| Summary: | The role of the Angiopoietin-1 (Ang1)-Tie2 pathway in pulmonary arterial hypertension (PAH) is controversial, with one group suggesting that increased Ang1-Tie2 activity may even play a causal role in PAH progression. However, Ang1 has been well-characterized as a homeostatic factor, which prevents endothelial cell (EC) activation and injury through Tie2 receptor stimulation. Furthermore, we have previously demonstrated that lung Tie2 expression and activity are significantly downregulated in experimental PAH, implicating reduced Tie2 signaling in the pathogenesis of this disease. Thus, we hypothesized that manipulations to increase Ang1-Tie2 signaling will protect against lung EC injury and onset of PAH, whereas a loss-of-function of this pathway in Tie2-deficient mice (Tie2+/-) would predispose to PAH.
Approximately 13% of Tie2+/- mice developed spontaneous elevation in right ventricular systolic pressure (RVSP), although no significant difference was observed between the WT and Tie2+/- groups following chronic hypoxic exposure. Serotonin (5-HT) infusion for one week or daily interleukin-6 (IL-6) injection for two weeks produced substantial RVSP elevations in Tie2+/-, but not WT mice (P<0.05). Following one week of 5-HT or IL-6 treatment, there was a significant decrease in Ang1 protein expression in both WT and Tie2+/- mice (P<0.05). Similarly, following exposure of cultured pulmonary arterial smooth muscle cells to 5-HT or IL-6, Ang1 secretion was significantly decreased (P<0.01). Moreover, 5-HT or IL-6 exposure resulted in decreased Tie2 activity and increased apoptosis only in the lungs of Tie2+/- mice (P<0.01), and treatment with a pan-caspase inhibitor, Z-VAD, prevented PAH in 5-HT-treated Tie2-deficient mice. Together with in vitro studies showing that pulmonary arterial ECs subjected to Tie2-siRNA were more susceptible to apoptosis following 5-HT treatment, this strongly implicates EC death as a primary mechanism of PAH in the presence of reduced Tie2 activity. In addition, doxycycline-conditional, endothelial-targeted Ang1 binary transgenic mice showed no evidence of elevated RVSP under basal conditions and if anything, demonstrated a blunted response to 5-HT treatment compared to non-binary littermate controls.
Therefore, our findings support the importance of EC survival-signaling via the Ang1-Tie2 pathway as a protective mechanism in PAH and emphasize the pivotal role of EC apoptosis in the onset of this disease.
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