The
antibiotics AT2433-A1 and
AT2433-B1 are two indolocarbazole diglycosides related to the
antitumor drug rebeccamycin known to stabilize
topoisomerase I-
DNA complexes. This structural analogy prompted us to explore the binding of four indolocarbazole diglycosides with
DNA and their capacity to interfere with the DNA cleavage-reunion reaction catalyzed by
topoisomerase I. The molecular basis of the drug interaction with
double-stranded DNA and with purified
chromatin, with particular emphasis on the role of the
carbohydrate moiety, was investigated by means of complementary spectroscopic techniques, including surface plasmon resonance and electric linear dichroism. We compared the
DNA binding properties, sequence recognition, and effects on
topoisomerase I-mediated
DNA relaxation and cleavage of
AT2433-A1 bearing a 2,4-dideoxy-4-methylamino-L-xylose residue, its dechlorinated analog
AT2433-B1, the diastereoisomer
iso-AT2433-B1 with an inverted aminosugar residue, and compounds 5H-indolo[2,3-a]pyrrolo[3,4-c]
carbazole-5,7(6H)-dione, 12-beta-D-glucopyranosyl-12,13-dihydro-6-methyl (JDC-108) and 5H-indolo[2,3-a]pyrrolo[3, 4-c]
carbazole-5,7(6H)-dione, 12-(6-O-alpha-D-galacto-pyranosyl-beta-D-glucopyranosyl)-12,13-dihydro-6-methyl (JDC-277) with an uncharged mono- and
disaccharide, respectively. The two
antibiotics AT2433-A1 and
AT2433-B1 proved to be highly cytotoxic to
leukemia cells and this may be a consequence of their tight intercalative binding to
DNA, preferentially into GC-rich sequences as inferred from
DNase I footprinting studies and surface plasmon resonance measurements. Like the diastereoisomer
iso-AT2433-B1, they have no inhibitory effect on
topoisomerase I, in contrast to the uncharged diglycoside JDC-277, which stimulates DNA cleavage by the
enzyme mainly at TG sites, as observed with
camptothecin. Cytotoxicity measurements with CEM and CEM/C2 human
leukemia cell lines sensitive and resistant to
camptothecin, respectively, also suggested that
topoisomerase I contributes, at least partially, to the mechanism of action of the neutral diglycoside JDC-277 but not to that of the cationic AT2433 compounds. Together, the results indicate that sequence-selective
DNA interaction and
topoisomerase I inhibition is controlled to a large extent by the stereochemistry of the diglycoside moiety.