Inhibition of hepatic cell proliferation through the systematic dissection of Cdk2/cyclin E complexes in mice
Year: 2013
Type: dissertation
Abstract: Complexes of Cdk2 with E-type cyclins (CcnE1 and CcnE2) specifically regulate the onset of DNA replication in previously quiescent mammalian cells. Partial 2/3 hepatectomy (PH) in rodents is an established model to study this process in vivo as quiescent hepatocytes synchronously undergo 1-2 cell cycles after surgery. Surprisingly, previous studies from our lab demonstrated that unique depletion of CcnE1 or CcnE2 did not markedly impair hepatocyte proliferation after PH. Thus the initial aim of the present study was to characterize the role of Cdk2 for the hepatic cell cycle. As these analyses revealed that Cdk2 is also fully dispensable for cell cycle progression in hepatocytes, the underlying compensatory mechanisms were evaluated by detailed genetic dissection of the complete Cdk2/CcnE complexes with the ultimate aim to inhibit the hepatic cell cycle by blocking appropriate combinations of Cdk2 and E-cyclins. Hepatocyte-specific ablation of Cdk2 (Cdk2Deltahepa) resulted in accelerated induction and overall stronger expression of CcnE1 after PH. Elevated CcnE1 levels did not contribute to a non-canonical kinase activity but instead performed a kinase-independent function by facilitating the loading of minichromosome maintenance (MCM) proteins on chromatin. Accordingly, hepatocytes lacking both Cdk2 and CcnE1 showed severely impaired loading of MCM and were unable to activate the S-phase mediator CcnA2 which is a prerequisite for S-phase progression. As a consequence, mitogen-primed Cdk2-/-CcnE1-/- hepatocytes displayed severely impaired cell viability and dramatically reduced proliferative capability. In vivo, DNA replication and subsequent liver mass reconstitution following PH were substantially – but not completely blocked in Cdk2DeltahepaCcnE1-/- mice. Surprisingly, hepatocytes lacking Cdk2 and both E-cyclins did not show further impaired liver regeneration suggesting a negligible role of CcnE2 for cell cycle progression. In fact, Cdk2DeltahepaCcnE2-/- mice showed normal S-phase and liver regeneration. However, mice lacking both E-type cyclins exhibited strongly impaired liver regeneration pointing to some functional overlap of CcnE1 and CcnE2. In summary, the current study identifies for the first time combinations of Cdk2 and/or E-cyclins, which are largely essential for driving the hepatic cell cycle. This contributes to a better understanding of redundant mechanisms in the mammalian cell cycle machinery and may help to inhibit pathological liver cell proliferation e.g. in hepatocellular carcinoma.
Source: RWTH Publications (RWTH Aachen)
Authors: Wei Hu, Jürgen Bernhagen
Language: English
Subfield: Hepatology
Field: Medicine
Domain: Health Sciences
Open Access status: green
OpenAlex ID: W2206996818