The ubiquitin-proteasome pathway is crucial for controlling the abundance of several proteins, and it plays an essential role in maintaining normal cellular functions. Kelch-like ECH-associated protein 1 (KEAP1) is a Bric-a-Brac (BTB)–Kelch protein that functions as a substrate adaptor protein for a Cullin 3 (CUL3)/ Ring-Box 1 (RBX1)–dependent E3 ubiquitin ligase complex (Kobayashi et al., 2004
; Zhang et al., 2004
). The well-studied substrate for the KEAP1-CUL3 ubiquitin E3 ligase complex is nuclear factor erythroid-2–related factor 2 (NRF2) (Motohashi et al., 2004
). The KEAP1-CUL3 ubiquitin E3 ligase complex interacts with and conjugates ubiquitin onto the N-terminal Neh2 domain of NRF2 and promotes NRF2 degradation through the 26S ubiquitin proteasome pathway (Itoh et al., 1999
). While studying the role of KEAP1-NRF2 signaling pathway in human cancers, it was noticed that numerous different kinds of KEAP1 somatic mutations were detected in both lung cancer and gallbladder cancer (Ohta et al., 2008
; Padmanabhan et al., 2006
; Shibata et al., 2008
; Singh et al., 2006
). Moreover, loss of heterozygosity of KEAP1 genome regions (Singh et al., 2006
) were frequently found in both lung cancer cell lines and lung cancer tissues. These genomic alternations result in constitutive NRF2 activation which induces a couple of antioxidant-related enzyme to provide advantages of cancer cells for resistance to chemotherapeutic drugs (Shibata et al., 2008
; Singh et al., 2006
), implicating that KEAP1 might function as a tumor suppressor and loss of KEAP1 function confers tumorigenesis. Given that KEAP1 might function as a tumor suppressor protein to negatively regulate tumor progression and thus provide an advantage for chemotherapy, it is important to understand the KEAP1-involved signaling cascades.
IκB kinase β (IKKβ) functions as a fine-tuning controller of the nuclear factor (NF)–κB pathway. NF-κB activation governs the expression of myriad genes involved in the immune response, cell proliferation, angiogenesis, cell survival, tumor invasion, metastasis, and the epithelial-mesenchymal transition (Karin and Greten, 2005
; Perkins, 2007
). Additionally, IKKβ has been shown to promote tumorigenicity through phosphorylation-mediated inhibition of tumor suppressors and thus is considered an oncogenic kinase (Hu et al., 2004
; Lee and Hung, 2007
; Lee et al., 2007
). These findings strongly suggested that IKKβ-mediated signaling cascades link inflammation to cancer pathogenesis.
In the current study, we demonstrated that KEAP1 functions as an IKKβ E3 ubiquitin ligase. Depletion of KEAP1 led to the accumulation and stabilization of IKKβ and up-regulation of NF-κB–derived tumor angiogenic factors. A systematic analysis of CUL3, KEAP1, and RBX1 (E3 ubiquitin ligase complex) genomic loci revealed a high percentage of genome loss and missense mutations in human cancers that failed to facilitate IKKβ degradation. Thus, our results suggest that inactivation of the KEAP1-CUL3-RBX1 complex in human cancers may prevent IKKβ degradation and result in NF-κB activation and that induction of KEAP1 expression may have vital clinical implications for the treatment or prevention of inflammation-associated cancers.