INHIBITION OF DNA TOPOISOMERASE-I ACTIVITY BY HEPARAN-SULFATE AND MODULATION BY BASIC FIBROBLAST GROWTH-FACTOR

Eukaryotic DNA topoisomerase I catalyzes changes in the superhelical s tate of duplex DNA by transiently breaking single strands thereby allo wing relaxation of both positively and negatively supercoiled DNA. Top oisomerase I is a nuclear enzyme localized at active sites of transcri ption, and abnormal levels of the enzyme have been observed in a varie ty of neoplasms. Because the enzyme binds heparin and, given the prese nce of heparan sulfate within the nuclei of mammalian cells, we sought to investigate the interaction between topoisomerase I and sulfated g lycosaminoglycans isolated from normal and neoplastic human liver The results demonstrated that low concentrations (similar to 100 nM) of he paran sulfate from normal liver but not from its malignant counterpart effectively blocked relaxation of supercoiled DNA driven by either pu rified holoenzyme or topoisomerase I activity present in nuclear extra cts of three malignant cell lines. Heparin acted at even lower (simila r to 10 nM) concentrations. Moreover, we show that basic fibroblast gr owth factor could interfere with this heparan sulfate/heparin-driven i nhibition and that both basic fibroblast growth factor and heparin-bin ding sites co-localized in the nuclei of U937 leukemic cells. Our resu lts suggest that DNA topoisomerase I activity may be modulated in vivo by specific heparan sulfate moieties present in normal cells but mark edly reduced or absent in their transformed counterparts.