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

曲格列酮(Tro)阻断G1期的肝癌细胞MRP2(CMOAT)表达较低，提示从细胞内侧输出抗癌药物的可能性较低。TRO处理的细胞表现出Bclxl的表达增加，表明其抗细胞凋亡的能力增强。当细胞进入细胞周期的S时，MRP2的表达水平恢复，有助于DNA合成期的细胞保护。然后，我们使用另一种PPARγ配体吡格列酮(Pio)、CDK抑制剂和Skp2 siRNA检查了这些发现是否为曲格列酮治疗所特有。结果表明，Pio不降低MRP2的表达水平，与抗癌药物对肝癌细胞无协同杀伤作用。与抗癌药物联合应用时，tro对肝癌细胞有较强的协同杀伤作用。CKI和Skp2 siRNA诱导细胞死亡，对细胞具有协同抗癌作用。在裸鼠移植瘤模型中，tro与抗癌药物联用对肿瘤生长有轻微抑制作用，而pio则无此作用。这些发现表明，PPARγ配体对细胞周期的调节并不一定会增强传统抗癌药的细胞毒性。PPARGamma配体的这种局限性可能归因于通过表达BclxL和/或MRP2来保护细胞的细胞周期特异性机制。Skp2 siRNA与抗癌药物的结合可能更有前景。

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.