NF-kB: Regulation by Methylation

In normal cells exposed to stress, the central transcription factor NF-kB is activated only transiently, to modulate the activation of downstream immune responses. However, in most cancers, NFkB is abnormally activated constitutively, contributing thus to oncogenesis and tumor progression. Therefore, downregulating NF-kBactivity is an important goal of cancer treatment. In order to control NF-kB activity therapeutically, it is helpful to understand the molecular mechanisms that normally govern its activation and how dysregulated NF-kB activity may aid the development of disease. Recent evidence from our laboratories and others indicates that, in addition to various posttranslational modifications of NF-kB that have been observed previously, including phosphorylation, ubiquitination, and acetylation, NF-kB can be methylated reversibly on lysine or arginine residues by histonemodifying enzymes, including lysine and arginine methyl transferases and demethylases. Furthermore, these methylations are required to activate many downstream genes. Interestingly, amplifications and mutations of several such enzymes have been linked to cancer. We propose that some of these mutations may alter the methylation not only of histones but also of NF-kB, making them attractive therapeutic targets. Cancer Res; 75(18); 3692–5. 2015 AACR. Overview of NF-kB Signaling The transcription factor NF-kB plays a critical role in inflammation, oncogenesis, and tumor progression. Its family includes p65 (RelA), RelB, c-Rel, p50/p105 (NF-kB1), and p52/p100 (NF-kB2). All proteins of the family share aRel homology domain (RHD) at their N-termini, which is required for dimerization, nuclear targeting, binding to DNA, and interaction with the inhibitory IkB proteins (IkB; refs. 1, 2). A subgroup of NF-kB family members, the Rel proteins, including p65, RelB, and c-Rel, also contain an additional carboxy-terminal transactivation domain (TAD). The prototype of NF-kB is the heterodimer of p65 and p50 subunits. In unstimulated normal cells, NF-kB exists as inactive heterodimers or homodimers of p65 that are bound to IkB (1). The NF-kB family uses canonical and noncanonical activation pathways (3). In the canonical pathway, signals from proinflammatory cytokines such as IL1 activate IkB kinase (IKK), which phosphorylates IkBa, leading to its ubiquitination and degradation by proteasomes (3). The released NF-kB (mostly p65/p50 heterodimers) translocates into the nucleus, binds to DNA to regulate the expression of many inflammatory and prooncogenic genes (3). In the noncanonical pathway, phosphorylation of p100 on its C-terminus allows the processing of p100 to p52 (3). The freed NF-kB (mostly p52/RelB heterodimers) translocates into the nucleus and regulates the expression of genes whose products mostly regulate the development and maintenance of the secondary lymphoid organs (3). Posttranslational Modifications of NF-kB An important aspect of the complex regulation of NF-kB is multiple posttranslational modifications of the p65 subunit. These modifications include ubiquitination, phosphorylation, acetylation, sumoylation, and nitrosylation and, more recently, methylation (3–11). The nature and extent of these regulatory modifications can vary with different NF-kB stimulators and the samemodificationsmay even facilitate quite different effects (11). In this review, we focus on the methylation of p65, the major functional subunit of NF-kB. Methylation of NF-kB In the past few years, accumulated evidence suggests that histone-modifying enzymes not only modify the histone proteins, but also play a role in the modification of nonhistone proteins, such as NF-kB. This interesting set of observations reminds us of the economy of nature, which has empowered histone-modifying enzymes with the dual ability to control both the histone proteins—which directly affect chromatin conformation and function, and the nonhistone proteins—which directly drive gene expression. To date, methylations of both lysine and arginine have been identified on the p65 subunit of NF-kB (4–10). The six methylated K sites, K37, 218, 221, 310, 314, and 315 (Fig. 1A), are modified by different histone-modifying enzymes (Supplementary Table S1; refs. 4–10). We used a novel genetic approach to identify previously unknown regulators (4–7), discovering that the nuclear receptor binding SET domain protein 1 (NSD1) and the demethylase F-box leucine-rich protein 11 (FBXL11) regulate Department of Pharmacology and Toxicology, Indiana University School of Medicine, Indianapolis, Indiana. Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, Indiana. DepartmentofMedical andMolecularGenetics, Indiana University School of Medicine, Indianapolis, Indiana. Department of Cancer Biology, Cleveland Clinic, Cleveland, Ohio. Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/). Corresponding Authors: Tao Lu, Department of Pharmacology and Toxicology, Indiana University School of Medicine, 635 Barnhill Drive, Room MS-A507, Indianapolis, IN 46202. Phone: 317-278-0520; Fax: 317-274-7714; E-mail: [email protected]; or George R. Stark, Department of Cancer Biology, Cleveland Clinic, 9500 Euclid Avenue, Cleveland, OH 44195. Phone: 216-444-6062; Fax: 216-444-0512; E-mail: [email protected] doi: 10.1158/0008-5472.CAN-15-1022 2015 American Association for Cancer Research. Cancer Research Cancer Res; 75(18) September 15, 2015 3692 on January 6, 2018. © 2015 American Association for Cancer Research. cancerres.aacrjournals.org Downloaded from Published OnlineFirst September 3, 2015; DOI: 10.1158/0008-5472.CAN-15-1022