Translocator protein (18 kDa)/peripheral benzodiazepine receptor specific ligands induce microglia functions consistent with an activated state

Abstract

In the brain, translocator protein (18 kDa) (TSPO), previously called peripheral benzodiazepine receptor (PBR), is a glial protein that has been extensively used as a biomarker of brain injury and inflammation. However, the functional role of TSPO in glial cells is not well characterized. In this study, we show that the TSPO-specific ligands R-PK11195 (PK) and Ro5-4864 (Ro) increased microglia proliferation and phagocytosis with no effect on migration. Both ligands increased reactive oxygen species (ROS) production, and this effect may be mediated by NADPH-oxidase. PK and Ro also produced a small but detectable increase in IL-1β release. We also examined the effect of PK and Ro on the expression of pro-inflammatory genes and cytokine release in lipopolysaccharide (LPS) and adenosine triphosphate (ATP) activated microglia. PK or Ro had no effect on LPS-induced increase of pro-inflammatory genes, but they both decreased the ATP-induced increase of COX-2 gene expression. Ro, but not PK, enhanced the LPS-induced release of IL-1β. However, Ro decreased the ATP-induced release of IL-1β and TNF-α, and PK decreased the ATP-induced release of TNF-α. Exposure to Ro in the presence of LPS increased the number of apoptotic microglia, an effect that could be blocked by PK. These findings show that TSPO ligands modulate cellular functions consistent with microglia activation. Further, when microglia are activated, these ligands may have therapeutic potential by reducing the expression of pro-inflammatory genes and cytokine release. Finally, Ro-like ligands may be involved in the elimination of activated microglia via apoptosis.

The nitric oxide donor sodium nitroprusside requires the 18 kDa Translocator Protein to induce cell death

“…Cell death, including apoptosis, induced by various agents such as CoCl 2 , erucylphosphohomocholine (ErPC3), etc., includes activation of the TSPO leading to ROS generation at mitochondrial levels, oxidation of cardiolipins, collapse of the Dwm, caspase activation, and induction of DNA fragmentation as components of the mitochondrial apoptosis cascade in human glioblastoma U118MG cells, as described previously [13,[16][17][18]63]. These are processes that are also induced by NO [64][65][66][67].…”

Double Application of Translocator Protein Ligands and RAW Cells Inflammatory Milieu

“…Our in vitro study supports the view that a spectrum of cytokine changes occurs also as a direct result of ligand binding to TSPO. On the other hand, it is difficult to compare our results about no change in RNI and cytokine levels after a single application of Ro5-4864 and PK11195 with the results for rat microglial cells (Choi et al, 2011) and the murine immortalized microglial cell line BV2 (Bae et al, 2014) because the cell activation and the ligands application were the other way round.…”

Evidence for Complex Binding Profiles and Species Differences at the Translocator Protein (TSPO) (18 kDa)

“…Thus, while TSPO functional activity is traditionally denoted by pregnenolone production in rat C6 glioma cells at a ligand concentration of 40µM, this may not be a true indication of solely TSPO-mediated effects. More recent studies suggest that testing ligands at nanomolar concentrations consistent with their binding affinities mediate very different functional effects compared to those assays performed at micromolar concentrations, whereby effects may be TSPOindependent [24]. Alternatively, recent evidence suggests that TSPO ligand binding varies across different tissues [25].…”