apyrase and lactacystin

DNA topoisomerase I (Topo I) contributes to various important biological functions, and its activity is therefore likely regulated in response to different physiological conditions. Increases in both the synthesis and degradation of Topo I were previously shown to accompany phytohemagglutinin stimulation of proliferation in human peripheral T lymphocytes. The mechanism of this degradation of Topo I has now been investigated with both in vivo and in vitro assays. The activity of a nuclear protease that specifically degrades Topo I was induced in proliferating T lymphocytes. The full-length Topo I protein (100 kDa) was sequentially degraded to 97- and 82-kDa fragments both in vivo and in vitro. The initial site of proteolytic cleavage was mapped to the NH(2)-terminal region of the enzyme. The degradation of Topo I in vitro was inhibited by aprotinin or soybean trypsin inhibitor, suggesting that the enzyme responsible is a trypsin-like serine protease. Furthermore, Topo I degradation by this protease was Mg(2+)-dependent. The Topo I-specific protease activity induced during T lymphocytes proliferation was not detected in Jurkat (human T cell leukemia) cells and various other tested human cancer cell lines, possibly explaining why the abundance of Topo I is increased in tumor cells.

Topics: Acetylcysteine; Apyrase; Cell Division; Cell Line; Cell Nucleus; Cysteine Proteinase Inhibitors; Cytosol; DNA Topoisomerases, Type I; Humans; Immunoblotting; Jurkat Cells; Magnesium; Microfilament Proteins; Muscle Proteins; Phytohemagglutinins; Protein Biosynthesis; Recombinant Fusion Proteins; Serine Endopeptidases; T-Lymphocytes; Temperature; Trypsin; Tumor Cells, Cultured; Tumor Suppressor Protein p53