There is an increasing concern for the possible impact of environmental factors on genomic stability in human populations. We studied the aneuploidy-inducing capacity of pesticides of significant human exposure in cultured human cells and demonstrated that dichlorvos, an organophosphate insecticide classified as possible carcinogen, significantly induced CREST-positive micronuclei in binucleated lymphoblastoid cells (Mattiuzzo et al., 2006). To identify the mechanisms involved in the aneuploidy-inducing ability of this chemical, we investigated the influence of the drug on mitotic progression and spindle assembly. In both lymphoblasts and Hela cells Dichlorvos greatly increased mitotic index and inhibited the metaphase/anaphase transition: this resulted in a mitotic arrest at promethaphase with hyper-condensed chromosomes. Analysis of mitotic spindles and chromosome congression by immunofluorescence staining with anti- alpha tubulin and anti- Ser10 phospho H3 antibodies showed that the chemical was able to perturb spindle dynamics and chromosome behaviour. Spindle microtubules in treated cells were not organized in parallel fibres but collapsed in tubulin aggregates or formed distorted fibres that did not interact with the kinetochores. At higher doses Ser10 phospho H3-positive picnotic nuclei were associated to monopolar spindles, showing two close gamma tubulin signals. These results demonstrate that Dichlorvos mimics spindle poison effects, inducing mitotic arrest and altering the structure and the function of the mitotic apparatus in cultured human cells. In vivo analysis of mitotic progression showed that a large proportion of dichlorvos-treated cells did not progress beyond prometaphase. The few cells that completed mitosis, did so without chromatid separation producing polyploid or multinucleated cells. Finally, B tubulin in dichlorvos treated cells was heavily phoshorylated at a residue that is specific for tubulin not incorporated into microtubules (Fourest-Lieuvin et al., 2006), suggesting that the pesticide affects microtubule dynamics during mitosis. These effects of the chemical may be relevant for its potential carcinogenic activity.

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