Keap1: One stone kills three birds Nrf2, IKKβ and Bcl-2/Bcl-xL

Abstract

Oxidative stress, implicated in the etiology of cancer, results from an imbalance in the production of Reactive Oxygen Species (ROS) and cell’s own antioxidant defenses. As a oxidative stress sensor, Keap1 functions as both an adaptor for Cul3⋅Rbx1 E3 ligase complex mediated degradation of the transcription factor Nrf2, and a master regulator of cytoprotective gene expression. Although Nrf2 is a well known substrate for Keap1, the DGR domain of Keap1 has been reported also to bind other proteins directly or indirectly. IKKβ as positive regulator of NF-κB is also destabilized by Keap1, which resulted in inhibiting NF-κB-derived tumor promotion. In addition, anti-apoptotic Bcl-2/Bcl-xL protein was identified as another substrate for the Keap1-Cul3-E3 ligase complex. Keap1 led to the repression and destabilization of Bcl-2, decreased Bcl-2:Bax heterodimers and facilitated cancer cells apoptosis. Given that Keap1 might function as a tumor suppressor protein to mitigate tumor progression, the different kinds of Keap1 somatic mutations were detected in numerous cancer cells. Therefore, it is important to understand the Keap1-involved signaling cascades. This review primarily focuses on the prevention of tumorigenesis role of Keap1 through negative regulation of three substrates Nrf2, IKKβ and Bcl-2/Bcl-xL, with emphasis on the recent findings indicating the cancer guarder function of Keap1.

Introduction

Oxidative stress is induced by a vast range of factors including ionizing radiations (IRs), ultraviolet (UV) and other environmental carcinogens. Oxidative stress leads to the generation of Reactive Oxygen Species (ROS). ROS exerts oxidative stress in the cells which promotes damage to the cell structure including proteins, lipids, membranes and DNA, that plays a key role in the development of cancer [1], [2], [3]. Therefore, it is obvious that cells must constantly labor to prevent them from accumulation. Cellular defense system is comprised of several Phase II detoxification enzymes such as glutathione-S-transferases (GSTs), NADP(H):quinone oxidoreductase (NQO1), glutathione peroxidases (GPx), catalase, superoxide dismutases (SODs), epoxide hydrolase, heme oxygenase (HO-1), UDP-glucuronosyl transferases (UGTs), and gamma-glutamylcysteine synthetase [4], [5], [6], [7]. The concept of “Redox Regulation” in the field of cancer prevention is emerging to understand the mechanisms behind the pathogenesis of several disorders including tumor initiation and malignant transformation. Transcription factors/activators bind to specific consensus sequences (cis elements) in the promoter regions of downstream target/effector and subsequently transactivate or repress effector gene expression, resisting to oxidative stress or converting to less toxic and more readily excretable products [8], [9]. Therefore, differential regulation of these transcriptional activators, which in turn, regulate many target/effector genes, may provide an additional mechanism in anticancer processes and impact drug discovery and therapy for the inhibition of cancer.

A redox regulatory circuit includes an integrity set of elements in response to the extra stress stimulation: a signal and a sensor to trigger the adaptive process, a transducer in concert with a modulator of sensitivity, an effector, and a terminating device. Kelch-like ECH-associated protein-1(Keap1) or inhibitor of Nrf2 (INrf2) is a sensor of redox regulation. Keap1 possesses dual functions (I) “sense” a disturbance in the redox homeostasis and (II) switch its well known substrate nuclear factor erythroid-2-related factor 2 (Nrf2) mediated response on or off. The reactive cysteine in Keap1 may be oxidized to sulfenic acids, form intermolecular disulfide bridge, or be alkylated by electrophiles. The consequence of cysteine modification in Keap1 is a dramatic conformational change resulting in a dissociation of Nrf2 from the Keap1-Nrf2 complex [10], [11], [12], [13], [14]. Subsequently, Nrf2 transacts the downstream effector through the activation of antioxidant response element (ARE). The ARE was first identified as cis-element and found in the promoters of detoxifying enzyme genes such as glutathione S-transferases [15], [16], NAD(P)H:quinone oxidoreductases (NQOs) [17], [18], glutathione peroxidase-1 (GPx-1) and peroxiredoxin 1 (PRDX1) [19]. Therefore, Keap1 plays the dual roles: the sensor of oxidative stress and the regulator of transcription factor Nrf2.