More precise localization of nasal tumors associated with chronic exposure of F-344 rats to formaldehyde gas

Competitive inhibition of a non-natural cofactor dependent formaldehyde dehydrogenase by imidazole

OBJECTIVES

To better understand the unique inhibitory behavior of a non-natural cofactor preferred formaldehyde dehydrogenase (FalDH) mutant 9B2.

RESULTS

We described our serendipitous observation that 9B2 was reversibly inhibited by residual imidazole introduced during protein preparation, while the wild-type enzyme was not sensitive to imidazole. Kinetic analysis showed that imidazole was a competitive inhibitor of formaldehyde with a K_i of 16 μM and an uncompetitive inhibitor of Nicotinamide Cytosine Dinucleotide for 9B2, indicating that formaldehyde and imidazole were combined in the same position. Molecular docking results of 9B2 showed that imidazole could favorably bind very close to the nicotinamide moiety of the cofactor, where formaldehyde was expected to reside for catalysis, which was in line with a competitive inhibition.

CONCLUSION

The mutant 9B2 can be competitively inhibited by imidazole, suggesting that cautions should be taken to evaluate activities as protein mutants might attain unexpected sensitivity to a component in buffers for purification or activity assays.

Benchmark Dose Modeling Approaches for Volatile Organic Chemicals using a Novel Air-Liquid Interface In Vitro Exposure System

Inhalation is the most relevant route of volatile organic chemical (VOC) exposure; however, due to unique challenges posed by their chemical properties and poor solubility in aqueous solutions, in vitro chemical safety testing is predominantly performed using direct application dosing/submerged exposures. To address the difficulties in screening toxic effects of VOCs, our cell culture exposure system permits cells to be exposed to multiple concentrations at air-liquid interface (ALI) in a 24-well format. ALI exposure methods permit direct chemical-to-cell interaction with the test article at physiological conditions. In the present study, BEAS-2B and primary normal human bronchial epithelial cells (pHBEC) are used to assess gene expression, cytotoxicity, and cell viability responses to a variety of volatile chemicals including acrolein, formaldehyde, 1,3-butadiene, acetaldehyde, 1-bromopropane, carbon tetrachloride, dichloromethane, and trichloroethylene. BEAS-2B cells were exposed to all the test agents, while pHBECs were only exposed to the latter four listed above. The VOC concentrations tested elicited only slight cell viability changes in both cell types. Gene expression changes were analyzed using benchmark dose (BMD) modeling. The BMD for the most sensitive gene set was within one order of magnitude of the threshold-limit value reported by the American Conference of Governmental Industrial Hygienists, and the most sensitive gene sets impacted by exposure correlate to known adverse health effects recorded in epidemiologic and in vivo exposure studies. Overall, our study outlines a novel in vitro approach for evaluating molecular-based points-of-departure in human airway epithelial cell exposure to volatile chemicals.

Engineering formaldehyde dehydrogenase from Pseudomonas putida to favor nicotinamide cytosine dinucleotide

The enzyme formaldehyde dehydrogenase (FalDH) from Pseudomonas putida is of particular interest for biotechnological applications as it catalyses the oxidation of formaldehyde independent of glutathione. However, the consumption of a stoichiometric amount of nicotinamide adenine dinucleotide (NAD) can be challenging at the metabolic level as this may affect many other NAD-linked processes. A potential solution is to engineer FalDH to utilize non-natural cofactors. Here we devised FalDH variants to favor nicotinamide cytosine dinucleotide (NCD) by structure-guided modification of the binding pocket for the adenine moiety of NAD. Several mutants were obtained and the best one FalDH 9B2 had over 150-fold higher preference for NCD than NAD. Molecular docking analysis indicated that the cofactor binding pocket shrinked to better fit NCD, a smaller-sized cofactor. FalDH 9B2 together with other NCD-linked enzymes offer opportunities to assemble orthogonal pathways for biological conversion of C1 molecules.

An updated mode of action and human relevance framework evaluation for Formaldehyde-Related nasal tumors

Formaldehyde is a reactive aldehyde naturally present in all plant and animal tissues and a critical component of the one-carbon metabolism pathway. It is also a high production volume chemical used in the manufacture of numerous products. Formaldehyde is also one of the most well-studied chemicals with respect to environmental fate, biology, and toxicology-including carcinogenic potential, and mode of action (MOA). In 2006, a published MOA for formaldehyde-induced nasal tumors in rats concluded that nasal tumors were most likely driven by cytotoxicity and regenerative cell proliferation, with possible contributions from direct genotoxicity. In the past 15 years, new research has better informed the MOA with the publication of in vivo genotoxicity assays, toxicogenomic analyses, and development of ultra-sensitive methods to measure endogenous and exogenous formaldehyde-induced DNA adducts. Herein, we review and update the MOA for nasal tumors, with particular emphasis on the numerous studies published since 2006. These new studies further underscore the involvement of cytotoxicity and regenerative cell proliferation, and further inform the genotoxic potential of inhaled formaldehyde. The data lend additional support for the use of mechanistic data for the derivation of toxicity criteria and/or scientifically supported approaches for low-dose extrapolation for the risk assessment of formaldehyde.

A comprehensive review of mechanistic insights into formaldehyde-induced nasal cavity carcinogenicity

According to the International Agency for Research on Cancer classification, formaldehyde is a human carcinogen that targets the nasal cavity. In humans and rats, inhaled formaldehyde is primarily deposited in the nasal cavity mucosa, metabolized to the less toxic formic acid, and finally excreted into the urine or exhaled. Thus, formaldehyde-induced nasal carcinogenicity may be a direct effect of formaldehyde itself, although the underlying mechanisms remain unclear. With regard to cytotoxicity, degeneration and necrosis of nasal respiratory cells occur in rats after short exposure to formaldehyde. Cell proliferation is increased in the damaged cells, suggesting its critical roles both in the early stages and throughout the entire process of nasal carcinogenicity. Hyperplasia, squamous metaplasia, and dysplasia of the damaged epithelium frequently appear as morphological precursor lesions. With regard to genotoxicity, in addition to DNA-protein crosslinks, oxidative DNA damage also occurs in the exposed nasal mucosal cells. Sustained exposure to formaldehyde may cause nasal carcinogenicity through cytotoxicity and auxiliary genotoxicity. In this review, we discuss adverse outcome pathways through which cytotoxicity can lead to carcinogenicity and the development of integrated approaches for testing and assessment for nongenotoxic carcinogens.

Derivation of safe exposure levels for potential migration of formaldehyde into food

Polyoxymethylene (POM) is a polymer of formaldehyde used inter alia for kitchenware and food processing machines. By migration into food, consumers may be exposed to small additional amounts of formaldehyde in food. In order to address such potential exposures, Specific Migration Limits are derived using all studies with oral exposure in mammals and birds. The assessment is not only based on local irritation observed in a 2-year rat study that has previously served to calculate acceptable exposure levels, but also on systemic effects, namely on effects on the kidney in adult rats and testes in birds before sexual maturity. At the relatively high oral exposure levels (up to 2000 ppm in drinking water) long-term effects caused by formic acid, the first step metabolite of formaldehyde, such as acidosis, cannot be excluded. The lowest Specific Migration Limit of 2.74 mg/dm², corresponding to 16.5 mg formaldehyde/kg food, is based upon kidney effects in rats, leading to potential exposures that range between 2900 and 4400 times below the endogenous turnover of formaldehyde. Lastly, a recent migration study with POM showed that migration of formaldehyde into food simulants is over an order of magnitude below the lowest Specific Migration Limit derived herein.

Selecting Experimental Data for Use in Quantitative Risk Assessment: An Expert-Judgment Approach

In many cancer risk assessments the experimental data used in statistical modeling are selected by applying generic guidelines. The guidelines exclude use of some types of experimental data and often appear arbitrary since rules rather than scientific judgments guide selection of data. This paper implements an alternative approach in which data are selected based on the judgments of practicing scientists. Eight such scientists were identified through an explicit selection procedure to help select data for use in a dose-response assessment of formaldehyde. Judgments about appropriate data sets were then elicited in personal interviews using a formal interview protocol. Appropriate data sets were fit to the multistage model and used as the basis for low-dose extrapolation. Low-dose risk estimates are shown to be sensitive to the selection of data, especially the treatment of benign tumors. The recommendations of the experts also differ in some respects from the choices made in previously published risk assessments. This suggests that scientific judgment may be an appropriate method to augment guidelines when a broad range of data is available. The paper argues that the expert judgment approach has some advantages that are worth considering.

Comparative Risks of Aldehyde Constituents in Cigarette Smoke Using Transient Computational Fluid Dynamics/Physiologically Based Pharmacokinetic Models of the Rat and Human Respiratory Tracts

Computational fluid dynamics (CFD) modeling is well suited for addressing species-specific anatomy and physiology in calculating respiratory tissue exposures to inhaled materials. In this study, we overcame prior CFD model limitations to demonstrate the importance of realistic, transient breathing patterns for predicting site-specific tissue dose. Specifically, extended airway CFD models of the rat and human were coupled with airway region-specific physiologically based pharmacokinetic (PBPK) tissue models to describe the kinetics of 3 reactive constituents of cigarette smoke: acrolein, acetaldehyde and formaldehyde. Simulations of aldehyde no-observed-adverse-effect levels for nasal toxicity in the rat were conducted until breath-by-breath tissue concentration profiles reached steady state. Human oral breathing simulations were conducted using representative aldehyde yields from cigarette smoke, measured puff ventilation profiles and numbers of cigarettes smoked per day. As with prior steady-state CFD/PBPK simulations, the anterior respiratory nasal epithelial tissues received the greatest initial uptake rates for each aldehyde in the rat. However, integrated time- and tissue depth-dependent area under the curve (AUC) concentrations were typically greater in the anterior dorsal olfactory epithelium using the more realistic transient breathing profiles. For human simulations, oral and laryngeal tissues received the highest local tissue dose with greater penetration to pulmonary tissues than predicted in the rat. Based upon lifetime average daily dose comparisons of tissue hot-spot AUCs (top 2.5% of surface area-normalized AUCs in each region) and numbers of cigarettes smoked/day, the order of concern for human exposures was acrolein > formaldehyde > acetaldehyde even though acetaldehyde yields were 10-fold greater than formaldehyde and acrolein.

Lowest adverse effects concentrations (LOAECs) for formaldehyde exposure

In 2012 the Committee for Risk Assessment (RAC) of the European Chemicals Agency concluded that 2ppm formaldehyde represent a Lowest Observed Adverse Effect Concentration (LOAEC) for polypoid adenomas, histopathological lesions and cell proliferation. An analysis of all data shows that a LOAEC of 2ppm it is not justified for cell proliferation and polypoid adenomas. Higher values are also supported by a new statistical analysis. For histopathological lesions a NOAEC of 1ppm may be defined but the lesions at 2ppm cannot be regarded as pre-stages for tumour development. One major uncertainty exists: the description of polypoid adenomas and the lesions at 2ppm often is insufficient and diagnostic uncertainties can only be resolved by a re-evaluation according to modern histomorphological standards. Although the discrepancy between our assessment and that of RAC may seem rather small we feel the LOAECs proposed by RAC must be challenged taking into consideration the broad data base for formaldehyde and the potential impact of any published RAC opinion on the present discussions about appropriate occupational and indoor exposure limits.

Expression, purification, and characterization of formaldehyde dehydrogenase from Pseudomonas aeruginosa

As a member of zinc-containing medium-chain alcohol dehydrogenase family, formaldehyde dehydrogenase (FDH) can oxidize toxic formaldehyde to less active formate with NAD(+) as a cofactor and exists in both prokaryotes and eukaryotes. Most FDHs are well known to be glutathione-dependent in the catalysis of formaldehyde oxidation, but the enzyme from Pseudomonas putida is an exception, which is independent of glutathione. To identify novel glutathione-independent FDHs from other bacterial strains and facilitate the corresponding structural and enzymatic studies, high-level soluble expression and efficient purification of these enzymes need to be achieved. Here, we present molecular cloning, expression, and purification of the FDH from Pseudomonas aeruginosa, which is a Gram-negative pathogenic bacterium causing opportunistic human infection. The FDH of P. aeruginosa shows high sequence identity (87.97%) with that of P. putida. Our results indicated that coexpression with molecular chaperones GroES, GroEL, and Tig has significantly attenuated inclusion body formation and improved the solubility of the recombinant FDH in Escherichiacoli cells. A purification protocol including three chromatographic steps was also established to isolate the recombinant FDH to homogeneity with a yield of ∼3.2 mg from 1L of cell culture. The recombinant P. aeruginosa FDH was properly folded and biologically functional, as demonstrated by the mass spectrometric, crystallographic, and enzymatic characterizations of the purified proteins.

Air pollution toxicology--a brief review of the role of the science in shaping the current understanding of air pollution health risks

Human and animal toxicology has had a profound impact on our historical and current understanding of air pollution health effects. Early animal toxicological studies of air pollution had distinctively military or workplace themes. With the discovery that ambient air pollution episodes led to excess illness and death, there became an emergence of toxicological studies that focused on industrial air pollution encountered by the general public. Not only did the pollutants investigated evolve from ambient mixtures to individual pollutants but also the endpoints and outcomes evaluated became more sophisticated, resulting in our present state of the science. Currently, a large toxicological database exists for the effects of particulate matter and ozone, and we provide a focused review of some of the major contributions to the biological understanding for these two "criteria" air pollutants. A limited discussion of the toxicological advancements in the scientific knowledge of two hazardous air pollutants, formaldehyde and phosgene, is also included. Moving forward, the future challenge of air pollution toxicology lies in the health assessment of complex mixtures and their interactions, given the projected impacts of climate change and altered emissions on ambient conditions. In the coming years, the toxicologist will need to be flexible and forward thinking in order to dissect the complexity of the biological system itself, as well as that of air pollution in all its varied forms.

Guidance for the classification of carcinogens under the Globally Harmonised System of Classification and Labelling of Chemicals (GHS)

The United Nations Conference on Environment and Development (UNCED) has developed criteria for a globally harmonised system of classification and labelling of chemicals (GHS). With regard to carcinogenicity, GHS distinguishes between Category 1 ('known or presumed human carcinogens') and Category 2 ('suspected human carcinogens'). Category 1 carcinogens are divided into Category 1A ('known to have carcinogenic potential for humans'), based largely on human evidence, and 1B ('presumed to have carcinogenic potential for humans'), based largely on experimental animal data. Concerns have been raised that the criteria for applying these carcinogenicity classifications are not sufficiently well defined and potentially allow different conclusions to be drawn. The current document describes an attempt to reduce the potential for diverse conclusions resulting from the GHS classification system through the application of a series of questions during the evaluation of data from experiments with rodents; epidemiological data, which could lead to Category 1A, have not been considered. Answers to each question can lead either to a classification decision or to the next question, but this process should only be implemented in an environment of informed scientific opinion. The scheme is illustrated with five case studies. These questions are: (1) Has a relevant form of the substance been tested? (2) Is the study design relevant to human exposure? (3) Is there a substance-related response? (4) Is the target tissue exposure relevant to humans? (5) Can a mode of action be established? (6) Is the mode of action relevant to humans? (7) What is the potency?

Two-year inhalation study of carcinogenicity and chronic toxicity of 1,4-dioxane in male rats

Carcinogenicity and chronic toxicity of 1,4-dioxane were examined by inhalation exposure of 50 male F344 rats to 1,4-dioxane vapor at 0 (clean air), 50, 250, or 1250 ppm (v/v) for 6 h/day, 5 days/wk, and 104 wk. Survival rates of 250 and 1250 ppm-exposed groups were decreased near the end of the 2-yr exposure period, due probably to the occurrence of malignant tumors. A statistically significant but marginal decrement of terminal body weight (<10%) was found in the 1250 ppm-exposed group, suggesting slight systemic toxicity. Significant changes in plasma levels of AST, ALT, ALP, and gamma-GTP and relative weight of the liver occurred in the 1250 ppm-exposed group. Dose-dependent and statistically significant increases in incidences of nasal squamous cell carcinomas, hepatocellular adenomas, and peritoneal mesotheliomas were found primarily in the 1250 ppm-exposed group. The incidences of renal cell carcinomas, fibroadenomas in the mammary gland, and adenomas in the Zymbal gland were also increased dose-dependently. Preneoplastic lesions occurred in the nasal cavity and liver of the 1,4-dioxane-exposed groups. As nonneoplastic lesions, the significantly increased incidences of nuclear enlargement, atrophy, and respiratory metaplasia in the nasal cavity were noted at 50 ppm and above. A LOAEL (lowest observed adverse effect level) was determined at 50 ppm for the nasal endpoint of general chronic toxicity. This study provides clear evidence of carcinogenicity for 1,4-dioxane in male rats. A cytotoxic-proliferative and in vivo genotoxic mode of action is suggested to operate in 1,4-dioxane-induced carcinogenesis.

Effects of formaldehyde inhalation on the junctional proteins of nasal respiratory mucosa of rats

Exposure to formaldehyde, which is an organic compound, disturbs the integrity of nasal mucosa. In this study, we aimed to clarify the protein changes in the junctional complex of nasal mucosa of Wistar rats exposed to formaldehyde inhalation. The study was performed in 20 female Wistar rats. Rats were divided into two groups randomly. Control rats were allowed free access to standard rat chaw and tap water (n:10). Experimental group was exposed to formaldehyde vapor at 15ppm, 6h/day, 5 days/week for 12 weeks (n:10). Histological evaluation of the experimental model was determined by hematoxylin-eosin (HE) and periodic acid Schiff (PAS) stainings of paraffin-embedded nasal mucosa tissues and by electron microscopy. The effects of formaldehyde inhalation on the distribution of occludin, E-cadherin, and gamma-catenin were assessed by immunohistochemistry. The nasal mucosa of the experimental group was correlated with hypertrophy in goblet cell, degeneration in basal lamina, stratification of epithelium, and proliferation. Thickness of basal lamina and also local degenerative regions, vacuole increase in cytoplasmic areas, irregular forms of kinocilium and loss of sharpness in the kinocilium membrane were the findings at the ultrastructural level. The expressions of E-cadherin, occludin, gamma-catenin proteins in intercellular junctional complexes of rat nasal mucosa were also decreased in experimental group compared to control group. The findings of the present study indicated that formaldehyde vapor inhalation in the concentrations and duration of exposure used in the present experiment significantly decreased the density of structural proteins of the junctional complex in the nasoepithelium. It was suggested that, the formaldehyde inhalation could cause complete impairment of intercellular junctional complexes and disturb the tissue integrity in nasal mucosa at higher concentrations.

A method to integrate benchmark dose estimates with genomic data to assess the functional effects of chemical exposure

The use of genomic technology for assessing health risks associated with chemical exposure has significant potential, but its direct application has proven to be challenging for the toxicology and risk assessment communities. In this study, a method was established for analyzing dose-response microarray data using benchmark dose (BMD) calculations and gene ontology (GO) classification. Gene expression changes in the rat nasal epithelium following acute formaldehyde exposure were used as a case study. The gene expression data were first analyzed using a one-way ANOVA to identify genes that showed significant dose-response behavior. These genes were then fit to a series of four statistical models (linear, second-degree polynomial, third-degree polynomial, and power models) and the least complex model that best described the data was selected. The genes were matched to their associated GO categories, and the average BMD and benchmark dose lower confidence limit (BMDL) were calculated for each GO category. The results were used to identify doses at which individual cellular processes were altered. For the formaldehyde exposures, the BMD estimates for the GO categories related to cell proliferation and DNA damage were similar to those measured in previous studies using cell labeling indices and DNA-protein cross-links and consistent with the BMD estimated for rat nasal tumors. The method represents a significant advance in applying genomic information to risk assessment by allowing a comprehensive survey of molecular changes associated with chemical exposure and providing the capability to identify reference doses at which particular cellular processes are altered.

Formaldehyde and glutaraldehyde and nasal cytotoxicity: case study within the context of the 2006 IPCS Human Framework for the Analysis of a cancer mode of action for humans

Formaldehyde and glutaraldehyde cause toxicity to the nasal epithelium of rats and mice upon inhalation. In addition, formaldehyde above certain concentrations induces dose-related increases in nasal tumors in rats and mice, but glutaraldehyde does not. Using the 2006 IPCS human framework for the analysis of cancer mode of action (MOA), an MOA for formaldehyde was formulated and its relevance was tested against the properties of the noncarcinogenic glutaraldehyde. These compounds produce similar patterns of response in histopathology and in genotoxicity tests (although formaldehyde has been much more extensively tested studied). The MOA is based on the induction of sustained cytotoxicity and reparative cell proliferation induced by formaldehyde at concentrations that also induce nasal tumors upon long-term exposure. Data on dose dependency and temporal relationships of key events are consistent with this MOA. While a genotoxic MOA can never be ruled out for a compound that is clearly genotoxic, at least in vitro, the nongenotoxic properties fundamental to the proposed MOA can explain the neoplastic response in the nose and may be more informative than genotoxicity in risk assessment. It is not yet fully explained why glutaraldehyde remains noncarcinogenic upon inhalation, but its greater inherent toxicity may be a key factor. The dual aldehyde functions in glutaraldehyde are likely to produce damage resulting in fewer kinetic possibilities (particularly for proteins involved in differentiation control) and lower potential for repair (nucleic acids) than would be the case for formaldehyde. While there have been few studies of possible glutaraldehyde-associated cancer, the evidence that formaldehyde is a human carcinogen is strong for nasopharyngeal cancers, although less so for sinonasal cancers. This apparent discrepancy could be due in part to the classification of human nasal tumors with tumors of the sinuses, which would receive much less exposure to inhaled formaldehyde. Evaluation of the human relevance of the proposed MOA of formaldehyde in rodents is restricted by human data limitations, although the key events are plausible. It is clear that the human relevance of the formaldehyde MOA in rodents cannot be excluded on either kinetic or dynamic grounds.

The nose revisited: a brief review of the comparative structure, function, and toxicologic pathology of the nasal epithelium

The nose is a very complex organ with multiple functions that include not only olfaction, but also the conditioning (e.g., humidifying, warming, and filtering) of inhaled air. The nose is also a "scrubbing tower" that removes inhaled chemicals that may be harmful to the more sensitive tissues in the lower tracheobronchial airways and pulmonary parenchyma. Because the nasal airway may also be a prime target for many inhaled toxicants, it is important to understand the comparative aspects of nasal structure and function among laboratory animals commonly used in inhalation toxicology studies, and how nasal tissues and cells in these mammalian species may respond to inhaled toxicants. The surface epithelium lining the nasal passages is often the first tissue in the nose to be directly injured by inhaled toxicants. Five morphologically and functionally distinct epithelia line the mammalian nasal passages--olfactory, respiratory, squamous, transitional, and lymphoepithelial--and each nasal epithelium may be injured by an inhaled toxicant. Toxicant-induced epithelial lesions in the nasal passages of laboratory animals (and humans) are often site-specific and dependent on the intranasal regional dose of the inhaled chemical and the sensitivity of the nasal epithelial tissue to the specific chemical. In this brief review, we present examples of nonneoplastic epithelial lesions (e.g., cell death, hyperplasia, metaplasia) caused by single or repeated exposure to various inhaled chemical toxicants. In addition, we provide examples of how nasal maps may be used to record the character, magnitude and distribution of toxicant-induced epithelial injury in the nasal airways of laboratory animals. Intranasal mapping of nasal histopathology (or molecular and biochemical alterations to the nasal mucosa) may be used along with innovative dosimetric models to determine dose/response relationships and to understand if site-specific lesions are driven primarily by airflow, by tissue sensitivity, or by another mechanism of toxicity. The present review provides a brief overview of comparative nasal structure, function and toxicologic pathology of the mammalian nasal epithelium and a brief discussion on how data from animal toxicology studies have been used to estimate the risk of inhaled chemicals to human health.

Nasal dosimetry of inhaled gases and particles: where do inhaled agents go in the nose?

The anatomical structure of the nasal passages differs significantly among species, affecting airflow and the transport of inhaled gases and particles throughout the respiratory tract. Since direct measurement of local nasal dose is often difficult, 3-dimensional, anatomically accurate, computational models of the rat, monkey, and human nasal passages were developed to estimate regional transport and dosimetry of inhaled material. The computational models predicted that during resting breathing, a larger portion of inspired air passed through olfactory-lined regions in the rat than in the monkey or human. The models also predicted that maximum wall mass flux (mass per surface area per time) of inhaled formaldehyde in the nonsquamous epithelium was highest in monkeys (anterior middle turbinate) and similar in rats and humans (dorsal medial meatus in the rat and mid-septum in the human, near the squamous/nonsquamous epithelial boundary in both species). For particles that are 5 microm in aerodynamic diameter, preliminary simulations at minute volume flow rates predicted nasal deposition efficiencies of 92%, 11% and 25% in the rat, monkey, and human, respectively, with more vestibular deposition in the rat than in the monkey or human. Estimates such as these can be used to test hypotheses about mechanisms of toxicity and supply species-specific information for risk assessment, thus reducing uncertainty in extrapolating animal data to humans.

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.

Transcriptomic analysis of F344 rat nasal epithelium suggests that the lack of carcinogenic response to glutaraldehyde is due to its greater toxicity compared to formaldehyde

Formaldehyde is cytotoxic and carcinogenic to the rat nasal respiratory epithelium inducing tumors after 12 months. Glutaraldehyde is also cytotoxic but is not carcinogenic to nasal epithelium even after 24 months. Both aldehydes induce similar acute and subchronic histopathology that is characterized by inflammation, hyperplasia, and squamous metaplasia. Because early aldehyde-induced lesions are microscopically similar, we investigated whether transcriptional patterns using cDNA technology could explain the different cancer outcomes. Treatments included 1-, 5-, or 28-day exposure by nasal instillation of formaldehyde solution (400 mM) or glutaraldehyde solution (20 mM). Animals were euthanized and the nasal respiratory epithelium removed for gene expression analysis and a subset of rats treated for 28 days was processed for microscopic examination. RNA was isolated and processed for expression assessment using Clontech Atlas Toxicology II Arrays. Both aldehydes induced hyperplasia, squamous metaplasia, and inflammatory infiltrates with scattered apoptotic bodies in the epithelium covering luminal surfaces of the nasoturbinate, maxilloturbinate, and nasal septum. A subset of 80 genes that were the most variant between the treated and control included the functional categories of DNA repair and apoptosis. Hierarchical clustering discriminated chemical treatment effects after 5 days of exposure, with 6 clusters of genes distinguishing formaldehyde from glutaraldehyde. These data suggest that although both aldehydes induced similar short-term cellular phenotypes, gene expression could distinguish glutaraldehyde from formaldehyde. The gene expression patterns suggest that glutaraldehyde's lack of carcinogenicity may be due to its greater toxicity from lack of DNA-repair, greater mitochondrial damage, and increased apoptosis.

Meeting report: summary of IARC monographs on formaldehyde, 2-butoxyethanol, and 1-tert-butoxy-2-propanol

An international, interdisciplinary working group of expert scientists met in June 2004 to develop IARC Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Humans (IARC Monographs) on formaldehyde, 2-butoxyethanol, and 1-tert-butoxy-2-propanol. Each IARC Monograph includes a critical review of the pertinent scientific literature and an evaluation of an agent's potential to cause cancer in humans. After a thorough discussion of the epidemiologic, experimental, and other relevant data, the working group concluded that formaldehyde is carcinogenic to humans, based on sufficient evidence in humans and in experimental animals. In the epidemiologic studies, there was sufficient evidence that formaldehyde causes nasopharyngeal cancer, "strong but not sufficient" evidence of leukemia, and limited evidence of sinonasal cancer. The working group also concluded that 2-butoxyethanol and 1-tert-butoxy-2-propanol are not classifiable as to their carcinogenicity to humans, each having limited evidence in experimental animals and inadequate evidence in humans. These three evaluations and the supporting data will be published as Volume 88 of the IARC Monographs.

Zonal expression and activity of glutathione S-transferase enzymes in the mouse olfactory mucosa

The rodent olfactory mucosa is characterized by a mosaic of gene expression that is exhibited among various cell types. Olfactory sensitivity in these animals is conveyed through odorant receptor families that are distinctly expressed within various subsets of the olfactory neuron population. Receptor neurons that express a particular class of odorant receptors exhibit bilaterally symmetric zones, which generally define their location within the nasal cavity. Less characterized are zonal expression profiles of proteins among non-neuronal cell types of the olfactory mucosa. In this study, we survey the expression of three glutathione S-transferase (GST) isozymes (alpha, mu, and pi) in the mouse olfactory mucosa and characterize the zonal expression of the mu isozyme. Immunohistochemistry and Western blot analysis of the GST mu isozyme reveal that the lateral olfactory turbinates I, Ib, II, IIb, and III display a greater intensity of expression for GST mu, in comparison to the dorsal and septal regions of the mucosa. GST alpha and pi isozymes do not display any distinct zonal organization in olfactory tissue of the adult mouse. When the general substrate 1-chloro-2-4-dinitrobenzene (CDNB) was used to assess GST activity within the olfactory tissue, the lateral turbinate regions displayed a higher level of activity when compared to dorsal or septal regions. Analysis of GST mu expression in prenatal and early postnatal olfactory tissue also reveals a zonal expression of the isozyme. We relate the significance of these findings to metabolic topography and olfactory chemosensory function.

DNA-protein crosslinks and p53 protein expression in relation to occupational exposure to formaldehyde

BACKGROUND

Formaldehyde (FA) is classified as a probable human carcinogen.

AIMS

To examine DNA protein crosslinks (DPC) and p53, which are generally known to be involved in carcinogenesis, in peripheral blood lymphocytes of workers exposed to FA.

METHODS

DPC and p53 ("wild type" and mutant) were examined in peripheral blood lymphocytes of 186 workers exposed to FA (mean years of exposure = 16) and 213 unexposed workers. Every worker completed a questionnaire on demographic data, occupational and medical history, smoking, and hygiene.

RESULTS

The adjusted mean level of DPC in the exposed and the unexposed workers differed significantly. Adjustment was made for age, sex, years of education, smoking, and origin. Exposure to FA increased the risk of having a higher level of pantropic p53 above 150 pg/ml (OR 1.6, 95% CI 0.8 to 3.1). A significant positive correlation was found between the increase of pantropic p53 protein and mutant p53 protein, as well as between pantropic p53 >150 pg/ml and mutant p53 protein. In the exposed group a significantly higher proportion of p53 >150 pg/ml was found among workers with DPC >0.187 (55.7%) (0.187 = median level of DPC) than among workers with DPC < or =0.187 (33.3%). The risk of having pantropic p53 protein >150 pg/ml was determined mainly by levels of DPC. Workers with DPC above the median level had a significantly higher risk of having pantropic p53 >150 pg/ml (adjusted OR 2.5, 95% CI 1.2 to 5.4).

CONCLUSIONS

Results suggest that DPC and mutation in p53 may represent steps in FA carcinogenesis and a possible causal relation between DPC and mutation in p53. These biomarkers can be applied in the assessment of the development of cancer due to FA exposure.

Inhaled formaldehyde: exposure estimation, hazard characterization, and exposure-response analysis

Formaldehyde has been assessed as a Priority Substance under the Canadian Environmental Protection Act. Probabilistic estimates of exposure of the general population in Canada to formaldehyde in ambient and indoor air are presented. Critical health effects include sensory irritation and the potential to induce tumors in the upper respiratory tract (the nasal region in rodents and potentially the lungs of humans). The majority of the general population is exposed to airborne concentrations of formaldehyde less than those typically associated with sensory irritation (i.e., 0.1 mg/m3). Based primarily upon data derived from laboratory studies, the inhalation of formaldehyde under conditions that induce cytotoxicity and sustained regenerative proliferation within the respiratory tract is considered to present a carcinogenic hazard to humans. At airborne levels for which the prevalence of sensory irritation is minimal (i.e., 0.1 mg/m3), risks of respiratory-tract cancers for the general population estimated on the basis of a biologically motivated case-specific model are exceedingly low. This biologically motivated case-specific model incorporates two-stage clonal expansion and is supported by dosimetry calculations from computational fluid dynamics analyses of formaldehyde flux in various regions of the nose and single-path modeling for the lower respiratory tract. The degree of confidence in the underlying database and uncertainties in estimates of exposure and in characterization of hazard and dose response are delineated.

Normal gene expression in male F344 rat nasal transitional and respiratory epithelium

The nasal epithelium is an important target site for chemically-induced toxicity and carcinogenicity in rodents. Gene expression profiles were determined in order to provide normal baseline data for nasal transitional/respiratory epithelium from healthy rats. Cells lining the rat nasal passages were collected and gene expression analysis was performed using Clontech cDNA Rat Atlas 1.2 arrays (1185 genes). The percentages of genes within specific average expression ranges were 4.2% at 45,000-1000, 14.8% at 1000-200, 25.0% at 200-68, and 56.0% below 68. Nine out of a subset of ten genes were confirmed for relative signal intensity using quantitative real-time RT-PCR. The most highly expressed genes included those involved in phase I (e.g. cytochrome P450s) and phase II (e.g. glutathione S-transferases) xenobiotic metabolism, bioenergetics (e.g. cytochrome oxidase), osmotic balance (e.g. Na(+)/K(+) ATPase) and epithelial ionic homeostasis (e.g. ion channels). Such baseline data will contribute to further understanding the normal physiology of these cells and facilitate the interpretation of responses by the nasal epithelial cells to xenobiotic treatment or disease.

Dosimetry modeling of inhaled formaldehyde: comparisons of local flux predictions in the rat, monkey, and human nasal passages

Formaldehyde-induced nasal squamous cell carcinomas in rats and squamous metaplasia in rats and rhesus monkeys occur in specific regions of the nose with species-specific distribution patterns. Experimental approaches addressing local differences in formaldehyde uptake patterns and dose are limited by the resolution of dissection techniques used to obtain tissue samples and the rapid metabolism of absorbed formaldehyde in the nasal mucosa. Anatomically accurate, 3-dimensional computational fluid dynamics models of F344 rat, rhesus monkey, and human nasal passages were used to estimate and compare regional inhaled formaldehyde uptake patterns predicted among these species. Maximum flux values, averaged over a breath, in nonsquamous epithelium were estimated to be 2620, 4492, and 2082 pmol/(mm(2)-h-ppm) in the rat, monkey, and human respectively. Flux values predicted in sites where cell proliferation rates were measured as similar in rats and monkeys were also similar, as were fluxes predicted in a region of high tumor incidence in the rat nose and the anterior portion of the human nose. Regional formaldehyde flux estimates are directly applicable to clonal growth modeling of formaldehyde carcinogenesis to help reduce uncertainty in human cancer risk estimates.

Dosimetry modeling of inhaled formaldehyde: binning nasal flux predictions for quantitative risk assessment

Interspecies extrapolations of tissue dose and tumor response have been a significant source of uncertainty in formaldehyde cancer risk assessment. The ability to account for species-specific variation of dose within the nasal passages would reduce this uncertainty. Three-dimensional, anatomically realistic, computational fluid dynamics (CFD) models of nasal airflow and formaldehyde gas transport in the F344 rat, rhesus monkey, and human were used to predict local patterns of wall mass flux (pmol/[mm(2)-h-ppm]). The nasal surface of each species was partitioned by flux into smaller regions (flux bins), each characterized by surface area and an average flux value. Rat and monkey flux bins were predicted for steady-state inspiratory airflow rates corresponding to the estimated minute volume for each species. Human flux bins were predicted for steady-state inspiratory airflow at 7.4, 15, 18, 25.8, 31.8, and 37 l/min and were extrapolated to 46 and 50 l/min. Flux values higher than half the maximum flux value (flux median) were predicted for nearly 20% of human nasal surfaces at 15 l/min, whereas only 5% of rat and less than 1% of monkey nasal surfaces were associated with fluxes higher than flux medians at 0.576 l/min and 4.8 l/min, respectively. Human nasal flux patterns shifted distally and uptake percentage decreased as inspiratory flow rate increased. Flux binning captures anatomical effects on flux and is thereby a basis for describing the effects of anatomy and airflow on local tissue disposition and distributions of tissue response. Formaldehyde risk models that incorporate flux binning derived from anatomically realistic CFD models will have significantly reduced uncertainty compared with risk estimates based on default methods.

Investigations of the pathways of toxicity of methyl iodide in the rat nasal cavity

The monohalomethane methyl iodide (MeI) is a site specific toxin within the nasal cavity of the rat, selectively damaging the olfactory epithelium (OE) whilst respiratory epithelium (RE) is spared. The aim of this study was to investigate the rates and routes of metabolism of MeI within the nasal cavity, in order to understand the reasons for the observed site-selectivity. Cytosolic glutathione S-transferases (GSTs) of both OE and RE catalysed the conjugation of MeI with glutathione (GSH), but rates were 4-fold higher in OE than RE. The product of this reaction was confirmed as S-methyl GSH. In both OE and liver the GST catalysing the conjugation of MeI was shown to belong to the theta class. No cytochrome P450-dependent oxidation of MeI to formaldehyde could be detected in incubations containing hepatic or olfactory microsomes. Intact nasal turbinates were incubated with [14C]-MeI, and a dose- and time-dependent covalent binding of MeI to olfactory protein was demonstrated. The rates of protein methylation were found to be similar in OE and RE. Thus the only parameter that correlates with the site-selectivity of the observed lesion is the rate of conjugation of MeI with GSH. Whether toxicity is due to production of a reactive metabolite or GSH depletion per se, remains to be elucidated.

Oncogene and tumor suppressor gene alterations in nasal tumors

The development of nasal tumors in humans and rodents is likely mediated through the accumulation of genetic alterations in genes that regulate cell proliferation, cell death and differentiation (oncogenes and tumor suppressor genes). By examination of the relationship between genetic alterations that are known to occur in human cancers with those induced in rodent tumors with defined carcinogenic exposures, biologically relevant mechanistic linkages of molecular events leading to tumors in rodents and humans can be established. Molecular genetic studies on nasal squamous cell carcinomas (SCC) in rats thus far have indicated the presence of oncogenes unrelated to the ras oncogene family and that p53 mutation occurs at a high frequency among the nasal SCC examined. The finding of p53 mutations in rat nasal SCC and the high prevalence of p53 mutations among human SCC, indicates that a common molecular alteration is shared between rodent and human SCC.

Correlation of regional formaldehyde flux predictions with the distribution of formaldehyde-induced squamous metaplasia in F344 rat nasal passages

Squamous epithelium lines the nasal vestibule of the rat, rhesus monkey, and human. Respiratory, transitional, and olfactory epithelia line most areas posterior to the nasal vestibule. Inhaled formaldehyde gas induces squamous metaplasia posterior to the nasal vestibule and does not induce lesions in the nasal vestibule in rats and rhesus monkeys, indicating that squamous epithelium is resistant to irritant effects of formaldehyde and that squamous metaplasia may be an adaptive response. If squamous metaplasia is determined by formaldehyde dosimetry rather than by tissue-specific factors, squamous epithelium may be protective by absorbing less formaldehyde than other epithelial types. In a previous study, a three-dimensional, anatomically accurate computational fluid dynamics (CFD) model of the anterior F344 rat nasal passages was used to simulate inspiratory airflow and inhaled formaldehyde transport. The present study consisted of two related parts. First, the rat CFD model was used to test the hypothesis that the distribution of formaldehyde-induced squamous metaplasia is related to the location of high-flux regions posterior to squamous epithelium. Regional formaldehyde flux into nonsquamous epithelium predicted by the CFD model correlated with regional incidence of formaldehyde-induced squamous metaplasia on the airway perimeter of one cross-sectional level of the noses of F344 rats exposed to 10 and 15 ppm formaldehyde gas for 6 months. Formaldehyde flux into nonsquamous epithelium was estimated to vary by an order of magnitude depending on the degree of formaldehyde absorption by squamous epithelium. These results indicate that the degree to which squamous epithelium absorbs formaldehyde strongly affects the rate and extent of the progression of squamous metaplasia with continued exposure to formaldehyde. In the second part of this study, the CFD model was used to predict squamous metaplasia progression. Data needs for verification of this model prediction are considered. These results indicate that information on the permeability of squamous epithelium in rats, monkeys, and humans is important for accurate prediction of uptake in regions posterior to the nasal vestibule.

Chemically-induced nasal carcinogenesis and epithelial cell proliferation: a brief review

An increased rate of cell proliferation has long been recognized as an important factor in both human and experimental carcinogenesis, and may be a major risk factor for cancer development in a number of tissues. Limited information exists, however, regarding the relevance of increased cell proliferation and nasal cancer. Examples of toxicological studies utilizing nasal cell proliferation data as an important endpoint are briefly reviewed. Data for one of the most extensively studied chemicals, the weakly genotoxic carcinogen formaldehyde, support the contention that the concentration-response relationship for tumor incidence is a function of formaldehyde-induced target cell proliferation, in addition to other factors including target cell population size. The increasing importance of utilizing cell proliferation data in determining dose-response relationships and in biologically-based risk assessment models is discussed.

DNA-Protein Crosslinks and Sister Chromatid Exchanges as Biomarkers of Exposure to Formaldehyde

Formaldehyde is classified as a probable human carcinogen. DNA-protein crosslinks (DPCs) and sister chromatid exchanges (SCEs) may represent early lesions in the carcinogenic process. The authors examined the DPCs and SCEs in peripheral-blood lymphocytes of 12 and 13 workers exposed to formaldehyde and eight and 20 unexposed workers, respectively. The amounts of DPCs and SCEs in the exposed and the unexposed differed significantly after adjustment for smoking. There was a linear relationship between years of exposure and the amounts of DPC and SCE. The authors conclude that the data indicate a possible mechanism of carcinogenicity of formaldehyde, and that formaldehyde is mutagenic to humans. These results support the use of DPCs as a biomarker of occupational exposure to formaldehyde and to detect high-risk populations for secondary prevention.

DNA damage in nasal respiratory epithelium from children exposed to urban pollution

The nasal cavity is the most common portal of entry to the human body and a well-known target site for a wide range of air pollutants and chemically induced toxicity and carcinogenicity. DNA single-strand breaks (SSB) can be used as a biomarker of oxidant exposure and as an indicator of the carcinogenicity and mutagenicity of a substance. We examined the utility of using the alkaline single cell gel electrophoresis assay (SCGE) for measuring DNA damage in children's nasal epithelium exposed to air pollutants. We studied 148 children, ages 6-12, including 19 control children from a low polluted Pacific port and 129 children from Southwest Metropolitan Mexico City, an urban polluted area with high ozone concentrations year-round. Three sets of two nasal biopsies were taken in a 3-month period. All exposed children had upper respiratory symptoms and DNA damage in their nasal cells. Eleven- and twelve-year-olds had the most DNA damage, and more than 30% of children aged 9-12 exhibited patchy areas of squamous metaplasia over high-flow nasal regions. These areas had the greatest numbers of damaged DNA cells (P < or = 0.001) and a large number of DNA tails > 80 microns (P < 0.001) when compared to the contralateral macroscopically normal site in the same child. The youngest children with significantly less outdoor exposure displayed patchy areas of goblet cell hyperplasia and had the least DNA damage. These findings suggest that SCGE can be used to monitor DNA damage in children's nasal epithelium and, further, the identification of DNA damage in nasal proliferative epithelium could be regarded as a sentinel lesion, most likely due to severe and sustained cell injury.

Toxicological considerations in evaluating indoor air quality and human health: impact of new carpet emissions

This review article considers evidence regarding the toxicological impact of new carpet emissions on indoor air quality and human health. It compares emissions data from several studies and describes the dominant compounds found in those emissions. The toxicity of each these compounds is assessed for animal and human data, with a focus on inhalation exposure. Data for acute and chronic exposures are presented, and synergistic effects are considered. Differences and similarities between health responses caused by toxicity and/or by immunological reactions are discussed. Possible neurogenic pathways and associations between these and immune changes are considered as they might relate to inflammatory-based human reactions. Additionally, factors affecting human odor responses are described. The roles that a variety of psychological factors may also play in the etiology of potentially related phenomena, such as the sick building syndrome, pathogenic illness, and multiple chemical sensitivity, are considered. Gaps in the literature are identified within the article and suggestions for future research are offered. In particular, it is noted that few, if any, prior studies have evaluated both neurogenic and immune-mediated inflammation status within the same study. Based on the present information available, it is concluded that under normal environmental circumstances, VOC emissions from new carpets are sufficiently low such that they should not adversely affect indoor air quality or pose significant health risk to people.

Reactions of hydrated formaldehyde in nasal mucus

Formaldehyde is a well known toxic air impurity affecting the upper respiratory tract. It rapidly forms methylene glycol in water. Reactions of the hydrated formaldehyde with nasal mucus were studied by C-13 NMR spectroscopy. In the NMR spectra methylene glycol dominated and only minor signals from possible reactions were observed. This finding suggests that nasal mucus effectively protects nasal epithelium against formaldehyde.

A strategy for establishing mode of action of chemical carcinogens as a guide for approaches to risk assessments

The current standard approach for assessing carcinogenic potential is to conduct a near lifetime rodent pathology study with the high dose set to the maximum tolerated dose (MTD) of the test chemical. The linearized multistage model is then used as the default approach to estimate the potential human cancer risk at environmental elvels of the chemical. There is an increasing appreciation in the scientific and regulatory communities that chemical carcinogens differ dramatically in potency, exhibit a high degree of tissue and species specificity, and act through different modes of action. This paper advocates a decision tree strategy for classifying carcinogens that are acting primarily through genotoxic, cytotoxic, or mitogenic pathways. A primary concern is whether the chemical has direct genotoxic potential resulting from DNA reactivity or clastogenicity of the compound or its metabolite(s). Knowledge of the exposure-response curve for cytotoxicity is important because initiation and promotion events may occur secondary to a variety of associated activities such as regenerative cell proliferation. Mitogens indice direct stimulation of growth and may provide a selective growth advantage to spontaneously initiated precancerous cells. Of particular concern is the situation where pathological changes induced during the course of the treatment at high doses near the MTD are absent at lower, environmentally relevant, doses. If the tumor response is coincident with the preceding toxic response, it may not be justified to use the high-dose data in extrapolating to expected responses at low environmental exposures where no induced tissue abnormalities occur. Suggestions are presented for appropriate risk assessment approaches for different modes of action. Examples discussed are formaldehyde, a weakly genotoxic rodent nasal carcinogen; chloroform, a nongenotoxic-cytotoxic rodent liver and kidney carcinogen; and phenobarbital, a nongenotoxic-mitogenic rodent liver carcinogen.

Nasal diagrams: a tool for recording the distribution of nasal lesions in rats and mice

Knowledge of patterns of lesion distribution can provide insight into the relative roles played by regional tissue dose and local tissue susceptibility in toxic responses to xenobiotics in the nose and assist assessment of potential human risk. A consistent approach is needed for recording lesion distribution patterns in the complex nasal airways of rats and mice. The present work provides a series of diagrams of the nasal passages of the Fischer-344 rat and B6C3F1 mouse, designed for mapping nasal lesions. The diagrams present each of the major cross-sectional airway profiles, provide adequate space for nasal mucosal lesion recording, and are suitable for duplication in a commercial photocopier. Sagittal diagrams are also provided to permit transfer of lesion location data observed in transverse sections onto the long axis of the nose. The distribution of lesions induced by a selected range of xenobiotics is presented. Approaches to application of the diagrams and interpretation of results obtained are discussed in relation to factors responsible for lesion distribution in the nose and their relevance to interspecies extrapolation. A modified approach to anatomical classification of the ethmoturbinates of the rodent is also presented.

Cell proliferation and formaldehyde-induced respiratory carcinogenesis

Formaldehyde is a nasal carcinogen in the rat but the cancer risk this chemical poses for humans remains to be determined. Formaldehyde induces nonlinear, concentration-dependent increases in nasal epithelial cell proliferation and DNA-protein cross-link formation following short-term exposure. Presented in this review are results from a mechanistically based formaldehyde inhalation study in which an important endpoint was the measurement of cell proliferation indices in target sites for nasal tumor induction. Male Fischer 344 rats were exposed to 0, 0.7, 2, 6, 10, or 15 ppm formaldehyde for up to 2 years (6 hr/day, 5 day/week). Statistically significant increases in cell proliferation were confined to the 10 and 15 ppm groups, which remained elevated throughout the study. The concentration-dependent increases in cell proliferation correlated strongly with the tumor response curve, supporting the proposal that sustained increases in cell proliferation are an important component of formaldehyde carcinogenesis. The nonlinearity observed in formaldehyde-induced rodent nasal cancer is consistent with a high-concentration effect of regenerative cell proliferation of the target organ coupled with the genotoxic effects of formaldehyde. Cell kinetic data from these studies provide important information that may be utilized in the assessment of risk for humans exposed to formaldehyde.

A critical review of the toxicology of glutaraldehyde

Glutaraldehyde, a low molecular weight aldehyde, has been investigated for toxicity in humans and animals. Examination of this dialdehyde was indicated from previous studies with other aldehydes in which carcinogenicity of formaldehyde and toxicity of acetaldehyde and malonaldehyde have been disclosed. Information gaps concerning the actions of glutaraldehyde have been identified in this review and recommendations are suggested for additional short- and long-term studies. In particular, information regarding irritation of the respiratory tract, potential neurotoxicity, and developmental effects would assist in a complete hazard evaluation of glutaraldehyde. Further study related to disposition, metabolism, and reactions of glutaraldehyde may elucidate the mechanism of action.

Proliferative and neoplastic lesions in the rodent nasal cavity

Proliferative lesions in the rodent nasal cavity are reviewed; attempt was made to compare species affected, sex differences, strain differences, route of administration and tumor types occurring both spontaneously and after induction by different chemicals. This review is not meant to be all inclusive but to be representative of observed trends. Our general conclusions in this paper are that: 1) spontaneous nasal tumors in rodents are very rare; 2) spontaneous nasal tumors in rats are most often squamous cell tumors, whereas hemangiomas or respiratory adenomas predominate in mice and squamous cell tumors are rare; 3) rats are usually more susceptible to the induction of epithelial tumors of the nasal cavity than mice; 4) chemically-induced hemangiomas and hemangiosarcomas of the nasal cavity have only been reported in mice; 5) tumors of the olfactory epithelium are almost uniformly malignant and invasive, while nonsquamous tumors of the respiratory epithelium are typically less invasive; 6) chemically-induced tumors of the olfactory region, either mesenchymal or epithelial, do not always require an inhalation route of exposure but may occur by systemic targeting of this region; and 7) chemicals inducing tumors in the olfactory region often produce a variety of tumor morphologies in this location as well as squamous and polypoid tumors of the transitional region. More work will be needed to illucidate the mechanisms of nasal carcinogenesis and to further refine the current tumor classification system.

Studies of inspiratory airflow patterns in the nasal passages of the F344 rat and rhesus monkey using nasal molds: relevance to formaldehyde toxicity

For highly water soluble and reactive gases, such as formaldehyde, the reported distribution of nasal lesions in rats and rhesus monkeys following inhalation exposure may be attributable, at least in part, to regional gas uptake patterns that are a consequence of nasal airflow characteristics. Inspiratory nasal airflow was studied at flow rates across the physiologic range using a unidirectional dynamically similar water-dye siphon system in clear acrylic molds of the nasal airways of F344 rats and rhesus monkeys. In both species there were complex and inspiratory flow streams, exhibiting regions of simple laminar, complex secondary (vortices, eddies, swirling), and turbulent flows, with only minor effects of the volumetric flow rates studied on these flow patterns. There was a precise association between points of dye intake at the nostril with complex but generally coherent streaklines throughout the nose, indicating the potential for sensitive dependence of nasal airflow on nostril geometry. On the basis of these studies, a classification for the major airways (meatuses) in the nasal passages of rats and rhesus monkeys was proposed. The spiral shape of the anterior nasal airway of the rat was considered to play an important role in local mixing of inspired airstreams. In the rhesus monkey, the complex geometry of the nasal vestibule contributed to the formation of secondary flows and turbulence in the anterior nose, which represents a potentially important difference between rheusus monkeys and humans. There was a good correlation between routes of flow, regional secondary flows, turbulence, and impaction of airstreams on the airway wall, with the reported distribution of formaldehyde-induced nasal lesions in rats and rhesus monkeys. These studies support the proposal that nasal airflow patterns play an important role in the distribution of lesions induced by formaldehyde.

Toxic and neoplastic responses in the nasal passages: future research needs

It is evident that much remains to be learned about the nasal passages and their responses to toxic materials. For the nose of both laboratory animals and humans, information is needed in the areas of anatomy, physiology, biochemistry, neurobiology, physiopathology, and oncology. This article briefly discussed toxic and neoplastic responses of the nasal passages, and identified a number of issues and questions that provide potentially valuable areas for further research. It was stated that: (1) Histopathologic examination of the nose could profit from the development of a good all-purpose fixative. (2) A consistent and appropriate classification of nasal passageways, epithelia, and other structures is needed to avoid further confusion. (3) A workable scheme for lesion mapping is needed for routine description of lesion distribution in the nasal passages in rodent toxicology studies. (4) Quantitative data are needed concerning regional substrate specificities and kinetics of nasal enzymes in animals and humans for a wide range of enzymes responsible for metabolism of xenobiotics. Moreover, the following questions should be addressed in the future: (1) What is the nature of the progenitor cells in the olfactory epithelium, basal cells alone, or basal and ductular cells? (2) What determines the resistance of regenerated rat olfactory epithelium to subsequent methyl bromide exposure? (3) Can this resistance phenomenon be demonstrated with other olfactory toxicants and in other species? (4) What influence do cage contaminant gases have on olfactory research in laboratories using rodents? The authors also believe that, despite the fact that nasal airflow has been a subject of investigation for many years, much remains to be learned about this complex process. It is expected that the application of computer technology to mathematical modeling of nasal airflow and regional gas uptake will yield significant new information for the understanding of mechanisms responsible for the distribution of upper respiratory tract lesions in animals and humans. The combination of models of regional uptake, wall flux rates, critical biochemical events, nasal blood flow, and other features of nasal physiology, and integration of these models with lower respiratory tract models, will provide valuable tools for investigations of nasal pathology and toxicology. It was also stressed that the effects of toxicants on olfactory function in humans deserve more attention since, in some past studies, it was suggested that the protection afforded by current TLVs against olfactory toxicity may be marginal. A simple and sensitive olfactometric test of general application for toxicology testing in animals remains to be validated.(ABSTRACT TRUNCATED AT 400 WORDS)

Examination of potential mechanisms of carcinogenicity of 1,4-dioxane in rat nasal epithelial cells and hepatocytes

Several long-term studies with 1,4-dioxane (dioxane) have shown it to induce liver tumors in mice, and nasal and liver tumors in rats when administered in amounts from 0.5 to 1.8% in the drinking water (Argus et al. 1965; Kociba et al. 1974; National Cancer Institute, 1978). In order to examine potential mechanisms of action, chemically-induced DNA repair (as an indicator of DNA reactivity) and cell proliferation (as an indicator of promotional activity) were examined in nasal turbinate epithelial cells and hepatocytes of male Fischer-344 rats treated with dioxane. Neither dioxane nor 1,4-dioxane-2-one, one of the proposed metabolites, exhibited activity in the in vitro primary rat hepatocyte DNA repair assay, even from cells that had been isolated from animals given either 1 or 2% dioxane in the drinking water for 1 week to induce enzymes that might be responsible for producing genotoxic metabolites. No activity was seen in the in vivo hepatocyte DNA repair assay in animals given a single dose of up to 1000 mg/kg dioxane or up to 2% dioxane in the drinking water for 1 week. Treatment of rats with 1.0% dioxane in the drinking water for 5 days yielded no increase in liver/body weight nor induction of palmitoyl CoA oxidase, indicating that dioxane does not fit into the class of peroxisomal proliferating carcinogens. The percentage of cells in DNA synthesis phase (S-phase) was determined by administration of 3H-thymidine and subsequent quantitative histoautoradiography. The hepatic labeling index (LI) did not increase at either 24 or 48 h following a single dose of 1000 mg/kg dioxane.(ABSTRACT TRUNCATED AT 250 WORDS)

Histochemical localization of formaldehyde dehydrogenase in the rat

Formaldehyde dehydrogenase (FDH) activity has been demonstrated biochemically in the olfactory and respiratory mucosae and in the liver of the rat, but the cellular localization of this enzyme has not been investigated. A histochemical procedure was developed to permit cellular localization of FDH. This allowed us to examine the relationship between distribution of FDH and formaldehyde-induced toxicity. Cold-processed glycol methacrylate embedded tissues were used to localize FDH activity in the rat respiratory tract, kidney, liver, and brain. Five- or ten-micrometer tissue sections were incubated in a reaction mixture containing formaldehyde (HCHO), glutathione (GSH), NAD+, nitroblue tetrazolium, pyrazole, and disulfiram. A blue formazan precipitate was formed at the site of FDH activity. Epithelial cell cytoplasm of both the respiratory and the olfactory mucosae of the nose stained for FDH, and olfactory sensory cell nuclei were also positive. Underlying Bowman's and seromucous glands were weakly positive. The lung had FDH activity located mainly in the Clara cells of the airways, with only diffuse weak activity in the lung parenchyma. Liver had activity in the cytoplasm of the hepatocytes, while in the kidney FDH was most prominent in the brush border of the P2 segment of the proximal tubules. Brain white matter stained strongly for FDH, while in gray matter only the neuropil exhibited weak activity. Corresponding tissue sections were stained for sulfhydryls; these sections indicated that GSH is likely to be present in all cells with FDH activity. For the respiratory tract these results demonstrate distinct differences between the location of FDH activity and previously reported nonspecific aldehyde dehydrogenase activity in the nose (M. S. Bogdanffy, H. W. Randall, and K. T. Morgan, 1986, Toxicol. Appl. Pharmacol. 82, 560-567). While high aldehyde dehydrogenase activities were found in tissues with low toxicities due to acetaldehyde exposure and vice versa, FDH activity was observed in tissues whether or not they exhibited a toxic response to inhaled HCHO. While not able to account for the localized toxicity of HCHO, the presence of FDH and glutathione in the epithelial layer of the nasal cavity presents a barrier to inhaled formaldehyde at low concentrations and may partially explain the observed nonlinearity of HCHO toxicity.

Pathology and cell proliferation induced by intra-nasal instillation of aldehydes in the rat: comparison of glutaraldehyde and formaldehyde

The relative toxicities of formaldehyde and glutaraldehyde to the rat nasal epithelium were determined following intra-nasal instillation of aqueous solutions of these compounds into one nostril of male Fischer 344 (F-344) rats. Lesions identical in appearance to those resulting from acute inhalation exposure to formaldehyde were induced by both compounds in a concentration-dependent manner. Treatments included India ink or 1 M methylene blue (for instillation deposition studies); sterile saline (vehicle control); 40, 200, 400, and 800 mM formaldehyde; and 10, 20, and 40 mM glutaraldehyde. Dye-treated rats were sacrificed immediately, and nasal passages were examined to determine the localization of instilled materials. Three days after treatment, all other animals received a single ip injection of 5-bromo-2'-deoxyuridine 2 hr prior to sacrifice, and the nasal passages were prepared for histopathology and cell proliferation studies. While sterile saline and 10 mM glutaraldehyde induced no significant epithelial changes, 20 and 40 mM glutaraldehyde induced extensive lesions in the treated side of the nose. Aldehyde-induced lesions included inflammation, epithelial degeneration, respiratory epithelial hypertrophy, and squamous metaplasia in association with marked increases (3-8-fold) in labeling index for both compounds. Formaldehyde induced similar lesions but required concentrations of 200 mM or more to elicit a toxic response. Thus, glutaraldehyde is approximately an order of magnitude more toxic to the nasal epithelium than formaldehyde. These studies also indicate that the nose is very resistant to the aldehydes studied, requiring instillation of millimolar concentrations before toxic responses occurred.

Interactive effects of ozone and formaldehyde on the nasal respiratory lining epithelium in rats

The combined effects on the nasal epithelium of mixtures of ozone and formaldehyde at cytotoxic and noncytotoxic concentrations were examined. Male Wistar rats were exposed by inhalation during 22 h/d for 3 consecutive days to 0.3, 1.0, or 3.0 ppm formaldehyde, or to 0.2, 0.4, or 0.8 ppm ozone, or to mixtures of 0.4 ppm ozone and 0.3, 1.0, or 3.0 ppm formaldehyde, or to 1.0 ppm formaldehyde and 0.2, 0.4, or 0.8 ppm ozone, or they were sham-exposed to clean air. The noses were examined for pathological changes at six standard cross levels by light microscopy and for epithelial cell proliferation by counting [3H-methyl]thymidine-labeled cells at cross levels II and III. Ozone at 0.4 ppm or 0.8 ppm or formaldehyde at 3 ppm enhanced cell proliferation at cross level II at all locations, except for the epithelium of the septum, which was not affected by ozone. At cross level III ozone alone did not induce cell proliferation, but formaldehyde at 0.3 and 1 ppm tended to reduce cell proliferation while at 3 ppm proliferation was slightly stimulated. The combined exposure to 0.4 ppm ozone and 0.3 ppm formaldehyde induced less cell proliferation at cross levels II and III when compared with that of 0.4 ppm ozone alone. Less cell proliferation was also seen at cross level II when animals were exposed to 0.4 or 0.8 ppm ozone in combination with 1 ppm formaldehyde than when exposed to these ozone concentrations alone. A more than additive increase in cell proliferation was found at cross level II after exposure to 0.4 ppm ozone in combination with 3 ppm formaldehyde, and at cross level III in animals exposed to 0.4 ppm ozone and 1 or 3 ppm formaldehyde. Treatment-related histopathological nasal changes, such as disarrangement, loss of cilia, and hyper/metaplasia of the epithelium were seen at 0.2, 0.4, and 0.8 ppm ozone and at 3 ppm formaldehyde. Simultaneous exposure to both materials did not noticeably affect type, degree, and size of the microscopic nasal lesions.