The antileukemic
alkaloid,
fagaronine, is a potent differentiation inducer of various hematopoietic cell lines. We show here that
fagaronine is
a DNA base-pair
intercalator with a K(app) of 2.1 x 10(5) M-1 for
calf thymus DNA.
Fagaronine inhibits the catalytic activity of purified calf thymus
topoisomerase I as shown by relaxation of supercoiled plasmid
DNA followed by electrophoresis in neutral as well as in
chloroquine-containing
gels. The catalytic activity of
topoisomerase I is inhibited at concentrations above 30 microM.
Fagaronine also inhibits the catalytic activity of purified calf thymus
topoisomerase II at concentrations above 25 microM as shown by decatenation of
kinetoplast DNA.
Fagaronine stabilizes the covalent
DNA-
enzyme reaction intermediate (the cleavable complex) between
topoisomerase I and linear pBR322
DNA at concentrations up to 1 microM. Further increase of the
fagaronine concentration leads to a progressive decrease in the cleavable complex formation, which is totally inhibited at 100 microM. In contrast, up to 1 microM
fagaronine has no effect on cleavable complex formation between purified calf thymus
topoisomerase II and linear pBR322
DNA, whereas cleavable complex formation is inhibited at higher concentrations. Exposure to
fagaronine results in an increase in
DNA-
protein complex formation in intact P388 murine
leukemia cells. P388CPT5 cells, which have an altered
topoisomerase I activity, are 4-fold resistant to the growth inhibitory effects of
fagaronine compared to the parental cell line. Similarly, DC-3F/9-
OH-E Chinese hamster
fibrosarcoma cells, which have an altered
topoisomerase II activity, are about 5-fold resistant to the growth inhibitory effects of
fagaronine. We conclude that
fagaronine is an inhibitor of both
DNA topoisomerase I and II and propose that this might play a role in the cytotoxic activity.