Oncogene (2004) 23, 2275–2286 & 2004 Nature Publishing Group All rights reserved 0950-9232/04 $25.00 www.nature.com/onc REVIEW The c-Rel transcription factor and B-cell proliferation: a deal with the devil Thomas D Gilmore*,1, Demetrios Kalaitzidis1, Mei-Chih Liang1 and Daniel T Starczynowski1 1Department of Biology, Boston University, 5 Cummington Street, Boston, MA 02215, USA Activation of the Rel/NF-jB signal transduction pathway called the Rel homology domain, which has sequences has been associated with a varietyof animal and human required for DNA binding, dimerization, nuclear malignancies. However, among the Rel/NF-jB family localization, and inhibitor (IkB) binding. Sequences members, onlyc-Rel has been consistentlyshown to be C-terminal to the Rel homology domain in c-Rel, RelA, able to malignantlytransform cells in culture. In addition, and RelB contain transcriptional activation domains, c-rel has been activated bya retroviral promoter insertion and therefore dimers that contain these Rel/NF-kB in an avian B-cell lymphoma, and amplifications of REL members usually increase transcription of target genes. (human c-rel) are frequentlyseen in Hodgkin’s lympho- Although the Rel homology domain sequences of c-Rel mas and diffuse large B-cell lymphomas, and in some proteins across species are highly conserved, their follicular and mediastinal B-cell lymphomas. Phenotypic C-terminal transactivation domains are not. For exam- analysis of c-rel knockout mice demonstrates that c-Rel ple, the Rel homology domains of chicken and human has a normal role in B-cell proliferation and survival; c-Rel are approximately 85% identical, whereas their moreover, c-Rel nuclear activityis required for B-cell C-terminal transactivation domains are only about 10% development. Few mammalian model systems are avail- identical. Therefore, the DNA-binding specificities of able to studythe role of c-Rel in oncogenesis, and it is still c-Rel proteins across species are likely to be quite not clear what features of c-Rel endow it with its unique similar, but the properties and regulation of their oncogenic activityamong the Rel/NF- jB family. In any transactivation domains may vary. event, REL mayprovide an appropriate therapeutic target The activity of Rel/NF-kB dimers is also highly for certain human lymphoid cell malignancies. regulated. The primary method of Rel/NF-kB regula- Oncogene (2004) 23, 2275–2286. doi:10.1038/sj.onc.1207410 tion is through interaction with the family of IkB Published online 2February 2004 inhibitor proteins (Ghosh and Karin, 2002). Interaction of the Rel/NF-kB dimer with the IkB protein generally Keywords: c-Rel; Rel; NF-kB; B cell; lymphoma; gene sequesters the complex in the cytoplasm and blocks its amplification ability to bind DNA. Activation of the Rel/NF-kB pathway by any number of inducers activates an IkB kinase (IKK) complex, which then phosphorylates IkB. Phosphorylated IkB is then a preferred substrate for ubiquitination and degradation by the proteasome, Introduction which frees the Rel/NF-kB complex to enter the nucleus and bind to target gene promoters/enhancers. However, The c-Rel transcription factor is a member of the Rel/ in some cases, Rel/NF-kB proteins may also have effects NF-kB family, which also includes RelA (p65), RelB, on gene expression that are not mediated by direct DNA p50, and p52(Gilmore, 2003).As Rel/NF- kB transcrip- binding, such as by interacting with heterologous DNA- tion factors have been the subject of numerous reviews, bound transcription factors. For example, NF-kB this article will focus on the unique role of c-Rel in can enhance bcl-2 gene expression by associating on controlling B-cell proliferation, survival, and oncogen- the bcl-2 enhancer through CREB and Sp1 (Heckman esis. In a variety of cell types, c-Rel exists as et al., 2002). homodimers or heterodimers with p50, or infrequently Like other Rel/NF-kB proteins, c-Rel is also subject with RelA. Dimers containing c-Rel bind to a set of to regulation beyond that imposed by IkB proteins. related 9–10 bp DNA sequences (kB sites), and regulate Phosphorylation has been reported to affect c-Rel DNA the expression of numerous cellular genes, including binding (Neumann et al., 1992; Glineur et al., 2000; Liu many genes involved in lymphoid cell development, and Beller, 2002) and its C-terminal transactivation proliferation, and survival. activity (Martin et al., 2001). Moreover, even though the Rel/NF-kB proteins are related through an approxi- C-terminal transactivation domains of human and mately 300-amino-acid N-terminal domain usually chicken c-Rel are weakly, if at all, conserved (Gilmore et al., 2001), they both contain numerous sites of serine *Correspondence: TD Gilmore; E-mail: [email protected] phosphorylation (Mosialos et al., 1991; Martin et al., Received 6 November 2003; revised 30 November 2003; accepted 1 2001). In the case of REL, phosphorylation of different December 2003 sets of serine residues affects its transactivation activity c-Rel and B-cell proliferation/oncogenesis TD Gilmore et al 2276 in response to different stimuli, such as phorbol ester/ background of increased IkBa turnover in mature B ionomycin versus tumor necrosis factor (Martin and cells. Fresno, 2000; Martin et al., 2001). In addition, the C- c-rel knockout mice develop normally and have no terminal domain of REL has numerous sites of gross irregularities in hemopoiesis, but show immuno- ubiquitination (Chen et al., 1998), which has recently deficiencies (Ko¨ ntgen et al., 1995; Tumang et al., 1998). been shown to play a role in transactivation in other Although c-rel knockout mice show some alterations in transcription factors (Jin and Harper, 2003). T-cell function (Liou and Hsia, 2003), the primary defects in these mice occur in B cells. In particular, c-rel c-Rel plays a role in B-Cell proliferation and survival knockout mice have greatly reduced B-cell proliferation and reduced survival in response to mitogenic activa- The distinct phenotypes of knockout mice lacking tion, such as by treatment with LPS, anti-IgM, antigen, individual Rel/NF-kB family members and the different or CD40, and show reduced antibody production in expression patterns of these proteins indicate that each response to antigen (Ko¨ ntgen et al., 1995; Harling- family member has unique physiological functions. As McNabb et al., 1999; Tumang et al., 2002). The reduced such, a role for c-Rel in B-cell development, growth, and mitogen-induced proliferation is due to arrested cell- survival has been established by the analysis of c-Rel cycle progression in c-relÀ/À B cells (Hsia et al., 2002). expression and activity and by the characterization of However, c-Rel is not required for antigen-independent c-rel knockout mice. B-cell proliferation that occurs during B-cell depletion The first indication that c-Rel might have a unique (Cabatingan et al., 2002). Consistent with the B-cell role in lymphoid cells came from in situ hybridization proliferation and survival defects, c-rel null mice have studies of mouse embryos and adult tissues, wherein it small/irregular germinal centers and reduced marginal was found that c-rel mRNA was expressed primarily in zone B cells (Cariappa et al., 2000). Recently, Cheng developing hematopoietic tissues (Carrasco et al., 1994). et al. (2003) have shown that overexpression of both The c-Rel protein is expressed at all stages of B-cell cyclin E and Bcl-XL can restore BCR-induced cell-cycle development, but is expressed at the highest levels and is progression and survival in c-relÀ/À B cells; Bcl-XL is constitutively nuclear in mature B cells (Grumont and likely to be a direct target for c-Rel activation, whereas Gerondakis, 1994; Liou et al., 1994; Weih et al., 1994). cyclin E gene expression is likely induced by c-Rel- The nuclear kB site DNA-binding activity in mature B mediated activation of cell-cycle transcription factor lymphocytes consists mainly of p50–c-Rel heterodimers, E2F3a. which is different from pre-B cells that have largely The c-rel null phenotypes are exacerbated in nfkb1/ inducible p50–RelA complexes (Grumont and Geron- c-rel double knockouts, which have reduced numbers of dakis, 1994; Liou et al., 1994; Miyamoto et al., 1994b; follicular, marginal, and peritoneal B cells and have Kistler et al., 1998). Moreover, c-rel mRNA has recently mature B cells that fail to proliferate in response to been shown to be elevated in mature B cells as compared antigen due to a cell-cycle defect (Pohl et al., 2002). to plasma B cells (Tarte et al., 2003; Zhan et al., 2003), Adoptive transfer studies have shown that embryonic paralleling the protein expression data. liver cells lacking both RelA and c-Rel can still The preferential nuclear accumulation of c-Rel in repopulate all hematopoietic cell lineages, although at mature B cells is probably due, at least in part, to a somewhat reduced level compared to wild-type cells increased basal turnover of IkBa in these cells (Miya- (Grossmann et al., 1999), emphasizing that both moto et al., 1994a, 1998; Doerre and Corley, 1999; activating subunits of NF-kB are not required for Shumway et al., 1999; Fields et al., 2000). In addition, hematopoietic cell development. However, mature IgM- Tam et al. (2001) have shown that the nuclear and IgD-positive B cells were absent in these mice, accumulation of p50–c-Rel heterodimers in lieu of because of a survival defect as a result of reduced p50–RelA heterodimers in mature B cells can also be expression of antiapoptotic genes bcl-2 and A1 (Gross- attributed to the types of IkB interactions that occur mann et al., 2000).