Inactivation Of MYC Research Articles

Degradation of MYC by the mutant p53 reactivator drug, COTI-2 in breast cancer cells

TP53 (p53) and MYC are amongst the most frequently altered genes in cancer. Both are thus attractive targets for new anticancer therapies. Historically, however, both genes have proved challenging to target and currently there is no approved therapy against either. The aim of this study was to investigate the effect of the mutant p53 reactivating drug, COTI-2 on MYC. Total MYC, pSer62 MYC and pThr58 MYC were detected using Western blotting. Proteasome-mediated degradation was determined using the proteasome, inhibitor MG-132, while MYC half-life was measured using pulse chase experiments in the presence of cycloheximide. Cell proliferation was assessed using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method. Treatment of 5 mutant p53 breast cancer cell lines with COTI-2 resulted in dose-dependent MYC degradation. Addition of the proteasome inhibitor, MG132, rescued the degradation, suggesting that this proteolytic system was at least partly responsible for the inactivation of MYC. Using cycloheximide in pulse chase experiments, COTI-2 was found to reduce the half-life of MYC in 2 different mutant p53 breast cancer cell lines, i.e., from 34.8 to 18.6 min in MDA-MB-232 cells and from 29.6 to 20.3 min in MDA-MB-468 cells. Co-treatment with COTI-2 and the MYC inhibitor, MYCi975 resulted in synergistic growth inhibition in all 4 mutant p53 cell lines investigated. The dual ability of COTI-2 to reactivate mutant p53 and degrade MYC should enable this compound to have broad application as an anticancer drug.

Disruption of Multiple Overlapping Functions Following Stepwise Inactivation of the Extended Myc Network.

Myc, a member of the "Myc Network" of bHLH-ZIP transcription factors, supervises proliferation, metabolism, and translation. It also engages in crosstalk with the related "Mlx Network" to co-regulate overlapping genes and functions. We investigated the consequences of stepwise conditional inactivation of Myc and Mlx in primary and SV40 T-antigen-immortalized murine embryonic fibroblasts (MEFs). Myc-knockout (MycKO) and Myc × Mlx "double KO" (DKO)-but not MlxKO-primary MEFs showed rapid growth arrest and displayed features of accelerated aging and senescence. However, DKO MEFs soon resumed proliferating, indicating that durable growth arrest requires an intact Mlx network. All three KO MEF groups deregulated multiple genes and functions pertaining to aging, senescence, and DNA damage recognition/repair. Immortalized KO MEFs proliferated in Myc's absence while demonstrating variable degrees of widespread genomic instability and sensitivity to genotoxic agents. Finally, compared to primary MycKO MEFs, DKO MEFs selectively downregulated numerous gene sets associated with the p53 and retinoblastoma (Rb) pathways and G2/M arrest. Thus, the reversal of primary MycKO MEF growth arrest by either Mlx loss or SV40 T-antigen immortalization appears to involve inactivation of the p53 and/or Rb pathways.

Abstract GS1-08: The MYC oncogene suppresses tumor immune infiltration and function which is reversible with combinatorial immunotherapies

Abstract Immunotherapy has great potential to improve outcomes for breast cancer patients. Yet, deciding on how to rationally combine therapeutic agents and predict patient populations who would benefit remains challenging. Less than 20% of patients with metastatic breast cancer respond to current immunotherapy regiments, further demonstrating the need for research in this area. We postulate that oncogene activation leads to immune-related vulnerabilities which can be exploited to discover new immunotherapy combinations with improved efficacy. MYC is an oncogene that is frequently overexpressed in the majority of triple negative breast cancers (TNBCs), and is frequently found in disease recurrence, metastasis, and chemotherapy resistance. Despite the association of MYC with poor patient outcome, little is known about how MYC facilitates immune evasion in TNBC. Using TNBC patient datasets from the TCGA, we discovered that the MYC gene signature is anti-correlated with T-cell activation signatures and a subset of genes that regulate antigen processing and presentation with MHC-I. We further examined the effects of MYC using a MYC-driven mammary tumor mouse model (MTB-TOM). MYC activation leads to downregulation of MHC-I expression on tumor cells, poor CD8+ T cell infiltration and consequently poor response to anti-PD-L1 monotherapy. We demonstrate three methods to improve response to immunotherapy—by inactivation of MYC, by inducing an interferon response within the breast tumor, or by inducing MYC tumor synthetic lethality with a small molecule inhibitor of PIM kinases. Using these combination immunotherapy strategies, we observed that a majority of animals eradicated their breast tumors and demonstrated a durable anti-tumor response following subsequent tumor re-challenge. Our data suggest MYC is an indicator of whether a breast cancer patient will respond to immunotherapy. Analysis of the MYC gene signature as a predictor of patient response to pembrolizumab in combination with paclitaxel followed by AC in the neoadjuvant I-SPY 2 trial is on-going and will be presented. Our study is the first to describe oncogenic MYC downregulation of MHC-I in a TNBC model of breast cancer and to demonstrate translatable approaches to overcome MYC orchestrated immune evasion. Citation Format: Joyce V Lee, Filomena Housley, Christina Yau, Golzar Hemmati, Yibing Zhang, Susan Samson, Carole Baas, Hope Rugo, Mehrdad Matloubian, Andrei Goga. The MYC oncogene suppresses tumor immune infiltration and function which is reversible with combinatorial immunotherapies [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr GS1-08.

An Embryonic Diapause-like Adaptation with Suppressed Myc Activity Enables Tumor Treatment Persistence

Treatment-persistent residual tumors impede curative cancer therapy. To understand this cancer cell state we generated models of treatment persistence that simulate the residual tumors. We observe that treatment-persistent tumor cells in organoids, xenografts, and cancer patients adopt a distinct and reversible transcriptional program resembling that of embryonic diapause, a dormant stage of suspended development triggered by stress and associated with suppressed Myc activity and overall biosynthesis. In cancer cells, depleting Myc or inhibiting Brd4, a Myc transcriptional co-activator, attenuates drug cytotoxicity through a dormant diapause-like adaptation with reduced apoptotic priming. Conversely, inducible Myc upregulation enhances acute chemotherapeutic activity. Maintaining residual cells in dormancy after chemotherapy by inhibiting Myc activity or interfering with the diapause-like adaptation by inhibiting cyclin-dependent kinase 9 represent potential therapeutic strategies against chemotherapy-persistent tumor cells. Our study demonstrates that cancer co-opts a mechanism similar to diapause with adaptive inactivation of Myc to persist during treatment.

Abstract 2613: MYC controls DNA methylation on a global scale through DNMT3b upregulation in T-ALL and Burkitts Lymphoma

Abstract Background: Oncogenic MYC is implicated in 60-70% of all human cancers, including T-cell acute lymphoblastic leukemia (T-ALL) and Burkitts lymphoma (BL). Genetic inactivation of MYC can cause tumor regression through cellular senescence in mouse models (exhibiting oncogene addiction), making it a promising therapeutic target. However, a pharmacological MYC inhibitor remains elusive, thus these findings cannot be translated into clinic. We are the first to demonstrate that MYC can control DNA methyl-transferase (DNMT) expression and thus, genome-wide DNA methylation; as an alternative strategy to disrupt the MYC network indirectly, we have chosen to target MYC-dependent DNMTs as a tumor-cell specific strategy. Experimental Procedures and Results: Utilizing expression profiling in patient-derived cell lines and clinical specimens, we found DNMT 1, 3a, and 3b to be overexpressed in MYC-driven T-ALL and BL compared to non-malignant tissue. Using transgenic inducible “tet-off” MYC-models in mouse T-ALL (EµSRα-tTA;tet-o-MYC) and human Burkitt’s lymphoma (P493-6), we have shown that DNMT1 and DNMT3b expression depend on high MYC expression levels in T-ALL and BL. Using patient-derived cell lines, we’ve shown that depletion of endogenous MYC caused a reduction of both DNMT1 and DNMT3b expression. Chromatin Immunoprecipitation (ChIP) profiling for MYC binding, further establishes the DNMT1 and DNMT3b promoters as bona fide targets of the MYC oncogene. Furthermore, shRNA-mediated knockdown of DNMT3b or pharmacological inhibition using NanaomycinA, indicates a tumor promoter function in the context of MYC-driven tumor maintenance in T-ALL. To further elucidate the consequence on DNA methylation and gene expression upon DNMT3b loss of function, Reduced Representation Bisulfite Sequencing and RNA-sequencing was performed to analyze alterations in DNA methylation and gene expression, respectively. We found that DNMT3b maintains CpG island DNA methylation near promoters, responsible for gene silencing during tumor maintenance. Accompanying changes in DNA methylation, we identified approximately one thousand differentially expressed genes upon DNMT3b knockdown, with upregulated genes responsible for controlling differentiation, transport, and development; while downregulated genes controlled processes including signal transduction, apoptosis, and regulation of cellular proliferation. Conclusion: Together, we provide novel evidence that MYC directly controls global DNA methylation through deregulation of DNMT1 and DNMT3b expression to maintain gene programs for T-ALL tumor maintenance. Furthermore, we provide evidence that targeted inhibition of individual DNMTs may reveal alternative tumor-specific therapies for T-ALL, BL, and other MYC-driven cancers. Citation Format: Candace J. Poole, Atul Lodh, Jan van Riggelen. MYC controls DNA methylation on a global scale through DNMT3b upregulation in T-ALL and Burkitts Lymphoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2613.

MYC Functions As a Master Switch for Natural Killer Cell-Mediated Immune Surveillance of Lymphoid Malignancies

Background: The MYC oncogene drives T and B cell lymphoid malignancies, including Burkitt's lymphoma (BL) and Acute Lymphoblastic Leukemia (ALL). Such lymphomas are said to be “oncogene-addicted” to MYC. In order to develop targeted therapies for MYC-driven cancers, it is vital to understand how MYC regulates both cell autonomous and non-cell-autonomous processes, including host immunity.Approach: We have used a particularly tractable approach for studying the role of oncogenic MYC on the host immune system during lymphomagenesis through a tetracycline (tet)-system regulated transgenic mouse model of MYC-driven T cell lymphoma (SRα-tTA/Tet-O-MYC mice; Felsher and Bishop, Molecular Cell, 1999). By delineating the global immunological changes during primary MYC-driven T cell lymphomagenesis in SRα-tTA/Tet-O-MYC mice using mass cytometry (CyTOF) and CIBERSORT, we identified anti-tumor immune subsets that can be developed as therapies to treat MYC-driven lymphomas.Results: Amongst the immune subsets evaluated, our results demonstrated a specific systemic suppression of natural killer (NK) cell-mediated surveillance in SRα-tTA/Tet-O-MYC mice bearing overt MYC-driven T cell lymphomas. Inactivation of lymphoma-intrinsic MYC restores NK cell-mediated immune surveillance, suggesting that the regulation of NK cell-mediated surveillance may be integral to MYC-induced lymphomagenesis. We observed that lymphoma-specific MYC transcriptionally represses STAT1/2 and secretion of Type I Interferons (IFNs) required for normal NK cell homeostasis and NK cell-mediated immune surveillance in the tumor microenvironment. Concordantly, treating T cell lymphoma-bearing SRα-tTA/Tet-O-MYC mice with Type I IFN improves survival by rescuing NK cell production, and in part overriding MYC-mediated suppression of NK surveillance. We showed that human lymphomas with both high levels of MYC and low levels of STAT1/2 have lower NK surveillance, and are associated with poor prognosis. Finally, we established the therapeutic implications of our findings by showing that adoptive transfer of NK cells significantly delays lymphoma initiation, and recurrence after MYC inactivation; suggesting that NK cell-based therapy may be effective against MYC-driven lymphomas.Conclusion: Subversion of NK cell surveillance is integral to MYC-induced lymphomagenesis. Our studies provide a rationale for further developing the NK subset as a cell-based immunotherapy to effectively treat MYC-driven lymphomas in the future. DisclosuresNo relevant conflicts of interest to declare.

Shotgun proteomic analysis to study the decrease of xenograft tumor growth after rosemary extract treatment

The antiproliferative activity of Rosemary (Rosmarinus officinalis) has been widely studied in different in vitro and in vivo models, which demonstrate that rosemary extracts inhibit the cellular proliferation due to its ability to interact with a wide spectrum of molecular targets. However, a comprehensive proteomics study in vivo has not been carried out yet. In the present work, the effects of rosemary extract on xenograft tumor growth has been studied and, for the first time, a shotgun proteomic analysis based on nano-LC–MS/MS together with stable isotope dimethyl labeling (DML) has been applied to investigate the global protein changes in vivo. Our results show that the daily administration of a polyphenol-enriched rosemary extract reduces the progression of colorectal cancer in vivo with the subsequent deregulation of 74 proteins. The bioinformatic analysis of these proteins indicates that the rosemary extract mainly alters the RNA Post-Transcriptional Modification, the Protein Synthesis and the Amino Acid Metabolism functions and suggests the inactivation of the oncogene MYC. These results demonstrate the high utility of the proposed analytical methodology to determine, simultaneously, the expression levels of a large number of protein biomarkers and to generate new hypothesis about the molecular mechanisms of this extract in vivo.

KRAS-dependent suppression of MYC enhances the sensitivity of cancer cells to cytotoxic agents.

KRAS is the most commonly mutated oncogene, frequently associated with some of the deadliest forms of cancer. However, the need for potent and specific KRAS inhibitors remains unmet. Here, we evaluated the effects of selected cytotoxic agents on oncogenic KRAS signaling and drug response. The data provided new insights into the functional interaction between the KRAS and MYC pathways and revealed key differences between WT and mutant KRAS expressing cells. Systematic investigation of non-small cell lung cancer cell lines revealed that KRAS mutation can paradoxically increase the sensitivity of cells to cytotoxic agents. We identify MYC as a key regulator of the cellular stress responses and tumor cell viability as MYC expression was suppressed in drug-sensitive but not resistant cells. Furthermore, this suppression was driven by hyperactive KRAS/MAPK signaling. Our findings support a direct link between MYC and cancer cell viability, and raise the possibility that inactivation of MYC may be an effective therapeutic strategy for KRAS mutant tumors across various cancer types.

Abstract 1227: New small molecules targeting MYC:MAX interactions that inhibits tumor cell growth in a MYC-dependent manner

Abstract Deregulated expression of MYC family oncogenes MYC, MYCN and MYCL (here collectively referred to as MYC) occurs in many types of human tumors, and is often associated with aggressive tumor development and poor prognosis. In mouse tumor models, inactivation of MYC often leads to tumor regression with well-tolerated side effects, suggesting that MYC is a potential and suitable target for anti-cancer therapy. At present there are no drugs targeting MYC in the clinic, and transcription factors like MYC are considered difficult to target. However, MYC function is strictly dependent on interaction with cofactors, and targeting such interaction may be a plausible way to combat Myc. We have used cell-based screens utilizing Bimolecular Fluorescence Complementation (BiFC) and split Gaussia luciferase (G-Luc) to identify small molecule inhibitors of the interaction between MYC and its essential partner MAX. Several potent MYC:MAX interaction inhibitors have been identified in these screens and have been validated by other protein-protein interactions (PPI) assays such as in situ Proximity Ligation Assay (isPLA), Fluorescence Resonance Energy Transfer (FRET), Surface Plasmon Resonance (SPR) and coimmunoprecipitations. In general the identified molecules fall into two categories: those that inhibit MYC:MAX interactions not only in cells but also in vitro using purified MYC and MAX, and those that inhibit the interaction in cells but not in vitro, suggesting an indirect mode of action. Molecules of both categories inhibit cell growth and viability of a variety of tumor cells in culture with high efficacy in a MYC-dependent manner. So far, one of the molecules have been utilized for cancer treatment in mouse tumor models and found to significantly inhibit tumor growth and improve survival. These molecules have the potential to become important tools in the studies of MYC function in cells and in vivo as well as potentially basis for drug development for treatment of MYC-driven tumors. We have thus provided proof of principle that our screening systems are able to identify potent PPI inhibitors and we are at present setting up similar systems to screen for inhibitors of other MYC:cofactor interactions of relevance for specific tumor types. Citation Format: Alina Castell, Karin Ridderstråle, Qinzi Yan, Fan Zhang, Per Hydbring, Marcela Franco, Jacob Goodwin, Inga Müller, Siti Mariam Zakaria, Lars Johansson, Lars-Gunnar Larsson. New small molecules targeting MYC:MAX interactions that inhibits tumor cell growth in a MYC-dependent manner. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1227.

MYC-mediated synthetic lethality for treating tumors.

Deregulated c-MYC (hereafter MYC) is widely expressed in many human tumors. Myctransgenic mouse models produce diverse tumors. MYC is a strong driver of tumorigenesis and is required for tumor maintenance. MYC is therefore an attractive target for cancer treatment. However, genetic and pharmacological approaches for the targeted inactivation of MYC for the treatment of MYC-overexpressing tumors have been shown to be unsatisfactory. MYC expression is regulated by different mechanisms at transcriptional, post-transcriptional, and post-translational levels. Turnover of MYC protein is an important step that influences the expression and function of MYC. MYC turnover is predominantly controlled by the GSK3/FBW7 axis that is regulated by multiple elements. Small molecule inhibitors (SMIs) can influence the stability and activity of MYC protein by targeting the axis and its regulator elements. Wang et al. (2004) first introduced the concept of MYC-mediated synthetic lethality (MYC-SL) induced by TRAIL and DR5-agonists. Researchers have turned to synthetic lethality as a treatment strategy for MYC-overexpressing tumors. MYC function is closely associated with MYC levels. Two strategies have been developed to treat MYC-overexpressing tumors by upregulating or downregulating MYC. An SMI can induce MYC-SL by increasing MYC expression through the inactivation of the GSK3β/FBW7 axis and CK1. Elevated MYC levels lead to DNA damage, senescence, and apoptosis. An SMI can also induce MYC-SL by decreasing MYC expression through the activation of the GSK3β/FBW7 axis, the inactivation of PP2A inhibitors, and the inhibition of ARK5, AUK-A, Brd4, CDK1, CDK2, CHK1, and SAE1/2. Reduced MYC levels cause tumor regression. Some SMIs have entered phase I and II clinical trials. SMIs may be used in the near future to treat cancers co-overexpressing MYC and the corresponding MYC-SL genes.

Addiction to multiple oncogenes can be exploited to prevent the emergence of therapeutic resistance.

Many cancers exhibit sensitivity to the inhibition of a single genetic lesion, a property that has been successfully exploited with oncogene-targeted therapeutics. However, inhibition of single oncogenes often fails to result in sustained tumor regression due to the emergence of therapy-resistant cells. Here, we report that MYC-driven lymphomas frequently acquire activating mutations in β-catenin, including a previously unreported mutation in a splice acceptor site. Tumors with these genetic lesions are highly dependent on β-catenin for their survival and the suppression of β-catenin resulted in marked apoptosis causally related to a decrease in Bcl-xL expression. Using a novel inducible inhibitor of β-catenin, we illustrate that, although MYC withdrawal or β-catenin inhibition alone results in initial tumor regression, most tumors ultimately recurred, mimicking the clinical response to single-agent targeted therapy. Importantly, the simultaneous combined inhibition of both MYC and β-catenin promoted more rapid tumor regression and successfully prevented tumor recurrence. Hence, we demonstrated that MYC-induced tumors are addicted to mutant β-catenin, and the combined inactivation of MYC and β-catenin induces sustained tumor regression. Our results provide a proof of principle that targeting multiple oncogene addicted pathways can prevent therapeutic resistance.

Abstract 267: MYC maintains the self-renewal of cancer stem cells through the regulation of the glutathione redox system.

Abstract Cancer stem cells (CSCs) are a rare population of self-renewing cells that can persist even after extensive therapeutic intervention. The molecular mechanism of CSC self-renewal is not yet clear. Here we show that MYC is causally involved in CSC self-renewal through the regulation of glutathione (GSH) redox system. In a MYC-driven conditional transgenic mouse model of T-cell acute lymphoblastic leukemia (T-ALL), we identified Sca-1+ CSCs. We found that these CSCs express the glutathione (GSH) redox system related genes including glutamate cysteine ligase (GCL) and Pyruvate Dehydrogenase Kinase 1 (PDK1) but low levels of expression of the mitochondrial redox enzyme, Superoxide Dismutase 2 (SOD2). Even a brief period of MYC inactivation in the Sca1+ CSCs led to their apoptosis associated with the downregulation of GCL and PDK1, but the upregulation of SOD2, in turn resulting in ROS-generation and mitochondrial damage. Moreover, suppression of SOD2 by siRNA silencing or the use of the peroxide scavenger, ascorbic acid, prevented MYC inactivation from inducing apoptosis in the Sca-1+ CSCs. Finally, in human CSCs, including primary human Chronic Myeloid Leukemia (CML), and human colon cancers, we confirmed that MYC expression was required for high GCL, PDK1 and low SOD2. Even the brief inactivation of MYC in human CSCs resulted in the loss of their self-renewal, as well as the global chance in redox state by decrease in GCL, PDK1 and increase in SOD2. We conclude that high levels of continued MYC expression are essential for CSC self-renewal through regulation of redox state. Citation Format: Bikul Das, Rika Tsuchida, Reza Bayat-Mokhtari, Jason R. Gotlib, Renee Reijo-Pera, Herman Yeger, Felsher W. Dean. MYC maintains the self-renewal of cancer stem cells through the regulation of the glutathione redox system. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 267. doi:10.1158/1538-7445.AM2013-267 Note: This abstract was not presented at the AACR Annual Meeting 2013 because the presenter was unable to attend.

Roles of EDR1 in non-host resistance of Arabidopsis

Entry control of Arabidopsis thaliana against non-adapted powdery mildews largely depends on the PEN1 secretion pathway and the PEN2–PEN3 antifungal metabolite pathway, and is critical for non-host resistance. In a recent study, we reported that ENHANCED DISEASE RESISTANCE 1 (EDR1) plays a role in entry control against a non-adapted anthracnose fungus, which exhibits an infection style distinct from that of powdery mildews. Results obtained using edr1 pen2 double mutants indicate that the contribution of EDR1 to non-host resistance is independent of that of the PEN2-mediated defence pathway. Comparative transcript profiling revealed that EDR1 is critical for expression of four plant defensin genes. The MYC2-encoded transcription factor represses defensin expression. Inactivation of MYC fully restored defensin expression in edr1 mutants, implying that EDR1 cancels MYC2 function to regulate defensin expression. These findings indicate that EDR1 exerts a critical role in non-host resistance, in part by inducing antifungal peptide expression via interference in MYC2-mediated repressor function.

Lymphomas that recur after MYC suppression continue to exhibit oncogene addiction

The suppression of oncogenic levels of MYC is sufficient to induce sustained tumor regression associated with proliferative arrest, differentiation, cellular senescence, and/or apoptosis, a phenomenon known as oncogene addiction. However, after prolonged inactivation of MYC in a conditional transgenic mouse model of Eμ-tTA/tetO-MYC T-cell acute lymphoblastic leukemia, some of the tumors recur, recapitulating what is frequently observed in human tumors in response to targeted therapies. Here we report that these recurring lymphomas express either transgenic or endogenous Myc, albeit in many cases at levels below those in the original tumor, suggesting that tumors continue to be addicted to MYC. Many of the recurring lymphomas (76%) harbored mutations in the tetracycline transactivator, resulting in expression of the MYC transgene even in the presence of doxycycline. Some of the remaining recurring tumors expressed high levels of endogenous Myc, which was associated with a genomic rearrangement of the endogenous Myc locus or activation of Notch1. By gene expression profiling, we confirmed that the primary and recurring tumors have highly similar transcriptomes. Importantly, shRNA-mediated suppression of the high levels of MYC in recurring tumors elicited both suppression of proliferation and increased apoptosis, confirming that these tumors remain oncogene addicted. These results suggest that tumors induced by MYC remain addicted to overexpression of this oncogene.

Statins Radiosensitize Myc Over-expressing Prostate Cancer Cells

Studies have shown a link between HMG-CoA reductase inhibitors (statins) and decreased cancer risk in various cancers including prostate cancer. Recent clinical studies have shown improved outcome for men on statin therapy receiving definitive local therapy either with radiation therapy or following surgery compared to local therapy alone for prostate cancer. Isoprenoid signaling can be inhibited by statin treatment and in turn also result in targeted downregulation of c-Myc (or Myc). Inactivation of Myc has been shown to induce tumor regression in transgenic mouse models of different cancers.

Radiosensitization of MYC-overexpressing prostate cancer cells by statins.

26 Background: Studies have shown a link between HMG-CoA reductase inhibitors (statins) and decreased cancer risk in various cancers including prostate cancer. Recent clinical studies have shown improved outcome for men on statin therapy receiving definitive radiation therapy compared to radiation therapy alone for prostate cancer. The oncogene Ras can be inhibited by statin treatment and in turn also result in targeted downregulation of Myc. Inactivation of Myc has been shown to induce tumor regression in transgenic mouse models of different cancers. Methods: Utilizing both in vitro and in vivo models, we studied in MYC over-expressing prostate cancer cells statin induced cytotoxicity and radiosensitization. We studied the effect of statin treatment with and without radiation treatment on in vitro cell death and clonogenic survival and correlated effects to levels of MYC and phospho-MYC. We constructed mutant cell lines to specifically examine the MYC-dependent effects of statins on prostate cancer cell cytotoxicity and radiosensitization. We performed similar studies in vivo using subcutaneous tumor grafts. Results: Statin treatment alone of prostate cancer cells resulted in increased cell death and decreased clonegenic survival. Statin treatment further sensitized prostate cancer cells to ionizing radiation as shown through colony formation assays. These cellular effects were associated with decreased levels of active MYC as confirmed by western and phospho-western analysis. Statin-induced cell death and decreased MYC levels could be rescued with the HMG-CoA bypass product mevalonate. Finally, flank tumor graft experiments demonstrated the ability of statins and radiation to delay tumor growth in vivo. Conclusions: Our preliminary data have implicated MYC as a potential critical target for the cytotoxic effects of statins on prostate cancer cells. The prevalence of MYC overexpression in human prostate cancers is as high as 80%. Thus the ability of statins to radiosensitize MYC overexpressing prostate cancer cells may provide a mechanistic explanation to the improved outcomes for men taking statins while on radiation therapy and now may afford us a molecular biomarker to examine this phenomenon in the clinic. No significant financial relationships to disclose.

MYC Inactivation Elicits Oncogene Addiction through Both Tumor Cell-Intrinsic and Host-Dependent Mechanisms

Tumorigenesis is generally caused by genetic changes that activate oncogenes or inactivate tumor suppressor genes. The targeted inactivation of oncogenes can be associated with tumor regression through the phenomenon of oncogene addiction. One of the most common oncogenic events in human cancer is the activation of the MYC oncogene. The inactivation of MYC may be a general and effective therapy for human cancer. Indeed, it has been experimentally shown that the inactivation of MYC can result in dramatic and sustained tumor regression in lymphoma, leukemia, osteosarcoma, hepatocellular carcinoma, squamous carcinoma, and pancreatic carcinoma through a multitude of mechanisms, including proliferative arrest, terminal differentiation, cellular senescence, induction of apoptosis, and the shutdown of angiogenesis. Cell-autonomous and cell-dependent mechanisms have both been implicated, and recent results suggest a critical role for autocrine factors, including thrombospondin-1 and TGF-β. Hence, targeting the inactivation of MYC appears to elicit oncogene addiction and, thereby, tumor regression through both tumor cell-intrinsic and host-dependent mechanisms.

Abstract 3300: MYC regulation of a “poor prognosis” metastatic cancer cell state

Abstract Gene expression signatures are used clinically as prognostic tools to determine the risk of individual patients with localized breast tumors developing distant metastasis. However, we lack a clear understanding of whether these correlative biomarkers link to a common biological network that regulates metastasis. METHODS: In order to address this question, we analyzed thirteen different well-described (including two in clinical use) “poor outcome” cancer signatures in a cell line model of estrogen receptor positive breast cancer (MCF7) manipulated by diverse oncogenic stimuli. RESULTS: We find that the c-MYC oncoprotein coordinately regulates the expression of these thirteen different “poor outcome” cancer signatures. In addition, functional inactivation of MYC in human breast cancer cells specifically inhibits their distant metastasis in vivo and invasive behavior in vitro. CONCLUSION: These results suggest that MYC oncogene activity, as reflected by “poor prognosis” signature expression, may be necessary for the translocation of “poor outcome” human breast tumors to distant sites. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3300.

Myc controls transcriptional regulation of cardiac metabolism and mitochondrial biogenesis in response to pathological stress in mice

In the adult heart, regulation of fatty acid oxidation and mitochondrial genes is controlled by the PPARgamma coactivator-1 (PGC-1) family of transcriptional coactivators. However, in response to pathological stressors such as hemodynamic load or ischemia, cardiac myocytes downregulate PGC-1 activity and fatty acid oxidation genes in preference for glucose metabolism pathways. Interestingly, despite the reduced PGC-1 activity, these pathological stressors are associated with mitochondrial biogenesis, at least initially. The transcription factors that regulate these changes in the setting of reduced PGC-1 are unknown, but Myc can regulate glucose metabolism and mitochondrial biogenesis during cell proliferation and tumorigenesis in cancer cells. Here we have demonstrated that Myc activation in the myocardium of adult mice increases glucose uptake and utilization, downregulates fatty acid oxidation by reducing PGC-1alpha levels, and induces mitochondrial biogenesis. Inactivation of Myc in the adult myocardium attenuated hypertrophic growth and decreased the expression of glycolytic and mitochondrial biogenesis genes in response to hemodynamic load. Surprisingly, the Myc-orchestrated metabolic alterations were associated with preserved cardiac function and improved recovery from ischemia. Our data suggest that Myc directly regulates glucose metabolism and mitochondrial biogenesis in cardiac myocytes and is an important regulator of energy metabolism in the heart in response to pathologic stress.

MYC regulation of a “poor-prognosis” metastatic cancer cell state

Gene expression signatures are used in the clinic as prognostic tools to determine the risk of individual patients with localized breast tumors developing distant metastasis. We lack a clear understanding, however, of whether these correlative biomarkers link to a common biological network that regulates metastasis. We find that the c-MYC oncoprotein coordinately regulates the expression of 13 different "poor-outcome" cancer signatures. In addition, functional inactivation of MYC in human breast cancer cells specifically inhibits distant metastasis in vivo and invasive behavior in vitro of these cells. These results suggest that MYC oncogene activity (as marked by "poor-prognosis" signature expression) may be necessary for the translocation of poor-outcome human breast tumors to distant sites.