Folate-mediated one carbon (1C) metabolism supports a series of processes that are essential for the cell. Through a number of interlinked reactions happening in the cytosol and mitochondria of the cell, folate metabolism contributes to de novo purine and thymidylate synthesis, to the methionine cycle and redox defence. Targeting the folate metabolism gave rise to modern chemotherapy, through the introduction of antifolates to treat paediatric leukaemia. Since then, antifolates, such as methotrexate and pralatrexate have been used to treat a series of blood cancers in clinic. However, traditional antifolates have many deleterious side effects in normal proliferating tissue, highlighting the urgent need for novel strategies to more selectively target 1C metabolism. Notably, mitochondrial 1C enzymes have been shown to be significantly upregulated in various cancers, making them attractive targets for the development of new chemotherapeutic agents. In this article, we present a detailed overview of folate-mediated 1C metabolism, its importance on cellular level and discuss how targeting folate metabolism has been exploited in blood cancers. Additionally, we explore possible therapeutic strategies that could overcome the limitations of traditional antifolates.

ULK1 inhibition increases the sensitivity of leukemia stem cells to TKI therapy by modulating energy and redox balance.

Our understanding of cancer metabolism spans from its role in cellular energetics and supplying the building blocks necessary for proliferation, to maintaining cellular redox and regulating the cellular epigenome and transcriptome. Cancer metabolism, once thought to be solely driven by upregulated glycolysis, is now known to comprise of multiple pathways with great plasticity in response to extrinsic challenges. Furthermore, cancer cells can modify their surrounding niche during disease initiation, maintenance and metastasis, contributing to therapy resistance. Leukaemia is a paradigm model of stem cell driven cancer. Here, we review how leukaemia remodels the niche and rewires its metabolism with particular attention paid to therapy-resistant stem cells. Specifically, we aim to give a global, non-exhaustive overview of key metabolic pathways. By contrasting the metabolic rewiring required by myeloid leukaemic stem cells with that required for haematopoiesis and immune cell function, we highlight the metabolic features they share. This is a critical consideration when contemplating anti-cancer metabolic inhibitor options, especially in the context of anti-cancer immune therapies. Finally, we examine pathways that have not been studied in leukaemia but are critical in solid cancers in the context of metastasis and interaction with new niches. These studies also offer detailed mechanisms that have yet to be investigated in leukaemia. Given that cancer (and normal) cells can meet their energy requirements by not only upregulating metabolic pathways, but also utilising systemically available substrates, we aim to inform how interlinked these metabolic pathways are, both within leukaemic cells and between cancer cells and their niche.

Supporting cell proliferation through nucleotide biosynthesis is an essential requirement for cancer cells. Hence, inhibition of folate-mediated one carbon (1C) metabolism, which is required for nucleotide synthesis, has been successfully exploited in anti-cancer therapy. Here, we reveal that mitochondrial folate metabolism is upregulated in patient-derived leukaemic stem cells (LSCs). We demonstrate that inhibition of mitochondrial 1C metabolism through impairment of de novo purine synthesis has a cytostatic effect on chronic myeloid leukaemia (CML) cells. Consequently, changes in purine nucleotide levels lead to activation of AMPK signalling and suppression of mTORC1 activity. Notably, suppression of mitochondrial 1C metabolism increases expression of erythroid differentiation markers. Moreover, we find that increased differentiation occurs independently of AMPK signalling and can be reversed through reconstitution of purine levels and reactivation of mTORC1. Of clinical relevance, we identify that combination of 1C metabolism inhibition with imatinib, a frontline treatment for CML patients, decreases the number of therapy-resistant CML LSCs in a patient-derived xenograft model. Our results highlight a novel role for folate metabolism and purine sensing in stem cell fate decisions and leukaemogenesis.