Important Functions and Molecular Mechanisms of Mitochondrial Redox Signaling in Pulmonary Hypertension

Abstract

Mitochondria are important organelles that act as a primary site to produce reactive oxygen species (ROS). Additionally, mitochondria play a pivotal role in the regulation of Ca²⁺ signaling, fatty acid oxidation, and ketone synthesis. Dysfunction of these signaling molecules leads to the development of pulmonary hypertension (PH), atherosclerosis, and other vascular diseases. Features of PH include vasoconstriction and pulmonary artery (PA) remodeling, which can result from abnormal proliferation, apoptosis, and migration of PA smooth muscle cells (PASMCs). These responses are mediated by increased Rieske iron-sulfur protein (RISP)-dependent mitochondrial ROS production and increased mitochondrial Ca²⁺ levels. Mitochondrial ROS and Ca²⁺ can both synergistically activate nuclear factor κB (NF-κB) to trigger inflammatory responses leading to PH, right ventricular failure, and death. Evidence suggests that increased mitochondrial ROS and Ca²⁺ signaling leads to abnormal synthesis of ketones, which play a critical role in the development of PH. In this review, we discuss some of the recent findings on the important interactive role and molecular mechanisms of mitochondrial ROS and Ca²⁺ in the development and progression of PH. We also address the contributions of NF-κB-dependent inflammatory responses and ketone-mediated oxidative stress due to abnormal regulation of mitochondrial ROS and Ca²⁺ signaling in PH.

Keywords: Ca2+ signaling; ketones; mitochondrial ROS; pulmonary hypertension.

Figure 1

Schematic representation of mitROS generation and signaling; crosstalk between ROS and Ca²⁺ signaling in PASMCs. Mitochondria are the major source of ROS in pulmonary artery smooth muscle cells (PASMCs). During ATP synthesis in the electron transport chain (ETC), coupling between the proton gradient on either side of the inner mitochondrial membrane leads to the production of ROS. Briefly, electrons are transferred from nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2) to molecular oxygen. In this process, protons are pumped from the mitochondrial matrix into the intermembrane space, and oxygen is reduced to H₂O. Hypoxia increases the production of mitROS, contributing to the increase in [Ca²⁺]_i and to hypoxia-induced pulmonary vasoconstriction. The Rieske iron–sulfur protein (RISP), a catalytic subunit of the complex III of the mitochondrial ETC serves as a primary molecule in intracellular ROS generation in PASMCs, especially under hypoxic conditions. In addition, mitROS and vasoconstrictor agonists stimulate the PLCγ and PKCε signaling pathways via GPCR activation. PLCγ induces the formation of IP₃ and DAG, causing the opening of IP₃R1 and the release of Ca²⁺ from the sarcoplasmic reticulum (SR). Moreover, mitROS augment the activity of PKCε, which in turn stimulates NOX and promotes the formation of ROS in a process named ROS-induced ROS generation (RIRG). In addition, ROS enable the dissociation of FK506 binding protein 12.6 (FKBP12.6) from ryanodine receptor 2 (RyR2) favoring the opening of this channel and enhancing Ca²⁺ release. Furthermore, FKBP12.6 is physically bound to high conductance K⁺ channels (BK_Ca) and regulates their open probability. Finally, ROS upregulate voltage-dependent Ca²⁺ channels (VDCCs) which further contribute to the increase in [Ca²⁺]_i, leading to persistent vasoconstriction observed in PH.

Figure 2

Glutaminolysis and glutamate accumulation contribute to pulmonary hypertension (PH). Glutaminolysis is a mitochondrial process responsible for obtaining cellular energy from the breakdown of glutamine. In this cellular pathway, glutamine is converted into glutamate, aspartate, CO₂, pyruvate, lactate, alanine, and citrate. Initially, glutamine enters the pulmonary vascular cells via a glutamine transporter and is deaminated to glutamate by glutaminase (GLS1). Subsequently, glutamate is converted to α-ketoglutarate (α-KG) by glutamate dehydrogenase. α kg enters the tricarboxylic acid (TCA) cycle, where it is decarboxylated by α kg dehydrogenase to succinyl-CoA and CO₂, providing energy for proliferating cells. Accumulation of glutamate in pulmonary vascular cells promotes PH. In addition, stiffening of the extracellular matrix in remodeled pulmonary cells activates the transcriptional coactivators Yes-associated protein 1 (YAP) and TAZ, leading to upregulation of GLS1 and enhanced glutaminolysis. Furthermore, in remodeled pulmonary arteries, the N-methyl-d-aspartate receptor (NMDAR) is overexpressed and overactivated.

Figure 3

Fatty acid metabolism and its byproducts (ketones) play an important role in the development of right ventricular failure (RVF) and pulmonary arterial hypertension (PAH). Peroxisome proliferator-activated receptors (PPAR) belong to the superfamily of nuclear receptors that serve as ligand-activated transcription factors and consist of three members, PPARα, PPARγ, and PPARδ. These receptors together with retinoid X receptors (RXR) form heterodimers and bind to specific DNA sites to promote genetic transcription. PPARα is a master regulator of adipogenesis expressed mainly in adipose tissue and liver, as is PPARγ. Additionally, PPARγ regulates glucose and fatty acid metabolism in adipocytes, hepatocytes, skeletal muscle, and pancreatic β-cells. PPARγ agonists, such as pioglitazone, increase the expression of Cpt1b and Fabp4, proteins involved in fatty acid oxidation and transport in cardiomyocytes. These effects favor mitochondrial fatty acid oxidation and ATP production, leading to reversal of cardiac hypertrophy, fibrosis, and eliminating severe PAH. Furthermore, PPARδ stimulates fatty acid oxidation, decreases right ventricle hypertrophy and pulmonary congestion. In cardiac inflammation, PPARδ blocks nuclear factor κB (NF-κB) activation and inhibits tumor necrosis factor (TNF)-α synthesis.