Constitutive endothelin-1 overexpression promotes smooth muscle cell proliferation via an external autocrine loop

Endothelin-1 (ET-1) is a potent vasoconstrictor peptide originally purified from endothelial cell-conditioned medium. It has multiple biological activities and has been implicated in a number of human diseases, including hypertension and atherosclerosis. Contradictory reports have been published regarding whether ET-1 is a mitogen for vascular smooth muscle cells (SMC); thus, this issue is presently unresolved. In this study, we demonstrate that rat aortic SMC express functional endothelin cell surface receptors but do not proliferate when ET-1 is added to serum-free culture medium on every other day for a period of 1 week. To determine whether ET-1 could function in an autocrine manner to promote SMC growth, we transfected this same cell line with an ET-1 expression plasmid. Several independent lines expressing variable levels of ET-1 mRNA and biologically active ET-1 were obtained. Cell proliferation assays indicated that the transfected SMC line secreting the highest level of ET-1 had an enhanced growth rate when compared with untransfected or vector-alone transfected cells. The growth rate of this SMC line, but not of untransfected cells, was significantly reduced when the ETA receptor subtype-selective antagonist BQ-123 was included in the culture medium. These results indicate that constitutive ET-1 overexpression can promote SMC proliferation. Therefore, it is possible that under certain conditions ET-1 could be an important factor controlling SMC replication in vivo.