Angiopoietin/Tie2 pathway influences smooth muscle hyperplasia in idiopathic pulmonary hypertension.

RATIONALE Angiopoietins are involved in blood vessel maturation and remodeling. OBJECTIVES One consequence of endothelium-specific tyrosine kinase-2 (Tie2) receptor activation by angiopoietin-1 (Ang1) is the release of endothelium-derived growth factors that recruit vascular wall cells. We investigated this process in idiopathic pulmonary arterial hypertension (iPAH). METHODS Ang1, Ang2, and total and phosphorylated Tie2 expression (mRNA and protein) was evaluated in human lung specimens and in cultured pulmonary artery smooth muscle cells (PA-SMCs) and pulmonary endothelial cells (P-ECs) isolated from patients with iPAH and control subjects. Media collected from Ang1-treated P-ECs were assessed for their PA-SMC growth-promoting effect. MEASUREMENTS AND MAIN RESULTS Tie2 receptor was fourfold higher in lungs and P-ECs from patients with iPAH than in those from control subjects, with a parallel increase in phosphorylated lung Tie2 receptor. In contrast, Ang1 and Ang2 expression in lungs, P-ECs, and PA-SMCs did not differ. Incubation of PA-SMCs with medium collected from P-EC cultures induced marked proliferation, and this effect was stronger when using P-ECs from patients with iPAH than from control subjects. Ang1 pretreatment of P-ECs from either patients or control subjects induced a further increase in PA-SMC proliferation. Fluoxetine, an inhibitor of the mitogenic action of serotonin, reduced the growth-promoting effect of P-EC media. Ang1 added to P-ECs from patients with iPAH increased the production of endothelin-1 (ET-1) and serotonin, but not of platelet-derived growth factor-BB or epidermal growth factor, and increased the amount of mRNA encoding tryptophan hydroxylase-1 (the rate-limiting enzyme of serotonin synthesis), preproET-1, and ET-1-converting enzyme. CONCLUSIONS The Ang1/Tie2 pathway is potentiated in iPAH, contributing to PA-SMC hyperplasia via increased stimulation of endothelium-derived growth factors synthesis by P-ECs.