2B) and 48 hours (Fig. 3D) after induction of HBx expression. Furthermore, 4pX cells displayed a significant increase in HBx-dependent S phase entry 24 hours (Supporting
Fig. 2B)17 but not 48 hours (Fig. 3D) after induction of HBx expression. Additionally, transient transfection of Chang liver cells with the HBV wild-type and HBx-defective replicons did not induce changes in the cell cycle profile (Fig. 3C). Given that HBx promoted PTTG1 accumulation without significantly affecting cell cycle (p34X and HBV complete replicon-transfected Chang liver cells), these results indicated that the HBx-promoted PTTG1 accumulation was not dependent on cell cycle modifications. It is known that HBx transcriptionally induces the expression of viral and cellular genes by activating promoter regulatory sequences.2 To determine Daporinad cell line whether HBx modulates PTTG1 transcription, its messenger RNA (mRNA) levels were measured by means of quantitative RT-PCR
in p34x and 4pX cells. PTTG1 mRNA levels were unaffected by HBx expression in both p34X (Fig. 4A) and 4px (Supporting Fig. 3) cells. As expected,25 RT-PCR analysis revealed increased TNF-α mRNA levels upon induction of HBx (Fig. Protein Tyrosine Kinase inhibitor 4A). Additionally, we transiently transfected Hela cells with both pPTTG1–cyan fluorescent protein (CFP), an expression vector in which PTTG1-CFP transcription is controlled by the CMV promoter, and pHBx-hemagglutinin
(HA) plasmids. Western blot analysis using an anti–green fluorescent protein (GFP) Ab revealed that PTTG1-CFP was clearly accumulated in HBx-transfected cells (Fig. 4B). Interestingly, the effect of HBx was not observed when cells were cotransfected with the control plasmid pECFP-N1, coding only for the CFP protein. These results were further confirmed by cotransfecting Hela cells with wild-type or HBx-defective HBV replicons along with the pPTTG1-CFP vector (Fig. 4C). These results strongly suggested that PTTG1 accumulation induced by HBx was not mediated by transcriptional activation. We next examined whether HBx-induced PTTG1 up-regulation could be explained through changes on protein stability by analyzing click here PTTG1 levels after blocking protein synthesis with cycloheximide. Western blot analysis revealed that PTTG1 protein half-life increased in p34X cells after induction of HBx expression when compared with noninduced cells (Fig. 4D,E). Taken together, these results indicated that HBx promoted PTTG1 accumulation by modulating its degradation. Phosphorylation of PTTG1 leads to its ubiquitination and proteasomal degradation.10 Thus, we analyzed the levels of phosphorylated forms of PTTG1 in p34X cells treated with okadaic acid (OA), a protein phosphatase 2A (PP2A) inhibitor, and/or MG132, a proteasome inhibitor.