Purpose: Heat shock protein 90 (Hsp90) is a conserved chaperone involved in crucial signaling events in normal and malignant cells. Previous research suggests that tumor cells are particularly dependent on Hsp90 for survival as well as malignant progression. Hsp90 inhibitors which are derivates of the natural compound geldanamycin, such as the orally bioavailable 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17-DMAG), are currently being tested in clinical trials and small molecule inhibitors are in development. In this study we investigated the response of a panel of cervical carcinoma cell lines in vitro and in vivo to determine potential factors that might influence the sensitivity towards Hsp90 inhibition.
Methods: Cell viability, proliferation and drug-induced changes on Hsp90 chaperoned "client" factors were examined with focus on G2/M cell cycle regulators, and a comparison with immortalized and normal keratinocytes was performed. ME180 and CaSki cells were grown as subcutaneous xenografts in mice treated with 6-10 mg/kg 17-DMAG by oral gavage 2x/day on a chronic schedule. Tissue concentrations of 17-DMAG were measured by high performance liquid chromatography.
Results: Cell death during abnormal mitosis was observed within 48 h after treatment start. ME180 and CaSki showed more cell death at this time point than SiHa and HeLa, and higher levels of pre-treatment Akt activity. IC(50) values ranged between 17 and 37 nanoM geldanamycin (MTS). Keratinocytes were at least as sensitive as carcinoma cells. All cell lines responded with an increase of the G2/M fraction. Despite in vitro effectiveness and tissue concentrations of 1 microM, only a limited tumor growth reduction was observed with 17-DMAG given close to the maximum tolerated dose level. Lower levels of Hsp90 protein, a lower Akt activity and signs of tissue hypoxia were observed in xenografts compared to cell cultures.
Conclusions: We show here that Hsp90 inhibition effectively induces apoptosis and growth arrest in cervical carcinoma cells in vitro. Mitotic catastrophe was identified as one mechanism of cell death. In contrast, a limited efficacy of 17-DMAG was observed in subcutaneous xenograft models. Induction of a heat shock response has previously been implicated in resistance towards Hsp90 inhibition. Additional factors might be (1) an altered abundance and/or activity of primary (Hsp90) and secondary (e.g., Akt) target(s), (2) a narrow therapeutic range of 17-DMAG by oral application and (3) response-modifying factors within the tumor environment. The further development of synthetic Hsp90 inhibitors with increased therapeutic window is warranted.