Tissue regeneration repairs the fabric of the skin to maintain homeostasis after injury. The expression and proliferation of extracellular matrix (ECM) molecules in the dermis, mediated by a range of growth factors and cytokines, is a fundamental element of wound repair. Previous work focused on how these complex molecular mechanisms relate to the formation of raised dermal scars, including keloid and hypertrophic scars, characterized by excessive deposition of ECM molecules. However, the mechanisms in the wound repair pathway which lead to the differential expression and organization of ECM molecules observed in different types of scar tissue are not fully understood. To summarize what is known about the expression and composition of ECM molecules in abnormal scarring, an extensive search of the literature was conducted, focusing on keywords connected to skin scarring, hypertrophic scars and keloid disease. The transcription and translation of collagen I and III, fibronectin, laminin, periostin and tenascin are all increased in raised dermal scar tissue. However, hyaluronic acid, dermatopontin and decorin are decreased, and the expression and localisation of fibrillin and elastin fibres in the dermis are altered compared with normal skin and scars. Recent whole genome profiling and proteomic studies have led to the identification of regulatory elements with different expression profiles in hypertrophic and keloid tissue. If the mechanisms of raised dermal scar formation are to be elucidated and effective therapeutic treatments developed, an integrated approach to research is required, focussing on the interactions between ECM molecules, regulatory elements and pathways.

Extracellular matrix molecules implicated in hypertrophic and keloid scarring

Significance: Regulation of cell adhesions during tissue repair is fundamentally important for cell migration, proliferation, and protein production. All cells interact with extracellular matrix proteins with cell surface integrin receptors that convey signals from the environment into the nucleus, regulating gene expression and cell behavior. Integrins also interact with a variety of other proteins, such as growth factors, their receptors, and proteolytic enzymes. Re-epithelialization and granulation tissue formation are crucially dependent on the temporospatial function of multiple integrins. This review explains how integrins function in wound repair. Recent Advances: Certain integrins can activate latent transforming growth factor beta-1 (TGF-β1) that modulates wound inflammation and granulation tissue formation. Dysregulation of TGF-β1 function is associated with scarring and fibrotic disorders. Therefore, these integrins represent targets for therapeutic intervention in fibrosis. Critical Issues: In...

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Integrins in Wound Healing

Increased types I and III collagen and transforming growth factor-beta 1 mRNA and protein in hypertrophic burn scar

Keloids constitute an abnormal fibroproliferative wound healing response in which raised scar tissue grows excessively and invasively beyond the original wound borders. This review provides a comprehensive overview of several important themes in keloid research: namely keloid histopathology, heterogeneity, pathogenesis, and model systems. Although keloidal collagen versus nodules and α-SMA-immunoreactivity have been considered pathognomonic for keloids versus hypertrophic scars, conflicting results have been reported which will be discussed together with other histopathological keloid characteristics. Importantly, histopathological keloid abnormalities are also present in the keloid epidermis. Heterogeneity between and within keloids exists which is often not considered when interpreting results and may explain discrepancies between studies. At least two distinct keloid phenotypes exist, the superficial-spreading/flat keloids and the bulging/raised keloids. Within keloids, the periphery is often seen as the actively growing margin compared to the more quiescent center, although the opposite has also been reported. Interestingly, the normal skin directly surrounding keloids also shows partial keloid characteristics. Keloids are most likely to occur after an inciting stimulus such as (minor and disproportionate) dermal injury or an inflammatory process (environmental factors) at a keloid-prone anatomical site (topological factors) in a genetically predisposed individual (patient-related factors). The specific cellular abnormalities these various patient, topological and environmental factors generate to ultimately result in keloid scar formation are discussed. Existing keloid models can largely be divided into in vivo and in vitro systems including a number of subdivisions: human/animal, explant/culture, homotypic/heterotypic culture, direct/indirect co-culture, and 3D/monolayer culture. As skin physiology, immunology and wound healing is markedly different in animals and since keloids are exclusive to humans, there is a need for relevant human in vitro models. Of these, the direct co-culture systems that generate full thickness keloid equivalents appear the most promising and will be key to further advance keloid research on its pathogenesis and thereby ultimately advance keloid treatment. Finally, the recent change in keloid nomenclature will be discussed, which has moved away from identifying keloids solely as abnormal scars with a purely cosmetic association toward understanding keloids for the fibroproliferative disorder that they are.

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The Keloid Disorder: Heterogeneity, Histopathology, Mechanisms and Models.

Keloid disease is a benign, yet locally aggressive and recurrent cutaneous fibroproliferative condition characterised by excessive scarring. Unique to humans, keloids represent the end-point of a spectrum of abnormal wound healing, are aesthetically disfiguring and can cause major functional impairment. Its heterogeneous phenotype can confound clinical diagnosis leading to mismanagement. This review examines the histological morphology of keloid disease relative to the underlying pathobiology, places it in the context of other cutaneous fibroses and highlights gaps within the literature that hinder differential diagnosis. The pathological similarity to hypertrophic scarring, dermatofibrosarcoma protuberans, dermatofibroma and scleroderma emphasise the importance of detailing the architectural and cellular components of this unique entity. In the papillary dermis keloid tumours show a tongue-like advancing edge that resembles invasive tumour growth. A thickened but flattened epidermis, hyalinised haphazardly arranged collagen bundles that dominate the dermis with subsequent obliteration of the papillary-reticular boundary along with displacement and eventually destruction of skin appendages, exemplify additional hallmark findings associated with keloid disease. Compared to healthy skin, keloid scars show an increased type I/III collagen ratio, decreased fibrillin-1 and decorin expression, increased dermal cellularity and increased expression of fibronectin, versican, elastin and tenascin in the reticular dermis and hyaluronan and osteopontin in the epidermis. We illustrate these "pathognomonic" features of keloid disease by representative micrographs and discuss them in the context of inflammation, hypoxia and tension--as key elements of keloid disease. Finally, we highlight deficits within the keloid research literature as well as discuss important areas for future research in keloid histology.

Functional histopathology of keloid disease

The overproduction of fibronectin and type I collagen in keloids and hypertrophic scars implicates altered regulation of extracellular matrix components as an important aspect of these wound healing pathologies. However, little is known about the similarities and differences in extracellular matrix gene expression during normal and abnormal wound healing. This study compared the content of fibronectin messenger RNA and rates of fibronectin protein biosynthesis in fibroblasts derived from normal skin, normal scar, keloid, and hypertrophic scar. Fibronectin expression was enhanced in cells from both normal and abnormal wounds relative to cells from quiescent normal skin. Matched pairs of normal and keloid fibroblasts from the same individuals were also compared, and three of the four pairs showed higher fibronectin expression by the keloid cells at the levels of messenger RNA and protein synthesis. This was consistent with previous studies showing elevated steady state content of fibronectin in keloid cells relative to normal cells from the same individual. Fibronectin messenger RNA and protein content in the tissues from which these cells were derived was examined by in situ hybridization and immunohistochemistry. These studies revealed that in vivo, the steady state content of fibronectin messenger RNA and protein was highest in abnormal wounds, less in most normal scars, and lowest in normal skin. Thus, fibroblasts from keloids and hypertrophic scars overexpressed fibronectin in vivo relative to normal skin and normal scar and retain this characteristic in vitro relative to normal skin. Although normal scars contained little fibronectin protein and messenger RNA, cultured fibroblasts derived from these scars had contents of fibronectin messenger RNA and rates of biosynthesis in vitro similar to those of keloid fibroblasts. This indicates that the fibronectin regulatory pathway in scar fibroblasts is influenced by the tissue environment. These results are discussed with respect to the relationship of fibronectin expression in keloids, hypertrophic scars, and normal wounds in human beings.

Fibronectin gene expression differs in normal and abnormal human wound healing.

Tenascin is an extracellular matrix molecule with structural similarity to fibronectin. An increase in extracellular matrix content of both tenascin and fibronectin is associated with early wound healing and with various skin fibroses. However, the relationship of tenascin and fibronectin expression during scar remodeling and the formation of pathologic scars such as keloids is unknown. Expression of tenascin in normal and abnormal human scars was examined and compared with that of fibronectin by immunohistochemistry and in situ hybridization. Tenascin and fibronectin protein and messenger RNA contents were elevated in normal, mature scars relative to quiescent skin, similar to the situation during earlier stages of healing. Tenascin and fibronectin expression was further enhanced in keloids relative to normal skin and scar, and, as has been shown for fibronectin, tenascin expression in uninjured skin adjacent to keloids was indistinguishable from that in quiescent skin from unaffected individuals. These data suggest that tenascin and fibronectin gene expression are coordinated during later stages of normal wound healing and that a defect involving common regulatory elements for these genes is associated with the formation of keloids.

Gene expression of tenascin is altered in normal scars and keloids.

Keloids are pathological lesions characterized by enhanced expression of extracellular matrix molecules including fibronectin. A molecular dissection of the human fibronectin promoter was performed to identify DNA-protein interactions that correlate with altered fibronectin gene expression by keloid fibroblasts. DNA mobility shift patterns generated by nuclear extracts from skin, scar, and keloid fibroblasts were identical at a consensus CRE at -170 of the human fibronectin promoter whereas extracts from keloid fibroblasts formed complexes at a CRE/AP-1-like sequence at -415 that differed from those generated by skin and scar fibroblast extracts. The DNA-protein interactions identified at -415 were sensitive to altered serum concentrations in skin and scar but not keloid fibroblast cultures. The effects of forskolin and TGF-beta on fibronectin expression correlated with changes in the DNA-protein complexes assembled on the -170 and -415 cis elements, respectively. Oligonucleotides containing consensus CRE and AP-1 sequences did not compete for binding of nuclear proteins to the CRE/AP-1-like domain at -415, suggesting that this is a unique cis element. These studies indicate that the human fibronectin promoter contains two cis elements on which related but nonidentical complexes form. Alterations in the complexes interacting with the sequence at -415 may be responsible for the differences in fibronectin gene expression among quiescent skin, mature scar, and keloid fibroblasts.

Jill C. Sible

Papers

Fibronectin gene expression differs in normal and abnormal human wound healing.

Gene expression of tenascin is altered in normal scars and keloids.

DNA binding proteins from keloid fibroblasts form unique complexes with the human fibronectin promoter.