Background:Intact endothelial structure and function are critical for maintaining microcirculatory homeostasis.Dysfunction of the latter is an underlying cause of various organ pathologies.In a previous study,we showed that rhubarb,a traditional Chinese medicine,protected intestinal mucosal microvascular endothelial cells in rats with metastasizing septicemia.In this study,we investigated the effects and mechanisms of rhubarb on matrix metalloproteinase-9 (MMP9)-induced vascular endothelial (VE) permeability.Methods:Rhubarb monomers were extracted and purified by a series of chromatography approaches.The identity of these monomers was analyzed by hydrogen-1 nuclear magnetic resonance (NMR),carbon-13 NMR,and distortionless enhancement by polarization transfer magnetic resonance spectroscopy.We established a human umbilical vein endothelial cell (HUVEC) monolayer on a Transwell insert.We measured the HUVEC permeability,proliferation,and the secretion of VE-cadherin into culture medium using fluorescein isothiocyanate-dextran assay,3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide assay,and enzyme-linked immunosorbent assay,respectively,in response to treatment with MMP9 and/or rhubarb monomers.Results:A total of 21 rhubarb monomers were extracted and identified.MMP9 significantly increased the permeability of the HUVEC monolayer,which was significantly reduced by five individual rhubarb monomer (emodin,3,8-dihydroxy-1-methyl-anthraquinone-2-carboxylic acid,1-O-caffeoyl-2-(4-hydroxyl-O-cinnamoyl)-β-D-glucose,daucosterol linoleate,and rhein) or a combination of all five monomers (1 μmol/L for each monomer).Mechanistically,the five-monomer mixture at 1 μmol/L promoted HUVEC proliferation.In addition,MMP9 stimulated the secretion of VE-cadherin into the culture medium,which was significantly inhibited by the five-monomer mixture.Conclusions:The rhubarb mixture of emodin,3,8-dihydroxy-1-methyl-anthraquinone-2-carboxylic acid,1-O-caffeoyl-2-(4-hydroxyl-O -cinnamoyl)-β-D-glucose,daucosterol linoleate,and rhein,at a low concentration,antagonized the MMP9-induced HUVEC monolayer permeability by promoting HUVEC proliferation and reducing extracellular VE-cadherin concentrations.