Characterization of L Amino Acid Transporter 1 (LAT1) for 3-O-Methyl-6-¹⁸F-Fluoro-L-DOPA (OMFD) in Tumor Cells and Tumor Tissues

¹⁸F-labeled amino acids represent a promising class of imaging agents in tumors visualized by means of positron emission tomography (PET). Because of the high uptake it is still problematic to clear differentiate between tumors and inflammation.
The high enrichment in tumor tissues assumed the uptake of the tracer via a tumor-specific amino acid transporter, which is not or different expressed in inflammatory cells suggesting a different endowment of neutral amino acids. As previously shown, L-type amino acid transporter 1 (LAT1) is playing a key role because of its high up-regulation in malignant tumors. For the functional expression of LAT1 a single membrane-spanning protein, the heavy chain of 4F2 antigen (4F2hc), essentially forms a heterodimeric complex via disulfide bonds.
The present study investigated the amino acid transport mechanism of LAT1 for 3-O-methyl-6-¹⁸F-Fluoro-L-DOPA (OMFD), a novel ¹⁸F-labeled phenylalanine derivative, into tumor cells.
For molecular characterisation of L-type amino acid transporters focusing on the LAT1-4F2hc subtype we used two different tumor cells like FaDu (squamous cell carcinoma)/HT29 (colorectal adenocarcinoma) and tumor bearing mice performing quantitative RT-PCR, Western-Blot, and immunhistochemistry. In vitro uptake assays with HT29 and FaDu were performed with OMFD under physiological amino acid concentrations.
OMFD demonstrated a saturable and sodium- and energy-independent accumulation in vitro in different tumor cell lines, suggesting its uptake to be mediated exclusively by sodium-independent LAT1.
Our data emphasize the relevance of OMFD as a PET tracer for imaging of specific amino acid transport via LAT1 in tumors. Furthermore, the identification and characterization of tumor specific amino acid transporters like LAT1 will be a helpful tool for therapeutic implications. The inhibition of LAT1 activity in tumor cells could be effective in the inhibition of tumor cell growth by depriving tumor cells of essential amino acids, too.