Structures of BIRC6-client complexes provide a mechanism of SMAC-mediated release of caspases

Pyroptosis and chemical classification of pyroptotic agents

Pyroptosis, as a lytic-inflammatory type of programmed cell death, has garnered considerable attention due to its role in cancer chemotherapy and many inflammatory diseases. This review will discuss the biochemical classification of pyroptotic inducers according to their chemical structure, pyroptotic mechanism, and cancer type of these targets. A structure-activity relationship study on pyroptotic inducers is revealed based on the surveyed pyroptotic inducer chemotherapeutics. The shared features in the chemical structures of current pyroptotic inducer agents were displayed, including an essential cyclic head, a vital linker, and a hydrophilic tail that is significant for π-π interactions and hydrogen bonding. The presented structural features will open the way to design new hybridized classes or scaffolds as potent pyroptotic inducers in the future, which may represent a solution to the apoptotic-resistance dilemma along with synergistic chemotherapeutic advantage.

Mechanistic Insights Into the Assembly of Functional CRL3 Dimeric Complexes

The assembly of Cullin3-based RING E3 ubiquitin ligase (CRL3) complexes is orchestrated in two consecutive steps: the formation of the dimeric BTB domain core and the recruitment of CUL3-RBX1 subunits. Each step is tightly regulated to ensure the formation of complete and functional dimeric CRL3s. The first assembly step is regulated by two mechanisms: "co-co assembly" and proteasome-dependent degradation of aberrant heterodimers. The second step is facilitated by a conserved CUL3 N-terminal assembly (NA) motif. The CUL3 NA motif contributes to the assembly of CRL3s in two aspects: interacting with both BTB domain-containing protein protomers to facilitate complete dimeric assembly, and enhancing the stability of CRL3s by overcoming the tensions generated by conformational entropy during ubiquitin transfer. Given that all Cullin proteins contain N-terminal extensions, we postulate that these extensions, similar to the CUL3 NA motif-contributed assembly, play an important role in the functional regulation of CRLs and thus warrant further investigation.

Targeted protein degradation for cancer therapy

Targeted protein degradation (TPD) aims at reprogramming the target specificity of the ubiquitin-proteasome system, the major cellular protein disposal machinery, to induce selective ubiquitination and degradation of therapeutically relevant proteins. Since its conception over 20 years ago, TPD has gained a lot of attention mainly due to improvements in the design of bifunctional proteolysis targeting chimeras (PROTACs) and understanding the mechanisms underlying molecular glue degraders. Today, PROTACs are on the verge of a first clinical approval and recent structural and mechanistic insights combined with technological leaps promise to unlock the rational design of protein degraders, following the lead of lenalidomide and related clinically approved analogues. At the same time, the TPD universe is expanding at a record speed with the discovery of novel modalities beyond molecular glue degraders and PROTACs. Here we review the recent progress in the field, focusing on newly discovered degrader modalities, the current state of clinical degrader candidates for cancer therapy and upcoming design approaches.

TRIM52 maintains cellular fitness and is under tight proteolytic control by multiple giant E3 ligases

Tripartite motif 52 (TRIM52) exhibits strong positive selection in humans, yet is lost in many other mammals. In contrast to what one would expect for such a non-conserved factor, TRIM52 loss compromises cell fitness. We set out to determine the cellular function of TRIM52. Genetic and proteomic analyses revealed TRIM52 physically and functionally interacts with the DNA repair machinery. Our data suggest that TRIM52 limits topoisomerase 2 adducts, thereby preventing cell-cycle arrest. Consistent with a fitness-promoting function, TRIM52 is upregulated in various cancers, prompting us to investigate its regulatory pathways. We found TRIM52 to be targeted for ultra-rapid proteasomal degradation by the giant E3 ubiquitin ligases BIRC6, HUWE1, and UBR4/KCMF1. BIRC6 mono-ubiquitinates TRIM52, with subsequent extension by UBR4/KCMF1. These findings suggest a role for TRIM52 in maintaining genome integrity, and regulation of its own abundance through multi-ligase degradation.

Tumor-specific activated polymeric nanotuners disrupt positive feedback cycle of hypoxia and apoptosis evasion for potent cancer radiotherapy

Effective cancer radiotherapy is usually hindered by the self-perpetuating feedback cycle between tumor hypoxia and apoptosis evasion. Herein, a tumor-specific activated polymeric nanotuner is developed to boost radiotherapy outcomes by disrupting this vicious cycle. The designed nanotuner is composed of a proapoptotic peptide-engineered catalase core and a pH-detachable polymer shell. They can maintain the core-shell structure to against immune clearance and enzymatic degradation under the "turn-off" state. When reaching the tumor site, the nanotuners hold acid-responsive "turn-on" property by dissociating the polymeric shell, facilitating the tumor accumulation and cellular internalization of the exposed functional core. Subsequently, the internalized core of polymeric nanotuners efficiently decomposes endogenous hydrogen peroxide (H2O2) into oxygen (O2) for hypoxia alleviation, thus upregulating the expression of proapoptotic protein Smac. Furthermore, the apoptotic-inducing peptide modified on the core surface further boosts the Smac-induced apoptosis signal, intervening in tumor apoptosis evasion and ultimately realizing the efficient radiotherapeutic efficiency by blocking this vicious cycle. In vivo studies demonstrated that treatment with polymeric nanotuners remarkably enhances radiation-mediated tumor ablation without perceptible side effects. This study sheds light on the innovative attempt to specifically interfere with the feedback cycle in tumor radioresistance, pioneering the way for achieving safe and efficient cancer therapies.

Structural insights into the biochemical mechanism of the E2/E3 hybrid enzyme UBE2O

The E2/E3 hybrid enzyme UBE2O plays important roles in key biological events, but its autoubiquitination mechanism remains largely unclear. In this study, we determined the crystal structures of full-length (FL) UBE2O from Trametes pubescens (tp) and its ubiquitin-conjugating (UBC) domain. The dimeric FL-tpUBE2O structure revealed interdomain interactions between the conserved regions (CR1-CR2) and UBC. The dimeric intermolecular and canonical ubiquitin/UBC interactions are mechanistically important for UBE2O functions in catalyzing the formation of free polyubiquitin chains and substrate ubiquitination. Beyond dimerization, autoubiquitination within the CR1-CR2 domain also regulates tpUBE2O activity. Additionally, we show that tpUBE2O catalyzes the formation of all seven types of polyubiquitin chains in vitro. The CR1-CR2/UBC and canonical ubiquitin/UBC interactions are important for the polyubiquitination of AMP-activated protein kinase α2 (AMPKα2) by human UBE2O (hUBE2O), which leads to tumorigenesis. These structural insights lay the groundwork for understanding UBE2O's mechanisms and developing structure-based therapeutics targeting UBE2O.

The differential interactomes of the KRAS splice variants identify BIRC6 as a ubiquitin ligase for KRAS4A

Transcripts of the KRAS locus are alternatively spliced to generate two proteins, KRAS4A and KRAS4B, which differ in their membrane-targeting sequences. These splice variants have been conserved for more than 450 million years, suggesting non-overlapping functions driven by differential membrane association. Here, we use proximity labeling to map the differential interactomes of the KRAS splice variants. We find 24 and 10 proteins that interact specifically with KRAS4A or KRAS4B, respectively. The KRAS interacting protein most specific to KRAS4A is BIRC6, a large member of the inhibitor of apoptosis protein family unique in possessing E2/E3 ubiquitin ligase activity. We find that this interaction takes place on the Golgi apparatus and results in the mono- and di-ubiquitination of KRAS4A at lysines 128 and 147. Silencing BIRC6 diminishes GTP loading of and growth stimulation by KRAS4A but not KRAS4B. Thus, BIRC6 is a ubiquitin ligase that inhibits apoptosis and also modifies KRAS4A.

The correlation between Smac, IAPs and mitochondrial apoptosis, muscle tenderness during postmortem aging of Oula Tibetan sheep meat

Oula Tibetan sheep meat has rich nutritional value but relatively poor tenderness. Recently, apoptosis of muscle cells has gradually become a research hotspot for improving meat tenderness during postmortem aging. Smac can promote the decrease of IAPs expression in tumor cells, thereby inducing mitochondrial apoptosis. However, the relationship between Smac, IAPs and mitochondrial apoptosis, muscle tenderness during postmortem meat aging is still unclear. Thus, the aim of this work was to explore the relationship between Smac, IAPs and mitochondrial apoptosis as well as muscle tenderness during postmortem meat aging. Smac concentration, IAPs concentration, pH value, ATP content, SDH activity, MPTP opening degree, MMP, caspase-3/9 activity, apoptotic rate, MFI and shear force value of Oula Tibetan sheep meat were measured at different aging times and correlation analysis was performed. Correlation analysis revealed that Smac, IAPs were markedly related to mitochondrial apoptosis and muscle tenderness during postmortem aging of Tibetan sheep meat. The results suggest that Smac may regulate IAPs to promote mitochondrial apoptosis and muscle tenderization in Oula Tibetan sheep meat during postmortem aging.

SMAC mimetics induce human macrophages to phagocytose live cancer cells

Macrophages engulf apoptotic bodies and cellular debris as part of homeostasis, but they can also phagocytose live cells such as aged red blood cells. Pharmacologic reprogramming with the SMAC mimetic LCL161 in combination with T cell-derived cytokines can induce macrophages to phagocytose live cancer cells in mouse models. Here we extend these findings to encompass a wide range of monovalent and bivalent SMAC mimetic compounds, demonstrating that live cell phagocytosis is a class effect of these agents. We demonstrate robust phagocytosis of live pancreatic and breast cancer cells by primary human macrophages across a range of healthy donors. Unlike mouse macrophages where combination of SMAC mimetics with lymphotoxin enhanced phagocytosis, human macrophages were more efficiently polarized to phagocytose live cells by the combination of SMAC mimetics and IFNψ. We profiled phagocytic macrophages by transcriptional and proteomic methodologies, uncovering a positive feedback loop of autocrine TNFα production.

Noncanonical assembly, neddylation and chimeric cullin-RING/RBR ubiquitylation by the 1.8 MDa CUL9 E3 ligase complex

Ubiquitin ligation is typically executed by hallmark E3 catalytic domains. Two such domains, 'cullin-RING' and 'RBR', are individually found in several hundred human E3 ligases, and collaborate with E2 enzymes to catalyze ubiquitylation. However, the vertebrate-specific CUL9 complex with RBX1 (also called ROC1), of interest due to its tumor suppressive interaction with TP53, uniquely encompasses both cullin-RING and RBR domains. Here, cryo-EM, biochemistry and cellular assays elucidate a 1.8-MDa hexameric human CUL9-RBX1 assembly. Within one dimeric subcomplex, an E2-bound RBR domain is activated by neddylation of its own cullin domain and positioning from the adjacent CUL9-RBX1 in trans. Our data show CUL9 as unique among RBX1-bound cullins in dependence on the metazoan-specific UBE2F neddylation enzyme, while the RBR domain protects it from deneddylation. Substrates are recruited to various upstream domains, while ubiquitylation relies on both CUL9's neddylated cullin and RBR domains achieving self-assembled and chimeric cullin-RING/RBR E3 ligase activity.

Structural basis of substrate recognition and allosteric activation of the proapoptotic mitochondrial HtrA2 protease

The mitochondrial serine protease HtrA2 is a human homolog of the Escherichia coli Deg-proteins exhibiting chaperone and proteolytic roles. HtrA2 is involved in both apoptotic regulation via its ability to degrade inhibitor-of-apoptosis proteins (IAPs), as well as in cellular maintenance as part of the cellular protein quality control machinery, by preventing the possible toxic accumulation of aggregated proteins. In this study, we use advanced solution NMR spectroscopy methods combined with biophysical characterization and biochemical assays to elucidate the crucial role of the substrate recognizing PDZ domain. This domain regulates the protease activity of HtrA2 by triggering an intricate allosteric network involving the regulatory loops of the protease domain. We further show that divalent metal ions can both positively and negatively modulate the activity of HtrA2, leading to a refined model of HtrA2 regulation within the apoptotic pathway.

Non-canonical substrate recognition by the human WDR26-CTLH E3 ligase regulates prodrug metabolism

The yeast glucose-induced degradation-deficient (GID) E3 ubiquitin ligase forms a suite of complexes with interchangeable receptors that selectively recruit N-terminal degron motifs of metabolic enzyme substrates. The orthologous higher eukaryotic C-terminal to LisH (CTLH) E3 complex has been proposed to also recognize substrates through an alternative subunit, WDR26, which promotes the formation of supramolecular CTLH E3 assemblies. Here, we discover that human WDR26 binds the metabolic enzyme nicotinamide/nicotinic-acid-mononucleotide-adenylyltransferase 1 (NMNAT1) and mediates its CTLH E3-dependent ubiquitylation independently of canonical GID/CTLH E3-family substrate receptors. The CTLH subunit YPEL5 inhibits NMNAT1 ubiquitylation and cellular turnover by WDR26-CTLH E3, thereby affecting NMNAT1-mediated metabolic activation and cytotoxicity of the prodrug tiazofurin. Cryoelectron microscopy (cryo-EM) structures of NMNAT1- and YPEL5-bound WDR26-CTLH E3 complexes reveal an internal basic degron motif of NMNAT1 essential for targeting by WDR26-CTLH E3 and degron mimicry by YPEL5's N terminus antagonizing substrate binding. Thus, our data provide a mechanistic understanding of how YPEL5-WDR26-CTLH E3 acts as a modulator of NMNAT1-dependent metabolism.

A conserved N-terminal motif of CUL3 contributes to assembly and E3 ligase activity of CRL3KLHL22

The CUL3-RING E3 ubiquitin ligases (CRL3s) play an essential role in response to extracellular nutrition and stress stimuli. The ubiquitin ligase function of CRL3s is activated through dimerization. However, how and why such a dimeric assembly is required for its ligase activity remains elusive. Here, we report the cryo-EM structure of the dimeric CRL3KLHL22 complex and reveal a conserved N-terminal motif in CUL3 that contributes to the dimerization assembly and the E3 ligase activity of CRL3KLHL22. We show that deletion of the CUL3 N-terminal motif impairs dimeric assembly and the E3 ligase activity of both CRL3KLHL22 and several other CRL3s. In addition, we found that the dynamics of dimeric assembly of CRL3KLHL22 generates a variable ubiquitination zone, potentially facilitating substrate recognition and ubiquitination. These findings demonstrate that a CUL3 N-terminal motif participates in the assembly process and provide insights into the assembly and activation of CRL3s.

The ubiquitin codes in cellular stress responses

Ubiquitination/ubiquitylation, one of the most fundamental post-translational modifications, regulates almost every critical cellular process in eukaryotes. Emerging evidence has shown that essential components of numerous biological processes undergo ubiquitination in mammalian cells upon exposure to diverse stresses, from exogenous factors to cellular reactions, causing a dazzling variety of functional consequences. Various forms of ubiquitin signals generated by ubiquitylation events in specific milieus, known as ubiquitin codes, constitute an intrinsic part of myriad cellular stress responses. These ubiquitination events, leading to proteolytic turnover of the substrates or just switch in functionality, initiate, regulate, or supervise multiple cellular stress-associated responses, supporting adaptation, homeostasis recovery, and survival of the stressed cells. In this review, we attempted to summarize the crucial roles of ubiquitination in response to different environmental and intracellular stresses, while discussing how stresses modulate the ubiquitin system. This review also updates the most recent advances in understanding ubiquitination machinery as well as different stress responses and discusses some important questions that may warrant future investigation.

BIRC6 Modulates the Protein Stability of Axin to Regulate the Growth, Stemness, and Resistance of Renal Cancer Cells via the β-Catenin Pathway

The mechanism underlying the development of renal cell carcinoma (RCC) remains unclear, and effective prevention and therapeutic measures are lacking. BIRC6, a protein inhibitor of apoptosis, has attracted great interest. Our data indicated that overexpression of BIRC6 elevated cell growth, colony formation, migration, and invasion of cultured RCC cells, while siRNA knockdown of BIRC6 suppressed these processes. Additionally, BIRC6 was highly expressed in RCC clinical samples along with a downregulated level of Axin. Immunoprecipitation assays found that BIRC6 interacted with Axin and the two proteins colocalized within the cytoplasm of RCC cells. Overexpression of BIRC6 promoted the ubiquitination modification of Axin, while genetic knockdown of BIRC6 suppressed it. Furthermore, overexpression of BIRC6 significantly promoted the turnover of Axin, suggesting BIRC6's inhibitory effect on Axin protein stability. BIRC6 was also upregulated in cancer stem-like cells of RCC and increased the drug resistance of RCC cells against sunitinib. Western blotting assays showed that the overexpression of BIRC6 upregulated CXCR4 protein expression and activated the β-catenin pathway. Two cell lines were then constructed with BIRC6 overexpressed by lentiviruses. Pharmacological administration of a Wnt/β-catenin inhibitor, XAV-939, or genetic knockdown of β-catenin inhibited cell growth, tumor sphere formation, colony formation, migration, and invasion of BIRC6-overexpressed cells. In vivo administration of XAV-939 markedly suppressed the tumorigenesis of BIRC6-overexpressed RCC cells in nude mice. In conclusion, we propose that BIRC6 activates the β-catenin signaling pathway via mediating the ubiquitination and degradation of Axin, promoting the growth, stemness, and drug resistance of RCC cells. This project aims to elucidate the role of BIRC6 as a potential therapeutic target and provide new insights into the clinical treatment of RCC.

Structural mechanisms of autoinhibition and substrate recognition by the ubiquitin ligase HACE1

Ubiquitin ligases (E3s) are pivotal specificity determinants in the ubiquitin system by selecting substrates and decorating them with distinct ubiquitin signals. However, structure determination of the underlying, specific E3-substrate complexes has proven challenging owing to their transient nature. In particular, it is incompletely understood how members of the catalytic cysteine-driven class of HECT-type ligases (HECTs) position substrate proteins for modification. Here, we report a cryogenic electron microscopy (cryo-EM) structure of the full-length human HECT HACE1, along with solution-based conformational analyses by small-angle X-ray scattering and hydrogen-deuterium exchange mass spectrometry. Structure-based functional analyses in vitro and in cells reveal that the activity of HACE1 is stringently regulated by dimerization-induced autoinhibition. The inhibition occurs at the first step of the catalytic cycle and is thus substrate-independent. We use mechanism-based chemical crosslinking to reconstitute a complex of activated, monomeric HACE1 with its major substrate, RAC1, determine its structure by cryo-EM and validate the binding mode by solution-based analyses. Our findings explain how HACE1 achieves selectivity in ubiquitinating the active, GTP-loaded state of RAC1 and establish a framework for interpreting mutational alterations of the HACE1-RAC1 interplay in disease. More broadly, this work illuminates central unexplored aspects in the architecture, conformational dynamics, regulation and specificity of full-length HECTs.

Molecular mechanisms underlying the BIRC6-mediated regulation of apoptosis and autophagy

Procaspase 9 is the initiator caspase for apoptosis, but how its levels and activities are maintained remains unclear. The gigantic Inhibitor-of-Apoptosis Protein BIRC6/BRUCE/Apollon inhibits both apoptosis and autophagy by promoting ubiquitylation of proapoptotic factors and the key autophagic protein LC3, respectively. Here we show that BIRC6 forms an anti-parallel U-shaped dimer with multiple previously unannotated domains, including a ubiquitin-like domain, and the proapoptotic factor Smac/DIABLO binds BIRC6 in the central cavity. Notably, Smac outcompetes the effector caspase 3 and the pro-apoptotic protease HtrA2, but not procaspase 9, for binding BIRC6 in cells. BIRC6 also binds LC3 through its LC3-interacting region, probably following dimer disruption of this BIRC6 region. Mutation at LC3 ubiquitylation site promotes autophagy and autophagic degradation of BIRC6. Moreover, induction of autophagy promotes autophagic degradation of BIRC6 and caspase 9, but not of other effector caspases. These results are important to understand how the balance between apoptosis and autophagy is regulated under pathophysiological conditions.

SMAC Mimetics for the Treatment of Lung Carcinoma: Present Development and Future Prospects

BACKGROUND

Uncontrolled cell growth and proliferation, which originate from lung tissue often lead to lung carcinoma and are more likely due to smoking as well as inhaled environmental toxins. It is widely recognized that tumour cells evade the ability of natural programmed death (apoptosis) and facilitates tumour progression and metastasis. Therefore investigating and targeting the apoptosis pathway is being utilized as one of the best approaches for decades.

OBJECTIVE

This review describes the emergence of SMAC mimetic drugs as a treatment approach, its possibilities to synergize the response along with current limitations as well as future perspective therapy for lung cancer.

METHOD

Articles were analysed using search engines and databases namely Pubmed and Scopus.

RESULT

Under cancerous circumstances, the level of Inhibitor of Apoptosis Proteins (IAPs) gets elevated, which suppresses the pathway of programmed cell death, plus supports the proliferation of lung cancer. As it is a major apoptosis regulator, natural drugs that imitate the IAP antagonistic response like SMAC mimetic agents/Diablo have been identified to trigger cell death. SMAC i.e. second mitochondria activators of caspases is a molecule produced by mitochondria, stimulates apoptosis by neutralizing/inhibiting IAP and prevents its potential responsible for the activation of caspases. Various preclinical data have proven that these agents elicit the death of lung tumour cells. Apart from inducing apoptosis, these also sensitize the cancer cells toward other effective anticancer approaches like chemo, radio, or immunotherapies. There are many SMAC mimetic agents such as birinapant, BV-6, LCL161, and JP 1201, which have been identified for diagnosis as well as treatment purposes in lung cancer and are also under clinical investigation.

CONCLUSION

SMAC mimetics acts in a restorative way in the prevention of lung cancer.

Rational Prediction of PROTAC-Compatible Protein-Protein Interfaces by Molecular Docking

Proteolysis targeting chimeras (PROTACs) are heterobifunctional ligands that mediate the interaction between a protein target and an E3 ligase, resulting in a ternary complex, whose interaction with the ubiquitination machinery leads to target degradation. This technology is emerging as an exciting new avenue for therapeutic development, with several PROTACs currently undergoing clinical trials targeting cancer. Here, we describe a general and computationally efficient methodology combining restraint-based docking, energy-based rescoring, and a filter based on the minimal solvent-accessible surface distance to produce PROTAC-compatible PPIs suitable for when there is no a priori known PROTAC ligand. In a benchmark employing a manually curated data set of 13 ternary complex crystals, we achieved an accuracy of 92% when starting from bound structures and 77% when starting from unbound structures, respectively. Our method only requires that the ligand-bound structures of the monomeric forms of the E3 ligase and target proteins be given to run, making it general, accurate, and highly efficient, with the ability to impact early-stage PROTAC-based drug design campaigns where no structural information about the ternary complex structure is available.

A massive machine regulates cell death

Structural analysis reveals how the decision to induce apoptotic cell death is regulated.