Enhanced expression of calreticulin in the nucleus of radioresistant squamous carcinoma cells in response to ionizing radiation.

Ionizing radiation has been shown to modulate gene and protein expression as well as cellular signal transduction pathways. However, the biochemical and molecular mechanisms that underlie the cellular response to radiation are not fully understood. The effects of ionizing radiation on the expression of nuclear proteins have now been investigated in radiorestistant human head and neck squamous carcinoma cells (SQ-20B cells) using the techniques of two-dimensional PAGE, silver staining, and computer-assisted quantitative analysis. Radiation (600 cGy) induced the expression of 10 proteins and suppressed the expression of 5 proteins in the nuclei of SQ-20B cells as detected 4 h after treatment. Electroelution and NH2-terminal amino acid sequence analysis revealed that one of the radiation-induced proteins was the Ca(2+)-binding protein calreticulin. The expression of calreticulin was increased approximately 6-fold in the nuclei of irradiated SQ-20B cells. Calreticulin and the other proteins whose expression was affected by radiation may contribute to the radioresistance phenotype of SQ-20B cells.