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Studies on the mechanism of the carcinogenic activity of amitrole
F Mattioli1, L Robbiano, L Fazzuoli
1Institute of Pharmacology, University of Genoa, Italy.
Abstract:
Amitrole, a widely used herbicide found to produce thyroid and liver tumors in rodents and classified as possibly carcinogenic to humans, was investigated to acquire further information about its mechanism of action. A 20-hr exposure to amitrole concentrations ranging from 5.6 to 18 mM did not induce DNA fragmentation, as measured by the alkaline elution technique, in primary cultures of human thyroid follicular cells and of human liver cells. Under the same experimental conditions a minimal frequency of DNA breaks was detected in primary cultures of rat hepatocytes, but this event was presumably the unspecific consequence of a cytotoxic effect. In rats given amitrole with drinking water for 12 successive days at a daily dose of approximately 200 mg/kg, plasma levels of triiodothyronine and thyroxine displayed a progressive reduction, and a concurrent increase of both the mitotic index and frequency of S-phase cells revealing a clear-cut follicular cell hyperplasia was observed. In a group of these rats euthanized after 8 days of treatment any evidence of DNA fragmentation was absent in both thyroid and liver cells. Taken as a whole these results provide further evidence that the mechanism of amitrole carcinogenic activity is most likely nongenotoxic but due to hormone imbalance.
Insights
Amitrole, a possible human carcinogen, does not damage DNA directly. Its cancer-causing effects in rats are linked to hormone imbalance, not genetic damage.
Area of Science:
- Toxicology
- Carcinogenesis
- Endocrinology
Background:
- Amitrole is a herbicide linked to thyroid and liver tumors in rodents.
- It is classified as possibly carcinogenic to humans.
- The precise mechanism of amitrole's carcinogenicity requires further investigation.
Purpose of the Study:
- To investigate the mechanism of action of amitrole, specifically its genotoxic potential.
- To determine if amitrole induces DNA damage in human and rat cells.
- To explore the role of hormone imbalance in amitrole-induced carcinogenesis.
Main Methods:
- Primary cultures of human thyroid follicular cells and human liver cells were exposed to amitrole.
- Primary cultures of rat hepatocytes were exposed to amitrole.
- Rats were administered amitrole daily for 12 days, with thyroid and liver cells analyzed for DNA fragmentation and cell proliferation.
- Plasma levels of triiodothyronine and thyroxine were measured in treated rats.
Main Results:
- Amitrole did not induce DNA fragmentation in human thyroid or liver cells.
- Minimal DNA breaks in rat hepatocytes were attributed to cytotoxicity, not genotoxicity.
- Rats treated with amitrole showed reduced thyroid hormone levels and increased cell proliferation in thyroid follicular cells.
- No DNA fragmentation was observed in thyroid or liver cells of rats after 8 days of amitrole treatment.
Conclusions:
- Amitrole's carcinogenic activity is likely nongenotoxic.
- The mechanism of amitrole carcinogenicity is associated with hormone imbalance.
- These findings suggest that amitrole's risk may stem from endocrine disruption rather than direct DNA damage.