Signal transduction and glial cell modulation of cultured brain microvessel endothelial cell tight junctions

Noncontact coculture of postconfluent bovine brain microvessel endothelial cell (BMEC) monolayers with rat C6 glioma cells results in markedly decreased transmonolayer permeability measured by transendothelial electrical resistance (TER) and solute flux. An elevation in adenosine 3',5'-cyclic monophosphate (cAMP) in response to forskolin correlates with an increase in TER through a threshold event; however, unlike forskolin, the severalfold increase in TER induced by C6 cells is cAMP independent. Activation of protein kinase C (PKC) enhances the C6 cell-induced increase in TER, and PKC inhibition blocks C6 cell induction. Treatment of control or C6 cell-induced BMEC monolayers with pertussis toxin immediately and irreversibly obliterates TER, without an apparent change in guanosine 3',5'-cyclic monophosphate levels or viability, indicating the importance of a G protein-mediated event. A similar effect is observed with transforming growth factor-beta 1, and both responses are polarized, since the loss in TER is significantly faster in BMEC monolayers exposed basolaterally. These results suggest that astroglia modulate the blood-brain barrier through a cAMP-independent PKC-dependent pathway maintained by a G protein-coupled mechanism.