Cytotoxic effect of oyster mushroom Pleurotus ostreatus on human androgen-independent prostate cancer PC-3 cells

Twenty species of edible mushrooms and three purified mushroom polysaccharides were screened for their antitumor potential on human androgen-independent cancer PC-3 cells. A water-soluble extract (POE) prepared from the fresh oyster mushroom Pleurotus ostreatus produced the most significant cytotoxicity on PC-3 cells among the mushroom species tested. At the same time, POE induced a rapid apoptosis on PC-3 cells detected with annexin V-fluorescein isothiocyanate flow cytometry when the cells were exposed to POE (150 mu g/mL) for 2 hours. Induced apoptosis was also confirmed by DNA fragment terminal deoxynucleotidyl transferase-mediated X-dUTP nick end labeling staining while POE (200 mu g/mL) was added to PC-3 cells for 6 hours. Both cytotoxicity and induced apoptosis mediated by POE in PC-3 cells are dose-dependent. Interestingly, PC-3 cells appeared to be more sensitive to POE in anchorage-independent growth condition. Tumor colony-forming efficiency was dramatically reduced to 4.5% or 0.5% in POE (60 or 120 mu g/mL)-supplemented soft agar medium compared with that of POE-free medium (defined as 100%). Temperature in POE processing plays a decisive role for the cytotoxic activity. Bioactivity of POE was eliminated by exposure to high temperature (80 degrees C) for 2 hours; however, it remained stable at a series temperatures of below 40 degrees C. The active fraction POE-F2 was analyzed and identified by size exclusion of high performance liquid chromatography and the CellTiter 96 (R) AQueous Cell Proliferation Assay (Promega, Madison, WI). Since POE-F2 is also sensitive to heat and has strong 280 nm absorption, the results imply that active compounds recovered from P. ostreatus are water-soluble proteins or polypeptides.