Purpose: Rare cells within tumors with stem cell-like properties (cancer stem stem cells or CSCs) play a critical role in treatment resistance and cancer recurrence because CSCs are resistant to chemotherapy. Hence, elimination of CSCs will require new treatment approaches to improve the long-term outcomes for patients with cancer. We have recently discovered that breast CSCs require the essential amino acid methionine for cell survival. Methionine is converted by the enzyme MAT2A to S-adenosylmethionine (SAM), a universal methyl donor that regulates gene expression by DNA and histone methylation. We hypothesize that the methionine dependence of CSCs is a unique metabolic vulnerability that we can target by dietary methionine restriction (MR) and/or inhibiting the enzyme that converts methionine to SAM (MAT2A).
Experimental design and Results: We examined the effects of MR and/or MAT2A inhibition on the formation of CSC-enriched “mammospheres” and on the expression of CSC markers (CD44hi/C24low) in triple (ER/PR/HER2)-negative breast cancer (TNBC) cells. MR inhibited mammosphere formation, induced apoptosis and reduced the CD44hi/C24low CSC population. These effects of MR were partly rescued by addition of SAM, suggesting that CSCs are dependent on SAM for cell survival. MR resulted in robust induction of MAT2A mRNA and protein levels. Inhibition of MAT2A with cycloleucine, or by silencing MAT2A with specific siRNAs, reduced mammosphere formation and the CD44hi/C24low CSC population. Strikingly, the effects of cycloleucine or MAT2A silencing on cancer stem cells were dramatically potentiated by MR. Treatment of TNBC mammospheres with MR or cycloleucine suppressed the expression of transcriptional regulators of pluripotency and epithelial-to-mesenchymal transition, while the combination of MR and cycloleucine resulted in more robust reductions in the expression of these transcription factors than either intervention alone. Consistent with these effects, the combination of dietary MR and cycloleucine was more effective than either treatment alone at inhibiting primary tumor growth and lung metastases in a murine TNBC model.
Conclusion: Our findings point to previously unrecognized metabolic vulnerability of CSCs, namely, their intrinsic dependence on methionine and SAM for cell survival. MR induces expression of MAT2A, thereby providing an explanation for the robust synergy between MR and MAT2A inhibition/silencing in supressing CSC survival. Moreover, cycloleucine potentiates the activity of dietary MR in a murine metastatic TNBC model, providing proof-of-principle in vivo evidence for this novel metabolic approach to eradicate CSCs and thereby improve long-term clinical outcomes in poor-prognosis TNBC.
Citation Format: Elena Strekalova, Dmitry Malin, Dominik Hoelper, Peter Lewis, Vincent Cryns. Targeting methionine metabolism to eradicate cancer stem cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-247.