Summary
The vast majority of thrombin (>95%) is generated after clotting is completed, suggesting that thrombin formation serves purposes beyond
coagulation, such as tissue repair after vessel injury. Two types of vascular thrombin
binding sites exist: protease-activated receptors (PARs) and thrombomodulin (TM).
Their expression is low in contractile vascular smooth muscle cells (SMC), the dominating
subendothelial cell population, but becomes markedly up-regulated upon injury. In
human SMC, PAR-1, PAR-3, and PAR-4 mediate thrombin-induced proliferation, migration
and matrix biosynthesis as well as generation of inflammatory and growth-promoting
mediators. Thrombin-responsive PARs are transcriptionally down-regulated in human
vascular SMC by vasodilatory prostaglandins (PGI2/PGE2). For PAR-1 and PAR-3 this mechanism involves cAMP-dependent inactivation of the
transcription factor NFAT. The human PAR-4 promoter does not possess NFAT recognition
motifs suggesting involvement of other cAMP-regulated effectors. Unlike PARs, TM is
induced in SMC exposed to vasodilatory pros-taglandins. Enhanced thrombin binding
to TM might ameliorate PAR-mediated SMC stimulation. Also expressed in human SMC is
the endothelial protein C receptor (EPCR), which serves as an anchor to facilitate
generation of activated protein C (aPC) by TM-bound thrombin. Whether prostaglandins
affect aPC-generation is not known. In SMC, thrombin and aPC act synergistically via
PAR-1 to modify tissue remodelling, in contrast to their antagonistic interaction
in the coagulation pathways. Overall, this will contribute to plaque stability and
wound healing. The processes outlined here are likely to become clinically relevant
after up-regulation of vascular cyclooxygenase2, the rate limiting step in vascular
PGE2/PGI2 biosynthesis, such as in advanced atherosclerosis and acute coronary syndromes.
Keywords
Prostaglandins - thrombin - thrombomodulin - PAR - smooth muscle cell