Jordan Kuhlman

Background: Heart Failure with Preserved Ejection Fraction (HFpEF) is a rapidly growing subset of Heart Failure associated with significant morbidity and mortality.^1,2 One hypothesized mechanism is coronary microvascular dysfunction driven by dysregulation of Nitric Oxide pathways.¹ Of particular interest are the endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS) pathways, which regulate endothelial function and remodeling.^3,4 Historically, models failed to replicate HFpEF, but novel models have led to a greater understanding of iNOS’ role in coronary microvascular dysfunction.⁵ These discoveries provide necessary knowledge of molecular mechanisms driving HFpEF in order to develop new therapeutics to address this unmet need.

Objective: This study aims to explore the role of the IRE1α-Xbp1s axis and PI3K/Akt pathway in iNOS dysfunction in HFpEF pathogenesis.

Search Methods: Using PubMed, data was gathered from studies conducted between 2018 and 2024 with relevant keywords including “HFpEF,” “Nitric Oxide Synthase,” “IRE1α-Xbp1s,” “Microvascular Dysfunction,” and “Akt.”

Results: Mice given the HFD+L-NAME treatment strongly replicated symptoms in HFpEF patients marked by increase in pulmonary weight and exercise intolerance.⁵ Within these mice, there was significant dysregulation of the IRE1α-Xbp1s axis, a key component of the unfolded protein response (UPR), in cardiomyocytes.⁵ HFpEF models were observed to have a four-fold increase in aberrant S-Nitrosylation of IRE1α and marked decreased splicing of X-box binding protein 1 (Xbp1s) in mice and human tissue samples.⁵ This coincided with elevation of iNOS levels and a proinflammatory state associated with HFpEF.⁵ Furthermore, the HFD+L-NAME diet elicited cardiac CD4+ T-cell driven inflammation due to IRE1α-Xbp1s dysregulation.⁶ These CD4+ T Cells were found to survive longer and induce stress within coronary microvasculature.⁶ Moreover, the PI3K/Akt/mTOR pathway, essential for cardiomyocyte glucose utilization and cell signaling, was heavily impacted by iNOS.⁷ When exposed to high levels of iNOS, there was a two-fold increase in S-Nitrosylation of Akt and decreased phosphorylation of Akt.⁷ Further, Atf4 gene transcripts decreased significantly when exposed to these conditions.⁷ Both pathways returned to normal function when iNOS was inhibited, either through dietary changes or direct inhibition. In INOS-KO mice, S-Nitrosylation of IRE1α and Akt decreased which was associated with Xbp1s transcripts returning to basal levels and infiltration of CD4+ T cells decreasing in the coronary microvasculature. ^5-7

Conclusion: These findings underscore the central role of iNOS dysregulation in the development of HFpEF. The IRE1α-Xbp1s and PI3K/Akt/mTOR pathways are significantly impacted by elevated iNOS.^5&6 When iNOS was targeted for inhibition, these pathways returned to normal function. In mice, iNOS inhibition ameliorated symptoms of HFpEF. Mice recovered cardiac function, exercise ability and other symptoms such as pulmonary edema significantly improved.^5&6 Furthermore, lowering iNOS coincided with normal levels of IRE1α-Xbp1s and PI3K/Akt/mTOR transcripts and downstream signaling.^5-7 These findings pose an interesting mechanism for future therapeutic targets such as eNOS targeted micro-therapeutics and cardiac progenitor cells.^8&9 Overall, this study sheds light on the mechanisms underlying HFpEF pathogenesis for future therapeutic targeting.

Works Cited:

1. Redfield MM, Borlaug BA. Heart Failure With Preserved Ejection Fraction: A Review. JAMA. 2023;329(10):827-838. doi:10.1001/jama.2023.2020
2. Pfeffer MA, Shah AM, Borlaug BA. Heart Failure With Preserved Ejection Fraction In Perspective. Circ Res. 2019;124(11):1598-1617. doi:10.1161/CIRCRESAHA.119.313572
3. Borlaug BA, Sharma K, Shah SJ, Ho JE. Heart Failure With Preserved Ejection Fraction: JACC Scientific Statement. J Am Coll Cardiol. 2023;81(18):1810-1834. doi:10.1016/j.jacc.2023.01.049
4. Shah SJ, Lam CSP, Svedlund S, et al. Prevalence and correlates of coronary microvascular dysfunction in heart failure with preserved ejection fraction: PROMIS-HFpEF [published correction appears in Eur Heart J. 2019 Feb 7;40(6):541]. Eur Heart J. 2018;39(37):3439-3450. doi:10.1093/eurheartj/ehy531
5. Schiattarella GG, Altamirano F, Tong D, et al. Nitrosative stress drives heart failure with preserved ejection fraction. Nature. 2019;568(7752):351-356. doi:10.1038/s41586-019-1100-z
6. Smolgovsky S, Bayer AL, Kaur K, et al. Impaired T cell IRE1α/XBP1 signaling directs inflammation in experimental heart failure with preserved ejection fraction. J Clin Invest. 2023;133(24):e171874. Published 2023 Dec 15. doi:10.1172/JCI171874
7. Guo Y, Wen J, He A, et al. iNOS contributes to heart failure with preserved ejection fraction through mitochondrial dysfunction and Akt S-nitrosylation. J Adv Res. 2023;43:175-186. doi:10.1016/j.jare.2022.03.003
8. Chen G, Xu C, Gillette TG, et al. Cardiomyocyte-derived small extracellular vesicles can signal eNOS activation in cardiac microvascular endothelial cells to protect against Ischemia/Reperfusion injury. Theranostics. 2020;10(25):11754-11774. Published 2020 Sep 23. doi:10.7150/thno.43163
9. de Couto G, Mesquita T, Wu X, et al. Cell therapy attenuates endothelial dysfunction in hypertensive rats with heart failure and preserved ejection fraction. Am J Physiol Heart Circ Physiol. 2022;323(5):H892-H903. doi:10.1152/ajpheart.00287.2022