New Apoptosis-Inducing Sesquiterpenoids from the Mycelial Culture of Chinese Edible Fungus Pleurotus cystidiosus

Two new bisabolane-type sesquiterpenoids, pleuroton A (1) and pleuroton B (2), and three clitocybulol derivatives, clitocybulol D (3), clitocybulol E (4), and clitocybulol F (5), were obtained from the mycelial culture of edible fungus Pleurotus cystidiosus O. K. Mill by repeated column chromatography over RP-18, Sephadex LH-20, and silica gel. Their structures were determined according to nuclear magnetic resonance data, high-resolution electron impact mass spectrometry, and circular dichroism spectra. These new sesquiterpenoids exhibited significant cytotoxicity against two human prostate cancer DU-145 and C42B cells in the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. The median inhibitory concentration (IC50) of compounds 1, 2, 3, 4, and 5 was 174, 28, 233, 162, and 179 nM, respectively, against the DU-145 cell and was 104, 52, 163, 120, and 119 nM, respectively, against the C42B cell. Especially, pleuroton B (2) exhibited the strongest cytotoxity among these sesquiterpenoids, which was confirmed by the colony formation assay. Furthermore, pleuroton B (2) could trigger the apoptosis of DU-145 cells through the detection of apoptosis cells using annexin V-FITC staining by flow cytometry, the observation of condensed nuclei in the apoptosis cells, and the western blot analysis for the expression of apoptosis-related proteins Bcl-2, Bak, and Bax. Analysis of structure–activity relationships of these sesquiterpenoids revealed that the unusual functional moiety of pleuroton B should contribute to its significant bioactivity. These results display the pharmacological potential of P. cystidiosus.