Protective effects of Allium vegetables against
cancers have been shown extensively in experimental animals and epidemiologic studies. We investigated cell proliferation and the induction of apoptosis by
onion oil extracted from Allium cepa, a widely consumed Allium vegetable, in human
lung cancer A549 cells. GC/MS analysis suggested that propyl
sulfides but not allyl
sulfides are major
sulfur-containing constituents of
onion oil.
Onion oil at 12.5 mg/L significantly induced apoptosis (13% increase of apoptotic cells) as indicated by sub-G1
DNA content. It also caused cell cycle arrest at the G2/M phase; 25 mg/L
onion oil increased the percentage of G2/M cells almost 6-fold compared with the
dimethyl sulfoxide control. The action of
onion oil may occur via a
reactive oxygen species-dependent pathway because cell cycle arrest and apoptosis were blocked by the
antioxidants N-acetylcysteine and exogenous
glutathione. Marked collapse of the mitochondrial membrane potential suggested that dysfunction of the mitochondria may be involved in the oxidative burst and apoptosis induced by
onion oil. Expression of phospho-cdc2 and phospho-
cyclin B1 were downregulated by
onion oil, perhaps accounting for the G2/M arrest. Overall, these results suggest that
onion oil may exert chemopreventive action by inducing cell cycle arrest and apoptosis in
tumor cells.