Cand1-Mediated Adaptive Exchange Mechanism Enables Variation in F-Box Protein Expression.

Skp1⋅Cul1⋅F-box (SCF) ubiquitin ligase assembly is regulated by the interplay of substrate binding, reversible Nedd8 conjugation on Cul1, and the F-box protein (FBP) exchange factors Cand1 and Cand2. Detailed investigations into SCF assembly and function in reconstituted systems and Cand1/2 knockout cells informed the development of a mathematical model for how dynamical assembly of SCF complexes is controlled and how this cycle is coupled to degradation of an SCF substrate. Simulations predicted an unanticipated hypersensitivity of Cand1/2-deficient cells to FBP expression levels, which was experimentally validated. Together, these and prior observations lead us to propose the adaptive exchange hypothesis, which posits that regulation of the koff of an FBP from SCF by the actions of substrate, Nedd8, and Cand1 molds the cellular repertoire of SCF complexes and that the plasticity afforded by this exchange mechanism may enable large variations in FBP expression during development and in FBP gene number during evolution.

Regulation of Poly(A) Tail and Translation during the Somatic Cell Cycle.

Poly(A) tails are critical for mRNA stability and translation. However, recent studies have challenged this view, showing that poly(A) tail length and translation efficiency are decoupled in non-embryonic cells. Using TAIL-seq and ribosome profiling, we investigate poly(A) tail dynamics and translational control in the somatic cell cycle. We find dramatic changes in poly(A) tail lengths of cell-cycle regulatory genes like CDK1, TOP2A, and FBXO5, explaining their translational repression in M phase. We also find that poly(A) tail length is coupled to translation when the poly(A) tail is <20 nucleotides. However, as most genes have >20 nucleotide poly(A) tails, their translation is regulated mainly via poly(A) tail length-independent mechanisms during the cell cycle. Specifically, we find that terminal oligopyrimidine (TOP) tract-containing transcripts escape global translational suppression in M phase and are actively translated. Our quantitative and comprehensive data provide a revised view of translational control in the somatic cell cycle.

FBX4 mediates rapid cyclin D1 proteolysis upon DNA damage in immortalized esophageal epithelial cells.

It has been implied that deregulation of cyclin D1 turnover under stresses can facilitate genomic instability and trigger tumorigenesis. Much focus has been placed on identifying the E3 ligases responsible for mediating cyclin D1 degradation. However, the findings were quite controversial and cell type-dependent. Little is known about how cyclin D1 is regulated in precancerous cells upon DNA damage and which E3 ligases mediate the effects. Here we found cyclin D1 reduction is an early response to DNA damage in immortalized esophageal epithelial cells, with expression dropping to a low level within 1 h after γ-irradiation. Comparison of temporal expression of cyclin D1 upon DNA damage between immortalized NE083-hTERT and NE083-E6E7, the latter being p53/p21-defective, showed that DNA damage-induced rapid cyclin D1 reduction was p53-independent and occurred before p21 accumulation. Overexpression of cyclin D1 in NE083-E6E7 cells could attenuate G0/G1 cell cycle arrest at 1 h after irradiation. Furthermore, rapid reduction of cyclin D1 upon DNA damage was attributed to proteasomal degradation, as evidenced by data showing that proteasomal inhibition by MG132 blocked cyclin D1 reduction while cycloheximide facilitated it. Inhibition of ATM activation and knockdown of E3 ligase adaptor FBX4 reversed cyclin D1 turnover in immortalized NE083-hTERT cells. Further study showed that knockdown of FBX4 facilitated DNA breaks, as indicated by an increase in γ-H2AX foci in esophageal cancer cells. Taken together, the results substantiated a pivotal role of ATM and FBX4 in cyclin D1 proteolysis upon DNA damage in precancerous esophageal epithelial cells, implying that deregulation of the process may contribute to carcinogenesis of esophageal squamous cell carcinoma.

7-Ethoxyrosmanol alleviates hyperglycemia-induced vascular endothelial dysfunction by regulating FBXL7 expression.

The current therapeutic strategy for hyperglycemia is reasonable diet and appropriate exercise with drugs, whose outcome is unsatisfied. Therefore, we aimed to explore the new candidate drug 7-Ethoxyrosmanol (7ERM) on hyperglycemia-induced endothelial dysfunction. Human umbilical vein endothelial cells (HUVECs) were treated with different doses of 7ERM in the presence of 33 mM high glucose to measure cell injury, inflammation, and reactive oxygen species (ROS) production. Then F-box/LRR-repeat protein 7 (FBXL7) knockdown by siRNA or overexpressed by plasmid in HUVECs was assessed its effect in the protective role on hyperglycemia-induced endothelial dysfunction. 7ERM time-dependently increased high glucose-induced cell injury, the secretions of pro-inflammatory cytokines and ROS production in HUVECs. Moreover, high glucose time-dependently increased the FBXL7 expressions, which could be gradually inhibited by 7ERM. FBXL7 knockdown ameliorated high glucose-induced cell injury. On the contrary, FBXL7 over-expression inhibited the protective effect of 7ERM on cell injury. In conclusion, 7ERM effectively attenuates high glucose-induced endothelial dysfunction in HUVECs by regulating FBXL7 expression, indicating its potential as a therapeutic drug to treat hyperglycemia.

Overexpression of F-box only protein 31 predicts poor prognosis and deregulates p38α- and JNK-mediated apoptosis in esophageal squamous cell carcinoma.

F-box only protein 31 (FBXO31), a subunit of the Skp1-Cul1-F box ubiquitin ligase, plays a crucial role in DNA damage response and tumorigenesis. Yet its expression and function vary in different types of human cancer. The expression of FBXO31 in esophageal squamous cell carcinoma (ESCC) and its association with clinicopathological features is not well studied. The underlying mechanism by which deregulated FBXO31 contributes to ESCC tumorigenesis is largely unknown. By immunohistochemical analysis of a tissue microarray containing 85 cases of ESCC and matched adjacent noncancerous tissue and an additional 10 cases of ESCC tissue samples, we found that FBXO31 was overexpressed in ESCC, and that its expression was significantly correlated with histological grade (p = 0.04) and clinical stage (p = 0.022). Higher expression of FBXO31 was associated with poor prognosis in univariate (p = 0.013) and multivariate (p = 0.014) analyses. We found that FBXO31 functioned as an antiapoptotic molecule in ESCC cells exposed to different types of genotoxic stress. Knockdown of FBXO31 inhibited serum-starved cell viability and decreased tumorigenicity of ESCC cells. In addition, the antiapoptotic effects of FBXO31 were associated with deactivation of stress-induced MAPK p38α and JNK. Furthermore, in vitro and in vivo data showed that silencing of FBXO31-sensitized ESCC cells and tumors to cisplatin treatment. Taken together, in addition to revealing that FBXO31 is an independent prognostic marker for ESCC, our findings substantiate a novel regulatory role of FBXO31 in tumorigenesis and drug resistance of ESCC.

FBXO17 promotes cell proliferation through activation of Akt in lung adenocarcinoma cells.

BACKGROUND: The ubiquitin-proteasome pathway, mediated in part, by ubiquitin E3 ligases, is critical in regulating cellular processes such as cell proliferation, apoptosis, and migration. FBXO17 was recently identified as an F-box protein that targets glycogen synthase kinase-3ß to the E3 ubiquitin ligase protein complex for polyubiquitination and proteasomal degradation. Here, we identified that in several lung adenocarcinoma cell lines, FBXO17 cellular protein was detected at relatively high levels, as was expression in a subset of lung cancers. Hence, we investigated the effects of FBXO17 on cell proliferation. METHODS: Single cell RNA sequencing analysis was performed on a resection of a non-small cell lung carcinoma tumor to examine FBXO17 expression. Multiple lung cancer cell lines were immunoblotted, and The Cancer Genome Atlas was analyzed to determine if FBXO17 expression was amplified in a subset of lung cancers. A549 cells were transfected with empty vector or FBXO17-V5 plasmid and immunoblotted for Akt pathway mediators including PDK1, ERK1/2, ribosomal protein S6, and CREB. Cell proliferation and viability were analyzed by trypan blue exclusion, BrdU incorporation and an MTS-based fluorometric assay. Studies were also performed after transfecting with sifbxo17. Samples were used in an RNA microarray analysis to evaluate pathways affected by reduced FBXO17 gene expression. RESULTS: We observed that overexpression of FBXO17 increased A549 cell proliferation coupled with Akt activation. Ectopically expressed FBXO17 also increased ERK1/2 kinase activation and increased phosphorylation of RPS6, a downstream target of mTOR. We also observed an increased number of cells in S-phase and increased metabolic activity of lung epithelial cells expressing FBXO17. FBXO17 knockdown reduced Akt Ser 473 phosphorylation approaching statistical significance with no effect on Thr 308. However, ERK1/2 phosphorylation, cellular metabolic activity, and overall cell numbers were reduced. When we analyzed RNA profiles of A549 cells with reduced FBXO17 expression, we observed downregulation of several genes associated with cell proliferation and metabolism. CONCLUSIONS: These data support a role for FBXO17 abundance, when left unchecked, in regulating cell proliferation and survival through modulation of Akt and ERK kinase activation. The data raise a potential role for the F-box subunit in modulating tumorigenesis.

Ndd1 turnover by SCF(Grr1) is inhibited by the DNA damage checkpoint in Saccharomyces cerevisiae.

In Saccharomyces cerevisiae, Ndd1 is the dedicated transcriptional activator of the mitotic gene cluster, which includes thirty-three genes that encode key mitotic regulators, making Ndd1 a hub for the control of mitosis. Previous work has shown that multiple kinases, including cyclin-dependent kinase (Cdk1), phosphorylate Ndd1 to regulate its activity during the cell cycle. Previously, we showed that Ndd1 was inhibited by phosphorylation in response to DNA damage. Here, we show that Ndd1 is also subject to regulation by protein turnover during the mitotic cell cycle: Ndd1 is unstable during an unperturbed cell cycle, but is strongly stabilized in response to DNA damage. We find that Ndd1 turnover in metaphase requires Cdk1 activity and the ubiquitin ligase SCF(Grr1). In response to DNA damage, Ndd1 stabilization requires the checkpoint kinases Mec1/Tel1 and Swe1, the S. cerevisiae homolog of the Wee1 kinase. In both humans and yeast, the checkpoint promotes Wee1-dependent inhibitory phosphorylation of Cdk1 following exposure to DNA damage. While this is critical for checkpoint-induced arrest in most organisms, this is not true in budding yeast, where the function of damage-induced inhibitory phosphorylation is less well understood. We propose that the DNA damage checkpoint stabilizes Ndd1 by inhibiting Cdk1, which we show is required for targeting Ndd1 for destruction.

FBXW7 expression affects the response to chemoradiotherapy and overall survival among patients with oral squamous cell carcinoma: A single-center retrospective study.

FBXW7 (F-box and WD repeat domain containing-7) is a tumor suppressor protein that regulates the degradation of various oncoproteins in several malignancies. However, limited information is available regarding FBXW7 expression in oral squamous cell carcinoma. Therefore, this study aimed to determine the clinical significance of FBXW7 expression in oral squamous cell carcinoma. The FBXW7 expression patterns in oral squamous cell carcinoma and adjacent normal tissues from 15 patients who underwent radical resection were evaluated using quantitative real-time polymerase chain reaction and immunohistochemical staining. In addition, immunohistochemistry was performed using paraffin-embedded sections from biopsy specimens obtained from 110 patients with oral squamous cell carcinoma who underwent surgery after 5 fluorouracil-based chemoradiotherapy. The associations of FBXW7 expression with various clinicopathological features and prognosis were evaluated in these patients. As a results, in the 15 matched samples, the FBXW7 expression was significantly decreased in the oral squamous cell carcinoma tissues compared to that in the adjacent normal tissues. In the clinicopathological analysis, compared to high protein expression, low FBXW7 expression was found to significantly associate with a poor histological response to preoperative chemoradiotherapy. Kaplan-Meier curve analysis revealed that low FBXW7 expression was significantly associated with a poor prognosis, and FBXW7 expression was found to be an independent predictor of overall survival in the multivariate analysis. Our results suggest that FBXW7 may function as a tumor suppressor protein in oral squamous cell carcinoma. In addition, FBXW7 could be a potential biomarker for predicting not only the clinical response to chemoradiotherapy but also overall survival in patients with oral squamous cell carcinoma.

Altered expression of CG5961, a putative Drosophila melanogaster homologue of FBXO9, provides a new model of Parkinson disease.

F-box proteins act as the protein recognition component of the Skp-Cul-F-box class of ubiquitin ligases. Two members of a gene sub-family encoding these proteins, FBXO7 and FBXO32, have been implicated in the onset and progression of degenerative disease. FBXO7 is responsible for rare genetic forms of Parkinson disease, while FBXO32 has been implicated in muscle wasting. The third gene in this family, FBXO9, is related to growth signaling, but the role of this gene in degenerative disease pathways has not been thoroughly investigated. Characterizing the putative Drosophila melanogaster homologue of this gene, CG5961, enables modeling and analysis of the consequence of targeted alteration of gene function and the effects on the overall health of the organism. Comparison of the protein domains of Homo sapiens FBXO9 and the putative D. melanogaster homologue CG5961 revealed a high degree of conservation between the protein domains. Directed expression of CG5961 (via CG5961(EP)) and inhibition of CG5961 (through a stable RNAi transgene) in the developing D. melanogaster eye caused abnormalities in adult structures (ommatidia and inter-ommatidial bristles). Directed expression of either CG5961 or CG5961-RNAi in the dopaminergic neurons led to a reduced lifespan compared to that in lacZ controls. We showed that protein structures of CG5961 and FBXO9 are highly similar and studied the effects of altered expression of CG5961 in neuron-rich tissues. Our results suggest that CG5961 activity is necessary for the proper formation of neuronal tissue and that targeted alteration of gene expression in dopaminergic neurons leads to a reduced lifespan.

Genome-wide survey and expression analysis of F-box genes in chickpea.

BACKGROUND: The F-box genes constitute one of the largest gene families in plants involved in degradation of cellular proteins. F-box proteins can recognize a wide array of substrates and regulate many important biological processes such as embryogenesis, floral development, plant growth and development, biotic and abiotic stress, hormonal responses and senescence, among others. However, little is known about the F-box genes in the important legume crop, chickpea. The available draft genome sequence of chickpea allowed us to conduct a genome-wide survey of the F-box gene family in chickpea. RESULTS: A total of 285 F-box genes were identified in chickpea which were classified based on their C-terminal domain structures into 10 subfamilies. Thirteen putative novel motifs were also identified in F-box proteins with no known functional domain at their C-termini. The F-box genes were physically mapped on the 8 chickpea chromosomes and duplication events were investigated which revealed that the F-box gene family expanded largely due to tandem duplications. Phylogenetic analysis classified the chickpea F-box genes into 9 clusters. Also, maximum syntenic relationship was observed with soybean followed by Medicago truncatula, Lotus japonicus and Arabidopsis. Digital expression analysis of F-box genes in various chickpea tissues as well as under abiotic stress conditions utilizing the available chickpea transcriptome data revealed differential expression patterns with several F-box genes specifically expressing in each tissue, few of which were validated by using quantitative real-time PCR. CONCLUSIONS: The genome-wide analysis of chickpea F-box genes provides new opportunities for characterization of candidate F-box genes and elucidation of their function in growth, development and stress responses for utilization in chickpea improvement.

FBW7 increases drug sensitivity to cisplatin in human nasopharyngeal carcinoma by downregulating the expression of multidrug resistance-associated protein.

F-box/WD repeat-containing protein 7 (FBW7) is a member of the F-box protein family that regulates cell cycle progression and cell growth and differentiation. FBW7 also functions as a tumor suppressor. A cisplatin (CDDP)-based multidrug chemotherapy regimen is standard for nasopharyngeal carcinoma (NPC), but drug resistance is an increasing problem. Here, we evaluated the relationship between FBW7 and multidrug resistance-associated protein (MRP), and its correlation with drug resistance in NPC, and explored the mechanism underlying drug resistance to CDDP in this disease. We used cell viability assays, Western blotting, and small interfering RNA (siRNA) interference to investigate the underlying mechanism underlying CDDP resistance in a NPC cell line. The expression of FBW7 and MRP was detected by Western blotting after siRNA interference in the CDDP-resistant NPC cell line, CNE2-CDDP. The 3-(4 5-dimethyl-2-thiazolyl)-2 5-diphenyl-2-H-tetrazolium bromide (MTT) assay was used to evaluate drug sensitivity of various types of antitumor drugs, including paclitaxel (PCX), CDDP, fluorouracil (5-FU), and vincristine (VCR). We found that siRNA-mediated upregulation of FBW7 significantly increased CDDP chemosensitivity. The IC50 values of CDDP in siRNA-FBW7-CNE2-CDDP and FBW7-CNE2-CDDP-NC cells were 2.485 ± 0.155 and 4.867 ± 0.442 µmol/mL, respectively. The IC50 values of PCX, CDDP, 5-FU, and VCR were significantly decreased in siRNA-FBW7-CNE2 than in FBW7-CNE2-NC (3.46 ± 0.14 vs. 46.21 ± 6.03 µmol/mL; 3.76 ± 0.54 vs. 39.45 ± 0.96 µmol/mL; 2.14 ± 1.67 vs. 28.76 ± 1.89 µmol/mL; 4.43 ± 0.89 vs. 87.90 ± 3.45 µmol/mL, respectively). The IC50 of CDDP was significantly less in siRNA-FBW7-CNE2-CDDP than in FBW7-CNE2-CDDP-NC. The level of FBW7 expression in CNE2 cells was correlated with CDDP chemosensitivity. siRNA-mediated upregulation of FBW7 expression downregulated the expression of MRP, significantly increasing drug sensitivity in CNE2 cells.

Sequential expression of miR-182 and miR-503 cooperatively targets FBXW7, contributing to the malignant transformation of colon adenoma to adenocarcinoma.

Genetic changes in colon cancer are known to parallel the tissue abnormalities associated with the disease, namely adenoma and adenocarcinoma. The role of microRNA dysregulation in dysplastic progression, however, is not well understood. Here, we show that miR-182 and miR-503 undergo sequential up-regulation and drive the progression of colon adenoma to adenocarcinoma by cooperatively down-regulating the tumour suppressor FBXW7. We identified that increased expression of miR-182 is a feature of adenomas. A subsequent increase in miR-503 expression works cooperatively with miR-182 to induce transformation of an adenoma to adenocarcinoma. We show that introducing miR-503 into AAC1 cells, which are derived from a benign adenoma, confers tumourigenic potential. We also demonstrated that blocking both miR-182 and miR-503 in HCT116 colon cancer cells resulted in increased FBXW7 expression and significantly reduced tumour size in xenograft models. We confirmed relevance of these results in patients by examining the expression levels of miR-182 and miR-503 in over 200 colon cancer patients with 12 year survival outcome data. Decreased patient survival was correlated with elevated expression of both miRNAs, suggesting that elevated levels of both miR-182 and miR-503 define a novel prognostic biomarker for colon cancer patients. In conclusion, we show that a sequential expression of miR-182 and miR-503 in benign adenoma cooperatively regulates the tumour suppressor FBXW7, contributing to the malignant transformation of colon adenoma to adenocarcinoma and miR-182 and miR-503 may prove to be novel therapeutic targets. Array data are available at: http://www.oncomir.umn.edu/

The tumour suppressor C/EBPδ inhibits FBXW7 expression and promotes mammary tumour metastasis.

Inflammation and hypoxia are known to promote the metastatic progression of tumours. The CCAAT/enhancer-binding protein-δ (C/EBPδ, CEBPD) is an inflammatory response gene and candidate tumour suppressor, but its physiological role in tumourigenesis in vivo is unknown. Here, we demonstrate a tumour suppressor function of C/EBPδ using transgenic mice overexpressing the Neu/Her2/ERBB2 proto-oncogene in the mammary gland. Unexpectedly, this study also revealed that C/EBPδ is necessary for efficient tumour metastasis. We show that C/EBPδ is induced by hypoxia in tumours in vivo and in breast tumour cells in vitro, and that C/EBPδ-deficient cells exhibit reduced glycolytic metabolism and cell viability under hypoxia. C/EBPδ supports CXCR4 expression. On the other hand, C/EBPδ directly inhibits expression of the tumour suppressor F-box and WD repeat-domain containing 7 gene (FBXW7, FBW7, AGO, Cdc4), encoding an F-box protein that promotes degradation of the mammalian target of rapamycin (mTOR). Consequently, C/EBPδ enhances mTOR/AKT/S6K1 signalling and augments translation and activity of hypoxia-inducible factor-1α (HIF-1α), which is necessary for hypoxia adaptation. This work provides new insight into the mechanisms by which metastasis-promoting signals are induced specifically under hypoxia.

Clear cell carcinomas of the ovary: a multi-institutional study of 129 cases in Korea with prognostic significance of Emi1 and Galectin-3.

Accurate diagnosis of ovarian clear cell carcinoma (CCC) is important because of its poor prognosis with chemoresistance and a high recurrent rate. The clinicopathologic characteristics and prognostic significance of the cell cycle regulator [early mitotic inhibitor-1 (Emi1)] and galactoside-binding protein (Galectin-3) were evaluated. Among 155 CCCs from 18 hospitals in Korea between 1995 and 2006, 129 pure CCCs were selected with consensus using immunohistochemical stains for hepatocyte nuclear factor-1ß, Wilms' tumor protein, and estrogen receptor. The expressions of Emi1, Galectin-3, p53, and Ki-67 labeling index were analyzed with clinicopathologic parameters and the patient's survival. The mean age of the patients was 49.6 yr; the tumors were bilateral in 10.9%, and the average size was 12 cm. Adenofibromatous component was found in 7%, and endometriosis in 48.1% of the cases. Psammoma body was seen in 16.3%. Disease-free survival and overall survival rates were 78.3% and 79.1%, respectively. The International Federation of Obstetrics and Gynecology (FIGO) stage was the most important prognostic indicator. Emi1 expression (>5%) was seen in 23.3% of CCCs, and associated with high FIGO grades and poor overall survival (P<0.05). High Galectin-3 (≥80%) expression was seen in 59.7% of CCCs, and associated with FIGO stages III and IV, and high Ki-67 labeling index. High Ki-67 labeling index (≥50%) and p53 expression (≥50%) were seen in 27.1% and 18.6% of CCCs, respectively, but there was no clinicopathologic and prognostic significance. On the basis of the fact that the expression of Emi1 in CCC was correlated with a high histologic grade and worse overall survival, target therapy using inhibitors of Emi1 may be tried in the management of CCC patients with Emi1 expression.

MicroRNA-223 regulates cyclin E activity by modulating expression of F-box and WD-40 domain protein 7.

F-box and WD-40 domain protein 7 (Fbw7) provides substrate specificity for the Skp1-Cullin1-F-box protein (SCF) ubiquitin ligase complex that targets multiple oncoproteins for degradation, including cyclin E, c-Myc, c-Jun, Notch, and mammalian target of rapamycin (mTOR). Fbw7 is a bona fide tumor suppressor, and loss-of-function mutations in FBXW7 have been identified in diverse human tumors. Although much is known about targets of the Fbw7 ubiquitin ligase pathway, relatively little is known about the regulation of Fbw7 expression. We identified a panel of candidate microRNA regulators of Fbw7 expression within a study of gene expression alterations in primary erythroblasts obtained from cyclin E(T74A T393A) knock-in mice, which have markedly dysregulated cyclin E expression. We found that overexpression of miR-223, in particular, significantly reduces FBXW7 mRNA levels, increases endogenous cyclin E protein and activity levels, and increases genomic instability. We next confirmed that miR-223 targets the FBXW7 3'-untranslated region. We then found that reduced miR-223 expression in primary mouse embryonic fibroblasts leads to increased Fbw7 expression and decreased cyclin E activity. Finally, we found that miR-223 expression is responsive to acute alterations in cyclin E regulation by the Fbw7 pathway. Together, our data indicate that miR-223 regulates Fbw7 expression and provide the first evidence that activity of the SCF(Fbw7) ubiquitin ligase can be modulated directly by the microRNA pathway.

Reduced expression of ubiquitin ligase FBXW7 mRNA is associated with poor prognosis in breast cancer patients.

FBXW7 is a cell cycle regulatory gene that ubiquitinates positive cell cycle regulators such as c-Myc and cyclin E, allowing for cell cycle exit. Defects in the FBXW7 gene that lead to cell cycle re-entry and expedite the G1-S transition is thought to be one of the causes of cancer development. However, its clinical importance for breast cancer patients remains undetermined. This prompted us to investigate its expression level in breast cancer patients to establish its clinical significance. The expression level of FBXW7 mRNA was assessed in 186 cases of primary invasive breast cancer. Correlations between FBXW7 mRNA expression and clinicopathological factors, prognoses and immunohistochemical expression levels of Ki-67, FBXW7, c-Myc and cyclin E were analyzed. In vitro investigation of FBXW7 gene silencing in a breast cancer cell line was conducted. FBXW7 mRNA was expressed at significantly lower levels in patients with high histological grade and hormone receptor-negative tumors. Patients with lower FBXW7 mRNA expression had a poorer prognosis for breast cancer-specific survival than those with higher expression. A high Ki-67 labeling index and positive cyclin E protein expression were significantly correlated with lower FBXW7 mRNA expression. In vitro, silencing FBXW7 enhanced expression of c-Myc and cyclin E proteins and upregulated both cell proliferation and G1-S transition. In breast cancer, reduced FBXW7 mRNA expression may have independent prognostic potential through the enhanced function of cell cycle regulatory proteins.

Loss of FBXW7, a cell cycle regulating gene, in colorectal cancer: clinical significance.

This study focused on a cell cycle regulatory gene, FBXW7, which ubiquitinates c-Myc and cyclin E and promotes exit from the cell cycle. We determined the expression level of FBXW7 in colorectal cancer (CRC) cases, correlated those values with clinicopathologic features, and characterized the molecular mechanism of reduced expression of FBXW7 in CRC cells in vitro. FBXW7 mRNA and protein expression were evaluated in 93 CRC cases. Using CGH array, the copy number aberrations of the flanking region of FBXW7 were evaluated in another 130 CRC specimens. In vitro analysis of FBXW7 gene silencing in CRC cells was conducted. FBXW7 mRNA expression was significantly lower in tumor tissues than the corresponding normal tissues. The low FBXW7 expression group showed a significantly poorer prognosis than patients in the high expression group. A concordant relationship was observed between the incidence of FBXW7 repression and the genetic alteration. The incidence of genetic alteration was associated with the stage of disease progression. In vitro, FBXW7-specific siRNA enhanced expression of c-MYC and cyclin E proteins and up-regulated cell proliferation. Genetic alterations in tumors led to the loss of FBXW7 expression and increased cell proliferation. FBXW7 expression provides a prognostic factor for patients with CRC.

Somatic mutations of the CDC4 (FBXW7) gene in hereditary colorectal tumors.

OBJECTIVES: Cdc4 (Fbxw7) is a potential tumor suppressor that regulates ubiquitination and proteolysis of multiple targets such as cyclin E, c-Myc, c-Jun and Notch. CDC4 mutations were investigated in 194 colorectal carcinomas and adenomas for comparison between sporadic and hereditary cancers. METHODS: Mutations of the CDC4 gene were analyzed by PCR-SSCP and sequencing, and loss of heterozygosity (LOH) was analyzed by microsatellite marker analysis. RESULTS: Somatic CDC4 mutations were detected in 9% (3 of 33) of hereditary nonpolyposis colorectal cancer (HNPCC), 9% (3 of 33) of familial adenomatous polyposis (FAP) carcinomas, and 10% (7 of 73) of sporadic carcinomas. CDC4 mutations were also detected in adenomas at frequencies of 6% (2 of 31) and 4% (1 of 24) in FAP and sporadic cases, respectively. Frameshift mutations were observed in HNPCC tumors, while single-base substitutions predominantly occurred in FAP and sporadic tumors. LOH at the chromosome 4q region was rarely detected in tumors with CDC4 mutations. CONCLUSIONS: The results indicate that the frequency of CDC4 mutations in colorectal tumors is similar in patients with HNPCC and FAP compared to patients with sporadic carcinomas. Moreover, infrequent LOH suggests that the CDC4 gene does not follow the general 2-hit model.

FBXO7 sensitivity of phenotypic traits elucidated by a hypomorphic allele.

FBXO7 encodes an F box containing protein that interacts with multiple partners to facilitate numerous cellular processes and has a canonical role as part of an SCF E3 ubiquitin ligase complex. Mutation of FBXO7 is responsible for an early onset Parkinsonian pyramidal syndrome and genome-wide association studies have linked variants in FBXO7 to erythroid traits. A putative orthologue in Drosophila, nutcracker, has been shown to regulate the proteasome, and deficiency of nutcracker results in male infertility. Therefore, we reasoned that modulating Fbxo7 levels in a murine model could provide insights into the role of this protein in mammals. We used a targeted gene trap model which retained 4-16% residual gene expression and assessed the sensitivity of phenotypic traits to gene dosage. Fbxo7 hypomorphs showed regenerative anaemia associated with a shorter erythrocyte half-life, and male mice were infertile. Alterations to T cell phenotypes were also observed, which intriguingly were both T cell intrinsic and extrinsic. Hypomorphic mice were also sensitive to infection with Salmonella, succumbing to a normally sublethal challenge. Despite these phenotypes, Fbxo7 hypomorphs were produced at a normal Mendelian ratio with a normal lifespan and no evidence of neurological symptoms. These data suggest that erythrocyte survival, T cell development and spermatogenesis are particularly sensitive to Fbxo7 gene dosage.