Modulation of Ca2+‐dependent intercellular adhesion in bovine aortic and human umbilical vein endothelial cells by heparin‐binding growth factors

Abstract

Cultured endothelial cells have been shown to possess two mechanisms of intercellular adhesion: Ca²⁺‐dependent and Ca²⁺‐independent. We report here that growth of bovine aortic endothelial cells (BAEC) in complete medium containing purified basic fibroblast growth factor (bFGF, 6 ng/ml) results in loss of Ca²⁺‐dependent intercellular adhesion. In the presence of heparin (90 μg/ml), this effect is reproduced upon treatment with acidic fibroblast growth factor (aFGF, 6 ng/ml) or endothelial cell growth supplement (ECGS, 100 μg/ml), in both human umbilical vein endothelial cells (HUVEC) and BAEC. Treatment at these doses with aFGF in the absence of heparin or with heparin alone is without significant effect. Loss of Ca²⁺‐dependent adhesion following treatment of cells with heparin‐binding growth factors (HBGFs) is prevented by pre‐treatment of cell layers with cycloheximide. The Ca²⁺‐independent adhesion mechanism is unaffected by HBGF treatment. Exposure of endothelial cells to HBGFs, moreover, prevents the eventual establishment of quiescence in growing cultures and restimulates replication in confluent cultures that have reached a final density‐inhibited state. Addition of bFGF alone or aFGF + heparin at these doses results in a 4‐fold increase in DNA synthesis over untreated control cultures at saturation density as reflected by thymidine index. A single addition of bFGF (6 ng/ml) to untreated quiescent confluent BAEC monolayers results in an increase in ³H‐TdR incorporation reaching a peak at 22 hours with a parallel loss of Ca²⁺‐dependent adhesiveness. Fluorescent staining with rhodamine‐phalloidin demonstrates an altered distribution of polymerized F‐actin in the bFGF‐treated monolayers, marked by disruption of the dense peripheral microfilament bands retained by untreated confluent monolayers. Together, these results indicate that the mitogenic effect of HBGFs in cultured endothelial cells is associated with a “morphogenic” set of responses, perhaps dependent on breakdown of calcium‐dependent cell‐cell contacts.