Wound healing is a dynamic process that ultimately leads to restoration of tissue integrity and function. The pineal gland hormone melatonin is known for its anti-oncotic, anti- inflammatory and immuno-modulatory effects. However, its role in wound healing has not been established.
Since melatonin is synthesised endogenously, we primarily sought to investigate whether the melatonin receptors played a role in the wound healing process. Using immunohistochemical methods and Western blot analysis we observed that MT₁ was normally absent in the rat skin but was strongly expressed on day 1 to day 3 post wounding in the epidermis adjacent to the wound edge. MT₁ expression was restricted to the stratum granulosum and stratum spinosum layers of the epidermis in the rat wounds. MT₁ expression declined thereafter and became nonexistent by day 21 when the wound had completely healed. In contrast, MT₂ was constitutively expressed in all the layers of the normal rat epidermis. MT₂ expression gradually decreased at the injury site following wounding but returned to the normal profile by day 21. Aged rat epidermis showed similar MT₁ and MT₂ expression as adult rats. The profile of tissue distribution of MT₁ and MT₂ in the human epidermis was comparable to the rat epidermis. In the CVUs MT₁ and MT₂ localisation profiles resembled that of a healing wound, akin to a day 1 or day 3 rat dermal wound, during the inflammatory phase. Surprisingly, in contrast to all the tissues investigated, MT₁ was also localised in the stratum basale layer of the keloid epidermis. MT₂ localisation in the same keloid tissues however resembled normal human skin profiles.
Secondly, we determined the effects of exogenously administered melatonin, on scarring and wound healing, using a full thickness incisional model of wound healing in rats. Melatonin treatment significantly improved the quality of scarring by day 21. However, our findings would have been strengthened by a more explicit wound closure analysis, measurement of granulation tissue weight, tensile strength, hydroxyl proline content and immunohistochemical assessments of neutrophil infiltration, macrophages, fibroblasts, myofibroblasts and reepithelialization. The treatment also accelerated the angiogenic process and enhanced the VEGF protein profile. Arginase generates proline, the building block for collagen synthesis. Melatonin treatment increased arginase activity and consequently would increase collagen synthesis from day 1. An increase in NOS activity and therefore NO production is known to be detrimental during inflammation. However, various studies have also shown that the NO is essential for granulation tissue formation. Melatonin treatment significantly decreased iNOS activity during the acute inflammatory phase in this study, but significantly increased iNOS activity during the resolving phase. Other markers of inflammatory response and repair were also examined in this study. COX-2 has been shown to play an anti-inflammatory role and melatonin increased COX-2 activity and protein following wounding. SOD (the antioxidant enzyme) activity was also significantly increased during the chronic inflammatory phase on melatonin administration. HO-1 and HO-2 isoforms have also been previously demonstrated to participate in the repair process. Melatonin treatment increased up-regulation of both HO-1 and HO-2 protein expression in the wounded skin. A significant decrease in all the mitochondrial enzyme activities (except complex-II-III), was observed post wounding. Melatonin treatment restored the complex activities to near normal levels. Melatonin also protected mitochondrial membrane integrity and reduced oxidative stress as evidenced by the maintained level of aconitase and citrate synthase activities at near normal levels.
In vitro experiments using macrophage and fibroblast cell lines illustrated that melatonin may decrease NOS activity and protein profiles indirectly by stimulating arginase activity and thereby depleting the availability of arginine.
This study is the first to fully demonstrate the distribution of melatonin receptors in normal and abnormal wounds. Improvement in the quality of scarring in a rat model of wound healing on melatonin administration is promising but much more quantitative work and preclinical studies are required before melatonin advances into clinical assessment.
| Identifer | oai:union.ndltd.org:ADTP/208073 |
| Date | January 2008 |
| Creators | Pugazhenthi, Kamali, n/a |
| Publisher | University of Otago. Department of Pharmacology & Toxicology |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Kamali Pugazhenthi |
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