Role of cyclin inhibitor protein p21 in the inhibition of HCT116 human colon cancer cell proliferation by American ginseng (Panax quinquefolius) and its constituents

American ginseng and its ginsenoside constituents have been shown to exert anti-cancer effects although the mechanism of action remains unclear. The present study determined the effects of water-extracted ginseng (GE) or its ginsenoside (GF) and polysaccharide (PS) fractions on the proliferation of human colon cancer cells and examined the role of p21 in mediating these effects using wild-type and p21-/- HCT116 human colon carcinoma cells. Proliferation was inhibited by GE, GF, and PS in wild-type and p21-/- cells, and the p21-/- cells were more sensitive to these treatments. Wild type cells treated with GE were arrested in the G0/G1 phase of the cell cycle and the expression of p53 and p21 proteins was increased while phospho-MEK levels decreased. In contrast, cells deficient in p21 displayed reduced cell viability, elevated number of dead cells, and increased expression of Bax and cleaved caspase-3 proteins. Both polysaccharides and ginsenosides appear to be responsible for the anti-proliferative and proapoptotic effects of GE. This study suggests that p21 functions to arrest HCT116 wild-type cells treated with GE, while p21-deficient cells undergo cell death in a ginseng constituent-dependent manner.