Vascular inflammation regulates endothelial pathophenotypes, particularly in pulmonary arterial hypertension (PAH). Dysregulated lysosomal activity and cholesterol metabolism activate pathogenic inflammation, but their relevance to PAH is unclear. Nuclear receptor coactivator 7 ( NCOA7 ) deficiency in endothelium produced an oxysterol and bile acid signature through lysosomal dysregulation, promoting endothelial pathophenotypes. This oxysterol signature overlapped with a plasma metabolite signature associated with human PAH mortality. Mice deficient for endothelial Ncoa7 or exposed to an inflammatory bile acid developed worsened PAH. Genetic predisposition to NCOA7 deficiency was driven by single-nucleotide polymorphism rs11154337, which alters endothelial immunoactivation and is associated with human PAH mortality. An NCOA7-activating agent reversed endothelial immunoactivation and rodent PAH. Thus, we established a genetic and metabolic paradigm that links lysosomal biology and oxysterol processes to endothelial inflammation and PAH.

Deficiency of iron‑sulfur (FeS) clusters promotes metabolic rewiring of the endothelium and the development of pulmonary hypertension (PH) in vivo. Joining a growing number of FeS biogenesis proteins critical to pulmonary endothelial function, recent data highlighted that frataxin (FXN) reduction drives Fe-S-dependent genotoxic stress and senescence across multiple types of pulmonary vascular disease. Trinucleotide repeat mutations in the FXN gene cause Friedreich's ataxia, a disease characterized by cardiomyopathy and neurodegeneration. These tissue-specific phenotypes have historically been attributed to mitochondrial reprogramming and oxidative stress. Whether FXN coordinates both nuclear and mitochondrial processes in the endothelium is unknown. Here, we aim to identify the mitochondria-specific effects of FXN deficiency in the endothelium that predispose to pulmonary hypertension. Our data highlight an Fe-S-driven metabolic shift separate from previously described replication stress whereby FXN knockdown diminished mitochondrial respiration and increased glycolysis and oxidative species production. In turn, FXN-deficient endothelial cells had increased vasoconstrictor production (EDN1) and decreased nitric oxide synthase expression (NOS3). These data were observed in primary pulmonary endothelial cells after pharmacologic inhibition of FXN, mice carrying a genetic endothelial deletion of FXN, and inducible pluripotent stem cell-derived endothelial cells from patients with FXN mutations. Altogether, this study indicates FXN is an upstream driver of pathologic aberrations in metabolism and genomic stability. Moreover, our study highlights FXN-specific vasoconstriction in vivo, prompting future studies to investigate available and novel PH therapies in contexts of FXN deficiency.

The dynamic regulation of endothelial pathophenotypes in pulmonary hypertension (PH) remains undefined. Cellular senescence is linked to PH with intracardiac shunts; however, its regulation across PH subtypes is unknown. Since endothelial deficiency of iron-sulfur (Fe-S) clusters is pathogenic in PH, we hypothesized that a Fe-S biogenesis protein, frataxin (FXN), controls endothelial senescence. An endothelial subpopulation in rodent and patient lungs across PH subtypes exhibited reduced FXN and elevated senescence. In vitro, hypoxic and inflammatory FXN deficiency abrogated activity of endothelial Fe-S-containing polymerases, promoting replication stress, DNA damage response, and senescence. This was also observed in stem cell-derived endothelial cells from Friedreich's ataxia (FRDA), a genetic disease of FXN deficiency, ataxia, and cardiomyopathy, often with PH. In vivo, FXN deficiency-dependent senescence drove vessel inflammation, remodeling, and PH, whereas pharmacologic removal of senescent cells in Fxn-deficient rodents ameliorated PH. These data offer a model of endothelial biology in PH, where FXN deficiency generates a senescent endothelial subpopulation, promoting vascular inflammatory and proliferative signals in other cells to drive disease. These findings also establish an endothelial etiology for PH in FRDA and left heart disease and support therapeutic development of senolytic drugs, reversing effects of Fe-S deficiency across PH subtypes.

Pulmonary hypertension (PH) is a progressive, enigmatic disease of the lung vasculature that is clinically defined by elevated mean pulmonary arterial pressure and pulmonary vascular resistance that results in significant and often fatal right ventricular (RV) failure (1). The term “PH” encompasses multiple heterogeneous etiologies that are classified into five groups by the World Health Organization. Historically, studies have focused predominantly on the most severe subtype, Group 1 pulmonary arterial hypertension (PAH). Broadly, panvascular remodeling in PAH is attributed to injury or dysfunction within the pulmonary vessels, and a growing number of heritable mutations are being recognized as initiating triggers for this disease (1). Furthermore, a “sex paradox” has been described for this disease whereby women exhibit increased susceptibility to PAH but better survival as compared with men, for incompletely defined reasons (2). The vasodilatory medications currently used to treat PAH fail to prevent or reverse disease progression (3), making identification of novel therapeutic targets crucial. An increasing number of studies support a link between metabolic reprogramming and progressive tissue dysfunction in the pulmonary vasculature. For example, tissues from animal and human models of PH were shown to exhibit attenuated oxidative phosphorylation (OXPHOS) and increased glycolysis in aerobic conditions (4, 5), a phenomenon that was first described in proliferating cancer cells known as the Warburg effect (6, 7). Importantly, results obtained with emerging metabolic therapies, such as dichloroacetate, a drug previously used in patients with cancer and mitochondrial diseases (8), suggest that targeting metabolic dysfunction may be a reasonable strategy for certain patients with Group 1 PAH. However, evidence of genetic mutations that primarily drive metabolic reprogramming in Group 1 PAH is limited. Furthermore, any metabolic differences between female and male patients that influence PAH pathogenesis remain largely unknown. In recent years and in line with this “metabolic theory,” pulmonary endothelial deficiency of iron-sulfur (Fe-S) clusters— bioinorganic cofactors required for enzymatic redox function—has been shown to promote PH (9–11). Specifically, deficiencies of certain Fe-S biogenesis genes, ISCU1/2 (iron-sulfur duster assembly protein) and BOLA3 (BolA family member 3), were found to attenuate OXPHOS and ultimately drive PH in preclinical models (10, 11). Like many other Fe-S biogenesis genes, rare (but naturally occurring) human mutations in these genes have been linked to mitochondrial syndromes (12). In turn, such metabolic diseases, specifically driven by ISCU1/2 (10) and BOLA3 (13), have been associated with PH. Similarly, a mutation (Gly206Cys) near the Fe-S binding motif on the mitochondrial scaffolding NFU1 gene results in multiple mitochondrial dysfunctions syndrome 1 (MMDS1), which is often complicated by early death and PAH (14, 15). Other clinical links between PH and metabolic diseases have been reported as well, but proof that genetic mutations in any Fe-S biogenesis genes directly drive PH pathogenesis has been elusive. As such, the clinical classification of PH associated with metabolic syndromes in general has been relegated to WHO Group 5 PH (PH due to unknown causes), reflecting the fact that the multifactorial mechanisms underlying this association have yet to be defined. In a study presented in this issue of the Journal, Niihori and colleagues (pp. 231–242) obtained more definitive proof regarding the causative link between NFU1 mutations and PAH by generating homozygous NFU1 mutant Sprague Dawley rats (16). Compared with wild-type controls, homozygous females exhibited hemodynamic changes consistent with PAH, including increased RV systolic pressure (RVSP), Fulton’s index (a surrogate for RV hypertrophy), occlusive vessel remodeling, and vessel rarefaction. These changes were accompanied by decreased NFU1 hexamer oligomerization, activity of pyruvate dehydrogenase (PDH), and expression and activity of complexes I and II. Conversely, the majority of homozygous males did not exhibit increased RVSP or Fulton’s index, despite similar evidence of pulmonary vessel remodeling and a metabolic shift away from OXPHOS (i.e., decreased PDH and complex II expression and activity). The authors partially addressed these sex-based differences by assessing compensatory changes in Fe-S biogenesis and mitochondrial respiration. To this point, they found that homozygous males exhibited normal hexamer formation, increased ISCU1/2 expression, and increased activity of complexes III and IV. Overall, the NFU1 rat constitutes the first preclinical model of PAH driven by a human Fe-S biogenesis gene mutation, and thus allows previously elusive evidence to be obtained regarding the direct causative relationship between Fe-S biogenesis and this vascular disease (Figure 1). Interestingly, although patients with MMDS1 uniformly die at a young age, with failure to thrive and substantial neurologic dysfunction (13–15), the authors did not report whether these rats reproduce all of these phenotypic features, although certainly these rats remain viable to adulthood. Although this may suggest an incomplete recapitulation of MMDS1, the spontaneous

INTRODUCTION:Pulmonary hypertension (PH) is a deadly enigmatic disease with increasing prevalence. Cellular pathologic hallmarks of PH are driven at least partly by metabolic rewiring, but details are just emerging. The discovery that vascular matrix stiffening can mechanically activate the glutaminase (GLS) enzyme and serve as a pathogenic mechanism of PH has advanced our understanding of the complex role of glutamine in PH. It has also offered a novel therapeutic target for development as a next-generation drug for this disease. Area covered: This review discusses the cellular contribution of glutamine metabolism to PH together with the possible therapeutic application of pharmacologic GLS inhibitors in this disease. Expert opinion: Despite advances in our understanding of glutamine metabolism in PH, questions remain unanswered regarding the development of therapies targeting glutamine in PH. The comprehensive mechanisms by which glutamine metabolism rewiring influences pulmonary vascular cell behavior to drive PH are incompletely understood. Because glutamine metabolism exhibits a variety of functions in organ repair and homeostasis, a better understanding of the overall risk-benefit ratio of these strategies with long-term follow-up is needed. This knowledge should pave the way for the design of new strategies to prevent and hopefully even regress PH.

Background: Endothelial iron-sulfur (Fe-S) cluster deficiency promotes pulmonary hypertension (PH), but the pathogenic mechanisms are incompletely defined. Frataxin (FXN) controls Fe-S cluster asse...