Radiation therapy is commonly used to treat head and neck squamous cell carcinoma (HNSCC); however, recurrence results from the development of radioresistant cancer cells. Therefore, it is necessary to identify the underlying mechanisms of radioresistance in HNSCC. Previously, we showed that the inhibition of karyopherin-β1 (KPNB1), a factor in the nuclear transport system, enhances radiation-induced cytotoxicity, specifically in HNSCC cells, and decreases the localization of SCC-specific transcription factor ΔNp63. This suggests that ΔNp63 may be a KPNB1-carrying nucleoprotein that regulates radioresistance in HNSCC. Here, we determined whether ΔNp63 is involved in the radioresistance of HNSCC cells. Cell survival was measured by a colony formation assay. Apoptosis was assessed by annexin V staining and cleaved caspase-3 expression. The results indicate that ΔNp63 knockdown decreased the survival of irradiated HNSCC cells, increased radiation-induced annexin V+ cells, and cleaved caspase-3 expression. These results show that ΔNp63 is involved in the radioresistance of HNSCC cells. We further investigated which specific karyopherin-α (KPNA) molecules, partners of KPNB1 for nuclear transport, are involved in nuclear ΔNp63 expression. The analysis of nuclear ΔNp63 protein expression suggests that KPNA1 is involved in nuclear ΔNp63 expression. Taken together, our results suggest that ΔNp63 is a KPNB1-carrying nucleoprotein that regulates radioresistance in HNSCC.
Keywords: head and neck squamous cell carcinoma; ionizing radiation; karyopherin-α; karyopherin-β1; radiation-induced apoptosis; radioresistance; ΔNp63.