Morphogenesis of nasal tumors in rats exposed to hexamethylphosphoramide by inhalation

A reverse genetic screen in Drosophila using a deletion-inducing mutagen

A new reverse-genetics mutagenesis method that uses the crosslinking drug hexamethylphosphoramide to introduce small deletions has been used to generate and screen pools of mutagenized Drosophila, identifying two mutants.

We report the use of the cross-linking drug hexamethylphosphoramide (HMPA), which introduces small deletions, as a mutagen suitable for reverse genetics in the model organism Drosophila melanogaster. A compatible mutation-detection method based on resolution of PCR fragment-length polymorphisms on standard DNA sequencers is implemented. As the spectrum of HMPA-induced mutations is similar in a variety of organisms, it should be possible to transfer this mutagenesis and detection procedure to other model systems.

Nasal tumors in rats following long-term inhalation exposure to 1,4-dichlorobutene-2 (DCB)

This study was conducted to elucidate the time- and dose-response relationships of long-term, low-level 1,4-dichlorobutene-2 (DCB) inhalation exposure to nasal tumor induction in rats. Male Crl:CD BR rats were exposed 6 hours per day, 5 days week to 0, 0.1, 0.3, or 1.0 ppm DCB for up to 19 months; some rats were sacrificed at various time intervals during the study. After 19 months of exposure, surviving rats were held without treatment for an additional 5 months. Tissues from the respiratory tract, lymph nodes, and brain were evaluated microscopically. Compound-related non-neoplastic lesions were observed in the nasal cavities of rats in the 1.0 ppm group after three months of exposure and in the other two groups after twelve months of exposure. The lesions were progressive in severity and frequency. A statistically significant increase in benign nasal tumors (adenomas) occurred in rats from all three DCB-exposed groups. The adenomas occurred in the respiratory region of the nasal cavity and were first observed in the 1.0 ppm group at study month 10. Malignant nasal tumors occurred in the olfactory region of the nasal cavity and were statistically increased at 1.0 ppm.

Mitogenic responses of rat nasal epithelium to hexamethylphosphoramide inhalation exposure

Hexamethylphosphoramide (HMPA) is a rat nasal carcinogen that induces squamous cell carcinomas in the anterior portions of the nasal cavity following chronic inhalation exposures as low as 50 ppb. These tumors may arise as a result of P-450-mediated release of formaldehyde (HCHO), a known rat nasal carcinogen. The goal of this research was to investigate early responses of the nasal epithelium to inhaled HMPA. Rats were exposed nose-only to approximately 3 ppm HMPA for 6 h, and killed 18, 48, 96 or 144 h post-exposure. In a separate study, rats were exposed nose-only for 6 h for 1, 2, 3, or 5 consecutive days and killed 18 or 96 h post-exposure. With both single and repeated doses of HMPA, there was no evidence of cytotoxicity in the anterior nose. Olfactory degeneration and necrosis of the dorsal meatus, Bowman's glands and tips of the ethmoid turbinates increased in severity with repeated exposures to HMPA. Cell proliferation was assessed in levels of nasal tissue that included regions of squamous, respiratory, transitional and olfactory epithelium. Regional induction of cell proliferation was measured by BrdU incorporation, and reported as the number of labeled cells/mm basement membrane. At 18 h after a single exposure, there was an increase in cell proliferation in squamous epithelium, which returned to control levels within 48 h. A transitory increase in cell proliferation was observed regions of respiratory and transitional epithelium, although the response of each tissue, in terms of magnitude and peak time of response post-exposure, also differed. Along the dorsal meatus in Level 9, olfactory labeling initially decreased, returned to control levels by 96 h, but again declined at 144 h post-exposure. In repeat dose studies, the squamous epithelium response was variable 18 h post-exposure. For respiratory and transitional epithelium, increased cell proliferation 18 h post-exposure was correlated with increased dose (exposure) of HMPA. Cell proliferation responses following two or more exposures returned to near control levels within 96 h post-exposure. In conclusion, HMPA induced cell proliferation, but not cytotoxicity, in the anterior nose at approximately 3 ppm. These data suggest that HMPA induces proliferative, perhaps mitogenic, responses in the nasal epithelium, and this response may facilitate the fixation of low level genetic damage induced by liberated HCHO.

Evaluation of hexamethylphosphoramide for gene mutations in Salmonella typhimurium using plate incorporation, preincubation, and suspension assays

Hexamethylphosphoramide (HMPA), a potent rat nasal carcinogen by inhalation, and three of its metabolites, pentamethylphosphoramide (PMPA), trimethylphosphoramide (TriMPA), and formaldehyde (HCHO), were assessed in Salmonella typhimurium gene mutation assays using various protocols, including plate incorporation, preincubation and suspension assays. HMPA (tested up to 15,000 micrograms/plate) was not mutagenic in plate incorporation or preincubation assays with or without metabolic activation. HCHO was mutagenic in the plate incorporation and preincubation assays (tested up to 150 micrograms/plate). In suspension assays, however, HMPA (tested up to 40 mg/ml), PMPA (up to 44 mg/ml) and HCHO (up to 45 micrograms/ml), but not TriMPA (up to 29 mg/ml), were mutagenic. HMPA and PMPA were positive only with activation. HMPA's mutagenicity was optimized using a relatively high level of rat liver S9 protein (3.5 mg/plate) in the metabolic activation mixture. Semicarbazide, an HCHO trapping agent, added at concentrations up to 167 micrograms/ml, markedly inhibited the mutagenic activities of HMPA and PMPA suggesting that HCHO generation may play a role in their mutagenicity. These studies show that HMPA is mutagenic in a modified Salmonella typhimurium reverse mutation assay with metabolic activation. Successive N-demethylation of HMPA eventually eliminates the mutagenic activity which further suggests that HMPA's mutagenic activity is related to the release of HCHO.

Metabolism-dependent activation and toxicity of chemicals in nasal glands

The mucosae of the nasal passages contain a large amount of glands which express secretory proteins as well as phase I and phase II biotransformation enzymes. In this review the metabolic activation, covalent binding and toxicity of chemicals in the Bowman's glands in the olfactory mucosa, in the sero-mucous glands in the nasal septum and in the lateral nasal glands and maxillary glands around the maxillary sinuses are discussed. Light microscopic autoradiographic studies have demonstrated a selective covalent binding of nasal toxicants and carcinogens such as halogenated hydrocarbons and N-nitrosamines, especially in the Bowman's glands following a single systemic exposure, suggesting a high rate of metabolic activation of chemicals in these glands. Special attention is put on the herbicide dichlobenil which induces necrosis in the olfactory mucosa following a cytochrome-P450-mediated metabolic activation and covalent binding in the Bowman's glands.

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

Considerable interest and research have resulted from the finding that squamous cell carcinomas, polypoid adenomas, and a small number of other nasal neoplasms occurred in F-344 rats following chronic inhalation exposure to formaldehyde. These tumors were reported to originate in the anterior portion of the nasal cavity but their precise location in the nose was not determined. Histologic sections from the nasal passages of these rats have been reexamined and the location of each tumor has been recorded. The majority of squamous cell carcinomas occurred on the anterior portion of the lateral aspect of the nasoturbinate and adjacent lateral wall (57%) or the midventral nasal septum (26%). Polypoid adenomas were confined to a small region of the anterior nasal cavity and were restricted to the free margins of the naso-and maxilloturbinates and lateral wall adjacent to these margins. One neoplasm, considered to be the malignant counterpart of the polypoid adenoma, originated on the dorsal margin of the maxilloturbinate in the same region of the nose. Remaining neoplasms were generally too large or too poorly preserved for assessment of their site of origin. Mechanistic studies directed toward a better understanding of the nasal carcinogenicity of formaldehyde, or other nasal carcinogens. should take into account the anatomic sites of origin of the neoplasms whenever this can be determined.