Loss of clock gene mPer2 promotes liver fibrosis induced by carbon tetrachloride

Aim: The clock gene mPer2 controls circadian periods and plays a critical role in clock resetting and responses to drugs of abuse. Mice deficient in mPer2 exhibit a marked susceptibility to acute liver injury. Clinical observations have demonstrated the existence of a relationship between circadian rhythm and liver cirrhosis. Here, we sought direct evidence for clock function to liver fibrosis using mPer2-deficient mice. Methods: Hepatic fibrosis was induced in wild-type (WT) and mPer2-/- mice by repetitive intraperitoneal carbon tetrachloride (CCl(4)) injection. Masson trichrome staining and analysis of alpha-smooth muscle actin (alpha-SMA) immunohistochemistry were performed to show the collagen accumulation and the hepatic stellate cell (HSC) activation, respectively. The mRNA levels of fibrosis-related genes were monitored by quantitative real-time polymerase chain reaction. The protein level of TIMP-1 was determined by immunohistochemistry. Transferase deoxytidyl uridine end labeling, alpha-SMA double staining and 4',6'-diamidino-2-phenylindole dihydrochloride staining were performed to show HSC apoptosis in vivo and in vitro, respectively. Results: CCl(4) caused much more severe liver fibrosis and activated more HSC in mPer2 null mice as compared to WT animals. Meanwhile, mPer2 null mice exhibited less efficiency in fibrosis resolution. Apoptotic HSC were significantly fewer in mPer2 null mice compared with WT mice after CCl(4); transfected Per2 cDNA into cultured HSC resulted in more HSC apoptosis with upregulation of TRAIL-R2/DR5 expression. Conclusion: Loss of clock gene mPer2 predisposes liver fibrosis by increasing HSC activation and inhibiting HSC apoptosis.