Oncogene (2011) 30, 631–641 & 2011 Macmillan Publishers Limited All rights reserved 0950-9232/11 www.nature.com/onc REVIEW Emerging roles for the non-canonical IKKs in cancer RR Shen1 and WC Hahn1,2 1Department of Medical Oncology, Dana Farber Cancer Institute, Boston, MA, USA and 2Broad Institute of Harvard and MIT, Cambridge, MA, USA The IjB Kinase (IKK)-related kinases TBK1 and IKKe to form the mature p50 and p52 proteins. This have essential roles as regulators of innate immunity by processing is necessary for p50 and p52 to function as modulating interferon and NF-jB signaling. Recent work transcription factors (Dolcet et al., 2005). Distinct has also implicated these non-canonical IKKs in malignant NF-kB complexes are formed from combinations of transformation. IKKe is amplified in B30% of breast homo- and heterodimers of these family members cancers and transforms cells through the activation of (Bonizzi and Karin, 2004). NF-kB complexes are NF-jB. TBK1 participates in RalB-mediated inflamma- retained in the cytoplasm by a family of NF-kB-binding tory responses and cell survival, and is essential for the proteins known as the inhibitors of NF-kB(IkBs). survival of non-small cell lung cancers driven by oncogenic A variety of inflammatory stimulants initiate the KRAS. The delineation of target substrates and down- induction of NF-kB and trigger activation of the IkB stream activities for TBK1 and IKKe has begun to define Kinase (IKK) complex, which is composed of the their role(s) in promoting tumorigenesis. In this review, we catalytic kinases IKKa and IKKb, and the regulatory will highlight the mechanisms by which IKKe and TBK1 NF-kB essential modifier (NEMO, or IKKg) (Perkins, orchestrate pathways involved in inflammation and cancer. 2007). One important function of the IKK complex is to Oncogene (2011) 30, 631–641; doi:10.1038/onc.2010.493; mark IkB for phosphorylation and ubiquitination, published online 1 November 2010 which in turn, leads to proteasomal degradation. This activity facilitates the release and accumulation NF-kB Keywords: NF-kB; cancer; IKK dimers in the nucleus where a transcriptional program involving many target genes related to immune response are activated. Although the classical NF-kB pathway primarily involves the activation of RelA and p50 and is Introduction strictly facilitated by the IKK complex, an alternative pathway is active in B cells where NF-kB inducing The NF-kB pathway is a pivotal regulator of several kinase (NIK) promotes the activation of RelB and p52/ important physiological functions including the inflam- p100 complexes (Ghosh and Hayden, 2008). Genetic matory immune response, proliferation, cell survival experiments have demonstrated that IKKb is the and cell invasion. These activities are well-described principal IKK for the canonical NF-kB pathway, hallmarks of cancer, and NF-kB activation has been whereas IKKa has a more dominant role in the non- observed in a wide range of tumors (Dolcet et al., 2005; canonical pathway (Bonizzi and Karin, 2004; Paspar- Baud and Karin, 2009; Prasad et al., 2010), leading some akis et al., 2006; Perkins, 2007). In addition to the to suggest that NF-kB serves as a bridge between conventional IKKs, a related pair of non-canonical inflammation and cancer (Baldwin, 2001; Karin, 2006). kinases, IKKe (IKKi, encoded by IKBKE) and TBK1 Activation of NF-kB in cancer arises either from (NAK), have been identified as important mediators of extrinsic signals in the tumor microenvironment or both inflammatory and oncogenic signaling. from intrinsic dysregulation of the pathway within the tumor. In either case, several different components of the NF-kB signaling cascade may contribute to this IKKe and TBK1 function in inflammation activity in particular cancers. In mammals five NF-kB members exist including IKKe was simultaneously identified in a subtractive RelA (p65), RelB, c-Rel, p50/p105 (NF-kB1) and p52/ hybridization screen as a LPS-inducible gene and as a p100 (NF-kB2). Although RelA, RelB and cRel are PMA-inducible protein with a kinase domain that is synthesized as mature forms, p105 and p100 are 27% identical to IKKa and IKKb (Shimada et al., 1999; synthesized as longer precursor proteins that are cleaved Peters et al., 2000). Concurrently, TBK1, which exhibits 49% identity and 65% similarity to IKKe was identified Correspondence: Dr WC Hahn, Department of Medical Oncology, as an interaction partner with the scaffolding molecule, Dana Farber Cancer Institute, 44 Binney Street, Dana 1538, Boston, TRAF-associated NF-kB activator (TANK) (Pomerantz MA 02115, USA. E-mail: [email protected] and Baltimore, 1999). Both IKKe and TBK1 are Received 30 July 2010; revised 15 September 2010; accepted 18 comprised of an N-terminal kinase domain, an ubiquitin- September 2010; published online 1 November 2010 like domain, a C-terminal LZ and a HLH motif Roles for the non-canonical IKKs in cancer RR Shen and WC Hahn 632 455 483 738 743 double stranded RNA and DNA initiate signaling IKKα 1 Kinase domainLZ HLH NBD 745 through intracellular RNA and DNA sensors such as 15 301 599 638 RIG-I, MDA-5 and DAI (Andrejeva et al., 2004; 311 388 603 642 Yoneyama et al., 2004; Kawai et al., 2005; Meylan IKKβ 1 Kinase domainULD LZ HLH NBD 756 et al., 2005; Seth et al., 2005; Takaoka et al., 2007). IFNb also activates a toll-like receptor-independent 15 300 458 486 737 742 350 383 W445S 578 619 pathway by stimulating IKKe phosphorylation of STAT1 to facilitate binding with ISGF3, a complex ε IKK 1Kinase domain ULD LZ HLH 716 that serves as the transcriptional machinery important 9 300G417D 500 527 for activating a subset of interferon response genes 305 383 591 632 (Tenoever et al., 2007). Moreover, engagement of TBK1 1Kinase domain ULD LZ HLH 730 both toll-like receptor-dependent and -independent 9 300 499 527 P675L pathways recruits additional scaffolding molecules Figure 1 Structural comparison of the classical and non-canonical including FADD, TRADD, MAVS, NAP1, HSP90 IKKs. The major domains of each IKK kinase are depicted with and SINTBAD which are necessary for IKKe amino-acid numbers that correspond to the human proteins. The and TBK1-mediated interferon activation (Rothe kinase domain of IKKe exhibits 27% and 24% identity to IKKa et al., 1996; Balachandran et al., 2004; Hacker et al., and IKKb, respectively, and TBK1 shares 49% identity and 65% 2006; Oganesyan et al., 2006; Yang et al., 2006; Gatot similarity to IKKe. Somatic mutations of IKKe and TBK1 recently identified in lung adenocarcinomas are marked in red. ULD, et al., 2007; Guo and Cheng, 2007; Ryzhakov and Ubiquitin-like domain; LZ, leucine zipper; HLH, helix-loop-helix; Randow, 2007; Michallet et al., 2008). Thus, IKKe and NBD, NEMO-binding domain (May et al., 2004; Hiscott et al., TBK1 form several protein complexes, the composition 2006; Perkins, 2007; Kan et al., 2010). of which is dependent on the type of cellular stimuli. Ultimately, these signaling complexes share a role in activating interferon responses for achieving an antiviral state. (Figure 1). Despite their similarity in structure, TBK1 The ability of IKKe and TBK1 to activate the and IKKe exhibit differential expression patterns. interferon response is also dependent on post transla- TBK1, like IKKa and IKKb, is ubiquitously expressed tional modifications by proteasome-independent Lys-63 (Shimada et al., 1999). In contrast, IKKe expression is linked ubiquitin chains (Figure 2). Both IKKe and restricted to particular tissue compartments, with high- TBK1 have an ubiquitin like domain, and Lys-63 est levels detected in lymphoid tissues, peripheral blood ubiquitination of both kinases are promoted by TRAF3, lymphocytes and the pancreas (Shimada et al., 1999). which itself is Lys-63 ubiquitinated. Disruption of this Various epithelial-derived cell lines also exhibit IKKe activity by TAXBP1 and the deubiquitinase A20 ablates expression (Shimada et al., 1999; Gravel and Servant, the interferon response (Ikeda et al., 2007; Parvatiyar 2005; Honda et al., 2005; Bibeau-Poirier et al., 2006). et al., 2010). Both TBK1 and IKKe also mediate Mitogenic stimulation with LPS and TNFa can also ubiquitination of TANK by an unknown E3 ligase induce IKKe and TBK1 expression in a NF-kB (Gatot et al., 2007). Although TANK further serves as a dependent manner (Shimada et al., 1999; Kravchenko phosphorylation target of TBK1 and IKKe, TANK et al., 2003; Hemmi et al., 2004). With these partially ubiquitination seems to occur independently of this overlapping characteristics, IKKe and TBK1 are func- kinase activity. The mechanism by which TANK tionally more similar to each other than the canonical ubiquitination contributes to the activation of IKKe IKKs (Clement et al., 2008). and TBK1 is unknown. Lys-63 ubiquitination also has a role in the negative regulation of IKKe and TBK1 induced responses. For example, during- by RNA The non-canonical IKKs coordinate the interferon viruses, Lys-63 ubiquitination of MAVS is essential for response the recruitment of IKKe and leads to the inhibition of IKKe and TBK1 are critical inducers of interferon antiviral and NF-kB induced inflammatory genes (Paz signaling in response to viral infection (Fitzgerald et al., et al., 2009). Ultimately, these modifications will likely 2003; Sharma et al., 2003). Following activation of toll- dictate a dynamic system of regulating IKKe and like receptors by viral components, IKKe and TBK1 TBK1-mediated function in both inflammation and assemble with TRAF3 and TANK to phosphorylate cancer. interferon regulatory factors (IRFs) 3, 5 and 7 at multiple serine and threonine residues (Pomerantz and Baltimore, 1999; McWhirter et al., 2004; Mori et al., The non-canonical IKKs function as NF-kB effectors 2004; Caillaud et al., 2005; Cheng et al., 2006). This Although TBK1 and IKKe are not a part of the classical activity allows for heterodimerization and nuclear IKKa/b/g signaling complex, these kinases were origin- translocation of the IRFs and induction of pro- ally characterized as activators of NF-kB and target inflammatory and antiviral genes, including IFN-a/b multiple NF-kB members and effectors.