Dose response for formaldehyde-induced cytotoxicity in the human respiratory tract

Relative contributions of endogenous and exogenous formaldehyde to formation of deoxyguanosine monoadducts and DNA-protein crosslink adducts of DNA in rat nasal mucosa

Understanding the dose-response for formaldehyde-induced nasal cancer in rats is complicated by (1) the uneven distribution of inhaled formaldehyde across the interior surface of the nasal cavity and, (2) the presence of endogenous formaldehyde (endoF) in the nasal mucosa. In this work, we used computational fluid dynamics (CFD) modeling to predict flux of inhaled (exogenous) formaldehyde (exogF) from air into tissue at the specific locations where DNA adducts were measured. Experimental work has identified DNA-protein crosslink (DPX) adducts due to exogF and deoxyguanosine (DG) adducts due to both exogF and endoF. These adducts can be considered biomarkers of exposure for effects of endoF and exogF on DNA that may be part of the mechanism of tumor formation. We describe a computational model linking CFD-predicted flux of formaldehyde from air into tissue, and the intracellular production of endoF, with the formation of DPX and DG adducts. We assumed that, like exogF, endoF can produce DPX. The model accurately reproduces exogDPX, exogDG, and endoDG data after inhalation from 0.7 to 15 ppm. The dose-dependent concentrations of exogDPX and exogDG are predicted to exceed the concentrations of their endogenous counterparts at about 2 and 6 ppm exogF, respectively. At all concentrations examined, the concentrations of endoDPX and exogDPX were predicted to be at least 10-fold higher than that of their DG counterparts. The modeled dose-dependent concentrations of these adducts are suitable to be used together with data on the dose-dependence of cell proliferation to conduct quantitative modeling of formaldehyde-induced rat nasal carcinogenicity.

Formaldehyde: Another hormesis-inducing chemical

Formaldehyde (FA) is a naturally-occurring compound, produced endogenously in diverse living organisms. It also occurs widely in the environment due to anthropogenic (e.g. used as a chemical intermediate) and natural sources (e.g. a component of the volatile organic compounds blends emitted by plants). While FA is considered a potential carcinogen, living organisms have the ability to cope with FA, and some minimum endogenous levels of FA may be required for health. Recently, genetic engineering approaches transferring biological information from one organism to another led to increased assimilation of and conferred genetic-based tolerance to FA in plants-microorganisms systems. Here, we propose that FA commonly induces hormesis, a hypothesis that we confirm by collating evidence from various published studies with animals, plants, and microorganisms. The stimulation by low doses below the no-observed-adverse-effect-level (NOAEL) was modest in magnitude, in agreement with the general hormesis literature. In plants, among the endpoints showing hormesis were growth, lipid peroxidation, and photosynthetic pigments. In various animal cells, hormesis was observed in cell proliferation and viability, responses that were related to mechanisms, such as activation of phosphorylated ERK (extra-cellular signaling-regulated kinase) expression, acceleration of the process of cell division, and enhancement of the Warburg effect (i.e. use of glycolysis by tumor cells to produce energy for rapid growth). Hormetic in vitro responses were reported in several cancerous/tumorous cell lines, suggesting that FA has the potential to influence tumor promotion within a specific concentration range and biological context. These observations suggest that FA commonly acts in an hormetic manner with implications for study designs across a broad range of biological models and in the assessment of environmental and human risks associated with FA exposures.

Low-Temperature Oxidation of Hydrogen Sulfide and Formaldehyde Pollutants in Humid Air by UV Radiation at 184.95 and 253.65 nm

An experimental study of the oxidation of hydrogen sulfide and formaldehyde impurities in humid air by ultraviolet radiation at wavelengths of 184.95 and 253.65 nm has been carried out at a pressure of 1 atm, an initial temperature of 20 °C, a relative humidity of 90%, and a flow rate of the gas mixture of 4920 m³/h. The initial concentrations of hydrogen sulfide and formaldehyde in air ranged from 8 to 20 and from 2.9 to 7.2 mg/m³, respectively. The photochemical kinetic mechanism was proposed for a numerical simulation of the low-temperature photooxidation of hydrogen sulfide and formaldehyde pollutants as well as the formation of ozone in humid air. The mechanism consists of 7 and 4 photochemical reactions initiated by UV radiation at wavelengths of 184.95 and 253.65 nm, respectively, and 62 reversible individual chemical reactions involving 32 chemical species (radicals, atoms, and molecules). The obtained results of numerical simulation are in good agreement with experimental data.

Melamine and/or formaldehyde exposures affect steroidogenesis via alteration of StAR protein and testosterone synthetic enzyme expression in male mice

The reproductive effects of melamine and formaldehyde, either alone or in combination, on mature male Swiss mice were investigated. The animals were orally administered melamine (50mg/kg/day), formaldehyde (25mg/kg/day), a mixture of melamine and formaldehyde, or a vehicle control for 65 consecutive days. As a result, the deterioration of sperm characteristics and inhibition of testicular enzyme activity were observed in the melamine- and formaldehyde-exposed groups. In addition, testosterone and luteinizing hormone levels were significantly reduced in the melamine but not in the formaldehyde-exposed group, which correlated with down-regulation of transcription levels of steroidogenic-related genes. Histopathologically, both compounds caused lesions in the testes. However, the co-exposure reduced the induced alterations in spermatogenesis, steroidogenesis, and testicular architecture that were obviously observed in the melamine-exposed group. Consequently, we demonstrated that melamine exhibited more pronounced reproductive impact in comparison with formaldehyde. In addition, formaldehyde was able to substantially temper the melamine -induced reproductive toxic effect.

Formaldehyde Is a Potent Proteotoxic Stressor Causing Rapid Heat Shock Transcription Factor 1 Activation and Lys48-Linked Polyubiquitination of Proteins

Endogenous and exogenous formaldehyde (FA) has been linked to cancer, neurotoxicity, and other pathophysiologic effects. Molecular and cellular mechanisms that underlie FA-induced damage are poorly understood. In this study, we investigated whether proteotoxicity is an important, unrecognized factor in cell injury caused by FA. We found that irrespective of their cell cycle phases, all FA-treated human cells rapidly accumulated large amounts of proteins with proteasome-targeting K48-linked polyubiquitin, which was comparable with levels of polyubiquitination in proteasome-inhibited MG132 controls. Both nuclear and cytoplasmic proteins were damaged and underwent K48-polyubiquitination. There were no significant changes in the nonproteolytic K63-polyubiquitination of soluble and insoluble cellular proteins. FA also rapidly induced nuclear accumulation and Ser326 phosphorylation of the main heat shock-responsive transcription factor HSF1, which was not a result of protein polyubiquitination. Consistent with the activation of the functional heat shock response, FA strongly elevated the expression of HSP70 genes. In contrast to the responsiveness of the cytoplasmic protein damage sensor HSF1, FA did not activate the unfolded protein response in either the endoplasmic reticulum or mitochondria. Inhibition of HSP90 chaperone activity increased the levels of K48-polyubiquitinated proteins and diminished cell viability after FA treatment. Overall, our results indicate that FA is a strong proteotoxic agent, which helps explain its diverse pathologic effects, including injury in nonproliferative tissues.

In vitro cytotoxicity testing of airborne formaldehyde collected in serum-free culture media

The purpose of this study was to identify a suitable sampling model for on-site toxicity assessment of soluble air contaminants such as formaldehyde, a well known industrial and indoor air contaminant. The in vitro cytotoxicity of formaldehyde, the selected model for soluble air contaminants, was studied using the MTS (tetrazolium salt) assay in two carcinoma cell lines, A549 epithelial lung and HepG2 hepatocarcinoma, and in skin fibroblasts. The cytotoxic effects of airborne formaldehyde were evaluated using test atmospheres in concentrations below 10 ppm (12.3 mg/m3), generated by a dynamic diffusion method and bubbled (0.3 L/min) through serum-free culture media for one or four hours. Human cells were treated with formaldehyde air samples, and cell viability was determined after four hours incubation. In parallel, the concentration of airborne formaldehyde was monitored, using the 3500 NIOSH method. Cell viability of the HepG2 cells exposed to formaldehyde air samples (8.75 ppm-4 h) was reduced to less than 50% (31.69/1.24%). The HepG2 cell lines were found to be more sensitive (IC50=103.799/23.55 mg/L) to formaldehyde than both A549 cell lines (IC50=198.369/9.54 mg/L) and skin fibroblasts (IC50=196.689/36.73 mg/L) (PB/0.01). An average of 96.8% was determined for collection efficiency of formaldehyde in serum-free culture media. The results of this study suggest that absorption of soluble air contaminants, such as formaldehyde, in serum-free culture media can be used as a suitable sampling model for on-site toxicity assessments.

Effects of formalin on some biomarker activities of earthworms pre-exposed to temephos

Despite its negative effects, formalin has been often used for the expulsion of earthworms due to its high efficiency; however it is not known whether it will affect any significant measurable molecular processes in sampled earthworms. The aim of this research was to investigate effects of formalin on the activities of chosen molecular biomarkers in Eisenia andrei earthworms previously exposed to temephos. Additionally, the inhibitory effect of temephos, hitherto evaluated only on laboratory-bred earthworm species, was confirmed on two earthworm species obtained from their natural environment -Dendrobaena octaedra and Lumbricus rubellus. Earthworms were first exposed to the sub-lethal concentration of temephos for 2h and then to formalin 15 min in order to simulate the sampling procedure. Besides acetylcholinesterase (AChE) inhibition - a known biomarker of exposure to organophosphate insecticides - the concentration of oximes and the activities of catalase (CAT) and efflux pump were measured. Results showed that in all species temephos caused inhibition of AChE and CAT activity. Exposure of E. andrei to formalin caused inhibition of AChE, however after post-exposure to formalin for 15 min significant increase in AChE activity was recorded. Similar results were obtained with the measurement of oximes concentrations. Exposure to only formalin and combination of temephos (2h) and formalin (15 min) led to an increase in the CAT activity. The obtained results showed that exposure to formalin during the sampling could affect measured molecular biomarkers and also may change effects caused by exposure to temephos.

S-phase sensing of DNA-protein crosslinks triggers TopBP1-independent ATR activation and p53-mediated cell death by formaldehyde

We examined genotoxic signaling and cell fate decisions in response to a potent DNA-protein crosslinker formaldehyde (FA). DNA-protein crosslinks (DPC) are poorly understood lesions produced by bifunctional carcinogens and several cancer drugs. FA-treated human cells showed a rapid activation of ATR kinase that preferentially targeted the p53 transcription factor at low doses and CHK1 kinase at more severe damage, producing bell-shaped and sublinear responses, respectively. CHK1 phosphorylation was transient, and its loss was accompanied by increased p53 accumulation and Ser15 phosphorylation. Activation of p53 was insensitive to inhibition of mismatch repair and nucleotide and base excision repair, excluding the role of small DNA adducts in this response. The p53-targeted signaling was transcription-independent, absent in quiescent cells and specific to S-phase in cycling populations. Unlike other S-phase stressors, FA-activated p53 was functional transcriptionally, promoted apoptosis in lung epithelial cells and caused senescence in normal lung fibroblasts. FA did not induce ATR, RAD1 or RPA foci, and p53 phosphorylation was TopBP1-independent, indicating a noncanonical mode of ATR activation. Replication arrest by FA caused a dissociation of ATR from a chromatin-loaded MCM helicase but no PCNA monoubiquitination associated with stalled polymerases. These results suggest that unlike typical DNA adducts that stall DNA polymerases, replication inhibition by bulkier DPC largely results from blocking upstream MCM helicase, which prevents accumulation of ssDNA. Overall, our findings indicate that S-phase-specific, TopBP1-independent activation of the ATR-p53 axis is a critical stress response to FA-DPC, which has implications for understanding of FA carcinogenesis.

Toxicity of derivatives from semicarbazide-sensitive amine oxidase-mediated deamination of methylamine against Toxoplasma gondii after infection of differentiated 3T3-L1 cells

Adipose tissue plays an active role in normal metabolic homeostasis as well as in the development of human diseases such as atherosclerosis and diabetes. We report here antimicrobial activities of the metabolites from adipocytes. Specifically, semicarbazide-sensitive amine oxidase of differentiated 3T3-L1 cells was found to utilize methylamine for producing formaldehyde and hydrogen peroxide, accounting for the inhibition of infectivity of Toxoplasma gondii and its replication in these cells. This was demonstrated by the findings that semicarbazide-sensitive amine oxidase was extremely high in differentiated 3T3-L1 cells; and that the infection of these cells by T. gondii and its intracellular replication were decreased to 33% and 37% of the control, respectively, when methylamine was provided in micromolar concentrations as the substrate to the aminoxidase. Only one of the two reaction products expected was found inhibitory against T. gondii when added to the infected pre-adipocytes of 3T3-L1. Intracellular replication of this parasite was inhibited by formaldehyde in the range of 10-100 microM and stimulated by hydrogen peroxide at 1-10 microM. The finding indicates that T. gondii may be useful as a sensitive and convenient sentinel for screening agents toxic to eukaryotic cells.

Uncertainties in biologically-based modeling of formaldehyde-induced respiratory cancer risk: identification of key issues

In a series of articles and a health-risk assessment report, scientists at the CIIT Hamner Institutes developed a model (CIIT model) for estimating respiratory cancer risk due to inhaled formaldehyde within a conceptual framework incorporating extensive mechanistic information and advanced computational methods at the toxicokinetic and toxicodynamic levels. Several regulatory bodies have utilized predictions from this model; on the other hand, upon detailed evaluation the California EPA has decided against doing so. In this article, we study the CIIT model to identify key biological and statistical uncertainties that need careful evaluation if such two-stage clonal expansion models are to be used for extrapolation of cancer risk from animal bioassays to human exposure. Broadly, these issues pertain to the use and interpretation of experimental labeling index and tumor data, the evaluation and biological interpretation of estimated parameters, and uncertainties in model specification, in particular that of initiated cells. We also identify key uncertainties in the scale-up of the CIIT model to humans, focusing on assumptions underlying model parameters for cell replication rates and formaldehyde-induced mutation. We discuss uncertainties in identifying parameter values in the model used to estimate and extrapolate DNA protein cross-link levels. The authors of the CIIT modeling endeavor characterized their human risk estimates as "conservative in the face of modeling uncertainties." The uncertainties discussed in this article indicate that such a claim is premature.

Sensitivity analysis of biologically motivated model for formaldehyde-induced respiratory cancer in humans

Conolly et al. (2003, 2004) developed biologically motivated models of formaldehyde carcinogenicity in F344 rats and humans based on a two-stage clonal expansion model of cancer. Based on the human model, Conolly et al. (2004) claimed that cancer risks associated with inhaled formaldehyde are deminimis at relevant human exposure levels. However, they did not conduct a sensitivity analysis to evaluate the robustness of this conclusion. Here, we present a limited sensitivity analysis of the formaldehyde human model. We show that when the control animals from the National Toxicology Program (NTP) studies are replaced with control animals only from NTP inhalation studies, estimates of human risk are increased by 50-fold. When only concurrent control rats are used, the model does not provide any upper bound (UB) to human risk. No data went into the model on the effect of formaldehyde on the division rates and death rates of initiated cells. We show that slight numerical perturbations to the Conolly et al. assumptions regarding these rates can be made that are equally consistent with the underlying data used to construct the model, but produce estimates of human risk ranging anywhere from negative up to 10,000 times higher than those deemed by Conolly et al. to be 'conservative'. Thus, we conclude that estimates of human risk by Conolly et al. (2004) are extremely sensitive to modeling assumptions. This calls into question the basis for the Conolly et al. claim of de minimis human risk and suggests caution in using the model to derive human exposure standards for formaldehyde.

Modeling approaches for estimating the dosimetry of inhaled toxicants in children

Risk assessment of inhaled toxicants has typically focused upon adults, with modeling used to extrapolate dosimetry and risks from lab animals to humans. However, behavioral factors such as time spent playing outdoors may lead to more exposure to inhaled toxicants in children. Depending on the inhaled agent and the age and size of the child, children may receive a greater internal dose than adults because of greater ventilation rate per body weight or lung surface area, or metabolic differences may result in different tissue burdens. Thus, modeling techniques need to be adapted to children in order to estimate inhaled dose and risk in this potentially susceptible life stage. This paper summarizes a series of inhalation dosimetry presentations from the U.S. EPA's Workshop on Inhalation Risk Assessment in Children held on June 8-9, 2006 in Washington, DC. These presentations demonstrate how existing default models for particles and gases may be adapted for children, and how more advanced modeling of toxicant deposition and interaction in respiratory airways takes into account children's anatomy and physiology. These modeling efforts identify child-adult dosimetry differences in respiratory tract regions that may have implications for children's vulnerability to inhaled toxicants. A decision framework is discussed that considers these different approaches and modeling structures including assessment of parameter values, supporting data, reliability, and selection of dose metrics.

Cytotoxicity of the exhaust gas from a thermal reactor of MSWI baghouse ash

Baghouse ash from municipal solid waste incineration (MSWI) plant was heated from 25 degrees C to 800 degrees C under nitrogen in a fixed-bed reactor. The exhaust gas was passed sequentially through water, acetone and cyclohexane. The cytotoxicity testing of the three adsorbates was done with the MRC-5 cell line and the percentage cell survival was determined by 3-(4,5-dimethylthiazol-2-yl)-5(3-carboxymethoxyphenol)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) analysis. The highest level of toxicity of the exhaust gas was observed at 500 degrees C. The total cytotoxicity of the three adsorbates at any treatment temperature was found to be a function of the sum of organic carbon (TOC), inorganic carbon (IC) and molecular chlorine (Cl(2)), of which, molecular chlorine was quantitatively the greatest.

Evaluation of the health impact of lowering the formaldehyde occupational exposure limit for Quebec workers

This study aimed at assessing the impact on irritating effects of lowering the current occupational exposure limit (OEL) for formaldehyde in occupational settings in the Province of Quebec, Canada, from a 2 ppm ceiling value to 1, 0.75 or 0.3 ppm. This was achieved through (i) a re-assessment of the exposure-response relationship based on a pooled analysis of published controlled human studies on the incidence of the most sensitive effects related to acute formaldehyde exposure (irritation of the eyes, nose and throat) and (ii) application of this relationship to the data on current exposure to formaldehyde in industrial sectors of Quebec. Results show that the exposure-irritating effect relationship compiled by concentration ranges and by degree of severity was best described by quadratic regression. Considering the current distribution of formaldehyde exposure among the 143,491 Quebec workers concerned, eye irritation, the most sensitive effect, could affect 526 workers (0.367%) at a moderate degree and 50 workers (0.035%) at a severe degree. By reducing the OEL to 1, 0.75 and 0.3 ppm, the proportion of these effects estimated to be avoided would be 442/526 (84%), 526/526 (100%) and 526/526 (100%), respectively. Results for nose and throat irritation follow the same trend. The greatest gain would thus be obtained by respecting the current OEL; the additional gain was estimated to be negligible below 0.75 ppm. The level of 0.75 ppm can be considered as a safe level that allows protecting virtually all workers.

Vitamin E Protects Against Oxidative Damage Caused by Formaldehyde in the Liver and Plasma of Rats

It is well known that formaldehyde (FA) and reactive oxygen species (ROS) are cytotoxic and potentially carcinogenic. Although the individual effects of these reactants on cells have been investigated, the cytotoxicity exerted by the coexistence of FA and ROS is poorly understood. The present study was carried out to evaluate oxidant/antioxidant status and biochemical changes occurring after chronic formaldehyde toxicity in liver tissue and plasma of rats and protective effect of vitamin E (vit E) against oxidative damage. Eighteen rats were divided into three groups: (1) control rats, (2) rats treated with FA (FAt), and (3) rats treated with FA plus vit E (FAt + vit E) groups. After the treatment, the animals were sacrificed and liver tissues were removed for biochemical investigations. As a result, FA treatment significantly increased the levels of tissue malondialdehyde (MDA), protein carbonyl (PC), nitric oxide (NO) and the activity of xanthine oxidase enzyme (XO). On the other hand, FA exposure led to decrease in superoxide dismutase (SOD) and catalase (CAT) activities in liver tissues compared to control. FA caused significant decreases in total protein (TP) and albumin (ALB) whereas increases in aspartate transaminase (AST), alanine aminotransferase (AST), alkaline phosphatase (ALP) and interleukine-2 (IL-2) levels in plasma. Vit E treatment abolished these changes at a level similar to the control group. It was concluded that vit E treatment might be beneficial in preventing FA-induced liver tissue damage, and therefore have potential for clinical use.

Risk assessment of formaldehyde for the general population in Japan

Formaldehyde is used in the production of resins, molding compounds, photographic film, bactericide, and tissue preservative. The purpose of this study was to provide an up-to-date critical review of the information to the toxicological profile of formaldehyde, and to assess the risk of formaldehyde for the general population in Japan. Inhaled formaldehyde is an effective sensory irritant at a dosage of 0.5 ppm in mice. Following inhalation in laboratory animals, more than 6 ppm formaldehyde causes degenerative non-neoplastic effects in mice and monkeys and nasal tumors in rats. It is considered that formaldehyde induces genotoxic effects directly in vitro and secondarily in vivo. Sensory irritation of the eyes and respiratory tract in response to inhalation exposure to formaldehyde has been reported at 0.08 ppm and above in human study. Formaldehyde is carcinogenic at the site of contact as a consequence of epithelial cell regenerative proliferation resulting from cytotoxicity and mutation, based on studies in both animals and humans. Levels of formaldehyde in atmosphere detected in rural, suburban, and urban areas in Japan were 2.5-3.2 ppb from 1998 to 2003. The majority of the population is exposed to atmosphere concentrations of formaldehyde less than those associated with sensory irritation. The reference concentration of formaldehyde in atmosphere for the Japanese general population is recommended to be 0.01 ppm.

Nonmonotonic Dose-Response Relationships: Mechanistic Basis, Kinetic Modeling, and Implications for Risk Assessment

Dose-response curves for the first interaction of a chemical with a biochemical target molecule are usually monotonic; i.e., they increase or decrease over the entire dose range. However, for reactions of a complex biological system to a toxicant, nonmonotonic (biphasic) dose-effect relationships can be observed, showing a decrease at low dose followed by an increase at high dose, or vice versa. We present four examples to demonstrate that nonmonotonic dose-response relationships can result from superimposition of monotonic dose responses of component biological reactions. Examples include (i) a membrane-receptor model with receptor subtypes of different ligand affinity and opposing downstream effects (adenosine receptors A1 vs. A2), (ii) androgen receptor-mediated gene expression driven by homodimers, but not mixed-ligand dimers, (iii) repair of background DNA damage by enzymatic activity induced by adducts formed by a xenobiotic, (iv) rate of mutation as a consequence of DNA damage times rate of cell division, the latter being modulated by cell-cycle delay at low-level DNA damage, and cell-cycle acceleration due to regenerative hyperplasia at cytotoxic dose levels. Quantitative analyses based on biological models are shown, and factors that affect the degree of nonmonotonicity are identified. It is noted that threshold-type dose-response curves could in fact be nonmonotonic. Our analysis should promote a scientific discussion of biphasic dose responses and the concept termed "hormesis," and of default procedures for low-dose extrapolation in toxicological risk assessment.

Respiratory cancer risks associated with low-level nickel exposure: an integrated assessment based on animal, epidemiological, and mechanistic data

Increased lung and nasal cancer risks have been reported in several cohorts of nickel refinery workers, but in more than 90% of the nickel-exposed workers that have been studied there is little, if any evidence of excess risk. This investigation utilizes human exposure measurements, animal data from cancer bioassays of three nickel compounds, and a mechanistic theory of nickel carcinogenesis to reconcile the disparities in lung cancer risk among nickel-exposed workers. Animal data and mechanistic theory suggest that the apparent absence of risk in workers with low nickel exposures is due to threshold-like responses in lung tumor incidence (oxidic nickel), tumor promotion (soluble nickel), and genetic damage (sulfidic nickel). When animal-based lung cancer dose-response functions for these compounds are extrapolated to humans, taking into account interspecies differences in deposition and clearance, differences in particle size distributions, and human work activity patterns, the predicted risks at occupational exposures are remarkably similar to those observed in nickel-exposed workers. This provides support for using the animal-based dose-response functions to estimate occupational exposure limits, which are found to be comparable to those in current use.