Respiratory toxicity of diacetyl in C57BL/6 mice

Recognizing occupational effects of diacetyl: What can we learn from this history?

For half of the 30-odd years that diacetyl-exposed workers have developed disabling lung disease, obliterative bronchiolitis was unrecognized as an occupational risk. Delays in its recognition as an occupational lung disease are attributable to the absence of a work-related temporal pattern of symptoms; failure to recognize clusters of cases; complexity of exposure environments; and absence of epidemiologic characterization of workforces giving rise to case clusters. Few physicians are familiar with this rare disease, and motivation to investigate the unknown requires familiarity with what is known and what is anomalous. In pursuit of the previously undescribed risk, investigators benefited greatly from multi-disciplinary collaboration, in this case including physicians, epidemiologists, environmental scientists, toxicologists, industry representatives, and worker advocates. In the 15 years since obliterative bronchiolitis was described in microwave popcorn workers, α-dicarbonyl-related lung disease has been found in flavoring manufacturing workers, other food production workers, diacetyl manufacturing workers, and coffee production workers, alongside case reports in other industries. Within the field of occupational health, impacts include new ventures in public health surveillance, attention to spirometry quality for serial measurements, identifying other indolent causes of obliterative bronchiolitis apart from accidental over-exposures, and broadening the spectrum of diagnostic abnormalities in the disease. Within toxicology, impacts include new attention to appropriate animal models of obliterative bronchiolitis, pertinence of computational fluid dynamic-physiologically based pharmacokinetic modeling, and contributions to mechanistic understanding of respiratory epithelial necrosis, airway fibrosis, and central nervous system effects. In these continuing efforts, collaboration between laboratory scientists, clinicians, occupational public health practitioners in government and industry, and employers remains critical for improving the health of workers inhaling volatile α-dicarbonyl compounds.

Pathology, toxicology, and latency of irritant gases known to cause bronchiolitis obliterans disease: Does diacetyl fit the pattern?

Bronchiolitis obliterans (BO) is a rare disease involving concentric bronchiolar fibrosis that develops rapidly following inhalation of certain irritant gases at sufficiently high acute doses. While there are many potential causes of bronchiolar lesions involved in a variety of chronic lung diseases, failure to clearly define the clinical features and pathological characteristics can lead to ambiguous diagnoses. Irritant gases known to cause BO follow a similar pathologic process and time course of disease onset in humans. Studies of inhaled irritant gases known to cause BO (e.g., chlorine, hydrochloric acid, ammonia, nitrogen oxides, sulfur oxides, sulfur or nitrogen mustards, and phosgene) indicate that the time course between causal chemical exposures and development of clinically significant BO disease is typically limited to a few months. The mechanism of toxic action exerted by these irritant gases generally involves widespread and severe injury of the epithelial lining of the bronchioles that leads to acute respiratory symptoms which can include lung edema within days. Repeated exposures to inhaled irritant gases at concentrations insufficient to cause marked respiratory distress or edema may lead to adaptive responses that can reduce or prevent severe bronchiolar fibrotic changes. Risk of BO from irritant gases is driven substantially by toxicokinetics affecting concentrations occurring at the bronchiolar epithelium. Highly soluble irritant gases that cause BO like ammonia generally follow a threshold-dependent cytotoxic mechanism of action that at sufficiently high doses results in severe inflammation of the upper respiratory tract and the bronchiolar epithelium concurrently. This is followed by acute respiratory distress, pulmonary edema, and post inflammatory concentric fibrosis that become clinically obvious within a few months. In contrast, irritant gases with lower solubility like phosgene also follow a threshold-dependent mechanism of cytotoxicity action but can exhibit more insidious and isolated bronchiolar tissue damage with a similar latency to fibrosis. To date, animal and human studies on the highly soluble gas, diacetyl, have not identified a coherent pattern of pathology and latency that would be expected based on studies of other known causes of bronchiolitis obliterans disease.

Diacetyl and 2,3-pentanedione exposure of human cultured airway epithelial cells: Ion transport effects and metabolism of butter flavoring agents

Inhalation of butter flavoring by workers in the microwave popcorn industry may result in “popcorn workers' lung.” In previous in vivo studies rats exposed for 6 h to vapor from the flavoring agents, diacetyl and 2,3-pentanedione, acquired flavoring concentration-dependent damage of the upper airway epithelium and airway hyporeactivity to inhaled methacholine. Because ion transport is essential for lung fluid balance,we hypothesized that alterations in ion transport may be an early manifestation of butter flavoring-induced toxicity.We developed a system to expose cultured human bronchial/tracheal epithelial cells (NHBEs) to flavoring vapors. NHBEs were exposed for 6 h to diacetyl or 2,3-pentanedione vapors (25 or ≥ 60 ppm) and the effects on short circuit current and transepithelial resistance (Rt) were measured. Immediately after exposure to 25 ppm both flavorings reduced Na+ transport,without affecting Cl- transport or Na+,K+-pump activity. Rt was unaffected. Na+ transport recovered 18 h after exposure. Concentrations (100-360 ppm) of diacetyl and 2,3-pentanedione reported earlier to give rise in vivo to epithelial damage, and 60 ppm, caused death of NHBEs 0 h post-exposure. Analysis of the basolateral medium indicated that NHBEs metabolize diacetyl and 2,3-pentanedione to acetoin and 2-hydroxy-3-pentanone, respectively. The results indicate that ion transport is inhibited transiently in airway epithelial cells by lower concentrations of the flavorings than those that result in morphological changes of the cells in vivo or in vitro.

Characterization of naturally occurring airborne diacetyl concentrations associated with the preparation and consumption of unflavored coffee

Diacetyl, a suspected cause of respiratory disorders in some food and flavorings manufacturing workers, is also a natural component of roasted coffee. We characterized diacetyl exposures that would plausibly occur in a small coffee shop during the preparation and consumption of unflavored coffee. Personal (long- and short-term) and area (long-term) samples were collected while a barista ground whole coffee beans, and brewed and poured coffee into cups. Simultaneously, long-term personal samples were collected as two participants, the customers, drank one cup of coffee each per h. Air sampling and analyses were conducted in accordance with OSHA Method 1012. Diacetyl was detected in all long-term samples. The long-term concentrations for the barista and area samples were similar, and ranged from 0.013⿿0.016 ppm; long-term concentrations for the customers were slightly lower and ranged from 0.010⿿0.014 ppm. Short-term concentrations ranged from below the limit of detection (<0.0047 ppm)⿿0.016 ppm. Mean estimated 8 h time-weighted average (8 h TWA) exposures for the barista ranged from 0.007⿿0.013 ppm; these values exceed recommended 8 h TWA occupational exposure limits (OELs) for diacetyl and are comparable to long-term personal measurements collected in various food and beverage production facilities. The concentrations measured based on area sampling were comparable to those measured in the breathing zone of the barista, thus exceedances of the recommended OELs may also occur for coffee shop workers who do not personally prepare coffee (e.g., cashier, sanitation/maintenance). These findings suggest that the practicality and scientific basis of the recommended OELs for diacetyl merit further consideration.

Naturally occurring diacetyl and 2,3-pentanedione concentrations associated with roasting and grinding unflavored coffee beans in a commercial setting

Over the last decade, concerns have been raised about potential respiratory health effects associated with occupational exposure to the flavoring additives diacetyl and 2,3-pentanedione. Both of these diketones are also natural components of many foods and beverages, including roasted coffee. To date, there are no published studies characterizing workplace exposures to these diketones during commercial roasting and grinding of unflavored coffee beans. In this study, we measured naturally occurring diacetyl, 2,3-pentanedione, and respirable dust at a facility that roasts and grinds coffee beans with no added flavoring agents. Sampling was conducted over the course of three roasting batches and three grinding batches at varying distances from a commercial roaster and grinder. The three batches consisted of lightly roasted soft beans, lightly roasted hard beans, and dark roasted hard beans. Roasting occurred for 37 to 41 min, and the grinding process took between 8 and 11 min. Diacetyl, 2,3-pentanedione, and respirable dust concentrations measured during roasting ranged from less than the limit of detection (<LOD) to 0.0039 ppm, <LOD to 0.018 ppm, and <LOD to 0.31 mg/m³, respectively. During grinding, diacetyl, 2,3-pentanedione, and respirable dust concentrations ranged from 0.018 to 0.39 ppm, 0.0089 to 0.21 ppm, and <LOD to 1.7 mg/m³, respectively. For any given bean/roast combination and sample location, diketone concentrations during grinding were higher than those measured during roasting. During grinding, concentrations decreased with increased distance from the source. Measured concentrations of both diketones were higher during grinding of soft beans than hard beans. The results indicate that airborne concentrations of naturally occurring diacetyl and 2,3-pentanedione associated with unflavored coffee processing: (1) are similar to the concentrations that have been measured in food flavoring facilities; (2) are likely to exceed some recommended short-term occupational exposure limits, but; (3) based on previous analyses of exposure response relationships in animal studies, are far below the concentrations that are expected to cause even minimal responses in the human respiratory tract.

Occupational and environmental bronchiolar disorders

Occupational and environmental causes of bronchiolar disorders are recognized on the basis of case reports, case series, and, less commonly, epidemiologic investigations. Pathology may be limited to the bronchioles or also involve other components of the respiratory tract, including the alveoli. A range of clinical, functional, and radiographic findings, including symptomatic disease lacking abnormalities on noninvasive testing, poses a diagnostic challenge and highlights the value of surgical biopsy. Disease clusters in workplaces and communities have identified new etiologies, drawn attention to indolent disease that may otherwise have been categorized as idiopathic, and expanded the spectrum of histopathologic responses to an exposure. More sensitive noninvasive diagnostic tools, evidence-based therapies, and ongoing epidemiologic investigation of at-risk populations are needed to identify, treat, and prevent exposure-related bronchiolar disorders.

Evaluation of four α-diketones for toll-like receptor-4 (TLR-4) activation in a human transfected cell line

Toll-like receptor-4 (TLR-4) activity is upregulated in persons with fibrotic lung diseases secondary to chronic inflammatory conditions like Crohn's disease and rheumatoid arthritis. We hypothesized that α-diketones associated with fixed obstructive lung disease may activate TLR-4. We utilized a human embryonic kidney cell assay (HEK293) with human TLR-4 receptors to test for potential activation effects of 2,3-butandeione, 2,3-pentanedione, 2,3-hexanedione, and 2,3-heptanedione at test concentrations of 1, 10, 100, and 1000 µM. The assay detects NF-κB-induced expression of secreted alkaline phosphatase measured after 16 h incubation by a UV-VIS Spectrometer at 650 nm. Escherichia coli K12 lipopolysaccharide (LPS) at 0.5 ng/mL served as a positive control and was added with each test compound to evaluate combined effects. None of the tested α-diketones were found to exhibit cytotoxicity, agonism, or synergistic effects with LPS in the human TLR-4 assay up to 1000 µM. Screening of 2,3-butanedione for agonist activity using the HEK assay with mouse TLR-4 receptors exhibited cytotoxicity at 1000 µM, but no agonist activity. We conclude that the tested α-diketones at relatively high concentrations in vitro do not exhibit TLR-4 agonist or synergistic activity and, therefore, apparently do not directly induce inflammasome activation through this pathway in humans or mice.

Diacetyl: occurrence, analysis, and toxicity

Diacetyl possesses a butter-like flavor and has been widely used as a flavoring agent. It forms from sugars and lipids via various bacteria and heat treatment in various foods and beverages, such as milk. The toxicity of diacetyl, especially when inhaled, has recently attracted the attention not only of consumers but also of regulatory agencies. Even though accurate quantitative analysis of diacetyl is extremely important in evaluating its possible adverse effects, precise quantitative analysis of diacetyl in foods and beverages, as well as in ambient air, is considerably difficult because it is highly reactive and soluble in water. Among the many analytical methods developed for measuring diacetyl, preparation of 2,3-dimethylquinoxaline followed by gas chromatography has been most commonly used in the analysis of various foods, beverages, and air samples. This mini-review summarizes the formation mechanisms, analytical methods, occurrence, and toxicity of diacetyl.

Diacetyl and 2,3-pentanedione exposures associated with cigarette smoking: implications for risk assessment of food and flavoring workers

Diacetyl and 2,3-pentanedione inhalation have been suggested as causes of severe respiratory disease, including bronchiolitis obliterans, in food/flavoring manufacturing workers. Both compounds are present in many food items, tobacco, and other consumer products, but estimates of exposures associated with the use of these goods are scant. A study was conducted to characterize exposures to diacetyl and 2,3-pentanedione associated with cigarette smoking. The yields (μg/cigarette) of diacetyl and 2,3-pentanedione in mainstream (MS) cigarette smoke were evaluated for six tobacco products under three smoking regimens (ISO, Massachusetts Department of Public Health, and Health Canada Intense) using a standard smoking machine. Mean diacetyl concentrations in MS smoke ranged from 250 to 361 ppm for all tobacco products and smoking regimens, and mean cumulative exposures associated with 1 pack-year ranged from 1.1 to 1.9 ppm-years. Mean 2,3-pentanedione concentrations in MS smoke ranged from 32.2 to 50.1 ppm, and mean cumulative exposures associated with 1 pack-year ranged from 0.14 to 0.26 ppm-years. We found that diacetyl and 2,3-pentanedione exposures from cigarette smoking far exceed occupational exposures for most food/flavoring workers who smoke. This suggests that previous claims of a significant exposure-response relationship between diacetyl inhalation and respiratory disease in food/flavoring workers were confounded, because none of the investigations considered or quantified the non-occupational diacetyl exposure from cigarette smoke, yet all of the cohorts evaluated had considerable smoking histories. Further, because smoking has not been shown to be a risk factor for bronchiolitis obliterans, our findings are inconsistent with claims that diacetyl and/or 2,3-pentanedione exposure are risk factors for this disease.

The National Toxicology Program Web-based nonneoplastic lesion atlas: a global toxicology and pathology resource

Toxicologists and pathologists worldwide will benefit from a new, website-based, and completely searchable Nonneoplastic Lesion Atlas just released by the U.S. National Toxicology Program (NTP). The atlas is a much-needed resource with thousands of high-quality, zoomable images and diagnostic guidelines for each rodent lesion. Liver, gallbladder, nervous system, bone marrow, lower urinary tract and skin lesion images, and diagnostic strategies are available now. More organ and biological systems will be added with a total of 22 chapters planned for the completed project. The atlas will be used by the NTP and its many pathology partners to standardize lesion diagnosis, terminology, and the way lesions are recorded. The goal is to improve our understanding of nonneoplastic lesions and the consistency and accuracy of their diagnosis between pathologists and laboratories. The atlas is also a useful training tool for pathology residents and can be used to bolster any organization's own lesion databases. Researchers have free access to this online resource at www.ntp.niehs.nih.gov/nonneoplastic.

Interspecific bacterial sensing through airborne signals modulates locomotion and drug resistance

Bacteria use chemical signals to sense each other and to regulate various physiological functions. Although it is known that some airborne volatile organic compounds function as bacterial signalling molecules, their identities and effects on global gene expression and bacterial physiological processes remain largely unknown. Here we perform microarray analyses of Escherichia coli exposed to volatile organic compounds emitted from Bacillus subtilis. We find that 2,3-butanedione and glyoxylic acid mediate global changes in gene expression related to motility and antibiotic resistance. Volatile organic compound-dependent phenotypes are conserved among bacteria and are regulated by the previously uncharacterized ypdB gene product through the downstream transcription factors soxS, rpoS or yjhU. These results strongly suggest that bacteria use airborne volatile organic compounds to sense other bacteria and to change master regulatory gene activity to adapt.

Evaluation of the hypersensitivity potential of alternative butter flavorings

Concern has been raised over the association of diacetyl with lung disease clinically resembling bronchiolitis obliterans in food manufacturing workers. This has resulted in the need for identification of alternative chemicals to be used in the manufacturing process. Structurally similar chemicals, 2,3-pentanedione, 2,3-hexanedione, 3,4-hexanedione and 2,3-heptanedione, used as constituents of synthetic flavoring agents have been suggested as potential alternatives for diacetyl, however, immunotoxicity data on these chemicals are limited. The present study evaluated the dermal irritation and sensitization potential of diacetyl alternatives using a murine model. None of the chemicals were identified as dermal irritants when tested at concentrations up to 50%. Similar to diacetyl (EC3=17.9%), concentration-dependent increases in lymphocyte proliferation were observed following exposure to all four chemicals, with calculated EC3 values of 15.4% (2,3-pentanedione), 18.2% (2,3-hexanedione), 15.5% (3,4-hexanedione) and 14.1% (2,3-heptanedione). No biologically significant elevations in local or total serum IgE were identified after exposure to 25-50% concentrations of these chemicals. These results demonstrate the potential for development of hypersensitivity responses to these proposed alternative butter flavorings and raise concern about the use of structurally similar replacement chemicals. Additionally, a contaminant with strong sensitization potential was found in varying concentrations in diacetyl obtained from different producers.

Diacetyl increases sensory innervation and substance P production in rat trachea

Inhalation of diacetyl, a butter flavoring, causes airway responses potentially mediated by sensory nerves. This study examines diacetyl-induced changes in sensory nerves of tracheal epithelium. Rats (n = 6/group) inhaled 0-, 25-, 249-, or 346-ppm diacetyl for 6 hr. Tracheas and vagal ganglia were removed 1-day postexposure and labeled for substance P (SP) or protein gene product 9.5 (PGP9.5). Vagal ganglia neurons projecting to airway epithelium were identified by axonal transport of fluorescent microspheres intratracheally instilled 14 days before diacetyl inhalation. End points were SP and PGP9.5 nerve fiber density (NFD) in tracheal epithelium and SP-positive neurons projecting to the trachea. PGP9.5-immunoreactive NFD decreased in foci with denuded epithelium, suggesting loss of airway sensory innervation. However, in the intact epithelium adjacent to denuded foci, SP-immunoreactive NFD increased from 0.01 ± 0.002 in controls to 0.05 ± 0.01 after exposure to 346-ppm diacetyl. In vagal ganglia, SP-positive airway neurons increased from 3.3 ± 3.0% in controls to 25.5 ± 6.6% after inhaling 346-ppm diacetyl. Thus, diacetyl inhalation increases SP levels in sensory nerves of airway epithelium. Because SP release in airways promotes inflammation and activation of sensory nerves mediates reflexes, neural changes may contribute to flavorings-related lung disease pathogenesis.

Evaluation of pulmonary function within a cohort of flavorings workers

OBJECTIVES

We present a re-analysis of a recent Health Hazard Evaluation (HHE) that was performed by the US National Institute for Occupational Safety and Health (NIOSH) regarding the pulmonary status of workers at a flavorings manufacturing facility. This facility has used acetaldehyde, acetoin, benzaldehyde, butyric acid, diacetyl and many other flavoring chemicals for many years.

METHODS

Ten years of spirometry testing and job descriptions data on 112 workers were analyzed by the authors and by NIOSH. Using NIOSH's exposure assessment criteria, we compared the prevalence of restrictive findings (as determined by spirometry testing) in production workers to an internal control group that had reduced or no potential for exposure to flavoring chemicals. NIOSH used multiple linear regression to evaluate changes in pulmonary function by the exposure group. After our review of the NIOSH findings, we evaluated associations between longitudinal changes in pulmonary health and workplace exposures through the use of generalized estimating equations. We then compared our results to those obtained by NIOSH.

RESULTS

We found that the prevalence of pulmonary restriction was similar in production workers and internal controls. We found no relationship between the magnitude of exposure to flavorings chemicals and observed decrements in pulmonary function. Our findings were contrary to those reported by NIOSH, most likely because of how we accounted for the longitudinal nature of the spirometric data.

CONCLUSION

Many years of exposures to flavoring chemicals in this workplace, including diacetyl, were not found to produce an increased risk of abnormal spirometric findings.

Administration of bleomycin via the oropharyngeal aspiration route leads to sustained lung fibrosis in mice and rats as quantified by UTE-MRI and histology

Pulmonary fibrosis can be experimentally induced in small rodents by bleomycin. The antibiotic is usually administered via the intratracheal or intranasal routes. In the present study, we investigated the oropharyngeal aspiration of bleomycin as an alternative route for the induction of lung fibrosis in rats and mice. The development of lung injury was followed in vivo by ultrashort echo time magnetic resonance imaging (UTE-MRI) and by post-mortem analyses (histology of collagen, hydroxyproline determination, and qRT-PCR). In C57BL/6 mice, oropharyngeal aspiration of bleomycin led to more prominent lung fibrosis as compared to intranasal administration. Consequently, the oropharyngeal aspiration route allowed a dose reduction of bleomycin and, therewith, a model refinement. Moreover, the distribution of collagen after oropharyngeal aspiration of bleomycin was more homogenous than after intranasal administration: for the oropharyngeal aspiration route, fibrotic areas appeared all over the lung lobes, while for the intranasal route fibrotic lesions appeared mainly around the largest superior airways. Thus, oropharyngeal aspiration of bleomycin induced morphological changes that were more comparable to the human disease than the intranasal administration route did. Oropharyngeal aspiration of bleomycin led to a homogeneous fibrotic injury also in rat lungs. The present data suggest oropharyngeal aspiration of bleomycin as a less invasive means to induce homogeneous and sustained fibrosis in the lungs of mice and rats.

Newly recognized occupational and environmental causes of chronic terminal airways and parenchymal lung disease

With the introduction of new materials and changes in manufacturing practices, occupational health investigators continue to uncover associations between novel exposures and chronic forms of diffuse parenchymal lung disease and terminal airways disease. To discern exposure-disease relationships, clinicians must maintain a high index of suspicion for the potential toxicity of occupational and environmental exposures. This article details several newly recognized chronic parenchymal and terminal airways. Diseases related to exposure to indium, nylon flock, diacetyl used in the flavorings industry, nanoparticles, and the World Trade Center disaster are reviewed. Also reviewed are methods in worker surveillance and the potential use of biomarkers in the evaluation of exposure-disease relationships.

Popcorn flavoring effects on reactivity of rat airways in vivo and in vitro

"Popcorn workers' lung" is an obstructive pulmonary disease produced by inhalation of volatile artificial butter flavorings. In rats, inhalation of diacetyl, a major component of butter flavoring, and inhalation of a diacetyl substitute, 2,3-pentanedione, produce similar damage to airway epithelium. The effects of diacetyl and 2,3-pentanedione and mixtures of diacetyl, acetic acid, and acetoin, all components of butter flavoring, on pulmonary function and airway reactivity to methacholine (MCh) were investigated. Lung resistance (RL) and dynamic compliance (Cdyn) were negligibly changed 18 h after a 6-h inhalation exposure to diacetyl or 2,3-pentanedione (100-360 ppm). Reactivity to MCh was not markedly changed after diacetyl, but was modestly decreased after 2,3-pentanedione inhalation. Inhaled diacetyl exerted essentially no effect on reactivity to mucosally applied MCh, but 2,3-pentanedione (320 and 360 ppm) increased reactivity to MCh in the isolated, perfused trachea preparation (IPT). In IPT, diacetyl and 2,3-pentanedione (≥3 mM) applied to the serosal and mucosal surfaces of intact and epithelium-denuded tracheas initiated transient contractions followed by relaxations. Inhaled acetoin (150 ppm) exerted no effect on pulmonary function and airway reactivity in vivo; acetic acid (27 ppm) produced hyperreactivity to MCh; and exposure to diacetyl + acetoin + acetic acid (250 + 150 + 27 ppm) led to a diacetyl-like reduction in reactivity. Data suggest that the effects of 2,3-pentanedione on airway reactivity are greater than those of diacetyl, and that flavorings are airway smooth muscle relaxants and constrictors, thus indicating a complex mechanism.

Respiratory and olfactory cytotoxicity of inhaled 2,3-pentanedione in Sprague-Dawley rats

Flavorings-related lung disease is a potentially disabling disease of food industry workers associated with exposure to the α-diketone butter flavoring, diacetyl (2,3-butanedione). To investigate the hypothesis that another α-diketone flavoring, 2,3-pentanedione, would cause airway damage, rats that inhaled air, 2,3-pentanedione (112, 241, 318, or 354 ppm), or diacetyl (240 ppm) for 6 hours were sacrificed the following day. Rats inhaling 2,3-pentanedione developed necrotizing rhinitis, tracheitis, and bronchitis comparable to diacetyl-induced injury. To investigate delayed toxicity, additional rats inhaled 318 (range, 317.9-318.9) ppm 2,3-pentanedione for 6 hours and were sacrificed 0 to 2, 12 to 14, or 18 to 20 hours after exposure. Respiratory epithelial injury in the upper nose involved both apoptosis and necrosis, which progressed through 12 to 14 hours after exposure. Olfactory neuroepithelial injury included loss of olfactory neurons that showed reduced expression of the 2,3-pentanedione-metabolizing enzyme, dicarbonyl/L-xylulose reductase, relative to sustentacular cells. Caspase 3 activation occasionally involved olfactory nerve bundles that synapse in the olfactory bulb (OB). An additional group of rats inhaling 270 ppm 2,3-pentanedione for 6 hours 41 minutes showed increased expression of IL-6 and nitric oxide synthase-2 and decreased expression of vascular endothelial growth factor A in the OB, striatum, hippocampus, and cerebellum using real-time PCR. Claudin-1 expression increased in the OB and striatum. We conclude that 2,3-pentanedione is a respiratory hazard that can also alter gene expression in the brain.

The butter flavorant, diacetyl, forms a covalent adduct with 2-deoxyguanosine, uncoils DNA, and leads to cell death

Diacetyl (DA), a natural butter flavorant, is a causative agent for the lung disease obliterative bronchiolitis. Mutagenic properties of 1,2-dicarbonyls have previously been empirically linked to their possible interaction with DNA nucleobases. This study for the first time identifies chemically the adduct of DA with 2-deoxyguanosine. Selective reactivity of DA with 5'-TTTGTTTTT-3' over 5'-TTTTTTTTT-3' indicated its propensity to modify specifically the guanosine residue. Treatment of plasmid DNA, pBR322, with DA induced changes in electrophoretic mobility that are typical of ternary structure disruption. Such DNA nucleobase interaction of DA translated into increased apoptosis in DA-treated SH-SY5Y cells in a dose-dependent manner (IC(50) = 0.114 ± 0.0421 mM). The traditional carbonyl scavengers metformin, 2-thiobarbituric acid, and d-penicillamine protected cells from DA toxicity in proportion to their rates of reaction with DA, with d-penicillamine causing a maximal increase in the IC(50) to 5.23 ± 0.0992 mM when co-incubated with DA.

Bronchial and bronchiolar fibrosis in rats exposed to 2,3-pentanedione vapors: implications for bronchiolitis obliterans in humans

2,3-Pentanedione (PD) is a component of artificial butter flavorings. The use of PD is increasing since diacetyl, a major butter flavorant, was associated with bronchiolitis obliterans (BO) in workers and has been removed from many products. Because the toxicity of inhaled PD is unknown, these studies were conducted to characterize the toxicity of inhaled PD across a range of concentrations in rodents. Male and female Wistar-Han rats and B6C3F1 mice were exposed to 0, 50, 100, or 200 ppm PD 6 h/d, 5 d/wk for up to 2 wk. Bronchoalveolar lavage fluid (BALF) was collected after 1, 3, 5, and 10 exposures, and histopathology was evaluated after 12 exposures. MCP-1, MCP-3, CRP, FGF-9, fibrinogen, and OSM were increased 2- to 9-fold in BALF of rats exposed for 5 and 10 days to 200 ppm. In mice, only fibrinogen was increased after 5 exposures to 200 ppm. The epithelium lining the respiratory tract was the site of toxicity in all mice and rats exposed to 200 ppm. Significantly, PD also caused both intraluminal and intramural fibrotic airway lesions in rats. The histopathological and biological changes observed in rats raise concerns that PD inhalation may cause BO in exposed humans.

Occupational lung disease risk and exposure to butter-flavoring chemicals after implementation of controls at a microwave popcorn plant

OBJECTIVES

After an outbreak of severe lung disease among workers exposed to butter-flavoring chemicals at a microwave popcorn plant, we determined whether or not lung disease risk declined after implementation of exposure controls.

METHODS

National Institute for Occupational Safety and Health staff performed eight serial cross-sectional medical and industrial hygiene surveys at the plant from November 2000 through August 2003. Medical surveys included standardized questionnaires and spirometry testing. Industrial hygiene surveys measured levels of production-related air contaminants, including butter-flavoring chemicals such as diacetyl. All diacetyl concentrations above detectable limits were corrected for the effects of absolute humidity and days to sample extraction.

RESULTS

Ventilation and isolation of the production process resulted in one to three orders of magnitude reductions in diacetyl air concentrations in different areas of the plant. Workers with past high exposures had stable chest symptoms over time; nasal, eye, and skin irritation symptoms declined. New workers had lower symptom prevalences and higher lung function than workers with past high exposures, and they did not worsen over time. In workers who had at least three spirometry tests, those with past high exposures were more likely to experience rapid declines in lung function than new workers.

CONCLUSIONS

Implemented controls lowered exposures to butter-flavoring chemicals and decreased lung disease risk for much of the plant workforce. Some workers with continuing potential for intermittent, short-term peak and measurable time-weighted exposures remain at risk and should use respiratory protection and have regularly scheduled spirometry to detect rapid lung function declines that may be work-related. Close follow-up of such workers is likely to yield additional information on risks due to peak and time-weighted exposure levels.

A validated hybrid computational fluid dynamics-physiologically based pharmacokinetic model for respiratory tract vapor absorption in the human and rat and its application to inhalation dosimetry of diacetyl

Diacetyl vapor is associated with bronchiolar injury in man but primarily large airway injury in the rat. The goal of this study was to develop a physiologically based pharmacokinetic model for inspired vapor dosimetry and to apply the model to diacetyl. The respiratory tract was modeled as a series of airways: nose, trachea, main bronchi, large bronchi, small bronchi, bronchioles, and alveoli with tissue dimensions obtained from the literature. Airborne vapor was allowed to absorb (or desorb) from tissues based on mass transfer coefficients. Transfer of vapor within tissues was based on molecular diffusivity with direct reaction with tissue substrates and/or metabolism being allowed in each tissue compartment. In vitro studies were performed to provide measures of diacetyl metabolism kinetics and direct reaction rates allowing for the development of a model with no unassigned variables. Respiratory tract uptake of halothane, acetone, ethanol and diacetyl was measured in male F344 rat to obtain data for model validation. The human model was validated against published values for inspired vapor uptake. For both the human and rat models, a close concordance of model estimates with experimental measurements was observed, validating the model. The model estimates that limited amounts of inspired diacetyl penetrate to the bronchioles of the rat (<2%), whereas in the lightly exercising human, 24% penetration to the bronchioles is estimated. Bronchiolar tissue concentrations of diacetyl in the human are estimated to exceed those in the rat by 40-fold. These inhalation dosimetric differences may contribute to the human-rat differences in diacetyl-induced airway injury.

Severe airway epithelial injury, aberrant repair and bronchiolitis obliterans develops after diacetyl instillation in rats

BACKGROUND

Bronchiolitis obliterans (BO) is a fibrotic lung disease that occurs in a variety of clinical settings, including toxin exposures, autoimmunity and lung or bone marrow transplant. Despite its increasing clinical importance, little is known regarding the underlying disease mechanisms due to a lack of adequate small animal BO models. Recent epidemiological studies have implicated exposure to diacetyl (DA), a volatile component of artificial butter flavoring, as a cause of BO in otherwise healthy factory workers. Our overall hypothesis is that DA induces severe epithelial injury and aberrant repair that leads to the development of BO. Therefore, the objectives of this study were 1) to determine if DA, delivered by intratracheal instillation (ITI), would lead to the development of BO in rats and 2) to characterize epithelial regeneration and matrix repair after ITI of DA.

METHODS AND MAIN RESULTS

Male Sprague-Dawley rats were treated with a single dose of DA (125 mg/kg) or sterile water (vehicle control) by ITI. Instilled DA resulted in airway specific injury, followed by rapid epithelial regeneration, and extensive intraluminal airway fibrosis characteristic of BO. Increased airway resistance and lung fluid neutrophilia occurred with the development of BO, similar to human disease. Despite rapid epithelial regeneration after DA treatment, expression of the normal phenotypic markers, Clara cell secretory protein and acetylated tubulin, were diminished. In contrast, expression of the matrix component Tenascin C was significantly increased, particularly evident within the BO lesions.

CONCLUSIONS

We have established that ITI of DA results in BO, creating a novel chemical-induced animal model that replicates histological, biological and physiological features of the human disease. Furthermore, we demonstrate that dysregulated epithelial repair and excessive matrix Tenacin C deposition occur in BO, providing new insights into potential disease mechanisms and therapeutic targets.

Reaction of the butter flavorant diacetyl (2,3-butanedione) with N-α-acetylarginine: a model for epitope formation with pulmonary proteins in the etiology of obliterative bronchiolitis

The butter flavorant diacetyl (2,3-butanedione) is implicated in causing obliterative bronchiolitis in microwave popcorn plant workers. Because diacetyl modifies arginine residues, an immunological basis for its toxicity is under investigation. Reaction products of diacetyl with N-α-acetylarginine (AcArg) were determined as a model for hapten formation, with characterization by mass spectrometry, NMR, and HPLC with UV detection and radiodetection. Four products were identified by LC-MS, each with a positive ion of m/z 303 (diacetyl + AcArg); one pair displayed an additional ion at m/z 217 (AcArg), the other pair at m/z 285 (- H(2)O). Their (1)H-(13)C NMR correlation spectra were consistent with the addition of one or two of the guanidine nitrogens to form aminols. Open-chain pairs interconverted at pH 2, as did the cyclized, but all four interconverted at neutral pH. This is the first structural characterization of the covalent adducts between diacetyl and an arginine moiety.

Electron transfer as a potential cause of diacetyl toxicity in popcorn lung disease

Diacetyl, a butter-flavoring component, has recently attracted scientific and media attention because it has been implicated as an agent that induces popcorn lung disease in exposed plant workers. This disease, officially referred to as bronchiolitis obliterans, entails exposure-induced compromise to the lung's epithelial barrier function. In this review, we present a novel molecular mechanism (electron transfer, ET) designed to explain how diacetyl and its imine derivatives might interact to produce lung damage. We relate the fact that diacetyl and related compounds possess reduction potentials amenable to electron transfer (ET) in vivo. The electrochemical nature of these toxicants can potentially disrupt normal ET processes, generate reactive oxygen species (ROS), and participate in cell signaling events. Condensation of diacetyl with protein may also play a role in the toxicity caused by this compound. ET is a common feature of toxic substances, usually involving their metabolites which can operate per se or through reactions that generate ROS and oxidative stress (OS). Examples of agents capable of ET are quinone and metal compounds, aromatic nitro compounds, and iminium salts. Among compounds that generate ET, the alpha-dicarbonyl ET class, of which diacetyl is a member, is much less studied. This review emphasizes diacetyl as an agent that acts through oxidative processes to cause its effects. However, we also treat related substances that appear to act by a similar mechanism. This mechanism forms a theoretical framework capable of describing the mechanism by which diacetyl may induce its effects and is in accord with various physiological activities displayed by other alpha-dicarbonyl substances. Examples of substances that may act by mechanisms similar to that displayed by diacetyl include cyclohexane-1,2-dione, marinopyrroles, reactive carbonyl species, the bacterial signaling agent DPD, and advanced glycation end products.

Oropharyngeal aspiration: an alternative route for challenging in a mouse model of chemical-induced asthma

BACKGROUND

To assess the importance of the route of challenge in an existing mouse model of chemical-induced asthma, we replaced intranasal instillation by oropharyngeal aspiration. To our knowledge, oropharyngeal aspiration as a challenge route has not yet been investigated in a mouse model of chemical-induced asthma.

METHODS

On days 1 and 8, mice were dermally sensitized with toluene diisocyanate (TDI) (0.3%) [or vehicle (acetone/olive oil)] and on day 15 they received a single challenge, via oropharyngeal aspiration, with TDI (0.01%) or vehicle. One day after challenge, airway reactivity to methacholine was measured by a forced oscillation technique (FlexiVent) and total and differential cell counts, as well as levels of KC, IL-5, IL-17 and TNF-alpha, were assessed in the bronchoalveolar lavage (BAL) fluid. Lymphocytes from the auricular and mediastinal lymph nodes were cultured to determine the concanavaline A-induced secretion of IL-2, IL-4, IL-10, IL-13, IL-17 and IFN-gamma. Total serum IgE was measured.

RESULTS

In TDI-sensitized mice, a significant increase in airway reactivity was found after a single oropharyngeal challenge with TDI. BAL neutrophils and eosinophils were increased 7- and 5-fold, respectively. An upregulation of Th1 (IFN-gamma), Th2 (IL-4, IL-10, IL-13) and Th17 (IL-17) cytokines was found in the auricular lymph nodes, in the mediastinal lymph nodes only IL-4 was upregulated. The total serum IgE level in TDI-sensitized mice was significantly increased when compared to control mice.

CONCLUSION

We conclude that challenging mice via oropharyngeal aspiration mimics the characteristics of human asthma well, without the possible drawbacks of other techniques.

Inhalation dosimetry of diacetyl and butyric acid, two components of butter flavoring vapors

Occupational exposure to butter flavoring vapors (BFV) is associated with significant pulmonary injury. The goal of the current study was to characterize inhalation dosimetric patterns of diacetyl and butyric acid, two components of BFV, and to develop a hybrid computational fluid dynamic-physiologically based pharmacokinetic model (CFD-PBPK) to describe these patterns. Uptake of diacetyl and butyric acid vapors, alone and in combination, was measured in the upper respiratory tract of anesthetized male Sprague-Dawley rats under constant velocity flow conditions and the uptake data were used to validate the CFD-PBPK model. Diacetyl vapor (100 or 300 ppm) was scrubbed from the airstream with 76-36% efficiency at flows of 100-400 ml/min. Butryic acid (30 ppm) was scrubbed with >90% efficiency. Concurrent exposure to butyric acid resulted in a small but significant reduction of diacetyl uptake (36 vs. 31%, p < 0.05). Diacetyl was metabolized in nasal tissues in vitro, likely by diacetyl reductase, an enzyme known to be inhibited by butyric acid. The CFD-PBPK model closely described diacetyl uptake; the reduction in diacetyl uptake by butyric acid could be explained by inhibition of diacetyl reductase. Extrapolation to the human via the model suggested that inspired diacetyl may penetrate to the intrapulmonary airways to a greater degree in the human than in the rat. Thus, based on dosimetric relationships, extrapulmonary airway injury in the rat may be predictive of intrapulmonary airway injury in humans. Butyric acid may modulate diacetyl toxicity by inhibiting its metabolism and/or altering its inhalation dosimetric patterns.