Recent insight into the role of FBXW7 as a tumor suppressor

Critical roles of the E3 ubiquitin ligase FBW7 in B-cell response and the pathogenesis of experimental autoimmune arthritis

Proper regulation of B-cell function is essential for effective humoral immunity and maintenance of immune tolerance. Here, we found that FBW7 (F-box/WD40 repeat-containing protein 7) is highly expressed in germinal centre B and B1 cells, and confirmed that it has an intrinsic role in maintaining homeostasis of mature B cells and B-1 cells. FBW7 deletion led to an impairment of antibody response, and although germinal centre formation was not affected, antibody class-switch recombination and affinity maturation processes were defective. Likewise, memory immune response was severely impaired. Moreover, FBW7 ablation ameliorated the pathogenesis of an autoimmune disease model, collagen-induced arthritis, by reducing the production of anti-collagen II autoantibodies. Taken together, these data suggest that FBW7 may be an attractive target for developing new therapeutics for the treatment of autoimmune diseases.

Clinical significance of FBXW7 tumor suppressor gene mutations and expression in human colorectal cancer: a systemic review and meta-analysis

BACKGROUND

Various studies investigating the clinical significance of FBXW7 mutation and/or expression have yielded inconclusive results in colorectal cancer (CRC) patients. Therefore, the present meta-analysis summarizes previous evidence and evaluates the clinical significance, including the prognostic role, of FBXW7 status in CRCs.

METHODS

The meta-analysis was conducted by searching the databases of PubMed, China National Knowledge Infrastructure (CNKI), WANFANG data, Web of Science, Embase, and Web of Science. Pooled odds ratios (ORs) and hazard ratios (HRs) and corresponding 95% confidence intervals (CIs) were calculated to assess the relationships between FBXW7 status and clinicopathological features and survival in CRC, respectively.

RESULTS

Ten studies involving 4199 patients met the inclusion criteria and included in our meta-analysis. FBXW7 mutation/low expression was obviously correlated with advanced T stage (OR = 0.44, 95% CI: 0.27-0.74, P <  0.01) and lymph node metastasis (OR = 1.88, 95% CI: 1.40-2.53, P <  0.01), but was not associated with other parameters. Further investigation found that FBXW7 mutation/low expression predicted poor OS (HR = 1.25, 95% CI: 1.06-1.47, P <  0.01), but not DFS in CRC (HR = 1.04, 95% CI: 0.60-1.82, P = 0.88). Subgroup analysis found that FBXW7 status was obviously correlated with OS in cohorts recruited after 2009 (HR = 1.32, 95% CI: 1.17-1.50, P <  0.01), from eastern Asia (HR = 1.27, 95% CI: 1.04-1.55, P = 0.02), detected by immunohistochemistry/qRT-PCR (HR = 1.39, 95% CI: 1.22-1.59, P <  0.01), and analysed with multivariate method (HR = 1.47, 95% CI: 1.25-1.74, P <  0.01).

CONCLUSIONS

This study indicates that FBXW7 status, expression level especially, is associated with OS but not DFS in CRC. FBXW7 expression level may function as a prognostic biomarker in CRC.

Molecular Landscape of Vulvar Squamous Cell Carcinoma

Vulvar squamous cell carcinoma (VSCC) is a rare malignancy with dual pathogenesis, Human papillomavirus (HPV)-associated and HPV-independent, with a poorly explored molecular landscape. We aimed to summarize the findings of the series analyzing molecular hallmarks of this neoplasm. In January 2021, we conducted a comprehensive literature search using Pubmed Medline and Scopus to identify publications focused on genomic profiling of VSCC. Observational studies, including both prospective and retrospective designs, evaluating molecular alterations in VSCC were deemed eligible. A total of 14 studies analyzing 749 VSCC were identified. The study series were heterogeneous in HPV testing and sequencing strategies, included small sets of tumors and cancer genes, and commonly lacked survival analysis. Only one extensive targeted next-generation sequencing-based study comprised a large cohort of 280 VSCC. The mutated genes, their number, and frequencies were highly variable between the series. Overall, TP53 and CDKN2A, followed by PIK3CA, HRAS, and PTEN, were the most frequently studied and mutated genes. Mutations involved in the PI3K/AKT/mTOR pathway, including TP53, HRAS, KRAS, and PIK3CA, have been consistently reported across the studies. However, the role of individual mutations or pathways in the development of VSCC remains unclear. In conclusion, heterogeneity and the small sample size of available molecular series contribute to a limited view of the molecular landscape of VSCC. Large-scale genome- or exome-wide studies with robust HPV testing are necessary to improve the molecular characterization of VSCC.

Overcoming anti-cancer drug resistance via restoration of tumor suppressor gene function

The cytotoxic anti-cancer drugs cisplatin, paclitaxel, doxorubicin, 5-fluorouracil (5-FU), as well as targeted drugs including imatinib, erlotinib, and nivolumab, play key roles in clinical cancer treatment. However, the frequent emergence of drug resistance severely comprosises their anti-cancer efficacy. A number of studies indicated that loss of function of tumor suppressor genes (TSGs) is involved in the development of cancer drug resistance, apart from decreased drug influx, increased drug efflux, induction of anti-apoptosis mechanisms, alterations in tumor microenvironment, drug compartmentalization, enhanced DNA repair and drug inactivation. TSGs are involved in the pathogenesis of tumor formation through regulation of DNA damage repair, cell apoptosis, autophagy, proliferation, cell cycle progression, and signal transduction. Our increased understanding of TSGs in the past decades demonstrates that gene mutation is not the only reason that leads to the inactivation of TSGs. Loss of function of TSGs may be based on the ubiquitin-proteasome pathway, epigenetic and transcriptional regualtion, post-translation modifications like phosphorylation as well as cellular translocation of TSGs. As the above processes can constitute"druggable targets", these mechanisms provide novel therapeutic approaches in targeting TSGs. Some small molecule compounds targeting these approaches re-activated TSGs and reversed cancer drug resistance. Along this vein, functional restoration of TSGs is a novel and promising approach to surmount cancer drug resistance. In the current review, we draw a scenario based on the role of loss of function of TSGs in drug resistance, on mechanisms leading to inactivation of TSGs and on pharmacological agents acting on these mechanisms to overcome cancer drug resistance. This review discusses novel therapeutic strategies targeting TSGs and offers possible modalities to conquer cancer drug resistance.

The roles and regulation of the KLF5 transcription factor in cancers

Krüppel‐like factor 5 (KLF5) is a member of the KLF family. Recent studies have suggested that KLF5 regulates the expression of a large number of new target genes and participates in diverse cellular functions, such as stemness, proliferation, apoptosis, autophagy, and migration. In response to multiple signaling pathways, various transcriptional modulation and posttranslational modifications affect the expression level and activity of KLF5. Several transgenic mouse models have revealed the physiological and pathological functions of KLF5 in different cancers. Studies of KLF5 will provide prognostic biomarkers, therapeutic targets, and potential drugs for cancers.

Notch signaling in cancer: Complexity and challenges on the path to clinical translation

Notch receptors participate in a conserved pathway in which ligands expressed on neighboring cells trigger a series of proteolytic cleavages that allow the intracellular portion of the receptor to travel to the nucleus and form a short-lived transcription complex that turns on target gene expression. The directness and seeming simplicity of this signaling mechanism belies the complexity of the outcomes of Notch signaling in normal cells, which are highly context and dosage dependent. This complexity is reflected in the diverse roles of Notch in cancers of various types, in which Notch may be oncogenic or tumor suppressive and may have a wide spectrum of effects on tumor cells and stromal elements. This review provides an overview of the roles of Notch in cancer and discusses challenges to clinical translation of Notch targeting agents as well as approaches that may overcome these hurdles.

The FBXW7-NOTCH interactome: A ubiquitin proteasomal system-induced crosstalk modulating oncogenic transformation in human tissues

BACKGROUND

Ubiquitin ligases or E3 ligases are well programmed to regulate molecular interactions that operate at a post-translational level. Skp, Cullin, F-box containing complex (or SCF complex) is a multidomain E3 ligase known to mediate the degradation of a wide range of proteins through the proteasomal pathway. The three-dimensional domain architecture of SCF family proteins suggests that it operates through a novel and adaptable "super-enzymatic" process that might respond to targeted therapeutic modalities in cancer.

RECENT FINDINGS

Several F-box containing proteins have been characterized either as tumor suppressors (FBXW8, FBXL3, FBXW8, FBXL3, FBXO1, FBXO4, and FBXO18) or as oncogenes (FBXO5, FBXO9, and SKP2). Besides, F-box members like βTrcP1 and βTrcP2, the ones with context-dependent functionality, have also been studied and reported. FBXW7 is a well-studied F-box protein and is a tumor suppressor. FBXW7 regulates the activity of a range of substrates, such as c-Myc, cyclin E, mTOR, c-Jun, NOTCH, myeloid cell leukemia sequence-1 (MCL1), AURKA, NOTCH through the well-known ubiquitin-proteasome system (UPS)-mediated degradation pathway. NOTCH signaling is a primitive pathway that plays a crucial role in maintaining normal tissue homeostasis. FBXW7 regulates NOTCH protein activity by controlling its half-life, thereby maintaining optimum protein levels in tissue. However, aberrations in the FBXW7 or NOTCH expression levels can lead to poor prognosis and detrimental outcomes in patients. Therefore, the FBXW7-NOTCH axis has been a subject of intense study and research over the years, especially around the interactome's role in driving cancer development and progression. Several studies have reported the effect of FBXW7 and NOTCH mutations on normal tissue behavior. The current review attempts to critically analyze these mutations prognostic value in a wide range of tumors. Furthermore, the review summarizes the recent findings pertaining to the FBXW7 and NOTCH interactome and its involvement in phosphorylation-related events, cell cycle, proliferation, apoptosis, and metastasis.

CONCLUSION

The review concludes by positioning FBXW7 as an effective diagnostic marker in tumors and by listing out recent advancements made in cancer therapeutics in identifying protocols targeting the FBXW7-NOTCH aberrations in tumors.

Dysregulation of the Ubiquitin Proteasome System in Human Malignancies: A Window for Therapeutic Intervention

Simple Summary

The ubiquitin proteasome system (UPS) governs the non-lysosomal degradation of oxidized, damaged, or misfolded proteins in eukaryotic cells. Dysregulation of the UPS results in loss of ability to maintain protein quality through proteolysis, and is closely related to the development of various malignancies and tumorigenesis. Here, we provide a comprehensive general overview on the regulation and roles of UPS and discuss the mechanisms linking dysregulated UPS to human malignancies. Inhibitors developed against components of the UPS, which include U.S. Food and Drug Administration FDA-approved and those currently undergoing clinical trials, are also presented in this review.

Abstract

The ubiquitin proteasome system (UPS) governs the non-lysosomal degradation of oxidized, damaged, or misfolded proteins in eukaryotic cells. This process is tightly regulated through the activation and transfer of polyubiquitin chains to target proteins which are then recognized and degraded by the 26S proteasome complex. The role of UPS is crucial in regulating protein levels through degradation to maintain fundamental cellular processes such as growth, division, signal transduction, and stress response. Dysregulation of the UPS, resulting in loss of ability to maintain protein quality through proteolysis, is closely related to the development of various malignancies and tumorigenesis. Here, we provide a comprehensive general overview on the regulation and roles of UPS and discuss functional links of dysregulated UPS in human malignancies. Inhibitors developed against components of the UPS, which include U.S. Food and Drug Administration FDA-approved and those currently undergoing clinical trials, are also presented in this review.

METTL3-mediated m6A mRNA modification of FBXW7 suppresses lung adenocarcinoma

Background

FBXW7 m⁶A modification plays an important role in lung adenocarcinoma (LUAD) progression; however, the underlying mechanisms remain unclear.

Methods

The correlation between FBXW7 and various genes related to m⁶A modification was analyzed using The Cancer Genome Atlas database. The regulatory effects of METTL3 on FBXW7 mRNA m⁶A modification were examined in a cell model, and the underlying mechanism was determined by methylated RNA immunoprecipitation, RNA immunoprecipitation, luciferase reporter, and mutagenesis assays. In vitro experiments were performed to further explore the biological effects of METTL3-mediated FBXW7 m⁶A modification on LUAD development.

Results

Decreased FBXW7 expression was accompanied by downregulated METTL3 expression in human LUAD tissues and was associated with a worse prognosis for LUAD in The Cancer Genome Atlas database. m⁶A was highly enriched in METTL3-mediated FBXW7 transcripts, and increased m⁶A modification in the coding sequence region increased its translation. Functionally, METTL3 overexpression or knockdown affected the apoptosis and proliferation phenotype of LUAD cells by regulating FBXW7 m⁶A modification and expression. Furthermore, FBXW7 overexpression in METTL3-depleted cells partially restored LUAD cell suppression in vitro and in vivo.

Conclusions

Our findings reveal that METTL3 positively regulates FBXW7 expression and confirm the tumor-suppressive role of m⁶A-modified FBXW7, thus providing insight into its epigenetic regulatory mechanisms in LUAD initiation and development.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-021-01880-3.

FBXW7 mediates high glucose‑induced SREBP‑1 expression in renal tubular cells of diabetic nephropathy under PI3K/Akt pathway regulation

Diabetic nephropathy (DN) is a severe complication of diabetes mellitus and lipid metabolism abnormality serves a key role in the pathogenesis of DN. Sterol regulatory element-binding protein 1 (SREBP-1) overexpression mediates aberrant lipid accumulation in renal tubular cells of DN. However, the exact mechanism involved in increased SREBP-1 has not been fully elucidated. The aim of the present study was to explore the mechanism involved in SREBP-1 upregulation. Diabetic mice and high glucose-cultured HKC cells were chosen to detect the expression of FBXW7 and SREBP-1 using immunohistochemistry, western blotting and PCR. The present study demonstrated that F-box and WD repeat domain containing 7 (FBXW7) expression was decreased in renal tubular cells of diabetic mice. Moreover, the co-expression of FBXW7 and SREBP-1 was observed in renal tubular cells, but not in the glomeruli. High glucose-induced the downregulation of FBXW7 expression in in vitro cultured HKC cells, which was accompanied by SREBP-1 upregulation. In addition, overexpression of FBXW7 in HKC cells led to SREBP-1 downregulation. By contrast, knockdown of FBXW7 caused SREBP-1 upregulation in HKC cells. It was found that the PI3K/Akt signaling pathway was activated in high glucose-stimulated HKC cells, and inhibition of PI3K/Akt pathway using LY294002 increased FBXW7 expression and decreased SREBP-1 expression. Taken together, the present results suggested that FBXW7 mediated high glucose-induced SREBP-1 expression in renal tubular cells of DN, under the regulation of the PI3K/Akt signaling pathway.

MYC as a target for cancer treatment

The MYC gene which consists of 3 paralogs, C-MYC, N-MYC and L-MYC, is one of the most frequently deregulated driver genes in human cancer. Because of its high prevalence of deregulation and its causal role in cancer formation, maintenance and progression, targeting MYC is theoretically an attractive strategy for treating cancer. As a potential anticancer target, MYC was traditionally regarded as undruggable due to the absence of a suitable pocket for high-affinity binding by low molecular weight inhibitors. In recent years however, several compounds that directly or indirectly inhibit MYC have been shown to have anticancer activity in preclinical tumor models. Amongst the most detailed investigated strategies for targeting MYC are inhibition of its binding to its obligate interaction partner MAX, prevention of MYC expression and blocking of genes exhibiting synthetic lethality with overexpression of MYC. One of the most extensively investigated MYC inhibitors is a peptide/mini-protein known as OmoMYC. OmoMYC, which acts by blocking the binding of all 3 forms of MYC to their target promoters, has been shown to exhibit anticancer activity in a diverse range of preclinical models, with minimal side effects. Based on its broad efficacy and limited toxicity, OmoMYC is currently being developed for evaluation in clinical trials. Although no compound directly targeting MYC has yet progressed to clinical testing, APTO-253, which partly acts by decreasing expression of MYC, is currently undergoing a phase I clinical trial in patients with relapsed/refractory acute myeloid leukemia or myelodysplastic syndrome.

Negative feedback by conserved kinases patterns the degradation of Caenorhabditis elegans Raf in vulval fate patterning

Activation of a canonical EGFR-Ras-Raf-ERK cascade initiates patterning of multipotent vulval precursor cells (VPCs) of Caenorhabditis elegans We have previously shown that this pathway includes a negative-feedback component in which MPK-1/ERK activity targets the upstream kinase LIN-45/Raf for degradation by the SEL-10/FBXW7 E3 ubiquitin ligase. This regulation requires a Cdc4 phosphodegron (CPD) in LIN-45 that is conserved in BRAF. Here, we identify and characterize the minimal degron that encompasses the CPD and is sufficient for SEL-10-mediated, MPK-1-dependent protein degradation. A targeted screen of conserved protein kinase-encoding genes yielded gsk-3 (an ortholog of human GSK3B) and cdk-2 (a CDK2-related kinase) as required for LIN-45 degron-mediated turnover. Genetic analysis revealed that LIN-45 degradation is blocked at the second larval stage due to cell cycle quiescence, and that relief of this block during the third larval stage relies on activation of CDKs. Additionally, activation of MPK-1 provides spatial pattern to LIN-45 degradation but does not bypass the requirement for gsk-3 and cdk-2 This analysis supports a model whereby MPK-1/ERK, GSK-3/GSK3 and CDK-2/CDK2, along with SEL-10/FBXW7, constitute a regulatory network that exerts spatial and temporal control of LIN-45/Raf degradation during VPC patterning.

A Highlight of the Mechanisms of Immune Checkpoint Blocker Resistance

In recent years, as our understanding of tumor immunology is continuously improved, immunotherapy has come to the center stage of cancer therapy and is deemed as the most promising approach for cancer control. Although immunotherapy, particularly immune checkpoint blockade (ICB), has achieved a milestone in several types of tumors, the majority of cancer patients do not benefit from immunotherapy. The dismal outcome of cancer immunotherapy is mainly due to primary or acquired resistance arising from tumor immune evasion. Exploring the mechanisms of tumor immune evasion in the course of immunotherapy may identify biological targets to conquer tumor resistance to immunotherapy. In this review, we highlight tumor cell-intrinsic and -extrinsic factors that may underlie tumor resistance to immune checkpoint blockers. Targeting these factors in combination with immune checkpoint blockers points to the future direction of cancer immunotherapy.

Loss of Fbxw7 Impairs Development of and Induces Heterogeneous Tumor Formation in the Mouse Mammary Gland

Fbxw7 is an F-box protein that contributes to regulation of cell proliferation and cell fate determination as well as to tumor suppression in various tissues. In this study, we generated mice with mammary gland-specific ablation of Fbxw7 (Blg-Cre/Fbxw7 ^F/F mice) and found that most neonates born to mutant dams die soon after birth as a result of defective maternal lactation. The mammary gland of mutant dams was markedly atrophic and manifested both excessive cell proliferation and apoptosis in association with the accumulation of Notch1 and p63. Despite the hypoplastic nature of the mutant mammary gland, Blg-Cre/Fbxw7 ^F/F mice spontaneously developed mammary tumors that resembled basal-like carcinoma with marked intratumoral heterogeneity. Additional inactivation of Trp53 in Blg-Cre/Fbxw7 ^F/F mice further promoted onset and development of mammary tumors, suggesting that spontaneous mutation of Trp53 may facilitate transition of hypoplastic mammary lesions to aggressive cancer in mice lacking Fbxw7. RNA-sequencing analysis of epithelial- and mesenchymal-like cell lines from a Blg-Cre/Fbxw7 ^F/F mouse tumor revealed an increased mutation rate and structural alterations in the tumor and differential expression of upstream transcription factors including known targets of Fbxw7. Together, our results implicate Fbxw7 in the regulation of cell differentiation and in tumor suppression in the mammary gland. Loss of Fbxw7 increases mutation rate and chromosome instability, activates signaling pathways governed by transcription factors regulated by Fbxw7, and triggers the development of mammary tumors with prominent heterogeneity. SIGNIFICANCE: Mammary gland-specific ablation of Fbxw7 in mice results in defective gland development and spontaneous mammary tumor formation reminiscent of human basal-like carcinoma with intratumoral heterogeneity. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/24/5515/F1.large.jpg.

The Effect of Dysfunctional Ubiquitin Enzymes in the Pathogenesis of Most Common Diseases

Ubiquitination is a multi-step enzymatic process that involves the marking of a substrate protein by bonding a ubiquitin and protein for proteolytic degradation mainly via the ubiquitin–proteasome system (UPS). The process is regulated by three main types of enzymes, namely ubiquitin-activating enzymes (E1), ubiquitin-conjugating enzymes (E2), and ubiquitin ligases (E3). Under physiological conditions, ubiquitination is highly reversible reaction, and deubiquitinases or deubiquitinating enzymes (DUBs) can reverse the effect of E3 ligases by the removal of ubiquitin from substrate proteins, thus maintaining the protein quality control and homeostasis in the cell. The dysfunction or dysregulation of these multi-step reactions is closely related to pathogenic conditions; therefore, understanding the role of ubiquitination in diseases is highly valuable for therapeutic approaches. In this review, we first provide an overview of the molecular mechanism of ubiquitination and UPS; then, we attempt to summarize the most common diseases affecting the dysfunction or dysregulation of these mechanisms.

An Integrative Synthetic Biology Approach to Interrogating Cellular Ubiquitin and Ufm Signaling

Global identification of substrates for PTMs (post-translational modifications) represents a critical but yet dauntingly challenging task in understanding biology and disease pathology. Here we presented a synthetic biology approach, namely ‘YESS’, which coupled Y2H (yeast two hybrid) interactome screening with PTMs reactions reconstituted in bacteria for substrates identification and validation, followed by the functional validation in mammalian cells. Specifically, the sequence-independent Gateway^® cloning technique was adopted to afford simultaneous transfer of multiple hit ORFs (open reading frames) between the YESS sub-systems. In proof-of-evidence applications of YESS, novel substrates were identified for UBE3A and UFL1, the E3 ligases for ubiquitination and ufmylation, respectively. Therefore, the YESS approach could serve as a potentially powerful tool to study cellular signaling mediated by different PTMs.

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.

F-Box Proteins and Cancer

Controlled protein degradation is essential for the operation of a variety of cellular processes including cell division, growth, and differentiation. Identification of the relations between ubiquitin ligases and their substrates is key to understanding the molecular basis of cancer development and to the discovery of novel targets for cancer therapeutics. F-box proteins function as the substrate recognition subunits of S-phase kinase-associated protein 1 (SKP1)−Cullin1 (CUL1)−F-box protein (SCF) ubiquitin ligase complexes. Here, we summarize the roles of specific F-box proteins that have been shown to function as tumor promoters or suppressors. We also highlight proto-oncoproteins that are targeted for ubiquitylation by multiple F-box proteins, and discuss how these F-box proteins are deployed to regulate their cognate substrates in various situations.