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Background Hypertrophic scars and keloids, characterized by over-proliferation of fibroblasts and aberrant formation of the extracellular matrix (ECM), are considered fibrotic diseases. Accumulating evidence indicates that mesenchymal stem cells (MSCs) promote scar-free wound healing and inhibit fibrotic tissue formation, making them a potentially effective therapeutic treatment for hypertrophic scars and keloids. Objective To investigate the paracrine effects of bone marrow derived MSCs (BMSCs) on the biological behavior of hypertrophic scar fibroblasts (HSFs) and keloid fibroblasts (KFs). Methods Proliferative and profibrotic phenotype changes of the fibroblasts were analyzed by immunofluorescence staining, in-cell western blot, and real-time PCR. Results BMSC-conditioned medium inhibited HSF and KF proliferation and migration, but did not induce apoptosis. Interestingly, normal skin fibroblast-conditioned medium exhibited no inhibitory effects on HSF or KF proliferation and migration. Furthermore, BMSC-conditioned medium significantly decreased expression of profibrotic genes, including connective tissue growth factor, plasminogen activator inhibitor-1, transforming growth factor-β 1 , and transforming growth factor-β 2 , in HSFs and KFs at both transcriptional and translational levels. In contrast, the expression of antifibrotic genes, such as transforming growth factor-β 3 and decorin, was substantially enhanced under the same culture conditions. Finally, we observed that BMSC-conditioned medium suppressed the ECM synthesis in HSFs and KFs, as indicated by decreased expression of collagen I and fibronectin and low levels of hydroxyproline in cell culture supernatant. Conclusion These findings suggest that BMSCs attenuate the proliferative and profibrotic phenotype associated with HSFs and KFs and inhibit ECM synthesis through a paracrine signaling mechanism. [ABSTRACT FROM AUTHOR]