Propylene oxide in blood and soluble nonprotein thiols in nasal mucosa and other tissues of male Fischer 344/N rats exposed to propylene oxide vapors--relevance of glutathione depletion for propylene oxide-induced rat nasal tumors.

High concentrations of propylene oxide (PO) induced inflammation in the respiratory nasal mucosa (RNM) of rodents. Concentrations > or =300 ppm caused nasal tumors. In order to investigate if glutathione depletion could be relevant for these effects, we determined in PO exposed male Fischer 344/N rats PO in blood and soluble nonprotein SH-groups (NPSH) in RNM and other tissues. Rats were exposed once (6 h) to PO concentrations between 0 and 750 ppm, and repeatedly for up to 20 days (6 h, 5 days/week) to concentrations between 0 and 500 ppm. At the end of the exposures, PO in blood and NPSH in tissues were determined. PO in blood was dependent on concentration and duration of exposure. After the 1-day exposures, NPSH depletion was most distinctive (RNM > liver > lung). Compared to controls, NPSH levels were 43% at 50 ppm PO in RNM and 16% at > or =300 ppm in both RNM and liver. Lung NPSH fell linearly to 20% at 750 ppm. After repeated exposures over 3 and 20 days to 5, 25, 50, 300, and 500 ppm, NPSH losses were less pronounced. At both time points, NPSH were 90%, 70%, 50%, 30%, and 30% of the control values in RNM. Liver NPSH decreased to 80% and 50% at 300 and 500 ppm, respectively. After 20 days, lung NPSH declined to 70% (300 ppm) and 50% (500 ppm). We conclude that continuous, severe perturbation of GSH in RNM following repeated high PO exposures may lead to inflammatory lesions and cell proliferation, critical steps on the path towards tumorigenicity.

Effects of propylene oxide exposure on rat nasal respiratory cell proliferation.

Long-term exposure of rodents to propylene oxide (PO) induced inflammation, respiratory cell hyperplasia, and nasal tumors at concentrations >/= 300 ppm, suggesting a possible role for cytotoxicity and compensatory cell proliferation in PO carcinogenesis. In this study, the effects of PO exposure on histopathology and cell proliferation in nasal and hepatic tissues were studied in male F344 rats exposed by inhalation for 3 or 20 days (0, 5, 25, 50, 300, and 500 ppm). Histopathology revealed an increase in mucous cell hyperplasia in the anterior nasal passages after 20 days of exposure (>/=300 ppm). This was associated with the formation of goblet cell nests. Cell proliferation was measured in the respiratory epithelium (NRE; mucociliary and transitional) lining the anterior nasal passages, the nasopharyngeal meatus (NPM), and the liver using BrdU administered with 3-day osmotic pumps. Significant increases in cell proliferation occurred (>3.6-fold) in the mucociliary epithelium lining the anterior nasal cavity at and above 300 ppm for both exposure periods. In the mucociliary epithelium, the 20-day labeling was commonly associated with nests of goblet cells. Significant increases in cell proliferation (>2.3-fold) were observed in the transitional epithelium at 500 ppm after 3 days of exposure and at 300 and 500 ppm after 20 days of exposure. Significant increases in cell proliferation in the NPM (>2.8-fold) were evident at 500 ppm PO after 3 days and at 300 and 500 ppm PO after 20 days of exposure. No exposure-related changes in cell proliferation were observed in the liver. These studies demonstrate a clear concordance between the site and exposure concentration for tumor induction and those causing significant increases in cell proliferation in the rat nose.

Molecular dosimetry of N7-(2-hydroxypropyl)guanine in tissues of F344 rats after inhalation exposure to propylene oxide.

Propylene oxide (PO) is a high-volume chemical intermediate that causes a low incidence of nasal tumors in rodents exposed to high concentrations (> or =300 p.p.m.). PO reacts with DNA forming mainly N7-(2-hydroxypropyl)guanine (7-HPG). The exposure-dependent accumulation of 7-HPG in nasal respiratory epithelium (NRE), lung and liver was determined in male F344 rats exposed to PO (0, 5, 25, 50, 300 or 500 p.p.m.) by the inhalation route for 3 or 20 days (6 h/day; 5 days/week). These exposures ranged from low concentrations, such as those potentially occurring in the workplace, to high concentrations that proved to be carcinogenic in rodents. Analysis of 7-HPG in DNA by gas chromatography-high-resolution mass spectrometry (GC-HRMS) showed a linear response in 7-HPG for all three tissues after 3 days of exposure, and for NRE and lung after 20 days of exposure. A slightly sublinear response in 7-HPG was observed in liver after 20 days of exposure. For both exposure periods, the NRE had the highest concentration of 7-HPG, followed by lung and liver. The amount of 7-HPG in NRE was seven and 17 times higher than in lung and liver, respectively, for the 3 day exposures. For the 20 day exposures, the concentration of 7-HPG in NRE was six and 13 times higher than that in lung and liver, respectively, over the concentration range studied. These results demonstrate a much higher extent of DNA alkylation in the target tissue for carcinogenesis, than in non-target tissues. As PO-induced tumor formation was highly sublinear, occurring only at high vapor concentrations, whereas 7-HPG adducts were shown to be linearly dependent on airborne concentration, these results suggest that 7-HPG is not sufficient for PO nasal carcinogenesis and that other factors such as increased cell proliferation may be important in determining the tumor exposure response.

Exposure-dependent accumulation of N-(2-hydroxypropyl)valine in hemoglobin of F344 rats exposed to propylene oxide by the inhalation route.

The detection of hemoglobin adducts by mass spectrometry is a very sensitive and specific measurement of the extent of covalent binding of electrophilic chemicals. The exposure-dependent accumulation of N-(2-hydroxypropyl)valine (N-HPVal) in globin of rats exposed to propylene oxide (PO) (0, 5, 25, 50, 300 or 500 ppm) by the inhalation route was measured to assess the utility of Hb adducts as biomarkers of exposure. Analysis of N-HPVal by gas-chromatography tandem mass spectrometry showed a linear exposure-dependent response for adduct accumulation in globin of rats exposed to PO for 3 days (6 h/day). After 20 days of exposure (6 h/day; 5 days/week), the exposure-response curve was slightly sub-linear. DNA adducts had been measured in several organs of the same animals in a companion study. The dose-response for accumulation of DNA adducts was similar to that obtained for Hb adducts. However, the number of DNA adducts varied by 17-fold between different tissues. The highest number of DNA adducts was found in respiratory nasal tissue, followed by lung and then liver. These data demonstrate that hemoglobin adducts provide a sensitive dosimeter for systemic exposure, but cannot be used to predict the extent of DNA binding in individual tissues. Furthermore, the exposure-response curve for both hemoglobin and DNA adduct accumulation does not reflect the tumor incidence curve for PO, providing evidence that the assessment of risk to cancer is more complex than simple biomarker measurements. When the present rat data were compared with recent N-HPVal measurements in humans, similar binding was found.