Lactate‐mediated activation of GPR81 regulates BCR /Abl protein expression in chronic myeloid leukemia cells selected under low oxygen tension

Chronic myeloid leukemia (CML) is a stem cell‐driven neoplasia characterized by the expression of the constitutively active tyrosine kinase (TK) BCR/Abl. Under low oxygen, a condition that characterizes stem cell niches (SCNs) in vivo , the oncogenic BCR/Abl protein is suppressed. Consequently, leukemia stem cells (LSCs) residing within SCNs show resistance to TK inhibitors (TKIs), the first‐line therapy for CML, due to the lack of their molecular target. It is therefore important to deepen understanding of the mechanisms driving BCR/Abl protein suppression to design new strategies able to repress TKI‐resistant LSCs. Our previous studies showed that BCR/Abl protein suppression occurred when glucose approaches completed exhaustion in culture medium. As lactate is the main byproduct of glucose catabolism in low oxygen, in this study we addressed the role of lactate in regulating BCR/Abl protein expression. We found that treatment of CML cells with 2‐DG, which blocks glycolysis and thereby lactate production, or monocarboxylate transporter (MCT) inhibitors, which reduce lactate excretion, enhanced BCR/Abl protein expression and promoted maintenance of a BCR/Abl‐dependent/TKI‐sensitive stem cell phenotype. The effects of MCT inhibition were abolished when exogenous lactate was added to culture medium, resulting in the suppression of BCR/Abl protein expression. Treatment with 3‐hydroxy‐butyrate acid, an antagonist of the GPR81 plasma membrane ‘lactate’ receptor, prevented lactate‐driven BCR/Abl protein suppression, while the selective GPR81 agonist 3‐chloro‐5‐hydroxy‐BA counteracted the maintenance of BCR/Abl protein induced by MCT inhibition. Our results indicate that GPR81 engagement by extracellular lactate determines BCR/Abl protein suppression in low oxygen environments.