Tumor Necrosis Factor-Mediated Effects on Gamma-Glutamyl Transpeptidase Activity in Cloned Cerebral Endothelial Cells

Gamma-glutamyl transpeptidase (γ-GTP), a marker enzyme for blood brain barrier (BBB) function, was induced consistently by TNF (500 U/mL) in cerebral endothelial cells (cEC) compared with control cultures. The enzyme activity was found to be considerably lower in pulmonary artery cells, where, when treated with TNF (500 U/mL), the induction was not observed. Pre-incubation with zinc (1 μg/mL)-enriched media for 48 hours did not block TNF-mediated activation of γ-GTP. The increase in γ-GTP activity does not appear to be mediated through the cAMP signal transduction pathway as evidenced by the lack of TNF-mediated intracellular accumulation of cAMP. In addition, forskolin failed to influence γ-GTP enzyme activity, which further suggests that cAMP is not involved. Phospholipase A₂, an enzyme involved in the arachidonic acid pathway, appears to be implicated in TNF signalling across the membrane of cEC. Two inhibitors of phospholipase A₂, mepacrine and p-bromophenacyl-bromide (BPB), depressed the activity of γ-GTP in the presence of TNF. These data suggest that an important relationship exists between γ-GTP activity and TNF-induced dysfunction of the cerebral endothelium. This may have implications in understanding cytokine-mediated alterations in BBB function.