Deletion of cyclin E1 inhibits DEN induced hepatocarcinogenesis in mice

Transition of resting cells into the proliferative state of the cell cycle is associated with strong expression of E-type cyclins potentially contributing to early tumorigenesis. The aim of this study was to characterize constitutive cyclin E1 (CcnE1-/-) and cyclin E2 (CcnE2-/-) knockout mice in a model of chemically induced hepatocarcinogenesis. Single injection of the carcinogen diethyl-nitrosamine (DEN, 25mg/kg) was applied to CcnE1-/-, CcnE2-/- and wild-type (WT) animals at the age of 14 days to induce hepatocellular carcinoma (HCC). In addition, 6–8 weeks old WT, CcnE1-/- and CcnE2-/- mice were treated once with 200mg/kg DEN and analyzed from 0h up to 240h after injection for early tumorigenic effects. 22 weeks after DEN application, 40% of WT and CcnE2-/- mice, respectively, had small, macroscopic visible tumor nodules ≤ 1mm in diameter. In contrast, none of the CcnE1-/- mice displayed any macroscopic tumor nodule at this time point. 38 weeks after DEN treatment both WT controls and CcnE2-/- mice showed overall strong tumorigenesis with an average of 32 tumors per liver and an average tumor diameter of 6mm (±3mm), whereas only 50% of CcnE1-/- mice developed liver tumors with a significantly lower average frequency (3 tumors/liver) and size (diameter of 2mm (±1mm)). To explore the molecular mechanism leading to tumor inhibition in CcnE1-/- animals, we investigated the immediate response to DEN. CcnE1-/- mice performed twofold higher liver apoptosis, overall increased expression of the cell cycle inhibitor p21 and a strongly prolonged cell cycle arrest compared to CcnE2-/- mice and controls. In summary, we demonstrate that depletion of cyclin E1 but not of cyclin E2 protects from HCC formation through mechanisms involving delayed cell cycle progression and increased apoptosis. We conclude that cyclin E1 is an essential factor for tumour initiation which can not be compensated by cyclin E2. Cyclin E - HCC