title: p21(WAF1/CIP1) inhibits cell cycle progression but not G2/M-phase transition following methylmercury exposure.
abstract: 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 G1- and G2-phase checkpoint 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 microM 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 G2/M at 2 and 4 microM MeHg (p < or = 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 G1 at a higher fraction compared to the wild type (+/+) and heterozygous (+/-) cells (p < or = 0.05). These data support previous observations that MeHg inhibits cell cycle progression through delayed G2/M transition. Whereas the G2/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.
authors.affiliation: Department of Environmental Health, University of Washington, Seattle, Washington 98195, USA.
authors.forename: Ma Aileen C
authors.initials: MA
authors.lastname: Mendoza
authors.forename: Rafael A
authors.initials: RA
authors.lastname: Ponce
authors.forename: Ying C
authors.initials: YC
authors.lastname: Ou
authors.forename: Elaine M
authors.initials: EM
authors.lastname: Faustman
mesh_terms: D000818:Animals; D002453:Cell Cycle / Q000187:drug effects; D002455:Cell Division / Q000187:drug effects; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins / Q000502:physiology*; D005347:Fibroblasts; D005434:Flow Cytometry; D016195:G2 Phase / Q000187:drug effects*; D005838:Genotype; D008767:Methylmercury Compounds / Q000633:toxicity*; D051379:Mice; D008822:Mice, Transgenic
chemical_list: C493957:Cdkn1a protein, mouse; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D008767:Methylmercury Compounds