PPAR-α Ligands Inhibit H 2 O 2 -Mediated Activation of Transforming Growth Factor-β1 in Human Mesangial Cells

Abstract

Transforming growth factor-β1 (TGF-β1) mediates the development of glomerulosclerosis by stimulating mesangial cell production of extracellular matrix (ECM) proteins. TGF-β1 and several ECM genes are regulated by promoter O-tetradecanoylphorbol 13-acetate-responsive elements (TREs) that are transactivated by the activator protein-1 (AP-1) transcription factor complex. AP-1-TRE interactions are regulated by redox changes. Recently, peroxisome proliferator-activated receptors (PPARs) were shown to negatively regulate several transcription factor families. In these studies, we postulated that PPAR-α could antagonize TGF-β1 expression by cultured human mesangial cells (HMC). A TGF-β1 luciferase expression plasmid was transduced into HMC via recombinant deficient adenoviral vectors. The TGF-β1 promoter activity increased twofold (209%) following 18-h treatments with H₂O₂ (1,000 µM). Using RT-PCR, we demonstrated that HMC possess PPAR-α RNA, and PPAR-α protein was identified by immunohistochemistry. Pretreatment of cells with the PPAR-α ligands WY14643 (100-500 µM) or clofibrate (100-500 µM) dose-dependently inhibited oxidant-mediated induction of TGF-β1. This inhibition occurred without affecting the H₂O₂-mediated activation of the mitogen-activated protein kinase (MAPK) pathways extracellular regulated kinase, p38 MAPK, or Jun N-terminal kinase, which are responsible for the regulation of AP-1 phosphorylation. These studies are the first to identify PPAR-α expression by HMC. The results of these studies suggest that TGF-β1 expression mediated by oxidant stress may be suppressible by PPAR-α activation.