Hyun Jin Kwun

p53-dependent transcriptional repression of p21 waf1 by hepatitis C virus NS3

Hepatitis C virus (HCV) NS3 protein is known to affect normal cellular functions, such as cell proliferation and cell death, and to be involved, either directly or indirectly, in HCV hepatocarcinogenesis. In this study, we demonstrated that NS3 protein could specifically repress the promoter activity of p21 in a dose-dependent manner. The effect was not cell type-specific and was synergistic when combined with HCV core protein. Repression of the p21 promoter by NS3 was almost completely lost when p53 binding sites present on the p21 promoter were removed. Furthermore, p53 binding sites were sufficient to confer a strong NS3 responsiveness to an heterologous promoter, suggesting that NS3 represses the transcription of p21 by modulating the activity of p53. Although the NS3 protein domain required for the majority of p21 repression was located on the protease domain, the proteinase activity itself does not seem to be necessary for repression. Both transcription and protein stability of p53 were unaffected by NS3, suggesting that NS3 might repress transcription of p21 by inhibiting the regulatory activity of p53 via protein-protein interaction(s). Finally, the growth rate of NS3-expressing cell lines was at least twice as fast as that of the parent NIH 3T3 cells, indicating that the repression of p21 is actually reflected by the stimulation of cell growth.

Transcriptional repression of p21waf1 promoter by hepatitis B virus X protein via a p53-independent pathway

The X-gene product of hepatitis B virus (HBx) has been implicated in hepatitis B virus (HBV)-mediated hepatocellular carcinoma through its ability to induce liver cancer in some transgenic mice and to transactivate a variety of viral and cellular promoters. In this study, we demonstrated that the level of p21(waf1) RNA was decreased in the HBx-expressing cells and this effect was due to the transcriptional repression of the p21(waf1) gene by HBx via a p53-independent pathway. As the Sp1 binding sites of the p21(waf1) promoter were sufficient to confer HBx responsiveness to a previously non-responsive promoter, we suggested that HBx represses the transcription of p21(waf1) by downregulating the activity of Sp1. Because the tumor repressor p21(waf1) protein is a universal inhibitor of cyclin-CDK complexes and DNA replication that induces cell cycle arrest at the G1-S checkpoint, the repression of p21(waf1) by HBx might play an important role in a HBV-mediated pathogenesis.

Dual effects of hepatitis B virus X protein on the regulation of cell-cycle control depending on the status of cellular p53.

Despite the extensive studies on the roles of hepatitis B virus (HBV) X protein (HBx) in the development of hepatocellular carcinomas (HCCs), the mechanisms by which HBx contributes to HCC remain controversial. In this study, the effect of HBx on the G(1)-S checkpoint control depending on the status of p53 was compared. Transcription of p21(waf1/cip1) was activated by HBx in the presence of functional p53 in a dose-dependent manner. However, it was repressed by HBx when p53 was absent or present at a low level. Furthermore, the growth rate of the HBx-expressing NIH3T3 cell lines compared with that of the parental cells was decreased when p53 was upregulated by a DNA-damaging agent, cisplatin, whereas it increased approximately twofold when p53 was present at a very low level. Thus, the opposite effects of HBx on the regulation of the cell cycle depending on the status of p53 might be important to understand the progression of hepatic diseases in HBV-positive patients.

Hepatitis C virus core protein represses the p21 promoter through inhibition of a TGF-β pathway

The increased proliferation rate of hepatocytes is one of the major risk factors for the development of hepatocellular carcinoma. In this study, we investigated the mechanism by which hepatitis C virus (HCV) core protein represses transcription of the universal cyclin-dependent kinase inhibitor p21 gene in murine fibroblast NIH 3T3 cells. From the transient reporter assays of p21 promoter, we found that the TGF-β-responsive element (TβRE) located between −83 and −74 of the p21 promoter is responsible for the effect. The TGF-β-induced p21 promoter activity was specifically decreased by HCV core protein and in the presence of the inhibitory Smad7 the repression effect was almost completely abolished. Furthermore, HCV core protein stimulated the growth rate of NIH 3T3 cells and could overcome growth arrest by TGF-β but not by butyrate, suggesting that HCV core protein stimulates cell cycle progression by repressing p21 transcription through a TGF-β pathway.

Transactivation of the human endogenous retrovirus K long terminal repeat by herpes simplex virus type 1 immediate early protein 0

We found that LTR-directed transcription of the human endogenous retrovirus K can be induced by HSV-1 infection. The effect was mediated by the action of a HSV-1 immediate early protein, ICP0 and required the AP-1 binding site present on the HERV-K LTR. In addition, ICP0 could up-regulate AP-1 activity, suggesting that ICP0 increases transcription of HERV-K through AP-1 site. This effect might be important to understand both HERV-K- and HSV-1-mediated pathogenesis because HERV-K LTR represents an important class of retrotranspositional mutagens and also could provide a new regulatory element for the linked DNA sequences.

Transcriptional repression of cyclin-dependent kinase inhibitor p21 gene by phospholipase D1 and D2

Phospholipase D (PLD) is known to stimulate cell cycle progression and to transform murine fibroblast cells into tumorigenic forms, although the precise mechanisms are not elucidated. In this report, we demonstrated that both PLD1 and PLD2 repressed expression of cyclin‐dependent kinase inhibitor p21 gene in an additive manner. The phospholipase activity of PLDs was important for the effect. PLD1 repressed the p21 promoter by decreasing the level of p53, whereas PLD2 via a p53‐independent pathway through modulating Sp1 activity. Taken together, we suggest that PLD isozymes stimulate cell growth by repressing expression of p21 gene, which may ultimately lead to carcinogenesis.

Sodium Butyrate Alters Cell-Cell Interactions through Up-Regulation of E-Cadherin in Human Hepatocellular Carcinoma Cells

Sodium butyrate (NaBt), a naturally occurring short chain fatty acid derived from carbohydrate metabolism in the gut, is known to exhibit strong anti-cancer potentials in various human cancer cells; however, its action mechanism is poorly understood. In the present study, we demonstrated that NaBt up-regulates levels of E-cadherin, a key cell adhesion molecule implicated as a tumor suppressor, in a cell type-specific manner. Although levels of p21, a potential activator for E-cadherin expression, were also up-regulated by treatment with NaBt in several types of cells, it does not seem to be associated with the activation of E-cadherin in the NaBt-treated cells. Instead, the data from promoter analysis suggest that NaBt up-regulates expression of E-cadherin at the transcription level by enhancing its promoter strength via a CCAAT-box. The elevated E-cadherin in the presence of NaBt was primarily localized at the cell-cell contacts, converting Hep3B cells into a more differentiated form.

Cooperative stimulation of cisplatin-mediated apoptosis by hepatitis B virus X Protein and hepatitis C virus core Protein

The co-infection with hepatitis B virus (HBV) and hepatitis C Virus (HCV) is associated with a more severe liver disease and increased frequency in the development of hepatocellular carcinoma compared to those with single infection. Here, we demonstrated that HBV X protein (HBx) and HCV Core cooperatively up-regulated the level of p53 in human hepatoma HepG2 cells. The elevated p53 subsequently stimulated the expression of proapoptotic Bax whereas it repressed the expression of anti-apoptotic Bcl2. These effects, however, were not observed in p53-negative Hep3B cells. Consistently to their cooperative regulation of apoptotic effectors, HBx and HCV Core additively stimulated cisplatin-mediated apoptotic cell death of HepG2 but not of Hep3B cells. These results may help to explain the development of a more severe liver disease in patients co-infected with HBV and HCV as well as some contradictory results on the roles of HBx and Core in apoptosis.