Ccne1 Overexpression Causes Chromosome Instability in Liver Cells and Liver Tumor Development in Mice

BACKGROUND & AIMS

The CCNE1 locus, which encodes cyclin E1, is amplified in many types of cancer cells and is activated in hepatocellular carcinomas (HCCs) from patients infected with hepatitis B virus or adeno-associated virus type 2, due to integration of the virus nearby. We investigated cell-cycle and oncogenic effects of cyclin E1 overexpression in tissues of mice.

METHODS

We generated mice with doxycycline-inducible expression of Ccne1 (Ccne1^T mice) and activated overexpression of cyclin E1 from age 3 weeks onward. At 14 months of age, livers were collected from mice that overexpress cyclin E1 and nontransgenic mice (controls) and analyzed for tumor burden and by histology. Mouse embryonic fibroblasts (MEFs) and hepatocytes from Ccne1^T and control mice were analyzed to determine the extent to which cyclin E1 overexpression perturbs S-phase entry, DNA replication, and numbers and structures of chromosomes. Tissues from 4-month-old Ccne1^T and control mice (at that age were free of tumors) were analyzed for chromosome alterations, to investigate the mechanisms by which cyclin E1 predisposes hepatocytes to transformation.

RESULTS

Ccne1^T mice developed more hepatocellular adenomas and HCCs than control mice. Tumors developed only in livers of Ccne1^T mice, despite high levels of cyclin E1 in other tissues. Ccne1^T MEFs had defects that promoted chromosome missegregation and aneuploidy, including incomplete replication of DNA, centrosome amplification, and formation of nonperpendicular mitotic spindles. Whereas Ccne1^T mice accumulated near-diploid aneuploid cells in multiple tissues and organs, polyploidization was observed only in hepatocytes, with losses and gains of whole chromosomes, DNA damage, and oxidative stress.

CONCLUSIONS

Livers, but not other tissues of mice with inducible overexpression of cyclin E1, develop tumors. More hepatocytes from the cyclin E1-overexpressing mice were polyploid than from control mice, and had losses or gains of whole chromosomes, DNA damage, and oxidative stress; all of these have been observed in human HCC cells. The increased risk of HCC in patients with hepatitis B virus or adeno-associated virus type 2 infection might involve activation of cyclin E1 and its effects on chromosomes and genomes of liver cells.

Development and characterization of cholangioids from normal and diseased human cholangiocytes as an in vitro model to study primary sclerosing cholangitis

Primary sclerosing cholangitis (PSC) is an incurable, fibroinflammatory biliary disease for which there is no effective pharmacotherapy. We recently reported cholangiocyte senescence as an important phenotype in PSC while others showed that portal macrophages accumulate in PSC. Unfortunately, our ability to explore cholangiocyte senescence and macrophage accumulation has been hampered by limited in vitro models. Thus, our aim was to develop and characterize a three-dimensional (3D) model of normal and diseased bile ducts (cholangioids) starting with normal human cholangiocytes (NHC), senescent NHC (NHC-sen), and cholangiocytes from PSC patients. In 3D culture, NHCs formed spheroids of ~5000 cells with a central lumen of ~150 μm. By confocal microscopy and western blot, cholangioids retained expression of cholangiocyte proteins (cytokeratin 7/19) and markers of epithelial polarity (secretin receptor and GM130). Cholangioids are functionally active, and upon secretin stimulation, luminal size increased by ~80%. Cholangioids exposed to hydrogen peroxide exhibited cellular senescence and the senescence-associated secretory phenotype (SASP; increased IL-6, p21, SA-β-Gal, yH2A.x and p16 expression). Furthermore, cholangioids derived from NHC-sen or PSC patients were smaller and had slower growth than the controls. When co-cultured with THP-1 macrophages, the number of macrophages associated with NHC-sen or PSC cholangioids was five- to seven-fold greater compared to co-culture with non-senescent NHC. We observed that NHC-sen and PSC cholangioids release greater number of extracellular vesicles (EVs) compared to controls. Moreover, conditioned media from NHC-sen cholangioids resulted in an ~2-fold increase in macrophage migration. In summary, we developed a method to generate normal and diseased cholangioids, characterized them morphologically and functionally, showed that they can be induced to senescence and SASP, and demonstrated both EV release and macrophage attraction. This novel model mimics several features of PSC, and thus will be useful for studying the pathogenesis of PSC and potentially identifying new therapeutic targets.

Metformin requires 4E-BPs to induce apoptosis and repress translation of Mcl-1 in hepatocellular carcinoma cells

Metformin inhibits the mammalian target of rapamycin complex 1 (mTORC1) signaling pathway, which is frequently upregulated in hepatocellular carcinoma (HCC). Metformin has also been shown to induce apoptosis in this cancer. Here, we investigate whether metformin-induced apoptosis in HCC is mediated by the downstream mTORC1 effectors eukaryotic initiation factor 4E and (eIF4E)-binding proteins (4E-BPs). Further, we ask whether changes in 4E-BPs activity during metformin treatment negatively regulate translation of the anti-apoptotic myeloid cell leukemia 1 (Mcl-1) mRNA. A genetic HCC mouse model was employed to assess the ability of metformin to reduce tumor formation, induce apoptosis, and control 4E-BP1 activation and Mcl-1 protein expression. In parallel, the HCC cell line Huh7 was transduced with scrambled shRNA (control) or shRNAs targeting 4E-BP1 and 4E-BP2 (4E-BP knock-down (KD)) to measure differences in mRNA translation, apoptosis, and Mcl-1 protein expression after metformin treatment. In addition, immunohistochemical staining of eIF4E and 4E-BP1 protein levels was addressed in a HCC patient tissue microarray. We found that metformin decreased HCC tumor burden, and tumor tissues showed elevated apoptosis with reduced Mcl-1 and phosphorylated 4E-BP1 protein levels. In control but not 4E-BP KD Huh7 cells, metformin induced apoptosis and repressed Mcl-1 mRNA translation and protein levels. Immunostaining of HCC patient tumor tissues revealed a varying ratio of eIF4E/4E-BP1 expression. Our results propose that metformin induces apoptosis in mouse and cellular models of HCC through activation of 4E-BPs, thus tumors with elevated expression of 4E-BPs may display improved clinical chemopreventive benefit of metformin.

High cell surface death receptor expression determines type I versus type II signaling

Previous studies have suggested that there are two signaling pathways leading from ligation of the Fas receptor to induction of apoptosis. Type I signaling involves Fas ligand-induced recruitment of large amounts of FADD (FAS-associated death domain protein) and procaspase 8, leading to direct activation of caspase 3, whereas type II signaling involves Bid-mediated mitochondrial perturbation to amplify a more modest death receptor-initiated signal. The biochemical basis for this dichotomy has previously been unclear. Here we show that type I cells have a longer half-life for Fas message and express higher amounts of cell surface Fas, explaining the increased recruitment of FADD and subsequent signaling. Moreover, we demonstrate that cells with type II Fas signaling (Jurkat or HCT-15) can signal through a type I pathway upon forced receptor overexpression and that shRNA-mediated Fas down-regulation converts cells with type I signaling (A498) to type II signaling. Importantly, the same cells can exhibit type I signaling for Fas and type II signaling for TRAIL (TNF-α-related apoptosis-inducing ligand), indicating that the choice of signaling pathway is related to the specific receptor, not some other cellular feature. Additional experiments revealed that up-regulation of cell surface death receptor 5 levels by treatment with 7-ethyl-10-hydroxy-camptothecin converted TRAIL signaling in HCT116 cells from type II to type I. Collectively, these results suggest that the type I/type II dichotomy reflects differences in cell surface death receptor expression.