Serum-derived factors weaken the barrier properties of cultured porcine brain capillary endothelial cells in vitro

Cultured cerebral capillary endothelial cells are often used as a functional in vitro model of the blood–brain barrier (BBB) to determine drug uptake or to study barrier properties. Usually serum is supplemented to these cultures for cell proliferation. Here, we demonstrate the effect of serum and the serum-derived factors lysophosphatidic acid (LPA) and vascular endothelial growth factor (VEGF) on the barrier properties of cultured porcine brain capillary endothelial cells (PBCEC). Serum prevents tight junction formation of confluent PBCEC monolayers and moreover, opens already established tight junctions shown by decreasing transendothelial electrical resistances (TER). These effects are highly polarised with serum almost exclusively acting from the basolateral side of the cell culture. Immunocytochemistry of PBCEC revealed a delocalisation of the cell border lining tight junction proteins ZO-1, occludin and claudin-5 when serum was added. A serum fraction of 67 kDa was isolated by size-exclusion chromatography, identified as albumin and found to cause a serum-like decrease of the TER. However, fatty acid-free serum albumin does not develop this barrier weakening effect, indicating that small protein-bound factors might be responsible. For instance, serum-bound LPA demonstrated a TER-decreasing effect as well, but in contrast to serum mainly when added to the apical side of PBCEC. Addition of VEGF caused a serum-like decrease of the TER with the same polar effect; however, VEGF will be denatured by heat and could thus not be the heat-sensitive factor. Thus, we hypothesise that serum contains a variety of factors which weaken the tightness of a PBCEC monolayer from the apical side as expected but also from the basolateral side. Although the structure of the 67 kDa factor could not be analysed, this finding is of importance for in vitro models not only of the blood–brain barrier mostly using serum-containing media.