Glioblastoma Exhibits Inter-Individual Heterogeneity of TSPO and LAT1 Expression in Neoplastic and Parenchymal Cells

Abstract

Molecular imaging is essential for diagnosis and treatment planning for glioblastoma patients. Positron emission tomography (PET) with tracers for the detection of the solute carrier family 7 member 5 (SLC7A5; also known as the amino acid transporter light chain L system, LAT1) and for the mitochondrial translocator protein (TSPO) is successfully used to provide additional information on tumor volume and prognosis. The current approaches for TSPO-PET and the visualization of tracer ([18F] Fluoroethyltyrosine, FET) uptake by LAT1 (FET-PET) do not yet exploit the full diagnostic potential of these molecular imaging techniques. Therefore, we investigated the expression of TSPO and LAT1 in patient glioblastoma (GBM) samples, as well as in various GBM mouse models representing patient GBMs of different genetic subtypes. By immunohistochemistry, we found that TSPO and LAT1 are upregulated in human GBM samples compared to normal brain tissue. Next, we orthotopically implanted patient-derived GBM cells, as well as genetically engineered murine GBM cells, representing different genetic subtypes of the disease. To determine TSPO and LAT1 expression, we performed immunofluorescence staining. We found that both TSPO and LAT1 expression was increased in tumor regions of the implanted human or murine GBM cells when compared to the neighboring mouse brain tissue. While LAT1 was largely restricted to tumor cells, we found that TSPO was also expressed by microglia, tumor-associated macrophages, endothelial cells, and pericytes. The Cancer Genome Atlas (TCGA)-data analysis corroborates the upregulation of TSPO in a bigger cohort of GBM patient samples compared to tumor-free brain tissue. In addition, AIF1 (the gene encoding for the myeloid cell marker Iba1) was also upregulated in GBM compared to the control. Interestingly, TSPO, as well as AIF1, showed significantly different expression levels depending on the GBM genetic subtype, with the highest expression being exhibited in the mesenchymal subtype. High TSPO and AIF1 expression also correlated with a significant decrease in patient survival compared to low expression. In line with this finding, the expression levels for TSPO and AIF1 were also significantly higher in (isocitrate-dehydrogenase wild-type) IDHWT compared to IDH mutant (IDHMUT) GBM. LAT1 expression, on the other hand, was not different among the individual GBM subtypes. Therefore, we could conclude that FET- and TSPO-PET confer different information on pathological features based on different genetic GBM subtypes and may thus help in planning individualized strategies for brain tumor therapy in the future. A combination of TSPO-PET and FET-PET could be a promising way to visualize tumor-associated myeloid cells and select patients for treatment strategies targeting the myeloid compartment.

Imaging temozolomide-induced changes in the myeloid glioma microenvironment

“…Cancer tissue hijack and alter metabolic pathways to increase amino acid transport for cell proliferation and survival 48 , 49 . Therefore, [ 18 F]FET has been utilized as an imaging biomarker targeting tumor cells and tumor progression 50 , 51 .…”

The Role of Translocator Protein TSPO in Hallmarks of Glioblastoma

“…TSPO expression correlates with states of inflammation in the CNS [ 27 ] and is commonly used as a marker of inflammation in positron emission tomography (PET) studies [ 82 ]. TSPO expression is also increased in the surrounding microenvironment in GBM, including microglia and macrophages [ 66 ]. Various analyses revealed that TSPO ligands can modulate inflammatory and immune responses [ 83 , 84 , 85 , 86 , 87 ].…”

“…Interestingly, the upregulation of various pro-inflammatory genes [ 93 ] and M2 macrophage and neutrophilic gene signatures are significantly associated with the mesenchymal phenotype [ 42 ], which has the highest percentage of microglia, macrophage, and lymphocyte infiltration [ 94 ]. Given that TSPO expression is also associated with the mesenchymal subtype [ 66 ], it would be interesting to see if TSPO is involved in this regulation.…”

“…Interestingly, TSPO-PET not only visualizes the tumor mass but also activated microglia in TAM and potentially additional inflamed components of GBM [ 66 , 214 ]. In view of its suspected role in the immune escape, TSPO-PET could therefore also be a promising way to visualize the reactive microenvironment.…”

“…Several studies have found a strong correlation between TSPO expression levels and aggressive cancer phenotypes. Indeed, TSPO is frequently upregulated in gliomas with the highest expression in GBM [ 28 ] and the level of TSPO expression correlates with proliferative and apoptotic indices and prognosis [ 19 , 29 , 66 ]. Noteworthy, TSPO expression levels also differ significantly between molecular subtypes of GBM with the highest expression in the mesenchymal subtype [ 66 ].…”