PPARα inhibits TGF-β-induced β5 integrin transcription in vascular smooth muscle cells by interacting with Smad4

Integrins play an important role in vascular smooth muscle cell (VSMC) migration, a crucial event in the development of restenosis and atherosclerosis. Transforming growth factor-beta (TGF-beta) is highly expressed in restenotic and atherosclerotic lesions, and known to induce integrin expression. Peroxisome proliferator-activated receptor alpha (PPARalpha), a member of the nuclear receptor superfamily, regulates gene expression in a variety of vascular cells. We investigated the effects of PPARalpha ligands on TGF-beta-induced beta(3) and beta(5) integrin expression and potential interaction between PPARalpha and TGF-beta signaling. PPARalpha ligands WY-14643 (100 mumol/L) and 5,8,11,14-eicosatetranoic acid (ETYA, 50 mumol/L) inhibited TGF-beta-induced beta(5) integrin protein expression by 72 +/- 6.8% and 73 +/- 7.1%, respectively (both P<0.05). TGF-β-stimulated β(3) integrin expression was not affected by PPARα ligands. Both PPARα ligands also suppressed TGF-β-induced β(5) integrin mRNA levels. PPARα ligands inhibited TGF-β-inducible transcription of β(5) integrin by an interaction with a TGF-β response element between nucleotides -63 and -44, which contains a Sp1/Sp3 transcription factor binding site. Nuclear complexes binding to the TGF-β response region contained Sp1/Sp3 and TGF-β-regulated Smad 2, 3, and 4 transcription factors. TGF-β-stimulated Sp1/Smad4 nuclear complex formation was inhibited by WY-14643 and ETYA with a parallel induction of PPARα/Smad4 interactions. However, in vitro pull-down experiments failed to demonstrate direct binding between PPARa/Smad4. Both PPARa ligands blocked PDGF-directed migration of TGF-β-pretreated VSMCs, a process mediated, in part, by β(5) integrins. The present study demonstrates that PPARα activators inhibit TGF-β-induced β(5) integrin transcription in VSMCs through a novel indirect interaction between ligand-activated PPARα and the TGF-β-regulated Smad4 transcription factors. The full text of this article is available at http://www.circresaha.org.