Arsenite disrupts zinc-dependent TGFβ2-SMAD activity during murine cardiac progenitor cell differentiation

TGFβ2 (transforming growth factor-β2) is a key growth factor regulating epithelial to mesenchymal transition (EMT). TGFβ2 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. TGFβ 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 TGFβ2-mediated cardiac precursor differentiation and transformation. In this study, the molecular mechanism of acute arsenite toxicity on TGFβ2-induced Smad2/3 nuclear shuttling and TGFβ2-mediated cardiac EMT was investigated. A 4-h exposure to 5 μM arsenite blocks nuclear accumulation of Smad2/3 in response to TGFβ2 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 TGFβ2-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.