Chemotherapeutic Effect of Cell-cycle Modulation in Skp2-targeting Strategy for Liver Cancer

曲格列酮（Tro）G1期阻滞的肝癌细胞表达较少的MRP 2（cMOAT），表明从细胞内侧输出抗癌药物的潜力较低。Tro处理的细胞显示Bcl-xL表达增加，表明抗凋亡特性增加。当细胞进入S期时，MRP 2的表达水平恢复，有助于DNA合成期的细胞保护作用。然后，我们使用其他PPARgamma配体吡格列酮（Pio）、CDK抑制剂和Skp 2 siRNA检查这些发现是否对曲格列酮治疗具有特异性。结果表明，Pio并没有降低MRP 2的表达水平，表明与抗癌药物对肝癌细胞没有协同的细胞杀伤作用。而Tro与抗癌药物联合应用时，对肝癌细胞有较强的协同杀伤作用。CKI和Skp 2 siRNA诱导细胞死亡，显示对细胞的协同抗癌作用。在使用裸鼠的异种移植模型中，Tro与抗癌剂的组合轻微抑制肿瘤生长;然而，Pio没有。这些发现表明，PPARgamma配体对细胞周期的调节并不一定会增强常规抗癌药物的细胞毒性。PPARgamma配体的这种限制可能归因于通过表达Bcl-xL和/或MRP 2的细胞周期特异性细胞保护机制。Skp 2 siRNA与抗癌药物的联合应用可能更有前景。

The G1-arrested hepatoma cells by troglitazone (Tro) expressed less MRP2 (cMOAT), suggesting a lower potential in exporting anti-cancer agents from the inner side of the cells. The Tro-treated cells showed an increased expression of Bcl-xL, indicating an increased anti-apoptotic property. When the cells entered into the S phase of the cell cycle, the expression level of MRP2 recovered, contributing to cyto-protection in the period of DNA synthesis. Then, we examined whether or not these findings were specific for the troglitazone treatment, using the other PPARgamma ligand pioglitazone (Pio), CDK inhibitors, and Skp2 siRNA. As a result, Pio did not decrease the expression level of MRP2, showing no synergistic cyto-cidal effect with anti-cancer agents on the hepatoma cells. In contrast, Tro showed the strong synergistic cyto-cidal effect on the hepatoma cells, when combined with anti-cancer agents simultaneously. CKI and Skp2 siRNA induced cell death, showing synergistic anti-cancer effect on the cells. In xenograft model using nude mice, the combination of Tro with anti-cancer agents slightly inhibited the tumor growth ; however, Pio did not. These findings suggested the cell-cycle modulation by PPARgamma ligands did not necessarily strengthen the cytotoxicity of conventional anti-cancer agents. This limitation of the PPARgamma ligands might be attributable to the cell-cycle-specific mechanism for cell protection by expressing Bcl-xL and/or MRP2. Combination of Skp2 siRNA and anti-cancer agents may be more promising.