Growth-inhibitory effects of a β-glucan from the mycelium of Poria cocos on human breast carcinoma MCF-7 cells: Cell-cycle arrest and apoptosis induction

Abstract

Because of the reported immune-enhancing and antitumor activities of some mushroom polysaccharides, their applications as biological response modifiers have attracted significant attention. We have purified a water-soluble β-glucan PCM3-II, comprising mainly 1→3 and 1→4 linkages, from the mycelia of Poria cocos (Schw.) Wolf (Fu-ling). In this study, the growth-inhibitory effect of PCM3-II was further explored on the human breast carcinoma MCF-7 cells in vitro. The dose effect of PCM3-II was studied by incubating the breast cancer cells with 12.5-400 μg/ml of the glucan for 72 h. The MTT study showed that PCM3-II reduced proliferation and viability of the MCF-7 cells dose-dependently, so that the cancer-cell growth was decreased by 50% of the control level at 400 μg/ml of the glucan. The time effect of PCM3-II was then investigated by treating the breast cancer cells with 400 μg/ml of the glucan for 24, 48 and 72 h, respectively. Results from the flow cytometry study demonstrated that PCM3-II induced cell-cycle G1 arrest time-dependently and about 90% of the cells in cell cycle were accumulated at G1 phase after 72 h of treatment. The G1 arrest was associated with downregulations of the unscheduled cyclin D1 and cyclin E expressions in the breast cancer cells. Apoptosis was also induced by PCM3-II in the MCF-7 cells, so that the subG1 cells in DNA histogram of the flow cytometry were elevated by 5-fold of the control level at 48 h and by 24-fold at 72 h of treatment. The immunoblot study also showed that the glucan induced depletion of the antiapoptotic Bcl-2 protein, but not the proapoptotic Bax protein, so that the Bax/Bcl-2 ratio was elevated in the breast cancer cells at the time when the most prominent apoptosis was also observed. In conclusion, although the detailed mechanism for the antitumor activity of the P. cocos β-glucan still needs further investigation, this study provides preliminary insights into its mode of action and perspectives of its development as a water-soluble anti-tumor agent.