A novel inducible genetic murine model of hepatocyte senescence causes a massive hepatic oval cell response and complete hepatocyte replacement from endogenous progenitor cells [Abstract]

Background and aims: In severe advanced liver disease it is increasingly recognised that hepatic progenitor cells (HPCs) are a major source of parenchymal regeneration but may also be a cellular source of liver cancers. Murine liver injury models eliciting HPC activation frequently demonstrate relatively small and heterogeneous activation of HPCs, together with simultaneous hepatocyte proliferation. While purified HPCs from such models have been shown in transplant models to repopulate injured liver the full repopulation potential of endogenous HPC remains unclear. In order to investigate the role of HPCs directly it is necessary to specifically inhibit hepatocyte proliferation. Mdm2 is a key inhibitor of the tumour suppressor gene p53. P53 activation results in cell cycle arrest, therefore hepatocyte specific Mdm2 knockout using Cre Lox technology provides an opportunity to investigate HPC mediated liver regeneration. Methods: Hepatocyte specific loss of Mdm2 was achieved by i.p. administration of β-naphthoflavone to AhCre+ Mdm2fl/fl mice. Immunohistochemical analysis was performed for p21, p53, and the HPC marker panCK. Cellular proliferation was assessed using Ki67 and BrdU label incorporation. Serum was analysed for LFTs. Results: Hepatocytes, but not other non parenchymal hepatic cells, are p53hi and p21hi following induction of Mdm2 loss by recombination. Loss of hepatocyte proliferation and massive expansion of p53low HPCs rapidly occurs. This results in the production of phenotypically normal p53low/p21low hepatocytes and the restoration of normal liver function. PanCK+ progenitor cell expansion is over three times greater than is seen in CDE diet induced HPC expansion (Mean±SEM; Mdm2 212± 27.48 versus CDE 60.89 ± 17.59 versus control 13.85± 0.82). No cancers or significant fibrosis are observed following complete replacement of the hepatic parenchyma during long term follow up. Conclusions: Mdm2 knockout induces hepatocyte senescence and induces massive expansion of p53low HPCs resulting in complete regeneration of the liver parenchyma in the absence of long term sequelae. This novel model permits detailed examination of HPC mediated regeneration and demonstrates the physiological potential of HPCs and highlights their therapeutic potential