Hyperpolarized 13C probes for the one carbon metabolism

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Abstract One-carbon metabolism is a network of fundamental metabolic and biosynthetic pathways that occur in the cytoplasm and the mitochondria. Central to one-carbon metabolism is the folate cycle. This involves the activation and transfer of serine hydroxymethyl group to tetrahydrofolate intermediates and eventually formate, which is then converted back into serine to via the same intermediates. The folate cycle provides activated one carbon units for the biosynthesis of a variety of biomolecules necessary for normal cellular function and proliferation, most importantly, nucleotides. The activity of one-carbon metabolism is upregulated in cancer cells to support high rates of proliferation. The folate cycle is highly compartmentalized with separate sets of enzymes in the cytosol and in the mitochondria. When there is an increased need for one-carbon units for nucleotide biosynthesis, the cytosolic and mitochondrial folate cycles run in the opposite direction. The mitochondrial folate pathway produces formate, which is transported into the cytosol where it enters the folate cycle to produce activated one-carbon units for biosynthesis. In many types of cancer cells, the mitochondrial formate production exceeds the biosynthetic demand. Excess formate is excreted from these cells in a process known as formate overflow. Despite its great biological importance and central role in cancer, currently there is no clinically translatable, non–invasive imaging method for the real time monitoring of one carbon metabolism and formate overflow in vivo. In this R21 application we propose to use deuterated, 13C-labeled serines as hyperpolarized 13C-tracers to monitor one carbon metabolic activity and formate overflow in vivo by 13C magnetic resonance spectroscopic imaging. As serine is the main source of one carbon units, 13C-labeled serines are ideal probes for the mitochondrial folate cycle. Aim 1 will focus on the synthesis and in vitro characterization of deuterated, 13C-labeled serines as hyperpolarized 13C probes for the one carbon metabolism. In Aim 2, we will test the hyperpolarized probes in hepatoma cell cultures and in vivo in normal and orthotopic xenograft rat model for hepatocellular carcinoma. By the end of the two year funding period we expect to have demonstrated that upregulated mitochondrial folate pathway can be visualized in hepatoma bearing rats by 13C magnetic resonance spectroscopic imaging.