Previously, we demonstrated that PL induces cultured prostate cancer cells to undergo apoptosis 
. Here, using xenograft assay, we showed that PL is capable of eliciting an apoptotic response in vivo
to block the growth of the tumors formed by the inoculation of human prostate cancer DU145 or PC3 cells. The histochemistry staining analysis showed that the majority of the cells in the tumors isolated from PL-treated mice are apoptotic, manifested by positively stained with TUNEL and reacted with an anti-caspase 3 antibody. The occurrence of apoptosis was also demonstrated in cultured DU145 or PC3 cells after the treatment with PL for 48 in a dose dependent manner. Thus, our study, using in vivo
as well in vitro
assays, inidcates that PL treatment initiates caspase cascade to induce apoptosis in prostate cancer cells, which provide a strong evidence for the potential of PL as an anti-prostate cancer remedy.
It is known that PL is non-toxic in general 
. Previously, we reported that PL, at high doses, induces control lung epithelial cells to arrest in the G1
phase of the cell cycle by blocking the expression of cyclin D1
and its interaction with cell cycle-dependent kinases 4 and 6 
. In our current study, the data from histopathological analysis demonstrate that the injection of PL into nude mice inoculated with either DU145 or PC3 prostate cancer cells has no toxic effect on the mice, manifested by that the treatment plays no role in the growth or water consumption of the mice. Importantly, the liver samples from the treated mice are normal without degeneration or fibrosis. It is possible that, like in vitro
experiments, treatment with PL elicits cell checkpoint controls in normal tissues or organs, which is not harmful to the mice. In the inoculated tumors, the same treatment appears to be very much cytotoxic.
Programmed cell death is important machinery for excluding abnormal or cancerous cells. Activation of caspase family members is at the core of apoptosis, representing a point of intersection of various apoptotic pathways in various types of cancer cells, including prostate cancer cells. TNF or Fas/CD95 receptors, mitochondrial proteins (such as cytochrome c) and granzyme are able to induce cell death through activation of the caspase cascade 
. Damage to or stress in the ER or Golgi has been shown to be able to trigger apoptosis 
. Studies have shown that the unfolded protein response or lack of calcium is responsible for ER-mediated apoptosis 
. ER stress has been demonstrated to cause the translocation of certain caspases to the ER or Golgi and the execution of apoptosis there. Calcium released from the ER during times of ATP deficiency is an important element in apoptosis induced by ischemia-reperfusion injury. We have shown that PL treatment triggers caspase activation in cultured prostate cancer cells 
. Our in vivo
study indicates that PL treatment is also able to initiate apoptosis via activating caspase 3 that are the effector in caspase chain reaction. The investigation of whether other caspases are involved in the induction of apoptosis in vivo
is under way.
PL is one of well-established medicinal mushrooms that have been taken orally as a common health-promoting dietary supplement or an adjuvant to treat malignancies in Asia for many decades. The extraction from water-soluble PL fraction shows a relatively homogeneous molecular weight distribution on gel permeation HPLC and is estimated to be around 150–180 kD from the retention time on HPLC pullulan molecular markers 
. The main components of PL have been suggested to be various polysaccharides 
. It has also been shown that the water soluble fraction of PL is able to suppress tumors either indirectly by enhancing the host's immune system, or directly by inducing apoptosis in tumor cells 
. However, it is still unclear which compositions of polysaccharide in PL possess the anti-cancer effect.
In summary, the successful use of PL to treat prostate cancer requires the fully understanding of the mechanisms for how PL mediates anti-tumor activities in tumors. Our previous study demonstrated that PL, through activation of caspases, is able to induce apoptosis in cultured cancer cells. In the present investigation, using xenograft assay, we further demonstrate that the injection of PL into nude mice with formed subcutaneous tumors by inoculated with prostate cancer DU145 or PC3 cells induces majority of inoculated tumor cells to lose their viabilities. Thus, our in vivo data support the notion that PL, by switching on a tumor suppression-related machinery, can be developed as an efficient therapy to treat human malignancies, such as refractory prostate cancer.