P21 functions to maintain quiescence of p27-deficient hepatocytes.

Hepatocytes rarely proliferate in the healthy adult liver. We explored the roles of the cyclin kinase inhibitors p21 and p27 in maintaining hepatocyte quiescence. p27 is expressed throughout the wild-type liver, but the related protein p21 was not detected. However, p21 was detected in livers of p27-deficient mice. Increased p21 protein levels did not result from an increase in p21 mRNA expression, indicating that p21 expression is regulated post-transcriptionally. p21 protein levels increased in cultured primary hepatocytes treated with the proteasome inhibitor MG132 and cycloheximide, indicating that p21 expression is regulated at the level of protein stability in liver cells. Although increased expression of cyclin-dependent kinase (Cdk) 4, Cdk2, and proliferating cell nuclear antigen was detected in p27-deficient livers, increased hepatocyte proliferation was detected only in livers of mice deficient for both p21 and p27. In p27-deficient livers, p21 was found in complexes with Cdk2 and CdK4 and can compensate for the absence of p27. Our data indicate that cyclin kinase inhibitor activity is important for maintaining hepatocyte quiescence in the adult liver. Significant increases in p21 were detected in multiple tissues of mature p27-deficient mice compared with wild-type mice, suggesting that the ability of p21 to functionally substitute for p27 is not liver-specific.