Plasminogen activator/plasmin system: a major player in wound healing?

The role of the plasminogen activator/plasmin system in fibrinolysis has been well established. Indeed, clinicians worldwide have successfully utilized recombinant tissue-type plasminogen activator as first-line treatment of acute myocardial infarction for almost 2 decades. Outside the field of cardiology, there has been increasing excitement regarding the possible contribution of this system in many other important biological processes, including cell adhesion, cell migration, cell-cell signaling, tumor invasion and metastasis, ovulation, and wound healing. In this review, we present evidence in the current literature that the plasminogen activator/plasmin system does have a role in wound healing, looking at both normal and abnormal healing. Furthermore, the invaluable insights provided by numerous transgenic animal experiments are summarized.

Differential expression of urokinase-type plasminogen activator and plasminogen activator inhibitor-1 in early and late gestational mouse skin and skin wounds.

Early gestation fetal mouse skin heals without scars. Plasminogen activator inhibitor-1 (PAI-1) has been associated with postnatal organ fibrosis. We hypothesized that the relative balance between urokinase-type plasminogen activator (uPA) and PAI-1 expression in favor of uPA prevents scarring in early fetal skin wounds, whereas a change in favor of PAI-1 in late gestation results in wound scarring. To evaluate uPA and PAI-1 expression, 1-mm skin wounds were made in E14.5 and E18 mice and harvested 24, 48, or 96 hours postwounding. Aprotinin (2 mg/ml)-coated beads were injected into selected E14.5 wounds. Normal skin and skin wounds were evaluated for uPA, PAI-1, and collagen expression. We showed that in normal skin uPA level is higher in E14.5 than in E18 mice, while PAI-1 is lower in E14.5 than in E18 mice. After wounding, E14.5 wounds show a moderate increase in uPA and a minimal increase in PAI-1. E18 wounds show a transient increase in uPA but a significant, sustained increase in PAI-1. Addition of aprotinin to E14.5 wounds causes an increase in collagen deposition. We conclude that the differential expression of uPA and PAI-1 in the skin of early vs. late gestation mice may contribute to the degree of scar formation seen after cutaneous injury.

Increased plasminogen activator inhibitor-1 in keloid fibroblasts may account for their elevated collagen accumulation in fibrin gel cultures.

Proteolytic degradation of the provisional fibrin matrix and subsequent substitution by fibroblast-produced collagen are essential features of injury repair. Immunohistochemical studies revealed that although dermal fibroblasts of normal scars and keloids expressed both urokinase type plasminogen activator (uPA) and plasminogen activator inhibitor-1 (PAI-1), keloid fibroblasts had a much higher PAI-1 expression. In long-term three-dimensional fibrin gel cultures (the in vitro fibroplasia model), normal fibroblasts expressed moderate and modulated activity levels of uPA and PAI-1. In contrast, keloid fibroblasts expressed a persistently high level of PAI-1 and a low level of uPA. The high PAI-1 activity of keloid fibroblasts correlated with their elevated collagen accumulation in fibrin gel cultures. Substituting collagen for fibrin in the gel matrix resulted in increased uPA activity and reduced collagen accumulation of keloid fibroblasts. Furthermore, decreasing PAI-1 activity of keloid fibroblasts in fibrin gel cultures with anti-PAI-1-neutralizing antibodies also resulted in a reduction in collagen accumulation by keloid fibroblasts. Cumulatively, these results suggest that PAI-1 overexpression is a consistent feature of keloid fibroblasts both in vitro and in vivo, and PAI-1 may play a causative role in elevated collagen accumulation of keloid fibroblasts.