N-terminal proteolysis of the endothelin B receptor abolishes its ability to induce EGF receptor transactivation and contractile protein expression in vascular smooth muscle cells

Objective - The extracellular N terminus of the endothelin B ( ETB) receptor is cleaved by a metalloprotease in an agonist-dependent manner, but the physiological role of this N-terminal proteolysis is not known. In this study, we aimed to determine the functional role of the ETB receptor and of its N-terminal cleavage in vascular smooth muscle cells ( VSMCs). Methods and Results - VSMCs expressing either the full-length ETB receptor or an N-terminally truncated ETB receptor ( corresponding to the N-terminally cleaved receptor) were analyzed for ligand-induced mitogen-activated protein kinase activation and expression of contractile proteins. In VSMCs expressing the full-length ETB receptor, IRL1620 ( an ETB-selective agonist) induced a biphasic extracellular signal-regulated kinase 1/2 ( ERK1/2) activation and increased expression of contractile proteins ( smooth muscle myosin-1 [ SM-1]/SM-2, SM22 alpha, and alpha-actin). Interestingly, the second phase of ERK1/2 activation required metalloprotease activity, epidermal growth factor ( EGF) receptor transactivation, and predominantly activation of G(i) proteins. In contrast, in VSMCs expressing N-terminally truncated ETB receptors, IRL1620 did not elicit EGF transactivation and failed to increase contractile protein expression. Conclusions - This study is the first to show that stimulation of full-length ETB receptors promotes expression of contractile proteins and may thus participate in the differentiation of VSMCs.