PI3K-dependent phosphorylation of Fbw7 modulates substrate degradation and activity

Molecular insights and clinical implications for the tumor suppressor role of SCFFBXW7 E3 ubiquitin ligase

FBXW7 is one of the most well-characterized F-box proteins, serving as substrate receptor subunit of SKP1-CUL1-F-box (SCF) E3 ligase complexes. SCFFBXW7 is responsible for the degradation of various oncogenic proteins such as cyclin E, c-MYC, c-JUN, NOTCH, and MCL1. Therefore, FBXW7 functions largely as a major tumor suppressor. In keeping with this notion, FBXW7 gene mutations or downregulations have been found and reported in many types of malignant tumors, such as endometrial, colorectal, lung, and breast cancers, which facilitate the proliferation, invasion, migration, and drug resistance of cancer cells. Therefore, it is critical to review newly identified FBXW7 regulation and tumor suppressor function under physiological and pathological conditions to develop effective strategies for the treatment of FBXW7-altered cancers. Since a growing body of evidence has revealed the tumor-suppressive activity and role of FBXW7, here, we updated FBXW7 upstream and downstream signaling including FBXW7 ubiquitin substrates, the multi-level FBXW7 regulatory mechanisms, and dysregulation of FBXW7 in cancer, and discussed promising cancer therapies targeting FBXW7 regulators and downstream effectors, to provide a comprehensive picture of FBXW7 and facilitate the study in this field.

FBXW7 in breast cancer: mechanism of action and therapeutic potential

Breast cancer is one of the frequent tumors that seriously endanger the physical and mental well-being in women. F-box and WD repeat domain-containing 7 (FBXW7) is a neoplastic repressor. Serving as a substrate recognition element for ubiquitin ligase, FBXW7 participates in the ubiquitin-proteasome system and is typically in charge of the ubiquitination and destruction of crucial oncogenic proteins, further performing a paramount role in cell differentiation, apoptosis and metabolic processes. Low levels of FBXW7 cause abnormal stability of pertinent substrates, mutations and/or deletions in the FBXW7 gene have been reported to correlate with breast cancer malignant progression and chemoresistance. Given the lack of an effective solution to breast cancer's clinical drug resistance dilemma, elucidating FBXW7's mechanism of action could provide a theoretical basis for targeted drug exploration. Therefore, in this review, we focused on FBXW7's role in a range of breast cancer malignant behaviors and summarized the pertinent cellular targets, signaling pathways, as well as the mechanisms regulating FBXW7 expression. We also proposed novel perspectives for the exploitation of alternative therapies and specific tumor markers for breast cancer by therapeutic strategies aiming at FBXW7.

The Role of FBXW7 in Gynecologic Malignancies

The F-Box and WD Repeat Domain Containing 7 (FBXW7) protein has been shown to regulate cellular growth and act as a tumor suppressor. This protein, also known as FBW7, hCDC4, SEL10 or hAGO, is encoded by the gene FBXW7. It is a crucial component of the Skp1-Cullin1-F-box (SCF) complex, which is a ubiquitin ligase. This complex aids in the degradation of many oncoproteins, such as cyclin E, c-JUN, c-MYC, NOTCH, and MCL1, via the ubiquitin-proteasome system (UPS). The FBXW7 gene is commonly mutated or deleted in numerous types of cancer, including gynecologic cancers (GCs). Such FBXW7 mutations are linked to a poor prognosis due to increased treatment resistance. Hence, detection of the FBXW7 mutation may possibly be an appropriate diagnostic and prognostic biomarker that plays a central role in determining suitable individualized management. Recent studies also suggest that, under specific circumstances, FBXW7 may act as an oncogene. There is mounting evidence indicating that the aberrant expression of FBXW7 is involved in the development of GCs. The aim of this review is to give an update on the role of FBXW7 as a potential biomarker and also as a therapeutic target for novel treatments, particularly in the management of GCs.

Recent Insight on Regulations of FBXW7 and Its Role in Immunotherapy

SCF^FBXW7 E3 ubiquitin ligase complex is a crucial enzyme of the ubiquitin proteasome system that participates in variant activities of cell process, and its component FBXW7 (F-box and WD repeat domain–containing 7) is responsible for recognizing and binding to substrates. The expression of FBXW7 is controlled by multiple pathways at different levels. FBXW7 facilitates the maturity and function maintenance of immune cells via functioning as a mediator of ubiquitination-dependent degradation of substrate proteins. FBXW7 deficiency or mutation results in the growth disturbance and dysfunction of immune cell, leads to the resistance against immunotherapy, and participates in multiple illnesses. It is likely that FBXW7 coordinating with its regulators and substrates could offer potential targets to improve the sensitivity and effects of immunotherapy. Here, we review the mechanisms of the regulation on FBXW7 and its tumor suppression role in immune filed among various diseases (mostly cancers) to explore novel immune targets and treatments.

Overcoming Steroid Resistance in Pediatric Acute Lymphoblastic Leukemia-The State-of-the-Art Knowledge and Future Prospects

Acute lymphoblastic leukemia (ALL) is the most common malignancy among children. Despite the enormous progress in ALL therapy, resulting in achieving a 5-year survival rate of up to 90%, the ambitious goal of reaching a 100% survival rate is still being pursued. A typical ALL treatment includes three phases: remission induction and consolidation and maintenance, preceded by a prednisone prephase. Poor prednisone response (PPR) is defined as the presence of ≥1.0 × 10⁹ blasts/L in the peripheral blood on day eight of therapy and results in significantly frequent relapses and worse outcomes. Hence, identifying risk factors of steroid resistance and finding methods of overcoming that resistance may significantly improve patients' outcomes. A mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK-ERK) pathway seems to be a particularly attractive target, as its activation leads to steroid resistance via a phosphorylating Bcl-2-interacting mediator of cell death (BIM), which is crucial in the steroid-induced cell death. Several mutations causing activation of MAPK-ERK were discovered, notably the interleukin-7 receptor (IL-7R) pathway mutations in T-cell ALL and rat sarcoma virus (Ras) pathway mutations in precursor B-cell ALL. MAPK-ERK pathway inhibitors were demonstrated to enhance the results of dexamethasone therapy in preclinical ALL studies. This report summarizes steroids' mechanism of action, resistance to treatment, and prospects of steroids therapy in pediatric ALL.

Clinical significance of FBXW7 loss of function in human cancers

FBXW7 (F-Box and WD Repeat Domain Containing 7) (also referred to as FBW7 or hCDC4) is a component of the Skp1-Cdc53 / Cullin-F-box-protein complex (SCF/β-TrCP). As a member of the F-box protein family, FBXW7 serves a role in phosphorylation-dependent ubiquitination and proteasome degradation of oncoproteins that play critical role(s) in oncogenesis. FBXW7 affects many regulatory functions involved in cell survival, cell proliferation, tumor invasion, DNA damage repair, genomic instability and telomere biology. This thorough review of current literature details how FBXW7 expression and functions are regulated through multiple mechanisms and how that ultimately drives tumorigenesis in a wide array of cell types. The clinical significance of FBXW7 is highlighted by the fact that FBXW7 is frequently inactivated in human lung, colon, and hematopoietic cancers. The loss of FBXW7 can serve as an independent prognostic marker and is significantly correlated with the resistance of tumor cells to chemotherapeutic agents and poorer disease outcomes. Recent evidence shows that genetic mutation of FBXW7 differentially affects the degradation of specific cellular targets resulting in a distinct and specific pattern of activation/inactivation of cell signaling pathways. The clinical significance of FBXW7 mutations in the context of tumor development, progression, and resistance to therapies as well as opportunities for targeted therapies is discussed.

Tumor Suppressor FBXW7 and Its Regulation of DNA Damage Response and Repair

The proper DNA damage response (DDR) and repair are the central molecular mechanisms for the maintenance of cellular homeostasis and genomic integrity. The abnormality in this process is frequently observed in human cancers, and is an important contributing factor to cancer development. FBXW7 is an F-box protein serving as the substrate recognition component of SCF (SKP1-CUL1-F-box protein) E3 ubiquitin ligase. By selectively targeting many oncoproteins for proteasome-mediated degradation, FBXW7 acts as a typical tumor suppressor. Recent studies have demonstrated that FBXW7 also plays critical roles in the process of DDR and repair. In this review, we first briefly introduce the processes of protein ubiquitylation by SCF^FBXW7 and DDR/repair, then provide an overview of the molecular characteristics of FBXW7. We next discuss how FBXW7 regulates the process of DDR and repair, and its translational implication. Finally, we propose few future perspectives to further elucidate the role of FBXW7 in regulation of a variety of biological processes and tumorigenesis, and to design a number of approaches for FBXW7 reactivation in a subset of human cancers for potential anticancer therapy.

Implications of FBXW7 in Neurodevelopment and Neurodegeneration: Molecular Mechanisms and Therapeutic Potential

The ubiquitin-proteasome system (UPS) mediated protein degradation is crucial to maintain quantitive and functional homeostasis of diverse proteins. Balanced cellular protein homeostasis controlled by UPS is fundamental to normal neurological functions while impairment of UPS can also lead to some neurodevelopmental and neurodegenerative disorders. Functioning as the substrate recognition component of the SCF-type E3 ubiquitin ligase, FBXW7 is essential to multiple aspects of cellular processes via targeting a wide range of substrates for proteasome-mediated degradation. Accumulated evidence shows that FBXW7 is fundamental to neurological functions and especially implicated in neurodevelopment and the nosogenesis of neurodegeneration. In this review, we describe general features of FBXW7 gene and proteins, and mainly present recent findings that highlight the vital roles and molecular mechanisms of FBXW7 in neurodevelopment such as neurogenesis, myelination and cerebral vasculogenesis and in the pathogenesis of some typical neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease and Huntington’s disease. Additionally, we also provide a prospect on focusing FBXW7 as a potential therapeutic target to rescue neurodevelopmental and neurodegenerative impairment.

Single allele loss-of-function mutations select and sculpt conditional cooperative networks in breast cancer

The most common events in breast cancer (BC) involve chromosome arm losses and gains. Here we describe identification of 1089 gene-centric common insertion sites (gCIS) from transposon-based screens in 8 mouse models of BC. Some gCIS are driver-specific, others driver non-specific, and still others associated with tumor histology. Processes affected by driver-specific and histology-specific mutations include well-known cancer pathways. Driver non-specific gCIS target the Mediator complex, Ca++ signaling, Cyclin D turnover, RNA-metabolism among other processes. Most gCIS show single allele disruption and many map to genomic regions showing high-frequency hemizygous loss in human BC. Two gCIS, Nf1 and Trps1, show synthetic haploinsufficient tumor suppressor activity. Many gCIS act on the same pathway responsible for tumor initiation, thereby selecting and sculpting just enough and just right signaling. These data highlight ~1000 genes with predicted conditional haploinsufficient tumor suppressor function and the potential to promote chromosome arm loss in BC.

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.

Targeting p63 Upregulation Abrogates Resistance to MAPK Inhibitors in Melanoma

Targeting the MAPK pathway by combined inhibition of BRAF and MEK has increased overall survival in advanced BRAF-mutant melanoma in both therapeutic and adjuvant clinical settings. However, a significant proportion of tumors develop acquired resistance, leading to treatment failure. We have previously shown p63 to be an important inhibitor of p53-induced apoptosis in melanoma following genotoxic drug exposure. Here, we investigated the role of p63 in acquired resistance to MAPK inhibition and show that p63 isoforms are upregulated in melanoma cell lines chronically exposed to BRAF and MEK inhibition, with consequent increased resistance to apoptosis. This p63 upregulation was the result of its reduced degradation by the E3 ubiquitin ligase FBXW7. FBXW7 was itself regulated by MDM2, and in therapy-resistant melanoma cell lines, nuclear accumulation of MDM2 caused downregulation of FBXW7 and consequent upregulation of p63. Consistent with this, both FBXW7-inactivating mutations and MDM2 upregulation were found in melanoma clinical samples. Treatment of MAPK inhibitor-resistant melanoma cells with MDM2 inhibitor Nutlin-3A restored FBXW7 expression and p63 degradation in a dose-dependent manner and sensitized these cells to apoptosis. Collectively, these data provide a compelling rationale for future investigation of Nutlin-3A as an approach to abrogate acquired resistance of melanoma to MAPK inhibitor targeted therapy. SIGNIFICANCE: Upregulation of p63, an unreported mechanism of MAPK inhibitor resistance in melanoma, can be abrogated by treatment with the MDM2 inhibitor Nutlin-3A, which may serve as a strategy to overcome resistance.

Ehrlichia chaffeensis TRP120-mediated ubiquitination and proteasomal degradation of tumor suppressor FBW7 increases oncoprotein stability and promotes infection

Ehrlichia chaffeensis (E. chaffeensis) exploits evolutionarily conserved Notch and Wnt host cell signaling pathways to downregulate innate immune host defenses and promote infection. The multifunctional E. chaffeensis TRP120 effector which has HECT E3 ubiquitin ligase activity, interacts with the host nuclear tumor suppressor F-BOX and WD domain repeating-containing 7 (FBW7). FBW7 is the substrate recognition subunit of the Skp1-cullin-1-FBOX E3 ubiquitin (Ub) ligase complex (SCF) known to negatively regulate a network of oncoproteins (Notch, cyclin E, c-Jun, MCL1 and cMYC). In this study, we demonstrate that TRP120 and FBW7 colocalize strongly in the nucleus by confocal immunofluorescent microscopy and interactions between TRP120 and FBW7 FBOX and WD40 domains were demonstrated by ectopic expression and co-immunoprecipitation. Although FBW7 gene expression increased during E. chaffeensis infection, FBW7 levels significantly decreased (>70%) by 72 h post infection. Moreover, an iRNA knockdown of FBW7 coincided with increased E. chaffeensis infection and levels of Notch intracellular domain (NICD), phosphorylated c-Jun, MCL-1 and cMYC, which are negatively regulated by FBW7. An increase in FBW7 K48 ubiquitination was detected during infection by co-IP, and FBW7 degradation was inhibited in infected cells treated with the proteasomal inhibitor bortezomib. Direct TRP120 ubiquitination of native and recombinant FBW7 was demonstrated in vitro and confirmed by ectopic expression of TRP120 HECT Ub ligase catalytic site mutant. This study identifies the tumor suppressor, FBW7, as a TRP120 HECT E3 Ub ligase substrate, and demonstrates that TRP120 ligase activity promotes ehrlichial infection by degrading FBW7 to maintain stability of Notch and other oncoproteins involved in cell survival and apoptosis.

Recent insight into the role of FBXW7 as a tumor suppressor

FBXW7 (also known as Fbw7, Sel10, hCDC4, or hAgo) is a tumor suppressor and the most frequently mutated member of the F-box protein family in human cancers. FBXW7 functions as the substrate recognition component of an SCF-type E3 ubiquitin ligase. It specifically controls the proteasome-mediated degradation of many oncoproteins such as c-MYC, NOTCH, KLF5, cyclin E, c-JUN, and MCL1. In this review, we summarize the molecular and biological features of FBXW7 and its substrates as well as the impact of mutations of FBXW7 on cancer development. We also address the clinical potential of anticancer therapy targeting FBXW7.

A negative feedback loop between XBP1 and Fbw7 regulates cancer development

In cancer, activation of X-box binding protein (XBP1) has a critical role in tumorigenesis and cancer progression. Transcriptional regulatory mechanism of XBP1 in cancer development has been well known, however, regulation of ubiquitination and degradation of XBP1 has not been elucidated yet. Here we show that Fbw7, a substrate recognition component of the SKP1-Cullin-F-box-type E3 ligase, interacts with XBP1 in a phosphorylation-dependent manner, and facilitates XBP1 ubiquitination and protein degradation. Moreover, Fbw7 inhibits oncogenic pathways including NF-κB, AP1, and Myc induced by XBP1. Interestingly, XBP1 negatively regulates transcription of Fbw7 via a feedback mechanism through NF-κB/E2F-1 axis signaling pathway, suggesting that overexpression of XBP1s may contribute to low level of Fbw7 expression in human cancers. Therefore, a negative feedback loop between Fbw7 and XBP1 contributes to the regulation of tumor development and can be an attractive target for novel therapy in cancers.

Transcriptional Repression and Protein Degradation of the Ca2+-Activated K+ Channel KCa1.1 by Androgen Receptor Inhibition in Human Breast Cancer Cells

The large-conductance Ca²⁺-activated K⁺ channel K_Ca1.1 plays an important role in the promotion of breast cancer cell proliferation and metastasis. The androgen receptor (AR) is proposed as a therapeutic target for AR-positive advanced triple-negative breast cancer. We herein investigated the effects of a treatment with antiandrogens on the functional activity, activation kinetics, transcriptional expression, and protein degradation of K_Ca1.1 in human breast cancer MDA-MB-453 cells using real-time PCR, Western blotting, voltage-sensitive dye imaging, and whole-cell patch clamp recording. A treatment with the antiandrogen bicalutamide or enzalutamide for 48 h significantly suppressed (1) depolarization responses induced by paxilline (PAX), a specific K_Ca1.1 blocker and (2) PAX-sensitive outward currents induced by the depolarizing voltage step. The expression levels of K_Ca1.1 transcripts and proteins were significantly decreased in MDA-MB-453 cells, and the protein degradation of K_Ca1.1 mainly contributed to reductions in K_Ca1.1 activity. Among the eight regulatory β and γ subunits, LRRC26 alone was expressed at high levels in MDA-MB-453 cells and primary and metastatic breast cancer tissues, whereas no significant changes were observed in the expression levels of LRRC26 and activation kinetics of PAX-sensitive outward currents in MDA-MB-453 cells by the treatment with antiandrogens. The treatment with antiandrogens up-regulated the expression of the ubiquitin E3 ligases, FBW7, MDM2, and MDM4 in MDA-MB-453 cells, and the protein degradation of K_Ca1.1 was significantly inhibited by the respective siRNA-mediated blockade of FBW7 and MDM2. Based on these results, we concluded that K_Ca1.1 is an androgen-responsive gene in AR-positive breast cancer cells, and its down-regulation through enhancements in its protein degradation by FBW7 and/or MDM2 may contribute, at least in part, to the antiproliferative and antimetastatic effects of antiandrogens in breast cancer cells.

Physiological functions of FBW7 in cancer and metabolism

FBW7 is one of the most well characterized F-box proteins that serve as substrate recognition subunits of SCF (Skp1-Cullin 1-F-box proteins) E3 ubiquitin ligase complexes. SCF^FBW7 plays key roles in regulating cell cycle progression, differentiation, and stem cell maintenance largely through targeting a broad range of oncogenic substrates for proteasome-dependent degradation. The identification of an increasing number of FBW7 substrates for ubiquitination, and intensive in vitro and in vivo studies have revealed a network of signaling components controlled by FBW7 that contributes to metabolic regulation as well as its tumor suppressor role. Here we mainly focus on recent findings that highlight a critical role for FBW7 in cancer and metabolism.

Next generation sequencing and its applications in HPV-associated cancers

Approximately 18% of all human cancers have a viral etiology, and human papillomavirus (HPV) has been identified as one of the most prevalent viruses that plays causative role in nearly all cervical cancers and, in addition, in subset of head and neck, anal, penile and vulvar cancers. The recent introduction of next generation sequencing (NGS) and other omics approaches have resulted in comprehensive knowledge on the pathogenesis of HPV-driven tumors. Specifically, these approaches have provided detailed information on genomic HPV integration sites, disrupted genes and pathways, and common and distinct genetic and epigenetic alterations in different human HPV-associated cancers. This review focuses on HPV integration sites, its concomitantly disrupted genes and pathways and its functional consequences in both cervical and head and neck cancers. Integration of NGS data with other omics and clinical data is crucial to better understand the pathophysiology of each individual malignancy and, based on this, to select targets and to design effective personalized treatment options.

The stability of Fbw7α in M-phase requires its phosphorylation by PKC

Fbw7 is a tumor suppressor often deleted or mutated in human cancers. It serves as the substrate-recruiting subunit of a SCF ubiquitin ligase that targets numerous critical proteins for degradation, including oncoproteins and master transcription factors. Cyclin E was the first identified substrate of the SCF^Fbw7 ubiquitin ligase. In human cancers bearing FBXW7-gene mutations, deregulation of cyclin E turnover leads to its aberrant expression in mitosis. We investigated Fbw7 regulation in Xenopus eggs, which, although arrested in a mitotic-like phase, naturally express high levels of cyclin E. Here, we report that Fbw7α, the only Fbw7 isoform detected in eggs, is phosphorylated by PKC (protein kinase C) at a key residue (S18) in a manner coincident with Fbw7α inactivation. We show that this PKC-dependent phosphorylation and inactivation of Fbw7α also occurs in mitosis during human somatic cell cycles, and importantly is critical for Fbw7α stabilization itself upon nuclear envelope breakdown. Finally, we provide evidence that S18 phosphorylation, which lies within the intrinsically disordered N-terminal region specific to the α-isoform reduces the capacity of Fbw7α to dimerize and to bind cyclin E. Together, these findings implicate PKC in an evolutionarily-conserved pathway that aims to protect Fbw7α from degradation by keeping it transiently in a resting, inactive state.

The pseudophosphatase STYX targets the F-box of FBXW7 and inhibits SCFFBXW7 function

The F-box protein FBXW7 is the substrate-recruiting subunit of an SCF ubiquitin ligase and a major tumor-suppressor protein that is altered in several human malignancies. Loss of function of FBXW7 results in the stabilization of numerous proteins that orchestrate cell proliferation and survival. Little is known about proteins that directly regulate the function of this protein. In the current work, we have mapped the interactome of the enigmatic pseudophosphatase STYX We reasoned that a catalytically inactive phosphatase might have adopted novel mechanisms of action. The STYX interactome contained several F-box proteins, including FBXW7. We show that STYX binds to the F-box domain of FBXW7 and disables its recruitment into the SCF complex. Therefore, STYX acts as a direct inhibitor of FBXW7, affecting the cellular levels of its substrates. Furthermore, we find that levels of STYX and FBXW7 are anti-correlated in breast cancer patients, which affects disease prognosis. We propose the STYX-FBXW7 interaction as a promising drug target for future investigations.

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.

Pluripotency transcription factors in the pathogenesis of colorectal cancer and implications for prognosis

The cancer stem cell hypothesis argues that cancers depend on a specific type of cells, representing usually a small percentage of the total cancer cell population, termed cancer stem cells (or tumor-initiating cells) for their development and propagation. In colorectal cancer these cells express specific surface proteins such as CD133 and CD44 and can recapitulate the whole tumor. Besides expression of these surface markers, stem cells are associated with a network of pluripotency transcription factors, such as Oct4 and Sox2, which is present in them. Pluripotency factors are normally active in early development and characterize primitive cells, able to give rise to all different cell and tissue types of the three embryonic layers. In this review I will discuss the relationship of these factors with pathogenic lesions in colorectal cancer and their prognostic implications.

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.

Fbw7 and its counteracting forces in stem cells and cancer: Oncoproteins in the balance

Fbw7 is well characterised as a stem cell regulator and tumour suppressor, powerfully positioned to control proliferation, differentiation and apoptosis by targeting key transcription factors for ubiquitination and destruction. Evidence in support of these roles continues to accumulate from in vitro studies, mouse models and human patient data. Here we summarise the latest of these findings, highlighting the tumour-suppressive role of Fbw7 in multiple tissues, and the rare circumstances where Fbw7 activity can be oncogenic. We discuss mechanisms that regulate ubiquitination by Fbw7, including ubiquitin-specific proteases such as USP28 that counteract Fbw7 activity and thereby stabilise oncoproteins. Deubiquitination of key Fbw7 substrates to prevent their destruction is beginning to be appreciated as an important pro-tumourigenic mechanism. As the ubiquitin-proteasome system represents a largely untapped field for drug development, the interplay between Fbw7 and its counterpart deubiquitinating enzymes in tumours is likely to attract increasing interest and influence future treatment strategies.

HPV Infection of the Head and Neck Region and Its Stem Cells

The human papillomavirus (HPV) is an etiologic agent associated with the development of head and neck squamous carcinoma (HNSCC)-in particular, oropharyngeal squamous cell carcinoma. The HPV-positive HNSCC is characterized by genetic alterations, clinical progression, and therapeutic response, which are distinct from HPV-negative head and neck cancers, suggesting that virus-associated tumors constitute a unique entity among head and neck cancers. Malignant stem cells, or cancer stem cells, are a subpopulation of tumor cells that self-renew, initiate new tumors upon transplantation, and are resistant to therapy, and their discovery has revealed novel effects of oncovirus infection in cancer. In this review, we provide a virus-centric view and novel insights into HPV-positive head and neck pathogenesis. We discuss the influence of cancer stem cells, HPV oncoproteins, altered molecular pathways, and mutations in cancer initiation and cancer progression. We compiled a catalogue of the mutations associated with HPV-positive HNSCC, which may be a useful resource for genomic-based studies aiming to develop personalized therapies. We also explain recent changes in mass vaccination campaigns against HPV and the potential long-term impact of vaccinations on the prevention and treatment of HPV-positive head and neck cancers.

Regulation mechanism of Fbxw7-related signaling pathways (Review)

F-box and WD repeat domain-containing 7 (Fbxw7), the substrate-recognition component of SCFFbxw7 complex, is thought to be a tumor suppressor involved in cell growth, proliferation, differentiation and survival. Although an increasing number of ubiquitin substrates of Fbxw7 have been identified, the best characterized substrates are cyclin E and c-Myc. Fbxw7/cyclin E and Fbxw7/c-Myc pathways are tightly regulated by multiple regulators. Fbxw7 has been identified as a tumor suppressor in hepatocellular carcinoma. This review focused on the regulation of Fbxw7/cyclin E and Fbxw7/c-Myc pathways and discussed findings to gain a better understanding of the role of Fbxw7 in hepatocellular carcinoma.

Fbw7 and Usp28 - enemies and allies

The Usp28 deubiquitinase antagonizes Fbw7-mediated turnover of multiple oncoproteins, including Myc, Jun, and Notch, and promotes tumorigenesis in the intestine. Our recent study reveals that Usp28 also counteracts autocatalytic ubiquitination of Fbw7, suggesting a complex role for Usp28 in the regulation of Fbw7 activity and tumor development.

Dual regulation of Fbw7 function and oncogenic transformation by Usp28

Fbw7, the substrate recognition subunit of SCF(Fbw7) ubiquitin ligase, mediates the turnover of multiple proto-oncoproteins and promotes its own degradation. Fbw7-dependent substrate ubiquitination is antagonized by the Usp28 deubiquitinase. Here, we show that Usp28 preferentially antagonizes autocatalytic ubiquitination and stabilizes Fbw7, resulting in dose-dependent effects in Usp28 knockout mice. Monoallelic deletion of Usp28 maintains stable Fbw7 but drives Fbw7 substrate degradation. In contrast, complete knockout triggers Fbw7 degradation and leads to the accumulation of Fbw7 substrates in several tissues and embryonic fibroblasts. On the other hand, overexpression of Usp28 stabilizes both Fbw7 and its substrates. Consequently, both complete loss and ectopic expression of Usp28 promote Ras-driven oncogenic transformation. We propose that dual regulation of Fbw7 activity by Usp28 is a safeguard mechanism for maintaining physiological levels of proto-oncogenic Fbw7 substrates, which is equivalently disrupted by loss or overexpression of Usp28.

Defining the genomic landscape of head and neck cancers through next-generation sequencing

Next-generation sequencing (NGS) has revolutionized the field of genomics and improved our understanding of cancer biology. Advances have been achieved by sequencing tumor DNA and using matched normal DNA to filter out germ line variants to identify cancer-specific changes. The identification of high incidences of activating mutations in head and neck squamous cell carcinoma (HNSCC) amenable to drug targeting has been made, with clear distinctions between the mutational profile of HPV-positive and HPV-negative tumors. This wealth of new understanding undoubtedly ameliorates our understanding of HNSCC cancer biology and elucidates clear targets for drug targeting which will guide future personalized medicine.

Glucosamine, a naturally occurring amino monosaccharide, inhibits A549 and H446 cell proliferation by blocking G1/S transition

Uncontrolled proliferation is important in tumorigenesis. In the present study, the effects of glucosamine on lung cancer cell proliferation were investigated. The expression of cyclin E, one of the key cyclins in the G1/S transition, and Skp2, the ubiquitin ligase subunit that targets the negative cell cycle regulator, p27Kip1, were also assessed. Moreover, the underlying mechanisms of action of glucosamine were investigated in lung cancer cells. A549 and H446 cells were synchronized using thymidine in the presence or absence of glucosamine. The effect of glucosamine on lung cancer cell proliferation was determined by MTT assay. Cyclin E and p27Kip1 proteins and their phosphorylation levels were detected by western blot analysis. Furthermore, the effect of glucosamine on the cell cycle was evaluated by flow cytometry. Glucosamine was found to inhibit lung cancer cell proliferation and to suppress Skp2 and cyclin E expression. Notably, the phosphorylation levels of cyclin E (Thr62) and p27Kip1 (Thr187) were downregulated by glucosamine, and negatively correlated with degradation. Glucosamine was also found to arrest lung cancer cells in the G1/S phase. Thus, glucosamine may inhibit lung cancer cell proliferation by blocking G1/S transition through the inhibition of cyclin E and Skp2 protein expression.

Targeted next-generation sequencing of head and neck squamous cell carcinoma identifies novel genetic alterations in HPV+ and HPV- tumors

BACKGROUND

Human papillomavirus positive (HPV+) head and neck squamous cell carcinoma (HNSCC) is an emerging disease, representing a distinct clinical and epidemiological entity. Understanding the genetic basis of this specific subtype of cancer could allow therapeutic targeting of affected pathways for a stratified medicine approach.

METHODS

Twenty HPV+ and 20 HPV- laser-capture microdissected oropharyngeal carcinomas were used for paired-end sequencing of hybrid-captured DNA, targeting 3,230 exons in 182 genes often mutated in cancer. Copy number alteration (CNA) profiling, Sequenom MassArray sequencing and immunohistochemistry were used to further validate findings.

RESULTS

HPV+ and HPV- oropharyngeal carcinomas cluster into two distinct subgroups. TP53 mutations are detected in 100% of HPV negative cases and abrogation of the G1/S checkpoint by CDKN2A/B deletion and/or CCND1 amplification occurs in the majority of HPV- tumors.

CONCLUSION

These findings strongly support a causal role for HPV, acting via p53 and RB pathway inhibition, in the pathogenesis of a subset of oropharyngeal cancers and suggest that studies of CDK inhibitors in HPV- disease may be warranted. Mutation and copy number alteration of PI3 kinase (PI3K) pathway components appears particularly prevalent in HPV+ tumors and assessment of these alterations may aid in the interpretation of current clinical trials of PI3K, AKT, and mTOR inhibitors in HNSCC.

FBW7 regulates endothelial functions by targeting KLF2 for ubiquitination and degradation

F-box and WD repeat domain-containing 7 (FBW7), the substrate-binding subunit of E3 ubiquitin ligase SCF(FBW7) (a complex of SKP1, cullin-1 and FBW7), plays important roles in various physiological and pathological processes. Although FBW7 is required for vascular development, its function in the endothelium remains to be investigated. In this study, we show that FBW7 is an important regulator of endothelial functions, including angiogenesis, leukocyte adhesion and the endothelial barrier integrity. Using RNA interference, we found that the depletion of FBW7 markedly impairs angiogenesis in vitro and in vivo. We identified the zinc finger transcription factor Krüppel-like factor 2 (KLF2) as a physiological target of FBW7 in endothelial cells. Knockdown of FBW7 expression resulted in the accumulation of endogenous KLF2 protein in endothelial cells. FBW7-mediated KLF2 destruction was shown to depend on the phosphorylation of KLF2 via glycogen synthase kinase-3 (GSK3) at two conserved phosphodegrons. Mutating these phosphodegron motifs abolished the FBW7-mediated degradation and ubiquitination of KLF2. The siRNA-mediated knockdown of FBW7 showed that KLF2 is an essential target of FBW7 in the regulation of endothelial functions. Moreover, FBW7-mediated KLF2 degradation was shown to be critical for angiogenesis in teratomas and in zebrafish development. Taken together, our study suggests a role for FBW7 in the processes of endothelial cell migration, angiogenesis, inflammation and barrier integrity, and provides novel insights into the regulation of KLF2 stability in vivo.

Role of the ubiquitin ligase Fbw7 in cancer progression

Fbw7 is a member of F-box family proteins, which constitute one subunit of Skp1, Cul1, and F-box protein (SCF) ubiquitin ligase complex. SCF(Fbw7) targets a set of well-known oncoproteins, including c-Myc, cyclin E, Notch, c-Jun, and Mcl-1, for ubiquitylation and degradation. Fbw7 provides specificity of the ubiquitylation of these substrate proteins via recognition of a consensus phosphorylated degron. Through regulation of several important proteins, Fbw7 controls diverse cellular processes, including cell-cycle progression, cell proliferation, differentiation, DNA damage response, maintenance of genomic stability, and neural cell stemness. As reduced Fbw7 expression level and loss-of-function mutations are found in a wide range of human cancers, Fbw7 is generally considered as a tumor suppressor. However, the exact mechanisms underlying Fbw7-induced tumor suppression is unclear. This review focuses on regulation network, biological functions, and genetic alteration of Fbw7 in connection with its role in cancer development.

The ubiquitin-proteasome system and signal transduction pathways regulating Epithelial Mesenchymal transition of cancer

Epithelial to Mesenchymal transition (EMT) in cancer, a process permitting cancer cells to become mobile and metastatic, has a signaling hardwire forged from development. Multiple signaling pathways that regulate carcinogenesis enabling characteristics in neoplastic cells such as proliferation, resistance to apoptosis and angiogenesis are also the main players in EMT. These pathways, as almost all cellular processes, are in their turn regulated by ubiquitination and the Ubiquitin-Proteasome System (UPS). Ubiquitination is the covalent link of target proteins with the small protein ubiquitin and serves as a signal to target protein degradation by the proteasome or to other outcomes such as endocytosis, degradation by the lysosome or specification of cellular localization. This paper reviews signal transduction pathways regulating EMT and being regulated by ubiquitination.

Paracrine signaling through MYCN enhances tumor-vascular interactions in neuroblastoma

Neuroblastoma, a tumor of peripheral neural crest origin, numbers among the most common childhood cancers. Both amplification of the proto-oncogene MYCN and increased neoangiogenesis mark high-risk disease. Because angiogenesis is regulated by phosphatidylinositol 3-kinase (PI3K), we tested a clinical PI3K inhibitor, NVP-BEZ235, in MYCN-dependent neuroblastoma. NVP-BEZ235 decreased angiogenesis and improved survival in both primary human (highly pretreated recurrent MYCN-amplified orthotopic xenograft) and transgenic mouse models for MYCN-driven neuroblastoma. Using both gain- and loss-of-function approaches, we demonstrated that the antiangiogenic efficacy of NVP-BEZ235 depended critically on MYCN in vitro and in vivo. Thus, clinical PI3K/mammalian target of rapamycin inhibitors drove degradation of MYCN in tumor cells, with secondary paracrine blockade of angiogenesis. Our data demonstrated significantly improved survival in treated animals and suggest that NVP-BEZ235 should be tested in children with high-risk, MYCN-amplified neuroblastoma.