Hsc70/Stub1 promotes the removal of individual oxidatively stressed peroxisomes

ZLDI-8 facilitates pexophagy by ROS-mediated activation of TFEB and ATM in HeLa cells

Autophagy-mediated organelle quality control is vital for cellular homeostasis. However, the mechanisms underlying selective autophagy of peroxisomes, known as pexophagy, are less well understood than those of other organelles, such as mitochondria. In this study, we screened a phosphatase inhibitor library using a cell-based system and identified several potent pexophagy inducers, including ZLDI-8, a known inhibitor of lymphoid-specific tyrosine phosphatase. Notably, treatment with ZLDI-8 selectively induces the loss of peroxisomes without affecting other organelles, such as mitochondria, the endoplasmic reticulum, or the Golgi apparatus. The peroxisome loss induced by ZLDI-8 was significantly blocked in ATG5-knockout HeLa cells, confirming its dependence on autophagy. We further found that ZLDI-8 treatment increases both cellular and peroxisomal reactive oxygen species (ROS), which were effectively scavenged by N-acetylcysteine (NAC). The increase in peroxisomal ROS leads to the activation of ATM kinase and the dephosphorylation of TFEB. Moreover, ROS scavenging prevents all of these processes. Taken together, these findings demonstrate that ZLDI-8 induces pexophagy through a mechanism involving peroxisomal ROS-mediated activation of TFEB and ATM. This study provides valuable insights into the molecular mechanisms regulating selective peroxisome degradation and potential therapeutic strategies for targeting pexophagy.

E3 ubiquitin ligase CHIP alleviates H/R-induced myocardial injury by inhibiting pyroptosis

BACKGROUND

Hypoxia/Reoxygenation (H/R) injury to cardiomyocytes has adverse effects on the function, structure and prognosis of the heart. Studies have shown that H/R injury is closely related to pyroptosis. The inflammatory response induced by pyroptosis, leading to the death of cardiomyocytes. However, the specific mechanism of pyroptosis in myocardial H/R injury is not fully understood. In recent years, the important role of CHIP proteins in cardiovascular diseases has gradually attracted attention. Studies have found that CHIP protein can play an important role in the regulation of pyroptosis. However, its role in ameliorating H/R injury in cardiomyocytes has not been fully studied.

METHODS

An in vitro H/R model was constructed, and CHIP was knockdown and overexpression interfered simultaneously. The effect of CHIP on pyroptosis and its reduction of H/R-induced myocardial injury were verified by detection of cell viability, LDH, cell membrane integrity, ROS production, inflammatory factors (NLRP3, Caspase-1, ASC, IL-1β) and β-catenin/HSF1 signaling pathway.

RESULTS

In our study, we verified that the occurrence of oxidative stress and pyroptosis as well as cell damage was significantly increased in cardiomyocytes after H/R stimulation in vitro. After CHIP knockdown, pyroptosis of cardiomyocytes was further aggravated, accompanied by the down-regulation of HSF1/β-catenin signaling axis. These adverse changes were ameliorated after CHIP overexpression.

CONCLUSION

Our study confirmed that CHIP can alleviate H/R-induced myocardial injury by mediating pyroptosis, which may be achieved by regulating HSF1/β-catenin signaling pathway.

MSC-EV-transmitted HSPA8 alleviates cisplatin-induced ovotoxicity by regulating the MGARP/PRDX2 axis

Cisplatin (Cis) is among the most widely employed antitumour agents, although its clinical application is limited by self-induced multiple-organ toxicity. Previous studies have demonstrated the essential role of mitochondrial injury in the pathogenesis of Cis-induced ovotoxicity. Notably, mesenchymal stem cell-extracellular vesicles (MSC-EVs), potential cell-free therapeutic agents, exhibit pronounced advantages for the treatment of ovarian dysfunction. However, little is known about which core component contained in MSC-EVs plays a major role in repairing Cis-induced ovarian damage, and further, the potential mechanisms underlying the repair of mitochondrial damage remain unclear. Herein, our study first verified that MSC-EVs effectively ameliorate Cis-induced ovarian dysfunction by upregulating the level of mitochondrion-localized glutamic acid-rich protein (MGARP), after which MGARP repairs mitochondrial damage and inhibits cellular ROS production by combining with and suppressing the degradation of peroxiredoxin 2 (PRDX2) in granulosa cells (GCs). More importantly, our study further showed that heat shock protein family A member 8 (HSPA8) is indispensable for MenSC-EV-mediated improvement of Cis-induced ovotoxicity. This investigation provides novel insights into the molecular mechanisms by which MSCs alleviate Cis-induced ovotoxicity through improving mitochondrial dysfunction.

E3 ubiquitin ligase CHIP facilitates cAMP and cGMP signalling cross-talk by polyubiquitinating PDE9A

The carboxyl terminus of Hsc70-interacting protein (CHIP) is pivotal for managing misfolded and aggregated proteins via chaperone networks and degradation pathways. In a preclinical rodent model of CHIP-related ataxia, we observed that CHIP mutations lead to increased levels of phosphodiesterase 9A (PDE9A), whose role in this context remains poorly understood. Here, we investigated the molecular mechanisms underlying the role of PDE9A in CHIP-related ataxia and demonstrated that CHIP binds to PDE9A, facilitating its polyubiquitination and autophagic degradation. Conversely, dysfunctional CHIP disrupts this process, resulting in PDE9A accumulation, increased cGMP hydrolysis, and impaired PKG phosphorylation of CHIP at serine 19. This cascade further amplifies PDE9A accumulation, ultimately disrupting mitophagy and triggering neuronal apoptosis. Elevated PKA levels inhibit PDE9A degradation, further exacerbating this neuronal dysfunction. Notably, pharmacological inhibition of PDE9A via Bay 73-6691 or virus-mediated CHIP expression restored the balance of cGMP/cAMP signalling. These interventions protect against cerebellar neuropathologies, particularly Purkinje neuron mitophagy dysfunction. Thus, PDE9A upregulation considerably exacerbates ataxia associated with CHIP mutations, and targeting the interaction between PDE9A and CHIP is an innovative therapeutic strategy for CHIP-related ataxia.

Low shear stress induces vascular endothelial cells apoptosis via miR-330 /SOD2 /HSP70 signaling pathway

Atherosclerosis (AS) is a chronic disease initiated by vascular endothelial dysfunction, with low shear stress (SS) being a critical inducing factor in this dysfunction. Apoptosis, a form of programmed cell death, is closely associated with AS progression. However, the impact of low SS on endothelial apoptosis and its specific molecular mechanisms remains unclear. Our study revealed that low SS induces apoptosis in endothelial cells and contributes to endothelial dysfunction. Under low SS conditions, miR-330 expression was markedly upregulated, which subsequently targeted and inhibited SOD2 expression, leading to ROS accumulation and oxidative stress. Overexpression of SOD2 under low SS conditions markedly elevated HSP70 expression, contributing to endothelial homeostasis. However, when HSP70 expression was inhibited in the context of SOD2 overexpression, there was a significant increase in pro-apoptotic proteins (BAX and cleaved-caspase-3) and total apoptosis rate, along with a significant reduction in endothelial function markers such as nitric oxide and endothelial nitric oxide synthase. Notably, our experiments indicated no direct interaction between SOD2 and HSP70. Furthermore, inhibiting ROS production significantly raised HSP70 expression, suggesting that SOD2 regulates HSP70 in an indirect process involving ROS. In summary, our findings elucidate that low SS induces endothelial apoptosis and dysfunction through the miR-330/SOD2/HSP70 signaling pathway, providing valuable insights into AS intervention and prevention.

Autophagy promotes p72 degradation and capsid disassembly during the early phase of African swine fever virus infection

During viral infections, autophagy functions as a cell-intrinsic defense mechanism by facilitating the delivery of virions or viral components to the endosomal/lysosomal pathway for degradation. In this study, we report that internalized African swine fever virus (ASFV) virions enter autolysosomes during the early phase of viral infection. Autophagy selectively targets the major capsid protein p72 within the ASFV virion. The ASFV p72 protein undergoes modification through ubiquitination at the C-terminus, a process mediated by the E3 ubiquitin ligase Stub1. Subsequently, ubiquitinated p72 is recognized by the autophagy receptor SQSTM1/p62 through its ubiquitin-binding domain. Stub1 facilitates the ubiquitination and degradation of p72 in an HSPA8-dependent manner via selective autophagy. Autophagy plays a critical role in disassembling ASFV virions and further promotes the release of ASFV genomic DNA. These findings support the notion that autophagy is involved in and contributes to the capsid disassembly of ASFV, providing valuable insights into this essential viral process.IMPORTANCEAfrican swine fever (ASF), a highly contagious disease caused by the ASF virus (ASFV), affects domestic pigs and wild boars, with a mortality rate of up to 100%. The ASF epidemic poses a persistent threat to the global pig industry. Currently, no effective vaccines or antiviral drugs are available for prevention and control. In this study, we discovered that autophagy promotes the degradation of p72 and the disassembly of the capsid during the early phase of ASFV infection. Mechanically, Stub1 facilitates the polyubiquitination of ASFV p72 through the chaperone HSPA8. The polyubiquitinated p72 then interacts with the autophagy receptor SQSTM1/p62, leading to its degradation via the selective autophagy pathway. These findings reveal the mechanism of p72 degradation through autophagy and provide new insights into the capsid disassembly process of ASFV.

One BAG Does Not Fit All: Differences and Similarities of BAG Family Members in Mediating Central Nervous System Homeostasis

There is an increasing awareness that B-cell lymphoma 2 (Bcl-2)-associated athanogene (BAG) proteins play critical roles in maintaining neural homeostasis and that their dysregulation contributes to neurological disorders. This protein family, which comprises 9 members, is evolutionarily conserved, with each member having at least one BAG domain that binds to the nucleotide-binding domains of Hsp70 family members. Collectively, these proteins are essential for the proper functioning of the central nervous system (CNS). Although numerous studies have focused on a specific BAG protein, an understanding of how BAG family members may act cooperatively to maintain cellular homeostasis is needed. In this review, we give an overview of the BAG domain interactors Hsp72, Hsp70.2, CHIP, and METTL3, which are common to all BAG family members. This is followed by a concise description of each BAG family member, with a focus on its function in the CNS and dysfunction in neurological conditions. Finally, we discuss the intersection of the molecular functions of the different BAG family proteins by delineating similarities and differences and describing how their functions can be either complementary or competing. The information in this review provides a basic conceptual framework for analyzing the roles of a particular BAG family member in the CNS and neurological conditions. This review also provides a basis for examining how BAG family members can play either redundant or antagonistic roles that may modulate experimental outcomes.

The p97-UBXD8 complex maintains peroxisome abundance by suppressing pexophagy

Peroxisomes are vital organelles involved in key metabolic functions in eukaryotic cells. Their significance is highlighted by peroxisome biogenesis disorders; severe childhood diseases marked by disrupted lipid metabolism. One mechanism regulating peroxisome abundance is through selective ubiquitylation of peroxisomal membrane proteins that triggers peroxisome degradation via selective autophagy (pexophagy). However, the mechanisms regulating pexophagy remain poorly understood in mammalian cells. Here we show that the evolutionarily conserved AAA-ATPase p97 and its membrane embedded adaptor UBXD8 are essential for maintaining peroxisome abundance. From quantitative proteomic studies we reveal that loss of UBXD8 affects many peroxisomal proteins. We find depletion of UBXD8 results in a loss of peroxisomes in a manner that is independent of the known role of UBXD8 in ER associated degradation (ERAD). Loss of UBXD8 or inhibition of p97 increases peroxisomal turnover through autophagy and can be rescued by depleting key autophagy proteins or overexpressing the deubiquitylating enzyme USP30. Furthermore, we find increased ubiquitylation of the peroxisomal membrane protein PMP70 in cells lacking UBXD8 or p97. Collectively, our findings identify a new role for the p97-UBXD8 complex in regulating peroxisome abundance by suppressing pexophagy.

Mechanism and complex roles of HSC70/HSPA8 in viral entry

The process of viruses entering host cells is complex, involving multiple aspects of the molecular organization of the cell membrane, viral proteins, the interaction of receptor molecules, and cellular signaling. Most viruses depend on endocytosis for uptake, when viruses reach the appropriate location, they are released from the vesicles, undergo uncoating, and release their genomes. Heat shock cognate protein 70(HSC70): also known as HSPA8, a protein involved in mediating clathrin-mediated endocytosis (CME), is involved in various viral entry processes. In this mini-review, our goal is to provide a summary of the function of HSC70 in viral entry. Understanding the interaction networks of HSC70 with viral proteins helps to provide new directions for targeted therapeutic strategies against viral infections.

Mammalian pexophagy at a glance

Peroxisomes are highly plastic organelles that are involved in several metabolic processes, including fatty acid oxidation, ether lipid synthesis and redox homeostasis. Their abundance and activity are dynamically regulated in response to nutrient availability and cellular stress. Damaged or superfluous peroxisomes are removed mainly by pexophagy, the selective autophagy of peroxisomes induced by ubiquitylation of peroxisomal membrane proteins or ubiquitin-independent processes. Dysregulated pexophagy impairs peroxisome homeostasis and has been linked to the development of various human diseases. Despite many recent insights into mammalian pexophagy, our understanding of this process is still limited compared to our understanding of pexophagy in yeast. In this Cell Science at a Glance article and the accompanying poster, we summarize current knowledge on the control of mammalian pexophagy and highlight which aspects require further attention. We also discuss the role of ubiquitylation in pexophagy and describe the ubiquitin machinery involved in regulating signals for the recruitment of phagophores to peroxisomes.

Integrative multi-omics analysis reveals the crucial biological pathways involved in the adaptive response to NaCl stress in peanut seedlings

Plant growth is restricted by salt stress, which is a significant abiotic factor, particularly during the seedling stage. The aim of this study was to investigate the mechanisms underlying peanut adaptation to salt stress by transcriptomic and metabolomic analysis during the seedling stage. In this study, phenotypic variations of FH23 and NH5, two peanut varieties with contrasting tolerance to salt, changed obviously, with the strongest differences observed at 24 h. FH23 leaves wilted and the membrane system was seriously damaged. A total of 1470 metabolites were identified, with flavonoids being the most common (21.22%). Multi-omics analyses demonstrated that flavonoid biosynthesis (ko00941), isoflavones biosynthesis (ko00943), and plant hormone signal transduction (ko04075) were key metabolic pathways. The comparison of metabolites in isoflavone biosynthesis pathways of peanut varieties with different salt tolerant levels demonstrated that the accumulation of naringenin and formononetin may be the key metabolite leading to their different tolerance. Using our transcriptomic data, we identified three possible reasons for the difference in salt tolerance between the two varieties: (1) differential expression of LOC112715558 (HIDH) and LOC112709716 (HCT), (2) differential expression of LOC112719763 (PYR/PYL) and LOC112764051 (ABF) in the abscisic acid (ABA) signal transduction pathway, then (3) differential expression of genes encoding JAZ proteins (LOC112696383 and LOC112790545). Key metabolites and candidate genes related to improving the salt tolerance in peanuts were screened to promote the study of the responses of peanuts to NaCl stress and guide their genetic improvement.

Raltitrexed induces apoptosis through activating ROS-mediated ER stress by impeding HSPA8 expression in prostate cancer cells

Prostate cancer is the most common malignant tumor in males, which frequently develops into castration-resistant prostate cancer (CRPC). CRPC metastasis is the main reason for its high mortality rate. At present, it lacks effective treatment for patients with CRPC. Raltitrexed (RTX) has been shown to be effective in the treatment of colorectal cancer. However, the effect of RTX on prostate cancer and the underlying mechanism remain unknown. In the current study, we found that RTX could dose-dependently inhibit proliferation, migration, colony formation and induce apoptosis in DU145 and PC-3 cells. RTX also increased ROS generation in prostate cancer cells. Pretreatment with N-acetyl-L-cysteine (NAC) significantly prevented RTX-induced cell apoptosis and endoplasmic reticulum (ER) stress signaling activation in prostate cancer cells. Additionally, we found RTX-induced ROS generation and ER stress activation depended on the expression of heat shock protein family A member 8 (HSPA8). Over-expression of HSPA8 could alleviate RTX-induced cell apoptosis, ROS generation and ER stress signaling activation. Finally, our study also showed that RTX attenuated the tumor growth of prostate cancer in the DU145 xenograft model and significantly downregulated HSPA8 expression and activated ER stress signaling pathway in tumor tissues. Our study is the first to reveal that RTX induces prostate cancer cells apoptosis through inhibiting the expression of HSPA8 and further inducing ROS-mediated ER stress pathway action. This study suggests that RTX may be a novel promising candidate drug for prostate cancer therapy.

Inhibition of VHL by VH298 Accelerates Pexophagy by Activation of HIF-1α in HeLa Cells

Autophagy is a pivotal biological process responsible for maintaining the homeostasis of intracellular organelles. Yet the molecular intricacies of peroxisomal autophagy (pexophagy) remain largely elusive. From a ubiquitin-related chemical library for screening, we identified several inhibitors of the Von Hippel-Lindau (VHL) E3 ligase, including VH298, thereby serving as potent inducers of pexophagy. In this study, we observed that VH298 stimulates peroxisomal degradation by ATG5 dependently and escalates the ubiquitination of the peroxisomal membrane protein ABCD3. Interestingly, the ablation of NBR1 is similar to the curtailed peroxisomal degradation in VH298-treated cells. We also found that the pexophagy induced by VH298 is impeded upon the suppression of gene expression by the translation inhibitor cycloheximide. Beyond VHL inhibition, we discovered that roxadustat, a direct inhibitor of HIF-α prolyl hydroxylase, is also a potent inducer of pexophagy. Furthermore, we found that VH298-mediated pexophagy is blocked by silencing HIF-1α. In conclusion, our findings suggest that VH298 promotes pexophagy by modulating VHL-mediated HIF-α transcriptional activity.

Upregulated pexophagy limits the capacity of selective autophagy

Selective autophagy is an essential process to maintain cellular homeostasis through the constant recycling of damaged or superfluous components. Over a dozen selective autophagy pathways mediate the degradation of diverse cellular substrates, but whether these pathways can influence one another remains unknown. We address this question using pexophagy, the autophagic degradation of peroxisomes, as a model. We show in cells that upregulated pexophagy impairs the selective autophagy of both mitochondria and protein aggregates by exhausting the autophagy initiation factor, ULK1. We confirm this finding in cell models of the pexophagy-mediated form of Zellweger Spectrum Disorder, a disease characterized by peroxisome dysfunction. Further, we extend the generalizability of limited selective autophagy by determining that increased protein aggregate degradation reciprocally reduces pexophagy using cell models of Parkinson's Disease and Huntington's Disease. Our findings suggest that the degradative capacity of selective autophagy can become limited by an increase in one substrate.

CHIP protects against septic acute kidney injury by inhibiting NLRP3-mediated pyroptosis

Septic acute kidney injury (S-AKI), the most common type of acute kidney injury (AKI), is intimately related to pyroptosis and oxidative stress in its pathogenesis. Carboxy-terminus of Hsc70-interacting protein (CHIP), a U-box E3 ligase, modulates oxidative stress by degrading its targeted proteins. The role of CHIP in S-AKI and its relevance with pyroptosis have not been investigated. In this study, we showed that CHIP was downregulated in renal proximal tubular cells in lipopolysaccharide (LPS)-induced S-AKI. Besides, the extent of redox injuries in S-AKI was attenuated by CHIP overexpression or activation but accentuated by CHIP gene disruption. Mechanistically, our work demonstrated that CHIP interacted with and ubiquitinated NLRP3 to promote its proteasomal degradation, leading to the inhibition of NLRP3/ACS inflammasome-mediated pyroptosis. In summary, this study revealed that CHIP ubiquitinated NLRP3 to alleviate pyroptosis in septic renal injuries, suggesting that CHIP might be a potential therapeutic target for S-AKI.

Extracellular release in the quality control of the mammalian mitochondria

Mammalian cells release a wealth of materials to their surroundings. Emerging data suggest these materials can even be mitochondria with perturbed morphology and aberrant function. These dysfunctional mitochondria are removed by migrating cells through membrane shedding. Neuronal cells, cardiomyocytes, and adipocytes send dysfunctional mitochondria into the extracellular space for nearby cells to degrade. Various studies also indicate that there is an interplay between intracellular mitochondrial degradation pathways and mitochondrial release in handling dysfunctional mitochondria. These observations, in aggregate, suggest that extracellular release plays a role in quality-controlling mammalian mitochondria. Future studies will help delineate the various types of molecular machinery mammalian cells use to release dysfunctional mitochondria. Through the studies, we will better understand how mammalian cells choose between intracellular degradation and extracellular release for the quality control of mitochondria.

Chaperone-mediated autophagy in neurodegenerative diseases: mechanisms and therapy

Chaperone-mediated autophagy (CMA) is the selective degradation process of intracellular components by lysosomes, which is required for the degradation of aggregate-prone proteins and contributes to proteostasis maintenance. Proteostasis is essential for normal cell function and survival, and it is determined by the balance of protein synthesis and degradation. Because postmitotic neurons are highly susceptible to proteostasis disruption, CMA is vital for the nervous system. Since Parkinson's disease (PD) was first linked to CMA dysfunction, an increasing number of studies have shown that CMA loss, as seen during aging, occurs in the pathogenetic process of neurodegenerative diseases. Here, we review the molecular mechanisms of CMA, as well as the physiological function and regulation of this autophagy pathway. Following, we highlight its potential role in neurodegenerative diseases, and the latest advances and challenges in targeting CMA in therapy of neurodegenerative diseases.

HSPA8 regulates anti-bacterial autophagy through liquid-liquid phase separation

HSPA8 (heat shock protein family A (Hsp70) member 8) plays a significant role in the autophagic degradation of proteins, however, its effect on protein stabilization and anti-bacterial autophagy remains unknown. Here, it is discovered that HSPA8, as a binding partner of RHOB and BECN1, induce autophagy for intracellular bacteria clearance. Using its NBD and LID domains, HSPA8 physically binds to RHOB residues 1-42 and 89-118 as well as to BECN1 ECD domain, preventing RHOB and BECN1 degradation. Intriguingly, HSPA8 contains predicted intrinsically disordered regions (IDRs), and drives liquid-liquid phase separation (LLPS) to concentrate RHOB and BECN1 into HSPA8-formed liquid-phase droplets, resulting in improved RHOB and BECN1 interactions. Our study reveals a novel role and mechanism of HSPA8 in modulating anti-bacterial autophagy, and highlights the effect of LLPS-related HSPA8-RHOB-BECN1 complex on enhancing protein interaction and stabilization, which improves the understanding of autophagy-mediated defense against bacteria.

Crosstalk between Autophagy and RLR Signaling

Autophagy plays a homeostatic role in regulating cellular metabolism by degrading unwanted intracellular materials and acts as a host defense mechanism by eliminating infecting pathogens, such as viruses. Upon viral infection, host cells often activate retinoic acid-inducible gene I (RIG-I)-like receptor (RLR) signaling to induce the transcription of type I interferons, thus establishing the first line of the innate antiviral response. In recent years, numerous studies have shown that virus-mediated autophagy activation may benefit viral replication through different actions on host cellular processes, including the modulation of RLR-mediated innate immunity. Here, an overview of the functional molecules and regulatory mechanism of the RLR antiviral immune response as well as autophagy is presented. Moreover, a summary of the current knowledge on the biological role of autophagy in regulating RLR antiviral signaling is provided. The molecular mechanisms underlying the crosstalk between autophagy and RLR innate immunity are also discussed.

Enhanced Levels of Peroxisome-Derived H2O2 Do Not Induce Pexophagy but Impair Autophagic Flux in HEK-293 and HeLa Cells

Peroxisomes are functionally specialized organelles that harbor multiple hydrogen peroxide (H2O2)-producing and -degrading enzymes. Given that this oxidant functions as a major redox signaling agent, peroxisomes have the intrinsic ability to mediate and modulate H2O2-driven processes, including autophagy. However, it remains unclear whether changes in peroxisomal H2O2 (po-H2O2) emission impact the autophagic process and to which extent peroxisomes with a disturbed H2O2 metabolism are selectively eliminated through a process called “pexophagy”. To address these issues, we generated and validated HEK-293 and HeLa pexophagy reporter cell lines in which the production of po-H2O2 can be modulated. We demonstrate that (i) po-H2O2 can oxidatively modify multiple selective autophagy receptors and core autophagy proteins, (ii) neither modest nor robust levels of po-H2O2 emission act as a prime determinant of pexophagy, and (iii) high levels of po-H2O2 impair autophagic flux by oxidative inhibition of enzymes involved in LC3II formation. Unexpectedly, our analyses also revealed that the autophagy receptor optineurin can be recruited to peroxisomes, thereby triggering pexophagy. In summary, these findings lend support to the idea that, during cellular and organismal aging, peroxisomes with enhanced H2O2 release can escape pexophagy and downregulate autophagic activity, thereby perpetuating the accumulation of damaged and toxic cellular debris.

CHIP: A Co-chaperone for Degradation by the Proteasome and Lysosome

Protein homeostasis relies on a balance between protein folding and protein degradation. Molecular chaperones like Hsp70 and Hsp90 fulfill well-defined roles in protein folding and conformational stability via ATP-dependent reaction cycles. These folding cycles are controlled by associations with a cohort of non-client protein co-chaperones, such as Hop, p23, and Aha1. Pro-folding co-chaperones facilitate the transit of the client protein through the chaperone-mediated folding process. However, chaperones are also involved in proteasomal and lysosomal degradation of client proteins. Like folding complexes, the ability of chaperones to mediate protein degradation is regulated by co-chaperones, such as the C-terminal Hsp70-binding protein (CHIP/STUB1). CHIP binds to Hsp70 and Hsp90 chaperones through its tetratricopeptide repeat (TPR) domain and functions as an E3 ubiquitin ligase using a modified RING finger domain (U-box). This unique combination of domains effectively allows CHIP to network chaperone complexes to the ubiquitin-proteasome and autophagosome-lysosome systems. This chapter reviews the current understanding of CHIP as a co-chaperone that switches Hsp70/Hsp90 chaperone complexes from protein folding to protein degradation.

Membrane compartmentalisation of the ubiquitin system

We now have a comprehensive inventory of ubiquitin system components. Understanding of any system also needs an appreciation of how components are organised together. Quantitative proteomics has provided us with a census of their relative populations in several model cell types. Here, by examining large scale unbiased data sets, we seek to identify and map those components, which principally reside on the major organelles of the endomembrane system. We present the consensus distribution of > 50 ubiquitin modifying enzymes, E2s, E3s and DUBs, that possess transmembrane domains. This analysis reveals that the ER and endosomal compartments have a diverse cast of resident E3s, whilst the Golgi and mitochondria operate with a more restricted palette. We describe key functions of ubiquitylation that are specific to each compartment and relate this to their signature complement of ubiquitin modifying components.

Manipulating autophagic degradation in human diseases: from mechanisms to interventions

Targeted degradation, having emerged as a powerful and promising strategy in drug discovery in the past two decades, has provided a solution for many once undruggable targets involved in various diseases. While earlier targeted degradation tools, as exemplified by PROteolysis-TArgeting Chimera (PROTAC), focused on harnessing the ubiquitin-proteasome system, novel approaches that aim to utilize autophagy, a potent, lysosome-dependent degradation pathway, have also surfaced recently as promising modalities. In this review, we first introduce the mechanisms that establish selectivity in autophagy, which provides the rationales for autophagy-based targeted degradation; we also provide an overview on the panoply of cellular machinery involved in this process, an arsenal that could be potentially harnessed. On this basis, we propose four strategies for designing autophagy-based targeted degraders, including Tagging Targets, Directly Engaging Targets, Initiating Autophagy at Targets, and Phagophore-Tethering to Targets. We introduce the current frontiers in this field, including AUtophagy-TArgeting Chimera (AUTAC), Targeted Protein Autophagy (TPA), AUTOphagy-TArgeting Chimera (AUTOTAC, not to be confused with AUTAC), AuTophagosome TEthering Compound (ATTEC), and other experimental approaches as case studies for each strategy. Finally, we put forward a workflow for generating autophagy-based degraders and some important questions that may guide and inspire the process.

The Global mRNA Expression Profiles of Inhibiting PHGDH Induced Cisplatin Resistance in Gastric Cancer

<strong>Objective</strong>: Drug resistance is the main hindrance to improve the prognosis of patients with gastric cancer. Amino<br />acid metabolic reprograming is essential to satisfy the different requirements of cancer cells during drug resistance,<br />of which serine deprivation could promote resistance to cisplatin in gastric cancer. As the key enzyme in the de novo<br />biosynthesis of serine, phosphoglycerate dehydrogenase (PHGDH) inhibition could also induce cisplatin resistance in<br />gastric cancer. This study aims to reveal the potential mechanisms of drug resistance induced by PHGDH inhibition via<br />exploring the global mRNA expression profiles.<br /><strong>Materials and Methods</strong>: In this experimental study, the viability and the apoptotic rate of gastric cancer cells<br />were evaluated by using Cell Counting Kit-8 (CCK-8) analysis and flow cytometric determination, respectively. The<br />identification of differentially expressed genes (DEGs) was tested by mRNA-sequencing (mRNA-Seq) analysis. The<br />confirmation of sequencing results was verified using real-time quantitative reverse transcription polymerase chain<br />reaction (RT-qPCR).<br /><strong>Results:</strong> The inhibition of PHGDH significantly increased the viability and decreased the apoptotic rate induced by cisplatin<br />in gastric cancer cells. mRNA-Seq analysis revealed that the combined treatment of NCT503 reduced the number of DEGs<br />induced by cisplatin. Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Set Enrichment<br />Analysis (GSEA) showed that unfolded protein response, ECM receptor interaction and cell cycle signaling pathways were<br />modulated by NCT503 treatment. Hub genes were identified by using protein-protein interaction network modeling, of which E1A binding protein p300 (EP300) and heat shock protein family A (Hsp70) member 8 (HSPA8) act as the vital genes in cisplatin resistance induced by the inhibition of PHGDH.<br /><strong>Conclusion:</strong> These findings suggested that the inhibition of PHGDH promoted cisplatin resistance in gastric cancer<br />through various intercellular mechanisms. And appropriate serine supplementation or the modulation of EP300 and<br />HSPA8 may be of great help in overcoming cisplatin resistance in gastric cancer.

HSPA8 Single-Nucleotide Polymorphism Is Associated with Serum HSC70 Concentration and Carotid Artery Atherosclerosis in Nonalcoholic Fatty Liver Disease

There is an association between nonalcoholic fatty liver disease (NAFLD) and atherosclerosis, but the genetic risk of atherosclerosis in NAFLD remains unclear. Here, a single-nucleotide polymorphism (SNP) of the heat shock 70 kDa protein 8 (HSPA8) gene was analyzed in 123 NAFLD patients who had been diagnosed using a liver biopsy, and the NAFLD phenotype including the maximum intima–media thickness (Max-IMT) of the carotid artery was investigated. Patients with the minor allele (A/G or G/G) of rs2236659 showed a lower serum heat shock cognate 71 kDa protein concentration than those with the major A/A allele. Compared with the patients with the major allele, those with the minor allele showed a higher prevalence of hypertension and higher Max-IMT in men. No significant associations between the HSPA8 genotype and hepatic pathological findings were identified. In decision-tree analysis, age, sex, liver fibrosis, and HSPA8 genotype were individually associated with severe carotid artery atherosclerosis (Max-IMT ≥ 1.5 mm). Noncirrhotic men aged ≥ 65 years were most significantly affected by the minor allele of HSPA8. To predict the risk of atherosclerosis and cardiovascular disease, HSPA8 SNP genotyping might be useful, particularly for older male NAFLD patients.

Golgi quality control and autophagy

Organelles can easily be disrupted by intracellular and extracellular factors. Studies on ER and mitochondria indicate that a wide range of responses are elicited upon organelle disruption. One response thought to be of particular importance is autophagy. Cells can target entire organelles into autophagosomes for removal. This wholesale nature makes autophagy a robust means for eliminating compromised organelles. Recently, it was demonstrated that the Golgi apparatus is a substrate of autophagy. On the other hand, various reports have shown that components traffic away from the Golgi for elimination in an autophagosome-independent manner when the Golgi apparatus is stressed. Future studies will reveal how these different pieces of machinery coordinate to drive Golgi degradation. Quantitative measurements will be needed to determine how much autophagy contributes to the maintenance of the Golgi apparatus.

Hsp70 in Redox Homeostasis

Cellular redox homeostasis is precisely balanced by generation and elimination of reactive oxygen species (ROS). ROS are not only capable of causing oxidation of proteins, lipids and DNA to damage cells but can also act as signaling molecules to modulate transcription factors and epigenetic pathways that determine cell survival and death. Hsp70 proteins are central hubs for proteostasis and are important factors to ameliorate damage from different kinds of stress including oxidative stress. Hsp70 members often participate in different cellular signaling pathways via their clients and cochaperones. ROS can directly cause oxidative cysteine modifications of Hsp70 members to alter their structure and chaperone activity, resulting in changes in the interactions between Hsp70 and their clients or cochaperones, which can then transfer redox signals to Hsp70-related signaling pathways. On the other hand, ROS also activate some redox-related signaling pathways to indirectly modulate Hsp70 activity and expression. Post-translational modifications including phosphorylation together with elevated Hsp70 expression can expand the capacity of Hsp70 to deal with ROS-damaged proteins and support antioxidant enzymes. Knowledge about the response and role of Hsp70 in redox homeostasis will facilitate our understanding of the cellular knock-on effects of inhibitors targeting Hsp70 and the mechanisms of redox-related diseases and aging.

Accumulated ROS Activates HIF-1α-Induced Glycolysis and Exerts a Protective Effect on Sensory Hair Cells Against Noise-Induced Damage

Noise exposure causes noise-induced hearing loss (NIHL). NIHL exhibits loss of inner ear sensory hair cells and is often irreparable. Although oxidative stress is involved in hearing loss, the complex mechanisms involved in NIHL are unclear. Hypoxia-inducible factor 1α (HIF-1α) has been suggested to be essential for protecting sensory hair cells. Additionally, it has been shown that ROS is involved in modulating the stability of HIF-1α. To investigate the NIHL pathogenesis, we established a tert-butyl hydroperoxide (t-BHP)-induced oxidative stress damage model in hair-like HEI-OC1 cells and an NIHL model in C57BL/6 mice. Protein and mRNA expression were determined, and biochemical parameters including reactive oxygen species (ROS) accumulation, glucose uptake, adenosine triphosphat (ATP) production, and mitochondrial content were evaluated. In HEI-OC1 cells, t-BHP induced ROS accumulation and reduced mitochondrial content and oxygen consumption, but the ATP level was unaffected. Additionally, there was increased glucose uptake and lactate release along with elevated expression of HIF-1α, glucose transporter 1, and several glycolytic enzymes. Consistently, noise trauma induced oxidative stress and the expression of HIF-1α and glycolytic enzymes in mice. Thus, we concluded that ROS induced HIF-1α expression, which promoted glycolysis, suggesting a metabolic shift maintained the ATP level to attenuate hair cell damage in NIHL.

Hypermethylation of mitochondrial DNA facilitates bone metastasis of renal cell carcinoma

Kidney cancers including clear cell carcinoma (RCC) are identified with very vulnerable mitochondria DNA (mtDNA) and frequent epigenetic aberrations. Bone metastasis from RCC is prevalent and destructive. Bone marrow contains a quite hypoxic microenvironment that usually insitigate 50% of hypermethylation events in conferring a selective advantage for tumor growth. We hypothesized that hypermethylation of mtDNA in RCC cells would significantly contribute to bone metastatic tumor progression. Methylation-specific polymerase chain reaction assay (MSP) was adopted to measure the methylation status of D-loop region of mtDNA in 15 pairs of bone metastatic and primary RCC as well as tumor adjescent normal kidney tissues. mtDNA copy number was examined by the real-time quantitative polymerase chain reaction (qPCR). Western blotting analysis was used to measure the accumulation of several DNA methyltransferases (DNMTs) in the mitochondria and nucleus fractions of bone metastatic RCC cells. mRNA expression of mitochondria encoded genes was examined by RT-PCR. Reactive oxygen species (ROS), mitochondrial membrane potential and ATP content were measured using in vitro cells treated with de-methylation drug 5-Azacytidine (5-Aza). Non-invasive bioluminescent imaging was performed to monitor tumor occurrence in skeleton in mice. Our results showed that the D-loop region in bone metastatic tumor cells was markedly hypermethylated than those in primary RCC tumor cells, that is associated with a decreased mtDNA copy number and accumulation of DNMT1 in the mitochondria. The bone-tropism tumor colonization and progression of RCC cells was significantly suppressed by demethylating the D-loop region of mtDNA and reducing the intracellular level of ROS and ATP by 5-Aza treatment. In conclusion, our study provided a direct association between hypermethylation of mtDNA in RCC with bone metastastic tumor growth.

The Peroxisome-Autophagy Redox Connection: A Double-Edged Sword?

Peroxisomes harbor numerous enzymes that can produce or degrade hydrogen peroxide (H2O2). Depending on its local concentration and environment, this oxidant can function as a redox signaling molecule or cause stochastic oxidative damage. Currently, it is well-accepted that dysfunctional peroxisomes are selectively removed by the autophagy-lysosome pathway. This process, known as "pexophagy," may serve a protective role in curbing peroxisome-derived oxidative stress. Peroxisomes also have the intrinsic ability to mediate and modulate H2O2-driven processes, including (selective) autophagy. However, the molecular mechanisms underlying these phenomena are multifaceted and have only recently begun to receive the attention they deserve. This review provides a comprehensive overview of what is known about the bidirectional relationship between peroxisomal H2O2 metabolism and (selective) autophagy. After introducing the general concepts of (selective) autophagy, we critically examine the emerging roles of H2O2 as one of the key modulators of the lysosome-dependent catabolic program. In addition, we explore possible relationships among peroxisome functioning, cellular H2O2 levels, and autophagic signaling in health and disease. Finally, we highlight the most important challenges that need to be tackled to understand how alterations in peroxisomal H2O2 metabolism contribute to autophagy-related disorders.

The Pathophysiological Role of Heat Shock Response in Autoimmunity: A Literature Review

Within the last two decades, there has been increasing evidence that heat-shock proteins can have a differential influence on the immune system. They can either provoke or ameliorate immune responses. This review focuses on outlining the stimulatory as well as the inhibitory effects of heat-shock proteins 27, 40, 70, 65, 60, and 90 in experimental and clinical autoimmune settings.