Gene expression microarray analysis of rat nasal tissue with time and dose following formaldehyde inhalation

Formaldehyde (FA), an endogenous cellular aldehyde, is a rat nasal carcinogen. In this study, concentration- and exposure-duration transitions in FA mode of action (MOA) were examined with pharmacokinetic (PK) modeling for tissue formaldehyde acetal (FAcetal) and glutathione (GSH) and with histopathology and gene expression studies for tissue responses in nasal epithelium from rats exposed to 0, 0.7, 2, 6, 10 or 15 ppm FA 6 hr/day for 1, 4 or 13 weeks. The study had two goals. The first goal was to develop a basic PK model to estimate various forms of tissue formaldehyde and tissue glutathione (GSH). The second goal was to compare histopathology and gene expression changes in nasal tissues caused by inhalation of FA with changes in tissue FAcetal and free GSH calculated from the PK model. Patterns of gene expression varied with concentration and with duration. At 0.7 and 2 ppm, sensitive response genes (SRGs) - associated with cellular stress, thiol transport/reduction, inflammation, and cell proliferation - were similarly upregulated at all exposure durations. At 6 ppm and greater, gene expression changes showed enrichment of pathways involved in cell cycle, DNA repair, and apoptosis processes. ERBB, EGFR, WNT, TGF-β, Hedgehog, and Notch signaling were also enriched in differentially expressed genes. Benchmark doses (BMDs) for genes in significantly enriched pathways were lower at 13 weeks than at 1 or 4-week. The transcriptional and histological changes corresponded to PK model-predicted changes in free GSH at 0.7 and 2 ppm and in FAcetal at 6 ppm. DNA-replication stress, enhanced proliferation, metaplasia, and stem cell-niche activation appear to be associated with FA carcinogenesis at 6 ppm and above. Dose dependencies in MOA, the presence of high physiological FAcetal, and non-linear FAcetal/GSH tissue kinetics indicate that FA concentrations below 150 ppb (and probably any concentrations below irritant levels, i.e., ~ 1 ppm) would not increase cancer risks of inhaled FA in the nose or any other tissue. Closer examination of dose response relationships for endogenous compound toxicity could help guide biologically relevant approaches for chemical risk assessment. Eight week old male F344/CrlBR rats were exposed to formaldehyde through whole body inhalation. Whole-body exposures were performed at doses of 0, 0.7, 2, 6, 10, and 15 ppm (6 hours per day, 5 days per week). Inhalation animals were sacrified at 1, 4, and 13 weeks following initiation of exposure. Following sacrifice, tissue from the Level II region of the nose was dissected and digested with a mixture of proteases to remove the epithelial cells. The epithelial cells acquired from this section of the nose consisted primarily of transitional epithelium with some respiratory epithelium. Gene expression microarray analysis was performed on the epithelial cells.