Increased expression of fibroblast activation protein-alpha in keloid fibroblasts: implications for development of a novel treatment option

Keloid scars are common benign fibroproliferative reticular dermal lesions with unknown etiology and ill-defined management with high rate of recurrence post surgery. The progression of keloids is characterized by increased deposition of extracellular matrix proteins, invasion into the surrounding healthy skin and inflammation. Fibroblasts are considered to be the key cellular mediators of fibrogenesis in keloid scars. Fibroblast activation protein alpha (FAP-α) and dipeptidyl peptidase IV (DPPIV) are proteases located at the plasma membrane promoting cell invasiveness and tumor growth and have been previously associated with keloid scars. Therefore, in this study we analyzed in further detail the expression of FAP-α in keloid fibroblasts compared to control skin fibroblasts. Dermal fibroblasts were obtained from punch-biopsies from the active margin of four keloids and four control skin samples. Flow cytometry was used to analyze FAP-α expression and the CytoSelect^® 24-Well Collagen I Cell Invasion Assay was applied to study fibroblast invasion. Secretion of extracellular matrix (ECM) proteins was investigated by multiplexed particle-based flow cytometric assay and enzyme-linked immunosorbent assay. We found an increased expression of FAP-α in keloid fibroblasts compared to control skin fibroblasts (p < 0.001). Inhibition of FAP-α/DPPIV activity using the irreversible inhibitor H₂N-Gly-Pro diphenylphosphonate reduced the increased invasiveness of keloid fibroblasts (p < 0.001) indicating that keloid invasion may be partly FAP-α/DPPIV mediated. FAP-α/DPPIV inhibition had no effect, (a) on the synthesis of the ECM proteins procollagen type I C-terminal peptide and fibronectin, (b) on the production of fibroblast growth factor or vascular endothelial growth factor, (c) on the expression of the proinflammatory cytokines interleukin-6 (IL-6), interleukin 8 (IL-8) or monocyte chemotactic protein-1. These results suggest a potential role for FAP-α and DPPIV in the invasive behavior of keloids. FAP-α and DPPIV may increase the invasive capacity of keloid fibroblasts rather than by modulating inflammation or ECM production. Since FAP-α expression is restricted to reactive fibroblasts in wound healing and normal adult tissues are generally FAP-α negative, inhibiting FAP-α/DPPIV activity may be a novel treatment option to prevent keloid progression.