Tumor specific modulation of KU70/80 DNA binding activity in breast and bladder human tumor biopsies

The Ku70/80 heterodimer is the regulatory subunit of the DNA-dependent prot ein kinase (DNA-PK) and its DNA-binding activity mediates DNA double-strand breaks repair. Although Ku80 was recently proposed as a caretaker gene inv olved in the control of genome integrity, no data are available on Ku70/80 DNA-binding activity in human tumors. Heterodimer DNA-binding activity and protein expression were assayed by electrophoretic-mobility-shift-assay (EM SA) and Western blot analysis, in nuclear and cytoplasmic extracts from eig ht breast, seven bladder primary tumors and three metastatic nodes from bre ast cancers. Corresponding normal tissues of the same patients were used as controls. Ten out of 15 tumors showed nuclear Ku-binding activity 3-10 tim es higher than in the normal tissues, irrespective of bladder or breast ori gin. Conversely, in 5/15 primary tumors and in all the metastatic nodes ana lysed, nuclear Ku-activity was 1.5-4.5-fold lower than in the corresponding normal tissues. Cytoplasmic heterodimer activity significantly differed be tween tumor and normal tissues, displaying a 2-10-fold increase in neoplast ic tissues. Three different patterns combining both Ku expression and activ ity with tumor characteristics were identified. In low aggressive breast tu mors p70/p80 proteins were expressed in tumor but not in normal tissues. Th e heterodimer binding-activity matched the protein levels. In non-invasive bladder carcinomas no significant differences in protein expression between tumor and the corresponding normal tissues were found, however heterodimer binding-activity was increased in tumor samples. In breast and bladder tum ors, at the advanced stage and in node metastases, the binding activity was strongly reduced in tumor biopsies, however no differences mere demonstrat ed between normal and tumor protein levels, Our results suggest a different modulation of Ku70/80 DNA-binding activity in human neoplastic tissues, po ssibly related to tumor progression. Findings provide further data on tissu e-specific protein expression and post-translational regulation of heterodi mer activity.