Pathogenic Role of the CRL4 Ubiquitin Ligase in Human Disease

CUL4A silencing attenuates cervical carcinogenesis and improves Cisplatin sensitivity

CUL4A is an ubiquitin ligase deregulated in numerous pathologies including cancer and even hijacked by viruses for facilitating their survival and propagation. However, its role in Human papilloma virus (HPV)-mediated cervical carcinogenesis remains elusive. The UALCAN and GEPIA datasets were analyzed to ascertain the transcript levels of CUL4A in cervical squamous cell carcinoma and endocervical adenocarcinoma (CESC) patients. Subsequently, various biochemical assays were employed to explore the functional contribution of CUL4A in cervical carcinogenesis and to shed some light on its involvement in Cisplatin resistance in cervical cancer. Our UALCAN and GEPIA datasets analyses reveal elevated CUL4A transcript levels in cervical squamous cell carcinoma and endocervical adenocarcinoma (CESC) patients that correlate with adverse clinicopathological parameters such as tumor stage and lymph node metastasis. Kaplan-Meier plot and GEPIA assessment depict poor prognosis of CESC patients having high CUL4A expression. Varied biochemical assays illustrate that CUL4A inhibition severely curtails hallmark malignant properties such as cellular proliferation, migration, and invasion of cervical cancer cells. We also show that CUL4A knockdown in HeLa cells causes increased susceptibility and better apoptotic induction toward Cisplatin, a mainstay drug used in cervical cancer treatment. More interestingly, we find reversion of Cisplatin-resistant phenotype of HeLa cells and an augmented cytotoxicity towards the platinum compound upon CUL4A downregulation. Taken together, our study underscores CUL4A as a cervical cancer oncogene and illustrates its potential as a prognosis indicator. Our investigation provides a novel avenue in improving current anti-cervical cancer therapy and overcoming the bottle-neck of Cisplatin resistance.

CUL4A Ubiquitin Ligase Is an Independent Predictor of Overall Survival in Pancreatic Adenocarcinoma

BACKGROUND/AIM

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive malignancy with dismal prognosis. Genomic instability due to defects in cell-cycle regulation/mitosis or deficient DNA-damage repair is a major driver of PDAC progression with clinical relevance. Deregulation of licensing of DNA replication leads to DNA damage and genomic instability, predisposing cells to malignant transformation. While overexpression of DNA replication-licensing factors has been reported in several human cancer types, their role in PDAC remains largely unknown. We aimed here to examine the expression and prognostic significance of the DNA replication-licensing factors chromatin licensing and DNA replication factor 1 (CDT1), cell-division cycle 6 (CDC6), minichromosome maintenance complex component 7 (MCM7) and also of the ubiquitin ligase regulator of CDT1, cullin 4A (CUL4A), in PDAC.

MATERIALS AND METHODS

Expression levels of CUL4, CDT1, CDC6 and MCM7 were evaluated by immunohistochemistry in 76 formalin-fixed paraffin-embedded specimens of PDAC patients in relation to DNA-damage response marker H2AX, clinicopathological parameters and survival. We also conducted bioinformatics analysis of data from online available databases to corroborate our findings.

RESULTS

CUL4A and DNA replication-licensing factors were overexpressed in patients with PDAC and expression of CDT1 positively correlated with H2AX. Expression of CUL4A and CDT1 positively correlated with lymph node metastasis. Importantly, elevated CUL4A expression was associated with reduced overall survival and was an independent indicator of poor prognosis on multivariate analysis.

CONCLUSION

Our findings implicate CUL4A, CDT1, CDC6 and MCM7 in PDAC progression and identify CUL4A as an independent prognostic factor for this disease.

WDR5-HOTTIP Histone Modifying Complex Regulates Neural Migration and Dendrite Polarity of Pyramidal Neurons via Reelin Signaling

WD-repeat domain 5 (WDR5), a core component of histone methyltransferase complexes, is associated with Kabuki syndrome and Kleefstra syndrome that feature intellectual disability and neurodevelopmental delay. Despite its critical status in gene regulation and neurological disorders, the role of WDR5 in neural development is unknown. Here we show that WDR5 is required for normal neuronal placement and dendrite polarization in the developing cerebral cortex. WDR5 knockdown led to defects in both entry into the bipolar transition of pyramidal neurons within the intermediate zone and radial migration into cortical layers. Moreover, WDR5 deficiency disrupted apical and basal polarity of cortical dendrites. Aberrant dendritic spines and synapses accompanied the dendrite polarity phenotype. WDR5 deficiency reduced expression of reelin signaling receptors, ApoER and VdldR, which were associated with abnormal H3K4 methylation and H4 acetylation on their promoter regions. Finally, an lncRNA, HOTTIP, was found to be a partner of WDR5 to regulate dendritic polarity and reelin signaling via histone modification. Our results demonstrate a novel role for WDR5 in neuronal development and provide mechanistic insights into the neuropathology associated with histone methyltransferase dysfunction.

CRL4-DCAF8L2 E3 ligase promotes ubiquitination and degradation of BARD1

BARD1 is a tumor suppressor that is necessary for the functioning and stability of BRCA1, with which it forms a heterodimer and participates in the repair of DNA double-strand breaks. The cellular level of BARD1 and its interaction with BRCA1 are crucial for BRCA1/BARD1 function in homologous recombination and tumor suppression. However, the regulatory mechanism underpinning the stability of BARD1 is largely unclear. In this study, we identified DCAF8L2, a DDB1-Cullin associated factor (DCAF) associated with CRL4 E3 ligase, as a negative regulator of BARD1. Mechanistically, DCAF8L2 interacts with and targets BARD1 for ubiquitination and degradation. In addition, the interaction of DCAF8L2 with BARD1 through the RING domain could compete with the dimerization of BRCA1 and BARD1, leading to increased cellular uncoupling of BARD1 and BRCA1, subjecting the latter to degradation. The overexpression of DCAF8L2 compromises the homologous recombination process and confers cells with increased sensitivity to DNA damage. Furthermore, DCAF8L2 was aberrantly expressed in breast cancer cell lines. Our findings suggest that DCAF8L2 may play an oncogenic role in the pathogenesis of breast cancer, possibly by negative regulation of BARD1.

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.

CRL4ADTL degrades DNA-PKcs to modulate NHEJ repair and induce genomic instability and subsequent malignant transformation

Genomic instability induced by DNA damage and improper DNA damage repair is one of the main causes of malignant transformation and tumorigenesis. DNA double strand breaks (DSBs) are the most detrimental form of DNA damage, and nonhomologous end-joining (NHEJ) mechanisms play dominant and priority roles in initiating DSB repair. A well-studied oncogene, the ubiquitin ligase Cullin 4A (CUL4A), is reported to be recruited to DSB sites in genomic DNA, but whether it regulates NHEJ mechanisms of DSB repair is unclear. Here, we discovered that the CUL4A-DTL ligase complex targeted the DNA-PKcs protein in the NHEJ repair pathway for nuclear degradation. Overexpression of either CUL4A or DTL reduced NHEJ repair efficiency and subsequently increased the accumulation of DSBs. Moreover, we demonstrated that overexpression of either CUL4A or DTL in normal cells led to genomic instability and malignant proliferation. Consistent with the in vitro findings, in human precancerous lesions, CUL4A expression gradually increased with increasing malignant tendency and was negatively correlated with DNA-PKcs and positively correlated with γ-H2AX expression. Collectively, this study provided strong evidence that the CUL4A-DTL axis increases genomic instability and enhances the subsequent malignant transformation of normal cells by inhibiting NHEJ repair. These results also suggested that CUL4A may be a prognostic marker of precancerous lesions and a potential therapeutic target in 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.

The application of ubiquitin ligases in the PROTAC drug design

Protein ubiquitylation plays important roles in many biological activities. Protein ubiquitylation is a unique process that is mainly controlled by ubiquitin ligases. The ubiquitin-proteasome system (UPS) is the main process to degrade short-lived and unwanted proteins in eukaryotes. Many components in the UPS are attractive drug targets. Recent studies indicated that ubiquitin ligases can be employed as tools in proteolysis-targeting chimeras (PROTACs) for drug discovery. In this review article, we will discuss the recent progress of the application of ubiquitin ligases in the PROTAC drug design. We will also discuss advantages and existing problems of PROTACs. Moreover, we will propose a few principles for selecting ubiquitin ligases in PROTAC applications.

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.

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.

Stabilization of E2-EPF UCP protein is implicated in hepatitis B virus-associated hepatocellular carcinoma progression

Hepatitis B virus (HBV) X protein (HBx) is associated with hepatocarcinogenesis. E2-EPF ubiquitin carrier protein (UCP) catalyzes ubiquitination of itself and von Hippel-Lindau protein (pVHL) for degradation and associates with tumor growth and metastasis. However, it remains unknown whether HBx modulates the enzyme activity of UCP and thereby influences hepatocarcinogenesis. Here, we show that UCP is highly expressed in liver tissues of HBx-transgenic mice, but not non-transgenic mice. UCP was more frequently expressed in HBV-positive liver cancers than in HBV-negative liver cancers. HBx binds to UCP specifically and serotype independently, and forms a ternary complex with UCP and pVHL. HBx inhibits self-ubiquitination of UCP, but enhances UCP-mediated pVHL ubiquitination, resulting in stabilization of hypoxia-inducible factor-1α and -2α. HBx and UCP stabilize each other by mutually inhibiting their ubiquitination. HBx promotes cellular proliferation and metastasis via UCP. Our findings suggest that UCP plays a key role in HBV-related hepatocarcinogenesis.

Cul4a promotes zebrafish primitive erythropoiesis via upregulating scl and gata1 expression

CUL4A and CUL4B are closely related members in Cullin family and can each assemble a Cullin-RING E3 ligase complex (Cullin-RING Ligase 4A or 4B, CRL4A, or CRL4B) and participate in a variety of biological processes. Previously we showed that zebrafish cul4a, but not cul4b, is essential for cardiac and pectoral fin development. Here, we have identified cul4a as a crucial regulator of primitive erythropoiesis in zebrafish embryonic development. Depletion of cul4a resulted in a striking reduction of erythroid cells due to the inhibition of erythroid differentiation. Transcript levels for early hematopoietic regulatory genes including scl, lmo2, and gata1 are significantly reduced in cul4a-deficient embryos. Mechanistically, we demonstrated that scl and gata1, the central regulators of primitive hematopoiesis for erythroid determination, are transcriptionally upregulated by cul4a. These findings demonstrate an important role for cul4a in primitive erythropoiesis and may bear implications in regeneration medicine of anemia and related diseases.

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.

The emerging role for Cullin 4 family of E3 ligases in tumorigenesis

As a member of the Cullin-RING ligase family, Cullin-RING ligase 4 (CRL4) has drawn much attention due to its broad regulatory roles under physiological and pathological conditions, especially in neoplastic events. Based on evidence from knockout and transgenic mouse models, human clinical data, and biochemical interactions, we summarize the distinct roles of the CRL4 E3 ligase complexes in tumorigenesis, which appears to be tissue- and context-dependent. Notably, targeting CRL4 has recently emerged as a noval anti-cancer strategy, including thalidomide and its derivatives that bind to the substrate recognition receptor cereblon (CRBN), and anticancer sulfonamides that target DCAF15 to suppress the neoplastic proliferation of multiple myeloma and colorectal cancers, respectively. To this end, PROTACs have been developed as a group of engineered bi-functional chemical glues that induce the ubiquitination-mediated degradation of substrates via recruiting E3 ligases, such as CRL4 (CRBN) and CRL2 (pVHL). We summarize the recent major advances in the CRL4 research field towards understanding its involvement in tumorigenesis and further discuss its clinical implications. The anti-tumor effects using the PROTAC approach to target the degradation of undruggable targets are also highlighted.

Genomic investigation of porcine periweaning failure to thrive syndrome (PFTS)

Porcine periweaning failure to thrive syndrome (PFTS) can be defined by anorexia, lethargy, progressive debilitation and compulsive behaviours that occur in seemingly healthy pigs within two to threeweeks of weaning in the absence of any known infectious, nutritional, management or environmental factors. A genetic component has been hypothesised for this syndrome. In the present study, 119 commercial pigs (80 cases and 39 controls) were genotyped with the porcine 80K single nucleotide polymorphism-chip and were analysed with logistic regression and two Fixation Index-based approaches. The analyses revealed several regions on chromosomes 1, 3, 6 and 11 with moderate divergence between cases and controls, particularly three haplotypes on SSC3 and 11. The gene-based analyses of the candidate regions revealed the presence of genes that have been reported to be associated with phenotypes like PFST including depression (PDE10A) and intestinal villous atrophy (CUL4A). It is important to increase the effort of collecting more samples to improve the power of these analyses.

Role of HBx in hepatitis B virus persistence and its therapeutic implications

Chronic hepatitis B virus infection is a significant risk factor for cirrhosis and hepatocellular carcinoma. The HBx protein is required for virus replication, but the lack of robust infection models has hindered our understanding of HBx functions that could be targeted for antiviral purposes. We briefly review three properties of HBx: its binding to DDB1 and its regulation of cell survival and metabolism, to illustrate how a single viral protein can have multiple effects in a cell. We propose that different functions of HBx are needed, depending on the changing hepatocyte environment encountered during a chronic virus infection, and that these functions might serve as novel therapeutic targets for inhibiting hepatitis B virus replication and the development of associated diseases.

CUL4B promotes gastric cancer invasion and metastasis-involvement of upregulation of HER2

Cullin 4B (CUL4B) is a scaffold protein overexpressed in several solid malignancies. It is known to silence tumor suppressor through post-transcriptional manner. However, its clinical significance and underlying molecular mechanisms in gastric cancer (GC) remain largely unknown. In this study, we found that CUL4B was significantly overexpressed in GC tissues and its overexpression was correlated with lymph node metastasis and poor prognosis. Through gain- and loss-of-function experiments, we showed that CUL4B promotes GC cell invasion and epithelial-mesenchymal transition (EMT) in vitro, as well as tumor growth and metastasis in vivo. Mechanistically, we identified HER2 as a downstream target gene of CUL4B in GC. CUL4B unregulated HER2 expression via transcriptionally repressing miR-125a. Intriguingly, HER2 inhibitors significantly reversed CUL4B-induced EMT in vitro and partially blocked GC metastasis in tumor xenografts with CUL4B overexpression. Finally, we suggested the involvement of the PI3K/AKT pathway in CUL4B-induced HER2 upregulation in GC. In all, we proposed a model for a CUL4B-miR-125a-HER2 oncoprotein axis, which provided novel insight into how HER2 was activated and contributed to GC progression and metastasis.

CUL4A as a marker and potential therapeutic target in multiple myeloma

Multiple myeloma is the most common cause of death of hematological malignancy worldwide. Cullin 4A has been proposed as oncogene in several types of human cancer, but the expression and function of cullin 4A in multiple myeloma remain unclear. Here, we demonstrate that cullin 4A plays an oncogenic role in multiple myeloma development. The expression of cullin 4A was detected by quantitative real-time polymerase chain reaction in multiple myeloma patients and multiple myeloma cell lines. In addition, silencing of cullin 4A with small interfering RNA was performed in human multiple myeloma cells, and the impact on proliferation, cell cycle, apoptosis, migration, and invasion of the multiple myeloma cells was analyzed. We found that the level of cullin 4A in serum samples was significantly upregulated in patients with multiple myeloma compared with healthy control subjects. Knockdown of cullin 4A via small interfering RNA inhibited the proliferation of the multiple myeloma cell lines by delaying cell-cycle progression and increasing apoptosis. cullin 4A downregulation inhibited multiple myeloma cell migration and invasion in vitro. Our results suggested that cullin 4A could be a promising therapy target in multiple myeloma patients.

The role of cullin4B in human cancers

Cullin 4B (CUL4B) is a scaffold of the Cullin4B-Ring E3 ligase complex (CRL4B) that plays an important role in proteolysis and is implicated in tumorigenesis. Aberrant expression of CUL4B has been reported in various types of human diseases. Recently, studies have shown that CUL4B was overexpressed in a multitude of solid neoplasms and affect the expression of several tumor suppressor genes. In this review, we aim to summarize the biological function of CUL4B in order to better understand its pathogenesis in human cancers.

Cul4B is a novel prognostic marker in cholangiocarcinoma

Cullin 4B (Cul4B), a scaffold protein that assembles the ubiquitin ligase complex, is involved in a wide variety of physiological and developmental processes, such as cell cycle progression, DNA damage response and gene expression regulation. Cul4B is overexpressed in various solid tumors. However, the prognostic value and role of Cul4B in cholangiocarcinoma (CCA) is largely unknown. The present study demonstrated that Cul4B was overexpressed in 21 (26.6%) of 79 patients with intrahepatic CCA, and in 40 (28.6%) of 140 patients with extrahepatic CCA (EHCC). Kaplan-Meier survival analysis suggested that Cul4B expression is an unfavorable prognostic factor in EHCC patients. Notably, Cul4B and epidermal growth factor receptor expression define a subset of CCA patients with poor prognosis. In vitro data indicated that Cul4B promotes the proliferation, migration and invasion of CCA cells. Furthermore, Cul4B expression promotes the epithelial-mesenchymal transition (EMT) process in CCA cells. Finally, Cul4B repressed the expression of the tumor suppressor genes P16 and phosphatase and tensin homolog. Collectively, the results of the present study revealed an important role of Cul4B in CCA with respect to initiating EMT. Cul4B expression may serve as a prognostic marker for patients with EHCC.

CUL4A promotes cell invasion in gastric cancer by activating the NF-κB signaling pathway

Cullin 4A (CUL4A) overexpression has been reported to be involved in the carcinogenesis and progression of many malignant tumors. However, the role of CUL4A in the progression of gastric cancer (GC) remains unclear. In this study, we explored whether and how CUL4A regulates proinflammatory signaling to promote GC cell invasion. Our results showed that knockdown of CUL4A inhibited GC cell migration and invasion induced by lipopolysaccharide (LPS) stimulation. We also found that both CUL4A and nuclear factor-kappa B (NF-κB) protein expressions were enhanced by LPS stimulation in HGC27 GC cell lines. Furthermore, knockdown of CUL4A decreased the protein expression of NF-κB and mRNA expression of the downstream genes of the NF-κB pathway, such as matrix metalloproteinase (MMP) 2, MMP9, and interleukin-8. Our immunohistochemistry analysis on 50 GC tissue samples also revealed that CUL4A positively correlated with NF-κB expression. Taken together, our findings suggest that CUL4A may promote GC cell invasion by regulating the NF-κB signaling pathway and could be considered as a potential therapeutic target in patients with GC.

Proteomic analysis of the cullin 4B interactome using proximity-dependent biotinylation in living cells

Cullin 4B (CUL4B) mutations have been implicated in mental retardation and dopamine-related behaviors due to disruptions in their interaction with cullin-RING E3 ligases (CRLs). Thus, further identification of CUL4B substrates can increase the knowledge of protein homeostasis and illuminate the role of CUL4B in neuropsychiatric disease. However, the transient nature of the coupling between CUL4B and its substrates is difficult to detect in vivo using current approaches, thus hampers efforts to investigate functions of CRLs within unperturbed living systems. In this study, we sought to discover CUL4B interactants with or without dopamine stimulation. BirA (118G) proximity-dependent biotin labeling combined with LC-MS was employed to biotinylate and identify transient and weak interactants of CUL4B. After purification with streptavidin beads and identified by LC-MS, a total of 150 biotinylated proteins were identified at baseline condition, 53 of which are well-known CUL4B interactants. After dopamine stimulation, 29 proteins disappeared and were replaced by 21 different protein interactants. The altered CUL4B interactants suggest that CUL4B regulates protein turnover and homeostasis in response to dopamine stimulation. Our results demonstrate the potential of this approach to identify novel CUL4B-related molecules in respond to cellular stimuli, which may be applied to other types of signaling pathways.

CUL4A promotes proliferation and metastasis of colorectal cancer cells by regulating H3K4 trimethylation in epithelial-mesenchymal transition

Increasing evidence suggests that CUL4A, a ubiquitin ligase, is involved in the promotion of cancer malignancy and correlated with worse clinical prognosis in several kinds of human cancers. Although its effect and mechanism on the progression of colorectal cancer (CRC) remain unknown. Our clinical data show that CUL4A protein is overexpressed, positively associated with lymph nodes status, differentiation degree, tumor size, and poor prognosis in 80 CRC patients. CUL4A overexpression promotes cell proliferation and colony formation of CRC cells. Knockdown of CUL4A inhibits cell proliferation and migration. CUL4A can significantly promote the in vitro migration of CRC cells via induction of the epithelial–mesenchymal transition process. And the modulation of CUL4A expression altered the level of H3K4 trimethylation at the E-cadherin, N-cadherin, and vimentin gene promoters, which in turn transcriptionally regulated their expression. Moreover, knockdown of CUL4A also decreased the tumor volume and tumor weight in vivo. Together, our results reveal that CUL4A plays as an oncogene in CRC and may become a potential therapeutic target in the treatment of colorectal cancer.

Identification of a tumor suppressor network in T-cell leukemia

To identify novel cancer-related genes targeted by copy number alterations, we performed genomic profiling of T-cell acute lymphoblastic leukemia (T-ALL) cell lines. In 3/8, we identified a shared deletion at chromosomal position 2p16.3-p21. Within the minimally deleted region, we recognized several candidate tumor suppressor (TS) genes, including FBXO11 and FOXN2. An additional deletion at chromosome 14q23.2-q32.11 included FOXN3, highlighting this class of FOX genes as potential TS. Quantitative expression analyses of FBXO11, FOXN2, and FOXN3 confirmed reduced transcript levels in the identified cell lines. Moreover, reduced expression of these genes was also observed in about 7% of T-ALL patients, showing their clinical relevance in this malignancy. Bioinformatic analyses revealed concurrent reduction of FOXN2 and/or FOXN3 together with homeobox gene ZHX1. Consistently, experiments demonstrated that both FOXN2 and FOXN3 directly activated transcription of ZHX1. Taken together, we identified novel TS genes forming a regulatory network in T-cell development and leukemogenesis.

Guard the guardian: A CRL4 ligase stands watch over histone production

Histones are evolutionarily conserved proteins that together with DNA constitute eukaryotic chromatin in a defined stoichiometry. Core histones are dynamic scaffolding proteins that undergo a myriad of post-translational modifications, which selectively engage chromosome condensation, replication, transcription and DNA damage repair. Cullin4-RING ubiquitin E3 ligases are known to hold pivotal roles in a wide spectrum of chromatin biology ranging from chromatin remodeling and transcriptional repression, to sensing of cytotoxic DNA lesions. Our recent work uncovers an unexpected function of a CRL4 ligase upstream of these processes in promoting histone biogenesis. The CRL4^WDR23 ligase directly controls the activity of the stem-loop binding protein (SLBP), which orchestrates elemental steps of canonical histone transcript metabolism. We demonstrate that non-proteolytic ubiquitination of SLBP ensures sufficient histone reservoirs during DNA replication and is vital for genome integrity and cellular fitness.

Dysregulation of CUL4A and CUL4B Ubiquitin Ligases in Lung Cancer

The Cullin-RING ubiquitin ligase 4 (CRL4) is implicated in controlling cell cycle, DNA damage repair, and checkpoint response based on studies employing cell lines and mouse models. CRL4 proteins, including CUL4A and CUL4B, are often highly accumulated in human malignancies. Elevated CRL4 attenuates DNA damage repair and increases genome instability that is believed to facilitate tumorigenesis. However, this has yet to be evaluated in human patients with cancer. In our study, 352 lung cancer and 62 normal lung specimens of Asian origin were constructed into tissue microarrays of four distinct lung cancer subtypes. Expression of CUL4A, CUL4B, and their substrates was detected by immunohistochemistry and analyzed statistically for their prognostic value and association with DNA damage response and genomic instability. Our results show that both CUL4A and CUL4B are overexpressed in the majority of lung carcinomas (P_CUL4A <0.001 and P_CUL4B <0.001) and significantly associated with tumor size (P_CUL4A <0.001 and P_CUL4B = 0.002), lymphatic invasion (P_CUL4A = 0.004 and P_CUL4B <0.001), metastasis (P_CUL4A = 0.019 and P_CUL4B = 0.006), and advanced TNM stage (P_CUL4A <0.001 and P_CUL4B <0.001), which parallels gene amplification and abnormal activation of the canonical WNT signaling. Moreover, overexpression of CUL4A, but not CUL4B, is significantly associated with tobacco smoking (p = 0.01) and is inversely correlated with XPC and P21, both of which are substrates of CUL4A (P_CUL4A = 0.019 and P_CUL4B = 0.006). Higher levels of CUL4A or CUL4B are significantly associated with the overall survival of patients (P_CUL4A <0.001 and P_CUL4B <0.001) and progression-free survival (P_CUL4A <0.001 and P_CUL4B = 0.001). Our findings revealed that CUL4A and CUL4B are differentially associated with etiologic factors for pulmonary malignancies and are independent prognostic markers for the survival of distinct lung cancer subtypes.

A new layer of degradation mechanism for PR-Set7/Set8 during cell cycle

Set8 is critically involved in transcription regulation, cell cycle progression and genomic stability. Emerging evidence has revealed that E3 ubiquitin ligases such as CRL4^cdt2 and SCF^Skp2 regulate Set8 protein abundance. However, it is unclear whether other E3 ligase(s) could govern Set8 level for proper cell cycle progression in response to genotoxic stress such as UV irradiation. Recently, we report that the SCF^β-TRCP complex regulates Set8 protein stability by targeting it for ubiquitination and subsequent degradation. Notably, Set8 interacts with the SCF^β-TRCP E3 ligase complex. We further revealed a critical role of CKI in SCF^β-TRCP-mediated degradation of Set8. Mechanistically, CKI-mediated phosphorylation of Set8 at the S253 site promotes its destruction by SCF^β-TRCP. Importantly, SCF^β-TRCP-dependent Set8 destruction also contributes to the tight control of cell proliferation and cell cycle progression, in response to UV irradiation. Here, we summarize our new findings regarding the crucial role of β-TRCP in CKI-mediated Set8 degradation, which could provide new evidence to support that dysregulation of a tight regulatory network of Set8 could lead to aberrant cell cycle process.

Aberrant Expression of CUL4A Is Associated with IL-6/ STAT3 Activation in Colorectal Cancer Progression

BACKGROUND AND AIMS

Although it has been indicated that the cytokine interleukin-6 (IL-6) promotes colorectal cancer (CRC) tumorigenesis in tumor microenvironment, the mechanisms related to IL-6-induced tumor progression are still not well understood.

METHODS

First, the correlation between pSTAT3, CUL4A and ZEB1 was analyzed using immunocytochemistry. Logistic regression analysis was then used to observe the relationship between levels of pSTAT3, CUL4A and ZEB1 and clinicopathological characteristics. Finally, the mechanism of the effect of the expression level of pSTAT3, CUL4A and ZEB1 on cell invasion ability was verified by cell experiment.

RESULTS

We discovered that the increased expression levels of pSTAT3, CUL4A and ZEB1 had significant relationships in CRC patients. These up-regulated expression levels were also closely associated with CRC aggressiveness. Furthermore, in vitro, we discovered that expression levels of CUL4A and ZEB1 were significantly up-regulated when IL-6 stimulated. However, the CUL4A-knockdown, IL-6, could not induce expression of ZEB1. CHIP assay authenticated that pSTAT3 could bind to CUL4A promoter and worked as their transcription factors. We also demonstrated that IL-6 markedly increased the reporter activity using a luciferase reporter gene containing CUL4A promoter. Finally, silencing CUL4A blocked IL-6-driven invasion in matrigel invasion assay.

CONCLUSION

This study proposed that CUL4A played an oncogene role through ZEB1 in IL-6-induced colorectal carcinoma progression.

Cell Autonomous and Nonautonomous Function of CUL4B in Mouse Spermatogenesis

CUL4B ubiquitin ligase belongs to the cullin-RING ubiquitin ligase family. Although sharing many sequence and structural similarities, CUL4B plays distinct roles in spermatogenesis from its homologous protein CUL4A. We previously reported that genetic ablation ofCul4ain mice led to male infertility because of aberrant meiotic progression. In the present study, we generated Cul4bgerm cell-specific conditional knock-out (Cul4b(Vasa)),as well asCul4bglobal knock-out (Cul4b(Sox2)) mouse, to investigate its roles in spermatogenesis. Germ cell-specific deletion of Cul4bled to male infertility, despite normal testicular morphology and comparable numbers of spermatozoa. Notably, significantly impaired sperm mobility caused by reduced mitochondrial activity and glycolysis level were observed in the majority of the mutant spermatozoa, manifested by low, if any, sperm ATP production. Furthermore,Cul4b(Vasa)spermatozoa exhibited defective arrangement of axonemal microtubules and flagella outer dense fibers. Our mass spectrometry analysis identified INSL6 as a novel CUL4B substrate in male germ cells, evidenced by its direct polyubiquination and degradation by CUL4B E3 ligase. Nevertheless,Cul4bglobal knock-out males lost their germ cells in an age-dependent manner, implying failure of maintaining the spermatogonial stem cell niche in somatic cells. Taken together, our results show that CUL4B is indispensable to spermatogenesis, and it functions cell autonomously in male germ cells to ensure spermatozoa motility, whereas it functions non-cell-autonomously in somatic cells to maintain spermatogonial stemness. Thus, CUL4B links two distinct spermatogenetic processes to a single E3 ligase, highlighting the significance of ubiquitin modification during spermatogenesis.

Wild type HBx and truncated HBx: Pleiotropic regulators driving sequential genetic and epigenetic steps of hepatocarcinogenesis and progression of HBV-associated neoplasms

Hepatitis B virus (HBV) is one of the causative agents of hepatocellular carcinoma. The molecular mechanisms of tumorigenesis are complex. One of the host factors involved is apparently the long-lasting inflammatory reaction which accompanies chronic HBV infection. Although HBV lacks a typical viral oncogene, the HBx gene encoding a pleiotropic regulatory protein emerged as a major player in liver carcinogenesis. Here we review the tumorigenic functions of HBx with an emphasis on wild type and truncated HBx variants, and their role in the transcriptional dysregulation and epigenetic reprogramming of the host cell genome. We suggest that HBx acquired by the HBV genome during evolution acts like a cellular proto-onc gene that is activated by deletion during hepatocarcinogenesis. The resulting viral oncogene (v-onc gene) codes for a truncated HBx protein that facilitates tumor progression. Copyright © 2015 John Wiley & Sons, Ltd.

The role of cullin proteins in gastric cancer

The cullin proteins are a family of scaffolding proteins that associate with RING proteins and ubiquitin E3 ligases and mediate substrate-receptor bindings. Thus, cullin proteins regulate the specificity of ubiquitin targeting in the regulation of proteins involved in various cellular processes, including proliferation, differentiation, and apoptosis. There are seven cullin proteins that have been identified in eukaryotes: CUL1, CUL2, CUL3, CUL4A, CUL4B, CUL5, and CUL7/p53-associated parkin-like cytoplasmic protein. All of these proteins contain a conserved cullin homology domain that binds to RING box proteins. Cullin-RING ubiquitin ligase complexes are activated upon post-translational modification by neural precursor cell-expressed, developmentally downregulated protein 8. The aberrant expression of several cullin proteins has been implicated in many cancers though the significance in gastric cancer has been less well investigated. This review provides the first systematic discussion of the associations between all members of the cullin protein family and gastric cancer. Functional and regulatory mechanisms of cullin proteins in gastric carcinoma progression are also summarized along with a discussion concerning future research areas. Accumulating evidence suggests a critical role of cullin proteins in tumorigenesis, and a better understanding of the function of these individual cullin proteins and their targets will help identify potential biomarkers and therapeutic targets.

The CUL4B/AKT/β-Catenin Axis Restricts the Accumulation of Myeloid-Derived Suppressor Cells to Prohibit the Establishment of a Tumor-Permissive Microenvironment

Cancer progression requires a permissive microenvironment that shields cancer from the host immunosurveillance. The presence of myeloid-derived suppressor cells (MDSC) is a key feature of a tumor-permissive microenvironment. Cullin 4B (CUL4B), a scaffold protein in the Cullin 4B-RING E3 ligase complex (CRL4B), represses tumor suppressors through diverse epigenetic mechanisms and is overexpressed in many malignancies. We report here that CUL4B unexpectedly functions as a negative regulator of MDSC functions in multiple tumor settings. Conditional ablation of CUL4B in the hematopoietic system, driven by Tek-Cre, resulted in significantly enhanced accumulation and activity of MDSCs. Mechanistically, we demonstrate that the aberrant abundance of MDSCs in the absence of CUL4B was mediated by the downregulation of the AKT/β-catenin pathway. Moreover, CUL4B repressed the phosphatases PP2A and PHLPP1/2 that dephosphorylate and inactivate AKT to sustain pathway activation. Importantly, the CUL4B/AKT/β-catenin axis was downregulated in MDSCs of healthy individuals and was further suppressed in tumor-bearing mice and cancer patients. Thus, our findings point to a pro- and antitumorigenic role for CUL4B in malignancy, in which its ability to impede the formation of a tumor-supportive microenvironment may be context-specific.

Knockdown of CUL4B inhibits proliferation and promotes apoptosis of colorectal cancer cells through suppressing the Wnt/β-catenin signaling pathway

Colorectal cancer is one of the leading causes of cancer related deaths worldwide. Cullin 4B (CUL4B) is over-expressed in diverse cancer types. However, the function and precise molecular mechanism of CUL4B in colorectal cancer remains largely unknown. Therefore, in this study, we examined the expression of CUL4B in colorectal cancer cell lines and its effects on cellular proliferation and apoptosis, and the underlying mechanism was also explored. Our results showed that CUL4B was significantly overexpressed in colorectal cancer cell lines. Silencing CUL4B obviously inhibited proliferation and tumorigenicity of colorectal cancer cells both in vitro and in vivo, and it also promoted the apoptosis of colorectal cancer cells. Moreover, knockdown of CUL4B inhibited the expression of β-catenin, cyclin D1 and c-Myc in colorectal cancer cells. Taken together, these results showed that knockdown of CUL4B inhibit proliferation and promotes apoptosis of colorectal cancer cells through suppressing the Wnt/β-catenin signaling pathway. Therefore, CUL4B may represent a novel therapeutic target for colorectal cancer treatment.

Targeting Cullin-RING E3 ubiquitin ligases for drug discovery: structure, assembly and small-molecule modulation

In the last decade, the ubiquitin–proteasome system has emerged as a valid target for the development of novel therapeutics. E3 ubiquitin ligases are particularly attractive targets because they confer substrate specificity on the ubiquitin system. CRLs [Cullin–RING (really interesting new gene) E3 ubiquitin ligases] draw particular attention, being the largest family of E3s. The CRLs assemble into functional multisubunit complexes using a repertoire of substrate receptors, adaptors, Cullin scaffolds and RING-box proteins. Drug discovery targeting CRLs is growing in importance due to mounting evidence pointing to significant roles of these enzymes in diverse biological processes and human diseases, including cancer, where CRLs and their substrates often function as tumour suppressors or oncogenes. In the present review, we provide an account of the assembly and structure of CRL complexes, and outline the current state of the field in terms of available knowledge of small-molecule inhibitors and modulators of CRL activity. A comprehensive overview of the reported crystal structures of CRL subunits, components and full-size complexes, alone or with bound small molecules and substrate peptides, is included. This information is providing increasing opportunities to aid the rational structure-based design of chemical probes and potential small-molecule therapeutics targeting CRLs.

Clinical significance of CUL4A in human prostate cancer

Aberrant expression of the Cullin 4A (CUL4A) is found in many tumor types, but the functions and mechanism of CUL4A in prostate cancer (PCa) development and progression remain largely unknown. The aim of this study was to investigate the possible role of CUL4A in prostate tumorigenesis. Immunohistochemistry was used to examine CUL4A expression in human PCa tissues and BPH tissues. Cell proliferation was assessed by MTT, and migration and invasion were analyzed by Transwell and Matrigel assays after CUL4A knockdown in PCa in vitro. The results showed that CUL4A protein was overexpressed in 86.21 % of PCa tissues. CUL4A knockdown with siRNA in PCa cells decreased cell proliferation, migration, and invasion. Mechanistically, CUL4A could modulate the expression of P53 in PCa cells. Our results indicate that CUL4A overexpression play an oncogenic role in the pathogenesis of PCa, and CUL4A may be a potential therapeutic target for PCa.

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.

Intellectual disability-associated dBRWD3 regulates gene expression through inhibition of HIRA/YEM-mediated chromatin deposition of histone H3.3

Many causal mutations of intellectual disability have been found in genes involved in epigenetic regulations. Replication-independent deposition of the histone H3.3 variant by the HIRA complex is a prominent nucleosome replacement mechanism affecting gene transcription, especially in postmitotic neurons. However, how HIRA-mediated H3.3 deposition is regulated in these cells remains unclear. Here, we report that dBRWD3, the Drosophila ortholog of the intellectual disability gene BRWD3, regulates gene expression through H3.3, HIRA, and its associated chaperone Yemanuclein (YEM), the fly ortholog of mammalian Ubinuclein1. In dBRWD3 mutants, increased H3.3 levels disrupt gene expression, dendritic morphogenesis, and sensory organ differentiation. Inactivation of yem or H3.3 remarkably suppresses the global transcriptome changes and various developmental defects caused by dBRWD3 mutations. Our work thus establishes a previously unknown negative regulation of H3.3 and advances our understanding of BRWD3-dependent intellectual disability.

CUL4A contributes to the biology of basal-like breast tumors through modulation of cell growth and antitumor immune response

The CUL4A E3 ubiquitin ligase is involved in the regulation of many cellular processes and its amplification and/or overexpression has been observed in breast cancer. The 13q34 amplification, which is associated with the basal-like breast cancer subtype, has been proposed as one of the mechanism behind CUL4A up-regulation. However, the specific contribution of CUL4A to the biology of basal-like breast tumors has not yet been elucidated. In this work, by using cellular models of basal phenotype, we show the inhibitory effect of CUL4A silencing in the proliferation and growth of breast cancer cells both, in vitro and in vivo. We also demonstrate the transforming capacity of CUL4A exogenous overexpression in the 184B5 human mammary epithelial cells in vitro. Our results suggest a synergistic effect between CUL4A high levels and the activation of the RAS pathway in the tumorigenesis of basal-like breast cancer tumors. In addition, by using a proteomics approach we have defined novel candidate proteins and pathways that might mediate the oncogenic effect of CUL4A. In particular, we report a putative role of CUL4A in bypassing the immune system in breast cancer through the down-regulation of several molecules involved in the immune surveillance. These findings provide insight into the oncogenic properties of CUL4A in basal-like breast cancer and highlight the therapeutic opportunities to target CUL4A.

AMBRA1 interplay with cullin E3 ubiquitin ligases regulates autophagy dynamics

Autophagy maintains cellular homeostasis by degrading harmful or unnecessary intracellular components. How the autophagy response is induced rapidly and transiently remains largely unknown. We report that the E3 ubiquitin ligases Cullin-5 and Cullin-4 regulate the onset and termination of autophagy, respectively, by dynamically interacting with AMBRA1, a regulator of autophagy. Under normal conditions, Cullin-4 binding to AMBRA1 limits its protein abundance. Autophagy stimuli promote AMBRA1 stabilization by causing ULK1-dependent Cullin-4 release. Notably, Cullin-4/AMBRA1 dissociation is transient, and the re-established interaction triggers AMBRA1 degradation, terminating the autophagy response. Moreover, Cullin-4 inhibits the interaction between AMBRA1 and another Cullin E3 ligase. Indeed, upon Cullin-4 dissociation, AMBRA1 binds and inhibits Cullin-5, thus promoting the accumulation of the mTOR inhibitor DEPTOR. Through DEPTOR stabilization, AMBRA1 establishes a feedback loop that ensures the rapid onset of autophagy by enhancing mTOR inactivation. Our findings show that Cullin-mediated degradation of autophagy regulators temporally controls the autophagy response.

Hepatitis B virus HBx protein interactions with the ubiquitin proteasome system

The hepatitis B virus (HBV) causes acute and chronic hepatitis, and the latter is a major risk factor for the development of hepatocellular carcinoma (HCC). HBV encodes a 17-kDa regulatory protein, HBx, which is required for virus replication. Although the precise contribution(s) of HBx to virus replication is unknown, many viruses target cellular pathways to create an environment favorable for virus replication. The ubiquitin proteasome system (UPS) is a major conserved cellular pathway that controls several critical processes in the cell by regulating the levels of proteins involved in cell cycle, DNA repair, innate immunity, and other processes. We summarize here the interactions of HBx with components of the UPS, including the CUL4 adaptor DDB1, the cullin regulatory complex CSN, and the 26S proteasome. Understanding how these protein interactions benefit virus replication remains a challenge due to limited models in which to study HBV replication. However, studies from other viral systems that similarly target the UPS provide insight into possible strategies used by HBV.

CUL4A overexpression enhances lung tumor growth and sensitizes lung cancer cells to erlotinib via transcriptional regulation of EGFR

Background

CUL4A has been proposed as oncogene in several types of human cancer, but its clinical significance and functional role in human non-small cell lung cancer (NSCLC) remain unclear.

Methods

Expression level of CUL4A was examined by RT-PCR and Western blot. Forced expression of CUL4A was mediated by retroviruses, and CUL4A silencing by shRNAs expressing lentiviruses. Growth capacity of lung cancer cells was measured by MTT in vitro and tumorigenesis in vivo, respectively.

Results

We found that CUL4A was highly expressed in human lung cancer tissues and lung cancer cell lines, and this elevated expression positively correlated with disease progression and prognosis. Overexpression of CUL4A in human lung cancer cell lines increased cell proliferation, inhibited apoptosis, and subsequently conferred resistance to chemotherapy. On other hand, silencing CUL4A expression in NSCLC cells reduced proliferation, promoted apoptosis and resulted in tumor growth inhibition in cancer xenograft model. Mechanistically, we revealed CUL4A regulated EGFR transcriptional expression and activation, and subsequently activated AKT. Targeted inhibition of EGFR activity blocked these CUL4A induced oncogenic activities.

Conclusions

Our results highlight the significance of CUL4A in NSCLC and suggest that CUL4A could be a promising therapy target and a potential biomarker for prognosis and EGFR target therapy in NSCLC patients.

Electronic supplementary material

The online version of this article (doi:10.1186/1476-4598-13-252) contains supplementary material, which is available to authorized users.

Inositol hexakisphosphate kinase-1 mediates assembly/disassembly of the CRL4-signalosome complex to regulate DNA repair and cell death

Inositol polyphosphates containing an energetic pyrophosphate bond are formed primarily by a family of three inositol hexakisphosphate (IP6) kinases (IP6K1-3). The Cullin-RING ubiquitin ligases (CRLs) regulate diverse biological processes through substrate ubiquitylation. CRL4, comprising the scaffold Cullin 4A/B, the E2-interacting Roc1/2, and the adaptor protein damage-specific DNA-binding protein 1, is activated by DNA damage. Basal CRL4 activity is inhibited by binding to the COP9 signalosome (CSN). UV radiation and other stressors dissociate the complex, leading to E3 ligase activation, but signaling events that trigger signalosome dissociation from CRL4 have been unclear. In the present study, we show that, under basal conditions, IP6K1 forms a ternary complex with CSN and CRL4 in which IP6K1 and CRL4 are inactive. UV dissociates IP6K1 to generate IP7, which then dissociates CSN-CRL4 to activate CRL4. Thus, IP6K1 is a novel CRL4 subunit that transduces UV signals to mediate disassembly of the CRL4-CSN complex, thereby regulating nucleotide excision repair and cell death.

Enhanced LPS-induced peritonitis in mice deficiency of cullin 4B in macrophages

Cullin 4B (CUL4B), a member of the cullin protein family, is a scaffold protein of the CUL4B-RING-E3 ligase complex that ubiquitinates intracellular proteins.CUL4B's targets include cell cycle-regulated proteins and DNA replication-related molecules. In this study, we generated myeloid-specific Cul4b-deficient mice (Cul4b(f/y);LysM-Cre(KI/KI)) to investigate the influence of Cul4b deficiency on innate immunity, especially on the function of macrophages. Our results show that an intraperitoneal injection of lipopolysaccharide (LPS) led to a significant decrease in body weights and increased leukocyte infiltrates with increased chemokines in the peritoneal cavity of Cul4b(f/y);LysM-Cre(KI/KI) mice. However, the proinflammatory cytokines, IL-6 and TNF-α did not increase in LPS-injected Cul4b(f/y);LysM-Cre(KI/KI) mice. Furthermore, bone marrow-derived macrophages from Cul4b(f/y);LysM-Cre(KI/KI) mice secreted higher levels of chemokines but lower levels of TNF-α and IL-6 upon LPS stimulation. Of note, increased proliferation of Cul4b-deficient macrophages was also observed. These results show that myeloid-specific Cul4b deficiency worsens LPS-induced peritonitis. In addition, Cul4b deficiency leads to enhanced DNA replication and proliferation, increased production of chemokines but a decreased production of proinflammatory cytokines of macrophages. Our data highlight a new role of cullin family, CUL4B, in the immune system.

Regulation of TGF-β signaling, exit from the cell cycle, and cellular migration through cullin cross-regulation: SCF-FBXO11 turns off CRL4-Cdt2

Deregulation of the cell cycle and genome instability are common features of cancer cells and various mechanisms exist to preserve the integrity of the genome and guard against cancer. The cullin 4-RING ubiquitin ligase (CRL4) with the substrate receptor Cdt2 (CRL4 (Cdt2)) promotes cell cycle progression and prevents genome instability through ubiquitylation and degradation of Cdt1, p21, and Set8 during S phase of the cell cycle and following DNA damage. Two recently published studies report the ubiquitin-dependent degradation of Cdt2 via the cullin 1-RING ubiquitin ligase (CRL1) in association with the substrate specificity factor and tumor suppressor FBXO11 (CRL1 (FBXO11)). The newly identified pathway restrains the activity of CRL4 (Cdt2) on p21 and Set8 and regulates cellular response to TGF-β, exit from the cell cycle and cellular migration. Here, we show that the CRL1 (FBXO11) also promotes the degradation of Cdt2 during an unperturbed cell cycle to promote efficient progression through S and G 2/M phases of the cell cycle. We discuss how this new method of regulating the abundance of Cdt2 participates in various cellular activities.

A Cul4 E3 ubiquitin ligase regulates histone hand-off during nucleosome assembly

Nucleosome assembly following DNA replication and gene transcription is important to maintain genome stability and epigenetic information. Newly synthesized histones H3-H4 first bind histone chaperone Asf1 and are then transferred to other chaperones for nucleosome assembly. However, it is unknown how H3-H4 is transferred from the Asf1-H3-H4 complex to other chaperones because Asf1 binds H3-H4 with high affinity. Here, we show that yeast Rtt101(Mms1) E3 ubiquitin ligase preferentially binds and ubiquitylates new histone H3 acetylated at lysine 56. Inactivation of Rtt101 or mutating H3 lysine residues ubiquitylated by the Rtt101(Mms1) ligase impairs nucleosome assembly and promotes Asf1-H3 interactions. Similar phenotypes occur in human cells in which the ortholog of Rtt101(Mms1), Cul4A(DDB1), is depleted. These results indicate that the transfer of H3-H4 from the Asf1-H3-H4 complex to other histone chaperones is regulated by a conserved E3 ligase and provide evidence for crosstalk between histone acetylation and ubiquitylation in nucleosome assembly.

E3 ubiquitin ligase Cullin4B mediated polyubiquitination of p53 for its degradation

Controlled protein ubiquitination through E3 ubiquitin ligases and degradation via 26S proteasome machinery is required for orderly progression through cell cycle, chromatin remodeling, DNA repair, and development. Each cullin-dependent ubiquitin ligase (E3) complex can recruit various substrates for their degradation. Cullin 4A (CUL4A) and Cullin 4B (CUL4B) are members of cullin family proteins that mediate ubiquitin dependent proteolysis. Though, these two cul4 genes are functionally redundant, Cullin 4B is not a substitute for all the Cullin 4A functions. Published report has shown that CUL4A interacts with p53 and induces its decay. Although, CUL4A has been known to control several cellular processes, little is known about CUL4B functions. Therefore, in this study, we analyzed the role of CUL4B on p53 polyubiquitination. Our stable cell line and transient transfection studies show that CUL4B indeed interacts with p53 and induces its polyubiquitination. Importantly, both CUL4A and CUL4B overexpressing cells show almost equal levels of p53 polyubiquitination. Moreover, we observed an increased level of polyubiquitination on p53 in CUL4B overexpressing stable cell line upon treatment with siRNA specific for CUL4A indicating that CUL4B plays a vital role in p53 stability. In addition, we have observed the differential expression of CUL4B in various eukaryotic cell lines and mouse tissues suggesting the important role of CUL4B in various tissues. Together, these observations establish an important negative regulatory role of CUL4B on p53 stability.

CUL4A is overexpressed in human pituitary adenomas and regulates pituitary tumor cell proliferation

Cullin 4A (CUL4A) encodes a core subunit of an E3 ubiquitin ligase that targets proteins for ubiquitin-mediated degradation, and aberrant expression of the CUL4A is found in many tumor types. However, its roles and clinicopathologic significance in pituitary adenomas are not clear. The aim of this study was to investigate the possible role of CUL4A in pituitary tumorigenesis. Immunohistochemistry was used to examine CUL4A expression in human normal pituitaries and pituitary tumors with respect to various clinicopathologic factors in pituitary adenomas. Cell proliferation was assessed by MTT and colony formation, and migration and invasion were analyzed by Transwell and Matrigel assays after CUL4A overexpression or knockdown in pituitary tumor cells. Overexpression of CUL4A was frequently observed in pituitary adenomas compared with normal adenohypophysial tissue and significantly associated with tumor progressiveness and invasion. CUL4A overexpression in GH3 adenoma cells increased colony numbers, cell viability and cell invasion and silencing CUL4A in AtT20 adenoma cells decreased cell proliferation, migration and invasion. Mechanistically, CUL4A could modulate the expression of p53, p21, and p27 in pituitary tumor cells. In addition, high levels of CUL4A expression also significantly inversely correlated with the p53 protein level in human pituitary adenomas. Our results indicate that CUL4A enhances pituitary cell proliferation, migration and invasion and may thus contribute to pituitary tumor development and progression.

Involvement of CUL4A in regulation of multidrug resistance to P-gp substrate drugs in breast cancer cells

CUL4A encodes a core component of a cullin-based E3 ubiquitin ligase complex that regulates many critical processes such as cell cycle progression, DNA replication, DNA repair and chromatin remodeling by targeting a variety of proteins for ubiquitination and degradation. In the research described in this report we aimed to clarify whether CUL4A participates in multiple drug resistance (MDR) in breast cancer cells. We first transfected vectors carrying CUL4A and specific shCUL4A into breast cancer cells and corresponding Adr cells respectively. Using reverse transcription polymerase chain reactions and western blots, we found that overexpression of CUL4A in MCF7 and MDA-MB-468 cells up-regulated MDR1/P-gp expression on both the transcription and protein levels, which conferred multidrug resistance to P-gp substrate drugs, as determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays. On the other hand, silencing CUL4A in MCF7/Adr and MDA-MB-468/Adr cells led to the opposite effect. Moreover, ERK1/2 in CUL4A-overexpressing cells was highly activated and after treatment with PD98059, an ERK1/2-specific inhibitor, CUL4A-induced expression of MDR1/P-gp was decreased significantly. Lastly, immunohistochemistry in breast cancer tissues showed that P-gp expression had a positive correlation with the expression of CUL4A and ERK1/2. Thus, these results implied that CUL4A and ERK1/2 participated in multi-drug resistance in breast cancer through regulation of MDR1/P-gp expression.

Building and remodelling Cullin-RING E3 ubiquitin ligases

Cullin-RING E3 ubiquitin ligases (CRLs) control a plethora of biological pathways through targeted ubiquitylation of signalling proteins. These modular assemblies use substrate receptor modules to recruit specific targets. Recent efforts have focused on understanding the mechanisms that control the activity state of CRLs through dynamic alterations in CRL architecture. Central to these processes are cycles of cullin neddylation and deneddylation, as well as exchange of substrate receptor modules to re-sculpt the CRL landscape, thereby responding to the cellular requirements to turn over distinct proteins in different contexts. This review is focused on how CRLs are dynamically controlled with an emphasis on how cullin neddylation cycles are integrated with receptor exchange.

CRL4A-FBXW5-mediated degradation of DLC1 Rho GTPase-activating protein tumor suppressor promotes non-small cell lung cancer cell growth

DLC1 encodes a RhoA GTPase-activating protein and tumor suppressor lost in cancer by genomic deletion or epigenetic silencing and loss of DLC1 gene transcription. We unexpectedly identified non-small cell lung cancer (NSCLC) cell lines and tumor tissue that expressed DLC1 mRNA yet lacked DLC1 protein expression. We determined that DLC1 was ubiquitinated and degraded by cullin 4A-RING ubiquitin ligase (CRL4A) complex interaction with DDB1 and the FBXW5 substrate receptor. siRNA-mediated suppression of cullin 4A, DDB1, or FBXW5 expression restored DLC1 protein expression in NSCLC cell lines. FBXW5 suppression-induced DLC1 reexpression was associated with a reduction in the levels of activated RhoA-GTP and in RhoA effector signaling. Finally, FBXW5 suppression caused a DLC1-dependent decrease in NSCLC anchorage-dependent and -independent proliferation. In summary, we identify a posttranslational mechanism for loss of DLC1 and a linkage between CRL4A-FBXW5-associated oncogenesis and regulation of RhoA signaling.