Oxidative stress in wound healing –  cur­rent knowledge

Summary

Wound heal­­ing is a complex process based on a subtle coordination of bio­chemical and physiological interactions. Heal­­ing process itself and its quality are af­fected by numerous factors, both local (type, size, depth, and localization of the wound, bacterial contamination, microcirculation, oxygen supply, etc.) and systemic (age, comorbidities, smoking, nutritional status, etc.). Many studies, us­­ing various methodological approaches, focus on wound heal­­ing process at various levels. It is well known that reactive oxygen and nitrogen species play an important role in all phases of wound healing. Regardless increas­­ing knowledge about the role of oxidative stress in wound heal­­ing proces­s, the conclusions of research in this area are still rather contradictory. Therefore, aim of this paper is to sum­marize cur­rent knowledge about the role of oxidative stress in wound heal­­ing proces­s.

Keywords:

Wound healing – reactive oxygen species – reactive nitrogen species – oxidative stress

Introduction

Wound heal­­ing is a complex process based on a subtle coordination of bio­chemical and physiological interactions. Heal­­ing process in the wound starts by a tis­sue damage and is finished when a functional scar is formed. Heal­­ing process itself and its quality are af­fected at various levels by numerous factors, both local and systemic. Local factors include type, size, depth, and locaization of the wound, then also bacterial contamination, microcirculation, oxygen supply, etc. Systemic factors are age, comorbidities, nutritional status, and others. Chronic and non-heal­­ing wounds (e. g. pres­sure ulcers, diabetic ulcerations) still represent a major concern not only for patient and his fami­ly, but also for public health system due to a steadily grow­­ing is­sue of socioeconomic cost. Specific group of patients at increased risk for development of pres­sure ulcers and other dermatological complications are those after spinal cord injury [1,2]. Therefore, any therapeutic approach potentiat­­ing or accelerat­­ing wound heal­­ing process at any level is considered beneficial. Several models have been used to evaluate wound heal­­ing process from macroscopic down to molecular level, us­­ing various experimental approaches from in silico (computational model to understand wound heal­­ing theoretical­ly), in vitro (explain­­ing pathogenesis of wound healing), ex vivo (provid­­ing 3D model of skin explant), and in vivo (us­­ing ani­mal or human model) [3]. At present, one of the main topics in the theoretical research in wound heal­­ing is the role of oxidative stress in various phases of heal­­ing proces­s. It is widely believed that the amount of oxy­gen/ nitrogen radicals might be crucial for further direction of a heal­­ing proces­s. However, number of systematic studies present­­ing detailed insight into reactive oxygen species (ROS) /  nitrogen species (RNS) role in particular phases of wound heal­­ing is still limited. Aim of this article is to sum­marize in detail present knowledge about parameters of oxidative stress in particular phases of wound heal­­ing in order to provide an integrated, synthesized overview of the cur­rent knowledge.

Reactive oxygen and nitrogen species in wound healing –  a general view

Wound heal­­ing is one of the most complex bio­logical proces­ses. It involves the spatial and temporal synchronization of a variety of cell types with distinct roles in the phases of haemostasis, inflam­mation, growth, re-epithelialization, and remodel­ling [4,5]. The first phase “haemostasis” prevents exces­sive blood los­s; it triggers events that lead to local inflam­mation by neutrophils and then macrophages. The inflam­mation is fol­lowed by the performance of local tis­sue cel­ls, keratinocytes and fibroblasts. The former cel­ls first migrate into the injured area for the primary coverage and start to proliferate to recover the stratification. The latter transform to the myofibroblasts that are capable of produc­­ing extracel­lular matrix and of tis­sue contraction. Both cell migration of keratinocytes and fibroblasts-myofibroblasts conversion largely depend on the activity of a potent growth factor, transform­­ing growth factor β (TGFβ), although a set of growth factors are believed to orchestrate the whole process of tis­sue repair [6]. Changes in the microenvironment, includ­­ing alterations in mechanical forces, oxygen levels, chemokines, extracel­lular matrix, and growth factor synthesis directly af­fect cel­lular recruitment and activation, lead­­ing to impaired states of wound healing. Impaired wound healing, in turn, may lead to post-surgical complications frequently observed in elderly patients, chronic ulcers in diabetic patients, hindered and inef­fective pain management, etc. [7]. The mechanism of delayed wound heal­­ing has multifactorial causes, includ­­ing a prolonged inflam­matory stage, postponed proliferation and remodel­l­­ing stages. It has been reported that nuclear factor kappa B (NF-κB) regulates the gene expres­sion of several cytokines, such as interleukin-1beta, interleukin-6, tumor necrosis factor-alpha, and interleukin-10; inducible nitric oxide synthase (iNOS); chemotactic and matrix proteins; im­munological responses; and cell proliferation [8]. NF-κB can contribute to inflam­mation and fibroblast function, which are neces­sary components of incision and wound healing [9]. It has been shown that inhibition of these signal transduction pathways may provide novel strategies to prevent sepsis but may interfere with healing. The persistence of the inflam­matory reaction is as­sociated with oxidative stres­s, which is one of the most com­mon reasons for the delayed wound healing [10]. The increased production of free radicals and decreased antioxidant activities of enzymes, such as superoxide dismutase, glutathione peroxi­dase, heme oxygenase-1, and heme oxygenase-2 may aggravate the situation lead­­ing to a delay in diabetic wound healing [11]. All these events indicate a pivotal role of ROS in the orchestration of the normal wound heal­­ing response. On the other hand, ROS act as secondary mes­sengers to many im­munocytes and non-lymphoid cel­ls involved in the repair process and appear to be important in coordinat­­ing the recruitment of lymphoid cel­ls to the wound site and ef­fective tis­sue repair. ROS also pos­sess the ability to regulate the formation of blood ves­sels (angiogenesis) at the wound site and the optimal perfusion of blood into the wound-heal­­ing area [8]. ROS act in the host‘s defence through phagocytes that induce a ROS burst onto the pathogens present in wounds, lead­­ing to their destruction. Dur­­ing this period, exces­sive ROS leakage into the sur­round­­ing environment exhibits further bacteriostatic ef­fects. In light of these important roles of ROS in wound heal­­ing and the continued quest for therapeutic strategies to treat wounds, it is neces­sary to look for ways to manipulate with ROS as a promis­­ing avenue for improv­­ing wound-heal­­ing responses [12]. On the other hand, several applications of ROS in wound heal­­ing have been shown. Cold physical plasmas are particularly ef­fective in promot­­ing wound closure, ir­respective of its aetiology. These partial­ly ionized gases deliver a thera­peutic cocktail of ROS and RNS safely at body temperature and without genotoxic side ef­fects. Specifical­ly, molecular switches govern­­ing redox-mediated tis­sue response, the activation of the nuclear E2-related factor signal­ling, together with antioxidative and im­munomodulatory responses, and the stabilization of the scaf­fold­­ing function and actin network in dermal fibroblasts are emphasized in the light of wound healing [13]. This example shows the inconsistency of published results and the need for further research in the role of ROS in wound healing.

Reactive oxygen species are closely con­nected with nitric oxide and other RNS. There is very close interplay between them –  they can create com­mon forms of free radi­cals; in addition, ROS and RNS are able to partake in the modification of thiol groups, suggest­­ing that the final outcome will be dependent on the concentrations and locations of these molecules [14]. Nitric oxide itself is implicated in cel­lular and molecular events of wound healing, such as vasodilation, angiogenesis, inflam­mation, tis­sue fibrosis, or im­mune responses. Several studies suggested that NO synthesis is es­sential to the uncomplicated cutaneous wound healing. NO production is mediated by iNOS that is regulated independently of intracel­lular calcium elevations. Initial injury is fol­lowed by infiltration of inflam­matory cel­ls, that is, neutrophils and macrophages, fibroblast repopulation and its transformation to myofibroblast, and new ves­sel formation as well as keratinocyte migration and proliferation. The major source of TGFβ in a tis­sue under repair­­ing process is macrophage. Recruitment of macrophage to an injured tis­sue is stimulated by NO. It is therefore hypothesized that NO might af­fect the heal­­ing process of cutaneous injury [15]. The process of wound heal­­ing is completed by action of other molecules. Growth factors such as epidermal growth factor, fibroblast growth factor, TGF-beta1, and vascular endothelial growth factor and several molecules includ­­ing hypoxia-inducible factor-1alpha are involved in the heal­­ing process by stimulat­­ing and activat­­ing cell proliferation via activation of various reactions, such as angiogenesis, reepithelialisation, dif­ferentiation, and production of the extracel­lular matrix [16]. All above mentioned facts indicate often contradictory information about involvement of ROS and RNS in wound healing.

Conclusion

Reactive forms of oxygen and nitrogen –  basic oxidative stress parameters –  play an important role in all phases of wound healing. This “overview” of cur­rently available scientific information of­fers a framework for the exploration of the role of oxidative stress dur­­ing wound heal­­ing proces­s. Despite of grow­­ing attention in the field of oxidative stress research, conclusions of contemporary studies are still contradictory, therefore further intense work is needed to ful­ly understand its role in wound heal­­ing proces­s. Based on the present knowledge, it can be concluded that balanced ROS response will debride and disinfect a tis­sue and stimulate healthy tis­sue turnover; suppres­sed ROS will result in infection and an elevation in ROS will destroy otherwise healthy stromal tis­sue. Understand­­ing and anticipat­­ing the ROS function within a tis­sue will greatly enhance our pos­sibilities to orchestrate the proces­ses of wound healing.

The authors declare they have no potential conflicts of interest concerning drugs, products, or services used in the study.

The Editorial Board declares that the manu­script met the ICMJE “uniform requirements” for biomedical papers.

Accepted for review: 30. 6. 2019

Accepted for print: 11. 7. 2019

Petr Šín, MD, PhD

Department of Burns and Plastic Surgery

University Hospital Brno

Jihlavská 20

625 00 Brno

Czech Republic

e-mail: p.sin@seznam.cz