The role of mitochondrial dysfunction in macrophages on SiO2 -induced pulmonary fibrosis: A review

PGC-1 alpha regulates mitochondrial biogenesis to promote silica-induced pulmonary fibrosis

Silicosis is an incurable chronic fibrotic lung disease caused by long-term exposure to respirable silica particles. It is characterized by persistent inflammation and progressive fibrosis of lung tissues, which eventually leads to respiratory failure and seriously affects human health. The high incidence and mortality associated with silicosis have made the disease a widespread public health concern. However, its pathogenesis has not been fully elucidated. Mitochondrial biogenesis plays a crucial role under various fibrotic conditions. However, the mechanism of this process in silicosis is still unclear. Therefore, this study aimed to explore the influence of the PGC-1α gene on mitochondrial biogenesis in the development of silicosis. We established in vivo and in vitro silicosis models by exposing rats and rat type-2 alveolar epithelial cells (RLE-6TN) to silica. Our findings revealed alterations in the mitochondrial structure and function, decreased mitochondrial biogenesis, and reduced expression of mtDNA (Mitochondrial DNA) content. By upregulating the PGC-1α gene in RLE-6TN cells, we activated the PGC-1α- NRF1-TFAM signaling pathway, enhancing mitochondrial biogenesis, increasing citrate synthase and mtDNA content, improving mitochondrial function, and mitigating fibrosis. Our results indicate that the regulation of mitochondrial biogenesis can affect silicosis-induced fibrosis, highlighting the significance of reduced mitochondrial biogenesis in the progression of silicosis-induced fibrosis.

Role of macrophage ATP metabolism disorder in SiO2‑induced pulmonary fibrosis: a review

Silicosis, a chronic lung disease, results from prolonged inhalation of silica dust (SiO2) in occupational environments, and its pathogenesis remains incompletely elucidated. Studies have shown that alveolar macrophages (AMs) play a pivotal role in its development. These AMs phagocytose the inhaled SiO2, which leads to morphological, structural, and functional abnormalities that result in lung fibrosis. During this process, adenosine triphosphate (ATP) not only provides energy for the physiological and pathological activities but also acts as a key intracellular and extracellular signaling molecule and regulates cytokine synthesis and secretion. This complex process has not been systematically summarized. In this study, first, the current data on ATP metabolism in the development of SiO2-induced pulmonary fibrosis are introduced. ATP metabolism disorder, caused by impaired production, utilization, or distribution of ATP, disrupts macrophage energy homeostasis. Then, how ATP metabolism disorder affects macrophage morphology and function and the inflammatory and fibrotic processes of the lungs by activating the P2X7 receptor-mediated ATP signaling pathway are discussed. Finally, current therapeutic strategies targeting ATP metabolism disorder and ATP signaling pathways in silicosis are summarized. In conclusion, SiO2-induced ATP metabolism disorder indirectly accelerates the progression of silicosis fibrosis.

miR-207 Suppresses the Progression of SiO2-Induced Pulmonary Fibrosis by Targeting Smad3 to Regulate the TGF-β1/Smad3 Signaling Pathway in C57BL/6 Mice

Silicosis is a worldwide occupational disease characterized by irreversible pulmonary fibrosis. Recent studies have showed that microRNAs (miRNAs) may play a crucial role in silicosis progression by modulating fibrosis-related gene express. In this study, we selected miR-207 as our research subject because we found that miR-207 can be match with Smad3 using bioinformatic techniques, which might silence the key fibrosis-related TGF-β1/Smad3 signal pathway. In this study, the mice were given silica suspension (20 µg/µL, 80 µL) via nostril once a day for 16 days to establish silicosis models, and then were transfected with miR-207 mimic or inhibitor. The mice which were given phosphate-buffered saline (PBS) (80 µL) via nostril were used as control. All mice were killed on Day 45 after the first exposure to dust, after which their lungs were removed for pathological observation and to measure the hydroxyproline content. Then, real-time polymerase chain reaction and Western blot analysis were applied to detect the relative expression levels of TGF-β1/Smad3 signaling pathway indicators (TGF-β1, TGF-βR, and Smad3), and myofibroblast transformation indicators (α-SMA and Fn). Results showed that the lung pathological images of silicosis model group mice showed significant fibrosis, and TGF-β1, TGF-βR, Smad3, α-SMA, and Fn were all highly upregulated compared with the control group mice. Intervention with miR-207 mimics significantly inhibited pulmonary fibrosis in silicosis mice by downregulation of TGF-β1/Smad3 and inhibiting of myofibroblast formation. Whereas these phenomena were not observed in silicosis mice treated with miR-207 inhibitor. The results demonstrated that miR-207 can block the progression of SiO2-induced pulmonary fibrosis by targeting the TGF-β/Smad3 signaling pathway.

Silica-induced ROS in alveolar macrophages and its role on the formation of pulmonary fibrosis via polarizing macrophages into M2 phenotype: a review

Alveolar macrophages (AMs), the first line against the invasion of foreign invaders, play a predominant role in the pathogenesis of silicosis. Studies have shown that inhaled silica dust is recognized and engulfed by AMs, resulting in the production of large amounts of silica-induced reactive oxygen species (ROS), including particle-derived ROS and macrophage-derived ROS. These ROS change the microenvironment of the AMs where the macrophage phenotype is stimulated to swift from M0 to M1 and/or M2, and ultimately emerge as the M2 phenotype to trigger silicosis. This is a complex process accompanied by various molecular biological events. Unfortunately, the detailed processes and mechanisms have not been systematically described. In this review, we first systematically introduce the process of ROS induced by silica in AMs. Then, describe the role and molecular mechanism of M2-type macrophage polarization caused by silica-induced ROS. Finally, we review the mechanism of pulmonary fibrosis induced by M2 polarized AMs. We conclude that silica-induced ROS initiate the fibrotic process of silicosis by inducing macrophage into M2 phenotype, and that targeted intervention of silica-induced ROS in AMs can reprogram the macrophage polarization and ameliorate the pathogenesis of silicosis.

The multifaceted role of macrophage mitophagy in SiO2-induced pulmonary fibrosis: A brief review

Prolonged exposure to environments with high concentrations of crystalline silica (CS) can lead to silicosis. Macrophages play a crucial role in the pathogenesis of silicosis. In the process of silicosis, silica (SiO2) invades alveolar macrophages (AMs) and induces mitophagy which usually exists in three states: normal, excessive, and/or deficiency. Different mitophagy states lead to corresponding toxic responses, including successful macrophage repair, injury, necrosis, apoptosis, and even pulmonary fibrosis. This is a complex process accompanied by various cytokines. Unfortunately, the details have not been fully systematically summarized. Therefore, it is necessary to elucidate the role of macrophage mitophagy in SiO2-induced pulmonary fibrosis by systematic analysis on the literature reports. In this review, we first summarized the current data on the macrophage mitophagy in the development of SiO2-induced pulmonary fibrosis. Then, we introduce the molecular mechanism on how SiO2-induced mitophagy causes pulmonary fibrosis. Finally, we focus on introducing new therapies based on newly developed mitophagy-inducing strategies. We conclude that macrophage mitophagy plays a multifaceted role in the progression of SiO2-induced pulmonary fibrosis, and reprogramming the macrophage mitophagy state accordingly may be a potential means of preventing and treating pulmonary fibrosis.

Impact of Respiratory Dust on Health: A Comparison Based on the Toxicity of PM2.5, Silica, and Nanosilica

Respiratory dust of different particle sizes in the environment causes diverse health effects when entering the human body and makes acute or chronic damage through multiple systems and organs. However, the precise toxic effects and potential mechanisms induced by dust of different particle sizes have not been systematically summarized. In this study, we described the sources and characteristics of three different particle sizes of dust: PM2.5 (<2.5 μm), silica (<5 μm), and nanosilica (<100 nm). Based on their respective characteristics, we further explored the main toxicity induced by silica, PM2.5, and nanosilica in vivo and in vitro. Furthermore, we evaluated the health implications of respiratory dust on the human body, and especially proposed potential synergistic effects, considering current studies. In summary, this review summarized the health hazards and toxic mechanisms associated with respiratory dust of different particle sizes. It could provide new insights for investigating the synergistic effects of co-exposure to respiratory dust of different particle sizes in mixed environments.