Tripterine prevents endothelial barrier dysfunction by inhibiting endogenous peroxynitrite formation

Background and purpose: Tripterine is an inhibitor of heat shock protein 90 and an active component of Tripterygium wilfordii Hook F., which is used in traditional Chinese medicine to treat inflammatory diseases such as rheumatoid arthritis. We hypothesized that tripterine inhibits endogenous peroxynitrite formation and thereby prevents endothelial barrier dysfunction. Experimental approach: Effects of tripterine were investigated on endothelial barrier function, inducible nitric oxide synthase (iNOS) expression, nicotinamide adenine dinucleotide phasphate (NADPH) oxidase activity, 3-nitrotyrosine formation, protein phosphatase type 2A (PP2A) activity, activation of extracellular-regulated kinase (ERK), c-Jun terminal kinase (JNK) and Janus kinase (Jak2), and degradation of IkB in microvascular endothelial cells exposed to pro-inflammatory stimulus [lipopolysaccharide (LPS) + interferon g (IFNg)] and on vascular permeability in air pouches of mice injected with LPS + IFNg. Key results: LPS + IFNg caused an increase in monolayer permeability, induction of iNOS and NADPH oxidase type 1 (Nox1) proteins, formation of superoxide, nitric oxide and 3-nitrotyrosine, and increase in PP2A activity in endothelial cells. These effects of LPS + IFNg were diminished by tripterine (50-200 nM). Further, LPS + IFNg-induced expression of iNOS and Nox1 was attenuated by the mitogen-activated protein kinase kinase 1/2 (MEK1/2) inhibitor PD98059, the JNK inhibitor SP600125, the Jak2 inhibitor AG490 and the NFkB inhibitor MG132, but not by the p38 mitogen-activated protein kinase inhibitor SB203580. LPS + IFNg stimulated phosphorylation of ERK, JNK and Jak2, and degradation of IkB, but only Jak2 phosphorylation was sensitive to tripterine (50-200 nM). Further, tripterine diminished the increased vascular permeability in inflamed air pouches. Conclusion and implications: Our results indicate that, by preventing Jak2-dependent induction of iNOS and Nox1, tripterine inhibits peroxynitrite precursor synthesis, attenuates the increased activity of PP2A and consequently protects endothelial barrier function.