Paraquat increases connective tissue growth factor and collagen expression via angiotensin signaling pathway in human lung fibroblasts

Abstract

Survivors of paraquat poisoning are left with pulmonary fibrosis which results in a restrictive type of long-term pulmonary dysfunction. Connective tissue growth factor (CTGF) is a key growth factor that initiates tissue repair and underlies the development of lung fibrosis. Angiotensin (ANG) II may induce CTGF expression in the heart and kidney and plays an important role in the pathogenesis of lung fibrosis. The biological effects of ANG II are mediated by ANG II type 1 receptor (AT1R) and AT2R. The aims of this study were to investigate the effects of paraquat on ANG II, ANG II receptors, CTGF, and collagen expressions and to assess the role of ANG II receptors in paraquat-induced collagen synthesis in human lung fibroblasts (MRC-5). MRC-5 cells were incubated with various concentrations of paraquat with or without the ANG II receptor antagonist, saralasin. Paraquat increased ANG II production and AT1R mRNA and protein expression and decreased AT2R mRNA expression. Furthermore, paraquat treatment increased CTGF and collagen mRNA and protein expression in a dose-dependent manner and saralasin inhibited these effects. These results indicate that paraquat increases CTGF and collagen expression by activating angiotensin signaling pathway in human lung fibroblasts.

Introduction

Paraquat dichloride (1,1′-dimethyl-4,4′-bipyridilium dichloride; methyl viologen) is an effective and widely used herbicide. The intentional and accidental ingestion of commercial liquid formulations of paraquat has caused a large number of human fatalities in Taiwan during 1985 and 1997 (Satoh and Hosokawa, 2000). The lungs are one of the primary target organs in paraquat-induced toxicity in rats and humans (Chen and Lua, 2000, Dinis-Oliveira et al., 2008). The acute toxic effects of paraquat are pulmonary edema, hypoxia, and respiratory failure. Survivors of paraquat poisoning may be left with pulmonary fibrosis which results in a restrictive type of long-term pulmonary dysfunction (Yamashita et al., 2000).

Connective tissue growth factor (CTGF) is an important growth factor that initiates lung tissue repair and fibrosis (Bogatkevich et al., 2008). CTGF is a member of the CCN (CTGF, Cyr61/Cef10, Nov) family and was originally identified in conditioned media from human umbilical vein endothelial cells and mice fibroblasts; it has been implicated in fibroblast proliferation, cellular adhesion, angiogenesis, and extracellular matrix synthesis (Moussad and Brigstock, 2000). Angiotensin (ANG) II induces CTGF expression in the heart and kidney (Finckenberg et al., 2003, Rupérez et al., 2003) and plays an important role in the pathogenesis of lung fibrosis (Marshall et al., 2004). The biological effects of ANG II are mediated by its interaction with two distinct high-affinity G protein-coupled receptors now designated ANG II type 1 receptor (AT1R) and AT2R (DeGasparo et al., 2000). Most physiological and pathophysiological effects of ANG II are mediated via the AT1R (Iwanciw et al., 2003). Although CTGF has been reported to play a role in pulmonary fibrosis induced with bleomycin and hyperoxia (Lasky et al., 1998, Bonniaud et al., 2003, Chen et al., 2007), its relationship with ANG II has not yet been confirmed in paraquat-induced collagen production. The aims of this study were to investigate the effects of paraquat on ANG II, ANG II receptors, CTGF, and collagen expressions and to assess the role of ANG II receptors in paraquat-induced collagen synthesis in human lung fibroblasts.