The Effect of Suppressing Discoidin Domain Receptor Expression on Keloid Formation and Proliferation
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Functionally, DDRs are tyrosine kinase receptors, which are distinct from other receptor groups due to their extracellular domain that resembles discoidin8—a developmentally regulated lectin first described in the slime mold dictyostelium discoideum.12,13 Many studies have revealed that DDR1 is overexpressed in human tumors including breast, ovarian, esophageal, and brain tumors and are believed to be a tumor marker14; the reason is the expression of DDR1 were highly up-regulated as collagen receptor, which sense the actively remodeling of extracellular matrix, especially the collagen, resulting from the powerful erosion of malignant tumors. The authors hypothesize that DDR1 may be highly expressed in keloids for its excessive deposition of collagen and dermal tissue remodeling.
In this study, the role of DDR1 in keloids was evaluated. As expected, histological analysis demonstrated that the expression of DDR1 was widely and strongly distributed in keloid tissues. The extent of expression was highly increased on the keloid margins and less increased in the center. This showed that the expression of DDR1 was related to keloid infiltration. The authors assume that in the area of infiltration the higher extent of destruction in the ECM was related to the increased ECM remodeling and higher DDR1 expression. The expression of DDR1 was also higher in the normal skin around the keloid than the normal, healthy skin (data not shown), revealing that the person was genetically susceptible due to the keloid. However, in order to avoid new keloid formation, we cannot draw normal skin samples from patients already affected by keloid.
Additionally, when the cell culture was made from the tissue explants, the keloid fibroblasts were more easily cultured from the keloid margin. Thus, the keloid fibroblasts were from the area of keloid infiltration. In order to create further experiments, the expression of DDR1 was made by Western blot analysis between keloid fibroblasts and normal skin fibroblasts. The result showed that the expression of DDR1 was significantly higher in keloid dermal fibroblasts than in normal skin fibroblasts, demonstrating a difference between the function and phenotype of keloid fibroblasts and the normal skin fibroblasts. Others have also demonstrated this phenomenon.11
Next, we examined the production of collagen in keloid dermal fibroblasts after blocking the expression of DDR1. After liposome-encapsulation with DDR1-ASODN transfected into the keloids dermal fibroblasts, the relative expression of DDR1 mRNA was dramatically inhibited. This inhibition progressively increased with an increased concentration of DDR1-ASODN. The relative expressions of Col-1A and Col-3A mRNA levels also were significantly inhibited. Inhibition gradually increased as the final concentration of DDR1-ASODN increased. Using image analysis software, it was observed that the ratio of α (I) to α (III)-collagen mRNA decreased significantly as the concentration increased. Moreover, the production of collagen also decreased significantly after transfection. The extent of decrease continued as the concentration of DDR1-ASODN increased. These results suggest that blocking the expression of DDR1 would downregulate behavior of keloid dermal fibroblasts. However, this conclusion does not completely support the results of Chin et al7 who found that there was a gradual increase in collagen production that was concomitant with progressively lower expression of DDR1 in fetal rat fibroblasts during the characteristic fetal rat skin conversion from scarless phenotype to a scarring phenotype.
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