Regulation of endothelial thrombomodulin expression by inflammatory cytokines is mediated by activation of nuclear factor-kappa B.

Inflammation and thrombosis are increasingly recognized as interrelated biologic processes. Endothelial cell expression of thrombomodulin (TM), a key component of the anticoagulant protein C pathway, is potently inhibited by inflammatory cytokines. Because the mechanism underlying this effect is largely unknown, we investigated a potential role for the inflammatory transcription factor nuclear factor-kappa B (NF-kappaB). Blocking NF-kappaB activation effectively prevented cytokine-induced down-regulation of TM, both in vitro and in a mouse model of tumor necrosis factor-alpha (TNF-alpha)-mediated lung injury. Although the TM promoter lacks a classic NF-kappaB consensus site, it does contain tandem Ets transcription factor binding sites previously shown to be important for both constitutive TM gene expression and cytokine-induced repression. Using electrophoretic mobility shift assay and chromatin immunoprecipitation, we found that multiple Ets species bind to the TNF-alpha response element within the TM promoter. Although cytokine exposure did not alter Ets factor binding, it did reduce binding of p300, a coactivator required by Ets for full transcriptional activity. Overexpression of p300 also prevented TM repression by cytokines. We conclude that NF-kappaB is a critical mediator of TM repression by cytokines. Further evidence suggests a mechanism involving competition by NF-kappaB for limited pools of the transcriptional coactivator p300 necessary for TM gene expression.

Early loss of thrombomodulin expression impairs vein graft thromboresistance: implications for vein graft failure.

Thrombosis is the major cause of early vein graft failure. Our aim was to determine whether alterations in the expression of the anticoagulant proteins, thrombomodulin (TM) and the endothelial cell protein C receptor (EPCR), impair endothelial thromboresistance that may contribute to vein graft failure. Immunohistochemical staining of autologous rabbit vein graft sections revealed that the expression of TM, but not EPCR, was reduced significantly early after graft implantation. Western blot analysis revealed that TM expression was reduced by >95% during the first 2 weeks after implantation, with gradual but incomplete recovery by 42 days. This resulted in up to a 95% reduction in the capacity of the grafts to activate protein C and was associated with an increase in bound thrombin activity, which peaked on day 7 at 28.7 +/- 3.8 mU/cm(2) and remained elevated for more than 14 days. Restoration of TM expression using adenovirus vector-mediated gene transfer significantly enhanced the capacity of grafts to activate protein C and reduced bound thrombin activity on day 7 to levels comparable to that of normal veins (5.7 +/- 0.4 versus 5.2 +/- 1.1 mU/cm(2), respectively, P=0.74). Surprisingly, neointima formation was not affected by this inhibition of local thrombin activity. These data suggest that the early loss of TM expression significantly impairs vein graft thromboresistance and results in enhanced local thrombin generation. Although enhanced local thrombin generation may predispose to early vein graft failure due to thrombosis, it does not seem to contribute significantly to late vein graft failure due to neointimal hyperplasia.