Esophageal cancer tissues and adjacent normal mucosae in 13 patients with primary esophageal cancer were examined for quantitative differences in DNA-dependent protein kinase (DNA-PK) activity and for expressions of Ku70, Ku80 and DNA-PKcs proteins by Western blotting and immunohistochemistry. The tumor tissues showed higher DNA-PK activity than the normal mucosae. Protein levels of Ku70, Ku80 and DNA-PKcs correlated with DNA-PK activities in the tumor tissues. Immunohistochemical analysis

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... led that Ku70, Ku80 and DNA-PKcs located predominantly in the nuclei in both the tumor tissues and normal mucosae. In the normal epithelium, Ku70, Ku80 and DNA-PKcs were expressed only in the nuclei of the basal cell layers and not in those of the lumenal cell layers. In the tumor tissues, the expressions of DNA-PK proteins showed intratumoral heterogeneity. The different portions in the same tumor showed different expression levels of DNA-PK proteins, and even each tumor cell showed different expression levels. These results suggest that cell differentiation and tumor progression affect cellular DNA-PK protein levels and its activity. Furthermore, the intratumoral heterogeneity of DNA-PK protein expression in esophageal cancer cells/ tissues also suggests the difficulty in prediction of radio-or chemo-sensitivity of the tumor through estimation of DNA-PK activity/protein levels in tumor specimens.