PPAR-γ Ligands Repress TGFβ-Induced Myofibroblast Differentiation by Targeting the PI3K/Akt Pathway: Implications for Therapy of Fibrosis

Transforming growth factor beta (TGF beta) induced differentiation of human lung fibroblasts to myofibroblasts is a key event in the pathogenesis of pulmonary fibrosis. Although the typical TGF beta signaling pathway involves the Smad family of transcription factors, we have previously reported that peroxisome proliferator-activated receptor-gamma (PPAR-gamma) ligands inhibit TGF beta-mediated differentiation of human lung fibroblasts to myofibroblasts via a Smad-independent pathway. TGF beta also activates the phosphatidylinositol 3 kinase/protein kinase B (PI3K/Akt) pathway leading to phosphorylation of Akt(S473). Here, we report that PPAR-gamma ligands, 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid (CDDO) and 15-deoxy-(12,14)-15d-prostaglandin J(2) (15d-PGJ(2)), inhibit human myofibroblast differentiation of normal and idiopathic pulmonary fibrotic (IPF) fibroblasts, by blocking Akt phosphorylation at Ser473 by a PPAR-gamma-independent mechanism. The PI3K inhibitor LY294002 and a dominant-negative inactive kinase-domain mutant of Akt both inhibited TGF beta-stimulated myofibroblast differentiation, as determined by Western blotting for alpha-smooth muscle actin and calponin. Prostaglandin A(1) (PGA(1)), a structural analogue of 15d-PGJ(2) with an electrophilic center, also reduced TGF beta-driven phosphorylation of Akt, while CAY10410, another analogue that lacks an electrophilic center, did not; implying that the activity of 15d-PGJ(2) and CDDO is dependent on their electrophilic properties. PPAR-gamma ligands inhibited TGF beta-induced Akt phosphorylation via both post-translational and post-transcriptional mechanisms. This inhibition is independent of MAPK-p38 and PTEN but is dependent on TGF beta-induced phosphorylation of FAK, a kinase that acts upstream of Akt. Thus, PPAR-gamma ligands inhibit TGF beta signaling by affecting two pro-survival pathways that culminate in myofibroblast differentiation. Further studies of PPAR-gamma ligands and small electrophilic molecules may lead to a new generation of anti-fibrotic therapeutics.