bims-apauto Biomed News
on Apoptosis and autophagy
Issue of 2022‒02‒27
ten papers selected by
Su Hyun Lee
Seoul National University

  1. Autophagy. 2022 Feb 19. 1-2
      SQSTM1/p62 is an autophagy receptor, forming droplets to sequester intracellular polyubiquitinated cargo and mediate its delivery for autophagic clearance. SQSTM1 droplets can function as platforms to allow the formation of autophagosomes at their surfaces. It would be interesting to understand how SQSTM1-droplet formation is regulated. We have shown that inflammatory toxicity induces SQSTM1 cleavage by CASP6 at a novel cleavage site, D256. The C-terminal cleavage product is unlikely to be functional, because it is hardly detectable, possibly due to its rapid turnover. The SQSTM1 N-terminal cleavage product (SQSTM1-N) exerts a dominant-negative effect on SQSTM1-droplet production, in turn attenuating SQSTM1 droplets-based autophagosome formation. Our study suggests that the CASP6-SQSTM1 axis negatively regulates SQSTM1 droplets-based autophagy under certain stress conditions.
    Keywords:  Autophagosomes; CASP6; SQSTM1; autophagy; liquid droplets
  2. Autophagy. 2022 Feb 23. 1-3
      NLRP3 (NLR family pyrin domain containing 3) inflammasome is a potent mediator of inflammation due to its ability to produce the pro-inflammatory cytokines IL1B (interleukin 1 beta) and IL18 in response to numerous danger signals and pathogens. Mitophagy, a selective form of autophagy, restricts NLRP3 inflammasome activation by limiting the mitochondrial-derived danger signals. Here, we demonstrated that the adaptor protein APPL1 together with its interaction partner RAB5 in early endosomes negatively regulate NLRP3 inflammasome activation via induction of mitophagy in macrophages. Hematopoietic-deletion of Appl1 exacerbates systemic NLRP3 inflammasome activation in rodent models under obese or septic conditions. Our study identified a new regulatory network between early endosomes and mitochondria in control of NLRP3 inflammasome activation.
    Keywords:  APPL1; NLRP3 inflammasome; RAB5; early endosome; mitochondria; mitophagy
  3. Cell Rep. 2022 Feb 22. pii: S2211-1247(22)00070-5. [Epub ahead of print]38(8): 110354
      Excessive generation and accumulation of highly reactive oxidizing molecules causes oxidative stress and oxidative damage to cellular components. Accumulating evidence indicates that autophagy diminishes oxidative damage in cells and maintains redox homeostasis by degrading and recycling intracellular damaged components. Here, we show that TRAF6 E3 ubiquitin ligase and A20 deubiquitinase coordinate to regulate ATG9A ubiquitination and autophagy activation in cells responding to oxidative stress. The ROS-dependent TRAF6-mediated non-proteolytic, K48/63-linked ubiquitination of ATG9A enhances its association with Beclin 1 and the assembly of VPS34-UVRAG complex, thereby stimulating autophagy. Notably, expression of the ATG9A ubiquitination mutants impairs ROS-induced VPS34 activation and autophagy. We further find that lipopolysaccharide (LPS)-induced ROS production also stimulates TRAF6-mediated ATG9A ubiquitination. Ablation of ATG9A causes aberrant TLR4 endosomal trafficking and decreases IRF-3 phosphorylation in LPS-stimulated macrophages. Our findings provide important insights into how K48/K63-linked ubiquitination of ATG9A contributes to the regulation of oxidative stress-induced autophagy.
    Keywords:  ATG9A; TRAF6; VPS34 complex; autophagy; oxidative stress
  4. Autophagy. 2022 Feb 20. 1-12
      SQSTM1/p62-type selective macroautophagy/autophagy receptors cross-link poly-ubiquitinated cargo and autophagosomal LC3/Atg8 proteins to deliver them for lysosomal degradation. Consequently, loss of autophagy leads to accumulation of polyubiquitinated protein aggregates that are also frequently seen in various human diseases, but their physiological relevance is incompletely understood. Here, using a genetically non-redundant Drosophila model, we show that specific disruption of ubiquitinated protein autophagy and concomitant formation of polyubiquitinated aggregates has hardly any effect on bulk autophagy, proteasome activity and fly healthspan. We find that accumulation of ref(2)P/SQSTM1 due to a mutation that disrupts its binding to Atg8a results in the co-sequestering of Keap1 and thus activates the cnc/NFE2L2/Nrf2 antioxidant pathway. These mutant flies have increased tolerance to oxidative stress and reduced levels of aging-associated mitochondrial superoxide. Interestingly, ubiquitin overexpression in ref(2)P point mutants prevents the formation of large aggregates and restores the cargo recognition ability of ref(2)P, although it does not prevent the activation of antioxidant responses. Taken together, potential detrimental effects of impaired ubiquitinated protein autophagy are compensated by the aggregation-induced antioxidant response.
    Keywords:  Autophagic receptor; Drosophila; autophagy; longevity; oxidative stress
  5. RNA. 2022 Feb 24. pii: rna.079129.122. [Epub ahead of print]
      Cellular processes can be regulated at multiple levels, including transcriptional, post-transcriptional and post-translational mechanisms. We have recently shown that the small, non-coding vault RNA1-1 negatively riboregulates p62 oligomerisation in selective autophagy through direct interaction with the autophagic receptor. This function is highly specific for this Pol III transcript, but the determinants of this specificity and a mechanistic explanation of how vault RNA1-1 inhibits p62 oligomerisation are lacking. Here, we combine biochemical and functional experiments to answer these questions. We show that the PB1 domain and adjacent linker region of p62 (aa 1-122) are necessary and sufficient for specific vault RNA1-1 binding, and identify lysine 7 and arginine 21 as key hinges for p62 riboregulation. Chemical structure probing of vault RNA1-1 further reveals a central flexible loop within vault RNA1-1 that is required for the specific interaction with p62. Overall, our data provide molecular insight into how a small RNA riboregulates protein-protein interactions critical to the activation of specific autophagy.
    Keywords:  autophagy; p62; riboregulation; small ncRNA; vault RNA
  6. Clin Transl Oncol. 2022 Feb 26.
      Autophagy is a lysosomal degradation pathway that is constitutively active in almost every cell of our body at basal level. This self-eating process primarily serves to remove superfluous constituents of the cells and recycle the degraded products. Autophagy plays an essential role in cell homeostasis and can be enhanced in response to stressful conditions. Impairment in the regulation of the autophagic pathway is implicated in pathological conditions such as neurodegeneration, cardiac disorders, and cancer. However, the role of autophagy in cancer initiation and development is controversial and context-dependent. Evidence from various studies has shown that autophagy serves dual purpose and may assist in cancer progression or suppression. In the early stages of cancer initiation, autophagy acts as a quality control mechanism and prevents cancer development. When cancer is established and progresses to a later stage, autophagy helps in the survival of these cells through adaptation to stresses, including exposure to anti-cancer drugs. In this review, we highlight various studies on autophagic pathways and describe the role of autophagy in cancer, specifically acute myeloid leukemia (AML). We also discuss the prognostic significance of autophagy genes involved in AML leukemogenesis and implications in conferring resistance to chemotherapy.
    Keywords:  Acute myeloid leukemia; Autophagy; Autophagy-related protein (ATG); Drug resistance
  7. EMBO J. 2022 Feb 25. e108272
      Most cancer deaths result from progression of therapy resistant disease, yet our understanding of this phenotype is limited. Cancer therapies generate stress signals that act upon mitochondria to initiate apoptosis. Mitochondria isolated from neuroblastoma cells were exposed to tBid or Bim, death effectors activated by therapeutic stress. Multidrug-resistant tumor cells obtained from children at relapse had markedly attenuated Bak and Bax oligomerization and cytochrome c release (surrogates for apoptotic commitment) in comparison with patient-matched tumor cells obtained at diagnosis. Electron microscopy identified reduced ER-mitochondria-associated membranes (MAMs; ER-mitochondria contacts, ERMCs) in therapy-resistant cells, and genetically or biochemically reducing MAMs in therapy-sensitive tumors phenocopied resistance. MAMs serve as platforms to transfer Ca2+ and bioactive lipids to mitochondria. Reduced Ca2+ transfer was found in some but not all resistant cells, and inhibiting transfer did not attenuate apoptotic signaling. In contrast, reduced ceramide synthesis and transfer was common to resistant cells and its inhibition induced stress resistance. We identify ER-mitochondria-associated membranes as physiologic regulators of apoptosis via ceramide transfer and uncover a previously unrecognized mechanism for cancer multidrug resistance.
    Keywords:  ceramides; inter-organelle contacts; mitochondria-associated membranes; multidrug resistance; sphingolipids
  8. J Zhejiang Univ Sci B. 2022 Feb 15. pii: 1673-1581(2022)02-0089-13. [Epub ahead of print]23(2): 89-101
      Cancer is the leading cause of death worldwide. Drugs play a pivotal role in cancer treatment, but the complex biological processes of cancer cells seriously limit the efficacy of various anticancer drugs. Autophagy, a self-degradative system that maintains cellular homeostasis, universally operates under normal and stress conditions in cancer cells. The roles of autophagy in cancer treatment are still controversial because both stimulation and inhibition of autophagy have been reported to enhance the effects of anticancer drugs. Thus, the important question arises as to whether we should try to strengthen or suppress autophagy during cancer therapy. Currently, autophagy can be divided into four main forms according to its different functions during cancer treatment: cytoprotective (cell survival), cytotoxic (cell death), cytostatic (growth arrest), and nonprotective (no contribution to cell death or survival). In addition, various cell death modes, such as apoptosis, necrosis, ferroptosis, senescence, and mitotic catastrophe, all contribute to the anticancer effects of drugs. The interaction between autophagy and these cell death modes is complex and can lead to anticancer drugs having different or even completely opposite effects on treatment. Therefore, it is important to understand the underlying contexts in which autophagy inhibition or activation will be beneficial or detrimental. That is, appropriate therapeutic strategies should be adopted in light of the different functions of autophagy. This review provides an overview of recent insights into the evolving relationship between autophagy and cancer treatment.
    Keywords:  Autophagy; Cancer treatment; Cell death mode; Precision treatment
  9. Dis Model Mech. 2022 Feb 23. pii: dmm.049203. [Epub ahead of print]
      Breast Cancer Stem Cells (BCSCs) are a major cause of therapy resistance and tumour progression. Currently, their regulation is not entirely understood. Previous work from our lab demonstrated context-specific pro-tumorigenic role of AMP-activated protein kinase (AMPK) in breast cancer cell survival under anchorage-deprivation and mammosphere formation hallmarks of BCSCs. We therefore investigated the role of AMPK in the maintenance of BCSC state/function. AMPK depletion reduces serial sphere formation in vitro and tumour initiation in vivo. Intriguingly, tumour-derived cell analysis using stem cell markers and functional assays revealed that AMPK is required for the maintenance of BCSC population in vivo. AMPK promotes the expression of stemness genes like Nanog, Sox2 and Bmi1 through the transcriptional upregulation of Twist via promoter acetylation. Further, AMPK-driven stemness plays a critical role in resistance to doxorubicin. Significantly, we found that AMPK activity increased after chemotherapy in patient-derived tumour samples alongside an increase in stemness markers. Importantly, AMPK depletion sensitises mice tumours to doxorubicin treatment. Our work indicates that targeting AMPK in conjunction with regular chemotherapy is likely to reduce the stem cell pool and improve chemosensitivity in breast cancers.
    Keywords:  AMPK; Chemotherapy; Drug resistance; Stemness; Twist
  10. Discov Oncol. 2021 Dec 01. 12(1): 58
      In order to maintain a functional mitochondrial network, cells have developed a quality control mechanism, namely mitophagy. This process can be induced through different pathways. The most studied is the so-called PINK1/Parkin pathway, which is associated with ubiquitylation of several mitochondrial proteins that were initially found to be related to Parkinson's disease. Another type of mitophagy is known as receptor-mediated mitophagy, which includes proteins, such as BNIP3 and BNIP3L, also known as Nix. Through these two mechanisms, mitophagy fulfills its functions and maintains cellular homeostasis. Here, we summarize the current knowledge about the mechanisms of mitophagy regulation and their interplay with cancer progression as well as anticancer treatment.
    Keywords:  Autophagy; Cancer; Homeostasis; Mitophagy