Identifying a Ubiquitinated Adaptor Protein by a Viral E3 Ligase Through Co-immunoprecipitation.

Co-immunoprecipitation is a technique widely utilized to isolate protein complexes and study protein-protein interactions. Ubiquitinated proteins could be identified by combining co-immunoprecipitation with SDS-PAGE followed by immunoblotting. In this chapter, we use Herpes Simplex Virus 1 immediate-early protein ICP0-mediated polyubiquitination of p50 as an example to describe the method to identify a ubiquitinated adaptor protein by a viral E3 ligase by co-immunoprecipitation.

A Guideline Strategy for Identifying Genes/Proteins Regulating Antiviral Innate Immunity.

Antiviral innate immunity is a complicated system initiated by the induction of type I interferon (IFN-I) and downstream interferon-stimulated genes (ISGs) and is finely regulated by numerous positive and negative factors at different signaling adaptors. During this process, posttranslational modifications, especially ubiquitination, are the most common regulatory strategy used by the host to switch the antiviral innate signaling pathway and are mainly controlled by E3 ubiquitin ligases from different protein families. A comprehensive understanding of the regulatory mechanisms and a novel discovery of regulatory factors involved in the IFN-I signaling pathway are important for researchers to identify novel therapeutic targets against viral infectious diseases based on innate immunotherapy. In this section, we use the E3 ubiquitin ligase as an example to guide the identification of a protein belonging to the RING Finger (RNF) family that regulates the RIG-I-mediated IFN-I pathway through ubiquitination.

Citrullination modulation stabilizes HIF-1α to promote tumour progression.

Citrullination plays an essential role in various physiological or pathological processes, however, whether citrullination is involved in regulating tumour progression and the potential therapeutic significance have not been well explored. Here, we find that peptidyl arginine deiminase 4 (PADI4) directly interacts with and citrullinates hypoxia-inducible factor 1α (HIF-1α) at R698, promoting HIF-1α stabilization. Mechanistically, PADI4-mediated HIF-1αR698 citrullination blocks von Hippel-Lindau (VHL) binding, thereby antagonizing HIF-1α ubiquitination and subsequent proteasome degradation. We also show that citrullinated HIF-1αR698, HIF-1α and PADI4 are highly expressed in hepatocellular carcinoma (HCC) tumour tissues, suggesting a potential correlation between PADI4-mediated HIF-1αR698 citrullination and cancer development. Furthermore, we identify that dihydroergotamine mesylate (DHE) acts as an antagonist of PADI4, which ultimately suppresses tumour progression. Collectively, our results reveal citrullination as a posttranslational modification related to HIF-1α stability, and suggest that targeting PADI4-mediated HIF-1α citrullination is a promising therapeutic strategy for cancers with aberrant HIF-1α expression.

Timely lagging strand maturation relies on Ubp10 deubiquitylase-mediated PCNA dissociation from replicating chromatin.

Synthesis and maturation of Okazaki Fragments is an incessant and highly efficient metabolic process completing the synthesis of the lagging strands at replication forks during S phase. Accurate Okazaki fragment maturation (OFM) is crucial to maintain genome integrity and, therefore, cell survival in all living organisms. In eukaryotes, OFM involves the consecutive action of DNA polymerase Pol ∂, 5' Flap endonuclease Fen1 and DNA ligase I, and constitutes the best example of a sequential process coordinated by the sliding clamp PCNA. For OFM to occur efficiently, cooperation of these enzymes with PCNA must be highly regulated. Here, we present evidence of a role for the K164-PCNA-deubiquitylase Ubp10 in the maturation of Okazaki fragments in the budding yeast Saccharomyces cerevisiae. We show that Ubp10 associates with lagging-strand DNA synthesis machineries on replicating chromatin to ensure timely ligation of Okazaki fragments by promoting PCNA dissociation from chromatin requiring lysine 164 deubiquitylation.

Targeted protein degradation using chimeric human E2 ubiquitin-conjugating enzymes.

Proteins can be targeted for degradation by engineering biomolecules that direct them to the eukaryotic ubiquitination machinery. For instance, the fusion of an E3 ubiquitin ligase to a suitable target binding domain creates a 'biological Proteolysis-Targeting Chimera' (bioPROTAC). Here we employ an analogous approach where the target protein is recruited directly to a human E2 ubiquitin-conjugating enzyme via an attached target binding domain. Through rational design and screening we develop E2 bioPROTACs that induce the degradation of the human intracellular proteins SHP2 and KRAS. Using global proteomics, we characterise the target-specific and wider effects of E2 vs. VHL-based fusions. Taking SHP2 as a model target, we also employ a route to bioPROTAC discovery based on protein display libraries, yielding a degrader with comparatively weak affinity capable of suppressing SHP2-mediated signalling.

The deubiquitinating enzyme USP11 regulates breast cancer progression by stabilizing PGAM5.

Breast cancer is common worldwide. Phosphoglycerate mutase 5 (PGAM5) belongs to the phosphoglycerate mutase family and plays an important role in many cancers. However, research on its role in breast cancer remains unclear. The present investigation highlights the significant expression of PGAM5 in breast cancer and its essential role in cell proliferation, invasion, apoptosis and the regulation of ferroptosis in breast cancer cells. Overexpression or knockdown of ubiquitin-specific protease 11 (USP11) promotes or inhibits the growth and metastasis of breast cancer cells, respectively, in vitro and in vivo. Mechanistically, USP11 stabilizes PGAM5 via de-ubiquitination, protecting it from proteasome-mediated degradation. In addition, the USP11/PGAM5 complex promotes breast cancer progression by activating iron death-related proteins, indicating that the synergy between USP11 and PGAM5 may serve as a predictor of disease outcome and provide a new treatment strategy for breast cancer.

An Overview of the Deubiquitinase USP53: A Promising Diagnostic Marker and Therapeutic Target.

Ubiquitination and deubiquitination are important mechanisms to maintain normal physiological activities, and their disorders or imbalances can lead to various diseases. As a subgroup of deubiquitinases (DUBs), the ubiquitin-specific peptidase (USP) family is closely related to many biological processes. USP53, one of the family members, is widely expressed in human tissues and participates in a variety of life activities, such as cell apoptosis, nerve transmission, and bone remodeling. Mutations in the USP53 gene can cause cholestasis and deafness and may also be a potential cause of schizophrenia. Knockout of USP53 can alleviate neuropathic pain induced by chronic constriction injury. Loss of USP53 up-regulates RANKL expression, promotes the cytogenesis and functional activity of osteoclasts, and triggers osteodestructive diseases. USP53 plays a tumor-suppressive role in lung cancer, renal clear cell carcinoma, colorectal cancer, liver cancer, and esophageal cancer but reduces the radiosensitivity of cervical cancer and esophageal cancer to induce radioresistance. Through the in-depth combination of literature and bioinformatics, this review suggested that USP53 may be a good potential biomarker or therapeutic target for diseases.

Ubiquitylation: Sword and shield in the bacterial arsenal.

In two recent studies in Nature, Hör et al.1 and Chambers et al.2 report that ubiquitin-like conjugation in bacteria antagonizes phage replication.

Correlation Between the Expression of DNA Damage Repair Protein OGG1 and Ubiquitination Pathway Protein STUB1 in Pediatric Neuroblastoma.

BACKGROUND: Neuroblastoma, a pediatric malignancy, is significantly influenced by genetic factors. Prior research indicates that the OGG1 rs1052133 G > C polymorphism correlates with a decreased risk of neuroblastoma. METHODS: We analyzed 57 neuroblastoma and 21 adrenal samples, using immunohistochemistry to measure OGG1 and STUB1 expression levels. We conducted a survival analysis to explore relationship between the expressions and neuroblastoma prognosis. RESULTS: Notably higher OGG1 expression and significantly lower STUB1 expression in neuroblastoma. OGG1 levels were significantly correlated with patient age, tumor location, histological grade, Shimada classification, INSS stage, and risk category. A negative association was observed between OGG1 and STUB1 expressions. Higher OGG1 expression was linked to reduced PFS and OS. Lower STUB1 expression was associated with unfavorable PFS. Additionally, OGG1 expression and risk category emerged as independent predictors of prognosis. CONCLUSION: OGG1 potentially functions as an oncogene in NB, with its activity possibly modulated by STUB1 through the ubiquitination pathway.

Orientia tsutsugamushi Ank5 promotes NLRC5 cytoplasmic retention and degradation to inhibit MHC class I expression.

How intracellular bacteria subvert the major histocompatibility complex (MHC) class I pathway is poorly understood. Here, we show that the obligate intracellular bacterium Orientia tsutsugamushi uses its effector protein, Ank5, to inhibit nuclear translocation of the MHC class I gene transactivator, NLRC5, and orchestrate its proteasomal degradation. Ank5 uses a tyrosine in its fourth ankyrin repeat to bind the NLRC5 N-terminus while its F-box directs host SCF complex ubiquitination of NLRC5 in the leucine-rich repeat region that dictates susceptibility to Orientia- and Ank5-mediated degradation. The ability of O. tsutsugamushi strains to degrade NLRC5 correlates with ank5 genomic carriage. Ectopically expressed Ank5 that can bind but not degrade NLRC5 protects the transactivator during Orientia infection. Thus, Ank5 is an immunoevasin that uses its bipartite architecture to rid host cells of NLRC5 and reduce surface MHC class I molecules. This study offers insight into how intracellular pathogens can impair MHC class I expression.

STK40 inhibits trophoblast fusion by mediating COP1 ubiquitination to degrade P57Kip2.

BACKGROUND: The syncytiotrophoblast (SCT) layer in the placenta serves as a crucial physical barrier separating maternal-fetal circulation, facilitating essential signal and substance exchange between the mother and fetus. Any abnormalities in its formation or function can result in various maternal syndromes, such as preeclampsia. The transition of proliferative villous cytotrophoblasts (VCT) from the mitotic cell cycle to the G0 phase is a prerequisite for VCT differentiation and their fusion into SCT. The imprinting gene P57Kip2, specifically expressed in intermediate VCT capable of fusion, plays a pivotal role in driving this key event. Moreover, aberrant expression of P57Kip2 has been linked to pathological placental conditions and adverse fetal outcomes. METHODS: Validation of STK40 interaction with P57Kip2 using rigid molecular simulation docking and co-immunoprecipitation. STK40 expression was modulated by lentivirus in BeWo cells, and the effect of STK40 on trophoblast fusion was assessed by real-time quantitative PCR, western blot, immunofluorescence, and cell viability and proliferation assays. Co-immunoprecipitation, transcriptome sequencing, and western blot were used to determine the potential mechanisms by which STK40 regulates P57Kip2. RESULTS: In this study, STK40 has been identified as a novel interacting protein with P57Kip2, and its expression is down-regulated during the fusion process of trophoblast cells. Overexpressing STK40 inhibited cell fusion in BeWo cells while stimulating mitotic cell cycle activity. Further experiments indicated that this effect is attributed to its specific binding to the CDK-binding and the Cyclin-binding domains of P57Kip2, mediating the E3 ubiquitin ligase COP1-mediated ubiquitination and degradation of P57Kip2. Moreover, abnormally high expression of STK40 might significantly contribute to the occurrence of preeclampsia. CONCLUSIONS: This study offers new insights into the role of STK40 in regulating the protein-level homeostasis of P57Kip2 during placental development.

NDR1/FBXO11 promotes phosphorylation-mediated ubiquitination of ß-catenin to suppress metastasis in prostate cancer.

Background: Prostate cancer progression hinges on ß-catenin's stability and activity, a key factor in epithelial-mesenchymal transition (EMT) and metastasis. This study delves into NDR1-dependent phosphorylation's impact on ß-catenin via FBXO11, an E3 ubiquitin ligase, in prostate cancer cells. Methods: Human prostate cancer cell lines underwent various in vitro assays, including real-time PCR, Western blotting, immunoprecipitation, immunofluorescence, and protein stability assays, to explore ß-catenin's interactions and post-translational modifications. NDR1 modulation's in vivo efficacy was assessed using a nude mice lung metastasis model. Small-molecule screening identified a potential NDR1 activator, aNDR1, tested for its effects on metastasis via in vitro and in vivo assays. Results: NDR1 phosphorylated ß-catenin at Ser33/37, facilitating its interaction with FBXO11. This led to FBXO11-mediated ubiquitination and cytoplasmic degradation of ß-catenin, while the NDR1-FBXO11 complex impeded ß-catenin nuclear translocation by inducing JNK2 ubiquitination. Thus, NDR1 and FBXO11 jointly regulate ß-catenin activity in prostate cancer cells through dual phosphorylation-driven ubiquitination, potentially suppressing EMT. Reduced NDR1 expression inhibited FBXO11 and ß-catenin phosphorylation, diminishing ß-catenin and JNK2 ubiquitination, promoting EMT and enhancing prostate cancer cell metastasis. The inhibitory effects of aNDR1 on prostate cancer metastasis were validated. Conclusion: The NDR1/FBXO11 axis outlines a non-canonical ß-catenin degradation pathway crucial in regulating EMT and prostate cancer cell metastasis. NDR1 activation, particularly with aNDR1, could offer a promising therapeutic avenue against prostate cancer metastasis.

Recruitment of USP10 by GCS1 to deubiquitinate GRP78 promotes the progression of colorectal cancer via alleviating endoplasmic reticulum stress.

BACKGROUND: Long-term accumulation of misfolded proteins leads to endoplasmic reticulum (ER) stress in colorectal cancer (CRC). However, the precise pathways controlling the decision between survival and apoptosis in CRC are unclear. Therefore, in this study, we investigated the function and molecular mechanism of glucosidase I (GCS1) in regulating ER stress in CRC. METHODS: A public database was used to confirm the expression level of GCS1 in CRC and normal tissues. Clinical samples from our center were used to confirm the mRNA and protein expression levels of GCS1. Cell proliferation, migration, invasion, and apoptosis assays revealed the biological role of GCS1. Immunohistochemical techniques were used to evaluate the expression of key proteins in subcutaneous implanted tumors in nude mice, which provided further evidence for the biological function of GCS1 in promoting cancer in vivo. The results of coimmunoprecipitation-mass spectrometry analysis and immunofluorescence colocalization analysis the interaction between GCS1 and GRP78. In addition, the mechanism of action of USP10, GRP78, and GCS1 at the post- translational level was investigated. Finally, a tissue microarray was used to examine the connection between GCS1 and GRP78 expression and intracellular localization of these proteins using immunohistochemistry and immunofluorescence. RESULTS: The experimental results revealed that GCS1 was substantially expressed in CRC, with higher expression indicating a worse prognosis. Thus, GCS1 can enhance the proliferation and metastasis while inhibiting the apoptosis of CRC cells both in vivo and in vitro. Mechanistically, GCS1 binds to GRP78, recruits USP10 for deubiquitination of GRP78 to promote its degradation, and decreases ER stress-mediated apoptosis, increasing CRC cell proliferation and metastasis. CONCLUSIONS: In summary, GCS1 stimulates CRC growth and migration and reduces ER stress-mediated apoptosis via USP10-mediated deubiquitination of GRP78. Our findings identify a possible therapeutic target for CRC.

STUB1-mediated K63-linked ubiquitination of UHRF1 promotes the progression of cholangiocarcinoma by maintaining DNA hypermethylation of PLA2G2A.

BACKGROUND: Cholangiocarcinoma (CCA) is a highly malignant tumor characterized by a lack of effective targeted therapeutic strategies. The protein UHRF1 plays a pivotal role in the preservation of DNA methylation and works synergistically with DNMT1. Posttranscriptional modifications (PTMs), such as ubiquitination, play indispensable roles in facilitating this process. Nevertheless, the specific PTMs that regulate UHRF1 in CCA remain unidentified. METHODS: We confirmed the interaction between STUB1 and UHRF1 through mass spectrometry analysis. Furthermore, we investigated the underlying mechanisms of the STUB1-UHRF1/DNMT1 axis via co-IP experiments, denaturing IP ubiquitination experiments, nuclear‒cytoplasmic separation and immunofluorescence experiments. The downstream PLA2G2A gene, regulated by the STUB1-UHRF1/DNMT1 axis, was identified via RNA-seq.  The negative regulatory mechanism of PLA2G2A was explored via bisulfite sequencing PCR (BSP) experiments to assess changes in promoter methylation. The roles of PLA2G2A and STUB1 in the proliferation, invasion, and migration of CCA cells were assessed using the CCK-8 assay, colony formation assay, Transwell assay, wound healing assay and xenograft mouse model. We evaluated the effects of STUB1/UHRF1 on cholangiocarcinoma by utilizing a primary CCA mouse model. RESULTS: This study revealed that STUB1 interacts with UHRF1, resulting in an increase in the K63-linked ubiquitination of UHRF1. Consequently, this facilitates the nuclear translocation of UHRF1 and enhances its binding affinity with DNMT1. The STUB1-UHRF1/DNMT1 axis led to increased DNA methylation of the PLA2G2A promoter, subsequently repressing its expression. Increased STUB1 expression in CCA was inversely correlated with tumor progression and overall survival. Conversely, PLA2G2A functions as a tumor suppressor in CCA by inhibiting cell proliferation, invasion and migration. CONCLUSIONS: These findings suggest that the STUB1-mediated ubiquitination of UHRF1 plays a pivotal role in tumor progression by epigenetically silencing PLA2G2A, underscoring the potential of STUB1 as both a prognostic biomarker and therapeutic target for CCA.

Vangl2 suppresses NF-κB signaling and ameliorates sepsis by targeting p65 for NDP52-mediated autophagic degradation.

Van Gogh-like 2 (Vangl2), a core planar cell polarity component, plays an important role in polarized cellular and tissue morphology induction, growth development, and cancer. However, its role in regulating inflammatory responses remains elusive. Here, we report that Vangl2 is upregulated in patients with sepsis and identify Vangl2 as a negative regulator of The nuclear factor-kappaB (NF-κB) signaling by regulating the protein stability and activation of the core transcription component p65. Mice with myeloid-specific deletion of Vangl2 (Vangl2ΔM) are hypersusceptible to lipopolysaccharide (LPS)-induced septic shock. Vangl2-deficient myeloid cells exhibit enhanced phosphorylation and expression of p65, therefore, promoting the secretion of proinflammatory cytokines after LPS stimulation. Mechanistically, NF-κB signaling-induced-Vangl2 recruits E3 ubiquitin ligase PDLIM2 to catalyze K63-linked ubiquitination on p65, which serves as a recognition signal for cargo receptor NDP52-mediated selective autophagic degradation. Taken together, these findings demonstrate Vangl2 as a suppressor of NF-κB-mediated inflammation and provide insights into the crosstalk between autophagy and inflammatory diseases.

The Proteasome and Cul3-Dependent Protein Ubiquitination Is Required for Gli Protein-Mediated Activation of Gene Expression in the Hedgehog Pathway.

Many cellular processes are regulated by proteasome-mediated protein degradation, including regulation of signaling pathways and gene expression. Among the pathways regulated by the ubiquitin-proteasome system is the Hedgehog pathway and its downstream effectors, the Gli transcription factors. Here we provide evidence that proteasomal activity is necessary for maintaining the activation of the Hedgehog pathway, and this crucial event takes place at the level of Gli proteins. We undertook extensive work to demonstrate the specificity of the observed phenomenon by ruling out the involvement of primary cilium, impaired nuclear import, failed dissociation from Sufu, microtubule stabilization, and stabilization of Gli repressor forms. Moreover, we showed that proteasomal-inhibition-mediated Hedgehog pathway downregulation is not restricted to the NIH-3T3 cell line. We demonstrated, using CRISPR/Ca9 mutagenesis, that neither Gli1, Gli2, nor Gli3 are solely responsible for the Hedgehog pathway downregulation upon proteasome inhibitor treatment, and that Cul3 KO renders the same phenotype. Finally, we report two novel E3 ubiquitin ligases, Btbd9 and Kctd3, known Cul3 interactors, as positive Hedgehog pathway regulators. Our data pave the way for a better understanding of the regulation of gene expression and the Hedgehog signaling pathway.

L3MBTL1, a polycomb protein, promotes Osimertinib acquired resistance through epigenetic regulation of DNA damage response in lung adenocarcinoma.

Osimertinib is a third-generation epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (EGFR-TKI) approved for patients with EGFR T790M resistance mutations as first- or second-line treatment of EGFR-positive patients. Resistance to Osimertinib will inevitably develop, and the underlying mechanisms are largely unknown. In this study, we discovered that acquired resistance to Osimertinib is associated with abnormal DNA damage response (DDR) in lung adenocarcinoma cells. We discovered that the polycomb protein Lethal(3) Malignant Brain Tumor-Like Protein 1 (L3MBTL1) regulates chromatin structure, thereby contributing to DDR and Osimertinib resistance. EGFR oncogene inhibition reduced L3MBTL1 ubiquitination while stabilizing its expression in Osimertinib-resistant cells. L3MBTL1 reduction and treatment with Osimertinib significantly inhibited DDR and proliferation of Osimertinib-resistant lung cancer cells in vitro and in vivo. L3MBTL1 binds throughout the genome and plays an important role in EGFR-TKI resistance. It also competes with 53BP1 for H4K20Me2 and inhibits the development of drug resistance in Osimertinib-resistant lung cancer cells in vitro and in vivo. Our findings suggest that L3MBTL1 inhibition is a novel approach to overcoming EGFR-TKI-acquired resistance.

Equol exerts anti-tumor effects on choriocarcinoma cells by promoting TRIM21-mediated ubiquitination of ANXA2.

Choriocarcinoma is a malignant cancer that belongs to gestational trophoblastic neoplasia (GTN). Herein, serum metabolomic analysis was performed on 29 GTN patients and 30 healthy individuals to characterize the metabolic variations during GTN progression. Ultimately 24 differential metabolites (DMs) were identified, of which, Equol was down-regulated in GTN patients, whose VIP score is the 3rd highest among the 24 DMs. As an intestinal metabolite of daidzein, the anticancer potential of Equol has been demonstrated in multiple cancers, but not choriocarcinoma. Hence, human choriocarcinoma cell lines JEG-3 and Bewo were used and JEG-3-derived subcutaneous xenograft models were developed to assess the effect of Equol on choriocarcinoma. The results suggested that Equol treatment effectively suppressed choriocarcinoma cell proliferation, induced cell apoptosis, and reduced tumorigenesis. Label-free quantitative proteomics showed that 136 proteins were significantly affected by Equol and 20 proteins were enriched in Gene Ontology terms linked to protein degradation. Tripartite motif containing 21 (TRIM21), a E3 ubiquitin ligase, was up-regulated by Equol. Equol-induced effects on choriocarcinoma cells could be reversed by TRIM21 inhibition. Annexin A2 (ANXA2) interacted with TRIM21 and its ubiquitination was modulated by TRIM21. We found that TRIM21 was responsible for proteasome-mediated degradation of ANXA2 induced by Equol, and the inhibitory effects of Equol on the malignant behaviors of choriocarcinoma cells were realized by TRIM21-mediated down-regulation of ANXA2. Moreover, ß-catenin activation was inhibited by Equol, which also depended on TRIM21-mediated down-regulation of ANXA2. Taken together, Equol may be a novel candidate for the treatment for choriocarcinoma.

USP1-mediated deubiquitination of KDM1A promotes the malignant progression of triple-negative breast cancer.

Previous research has indicated the highly expressed lysine-specific histone demethylase 1A (KDM1A) in several human malignancies, including triple-negative breast cancer (TNBC). However, its detailed mechanisms in TNBC development remain poorly understood. The mRNA levels of KDM1A and Yin Yang 1 (YY1) were determined by RT-qPCR analysis. Western blot was performed to measure KDM1A and ubiquitin-specific protease 1 (USP1) protein expression. Cell proliferation, apoptosis, invasion, migration and stemness were evaluated by MTT assay, EdU assay, flow cytometry, transwell invasion assay, wound-healing assay and sphere-formation assay, respectively. ChIP and dual-luciferase reporter assays were conducted to determine the relationship between YY1 and KDM1A. Xenograft tumor experiment and IHC were carried out to investigate the roles of USP1 and KDM1A in TNBC development in vivo. The highly expressed KDM1A was demonstrated in TNBC tissues and cells, and KDM1A knockdown significantly promoted cell apoptosis, and hampered cell proliferation, invasion, migration, and stemness in TNBC cells. USP1 could increase the stability of KDM1A via deubiquitination, and USP1 depletion restrained the progression of TNBC cells through decreasing KDM1A expression. Moreover, YY1 transcriptionally activated KDM1A expression by directly binding to its promoter in TNBC cells. Additionally, USP1 inhibition reduced KDM1A expression to suppress tumor growth in TNBC mice in vivo. In conclusion, YY1 upregulation increased KDM1A expression via transcriptional activation. USP1 stabilized KDM1A through deubiquitination to promote TNBC progression.

ER-associated degradation ligase HRD1 links ER stress to DNA damage repair by modulating the activity of DNA-PKcs.

Proteostasis and genomic integrity are respectively regulated by the endoplasmic reticulum-associated protein degradation (ERAD) and DNA damage repair signaling pathways, with both pathways essential for carcinogenesis and drug resistance. How these signaling pathways coordinate with each other remains unexplored. We found that ER stress specifically induces the DNA-PKcs-regulated nonhomologous end joining (NHEJ) pathway to amend DNA damage and impede cell death. Intriguingly, sustained ER stress rapidly decreased the activity of DNA-PKcs and DNA damage accumulated, facilitating a switch from adaptation to cell death. This DNA-PKcs inactivation was caused by increased KU70/KU80 protein degradation. Unexpectedly, the ERAD ligase HRD1 was found to efficiently destabilize the classic nuclear protein HDAC1 in the cytoplasm, by catalyzing HDAC1's polyubiquitination at lysine 74, at a late stage of ER stress. By abolishing HDAC1-mediated KU70/KU80 deacetylation, HRD1 transmits ER signals to the nucleus. The resulting enhanced KU70/KU80 acetylation provides binding sites for the nuclear E3 ligase TRIM25, resulting in the promotion of polyubiquitination and the degradation of KU70/KU80 proteins. Both in vitro and in vivo cancer models showed that genetic or pharmacological inhibition of HADC1 or DNA-PKcs sensitizes colon cancer cells to ER stress inducers, including the Food and Drug Administration-approved drug celecoxib. The antitumor effects of the combined approach were also observed in patient-derived xenograft models. These findings identify a mechanistic link between ER stress (ERAD) in the cytoplasm and DNA damage (NHEJ) pathways in the nucleus, indicating that combined anticancer strategies may be developed that induce severe ER stress while simultaneously inhibiting KU70/KU80/DNA-PKcs-mediated NHEJ signaling.