Critical cholangiocarcinogenesis control by cryptochrome clock genes

A coordinated network of molecular circadian clocks in individual cells generates 24-hr rhythms in liver metabolism and proliferation. Circadian disruption through chronic jet lag or Per2 clock gene mutation was shown to accelerate hepatocarcinoma development in mice. As divergent effects were reported for clock genes Per and Cry regarding xenobiotic toxicity, we questioned the role of Cry1 and Cry2 in liver carcinogenesis. Male WT and Cry1(-/-)Cry2(-/-) mice (C57Bl/6 background) were chronically exposed to diethylnitrosamine (DEN) at ZT11. Rest-activity and body temperature rhythms were monitored using an implanted radiotransmitter. Serum aspartate and alanine aminotransferases (AST and ALT) were determined on four occasions during the progression stage. After 7 months, serum alkaline phosphatases (ALP) were determined, and livers were sampled for microscopic tumor nodule counting and histopathology. Five months after initiation of DEN treatment, we found that Cry1(-/-)Cry2(-/-) mice developed severe liver dysplasia, as evident from the increased AST, ALT and ALP levels, as compared to WT mice. DEN exposure induced primary liver cancers in nearly fivefold as many Cry1(-/-)Cry2(-/-) mice as compared to WT mice (p=0.01). Microscopic study revealed no difference in the average number of hepatocarcinomas and a nearly eightfold increase in the average number of cholangiocarcinomas in Cry1(-/-)Cry2(-/-) mice, as compared to WT mice. This study validated the hypothesis that molecular circadian clock disruption dramatically increased chemically induced liver carcinogenesis. In addition, the pronounced shift toward cholangiocarcinoma in DEN exposed Cry1(-/-)Cry2(-/-) mice revealed a critical role of the Cry clock genes in bile duct carcinogenesis. What's new? Shift-work that involves circadian disruption has been declared probably carcinogenic to humans. Here, the relevance of the circadian clock in liver carcinogenesis was investigated through exposing Cry1/Cry2-deficient mice to hepatic carcinogen diethylnitrosamine (DEN). Cry1/Cry2 deficiency increased liver carcinogenesis nearly fivefold, validating the hypothesis that molecular circadian clock disruption dramatically increased chemically induced liver carcinogenesis. In addition, the pronounced shift in tumor spectrum from hepatocarcinoma to cholangiocarcinoma in DEN-exposed Cry1(-/-)Cry2(-/-) mice revealed a critical role of the Cry clock genes in bile duct carcinogenesis. The findings emphasize the role of the two clock genes in the yet poorly understood physiopathology of cholangiocarcinogenesis.