Arsenite Disrupts Zinc-Dependent TGFβ2-SMAD Activity During Murine Cardiac Progenitor Cell Differentiation

TGF beta 2 (transforming growth factor-beta 2) is a key growth factor regulating epithelial to mesenchymal transition (EMT). TGF beta 2 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. TGF beta signaling is dependent on a dynamic on and off switch in Smad activity. Arsenite exposure of 1.34 mu M for 24-48 h has been reported to disrupt Smad phosphorylation leading to deficits in TGF beta 2-mediated cardiac precursor differentiation and transformation. In this study, the molecular mechanism of acute arsenite toxicity on TGF beta 2-induced Smad2/3 nuclear shuttling and TGF beta 2-mediated cardiac EMT was investigated. A 4-h exposure to 5 mu M arsenite blocks nuclear accumulation of Smad2/3 in response to TGF beta 2 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 TGF beta 2-Smad signaling is likely due to the loss of Zn2+ cofactor in Smad proteins, as Zn2+ supplementation reverses the disruption in Smad2/3 nuclear translocation and transcriptional activity by arsenite. This coincides with Zn2+ supplementation rescuing arsenite-mediated deficits in cardiac EMT. Thus, zinc partially protects cardiac EMT from developmental toxicity by arsenite.