山竹(α-mangostin)對於人類惡性膠狀母細胞瘤之療效研究

Abstract

Glioblastoma multiforme (GBM) is the most common primary brain cancer and one of the most lethal primary malignancies in humans. Glioblastoma patients have a poor prognosis, with a patient survival rate at five years after diagnosis of less than 5% and a median survival of only 14.6 months. Glioblastoma is highly resistant to radio- and chemotherapy, and there is no effective cure for patients. Effective chemopreventive treatment for brain cancer would have a tremendous impact on brain cancer morbidity and mortality. Autophagic cell death is an important physiological process of all eukaryotic cells. Autophagic cell death is characterized by massive degradation of cellular contents, including portions of the cytoplasm and intracellular organelles, by means of complicated intracellular membrane/vesicle reorganization and lysosomal hydrolysis. Autophagic cell death is important for development and stress responses and has also been observed in several human diseases, including neurodegenerative and muscular disorders as well as resistance to pathogens. Furthermore, like apoptosis, autophagic cell death is suppressed in malignant tumors. A number of studies have reported that autophagy is activated in response to γ-irradiation and various anticancer therapies, particularly in apoptosis-deficient cells. Several molecular and cell signaling pathways have been implicated in regulating autophagy, such as the BECN1, AMPK (AMP-activated protein kinase), MAPK (mitogen-activated kinase) and PI3K-AKT-mTOR (phosphatidylinositol 3-kinase/AKT/mammalian target of rapamycin) pathways. However, the detailed mechanisms of the autophagic cell-signaling pathway are still poorly understood. α-Mangostin (5-hydroxy-2-methyl-1,4-naphthoquinone) is a quinonoid constituent isolated from mangosteen, a tropical fruit native to southeast Asia. It has been shown to possess antioxidants and stabilize digestion. α-Mangostin also has anticancer and antiproliferative properties in leukemia as well as prostate, breast and colorectal cancers. However, the effect of α-mangostin on brain cancer and the molecular mechanism of its effect have not yet been fully determined. In this study, we determined the effect of α-mangostin on cell growth inhibition using in vitro and in vivo experimental models. We measured its effect on cell viability and autophagic cell death in two human brain cancer cell lines, GBM8401 and DBTRG-05MG. Furthermore, to establish α-mangostin’s anticancer mechanism, we determined the levels of autophagy-related molecules, which are strongly associated with the cell death signal transduction pathway and also affect the sensitivity of tumor cells to anticancer agents. This study is the first to investigate the anticancer effects of α-mangostin in human glioblastoma cells. α-Mangostin decreases cell viability by inducing autophagic cell death, but not apoptosis. Pretreatment of cells with the autophagy inhibitors 3-methyladenine (3-MA) and bafilomycin or knockdown Beclin-1, resulted in the suppression of α-mangostin-mediated cell death. We also found that Liver Kinase B1 (LKB1)/AMP-activated protein kinase (AMPK) signaling is a critical mediator of α-mangostin-induced inhibition of cell growth. Activation of AMPK induces α-mangostin-mediated phosphorylation of raptor, which subsequently associates with 14-3-3γ and results in loss of mTORC1 activity. The phosphorylation of both of mTORC1’s downstream targets, p70 ribosomal protein S6 kinase (p70S6 kinase) and 4E-BP1, is also diminished by activation of AMPK. Furthermore, the inhibition of AMPK expression with shRNAs or an inhibitor of AMPK reduced α-mangostin induced autophagy and raptor phosphorylation, supporting the theory that activation of AMPK is beneficial to autophagy. Further investigation revealed that α-mangostin also induced autophagic cell death in transplanted glioblastoma in nude mice. Together, these results suggest a critical role for AMPK activation in the α-mangostin-induced autophagy of human glioblastoma cells.