Arsenic Inhibits Autophagic Flux, Activating the Nrf2-Keap1 Pathway in a p62-Dependent Manner

Abstract

The Nrf2-Keap1 signaling pathway is a protective mechanism promoting cell survival. Activation of the Nrf2 pathway by natural compounds has been proven to be an effective strategy for chemoprevention. Interestingly, a cancer-promoting function of Nrf2 has recently been observed in many types of tumors due to deregulation of the Nrf2-Keap1 axis, which leads to constitutive activation of Nrf2. Here, we report a novel mechanism of Nrf2 activation by arsenic that is distinct from that of chemopreventive compounds. Arsenic deregulates the autophagic pathway through blockage of autophagic flux, resulting in accumulation of autophagosomes and sequestration of p62, Keap1, and LC3. Thus, arsenic activates Nrf2 through a noncanonical mechanism (p62 dependent), leading to a chronic, sustained activation of Nrf2. In contrast, activation of Nrf2 by sulforaphane (SF) and tert-butylhydroquinone (tBHQ) depends upon Keap1-C151 and not p62 (the canonical mechanism). More importantly, SF and tBHQ do not have any effect on autophagy. In fact, SF and tBHQ alleviate arsenic-mediated deregulation of autophagy. Collectively, these findings provide evidence that arsenic causes prolonged activation of Nrf2 through autophagy dysfunction, possibly providing a scenario similar to that of constitutive activation of Nrf2 found in certain human cancers. This may represent a previously unrecognized mechanism underlying arsenic toxicity and carcinogenicity in humans.

Antioncogenic and Oncogenic Properties of Nrf2 in Arsenic-induced Carcinogenesis

“…The p62/sequestosome 1 protein is a multifunction, ubiquitin-binding adapter protein that serves multiple cellular functions for autophagy, apoptosis, reactive oxygen species (ROS) signaling, and cancer development (2,3). The accumulated p62 competitively binds to the Nrf2-binding site of Keap1 and promotes the constitutive activation of Nrf2 transcriptional programs (4). Thus, p62 accumulation activates Nrf2 and Nrf2 target gene expression (5,6).…”

“…It has been shown that inducible Nrf2 activation is important in the anticancer effects of many natural compounds, including phenethyl isothiocyanate, curcumin, and resveratrol (12 859-323-1059; E-mail: xshi5@email.uky.edu. 4 The abbreviations used are: Nrf2, nuclear factor erythroid 2-related factor; ROS, reactive oxygen species; ARE, antioxidant response element; NF-B, nuclear factor B; Keap1, kelch-like ECH-associated protein 1; LC3, microtubule-associated protein 1 light chain 3; ABT-263, 4-(4- -induced carcinogenesis (cell transformation) by an up-regulation of antioxidants to reduce As 3ϩ -induced ROS; however, once a cell is transformed, Nrf2 is oncogenic by inducing apoptosis resistance. This study also will examine how constitutive activation of Nrf2 contributes to apoptosis resistance as well as a decrease in ROS generation and tumorigenesis in transformed cells.…”

“…The nuclear factor erythroid 2-related factor (Nrf2) 4 is a master regulator of cellular antioxidant responses. Under unstressed conditions, Nrf2 is degraded by proteasomal machinery.…”

Realgar (As4S4) nanoparticles and arsenic trioxide (As2O3) induced autophagy and apoptosis in human melanoma cells in vitro

“…One of possible explanation can be found in the article of T. Schwerdtle et al [46] about S(ulphur)-containing arsenic metabolites that modulated total cellular glutathione amount at picomolar concentrations. Recently it was shown that sodium arsenite treatment of normal cells induces prolonged activation of Nrf2 through autophagy dysfunction [29]. This observation corresponds to the fact found in certain human cancer types, where high sustained level of Nrf2 provides the resistance to treatment.…”

“…Metabolism of inorganic arsenic contributes to alteration of large number of transcription factors [28][29][30][31]. Among them Nrf2 plays a crucial role in the cellular protection against oxidative stress regulating the cellular antioxidant response.…”