PPAR-γ ligands repress TGFβ-induced myofibroblast differentiation by targeting the PI3K/Akt pathway: Implications for therapy of fibrosis

Abstract

Transforming growth factor beta (TGFβ) induced differentiation of human lung fibroblasts to myofibroblasts is a key event in the pathogenesis of pulmonary fibrosis. Although the typical TGFβ signaling pathway involves the Smad family of transcription factors, we have previously reported that peroxisome proliferator-activated receptor-γ(PPAR-γ) ligands inhibit TGFβ-mediated differentiation of human lung fibroblasts to myofibroblasts via a Smad-independent pathway. TGFβ also activates the phosphatidylinositol 3 kinase/protein kinase B (PI3K/Akt) pathway leading to phosphorylation of AktS473. Here, we report that PPAR-γ ligands, 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid (CDDO) and 15-deoxy-(12,14)-15dprostaglandin J2 (15d-PGJ2), inhibit human myofibroblast differentiation of normal and idiopathic pulmonary fibrotic (IPF) fibroblasts, by blocking Akt phosphorylation at Ser473 by a PPAR-γ-independent mechanism. The PI3K inhibitor LY294002 and a dominant-negative inactive kinase-domain mutant of Akt both inhibited TGFβ-stimulated myofibroblast differentiation, as determined by Western blotting for α-smooth muscle actin and calponin. Prostaglandin A1 (PGA1), a structural analogue of 15d-PGJ2 with an electrophilic center, also reduced TGFβ-driven phosphorylation of Akt, while CAY10410, another analogue that lacks an electrophilic center, did not; implying that the activity of 15d-PGJ2 and CDDO is dependent on their electrophilic properties. PPAR-γ ligands inhibited TGFβ-induced Akt phosphorylation via both posttranslational and post-transcriptional mechanisms. This inhibition is independent of MAPK-p38 and PTEN but is dependent on TGFβ-induced phosphorylation of FAK, a kinase that acts upstream of Akt. Thus, PPAR-γ ligands inhibit TGFβ signaling by affecting two pro-survival pathways that culminate in myofibroblast differentiation. Further studies of PPAR-γ ligands and small electrophilic molecules may lead to a new generation of anti-fibrotic therapeutics. © 2011 Kulkarni et al.