Introduction: Pulmonary hypertension due to left heart disease (PH-LHD) is a common and currently untreatable complication of chronic heart failure. With the growing prevalence of heart failure with preserved ejection fraction (HFpEF), PH-HFpEF has emerged as a particularly severe complication, associated with increased morbidity and mortality. However, its underlying mechanisms remain poorly understood. Recent evidence implicates Nicotinamide Nucleotide Transhydrogenase (NNT) – a mitochondrial enzyme responsible for generating NADPH for reactive oxygen species (ROS) clearance – as a central player in the pathogenesis of HFpEF. Here, we investigated the role of NNT in PH-HFpEF-associated meta-inflammation, hypothesizing that C57BL/6N mice with functioning NNT develop milder PH-HFpEF phenotypes compared to C57BL/6J mice lacking NNT.
Methods: Eight-week-old male C57BL/6J mice (n=11), lacking functional NNT, and C57BL/6N mice (n=11), with intact NNT, were subjected to a high-fat diet combined with L-NAME (0.5g/L) ad libitum for 10 weeks to induce HFpEF. For PH induction, mice were exposed to chronic hypoxia (10% O2) for additional two weeks. Naïve C57BL/6J (n=11) and C57BL/6N (n=11) mice served as controls. In a separate cohort, mice of both genotypes (n=8) were exposed to chronic hypoxia only (10% for five weeks) to evaluate NNT effects independent of HFpEF-associated metabolic stress. All animals underwent comprehensive evaluation including biventricular echocardiography, invasive hemodynamic assessment and multicolor flow cytometry.
Results: PH-HFpEF mice showed increased RV systolic pressures and signs of diastolic dysfunction. Compared to NNT-deficient C57BL/6J mice, C57BL/6N mice with intact NNT displayed lower RV systolic pressures and improved ventricular relaxation, evident as decreased left ventricular end-diastolic pressure after PH-HFpEF induction. Gravimetric analysis revealed reduced RV hypertrophy and pulmonary congestion in C57BL/6N mice exposed to a PH-HFpEF phenotype, as indicated by lower Fulton’s index and lung-weight-to-tibia-length ratios. Mechanistically, PH-HFpEF C57BL/6N mice showed reduced LV macrophage recruitment when compared to C57BL/6J mice, suggesting an attenuated cardiac immune cell expansion. Interestingly, under chronic hypoxia only, NNT status did not affect cardiopulmonary phenotypes, indicating that the observed changes are specific to metabolic stress in PH-HFpEF.
Conclusion: Overall, our data suggest that intact NNT in C57BL/6N mice is associated with ameliorated PH-HFpEF phenotypes compared to C57BL/6J mice, mechanistically associated with reduced immune cell recruitment. Ongoing experiments explore how NNT status impacts leukocyte function in HFpEF-associated metabolic stress.
Novelty and Clinical Relevance:
This study is the first evidence that NNT status modulates immune cell responses and disease severity in PH-HFpEF. Clinically, reduced NNT activity has been reported in patients with heart failure, aligning with our findings of NNT’s protective role. However, conflicting reports linking elevated NNT activity to worse outcomes in HFpEF suggest that its impact may depend on disease context. Further studies are required to define NNT’s potential as a therapeutic target or prognostic biomarker in cardiopulmonary diseases associated with sterile inflammation.
