Abstract
Methionine dependency of tumor growth, although not well-understood, is detectable by 11C-methionine positron emission tomography and may contribute to the aggressivity of glioblastomas (GBM) and meningiomas. Cytosolic folate cycle is required for methionine synthesis. Its dysregulation may influence cell reprogramming towards pluripotency. We evaluated methionine-dependent growth of monolayer (ML) cells and stem cell-like tumor spheres (TS) derived from 4 GBM (U251, U87, LN299, T98G) and 1 meningioma (IOMM-LEE) cell lines. Our data showed that for all cell lines studied, exogenous methionine is required for TS formation but not for ML cells proliferation. Furthermore, for GBM cell lines, regardless of the addition of folate cycle substrates (folic acid and formate), the level of 3 folate isoforms, 5-methytetrahydrofolate, 5,10-methenyltetrahydrofolate, and 10-formyltetrahydrofolate, were all downregulated in TS relative to ML cells. Unlike GBM cell lines, in IOMM-LEE cells, 5-methyltetrahydrofolate was actually more elevated in TS than ML, and only 5,10-methenyltetrahydrofolate and 10-formyltetrahydrofolate were downregulated. The functional significance of this variation in folate cycle repression was revealed by the finding that Folic Acid and 5-methyltetrahydrofolate promote the growth of U251 TS but not IOMM-LEE TS. Transcriptome-wide sequencing of U251 cells revealed that DHFR, SHMT1, and MTHFD1 were downregulated in TS vs ML, in concordance with the low activity cytosolic folate cycle observed in U251 TS. In conclusion, we found that a repressed cytosolic folate cycle underlies the methionine dependency of GBM and meningioma cell lines and that 5-methyltetrahydrofolate is a key metabolic switch for glioblastoma TS formation. The finding that folic acid facilitates TS formation, although requiring further validation in diseased human tissues, incites to investigate whether excessive folate intake could promote cancer stem cells formation in GBM patients.
Highlights
Methionine dependency is a common feature of most cancer cells[1]
Methionine is required for the growth of tumor spheres but not monolayer cells derived from U251, T98G, U87, and LN299 glioblastoma cell lines
Differentiated monolayer cells (ML) and tumor spheres (TS) could be derived from 4 glioblastoma (GBM) cell lines U251, T98G, U87, and LN299 cultured in a methionine free-medium to which exogenous methionine was added in incremental amounts up to 0.5 ML Methionine (mM) (Fig. 1b)
Summary
Methionine dependency is a common feature of most cancer cells[1]. It is characterized by the cells’ inability to grow without methionine even in the presence of its precursor homocysteine[2]. Glioblastoma (GBM) is one of the most aggressive brain tumors and current therapeutic strategies remain ineffective in delaying its progression[3]. GBM is composed of heterogeneous tumor tissues[4], and is enriched with self-renewing stem-like cells[5,6,7]. Methionine dependency might flag the aggressive grade of these brain tumors and explain their increased uptake in positron emission tomography of 11C-methionine[9,10,11]
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