p21WAF1/CIP1 inhibits cell cycle progression but not G2/M-phase transition following methylmercury exposure

Methylmercury (MeHg) is an environmentally prevalent organometal that is particularly toxic to the developing central nervous system (CNS). Prenatal MeHg exposure is associated with reduced brain size and weight and a reduced number of neurons, which have been associated with impaired cell proliferation. We evaluate the role of p21, a cell cycle protein involved in the G(1)- and G(2)-phase check point control, in the cell cycle inhibition induced by MeHg. Primary mouse embryonic fibroblasts (MEFs) of different p21 genotypes (wild-type, heterozygous, and null) were isolated at day 14 of gestation and treated at passages 4-6 with either 0, 2, 4, or 6 muM MeHg or 50 nM colchicine for 24 h. Changes in cell cycle distribution after continuous toxicant treatment were analyzed by DNA content-based flow cytometry using DAPI. MeHg induced an increase in the proportion of cells in G(2)/M at 2 and 4 muM MeHg (p less than or equal to 0.05) irrespective of p21 genotype. Effects of MeHg on cell cycle progression were subsequently evaluated using BrdU-Hoechst flow cytometric analysis. Inhibition of cell cycle progression was observed in all p21 genotypes after continuous exposure to MeHg for 24 and 48 h. p21 null (-/-) cells reached the second-round G(1) at a higher fraction compared to the wild type (+/+) and heterozygous (+/-) cells (p less than or equal to 0.05). These data support previous observations that MeHg inhibits cell cycle progression through delayed G(2)/M transition. Whereas the G(2)/M accumulation induced by MeHg was independent of p21 status, a greater proportion of p21(-/-) cells were able to complete one round of cell division in the presence of MeHg compared to p21(+/-) or p21(+/+) cells. These data suggest a role for p21 in retarding cell cycle progression, but not mitotic inhibition, following exposure to MeHg. (C) 2002 Elsevier Science (USA).