Transdifferentiated circulating monocytes release exosomes containing 14-3-3 proteins with matrix metalloproteinase-1 stimulating effect for dermal fibroblasts

Fibroblasts are major cellular components of healing wounds. In this regard, it remains to be fully understood how different paracrine signals may influence the final collagen/matrix metalloproteinase (MMP) balance in resident fibroblasts. Our previous reports have demonstrated that circulating stem cells and monocytes can be transdifferentiated into keratinocyte-like cells under certain culture conditions. These transformed cells are able to stimulate MMP-1 expression in dermal fibroblasts. However, the underlying mechanism of this cell-to-cell interaction is unknown. This study describes exosomes as a major delivery system that keratinocyte-like cells use to release proteins into the conditioned media. The exosomes exhibited distinctive size, density, and saucer-like morphology. Using PKH-26 and GFP-adenovirus infection, we demonstrated that exosomes are able to fuse and then release their protein content into dermal fibroblasts. Mass spectrometry and Western blotting identified five 14-3-3 isoforms (beta, gamma, epsilon, tau, and zeta) as MMP-1 stimulating factors for dermal fibroblasts. Immunoprecipation assays confirmed that these 14-3-3 isoforms account for almost the entire MMP-1 up-regulation induced by exosomes. In summary, our results demonstrated that circulating monocytes stimulated to be transformed into keratinocyte-like cells could promote an anti-fibrogenic commitment of dermal fibroblasts via exosomal 14-3-3 proteins.