Cullin1 Research Articles

The Biology of F-box Proteins: The SCF Family of E3 Ubiquitin Ligases.

F-box proteins function as substrate adaptors for the S-phase kinase-associated protein 1 (SKP1)-cullin 1 (CUL1)-F-box protein (SCF) ubiquitin ligase complexes, which mediate the proteasomal degradation of a diverse range of regulatory proteins. 20years since the F-box protein family has been discovered, our understanding of substrate-recognition regulation and the roles F-box proteins play in cellular processes has continued to expand. Here, we provide an introduction to the discovery and classification of F-box proteins, the overall structural assembly of SCF complexes, the varied mechanisms by which F-box proteins recognize their substrates, and the role F-box proteins play in diseases and their potentials in targeted therapies.

CUL1 Knockdown Attenuates the Adhesion, Invasion, and Migration of Triple-Negative Breast Cancer Cells via Inhibition of Epithelial-Mesenchymal Transition.

Cullin-1 (CUL1) is an important factor for tumor growth and a potential therapeutic target for breast cancer therapy, but the molecular mechanism in triple-negative breast cancer (TNBC) is unknown. In the present study, CUL1 shRNA was transfected into BT549 and MDA-MB-231 breast cancer cells. Cell morphology, adhesion, invasion, and migration assays were carried out in the CUL1 knockdown cells. Additionally, protein expression levels of epithelial-mesenchymal transition (EMT)-related factors, Akt phosphorylation at S473 (pAkt), glycogen synthase kinase-3β phosphorylation at ser9 (pGSK3β), cytoplasmic and nuclear β-catenin, and epidermal growth factor receptor phosphorylation at Tyr1068 (pEGFR) were detected by Western blot analysis. CUL1 knockdown significantly suppressed the adhesion, invasion and migration capabilities of the cells, and decreased the expression of Snail1/2, ZEB1/2, Twist1/2, Vimentin, and increased the expression of Cytokeratin 18 (CK18). Moreover, CUL1 knockdown significantly downregulated the phosphorylated levels of Akt, GSK3β, and EGFR, inhibiting the translocation of β-catenin from the cytoplasm to the nucleus. The results indicate that CUL1 knockdown prohibited the metastasis behaviors of breast cancer cells through downregulation (dephosphorylation) of the EMT signaling pathways of EGFR and Akt/GSK3β/β-catenin in breast cancer. These results strongly suggested that reinforcement of the EMT might be a key for CUL1 to accelerate TNBC metastasis.

Cloning and expression analysis of the StCUL1 gene in potato

The Cullin-RING E3 ubiquitin ligase (CRL) complex is the most common E3 ligase, and the SCF complex (CRL1) has the most diverse functions. Cullin1(CUL1) is a scaffolding protein for assembly of the complex. SCF has been shown to participate in the non-biological stress response pathways. In this study, a classic CUL1 protein was identified in Solanum tuberosum, StCUL1. A full-length cDNA of the StCUL1 gene was obtained from ED13 (a potato variety) by Ralstonia solanacearum inoculation using the RACE method. Sequence analysis indicated that the gene comprised 2662 bp, with an open reading frame of 2229 bp encoding 743 amino acids. The expression levels of the StCUL1 gene in potato treated with R. solanacearum and exogenous hormones (such as salicylic acid, jasmonic acid methyl ester and abscisic acid) at different time points were determined by real-time PCR. The results indicated that StCUL1 was induced not only by pathogenic bacteria, but also by exogenous hormones, with sustained high expression. However, there were some differences in the modes of expression. Tissue localization analysis indicated that its expression was tissue specific, and it was mainly in the phloem of the vascular system of stems and leaves.

Cullin 1 is not associated with late-onset preeclampsia

BackgroundLate-onset preeclampsia (PE) is preeclampsia occurring after 34 weeks of gestational age or later. Cullin 1 (CUL1), a proangiogenic protein, is expressed in the placenta, where an imbalance between proangiogenic and antiangiogenic proteins during gestation can cause disturbance of trophoblast invasion. This defect results in vascular ischemia that may produce preeclampsia. The objective of this study was to determine the correlation between CUL1 as proangiogenic factor and late-onset preeclampsia. MethodsThis study was of analytical observational cross-sectional design and involved 44 preeclampsia patients with ³34 weeks of gestational age (late-onset PE). The CUL1 level in the subjects’ sera, taken before they gave birth, and in homogenates of their placenta, obtained per vaginam or by cesarean section, were examined by the ELISA technique. Statistical analysis was performed with the Spearman correlation test with significant p value of <0.05.ResultsMedian maternal age was 31 years and median gestational age was 37 weeks. Median serum CUL1 was 41.78 pg/mL and median placental homogenate CUL1 was 32.24 pg per milligram of total placental tissue protein. There was no significant correlation between serum CUL1 level and late-onset preeclampsia (r=-0.281; p=0.065). There was also no significant correlation between placental CUL1 level and late-onset preeclampsia (r=-0.166; p=0.281).ConclusionSerum CUL1 and placental CUL1 were not correlated with late-onset preeclampsia. However, this study indicated that low serum CUL1 tends to prolong gestational age in preeclampsia.

Cullin1 binds and promotes NLRP3 ubiquitination to repress systematic inflammasome activation

Activation of the NACHT, leucine-rich repeat, and pyrin domains-containing protein 3 (collectively known as NLRP3) inflammasome plays a key role in host immune response, which is the first line of defense against cellular stresses and pathogen infections. However, excessive inflammasome activation damages host cells, and therefore it must be precisely controlled. Here, we discover that Cullin1 (CUL1), a key component of the Skp1-Cullin1-F-box E3 ligase, plays a critical role in controlling the NLRP3 inflammasome. CUL1 represses inflammasome assembly in cultured cells, suppresses NLRP3 function in human monocytic cell line macrophages, and attenuates inflammatory responses in mouse model. Detailed studies demonstrate that CUL1 interacts with NLRP3 and promotes NLRP3 ubiquitination, but not protein degradation, to repress the NLRP3 inflammasome activation. Moreover, upon inflammatory stimuli, including ATP and nigericin treatments, CUL1 disassociates from NLRP3 to release the repression of the NLRP3 inflammasome. Thus, this study reveals a distinct and unique mechanism underlying the control of systematic activation of the NLRP3 inflammasome.-Wan, P., Zhang, Q., Liu, W., Jia, Y., Ai, S., Wang, T., Wang, W., Pan, P., Yang, G., Xiang, Q., Huang, S., Yang, Q., Zhang, W., Liu, F., Tan, Q., Zhang, W., Wu, K., Liu, Y., Wu, J. Cullin1 binds and promotes NLRP3 ubiquitination to repress systematic inflammasome activation.

SEMA3B but Not CUL1 as Marker for Pre-Eclampsia Progression.

BackgroundAn imbalance between pro- and anti-angiogenic factors contributes to impaired trophoblast invasion during pregnancy, leading to failure of uterine spiral artery remodeling, blood vessel ischemia, and pre-eclampsia (PE). Anti-angiogenic semaphorin 3B (SEMA3B) and pro-angiogenic cullin 1 (CUL1) are expressed in both the placenta and maternal blood. The present study investigated correlations between serum and placental SEMA3B as well as CUL1 levels in late-onset PE.MethodsThis cross-sectional study included 50 patients with late-onset (≥ 32 weeks gestation) PE. Maternal serum was obtained before delivery, and placentas were obtained immediately after delivery. SEMA3B and CUL1 levels were evaluated by ELISA. Results were statistically analysed by Spearman correlation test, with a P < 0.05 considered statistically significant.ResultsWhile elevated serum SEMA3B levels significantly correlated with increased placental SEMA3B levels in late-onset PE (R = 0.620, P = 0.000), alteration of serum CUL1 levels did not correlate with alteration of placental CUL1.ConclusionAlteration of circulating maternal SEMA3B, but not CUL1, levels can potentially be used to monitor PE progression during pregnancy.

O-GlcNAcylation confers protection against Staphylococcus aureus infection in Caenorhabditis elegans through ubiquitination.

Glycosylation is one of the most prevalent post-translational modifications in biological systems. In Caenorhabditis elegans, O-GlcNAcylation has been shown to be actively involved in the regulation of dauer formation and detoxification of toxins secreted by invading pathogens. On this backdrop, the present study is focused on understanding the role of O-GlcNAcylation in C. elegans during Staphylococcus aureus infection using a gel based proteomic approach. Results of time course killing assays with wild-type and mutants of glycosylation and comparison of results revealed an increase in the survival of the C. elegans oga-1 mutant when compared to wild-type N2 and the ogt-1 mutant. Increased survival of C. elegans N2 upon S. aureus infection in the presence of O-(2-acetamido-2-deoxy-d-glucopyranosylidenamino) N-phenylcarbamate (PUGNAc-an OGA inhibitor) further confirmed the involvement of O-GlcNAcylation in protecting C. elegans from infection. The two-dimensional gel-based proteomic analysis of the control and S. aureus infected C. elegans oga-1 mutant followed by mass spectrometric identification of differentially expressed proteins has been carried out. The results revealed that key proteins involved in ubiquitination such as Cullin-1 (CUL-1), Cullin-3 (CUL-3), BTB and MATH domain-containing protein 15 (BATH-15), ubiquitin-conjugating enzyme E2 variant 3 (UEV-3) and probable ubiquitin-conjugating enzyme E2 7 (UBC-7) are upregulated. Real-time PCR analysis further confirms the upregulation of genes encoding the above-mentioned proteins which are involved in the ubiquitin-mediated pathways in C. elegans. In addition, treatment of C. elegans wild-type N2 and the oga-1 mutant with PUGNAc + suramin and suramin (an ubiquitination inhibitor), respectively has resulted in increased sensitivity to S. aureus infection. Hence, it is presumed that upregulation of proteins involved in the ubiquitination pathway could be the key regulatory mechanism responsible for the enhanced survival of the oga-1 mutant during S. aureus infection.

Evolutionary history of two pollen self-incompatibility factors reveals alternate routes to self-compatibility within Solanum.

Self-incompatibility (SI) prevents self-fertilization and reduces inbreeding. While SI is common in plants, transitions to self-compatibility (SC) occur frequently. Little is known about the genetic changes and evolutionary steps underlying these shifts. In the Solanaceae, SI is gametophytic, with specificity determined by S-RNases in the pistil and S-locus F-box proteins (SLFs) in pollen. We examined the role of two pollen factors, Cullin1 (CUL1) and SLF-23, in SI → SC transitions in wild tomato species from the Arcanum species group (Solanum arcanum, S. neorickii, and S. chmielewskii). Pollen compatibility was assessed on tester lines that reject pollen lacking functional SLF-23 or CUL1. Complementation tests, gene sequencing, and phylogenetic analyses were used to characterize both functional and nonfunctional alleles. We found evidence for multiple independent SI → SC transitions. In S. arcanum and S. chmielewskii, SC is caused by loss of pistil S-RNase activity, while in S. neorickii SC is associated with expression of a functional SLF-23 that recognizes the S9 type S-RNase expressed in its pistils. Interestingly, we found identical deletion mutations in CUL1 exon 7 of S. chmielewskii as previously seen in S. habrochaites. Mating system transitions in the Arcanum group have occurred via both pistil loss-of-function and pollen gain-of-function SC mutations. Mutations common to S. chmielewskii and S. habrochaites must have arisen in a common ancestor, possibly to the entire tomato clade, then became fixed in different lineages after loss of pistil-side SI function.

Cullin-1 regulates MG63 cell proliferation and metastasis and is a novel prognostic marker of osteosarcoma.

There is no reliable marker available for early detection, diagnostic confirmation or disease prognosis of osteosarcoma. Cullin-1 (CUL1) is a newly reported tumor-related gene, and we aimed to unravel its role in osteosarcoma. We used immunohistochemistry to analyze the correlation between CUL1 expression and clinicopathological variables and patient survival. To evaluate the function of CUL1, a group of 28 osteosarcoma patients were recruited for this study. The role of regulation of CUL1 in osteosarcoma was studied in vitro and in vivo. In addition, we further investigated the signaling pathway of CUL1 in osteosarcoma progression. We first discovered that CUL1 expression was up-regulated in human osteosarcoma tissues and inversely correlated with osteosarcoma differentiation. In addition, CUL1 promotes osteosarcoma cell proliferation in vitro and in vivo. We also found that CUL1 promotes osteosarcoma cell invasion and metastasis in vitro and in vivo. High levels of CUL1 promote osteosarcoma progression via up-regulation of MMP9 expression. Our results demonstrate that increased CUL1 expression is significantly correlated with poor prognosis of patients with osteosarcoma. CUL1 might be an important marker and a therapeutic target for osteosarcoma.

Enhanced Stabilization of MCL1 by the Human T-Cell Leukemia Virus Type 1 bZIP Factor Is Modulated by Blocking the Recruitment of Cullin 1 to the SCF Complex.

Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus that is the etiological agent of adult T-cell leukemia (ATL). The HTLV-1 basic leucine zipper factor (HBZ), which is encoded by the minus strand of the provirus, is constitutively expressed in all ATL patient cells and likely contributes to the development and maintenance of ATL. Furthermore, the overexpression of the myeloid cell leukemia 1 (MCL1) protein is frequently observed in hematological cancers as well as several other types of cancers. Here, we found that the expression of HBZ in cells stabilized MCL1 protein expression and suppressed the MCL1-mediated release of cytochrome c from the mitochondria. This effect was mediated by inhibition of the ubiquitin-dependent degradation of MCL1. In a serial binding assay, HBZ interacted with cullin 1 (CUL1) through a head-to-tail interaction. The association between CUL1 and Skp1, which serves as the molecular scaffold for the components of SCF ubiquitin ligase complexes, was markedly repressed in the presence of HBZ. Mechanistic analysis indicated that HBZ abrogated the CUL1 association with Skp1, which in turn promoted the cellular expression of MCL1. This novel function of HBZ likely plays a role in the viral pathogenesis of HTLV-1 and provides important insights into our understanding of the development of ATL.

Mutations in two pollen self-incompatibility factors in geographically marginal populations of Solanum habrochaites impact mating system transitions and reproductive isolation.

Self-incompatibility (SI) is a mechanism that prevents inbreeding in many plant species. The mutational breakdown of SI occurs frequently, yet relatively little is known about the evolutionary steps involved in the progressive loss of pistil and pollen SI function. In Solanaceae, SI is the S-RNase-based gametophytic type. We used SI and SC populations of the wild tomato species Solanum habrochaites to study natural variation for two pollen SI factors: a Cullin1 (CUL1) protein and an S-locus F-box protein (SLF-23). Pollen compatibility was assessed on an allotriploid tester line encoding an S-RNase recognized by SLF-23. Both pollen factors are required for compatibility on this tester line. Complementation tests and gene sequencing were used to identify mutations in CUL1 or SLF-23. We detected loss-of-function mutations in CUL1 and/or SLF-23 in SC populations collected near the northern and southern geographic margins of this taxon's natural range. Nonmarginal SC and all SI accessions expressed mostly functional alleles of these pollen factors. Comparison of the CUL1 sequences identified several shared deletion mutations present in both northern and southern margin SC accessions. Loss-of-function mutations in CUL1 and SLF-23 likely became fixed relatively late during SI to SC transitions, after loss of pistil SI function. Mutations in CUL1 establish unilateral incompatibility with SI populations and strengthen reproductive isolation. Point mutations common to northern and southern SC biotypes likely derive from shared ancestral variants found in more central SI populations.

Expression of CUL1 correlates with tumour-grade and recurrence in urothelial carcinoma.

Urothelial carcinoma (UC) is a major health problem in the general population. We aimed to evaluate the function of Cullin-1 (CUL1) and unravel its underlying molecular mechanism to develop novel treatment options equivalent to UC. To evaluate the function of CUL1, a group of 132 pairs of UC patients were recruited for this study. UC tissues and their adjacent noncancerous tissues (NCTs) were collected between 2008 and 2009. We used immunohistochemistry to analyse the correlation between CUL1 expression and clinicopathologic variables and patient survival. CUL1 was dramatically overexpressed in high-grade UC tissues compared with low-grade UC tissues. CUL1 up-regulation in recurrence cases in comparison with the non-recurrence cases. CUL1 expression upregulated in human UC tissues versus NCTs. CUL1 protein expression associated with androgen receptor. Our results demonstrate that increased CUL1 expression is significantly correlated with poor prognosis of patients with UC. CUL1 might be an important marker and a therapeutic target for UC.

The Putative O-Linked N-Acetylglucosamine Transferase SPINDLY Inhibits Class I TCP Proteolysis to Promote Sensitivity to Cytokinin.

Arabidopsis (Arabidopsis thaliana) SPINDLY (SPY) is a putative serine and threonine O-linked N-acetylglucosamine transferase (OGT). While SPY has been shown to suppress gibberellin signaling and to promote cytokinin (CK) responses, its catalytic OGT activity was never demonstrated and its effect on protein fate is not known. We previously showed that SPY interacts physically and functionally with TCP14 and TCP15 to promote CK responses. Here, we aimed to identify how SPY regulates TCP14/15 activities and how these TCPs promote CK responses. We show that SPY activity is required for TCP14 stability. Mutation in the putative OGT domain of SPY (spy-3) stimulated TCP14 proteolysis by the 26S proteasome, which was reversed by mutation in CULLIN1 (CUL1), suggesting a role for SKP, CUL1, F-box E3 ubiquitin ligase in TCP14 proteolysis. TCP14 proteolysis in spy-3 suppressed all TCP14 misexpression phenotypes, including the enhanced CK responses. The increased CK activity in TCP14/15-overexpressing flowers resulted from increased sensitivity to the hormone and not from higher CK levels. TCP15 overexpression enhanced the response of the CK-induced synthetic promoter pTCS to CK, suggesting that TCP14/15 affect early steps in CK signaling. We propose that posttranslational modification of TCP14/15 by SPY inhibits their proteolysis and that the accumulated proteins promote the activity of the CK phosphorelay cascade in developing Arabidopsis leaves and flowers.

Unilateral incompatibility gene ui1.1 encodes an S-locus F-box protein expressed in pollen of Solanum species

Unilateral interspecific incompatibility (UI) is a postpollination, prezygotic reproductive barrier that prevents hybridization between related species when the female parent is self-incompatible (SI) and the male parent is self-compatible (SC). In tomato and related Solanum species, two genes, ui1.1 and ui6.1, are required for pollen compatibility on pistils of SI species or hybrids. We previously showed that ui6.1 encodes a Cullin1 (CUL1) protein. Here we report that ui1.1 encodes an S-locus F-box (SLF) protein. The ui1.1 gene was mapped to a 0.43-cM, 43.2-Mbp interval at the S-locus on chromosome 1, but positional cloning was hampered by low recombination frequency. We hypothesized that ui1.1 encodes an SLF protein(s) that interacts with CUL1 and Skp1 proteins to form an SCF-type (Skp1, Cullin1, F-box) ubiquitin E3 ligase complex. We identified 23 SLF genes in the S. pennellii genome, of which 19 were also represented in cultivated tomato (S. lycopersicum). Data from recombination events, expression analysis, and sequence annotation highlighted 11 S. pennellii genes as candidates. Genetic transformations demonstrated that one of these, SpSLF-23, is sufficient for ui1.1 function. A survey of cultivated and wild tomato species identified SLF-23 orthologs in each of the SI species, but not in the SC species S. lycopersicum, S. cheesmaniae, and S. galapagense, pollen of which lacks ui1.1 function. These results demonstrate that pollen compatibility in UI is mediated by protein degradation through the ubiquitin-proteasome pathway, a mechanism related to that which controls pollen recognition in SI.

Untethering the TIR1 auxin receptor from the SCF complex increases its stability and inhibits auxin response.

Plant genomes encode large numbers of F-box proteins (FBPs), the substrate recognition subunit of SKP1-CULLIN-F-box (SCF) ubiquitin ligases. There are ~700 FBPs in Arabidopsis, most of which are uncharacterized. TIR1 is among the best-studied plant FBPs and functions as a receptor for the plant hormone auxin. Here we use a yeast two-hybrid system to identify novel TIR1 mutants with altered properties. The analysis of these mutants reveals that TIR1 associates with the CULLIN1 (CUL1) subunit of the SCF through the N-terminal H1 helix of the F-box domain. Mutations that untether TIR1 from CUL1 stabilize the FBP and cause auxin resistance and associated growth defects, probably by protecting TIR1 substrates from degradation. Based on these results we propose that TIR1 is subject to autocatalytic degradation when assembled into an SCF. Further, our results suggest a general method for determining the physiological function of uncharacterized FBPs. Finally, we show that a key amino acid variation in the F-box domain of auxin signalling F-box (AFB1), a closely related FBP, reduces its ability to form an SCF, resulting in an increase in AFB1 levels.

Cullin1 is a novel prognostic marker and regulates the cell proliferation and metastasis in colorectal cancer.

To investigate the precise function of Cullin1 (CUL1) in colorectal cancer (CRC). Immunohistochemistry was performed to test the expression of CUL1 on a CRC tissue microarray containing the tumor and corresponding normal tissues. Simultaneously, the correlation of CUL1 expression with clinicopathological parameters and survival was evaluated. CUL1 was over-expressed or knocked down in HCT116 and SW480 cells, then the cell proliferation, migration and invasion assays in vitro and in vivo were performed. In this study, we found that CUL1 expression was significantly up-regulated in CRC compared with normal colon tissues. High CUL1 expression was positively associated with lymph node metastasis (P = 0.007) and tumor diameter (P = 0.052). Multivariate Cox regression analysis revealed that high CUL1 expression was an independent unfavorable prognostic factor for CRC patients (HR = 13.9, 95% confidence interval = 5.89-32.6, P < 0.001). Moreover, we found that CUL1 over-expression induced CRC cell proliferation and the growth of xenografts in nude mice via the changing of cell-cycle proteins. In addition, increased CUL1 expression in CRC cells significantly promoted cell migration and invasion abilities in vitro and peritoneal metastasis in vivo through inducing high expression of MMPs. Our findings imply that CUL1 may serve as promising prognostic markers in CRC patients.

Identification of SFBB-containing canonical and noncanonical SCF complexes in pollen of apple (Malus × domestica).

Gametophytic self-incompatibility (GSI) of Rosaceae, Solanaceae and Plantaginaceae is controlled by a single polymorphic S locus. The S locus contains at least two genes, S-RNase and F-box protein encoding gene SLF/SFB/SFBB that control pistil and pollen specificity, respectively. Generally, the F-box protein forms an E3 ligase complex, SCF complex with Skp1, Cullin1 (CUL1) and Rbx1, however, in Petunia inflata, SBP1 (S-RNase binding protein1) was reported to play the role of Skp1 and Rbx1, and form an SCFSLF-like complex for ubiquitination of non-self S-RNases. On the other hand, in Petunia hybrida and Petunia inflata of Solanaceae, Prunus avium and Pyrus bretschneideri of Rosaceae, SSK1 (SLF-interacting Skp1-like protein1) is considered to form the SCFSLF/SFB complex. Here, we isolated pollen-expressed apple homologs of SSK1 and CUL1, and named MdSSK1, MdCUL1A and MdCUL1B. MdSSK1 was preferentially expressed in pollen, but weakly in other organs analyzed, while, MdCUL1A and MdCUL1B were almost equally expressed in all the organs analyzed. MdSSK1 transcript abundance was significantly (>100 times) higher than that of MdSBP1. In vitro binding assays showed that MdSSK1 and MdSBP1 interacted with MdSFBB1-S 9 and MdCUL1, and MdSFBB1-S 9 interacted more strongly with MdSSK1 than with MdSBP1. The results suggest that both MdSSK1-containing SCFSFBB1 and MdSBP1-containing SCFSFBB1-like complexes function in pollen of apple, and the former plays a major role.

Mechanisms and function of substrate recruitment by F-box proteins

S phase kinase-associated protein 1 (SKP1)-cullin 1 (CUL1)-F-box protein (SCF) ubiquitin ligase complexes use a family of F-box proteins as substrate adaptors to mediate the degradation of a large number of regulatory proteins involved in diverse processes. The dysregulation of SCF complexes and their substrates contributes to multiple pathologies. In the 14 years since the identification and annotation of the F-box protein family, the continued identification and characterization of novel substrates has greatly expanded our knowledge of the regulation of substrate targeting and the roles of F-box proteins in biological processes. Here, we focus on the evolution of our understanding of substrate recruitment by F-box proteins, the dysregulation of substrate recruitment in disease and potential avenues for F-box protein-directed disease therapies.

The Novel Ubiquitin Ligase Complex, SCFFbxw4, Interacts with the COP9 Signalosome in an F-Box Dependent Manner, Is Mutated, Lost and Under-Expressed in Human Cancers

Identification of novel proteins that can potentially contribute to carcinogenesis is a requisite venture. Herein, we report the first biochemical characterization of the novel F-box and WD40 containing protein, FBXW4. We have identified interacting protein partners and demonstrated that FBXW4 is part of a ubiquitin ligase complex. Furthermore, the Fbxw4 locus is a common site of proviral insertion in a variety of retroviral insertional mutagenesis murine cancer models and Fbxw4 mRNA is highly expressed in the involuting murine mammary gland. To begin to characterize the biochemical function of Fbxw4, we used proteomic analysis to demonstrate that Fbxw4 interacts with Skp1 (SKP1), Cullin1 (CUL1), Ring-box1 (RBX1) and all components of the COP9 signalosome. All of these interactions are dependent on an intact F-box domain of Fbxw4. Furthermore, Fbxw4 is capable of interacting with ubiquitinated proteins within cells in an F-box dependent manner. Finally, we demonstrate that FBXW4 is mutated, lost and under-expressed in a variety of human cancer cell lines and clinical patient samples. Importantly, expression of FBXW4 correlates with survival of patients with non-small cell lung cancer. Taken together, we suggest that FBXW4 may be a novel tumor suppressor that regulates important cellular processes.

CUL1 promotes trophoblast cell invasion at the maternal–fetal interface

Human trophoblast progenitor cells differentiate via two distinct pathways, to become the highly invasive extravillous cytotrophoblast (CTB) cells (EVT) or fuse to form syncytiotrophoblast. Inadequate trophoblast differentiation results in poor placenta perfusion, or even complications such as pre-eclampsia (PE). Cullin1 (CUL1), a scaffold protein in cullin-based ubiquitin ligases, plays an important role in early embryonic development. However, the role of CUL1 in trophoblast differentiation during placenta development has not been examined. Here we show that CUL1 was expressed in CTB cells and EVT in the first trimester human placentas by immunohistochemistry. CUL1 siRNA significantly inhibited outgrowth of extravillous explants in vitro, as well as invasion and migration of HTR8/SVneo cells of EVT origin. This inhibition was accompanied by decreased gelatinolytic activities of matrix metalloproteinase (MMP)-9 and increased expression of tissue inhibitors of MMPs (TIMP-1 and -2). Consistently, exogenous CUL1 promoted invasion and migration of HTR8/SVneo cells. Notably, CUL1 was gradually decreased during trophoblast syncytialization and CUL1 siRNA significantly enhanced forskolin-induced fusion of choriocarcinoma BeWo cells. CUL1 protein levels in human pre-eclamptic placental villi were significantly lower as compared to their matched control placentas. Taken together, our results suggest that CUL1 promotes human trophoblast cell invasion and dysregulation of CUL1 expression may be associated with PE.