Molecular mechanisms of Nrf2 regulation and how these influence chemical modulation for disease intervention

Abstract

Nrf2 (nuclear factor erytheroid-derived-2-like 2) transcriptional programmes are activated by a variety of cellular stress conditions to maintain cellular homoeostasis. Under non-stress conditions, Nrf2 is under tight regulation by the ubiquitin proteasome system (UPS). Detailed mechanistic investigations have shown the Kelch-like ECH-associated protein 1 (Keap1)-cullin3 (Cul3)-ring-box1 (Rbx1) E3-ligase to be the primary Nrf2 regulatory system. Recently, both beta-transducin repeat-containing E3 ubiquitin protein ligase (β-TrCP) and E3 ubiquitin-protein ligase synoviolin (Hrd1) have been identified as novel E3 ubiquitin ligases that negatively regulate Nrf2 through Keap1-independent mechanisms. In addition to UPS-mediated regulation of Nrf2, investigations have revealed a cross-talk between Nrf2 and the autophagic pathway resulting in activation of Nrf2 in a non-canonical manner. In addition to regulation at the protein level, Nrf2 was recently shown to be regulated at the transcriptional level by oncogenic K-rat sarcoma (Ras). A consequence of these differential regulatory mechanisms is the dual role of Nrf2 in cancer: the canonical, protective role and the non-canonical 'dark-side' of Nrf2. Based on the protective role of Nrf2, a vast effort has been dedicated towards identifying novel chemical inducers of Nrf2 for the purpose of chemoprevention. On the other hand, upon malignant transformation, some cancer cells have a constitutively high level of Nrf2 offering a growth advantage, as well as rendering cancer cells resistant to chemotherapeutics. This discovery has led to a new paradigm in cancer treatment; the initially counterintuitive use of Nrf2 inhibitors as adjuvants in chemotherapy. Herein, we will discuss the mechanisms of Nrf2 regulation and how this detailed molecular understanding can be leveraged to develop Nrf2 modulators to prevent diseases, mitigate disease progression or overcome chemoresistance.

Keywords: Hrd1; Kelch-like ECH-associated protein 1 (Keap1); chemoprevention/chemoresistance; nuclear factor erytheroid-derived-2-like 2 (Nrf2); reactive oxygen species (ROS); ubiquitin proteasome system (UPS).

Figure 1. The canonical Nrf2 regulatory pathway

(A) The domain architecture of Nrf2 and known functions of the individual domains. (B) The canonical, Keap1–Cul3–Rbx1-mediated Nrf2 regulatory pathway. Upon cellular insult, cysteine residues in Keap1 are modified and the activity of the E3 ubiquitin ligase is suppressed. Nrf2 levels rise and Nrf2 enters the nucleus, where it dimerizes with Maf and turns on ARE-containing genes.

Figure 2. The three E3 ubiquitin ligases for Nrf2

(A) Keap1–Cul3–Rbx1 E3 ubiquitin ligase. (B) β-TrCP–Skp1–Cul1–Rbx1 E3 ubiquitin ligase. (C) The Hrd1 E3 ubiquitin ligase.

Figure 3. The three modes of Nrf2 activation in health and disease

(A) The canonical mechanism of Nrf2 regulation in a normal cell. (B) Misregulated Nrf2 signalling in a cancer cell that leads to constitutive activation of Nrf2. The molecular events underlying this misregulation are discussed in the text. (C) Prolonged Nrf2 signalling from compromised autophagy. This is probably a major contributing factor in arsenic-mediated carcinogenesis.