It is known that polysaccharides extracted from the Phellinus linteus (PL) mushroom possess antitumour activity. We previously have demonstrated that high doses of PL render murine or human lung cancer cells susceptible to apoptosis. However, the molecular mechanisms of PL-mediated apoptosis have not been fully explored. In this study, we demonstrate that LNCaP cells expressing the androgen receptor (AR) are highly susceptible to apoptosis in response to treatment with high doses of PL. In this process, caspase 8 and its downstream effectors (such as BID), as well as ER stress-related, apoptotic signalling, are activated. In contrast, a moderate amount of apoptosis occurs in PC3 cells (that lack AR) after the same treatment, which does not activate ER-mediated apoptotic signalling. We also show that, in the process of PL-induced apoptosis, caspase 2 is induced in LNCaP cells, but not in PC3 cells. However, LNCaP cells that express a mutated AR or LNCaP cells treated with a caspase 2 inhibitor blocked ER stress-induced apoptotic signals. The magnitudes of the induction of apoptosis in these cells are comparable with what occurred in the PC3 cells. The data demonstrate that high doses of PL activate the AR-dependent and independent apoptotic pathways. Our study also suggests that caspase 2 is a key target in the determination of the susceptibility of prostate cancer cells to PL-induced apoptosis. Phellinus linteus (PL) is among a number of well-known medicinal mushrooms from Asian countries, which have been taken orally since ancient times as a health-promoting dietary supplement and an adjuvant to combat viral and bacterial infections. PL, after purification, shows a relatively homogeneous molecular weight distribution on gel permeation HPLC and is estimated to be around 150 kDa from the retention time on HPLC pullulan molecular markers (Song et al, 1995). The main components of PL are polysaccharides (Song et al, 1995;Lorenzen and Anke, 1998;Borchers et al, 1999;Han et al, 1999). Many studies demonstrated that polysaccharides from various substances, including PL, are remarkably effective in inhibiting the growth of tumours without toxic side effects. Studies also showed that polysaccharides in PL are able to suppress tumours, either indirectly by enhancing the host's immune system or directly by inducing apoptosis in tumour cells (Chihara et al, 1969;Chung et al, 1982;Cun et al, 1994;Wasser, 2002;Collins et al, 2006;Guo et al, 2006). Therefore, the antitumour, antiangiogenic and immunomodulatory effects of PL are potential areas for developing novel pharmaceutical products. However, the underlying molecular mechanisms of the antitumour effects of PL have not yet been fully explored.Changes in the androgen receptor (AR) have been indicated to contribute to the development of prostate cancer and are a serious challenge to effective treatment. Mutations in the AR can increase its affinity for ligand binding, permitting activation by nonandrogenic hormones or even antagonists (Nelson et al, 2003;Debes and Tindall, ...