Overexpression of endogenous lipoic acid synthase attenuates pulmonary fibrosis induced by crystalline silica in mice

Highlights

• The antioxidant defense was strengthen including increased NRF2 and LIAS production in Lias^H/H mice.

• The Lias^H/H mouse model overexpression of lipoic acid synthase gene retarded the development of silica-induced pulmonary fibrosis through the suppression of oxidative stress and inflammatory processes.

• Strengthen antioxidant defense by increased lipoic acid synthase is a potential strategy for protection against silica-induced pulmonary fibrosis.

Abstract

Oxidative stress and inflammatory processes are proposed to mediate the development of silicosis. However, antioxidant therapy has not produced consistent results during the treatment of silicosis. α-Lipoic acid synthesized by lipoic acid synthase is a powerful anti-oxidant and helps protect mitochondria. Thus far, the effect of endogenous α-Lipoic acid on silicosis has not been elucidated yet. We established an experimental model of silicosis with wildtype and Lias^H/H mice, a new antioxidant mouse model which has overexpressed Lias gene (∼150 %) relative to its wild type counterpart. We systemically examined main pathological changes of pulmonary fibrosis, and explored α-lipoic acid effects on oxidative stress, inflammatory and pulmonary fibrosis biomarkers in silica-instillated mice. In Lias^H/H mice over-expression of lipoic acid alleviated the severity of major pathological alterations in the early stage of pulmonary fibrosis induced by silica compared with wild type mice. Silica significantly increased oxidative stress in both wild type and Lias^H/H mice. The antioxidant defense was strengthen including increased NRF2 and LIAS production in Lias^H/H mice. Relieved oxidative stress resulted in decreased inflammatory response and secretion of chemokines. Lias^H/H mice reduced chronic inflammatory response and inhibition of NF-κB activity after silica instillation. The Lias^H/H mouse model overexpression of lipoic acid synthase gene retarded the development of silica-induced pulmonary fibrosis. Strengthen antioxidant defense by increased lipoic acid synthase is a potential strategy for protection against silica-induced pulmonary fibrosis.

Introduction

Silicosis is a lethal disease characterized by chronic lung inflammation and pulmonary interstitial fibrosis (Riley and Urbine, 2019). It is caused by the inhalation of respirable crystalline silica dust. A large number of workers are exposed to silica in many countries, especially in India, China, Turkey, and South Africa (Akgun and Ergan, 2018; Smith and Blom, 2019). Thus, silicosis remains the most common occupational disorder in these countries. More than 20,000 new silicosis and coal worker pneumoconiosis cases occur every year in China (Han et al., 2018).

At present, the prevention of silicosis worldwide has been unsuccessful, and no specific treatment exists, although a small minority of patients may be offered lung transplantation (The Lancet Respiratory Medicine., 2019). Silica results in the production of reactive oxygen species (ROS) by the alveolar macrophages both directly and indirectly. Antioxidant therapy has been proved effective for silicosis in animal experiments (Huang et al., 2019; Nakashima et al., 2018; Yang et al., 2016). Our previous study showed that the antioxidant N-acetylcysteine alleviates silica-induced lung fibrosis in rats by down-regulating ROS and interfering with mitochondrial apoptosis signaling (Zhang et al., 2014). However, antioxidant treatments have produced conflicting results in clinical trials of the treatment of pulmonary fibrosis and other chronic diseases (Idiopathic Pulmonary Fibrosis Clinical Research et al., 2014; Moser and Chun, 2016). We speculate that the endogenous antioxidant levels of the subjects were different, which may greatly affect the results of clinical trials aimed at examining the impact of antioxidant therapy on the disease course. Moreover, laboratory animals have seldom been exposed to exogenous excessive oxidative stress in an optimum environment, and similar genetic variation leads to sustainable inter-individual variations at the basic endogenous antioxidant levels (Tsai et al., 2006). It is therefore necessary to study the effects of different endogenous antioxidant levels on antioxidant therapy. We therefore selected genetically modified mice with high endogenous antioxidant levels as an antioxidant mouse model (Xu et al., 2016).

When silica particles are inhaled, they are deposited in the alveoli and are ingested by alveolar macrophages (AMs), which initiate oxidative stress through the formation of ROS (Castranova, 1994; Du et al., 2019; Joshi et al., 2015a). Siloxil radicals generated after inhalation of freshly fractured crystalline-silica can lead to enhanced pulmonary injury and the development of silicosis. It suggests that increased radical generation from fresh quartz dust exposure may lead to pulmonary inflammation and damage (Vallyathan et al., 1995).

It is well-known that the mitochondria are the major cellular sources of ROS and the most immediate target of ROS (Lyu et al., 2019). The oxidative damage of mitochondria plays an essential role in the development of silicosis (Harijith et al., 2014; Joshi et al., 2015b; Zhang et al., 2019). We postulated that the oxidative stress in silicosis is due to an imbalance between the production of ROS and the scavenging capacity of the cellular antioxidant systems. Alpha-Lipoic acid (1,2-dithiolane-3-pentanoic acid, ALA) is a unique antioxidant, synthesized by lipoic acid synthase (Lias) in mitochondria. Studies have shown that the oxidation-reduction potential values of GSH/GSSG are considerably more positive than that of the dihydrolipoic acid/lipoic acid system (Jocelyn, 1967). Therefore, ALA has a powerful antioxidant capacity and can regenerate other antioxidants. ALA has been described as a “mitochondrial nutrient” and ALA supplementation alleviates oxidative stress-related diseases including diabetes, obesity, Alzheimer’s disease, multiple sclerosis in patients and animal models (Park et al., 2008). The clinical effects of ALA were assessed in organ transplantation. ALA can be protective against oxidative stress and inflammation in post-transplant patients (Ambrosi et al., 2016; Casciato et al., 2018). It may protect nearby mitochondrial enzymes and directly scavenge a variety of ROS from oxidative damage. Thus, ALA could have better efficacy for mitochondrial protection against silicosis than other antioxidants.

Lipoic acid synthase (LIAS) is a key enzyme in lipoic acid synthesis. Moreover, genetically modifying the Lias gene changes its antioxidant capacity and consequently affects the development of diseases (Yi et al., 2012). In this study, we used hypermorphic antioxidant Lias^H/H mice by modifying the 3’-UTR of the Lias gene and sought to determine whether overexpression of the Lias gene could attenuate pulmonary fibrosis in mice with silicosis. Our data demonstrated that Lias^H/H mice with increased endogenous antioxidant levels alleviated the major pathological alterations of pulmonary fibrosis.