Hallmarks of Pulmonary Hypertension: Mesenchymal and Inflammatory Cell Metabolic Reprogramming

Abstract

Significance: The molecular events that promote the development of pulmonary hypertension (PH) are complex and incompletely understood. The complex interplay between the pulmonary vasculature and its immediate microenvironment involving cells of immune system (i.e., macrophages) promotes a persistent inflammatory state, pathological angiogenesis, and fibrosis that are driven by metabolic reprogramming of mesenchymal and immune cells. Recent Advancements: Consistent with previous findings in the field of cancer metabolism, increased glycolytic rates, incomplete glucose and glutamine oxidation to support anabolism and anaplerosis, altered lipid synthesis/oxidation ratios, increased one-carbon metabolism, and activation of the pentose phosphate pathway to support nucleoside synthesis are but some of the key metabolic signatures of vascular cells in PH. In addition, metabolic reprogramming of macrophages is observed in PH and is characterized by distinct features, such as the induction of specific activation or polarization states that enable their participation in the vascular remodeling process.

Critical issues: Accumulation of reducing equivalents, such as NAD(P)H in PH cells, also contributes to their altered phenotype both directly and indirectly by regulating the activity of the transcriptional co-repressor C-terminal-binding protein 1 to control the proliferative/inflammatory gene expression in resident and immune cells. Further, similar to the role of anomalous metabolism in mitochondria in cancer, in PH short-term hypoxia-dependent and long-term hypoxia-independent alterations of mitochondrial activity, in the absence of genetic mutation of key mitochondrial enzymes, have been observed and explored as potential therapeutic targets.

Future directions: For the foreseeable future, short- and long-term metabolic reprogramming will become a candidate druggable target in the treatment of PH. Antioxid. Redox Signal. 28, 230-250.

Keywords: aerobic glycolysis; hypoxia; metabolism; mitochondria; pulmonary hypertension.

FIG. 1.

Borrowing from and noticing the similarities with the founding concepts of the hallmarks of cancer (78), here we propose and summarize the hallmarks of PH. ECM, extracellular matrix; HIF, hypoxia-inducible factor; PH, pulmonary hypertension.

FIG. 2.

Phosphorylation of PDH and gene expression in vascular tissue is regulated by G6PD. Phosphorylation of PDH subunits (E1–E3) was determined by In Cell Western blot assay kit (PhosphoPDH In-Cell ELISA Kit [IR]; Abcam, MA) in PASMCs cultured in normoxia (21% O₂) and hypoxia (3% O₂) treated with or without DHEA (100 μM). (A) As shown, phosphorylation of PDH, which inhibits acetyl CoA formation from pyruvate, was increased in hypoxic PASMCs and DHEA decreased hypoxia-induced phosphorylation of PDH. (B) Expression of genes critical in endothelial cells and PASMCs growth and function is modulated in vascular tissue of G6PD-deficient as compared with wild-type mice. *p < 0.05 T-test. DHEA, dehydroepiandrosterone; G6PD, glucose 6-phosphate dehydrogenase; PASMCs, pulmonary artery smooth muscle cells; PDH, pyruvate dehydrogenase.

FIG. 3.

G6PD inhibition increased phosphorylation of inositol triphosphate receptor and decreased angiotensin II-induced bovine PA contractions. (A) A representative Western blot of four different experiments demonstrates total and phosphorylated inositol triphosphate receptor (IP3R) at Ser1755, a PKG phosphorylation site in consensus GRRESLTS sequence (67), in angiotensin II (Ang II; 10 μM)-treated bovine PA in absence and presence of DHEA (100 μM) in hypoxia. (B) Summary data demonstrate that DHEA increased phospho-to-total IP3R ratio. (C) DHEA did not decrease Ang II-induced IP3 levels in bovine PAs. (D) Bovine PAs were precontracted with Ang II in hypoxia (0% O₂). *p < 0.05 vs control T-test; ^#p < 0.05 vs Ang II T-test. DHEA treatment blocked Ang II-induced peak contraction. PA, pulmonary artery; PKG, protein kinase G.

FIG. 4.

Decreased oxygen availability under hypoxic conditions promotes succinate accumulation owing to lack of oxygen as the final acceptor of electron at complex IV and reverse activity of succinate dehydrogenase at complex II. On reperfusion, rapid oxidation of succinate promotes a reverse-electron flow through complex I of the electron transport chain, a phenomenon that favors the accumulation of reactive oxygen species.

FIG. 5.

Transcriptional regulation by CtBP1 in response to altered NADH/NAD⁺ homeostasis in PH (108). The crystal structure shows CtBP1 in complex with substrate MTOB (pdb: 4LCE). CtBP1, C-terminal-binding protein 1; MTOB, 4-methylthio 2-oxobutyric acid.

FIG. 6.

Metabolic synergy between adventitial fibroblasts and macrophages in the vascular remodeling process in PH. In adventitial fibroblasts, epigenetic changes reflected in decreased miRNA124 expression enable increased PTBP1 signaling, resulting in increased expression of PKM2, which drives lactate formation and increases aerobic glycolysis while reducing TCA cycle. Increased glycolysis results in increased free NADH, which activates the transcriptional repressor CtBP1. Together, these alterations promote proliferation and apoptosis resistance concomitantly with an increased production of lactate, succinate, citrulline, IL-6, and other pro-inflammatory mediators, while suppressing anti-inflammatory mediator production. Adventitial macrophages respond to the increased concentrations of fibroblast-derived metabolites in the microenvironment whereby IL6, lactate, and succinate drive activation of STAT3 and HIF1, which, in turn, drive metabolic reprogramming similar to that in adventitial fibroblasts; utilization of citrulline feeds polyanine production by Arginase, which, in turn, is increased in response to IL6 and lactate. Thus, exchange of substrates and metabolites between macrophages and fibroblasts enables persistent metabolic reprogramming, cellular activation, and proliferation. PTBP1, polypyrimidine-tract binding protein 1; TCA, tricarboxylic acid.

FIG. 7.

An overview of mitochondrial metabolism and its role in redox and energy homeostasis in PH. GDH, glutamate dehydrogenase; GSH, glutathione; PHD, prolyl hydroxylase domain; OAA, oxaloacetate; NOS, nitric oxide synthase; NOX, NADPH oxidase.

FIG. 8.

A summary of metabolic pathways and potential therapeutic interventions in PH, as reviewed in this article. CHC, α-cyano-4-hydroxycinnamic acid; DCA, dichloroacetate; PDK, pyruvate dehydrogenase kinase.