Tumor suppression by the Fbw7 ubiquitin ligase: mechanisms and opportunities

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.

Involvement of F-box proteins in esophageal cancer (Review)

The F-box proteins (FBPs) in esophageal tumorigenesis are pivotal as they govern a broad array of basic physiological responses including cell growth, cell death and DNA damage repair. Esophageal cancer (EC) is a common and highly aggressive cancer worldwide. Aberrant stabilization of crucial proteins participates in esophageal tumorigenesis. Recently, growing evidence has shown that FBPs play a critical role in oncogenesis, invasion, metastasis and prognosis assessment of EC. In this review we summarized published data on the roles of known FBPs, their respective substrates and the key signaling pathways, in the development of EC, aiming to uncover new ways for the rational design of targeted therapies in EC.

FBXW7 Facilitates Nonhomologous End-Joining via K63-Linked Polyubiquitylation of XRCC4

FBXW7 is a haploinsufficient tumor suppressor with loss-of-function mutations occurring in human cancers. FBXW7 inactivation causes genomic instability, but the mechanism remains elusive. Here we show that FBXW7 facilitates nonhomologous end-joining (NHEJ) repair and that FBXW7 depletion causes radiosensitization. In response to ionizing radiation, ATM phosphorylates FBXW7 at serine 26 to recruit it to DNA double-strand break (DSB) sites, whereas activated DNA-PKcs phosphorylates XRCC4 at serines 325/326, which promotes binding of XRCC4 to FBXW7. SCF(FBXW7) E3 ligase then promotes polyubiquitylation of XRCC4 at lysine 296 via lysine 63 linkage for enhanced association with the Ku70/80 complex to facilitate NHEJ repair. Consistent with these findings, a small-molecule inhibitor that abrogates XRCC4 polyubiquitylation reduces NHEJ repair. Our study demonstrates one mechanism by which FBXW7 contributes to genome integrity and implies that inactivated FBXW7 in human cancers could be a strategy for increasing the efficacy of radiotherapy.

E3 Ubiquitin Ligase Fbw7 Negatively Regulates Osteoblast Differentiation by Targeting Runx2 for Degradation

Runx2, a master regulator of osteoblast differentiation, is tightly regulated at both transcriptional and post-translational levels. Post-translational modifications such as phosphorylation and ubiquitination have differential effects on Runx2 functions. Here, we show that the reduced expression and functions of Runx2 upon its phosphorylation by GSK3β are mediated by its ubiquitin-mediated degradation through E3 ubiquitin ligase Fbw7α. Fbw7α through its WD domain interacts with Runx2 both in a heterologous (HEK293T cells) system as well as in osteoblasts. GSK3β was also present in the same complex as determined by co-immunoprecipitation. Furthermore, overexpression of either Fbw7α or GSK3β was sufficient to down-regulate endogenous Runx2 expression and function; however, both failed to inhibit endogenous Runx2 when either of them was depleted in osteoblasts. Fbw7α-mediated inhibition of Runx2 expression also led to reduced Runx2 transactivation and osteoblast differentiation. In contrast, inhibition of Fbw7α restored Runx2 levels and promoted osteoblast differentiation. We also observed reciprocal expression levels of Runx2 and Fbw7α in models of bone loss such as lactating (physiological bone loss condition) and ovariectomized (induction of surgical menopause) animals that show reduced Runx2 and enhanced Fbw7α, whereas this was reversed in the estrogen-treated ovariectomized animals. In addition, methylprednisolone (a synthetic glucocorticoid) treatment to neonatal rats showed a temporal decrease in Runx2 with a reciprocal increase in Fbw7 in their calvarium. Taken together, these data demonstrate that Fbw7α negatively regulates osteogenesis by targeting Runx2 for ubiquitin-mediated degradation in a GSK3β-dependent manner and thus provides a plausible explanation for GSK3β-mediated bone loss as described before.

The role of the ubiquitin proteasome system in cerebellar development and medulloblastoma

Cerebellar granule cells precursors are derived from the upper rhombic lip and migrate tangentially independent of glia along the subpial stream pathway to form the external germinal zone. Postnatally, granule cells migrate from the external germinal zone radially through the Purkinje cell layer, guided by Bergmann glia fibers, to the internal granular cell layer.

Medulloblastomas (MBs) are the most common malignant childhood brain tumor. Many of these tumors develop from precursor cells of the embryonic rhombic lips. Four main groups of MB are recognized. The WNT group of MBs arise primarily from the lower rhombic lip and embryonic brainstem. The SHH group of MBs originate from cerebellar granule cell precursors in the external germinal zone of the embryonic cerebellum. The cellular origins of type 3 and type 4 MBs are not clear.

Several ubiquitin ligases are revealed to be significant factors in development of the cerebellum as well as in the initiation and maintenance of MBs. Proteasome dysfunction at a critical stage of development may be a major factor in determining whether progenitor cells which are destined to become granule cells differentiate normally or become MB cells. We propose the hypothesis that proteasomal activity is essential to regulate the critical transition between proliferating granule cells and differentiated granule cells and that proteasome dysfunction may lead to MB. Proteasome dysfunction could also account for various mutations in MBs resulting from deficiencies in DNA checkpoint and repair mechanisms prior to development of MBs.

Data showing a role for the ubiquitin ligases β-TrCP, FBW7, Huwe1, and SKP2 in MBs suggest the possibility of a classification of MBs based on the expression (over expression or under expression) of specific ubiquitin ligases which function as oncogenes, tumor suppressors or cell cycle regulators.

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.

A new mitochondrial pool of cyclin E, regulated by Drp1, is linked to cell-density-dependent cell proliferation

The regulation and function of the crucial cell cycle regulator cyclin E (CycE) remains elusive. Unlike other cyclins, CycE can be uniquely controlled by mitochondrial energetics, the exact mechanism being unclear. Using mammalian cells (in vitro) and Drosophila (in vivo) model systems in parallel, we show that CycE can be directly regulated by mitochondria through its recruitment to the organelle. Active mitochondrial bioenergetics maintains a distinct mitochondrial pool of CycE (mtCycE) lacking a key phosphorylation required for its degradation. Loss of the mitochondrial fission protein dynamin-related protein 1 (Drp1, SwissProt O00429 in humans) augments mitochondrial respiration and elevates the mtCycE pool allowing CycE deregulation, cell cycle alterations and enrichment of stem cell markers. Such CycE deregulation after Drp1 loss attenuates cell proliferation in low-cell-density environments. However, in high-cell-density environments, elevated MEK-ERK signaling in the absence of Drp1 releases mtCycE to support escape of contact inhibition and maintain aberrant cell proliferation. Such Drp1-driven regulation of CycE recruitment to mitochondria might be a mechanism to modulate CycE degradation during normal developmental processes as well as in tumorigenic events.

Deregulation of F-box proteins and its consequence on cancer development, progression and metastasis

F-box proteins are substrate receptors of the SCF (SKP1-Cullin 1-F-box protein) E3 ubiquitin ligase that play important roles in a number of physiological processes and activities. Through their ability to assemble distinct E3 ubiquitin ligases and target key regulators of cellular activities for ubiquitylation and degradation, this versatile group of proteins is able to regulate the abundance of cellular proteins whose deregulated expression or activity contributes to disease. In this review, we describe the important roles of select F-box proteins in regulating cellular activities, the perturbation of which contributes to the initiation and progression of a number of human malignancies.

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.

Dysregulation of ubiquitin ligases in cancer

Ubiquitin ligases (UBLs) are critical components of the ubiquitin proteasome system (UPS), which governs fundamental processes regulating normal cellular homeostasis, metabolism, and cell cycle in response to external stress signals and DNA damage. Among multiple steps of the UPS system required to regulate protein ubiquitination and stability, UBLs define specificity, as they recognize and interact with substrates in a temporally- and spatially-regulated manner. Such interactions are required for substrate modification by ubiquitin chains, which marks proteins for recognition and degradation by the proteasome or alters their subcellular localization or assembly into functional complexes. UBLs are often deregulated in cancer, altering substrate availability or activity in a manner that can promote cellular transformation. Such deregulation can occur at the epigenetic, genomic, or post-translational levels. Alterations in UBL can be used to predict their contributions, affecting tumor suppressors or oncogenes in select tumors. Better understanding of mechanisms underlying UBL expression and activities is expected to drive the development of next generation modulators that can serve as novel therapeutic modalities. This review summarizes our current understanding of UBL deregulation in cancer and highlights novel opportunities for therapeutic interventions.

F-box protein interactions with the hallmark pathways in cancer

F-box proteins (FBP) are the substrate specifying subunit of Skp1-Cul1-FBP (SCF)-type E3 ubiquitin ligases and are responsible for directing the ubiquitination of numerous proteins essential for cellular function. Due to their ability to regulate the expression and activity of oncogenes and tumour suppressor genes, FBPs themselves play important roles in cancer development and progression. In this review, we provide a comprehensive overview of FBPs and their targets in relation to their interaction with the hallmarks of cancer cell biology, including the regulation of proliferation, epigenetics, migration and invasion, metabolism, angiogenesis, cell death and DNA damage responses. Each cancer hallmark is revealed to have multiple FBPs which converge on common signalling hubs or response pathways. We also highlight the complex regulatory interplay between SCF-type ligases and other ubiquitin ligases. We suggest six highly interconnected FBPs affecting multiple cancer hallmarks, which may prove sensible candidates for therapeutic intervention.

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.

The emerging roles of F-box proteins in pancreatic tumorigenesis

The role of F-box proteins in pancreatic tumorigenesis is emerging owing to their pivotal and indispensable roles in cell differentiation, cell cycle regulation and proliferation. In this review, we will focus on β-TrCP (β-transducin repeat-containing protein) and two other prototypical mammalian F-box proteins, Fbxw7 and Fbxw8, in pancreatic tumorigenesis and progression. We will highlight the functions and regulation of these F-box proteins, their respective substrates and cross-talks with other key signaling pathways, such as the Ras-Raf-Mek-Erk, Hedgehog, NFκB, TGF-β, Myc and HPK1 signaling pathways in pancreatic 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.

Cell cycle-dependent ubiquitylation and destruction of NDE1 by CDK5-FBW7 regulates ciliary length

Primary cilia start forming within the G1 phase of the cell cycle and continue to grow as cells exit the cell cycle (G0). They start resorbing when cells re-enter the cell cycle (S phase) and are practically invisible in mitosis. The mechanisms by which cilium biogenesis and disassembly are coupled to the cell cycle are complex and not well understood. We previously identified the centrosomal phosphoprotein NDE1 as a negative regulator of ciliary length and showed that its levels inversely correlate with ciliogenesis. Here, we identify the tumor suppressor FBW7 (also known as FBXW7, CDC4, AGO, or SEL-10) as the E3 ligase that mediates the destruction of NDE1 upon entry into G1. CDK5, a kinase active in G1/G0, primes NDE1 for FBW7-mediated recognition. Cells depleted of FBW7 or CDK5 show enhanced levels of NDE1 and a reduction in ciliary length, which is corrected in cells depleted of both FBW7 or CDK5 and NDE1. These data show that cell cycle-dependent mechanisms can control ciliary length through a CDK5-FBW7-NDE1 pathway.

Novel Genes Involved in Controlling Specification of Drosophila FMRFamide Neuropeptide Cells

The expression of neuropeptides is often extremely restricted in the nervous system, making them powerful markers for addressing cell specification . In the developing Drosophila ventral nerve cord, only six cells, the Ap4 neurons, of some 10,000 neurons, express the neuropeptide FMRFamide (FMRFa). Each Ap4/FMRFa neuron is the last-born cell generated by an identifiable and well-studied progenitor cell, neuroblast 5-6 (NB5-6T). The restricted expression of FMRFa and the wealth of information regarding its gene regulation and Ap4 neuron specification makes FMRFa a valuable readout for addressing many aspects of neural development, i.e., spatial and temporal patterning cues, cell cycle control, cell specification, axon transport, and retrograde signaling. To this end, we have conducted a forward genetic screen utilizing an Ap4-specific FMRFa-eGFP transgenic reporter as our readout. A total of 9781 EMS-mutated chromosomes were screened for perturbations in FMRFa-eGFP expression, and 611 mutants were identified. Seventy-nine of the strongest mutants were mapped down to the affected gene by deficiency mapping or whole-genome sequencing. We isolated novel alleles for previously known FMRFa regulators, confirming the validity of the screen. In addition, we identified novel essential genes, including several with previously undefined functions in neural development. Our identification of genes affecting most major steps required for successful terminal differentiation of Ap4 neurons provides a comprehensive view of the genetic flow controlling the generation of highly unique neuronal cell types in the developing nervous system.

Fbxw7 suppresses cancer metastasis by inhibiting niche formation

Fbxw7 has been identified as an oncosuppressor protein in many types of cancer. We have recently shown that loss of Fbxw7 in bone marrow-derived stromal cells (BMSCs) promotes cancer metastasis by increasing production of the chemokine CCL2, which attracts monocytic myeloid-derived suppressor cells (Mo-MDSCs) and macrophages to the metastatic niche.

CD166 positively regulates MCAM via inhibition to ubiquitin E3 ligases Smurf1 and βTrCP through PI3K/AKT and c-Raf/MEK/ERK signaling in Bel-7402 hepatocellular carcinoma cells

Both Cluster of Differentiation 166 (CD166) and Melanoma Cell Adhesion Molecule (MCAM) play critical roles in maintaining transformative phenotype of Hepatocellular Carcinoma (HCC) cells. However, the relationship between these two membrane proteins remains unknown. Here, we found that CD166 has a positive impact on the expression of MCAM, while MCAM has no feedback on CD166. Tissue microarray analysis (TMA) also showed a positive correlation between CD166 and MCAM. Depletion of CD166-induced anti-carcinogenic phenotype could be reversed by overexpression of MCAM, suggesting MCAM is functional important in the CD166-induced liver tumorigenesis. Furthermore, we found CD166 regulates MCAM mainly through protecting MCAM from ubiquitin-mediated protein degradation. Mechanically, CD166 down-regulated two ubiquitin E3 ligases, βTrCP and Smurf1, which play critical roles in the destability of MCAM protein. In addition, overexpression of βTrCP and Smurf1-reduced transformative phenotype could be partially reversed by MCAM, providing evidence that MCAM is a target of βTrCP and Smurf1. Moreover, we identified c-Raf/MEK/ERK signaling acts as a downstream effecter of CD166/PI3K/AKT axis to stimulate ubiquitination and destability of βTrCP and Smurf1. Taken together, we establish a model that CD166 regulates MCAM through a signaling flow from activation of PI3K/AKT and c-Raf/MEK/ERK signaling to the inhibition of potential MCAM ubiquitin E3 ligases, βTrCP and Smurf1, blockage of this signaling cascade may be useful in the treatment of CD166 and MCAM-dependent HCC.

Stem cell and progenitor fate in the mammalian intestine: Notch and lateral inhibition in homeostasis and disease

The control of cell fate decisions is vital to build functional organs and maintain normal tissue homeostasis, and many pathways and processes cooperate to direct cells to an appropriate final identity. Because of its continuously renewing state and its carefully organised hierarchy, the mammalian intestine has become a powerful model to dissect these pathways in health and disease. One of the signalling pathways that is key to maintaining the balance between proliferation and differentiation in the intestinal epithelium is the Notch pathway, most famous for specifying distinct cell fates in adjacent cells via the evolutionarily conserved process of lateral inhibition. Here, we will review recent discoveries that advance our understanding of how cell fate in the mammalian intestine is decided by Notch and lateral inhibition, focusing on the molecular determinants that regulate protein turnover, transcriptional control and epigenetic regulation.

A novel synthetic chemistry approach to linkage-specific ubiquitin conjugation

Ubiquitination is of great importance as the post-translational modification of proteins with ubiquitin, or ubiquitin chains, facilitates a number of vital cellular processes. Herein we present a facile method of preparing various ubiquitin conjugates under mild conditions using michael acceptors based on dibromo-maleimides and dibromo-pyridazinediones.

FBXW7 modulates cellular stress response and metastatic potential through ​HSF1 post-translational modification

​Heat-shock factor 1 (​HSF1) orchestrates the heat-shock response in eukaryotes. Although this pathway has evolved to help cells adapt in the presence of challenging conditions, it is co-opted in cancer to support malignancy. However, the mechanisms that regulate ​HSF1 and thus cellular stress response are poorly understood. Here we show that the ubiquitin ligase ​FBXW7α interacts with ​HSF1 through a conserved motif phosphorylated by ​GSK3β and ​ERK1. ​FBXW7α ubiquitylates ​HSF1 and loss of ​FBXW7α results in impaired degradation of nuclear ​HSF1 and defective heat-shock response attenuation. ​FBXW7α is either mutated or transcriptionally downregulated in melanoma and ​HSF1 nuclear stabilization correlates with increased metastatic potential and disease progression. ​FBXW7α deficiency and subsequent ​HSF1 accumulation activates an invasion-supportive transcriptional program and enhances the metastatic potential of human melanoma cells. These findings identify a post-translational mechanism of regulation of the ​HSF1 transcriptional program both in the presence of exogenous stress and in cancer.

Targeting the ubiquitin pathway for cancer treatment

Proteasome-mediated degradation is a common mechanism by which cells renew their intracellular proteins and maintain protein homeostasis. In this process, the E3 ubiquitin ligases are responsible for targeting specific substrates (proteins) for ubiquitin-mediated degradation. However, in cancer cells, the stability and the balance between oncoproteins and tumor suppressor proteins are disturbed in part due to deregulated proteasome-mediated degradation. This ultimately leads to either stabilization of oncoprotein(s) or increased degradation of tumor suppressor(s), contributing to tumorigenesis and cancer progression. Therefore, E3 ubiquitin ligases including the SCF types of ubiquitin ligases have recently evolved as promising therapeutic targets for the development of novel anti-cancer drugs. In this review, we highlighted the critical components along the ubiquitin pathway including E1, E2, various E3 enzymes and DUBs that could serve as potential drug targets and also described the available bioactive compounds that target the ubiquitin pathway to control various cancers.