The CUL4 enigma: culling DNA repair factors

Targeting NEDDylation as a Novel Approach to Improve the Treatment of Head and Neck Cancer

Head and neck cancer is diagnosed in nearly 900,000 new patients worldwide each year. Despite this alarming number, patient outcomes, particularly for those diagnosed with late-stage and human papillomavirus (HPV)-negative disease, have only marginally improved in the last three decades. New therapeutics that target novel pathways are desperately needed. NEDDylation is a key cellular process by which NEDD8 proteins are conjugated to substrate proteins in order to modulate their function. NEDDylation is closely tied to appropriate protein degradation, particularly proteins involved in cell cycle regulation, DNA damage repair, and cellular stress response. Components of the NEDDylation pathway are frequently overexpressed or hyperactivated in many cancer types including head and neck cancer, which contribute to disease progression and drug resistance. Therefore, targeting NEDDylation could have a major impact for malignancies with alterations in the pathway, and this has already been demonstrated in preclinical studies and clinical trials. Here, we will survey the mechanisms by which aberrant NEDDylation contributes to disease pathogenesis and discuss the potential clinical implications of inhibiting NEDDylation as a novel approach for the treatment of head and neck cancer.

DDB1- and CUL4-associated factor 8 plays a critical role in spermatogenesis

Cullin-RING E3 ubiquitin ligase (CRL)-4 is a member of the large CRL family in eukaryotes. It plays important roles in a wide range of cellular processes, organismal development, and physiological and pathological conditions. DDB1- and CUL4-associated factor 8 (DCAF8) is a WD40 repeat-containing protein, which serves as a substrate receptor for CRL4. The physiological role of DCAF8 is unknown. In this study, we constructed Dcaf8 knockout mice. Homozygous mice were viable with no noticeable abnormalities. However, the fertility of Dcaf8-deficient male mice was markedly impaired, consistent with the high expression of DCAF8 in adult mouse testis. Sperm movement characteristics, including progressive motility, path velocity, progressive velocity, and track speed, were significantly lower in Dcaf8 knockout mice than in wild-type (WT) mice. However, the total motility was similar between WT and Dcaf8 knockout sperm. More than 40% of spermatids in Dcaf8 knockout mice showed pronounced morphological abnormalities with typical bent head malformation. The acrosome and nucleus of Dcaf8 knockout sperm looked similar to those of WT sperm. In vitro tests showed that the fertilization rate of Dcaf8 knockout mice was significantly reduced. The results demonstrated that DCAF8 plays a critical role in spermatogenesis, and DCAF8 is a key component of CRL4 function in the reproductive system.

Expanding molecular roles of UV-DDB: Shining light on genome stability and cancer

UV-damaged DNA binding protein (UV-DDB) is a heterodimeric complex, composed of DDB1 and DDB2, and is involved in global genome nucleotide excision repair. Mutations in DDB2 are associated with xeroderma pigmentosum complementation group E. UV-DDB forms a ubiquitin E3 ligase complex with cullin-4A and RBX that helps to relax chromatin around UV-induced photoproducts through the ubiquitination of histone H2A. After providing a brief historical perspective on UV-DDB, we review our current knowledge of the structure and function of this intriguing repair protein. Finally, this article discusses emerging data suggesting that UV-DDB may have other non-canonical roles in base excision repair and the etiology of cancer.

Treatment of Lymphoid and Myeloid Malignancies by Immunomodulatory Drugs

Thalidomide and its derivatives (lenalidomide, pomalidomide, avadomide, iberdomide hydrochoride, CC-885 and CC-90009) form the family of immunomodulatory drugs (IMiDs). Lenalidomide (CC5013, Revlimid®) was approved by the US FDA and the EMA for the treatment of multiple myeloma (MM) patients, low or intermediate-1 risk transfusion-dependent myelodysplastic syndrome (MDS) with chromosome 5q deletion [del(5q)] and relapsed and/or refractory mantle cell lymphoma following bortezomib. Lenalidomide has also been studied in clinical trials and has shown promising activity in chronic lymphocytic leukemia (CLL) and non-Hodgkin lymphoma (NHL). Lenalidomide has anti-inflammatory effects and inhibits angiogenesis. Pomalidomide (CC4047, Imnovid® [EU], Pomalyst® [USA]) was approved for advanced MM insensitive to bortezomib and lenalidomide. Other IMiDs are in phases 1 and 2 of clinical trials. Cereblon (CRBN) seems to have an important role in IMiDs action in both lymphoid and myeloid hematological malignancies. Cereblon acts as the substrate receptor of a cullin-4 really interesting new gene (RING) E3 ubiquitin ligase CRL4CRBN. This E3 ubiquitin ligase in the absence of lenalidomide ubiquitinates CRBN itself and the other components of CRL4CRBN complex. Presence of lenalidomide changes specificity of CRL4CRBN which ubiquitinates two transcription factors, IKZF1 (Ikaros) and IKZF3 (Aiolos), and casein kinase 1α (CK1α) and marks them for degradation in proteasomes. Both these transcription factors (IKZF1 and IKZF3) stimulate proliferation of MM cells and inhibit T cells. Low CRBN level was connected with insensitivity of MM cells to lenalidomide. Lenalidomide decreases expression of protein argonaute-2, which binds to cereblon. Argonaute-2 seems to be an important drug target against IMiDs resistance in MM cells. Lenalidomide decreases also basigin and monocarboxylate transporter 1 in MM cells. MM cells with low expression of Ikaros, Aiolos and basigin are more sensitive to lenalidomide treatment. The CK1α gene (CSNK1A1) is located on 5q32 in commonly deleted region (CDR) in del(5q) MDS. Inhibition of CK1α sensitizes del(5q) MDS cells to lenalidomide. CK1α mediates also survival of malignant plasma cells in MM. Though, inhibition of CK1α is a potential novel therapy not only in del(5q) MDS but also in MM. High level of full length CRBN mRNA in mononuclear cells of bone marrow and of peripheral blood seems to be necessary for successful therapy of del(5q) MDS with lenalidomide. While transfusion independence (TI) after lenalidomide treatment is more than 60% in MDS patients with del(5q), only 25% TI and substantially shorter duration of response with occurrence of neutropenia and thrombocytopenia were achieved in lower risk MDS patients with normal karyotype treated with lenalidomide. Shortage of the biomarkers for lenalidomide response in these MDS patients is the main problem up to now.

Autophagic UVRAG Promotes UV-Induced Photolesion Repair by Activation of the CRL4(DDB2) E3 Ligase

UV-induced DNA damage, a major risk factor for skin cancers, is primarily repaired by nucleotide excision repair (NER). UV radiation resistance-associated gene (UVRAG) is a tumor suppressor involved in autophagy. It was initially isolated as a cDNA partially complementing UV sensitivity in xeroderma pigmentosum (XP), but this was not explored further. Here we show that UVRAG plays an integral role in UV-induced DNA damage repair. It localizes to photolesions and associates with DDB1 to promote the assembly and activity of the DDB2-DDB1-Cul4A-Roc1 (CRL4(DDB2)) ubiquitin ligase complex, leading to efficient XPC recruitment and global genomic NER. UVRAG depletion decreased substrate handover to XPC and conferred UV-damage hypersensitivity. We confirmed the importance of UVRAG for UV-damage tolerance using a Drosophila model. Furthermore, increased UV-signature mutations in melanoma correlate with reduced expression of UVRAG. Our results identify UVRAG as a regulator of CRL4(DDB2)-mediated NER and suggest that its expression levels may influence melanoma predisposition.

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.

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.

Cullin 4A is associated with epithelial to mesenchymal transition and poor prognosis in perihilar cholangiocarcinoma

AIM

To explore the functional role of cullin 4A (CUL4A), a core subunit of E3 ubiquitin ligase, in perihilar cholangiocarcinoma (PHCC).

METHODS

The expression of CUL4A in PHCC cell lines was evaluated by Western blot and quantitative reverse transcription-polymerase chain reaction. Immunohistochemistry (IHC) was adopted to investigate the relationship between CUL4A expression and clinicopathological characteristics of PHCC. Univariate analysis and multivariate regression analysis were performed to analyze the risk factors related to overall survival (OS) and progression-free survival (PFS) of PHCC patients. Wound healing, Transwell and Matrigel assays were utilized to explore the function of CUL4A in PHCC metastasis. Furthermore, expression of epithelial to mesenchymal transition (EMT) markers was verified in cells with CUL4A knockdown or overexpression. The relationship between CUL4A expression and E-cadherin expression was also analyzed by IHC assay. Finally, the role of ZEB1 in regulating CUL4A mediated PHCC was detected by IHC, Western blot, Transwell and Matrigel assays.

RESULTS

CUL4A overexpression was detected in PHCC cell lines and clinical specimens. Clinicopathological analysis revealed a close correlation between CUL4A overexpression and tumour differentiation, T, N and TNM stages in PHCC. Kaplan-Meier analysis revealed that high CUL4A expression was correlated with poor OS and PFS of PHCC patients. Univariate analysis identified the following four parameters as risk factors related to OS rate of PHCC: T, N, TNM stages and high CUL4A expression; as well as three related to PFS: N stage, TNM stage and high CUL4A expression. Further multivariate logistic regression analysis identified high CUL4A expression as the only independent prognostic factor for PHCC. Moreover, CUL4A silencing in PHCC cell lines dramatically inhibited metastasis and the EMT. Conversely, CUL4A overexpression promoted these processes. Mechanistically, ZEB1 was discovered to regulate the function of CUL4A in promoting the EMT and metastasis.

CONCLUSION

CUL4A is an independent prognostic factor for PHCC, and it can promote the EMT by regulating ZEB1 expression. CUL4A may be a potential therapeutic target for PHCC.

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.

Loss of CUL4A expression is underlying cisplatin hypersensitivity in colorectal carcinoma cells with acquired trabectedin resistance

Background:

Colorectal carcinoma (CRC) is the third most common cancer worldwide. Platinum-based anticancer compounds still constitute one mainstay of systemic CRC treatment despite limitations due to adverse effects and resistance development. Trabectedin has shown promising antitumor effects in CRC, however, again resistance development may occur. In this study, we aimed to develop strategies to circumvent or even exploit acquired trabectedin resistance in novel CRC treatment regimens.

Methods:

Human HCT116 CRC cells were selected for acquired trabectedin resistance in vitro and characterised by cell biological as well as bioinformatic approaches. In vivo xenograft experiments were conducted.

Results:

Selection of HCT116 cells for trabectedin resistance resulted in p53-independent hypersensitivity of the selected subline against cisplatin. Bioinformatic analyses of mRNA microarray data suggested deregulation of nucleotide excision repair and particularly loss of the ubiquitin ligase CUL4A in trabectedin-selected cells. Indeed, transient knockdown of CUL4A sensitised parental HCT116 cells towards cisplatin. Trabectedin selected but not parental HCT116 xenografts were significantly responsive towards cisplatin treatment.

Conclusions:

Trabectedin selection-mediated CUL4A loss generates an Achilles heel in CRC cancer cells enabling effective cisplatin treatment. Hence, inclusion of trabectedin in cisplatin-containing cancer treatment regimens might cause profound synergism based on reciprocal resistance prevention.

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.

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.

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.

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.

Chromatin retention of DNA damage sensors DDB2 and XPC through loss of p97 segregase causes genotoxicity

DNA damage recognition subunits like DDB2 and XPC protect the human skin from ultraviolet (UV) light-induced genome instability and cancer, as demonstrated by the devastating inherited syndrome xeroderma pigmentosum. Here, we show that the beneficial DNA repair response triggered by these two genome caretakers critically depends on a dynamic spatiotemporal regulation of their homeostasis. The prolonged retention of DDB2 and XPC in chromatin, due to a failure to readily remove both recognition subunits by the ubiquitin-dependent p97/VCP/Cdc48 segregase complex, leads to impaired DNA excision repair of UV lesions. Surprisingly, the ensuing chromosomal aberrations in p97-deficient cells are alleviated by a concomitant down regulation of DDB2 or XPC. Also, genome instability resulting from an excess of DDB2 persisting in UV-irradiated cells is prevented by concurrent p97 over-expression. Our findings demonstrate that DNA damage sensors and repair initiators acquire unexpected genotoxic properties if not controlled by timely extraction from chromatin.

Phosphoproteomics study on the activated PKCδ-induced cell death

The proteolytic activation of protein kinase Cδ (PKCδ) generates a catalytic fragment called PKCδ-CF, which induces cell death. However, the mechanisms underlying PKCδ-CF-mediated cell death are largely unknown. On the basis of an engineering leukemic cell line with inducible expression of PKCδ-CF, here we employ SILAC-based quantitative phosphoproteomics to systematically and dynamically investigate the overall phosphorylation events during cell death triggered by PKCδ-CF expression. Totally, 3000 phosphorylation sites were analyzed. Considering the fact that early responses to PKCδ-CF expression initiate cell death, we sought to identify pathways possibly related directly with PKCδ by further analyzing the data set of phosphorylation events that occur in the initiation stage of cell death. Interacting analysis of this data set indicates that PKCδ-CF triggers complicated networks to initiate cell death, and motif analysis and biochemistry verification reveal that several kinases in the downstream of PKCδ conduct these networks. By analysis of the specific sequence motif of kinase-substrate, we also find 59 candidate substrates of PKCδ from the up-regulated phosphopeptides, of which 12 were randomly selected for in vitro kinase assay and 9 were consequently verified as substrates of PKCδ. To our greatest understanding, this study provides the most systematic analysis of phosphorylation events initiated by the cleaved activated PKCδ, which would vastly extend the profound understanding of PKCδ-directed signal pathways in cell death. The MS data have been deposited to the ProteomeXchange with identifier PXD000225.

Epidermal growth factor receptor protects proliferating cell nuclear antigen from cullin 4A protein-mediated proteolysis

Proliferating cell nuclear antigen (PCNA) is an essential component for DNA synthesis upon growth stimulation. It has been shown that phosphorylation of PCNA at Tyr-211 by the EGF receptor (EGFR) protects PCNA from polyubiquitylation and degradation, whereas blocking phosphorylation induces ubiquitylation-mediated degradation of the chromatin-bound, but not the -unbound, PCNA, and suppresses cell proliferation. However, the ubiquitin E3 ligase linking growth signaling to the proteolysis of PCNA and the underlying regulatory mechanism remain to be identified. Here we show that, in the absence of Tyr-211 phosphorylation, PCNA is subject to polyubiquitylation at Lys-164 by the CUL4A E3 ligase, resulting in the degradation of PCNA. Mutation of Lys-164 to arginine prevents PCNA ubiquitylation and rescues the degradation of the K164R/Y211F PCNA double mutant. Activation of EGFR inhibits the interaction of PCNA with CUL4A, whereas inhibition of EGFR leads to increased CUL4A-PCNA interaction and CUL4A-dependent ubiquitin-mediated degradation of PCNA. Substitution of endogenous PCNA with the Y211F mutant PCNA conveys enhanced sensitization to EGFR inhibition. Our findings identify CUL4A as the ubiquitin ligase linking the down-regulation of cell surface receptor tyrosine kinase to the nuclear DNA replication machinery in cancer cells.

CUL4B-deficiency in humans: understanding the clinical consequences of impaired Cullin 4-RING E3 ubiquitin ligase function

Cullin's encode the structural components for one of the most abundant E3 ubiquitin ligase families in eukaryotes accounting for as many as 400 distinct E3 ubiquitin ligases. Because of their modular assembly involving combinations of multiple distinct adaptor and substrate receptor proteins, it comes as no surprise that these E3's are implicated in a plethora of fundamental biochemical processes ranging from DNA replication and repair to transcription and development. Herein, we focus on one member of the cullin family, namely the Cullin 4-RING E3 ligases (CRL4's). More specifically, we overview what has been learned about some of the functions of CRL4's from various model systems. We discuss the unexpected association of defective CUL4B with syndromal X-linked mental retardation in humans and speculate on the biochemical consequences and clinical implications of defective CRL4 function. In particular, mutations in CUL4B highlight a previously unappreciated role for CRL4's in neuronal function and cognition in humans.

COP9 signalosome function in the DDR

The COP9 signalosome (CSN) is a platform for protein communication in eukaryotic cells. It has an intrinsic metalloprotease that removes the ubiquitin (Ub)-like protein Nedd8 from cullins. CSN-mediated deneddylation regulates culling-RING Ub ligases (CRLs) and controls ubiquitination of proteins involved in DNA damage response (DDR). CSN forms complexes with CRLs containing cullin 4 (CRL4s) which act on chromatin playing crucial roles in DNA repair, checkpoint control and chromatin remodeling. Furthermore, via associated kinases the CSN controls the stability of DDR effectors such as p53 and p27 and thereby the DDR outcome. DDR is a protection against cancer and deregulation of CSN function causes cancer making it an attractive pharmacological target. Here we review current knowledge on CSN function in DDR.