Peroxisome proliferator-activated receptor-γ agonist troglitazone suppresses transforming growth factor-β1 signalling through miR-92b upregulation-inhibited Axl expression in human keloid fibroblasts in vitro.

Keloid, a skin benign tumor, is characterized by overgrowth of fibroblasts and the excessive deposition of extracellular matrix in wounded skin. Peroxisome proliferator-activated receptor-γ (PPAR-γ) agonist was recently evaluated to inhibit fibrosis. This study explored the underlying mechanisms. Fibroblasts isolated from 25 keloid patients (KFs) and fibroblasts isolated from healthy controls (NSFBs) were also subjected to treatment with PPAR-γ agonist troglitazone and antagonist GW9662 or for transfection with miR-92 mimics or inhibitor, Axl siRNA, and miR-92b or Axl promoter constructs, as well as being subjected to qRT-PCR, ELISA, Western blot, protein array, luciferase, and ChIP assays. The data demonstrated that TGF-β1 and Axl proteins were significantly elevated in samples from keloid patients, while troglitazone treatment significantly reduced levels of TGF-β1 and Axl mRNA and proteins in KFs. Moreover, knockdown of Axl expression reduced expression of TGF-β1 and its pathway genes (such as α-SMA and Snail). PPAR-γ regulation of Axl expression was through transcriptional activation of miR-92b. miR-92b expression downregulated Axl expression at both mRNA and protein levels, whereas GW9662 completely reversed the inhibitory effects of miR-92b mimics on Axl expression. Gene ontology analysis of miR-92b targeting genes showed that TGF-β and Axl were both potential targets of miR-92b, as confirmed by luciferase assay. These findings demonstrated that PPAR-γ-induced miR-92b expression inhibited Axl expression and in turn reduced expression of TGF-β1 and the downstream genes in KFs, suggesting that targeting of this novel gene pathway may be useful for therapeutic control of fibrosis or keloid.