Triiodothyronine Attenuates Silica-Induced Oxidative Stress, Inflammation, and Apoptosis via Thyroid Hormone Receptor α in Differentiated THP-1 Macrophages

Alveolar macrophage (AM) injury and inflammatory response are key processes in pathological damage caused by silica. However, the role of triiodothyronine (T3) in silica-induced AM oxidative stress, inflammation, and mitochondrial apoptosis remained unknown. To investigate the possible effects and underlying mechanism of T3 in silica-induced macrophage damage, differentiated human acute monocytic leukemia cells (THP-1) were exposed to different silica concentrations (0, 50, 100, 200, and 400 μg/mL) for 24 h. Additionally, silica-activated THP-1 macrophages were treated with gradient-dose T3 (0, 5, 10, 20, and 40 nM) for 24 h. To illuminate the potential mechanism, we used short hairpin RNA to knock down the thyroid hormone receptor α (TRα) in the differentiated THP-1 macrophages. The results showed that T3 decreased lactate dehydrogenase and reactive oxygen species levels, while increasing cell viability and superoxide dismutase in silica-induced THP-1 macrophages. In addition, silica increased the expression of interleukin 1 beta (IL-1β), interleukin 6 (IL-6), and tumor necrosis factor-α (TNF-α), and T3 treatment reduced those pro-inflammatory cytokines secretion. Compared with silica-alone treated groups, cells treated with silica and T3 restored the mitochondrial membrane potential loss and had reduced levels of cytochrome c and cleaved caspase-3 expressions. Lastly, we observed that TRα-knockdown inhibited the protective effects of T3 silica-induced THP-1 macrophages. Together, these findings revealed that T3 could serve as a potential therapeutic target for protection against silica-induced oxidative stress, inflammatory response, and mitochondrial apoptosis, which are mediated by the activation of the T3/TRα signal pathway.