The role of LAT1 in ¹⁸F-DOPA uptake in malignant gliomas

Positron emission tomography (PET) imaging with the amino acid tracer 6-¹⁸F-fluoro-l-3,4-dihydroxy-phenylalanine (¹⁸F-DOPA) may provide better spatial and functional information in human gliomas than CT or MRI alone. The l-type amino acid transporter 1 (LAT1) is responsible for membrane transport of large neutral amino acids in normal cells. This study assessed the relationship between LAT1 expression and ¹⁸F-DOPA uptake in human astrocytomas. Endogenous LAT1 expression was measured in established glioblastoma (GBM) cell lines and primary GBM xenografts using Western blotting and quantitative reverse transcription polymerase chain reaction (qRT-PCR). Uptake of ¹⁸F-DOPA was approximated in vitro using ³H-l-DOPA as an analog. Uptake of ³H-l-DOPA was assessed in cells expressing LAT1 shRNA or LAT1 siRNA and compared to non-targeted (NT) control shRNA or siRNA sequences, respectively. To demonstrate the clinical relevance of these findings, LAT1 immunofluorescence staining was compared with corresponding regions of ¹⁸F-DOPA PET uptake in patients with newly diagnosed astrocytomas. LAT1 mRNA and protein expression varies in GBM, and the extent of ³H-l-DOPA uptake was positively correlated with endogenous LAT1 expression. Stable shRNA-mediated LAT1 knockdown in T98 and GBM28 reduced ³H-l-DOPA uptake relative to NT shRNA by 57 (P < 0.0001) and 52 % (P < 0.001), respectively. Transient siRNA-mediated LAT1 knockdown in T98 reduced ³H-l-DOPA uptake relative to NT siRNA up to 68 % (P < 0.01). In clinical samples, LAT1 expression positively correlated with ¹⁸F-DOPA PET uptake (P = 0.04). Expression of LAT1 is strongly associated with ³H-l-DOPA uptake in vitro and ¹⁸F-DOPA uptake in patient biopsy samples. These results define LAT1 as a key determinant of ¹⁸F-DOPA accumulation in GBM.