Transport of D-[1-14C]-amino acids into Chinese hamster ovary (CHO-K1) cells: implications for use of labeled D-amino acids as molecular imaging agents

Introduction: The fact that D-amino acids have been found in various tissues and are involved in various functions is a clue to how to develop new imaging agents. We examined D-amino acid transport mechanisms in Chinese hamster ovary (CHO-K1) cells because CHO-K1 cells are widely used in biomedical studies and are thought to be useful for expression of genes involved in metabolism of D-amino acids. Methods: Uptake experiments were performed. CHO-K1 cells cultured in 60-mm plastic culture dishes under ordinary culture conditions were incubated with 18.5 kBq of radiolabeled amino acid in 2 ml of phosphate-buffered-saline-based uptake solution at 37oC. The following radiolabeled amino acid tracers were used: D-[1-14C]-alanine, L-[1-14C]-alanine, D-[1-14C]-serine, L-[1-14C]-serine, D-[1-14C]-methionine, L-[1-14C]-methionine, D-[1-14C]-phenylalanine, L-[1-14C]-phenylalanine, D-[1-14C]-leucine, L-[1-14C]-leucine, D-[1-14C]-valine, L-[1-14C]-valine, D-[1-14C]-tyrosine, L-[1-14C]-tyrosine, D-[1-14C]-glutamic acid, L-[1-14C]-glutamic acid, D-[1-14C]-lysine, L-[1-14C]-lysine, D-[1-14C]-arginine and L-[1-14C]-arginine. We tested the inhibitory effects of the following compounds (1.0 mM) on transport: 2-(methylamino)isobutyric acid (a specific inhibitor of system A, in Na+-containing uptake solution) and 2-amino-bicyclo[2,2,1]heptane-2-carboxylic acid (a specific inhibitor of system L, in Na+-free uptake solution). Results: D-[1-14C]-methionine, D-[1-14C]-phenylalanine and D-[1-14C]-tyrosine accumulated mainly via system L. D-[1-14C]-alanine and D-[1-14C]-serine accumulated primarily via system ASC. High uptake of D-[1-14C]-alanine, D-[1-14C]-methionine, D-[1-14C]-phenylalanine and D-[1-14C]-leucine was observed. The uptake of radiolabeled serine, valine, tyrosine, glutamic acid and arginine into CHO-K1 was highly stereoselective for L-isomers. Conclusions: We observed high uptake of D-[1-14C]-alanine via system ASC (most likely alanine-serine-cysteine-selective amino acid transporter-1) and high uptake of D-[1-14C]-methionine and D-[1-14C]-phenylalanine via system L (most likely L-type amino acid transporter-1)