Metabolic dysfunction-associated steatotic liver disease (MASLD) and type 2 diabetes mellitus (T2DM) have emerged as significant global health issues due to alterations in lifestyle and dietary habits [1], 2]. The inflammatory variant of metabolic dysfunction-associated steatohepatitis (MASH) may result in cirrhosis and hepatocellular carcinoma. The coexistence or exacerbation of T2DM and MASLD increases the risk of cardiovascular and cerebrovascular complications. Notably, T2DM is a key driver of MASLD progression toward advanced fibrosis and cirrhosis. Despite their growing burden, effective disease-modifying interventions remain limited. In the recent issue of Signal Transduction and Targeted Therapy, Li et al. investigate the effects of inorganic nitrate supplementation on macrophage metabolic reprogramming and systemic metabolic homeostasis [3] (Figure 1).
Figure 1:
Metabolic pathways of nitrate-Sialin-cathepsin L axis affecting hepatic immune homeostasis. Nitrate, commonly found in leafy green vegetables, can act as a “homeostatic remodeler” when ingested exogenously. It reduces pro-inflammatory macrophages (CD11C+) and increases anti-inflammatory macrophages (CD206+), thereby rebalancing bone marrow-derived macrophages. Sialin is not merely a nitrate transporter-it serves as a key target through which sodium nitrate exerts its homeostatic remodeling effects. Nitrate facilitates the interaction between Sialin and Rel, thereby retaining Rel in the cytoplasm and inhibiting its nuclear translocation. This blocks Rel binding to the CtsL promoter, leading to downregulation of CtsL expression. This process promotes the transcriptional activity of Nrf2, which in turn suppresses the polarization of CD11C+ M1-type macrophages and enhances the polarization of CD206+ M2-type macrophages. Ultimately, this alleviates inflammation, maintains functional balance in liver tissue, and promotes metabolic homeostasis. This mechanism offers novel insights into the prevention and treatment of metabolic diseases such as MASLD and T2DM. Abbreviations: NO3 −, nitrate; CtsL, cathepsin L; Nrf2: nuclear factor erythroid 2-related factor 2; MASLD, metabolic dysfunction-associated steatotic liver disease; T2DM, type 2 diabetes mellitus. This figure was created using BioRender.
Dietary nitrate enhances nitric oxide (NO) bioavailability via the nitrate-nitrite-NO pathway, complementing the classical nitric oxide synthase (NOS) system. Through entero-salivary circulation and enzymatic reduction in blood and tissues, nitrate and nitrite can be converted into bioactive NO. Following recognition of nitrate as a physiological signaling precursor instead of being a dietary risk factor, interest has expanded toward the broader metabolic effects of this anion.
Previous studies mostly focused on vascular function and exercise performance. However, there is growing evidence linking dietary nitrate to metabolic regulation. Population-based studies have indicated correlations between circulating nitrate and liver enzymes [4], but other research proposes that vegetable-derived nitrate may confer preventive effects against fatty liver disease [5]. In experimental animals, nitrate supplementation mitigates hepatic steatosis, enhances glucose and lipid metabolism, and alleviates age-related metabolic decline [6]. These results suggest a context-dependent but potentially beneficial role of nitrate in metabolic equilibrium.
In the research of Li et al., the results show that dietary nitrate alleviates MASLD-like symptoms in mice fed choline-deficient high-fat, methionine/choline-deficient, or Western diets [3]. Nitrate alleviates hepatic injury and improves glucose and lipid metabolism. The metabolic health benefits of sodium nitrate were unchanged despite the removal of bacteria-derived NO from mice. This finding challenges the idea that nitrate only works by making NO and suggests a way for nitrate to work without the help of bacteria.
Prior studies indicated that initiating the nitrate-nitrite-NO pathway enhances AMP-activated protein kinase in mice, reduces oxidative stress, regulates lipid synthesis, inhibits fatty acid oxidation, and maintains stable glucose levels [7]. However, above research mostly focused on hepatocytes. Li et al. examined immune cells within the liver, concentrating mainly on bone marrow-derived macrophages (MoMFs). MoMFs may be induced to exhibit pro-inflammatory traits, resulting in tissue damage and metabolic dysfunction in pathological liver fibrosis. Thus, MoMFs play an important role in the development of MASLD and MASH.
The authors demonstrated that sodium nitrate can polarize MoMFs toward a CD206+ state, which is closely associated with metabolic homeostasis in both laboratory and animal studies. Further research confirmed that even after removing microbiota-derived NO and without a significant increase in intracellular NO concentrations, sodium nitrate still polarizes MoMFs toward the CD206+ state. The data provided herein challenge the traditional perspective that nitrate predominantly exert their effects through redox pathways reliant on NO. Treatment with nitrate could increase antioxidant capacity and activate Nrf2-related pathways, which are known to link redox regulation with inflammatory control [8].
Further research on how nitrate regulates MoMF indicates that the nitrate transporter Sialin is a key regulatory component. Sialin may act as a signaling or sensing node through intracellular signaling pathways or protein-protein interactions [9]. In plants, nitrate sensing is mediated by proteins with dual transport and sensing functions, such as nitrate transporter 1.1 (NRT1.1) and the transcription factor NIN-like protein 7 (NLP7), which serve as genuine nitrate sensors by linking nitrate transport with sensing [10]. Studies indicate that Sialin may serve as the mammalian equivalent of plant nitrate receptors.
Sialin is not confined to the plasma membrane but is extensively distributed among several organelles, including mitochondria, endosomes, the endoplasmic reticulum, and lysosomes [9]. Thus, Sialin may serve a function in nitrate signaling beyond its role in nitrate transport in mammalian cells. Recent studies indicated that nitrate activates a Sialin-dependent sensory neuropeptide pathway in sensory neurons to enhance mucosal regeneration [11]. Nitrate protects mitochondrial membrane integrity by promoting the formation of Sialin-FKBP8 (FK506-binding protein 8) granules. Nitrate-Sialin axis could mitigate mitochondrial damage in salivary epithelial cells [12].
This study found a significant reduction in Sialin expression in liver macrophages of MASLD patients. Nitrate intervention significantly upregulated Sialin expression in macrophages in the MASLD mouse model. Therefore, the nitrate-Sialin axis may play an important role in systemic metabolic homeostasis. In vitro experiments did not detect a direct protective effect of nitrate on steatotic hepatocytes, suggesting that nitrate primarily does not regulate hepatocyte metabolic homeostasis through Sialin. Using an Slc17a5-sgRNA double-cell embryo mouse model, it was demonstrated that nitrate requires Sialin to exert its regulatory effects on MoMFs and prevent metabolic disorders. In vitro, knocking out Sialin also weakened nitrate’s regulatory effects on MoMFs. These findings indicate that the nitrate-Sialin axis could remodel the immune microenvironment. Sialin serves not only as a nitrate transporter but also as a pivotal regulator of immune metabolism.
This study enhances our comprehension of nitrate biology by elucidating a nitrate-Sialin axis that governs metabolic homeostasis via macrophage-centric mechanisms. The research underscores the therapeutic potential of interventions in metabolic diseases by focusing on Sialin and its downstream effectors, such as cathepsin L (CtsL) and Nrf2. Nrf2 has recently been recognized as a nuclear phosphoinositide effector that orchestrates nuclear stress responses. It suggests that nitrate signaling through Sialin may converge with the nuclear phosphoinositide signaling pathway and related stress responses [13], [14], [15]. This conceptual link underscores the significance of inorganic nutrients in cellular signaling and stress adaptation.
This study has certain limitations. Currently, most experiments on nitrate intake use beetroot consumption to conduct related short-term clinical trials, while there is still insufficient medium- to long-term follow-up research on nitrate-enriched supplements in metabolic diseases (such as fatty liver). In addition, the lack of specific, reliable nitrate probes for mammalian systems limits the precise assessment of nitrate dynamics and signaling. Finally, Sialin, as a regulatory node in metabolic homeostasis, highlights the potential to promote dietary nitrate conversion, providing a theoretical basis for the development of targeted therapies.
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Acknowledgments
Figure 1 is generated using BioRender.
Supplementary Material
This article contains supplementary material (https://doi.org/10.1515/mr-2026-0011).
Footnotes
Research ethics: Not applicable.
Informed consent: Not applicable.
Author contributions: M.C. and S.W. provided overall direction, secured funding, and supervised the development of the commentary. Y.F., Y.S., S.W., and M.C. drafted the manuscript, and W.Z., Y.H., and S.L. offered valuable insights and contributed to its refinement. Y.S. and Y.F. prepared the figure.
Use of Large Language Models, AI and Machine Learning Tools: During the preparation of this manuscript, the author used ChatGPT to improve clarity and narrative flow. All outputs were carefully reviewed and edited as necessary, and the author assumes full responsibility for the accuracy, integrity, and final content of the published article.
Conflict of interest: The authors declare no competing interests.
Research funding: S.W. is supported by grants from the Beijing Municipal Government (Beijing Laboratory of Oral Health, PXM2021_014226_000041 and PXM2021_014226_000020; Beijing Scholar Program-PXM2018_014226_000021), the National Natural Science Foundation of China (82030031 and 92149301), and the Chinese Research Unit of Tooth Development and Regeneration, Academy of Medical Sciences (2019-12M-5-031). M.C. is supported by grants from the National Natural Science Foundation of China (32541042 and 32400577), Guangdong Province Basic and Applied Basic Research Foundation (2023A1515110237), the Science and Technology Foundation of Shenzhen (JCYJ20240813094605008), and Guangdong Province Higher Education Teaching Quality and Reform Project (SJZLGC202417). Y.F. is supported by a grant from the National Natural Science Foundation of China (824B2025). Y.H. is supported by grants from the Joint Funds for the innovation of science and Technology, Fujian province (2025Y9410) and the Startup Fund for scientific research, Fujian Medical University (2024QH1166). S.L. is supported by grants from the National Natural Science Foundation of China (82201054) and the Group Notice Young Elite Scientists Sponsorship Program of the Beijing High Innovation Plan.
Data availability: Not applicable.
Contributor Information
Songlin Wang, Email: slwang@ccmu.edu.cn.
Mo Chen, Email: chenm7@sustech.edu.cn.
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