Transport of 3-Fluoro-l-α-Methyl-Tyrosine by Tumor-Upregulated L-Type Amino Acid Transporter 1: A Cause of the Tumor Uptake in PET

Inhibitory effects of FAMT on l-¹⁴C-leucine uptake in S2-LAT1 and S2-LAT2 cells. (A and B) Concentration-dependent inhibition of l-¹⁴C-leucine uptake by FAMT in S2-LAT1 cells (A) and S2-LAT2 cells (B). Uptake of 1 μM l-¹⁴C-leucine was measured in presence of varied concentrations of FAMT. Uptake values were fitted to inhibition curves. IC₅₀ of FAMT on LAT1-mediated l-¹⁴C-leucine uptake was 26.9 μM. (C and D) Kinetics of inhibition of LAT1-mediated l-¹⁴C-leucine uptake by FAMT. Uptake of l-¹⁴C-leucine (1–100 μM) was measured in absence or presence of FAMT, and plotted against l-¹⁴C-leucine concentration to fit to Michaelis–Menten curves (C). Double reciprocal plot analysis was performed on inhibitory effect of 50 μM FAMT (D). Two lines fit to competitive inhibition with FAMT’s K_i value of 29.5 μM. Leu = leucine.

Efflux of l-¹⁴C-leucine induced by FAMT in S2-LAT1 and S2-LAT2 cells. (A and B) Efflux of preloaded l-¹⁴C-leucine from S2-LAT1 cells (A) and S2-LAT2 cells (B) in absence ((−)) or presence of 40 μM l-tyrosine and FAMT. (C) Concentration dependence of effect of FAMT on induction of l-¹⁴C-leucine efflux from S2-LAT1 cells. Efflux of preloaded l-¹⁴C-leucine from S2-LAT1 cells was measured for 1 min in presence of extracellularly applied FAMT. Radioactivity of l-¹⁴C-leucine released from cells in presence of FAMT was subtracted from that in absence of FAMT ((−) in A). Efflux values were fit to Michaelis–Menten curve. Inset shows Eadie–Hofstee plot of l-¹⁴C-leucine efflux, which was used to determine kinetic parameters. Apparent K_m of 27.5 μM and V_max of 27.7% radioactivity/min were obtained. *P < 0.05. **P < 0.01. Leu = leucine; NS = not significant.

Interaction of 2-FAMT with LAT1 and LAT2. (A and B) Concentration-dependent inhibition of l-¹⁴C-leucine uptake by 2-FAMT in S2-LAT1 cells (A) and S2-LAT2 cells (B). Uptake of 1 μM l-¹⁴C-leucine was measured in presence of varied concentrations of 2-FAMT. Uptake values were fitted by nonlinear regression to inhibition curve. IC₅₀ of 2-FAMT on LAT1-mediated l-¹⁴C-leucine uptake was 28.8 μM. (C and D) Efflux of preloaded l-¹⁴C-leucine from S2-LAT1 cells (C) and S2-LAT2 cells (D) in absence ((−)) or presence of 40 μM 2-FAMT. **P < 0.01. Leu = leucine.

Inhibitory effects of tyrosine-related compounds and l-methionine on l-¹⁴C-leucine uptake in S2-LAT1 and S2-LAT2 cells. Uptake of 1 μM l-¹⁴C-leucine was measured for 1 min in absence ((−)) or presence of 200 μM tyrosine-related compounds and l-methionine in S2-LAT1 and S2-LAT2 cells. Uptake values were expressed as percentage of control l-¹⁴C-leucine uptake measured in absence of inhibitors. Leu = leucine.

Efflux of l-¹⁴C-leucine induced by tyrosine-related compounds and l-methionine in S2 LAT1 and S2-LAT2 cells. Efflux of preloaded l-¹⁴C-leucine from S2-LAT1 and S2-LAT2 cells was measured for 1 min in absence or presence of extracellularly applied indicated compounds (40 μM). Radioactivity released from cells in presence of test compounds was subtracted from radioactivity in their absence. Leu = leucine.