Abstract SY12-03: MYC-mediated metabolic vulnerabilities and the circadian clock

The MYC oncogene belongs to the family of MYC genes, including MYCN (N-MYC) and MYCL (L-MYC), which are linked to human cancers such as Burkitt's lymphoma, neuroblastoma and lung cancer, respectively. MYC proteins belong to the MAX-MLX network of heterodimeric transcription factors that bind ‘E-boxes’ (5’-CACGTG-3’) to regulate genes involved in cell proliferation, differentiation and metabolism. MYC is downstream of many signal transduction pathways in normal cells; when stimulated through pathways such as tyrosine receptor kinases, MYC is induced to produce the MYC protein that dimerizes with MAX to bind DNA and regulate a transcriptional program of metabolism, cell growth and proliferation (1). MYC is under the control of growth factor signaling as well as nutrient availability, such that nutrient deprivation could result in diminished normal MYC expression and cell growth arrest. By contrast, deregulation of MYC in normal cells results in MYC binding not only to high affinity physiological binding sites but also invading into low-affinity binding sites and enhancers, resulting in an imbalance amplification of gene expression that triggers stress and cell death through activation of checkpoints such as p53. Elimination of p53 during tumorigenesis, however, can unleash MYC's transcriptional power to drive deregulated cell growth that renders cells addicted to nutrients, such that deprivation of glucose or glutamine results in cell death. The conceptual framework has been exploited to target metabolism for therapy against MYC-driven cancers, particularly aiming at glycolysis and glutaminolysis - metabolic processes that are increased by MYC. While lactate dehydrogenase A (LDHA) inhibition could curb MYC-mediated tumorigenesis (2), glutaminolysis remains an alternative survival pathway that could be targeted through inhibition of glutaminase (3-9). Glutaminase is normally expressed from two different genes: GLS that is normally expressed in kidney and brain, and GLS2 that is normally expressed in liver. Indeed, we have shown that knockdown of glutaminase (GLS) or inhibition with a small molecule (BPTES) diminishes the progression of a MYC-inducible human lymphoma xenograft model (3-7). We further documented that glutaminase (Gls) is induced by MYC and is required for early tumor development in a MYC-inducible model of mouse liver cancer (8), which displayed a decrease in the expression of normal liver Gls2. In this regard, the isoform switch from Gls2 to Gls1 in mouse (and also human) liver cancer renders tumors vulnerable to loss of one copy of Gls, which delayed tumorigenesis. We further showed that treatment with BPTES as a single agent was sufficient to prolong survival of mice bearing these MYC-induced liver cancers, providing proof-of-concept that targeting a single enzyme, in this case GLS, could change the course of the disease (8).

Targeting GLS, however, has limitations, since the oncogenotypes of cancers result in different re-wiring of metabolism that is best illustrated by metabolomics studies of different mouse models of cancer (10). The MET oncogene-driven liver cancer model largely relies on glucose, whereas MYC oncogene-driven liver and lung cancers rely on both glutamine and glucose. As such, glutaminase inhibition should be considered in the context of the oncogenotype and metabolic profile of specific cancer types. Further, targeting metabolism could be constrained by the circadian regulation of cellular metabolism, rendering proliferating normal cells more vulnerable to inhibition of metabolism at specific times of the day. In this regard, we tested the hypothesis that high oncogenic MYC would ectopically invade the circadian Clock regulated genes that are also driven by E-boxes (11). The cell intrinsic clock machinery comprises of the central Clock-Bmal1 transcription factor, which induces Rev-erb’s, Cry’s, and Per's that in turn negatively regulated Bmal1 expression or Clock-Bmal1 levels, resulting in a circadian oscillation of Clock-Bmal1 function. Oscillation of the central clock transcription, which drives metabolic genes with E-boxes, results in oscillatory metabolism. Indeed, using an inducible MYC-ER system, we demonstrate that MYC could activate the negative clock regulators PER, CRY, and REV-ERBs and documented that the suppression of Bmal1 expression by MYC is mediated transcriptionally through REV-ERBs. Metabolic profiling in time-series experiments reveals oscillation of glucose and glutamine metabolic in the MYC-OFF state in U2OS cells that have very low basal endogenous MYC expression. In the MYC-ON state, intracellular glucose levels cease to oscillate and are barely detectable by NMR. Additional evidence suggests that it is converted toward lactate and biomass for cell growth. MYC, hence, can invade and disrupt the molecular circadian clock as well as metabolism, in favor of an anabolic growth program.

These observations have key implications for targeting metabolic enzymes that have been documented as being induced by Clock-Bmal1 and MYC. For example, ornithine decarboxylase (ODC) and nicotinamide phosphoribosyltransferase (NAMPT) are common targets of MYC and Clock-Bmal1. Hence, we hypothesize that the dose-limiting toxicity of NAMPT inhibition, being thrombocytopenia (12), could be curbed in a lymphoma xenograft model by applying chronotherapy. In this regard, our preliminary studies illustrate that while NAMPT inhibition at two different times of the day resulted in similar efficacy in reducing lymphoma xenograft growth, one of administration time resulted in thrombocytopenia while the other time of drug administration was indistinguishable from the control treated animals (unpublished). These observations indicate that the combination of identifying metabolic vulnerabilities of MYC-driven cancer along with understanding circadian could strategically guide the use of metabolism-targeted drugs in the clinic through oncogenotyping and potentially chronotherapy.

1.Stine ZE, et al. MYC, Metabolism, and Cancer. Cancer Discov. 2015 Oct;5(10):1024-39.

2. Le A, et al. Inhibition of lactate dehydrogenase A induces oxidative stress and inhibits tumor progression. Proc Natl Acad Sci U S A. 2010 Feb 2;107(5):2037-42.

3. Gao P, et al. c-Myc suppression of miR-23a/b enhances mitochondrial glutaminase expression and glutamine metabolism. Nature. 2009 Apr 9;458(7239):762-5.

4. Wang JB, et al. Targeting mitochondrial glutaminase activity inhibits oncogenic transformation. Cancer Cell. 2010 Sep 14;18(3):207-19.

5. Le A, et al. Glucose-independent glutamine metabolism via TCA cycling for proliferation and survival in B cells. Cell Metab. 2012 Jan 4;15(1):110-21.

6. Shukla K, et al. Design, synthesis, and pharmacological evaluation of bis-2-(5-phenylacetamido-1,2,4-thiadiazol-2-yl)ethyl sulfide 3 (BPTES) analogs as glutaminase inhibitors. J Med Chem. 2012 Dec 13;55(23):10551-63.

7. Dutta P, et al. Evaluation of LDH-A and glutaminase inhibition in vivo by hyperpolarized 13C-pyruvate magnetic resonance spectroscopy of tumors. Cancer Res. 2013 Jul 15;73(14):4190-5.

8. Xiang Y, et al. Targeted inhibition of tumor-specific glutaminase diminishes cell-autonomous tumorigenesis. J Clin Invest. 2015 Jun;125(6):2293-306.

9. Shroff EH, et al. MYC oncogene overexpression drives renal cell carcinoma in a mouse model through glutamine metabolism. Proc Natl Acad Sci U S A. 2015 May 26;112(21):6539-44.

10. Yuneva MO, et al. The metabolic profile of tumors depends on both the responsible genetic lesion and tissue type. Cell Metab. 2012 Feb 8;15(2):157-70.

11. Altman BJ, et al. MYC Disrupts the Circadian Clock and Metabolism in Cancer Cells. Cell Metab. 2015 Sep 16. pii: S1550-4131(15)00460-X. doi: 10.1016/j.cmet.2015.09.003.

12. von Heideman A, et al. Safety and efficacy of NAD depleting cancer drugs: results of a phase I clinical trial of CHS 828 and overview of published data. Cancer Chemother Pharmacol. 2010 May;65(6):1165-72.

Citation Format: Chi Van Dang. MYC-mediated metabolic vulnerabilities and the circadian clock. [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 SY12-03.

Abstract IA05: Targeting MYC-mediated cancer metabolism

Myc, as a transcription factor capable of amplifying gene expression, frequently activates genes involved in cell metabolism to support cell growth and proliferation. Because many metabolic genes are ambiently expressed, it is not surprising in retrospect, that many metabolic genes are targets for Myc-mediated gene expression amplification. Collectively, Myc drives the expression of genes that are involved in glucose uptake, glycolysis, amino acid uptake - particularly glutamine- and catabolism, supporting increased protein synthesis required for a growing cell. In this regard, Myc stimulates many genes involved in nucleotide synthesis, ribosome biogenesis and translation. Furthermore, Myc can stimulate mitochondrial biogenesis and lipogenesis. When Myc is ectopically expressed in oncogenic settings where checkpoints are lost, MYC-transformed cells are constitutively driven to undergo ribosome biogenesis and growth, rendering them addicted to nutrients such as glucose or glutamine. Hence, targeting enzymes that are involved in glycolysis and/or glutaminolysis provides a strategy to treat MYC-driven cancers. We provide proof-of-concept that targeting glutaminolysis in GEMM or xenograft models is feasible and therefore provide a foundation for further studies.

Citation Format: Chi Van Dang. Targeting MYC-mediated cancer metabolism. [abstract]. In: Proceedings of the AACR Special Conference on Myc: From Biology to Therapy; Jan 7-10, 2015; La Jolla, CA. Philadelphia (PA): AACR; Mol Cancer Res 2015;13(10 Suppl):Abstract nr IA05.

Abstract 1879: Myc induces lipogenesis and suppresses fatty acid oxidation in human P493-6 B-lymphoma cells

Myc has been known to control cell growth and proliferation through regulating thousands of genes, of which a substantial amount are involved in metabolism. We have previously shown that Myc increases ribosomal biogenesis, nucleotide metabolism, mitochondrial biogenesis, glycolysis and glutaminolysis in cancer cells. However, Myc's role in fatty acid metabolism is not well-documented, notwithstanding the expectation that Myc induction of cell growth must be accompanied by membrane biogenesis. Other studies have shown that fatty acids can be utilized as energy fuel by cancer cells or as building blocks for proliferation. The P493 human B cell line contains a tetracycline (Tet)-repressible human MYC transgene, permitting robust control of cell growth and proliferation through MYC. Having previously documented that Myc stimulates glucose and glutamine metabolism, we sought to determine whether Myc affects P493 use of fatty acids for oxidation and fatty acid synthesis. We found that resting P493 cells, in which ectopic MYC was off, can use labeled palmitate for respiration, while Myc activated P493 cells demonstrated an inability to oxidize palmitate. We also observe through microarray analysis and RT-PCR that Myc increases the levels of mRNAs for lipogenesis genes. Chromatin immunoprecipitation (ChIP) with an anti-Myc antibody documents that Myc binds to the promoters of lipogenesis genes. Moreover, immunoblot analyses with antibodies available for selected enzymes also indicate that Myc increases the protein levels of ACACA and FASN. Uniformly labeled 13C-glucose and 13C,15N-glutamine were found to be incorporated into lipids when Myc was induced in P493 cells. Cerulenin, an inhibitor of FASN, dramatically inhibits Myc-induced P493 cell proliferation. Similarly, TOFA-inhibition of ACACA suppresses P493 cell proliferation as well. Our data suggest that in P493 Burkitt lymphoma model cells, Myc inhibits fatty acid oxidation and upregulates fatty acid synthesis through transcription of target genes.

Citation Format: Arvin Matthew Gouw, Anthony Mancuso, Annie Lee Hsieh, Brian Altman, Michael Wolfgang, Chi Van Dang. Myc induces lipogenesis and suppresses fatty acid oxidation in human P493-6 B-lymphoma cells. [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 1879. doi:10.1158/1538-7445.AM2013-1879

Abstract 852: Novel role and mechanism of the LDHA protein as a survival factor for cancer cells

Rationale: Lactate dehydrogenase A (LDHA), which catalyzes the conversion of pyruvate to lactate, serves as a poor prognostic serum marker for many different types of human cancers; however, it is not known whether this is simply the result of LDHA being released from dying cancer cells in patients with high tumor burden or whether LDHA could also be a signaling molecule that portends poor clinical outcome. Based on the observation that extracellular LDHA was identified as the key survival factor for cardiomyocytes exposed to oxidative stress, we sought to determine whether extracellular LDHA can act as a cancer cell survival factor and hence contributes to poor clinical outcome. We reasoned that LDHA released from the damaged cells could activate intracellular signaling pathways in surrounding tumor cells by means of membrane receptors, ultimately protecting those surrounding tumor cells from oxidative stress-induced cell death.

Methods: To investigate our hypothesis, recombinant LDHA and its isoform LDHB was produced by transforming E. coli strain BL21(DE3) with pET-SUMO expression vector (Clontech) containing full-length human LDHA or LDHB cDNA. The resulting recombinant proteins were purified using TALON Metal Affinity Resin and used for 16h pre-treatment of Ramos human Burkitt's lymphoma and A6L human pancreatic cancer cells cultured in full serum containing medium. The amount of recombinant proteins relative to serum albumin is estimated to be less 1% and hence any effects of LDHA could not be simply due to non-specific protein concentration effect. Following the pre-treatment procedure, these cancer cells were subjected to oxidative stress induced by hydrogen peroxide (H2O2) at 100 μM for 24h. Cells were then counted in a hemacytometer using trypan blue dye to exclude dead cells.

Results: We observed that while LDHB had no effect on cell death induced by H2O2 as compared to the non-pretreatment group, LDHA can act as a protective factor against oxidative stress induced by H2O2, and ultimately cell death. We also investigated the mechanism involved in the protective role of LDHA by using recombinant LDHA as a molecular probe against membrane proteins that were extracted from cancer cells.

Conclusion: Our study defines the novel role and mechanism of LDHA protein as a survival factor in cancer cells, in addition to its enzymatic activity, which may explain the prognostic significance of extracellular LDHA. Furthermore, this study can lead to the identification of new membrane receptor(s) for LDHA which may provide new targets for cancer therapy.

Citation Format: Brad Poore, Joshua Kee Park, Iman Afif, Laura Shingleton, Chi Van Dang, Anne Le. Novel role and mechanism of the LDHA protein as a survival factor for cancer cells. [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 852. doi:10.1158/1538-7445.AM2013-852

Therapeutic targeting of Myc-reprogrammed cancer cell metabolism

Studies from many laboratories document that the MYC oncogene produces a pleiotropic transcription factor, Myc, which influences genes driven by all three RNA polymerases to orchestrate nutrient import with biomass accumulation for cell division. Myc has been shown to activate genes involved in glycolysis, glutaminolysis, and mitochondrial biogenesis to provide ATP and anabolic substrates for cell mass accumulation. Myc stimulates ribosome biogenesis and orchestrates the energetic demand for biomass accumulation through its regulation of glucose and glutamine import and metabolism. When normal cells are deprived of nutrients, endogenous MYC expression diminishes and cells withdraw from the cell cycle. However, ectopic MYC-driven cancer cells are locked in a state of deregulated biomass accumulation, which renders them addicted to glucose and glutamine. This addictive state can be exploited for cancer therapy, because nutrient deprivation kills Myc-driven cells and inhibition of the Myc targets, lactate dehydrogenase A or glutaminase, diminishes tumor xenograft growth in vivo.

Emerging Concepts in the Analysis of Transcriptional Targets of the MYC Oncoprotein: Are the Targets Targetable?

Activation of the MYC oncoprotein is among the most ubiquitous events in human cancer. MYC functions in part as a sequence-specific regulator of transcription. Although early searches for direct downstream target genes that explain MYC's potent biological activity were met with enthusiasm, the postgenomic decade has brought the realization that MYC regulates the transcription of not just a manageably small handful of target genes but instead up to 15% of all active loci. As the dust has begun to settle, two important concepts have emerged that reignite hope that understanding MYC's downstream targets might still prove valuable for defining critical nodes for therapeutic intervention in cancer patients. First, it is now clear that MYC target genes are not a random sampling of the cellular transcriptome but instead fall into specific, critical biochemical pathways such as metabolism, chromatin structure, and protein translation. In retrospect, we should not have been surprised to discover that MYC rewires cell physiology in a manner designed to provide the tumor cell with greater biosynthetic properties. However, the specific details that have emerged from these studies are likely to guide the development of new clinical tools and strategies. This raises the second concept that instills renewed optimism regarding MYC target genes. It is now clear that not all MYC target genes are of equal functional relevance. Thus, it may be possible to discern, from among the thousands of potential MYC target genes, those whose inhibition will truly debilitate the tumor cell. In short, targeting the targets may ultimately be a realistic approach after all.

Abstract 5453: Inhibition of lactate dehydrogenase A induces oxidative stress and inhibits tumor progression

As the result of genetic alterations and tumor hypoxia, many cancer cells avidly take up glucose and generate lactate through lactate dehydrogenase A (LDHA), which is encoded by a target gene of c-Myc and HIF-1. Previous studies with reduction of LDHA expression indicate that LDHA is involved in tumor initiation, but its role in tumor maintenance and progression has not been established. Furthermore, how reduction of LDHA expression by interference or antisense RNA inhibits tumorigenesis is not well understood. Here, we report that reduction of LDHA by siRNA or its inhibition by a small molecule inhibitor (FX11) reduced ATP levels and induced significant oxidative stress and cell death that could be partially reversed by the antioxidant N-acetylcysteine. Furthermore, we document that FX11 inhibited the progression of sizable human lymphoma and pancreatic cancer xenografts. When used in combination with the NAD+ synthesis inhibitor FK866, FX11 induced lymphoma regression. Hence, inhibition of LDHA with FX11 is an achievable and tolerable treatment for LDHA-dependent tumors. Our studies document a therapeutic approach to the Warburg effect and demonstrate that oxidative stress and metabolic phenotyping of cancers are critical aspects of cancer biology to consider for the therapeutic targeting of cancer energy metabolism.

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 5453.

Nuclear MYC protein overexpression is an early alteration in human prostate carcinogenesis

The MYC onco-protein is a transcription factor that regulates cell proliferation, metabolism, protein synthesis, mitochondrial function and stem cell renewal. A region on chromosome 8q24 encompassing the MYC locus is amplified in prostate cancer, but this occurs mostly in advanced disease suggesting that MYC alterations occur late in prostate cancer. In contrast, MYC mRNA is elevated in most prostate cancers, even those of relatively low stage and grade (eg Gleason score 6) suggesting that MYC plays a role in initiation. However, since MYC protein levels are tightly regulated, elevated MYC mRNA does not necessarily imply elevated MYC protein. Thus, it is critical to determine whether MYC protein is elevated in human prostate cancer, and if so, at what stage of the disease this elevation occurs. Prior studies of MYC protein localization have been hampered by lack of suitable antibodies and controls. We utilized a new anti-MYC antibody coupled with genetically defined control experiments to localize MYC protein within human tissue microarrays consisting of normal, atrophy, PIN, primary adenocarcinoma, and metastatic adenocarcinoma. Nuclear overexpression of MYC protein occurred frequently in luminal cells of PIN, as well as in most primary carcinomas and metastatic disease. MYC protein did not correlate with gain of 8q24, suggesting alternative mechanisms for MYC overexpression. These results provide evidence that upregulation of nuclear MYC protein expression is a highly prevalent and early change in prostate cancer and suggest that increased nuclear MYC may be a critical oncogenic event driving human prostate cancer initiation and progression.

The interplay between MYC and HIF in the Warburg effect

c-MYC and the hypoxia-inducible factors (HIFs) are critical factors for tumorigenesis in a large number of human cancers. While the normal function of MYC involves the induction of cell proliferation and enhancement of cellular metabolism, the function of HIF, particularly HIF-1, involves adaptation to the hypoxic microenvironment, including activation of anaerobic glycolysis. When MYC-dependent tumors grow, the hypoxic tumor microenvironment elevates the levels of HIF, such that oncogenic MYC and HIF collaborate to enhance the cancer cell's metabolic needs through increased uptake of glucose and its conversion to lactate. HIF is also able to attenuate mitochondrial respiration through the induction of pyruvate dehydrogenase kinase 1 (PDK1), which in part accounts for the Warburg effect that describes the propensity for cancers to avidly take up glucose and convert it to lactate with the concurrent decrease in mitochondrial respiration. Target genes that are common to both HIF and MYC, such as PDK1, LDHA, HK2, and TFRC, are therefore attractive therapeutic targets, because their coordinate induction by HIF and MYC widens the therapeutic window between cancer and normal tissues.

Myc target transcriptomes

The c-Myc oncogenic transcription factor plays a central role in many human cancers through the regulation of gene expression. Although the molecular mechanisms by which c-Myc and its obligate partner, Max, regulate gene expression are becoming better defined, genes or transcriptomes that c-Myc regulate are just emerging from a variety of different experimental approaches. Studies of individual c-Myc target genes and their functional implications are now complemented by large surveys of c-Myc target genes through the use of subtraction cloning, DNA microarray analysis, serial analysis of gene expression (SAGE), chromatin immunoprecipitation, and genome marking methods. To fully appreciate the differences between physiological c-Myc function in normal cells and deregulated c-Myc function in tumors, the challenge now is to determine how the authenticated transcriptomes effect the various phenotypes induced by c-Myc and to define how c-Myc transcriptomes are altered by the Mad family of proteins.

A novel c-Myc-responsive gene, JPO1, participates in neoplastic transformation

We have identified a novel c-Myc-responsive gene, named JPO1, by representational difference analysis. JPO1 responds to two inducible c-Myc systems and behaves as a direct c-Myc target gene. JPO1 mRNA expression is readily detectable in the thymus, small intestine, and colon, whereas expression is relatively low in spleen, bone marrow, and peripheral leukocytes. We cloned a full-length JPO1 cDNA that encodes a 47-kDa nuclear protein. To determine the role of JPO1 in Myc-mediated cellular phenotypes, stable Rat1a fibroblasts overexpressing JPO1 were tested and compared with transformed Rat1a-Myc cells. Although JPO1 has a diminished transforming activity as compared with c-Myc, JPO1 complements a transformation-defective Myc Box II mutant in the Rat1a transformation assay. This complementation provides evidence for a genetic link between c-Myc and JPO1. Similar to c-Myc, JPO1 overexpression enhances the clonogenicity of CB33 human lymphoblastoid cells in methylcellulose assays. These observations suggest that JPO1 participates in c-Myc-mediated transformation, supporting an emerging concept that c-Myc target genes constitute nodal points in a network of pathways that lead from c-Myc to various Myc-related phenotypes and ultimately to tumorigenesis.

Characterization of nucleophosmin (B23) as a Myc target by scanning chromatin immunoprecipitation

The genetic program through which a specific transcription factor regulates a biological response is fundamental to our understanding how instructions in the genome are implemented. The emergence of DNA microarray technology for gene expression analysis has generated vast numbers of target genes resulting from specific transcription factor activity. We use the oncogenic transcription factor c-Myc as proof-of-principle that human genome sequence analysis and scanning of a specific gene by chromatin immunoprecipitation can be coupled to identify target transcription factor binding sequences. We focused on nucleophosmin, also known as B23, which was identified as a candidate Myc-responsive gene from a subtractive hybridization screen, and we found that sequences in intron 1, and not 5' sequences in the proximal promoter, are bound by c-Myc in vivo. Hence, a scanning chromatin immunoprecipitation (SChIP) strategy is useful in analyzing functional transcription factor-binding sites.

Arsenic inhibition of telomerase transcription leads to genetic instability

Arsenic is effective in the treatment of acute promyelocytic leukemia. Paradoxically, it is also carcinogenic. In the process of elucidating a mechanism of arsenic resistance in a leukemia cell line, NB4, we discovered that arsenic exposure causes chromosomal abnormalities, with a preponderance of end-to-end fusions. These chromosomal end fusions suggested that telomerase activity may be inhibited by arsenic. We found that arsenic inhibits transcription of the hTERT gene, which encodes the reverse transcriptase subunit of human telomerase. This effect may in part be explained by decreased c-Myc and Sp1 transcription factor activities. Decreased telomerase activity leads to chromosomal end lesions, which promote either genomic instability and carcinogenesis or cancer cell death. These phenomena may explain the seemingly paradoxical carcinogenic and antitumor effects of arsenic.

c-myc box II mutations in Burkitt's lymphoma-derived alleles reduce cell-transformation activity and lower response to broad apoptotic stimuli

In addition to c-myc rearrangement, over 50% of Burkitt's lymphoma cases present clustered mutations in exon 2, where many of the functional activities of c-Myc protein are based. This report describes the functional consequences induced by tumour-derived c-myc mutations located in c-myc box II. Two mutated alleles were studied, focusing on the P138C mutation, and compared to wild-type c-myc. The c-Myc transformation, transactivation and apoptosis activities were explored based on cells over-expressing c-Myc. While the transcriptional activation activity was not affected, our experiments exploring the anchorage-independent growth capacity of c-Myc-transfected Rat1a cells showed that c-Myc box II mutants were less potent than wild-type c-Myc in promoting cell transformation. Considering the possibility that these mutations could be interfering with the ability of c-Myc to promote apoptosis, we tested c-Myc-transfected Rat1a fibroblasts under several conditions: serum deprivation-, staurosporine- and TNFalpha-induced cell death. Interestingly, the mutated alleles were characterized by an overall decrease in ability to mediate apoptosis. Our study indicates that point mutations located in c-Myc box II can decrease the ability of the protein to promote both transformation and apoptosis without modifying its transactivating activity.

Translocations involving c-myc and c-myc function

c-MYC is the prototype for oncogene activation by chromosomal translocation. In contrast to the tightly regulated expression of c-myc in normal cells, c-myc is frequently deregulated in human cancers. Herein, aspects of c-myc gene activation and the function of the c-Myc protein are reviewed. The c-myc gene produces an oncogenic transcription factor that affects diverse cellular processes involved in cell growth, cell proliferation, apoptosis and cellular metabolism. Complete removal of c-myc results in slowed cell growth and proliferation, suggesting that while c-myc is not required for cell proliferation, it acts as an integrator and accelerator of cellular metabolism and proliferation.

Runner's anemia

Macrocytic anemia occurring in patients with fatigue suggests numerous diagnoses, ranging from nutritional deficiencies to a myelodysplastic syndrome. A careful history-taking is critically important for recognition of runner's anemia, which is due to plasma volume expansion, with hemolysis from the pounding of feet on pavement, and hemoglobinuria. Gastrointestinal blood loss may also contribute to anemia in long-distance runners. Early recognition of runner's anemia in patients with a complex presentation of anemia is important in circumventing many diagnostic tests. Runner's anemia should be considered when, amidst a constellation of signs and symptoms, mild anemia is well tolerated by an avid runner.

Hypoxia inhibits G1/S transition through regulation of p27 expression

Mammalian cellular responses to hypoxia include adaptive metabolic changes and a G1 cell cycle arrest. Although transcriptional regulation of metabolic genes by the hypoxia-induced transcription factor (HIF-1) has been established, the mechanism for the hypoxia-induced G1 arrest is not known. By using genetically defined primary wild-type murine embryo fibroblasts and those nullizygous for regulators of the G1/S checkpoint, we observed that the retinoblastoma protein is essential for the G1/S hypoxia-induced checkpoint, whereas p53 and p21 are not required. In addition, we found that the cyclin-dependent kinase inhibitor p27 is induced by hypoxia, thereby inhibiting CDK2 activity and forestalling S phase entry through retinoblastoma protein hypophosphorylation. Reduction or absence of p27 abrogated the hypoxia-induced G1 checkpoint, suggesting that it is a key regulator of G1/S transition in hypoxic cells. Intriguingly, hypoxic induction of p27 appears to be transcriptional and through an HIF-1-independent region of its proximal promoter. This demonstration of the molecular mechanism of hypoxia-induced G1/S regulation provides insight into a fundamental response of mammalian cells to low oxygen tension.

A strategy to identify differentially expressed genes using representational difference analysis and cDNA arrays

Representational difference analysis (RDA) combined with cDNA arrays is an effective approach to identify differentially expressed genes. To identify differentially expressed genes in c-Myc transgenic mouse liver, we compared the virtues of probing commercially available cDNA arrays with either radiolabeled cDNA pools or radiolabeled difference products (DP2) derived from RDA using c-Myc transgenic and normal mouse liver. Probing commercial and custom arrays with DP2 products led to the identification of transcripts of low abundance that were missed when the arrays were initially probed with PCR-amplified cDNA pools. Although DP2 probes also detected abundant transcripts that are highly differentially expressed, they failed to identify abundant transcripts with low differential expression that were detected with cDNA pools. The combined use of radiolabeled cDNA and DP2 products to probe arrays allows a more comprehensive identification of differentially expressed transcripts that are abundant or rare. Our method has the additional benefit of eliminating false-positive transcripts that lack true differential expression and frequently contaminate DP2 pools. Using this method we identified 16 differentially expressed genes in c-Myc transgenic liver, one of which is novel.

Tumor induction by the c-Myc target genes rcl and lactate dehydrogenase A

The characterization of c-Myc target genes, such as rcl and lactate dehydrogenase A (LDH-A), is critical for understanding the mechanisms of c-Myc-induced cell transformation and tumorigenesis. We have previously demonstrated that Rcl induces anchorage-independent growth in Ratla fibroblasts and that LDH-A is required for cell transformation by c-Myc. In this study, we report that Rcl and LDH-A act synergistically to induce anchorage-independent growth. Cells expressing both Rcl and LDH-A form tumors after s.c. injection into nude mice, although neither Rcl or LDH-A overexpression alone induces tumorigenesis. The inability of Rcl and LDH-A to fully recapitulate c-Myc activity, however, indicates that other c-Myc target genes participate in tumorigenesis. In addition, cells that coexpress Rcl and vascular endothelial growth factor are more comparable with c-Myc overexpressing cells in their ability to form tumors in nude mice. These findings confirm Rcl and LDH-A as critical components of the cell transformation program induced by c-Myc and suggest that Rcl is tumorigenic in cells that are provided with a permissive metabolic milieu.

Identification of c-myc responsive genes using rat cDNA microarray

c-Myc functions through direct activation or repression of transcription. Using cDNA microarray analysis, we have identified c-Myc-responsive genes by comparing gene expression profiles between c-myc null and c-myc wild-type rat fibroblast cells and between c-myc null and c-myc null cells reconstituted with c-myc. From a panel of 4400 cDNA elements, we found 198 genes responsive to c-myc when comparing wild-type or reconstituted cells with the null cells. The plurality of the named c-Myc-responsive genes that were up-regulated, including 30 ribosomal protein genes, are involved in macromolecular synthesis and metabolism, suggesting a major role of c-Myc in the regulation of protein synthetic and metabolic pathways. When ectopically overexpressed, c-Myc induced a different and smaller set of c-Myc-responsive genes as compared with the physiologically expressed c-Myc condition. Thus, these results from expression profiling suggest a new primary function for c-Myc and raise the possibility that the physiological and transforming functions of c-myc may be separable.

Induction of ribosomal genes and hepatocyte hypertrophy by adenovirus-mediated expression of c-Myc in vivo

Overexpression of c-Myc in immortalized cells increases cell proliferation, inhibits cell differentiation, and promotes cell transformation. Recent evidence suggests that these effects, however, do not necessarily occur when c-Myc is overexpressed in primary mammalian cells. We sought to determine the immediate effects of transient overexpression of c-Myc in primary cells in vivo by using recombinant adenovirus to overexpress human MYC in mouse liver. Mice were intravenously injected with adenoviruses encoding MYC (Ad/Myc), E2F-1 (Ad/E2F-1), or beta-galactosidase (Ad/LacZ). Transgene expression was detectable 4 days after injection. Expression of ectopic c-Myc was immediately accompanied by enlarged and dysmorphic hepatocytes in the absence of significant cell proliferation or apoptosis. These findings were not present in the livers of mice injected with Ad/E2F-1 or Ad/LacZ. Prominent hepatocyte nuclei and nucleoli were associated with the up-regulation of large- and small-subunit ribosomal and nucleolar genes, suggesting that c-Myc may induce their expression to increase cell mass. Our studies support a role for c-Myc in the in vivo control of vertebrate cell size and metabolism independent of cell proliferation.

Deregulation of glucose transporter 1 and glycolytic gene expression by c-Myc

Unlike normal mammalian cells, which use oxygen to generate energy, cancer cells rely on glycolysis for energy and are therefore less dependent on oxygen. We previously observed that the c-Myc oncogenic transcription factor regulates lactate dehydrogenase A and induces lactate overproduction. We, therefore, sought to determine whether c-Myc controls other genes regulating glucose metabolism. In Rat1a fibroblasts and murine livers overexpressing c-Myc, the mRNA levels of the glucose transporter GLUT1, phosphoglucose isomerase, phosphofructokinase, glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase, and enolase were elevated. c-Myc directly transactivates genes encoding GLUT1, phosphofructokinase, and enolase and increases glucose uptake in Rat1 fibroblasts. Nuclear run-on studies confirmed that the GLUT1 transcriptional rate is elevated by c-Myc. Our findings suggest that overexpression of the c-Myc oncoprotein deregulates glycolysis through the activation of several components of the glucose metabolic pathway.

Identification of CDK4 as a target of c-MYC

The prototypic oncogene c-MYC encodes a transcription factor that can drive proliferation by promoting cell-cycle reentry. However, the mechanisms through which c-MYC achieves these effects have been unclear. Using serial analysis of gene expression, we have identified the cyclin-dependent kinase 4 (CDK4) gene as a transcriptional target of c-MYC. c-MYC induced a rapid increase in CDK4 mRNA levels through four highly conserved c-MYC binding sites within the CDK4 promoter. Cell-cycle progression is delayed in c-MYC-deficient RAT1 cells, and this delay was associated with a defect in CDK4 induction. Ectopic expression of CDK4 in these cells partially alleviated the growth defect. Thus, CDK4 provides a direct link between the oncogenic effects of c-MYC and cell-cycle regulation.