Upregulation of Steroidogenic Acute Regulatory Protein by Hypoxia Stimulates Aldosterone Synthesis in Pulmonary Artery Endothelial Cells to Promote Pulmonary Vascular Fibrosis

Abstract

Background— The molecular mechanism(s) regulating hypoxia-induced vascular fibrosis are unresolved. Hyperaldosteronism correlates positively with vascular remodeling in pulmonary arterial hypertension, suggesting that aldosterone may contribute to the pulmonary vasculopathy of hypoxia. The hypoxia-sensitive transcription factors c-Fos/c-Jun regulate steroidogenic acute regulatory protein (StAR), which facilitates the rate-limiting step of aldosterone steroidogenesis. We hypothesized that c-Fos/c-Jun upregulation by hypoxia activates StAR-dependent aldosterone synthesis in human pulmonary artery endothelial cells (HPAECs) to promote vascular fibrosis in pulmonary arterial hypertension. Methods and Results— Patients with pulmonary arterial hypertension, rats with Sugen/hypoxia–pulmonary arterial hypertension, and mice exposed to chronic hypoxia expressed increased StAR in remodeled pulmonary arterioles, providing a basis for investigating hypoxia–StAR signaling in HPAECs. Hypoxia (2.0% FiO 2 ) increased aldosterone levels selectively in HPAECs, which was confirmed by liquid chromatography–mass spectrometry. Increased aldosterone by hypoxia resulted from enhanced c-Fos/c-Jun binding to the proximal activator protein-1 site of the StAR promoter in HPAECs, which increased StAR expression and activity. In HPAECs transfected with StAR–small interfering RNA or treated with the activator protein-1 inhibitor SR-11302 [3-methyl-7-(4-methylphenyl)-9-(2,6,6-trimethylcyclohexen-1-yl)nona-2,4,6,8-tetraenoic acid], hypoxia failed to increase aldosterone, confirming that aldosterone biosynthesis required StAR activation by c-Fos/c-Jun. The functional consequences of aldosterone were confirmed by pharmacological inhibition of the mineralocorticoid receptor with spironolactone or eplerenone, which attenuated hypoxia-induced upregulation of the fibrogenic protein connective tissue growth factor and collagen III in vitro and decreased pulmonary vascular fibrosis to improve pulmonary hypertension in vivo. Conclusion— Our findings identify autonomous aldosterone synthesis in HPAECs attributable to hypoxia-mediated upregulation of StAR as a novel molecular mechanism that promotes pulmonary vascular remodeling and fibrosis.