title: Putative transport mechanism and intracellular fate of trans-1-amino-3-18F-fluorocyclobutanecarboxylic acid in human prostate cancer.
abstract: Trans-1-amino-3-(18)F-fluorocyclobutanecarboxylic acid (anti-(18)F-FACBC) is an amino acid PET tracer that has shown promise for visualizing prostate cancer. Therefore, we aimed to clarify the anti-(18)F-FACBC transport mechanism in prostate cancer cells. We also studied the fate of anti-(18)F-FACBC after it is transported into cells.
  
  
  METHODS
  For convenience, because of their longer half-lives, (14)C compounds were used instead of (18)F-labeled tracers. Trans-1-amino-3-fluoro-1-(14)C-cyclobutanecarboxylic acid ((14)C-FACBC) uptake was examined in human prostate cancer DU145 cells with the following substrates of amino acid transporters: α-(methylamino) isobutyric acid (a system A-specific substrate) and 2-amino-2-norbornanecarboxylic acid (a system L-specific substrate). The messenger RNA expression of amino acid transporters in human prostate cancer specimens was analyzed by complementary DNA microarray and quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR). Gene expression in DU145 cells was analyzed by qRT-PCR. We also examined the knockdown effect of the amino acid transporters system ASC transporter 2 (ASCT2) and sodium-coupled neutral amino acid transporter 2 (SNAT2) on (14)C-FACBC uptake. In addition, the possibility of (14)C-FACBC incorporation into proteins was examined.
  
  
  RESULTS
  (14)C-FACBC uptake by DU145 cells was markedly decreased to approximately 20% in the absence of Na(+), compared with that in its presence, indicating that Na(+)-dependent transporters are mainly responsible for the uptake of this tracer. Moreover, 2-amino-2-norbornanecarboxylic acid inhibited the transport of (14)C-FACBC to the basal level in Na(+)-free buffer. In contrast, α-(methylamino) isobutyric acid did not inhibit (14)C-FACBC accumulation in DU145 cells. Human prostate tumor specimens and DU145 cells had similar messenger RNA expression patterns of amino acid transporter genes. Although SNAT2 and ASCT2 are 2 major amino acid transporters expressed in prostate tumor tissues and DU145 cells, ASCT2 knockdown using small interfering RNA was more effective in lowering (14)C-FACBC transport than SNAT2. Almost all intracellular (14)C-FACBC was recovered from the nonprotein fraction.
  
  
  CONCLUSIONS
  ASCT2, which is a Na(+)-dependent amino acid transporter, and to a lesser extent Na(+)-independent transporters play a role in the uptake of (14)C-FACBC by DU145 cells. Among the Na(+)-independent transporters, system L transporters are also involved in the transport of (14)C-FACBC. Moreover, (14)C-FACBC is not incorporated into proteins in cells. These findings suggest a possible mechanism of anti-(18)F-FACBC PET for prostate cancer.
authors.affiliation: Division of Health Sciences, Graduate School of Medical Science, Kanazawa University, Ishikawa, Japan.
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Okudaira
authors.forename: Naoto
authors.initials: N
authors.lastname: Shikano
authors.forename: Ryuichi
authors.initials: R
authors.lastname: Nishii
authors.forename: Tohru
authors.initials: T
authors.lastname: Miyagi
authors.forename: Mitsuyoshi
authors.initials: M
authors.lastname: Yoshimoto
authors.forename: Masato
authors.initials: M
authors.lastname: Kobayashi
authors.forename: Kazuyo
authors.initials: K
authors.lastname: Ohe
authors.forename: Takeo
authors.initials: T
authors.lastname: Nakanishi
authors.forename: Ikumi
authors.initials: I
authors.lastname: Tamai
authors.forename: Mikio
authors.initials: M
authors.lastname: Namiki
authors.forename: Keiichi
authors.initials: K
authors.lastname: Kawai
mesh_terms: D026962:Amino Acid Transport System ASC / Q000172:deficiency / Q000235:genetics / Q000378:metabolism; D001667:Binding, Competitive; D001692:Biological Transport; D002264:Carboxylic Acids / Q000378:metabolism*; D045744:Cell Line, Tumor; D003503:Cyclobutanes / Q000378:metabolism*; D020869:Gene Expression Profiling; D006801:Humans; D042541:Intracellular Space / Q000378:metabolism*; D008297:Male; D015778:Minor Histocompatibility Antigens; D020411:Oligonucleotide Array Sequence Analysis; D011471:Prostatic Neoplasms / Q000235:genetics / Q000378:metabolism* / Q000473:pathology*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D034741:RNA, Small Interfering / Q000235:genetics
chemical_list: D026962:Amino Acid Transport System ASC; D002264:Carboxylic Acids; D003503:Cyclobutanes; D015778:Minor Histocompatibility Antigens; D012333:RNA, Messenger; D034741:RNA, Small Interfering; C420506:SLC1A5 protein, human; C117460:fluciclovine F-18