Transforming growth factor-beta (TGF-beta) isoforms are multifunctional cytokines that play an important role in wound healing. Transgenic mice overexpressing TGF-beta in the skin under control of epidermal-specific promoters have provided models to study the effects of increased TGF-beta on epidermal cell growth and cutaneous wound repair. To date, most of these studies used transgenic mice that overexpress active TGF-beta in the skin by modulating the latency-associated-peptide to prevent its association with active TGF-beta. The present study is the first to use transgenic mice that overexpress the natural form of latent TGF-beta 1 in the epidermis, driven by the keratin 14 gene promoter to investigate the effects of locally elevated TGF-beta 1 on the healing of partial-thickness burn wounds made on the back of the mice using a CO(2) laser. Using this model, we demonstrated activation of latent TGF-beta after wounding and determined the phenotypes of burn wound healing. We found that introduction of the latent TGF-beta1 gene into keratinocytes markedly increases the release and activation of TGF-beta after burn injury. Elevated local TGF-beta significantly inhibited wound re-epithelialization in heterozygous (42% closed versus 92% in controls, P < 0.05) and homozygous (25% versus 92%, P < 0.01) animals at day 12 after wounding. Interestingly, expression of type I collagen mRNA and hydroxyproline significantly increased in the wounds of transgenic mice, probably as a result of a paracrine effect of the transgene.
Transforming growth factor-beta (TGF-beta) is known to affect nearly every aspect of wound repair. Many of the effects have been extensively investigated; however, the primary effect of endogenously derived TGF-beta on wound reepithelialization is still not completely understood. To examine this, two types of wounds were made on a transgenic mouse over-expressing TGF-beta1. Full-thickness back wounds were made to compare the wound healing process in the presence of compensatory healing mechanisms. Superficial partial-thickness ear wounds involving only the epidermis were made to determine the effect of TGF-beta on reepithelialization. In the partial-thickness ear wounds, at later time points, the transgenic group had smaller epithelial gaps than the wild-type mice. A greater number of actively proliferating cells, as determined by bromodeoxyuridine incorporation, was also found in the transgenic mice at post-injury day 8. These results show that TGF-beta1 stimulates the rate of reepithelialization at later time points in partial-thickness wounds. However, in the full-thickness back wounds, the transgenic animals exhibited a slower reepithelialization rate at all time points and the number of bromodeoxyuridine-positive cells was fewer. Our findings would suggest that the overexpression of TGF-beta1 speeds the rate of wound closure in partial-thickness wounds by promoting keratinocyte migration. In full-thickness wounds, however, the overexpression of TGF-beta1 slows the rate of wound reepithelialization.
Transforming growth factor-beta1 is a fibrogenic cytokine that is important in the development of fibroproliferative disorders of the skin after injury. To investigate the role of transforming growth factor-beta1 produced by keratinocytes during wound healing, a plasmid with the human transforming growth factor-beta1 gene coupled with the keratin 14 promoter (pG3Z: K14-TGF-beta1) was constructed. The construct was tested successfully in vitro before being used to generate transgenic animals, which were subsequently bred into homozygous and heterozygous lines. Genotype screening of founders and progeny was performed by Southern blotting and targeting of the transgene to the epidermis by the keratin 14 promoter was shown by reverse transcription polymerase chain reaction. The major phenotypic change observed in the transgenic animals was "scruffiness" of the fur attributed to transgene expression in the skin, seen primarily in the homozygous line. A significant reduction in the rate of reepithelialization of full-thickness excisional wounds of dorsal skin was seen in homozygous animals compared with normal litter-mate controls at day 7 (p < 0.05, Fisher's Exact test) and day 9 (p < 0.01) postwounding. Wounds in heterozygous animals also healed more slowly at day 9 (p < 0.01). Northern analysis of mRNA extracted from the wounds showed increased human transforming growth factor-beta1 message levels in homozygous and heterozygous animals, maximal at day 5. Significant increases in transforming growth factor-beta1 activity in healing wounds measured using the plasminogen activator inhibitor-1/luciferase assay were found in the transgenic strains at day 9 postinjury as compared with the normal litter-mate control mice (p < 0.001, ANOVA). Type I procollagen mRNA expression was higher in the homozygous and heterozygous animals, with the highest levels reached at day 9. By day 5 postwounding, biopsies of both homozygous and heterozygous tissues were significantly higher in collagen as compared with wounds in control animals (p < 0.05, ANOVA). Based on these data, the K14-TGF-beta1 transgenic mouse shows that excessive latent transforming growth factor-beta1 produced in the epidermal layer of the skin delays reepithelialization in excisional wounds but subsequently the cells of the epidermis stimulate dermal fibroblasts leading to fibrosis through a paracrine mechanism.
This study was conducted to assess the ability of a dermal cream containing liposome-encapsulated interferon- alpha2b (IFN-alpha2b) (LIPO+IFN) to improve hypertrophic scarring in open and reepithelialized dermal wounds in a rabbit fibrotic ear model. Full-thickness skin wounds were made in New Zealand white rabbits, and were either left untreated, treated on day 16 postsurgery (open wound), or treated on day 23 postsurgery (reepithelialized wound) with either LIPO+IFN or liposome alone (LIPO). The conditions of the wounds were monitored until day 35 postsurgery, when hypertrophic scar formation reached its peak. Dry wound weight, scar thickness, hypertrophic index (HI), and tissue cellularity of treated and untreated wounded tissue samples were evaluated as an index for scar formation. The results of this study showed that reepithelialized wounds treated with LIPO+IFN and to a lesser extent with LIPO alone were reduced in thickness, HI, and cellularity compared with untreated control wounds or LIPO+IFN-treated open wounds. Dry wound weight was also reduced but not significantly. The findings of this study suggest that LIPO+IFN is more effective than using LIPO alone in reducing the scar formation in a rabbit fibrotic ear model. Further investigation is required to confirm these results.
JCR:LA-cp/cp obese rats and their lean controls were evaluated as a type 2 diabetic wound healing model and the healing quality was characterized. This model of insulin resistance has been used extensively to study atherosclerosis but has not previously been used to study wound healing. Six circular excisional wounds were made on the dorsum of each rat and followed to day 21. Tracings of the wounds were made and used to assess the rate of wound closure. Planimetry showed a significantly diminished contraction of wounds in obese rats, but no significant difference in reepithelialization was observed. Collagen content was determined from the hydroxyproline content in wounded and unwounded skin. There were significantly lower levels of hydroxyproline in the wounds of obese compared to lean animals at day 21. Histology showed adipose tissue in place of dermal tissue in the JCR:LA-cp/cp rat in both unwounded tissue and in the wound at day 21. Active transforming growth factor-beta 1 (TGF-beta 1) was measured in the serum using the plasminogen activator inhibitor-1/luciferase assay and serum total TGF-beta was measured using an enzyme-linked immunosorbent assay. Active TGF-beta was significantly higher in the serum of obese animals compared with lean animals, while total TGF-beta 1 was not significantly different between the groups. Both active and total TGF-beta was measured in tissue sections using the plasminogen activator inhibitor-1/luciferase assay. There was no significant difference in active TGF-beta between genotypes, while obese rats had significantly higher levels of total TGF-beta at day 21. These results indicate a deficiency in wound healing in obese animals characterized by decreased wound contraction, decreased collagen production, and changes in histology. The JCR:LA-cp rat develops insulin resistance, atherosclerosis and early type 2 diabetes and may be a good model for impairment of wound healing in humans with metabolic syndrome.
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