Paraquat Toxicity Induced by Voltage-dependent Anion Channel 1 Acts as an NADH-dependent Oxidoreductase

Abstract

Paraquat (PQ), a herbicide used worldwide, causes fatal injury to organs upon high dose ingestion. Treatments for PQ poisoning are unreliable, and numerous deaths have been attributed inappropriate usage of the agent. It is generally speculated that a microsomal drug-metabolizing enzyme system is responsible for PQ toxicity. However Paraquat (PQ 2 ; methyl viologen, 1,1Ј-dimethyl-4,4Ј-bipyridinium dichloride) is an effective herbicide used in more than 120 countries (1). Although it is classified as a low hazard compound, PQ is hazardous when used improperly and has been found responsible for thousands of deaths worldwide because of intentional overdose and high levels of occupational and accidental exposure especially in developing countries (1). Direct exposure to PQ causes severe irritation to the eyes and skin, and ingestion of concentrated products may result in fatal injury to lungs because of edema, hemorrhage, and subsequent fibrosis as well as damage to other organs (2). Additionally, PQ has emerged as a risk factor for Parkinson disease (3). The acute toxicity of PQ in mammals is mediated by reactive oxygen species (ROS) produced by a cyclic oxidation-reduction reaction (4). It is generally speculated that NADPH-cytochrome P450 reductase in microsomal drug-metabolizing enzyme systems is responsible for the production of ROS (5). However, we previously observed that the initial ultrastructural alterations associated with PQ exposure occurred only in mitochondria and not in the endoplasmic reticulum in pulmonary cells in vivo (6) and in vitro (7). In addition, several reports have suggested the cytotoxicity of PQ via mitochondrial dysfunction (8 -10). Despite the development of a number of treatments for PQ poisoning, the efficacy and reliability of currently available treatments have remained limited because of an insufficient understanding of PQ cytotoxicity (2).We recently discovered that active NADH-dependent oxidoreductase located on the mitochondrial outer membrane reduced PQ to a radical form that spontaneously formed superoxide anion (O 2 . ) and destroyed mitochondria (11-13). Furthermore, we demonstrated that 1) PQ was initially metabolized to monopyridone in the cytosol and subsequently hydroxylated by the microsomes and 2) the induction of drugmetabolizing enzymes and the administration of a ROS scavenger reduced PQ toxicity in mice (11,14). These results indicate that the mitochondrial system, not the microsomal system, is responsible for PQ toxicity. We verified that enzymes in the electron transport chain and NADH-cytochrome b 5 reductase, an NADH-dependent oxidoreductase in the outer membrane, were not involved in this reaction (11,12). A voltage-dependent anion channel (VDAC), an abundant pore-forming protein in the outer membrane, exerts numerous physiological functions as a channel; it regulates both the metabolite flux of mitochondria and transmembrane potential, and plays a role in apoptosis. Recently, it was reported that NADH regulates VDAC func-*

The antidote effect of quinone oxidoreductase 2 inhibitor against paraquat‐induced toxicity in vitro and in vivo

“…Our data suggest that PQ‐induced toxicity is mediated via QR2 activity. Several other oxidoreductases have also been implicated in PQ bioactivation, such as NADPH oxidases of the Nox family (Miller et al ., 2007; Cristovao et al ., 2009), a DPI‐sensitive thioredoxin reductase (Gray et al ., 2007), as well as a mitochondrial NADH‐dependent reductase (Shimada et al ., 2009). However, in spite of a wide specificity of QR2 in terms of substrates and co‐substrates, it is unlikely that QR2 directly catalyses the conversion of the PQ di‐cation into a PQ radical, thus using PQ as a substrate in a mechanism analogous to the diaphorases listed above.…”

“…Several NAD(P)H‐dependent diaphorases have been identified as PQ‐reducing enzymes. These include, cytoplasmic NADPH oxidases of the Nox family (Miller et al ., 2007; Cristovao et al ., 2009), a diphenylene iodonium (DPI)‐sensitive thioredoxin reductase, highly expressed in alveolar epithelium (Gray et al ., 2007), mitochondrial oxidoreductases present in the electron transport Complex I (Cocheme and Murphy, 2008) and Complex III (Castello et al ., 2007; Drechsel and Patel, 2009) or a mitochondrial NADH‐dependent reductase VDAC1 (Shimada et al ., 2009). Finally, it cannot be excluded that oxidoreductases that preferentially use an alternative electron donor to NAD(P)H, are also involved in PQ‐induced cytotoxicity.…”

Voltage‐dependent anion‐selective channel (VDAC) in the plasma membrane

“…Although it seems unlikely that VDAC1 would function as a reductase in mitochondria, there have been two recent reports claiming that mitochondrial VDAC1 is functional as an NADH-dependent reductase. A first report suggested that mitochondrial VDAC1's reductase activity is responsible for the reductive activation of quinone anticancer drugs like 2-methyl-furanonaphthoquinone and 2-methyl-5-hydroxy-furanonaphthoquinone [47]; a second report claimed that it is VDAC1's NADH:paraquat reductase activity in the mitochondria that is responsible for paraquat toxicity [48]. As discussed above, NADH can regulate the gating of VDAC [36] and adenosine nucleotides can be transported by the molecule [49].…”

Regulation of PINK1-Parkin-mediated mitophagy

“…109 Whether this phenotype reflects the observed defects in pink1 and parkin mutants, and moreover, whether these mutants genetically interact is unclear; however, certain pieces of information support the concept of VDAC1 playing a role in PD pathogenesis, such as VDAC1-dependent ROS generation and release in the context of paraquat and dopamine toxicity. 110,111 In addition, a role of VDAC1 in mitophagy has already been suggested. Furthermore, VDAC1 has been proposed to contribute also to other neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) 112 and Alzheimer disease (AD) (reviewed in ref.…”