Background and aims The cyclin-dependent kinase inhibitor p21 has been implicated as a tumour suppressor. Moreover, recent genetic studies suggest that p21 might be a potential therapeutic target to improve regeneration in chronic diseases. The aim of this study was to delineate the role of p21 in chronic liver injury and to specify its role in hepatocarcinogenesis in a mouse model of chronic cholestatic liver injury.

Methods The degree of liver injury, regeneration and tumour formation was assessed in Mdr2^−/− mice and compared with Mdr2/ p21^−/− mice. Moreover, the role of p21 was evaluated in hepatoma cells in vitro and in human hepatocellular carcinoma (HCC).

Results Mdr2^−/− mice developed HCCs as a consequence of chronic inflammatory liver injury. In contrast, tumour development was profoundly delayed in Mdr2/ p21^−/− mice. Delayed tumour development was accompanied by markedly impaired liver regeneration in Mdr2/ p21^−/− mice. Moreover, the regenerative capacity of the Mdr2/ p21^−/− livers in response to partial hepatectomy declined with age in these mice. Hepatocyte transplantation experiments revealed that impaired liver regeneration was due to intrinsic factors within the cells and changes in the Mdr2/ p21^−/− microenvironment. In human HCCs, a subset of tumours expressed p21, which was associated with a significant shorter patient survival.

Conclusions We provide experimental evidence that p21 is required for sustained liver regeneration and tumour development in chronic liver injury indicating that p21 needs to be tightly regulated in order to balance liver regeneration and cancer risk. Moreover, we identify p21 as a negative prognostic marker in human HCC.