64 - Low Dose Cadmium Stimulates Myofibroblast Differentiation and Lung Fibrosis by Activation of SMAD and Nuclear Thioredoxin-1

Abstract

Background Toxic effects of cadmium (Cd) from cigarette smoke and occupational sources are well studied, yet low-level Cd also occurs in human diet. Cd accumulates in vivo due to a twenty-year biological half-life. Increasing evidence suggests that exposure to dietary Cd can impact human health by potentiating inflammatory mechanism. Submicromolar Cd can alter cellular redox control mechanism, contributing to stimulation of pulmonary diseases. The pathogenesis of idiopathic pulmonary fibrosis involves excessive myofibroblast activation which leads to increased contractures in lung. Substantial evidence supports that myofibroblast differentiation is associated with disruption of redox homeostasis, yet the mechanistic effects of exogenous exposure remains unknown. Objectives To test our hypothesis that low dietary levels of Cd stimulate pulmonary fibrosis via activation of SMAD transcription factor and stimulation of myofibroblast differentiation. Results Using human fetal lung fibroblasts (HFLF), we found that two differentiation markers, α-Smooth-Muscle-actin (α-SM-actin) and Extra-Domain-A-containing fibronectin (ED-A FN), were significantly elevated in transcriptional and translational levels by low-dose Cd (0.5, 1.0 and 2.0 μM). Under the same Cd treatment, an increased formation of stress fibers was observed in HFLF. To examine the effects of low-dose Cd on SMAD activation and redox regulatory mechanism, HFLF was transfected with nuclear exporting or nuclear localized signal conjugated thioredoxin (Trx)-1 to express in cytoplasm or nucleus, respectively. The results show that Cd increased SMAD activity by 3 fold and stimulated translocation of Trx1 from cytoplasm to nucleus. Overexpression of Trx1 in cytoplasm diminished Cd induced SMAD activation, while overexpression of Trx1 in nuclei mimicked the Cd effect. This suggests that low-dose Cd stimulates differentiation of HFLF to myofibroblast by mechanism involving nuclear translocation of Trx1 followed by activation of SMAD and subsequent gene expression of α-SM-actin and ED-A FN for differentiation and fibrosis. Conclusion Low dietary exposure to Cd presents a potential risk for pulmonary fibrosis by affecting redox signaling mechanism. The improved understanding of redox regulation mechanism in differentiation and fibrosis can provide therapeutic targets to maintain pulmonary health.