The role of kynurenine pathway metabolism in pulmonary arterial hypertension

/Abstract Pulmonary arterial hypertension (PAH) is characterized by pulmonary vascular remodeling and increased pulmonary vascular resistance. Most therapies for PAH work by reducing pulmonary vascular resistance, without addressing the underlying cellular and molecular mechanisms driving pathobiology. Our prior work in metabolomics has implicated upregulated kynurenine pathway (KP) metabolism as an early, persistent, and potentially causal feature contributing to PAH pathobiology. Our preliminary data suggest upregulation of tryptophan 2,3 dioxygenase (TDO), an enzyme that converts tryptophan to kynurenine, occurs in an expanded fibroblast population in the PAH lung, and increased TDO drives increased kynurenine production in PAH. Kynurenine is an endogenous activator of the transcription factor (TF) AHR (aryl hydrocarbon receptor) in other biologic contexts. Separately, exogenous AHR activation was recently proven necessary and sufficient for PAH development in a preclinical model, and AHR activity is increased in human PAH. However, the downstream effects of upregulated KP metabolism in PAH are undefined, and the endogenous upstream activator(s) of AHR TF activity in human PAH are unknown. We hypothesize that TDO-derived kynurenine is the endogenous activator of AHR in human PAH, and that kynurenine-induced AHR activity in PAH promotes transcriptional programs that contribute to pulmonary vascular remodeling and resistance. Because pharmacologic TDO inhibitors are under study for other applications, interrogation of a TDO-kynurenine-AHR axis may reveal an opportunity for repurposing of a therapy to PAH that addresses fundamental pathobiology. We propose 3 Specific Aims. Aim 1 establishes a prospective observational cohort of PAH patients in which we will measure longitudinal KP metabolites and AHR activity over the treatment course. We will apply a causal inference framework to test AHR activity as the causal mediator of KP metabolite effects on pulmonary vascular resistance and clinical outcomes. In Aim 2, we will perform single-cell RNA sequencing in archived PAH lung specimens to investigate the TDO-expressing fibroblast population using high-resolution clustering, co-expression analyses, and trajectory inference. We will perform cell-cell ligand-receptor signaling and TF activity analysis on high-resolution clusters to identify activators of AHR-mediated transcriptional programs. Our proposed use of archived tissues presents a unique opportunity to analyze KP-AHR cell type-specific transcriptional associations with pulmonary vascular resistance and other patient-level clinical data. In Aim 3, we will perform complementary gain- and loss- of-function experiments in rodent models of PAH to examine effects of TDO forced overexpression and inhibition on KP metabolism and AHR-mediated gene expression in isolated pulmonary vascular cells (e.g., fibroblasts and endothelial cells) in vitro. Further, we will test the molecular and phenotypic effects of TDO inhibition in our animal models in vivo. Should our overall hypothesis be confirmed, the proposed studies will provide a robust evidentiary foundation for early-phase trials investigating KP inhibition as a disease-modifying strategy in PAH. Project Number: 1R01HL175014-01A1 | Fiscal Year: 2025 | NIH Institute/Center: National Heart Lung and Blood Institute (NHLBI) | Principal Investigator: Catherine Simpson | Institution: JOHNS HOPKINS UNIVERSITY, BALTIMORE, MD | Award Amount: $514,484 | Activity Code: R01 | Study Section: Pulmonary Vascular Disease and Physiology Study Section [PVP] View on NIH RePORTER: https://reporter.nih.gov/project-details/1R01HL17501401A1