Differential modulation of claudin 4 expression and myosin light chain phosphorylation by thyroid function in lung injury

Background Trauma and ventilator‐induced lung injury is often associated with endothelial–epithelial barriers breakdown, which may lead to multiple system organ failure (MSOF) and death in critically ill patients. Although molecular mechanism involved in MSOF is not known, junctional opening is believed to happen. In vitro, thyroid hormones inhibit myosin light chain (MLC) phosphorylation and may, thus, inhibit cellular contraction and junctional opening. Trauma is also associated with tissue hypo‐thyroid state. Therefore, we examined the effects of thyroid function on expression of phospho‐MLC (pp‐MLC) and claudin 4 (Clud4), key proteins involved in regulation of junctional tightness, in lung injury. Methods Rats were rendered hypo‐thyroid (Hypo) or hyperthyroid (Hyper) by adding methimazole or levo‐thyroxine, respectively, to their drinking water. Untreated euthyroid (Eue) animals were used as control. Lung pp‐MLC and Clud4 proteins were assessed by western blotting and in situ immunodetection, respectively. Lung injury was induced by high tidal volume mechanical ventilation. Results Lung injury was significantly enhanced in Hypo animals and attenuated in Hyper animals. Parallel changes in expression of lung pp‐MLC were detected. Alterations in lung histomorphology correlated with the level of pp‐MLC. Expression of alveolar and bronchiolar Clud4 protein was differentially affected by the state of thyroid gland. Conclusions Our data suggest that thyroid function plays significant role in lung injury perhaps by modulating expression of the proteins involved in junctional tightness. Besides, they strongly support the idea that the tissue hypo‐thyroid state may contribute to endothelial–epithelial barriers breakdown associated with trauma.