Metabolic Profiling of Right Ventricular-Pulmonary Vascular Function Reveals Circulating Biomarkers of Pulmonary Hypertension

Abstract

Background: Pulmonary hypertension and associated right ventricular (RV) dysfunction are important determinants of morbidity and mortality, which are optimally characterized by invasive hemodynamic measurements.

Objectives: This study sought to determine whether metabolite profiling could identify plasma signatures of right ventricular-pulmonary vascular (RV-PV) dysfunction.

Methods: We measured plasma concentrations of 105 metabolites using targeted mass spectrometry in 71 individuals (discovery cohort) who underwent comprehensive physiological assessment with right-sided heart catheterization and radionuclide ventriculography at rest and during exercise. Our findings were validated in a second cohort undergoing invasive hemodynamic evaluations (n = 71), as well as in an independent cohort with or without known pulmonary arterial (PA) hypertension (n = 30).

Results: In the discovery cohort, 21 metabolites were associated with 2 or more hemodynamic indicators of RV-PV function (i.e., resting right atrial pressure, mean PA pressure, pulmonary vascular resistance [PVR], and PVR and PA pressure-flow response [ΔPQ] during exercise). We identified novel associations of RV-PV dysfunction with circulating indoleamine 2,3-dioxygenase (IDO)-dependent tryptophan metabolites (TMs), tricarboxylic acid intermediates, and purine metabolites and confirmed previously described associations with arginine-nitric oxide metabolic pathway constituents. IDO-TM levels were inversely related to RV ejection fraction and were particularly well correlated with exercise PVR and ΔPQ. Multisite sampling demonstrated transpulmonary release of IDO-TMs. IDO-TMs also identified RV-PV dysfunction in a validation cohort with known risk factors for pulmonary hypertension and in patients with established PA hypertension.

Conclusions: Metabolic profiling identified reproducible signatures of RV-PV dysfunction, highlighting both new biomarkers and pathways for further functional characterization.

Keywords: exercise; hemodynamics; metabolism; pulmonary circulation.

Copyright © 2016 American College of Cardiology Foundation. Published by Elsevier Inc. All rights reserved.

Figures

FIGURE 1. Metabolic Heat Map

β-Coefficients and p values generated from regression analyses depict the relationship of each metabolite (log transformed) with hemodynamic measurements. *p

FIGURE 1. Metabolic Heat Map

β-Coefficients and p values generated from regression analyses depict the relationship of each metabolite (log transformed) with hemodynamic measurements. *p

FIGURE 2. Metabolite-ΔPQ Relationship

The highest quartile of indoleamine 2,3-dioxygenase (IDO) tryptophan metabolite (TM) score exhibited a much greater ΔPQ during exercise than the lower quartiles. ΔPQ data = median ± interquartile range of change in mean pulmonary arterial pressure relative to change in cardiac output.

FIGURE 3. Transpulmonary Metabolite Release

For gradients of indoleamine 2,3-dioxygenase tryptophan metabolites across the pulmonary circulation, the average ± SD percent difference in metabolite levels in the radial arterial samples, compared with the pulmonary arterial samples, is indicated for subjects with pulmonary vascular resistance (PVR) ≤2 Wood units (diamonds) and >2 Wood units (squares). *p < 0.05 for PVR ≤2 Wood units versus PVR >2 Wood units; †p < 0.05 for PVR >2 Wood units versus no difference.

FIGURE 4. Animal Model Studies

(A) Indoleamine 2,3-dioxygenase (IDO1) messenger ribonucleic acid (mRNA) was up-regulated in the lungs of mice with hypoxia-induced pulmonary hypertension (PH) (n = 10) compared with control mice (n = 10). *p = 0.01. (B) IDO activity, determined by the ratio of IDO substrate and product (kynurenine/tryptophan [K/T]), was higher in lungs of mice with hypoxia-induced PH (n = 12) than in control mice (n = 10). ***p = 0.002. (C) A consistent pattern of elevation in metabolites downstream of IDO was seen in the plasma of mice with hypoxia-induced PH (n = 12) versus control mice (n = 10). *p < 0.05.

FIGURE 5. ROC Analysis

Receiver-operating characteristic (ROC) curves and area under the curve (with 95% confidence interval) values illustrate the ability of indoleamine 2,3-dioxygenase tryptophan metabolite levels to distinguish (A) individuals with and without right ventricular–pulmonary vascular dysfunction in the validation dyspnea cohort stratified by the presence or absence of pulmonary arterial hypertension and (B) individuals with known pulmonary arterial hypertension from control subjects in an independent validation cohort.

CENTRAL ILLUSTRATION. Metabolic Signatures of Pulmonary Hypertension

Metabolic profiling identified novel associations of right ventricular–pulmonary vascular (RV-PV) dysfunction involving potential new biomarkers and pathways for future research. Arg-NO = arginine nitric oxide; IDO = indoleamine 2-dioxygenase; LV = left ventricle; PAP = pulmonary arterial pressure; PAP/Qex = pulmonary arterial pressure flow relationship during exercise; PVR = pulmonary vascular resistance; PVRrest/ex = pulmonary vascular resistance during rest and exercise; RAP = right atrial pressure; RV = right ventricle; RV-PV = right ventricular-pulmonary vascular; TCA = tricarboxylic acid; Vasc SMC = vascular smooth muscle cell.