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Relationship of carcinogenicity and cellular proliferation induced by mutagenic noncarcinogens vs carcinogens. III. Organophosphate pesticides vs tris(2,3-dibromopropyl)phosphate.

Our laboratory has been examining the mechanisms whereby chemicals produce mutagenicity in short-term in vitro assays yet fail to produce carcinogenesis in 2-year rodent bioassays. Previous studies indicated that some mutagenic hepatocarcinogens increased cell proliferation in the target organ, the liver, while other structurally related mutagens that were noncarcinogenic failed to do so. We demonstrate in this report that another mutagenic carcinogen, tris(2,3-dibromopropyl phosphate), increased cell proliferation that was localized in the outer medulla of the kidney. This was also the target site for carcinogenesis in a 2-year bioassay and is another example of the association between chemically induced cell proliferation and carcinogenesis. This study also reports the absence of increased cell proliferation in the liver or kidney after exposure in the diet to the mutagenic organophosphate insecticides dimethoate, dioxathion, and dichlorvos following dietary exposure for 2 weeks at the same dose levels and routes of exposure that did not increase the tumor incidence in either organ in 2-year carcinogenesis assays. The present studies support the tenet that chemically induced cell proliferation may be a necessary prerequisite for chemical carcinogenesis, since in rat liver and kidney there was neither cell proliferation after 2 weeks nor tumor development after 2 years dietary exposure to the mutagenic organophosphate insecticides dimethoate, dioxathion, and dichlorvos.