Interactions between TGF-β1, canonical WNT/β-catenin pathway and PPAR γ in radiation-induced fibrosis

Radiation therapy induces DNA damage and inflammation leading to fibrosis. Fibrosis can occur 4 to 12 months after radiation therapy. This process worsens with time and years. Radiation-induced fibrosis is characterized by fibroblasts proliferation, myofibroblast differentiation, and synthesis of collagen, proteoglycans and extracellular matrix. Myofibroblasts are non-muscle cells that can contract and relax. Myofibroblasts evolve towards irreversible retraction during fibrosis process. In this review, we discussed the interplays between transforming growth factor-beta 1 (TGF-beta 1), canonical WNT/beta-catenin pathway and peroxisome proliferator-activated receptor gamma (PPAR gamma) in regulating the molecular mechanisms underlying the radiation-induced fibrosis, and the potential role of PPAR gamma agonists. Overexpression of TGF-beta and canonical WNT/beta-catenin pathway stimulate fibroblasts accumulation and myofibroblast differentiation whereas PPAR gamma expression decreases due to the opposite interplay of canonical WNT/beta-catenin pathway. Both TGF-beta 1 and canonical WNT/beta-catenin pathway stimulate each other through the Smad pathway and non-Smad pathways such as phosphatidylinositol 3-kinase/serine/threonine kinase (PI3K/Akt) signaling. WNT/beta-catenin pathway and PPAR gamma interact in an opposite manner. PPAR gamma agonists decrease beta-catenin levels through activation of inhibitors of the WNT pathway such as Smad7, glycogen synthase kinase-3 (GSK-3 beta) and dickkopf-related protein 1 (DKK1). PPAR. agonists also stimulate phosphatase and tensin homolog (PTEN) expression, which decreases both TGF-beta 1 and PI3K/Akt pathways. PPAR gamma agonists by activating Smad7 decrease Smads pathway and then TGF-beta signaling leading to decrease radiation-induced fibrosis. TGF-beta 1 and canonical WNT/beta-catenin pathway promote radiation-induced fibrosis whereas PPAR gamma agonists can prevent radiation-induced fibrosis.