MDA-9/Syntenin regulates protective autophagy in anoikis-resistant glioma stem cells

Abstract

<p id="d1897200e346">Gliomas exhibit high proportions of glioma stem cells (GSCs), anoikis resistance, increased brain parenchyma invasion, and resistance to therapy with high recurrence. GSCs display protective autophagy, a self-mediated lysosomal degradation process that balances sources of energy at critical times of stress. Protective autophagy in GSCs promotes resistance to anoikis, programmed death resulting from growth in an anchorage-independent manner. MDA-9 is critical in maintaining protective autophagy in GSCs, thereby contributing to anoikis-resistance. MDA-9 regulates critical molecules, including BCL2 and EGFR, which control autophagy. A link between MDA-9 and protective autophagy and anoikis-resistance identifies an Achilles’ heel of glioblastoma multiforme that may be exploited to define enhanced therapies with improved prognosis and decreased recurrence. Accordingly, targeting MDA-9 may represent a viable therapeutic strategy for glioblastoma multiforme. </p><p class="first" id="d1897200e349">Glioma stem cells (GSCs) comprise a small subpopulation of glioblastoma multiforme cells that contribute to therapy resistance, poor prognosis, and tumor recurrence. Protective autophagy promotes resistance of GSCs to anoikis, a form of programmed cell death occurring when anchorage-dependent cells detach from the extracellular matrix. In nonadherent conditions, GSCs display protective autophagy and anoikis-resistance, which correlates with expression of melanoma differentiation associated gene-9/Syntenin (MDA-9) (syndecan binding protein; SDCBP). When MDA-9 is suppressed, GSCs undergo autophagic death supporting the hypothesis that MDA-9 regulates protective autophagy in GSCs under anoikis conditions. MDA-9 maintains protective autophagy through phosphorylation of BCL2 and by suppressing high levels of autophagy through EGFR signaling. MDA-9 promotes these changes by modifying FAK and PKC signaling. Gain-of-function and loss-of-function genetic approaches demonstrate that MDA-9 regulates pEGFR and pBCL2 expression through FAK and pPKC. EGFR signaling inhibits autophagy markers (ATG5, Lamp1, LC3B), helping to maintain protective autophagy, and along with pBCL2 maintain survival of GSCs. In the absence of MDA-9, this protective mechanism is deregulated; EGFR no longer maintains protective autophagy, leading to highly elevated and sustained levels of autophagy and consequently decreased cell survival. In addition, pBCL2 is down-regulated in the absence of MDA-9, leading to cell death in GSCs under conditions of anoikis. Our studies confirm a functional link between MDA-9 expression and protective autophagy in GSCs and show that inhibition of MDA-9 reverses protective autophagy and induces anoikis and cell death in GSCs. </p>