The Clock gene is a core clock factor that plays an essential role in generating circadian rhythms. In the present study, it was investigated whether the Clock gene affects the response to diethylnitrosamine (DEN)-induced cytotoxicity using mouse primary hepatocytes. DEN-induced cytotoxicity, after 24h exposure, was caused by apoptosis in hepatocytes isolated from wild-type mouse. On the other hand, Clock mutant mouse (Clk/Clk) hepatocytes showed resistance to apoptosis. Because apoptosis is an important pathway for suppressing carcinogenesis after genomic DNA damage, the mechanisms that underlie resistance to DEN-induced apoptosis were examined in Clk/Clk mouse hepatocytes. The mRNA levels of metabolic enzymes bioactivating DEN and apoptosis-inducing factors before DEN exposure were lower in Clk/Clk cells than in wild-type cells. The accumulation of p53 and Ser15 phosphorylated p53 after 8h DEN exposure was seen in wild-type cells but not in Clk/Clk cells. Caspase-3/7 activity was elevated during 24h DEN exposure in wild-type cells but not in Clk/Clk cells. In addition, resistance to DEN-induced apoptosis in Clk/Clk cells affected the cell viability. These studies suggested that the lower expression levels of metabolic enzymes bioactivating DEN and apoptosis inducing factors affected the resistance to DEN-induced apoptosis in Clk/Clk cells, and the Clock gene plays an important role in cytotoxicity induced by DEN.
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