Regulation and functions of the protein C anticoagulant pathway

The protein C pathway plays a critical role in the negative regulation of t he blood clotting process. We recently identified an endothelial cell recep tor for protein C/activated protein C (APC). The receptor is localized almo st exclusively on endothelial cells of large vessels and Is present at only trace levels or indeed absent from capillaries in most tissues. Patients w ith sepsis or lupus erythematosus exhibit elevated levels of plasma EPCR wh ich migrates on gels as a single band and is fully capable of binding prote in C/APC. There is no correlation with thrombomodulin levels, probably due to different vascular localizations and/or cellular release mechanisms. To understand the mechanisms by which EPCR plasma levels are elevated, we exam ined EPCR mRNA expression in a rat endotoxin shock model. The EPCR mRNA gen e exhibited an early immediate gene response to endotoxin with the mRNA lev els increasing nearly 4 fold in the first 3-6 hrs, before returning toward baseline. Plasma levels of EPCR also rose about 4 fold with little change i n tissue EPCR levers. Both processes were markedly attenuated by hirudin su ggesting that thrombin was responsible for increases in mRNA and plasma EPC R levels. At the level of mRNA, the induction is mediated by a thrombin res ponse element in the 5' flanking region of the gene. Direct thrombin infusi on and cell culture experiments support this contention. On endothelium, th rombin is capable of releasing cell surface EPCR and this process is blocke d by the metalloproteinase inhibitor orthophenanthroline. Taken together th ese studies indicate that elevation in soluble plasma EPCR reflects endothe lial cell activation in the larger vessels and is likely to be an indicatio n of local thrombin generation near these vessel surfaces. (C) 1999, Ferrat a Storti Foundation.