Induction of the Antioxidant Response by the Transcription Factor NRF2 Increases Bioactivation of the Mutagenic Air Pollutant 3-Nitrobenzanthrone in Human Lung Cells

Aldo-keto reductase (AKR) superfamily website and database: An update

The aldo-keto reductase (AKR) superfamily is a large family of proteins found across the kingdoms of life. Shared features of the family include 1) structural similarities such as an (α/β)8-barrel structure, disordered loop structure, cofactor binding site, and a catalytic tetrad, and 2) the ability to catalyze the nicotinamide adenine dinucleotide (phosphate) reduced (NAD(P)H)-dependent reduction of a carbonyl group. A criteria of family membership is that the protein must have a measured function, and thus, genomic sequences suggesting the transcription of potential AKR proteins are considered pseudo-members until evidence of a functionally expressed protein is available. Currently, over 200 confirmed AKR superfamily members are reported to exist. A systematic nomenclature for the AKR superfamily exists to facilitate family and subfamily designations of the member to be communicated easily. Specifically, protein names include the root "AKR", followed by the family represented by an Arabic number, the subfamily-if one exists-represented by a letter, and finally, the individual member represented by an Arabic number. The AKR superfamily database has been dedicated to tracking and reporting the current knowledge of the AKRs since 1997, and the website was last updated in 2003. Here, we present an updated version of the website and database that were released in 2023. The database contains genetic, functional, and structural data drawn from various sources, while the website provides alignment information and family tree structure derived from bioinformatics analyses.

Pharmacological effects of methysticin and L-sulforaphane through the Nrf2/ARE signaling pathway in MLO-Y4 osteocytes: in vitro study

BACKGROUND

Oxidative stress plays a crucial role in the pathogenesis of many skeletal diseases by inducing osteocyte death. The transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) is a master regulator of various antioxidant gene expressions through antioxidant response element (ARE) against cellular oxidative stress and can be induced by various stimulants, including the phytochemicals methysticin (MET) and L-sulforaphane (SFN). This study aimed to establish an osteocyte in vitro model to investigate the pharmacological effects of MET and SFN on the Nrf2/ARE pathway.

METHODS

MLO-Y4 murine osteocytes and the stably transduced MLO-Y4-SIN-lenti-ARE reporter gene cell line were used. MET and SFN were used as Nrf2 inducers. The cytotoxicity of MET, SFN, and hydrogen peroxide (H2O2) was evaluated using the CytoTox-Glo™ Assay. Time- and dose-dependent ARE induction was examined by Monoluciferase Assay. The mRNA and protein expressions of Nrf2 target markers, such as heme-oxygenase 1 (Ho-1), NADPH quinone dehydrogenase 1 (Nqo1), and thioredoxin reductase 1 (Txnrd1), were detected by RT-qPCR, Western Blot, and immunofluorescence staining, respectively. Osteogenesis markers, osteopontin, and osteocalcin were compared with and without treatment by immunofluorescence staining.

RESULTS

The experimental data showed that MET and SFN induced ARE activity in a time- and dose-dependent manner and increased the mRNA and protein expression of antioxidant markers compared to vehicle-treated controls. The protein expression of osteopontin and osteocalcin in the samples treated with SFN were significantly higher than without treatment, and the number of cell death treated with SFN was significantly lower than without treatment under H2O2-induced stress conditions.

CONCLUSIONS

Nrf2 inducers MET and SFN increased the mRNA expression of antioxidant genes through the Nrf2/ARE pathway in osteocytes. Notably, SFN increased the protein expression of osteocyte-associated osteogenic markers and suppressed cell death under H2O2-induced stress condition. Thus, Nrf2 stimulators can exert stress-relieving and osteogenic effects on osteocytes.

Angular dihydropyranocoumarins from the flowers of Peucedanum japonicum and their aldo-keto reductase inhibitory activities

Twenty-one angular dihydropyranocoumarins and a linear furanocoumarin, including four previously undescribed compounds (1-4), were isolated from the flowers of Peucedanum japonicum (Umbelliferae). The structures of 1-4, along with their absolute stereochemistry, were determined to be (3'S,4'S)-3'-O-propanoyl-4'-O-(3‴-methyl-2‴-butenoyl)khellactone (1), (3'S,4'S)-3'-O-propanoyl-4'-O-(2‴-methyl-2‴Z-butenoyl)khellactone (2), (3'S,4'S)-3'-O-propanoyl-4'-O-(2‴-methylbutanoyl)khellactone (3), and (3'S,4'S)-3'-O-(2″-methylpropanoyl)-4'-O-(3‴-methyl-2‴-butenoyl)khellactone (4) using one- and two-dimensional nuclear magnetic resonance, high-resolution electrospray ionization mass spectroscopy, and electronic circular dichroism spectroscopy. In addition, the absolute configuration of the three angular dihydropyranocoumarins (5-7) was determined for the first time in this study. Among the previously reported compounds isolated in this study, 8 and 9 were isolated for the first time from the genus Peucedanum, whereas 10 and 11 were previously unreported and had not been isolated from P. japonicum to date. Furthermore, all isolated compounds were evaluated for their aldo-keto reductase 1C1 inhibitory activities on A549 human non-small-cell lung cancer cells. Compounds 10 and 12 exhibited substantial AKR1C1 inhibitory activities with IC50 values of 35.8 ± 0.9 and 44.2 ± 1.5 μM, respectively.

Nrf2 depletion in the context of loss-of-function Keap1 leads to mitolysosome accumulation

Transcription factor nuclear factor erythroid 2 p45-related factor 2 (Nrf2) is the principal determinant of the cellular redox homeostasis, contributing to mitochondrial function, integrity and bioenergetics. The main negative regulator of Nrf2 is Kelch-like ECH associated protein 1 (Keap1), a substrate adaptor for Cul3/Rbx1 ubiquitin ligase, which continuously targets Nrf2 for ubiquitination and proteasomal degradation. Loss-of-function mutations in Keap1 occur frequently in lung cancer, leading to constitutive Nrf2 activation. We used the human lung cancer cell line A549 and its CRISPR/Cas9-generated homozygous Nrf2-knockout (Nrf2-KO) counterpart to assess the role of Nrf2 on mitochondrial health. To confirm that the observed effects of Nrf2 deficiency are not due to clonal selection or long-term adaptation to the absence of Nrf2, we also depleted Nrf2 by siRNA (siNFE2L2), thus creating populations of Nrf2-knockdown (Nrf2-KD) A549 cells. Nrf2 deficiency decreased mitochondrial respiration, but increased the mitochondrial membrane potential, mass, DNA content, and the number of mitolysosomes. The proportion of ATG7 and ATG3 within their respective LC3B conjugates was increased in Nrf2-deficient cells with mutant Keap1, whereas the formation of new autophagosomes was not affected. Thus, in lung cancer cells with loss-of-function Keap1, Nrf2 facilitates mitolysosome degradation thereby ensuring timely clearance of damaged mitochondria.

The Multi-Faceted Consequences of NRF2 Activation throughout Carcinogenesis

The oxidative balance of a cell is maintained by the Kelch-like ECH-associated protein 1 (KEAP1)/nuclear factor erythroid 2-related factor 2 (NRF2) pathway. This cytoprotective pathway detoxifies reactive oxygen species and xenobiotics. The role of the KEAP1/NRF2 pathway as pro-tumorigenic or anti-tumorigenic throughout stages of carcinogenesis (including initiation, promotion, progression, and metastasis) is complex. This mini review focuses on key studies describing how the KEAP1/NRF2 pathway affects cancer at different phases. The data compiled suggest that the roles of KEAP1/NRF2 in cancer are highly dependent on context; specifically, the model used (carcinogen-induced vs genetic), the tumor type, and the stage of cancer. Moreover, emerging data suggests that KEAP1/NRF2 is also important for regulating the tumor microenvironment and how its effects are amplified either by epigenetics or in response to co-occurring mutations. Further elucidation of the complexity of this pathway is needed in order to develop novel pharmacological tools and drugs to improve patient outcomes.

Role of Human Aldo-Keto Reductases and Nuclear Factor Erythroid 2-Related Factor 2 in the Metabolic Activation of 1-Nitropyrene via Nitroreduction in Human Lung Cells

1-Nitropyrene (1-NP) is a constituent of diesel exhaust and classified as a group 2A probable human carcinogen. The metabolic activation of 1-NP by nitroreduction generates electrophiles that can covalently bind DNA to form mutations to contribute to cancer causation. NADPH-dependent P450 oxidoreductase (POR), xanthine oxidase (XO), aldehyde oxidase (AOX), and NAD(P)H/quinone oxidoreductase 1 (NQO1) may catalyze 1-NP nitroreduction. We recently found that human recombinant aldo-keto reductases (AKRs) 1C1-1C3 catalyze 1-NP nitroreduction. NQO1 and AKR1C1-1C3 are genes induced by nuclear factor erythroid 2-related factor 2 (NRF2). Despite this knowledge, the relative importance of these enzymes and NRF2 to 1-NP nitroreduction is unknown. We used a combination of pharmacological and genetic approaches to assess the relative importance of these enzymes and NRF2 in the aerobic nitroreduction of 1-NP in human bronchial epithelial cells, A549 and HBEC3-KT. 1-NP nitroreduction was assessed by the measurement of 1-aminopyrene (1-AP), the six-electron reduced metabolite of 1-NP, based on its intrinsic fluorescence properties (λex and λem). We found that co-treatment of 1-NP with salicylic acid, an AKR1C1 inhibitor, or ursodeoxycholate, an AKR1C2 inhibitor, for 48 h decreased 1-AP production relative to 1-NP treatment alone (control) in both cell lines. R-Sulforaphane or 1-(2-cyano-3,12,28-trioxooleana-1,9(11)-dien-28-yl)-1H-imidazole (CDDO-Im), two NRF2 activators, each increased 1-AP production relative to control only in HBEC3-KT cells, which have inducible NRF2. Inhibitors of POR, NQO1, and XO failed to modify 1-AP production relative to control in both cell lines. Importantly, A549 wild-type cells with constitutively active NRF2 produced more 1-AP than A549 cells with heterozygous expression of NFE2L2/NRF2, which were able to produce more 1-AP than A549 cells with homozygous knockout of NFE2L2/NRF2. Together, these data show dependence of 1-NP metabolic activation on AKR1Cs and NRF2 in human lung cells. This is the second example whereby NFE2L2/NRF2 is implicated in the carcinogenicity of diesel exhaust constituents.

Nitroreduction: A Critical Metabolic Pathway for Drugs, Environmental Pollutants, and Explosives

Nitro group containing xenobiotics include drugs, cancer chemotherapeutic agents, carcinogens (e.g., nitroarenes and aristolochic acid) and explosives. The nitro group undergoes a six-electron reduction to form sequentially the nitroso-, N-hydroxylamino- and amino-functional groups. These reactions are catalyzed by nitroreductases which, rather than being enzymes with this sole function, are enzymes hijacked for their propensity to donate electrons to the nitro group either one at a time via a radical mechanism or two at time via the equivalent of a hydride transfer. These enzymes include: NADPH-dependent flavoenzymes (NADPH: P450 oxidoreductase, NAD(P)H-quinone oxidoreductase), P450 enzymes, oxidases (aldehyde oxidase, xanthine oxidase) and aldo-keto reductases. The hydroxylamino group once formed can undergo conjugation reactions with acetate or sulfate catalyzed by N-acetyltransferases or sulfotransferases, respectively, leading to the formation of intermediates containing a good leaving group which in turn can generate a nitrenium or carbenium ion for covalent DNA adduct formation. The intermediates in the reduction sequence are also prone to oxidation and produce reactive oxygen species. As a consequence, many nitro-containing xenobiotics can be genotoxic either by forming stable covalent adducts or by oxidatively damaging DNA. This review will focus on the general chemistry of nitroreduction, the enzymes responsible, the reduction of xenobiotic substrates, the regulation of nitroreductases, the ability of nitrocompounds to form DNA adducts and act as mutagens as well as some future directions.

Intervening Effects and Molecular Mechanism of Quercitrin on PCV2-Induced Histone Acetylation, Oxidative Stress and Inflammatory Response in 3D4/2 Cells

Porcine circovirus type 2 (PCV2) is the main pathogen causing porcine circovirus-associated diseases (PCVD/PCVADs), and infection of the host induces immunosuppression. Since quercitrin (QUE) has anti-inflammatory and antiviral activity, it is worth exploiting in animal diseases. In this study, the interventional effects and the molecular mechanism of QUE on PCV2-induced oxidative stress and inflammatory responses in 3D4/2 cells and the modulation of histone acetylation modifications were investigated. The ROS production was measured by DCFH-DA fluorescent probes. HAT and HDAC enzyme activity were determined by ELISA. Histone acetylation, oxidative stress and inflammation-related gene expression levels were measured by q-PCR. Histone H3 and H4 (AcH3 and AcH4) acetylation, oxidative stress and inflammation-related protein expression levels were measured by Western blot. The results showed that QUE treatment at different concentrations on PCV2-infected 3D4/2 cells was able to attenuate the production of ROS. Moreover, QUE treatment could also intervene in oxidative stress and decrease the enzyme activity of HAT and the mRNA expression level of HAT1, while it increased the enzyme activity of HDAC and HDAC1 mRNA expression levels and downregulated histone H3 and H4 (AcH3 and AcH4) acetylation modification levels. In addition, QUE treatment even downregulated the mRNA expression levels of IL-6, IL-8, IκB, AKT and p38, but upregulated the mRNA expression levels of IL-10, SOD, GPx1, p65, Keap1, Nrf2, HO-1 and NQO1. As to protein expression, QUE treatment downregulated the levels of iNOS, p-p65 and IL-8 as well as the phosphorylation expression of IκB and p38, while it upregulated the levels of HO-1 and NQO1. It was shown that QUE at 25, 50 or 100 μmol/L regulated p38MAPK and PI3K/AKT signaling pathways by downregulating cellular histone acetylation modification levels while inhibiting the NF-κB inflammatory signaling pathway and activating the Nrf2/HO-1 antioxidant signaling pathway, thus regulating the production of inflammatory and antioxidant factors and exerting both anti-inflammatory and antioxidant effects.

The synthetic triterpenoids CDDO-TFEA and CDDO-Me, but not CDDO, promote nuclear exclusion of BACH1 impairing its activity

The transcription factor BACH1 is a potential therapeutic target for a variety of chronic conditions linked to oxidative stress and inflammation, as well as cancer metastasis. However, only a few BACH1 degraders/inhibitors have been described. BACH1 is a transcriptional repressor of heme oxygenase 1 (HMOX1), which is positively regulated by transcription factor NRF2 and is highly inducible by derivatives of the synthetic oleanane triterpenoid 2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oic acid (CDDO). Most of the therapeutic activities of these compounds are due to their anti-inflammatory and antioxidant properties, which are widely attributed to their ability to activate NRF2. However, with such a broad range of action, these compounds have other molecular targets that have not been fully identified and could also be of importance for their therapeutic profile. Herein we identified BACH1 as a target of two CDDO-derivatives (CDDO-Me and CDDO-TFEA), but not of CDDO. While both CDDO and CDDO-derivatives activate NRF2 similarly, only CDDO-Me and CDDO-TFEA inhibit BACH1, which explains the much higher potency of these CDDO-derivatives as HMOX1 inducers compared with unmodified CDDO. Notably, we demonstrate that CDDO-Me and CDDO-TFEA inhibit BACH1 via a novel mechanism that reduces BACH1 nuclear levels while accumulating its cytoplasmic form. In an in vitro model, both CDDO-derivatives impaired lung cancer cell invasion in a BACH1-dependent and NRF2-independent manner, while CDDO was inactive. Altogether, our study identifies CDDO-Me and CDDO-TFEA as dual KEAP1/BACH1 inhibitors, providing a rationale for further therapeutic uses of these drugs.

AKR1C1 connects autophagy and oxidative stress by interacting with SQSTM1 in a catalytic-independent manner

Targeting autophagy might be a promising anticancer strategy; however, the dual roles of autophagy in cancer development and malignancy remain unclear. NSCLC (non-small cell lung cancer) cells harbour high levels of SQSTM1 (sequestosome 1), the autophagy receptor that is critical for the dual roles of autophagy. Therefore, mechanistic insights into SQSTM1 modulation may point towards better approaches to treat NSCLC. Herein, we used multiple autophagy flux models and autophagy readouts to show that aldo-keto reductase family 1 member C1 (AKR1C1), which is highly expressed in NSCLC, promotes autophagy by directly binding to SQSTM1 in a catalytic-independent manner. This interaction may be strengthened by reactive oxygen species (ROS), important autophagy inducers. Further mechanistic research demonstrated that AKR1C1 interacts with SQSTM1 to augment SQSTM1 oligomerization, contributing to the SQSTM1 affinity for binding cargo. Collectively, our data reveal a catalytic-independent role of AKR1C1 for interacting with SQSTM1 and promoting autophagy. All these findings not only reveal a novel functional role of AKR1C1 in the autophagy process but also indicate that modulation of the AKR1C1-SQSTM1 interaction may be a new strategy for targeting autophagy.

Genetic and epigenetic regulation of the NRF2-KEAP1 pathway in human lung cancer

Electrophilic and oxidative stress is caused when homeostatic mechanisms are disrupted. A major defense mechanism involves the activation of the nuclear factor erythroid 2-related factor 2 (NRF2) transcription factor encoded by the NFE2L2 gene, which can accelerate the detoxification of electrophilic carcinogens and prevent cancer and on the other hand in certain exposure contexts may exacerbate the carcinogenic process. NRF2-target genes activated under these conditions can be used as biomarkers of stress signalling, while activation of NRF2 can also reveal the epigenetic mechanisms that modulate NFE2L2 expression. Epigenetic mechanisms that regulate NFE2L2 and the gene for its adaptor protein KEAP1 include DNA methylation, histone modifications and microRNA. Understanding the activation of the NRF2-KEAP1 signalling pathway in human lung cancer, its epigenetic regulation and its role in oncogenesis is the subject of this review.

Aldo-Keto Reductases and Cancer Drug Resistance

Human aldo-keto reductases (AKRs) catalyze the NADPH-dependent reduction of carbonyl groups to alcohols for conjugation reactions to proceed. They are implicated in resistance to cancer chemotherapeutic agents either because they are directly involved in their metabolism or help eradicate the cellular stress created by these agents (e.g., reactive oxygen species and lipid peroxides). Furthermore, this cellular stress activates the Nuclear factor-erythroid 2 p45-related factor 2 (NRF2)-Kelch-like ECH-associated protein 1 pathway. As many human AKR genes are upregulated by the NRF2 transcription factor, this leads to a feed-forward mechanism to enhance drug resistance. Resistance to major classes of chemotherapeutic agents (anthracyclines, mitomycin, cis-platin, antitubulin agents, vinca alkaloids, and cyclophosphamide) occurs by this mechanism. Human AKRs also catalyze the synthesis of androgens and estrogens and the elimination of progestogens and are involved in hormonal-dependent malignancies. They are upregulated by antihormonal therapy providing a second mechanism for cancer drug resistance. Inhibitors of the NRF2 system or pan-AKR1C inhibitors offer promise to surmount cancer drug resistance and/or synergize the effects of existing drugs. SIGNIFICANCE STATEMENT: Aldo-keto  reductases (AKRs) are overexpressed in a large number of human tumors and mediate resistance to cancer chemotherapeutics and antihormonal therapies. Existing drugs and new agents in development may surmount this resistance by acting as specific AKR isoforms or AKR pan-inhibitors to improve clinical outcome.

Peroxisome Proliferator-Activated Receptor-Gamma (PPAR-ɣ): Molecular Effects and Its Importance as a Novel Therapeutic Target for Cerebral Ischemic Injury

Cerebral ischemic injury is a leading cause of death and long-term disability throughout the world. Peroxisome proliferator-activated receptor gamma (PPAR-ɣ) is a ligand-activated nuclear transcription factor that is a member of the PPAR family. PPAR-ɣ has been shown in several in vitro and in vivo models to prevent post-ischemic inflammation and neuronal damage by negatively controlling the expression of genes modulated by cerebral ischemic injury, indicating a neuroprotective effect during cerebral ischemic injury. A extensive literature review of PubMed, Medline, Bentham, Scopus, and EMBASE (Elsevier) databases was carried out to understand the nature of the extensive work done on the mechanistic role of Peroxisome proliferator activated receptor gamma and its modulation in Cerebral ischemic injury. PPAR-ɣ can interact with specific DNA response elements to control gene transcription and expression when triggered by its ligand. It regulates lipid metabolism, improves insulin sensitivity, modulates antitumor mechanisms, reduces oxidative stress, and inhibits inflammation. This review article provides insights on the current state of research into the neuroprotective effects of PPAR-ɣ in cerebral ischemic injury, as well as the cellular and molecular mechanisms by which these effects are modulated, such as inhibition of inflammation, reduction of oxidative stress, suppression of pro-apoptotic production, modulation of transcription factors, and restoration of injured tissue through neurogenesis and angiogenesis.

Modulating glutathione thiol status alters pancreatic β-cell morphogenesis in the developing zebrafish (Danio rerio) embryo

Emerging evidence suggests that redox-active chemicals perturb pancreatic islet development. To better understand potential mechanisms for this, we used zebrafish (Danio rerio) embryos to investigate roles of glutathione (GSH; predominant cellular redox buffer) and the transcription factor Nrf2a (Nfe2l2a; zebrafish Nrf2 co-ortholog) in islet morphogenesis. We delineated critical windows of susceptibility to redox disruption of β-cell morphogenesis, interrogating embryos at 24, 48 and 72 h post fertilization (hpf) and visualized Nrf2a expression in the pancreas using whole-mount immunohistochemistry at 96 hpf. Chemical GSH modulation at 48 hpf induced significant islet morphology changes at 96 hpf. Pro-oxidant exposures to tert-butylhydroperoxide (77.6 μM; 10-min at 48 hpf) or tert-butylhydroquinone (1 μM; 48-56 hpf) decreased β-cell cluster area at 96 hpf. Conversely, exposures to antioxidant N-acetylcysteine (bolsters GSH pools; 100 μM; 48-72 hpf) or sulforaphane (activates Nrf2a; 20 μM; 48-72 hpf) significantly increased islet areas. Nrf2a was also stabilized in β-cells: 10-min exposures to 77.6 μM tert-butylhydroperoxide significantly increased Nrf2a protein compared to control islet cells that largely lack stabilized Nrf2a; 10-min exposures to higher (776 μM) tert-butylhydroperoxide concentration stabilized Nrf2a throughout the pancreas. Using biotinylated-GSH to visualize in situ protein glutathionylation, islet cells displayed high protein glutathionylation, indicating oxidized GSH pools. The 10-min high (776 μM) tert-butylhydroperoxide exposure (induced Nrf2a globally) decreased global protein glutathionylation at 96 hpf. Mutant fish expressing inactive Nrf2a were protected against tert-butylhydroperoxide-induced abnormal islet morphology. Our data indicate that disrupted redox homeostasis and Nrf2a stabilization during pancreatic β-cell development impact morphogenesis, with implications for disease states at later life stages. Our work identifies a potential molecular target (Nrf2) that mediates abnormal β-cell morphology in response to redox disruptions. Moreover, our findings imply that developmental exposure to exogenous stressors at distinct windows of susceptibility could diminish the reserve redox capacity of β-cells, rendering them vulnerable to later-life stresses and disease.

Nitrative DNA damage in lung epithelial cells exposed to indium nanoparticles and indium ions

Indium compounds have been widely used in manufacturing displays of mobile phones, computers and televisions. However, inhalation exposure to indium compounds causes interstitial pneumonia in exposed workers and lung cancer in experimental animals. 8-Nitroguanine (8-nitroG) is a mutagenic DNA lesion formed under inflammatory conditions and may participate in indium-induced carcinogenesis. In this study, we examined 8-nitroG formation in A549 cultured human lung epithelial cells treated with indium compounds, including nanoparticles of indium oxide (In₂O₃) and indium-tin oxide (ITO), and indium chloride (InCl₃). We performed fluorescent immunocytochemistry to examine 8-nitroG formation in indium-exposed A549 cells. All indium compounds significantly increased 8-nitroG formation in A549 cells at 5 ng/ml after 4 h incubation. 8-NitroG formation was largely reduced by 1400 W, methyl-β-cyclodextrin (MBCD) and monodansylcadaverine (MDC), suggesting the involvement of nitric oxide synthase and endocytosis. 8-NitroG formation in A549 cells was also largely suppressed by small interfering RNA (siRNA) for high-mobility group box-1 (HMGB1), receptor for advanced glycation and end products (AGER, RAGE) and Toll-like receptor 9 (TLR9). These results suggest that indium compounds induce inflammation-mediated DNA damage in lung epithelial cells via the HMGB1-RAGE-TLR9 pathway. This mechanism may contribute to indium-induced genotoxicity in the respiratory system.