E-type cyclins (CcnE) control the transition of quiescent cells into the cell cycle. Two E-type cyclins, CcnE1 and CcnE2 have been described. A variety of human cancers, including hepatocellular carcinoma (HCC), overexpress CcnE and this is frequently associated with reduced patient survival. The aim of the present study was to dissect the role of CcnE1 and CcnE2 for hepatocarcinogenesis induced by the carcinogen diethylnitrosamine (DEN) using CcnE1 and CcnE2 knockout mice. The central question was how the genetic loss of CcnE1 or CcnE2 would affect tumor initiation and/or progression. The study revealed several unexpected findings. CcnE2-/- mice developed liver tumors of similar number and size compared to wild type (WT) animals demonstrating for the first time that CcnE2 is dispensable for development of liver cancer. Surprisingly, CcnE1-deficient animals were mostly resistant to HCC induction and showed poor tumor growth. Therefore the present data suggests that CcnE1 – but not its homologue CcnE2 – is essential for initiation and progression of liver cancer. The molecular mechanisms underlying these findings were further investigated in a model of DEN-induced acute liver injury, which reflected immediate early events of cell transformation and cellular signaling. DEN-mediated liver injury was genotoxic and triggered a DNA damage response pathway (DDR) and activation of Jun kinases (JNK) already 24 and 48 hours after treatment in all genotypes. Interestingly, CcnE1-deficient livers revealed transient hyper-activation of JNK and more importantly a stronger and prolonged expression of the tumor suppressor p53. In good agreement, CcnE1-/- livers showed also prolonged cell cycle arrest associated with increased expression of cell cycle inhibitors p21 and p27 following DEN challenge. Thus expression of CcnE1 was shown to be essential to overcome the cell cycle arrest and DDR of hepatocytes immediately after mutagenic treatment. In another approach knockout mice with a hepatocyte-specific deletion of Cdk2 (Cdk2 delta hepa) and Cdk2 delta hepa CcnE2-/- double knockout mice were generated and subjected to DEN treatment. Importantly, these strains were also strongly protected from HCC formation similar to CcnE1-/- mice. In summary, the present study demonstrated for the first time that CcnE1 is an essential oncogene in the liver and drives hepatocarcinogenesis and tumor growth in a Cdk2-dependent manner
