Phosphorylation regulates c-Myc's oncogenic activity in the mammary gland

How to Choose a Mouse Model of Breast Cancer, a Genomic Perspective

Human breast cancer is a heterogeneous disease with numerous subtypes that have been defined through immunohistological, histological, and gene expression patterns. The diversity of breast cancer has made the study of its various underlying causes complex. To facilitate the examination of particular facets of breast cancer, mouse models have been generated, ranging from carcinogen induced models to genetically engineered mice. While mouse models have been generated to mimic the initiating event, including p53 loss, BRCA loss, or overexpression of HER2 / Neu / erbB2, other genomic events are often not well characterized. However, these secondary genetic events are often critical to the mouse tumor evolution, subtype, and outcome, just as they are in human breast cancer. As such, these other genomic events are a critical component of what models are chosen to study specific subtypes of human breast cancer. Here we review the genomic analyses that have been completed for various genetically engineered mouse models, how they compare to human breast cancer, and detail how this information can be used in choosing a mouse model for analysis.

Antigen Complexed with a TLR9 Agonist Bolsters c-Myc and mTORC1 Activity in Germinal Center B Lymphocytes

The germinal center (GC) is the anatomical site where humoral immunity evolves. B cells undergo cycles of proliferation and selection to produce high-affinity Abs against Ag. Direct linkage of a TLR9 agonist (CpG) to a T-dependent Ag increases the number of GC B cells. We used a T-dependent Ag complexed with CpG and a genetic model for ablating the TLR9 signaling adaptor molecule MyD88 specifically in B cells (B-MyD88^- mice) together with transcriptomics to determine how this innate pathway positively regulates the GC. GC B cells from complex Ag-immunized B-MyD88^- mice were defective in inducing gene expression signatures downstream of c-Myc and mTORC1. In agreement with the latter gene signature, ribosomal protein S6 phosphorylation was increased in GC B cells from wild-type mice compared with B-MyD88^- mice. However, GC B cell expression of a c-Myc protein reporter was enhanced by CpG attached to Ag in both wild-type and B-MyD88^- mice, indicating a B cell-extrinsic effect on c-Myc protein expression combined with a B cell-intrinsic enhancement of gene expression downstream of c-Myc. Both mTORC1 activity and c-Myc are directly induced by T cell help, indicating that TLR9 signaling in GC B cells either enhances their access to T cell help or directly influences these pathways to further enhance the effect of T cell help. Taken together, these findings indicate that TLR9 signaling in the GC could provide a surrogate prosurvival stimulus, "TLR help," thus lowering the threshold for selection and increasing the magnitude of the GC response.

Modelling the MYC-driven normal-to-tumour switch in breast cancer

The potent MYC oncoprotein is deregulated in many human cancers, including breast carcinoma, and is associated with aggressive disease. To understand the mechanisms and vulnerabilities of MYC-driven breast cancer, we have generated an in vivo model that mimics human disease in response to MYC deregulation. MCF10A cells ectopically expressing a common breast cancer mutation in the phosphoinositide 3 kinase pathway (PIK3CA^H1047R) led to the development of organised acinar structures in mice. Expressing both PIK3CA^H1047R and deregulated MYC led to the development of invasive ductal carcinoma. Therefore, the deregulation of MYC expression in this setting creates a MYC-dependent normal-to-tumour switch that can be measured in vivo. These MYC-driven tumours exhibit classic hallmarks of human breast cancer at both the pathological and molecular level. Moreover, tumour growth is dependent upon sustained deregulated MYC expression, further demonstrating addiction to this potent oncogene and regulator of gene transcription. We therefore provide a MYC-dependent model of breast cancer, which can be used to assay invivo tumour signalling pathways, proliferation and transformation from normal breast acini to invasive breast carcinoma. We anticipate that this novel MYC-driven transformation model will be a useful research tool to better understand the oncogenic function of MYC and for the identification of therapeutic vulnerabilities.

Summary: We present a MYC-driven transformation model of breast cancer that recapitulates the disease in vivo and which can be used to identify MYC-dependent cancer vulnerabilities.

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.

CD110 promotes pancreatic cancer progression and its expression is correlated with poor prognosis

PURPOSE

This study aimed at investigating the function and significance of CD110 expression in pancreatic cancer.

METHODS

We performed immunohistochemical staining for CD110 expression in tumor samples from 86 patients with pancreatic cancer. We evaluated clinical outcomes and other clinicopathological factors to determine the significance of CD110 on survival and liver metastasis. We examine thrombopoietin-CD110 signaling in cancer cell extravasation in vitro and in vivo. We investigated the effects of CD110 knockdown on liver metastasis in a splenic xenograft mouse model.

RESULTS

CD110 expression in cancer cells was associated with low-histological-grade invasive ductal carcinoma, and patients with high CD110 expression had poorer prognosis (P = 0.0003). High CD110 expression was an independent predictor of liver metastasis (P = 0.0422). Knockdown of CD110 expression significantly attenuated cell migration and invasion. Treatment with thrombopoietin promoted pancreatic cancer cell extravasation. In the presence of thrombopoietin, CD110 increased cell viability through the activation of the ERK-MYC signaling pathway. Knockdown of CD110 expression inhibited liver metastases in the mouse model.

CONCLUSIONS

CD110 promotes pancreatic cancer progression and it may serve as a predictive factor for liver metastasis.

Myc and Loss of p53 Cooperate to Drive Formation of Choroid Plexus Carcinoma

Choroid plexus carcinoma (CPC) is a rare brain tumor that occurs most commonly in very young children and has a dismal prognosis despite intensive therapy. Improved outcomes for patients with CPC depend on a deeper understanding of the mechanisms underlying the disease. Here we developed transgenic models of CPCs by activating the Myc oncogene and deleting the Trp53 tumor suppressor gene in murine neural stem cells or progenitors. Murine CPC resembled their human counterparts at a histologic level, and like the hypodiploid subset of human CPC, exhibited multiple whole-chromosome losses, particularly of chromosomes 8, 12, and 19. Analysis of murine and human CPC gene expression profiles and copy number changes revealed altered expression of genes involved in cell cycle, DNA damage response, and cilium function. High-throughput drug screening identified small molecule inhibitors that decreased the viability of CPC. These models will be valuable tools for understanding the biology of choroid plexus tumors and for testing novel approaches to therapy. SIGNIFICANCE: This study describes new mouse models of choroid plexus carcinoma and uses them to investigate the biology and therapeutic responsiveness of this highly malignant pediatric brain tumor.

Intestinal insulin/IGF1 signalling through FoxO1 regulates epithelial integrity and susceptibility to colon cancer

Obesity promotes the development of insulin resistance and increases the incidence of colitis-associated cancer (CAC), but whether a blunted insulin action specifically in intestinal epithelial cells (IECs) affects CAC is unknown. Here, we show that obesity impairs insulin sensitivity in IECs and that mice with IEC-specific inactivation of the insulin and IGF1 receptors exhibit enhanced CAC development as a consequence of impaired restoration of gut barrier function. Blunted insulin signalling retains the transcription factor FOXO1 in the nucleus to inhibit expression of Dsc3, thereby impairing desmosome formation and epithelial integrity. Both IEC-specific nuclear FoxO1ADA expression and IEC-specific Dsc3 inactivation recapitulate the impaired intestinal integrity and increased CAC burden. Spontaneous colonic tumour formation and compromised intestinal integrity are also observed upon IEC-specific coexpression of FoxO1ADA and a stable Myc variant, thus suggesting a molecular mechanism through which impaired insulin action and nuclear FOXO1 in IECs promotes CAC.

Loss of FOXP3 and TSC1 Accelerates Prostate Cancer Progression through Synergistic Transcriptional and Posttranslational Regulation of c-MYC

Although c-MYC and mTOR are frequently activated proteins in prostate cancer, any interaction between the two is largely untested. Here, we characterize the functional cross-talk between FOXP3-c-MYC and TSC1-mTOR signaling during tumor progression. Deletion of Tsc1 in mouse embryonic fibroblasts (MEF) decreased phosphorylation of c-MYC at threonine 58 (pT58) and increased phosphorylation at serine 62 (pS62), an observation validated in prostate cancer cells. Conversely, inhibition of mTOR increased pT58 but decreased pS62. Loss of both FOXP3 and TSC1 in prostate cancer cells synergistically enhanced c-MYC expression via regulation of c-Myc transcription and protein phosphorylation. This crosstalk between FOXP3 and TSC1 appeared to be mediated by both the mTOR-4EBP1-c-MYC and FOXP3-c-MYC pathways. In mice, Tsc1 and Foxp3 double deletions in the prostate led to prostate carcinomas at an early age; this did not occur in these mice with an added c-Myc deletion. In addition, we observed synergistic antitumor effects of cotreating mice with inhibitors of mTOR and c-MYC in prostate cancer cells and in Foxp3 and Tsc1 double-mutant mice. In human prostate cancer, loss of nuclear FOXP3 is often accompanied by low expression of TSC1. Because loss of FOXP3 transcriptionally induces c-Myc expression and loss of TSC1 activates mTOR signaling, these data suggest cross-talk between FOXP3-c-MYC and TSC1-mTOR signaling that converges on c-MYC to regulate tumor progression. Coadministration of c-MYC and mTOR inhibitors may overcome the resistance to mTOR inhibition commonly observed in prostate cancer cells. SIGNIFICANCE: These results establish the principle of a synergistic action of TSC1 and FOXP3 during prostate cancer progression and provide new therapeutic targets for patients who have prostate cancer with two signaling defects.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/79/7/1413/F1.large.jpg.

p53-independent role of MYC mutant T58A in the proliferation and apoptosis of breast cancer cells

Myc proto-oncogene (MYC) is an oncoprotein that promotes proliferation and apoptosis. MYC mutations frequently disrupt the apoptotic processes during tumorigenesis. In the present study, the effects of the MYC point mutation T58A on the progression of a cellular tumor antigen p53 (p53)^-/- human breast cancer cell line was analyzed, and the mechanism of p53-independent MYC-induced apoptosis was investigated. HCC1937 cells were transfected with mutant (T58A) or wild-type (WT) MYC using lentiviral vectors. The proliferation of transfected cells was evaluated by colony formation and MTT assays, and apoptosis was analyzed by flow cytometry and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assays. WT MYC was transfected into HCC1937 cells exhibiting p14/p21 silencing through lentivirus-mediated RNA interference. The expression levels of Bim were detected by reverse transcription-quantitative polymerase chain reaction and western blot analyses. Mutant MYC proteins retained the ability to stimulate the proliferation of HCC1937 cells, although they were defective at promoting apoptosis due to a failure to induce the Bcl-2 homology 3 domain-only protein Bim. When p14 was silenced, the effects of mutant MYC on proliferation and apoptosis were weakened. When p21 was silenced, the effects of mutant MYC were strengthened. Breast cancer-derived T58A MYC mutations are unable to activate Bim due to their failure to regulate p14/p21. It was concluded that mutant MYC was more effective compared with WT MYC at promoting the progression of breast cancer.

Post-translational modification localizes MYC to the nuclear pore basket to regulate a subset of target genes involved in cellular responses to environmental signals

In this study, Su et al. investigate how post-translational modifications of Myc that affect stability and oncogenic activity regulate its function. They show that Ser62 phosphorylation and PIN1-mediated isomerization of MYC dynamically regulate the spatial distribution of MYC in the nucleus, promoting its association with the inner basket of the nuclear pore in response to proliferative signals, where it recruits the histone acetyltransferase GCN5 to bind and regulate local gene acetylation and expression, thus providing new insights into how post-translational modification of MYC controls its spatial activity.

The transcription factor MYC (also c-Myc) induces histone modification, chromatin remodeling, and the release of paused RNA polymerase to broadly regulate transcription. MYC is subject to a series of post-translational modifications that affect its stability and oncogenic activity, but how these control MYC's function on the genome is largely unknown. Recent work demonstrates an intimate connection between nuclear compartmentalization and gene regulation. Here, we report that Ser62 phosphorylation and PIN1-mediated isomerization of MYC dynamically regulate the spatial distribution of MYC in the nucleus, promoting its association with the inner basket of the nuclear pore in response to proliferative signals, where it recruits the histone acetyltransferase GCN5 to bind and regulate local gene acetylation and expression. We demonstrate that PIN1-mediated localization of MYC to the nuclear pore regulates MYC target genes responsive to mitogen stimulation that are involved in proliferation and migration pathways. These changes are also present at the chromatin level, with an increase in open regulatory elements in response to stimulation that is PIN1-dependent and associated with MYC chromatin binding. Taken together, our study indicates that post-translational modification of MYC controls its spatial activity to optimally regulate gene expression in response to extrinsic signals in normal and diseased states.

Generating a recombinant phosphothreonine-binding domain for a phosphopeptide of the human transcription factor, c-Myc

Transcription factor c-Myc is an oncoprotein that is regulated at the post-translational level through phosphorylation of two conserved residues, Serine 62 (Ser62) and Threonine 58 (Thr58). A highly specific tool capable of recognizing Myc via pThr58 is needed to monitor activation and localization. Through phage display, we have isolated 10 engineered Forkhead-associated (FHA) domains that selectively bind to a phosphothreonine (pThr)-containing peptide (53-FELLPpTPPLSPS-64) segment of human c-Myc. One domain variant was observed to bind to the Myc-pThr58 peptide with a K_D value of 800 nM and had >1000-fold discrimination between the phosphorylated and non-phosphorylated peptide. The crystal structure of the engineered FHA Myc-pThr-binding domain (Myc-pTBD) was solved in complex with its cognate ligand. The Myc-pTBD was observed to be structurally similar to the yeast Rad9 FHA1 domain, except that its β4-β5 and β10-β11 loops form a hydrophobic pocket to facilitate the interaction between the domain and the peptide ligand. The Myc-pTBD's specificity for its cognate ligand was demonstrated to be on a par with 3 commercial polyclonal antibodies, suggesting that this recombinant reagent is a viable alternative to antibodies for monitoring Myc regulation.

Proteomics, Post-translational Modifications, and Integrative Analyses Reveal Molecular Heterogeneity within Medulloblastoma Subgroups

There is a pressing need to identify therapeutic targets in tumors with low mutation rates such as the malignant pediatric brain tumor medulloblastoma. To address this challenge, we quantitatively profiled global proteomes and phospho-proteomes of 45 medulloblastoma samples. Integrated analyses revealed that tumors with similar RNA expression vary extensively at the post-transcriptional and post-translational levels. We identified distinct pathways associated with two subsets of SHH tumors, and found post-translational modifications of MYC that are associated with poor outcomes in group 3 tumors. We found kinases associated with subtypes and showed that inhibiting PRKDC sensitizes MYC-driven cells to radiation. Our study shows that proteomics enables a more comprehensive, functional readout, providing a foundation for future therapeutic strategies.

Metabolic Reprogramming by Dual AKT/ERK Inhibition through Imipridones Elicits Unique Vulnerabilities in Glioblastoma

Purpose: The goal of this study is to enhance the efficacy of imipridones, a novel class of AKT/ERK inhibitors that displayed limited therapeutic efficacy against glioblastoma (GBM).Experimental Design: Gene set enrichment, LC/MS, and extracellular flux analyses were used to determine the mechanism of action of novel imipridone compounds, ONC206 and ONC212. Orthotopic patient-derived xenografts were utilized to evaluate therapeutic potency.Results: Imipridones reduce the proliferation of patient-derived xenograft and stem-like glioblastoma cell cultures in vitro and in multiple xenograft models in vivo ONC212 displayed the highest potency. High levels of c-myc predict susceptibility to growth inhibition and apoptosis induction by imipridones and increased host survival in orthotopic patient-derived xenografts. As early as 1 hour, imipridones elicit on-target inhibition, followed by dephosphorylation of GSK3β at serine 9. GSK3β promotes phosphorylation of c-myc at threonine 58 and enhances its proteasomal degradation. Moreover, inhibition of c-myc by BRD4 antagonists sensitizes for imipridone-induced apoptosis in stem-like GBM cells in vitro and in vivo Imipridones affect energy metabolism by suppressing both glycolysis and oxidative phosphorylation, which is accompanied by a compensatory activation of the serine-one carbon-glycine (SOG) pathway, involving the transcription factor ATF4. Interference with the SOG pathway through novel inhibitors of PHGDH results in synergistic cell death induction in vitro and in vivo Conclusions: These results suggest that c-myc expression predicts therapeutic responses to imipridones and that imipridones lead to suppression of tumor cell energy metabolism, eliciting unique metabolic vulnerabilities that can be exploited for clinical relevant drug combination therapies. Clin Cancer Res; 24(21); 5392-406. ©2018 AACR.

Downregulation of PIK3CA via antibody-esiRNA-complexes suppresses human xenograft tumor growth

Precision cancer therapy requires on the one hand detailed knowledge about a tumor’s driver oncogenes and on the other hand an effective targeted therapy that specifically inhibits these oncogenes. While the determination of genomic landscape of a tumor has reached a very precise level, the respective therapy options are scarce. The application of small inhibitory (si) RNAs is a promising field of investigation. Here, we present the effective in vivo-treatment of colorectal cancer (CRC) xenograft tumors with antibody-complexed, endoribonuclease-prepared small inhibitory (esi)RNAs. We chose heterogeneous endoribonuclease-prepared siRNA pools (esiRNAs) against the frequently mutated genes PIK3CA and KRAS and coupled them to the anti-EGFR antibody cetuximab, which was internalized specifically into the tumor cells. esiRNA pools have been shown to exhibit superior specificity in target gene knockdown compared to classic siRNAs. We identified a significant decrease in tumor growth upon this treatment due to decreased tumor cell proliferation. The ex vivo-analysis of the xenograft CRC tumors revealed the expected downregulation of the intended direct targets PIK3CA and KRAS on protein level. Moreover, known downstream targets for EGFR signaling such as p-ERK, p-AKT, and c-MYC were decreased as well. We therefore propose the use of antibody-esiRNA complexes as a novel experimental treatment option against key components of the EGFR signaling cascade.

MYC-induced metabolic stress and tumorigenesis

The MYC oncogene is commonly altered across human cancers. Distinct from the normal MYC proto-oncogene, which is under tight transcriptional, translational, and post-translational control, deregulated oncogenic MYC drives imbalanced, non-linear amplification of transcription that results in oncogenic 'stress.' The term 'stress' had been a euphemism for our lack of mechanistic understanding, but synthesis of many studies over the past decade provides a more coherent picture of oncogenic MYC driving metastable cellular states, particularly altered metabolism, that activate and depend on cellular stress response pathways to allow for continued growth and survival. Both deregulated metabolism and these stress response pathways represent vulnerabilities that can be exploited therapeutically.

Combined HDAC and Bromodomain Protein Inhibition Reprograms Tumor Cell Metabolism and Elicits Synthetic Lethality in Glioblastoma

Purpose: Glioblastoma remains a challenge in oncology, in part due to tumor heterogeneity.Experimental Design: Patient-derived xenograft and stem-like glioblastoma cells were used as the primary model systems.Results: Based on a transcriptome and subsequent gene set enrichment analysis (GSEA), we show by using clinically validated compounds that the combination of histone deacetylase (HDAC) inhibition and bromodomain protein (BRD) inhibition results in pronounced synergistic reduction in cellular viability in patient-derived xenograft and stem-like glioblastoma cells. Transcriptome-based GSEA analysis suggests that metabolic reprogramming is involved with synergistic reduction of oxidative and glycolytic pathways in the combination treatment. Extracellular flux analysis confirms that combined HDAC inhibition and BRD inhibition blunts oxidative and glycolytic metabolism of cancer cells, leading to a depletion of intracellular ATP production and total ATP levels. In turn, energy deprivation drives an integrated stress response, originating from the endoplasmic reticulum. This results in an increase in proapoptotic Noxa. Aside from Noxa, we encounter a compensatory increase of antiapoptotic Mcl-1 protein. Pharmacologic, utilizing the FDA-approved drug sorafenib, and genetic inhibition of Mcl-1 enhanced the effects of the combination therapy. Finally, we show in orthotopic patient-derived xenografts of GBM, that the combination treatment reduces tumor growth, and that triple therapy involving the clinically validated compounds panobinostat, OTX015, and sorafenib further enhances these effects, culminating in a significant regression of tumors in vivoConclusions: Overall, these results warrant clinical testing of this novel, efficacious combination therapy. Clin Cancer Res; 24(16); 3941-54. ©2018 AACR.

The Human Cytomegalovirus Strain DB Activates Oncogenic Pathways in Mammary Epithelial Cells

Background

Human cytomegalovirus (HCMV) establishes a persistent life-long infection and increasing evidence indicates HCMV infection can modulate signaling pathways associated with oncogenesis. Breast milk is an important route of HCMV transmission in humans and we hypothesized that mammary epithelial cells could be one of the main cellular targets of HCMV infection.

Methods

The infectivity of primary human mammary epithelial cells (HMECs) was assessed following infection with the HCMV-DB strain, a clinical isolate with a marked macrophage-tropism. The impact of HCMV-DB infection on expression of p53 and retinoblastoma proteins, telomerase activity and oncogenic pathways (c-Myc, Akt, Ras, STAT3) was studied. Finally the transformation of HCMV-DB infected HMECs was evaluated using soft agar assay. CTH cells (CMV Transformed HMECs) were detected in prolonged cultures of infected HMECs. Tumor formation was observed in NOD/SCID Gamma (NSG) mice injected with CTH cells. Detection of long non coding RNA4.9 (lncRNA4.9) gene was assessed in CTH cells, tumors isolated from xenografted NSG mice and biopsies of patients with breast cancer using qualitative and quantitative PCR.

Results

We found that HCMV, especially a clinical strain named HCMV-DB, infects HMECs in vitro. The clinical strain HCMV-DB replicates productively in HMECs as evidenced by detection of early and late viral transcripts and proteins. Following infection of HMECs with HCMV-DB, we observed the inactivation of retinoblastoma and p53 proteins, the activation of telomerase activity, the activation of the proto-oncogenes c-Myc and Ras, the activation of Akt and STAT3, and the upregulation of cyclin D1 and Ki67 antigen. Colony formation was observed in soft agar seeded with HCMV-DB-infected HMECs. Prolonged culture of infected HMECs resulted in the development of clusters of spheroid cells that we called CTH cells (CMV Transformed HMECs). CTH cells when injected in NOD/SCID Gamma (NSG) mice resulted in the development of tumors. We detected in CTH cells the presence of a HCMV signature corresponding to a sequence of the long noncoding RNA4.9 (lncRNA4.9) gene. We also found the presence of the HCMV lncRNA4.9 sequence in tumors isolated from xenografted NSG mice injected with CTH cells and in biopsies of patients with breast cancer using qualitative and quantitative PCR.

Conclusions

Our data indicate that key molecular pathways involved in oncogenesis are activated in HCMV-DB-infected HMECs that ultimately results in the transformation of HMECs in vitro with the appearance of CMV-transformed HMECs (CTH cells) in culture. CTH cells display a HCMV signature corresponding to a lncRNA4.9 genomic sequence and give rise to fast growing triple-negative tumors in NSG mice. A similar lncRNA4.9 genomic sequence was detected in tumor biopsies of patients with breast cancer.

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The infection of primary human mammary epithelial cells (HMECs) with the HCMV-DB strain results in a pro-oncogenic cellular environment.

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HCMV-DB transforms primary HMECs in vitro as measured by a soft agar assay.

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Prolonged culture of HMECs infected with HCMV-DB results in the appearance of clusters of spheroid cells that we called CTH cells (CMV Transformed HMECs).

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CTH cells when injected in NOD/SCID Gamma mice resulted in the development of breast tumor.

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The HCMV lncRNA4.9 sequence was detected in CTH cells, in tumors isolated from xenografted NSG mice injected with CTH cells and in biopsies of patients with breast cancer.

Research in Context: Worldwide breast cancer is the most common cancer diagnosed among women. Etiological factors involved in breast cancer include genetic and environmental risk factors and among these latter viruses could be involved with close to one-fifth of all cancers in the world caused by infectious agents. We found that the cytomegalovirus strain DB, a member of the herpesvirus family, activates oncogenic pathways in infected mammary epithelial cells, transforms these cells in culture and favors the appearance of tumors in xenografted mice. Our findings might lead to a better understanding of the pathogenesis of breast cancer.

A new genetically engineered mouse model of choroid plexus carcinoma

Choroid plexus carcinomas (CPCs) are highly malignant brain tumours predominantly found in children and associated to poor prognosis. Improved therapy for these cancers would benefit from the generation of animal models. Here we have created a novel mouse CPC model by expressing a stabilised form of c-Myc (MycT58A) and inactivating Trp53 in the choroid plexus of newborn mice. This induced aberrant proliferation of choroid plexus epithelial cells, leading to aggressive tumour development and death within 150 days. Choroid plexus tumours occurred with a complete penetrance in all brain ventricles, with prevalence in the lateral and fourth ventricles. Histological and cellular analysis indicated that these tumours were CPCs resembling their human counterparts. Comparison of gene expression profiles of CPCs and non-neoplastic tissues revealed profound alterations in cell cycle regulation and DNA damage responses, suggesting that dysregulation of cell division and DNA checkpoint pathways may represent key vulnerabilities. This novel animal model of CPC provides an invaluable tool to elucidate the mechanism of CPC formation and to develop successful therapies against this devastating paediatric cancer.

A BET Bromodomain Inhibitor Suppresses Adiposity-Associated Malignant Transformation

Almost half a million of all new cancers have been attributed to obesity and epidemiologic evidence implicates visceral adipose tissue (VAT) and high-fat diets (HFD) in increasing cancer risk. We demonstrated that VAT-derived fibroblast growth factor 2 (FGF2) from mice fed an HFD or obese individuals stimulates the malignant transformation of epithelial cells. Mechanism-based strategies to prevent this VAT-enhanced tumorigenesis have not been explored. Clinical studies have indicated that bromodomain inhibitors have considerable potential as therapeutic agents for cancer by inhibiting the activity of several oncogenes, including c-Myc; however, their chemopreventive activity is unknown. We show herein that mice with visceral adiposity have elevated nuclear c-Myc expression in their epidermis. We hypothesized that the bromodomain inhibitor I-BET-762 (I-BET) would have efficacy in the prevention of malignant transformation by VAT and FGF2. We tested this hypothesis using our novel models of VAT-stimulated transformation in vitro and FGF2- stimulated tumor formation in vivo We found that I-BET significantly attenuates VAT and FGF2-stimulated transformation and inhibits VAT-induced c-Myc protein expression in several skin and breast epithelial cell lines. Moreover, I-BET attenuated tumor growth significantly in FGF2-treated nude mice. Work is ongoing to determine the role of visceral adiposity in c-Myc activity in several tissues and determine the inhibitory effect of I-BET on VAT-promoted tumors in vivoCancer Prev Res; 11(3); 129-42. ©2017 AACRSee related editorial by Berger and Scacheri, p. 125.

Melanoma Therapeutic Strategies that Select against Resistance by Exploiting MYC-Driven Evolutionary Convergence

Diverse pathways drive resistance to BRAF/MEK inhibitors in BRAF-mutant melanoma, suggesting that durable control of resistance will be a challenge. By combining statistical modeling of genomic data from matched pre-treatment and post-relapse patient tumors with functional interrogation of >20 in vitro and in vivo resistance models, we discovered that major pathways of resistance converge to activate the transcription factor, c-MYC (MYC). MYC expression and pathway gene signatures were suppressed following drug treatment, and then rebounded during progression. Critically, MYC activation was necessary and sufficient for resistance, and suppression of MYC activity using genetic approaches or BET bromodomain inhibition was sufficient to resensitize cells and delay BRAFi resistance. Finally, MYC-driven, BRAFi-resistant cells are hypersensitive to the inhibition of MYC synthetic lethal partners, including SRC family and c-KIT tyrosine kinases, as well as glucose, glutamine, and serine metabolic pathways. These insights enable the design of combination therapies that select against resistance evolution.

The tumor suppressor phosphatase PP2A-B56α regulates stemness and promotes the initiation of malignancies in a novel murine model

Protein phosphatase 2A (PP2A) is a ubiquitously expressed Serine-Threonine phosphatase mediating 30–50% of protein phosphatase activity. PP2A functions as a heterotrimeric complex, with the B subunits directing target specificity to regulate the activity of many key pathways that control cellular phenotypes. PP2A-B56α has been shown to play a tumor suppressor role and to negatively control c-MYC stability and activity. Loss of B56α promotes cellular transformation, likely at least in part through its regulation of c-MYC. Here we report generation of a B56α hypomorph mouse with very low B56α expression that we used to study the physiologic activity of the PP2A-B56α phosphatase. The predominant phenotype we observed in mice with B56α deficiency in the whole body was spontaneous skin lesion formation with hyperproliferation of the epidermis, hair follicles and sebaceous glands. Increased levels of c-MYC phosphorylation on Serine62 and c-MYC activity were observed in the skin lesions of the B56α^hm/hm mice. B56α deficiency was found to increase the number of skin stem cells, and consistent with this, papilloma initiation was accelerated in a carcinogenesis model. Further analysis of additional tissues revealed increased inflammation in spleen, liver, lung, and intestinal lymph nodes as well as in the skin lesions, resembling elevated extramedullary hematopoiesis phenotypes in the B56α^hm/hm mice. We also observed an increase in the clonogenicity of bone marrow stem cells in B56α^hm/hm mice. Overall, this model suggests that B56α is important for stem cells to maintain homeostasis and that B56α loss leading to increased activity of important oncogenes, including c-MYC, can result in aberrant cell growth and increased stem cells that can contribute to the initiation of malignancy.

microRNA-206 impairs c-Myc-driven cancer in a synthetic lethal manner by directly inhibiting MAP3K13

c-Myc (Myc) is one of the most frequently dysregulated oncogenic transcription factors in human cancer. By functionally screening a microRNA (miR) library, we identified miR-206 as being a synthetic lethal in Myc over-expressing human cancer cells. miR-206 inhibited MAP3K13, which resulted in Myc protein de-stabilization, and an inhibition of anchorage-independent growth and in vivo tumorigenesis by Myc over-expressing human cancer cells. Eliminating MAP3K13 by shRNA recapitulated the effects caused by miR-206, thus supporting the idea that miR-206's effect on Myc was mediated through MAP3K13. Conversely, enforced expression of MAP3K13 stabilized Myc by promoting its N-terminal phosphorylation and enhancing its transcriptional activity. Gene expression analyses of breast cancers expressing high levels of Myc indicated that low miR-206 expression and high MAP3K13 expression correlated with poor patient survival. The critical link between miR-206 and MAP3K13 in the development of Myc over-expressing human cancers suggests potential points of therapeutic intervention for this molecular sub-category.

MYC regulates ductal-neuroendocrine lineage plasticity in pancreatic ductal adenocarcinoma associated with poor outcome and chemoresistance

Intratumoral phenotypic heterogeneity has been described in many tumor types, where it can contribute to drug resistance and disease recurrence. We analyzed ductal and neuroendocrine markers in pancreatic ductal adenocarcinoma, revealing heterogeneous expression of the neuroendocrine marker Synaptophysin within ductal lesions. Higher percentages of Cytokeratin-Synaptophysin dual positive tumor cells correlate with shortened disease-free survival. We observe similar lineage marker heterogeneity in mouse models of pancreatic ductal adenocarcinoma, where lineage tracing indicates that Cytokeratin-Synaptophysin dual positive cells arise from the exocrine compartment. Mechanistically, MYC binding is enriched at neuroendocrine genes in mouse tumor cells and loss of MYC reduces ductal-neuroendocrine lineage heterogeneity, while deregulated MYC expression in KRAS mutant mice increases this phenotype. Neuroendocrine marker expression is associated with chemoresistance and reducing MYC levels decreases gemcitabine-induced neuroendocrine marker expression and increases chemosensitivity. Altogether, we demonstrate that MYC facilitates ductal-neuroendocrine lineage plasticity in pancreatic ductal adenocarcinoma, contributing to poor survival and chemoresistance.

Stabilization of the c-Myc Protein by CAMKIIγ Promotes T Cell Lymphoma

Although high c-Myc protein expression is observed alongside MYC amplification in some cancers, in most cases protein overexpression occurs in the absence of gene amplification, e.g., T cell lymphoma (TCL). Here, Ca2+/calmodulin-dependent protein kinase II γ (CAMKIIγ) was shown to stabilize the c-Myc protein by directly phosphorylating it at serine 62 (S62). Furthermore, CAMKIIγ was shown to be essential for tumor maintenance. Inhibition of CAMKIIγ with a specific inhibitor destabilized c-Myc and reduced tumor burden. Importantly, high CAMKIIγ levels in patient TCL specimens correlate with increased c-Myc and pS62-c-Myc levels. Together, the CAMKIIγ:c-Myc axis critically influences the development and maintenance of TCL and represents a potential therapeutic target for TCL.

The MYC mRNA 3'-UTR couples RNA polymerase II function to glutamine and ribonucleotide levels

Deregulated expression of MYC enhances glutamine utilization and renders cell survival dependent on glutamine, inducing "glutamine addiction". Surprisingly, colon cancer cells that express high levels of MYC due to WNT pathway mutations are not glutamine-addicted but undergo a reversible cell cycle arrest upon glutamine deprivation. We show here that glutamine deprivation suppresses translation of endogenous MYC via the 3'-UTR of the MYC mRNA, enabling escape from apoptosis. This regulation is mediated by glutamine-dependent changes in adenosine-nucleotide levels. Glutamine deprivation causes a global reduction in promoter association of RNA polymerase II (RNAPII) and slows transcriptional elongation. While activation of MYC restores binding of MYC and RNAPII function on most promoters, restoration of elongation is imperfect and activation of MYC in the absence of glutamine causes stalling of RNAPII on multiple genes, correlating with R-loop formation. Stalling of RNAPII and R-loop formation can cause DNA damage, arguing that the MYC 3'-UTR is critical for maintaining genome stability when ribonucleotide levels are low.

MYC Modulation around the CDK2/p27/SKP2 Axis

MYC is a pleiotropic transcription factor that controls a number of fundamental cellular processes required for the proliferation and survival of normal and malignant cells, including the cell cycle. MYC interacts with several central cell cycle regulators that control the balance between cell cycle progression and temporary or permanent cell cycle arrest (cellular senescence). Among these are the cyclin E/A/cyclin-dependent kinase 2 (CDK2) complexes, the CDK inhibitor p27^KIP1 (p27) and the E3 ubiquitin ligase component S-phase kinase-associated protein 2 (SKP2), which control each other by forming a triangular network. MYC is engaged in bidirectional crosstalk with each of these players; while MYC regulates their expression and/or activity, these factors in turn modulate MYC through protein interactions and post-translational modifications including phosphorylation and ubiquitylation, impacting on MYC's transcriptional output on genes involved in cell cycle progression and senescence. Here we elaborate on these network interactions with MYC and their impact on transcription, cell cycle, replication and stress signaling, and on the role of other players interconnected to this network, such as CDK1, the retinoblastoma protein (pRB), protein phosphatase 2A (PP2A), the F-box proteins FBXW7 and FBXO28, the RAS oncoprotein and the ubiquitin/proteasome system. Finally, we describe how the MYC/CDK2/p27/SKP2 axis impacts on tumor development and discuss possible ways to interfere therapeutically with this system to improve cancer treatment.

Critical role of Myc activation in mouse hepatocarcinogenesis induced by the activation of AKT and RAS pathways

MYC activation at modest levels has been frequently found in hepatocellular carcinoma. However, its significance in hepatocarcinogenesis has remained obscure. Here we examined the role of Myc activation in mouse liver tumours induced by hepatocytic expression of myristoylated AKT (AKT) and/or mutant HRAS^V12 (HRAS) via transposon-mediated gene integration. AKT or HRAS alone required 5 months to induce liver tumours, whereas their combination generated hepatocellular carcinoma within 8 weeks. Co-introduction of AKT and HRAS induced lipid-laden preneoplastic cells that grew into nodules composed of tumour cells with or without intracytoplasmic lipid, with the latter being more proliferative and associated with spontaneous Myc expression. AKT/HRAS-induced tumorigenesis was almost completely abolished when MadMyc, a competitive Myc inhibitor, was expressed simultaneously. The Tet-On induction of MadMyc in preneoplastic cells significantly inhibited the progression of AKT/HRAS-induced tumours; its induction in transformed cells suppressed their proliferative activity with alterations in lipid metabolism and protein translation. Transposon-mediated Myc overexpression facilitated tumorigenesis by AKT or HRAS, and when it was co-introduced with AKT and HRAS, diffusely infiltrating tumours without lipid accumulation developed as early as 2 weeks. Examination of the dose-responses of Myc in the enhancement of AKT/HRAS-induced tumorigenesis revealed that a reduction to one-third retained enhancing effect but three-times greater introduction damped the process with increased apoptosis. Myc overexpression suppressed the mRNA expression of proteins involved in the synthesis of fatty acids, and when combined with HRAS introduction, it also suppressed the mRNA expression of proteins involved in their degradation. Finally, the MYC-positive human hepatocellular carcinoma was characterized by the cytoplasm devoid of lipid accumulation, prominent nucleoli and a higher proliferative activity. Our results demonstrate that in hepatocarcinogenesis induced by both activated AKT and HRAS, activation of endogenous Myc is an enhancing factor and adequate levels of Myc deregulation further facilitate the process with alterations in cellular metabolism.

MYCbase: a database of functional sites and biochemical properties of Myc in both normal and cancer cells

Background

Myc is an essential gene having multiple functions such as in cell growth, differentiation, apoptosis, genomic stability, angiogenesis, and disease biology. A large number of researchers dedicated to Myc biology are generating a substantial amount of data in normal and cancer cells/tissues including Burkitt’s lymphoma and ovarian cancer.

Results

MYCbase (http://bicresources.jcbose.ac.in/ssaha4/mycbase) is a collection of experimentally supported functional sites in Myc that can influence the biological cellular processes. The functional sites were compiled according to their role which includes mutation, methylation pattern, post-translational modifications, protein-protein interactions (PPIs), and DNA interactions. In addition, biochemical properties of Myc are also compiled, which includes metabolism/pathway, protein abundance, and modulators of protein-protein interactions. The OMICS data related to Myc- like gene expression, proteomics expression using mass-spectrometry and miRNAs targeting Myc were also compiled in MYCbase. The mutation and pathway data from the MYCbase were analyzed to look at the patterns and distributions across different diseases. There were few proteins/genes found common in Myc-protein interactions and Myc-DNA binding, and these can play a significant role in transcriptional feedback loops.

Conclusion

In this report, we present a comprehensive integration of relevant information regarding Myc in the form of MYCbase. The data compiled in MYCbase provides a reliable data resource for functional sites at the residue level and biochemical properties of Myc in various cancers.

MYC and PVT1 synergize to regulate RSPO1 levels in breast cancer

Copy number gain of the 8q24 region including the v-myc avian myelocytomatosis viral oncogene homolog (MYC) oncogene has been observed in many different cancers and is associated with poor outcomes. While the role of MYC in tumor formation has been clearly delineated, we have recently shown that co-operation between adjacent long non-coding RNA plasmacytoma variant transcription 1 (PVT1) and MYC is necessary for tumor promotion. Chromosome engineered mice containing an increased copy of Myc-Pvt1 (Gain Myc-Pvt1) accelerates mammary tumors in MMTV-Neu mice, while single copy increase of each is not sufficient. In addition, mammary epithelium from the Gain Myc-Pvt1 mouse show precancerous phenotypes, notably increased DNA replication, elevated -H2AX phosphorylation and increased ductal branching. In an attempt to capture the molecular signatures in pre-cancerous cells we utilized RNA sequencing to identify potential targets of supernumerary Myc-Pvt1 cooperation in mammary epithelial cells. In this extra view we show that an extra copy of both Myc and Pvt1 leads to increased levels of Rspo1, a crucial regulator of canonical β-catenin signaling required for female development. Human breast cancer tumors with high levels of MYC transcript have significantly more PVT1 transcript and RSPO1 transcript than tumors with low levels of MYC showing that the murine results are relevant to a subset of human tumors. Thus, this work identifies a key mechanism in precancerous and cancerous tissue by which a main player in female differentiation is transcriptionally activated by supernumerary MYC and PVT1, leading to increased premalignant features, and ultimately to tumor formation.

Far-Red and Near-Infrared Seminaphthofluorophores for Targeted Pancreatic Cancer Imaging

Molecular probes that selectively highlight pancreatic cancer (PC) tissue have the potential to improve pancreatic ductal adenocarcinoma (PDAC) margin assessment through the selective highlighting of individual PC cells. Herein, we report a simple and unique family of systematically modified red and near-infrared fluorescent probes that exhibit a field-effect-derived redshift. Two of thirteen probes distributed to the normal mouse pancreas following systemic administration. One selectively accumulated in genetically modified mouse models of PDAC. The probe exhibited intracellular accumulation and enabled visualization of four levels of the structure, including the whole organ, resected tissue, individual cells, and subcellular organelles. In contrast to the small-molecule probes reported previously, it possesses an inherent affinity toward PDAC cells and thus does not require conjugation to any targeting agent. The fluorescent probe can thus promote new strategies not only for precision image-guided surgery, but also for PC detection, monitoring of therapeutic outcomes, and basic research.

Prognostic impact of MYC protein expression in central nervous system diffuse large B-cell lymphoma: comparison with MYC rearrangement and MYC mRNA expression

The prognostic role of MYC has been well documented in non-central nervous system diffuse large B-cell lymphoma; however, it remains controversial in central nervous system diffuse large B-cell lymphoma. To investigate the prognostic value of MYC, we analyzed the MYC protein expression by immunohistochemistry, mRNA expression by RNA in situ hybridization, and gene status by fluorescence in situ hybridization in 74 cases of central nervous system diffuse large B-cell lymphoma. Moreover, we examined the correlation between MYC translocation, mRNA expression, and protein expression. The mean percentage of MYC immunopositive cells was 49%. Using a 44% cutoff value, 49 (66%) cases showed MYC protein overexpression. The result of mRNA in situ hybridization using the RNA scope technology was obtained using the H-scoring system; the median value was 34.2. Using the cutoff value of 63.5, 16 (22%) cases showed MYC mRNA overexpression. MYC gene rearrangement was detected in five out of 68 (7%) cases. MYC translocation showed no statistically significant correlation with mRNA expression; however, all MYC translocation-positive cases showed MYC protein overexpression, with a higher mean percentage of MYC protein expression than that of translocation-negative cases (78 vs 48%, P=0.001). The level of MYC mRNA expression was moderately correlated with the level of MYC protein expression (P<0.001). The mean percentage of MYC protein expression in the high MYC mRNA group was higher than that in the low MYC mRNA group (70 vs 47%, P<0.001). A univariate analysis showed that age over 60 years, Eastern Cooperative Oncology Group (ECOG) performance status ≥2 and MYC protein overexpression were significantly associated with an increased risk of death. MYC translocation and MYC mRNA expression had no prognostic significance. On multivariate analysis, MYC protein overexpression and ECOG score retained prognostic significance.

Deubiquitinase USP13 maintains glioblastoma stem cells by antagonizing FBXL14-mediated Myc ubiquitination

Fang et al. show that the deubiquitinase USP13 stabilizes c-Myc in glioblastoma stem cells (GSCs) by counteracting FBXL14-mediated Myc ubiquitination. c-Myc stabilization maintains GSC self-renewal and tumorigenic potential.

Glioblastoma is the most lethal brain tumor and harbors glioma stem cells (GSCs) with potent tumorigenic capacity. The function of GSCs in tumor propagation is maintained by several core transcriptional regulators including c-Myc. c-Myc protein is tightly regulated by posttranslational modification. However, the posttranslational regulatory mechanisms for c-Myc in GSCs have not been defined. In this study, we demonstrate that the deubiquitinase USP13 stabilizes c-Myc by antagonizing FBXL14-mediated ubiquitination to maintain GSC self-renewal and tumorigenic potential. USP13 was preferentially expressed in GSCs, and its depletion potently inhibited GSC proliferation and tumor growth by promoting c-Myc ubiquitination and degradation. In contrast, overexpression of the ubiquitin E3 ligase FBXL14 induced c-Myc degradation, promoted GSC differentiation, and inhibited tumor growth. Ectopic expression of the ubiquitin-insensitive mutant T58A–c-Myc rescued the effects caused by FBXL14 overexpression or USP13 disruption. These data suggest that USP13 and FBXL14 play opposing roles in the regulation of GSCs through reversible ubiquitination of c-Myc.

Stabilization of nuclear oncoproteins by RNF4 and the ubiquitin system in cancer

RNF4, a SUMO-targeted ubiquitin ligase, stabilizes a selected group of oncoproteins. It potentiates oncoprotein activity and serves as a positive feedback agonist of Wnt and Notch pathways. RNF4 is essential for cancer cell survival and its levels are elevated in human cancers, correlating with poor outcome in a subset of cancer patients.

PIM1 kinase inhibition as a targeted therapy against triple-negative breast tumors with elevated MYC expression

Triple-negative breast cancer (TNBC), in which cells lack expression of the estrogen receptor (ER), the progesterone receptor (PR) and the ERBB2 (also known as HER2) receptor, is the breast cancer subtype with the poorest outcome. No targeted therapy is available against this subtype of cancer owing to a lack of validated molecular targets. We previously reported that signaling involving MYC-an essential, pleiotropic transcription factor that regulates the expression of hundreds of genes-is disproportionally higher in triple-negative (TN) tumors than in receptor-positive (RP) tumors. Direct inhibition of the oncogenic transcriptional activity of MYC has been challenging to achieve. Here, by conducting a shRNA screen targeting the kinome, we identified PIM1, a non-essential serine-threonine kinase, in a synthetic lethal interaction with MYC. PIM1 expression was higher in TN tumors than in RP tumors and was associated with poor prognosis in patients with hormone- and HER2-negative tumors. Small-molecule PIM kinase inhibitors halted the growth of human TN tumors with elevated MYC expression in patient-derived tumor xenograft (PDX) and MYC-driven transgenic mouse models of breast cancer by inhibiting the oncogenic transcriptional activity of MYC and restoring the function of the endogenous cell cycle inhibitor, p27. Our findings warrant clinical evaluation of PIM kinase inhibitors in patients with TN tumors that have elevated MYC expression.

PIM1 kinase regulates cell death, tumor growth and chemotherapy response in triple-negative breast cancer

Triple-negative breast cancers (TNBCs) have poor prognosis and lack targeted therapies. Here we identified increased copy number and expression of the PIM1 proto-oncogene in genomic data sets of patients with TNBC. TNBC cells, but not nonmalignant mammary epithelial cells, were dependent on PIM1 for proliferation and protection from apoptosis. PIM1 knockdown reduced expression of the anti-apoptotic factor BCL2, and dynamic BH3 profiling of apoptotic priming revealed that PIM1 prevents mitochondrial-mediated apoptosis in TNBC cell lines. In TNBC tumors and their cellular models, PIM1 expression was associated with several transcriptional signatures involving the transcription factor MYC, and PIM1 depletion in TNBC cell lines decreased, in a MYC-dependent manner, cell population growth and expression of the MYC target gene MCL1. Treatment with the pan-PIM kinase inhibitor AZD1208 impaired the growth of both cell line and patient-derived xenografts and sensitized them to standard-of-care chemotherapy. This work identifies PIM1 as a malignant-cell-selective target in TNBC and the potential use of PIM1 inhibitors for sensitizing TNBC to chemotherapy-induced apoptotic cell death.

Deubiquitinating c-Myc: USP36 steps up in the nucleolus

Ubiquitination plays a key and complex role in the regulation of c-Myc stability, transactivation, and oncogenic activity. c-Myc is ubiquitinated by a number of ubiquitin ligases (E3s), such as SCF(Fbw7) and SCF(Skp2). Depending on the E3s, ubiquitination can either positively or negatively regulate c-Myc levels and activity. Meanwhile, c-Myc ubiquitination can be reversed by deubiquitination. An early study showed that USP28 deubiquitinates c-Myc via interacting with Fbw7α whereas a recent study reveals that USP37 deubiquitinates c-Myc independently of Fbw7 and c-Myc phosphorylation. Consequently, both USP28 and USP37 stabilize c-Myc and enhance its activity. We recently found the nucleolar USP36 as a novel c-Myc deubiquitinase that controls the end-point of c-Myc degradation pathway in the nucleolus. Here we briefly review the current understanding of ubiquitination and deubiquitination regulation of c-Myc and further discuss the USP36-c-Myc regulatory pathway.

MYC-nick promotes cell migration by inducing fascin expression and Cdc42 activation

MYC-nick is a cytoplasmic, transcriptionally inactive member of the MYC oncoprotein family, generated by a proteolytic cleavage of full-length MYC. MYC-nick promotes migration and survival of cells in response to chemotherapeutic agents or withdrawal of glucose. Here we report that MYC-nick is abundant in colonic and intestinal tumors derived from mouse models with mutations in the Wnt, TGF-β, and PI3K pathways. Moreover, MYC-nick is elevated in colon cancer cells deleted for FBWX7, which encodes the major E3 ligase of full-length MYC frequently mutated in colorectal cancers. MYC-nick promotes the migration of colon cancer cells assayed in 3D cultures or grown as xenografts in a zebrafish metastasis model. MYC-nick accelerates migration by activating the Rho GTPase Cdc42 and inducing fascin expression. MYC-nick, fascin, and Cdc42 are frequently up-regulated in cells present at the invasive front of human colorectal tumors, suggesting a coordinated role for these proteins in tumor migration.

Epidermal CYLD inactivation sensitizes mice to the development of sebaceous and basaloid skin tumors

The deubiquitinase-encoding gene Cyld displays a dominant genetic linkage to a wide spectrum of skin-appendage tumors, which could be collectively designated as CYLD mutant-syndrome (CYLD^m-syndrome). Despite recent advances, little is understood about the molecular mechanisms responsible for this painful and difficult-to-treat skin disease. Here, we generated a conditional mouse model with epidermis-targeted expression of a catalytically deficient CYLD^m through K14-Cre-mediated deletion of exon 9 (hereafter refer to Cyld^EΔ9/Δ9 ). Cyld^EΔ9/Δ9 mice were born alive but developed hair and sebaceous gland abnormalities and dental defects at 100% and 60% penetrance, respectively. Upon topical challenge with DMBA/TPA, these animals primarily developed sebaceous and basaloid tumors resembling human CYLD^m-syndrome as opposed to papilloma, which is most commonly induced in WT mice by this treatment. Molecular analysis revealed that TRAF6-K63-Ubiquitination (K63-Ub), c-Myc-K63-Ub, and phospho-c-Myc (S62) were markedly elevated in Cyld^EΔ9/Δ9 skin. Topical treatment with a pharmacological c-Myc inhibitor induced sebaceous and basal cell apoptosis in Cyld^EΔ9/Δ9 skin. Consistently, c-Myc activation was readily detected in human cylindroma and sebaceous adenoma. Taken together, our findings demonstrate that Cyld^EΔ9/Δ9 mice represent a disease-relevant animal model and identify TRAF6 and c-Myc as potential therapeutic targets for CYLD^m-syndrome.

Clinical and Biologic Significance of MYC Genetic Mutations in De Novo Diffuse Large B-cell Lymphoma

PURPOSE

MYC is a critical driver oncogene in many cancers, and its deregulation in the forms of translocation and overexpression has been implicated in lymphomagenesis and progression of diffuse large B-cell lymphoma (DLBCL). The MYC mutational profile and its roles in DLBCL are unknown. This study aims to determine the spectrum of MYC mutations in a large group of patients with DLBCL, and to evaluate the clinical significance of MYC mutations in patients with DLBCL treated with rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP) immunochemotherapy.

EXPERIMENTAL DESIGN

We identified MYC mutations in 750 patients with DLBCL using Sanger sequencing and evaluated the prognostic significance in 602 R-CHOP-treated patients.

RESULTS

The frequency of MYC mutations was 33.3% at the DNA level (mutations in either the coding sequence or the untranslated regions) and 16.1% at the protein level (nonsynonymous mutations). Most of the nonsynonymous mutations correlated with better survival outcomes; in contrast, T58 and F138 mutations (which were associated with MYC rearrangements), as well as several mutations occurred at the 3' untranslated region, correlated with significantly worse survival outcomes. However, these mutations occurred infrequently (only in approximately 2% of DLBCL). A germline SNP encoding the Myc-N11S variant (observed in 6.5% of the study cohort) was associated with significantly better patient survival, and resulted in reduced tumorigenecity in mouse xenografts.

CONCLUSIONS

Various types of MYC gene mutations are present in DLBCL and show different impact on Myc function and clinical outcomes. Unlike MYC gene translocations and overexpression, most MYC gene mutations may not have a role in driving lymphomagenesis. Clin Cancer Res; 22(14); 3593-605. ©2016 AACR.

Signaling Pathway of GP88 (Progranulin) in Breast Cancer Cells: Upregulation and Phosphorylation of c-myc by GP88/Progranulin in Her2-Overexpressing Breast Cancer Cells

Her2 is a receptor tyrosine kinase overexpressed in 25% of breast tumors. We have shown that the 88 kDa autocrine growth and survival factor GP88 (progranulin) stimulated Her2 phosphorylation and proliferation and conferred Herceptin resistance in Her2-overexpressing cells. Herein, we report that GP88 stimulates c-myc phosphorylation and upregulates c-myc levels in Her2-overexpressing cells. c-myc phosphorylation and upregulation by GP88 were not observed in non-Her2-overexpressing breast cancer cells. c-myc activation was inhibited upon treatment with ERK, PI3 kinase, and c-src pathway inhibitors, U0126, LY294002, and PP2. GP88 also stimulated c-src phosphorylation, a known upstream regulator of c-myc. Thus, we describe here a signaling pathway for GP88 in Her2-overexpressing cells, with GP88 stimulating Src phosphorylation, followed by phosphorylation and upregulation of c-myc. These data would suggest that targeting GP88 could provide a novel treatment approach in breast cancer.

Regulation of cancer cell death by a novel compound, C604, in a c-Myc-overexpressing cellular environment

The proto-oncogene c-Myc has been implicated in a variety of cellular processes, such as proliferation, differentiation and apoptosis. Several c-Myc targets have been studied; however, selective regulation of c-Myc is not easy in cancer cells. Herein, we attempt to identify chemical compounds that induce cell death in c-Myc-overexpressing cells (STF-cMyc and STF-Control) by conducting MTS assays on approximately 4000 chemical compounds. One compound, C604, induced cell death in STF-cMyc cells but not STF-Control cells. Apoptotic proteins, including caspase-3 and poly(ADP-ribose) polymerase (PARP), were cleaved in C604-treated STF-cMyc cells. In addition, SW620, HCT116 and NCI-H23 cells, which exhibit higher basal levels of c-Myc, underwent apoptotic cell death in response to C604, suggesting a role for C604 as an inducer of apoptosis in cancer cells with c-Myc amplification. C604 induced cell cycle arrest at the G2/M phase in cells, which was not affected by apoptotic inhibitors. Interestingly, C604 induced accumulation of c-Myc and Cdc25A proteins. In summary, a chemical compound was identified that may induce cell death in cancer cells with c-Myc amplification specifically through an apoptotic pathway.

Pre-Anchoring of Pin1 to Unphosphorylated c-Myc in a Fuzzy Complex Regulates c-Myc Activity

Hierarchic phosphorylation and concomitant Pin1-mediated proline isomerization of the oncoprotein c-Myc controls its cellular stability and activity. However, the molecular basis for Pin1 recognition and catalysis of c-Myc and other multisite, disordered substrates in cell regulation and disease is unclear. By nuclear magnetic resonance, surface plasmon resonance, and molecular modeling, we show that Pin1 subdomains jointly pre-anchor unphosphorylated c-Myc1-88 in the Pin1 interdomain cleft in a disordered, or "fuzzy", complex at the herein named Myc Box 0 (MB0) conserved region N-terminal to the highly conserved Myc Box I (MBI). Ser62 phosphorylation in MBI intensifies previously transient MBI-Pin1 interactions in c-Myc1-88 binding, and increasingly engages Pin1PPIase and its catalytic region with maintained MB0 interactions. In cellular assays, MB0 mutated c-Myc shows decreased Pin1 interaction, increased protein half-life, but lowered rates of Myc-driven transcription and cell proliferation. We propose that dynamic Pin1 recognition of MB0 contributes to the regulation of c-Myc activity in cells.

Disorder, oscillatory dynamics and state switching: the role of c-Myc

In this paper, using the intrinsically disordered oncoprotein Myc as an example, we present a mathematical model to help explain how protein oscillatory dynamics can influence state switching. Earlier studies have demonstrated that, while Myc overexpression can facilitate state switching and transform a normal cell into a cancer phenotype, its downregulation can reverse state-switching. A fundamental aspect of the model is that a Myc threshold determines cell fate in cells expressing p53. We demonstrate that a non-cooperative positive feedback loop coupled with Myc sequestration at multiple binding sites can generate bistable Myc levels. Normal quiescent cells with Myc levels below the threshold can respond to mitogenic signals to activate the cyclin/cdk oscillator for limited cell divisions but the p53/Mdm2 oscillator remains nonfunctional. In response to stress, the p53/Mdm2 oscillator is activated in pulses that are critical to DNA repair. But if stress causes Myc levels to cross the threshold, Myc inactivates the p53/Mdm2 oscillator, abrogates p53 pulses, and pushes the cyclin/cdk oscillator into overdrive sustaining unchecked proliferation seen in cancer. However, if Myc is downregulated, the cyclin/cdk oscillator is inactivated and the p53/Mdm2 oscillator is reset and the cancer phenotype is reversed.

Identification of a Novel Proto-oncogenic Network in Head and Neck Squamous Cell Carcinoma

BACKGROUND

The developmental transcription factor Grainyhead-like 3 (GRHL3) plays a critical tumor suppressor role in the mammalian epidermis through direct regulation of PTEN and the PI3K/AKT/mTOR signaling pathway. GRHL3 is highly expressed in all tissues derived from the surface ectoderm, including the oral cavity, raising a question about its potential role in suppression of head and neck squamous cell carcinoma (HNSCC).

METHODS

We explored the tumor suppressor role of Grhl3 in HNSCC using a conditional knockout (Grhl3 (∆/-) /K14Cre (+) ) mouse line (n = 26) exposed to an oral chemical carcinogen. We defined the proto-oncogenic pathway activated in the HNSCC derived from these mice and assessed it in primary human HNSCC samples, normal oral epithelial cell lines carrying shRNA to GRHL3, and human HNSCC cell lines. Data were analyzed with two-sided chi square and Student's t tests.

RESULTS

Deletion of Grhl3 in oral epithelium in mice did not perturb PTEN/PI3K/AKT/mTOR signaling, but instead evoked loss of GSK3B expression, resulting in stabilization and accumulation of c-MYC and aggressive HNSCC. This molecular signature was also evident in a subset of primary human HNSCC and HNSCC cell lines. Loss of Gsk3b in mice, independent of Grhl3, predisposed to chemical-induced HNSCC. Restoration of GSK3B expression blocked proliferation of normal oral epithelial cell lines carrying shRNA to GRHL3 (cell no., Day 8: Scramble ctl, 616±21.8 x 10(3) vs GRHL3-kd, 1194±44 X 10(3), P < .001; GRHL3-kd vs GRHL3-kd + GSK3B, 800±98.84 X 10(3), P = .003) and human HNSCC cells.

CONCLUSIONS

We defined a novel molecular signature in mammalian HNSCC, suggesting new treatment strategies targeting the GRHL3/GSK3B/c-MYC proto-oncogenic network.

Serine 62-Phosphorylated MYC Associates with Nuclear Lamins and Its Regulation by CIP2A Is Essential for Regenerative Proliferation

An understanding of the mechanisms determining MYC's transcriptional and proliferation-promoting activities in vivo could facilitate approaches for MYC targeting. However, post-translational mechanisms that control MYC function in vivo are poorly understood. Here, we demonstrate that MYC phosphorylation at serine 62 enhances MYC accumulation on Lamin A/C-associated nuclear structures and that the protein phosphatase 2A (PP2A) inhibitor protein CIP2A is required for this process. CIP2A is also critical for serum-induced MYC phosphorylation and for MYC-elicited proliferation induction in vitro. Complementary transgenic approaches and an intestinal regeneration model further demonstrated the in vivo importance of CIP2A and serine 62 phosphorylation for MYC activity upon DNA damage. However, targeting of CIP2A did not influence the normal function of intestinal crypt cells. These data underline the importance of nuclear organization in the regulation of MYC phosphorylation, leading to an in vivo demonstration of a strategy for inhibiting MYC activity without detrimental physiological effects.

Repression of SRF target genes is critical for Myc-dependent apoptosis of epithelial cells

Oncogenic levels of Myc expression sensitize cells to multiple apoptotic stimuli, and this protects long-lived organisms from cancer development. How cells discriminate physiological from supraphysiological levels of Myc is largely unknown. Here, we show that induction of apoptosis by Myc in breast epithelial cells requires association of Myc with Miz1. Gene expression and ChIP-Sequencing experiments show that high levels of Myc invade target sites that lack consensus E-boxes in a complex with Miz1 and repress transcription. Myc/Miz1-repressed genes encode proteins involved in cell adhesion and migration and include several integrins. Promoters of repressed genes are enriched for binding sites of the serum-response factor (SRF). Restoring SRF activity antagonizes Myc repression of SRF target genes, attenuates Myc-induced apoptosis, and reverts a Myc-dependent decrease in Akt phosphorylation and activity, a well-characterized suppressor of Myc-induced apoptosis. We propose that high levels of Myc engage Miz1 in repressive DNA binding complexes and suppress an SRF-dependent transcriptional program that supports survival of epithelial cells.

TP53 Silencing Bypasses Growth Arrest of BRAFV600E-Induced Lung Tumor Cells in a Two-Switch Model of Lung Tumorigenesis

Lung carcinogenesis is a multistep process in which normal lung epithelial cells are converted to cancer cells through the sequential acquisition of multiple genetic or epigenetic events. Despite the utility of current genetically engineered mouse (GEM) models of lung cancer, most do not allow temporal dissociation of the cardinal events involved in lung tumor initiation and cancer progression. Here we describe a novel two-switch GEM model for BRAF(V600E)-induced lung carcinogenesis allowing temporal dissociation of these processes. In mice carrying a Flp recombinase-activated allele of Braf (Braf(FA)) in conjunction with Cre-regulated alleles of Trp53, Cdkn2a, or c-MYC, we demonstrate that secondary genetic events can promote bypass of the senescence-like proliferative arrest displayed by BRAF(V600E)-induced lung adenomas, leading to malignant progression. Moreover, restoring or activating TP53 in cultured BRAF(V600E)/TP53(Null) or BRAF(V600E)/INK4A-ARF(Null) lung cancer cells triggered a G1 cell-cycle arrest regardless of p19(ARF) status. Perhaps surprisingly, neither senescence nor apoptosis was observed upon TP53 restoration. Our results establish a central function for the TP53 pathway in restricting lung cancer development, highlighting the mechanisms that limit malignant progression of BRAF(V600E)-initiated tumors.

Telomerase regulates MYC-driven oncogenesis independent of its reverse transcriptase activity

Constitutively active MYC and reactivated telomerase often coexist in cancers. While reactivation of telomerase is thought to be essential for replicative immortality, MYC, in conjunction with cofactors, confers several growth advantages to cancer cells. It is known that the reactivation of TERT, the catalytic subunit of telomerase, is limiting for reconstituting telomerase activity in tumors. However, while reactivation of TERT has been functionally linked to the acquisition of several "hallmarks of cancer" in tumors, the molecular mechanisms by which this occurs and whether these mechanisms are distinct from the role of telomerase on telomeres is not clear. Here, we demonstrated that first-generation TERT-null mice, unlike Terc-null mice, show delayed onset of MYC-induced lymphomagenesis. We further determined that TERT is a regulator of MYC stability in cancer. TERT stabilized MYC levels on chromatin, contributing to either activation or repression of its target genes. TERT regulated MYC ubiquitination and proteasomal degradation, and this effect of TERT was independent of its reverse transcriptase activity and role in telomere elongation. Based on these data, we conclude that reactivation of TERT, a direct transcriptional MYC target in tumors, provides a feed-forward mechanism to potentiate MYC-dependent oncogenesis.