Key role for p27Kip1, retinoblastoma protein Rb, and MYCN in polyamine inhibitor-induced G1 cell cycle arrest in MYCN-amplified human neuroblastoma cells

Study on the role of MYCN in retinoblastoma by inhibiting p53 and activating wnt/βcatenin/Fra-1 signaling pathway by reducing DKK3

Retinoblastoma (RB) is a pediatric malignancy, typically diagnosed at birth or during early childhood. The pathogenesis of RB is marked by the amplification of the Basic Helix-Loop-Helix (BHLH) Transcription Factor MYCN, which serves as a transcriptional regulator capable of binding to Dickkopf 3 (DKK3). However, the precise role of DKK3 in the malignant progression of RB cells caused by MYCN remains elusive. In the present study, the expression of MYCN was either overexpressed or interfered in RB cells. Subsequently, the expression level of DKK3 was assessed through quantitative real-time polymerase chain reaction and western blot analysis. Cell proliferation was evaluated using the Cell Counting Kit-8 assay and 5-ethynyl-2'-deoxyuridine staining, while cell cycle progression and apoptosis were analyzed by flow cytometry and western blot analysis, respectively. Additionally, the expression of proteins involved in the Wnt/β-catenin/Fra-1/p53 signaling pathway was evaluated via western blot analysis. To gain further insights, Wnt agonists and the P53 inhibitor PFT-α were introduced into exploration. The current investigation revealed a negative correlation between the expression levels of MYCN and DKK3 in RB cells. Additionally, DKK3 overexpression inhibited cell proliferation, promoted cell apoptosis, and arrested cell cycle in RB cells with high expression of MYCN. Moreover, enhanced DKK3 expression inhibited proliferation, promoted cell cycle arrest and apoptosis of RB cells by modulating the wnt/βcatenin/Fra-1/p53 signaling pathway. Furthermore, in vivo experiments revealed that overexpression of DKK3 inhibits the growth of RB tumors. Collectively, our findings elucidate that MYCN stimulates the Wnt/β-catenin/Fra-1 pathway by suppressing DKK3 expression, ultimately suppressing p53 activity and contributing to malignant progression of RB.

US Food and Drug Administration Approval Summary: Eflornithine for High-Risk Neuroblastoma After Prior Multiagent, Multimodality Therapy

On December 13, 2023, the US Food and Drug Administration (FDA) approved eflornithine (IWILFIN, US WorldMeds) to reduce the risk of relapse in adult and pediatric patients with high-risk neuroblastoma who have demonstrated at least a partial response to prior multiagent, multimodality therapy including anti-GD2 immunotherapy. The approval was based on an externally controlled trial (ECT) consisting of a single-arm trial, study 3(b), compared with an external control (EC) derived from a National Cancer Institute/Children's Oncology Group-sponsored clinical trial (Study ANBL0032) and supported by confirmatory evidence. In the protocol-specified primary analysis, the event-free survival hazard ratio (HR) was 0.48 (95% CI, 0.27 to 0.85) and overall survival HR was 0.32 (95% CI, 0.15 to 0.70). The most common adverse reactions (≥5%) were hearing loss, otitis media, pyrexia, pneumonia, and diarrhea. Notably, this is the first oncology drug approval which relies on an ECT as the primary clinical data to support substantial evidence of effectiveness. This was made possible by a distinctly high-quality, comparable EC data set with consistent treatment effect estimations demonstrated in multiple sensitivity and supportive analyses. Eflornithine's manageable safety profile and strong nonclinical and mechanistic data provided further support for the approval, and the evidentiary package was evaluated in the context of high unmet need in a rare, life-threatening cancer.

DFMO inhibition of neuroblastoma tumorigenesis

BACKGROUND

Most high-risk neuroblastoma patients who relapse succumb to disease despite the existing therapy. We recently reported increased event-free and overall survival in neuroblastoma patients receiving difluoromethylornithine (DFMO) during maintenance therapy. The effect of DFMO on cellular processes associated with neuroblastoma tumorigenesis needs further elucidation. Previous studies have shown cytotoxicity with IC50 values >5-15 mM, these doses are physiologically unattainable in patients, prompting further mechanistic studies at therapeutic doses.

METHODS

We characterized the effect of DFMO on cell viability, cell cycle, apoptosis, neurosphere formation, and protein expression in vitro using five established neuroblastoma cell lines (BE2C, CHLA-90, SHSY5Y, SMS-KCNR, and NGP) at clinically relevant doses of 0, 50, 100, 500, 1000, and 2500 μM. Limiting Dilution studies of tumor formation in murine models were performed. Statistical analysis was done using GraphPad and the level of significance set at p = 0.05.

RESULTS

There was not a significant loss of cell viability or gain of apoptotic activity in the in vitro assays (p > 0.05). DFMO treatment initiated G1 to S phase cell cycle arrest. There was a dose-dependent decrease in frequency and size of neurospheres and a dose-dependent increase in beta-galactosidase activity in all cell lines. Tumor formation was decreased in xenografts both with DFMO-pretreated cells and in mice treated with DFMO.

CONCLUSION

DFMO treatment is cytostatic at physiologically relevant doses and inhibits tumor initiation and progression in mice. This study suggests that DFMO, inhibits neuroblastoma by targeting cellular processes integral to neuroblastoma tumorigenesis at clinically relevant doses.

Drivers Underlying Metastasis and Relapse in Medulloblastoma and Targeting Strategies

Medulloblastomas comprise a molecularly diverse set of malignant pediatric brain tumors in which patients are stratified according to different prognostic risk groups that span from very good to very poor. Metastasis at diagnosis is most often a marker of poor prognosis and the relapse incidence is higher in these children. Medulloblastoma relapse is almost always fatal and recurring cells have, apart from resistance to standard of care, acquired genetic and epigenetic changes that correlate with an increased dormancy state, cell state reprogramming and immune escape. Here, we review means to carefully study metastasis and relapse in preclinical models, in light of recently described molecular subgroups. We will exemplify how therapy resistance develops at the cellular level, in a specific niche or from therapy-induced secondary mutations. We further describe underlying molecular mechanisms on how tumors acquire the ability to promote leptomeningeal dissemination and discuss how they can establish therapy-resistant cell clones. Finally, we describe some of the ongoing clinical trials of high-risk medulloblastoma and suggest or discuss more individualized treatments that could be of benefit to specific subgroups.

Bachmann-Bupp syndrome and treatment

Bachmann-Bupp syndrome (BABS) is a neurodevelopmental disorder characterized by developmental delay, hypotonia, and varying forms of non-congenital alopecia. The condition is caused by 3'-end mutations of the ornithine decarboxylase 1 (ODC1) gene, which produce carboxy (C)-terminally truncated variants of ODC, a pyridoxal 5'-phosphate-dependent enzyme. C-terminal truncation of ODC prevents its ubiquitin-independent proteasomal degradation and leads to cellular accumulation of ODC enzyme that remains catalytically active. ODC is the first rate-limiting enzyme that converts ornithine to putrescine in the polyamine pathway. Polyamines (putrescine, spermidine, spermine) are aliphatic molecules found in all forms of life and are important during embryogenesis, organogenesis, and tumorigenesis. BABS is an ultra-rare condition with few reported cases, but it serves as a convincing example for drug repurposing therapy. α-Difluoromethylornithine (DFMO, also known as eflornithine) is an ODC inhibitor with a strong safety profile in pediatric use for neuroblastoma and other cancers as well as West African sleeping sickness (trypanosomiasis). Patients with BABS have been treated with DFMO and have shown improvement in hair growth, muscle tone, and development.

N-Myc transcriptionally activates Skp2 to suppress p27 expression in small cell lung cancer

BACKGROUND

Small cell lung cancer (SCLC) is characterized by a high proliferative rate, a strong predilection for early metastasis and poor prognosis. Novel SCLC biomarkers are urgently required to improve current diagnostic and treatment modalities. MYCN encodes the proto-oncogene N-Myc that is overexpressed in SCLC, but its downstream effectors are poorly characterized. Here, we investigated the role of the N-Myc/Skp2/p27 axis during SCLC progression.

METHODS

Immunohistochemistry (IHC) and western blotting were performed to evaluate N-Myc/Skp2/p27 expression. SCLC cell apoptosis was investigated through TUNEL staining. Wound healing and transwell assays were performed to detect the migratory and invasive potential of SCLC cells. N-Myc and Skp2 binding was confirmed through luciferase reporter and ChIP assays. Xenograft models were developed to investigate the function of Skp2 during SCLC tumor growth in vivo.

RESULTS

N-Myc and Skp2 were overexpressed in SCLC, whilst p27 expression was suppressed. Skp2 facilitated SCLC progression by protecting cells from apoptosis and facilitating cell migration and invasion. N-Myc was found to bind to the promoter region of Skp2 to enhance its expression. Skp2 enhanced tumor growth in vivo through the suppression of p27. Skp2 silencing reversed the pro-oncogenic effects of N-myc in SCLC tumors.

CONCLUSION

We show that N-Myc enhances Skp2 to regulate p27 expression during SCLC progression. We therefore highlight the N-Myc/Skp2/p27 axis as a novel diagnostic and much-needed therapeutic target in SCLC.

DFMO Carbon Dots for Treatment of Neuroblastoma and Bioimaging

Neuroblastoma (NB) is a pediatric malignancy affecting the peripheral nervous system. Despite recent advancements in treatment, many children affected with NB continue to submit to this illness, and new therapeutic strategies are desperately needed. In recent years, studies of carbon dots (CDs) as nanocarriers have mostly focused on the delivery of anticancer agents because of their biocompatibility, good aqueous dissolution, and photostability. Their fluorescence properties, surface functionalities, and surface charges differ on the basis of the type of precursors used and the synthetic approach implemented. At present, most CDs are used as nanocarriers by directly linking them either covalently or electrostatically to drug molecules. Though most modern CDs are synthesized from large carbon macromolecules and conjugated to anticancerous drugs, constructing CDs from the anticancerous drugs and precursors themselves to increase antitumoral activity requires further investigation. Herein, CDs were synthesized using difluoromethylornithine (DFMO), an irreversible ornithine decarboxylase inhibitor commonly used in high-risk neuroblastoma treatment regiments. In this study, NB cell lines, SMS-KCNR and SK-N-AS, were treated with DFMO, the newly synthesized DFMO CDs, and conventional DFMO conjugated to black carbon dots. Bioimaging was done to determine the cellular localization of a fluorescent drug over time. The mobility of DNA mixed with DFMO CDs was evaluated by gel electrophoresis. DFMO CDs were effectively synthesized from DFMO precursor and characterized using spectroscopic methods. The DFMO CDs effectively reduced cell viability with increasing dose. The effects were dramatic in the N-MYC-amplified line SMS-KCNR at 500 μM, which is comparable to high doses of conventional DFMO at a 60-fold lower concentration. In vitro bioimaging as well as DNA electrophoresis showed that synthesized DFMO CDs were able to enter the nucleus of neuroblastoma cells and neuronal cells and interact with DNA. Our new DFMO CDs exhibit a robust advantage over conventional DFMO because they induce comparable reductions in viability at a dramatically lower concentration.

Pharmacological targeting of polyamine and hypusine biosynthesis reduces tumor activity of endometrial cancer

Endometrial cancer (EC) is a common and deadly cancer in women and novel therapeutic approaches are urgently needed. Polyamines (putrescine, spermidine, spermine) are critical for mammalian cell proliferation and MYC coordinately regulates polyamine metabolism through ornithine decarboxylase (ODC). ODC is a MYC target gene and rate-limiting enzyme of polyamine biosynthesis and the FDA-approved anti-protozoan drug α-difluoromethylornithine (DFMO) inhibits ODC activity and induces polyamine depletion that leads to tumor growth arrest. Spermidine is required for the hypusine-dependent activation of eukaryotic translation initiation factors 5A1 (eIF5A1) and 5A2 (eIF5A2) and connects the MYC/ODC-induced deregulation of spermidine to eIF5A1/2 protein translation, which is increased during cancer cell proliferation. We show that the eIF5A1 is significantly upregulated in EC cells compared to control cells (p = 0.000038) and that combined pharmacological targeting of ODC and eIF5A hypusination with cytostatic drugs DFMO and N1-guanyl-1,7-diaminoheptane (GC7), respectively, reduces eIF5A1 activation and synergistically induces apoptosis in EC cells. In vivo, DFMO/GC7 suppressed xenografted EC tumor growth in mice more potently than each drug alone compared to control (p = 0.002) and decreased putrescine (p = 0.045) and spermidine levels in tumor tissues. Our data suggest DFMO and GC7 combination therapy may be useful in the treatment or prevention of EC.

Difluoromethylornithine Induces Apoptosis through Regulation of AP-1 Signaling via JNK Phosphorylation in Epithelial Ovarian Cancer

Difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase (ODC), has promising activity against various cancers and a tolerable safety profile for long-term use as a chemopreventive agent. However, the anti-tumor effects of DFMO in ovarian cancer cells have not been entirely understood. Our study aimed to identify the effects and mechanism of DFMO in epithelial ovarian cancer cells using SKOV-3 cells. Treatment with DFMO resulted in a significantly reduced cell viability in a time- and dose-dependent manner. DFMO treatment inhibited the activity and downregulated the expression of ODC in ovarian cancer cells. The reduction in cell viability was reversed using polyamines, suggesting that polyamine depletion plays an important role in the anti-tumor activity of DFMO. Additionally, significant changes in Bcl-2, Bcl-xL, Bax protein levels, activation of caspase-3, and cleavage of poly (ADP-ribose) polymerase were observed, indicating the apoptotic effects of DFMO. We also found that the effect of DFMO was mediated by AP-1 through the activation of upstream JNK via phosphorylation. Moreover, DFMO enhanced the effect of cisplatin, thus showing a possibility of a synergistic effect in treatment. In conclusion, treatment with DFMO alone, or in combination with cisplatin, could be a promising treatment for ovarian cancer.

Probenecid increases renal retention and antitumor activity of DFMO in neuroblastoma

BACKGROUND

Neuroblastoma (NB) is the most common extracranial solid tumor in children. Interference with the polyamine biosynthesis pathway by inhibition of MYCN-activated ornithine decarboxylase (ODC) is a validated approach. The ODC inhibitor α-difluoromethylornithine (DFMO, or Eflornithine) has been FDA-approved for the treatment of trypanosomiasis and hirsutism and has advanced to clinical cancer trials including NB as well as cancer-unrelated human diseases. One key challenge of DFMO is its rapid renal clearance and the need for high and frequent drug dosing during treatment.

METHODS

We performed in vivo pharmacokinetic (PK), antitumorigenic, and molecular studies with DFMO/probenecid using NB patient-derived xenografts (PDX) in mice. We used LC-MS/MS, HPLC, and immunoblotting to analyze blood, brain tissue, and PDX tumor tissue samples collected from mice.

RESULTS

The organic anion transport 1/3 (OAT 1/3) inhibitor probenecid reduces the renal clearance of DFMO and significantly increases the antitumor activity of DFMO in PDX of NB (P < 0.02). Excised tumors revealed that DFMO/probenecid treatment decreases polyamines putrescine and spermidine, reduces MYCN protein levels and dephosphorylates retinoblastoma (Rb) protein (p-Rb^Ser795), suggesting DFMO/probenecid-induced cell cycle arrest.

CONCLUSION

Addition of probenecid as an adjuvant to DFMO therapy may be suitable to decrease overall dose and improve drug efficacy in vivo.

Polyamines: Functions, Metabolism, and Role in Human Disease Management

Putrescine, spermine, and spermidine are the important polyamines (PAs), found in all living organisms. PAs are formed by the decarboxylation of amino acids, and they facilitate cell growth and development via different cellular responses. PAs are the integrated part of the cellular and genetic metabolism and help in transcription, translation, signaling, and post-translational modifications. At the cellular level, PA concentration may influence the condition of various diseases in the body. For instance, a high PA level is detrimental to patients suffering from aging, cognitive impairment, and cancer. The levels of PAs decline with age in humans, which is associated with different health disorders. On the other hand, PAs reduce the risk of many cardiovascular diseases and increase longevity, when taken in an optimum quantity. Therefore, a controlled diet is an easy way to maintain the level of PAs in the body. Based on the nutritional intake of PAs, healthy cell functioning can be maintained. Moreover, several diseases can also be controlled to a higher extend via maintaining the metabolism of PAs. The present review discusses the types, important functions, and metabolism of PAs in humans. It also highlights the nutritional role of PAs in the prevention of various diseases.

Emerging Role of ODC1 in Neurodevelopmental Disorders and Brain Development

Ornithine decarboxylase 1 (ODC1 gene) has been linked through gain-of-function variants to a rare disease featuring developmental delay, alopecia, macrocephaly, and structural brain anomalies. ODC1 has been linked to additional diseases like cancer, with growing evidence for neurological contributions to schizophrenia, mood disorders, anxiety, epilepsy, learning, and suicidal behavior. The evidence of ODC1 connection to neural disorders highlights the need for a systematic analysis of ODC1 genotype-to-phenotype associations. An analysis of variants from ClinVar, Geno2MP, TOPMed, gnomAD, and COSMIC revealed an intellectual disability and seizure connected loss-of-function variant, ODC G84R (rs138359527, NC_000002.12:g.10444500C > T). The missense variant is found in ~1% of South Asian individuals and results in 2.5-fold decrease in enzyme function. Expression quantitative trait loci (eQTLs) reveal multiple functionally annotated, non-coding variants regulating ODC1 that associate with psychiatric/neurological phenotypes. Further dissection of RNA-Seq during fetal brain development and within cerebral organoids showed an association of ODC1 expression with cell proliferation of neural progenitor cells, suggesting gain-of-function variants with neural over-proliferation and loss-of-function variants with neural depletion. The linkage from the expression data of ODC1 in early neural progenitor proliferation to phenotypes of neurodevelopmental delay and to the connection of polyamine metabolites in brain function establish ODC1 as a bona fide neurodevelopmental disorder gene.

Allicin, a Potent New Ornithine Decarboxylase Inhibitor in Neuroblastoma Cells

The natural product allicin is a reactive sulfur species (RSS) from garlic (Allium sativum L.). Neuroblastoma (NB) is an early childhood cancer arising from the developing peripheral nervous system. Ornithine decarboxylase (ODC) is a rate-limiting enzyme in the biosynthesis of polyamines, which are oncometabolites that contribute to cell proliferation in NB and other c-MYC/MYCN-driven cancers. Both c-MYC and MYCN directly transactivate the E-box gene ODC1, a validated anticancer drug target. We identified allicin as a potent ODC inhibitor in a specific radioactive in vitro assay using purified human ODC. Allicin was ∼23 000-fold more potent (IC₅₀ = 11 nM) than DFMO (IC₅₀ = 252 μM), under identical in vitro assay conditions. ODC is a homodimer with 12 cysteines per monomer, and allicin reversibly S-thioallylates cysteines. In actively proliferating human NB cells allicin inhibited ODC enzyme activity, reduced cellular polyamine levels, inhibited cell proliferation (IC₅₀ 9-19 μM), and induced apoptosis. The natural product allicin is a new ODC inhibitor and could be developed for use in conjunction with other anticancer treatments, the latter perhaps at a lower than usual dosage, to achieve drug synergism with good prognosis and reduced adverse effects.

Inhibition of N-myc expression sensitizes human neuroblastoma IMR-32 cells expressing caspase-8 to TRAIL

Objectives

This study aims to explore the roles of N‐myc and caspase‐8 in TRAIL‐resistant IMR‐32 cells which exhibit MYCN oncogene amplification and lack caspase‐8 expression.

Materials and methods

We established N‐myc–downregulated IMR‐32 cells using shRNA lentiviral particles targeting N‐myc and examined the effect the N‐myc inhibition on TRAIL susceptibility in human neuroblastoma IMR‐32 cells expressing caspase‐8.

Results

Cisplatin treatment in IMR‐32 cells increased the expression of death receptor 5 (DR5; TRAIL‐R2), but not other receptors, via downregulation of NF‐κB activity. However, the cisplatin‐mediated increase in DR5 failed to induce cell death following TRAIL treatment. Furthermore, interferon (IFN)‐γ pretreatment increased caspase‐8 expression in IMR‐32 cells, but cisplatin failed to trigger TRAIL cytotoxicity. We downregulated N‐myc expression in IMR‐32 cells using N‐myc–targeting shRNA. These cells showed decreased growth rate and Bcl‐2 expression accompanied by a mild collapse in the mitochondrial membrane potential as compared with those treated with scrambled shRNA. TRAIL treatment in N‐myc–negative cells expressing caspase‐8 following IFN‐γ treatment significantly triggered apoptotic cell death. Concurrent treatment with cisplatin enhanced TRAIL‐mediated cytotoxicity, which was abrogated by an additional pretreatment with DR5:Fc chimera protein.

Conclusions

N‐myc and caspase‐8 expressions are involved in TRAIL susceptibility in IMR‐32 cells, and the combination of treatment with cisplatin and TRAIL may serve as a promising strategy for the development of therapeutics against neuroblastoma that is controlled by N‐myc and caspase‐8 expression.

Crocin inhibits proliferation and induces apoptosis through suppressing MYCN expression in retinoblastoma

The pathogenetic mechanisms of retinoblastoma are still not yet fully elucidated, putting limits to efficacious treatment. Crocin is the main component of saffron, which exhibits significant antitumorigenic properties. The aim of this paper is to investigate the effect of crocin on retinoblastoma. The effects of crocin on the proliferation of human retinoblastoma cells were determined by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, cell number assay, and colony formation assay. Cell apoptosis induced by crocin was measured by flow cytometry analysis. Cleaved poly(ADP-ribose) polymerase and cleaved caspase-3 were tested by western blot analysis. The expression levels of MYCN were assessed by western blot and quantitative polymerase chain reaction and the stability of MYCN messenger RNA was determined by in vitro RNA degradation assays. We found that crocin significantly inhibited the cell proliferation and clonogenicity and induced cell apoptosis in Y79 and WERI-RB-1 cells. In addition, crocin treatment significantly reduced the expression and the stability of MYCN. Besides, overexpression of MYCN rescued the inhibitory effect of crocin in Y79 cells. Our findings suggest that crocin exhibits antitumorigenic effects in human retinoblastoma cell lines through a MYCN-dependent manner, which may provide guidance to logical therapeutic designs in prevention and treatment of retinoblastoma.

Polyamine synthesis as a target of MYC oncogenes

This paper is in recognition of the 100th birthday of Dr. Herbert Tabor, a true pioneer in the polyamine field for over 70 years, who served as the editor-in-chief of the Journal of Biological Chemistry from 1971 to 2010. We review current knowledge of MYC proteins (c-MYC, MYCN, and MYCL) and focus on ornithine decarboxylase 1 (ODC1), an important bona fide gene target of MYC, which encodes the sentinel, rate-limiting enzyme in polyamine biosynthesis. Although notable advances have been made in designing inhibitors against the "undruggable" MYCs, their downstream targets and pathways are currently the main avenue for therapeutic anticancer interventions. To this end, the MYC-ODC axis presents an attractive target for managing cancers such as neuroblastoma, a pediatric malignancy in which MYCN gene amplification correlates with poor prognosis and high-risk disease. ODC and polyamine levels are often up-regulated and contribute to tumor hyperproliferation, especially of MYC-driven cancers. We therefore had proposed to repurpose α-difluoromethylornithine (DFMO), an FDA-approved, orally available ODC inhibitor, for management of neuroblastoma, and this intervention is now being pursued in several clinical trials. We discuss the regulation of ODC and polyamines, which besides their well-known interactions with DNA and tRNA/rRNA, are involved in regulating RNA transcription and translation, ribosome function, proteasomal degradation, the circadian clock, and immunity, events that are also controlled by MYC proteins.

Novel Therapies for Relapsed and Refractory Neuroblastoma

While recent increases in our understanding of the biology of neuroblastoma have allowed for more precise risk stratification and improved outcomes for many patients, children with high-risk neuroblastoma continue to suffer from frequent disease relapse, and despite recent advances in our understanding of neuroblastoma pathogenesis, the outcomes for children with relapsed neuroblastoma remain poor. These children with relapsed neuroblastoma, therefore, continue to need novel treatment strategies based on a better understanding of neuroblastoma biology to improve outcomes. The discovery of new tumor targets and the development of novel antibody- and cell-mediated immunotherapy agents have led to a large number of clinical trials for children with relapsed neuroblastoma, and additional clinical trials using molecular and genetic tumor profiling to target tumor-specific aberrations are ongoing. Combinations of these new therapeutic modalities with current treatment regimens will likely be needed to improve the outcomes of children with relapsed and refractory neuroblastoma.

A dual regulatory circuit consisting of S-adenosylmethionine decarboxylase protein and its reaction product controls expression of the paralogous activator prozyme in Trypanosoma brucei

Polyamines are essential for cell growth of eukaryotes including the etiologic agent of human African trypanosomiasis (HAT), Trypanosoma brucei. In trypanosomatids, a key enzyme in the polyamine biosynthetic pathway, S-adenosylmethionine decarboxylase (TbAdoMetDC) heterodimerizes with a unique catalytically-dead paralog called prozyme to form the active enzyme complex. In higher eukaryotes, polyamine metabolism is subject to tight feedback regulation by spermidine-dependent mechanisms that are absent in trypanosomatids. Instead, in T. brucei an alternative regulatory strategy based on TbAdoMetDC prozyme has evolved. We previously demonstrated that prozyme protein levels increase in response to loss of TbAdoMetDC activity. Herein, we show that prozyme levels are under translational control by monitoring incorporation of deuterated leucine into nascent prozyme protein. We furthermore identify pathway factors that regulate prozyme mRNA translation. We find evidence for a regulatory feedback mechanism in which TbAdoMetDC protein and decarboxylated AdoMet (dcAdoMet) act as suppressors of prozyme translation. In TbAdoMetDC null cells expressing the human AdoMetDC enzyme, prozyme levels are constitutively upregulated. Wild-type prozyme levels are restored by complementation with either TbAdoMetDC or an active site mutant, suggesting that TbAdoMetDC possesses an enzyme activity-independent function that inhibits prozyme translation. Depletion of dcAdoMet pools by three independent strategies: inhibition/knockdown of TbAdoMetDC, knockdown of AdoMet synthase, or methionine starvation, each cause prozyme upregulation, providing independent evidence that dcAdoMet functions as a metabolic signal for regulation of the polyamine pathway in T. brucei. These findings highlight a potential regulatory paradigm employing enzymes and pseudoenzymes that may have broad implications in biology.

Trypanosoma brucei is a single-celled eukaryotic pathogen and the causative agent of human African trypanosomiasis (HAT). Polyamines are organic polycations that are essential for growth in T. brucei to facilitate protein translation and to maintain redox homeostasis. The pathway is the target of eflornithine, a current frontline therapy for treatment of HAT. Polyamine biosynthetic enzymes are regulated at multiple levels in mammals (e.g. transcription, translation and protein turnover), but in contrast, T. brucei lacks these mechanisms. Instead in T. brucei a central enzyme in polyamine metabolism called AdoMetDC must form a complex with a sister protein (termed a pseudoenzyme) to be active. Herein, we show that cellular levels of this sister protein we call prozyme are in turn feedback regulated by both AdoMetDC and by its reaction product in response to cell treatments that reduce pathway output. This regulatory paradigm highlights how pseudoenzymes can evolve to play an important role in metabolic pathway regulation and in organismal fitness.

Polyamine Biosynthetic Pathway as a Drug Target for Osteosarcoma Therapy

Osteosarcoma (OS) is the most common bone tumor in children. Polyamines (PAs) are ubiquitous cations involved in many cell processes including tumor development, invasion and metastasis. In other pediatric cancer models, inhibition of the PA biosynthesis pathway with ornithine decarboxylase (ODC) inhibitor alpha-difluoromethylornithine (DFMO) results in decreased cell proliferation and differentiation. In OS, the PA pathway has not been evaluated. DFMO is an attractive, orally administered drug, is well tolerated, can be given for prolonged periods, and is already used in pediatric patients. Three OS cell lines were used to study the cellular effects of PA inhibition with DFMO: MG-63, U-2 OS and Saos-2. Effects on proliferation were analyzed by cell count, flow cytometry-based cell cycle analysis and RealTime-Glo™ MT Cell Viability assays. Intracellular PA levels were measured with high-performance liquid chromatography (HPLC). Western blot analysis was used to evaluate cell differentiation. DFMO exposure resulted in significantly decreased cell proliferation in all cell lines. After treatment, intracellular spermidine levels were drastically decreased. Cell cycle arrest at G₂/M was observed in U-2 OS and Saos-2. Cell differentiation was most prominent in MG-63 and U-2 OS as determined by increases in the terminal differentiation markers osteopontin and collagen 1a1. Cell proliferation continued to be suppressed for several days after removal of DFMO. Based on our findings, DFMO is a promising new adjunct to current osteosarcoma therapy in patients at high risk of relapse, such as those with poor necrosis at resection or those with metastatic or recurrent osteosarcoma. It is a well-tolerated oral drug that is currently in phase II clinical trials in pediatric neuroblastoma patients as a maintenance therapy. The same type of regimen may also improve outcomes in osteosarcoma patients in whom there have been essentially no medical advances in the last 30 years.

The old and new biochemistry of polyamines

Polyamines are ubiquitous positively charged amines found in all organisms. These molecules play a crucial role in many biological functions including cell growth, gene regulation and differentiation. The three major polyamines produced in all mammalian cells are putrescine, spermidine and spermine. The intracellular levels of these polyamines depend on the interplay of the biosynthetic and catabolic enzymes of the polyamine and methionine salvage pathway, as well as the involvement of polyamine transporters. Polyamine levels are observed to be high in cancer cells, which contributes to malignant transformation, cell proliferation and poor patient prognosis. Considering the critical roles of polyamines in cancer cell proliferation, numerous anti-polyaminergic compounds have been developed as anti-tumor agents, which seek to suppress polyamine levels by specifically inhibiting polyamine biosynthesis, activating polyamine catabolism, or blocking polyamine transporters. However, in terms of the development of effective anti-cancer therapeutics targeting the polyamine system, these efforts have unfortunately resulted in little success. Recently, several studies using the iron chelators, O-trensox and ICL670A (Deferasirox), have demonstrated a decline in both iron and polyamine levels. Since iron levels are also high in cancer cells, and like polyamines, are required for proliferation, these latter findings suggest a biochemically integrated link between iron and polyamine metabolism.

Genetic susceptibility to neuroblastoma: current knowledge and future directions

Neuroblastoma, a malignancy of the developing peripheral nervous system that affects infants and young children, is a complex genetic disease. Over the past two decades, significant progress has been made toward understanding the genetic determinants that predispose to this often lethal childhood cancer. Approximately 1-2% of neuroblastomas are inherited in an autosomal dominant fashion and a combination of co-morbidity and linkage studies has led to the identification of germline mutations in PHOX2B and ALK as the major genetic contributors to this familial neuroblastoma subset. The genetic basis of "sporadic" neuroblastoma is being studied through a large genome-wide association study (GWAS). These efforts have led to the discovery of many common susceptibility alleles, each with modest effect size, associated with the development and progression of sporadic neuroblastoma. More recently, next-generation sequencing efforts have expanded the list of potential neuroblastoma-predisposing mutations to include rare germline variants with a predicted larger effect size. The evolving characterization of neuroblastoma's genetic basis has led to a deeper understanding of the molecular events driving tumorigenesis, more precise risk stratification and prognostics and novel therapeutic strategies. This review details the contemporary understanding of neuroblastoma's genetic predisposition, including recent advances and discusses ongoing efforts to address gaps in our knowledge regarding this malignancy's complex genetic underpinnings.

Alpha-Difluoromethylornithine, an Irreversible Inhibitor of Polyamine Biosynthesis, as a Therapeutic Strategy against Hyperproliferative and Infectious Diseases

The fluorinated ornithine analog α-difluoromethylornithine (DFMO, eflornithine, ornidyl) is an irreversible suicide inhibitor of ornithine decarboxylase (ODC), the first and rate-limiting enzyme of polyamine biosynthesis. The ubiquitous and essential polyamines have many functions, but are primarily important for rapidly proliferating cells. Thus, ODC is potentially a drug target for any disease state where rapid growth is a key process leading to pathology. The compound was originally discovered as an anticancer drug, but its effectiveness was disappointing. However, DFMO was successfully developed to treat African sleeping sickness and is currently one of few clinically used drugs to combat this neglected tropical disease. The other Food and Drug Administration (FDA) approved application for DFMO is as an active ingredient in the hair removal cream Vaniqa. In recent years, renewed interest in DFMO for hyperproliferative diseases has led to increased research and promising preclinical and clinical trials. This review explores the use of DFMO for the treatment of African sleeping sickness and hirsutism, as well as its potential as a chemopreventive and chemotherapeutic agent against colorectal cancer and neuroblastoma.

Synergistic drug combination GC7/DFMO suppresses hypusine/spermidine-dependent eIF5A activation and induces apoptotic cell death in neuroblastoma

The eukaryotic initiation factor 5A (eIF5A), which contributes to several crucial processes during protein translation, is the only protein that requires activation by a unique post-translational hypusine modification. eIF5A hypusination controls cell proliferation and has been linked to cancer. eIF5A hypusination requires the enzymes deoxyhypusine synthase (DHPS) and deoxyhypusine hydroxylase and uniquely depends on the polyamine (PA) spermidine as the sole substrate. Ornithine decarboxylase (ODC) is the rate-limiting enzyme in PA biosynthesis. Both ODC and PAs control cell proliferation and are frequently dysregulated in cancer. Since only spermidine can activate eIF5A, we chose the hypusine-PA nexus as a rational target to identify new drug combinations with synergistic antiproliferative effects. We show that elevated mRNA levels of the two target enzymes DHPS and ODC correlate with poor prognosis in a large cohort of neuroblastoma (NB) tumors. The DHPS inhibitor GC7 (N1-guanyl-1,7-diaminoheptane) and the ODC inhibitor α-difluoromethylornithine (DFMO) are target-specific and in combination induced synergistic effects in NB at concentrations that were not individually cytotoxic. Strikingly, while each drug alone at higher concentrations is known to induce p21/Rb- or p27/Rb-mediated G1 cell cycle arrest, we found that the drug combination induced caspase 3/7/9, but not caspase 8-mediated apoptosis, in NB cells. Hypusinated eIF5A levels and intracellular spermidine levels correlated directly with drug treatments, signifying specific drug targeting effects. This two-pronged GC7/DFMO combination approach specifically inhibits both spermidine biosynthesis and post-translational, spermidine-dependent hypusine-eIF5A activation, offering an exciting clue for improved NB drug therapy.

Polyamines and Cancer

This chapter provides an overview of how the polyamine pathway has been exploited as a target for the treatment and prevention of multiple forms of cancer, since this pathway is disrupted in all cancers. It is divided into three main sections. The first explores how the polyamine pathway has been targeted for chemotherapy, starting from the first drug to target it, difluoromethylornithine (DFMO) to the large variety of polyamine analogues that have been synthesised and tested throughout the years with all their potentials and pitfalls. The second section focuses on the use of polyamines as vectors for drug delivery. Knowing that the polyamine transport system is upregulated in cancers and that polyamines naturally bind to DNA, a range of polyamine analogues and polyamine-like structures have been synthesised to target epigenetic regulators, with encouraging results. Furthermore, the use of polyamines as transport vectors to introduce toxic/bioactive/fluorescent agents more selectively to the intended target in cancer cells is discussed. The last section concentrates on chemoprevention, where the different strategies that have been undertaken to interfere with polyamine metabolism and function for antiproliferative intervention are outlined and discussed.

Ornithine decarboxylase as a therapeutic target for endometrial cancer

Ornithine Decarboxylase (ODC) a key enzyme in polyamine biosynthesis is often overexpressed in cancers and contributes to polyamine-induced cell proliferation. We noted ubiquitous expression of ODC1 in our published endometrial cancer gene array data and confirmed this in the cancer genome atlas (TCGA) with highest expression in non-endometrioid, high grade, and copy number high cancers, which have the worst clinical outcomes. ODC1 expression was associated with worse overall survival and increased recurrence in three endometrial cancer gene expression datasets. Importantly, we confirmed these findings using quantitative real-time polymerase chain reaction (qRT-PCR) in a validation cohort of 60 endometrial cancers and found that endometrial cancers with elevated ODC1 had significantly shorter recurrence-free intervals (KM log-rank p = 0.0312, Wald test p = 5.59e-05). Difluoromethylornithine (DFMO) a specific inhibitor of ODC significantly reduced cell proliferation, cell viability, and colony formation in cell line models derived from undifferentiated, endometrioid, serous, carcinosarcoma (mixed mesodermal tumor; MMT) and clear cell endometrial cancers. DFMO also significantly reduced human endometrial cancer ACI-98 tumor burden in mice compared to controls (p = 0.0023). ODC-regulated polyamines (putrescine [Put] and/or spermidine [Spd]) known activators of cell proliferation were strongly decreased in response to DFMO, in both tumor tissue ([Put] (p = 0.0006), [Spd] (p<0.0001)) and blood plasma ([Put] (p<0.0001), [Spd] (p = 0.0049)) of treated mice. Our study indicates that some endometrial cancers appear particularly sensitive to DFMO and that the polyamine pathway in endometrial cancers in general and specifically those most likely to suffer adverse clinical outcomes could be targeted for effective treatment, chemoprevention or chemoprevention of recurrence.

Heterogeneity in retinoblastoma: a tale of molecules and models

Retinoblastoma, an intraocular pediatric cancer, develops in the embryonic retina following biallelic loss of RB1. However, there is a wide range of genetic and epigenetic changes that can affect RB1 resulting in different clinical outcomes. In addition, other transformations, such as MYCN amplification, generate particularly aggressive tumors, which may or may not be RB1 independent. Recognizing the cellular characteristics required for tumor development, by identifying the elusive cell-of-origin for retinoblastoma, would help us understand the development of these tumors. In this review we summarize the heterogeneity reported in retinoblastoma on a molecular, cellular and tissue level. We also discuss the challenging heterogeneity in current retinoblastoma models and suggest future platforms that could contribute to improved understanding of tumor initiation, progression and metastasis in retinoblastoma, which may ultimately lead to more patient-specific treatments.

The functional variant rs34330 of CDKN1B is associated with risk of neuroblastoma

The genetic aetiology of sporadic neuroblastoma is still largely unknown. We have identified diverse neuroblastoma susceptibility loci by genomewide association studies (GWASs); however, additional SNPs that likely contribute to neuroblastoma susceptibility prompted this investigation for identification of additional variants that are likely hidden among signals discarded by the multiple testing corrections used in the analysis of genomewide data. There is evidence suggesting the CDKN1B, coding for the cycle inhibitor p27Kip1, is involved in neuroblastoma. We thus assess whether genetic variants of CDKN1B are associated with neuroblastoma. We imputed all possible genotypes across CDKN1B locus on a discovery case series of 2101 neuroblastoma patients and 4202 genetically matched controls of European ancestry. The most significantly associated rs34330 was analysed in an independent Italian cohort of 311 cases and 709 controls. In vitro functional analysis was carried out in HEK293T and in neuroblastoma cell line SHEP‐2, both transfected with pGL3‐CDKN1B‐CC or pGL3‐CDKN1B‐TT constructs. We identified an association of the rs34330 T allele (‐79C/T) with the neuroblastoma risk (P_combined = 0.002; OR = 1.17). The risk allele (T) of this single nucleotide polymorphism led to a lower transcription rate in cells transfected with a luciferase reporter driven by the polymorphic p27Kip1 promoter (P < 0.05). Three independent sets of neuroblastoma tumours carrying ‐79TT genotype showed a tendency towards lower CDKN1B mRNA levels. Our study shows that a functional variant, associated with a reduced CDKN1B gene transcription, influences neuroblastoma susceptibility.

Targeting polyamine metabolism for cancer therapy and prevention

The chemically simple, biologically complex eukaryotic polyamines, spermidine and spermine, are positively charged alkylamines involved in many crucial cellular processes. Along with their diamine precursor putrescine, their normally high intracellular concentrations require fine attenuation by multiple regulatory mechanisms to keep these essential molecules within strict physiologic ranges. Since the metabolism of and requirement for polyamines are frequently dysregulated in neoplastic disease, the metabolic pathway and functions of polyamines provide rational drug targets; however, these targets have been difficult to exploit for chemotherapy. It is the goal of this article to review the latest findings in the field that demonstrate the potential utility of targeting the metabolism and function of polyamines as strategies for both chemotherapy and, possibly more importantly, chemoprevention.

All-Trans-Retinoic Acid Stimulates Overexpression of Tumor Protein D52 (TPD52, Isoform 3) and Neuronal Differentiation of IMR-32 Cells

Tumor protein D52 (TPD52), a proto-oncogene is overexpressed in a variety of epithelial carcinomas and plays an important role in cell proliferation, migration, and cell death. In the present study we found that the treatment of IMR-32 neuroblastoma (NB) cells with retinoic acid (RA) stimulates an increase in expression of TPD52. TPD52 expression is detectable after 72 h, can be maintained till differentiation of NB cells suggesting that TPD52 is involved in differentiation. Here, we demonstrate that TPD52 is essential for RA to promote differentiation of NB cells. Our results show that exogenous expression of EGFP-TPD52 in IMR-32 cells resulted cell differentiation even without RA. RA by itself and with overexpression of TPD52 can increase the ability of NB cells differentiation. Interestingly, transfection of IMR-32 cells with a specific small hairpin RNA for efficient knockdown of TPD52 attenuated RA induced NB cells differentiation. Transcriptional and translational level expression of neurotropic (BDNF, NGF, Nestin) and differentiation (β III tubulin, NSE, TH) factors in NB cells with altered TPD52 expression and/or RA treatment confirmed essential function of TPD52 in cellular differentiation. Furthermore, we show that TPD52 protects cells from apoptosis and arrest cell proliferation by varying expression of p27Kip1, activation of Akt and ERK1/2 thus promoting cell differentiation. Additionally, inhibition of STAT3 activation by its specific inhibitor arrested NB cells differentiation by EGFP-TPD52 overexpression with or without RA. Taken together, our data reveal that TPD52 act through activation of JAK/STAT signaling pathway to undertake NB cells differentiation induced by RA. J. Cell. Biochem. 118: 4358-4369, 2017. © 2017 Wiley Periodicals, Inc.

Neuroblastoma treatment in the post-genomic era

Neuroblastoma is an embryonic malignancy of early childhood originating from neural crest cells and showing heterogeneous biological, morphological, genetic and clinical characteristics. The correct stratification of neuroblastoma patients within risk groups (low, intermediate, high and ultra-high) is critical for the adequate treatment of the patients.

High-throughput technologies in the Omics disciplines are leading to significant insights into the molecular pathogenesis of neuroblastoma. Nonetheless, further study of Omics data is necessary to better characterise neuroblastoma tumour biology. In the present review, we report an update of compounds that are used in preclinical tests and/or in Phase I-II trials for neuroblastoma. Furthermore, we recapitulate a number of compounds targeting proteins associated to neuroblastoma: MYCN (direct and indirect inhibitors) and downstream targets, Trk, ALK and its downstream signalling pathways. In particular, for the latter, given the frequency of ALK gene deregulation in neuroblastoma patients, we discuss on second-generation ALK inhibitors in preclinical or clinical phases developed for the treatment of neuroblastoma patients resistant to crizotinib.

We summarise how Omics drive clinical trials for neuroblastoma treatment and how much the research of biological targets is useful for personalised medicine. Finally, we give an overview of the most recent druggable targets selected by Omics investigation and discuss how the Omics results can provide us additional advantages for overcoming tumour drug resistance.

Polyamine Antagonist Therapies Inhibit Neuroblastoma Initiation and Progression

PURPOSE

Deregulated MYC drives oncogenesis in many tissues yet direct pharmacologic inhibition has proven difficult. MYC coordinately regulates polyamine homeostasis as these essential cations support MYC functions, and drugs that antagonize polyamine sufficiency have synthetic-lethal interactions with MYC Neuroblastoma is a lethal tumor in which the MYC homologue MYCN, and ODC1, the rate-limiting enzyme in polyamine synthesis, are frequently deregulated so we tested optimized polyamine depletion regimens for activity against neuroblastoma.

EXPERIMENTAL DESIGN

We used complementary transgenic and xenograft-bearing neuroblastoma models to assess polyamine antagonists. We investigated difluoromethylornithine (DFMO; an inhibitor of Odc, the rate-limiting enzyme in polyamine synthesis), SAM486 (an inhibitor of Amd1, the second rate-limiting enzyme), and celecoxib (an inducer of Sat1 and polyamine catabolism) in both the preemptive setting and in the treatment of established tumors. In vitro assays were performed to identify mechanisms of activity.

RESULTS

An optimized polyamine antagonist regimen using DFMO and SAM486 to inhibit both rate-limiting enzymes in polyamine synthesis potently blocked neuroblastoma initiation in transgenic mice, underscoring the requirement for polyamines in MYC-driven oncogenesis. Furthermore, the combination of DFMO with celecoxib was found to be highly active, alone, and combined with numerous chemotherapy regimens, in regressing established tumors in both models, including tumors harboring highest risk genetic lesions such as MYCN amplification, ALK mutation, and TP53 mutation with multidrug resistance.

CONCLUSIONS

Given the broad preclinical activity demonstrated by polyamine antagonist regimens across diverse in vivo models, clinical investigation of such approaches in neuroblastoma and potentially other MYC-driven tumors is warranted. Clin Cancer Res; 22(17); 4391-404. ©2016 AACR.

Transcriptional co-activator TAZ sustains proliferation and tumorigenicity of neuroblastoma by targeting CTGF and PDGF-β

Neuroblastoma is a common childhood malignant tumor originated from the neural crest-derived sympathetic nervous system. A crucial event in the pathogenesis of neuroblastoma is to promote proliferation of neuroblasts, which is closely related to poor survival. However, mechanisms for regulation of cell proliferation and tumorigenicity in neuroblastoma are not well understood. Here, we report that overexpression of TAZ in neuroblastoma BE(2)-C cells causes increases in cell proliferation, self renewal and colony formation, which was restored back to its original levels by knockdown of TAZ in TAZ-overexpression cells. Inhibition of endogenous TAZ attenuated cell proliferation, colony formation and tumor development in neuroblastoma SK-N-AS cell, which could be rescued by re-introduction of TAZ into TAZ-knockdown cells. In addition, we found that overexpressing TAZ-mediated induction of CTGF and PDGF-β expression, cell proliferation and colony formation were inhibited by knocking down CTGF and PDGF-β with siRNA in TAZ-overexpressing cell. Overall, our findings suggested that TAZ plays an essential role in regulating cell proliferation and tumorigenesis in neuroblastoma cells. Thus, TAZ seems to be a novel and promising target for the treatment of neuroblastoma.

Cell Proliferation in Neuroblastoma

Neuroblastoma, the most common extracranial solid tumor of childhood, continues to carry a dismal prognosis for children diagnosed with advanced stage or relapsed disease. This review focuses upon factors responsible for cell proliferation in neuroblastoma including transcription factors, kinases, and regulators of the cell cycle. Novel therapeutic strategies directed toward these targets in neuroblastoma are discussed.

Targeting ornithine decarboxylase reverses the LIN28/Let-7 axis and inhibits glycolytic metabolism in neuroblastoma

LIN28 has emerged as an oncogenic driver in a number of cancers, including neuroblastoma (NB). Overexpression of LIN28 correlates with poor outcome in NB, therefore drugs that impact the LIN28/Let-7 pathway could be beneficial in treating NB patients. The LIN28/Let-7 pathway affects many cellular processes including the regulation of cancer stem cells and glycolytic metabolism. Polyamines, regulated by ornithine decarboxylase (ODC) modulate eIF-5A which is a direct regulator of the LIN28/Let-7 axis. We propose that therapy inhibiting ODC will restore balance to the LIN28/Let-7 axis, suppress glycolytic metabolism, and decrease MYCN protein expression in NB. Difluoromethylornithine (DFMO) is an inhibitor of ODC in clinical trials for children with NB. In vitro experiments using NB cell lines, BE(2)-C, SMS-KCNR, and CHLA90 show that DFMO treatment reduced LIN28B and MYCN protein levels and increased Let-7 miRNA and decreased neurosphere formation. Glycolytic metabolic activity decreased with DFMO treatment in vivo. Additionally, sensitivity to DFMO treatment correlated with LIN28B overexpression (BE(2)-C>SMS-KCNR>CHLA90). This is the first study to demonstrate that DFMO treatment restores balance to the LIN28/Let-7 axis and inhibits glycolytic metabolism and neurosphere formation in NB and that PET scans may be a meaningful imaging tool to evaluate the therapeutic effects of DFMO treatment.

Translational development of difluoromethylornithine (DFMO) for the treatment of neuroblastoma

Neuroblastoma is a childhood tumor in which MYC oncogenes are commonly activated to drive tumor progression. Survival for children with high-risk neuroblastoma remains poor despite treatment that incorporates high-dose chemotherapy, stem cell support, surgery, radiation therapy and immunotherapy. More effective and less toxic treatments are sought and one approach under clinical development involves re-purposing the anti-protozoan drug difluoromethylornithine (DFMO; Eflornithine) as a neuroblastoma therapeutic. DFMO is an irreversible inhibitor of ornithine decarboxylase (Odc), a MYC target gene, bona fide oncogene, and the rate-limiting enzyme in polyamine synthesis. DFMO is approved for the treatment of Trypanosoma brucei gambiense encephalitis ("African sleeping sickness") since polyamines are essential for the proliferation of these protozoa. However, polyamines are also critical for mammalian cell proliferation and the finding that MYC coordinately regulates all aspects of polyamine metabolism suggests polyamines may be required to support cancer promotion by MYC. Pre-emptive blockade of polyamine synthesis is sufficient to block tumor initiation in an otherwise fully penetrant transgenic mouse model of neuroblastoma driven by MYCN, underscoring the necessity of polyamines in this process. Moreover, polyamine depletion regimens exert potent anti-tumor activity in pre-clinical models of established neuroblastoma as well, in combination with numerous chemotherapeutic agents and even in tumors with unfavorable genetic features such as MYCN, ALK or TP53 mutation. This has led to the testing of DFMO in clinical trials for children with neuroblastoma. Current trial designs include testing lower dose DFMO alone (2,000 mg/m(2)/day) starting at the completion of standard therapy, or higher doses combined with chemotherapy (up to 9,000 mg/m(2)/day) for patients with relapsed disease that has progressed. In this review we will discuss important considerations for the future design of DFMO-based clinical trials for neuroblastoma, focusing on the need to better define the principal mechanisms of anti-tumor activity for polyamine depletion regimens. Putative DFMO activities that are both cancer cell intrinsic (targeting the principal oncogenic driver, MYC) and cancer cell extrinsic (altering the tumor microenvironment to support anti-tumor immunity) will be discussed. Understanding the mechanisms of DFMO activity are critical in determining how it might be best leveraged in upcoming clinical trials. This mechanistic approach also provides a platform by which iterative pre-clinical testing using translational tumor models may complement our clinical approaches.

Effect of sulfasalazine on human neuroblastoma: analysis of sepiapterin reductase (SPR) as a new therapeutic target

Background

Neuroblastoma (NB) is an aggressive childhood malignancy in children up to 5 years of age. High-stage tumors frequently relapse even after aggressive multimodal treatment, and then show therapy resistance, typically resulting in patient death. New molecular-targeted compounds that effectively suppress tumor growth and prevent relapse with more efficacy are urgently needed. We and others previously showed that polyamines (PA) like spermidine and spermine are essential for NB tumorigenesis and that DFMO, an inhibitor of the key PA synthesis gene product ODC, is effective both in vitro and in vivo, securing its evaluation in NB clinical trials. To find additional compounds interfering with PA biosynthesis, we tested sulfasalazine (SSZ), an FDA-approved salicylate-based anti-inflammatory and immune-modulatory drug, recently identified to inhibit sepiapterin reductase (SPR). We earlier presented evidence for a physical interaction between ODC and SPR and we showed that RNAi-mediated knockdown of SPR expression significantly reduced native ODC enzyme activity and impeded NB cell proliferation.

Methods

Human NB mRNA expression datasets in the public domain were analyzed using the R2 platform. Cell viability, isobologram, and combination index analyses as a result of SSZ treatment with our without DFMO were carried out in NB cell cultures. Molecular protein-ligand docking was achieved using the GRAMM algorithm. Statistical analyses were performed with the Kruskal-Wallis test, 2log Pearson test, and Student’s t test.

Results

In this study, we show the clinical relevance of SPR in human NB tumors. We found that high SPR expression is significantly correlated to unfavorable NB characteristics like high age at diagnosis, MYCN amplification, and high INSS stage. SSZ inhibits the growth of NB cells in vitro, presumably due to the inhibition of SPR as predicted by computational docking of SSZ into SPR. Importantly, the combination of SSZ with DFMO produces synergistic antiproliferative effects in vitro.

Conclusions

The results suggest the use of SSZ in combination with DFMO for further experiments, and possible prioritization as a novel therapy for the treatment of NB patients.

A Phase I Trial of DFMO Targeting Polyamine Addiction in Patients with Relapsed/Refractory Neuroblastoma

BACKGROUND

Neuroblastoma (NB) is the most common cancer in infancy and most frequent cause of death from extracranial solid tumors in children. Ornithine decarboxylase (ODC) expression is an independent indicator of poor prognosis in NB patients. This study investigated safety, response, pharmacokinetics, genetic and metabolic factors associated with ODC in a clinical trial of the ODC inhibitor difluoromethylornithine (DFMO) ± etoposide for patients with relapsed or refractory NB.

METHODS AND FINDINGS

Twenty-one patients participated in a phase I study of daily oral DFMO alone for three weeks, followed by additional three-week cycles of DFMO plus daily oral etoposide. No dose limiting toxicities (DLTs) were identified in patients taking doses of DFMO between 500-1500 mg/m2 orally twice a day. DFMO pharmacokinetics, single nucleotide polymorphisms (SNPs) in the ODC gene and urinary levels of substrates for the tissue polyamine exporter were measured. Urinary polyamine levels varied among patients at baseline. Patients with the minor T-allele at rs2302616 of the ODC gene had higher baseline levels (p=0.02) of, and larger decreases in, total urinary polyamines during the first cycle of DFMO therapy (p=0.003) and had median progression free survival (PFS) that was over three times longer, compared to patients with the major G allele at this locus although this last result was not statistically significant (p=0.07). Six of 18 evaluable patients were progression free during the trial period with three patients continuing progression free at 663, 1559 and 1573 days after initiating treatment. Median progression-free survival was less among patients having increased urinary polyamines, especially diacetylspermine, although this result was not statistically significant (p=0.056).

CONCLUSIONS

DFMO doses of 500-1500 mg/m2/day are safe and well tolerated in children with relapsed NB. Children with the minor T allele at rs2302616 of the ODC gene with relapsed or refractory NB had higher levels of urinary polyamine markers and responded better to therapy containing DFMO, compared to those with the major G allele at this locus. These findings suggest that this patient subset may display dependence on polyamines and be uniquely susceptible to therapies targeting this pathway.

TRIAL REGISTRATION

Clinicaltrials.gov NCT#01059071.

Polyamine metabolism is sensitive to glycolysis inhibition in human neuroblastoma cells

Polyamines are essential for cell proliferation, and their levels are elevated in many human tumors. The oncogene n-myc is known to potentiate polyamine metabolism. Neuroblastoma, the most frequent extracranial solid tumor in children, harbors the amplification of n-myc oncogene in 25% of the cases, and it is associated with treatment failure and poor prognosis. We evaluated several metabolic features of the human neuroblastoma cell lines Kelly, IMR-32, and SK-N-SH. We further investigated the effects of glycolysis impairment in polyamine metabolism in these cell lines. A previously unknown linkage between glycolysis impairment and polyamine reduction is unveiled. We show that glycolysis inhibition is able to trigger signaling events leading to the reduction of N-Myc protein levels and a subsequent decrease of both ornithine decarboxylase expression and polyamine levels, accompanied by cell cycle blockade preceding cell death. New anti-tumor strategies could take advantage of the direct relationship between glucose deprivation and polyamine metabolism impairment, leading to cell death, and its apparent dependence on n-myc. Combined therapies targeting glucose metabolism and polyamine synthesis could be effective in the treatment of n-myc-expressing tumors.

MycN promotes TRPM7 expression and cell migration in neuroblastoma through a process that involves polyamines

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MycN expression correlates with TRPM7 expression in neuroblastoma (NB) tumors.

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Expression of the transmembrane protein TRPM7 correlates with lower overall survival in NB tumors.

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MycN promotes TRPM7 mRNA and protein expression and increases TRPM7 channel activity.

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TRPM7 regulates NB cell migration.

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Polyamines regulate TRPM7 expression.

Neuroblastoma is an extra-cranial solid cancer in children. MYCN gene amplification is a prognostic indicator of poor outcome in neuroblastoma. Recent studies have shown that the multiple steps involved in cell migration are dependent on the availability of intracellular calcium (Ca²⁺). Although significant advances have been made in understanding the role of Ca²⁺ during migration, little has been achieved towards understanding its impact on the progression of diseases such as cancer. Interestingly, previous studies showed that cancer cell migration is regulated by TRPM7, a calcium-permeable ion channel. The objective of the current study was to elucidate the mechanism by which MycN promotes NB cell migration and the mechanism regulating TRPM7 expression. The results showed that MycN increased TRPM7 expression, induced TRPM7 channel activity, increased intracellular Ca²⁺ signaling, and promoted cell migration in NB cells. The results also showed that inhibition or down-regulation of ornithine decarboxylase (ODC) inhibited TRPM7 expression, a process that was reversed by spermidine. Overall, this study provides evidence that MycN promotes TRPM7 expression and cell migration through a mechanism that involves ODC synthesis of polyamines.

Deoxyhypusine synthase (DHPS) inhibitor GC7 induces p21/Rb-mediated inhibition of tumor cell growth and DHPS expression correlates with poor prognosis in neuroblastoma patients

PURPOSE

Neuroblastoma (NB) is an aggressive pediatric malignancy that typically occurs in infants and children under the age of 5 years. High-stage tumors relapse frequently even after aggressive multimodal treatment, resulting in therapy resistance and eventually in patient death. Clearly, new biologically-targeted drugs are needed that more efficiently suppress tumor growth and prevent relapse. We and others previously showed that polyamines such as spermidine play an essential role in NB tumorigenesis and that DFMO, an inhibitor of the central polyamine synthesis gene ODC, is effective in vitro and in vivo, prompting its evaluation in NB clinical trials. However, the specific molecular actions of polyamines remain poorly defined. Spermidine and deoxyhypusine synthase (DHPS) are essential components in the hypusination-driven post-translational activation of eukaryotic initiation factor 5A (eIF5A).

METHODS

We assessed the role of DHPS in NB and the impact of its inhibition by N(1)-guanyl-1,7-diaminoheptane (GC7) on tumor cell growth using cell proliferation assays, Western blot, immunofluorescence microscopy, and Affymetrix micro-array mRNA expression analyses in NB tumor samples.

RESULTS

We found that GC7 inhibits NB cell proliferation in a dose-dependent manner, through induction of the cell cycle inhibitor p21 and reduction of total and phosphorylated Rb proteins. Strikingly, high DHPS mRNA expression correlated significantly with unfavorable clinical parameters, including poor patient survival, in a cohort of 88 NB tumors (all P < 0.04).

CONCLUSIONS

These results suggest that spermidine and DHPS are key contributing factors in NB tumor proliferation through regulation of the p21/Rb signaling axis.

Cyclin-dependent kinases regulate apoptosis of intestinal epithelial cells

Homeostasis of the gastrointestinal epithelium is dependent upon a balance between cell proliferation and apoptosis. Cyclin-dependent kinases (Cdks) are well known for their role in cell proliferation. Previous studies from our group have shown that polyamine-depletion of intestinal epithelial cells (IEC-6) decreases cyclin-dependent kinase 2 (Cdk2) activity, increases p53 and p21Cip1 protein levels, induces G1 arrest, and protects cells from camptothecin (CPT)-induced apoptosis. Although emerging evidence suggests that members of the Cdk family are involved in the regulation of apoptosis, their roles directing apoptosis of IEC-6 cells are not known. In this study, we report that inhibition of Cdk1, 2, and 9 (with the broad range Cdk inhibitor, AZD5438) in proliferating IEC-6 cells triggered DNA damage, activated p53 signaling, inhibited proliferation, and induced apoptosis. By contrast, inhibition of Cdk2 (with NU6140) increased p53 protein and activity, inhibited proliferation, but had no effect on apoptosis. Notably, AZD5438 sensitized, whereas, NU6140 rescued proliferating IEC-6 cells from CPT-induced apoptosis. However, in colon carcinoma (Caco-2) cells with mutant p53, treatment with either AZD5438 or NU6140 blocked proliferation, albeit more robustly with AZD5438. Both Cdk inhibitors induced apoptosis in Caco-2 cells in a p53-independent manner. In serum starved quiescent IEC-6 cells, both AZD5438 and NU6140 decreased TNF-α/CPT-induced activation of p53 and, consequently, rescued cells from apoptosis, indicating that sustained Cdk activity is required for apoptosis of quiescent cells. Furthermore, AZD5438 partially reversed the protective effect of polyamine depletion whereas NU6140 had no effect. Together, these results demonstrate that Cdks possess opposing roles in the control of apoptosis in quiescent and proliferating cells. In addition, Cdk inhibitors uncouple proliferation from apoptosis in a p53-dependent manner.

Polyamine transport inhibitors: design, synthesis, and combination therapies with difluoromethylornithine

The development of polyamine transport inhibitors (PTIs), in combination with the polyamine biosynthesis inhibitor difluoromethylornithine (DFMO), provides a method to target cancers with high polyamine requirements. The DFMO+PTI combination therapy results in sustained intracellular polyamine depletion and cell death. A series of substituted benzene derivatives were evaluated for their ability to inhibit the import of spermidine in DFMO-treated Chinese hamster ovary (CHO) and L3.6pl human pancreatic cancer cells. Several design features were discovered which strongly influenced PTI potency, sensitivity to amine oxidases, and cytotoxicity. These included changes in (a) the number of polyamine chains appended to the ring system, (b) the polyamine sequence, (c) the attachment linkage of the polyamine to the aryl core, and (d) the presence of a terminal N-methyl group. Of the series tested, the optimal design was N(1),N(1'),N(1″)-(benzene-1,3,5-triyltris(methylene))tris(N(4)-(4-(methylamino)butyl)butane-1,4-diamine, 6b, which contained three N-methylhomospermidine motifs. This PTI exhibited decreased sensitivity to amine oxidases and low toxicity as well as high potency (EC50 = 1.4 μM) in inhibiting the uptake of spermidine (1 μM) in DFMO-treated L3.6pl human pancreatic cancer cells.

S-adenosylmethionine limitation induces p38 mitogen-activated protein kinase and triggers cell cycle arrest in G1

The primary methyl group donor S-adenosylmethionine (SAM) is important for a plethora of cellular pathways including methylation of nucleic acids, proteins, and the 5' cap structure of mRNAs, as well as biosynthesis of phospholipids and polyamines. In addition, because it is the cofactor for chromatin methylation, SAM is an important metabolite for the establishment and maintenance of epigenetic marks. Here, we demonstrate that cells halt proliferation when SAM levels become low. Cell cycle arrest occurs primarily in the G1 phase of the cell cycle and is accompanied by activation of the mitogen-activated protein kinase p38 (MAPK14) and subsequent phosphorylation of MAPK-activated protein kinase-2 (MK2). Surprisingly, Cdk4 activity remains high during cell cycle arrest, whereas Cdk2 activity decreases concomitantly with cyclin E levels. Cell cycle arrest was induced by both pharmacological and genetic manipulation of SAM synthesis through inhibition or downregulation of methionine adenosyltransferase, respectively. Depletion of methionine, the precursor of SAM, from the growth medium induced a similar cell cycle arrest. Unexpectedly, neither methionine depletion nor inhibition of methionine adenosyltransferase significantly affected mTORC1 activity, suggesting that the cellular response to SAM limitation is independent from this major nutrient-sensing pathway. These results demonstrate a G1 cell cycle checkpoint that responds to limiting levels of the principal cellular methyl group donor S-adenosylmethionine. This metabolic checkpoint might play important roles in maintenance of epigenetic stability and general cellular integrity.

Novel interaction of ornithine decarboxylase with sepiapterin reductase regulates neuroblastoma cell proliferation

Ornithine decarboxylase (ODC) is the sentinel enzyme in polyamine biosynthesis. Both ODC and polyamines regulate cell division, proliferation, and apoptosis. Sepiapterin reductase (SPR) catalyzes the last step in the biosynthesis of tetrahydrobiopterin (BH4), an essential cofactor of nitric oxide synthase, and has been implicated in neurological diseases but not yet in cancer. In this study, we present compelling evidence that native ODC and SPR physically interact, and we defined the individual amino acid residues involved in both enzymes using in silico protein-protein docking simulations. The resulting heterocomplex is a surprisingly compact structure, featuring two energetically and structurally equivalent binding modes both in monomer and in dimer conformations. The novel interaction between ODC and SPR proteins was confirmed under physiological conditions by co-immunoprecipitation and co-localization in neuroblastoma (NB) cells. Importantly, we showed that siRNA (small interfering RNA)-mediated knockdown of SPR expression significantly reduced endogenous ODC enzyme activity in NB cells, thus demonstrating the biological relevance of the ODC-SPR interaction. Finally, in a cohort of 88 human NB tumors, we found that high SPR mRNA expression correlated significantly with poor survival prognosis using a Kaplan-Meier analysis (log-rank test, P=5 × 10(-4)), suggesting an oncogenic role for SPR in NB tumorigenesis. In conclusion, we showed that ODC binds SPR and thus propose a new concept in which two well-characterized biochemical pathways converge via the interaction of two enzymes. We identified SPR as a novel regulator of ODC enzyme activity and, based on clinical evidence, present a model in which SPR drives ODC-mediated malignant progression in NB.

AMXT-1501, a novel polyamine transport inhibitor, synergizes with DFMO in inhibiting neuroblastoma cell proliferation by targeting both ornithine decarboxylase and polyamine transport

Neuroblastoma (NB) is associated with MYCN oncogene amplification occurring in approximately 30% of NBs and is associated with poor prognosis. MYCN is linked to a number of genes including ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis. ODC expression is elevated in many forms of cancer including NB. Alpha-difluoromethylornithine (DFMO), an ODC inhibitor, is currently being used in a Phase I clinical trial for treatment of NB. However, cancer cells treated with DFMO may overcome their polyamine depletion by the uptake of polyamines from extracellular sources. A novel polyamine transport inhibitor, AMXT-1501, has not yet been tested in NB. We propose that inhibiting ODC with DFMO, coupled with polyamine transport inhibition by AMXT-1501 will result in enhanced NB growth inhibition. Single and combination drug treatments were conducted on three NB cell lines. DFMO IC50 values ranged from 20.76 to 33.3 mM, and AMXT-1501 IC50 values ranged from 14.13 to 17.72 µM in NB. The combination treatment resulted in hypophosphorylation of retinoblastoma protein (Rb), suggesting growth inhibition via G1 cell cycle arrest. Increased expression of cleaved PARP and cleaved caspase 3 in combination-treated cells starting at 48 hr suggested apoptosis. The combination treatment depleted intracellular polyamine pools and decreased intracellular ATP, further verifying growth inhibition. Given the current lack of effective therapies for patients with relapsed/refractory NB and the preclinical effectiveness of DFMO with AMXT-1501, this combination treatment provides promising preclinical results. DFMO and AMXT-1501 may be a potential new therapy for children with NB.

Product feedback regulation implicated in translational control of the Trypanosoma brucei S-adenosylmethionine decarboxylase regulatory subunit prozyme

Human African sleeping sickness (HAT) is caused by the parasitic protozoan Trypanosoma brucei. Polyamine biosynthesis is an important drug target in the treatment of HAT. Previously we showed that trypanosomatid S-adenosylmethionine decarboxylase (AdoMetDC), a key enzyme for biosynthesis of the polyamine spermidine, is activated by heterodimer formation with an inactive paralogue termed prozyme. Furthermore, prozyme protein levels were regulated in response to reduced AdoMetDC activity. Herein we show that T. brucei encodes three prozyme transcripts. The 3'UTRs of these transcripts were mapped and chloramphenicol acetyltransferase (CAT) reporter constructs were used to identify a 1.2 kb region that contained a 3'UTR prozyme regulatory element sufficient to upregulate CAT protein levels (but not RNA) upon AdoMetDC inhibition, supporting the hypothesis that prozyme expression is regulated translationally. To gain insight into trans-acting factors, genetic rescue of AdoMetDC RNAi knock-down lines with human AdoMetDC was performed leading to rescue of the cell growth block, and restoration of prozyme protein to wild-type levels. Metabolite analysis showed that prozyme protein levels were inversely proportional to intracellular levels of decarboxylated AdoMet (dcAdoMet). These data suggest that prozyme translation may be regulated by dcAdoMet, a metabolite not previously identified to play a regulatory role.

DFMO/eflornithine inhibits migration and invasion downstream of MYCN and involves p27Kip1 activity in neuroblastoma

Neuroblastoma (NB) is the most common extracranial pediatric tumor. NB patients over 18 months of age at the time of diagnosis are often in the later stages of the disease, present with widespread dissemination, and often possess MYCN tumor gene amplification. MYCN is a transcription factor that regulates the expression of a number of genes including ornithine decarboxylase (ODC), a rate-limiting enzyme in the biosynthesis of polyamines. Inhibiting ODC in NB cells produces many deleterious effects including G₁ cell cycle arrest, inhibition of cell proliferation, and decreased tumor growth, making ODC a promising target for drug interference. DFMO treatment leads to the accumulation of the cyclin-dependent kinase inhibitor p27^Kip1 protein and causes p27^Kip1/Rb-coupled G₁ cell cycle arrest in MYCN-amplified NB tumor cells through a process that involves p27^Kip1 phosphorylation at residues Ser10 and Thr198. While p27^Kip1 is well known for its role as a cyclin-dependent kinase inhibitor, recent studies have revealed a novel function of p27^Kip1 as a regulator of cell migration and invasion. In the present study we found that p27^Kip1 regulates the migration and invasion in NB and that these events are dependent on the state of phosphorylation of p27^Kip1. DFMO treatments induced MYCN protein downregulation and phosphorylation of Akt/PKB (Ser473) and GSK3-β (Ser9), and polyamine supplementation alleviated the DFMO-induced effects. Importantly, we provide strong evidence that p27^Kip1 mRNA correlates with clinical features and the survival probability of NB patients.

Critical roles of Myc-ODC axis in the cellular transformation induced by myeloproliferative neoplasm-associated JAK2 V617F mutant

The acquired mutation (V617F) of Janus kinase 2 (JAK2) is observed in the majority of patients with myeloproliferative neoplasms (MPNs). In the screening of genes whose expression was induced by JAK2 (V617F), we found the significant induction of c-Myc mRNA expression mediated by STAT5 activation. Interestingly, GSK-3β was inactivated in transformed Ba/F3 cells by JAK2 (V617F), and this enhanced the protein expression of c-Myc. The enforced expression of c-Myc accelerated cell proliferation but failed to inhibit apoptotic cell death caused by growth factor deprivation; however, the inhibition of GSK-3β completely inhibited the apoptosis of cells expressing c-Myc. Strikingly, c-Myc T58A mutant exhibited higher proliferative activity in a growth-factor-independent manner; however, this mutant failed to induce apoptosis. In addition, knockdown of c-Myc significantly inhibited the proliferation of transformed cells by JAK2 (V617F), suggesting that c-Myc plays an important role in oncogenic activity of JAK2 (V617F). Furthermore, JAK2 (V617F) induced the expression of a target gene of c-Myc, ornithine decarboxylase (ODC), known as the rate-limiting enzyme in polyamine biosynthesis. An ODC inhibitor, difluoromethylornithine (DFMO), prevented the proliferation of transformed cells by JAK2 (V617F). Importantly, administration of DFMO effectively delayed tumor formation in nude mice inoculated with transformed cells by JAK2 (V617F), resulting in prolonged survival; therefore, ODC expression through c-Myc is a critical step for JAK2 (V617F)-induced transformation and DFMO could be used as effective therapy for MPNs.

Targeted expression of ornithine decarboxylase antizyme prevents upper aerodigestive tract carcinogenesis in p53-deficient mice

Upper aerodigestive tract (UADT) cancers of the oral cavity and esophagus are a significant global health burden, and there is an urgent need to develop relevant animal models to identify chemopreventive and therapeutic strategies to combat these diseases. Antizyme (AZ) is a multifunctional negative regulator of cellular polyamine levels, and here, we evaluate the susceptibility of keratin 5 (K5)-AZ transgenic mice to tumor models that combine chemical carcinogenesis with dietary and genetic risk factors known to influence human susceptibility to UADT cancer and promote UADT carcinogenesis in mice. First, p53(+/-) and K5-AZ/p53(+/-) (AZ/p53(+/-)) mice were placed on a zinc-deficient (ZD) or zinc-sufficient (ZS) diet and chronically exposed to 4-nitroquinoline 1-oxide. Tongue tumor incidence, multiplicity and size were substantially reduced in both ZD and ZS AZ/p53(+/-) mice compared with p53(+/-). AZ expression also reduced progression to carcinoma in situ or invasive carcinoma and decreased expression of the squamous cell carcinoma biomarkers K14, cyclooxygenase-2 and metallothionein. Next, AZ-expressing p53(+/-) and p53 null mice were placed on the ZD diet and treated with a single dose of N-nitrosomethylbenzylamine. Regardless of p53 status, forestomach (FST) tumor incidence, multiplicity and size were greatly reduced with AZ expression, which was also associated with a significant decrease in FST epithelial thickness along with reduced proliferation marker K6 and increased differentiation marker loricrin. These studies demonstrate the powerful tumor suppressive effects of targeted AZ expression in two distinct and unique mouse models and validate the polyamine metabolic pathway as a target for chemoprevention of UADT cancers.