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Targeting autophagy may serve as a promising strategy for cancer therapy. Ganoderma lucidum polysaccharide (GLP) has been shown to exert promising anti-cancer effects. However, the underlying mechanisms remain elusive. Whether GLP regulates autophagy in cancer has never been reported. In this study, GLP induced the initiation of autophagy in colorectal cancer (CRC) HT-29 and HCT116 cells, as evidenced by enhanced level of LC3-II protein, GFP-LC3 puncta, and increased formation of double membrane vacuoles. However, GLP treatment caused marked increase of p62 expression. Addition of late stage autophagy inhibitor, chloroquine (CQ), further enhanced LC3-II and p62 level, as well as increased autophagosome accumulation, suggesting a blockage of autophagic flux by GLP in CRC cells. We then found GLP blocked autophagosome and lysosome fusion as determined by mRFP-GFP-LC3 colocalization analysis. Mechanistic study revealed that GLP-induced disruption of autophagosome-lysosome fusion is due to reduced lysosome acidification and lysosomal cathepsin activities. Cell viability and flow cytometry assays revealed that GLPinduced autophagosome accumulation is responsible for GLP-induced apoptosis in CRC cells. In line with this, inhibition of autophagy initiation by 3-methyladenine (3-MA), an early stage autophagy inhibitor, attenuated GLPinduced apoptosis. In contrast, suppression of autophagy at late stage by CQ enhanced the anti-cancer effect of GLP. Furthermore, we demonstrated that GLP-induced autophagosome accumulation and apoptosis is mediated via MAPK/ERK activation. Finally, GLP inhibited tumor growth and also inhibited autophagic flux in vivo. These results unveil new molecular mechanism underlying anti-cancer effects of GLP, suggesting that GLP is a potent autophagy inhibitor and might be useful in anticancer therapy.

The sporoderm-broken spores of Ganoderma lucidum (G. lucidum) polysaccharide (BSGLP) have been demonstrated to inhibit carcinogenesis in several types of cancer. However, to the best of our knowledge, the anticancer effects of polysaccharides extracted from the newly developed sporoderm-removed spores of G. lucidum (RSGLP) have not been assessed. The present study first compared the anticancer effects of RSGLP and BSGLP in three gastric cancer cell lines and it was found that RSGLP was more potent than BSGLP in decreasing gastric cancer cell viability. RSGLP significantly induced apoptosis in AGS cells, accompanied by downregulation of Bcl-2 and pro-caspase-3 expression levels, and upregulation of cleaved-PARP. Furthermore, RSGLP increased LC3-II and p62 expression, indicative of induction of autophagy and disruption of autophagic flux in AGS cells. These results were further verified by combined treatment of AGS cells with the late-stage autophagy inhibitor chloroquine, or early-stage autophagy inducer rapamycin. Adenoviral transfection with mRFP-GFP-LC3 further confirmed that autophagic flux was inhibited by RSGLP in AGS cells. Finally, the present study demonstrated that the RSGLP-induced autophagy and disruption of autophagic flux disruption was, at least in part, responsible for RSGLP-induced apoptosis in AGS cells. The results of the present study demonstrated for the first time that RSGLP is more effective than BSGLP in inhibiting gastric cancer cell viability, and RSGLP may serve as a promising autophagy inhibitor in the management of gastric cancer.

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