System l amino acid transporter LAT1 accumulates O-(2-fluoroethyl)-l-tyrosine (FET)

Abstract

O-(2-fluoroethyl)-L-tyrosine (FET) labeled with fluorine-18 is an important and specific tracer for diagnostics of glioblastoma via positron emission tomography (PET). However, the mechanism of its quite specific accumulation in tumor tissue has not been understood so far. In this work we demonstrate that [(3)H]L-tyrosine is primarily transported by the system L transporter LAT1 in human LN229 glioblastoma cells. FET reduced tyrosine transport, suggesting that it shares the same uptake pathway. More importantly, accumulation of FET was significantly reduced after siRNA-mediated downregulation of LAT1. Xenopus laevis oocytes expressing human LAT1 together with the glycoprotein 4F2hc (necessary to pull LAT-1 to the plasma membrane) exhibited a similar accumulation of FET as observed in glioblastoma cells. In contrast, no accumulation was observed in control oocytes, not overexpressing an exogenous transporter. Because LAT1 works exclusively as an exchanger of amino acids, substrates at one side of the membrane stimulate exchange against substrates at the other side. Extracellular FET stimulated the efflux of intracellular [(3)H]L-leucine, demonstrating that FET is indeed an influx substrate for LAT1. However, FET injected into oocytes was not able to stimulate uptake of extracellular [(3)H]L-leucine, indicating that FET is not a good efflux substrate. Our data, therefore, suggest that FET is trapped within cells due to the asymmetry of its intra- and extracellular recognition by LAT1. If also found for other transporters in tumor cells, asymmetric substrate recognition may be further exploited for tumor-specific accumulation of PET-tracers and/or other tumor-related drugs.

Epileptic Activity Increases Cerebral Amino Acid Transport Assessed by 18F-Fluoroethyl-l-Tyrosine Amino Acid PET: A Potential Brain Tumor Mimic

“…Previous studies have shown that 18 F-FET does not participate in specific metabolic pathways and is transported predominantly via the system L amino acid transporters LAT1 and LAT2 (3,4). In normal cerebral cortex, LAT1 protein is only moderately expressed and LAT2 protein is absent (24).…”

“…18 F-FET uptake is primarily mediated by the system L amino acid transporter subtypes LAT1 and LAT2 (3,4). These transporters are heterodimers consisting of a light (LAT1, LAT2) and a heavy chain (CD98) and function as obligatory stereospecific exchanger (antiporter) of neutral amino acids (3,4).…”

Serial 18F-FET PET Imaging of Primarily 18F-FET–Negative Glioma: Does It Make Sense?

“…Particularly, it remains unclear which cellular mechanisms lead to the phenomenon of missing intracellular 18 F-FET uptake in 18 F-FET-negative gliomas. One might speculate that 18 F-FET-negative gliomas are characterized either by a low expression or by low activity of the large neutral amino acid transporter at the tumor cells or at the brain capillary endothelial cells (24) responsible for intracellular uptake of amino acids (25). Besides, one might hypothesize that a rather low need of amino acids due to low protein synthesis of a glioma might lead to 18 F-FET negativity, which would fit to the reported better outcome in patients without primary tumoral 18 F-FET uptake compared with 18 F-FET-enhancing tumors (26), but this hypothesis appears controversial as another study did not show a better prognosis in patients without tumoral 18 F-FET uptake (10).…”

Determination of an Optimal Pharmacokinetic Model of 18F-FET for Quantitative Applications in Rat Brain Tumors

“…mechanisms for muscles and the tumor. In this case, all tissues have similar values of K 1 , but k 2 and k 3 are generally lower for muscles than for the tumor or brain, indicating that 18 F-FET tends to remain in the reversible compartment (most likely the extracellularextravascular space (30)). On the other hand, values of k 3 are highest in the tumor, suggesting that the principal accumulation mechanism of 18 F-FET in F98 glioblastoma is internalization.…”

“…In contrast, for muscles, 18 F-FET internalization may be reversible such that including k 4 was deemed preferable, even though it is small and highly variable between animals (Table 3). Because different tumors use different 18 F-FET uptake mechanisms (4), these results apply to the F98 glioblastoma, but tumors with similar energy-independent 18 F-FET uptake mechanisms, such as human gliomas (30), are likely to show similar kinetics, although v b is expected to be significant in humans.…”