Despite long-standing knowledge about the relationship between thrombosis and atherosclerosis, the specific role of thrombin in modulating atherosclerosis and the response to vascular injury is not well understood. Thrombin receptor stimulation in vitro signals many cellular events that are associated with the response to vascular injury (atherosclerosis) in vivo. Proliferation of smooth muscle cells (SMCs) is an important component of the response to vascular injury. We have previously shown that human alpha-thrombin and the 14-amino acid human thrombin receptor-activating peptide (huTRAP-14) stimulate proliferation of cultured rat aortic SMCs. However, thrombin-induced SMC proliferation demonstrates delayed kinetics relative to platelet-derived growth factor (PDGF-BB, another potent SMC mitogen). Several mechanisms may be responsible for these delayed kinetics in vitro, including production of necessary secondary growth factors and thrombin-induced upregulation of its receptor. In vivo studies have demonstrated that thrombin inhibition limits the response to vascular injury in a hypercholesterolemic rabbit model of focal femoral atherosclerosis. However, this effect does not appear to be mediated by effects on early SMC proliferation. In this discussion, we will address the mechanisms of thrombin-induced SMC proliferation in vitro and apply this knowledge to our understanding of the role of thrombin inhibition in limiting the response to vascular injury in vivo.

The potent growth factors and chemoattractants α-thrombin and transforming growth factor-β₁ (TGF-β₁) have both been identified at sites of arterial injury, however the interaction between these two factors has not been defined. By Northern hybridization analyses, accumulation of both a 1.9- and a 2.4-kb transcript of TGF-β₁ were detected and occurred in a time- and dose-dependent fashion following α-thrombin stimulation of cultured vascular smooth muscle cells (VSMC). This induction of TGF-β₁ mRNA required the proteolytic activity of thrombin and was mimicked by a thrombin-receptor-(TR)-activating peptide or TRAP (SFFLRNP). Increases in α-thrombin-induced TGF-β₁ message expression were insensitive to cycloheximide, but sensitive to actinomycin D. Furthermore, the induction of TGF-β₁. mRNA expression correlated with the production of latent TGF-β₁ protein in α-thrombin-conditioned media. In summary, α-thrombin stimulation of VSMC induces transcriptional activation of the TGF-β₁ gene through proteolytic activation of the cloned seven-transmembrane TR resulting in the formation of latent TGF-β₁ protein. These results demonstrate a potential mechanism whereby α-thrombin may modulate the vascular response to injury through TGF-β₁-dependent mechanisms.

Thrombin has been implicated in vascular smooth muscle cell (VSMC) proliferation after vessel injury. Its proliferative effects, which are mediated via proteolytic activation of a receptor similar or identical to the cloned thrombin receptor (TR), have markedly delayed kinetics. The present study demonstrates that, despite rapid thrombin receptor activation and similar time to S phase entry compared with classic polypeptide growth factors, prolonged thrombin exposure is required to promote maximal VSMC mitogenesis. Flow cytometric analysis of thrombin-stimulated cells revealed that thrombin induced a progressive increase in the growth fraction over 3 days in culture, an effect that was blocked by hirudin even late after thrombin addition. Northern blot hybridization after thrombin stimulation demonstrated that thrombin upregulates TR mRNA expression within 6 h. These findings indicate that VSMC proliferate in response to prolonged thrombin exposure and suggest that the mitogenic delay may involve not only the thrombin-dependent synthesis and activation of newly made TR but also the progressive thrombin-dependent recruitment of cells into the growth fraction.

Background-A 2-hour infusion of r-hirudin at the time of balloon angioplasty limits restenosis in atherosclerotic rabbits.Because thrombin activity in the vessel wall after angioplasty remains high for 48 to 72 hours, we hypothesized that a second infusion of hirudin at 24 hours would reduce restenosis more than early treatment alone. Methods and Results-Femoral atherosclerosis was induced in 35 rabbits by air desiccation injury and a high-cholesterol diet. At the time of angioplasty, rabbits were randomly assigned to 1 of 4 groups: controls: heparin bolus, saline infusion at 24 hours; early hirudin: hirudin bolusϩ2 hours' infusion, saline infusion at 24 hours; delayed hirudin: heparin bolus, hirudin infusionϮbolus at 24 hours; and earlyϩdelayed hirudin: hirudin bolusϩ2 hours' infusion, hirudin infusionϮbolus at 24 hours. Rabbits were euthanized after 28 days. The earlyϩdelayed hirudin treatment group had less loss of minimal lumen diameter by angiography at 28 days. By histomorphometry, cross-sectional area narrowing by plaque was least in the earlyϩdelayed treatment group compared with controls (Pϭ0.0001), early hirudin (Pϭ0.01), or delayed hirudin (Pϭ0.001). The earlyϩdelayed hirudin group also had a significant reduction in absolute plaque area and an improvement in lumen area compared with the other groups. No differences were observed between treatment groups with respect to the cross-sectional area encompassed by the internal or external elastic laminae. Conclusions-Combined earlyϩdelayed administration of hirudin significantly reduces angiographic restenosis and cross-sectional area narrowing by plaque compared with early or late treatment alone. These results suggest that restenosis after balloon angioplasty is markedly influenced by thrombin-mediated events not only occurring early but also extending beyond the first 24 hours in this model. (Circulation. 1998;98:2301-2306.)