Diving Deep Into Healing: The Promising Role of Fish Skin in Wound Recovery

Fish skin has emerged as a potential candidate for improving wound healing due to its notable results in human trials, in which it has been directly applied as a dressing on wounds. The current review explores the mechanisms by which fish skin can boost the wound healing process. The natural wound healing process involves inflammation at the wound site to initiate tissue repair. The body balances this inflammation through interleukin signaling, and imbalances can cause chronic wounds or scarring. The wound site also secretes epidermal growth factor, which activates the Ras/Raf/MEK/ERK and PI3K/Akt pathways. These pathways promote angiogenesis (ie, replacing injured blood vessels) and epithelialization (ie, replacing injured skin). Delays in these pathways increase the healing time. The rich contents of omega-3, collagen, and selenium in fish skin boost wound healing by inhibiting compounds that can cause over-inflammation during interleukin signaling. They also upregulate the Ras/Raf/MEK/ERK and PI3K pathways by altering lipid composition (via omega-3), binding with collagen receptors (via collagen), and modulating selenoproteins (via selenium). The mechanisms discussed in this review support the finding that fish skin is a promising candidate with a strong potential to naturally boost the wound healing process in clinical settings. Continued investigation into the application of fish skin as a practical and commercial wound healing agent is warranted. Future study of additional wound healing properties of fish skin, such as microbial protection of open wounds, is recommended.

Abbreviations

5-LOX, 5-lipoxygenase; AA, arachidonic acid; COX-2, cyclooxygenase-2; DDR, discoidin domain receptor; DHA, docosahexaenoic acid; ECM, extracellular matrix; EGF, epidermal growth factor; EGFR, EGF receptor; EPA, eicosapentaenoic acid; GPx, glutathione peroxidase; IL, interleukin; MAPK, mitogen-activated protein kinase; mTORC, mammalian target of rapamycin complex; NFκB, nuclear factor-κB; PI3K, phosphoinositide 3-kinase; PIP2, phosphatidylinositol 4,5-bisphosphate; PIP3, phosphatidylinositol 3,4,5-trisphosphate; PTP, protein tyrosine phosphatases; Ras/Raf/MEK/ERK, ras protein, raf protein, MEK (a dual threonine and tyrosine recognition kinase), extracellular signal-regulated kinase; RTK, receptor tyrosine kinase; SelK, selenoprotein K; SOS, Son of Sevenless; TGF-β, transforming growth factor β; TNF-α, tumor necrosis factor α; Treg, regulatory T cell; TrxR, thioredoxin reductase; VEGF, vascular endothelial growth factor; VEGFR, VEGF receptor.

Introduction

Wound healing involves a series of physiological actions in the body that aim to restore the function and integrity of damaged skin.^1,2 Failure of the natural wound healing process can lead to chronic wound complications, such as infection and amputation, that significantly affect quality of life.^3-5 The key events of the wound healing process are inflammation, angiogenesis, and epithelialization, and various biochemical pathways are responsible for regulating these events.⁵ A disturbance in these events, which may be due to a weak immune system, comorbidities, or advanced age (ie, being 65 years or more in age), can lead to uncontrolled inflammation (resulting in excessive scarring), poor angiogenesis or growth of new blood vessels (resulting in delayed healing), and poor epithelialization or growth of new skin (resulting in increased risk of infection).^6-8 To counter these risks, research is underway on substances that can support normal wound healing and reduce the risk of wound complications.^5,6,9,10 One such substance that should be given due consideration is fish skin. Previous studies have determined that fish skin contains high amounts of omega-3, collagen, and selenium, and that applying fish skin on wounds markedly improves wound healing parameters.

The possible mechanisms of action behind these successful findings are discussed in this review, in the interest of promoting continued study of fish skin and the development of a practically beneficial and commercially available healing agent. The pathways in the normal wound healing process that regulate inflammation, angiogenesis, and epithelialization are also discussed, as are the means by which the omega-3, collagen, and selenium content in fish skin positively influences each pathway, and in turn, the wound healing process.

The normal mechanism of wound healing

Inflammation and the IL-10 signaling pathway. Inflammation is the body’s innate response to injury or infection, designed to eliminate pathogens, remove cellular debris, and initiate tissue repair.^4,5,11,12 However, prolonged or excessive inflammation can impede wound healing and lead to chronic wounds and excessive scarring.⁴ Thus, the timely activation of anti-inflammatory mechanisms is crucial for efficient wound healing.^13-15 Pro- and anti-inflammatory cytokines in the interleukin signaling pathway are balanced during the anti-inflammatory response.¹³

To ensure that cytokine-driven inflammation does not overwhelm a wound site, M2 macrophages secrete IL-10 and TGF-β.^16-18 TGF-β induces the differentiation of Tregs, which suppress excessive immune responses by reducing the activity of effector T cells. TGF-β also produces IL-10.¹⁰ IL-10 reduces pro-inflammatory cytokines IL-1β, IL-6, and TNF-α by inhibiting activation of MAPK and nuclear factor-κB.^19,20

In summary, the body requires pro-inflammatory cytokines to cause inflammation at a wound site so that the body can become aware of the wound and start its natural healing process. However, anti-inflammatory cytokines also need to be activated so that pro-inflammatory cytokines do not cause unchecked inflammation, a common finding in delayed wound healing. The components of fish skin increase the activation of anti-inflammatory cytokines, as discussed in “The Role of Fish Skin in Improving Wound Healing” section.

Angiogenesis and the Ras/Raf/MEK/ERK pathway. Upon wound injury, EGF is released by platelets, macrophages, and keratinocytes.²¹ EGF binds and activates EGFR, which begins a conformational change resulting in trans-autophosphorylation.^22,23 The autophosphorylation event creates docking sites for signaling proteins, including Src homology 2 and growth factor receptor-bound protein 2, thereby initiating 2 important biochemical pathways.^24-26

When growth factor receptor-bound protein 2 binds to EGFR, SOS is activated. SOS catalyzes the exchange of a guanosine diphosphate molecule for a guanosine triphosphate molecule to activate Ras, which then causes Raf, MEK (a dual threonine and tyrosine recognition kinase), and ERK (part of the MAPK family), to be activated in a specific order.^27-29 In the nucleus, ERK moves and phosphorylates transcription factors like c-Jun. These elements enhance the expression of genes essential for the migration and proliferation of new epithelial cells in the wound bed.³⁰

Similar to EGFR, VEGF activates its receptor (ie, VEGFR) to initiate the Ras/Raf/MEK/ERK pathway, thereby activating the transcription factors responsible for the migration and proliferation of endothelial cells, as well as tube formation, a crucial element of angiogenesis in the wound bed.^31-34

Every time an EGFR or VEGFR molecule is activated, a series of biochemical steps, collectively known as the Ras/Raf/MEK/ERK pathway, can successfully occur at the wound site to eventually allow new blood vessels to replace injured ones. The new blood vessels also provide nutrients that enable faster wound recovery. As discussed in “The Role of Fish Skin in Improving Wound Healing” section, the components of fish skin increase the rate of activation of EGFR and VEGFR, thereby increasing the rate at which new healthy blood vessels are formed.

Epithelialization and the PI3K/Akt pathway. Simultaneously, EGFR and VEGFR trigger the activation of PI3K, which phosphorylates PIP2 to generate PIP3.^35,36 PIP3 then activates Akt (also known as protein kinase B) by phosphorylating it at the threonine 308 residue by phosphoinositide-dependent kinase-1 and the serine 473 residue by mTORC2.^37,38 Akt creates an environment that is favorable for new cell growth in 2 ways: it inactivates proapoptotic proteins (ie, BAD and Bcl-2) and proapoptotic enzymes (ie, caspases).³⁹ It also activates mTORC1, which modulates the eukaryotic translation initiation factor 4E-binding protein 1 and ribosomal protein S6 kinase.^40,41 This cascade promotes endothelial cell proliferation and stimulates protein synthesis for new cell growth.

Akt also boosts angiogenesis. Endothelial nitric oxide synthase and glycogen synthase kinase-3 are phosphorylated and modulated by Akt to increase endothelial cell proliferation and tube formation.^38,42-44

Increasing the rate of activation of EGFR and VEGFR also increases the rate by which new cells grow and cover injured skin. Because fish skin components upregulate EGFR and VEGFR, they also upregulate the resulting series of biochemical steps, collectively known as the PI3K/Akt pathway, which increases the amount of healthy skin cells entering the wound site and the amount of protein being made available to synthesize new cells.

The Role of Fish Skin in Improving Wound Healing

Fish skin has emerged as a promising candidate for boosting the natural wound healing process due to its uniquely rich levels of omega-3, collagen, and selenium.^45-48 This section discusses how these 3 components interact with the pathways involved in wound recovery (ie, IL-10 signaling, Ras/Raf/MEK/ERK, and PI3K/Akt) to improve healing.

Omega-3

Omega-3 fatty acid content, in particular EPA and DHA, can vary in fish skin depending on fish diet and species. Fatty fish species, such as salmon, mackerel, and sardines, are widely recognized as rich sources of omega-3 fatty acids.^48-50

Role in the IL-10 signaling pathway: inhibition of 3 pro-inflammatory compounds and creation of 4 anti-inflammatory compounds. EPA and DHA can competitively inhibit cyclooxygenase-2 (COX-2), which is responsible for converting AA into pro-inflammatory prostaglandins^51,52 (Figure 1). Additionally, EPA competes with AA as a substrate for 5-LOX. 5-LOX converts AA into leukotrienes, which are potent inflammatory mediators.^53,54 EPA substitution results in the formation of anti-inflammatory resolvins and protectins instead. Both EPA and DHA also suppress NFκB activation by inhibiting the degradation of its inhibitory protein, inhibitor kappa B-alpha, thus preventing its translocation into the nucleus and subsequent expression of pro-inflammatory genes.⁵⁵ Moreover, EPA and DHA serve as a starting point for the creation of specific pro-resolving lipid mediators, including lipoxins and maresins, which are responsible for suppressing IL-1β and IL-6.^56,57

Role in the Ras/Raf/MEK/ERK and PI3K/Akt pathways: activation of Ras and PI3K by altering lipid composition of cell membranes. EPA and DHA can increase epithelialization and angiogenesis by increasing the activation of EGFR and VEGFR, respectively, which can then activate both pathways.^58,59 This effect is exerted by replacing the fatty acids of the existing phospholipids in cell membranes with omega-3 fatty acids via acyltransferases and phospholipases.^60-62 Because omega-3 fatty acids have longer and more unsaturated fatty acid tails, the increased unsaturation creates a more fluid and flexible membrane structure.^62,63 This structure increases the clustering of transmembrane receptors and enhances receptor-receptor interactions, resulting in increased activity of VEGFR and EGFR.^62-66

Collagen

The ECM of fish skin is laden with collagen, particularly type I collagen that is notably similar in structure to human collagen.⁶⁷ Fish collagen has a high degree of purity (approximately 70%), depending on the season and the age and species of the fish.^67,68 Different fish species and even different portions of the same species may have varying total collagen contents. Some of the highest content (54.2% w/w) has been found in the skin of Nile tilapia (Oreochromis niloticus).^69,70

Role in the IL-10 signaling pathway: inhibition of 5 pro-inflammatory compounds and activation of 2 anti-inflammatory compounds. Collagen can sequester pro-inflammatory TNF-α and IL-1β by binding them to collagen fibers, thereby blocking their interaction with cellular receptors of pro-inflammatory signaling^71-73 (Figure 2). Matrix metalloproteinases, which are responsible for breaking down ECM and producing pro-inflammatory cytokines, are naturally inhibited by collagen.^74-76 Similar to omega-3 fatty acids, collagen-derived peptides inhibit COX-2 expression.⁷⁷ Additionally, collagen-derived peptides, including glycine-proline-hydroxyproline, inhibit NFκB.⁷¹ Collagen also influences macrophage polarization to shift the balance from pro-inflammatory M1 to anti-inflammatory M2 macrophages.^78,79 Instead, collagen promotes the activation of Tregs.⁸⁰ Both activities upregulate the production of IL-10, as discussed previously.^78-80

Role in the Ras/Raf/MEK/ERK and PI3K/Akt pathways: activation of Ras and PI3K by binding to collagen receptors. Collagen binds to integrins, a family of transmembrane collagen receptors, on both epithelial and endothelial cells.⁸¹ This activates focal adhesion kinase and Src family kinases, which phosphorylate and activate downstream effectors, including Ras and PI3K.^81,82 Similarly, in epithelial cells, collagen binds to integrins and DDRs, which are non-integrin collagen receptors that are members of the RTK family and includes EGFR.^83,84 Both integrins (eg, α1β1, α2β1) and DDRs mediate the attachment of endothelial cells to the postinjury ECM.^84-87

Selenium

Selenium is an essential trace element present in fish skin and can exist in different chemical forms, including selenocysteine, selenomethionine, and various selenoproteins.⁸⁸ Selenocysteine and selenomethionine are amino acid analogues of cysteine and methionine, respectively, with a selenium atom replacing the sulfur atom in the side chain. These selenium-containing amino acids can be incorporated into the primary structure of proteins during translation, leading to the synthesis of selenoproteins, which play several critical roles in wound healing^88-90 (Figure 3).

Role in the Ras/Raf/MEK/ERK pathway: activation of Raf through selenoprotein SelK. Selenium directly influences the Ras/Raf/MEK/ERK pathway through SelK. SelK interacts with the Raf-1 kinase to enhance its binding to Ras and thereby initiate Raf-1 activation. This, in turn, leads to the phosphorylation and activation of MEK and ERK. Furthermore, selenium supplementation has been shown to increase the expression of Ras and Raf-1 in certain cell types.^91,92

Role in the PI3K/Akt pathway: activation of PI3K through selenoprotein TrxR. Similarly, selenium influences the PI3K pathway by being an essential cofactor for selenoprotein TrxR.^93,94 TrxR, through its redox activity, affects the formation and stability of RTK dimers.^94-96 RTKs are upstream regulators of PI3K. When activated by ligand binding, they undergo dimerization and autophosphorylation, leading to the recruitment of PI3K and initiation of the PI3K pathway.⁹⁷ Additionally, it has been noted that selenium administration increases the expression and phosphorylation of Akt, thereby amplifying the signaling output of the PI3K pathway.^97-99

Role in all pathways: modulating key enzymes for successful pathway activation. Selenium is a cofactor mandatory for the activation of GPx, a selenoprotein antioxidant enzyme that neutralizes free radicals to minimize oxidative stress.¹⁰⁰ It has been observed that a strong messenger RNA expression of GPx1 during the inflammatory process aids wound healing. The neutralizing activity of selenium facilitates maintaining a favorable environment for VEGF expression, endothelial cell proliferation, and the development of capillary-like structures.^100,101 Selenium has also been shown to inhibit PTPs, a class of signaling enzymes that dephosphorylate phosphotyrosine residues on proteins involved in signaling pathways, including EGFR, in order to deactivate them.¹⁰² By inhibiting PTP activity, selenium thereby prolongs the phosphorylation and activation of proteins in the Ras/Raf/MEK/ERK and PI3K pathways, allowing for sustained signaling to activate the pathways (and related genes) responsible for successful wound healing.¹⁰³

Limitations

As with all review articles, this article is limited by the complete reliance on previously published research. However, the authors of the current article ensured the incorporation of a variety of primary sources (original research articles) and secondary sources (review articles) when synthesizing the text in this manuscript, with preference given to recently published and peer-reviewed articles.

Conclusion

Fish skin is a promising potential candidate to improve wound healing due to its uniquely high omega-3, collagen, and selenium content. These components have shown to boost the pathways that regulate the natural wound healing process (IL-10 signaling, Ras/Raf/MEK/ERK, and PI3K pathways), which in turn boosts the natural wound healing process. Notable cellular effects of omega-3, collagen, and selenium include the inhibition of pro-inflammatory components and creation of anti-inflammatory compounds involved in interleukin signaling. Additional notable effects include the upregulation of Ras and PI3K activation in the Ras/Raf/MEK/ERK and PI3K pathways, respectively, through altering lipid composition (via omega-3), binding to collagen receptors (via collagen), and modulating selenoproteins (via selenium). The mechanisms discussed herein emphasize the necessity to investigate the application of fish skin as a practical and commercial wound healing agent in clinical settings. Additional wound healing properties of fish skin, such as microbial protection of open wounds, should be studied in the future to ascertain their potential.

Acknowledgements

Affiliations: ¹Department of Diet & Nutritional Sciences, Rashid Latif Khan University, Lahore, Punjab, Pakistan ²University Institute of Diet & Nutritional Sciences, The University of Lahore, Lahore, Punjab, Pakistan ³Institute of Food Sciences and Nutrition, University of Sargodha, Sargodha, Punjab, Pakistan

ORCID: Murtaza, 0000-0002-0893-7397

Disclosure: The authors disclose no financial or other conflicts of interest.

Correspondence: Sarosh Malik, MS; The University of Lahore, 186 D Punjab Cooperative Housing Society Ghazi Road Lahore Cantt, Lahore, Punjab 54000 Pakistan; saroshmalik12@gmail.com

Manuscript Accepted: April 26, 2024

How Do I Cite This?

Basharat S, Malik S, Samad HUA, Murtaza MA. Diving deep into healing: the promising role of fish skin in wound recovery. Wounds. 2024;36(8):274-280. doi:10.25270/wnds/23134

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