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

Abstract

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.

MDA-9/Syntenin (SDCBP) Is a Critical Regulator of Chemoresistance, Survival and Stemness in Prostate Cancer Stem Cells

“…We now demonstrate that MDA-9 may be such a crucial element, which has a central regulatory role in the survival and maintenance as well as chemoresistance of PCSCs. MDA-9 physically interacts with IGF-1R, thereby regulating STAT3 phosphorylation at Tyr-705 [46,68]. By regulating STAT3 and C-myc, MDA-9 contributes to the observed resistance to docetaxel and trichostatin A (Figures 3 and 4, Supplementary Figures S2, S3 and S5).…”

“…mda-9 silencing with intra-tumoral injection of shmda-9 virus in subcutaneous DU-145 PCSC-derived tumors also resulted in smaller tumors, with decreased tumor growth kinetics and PCSC populations in the intra-tumoral treated groups ( Figure 2C,D). The PCSCs are grown in anoikis conditions in vitro, and these cells are sensitive to anoikis post mda-9 kd [46]. This may be the reason why pretreated PCSCs did not form tumors, but intra-tumoral mda-9 kd PCSCs were able to form small tumors.…”

“…2 × 10 5 cells were infected with either Ad.5/3.shcon or Ad.5/3.shmda-9. The cells were transfected post-infection with an RBPJK-responsive luciferase reporter construct using Lipofectamine 2000 [46]. Cells were lysed and the resultant relative luciferase activity was measured using a Dual-Luciferase Reporter Assay system (Promega, Madison, WI, USA) (manufacturer's instructions).…”

“…MDA-9 is a widely distributed cytosolic protein that interacts with a gamut of crucial regulatory proteins, such as SRC, FAK and EGFR, through its amino, carboxy-and PDZ domains, thereby contributing significantly to cancer evolution [42][43][44][45][46]. MDA-9 plays a pivotal role in cancer progression and recent studies indicated that it can serve as a diagnostic marker of tumor aggression and grade in several cancer types [42][43][44]47,48].…”

Cancer stem cells (CSCs) in cancer progression and therapy

“…Hypoxia through activation of autophagy process in CSCs is able to compensate ATP deficiency, to provide metabolic nutrients, to promote survival in CSCs, and to upregulate stemness genes (Dianat-Moghadam et al, 2018). The protective autophagy process in CSCs is for promoting their resistance to anoikis that is considered as a form of programmed cell death occurring after detachment of anchorage-dependent cells from the ECM (Talukdar et al, 2018a). Autophagy in CSCs can be targeted by either inhibition of this process (Baquero et al, 2018) or by its overactivation (Talukdar et al, 2018b).…”