In vivo assessment of increased oxidation of branched-chain amino acids in glioblastoma

Altered branched-chain amino acids (BCAAs) metabolism is a distinctive feature of various cancers and plays an important role in sustaining tumor proliferation and aggressiveness. Despite the therapeutic and diagnostic potentials, the role of BCAA metabolism in cancer and the activities of associated enzymes remain unclear. Due to its pivotal role in BCAA metabolism and rapid cellular transport, hyperpolarized C-13-labeled alpha-ketoisocaproate (KIC), the alpha-keto acid corresponding to leucine, can assess both BCAA aminotransferase (BCAT) and branched-chain alpha-keto acid dehydrogenase complex (BCKDC) activities via production of [1-C-13] leucine or (CO2)-C-13 (and thus (HCO3-)-C-13), respectively. Here, we investigated BCAA metabolism of F98 rat glioma model in vivo using hyperpolarized C-13-KIC. In tumor regions, we observed a decrease in C-13-leucine production from injected hyperpolarized C-13-KIC via BCAT compared to the contralateral normal-appearing brain, and an increase in (HCO3-)-C-13, a catabolic product of KIC through the mitochondrial BCKDC. A parallel ex vivo C-13 NMR isotopomer analysis following steady-state infusion of [U-C-13] leucine to glioma-bearing rats verified the increased oxidation of leucine in glioma tissue. Both the in vivo hyperpolarized KIC imaging and the leucine infusion study indicate that KIC catabolism is upregulated through BCAT/BCKDC and further oxidized via the citric acid cycle in F98 glioma.