A NOVEL P53 BYSTANDER EFFECT INDUCES GLIOBLASTOMA CELL DEATH THROUGH A GLYCOSYLATION-DEPENDENT MECHANISM

BACKGROUND: Mutations in the TP53 tumor suppressor gene are required for the development of many malignancies, including brain tumors. Yet, the molecular mechanisms underlying p53's anti-tumor effects remain incompletely understood. In the present study we demonstrate the existence of a novel cell-extrinsic mechanism whereby p53 can antagonize tumor growth. METHODS: To activate p53-dependent DNA damage, cells were either untreated (Ctrl) or treated with 25 mu M etoposide (VP16), ultraviolet light (UV; 60 mJ per cm2) or gamma -radiation (5 Gy). Gene and protein expression were analyzed by Northern Blot, Western blot and/or RT/PCR analyses. Standard apoptosis assays, crystal violet cytotoxicity assays, soft-agar colony formation assays and in vivo tumorigenicity experiments were used. RESULTS: We found that following DNA damage, cells with activated p53 can control the viability of adjacent tumor cells through secretion of galectin-3 (Gal-3), a beta -galactoside-binding lectin. Gal-3 specifically induced the killing of bystander cancer cells but not normal cells by paracrine induction of caspase 9-mediated apoptosis. Induction of Gal3 secretion in tumor cells could also antagonize glioblastoma tumor growth in subcutaneous and intracranial models. Mechanistically, p53 indirectly induced Gal-3 export through transcriptional regulation of p53-tumor suppressor activated pathway-6 (TSAP6), a key component of the non-classical exosome-mediated secretory pathway. Investigation into the mechanism of tumor-specific killing by Gal-3 revealed preferential binding to beta 1 integrin on the surface of tumor cells due to differential post-translational N-glycosylation resulting from the coordinated upregulation of several glycosyltransferases in the tumor cells. CONCLUSIONS: The bystander apoptosis effect defined here is significant because it represents a novel paracrine tumor suppressive mechanism for p53, which increases our understanding of p53 function and has therapeutic implications. It is important for the optimization of chemo- and radiotherapies as well as reactivation of p53 through small molecule or gene therapies. These data also suggest that Gal-3 has potential as an anti-cancer therapeutic. This work was supported by NIH R01 CA163722. SECONDARY CATEGORY: n/a.