bims-apauto Biomed News
on Apoptosis and autophagy
Issue of 2021‒10‒31
six papers selected by
Su Hyun Lee
Seoul National University

  1. Cell Death Dis. 2021 Oct 25. 12(11): 997
      The autophagy-lysosome pathway and apoptosis constitute vital determinants of cell fate and engage in a complex interplay in both physiological and pathological conditions. Central to this interplay is the archetypal autophagic cargo adaptor p62/SQSTM1/Sequestosome-1 which mediates both cell survival and endoplasmic reticulum stress-induced apoptosis via aggregation of ubiquitinated caspase-8. Here, we investigated the role of p62-mediated apoptosis in head and neck squamous cell carcinoma (HNSCC), which can be divided into two groups based on human papillomavirus (HPV) infection status. We show that increased autophagic flux and defective apoptosis are associated with radioresistance in HPV(-) HNSCC, whereas HPV(+) HNSCC fail to induce autophagic flux and readily undergo apoptotic cell death upon radiation treatments. The degree of radioresistance and tumor progression of HPV(-) HNSCC respectively correlated with autophagic activity and cytosolic levels of p62. Pharmacological activation of the p62-ZZ domain using small molecule ligands sensitized radioresistant HPV(-) HNSCC cells to ionizing radiation by facilitating p62 self-polymerization and sequestration of cargoes leading to apoptosis. The self-polymerizing activity of p62 was identified as the essential mechanism by which ubiquitinated caspase-8 is sequestered into aggresome-like structures, without which irradiation fails to induce apoptosis in HNSCC. Our results suggest that harnessing p62-dependent sequestration of ubiquitinated caspase-8 provides a novel therapeutic avenue in patients with radioresistant tumors.
  2. Cell Mol Life Sci. 2021 Oct 30.
      Lysosomes are single membrane-bound organelles containing acid hydrolases responsible for the degradation of cellular cargo and maintenance of cellular homeostasis. Lysosomes could originate from pre-existing endolysosomes or autolysosomes, acting as a critical juncture between autophagy and endocytosis. Stress that triggers lysosomal membrane permeabilization can be altered by ESCRT complexes; however, irreparable damage to the membrane results in the induction of a selective lysosomal degradation pathway, specifically lysophagy. Lysosomes play an indispensable role in different types of autophagy, including microautophagy, macroautophagy, and chaperone-mediated autophagy, and various cell death pathways such as lysosomal cell death, apoptotic cell death, and autophagic cell death. In this review, we discuss lysosomal reformation, maintenance, and degradation pathways following the involvement of the lysosome in autophagy and cell death, which are related to several pathophysiological conditions observed in humans.
    Keywords:  Autolysosome; Autophagic cell death; Autophagic lysosome reformation; Autophagy; Lysosome
  3. EMBO J. 2021 Oct 26. e103718
      Mixed lineage kinase domain-like (MLKL) is the executioner in the caspase-independent form of programmed cell death called necroptosis. Receptor-interacting serine/threonine protein kinase 3 (RIPK3) phosphorylates MLKL, triggering MLKL oligomerization, membrane translocation and membrane disruption. MLKL also undergoes ubiquitylation during necroptosis, yet neither the mechanism nor the significance of this event has been demonstrated. Here, we show that necroptosis-specific multi-mono-ubiquitylation of MLKL occurs following its activation and oligomerization. Ubiquitylated MLKL accumulates in a digitonin-insoluble cell fraction comprising organellar and plasma membranes and protein aggregates. Appearance of this ubiquitylated MLKL form can be reduced by expression of a plasma membrane-located deubiquitylating enzyme. Oligomerization-induced MLKL ubiquitylation occurs on at least four separate lysine residues and correlates with its proteasome- and lysosome-dependent turnover. Using a MLKL-DUB fusion strategy, we show that constitutive removal of ubiquitin from MLKL licences MLKL auto-activation independent of necroptosis signalling in mouse and human cells. Therefore, in addition to the role of ubiquitylation in the kinetic regulation of MLKL-induced death following an exogenous necroptotic stimulus, it also contributes to restraining basal levels of activated MLKL to avoid unwanted cell death.
    Keywords:  DUB-fusion; MLKL; membranes; necroptosis; ubiquitylation
  4. Autophagy. 2021 Oct 27.
      Mice deficient for GHR (growth hormone receptor; ghr KO) have a dramatic lifespan extension, and elevated levels of hepatic chaperone-mediated autophagy (CMA). Using quantitative proteomics to identify protein changes in purified liver lysosomes and whole liver lysates, we provide evidence that elevated CMA in ghr KO mice downregulates proteins involved in ribosomal structure, translation initiation and elongation, and nucleocytosolic acetyl-coA production. Following up on these initial proteomics findings, we used a cell culture approach to show that CMA is necessary and sufficient to regulate the abundance of ACLY and ACSS2, the two enzymes that produce nucleocytosolic (but not mitochondrial) acetyl-coA. Inhibition of CMA in NIH3T3 cells has been shown to lead to aberrant accumulation of lipid droplets. We show that this lipid droplet phenotype is rescued by knocking down ACLY or ACSS2, suggesting that CMA regulates lipid droplet formation by controlling ACLY and ACSS2. This evidence leads to a model of how constitutive activation of CMA can shape specific metabolic pathways in long-lived endocrine mutant mice.
    Keywords:  aging; autophagy; growth hormone; metabolism; proteomics
  5. Mol Cancer. 2021 Oct 27. 20(1): 140
      Autophagy is best known for its role in organelle and protein turnover, cell quality control, and metabolism. The autophagic machinery has, however, also adapted to enable protein trafficking and unconventional secretory pathways so that organelles (such as autophagosomes and multivesicular bodies) delivering cargo to lysosomes for degradation can change their mission from fusion with lysosomes to fusion with the plasma membrane, followed by secretion of the cargo from the cell. Some factors with key signalling functions do not enter the conventional secretory pathway but can be secreted in an autophagy-mediated manner.Positive clinical results of some autophagy inhibitors are encouraging. Nevertheless, it is becoming clear that autophagy inhibition, even within the same cancer type, can affect cancer progression differently. Even next-generation inhibitors of autophagy can have significant non-specific effects, such as impacts on endosome-related secretory pathways and secretion of extracellular vesicles (EVs). Many studies suggest that cancer cells release higher amounts of EVs compared to non-malignant cells, which makes the effect of autophagy inhibitors on EVs secretion highly important and attractive for anticancer therapy. In this review article, we discuss how different inhibitors of autophagy may influence the secretion of EVs and summarize the non-specific effects of autophagy inhibitors with a focus on endosome-related secretory pathways. Modulation of autophagy significantly impacts not only the quantity of EVs but also their content, which can have a deep impact on the resulting pro-tumourigenic or anticancer effect of autophagy inhibitors used in the antineoplastic treatment of solid cancers.
    Keywords:  Amphisomes; Autophagy; Autophagy inhibitors; Cancer; Endosomes; Exosomes; Extracellular vesicles; Multivesicular bodies; Non-conventional secretory pathways
  6. Cell Rep. 2021 Oct 26. pii: S2211-1247(21)01369-3. [Epub ahead of print]37(4): 109899
      Although commonly associated with autophagosomes, LC3 can also be recruited to membranes by covalent lipidation in a variety of non-canonical contexts. These include responses to ionophores such as the M2 proton channel of influenza A virus. We report a subtractive CRISPR screen that identifies factors required for non-canonical LC3 lipidation. As well as the enzyme complexes directly responsible for LC3 lipidation in all contexts, we show the RALGAP complex is important for M2-induced, but not ionophore drug-induced, LC3 lipidation. In contrast, ATG4D is responsible for LC3 recycling in M2-induced and basal LC3 lipidation. Identification of a vacuolar ATPase subunit in the screen suggests a common mechanism for non-canonical LC3 recruitment. Influenza-induced and ionophore drug-induced LC3 lipidation lead to association of the vacuolar ATPase and ATG16L1 and can be antagonized by Salmonella SopF. LC3 recruitment to erroneously neutral compartments may therefore represent a response to damage caused by diverse invasive pathogens.
    Keywords:  ATG16L1; ATG4D; RALGAP; autophagy; influenza; v-ATPase