The Pterogyne nitens (Fabaceae) tree, native to South America, has been found to produce
guanidine alkaloids as well as bioactive
flavonols such as
kaempferol,
quercetin, and
rutin. In the present study, we examined the possibility of interaction between human
ATP-binding cassette (
ABC) transporter ABCB1 and four
guanidine alkaloids isolated from P. nitens (i.e.,
galegine,
nitensidine A,
pterogynidine, and
pterogynine) using human T cell lymphoblast-like
leukemia cell line CCRF-CEM and its multi-
drug resistant (MDR) counterpart CEM/ADR5000. In XTT assays, CEM/ADR5000 cells were resistant to the four
guanidine alkaloids compared to CCRF-CEM cells, although the four
guanidine alkaloids exhibited some level of cytotoxicity against both CCRF-CEM and CEM/ADR5000 cells. In
ATPase assays, three of the four
guanidine alkaloids were found to stimulate the
ATPase activity of ABCB1. Notably,
nitensidine A was clearly found to stimulate the
ATPase activity of ABCB1 as strongly as the control drug,
verapamil. Furthermore, the cytotoxic effect of
nitensidine A on CEM/ADR5000 cells was synergistically enhanced by
verapamil.
Nitensidine A inhibited the extrusion of
calcein by ABCB1. In the present study, the possibility of interaction between ABCB1 and two synthetic
nitensidine A analogs (nitensidine AT and AU) were examined to gain insight into the mechanism by which
nitensidine A stimulates the
ATPase activity of ABCB1. The ABCB1-dependent
ATPase activity stimulated by
nitensidine A was greatly reduced by substituting
sulfur (S) or
oxygen (O) for the imino
nitrogen atom (N) in
nitensidine A. Molecular docking studies on human ABCB1 showed that,
guanidine alkaloids from P. nitens dock to the same binding pocket as
verapamil.
Nitensidine A and its analogs exhibit similar binding energies to
verapamil. Taken together, this research clearly indicates that
nitensidine A is a novel substrate for ABCB1. The present results also suggest that the number, binding site, and polymerization degree of the isoprenyl moiety in the
guanidine alkaloids and the imino
nitrogen atom cooperatively contribute to their stimulation of ABCB1's
ATPase activity.