Redox biology in pulmonary arterial hypertension (2013 Grover Conference Series)

Abstract

Through detailed interrogation of the molecular pathways that contribute to the development of pulmonary arterial hypertension (PAH), the separate but related processes of oxidative stress and cellular metabolic dysfunction have emerged as being critical pathogenic mechanisms that are as yet relatively untargeted therapeutically. In this review, we have attempted to summarize some of the important existing studies, to point out areas of overlap between oxidative stress and metabolic dysfunction, and to do so under the unifying heading of redox biology. We discuss the importance of precision in assessing oxidant signaling versus oxidant injury and why this distinction matters. We endeavor to advance the discussion of carbon-substrate metabolism beyond a focus on glucose and its fate in the cell to encompass other carbon substrates and some of the murkiness surrounding our understanding of how they are handled in different cell types. Finally, we try to bring these ideas together at the level of the mitochondrion and to point out some additional points of possible cognitive dissonance that warrant further experimental probing. The body of beautiful science regarding the molecular and cellular details of redox biology in PAH points to a future that includes clinically useful therapies that target these pathways. To fully realize the potential of these future interventions, we hope that some of the issues raised in this review can be addressed proactively.

Keywords: 3-nitrotyrosine; Warburg; bone morphogenetic protein receptor 2; nitrosylation.

Figure 1

Reactive oxygen species (ROSs) and products of redox reactions exist along a spectrum from oxidant injury to oxidant signaling. The ROSs that are most commonly studied under the umbrella of “oxidative stress” can be separated on the basis of how they behave as injurious stimuli versus how they behave as signaling species/second messengers. A partial listing is shown in this figure. For example, the hydroxyl radical covalently reacts essentially at the diffusion limit with any nearby macromolecule (lipid, protein, or nucleic acid), kinetics that are too rapid for regulated signaling and that functionally can only drive oxidant injury. By contrast, at the other end of the spectrum is nitric oxide, a free radical that is quite chemically stable and has a very well established role as a bona fide second messenger. Similarly, the products of free-radical reactions with macromolecules can be ordered along a similar spectrum. Products such as F₂-isoprostanes and protein carbonyls have predominantly been shown to be markers of oxidant injury (although F₂-isoprostanes can signal, there does not appear to be tightly regulated production and degradation that would qualify them as an oxidant signal). However, production of 8-oxoguanosine has been shown to exhibit regulated production to control gene expression. GSSG: glutathione disulfide; HNE: hydroxynonenal.

Figure 2

Summary of the Fenton reaction and the Haber-Weiss cycle. The Fenton reaction and the Haber-Weiss cycle are often invoked as important sources of damaging reactive oxygen species (particularly the hydroxyl radical) in vivo. There is some debate as to whether either of these processes proceeds with any appreciable efficiency in living cells and tissues.

Figure 3

Pulmonary arterial hypertension–causing bone morphogenetic protein receptor-2 (BMPR2) mutations drive tyrosine nitration of specific targets. In both murine pulmonary microvascular endothelial cells (PMVECs) and skin fibroblasts from patients, expression of any one of several mutant BMPR2 isoforms is sufficient to drive nitration of tyrosines in multiple protein targets, as shown in this Western blot from total protein lysates probed with an anti-nitrotyrosine antibody. Fold increase is indicated by the numbers and calculated as a ratio of the densitometry in the BMPR2 mutant sample to that in control/wild-type cells (average densitometry for the two, in the case of the two different BMPR2 mutations represented by the human fibroblasts). Of note, bands appear as a ladder in both wild-type and mutant cells, indicating that tyrosine nitration is not a stochastic event. Further, only certain bands (arrows) show increased intensity in the BMPR2 mutant cells, suggesting further specificity.

Figure 4

Schematic of Randle’s cycle. A reciprocal relationship exists in most cells between glucose oxidation and fatty acid oxidation. As shown here and discussed in the text, acetyl coenzyme A (CoA) and citrate feed back to inhibit glucose oxidation, mainly at pyruvate dehydrogenase (PDH) and phosphofructokinase (PFK) and indirectly at hexokinase and glucose transporter 4. Malonyl-CoA will feed back to inhibit fatty acid oxidation through inhibition of carnitine palmitoyltransferase 1 (CPT1). Note that a number of enzymes (e.g., adenosine triphosphate citrate lyase) have been left out to allow focus on the key nodes of regulation between the two pathways that constitute Randle’s cycle.

Figure 5

Major sites for superoxide generation in the mitochondrial electron transport system. Shown are the major sites for free-radical leak, the overall direction of electron flow, and the overall direction of proton flow in the mitochondrion. ADP: adenosine diphosphate; ATP: adenosine triphosphate; CoQ: coenzyme Q/ubiquinol/ubiquinone; Cx: complex; cyt c: cytochrome c; ETF: electron transport flavoprotein system; FAO: fatty acid oxidation; Pi: phosphate ion.