Genetic fine analysis of the pro-fibrotic and pro-carcinogenic functions of Cyclin E1 in the liver

Question:

Hepatocellular carcinoma (HCC) is one of the most severe tumor diseases with increasing incidence and limited treatment options. We have recently shown that the G1/S-phase cyclin, Cyclin E1 (CcnE1), is essential for the onset of liver fibrosis and the initiation of HCC. However, the precise contribution of CcnE1 in different cell compartments besides hepatocytes is incompletely understood. In the present study, we examined the role of CcnE1 in hepatic stellate cells (HSCs) for liver fibrogenesis, HCC initiation and progression.

Methods:

In order to delete CcnE1 specifically in HSCs we crossed conditional CcnE1 knockout mice (CcnE1^f/f) with transgenic mice expressing cre-recombinase under the control of the L-rat promoter resulting in mice lacking CcnE1 in HSCs (CcnE1^ΔHSC). Induction of fibrosis and HCC was performed using the DEN/CCl₄ model. Briefly, mice were injected with a single dose of diethylnitrosamine at the age of 14 days followed by a weekly injection of carbon tetrachloride (CCl₄) beginning from the age of 6 weeks for up to 18 weeks. Mice were analyzed for liver fibrosis formation by histologic Sirius red staining. HCC initiation was determined by quantification of histologic pre-malignant lesions; as a measure of HCC progression, the number and size of macroscopic tumor nodules was evaluated.

Results:

Mice with deletion of CcnE1 exclusively in HSCs revealed significantly reduced liver fibrosis in comparison to cre-negative littermates after DEN/CCl₄ treatment. However, CcnE1^ΔHSC mice did surprisingly not show a reduced number or size of pre-malignant lesions in liver sections as initially hypothesized. In line, CcnE1^ΔHSC mice revealed a similar number and size of macroscopic tumor nodules.

Conclusions:

We provide evidence that HSCs are the major effector cells for the previously shown pro-fibrotic function of Cyclin E1. However, inhibition Cyclin E1 in HSCs is not sufficient to prevent onset of liver cancer suggesting that the postulated HCC-inducing impact of activated HSCs has been over-estimated. We conclude that a theoretical anti-Cyclin E1 therapy of patients with liver fibrosis and high risk for HCC development could be beneficial provided that Cyclin E1 will be inhibited in most liver cells including HSCs and transformed hepatocytes. In summary, our study contributes to a better understanding of the complex, cell type-specific roles of CcnE1 for the pathological sequence from liver fibrosis to HCC.