Stroma-driven horizontal transfer of TCA proteins mediates metabolic plasticity and imatinib resistance in leukemia

Leukemia cells residing in the bone marrow often exhibit resistance to tyrosine kinase inhibitors. Metabolic rearrangement of cancer cells has recently gained particular attention as a possible cause of adaptation and insensitivity to drug treatment. We demonstrated here that stromal cells directly transferred the membrane vesicles together with proteins related to the tricarboxylic acid (TCA/Krebs) cycle to leukemic cells. This transfer was dependent on direct cell-to-cell contact and led to increased metabolic plasticity. In addition, co-culture increased activities of the TCA cycle, oxidative phosphorylation and oxidative capacity, therefore protected from loss of metabolic homeostasis and increased oxidative stress in response to imatinib. As a result of co-culture with stroma, the reductions in OXPHOS, mitochondria-related parameters and the maximal respiration observed in imatinib-treated leukemic cells, were substantially less present, and the spare respiratory capacity parameter was even higher compared to control cells. Metabolome profiling revealed that co-cultured leukemic cells treated with imatinib exhibit higher levels of TCA-related metabolites such as isocitric acid, L-malic acid ketoglutaric acid and cis-aconitic acid, as well as lower level of oxidative stress. The co-culture with rho0 stromal cells and analysis of horizontal transfer of GFP-positive mitochondria excluded transfer of mitochondria and their oxidative phosphorylation status as important for the stroma-driven metabolic protection. Altogether, our data provide insight into the novel mechanism of the bone marrow-mediated protection of leukemic cells, associated with metabolic adaptation to imatinib treatment. Metabolic plasticity as a resistance driver has been indicated in leukemia stem cells, however our data indicate that the presence of stromal cells may provide such support to all leukemic cells. In conclusion, we postulate that elements involved in the TCA-related metabolic plasticity in leukemia can be targeted to achieve a therapeutic effect and to overcome the resistance caused by the bone marrow microenvironment.