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THE VALENCE AND METHYLATION STATE OF ARSENIC DETERMINES ITS POTENCY IN INTERACTION WITH THE MITOTIC APPARATUS
Citation:
Kligerman, A D., C L. Doerr, AND A H. Tennant. THE VALENCE AND METHYLATION STATE OF ARSENIC DETERMINES ITS POTENCY IN INTERACTION WITH THE MITOTIC APPARATUS. Presented at 3rd Conference on Metal Toxicity and Carcinogenesis, Morgantown, WV, Sept. 12-15, 2004.
Description:
We have previously shown that the cytotoxic and genotoxic potency of arsenicals is dependent upon their valence and methylation state. Trivalent methylated arsenicals are much more potent DNA damaging agents than are their inorganic and pentavalent counterparts. Furthermore, their activities are directed toward chromosome breakage, DNA damage, and large deletion-type mutations while apparently not causing gene mutations. During the course of these studies, we noticed that many of the arsenicals induced "c-type" anaphases consisting of small condensed chromosomes characteristic of spindle poisons. To investigate this further, we exposed human lymmhoblasts for 6 hours to six arsenicals: sodium arsenate (NaAsV), sodium arsenite (NaAsIII), monomethylarsonic acid (MMAV), monomethylarsonous acid (MMAIII), dimethylarsinic acid (DMAV), and dimethylarsinous acid (DMAIII). After exposure slides were prepared, and the mitotic indices (MI) were calculated by scoring the number of metaphase figures per 2000 cells. To normalize the data for each experiment, the MI for each treatment was compared to either the control MI (no mitotic inhibitor) or a positive control (0.5 ?g/ml demecolcine [DC]). NaAsV caused a small but significant increase in MI, which at the maximum concentration (10 mM) had about twice the MI seen with the control but less than 50% of that caused by DC. MMAV also was weak causing a slight increase in MI that just reached statistical significance. In contrast, DMAV caused a significant increase in MI producing ~75% the MI of DC and ~4 times the MI of the control. NaAsIII had no significant effect on MI and was quite toxic. MMAIII induced more than a twofold increase in MI compared to the control, which was about 40% that caused by DC. On a micromolar basis, MMAIII was the most potent of the arsenicals tested. DMAIII gave inconsistent results, being potent at times, and showing minor effects in other experiments. In addition, because the cells have some potential to modify both the valence and methylation state of the arsenicals, we exposed tubulin directly to each arsenical and spectrophotometrically measured the effect on polymerization. None of the pentavalent arsenicals had a substantial effect on the polymerization or inhibition of spontaneous polymerization of tubulin. In contrast, all of the trivalent arsenicals were active, with inhibition of polymerization seen with NaAsIII at 1 mM and above, and MMAIII and DMAIII at 10 ?M and above. Taken together, these results give a complex picture of how arsenicals may affect cells. Some of the arsenicals added to the cell culture are metabolized to the more highly tubulin-reactive methylated trivalent arsenicals. In addition, some of the trivalent arsenicals may act in several, and at times, contradictory directions. Some are cytotoxic, which may prevent the cells from cycling, thereby reducing the MI. Simultaneously, at lower concentrations, these same arsenicals may react with the spindle and cause an apparent increase in MI by blocking the cells at metaphase or anaphase. Thus, the metabolites of arsenic are active not only as chromosome breaking and DNA damaging agents, but they can affect cell division through action on the spindle potentially leading to aneuploidy, a common driving force in cellular instability and to tumor formation. [This is an abstract of a proposed presentation and does not necessarily reflect EPA policy].