ICBP90, a novel human CCAAT binding protein, involved in the regulation oftopoisomerase II alpha expression

The one-hybrid system with an inverted CCAAT box as the DNA target sequence was used to identify proteins acting on key DNA sequences of the promoter of the topoisomerase II alpha gene. Screening of cDNA libraries from the le ukemia Jurkat cell line and from the adult human thymus resulted in the iso lation of a novel protein of 793 amino acids (89,758 Da). This protein has in vitro CCAAT binding properties and has been called ICBP90. Adult thymus, fetal thymus, fetal liver, and bone marrow, known as active tissues in ter ms of cell proliferation, are the tissues richest in ICBP90 mRNA. In contra st, highly differentiated tissues and cells such as the central nervous sys tem and peripheral leukocytes are free of ICBP90 mRNA. Western blotting exp eriments showed a simultaneous expression of topoisomerase II alpha and ICB P90 in proliferating human lung fibroblasts, Simultaneous expression of bot h proteins has also been observed in HeLa cells, but in both proliferating and confluent cells. Overexpression of ICBP90 in COS-1-transfected cells in duced an enhanced expression of endogenous topoisomerase II alpha. Immunohi stochemistry experiments showed that topoisomerase II alpha and ICBP90 were coexpressed in proliferating areas of paraffin-embedded human appendix tis sues and in high-grade breast carcinoma tissues. We have identified ICBP90, which is a novel CCAAT binding protein, and our results suggest that it ma g be involved in topoisomerase II alpha expression. ICBP90 may also be usef ul as a new proliferation marker for cancer tissues.