M2 Muscarinic Receptor Stimulation Induces Autophagy in Human Glioblastoma Cancer Stem Cells via mTOR Complex-1 Inhibition

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Abstract
Background: Although autophagy is a pro-survival process of tumor cells, in particular conditions and when differently regulated by specific signals it can stimulate cell death. We previously demonstrated that the selective stimulation of the M2 muscarinic receptor subtype (M2 mAChR) negatively controls cell proliferation and survival and causes oxidative stress and cytotoxic and genotoxic effects in both GBM cell lines and GBM stem cells (GSCs). In this work, we have evaluated whether autophagy was induced as a downstream mechanism of the observed cytotoxic processes induced by M2 mAChR activation by the orthosteric agonist APE or the dualsteric agonist N-8-Iper. Methods: To assess the activation of autophagy, we analyzed the expression of LC3B by Western blot analysis and in LC3B-EGFP transfected cell lines. Apoptosis was assessed by Caspases 3 and 9 protein expression. Results: Our data indicate that activation of M2 mAChR by N-8-Iper promotes autophagy in both U251 and GB7 cells lines as suggested by the LC3B-II expression level and analysis of the transfected cells by fluorescence microscopy. Autophagy induction by M2 mAChRs is regulated by the decreased activity of the PI3K/AKT/mTORC1 pathway and upregulated by the pAMPK expression. Downstream autophagy activation, the increase of apoptosis was also observed in both cell lines after treatment with the two M2 agonists. Conclusions: N-8-Iper treatment causes autophagy via pAMPK upregulation, followed by apoptosis in both investigated cell lines. In contrast, the absence of autophagy in APE-treated GSC cells seems to indicate that cell death could be triggered by mechanisms alternative to those observed for N-8-Iper.

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