The effect of inflammatory cytokines and coagulation factors on Von Willebrand factor synthesis and cleavage
Allers, Werner Ernst
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When injured, endothelial cells secrete inflammatory cytokines such as interleukin-6 (IL-6), interleukin-8 (IL-8), and tumour necrosis factor-α (TNF-α). These inflammatory cytokines stimulate the endothelial release of ultra large Von Willebrand factor (ULVWF) multimers that bind platelets to form thrombi in small vessels. The interaction between thrombosis and inflammation is not fully elucidated. A disintegrin-like metalloprotease with thrombospondin type I repeats-13 (ADAMTS-13) is freshly released from Weibel-Palade bodies of endothelial cells into the plasma and it cleaves the ultra large and hyperactive VWF multimers into smaller and less active forms. These VWF multimers mediate the initial adhesion of activated platelets, the first step in both inflammation and thrombosis. This process may be affected by the amount of ULVWF released and the processing capacity of ADAMTS-13. Little is known about the initial onset of HIV-associated TTP, a fatal thrombotic disease that is characterised by the absence of ADAMTS-13. The mechanisms underlying the initial onset and/or burst of TTP episodes still remain poorly understood. Interrelated components, such as coagulation factors and inflammatory cytokines, all contribute to the development of TTP, since increased levels of cytokines interleukin-6 and tumour necrosis factor and the coagulation factor, tissue factor is measured in these patients. Therefore, we hypothesised that certain inflammatory cytokines and coagulation factors released during inflammation may stimulate the release of VWF simultaneously while inhibiting the synthesis of ADAMTS-13, which results in an acquired deficiency of plasma ADAMTS-13 and ultimately in a TTP episode. Our aim was to examine the effects of inflammatory cytokines and coagulation factors such as tissue factor and thrombin as well as combinations thereof on the release and cleavage of ULVWF by cultured human umbilical vein endothelial cells (HUVECs). HUVECs were treated with cytokines, IL-6, IL-8, and TNF-α and coagulation factors, tissue factor and thrombin, and their combinations, for 24 hours under static conditions. The cells were then exposed to a shear stress of 2.5 dyne/cm2 to expose the VWF cleaving sites. The VWF, VWF propeptide and ADAMTS-13 secretion were measured by an ELISA technique. ADAMTS-13 content was measured using Western blot technology with densitometry. All treatments and their combinations, excluding IL-6, significantly stimulated the release of VWF and VWF propeptide from HUVECs. The VWF propeptide levels were constantly higher than the major VWF protein levels suggesting that the measurement of the VWF propeptide levels may be a better representation of the amount of VWF secreted from endothelial cells. Tissue factor alone and in combination with inflammatory cytokines increase the amount of VWF release from endothelial cells substantially. This correlates with the situation in thrombotic patients with inflammation where extremely high VWF levels are measured. Densitometric analysis of the Western blots indicated that lower levels of ADAMTS-13 secretion were found with all treatments. These results suggest that inflammatory cytokines such as IL-8 and TNF-α, coagulation factors such as thrombin and tissue factor, as well as combinations thereof, stimulate the release of ULVWF and inhibit the release of ADAMTS-13 in HUVECs, resulting in the accumulation of hyperreactive ULVWF in plasma and on the surface of endothelial cells to induce platelet aggregation and adhesion on the vascular endothelium. Our study may offer a logical explanation of how systemic inflammation and thrombosis might trigger the onset and/or burst of TTP in patients with HIV-associated TTP.