Endothelin-1 Stimulation of Proteoglycan Synthesis in Vascular Smooth Muscle is Mediated by Endothelin Receptor Transactivation of the Transforming Growth Factor-β Type I Receptor

We utilized human vascular smooth muscle cells to address the question if a G-protein-coupled receptor, the endothelin (ET) receptor, could transactivate a serine/threonine kinase receptor, specifically the transforming growth factor (TGF)-beta receptor, T beta RI. Functionality of the interaction was addressed by studying endothelin-1-stimulated proteoglycan synthesis. Signaling molecules were assessed by Western blotting and proteoglycan synthesis by [S-35] sulfate and S-35-met/cys incorporation and molecular size by SDS-PAGE. Endothelin-1 treatment led to a time-and concentration-dependent increase in cytosolic phosphoSmad2C, which was inhibited by the mixed endothelin receptor antagonist bosentan and the TbRI antagonist SB431542. Endothelin-1 treatment led to a time-dependent increase in nuclear phosphoSmad2C. Endothelin-1-stimulated proteoglycan synthesis was partially inhibited (40%) by SB431542 and completely blocked by bosentan. The effect of endothelin-1 to stimulate an increase in glycosaminoglycan size on biglycan was also blocked in a concentration-dependent manner by SB431542. These data extend the current paradigm of G-protein coupled receptor signaling to include the transactivation of the serine kinase receptor for TGF-beta (T beta RI). This response should be considered in the context of response to endothelin- 1, and the options for therapeutically targeting endothelin-1 are accordingly broadened to include downstream signaling otherwise associated with TGF-beta receptor activation.