Citrus
limonoid glucosides, a family of fruit bioactive compounds, were postulated to have
free radical-scavenging and apoptosis-inducing properties against certain types of
cancers. Four highly purified
limonoid glucosides, limoin 17beta D-glucopypranoside (LG),
obacunone 17beta D-glucopyranoside (OG), nomilinic
acid 17beta D-glucopyranoside (NAG), and deacetylnomilinic
acid 17beta D-glucopyranoside (DNAG) were tested for
superoxide radical (O(2)(-))-quenching activity and cytotoxic action against undifferentiated human SH-SY5Y
neuroblastoma cells in culture. All 4 scavenged O(2)(-) as measured by inhibition of
pyrogallol decomposition in a spectrophotometric assay. Quenching by NAG in particular emulated an equivalent concentration of
vitamin C. When added to the medium of SH-SY5Y cells in culture, micromolar amounts of LG and OG, compared with untreated controls, caused a cessation of cell growth and rapid cell death (P < 0.001); NAG and DNAG were better tolerated, but nonetheless toxic as well. Cytotoxicity was related to a concentration- and time-dependent increase in
caspase 3/7 activity, suggesting that
limonoid glucosides were capable of inducing apoptosis. Arrested cell growth and the induction of apoptosis were confirmed by flow cytometry and DNA fragmentation analysis. Importantly,
caspase induction at 12 h correlated with cell survival at 24 h (P = 0.046), suggesting that apoptosis was the primary cause of cell death. We conclude that citrus
limonoid glucosides are toxic to SH-SY5Y
cancer cells. Cytotoxicity is exerted through apoptosis by an as yet unknown mechanism of induction. Individual
limonoid glucosides differ in efficacy as
anticancer agents, and this difference may reside in structural variations in the A ring of the
limonoid molecule.