| Colon carcinoma is the second largest cause of cancer-related mortality in the United States. Diet has been recognized as an important modifier of colon cancer development for decades. Specifically, dietary patterns that include legumes, such as soybeans, may reduce colon cancer progression. High concentrations of the B-group soyasaponins are found in many legumes, and these compounds are bioavailable to the colonic epithelium. Therefore, B-group soyasaponins may contribute to the lower incidence of colon cancer observed in individuals that consume legumes. Consequently, the impact of a purified fraction of triterpenoid B-group soyasaponins isolated from soybeans was evaluated in cultures of HCT-15 human colon adenocarcinoma cells. Treatment of cells with the soyasaponins at concentrations of 25-500 ppm significantly reduced viable cell numbers after 24 and 48 hours of exposure, an effect that was specific to cancer cells. Treatment of cells with 25 and 100 ppm of soyasaponins resulted in a transient accumulation of cells in the S-phase of the cell cycle that was associated with a significant reduction of cyclin dependant kinase-2 activity. The reduction in cell numbers was mediated, in part, through sustained autophagy, the hallmark of Type II programmed cell death. When examined by transmission electron microscopy, soyasaponin-treated cells exhibited an approximate 4.5-fold increase in cell morphologies characteristic of Type II programmed cell death. In addition, enhanced autophagic capacity of cells exposed to B-group soyasaponins was demonstrated through increases in both incorporation of monodansylcadaverine, a biochemical marker of autophagic vacuoles, and concentrations of the autophagy-specific microtubule associated protein light chain-3 protein. Exposure to B-group soyasaponins resulted in modulation of two signaling pathways that have previously been implicated in controlling the induction of autophagic cell death in mammalian cancer cells. First, treatment with B-group soyasaponins suppressed Akt activity, which was associated with a reduction in the activating phosphorylation of the Akt-serine473 residue. In addition, ERK1/2 activity was significantly increased, and was determined to be necessary for B-group soyasaponin-induced autophagy. These findings suggest that B-group soyasaponins may be another colon-cancer suppressive component of legumes that warrant further examination as a potential chemopreventive phytochemicals. |
