Intracellular signaling of tumor necrosis factor-alpha in brain microvascular endothelial cells is mediated by a protein tyrosine kinase and protein kinase C-dependent pathway

Blood-Brain Barrier Dysfunction Amplifies the Development of Neuroinflammation: Understanding of Cellular Events in Brain Microvascular Endothelial Cells for Prevention and Treatment of BBB Dysfunction

Neuroinflammation is involved in the onset or progression of various neurodegenerative diseases. Initiation of neuroinflammation is triggered by endogenous substances (damage-associated molecular patterns) and/or exogenous pathogens. Activation of glial cells (microglia and astrocytes) is widely recognized as a hallmark of neuroinflammation and triggers the release of proinflammatory cytokines, leading to neurotoxicity and neuronal dysfunction. Another feature associated with neuroinflammatory diseases is impairment of the blood-brain barrier (BBB). The BBB, which is composed of brain endothelial cells connected by tight junctions, maintains brain homeostasis and protects neurons. Impairment of this barrier allows trafficking of immune cells or plasma proteins into the brain parenchyma and subsequent inflammatory processes in the brain. Besides neurons, activated glial cells also affect BBB integrity. Therefore, BBB dysfunction can amplify neuroinflammation and act as a key process in the development of neuroinflammation. BBB integrity is determined by the integration of multiple signaling pathways within brain endothelial cells through intercellular communication between brain endothelial cells and brain perivascular cells (pericytes, astrocytes, microglia, and oligodendrocytes). For prevention of BBB disruption, both cellular components, such as signaling molecules in brain endothelial cells, and non-cellular components, such as inflammatory mediators released by perivascular cells, should be considered. Thus, understanding of intracellular signaling pathways that disrupt the BBB can provide novel treatments for neurological diseases associated with neuroinflammation. In this review, we discuss current knowledge regarding the underlying mechanisms involved in BBB impairment by inflammatory mediators released by perivascular cells.

Interferon beta-1a downregulates TNFalpha-induced intercellular adhesion molecule 1 expression on brain microvascular endothelial cells through a tyrosine kinase-dependent pathway

TNFalpha (100 U/ml, 24 h) upregulated intercellular adhesion molecule 1 (ICAM1) expression on brain microvascular endothelial cell (BMEC) culture. The tyrosine kinase (TK) inhibitor genestein (100 microgram/ml), the protein kinase C (PKC) inhibitor staurosporin (1 nM), and interferon (IF) beta-1a (1000 U/ml) antagonized TNFalpha effect. When an ineffective dose of IFbeta-1a (100 U/ml) was challenged with ineffective doses of either genestein (10 microgram/ml) or staurosporin (0.1 nM), the combination IFbeta-1a-genestein significantly reduced TNFalpha-induced ICAM1 expression whereas IFbeta-1a-staurosporin did not. These findings indicate that a TK- rather than a PKC-dependent mechanism is involved in the modulation of TNFalpha response by IFbeta-1a on BMECs.

The antiangiogenic factor 16K PRL induces programmed cell death in endothelial cells by caspase activation

We asked whether the antiangiogenic action of 16K human PRL (hPRL), in addition to blocking mitogen-induced vascular endothelial cell proliferation, involved activation of programmed cell death. Treatment with recombinant 16K hPRL increased DNA fragmentation in cultured bovine brain capillary endothelial (BBE) and human umbilical vein endothelial (HUVE) cells in a time- and dose-dependent fashion, independent of the serum concentration. The activation of apoptosis by 16K hPRL was specific for endothelial cells, and the activity of the peptide could be inhibited by heat denaturation, trypsin digestion, and immunoneutralization, but not by treatment with the endotoxin blocker, polymyxin-B. 16K hPRL-induced apoptosis was correlated with the rapid activation of caspases 1 and 3 and was blocked by pharmacological inhibition of caspase activity. Caspase activation was followed by inactivation of two caspase substrates, poly(ADP-ribose) polymerase (PARP) and the inhibitor of caspase-activated deoxyribonuclease (DNase) (ICAD). Furthermore, 16K hPRL increased the conversion of Bcl-X to its proapoptotic form, suggesting that the Bcl-2 protein family may also be involved in 16K hPRL-induced apoptosis. These findings support the hypothesis that the antiangiogenic action of 16K hPRL includes the activation of programmed cell death of vascular endothelial cells.

Inhibition of protein kinase C counteracts TNFalpha-induced intercellular adhesion molecule 1 expression and fluid phase endocytosis on brain microvascular endothelial cells

TNFalpha (100 U/ml, 24 h) upregulated intercellular adhesion molecule 1 (ICAM1) expression and fluid phase endocytosis (FPE) of horseradish peroxidase on brain microvascular endothelial cell (BMEC) culture. The protein kinase C (PKC) inhibitor staurosporin (0. 5-10 nM) antagonized ICAM1 expression and FPE due to TNFalpha, whereas the protein kinase A inhibitor H89 (0.5-10 nM) did not. These findings indicate that a PKC-dependent mechanism may affect TNFalpha signalling on different barrier properties of BMECs.

ICAM 1 expression and fluid phase endocytosis of cultured brain microvascular endothelial cells following exposure to interferon beta-1a and TNFalpha

We have studied the effect of interferon (IF) beta-1a on the basal and TNFalpha-induced intercellular adhesion molecule 1 (ICAM 1) expression and fluid phase endocytosis (FPE) of horseradish peroxidase in cultured rat brain microvascular endothelial cells. Neither basal ICAM 1 expression nor basal FPE were significantly affected by 24-72 h exposure to 1000 U/ml IFbeta-1a. ICAM 1 induction and FPE enhancement caused by 100 U/ml TNFalpha for 24 h was not influenced by simultaneous administration of 1000 U/ml IFbeta-1a. Treatment of cultures with IFbeta-1a for 48 h followed by 24-h coincubation with TNFalpha (100 U/ml) and IFbeta-1a (1000 U/ml) resulted in significant downregulation of TNFalpha-induced ICAM 1 expression and FPE. Downregulation of TNFalpha-induced ICAM 1 expression was not observed when combined treatment with TNFalpha (100 U/ml) and IFbeta-1a (1000 U/ml) for 24 h was followed by 48 h exposure to IFbeta-1a. We concluded that the blood-brain barrier endothelium may be a target of IFbeta-1a. Further, these in vitro findings may correlate with the results of recent clinical trials indicating that chronic treatment of relapsing remitting multiple sclerosis with IFbeta-1a prevents both clinical exacerbations and the appearance on Magnetic Resonance Imaging of new lesions enhanced by gadolinium which is taken up by increased transendothelial fluid phase vesicular transport.