Growth arrest by troglitazone is mediated by p27Kip1 accumulation, which results from dual inhibition of proteasome activity and Skp2 expression in human hepatocellular carcinoma cells



In our study, we examined whether human hepatocellular carcinoma (HCC) expresses peroxisome proliferator-activated receptor γ (PPARγ) and the effects of PPAR γ activation by its selective ligands on cell growth and cell invasion in HCC cells. RT-PCR and Western blot analysis revealed that HCC-derived cell lines, HepG2 and HLF, express PPARγ mRNA and protein. Luciferase assay in HLF cells showed that troglitazone, a selective ligand for PPAR γ, transactivated the transcription of a peroxisome proliferator response element-driven promoter in a dose-dependent manner, suggesting that the expressed PPARγ functions as a transcriptional factor. Not only troglitazone but pioglitazone dose-dependently inhibited cell growth in HepG2 and HLF cells. Invasion assay using a transwell chamber demonstrated that troglitazone also inhibited cell invasion in HCC cells. To examine the mechanism of the troglitazone-induced growth inhibition, we determined p27Kip1, a cyclin dependent kinase inhibitor, expression by Western blot analysis in troglitazone-treated HLF cells. Troglitazone increased p27Kip1 in time- and dose-dependent manners, suggesting that p27Kip1 may be involved in the growth inhibition by troglitazone in HLF cells. To further examine the mechanism of the troglitazone-induced p27Kip1 protein accumulation, 2 major systems for regulation of p27Kip1 protein, proteasome activity and Skp2, an F-box protein that targets p27Kip1 for degradation, were evaluated. Troglitazone potently inhibited proteasome activity and decreased Skp2 protein levels. All these results suggest that human HCC cells express functional PPAR γ and PPARγ activation resulted in growth inhibition. The growth inhibition was mediated by p27Kip1 accumulation, which is induced by both inhibition of ubiquitylation of p27Kip1 and reduction of degradation activity of p27Kip1 by proteasome. © 2003 Wiley-Liss, Inc.