Disruption of transforming growth factor β signaling and profibrotic responses in normal skin fibroblasts by peroxisome proliferator–activated receptor γ

Abstract

Objective. In fibroblasts, transforming growth factor ␤ (TGF␤) stimulates collagen synthesis and myofibroblast transdifferentiation through the Smad intracellular signal transduction pathway. TGF␤-mediated fibroblast activation is the hallmark of scleroderma and related fibrotic conditions, and disrupting the intracellular TGF␤/Smad signaling may provide a novel approach to controlling fibrosis. Because of its potential role in modulating inflammatory and fibrotic responses, we examined the expression of the nuclear hormone receptor peroxisome proliferator-activated receptor ␥ (PPAR␥) in normal skin fibroblasts and its effect on TGF␤-induced cellular responses.Methods. The expression and activity of PPAR␥ in normal dermal fibroblasts were examined by Northern and Western blot analyses, immunocytochemistry, flow cytometry, and transient transfections with reporter constructs. The same approaches were used to evaluate the effects of PPAR␥ activation by naturally occurring and synthetic ligands on collagen synthesis and ␣-smooth muscle actin (␣-SMA) expression. Modulation of Smad-mediated transcriptional responses was examined by transient transfection assays using wild-type and dominant-negative PPAR␥ expression constructs. Abnormal synthesis and tissue accumulation of collagen are hallmarks of scleroderma and are responsible for the damage and failure of affected organs. Lesional scleroderma fibroblasts display an activated phenotype characterized by accelerated transcription of genes coding for collagen and other extracellular matrix proteins, increased expression of cell surface receptors for transforming growth factor ␤ (TGF␤), and sustained production of TGF␤, connective tissue growth factor, Supported by grants from the NIH (AR-46390 and AR-42309) and the Scleroderma Foundation. Results

Pentoxifylline Attenuates Tubulointerstitial Fibrosis by Blocking Smad3/4-Activated Transcription and Profibrogenic Effects of Connective Tissue Growth Factor

“…In the absence of effect on nuclear translocation and DNA binding of Smad3/4 complexes, cAMP-elevating agents can interfere with Smad3/4-dependent gene transcription in nucleus via reduction of Smad3/4-CBP/p300 interactions and possible sequestration of CBP and p300 by phosphorylated CREB in a PKA-dependent manner. Smad mediates TGF-␤1 signaling to the activation of ␣-SMA transcription in mouse AKR-2B fibroblasts and normal skin fibroblasts (43,45). However, the activation of Smad is not enough to induce ␣-SMA expression in NRK-52E because such expression was blocked by antisense CTGF gene transfection, a finding in agreement with that in human proximal tubular Figure 11.…”

“…epithelial cells (32). Interference with TGF-␤1-induced Smad signaling by peroxisome proliferator-activated receptor ␥ activation or hepatocyte growth factor treatment has been shown to downregulate ␣-SMA expression in normal skin fibroblasts and human kidney epithelial cells (34,45). Thus, we speculate that PTX inhibits TGF-␤1-induced expression of ␣-SMA in NRK-52E by inhibiting CTGF expression through blocking Smad-dependent gene transcription.…”

The Synthetic Peroxisome Proliferator-Activated Receptor-γ Agonist Ciglitazone Attenuates Neuroinflammation and Accelerates Encapsulation in Bacterial Brain Abscesses

“…This finding was revealed by enhanced fibronectin deposition and the earlier appearance of myofibroblasts expressing ␣-smooth muscle actin. This finding is at direct odds with other literature demonstrating that PPAR-␥ agonists attenuate myofibroblast differentiation or fibrosis in models of pathological fibrosis such as scleroderma or pulmonary/hepatic fibrosis (52)(53)(54)(55)(56). However, it is important to acknowledge a fundamental difference between these models of excessive fibrosis vs brain abscess in which a highly regulated fibrotic wall forms in a region-specific manner.…”

“…In contrast, in models of fibrosis, the actions of PPAR-␥ ligands have, in large part, been shown to be receptordependent (52)(53)(54). Therefore, it is highly likely that ciglitazone may exert both receptor-dependent and -independent effects in brain abscesses through its ability to accelerate fibrosis and inhibit inflammatory responses, respectively.…”

The contribution of transforming growth factor- and epidermal growth factor signalling to airway remodelling in chronic asthma

“…It was further reported that PPAR-c could physically interact with Smad3, highlighting a potential mechanism of action for the ligand. More recent studies have shown that PPAR-c ligands can prevent TGF-b 1 -induced fibronectin in mesangial cells [206] and skin fibroblasts [207].…”