Effects of arsenic on modification of promyelocytic leukemia (PML): PML responds to low levels of arsenite

Mutations at proximal cysteine residues in PML impair ATO binding by destabilizing the RBCC domain

Acute promyelocytic leukemia (APL) is characterized by the fusion gene promyelocytic leukemia-retinoic acid receptor-alpha (PML-RARA) and is conventionally treated with arsenic trioxide (ATO). ATO binds directly to the RING finger, B-box, coiled-coil (RBCC) domain of PML and initiates degradation of the fusion oncoprotein PML-RARA. However, the mutational hotspot at C212-S220 disrupts ATO binding, leading to drug resistance in APL. Therefore, structural consequences of these point mutations in PML that remain uncertain require comprehensive analysis. In this study, we investigated the structure-based ensemble properties of the promyelocytic leukemia-RING-B-box-coiled-coil (PML-RBCC) domains and ATO-resistant mutations. Oligomeric studies reveal that PML-RBCC wild-type and mutants C212R, S214L, A216T, L217F, and S220G predominantly form tetramers, whereas mutants C213R, A216V, L218P, and D219H tend to form dimers. The stability of the dimeric mutants was lower, exhibiting a melting temperature (Tm) reduction of 30 °C compared with the tetrameric mutants and wild-type PML protein. Furthermore, the exposed surface of the C213R mutation rendered it more prone to protease digestion than that of the C212R mutation. The spectroscopic analysis highlighted ATO-induced structural alterations in S214L, A216V, and D219H mutants, in contrast to C213R, L217F, and L218P mutations. Moreover, the computational analysis revealed that the ATO-resistant mutations C213R, A216V, L217F, and L218P caused changes in the size, shape, and flexibility of the PML-RBCC wild-type protein. The mutations C213R, A216V, L217F, and L218P destabilize the wild-type protein structure due to the adaptation of distinct conformational changes. In addition, these mutations disrupt several hydrogen bonds, including interactions involving C212, C213, and C215, which are essential for ATO binding. The local and global structural features induced by these mutations provide mechanistic insight into ATO resistance and APL pathogenesis.

Effects of arsenic on the topology and solubility of promyelocytic leukemia (PML)-nuclear bodies

Promyelocytic leukemia (PML) proteins are involved in the pathogenesis of acute promyelocytic leukemia (APL). Trivalent arsenic (As3+) is known to cure APL by binding to cysteine residues of PML and enhance the degradation of PML-retinoic acid receptor α (RARα), a t(15;17) gene translocation product in APL cells, and restore PML-nuclear bodies (NBs). The size, number, and shape of PML-NBs vary among cell types and during cell division. However, topological changes of PML-NBs in As3+-exposed cells have not been well-documented. We report that As3+-induced solubility shift underlies rapid SUMOylation of PML and late agglomeration of PML-NBs. Most PML-NBs were toroidal and granular dot-like in GFPPML-transduced CHO-K1 and HEK293 cells, respectively. Exposure to As3+ and antimony (Sb3+) greatly reduced the solubility of PML and enhanced SUMOylation within 2 h in the absence of changes in the number and size of PML-NBs. However, the prolonged exposure to As3+ and Sb3+ resulted in agglomeration of PML-NBs. Exposure to bismuth (Bi3+), another Group 15 element, did not induce any of these changes. ML792, a SUMO activation inhibitor, reduced the number of PML-NBs and increased the size of the NBs, but had little effect on the As3+-induced solubility change of PML. These results warrant the importance of As3+- or Sb3+-induced solubility shift of PML for the regulation intranuclear dynamics of PML-NBs.

SUMOylation regulates the number and size of promyelocytic leukemia-nuclear bodies (PML-NBs) and arsenic perturbs SUMO dynamics on PML by insolubilizing PML in THP-1 cells

The functional roles of protein modification by small ubiquitin-like modifier (SUMO) proteins are not well understood compared to ubiquitination. Promyelocytic leukemia (PML) proteins are good substrates for SUMOylation, and PML-nuclear bodies (PML-NBs) may function as a platform for the PML SUMOylation. PML proteins are rapidly modified both with SUMO2/3 and SUMO1 after exposure to arsenite (As³⁺) and SUMOylated PML are further ubiquitinated and degraded by proteasomes. However, effects of As³⁺ on SUMO dynamics on PML-NBs are not well investigated. In the present study, we report that (1) the number and size of PML-NBs were regulated by SUMO E1-activating enzyme, (2) SUMO2/3 co-localized with PML irrespective of As³⁺ exposure and was restricted to PML-nuclear bodies (PML-NBs) via covalent binding in response to As³⁺, and (3) As³⁺-induced biochemical changes in PML were not modulated by ubiquitin-proteasome system (UPS) in THP-1 cells. Undifferentiated and differentiated THP-1 cells responded to As³⁺ similarly and PML proteins were changed from the detergent soluble to the insoluble form and further SUMOylated with SUMO2/3 and SUMO1. ML792, a SUMO E1 inhibitor, decreased the number of PML-NBs and reciprocally increased the size irrespective of exposure to As³⁺, which itself slightly decrease both the number and size of PML-NBs. TAK243, a ubiquitin E1 inhibitor, did not change the PML-NBs, while SUMOylated proteins accumulated in the TAK243-exposed cells. Proteasome inhibitors did not change the As³⁺-induced SUMOylation levels of PML. Co-localization and further restriction of SUMO2/3 to PML-NBs were confirmed by PML-transfected CHO-K1 cells. Collectively, SUMOylation regulates PML-NBs and As³⁺ restricts SUMO dynamics on PML by changing its solubility.

Biotransformation of arsenic and toxicological implication of arsenic metabolites

Arsenic is a well-known environmental carcinogen and chronic exposure to arsenic through drinking water has been reported to cause skin, bladder and lung cancers, with arsenic metabolites being implicated in the pathogenesis. In contrast, arsenic trioxide (As₂O₃) is an effective therapeutic agent for the treatment of acute promyelocytic leukemia, in which the binding of arsenite (iAs^III) to promyelocytic leukemia (PML) protein is the proposed initial step. These findings on the two-edged sword characteristics of arsenic suggest that after entry into cells, arsenic reaches the nucleus and triggers various nuclear events. Arsenic is reduced, conjugated with glutathione, and methylated in the cytosol. These biotransformations, including the production of reactive metabolic intermediates, appear to determine the intracellular dynamics, target organs, and biological functions of arsenic.

Cellular uptake of paraquat determines subsequent toxicity including mitochondrial damage in lung epithelial cells

Paraquat (PQ) is one of the commonly used herbicides in the world, despite its high toxicity. The ingestion of PQ accidentally or intentionally causes severe damage in diverse organs including the lung. Pulmonary fibrosis triggered by PQ accumulation in the lung epithelial cells is one of the major causes of death. This study investigated the intracellular accumulation of PQ, reactive oxygen species (ROS) generation and mitochondrial injury using two lung epithelial cell lines A549 and BEAS-2B (BEAS). Although A549 exhibit greater resistance to oxidative stress than BEAS, a cytotoxicity assay for PQ demonstrated that EC₅₀ for lethality in A549 was 7 times lower than that in BEAS. When exposed to PQ at a concentration around EC₅₀ for lethality, the amount of ROS generated in A549 was as low as that in BEAS. Conversely, the cellular concentration of PQ in A549 after exposure was higher than that in BEAS, which suggests a distinct difference in the susceptibility to PQ between these cell lines. After a 16 h exposure to PQ, mitochondrial membrane potential (MMP) decreased in A549, but decreased only slightly in BEAS even following a 30 h exposure. PQ-exposed A549 reduced an accumulation of PTEN-induced kinase 1 (PINK1), which works in degradation of damaged mitochondria, following the decrease of MMP, whereas PQ did not decline the PINK1 in BEAS. These results suggest that mitochondrial dysfunction due to cellular accumulation of PQ might contribute to the PQ-provoked toxicity more than the ROS generation in the lung epithelial cells.

Ataxia telangiectasia and rad3 related (ATR)-promyelocytic leukemia protein (PML) pathway of the DNA damage response in the brain of rats administered arsenic trioxide

To examine the in vivo responses of promyelocytic leukemia protein (PML) to arsenic, rats (male, 6 weeks old, Sprague Dawley) were administered a single intraperitoneal dose of 5 mg/kg arsenic trioxide (ATO). The protein was examined in the heart, lung, liver, and brain 6 and 48 hours after administration: a significant response of PML was observed in the brain. Oxidative DNA modification was also observed in the brain as revealed by increased immunoreactivity to anti-8-hydroxy-2’-deoxyguanosine (8-OHdG) antibody. In contrast, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) stain reactivity was only slightly increased, suggesting oxidative cellular stress without apoptotic cell death in the ATO-administered rat brain. Among the DNA damage response pathways, the ATR-Chk1 axis was activated, while the ATM-Chk2 axis was not, implying that the PML response is associated with activation of the ATR-Chk1 DNA repair pathway in the brain.

Phosphate enhances Fgf23 expression through reactive oxygen species in UMR-106 cells

Fibroblast growth factor 23 (FGF23) has been shown to work as a phosphotropic hormone. Although FGF23 reduces the serum phosphate level, it has not been established that phosphate directly regulates FGF23 production. In this study, we investigated whether phosphate can enhance Fgf23 expression using the rat osteoblastic cell line UMR-106, which has been shown to express Fgf23 in response to 1,25-dihydroxyvitamin D [1,25(OH)2D]. Phosphate increased Fgf23 expression in a dose- and time-dependent manner in the presence of 1,25(OH)2D. Phosphate also increased Fgf23 promoter activity, but showed no effect on the half-life of Fgf23 messenger RNA. Phosphonoformic acid and PD98059, an inhibitor of MEK, inhibited the effects of phosphate on Fgf23 expression and promoter activity. In addition, phosphate enhanced production of reactive oxygen species (ROS) in UMR-106 cells, and hydrogen peroxide enhanced FGF23 production in a dose- and time-dependent manner. Hydrogen peroxide also enhanced Elk1 reporter activity, a target of the MEK-extracellular-signal-regulated kinase (ERK) pathway. Furthermore, the effect of phosphate on ROS production and Fgf23 expression was inhibited by apocynin, an inhibitor of NADPH oxidase. These results indicate that phosphate directly enhances Fgf23 transcription without affecting the stability of Fgf23 messenger RNA by stimulating NADPH-induced ROS production and the MEK-ERK pathway in UMR-106 cells.