Targeting cancer stem cells in medulloblastoma by inhibiting AMBRA1 dual function in autophagy and STAT3 signalling

Medulloblastoma (MB) is a childhood malignant brain tumour comprising four main subgroups characterized by different genetic alterations and rate of mortality. Among MB subgroups, patients with enhanced levels of the c-MYC oncogene (MBGroup3) have the poorest prognosis. Here we identified a previously unrecognized role of the pro-autophagy factor AMBRA1 in regulating MB. We demonstrated that AMBRA1 expression levels strongly depend on c-MYC expression and correlate with patient poor prognosis; also, knockdown of AMBRA1 reduces MB stem potential, growth and migration of MBGroup3 cells in vitro and in vivo. At a molecular level, AMBRA1 mediates these effects by suppressing SOCS3, an inhibitor of STAT3 activation. In turn, active STAT3 increases c-MYC expression that, in a positive feedback loop, sustains AMBRA1 transcription. Importantly, pharmacological inhibition of autophagy profoundly affects stem potential and metastasization of MBGroup3 cells in vitro and in vivo, and a combined anti-autophagy and anti-STAT3 approach impacts MBGroup3 cell survival. Taken together, our data identified the c-MYC/AMBRA1/STAT3 axis as a strong oncogenic signalling pathway with significance for both patient stratification strategies and targeted treatments of MBGroup3. Characterization of the global gene expression changes induced by AMBRA1 or by ATG7 siRNA-mediated downregulation