Cyclin E1 controls proliferation of hepatic stellate cells and is essential for liver fibrogenesis in mice

Abstract

Liver fibrogenesis is associated with the transition of quiescent hepatocytes and hepatic stellate cells (HSC) into the cell cycle. Exit from quiescence is controlled by E-type cyclins (CcnE1, CcnE2). Thus, the aim of the current study was to investigate the contribution of E-type cyclins for liver fibrosis in man and mice. Expression of CcnE1, but not of its homologue CcnE2 was induced in fibrotic and cirrhotic livers from human patients with different etiologies and in murine wildtype (WT) livers after periodical administration of the pro-fibrotic toxin carbon tetrachloride (CCl4). To further evaluate the potential function of E-type cyclins for liver fibrogenesis, we repetitively treated constitutive CcnE1−/− and CcnE2−/− knockout mice with CCl4 to induce liver fibrosis. Interestingly, CcnE1−/− mice were protected against CCl4–mediated liver fibrogenesis as evidenced by reduced collagen type I α1 expression and lack of septum formation. In contrast, CcnE2−/− mice showed accelerated fibrogenesis following CCl4 treatment. We isolated primary HSC from WT, CcnE1−/− and CcnE2−/− mice and analyzed their activation, proliferation and survival in vitro. CcnE1 expression in WT HSC was maximal when they started to proliferate, but decreased after the cells transdifferentiated into myofibroblasts. CcnE1−/− HSC showed dramatically impaired survival, cell cycle arrest and strongly reduced expression of alpha-smooth muscle actin, indicating deficient HSC activation. In contrast, CcnE2-deficient HSC expressed elevated level of CcnE1 and showed enhanced cell cycle activity and proliferation compared to WT cells. Conclusions CcnE1 and CcnE2 have antagonistic roles in liver fibrosis. CcnE1 is indispensable for activation, proliferation and survival of HSC and thus promotes synthesis of extracellular matrix and liver fibrogenesis.

Vitamin C promotes the proliferation of human adipose-derived stem cells via p53-p21 pathway

“…21 It has been reported that cyclin E1 can interact with CDK2 and activate CDK2 at the G 1 /S transition of the cell cycle. 22,23 In the present study, protein gel blotting analysis indicated that the expression levels of cyclin E1 and CDK2 proteins were upregulated after 12 h of incubation with vitamin C, which suggests that cyclin E1 and CDK2 may involved in the transition of cell cycle from G 1 to S phase in the ADSCs. The p53 protein is an important regulative factor of many processes necessary for the proper cellular function, and is also responsible for the regulation of cell cycle.…”

Stellate Cells and Hepatic Fibrosis

“…Interestingly, in this model, c-myc was not required for cyclin E1 mediated HSC activation. Several other levels of control may exist for cyclin E1, as it appears to be negatively regulated by cyclin E2 [141] and miR-195 [142]. In a human immortalized HSC line (LX-2) in culture, spontaneous activation is associated with a reduction in miR-195, an increase in cyclin E1, and entry into the cell cycle with subsequent proliferation [142].…”

“…However, in view of how HSCs and a fibrotic microenvironment promote HCC [14], some of the observed results in the in vivo model may have been due to reduced c-myc expression and attenuated HSC activation, but again, these parameters were not directly measured. Cyclin E1 is known to be under the control of c-myc and is critical for HSC activation [141]. As with c-myc, cyclin E1 is highly expressed in cirrhotic human liver tissue and not only promotes the transdifferentiation of quiescent HSCs but drives the proliferation of activated myofibroblasts.…”

Overexpression of c-myc in hepatocytes promotes activation of hepatic stellate cells and facilitates the onset of liver fibrosis

“…HSC were isolated from adult male mice in a C57BL/6 background weighting approximately 25 g following the collagenase method as described recently [17]. Freshly isolated HSC were washed and plated in 6-well plates (Falcon) at a density of 150,000 cells per well and cultivated in DMEM supplemented with 10% FCS 4 mmol/l Lglutamine and 4 mmol/l penicillin/streptomycin (PAA, Pasching, Austria).…”

“…Thus, regeneration of hepatocytes and activation of HSCs during chronic liver injury involves transition of these normally quiescent cells into the cell cycle leading to cell proliferation. We recently demonstrated that the cell cycle mediator cyclin E1 is essential for proliferation and activation of HSC but less relevant for proliferation of regenerating hepatocytes [17,18]. However, the role of c-myc -which is thought to act upstream of cyclin E1 -for initiation and progression of liver fibrosis is largely unknown.…”