The role of E-type cyclins for hepatic fibrogenesis in mice

Background and Aims: It has been recently stated, that Cyclin E1 and E2 are dispensable in continuously dividing cells, but essential for cell cycle reentry of quiescent cells. The aim of this study was to investigate the role of E-type cyclins for cells proliferation during fibrogenesis. Methods: For our studies hepatocyte-specific c-myc transgenic animals (myc) containing a knockout for either cyclin E1 (E1KO) or E2 (E2KO) were generated and analyzed after 9, 20, 40 and 60 weeks for histological and molecular markers of cell cycle activity (cyclin E1, E2, BrdU incorporation), apoptosis (TUNEL) and fibrosis (collagen1, alpha-smooth-muscle-actin (α-SMA)). To chemically induce liver fibrosis we injected 0,6ml/kg CCl4 i.p. into wild-type (WT) and E1KO mice 2 times per week for up to 4 weeks. Results: We recently found that cyclin E1 is a key player for proliferation whereas cyclin E2 is mainly a negative regulator of cell cycle following 2/3 hepatectomy. The transcription factor c-myc is a known regulator of cyclin E1. In myc mice we found significantly increased levels of cyclin E1 already at the age of 9 weeks which further increased time dependently. On the contrary, cyclin E2 mRNA levels were initially lower in myc mice compared to the WT group and remained constant for all time points investigated. In myc and myc E2KO mice we found strong collagen1 expression and fiber deposition in the liver already at the age of 9 weeks but also after 20, 40 and 60 weeks indicating early modifications related to liver fibrosis. In contrast myc E1KO mice had lower collagen 1 expression compared to myc animals for all time points investigated indicating that depletion of cyclin E1 may reduce fibrogenic effects. To prove this idea, we applied the CCl4 model to cyclin E1KO mice and WT controls. Interestingly, expression of collagen1 and α-SMA in E1KO mice was strongly reduced compared to controls. In addition, in situ staining of collagen1 and α-SMA revealed strong fibrogenic effects in WT animals but only moderate signs of fibrosis in E1KO mice. Conclusion: Our results suggest that cell cycle activation via cyclin E1 may promote fibrogenesis. Therefore cyclin E1 is a pro-fibrotic factor whereas cyclin E2 potentially antagonizes this effect.