MicroRNA regulatory networks reflective of polyhexamethylene guanidine phosphate-induced fibrosis in A549 human alveolar adenocarcinoma cells

Synergistic interaction of co-exposure to humidifier disinfectant chemicals CMIT/MIT and PHMG in lung injury

A number of biocidal disinfectant chemicals are used as household products to prevent spread of pathogens. People are commonly exposed to multiple chemicals through those disinfectants. However, effects of interactions (e.g., synergism) between disinfectants on human health outcomes have been rarely studied. In this study, we aimed to investigate associations of a mixture of chloromethylisothiazolinone/methylisothiazolinone (CMIT/MIT) and polyhexamethylene guanidine (PHMG), which had been used as humidifier disinfectants (HDs) in South Korea, with HD-associated lung injury (HDLI) in a Korean population (n = 4058) with HD exposure through use of HD products. Exposure to HD was retrospectively assessed by an interview-based standardized survey, and HDLI was determined by clinical assessment. After adjusting for covariates, PHMG-specific exposure indices (e.g., amount of use, indoor air concentration, and weekly exposure level) were dose-dependently associated with HDLI (their odds ratios for the comparison of third tertile versus first tertile were 1.95, 1.77, and 2.16, respectively). CMIT/MIT exposure was not observed to have a significant association with HDLI in a single chemical exposure model; however, associations between PHMG exposure and HDLI were strengthened by co-exposure to CMIT/MIT in combined chemical exposure models, where synergistic interactions between CMIT/MIT use and PHMG indices (amount of use and weekly exposure level) were observed (p-interaction in additive scale: 0.02 and 0.03, respectively). Our findings imply that adverse effects of PHMG exposure on lung injury among HD users might be worsened by co-exposure to CMIT/MIT. Given that plenty of household products contain disinfectants on global markets, epidemiological and toxicological investigations are warranted on interaction effects of co-exposure to disinfectants.

A multiplex inhalation platform to model in situ like aerosol delivery in a breathing lung-on-chip

Prolonged exposure to environmental respirable toxicants can lead to the development and worsening of severe respiratory diseases such as asthma, chronic obstructive pulmonary disease (COPD) and fibrosis. The limited number of FDA-approved inhaled drugs for these serious lung conditions has led to a shift from in vivo towards the use of alternative in vitro human-relevant models to better predict the toxicity of inhaled particles in preclinical research. While there are several inhalation exposure models for the upper airways, the fragile and dynamic nature of the alveolar microenvironment has limited the development of reproducible exposure models for the distal lung. Here, we present a mechanistic approach using a new generation of exposure systems, the Cloud α AX12. This novel in vitro inhalation tool consists of a cloud-based exposure chamber (VITROCELL) that integrates the breathing ^AXLung-on-chip system (AlveoliX). The ultrathin and porous membrane of the AX12 plate was used to create a complex multicellular model that enables key physiological culture conditions: the air-liquid interface (ALI) and the three-dimensional cyclic stretch (CS). Human-relevant cellular models were established for a) the distal alveolar-capillary interface using primary cell-derived immortalized alveolar epithelial cells (^AXiAECs), macrophages (THP-1) and endothelial (HLMVEC) cells, and b) the upper-airways using Calu3 cells. Primary human alveolar epithelial cells (^AXhAEpCs) were used to validate the toxicity results obtained from the immortalized cell lines. To mimic in vivo relevant aerosol exposures with the Cloud α AX12, three different models were established using: a) titanium dioxide (TiO2) and zinc oxide nanoparticles b) polyhexamethylene guanidine a toxic chemical and c) an anti-inflammatory inhaled corticosteroid, fluticasone propionate (FL). Our results suggest an important synergistic effect on the air-blood barrier sensitivity, cytotoxicity and inflammation, when air-liquid interface and cyclic stretch culture conditions are combined. To the best of our knowledge, this is the first time that an in vitro inhalation exposure system for the distal lung has been described with a breathing lung-on-chip technology. The Cloud α AX12 model thus represents a state-of-the-art pre-clinical tool to study inhalation toxicity risks, drug safety and efficacy.

Health Effects Associated With Humidifier Disinfectant Use: A Systematic Review for Exploration

BACKGROUND

It has been 10 years since the outbreak of lung disease caused by humidifier disinfectants in Korea, but the health effects have not yet been summarized. Therefore, this study aims to systematically examine the health effects of humidifier disinfectants that have been discovered so far.

METHODS

All literature with humidifier disinfectants and their representative components as the main words were collected based on the web, including PubMed, Research Information Sharing Service, and government publication reports. A total of 902 studies were searched, of which 196 were selected. They were divided into four groups: published human studies (group 1), published animal and cytotoxicology studies (group 2), technical reports (group 3), and gray literature (group 4).

RESULTS

Out of the 196 studies, 97 (49.5%) were published in peer-reviewed journals as original research. Group 1 consisted of 49 articles (50.5%), while group 2 consisted of 48 articles (49.5%). Overall, respiratory diseases such as humidifier disinfectant associated lung injury, interstitial lung disease, and asthma have a clear correlation, but other effects such as liver, heart, thymus, thyroid, fetal growth, metabolic abnormalities, and eyes are observed in toxicological experimental studies, but have not yet been identified in epidemiologic studies.

CONCLUSION

The current level of evidence does not completely rule out the effects of humidifier disinfectants on extrapulmonary disease. Based on the toxicological evidence so far, it is required to monitor the population of humidifier disinfectant exposure continuously to see if similar damage occurs.

Reprogrammed lung epithelial cells by decrease of miR-451a in extracellular vesicles contribute to aggravation of pulmonary fibrosis

Extracellular vesicles (EVs) play novel roles in homeostasis through cell-to-cell communication in human airways via transferring miRNAs. However, the contribution of EV miRNAs to pulmonary phenotypic homeostasis is not clearly understood. Hence, the aim of this study was to elucidate the functional role of miRNAs obtained from epithelium-derived EVs in lung fibrogenesis. Pulmonary fibrosis was induced by exposure of polyhexamethylene guanidine phosphate (PHMG-p)-instilled mice. In histopathological changes, a clear phenotypic change was observed in bronchial epithelium. For figuring out the role of EVs derived from conditioned media of untreated cells (EV-Con) and PHMG-p-treated BEAS-2B (EV-PHMG), significant increase in EVs released from PHMG-p-treated BEAS-2B was detected. Functional analysis with targets of differentially expressed miRNAs in EVs was annotated to epithelial-mesenchymal transition (EMT). Especially, the most abundant miRNA, miR-451a, was downregulated in EV of PHMG-p-treated BEAS-2B cells. We found that odd-skipped related 1 (OSR1) was a putative target for miR-451a, which had been known as a transcription factor of several fibrosis-associated genes. Transfer of decreased miR-451a via EV-PHMG upregulated OSR1 and induced EMT compared to Con-EV-treated cells. In pulmonary fibrosis mice, miR-451a levels were significantly reduced in EV derived from bronchoalveolar lavage fluid and OSR1 expression was increased in lung tissues of mice with PHMG-p exposure. MiR-451a-transfected EVs markedly alleviated fibrogenesis in the PHMG-p-exposed lungs. Low level of miR-451a in EVs modulated EMT and fibrogenesis in recipient cells by increasing OSR1 levels in vitro and in vivo. Our results suggest that transferring EV miR-451a induces anti-fibrotic autocrine effect by downregulating its target, OSR1 maintaining pulmonary homeostasis disrupted by PHMG-p exposure, which can be a potential therapeutic target.

MTF1 Is Essential for the Expression of MT1B, MT1F, MT1G, and MT1H Induced by PHMG, but Not CMIT, in the Human Pulmonary Alveolar Epithelial Cells

The inhalation of humidifier disinfectants (HDs) is linked to HD-associated lung injury (HDLI). Polyhexamethylene guanidine (PHMG) is significantly involved in HDLI, but the correlation between chloromethylisothiazolinone (CMIT) and HDLI remains ambiguous. Additionally, the differences in the molecular responses to PHMG and CMIT are poorly understood. In this study, RNA sequencing (RNA-seq) data showed that the expression levels of metallothionein-1 (MT1) isoforms, including MT1B, MT1E, MT1F, MT1G, MT1H, MT1M, and MT1X, were increased in human pulmonary alveolar epithelial cells (HPAEpiCs) that were treated with PHMG but not in those treated with CMIT. Moreover, upregulation of MT1B, MT1F, MT1G, and MT1H was observed only in PHMG-treated HPAEpiCs. The protein expression level of metal regulatory transcription factor 1 (MTF1), which binds to the promoters of MT1 isoforms, was increased in PHMG-treated HPAEpiCs but not in CMIT-treated HPAEpiCs. However, the expression of early growth response 1 (EGR1) and nuclear receptor superfamily 3, group C, member 1 (NR3C1), other transcriptional regulators involved in MT1 isomers, were increased regardless of treatment with PHMG or CMIT. These results suggest that MTF1 is an essential transcription factor for the induction of MT1B, MT1F, MT1G, and MT1H by PHMG but not by CMIT.

Critical window of exposure of CMIT/MIT with respect to developmental effects on zebrafish embryos: Multi-level endpoint and proteomics analysis

Systemic toxicity, particularly, developmental defects of humidifier disinfectant chemicals that have caused lung injuries in Korean children, remains to be elucidated. This study evaluated the mechanisms of the adverse effects of 5-chloro-2-methyl-4-isothiazoline-3-one/2methyl-4-isothiazolin-3-one (CMIT/MIT), one of the main biocides of the Korean tragedy, and identify the most susceptible developmental stage when exposed in early life. To this end, the study was designed to analyze several endpoints (morphology, heart rate, behavior, global DNA methylation, gene expressions of DNA methyl-transferases (dnmts) and protein profiling) in exposed zebrafish (Danio rerio) embryos at various developmental stages. The results showed that CMIT/MIT exposure causes bent tail, pericardial edema, altered heart rates, global DNA hypermethylation and significant alterations in the locomotion behavior. Consistent with the morphological and physiological endpoints, proteomics profiling with bioinformatics analysis suggested that the suppression of cardiac muscle contractions and energy metabolism (oxidative phosphorylation) were possible pivotal underlying mechanisms of the CMIT/MIT mediated adverse effects. Briefly, multi-level endpoint analysis indicated the most susceptible window of exposure to be ≤ 6 hpf followed by ≤ 48 hpf for CMIT/MIT. These results could potentially be translated to a risk assessment of the developmental exposure effects to the humidifier disinfectants.

Prediction of acute inhalation toxicity using cytotoxicity data from human lung epithelial cell lines

Recent research on in vitro systems has focused on mimicking the in vivo situation of cells within the respiratory system. However, few studies have predicted inhalation toxicity using conventional and simple submerged two-dimensional (2D) cell culture models. We investigated the conventional submerged 2-D cell culture model as a method for the prediction of acute inhalation toxicity. Median lethal concentration (LC₅₀ ) (rat, inhalation, 4 h) and half maximal inhibitory concentration (IC₅₀ ) (lung or bronchial cell, 24 h) data for 59 substances were obtained from the literature and by experiments. Cytotoxicity assays were performed on 44 substances with reported LC₅₀ , but without IC₅₀ , data to obtain the IC₅₀ values. A weak correlation was observed between the IC₅₀ and LC₅₀ of all substances. Semi-volatile organic compounds (SVOCs) and non-VOCs (NVOCs) (16 substances) with a water solubility of ≥1 g/L were strongly correlated between 24-h IC₅₀ and 4-h LC₅₀ , and this had an excellent predictive ability to distinguish between Categories 1-3 and 4 (Globally Harmonized System classification for acute inhalation toxicity). Our results suggest that the submerged 2-D cell culture model may be used to predict in vivo acute inhalation toxicity for substances with a water solubility of ≥1 g/L in SVOCs and NVOCs.

The past, present, and future of humidifier disinfectant-associated interstitial lung diseases in children

Exposure to environmental factors can cause interstitial lung diseases (ILDs); however, such types of ILDs are rare. From 2007 to 2011, an ILD epidemic occurred in South Korea owing to inhalational exposure to toxic chemicals in humidifier disinfectants (HDs). HD-associated ILDs (HD-ILDs) are characterized by rapidly progressing respiratory failure with pulmonary fibrosis and a high mortality rate of 43.8%-58.0%. Although 18.1%-31.1% of the general population used HDs, only a small proportion of HD users were diagnosed with HD-ILDs. This finding suggests that investigation of the pathophysiologies underlying HD-ILDs is needed in addition to the identification of susceptibility to HD-ILDs. Further, there have been several concerns regarding the diverse health effects of exposure to toxic chemicals in HDs, including those that have not been identified, and long-term prognoses in terms of pulmonary function and residual pulmonary lesions observed on follow-up chest images. In this review, we summarize the clinical features, pathologic findings, and changes in radiologic findings over time in patients with HD-ILDs and the results of previous experimental research on the mechanisms underlying the effects of toxic chemicals in HDs. Studies are currently underway to identify the pathophysiologies of HD-ILDs and possible health effects of exposure to HDs along with the development of targeted therapeutic strategies. The experience of identification of HD-ILDs has encouraged stricter control of safe chemicals in everyday life.

MicroRNAs and Xenobiotic Toxicity: An Overview

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miRNAs are key regulators of gene expression at both transcription and translation.

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The role of miRNAs in xenobiotic toxicity and its potential as biomarkers are being explored.

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In spite of numerous studies, the complex mechanism of miRNA biogenesis and its regulation remains unclear.

The advent of new technologies has paved the rise of various chemicals that are being employed in industrial as well as consumer products. This leads to the accumulation of these xenobiotic compounds in the environment where they pose a serious threat to both target and non-target species. miRNAs are one of the key epigenetic mechanisms that have been associated with toxicity by modulating the gene expression post-transcriptionally. Here, we provide a comprehensive view on miRNA biogenesis, their mechanism of action and, their possible role in xenobiotic toxicity. Further, we review the recent in vitro and in vivo studies involved in xenobiotic exposure induced miRNA alterations and the mRNA-miRNA interactions. Finally, we address the challenges associated with the miRNAs in toxicological studies.

NOTCH1 Pathway is Involved in Polyhexamethylene Guanidine-Induced Humidifier Disinfectant Lung Injuries

An outbreak of fatal humidifier disinfectant lung injuries (HDLI) occurred in Korea. Human studies on mechanisms underlying HDLI have yet to be conducted. This study aimed to investigate methylation changes and their potential role in HDLI after exposure to HDs containing polyhexamethylene guanidine-phosphate. DNA methylation analysis was performed in blood samples from 10 children with HDLI and 10 healthy children using Infinium Human MethylationEPIC BeadChip. Transcriptome analysis was performed using lung tissues from 5 children with HDLI and 5 controls. Compared to healthy controls, 92 hypo-methylated and 79 hyper-methylated CpG sites were identified in children with HDLI at the statistical significance level of |Δβ|>0.2 and p<0.05. NOTCH1 was identified as a candidate network hub gene in cases. NOTCH1 transcripts significantly increased in lung tissues from HDLI cases compared to unexposed controls (p=0.05). NOTCH1 may play an important role in pulmonary fibrosis of HDLI.

Exposure to humidifier disinfectants induces developmental effects and disrupts thyroid endocrine systems in zebrafish larvae

Humidifier disinfectants have been widely used in Korea to prevent the growth of microorganisms in humidifier water. However, their use has been banned since 2011 after epidemiological studies reported humidifier disinfectant induced lung injury. In the present study, the developmental effects of exposure to two humidifier disinfectants (Oxy® and Wiselect) and their main component, polyhexamethylene guanidine (PHMG)-phosphate, were investigated in zebrafish embryos/larvae for seven days. The effects on triiodothyronine (T3) and thyroxine (T4) hormones, reactive oxygen species (ROS) generation, antioxidant enzyme activities, and changes in expression of the genes related to the hypothalamus-pituitary-thyroid (HPT) axis and oxidative stress were also investigated. Zebrafish embryos exposed to the highest concentration (amounts recommended for use by the manufacturers) of all tested humidifier disinfectants showed an increase in embryo coagulation, leading to death without hatching. Exposure to Oxy® and Wiselect resulted in significantly decreased body length, increased ROS generation and antioxidant enzyme activities, decreased T4, and up-regulated genes related to the HPT axis (trh, trβ, and tpo) and oxidative damage (sod2 and gpx1b). The humidifier disinfectants and PHMG-phosphate could induce oxidative stress and disrupt thyroid hormone systems in zebrafish, leading to developmental retardation when used at sub-lethal concentrations. Potential effects of long-term exposure to humidifier disinfectants and mixture effects of several major components deserve further investigation.

Polyhexamethyleneguanidine Phosphate-Induced Cytotoxicity in Liver Cells Is Alleviated by Tauroursodeoxycholic Acid (TUDCA) via a Reduction in Endoplasmic Reticulum Stress

Polyhexamethyleneguanidine phosphate (PHMG-P) is a widely used polymeric antimicrobial agent known to induce significant pulmonary toxicity. Several studies have reported that the liver also can be a target organ of polyhexamethyleneguanidine (PHMG) toxicity, but the exact effect of this compound on liver cells is not well understood. To identify the mechanism of PHMG hepatotoxicity, HepG2 cells were exposed to PHMG-P for 72 h. The cell viability was significantly decreased by PHMG-P in a time- and concentration-dependent manner. The mitochondrial membrane potential was markedly reduced by PHMG-P and the apoptotic signaling cascade was activated. The increases observed in C/EBP homologous protein (CHOP), p-IRE, and p-JNK levels in PHMG-P-treated cells indicated the induction of endoplasmic reticulum stress. To verify the role of ER stress in PHMG-P-induced cytotoxicity, HepG2 cells were pretreated with the chemical chaperone, tauroursodeoxycholic acid (TUDCA) and then co-treated with TUDCA and PHMG-P for 24 h. Interestingly, TUDCA inhibited PHMG-P-induced ER stress and cytotoxicity in a dose-dependent manner. The apoptotic cell death and mitochondrial depolarization were also prevented by TUDCA. The proteins involved in the apoptotic pathway were all normalized to their control levels in TUDCA-treated cells. In conclusion, the results suggest that PHMG-P induced significant cytotoxicity in liver cells and ER stress-mediated apoptosis, which may be an important mechanism mediating this hepatotoxicity.

Akt and Notch pathways mediate polyhexamethylene guanidine phosphate-induced epithelial-mesenchymal transition via ZEB2

Polyhexamethylene guanidine phosphate (PHMG-p), an antimicrobial additive, was used as a humidifier disinfectant in Korea and caused severe lung injuries, including lung fibrosis, in hundreds of victims. As PHMG-p-induced lung fibrosis is different from that induced by known fibrogenic agents such as bleomycin, it is important to understand the molecular mechanisms underlying this effect. A recent study showed that epithelial-mesenchymal transition (EMT) could play key roles in PHMG-p-induced pulmonary fibrosis. Therefore, we aimed to characterize the molecular mechanisms associated with PHMG-p-induced EMT. We observed EMT, macrophage infiltration, and fibrosis in mouse lung tissues after intratracheal instillation of PHMG-p. Furthermore, PHMG-p-induced EMT was observed in A549 cells by the evaluation of cell morphology and quantitation of mRNA and protein expression. The use of EMT inhibitors revealed that PHMG-p induced EMT through the activation of Akt and Notch signaling. Moreover, the transcription factor ZEB2 was observed in PHMG-p-treated A549 cells and mouse lungs. The results indicated that upstream regulators, including Akt and Notch 1, acted as intracellular effectors that triggered ZEB2 expression after exposure to PHMG-p. Attenuation of PHMG-p-induced EMT following inhibition or silencing of Akt and Notch signaling or ZEB2 implied that PHMG-p-induced EMT was a result of Akt, Notch, and ZEB2 activation. Our findings showed that PHMG-p induced EMT through Akt/Notch signaling pathways and that ZEB2 played an important role in PHMG-p-induced lung toxicity. This study will help to understand the mechanisms of action of PHMG-p associated with lung fibrogenesis.

Polyhexamethylene guanidine phosphate-induced ROS-mediated DNA damage caused cell cycle arrest and apoptosis in lung epithelial cells

Polyhexamethylene guanidine phosphate (PHMG-p) is an active ingredient of humidifier disinfectants and causes severe lung injury resulting in pulmonary fibrosis. Current evidence indicates that pulmonary fibrosis is initiated as a result of epithelial damage, which can lead to an inflammatory response and fibrotic cell infiltration; however, the toxic mechanism of PHMG-p on the epithelium is still unknown. In this study, the toxic response of PHMG-p on human lung epithelial cells was evaluated, and its mechanisms associated with reactive oxygen species (ROS), DNA damage, and its relationship with p53 activation were investigated. The toxic responses of epithelial cells were assessed by flow cytometry analysis and western blot analysis. The results revealed that PHMG-p induced G1/S arrest and apoptosis in A549 cells. Interestingly, p53 was activated by PHMG-p treatment and p53 knockdown suppressed PHMG-p-induced apoptosis and cell cycle arrest. PHMG-p promoted ROS generation and consequently increased the expression of DNA damage markers such as ATM and H2AX phosphorylation. The antioxidant N-acetylcysteine reduced the expression of phosphorylated ATM and H2AX, and the ATM inhibitor, caffeine, inhibited p53 activation. Taken together, our results demonstrate that PHMG-p triggered G1/S arrest and apoptosis through the ROS/ATM/p53 pathway in lung epithelial cells.

Guanidine-based disinfectants, polyhexamethylene guanidine-phosphate (PHMG-P), polyhexamethylene biguanide (PHMB), and oligo(2-(2-ethoxy)ethoxyethyl guanidinium chloride (PGH) induced epithelial-mesenchymal transition in A549 alveolar epithelial cells

Abstracts Objective: The major active ingredient of humidifier disinfectant, polyhexamethylene guanidine-phosphate (PHMG-P), caused hundreds of deaths with pulmonary fibrosis. However, structurally similar guanidine-based disinfectants are still in use in various fields. Moreover, as they are precursors of excellent antimicrobial compounds, new chemicals with guanidine-based structures have been synthesized and introduced. In this study, we evaluated pulmonary fibrotic responses induced by PHMG-P, polyhexamethylene biguanide (PHMB), and oligo(2-(2-ethoxy)ethoxyethyl guanidinium chloride (PGH) and their toxicity mechanisms in type II alveolar epithelial A549 cells. Materials and methods: Cellular damage was compared by using the cytotoxicity test (WST-1 assay) and plasma membrane toxicity tests (Lactate dehydrogenase leakage detection assay and plasma membrane staining). As a measure of fibrotic response, induction of the epithelial-mesenchymal transition (EMT) was evaluated by measuring E-cadherin and α-smooth muscle actin (α-SMA) protein expression (epithelial and mesenchymal marker, respectively). Results: All tested compounds showed membrane damage; PHMG-P and PGH induced the highest and lowest damage, respectively. Moreover, they induced EMT when the test chemicals were treated with similar cytotoxic concentrations. Conclusions: Our study indicates that three guanidine-based disinfectants are potential fibrosis-inducing chemicals that induce EMT through cellular damage. Therefore, use of guanidine-based polymers should be strictly regulated by considering their potential adverse effects on the lungs.

Polyhexamethylene guanidine phosphate-induced upregulation of MUC5AC via activation of the TLR-p38 MAPK and JNK axis

Epidemiological and toxicological studies indicate that polyhexamethylene guanidine phosphate (PHMG-p) is a guanidine-based cationic disinfectant strongly associated with interstitial lung diseases. As individuals exposed to aerosolized PHMG-p complain of respiratory problems (asthma and rhinitis), whether PHMG-p can cause respiratory diseases other than interstitial fibrosis should be investigated. MUC5AC, the predominant mucin gene expressed in airways, is associated with obstructive respiratory disease pathogenesis. Therefore, in this study, we elucidated the relationship between PHMG-p and MUC5AC overexpression. First, in immunofluorescence studies, the bronchial epithelia of mice intratracheally administrated PHMG-p appeared to be sloughing and tethered by MUC5AC. Second, Calu-3 cells exposed to PHMG-p showed concentration-dependent increases in MUC5AC mRNA and protein expression. c-Jun N-terminal kinase (JNK), p38, and c-jun were phosphorylated in cells exposed to PHMG-p. SP600125 and SB203580, JNK and p38 inhibitors, respectively, reduced the upregulation of MUC5AC by PHMG-p in Calu-3 cells. When toll-like receptor (TLR)2, 4, and 6 were silenced, PHMG-p-induced JNK and p38 phosphorylation decreased. Furthermore, TLR2-, 4-, and 6-silenced cells showed reduced levels of MUC5AC mRNA and protein induced by PHMG-p, with TLR6 knockdown showing the greatest effect. In conclusion, PHMG-p induced MUC5AC overexpression via activation of the TLR-p38 MAPK and JNK axis.

A review of current studies on cellular and molecular mechanisms underlying pulmonary fibrosis induced by chemicals

Several studies showed that the inflammatory and fibrotic responses induced by polyhexamethylene guanidine phosphate (PHMG-p) were similar to those observed for idiopathic pulmonary fibrosis in South Korea in 2011. "Omic" technologies can be used to understand the mechanisms underlying chemical-induced diseases. Studies to determine the toxicity of chemicals may facilitate understanding of the mechanisms underlying the development of pulmonary fibrosis at a molecular level; thus, such studies may provide information about the toxic characteristics of various substances. In this review, we have outlined the cellular and molecular mechanisms underlying idiopathic pulmonary fibrosis and described pulmonary fibrosis induced by various chemicals, including bleomycin, paraquat, and PHMG-p, based on the results of studies performed to date.