International Journal of Molecular Sciences Review Molecular Changes Underlying Hypertrophic Scarring Following Burns Involve Specific Deregulations at All Wound Healing Stages (Inflammation, Proliferation and Maturation) Matúš Comaˇ 1,2, Lucia Fröhlichová 3, Lukáš Urban 1,4, Robert Zajíˇcek 5, Tomáš Urban 5, Pavol Szabo 6,7 , Štˇepán Novák 6,8, Vitaly Fetissov 5, Barbora Dvoˇránková 6,7, Karel Smetana Jr. 6,7,* and Peter Gál 1,4,5,* 1 Department of Biomedical Research, East-Slovak Institute of Cardiovascular Diseases, Inc., 040 11 Košice, Slovakia; [email protected] (M.C.);ˇ [email protected] (L.U.) 2 Department of Pharmacology, Faculty of Medicine, Pavol Jozef Šafárik University, 040 11 Košice, Slovakia 3 Department of Pathology, Louise Pasteur University Hospital, 041 90 Košice, Slovakia; [email protected] 4 Center of Clinical and Preclinical Research MediPark, Pavol Jozef Šafárik University, 040 11 Košice, Slovakia 5 Prague Burn Centre, Third Faculty of Medicine, Charles University and University Hospital Královské Vinohrady, 100 34 Prague, Czech Republic; [email protected] (R.Z.); [email protected] (T.U.); [email protected] (V.F.) 6 Institute of Anatomy, First Faculty of Medicine, Charles University, 128 00 Prague, Czech Republic; [email protected] (P.S.); [email protected] (Š.N.); [email protected] (B.D.) 7 BIOCEV, 252 50 Vestec, Czech Republic 8 Department of Otorhinolaryngology, Head and Neck Surgery, First Faculty of Medicine, Charles University and University Hospital Motol, 150 06 Prague, Czech Republic * Correspondence: [email protected] (K.S.J.); [email protected] (P.G.); Fax: +421-55-789-1613 (P.G.) Citation: Coma,ˇ M.; Fröhlichová, L.; Urban, L.; Zajíˇcek,R.; Urban, T.; Abstract: Excessive connective tissue accumulation, a hallmark of hypertrophic scaring, results Szabo, P.; Novák, Š.; Fetissov, V.; in progressive deterioration of the structure and function of organs. It can also be seen during Dvoˇránková,B.; Smetana, K., Jr.; et al. Molecular Changes Underlying tumor growth and other fibroproliferative disorders. These processes result from a wide spectrum Hypertrophic Scarring Following of cross-talks between mesenchymal, epithelial and inflammatory/immune cells that have not Burns Involve Specific Deregulations yet been fully understood. In the present review, we aimed to describe the molecular features of at All Wound Healing Stages fibroblasts and their interactions with immune and epithelial cells and extracellular matrix. We also (Inflammation, Proliferation and compared different types of fibroblasts and their roles in skin repair and regeneration following burn Maturation). Int. J. Mol. Sci. 2021, 22, injury. In summary, here we briefly review molecular changes underlying hypertrophic scarring 897. https://doi.org/10.3390/ following burns throughout all basic wound healing stages, i.e. during inflammation, proliferation ijms22020897 and maturation. Received: 8 January 2021 Keywords: wound healing; skin; burn; pathological scar; cell interaction; stem cell Accepted: 14 January 2021 Published: 18 January 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- Each year, over 100 million patients in the developed world form scars as a result of iations. surgery and/or trauma [1]. Hypotrophic, atrophic (e.g., acne scars) and linear normotrophic maturated scars have little to no effect on the patient’s quality of life (Figure1)[ 2]. However, pruritus, pain and contractures often related to extreme scarring, such as hypertrophic or keloid scarring, dramatically affect the patient’s physical and psychological state [3–5]. In particular, hypertrophic and keloid scarring are of the highest clinical relevance in Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. burn treatment and esthetic medicine [1]. Since hypertrophic and keloid scarring belong This article is an open access article to the family of fibroproliferative disorders, these scars greatly differ from hypotrophic, distributed under the terms and atrophic and normotrophic scars in their histological features [6]. Clinical differentiation conditions of the Creative Commons between hypertrophic and keloid scars is often challenging. Proper identification of the Attribution (CC BY) license (https:// scar type is necessary for appropriate management of scar formation and cosmetic surgery creativecommons.org/licenses/by/ approach [7]. Insight into the complex molecular mechanisms involved in extreme scarring 4.0/). reviewed in this article is the key to understand the complicated pathophysiology of Int. J. Mol. Sci. 2021, 22, 897. https://doi.org/10.3390/ijms22020897 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 2 of 21 Int. J. Mol. Sci. 2021, 22, 897 2 of 20 formation and cosmetic surgery approach [7]. Insight into the complex molecular mechanisms involved in extreme scarring reviewed in this article is the key to under- excessivestand the scarring complicated and pathophysiology related fibroproliferative of excessive disorders. scarring and In relate this review,d fibroprolifera- we focus on fibroblasts,tive disorders. the key In cellularthis review, component we focus inon wound fibroblasts, healing. the key We cellular compare component their function in in normalwound and healing. hypertrophic We compare wound their healingfunction duringin normal chronic and hypertrophic inflammation, wound as healing well as their during chronic inflammation, as well as their production and remodeling of extracellular production and remodeling of extracellular matrix (ECM). matrix (ECM). FigureFigure 1. Clinical 1. Clinical photos photo ofs normal, of normal, keloid keloid and and hypertrophic hypertrophic scar scars:s: n normalormal fine fine line line scar scar following following a surgical a surgical wound wound (left); (left); keloid scar, which extends beyond the original wound (middle); and hypertrophic scar, which does not extend beyond keloidthe scar, initial which site extendsof the skin beyond lesion theassociated original with wound burn injury (middle (right); and). hypertrophic scar, which does not extend beyond the initial site of the skin lesion associated with burn injury (right). 2. Inflammatory Phase of Wound Healing 2. InflammatoryWound healing Phase is ofa very Wound complex Healing process involving activation of different cell types, productionWound healing of various is a cytokines, very complex chemoki processnes and involving many factors activation affecting of different immune cell re- types, productionsponses, and of various is thus cytokines,prone to errors chemokines [8]. Possibly and arising many factorsimbalances affecting in proinflammatory immune responses, andand is thusanti-inflammatory prone to errors signaling [8]. Possibly molecules arising lead to imbalances an excessively in proinflammatory strong and prolonged and anti- inflammatory response causing pathological scarring [9,10]. Such scarring was observed inflammatory signaling molecules lead to an excessively strong and prolonged inflamma- following cutaneous injury or/and irritation, including trauma, insect bite, burn injury tory response causing pathological scarring [9,10]. Such scarring was observed following grade 2b, surgery, vaccination, skin piercing, acne, and virus infections. On the other cutaneoushand, pathological injury or/and scarring irritation, was not observed including following trauma, superficial insect bite, injuries burn not injury reaching grade 2b, surgery,reticular vaccination, dermis (Fig skinure piercing,2), suggesting acne, a and critical virus role infections. of deeply On localized the other inflammation hand, patholog- ical[11]. scarring Even wasthough not inflammatory observed following responses superficial begin immediately injuries not after reaching injury, reticularkeloid and dermis (Figurehypertrophic2), suggesting scars may a critical form weeks role of or deeply even years localized after inflammationinjury [12]. By [this11]. time, Even the though inflammatorywound is closed responses and the begin epidermis immediately fully regenerated. after injury, The keloidpathological and hypertrophicscar therefore scars mayresults form as weeks a product or evenof chronic years inflammation after injury of [ 12reticular]. By thisdermis time, and the continuous wound aberrant is closed and thedermal epidermis matrix fully production regenerated. [11,13]. The During pathological the chronic scar phase, therefore inflammation results of as reticular a product of chronicdermis inflammation and subsequent of reticularscarring can dermis be further and continuous accelerated aberrantby recurrent dermal trauma matrix or me- produc- tionchanical [11,13 ].damage During to thethe chronicdermis layer phase, [14– inflammation16]. In general, oflong reticular and systemic dermis inflammation and subsequent persistency is typical of burn injuries, particularly in injured patients with severe skin scarring can be further accelerated by recurrent trauma or mechanical damage to the dermis loss and/or deep skin defects [17]. layer [14–16]. In general, long and systemic inflammation persistency is typical of burn It has been generally accepted that skin wound healing runs in four characteristic injuries,stages: particularly hemostasis, ininflammation, injured patients proliferation, with severe and skinmaturation/remod loss and/or deepeling. skinThe defectsinitial [17]. phaseIt has called