The collapse of the mechanisms which stably maintain M phase chromosome and its biological effect

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Ortho-phenyl phenol (OPP) and its hepatic metabolite, phenyl hydroquinone (PHQ), are broad-spectrum fungicides and antibacterial agents. OPP and PHQ tested negative in an Ames system and positive with respect to the formation of tumors in the urinary bladder in rats when administered in diet, showing attributes of a non-genotoxic carcinogen. It has also been demonstrated that OPP and PHQ do not bind or cleave DNA in vivo or in vitro, rather dose-dependent protein binding in OPP-treated rats was observed. OPP and PHQ, however, generate chromosomal aberrations including aneuploidy. Thus, the steps by which non-genotoxic carcinogens exert their effects need to be elucidated. In this study, we used an assay of loss of heterozygosity (LOH) in Saccharomyces cerevisiae and cultured human cells to determine the biological effects of OPP and PHQ. LOH was found to be induced by OPP and PHQ because of a functional chromosome loss : aneuploidy. PHQ bound to and interfered with the depolymerization of tubulin in vitro. We further demonstrate that PHQ can arrest the cell cycle at the G2/M transition as a result of the stabilization of Swe1 (Wee1 homolog), probably leading to inactivation of the Cdc28 (Cdk1/Cdc2 homolog). Furthermore, Hog1 (p38 MAPK homolog) was robustly phosphorylated by PHQ, which can stabilize Swe1. On the other hand, Chk1 and Rad53 were not phosphorylated by PHQ, indicating that Mec1/Tel1 DNA damage checkpoint was not functional. Mutation of swe1 and hog1 abolished the PHQ-induced arrest at the G2/M transition and became resistant to PHQ lethality and aneuploidy formation. These results suggest that PHQ-induced G2/M transition checkpoint which is activated by the Hog1-Swe1 pathway plays a role in the formation of aneuploidy. We argue that OPP and PHQ activate MAPK pathway arrested cell cycle at G2/M transition and caused aneuploidy.