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.