Cellular Ubiquitin Research Articles

E6AP is essential for the proliferation of HPV-positive cancer cells by preventing senescence.

Oncogenic types of human papillomaviruses (HPVs) are major human carcinogens. The formation of a trimeric complex between the HPV E6 oncoprotein, the cellular ubiquitin ligase E6AP and the p53 tumor suppressor protein leads to proteolytic p53 degradation and plays a central role for HPV-induced cell transformation. We here uncover that E6AP silencing in HPV-positive cancer cells ultimately leads to efficient induction of cellular senescence, revealing that E6AP acts as a potent anti-senescent factor in these cells. Thus, although the downregulation of either E6 or E6AP expression also acts partially pro-apoptotic, HPV-positive cancer cells surviving E6 repression proliferate further, whereas they become irreversibly growth-arrested upon E6AP repression. We moreover show that the senescence induction following E6AP downregulation is mechanistically highly dependent on induction of the p53/p21 axis, other than the known pro-senescent response of HPV-positive cancer cells following combined downregulation of the viral E6 and E7 oncoproteins. Of further note, repression of E6AP allows senescence induction in the presence of the anti-senescent HPV E7 protein. Yet, despite these mechanistic differences, the pathways underlying the pro-senescent effects of E6AP or E6/E7 repression ultimately converge by being both dependent on the cellular pocket proteins pRb and p130. Taken together, our results uncover a hitherto unrecognized and potent anti-senescent function of the E6AP protein in HPV-positive cancer cells, which is essential for their sustained proliferation. Our results further indicate that interfering with E6AP expression or function could result in therapeutically desired effects in HPV-positive cancer cells by efficiently inducing an irreversible growth arrest. Since the critical role of the E6/E6AP/p53 complex for viral transformation is conserved between different oncogenic HPV types, this approach could provide a therapeutic strategy, which is not HPV type-specific.

Proteolysis-targeting influenza vaccine strains induce broad-spectrum immunity and in vivo protection.

Generating effective live vaccines from intact viruses remains challenging owing to considerations of safety and immunogenicity. Approaches that can be applied in a systematic manner are needed. Here we created a library of live attenuated influenza vaccines by using diverse cellular E3 ubiquitin ligases to generate proteolysis-targeting (PROTAR) influenza A viruses. PROTAR viruses were engineered to be attenuated by the ubiquitin-proteasome system, which mediates viral protein degradation in conventional host cells, but allows efficient replication in engineered cell lines for large-scale manufacturing. Depending on the degron-E3 ligase pairs, viruses showed varying degrees of attenuation. In animal models, PROTAR viruses were highly attenuated and elicited robust, broad, strain-dependent humoral, mucosal and cellular immunity. In addition, they provided cross-reactive protection against homologous and heterologous viral challenges. This study provides a systematic approach for developing safe and effective vaccines, with potential applications in designing live attenuated vaccines against other pathogens.

Stress granule formation is regulated by signaling machinery involving Sch9/Ypk1, sphingolipids, and Ubi4.

Rationale: Stress granules (SGs) are membraneless organelles that are formed in response to various stresses. Multiple cellular processes have been reported to be involved in SG formation. However, the signaling cascades that coordinate SG formation remain to be elucidated. Methods: By performing two high-content imaging-based phenomic screens, we identified multiple signaling components that form a possible signal transduction pathway that regulates SG formation. Results: We found that Sch9 and Ypk1 function in an early step of SG formation, leading to a decrease in intermediate long-chain base sphingolipids (LCBs). This further downregulates the polyubiquitin precursor protein Ubi4 through upregulating the deubiquitinase Ubp3. Decreased levels of cellular free ubiquitin may subsequently facilitate Lsm7 phase separation and thus trigger SG formation. Conclusion: The signaling pathway identified in this work, together with its conserved components, provides valuable clues for understanding the mechanisms underlying SG formation and SG-associated human diseases.

Identification of suitable target/E3 ligase pairs for PROTAC development using a rapamycin-induced proximity assay (RiPA).

The development of proteolysis targeting chimeras (PROTACs), which induce the degradation of target proteins by bringing them into proximity with cellular E3 ubiquitin ligases, has revolutionized drug development. While the human genome encodes more than 600 different E3 ligases, current PROTACs use only a handful of them, drastically limiting their full potential. Furthermore, many PROTAC development campaigns fail because the selected E3 ligase candidates are unable to induce degradation of the particular target of interest. As more and more ligands for novel E3 ligases are discovered, the chemical effort to identify the best E3 ligase for a given target is exploding. Therefore, a genetic system to identify degradation-causing E3 ligases and suitable target/E3 ligase pairs is urgently needed. Here, we used the well-established dimerization of the FKBP12 protein and FRB domain by rapamycin to bring the target protein WDR5 into proximity with candidate E3 ligases. Strikingly, this rapamycin-induced proximity assay (RiPA) revealed that VHL, but not Cereblon, is able to induce WDR5 degradation - a finding previously made by PROTACs, demonstrating its predictive power. By optimizing the steric arrangement of all components and fusing the target protein with a minimal luciferase, RiPA can identify the ideal E3 for any target protein of interest in living cells, significantly reducing and focusing the chemical effort in the early stages of PROTAC development.

Identification of suitable target/E3 ligase pairs for PROTAC development using a rapamycin-induced proximity assay (RiPA)

The development of proteolysis targeting chimeras (PROTACs), which induce the degradation of target proteins by bringing them into proximity with cellular E3 ubiquitin ligases, has revolutionized drug development. While the human genome encodes more than 600 different E3 ligases, current PROTACs use only a handful of them, drastically limiting their full potential. Furthermore, many PROTAC development campaigns fail because the selected E3 ligase candidates are unable to induce degradation of the particular target of interest. As more and more ligands for novel E3 ligases are discovered, the chemical effort to identify the best E3 ligase for a given target is exploding. Therefore, a genetic system to identify degradation-causing E3 ligases and suitable target/E3 ligase pairs is urgently needed. Here, we used the well-established dimerization of the FKBP12 protein and FRB domain by rapamycin to bring the target protein WDR5 into proximity with candidate E3 ligases. Strikingly, this rapamycin-induced proximity assay (RiPA) revealed that VHL, but not Cereblon, is able to induce WDR5 degradation - a finding previously made by PROTACs, demonstrating its predictive power. By optimizing the steric arrangement of all components and fusing the target protein with a minimal luciferase, RiPA can identify the ideal E3 for any target protein of interest in living cells, significantly reducing and focusing the chemical effort in the early stages of PROTAC development.

USP33 promotes caerulein-induced apoptotic, oxidative and inflammatory injuries in acute pancreatitis by deubiquitinating TRAF3.

Background: Tumor necrosis factor receptor associated factor 3 (TRAF3) and deubiquitinating enzyme ubiquitin-specific protease 33 (USP33) have been identified to play important roles in inflammatory diseases, including acute pancreatitis (AP). Here, we aimed to explore whether USP33 affected AP progression by affecting TRAF3 expression through deubiquitination.Methods: Caerulein-treated HPDE6-C7 cells were used to mimic AP conditions in vitro. Levels of mRNAs and proteins were examined by qRT-PCR and Western blot. Cell proliferation and apoptosis were evaluated using CCK-8 assay, EdU assay and flow cytometry. Cell oxidative stress was assessed by detecting the production of superoxide dismutase and malonaldehyde. ELISA analysis detected IL-6 and TNF-α levels. Macrophage M1 polarization was evaluated by flow cytometry. Cellular ubiquitination analyzed the ubiquitination effect on TRAF3. Protein interaction between USP33 and TRAF3 was identified by immunofluorescence staining.Results: Caerulein dose-dependently induced apoptosis, oxidative stress, and inflammatory response in HPDE6-C7 cells and promoted macrophage M1 polarization to enhance inflammation (P < 0.05). TRAF3 was highly expressed in AP patients (3.5±1.10 vs. 1.0 ±0.74, P < 0.05) and caerulein-induced HPDE6-C7 cells (3.3 ±0.34 vs. 1.0 ±0.10, P < 0.05). Knockdown of TRAF3 protected HPDE6-C7 cells from caerulein-induced apoptotic, oxidative and inflammatory injuries. Mechanistically, USP33 interacted with TRAF3 and induced TRAF3 deubiquitination to up-regulate its expression (P < 0.05). Further analyses showed that USP33 knockdown reversed caerulein-induced apoptosis, oxidative stress and inflammation in HPDE6-C7 cells by TRAF3 (P < 0.05). Moreover, USP33-TRAF3 activated the NF-κB pathway (P < 0.05).Conclusion: USP33 promoted caerulein-induced apoptosis, oxidative stress and inflammation in pancreatic ductal cells by deubiquitinating TRAF3, indicating a novel insight into the pathogenesis of AP.

Signatures of Six Autophagy-Related Genes as Diagnostic Markers of Thyroid-Associated Ophthalmopathy and Their Correlation With Immune Infiltration.

Thyroid-associated ophthalmopathy (TAO) is one of the most complex autoimmune diseases in endocrinology areas. Autophagy-related genes may be involved in the pathophysiology of TAO. This study aims to reveal key genes associated with autophagy in the pathogenesis and the potential diagnostic markers for TAO. We obtained autophagy-related differential genes (AR-DEGs) and their expression in TAO patients and controls. Gene ontology analysis (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis were used to perform the enrichment analysis of AR-DEGs. LASSO regression, support vector machine recursive feature elimination, and random forest were performed to screen for disease signature genes (DSGs), which were further validated in another independent validation dataset. We used the receiver operating characteristic for the evaluation of the diagnostic efficacy of DSGs and also established a nomogram. The relative proportion of immune infiltration was calculated using the CIBERSORT algorithm, and the relationship between the identified gene markers and the level of infiltrating immune cells was explored. We identified 24 AR-DEGs, which were primarily enriched in cellular catabolic regulation, autophagosome membrane, and ubiquitin protein ligase binding in GO analysis, while KEGG analysis highlighted autophagy as the main enriched pathway. Six DSGs were identified by three algorithms. They were validated in another independent validation dataset. The combined six-gene model also showed good diagnostic efficacy (AUC = 0.948). We further plotted the nomogram with better diagnostic efficacy. Immuno-infiltration analysis and correlation analysis demonstrated that six DSGs were significantly correlated with the infiltrating immune cells. We identified several biological processes and pathways for the enrichment of AR-DEGs. Six DSGs were identified, which showed great potential to become critical molecules in the diagnosis of TAO, and these DSGs showed a correlation with infiltrating immune cells.

FBXO22 inhibits colitis and colorectal carcinogenesis by regulating the degradation of the S2448-phosphorylated form of mTOR

Inflammatory bowel disease (IBD) is a considerable threat to human health with a significant risk for colorectal cancer (CRC). However, currently, both the molecular pathogenesis and therapeutic treatment of IBD remain limited. In this report, using both systemic and intestinal epithelium-specific gene knockout mouse models, we demonstrate that FBXO22, a substrate receptor within the SKP1-Cullin 1-F-box family of E3 ubiquitin ligases, plays an inhibitory role in the Azoxymethane/Dextran Sodium Sulfate-induced colorectal inflammatory responses and CRC. FBXO22 targets the serine 2448-phosphorylated form of mammalian mechanistic target of rapamycin (pS2448-mTOR) for ubiquitin-dependent degradation. This proteolytic targeting effect is established based on multiple lines of evidence including the results of colon tissue immunoblots, analysis of cultured cells with altered abundance of FBXO22 by depletion or overexpression, comparison of protein decay rate, effects on mTOR substrates S6K1 and 4E-BP1, analysis of protein-protein interactions, phosphor-peptide binding and competition, as well as reconstituted and cellular ubiquitination. Finally, we have shown that mTOR inhibitor rapamycin (RAPA) was able to alleviate the effects of fbxo22 deletion on colorectal inflammatory response and CRC. These RAPA effects are correlated with the ability of RAPA to inhibit pS2448-mTOR, pS6K1, and p4E-BP1. Collectively, our data support a suppressive role for FBXO22 in colorectal inflammation signaling and CRC initiation by targeting pS2448-mTOR for degradation.

Caspase-2 is a condensate-mediated deubiquitinase in protein quality control

Protein ubiquitination plays a critical role in protein quality control in response to cellular stress. The excessive accumulation of ubiquitinated conjugates can be detrimental to cells and is recognized as a hallmark of multiple neurodegenerative diseases. However, an in-depth understanding of how the excessive ubiquitin chains are removed to maintain ubiquitin homeostasis post stress remains largely unclear. Here we found that caspase-2 (CASP2) accumulates in a ubiquitin and proteasome-positive biomolecular condensate, which we named ubstressome, following stress and functions as a deubiquitinase to remove overloaded ubiquitin chains on proteins prone to misfolding. Mechanistically, CASP2 binds to the poly-ubiquitinated conjugates through its allosteric ubiquitin-interacting motif-like region and decreases overloaded ubiquitin chains in a protease-dependent manner to promote substrate degradation. CASP2 deficiency in mice results in excessive accumulation of poly-ubiquitinated TAR DNA-binding protein 43, leading to motor defects. Our findings uncover a stress-evoked deubiquitinating activity of CASP2 in the maintenance of cellular ubiquitin homeostasis, which differs from the well-known roles of caspase in apoptosis and inflammation. These data also reveal unrecognized protein quality control functions of condensates in the removal of stress-induced ubiquitin chains.

CXCR4 up-regulation mediated by USP1 deubiquitination promotes the tumorigenesis and immune escape in esophageal squamous-cell carcinoma.

CXC chemokine receptor 4 (CXCR4) and ubiquitin specific protease 1 (USP1) have been reported to involve in the tumorigenesis of esophageal squamous-cell carcinoma (ESCC). Here, we investigated whether USP1 induced CXCR4 deubiquitination in regulating ESCC progression. MTT assay, 5-ethynyl-2'-deoxyuridine assay, flow cytometry, transwell assay and ELISA analysis were used to detect cell oncogenic phenotypes, macrophage phenotypes, inflammatory cytokines production, the cytotoxicity of cytokine-induced killer (CIK) cells and CD8 + T cell apoptosis. Protein interaction was determined by immunoprecipitation assay. Cellular ubiquitination detected the ubiquitination effect on CXCR4. A mouse xenograft model was established for in vivo experiments. CXCR4 was highly expressed in ESCC tissues and cells. Functionally, CXCR4 silencing suppressed ESCC cell proliferation, invasion, and induced cell apoptosis. Moreover, CXCR4 deficiency suppressed cancer cell immune escape by suppressing macrophage M2 polarization, elevating inflammatory cytokines produced by PBMCs, enhancing the cytotoxicity of CIK cells, and suppressing CD8 + T cell apoptosis. A high USP1 expression was observed in ESCC, USP1 interacted with CXCR4 and enhanced its protein stability through deubiquitination. USP1 silencing suppressed ESCC cell proliferation, invasion, and immune escape, which were reversed by CXCR4 overexpression. In vivo assay showed that USP1 deficiency impeded tumor growth by regulating CXCR4. Besides, fused in sarcoma (FUS) was confirmed to bind to USP1 and stabilized its mRNA expression, and could regulate CXCR4 via USP1. In conclusion, USP1 stabilized CXCR4 by removing ubiquitination on CXCR4, thereby promoting ESCC cell proliferation, invasion, and immune escape in vitro, and tumor growth in vivo.

Design of linked-domain protein inhibitors of UBE2D as tools to study cellular ubiquitination.

Ubiquitin (Ub) is a post-translational modification that largely controls proteostasis through mechanisms spanning transcription, translation, and notably, protein degradation. Ub conjugation occurs through a hierarchical cascade of three enzyme classes (E1, E2, and E3s) involving >1000 proteins that regulate the ubiquitination of proteins. The E2 Ub-conjugating enzymes are the midpoint, yet their cellular roles remain under-characterized, partly due to a lack of inhibitors. For example, the cellular roles of the promiscuous E2 UBE2D/UBCH5 are not well described. Here, we develop a highly selective, multivalent, engineered protein inhibitor for the UBE2D family that simultaneously targets the RING- and backside-binding sites. In HeLa cells, these inhibitors phenocopy knockdown of UBE2D by reducing the IC50 to cisplatin and whole-cell proteomics reveal an increased abundance of ~20% of the identified proteins, consistent with reduced Ub degradation and proteotoxic stress. These precision tools will enable new studies probing UBE2D's central role in proteome management.

CRL4-DCAF1 Ubiquitin Ligase Dependent Functions of HIV Viral Protein R and Viral Protein X.

The Human Immunodeficiency Virus (HIV) encodes several proteins that contort the host cell environment to promote viral replication and spread. This is often accomplished through the hijacking of cellular ubiquitin ligases. These reprogrammed complexes initiate or enhance the ubiquitination of cellular proteins that may otherwise act to restrain viral replication. Ubiquitination of target proteins may alter protein function or initiate proteasome-dependent destruction. HIV Viral Protein R (Vpr) and the related HIV-2 Viral Protein X (Vpx), engage the CRL4-DCAF1 ubiquitin ligase complex to target numerous cellular proteins. In this review we describe the CRL4-DCAF1 ubiquitin ligase complex and its interactions with HIV Vpr and Vpx. We additionally summarize the cellular proteins targeted by this association as well as the observed or hypothesized impact on HIV.

Crop antiviral defense: Past and future perspective.

Viral pathogens not only threaten the health and life of humans and animals but also cause enormous crop yield losses and contribute to global food insecurity. To defend against viral pathogens, plants have evolved an intricate immune system to perceive and cope with such attacks. Although most of the fundamental studies were carried out in model plants, more recent research in crops has provided new insights into the antiviral strategies employed by crop plants. We summarize recent advances in understanding the biological roles of cellular receptors, RNA silencing, RNA decay, hormone signaling, autophagy, and ubiquitination in manipulating crop host-mediated antiviral responses. The potential functions of circular RNAs, the rhizosphere microbiome, and the foliar microbiome of crops in plant-virus interactions will be fascinating research directions in the future. These findings will be beneficial for the development of modern crop improvement strategies.

The role of RNA viruses in human cancers

Many RNA viruses have been reported to be oncogenic (or carcinogenic) in a variety of animal and human cancers. The increase in the incidence and prevalence of cancer-causing viruses in human populations can be known as a key precursor to the development of various cancers. The retrovirus family and Hepatitis C virus (HCV) are also reported to cause cancer. Viral oncoproteins such as Tax of HTLV 1 interacts with cellular ubiquitination complex such as cyclindromatosis tumor suppressor, ubiquitin-specific proteases 7, 11, 15 and 20, A-20 and signal-transducing adaptor molecule binding protein-like-1 in order to improve the cellular signaling pathways. The viral oncoproteins binding to DUB, leading to proliferation of virus-infected cells and cell transformation. Proto-oncogenes (c-onc genes) are the cellular form of v-onc genes. The activation of c-onc genes leads to cell growth. C-onc genes are transformed into an oncogenic form by viral infection. C-onc genes play some roles such as protein kinases, growth factors, growth factor receptors, and DNA binding proteins. The study of transforming retroviruses and their oncogenes and the multiple mechanisms deployed by other RNA viruses to use the growth-suppressive and proapoptotic function of tumor suppressor genes has been added to our current understanding of cancer biology. Oncogenic RNA viruses are important experimental models to study molecular investigation such as cellular networks, including the discovery of oncogenes and tumor suppressors. Understanding of different strategies of RNA viruses as well as the function of their proteins helps to make more extensive plans regarding the adoption of follow-up, prevention and treatment strategies in cancer patients caused by viral origin.

Targeting MCL1 with Sanggenon C overcomes MCL1-driven adaptive chemoresistance via dysregulation of autophagy and endoplasmic reticulum stress in cervical cancer

Cervical cancer ranks as one of the most prevalent malignancies among women worldwide and poses a significant threat to health and quality of life. MCL1 is an antiapoptotic protein closely linked to tumorigenesis, drug-resistance and poor prognosis in various cancers. Sanggenon C, a natural flavonoid derived from Morus albal., exhibits multiple activities, including anti-oxidant, anti-inflammatory, antivirus, and antitumor properties. However, the molecular mechanisms by which Sanggenon C exerts antitumor effects on in cervical cancer remain unclear. To investigate the oncogenic role of MCL1 and elucidate the antitumor activity of Sanggenon C, along with its molecular mechanisms, in cervical cancer. In vitro, the effects of Sanggenon C on proliferation, the cell cycle, apoptosis, and autophagy were explored. Transcriptome sequencing was employed to analyze critical genes and pathways. The expression of genes or proteins was evaluated via immunofluorescence, qRT-PCR, immunohistochemistry, and Western blotting. To identify targets of Sanggenon C, various techniques such as clinical database analysis, molecular docking, cellular thermal shift assays, co-immunoprecipitation, and ubiquitination assays were utilized. Additionally, Xenograft mouse models were established to further investigate Sanggenon C as a novel MCL1 inhibitor and its anti-tumor activity in vivo. Our investigation reveals that Sanggenon C effectively inhibits cervical cancer cell proliferation both in vitro and in vivo. Furthermore, Sanggenon C induces endoplasmic reticulum stress and triggers protective autophagy via activation of the ATF4-DDIT3-TRIB3-AKT-MTOR signaling axis. Furthermore, Sanggenon C specifically targets MCL1 to exert its antitumor effects by modulating MCL1 protein stability through SYVN1-mediated ubiquitination. Notably, MCL1 overexpression attenuates the Sanggenon C-induced decrease in cell viability and apoptosis. Our study further characterizes the role of MCL1 in cisplatin resistance and identifies MCL1 as a promising target for Sanggenon C, which effectively inhibits proliferation and induces apoptosis in cisplatin-resistant cervical cancer cells. Importantly, combining Sanggenon C with an autophagy inhibitor represents a promising strategy to enhance therapeutic outcomes in cisplatin-resistant cervical cancer cells. Our findings demonstrates that Sanggenon C induces endoplasmic reticulum stress and highlights the potential of targeting MCL1 to exploit vulnerabilities in drug-resistant cervical cancer cells. Sanggenon C emerges as a promising therapeutic agent against MCL1-driven adaptive chemoresistance through disruption of autophagy and endoplasmic reticulum stress in cervical cancer.

MRPL35 Induces Proliferation, Invasion, and Glutamine Metabolism in NSCLC Cells by Upregulating SLC7A5 Expression.

Mitochondrial ribosomal protein L35 (MRPL35) has been reported to contribute to the growth of non-small cell lung cancer (NSCLC) cells. However, the functions and mechanisms of MRPL35 on glutamine metabolism in NSCLC remain unclear. The detection of mRNA and protein of MRPL35, ubiquitin-specific protease 39 (USP39), and solute carrier family 7 member 5 (SLC7A5) was conducted using qRT-PCR and western blotting. Cell proliferation, apoptosis, and invasion were evaluated using the MTT assay, EdU assay, flow cytometry, and transwell assay, respectively. Glutamine metabolism was analyzed by detecting glutamine consumption, α-ketoglutarate level, and glutamate production. Cellular ubiquitination analyzed the deubiquitination effect of USP39 on MRPL35. An animal experiment was conducted for invivo analysis. MRPL35 was highly expressed in NSCLC tissues and cell lines, and high MRPL35 expression predicted poor outcome in NSCLC patients. Invitro analyses suggested that MRPL35 knockdown suppressed NSCLC cell proliferation, invasion, and glutamine metabolism. Moreover, MRPL35 silencing hindered tumor growth invivo. Mechanistically, USP39 stabilized MRPL35 expression by deubiquitination and then promoted NSCLC cell proliferation, invasion, and glutamine metabolism. In addition, MRPL35 positively affected SLC7A5 expression in NSCLC cells invitro and invivo. Moreover, the anticancer effects of MRPL35 silencing could be rescued by SLC7A5 overexpression in NSCLC cells. MRPL35 expression was stabilized by USP39-induced deubiquitination in NSCLC cells, and knockdown of MRPL35 suppressed NSCLC cell proliferation, invasion, and glutamine metabolism invitro and impeded tumor growth invivo by upregulating SLC7A5, providing a promising therapeutic target for NSCLC.

Amphiphilic Affibody-PROTAC conjugate Self-Assembled nanoagents for targeted cancer therapy

Amphiphilic Affibody-PROTAC conjugate Self-Assembled nanoagents for targeted cancer therapy

Small molecule induced STING degradation facilitated by the HECT ligase HERC4

Stimulator of interferon genes (STING) is a central component of the cytosolic nucleic acids sensing pathway and as such master regulator of the type I interferon response. Due to its critical role in physiology and its’ involvement in a variety of diseases, STING has been a focus for drug discovery. Targeted protein degradation (TPD) has emerged as a promising pharmacology for targeting previously considered undruggable proteins by hijacking the cellular ubiquitin proteasome system (UPS) with small molecules. Here, we identify AK59 as a STING degrader leveraging HERC4, a HECT-domain E3 ligase. Additionally, our data reveals that AK59 is effective on the common pathological STING mutations, suggesting a potential clinical application of this mechanism. Thus, these findings introduce HERC4 to the fields of TPD and of compound-induced degradation of STING, suggesting potential therapeutic applications.

PROTAC-Mediated Dual Degradation of BCL-xL and BCL-2 Is a Highly Effective Therapeutic Strategy in Small-Cell Lung Cancer.

BCL-xL and BCL-2 are validated therapeutic targets in small-cell lung cancer (SCLC). Targeting these proteins with navitoclax (formerly ABT263, a dual BCL-xL/2 inhibitor) induces dose-limiting thrombocytopenia through on-target BCL-xL inhibition in platelets. Therefore, platelet toxicity poses a barrier in advancing the clinical translation of navitoclax. We have developed a strategy to selectively target BCL-xL in tumors, while sparing platelets, by utilizing proteolysis-targeting chimeras (PROTACs) that hijack the cellular ubiquitin proteasome system for target ubiquitination and subsequent degradation. In our previous study, the first-in-class BCL-xL PROTAC, called DT2216, was shown to have synergistic antitumor activities when combined with venetoclax (formerly ABT199, BCL-2-selective inhibitor) in a BCL-xL/2 co-dependent SCLC cell line, NCI-H146 (hereafter referred to as H146), in vitro and in a xenograft model. Guided by these findings, we evaluated our newly developed BCL-xL/2 dual degrader, called 753b, in three BCL-xL/2 co-dependent SCLC cell lines and the H146 xenograft models. 753b was found to degrade both BCL-xL and BCL-2 in these cell lines. Importantly, it was considerably more potent than DT2216, navitoclax, or DT2216 + venetoclax in reducing the viability of BCL-xL/2 co-dependent SCLC cell lines in cell culture. In vivo, 5 mg/kg weekly dosing of 753b was found to lead to significant tumor growth delay, similar to the DT2216 + venetoclax combination in H146 xenografts, by degrading both BCL-xL and BCL-2. Additionally, 753b administration at 5 mg/kg every four days induced tumor regressions. At this dosage, 753b was well tolerated in mice, without observable induction of severe thrombocytopenia as seen with navitoclax, and no evidence of significant changes in mouse body weights. These results suggest that the BCL-xL/2 dual degrader could be an effective and safe therapeutic for a subset of SCLC patients, warranting clinical trials in future.

SIAH1 modulates antiviral immune responses by targeting deubiquitinase USP19.

Tight control of the type I interferon (IFN) signaling pathway is critical for maintaining host innate immune responses, and the ubiquitination and deubiquitination of signaling molecules are essential for signal transduction. Deubiquitinase ubiquitin-specific protein 19 (USP19) is known to be involved in deubiquitinating Beclin1, TRAF3, and TRIF for downregulation of the type I IFN signaling. Here, we show that SIAH1, a cellular E3 ubiquitin ligase that is involved in multicellular pathway, is a potent positive regulator of virus-mediated type I IFN signaling that maintains homeostasis within the antiviral immune response by targeting USP19. In the early stages of virus infection, stabilized SIAH1 directly interacts with the USP19 and simultaneously mediates K27-linked ubiquitination of 489, 490, and 610 residues of USP19 for proteasomal degradation. Additionally, we found that USP19 specifically interacts with MAVS and deubiquitinates K63-linked ubiquitinated MAVS for negative regulation of type I IFN signaling. Ultimately, we identified that SIAH1-mediated degradation of USP19 reversed USP19-mediated deubiquitination of MAVS, Beclin1, TRAF3, and TRIF, resulting in the activation of antiviral immune responses. Taken together, these findings provide new insights into the molecular mechanism of USP19 and SIAH1, and suggest a critical role of SIAH1 in antiviral immune response and homeostasis.