Persistent infection with hepatitis C virus (HCV) induces tumorigenicity in hepatocytes.To gain insight into the mechanisms underlying this process, we generated monoclonal antibodies on a genome-wide scale against an HCV-expressing human hepatoblastoma-derived cell line, RzM6-LC, showing augmented tumorigenicity. We identified 3␤-hydroxysterol ⌬24-reductase (DHCR24) from this screen and showed that its expression reflected tumorigenicity. HCV induced the DHCR24 overexpression in human hepatocytes. Ectopic or HCV-induced DHCR24 overexpression resulted in resistance to oxidative stress-induced apoptosis and suppressed p53 activity. DHCR24 overexpression in these cells paralleled the increased interaction between p53 and MDM2 (also known as HDM2), a p53-specific E3 ubiquitin ligase, in the cytoplasm. Persistent DHCR24 overexpression did not alter the phosphorylation status of p53 but resulted in decreased acetylation of p53 at lysine residues 373 and 382 in the nucleus after treatment with hydrogen peroxide. Taken together, these results suggest that DHCR24 is elevated in response to HCV infection and inhibits the p53 stress response by stimulating the accumulation of the MDM2-p53 complex in the cytoplasm and by inhibiting the acetylation of p53 in the nucleus. Hepatitis C virus (HCV)5 is composed of a single-stranded RNA genome of positive polarity (1). Translation of viral proteins is initiated from an internal ribosome entry site (2) and results in a single polypeptide that is subsequently cleaved by host and viral proteases to yield viable proteins (3). The HCV genome does not rely on canonical translation factors and can readily establish chronic infection without integrating into the host genome, resulting in hepatic steatosis and hepatocellular carcinoma (HCC) (4). More than 170 million people worldwide are infected with HCV (5); chronic HCV infection and aging are the major risk factors for HCC (6 -8). Liver cancer is the fifth most common cause of cancer mortality worldwide (9). The frequent inactivation of p53 in human HCC suggests that the loss of p53-dependent apoptosis may promote hepatocarcinogenesis (10). Chronic HCV infection results in chronic liver inflammation and induces endoplasmic reticulum stress and oxidative stress, which are thought to induce hepatocarcinogenesis (11, 12). The mechanistic details underlying HCC development are not fully understood. To gain insight into the molecular mechanisms underlying HCV-induced pathogenesis, we previously established RzM6 cells (13), a human hepatoblastoma (HepG2)-derived cell line in which expression of the full-length HCV genome is controlled by a Cre/loxP system. Expression of the HCV genome promoted anchorage-independent growth of RzM6 cells after 44 days of culture from the onset of HCV expression (RzM6-44d cells) but not in RzM6 cells after 0 days (RzM6-0d cells) (13). In the present study, we generated monoclonal antibodies against RzM6 cells cultured for longer than 44 days (RzM6-LC cells) and then screened the antibodies for their ability to...

Abstract. The fine structure and ATPase activity of the mitotic spindle in neoblasts of planaria were examined. In neoblasts, the cells have a large nucleus and nucleolus. Mitochondria are aggregated around the nucleus with chromatoid bodies adjacent. The cytoplasm contains little endoplasmic reticulum (ER) and few Golgi bodies but many free ribosomes, forming polysomes, can be seen throughout the cytoplasmic and spindle ground areas. In addition, centriolar bodies, atypical centrioles, can also be recognized in the cytoplasm. Ceils in the G 2 stage contain a pair of electron-dense bodies, both consisting of fibrogranules but differing from each other in fine structure and, in the mitotic stage, only one fibrogranular body can be recognized at each pole. ATPase activity was detected in the centriolar bodies in the G 2 and mitotic stages and in the ground area of the cytoplasm and spindle apparatus filled by free ribosomes. The activity associated with the microtubules differed with the developmental stage.