p53-independent induction of p21WAF1/CIP1 expression in pericentral hepatocytes following carbon tetrachloride intoxication.

The cyclin-dependent kinase, proliferating cell nuclear antigen, and stress-activated protein kinase/c-jun NH2 terminal kinase inhibitor p21WAF1/CIP1 can induce G1 arrest, and its expression coincides with the cessation of replication in many systems. We examined expression of p21 during the early stages of carbon tetrachloride intoxication in the mouse liver and observed a dramatic increase in p21 RNA levels between 4 and 8 h after administration. p21 expression, visualized by in situ hybridization, is induced in pericentral hepatocytes before carbon tetrachloride-induced necrosis. Examination of c-fos and c-myc expression patterns confirm that these immediate-early genes are induced in similar regions of the mouse liver. p21 induction is not dependent on p53; we observed similar levels and localization of p21 in wild-type and p53 null animals. Immunohistochemical localization of p21 and CCAAT/enhancer-binding protein expression shows that p21 protein accumulation is limited to a subset of CCAAT/enhancer-binding protein-positive hepatocytes. A second peak of periportal and intermediate zone-specific p21 gene expression, appearing 1-2 days after injection, is also p53 independent and may represent cell cycle checkpoints or postmitotic growth arrest. Sporadic p21 expression was also detected in pairs of hepatocytes distributed throughout the liver acini in healthy animals. Together, these data suggest several roles for p21 in the liver in response to toxicity, regeneration, and growth inhibition.