Abstract:
Environmental exposure to methylmercury (MeHg) has been recognized as a public health concern. MeHg has been shown to exert selective damage to the central nervous system after both adult and fetal exposures. The fetal brain seems to exhibit a higher sensitivity to MeHg. However, the mechanism of toxicity is still poorly understood. Damage to the developing brain has been attributed to MeHg inhibition of cell proliferation. The effects of MeHg on cell proliferation, particularly on the cell cycle, has been previously studied. In the present study, the role of p21 and p53, key proteins that regulate the cell cycle, were evaluated.
Fibroblasts were isolated from p21 or p53 transgenic embryos at day 14 of gestation. Cells of different genotypes (wild type, heterozygous and null) relative to p21 and p53 were cultured. Early passage (4-6) cells were treated with a 0,2,4 and 6 mM MeHg for 24 hrs. Colchicine, a known mitotic inhibitor was used a positive control. Changes in cell cycle distribution after continuous MeHg treatment were analyzed by DNA content-based flow cytometry using DAPI. MeHg induced an increase in the proportion of cells in G2/M at 2 and 4 mM. the observed G2/M accumulation after MeHg treatment was not dependent of p21 or p53 genotype. Effects of MeHg on cell cycle progression were also evaluated in the p21 genotypes using BrdU-Hoechst flow cytometric analysis. Inhibition of cell cycling was observed in all p21 genotypes after continuous exposure to MeHg for 24 and 48 hours. MeHg-induced changes in expression of cell cycle-related genes were also studied. Northern blot hybridization was performed to measure differences in expression of the p53-regulated genes p21, GADD45 and Hsp72 after MeHg treatment. Our results suggest that MeHg-induced changes in p21, GADD45 and Hsp72 mRNA expression may be independent of p53.
The present study confiRMS previous observations that MeHg inhibits cell cycle progression through delayed G2-M transition. Furthermore, the G2/M accumulation induced by MeHg is also independent of p21 and p53. More studies on other cell cycle regulation pathways will provide further understanding of the effects of MeHg on cell proliferation.