KPNA6 (Importin α7)-Mediated Nuclear Import of Keap1 Represses the Nrf2-Dependent Antioxidant Response

Abstract

The transcription factor Nrf2 has emerged as a master regulator of cellular redox homeostasis. As an adaptive response to oxidative stress, Nrf2 activates the transcription of a battery of genes encoding antioxidants, detoxification enzymes, and xenobiotic transporters by binding the cis-antioxidant response element in the promoter regions of genes. The magnitude and duration of inducible Nrf2 signaling is delicately controlled at multiple levels by Keap1, which targets Nrf2 for redox-sensitive ubiquitin-mediated degradation in the cytoplasm and exports Nrf2 from the nucleus. However, it is not clear how Keap1 gains access to the nucleus. In this study, we show that Keap1 is constantly shuttling between the nucleus and the cytoplasm under physiological conditions. The nuclear import of Keap1 requires its C-terminal Kelch domain and is independent of Nrf1 and Nrf2. We have determined that importin ␣7, also known as karyopherin ␣6 (KPNA6), directly interacts with the Kelch domain of Keap1. Overexpression of KPNA6 facilitates Keap1 nuclear import and attenuates Nrf2 signaling, whereas knockdown of KPNA6 slows down Keap1 nuclear import and enhances the Nrf2-mediated adaptive response induced by oxidative stress. Furthermore, KPNA6 accelerates the clearance of Nrf2 protein from the nucleus during the postinduction phase, therefore promoting restoration of the Nrf2 protein to basal levels. These findings demonstrate that KPNA6-mediated Keap1 nuclear import plays an essential role in modulating the Nrf2-dependent antioxidant response and maintaining cellular redox homeostasis.Maintaining cellular redox homeostasis is required for proper functioning of the cell. Oxidative stress, characterized by excessive reactive oxygen species (ROS), is associated with the toxicity of many environmental insults and the pathogenesis of age-related diseases, such as cancer and neurodegenerative disorders (12,14,33,34,45). On the other hand, prolonged activation of cellular defense systems by genetic mutations in regulatory molecules is a common strategy adapted by cancer cells to promote malignant growth against deleterious microenvironments (7, 31). Mammalian intracellular redox homeostasis is maintained mainly through transcriptional control of a battery of antioxidant genes in response to oxidative stress. This requires that the transcription of those antioxidant genes is immediately induced in the presence of excess ROS and is quickly reduced back to basal levels once cells return to redox homeostasis. As a key component of such a control system, the antioxidant response element (ARE) is a conservative cis-acting element found in the promoter region of many genes encoding antioxidants and detoxification enzymes. The corresponding trans-acting factor for the ARE is a transcription factor named nuclear factor erythroid 2-related factor 2 (Nrf2) (28,29,44). The Nrf2-ARE system is responsible for both basal and inducible expression of many genes involved in the antioxidant response, such as NAD(P)H quinone oxidoreductase 1 (NQO1), heme...

Src Subfamily Kinases Regulate Nuclear Export and Degradation of Transcription Factor Nrf2 to Switch Off Nrf2-mediated Antioxidant Activation of Cytoprotective Gene Expression

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

“…Therefore, there is a tight relationship between Src kinase-mediated phosphorylation of Nrf2Y568 and Keap1 nuclear-cytoplasmic shuttling via PTMa in the case of nuclear elimination of Nrf2. Additionally, an alpha karyopherin KPNA6 (also known as importin a 7), a nucleocytoplasmic transport adaptor, was also reported as a specific nuclear import adaptor protein for Keap1 [59]. Keap1 constitutively shuttles between the nucleus and the cytoplasm under basal conditions.…”