A metabolic trade-off between malignant plasma cells and mesenchymal stromal cells sustains multiple myeloma growth

: Multiple myeloma (MM) is a glutamine (Gln)-auxotroph and Gln-addicted cancer, with Gln synthetase (GS)-deficient MM cells avidly taking up extracellular Gln to sustain their metabolism. Thus, MM cells create a peculiar metabolic niche in the patients' bone marrow (BM), where low levels of Gln contribute to the osteolytic bone lesions by inhibiting the osteoblastic differentiation of mesenchymal stromal cells (MSCs). The effects of the altered MM metabolic niche on other BM cell populations remain to be clarified. We demonstrate here that MM cells secrete high amounts of glutamate through the exchange transporter SLC7A11/xCT. In turn, BM MSCs, but neither MM cells nor osteoblasts (OBs), actively take up extracellular glutamate through the transporter EAAT3 (SLC1A1), whose expression decreases during osteogenesis. GS-positive MSCs secrete Gln, a process boosted by extracellular glutamate in undifferentiated MSCs, but not in differentiated OBs. Coculture of MSCs with MM cells promotes the expression of the bidirectional transporter SNAT5 (SLC38A5), suggesting its involvement in Gln efflux. Consistently, MSCs, derived from either patients with MM or healthy donors, sustain MM growth in a low-Gln environment, an effect suppressed by the inhibition or silencing of glutamate uptake or Gln synthesis. In conclusion, a metabolic cycle occurs in MM BM microenvironment, where Gln-auxotroph MM cells extrude glutamate that is converted into Gln by MSC, sustaining in turn MM anabolism through Gln secretion. The inhibition of this metabolic trade-off impairs MM cell growth, thus highlighting novel potential, niche-oriented therapeutic targets.