Polµ deficiency increases resistance to oxidative damage and delays liver aging.

Polµ is an error-prone PolX polymerase that contributes to classical NHEJ DNA repair. Mice lacking Polµ (Polµ(-/-)) show altered hematopoiesis homeostasis and DSB repair and a more pronounced nucleolytic resection of some V(D)J junctions. We previously showed that Polµ(-/-) mice have increased learning capacity at old ages, suggesting delayed brain aging. Here we investigated the effect of Polµ(-/-) deficiency on liver aging. We found that old Polµ(-/-) mice (>20 month) have greater liver regenerative capacity compared with wt animals. Old Polµ(-/-) liver showed reduced genomic instability and increased apoptosis resistance. However, Polµ(-/-) mice did not show an extended life span and other organs (e.g., heart) aged normally. Our results suggest that Polµ deficiency activates transcriptional networks that reduce constitutive apoptosis, leading to enhanced liver repair at old age.

p21 promotes sustained liver regeneration and hepatocarcinogenesis in chronic cholestatic liver injury.

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.