Down-regulation of occludin expression in astrocytes by tumour necrosis factor (TNF) is mediated via TNF type-1 receptor and nuclear factor-kappa B activation

Tight junctions form the diffusion barrier of brain microcapillary endothel ial cells and support cell polarity. Also astrocytes express tight junction components such as occludin, claudin-1, ZO-1 and ZO-2, but do not establis h a permeability barrier. However, little is known about the function and r egulation of these molecules in astrocytes. We studied the impact of tumour necrosis factor (TNF) on occludin and ZO-1 expression in astrocytes. TNF d ecreased occludin, but not ZO-1 expression, in brain microcapillary endothe lial cells, as well as in epithelial cells. occludin expression was not inf luenced by TNF. Removal of TNF from astrocytes restored the basal level of occludin. Down-regulation was inhibited by caffeic acid phenethyl ester, a specific inhibitor of nuclear factor-kappaB (NF-kappaB) activation. Exposur e of astrocytes isolated from mice deficient in either TNF type-1 receptor (TNFR1), TNF type-2 receptor (TNFR2), or both, respectively, revealed that downregulation was mediated entirely by TNFR1. ZO-1, which can interact wit h occludin, was found to cc-precipitate connexin43, but not occludin, These findings demonstrate that TNF selectively down-regulates occludin in astro cytes, but not in cells forming established tight junctions, through TNFR1 and suggest that NF-kappaB is involved as a negative regulator.