Abstract
Expression of the endothelin (ET)-1 gene is subject to complex regulation by numerous factors, among which the cytokine transforming growth factor-β (TGF-β) is one of the most important. TGF-β action is based on the activation of the Smad signaling pathway. Smad proteins activate transcription of the gene by cooperation with activator protein-1 (AP-1) at specific sites on the ET-1 promoter. Smad signaling pathway is initiated by binding of the cytokine to a heteromeric complex of type I and type II receptors. Signal is then propagated to the nucleus by specific members of the Smad family. Most cell types contain a type I receptor known as ALK5. However, endothelial cells are unique because they coexpress an additional type I receptor named ALK1. These forms do not constitute redundant receptors with the same function, but they actually activate different Smad-mediated expression programs that lead to specific endothelial phenotypes. TGF-β/ALK5/Smad3 pathway is associated to a mature endothelium because it leads to inhibition of cell migration/proliferation. Conversely, TGF-β/ALK1/Smad5 activates both processes and is more related to the angiogenic state. We have analyzed the TGF-β receptor subtype requirements for the activation of the ET-1 gene. For that purpose, we have overexpressed type I receptor and Smad isoforms in endothelial cells and analyzed the effect on ET-1 expression. Our experiments indicate that TGF-β induces ET-1 expression preferentially through the activation of the ALK5/Smad3 pathway and, therefore, the expression of the vaso-constrictor may be associated to a quiescent and mature endothelial phenotype.
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