The plasminogen activator (PA) system has been shown to be intimately involved in wound healing. However, the role of this system in the initiation and resolution of inflammation during healing process remained to be determined. The aims of this thesis were to investigate the molecular mechanism underlying the interaction between the PA system and the inflammatory system during wound healing and to explore the therapeutic potential of plasminogen in various wound-healing models.
The role of plasminogen in the inflammatory phase of the healing process of acute and diabetic wounds was studied first. Our data showed that administration of additional plasminogen to wild-type mice accelerates the healing of acute wounds. After injury, both endogenous and exogenous plasminogen are bound to inflammatory cells and are transported to the wound site, which leads to activation of inflammatory cells. In diabetic db/db mice, wound-specific accumulation of plasminogen does not take place and the inflammatory response is impaired. However, when additional plasminogen is injected, plasminogen accumulates in the wound, the inflammatory response is enhanced, the signal transduction cascade is activated and the healing rate is significantly increased. These results indicate that administration of plasminogen may be a novel therapeutic strategy to treat different types of wounds, especially chronic wounds in diabetes.
The role of plasminogen at the later stage of wound healing was also studied in plasminogen-deficient mice. Our data showed that even if re-epithelialization is achieved in these mice, a prolonged inflammatory phase with abundant neutrophil accumulation and persistent fibrin deposition is observed at the wound site. These results indicate that plasminogen is also essential for the later phases of wound healing by clearing fibrin and resolving inflammation.
The functional role of two physiological PAs during wound healing was further studied in a tympanic membrane (TM) wound-healing model. Our data showed that the healing process was clearly delayed in urokinase-type PA (uPA)-deficient mice but not in tissue-type PA (tPA)-deficient mice. Less pronounced keratinocyte migration, abundant neutrophil accumulation and persistent fibrin deposition were observed in uPA-deficient mice. These results indicate that uPA plays a central role in the generation of plasmin during the healing of TM perforations.
Finally the therapeutic potential of plasminogen in the TM wound-healing model was studied. Our data showed that local injection of plasminogen restores the ability to heal TM perforations in plasminogen-deficient mice in a dose-dependent manner. Plasminogen supplementation also potentiates healing of acute TM perforations in wild-type mice, independent of the administration method used. A single local injection of plasminogen in plasminogen-deficient mice can initiate healing of chronic TM perforations resulting in a closed TM with a continuous but rather thick outer keratinocyte layer. Three plasminogen injections lead to a completely healed TM with a thin keratinizing squamous epithelium covering a connective tissue layer that can start to reorganize and further mature to its normal appearance. In conclusion, our results suggest that plasminogen is a promising drug candidate for the treatment of chronic TM perforations in humans.
Taken together, our data indicate that plasminogen is a novel inflammatory regulator that promotes wound healing.