Cyclin E1 and Cdk2 in Hepatic Stellate Cells are critical for initiation and progression of liver fibrosis and cancer

Question Hepatocellular carcinoma (HCC) is one of the most severe tumor diseases with increasing incidence and limited treatment options. Typically, HCC develops during a multistep process involving chronic liver inflammation and liver fibrosis. The latter is characterized by accumulation of extracellular collagen produced by Hepatic Stellate Cells (HSCs). This process involves cell cycle re-entry and proliferation of the normally quiescent HSCs. The cell cycle is regulated by cyclins and associated cyclin-dependent kinases (Cdks). In the present study, we examined the role of Cyclin E1 (CcnE1) and Cdk2 in HSCs for liver fibrogenesis and hepatocarcinogenesis.

Methods We crossed conditional CcnE1 (CcnE1^f/f) or Cdk2 (Cdk2^f/f) knockout mice with transgenic mice expressing cre-recombinase under the control of the L-rat promoter resulting in mice lacking CcnE1 or Cdk2 specifically in HSCs (CcnE1^∆HSC or Cdk2^∆HSC). Cdk2^∆HSC and CcnE1^∆HSC mice were challenged with CCl₄ for 6 weeks and subsequently investigated for liver fibrosis. Induction of fibrosis and HCC in CcnE1^∆HSC mice was performed using the DEN/CCl₄ model.

Results Genetic ablation of Cdk2 or Cyclin E1 specifically in HSCs significantly reduced collagen accumulation and fiber formation in the liver after CCl₄ treatment when compared to cre-negative littermates. Accordingly, Cdk2^∆HSC and CcnE1^∆HSC mice showed a significantly reduced HSC activation in the liver, as determined by aSMA and col1a1 gene expression. This suggests that cell cycle re-activation of naïve HSCs in vivo requires functional Cdk2 presumably in complex with Cyclin E1. CcnE1^∆HSC mice, challenged with DEN/CCL₄ showed a reduced number and size of dysplastic lesions when compared to cre- littermates, which was again associated with decreased HSC activation even after long-term observation.

Conclusion We provide evidence that the pro-fibrotic properties of HSCs depend on functional Cdk2 and CcnE1. In addition, inhibition of Cyclin E1 in HSCs is sufficient to reduce initiation and progression of HCC in mice at least in the DEN/CCL₄ model. We conclude that a HSC-specific therapeutic targeting of Cyclin E1 or Cdk2 in patients with liver fibrosis and high risk for HCC development could be beneficial. In summary, our study contributes to a better understanding of the complex, cell type-specific roles of CcnE1 and Cdk2 for the pathological sequence from liver fibrosis to HCC.

Publication History

Article published online:
04 January 2021