6. Reaction to Exacerbated TGF-β Levels in Wound Healing
Based on what has been discussed above, it is plausible that excessive TGF-β levels occurring only in the epidermal layer at the edge of the ulcer might contribute to the chronification of the wound. This notion is supported by concurrent evidence obtained from animal and human study models. Constitutive and high expression of integrin αvβ6 has been linked to a chronic wound state
[101][62]. Moreover, overexpression of this integrin has been associated with elevated TGF-β1 levels and spontaneous ulceration in mice
[101][62]. Studies using transgenic mice expressing different TGF-β1 constructs reveal skin alterations similar to those found in chronic wounds. The first model ever described implemented a construct expressing constitutively active TGF-β1 under the control of the human keratin-1 promoter. These animals showed an altered phenotype characterized by restricted mobility and impaired breathing and died within a day after birth. Analysis of their skin revealed hyperkeratosis and decreased proliferation in the epidermis
[102][63]. Later models expanded on this evidence as those mice reached adulthood. This was achieved by implementing complete TGF-β1 sequences along with either human keratin-5 (hK5) or keratin-14 (hK14) promoters, which restrict transgene expression to the basal keratinocyte layer
[31]. These advanced models differed greatly in their phenotype, ranging from hK14 shabby aspect
[49][64] to hK5 scaly and erythematous skin similar to psoriatic erythroderma
[48][65]. Notably, while TGF-β1 levels in unwounded skin of hK14 mice were similar to TGF-β1 levels in wild type
[49][64], persistent levels similar to those occurring in acute wounds were reported for the hK5 models
[48,50][65][66]. At the microscope, hK5 mice showed altered epidermis with signs of acanthosis (hyperplastic epidermis), diminished granular layer, and thickened stratum corneum, in some cases also showing abnormal basal and follicular keratinocyte proliferation in the form of hyperplasia and hyperkeratosis
[48,50][65][66]. Moreover, some of these models developed spontaneous ulcerations in friction areas
[36]. Interestingly, regardless of the promoter used, all these mice models developed significant delay in full-thickness wound recovery in comparison with non-transgenic mice
[36,49,65,103][36][64][67][68]. By contrast, results obtained from TGF-β1 knockout mice showed regular wound healing and, in some cases, accelerated recovery
[69,104][69][70]. In sum, the evidence obtained in transgenic mice constitutively overexpressing TGF-β1 in keratinocytes supports the hypothesis that persistent TGF-β signaling might not benefit wound healing and might constitute a relevant element in the pathogenesis of chronic wounds.
7. Epithelial to Mesenchymal Transition During Wound Healing and the Involvement of TGF-β
Though devoid of proliferation, keratinocytes at the edge of acute wounds experience changes similar to epithelial–mesenchymal transition (EMT). The concept of EMT implies a dramatic phenotypical switch providing epithelial cells with unusual capacities, like high mobility, the ability to surpass basement membranes, and resistance to apoptosis
[105][71]. These are considered unequivocal traits of advanced states in tumor transformation and are necessary for the progression of epithelium-derived cancer in terms of invasiveness and metastatic potential
[105][71]. Diverse molecular processes engage in this phenomenon and contribute to its development. These processes include signals from growth factor and cytokines, including EGF, FGF, HGF, KGF, and TGF-β, promoting the activation of transcription factors, reorganization of cell architecture, expression of specific surface proteins, production of ECM-degrading enzymes, and modulation of specific microRNAs
[105][71]. Biased response to TGF-β is considered a major mechanism for EMT in cancer, this being the subject matter of numerous reviews
[106,107][72][73]. Indeed, biased responses affect the expression of key cell adherence and migration markers, among other known effects
[106,107,108,109,110][72][73][74][75][76]. These include Smad canonical signaling driving the expression of key transcription factors such as Slug or Snail
[111][77], which mediate reduced E-cadherin detection and induction of mesenchymal markers such as vimentin, the two latter effects being recognized EMT hallmarks
[112][78]. Concurrent signaling involving MAP-kinases has also been suggested to contribute to this phenomenon
[113][79].
The transition phenomenon described above develops less intensely in skin wounds. In that context, keratinocytes are reported to experience temporal transdifferentiation involving cytoskeleton reorganization, loss of cell polarity, and partial dissociation of adhesion structures
[114][80]. This is supposed to allow cells to minimize attachment to the basal membrane while elongating and acquiring motility, seeking to re-establish epithelial coherence
[115][81]. While these transdifferentiation dynamics are fairly well understood, no consensus exists on how keratinocyte activities coordinate for re-epithelization. Several models have been proposed for the discussion on whether proliferation at the wound edge and migration occurs in the basal, suprabasal, or both layers
[115][81]. Moreover, specifically for migration, the considerations in the available literature regarding keratinocyte transdifferentiation during wound healing are mostly based on what is known about TGF-β and EMT during development and cancer
[38,114,116][38][80][82]. Interestingly, in light of the accumulated evidence, different review authors point to the existence of intermediate EMT states
[117,118][83][84]. Though it is still being discussed, molecular characterization of these intermediate states might be helpful for wound healing research.
As mentioned before, there is a lack of specific knowledge on TGF-β and EMT in skin wound healing. A recently published study (2019) demonstrated the implication of both canonical and non-canonical TGF-β1 signaling for proper keratinocyte transdifferentiation and successful wound closure in the axolotl (
Ambystoma mexicanum), since exposure to selective inhibitors for each pathway resulted in delayed re-epithelization
[119][85]. Compared to humans, the axolotl shows accelerated wound closure also characterized by no scarring, both achieved through a molecular mechanism which involves TGF-β signaling
[120][86]. In fact, several studies show how TGF-β signaling is necessary for tissue regeneration in other lower vertebrate models, including
Xenopus and zebrafish
[121,122][87][88]. This evidence provides unique hints on the role of TGF-β in keratinocyte transdifferentiation; however, the axolotl model itself is distant from human skin. At the molecular level, this is evidenced by the fact that only TGF-β1, but not TGF-β2 or TGF-β3, is detected in its regenerating tissues
[123][89]. Moreover, the axolotl develops shorter TGF-β1 induction and scarce leukocyte infiltration during wound healing
[124][90]. Interestingly, it has been suggested that the regenerative capacities shown by some lower vertebrates might be related to their neotenic potential (i.e., retaining typical traits of early stages of life)
[125][91]. To that extent, it is well established that early-gestation human skin wounds repair quickly and without scar formation
[126][92]. The mechanisms leading to this resolution of fetal wounds remain unknown. However, reviewed evidence on this issue points to TGF-β as the main factor involved, as in fetal skin, only TGF-β3 expression is found to be increased while TGF-β1 levels remain steady, in clear contrast with what is found in adults
[127][93]. In that sense, studies performed on mice and rats provide evidence of a contrasting TGF-β1/TGF-β3 ratio between the skin and the oral mucosa during wound healing
[128,129][94][95]. This observation is highly interesting, as oral mucosal wounds are indeed known to heal faster and with minimal scarring in comparison with skin wounds
[130][96]. Interestingly, although evidence available in humans is restricted, a recent study analyzing oral scars appearing after oral tumor removal suggests a pattern of increased TGF-β1 detection
[131][97]. Indeed, the treatment of adult skin wounds with exogenous TGF-β3 or, alternatively, with neutralizing antibodies for TGF-β1 and TGF-β2 has been suggested to reduce scar formation and improve aesthetics after healing
[89,132][50][98]. Moreover, treatment of fetal wounds with TGF-β1 results in scarification
[133][99]. Altogether, this evidence suggests that relative fractions of TGF-β isoforms, rather than absolute amounts, may direct the evolution of the wound through their impact on the regulation of gene expression and the release of cell mediators driving leukocyte recruitment, keratinocyte activation, or ECM deposition.