RESUMEN
TGFß2 (transforming growth factor-ß2) is a key growth factor regulating epithelial to mesenchymal transition (EMT). TGFß2 triggers cardiac progenitor cells to differentiate into mesenchymal cells and give rise to the cellular components of coronary vessels as well as cells of aortic and pulmonary valves. TGFß signaling is dependent on a dynamic on and off switch in Smad activity. Arsenite exposure of 1.34 µM for 24-48 h has been reported to disrupt Smad phosphorylation leading to deficits in TGFß2-mediated cardiac precursor differentiation and transformation. In this study, the molecular mechanism of acute arsenite toxicity on TGFß2-induced Smad2/3 nuclear shuttling and TGFß2-mediated cardiac EMT was investigated. A 4-h exposure to 5 µM arsenite blocks nuclear accumulation of Smad2/3 in response to TGFß2 without disrupting Smad phosphorylation or nuclear importation. The depletion of nuclear Smad is restored by knocking-down Smad-specific exportins, suggesting that arsenite augments Smad2/3 nuclear exportation. The blockage in TGFß2-Smad signaling is likely due to the loss of Zn(2+) cofactor in Smad proteins, as Zn(2+) supplementation reverses the disruption in Smad2/3 nuclear translocation and transcriptional activity by arsenite. This coincides with Zn(2+) supplementation rescuing arsenite-mediated deficits in cardiac EMT. Thus, zinc partially protects cardiac EMT from developmental toxicity by arsenite.