Heterogeneous expression of Ku70 in human tissues is associated with morphological and functional alterations of the nucleus.

Ku70 is a subunit of DNA-protein kinase complex and involved in diverse intranuclear events including the repair of double-stranded DNA breaks. Ku70 is rich in the interphase nucleus of cultured cells. In human tissues, however, the distribution of Ku70 has not yet been systematically examined. To characterize the difference of Ku70 distribution between cells of human tissues and cultured cells, the expression of Ku70 was examined in various normal and neoplastic human tissues by immunohistochemistry and immunoblot. In addition, the role of Ku70 in the cellular response against ionizing radiation (IR) was analysed in fibroblasts after exposure to 5 Gy IR and apoptotic indices were examined in Ku70-overexpressed fibroblasts from an ataxia telangiectasia patient and in normal fibroblasts, before and after irradiation. In contrast to cultured cells, Ku70 was not detected in some interphase cells of human tissues and was distributed heterogeneously, even in the same nucleus. Ku70 expression was strikingly low in terminally differentiated cells such as neutrophils, eosinophils, glomerular capillary endothelial cells and fibroblasts, and was absent in spermatids. In spermatocytes, Ku70 was tightly integrated with chromosome filaments, unlike other somatic cells under mitosis. After exposure to IR, Ku70 expression was not increased in ataxia telangiectasia fibroblasts, but was significantly increased in normal fibroblasts. Most of the increased Ku70 was of soluble nuclear protein fraction. Furthermore, overexpression of Ku70 increased radiation resistance both in ataxia telangiectasia fibroblasts and normal fibroblasts. The presented data indicate that the distribution of Ku70 in cells of human tissues is closely associated with the cell cycle, cellular differentiation, nuclear shape and the process of repair of DNA damage caused by IR.