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Institute of Cell Biology, Department of Biology, ETH Zurich, Zurich, Switzerland; and Cancer Research UK, London Research Institute, London, United Kingdom
ABSTRACT I. INTRODUCTION II. PLATELET-DERIVED GROWTH FACTOR FAMILY A. Expression of PDGF at the Wound Site B. Inhibition of PDGF Action in Healing Skin Wounds III. FIBROBLAST GROWTH FACTOR FAMILY A. Expression of FGFs in Healing Skin Wounds B. A Role for FGF2 in Wound Repair C. FGF Receptor Signaling Is Important for Reepithelialization D. FGF7-Deficient Mice Show No Defect in Wound Healing IV. EPIDERMAL GROWTH FACTOR FAMILY A. Expression of EGF, TGF-{alpha}, and HB-EGF at the Wound Site B. Expression of EGF Receptors at the Wound Site C. Ectodomain Shedding of EGF Receptor Ligands Is Required for Keratinocyte Migration During Wound Healing D. Wound Healing in Mice Deficient in TGF-{alpha} E. A Role of Neu Differentiation Factor in Wound Repair? V. VASCULAR ENDOTHELIAL GROWTH FACTOR FAMILY A. Expression of VEGF-A and Its Receptors in Skin Wounds B. A Role for VEGF-A in Wound Angiogenesis C. Lack of PLGF Results in Impaired Wound Angiogenesis D. Expression of VEGF-C and Its Receptor in Healing Skin Wounds VI. ANGIOPOIETINS A. Expression of Angiopoietins and Their Receptor in Healing Skin Wounds VII. INSULIN-LIKE GROWTH FACTORS A. Expression of IGFs and Their Receptors in Skin Wounds B. Impaired Wound Healing Is Associated With Abnormal Expression of IGFs and Their Receptors VIII. SCATTER FACTORS A. Overexpression of HGF Enhances Granulation Tissue Formation and Wound Angiogenesis B. Expression of MSP at the Wound Site C. MSP Is Dispensable for Wound Repair IX. NERVE GROWTH FACTOR A. Expression of NGF in Skin Wounds B. Multiple Roles for NGF in Wound Healing? X. TRANSFORMING GROWTH FACTOR-{beta} A. Expression of TGF-{beta} at the Wound Site B. Neutralizing Antibodies to TGF-{beta}1 and -{beta}2 Reduce Scarring C. TGF-{beta}1-Deficient Mice Show Severely Impaired Late-Stage Wound Repair D. Immunosuppressive Approaches Allow the Study of TGF-{beta}1 Function in Adult Wounds E. TGF-{beta}1 Overexpression Studies Yield Contrasting Results, Dependent on the Transgenic Strategy F. Mice Expressing a Dominant-Negative Type II TGF-{beta} Receptor in the Epidermis Show Accelerated Reepithelialization and Reduced Keratinocyte Apoptosis G. Impaired Wound Healing in Mice Lacking the TGF-{beta} Type II Receptor in Fibroblasts H. Accelerated Cutaneous Wound Healing With an Increased Rate of Reepithelialization and Reduced Inflammation in Smad3-Null Mice XI. ACTIVINS A. Increased Expression of Activin After Skin Injury B. Overexpression of Activin in the Epidermis of Trangenic Mice Enhances Wound Repair and Scarring C. Impaired Wound Healing in Transgenic Mice Overexpressing the Activin Antagonist Follistatin in the Epidermis XII. BONE MORPHOGENETIC PROTEINS A. Expression of BMPs at the Wound Site B. Delayed Reepithelialization in Transgenic Mice Overexpressing BMP-6 in the Epidermis XIII. CONNECTIVE TISSUE GROWTH FACTOR/CYSTEINE-RICH 61/NEPHROBLASTOMA OVEREXPRESSED (CNN) FAMILY A. Expression of CTGF in Skin Wounds B. Expression of Cyr61 in Skin Wounds XIV. CHEMOKINES A. A Role for Macrophage Chemoattractant Protein in the Regulation of Inflammation, Granulation Tissue Formation, and Reepithelialization B. Macrophage Inflammatory Protein 1{alpha}: A Chemoattractant for Macrophages in the Healing Wound? C. Growth-Related Oncogene-{alpha} Regulates Macrophage Infiltration Into Healing Wounds D. Interleukin-8 Stimulates Inflammation but Inhibits Wound Contraction E. Impaired Wound Healing in CXCR2 Knock-out Mice F. Overexpression of Interferon-{gamma}-Inducible Protein 10 in the Epidermis of Transgenic Mice Stimulates Inflammation but Inhibits Reepithelialization G. Multiple Functions of Chemokines in Wound Repair XV. PROINFLAMMATORY CYTOKINES A. Expression of Proinflammatory Cytokines in Skin Wounds B. IL-6 Knock-out Mice Show Severe Deficits in Cutaneous Wound Repair C. STAT-3-Mediated Transduction of Cytokine Signals Is Important for Wound Repair D. Accelerated Wound Healing in TNF Receptor p55-Deficient Mice XVI. GRANULOCYTE-MACROPHAGE COLONY STIMULATING FACTOR A. Overexpression of GM-CSF in the Epidermis of Transgenic Mice Accelerates Wound Reepithelialization XVII. LEPTIN A. Systemic and Topical Application of Leptin Accelerates Wound Repair B. Expression of Leptin Receptors at the Wound Site XVIII. INTERLEUKIN-10 A. Expression of IL-10 at the Wound Site B. IL-10 Inhibits Inflammation and Scar Formation XIX. TEMPORAL AND SPATIAL INTERACTION OF DIFFERENT GROWTH FACTORS AT THE WOUND SITE) XX. CONCLUSIONS
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I. INTRODUCTION |
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In addition to the importance of cell-cell and cell-matrix interactions, all stages of the repair process are controlled by a wide variety of different growth factors and cytokines. Multiple studies have demonstrated a beneficial effect of many of these growth factors, e.g., platelet-derived growth factors (PDGFs), fibroblast growth factors (FGFs), and granulocyte-macrophage colony stimulating factor (GM-CSF) on the healing process, both in animal models and also in patients suffering from different types of wound healing disorders (1, 79, 107, 115, 196). However, the roles of endogenous growth factors in the healing response have been only partially elucidated, and in most cases, the suggested function of these molecules is based on descriptive expression studies and/or functional cell culture data. However, in vivo functions of many growth factors remain largely unconfirmed.
The development of transgenic and knock-out mouse technologies has provided new insights into the function of many different genes during embryonic development. These technologies allow gain of function experiments (overexpression of genes) as well as loss of function experiments (gene knock-outs by homologous recombination in embryonic stem cells or overexpression of dominant-negative mutants). Most importantly, spatial and temporal control of gene ablation or overexpression, using both inducible and cre-lox technologies, makes it possible to determine the functions of proteins formerly precluded due to embryonic lethality. A large number of viable genetically modified mice are now available that can be used to elucidate the role of the deleted, mutated, or overexpressed genes in different types of repair processes. Indeed, the past years have seen an exponential growth in the number of genetically modified mice that were used for wound healing experiments, and these studies have provided interesting, and often unexpected, results concerning the in vivo function of growth factors in wound repair (see http://icbxs.biol.ethz.ch/members/grose/woundtransgenic/home.html). In this review, we summarize the reported expression and function of endogenous growth factors and cytokines in cutaneous wound repair. Results of experiments with exogenous growth factors for the treatment of wound repair are only mentioned briefly, and reviews are cited wherever possible. In addition, we focus on those growth factors and cytokines for which results from functional in vivo studies are available.
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II. PLATELET-DERIVED GROWTH FACTOR FAMILY |
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- and a 
-chain (120, 121). PDGF was the first growth factor shown to be chemotactic for cells migrating into the healing skin wound, such as neutrophils, monocytes, and fibroblasts. In addition, it enhances proliferation of fibroblasts and production of extracellular matrix by these cells. Finally, it stimulates fibroblasts to contract collagen matrices and induces the myofibroblast phenotype in these cells (56, 121). Thus it has long been suggested to be a major player in wound healing. Indeed, a series of experimental and clinical studies have demonstrated a beneficial effect of PDGF for the treatment of wound healing disorders (121). Furthermore, PDGF was the first growth factor to be approved for the treatment of human ulcers (80, 169).
A. Expression of PDGF at the Wound Site
In addition to its therapeutical potency, a series of studies suggest an important role of endogenous PDGF in the repair process. Upon injury, PDGF is released in large amounts from degranulating platelets (233), and it is present in wound fluid, particularly early after injury (35, 116, 173, 183, 204, 255, 275, 281). Furthermore, expression of PDGFs and their receptors has been demonstrated in various cells of murine, pig, and human wounds using in situ hybridization and immunohistochemistry (5, 6, 21, 223, 294). The patterns of PDGF and PDGF receptor expression suggest a paracrine mechanism of action, since the ligands are predominantly expressed in the epidermis, whereas the receptors are found in the dermis and the granulation tissue. Interestingly, expression of PDGFs and their receptors was reduced in wounds of healing-impaired genetically diabetic db/db mice and glucocorticoid-treated mice (19, 21), indicating that a certain expression level of PDGFs and their receptors is essential for normal repair. This hypothesis was supported by the finding that impaired wound healing in aged mice is associated with a delay in appearance of PDGF A and B isoforms, and - and -receptors (10). Finally, the levels of PDGF in nonhealing human dermal ulcers were strongly reduced compared with surgically created acute wounds (216), further supporting an important role of PDGF for normal healing.
On the other hand, augmented PDGF production might be involved in the pathogenesis of hypertrophic scars and keloids as suggested by the potent effect of PDGF on fibroblast proliferation and extracellular matrix production by these cells (see above), the presence of enhanced levels of this growth factor in hypertrophic scar tissue (198), and the increased responsiveness of keloid fibroblasts to PDGF (114).
B. Inhibition of PDGF Action in Healing Skin Wounds
Based on its expression pattern in the healing wound and its known in vitro activities, PDGF has been suggested to have two major but distinct roles in wound repair: an early function to stimulate fibroblast proliferation and a later function to induce the myofibroblast phenotype (56). This hypothesis was supported by the finding that addition of neutralizing PDGF antibodies to human wound fluid caused a 45% reduction in the mitogenic effect of the wound fluid for cultured fibroblasts (143). However, a recent study demonstrated that the PDGF-B chain of hematopoietic origin is not necessary for granulation tissue formation and that its absence even enhances vascularization (42). In this study, the authors prepared hematopoietic chimeras, in which the hematopoietic system of a normal adult mouse was replaced by that of a PDGF B-chain -/- donor. In these chimeras the extent of local granulation tissue was not affected, and vascularization was increased. These findings suggest that the production of PDGF by other cell types in the wound is sufficient for normal healing. The use of neutralizing antibodies for wound healing studies or analysis of tissue-specific PDGF or PDGF receptor knock-out mice will help to further clarify the role of endogenous PDGF in wound repair.
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III. FIBROBLAST GROWTH FACTOR FAMILY |
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Most members of the FGF family have a broad mitogenic spectrum. They stimulate proliferation of various cells of mesodermal, ectodermal, and also endodermal origin. The only exception is FGF7 (keratinocyte growth factor, KGF), which seems to be specific for epithelial cells, at least in the adult organism (289). In addition to their mitogenic effects, FGFs also regulate migration and differentiation of their target cells, and some FGFs have been shown to be cytoprotective and to support cell survival under stress conditions (17, 206, 289).
Numerous in vivo effects of FGFs have been demonstrated, which suggest a role of these growth factors in wound repair. In particular, FGF1 and FGF2 were shown to stimulate angiogenesis in various assay systems (226). Furthermore, FGFs are mitogenic for several cell types present at the wound site, including fibroblasts and keratinocytes (1). Thus FGFs are clear candidates for contributing to the wound healing response, and this hypothesis has been corroborated by a number of studies where local application of FGF1, FGF2, FGF4, FGF7, or FGF10 stimulated tissue repair (1, 289).
A. Expression of FGFs in Healing Skin Wounds
Some FGFs have been detected at the wound site, indicating that the endogenous proteins are also regulators of wound healing. FGF2 was found in human and porcine wound fluid, particularly at early stages after injury (35, 50, 61, 106, 199, 281). Using immunohistochemistry, this FGF has been localized in injured skin. In a mouse incisional wound model, FGF2 was found extracellularly at the surface of the wound and within the dermis adjacent to the wound. Interestingly, this staining pattern was only seen in wounds of adult mice but not in fetal wounds where FGF2 immunoreactivity was undetectable. It was suggested that this difference could explain at least in part the reduced amount of capillary formation seen in the fetal versus adult wounds (294). In a full-thickness excisional wound model in mice, FGF2 was associated with hair bulbs at the wound edge and with basal keratinocytes of the normal and hyperproliferative wound epidermis (152). In rat burn wounds, FGF2 immunoreactivity was detected in the regenerated epidermis, in a bandlike zone near the regenerated epidermis, in renewed capillaries, and in cells infiltrating in the granulation tissue (145). Finally, a diffuse extracellular staining was seen at the edge of human burn wounds (101). The observed differences are probably due to species-specific differences or to different cross-reactivities of the antibodies with other members of the FGF family. To overcome this problem, two groups determined the expression of FGFs during wound healing at the mRNA level. Using in situ hybridization, Antoniades et al. (7) found upregulation of FGF1 and FGF2 expression in keratinocytes of porcine wound epidermis. Werner et al. (291) determined the mRNA levels of different FGFs in full-thickness excisional mouse wounds by RNase protection assay. Expression of FGF1, FGF2, FGF5, and FGF7 was found in normal and wounded skin, and expression of all these FGFs increased after skin injury. The most dramatic effect was seen with FGF7, which was more than 100-fold upregulated within 24 h after wounding. The strong upregulation of FGF7 expression was subsequently also confirmed for acute human wounds (172). In both mouse and human wounds, FGF7 mRNA was predominantly detected in dermal fibroblasts adjacent to the wound and in fibroblasts of the granulation tissue (172, 291). In addition, 
T cell receptor-bearing dendritic epidermal T cells (DETCs) were recently identified as a major source of FGF7 in murine skin wounds (134). Finally, FGF10 (KGF-2) was also shown to be expressed in mouse wounds (20, 134, 267), although upregulation of this type of FGF was only found in one study using RT-PCR (267), but not in another study where expression was determined by RNase protection assay (20). Similar to FGF7, FGF10 was predominantly expressed by DETCs (134) and fibroblasts (unpublished data).
In addition to the ligands, all FGF receptors are expressed in normal and wounded mouse skin (291; Werner, unpublished data). FGFR2IIIb, the only high-affinity receptor for FGF7, is expressed in keratinocytes of the normal and wounded epidermis as well as in hair follicles of murine, porcine, and human wounds (69, 172; Werner, unpublished data), and FGFR1 was found in the regenerating epidermis as well as in blood vessels of rat burn wounds (269).
Three different studies demonstrated a correlation between reduced FGF expression/responsiveness and wound healing disorders. Thus the mRNA levels of FGF1, FGF2, and FGF7 were reduced during wound healing in healing-impaired genetically diabetic mice compared with control mice (290). Furthermore, impaired would healing was seen in aged mice, and this impairment was associated with reduced levels of FGF2 and with a reduced angiogenic response in the skin of these mice upon addition of FGF2 (265). Finally, a member of the FGF family, most likely FGF2, was identified in a search for woundregulated proteins (250). Expression of this FGF was found to be upregulated after injury in normal but not in diabetic rats.
B. A Role for FGF2 in Wound Repair
To provide functional evidence for a role of FGF2 in wound repair, Broadley et al. (37) used a neutralizing polyclonal antibody that was raised against human FGF2. They incorporated the purified IgG into pellets, which were placed in the center of a polyvinyl alcohol sponge disk, and the disks were then implanted subcutaneously under ventral panniculus carnosus of rats. The continuous release of the antibody caused a striking reduction in cellularity and vascularization compared with the granulation tissue formed in the control IgG sponges. In addition, DNA, protein, and collagen levels in the anti-FGF2 sponges were reduced by 
25–35% relative to control at day 7 after implantation. This study strongly suggested an important role of endogenous FGF2 in wound repair, although cross-reactivity of this antibody with other members of the FGF family could not be excluded. The role of FGF2 in wound repair was finally clarified when FGF2 null mice were used for wound healing studies. Interestingly, these mice appeared superficially indistinguishable from wild-type littermates. However, when they were challenged by full-thickness excisional wounding, they showed delayed healing (208). In addition to a retardation in the rate of reepithelialization, mice null for FGF2 showed reduced collagen deposition at the wound site, and they had thicker scabs. In contrast, no wound healing abnormalities were observed in FGF1 knock-out mice, and in FGF1/FGF2 double knock-out mice, the defects were similar in extent to those seen in the FGF2 null animals (188). These results demonstrate that FGF1 is dispensable for wound healing in mice.
C. FGF Receptor Signaling Is Important for Reepithelialization
In addition to FGF2, several studies have provided evidence for an important role of FGF7 and its receptor (FGFR2IIIb) in cutaneous wound repair. The strong upregulation of this FGF in fibroblasts and DETCs after skin injury and the expression of its receptor in keratinocytes (see above) suggested that FGF7 stimulates wound reepithelialization in a paracrine manner. To test this hypothesis, transgenic mice were generated that express a dominant-negative FGFR2IIIb mutant in the epidermis (292). The mutant receptor lacks a functional tyrosine kinase domain and, upon ligand binding, forms nonfunctional heterodimers with full-length wild-type receptors, thereby blocking signal transduction (278, 279). The truncated FGFR2IIIb is known to bind FGF7, FGF10, FGF1, FGF3, and, although with lower affinity, also FGF2 (130, 207). Therefore, it should inhibit the action of all these ligands. The skin of the animals expressing the dominant-negative receptor mutant was characterized by epidermal atrophy, disorganization of the epidermis, hair follicle abnormalities, and dermal hyperthickening (292). Histological analysis of full-thickness excisional wounds revealed a severe delay in wound reepithelialization in the transgenic mice compared with control littermates. At day 5 after injury, the number of proliferating keratinocytes in the hyperproliferative epithelium was 80–90% reduced compared with control mice. These results demonstrated an important role for FGF receptor signaling in wound repair, although the type of FGF that is responsible for this defect was not defined by this study.
D. FGF7-Deficient Mice Show No Defect in Wound Healing
To further determine the role of FGF7 in development and repair, Guo et al. (113) used embryonic stem cell technology to generate mice lacking FGF7. Their knock-out mice revealed no obvious defects, with the exception of the fur, which appeared matted and greasy, especially in male animals. Most surprisingly, the healing process of full-thickness incisional wounds was not obviously affected by the lack of FGF7, and the proliferation rate of the keratinocytes at the wound edge was not altered. These data demonstrate that incisional wounds can heal in the absence of FGF7. It would, however, be interesting to study the healing process of excisional wounds in these animals, since the extent of reepithelialization is much higher in excisional than in incisional wounds.
The lack of obvious phenotypic abnormalities in the FGF7 null mice is contradictory to the results obtained with the dominant-negative FGFR2IIIb mutant (see above). Although it might be possible that FGF7 is indeed not involved in reepithelialization of skin wounds, this seems unlikely, since the pattern of FGF7 expression correlates well with its postulated functions in normal and wounded skin. The most likely explanation for the discrepancies between the knock-out and the dominant-negative receptor results is a redundancy in ligand signaling. Although FGF7 might normally be the most important ligand of FGFR2IIIb in the skin, the lack of this gene could be compensated for by other known ligands of this receptor. More recent data suggest that FGF10 is the principal candidate for effecting this compensation, since it is also expressed in normal and wounded skin (20, 134, 267). Furthermore, mice lacking DETCs have a significant delay in wound reepithelialization, most likely due to the lack of DETC-derived FGF7 and FGF10 in the healing wound (134). Studies using neutralizing FGF7 and/or FGF10 antibodies during wound repair should help to further clarify the roles of FGF7 and FGF10 in the healing process. The tissue-specific knockout of FGFR2IIIb, as well as double knock-outs of different ligands of this receptor, will shed more light on the role of FGFR2IIIb and the various types of FGF in normal and wounded skin.
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IV. EPIDERMAL GROWTH FACTOR FAMILY |
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(TGF-), heparin-binding EGF (HB-EGF), amphiregulin, epiregulin, betacellulin, neuregulins, the recently discovered epigen, as well as proteins encoded by Vaccinia virus and other poxviruses (263, 276, 303). In addition, more distantly related proteins known as neuregulins (heregulins, neu differentiation factors, NDF 1–4) can also bind to some EGF receptor family members (303). All these growth factors exert their functions by binding to four different high-affinity receptors, EGFR/ErbB1, HER2/ErbB2, HER3/ErbB3, and HER4/ErbB4 (Fig. 2A). Upon ligand binding, these receptors form homo- or heterodimers (303). Overexpression of these receptors, in particular of HER2, is often found in human cancers and is likely to have a causative role in tumorigenesis. In addition, a series of experimental and clinical studies have demonstrated a positive effect of EGF, TGF-, and HB-EGF on wound repair, suggesting that the endogenous growth factors are also involved in the healing process (107, 240, 259).
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A. Expression of EGF, TGF-, and HB-EGF at the Wound Site
First evidence for a role of EGF receptor ligands in wound healing came from the analysis of wound fluid. Grotendorst et al. (111) detected EGF-like factors in wound fluid collected from rats. Acid extracts from this type of wound fluid contained a chemotactic activity for endothelial cells that was neutralized with anti-EGF antisera (111). In addition, substantial levels of EGF and TGF- were found in wound fluid from skin graft donor site wounds in patients with small to moderatesized burn injuries (106). The result was confirmed for TGF- in another study (204). However, this group detected only very low levels of EGF in the same type of wound fluid. Several publications report on the presence of HB-EGF in wound fluid. Thus this growth factor was shown to be present at high levels in human burn wound fluid (184). In addition, HB-EGF was identified as the major heparin-binding growth factor in wound fluid of porcine partial-thickness excisional wounds (173). Because HB-EGF is mitogenic for fibroblasts and keratinocytes, it was suggested to play an important role in reepithelialization and granulation tissue formation. Interestingly, it was shown to act synergistically with insulin-like growth factor (IGF) I, another growth factor present in wound fluid, in stimulating keratinocyte proliferation in vitro (174).
In a search for the cellular source of these EGFR ligands in wounds, Rappolee et al. (219) detected TGF- mRNA in isolated wound macrophages. With the use of in situ hybridization and immunohistochemistry, this growth factor was also detected in eosinophils in a rabbit cutaneous open wound model and also in hamster wounds (272, 297). In addition, epidermal keratinocytes at the wound edge as well as hair follicle epithelial cells were identified as a source of TGF- in partial-thickness murine burn wounds, in particular during the phase of keratinocyte proliferation (65). EGF immunoreactivity was found to be associated with the presence of wound inflammatory cells and wound fibroblasts in early rat CO2 laser wounds (304). Finally, HB-EGF was localized in the advancing epithelial margin, islands of regenerating epithelium within human burn wounds, and in eccrine sweat glands (184). In another study, the same growth factor was detected in marginal surface keratinocytes and hair follicle epithelial cells of murine partial-thickness burn wounds, with maximal levels being found during the period of keratinocyte proliferation (65).
B. Expression of EGF Receptors at the Wound Site
EGF, TGF-, and HB-EGF exert their function via binding to the EGFR, a transmembrane protein tyrosine kinase that is expressed on many different cell types. Consistent with the expression of the ligands at the wound site, EGFR mRNA and protein were also detected in healing wounds. With the use of enzyme-linked immunosorbent assay and histological methods, an increase in the number of immunoreactive receptors was found in a tape stripping wound model before an increase in epidermal thickness. This early increase was followed by a decline in EGFR levels, which was followed by a decline in epidermal thickness (262). This expression pattern suggested a role of the EGFR in reepithelialization of skin wounds. In early human full- and partial-thickness burn wounds, EGFR was detected in undifferentiated, marginal keratinocytes, in keratinocytes of the hyperproliferative wound epidermis and hair follicles, as well as in sweat ducts and sebaceous glands (288). At later stages after injury, immunoreactive EGFR was still detected in the hyperthickened wound epidermis and in all appendages, but was absent from leading epithelial margins (288). This expression pattern of the EGFR in human burn wounds provided further evidence for a role of EGFR signaling in reepithelialization. In addition, the observed delayed appearance of EGF and EGF receptors in incisional wounds of aged mice compared with young mice (10) further suggests a functional role of these proteins in the healing process.
C. Ectodomain Shedding of EGF Receptor Ligands Is Required for Keratinocyte Migration During Wound Healing
In addition to these correlative data, recent functional studies revealed an important role of EGFR ligands in wound repair. All EGFR ligands are synthesized as membrane-anchored forms, which are proteolytically processed to the bioactive soluble forms (180). Interestingly, the transmembrane forms are also able to stimulate the growth of adjacent cells in a juxtacrine manner, indicating that both transmembrane and soluble forms might play a role in wound healing. However, processed HB-EGF was detected in wound fluid (173), suggesting that ligand shedding could play an important role in wound healing. Indeed, in vitro scratch wounding of a keratinocyte monolayer induced shedding of EGFR ligands, particularly of HB-EGF. Shedding was inhibited by the compound OSU8–1, and this in turn suppressed keratinocyte migration. Most interestingly, the application of this compound to full-thickness mouse wounds caused a strong retardation of reepithelialization as a result of impaired keratinocyte migration. This inhibition was reversed by addition of recombinant soluble HB-EGF along with OSU8–1 (273). These results indicate an important role of EGFR ligand shedding for keratinocyte migration in vitro and in vivo.
D. Wound Healing in Mice Deficient in TGF-
Based on the presence of TGF- in wound fluid (see above), its strong upregulation early after injury (113), and the beneficial effect of exogenous TGF- for wound healing, TGF- was expected to play an important role in the repair process. To test this possibility, two groups generated mice lacking this growth factor (164, 171). Surprisingly, these mice appeared normal with the exception of eye abnormalities and waviness of whiskers and fur. The epidermis of these animals was indistinguishable from that of control mice. Most interestingly, no significant wound healing abnormalities were observed in these mice, whereby two different wound models (full-thickness back skin excisions and tail amputation) were used. However, one group observed more variability in the rate of wound closure in TGF--deficient mice (164), suggesting that the lack of this mitogen can be compensated for to a variable extent by other growth factors. Such compensation could be achieved by other EGFR ligands, in particular HB-EGF. This hypothesis is supported by the severe phenotypic abnormalities of mice lacking the EGF receptor (187, 252) and of transgenic mice expressing a dominant-negative EGF receptor in the epidermis (193), although the wound-healing process in these animals has not been analyzed yet. In contrast, the lack of TGF- is unlikely to be compensated for exclusively by FGF7, since incisional wound healing also appeared normal in mice lacking both TGF- and FGF7 (113).
Although these initial studies suggested that TGF- is dispensable for wound healing, a more detailed analysis revealed a role of this factor in the early phase of reepithelialization (146). These investigators generated full-thickness head wounds and partial-thickness ear wounds in the TGF- knock-out mice. In the ear model, where healing is mainly achieved by reepithelialization, the knock-out mice had significantly larger epithelial gaps compared with control animals at days 3 and 5 after injury, and the epithelial thickness was reduced at these time points. However, wounds of both genotypes were completely reepithelialized at day 8 postwounding. In contrast, head wounds that heal by reepithelialization and granulation tissue formation were indistinguishable in TGF- null mice and control animals. These data suggest a role of TGF- in the early phase of reepithelialization, but the lack of this factor is compensated if healing is accompanied by granulation tissue formation. These results demonstrate the importance of the chosen wound model for the analysis of growth factor function in wound repair.
E. A Role of Neu Differentiation Factor in Wound Repair?
In addition to the EGF receptor ligands, Neu differentiation factor (NDF) might also play a role in the regulation of wound repair. Thus recombinant NDF-2 stimulated epidermal migration, epidermal thickness, and keratinocyte differentiation in a rabbit ear model of excisional wound repair (68). Endogenous NDF was found to be upregulated during the healing process of full-thickness excisional wounds, possibly as a response to increased levels of FGF7 and HGF which were found to be potent inducers of NDF expression in cultured keratinocytes (46). With the use of in situ hybridization, NDF -isoforms were found to be expressed in dermal fibroblasts of wounded and unwounded rabbit ear skin. HER2 and HER3 receptors, which mediate the function of NDF, were expressed in unwounded epidermis and dermal adnexa. After injury, expression of HER2 decreased in the wound neoepidermis, while neoepidermal HER3 expression was strongly upregulated (68). These results suggest that NDF stimulates keratinocyte migration during cutaneous wound repair in a paracrine manner.
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V. VASCULAR ENDOTHELIAL GROWTH FACTOR FAMILY |
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A. Expression of VEGF-A and Its Receptors in Skin Wounds
In support for a role of VEGF-A in wound repair, expression of this gene was shown to be strongly induced after cutaneous injury, with keratinocytes and macrophages being the major producers (40, 90). In addition, its receptors were detected on blood vessels of the granulation tissue (153, 213). This expression pattern suggested that VEGF-A stimulates wound angiogenesis in a paracrine manner. The important role of VEGF-A for the healing process was supported in several studies where reduced expression of VEGF-A or its accelerated degradation were found to be associated with wound healing defects (90, 140, 153, 265). Furthermore, treatment of ischemic wounds with VEGF-A or VEGF-A-overexpressing fibroblasts accelerated the healing process (33, 55), and adenovirus-mediated VEGF-165 gene transfer enhanced wound healing in diabetic mice by promoting angiogenesis (232).
B. A Role for VEGF-A in Wound Angiogenesis
The important role of VEGF-A in wound healing was recently revealed in a study where application of neutralizing VEGF-A antibodies caused a striking reduction in wound angiogenesis, fluid accumulation, and granulation tissue formation in a pig wound model (124). Furthermore, the angiogenic activity present in human wound fluid from later time points after injury was strongly inhibited by VEGF neutralization (200). Finally, retroviral delivery of a dominant-negative VEGFR-2 to murine skin wounds caused a strong reduction in angiogenesis and granulation tissue formation (277). However, wound closure was not affected in these animals due to increased wound contraction. These findings support the important role of endogenous VEGF in wound angiogenesis, although functional VEGFR2 signaling is obviously not critical for normal closure of acute excisional wounds.
C. Lack of PLGF Results in Impaired Wound Angiogenesis
In addition to VEGF-A, PLGF was recently identified as a regulator of wound angiogenesis. Expression of PLGF mRNA and protein was strongly upregulated in migrating keratinocytes of acute human skin wounds. Furthermore, endothelial cells of capillaries adjacent to the wound expressed PLGF (84). This upregulation appears to be of functional importance, since PLGF knockout mice were characterized by impaired wound healing as a result of a defect in angiogenesis (45). Interestingly, a synergy between VEGF-A and PLGF was detected in these studies, indicating that the presence of both growth factors is important for normal wound angiogenesis.
D. Expression of VEGF-C and Its Receptor in Healing Skin Wounds
Besides the formation of new blood vessels, lymphangiogenesis occurs during the healing of skin wounds. Several groups have shown the formation of lymphatic vessels to be regulated via VEGFR-3 and its ligands VEGF-C and VEGF-D (142). In a recent study using a pig wound model, VEGFR-3-positive lymphatic vessels were found in the wound granulation tissue (209). These vessels appeared in the wound concurrently with blood vessels but regressed earlier. The responsible ligand is probably VEGF-C, which is expressed in normal and wounded mouse skin (unpublished data). Interestingly, a relative absence of lymphatic vessels was found in chronic human wounds (209), which might be one of the reasons for their impaired healing. Taken together, members of the VEGF family are likely to be major regulators of angiogenesis and lymphangiogenesis not only during development but also during cutaneous wound repair.
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VI. ANGIOPOIETINS |
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A. Expression of Angiopoietins and Their Receptor in Healing Skin Wounds
First evidence for a role of angiopoietins in wound healing came from studies by Wong et al. (296), who demonstrated upregulation of Tie2 protein and mRNA in rat and mouse skin wounds, respectively. Moreover, Tie2 was found to be tyrosine-phosphorylated in the healing wound, indicating active downstream signaling. In addition to the receptor, two groups demonstrated expression of angiopoietins-1 and -2 in normal and wounded mouse skin. Whereas angiopoietin-1 expression was not affected by skin injury, angiopoietin-2 expression was transiently upregulated during the period of granulation tissue formation in normal mice (30, 140). In healing-impaired genetically diabetic mice, the period of angiopoietin-2 upregulation was extended (140). Thus wounds in diabetic mice are characterized by high levels of angiopoietin-2 but low levels of VEGF-A (90), a situation that has been suggested to lead to blood vessel regression during tumorigenesis (123). These findings suggest that the strongly impaired angiogenic response in diabetic animals could result from an imbalance in the levels of VEGF-A and angiopoietins.
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VII. INSULIN-LIKE GROWTH FACTORS |
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A. Expression of IGFs and Their Receptors in Skin Wounds
Several groups demonstrated expression of IGF-I and IGF-II in wounds of different species. Thus IGF-I was found in rat and porcine wound fluid (174, 230, 260), and minimal degradation of this protein was observed (231). In an attempt to localize IGFs and their receptors at the wound site, one group used a rat ear freeze-thaw injury model to study IGF-I expression by immunohistochemistry (135). In normal skin, only a few cells in the dermis and epidermis expressed this protein. However, all epidermal cells as well as macrophages and some other inflammatory cells were positive within 1–3 days after wounding. Others used an incisional wound model as well as a subcutaneous sponge implant model to determine expression of IGF-I and IGF-II in the wound (97). Interestingly, the mRNA levels of both IGFs increased significantly after injury in both models. Increased IGF-I mRNA levels but unaltered IGF-I receptor expression were observed in a rat wound model where steel wire mesh cylinders were implanted in the subcutaneous tissue of the back (260). Finally, in situ hybridization studies on porcine wounds revealed expression of IGF-I, IGF-I receptor, and IGF-II receptor mRNAs in epithelial cells of normal and wounded skin. In this study, however, no major differences between nonwounded and wounded skin were observed (7).
B. Impaired Wound Healing Is Associated With Abnormal Expression of IGFs and Their Receptors
Several studies suggest a role of the IGF system in the wound healing abnormalities associated with diabetes and glucocorticoid treatment. Thus one group found that streptozotocin-induced diabetes in rats caused a 42% reduction in wound fluid IGF-I levels (27). Others analyzed the expression of IGF-I and IGF-II during wound healing in normal and genetically diabetic mice (39). The normal induction of IGF-I mRNA expression was severely delayed and reduced in diabetic mice. Delayed induction was also seen for IGF-II, although peak concentrations of IGF-II mRNA were higher in diabetic compared with control mice. Consistent with the RNA data, a delayed appearance of the proteins was noted in diabetic animals. In another study, subcutaneously implanted polyvinyl sponges and stainless steel mesh chamber models were used to analyze the levels of IGF-I, IGF-I receptor, and IGFBP3 mRNAs in wound tissue of healing-impaired diabetic and glucocorticoid-treated rats (26). Interestingly, expression of all these genes was strongly reduced in the healing-impaired animals, further supporting the importance of the IGF system for normal healing. These findings are likely to be important for the pathogenesis of chronic human wounds, since IGF-I protein was absent in the basal layer of the epidermis and in fibroblasts of diabetic patients but not of healthy control patients. Furthermore, it was absent in the basal keratinocyte layer at the edge of human diabetic foot ulcers (28). Taken together, these studies suggest that reduced expression of IGFs and/or their receptors leads to impaired wound healing, although this hypothesis has yet to be confirmed by functional studies.
On the other hand, enhanced expression of IGF-I might lead to excessive scarring as suggested by the observed overexpression of IGF-I in postburn hypertrophic scar tissue compared with control skin. Because IGF-I was shown to increase the expression of the pro alpha 1(I) chain of type I procollagen and the pro alpha 1 (III) chain of type III procollagen in cultured dermal fibroblasts, these findings indicate a causative role of elevated IGF-I levels in the pathogenesis of hypertrophic scars (98).
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VIII. SCATTER FACTORS |
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HGF was independently discovered as a powerful mitogen for hepatocytes and as a stimulator of dissociation of epithelial cells. Due to these features it was designated HGF or SF. It is predominantly produced by cells of mesenchymal origin and acts via a high-affinity transmembrane tyrosine kinase receptor (MET) on various cell types. In addition, heparan sulfate proteoglycans act as low-affinity receptors for HGF and allow accumulation of the ligand in the proximity of its target cells (59). Because HGF stimulates migration, proliferation, and matrix metalloproteinase production of keratinocytes (78, 182), as well as new blood vessel formation (43), it has been suggested to play a role in cutaneous wound repair.
MSP is a liver-derived serum protein that regulates proliferation and differentiation of various cell types. In the serum MSP is predominantly present in the inactive precursor form, whereas active MSP is only generated at the surface of its target cells. The latter express RON, the only known high-affinity receptor for this protein. It is present on many different cell types, including macrophages and keratinocytes, suggesting a function of MSP in wound repair (155, 253).
A. Overexpression of HGF Enhances Granulation Tissue Formation and Wound Angiogenesis
Expression of HGF and its receptor MET was found to be strongly upregulated in keratinocytes of the wound epidermis as well as in several cell types in the granulation tissue during the healing of excisional wounds in rats (63). This upregulation is likely to be of functional importance, since transgenic mice overexpressing HGF under the control of the metallothionein promoter were characterized by enhanced granulation tissue formation after full-thickness excisional wounding, and the number of blood vessels in the granulation tissue was strongly increased. This effect on wound angiogenesis seems to be at least partially mediated via VEGF-A, since the latter was overexpressed in these transgenic mice (274). In contrast, reepithelialization was obviously not affected by overexpression of HGF. These results revealed important activities of HGF during wound healing, although the role of the endogenous protein in the healing process remains to be determined.
B. Expression of MSP at the Wound Site
First evidence for a role of MSP in wound healing came from studies by Nanney et al. (195), who demonstrated the presence of MSP in wound exudates of burn patients. Interestingly, a large percentage of the wound exudate-derived MSP was found to be in the active form, and MSP was shown to be responsible for the stimulatory effect of wound exudate on macrophages. In the same study, a marked upregulation of RON expression was demonstrated in burn wound epidermis and accessory structure as well as on macrophages and capillaries of the granulation tissue (195). Because MSP stimulates macrophage pinocytosis and phagocytosis in vitro (253), this study suggested that MSP may enhance macrophage-dependent wound debridement. In another study, the localization of MSP and RON was determined in full-thickness excisional wounds in rats (63). MSP-positive cells were identified by immunofluorescence at the wound edge as well as in cells within the wounds, and some of them were shown to be monocytes. In addition, RON was detected in the granulation tissue, but not in the wound epidermis.
C. MSP Is Dispensable for Wound Repair
To determine the role of MSP in cutaneous wound repair, mice lacking the msp gene were generated (24). Although these animals were characterized by delayed macrophage activation, no macroscopic differences in the healing of incisional wounds were observed. However, it is still possible that these mice have subtle wound healing abnormalities that are only detectable upon histological and/or molecular analysis.
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IX. NERVE GROWTH FACTOR |
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A. Expression of NGF in Skin Wounds
A role of endogenous NGF in wound healing was further supported by studies of Constantinou et al. (60), who found a marked increase in NGF levels after wounding of neonatal but not of adult rats. Subsequently, a rise in serum NGF levels after generation of full-thickness wounds in mice was demonstrated, which was shown to be due to release of NGF from the salivary gland (181). In addition, NGF levels also increased at the wound site in the same wound model, and NGF mRNA was detected in newly formed epithelial cells at the wound edge and in granulation tissue fibroblasts (181). A particular high expression of NGF was found in myofibroblasts within the granulation tissue of rat wounds, with much higher levels being found in myofibroblasts of neonatal compared with adult animals (118).
B. Multiple Roles for NGF in Wound Healing?
Due to its potent effects on sensory nerves, the major function of NGF in the wound tissue appears to be the stimulation of nerve ingrowth. This hypothesis is supported by results obtained in an in vitro coculture model, which demonstrated a potent effect of adult rat dorsal foot skin on dorsal root ganglia neurite outgrowth. This function was blocked by neutralizing antibodies to NGF (224). However, the activity in wounded neonatal skin was not blocked by these antibodies, suggesting the presence of other factors in neonatal wounds that induce neurite outgrowth (224).
Because innervation has been shown to be essential for normal wound healing (117 and references therein), the stimulatory effect of NGF on the wound repair process is likely to be at least partially due to its effect on nerves. This might be of particular importance in diabetic patients who suffer from peripheral neuropathy, which often results in impaired wound healing. Indeed, NGF administration was shown to protect against experimental diabetic sensory neuropathy (8), and NGF depletion was found in keratinocytes in diabetic human skin (4), suggesting that NGF might be helpful for the treatment of diabetic foot ulcers.
