HSP90 inhibition as a means of radiosensitizing resistant, aggressive soft tissue sarcomas

• Intrinsic radioresistance of soft tissue sarcoma cell lines as measured by clonogenic survival varies substantially.
• Scores of radioresistance were extracted by principal component analysis of clonogenic survival data.
• mRNA levels of HSP90 and its clients ATR, ATM, and NBS1 reveal strong correlations with PCA-derived radioresistance scores.
• HSP90 inhibition preferentially sensitizes radioresistant sarcoma cells.
• Radiosensitization is accompanied by client protein degradation and delayed γ-H2AX foci clearance.
• HSP90 inhibitor concentrations sufficient for radiosensitization do not affect irradiation-induced apoptosis or necrosis.
• HSP90 inhibitor-mediated radiosensitization is accompanied by enforced basal and irradiation-induced senescence.
• Soft tissue sarcomas with particularly poor prognosis reveal elevated levels of HSP90, ATR, ATM, and NBS1.
• These aggressive soft tissue sarcomas might preferentially respond to HSP90 inhibition in combination with radiotherapy.

Radiotherapy is an essential part of multi-modal treatment for soft tissue sarcomas. Treatment failure is commonly attributed to radioresistance, but comprehensive analyses of radiosensitivity are not available, and suitable biomarkers or candidates for targeted radiosensitization are scarce. Here, we systematically analyzed the intrinsic radioresistance of a panel of soft tissue sarcoma cell lines, and extracted scores of radioresistance by principal component analysis (PCA). To identify molecular markers of radioresistance, transcriptomic profiling of DNA damage response regulators was performed. The expression levels of HSP90 and its clients ATR, ATM, and NBS1 revealed strong, positive correlations with the PCA-derived radioresistance scores. Their functional involvement was addressed by HSP90 inhibition, which preferentially sensitized radioresistant sarcoma cells and was accompanied by delayed γ-H2AX foci clearance and HSP90 client protein degradation. The induction of apoptosis and necrosis was not significantly enhanced, but increased levels of basal and irradiation-induced senescence upon HSP90 inhibition were detected. Finally, evaluation of our findings in the TCGA soft tissue sarcoma cohort revealed elevated expression levels of HSP90, ATR, ATM, and NBS1 in a relevant subset of cases with particularly poor prognosis, which might preferentially benefit from HSP90 inhibition in combination with radiotherapy in the future.