Tripartite motif (TRIM) family proteins, also named RBCC proteins, are characterized by three highly conserved domains, RING (really interesting new gene) finger, one or two B-box domains, and a coiled-coil region (McNab F W, et al., 2011). A number of TRIM family proteins exhibit E3 ubiquitin ligase activity, which is attributed to their RING domain. TRIM family proteins are involved in many cellular processes, including apoptosis, proliferation, oncogenesis, differentiation, intracellular trafficking, and innate immune responses (Ozato K, et al., 2008). In addition, many TRIM proteins have been reported to be able to regulate innate immunity through management of the nuclear factor-kappa B (NF-κB) pathway (Gong J, et al., 2011; Poole E, et al., 2009; Tian Y, et al., 2007; Zha J, et al., 2006; Zurek B, et al., 2012). TRIM30α can target transforming growth factor-β activated kinase (TAK) 1 binding protein 2 (TAB 2) and TAB 3 for degradation, and can negatively regulate the long terminal repeat (LTR)-mediated signal pathway (Shi M, et al., 2008). In glioblastoma, TRIM19, also named promyelocytic leukemia protein (PML), can inhibit the activation of the NF-κB pathway to activate caspase-8, then induce apoptosis (Kuwayama K, et al., 2009).
NF-κB is an important transcription factor that participates in cellular response to apoptosis, cellular proliferation, inflammation, and immune response. IκBα, one of the prototypical inhibitors of κB (IκB) proteins, plays an important role in mediating the canonical NF-κB pathway activation (Hayden M S, et al., 2008). Canonical NF-κB pathway activation requires phosphorylation of the IκB kinase (IKK) complex and the subsequent disassociation of IκBα from NF-κB. Moreover, IKK complex activation is regulated by tumor necrosis factor (TNF) receptorassociated factor 6 (TRAF6) and its downstream TRAF6-regulated IKK activator (TRIKA)1 and TRIKA2 protein complex (Wang C, et al., 2001). TRAF6, which has been characterized as an E3 ubiquitin ligase, can catalyze the lysine-63 (K63) polyubiquitination reaction that triggers IKK activation (Cao Z, et al., 1996). Furthermore, TRAF6 overexpression can activate the NF-κB pathway. This TRAF6-stimulated activation requires TRIKA1, a dimeric ubiquitin-conjugating enzyme complex composed of Ubc13 and Uev1A (Wang C, et al., 2001), and TRIKA2, another kinase complex consisting of TAK1, TAB 1, and TAB 2 (Besse A, et al., 2007). Activation of TAK1 kinase is induced by polyubiquitinated TRAF6. TAB 2, as a receptor, can preferentially bind to K63-linked polyubiquitin chains through a highly conserved zinc finger (ZnF) domain, which can facilitate TRAF6 self-ubiquitination and promote its assembly with IKK (Kanayama A, et al., 2004; Kishida S, et al., 2005).
TRIM22, a member of the TRIM/RBCC family, was first identified as an interferon-inducible gene in Daudi cells (Tissot C, et al., 1995). Studies have shown that TRIM22 is implicated in innate immunity (Barr S D, et al., 2008; Di Pietro A, et al., 2013; Gao B, et al., 2009), and TRIM22 has been identified as a p53 target gene involved in hematopoietic differentiation and T-lymphocyte activation (Obad S, et al., 2007; Obad S, et al., 2007). Recently, it was reported that TRIM22 overexpression activated the NF-κB pathway in unstimulated macrophage cell lines, and that this activation could be blocked by the IκBα inhibitorpyrollidine dithiocarbamate (PDTC) (Yu S, et al., 2011). However, the precise mechanisms of this activation are still unclear.
In the current study, we attempted to clarify this mechanism. We found that overexpressed TRIM22 negatively regulated the TRAF6-stimulated NF-κB pathway in the human embryonic kidney cell line HEK293T. TRIM22 inhibited TRAF6 self-ubiquitination, and this could be partially rescued by a TRIM22 RING domain deletion mutant. Additional experiments showed that TRIM22 was able to interact with TAB 2 and promote TAB 2 degradation, which might be responsible for inhibiting TRAF6 selfubiquitination.
TRIM22 Inhibits the TRAF6-stimulated NF-κB Pathway by Targeting TAB2 for Degradation
- Received Date: 19 May 2013
- Accepted Date: 27 May 2013
- Published Date: 27 June 2013
Abstract: Tripartite motif containing 22 (TRIM22), a member of the TRIM/RBCC family, has been reported to activate the nuclear factor-kappa B (NF-κB) pathway in unstimulated macrophage cell lines, but the detailed mechanisms governing this activation remains unclear. We investigated this mechanism in HEK293T cells. We found that overexpression of TRIM22 could activate the NF-κB pathway and conversely, could inhibit the tumor necrosis factor receptor-associated factor 6 (TRAF6)-stimulated NF-κB pathway in HEK293T cells. Further experiments showed that TRIM22 could decrease the self-ubiquitination of TRAF6, and interact with and degrade transforming growth factor-β activated kinase 1 binding protein 2 (TAB2), and that these effects could be partially rescued by a TRIM22 RING domain deletion mutant. Collectively, our data indicate that overexpression of TRIM22 may negatively regulate the TRAF6-stimulated NF-κB pathway by interacting with and degrading TAB2.