Positive and negative regulation of the human thymidine kinase promoter mediated by CCAAT binding transcription factors NF-Y/CBF, dbpA, and CDP/cut.

The proximal CCAAT element located 38 bp upstream of the transcription initiation site contributes to the human thymidine kinase (htk) promoter activity, because site-directed mutagenesis of a 10-bp region containing this CCAAT motif (TKC1) reduced the promoter activity by 55%. Through binding site competitions and antigenic cross-reactivity, the major factor that binds TKC1 from both HeLa and hamster nuclear extracts is identified as NF-Y/CBF. In serum-stimulated cells, the binding of NF-Y/CBF to TKC1 increased gradually, reaching a plateau at the S phase. In cell transfection assays, a dominant-negative mutant of NF-Y/CBF inhibited the htk promoter in a dosage-dependent manner, providing direct evidence that NF-Y/CBF is required for maximal htk promoter activity. Recently, it has been demonstrated that the site occupied by NF-Y/CBF also binds the serum-inducible dbpA and dpbB. We show here that recombinant dbpA interacts with the htk promoter, and overexpression of dbpA can stimulate htk promoter activity mediated through TCK1. In contrast, CDP/cut, the CCAAT displacement protein with known repressor property, binds the htk promoter through both the proximal and distal CCAAT elements. Our discovery that CDP/cut binds the htk promoter primarily in quiescent cells and that overexpression of CDP/cut inhibits htk promoter activity provides an explanation for the reported dramatic increase in htk promoter activity in serum-starved cells when both CCAAT elements were mutated. Thus, a combination of suppression in quiescent cells and activation in serum-stimulated cells mediated through various CCAAT-binding proteins may account in part for the induction of htk promoter activity as quiescent cells reenter the cell cycle.