Absence of E-cyclins E1 and E2 affects cell cycle progression, fibrogenesis and tumorgenesis in the murine liver

Recent studies revealed that cyclin E1 and E2 are dispensable for continuously dividing cells, but are important for endoreplication and essential for cell cycle re-entry of quiescent fibroblasts. The aim of this study was to explore the role of E-type cyclins during liver regeneration and to study the influence of cyclin E depletion on fibrogenesis and HCC development in murine c-myc tumor models.

For our studies on liver regeneration we performed 70% partial hepatectomy (PH) in cyclin E1 and E2 knockout mice (KO) and wildtype (WT) controls. For studies on fibrogenesis and tumorgenesis, hepatocyte-specific c-myc transgenic animals (alb-myc^tg) were crossed with cyclin E1 or E2 KO mice and analyzed after 9, 20, 40 and 60 weeks for histological and molecular markers of fibrosis and HCC.

Following PH hepatocyte proliferation was only slightly delayed in cyclin E1KO animals but showed an earlier onset of liver regeneration in cyclin E2KO mice with higher proliferation and hepatomegaly as found seven days after PH. In addition, cyclin E-cdk2 kinase activity was also strongly prolonged in these animals. In E1KO mice we observed a prevalence of diploid cells and only a low number of cells with higher polyploidy index after PH. In contrast, E2KO mice showed much higher polyploidy levels (>4n) from 72h to 96h post PH indicating higher endoreplication.

In alb-myc^tg mice we found significantly high 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 alb-myc^tg mice compared to the WT group and remained constant for all time points investigated. In alb-myc^tg and alb-myc^tg E2KO mice we found strong collagen I expression already at the age of 9 weeks, whereas in alb-myc^tg E1KO mice the level of collagen 1 was lower compared to alb-myc^tg animals for all time points investigated. Of notice, lower fibrosis in alb-myc^tg E1KO mice was associated with a significantly lower tumor incidence in these animals (33% compared to 60% in alb-myc^tg mice at the age of 70 weeks).

We conclude that cyclin E1 is the key player for G1-S phase transition during liver regeneration and seems to play in outstanding role for proliferating cells during fibrogenesis and tumorgenesis. Depletion of cyclin E1 can partly be compensated by cyclin E2. However, the main function of cyclin E2 seems to be the negative regulation of cyclin E1 activity.