Circ_0011058 alleviates RA pathology through the circ_0011058/miR-335-5p/CUL4B signal axis

Our previous study found that Cullin 4B (CUL4B) inhibited rheumatoid arthritis (RA) pathology through glycogen synthase kinase-3beta (GSK3β)/canonical Wnt signalling pathway. In this work, pre-experiment and bioinformatics analysis suggested that circ_0011058 may lead to the up-regulation of CUL4B expression by inhibiting miR-335-5p. Therefore, we studied whether circ_0011058 can promote the expression of CUL4B through sponging the miR-335-5p and further promote the pathological development of RA. Bioinformatics prediction, real-time quantitative PCR (RT-qPCR), western blot (WB), double luciferase reporter gene and other relevant methods were used to study the inhibition of circ_0011058 on RA pathology and its molecular mechanism. Results showed that the expression of circ_0011058 was significantly increased in adjuvant arthritis (AA) rats and RA fibroblast-like synoviocytes (FLS). The knockout of circ_0011058 inhibited the proliferation of AA FLS and RA FLS, decreased the levels of interleukin-1 beta (IL-1β), interleukin 6 (IL-6), interleukin 8 (IL-8), and inhibited the expression of matrix metalloproteinase 3 (MMP3), fibronectin, which showed that circ_0011058 had a strong role in promoting RA pathology. Furthermore, miR-335-5p expression was reduced in AA rats and RA FLS. The highly expressed circ_0011058 directly sponged the miR-335-5p, which led to the increase of CUL4B expression and promoted the activation of the GSK3β/canonical signalling pathway. Finally, we confirmed that miR-335-5p mediated the roles of circ_0011058 in promoting RA pathological development, which showed that the circ_0011058/miR-335-5p/CUL4B signal axis was involved in RA pathology. This work was of great significance for clarifying the roles of circ_0011058 in RA pathology, and further work was needed to establish whether circ_0011058 was a potential therapeutic target or diagnostic marker for RA.

CUL4B enhances the malignant phenotype of esophageal squamous cell carcinoma by suppressing TGFBR3 expression

Cullin 4B (CUL4B), which acts as a scaffold protein in CUL4B-RING ubiquitin ligase complexes (CRL4B), is frequently overexpressed in cancer and represses tumor suppressors through epigenetic mechanisms. However, the expression and function of CUL4B in esophageal squamous cell carcinoma (ESCC) have not been well illustrated. In this study, we show that upregulation of CUL4B in ESCC cells enhances proliferation, invasion and cisplatin (CDDP)-resistance, while knockdown of CUL4B significantly represses the malignant activities. Mechanistically, we demonstrate that CUL4B promotes proliferation and migration of ESCC cells through inhibiting expression of transforming growth factor beta receptor III (TGFBR3). CRL4B complex binds to the promoter of TGFBR3, and represses its transcription by catalyzing monoubiquitination at H2AK119 and coordinating with PRC2 and HDAC complexes. Taken together, our findings establish a critical role for the CUL4B/TGFBR3 axis in the regulation of ESCC malignancy.

Knockdown of circMFN2 inhibits cell progression and glycolysis by miR-198/CUL4B pathway in ovarian cancer

Circular RNA (circRNA) regulates malignant tumors, including ovarian cancer (OC). The present research study aimed to reveal the biological mechanism of circRNA mitofusin 2 (circMFN2) in OC. Cell biological behaviors were investigated using clonogenicity assay, EdU assay, transwell assay, and flow cytometry analysis. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) and western blot analysis were implemented to detect the levels of circMFN2, miR-198, Cullin 4B (CUL4B), and apoptosis-related proteins. Glycolysis was assessed by glucose assay kit, lactate assay kit, and ATP level detection kit. The relationships among miR-198, circMFN2, and CUL4B were verified by dual-luciferase reporter assay and RNA immunoprecipitation assay. The xenograft mice model was used to analyze tumor growth in vivo. The expression of circMFN2 and CUL4B was increased, while miR-330-5p was decreased in OC tissues or cells. The absence of CircMFN2 hindered cell proliferation, migration, invasion, and glycolysis and promoted apoptosis in OC cells. We found that circMFN2 promoted CUL4B expression via sponging miR-198. MiR-198 depletion reversed circMFN2 knockdown-induced effects in OC cells. Furthermore, CUL4B overexpression overturned the inhibitory effect of miR-198 in OC cells. And the absence of circMFN2 inhibited tumor growth in vivo. CircMFN2 repressed OC progression by regulating the miR-198/CUL4B axis.

Depletion of CUL4B in macrophages ameliorates diabetic kidney disease via miR-194-5p/ITGA9 axis

Diabetic kidney disease (DKD) is the most prevalent chronic kidney disease. Macrophage infiltration in the kidney is critical for the progression of DKD. However, the underlying mechanism is far from clear. Cullin 4B (CUL4B) is the scaffold protein in CUL4B-RING E3 ligase complexes. Previous studies have shown that depletion of CUL4B in macrophages aggravates lipopolysaccharide-induced peritonitis and septic shock. In this study, using two mouse models for DKD, we demonstrate that myeloid deficiency of CUL4B alleviates diabetes-induced renal injury and fibrosis. In vivo and in vitro analyses reveal that loss of CUL4B suppresses migration, adhesion, and renal infiltration of macrophages. Mechanistically, we show that high glucose upregulates CUL4B in macrophages. CUL4B represses expression of miR-194-5p, which leads to elevated integrin α9 (ITGA9), promoting migration and adhesion. Our study suggests the CUL4B/miR-194-5p/ITGA9 axis as an important regulator for macrophage infiltration in diabetic kidneys.

Proteomic analysis of DZIP3 interactome and its role in proliferation and metastasis in gastric cancer cells

Gastric cancer is a serious malignant tumor in the world, accounting for the third cause of cancer death worldwide. The pathogenesis of gastric cancer is very complex, in which epigenetic inheritance plays an important role. In our study, we found that DZIP3 was significantly up-regulated in gastric cancer tissues as compared to adjacent normal tissue, which suggested it may be play a crucial part in gastric cancer. To clarify the mechanism of it, we further analyzed the interacting proteome and transcriptome of DZIP3. An association between DZIP3 and some epigenetic regulators, such as CUL4B complex, was verified. We also present the first proteomic characterization of the protein-protein interaction (PPI) network of DZIP3. Then, the transcriptome analysis of DZIP3 demonstrated that knockdown DZIP3 increased a cohort of genes, including SETD7 and ZBTB4, which have essential role in tumors. We also revealed that DZIP3 promotes proliferation and metastasis of gastric cancer cells. And the higher expression of DZIP3 is positively associated with the poor prognosis of several cancers. In summary, our study revealed a mechanistic role of DZIP3 in promoting proliferation and metastasis in gastric cancer, supporting the pursuit of DZIP3 as a potential target for gastric cancer therapy.

Circ_0021573 acts as a competing endogenous RNA to promote the malignant phenotypes of human ovarian cancer cells

Circular RNAs (circRNAs) have been reported to be implicated in the tumorigenesis and progression of ovarian cancer. Here, the study was designed to explore the activity of human circ_0021573 in ovarian cancer pathogenesis and its regulation through the competing endogenous RNA (ceRNA) crosstalk. Circ_0021573, microRNA (miR)- 936, and cullin 4B (CUL4B) were quantified by qRT-PCR and western blot. Cell proliferation ability was detected by XTT, 5-Ethynyl-2'-Deoxyuridine (EdU), and colony formation assays. Cell apoptosis, migration, and invasion were assessed by flow cytometry, wound-healing, and transwell assays, respectively. Dual-luciferase reporter and RNA immunoprecipitation (RIP) assays were used to evaluate the direct relationship between miR-936 and circ_0021573 or CUL4B 3'UTR. Xenograft studies were applied to assess the role of circ_0021573 in tumor growth. Our data showed that circ_0021573 expression is enhanced in human ovarian cancer. Inhibition of circ_0021573 impedes cell proliferation, migration, and invasion and promotes apoptosis in vitro, as well as diminishes tumor growth in vivo. Mechanistically, circ_0021573 contains a miR-936 binding site, and miR-936 is a relevant mediator of circ_0021573 regulation. MiR-936 direct targets and inhibits CUL4B. MiR-936-mediated suppression of CUL4B hinders cell proliferation, migration, and invasion and accelerates apoptosis in vitro.. These data suggested that circ_0021573 might promote the malignant phenotypes of ovarian cancer cells by functioning as a ceRNA for miR-936 to induce CUL4B, which provided a promising target for the prevention and inhibition of ovarian cancer.

Exosomal circKDM4A Induces CUL4B to Promote Prostate Cancer Cell Malignancy in a miR-338-3p-Dependent Manner

Circular RNA lysine demethylase 4A (circKDM4A) is also named circ_0012098 and its abnormal expression has been confirmed in serum exosomes of prostate cancer (PC) patients. However, whether PC progression involves the exosomal circ_0012098 remains unknown. RNA expression of circKDM4A, microRNA-338-3p (miR-338-3p) and cullin 4B (CUL4B) was detected by quantitative real-time polymerase chain reaction. Protein expression was checked by Western blot. The positive expression rate of nuclear proliferation marker (ki-67) was analyzed by immunohistochemistry assay. Dual-luciferase reporter assay and RNA immunoprecipitation assay were used to identify the interaction between miR-338-3p and circKDM4A or CUL4B. Mouse model assay was performed to determine the effect of exosomal circKDM4A on tumorigenesis in vivo. CircKDM4A expression was significantly upregulated in the serum exosomes from PC patients compared with the exosomes from healthy volunteers. Exosomes treatment promoted the proliferation, migration and invasion of PC cells but inhibited apoptosis; however, these effects were attenuated after circKDM4A knockdown. Meanwhile, circKDM4A depletion restored exosome-increased circKDM4A expression. Additionally, circKDM4A acted as a miR-338-3p sponge, and miR-338-3p bound to CUL4B in PC cells. CircKDM4A regulated the effect of exosome-induced PC cell malignancy by interacting with miR-338-3p and CUL4B. Moreover, circKDM4A silencing relieved exosome-induced tumor growth in vivo. Exosomal circKDM4A promoted PC malignant progression by the miR-338-3p/CUL4B axis, providing a therapeutic target for PC.

PHIP-associated Chung-Jansen syndrome: Report of 23 new individuals

In 2016 and 2018, Chung, Jansen and others described a new syndrome caused by haploinsufficiency of PHIP (pleckstrin homology domain interacting protein, OMIM *612,870) and mainly characterized by developmental delay (DD), learning difficulties/intellectual disability (ID), behavioral abnormalities, facial dysmorphism and obesity (CHUJANS, OMIM #617991). So far, PHIP alterations appear to be a rare cause of DD/ID. "Omics" technologies such as exome sequencing or array analyses have led to the identification of distinct types of alterations of PHIP, including, truncating variants, missense substitutions, splice variants and large deletions encompassing portions of the gene or the entire gene as well as adjacent genomic regions. We collected clinical and genetic data of 23 individuals with PHIP-associated Chung-Jansen syndrome (CHUJANS) from all over Europe. Follow-up investigations (e.g. Sanger sequencing, qPCR or Fluorescence-in-situ-Hybridization) and segregation analysis showed either de novo occurrence or inheritance from an also (mildly) affected parent. In accordance with previously described patients, almost all individuals reported here show developmental delay (22/23), learning disability or ID (22/23), behavioral abnormalities (20/23), weight problems (13/23) and characteristic craniofacial features (i.e. large ears/earlobes, prominent eyebrows, anteverted nares and long philtrum (23/23)). To further investigate the facial gestalt of individuals with CHUJANS, we performed facial analysis using the GestaltMatcher approach. By this, we could establish that PHIP patients are indistinguishable based on the type of PHIP alteration (e.g. missense, loss-of-function, splice site) but show a significant difference to the average face of healthy individuals as well as to individuals with Prader-Willi syndrome (PWS, OMIM #176270) or with a CUL4B-alteration (Intellectual developmental disorder, X-linked, syndromic, Cabezas type, OMIM #300354). Our findings expand the mutational and clinical spectrum of CHUJANS. We discuss the molecular and clinical features in comparison to the published individuals. The fact that some variants were inherited from a mildly affected parent further illustrates the variability of the associated phenotype and outlines the importance of a thorough clinical evaluation combined with genetic analyses for accurate diagnosis and counselling.

CUL4B-associated epilepsy: Report of a novel truncating variant promoting drug-resistant seizures and systematic review of the literature

BACKGROUND

Cabezas syndrome is a rare X-linked disease caused by mutations in CUL4B and characterized by developmental delay/intellectual disability, somatic dysmorphisms, behavioural disorder, ataxia/tremors. Although seizures have been formerly reported, their clinical semiology, EEG features and long-term outcome are largely unknown.

PURPOSE

This study aims to expand knowledge on epilepsy associated with Cabezas syndrome and to understand whether different types of variants in the CUL4B gene or brain MRI abnormalities may influence seizure onset and epilepsy course.

METHODS

With this in mind, we characterised the epileptic phenotype of a 17-year-old adolescent harbouring a CUL4B novel variant and performed a systematic literature review of CUL4B-associated seizures, analysing mutation types and neuroimaging features as epilepsy predictors.

RESULTS

Our case observation indicates that CUL4B-associated epilepsy may also be drug-resistant and persist beyond infancy. Literature analysis shows that 43% of CUL4B patients develop seizures, with no statistically significant differences in epilepsy development according to mutation type and neuroimaging features.

CONCLUSION

Our study extends knowledge of CUL4B-associated epilepsy, offering new insights into disease progression.

Aurora kinase a promotes the progression of papillary thyroid carcinoma by activating the mTORC2-AKT signalling pathway

BACKGROUND

Treatment failure is the main cause of death from papillary thyroid carcinoma (PTC). It is urgent to look for new intervention targets and to develop new therapies for treating PTC. Aurora-A kinase (AURKA) functionally regulates cell mitosis and is closely related to the occurrence and development of a variety of tumours. However, the expression and potential functions of AURKA in PTC remain largely elusive.

RESULTS

Clinicopathologically, AURKA is highly expressed in PTC tissues compared to normal tissues and is correlated with lymph node metastasis, TNM stage and patient prognosis. Biologically, AURKA functions as an oncoprotein to promote the proliferation and migration of PTC cells. Mechanistically, AURKA directly binds to SIN1 and compromises CUL4B-based E3 ligase-mediated ubiquitination and subsequent degradation of SIN1, leading to hyperactivation of the mTORC2-AKT pathway in PTC cells.

CONCLUSIONS

We found that AURKA plays critical roles in regulating the progression of PTC by activating the mTORC2-AKT pathway, highlighting the potential of targeting AURKA to treat PTC.

CUL4B increases platinum-based drug resistance in colorectal cancer through EMT: A study in its mechanism

Platinum-based chemotherapy drugs play a very important role in the treatment of patients with advanced colorectal cancer, but the drug resistance of platinum-based chemotherapy drugs is an important topic that puzzles us. If we can find mechanisms of resistance, it will be revolutionary for us. We analysed the differential genes, core genes and their enrichment pathways in platinum-resistant and non-resistant patients through a public database. Platinum-resistant cell lines were cultured in vitro for in vitro colony and Transwell analysis. Tumorigenesis analysis of nude mice in vivo. Verify the function of core genes. Through differential gene and enrichment analysis, we found that CUL4B was the main factor affecting platinum drug resistance and EMT. Our hypothesis was further verified by in vitro drug-resistant and wild-type cell lines and in vivo tumorigenesis analysis of nude mice. CUL4B leads to platinum drug resistance in colorectal cancer by affecting tumour EMT.

Insulin alleviates LPS-induced ARDS via inhibiting CUL4B-mediated proteasomal degradation and restoring expression level of Na,K-ATPase α1 subunit through elevating HCF-1

Acute respiratory distress syndrome (ARDS) is a critical disease with a high mortality rate, characterized by obstinate hypoxemia caused by accumulation of alveolar fluid and excessive uncontrolled inflammation. Na,K-ATPase α1 (ATP1A1) subunit is an important component of Na,K-ATPase that transports Na⁺ and K⁺ and scavenges alveolar fluid. The function of Na,K-ATPase is always impaired during ARDS and results in more severe symptoms of ARDS. However, the regulatory mechanism of Na,K-ATPase after ARDS remains unclear. Here, we revealed ATP1A1 was downregulated post-transcriptionally by an E3 ligase component CUL4B mediated proteasomal degradation. Moreover, we found insulin could inhibit the upregulation of CUL4B in an insulin receptor cofactor HCF-1-dependent manner. Our study resolved the molecular mechanism underlying the clearance impairment of alveolar fluid and provided a clue for the usage of insulin as a potential therapeutic medicine for ARDS.

Calcifying pseudoneoplasms of the neuraxis (CAPNON). A case report

Calcifying pseudoneoplasms of the neuraxis (CAPNON) are rare, slow-growing, benign lesions occurring throughout the neuroaxis that are frequently misdiagnosed and overlooked by clinicians. Here, we report a case of a 56-year-old woman who presented with a history of recurrent headache for the previous six years. Magnetic resonance imaging (MRI) revealed a 2.3-cm-sized solid mass in the right frontal lobe that was surrounded by marked edematous areas. The lesion demonstrated dense calcification and avid enhancement. The lesion was initially diagnosed as oligodendroglioma, and then found to be CAPNON based on histopathology of a surgically resected tissue. Genetic analysis revealed a nonsense mutation in the CUL4B gene. The patient's condition appeared to reflect a reactive, rather than neoplastic, process. Clinicians should be prepared to detect such pseudotumors histopathologically in order to avoid unnecessary differential tests of neoplastic or infectious diseases, as well as potentially harmful therapies.

Inhibition of MicroRNA miR-101-3p on prostate cancer progression by regulating Cullin 4B (CUL4B) and PI3K/AKT/mTOR signaling pathways

To probe into the efffects of miR-101-3p via regulating CUL4B within PI3K/AKT/mTOR signaling pathway on progression of prostate cancer (PCA). Western blot and qRT-PCR were adopted to detect CUL4B and miR-101-3p expressions in 75 cases with PCA . The cellular strains of PCA (LNCaP and PC3) were chose as the objects to check the targeting correlation between CUL4B and miR-101-3p through dual-luciferase reporter experiments. LNCaP cells and PC3 cells were randomly divided into the blank group, miR-101-3p mimic group, siRNA negative control (NC) group, CUL4B siRNA group and CUL4B siRNA plus the miR-101-3p inhibitor group. Cellular bioactivity measurement was done via Cell-Light EDU, MTT, Annexin-V-FITC/PI, scratch-heal experiments and invasion tests of Transwell. MiR-101-3p expression was decreased more signally in tumor tissues than in normal tissues adjacent to the cancer. MiR-101-3p inhibited cellular proliferating, migrating and invasion. Nevertheless, it promoted cellular apoptosis, up-regulated apoptotic proteins as well as down-regulated anti-apoptotic proteins. CUL4B siRNA and miR-101-3p simulation were similar in terms of their outcomes. Nonetheless, these results could be reversed through the miR-101-3p inhibitor. Besides, CUL4B siRNA and the simulation halted a serious of PI3K signal in PCA cells. MiR-101-3p expression was down-regulated in PCA patients. CUL4B was upregulated in PCA patients. Moreover, miR-101-3p suppressed cellular invasion, migration, proliferation and led to cellular apoptosis, which might be related to the PI3K/AKT/mTOR signaling pathway suppression. Finally, we found, MiR-101-3P suppressed PCA progression via aiming for CUL4B, which may offer the new molecular target for PCA clinical treatment.

[CRL4B complex promotes the development of pancreatic cancer by inhibiting secreted frizzled related protein 1]

Objective: To investigate the role of CUL4B-RING E3 ubiquitin ligase (CRL4B) complex in pancreatic tumorigenesis and the molecular mechanism. Methods: Pancreatic cells were divided into control group (transfected with negative control lentivirus), shCUL4B group (transfected with CUL4B lentivirus), shDDB1 group [transfected with DNA damage binding protein 1 (DDB1) lentivirus], and shCUL4B+ siSFRP1 group (transfected with CUL4B lentivirus and SFRP1-siRNA). RNA-seq was performed in pancreatic cancer cell lines with CUL4B and DDB1 knocked down respectively, to identify the target genes regulated by CRL4B complex. Real-time fluorescent quantitative polymerase chain reaction (qRT-PCR) was used to detect the mRNA expression levels of target genes. Chromatin immunoprecipitation (ChIP) assay was used to identify the target genes directly regulated by CUL4B and DDB1. Western blot was used to detect the protein expression levels of the epithelial-mesenchymal transition (EMT) markers. The EdU cell proliferation test was used to detect cell proliferation ability. The scratch repair test and Transwell cell invasion test were used to detect cell migration and invasion ability. Finally, the sequencing data of pancreatic cancer-related tumor samples and normal samples in GEO, TCGA and GTEx databases were used to analyze the expression correlations of CUL4B, DDB1 and their downstream target genes. Results: RNA-seq results showed that target genes regulated by CRL4B complex involved in a number of malignant tumor-related signaling pathways. qRT-PCR results verified that the mRNA expression levels of the target genes of CUL4B or DDB1 knockdown groups were higher than those of the control group, and the difference was statistically significant (P<0.05). ChIP-PCR results showed that CRL4B complex directly bound to the promoter regions of the target genes, NME1 and SFRP1, and the enrichment of monoubiquitination of lysine at 119 of histone H2A (H2AK119ub1) in the promoter region of target gene was reduced after CUL4B knockdown. The proliferation rate in PANC-1 cell line of the control group was (32.10±3.58)%, higher than (13.95±1.66)% in the shCUL4B group and (22.38±0.77)% in the shCUL4B+ siSFRP1 group (P<0.05). The proliferation rate in AsPC-1 cell line of the control group was (35.47±7.80)%, higher than (19.60±3.58)% in the shCUL4B group and (30.09±0.81)% in the shCUL4B+ siSFRP1 group (P<0.05). The scratch repair experiment showed that the migration rate of PANC-1 cell line control group was (53.18±3.70)%, higher than that (17.46±2.62)% in the shCUL4B group and (44.99±9.18)% in the shCUL4B + siSFRP1 group (P<0.05). Western blot showed the expression levels of epithelial markers including α-catenin and γ-catenin in the control group were 1.00±0.03 and 1.01±0.11, respectively, lower than 1.44±0.01 and 1.21±0.06 in the shCUL4B group (P<0.05). The expression levels of mesenchymal markers including fibronectin and vimentin in the control group were 1.01±0.14 and 1.02±0.18, respectively, higher than 1.53±0.13 and 1.22±0.07 in the shCUL4B+ siSFRP1 group (P<0.05). The cell metastasis rate of the control group was (100.00±3.96)%, higher than the (35.49±0.34)% in the shCUL4B group and (107.06±2.77)% in the shCUL4B+ siSFRP1 group, the difference was statistically significant (P<0.05). The expressions of CUL4B and DDB1 were significantly upregulated in the pancreatic cancer tissues, and were negatively correlated with the expression of SFRP1 (r=-0.342 and r=-0.264, respectively). Conclusions: CRL4B complex inhibits the transcription of target gene SFRP1 and promotes the development of pancreatic cancer. Moreover, CRL4B complex is upregulated in pancreatic cancer, which provide a potential of therapeutic target for pancreatic cancer.

Cullin-4B promotes cell proliferation and invasion through inactivation of p53 signaling pathway in colorectal cancer

Cullin 4B (CUL4B) is a member of the Cullin RING E3 ligase family, which is found to be overexpressed in multiple cancers, thus facilitating tumorigenesis and progression. However, the correlation between CUL4B and p53 in colorectal cancer cells (CRC) remains to be further elucidated. In this study, we newly identified that CUL4B functions as a negative regulator of p53, thereby facilitating CRC tumorigenesis and progression. Our data has demonstrated that CUL4B was frequently overexpressed in CRC tissues, and its upregulation was closely correlated with disease progression and poor prognosis. Moreover, CUL4B knockdown suppressed cell proliferation, invasion and epithelial-mesenchymal transition (EMT) of CRC cells. Mechanistically, CUL4B depletion increased the expression of p53 protein and its downstream targets p21, PUMA and MDM2. Furthermore, CUL4B depletion prolonged the half-life of p53 protein, and CUL4B is a binding partner of MDM2. In conclusion, our study shed new lights on the complex regulatory network between CUL4B and p53, and clarifies this CUL4B-p53 axis contributes greatly to CRC tumorigenesis and progression.

LncRNA SNHG12 regulates the miR-101-3p/CUL4B axis to mediate the proliferation, migration and invasion of non-small cell lung cancer

Mounting evidence has shown that long noncoding RNAs (lncRNAs) play critical roles in carcinogenesis and tumor progression. SNHG12 has been identified in multiple types of malignant tumors. However, the role of SNHG12 in human non-small cell lung cancer (NSCLC) is poorly characterized, and the relevant underlying mechanism remains unclear. The expression levels of SNHG12, miR-101-3p, and CUL4B in collected human NSCLC tumor tissues and NSCLC cell lines were tested via qRT-PCR. Then, NSCLC cellular proliferation, migration and invasion were determined, followed by MTT, scratch and Transwell assays. Dual-luciferase reporter assays and RNA pulldown assays were adopted to explore the target site. Moreover, western blotting was performed to detect the relevant protein expression concerning the CUL4B/PI3K/AKT pathway. This study clarified that SNHG12 knockdown significantly reduced proliferation, migration, invasion and EMT of NSCLC cells. Our data indicated that SNHG12 targeted and negatively regulated miR-101-3p, and this depletion reversed the inhibitory effect of si-SNHG12 on NSCLC cells. Furthermore, CUL4B was confirmed as a functional target of miR-101-3p, and its knockdown resulted in a strong alleviation of the NSLCL cell phenotype, which was enhanced by the silencing of miR-101-3p. Mechanistically, we found that SNHG12 regulated miR-101-3p to modulate the PI3K/AKT pathway mediated by CUL4B.These observations suggested that lncRNA SNHG12-mediated miR-101-3p downregulation regulated the malignant phenotype of NSCLC cells by targeting CUL4B through the PI3K/AKT pathway, which may present a path to novel therapeutic strategies for NSCLC therapy.

CUL4B facilitates HBV replication by promoting HBx stabilization

OBJECTIVE

Hepatitis B virus (HBV) infection is a major public health problem worldwide. However, the regulatory mechanisms underlying HBV replication remain unclear. Cullin 4B-RING ubiquitin E3 ligase (CRL4B) is involved in regulating diverse physiological and pathophysiological processes. In our study, we aimed to explain the role of CUL4B in HBV infection.

METHODS

Cul4b transgenic mice or conditional knockout mice, as well as liver cell lines with CUL4B overexpression or knockdown, were used to assess the role of CUL4B in HBV replication. Immunoprecipitation assays and immunofluorescence staining were performed to study the interaction between CUL4B and HBx. Cycloheximide chase assays and in vivo ubiquitination assays were performed to evaluate the half-life and the ubiquitination status of HBx.

RESULTS

The hydrodynamics-based hepatitis B model in Cul4b transgenic or conditional knockout mice indicated that CUL4B promoted HBV replication (P < 0.05). Moreover, the overexpression or knockdown system in human liver cell lines validated that CUL4B increased HBV replication in an HBx-dependent manner. Importantly, immunoprecipitation assays and immunofluorescence staining showed an interaction between CUL4B and HBx. Furthermore, CUL4B upregulated HBx protein levels by inhibiting HBx ubiquitination and proteasomal degradation (P < 0.05). Finally, a positive correlation between CUL4B expression and HBV pgRNA level was observed in liver tissues from HBV-positive patients and HBV transgenic mice.

CONCLUSIONS

CUL4B enhances HBV replication by interacting with HBx and disrupting its ubiquitin-dependent proteasomal degradation. CUL4B may therefore be a potential target for anti-HBV therapy.

CUL4B Promotes Breast Carcinogenesis by Coordinating with Transcriptional Repressor Complexes in Response to Hypoxia Signaling Pathway

Cullin4B (CUL4B) is a scaffold protein of the CUL4B-Ring E3 ligase (CRL4B) complex. However, the role of CUL4B in the development of breast cancer remains poorly understood. Here it is shown that CRL4B interacts with multiple histone deacetylase (HDAC)-containing corepressor complexes, including MTA1/NuRD, SIN3A, CoREST, and NcoR/SMRT complexes. It is demonstrated that CRL4B/NuRD(MTA1) complexes cooccupy the E-cadherin and AXIN2 promoters, and could be recruited by transcription factors including Snail and ZEB2 to promote cell invasion and tumorigenesis both in vitro and in vivo. Remarkably, CUL4B responded to transformation and migration/invasion stimuli and is essential for multiple epithelial-mesenchymal transition (EMT) signaling pathways such as hypoxia. Furthermore, the transcription of CUL4B is directedly activated by hypoxia-inducible factor 1α (HIF1α) and repressed by the ERα-GATA3 axis. Overexpressing of CUL4B successfully induced CSC-like properties. Strikingly, CUL4B expression is markedly upregulated during breast cancer progression and correlated with poor prognosis. The results suggest that CUL4B lies at a critical crossroads between EMT and stem cell properties, supporting CUL4B as a potential novel target for the development of anti-breast cancer therapy.

CUL4B renders breast cancer cells tamoxifen-resistant via miR-32-5p/ER-α36 axis

Tamoxifen (TAM) resistance is a significant clinical challenge in endocrine therapies for estrogen receptor (ER)-positive breast cancer patients. Cullin 4B (CUL4B), which acts as a scaffold protein in CUL4B-RING ubiquitin ligase complexes (CRL4B), is frequently overexpressed in cancer and represses tumor suppressors through diverse epigenetic mechanisms. However, the role and the underlying mechanisms of CUL4B in regulating drug resistance remain unknown. Here, we showed that CUL4B promotes TAM resistance in breast cancer cells through a miR-32-5p/ER-α36 axis. We found that upregulation of CUL4B correlated with decreased TAM sensitivity of breast cancer cells, and knockdown of CUL4B or expression of a dominant-negative CUL4B mutant restored the response to TAM in TAM-resistant MCF7-TAM^R and T47D-TAM^R cells. Mechanistically, we demonstrated that CUL4B renders breast cancer cells TAM-resistant by upregulating ER-α36 expression, which was mediated by downregulation of miR-32-5p. We further showed that CRL4B epigenetically represses the transcription of miR-32-5p by catalyzing monoubiquitination at H2AK119 and coordinating with PRC2 and HDAC complexes to promote trimethylation at H3K27 at the promoter of miR-32-5p. Pharmacologic or genetic inhibition of CRL4B/PRC2/HDAC complexes significantly increased TAM sensitivity in breast cancer cells in vitro and in vivo. Taken together, our findings thus establish a critical role for the CUL4B-miR-32-5p-ER-α36 axis in the regulation of TAM resistance and have important therapeutic implications for combined application of TAM and the inhibitors of CRL4B/PRC2/HDAC complex in breast cancer treatment. © 2021 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

CUL4B Promotes Temozolomide Resistance in Gliomas by Epigenetically Repressing CDNK1A Transcription

Resistance to temozolomide (TMZ), the first-line chemotherapeutic drug for glioblastoma (GBM) and anaplastic gliomas, is one of the most significant obstacles in clinical treatment. TMZ resistance is regulated by complex genetic and epigenetic networks. Understanding the mechanisms of TMZ resistance can help to identify novel drug targets and more effective therapies. CUL4B has been shown to be upregulated and promotes progression and chemoresistance in several cancer types. However, its regulatory effect and mechanisms on TMZ resistance have not been elucidated. The aim of this study was to decipher the role and mechanism of CUL4B in TMZ resistance. Western blot and public datasets analysis showed that CUL4B was upregulated in glioma specimens. CUL4B elevation positively correlated with advanced pathological stage, tumor recurrence, malignant molecular subtype and poor survival in glioma patients receiving TMZ treatment. CUL4B expression was correlated with TMZ resistance in GBM cell lines. Knocking down CUL4B restored TMZ sensitivity, while upregulation of CUL4B promoted TMZ resistance in GBM cells. By employing senescence β-galactosidase staining, quantitative reverse transcription PCR and Chromatin immunoprecipitation experiments, we found that CUL4B coordinated histone deacetylase (HDAC) to co-occupy the CDKN1A promoter and epigenetically silenced CDKN1A transcription, leading to attenuation of TMZ-induced senescence and rendering the GBM cells TMZ resistance. Collectively, our findings identify a novel mechanism by which GBM cells develop resistance to TMZ and suggest that CUL4B inhibition may be beneficial for overcoming resistance.

A 22.5 kb deletion in CUL4B causing Cabezas syndrome identified using CNV approach from WES data

Detecting clinical grade CNV based on WES is being improved in the NGS era.

Circ_0015756 promotes the progression of ovarian cancer by regulating miR-942-5p/CUL4B pathway

BACKGROUND

Ovarian cancer (OC) is the gynecologic cancer with the highest mortality. Circular RNAs (circRNAs) play a vital role in the development and progression of cancer. This study aimed to explore the potential role of circ_0015756 in OC and its molecular mechanism.

METHODS

The levels of circ_0015756, microRNA-942-5p (miR-942-5p) and Cullin 4B (CUL4B) were determined by quantitative real-time PCR (qRT-PCR) or Western blot assay. Cell proliferation, apoptosis, migration and invasion were assessed by Cell Counting Kit-8 (CCK-8), colony formation assay, flow cytometry and transwell assay. The levels of proliferation-related and metastasis-related proteins were measured by Western blot assay. The relationship between miR-942-5p and circ_0015756 or CUL4B was verified by dual-luciferase reporter assay, RNA immunoprecipitation assay and RNA pull-down assay. Xenograft assay was used to analyze tumor growth in vivo.

RESULTS

Circ_0015756 and CUL4B levels were increased, while miR-942-5p level was decreased in OC tissues and cells. Depletion of circ_0015756 suppressed proliferation, migration and invasion and promoted apoptosis in OC cells. Down-regulation of circ_0015756 hindered OC cell progression via modulating miR-942-5p. Also, up-regulation of miR-942-5p impeded OC cell development by targeting CUL4B. Mechanistically, circ_0015756 up-regulated CUL4B via sponging miR-942-5p. Moreover, circ_0015756 silencing inhibited tumor growth in vivo.

CONCLUSION

Knockdown of circ_0015756 suppressed OC progression via regulating miR-942-5p/CUL4B axis, suggesting that circ_0015756 might be a potential therapeutic target for ovarian cancer.

CUL4B promotes aggressive phenotypes of renal cell carcinoma via upregulating c-Met expression

Cullin 4B (CUL4B), encoding a scaffold protein in Cullin RING ubiquitin-ligase complexes (CRL4B), is overexpressed and serves as an oncogene in various solid tumors. However, the roles and the underlying mechanisms of CUL4B in renal cell carcinoma (RCC) are still unknown. In this study, we demonstrated that CUL4B was significantly upregulated in RCC cells and clinical specimens, and its overexpression was correlated with poor survival of RCC patients. Knockdown of CUL4B resulted in the inhibition of proliferation, migration and invasion of RCC cells. Furthermore, we found that the expression of CUL4B is positively correlated with c-Met expression in RCC cells and tissues. Konckdown of c-Met or treatment with c-Met inhibitor, SU11274, could block the increase in cell proliferation, migration and invasion induced by CUL4B-overexpression. We also showed that CUL4B overexpression significantly accelerated xenograft tumor growth, and administration of SU11274 could also abrogate the accelerated tumor growth induced by CUL4B overexpression in vivo. These findings shed light on the contribution of CUL4B to tumorigenesis in RCC via activating c-Met signaling and its therapeutic implications in RCC patients.

Importance of Cullin4 Ubiquitin Ligase in Malignant Pleural Mesothelioma

Neurofibromatosis type 2 (NF2), the tumor suppressor frequently lost in malignant pleural mesothelioma (MPM), suppresses tumorigenesis in part by inhibiting the Cullin4 ubiquitin ligase (CUL4) complex in the nucleus. Here, we evaluated the importance of CUL4 in MPM progression and tested the efficacy of cullin inhibition by pevonedistat, a small molecule inhibiting cullin neddylation. CUL4 paralogs (CUL4A and CUL4B) were upregulated in MPM tumor specimens compared to nonmalignant pleural tissues. High gene and protein expressions of CUL4B was associated with a worse progression-free survival of MPM patients. Among 13 MPM cell lines tested, five (38%) were highly sensitive to pevonedistat (half maximal inhibitory concentration of cell survival IC₅₀ < 0.5 µM). This remained true in a 3D spheroid culture. Pevonedistat treatment caused the accumulation of CDT1 and p21 in both sensitive and resistant cell lines. However, the treatment induced S/G2 cell cycle arrest and DNA rereplication predominantly in the sensitive cell lines. In an in vivo mouse model, the pevonedistat treatment significantly prolonged the survival of mice bearing both sensitive and resistant MPM tumors. Pevonedistat treatment reduced growth in sensitive tumors but increased apoptosis in resistant tumors. The mechanism in the resistant tumor model may be mediated by reduced macrophage infiltration, resulting from the suppression of macrophage chemotactic cytokines, C-C motif chemokine ligand 2 (CCL2), expression in tumor cells.

Genome-first approach for the characterization of a complex phenotype with combined NBAS and CUL4B deficiency

Biallelic variants in neuroblastoma-amplified sequence (NBAS) cause an extremely broad spectrum of phenotypes. Clinical features range from isolated recurrent episodes of liver failure to multisystemic syndrome including short stature, skeletal osteopenia and dysplasia, optic atrophy, and a variable immunological, cutaneous, muscular, and neurological abnormalities. Hemizygous variants in CUL4B cause syndromic X-linked intellectual disability characterized by limitations in intellectual functions, developmental delays in gait, cognitive, and speech functioning, and other features including short stature, dysmorphism, and cerebral malformations. In this study, we report on a 4.5-month-old preterm infant with a complex phenotype mainly characterized by placental-related severe intrauterine growth restriction, post-natal growth failure with spontaneous bone fractures, which led to a suspicion of osteogenesis imperfecta, and lethal bronchopulmonary dysplasia with pulmonary hypertension. Whole exome sequencing identified compound heterozygosity for a known frameshift and a novel missense variant in NBAS and hemizygosity for a known CUL4B nonsense mutation. In vitro functional studies on the novel NBAS missense substitution demonstrated altered Golgi-to-endoplasmic reticulum retrograde vesicular trafficking and reduced collagen secretion, likely explaining part of the patient's phenotype. We also provided a comprehensive overview of the phenotypic features of NBAS and CUL4B deficiency, thus updating the recently emerging NBAS genotype-phenotype correlations. Our findings highlight the power of a genome-first approach for an early diagnosis of complex phenotypes.

Raptor-Mediated Proteasomal Degradation of Deamidated 4E-BP2 Regulates Postnatal Neuronal Translation and NF-κB Activity

The translation initiation repressor 4E-BP2 is deamidated in the brain on asparagines N99/N102 during early postnatal brain development. This post-translational modification enhances 4E-BP2 association with Raptor, a central component of mTORC1 and alters the kinetics of excitatory synaptic transmission. We show that 4E-BP2 deamidation is neuron specific, occurs in the human brain, and changes 4E-BP2 subcellular localization, but not its disordered structure state. We demonstrate that deamidated 4E-BP2 is ubiquitinated more and degrades faster than the unmodified protein. We find that enhanced deamidated 4E-BP2 degradation is dependent on Raptor binding, concomitant with increased association with a Raptor-CUL4B E3 ubiquitin ligase complex. Deamidated 4E-BP2 stability is promoted by inhibiting mTORC1 or glutamate receptors. We further demonstrate that deamidated 4E-BP2 regulates the translation of a distinct pool of mRNAs linked to cerebral development, mitochondria, and NF-κB activity, and thus may be crucial for postnatal brain development in neurodevelopmental disorders, such as ASD.

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Deamidated 4E-BP2 occurs in neurons and is susceptible to ubiquitination/degradation

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mTORC1 or glutamate receptor inhibition stabilizes deamidated 4E-BP2

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A Raptor-CUL4B ubiquitin ligase complex binds to deamidated 4E-BP2

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Deamidated 4E-BP2 regulates postnatal brain translation and NF-κB activity

CREB acts as a common transcription factor for major epigenetic repressors; DNMT3B, EZH2, CUL4B and E2F6

CREB signaling is known for several decades, but how it regulates both positive and negative regulators of cell proliferation is not well understood. On the other hand functions of major epigenetic repressors such as DNMT3B, EZH2 and CUL4B for their repressive epigenetic modifications on chromatin have also been well studied. However, there is very limited information available on how these repressors are regulated at their transcriptional level. Here, using computational tools and molecular techniques including site directed mutagenesis, promoter reporter assay, chromatin immunoprecipitation (ChIP), we identified that CREB acts as a common transcription factor for DNMT3B, EZH2, CUL4B and E2F6. ChIP assay revealed that pCREB binds to promoters of these repressors at CREs and induce their transcription. As expected, the expression of these repressors and their associated repressive marks particularly H3K27me3 and H2AK119ub are increased and decreased upon CREB overexpression and knock-down conditions respectively in the cancer cells indicating that CREB regulates the functions of these repressors by activating their transcription. Since CREB and these epigenetic repressors are overexpressed in various cancer types, our findings showed the molecular relationship between them and indicate that CREB is an important therapeutic target for cancer therapy.

Circ_0074027 Contributes to Nonsmall Cell Lung Cancer Progression by Upregulating CUL4B Expression Through miR-335-5p

Background: Circular RNAs (circRNAs) are crucial regulators in human cancers, including nonsmall cell lung cancer (NSCLC). In this study, we aim to explore the biological functions and molecular mechanisms of circ_0074027 in NSCLC. Methods: Quantitative real-time polymerase chain reaction (qRT-PCR) was employed to determine the expression of circ_0074027, paired like homeodomain 1 (PITX1) mRNA, microRNA-335-5p (miR-335-5p), and cullin 4B (CUL4B) mRNA. The feature of circ_0074027 was analyzed by RNase R digestion assay. Flow cytometry analysis was adopted to analyze cell cycle and cell apoptosis. Cell counting kit-8 (CCK-8) assay and colony formation assay were performed to assess cell proliferation. Western blot assay was conducted to measure protein levels. Dual-luciferase reporter and RNA pull-down assays were carried out to verify the relationships among circ_0074027, miR-335-5p, and CUL4B. The murine xenograft model was established to investigate the role of circ_0074027 in vivo. Results: High expression of circ_0074027 was found in NSCLC tissues and cells. Circ_0074027 knockdown suppressed cell viability, cell cycle process, and colony formation and promoted apoptosis in NSCLC cells in vitro and inhibited tumor growth in vivo. Circ_0074027 acted as a sponge of miR-335-5p. The effect of circ_0074027 knockdown on NSCLC progression was weakened by miR-335-5p inhibition. Moreover, CUL4B was a target gene of miR-335-5p. CUL4B overexpression reversed the inhibitory effects on cell viability, cell cycle process, and colony formation and the promotional effect on cell apoptosis caused by miR-335-5p in NSCLC. Conclusion: Circ_0074027 facilitated NSCLC cell progression through regulating miR-335-5p/CUL4B axis.

Downregulation of lncRNA ZEB1-AS1 Represses Cell Proliferation, Migration, and Invasion Through Mediating PI3K/AKT/mTOR Signaling by miR-342-3p/CUL4B Axis in Prostate Cancer

Background: Prostate cancer (PCa) is the second most common cancer among men, threatening men's health and life. Long noncoding RNA Zinc-finger E-box binding homeobox 1 antisense gene 1 (ZEB1-AS1) and Cullin 4B (CUL4B) were reported to be connected with the tumorigenesis of PCa. However, it is unclear whether ZEB1-AS1 regulates the expression of CUL4B in PCa. Materials and Methods: The levels of ZEB1-AS1 and CUL4B in PCa tissues and cells were evaluated by quantitative real-time polymerase chain reaction. Protein levels of CUL4B, p21, CyclinD1, matrix metalloprotease 9 (MMP9), E-cadherin, phosphorylated-phosphatidylinositol 3 kinase (p-PI3K), PI3K phosphorylated protein kinase B (p-AKT), AKT, p-mTOR and mammalian target of rapamycin (mTOR) in PCa tissues or cells were assessed by Western blot analysis. The proliferation, migration, and invasion abilities of PCa cells were determined with 3-(4, 5-dimethylthiazol-2-YI)-2,5-diphenyltetrazolium bromide (MTT) or transwell assay. The interaction between ZEB1-AS1 or CUL4B and microRNA-342-3p (miR-342-3p) was predicted using starBase v2.0 database and confirmed by the dual-luciferase reporter assay. Results: ZEB1-AS1 and CUL4B were upregulated and miR-342-3p was downregulated in PCa tissues and cells. Both ZEB1-AS1 and CUL4B inhibition constrained proliferation, migration, and invasion of PCa cells. Moreover, the elevation of CUL4B reversed the effects of ZEB1-AS1 silencing on the proliferation, migration, and invasion of PCa cells. Importantly, ZEB1-AS1 modulated CUL4B expression by sponging miR-342-3p in PCa cells. Besides, ZEB1-AS1 mediated PI3K/AKT/mTOR signal pathway by miR-342-3p/CUL4B axis in PCa cells. Conclusion: ZEB1-AS1 modulated PCa progression through mediating PI3K/AKT/mTOR signaling by miR-342-3p/CUL4B axis, providing a possible strategy for the treatment of PCa.

Upregulation of Cullin 4B Promotes Gastric Cancer and Predicts Poor Prognosis

Aim

Cullin 4B (CUL4B) is a member of the cullin ubiquitin-ligase family, which participates in proteolysis. Aberrant CUL4B expression has been shown in many malignancies. This study aimed to elucidate oncogenic role of CUL4B in gastric cancer (GC).

Methods

CUL4B expression in GC tissues was examined by RT-PCR and immunohistochemistry. The proliferation, invasion and tumorigenicity of GC cells with CUL4B overexpression or knockdown were evaluated.

Results

CUL4B expression significantly increased in GC tissues, and was correlated to UICC stage and differentiation of GC, as well as poor overall survival and disease-free survival. Both univariate and multivariate analysis identified CUL4B as an independent predictor for GC patient prognosis. In addition, CUL4B promoted GC cell proliferation and invasion in vitro and tumor formation in vivo.

Conclusion

CUL4B is overexpressed to promote GC development and progression. CUL4B is a promising prognostic marker and therapeutic target for GC.

Small molecule NSC1892 targets the CUL4A/4B-DDB1 interactions and causes impairment of CRL4DCAF4 E3 ligases to inhibit colorectal cancer cell growth

Cullin 4A and 4B (CUL4A and 4B) function as oncogenes in colorectal cancer (CRC) cells. Both of them conservatively associate with DNA damage-binding protein 1 (DDB1) and DDB1-CUL4-associated factor 4 (DCAF4) to form Cullin-RING E3 ligases known as CRL4^DCAF4, which specifically ubiquitinate and degrade tumor suppressor ST7 (suppression of tumorigenicity 7). Knockdown either CUL4A/4B or DDB1 significantly inhibits tumor cell growth in vitro and in vivo. Thus, targeting these CRL4^DCAF4 components and their interactions may be an effective strategy for the therapy of CRC. In this study, we developed an in vitro AlphaScreen assay to identify small molecules targeting the CUL4A-DDB1 interaction. We obtained a compound NSC1892, which strongly disrupted the CUL4A-DDB1 interaction (IC₅₀ = 1.8 μM). Oncogenic phenotype analyses indicated that NSC1892 showed significant cytotoxicity to decrease cell proliferation, colony formation and invasion in CRC cells. Biochemical analyses demonstrated that NSC1892 treatment did not change CUL4A and CUL4B protein levels, but caused the degradation of DDB1, thereby leading to the impaired assembly of CRL4^DCAF4 E3 ligases and resulting in the accumulation of ST7. The administration of NSC1892 in mice also significantly inhibited tumor growth through degrading DDB1 and accumulating ST7. Interestingly, NSC1892 also showed promising cytotoxicity to decrease the growth of other CUL4A- or CUL4B-overexpressing tumor cells such as SKOV3 ovarian cells and Saos2 osteosarcoma cells. Our results provide a new avenue for the development of a therapeutic compound targeting tumors through disrupting the CUL4-DDB1 interaction.

The CARM1-p300-c-Myc-Max (CPCM) transcriptional complex regulates the expression of CUL4A/4B and affects the stability of CRL4 E3 ligases in colorectal cancer

The transcription factor c-Myc and two cullin family members CUL4A/4B function as oncogenes in colorectal cancer. Our recent publication reveals that c-Myc specifically activates the expression of CUL4A/4B through binding to their promoters. However, the underlying mechanism of how c-Myc actions in this process is still unknown. Using mass spectrometry and immunoprecipitation assays, we identified c-Myc formed a transcriptional complex with its partner Max (Myc-associated factor X), a histone acetyltransferase p300 and a coactivator associated arginine methyltransferase 1 (CARM1) in the present study. Knockdown or overexpression of the components of CARM1-p300-c-Myc-Max (CPCM) complex resulted in a decrease or increase of CUL4A/4B levels, respectively. Individual knockdown or inhibition of CPCM components decreased cell proliferation, colony formation, and cell invasion. Biochemically, knockdown or inhibition of CPCM components decreased their occupancies on the promoters of CUL4A/4B and resulted in their downregulation. Importantly, inhibition of CPCM components also caused a decrease of CRL4 E3 ligase activities and eventually led to an accumulation of ST7 (suppression of tumorigenicity 7), the specific substrate of CRL4 E3 ligases in colorectal cancer. Moreover, the in vivo tumor formation results indicated that knockdown or inhibition of CPCM components significantly decreased the tumor volumes. Together, our results suggest that the CPCM complex mediates explicitly the expression of CUL4A/4B, and thus affects the stability of CRL4 E3 ligases and the ubiquitination of ST7. These results provide more options by targeting the CPCM components to inhibit tumor growth in the therapy of colorectal cancer.

CUL4B negatively regulates Toll-like receptor-triggered proinflammatory responses by repressing Pten transcription

Toll-like receptors (TLRs) play critical roles in innate immunity and inflammation. The molecular mechanisms by which TLR signaling is fine-tuned remain to be completely elucidated. Cullin 4B (CUL4B), which assembles the CUL4B-RING E3 ligase complex (CRL4B), has been shown to regulate diverse developmental and physiological processes by catalyzing monoubiquitination for histone modification or polyubiquitination for proteasomal degradation. Here, we identified the role of CUL4B as an intrinsic negative regulator of the TLR-triggered inflammatory response. Deletion of CUL4B in macrophages increased the production of proinflammatory cytokines and decreased anti-inflammatory cytokine IL-10 production in response to pathogens that activate TLR3, TLR4, or TLR2. Myeloid cell-specific Cul4b knockout mice were more susceptible to septic shock when challenged with lipopolysaccharide, polyinosinic-polycytidylic acid or Salmonella typhimurium infection. We further demonstrated that enhanced TLR-induced inflammatory responses in the absence of CUL4B were mediated by increased GSK3β activity. Suppression of GSK3β activity efficiently blocked the TLR-triggered increase in proinflammatory cytokine production and attenuated TLR-triggered death in Cul4b mutant mice. Mechanistically, CUL4B was found to negatively regulate TLR-triggered signaling by epigenetically repressing the transcription of Pten, thus maintaining the anti-inflammatory PI3K-AKT-GSK3β pathway. The upregulation of PTEN caused by CUL4B deletion led to uncontrolled GSK3β activity and excessive inflammatory immune responses. Thus, our findings indicate that CUL4B functions to restrict TLR-triggered inflammatory responses through regulating the AKT-GSK3β pathway.

Small molecule TSC01682 inhibits osteosarcoma cell growth by specifically disrupting the CUL4B-DDB1 interaction and decreasing the ubiquitination of CRL4B E3 ligase substrates

The direct interaction between Cullin 4B (CUL4B) and DNA damage-binding protein 1 (DDB1) is required for the assembly of Cullin4B-RING E3 ligase complex (CRL4B), which are involved in the tumorigenesis of osteosarcoma through ubiquitinating and degrading multiple tumor suppressors and cell cycle regulators. Thus, targeting CUL4B-DDB1 interaction to prevent the assembly of CRL4B may be a potent approach to inhibit osteosarcoma cell growth. In the present study, we identified six naturally-sourced small molecules that can specifically disrupt the CUL4B-DDB1 interaction using an in vitro high-throughput screening (HTS) system in yeast. We focused our investigation on revealing the molecular effects of TSC01682, the most active compound capable of inhibiting osteosarcoma cell growth. Biochemically, TSC01682 significantly repressed the CUL4B-DDB1 interaction in both yeast cells and osteosarcoma cells. Moreover, TSC01682 treatment in osteosarcoma cells also caused a decrease of other CRL4B components including CUL4-associated factor 11 (DCAF11) and DCAF13, but an increase of two CRL4B substrates including cyclin-dependent kinase inhibitor 1A (CDKN1A, also known as p21) and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) through inhibiting their ubiquitination. Consistent with these molecular changes, TSC01682 treatment significantly inhibited cell proliferation, colony formation, invasion, and in vivo tumor growth. Collectively, our results suggest that TSC01682 is a potent compound capable of disrupting the CUL4B-DDB1 interaction, and it may be developed as a chemotherapeutic drug for osteosarcoma treatment.

NCBP1 promotes the development of lung adenocarcinoma through up-regulation of CUL4B

Lung cancer is the most frequent cancer type and is the leading cause of tumour‐associated deaths worldwide. Nuclear cap‐binding protein 1 (NCBP1) is necessary for capped RNA processing and intracellular localization. It has been reported that silencing of NCBP1 resulted in cell growth reduction in HeLa cells. Nevertheless, its clinical significance and underlying molecular mechanisms in non–small‐cell lung cancer remain unclear. In this study, we found that NCBP1 was significantly overexpressed in lung cancer tissues and several lung cancer cell lines. Through knockdown and overexpression experiments, we showed that NCBP1 promoted lung cancer cell growth, wound healing ability, migration and epithelial‐mesenchymal transition. Mechanistically, we found that cullin 4B (CUL4B) was a downstream target gene of NCBP1 in NSCLC. NCBP1 up‐regulated CUL4B expression via interaction with nuclear cap‐binding protein 3 (NCBP3). CUL4B silencing significantly reversed NCBP1‐induced tumorigenesis in vitro. Based on these findings, we propose a model involving the NCBP1‐NCBP3‐CUL4B oncoprotein axis, providing novel insight into how CUL4B is activated and contributes to LUAD progression.

CUL4B regulates cancer stem-like traits of prostate cancer cells by targeting BMI1 via miR200b/c

BACKGROUND

Cancer stem-like traits contribute to prostate cancer (PCa) progression and metastasis. Cullin 4B (CUL4B) is a member of the ubiquitin E3 ligase family and overexpressed in several solid malignancies including PCa. CUL4B has been suggested to be an oncogene through epigenetic repression of tumor suppressors. However, the link between CUL4B expression and cancer stem-like phenotype remains unclear.

METHODS

Western blot analysis, sphere formation, and colony formation assays were used to examine the effect of CUL4B on cancer stem-like traits in PCa cells. Mechanically, bioinformatic analysis was utilized to evaluate whether BMI1 was a target of CUL4B. Moreover, real-time polymerase chain reaction, chromatin immunoprecipitation, and luciferase reporter assays were performed to identify microRNAs regulated by CUL4B. Finally, Western blot assay was used to validate the regulation of CUL4B, miR200b, and miR200c (miR200b/c) on the stem-like characteristics of PCa cells.

RESULTS

CUL4B promotes PCa pluripotency-associated markers expression, sphere formation, and anchorage-independent growth ability in vitro. Mechanically, CUL4B upregulates BMI1 expression via epigenetically repressing miR200b/c expression. In addition, miR200b/c could partially reverse CUL4B-induced BMI1 and pluripotency-associated marker expression.

CONCLUSIONS

Our study revealed that CUL4B regulates cancer stem-like traits of prostate cancer cells by targeting BMI1 via miR200b/c, which might give novel insight into how CUL4B promotes PCa progression through regulating cancer stem-like traits.

CUL4B promotes aggressive phenotypes of HNSCC via the activation of the Wnt/β-catenin signaling pathway

Previous studies have revealed that CUL4B is overexpressed in various types of cancer and that its overexpression is related to the progression and metastasis of tumors. However, the biological functions of CUL4B in the progression of head and neck squamous cell carcinoma (HNSCC) are still not well understood. In the current study, we aimed to determine the changes in biological functions and molecular events that are related to CUL4B overexpression. Interestingly, our results showed that CUL4B is upregulated in HNSCC and that its upregulation is associated with poor survival and worse histological grade. Overexpression of CUL4B promoted cancer cell growth, invasion, and migration, as well as epithelial-mesenchymal transition, whereas the loss of CUL4B abrogated these malignant phenotypes. Moreover, our mechanistic investigations suggest that the Wnt/β-catenin signaling pathway was activated by CUL4B overexpression. Treatment with a Wnt/β-catenin inhibitor decreased CUL4B-induced migration and invasion, establishing a key role of Wnt/β-catenin signaling in mediating the effects of CUL4B expression. Together, these results demonstrate a key contribution of CUL4B overexpression in the malignant behavior of HNSCC cells, at least in part through the stimulation of angiogenesis and the activation of the Wnt/β-catenin signaling pathway.

Inflammation-dependent overexpression of c-Myc enhances CRL4DCAF4 E3 ligase activity and promotes ubiquitination of ST7 in colitis-associated cancer

Inflammation is well known as an important driver of the initiation of colitis-associated cancer (CAC). Some cytokines, such as IL-6 and TNF-α can activate expression of the oncogene c-Myc (MYC) and regulate its downstream effects. Cullin-RING E3 Ligases (CRLs) are emerging as master regulators controlling tumorigenesis. Here, we demonstrate that two cullin genes, CUL4A and CUL4B, but not other members, are specifically overexpressed in CAC tumour samples and positively correlate with levels of the proinflammatory cytokines IL-1β and IL-6. In vitro experiments revealed that the transcription factor c-Myc can specifically activate the expression of CUL4A and CUL4B by binding to a conserved site (CACGTG) located in their promoters. Additionally, we found that both CUL4A and CUL4B can form an E3 complex with DNA damage-binding protein 1 (DDB1) and DDB1-CUL4-associated factor 4 (DCAF4). In vitro and in vivo ubiquitination analyses indicate that CRL4^DCAF4 E3 ligase specifically directs degradation of ST7 (suppression of tumorigenicity 7). Overexpression of c-Myc in human colon epithelial cells resulted in the accumulation of CUL4A, CUL4B and DCAF4, but degradation of ST7. In contrast, knockdown of c-Myc, CUL4A or CUL4B in the colon adenocarcinoma cell line HT29 caused accumulation of ST7 and inhibition of cell proliferation, colony formation ability and in vivo tumour growth. Collectively, our results provide in vitro and in vivo evidence that c-Myc regulates CRL4^DCAF4 E3 ligase activity to mediate ubiquitination of ST7, whose presence is physiologically essential for CAC tumorigenesis. © 2019 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Abnormal level of CUL4B-mediated histone H2A ubiquitination causes disruptive HOX gene expression

Background

Neural tube defects (NTDs) are common birth defects involving the central nervous system. Recent studies on the etiology of human NTDs have raised the possibility that epigenetic regulation could be involved in determining susceptibility to them.

Results

Here, we show that the H2AK119ub1 E3 ligase CUL4B is required for the activation of retinoic acid (RA)-inducible developmentally critical homeobox (HOX) genes in NT2/D1 embryonal carcinoma cells. RA treatment led to attenuation of H2AK119ub1 due to decrease in CUL4B, further affecting HOX gene regulation. Furthermore, we found that CUL4B interacted directly with RORγ and negatively regulated its transcriptional activity. Interestingly, knockdown of RORγ decreased the expression of HOX genes along with increased H2AK119ub1 occupancy levels, at HOX gene sites in N2/D1 cells. In addition, upregulation of HOX genes was observed along with lower levels of CUL4B-mediated H2AK119ub1 in both mouse and human anencephaly NTD cases. Notably, the expression of HOXA10 genes was negatively correlated with CUL4B levels in human anencephaly NTD cases.

Conclusions

Our results indicate that abnormal HOX gene expression induced by aberrant CUL4B-mediated H2AK119ub1 levels may be a risk factor for NTDs, and highlight the need for further analysis of genome-wide epigenetic modifications in NTDs.

Electronic supplementary material

The online version of this article (10.1186/s13072-019-0268-7) contains supplementary material, which is available to authorized users.

CUL4B/miR-33b/C-MYC axis promotes prostate cancer progression

BACKGROUND

Cullin 4B (CUL4B), a scaffold protein that assembles CRL4B ubiquitin ligase complexes, is overexpressed in many types of solid tumors and contributes to epigenetic silencing of tumor suppressors. However, its clinical significance and underlying molecular mechanisms in prostate cancer (PCa) remain unknown.

METHODS

The clinical significance of CUL4B in PCa was characterized by in silico method. RT-qPCR and Western blot were used to study the transcript and protein expression levels of CUL4B and C-MYC. Bioinformatics tools, chromatin immunoprecipitation (ChIP) and luciferase reporter assay were utilized to identify and characterize the microRNAs (miRNAs) regulated by CUL4B. The biological function of CUL4B and miR-33b-5p was evaluated by MTS, transwell, and wound healing assays, accordingly.

RESULTS

CUL4B is significantly overexpressed in PCa tissues compared with benign prostatic tissues and its overexpression is correlated with poor prognosis. CUL4B promotes proliferation and aggressiveness of PCa cells in vitro. Mechanistically, we demonstrate that CUL4B upregulates the expression of C-MYC at post-transcriptional level through epigenetic silencing of miR-33b-5p. Importantly, CUL4B-induced oncogenic activity in PCa by targeting C-MYC is repressed by miR-33b-5p.

CONCLUSIONS

Our results suggested a novel CUL4B/miR-33b/C-MYC axis implicated in PCa cell growth and progression. This might provide novel insight into how CUL4B contributed to PCa aggressiveness and progression.

CRL4BRBBP7 targets HUWE1 for ubiquitination and proteasomal degradation

The E3 ubiquitin ligase HUWE1/Mule/ARF-BP1 plays an important role in diverse biological processes including DNA damage repair and apoptosis. Our previous study has shown that in response to DNA damage HUWE1 was downregulated in CUL4B-mediated ubiquitination and subsequent proteasomal degradation, and CUL4B-mediated regulation of HUWE1 was important for cell survival upon DNA damage. CUL4B is a core component of the CUL4B Ring ligase complexes containing ROC1, DDB1 and a DDB1-Cullin Associated Factors (DCAFs), the latter of which are DDB1-binding WD40 adaptors critical for substrate recognition and recruitment. However, the identity of DCAF in CRL4B that mediates degradation of HUWE1 remains elusive. Here we report that RBBP7 is the DCAF in the CRL4B complex bridging the DDB1-CUL4B-ROC1 to HUWE1. Loading of HUWE1 to the E3 ubiquitin ligase complex resulted in its polyubiquitination, and consequently its proteasome mediated degradation. Overexpression of RBBP7 promoted HUWE1 protein degradation, while depletion of RBBP7 stabilized HUWE1, and hence accelerated the degradation of MCL-1 and BRCA1, two substrates of HUWE1 that are critical in apoptosis and DNA damage repair. Taken together, these data reveal CRL4B^RBBP7 is the E3 ligase responsible for the proteasomal degradation of HUWE1, and further provide a potential strategy for cancer therapy by targeting HUWE1 and the CUL4B E3 ligase complex.

CUL4B promotes metastasis and proliferation in pancreatic cancer cells by inducing epithelial-mesenchymal transition via the Wnt/β-catenin signaling pathway

This study determines whether cullin 4B (CUL4B) promotes pancreatic cancer (PC) metastasis by inducing epithelial-mesenchymal transition (EMT) via the Wnt/β-catenin signaling pathway. A total of 64 PC patients were enrolled in this study. Human PC cell lines were distributed into blank, negative control, shCUL4B, PLOC, PLOC-CUL4B, and PLOC-CUL4B + siRNA-β-catenin groups. The expressions of CUL4B, Wnt/β-catenin signaling pathway-related proteins, and EMT-related proteins were determined using RT-qPCR and Western blotting. The positive expressions of CUL4B and β-catenin protein in tissues were detected by immunohistochemistry. MTT assay and flow cytometry was performed for cell proliferation and cell cycle, scratch test, and transwell assay for cell migration and invasion ability. CUL4B and β-catenin were expressed at a higher level in PC tissues than in paracancerous tissues though paracancerous tissues had higher expressions of CUL4B and β-catenin than normal tissues. The PLOC-CUL4B group showed increased CUL4B, Wnt, β-catenin, LEF-1, c-Jun, Cyclin D1, N-cadherin, Vimentin, Snail, and ZEB1 expression; decreased E-cadherin expression; accelerated cell proliferation; increased S-phase cell percentages; increased cell migration ability; more liver metastases; and enlarged tumor than the PLOC and PLOC-CUL4B + siRNA-β-catenin groups. The shCUL4B group showed decreased CUL4B, Wnt, β-catenin, LEF-1, c-Jun, Cyclin D1, N-cadherin, Vimentin, Snail, and ZEB1 expression; increased E-cadherin expression; decelerated cell proliferation; decreased S-phase cell percentages; reduced cell migration ability; less liver metastases; and decreased tumor weight than the blank and negative control groups. We demonstrate that CUL4B promotes PC metastasis by inducing EMT via the Wnt/β-catenin signaling pathway. Therefore, CUL4B might be the clinical target for treating PC.

MicroRNA-300 Regulates the Ubiquitination of PTEN through the CRL4BDCAF13 E3 Ligase in Osteosarcoma Cells

Cullins, critical members of the cullin-RING ubiquitin ligases (CRLs), are often aberrantly expressed in different cancers. However, the underlying mechanisms regarding aberrant expression of these cullins and the specific substrates of CRLs in different cancers are mostly unknown. Here, we demonstrate that overexpressed CUL4B in human osteosarcoma cells forms an E3 complex with DNA damage binding protein 1 (DDB1) and DDB1- and CUL4-associated factor 13 (DCAF13). In vitro and in vivo analyses indicated that the CRL4B^DCAF13 E3 ligase specifically recognized the tumor suppressor PTEN (phosphatase and tensin homolog deleted on chromosome 10) for degradation, and disruption of this E3 ligase resulted in PTEN accumulation. Further analyses indicated that miR-300 directly targeted the 3' UTR of CUL4B, and DNA hypermethylation of a CpG island in the miR-300 promoter region contributed to the downregulation of miR-300. Interestingly, ectopic expression of miR-300 or treatment with 5-AZA-2'-deoxycytidine, a DNA methylation inhibitor, decreased the stability of CRL4B^DCAF13 E3 ligase and reduced PTEN ubiquitination. By applying in vitro screening to identify small molecules that specifically inhibit CUL4B-DDB1 interaction, we found that TSC01131 could greatly inhibit osteosarcoma cell growth and could disrupt the stability of the CRL4B^DCAF13 E3 ligase. Collectively, our findings shed new light on the molecular mechanism of CUL4B function and might also provide a new avenue for osteosarcoma therapy.

CUL4B promotes bladder cancer metastasis and induces epithelial-to-mesenchymal transition by activating the Wnt/β-catenin signaling pathway

Increased expression of cullin 4B (CUL4B) is linked to progression in several cancers. This study aims to explore the effects of CUL4B on bladder cancer (BC) metastasis and epithelial-to-mesenchymal transition (EMT) and potential correlation to the Wnt/β-catenin signaling pathway. We collected BC tissues and adjacent normal tissues from 124 BC patients. Quantitative real-time polymerase chain reaction (qRT-PCR) and western blotting were employed in order to detect the expression of Wnt/β-catenin signaling pathway-related proteins and EMT markers. MTT and Transwell assays were used in order to measure cell proliferation, migration, and invasion. BC 5637 cells were transfected with control, siRNA scramble control (siRNA-NC), si-CUL4B, and CUL4B or Wnt inhibitory factor 1 (WIF-1) overexpression constructs. Levels of CUL4B mRNA and protein were increased in BC tissues in comparison with the adjacent normal tissues. CUL4B expression was negatively correlated with the expression of E-cadherin and positively correlated to the expression of N-cadherin and Vimentin. Compared to the control group, levels of β-catenin, cyclinD1, c-myc, MMP7, and EMT markers were reduced, whereas phosphorylated GSK3β^Ser9 and E-cadherin levels were increased in the si-CUL4B and WIF-1 groups. In addition, cell proliferation, migration, and invasion abilities were also increased. Increasing CUL4B expression had the opposite effect. These findings suggest that CUL4B induces EMT and promotes metastasis of BC by activating the Wnt/β-catenin signaling pathway.

A new CUL4B variant associated with a mild phenotype and an exceptional pattern of leukoencephalopathy

Cabezas type of X-linked syndromic intellectual disability (MRXSC; MIM300354) is a rare X-linked recessive intellectual disability characterized primarily by intellectual disability, short stature, hypogonadism, and gait abnormalities. It is caused by a wide spectrum of hemizygous variants in CUL4B. In a 10-year-old boy with an exceptional leukoencephalopathy pattern, we identified a new missense variant p.Leu329Gln in CUL4B using "Mendeliome" sequencing. However, his phenotype does not include the severe characteristics currently known for MRXSC. We discuss the divergent phenotype and propose a potential connection between the different CUL4B variants and corresponding phenotypes in the context of the current literature as well as 3D homology modeling.

Dysregulation of the miR-194-CUL4B negative feedback loop drives tumorigenesis in non-small-cell lung carcinoma

Cullin 4B (CUL4B), a scaffold protein that assembles CRL4B ubiquitin ligase complexes, is overexpressed in many types of cancers and represses many tumor suppressors through epigenetic mechanisms. However, the mechanisms by which CUL4B is upregulated remain to be elucidated. Here, we show that CUL4B is upregulated in non‐small‐cell lung carcinoma (NSCLC) tissues and is critically required for cell proliferation and migration in vitro and for xenograft tumor formation in vivo. We found that microRNA‐194 (miR‐194) and CUL4B protein were inversely correlated in cancer specimens and demonstrated that miR‐194 could downregulate CUL4B by directly targeting its 3′‐UTR. We also showed that CUL4B could be negatively regulated by p53 in a miR‐194‐dependent manner. miR‐194 was further shown to attenuate the malignant phenotype of lung cancer cells by downregulating CUL4B. Interestingly, CRL4B also epigenetically represses miR‐194 by catalyzing monoubiquitination at H2AK119 and by coordinating with PRC2 to promote trimethylation at H3K27 at the gene clusters encoding miR‐194. RBX1, another component in CRL4B complex, is also targeted by miR‐194 in NSCLC cells. Our results thus establish a double‐negative feedback loop between miR‐194 and CRL4B, dysregulation of which contributes to tumorigenesis. The function of miR‐194 as a negative regulator of CUL4B has therapeutic implications in lung cancer.

Variants in CUL4B are associated with cerebral malformations

Variants in cullin 4B (CUL4B) are a known cause of syndromic X-linked intellectual disability. Here, we describe an additional 25 patients from 11 families with variants in CUL4B. We identified nine different novel variants in these families and confirmed the pathogenicity of all nontruncating variants. Neuroimaging data, available for 15 patients, showed the presence of cerebral malformations in ten patients. The cerebral anomalies comprised malformations of cortical development (MCD), ventriculomegaly, and diminished white matter volume. The phenotypic heterogeneity of the cerebral malformations might result from the involvement of CUL-4B in various cellular pathways essential for normal brain development. Accordingly, we show that CUL-4B interacts with WDR62, a protein in which variants were previously identified in patients with microcephaly and a wide range of MCD. This interaction might contribute to the development of cerebral malformations in patients with variants in CUL4B.

Distinct and overlapping functions of the cullin E3 ligase scaffolding proteins CUL4A and CUL4B

The cullin 4 subfamily of genes includes CUL4A and CUL4B, which share a mostly identical amino acid sequence aside from the elongated N-terminal region in CUL4B. Both act as scaffolding proteins for modular cullin RING ligase 4 (CRL4) complexes which promote the ubiquitination of a variety of substrates. CRL4 function is vital to cells as loss of both genes or their shared substrate adaptor protein DDB1 halts proliferation and eventually leads to cell death. Due to their high structural similarity, CUL4A and CUL4B share a substantial overlap in function. However, in some cases, differences in subcellular localization, spatiotemporal expression patterns and stress-inducibility preclude functional compensation. In this review, we highlight the most essential functions of the CUL4 genes in: DNA repair and replication, chromatin-remodeling, cell cycle regulation, embryogenesis, hematopoiesis and spermatogenesis. CUL4 genes are also clinically relevant as dysregulation can contribute to the onset of cancer and CRL4 complexes are often hijacked by certain viruses to promote viral replication and survival. Also, mutations in CUL4B have been implicated in a subset of patients suffering from syndromic X-linked intellectual disability (AKA mental retardation). Interestingly, the antitumor effects of immunomodulatory drugs are caused by their binding to the CRL4CRBN complex and re-directing the E3 ligase towards the Ikaros transcription factors IKZF1 and IKZF3. Because of their influence over key cellular functions and relevance to human disease, CRL4s are considered promising targets for therapeutic intervention.

CUL4B: a novel epigenetic driver in Wnt/β-catenin-dependent hepatocarcinogenesis

Emerging evidence indicates that Cullin 4B (CUL4B), a major component of ubiquitin ligase complexes, is over-expressed in diverse cancer types with pro-tumourigenic effects. In this issue of the Journal of Pathology, Yuan and colleagues [6] elucidated the oncogenic activity of CUL4B in hepatocellular carcinoma (HCC) and delineated its role in driving Wnt/β-catenin signalling. In addition to the stabilization of β-catenin protein against proteasomal degradation, CUL4B also acts in concert with enhancer of Zeste homologue 2 (EZH2) to concordantly silence multiple Wnt inhibitors. These findings provide significant mechanistic insights into the epigenetic activation of the Wnt/β-catenin pathway in HCC and shed light on the functional importance of ubiquitination in this intricate regulatory system.