Endoplasmic Reticulum Stress Triggers Autophagy in Malignant Glioma Cells Undergoing Cyclosporine A-induced Cell Death

Autophagy is a conserved, self-digestion process that is activated in response to nutrient limitation but acting also as an alternative death mechanism under certain conditions. It is accompanied by the progressive formation of vesicle structures from autophagosomes to autophagolysosomes orchestrated by autophagy effectors (Atg proteins) and modulators (that is, mTOR-mammalian target of rapamycin as a negative regulator). Malignant gliomas are highly resistant to current therapies that induce apoptosis, thus induction of the alternative cell death is an attractive strategy. We demonstrate that cyclosporine A (CsA, an immunophilin/calcineurin inhibitor) induces cell death with some apoptotic features but also accompanied by the appearance of numerous cytoplasmic vacuoles, immunostained for endoplasmic reticulum (ER) and autophagy markers. The induction of ER stress in glioma cells by CsA was evidenced by detection of unfolded protein response activation (phosphorylation of PERK, accumulation of IRE1α) and accumulation of ER stress-associated proteins (BIP and CHOP). Formation of the acidic vesicular organelles, increase of autophagic vacuoles, GFP-LC3 punctation (microtubule-associated protein light chain 3) and LC3-II accumulation upon CsA treatment confirmed activation of autophagy. Decrease of phosphorylation of 4E-BP1, p70S6K1 and its downstream target S6 ribosomal protein demonstrate inhibition of mTOR signaling by CsA. Salubrinal and silencing of PERK and IRE1α partially blocked CsA-induced accumulation of LC3-II. It suggests that ER stress precedes CsA-induced autophagy. Surprisingly, silencing of autophagy effectors ULK1, Atg5 or Atg7 increased the level of active caspases 3, 7 and PARP degradation in CsA-treated cells. Our results demonstrate that CsA induces both apoptosis and autophagy in malignant glioma cells via induction of ER stress and inhibition of mTOR/p70S6K1 pathway, however autophagy is cytoprotective in this context.