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

Matrix Metalloproteinase-9 gene induction by a truncated oncogenic NF-kappaB2 protein involves the recruitment of MLL1 and MLL2 H3K4 histone methyltransferase complexes

Constitutive nuclear factor (NF)-kappaB activation in haematological malignancies is caused in several cases by loss of function mutations within the coding sequence of NF-kappaB inhibitory molecules such as IkappaBalpha or p100. Hut-78, a truncated form of p100, constitutively generates p52 and contributes to the development of T-cell lymphomas but the molecular mechanism underlying this oncogenic potential remains unclear. We show here that MMP9 gene expression is induced through the alternative NF-kappaB-activating pathway in fibroblasts and also on Hut-78 or p52 overexpression in fibroblasts as well as in lymphoma cells. p52 is critical for Hut-78-mediated MMP9 gene induction as a Hut-78 mutant as well as other truncated NF-kappaB2 proteins that are not processed into p52 failed to induce the expression of this metalloproteinase. Conversely, MMP9 gene expression is impaired in p52-depleted HUT-78 cells. Interestingly, MLL1 and MLL2 H3K4 methyltransferase complexes are tethered by p52 on the MMP9 but not on the IkappaBalpha promoter, and the H3K4 trimethyltransferase activity recruited on the MMP9 promoter is impaired in p52-depleted HUT-78 cells. Moreover, MLL1 and MLL2 are associated with Hut-78 in a native chromatin-enriched extract. Thus, we identified a molecular mechanism by which the recruitment of a H3K4 histone methyltransferase complex on the promoter of a NF-kappaB-dependent gene induces its expression and potentially the invasive potential of lymphoma cells harbouring constitutive activity of the alternative NF-kappaB-activating pathway.

Deletion analysis and alternative splicing define a transactivation inhibitory domain in human oncoprotein REL

Misregulation of REL, a nuclear factor-kappaB family transcription factor, has been implicated in several human lymphoid malignancies. REL has a conserved N-terminal DNA-binding/dimerization domain called the Rel homology domain (RHD) and a C-terminal transactivation domain (TAD). Here, we define the sequences (amino acids (aa) 323-422) between the RHD and TAD as a REL inhibitory domain (RID) because deletion of these sequences increases both REL transactivation and DNA binding. Furthermore, we have characterized two REL mRNA splice variants that encode proteins with alterations near RID: one lacking exon 9 sequences (aa 308-330; RELDelta9) and one with an exonized Alu fragment insertion of 32 aa after aa 307 (REL+Alu). Deletion of RID or exon 9-encoded sequences increases transactivation by GAL4-REL by approximately threefold. Moreover, deletion of RID or exon 9 sequences increases transactivation by full-length REL from certain kappaB site-containing promoters and increases DNA binding by REL. Deletion of RID does not affect REL's ability to transform chicken spleen cells. Reverse transcriptase-polymerase chain reaction analysis of mRNA from both primary lymphoma samples and several transformed tissue culture cell lines indicates that the RELDelta9 splice variant is preferentially expressed in lymphoma, suggesting that the REL transcript lacking exon 9 could serve as a marker for certain types of lymphoid tumors.

Constitutive production of NF-kappaB2 p52 is not tumorigenic but predisposes mice to inflammatory autoimmune disease by repressing Bim expression

Normal development of the immune system requires regulated processing of NF-kappaB2 p100 to p52, which activates NF-kappaB2 signaling. Constitutive production of p52 has been suggested as a major mechanism underlying lymphomagenesis induced by NF-kappaB2 mutations, which occur recurrently in a variety of human lymphoid malignancies. To test the hypothesis, we generated transgenic mice with targeted expression of p52 in lymphocytes. In contrast to their counterparts expressing the tumor-derived NF-kappaB2 mutant p80HT, which develop predominantly B cell tumors, p52 transgenic mice are not prone to lymphomagenesis. However, they are predisposed to inflammatory autoimmune disease characterized by multiorgan infiltration of activated lymphocytes, high levels of autoantibodies in the serum, and immune complex glomerulonephritis. p52, but not p80HT, represses Bim expression, leading to defects in apoptotic processes critical for elimination of autoreactive lymphocytes and control of immune response. These findings reveal distinct signaling pathways for actions of NF-kappaB2 mutants and p52 and suggest a causal role for sustained NF-kappaB2 activation in the pathogenesis of autoimmunity.

NF-kappaB2 mutation targets TRAF1 to induce lymphomagenesis

The NF-kappaB2 gene is recurrently mutated in human lymphoid malignancies. However, a causal relationship between NF-kappaB2 mutation and lymphomagenesis has not been established. It is also unclear how the mutation may lead to lymphoid malignancies. We report the generation of transgenic mice with targeted expression of p80HT, a lymphoma-associated NF-kappaB2 mutant, in lymphocytes. The transgenic mice display a marked expansion of peripheral B cell populations and develop predominantly small B cell lymphomas. p80HT expression has no apparent effect on the proliferation of B cells, but renders them specifically resistant to apoptosis induced by cytokine deprivation and mitogenic stimulation. Lymphocytes and lymphoma cells from p80HT mice express high levels of TRAF1, an antiapoptotic protein also implicated in lymphoid malignancies. p80HT binds the TRAF1 promoter in vivo and activates TRAF1 transcription. Moreover, TRAF1 knockdown abrogates the antiapoptotic activity of p80HT and TRAF1 deficiency reestablishes B cell homeostasis in p80HT mice. These findings demonstrate NF-kappaB2 mutation as an oncogenic event in vivo and suggest a molecular pathway for TRAF1 activation in the pathogenesis of lymphomas.

Mutations in the NF-kappaB signaling pathway: implications for human disease

The nuclear factor-kappa B (NF-kappaB) signaling pathway is a multi-component pathway that regulates the expression of hundreds of genes that are involved in diverse and key cellular and organismal processes, including cell proliferation, cell survival, the cellular stress response, innate immunity and inflammation. Not surprisingly, mis-regulation of the NF-kappaB pathway, either by mutation or epigenetic mechanisms, is involved in many human and animal diseases, especially ones associated with chronic inflammation, immunodeficiency or cancer. This review describes human diseases in which mutations in the components of the core NF-kappaB signaling pathway have been implicated and discusses the molecular mechanisms by which these alterations in NF-kappaB signaling are likely to contribute to the disease pathology. These mutations can be germline or somatic and include gene amplification (e.g., REL), point mutations and deletions (REL, NFKB2, IKBA, CYLD, NEMO) and chromosomal translocations (BCL-3). In addition, human genetic diseases are briefly described wherein mutations affect protein modifiers or transducers of NF-kappaB signaling or disrupt NF-kappaB-binding sites in promoters/enhancers.

Mutations of tumor necrosis factor α-responsive serine residues within the C-terminal transactivation domain of human transcription factor REL enhance its in vitro transforming ability

The human c-rel gene (REL), encoding an NF-kappaB transcription factor, is amplified or mutated in several human B-cell lymphomas and can transform chicken lymphoid cells in vitro. We have previously shown that certain deletions of C-terminal transactivation sequences enhance REL's transforming ability in chicken spleen cells. In this report, we have analysed the effect of single amino-acid changes at select serine residues in the C-terminal transactivation domain on REL's transforming ability. Mutation of either of two TNFalpha-inducible serine residues (Ser460 and Ser471) to nonphosphorylatable residues (alanine, asparagine, phenylalanine) made REL more efficient at transforming chicken spleen cells in vitro. In contrast, mutation of Ser471 to a phosphorylation mimetic aspartate residue impaired REL's transforming ability, even though it increased REL's inherent transactivation ability as a GAL4-fusion protein. Alanine mutations of several other serine residues within the transactivation domain did not substantially affect REL's transforming ability. Transactivation by GAL4-REL fusion proteins containing either transformation enhancing or nonenhancing mutations at serine residues was generally similar to wild-type GAL4-REL. However, more transforming mutants with mutations at either Ser460 or Ser471 differed from wild-type REL in their ability to transactivate certain kappaB-site reporter genes. In particular, the SOD2 promoter, encoding manganese superoxide dismutase, was activated less strongly by the more transforming REL mutant REL-S471N in transient assays, but REL-S471N-transformed chicken spleen cells had increased levels of MnSOD protein as compared to wild-type REL-transformed cells. Taken together, our results show that mutations of certain serine residues can enhance REL's transforming ability in vitro and suggest that these mutations increase REL-mediated transformation by altering REL's ability to modulate the expression of select target genes. Furthermore, phosphorylation of Ser471 may be involved in REL-mediated modulation of transformation-specific target gene expression. Lastly, these results suggest that similar mutations in the REL transactivation domain contribute to the development of certain human B-cell lymphomas.

Discreet mutations from c-Rel to v-Rel alter kappaB DNA recognition, IkappaBalpha binding, and dimerization: implications for v-Rel oncogenicity

The avian Rev-T retrovirus encodes the oncoprotein v-Rel, a member of the Rel/nuclear factor (NF)-kappaB transcription factor family. The aggressive oncogenic potential of v-Rel has arisen from multiple mutations within the coding sequence of the avian cellular protein c-Rel. In this study, using quantitative biochemical experiments, we have tested the role of a limited set of alterations between v-Rel and c-Rel located within the Rel homology region (RHR) of the family that might confer functional differences. Our results show that only a set of six mutations within the RHR of v-Rel are responsible for its ability to bind to a broad spectrum of kappaB-DNA that are normally regulated by distinct NF-kappaB dimers. We also observe that both v-Rel homodimer and p50/v-Rel heterodimer bind IkappaBalpha weakly compared to other cellular Rel/NF-kappaB dimers with transcription activation potential. We suggest that the ability of v-Rel homodimer to deregulate subunit-specific gene expression and its ability to evade IkappaB inhibition are crucial to its strong oncogenic potential.

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.

Rel/NF-kappa B/I kappa B signal transduction in the generation and treatment of human cancer

The Rel/NF-kappa B family is a group of structurally-related, tightly-regulated transcription factors that control the expression of a multitude of genes involved in key cellular and organismal processes. The Rel/NF-kappa B signal transduction pathway is misregulated in a variety of human cancers, especially ones of lymphoid cell origin, due either to genetic changes (such as chromosomal rearrangements, amplifications, and mutations) or to chronic activation of the pathway by epigenetic mechanisms. Constitutive activation of the Rel/NF-kappa B pathway can contribute to the oncogenic state in several ways, for example, by driving proliferation, by enhancing cell survival, or by promoting angiogenesis or metastasis. In many cases, inhibition of Rel/NF-kappa B activity reverses all or part of the malignant state. Thus, the Rel/NF-kappa B pathway has received much attention as a focal point for clinical intervention.

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.

LIM domain protein Trip6 has a conserved nuclear export signal, nuclear targeting sequences, and multiple transactivation domains

Trip6 is a member of a subfamily of LIM domain proteins, including also zyxin, LPP, Ajuba, and Hic-5, which localize primarily to focal adhesion plaques. However, in this report, we demonstrate that Trip6 is largely in the nucleus in cells treated with leptomycin B, suggesting that Trip6 shuttles between nuclear and cytoplasmic compartments and that nuclear export of Trip6 is dependent on Crm1. Consistent with this finding, we have identified a nuclear export signal (NES) in Trip6, and mutation of this NES also results in sequestration of Trip6 in the nucleus. Addition of the Trip6 NES to the nuclear v-Rel oncoprotein redirects v-Rel to the cytoplasm. Trip6 also has at least two sequences that can direct cytoplasmic beta-galactosidase to the nucleus. Using GAL4 fusion proteins and reporter gene assays, we demonstrate that Trip6 has multiple transactivation domains, including one that appears to overlap with sequences of the NES. In vitro- or in vivo-synthesized Trip6, however, does not bind to DNA-cellulose. Taken together, these results are consistent with Trip6, and other members of this LIM protein family, having a role in relaying signals between focal adhesion plaques and the nucleus.

Envelope-dependent transactivation by the retroviral oncoprotein v-Rel is required for efficient malignant transformation of chicken spleen cells

The retroviral oncoprotein v-Rel is a chimeric protein that has 11 helper virus-derived Envelope (Env) amino acids (aa) at its N terminus. Within these N-terminal Env aa of v-Rel there are three aa substitutions compared to the Rev-A helper virus Env. These aa substitutions have previously been shown to impart a number of unique properties onto v-Rel, including increased transforming and transactivating ability. In this study, we have analysed the sequence requirements for the Env aa to influence several properties of v-Rel. Phe residues at aa 3 and 9 are critical for an N-terminal transactivation function of v-Rel, and the analysis of several Env mutants demonstrates that transactivation ability parallels the transforming ability of v-Rel. Substitutions of conservative aa, such as leucine and tyrosine, for Phe 3 and 9 are tolerated for transactivation in chicken embryo fibroblasts and for transformation of chicken spleen cells. In contrast, the substitution of 10 Phe residues at the N terminus of v-Rel does not enable transactivation, indicating that a distinct structure surrounding Phe-3 and Phe-9 is essential for v-Rel function. We also show that the addition of the v-Rel Env aa to the N terminus of human c-Rel can enable it to activate transcription. Taken together, these results indicate that Phe residues at positions 3 and 9 have been selected for their ability to enhance the oncogenicity of v-Rel by increasing its ability to activate transcription.