Fbw7 and p53 cooperatively suppress advanced and chromosomally unstable intestinal cancer. Mol Cell Biol. 2012;32:2160-2167. [PMID: 22473991 DOI: 10.1128/mcb.00305-12]

MYCN in human development and diseases

Somatic mutations in MYCN have been identified across various tumors, playing pivotal roles in tumorigenesis, tumor progression, and unfavorable prognoses. Despite its established notoriety as an oncogenic driver, there is a growing interest in exploring the involvement of MYCN in human development. While MYCN variants have traditionally been associated with Feingold syndrome type 1, recent discoveries highlight gain-of-function variants, specifically p.(Thr58Met) and p.(Pro60Leu), as the cause for megalencephaly-polydactyly syndrome. The elucidation of cellular and murine analytical data from both loss-of-function (Feingold syndrome model) and gain-of-function models (megalencephaly-polydactyly syndrome model) is significantly contributing to a comprehensive understanding of the physiological role of MYCN in human development and pathogenesis. This review discusses the MYCN's functional implications for human development by reviewing the clinical characteristics of these distinct syndromes, Feingold syndrome, and megalencephaly-polydactyly syndrome, providing valuable insights into the understanding of pathophysiological backgrounds of other syndromes associated with the MYCN pathway and the overall comprehension of MYCN's role in human development.

RNF126, 168 and CUL1: The Potential Utilization of Multi-Functional E3 Ubiquitin Ligases in Genome Maintenance for Cancer Therapy

Ubiquitination is a post-translational modification (PTM) that is involved in proteolysis, protein-protein interaction, and signal transduction. Accumulation of mutations and genomic instability are characteristic of cancer cells, and dysfunction of the ubiquitin pathway can contribute to abnormal cell physiology. Because mutations can be critical for cells, DNA damage repair, cell cycle regulation, and apoptosis are pathways that are in close communication to maintain genomic integrity. Uncontrolled cell proliferation due to abnormal processes is a hallmark of cancer, and mutations, changes in expression levels, and other alterations of ubiquitination factors are often involved. Here, three E3 ubiquitin ligases will be reviewed in detail. RNF126, RNF168 and CUL1 are involved in DNA damage response (DDR), DNA double-strand break (DSB) repair, cell cycle regulation, and ultimately, cancer cell proliferation control. Their involvement in multiple cellular pathways makes them an attractive candidate for cancer-targeting therapy. Functional studies of these E3 ligases have increased over the years, and their significance in cancer is well reported. There are continuous efforts to develop drugs targeting the ubiquitin pathway for anticancer therapy, which opens up the possibility for these E3 ligases to be evaluated for their potential as a target protein for anticancer therapy.

Experimental Murine Models for Colorectal Cancer Research

Colorectal cancer (CRC) is the third most prevalent malignancy worldwide and in both sexes. Numerous animal models for CRC have been established to study its biology, namely carcinogen-induced models (CIMs) and genetically engineered mouse models (GEMMs). CIMs are valuable for assessing colitis-related carcinogenesis and studying chemoprevention. On the other hand, CRC GEMMs have proven to be useful for evaluating the tumor microenvironment and systemic immune responses, which have contributed to the discovery of novel therapeutic approaches. Although metastatic disease can be induced by orthotopic injection of CRC cell lines, the resulting models are not representative of the full genetic diversity of the disease due to the limited number of cell lines suitable for this purpose. On the other hand, patient-derived xenografts (PDX) are the most reliable for preclinical drug development due to their ability to retain pathological and molecular characteristics. In this review, the authors discuss the various murine CRC models with a focus on their clinical relevance, benefits, and drawbacks. From all models discussed, murine CRC models will continue to be an important tool in advancing our understanding and treatment of this disease, but additional research is required to find a model that can correctly reflect the pathophysiology of CRC.

Mutational Status of SMAD4 and FBXW7 Affects Clinical Outcome in TP53-Mutated Metastatic Colorectal Cancer

Next-generation sequencing (NGS) provides a molecular rationale to inform prognostic stratification and to guide personalized treatment in cancer patients. Here, we determined the prognostic and predictive value of actionable mutated genes in metastatic colorectal cancer (mCRC). Among a total of 294 mCRC tumors examined by targeted NGS, 200 of them derived from patients treated with first-line chemotherapy plus/minus monoclonal antibodies were included in prognostic analyses. Discriminative performance was assessed by time-dependent estimates of the area under the curve (AUC). The most recurrently mutated genes were TP53 (64%), KRAS or NRAS (49%), PIK3CA (15%), SMAD4 (14%), BRAF (13%), and FBXW7 (9.5%). Mutations in FBXW7 correlated with worse OS rates (p = 0.036; HR, 2.24) independently of clinical factors. Concurrent mutations in TP53 and FBXW7 were associated with increased risk of death (p = 0.02; HR, 3.31) as well as double-mutated TP53 and SMAD4 (p = 0.03; HR, 2.91). Analysis of the MSK-IMPACT mCRC cohort (N = 1095 patients) confirmed the same prognostic trend for the previously identified mutated genes. Addition of the mutational status of these genes upon clinical factors resulted in a time-dependent AUC of 87%. Gene set enrichment analysis revealed specific molecular pathways associated with SMAD4 and FBXW7 mutations in TP53-defficient tumors. Conclusively, SMAD4 and FBXW7 mutations in TP53-altered tumors were predictive of a negative prognostic outcome in mCRC patients treated with first-line regimens.

FBXW7 and the Hallmarks of Cancer: Underlying Mechanisms and Prospective Strategies

FBXW7, a member of the F-box protein family within the ubiquitin–proteasome system, performs an indispensable role in orchestrating cellular processes through ubiquitination and degradation of its substrates, such as c-MYC, mTOR, MCL-1, Notch, and cyclin E. Mainly functioning as a tumor suppressor, inactivation of FBXW7 induces the aberrations of its downstream pathway, resulting in the occurrence of diseases especially tumorigenesis. Here, we decipher the relationship between FBXW7 and the hallmarks of cancer and discuss the underlying mechanisms. Considering the interplay of cancer hallmarks, we propose several prospective strategies for circumventing the deficits of therapeutic resistance and complete cure of cancer patients.

Quantifying chromosomal instability from intratumoral karyotype diversity using agent-based modeling and Bayesian inference

Chromosomal instability (CIN)-persistent chromosome gain or loss through abnormal mitotic segregation-is a hallmark of cancer that drives aneuploidy. Intrinsic chromosome mis-segregation rate, a measure of CIN, can inform prognosis and is a promising biomarker for response to anti-microtubule agents. However, existing methodologies to measure this rate are labor intensive, indirect, and confounded by selection against aneuploid cells, which reduces observable diversity. We developed a framework to measure CIN, accounting for karyotype selection, using simulations with various levels of CIN and models of selection. To identify the model parameters that best fit karyotype data from single-cell sequencing, we used approximate Bayesian computation to infer mis-segregation rates and karyotype selection. Experimental validation confirmed the extensive chromosome mis-segregation rates caused by the chemotherapy paclitaxel (18.5 ± 0.5/division). Extending this approach to clinical samples revealed that inferred rates fell within direct observations of cancer cell lines. This work provides the necessary framework to quantify CIN in human tumors and develop it as a predictive biomarker.

The Potential Role of Genomic Signature in Stage II Relapsed Colorectal Cancer (CRC) Patients: A Mono-Institutional Study

Purpose

The absolute benefit of adjuvant chemotherapy in stage II CRC is only 3–4%. The identification of biomarkers through molecular profiling could identify patients who will more benefit from adjuvant chemotherapy.

Patients and Methods

This retrospective analysis examined tissue blocks from 17 patients affected by relapsed stage II CRC, whose comprehensive genomic profiling of tumors was conducted through next-generation sequencing (NGS) via Roche-FoundationOne^®.

Results

Mutations were found in APC (76.5%), TP53 (58.8%) and KRAS (52.9%). Only KRAS wild-type samples showed FBXW7. APC frameshift mutations and MLH1 splice variant were conversely significant correlated (7% v 93%, P = 0.014). The median number of gene mutations reported was 6 (range 2–14). The TP53 mutation was associated most frequently with lung metastasis (P = 0.07) and high tumor budding (P = 0.03). Despite no statistical significance, lung recurrence, LVI/Pni, MSI and more than 6 genetic mutations were correlated to worse DFS and OS. Patients carried co-mutations of TP53-FBXW7 reported the worse DFS (4 v 14 months) and OS (4 v 65 months) compared to the other patients.

Conclusion

According to the present analysis, the setting of relapsed CRC emerges as one of the fields of greatest utility for NGS, looking at personalized cancer care.

Fbw7 Inhibits the Progression of Activated B-Cell Like Diffuse Large B-Cell Lymphoma by Targeting the Positive Feedback Loop of the LDHA/lactate/miR-223 Axis

Background

F-box and WD repeat domain-containing 7 (Fbw7) is well known as a tumor suppressor and ubiquitin ligase which targets a variety of oncogenic proteins for proteolysis. We previously reported that Fbw7 promotes apoptosis in diffuse large B-cell lymphoma (DLBCL) through Fbw7-mediated ubiquitination of Stat3. This study aimed to identify the mechanism of Fbw7-mediated aerobic glycolysis reprogramming in DLBCL.

Methods

Expression levels of Fbw7 and Lactate Dehydrogenase A (LDHA) in human DLBCL samples were evaluated by immunohistochemistry. Crosstalk between Fbw7 and LDHA signaling was analyzed by co-immunoprecipitation, ubiquitination assay, western blotting and mRNA quanlitative analyses. In vitro and in vivo experiments were used to assess the effect of the Fbw7-mediated LDHA/lactate/miR-223 axis on DLBCL cells growth.

Results

Fbw7 could interact with LDHA to trigger its ubiquitination and degradation. Inversely, lactate negatively regulated Fbw7 via trigging the expression of miR-223, which targeted Fbw7 3’-UTR to inhibit its expression. In vivo and in vitro experiments revealed that miR-223 promoted tumor growth and that the effects of miR-223 on tumor growth were primarily related to the inhibition of Fbw7-mediated LDHA’s ubiquitination.

Conclusions

We demonstrated that the ubiquitin-ligase Fbw7 played a key role in LDHA-related aerobic glycolysis reprogramming in DLBCL. Our study uncovers a negative functional loop consisting of a Fbw7-mediated LDHA/lactate/miR-223 axis, which may support the future ABC-DLBCL therapy by targeting LDHA-related inhibition.

Ubiquitination-Proteasome System (UPS) and Autophagy Two Main Protein Degradation Machineries in Response to Cell Stress

In response to environmental stimuli, cells make a series of adaptive changes to combat the injury, repair the damage, and increase the tolerance to the stress. However, once the damage is too serious to repair, the cells will undergo apoptosis to protect the overall cells through suicidal behavior. Upon external stimulation, some intracellular proteins turn into unfolded or misfolded protein, exposing their hydrophobic regions to form protein aggregation, which may ultimately produce serious damage to the cells. Ubiquitin plays an important role in the degradation of these unnatural proteins by tagging with ubiquitin chains in the ubiquitin-proteasome or autophagy system. If the two processes fail to eliminate the abnormal protein aggregates, the cells will move to apoptosis and death. Dysregulation of ubiquitin-proteasome system (UPS) and autophagy may result in the development of numerous diseases. This review focuses on the molecular mechanisms of UPS and autophagy in clearance of intracellular protein aggregates, and the relationship between dysregulation of ubiquitin network and diseases.

Pro-angiogenic effect of PC-3 exosomes in endothelial cells in vitro

The progression to a castration-resistant prostate cancer can occur after treatment with androgen deprivation therapy, resulting in poor prognosis and ineffective therapy response. Hormone dependence transition has been associated with increased tumor vascularization. Considering that exosomes are important components in communication between tumor cells and the microenvironment, we examined the angiogenic potential of exosomes released from Pca cell lines with distinctive profiles of androgen response through exosomes isolation, microscopy and uptake, functional assays follow up by microarray, RT-qPCR and bioinformatics analysis. HUVEC cells treated with PC-3 exosomes (androgen independent) showed increased invasion and tube formation ability. In order to identify microRNAs (miRNAs) related to the angiogenic response, the characterization of exosomal miRNA profile was performed. As result we suggest that the miR-27a-3p could be involved in the pro-angiogenic effect of PC-3 exosomes.

FBW7 suppresses ovarian cancer development by targeting the N6-methyladenosine binding protein YTHDF2

Background

The tumor suppressor FBW7 is the substrate recognition component of the SCF E3-ubiquitin ligase complex that mediates proteolytic degradation of various oncogenic proteins. However, the role of FBW7 in ovarian cancer progression remains inadequately understood.

Methods

IP-MASS, co-IP, immunohistochemistry, and western blotting were used to identify the potential substrate of FBW7 in ovarian cancer. The biological effects of FBW7 were investigated using in vitro and in vivo models. LC/MS was used to detect the m⁶A levels in ovarian cancer tissues. MeRIP-Seq and RNA-Seq were used to assess the downstream targets of YTHDF2.

Results

We unveil that FBW7 is markedly down-regulated in ovarian cancer tissues and its high expression is associated with favorable prognosis and elevated m⁶A modification levels. Consistently, ectopic FBW7 inhibits ovarian cancer cell survival and proliferation in vitro and in vivo, while ablation of FBW7 empowers propagation of ovarian cancer cells. In addition, the m⁶A reader protein, YTHDF2, is identified as a novel substrate for FBW7. FBW7 counteracts the tumor-promoting effect of YTHDF2 by inducing proteasomal degradation of the latter in ovarian cancer. Furthermore, YTHDF2 globally regulates the turnover of m⁶A-modified mRNAs, including the pro-apoptotic gene BMF.

Conclusions

Our study has demonstrated that FBW7 suppresses tumor growth and progression via antagonizing YTHDF2-mediated BMF mRNA decay in ovarian cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12943-021-01340-8.

Ubiquitin-proteasome system (UPS) as a target for anticancer treatment

The ubiquitin-proteasome system (UPS) plays an important role in the cellular processes for protein quality control and homeostasis. Dysregulation of the UPS has been implicated in numerous diseases, including cancer. Indeed, components of UPS are frequently mutated or abnormally expressed in various cancers. Since Bortezomib, a proteasome inhibitor, received FDA approval for the treatment of multiple myeloma and mantle cell lymphoma, increasing numbers of researchers have been seeking drugs targeting the UPS as a cancer therapeutic strategy. Here, we introduce the essential component of UPS, including ubiquitinating enzymes, deubiquitinating enzymes and 26S proteasome, and we summarize their targets and mechanisms that are crucial for tumorigenesis. In addition, we briefly discuss some UPS inhibitors, which are currently in clinical trials as cancer therapeutics.

Loss of Fbxw7 triggers mammary tumorigenesis associated with E2F/c-Myc activation and Trp53 mutation

Fbw7 is a tumor suppressor that regulates the degradation of oncogenic substrates such as c-Jun, c-Myc, Notch1 intracellular domain (ICD), and cyclin E by functioning as the substrate recognition protein in the Skp1-Cullin-F-box (SCF) ubiquitin ligase complex. Consequently, low expression or loss of FBXW7 in breast cancer has been hypothesized to result in the accumulation of oncogenic transcription factors that are master regulators of proliferation, apoptosis, and ultimately transformation. Despite this, the direct effect of Fbw7 loss on mammary gland morphology and tumorigenesis has not been examined. Here, we demonstrate that conditional deletion of Fbxw7 in murine mammary tissue initiates breast tumor development and also results in lactation and involution defects. Further, while Fbxw7 loss results in the overexpression of Notch1-ICD, c-Jun, cyclin E, and c-Myc, the downstream transcription factor pathways associated with c-Myc and cyclin E are the most dysregulated, including at the single-cell level. These pathways are dysregulated early after Fbxw7 loss, and their sustained loss results in tumorigenesis and reinforced c-Myc and cyclin E-E2F pathway disruption. We also find that loss of Fbxw7 is linked to the acquisition of Trp53 mutations, similar to the mutational spectrum observed in patients. Our results demonstrate that the loss of Fbxw7 promotes the acquisition of Trp53 mutations and that the two cooperate in breast tumor development. Targeting c-Myc, E2F, or p53 may therefore be a beneficial treatment strategy for FBXW7-altered breast cancer patients.

Function and regulation of F-box/WD repeat-containing protein 7

The ubiquitin-proteasome system is an important post-translational modification system involved in numerous biological processes, such as cell cycle regulation, gene transcription, signal transduction, apoptosis, differentiation and development. F-box/WD repeat-containing protein 7 (FBXW7) is one of the most studied F-box (FBX) proteins, serving as substrate recognition component of S phase kinase-associated protein 1-Cullin 1-FBX protein complexes. As a tumor suppressor, FBXW7 recognizes numerous proto-oncoproteins and promotes their ubiquitination and subsequent proteasomal degradation. FBXW7 is regulated at different levels, leading to tunable and specific control of the activity and abundance of its substrates. Therefore, genetic mutations or decreases in its expression serve an important biological role in tumor development. In-depth studies and identification of additional substrates targeted by FBXW7 have suggested a signaling network regulated by FBXW7, including its tumor-inhibitory role. The present review focused on the role of FBXW7 in tumor suppression and its application in cancer therapy.

Mouse models of colorectal cancer: Past, present and future perspectives

Colorectal cancer (CRC) is the third most common diagnosed malignancy among both sexes in the United States as well as in the European Union. While the incidence and mortality rates in western, high developed countries are declining, reflecting the success of screening programs and improved treatment regimen, a rise of the overall global CRC burden can be observed due to lifestyle changes paralleling an increasing human development index. Despite a growing insight into the biology of CRC and many therapeutic improvements in the recent decades, preclinical in vivo models are still indispensable for the development of new treatment approaches. Since the development of carcinogen-induced rodent models for CRC more than 80 years ago, a plethora of animal models has been established to study colon cancer biology. Despite tenuous invasiveness and metastatic behavior, these models are useful for chemoprevention studies and to evaluate colitis-related carcinogenesis. Genetically engineered mouse models (GEMM) mirror the pathogenesis of sporadic as well as inherited CRC depending on the specific molecular pathways activated or inhibited. Although the vast majority of CRC GEMM lack invasiveness, metastasis and tumor heterogeneity, they still have proven useful for examination of the tumor microenvironment as well as systemic immune responses; thus, supporting development of new therapeutic avenues. Induction of metastatic disease by orthotopic injection of CRC cell lines is possible, but the so generated models lack genetic diversity and the number of suited cell lines is very limited. Patient-derived xenografts, in contrast, maintain the pathological and molecular characteristics of the individual patient’s CRC after subcutaneous implantation into immunodeficient mice and are therefore most reliable for preclinical drug development – even in comparison to GEMM or cell line-based analyses. However, subcutaneous patient-derived xenograft models are less suitable for studying most aspects of the tumor microenvironment and anti-tumoral immune responses. The authors review the distinct mouse models of CRC with an emphasis on their clinical relevance and shed light on the latest developments in the field of preclinical CRC models.

Skp, Cullin, F-box (SCF)-Met30 and SCF-Cdc4-Mediated Proteolysis of CENP-A Prevents Mislocalization of CENP-A for Chromosomal Stability in Budding Yeast

Restricting the localization of the histone H3 variant CENP-A (Cse4 in yeast, CID in flies) to centromeres is essential for faithful chromosome segregation. Mislocalization of CENP-A leads to chromosomal instability (CIN) in yeast, fly and human cells. Overexpression and mislocalization of CENP-A has been observed in many cancers and this correlates with increased invasiveness and poor prognosis. Yet genes that regulate CENP-A levels and localization under physiological conditions have not been defined. In this study we used a genome-wide genetic screen to identify essential genes required for Cse4 homeostasis to prevent its mislocalization for chromosomal stability. We show that two Skp, Cullin, F-box (SCF) ubiquitin ligases with the evolutionarily conserved F-box proteins Met30 and Cdc4 interact and cooperatively regulate proteolysis of endogenous Cse4 and prevent its mislocalization for faithful chromosome segregation under physiological conditions. The interaction of Met30 with Cdc4 is independent of the D domain, which is essential for their homodimerization and ubiquitination of other substrates. The requirement for both Cdc4 and Met30 for ubiquitination is specifc for Cse4; and a common substrate for Cdc4 and Met30 has not previously been described. Met30 is necessary for the interaction between Cdc4 and Cse4, and defects in this interaction lead to stabilization and mislocalization of Cse4, which in turn contributes to CIN. We provide the first direct link between Cse4 mislocalization to defects in kinetochore structure and show that SCF-mediated proteolysis of Cse4 is a major mechanism that prevents stable maintenance of Cse4 at non-centromeric regions, thus ensuring faithful chromosome segregation. In summary, we have identified essential pathways that regulate cellular levels of endogenous Cse4 and shown that proteolysis of Cse4 by SCF-Met30/Cdc4 prevents mislocalization and CIN in unperturbed cells.

Genetic material on each chromosome must be faithfully transmitted to the daughter cell during cell division and chromosomal instability (CIN) results in aneuploidy, a hallmark of cancers. The kinetochore (centromeric DNA and associated proteins) regulates faithful chromosome segregation. Restricting the localization of CENP-A (Cse4 in yeast) to kinetochores is essential for chromosomal stability. Mislocalization of CENP-A contributes to CIN in yeast, fly and human cells and is observed in cancers where it correlates with increased invasiveness and poor prognosis. Hence, identification of pathways that regulate CENP-A levels will help us understand the correlation between CENP-A mislocalization and aneuploidy in cancers. We used a genetic screen to identify essential genes for Cse4 homeostasis and identified a major ubiquitin-dependent pathway where both nuclear F-box proteins, Met30 and Cdc4 of the SCF complex, cooperatively regulate proteolysis of Cse4 to prevent its mislocalization and CIN under physiological conditions. Our studies define a role for SCF-mediated proteolysis of Cse4 as a critical mechanism to ensure faithful chromosome segregation. These studies are significant because mutations in human homologs of Met30 (β-TrCP) and Cdc4 (Fbxw7) have been implicated in cancers, and future studies will determine if SCF-mediated proteolysis of CENP-A prevents its mislocalization for chromosomal stability in human cells.

Knockout Mouse Models Provide Insight into the Biological Functions of CRL1 Components

The CRL1 complex, also known as the SCF complex, is a ubiquitin ligase that in mammals consists of an adaptor protein (SKP1), a scaffold protein (CUL1), a RING finger protein (RBX1, also known as ROC1), and one of about 70 F-box proteins. Given that the F-box proteins determine the substrate specificity of the CRL1 complex, the variety of these proteins allows the generation of a large number of ubiquitin ligases that promote the degradation or regulate the function of many substrate proteins and thereby control numerous key cellular processes. The physiological and pathological functions of these many CRL1 ubiquitin ligases have been studied by the generation and characterization of knockout mouse models that lack specific CRL1 components. In this chapter, we provide a comprehensive overview of these mouse models and discuss the role of each CRL1 component in mouse physiology and pathology.

Regulation of the p53 Family Proteins by the Ubiquitin Proteasomal Pathway

The tumor suppressor p53 and its homologues, p63 and p73, play a pivotal role in the regulation of the DNA damage response, cellular homeostasis, development, aging, and metabolism. A number of mouse studies have shown that a genetic defect in the p53 family could lead to spontaneous tumor development, embryonic lethality, or severe tissue abnormality, indicating that the activity of the p53 family must be tightly regulated to maintain normal cellular functions. While the p53 family members are regulated at the level of gene expression as well as post-translational modification, they are also controlled at the level of protein stability through the ubiquitin proteasomal pathway. Over the last 20 years, many ubiquitin E3 ligases have been discovered that directly promote protein degradation of p53, p63, and p73 in vitro and in vivo. Here, we provide an overview of such E3 ligases and discuss their roles and functions.

The WD40 domain of FBXW7 is a poly(ADP-ribose)-binding domain that mediates the early DNA damage response

FBXW7, a classic tumor suppressor, is a substrate recognition subunit of the Skp1-cullin-F-box (SCF) ubiquitin ligase that targets oncoproteins for ubiquitination and degradation. We recently found that FBXW7 is recruited to DNA damage sites to facilitate nonhomologous end-joining (NHEJ). The detailed underlying molecular mechanism, however, remains elusive. Here we report that the WD40 domain of FBXW7, which is responsible for substrate binding and frequently mutated in human cancers, binds to poly(ADP-ribose) (PAR) immediately following DNA damage and mediates rapid recruitment of FBXW7 to DNA damage sites, whereas ATM-mediated FBXW7 phosphorylation promotes its retention at DNA damage sites. Cancer-associated arginine mutations in the WD40 domain (R465H, R479Q and R505C) abolish both FBXW7 interaction with PAR and recruitment to DNA damage sites, causing inhibition of XRCC4 polyubiquitination and NHEJ. Furthermore, inhibition or silencing of poly(ADP-ribose) polymerase 1 (PARP1) inhibits PAR-mediated recruitment of FBXW7 to the DNA damage sites. Taken together, our study demonstrates that the WD40 domain of FBXW7 is a novel PAR-binding motif that facilitates early recruitment of FBXW7 to DNA damage sites for subsequent NHEJ repair. Abrogation of this ability seen in cancer-derived FBXW7 mutations provides a molecular mechanism for defective DNA repair, eventually leading to genome instability.

N6-Isopentenyladenosine Inhibits Colorectal Cancer and Improves Sensitivity to 5-Fluorouracil-Targeting FBXW7 Tumor Suppressor

N6-isopentenyladenosine has been shown to exert potent in vitro antitumor activity on different human cancers, including colorectal cancer. Although some potential biochemical targets have been identified, its precise mechanism of action remains unclear. We found that N6-isopentenyladenosine affects colorectal cancer proliferation in in vitro models carrying different mutational status of FBXW7 and TP53 genes, and in HCT116 xenografts in SCID mice, by increasing the expression of the well-established tumor suppressor FBXW7, a component of the SCF-E3 ubiquitin ligase complex that promotes degradation of various oncoproteins and transcription factors, such as c-Myc, SREBP and Mcl1. Corroborating our previous studies, we identified for the first time the FBXW7/SREBP/FDPS axis as a target of the compound. Pull down of ubiquitinated proteins, immunoprecipitation and luciferase assays, reveal that through the increase of FBXW7/c-Myc binding, N6-isopentenyladenosine induces the ubiquitination of c-Myc, inhibiting its transcriptional activity. Moreover, in FBXW7- and TP53-wild type cells, N6-isopentenyladenosine strongly synergizes with 5-Fluorouracil to inhibit colon cancer growth in vitro. Our results provide novel insights into the molecular mechanism of N6-isopentenyladenosine, revealing its multi-targeting antitumor action, in vitro and in vivo. Restoring of FBXW7 tumor-suppressor represents a valid therapeutic tool, enabling N6-isopentenyladenosine as optimizable compound for patient-personalized therapies in colorectal cancer.

The Tumor Microenvironment in Colorectal Cancer Therapy

The current standard-of-care for metastatic colorectal cancer (mCRC) includes chemotherapy and anti-angiogenic or anti-epidermal growth factor receptor (EGFR) monoclonal antibodies, even though the addition of anti-angiogenic agents to backbone chemotherapy provides little benefit for overall survival. Since the approval of anti-angiogenic monoclonal antibodies bevacizumab and aflibercept, for the management of mCRC over a decade ago, extensive efforts have been devoted to discovering predictive factors of the anti-angiogenic response, unsuccessfully. Recent evidence has suggested a potential correlation between angiogenesis and immune phenotypes associated with colorectal cancer. Here, we review evidence of interactions between tumor angiogenesis, the immune microenvironment, and metabolic reprogramming. More specifically, we will highlight such interactions as inferred from our novel immune-metabolic (IM) signature, which groups mCRC into three distinct clusters, namely inflamed-stromal-dependent (IM Cluster 1), inflamed-non stromal-dependent (IM Cluster 2), and non-inflamed or cold (IM Cluster 3), and discuss the merits of the IM classification as a guide to new immune-metabolic combinatorial therapeutic strategies in mCRC.

Abrogation of FBW7α-dependent p53 degradation enhances p53's function as a tumor suppressor

The gene encoding the tumor suppressor p53 is mutated in most cancers. p53 expression is known to be tightly controlled by several E3 ligases. Here, we show that F-box and WD repeat domain-containing 7α (FBW7α), the substrate-recognition component of the SCF^FBW7 multiprotein E3 ligase complex, targets both WT and tumor-derived mutants of p53 for proteasomal degradation in multiple human cancer cell lines (HCT116 and U2OS). We found that lack of FBW7α stabilizes p53 levels, thereby increasing its half-life. p53 ubiquitylation and subsequent degradation require the F-box and the C-terminal WD40 repeats in FBW7α. The polyubiquitylation of p53 occurred via Lys-48 linkage and involved phosphorylation on p53 at Ser-33 and Ser-37 by glycogen synthase kinase 3β (GSK3β) and DNA-dependent protein kinase (DNA-PK), respectively. These phosphorylation events created a phosphodegron that enhanced p53 binding to FBW7α, allowing for the attachment of polyubiquitin moieties at Lys-132 in p53. FBW7α-dependent p53 polyubiquitylation apparently occurred during and immediately after DNA double-strand breaks induced by either doxorubicin or ionizing radiation. Accordingly, in cells lacking FBW7α, p53 induction was enhanced after DNA damage. Phosphodegron-mediated polyubiquitylation of p53 on Lys-132 had functional consequences, with cells in which FBW7α-mediated p53 degradation was abrogated exhibiting enhancement of their tumorigenic potential. We conclude that p53, which previously has been reported to transactivate FBW7, is also targeted by the same E3 ligase for degradation, suggesting the presence of a regulatory feedback loop that controls p53 levels and functions during DNA damage.

LSD1 destabilizes FBXW7 and abrogates FBXW7 functions independent of its demethylase activity

FBXW7 acts as a typical tumor suppressor, with loss-of-function alterations in human cancers, by promoting ubiquitylation and degradation of many oncoproteins. Lysine-specific demethylase 1 (LSD1) is a well-characterized histone demethylase. Whether LSD1 has demethylase-independent activity remains elusive. Here we report that LSD1 directly binds to FBXW7 to destabilize FBXW7 independent of its demethylase activity. Specifically, LSD1 is a pseudosubstrate of FBXW7 and LSD1-FBXW7 binding does not trigger LSD1 ubiquitylation, but instead promotes FBXW7 self-ubiquitylation by preventing FBXW7 dimerization. The self-ubiquitylated FBXW7 is subjected to degradation by proteasome as well as lysosome in a manner dependent on autophagy protein p62/SQSTM1. Biologically, LSD1 destabilizes FBXW7 to abrogate its functions in growth suppression, nonhomologous end-joining repair, and radioprotection. Collectively, our study revealed a previously unknown activity of LSD1, which likely contributes to its oncogenic function. Targeting LSD1 protein, not only its demethylase activity, might be a unique approach for LSD1-based drug discovery for anticancer application.

Approaches to functionally validate candidate genetic variants involved in colorectal cancer predisposition

Most next generation sequencing (NGS) studies identified candidate genetic variants predisposing to colorectal cancer (CRC) but do not tackle its functional interpretation to unequivocally recognize a new hereditary CRC gene. Besides, germline variants in already established hereditary CRC-predisposing genes or somatic variants share the same need when trying to categorize those with relevant significance. Functional genomics approaches have an important role in identifying the causal links between genetic architecture and phenotypes, in order to decipher cellular function in health and disease. Therefore, functional interpretation of identified genetic variants by NGS platforms is now essential. Available approaches nowadays include bioinformatics, cell and molecular biology and animal models. Recent advances, such as the CRISPR-Cas9, ZFN and TALEN systems, have been already used as a powerful tool with this objective. However, the use of cell lines is of limited value due to the CRC heterogeneity and its close interaction with microenvironment. Access to tridimensional cultures or organoids and xenograft models that mimic the in vivo tissue architecture could revolutionize functional analysis. This review will focus on the application of state-of-the-art functional studies to better tackle new genes involved in germline predisposition to this neoplasm.

FBXW7 in Cancer: What Has Been Unraveled Thus Far?

: The FBXW7 (F-box with 7 tandem WD40) protein encoded by the gene FBXW7 is one of the crucial components of ubiquitin ligase called Skp1-Cullin1-F-box (SCF) complex that aids in the degradation of many oncoproteins via the ubiquitin-proteasome system (UPS) thus regulating cellular growth. FBXW7 is considered as a potent tumor suppressor as most of its target substrates can function as potential growth promoters, including c-Myc, Notch, cyclin E, c-JUN, and KLF5. Its regulators include p53, C/EBP-δ, Numb, microRNAs, Pin 1, Hes-5, BMI1, Ebp2. Mounting evidence has indicated the involvement of aberrant expression of FBXW7 for tumorigenesis. Moreover, numerous studies have also shown its role in cancer cell chemosensitization, thereby demonstrating the importance of FBXW7 in the development of curative cancer therapy. This comprehensive review emphasizes on the targets, functions, regulators and expression of FBXW7 in different cancers and its involvement in sensitizing cancer cells to chemotherapeutic drugs.

Tolerance of Chromosomal Instability in Cancer: Mechanisms and Therapeutic Opportunities

Chromosomal instability (CIN) is the result of ongoing changes in the number (aneuploidy) and structure of chromosomes. CIN is induced by chromosome missegregation in mitosis and leads to karyotypic diversity within the cancer cell population, thereby adding to intratumor heterogeneity. Regardless of the overall pro-oncogenic function of CIN, its onset is typically detrimental for cell fitness and thus tumors must develop CIN-tolerance mechanisms in order to propagate. There is overwhelming genetic and functional evidence linking mutations in the tumor suppressor TP53 with CIN-tolerance. However, the pathways leading to p53 activation following chromosome missegregation remain controversial. Recently, additional mechanisms have been identified in CIN-surveillance, resulting in a more complex network of pathways acting independently or in cooperation with p53. Tolerance might also be achieved by modifying aspects of the cancer cell physiology in order to attenuate CIN or by adaptation to the consequences of aneuploid karyotypes. In this review, we summarize the current knowledge about p53-dependent and -independent mechanisms of CIN-tolerance in cancer, the adaptations observed in CIN cells buffering CIN levels, its consequences for cellular homeostasis, and the potential of exploiting these adaptations in order to design new cancer therapies.

FBXW7: a critical tumor suppressor of human cancers

The ubiquitin-proteasome system (UPS) is involved in multiple aspects of cellular processes, such as cell cycle progression, cellular differentiation, and survival (Davis RJ et al., Cancer Cell 26:455-64, 2014; Skaar JR et al., Nat Rev Drug Discov 13:889-903, 2014; Nakayama KI and Nakayama K, Nat Rev Cancer 6:369-81, 2006). F-box and WD repeat domain containing 7 (FBXW7), also known as Sel10, hCDC4 or hAgo, is a member of the F-box protein family, which functions as the substrate recognition component of the SCF E3 ubiquitin ligase. FBXW7 is a critical tumor suppressor and one of the most commonly deregulated ubiquitin-proteasome system proteins in human cancer. FBXW7 controls proteasome-mediated degradation of oncoproteins such as cyclin E, c-Myc, Mcl-1, mTOR, Jun, Notch and AURKA. Consistent with the tumor suppressor role of FBXW7, it is located at chromosome 4q32, a genomic region deleted in more than 30% of all human cancers (Spruck CH et al., Cancer Res 62:4535-9, 2002). Genetic profiles of human cancers based on high-throughput sequencing have revealed that FBXW7 is frequently mutated in human cancers. In addition to genetic mutations, other mechanisms involving microRNA, long non-coding RNA, and specific oncogenic signaling pathways can inactivate FBXW7 functions in cancer cells. In the following sections, we will discuss the regulation of FBXW7, its role in oncogenesis, and the clinical implications and prognostic value of loss of function of FBXW7 in human cancers.

Knockdown of FBXO39 inhibits proliferation and promotes apoptosis of human osteosarcoma U-2OS cells

F-box proteins are essential components of the Skp-cullin-F-box complex (a type of E3 ubiquitin ligase), and participate in cell cycle and immune responses through the ubiquitin proteasome system. F-box protein 39 (FBXO39) belongs to the F-box family, which has been reported to be associated with cancer oncogenesis and progression. The present study aimed to investigate the role of FBXO39 in osteosarcoma (OS) cell proliferation and apoptosis in vitro. It was demonstrated that U-2OS cells exhibited high expression of FBXO39 compared with HOS and SaOS-2 osteosarcoma cells. Thus, knockdown of FBXO39 was performed using lentivirus-mediated short hairpin RNA (shRNA) transfection to validate the effect of FBXO39 in U-2OS cells. Western blotting and RT-qPCR analysis were used to confirm the efficiency of infection by analyzing the expression level of FBXO39. Using Celigo-based cell counting and MTT assays, it was demonstrated that FBXO39 knockdown significantly reduced the rate of cell proliferation compared with control. Caspase 3/7 activity assays and fluorescence-activated cell sorting confirmed the induction of apoptosis in U-2OS cells following FBXO39 knockdown. In conclusion, it was demonstrated that FBXO39 knockdown may significantly inhibit proliferation and promote apoptosis of U-2OS cells. Thus, FBXO39 may serve an important role in OS progression.

FBW7 suppresses metastasis of colorectal cancer by inhibiting HIF1α/CEACAM5 functional axis

F-box and WD repeat domain-containing 7 (FBW7) functions as a major tumor suppressor by targeting oncoproteins for degradations. FBW7 has been reported to be one of the most frequently mutated genes in colorectal cancer (CRC). However, its roles and possible mechanisms in the development of CRC are still unclear. In the present study, we adopted immunohistochemistry staining in tissue microarray (TMA), consisting of 276 samples from stage I-IV CRC patients, and analyzed the correlation between FBW7 expression and clinicopathological parameters, as well as overall survival (OS) and disease-free survival (DFS). The impact of FBW7 on migration was further validated in vitro. Whole-genome expression microarray (GEO,accession numbers GSE76443), was then analyzed to find the possible target of FBW7. The results were verified by functional experiments in vitro and IHC staining of TMA. Finally, luciferase and chromatin immunoprecipitation (ChIP) assays were carried out to identify the possible mechanisms. The expression level of FBW7 in TMA was negatively correlated with serum CEA level, venous invasion, N stage and M stage, and positively associated with the survival of CRC patients(P<0.05). Ectopic FBW7 expression significantly suppressed migration of colon cancer cells in vitro. GEO analysis revealed that decreased FBW7 significantly correlated with increased level of CEACAM5, which encoded CEA. The correlation was verified by IHC of TMA and silencing CEACAM5 inhibited migration in vitro. Mechanistically, we demonstrated that CEACAM5 was a HIF1α target gene and that FBW7 regulated CEACAM5 in a HIF1α-dependent manner. In conclusion, our results revealed that FBW7 suppressed migration through regulation of the HIF1α/CEACAM5 axis in colorectal cancer. Therefore, our study sheds novel lights on the impact of FBW7 on HIF1α/CEACAM5 signaling axis and constitutes potential prognostic predictors and therapeutic targets for CRC.

Ubiquitin ligases in oncogenic transformation and cancer therapy

The cellular response to external stress signals and DNA damage depends on the activity of ubiquitin ligases (E3s), which regulate numerous cellular processes, including homeostasis, metabolism and cell cycle progression. E3s recognize, interact with and ubiquitylate protein substrates in a temporally and spatially regulated manner. The topology of the ubiquitin chains dictates the fate of the substrates, marking them for recognition and degradation by the proteasome or altering their subcellular localization or assembly into functional complexes. Both genetic and epigenetic alterations account for the deregulation of E3s in cancer. Consequently, the stability and/or activity of E3 substrates are also altered, in some cases leading to downregulation of tumour-suppressor activities and upregulation of oncogenic activities. A better understanding of the mechanisms underlying E3 regulation and function in tumorigenesis is expected to identify novel prognostic markers and to enable the development of the next generation of anticancer therapies. This Review summarizes the oncogenic and tumour-suppressor roles of selected E3s and highlights novel opportunities for therapeutic intervention.

Notch Signaling in Development, Tissue Homeostasis, and Disease

Notch signaling is an evolutionarily highly conserved signaling mechanism, but in contrast to signaling pathways such as Wnt, Sonic Hedgehog, and BMP/TGF-β, Notch signaling occurs via cell-cell communication, where transmembrane ligands on one cell activate transmembrane receptors on a juxtaposed cell. Originally discovered through mutations in Drosophila more than 100 yr ago, and with the first Notch gene cloned more than 30 yr ago, we are still gaining new insights into the broad effects of Notch signaling in organisms across the metazoan spectrum and its requirement for normal development of most organs in the body. In this review, we provide an overview of the Notch signaling mechanism at the molecular level and discuss how the pathway, which is architecturally quite simple, is able to engage in the control of cell fates in a broad variety of cell types. We discuss the current understanding of how Notch signaling can become derailed, either by direct mutations or by aberrant regulation, and the expanding spectrum of diseases and cancers that is a consequence of Notch dysregulation. Finally, we explore the emerging field of Notch in the control of tissue homeostasis, with examples from skin, liver, lung, intestine, and the vasculature.

Functional significance and therapeutic implication of ring-type E3 ligases in colorectal cancer

Accumulative studies revealed that E3 ubiquitin ligases have important roles in colorectal carcinogenesis. The pathogenic mechanisms of colorectal cancer (CRC) initiation and progression are complex and heterogeneous, involving somatic mutations, abnormal gene fusion, deletion or amplification and epigenetic alteration, which may cause aberrant expression or altered function of E3 ligases in CRC. Defects of E3 ligases have been reported to be involved in the molecular etiology and pathogenesis of CRC. The aberrant expressed E3 ligases can function as either oncogenes or tumor suppressors depending on ubiquiting target substrates in CRC. Recently, considerable progress has been made in our understanding of the potential roles of E3 ligase-mediated ubiquitylation in colorectal carcinogenesis. There are mainly two subtypes of E3 ubiquitin ligases in humans, as defined by the presence of either a HECT domain or a RING finger domain on the basis of structural similitude. Most cancer-associated E3 ligases participate in regulating the cell cycle, apoptosis, gene transcription, cell signaling and DNA repair, the critical parts of CRC tumorigenesis. In this review, we have provided a comprehensive summary of abnormally expressed E3 ligases and their related pivotal mechanistic effects in CRC. In particular, we have highlighted the function of RING-type E3 ubiquitin enzymes in modulating cancer signaling pathways, immunity and tumor microenvironment in CRC development and progression; their mechanism(s) of action in CRC involving both ubiquitylation-dependent and ubiquitylation-independent effects; and the potential of RING E3 ligases as molecular biomarkers for predicting patient prognosis and as therapeutic targets in CRC. A better understanding of E3 ligase-mediated substrates' ubiquitylation involved in the development of CRC will provide new insights into the pathophysiology mechanisms of CRC, and unravel novel prognostic markers and therapeutic strategies for CRC.

The Blebbishield Emergency Program Overrides Chromosomal Instability and Phagocytosis Checkpoints in Cancer Stem Cells

Genomic instability and immune evasion are hallmarks of cancer. Apoptotic cancer stem cells can evade cell death by undergoing cellular transformation by constructing "blebbishields" from apoptotic bodies. In this study, we report a novel linkage between genomic instability and phagocytosis evasion that is coordinated by the blebbishield emergency program. Blebbishield emergency program evaded genomic instability checkpoint, expressed genomic instability-associated genes at distinct phases of cellular transformation, exhibited chromosomal instability, and promoted increase in nuclear size. Blebbishields fused with immune cells to evade phagocytosis, and the resultant hybrid cells exhibited increased migration, tumorigenesis, metastasis, red blood cell recruitment to tumors, and induced hepatosplenomegaly with signatures of genomic instability, blebbishield emergency program, and phagocytosis evasion to offer poor prognosis. Overall, our data demonstrate that the blebbishield emergency program drives evasion of chromosomal instability and phagocytosis checkpoints by apoptotic cancer stem cells. Cancer Res; 77(22); 6144-56. ©2017 AACR.

FBXW7 expression is associated with prognosis and chemotherapeutic outcome in Chinese patients with gastric adenocarcinoma

Background

FBXW7, a component of the Skp-Cullin1-F-box, mediates target protein recognition. It is a tumor suppressor gene that plays a role in the regulation of cell cycle exit and reentry via c-Myc, c-Jun and Notch degradation. There are few studies, particularly involving a large patient cohort, that have evaluated FBXW7 during gastric cancer progression.

Methods

Our study aimed to evaluate the value of FBXW7 as a clinical marker in gastric adenocarcinoma (GC) patients including a subset treated with postoperative chemotherapy. Quantitative reverse transcription PCR (qRT-PCR) assay was used to measure FBXW7 transcript levels in tumors paired with normal gastric tissue in 24 gastric adenocarcinoma patients. Subsequently, 546 additional GC samples were evaluated from patients that underwent radical gastrectomy, including 118 early stage cases(Stage I) and 428 advanced stage cases (Stages II or III). Amongst the advanced stage patient cases evaluated, 347 received postoperative adjuvant chemotherapy. All 546 gastric adenocarcinoma cases were then evaluated by tissue microarray and immunohistochemistry (IHC) for FBXW7 expression. Clinicopathological features and diagnoses were confirmed by histopathologic evaluation and review of clinical data. Overall survival (OS) was then evaluated in the 546 gastric cancer patients.

Results

By immunohistologic evaluation, low expression of FBXW7 in primary gastric cancer significantly correlated with poor differentiation of tumor cells. Moreover, low FBXW7 expression was associated with worse survival as well as worse adjuvant chemotherapy response.

Conclusion

Our findings suggest that FBXW7 may serve as an important predictor in chemotherapeutic responses.

BCL9L Dysfunction Impairs Caspase-2 Expression Permitting Aneuploidy Tolerance in Colorectal Cancer

Chromosomal instability (CIN) contributes to cancer evolution, intratumor heterogeneity, and drug resistance. CIN is driven by chromosome segregation errors and a tolerance phenotype that permits the propagation of aneuploid genomes. Through genomic analysis of colorectal cancers and cell lines, we find frequent loss of heterozygosity and mutations in BCL9L in aneuploid tumors. BCL9L deficiency promoted tolerance of chromosome missegregation events, propagation of aneuploidy, and genetic heterogeneity in xenograft models likely through modulation of Wnt signaling. We find that BCL9L dysfunction contributes to aneuploidy tolerance in both TP53-WT and mutant cells by reducing basal caspase-2 levels and preventing cleavage of MDM2 and BID. Efforts to exploit aneuploidy tolerance mechanisms and the BCL9L/caspase-2/BID axis may limit cancer diversity and evolution.

Runx3 in Immunity, Inflammation and Cancer

In this chapter we summarize the pros and cons of the notion that Runx3 is a major tumor suppressor gene (TSG). Inactivation of TSGs in normal cells provides a viability/growth advantage that contributes cell-autonomously to cancer. More than a decade ago it was suggested that RUNX3 is involved in gastric cancer development, a postulate extended later to other epithelial cancers portraying RUNX3 as a major TSG. However, evidence that Runx3 is not expressed in normal gastric and other epithelia has challenged the RUNX3-TSG paradigm. In contrast, RUNX3 is overexpressed in a significant fraction of tumor cells in various human epithelial cancers and its overexpression in pancreatic cancer cells promotes their migration, anchorage-independent growth and metastatic potential. Moreover, recent high-throughput quantitative genome-wide studies on thousands of human samples of various tumors and new investigations of the role of Runx3 in mouse cancer models have unequivocally demonstrated that RUNX3 is not a bona fide cell-autonomous TSG. Importantly, accumulating data demonstrated that RUNX3 functions in control of immunity and inflammation, thereby indirectly influencing epithelial tumor development.

Fbxw7 Deletion Accelerates KrasG12D-Driven Pancreatic Tumorigenesis via Yap Accumulation

Pancreatic cancers driven by KRAS mutations require additional mutations for tumor progression. The tumor suppressor FBXW7 is altered in pancreatic cancers, but its contribution to pancreatic tumorigenesis is unknown. To determine potential cooperation between Kras mutation and Fbxw7 inactivation in pancreatic tumorigenesis, we generated P48-Cre;LSL-Kras^G12D;Fbxw7^fl/fl (KFC^fl/fl) compound mice. We found that KFC^fl/fl mice displayed accelerated tumorigenesis: all mice succumbed to pancreatic ductal adenocarcinoma (PDA) by 40 days of age, with PDA onset occurring by 2 weeks of age. PDA in KFC^fl/fl mice was preceded by earlier onset of acinar-to-ductal metaplasia (ADM) and pancreatic intraepithelial neoplasia (PanIN) lesions, and associated with chromosomal instability and the accumulation of Fbxw7 substrates Yes-associated protein (Yap), c-Myc, and Notch. Using KFC^fl/fl and FBXW7-deficient human pancreatic cancer cells, we found that Yap silencing attenuated growth promotion by Fbxw7 deletion. Our data demonstrate that Fbxw7 is a potent suppressor of Kras^G12D-induced pancreatic tumorigenesis due, at least in part, to negative regulation of Yap.

FBXW7 is involved in the acquisition of the malignant phenotype in epithelial ovarian tumors

FBXW7 is a ubiquitin ligase that mediates ubiquitylation of oncoproteins, such as c‐Myc, cyclin E, Notch and c‐Jun. FBXW7 is a known tumor‐suppressor gene, and mutations in FBXW7 have been reported in various human malignancies. In this study, we examined the sequences of the FBXW7 and p53 genes in 57 ovarian cancer clinical samples. Interestingly, we found no FBXW7 mutations associated with amino acid changes. We also investigated FBXW7 expression levels in 126 epithelial ovarian tumors. FBXW7 expression was negatively correlated with the malignant potential of ovarian tumors. That is to say, FBXW7 expression levels in ovarian cancer samples were significantly lower than those in borderline and benign tumors (P < 0.01). FBXW7 expression levels in serous carcinoma samples were the lowest among four major histological subtypes. In addition, p53‐mutated ovarian cancer samples showed significantly lower levels of FBXW7 expression compared with p53 wild‐type cancer samples (P < 0.001). DNA methylation arrays and bisulfite PCR sequencing experiments revealed that 5′‐upstream regions of FBXW7 gene in p53‐mutated samples were significantly higher methylated compared with those in p53 wild‐type samples (P < 0.01). This data indicates that p53 mutations might suppress FBXW7 expression through DNA hypermethylation of FBXW7 5′‐upstream regions. Thus, FBXW7 expression was downregulated in ovarian cancers, and was associated with p53 mutations and the DNA methylation status of the 5′‐upstream regions of FBXW7.

MYC-nick promotes cell migration by inducing fascin expression and Cdc42 activation

MYC-nick is a cytoplasmic, transcriptionally inactive member of the MYC oncoprotein family, generated by a proteolytic cleavage of full-length MYC. MYC-nick promotes migration and survival of cells in response to chemotherapeutic agents or withdrawal of glucose. Here we report that MYC-nick is abundant in colonic and intestinal tumors derived from mouse models with mutations in the Wnt, TGF-β, and PI3K pathways. Moreover, MYC-nick is elevated in colon cancer cells deleted for FBWX7, which encodes the major E3 ligase of full-length MYC frequently mutated in colorectal cancers. MYC-nick promotes the migration of colon cancer cells assayed in 3D cultures or grown as xenografts in a zebrafish metastasis model. MYC-nick accelerates migration by activating the Rho GTPase Cdc42 and inducing fascin expression. MYC-nick, fascin, and Cdc42 are frequently up-regulated in cells present at the invasive front of human colorectal tumors, suggesting a coordinated role for these proteins in tumor migration.

Oncogenic mutations in the FBXW7 gene of adult T-cell leukemia patients

Human T-cell leukemia virus type 1 (HTLV-I) is associated with adult T-cell leukemia (ATL), an aggressive lymphoproliferative disease with a dismal prognosis. We have previously described the presence of Notch1 activating mutations and constitutive Notch1 signaling in patients with acute ATL. In this study, we report a high frequency of F-box and WD repeat domain containing 7 (FBXW7)/hCDC4 mutations within the WD40 substrate-binding domain in 8 of 32 acute ATL patients (25%). Functionally, ATL FBXW7 mutants lost their ability to interact with intracellular Notch (NICD), resulting in increased protein stability and constitutive Notch1 signaling. Consistent with the loss-of-function found in ATL patients, expression of WT FBXW7 in several patient-derived ATL lines demonstrated strong tumor-suppressor activity characterized by reduced proliferation of ATL cells. Remarkably, two FBXW7 mutants, D510E and D527G, demonstrated oncogenic activity when expressed in the presence of HTLV-I Tax, mutated p53 R276H, or c-Myc F138C found in human cancers. Transforming activity was further demonstrated by the ability of the FBXW7 D510E mutant to provide IL-2-independent growth of Tax-immortalized human T cells and increase the tumor formation in a xenograft mouse model of ATL. This study suggests that FBXW7, normally a tumor suppressor, can act as an oncogene when mutated and may play an important role in the pathogenesis of ATL.

Deregulated ERK1/2 MAP kinase signaling promotes aneuploidy by a Fbxw7β-Aurora A pathway

Aneuploidy is a common feature of human solid tumors and is often associated with poor prognosis. There is growing evidence that oncogenic signaling pathways, which are universally dysregulated in cancer, contribute to the promotion of aneuploidy. However, the mechanisms connecting signaling pathways to the execution of mitosis and cytokinesis are not well understood. Here, we show that hyperactivation of the ERK1/2 MAP kinase pathway in epithelial cells impairs cytokinesis, leading to polyploidization and aneuploidy. Mechanistically, deregulated ERK1/2 signaling specifically downregulates expression of the F-box protein Fbxw7β, a substrate-binding subunit of the SCF(Fbxw7) ubiquitin ligase, resulting in the accumulation of the mitotic kinase Aurora A. Reduction of Aurora A levels by RNA interference or pharmacological inhibition of MEK1/2 reverts the defect in cytokinesis and decreases the frequency of abnormal cell divisions induced by oncogenic H-Ras(V12). Reciprocally, overexpression of Aurora A or silencing of Fbxw7β phenocopies the effect of H-Ras(V12) on cell division. In vivo, conditional activation of MEK2 in the mouse intestine lowers Fbxw7β expression, resulting in the accumulation of cells with enlarged nuclei. We propose that the ERK1/2/ Fbxw7β/Aurora A axis identified in this study contributes to genomic instability and tumor progression.

Fbxw7-associated drug resistance is reversed by induction of terminal differentiation in murine intestinal organoid culture

Colorectal cancer (CRC) is one of the top three cancer-related causes of death worldwide. FBXW7 is a known tumor-suppressor gene, commonly mutated in CRC and in a variety of other epithelial tumors. Low expression of FBXW7 is also associated with poor prognosis. Loss of FBXW7 sensitizes cancer cells to certain drugs, while making them more resistant to other types of chemotherapies. However, is not fully understood how epithelial cells within normal gut and primary tumors respond to potential cancer therapeutics. We have studied genetically engineered mice in which the fbxw7 gene is conditionally knocked-out in the intestine (fbxw7^∆G). To further investigate the mechanism of Fbxw7-action, we grew intestinal crypts from floxed-fbxw7 (fbxw7^fl/fl) and fbxw7^ΔG mice, in a Matrigel-based organoid (mini-gut) culture. The fbxw7^ΔG organoids exhibited rapid budding events in the crypt region. Furthermore, to test organoids for drug response, we exposed day 3 intestinal organoids from fbxw7^fl/fl and fbxw7^∆G mice, to various concentrations of 5-fluorouracil (5-FU) for 72 hours. 5-FU triggers phenotypic differences in organoids including changing shape, survival, resistance, and death. 5-FU however, rescues the drug-resistance phenotype of fbxw7^ΔG through the induction of terminal differentiation. Our results support the hypothesis that a differentiating therapy successfully targets FBXW7-mutated CRC cells.

FBXW7-mutated colorectal cancer cells exhibit aberrant expression of phosphorylated-p53 at Serine-15

FBXW7 mutations occur in a variety of human cancers including colorectal cancer (CRC). Elucidating its mechanism of action has become crucial for cancer therapy; however, it is also complicated by the fact that FBXW7 can influence many pathways due to its role as an E3-ubiquitin ligase in proteasome degradation. FBXW7 and TP53 are tumour suppressors intensively implicated in colorectal carcinogenesis. Deletion mutations in these two genes in animal models mark the progression from adenoma to carcinoma. Although still largely unknown, the last defense mechanism against CRC at the molecular level could be through a synergistic effect of the two genes. The underlying mechanism requires further investigation. In our laboratory, we have used a phospho-kinase profiler array to illustrate a potential molecular link between FBXW7 and p53 in CRC cells. In vitro and in vivo assessments demonstrated aberrant induction of phosphorylated p53 at Serine 15 [phospho-p53(Ser15)] in human FBXW7-deficient CRC cells as compared to their FBXW7-wild-type counterparts. FBXW7 loss in HCT116 cells promoted resistance to oxaliplatin. Immunoblotting data further confirmed that reduction of phospho-p53(Ser15) may contribute to the decreased efficacy of therapy in FBXW7-mutated CRC cells. The findings may suggest the applicability of phospho-p53(Ser15) as an indicative marker of FBXW7-mutations. Phospho-p53(Ser15) regulation by FBXW7 E3-ligase activity could provide important clues for understanding FBXW7 behavior in tumour progression and grounds for its clinical applicability thereafter.

Fbxw7 Tumor Suppressor: A Vital Regulator Contributes to Human Tumorigenesis

Rapidly accumulating data indicate that F-box/WD repeat-containing protein 7 (Fbxw7) is one of the most frequently mutated genes in human cancers and regulates a network of crucial oncoproteins. These studies have generated important new insights into tumorigenesis and may soon enable therapies targeting the Fbxw7 pathway. We searched PubMed, Embase, and ISI Web of Science databases (1973-2015, especially recent 5 years) for articles published in the English language using the key words "Fbxw7," "Fbw7," "hCDC4," and "Sel-10," and we reviewed recent developments in the search for Fbxw7. Fbxw7 coordinates the ubiquitin-dependent proteolysis of several critical cellular regulators, thereby controlling essential processes, such as cell cycle, differentiation, and apoptosis. Fbxw7 contains 3 isoforms (Fbxw7α, Fbxw7β, and Fbxw7γ), and they are differently regulated in subtract recognition. Besides those, Fbxw7 activity is controlled at different levels, resulting in specific and tunable regulation of the abundance and activity of its substrates in a variety of human solid tumor types, including glioma malignancy, nasopharyngeal carcinoma, osteosarcoma, melanoma as well as colorectal, lung, breast, gastric, liver, pancreatic, renal, prostate, endometrial, and esophageal cancers. Fbxw7 is strongly associated with tumorigenesis, and the mechanisms and consequences of Fbxw7 deregulation in cancers may soon enable the development of novel therapeutic approaches.

Regulating Fbw7 on the road to cancer

The F-box protein Fbw7 targets for degradation critical cellular regulators, thereby controlling essential processes in cellular homeostasis, including cell cycle, differentiation and apoptosis. Most Fbw7 substrates are strongly associated with tumorigenesis and Fbw7 can either suppress or promote tumor development in mouse models. Fbw7 activity is controlled at different levels, resulting in specific and tunable regulation of the abundance and activity of its substrates. Here we highlight recent studies on the role of Fbw7 in controlling tumorigenesis and on the mechanisms that modulate Fbw7 function.

Emerging roles for the FBXW7 ubiquitin ligase in leukemia and beyond

Protein degradation plays key roles in diverse pathways in cell division, growth and differentiation. Aberrant stabilization of crucial proteins participating in oncogenic pathways is often observed in cancer. The importance of proper protein turnover is exemplified by the SCF(Fbxw7) ubiquitin ligase, which is frequently mutated in human cancer, including T cell acute lymphoblastic leukemia. Recent studies have revealed novel substrates of Fbxw7 and shed light on its role on differentiation of stem cells and expansion of stem-cell-like cells driving tumorigenesis. Detailed understanding of the contribution of the Fbxw7-regulated network of proteins in initiation and progression of cancer will facilitate the identification of candidate intervention targets in human cancer.

Dependence of Human Colorectal Cells Lacking the FBW7 Tumor Suppressor on the Spindle Assembly Checkpoint

FBW7 (F-box and WD repeat domain containing 7), also known as FBXW7 or hCDC4, is a tumor suppressor gene mutated in a broad spectrum of cancer cell types. As a component of the SCF E3 ubiquitin ligase, FBW7 is responsible for specifically recognizing phosphorylated substrates, many important for tumor progression, and targeting them for ubiquitin-mediated degradation. Although the role of FBW7 as a tumor suppressor is well established, less well studied is how FBW7-mutated cancer cells might be targeted for selective killing. To explore this further, we undertook a genome-wide RNAi screen using WT and FBW7 knockout colorectal cell lines and identified the spindle assembly checkpoint (SAC) protein BUBR1, as a candidate synthetic lethal target. We show here that asynchronous FBW7 knockout cells have increased levels of mitotic APC/C substrates and are sensitive to knockdown of not just BUBR1 but BUB1 and MPS1, other known SAC components, suggesting a dependence of these cells on the mitotic checkpoint. Consistent with this dependence, knockdown of BUBR1 in cells lacking FBW7 results in significant cell aneuploidy and increases in p53 levels. The FBW7 substrate cyclin E was necessary for the genetic interaction with BUBR1. In contrast, the establishment of this dependence on the SAC requires the deregulation of multiple substrates of FBW7. Our work suggests that FBW7 knockout cells are vulnerable in their dependence on the mitotic checkpoint and that this may be a good potential target to exploit in FBW7-mutated cancer cells.

Frequency of mitogen-activated protein kinase and phosphoinositide 3-kinase signaling pathway pathogenic alterations in EUS-FNA sampled malignant lymph nodes in rectal cancer with theranostic potential

BACKGROUND

Targeted next-generation sequencing has the potential to stratify a tumor by molecular subtype and aid the development of a biomarker profile for prognostic risk stratification and theranostic potential.

OBJECTIVE

To assess the frequency and distribution of pathogenic alterations in malignant lymph node cytology specimens.

DESIGN

Multigene molecular profiling of archived malignant EUS-FNA lymph node cytology specimens using the Ion Ampliseq Cancer Hotspot Panel v2, which targets at least 2855 possible mutations within 50 cancer-associated genes.

SETTING

Single tertiary referral center.

PATIENTS

Sporadic, treatment naive, locally advanced primary rectal cancer by EUS-FNA (n = 76) who subsequently completed neoadjuvant therapy with on-site oncologic surgery.

MAIN OUTCOME MEASUREMENTS

The frequency and distribution of pathogenic alterations in malignant lymph node cytology specimens by the mitogen-activated protein kinase (MAPK) or phosphoinositide 3-kinase (PI3K) signaling pathways, by KRAS or NRAS wild-type lymph node status, by extramesenteric lymph node status, and by a complete pathologic response status.

RESULTS

Eleven patients (14.5%) were 50-gene panel wild-type. Sixty-five patients had 139 pathogenic alterations (2 [1-3] per patient) in 13 of 50 evaluated genes. The following represent a spectrum of identified alterations: TP53 (n = 52; 68.4%), APC (n = 36; 47.4%), KRAS (n = 22; 28.9%), FBXW7 (n = 8; 10.5%), NRAS (n = 6; 7.9%), PIK3CA (n = 4; 5.3%), SMAD4 (n = 3; 3.9%), and BRAF (n = 3; 3.9%). Pathogenic alterations were identified in the MAPK and PI3K signaling pathways in 41% and 5% of patients, respectively.

LIMITATIONS

Findings were limited to a 50 cancer-associated gene analysis.

CONCLUSIONS

Molecular EUS lymph node assessments using cancer "hotspot" panels can identify pathogenic alteration frequency and distribution and have theranostic potential for individualized patient care.

Genetic mutations in human rectal cancers detected by targeted sequencing

Colorectal cancer (CRC) is widespread with significant mortality. Both inherited and sporadic mutations in various signaling pathways influence the development and progression of the cancer. Identifying genetic mutations in CRC is important for optimal patient treatment and many approaches currently exist to uncover these mutations, including next-generation sequencing (NGS) and commercially available kits. In the present study, we used a semiconductor-based targeted DNA-sequencing approach to sequence and identify genetic mutations in 91 human rectal cancer samples. Analysis revealed frequent mutations in KRAS (58.2%), TP53 (28.6%), APC (16.5%), FBXW7 (9.9%) and PIK3CA (9.9%), and additional mutations in BRAF, CTNNB1, ERBB2 and SMAD4 were also detected at lesser frequencies. Thirty-eight samples (41.8%) also contained two or more mutations, with common combination mutations occurring between KRAS and TP53 (42.1%), and KRAS and APC (31.6%). DNA sequencing for individual cancers is of clinical importance for targeted drug therapy and the advantages of such targeted gene sequencing over other NGS platforms or commercially available kits in sensitivity, cost and time effectiveness may aid clinicians in treating CRC patients in the near future.