PPARgamma ligands suppress the feedback loop between E2F2 and cyclin-E1.

PPARgamma is a nuclear hormone receptor that plays a key role in the induction of peroxisome proliferation. A number of studies showed that PPARgamma ligands suppress cell cycle progression; however, the mechanism remains to be determined. Here, we showed that PPARgamma ligand troglitazone inhibited G1/S transition in colon cancer cells, LS174T. Troglitazone did not affect on either expression of CDK inhibitor (p18) or Wnt signaling pathway, indicating that these pathways were not involved in the troglitazone-dependent cell cycle arrest. GeneChip and RT-PCR analyses revealed that troglitazone decreased mRNA levels of cell cycle regulatory factors E2F2 and cyclin-E1 whose expression is activated by E2F2. Down-regulation of E2F2 by troglitazone results in decrease of cyclin-E1 transcription, which could inhibit phosphorylation of Rb protein, and consequently evoke the suppression of E2F2 transcriptional activity. Thus, we propose that troglitazone suppresses the feedback loop containing E2F2, cyclin-E1, and Rb protein.

Heat shock protein 90 inhibitor 17-allylamino-17-demethoxygeldanamycin potentiates the radiation response of tumor cells grown as monolayer cultures and spheroids by inducing apoptosis.

Activation of the PI3K-Akt pathway is known to induce tumor radioresistance. In the current study, we examined the ability of 17AAG, which decreases the levels of Hsp90 client proteins including components of the PI3K-Akt pathway, to sensitize radioresistant human squamous cell carcinoma cells to X-irradiation. Human squamous cell carcinoma cell lines (SQ20B, SCC61 and SCC13) were incubated for 16 h at 37 degrees C in medium containing 17AAG. Radiation sensitivity was determined by clonogenic assays, and protein levels were examined by western blotting. Apoptosis was determined in monolayer cells by AO/EB double staining and in spheroids using the TdT-mediated dUTP nick end labeling assay. 17AAG (0.2 microM) enhanced the radiosensitivity more effectively in radioresistant SQ20B and SCC13 cells than in radiosensitive SCC61 cells. However, in all three cell lines, 17AAG increased radiation-induced apoptosis by reducing the expression of EGFR and ErbB-2 and inhibiting the phosphorylation of Akt. Furthermore, 17AAG (1 microM) sensitized SQ20B spheroids to radiation, and inhibition of Akt activation by 17AAG increased radiation-induced apoptosis in spheroids. The findings suggest that 17AAG effectively sensitizes radioresistant cells to radiation by inhibiting the PI3K-Akt pathway. Targeting the PI3K-Akt pathway with 17AAG could be a useful strategy for radiosensitization of carcinomas.

Preferential sensitization of tumor cells to radiation by heat shock protein 90 inhibitor geldanamycin.

The purpose of this study was to investigate the radiosensitizing effect of geldanamycin (GA), an inhibitor of heat shock protein 90, on tumour cells and normal cells. We tested the effect of a combination of GA and radiation on cell survival, PI3K/Akt-related proteins and apoptosis induction. GA sensitized tumour cells to radiation in preference to normal cells. In addition, a combination of radiation and GA abolished Akt activities and strongly enhanced the induction of apoptosis in tumour cells which depend on Akt protein activities for cell survival. The present data support the hypothesis that GA sensitizes tumour cells by modulating the balance among mitogenic, antiproliferative and apoptotic pathways. Targeting Hsp90 in tumour cells may lead to the development of new radiosensitizing strategies in radiotherapy.