Geldanamycin, an inhibitor of Hsp90, sensitizes human tumour cells to radiation

Purpose: The effects of the heat shock protein 90 (Hsp90) inhibitor geldanamycin (GA) were examined on the radiosensitivity and signal transduction pathways in human tumour cell lines. Materials and methods: Two human cell lines, SQ-5 and DLD-1, derived from lung carcinoma and colon adenocarcinoma, respectively, were incubated for 16h at 37degreesC in medium containing 0.2 muM GA. The cells were then irradiated with X-rays and incubated with GA for a further 8 h. Radiation sensitivity was determined by clonogenic assays and protein levels were examined by Western blotting. Results: GA radiosensitized both cell lines, but potentiated X-ray sensitivity more in SQ-5 than in DLD-1 cells. It was found that GA depleted EGFR and ErbB-2 in DLD-1 cells and depleted only ErbB-2 in SQ-5 cells. GA also reduced the expression of Akt and phosphorylated Akt (pAkt) expression in SQ-5 cells. In addition, the ratio(%) of apoptotic cells and poly [ADP-ribose] polymerase cleavage increased in SQ-5 but not in DLD-1 cells after exposure to GA and X-ray irradiation. The findings suggest that GA enhances the radiation sensitivity of human tumour cells by inhibiting the EGFR signal transduction system and the Akt signalling pathway. Conclusion: Targeting Hsp90 with GA provides a promising experimental strategy for radiosensitization of carcinoma.