Published OnlineFirst September 29, 2009; DOI: 10.1158/1078-0432.CCR-08-3213 Molecular Pathways Disrupting Polyamine Homeostasis as a Therapeutic Strategy for Neuroblastoma Nicholas F. Evageliou and Michael D. Hogarty Abstract MYC genes are deregulated in a plurality of human cancers. Through direct and indirect mechanisms, the MYC network regulates the expression of >15% of the human genome, including both protein-coding and noncoding RNAs. This complexity has complicated efforts to define the principal pathways mediating MYC's oncogenic activ- ity. MYC plays a central role in providing for the bioenergetic and biomass needs of proliferating cells, and polyamines are essential cell constituents supporting many of these functions. The rate-limiting enzyme in polyamine biosynthesis, ODC, is a bona fide MYC target, as are other regulatory enzymes in this pathway. A wealth of data link enhanced polyamine biosynthesis to cancer progression, and polyamine depletion may limit the malignant transformation of preneoplastic lesions. Studies with transgen- ic cancer models also support the finding that the effect of MYC on tumor initiation and progression can be attenuated through the repression of polyamine production. High- risk neuroblastomas (an often lethal embryonal tumor in which MYC activation is paramount) deregulate numerous polyamine enzymes to promote the expansion of intracellular polyamine pools. Selective inhibition of key enzymes in this pathway, e.g., using DFMO and/or SAM486, reduces tumorigenesis and synergizes with chemotherapy to regress tumors in preclinical models. Here, we review the potential clinical application of these and additional polyamine depletion agents to neuroblastoma and other advanced cancers in which MYC is operative. (Clin Cancer Res 2009;15(19):5956–61) Background in neural stem cells, for example, leads to an aberrant nuclear structure mimicking a heterochromatin state accompanied by The MYC proto-oncogenes, which include MYC, MYCN, and widespread histone modifications (5). Such higher-order regu- MYCL, are among the most frequently deregulated genes in latory activities may explain MYC's profound influence on tran- cancer. The MYC proto-oncogenes encode highly homolo- scription and the diversity in putative target genes identified gous basic-helix-loop-helix leucine zipper transcription factors across different model systems. that are biologically redundant, but differentially expressed spa- MYC activity is tightly regulated through transcriptional and tiotemporally. MYC genes function through heterodimerization posttranslational mechanisms, with rapid degradation of Myc with Max and operate within a network of related proteins to protein in concert with cell cycle exit. In many cancers, MYC regulate transcription through interactions at E-box sequences genes are deregulated through genomic translocation or ampli- within promoters of diverse target genes (1). Many estimates fication events that lead to supraphysiologic Myc expression. have the MYC network governing the expression of > 15% of Although mutations in Myc have been identified in Burkitt's all human genes (2, 3) and a growing roster of noncoding RNAs lymphoma cells (accompanying rather than replacing activating (4). A simplified gene-specific model of transcriptional regula- translocations; ref. 6), Myc oncogenesis typically results from tion has been expanded with the appreciation that MYC genes deregulated overexpression of wild-type protein. Such large- also contribute to global chromatin regulation. Loss of MYCN scale biological reprogramming of cells through enforced expression of this promiscuous transactivator and chromatin regulator is highly oncogenic, and the ubiquity of MYC activa- Authors' Affiliations: Division of Oncology, The Children's Hospital of tion across tumor types makes it an attractive cancer target. Philadelphia and Department of Pediatrics, University of Pennsylvania Inhibiting grossly deregulated transcription factors remains a School of Medicine, Philadelphia, Pennsylvania daunting therapeutic challenge, yet pharmaceutical successes Received 5/4/09; revised 6/3/09; accepted 6/3/09; published OnlineFirst 9/29/09. continue to whittle away at the list of domains considered “un- Grant support: The author's laboratory is supported by National Institutes druggable”; thus, direct Myc antagonism may be an achievable of Health CA97323 and the Richard and Sheila Sanford Chair in Pediatric Oncology (M.D. Hogarty), and by the Children's Neuroblastoma Cancer goal. However, a secondary concern for such an approach is Foundation (N.F. Evageliou). that systemic interference with Myc might be quite toxic, as Requests for reprints: Michael D. Hogarty, Division of Oncology, The Chil- it is indispensable for cell cycle entry in response to mitogenic dren's Hospital of Philadelphia, 9 North ARC, Suite 902C, 3615 Civic Center signals. This fear has been partially allayed by evidence that a Boulevard, Philadelphia, PA 19104-4318. Phone: 215-590-3931; Fax: 215- 590-3770; E-mail: [email protected]. profound dominant-negative Myc contruct can be activated F 2009 American Association for Cancer Research. globally in mice without undue toxicity (7). An alternative ap- doi:10.1158/1078-0432.CCR-08-3213 proach to interfering directly with Myc is to define the critical Clin Cancer Res 2009;15(19) October 1, 20095956 www.aacrjournals.org Downloaded from clincancerres.aacrjournals.org on October 1, 2021. © 2009 American Association for Cancer Research. Published OnlineFirst September 29, 2009; DOI: 10.1158/1078-0432.CCR-08-3213 Polyamine Depletion in Neuroblastoma downstream pathways necessary for its oncogenic activity. (OAZ1, OAZ2, OAZ3) are themselves responsive to intracellular Among these may be more immediately tractable drug targets polyamines to provide a negative feedback loop (20). Further- that exploit cancer-specific aspects of Myc activity with a greater more, two mammalian antizyme inhibitors have been identi- therapeutic index. fied [AZIN1 (ref. 21) and, more recently, ADC (ref. 22)] that An improved understanding of Myc biology has emerged encode enzymatically inactive Odc homologs that compete to from high-dimensional assays that provide global transcrip- neutralize the antizymes and therefore constitute a positive reg- tome and/or Myc-chromatin binding site data. These platforms ulator of Odc activity (23). This level of control underscores the have generated daunting lists of genes and chromatin binding importance of modulating Odc activity to ensure cellular fitness. sites that underscore the widespread involvement of Myc in Polyamine catabolism occurs via the acetylation of spermi- diverse biological processes. Still, patterns are discernible with- dine or spermine by the readily inducible spermidine/ in this complexity. The most conserved set of MYC target genes spermine-N-acetyltransferase, SAT1 (24). Acetylated polyamines functions in ribosomal biogenesis and protein metabolism may be exported from the cell through specific transmembrane and processing, and this is true for both MYC (3) and MYCN solute carriers to reduce intracellular levels (25). Alternatively, (8). Additionally, programs that direct carbon assimilation, an- acetylated spermine and spermidine may be converted through abolic pathways, and bioenergetics are all targeted by Myc (3). PAOX oxidase activity to spermidine and putrescine, respective- Thus, Myc orchestrates a program redirecting metabolism to ly, whereas SMOX oxidase activity can convert spermine to sper- provide for the energetic needs of the cell through augmented midine directly. These conversions allow for homeostatic aerobic glycolysis (9) and glutaminolysis (10), as well as the control over the repertoire of natural polyamines, which may biomass needs through enhanced synthesis and processing of be important for maintaining functions unique to select polya- RNA, DNA, protein, lipid, and polyamine precursors. mines [such as protein translation, in which spermidine acts as a Polyamines are multifunctional polycations found in nearly all gate-keeper by regulating the activity of eIF5A (ref. 26); Fig. 2]. living organisms. Polyamines support biological processes Finally, an as yet uncharacterized energy-dependent polyamine through the stabilization of anionic macromolecules and mod- transporter functions to selectively import polyamines present ulate DNA:protein and protein:protein interactions. A detailed in the microenvironment through dietary intake, export from understanding of polyamine activities is hampered by the fact neighboring cells, or synthesis by intestinal flora. This pathway that they participate in mainly transient ionic interactions that can restore polyamine levels under conditions of biosynthetic are difficult to study. Still, polyamine homeostasis is essential blockade and may potentially undermine therapeutic efforts for cell survival, and depletion activates cellular checkpoints to diminish intracellular polyamines. that constrain proliferation or induce apoptosis (11). Reduced This multitude of regulatory controls highlights the critical polyamines are seen in postmitotic and senescent cells, whereas need for polyamine homeostasis, and as polyamines operate enhanced polyamine biosynthesis accompanies normal as well downstream of Myc to support proliferation, they provide an as oncogenic proliferation (12, 13). That polyamine biosynthe- intriguing cancer target. Indeed, substantial effort has been sis may be instructive in the cancer process, rather