Surface translocator protein 18 kDa (TSPO) localization on immune cells upon stimulation with LPS and in ART-treated HIV+ subjects

Abstract

Translocator protein 18 kDa (TSPO) is a well‐known outer mitochondrial membrane protein and it is widely used as a biomarker of neuroinflammation and brain injury. Although it is thought that TSPO plays key roles in a multitude of host cell functions, including steroid biosynthesis, apoptosis, generation of reactive oxygen species, and proliferation, some of these functions have recently been questioned. Here, we report the unexpected finding that circulating immune cells differentially express basal levels of TSPO on their cell surface, with a high percentage of monocytes and neutrophils expressing cell surface TSPO. In vitro stimulation of monocytes with LPS significantly increases the frequency of cells with surface TSPO expression in the absence of altered gene expression. Importantly, the LPS increase in TSPO cell surface expression in monocytes appears to be selective for LPS because two other distinct monocyte activators failed to increase the frequency of cells with surface TSPO. Finally, when we quantified immune cell TSPO surface expression in antiretroviral therapy‐treated HIV+ donors, a chronic inflammatory disease, we found significant increases in the frequency of TSPO surface localization, which could be pharmacologically suppressed with ∆9‐tetrahydrocannabinol. These findings suggest that cell surface TSPO in circulating leukocytes could serve as a peripheral blood‐based biomarker of inflammation.

Targeting TSPO Reduces Inflammation and Apoptosis in an In Vitro Photoreceptor-Like Model of Retinal Degeneration

“…Treatment with PIGAs completely restored the physiological localization of TSPO at the level of mitochondria (Figure C, arrows), likely preserving mitochondrial bioenergetics and avoiding the triggering of cell programmed death processes . Our data demonstrate, for the first time, the TSPO expression in photoreceptor-like cells and lead us to hypothesize a possible direct action of TSPO in the control not only of inflammation but also of apoptosis and proliferation of this cell line …”

“…The literature on TSPO reports that this transporter is usually overexpressed in neuroinflammation; however, recent studies demonstrated a cell-specific increase in TSPO, and that this increase at the cell surface did not correlate with an increase of the intracellular concentration of the protein or of its mRNA levels . The data reported here indicate that in our 661 W cell model of LPS-induced neuroinflammation, there was no increase in total TSPO protein levels (Figure S1), but the transporter changed its localization in the cell by altering the ratio of the percentage of its expression at the nuclear and mitochondrial levels (Figure A,B).…”

“…28 The literature on TSPO reports that this transporter is usually overexpressed in neuroinflammation; 20 however, recent studies demonstrated a cell-specific increase in TSPO, and that this increase at the cell surface did not correlate with an increase of the intracellular concentration of the protein or of its mRNA levels. 39 The data reported here indicate that in our 661 W cell model of LPS-induced neuroinflammation, there was no increase in total TSPO protein levels (Figure S1), but the transporter changed its localization in the cell by altering the ratio of the percentage of its expression at the nuclear and mitochondrial levels (Figure 7A,B). In Figure 7C, it can be observed that cells treated with PIGAs show a staining pattern more similar to the unstressed/untreated control cells (CTRL), where TSPO is localized at the level of the mitochondrial membrane, whereas in the cells stressed and treated with vehicle alone (DMSO), the colocalization was completely lost, and TSPO localized mainly at the peri-nuclear level (vehicle alone treated cell, DMSO, green staining).…”

“…41 Our data demonstrate, for the first time, the TSPO expression in photoreceptor-like cells and lead us to hypothesize a possible direct action of TSPO in the control not only of inflammation but also of apoptosis and proliferation of this cell line. 39 Finally, to assess whether the protective activity of TSPO ligands was, at least in part, mediated by the production of antiinflammatory steroids, 661 W cells were cotreated with the ligands (3 μM) and the inhibitor of pregnenolone synthesis (10 μM), SU10603 (inhibitor of 17α-hydroxylase/C17−20 lyase (P450c17 or CYP17A1)), which prevents the conversion of pregnenolone into deidroepiandrosterone (DHEA). 36,42 From the bar graph shown in Figure 8, it is possible to observe a significant reduction in the protective activity of all TSPO ligands, evidencing, in particular for PIGA823 and PIGA1138 (highly steroidogenic), cell viability values comparable to those of the stressed/untreated group (DMSO), indicating an almost total suppression of their efficacy.…”

Osteopontin/secreted phosphoprotein‐1 harnesses glial‐, immune‐, and neuronal cell ligand‐receptor interactions to sense and regulate acute and chronic neuroinflammation

“…Interestingly, immune cells express TSPO receptors through which their regulatory activity can be influenced by the binding and downstream signaling of benzodiazepines 150,151 . In addition, a recent study reported increased expression of the TSPO receptor on immune cells in treated HIV+ people suggesting a possible association with inflammatory signaling 152 …”