Role of Caspases in Cytokine-Induced Barrier Breakdown in Human Brain Endothelial Cells

During neuroinflammation, cytokines such as TNF-α and IFN-γ secreted by activated leukocytes and/or CNS resident cells have been shown to alter the phenotype and function of brain endothelial cells (BECs) leading to blood-brain barrier breakdown. In this study, we show that the human BEC line hCMEC/D3 expresses the receptors for TNF-α, TNF receptor 1 and TNF receptor 2, and for IFN-γ. BEC activation with TNF-α alone or in combination with IFN-γ induced endothelial leakage of paracellular tracers. At high cytokine concentrations (10 and 100 ng/ml), this effect was associated with caspase-3/7 activation and apoptotic cell death as evidenced by annexin V staining and DNA fragmentation (TUNEL) assays. In addition, inhibition of JNK and protein kinase C activation at these doses partially prevented activation of caspase-3/7, although only JNK inhibition was partially able to prevent the increase in BEC paracellular permeability induced by cytokines. By contrast, lower cytokine concentrations (1 ng/ml) also led to effector caspase activation, increased paracellular flux, and redistribution of zonula occludens-1 and VE-cadherin but failed to induce apoptosis. Under these conditions, specific caspase-3 and caspase-9, but not caspase-8, inhibitors partially blocked cytokine-induced disruption of tight and adherens junctions and BEC paracellular permeability. Our results suggest that the concentration of cytokines in the CNS endothelial microenvironment determines the extent of caspase-mediated barrier permeability changes, which may be generalized as a result of apoptosis or more subtle as a result of alterations in the organization of junctional complex molecules.