ICBP90, a novel human CCAAT binding protein, involved in the regulation of topoisomerase IIalpha 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 IIalpha gene. Screening of cDNA libraries from the leukemia Jurkat cell line and from the adult human thymus resulted in the isolation 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 terms of cell proliferation, are the tissues richest in ICBP90 mRNA. In contrast, highly differentiated tissues and cells such as the central nervous system and peripheral leukocytes are free of ICBP90 mRNA. Western blotting experiments showed a simultaneous expression of topoisomerase IIalpha and ICBP90 in proliferating human lung fibroblasts. Simultaneous expression of both proteins has also been observed in HeLa cells, but in both proliferating and confluent cells. Overexpression of ICBP90 in COS-1-transfected cells induced an enhanced expression of endogenous topoisomerase IIalpha. Immunohistochemistry experiments showed that topoisomerase IIalpha and ICBP90 were coexpressed in proliferating areas of paraffin-embedded human appendix tissues and in high-grade breast carcinoma tissues. We have identified ICBP90, which is a novel CCAAT binding protein, and our results suggest that it may be involved in topoisomerase IIalpha expression. ICBP90 may also be useful as a new proliferation marker for cancer tissues.