Adult cardiac valve endothelial cells (VEC) undergo endothelial to mesenchymal transformation (EndMT) in response to transforming growth factor-beta (TGF beta). EndMT has been proposed as a mechanism to replenish interstitial cells that reside within the leaflets and further, as an adaptive response that increases the size of mitral valve leaflets after myocardial infarction. To better understand valvular EndMT, we investigated TGF beta-induced signaling in mitral VEC, and carotid artery endothelial cells (CAEC) as a control. Expression of EndMT target genes cc-smooth muscle actin (alpha-SMA), Snail, Slug, and MMP-2 were used to monitor EndMT. We show that TGF beta-induced EndMT increases phosphorylation of ERK (p-ERK), and this is blocked by Losartan, an FDA-approved antagonist of the angiotensin II type 1 receptor (AT1), that is known to indirectly inhibit phosphorylation of ERK (p-ERK). Blocking TGF-beta-induced p-ERK directly with the MEK1/2 inhibitor RDEA119 was sufficient to prevent EndMT. In mitral VECs, TGF beta had only modest effects on phosphorylation of the canonical TGF-beta signaling mediator mothers against decapentaplegic homolog 3 (SMAD3). These results indicate a predominance of the non-canonical p-ERK pathway in TGF beta-mediated EndMT in mitral VECs. AT1 and angiotensin II type 2 (AT2) were detected in mitral VEC, and high concentrations of angiotensin II (AngII) stimulated EndMT, which was blocked by Losartan. The ability of Losartan or MEK1/2 inhibitors to block EndMT suggests these drugs may be useful in manipulating EndMT to prevent excessive growth and fibrosis that occurs in the leaflets after myocardial infarction. (C) 2014 Elsevier Inc. All rights reserved.