We have investigated the mechanism of topoisomerase I inhibition by an indolocarbazole derivative, R-3, The compound is cytotoxic to P388 leukemia cells, but not to P388CPT5 camptothecin-resistant cells having a deficient topoisomerase I. R-3 can behave both as a specific topoisomerase I inhibitor trapping the cleavable complexes and as a nonspecific inhibitor of a DNA-processing enzyme acting via DNA binding. In addition, the drug is a potent inhibitor of the kinase activity of topoisomerase I, Unlike camptothecin, R-3 completely inhibits the phosphorylation of SF2/ASF, a member of the SR protein family, in the absence of DNA, The inhibitory effect is also observed using mutant enzyme Y723F that lacks DNA cleavage/religation activity but does not affect phosphotransferase activity, indicating, therefore, that R-3 acts independently at both DNA cleavage and protein kinase sites. R-3 is the only compound known thus far that interferes specifically with the kinase activity of topoisomerase I and not with other kinases, such as protein kinase C and the cdc2 kinase, The study reinforces the view that topoisomerase I is a dual enzyme with a DNA cleavage site juxtaposed to a functionally independent kinase site and shows for the first time that indolocarbazole drugs can inhibit both the DNA cleavage/religation and kinase activities of the enzyme.
Poisoning of topoisomerase I by an antitumor indolocarbazole drug: Stabilization of topoisomerase I-DNA covalent complexes and specific inhibition of the protein kinase activity
Labourier, E.; Riou, J. F.; Prudhomme, M.; Carrasco, C.; Bailly, C.; Tazi, J.
1999-01-01 / vol 59 / pages 52-55
sr proteins; site; cleavage; compound 6-n-formylamino-12,13-dihydro-1,11-dihydroxy-13-(beta-d-glucopyranosyl); derivatives; induction; rebeccamycin