ER stress (PERK/eIF2α phosphorylation) mediates the polyglutamine-induced LC3 conversion, an essential step for autophagy formation

Abstract

Expanded polyglutamine 72 repeat (polyQ72) aggregates induce endoplasmic reticulum (ER) stress-mediated cell death with caspase-12 activation and vesicular formation (autophagy). We examined this relationship and the molecular mechanism of autophagy formation. Rapamycin, a stimulator of autophagy, inhibited the polyQ72-induced cell death with caspase-12 activation. PolyQ72, but not polyQ11, stimulated Atg5-Atg12-Atg16 complex-dependent microtubule-associated protein 1 (MAP1) light chain 3 (LC3) conversion from LC3-I to -II, which plays a key role in autophagy. The eucaryotic translation initiation factor 2 a (eIF2a) A/A mutation, a knock-in to replace a phosphorylatable Ser 51 with Ala 51 , and dominant-negative PERK inhibited polyQ72-induced LC3 conversion. PolyQ72 as well as ER stress stimulators upregulated Atg12 mRNA and proteins via eIF2a phosphorylation. Furthermore, Atg5 deficiency as well as the eIF2a A/A mutation increased the number of cells showing polyQ72 aggregates and polyQ72-induced caspase-12 activation. Thus, autophagy formation is a cellular defense mechanism against polyQ72-induced ER-stress-mediated cell death by degrading polyQ72 aggregates, with PERK/eIF2a phosphorylation being involved in polyQ72-induced LC3 conversion.

Autophagy promotes necrosis in apoptosis-deficient cells in response to ER stress

“…When treated with ER stress, atg5 À/À cells died faster than atg5 þ / þ cells (Supplementary Figure S5D), consistent with recent reports that autophagy acts as a survival mechanism to suppress apoptosis in response to ER stress. 16,17 As anticipated, Bcl-xL protected both atg5 þ / þ and atg5 À/À cells from acute apoptosis. In response to prolonged ER stress the Bcl-xLoverexpressing cells progressively died.…”

Autophagy and ageing: Insights from invertebrate model organisms

“…3; Durieux et al, 2011). In mammals, accumulation of misfolded proteins in the ER induces phosphorylation of the translation initiation factor eIF2a by the PERK kinase (pancreatic endoplasmic reticulum eIF2a kinase), a sensor of misfolded proteins in the ER, and also stimulates autophagy (Kouroku et al, 2007). In addition to ER stress, eIF2a phosphorylation is induced by other stress stimuli, such as amino-acid deprivation (via the GCN2 kinase), and the presence of dsRNA (via the PKR kinase).…”

Extracellular matrix regulation of autophagy

“…In cells starved for nutrients or growth factors, autophagy produces critical nutrients and energy that enhance cell survival through the breakdown of cytosolic components. Recent studies demonstrate that other stresses, including hypoxia, growth factor withdrawal, and ER stress, also induce autophagy to enhance cell survival [10][11][12]. Although autophagy is classically recognized as an adaptive response, basal levels of autophagy are required to maintain homeostasis by preventing the build-up of damaged proteins and organelles.…”

“…Notably, ER stress, induced via chemicals or the expression of toxic polyglutamine repeat proteins, elicits autophagosome formation that depends on both PERK activation and eIF2α phosphorylation. [12]. Further studies using cells deficient in components of this pathway will elucidate if the ISR is responsible for autophagy induction during ECM detachment.…”