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DNA topoisomerase I (Topo I) breaks and rejoins one strand of DNA, and DNA topoisomerase (Topo II) breaks and rejoins both strands of DNA. Nuclei of human cells contain two type I enzymes, Topo I and Topo IIIα, and two type II enzymes, Topo IIα and Topo IIβ. We are investigating the involvement of these enzymes in transcription in HuT 78 cells, a human T cell leukemia line that expresses CD 25, a high affinity receptor for interleukin‐2 (IL‐2). The specific activities of both Topo I and Topo II increased several fold in nuclear extracts prepared from HuT 78 cells within the first 12 hrs. after treatment with IL‐2 suggesting that both Topo I and Topo II function in transcription in activated human T cells. We are using specific monoclonal antibodies and antitumor drugs to determine which of the four human DNA topoisomerases are activated upon mitogenesis of HuT 78 by IL‐2. Evidence to date suggests that Topo I is the principal swivelase for transcription and Topo IIβ, a component of the chromosomal scaffold, is the main enzyme that breaks and rejoins DNA in order to make a loop of chromatin accessible for transcription. This work was supported by 5G11 HD052388‐02 and CA 74388.

The activities of DNA topoisomerase I and DNA topoisomerase II were measured in HuT 78 cells, a human T cell lymphoma cell line, after treatment of the cells with interleukin‐2 (IL‐2). HuT 78 cells were treated with 1,000 units IL‐2/ml, and extracts of the nuclei were prepared from 0 to 12 hours after treatment. The activity of DNA topoisomerase I was assayed by relaxation of supercoiled pBR322 DNA. The activity of DNA topoisomerase II was assayed by unknotting of P4 DNA, decatenation of kinetoplast DNA, and cleavable complex formation. The decatenation activity was sensitive to etoposide, an inhibitor of DNA topoisomerase II. The specific activities of both enzymes were determined in units/mg protein. The specific activities of both DNA topoisomerase I and DNA topoisomerase II increased 3‐ to 11‐fold in nuclear extracts prepared from HuT 78 cells in three transient, concomitant peaks observed at 0.5, 4, and 10 hrs after treatment with IL‐2. The specific activities of both enzymes returned to baseline values after each peak. These results indicate that the activities of DNA topoisomerase I and DNA topoisomerase II are extensively regulated in activated human T cells, and they suggest that both DNA topoisomerase I and DNA topoiosmerase II function in both transcription and DNA replication in IL‐2‐activated T cells. This work was supported by grants from the Morrison Trust and the NIH (HD 52388).