The Antioxidant Transcription Factor Nrf2 Negatively Regulates Autophagy and Growth Arrest Induced by the Anticancer Redox Agent Mitoquinone

Abstract

Mitoquinone (MitoQ) is a synthetically modified, redox-active ubiquinone compound that accumulates predominantly in mitochondria. We found that MitoQ is 30-fold more cytotoxic to breast cancer cells than to healthy mammary cells. MitoQ treatment led to irreversible inhibition of clonogenic growth of breast cancer cells through a combination of autophagy and apoptotic cell death mechanisms. Relatively limited cytotoxicity was seen with the parent ubiquinone coenzyme Q 10. Inhibition of cancer cell growth by MitoQ was associated with G 1 /S cell cycle arrest and phosphorylation of the checkpoint kinases Chk1 and Chk2. The possible role of oxidative stress in MitoQ activity was investigated by measuring the products of hydroethidine oxidation. Increases in ethidium and dihydroethidium levels, markers of one-electron oxidation of hydroethidine, were observed at cytotoxic concentrations of MitoQ. Keap1, an oxidative stress sensor protein that regulates the antioxidant transcription factor Nrf2, underwent oxidation, degradation, and dissociation from Nrf2 in MitoQ-treated cells. Nrf2 protein levels, nuclear localization, and transcriptional activity also increased following MitoQ treatment. Knockdown of Nrf2 caused a 2-fold increase in autophagy and an increase in G 1 cell cycle arrest in response to MitoQ but had no apparent effect on apoptosis. The Nrf2-regulated enzyme NQO1 is partly responsible for controlling the level of autophagy. Keap1 and Nrf2 act as redox sensors for oxidative perturbations that lead to autophagy. MitoQ and similar compounds should be further evaluated for novel anticancer activity.Mitochondria are central to many cellular processes, including energy production and apoptosis. A by-product of electron transfer in the mitochondria is the formation of superoxide.

Knockdown of Nrf2 enhances autophagy induced by temozolomide in U251 human glioma cell line

“…This indicated that Nrf2 was responsible for regulating basal levels of autophagy in U251 cells. Our finding was in concordance with previous reports in other tumors (35). Nevertheless, from our point of view, further investigation still needs to be done because Nrf2-responsive genes that stimulate autophagy are not well known.…”

“…Nevertheless, how Nrf2 regulates autophagy remains unknown. Others have also reported that Nrf2 negatively regulated autophagy induced by anticancer redox agent MitoQ and the Nrf2-regulated enzyme NQO1 was partly responsible for adjusting the level of autophagy (35).…”

Basic Principles and Emerging Concepts in the Redox Control of Transcription Factors

“…They comprise enzymes that directly or indirectly exert antioxidant functions (5,21,111,202,264,373,491), molecular chaperons (265), and proteins that enhance GSH synthesis and regeneration (164,174,210,276,330,336), phase 2 detoxication, drug metabolism (169,210,326,392,424,479), recognition, repair, and removal of damaged proteins (191,264,265,393), and nucleotide excision repair (8), further proteins that regulate the expression of other transcription factors, growth factors, and receptors (113,163), and inhibit cytokine-mediated inflammation (6,209,245,385,503) and autophagy (394). With its broad range of target genes, Nrf2 is certainly one of the most important transcription factors that protect the organism against exogenous stressors, be they poisonous food ingredients, physical damage, or infection.…”

Nuclear factor erythroid-2 related factor 2 Nrf2 -mediated protein quality control in cardiomyocytes

“…Nrf2 appears to be involved in the regulation of protein aggregation in autophagic substrate selection (43) as well as autophagy per se (44)(45)(46)(47). Although it remains unclear whether the observed increased steady state levels of autophagosomes in Nrf2-deficient breast cancer cells, endothelial cells, and airway epithelial cells are caused by an impairment of autophagosome clearance (44,46,47), it has been demonstrated that Nrf2 is a critical mediator of autophagic clearance of ubiquitinated protein aggregates in macrophages (45).…”

“…Although it remains unclear whether the observed increased steady state levels of autophagosomes in Nrf2-deficient breast cancer cells, endothelial cells, and airway epithelial cells are caused by an impairment of autophagosome clearance (44,46,47), it has been demonstrated that Nrf2 is a critical mediator of autophagic clearance of ubiquitinated protein aggregates in macrophages (45). In addition, a recent study showed that Nrf2 is capable of clearing phosphorylated tau by induction of nuclear dot protein 52 (NDP52), an autophagy adaptor protein (also known as CALCOCO2) in the presence of autophagy stimulator thereby potentially contributing to brain protection (48).…”