Loss of Macroautophagy Promotes or Prevents Fibroblast Apoptosis Depending on the Death Stimulus

Abstract

Macroautophagy has been implicated as a mechanism of cell death. However, the relationship between this degradative pathway and cell death is unclear as macroautophagy has been shown recently to protect against apoptosis. To better define the interplay between these two critical cellular processes, we determined whether inhibition of macroautophagy could have both pro-apoptotic and anti-apoptotic effects in the same cell. Embryonic fibroblasts from mice with a knock-out of the essential macroautophagy gene atg5 were treated with activators of the extrinsic and intrinsic death pathways. Loss of macroautophagy sensitized these cells to caspase-dependent apoptosis from the death receptor ligands Fas and tumor necrosis factor-␣ (TNF-␣). Atg5 ؊/؊ mouse embryonic fibroblasts had increased activation of the mitochondrial death pathway in response to Fas/TNF-␣ in concert with decreased ATP levels. Fas/TNF-␣ treatment failed to up-regulate macroautophagy, and in fact, decreased activity at late time points. In contrast to their sensitization to Fas/TNF-␣, Atg5؊/؊ cells were resistant to death from menadione and UV light. In the absence of macroautophagy, an up-regulation of chaperone-mediated autophagy induced resistance to these stressors. These results demonstrate that inhibition of macroautophagy can promote or prevent apoptosis in the same cell and that the response is governed by the nature of the death stimulus and compensatory changes in other forms of autophagy. Experimental findings that an inhibition of macroautophagy blocks apoptosis do not prove that autophagy mediates cell death as this effect may result from the protective up-regulation of other autophagic pathways such as chaperonemediated autophagy.The relationship between the cellular processes of macroautophagy, quantitatively the most important form of autophagy in mammalian cells, and cell death remains unclear (1). Experimental evidence from several model systems suggests that autophagy is a mechanism of cell death. These findings have led to the concept that programmed cell death can result from one of two distinct pathways, apoptosis or autophagy (2). These two forms of cell death can be distinguished by their morphological differences. Apoptosis is marked by cytoskeletal collapse that leads to cellular condensation and caspase-dependent DNA fragmentation but the relative preservation of organelles. In contrast, the critical initiating feature of autophagic cell death is not cytoskeletal breakdown but organelle degradation. Macroautophagy has been implicated as a mechanism of cell death from a variety of factors including radiation, tumor necrosis factor-␣ (TNF-␣), 3 viruses, and low potassium (3-6). This conclusion is supported by two findings: 1) the presence of increased numbers of autophagosomes in cells responding to these death stimuli, indicative of an up-regulation of macroautophagy that may kill the cell; and 2) the prevention of cell death by inhibition of macroautophagy with 3-methyladenine (3-MA) or RNA interference against autophagy-re...

DAP-kinase is a mediator of endoplasmic reticulum stress-induced caspase activation and autophagic cell death

“…3,15 In studies using mammalian cells in culture which were exposed to various ER stress-inducing drugs or polyglutamine aggregates, autophagy was alternatively described as either death protective or death promoting. 4,14,30,31 It has been proposed that autophagy contributes to cell death in nontransformed cells, while it serves to attenuate apoptotic cell death in transformed cancer cell lines. 4 Here, we found that the ER-induced autophagy in primary fibroblasts had prodeath characteristics.…”

Autophagy: A Crucial Moderator of Redox Balance, Inflammation, and Apoptosis in Lung Disease

“…In contrast, genetic knockdown of Atg5 was shown to sensitize hepatocytes to cell killing following exposure to menadione, associated with ATP depletion and activation of the c-Jun-NH 2 -terminal kinase ( JNK) pathway (181), whereas Atg5 -/ -embryo fibroblasts resisted menadione-induced apoptosis (161,180). The explanation for variable results observed between experimental models is not entirely clear but may include cell-type and inducer-specific effects, dosedependent effects, off-target activities of autophagy proteins and inhibitor compounds, and differential availability of compensatory mechanisms.…”

Regulation of lipid stores and metabolism by lipophagy

“…69 Although early evidence suggested that autophagy is a form of cell death, more recent investigations support a predominant if not exclusive function for autophagy as a pro-survival pathway. 70,71 The metabolism of lipids by lipophagy suggests several new mechanisms by which autophagy may act to prevent cell death. Potential cellprotective effects of lipophagy include the breakdown of lipids to supply energy to maintain cell function and prevent ATP depletion that leads to necrosis or apoptosis from mitochondrial dysfunction and the ability to promote the metabolism of potentially toxic lipid molecules (Figure 3).…”