Cell adhesion molecules (CAM) expressed by vascular endothelium in response to cytokine stimulation play a key role in leukocyte adhesion to endothelium during the inflammatory response. Tripterine, a chemical compound of the Chinese plant Tripterygium wilfordii Hook f, displays anti-inflammatory properties in several animal models. However, mechanisms of its action are poorly understood. In the present study, we show that in inflammatory conditions, mimicked by tumor necrosis factor alpha (TNF-alpha) stimulation, pretreatment for 6 h with tripterine at nontoxic concentrations of 20-200 nM inhibits the expression of E-selectin, vascular cell adhesion molecule (CAM)-1 (VCAM-1), and intercellular adhesion molecule-1 (ICAM-1) in human umbilical vein endothelial cells (HUVEC) in a dose-dependent manner. Tripterine (200 nM) almost completely inhibits expression of VCAM-1 [50% inhibitory concentration (IC50) = 52 nM] and ICAM-1 (IC50 = 51 nM) and 73% of E-selectin (IC50 = 94 nM). This inhibition effect is prominent, compared with that of dexamethasone, ibuprofen, methotrexate, or probucol, which revealed a much weaker inhibition at doses as high as 1 mM. Effects on endothelial CAM of other proinflammatory cytokines, such as interleukin-1beta and interferon-gamma, were also inhibited significantly by tripterine. Moreover, significant inhibition was equally observable in postincubation experiments. In addition, tripterine inhibited adhesion of human monocytes and T lymphocytes to TNF-alpha-stimulated HUVEC. Finally, tripterine inhibited TNF-alpha-driven CAM mRNA transcription and nuclear factor-kappaB nuclear (NF-kappaB) translocation. Hence, we describe a new mechanism of tripterine's anti-inflammatory action obtained at nanomolar concentrations, owing to the negative regulation of cytokine-induced adhesion molecule expression and adhesiveness in human endothelium.
Expression of the inducible isoform of nitric oxide synthase (iNOS) is stimulated by cytokines in human epithelial cells. This work indicates that incubation of human umbilical cord endothelial cells with combinations of interleukin-1beta, tumor necrosis factor alpha, and interferon-gamma stimulated the synthesis of iNOS mRNA, as detected by reverse transcriptase-polymerase chain reaction. It is important to note that 50, 100, and 200 microM hydrogen peroxide was able to stimulate iNOS directly. Furthermore, 100 microM H2O2 enhanced synthesis of the oxidation products, nitrite (NO2-) and nitrate (NO3-) at 12 and 36 h. iNOS protein, detected by Western blot analysis, as well as L-citrulline levels, were also increased. When endothelial cell monolayers were incubated for 1 h with 100 microM H2O2 and subsequently with cytokines, iNOS mRNA was further augmented. Under the same conditions, we regularly observed an inhibition (25%) of intercellular adhesion molecule-1 (ICAM-1/CD54) expression. The latter was reversed when the NOS inhibitor N(G)-monomethyl-L-arginine was added, as shown by flow cytometry. These data suggest a specific effect of endogenous hydroperoxides on the biosynthesis and processing of the human endothelial iNOS isoform. We propose that H2O2 induces a temporary NO-dependent modulation of adhesion molecule expression to limit the tissue destruction that accompanies the vascular recruitment of leukocytes.
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