The ubiquitin ligase FBXW7 modulates leukemia-initiating cell activity by regulating MYC stability

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

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

Germinal Centre B Cell Functions and Lymphomagenesis: Circuits Involving MYC and MicroRNAs

BACKGROUND

The transcription factor MYC regulates several biological cellular processes, and its target gene network comprises approximately 15% of all human genes, including microRNAs (miRNAs), that also contribute to MYC regulatory activity. Although miRNAs are emerging as key regulators of immune functions, the specific roles of miRNAs in the regulation/dysregulation of germinal centre B-cells and B-cell lymphomas are still being uncovered. The regulatory network that integrates MYC, target genes and miRNAs is a field of intense study, highlighting potential pathways to be explored in the context of future clinical approaches.

METHODS

The scientific literature that is indexed in PUBMED was consulted for publications involving MYC and miRNAs with validated bioinformatics analyses or experimental protocols. Additionally, seminal studies on germinal centre B-cell functions and lymphomagenesis were reported.

CONCLUSIONS

This review summarizes the interactions between MYC and miRNAs through regulatory loops and circuits involving target genes in germinal centre B-cell lymphomas with MYC alterations. Moreover, we provide an overview of the understanding of the regulatory networks between MYC and miRNAs, highlighting the potential implication of this approach for the comprehension of germinal centre B-cell lymphoma pathogenesis. Therefore, circuits involving MYC, target genes and miRNAs provide novel insight into lymphomagenesis that could be useful for new improved therapeutic strategies.

Recent Advances in the Targeting of Epigenetic Regulators in B-Cell Non-Hodgkin Lymphoma

In the last 10 years, major advances have been made in the diagnosis and development of selective therapies for several blood cancers, including B-cell non-Hodgkin lymphoma (B-NHL), a heterogeneous group of malignancies arising from the mature B lymphocyte compartment. However, most of these entities remain incurable and current treatments are associated with variable efficacy, several adverse events, and frequent relapses. Thus, new diagnostic paradigms and novel therapeutic options are required to improve the prognosis of patients with B-NHL. With the recent deciphering of the mutational landscapes of B-cell disorders by high-throughput sequencing, it came out that different epigenetic deregulations might drive and/or promote B lymphomagenesis. Consistently, over the last decade, numerous epigenetic drugs (or epidrugs) have emerged in the clinical management of B-NHL patients. In this review, we will present an overview of the most relevant epidrugs tested and/or used so far for the treatment of different subtypes of B-NHL, from first-generation epigenetic therapies like histone acetyl transferases (HDACs) or DNA-methyl transferases (DNMTs) inhibitors to new agents showing selectivity for proteins that are mutated, translocated, and/or overexpressed in these diseases, including EZH2, BET, and PRMT. We will dissect the mechanisms of action of these epigenetic inhibitors, as well as the molecular processes underlying their lack of efficacy in refractory patients. This review will also provide a summary of the latest strategies being employed in preclinical and clinical settings, and will point out the most promising lines of investigation in the field.

Pharmacological Targeting of BET Bromodomain Proteins in Acute Myeloid Leukemia and Malignant Lymphomas: From Molecular Characterization to Clinical Applications

Alterations in protein-protein and DNA-protein interactions and abnormal chromatin remodeling are a major cause of uncontrolled gene transcription and constitutive activation of critical signaling pathways in cancer cells. Multiple epigenetic regulators are known to be deregulated in several hematologic neoplasms, by somatic mutation, amplification, or deletion, allowing the identification of specific epigenetic signatures, but at the same time providing new therapeutic opportunities. While these vulnerabilities have been traditionally addressed by hypomethylating agents or histone deacetylase inhibitors, pharmacological targeting of bromodomain-containing proteins has recently emerged as a promising approach in a number of lymphoid and myeloid malignancies. Indeed, preclinical and clinical studies highlight the relevance of targeting the bromodomain and extra-terminal (BET) family as an efficient strategy of target transcription irrespective of the presence of epigenetic mutations. Here we will summarize the main advances achieved in the last decade regarding the preclinical and clinical evaluation of BET bromodomain inhibitors in hematologic cancers, either as monotherapies or in combinations with standard and/or experimental agents. A mention will finally be given to the new concept of the protein degrader, and the perspective it holds for the design of bromodomain-based therapies.

Loss of Fbxw7 synergizes with activated Akt signaling to promote c-Myc dependent cholangiocarcinogenesis

BACKGROUND & AIMS

The ubiquitin ligase F-box and WD repeat domain-containing 7 (FBXW7) is recognized as a tumor suppressor in many cancer types due to its ability to promote the degradation of numerous oncogenic target proteins. Herein, we aimed to elucidate its role in intrahepatic cholangiocarcinoma (iCCA).

METHODS

Herein, we first confirmed that FBXW7 gene expression was reduced in human iCCA specimens. To identify the molecular mechanisms by which FBXW7 dysfunction promotes cholangiocarcinogenesis, we generated a mouse model by hydrodynamic tail vein injection of Fbxw7ΔF, a dominant negative form of Fbxw7, either alone or in association with an activated/myristylated form of AKT (myr-AKT). We then confirmed the role of c-MYC in human iCCA cell lines and its relationship to FBXW7 expression in human iCCA specimens.

RESULTS

FBXW7 mRNA expression is almost ubiquitously downregulated in human iCCA specimens. While forced overexpression of Fbxw7ΔF alone did not induce any appreciable abnormality in the mouse liver, co-expression with AKT triggered cholangiocarcinogenesis and mice had to be euthanized by 15 weeks post-injection. At the molecular level, a strong induction of Fbxw7 canonical targets, including Yap, Notch2, and c-Myc oncoproteins, was detected. However, only c-MYC was consistently confirmed as a FBXW7 target in human CCA cell lines. Most importantly, selected ablation of c-Myc completely impaired iCCA formation in AKT/Fbxw7ΔF mice, whereas deletion of either Yap or Notch2 only delayed tumorigenesis in the same model. In human iCCA specimens, an inverse correlation between the expression levels of FBXW7 and c-MYC transcriptional activity was observed.

CONCLUSIONS

Downregulation of FBXW7 is ubiquitous in human iCCA and cooperates with AKT to induce cholangiocarcinogenesis in mice via c-Myc-dependent mechanisms. Targeting c-MYC might represent an innovative therapy against iCCA exhibiting low FBXW7 expression.

LAY SUMMARY

There is mounting evidence that FBXW7 functions as a tumor suppressor in many cancer types, including intrahepatic cholangiocarcinoma, through its ability to promote the degradation of numerous oncoproteins. Herein, we have shown that the low expression of FBXW7 is ubiquitous in human cholangiocarcinoma specimens. This low expression is correlated with increased c-MYC activity, leading to tumorigenesis. Our findings suggest that targeting c-MYC might be an effective treatment for intrahepatic cholangiocarcinoma.

Insulin-like growth factor (IGF) signaling in T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive cancer, characterized by an uncontrolled expansion and accumulation of T-cell progenitors. During leukemic progression, immature T cells grow abnormally and occupy the bone marrow compartment, thereby interfering with the production of normal blood cells. Pediatric T-ALL is curable with intensive chemotherapy, but there are significant, long-term side effects and ~20% of patients suffer relapse for which there are limited treatment options. Adult T-ALL in contrast is largely incurable and refractory/relapsed disease is common despite multi-agent chemotherapy (5-year overall survival of ~40%), and thus new therapeutic targets are needed. We have reported previously on the role of insulin-like growth factor (IGF) signaling in T-ALL, and shown that it exerts potent phenotypes in both leukemia stem cell and bulk tumor cell populations. Modulators of IGF signaling may thus prove useful in improving outcomes in patients with T-ALL. In this review, we summarize the most recent findings relating to IGF signaling in T-ALL and outline therapeutic options using clinically relevant IGF signaling modulators.

MicroRNA-27a (miR-27a) in Solid Tumors: A Review Based on Mechanisms and Clinical Observations

MicroRNAs (miRNAs) are a family of highly conserved, non-coding single-stranded RNAs transcribed as ~70 nucleotide precursors to an 18–22 nucleotide product (1). miRNAs can silence their homologous target genes at the post-transcriptional level, and these genes have been revealed to play an important role in tumorigenesis, invasion and metastasis (2). MicroRNA-27a (miR-27a), transcripted by miR-27a gene, has proved to implicate with many kinds of solid tumors, showing potential as a useful biomarker or drug target for clinical application. However, even though miR-27a has been reported in many cancers, the mechanism and signal pathways of miR-27 in oncogenesis, invasion, and metastasis are still obscure. Moreover, recent studies show that miR-27a pays an important role in epithelial-mesenchymal-transition, regulating tumor immune response, and chemoresistance. In this review, we summarize the current literature, demonstrate the established link between miR-27a and tumorigenesis, and focus on recently identified mechanisms. The review also aims to demonstrate the potential of miR-27a as a diagnostic and/or prognostic biomarker in solid tumors and to discuss the possibilities of targeted therapy and drug design.

FBXW7 Mutations Promote Cell Proliferation, Migration, and Invasion in Cervical Cancer

Aim: Cervical cancer is the most common gynecological cancer. Recent studies have revealed that the F-box and WD repeat domain containing 7 (FBXW7) gene, which encodes a subunit of Skp1-Cul1-F-box protein (SCF) ubiquitin ligase, is frequently mutated in cervical squamous cell carcinomas. In this study, we investigated whether Chinese cervical cancer cells also harbor these mutations. Methods: Using PCR and sequencing assays, a total of 190 specimens from Han Chinese patients with cervical cancer were analyzed for FBXW7 mutations. Results: Two FBXW7 mutations (p.R479P and p.L443H), were identified from a study of 145 (1.4%) cervical squamous cell carcinomas. The p.L443H somatic mutation has not been previously reported. Functional assays showed that both of these FBXW7 mutations could promote cell proliferation, migration, and invasion. Conclusion: A low frequency (1.4%) of cervical squamous cell carcinomas were identified with FBXW7 mutations. We did, however, identify a novel FBXW7 mutation. Our results also demonstrated that the identified FBXW7 mutations could promote cell proliferation, migration, and invasion in cervical cancer cells.

Activation of hedgehog signaling associates with early disease progression in chronic lymphocytic leukemia

Targeted sequencing of 103 leukemia-associated genes in leukemia cells from 841 treatment-naive patients with chronic lymphocytic leukemia (CLL) identified 89 (11%) patients as having CLL cells with mutations in genes encoding proteins that putatively are involved in hedgehog (Hh) signaling. Consistent with this finding, there was a significant association between the presence of these mutations and the expression of GLI1 (χ2 test, P < .0001), reflecting activation of the Hh pathway. However, we discovered that 38% of cases without identified mutations also were GLI1+ Patients with GLI1+ CLL cells had a shorter median treatment-free survival than patients with CLL cells lacking expression of GLI1 independent of IGHV mutation status. We found that GANT61, a small molecule that can inhibit GLI1, was highly cytotoxic for GLI1+ CLL cells relative to that of CLL cells without GLI1. Collectively, this study shows that a large proportion of patients have CLL cells with activated Hh signaling, which is associated with early disease progression and enhanced sensitivity to inhibition of GLI1.

Strategies to Overcome Resistance Mechanisms in T-Cell Acute Lymphoblastic Leukemia

Chemoresistance is a major cause of recurrence and death from T-cell acute lymphoblastic leukemia (T-ALL), both in adult and pediatric patients. In the majority of cases, drug-resistant disease is treated by selecting a combination of other drugs, without understanding the molecular mechanisms by which malignant cells escape chemotherapeutic treatments, even though a more detailed genomic characterization and the identification of actionable disease targets may enable informed decision of new agents to improve patient outcomes. In this work, we describe pathways of resistance to common chemotherapeutic agents including glucocorticoids and review the resistance mechanisms to targeted therapy such as IL7R, PI3K-AKT-mTOR, NOTCH1, BRD4/MYC, Cyclin D3: CDK4/CDK6, BCL2 inhibitors, and selective inhibitors of nuclear export (SINE). Finally, to overcome the limitations of the current trial-and-error method, we summarize the experiences of anti-cancer drug sensitivity resistance profiling (DSRP) approaches as a rapid and relevant strategy to infer drug activity and provide functional information to assist clinical decision one patient at a time.

E3 ubiquitin ligases in B-cell malignancies

Ubiquitylation is a post-translational modification (PTM) that controls various cellular signaling pathways. It is orchestrated by a three-step enzymatic cascade know as the ubiquitin proteasome system (UPS). E3 ligases dictate the specificity to the substrates, primarily leading to proteasome-dependent degradation. Deregulation of the UPS components by various mechanisms contributes to the pathogenesis of cancer. This review focuses on E3 ligase-substrates pairings that are implicated in B-cell malignancies. Understanding the molecular mechanism of specific E3 ubiquitin ligases will present potential opportunities for the development of targeted therapeutic approaches.

CAFs secreted exosomes promote metastasis and chemotherapy resistance by enhancing cell stemness and epithelial-mesenchymal transition in colorectal cancer

BACKGROUND

Cancer associated fibroblasts (CAFs) are key stroma cells that play dominant roles in tumor progression. However, the CAFs-derived molecular determinants that regulate colorectal cancer (CRC) metastasis and chemoresistance have not been fully characterized.

METHODS

CAFs and NFs were obtained from fresh CRC and adjacent normal tissues. Exosomes were isolated from conditioned medium and serum of CRC patients using ultracentrifugation method and ExoQuick Exosome Precipitation Solution kit, and characterized by transmission electronic microscopy, nanosight and western blot. MicroRNA microarray was employed to identify differentially expressed miRNAs in exosomes secreted by CAFs or NFs. The internalization of exosomes, transfer of miR-92a-3p was observed by immunofluorescence. Boyden chamber migration and invasion, cell counting kit-8, flow cytometry, plate colony formation, sphere formation assays, tail vein injection and primary colon cancer liver metastasis assays were employed to explore the effect of NFs, CAFs and exosomes secreted by them on epithelial-mesenchymal transition, stemness, metastasis and chemotherapy resistance of CRC. Luciferase report assay, real-time qPCR, western blot, immunofluorescence, and immunohistochemistry staining were employed to explore the regulation of CRC metastasis and chemotherapy resistance by miR-92a-3p, FBXW7 and MOAP1.

RESULTS

CAFs promote the stemness, epithelial-mesenchymal transition (EMT), metastasis and chemotherapy resistance of CRC cells. Importantly, CAFs exert their roles by directly transferring exosomes to CRC cells, leading to a significant increase of miR-92a-3p level in CRC cells. Mechanically, increased expression of miR-92a-3p activates Wnt/β-catenin pathway and inhibits mitochondrial apoptosis by directly inhibiting FBXW7 and MOAP1, contributing to cell stemness, EMT, metastasis and 5-FU/L-OHP resistance in CRC. Clinically, miR-92a-3p expression is significantly increased in CRC tissues and negatively correlated with the levels of FBXW7 and MOAP1 in CRC specimens, and high expression of exosomal miR-92a-3p in serum was highly linked with metastasis and chemotherapy resistance in CRC patients.

CONCLUSIONS

CAFs secreted exosomes promote metastasis and chemotherapy resistance of CRC. Inhibiting exosomal miR-92a-3p provides an alternative modality for the prediction and treatment of metastasis and chemotherapy resistance in CRC.

Emerging role of F-box proteins in the regulation of epithelial-mesenchymal transition and stem cells in human cancers

Emerging evidence shows that epithelial-mesenchymal transition (EMT) plays a crucial role in tumor invasion, metastasis, cancer stem cells, and drug resistance. Data obtained thus far have revealed that F-box proteins are critically involved in the regulation of the EMT process and stem cell differentiation in human cancers. In this review, we will briefly describe the role of EMT and stem cells in cell metastasis and drug resistance. We will also highlight how numerous F-box proteins govern the EMT process and stem cell survival by controlling their downstream targets. Additionally, we will discuss whether F-box proteins involved in drug resistance are associated with EMT and cancer stem cells. Targeting these F-box proteins might be a potential therapeutic strategy to reverse EMT and inhibit cancer stem cells and thus overcome drug resistance in human cancers.

Homeodomain-interacting protein kinase 2 suppresses proliferation and aerobic glycolysis via ERK/cMyc axis in pancreatic cancer

OBJECTIVES

To investigate the roles of the homeodomain-interacting protein kinase (HIPK) family of proteins in pancreatic cancer prognosis and the possible molecular mechanism.

MATERIALS AND METHODS

The expression of HIPK family genes and their roles in pancreatic cancer prognosis were analysed by using The Cancer Genome Atlas (TCGA). The roles of HIPK2 in pancreatic cancer proliferation and glycolysis were tested by overexpression of HIPK2 in pancreatic cancer cells, followed by cell proliferation assay, glucose uptake analysis and Seahorse extracellular flux analysis. The mechanism of action of HIPK2 in pancreatic cancer proliferation and glycolysis was explored by examining its effect on the ERK/cMyc axis.

RESULTS

Decreased HIPK2 expression indicated worse prognosis of pancreatic cancer. Overexpression of HIPK2 in pancreatic cancer cells decreased cell proliferation and attenuated aerobic glycolysis, which sustained proliferation of cancer cells. HIPK2 decreased cMyc protein levels and expression of cMyc-targeted glycolytic genes. cMyc was a mediator that regulated HIPK2-induced decrease in aerobic glycolysis. HIPK2 regulated cMyc protein stability via ERK activation, which phosphorylated and controlled cMyc protein stability.

CONCLUSIONS

HIPK2 suppressed proliferation of pancreatic cancer in part through inhibiting the ERK/cMyc axis and related aerobic glycolysis.

Super-enhancers in cancer

Cancer is fueled by the aberrant activity of oncogenic and tumor suppressive pathways. Transcriptional dysregulation of these pathways play a major role both in the genesis and development of cancer. Dysregulation of transcriptional programs can be mediated by genetic and epigenetic alterations targeting both protein coding genes and non-coding regulatory elements like enhancers and super-enhancers. Super-enhancers, characterized as large clusters of enhancers in close proximity, have been identified as essential oncogenic drivers required for the maintenance of cancer cell identity. As a result, cancer cells are often addicted to the super-enhancer driven transcriptional programs. Furthermore, pharmacological inhibitors targeting key components of super-enhancer assembly and activation have shown great promise in reducing tumor growth and proliferation in several pre-clinical tumor models. This article reviews the current understanding of super-enhancer assembly and activation, the different mechanisms by which cancer cells acquire oncogenic super-enhancers and, finally, the potential of targeting super-enhancers as future therapeutics.

Overcoming BET Inhibitor Resistance in Malignant Peripheral Nerve Sheath Tumors

PURPOSE

BET bromodomain inhibitors have emerged as a promising therapy for numerous cancer types in preclinical studies, including neurofibromatosis type 1 (NF1)-associated malignant peripheral nerve sheath tumor (MPNST). However, potential mechanisms underlying resistance to these inhibitors in different cancers are not completely understood. In this study, we explore new strategy to overcome BET inhibitor resistance in MPNST.Experimental Design: Through modeling tumor evolution by studying genetic changes underlying the development of MPNST, a lethal sarcoma with no effective medical treatment, we identified a targetable addiction to BET bromodomain family member BRD4 in MPNST. This served as a controlled model system to delineate mechanisms of sensitivity and resistance to BET bromodomain inhibitors in this disease.

RESULTS

Here, we show that a malignant progression-associated increase in BRD4 protein levels corresponds to partial sensitivity to BET inhibition in MPNST. Strikingly, genetic depletion of BRD4 protein levels synergistically sensitized MPNST cells to diverse BET inhibitors in culture and in vivo.

CONCLUSIONS

Collectively, MPNST sensitivity to combination genetic and pharmacologic inhibition of BRD4 revealed the presence of a unique addiction to BRD4 in MPNST. Our discovery that a synthetic lethality exists between BET inhibition and reduced BRD4 protein levels nominates MPNST for the investigation of emerging therapeutic interventions such as proteolysis-targeting chimeras (PROTACs) that simultaneously target bromodomain activity and BET protein abundance.

FBXW7 in Cancer: What Has Been Unraveled Thus Far?

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

USP7 Cooperates with NOTCH1 to Drive the Oncogenic Transcriptional Program in T-Cell Leukemia

PURPOSE

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive disease, affecting children and adults. Chemotherapy treatments show high response rates but have debilitating effects and carry risk of relapse. Previous work implicated NOTCH1 and other oncogenes. However, direct inhibition of these pathways affects healthy tissues and cancer alike. Our goal in this work has been to identify enzymes active in T-ALL whose activity could be targeted for therapeutic purposes.

EXPERIMENTAL DESIGN

To identify and characterize new NOTCH1 druggable partners in T-ALL, we coupled studies of the NOTCH1 interactome to expression analysis and a series of functional analyses in cell lines, patient samples, and xenograft models.

RESULTS

We demonstrate that ubiquitin-specific protease 7 (USP7) interacts with NOTCH1 and controls leukemia growth by stabilizing the levels of NOTCH1 and JMJD3 histone demethylase. USP7 is highly expressed in T-ALL and is transcriptionally regulated by NOTCH1. In turn, USP7 controls NOTCH1 levels through deubiquitination. USP7 binds oncogenic targets and controls gene expression through stabilization of NOTCH1 and JMJD3 and ultimately H3K27me3 changes. We also show that USP7 and NOTCH1 bind T-ALL superenhancers, and inhibition of USP7 leads to a decrease of the transcriptional levels of NOTCH1 targets and significantly blocks T-ALL cell growth in vitro and in vivo.

CONCLUSIONS

These results provide a new model for USP7 deubiquitinase activity through recruitment to oncogenic chromatin loci and regulation of both oncogenic transcription factors and chromatin marks to promote leukemia. Our studies also show that targeting USP7 inhibition could be a therapeutic strategy in aggressive leukemia.

aPKC controls endothelial growth by modulating c-Myc via FoxO1 DNA-binding ability

Strict regulation of proliferation is vital for development, whereas unregulated cell proliferation is a fundamental characteristic of cancer. The polarity protein atypical protein kinase C lambda/iota (aPKCλ) is associated with cell proliferation through unknown mechanisms. In endothelial cells, suppression of aPKCλ impairs proliferation despite hyperactivated mitogenic signaling. Here we show that aPKCλ phosphorylates the DNA binding domain of forkhead box O1 (FoxO1) transcription factor, a gatekeeper of endothelial growth. Although mitogenic signaling excludes FoxO1 from the nucleus, consequently increasing c-Myc abundance and proliferation, aPKCλ controls c-Myc expression via FoxO1/miR-34c signaling without affecting its localization. We find this pathway is strongly activated in the malignant vascular sarcoma, angiosarcoma, and aPKC inhibition reduces c-Myc expression and proliferation of angiosarcoma cells. Moreover, FoxO1 phosphorylation at Ser218 and aPKC expression correlates with poor patient prognosis. Our findings may provide a potential therapeutic strategy for treatment of malignant cancers, like angiosarcoma.

The cell polarity regulator aPKC is associated with cell proliferation but the precise mechanism are unknown. Here, the authors find that aPKC lambda phosphorylates the FoxO1 transcription factor, a gatekeeper of endothelial growth, during both angiogenesis and angiosarcomas.

Next-generation sequencing with a 54-gene panel identified unique mutational profile and prognostic markers in Chinese patients with myelofibrosis

Current prognostication in myelofibrosis (MF) is based on clinicopathological features and mutations in a limited number of driver genes. The impact of other genetic mutations remains unclear. We evaluated for mutations in a myeloid panel of 54 genes using next-generation sequencing. Multivariate Cox regression analysis was used to determine prognostic factors for overall survival (OS) and leukaemia-free survival (LFS), based on mutations of these genes and relevant clinical and haematological features. One hundred and one patients (primary MF, N = 70; secondary MF, N = 31) with a median follow-up of 49 (1-256) months were studied. For the entire cohort, inferior OS was associated with male gender (P = 0.04), age > 65 years (P = 0.04), haemoglobin < 10 g/dL (P = 0.001), CUX1 mutation (P = 0.003) and TP53 mutation (P = 0.049); and inferior LFS was associated with male gender (P = 0.03), haemoglobin < 10 g/dL (P = 0.04) and SRSF2 mutations (P = 0.008). In primary MF, inferior OS was associated with male gender (P = 0.03), haemoglobin < 10 g/dL (P = 0.002), platelet count < 100 × 10⁹/L (P = 0.02), TET2 mutation (P = 0.01) and CUX1 mutation (P = 0.01); and inferior LFS was associated with haemoglobin < 10 g/dL (P = 0.02), platelet count < 100 × 10⁹/L (P = 0.02), TET2 mutations (P = 0.01) and CUX1 mutations (P = 0.04). These results showed that clinical and haematological features and genetic mutations should be considered in MF prognostication.

The MYC Enhancer-ome: Long-Range Transcriptional Regulation of MYC in Cancer

MYC is one of the most important oncogenes in cancer. Indeed, MYC is upregulated in 50-60% of all tumors. MYC overexpression can be achieved through a variety of mechanisms, including gene duplications, chromosomal translocations, or somatic mutations leading to increased MYC stability. However, recent studies have identified numerous tissue-specific noncoding enhancers of MYC that play major roles in cancer, highlighting long-range transcriptional regulation of MYC as a critical novel mechanism leading to MYC hyperactivation and as a potential target for new therapeutic strategies in the near future. Here we summarize the regions and mechanisms involved in the long-range transcriptional regulation of MYC, underscoring the relevance of MYC enhancers both in normal physiological development and in MYC-driven cancer initiation and progression.

Gastric Cancer Cell Lines Have Different MYC-Regulated Expression Patterns but Share a Common Core of Altered Genes

MYC is an oncogene responsible for excessive cell growth in cancer, enabling transcriptional activation of genes involved in cell cycle regulation, metabolism, and apoptosis, and is usually overexpressed in gastric cancer (GC). By using siRNA and Next-Generation Sequencing (NGS), we identified MYC-regulated differentially expressed Genes (DEGs) in three Brazilian gastric cancer cell lines representing the histological subtypes of GC (diffuse, intestinal, and metastasis). The DEGs were picked using Sailfish software, followed by Gene Set Enrichment Analysis (GSEA) and Kyoto Encyclopedia of Gene and Genome (KEGG) pathway analysis using KEGG. We found 11 significantly enriched gene sets by using enrichment score (ES), False Discovery Rate (FDR), and nominal P-values. We identified a total of 5.471 DEGs with correlation over (80%). In diffuse-type and in metastatic GC cell lines, MYC-silencing caused DEGs downregulation, while the intestinal-type GC cells presented overall DEGs upregulation after MYC siRNA depletion. We were able to detect 11 significant gene sets when comparing our samples to the hallmark collection of gene expression, enriched mostly for the following hallmarks: proliferation, pathway, signaling, metabolic, and DNA damage response. When we analyzed our DEGs considering KEGG metabolic pathways, we found 12 common branches covering a wide range of biological functions, and three of them were common to all three cell lines: ubiquitin-mediated proteolysis, ribosomes, and system and epithelial cell signaling in Helicobacter pylori infection. The GC cell lines used in this study share 14 MYC-regulated genes, but their gene expression profile is different for each histological subtype of GC. Our results present a computational analysis of MYC-related signatures in GC, and we present evidence that GC cell lines representing distinct histological subtypes of this disease have different MYC-regulated expression profiles but share a common core of altered genes. This is an important step towards the understanding of MYC's role in gastric carcinogenesis and an indication of probable new drug targets in stomach cancer.

SCFFBW7-mediated degradation of Brg1 suppresses gastric cancer metastasis

Brg1/SMARCA4 serves as the ATPase and the helicase catalytic subunit for the multi-component SWI/SNF chromatin remodeling complex, which plays a pivotal role in governing chromatin structure and gene transcription. However, the upstream signaling pathways regulating Brg1 protein stability and its physiological contribution to carcinogenesis remain largely elusive. Here we report that Brg1 is a bona fide ubiquitin substrate of SCF^FBW7. We reveal that CK1δ phosphorylates Brg1 at Ser31/Ser35 residues to facilitate the binding of Brg1 to FBW7, leading to ubiquitination-mediated degradation. In keeping with a tumor suppressive role of FBW7 in human gastric cancer, we find an inverse correlation between FBW7 and Brg1 expression in human gastric cancer clinical samples. Mechanistically, we find that stabilization of Brg1 in gastric cancer cells suppresses E-cadherin expression, subsequently promoting gastric cancer metastasis. Hence, this previously unknown FBW7/Brg1 signaling axis provides the molecular basis and the rationale to target Brg1 in FBW7-compromised human gastric cancers.

Targeting the HTLV-I-Regulated BATF3/IRF4 Transcriptional Network in Adult T Cell Leukemia/Lymphoma

Adult T cell leukemia/lymphoma (ATLL) is a frequently incurable disease associated with the human lymphotropic virus type I (HTLV-I). RNAi screening of ATLL lines revealed that their proliferation depends on BATF3 and IRF4, which cooperatively drive ATLL-specific gene expression. HBZ, the only HTLV-I encoded transcription factor that is expressed in all ATLL cases, binds to an ATLL-specific BATF3 super-enhancer and thereby regulates the expression of BATF3 and its downstream targets, including MYC. Inhibitors of bromodomain-and-extra-terminal-domain (BET) chromatin proteins collapsed the transcriptional network directed by HBZ and BATF3, and were consequently toxic for ATLL cell lines, patient samples, and xenografts. Our study demonstrates that the HTLV-I oncogenic retrovirus exploits a regulatory module that can be attacked therapeutically with BET inhibitors.

FBXW7: a critical tumor suppressor of human cancers

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

Oncogenic hijacking of the stress response machinery in T cell acute lymphoblastic leukemia

Cellular transformation is accompanied by extensive re-wiring of many biological processes leading to augmented levels of distinct types of cellular stress, including proteotoxic stress. Cancer cells critically depend on stress-relief pathways for their survival. However, the mechanisms underlying the transcriptional initiation and maintenance of the oncogenic stress response remain elusive. Here, we show that the expression of heat shock transcription factor 1 (HSF1) and the downstream mediators of the heat shock response is transcriptionally upregulated in T-cell acute lymphoblastic leukemia (T-ALL). Hsf1 ablation suppresses the growth of human T-ALL and eradicates leukemia in mouse models of T-ALL, while sparing normal hematopoiesis. HSF1 drives a compact transcriptional program and among the direct HSF1 targets, specific chaperones and co-chaperones mediate its critical role in T-ALL. Notably, we demonstrate that the central T-ALL oncogene NOTCH1 hijacks the cellular stress response machinery by inducing the expression of HSF1 and its downstream effectors. The NOTCH1 signaling status controls the levels of chaperone/co-chaperone complexes and predicts the response of T-ALL patient samples to HSP90 inhibition. Our data demonstrate an integral crosstalk between mediators of oncogene and non-oncogene addiction and reveal critical nodes of the heat shock response pathway that can be targeted therapeutically.

Targeting the Noncoding Genome: Superenhancers Meet Their Kryptonite

<b/> In this study, McKeown and colleagues carried out a genome-wide characterization and stratification of the enhancer landscape in acute myeloid leukemia (AML). The authors' analysis led to the discovery of a novel RARA superenhancer found in a subset of patients with AML, rendering these leukemia cells highly sensitive to SY-1425, a highly potent RARA agonist able to induce myeloid differentiation in these high-expressing RARA AML subtypes. Cancer Discov; 7(10); 1065-6. ©2017 AACR.See related article by McKeown et al., p. 1136.

Mixed Mesonephric Adenocarcinoma and High-grade Neuroendocrine Carcinoma of the Uterine Cervix: Case Description of a Previously Unreported Entity With Insights Into Its Molecular Pathogenesis

Human papillomavirus (HPV)-negative cervical carcinomas are uncommon and typically encompass unusual histologic subtypes. Mesonephric adenocarcinoma is one such subtype. Mesonephric tumors in the female genital tract are thought to arise from Wolffian remnants, and are extremely rare tumors with widely variable morphology. Sarcomatoid dedifferentiation has been previously described in a few cases, but other forms of dedifferentiation have not been reported. Neuroendocrine carcinoma of the cervix (e.g. small cell carcinoma) is associated with HPV infection, typically HPV 18. These tumors often arise in association with a conventional epithelial component such as squamous cell carcinoma or usual-type endocervical adenocarcinoma. We describe a case of mesonephric adenocarcinoma of the uterine cervix associated with an HPV-negative high-grade neuroendocrine carcinoma at the morphologic and immunophenotypic level, for which we performed targeted massively parallel sequencing analysis of the 2 elements. Both components shared identical mutations in U2AF1 p.R156H (c.467G>A) and GATA3 p.M422fs (c.1263dupG), as well as MYCN amplification. In addition, the neuroendocrine carcinoma harbored TP53 and MST1R mutations not present in the mesonephric carcinoma. Our data suggest a clonal origin of the 2 components of this rare entity, rather than a collision tumor.

Pan-cancer transcriptional signatures predictive of oncogenic mutations reveal that Fbw7 regulates cancer cell oxidative metabolism

The Fbw7 (F-box/WD repeat-containing protein 7) ubiquitin ligase targets multiple oncoproteins for degradation and is commonly mutated in cancers. Like other pleiotropic tumor suppressors, Fbw7's complex biology has impeded our understanding of how Fbw7 mutations promote tumorigenesis and hindered the development of targeted therapies. To address these needs, we employed a transfer learning approach to derive gene-expression signatures from The Cancer Gene Atlas datasets that predict Fbw7 mutational status across tumor types and identified the pathways enriched within these signatures. Genes involved in mitochondrial function were highly enriched in pan-cancer signatures that predict Fbw7 mutations. Studies in isogenic colorectal cancer cell lines that differed in Fbw7 mutational status confirmed that Fbw7 mutations increase mitochondrial gene expression. Surprisingly, Fbw7 mutations shifted cellular metabolism toward oxidative phosphorylation and caused context-specific metabolic vulnerabilities. Our approach revealed unexpected metabolic reprogramming and possible therapeutic targets in Fbw7-mutant cancers and provides a framework to study other complex, oncogenic mutations.

Hepatic F-Box Protein FBXW7 Maintains Glucose Homeostasis Through Degradation of Fetuin-A

Type 2 diabetes mellitus (T2DM) has become one of the most serious and long-term threats to human health. However, the molecular mechanism that links obesity to insulin resistance remains largely unknown. Here, we show that F-box and WD repeat domain-containing 7 (FBXW7), an E3 ubiquitin protein ligase, is markedly downregulated in the liver of two obese mouse models and obese human subjects. We further identify a functional low-frequency human FBXW7 coding variant (p.Ala204Thr) in the Chinese population, which is associated with elevated blood glucose and T2DM risk. Notably, mice with liver-specific knockout of FBXW7 develop hyperglycemia, glucose intolerance, and insulin resistance even on a normal chow diet. Conversely, overexpression of FBXW7 in the liver not only prevents the development of high-fat diet-induced insulin resistance but also attenuates the disease signature of obese mice. Mechanistically, FBXW7 directly binds to hepatokine fetuin-A to induce its ubiquitination and subsequent proteasomal degradation, comprising an important mechanism maintaining glucose homeostasis. Thus, we provide evidence showing a beneficial role of FBXW7 in glucose homeostasis.

Fbxw11 promotes the proliferation of lymphocytic leukemia cells through the concomitant activation of NF-κB and β-catenin/TCF signaling pathways

The ubiquitin–proteasome system (UPS) participates in both physiological and pathological processes through the posttranslational regulation of intracellular signal transduction pathways. F-box and WD-40 domain protein 11 (Fbxw11) is a component of the SCF (Skp1–Cul1–F-box) E3 ubiquitin ligase complex. Fbxw11 regulates various signal transduction pathways, and it may have pathological roles in tumorigenesis. However, the role of Fbxw11 in the development of leukemia and the underlying mechanisms remain largely unknown. In this study, Fbxw11 expression was aberrantly upregulated in patients with lymphocytic leukemia. Its expression was dramatically decreased in patients who achieved complete remission (CR) after chemotherapy. The high level of Fbxw11 expression in L1210 lymphocytic leukemia cells stimulated cell proliferation in vitro and tumor formation in vivo. The effects were mediated by the stimulation of cell cycle progression rather than the induction of apoptosis. Furthermore, a bioinformatics analysis suggested concomitant activation of the NF-κB and β-catenin/TCF signaling pathways, which were confirmed by reporter gene assays. Moreover, blocking experiments suggested the involvement of both pathways in the growth-promoting effects of Fbxw11. Our results reveal the role of Fbxw11 in lymphocytic leukemia cells and imply that Fbxw11 may serve as a potential molecular target for the treatment of lymphocytic leukemia.

The deubiquitinase USP9X regulates FBW7 stability and suppresses colorectal cancer

The tumor suppressor FBW7 targets oncoproteins such as c-MYC for ubiquitylation and is mutated in several human cancers. We noted that in a substantial percentage of colon cancers, FBW7 protein is undetectable despite the presence of FBW7 mRNA. To understand the molecular mechanism of FBW7 regulation in these cancers, we employed proteomics and identified the deubiquitinase (DUB) USP9X as an FBW7 interactor. USP9X antagonized FBW7 ubiquitylation, and Usp9x deletion caused Fbw7 destabilization. Mice lacking Usp9x in the gut showed reduced secretory cell differentiation and increased progenitor proliferation, phenocopying Fbw7 loss. In addition, Usp9x inactivation impaired intestinal regeneration and increased tumor burden in colitis-associated intestinal cancer. c-Myc heterozygosity abrogated increased progenitor proliferation and tumor burden in Usp9x-deficient mice, suggesting that Usp9x suppresses tumor formation by regulating Fbw7 protein stability and thereby reducing c-Myc. Thus, we identify a tumor suppressor mechanism in the mammalian intestine that arises from the posttranslational regulation of FBW7 by USP9X independent of somatic FBW7 mutations.

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.

Loss of Capicua alters early T cell development and predisposes mice to T cell lymphoblastic leukemia/lymphoma

Capicua (CIC) regulates a transcriptional network downstream of the RAS/MAPK signaling cascade. In Drosophila, CIC is important for many developmental processes, including embryonic patterning and specification of wing veins. In humans, CIC has been implicated in neurological diseases, including spinocerebellar ataxia type 1 (SCA1) and a neurodevelopmental syndrome. Additionally, we and others have reported mutations in CIC in several cancers. However, whether CIC is a tumor suppressor remains to be formally tested. In this study, we found that deletion of Cic in adult mice causes T cell acute lymphoblastic leukemia/lymphoma (T-ALL). Using hematopoietic-specific deletion and bone marrow transplantation studies, we show that loss of Cic from hematopoietic cells is sufficient to drive T-ALL. Cic-null tumors show up-regulation of the KRAS pathway as well as activation of the NOTCH1 and MYC transcriptional programs. In sum, we demonstrate that loss of CIC causes T-ALL, establishing it as a tumor suppressor for lymphoid malignancies. Moreover, we show that mouse models lacking CIC in the hematopoietic system are robust models for studying the role of RAS signaling as well as NOTCH1 and MYC transcriptional programs in T-ALL.

Ubiquitin ligases in oncogenic transformation and cancer therapy

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

Establishment and analysis of a novel mouse line carrying a conditional knockin allele of a cancer-specific FBXW7 mutation

F-box and WD40 domain protein 7 (FBXW7) is a component of the SKP1-CUL1-F-box protein (SCF) complex that mediates the ubiquitination of diverse oncogenic target proteins. The exploration of FBXW7 mutations in human primary cancer has revealed three mutation hotspots at conserved arginine residues (Arg⁴⁶⁵, Arg⁴⁷⁹, and Arg⁵⁰⁵) in the WD40 domain, which are critical for substrate recognition. To study the function of human FBXW7^R465C, the most frequent mutation in human malignancies, we generated a novel conditional knockin mouse line of murine Fbxw7^R468C corresponding to human FBXW7^R465C. Systemic heterozygous knockin of the Fbxw7^R468C mutation resulted in perinatal lethality due to defects in lung development, and occasionally caused an eyes-open at birth phenotype and cleft palate. Furthermore, mice carrying liver-specific heterozygous and homozygous Fbxw7^R468C alleles cooperated with an oncogenic Kras mutation to exhibit bile duct hyperplasia within 8 months of birth and cholangiocarcinoma-like lesions within 8 weeks of birth, respectively. In addition, the substrates affected by the mutant Fbxw7 differed between the embryos, embryonic fibroblasts, and adult liver. This novel conditional knockin Fbxw7^R468C line should be useful to gain a more profound understanding of carcinogenesis associated with mutation of FBXW7.

The BET bromodomain inhibitor CPI203 overcomes resistance to ABT-199 (venetoclax) by downregulation of BFL-1/A1 in in vitro and in vivo models of MYC+/BCL2+ double hit lymphoma

High-grade B-cell lymphoma with MYC and BCL2 and/or BCL6 rearrangements, mostly known as double-hit lymphoma (DHL), is a rare entity characterized by morphologic and molecular features between Burkitt lymphoma and the clinically manageable diffuse large B-cell lymphoma (DLBCL). DHL patients usually undergo a rapidly progressing clinical course associated with resistance to standard chemo-immunotherapy. As a consequence, the prognosis of this entity is particularly poor with a median overall survival inferior to 1 year. ABT-199 (venetoclax) is a potent and selective small-molecule antagonist of BCL-2 recently approved for the treatment of a specific subtype of lymphoid neoplasm. In this study, we demonstrate that single-agent ABT-199 efficiently displaces BAX from BCL-2 complexes but fails to maintain a significant antitumor activity over time in most MYC+/BCL2+DHL cell lines and primary cultures, as well as in a xenograft mouse model of the disease. We further identify the accumulation of the BCL2-like protein BFL-1 to be a major mechanism involved in acquired resistance to ABT-199. Noteworthy, this phenomenon can be counteracted by the BET bromodomain inhibitor CPI203, since gene expression profiling identifies BCL2A1, the BFL-1 coding gene, as one of the top apoptosis-related gene modulated by this compound. Upon CPI203 treatment, simultaneous downregulation of MYC and BFL-1 further overcomes resistance to ABT-199 both in vitro and in vivo, engaging synergistic caspase-mediated apoptosis in DHL cultures and tumor xenografts. Together, these findings highlight the relevance of BFL-1 in DH lymphoma-associated drug resistance and support the combined use of a BCL-2 antagonist and a BET inhibitor as a promising therapeutic strategy for patients with aggressive DHL.

Deubiquitinating enzyme PSMD14 promotes tumor metastasis through stabilizing SNAIL in human esophageal squamous cell carcinoma

The epithelial-mesenchymal transition (EMT) transcription factor SNAIL is associated with distant metastasis and poor prognosis of esophageal squamous cell carcinoma (ESCC) patients. The proteolysis of SNAIL is mediated by the ubiquitin-proteasome system. Several E3 ligases have been characterized to promote SNAIL ubiquitination and degradation. However, the reverse process - deubiquitination of SNAIL remains largely unknown. In this study, we performed a mass spectrometry to examine the interaction between SNAIL and deubiquitinating enzyme(s). Subsequently, the deubiquitinating enzyme PSMD14 was identified to target SNAIL for deubiquitination and stabilization. Furthermore, knockdown of PSMD14 significantly blocks SNAIL-induced EMT and then suppresses tumor cell migration and invasion in vitro and tumor metastasis in vivo. In addition, the high expression level of PSMD14 predicts poor prognosis for esophageal cancer patients. These findings suggest PSMD14 as a bona fide deubiquitinating enzyme to regulate SNAIL at the post-translational level and provide a promising therapeutic strategy against tumor metastasis of esophageal cancer.

FBW7 regulates DNA interstrand cross-link repair by modulating FAAP20 degradation

Mutations that deregulate protein degradation lead to human malignancies. The SCF ubiquitin E3 ligase complex degrades key oncogenic regulators, thereby limiting their oncogenic potential. FBW7 is a substrate recognition subunit of SCF^FBW7 and is among the most commonly mutated ubiquitin-proteasome system proteins in cancer. FBW7-mutated cancer cells display increased genome instability, but the molecular mechanism by which FBW7 preserves genome integrity remains elusive. Here, we demonstrate that SCF^FBW7 regulates the stability of FAAP20, a critical component of the Fanconi anemia (FA) DNA interstrand cross-link (ICL) repair pathway. Phosphorylation of the FAAP20 degron motif by GSK3β provides a platform for recognition and polyubiquitination of FAAP20 by FBW7, and its subsequent degradation by the proteasome. Accordingly, enhanced GSK3β-FBW7 signaling disrupts the FA pathway. In cells expressing non-phosphorylatable FAAP20 mutant, the turnover of its binding partner, FANCA, is deregulated in the chromatin during DNA ICL repair, and the FA pathway is compromised. We propose that FAAP20 degradation, which is prompted by its phosphorylation, controls the dynamics of the FA core complex required for completing DNA ICL repair. Together, this study provides insights into how FBW7-mediated proteolysis regulates genome stability and how its deregulation is associated with tumorigenesis.

Somatic Superenhancer Duplications and Hotspot Mutations Lead to Oncogenic Activation of the KLF5 Transcription Factor

The Krüppel-like family of transcription factors plays critical roles in human development and is associated with cancer pathogenesis. Krüppel-like factor 5 gene (KLF5) has been shown to promote cancer cell proliferation and tumorigenesis and to be genomically amplified in cancer cells. We recently reported that the KLF5 gene is also subject to other types of somatic coding and noncoding genomic alterations in diverse cancer types. Here, we show that these alterations activate KLF5 by three distinct mechanisms: (i) Focal amplification of superenhancers activates KLF5 expression in squamous cell carcinomas; (ii) Missense mutations disrupt KLF5-FBXW7 interactions to increase KLF5 protein stability in colorectal cancer; (iii) Cancer type-specific hotspot mutations within a zinc-finger DNA binding domain of KLF5 change its DNA binding specificity and reshape cellular transcription. Utilizing data from CRISPR/Cas9 gene knockout screening, we reveal that cancer cells with KLF5 overexpression are dependent on KLF5 for their proliferation, suggesting KLF5 as a putative therapeutic target.Significance: Our observations, together with previous studies that identified oncogenic properties of KLF5, establish the importance of KLF5 activation in human cancers, delineate the varied genomic mechanisms underlying this occurrence, and nominate KLF5 as a putative target for therapeutic intervention in cancer. Cancer Discov; 8(1); 108-25. ©2017 AACR.This article is highlighted in the In This Issue feature, p. 1.

Notch in Leukemia

Notch is commonly activated in lymphoid malignancies through ligand-independent and ligand-dependent mechanisms. In T-cell acute lymphoblastic leukemia/lymphoma (T-ALL), ligand-independent activation predominates. Negative Regulatory Region (NRR) mutations trigger supraphysiological Notch1 activation by exposing the S2 site to proteolytic cleavage in the absence of ligand. Subsequently, cleavage at the S3 site generates the activated form of Notch, intracellular Notch (ICN). In contrast to T-ALL, in mature lymphoid neoplasms such as chronic lymphocytic leukemia (CLL), the S2 cleavage site is exposed through ligand-receptor interactions. Thus, agents that disrupt ligand-receptor interactions might be useful for treating these malignancies. Notch activation can be enhanced by mutations that delete the C-terminal proline (P), glutamic acid (E), serine (S), and threonine (T) (PEST) domain. These mutations do not activate the Notch pathway per se, but rather impair degradation of ICN. In this chapter, we review the mechanisms of Notch activation and the importance of Notch for the genesis and maintenance of lymphoid malignancies. Unfortunately, targeting the Notch pathway with pan-Notch inhibitors in clinical trials has proven challenging. These clinical trials have encountered dose-limiting on-target toxicities and primary resistance. Strategies to overcome these challenges have emerged from the identification and improved understanding of direct oncogenic Notch target genes. Other strategies have arisen from new insights into the "nuclear context" that selectively directs Notch functions in lymphoid cancers. This nuclear context is created by factors that co-bind ICN at cell-type specific transcriptional regulatory elements. Disrupting the functions of these proteins or inhibiting downstream oncogenic pathways might combat cancer without the intolerable side effects of pan-Notch inhibition.

Fbxo4-mediated degradation of Fxr1 suppresses tumorigenesis in head and neck squamous cell carcinoma

The Fbxo4 tumour suppressor is a component of an Skp1-Cul1-F-box E3 ligase for which two substrates are known. Here we show purification of SCFFbxo4 complexes results in the identification of fragile X protein family (FMRP, Fxr1 and Fxr2) as binding partners. Biochemical and functional analyses reveal that Fxr1 is a direct substrate of SCFFbxo4. Consistent with a substrate relationship, Fxr1 is overexpressed in Fbxo4 knockout cells, tissues and in human cancer cells, harbouring inactivating Fbxo4 mutations. Critically, in head and neck squamous cell carcinoma, Fxr1 overexpression correlates with reduced Fbxo4 levels in the absence of mutations or loss of mRNA, suggesting the potential for feedback regulation. Direct analysis reveals that Fbxo4 translation is attenuated by Fxr1, indicating the existence of a feedback loop that contributes to Fxr1 overexpression and the loss of Fbxo4. Ultimately, the consequence of Fxr1 overexpression is the bypass of senescence and neoplastic progression.

T-cell Acute Lymphoblastic Leukemia Cells Display Activation of Different Survival Pathways

T-cell acute lymphoblastic leukemia (T-ALL) is a disease of the blood affecting T-lymphocytes. Although notable improvements have been achieved in T-ALL treatment, half of the adult T-ALL patients still experience treatment failure. In order to develop a targeted therapy, we need a better understanding of T-ALL pathogenesis. In this study, we used patient-derived cell lines which display resistance to glucocorticoids. We observed that different cell lines are dependent on different survival signaling pathways. Aberrant activation of AKT, p38, S6K or ERK signaling was not found to the same degree in all cell lines studied. To understand the molecular differences in T-ALL cells, we compared gene expression and somatic mutations. Gene set enrichment analysis showed enrichment of the mTORC1, MAPK or TGF-beta signaling pathways. Loss-of-function mutations in the TP53 and FBXW7 genes were identified in all cell lines investigated. Thus, we suggest that T-ALL cells from different patients are addicted to different mutations and thereby to different signaling pathways. Therefore, understanding the enrichment of molecular pathways for each individual patient will provide us with a more precise and specific treatment plan.

Cancer stem cells and differentiation therapy

Cancer stem cells can generate tumors from only a small number of cells, whereas differentiated cancer cells cannot. The prominent feature of cancer stem cells is its ability to self-renew and differentiate into multiple types of cancer cells. Cancer stem cells have several distinct tumorigenic abilities, including stem cell signal transduction, tumorigenicity, metastasis, and resistance to anticancer drugs, which are regulated by genetic or epigenetic changes. Like normal adult stem cells involved in various developmental processes and tissue homeostasis, cancer stem cells maintain their self-renewal capacity by activating multiple stem cell signaling pathways and inhibiting differentiation signaling pathways during cancer initiation and progression. Recently, many studies have focused on targeting cancer stem cells to eradicate malignancies by regulating stem cell signaling pathways, and products of some of these strategies are in preclinical and clinical trials. In this review, we describe the crucial features of cancer stem cells related to tumor relapse and drug resistance, as well as the new therapeutic strategy to target cancer stem cells named "differentiation therapy."

Oncogenic Notch signaling in T-cell and B-cell lymphoproliferative disorders

PURPOSE OF REVIEW

This article highlights recent discoveries about Notch activation and its oncogenic functions in lymphoid malignancies, and discusses the therapeutic potential of Notch inhibition.

RECENT FINDINGS

NOTCH mutations arise in a broad spectrum of lymphoid malignancies and are increasingly scrutinized as putative therapeutic targets. In T-cell acute lymphoblastic leukemia (T-ALL), NOTCH1 mutations affect the extracellular negative regulatory region and lead to constitutive Notch activation, although mutated receptors remain sensitive to Notch ligands. Other NOTCH1 mutations in T-ALL and NOTCH1/2 mutations in multiple B-cell malignancies truncate the C-terminal proline (P), glutamic acid (E), serine (S), threonine (T)-rich (PEST) domain, leading to decreased Notch degradation after ligand-mediated activation. Thus, targeting Notch ligand-receptor interactions could provide therapeutic benefits. In addition, we discuss recent reports on clinical testing of Notch inhibitors in T-ALL that influenced contemporary thinking on the challenges of targeting Notch in cancer. We review advances in the laboratory to address these challenges in regards to drug targets, the Notch-driven metabolome, and the sophisticated protein-protein interactions at Notch-dependent superenhancers that underlie oncogenic Notch functions.

SUMMARY

Notch signaling is a recurrent oncogenic pathway in multiple T- and B-cell lymphoproliferative disorders. Understanding the complexity and consequences of Notch activation is critical to define optimal therapeutic strategies targeting the Notch pathway.