[Hepatitis B virus X protein-regulated expression of Plk1]

OBJECTIVE

To investigate the ability and underlying mechanism of hepatitis B virus X protein (HBx) regulation of Polo-like kinase 1 (Plk1) expression.

METHODS

The human HCC cell line HepG2 was transfected (transiently and stably) with an HBx plasmid expression vector (pCMV-HA-HBx) or empty plasmid vector (control), with and without expression plasmids with the Plk1 promoter. Effects on Plk1 expression were assessed by western blotting. Functional effects on the Plk1 promoter were assessed by luciferase reporter assay. Effects on the mRNA level of Plk1 in S phase HepG2 cells were assessed by quantitative real-time reverse transcriptase polymerase chain reaction. After blocking protein synthesis by treatment with cycloheximide (CHX), the turnover rate of Plk1 was assessed by western blotting. Lastly, the effect of HBx on cell cycle was assessed by flow cytometry.

RESULTS

HBx did not increase the protein expression of Plk1 in non-synchronized HepG2 cells, but did significantly up-regulate the Plk1 protein level in the synchronized S phase cells (P = 0.026 and P = 0.003, respectively). Ectopic expression of HBx did not increase the mRNA level of Plk1 in HepG2 cells, but did inhibit the degradation of Plk1, as evidenced by an increased half-life of Plk1 protein (from 30 to 90 minutes). The HBx-expressing HepG2 cells showed more frequent entry into the S or G(2)/M phase than the control cells (31.65% vs. 24.56% or 9.43% vs. 4.47%, respectively) and less in the G(0)/G(1) phase (decrease from 70.97% to 58.92% for the HBx-expressing HepG2 cells).

CONCLUSION

HBx is able to up-regulate the expression of Plk1 in HepG2 cells by a mechanism involving stabilization of the Plk1 protein primarily in the S phase of the cell cycl.

Genetic resistance to chemical carcinogen-induced preneoplastic hepatic lesions in DRH strain rats

DRH strain rats were established by inbreeding a closed colony of Donryu rats continuously fed the chemical hepatocarcinogen 3'-methyl-4-dimethylaminoazobenzene for over 10 years. They are highly resistant to chemical induction of liver cancer and preneoplastic lesions. We studied the genetic basis of DRH resistance to preneoplastic lesions by analyzing 108 (F344 x DRH)F2 male rats fed 3'-methyl-4-dimethylaminoazobenzene for 7 weeks. Five parameters of preneoplastic liver lesions were selected for quantitative analysis: (a) number of glutathione S-transferase placental form-positive foci per unit area of liver section; (b) percentage area occupied by the foci; (c) average size of foci; (d) glutathione S-transferase placental form mRNA level; and (e) gamma-glutamyltranspeptidase mRNA level. Furthermore, O6-methylguanine DNA methyltransferase and mannose 6-phosphatase/insulin-like growth factor 2 receptor mRNA levels were quantified. Composite interval mapping analysis showed that there were two remarkably significant clusters of quantitative trait loci affecting preneoplastic liver lesions on chromosomes 1 and 4. These clusters were designated collectively as Drh1 and Drh2, respectively. The functions of the recessive DRH allele of Drh1 and the semidominant DRH allele of Drh2 were to suppress the phenotypes of precancerous lesions. Each cluster showed two to three subpeaks in linkage likelihood plots, suggesting the presence of several closely linked quantitative trait loci affecting preneoplastic lesions. Possible candidate genes at each locus will be discussed. Expression of O6-methylguanine DNA methyltransferase and mannose 6-phosphatase/insulin-like growth factor 2 receptor did not affect DRH resistance to hepatocarcinogenesis, although they were polymorphic between DRH and F344 rats.

Human cord blood-derived primitive progenitors are enriched in CD34+c-kit- cells: correlation between long-term culture-initiating cells and telomerase expression

We studied the functional characteristics of subpopulations of cord blood-derived CD34+ cells expressing different levels of CD38 and c-kit antigens, using clonal cell culture and long-term culture with allogeneic bone marrow stromal cells or the MS-5 murine stromal cell line to assay long-term culture-initiating cells (LTC-IC) in each subpopulation. To investigate the capacity for replication, proliferation, and differentiation of each subpopulation of CD34+ cells, we also studied the correlation between LTC-IC and telomerase activity. After 5 weeks of coculture, LTC-IC accounted for one out of 32 CD34+CD38- cells and one out of 33 CD34+c-kit- cells. In contrast, the frequency of LTC-IC was low in their antigen-positive counterparts (one per 84 CD34+CD38+ cells, one per 90 CD34+c-kit(low) cells, and very low among CD34+c-kit(high) cells). It was noteworthy that some LTC-IC derived from CD34+CD38- as well as CD34+c-kit- cells generated colony-forming cells (CFCs) after up to 9 weeks of coculture. Telomerase activity was consistently low in CD34+CD38- and CD34+c-kit- cells compared to CD38+ or c-kit(high or low) cells, suggesting that CD34+CD38- or c-kit- cells are likely to be more quiescent. These results suggest that the CD34+CD38- and CD34+c-kit- cell populations are primitive stem/progenitor cells, and that the telomerase activity of these cells correlates with their proliferative capacity as well as their stage of differentiation.

Transgenic mouse models of human gastric and hepatic carcinomas

The development of human cancer is a complex process which has been difficult to define in vivo. The use of animal models of human cancer may prove useful in elucidating the mechanisms associated with malignant transformation. Transgenic mice with either the adenovirus 12 (Ad12) E1a/E1b genes or the human hepatitis B virus (HBV) HBx gene were developed. Expression of these viral genes resulted in the development of malignant tumors in restricted tissues; in the case of the HBx transgenic mice, hepatocellular carcinomas and in the E1a/E1b transgenic mice, gastric carcinomas. With the E1a/E1b transgenic mice, tumors were found to arise near the junction between the squamous and columnar epithelia, as found in several human cancers, including cervical and esophageal carcinomas, and thus appear to be an ideal animal model for determining why the squamocolumnar junction is such a hot spot for the development of human tumors of epithelial derivation. The HBx transgenic mice showed progressive changes in the liver, beginning with preneoplastic lesions, through benign adenomas, and finally to malignant carcinomas. These mice appear particularly suited for defining epigenetic rather than genetic events underlying the progression of human cancers. These transgenic models address two fundamental observations which are becoming increasingly important for our understanding of the mechanism of carcinogenesis.