Homeostasis of the epidermal barrier layer: a theory of how occlusion reduces hypertrophic scarring.

The mechanism of hypertrophic scar reduction using silicone gel sheeting remains elusive. We hypothesize that the decrease in scar formation is due to occlusion and homeostasis of the barrier layer. Using an established model of hypertrophic scarring, rabbits were divided into four groups and scars were tape-stripped or occluded with Kelocote, Cavilon, or Indermil, with each rabbit serving as its own internal control. All wounds were harvested on day 28 and examined histologically to measure the scar elevation index (SEI), epithelial thickness, and cellularity. Immunohistochemistry fluorescence was used to quantify inflammation in the dermis. Transepidermal water loss (TEWL) was measured for each occlusive agent and tape stripping. Ultrastructural analysis was performed by electron microscopy. Kelocote, Cavilon, and Indermil all significantly decreased SEI when compared with controls. Each of the occlusive treatments was shown to decrease TEWL while tape stripping increased TEWL. Tape stripping significantly increased the SEI, epithelial thickness, and cellularity. Immunostaining for macrophages showed increased density of inflammatory cells in the tape-stripped scars. Under electron microscopy, the tape-stripped wounds displayed extensive inflammation and keratinocyte damage. Both unwounded skin and occlusion-treated scars did not display these characteristics. In conclusion, hypertrophic scarring was reduced regardless of occlusive method used. Furthermore, repeated disruption of the permeability barrier by tape stripping led to an increase in scarring. Ultrastructural analysis suggests that occluded wounds may be in an advanced state of wound repair. Occlusion may mediate its effects through establishing homeostasis of the epidermal barrier layer.

Quantifying tissue level ischemia: hypoxia response element-luciferase transfection in a rabbit ear model.

Ischemia is a common underlying factor in a number of pathologic conditions ranging from cardiac dysfunction to delayed wound healing. Previous efforts have shown the resulting hypoxia activates the hypoxia inducible factor, a transcription factor with signaling effects through an intranuclear hypoxia response element (HRE). We hypothesized that ischemic conditions should activate these hypoxic signaling pathways in a measurable manner. We tested our hypothesis using variations of an established rabbit ear ischemic wound model and an HRE-luciferase-reporter gene construct. This plasmid construct was transfected into the ears of young, female New Zealand White rabbits, harvested at day 7 and processed to yield a reactive solution. Luminometry was used to quantify luciferase expression in each solution as a marker for HRE activation in each wound. Quantitative readings of hypoxic signaling as measured by luminescence yielded profound and statistically significant differences between the various ischemic models. Our results suggest that the biologic systems for hypoxic signaling can be used to detect local ischemia. HRE-luciferase transfection is an effective tool for quantifying the degree of tissue hypoxia. The caudal ischemic rabbit ear model showed significantly higher levels of hypoxia. Use of a validated model that produces sufficient tissue levels of hypoxia is recommended for meaningful study of ischemic wound healing.

Antisense inhibition of connective tissue growth factor (CTGF/CCN2) mRNA limits hypertrophic scarring without affecting wound healing in vivo.

Augmented expression of connective tissue growth factor (CTGF/CCN2) is observed in healing wounds and in a variety of fibrotic disorders. It appears to enhance many of the effects of transforming growth factor-beta and has been shown to have independent fibrogenic functions. Despite these observations, its importance to dermal wound healing and the transition from wound to scar remains poorly defined. In this study, we use established rabbit models to evaluate the roles of CTGF in dermal wound healing and hypertrophic scarring. We show that CTGF mRNA demonstrates persistent up-regulation in hypertrophic scars. Treatment of wounds with antisense oligonucleotides to CTGF has no measurable effect on early wound closure. However, antisense therapy significantly limits subsequent hypertrophic scarring. Inhibition of CTGF is associated with a marked reduction in the number of myofibroblasts in scars and decreased transcription of TIMP-1 and types I and III collagen. These findings confirm CTGF to be a key mediator of hypertrophic scarring in this model. Its effect on myofibroblasts in this setting suggests a mechanism whereby it plays this role. Its limited participation in early healing implies that it may be a useful and specific target for modulating hypertrophic scarring following injury.

Temporal expression of the transforming growth factor-Beta pathway in the rabbit ear model of wound healing and scarring.

BACKGROUND: Despite numerous studies that have investigated the cellular and molecular mechanisms underlying scar formation, this process still remains poorly understood. The importance of transforming growth factor-beta (TGF-beta) in these processes has been well recognized, and this study sought to define the temporal expression of the key members in this pathway in a well-established, clinically relevant, rabbit ear model of hypertrophic scarring. STUDY DESIGN: Seven-millimeter (hypertrophic) and 5-mm (nonhypertrophic) punch wounds were made on the ears of 12 rabbits. Wounds were harvested at days 0, 7, 15, 28, and 40. RESULTS: There were no appreciable histologic differences between the 5- and 7-mm wounds at days 7 and 15. At day 28, however, the 7-mm scars were considerably more hypertrophic compared with the 5-mm control scars (p<0.001). The mRNA levels of TGF-beta1 and collagen Ialpha2 were notably higher in the hypertrophic 7-mm scars at day 28 than in the nonhypertrophic 5-mm scars (p<0.03). Although not pronounced, levels of TGF-beta2 were higher in the hypertrophic scars. There were no other statistically significant differences between the 7- and 5-mm scars. CONCLUSIONS: Elevated levels of TGF-beta1, and possibly TGF-beta2, are associated with hypertrophic scar formation.

The temporal effects of anti-TGF-beta1, 2, and 3 monoclonal antibody on wound healing and hypertrophic scar formation.

BACKGROUND: A number of studies have implicated transforming growth factor (TGF)-beta1, 2, and 3 (TGF-beta) in wound healing and hypertrophic scarring. We propose that TGF-beta has a temporal effect on these processes. To test this hypothesis, we applied anti-TGF beta1, 2, and 3 monoclonal antibody topically to our dermal ulcer model in the rabbit ear. STUDY DESIGN: Rabbit ear wounds were treated intradermally with anti-TGF-beta1, 2, and 3 antibody at early, middle, and late time points. Treated and untreated control wounds were harvested at various time points and examined histologically to quantify wound healing and scar hypertrophy. Real-time polymerase chain reaction was performed to determine TGF-beta mRNA expression in the treated and control wounds. RESULTS: The early treatment group demonstrated decreased new epithelium and granulation tissue (p < 0.05 versus controls). Scars harvested on days 28 and 40 displayed no difference in scar hypertrophy. Both the middle and late treatment groups demonstrated a significant decrease in scar hypertrophy (p < 0.05). CONCLUSIONS: Treated wounds from the early treatment group displayed delayed wound healing, with no reduction in scar hypertrophy. Later treatment of wounds with the same antibody, beginning 7 days after wounding, resulted in a reduction in scar hypertrophy. These results support our hypothesis and clearly demonstrate that TGF-beta1, 2, and 3 have differential temporal effects during the wound-healing process, and are important for optimal wound healing in the first week after wounding; beyond 1 week, TGF-beta1, 2, and 3 play a critical role in hypertrophic scar formation.

Fibulin-5 promotes wound healing in vivo.

BACKGROUND: Fibulin-5 is a recently discovered multifunctional extracellular matrix protein that mediates endothelial cell adhesion through integrin ligation, regulates cell growth and motility in a context-specific manner, and prevents elastinopathy in vivo. Because fibulin-5 expression is induced dramatically in endothelial and smooth muscle cells in response to mechanical injury, my colleagues and I studied its role in dermal wound healing. STUDY DESIGN: We have used a gene therapy approach that used retroviral gene transfer to deliver fibulin-5 to the dermal wound milieu. Surgical generation of rabbit ear full-thickness dermal ulcers was performed in six female New Zealand white rabbits (6 months old). Wounds were injected with retrovirus containing either sense or antisense fibulin-5 or control vectors. Wounds were harvested on postwounding day 8 and analyzed by confocal microscopy, in situ hybridization, and histology. RESULTS: We report that fibulin-5 promotes wound healing in vivo. Wounds infected with fibulin-5 showed a considerable net increase (approximately 50%) in both newly formed granulation tissue volume and wound closure. Fibulin-5 expression stimulated substantial expression of collagen in dermal wounds. CONCLUSIONS: Taken together, our findings provide the first known example of overexpression of one extracellular matrix protein (fibulin-5) enhancing expression of another (type I collagen) in vivo. Our findings also demonstrate a novel role for fibulin-5 and suggest that altering extracellular matrix protein production through gene therapy may provide a novel means to promote wound healing.

Use of hypoxia-inducible factor signal transduction pathway to measure O2 levels and modulate growth factor pathways.

Tissue PO2 levels are known to directly modulate numerous processes involved in the reparative response to cutaneous tissue injury, including cell differentiation and migration, extracellular matrix synthesis and maturation, and effectiveness of endogenous and exogenous growth factors. Oxygen is therefore likely the critical variable determining the healing capabilities of any tissue. Significant advances in the understanding of cutaneous wound healing progressed with advances in the measurement of tissue PO2, which has advanced over the past several decades from implantable probes to now include molecular tools such as the transcription factor hypoxia inducible factor-1 (HIF-1). HIF-1 modulates the expression of genes that drive the cellular adaptive response to hypoxia and possess the HIF-1 binding sequence named hypoxia response element within their promoter sequence. Molecular biology techniques are now allowing exploitation of the HIF-1/hypoxia response element pathway to drive the expression of potential vulnerary ectopic genes. Here we show the utility of the hypoxia response element for hypoxia-driven expression of the transforming growth factor-beta-signaling component Smad3 in vitro and the in vivo detection of ischemic hypoxia using luciferase. Smad3 is a positive effector of transforming growth factor-beta superfamily signal transduction. Such approaches are the latest evolution of work championed by Hunt and colleagues over the past 4 decades.