The Fbw7 tumor suppressor regulates glycogen synthase kinase 3 phosphorylation-dependent c-Myc protein degradation

The FBXW7-SHOC2-Raptor Axis Controls the Cross-Talks between the RAS-ERK and mTORC1 Signaling Pathways

FBXW7 is a tumor suppressive E3 ligase, whereas RAS-ERK and mechanistic target of rapamycin kinase (mTORC1) are two major oncogenic pathways. Whether and how FBXW7 regulates these two oncogenic pathways are unknown. Here, we showed that SHOC2, a RAS activator, is a FBXW7 substrate. Growth stimuli trigger SHOC2 phosphorylation on Thr⁵⁰⁷ by the mitogen-activated protein kinase (MAPK) signal, which facilitates FBXW7 binding for ubiquitylation and degradation. FBXW7-mediated SHOC2 degradation terminates the RAS-MAPK signals and inhibits proliferation. Furthermore, SHOC2 selectively binds to Raptor to competitively inhibit the Raptor-mTOR binding to inactivate mTORC1 and induce autophagy, whereas Raptor binding of SHOC2 inhibits the SHOC2-RAS binding to block the MAPK pathway and proliferation. Finally, SHOC2 is overexpressed in pancreatic cancer, which correlated with poor patient survival. SHOC2 mutations were found in lung cancer tissues with gain-of-function activity. Collectively, the SHOC2-Raptor interaction triggers negative cross-talk between RAS-ERK and mTORC1 pathways, whereas FBXW7 regulates both pathways by targeting SHOC2 for ubiquitylation and degradation.

Beta-Tocotrienol Exhibits More Cytotoxic Effects than Gamma-Tocotrienol on Breast Cancer Cells by Promoting Apoptosis via a P53-Independent PI3-Kinase Dependent Pathway

Studies on tocotrienols have progressively revealed the benefits of these vitamin E isoforms on human health. Beta-tocotrienol (beta-T3) is known to be less available in nature compared to other vitamin E members, which may explain the restricted number of studies on beta-T3. In the present study, we aim to investigate the anti-proliferative effects and the pro-apoptotic mechanisms of beta-T3 on two human breast adenocarcinoma cell lines MDA-MB-231 and MCF7. To assess cell viability, both cell lines were incubated for 24 and 48 h, with different concentrations of beta-T3 and gamma-T3, the latter being a widely studied vitamin E isoform with potent anti-cancerous properties. Cell cycle progression and apoptosis induction upon treatment with various concentrations of the beta-T3 isoform were assessed. The effect of beta-T3 on the expression level of several apoptosis-related proteins p53, cytochrome C, cleaved-PARP-1, Bax, Bcl-2, and caspase-3, in addition to key cell survival proteins p-PI3K and p-GSK-3 α/β was determined using western blot analysis. Beta-tocotrienol exhibited a significantly more potent anti-proliferative effect than gamma-tocotrienol on both cell lines regardless of their hormonal receptor status. Beta-T3 induced a mild G1 arrest on both cell lines, and triggered a mitochondrial stress-mediated apoptotic response in MDA-MB-231 cells. Mechanistically, beta-T3′s anti-neoplastic activity involved the downregulation of phosphorylated PI3K and GSK-3 cell survival proteins. These findings suggest that vitamin E beta-T3 should be considered as a promising anti-cancer agent, more effective than gamma-T3 for treating human breast cancer and deserves to be further studied to investigate its effects in vitro and on other cancer types.

Xanthohumol suppresses glioblastoma via modulation of Hexokinase 2 -mediated glycolysis

Deregulation of aerobic glycolysis is a common phenomenon in human cancers, including glioblastoma (GBM). In the present study, we demonstrated that the natural compound xanthohumol has a profound anti-tumor effect on GBM via direct inhibition of glycolysis. Xanthohumol suppressed cell proliferation and colony formation of GBM cells, and significantly impaired glucose metabolism via inhibiting Hexokinase 2 (HK2) expression. We demonstrated that down-regulation of c-Myc was required for xanthohumol-induced decrease of HK2. Xanthohumol destabilization of c-Myc, and promoted FBW7-mediated ubiquitination of c-Myc. Xanthohumol attenuated Akt activity and inhibited the activation of GSK3β, resulted in c-Myc degradation. Overexpression of Myr-Akt1 significantly rescued xanthohumol-mediated c-Myc inhibition and glycolysis suppression. Finally, the xanthohumol-mediated down-regulation of the PI3-K/Akt-GSK3beta-FBW7 signaling axis promoted the destabilization of c-Myc. Finally, the animal results demonstrated that xanthohumol substantially inhibited tumor growth in vivo. Collectively, xanthohumol appears to be a promising new anti-tumor agent with the therapeutic potential for GBM.

Destruction of a Microtubule-Bound MYC Reservoir during Mitosis Contributes to Vincristine's Anticancer Activity

Tightly regulated activity of the transcription factor MYC is essential for orderly cell proliferation. Upon deregulation, MYC elicits and promotes cancer progression. Proteasomal degradation is an essential element of MYC regulation, initiated by phosphorylation at Serine62 (Ser62) of the MB1 region. Here, we found that Ser62 phosphorylation peaks in mitosis, but that a fraction of nonphosphorylated MYC binds to the microtubules of the mitotic spindle. Consequently, the microtubule-destabilizing drug vincristine decreases wild-type MYC stability, whereas phosphorylation-deficient MYC is more stable, contributing to vincristine resistance and induction of polyploidy. PI3K inhibition attenuates postmitotic MYC formation and augments the cytotoxic effect of vincristine. IMPLICATIONS: The spindle's function as a docking site for MYC during mitosis may constitute a window of specific vulnerability to be exploited for cancer treatment.

Targeting MYC Proteins for Tumor Therapy

Targeting the function of MYC oncoproteins holds the promise of achieving conceptually new and effective anticancer therapies that can be applied to a broad range of tumors. The nature of the target however—a broadly, possibly universally acting transcription factor that has no enzymatic activity and is largely unstructured unless complexed with partner proteins—has so far defied the development of clinically applicable MYC-directed therapies. At the same time, lingering questions about exactly which functions of MYC proteins account for their pervasive oncogenic role in human tumors and need to be targeted have prevented the development of effective therapies using surrogate targets that act in critical MYC-dependent pathways. In this review, we therefore argue that rigorous testing of critical oncogenic functions and protein/protein interactions and new chemical approaches to target them are necessary to successfully eradicate MYC-driven tumors.

The master regulators Myc and p53 cellular signaling and functions in polycystic kidney disease

The transcription factors Myc and p53 associated with oncogenesis play determinant roles in a human genetic disorder, autosomal dominant polycystic kidney disease (ADPKD), that was coined early in ADPKD etiology a «neoplasia in disguise ». These factors are interdependent master cell regulators of major biological processes including proliferation, apoptosis, cell growth, metabolism, inflammation, fibrosis and differentiation that are all modulated in ADPKD. Myc and p53 proteins evolved to respond and carry out overlapping functions via opposing mechanisms of action. Studies in human ADPKD kidneys, caused by mutations in the PKD1 or PKD2 genes, reveal reduced p53 expression and high expression of Myc in the cystic tubular epithelium. Myc and p53 via direct interaction act respectively, as transcriptional activator and repressor of PKD1 gene expression, consistent with increased renal PKD1 levels in ADPKD. Mouse models generated by Pkd1 and Pkd2 gene dosage dysregulation reproduce renal cystogenesis with activation of Myc expression and numerous signaling pathways, strikingly similar to those determined in human ADPKD. In fact, upregulation of renal Myc expression is also detected in virtually all non-orthologous animal models of PKD. A definitive causal connection of Myc with cystogenesis was established by renal overexpression of Myc in transgenic mice that phenocopies human ADPKD. The network of activated signaling pathways in human and mouse cystogenesis individually or in combination can target Myc as a central node of PKD pathogenesis. One or many of the multiple functions of Myc upon activation can play a role in every phases of ADPKD development and lend credence to the notion of "Myc addiction" for cystogenesis. We propose that the residual p53 levels are conducive to an ADPKD biological program without cancerogenesis while a "p53 dependent annihilation" mechanism would be permissive to oncogenesis. Of major importance, Myc ablation in orthologous mouse models or direct inhibition in non-orthologous mouse model significantly delays cystogenesis consistent with pharmacologic or genetic inhibition of Myc upstream regulator or downstream targets in the mouse. Together, these studies on PKD proteins upon dysregulation not only converged on Myc as a focal point but also attribute to Myc upregulation a causal and « driver » role in pathogenesis. This review will present and discuss our current knowledge on Myc and p53, focused on PKD mouse models and ADPKD.

The Genetics and Mechanisms of T-Cell Acute Lymphoblastic Leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematologic malignancy derived from early T-cell progenitors. The recognition of clinical, genetic, transcriptional, and biological heterogeneity in this disease has already translated into new prognostic biomarkers, improved leukemia animal models, and emerging targeted therapies. This work reviews our current understanding of the molecular mechanisms of T-ALL.

FBXW4 Is Highly Expressed and Associated With Poor Survival in Acute Myeloid Leukemia

The F-box and WD repeat domain-containing (FBXW) proteins play an important role in ubiquitin proteasome by inducing protein degradation. Ten FBXW proteins have been identified in humans. The functions of FBXW proteins, like FBXW7, have been well-established in many human cancers. However, little is known about their transcriptional expression profiles and relationship with prognosis in acute myeloid leukemia (AML). Here we investigated the roles of FBXW proteins in AML by analyzing their mRNA expression profiles and association with clinical features using data from EMBL-EBI, the Cancer Cell Line Encyclopedia, Gene Expression Profiling Interactive Analysis, and cBioPortal databases. Our results showed that the mRNA level of FBXW proteins were highly detected by microarray in 14 AML cell lines, although there were no obvious differences. The expression of FBXW4 was significantly higher in AML patients compared with that in normal controls (P < 0.01). Patients whose age was ≥60 years old had a higher FBXW4 expression when compared with those who were <60 years old (P < 0.05). Cytogenetic favorable-risk group patients had a much lower FBXW4 expression than the intermediate- and poor-risk group patients (P < 0.0001). Moreover, patients with high FBXW4 expression exhibited significantly shorter event-free survival (EFS) and overall survival (OS) than those with low FBXW4 expression (median EFS: 5.3 vs. 10.0 months, P = 0.025; median OS: 8.1 vs. 19.0 months, P= 0.015). A multivariate analysis indicated that high FBXW4 expression was an independent risk factor for poor EFS in AML patients who received intensive chemotherapy followed by allo-SCT. In summary, our data suggested that FBXW4 is aberrantly expressed in AML and high FBXW4 expression might be a poor prognostic biomarker; future functional and mechanistic studies will further illuminate the roles of FBXW4 in AML.

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.

MYC's Fine Line Between B Cell Development and Malignancy

The transcription factor MYC is transiently expressed during B lymphocyte development, and its correct modulation is essential in defined developmental transitions. Although temporary downregulation of MYC is essential at specific points, basal levels of expression are maintained, and its protein levels are not completely silenced until the B cell becomes fully differentiated into a plasma cell or a memory B cell. MYC has been described as a proto-oncogene that is closely involved in many cancers, including leukemia and lymphoma. Aberrant expression of MYC protein in these hematological malignancies results in an uncontrolled rate of proliferation and, thereby, a blockade of the differentiation process. MYC is not activated by mutations in the coding sequence, and, as reviewed here, its overexpression in leukemia and lymphoma is mainly caused by gene amplification, chromosomal translocations, and aberrant regulation of its transcription. This review provides a thorough overview of the role of MYC in the developmental steps of B cells, and of how it performs its essential function in an oncogenic context, highlighting the importance of appropriate MYC regulation circuitry.

FGF Trapping Inhibits Multiple Myeloma Growth through c-Myc Degradation-Induced Mitochondrial Oxidative Stress

Multiple myeloma, the second most common hematologic malignancy, frequently relapses because of chemotherapeutic resistance. Fibroblast growth factors (FGF) act as proangiogenic and mitogenic cytokines in multiple myeloma. Here, we demonstrate that the autocrine FGF/FGFR axis is essential for multiple myeloma cell survival and progression by protecting multiple myeloma cells from oxidative stress-induced apoptosis. In keeping with the hypothesis that the intracellular redox status can be a target for cancer therapy, FGF/FGFR blockade by FGF trapping or tyrosine kinase inhibitor impaired the growth and dissemination of multiple myeloma cells by inducing mitochondrial oxidative stress, DNA damage, and apoptotic cell death that were prevented by the antioxidant vitamin E or mitochondrial catalase overexpression. In addition, mitochondrial oxidative stress occurred as a consequence of proteasomal degradation of the c-Myc oncoprotein that led to glutathione depletion. Accordingly, expression of a proteasome-nondegradable c-Myc protein mutant was sufficient to avoid glutathione depletion and rescue the proapoptotic effects due to FGF blockade. These findings were confirmed on bortezomib-resistant multiple myeloma cells as well as on bone marrow-derived primary multiple myeloma cells from newly diagnosed and relapsed/refractory patients, including plasma cells bearing the t(4;14) translocation obtained from patients with high-risk multiple myeloma. Altogether, these findings dissect the mechanism by which the FGF/FGFR system plays a nonredundant role in multiple myeloma cell survival and disease progression, and indicate that FGF targeting may represent a therapeutic approach for patients with multiple myeloma with poor prognosis and advanced disease stage. SIGNIFICANCE: This study provides new insights into the mechanisms by which FGF antagonists promote multiple myeloma cell death. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/11/2340/F1.large.jpg.

Target gene-independent functions of MYC oncoproteins

Oncoproteins of the MYC family are major drivers of human tumorigenesis. Since a large body of evidence indicates that MYC proteins are transcription factors, studying their function has focused on the biology of their target genes. Detailed studies of MYC-dependent changes in RNA levels have provided contrasting models of the oncogenic activity of MYC proteins through either enhancing or repressing the expression of specific target genes, or as global amplifiers of transcription. In this Review, we first summarize the biochemistry of MYC proteins and what is known (or is unclear) about the MYC target genes. We then discuss recent progress in defining the interactomes of MYC and MYCN and how this information affects central concepts of MYC biology, focusing on mechanisms by which MYC proteins modulate transcription. MYC proteins promote transcription termination upon stalling of RNA polymerase II, and we propose that this mechanism enhances the stress resilience of basal transcription. Furthermore, MYC proteins coordinate transcription elongation with DNA replication and cell cycle progression. Finally, we argue that the mechanism by which MYC proteins regulate the transcription machinery is likely to promote tumorigenesis independently of global or relative changes in the expression of their target genes.

Mission Possible: Advances in MYC Therapeutic Targeting in Cancer

MYC is a master transcriptional regulator that controls almost all cellular processes. Over the last several decades, researchers have strived to define the context-dependent transcriptional gene programs that are controlled by MYC, as well as the mechanisms that regulate MYC function, in an effort to better understand the contribution of this oncoprotein to cancer progression. There are a wealth of data indicating that deregulation of MYC activity occurs in a large number of cancers and significantly contributes to disease progression, metastatic potential, and therapeutic resistance. Although the therapeutic targeting of MYC in cancer is highly desirable, there remain substantial structural and functional challenges that have impeded direct MYC-targeted drug development and efficacy. While efforts to drug the ‘undruggable’ may seem futile given these challenges and considering the broad reach of MYC, significant strides have been made to identify points of regulation that can be exploited for therapeutic purposes. These include targeting the deregulation of MYC transcription in cancer through small-molecule inhibitors that induce epigenetic silencing or that regulate the G-quadruplex structures within the MYC promoter. Alternatively, compounds that disrupt the DNA-binding activities of MYC have been the long-standing focus of many research groups, since this method would prevent downstream MYC oncogenic activities regardless of upstream alterations. Finally, proteins involved in the post-translational regulation of MYC have been identified as important surrogate targets to reduce MYC activity downstream of aberrant cell stimulatory signals. Given the complex regulation of the MYC signaling pathway, a combination of these approaches may provide the most durable response, but this has yet to be shown. Here, we provide a comprehensive overview of the different therapeutic strategies being employed to target oncogenic MYC function, with a focus on post-translational mechanisms.

PROTACs: A novel strategy for cancer therapy

Chemotherapeutic strategy has been widely used for treating malignance by targeting irregular expressed or mutant proteins with small molecular inhibitors (SMIs) or monoclonal antibodies (mAbs). However, most intracellular proteins lack of active sites or antigens where SMIs or mAbs bind with, and are called as non-druggable targets for a long time. From the first year of this century, PROteolysis-TArgeting Chimeras (PROTACs) has emerged to be a promising approach for proteins, including those non-druggable ones, such as transcriptional factors and scaffold proteins. The first generation of peptide-based PROTACs adopts β-TrCP and VHL as E3 ligases, but the cellular permeability and chemical stability issues restrict their clinical application. The second generation of small molecule-based PROTACs adopts MDM2, VHL, IAPs and Cereblon as E3 ligases have been tensely studied. To date, the targets of PROTACs including those overexpressed oncogenic proteins such as ER, AR and BRDs, disease-relevant fusion proteins such as NPM/EML4-ALK and BCR-ABL, cancer-driven mutant proteins such as EGFR, kinases such as CDKs and RTKs. The major disadvantage of PROTACs is the noncancer specificity and relative higher toxicity, due to its catalytic role. To overcome this, we and other have recently developed several similar light-controllable PROTACs, termed as the third generation controllable PROTACs. The degradation of targets by those PROTACs can be triggered by UVA or visible light, providing a tool box for further PROTACs design. Here in this review, we introduce the historical milestones and prospective for further PROTACs development in clinical use.

Direct Phosphorylation and Stabilization of MYC by Aurora B Kinase Promote T-cell Leukemogenesis

Deregulation of MYC plays an essential role in T cell acute lymphoblastic leukemia (T-ALL), yet the mechanisms underlying its deregulation remain elusive. Herein, we identify a molecular mechanism responsible for reciprocal activation between Aurora B kinase (AURKB) and MYC. AURKB directly phosphorylates MYC at serine 67, counteracting GSK3β-directed threonine 58 phosphorylation and subsequent FBXW7-mediated proteasomal degradation. Stabilized MYC, in concert with T cell acute lymphoblastic leukemia 1 (TAL1), directly activates AURKB transcription, constituting a positive feedforward loop that reinforces MYC-regulated oncogenic programs. Therefore, inhibitors of AURKB induce prominent MYC degradation concomitant with robust leukemia cell death. These findings reveal an AURKB-MYC regulatory circuit that underlies T cell leukemogenesis, and provide a rationale for therapeutic targeting of oncogenic MYC via AURKB inhibition.

The Evi5 oncogene promotes laryngeal cancer cells proliferation by stabilizing c-Myc protein

Background

The Ecotropic viral integration site 5 (Evi5) is recognized as a potential oncogene and a cell cycle regulator. Evi5 regulates the abundance of Emi1, an inhibitor of the anaphase-promoting complex/cyclosome, to govern mitotic fidelity. Evi5 has been shown to be dysregulated in several cancer types. However, the expression and biological function of Evi5 in human laryngeal squamous cell carcinoma (LSCC) are still unknown.

Methods

Clustered regularly interspaced short palindromic repeats (CRISPR)-based gene editing was used to generate Evi5 knockout (KO) LSCC cells. The proliferation and cell cycle distribution of LSCC cells was determined. The effect of Evi5 on LSCC tumor growth in vivo was studied in a tumor xenograft model in mice. The interaction between Evi5 and c-Myc was detected by immunoprecipitation (IP) assay. Luciferase assay was used to determine the transcriptional activity of c-Myc.

Results

Here, we show that Evi5 controls LSCC tumorigenesis via the stabilization of c-MYC oncogene. CRISPR-mediated knockout (KO) of Evi5 decreased the proliferation and decreased colony formation ability of LSCC cells. Knockout of Evi5 caused increased G1 phase and decreased S phase cells. In the tumor-bearing nude mice, The transplanted tumors originated from Evi5-KO TU212 cells were significantly decreased when compared with control TU212 cells. At the molecular level, we found that Evi5 interacted with c-MYC and Evi5 antagonized E3 ligase FBXW7-mediated ubiquitination and degradation of c-Myc protein, and promoted c-Myc-dependent transactivation.

Conclusion

Given the critical role of c-Myc in tumorigenesis, our data suggest that Evi5 is a potential therapeutic target in LSCC, and inhibition of Evi5 should be a prospective strategy for LSCC therapy.

UBR5 Is Coamplified with MYC in Breast Tumors and Encodes an Ubiquitin Ligase That Limits MYC-Dependent Apoptosis

For maximal oncogenic activity, cellular MYC protein levels need to be tightly controlled so that they do not induce apoptosis. Here, we show how ubiquitin ligase UBR5 functions as a molecular rheostat to prevent excess accumulation of MYC protein. UBR5 ubiquitinates MYC and its effects on MYC protein stability are independent of FBXW7. Silencing of endogenous UBR5 induced MYC protein expression and regulated MYC target genes. Consistent with the tumor suppressor function of UBR5 (HYD) in Drosophila, HYD suppressed dMYC-dependent overgrowth of wing imaginal discs. In contrast, in cancer cells, UBR5 suppressed MYC-dependent priming to therapy-induced apoptosis. Of direct cancer relevance, MYC and UBR5 genes were coamplified in MYC-driven human cancers. Functionally, UBR5 suppressed MYC-mediated apoptosis in p53-mutant breast cancer cells with UBR5/MYC coamplification. Furthermore, single-cell immunofluorescence analysis demonstrated reciprocal expression of UBR5 and MYC in human basal-type breast cancer tissues. In summary, UBR5 is a novel MYC ubiquitin ligase and an endogenous rheostat for MYC activity. In MYC-amplified, and p53-mutant breast cancer cells, UBR5 has an important role in suppressing MYC-mediated apoptosis priming and in protection from drug-induced apoptosis. SIGNIFICANCE: These findings identify UBR5 as a novel MYC regulator, the inactivation of which could be very important for understanding of MYC dysregulation on cancer cells. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/7/1414/F1.large.jpg.

The PI3K-AKT network at the interface of oncogenic signalling and cancer metabolism

The altered metabolic programme of cancer cells facilitates their cell-autonomous proliferation and survival. In normal cells, signal transduction pathways control core cellular functions, including metabolism, to couple the signals from exogenous growth factors, cytokines or hormones to adaptive changes in cell physiology. The ubiquitous, growth factor-regulated phosphoinositide 3-kinase (PI3K)-AKT signalling network has diverse downstream effects on cellular metabolism, through either direct regulation of nutrient transporters and metabolic enzymes or the control of transcription factors that regulate the expression of key components of metabolic pathways. Aberrant activation of this signalling network is one of the most frequent events in human cancer and serves to disconnect the control of cell growth, survival and metabolism from exogenous growth stimuli. Here we discuss our current understanding of the molecular events controlling cellular metabolism downstream of PI3K and AKT and of how these events couple two major hallmarks of cancer: growth factor independence through oncogenic signalling and metabolic reprogramming to support cell survival and proliferation.

E3 Ubiquitin Ligase Fbw7 Regulates the Survival of Mature B Cells

Mature naive B cells expressing BCRs of the IgM and IgD isotypes respond to Ag in secondary lymphoid organs. However, the vast majority of B cells do not undergo productive Ag encounter and have finite life spans dependent on survival signals propagated by the BCR and the BAFFR. In this study, we show that the E3 ubiquitin ligase Fbw7 is required for the maintenance of mature B cell populations in mice. BCR stimulation of B cells induced substantial apoptosis along with proliferative and growth defects upon the loss of Fbw7. Analysis of B cell proteomes revealed aberrant signaling patterns, including lower Bcl2 and diminished NF-κB signaling. Further, excessive accumulation of Fbw7 substrate c-Myc, increased Bim expression, and loss of PI3K signaling mediated apoptosis downstream of BCR signaling. In accordance, strong prosurvival signals delivered through ectopic expression of BCL2 in B cells could largely rescue apoptotic cells in the absence of Fbw7. Overall, this study reveals an unexpected role for Fbw7 in the survival and fitness of mature B cells.

Exosome-encapsulated miRNAs contribute to CXCL12/CXCR4-induced liver metastasis of colorectal cancer by enhancing M2 polarization of macrophages

Tumor-associated macrophages (TAMs) are important immunocytes associated with cancer metastasis. However, whether TAMs play a dominant role in mediating CXCL12/CXCR4-induced liver metastasis of colorectal cancer (CRC) remains unexplored. Herein, we found that CD206⁺ TAMs, which infiltrated at the invasive front, were correlated with CXCR4 expression and liver metastasis of CRC in clinical specimens. Several miRNAs (miR-25-3p, miR-130b-3p, miR-425-5p), upregulated in CRC cells by activation of the CXCL12/CXCR4 axis, could be transferred to macrophages via exosomes. These exosomal miRNAs induced M2 polarization of macrophages by regulating PTEN through activation of PI3K/Akt signaling pathway. In turn, M2 polarized macrophages promoted cancer metastasis by enhancing epithelial-mesenchymal transition (EMT) and secreting vascular endothelial growth factor (VEGF). Co-culture of CRC cells with macrophages transfected with these miRNAs or treated with exosomes enhanced their metastatic capacity both in vitro and in vivo. Clinically, the serum levels of exosomal miR-25-3p, miR-130b-3p and miR-425-5p were correlated with progression and metastasis of CRC. In conclusion, these results reveal a crucial role of exosomal miRNAs in mediating the crosstalk between CXCR4 overexpressing cancer cells and TAMs, providing potential therapeutic targets for circumventing liver metastasis of CRC.

FBW7 in hematological tumors

F-box and WD repeat domain-containing protein 7 (FBW7), also known as FBXW7, AGO or hCDC4, is an F-box protein with seven tandem WD40 repeats. FBW7 is a key substrate recognition subunit of the Skp1-Cul1-F-box-protein E3 ubiquitin ligase. FBW7 targets for ubiquitination and destruction of numerous crucial transcription factors and protooncogenes, including cyclin E, c-Myc, c-Jun, Notch and MCL-1. FBW7 is a well-characterized tumor suppressor, and its gene is frequently mutated or deleted in various types of human cancer, including colorectal cancer, gastric cancer, ovarian cancer and different types of leukemia. Accumulating evidence indicates that the aberrant expression of FBW7 is involved in the development of hematological tumors, including T cell acute lymphoblastic leukemia, adult T cell leukemia/lymphoma, chronic lymphocytic leukemia and multiple myeloma. The present review will describe the latest findings on the role of FBW7 in hematological tumors, in order to identify a novel target for future therapies.

Tumor Necrosis Factor Receptor-Associated Factor 6 Promotes Hepatocarcinogenesis by Interacting With Histone Deacetylase 3 to Enhance c-Myc Gene Expression and Protein Stability

The oncogene c-Myc is aberrantly expressed and plays a key role in malignant transformation and progression of hepatocellular carcinoma (HCC). Here, we report that c-Myc is significantly up-regulated by tumor necrosis factor receptor-associated factor 6 (TRAF6), an E3 ubiquitin ligase, in hepatocarcinogenesis. High TRAF6 expression in clinical HCC samples correlates with poor prognosis, and the loss of one copy of the Traf6 gene in Traf6^+/- mice significantly impairs liver tumorigenesis. Mechanistically, TRAF6 first interacts with and ubiquitinates histone deacetylase 3 (HDAC3) with K63-linked ubiquitin chains, which leads to the dissociation of HDAC3 from the c-Myc promoter and subsequent acetylation of histone H3 at K9, thereby epigenetically enhancing the mRNA expression of c-Myc. Second, the K63-linked ubiquitination of HDAC3 impairs the HDAC3 interaction with c-Myc and promotes c-Myc protein acetylation, which thereby enhances c-Myc protein stability by inhibiting carboxyl terminus of heat shock cognate 70-kDa-interacting protein-mediated c-Myc ubiquitination and degradation. Importantly, TRAF6/HDAC3/c-Myc signaling is also primed in hepatitis B virus-transgenic mice, unveiling a critical role for a mechanism in inflammation-cancer transition. In clinical specimens, TRAF6 positively correlates with c-Myc at both the mRNA and protein levels, and high TRAF6 and c-Myc expression is associated with an unfavorable prognosis, suggesting that TRAF6 collaborates with c-Myc to promote human hepatocarcinogenesis. Consistently, curbing c-Myc expression by inhibition of TRAF6 activity with a TRAF6 inhibitor peptide or the silencing of c-Myc by small interfering RNA significantly suppressed tumor growth in mice. Conclusion: These findings demonstrate the oncogenic potential of TRAF6 during hepatocarcinogenesis by modulating TRAF6/HDAC3/c-Myc signaling, with potential implications for HCC therapy.

Ubiquitin-Regulated Cell Proliferation and Cancer

Ubiquitin ligases (E3) play a crucial role in the regulation of different cellular processes such as proliferation and differentiation via recognition, interaction, and ubiquitination of key cellular proteins in a spatial and temporal regulated manner. The type of ubiquitin chain formed determines the fate of the substrates. The ubiquitinated substrates can be degraded by the proteasome, display altered subcellular localization, or can suffer modifications on their interaction with functional protein complexes. Deregulation of E3 activities is frequently found in various human pathologies, including cancer. The illegitimated or accelerated degradation of oncosuppressive proteins or, inversely, the abnormally high accumulation of oncoproteins, contributes to cell proliferation and transformation. Anomalies in protein abundance may be related to mutations that alter the direct or indirect recognition of proteins by the E3 enzymes or alterations in the level of expression or activity of ubiquitin ligases. Through a few examples, we illustrate here the complexity and diversity of the molecular mechanisms related to protein ubiquitination involved in cell cycle regulation. We will discuss the role of ubiquitin-dependent degradation mediated by the proteasome, the role of non-proteolytic ubiquitination during cell cycle progression, and the consequences of this deregulation on cellular transformation. Finally, we will highlight the novel opportunities that arise from these studies for therapeutic intervention.

Aurora A Kinase Inhibition Destabilizes PAX3-FOXO1 and MYCN and Synergizes with Navitoclax to Induce Rhabdomyosarcoma Cell Death

The clinically aggressive alveolar rhabdomyosarcoma (RMS) subtype is characterized by expression of the oncogenic fusion protein PAX3-FOXO1, which is critical for tumorigenesis and cell survival. Here, we studied the mechanism of cell death induced by loss of PAX3-FOXO1 expression and identified a novel pharmacologic combination therapy that interferes with PAX3-FOXO1 biology at different levels. Depletion of PAX3-FOXO1 in fusion-positive (FP)-RMS cells induced intrinsic apoptosis in a NOXA-dependent manner. This was pharmacologically mimicked by the BH3 mimetic navitoclax, identified as top compound in a screen from 208 targeted compounds. In a parallel approach, and to identify drugs that alter the stability of PAX3-FOXO1 protein, the same drug library was screened and fusion protein levels were directly measured as a read-out. This revealed that inhibition of Aurora kinase A most efficiently negatively affected PAX3-FOXO1 protein levels. Interestingly, this occurred through a novel specific phosphorylation event in and binding to the fusion protein. Aurora kinase A inhibition also destabilized MYCN, which is both a functionally important oncogene and transcriptional target of PAX3-FOXO1. Combined treatment with an Aurora kinase A inhibitor and navitoclax in FP-RMS cell lines and patient-derived xenografts synergistically induced cell death and significantly slowed tumor growth. These studies identify a novel functional interaction of Aurora kinase A with both PAX3-FOXO1 and its effector MYCN, and reveal new opportunities for targeted combination treatment of FP-RMS. SIGNIFICANCE: These findings show that Aurora kinase A and Bcl-2 family proteins are potential targets for FP-RMS.

Crosstalks of GSK3 signaling with the mTOR network and effects on targeted therapy of cancer

The introduction of therapeutics targeting specific tumor-promoting oncogenic or non-oncogenic signaling pathways has revolutionized cancer treatment. Mechanistic (previously mammalian) target of rapamycin (mTOR), a highly conserved Ser/Thr kinase, is a central hub of the phosphatidylinositol 3-kinase (PI3K)/Akt/mTOR network, one of the most frequently deregulated signaling pathways in cancer, that makes it an attractive target for therapy. Numerous mTOR inhibitors have progressed to clinical trials and two of them have been officially approved as anticancer therapeutics. However, mTOR-targeting drugs have met with a very limited success in cancer patients. Frequently, the primary impediment to a successful targeted therapy in cancer is drug-resistance, either from the very beginning of the therapy (innate resistance) or after an initial response and upon repeated drug treatment (evasive or acquired resistance). Drug-resistance leads to treatment failure and relapse/progression of the disease. Resistance to mTOR inhibitors depends, among other reasons, on activation/deactivation of several signaling pathways, included those regulated by glycogen synthase kinase-3 (GSK3), a protein that targets a vast number of substrates in its repertoire, thereby orchestrating many processes that include cell proliferation and survival, metabolism, differentiation, and stemness. A detailed knowledge of the rewiring of signaling pathways triggered by exposure to mTOR inhibitors is critical to our understanding of the consequences such perturbations cause in tumors, including the emergence of drug-resistant cells. Here, we provide the reader with an updated overview of intricate circuitries that connect mTOR and GSK3 and we relate them to the efficacy (or lack of efficacy) of mTOR inhibitors in cancer cells.

FBXW7-mediated stability regulation of signal transducer and activator of transcription 2 in melanoma formation

The physiological relevance of STAT2 (a member of STAT family) in melanoma formation is clearly shown using a human skin tissue array. Moreover, FBXW7-mediated STAT2 protein stability regulation via ubiquitination is shown to play an essential role in melanoma cell proliferation in monolayer and anchorage-independent 3D culture systems. The molecular mechanisms that regulate STAT2 protein stability by FBXW7 include the interaction between CCD and DBD domains of STAT2 and the WD40 domain of FBXW7. STAT2 phosphorylation at the putative degron motifs that contain Ser381, Thr385, and Ser393 might be mediated by GSK3β. These serve as critical amino acids that form hydrogen bonds with the WD40 domain of FBXW7. Thus, the FBXW7–STAT2 signaling axis is an important target for melanoma treatment.

In this study, we provide critical evidence that STAT2 stability regulation plays an essential role in melanoma cell proliferation and colony growth. We found that the interaction of FBXW7 and STAT2 induced STAT2 destabilization via a ubiquitination-mediated proteasomal degradation pathway. Notably, GSK3β-mediated STAT2 phosphorylation facilitated STAT2–FBXW7 interactions via the DNA binding domain of STAT2 and domains 1, 2, 6, and 7 of FBXW7 WD40. Importantly, the inverse correlation between protein levels of STAT2 and FBXW7 were observed not only in human melanoma cells but also in a human skin cancer tissue array. The relationship between protein levels of STAT2 and FBXW7, cell proliferation, and colony growth were similarly observed in the melanoma cell lines SK-MEL-2, -5, and -28. Moreover, STAT2 knockdown in melanoma cells suppressed melanoma cell proliferation and colony formation. These data demonstrated that FBXW7-mediated STAT2 stability regulation plays an essential role in melanoma cell proliferation and cancer growth.

The many substrates and functions of NEDD4-1

Tumorigenesis, tumor growth, and prognosis are highly related to gene alterations and post-translational modifications (PTMs). Ubiquitination is a critical PTM that governs practically all aspects of cellular function. An increasing number of studies show that E3 ubiquitin ligases (E3s) are important enzymes in the process of ubiquitination that primarily determine substrate specificity and thus need to be tightly controlled. Among E3s, neural precursor cell expressed developmentally downregulated 4-1 (NEDD4-1) has been shown to play a critical role in modulating the proliferation, migration, and invasion of cancer cells and the sensitivity of cancer cells to anticancer therapies via regulating multiple substrates. This review discusses some significant discoveries on NEDD4-1 substrates and the signaling pathways in which NEDD4-1 participates. In addition, we introduce the latest potential therapeutic strategies that inhibit or activate NEDD4-1 activity using small molecules. NEDD4-1 likely acts as a novel drug target or diagnostic marker in the battle against cancer.

TAZ functions as a tumor suppressor in multiple myeloma by downregulating MYC

Multiple myeloma (MM) is an incurable blood cancer that is often characterized by amplification and overexpression of the MYC oncogene. Despite efforts, direct targeting of MYC is not yet possible; therefore, alternative strategies to inhibit MYC activity are necessary. TAZ is a transcriptional coactivator downstream of the Hippo-signaling pathway that functions as an oncogene in many solid tumors. However, its role in hematological malignancies is largely unexplored. Here, we show that, in contrast to solid tumors, expression of TAZ is lower in hematological malignancies, and that high expression of TAZ correlates with better patient outcomes. We further show that TAZ is hypermethylated in MM patient samples and in a panel of MM cell lines. Genetic overexpression of TAZ or pharmacological upregulation of TAZ by treatment with the demethylating agent decitabine induces apoptosis. Importantly, TAZ-induced apoptosis is independent of canonical Hippo components LATS1 or the TEA-domain family of transcription factors. Instead, RNA-sequencing analysis revealed that overexpression of TAZ represses a MYC transcriptional program and we show that increased TAZ expression correlates with decreased MYC expression in both cell-line models and patient samples. Furthermore, promoter derepression of TAZ expression sensitizes MM cell lines through a reciprocal reduction in MYC expression using additional therapeutics such as bortezomib, trichostatin A, and panobinostat. Our findings uncover an unexpected role for TAZ in MM tumorigenesis and provide a compelling rationale for exploring the therapeutic potential of upregulating TAZ expression to restore sensitivity to specific therapeutics in MM.

GSK3β-SCFFBXW7α mediated phosphorylation and ubiquitination of IRF1 are required for its transcription-dependent turnover

IRF1 (Interferon Regulatory Factor-1) is the prototype of the IRF family of DNA binding transcription factors. IRF1 protein expression is regulated by transient up-regulation in response to external stimuli followed by rapid degradation via the ubiquitin-proteasome system. Here we report that DNA bound IRF1 turnover is promoted by GSK3β (Glycogen Synthase Kinase 3β) via phosphorylation of the T181 residue which generates a phosphodegron for the SCF (Skp-Cul-Fbox) ubiquitin E3-ligase receptor protein Fbxw7α (F-box/WD40 7). This regulated turnover is essential for IRF1 activity, as mutation of T181 results in an improperly stabilized protein that accumulates at target promoters but fails to induce RNA-Pol-II elongation and subsequent transcription of target genes. Consequently, the anti-proliferative activity of IRF1 is lost in cell lines expressing T181A mutant. Further, cell lines with dysfunctional Fbxw7 are less sensitive to IRF1 overexpression, suggesting an important co-activator function for this ligase complex. As T181 phosphorylation requires both DNA binding and RNA-Pol-II elongation, we propose that this event acts to clear ‘spent’ molecules of IRF1 from transcriptionally engaged target promoters.

Multiple Targets of the Canonical WNT/β-Catenin Signaling in Cancers

Canonical WNT/β-catenin signaling is involved in most of the mechanisms that lead to the formation and development of cancer cells. It plays a central role in three cyclic processes, which are the cell division cycle, the immune cycle, and circadian rhythms. When the canonical WNT pathway is upregulated as in cancers, the increase in β-catenin in the nucleus leads to activation of the expression of numerous genes, in particular CYCLIN D1 and cMYC, where the former influences the G1 phase of the cell division cycle, and the latter, the S phase. Every stage of the immune cycle is disrupted by the canonical WNT signaling. In numerous cancers, the dysfunction of the canonical WNT pathway is accompanied by alterations of the circadian genes (CLOCK, BMAL1, PER). Induction of these cyclic phenomena leads to the genesis of thermodynamic mechanisms that operate far from equilibrium, and that have been called “dissipative structures.” Moreover, upregulation of the canonical WNT/β-catenin signaling is important in the myofibroblasts of the cancer stroma. Their differentiation is controlled by the canonical WNT /TGF-β1 signaling. Myofibroblasts present ultraslow contractile properties due to the presence of the non-muscle myosin IIA. Myofibroblats also play a role in the inflammatory processes, often found in cancers and fibrosis processes. Finally, upregulated canonical WNT deviates mitochondrial oxidative phosphorylation toward the Warburg glycolysis metabolism, which is characteristic of cancers. Among all these cancer-generating mechanisms, the upregulated canonical WNT pathway would appear to offer the best hope as a therapeutic target, particularly in the field of immunotherapy.

Small-Molecule MYC Inhibitors Suppress Tumor Growth and Enhance Immunotherapy

Small molecules that directly target MYC and are also well tolerated in vivo will provide invaluable chemical probes and potential anti-cancer therapeutic agents. We developed a series of small-molecule MYC inhibitors that engage MYC inside cells, disrupt MYC/MAX dimers, and impair MYC-driven gene expression. The compounds enhance MYC phosphorylation on threonine-58, consequently increasing proteasome-mediated MYC degradation. The initial lead, MYC inhibitor 361 (MYCi361), suppressed in vivo tumor growth in mice, increased tumor immune cell infiltration, upregulated PD-L1 on tumors, and sensitized tumors to anti-PD1 immunotherapy. However, 361 demonstrated a narrow therapeutic index. An improved analog, MYCi975 showed better tolerability. These findings suggest the potential of small-molecule MYC inhibitors as chemical probes and possible anti-cancer therapeutic agents.

The expression characteristics of FBXW7 in human testis suggest its function is different from that in mice

F-box and WD domain protein 7 (FBXW7) is reported to bind with c-Myc in mouse spermatogonial stem cells, regulating self-renewal; however, the pattern and stage of expression of FBXW7 in human testes are unclear. In the present study, we examined the expression of human FBXW7 in adult testis, and analyzed fixed sections from adult testes and fetal testes to determine the cell type-specific expression pattern of FBXW7. The results showed that FBXW7α and FBXW7β genes are expressed in the testis; however, only FBXW7α protein could be detected. FBXW7 was not detected in human spermatogonial stem cells. Interestingly, FBXW7 was mainly expressed in the cell nuclei of later stage germ cells and differentiated somatic cells. We also observed high FBXW7 expression in human fetal germ cells, particularly in prespermatogonia. Our results raised the possibility that FBXW7 has different functions in humans and mice. The cell type-specific expression pattern of FBXW7 suggests that it performs regulatory functions during the late stage of human spermatogenesis instead of being involved in the self-renewal of spermatogonial stem cells.

SCF(FBXW7)-mediated degradation of p53 promotes cell recovery after UV-induced DNA damage

Eukaryotic cells have developed sophisticated mechanisms to ensure the integrity of the genome and prevent the transmission of altered genetic information to daughter cells. If this control system fails, accumulation of mutations would increase risk of diseases such as cancer. Ubiquitylation, an essential process for protein degradation and signal transduction, is critical for ensuring genome integrity as well as almost all cellular functions. Here, we investigated the role of the SKP1-Cullin-1-F-box protein (SCF)-[F-box and tryptophan-aspartic acid (WD) repeat domain containing 7 (FBXW7)] ubiquitin ligase in cell proliferation by searching for targets implicated in this process. We identified a hitherto-unknown FBXW7-interacting protein, p53, which is phosphorylated by glycogen synthase kinase 3 at serine 33 and then ubiquitylated by SCF(FBXW7) and degraded. This ubiquitylation is carried out in normally growing cells but primarily after DNA damage. Specifically, we found that SCF(FBXW7)-specific targeting of p53 is crucial for the recovery of cell proliferation after UV-induced DNA damage. Furthermore, we observed that amplification of FBXW7 in wild-type p53 tumors reduced the survival of patients with breast cancer. These results provide a rationale for using SCF(FBXW7) inhibitors in the treatment of this subset of tumors.-Galindo-Moreno, M., Giráldez, S., Limón-Mortés, M. C., Belmonte-Fernández, A., Reed, S. I., Sáez, C., Japón, M. Á., Tortolero, M., Romero, F. SCF(FBXW7)-mediated degradation of p53 promotes cell recovery after UV-induced DNA damage.

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

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

Drugging MYCN Oncogenic Signaling through the MYCN-PA2G4 Binding Interface

MYCN is a major driver for the childhood cancer, neuroblastoma, however, there are no inhibitors of this target. Enhanced MYCN protein stability is a key component of MYCN oncogenesis and is maintained by multiple feedforward expression loops involving MYCN transactivation target genes. Here, we reveal the oncogenic role of a novel MYCN target and binding protein, proliferation-associated 2AG4 (PA2G4). Chromatin immunoprecipitation studies demonstrated that MYCN occupies the PA2G4 gene promoter, stimulating transcription. Direct binding of PA2G4 to MYCN protein blocked proteolysis of MYCN and enhanced colony formation in a MYCN-dependent manner. Using molecular modeling, surface plasmon resonance, and mutagenesis studies, we mapped the MYCN-PA2G4 interaction site to a 14 amino acid MYCN sequence and a surface crevice of PA2G4. Competitive chemical inhibition of the MYCN-PA2G4 protein-protein interface had potent inhibitory effects on neuroblastoma tumorigenesis in vivo. Treated tumors showed reduced levels of both MYCN and PA2G4. Our findings demonstrate a critical role for PA2G4 as a cofactor in MYCN-driven neuroblastoma and highlight competitive inhibition of the PA2G4-MYCN protein binding as a novel therapeutic strategy in the disease. SIGNIFICANCE: Competitive chemical inhibition of the PA2G4-MYCN protein interface provides a basis for drug design of small molecules targeting MYC and MYCN-binding partners in malignancies driven by MYC family oncoproteins.

Max deletion destabilizes MYC protein and abrogates Eµ-Myc lymphomagenesis

Although MAX is regarded as an obligate dimerization partner for MYC, its function in normal development and neoplasia is poorly defined. We show that B-cell-specific deletion of Max has a modest effect on B-cell development but completely abrogates Eµ-Myc-driven lymphomagenesis. While Max loss affects only a few hundred genes in normal B cells, it leads to the global down-regulation of Myc-activated genes in premalignant Eµ-Myc cells. We show that the balance between MYC-MAX and MNT-MAX interactions in B cells shifts in premalignant B cells toward a MYC-driven transcriptional program. Moreover, we found that MAX loss leads to a significant reduction in MYC protein levels and down-regulation of direct transcriptional targets, including regulators of MYC stability. This phenomenon is also observed in multiple cell lines treated with MYC-MAX dimerization inhibitors. Our work uncovers a layer of Myc autoregulation critical for lymphomagenesis yet partly dispensable for normal development.

LncRNA GLCC1 promotes colorectal carcinogenesis and glucose metabolism by stabilizing c-Myc

Long non-coding RNAs (lncRNAs) contribute to colorectal cancer (CRC). However, the role of lncRNAs in CRC metabolism, especially glucose metabolism remains largely unknown. In this study, we identify a lncRNA, GLCC1, which is significantly upregulated under glucose starvation in CRC cells, supporting cell survival and proliferation by enhancing glycolysis. Mechanistically, GLCC1 stabilizes c-Myc transcriptional factor from ubiquitination by direct interaction with HSP90 chaperon and further specifies the transcriptional modification pattern on c-Myc target genes, such as LDHA, consequently reprogram glycolytic metabolism for CRC proliferation. Clinically, GLCC1 is associated with tumorigenesis, tumor size and predicts poor prognosis. Thus, GLCC1 is mechanistically, functionally, and clinically oncogenic in colorectal cancer. Targeting GLCC1 and its pathway may be meaningful for treating patients with colorectal cancer.

Site-specific phosphorylation of Fbxw7 by Cdk5/p25 and its resulting decreased stability are linked to glutamate-induced excitotoxicity

Cyclin-dependent kinase 5 (Cdk5) is a serine/threonine protein kinase that regulates brain development and neurodegeneration. Cdk5 is activated by p25 that is generated from calpain-dependent cleavage of p35. The generation of p25 is responsible for the aberrant hyper-activation of Cdk5, which causes neurodegeneration. Using in vitro assays, we discovered that F-box/WD repeat-containing protein 7 (Fbxw7) is a new substrate of Cdk5. Additionally, Cdk5-dependent phosphorylation of Fbxw7 was detected in the presence of p25, and two amino acid residues (S349 and S372) were determined to be major phosphorylation sites. This phosphorylation was eventually linked to decreased stability of Fbxw7. Using a culture model of cortical neurons challenged with glutamate, we confirmed that decreased stability of Fbxw7 was indeed Cdk5-dependent. Furthermore, diminished levels of Fbxw7 led to increased levels of transcription factor AP-1 (c-Jun), a known substrate of Fbxw7. Given that previous reports demonstrate that c-Jun plays a role in accelerating neuronal apoptosis in these pathological models, our data support the concepts of a molecular cascade in which Cdk5-mediated phosphorylation of Fbxw7 negatively regulates Fbxw7 expression, thereby contributing to neuronal cell death following glutamate-mediated excitotoxicity.

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

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

Writing and erasing MYC ubiquitination and SUMOylation

The transcription factor c-MYC (MYC thereafter) controls diverse transcription programs and plays a key role in the development of many human cancers. Cells develop multiple mechanisms to ensure that MYC levels and activity are precisely controlled in normal physiological context. As a short half-lived protein, MYC protein levels are tightly regulated by the ubiquitin proteasome system. Over a dozen of ubiquitin ligases have been found to ubiquitinate MYC whereas a number of deubiquitinating enzymes counteract this process. Recent studies show that SUMOylation and deSUMOylation can also regulate MYC protein stability and activity. Interestingly, evidence suggests an intriguing crosstalk between MYC ubiquitination and SUMOylation. Deregulation of the MYC ubiquitination-SUMOylation regulatory network may contribute to tumorigenesis. This review is intended to provide the current understanding of the complex regulation of the MYC biology by dynamic ubiquitination and SUMOylation and their crosstalk.

SCFFBXW7/GSK3β-Mediated GFI1 Degradation Suppresses Proliferation of Gastric Cancer Cells

Gastric cancer is the third leading cause of cancer-related death worldwide. The regulatory mechanisms underlying gastric cancer cell proliferation are largely unclear. Here, we show that the transcription factor GFI1 is associated with advanced clinical gastric cancer progression and promoted gastric cancer cell proliferation partially through inhibition of gastrokine-2 (GKN2) transcription. GFI1 was a degrading substrate of FBXW7, whose loss was observed in gastric cancer. Mechanistically, GSK3β-mediated GFI1 S94/S98 phosphorylation triggered its interaction with FBXW7, resulting in SCFFBXW7-mediated ubiquitination and degradation. A nondegradable GFI1 S94A/S98A mutant was more potent in driving gastric cancer cell proliferation and tumorigenesis than wild-type GFI1. Overall, this study reveals the oncogenic role of GFI1 in gastric cancer and provides mechanistic insights into the tumor suppressor function of FBXW7. SIGNIFICANCE: These findings demonstrate the oncogenic role of the transcription factor GFI1 and the tumor suppressive function of FBXW7 in gastric cancer.

FBXO45-MYCBP2 regulates mitotic cell fate by targeting FBXW7 for degradation

Cell fate decision upon prolonged mitotic arrest induced by microtubule-targeting agents depends on the activity of the tumor suppressor and F-box protein FBXW7. FBXW7 promotes mitotic cell death and prevents premature escape from mitosis through mitotic slippage. Mitotic slippage is a process that can cause chemoresistance and tumor relapse. Therefore, understanding the mechanisms that regulate the balance between mitotic cell death and mitotic slippage is an important task. Here we report that FBXW7 protein levels markedly decline during extended mitotic arrest. FBXO45 binds to a conserved acidic N-terminal motif of FBXW7 specifically under a prolonged delay in mitosis, leading to ubiquitylation and subsequent proteasomal degradation of FBXW7 by the FBXO45-MYCBP2 E3 ubiquitin ligase. Moreover, we find that FBXO45-MYCBP2 counteracts FBXW7 in that it promotes mitotic slippage and prevents cell death in mitosis. Targeting this interaction represents a promising strategy to prevent chemotherapy resistance.

Fbxw7 increases CCL2/7 in CX3CR1hi macrophages to promote intestinal inflammation

Resident and inflammatory mononuclear phagocytes (MPh) with functional plasticity in the intestine are critically involved in the pathology of Inflammatory Bowel Diseases (IBD), in which the mechanism remains incompletely understood. In the present study, we found that increased expression of E3 ligase FBXW7 in the inflamed intestine was significantly correlated to IBD severity in both human diseases and mice model. Myeloid-Fbxw7 deficiency protected mice from dextran sodium sulfate (DSS) and 2,6,4-trinitrobenzene sulfonic acid (TNBS) induced colitis. Fbxw7 deficiency resulted in decreased production of chemokines CCL2 and CCL7 by colonic CX3CR1hi resident macrophages and reduced accumulation of CX3CR1int pro-inflammatory MPh in colitis colon tissue. Mice received AAV-shFbxw7 administration showed significantly improved survival rate and alleviated colitis. Mechanisms screening demonstrated that FBXW7 suppresses H3K27me3 modification and promotes Ccl2 and Ccl7 expression via degradation of histone-lysine N-methyltransferase EZH2 in macrophages. Taken together, our results indicate that FBXW7 degrades EZH2 and increases Ccl2/Ccl7 in CX3CR1hi macrophages, which promotes the recruiting CX3CR1int pro-inflammatory MPh into local colon tissues with colitis. Targeting FBXW7 might represent a potential therapeutic approach for intestine inflammation intervention.

E3 ligase Fbw7 participates in oxidative stress‑induced myocardial cell injury via interacting with Mcl‑1

Oxidative stress participates in several heart diseases and is an important mechanism contributing to the pathological alterations of myocardial cell injury. In recent years, ubiquitylation has been demonstrated to be an important biochemical reaction associated with apoptosis. To investigate the effects and interactions of the E3 ligase F-box and WD repeat domain containing 7 (Fbw7) and MCL1 apoptosis regulator, BCL2 family member (Mcl-1) in myocardial cells during oxidative stress, Cell Counting Kit-8, flow cytometry, western blot, reactive oxygen species and co-immunoprecipitation assays were conducted. The current study revealed that Fbw7 may facilitate apoptosis via the Mcl-1-Bax pathway in oxidative stress-induced myocardial H9c2 cell injury. Mcl-1 inhibits the functions of Bcl-2 family members, including the mitochondrial apoptosis factor Bax, to maintain cell viability; however, the present study suggested that Fbw7 may degrade Mcl-1 and impaired this process. Therefore, it may be hypothesized that Fbw-7 promotes myocardial cell injury via interacting with Mcl-1.

The MAP3K13-TRIM25-FBXW7α axis affects c-Myc protein stability and tumor development

c-Myc (Myc) is a master transcription factor that is often deregulated and highly expressed by at least 50% of cancers. In many cases, Myc protein levels correlate with resistance to therapy and poor prognosis. However, effective direct inhibition of Myc by pharmacologic approaches has remained unachievable. Here, we identify MAP3K13 as a positive regulator of Myc to promote tumor development. Our findings show that MAP3K13 upregulation is predictive of poor outcomes in patients with hepatocellular carcinoma (HCC). Mechanistically, MAP3K13 phosphorylates the E3 ubiquitin ligase TRIM25 at Ser₁₂ to decrease its polyubiquitination and proteasomal degradation. This newly stabilized TRIM25 then directly ubiquitinates Lys₄₁₂ of FBXW7α, a core subunit of the SKP1-Cullin-F-box (SCF) ubiquitin ligase complex involved in Myc ubiquitination, thereby stabilizing Myc. Together, these results reveal a novel regulatory pathway that supervises Myc protein stability via the MAP3K13-TRIM25-FBXW7α signaling axis. In addition, they provide a potential therapeutic target in Myc over-expressing human cancers.

The role of aurora A and polo-like kinases in high-risk lymphomas

High-risk lymphomas (HRLs) are associated with dismal outcomes and remain a therapeutic challenge. Recurrent genetic and molecular alterations, including c-myc expression and aurora A kinase (AAK) and polo-like kinase-1 (PLK1) activation, promote cell proliferation and contribute to the highly aggressive natural history associated with these lymphoproliferative disorders. In addition to its canonical targets regulating mitosis, the AAK/PLK1 axis directly regulates noncanonical targets, including c-myc. Recent studies demonstrate that HRLs, including T-cell lymphomas and many highly aggressive B-cell lymphomas, are dependent upon the AAK/PLK1 axis. Therefore, the AAK/PLK1 axis has emerged as an attractive therapeutic target in these lymphomas. In addition to reviewing these recent findings, we summarize the rationale for targeting AAK/PLK1 in high-risk and c-myc-driven lymphoproliferative disorders.

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

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