Tumor cells with low proteasome subunit expression predict overall survival in head and neck cancer patients

Background: Experimental and clinical data suggest that solid cancers contain treatment-resistant cancer stem cells that will impair treatment efficacy. The objective of this study was to investigate if head and neck squamous cell carcinoma (HNSCC) also contain cancer stem cells that can be identified by low 26S proteasome activity and if their presence correlates to clinical outcome. Methods: Human HNSCC cells, engineered to report lack of proteasome activity based on accumulation of a fluorescent fusion protein, were separated based on high (ZsGreen-cODC(neg)) or low (ZsGreen-cODC(pos)) proteasome activity. Self-renewal capacity, tumorigenicity and radioresistance were assessed. Proteasome subunit expression was analyzed in tissue microarrays and correlated to survival and locoregional cancer control of 174 patients with HNSCC. Results: HNSCC cells with low proteasome activity showed a significantly higher self-renewal capacity and increased tumorigenicity. Irradiation enriched for ZsGreen-cODC(pos) cells. The survival probability of 82 patients treated with definitive radio-or chemo-radiotherapy exhibiting weak, intermediate, or strong proteasome subunit expression were 21.2, 28.8 and 43.8 months (p = 0.05), respectively. Locoregional cancer control was comparably affected. Conclusions: Subpopulations of HNSCC display stem cell features that affect patients' tumor control and survival. Evaluating cancer tissue for expression of the proteasome subunit PSMD1 may help identify patients at risk for relapse.