Progression of alachlor‐induced olfactory mucosal tumours

Abstract

Summary. Alachlor is an herbicide used primarily in the production of corn (maize), peanuts, and soybeans and is associated with cancer of the nasal cavity, thyroid, and stomach in rats. Previous work from our laboratory demonstrated that the nasal cavity tumours originate from the olfactory mucosa, and that neoplasms were present following 6 months of exposure (126 mg/kg/day in the diet). The studies presented herein were conducted to determine more precisely the earliest time point at which alachlor-induced tumours were present, and to describe the histological changes that occur en route to tumour formation. We determined that dramatic histological changes, including respiratory metaplasia of the olfactory mucosa, were present following 3 months of exposure, and the earliest alachlor-induced olfactory mucosal tumours were detected following 5 months of treatment. Because alachlor is positive in short-term mutagenicity assays with olfactory mucosal activation, and because of the relatively short time-to-tumour formation observed with alachlor, we also conducted a`stop' study in which rats were treated with alachlor for 1 month and then held without further treatment for an additional 5 months. This study demonstrated that abbreviated alachlor exposure did not result in subsequent tumour formation within the 6-month observation period.

Strain-Specific Effects of Alachlor on Murine Olfactory Mucosal Responses

“…The results of the current study, together with previous work in the rat (Genter et al, 2000(Genter et al, , 2002a(Genter et al, , 2002b confirm that there is a marked species difference in response to alachlor between rats and mice. Rats treated chronically with alachlor (126 mg/kg/d) developed treatment-related olfactory tumors with ≥5 months of treatment.…”

“…Rats treated chronically with alachlor (126 mg/kg/d) developed treatment-related olfactory tumors with ≥5 months of treatment. The progression of these tumors to more malignant phenotypes was accompanied by cytoplasmic accumulation and nuclear localization of β-catenin, and an observation that is suggestive of activation of Wnt signaling (U.S. EPA, 1985;Genter et al, 2000Genter et al, , 2002aGenter et al, , 2002b. In contrast, in the present study, we found no neoplastic changes in any of the 3 strains examined (treated at a higher dose of 260 mg/kg/d).…”

“…We found that these, too, were resistant, even with >1 year of exposure (260 mg/kg/d) in the C57BL/6J mice. At the same time point, 14 of 15 alachlor-treated (126 mg/kg/d) rats had 1 or more olfactory neoplasms (Genter et al, 2002a).…”

Comparison of rat olfactory mucosal responses to carcinogenic and non-carcinogenic chloracetanilides

“…The 10 d time point was chosen based on previous data that showed significant dysregulation of olfactory mucosal antioxidant levels with alachlor treatment at very early treatment time points (Burman et al, 2003); further, oxidative stress has been associated with upregulation of matrix metalloproteinases in other model systems (Siwik et al, 2001; Uemura et al, 2001). The 2 mo time point was selected based on prior data from our lab that the earliest histological changes seen in the OM of alachlor-treated rats occurred following 3 mo of treatment (Genter et al, 2002a). Euthanasia was via carbon dioxide asphyxiation, and tissues were snap frozen on dry ice and maintained at –80°C until analysis.…”

“…following 2 mo of chloracetanilide treatment, is prior to the development of any histological changes (Genter et al, 2002a), because we wanted to monitor very early changes in gene expression that might predict a carcinogenic response. Frozen olfactory tissue (50 – 100 mg) was homogenized in Tri Reagent (Molecular Research Center, Cincinnati, OH) and RNA was isolated from the homogenate per the manufacturer’s protocol.…”