Background: Pulmonary hypertension (PH) is a vascular disease that causes increased pulmonary arterial pressure, right heart failure, and death. We have shown iron-sulfur (Fe-S) cluster deficiency due to repression of ISCU1/2 promotes mitochondrial dysfunction and PH. Frataxin (FXN), a binding partner of ISCU1/2, is crucial to Fe-S cluster assembly. Genetic FXN deficiency causes Friedreich’s ataxia, a disease of neurologic and cardiovascular dysfunction. The latter is often accompanied by PH, but the molecular etiology is unclear. Thus, there may be a direct role for FXN in PH.
Hypothesis: FXN deficiency, due to genetic or acquired triggers, disrupts endothelial metabolism to promote PH.
Methods: FXN expression was modulated in human pulmonary arterial endothelial cells (PAECs) by gene transfection and by exposure to the BET bromodomain inhibitor I-BET151, hypoxia, and IL-1β. Fe-S clusters were quantified by fluorescent sensor; glycolytic flux was measured by Seahorse assay. Phenotypic changes, such as apoptosis, migration, vasomotor gene expression, and angiogenesis, were measured. In vivo, cell-specific conditional FXN knockout mice and mice delivered FXN siRNA to vascular endothelium were studied.
Results: In PAECs, hypoxia (0.48-fold change ± 0.05, P<0.01) and IL-1β (0.47-fold change ± 0.01, P<0.05) down-regulated FXN expression. I-BET151 inhibition of bromodomain-containing protein 4 restored FXN levels following hypoxia (1.98-fold change ± 0.11, P<0.01) and IL-1β (2.5-fold change ± 0.15, P<0.01). FXN deficiency decreased Fe-S cluster assembly (0.57-fold change ± 0.05, P<0.01) and increased glycolysis (2-fold change ± 0.05, P<0.01), leading to increased apoptosis, decreased migration, altered effectors of vasomotor tone, and decreased angiogenesis in vitro. Endothelial-specific FXN knockdown in chronically hypoxic mice promoted hemodynamic (RVSP 34.31 mmHg ± 0.18 v. 29.77 ± 0.93, P<0.05) and histologic indices of PH in vivo.
Conclusion: FXN deficiency induced by hypoxia and IL-1β promotes endothelial-specific metabolic changes, leading to PH development in vivo. Our results may provide a target for diagnostic and therapeutic intervention for PH and may guide genetic identification of a novel cohort of patients at risk for PH.