Autophagy: an adaptive physiological countermeasure to cellular senescence and ischaemia/reperfusion-associated cardiac arrhythmias

Abstract

Oxidative stress placed on tissues that involved in pathogenesis of a disease activates compensatory metabolic changes, such as DNA damage repair that in turn causes intracellular accumulation of detritus and 'proteotoxic stress', leading to emergence of 'senescent' cellular phenotypes, which express high levels of inflammatory mediators, resulting in degradation of tissue function. Proteotoxic stress resulting from hyperactive inflammation following reperfusion of ischaemic tissue causes accumulation of proteinaceous debris in cells of the heart in ways that cause potentially fatal arrhythmias, in particular ventricular fibrillation (VF). An adaptive response to VF is occurrence of autophagy, an intracellular bulk degradation of damaged macromolecules and organelles that may restore cellular and tissue homoeostasis, improving chances for recovery. Nevertheless, depending on the type and intensity of stressors and inflammatory responses, autophagy may become pathological, resulting in excessive cell death. The present review examines the multilayered defences that cells have evolved to reduce proteotoxic stress by degradation of potentially toxic material beginning with endoplasmic reticulum-associated degradation, and the unfolded protein response, which are mechanisms for removal from the endoplasmic reticulum of misfolded proteins, and then progressing through the stages of autophagy, including descriptions of autophagosomes and related vesicular structures which process material for degradation and autophagy-associated proteins including Beclin-1 and regulatory complexes. The physiological roles of each mode of proteotoxic defence will be examined along with consideration of how emerging understanding of autophagy, along with a newly discovered regulatory cell type called telocytes, may be used to augment existing strategies for the prevention and management of cardiovascular disease.

Keywords: autophagy; cardiovascular system; proteotoxic stress.

Figure 1

Progression of major macromolecular events in autophagic process. mTORC‐mediated repression of autophagy is relieved by decreased nutrient availability and pro‐autophagic downstream signalling thorough ULK1 and Beclin1–PI3KC3. The lipid mediator PI3P produced by Beclin1–PI3KC3 triggers WIP12/DFCP1‐mediated assembly of the Atg12– Atg5– Atg16 complex, in turn stimulating Atg8 (LC3)‐induced progression of the degradation of targeted materials.

Figure 2

Endoplasmic reticulum‐associated degradation (ERAD) and unfolded protein responses (UPR). Nascent polypeptides emerging into the ER as they are translated from their associated ribosomes undergo folding through linkage of cysteine residues by protein disulphide isomerases (PDIs). N‐terminal glycosylation forms binding sites for calnexin (CNX) and calreticulin (CRT). These chaperones along with Grp78 (BiP) direct misfolded proteins into retrotranslocation out of the ER, into degradation via the ubiquitin–proteasome pathway. ERAD constitutes a cell's first‐line countermeasure to the accumulation of potentially harmful metabolic detritus. Misfolded proteins displace GRP78/BiP from PERK, IRE1 and ATF6, thereby activating each factor. The activation of IRE1 mediates XBP1 splicing (sXBP1) and enhances lipids, chaperone ERAD proteins and also promotes autophagy. PERK activation globally reduces protein synthesis via eIF2a phosphorylation, and ATF6 is cleaved to nATF6, promoting expression of a variety of UPR target genes.

Figure 3

Regulation of and by the ULK1 complex during autophagy activation. Amino acid, glucose and serum starvation activate the ULK1 complex via separate signalling pathways. Autophosphorylation of ULK1 may maintain it in a conformation that is favourable for autophagy. Adapted from Wirth M, Joachim J, Tooze S. Autophagosome formation—The role of ULK1 and Beclin1–PI3KC3 complexes in setting the stage. Seminars in Cancer Biology 23 (2013) 301–309.

Figure 4

LC3B content in VF and non‐VF post‐IR heart tissue. (A) Cardiac LC3B‐I expression evaluated by Western immunoblotting and expressed relative to GAPDH content; (B) cardiac LC3B‐I expression evaluated by Western immunoblotting and expressed relative to GAPDH content. (C) Cardiac LC3B‐II/LC3B‐I ratio evaluated by Western immunoblotting (data are presented as n = 4 per group, *P < 0.05 No VF versus VF hearts.) Adapted from Meyer G, Czompa A, Reboul C, Csepanyi E, Czegledi A, Bak I, Balla G, Balla J, Tosaki A, Lekli I. The cellular autophagy markers Beclin‐1 and LC3B‐II are increased during reperfusion in fibrillated mouse hearts. Curr Pharm Des. 2013; 19 (39): 6912–8.