bims-apauto Biomed News
on Apoptosis and autophagy
Issue of 2021‒02‒07
ten papers selected by
Su Hyun Lee
Seoul National University

  1. Autophagy. 2021 Feb 02.
      Macroautophagy is a catabolic process critical for the degradation of intracellular material, but its physiological functions in vertebrates are not fully understood. Here, we discuss our recent finding that macroautophagy plays a role in lamellar body maturation. The lamellar body is a lysosome-related organelle and stores phospholipid-containing surfactant complexes that reduce the surface tension of the air-water interface in order to inflate the airspace in lungs and swim bladders. In the epithelial cells of these organs, autophagosomes fuse with immature lamellar bodies to increase their size and lipid contents. This function is essential for respiration after birth in mice and for maintaining buoyancy in zebrafish. These findings unveil a novel function of macroautophagy in the maturation of surfactant-containing lamellar bodies.
    Keywords:  autophagy; lamellar body; lung; lysosome-related organelle; swim bladder; zebrafish
  2. Mol Cell. 2021 Jan 28. pii: S1097-2765(21)00009-5. [Epub ahead of print]
      Autophagy deficiency in fed conditions leads to the formation of protein inclusions highlighting the contribution of this lysosomal delivery route to cellular proteostasis. Selective autophagy pathways exist that clear accumulated and aggregated ubiquitinated proteins. Receptors for this type of autophagy (aggrephagy) include p62, NBR1, TOLLIP, and OPTN, which possess LC3-interacting regions and ubiquitin-binding domains (UBDs), thus working as a bridge between LC3/GABARAP proteins and ubiquitinated substrates. However, the identity of aggrephagy substrates and the redundancy of aggrephagy and related UBD-containing receptors remains elusive. Here, we combined proximity labeling and organelle enrichment with quantitative proteomics to systematically map the autophagic degradome targeted by UBD-containing receptors under basal and proteostasis-challenging conditions in human cell lines. We identified various autophagy substrates, some of which were differentially engulfed by autophagosomal and endosomal membranes via p62 and TOLLIP, respectively. Overall, this resource will allow dissection of the proteostasis contribution of autophagy to numerous individual proteins.
    Keywords:  APEX2; SQSTM1/p62; TOLLIP; aggrephagy; autophagosomes; autophagy; endosomal microautophagy; proteostasis imbalance; proximity labeling; selective autophagy receptors
  3. Life Sci. 2021 Feb 01. pii: S0024-3205(21)00124-7. [Epub ahead of print] 119139
      AIMS: Complicated mechanisms in cancer cells have been restricting the medicinal value of resveratrol (Res). The mechanisms by which Res exerts its anti-tumor activity in lung cancer cells have diverged among reports in recent years, whether cells choose to undergo autophagic cell death or apoptosis remains controversial. Yet, whether Res-induced autophagic cell death transforms into apoptosis is still unknown, and by which autophagy regulates programmed cell death is still undefined.MAIN METHODS: Here, A549 cells were treated with Res to investigate the mechanisms of autophagy and apoptosis using western blot, immunofluorescence staining for LC3B.
    KEY FINDINGS: Non-canonical autophagy was induced by Res-treatment in a Beclin-1- and ATG5-independent manner, with apoptosis being activated simultaneously. Autophagy induced by Res was activated by rapamycin with decreased apoptosis, suggesting that autophagy may serve as a protective pathway in cells. Mitophagy was found to be induced by Res using fluorescence co-localization of mitochondria with lysosomes. Subsequently, it was identified that mitophagy was mediated by LC3B/p62 interaction and could be inhibited by LC3B knockout and p62 knockdown following increased apoptosis.
    SIGNIFICANCE: In conclusion, the current results demonstrate that Res-induced non-canonical autophagy in A549 lung cancer cells with apoptosis activation simultaneously, while LC3B/p62-mediated mitophagy protects tumor cells against apoptosis, providing novel mechanisms about the critical role of mitophagy in regulating cell fate.
    Keywords:  A549; Apoptosis; Autophagy; LC3B; Mitophagy; Resveratrol; p62
  4. J Int Med Res. 2021 Feb;49(2): 300060520986355
      OBJECTIVE: Accumulating evidence illustrates that sirtuins (SIRTs) regulate autophagy and apoptosis in cancer cells; however, the role of SIRT5 in gastric cancer (GC) cells remains unknown. In this study, we examined the role of SIRT5 in GC cells.METHODS: We detected SIRT5 protein levels in freshly collected samples from patients with GC. Next, we studied the function of SIRT5 in autophagy. Furthermore, the signaling pathway through which SIRT5 enhanced autophagy in GC cells was detected. In addition, we established a GC cell apoptosis model to analyze the role of SIRT5 in apoptosis.
    RESULTS: SIRT5 expression was downregulated in GC tissues. We discovered that SIRT5 promoted autophagy in GC cells. We demonstrated that SIRT5 enhanced autophagy in GC cells via the AMP-activated protein kinase-mammalian target of rapamycin signaling pathway. In addition, SIRT5 was degraded during apoptosis in GC cells. Meanwhile, we observed that calpains and caspase-related proteins were associated with SIRT5-related GC cell apoptosis.
    CONCLUSIONS: SIRT5 is a crucial regulator of autophagy and apoptosis in GC cell lines that can maintain the balance of autophagy and apoptosis.
    Keywords:  AMP-activated protein kinase; Sirtuin 5; apoptosis; autophagy; calpain; caspase; gastric cancer; mammalian target of rapamycin
  5. FASEB J. 2021 Feb;35(2): e21361
      Bcl-2-associated athanogen-6 (BAG6) is a nucleocytoplasmic shuttling protein involved in protein quality control. We previously demonstrated that BAG6 is essential for autophagy by regulating the intracellular localization of the acetyltransferase EP300, and thus, modifying accessibility to its substrates (TP53 in the nucleus and autophagy-related proteins in the cytoplasm). Here, we investigated BAG6 localization and function in the cytoplasm. First, we demonstrated that BAG6 is localized in the mitochondria. Specifically, BAG6 is expressed in the mitochondrial matrix under basal conditions, and translocates to the outer mitochondrial membrane after mitochondrial depolarization with carbonyl cyanide m-chlorophenyl hydrazine, a mitochondrial uncoupler that induces mitophagy. In SW480 cells, the deletion of BAG6 expression abrogates its ability to induce mitophagy and PINK1 accumulation. On the reverse, its ectopic expression in LoVo colon cancer cells, which do not express endogenous BAG6, reduces the size of the mitochondria, induces mitophagy, leads to the activation of the PINK1/PARKIN pathway and to the phospho-ubiquitination of mitochondrial proteins. Finally, BAG6 contains two LIR (LC3-interacting Region) domains specifically found in receptors for selective autophagy and responsible for the interaction with LC3 and for autophagosome selectivity. Site-directed mutagenesis showed that BAG6 requires wild-type LIRs domains for its ability to stimulate mitophagy. In conclusion, we propose that BAG6 is a novel mitophagy receptor or adaptor that induces PINK1/PARKIN signaling and mitophagy in a LIR-dependent manner.
    Keywords:  BAG6; mitophagy; receptor; signaling
  6. Signal Transduct Target Ther. 2021 Feb 03. 6(1): 49
      Ferroptosis is an iron-dependent cell death, which is different from apoptosis, necrosis, autophagy, and other forms of cell death. The process of ferroptotic cell death is defined by the accumulation of lethal lipid species derived from the peroxidation of lipids, which can be prevented by iron chelators (e.g., deferiprone, deferoxamine) and small lipophilic antioxidants (e.g., ferrostatin, liproxstatin). This review summarizes current knowledge about the regulatory mechanism of ferroptosis and its association with several pathways, including iron, lipid, and cysteine metabolism. We have further discussed the contribution of ferroptosis to the pathogenesis of several diseases such as cancer, ischemia/reperfusion, and various neurodegenerative diseases (e.g., Alzheimer's disease and Parkinson's disease), and evaluated the therapeutic applications of ferroptosis inhibitors in clinics.
  7. FEBS J. 2021 Feb 06.
      Mutations in OPTN are associated with glaucoma, an eye disease, and also with amyotrophic lateral sclerosis (ALS), a motor neuron disease. A 2bp insertion in OPTN (691_692insAG or 2bpIns-OPTN) is associated with both glaucoma and ALS. This mutation results in frameshift after 127 amino acids, giving rise to a protein with C-terminal aberrant sequence. We have explored the mechanism of induction of cell death by this mutant in a motor neuron cell line, NSC-34, and also in a retinal cell line, 661W. Compared to wild type OPTN, this mutant induced more cell death in NSC-34 and 661W cells. This mutant localizes predominantly in the nucleus whereas normal OPTN localizes in the cytoplasm. Deletion analysis of 2bpIns-OPTN showed that the aberrant sequence was not essential for cell death induction. This mutant interacts with Tbk1 but not with OPTN and activates Tbk1. This mutant induced ER stress in NSC-34 cells as seen by induction of CHOP and some other genes. Induction of CHOP, autophagosomal protein LC3-II and cell death by this mutant were abrogated by Tbk1 knockdown, and also by 4-phenylbutyric acid, that inhibits ER stress. Induction of CHOP and cell death by 2bpIns-OPTN was autophagy dependent as shown by the effect of Atg5 knockdown. This mutant caused increased formation of LC3-positive aggregates. Treatment of cells with autophagy inducer rapamycin reduced LC3-positive aggregates, CHOP and cell death induced by 2bpIns-OPTN. These results suggest that constitutive activation of Tbk1 by 2bpIns-OPTN leads to impaired autophagy that results in ER stress and cell death.
    Keywords:  Amyotrophic lateral sclerosis; ER stress; Glaucoma; Optineurin; Tbk1
  8. J Cell Biochem. 2021 Jan 31.
      Recent advances in the yeast Saccharomyces cerevisiae and higher eukaryotes have been increasingly connecting lipid droplet (LD) dynamics to the regulation of autophagy. In this review we will discuss implications that connect LD de novo synthesis and LD mobilization to autophagy and how autophagy is regulated by these mechanisms. Elucidating these connections might pose a chance to further understand autophagy induction and membrane biogenesis for the growing autophagosome under different conditions. Increasing our understanding of these mechanisms might provide a chance to understand several conditions that might be related to LD dysregulation and, possibly, as a consequence of this, dysregulation of autophagy.
    Keywords:  autophagy; lipid droplets; lipolysis; lipophagy; mammals; yeast
  9. Pancreatology. 2021 Jan 22. pii: S1424-3903(21)00034-X. [Epub ahead of print]
      Pancreatic cancer is the fourth most common cause of cancer-associated death in western countries, where the incidence and number of deaths are increasing every year. Intrinsic or acquired resistance of tumor cells to chemotherapy agents is the major reason for failure of traditional cancer treatment. Several factors are implicated in this impressive resistance; however, of these, it is important to highlight the extensive cellular heterogeneity of these tumors. This heterogeneity is linked to a wide range of sensitivity that different clones in the same tumor display to chemotherapeutic agents. Accordingly, recent findings in this field have discovered new therapeutic targets in order to develop new combinatory treatments, as well as to induce several cell death pathways and reduce therapy-threshold and likelihood of future resistance. Accordingly, recent research has focused on targeting mitochondria, an organelle with key roles regulating cell death signaling pathways, such as apoptosis, necroptosis, autophagy, ferroptosis, or parthanatos. These findings - identifying new compounds, alone or in combination, that can target pancreatic ductal adenocarcinoma cell resistance - could be the key to future treatments.
    Keywords:  Apoptosis; Autophagy; Cell death; Chemoresistance; Ferroptosis; Immunogenic cell death; Necroptosis; Pancreas cancer; Parthanatos; Pyroptosis
  10. Cell Death Discov. 2021 Feb 01. 7(1): 26
      Increased sialylation is one of the hallmarks of ovarian cancer (OC) but its relation with programmed cell death is not known. Here we explored the molecular interplay between autophagy, apoptosis/anoikis, and aberrant-expression of the PI3K-Akt/mTOR pathway in the context of sialidase. OC is accompanied by low expression of cytosolic sialidase (Neu2) and ~10-fold more α2,6- than α2,3-linked sialic acids found through qPCR, western blot, and flow cytometry. Interestingly, Neu2 overexpression cleaved α2,6- and α2,3-linked sialic acids and reduced cell viability. Several autophagy-related molecules like LC3B/Atg3/Atg5/Atg7/Atg12/Atg16L1/Beclin1 were upregulated upon Neu2 overexpression. Atg5, a crucial protein for autophagosome formation, was desialylated by overexpressed Neu2. Desialylated Atg5 now showed enhanced association both with Atg12 and Atg16L1 leading to more autophagosome formation. Neu2-overexpressing cells exhibited extrinsic pathway-mediated apoptosis as reflected the in activation of Fas/FasL/FADD/Bid/caspase 8/caspase 6/caspase 3/PARP cleavage. There was also increased Bax, reduced Bcl2, and several cell-cycle molecules (CDK2/CDK4/CDK6/cyclin-B1/cyclin-E). Inhibition of autophagy using bafilomycin A1 or Beclin1 siRNA leads to reversal of Neu2-induced apoptosis suggesting their possible relationship. Additionally, overexpressed Neu2 inhibited growth factor-mediated signaling molecules involved in the PI3K/Akt-mTOR pathway probably through their desialylation. Furthermore, overexpressed Neu2 inhibited epithelial (ZO-1/Claudin1), mesenchymal (snail/slug), and cell-adhesion (integrin-β3/focal-adhesion kinase) molecules suggesting anchorage-dependent cell death (anoikis). Such changes were absent in the presence of bafilomycin A1 indicating the involvement of autophagy in Neu2-induced anoikis. The physiological relevance of our in vitro observations was further confirmed in the OC xenograft model. Taken together, it is the first report demonstrating that Atg5 is a sialoglycoprotein having α2,6- and α2,3-linked sialic acids and its desialylation by overexpressed Neu2 leads to its activation for autophagosome formation, which induced apoptosis/anoikis in OC.