Activation of STING signaling accelerates skin wound healing

The cGAS-STING pathway: a therapeutic target in diabetes and its complications

Diabetic wound healing (DWH) represents a major complication of diabetes where inflammation is a key impediment to proper healing. The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) signaling pathway has emerged as a central mediator of inflammatory responses to cell stress and damage. However, the contribution of cGAS-STING activation to impaired healing in DWH remains understudied. In this review, we examine the evidence that cGAS-STING-driven inflammation is a critical factor underlying defective DWH. We summarize studies revealing upregulation of the cGAS-STING pathway in diabetic wounds and discuss how this exacerbates inflammation and senescence and disrupts cellular metabolism to block healing. Partial pharmaceutical inhibition of cGAS-STING has shown promise in damping inflammation and improving DWH in preclinical models. We highlight key knowledge gaps regarding cGAS-STING in DWH, including its relationships with endoplasmic reticulum stress and metal-ion signaling. Elucidating these mechanisms may unveil new therapeutic targets within the cGAS-STING pathway to improve healing outcomes in DWH. This review synthesizes current understanding of how cGAS-STING activation contributes to DWH pathology and proposes future research directions to exploit modulation of this pathway for therapeutic benefit.

Multifaceted roles of mitochondria in wound healing and chronic wound pathogenesis

Mitochondria are intracellular organelles that play a critical role in numerous cellular processes including the regulation of metabolism, cellular stress response, and cell fate. Mitochondria themselves are subject to well-orchestrated regulation in order to maintain organelle and cellular homeostasis. Wound healing is a multifactorial process that involves the stringent regulation of several cell types and cellular processes. In the event of dysregulated wound healing, hard-to-heal chronic wounds form and can place a significant burden on healthcare systems. Importantly, treatment options remain limited owing to the multifactorial nature of chronic wound pathogenesis. One area that has received more attention in recent years is the role of mitochondria in wound healing. With regards to this, current literature has demonstrated an important role for mitochondria in several areas of wound healing and chronic wound pathogenesis including metabolism, apoptosis, and redox signalling. Additionally, the influence of mitochondrial dynamics and mitophagy has also been investigated. However, few studies have utilised patient tissue when studying mitochondria in wound healing, instead using various animal models. In this review we dissect the current knowledge of the role of mitochondria in wound healing and discuss how future research can potentially aid in the progression of wound healing research.

GEO data mining identifies potential immune-related genes in hypertrophic scar and verities in a rabbit model

Objective

Hypertrophic scar (HTS), the secondary major abnormal tissue after wound healing, is the most frequent and severe type of skin scar. Dysregulated immune response plays an important role in HTS formation. In this study, we identified the potential immune-related genes in HTS and explored their potential therapeutic significance.

Methods

We first screened out the potential immune-related genes in HTS microarrays via bioinformatics analysis using public datasets. We then constructed a rabbit model of ear scar to investigate the morphological features of HTS and verify the basic expression of potential immune-related genes in HTS tissue. Finally, we used AlphaFold to determine the protein homology between human and rabbit scar tissues.

Results

Bioinformatics analysis revealed 22 differentially expressed genes (DEGs) and a single differential infiltration of immune cells (naïve B cells) in HTS and normal tissues. Six of the DEGs were correlated with naïve B cell numerically. CCL2, PLXDC2 and FOXF2 were expressed in rabbit ear scar model. PLXDC2 and FOXF2 showed relatively high homology between human and rabbit scar tissues.

Conclusions

PLXDC2 and FOXF2, both closely related to immune cell infiltration and specifically expressed in HTS, represent potential therapeutic targets in HTS.

The Crucial Roles and Research Advances of cGAS-STING Pathway in Cutaneous Disorders

The cGAS-STING signaling pathway senses the presence of cytosolic DNA, induces strong type I interferon responses, and enhances inflammatory cytokine production, placing it as an important axis in infection, autoimmunity, and tumor immunity. Recent studies have shown that the abnormalities and/or dysfunctions of cGAS-STING signaling are closely related to the pathogenesis of skin diseases and/or cancers. Additionally, a variety of new therapeutics targeting the cGAS-STING signaling are in development for the treatment of skin disorders. However, the precise molecular mechanisms of cGAS-STING-mediated cutaneous disorders have not been fully elucidated. In this review, we will summarize the regulatory roles and mechanisms of cGAS-STING signaling in skin disorders and recent progresses of cGAS-STING-related drugs as well as their potential clinical applications.

STING Agonist-Induced Skin Inflammation Is Exacerbated with Prior Systemic Innate Immune Activation

Activation of the Stimulator of Interferon Genes (STING) protein has paradoxical outcomes in skin disease. STING activation exacerbates psoriatic skin disease and delays wound healing in diabetic mice, yet it also facilitates wound healing in normal mice. To address the role of localized STING activation in the skin, mice were injected subcutaneously with a STING agonist, diamidobenzimidazole STING Agonist-1 (diAbZi). The effect of a prior inflammatory stimulus on STING activation was addressed by pre-treating mice intraperitoneally with poly (I:C). The skin at the injection site was evaluated for local inflammation, histopathology, immune cell infiltration, and gene expression. Serum cytokine levels were measured to assess systemic inflammatory responses. Localized diABZI injection induced severe skin inflammation with erythema, scaling, and induration. However, the lesions were self-limiting and resolved within 6 weeks. At the peak of inflammation, the skin showed epidermal thickening, hyperkeratosis, and dermal fibrosis. Neutrophils, CD3 T cells, and F4/80 macrophages were present in the dermis and subcutaneous layers. Gene expression was consistent with increased local interferon and cytokine signaling. Interestingly, the poly (I:C)-pre-treated mice showed higher serum cytokine responses and developed worse inflammation with delayed wound resolution. Our study demonstrates that prior systemic inflammation amplifies STING-mediated inflammatory responses and skin disease.

How Fragile We Are: Influence of Stimulator of Interferon Genes (STING) Variants on Pathogen Recognition and Immune Response Efficiency

The stimulator of interferon genes (STING) protein is a cornerstone of the human immune response. Its activation by cGAMP in the presence of cytosolic DNA stimulates the production of type I interferons and inflammatory cytokines. In the human population, several STING variants exist and exhibit dramatic differences in their activity, impacting the efficiency of the host defense against infections. Understanding the molecular mechanisms of these variants opens perspectives for personalized medicine treatments against diseases such as viral infections, cancers, or autoinflammatory diseases. Through microsecond-scale molecular modeling simulations, contact analyses, and machine learning techniques, we reveal the dynamic behavior of four STING variants (wild type, G230A, R293Q, and G230A/R293Q) and rationalize the variability of efficiency observed experimentally. Our results show that the decrease in STING activity is linked to a stiffening of key structural elements of the binding cavity together with changes in the interaction patterns within the protein.

STING activation promotes inflammatory response and delays skin wound healing in diabetic mice

Sustained inflammatory responses delay wound repair in diabetic skin. The stimulator of interferon genes (STING) plays a vital role in the innate immune responses. However, its function in diabetic skin wound repair, and the underlying mechanism remains unclear. Here, we reported that STING activation is a pathogenic marker that correlates with delayed wound repair in diabetic skin. Firstly, we found that STING expression is enhanced in the epidermis of STZ induced diabetes mouse model and db/db mouse model. Consistently, we also found that STING expression was upregulated in keratinocytes with the high-glucose (HG) treatment. Moreover, silencing of STING accelerated wound healing in vitro. In vivo, inhibition of STING by c176 inhibited inflammatory response in the epidermis and accelerated wound healing in diabetic skin. In addition, we found that autophagy dysfunction is correlated with the expression of STING in epidermis of diabetic mice. Induction of autophagy by rapamycin significantly reduced STING expression in keratinocytes. Collectively, these results indicated that defects of autophagy might lead to the activation of STING and finally delay the diabetic wound healing.

Platelet-Released Growth Factors Influence Wound Healing-Associated Genes in Human Keratinocytes and Ex Vivo Skin Explants

Platelet-released growth factors (PRGFs) or other thrombocyte concentrate products, e.g., Platelet-Rich Fibrin (PRF), have become efficient tools of regenerative medicine in many medical disciplines. In the context of wound healing, it has been demonstrated that treatment of chronic or complicated wounds with PRGF or PRF improves wound healing in the majority of treated patients. Nevertheless, the underlying cellular and molecular mechanism are still poorly understood. Therefore, we aimed to analyze if PRGF-treatment of human keratinocytes caused the induction of genes encoding paracrine factors associated with successful wound healing. The investigated genes were Semaphorin 7A (SEMA7A), Angiopoietin-like 4 (ANGPLT4), Fibroblast Growth Factor-2 (FGF-2), Interleukin-32 (IL-32), the CC-chemokine-ligand 20 (CCL20), the matrix-metalloproteinase-2 (MMP-2), the chemokine C-X-C motif chemokine ligand 10 (CXCL10) and the subunit B of the Platelet-Derived Growth Factor (PDGFB). We observed a significant gene induction of SEMA7A, ANGPLT4, FGF-2, IL-32, MMP-2 and PDGFB in human keratinocytes after PRGF treatment. The CCL20- and CXCL10 gene expressions were significantly inhibited by PRGF therapy. Signal transduction analyses revealed that the PRGF-mediated gene induction of SEMA7A, ANGPLT4, IL-32 and MMP-2 in human keratinocytes was transduced via the IL-6 receptor pathway. In contrast, EGF receptor signaling was not involved in the PRGF-mediated gene expression of analyzed genes in human keratinocytes. Additionally, treatment of ex vivo skin explants with PRGF confirmed a significant gene induction of SEMA7A, ANGPLT4, MMP-2 and PDGFB. Taken together, these results describe a new mechanism that could be responsible for the beneficial wound healing properties of PRGF or related thrombocytes concentrate products such as PRF.

Possible involvement of CXC motif chemokine ligand 10 in exercise-induced collagen production of mouse dermal fibroblasts

Evidence suggests that exercise can regulate skin functions such as promoting wound healing and inhibiting aging. Physical exercise modulates the secretion of proteins and peptides from skeletal muscles, called myokines, which play a role in transmitting exercise signals throughout the body. Therefore, exercise-regulated myokines may play a role in controlling skin functions; however, the precise mechanisms remain elusive. In this study, we focused on the recently identified CXC motif chemokine ligand 10 (CXCL10), an exercise-reduced myokine, and attempted to elucidate its role in regulating collagen synthesis in dermal fibroblasts. Mouse C2C12 myotubes were stimulated with or without electrical pulse stimulation (EPS) to induce contraction for 24 h, and conditioned medium was collected (EPS-CM or Ctrl-CM, respectively). The reduction in CXCL10 concentration by EPS was confirmed using ELISA. Next, mouse dermal fibroblasts were isolated from the dorsal skin of C57BL6/J mice (2 weeks old) and were stimulated with Ctrl-CM or EPS-CM for 24 h. EPS-CM treatment significantly increased collagen production compared to Ctrl-CM treatment. Even in the Ctrl-CM condition, the addition of an antagonist for CXCR3 (CXCL10 receptor) increased collagen production. In contrast, recombinant CXCL10 abolished EPS-CM-dependent collagen induction. Overall, this study raises the possibility that CXCL10 secretion from skeletal muscles may control collagen production in mouse dermal fibroblasts.

Therapeutic candidates for keloid scars identified by qualitative review of scratch assay research for wound healing

The scratch assay is an in vitro technique used to analyze cell migration, proliferation, and cell-to-cell interaction. In the assay, cells are grown to confluence and then ‘scratched’ with a sterile instrument. For the cells in the leading edge, the resulting polarity induces migration and proliferation in attempt to ‘heal’ the modeled wound. Keloid scars are known to have an accelerated wound closure phenotype in the scratch assay, representing an overactivation of wound healing. We performed a qualitative review of the recent literature searching for inhibitors of scratch assay activity that were already available in topical formulations under the hypothesis that such compounds may offer therapeutic potential in keloid treatment. Although several shortcomings in the scratch assay literature were identified, caffeine and allicin successfully inhibited the scratch assay closure and inflammatory abnormalities in the commercially available keloid fibroblast cell line. Caffeine and allicin also impacted ATP production in keloid cells, most notably with inhibition of non-mitochondrial oxygen consumption. The traditional Chinese medicine, shikonin, was also successful in inhibiting scratch closure but displayed less dramatic impacts on metabolism. Together, our results partially summarize the strengths and limitations of current scratch assay literature and suggest clinical assessment of the therapeutic potential for these identified compounds against keloid scars may be warranted.

Systemic and topical administration of spermidine accelerates skin wound healing

BACKGROUND

The skin wound healing process is regulated by various cytokines, chemokines, and growth factors. Recent reports have demonstrated that spermine/spermidine (SPD) promote wound healing through urokinase-type plasminogen activator (uPA)/uPA receptor (uPAR) signaling in vitro. Here, we investigated whether the systemic and topical administration of SPD would accelerate the skin wound-repair process in vivo.

METHODS

A skin wound repair model was established using C57BL/6 J mice. SPD was mixed with white petrolatum for topical administration. For systemic administration, SPD mixed with drinking water was orally administered. Changes in wound size over time were calculated using digital photography.

RESULTS

Systemic and topical SPD treatment significantly accelerated skin wound healing. The administration of SPD promoted the uPA/uPAR pathway in wound sites. Moreover, topical treatment with SPD enhanced the expression of IL-6 and TNF-α in wound sites. Scratch and cell proliferation assays revealed that SPD administration accelerated scratch wound closure and cell proliferation in vitro.

CONCLUSION

These results indicate that treatment with SPD promotes skin wound healing through activation of the uPA/uPAR pathway and induction of the inflammatory response in wound sites. The administration of SPD might contribute to new effective treatments to accelerate skin wound healing. Video Abstract.

DNA damage-triggered activation of cGAS-STING pathway induces apoptosis in human keratinocyte HaCaT cells

Exposure to ultraviolet B (UVB) from sunlight causes DNA damage, serious cellular inflammation and aging, and even cell death in the skin, commonly known as sunburn, leading to cutaneous tissue disorders. DNA damage can be sensed as a danger-associated molecular pattern (DAMP) by the innate immune system. It has not been studied, however, whether cGAS-STING activation is involved in the apoptosis induced by UVB irradiation or by cisplatin treatment. Here we report the findings that within hours of DNA damages keratinocytes show an innate immune response, which involves the activation of cGAS-STING; a cytosolic DNA receptor, cGAS (cyclic guanosine monophosphate-adenosine monophosphate synthase), cyclic GMP-AMP (cGAMP) synthase, and DNA sensing adaptor, STING (protein stimulator of interferon genes). Either UVB irradiation or cisplatin treatment can cause DNA damages, releasing fragmented DNA from nucleus and/or mitochondria. Roles of cGAS-STING were examined in the HaCaT cells with DNA damages caused by UVB irradiation or cisplatin treatment. Silencing STING by siRNA rescued HaCaT cells from UVB or cisplatin-induced apoptosis. NF-κB, one of the major downstream components of STING pathway, which usually regulates the classical STING apoptotic pathway, was translocated to nucleus in the HaCaT cells irradiated with UVB. This translocation was attenuated by STING silencing. Treatment with BAY, an inhibitor of NF-κB pathway, blocked UVB-induced apoptosis. cGAS-STING-mediated production of IFNβ was induced by nuclear translocation of interferon regulatory factor 3 (IRF3). UVB irradiation inceased the nuclear translocation of IRF3, accompanied by enhanced expression level of IFNβ mRNA. The nuclear translocation of IRF3 and expression of IFNβ mRNA were attenuated by STING silencing. Treatment with MRT67307, an inhibitor of TBK1-IRF3-IFNβ pathway, blocked UVB-induced apoptosis. Therefore, we conclude that NF-κB pathway and IFNβ pathway residing in the downstream of STING are resposible for apoptosis of UVB-irradiated or cisplatin-treated HaCaT cells.

The Immune Functions of Keratinocytes in Skin Wound Healing

As the most dominant cell type in the skin, keratinocytes play critical roles in wound repair not only as structural cells but also exerting important immune functions. This review focuses on the communications between keratinocytes and immune cells in wound healing, which are mediated by various cytokines, chemokines, and extracellular vesicles. Keratinocytes can also directly interact with T cells via antigen presentation. Moreover, keratinocytes produce antimicrobial peptides that can directly kill the invading pathogens and contribute to wound repair in many aspects. We also reviewed the epigenetic mechanisms known to regulate keratinocyte immune functions, including histone modifications, non-protein-coding RNAs (e.g., microRNAs, and long noncoding RNAs), and chromatin dynamics. Lastly, we summarized the current evidence on the dysregulated immune functions of keratinocytes in chronic nonhealing wounds. Based on their crucial immune functions in skin wound healing, we propose that keratinocytes significantly contribute to the pathogenesis of chronic wound inflammation. We hope this review will trigger an interest in investigating the immune roles of keratinocytes in chronic wound pathology, which may open up new avenues for developing innovative wound treatments.