Analysis of IFN-gamma-induced cell cycle arrest and cell death in hepatocytes.

The mechanism by which IFN-gamma induces cell cycle arrest and cell death in primary cultured hepatocytes was examined. The cell death exhibits apoptotic characters such as the appearance of apoptotic bodies and DNA fragmentation. IFN-gamma induced cell cycle arrest at the initial stage, followed by cell death. A protein synthesis inhibitor, cycloheximide, significantly inhibited cell death, implying that IFN-gamma induces de novo proteins involved in the death of hepatocytes. One of the most important apoptosis-related proteins, p53, was induced by IFN-gamma in hepatocytes in a dose- and time-dependent manner. Northern blot analysis demonstrated that IFN-gamma enhanced p53 mRNA expression as well as p21(WAF1/Cip1/Sdi1) mRNA expression, which is mediated by the increased expression of the p53 protein. Interestingly, IFN-gamma also induced cell death in p53-deficient hepatocytes. The cell death occurred rather earlier in p53-deficient cells than in normal hepatocytes. However, the cell death was not accompanied by apoptotic bodies. Therefore, IFN-gamma-induced hepatocyte cell death is p53-independent, and p53 may contribute to the apoptotic characters. In conclusion, IFN-gamma is supposed to cause cell cycle arrest by inducing p53 and p21(WAF1/Cip1/Sdi1), and it was demonstrated that IFN-gamma induces p53-independent cell death in primary cultured hepatocytes.