Induction of the Blood/Brain‐Barrier‐Associated Enzyme Alkaline Phosphatase in Endothelial Cells from Cerebral Capillaries is Mediated Via cAMP

Abstract

The mammalian blood/brain barrier is located at the endothelial cells of the cerebral capillaries. Alkaline phosphatase is associated to a very large extent with these cells and has been established as a marker enzyme for a differentiated blood/brain barrier phenotype in vivo and in vitro. Nevertheless cultured brain capillary endothelial cells (BCEC) lose this marker enzyme because of a cessation of de novo synthesis. Since astrocytes have been shown to possess the capability to re-induce the enzymic activity of alkaline phosphatase in BCEC in vitro we were interested in the second messengers involved in the signal-transduction mechanism of this induction in BCEC. For this reason we treated cultured porcine BCEC with a water-soluble and membrane-permeable analogue of cAMP, 8-(4-chlorophenylthio)-cAMP (C1PhS-cAMP) in the absence of astrocytes. By means of enzymic activity assays we were able to show that within three days the activity of alkaline phosphatase increased up to sixfold compared with the controls. The total activity of alkaline phosphatase in C1PhS-cAMP-treated BCEC was comparable to that of freshly isolated cells. Addition of cycloheximide inhibited the alkaline phosphatase activity increase. We conclude that cAMP is one of the second messengers involved in the induction of alkaline phosphatase activity in BCEC in vitro.