Protective Action of 3,5-Diiodo-L-Thyronine on Cigarette Smoke-Induced Mitochondrial Dysfunction in Human Alveolar Epithelial Cells

Background: Cigarette smoke (CS) is a major risk factor for chronic lung conditions. Oxidative stress and mitochondrial dysfunction play a crucial role in CS-induced pulmonary injury. 3,5-Diiodothyronine (T2) affects energy metabolism, having mitochondria as a major target. However, the underlying mechanisms of T2 related to lung diseases are poorly understood. Aims: To investigate the protective action of T2 on CS-induced mitochondrial dysfunction in an in vitro model of human epithelial alveolar cells. Methods: ATP synthesis and cytochrome c oxidase (COX) activity, as a marker of mitochondrial function, was assessed in A549 cells pretreated with T2 and exposed to CS using a bioluminescence assay and an Oroboros 2k-Oxygraph system, respectively. An evaluation of the oxidative status was conducted by assessing superoxide radical production, superoxide dismutase (SOD) activity, and H2O2 levels. Moreover, we investigated the mitochondrial mass via Mito-Tracker Green (MTG) staining and flow cytometry analysis. Results: CS significantly reduced ATP production. T2 pretreatment was found to prevent CS-induced impairments in ATP synthesis, enhancing COX activity. Additionally, the 2 h T2 pretreatment of CS-exposed cells mitigated CS-induced oxidative stress, thereby enhancing SOD activity and reducing the superoxide anion and H2O2 levels. Finally, MTG labeling was correlated with CS-induced mitochondrial mass gain, which is associated with cell senescence. Unexpectedly, T2 was not able to significantly prevent this mass increment, probably due to its rapid mode of action. Conclusions: Our results provide new insights into the protective effects of T2 against CS-induced mitochondrial damage.