Enhanced Tumor Cell Radiosensitivity and Abrogation of G2 and S Phase Arrest by the Hsp90 Inhibitor 17-(Dimethylaminoethylamino)-17-demethoxygeldanamycin

Abstract

Purpose: Because of the potential for affecting multiple signaling pathways, inhibition of Hsp90 may provide a strategy for enhancing tumor cell radiosensitivity. Therefore, we have investigated the effects of the orally bioavailable Hsp90 inhibitor 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17-DMAG) on the radiosensitivity of human tumor cells in vitro and grown as tumor xenografts.Experimental Design: The effect of 17-DMAG on the levels of three proteins (Raf-1, ErbB2, and Akt) previously implicated in the regulation of radiosensitivity was determined in three human solid tumor cell lines. A clonogenic assay was then used to evaluate cell survival after exposure to 17-DMAG followed by irradiation. For mechanistic insight, the G 2 -and S-phase checkpoints were evaluated in 17-DMAG-treated cells. Finally, the effect of in vivo administration of 17-DMAG in combination with radiation on the growth rate of xenograft tumors was determined.Results: 17-DMAG exposure reduced the levels of the three radiosensitivity-associated proteins in a cell line-specific manner with ErbB2 being the most susceptible. Corresponding concentrations of 17-DMAG enhanced the radiosensitivity of each of the tumor cell lines. This sensitization seemed to be the result of a 17-DMAG-mediated abrogation of the G 2 -and S-phase cell cycle checkpoints. The oral administration of 17-DMAG to mice bearing tumor xenografts followed by irradiation resulted in a greater than additive increase in tumor growth delay.Conclusions: These data indicate that 17-DMAG enhances the in vitro and in vivo radiosensitivity of human tumor cells. The mechanism responsible seems to involve the abrogation of radiation-induced G 2 -and S-phase arrest.

ErbB3 Expression Predicts Tumor Cell Radiosensitization Induced by Hsp90 Inhibition

“…A number of different treatment protocols using Hsp90 inhibitors, including 17DMAG, have been reported to enhance the radiosensitivity of human tumor cell lines (8)(9)(10)(11)(12). The radioresponse of three cell lines (DU145 prostate carcinoma, PSN1 pancreatic carcinoma, and MiaPaCa pancreatic carcinoma) exposed to 50 nmol/L 17DMAG for 16 hours prior to irradiation are shown in Fig.…”

“…17DMAG, in contrast to 17AAG, is water-soluble, orally bioavailable, and does not seem to undergo extensive metabolism to toxic species (20,21). In these studies, 17DMAG was found to enhance the in vitro radiosensitivity of three human cell lines originating from three different tumor histologies and to result in a greater than additive increase in radiation-induced tumor growth delay in a human tumor xenograft model (8). Whereas a number of radioresponse-associated proteins decreased after 17DMAG exposure, the radiosensitization appeared to correlate best with a decrease in the levels of ErbB2 (8).…”

“…As a potential molecular target for tumor cell radiosensitization, a number of groups, including our own, have investigated the chaperone protein Hsp90 (8)(9)(10)(11)(12). In contrast to other molecular chaperones, Hsp90 has a relatively high selectivity for signaling proteins (13).…”

“…In these studies, 17DMAG was found to enhance the in vitro radiosensitivity of three human cell lines originating from three different tumor histologies and to result in a greater than additive increase in radiation-induced tumor growth delay in a human tumor xenograft model (8). Whereas a number of radioresponse-associated proteins decreased after 17DMAG exposure, the radiosensitization appeared to correlate best with a decrease in the levels of ErbB2 (8). To further investigate the general relationship between Hsp90 inhibition and radiosensitization, we used 17 DMAG as a representative of clinically applicable Hsp90 inhibitors and have evaluated its effects on the radiosensitivity of a number of additional human tumor cell lines.…”

Hsp90-inhibitor geldanamycin abrogates G2 arrest in p53-negative leukemia cell lines through the depletion of Chk1

“…Furthermore, several agents inactivating Chk1 are reported to abrogate G 2 arrest and simultaneously sensitize tumor cells to DNAdamaging agents (Kohn et al, 2003;Kawabe, 2004). Recently, Chk1 has been identified as an Hsp90 client and 17-AAG-mediated depletion of Chk1 has been shown to sensitize tumor cells to replication stress induced by nucleoside analog gemcitabine (Arlander et al, 2003;Bull et al, 2004).…”