Poisoning of topoisomerase I by an antitumor indolocarbazole drug: stabilization of topoisomerase I-DNA covalent complexes and specific inhibition of the protein kinase activity.
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.
Labourier
E
E
Institut de Génétique Moléculaire, Université de Montpellier II, Montpellier, France.
Riou
J F
JF
Prudhomme
M
M
Carrasco
C
C
Bailly
C
C
Tazi
J
J
D000818:Animals; D000970:Antineoplastic Agents / Q000633:toxicity*; D002227:Carbazoles / Q000633:toxicity*; D004247:DNA / Q000378:metabolism; D004264:DNA Topoisomerases, Type I / Q000187:drug effects* / Q000378:metabolism; D007211:Indoles / Q000633:toxicity*; D051379:Mice; D047428:Protein Kinase Inhibitors; D014407:Tumor Cells, Cultured
D000970:Antineoplastic Agents; D002227:Carbazoles; D007211:Indoles; D047428:Protein Kinase Inhibitors; D004247:DNA; D004264:DNA Topoisomerases, Type I