Thrombomodulin and the vascular endothelium: insights into functional, regulatory, and therapeutic aspects

Abstract

Thrombomodulin (TM) is a 557-amino acid protein with a broad cell and tissue distribution consistent with its wide-ranging physiological roles. When expressed on the lumenal surface of vascular endothelial cells in both large vessels and capillaries, its primary function is to mediate endothelial thromboresistance. The complete integral membrane-bound protein form displays five distinct functional domains, although shorter soluble (functional) variants comprising the extracellular domains have also been reported in fluids such as serum and urine. TM-mediated binding of thrombin is known to enhance the specificity of the latter serine protease toward both protein C and thrombin activatable fibrinolysis inhibitor (TAFI), increasing their proteolytic activation rate by almost three orders of magnitude with concomitant anticoagulant, antifibrinolytic, and anti-inflammatory benefits to the vascular wall. Recent years have seen an abundance of research into the cellular mechanisms governing endothelial TM production, processing, and regulation (including flow-mediated mechanoregulation)--from transcriptional and posttranscriptional (miRNA) regulation of TM gene expression, to posttranslational processing and release of the expressed protein--facilitating greater exploitation of its therapeutic potential. The goal of the present paper is to comprehensively review the endothelial/TM system from these regulatory perspectives and draw some fresh conclusions. This paper will conclude with a timely examination of the current status of TM's growing therapeutic appeal, from novel strategies to improve the clinical efficacy of recombinant TM analogs for resolution of vascular disorders such as disseminated intravascular coagulation (DIC), to an examination of the complex pleiotropic relationship between statin treatment and TM expression.

Keywords: cyclic strain; endothelium; miRNA; therapeutic; thrombomodulin.

Fig. 1.

Predicted response elements within the 5′-flanking promoter region of the thrombomodulin gene. Consensus motifs and corresponding response element are highlighted. Key: KLF-2, Kruppel-like factor-2; HIF-1, hypoxia inducible factor-1; ETS-1, v-Ets E26 erythoblastosis; AP-1, activating protein-1; CRE, cAMP response element; SP-1, specificity protein-1; GRE, glucocorticoid response element; Sequence shown is for the rat thrombomodulin promoter (GenBank AF022742.1) Promoter, 1–708; ATG start site, 900–902.

Fig. 2.

Impact of equibiaxial cyclic strain on thrombomodulin (TM) expression and release in human aortic endothelial cells (HAECs). Cyclic strain was applied to HAECs for 24 h at 60 cycles/min (cardiac waveform) by Flexercell TensionPlus FX-4000T system (Flexercell International). Cells and media were harvested and monitored for TM levels by multiplex ELISA using a 4-plex human vascular injury detection panel (MesoScale Discovery). Histograms display effect of cyclic strain on TM levels in media (A) and cells (B). *P ≤ 0.05 vs. low-strain (2.5%) control. Inset above B shows typical ELISA standard curve for TM (electrochemiluminescent signal at 620 nm vs. TM concentration). In response to elevated chronic cyclic strain (12.5%), TM levels are observed to decrease in cells, while elevated levels of TM shedding into media are observed at high strain (F. A. Martin and P. M. Cummins, unpublished observations).