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: a-(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 1 , compared with that in its presence, indicating that Na 1 -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 1 -free buffer. In contrast, a-(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. Conclusion: ASCT2, which is a Na 1 -dependent amino acid transporter, and to a lesser extent Na 1 -independent transporters play a role in the uptake of 14 C-FACBC by DU145 cells. Among the Na 1 -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.

Relationship between [ 14 C]MeAIB uptake and amino acid transporter family gene expression levels or proliferative activity in a pilot study in human carcinoma cells: Comparison with [ 3 H]methionine uptake

“…The correlation between the uptake of radiotracers and the gene expression of amino acid transporter in tumor cells has been evaluated in a number of recent nuclear medicine studies. [25,44] have all been recently reported. However, it should be borne in mind that gene expression levels may not directly correlate with protein levels.…”

“…Overexpression of SNAT was observed in gastric cancer [58], human hepatocellular carcinoma [59], breast cancer [60,61], hilar cholangiocarcinoma [62], C6 glioma [63], prostate cancer [25], HeLa epithelial cervical cancer cells, and 143B osteosarcoma cells [64]; and SNAT expression in stomach and breast cancers is also related to Ki-67 [61] and PCNA (proliferating cell nuclear antigen) expression [58] as indicated by proliferative activity.…”

Update on 18F-Fluciclovine PET for Prostate Cancer Imaging

“…Anti-1-amino-3-[ 18 F]-flurocyclobutane-1-carboxylic acid (18F-fluciclovine) is a non-naturally occurring amino acid first synthesized by Dr. Mark Goodman, and initially developed for the evaluation of cerebral gliomas (8). 18F-fluciclovine transport is primarily mediated by sodium-dependent amino acid transporters, specifically system ASC (ASCT2), with contribution by sodium-independent system L (LAT1) (5). Because these amino acid transporters most involved in 18F-fluciclovine transport mediate influx and efflux of amino acids, washout of radiotracer occurs over time.…”

“…Several amino acid transporter systems are over expressed in prostate cancer; specifically large neutral amino acid transporters (system L: LAT1, LAT3, and LAT4) and alanine-serine-cysteine transporters (system ASC: ASCT1 and ASCT2) (5,6). Of these, LAT3, ASCT1, and ASCT2 are upregulated with androgen simulation and LAT1 and ASCT2 are associated with a more aggressive tumor phenotype (7).…”

A Putative Role of Organic Anion Transporting Polypeptides (OATPs) In Cell Survival of Hormone-Dependent Breast and Prostate Cancers

“…Besides, several transporter proteins for amino acids such as leucine and glutamine have been well documented to be upregulated in many types of cancer cells, [3,4] showing their critical role in nutrient signaling to mTOR and cell growth [5]. In addition to amino acid transporter, we have shown that enhanced activity of peptide transport in cancer cells [6,7].…”