E3 Ubiquitin Ligase Siah2 Research Articles

ClickRNA-PROTAC for Tumor-Selective Protein Degradation and Targeted Cancer Therapy.

Proteolysis-targeting chimeras (PROTACs) show promise in tumor treatment. However, the E3 ligases VHL and CRBN, commonly used in PROTAC, are highly expressed in only a few tumors, thus limiting the application scope and efficacy of PROTAC drugs. Furthermore, the lack of tumor specificity in PROTAC drugs can result in toxic side effects. Therefore, there is an urgent need to develop tumor-selective PROTAC drugs that do not rely on endogenous E3 ligases. In this study, we introduce the ClickRNA-PROTAC system, which involves the expression of a fusion protein of the E3 ubiquitin ligase SIAH1 and SNAPTag through mRNA transfection and recruits the protein of interest (POI) using bio-orthogonal click chemistry. ClickRNA-PROTAC can effectively degrade various proteins such as BRD4, KRAS, and NFκB simply by replacing the warhead molecules. By employing a tumor-specific mRNA-responsive translation strategy, ClickRNA-PROTAC can selectively degrade POIs in tumor cells. Furthermore, ClickRNA-PROTAC demonstrated strong efficacy in targeted cancer therapy in a xenograft mouse model of adrenocortical carcinoma. In conclusion, this approach offers several advantages, including independence from endogenous E3 ubiquitin ligases, tumor specificity, and programmability, thereby paving the way for the development of PROTAC drugs.

Siah2- and LRSAM1-mediated K63-linked ubiquitination of snakehead vesiculovirus nucleoprotein facilitates viral replication.

Ubiquitination of viral protein plays an important role in viral replication. However, the ubiquitination of the nucleoprotein (N) of negative-strand RNA viruses has rarely been investigated. This study aimed at investigating the ubiquitination of the N protein of a fish rhabdovirus SHVV (snakehead vesiculovirus), identifying the related host E3 ubiquitin ligases, and determining the role of SHVV N ubiquitination and host E3 ubiquitin ligases in viral replication. We found that SHVV N was ubiquitinated mainly via K63-linked ubiquitination, which was mediated by host E3 ubiquitin ligases Siah2 (Siah E3 ubiquitin protein ligase 2) and LRSAM1 (leucine-rich repeat and sterile alpha motif containing 1). The data suggested that Siah2 and LRSAM1 were hijacked by SHVV to ubiquitinate the N protein for viral replication, which exhibited novel anti-SHVV targets for drug design.

Proteome-wide identification and functional analysis of ubiquitinated proteins in Hepa1-6 cells by knockdown of E3 ubiquitin ligase SIAH1.

Hepatocellular carcinoma (HCC) is an aggressive malignancy that poses a serious threat to human life. The conventional therapies for HCC cannot substantially improve overall survival (OS), disease duration, and prognosis. Therefore, it is important to study the underlying mechanism of HCC and seek better methods for HCC prevention and treatment. Ubiquitination is a post-translational modification that modulates great cellular function by cooperating with E1, E2, and E3 ligases. Yet, the ubiquitination and lysine residues in HCC are still elusive. Seven in absentia homolog 1 (SIAH1), as an important E3 ubiquitin ligase, regulates ubiquitin-mediated proteolysis to function as a tumor suppressor in HCC. In the present study, we downregulated SIAH1 in the mouse HCC cell line Hepa1-6 and studied its function by using proteome-wide identification. SIAH1 was knocked down by SIAH1 short hairpin RNA (shRNA) in mouse HCC cell line Hepa1-6 cells, and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis was conducted to analyze the ubiquitinated proteins. Functional analysis was performed using Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) enrichment. The systematic profiling showed a total of 550 differently expressed proteins (DEPs), including 263 upregulated DEPs and 287 downregulated DEPs. Considering the amino acid sequences around the modified lysine residues, seven proteins were identified as conserved ubiquitination motifs in the peptides. The ubiquitinated proteins were mainly distributed in the cytoplasm, nucleus, and plasma membrane. Functional analysis suggested that the ubiquitinated proteins were mostly enriched in the nucleus, cytoplasm, and extracellular space; in addition, the ubiquitinated proteins were mostly attributed to the protein binding, and disease. The ubiquitinated proteins modulate HCC by mapping lysine modification sites. The use of high-throughput characterization to identify novel and specific targets associated with SIAH1 is of great significance in terms of functional weight. The results obtained in this paper from the analysis of proteomic data provided novel insights into ubiquitination regulation in HCC, which pave the way for further research and mechanism discovery of HCC.

Abstract 5273: The E3 ubiquitin ligase SIAH2 promotes clear cell renal cell carcinoma epithelial-mesenchymal transition via regulation of SETD2 stability

Abstract Introduction & Objective: SETD2, a major histone H3K36 trimethyltransferase, has been shown to play an important role in multiple biological processes. Mutations in SETD2 have been evidenced in several types of cancer, including clear cell renal cell carcinoma (ccRCC). Previous studies reported the potential involvement of SETD2 in driving ccRCC tumorigenesis. However, the mechanism by which SETD2 causes cell metastasis remains poorly understood. Methods: Genome-wide CRISPR-Cas9-based screens and immunoprecipitation-mass spectrometry were used to identify candidates for regulating SETD2 stability. Immunohistochemistry staining of the ccRCC tissue array and bioinformatics analysis of TCGA database were used to investigate the clinical relevance of SETD2 in ccRCC. The gene expression was detected by qRT-PCR, and the protein level was detected by western blotting. Wound healing and transwell migration assay were used to explore the cell migration ability. SIAH2 was deleted from ccRCC cell lines using CRISPR/Cas9-mediated gene editing. Furthermore, we used BALB/c-nude mice to construct xenograft mouse models and pulmonary metastasis models. Moreover, RNA-seq, H3K36me3 CUT&Tag and RNA N6-methyladenosine-IP were performed to uncover the mechanism. Results: SETD2 mutant or low expression was correlated with poor prognosis in patients with ccRCC based on TCGA data and tissue array. Through the CRISPR/Cas9 screening system and immunoprecipitation-mass spectrometry, we identified the E3 ubiquitin ligase SIAH2 that regulates the stability of SETD2. Then, we verified that the protein stability of SETD2 was downregulated by SIAH2 in a dose-dependent manner and further confirmed that the SIAH2 promotes SETD2 degradation in a proteasome-dependent pathway. In addition, we constructed a dominant negative mutant of SIAH2 and proved the degradation dependent on its enzymatic activity. Moreover, we showed that SIAH2 knockout increased the protein level of SETD2. In SETD2 stable knockdown ccRCC cell lines, loss of SETD2 enhanced the capability of cell migration, while knockdown of SIAH2 resulted in the rescue of SETD2 deficient effect. Moreover, SETD2 ablation significantly decreases H3K36me3 peaks of E-cadherin, further reducing the m6A signal and stability of E-cadherin mRNA. Conclusion: In this study, we firstly revealed the mechanism that E3 ubiquitin ligase SIAH2 interacted with SETD2 and promoted the ubiquitination degradation of SETD2. Moreover, SETD2 ablation in ccRCC cells enhanced EMT through impaired epigenetic regulation of E-cadherin, suggesting SIAH2/SETD2/E-cadherin axis as a potential therapeutic target for ccRCC. Citation Format: Mengxue Yu, Lingao Ju, Gang Wang. The E3 ubiquitin ligase SIAH2 promotes clear cell renal cell carcinoma epithelial-mesenchymal transition via regulation of SETD2 stability. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5273.

Structural Basis of the Interaction between Human Axin2 and SIAH1 in the Wnt/β-Catenin Signaling Pathway.

The scaffolding protein Axin is an important regulator of the Wnt signaling pathway, and its dysfunction is closely related to carcinogenesis. Axin could affect the assembly and dissociation of the β-catenin destruction complex. It can be regulated by phosphorylation, poly-ADP-ribosylation, and ubiquitination. The E3 ubiquitin ligase SIAH1 participates in the Wnt pathway by targeting various components for degradation. SIAH1 is also implicated in the regulation of Axin2 degradation, but the specific mechanism remains unclear. Here, we verified that the Axin2-GSK3 binding domain (GBD) was sufficient for SIAH1 binding by the GST pull-down assay. Our crystal structure of the Axin2/SIAH1 complex at 2.53 Å resolution reveals that one Axin2 molecule binds to one SIAH1 molecule via its GBD. These interactions critically depend on a highly conserved peptide 361EMTPVEPA368 within the Axin2-GBD, which forms a loop and binds to a deep groove formed by β1, β2, and β3 of SIAH1 by the N-terminal hydrophilic amino acids Arg361 and Thr363 and the C-terminal VxP motif. The novel binding mode indicates a promising drug-binding site for regulating Wnt/β-catenin signaling.

Methods to Evaluate the Effects of HAT/KAT Inhibition on SIAH2-Driven Reactive Oxygen Species Generation in Helicobacter pylori-Infected Gastric Epithelial Cells.

Helicobacter pylori infection is one of the leading factors that promotes, among other diseases, gastric cancer (GC). Infection of gastric epithelial cells (GECs) by H. pylori enhances the expression as well as acetylation of the E3 ubiquitin ligase SIAH2 which promotes GC progression. The histone acetyltransferase (HAT) activity of p300 catalyzes SIAH2 acetylation following H. pylori infection. Since reactive oxygen species (ROS) generation in H. pylori-infected GECs accelerates GC progression, acetylation-mediated SIAH2 regulation might be a crucial modifier of ROS generation in the infected GECs. Here, we describe a compendium of methods to evaluate the effects of HAT/lysine acetyl transferase (KAT) inhibitors (HAT/KATi) on SIAH2-mediated ROS regulation in H. pylori-infected GECs.

Hypoxia-induced proteasomal degradation of DBC1 by SIAH2 in breast cancer progression

DBC1 has been characterized as a key regulator of physiological and pathophysiological activities, such as DNA damage, senescence, and tumorigenesis. However, the mechanism by which the functional stability of DBC1 is regulated has yet to be elucidated. Here, we report that the ubiquitination-mediated degradation of DBC1 is regulated by the E3 ubiquitin ligase SIAH2 and deubiquitinase OTUD5 under hypoxic stress. Mechanistically, hypoxia promoted DBC1 to interact with SIAH2 but not OTUD5, resulting in the ubiquitination and subsequent degradation of DBC1 through the ubiquitin-proteasome pathway. SIAH2 knockout inhibited tumor cell proliferation and migration, which could be rescued by double knockout of SIAH2/CCAR2. Human tissue microarray analysis further revealed that the SIAH2/DBC1 axis was responsible for tumor progression under hypoxic stress. These findings define a key role of the hypoxia-mediated SIAH2-DBC1 pathway in the progression of human breast cancer and provide novel insights into the metastatic mechanism of breast cancer.

Siah2-GRP78 interaction regulates ROS and provides a proliferative advantage to Helicobacter pylori-infected gastric epithelial cancer cells.

Helicobacter pylori-mediated gastric carcinogenesis involves upregulation of the E3 ubiquitin ligase Siah2 and its phosphorylation-mediated stabilization. This study elucidates a novel mechanism of oxidative stress regulation by phosphorylated Siah2 in H. pylori-infected gastric epithelial cancer cells (GECs). We identify that H. pylori-mediated Siah2 phosphorylation at the 6th serine residue (P-S6-Siah2) enhances proteasomal degradation of the 78-kDa glucose-regulated protein (GRP78) possessing antioxidant functions. S6 phosphorylation stabilizes Siah2 and P-S6-Siah2 potentiates H. pylori-mediated reactive oxygen species (ROS) generation. However, infected S6A phospho-null Siah2-expressing cells have decreased cellular GRP78 level as surprisingly these cells release GRP78 to a higher extent and accumulate significantly higher ROS than the wild type (WT) Siah2 construct-expressing cells. Ectopic expression of GRP78 prevents the loss of mitochondrial membrane potential and cellular ROS accumulation caused by H. pylori. H. pylori-induced mitochondrial damage and mitochondrial membrane potential loss are potentiated in Siah2-overexpressing cells but these effects are further enhanced in S6A-expressing cells. This study also confirms that while phosphorylation-mediated Siah2 stabilization optimally upregulates aggresome accumulation, it suppresses autophagosome formation, thus decreasing the dependency on the latter mechanism in regulating cellular protein abundance. Disruption of the phospho-Siah2-mediated aggresome formation impairs proliferation of infected GECs. Thus, Siah2 phosphorylation has diagnostic and therapeutic significance in H. pylori-mediated gastric cancer (GC).

FRK inhibits glioblastoma progression via phosphorylating YAP and inducing its ubiquitylation and degradation by Siah1.

We previously report that yes-associated protein (YAP), the core downstream effector of Hippo pathway, promotes the malignant progression of glioblastoma (GBM). However, although classical regulatory mechanisms of YAP are well explored, how YAP is modulated by the Hippo-independent manner remains poorly understood. Meanwhile, the nonreceptor tyrosine kinase Fyn-related kinase (FRK), which exhibits low expression and possesses tumor suppressor effects in GBM, is reported to be involved in regulation of protein phosphorylation. Here, we examined whether FRK could impede tumor progression by modulating YAP activities. Human GBM cells and intracranial GBM model were used to assess the effects of FRK and YAP on the malignant biological behaviors of GBM. Immunoblotting and immunohistochemistry were used to detect the expression of core proteins in GBM tissues. Co-immunoprecipitation, proximity ligation assay, luciferase assay and ubiquitination assay were utilized to determine the protein-protein interactions and related molecular mechanisms. The expression levels of FRK and YAP were inversely correlated with each other in glioma tissues. In addition, FRK promoted the ubiquitination and degradation of YAP, leading to tumor suppression in vitro and in vivo. Mechanistically, FRK interacted with and phosphorylated YAP on Tyr391/407/444, which recruited the classical E3 ubiquitin ligase Siah1 to catalyze ubiquitination and eventually degradation of YAP. Siah1 is required for YAP destabilization initiated by FRK. We identify a novel mechanism by which FRK orchestrates tumor-suppression effect through phosphorylating YAP and inducing its ubiquitination by Siah1. FRK-Siah1-YAP signaling axis may serve as a potential therapeutic target for GBM treatment.

Abstract 676: ZEB1 acetylation regulates dimerization and protein stability to promote lung adenocarcinoma progression and metastasis

Abstract Lung cancer is the leading cause of cancer-related death worldwide due to the ability of cancer cells to metastasize. Therefore, it is essential to expand our current knowledge of the biological processes that contribute to metastasis to guide the discovery of novel therapeutic modalities. The Epithelial-to-mesenchymal transition (EMT) is a mechanism for metastasis, which changes polarized epithelial cells into invasive mesenchymal cells. High expression of the Zinc finger E-box binding homeobox 1 (ZEB1) transcription factor is correlated to poor outcomes in cancer, including therapeutic resistance and EMT-mediated metastasis. ZEB1 has a predicted molecular weight of 125kDa; however, multiple groups have reported discrepancies in the observed molecular weight (approximately 190-250kDa). This has been attributed to dimerization mediated by post-translational modifications (PTMs). Therefore, we performed mass spectrometry and identified a novel PTM - K811 acetylation - that may regulate ZEB1 dimerization and function. To define the role of ZEB1 acetylation, we generated ZEB1 acetyl mimetic (K811Q) and deficient (K811R) mutants in a panel of lung adenocarcinoma cell lines. We hypothesize that ZEB1 acetylation regulates dimerization and protein stability to promote lung adenocarcinoma progression and metastasis. We determined that the acetyl-deficient mutant (125kDa) exhibits a decreased half-life compared to WT and acetylated ZEB1, suggesting that disruption of acetylation hinders dimerization and protein stability. However, the acetyl-mimetic mutant (250kDa) protects ZEB1 from proteasomal degradation brought by the action of the E3 ubiquitin ligase SIAH1. siRNA-mediated silencing of SIAH1 increased the mRNA and protein levels of ZEB1 WT and both acetylation mutants. Importantly, silencing of SIAH1 markedly increased the half-life of 250kDa ZEB1 WT from 18.0hrs to 25.4hrs and acetyl-mimetic from 12.3hrs to 24.0hrs, as well as 125kDa ZEB1 acetyl-deficient from 9.7hrs to 18.3hrs. Although we determined that 250kDa ZEB1 is significantly more stable than 125kDa ZEB1, the increase in the stability by silencing SIAH1 contributes to ZEB1 acetyl-deficient binding at the promoter of its target genes mir200c-141 and SEMA3F. 250kDa ZEB1 WT and acetyl-mimetic recruits the histone deacetylases (HDACs) containing the nucleosome remodeling and deacetylase (NuRD) complex, to promote lung adenocarcinoma invasion and metastasis compared to ZEB1 acetyl-deficient. We further established that the stability of ZEB1 is integral to promoting these mesenchymal features. Our results suggest that ZEB1_K811ac regulates its dimerization and stability to promote lung adenocarcinoma via EMT. Future studies are focused on understanding the role of ZEB1 protein stability to recruit NuRD co-repressor and subsequently mediate tumorigenesis and metastasis. Citation Format: Mabel G. Perez-Oquendo, Roxsan Manshouri, Don Gibbons. ZEB1 acetylation regulates dimerization and protein stability to promote lung adenocarcinoma progression and metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 676.

M6A-induced repression of SIAH1 facilitates alternative splicing of androgen receptor variant 7 by regulating CPSF1

Androgen receptor splice variant 7 (AR-v7), a constitutively active transcription factor, plays a crucial role in the progression of castration-resistant prostate cancer (CRPC). Here, we found that the cleavage and polyadenylation specificity factor 1 (CPSF1) (the largest subunit of the multi-protein cleavage and polyadenylation specificity complex), regulated by the E3 ubiquitin ligases SIAH1, promoted AR-v7 expression. The data from microarray-based analysis and clinical specimen-based analysis showed that SIAH1 expression was decreased in PCa and was negatively correlated with aggressive phenotypes of PCa. SIAH1 repressed PCa cell growth and invasion under castrate conditions. SIAH1 directly interacted with CPSF1 and promoted ubiquitination and degradation of CPSF1. CPSF1 expression was negatively correlated with SIAH1 expression, but positively with PCa progression. CPSF1 overexpression switched the AR splicing pattern and facilitated the generation of the oncogenic isoform (AR-v7) by binding to the AAUAAA polyadenylation signal contained in AR-cryptic exon CE3. Functionally, SIAH1 acted as a tumor suppressor in PCa pathogenesis by repressing CPSF1-mediated AR-v7 generation. Finally, we demonstrated that m6A methylation was concerned with the repression of SIAH1 in PCa. Our results define SIAH1/CPSF1/AR-v7 axis as a regulatory factor of PCa progression, providing a promising target for treating PCa.

Abstract LBA023: Regulation and function of ZEB1 dimerization in lung adenocarcinoma progression and metastasis

Abstract Lung cancer is the leading cause of cancer-related death worldwide due to the ability of cancer cells to metastasize. Therefore, it is essential to expand our current knowledge of the biological processes that contribute to metastasis to guide the discovery of novel therapeutic modalities. The Epithelial-to-mesenchymal transition (EMT) is a mechanism for metastasis, which changes polarized epithelial cells into invasive mesenchymal cells. High expression of the Zinc finger E-box binding homeobox 1 (ZEB1) transcription factor is correlated to poor outcomes in cancer, including therapeutic resistance and EMT-mediated metastasis. ZEB1 has a predicted molecular weight of 125kDa; however, multiple groups have reported discrepancies in the observed molecular weight (approximately 190-250kDa). This has been attributed to dimerization mediated by post-translational modifications (PTMs). Therefore, we performed mass spectrometry and identified a novel PTM - K811 acetylation - that may regulate ZEB1 dimerization and function. To define the role of ZEB1 acetylation, we generated ZEB1 acetyl mimetic (K811Q) and deficient (K811R) mutants in the 393P lung adenocarcinoma cell line. We hypothesize that ZEB1 acetylation regulates dimerization and protein stability to promote lung adenocarcinoma progression and metastasis. We determined that the acetyl-deficient mutant (125kDa) exhibits a decreased half-life compared to WT and acetylated ZEB1, suggesting that disruption of acetylation hinders dimerization and protein stability. However, the acetyl-mimetic mutant (250kDa) protects ZEB1 from proteasomal degradation brought by the action of the E3 ubiquitin ligase SIAH1. siRNA-mediated silencing of SIAH1 increased the mRNA and protein levels of ZEB1 WT and both acetylation mutants. Importantly, silencing of SIAH1 markedly increased the half-life of 250kDa ZEB1 WT from 28.03hrs to 44.37hrs and acetyl-mimetic from 19.43hrs to 41.25hrs, as well as 125kDa ZEB1 acetyl-deficient from 9.38hrs to 12.70hrs. Although, we determined that 250kDa ZEB1 is significantly more stable than 125kDa ZEB1, the increase in the stability by silencing SIAH1 contributes to ZEB1 acetyl-deficient binding at the promoter of its target genes mir200c-141 and SEMA3F by Chromatin Immunoprecipitation. 250kDa ZEB1 WT and acetyl-mimetic promotes lung adenocarcinoma invasion and metastasis compared to ZEB1 acetyl-deficient. We further established that the stability of ZEB1 is integral to promoting these mesenchymal features. Our results suggest that PTMs (acetylation and ubiquitination) regulate ZEB1 to promote its stability and progression of lung adenocarcinoma via EMT. Future studies are focused on further understanding the role of ZEB1 dimerization to recruit co-repressors and subsequently mediate tumorigenesis, invasion and metastasis Citation Format: Mabel Perez-Oquendo, Roxsan Manshouri, Don L. Gibbons. Regulation and function of ZEB1 dimerization in lung adenocarcinoma progression and metastasis [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr LBA023.

Targeting ADT-Induced Activation of the E3 Ubiquitin Ligase Siah2 to Delay the Occurrence of Castration-Resistant Prostate Cancer.

Siah2 is an E3 ubiquitin ligase that targets androgen receptor (AR) and plays an important role in the development of castration-resistant prostate cancer (CRPC). However, the regulation of Siah2 in prostate cancer (PCa) is largely unknown. In this study, we used AR-dependent and -independent cells lines to investigate the cellular roles of AR and androgen deprivation therapy (ADT) on Siah2 protein levels and E3 ligase activity using Western blotting and co-immunoprecipitation. We also validated our findings using patient samples taken before and after ADT. Finally, we used xenograft tumor models to test the effects of ADT combined with vitamin K3 (Vit K3) on tumor growth in vivo. Our results showed that AR stabilizes Siah2 protein by attenuating its self-ubiquitination and auto-degradation, likely by blocking its E3 ubiquitin ligase activity. Conversely, ADT decreased Siah2 protein expression but enhanced its E3 ligase activity in PCa cells. Notably, the findings that ADT decreasing Siah2 protein expression were verified in a series of paired PCa samples from the same patient. Additionally, we found that ADT-induced Siah2 activation could be abolished by Vit K3. Strikingly, ADT combined with Vit K3 treatment delayed the occurrence of CRPC and dramatically inhibited the growth of tumor xenografts compared with ADT treatment alone. AR is an inhibitor of Siah2 in PCa, and ADT leads to the continuous activation of Siah2, which may contribute to CRPC. Finally, ADT+Vit K3 may be a potential approach to delay the occurrence of CRPC.

DBC1, p300, HDAC3, and Siah1 coordinately regulate ELL stability and function for expression of its target genes

Among all of the Super Elongation Complex (SEC) components, ELL1 (also known as ELL) is the only bona fide elongation factor that directly stimulates transcription elongation by RNA polymerase II. However, the mechanism(s) of functional regulation of ELL1 (referred to as ELL hereafter), through its stabilization, is completely unknown. Here, we report a function of human DBC1 in regulating ELL stability involving HDAC3, p300, and Siah1. Mechanistically, we show that p300-mediated site-specific acetylation increases, whereas HDAC3-mediated deacetylation decreases, ELL stability through polyubiquitylation by the E3 ubiquitin ligase Siah1. DBC1 competes with HDAC3 for the same binding sites on ELL and thus increases its acetylation and stability. Knockdown of DBC1 reduces ELL levels and expression of a significant number of genes, including those involved in glucose metabolism. Consistently, Type 2 diabetes patient-derived peripheral blood mononuclear cells show reduced expression of DBC1 and ELL and associated key target genes required for glucose homeostasis. Thus, we describe a pathway of regulating stability and functions of key elongation factor ELL for expression of diverse sets of genes, including ones that are linked to Type 2 diabetes pathogenesis.

The E3 Ubiquitin Ligase SIAH1 Targets MyD88 for Proteasomal Degradation During Dengue Virus Infection.

The dengue virus presents a serious threat to human health globally and can cause severe, even life-threatening, illness. Dengue virus (DENV) is endemic on all continents except Antarctica, and it is estimated that more than 100 million people are infected each year. Herein, we further mine the data from a previously described screen for microRNAs (miRNAs) that block flavivirus replication. We use miR-424, a member of the miR-15/16 family, as a tool to further dissect the role of host cell proteins during DENV infection. We observed that miR-424 suppresses expression of the E3 ubiquitin ligase SIAH1, which is normally induced during dengue virus 2 (DENV2) infection through activation of the unfolded protein response (UPR). Specific siRNA-mediated knockdown of SIAH1 also results in inhibition of DENV replication, demonstrating that this target is at least partly responsible for the antiviral activity of miR-424. We further show that SIAH1 binds to and ubiquitinates the innate immune adaptor protein MyD88 and that the antiviral effect of SIAH1 knockdown is reduced in cells in which MyD88 has been deleted by CRISPR/Cas9 gene editing. Additionally, MyD88-dependent signaling, triggered either by DENV2 infection or the Toll-like receptor 7 (TLR7) ligand imiquimod, is increased in cells in which SIAH1 has been knocked down by miR-424 or a SIAH1-specific siRNA. These observations suggest an additional pathway by which DENV2 harnesses aspects of the UPR to dampen the host innate immune response and promote viral replication.

Insights into the Regulatory Roles of E3 Ubiquitin Ligases Associated with VHL-HIF Axis in Clear Cell Renal Cell Carcinoma

Renal cell carcinoma (RCC) accounts for up to 85% to 90% of all kidney cancers. Clear cell RCC (ccRCC), the major subtype of RCC, is mainly characterized by the inactivation of the tumor suppressor gene VHL. pVHL as an E3 ubiquitin ligase targets the hydroxylated form of HIF-α for proteasomal degradation. The loss of VHL function leads to HIF-α aggregation as the main mechanism of ccRCC. Recently, the regulations of ccRCC through other E3 ubiquitin ligases are emerging. Moreover, most of them are associated with the VHL-HIF axis. In this review, we mainly focus on seven E3 ubiquitin ligases JADE1, SIAH1, CHIP, FBXW7, MDM2, SPOP, and HAF. Based on reported researches of these ligases on ccRCC, they are divided into two groups: JADE1, SIAH1, CHIP, and FBXW7 that negatively regulate the growth of ccRCC; MDM2, SPOP, and HAF that promote ccRCC progression. In addition, we further verify these possible links between these E3 ligases and VHL-HIF axis in ccRCC based on The Cancer Genome Atlas RNA-seq and Clinical data. Understanding the mechanisms by which these ligases regulate ccRCC, especially the interplay between these ligases and VHL-HIF axis may enable the development of novel therapeutic approaches for ccRCC. Overall, the present review reveals the potential mechanism by which seven ligases (JADE1, SIAH1, CHIP, FBXW7, MDM2, SPOP, and HAF) regulate ccRCC progression in detail, especially their relationship to VHL-HIF axis.

The SIAH1-HIPK2-p53ser46 Damage Response Pathway is Involved in Temozolomide-Induced Glioblastoma Cell Death.

Patients suffering from glioblastoma have a dismal prognosis, indicating the need for new therapeutic targets. Here we provide evidence that the DNA damage kinase HIPK2 and its negative regulatory E3-ubiquitin ligase SIAH1 are critical factors controlling temozolomide-induced cell death. We show that HIPK2 downregulation (HIPK2kd) significantly reduces the level of apoptosis. This was not the case in glioblastoma cells expressing the repair protein MGMT, suggesting that the primary DNA lesion responsible for triggering HIPK2-mediated apoptosis is O6 -methylguanine. Upon temozolomide treatment, p53 becomes phosphorylated whereby HIPK2kd had impact exclusively on ser46, but not ser15. Searching for the transcriptional target of p-p53ser46, we identified the death receptor FAS (CD95, APO-1) being involved. Thus, the expression of FAS was attenuated following HIPK2kd, supporting the conclusion that HIPK2 regulates temozolomide-induced apoptosis via p-p53ser46-driven FAS expression. This was substantiated in chromatin-immunoprecipitation experiments, in which p-p53ser46 binding to the Fas promotor was regulated by HIPK2. Other pro-apoptotic proteins such as PUMA, NOXA, BAX, and PTEN were not affected in HIPK2kd, and also double-strand breaks following temozolomide remained unaffected. We further show that downregulation of the HIPK2 inactivator SIAH1 significantly ameliorates temozolomide-induced apoptosis, suggesting that the ATM/ATR target SIAH1 together with HIPK2 plays a proapoptotic role in glioma cells exhibiting p53wt status. A database analysis revealed that SIAH1, but not SIAH2, is significantly overexpressed in glioblastomas. IMPLICATIONS: The identification of a novel apoptotic pathway triggered by the temozolomide-induced DNA damage O6 -methylguanine supports the role of p53 in the decision between survival and death and suggests SIAH1 and HIPK2 as new therapeutic targets.

Interaction between the cellular E3 ubiquitin ligase SIAH-1 and the viral immediate-early protein ICP0 enables efficient replication of Herpes Simplex Virus type 2 in vivo.

Herpes Simplex Virus type 2 (HSV-2) is a neurotropic human pathogen. Upon de novo infection, the viral infected cell protein 0 (ICP0) is immediately expressed and interacts with various cellular components during the viral replication cycle. ICP0 is a multifunctional regulatory protein that has been shown to be important for both efficient viral replication and virus reactivation from latency. In particular, as previously demonstrated in transfected tissue culture models, ICP0 interacts with the cellular E3 ubiquitin ligase SIAH-1, which targets ICP0 for proteasomal degradation. However, the consequence of this virus-host interaction during the establishment of HSV-2 infection in vivo has not yet been elucidated. Here we confirmed that ICP0 of HSV-2 interacts with SIAH-1 via two conserved PxAxVxP amino acid binding motifs. We also demonstrate in vitro that a SIAH-1 binding-deficient HSV-2 strain, constructed by homologous recombination technology, exhibits an attenuated growth curve and impaired DNA and protein synthesis. This attenuated phenotype was also confirmed in an in vivo ocular infection mouse model. Specifically, viral load of the SIAH-1 binding-deficient HSV-2 mutant was significantly reduced in the trigeminal ganglia and brain stem at day 5 and 7 post infection. Our findings indicate that the interplay between ICP0 and SIAH-1 is important for efficient HSV-2 replication in vivo, thereby affecting viral dissemination kinetics in newly infected organisms, and possibly revealing novel targets for antiviral therapy.

Testin and filamin-C downregulation by acetylated Siah2 increases invasiveness of Helicobacter pylori-infected gastric cancer cells.

Helicobacter pylori is the strongest known risk-factor for gastric cancer. However, its role in gastric cancer metastasis remains unclear. Previously we have reported that H. pylori promotes gastric cancer invasiveness by stabilizing the E3 ubiquitin ligase Siah2 which is mediated by Siah2 acetylation at Lys 139 (K139) residue. Here we identify that cell adhesion-related proteins testin (TES) and filamin-C (FLN-C) interact with Siah2 and get proteasomally degraded. The efficiency of TES and FLN-C degradation is significantly potentiated by K139-acetylated Siah2 (ac-K139 Siah2) in infected gastric cancer cells (GCCs). ac-Siah2-mediated downregulation of TES and FLN-C disrupts filopodia structures but promotes lamellipodia formation and enhances invasiveness and migration of infected GCCs. Since H. felis-infected mice as well as human gastric cancer biopsy samples also show high level of ac-K139 Siah2 and downregulated TES and FLN-C, we believe that acetylation of Siah2 is an important checkpoint that can be useful for therapeutic intervention.

PIWIL2 suppresses Siah2-mediated degradation of HDAC3 and facilitates CK2\u03b1-mediated HDAC3 phosphorylation

HDAC3 is involved in deacetylation of histone and non-histone proteins, having a key role in the regulation of gene transcription and also in the process of tumorgenesis. However, how HDAC3 is regulated in cancer remains largely unclear. Here, we showed that PIWIL2 can interact with HDAC3, leading to stabilization of HDAC3 from ubiquitin-mediated degradation by competitive association with E3 ubiquitin ligase Siah2. Furthermore, we found that expression of PIWIL2 enhanced HDAC3 activity via CK2α. PIWIL2 facilitated the interaction between HDAC3 and CK2α, thus exhibiting a promotion on the HDAC3 phosphorylation by CK2α. Further work showed that PIWIL2 could promote cell proliferation and suppress cell apoptosis via regulating HDAC3. Our present study firstly revealed that PIWIL2 can play a role in HDAC3-mediated epigenetic regulation on cancer cell proliferation and apoptosis. These findings provide a novel insight into the roles of PIWIL2 in tumorigenesis.