Regulation of Cigarette Smoke (CS)-Induced Autophagy by Nrf2

Abstract

Cigarette smoke (CS) has been reported to induce autophagy in airway epithelial cells. The subsequent autophagic cell death has been proposed to play an important pathogenic role in chronic obstructive pulmonary disease (COPD); however, the underlying molecular mechanism is not entirely clear. Using CS extract (CSE) as a surrogate for CS, we found that it markedly increased the expressions of both LC3B-I and LC3B-II as well as autophagosomes in airway epithelial cells. This is in contrast to the common autophagy inducer (i.e., starvation) that increases LC3B-II but reduces LC3B-I. Further studies indicate that CSE regulated LC3B at transcriptional and post-translational levels. In addition, CSE, but not starvation, activated Nrf2-mediated adaptive response. Increase of cellular Nrf2 by either Nrf2 overexpression or the knockdown of Keap1 (an Nrf2 inhibitor) significantly repressed CSE-induced LC3B-I and II as well as autophagosomes. Supplement of NAC (a GSH precursor) or GSH recapitulated the effect of Nrf2, suggesting the increase of cellular GSH level is responsible for Nrf2 effect on LC3B and autophagosome. Interestingly, neither Nrf2 activation nor GSH supplement could restore the repressed activities of mTOR or its downstream effctor-S6K. Thus, the Nrf2-dependent autophagy-suppression was not due to the re-activation of mTOR-the master repressor of autophagy. To search for the downstream effector of Nrf2 on LC3B and autophagosome, we tested Nrf2-dependent genes (i.e., NQO1 and P62) that are also increased by CSE treatment. We found that P62, but not NQO1, could mimic the effect of Nrf2 activation by repressing LC3B expression. Thus, Nrf2->P62 appears to play an important role in the regulation of CSE-induced LC3B and autophagosome.

Histone deacetylase 6–mediated selective autophagy regulates COPD-associated cilia dysfunction

“…HDAC6 expression was consistently elevated in Nrf2 -/-mice tissues and cells, which also displayed increased protein ubiquitination. Interestingly, NRF2 inhibition increases autophagy (71), and increasing NRF2 represses CSEinduced autophagosome formation (72). These results suggest that HDAC6 expression is increased by oxidative and proteostatic stress and may be counterregulated by NRF2.…”

Short- and Long- Term Effects of Cigarette Smoke Exposure on Glutathione Homeostasis in Human Bronchial Epithelial Cells

“…The observed mortality of 16HBE cells after CSE exposure, that has been observed also in other cell types [43], could be due to a different form of cell death, i.e. an autophagic cell death, whose morphological and biomedical features are distinct from other cell death pathways [44] and that has been demonstrated to be induced by cigarette smoke in airway epithelial cells [45]. Glutathione is the most abundant intracellular antioxidant thiol and is essential to redox protection during oxidative stress.…”

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).…”