DNA topoisomerase II (topo II) is an essential nuclear enzyme involved in major cellular functions such as DNA replication, transcription, recombination, and mitosis. While an elevated level of topo II␣ is associated with cell proliferation, wild-type (wt) p53 inhibits the expression of various growth-stimulatory genes. To determine if p53 downregulates topo II␣ gene expression, a murine cell line, (10)1val, that expresses a temperature-sensitive p53 was utilized. The (10)1val cells had significantly lower levels of topo II␣ mRNA and protein following incubation for 24 h at 32؇C (p53 with wt conformation) than at 39؇C (p53 with mutant conformation). The effect of p53 on the human topo II␣ gene promoter was determined by using luciferase reporter plasmids containing varying lengths of the topo II␣ promoter transiently cotransfected into p53-deficient (10)1 cells together with wt or mutant p53 expression plasmids. Transcription from the full-length (bp ؊557 to ؉90) topo II␣ promoter was decreased 15-fold by wt p53 in a concentration-dependent manner, whereas mutant p53 exerted much weaker inhibition. Consecutive deletion of the five inverted CCAAT elements (ICEs) from the topo II␣ promoter reduced both the basal promoter activity and wt p53-induced suppression. Transcription of the minimal promoter (؊32 to ؉90), which contains no ICE, was slightly stimulated by wt or mutant p53 expression. When point mutations were introduced into the most proximal ICE (؊68), the inhibitory effect of wt p53 was alleviated and stimulation of topo II␣ expression resulted. Our study suggests that wt p53 functions as a transcriptional repressor of topo II␣ gene expression, possibly through a functional interaction with specific ICEs. Inactivation of wt p53 may reduce normal regulatory suppression of topo II␣ and contribute to abortive cell cycle checkpoints, accelerated cell proliferation, and alterations in genomic stability associated with neoplasia.DNA topoisomerase II (topo II) is a ubiquitous enzyme that can alter the topological state of DNA and untangle intertwined DNA helices (for reviews, see references 79 and 80). As such, topo II plays an essential role in several cellular events, such as replication (20), chromatin condensation (76), and sister chromatid segregation (20,76). Functioning as a homodimer, topo II binds double-stranded DNA and attaches covalently to both strands of the helix, resulting in DNA breaks in each strand. Upon binding of ATP to the topo II-DNA complex, a second DNA helix can pass through the cleavable complex, and this is followed by hydrolysis of ATP and resealing of the cleaved double-stranded DNA (7, 10). The ability of topo II to pass duplex DNA molecules allows the enzyme to separate fully replicated DNA molecules prior to chromosome segregation (61). There are two topo II isoforms, topo II␣ (170 kDa) and topo II␤ (180 kDa), present in mammalian cells. Although both enzymes are closely related in structure (72% identical amino acid residues [42]), they differ in important biochemical and pharmac...

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