To address this hypothesis, we treated CcnE1−/− and CcnE2−/− mice acute or chronically with CCl4 and analyzed the effect on the proliferative response of hepatocytes and nonparenchymal liver cells. Ubiquitous ablation of CcnE1 in this fibrosis model revealed several unexpected findings defining CcnE1 as an essential profibrotic mediator. After acute toxic liver injury, the overall proliferative response of CcnE1-deficient hepatic cells was dramatically impaired. Several studies, including our own work, demonstrated that CcnE1
is dispensable for the proliferation of continuously cycling cells and regenerating hepatocytes after surgical partial liver resection.9, 11, 19 From our present data, it is now evident that the requirement of proliferating hepatocytes for CcnE1 depends on the proliferation stimulus. Though CcnE1 is dispensable for hepatocyte proliferation in a proinflammatory environment (e.g., hepatectomy), Birinapant purchase it is apparently essential after toxic liver injury (i.e., CCl4) in vivo. In agreement with this hypothesis, we recently observed a prolonged cell-cycle arrest of CcnE1−/−
hepatocytes in vivo after treatment with the hepatotoxic agent, diethylnitrosamine (data not shown). Intriguingly, constitutive ablation of CcnE1—but not inhibition of CcnE2—protected from CCl4-mediated liver fibrosis, which was related to impaired cell-cycle activity of nonparenchymal liver cells. Hence, we focused on HSCs because this cell population is central for the process of liver fibrosis find more as the major source of ECM proteins.20 A recent study suggested that down-regulation of CcnE1 is related to the delayed cell-cycle progression of the human HSC Mirabegron line, LX-2.21 In line with these findings, we demonstrated that complete ablation of CcnE1 induces a dramatic cell-cycle arrest of HSCs and hypersensitivity to apoptosis and overall poor survival. Although the mechanism triggering apoptosis in CcnE1-deficient cells remains elusive, HSC apoptosis clearly acts as an
antifibrotic.22 Thus, reduced liver fibrosis in CCl4-treated CcnE1−/− mice is most likely explained by impaired viability and cell-cycle arrest of HSCs after profibrogenic stimulation. Our study also revealed an unexpected role of CcnE2 for liver fibrogenesis. In line with our earlier studies,11 loss of CcnE2 resulted in accelerated gene activation of CcnE1 in hepatocytes and HSCs, suggesting that CcnE2 is an inhibitor of CcnE1 expression. At present, the underlying mechanism is unclear; however, CcnE2 was shown to be 1.5- to 10-fold more highly expressed, compared to CcnE1, in at least three independent studies.23 We speculate that CcnE2 might sequester, and thus inactivate, transcriptional activators of CcnE1. Besides up-regulating CcnE1, loss of CcnE2 resulted in early liver fibrogenesis and, more important, in accelerated HSC activation and proliferation.