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We have recently demonstrated that relaxin (RLX) acts as compensatory mediator in human heart failure. RLX inhibits the stimulation of endothelin-1, the most potent vasoconstrictor in heart failure. Upregulation of the endothelin type-B receptor (ETB ), which mediates endothelin-1 clearance and endothelial release of NO, represents a pivotal mode of RLX action. However, signal transduction and abundance of this phenomenon are unknown. Therefore, we investigated RLX-induced regulation of ETB in human umbilical vein endothelial, epithelial (HeLa), and vascular smooth muscle cells. In human umbilical vein endothelial cells and HeLa cells, but not in human vascular smooth muscle cells, RLX upregulated ETB expression and activated extracellular signal–regulated kinase-1/2 (ERK-1/2) and nuclear factor-κB (NF-κB), a transcription factor. PD-98059, a selective inhibitor of the mitogen-activated protein kinase kinase-1 (MEK-1)–ERK-1/2 pathway, abolished ERK-1/2 and NF-κB activation and ETB upregulation. NF-κB inhibition also prevented RLX-mediated ETB stimulation. In NF-κB-luciferase reporter assays, we demonstrated complete inhibition of RLX-induced NF-κB activation in cells transfected with dominant-negative Raf-1, MEK-1, or ERK-1/2 constructs, whereas dominant-negative Ras had no effect. In rat aorta and mesenteric artery, RLX pretreatment, in an ETB -dependent fashion, mitigated the maximum contractile response to endothelin-1, by 38±4% and 43±6%, and the endothelin-1 sensitivity (−log[EC50 ]: aorta, 8.2±0.2 for vehicle versus 7.2±0.2 for RLX; mesenteric artery, 8.0±0.2 for vehicle versus 7.1±0.1 for RLX). RLX pretreatment augmented the dilator effect of the ETB agonist endothelin-3 by 100±8% and 133±13%. In conclusion, RLX stimulates endothelial and epithelial ETB via a Ras-independent Raf-1–MEK-1–ERK-1/2 pathway that activates NF-κB. On vascular smooth muscle cells, ETB , a contributor to endothelin-mediated vasoconstriction, remains unaffected. This renders RLX a functional endothelin-1 antagonist.

Relaxin, a Pregnancy Hormone, Is a Functional Endothelin-1 Antagonist