogenic phenotype upon the Rel homology domain (RHD) of RelA, but not to the more divergent RHDs of p50/NF-kappaB1, p52/NF-kappaB2, or RelB. Collectively, our results highlight important differences in the intrinsic oncogenic activity of mammalian c-Rel and RelA proteins, and indicate that critical determinants of their differential oncogenicity reside in their divergent transactivation domains. These findings provide experimental evidence for a role of mammalian Rel/NF-kappaB factors in leukemia/lymphomagenesis in an in vivo animal model, and are consistent with the implication of c-rel in many human lymphomas.

Deletion of either C-terminal transactivation subdomain enhances the in vitro transforming activity of human transcription factor REL in chicken spleen cells

The REL gene is amplified in many human B-cell lymphomas and we have previously shown that expression of REL from a retroviral vector can malignantly transform chicken spleen cells in vitro. To identify REL protein functions necessary for malignant transformation, we have performed deletion analysis on REL sequences encoding residues of two C-terminal subdomains that are involved in transcriptional activation. We find that deletion of both C-terminal transactivation subdomains abolishes the ability of REL to transform chicken spleen cells in vitro. In contrast, deletion of either transactivation subdomain alone, which reduces the transactivation ability of REL, enhances the transforming activity of REL. Transforming REL mutants missing C-terminal sequences can also be selected at a low frequency in vitro. The REL transactivation domain can be functionally replaced in transformation assays by a portion of the VP16 transactivation domain that activates at a level similar to REL-transforming mutants. We also find that deletion of 29 C-terminal amino acids causes the subcellular localization of REL to change from cytoplasmic to nuclear in chicken embryo fibroblasts. In contrast, wild-type REL and all transforming REL mutants are located primarily in the cytoplasm of transformed spleen cells. Nevertheless, treatment of transformed spleen cells with leptomycin B causes wild-type REL and two REL mutants to relocalize to the nucleus, and nuclear extracts from these transformed cells contain REL DNA-binding activity. Taken together, these results suggest the following: (1) that REL must activate transcription to transform cells in vitro; (2) that a reduced level of transactivation enhances the oncogenicity of REL; (3) that REL shuttles from the cytoplasm to the nucleus in transformed chicken spleen cells; and (4) that mutations in REL, in addition to amplifications, could activate its oncogenicity in human lymphomas.

The human B-cell lymphoma cell line RC-K8 has multiple genetic alterations that dysregulate the Rel/NF-kappaB signal transduction pathway

The human large B-cell lymphoma cell line RC-K8 has a rearranged REL locus that directs the production of a chimeric protein, termed REL-NRG (Non-Rel Gene). In this study, we show that RC-K8 cells have constitutively nuclear heterodimeric and homodimeric DNA-binding complexes that consist of p50, REL, and REL-NRG. In vitro, IkappaBalpha can block the DNA-binding activity of wild-type REL homodimers but not REL-NRG homodimers. In vivo, REL-NRG cannot activate transcription of a kappaB site reporter plasmid, suggesting that it is a transcription repressing or blocking REL protein. By Western blotting, no IkappaBalpha protein can be detected in extracts of RC-K8 cells. The absence of IkappaBalpha protein in RC-K8 cells appears to be due to mutations that cause premature termination of translation in three of the four copies of the IKBA gene in RC-K8 cells. Re-expression of wild-type IkappaBalpha or a super-repressor form of IkappaBalpha in RC-K8 cells is cytotoxic; in contrast, expression of a dominant-negative form of IkappaB kinase does not affect the growth of RC-K8 cells. By cDNA microarray analysis, a number of previously identified Rel/NF-kappaB target genes are overexpressed in RC-K8 cells, consistent with there being transcriptionally active REL complexes. Taken together, our results suggest that the growth of RC-K8 cells is dependent on the activity of nuclear wild-type REL dimers, while the contribution of REL-NRG to the transformed state of RC-K8 cells is less clear. Nevertheless, the RC-K8 cell line is the first tumor cell line identified with mutations in genes encoding multiple proteins in the Rel/NF-kappaB signal transduction pathway.

Mutant envelope residues confer a transactivation function onto N-terminal sequences of the v-Rel oncoprotein

The retroviral oncoprotein v-Rel is a member of the Rel/ NF-kappaB family of transcription factors. v-Rel has multiple changes as compared to the proto-oncoprotein c-Rel, and these changes render v-Rel highly oncogenic in avian lymphoid cells. Previous results have shown that three mutant residues in the eleven helper virus-derived Envelope (Env) amino acids (aa) at the N-terminus of v-Rel are required for its full oncogenicity. In this report, we show that these mutant Env aa also enable sequences in the N-terminal half of v-Rel to activate transcription in yeast and chicken cells, under conditions where the analogous sequences from c-Rel either do not or only weakly activate transcription. Removal of the Env aa from v-Rel or site-directed mutations that revert the three mutant residues to the residues present in the Rev-A helper virus Env protein abolish this transactivation ability of v-Rel. Addition of mutant Env aa onto c-Rel is not sufficient to fully restore the transactivation function; other sequences in the N-terminal half of v-Rel are needed for full transactivating ability. A C terminally-truncated form of NF-kappaB p100 (p85), produced in HUT-78 human leukemic cells, also activates transcription in yeast, under conditions where the normal p52 and p100 proteins do not. Furthermore, transcriptional activation by p85 in yeast is likely to occur through N-terminal sequences. Taken together, these results are consistent with a model in which transactivation by N-terminal Rel Homology (RH) domain sequences in oncogenic Rel family proteins is influenced by sequences outside the RH domain.

HMG boxes of DSP1 protein interact with the rel homology domain of transcription factors

Formation of the dorsoventral axis in Drosophila melanogaster is mediated through control of the expression of several genes by the morphogen Dorsal. In the ventral part of the embryo Dorsal activates twist and represses zen amongst others. Recently, several proteins have been shown to assist Dorsal in the repression of zen, one of which is DSP1, a HMG box protein that was isolated as a putative co-repressor of Dorsal. In this report we used a DSP1 null mutant to ascertain in vivo the involvement of DSP1 in Dorsal-mediated repression of zen but not in the activation of twist. We show that Dorsal has the ability to interact with DSP1 in vitro as well as with rat HMG1. Using truncated versions of the proteins we located the domains of interaction as being the HMG boxes for DSP1 and HMG1 and the Rel domain for Dorsal. Finally, studies of the zen DNA binding properties of Dorsal and another related Rel protein (Gambif1 from Anopheles gambiae) revealed that their DNA binding affinities were increased in the presence of DSP1 and HMG1.

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.

In vitro-translated diphtheria toxin A chain inhibits translation in wheat germ extracts: analysis of biologically active, caspase-3-resistant diphtheria toxin mutants

The diphtheria toxin A chain (DTA) is a potent cytocidal agent that inactivates elongation factor 2. This activity of DTA inhibits protein synthesis and rapidly leads to cell death through apoptosis. In this paper, we have developed a simple in vitro assay for DTA activity in which in vitro-translated DTA is used to inhibit the translation of proteins in wheat germ extracts. Inhibition of translation by DTA is dependent on cofactor NAD+, and the analysis of an attenuated DTA mutant indicates that this in vitro assay accurately reflects the in vivo activity of DTA. We have also identified aspartic acid at residue 8 (Asp-8) of DTA as a site of cleavage by the cell-death protease caspase-3. Cleavage of DTA by caspase-3 inactivates its ability to inhibit translation in wheat germ extracts. Conservative mutations at Asp-8 render DTA resistant to cleavage by caspase-3, but only slightly affect the ability of DTA to inhibit translation in vitro. Moreover, caspase-3-resistant DTA mutants are toxic in cells in tissue culture. The in vitro assay that we describe here will be useful for the rapid analysis of DTA activity and the development of DTA mutants with altered biological properties that may be of therapeutic value. Lastly, these studies serve as a prototype for the creation of caspase-resistant effector molecules.

Dorsal-B, a splice variant of the Drosophila factor Dorsal, is a novel Rel/NF-kappaB transcriptional activator

The Drosophila transcription factor Dorsal, a member of the Rel/NF-kappaB family of proteins, plays a key role in the establishment of dorsoventral polarity in the early embryo and is also involved in the immune response. Here, we present evidence that the primary transcript of dorsal can be alternatively spliced, generating Dorsal-B, a new Rel/NF-kappaB family member. Dorsal and Dorsal-B are identical in the N-terminal region, which comprises both a DNA-binding domain and a dimerization domain. However, Dorsal-B lacks the nuclear localization signal located at the end of the Rel domain of Dorsal and is totally divergent in the C-terminal portion. Although Dorsal-B by itself is not able to induce the expression of a kappaB-controlled Luciferase reporter gene, we demonstrate that its C-terminal portion has transactivating properties. Analysis of the dorsal-B expression pattern indicates that the splicing is tissue-specific and excludes a putative role in early embryogenesis. However, dorsal-B synthesis is enhanced upon septic injury, and this challenge induces a nuclear accumulation of the protein in fat body cells suggesting that it may be involved in the immune response.

Mapping of a serine-rich domain essential for the transcriptional, antiapoptotic, and transforming activities of the v-Rel oncoprotein

The v-Rel oncoprotein belongs to the Rel/NF-kappaB family of transcription factors and induces aggressive lymphomas in chickens and transgenic mice. Current models for cell transformation by v-Rel invoke the combined activation of gene expression and the dominant inhibition of transcription mediated by its cellular homologs. Here, we mapped a serine-rich transactivation domain in the C terminus of v-Rel that is necessary for its biological activity. Specific serine-to-alanine substitutions within this region impaired the transcriptional activity of v-Rel, whereas a double mutant abolished its function. In contrast, substitutions with phosphomimetic aspartate residues led to a complete recovery of the transcriptional potential. The transforming activity of v-Rel mutants correlated with their ability to inhibit programmed cell death. The transforming and antiapoptotic activities of v-Rel were abolished by defined Ser-to-Ala mutations and restored by most Ser-to-Asp substitutions. However, one Ser-to-Asp mutant showed wild-type transactivation ability but failed to block apoptosis and to transform cells. These results show that the transactivation function of v-Rel is necessary but not sufficient for cell transformation, adding an important dimension to the transformation model. It is possible that defined protein-protein interactions are also required to block apoptosis and transform cells. Since v-Rel is an acutely oncogenic member of the Rel/NF-kappaB family, our data raise the possibility that phosphorylation of its serine-rich transactivation domain may regulate its unique biological activity.

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.

AP-1 factors play an important role in transformation induced by the v-rel oncogene

v-rel is the oncogenic member of the Rel/NF-kappaB family of transcription factors. The mechanism by which v-Rel induces transformation of avian lymphoid cells and fibroblasts is not precisely known. However, most models propose that v-rel disrupts the normal transcriptional regulatory network. In this study we evaluated the role of AP-1 family members in v-Rel-mediated transformation. The overexpression of v-Rel, c-Rel, and c-Rel delta resulted in a prolonged elevation of c-fos and c-jun expression and in a sustained repression of fra-2 at both the mRNA and protein levels in fibroblasts and lymphoid cells. Moreover, the transforming abilities of these Rel proteins correlated with their ability to alter the expression of these AP-1 factors. v-Rel exhibited the most pronounced effect, whereas c-Rel, with poor transforming ability, elicited only moderate changes in AP-1 levels. Furthermore, c-Rel delta, which exhibits enhanced transforming potential relative to c-Rel, induced intermediate changes in AP-1 expression. To directly evaluate the role of AP-1 family members in the v-Rel transformation process, a supjun-1 transdominant mutant was used. The supjun-1 mutant functions as a general inhibitor of AP-1 activity by inhibiting AP-1-mediated transactivation and by reducing AP-1 DNA-binding activity. Coinfection or sequential infection of fibroblasts or lymphoid cells with viruses carrying rel oncogenes and supjun-1 resulted in a reduction of the transformation efficiency of the Rel proteins. The expression of supjun-1 inhibited the ability of v-Rel transformed lymphoid cells and fibroblasts to form colonies in soft agar by over 70%. Furthermore, the expression of supjun-1 strongly interfered with the ability of v-Rel to morphologically transform avian fibroblasts. This is the first report showing that v-Rel might execute its oncogenic potential through modulating the activity of early response genes.

Multiple hemopoietic defects and lymphoid hyperplasia in mice lacking the transcriptional activation domain of the c-Rel protein

The c-rel protooncogene encodes a member of the Rel/nuclear factor (NF)-kappaB family of transcriptional factors. To assess the role of the transcriptional activation domain of c-Rel in vivo, we generated mice expressing a truncated c-Rel (Deltac-Rel) that lacks the COOH-terminal region, but retains a functional Rel homology domain. Mice with an homozygous mutation in the c-rel region encoding the COOH terminus of c-Rel (c-relDeltaCT/DeltaCT) display marked defects in proliferative and immune functions. c-relDeltaCT/DeltaCT animals present histopathological alterations of hemopoietic tissues, such as an enlarged spleen due to lymphoid hyperplasia, extramedullary hematopoiesis, and bone marrow hypoplasia. In older c-relDeltaCT/DeltaCT mice, lymphoid hyperplasia was also detected in lymph nodes, liver, lung, and stomach. These animals present a more severe phenotype than mice lacking the entire c-Rel protein. Thus, in c-relDeltaCT/DeltaCT mice, the lack of c-Rel activity is less efficiently compensated by other NF-kappaB proteins.

The reverse two-hybrid system: a genetic scheme for selection against specific protein/protein interactions

The yeast two-hybrid system is a powerful experimental approach for the characterization of protein/ protein interactions. A unique strength of the yeast two-hybrid system is the provision for genetic selection techniques that enable the identification of specific protein/protein interactions. We now report the development of a modified yeast two-hybrid system which enables genetic selection against a specific protein/protein interaction. This reverse two-hybrid system utilizes a yeast strain which is resistant to cycloheximide due to the presence of a mutant cyh2 gene. This strain also contains the wild-type CYH2 allele under the transcriptional control of the Gal1 promoter. Expression of the wild-type Gal4 protein is sufficient to restore growth sensitivity to cycloheximide. Growth sensitivity towards cycloheximide is also restored by the coexpression of the avian c-Rel protein and its I kappa B alpha counterpart, p40, as Gal4 fusion proteins. Restoration of growth sensitivity towards cycloheximide requires the association of c-Rel and p40 at the Gal1 promoter and correlates with the ability of the c-Rel/p40 interaction to activate expression from the Gal1 promoter. A genetic selection scheme against specific protein/protein interactions may be a valuable tool for the analysis of protein/protein interactions.

The v-rel oncogene promotes malignant T-cell leukemia/lymphoma in transgenic mice

The oncogene product from the avian reticuloendotheliosis virus strain T, v-Rel, is a member of the Rel/ NF-kappa B family of transcription factors. The mechanism by which v-Rel induces oncogenic transformation remains unclear. Several attempts to transform mammalian cells with v-Rel have failed, suggesting that v-Rel transformation may be a species-specific event. However, here we demonstrate that v-Rel, but not a truncated c-Rel, expressed under the control of the lck promoter, efficiently induced malignancies in transgenic mice. Most of the animals died before 10 months of age and developed immature, multicentric aggressive T-cell leukemia/lymphomas. Most tumors contain CD4+CD8+ cells or CD4-CD8+ cells, which have an immature rather than a mature peripheral phenotype. No tumor development was observed in control littermates and transgenic mice expressing a truncated form of c-Rel. Tumor formation was correlated with the presence of constitutive p50/v-Rel DNA binding activity and overexpression of several kappa B-regulated genes in v-rel transgenic thymocytes. However, v-Rel is also transforming in transgenic thymocytes lacking p50, indicating that p50/v-Rel heterodimer formation is not essential for the transforming activity of v-Rel. The transforming activity of v-Rel in p50 null mice has been identified as v-Rel/v-Rel homodimers. Since tumors represent immature T-lymphocytes, constitutive v-Rel expression appears to be leukemogenic at earlier stages of T-cell development. These v-Rel mice should aid in the study of lymphoma development, T-cell development and NF-kappa B regulation.

Moloney murine leukemia virus activates NF-kappa B

Nonacutely transforming retroviruses, such as Moloney murine leukemia virus (M-MuLV), differ from transforming viruses in their mechanisms of tumor induction. While the transforming viruses cause tumors by transduction of oncogenes, the leukemia retroviruses, lacking oncogenes, employ other mechanisms, including promoter insertion and enhancer activation. Although these two mechanisms occur in many tumors induced by leukemia viruses, a substantial proportion of such tumors do not show site-specific proviral insertions. Thus, other, unidentified virus-driven mechanisms may participate in tumorigenesis. In these studies, we show that infection of cells by M-MuLV activates expression of Rel family transcription factors. In murine cells chronically infected with M-MuLV, gel shift analyses with kappaB DNA-binding motifs from the murine immunoglobulin kappa light chain enhancer demonstrated induction of at least two distinct kappaB enhancer-binding complexes. Supershifting and immunoblotting analyses defined p50, p52, RelB, and c-Rel subunits as constituents of these virus-induced protein complexes. Transient transfections performed with kappaB-dependent reporter plasmids showed transcriptional activation in M-MuLV-infected cells relative to uninfected cells. Induction of Rel/NF-kappaB transcription factor activity by M-MuLV infection may prove relevant to the mechanism of M-MuLV-induced leukemia.

PEST-dependent cytoplasmic retention of v-Rel by I(kappa)B-alpha: evidence that I(kappa)B-alpha regulates cellular localization of c-Rel and v-Rel by distinct mechanisms

Association of c-Rel with the inhibitor of kappaB-alpha (IkappaB-alpha) protein regulates both cellular localization and DNA binding. The ability of v-Rel, the oncogenic viral counterpart of avian c-Rel, to evade regulation by p40, the avian IkappaB-alpha protein, contributes to v-Rel-mediated oncogenesis. The yeast two-hybrid system was utilized to dissect Rel:IkappaB-alpha interactions in vivo. We find that distinct domains in c-Rel and v-Rel are required for association with p40. Furthermore, while the ankyrin repeat domain of p40 is sufficient for association with c-Rel, both the ankyrin repeat domain and the PEST domain are required for association with v-Rel. Two amino acid differences between c-Rel and v-Rel that are principally responsible for PEST-dependent association of v-Rel with p40 were identified. These same amino acids were principally responsible for PEST-dependent cytoplasmic retention of v-Rel by p40. The presence of mutations in c-Rel that were sufficient to confer PEST-dependent association of the mutant c-Rel protein with p40 did not increase the weak oncogenicity of c-Rel. However, the introduction of these two c-Rel-derived amino acids into v-Rel markedly reduced the oncogenicity of v-Rel. Deletion of the NLS of either c-Rel or v-Rel did not abolish association with p40, but did confer PEST-dependent association of c-Rel with p40. Surprisingly, deletion of the nuclear localization signal in v-Rel did not affect oncogenicity by v-Rel. Analysis of several mutant c-Rel and v-Rel proteins demonstrated that association of Rel proteins with p40 is necessary but not sufficient for cytoplasmic retention. These results are not consistent with the hypothesis that p40 regulates cellular localization of v-Rel and c-Rel by the same mechanism. Rather, these results support the hypothesis that p40 regulates cellular localization of v-Rel and c-Rel by distinct mechanisms.

Interaction of the v-Rel oncoprotein with NF-kappaB and IkappaB proteins: heterodimers of a transformation-defective v-Rel mutant and NF-2 are functional in vitro and in vivo

The v-Rel oncoprotein of the avian Rev-T retrovirus is a member of the Rel/NF-kappa B family of transcription factors. The mechanism by which v-Rel malignantly transforms chicken spleen cells is not precisely known. To gain a better understanding of functions needed for transformation by v-Rel, we have now characterized the activities of mutant v-Rel proteins that are defective for specific protein-protein interactions. Mutant v-delta NLS, which has a deletion of the primary v-Rel nuclear localizing sequence, does not interact efficiently with I kappa B-alpha but still transforms chicken spleen cells approximately as well as wild-type v-Rel, indicating that interaction with I kappa B-alpha is not essential for the v-Rel transforming function. A second v-Rel mutant, v-SPW, has been shown to be defective for the formation of homodimers, DNA binding, and transformation. However, we now find that v-SPW can form functional DNA-binding heterodimers in vitro and in vivo with the cellular protein NF-kappa B p-52. Most strikingly, coexpression of v-SPW and p52 from a retroviral vector can induce the malignant transformation of chicken spleen cells, whereas expression of either protein alone cannot. Our results are most consistent with a model wherein Rel homodimers or heterodimers must bind DNA and alter gene expression in order to transform lymphoid 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.

Mice lacking the c-rel proto-oncogene exhibit defects in lymphocyte proliferation, humoral immunity, and interleukin-2 expression

The c-rel proto-oncogene, which is expressed predominantly in hemopoietic cells encodes a subunit of the NF-kappa B-like family of transcription factors. In mice with an inactivated c-rel gene, whereas development of cells from all hemopoietic lineages appeared normal, humoral immunity was impaired and mature B and T cells were found to be unresponsive to most mitogenic stimuli. Phorbol ester and calcium ionophore costimulation, in contrast to certain membrane receptor-mediated signals, overcame the T cell-proliferative defect, demonstrating that T cell proliferation occurs by Rel-dependent and -independent mechanisms. The ability of exogenous interleukin-2 to restore T Cell, but not B cell, proliferation indicates that Rel regulates the expression of different genes in B and T cells that are crucial for cell division and immune function.

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

The c-rel proto-oncogene encodes a member of the Rel/NF-kappa B family of transcription factors. The oncogenic viral form, v-rel, transduced by avian reticuloendotheliosis virus T, induces lymphoid tumors. v-Rel transformation may be mediated directly by binding of v-Rel to cognate DNA sites, resulting in altered gene expression, and/or indirectly by releasing Rel/NF-kappa B transcription factors from cytoplasmic retention molecules, resulting in their translocation to the nucleus and the inappropriate expression of genes under kappa B control. v-Rel-transformed cell lines of different phenotypes contained v-Rel as well as endogenous kappa B DNA-binding proteins in nuclear extracts. Kinetic analysis with avian leukosis virus-transformed B-cell lines expressing v-Rel or c-Rel indicated that the presence of endogenous kappa B DNA-binding proteins in the nucleus is temporally correlated with the relocalization of v-Rel to the cytoplasm. Supershift analysis of these DNA-binding complexes revealed that v-Rel was present in all of the nuclear DNA-binding complexes heterodimerized with c-Rel, NF-kappa B1, and other proteins. In contrast, c-Rel-transformed cells exhibited a less-complex pattern of nuclear kappa B DNA-binding complexes, and the nuclear appearance of these endogenous complexes was not observed. Studies with c-/v-Rel hybrids suggest that the induction of the endogenous kappa B DNA-binding complexes is the result of the mutations in the C-terminal region of the Rel homology (RH) domain of v-Rel. Moreover, v-Rel differed from c-Rel in its DNA-binding specificity. The altered DNA-binding specificity of v-Rel was associated with mutations located in the N-terminal part of the RH domain of v-Rel. These results suggest that two different regions of v-Rel (both located in the RH domain) influence the formation of kappa B DNA-binding complexes differently.

Cell type-specific transactivation of the VCAM-1 promoter through an NF-kappa B enhancer motif

Cytokine activation of vascular cell adhesion molecule-1 (VCAM-1) gene expression by endothelial cells is an important feature in a variety of vascular inflammatory responses. Cytokines transcriptionally activate the VCAM-1 promoter in endothelial cells at least in part through two closely linked NF-kappa B enhancer motifs, kappa L-kappa R (positions -77 and -63). However, cytokine activation of the dimeric NF-kappa B transcriptional factor (p50+p65 subunits) occurs in almost all cell types, whereas VCAM-1 gene expression exhibits a cell type-specific pattern of expression. Tumor necrosis factor-alpha markedly transactivated a transiently transfected minimal kappa L-kappa R motif-driven VCAM-1 promoter, p85VCAMCAT, in passaged human vascular endothelial cells but not in the human epithelial cell line, HeLa suggesting that cell type-specific factors may function through the kappa L-kappa R motif. Both cell types exhibited similar inductions of NF-kappa DNA binding activity and transcriptional activity. However, co-transfection of HeLa cells with p65 and p50 expression vectors demonstrated that the minimal VCAM-1 promoter was effectively transactivated by p65 alone but that additional co-expression of p50 blocked this activity. Furthermore, cytokine activation of the minimal VCAM-1 promoter in HeLa cells was recovered by inhibition of p50 expression using antisense oligonucleotide. These studies suggest that the NF-kappa B(p50+p65 heterodimer) does not support transactivation of the VCAM-1 promoter with the p50 subunit potentially playing a significant inhibitory role in suppressing cytokine activation of VCAM-1. In addition, p65 associated transcriptional factors other than NF-kappa B may serve as positive, cytokine-inducible, cell type-specific regulators of VCAM-1 gene expression.

A dopamine-responsive domain in the N-terminal sequence of Pit-1. Transcriptional inhibition in endocrine cell types

The POU transcription factor Pit-1 activates the prolactin gene in pituitary lactotrophs and may integrate responses of the gene to external signals. To study the role of Pit-1 in dopaminergic inhibition of the prolactin gene, we transiently transfected Pit-1 and dopamine D2 receptor vectors into a series of heterologous cell lines and examined dopamine regulation of the prolactin gene promoter. Regulation was Pit-1-dependent in all cell lines tested. Moreover, dopamine responsiveness was cell type-specific: stimulatory in fibroblasts (COS-7) and muscle-type cells (P19/Me2SO-induced) and inhibitory in pancreatic endocrine (RIN, InR1-G9) and neural-like (P19/retinoic acid-induced) cells. Because dopaminergic responses in Pit-1-transfected RIN cells paralleled those in pituitary GH4 cells, the islet cell line was used to test for sequences in Pit-1 that mediate negative hormone signals. Dopamine responsiveness of the Pit-1 transactivation domain (residues 8-80) was examined using a chimeric LexA construct. LxPit-1, LxSp1, and Lx-glucocorticoid receptor fusions all activated basal transcription, but only LxPit-1 was regulated by dopamine. Regulatory responses of LxPit-1 and full-length Pit-1 were quantitatively similar. In addition, gain-of-function G alpha mutants that inhibit Pit-1-dependent promoters in GH4 cells also suppressed selectively Pit-1- or LxPit-1-dependent promoters in RIN cells. This demonstrates that Pit-1 can function as a specific target for distinct inhibitory G protein signals. Interestingly, Pit-1 sequences N-terminal to the DNA-binding POU domain appear to be sufficient in mediating regulation by these pathways.

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.

The relocalization of v-Rel from the nucleus to the cytoplasm coincides with induction of expression of Ikba and nfkb1 and stabilization of I kappa B-alpha

The v-Rel oncogene induces the expression of major histocompatibility complex class I and II proteins and the interleukin-2 receptor more efficiently than does c-Rel (R. Hrdlicková, J. Nehyba, and E. H. Humphries, J. Virol. 68:308-319, 1994). The kinetics with which these immunoregulatory receptors are induced in B- and T-lymphoid cell lines and chicken embryo fibroblast cultures expressing c-Rel or v-Rel have been examined. v-Rel induced the expression of major histocompatibility complex classes I and II and interleukin-2 receptor more efficiently than did c-Rel at later times after infection. In all three cell types, this increased efficiency was accompanied by a shift in the majority of v-Rel from the nucleus of the cytoplasm. The concomitant relocalization of v-Rel was also demonstrated during the in vitro transformation of spleen cells. The translocation coincided with increased steady-state levels of I kappa B-alpha. Coninfection by retroviral vectors expressing v-Rel, I kappa B-alpha, or NF-kappa B1 demonstrated that either I kappa B-alpha can contribute to the shift of v-Rel to the cytoplasmic compartment. The induction of nfkb1 and Ikba mRNA and the stabilization of I kappa B-alpha by v-Rel were shown to be responsible for these effects. In comparison with c-Rel, the expression of v-Rel was associated with lower levels of transcription of these genes. However, the ability of v-Rel to stabilize I kappa B-alpha remained unchanged. The ability of v-Rel to stabilize I kappa B-alpha but poorly induce Ikba mRNA expression relative to c-Rel may play a role in regulating gene expression, thereby leading to transformation.

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.

Interaction of the v-Rel oncoprotein with cellular transcription factor Sp1

We previously showed that v-Rel, the oncoprotein of the avian retrovirus Rev-T, can increase expression from promoters containing binding sites for the cellular transcription factor Sp1 in chicken embryo fibroblasts (S. Sif, A.J. Capobianco, and T.D. Gilmore, Oncogene 8:2501-2509, 1993). In those experiments, v-Rel appeared to increase the transactivating function of Sp1; that is, v-Rel stimulated transactivation by a GAL4-Sp1 protein that lacked the Sp1 DNA-binding domain. We have now shown that in vitro-synthesized v-Rel and GAL4-Sp1 form a complex that can be immunoprecipitated with either anti-Sp1 or anti-v-Rel antiserum. We have also shown that a glutathione S-transferase (GST)-Sp1 fusion protein can specifically interact with in vitro-translated v-Rel and with in vivo-synthesized v-Rel from transformed chicken spleen cells. In addition, we have found that the abilities of wild-type and two mutant forms of v-Rel to increase transactivation by Sp1 in vivo correlate with their abilities to interact with Sp1 in vitro. The sequences important for the interaction of v-Rel with Sp1 in vitro have been mapped to the first 147 amino acids of v-Rel. Other Rel proteins, such as c-Rel, RelA, p52, and p50, were also able to form a complex with Sp1 in vitro. These results suggest that v-Rel increases expression from Sp1 site-containing promoters by functionally interacting with Sp1 and that cellular Rel proteins and Sp1 are likely to interact to influence transcription from natural promoters.

An interaction between the DNA-binding domains of RelA(p65) and Sp1 mediates human immunodeficiency virus gene activation

Induction of human immunodeficiency virus type 1 (HIV-1) gene expression in stimulated T cells has been attributed to the activation of the transcription factor NF-kappa B. The twice-repeated kappa B sites within the HIV-1 long terminal repeat are in close proximity to three binding sites for Sp1. We have previously shown that a cooperative interaction of NF-kappa B with Sp1 is required for the efficient stimulation of HIV-1 transcription. In this report, we define the domains of each protein responsible for this effect. Although the transactivation domains seemed likely to mediate this interaction, we find, surprisingly, that this interaction occurs through the putative DNA-binding domains of both proteins. Sp1 specifically interacted with the amino-terminal region of RelA(p65). Similarly, RelA bound directly to the zinc finger region of Sp1. This interaction was specific and resulted in cooperative DNA binding to the kappa B and Sp1 sites in the HIV-1 long terminal repeat. Furthermore, the amino-terminal region of RelA did not associate with several other transcription factors, including MyoD, E12, or Kox15, another zinc finger protein. These findings suggest that the juxtaposition of DNA-binding sites promotes a specific protein interaction between the DNA-binding regions of these transcription factors. This interaction is required for HIV transcriptional activation and may provide a mechanism to allow for selective activation of kappa B-regulated genes.

An interaction between the DNA-binding domains of RelA(p65) and Sp1 mediates human immunodeficiency virus gene activation

Induction of human immunodeficiency virus type 1 (HIV-1) gene expression in stimulated T cells has been attributed to the activation of the transcription factor NF-kappa B. The twice-repeated kappa B sites within the HIV-1 long terminal repeat are in close proximity to three binding sites for Sp1. We have previously shown that a cooperative interaction of NF-kappa B with Sp1 is required for the efficient stimulation of HIV-1 transcription. In this report, we define the domains of each protein responsible for this effect. Although the transactivation domains seemed likely to mediate this interaction, we find, surprisingly, that this interaction occurs through the putative DNA-binding domains of both proteins. Sp1 specifically interacted with the amino-terminal region of RelA(p65). Similarly, RelA bound directly to the zinc finger region of Sp1. This interaction was specific and resulted in cooperative DNA binding to the kappa B and Sp1 sites in the HIV-1 long terminal repeat. Furthermore, the amino-terminal region of RelA did not associate with several other transcription factors, including MyoD, E12, or Kox15, another zinc finger protein. These findings suggest that the juxtaposition of DNA-binding sites promotes a specific protein interaction between the DNA-binding regions of these transcription factors. This interaction is required for HIV transcriptional activation and may provide a mechanism to allow for selective activation of kappa B-regulated genes.

Evolution of the oncogenic potential of v-rel: rel-induced expression of immunoregulatory receptors correlates with tumor development and in vitro transformation

v-rel is a viral oncogene that evolved from turkey c-rel, an NF-kappa B-related transcription factor. Numerous structural alterations record the evolutionary selection of v-rel and distinguish it from c-rel. To evaluate the biological significance of these alterations, we constructed a set of five c/v-rel hybrids in which three mutation clusters (c-Rel amino acids 1 to 97,222 to 302, and 328 to 598) were differentially distributed. These constructs, in addition to parental v-rel and c-rel and two C-terminal deletion mutants of c-rel, were expressed from a retroviral vector. An analysis of cells infected with each of the nine viruses revealed that mutations in all three domains contributed to the ability of v-rel to induce two endogenous c-rel target genes, major histocompatibility complex (MHC) class I and class II, in the B-cell line DT95 as well as MHC class II in normal splenocytes. The analysis revealed a strong nonlinear correlation between the ability of a Rel protein to induce expression of MHC proteins and its capacity to produce splenic tumors and establish in vitro transformation. This correlation is consistent with the hypothesis that v-rel transforms by constitutively altering expression of genes regulated by c-rel and in this way simulates events associated with immune response-linked proliferation of cells of hematopoietic origin. Further, the 16 carboxy-terminal amino acids of c-Rel were identified as a domain responsible for producing a cytotoxic and/or cytostatic effect in DT95. Because this effect is likely to differentially influence induction of MHC expression and tumorigenesis/transformation, it may represent one factor that contributes to the nonlinearity of their correlation.

Transformation of avian fibroblasts overexpressing the c-rel proto-oncogene and a variant of c-rel lacking 40 C-terminal amino acids

The v-rel oncogene was derived from the c-rel proto-oncogene, which encodes a transcriptional activator. Expression of v-rel transforms avian hematopoietic cells and fibroblasts. Here we report that overexpression (via a replication-competent retroviral vector) of full-length c-Rel as well as a 40-amino-acid, carboxy-terminal deletion construct of c-Rel (c-Rel delta) resulted in the morphological transformation of chicken embryo fibroblasts (CEFs). Subcellular localization of Rel polypeptides in these transformed cells as determined by immunofluorescence and immunoprecipitation revealed their presence in both the nucleus and the cytoplasm, with the majority of Rel polypeptides showing cytoplasmic localization. Cytoplasmic localization could be due to interaction with I kappa B molecules, and in fact, the overexpression of c-Rel or the C-terminal deletion construct of c-Rel resulted in an increase in the levels of mRNA encoding the avian I kappa B protein pp40 and the avian homolog of the NF-kappa B protein, p105. However, expression of v-Rel resulted in the induction of pp40 mRNA only. While c-Rel was a weak activator of kappa B-mediated transcription of a reporter construct in transformed CEFs, v-Rel and c-Rel delta were transcriptional repressors. However, in spite of these differences, all of these proteins resulted in the transformation of CEFs.

In vivo evolution of c-rel oncogenic potential

The c-rel proto-oncogene belongs to the NF-kappa B/rel and I kappa B gene families, which regulate several inducible processes, including self-defense/repair and embryogenesis. Transduction of the c-rel transcription factor by the avian retrovirus resulted in the formation of a highly oncogenic virus, reticuloendotheliosis virus strain T (REV-T), that encodes the oncogene v-rel. To examine the oncogenic potential of c-rel, we inserted it into a REV-T-based retroviral vector, rescued virus [REV-C(CSV)], and infected 1-day-old chicks. All birds developed tumors, and all cell lines established from REV-C-induced tumors expressed c-rel proteins that lacked C-terminal sequences. These proteins, responsible for both in vivo and in vitro cell proliferation, were apparently selected for their oncogenic potential. In order to examine the cooperation of C-terminal deletions with other oncogenic alterations in vivo, point mutations present in the N-terminal and middle regions of v-rel were analyzed by a similar protocol. The data obtained support four conclusions. (i) c-rel proteins bearing any of three single-amino-acid mutations present in the N-terminal portion of v-rel were sufficiently oncogenic to induce tumor development in the absence of additional mutations. (ii) Combining a mutation from the N-terminal region of v-rel with a deletion of the C-terminal sequences of c-rel increases the oncogenicity of the protein in an additive manner. (iii) Mutations present in the middle of v-rel cooperated synergistically with C-terminal deletions to produce highly transforming viruses. (iv) Deletion of c-rel produced a variety of transforming rel proteins with sizes that extended from 42 to 65 kDa. The most frequently isolated rel deletion was 62 kDa in size. To examine the basis for the selection of different rel mutants, their ability to induce immunoregulatory surface receptors was analyzed. The data revealed a correlation between the induction capacity of these mutants and their corresponding contribution to in vivo tumorigenic potential. Moreover, an analysis of the subcellular localization of different rel proteins revealed an inverse correlation between the size of the protein and the proportion in the nucleus of lymphoid cells.

v-rel induces expression of three avian immunoregulatory surface receptors more efficiently than c-rel

The c-rel gene is a member of NF-kappa B/rel family of transcription factors that regulate expression of a variety of immunoregulatory molecules. The viral oncogene, v-rel, is a truncated and mutated form of the turkey c-rel gene expressed by reticuloendotheliosis virus, strain T. In this study, we demonstrated that three avian immunoregulatory receptors, major histocompatibility (MHC) antigens class I and class II as well as the interleukin-2 receptor (IL-2R), were induced on the surface of splenic tumor cells isolated from chickens infected with reticuloendotheliosis virus, strain T. All cell lines derived from splenic tumors expressed these three proteins. Their expression also correlated with the appearance of endogenous c-rel during a graft-versus-host reaction. In vitro, both c-rel and v-rel induced MHC class I, MHC class II, and IL-2R on an avian B-lymphoid cell line, DT95, and a T-lymphoid cell line, MSB-1. Quantitative kinetic analysis demonstrated both the accumulation of MHC class II mRNA and the appearance of surface MHC class II protein in response to the synthesis of either v-rel or c-rel. We show that v-rel induced the expression of MHC class II in the avian B-cell lines DT40 and DT95 more rapidly than c-rel and that, several weeks after infection, v-rel induced MHC class II as much as 50-fold more efficiently than c-rel. Finally, in vitro infection of splenocytes with retroviruses that express v-rel or c-rel induced MHC class I, MHC class II, and IL-2R expression. Quantitative analysis confirmed that p59v-rel was consistently more efficient at inducing expression of all three immunoregulatory receptors than exogenous p68c-rel. These data suggest that during tumor development, v-rel functions to induce (or suppress) the expression of genes similarly induced (or suppressed) by c-rel. The observations reported in this study are not in agreement with a model in which v-rel promotes tumor development by functioning as a dominant negative mutant of c-rel. In contrast, these findings support the hypothesis that lymphocyte immortalization and tumor development are the result, at least in part, of the capacity of v-rel to function as a dominant positive mutant that induces expression of genes normally regulated by c-rel.

Functional interaction of the v-Rel and c-Rel oncoproteins with the TATA-binding protein and association with transcription factor IIB

Rel family proteins regulate the expression of genes linked to kappa B-binding motifs. Little is known, however, of the mechanism by which they enhance transcription. We have investigated the ability of the v-Rel and c-Rel oncoproteins to interact with components of the basal transcription machinery. Here we report that both the acidic transcription activation domain mapping to the unique C terminus of chicken c-Rel and the F9 cell-specific activation region common to both v-Rel and c-Rel interact with the TATA-binding protein (TBP) and transcription factor IIB (TFIIB) in vitro and in vivo. We also demonstrate that TPB interaction with Rel activation regions leads to synergistic activation of transcription of a kappa B-linked reporter gene. Combined with the observation that the mouse c-Rel and human RelA proteins also interact with TBP and TFIIB in vitro, these results suggest that association with basal transcription factors is important for the transcriptional activities of Rel family proteins.

Functional interaction of the v-Rel and c-Rel oncoproteins with the TATA-binding protein and association with transcription factor IIB

Rel family proteins regulate the expression of genes linked to kappa B-binding motifs. Little is known, however, of the mechanism by which they enhance transcription. We have investigated the ability of the v-Rel and c-Rel oncoproteins to interact with components of the basal transcription machinery. Here we report that both the acidic transcription activation domain mapping to the unique C terminus of chicken c-Rel and the F9 cell-specific activation region common to both v-Rel and c-Rel interact with the TATA-binding protein (TBP) and transcription factor IIB (TFIIB) in vitro and in vivo. We also demonstrate that TPB interaction with Rel activation regions leads to synergistic activation of transcription of a kappa B-linked reporter gene. Combined with the observation that the mouse c-Rel and human RelA proteins also interact with TBP and TFIIB in vitro, these results suggest that association with basal transcription factors is important for the transcriptional activities of Rel family proteins.

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.

Identification of a rel-related protein in the nucleus during the S phase of the cell cycle

The c-rel proto-oncogene encodes a 75-kDa protein (p75c-rel) which is present in the cytosol of chick embryo fibroblasts (CEF) associated with a distinct set of cellular proteins with molecular masses of 40, 115, and 124 kDa. CEF cultures arrested in S phase of the cell cycle, or enriched for G2 or mitotic cells, were examined to determine whether the expression of c-rel was altered during the cell cycle. Levels of p75c-rel remained constant in all portions of the cell cycle examined; however, a Rel-related protein with an apparent molecular mass of 64 kDa was detected in nuclei of S-phase cells. As cells enter G2, the level of this protein in the nucleus decreases. This protein reacts with antiserum generated against the carboxy terminus of p75c-rel in radioimmunoprecipitations and Western immunoblot experiments and was also detected in a Western immunoblot with antiserum generated against the first 161 amino acids of pp59v-rel. Thus, unlike other Rel/NF-kappa B family members, p64 has carboxy-terminal homology with c-Rel. The majority of peptides generated by partial proteolytic cleavage of p64 are shared with peptides generated by digestion of p75c-rel and/or pp59v-rel. We suggest that this protein represents a new member of the Rel family of transcription factors and is located in the nucleus of avian fibroblasts during S phase of the cell cycle.

Identification of a rel-related protein in the nucleus during the S phase of the cell cycle

The c-rel proto-oncogene encodes a 75-kDa protein (p75c-rel) which is present in the cytosol of chick embryo fibroblasts (CEF) associated with a distinct set of cellular proteins with molecular masses of 40, 115, and 124 kDa. CEF cultures arrested in S phase of the cell cycle, or enriched for G2 or mitotic cells, were examined to determine whether the expression of c-rel was altered during the cell cycle. Levels of p75c-rel remained constant in all portions of the cell cycle examined; however, a Rel-related protein with an apparent molecular mass of 64 kDa was detected in nuclei of S-phase cells. As cells enter G2, the level of this protein in the nucleus decreases. This protein reacts with antiserum generated against the carboxy terminus of p75c-rel in radioimmunoprecipitations and Western immunoblot experiments and was also detected in a Western immunoblot with antiserum generated against the first 161 amino acids of pp59v-rel. Thus, unlike other Rel/NF-kappa B family members, p64 has carboxy-terminal homology with c-Rel. The majority of peptides generated by partial proteolytic cleavage of p64 are shared with peptides generated by digestion of p75c-rel and/or pp59v-rel. We suggest that this protein represents a new member of the Rel family of transcription factors and is located in the nucleus of avian fibroblasts during S phase of the cell cycle.

GAL4-I kappa B alpha and GAL4-I kappa B gamma activate transcription by different mechanisms

I kappa B proteins regulate Rel/NF-kappa B transcription complexes through a direct protein-protein interaction. In addition, we have previously shown that certain I kappa B proteins (I kappa B alpha and I kappa B gamma) can act as activators of transcription when fused to the DNA-binding domain of GAL4. We now show that a mutant chicken I kappa B alpha protein that cannot interact with Rel proteins in vitro did not activate transcription when fused to GAL4 in chicken embryo fibroblasts (CEF) and Saccharomyces cerevisiae, and did not inhibit growth in yeast; in contrast, an I kappa B alpha mutant that can still interact in vitro with Rel proteins activated transcription in both CEF and yeast and inhibited growth in yeast. In CEF, GAL4-I kappa B alpha mediated transcription activation was inhibited by co-transfection with an expression vector for a RelA (p65) protein that contained sequences needed for interaction with I kappa B alpha but that was deleted of its transcription activation domain. Therefore, it appears that GAL4-I kappa B alpha activates transcription by interacting with an endogenous Rel family protein in CEF. In contrast, the activation domain from I kappa B gamma behaved as a genuine acidic activator of transcription and did not inhibit growth when expressed in yeast. Since transcription activation and growth inhibition by GAL4-I kappa B alpha mutants in yeast correlated with their ability to interact with vertebrate Rel proteins, our results suggest that these activities of GAL4-I kappa B alpha are mediated through interaction with a Rel-like protein in yeast, which is important for cell growth.

Conservation of transcriptional activation functions of the NF-kappa B p50 and p65 subunits in mammalian cells and Saccharomyces cerevisiae

The NF-kappa B transcription factor complex is composed of a 50-kDa (p50) and a 65-kDa (p65) subunit. Both subunits bind to similar DNA motifs and elicit transcriptional activation as either homo- or heterodimers. By using chimeric proteins that contain the DNA binding domain of the yeast transcriptional activator GAL4 and subdomains of p65, three distinct transcriptional activation domains were identified. One domain was localized to a region of 42 amino acids containing a potential leucin zipper structure, consistent with earlier reports. Two other domains, both acidic and rich in prolines, were also identified. Of perhaps more significance, the same minimal activation domains that were functional in mammalian cells were also functional in the yeast Saccharomyces cerevisiae. Coexpression of the NF-kappa B inhibitory molecule, I kappa B, reduced the transcriptional activity of p65 significantly, suggesting the ability of I kappa B to function in a similar manner in S. cerevisiae. Surprisingly, while the conserved rel homology domain of p65 demonstrated no transcriptional activity in either mammalian cells or S. cerevisiae, the corresponding domain in p50 was a strong transcriptional activator in S. cerevisiae. The observation that similar domains elicit transcriptional activation in mammalian cells and S. cerevisiae demonstrates strong conservation of the transcriptional machinery required for NF-kappa B function and provides a powerful genetic system to study the transcriptional mechanisms of these proteins.

Differential pp40I kappa B-beta inhibition of DNA binding by rel proteins

Regulation of gene expression by members of the NF-kappa B/rel transcription factor family is a central component of signal transduction pathways utilized by many cellular processes, including lymphocyte activation, embryonic development, and oncogenesis. The members of the NF-kappa B/rel transcription factor family are regulated by association with a family of inhibitor (I kappa B) proteins (I kappa B) proteins. To address the importance of the association between rel and I kappa B proteins for oncogenesis by rel proteins, we characterized rel-I kappa B interactions in chicken embryo fibroblasts (CEF) infected with retroviral vectors encoding the avian c-rel (p68c-rel), v-rel (p59v-rel), and I kappa B-beta (pp40I kappa B-beta) proteins. In these experiments, the p59v-rel:pp40I kappa B-beta ratio in coinfected CEF was nearly identical to the p59v-rel:pp40I kappa B-beta ratio in v-rel-transformed cells. The avian I kappa B-beta protein, pp40I kappa B-beta, was able to associate with both the nononcogenic p68c-rel and the oncogenic p59v-rel. Association of p68c-rel with pp40I kappa B-beta in coinfected CEF resulted in inhibition of the DNA-binding activity of p68c-rel. Anti-pp40I kappa B-beta serum was able to restore DNA binding to p68c-rel in the presence of high levels of pp40I kappa B-beta, indicating that pp40I kappa B-beta functions in a trans-acting manner to inhibit DNA binding by p68c-rel. In contrast, sequence-specific DNA binding by the oncogenic v-rel protein, p59v-rel, was not abolished by pp40I kappa B-beta in coinfected CEF. Anti-pp40I kappa B-beta serum did not immunoprecipitate the p59v-rel-DNA adduct or alter the electrophoretic mobility of the p59v-rel-DNA adduct, consistent with the idea that pp40I kappa B-beta and DNA are competitive inhibitors for the same or overlapping domains on rel proteins. Internal v-rel-derived sequences were identified that are responsible for loss of pp40I kappa B-beta-mediated inhibition of DNA binding by p59v-rel. Loss of pp40I kappa B-beta-mediated inhibition of DNA binding by recombinant v/c-rel proteins was not sufficient for oncogenic activation of c-rel. Instead, removal of C-terminal c-rel-derived sequences in addition to loss of pp40I kappa B-beta-mediated inhibition of DNA binding was required for oncogenic activation of c-rel. These results demonstrate the presence of an interaction between internal and C-terminal regions of the c-rel protein that is important for the ability of c-rel to regulate the proliferation of lymphoid cells.

Differential pp40I kappa B-beta inhibition of DNA binding by rel proteins

Regulation of gene expression by members of the NF-kappa B/rel transcription factor family is a central component of signal transduction pathways utilized by many cellular processes, including lymphocyte activation, embryonic development, and oncogenesis. The members of the NF-kappa B/rel transcription factor family are regulated by association with a family of inhibitor (I kappa B) proteins (I kappa B) proteins. To address the importance of the association between rel and I kappa B proteins for oncogenesis by rel proteins, we characterized rel-I kappa B interactions in chicken embryo fibroblasts (CEF) infected with retroviral vectors encoding the avian c-rel (p68c-rel), v-rel (p59v-rel), and I kappa B-beta (pp40I kappa B-beta) proteins. In these experiments, the p59v-rel:pp40I kappa B-beta ratio in coinfected CEF was nearly identical to the p59v-rel:pp40I kappa B-beta ratio in v-rel-transformed cells. The avian I kappa B-beta protein, pp40I kappa B-beta, was able to associate with both the nononcogenic p68c-rel and the oncogenic p59v-rel. Association of p68c-rel with pp40I kappa B-beta in coinfected CEF resulted in inhibition of the DNA-binding activity of p68c-rel. Anti-pp40I kappa B-beta serum was able to restore DNA binding to p68c-rel in the presence of high levels of pp40I kappa B-beta, indicating that pp40I kappa B-beta functions in a trans-acting manner to inhibit DNA binding by p68c-rel. In contrast, sequence-specific DNA binding by the oncogenic v-rel protein, p59v-rel, was not abolished by pp40I kappa B-beta in coinfected CEF. Anti-pp40I kappa B-beta serum did not immunoprecipitate the p59v-rel-DNA adduct or alter the electrophoretic mobility of the p59v-rel-DNA adduct, consistent with the idea that pp40I kappa B-beta and DNA are competitive inhibitors for the same or overlapping domains on rel proteins. Internal v-rel-derived sequences were identified that are responsible for loss of pp40I kappa B-beta-mediated inhibition of DNA binding by p59v-rel. Loss of pp40I kappa B-beta-mediated inhibition of DNA binding by recombinant v/c-rel proteins was not sufficient for oncogenic activation of c-rel. Instead, removal of C-terminal c-rel-derived sequences in addition to loss of pp40I kappa B-beta-mediated inhibition of DNA binding was required for oncogenic activation of c-rel. These results demonstrate the presence of an interaction between internal and C-terminal regions of the c-rel protein that is important for the ability of c-rel to regulate the proliferation of lymphoid cells.

Conservation of transcriptional activation functions of the NF-kappa B p50 and p65 subunits in mammalian cells and Saccharomyces cerevisiae

The NF-kappa B transcription factor complex is composed of a 50-kDa (p50) and a 65-kDa (p65) subunit. Both subunits bind to similar DNA motifs and elicit transcriptional activation as either homo- or heterodimers. By using chimeric proteins that contain the DNA binding domain of the yeast transcriptional activator GAL4 and subdomains of p65, three distinct transcriptional activation domains were identified. One domain was localized to a region of 42 amino acids containing a potential leucin zipper structure, consistent with earlier reports. Two other domains, both acidic and rich in prolines, were also identified. Of perhaps more significance, the same minimal activation domains that were functional in mammalian cells were also functional in the yeast Saccharomyces cerevisiae. Coexpression of the NF-kappa B inhibitory molecule, I kappa B, reduced the transcriptional activity of p65 significantly, suggesting the ability of I kappa B to function in a similar manner in S. cerevisiae. Surprisingly, while the conserved rel homology domain of p65 demonstrated no transcriptional activity in either mammalian cells or S. cerevisiae, the corresponding domain in p50 was a strong transcriptional activator in S. cerevisiae. The observation that similar domains elicit transcriptional activation in mammalian cells and S. cerevisiae demonstrates strong conservation of the transcriptional machinery required for NF-kappa B function and provides a powerful genetic system to study the transcriptional mechanisms of these proteins.