Serine-Glycine-One-Carbon Metabolism: The Hidden Achilles Heel of MYCN-Amplified Neuroblastoma? Free

MYCN is a member of the MYC family of proto-oncogenes (c-MYC, MYCN, and MYCL), which encode helix-loop-helix-zipper transcription factors that play a pivotal role in the control of cellular processes such as cell division, differentiation, apoptosis, and metabolism. MYCN is fundamental in the development of the peripheral and central nervous systems and has a very restricted spatial and temporal expression pattern during embryonic development, after which, it is downregulated and only expressed in certain adult tissues. Deregulation of MYC genes is found in a large number of human cancers where activation of the MYC transcriptional network contributes to tumor initiation and maintenance. MYCN amplification and/or overexpression can be found in rhabdomyosarcoma, medulloblastoma, astrocytoma, Wilms' tumor, prostate cancer, small-cell lung cancer, and most notably in childhood neuroblastoma, where it strongly correlates with poor prognosis, therapy resistance, and unfavorable survival outcome (1).

Neuroblastoma is the most common extracranial pediatric solid tumor and represents 15% of all childhood cancer–related deaths. The majority of cases are diagnosed in children under the age of 5 years. Neuroblastoma is a very heterogeneous disease that ranges from low-risk tumors that spontaneously differentiate and regress with minimal or no therapy to aggressive high-risk tumors with a poor prognosis that frequently metastasize and show resistance to therapy. Children diagnosed with high-risk tumors undergo intensive treatments and only about half of them survive. Moreover, survivors often suffer lifetime-long severe side effects derived from the elevated toxicity of the chemotherapeutic drugs.

Although inhibiting MYC proteins would be a powerful approach to treat many cancer types, this strategy has proved challenging because MYC proteins have a flexible structure and lack effective binding pockets on their surface, thus making them very difficult targets. Still, many efforts have been made to develop compounds that affect MYC transcription and protein dimerization or stability, but despite successful in vitro results, most molecules lack in vivo efficacy or have shown elevated toxicity and/or low specificity. Together, these data suggest that targeting pathways activated downstream of these oncoproteins may represent a more promising therapeutic avenue.

One key function of MYC proteins is to promote cell growth and proliferation, which requires metabolic adaptations to fully support the biosynthetic demand for growth and maintenance of cell homeostasis. The interconnection between oncogenic drivers, including MYC proteins, and rewired cancer metabolism has been intensely studied during recent years. The strong dependency of tumors on specific metabolic pathways supports the view that constitutive oncogenic growth makes them more susceptible to metabolic interventions. It is well known that c-MYC regulates expression of several enzymes involved in glucose metabolism such as the glucose transporter, GLUT1, hexokinase 2, and phosphofructokinase (2). Via upregulation of the genes encoding these proteins, c-MYC contributes directly to the reprograming of glucose metabolism toward aerobic glycolysis instead of oxidative phosphorylation (Warburg effect) and the ability of tumor cells to convert glucose to pyruvate even in the presence of oxygen and fully functioning mitochondria. Oncogenic c-MYC has also been linked to increased glutaminolysis through coordinated transcriptional and posttranscriptional programs (2). Although less is known about MYCN and metabolism, recent publications have shown that MYCN overexpression enhances glutaminolysis in neural crest progenitors and that MYCN-dependent glutaminolysis renders neuroblastoma cells more susceptible to agents that induce oxidative stress (3). Furthermore, we previously showed that MYCN causes a metabolic shift and accumulation of lipid droplets due to inhibition of β-oxidation in neuroblastoma (4).

In this issue of Cancer Research, Xia and colleagues report that MYCN-induced metabolic reprograming confers neuroblastoma cell dependency on the serine-glycine-one-carbon biosynthetic pathway (SGOC), revealing a vulnerability that could be exploited as a selective therapeutic strategy for MYCN-amplified tumors (5).

One-carbon metabolism, which involves the folate and methionine cycles, occurs in the cytoplasm and in the mitochondria through the catabolism of different carbon sources to derive one-carbon units to be used in essential cellular functions (6). These include the generation of substrates for the synthesis of macromolecules, such as nucleotides and lipids, redox maintenance, and methylation reactions. The importance of this pathway in cancer was discovered more than 70 years ago, and today antifolates are widely used as chemotherapeutic agents.

Cells mainly derive one-carbon units from the nonessential amino acids serine and glycine. Serine is an important one-carbon donor to the folate cycle that can be obtained exogenously or synthesized de novo from glucose via the phosphoserine pathway. Phosphoglycerate dehydrogenase (PHGDH) catalyzes the -committed step in this pathway, the transformation of the glycolytic intermediate, 3-phosphoglycerate, into phosphohydroxy-pyruvate. Glycine can be produced from serine via the serine hydroxymethyltransferases (SHMT) or threonine.

SHMTs, especially the mitochondrial isoform SHMT2, transfer one-carbon unit from serine to tetrahydrofolate (THF) to form methylene-THF that initiates the folate cycle that then is coupled to the methionine cycle. Adenylation of methionine produces S-adenosyl methionine, the methyl donor for both DNA and histone methylation reactions that influence the epigenetic landscape. Several studies have linked deregulation of enzymes in the SGOC network to different types of cancer, highlighting the importance of SGOC metabolism in tumor progression (7).

In their previous work using the TH-MYCN mouse, a transgenic model of high-risk neuroblastoma with MYCN overexpression, Liu and colleagues showed that the cholesterol and serine–glycine synthesis pathways are essential for maintaining neuroblastoma sphere-forming cells. Importantly, they found that higher expression of genes from these pathways was significantly associated with advanced stages and lower event-free survival in patients with neuroblastoma (8).

In this study, Xia and colleagues study MYCN regulation of SOGC metabolism and the consequences of PHGDH inhibition in neuroblastoma more in depth. They observed that SGOC genes are direct transcriptional targets of MYCN and that overexpression of MYCN in non-MYC–amplified neuroblastoma cells increased the expression of the SGOC genes at mRNA and protein levels. Furthermore, as expected, this effect was accompanied by increased cell proliferation. Importantly, the authors identified a positive feedback loop between MYCN and the transcription factor 4 (ATF4) that promotes transcriptional activation of the SGOC metabolic pathway in MYCN-amplified neuroblastoma cells. MYCN activated the transcription of ATF4 that in turn stabilized MYCN protein levels by blocking FBXW7-mediated MYCN ubiquitination and further degradation (5).

During the last decade, there has been significant progress in establishing the expression, function, and regulation of PHGDH, the first enzyme in the serine synthesis pathway. Amplification of PHGDH in tumors leads to a higher production of serine from glucose and is associated with specific subsets of breast cancer, lung adenocarcinoma, and melanomas (9).

Xia and colleagues treated a panel of neuroblastoma cells with two small-molecule inhibitors of PHGDH, NCT-503 and CBR-5884. These inhibitors markedly affected the growth and survival of MYCN-amplified cells, with no significant effect on non-MYCN–amplified cells. Furthermore, exogenous MYCN overexpression was sufficient to sensitize non-MYCN–amplified neuroblastoma cells to NCT-503, suggesting that MYCN might impose a dependence on the SGOC metabolic pathway for survival and proliferation. In addition, the authors report that PHGDH inhibition induced metabolic stress in MYCN-amplified neuroblastoma cells, leading to G₁ arrest and autophagy. This data support a recent publication from Sharif and colleagues showing that PHGDH inhibition induces autophagy and importantly, promotes multilineage differentiation in embryonal carcinoma stem-like cells (10).

Collectively, the findings by Xia and colleagues highlight the important role of serine/glycine biosynthesis in MYCN-amplified neuroblastoma and the link between PHGDH and tumor aggressiveness and stemness. A better understanding of metabolic dependencies of tumors opens a therapeutic window that can be exploited for novel treatment strategies that selectively target cancer cells. Together, the results by Xia and colleagues suggest that targeting MYCN-activated metabolic pathways, such as SGOC metabolism, may represent an effective and less toxic approach to improve the outcome of children with aggressive neuroblastoma, as well as other MYCN-driven tumors.

No potential conflicts of interest were disclosed.