Inhibition of AMPK-dependent autophagy enhances in vitro antiglioma effect of simvastatin

Abstract

The role of autophagy, a process in which the cell self-digests its own components, was investigated in glioma cell death induced by the hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase-inhibiting drug simvastatin. Induction of autophagy and activation of autophagy-regulating signalling pathways were analyzed by immunoblotting. Flow cytometry/fluorescent microscopy was used to assess autophagy-associated intracellular acidification and apoptotic markers (phosphatidylserine exposure, DNA fragmentation and caspase activation). Cell viability was determined by crystal violet, MTT or LDH release assay. Simvastatin treatment of U251 and C6 glioma cell lines caused the appearance of autophagolysosome-like intracytoplasmic acidic vesicles. The induction of autophagy in U251 cells was confirmed by the upregulation of autophagosome-associated LC3-II and pro-autophagic beclin-1, as well as by the downregulation of the selective autophagic target p62. Simvastatin induced the activation of AMP-activated protein kinase (AMPK) and its target Raptor, while simultaneously downregulating activation of Akt. Mammalian target of rapamycin (mTOR), a major AMPK/Akt downstream target and a major negative autophagy regulator, and its substrate p70 S6 kinase 1 were also inhibited by simvastatin. Mevalonate, the product of HMG-CoA reductase enzymatic activity, AMPK siRNA or pharmacological inactivation of AMPK with compound C suppressed, while the inhibitors of Akt (10-DEBC hydrochloride) and mTOR (rapamycin) mimicked autophagy induction by simvastatin. Inhibition of autophagy with bafilomycin A1, 3-methyladenine and LC3β shRNA, as well as AMPK inhibition with compound C or AMPK siRNA, markedly increased apoptotic death of simvastatin-treated U251 cells. These data suggest that inhibition of AMPK-dependent autophagic response might sensitize glioma cells to statin-induced apoptotic death.