Andrographolide‐Mediated Protection of Barrier Integrity and Permeability is Mediated Through NLRP3 Inflammasome Inhibition in Blood‐Brain Barrier Endothelial Cells

Abstract

The blood‐brain barrier (BBB) that consists of the inter‐endothelial tight junctions, acts as the major blood‐brain interface and regulator of brain microvascular hyperpermeability. In the brain, the loss of BBB integrity and hyperpermeability leads to cerebral edema formation and intracranial pressure elevation in traumatic brain injury and ischemic strokes. Recent studies demonstrated that reactive oxygen species are potential signals for the NLR family pyrin domain containing 3 (NLRP3) inflammasome activation that leads to interleukin‐1β (IL‐1β) secretion. IL‐1β plays a major role in promoting BBB hyperpermeability in various brain pathologies, including TBI. Furthermore, recent studies from our lab have demonstrated a tri‐phasic role for hydrogen peroxide (H2O2) in BBB endothelial cells where it promotes, angiogenesis, hyperpermeability and apoptosis in a tri‐phasic and concentration dependent manner. Hydrogen peroxide plays a major role physiologically as the second messenger and pathologically as an inducer of oxidative stress and associated cellular signaling. We hypothesized that in BBB endothelial cells, H2O2‐induced barrier dysfunction and hyperpermeability are NLRP3 inflammasome‐dependent. The main objective of our study was to evaluate the role of NLRP3 inflammasome signaling in H2O2‐mediated barrier dysfunction and hyperpermeability in human brain microvascular endothelial cells and to test whether andrographolide, a labdane diterpenoid possessing potent anti‐inflammatory properties will attenuate such effects. Th effect of H2O2 treatment on tight junction integrity/permeability was studied using zonula occludens‐1 (ZO‐1) immunofluorescence localization, ZO‐1 immunoblot analysis, Transwell monolayer permeability assay using FITC‐dextran (10‐kDa) as a fluorescent marker and cell viability/apoptosis assay. The effect of NLRP3 inhibitor (MCC950) and andrographolide on H2O2‐induced hyperpermeability was studied using Transwell permeability assay. Our results show that, CRISPR/Cas‐9‐mediated knockdown of ZO‐1 resulted in monolayer hyperpermeability demonstrating the significance of ZO‐1 in regulating barrier functions. CRISPR‐based activation of NLRP3 induced monolayer hyperpermeability whereas CRISPR/Cas‐9‐based NLRP3 knockdown had no significant effect on permeability. H2O2 (10µM) induced monolayer hyperpermeability and the effect was decreased by MCC950 and andrographolide treatment. Hydrogen peroxide induced cathepsin B (an activator of the NLRP3 pathway) activity significantly, and the effect was decreased by andrographolide treatment. H2O2‐mediated barrier dysfunction and hyperpermeability were not due to changes in cell viability/apoptosis. These results suggest that NLRP3 is a mediator of H2O2‐induced barrier dysfunction and hyperpermeability in human BBB endothelial cells and andrographolide is an effective inhibitor of this pathway.