mRNA-induction and cytokine release during in vitro exposure of human nasal respiratory epithelia to methyl methacrylate

No acute effects of an exposure to 50 ppm methyl methacrylate on the upper airways

PURPOSE

The German MAK value of methyl methacrylate has been fixed at 50 ppm. The aim of this study was to evaluate possible acute effects of an exposure to 50 ppm methyl methacrylate on the upper airways of human subjects.

METHODS

Twenty healthy subjects were exposed to 50 ppm methyl methacrylate and to air (sham) in an exposure chamber for 4 h according to a crossover design. Symptoms were assessed by the SPES questionnaire. Olfactory thresholds for n-butanol and mucociliary transport time were measured before and after exposure. Concentrations of interleukin 1ß and interleukin 8 were determined in nasal secretions taken after exposure. mRNA levels of interleukins 1ß, 6 and 8, tumor necrosis factor α, granulocyte-macrophage colony-stimulating factor, monocyte chemotactic protein 1, and cyclooxygenases 1 and 2 were measured in nasal epithelial cells, obtained after exposure. Possible effects were investigated by semiparametric and parametric crossover analyses.

RESULTS

The score of the item "irritation to the nose" was slightly elevated following exposure to methyl methacrylate (p ≤ 0.01). Olfactory functioning was not impaired. Mucociliary transport time did not change. Neither concentrations of interleukins in nasal secretions nor mRNA levels were elevated.

CONCLUSION

Only minor irritating effects on the nose were observed. The acute exposure to 50 ppm methyl methacrylate did not cause any adverse effects. However, the results cannot be extrapolated to chronic exposure.

Hypothesis-based weight-of-evidence evaluation of methyl methacrylate olfactory effects in humans and derivation of an occupational exposure level

Over 40 years of scientific evidence indicates that methyl methacrylate (MMA) causes olfactory effects in rodents that are relevant to humans. More recent scientific studies have focused on understanding the apparent lack of species concordance between the rodent and human studies. Toxicokinetic studies and a physiologically based pharmacokinetic (PBPK) model describing inhalation dosimetry of MMA in the upper respiratory tract (URT) of rats and humans point to differences in nasal morphology and biochemistry that could explain and reconcile these differences as species-specific manifestations of a common toxicological process. We have applied the hypothesis-based weight-of-evidence (HBWoE) approach to evaluate the concordance of the available data and the hypothesis that the observed difference in sensitivity between rats and humans may be the expected result of physiological and biochemical differences. Our WoE analysis indicates that when the several lines of evidence (i.e., animal, human, mode-of-action, and toxicokinetics data) are integrated, they inform interpretation of one another and, overall, support use of the human data for derivation of an MMA occupational exposure level (OEL) of 50 ppm.

Methyl methacrylate and respiratory sensitization: a critical review

Methyl methacrylate (MMA) is a respiratory irritant and dermal sensitizer that has been associated with occupational asthma in a small number of case reports. Those reports have raised concern that it might be a respiratory sensitizer. To better understand that possibility, we reviewed the in silico, in chemico, in vitro, and in vivo toxicology literature, and also epidemiologic and occupational medicine reports related to the respiratory effects of MMA. Numerous in silico and in chemico studies indicate that MMA is unlikely to be a respiratory sensitizer. The few in vitro studies suggest that MMA has generally weak effects. In vivo studies have documented contact skin sensitization, nonspecific cytotoxicity, and weakly positive responses on local lymph node assay; guinea pig and mouse inhalation sensitization tests have not been performed. Cohort and cross-sectional worker studies reported irritation of eyes, nose, and upper respiratory tract associated with short-term peaks exposures, but little evidence for respiratory sensitization or asthma. Nineteen case reports described asthma, laryngitis, or hypersensitivity pneumonitis in MMA-exposed workers; however, exposures were either not well described or involved mixtures containing more reactive respiratory sensitizers and irritants. The weight of evidence, both experimental and observational, argues that MMA is not a respiratory sensitizer.

Early stress response of human nasal respiratory epithelia after exposure to 1-methoxypropanol-2

To evaluate the impact of 1-methoxypropanol-2 (MEP) for the stimulation of an inflammatory response in human respiratory mucosa, we exposed 22 primary cell cultures of nasal respiratory epithelia of healthy individuals to MEP concentrations at the level of the German MAK-value (100 ppm) and to the 10-fold concentration (1000 ppm). After 4 and 24h we analyzed the transcription of TNF-alpha, IL-1beta, IL-6, IL-8, MCP-1, GMCSF, Cox-1 and Cox-2 by quantitative PCR as well as the release of the respective cytokines by ELISA. At both MEP concentrations we observed a significant increase of TNF-alpha-, IL-1beta-, IL-6- and Cox-2-transcripts after 4h. After 24h cytokine transcription of TNF-alpha, IL-1beta and IL-6 was normalized, but Cox-2 remained elevated. On the protein level IL-1beta as well as granulocyte macrophages colony stimulating factor (GM-CSF) were decreased after 4h or 24h and uniquely IL-8 levels were increased after 4h. Our data suggest that MEP induces the transcription of genes encoding proinflammatory cytokines and mediators but largely not translation of those. Considering these in vitro data, existing exposure limits seem to be safe with respect to inflammatory responses of the upper respiratory tract. However, the effects of long-term exposures to MEP should be watched closely.