NOX-NOS crosstalk in the liver-brain axis: Novel insights for redox regulation and neurodegenerative diseases

Abstract

The liver-brain axis is an emerging concept linking liver dysfunction and brain disease. Hepatic metabolic abnormalities induce systemic oxidative stress and endothelial dysfunction, which contribute to central nervous system (CNS) inflammation and neurodegeneration. Redox regulation plays a key role in the liver-brain axis, with NADPH oxidase (NOX) and nitric oxide synthase (NOS) being involved in the generation of various reactive oxygen species (ROS) and reactive nitrogen species (RNS), respectively, thereby inducing oxidative stress and disrupting the NADPH/NADP balance. Dysregulation of NOX-NOS cross-signaling not only amplifies oxidative stress, but also disrupts endothelial homeostasis and exacerbates neuroinflammation, leading to progressive neurodegeneration. For instance, reactive carbonyl species such as methylglyoxal (MGO) and acrolein can upregulate NOX isoforms and stimulate NLRP (NOD like receptor protein) inflammasomes activation, illustrating disease-relevant links between hepatic redox imbalance and CNS pathology. Mechanistically, superoxide (O₂•^-) generated by NOX readily reacts with nitric oxide (•NO) derived from NOS to form peroxynitrite (ONOO^-), a highly reactive oxidant that exacerbates vascular and neuronal injury. Despite extensive research on NOX and NOS, their interactive contributions to redox imbalance and the progression of neurodegenerative diseases remain poorly understood. In this review, we introduce the NOX-NOS axis as a key regulator of the liver-brain axis, and highlight the roles of NOX and NOS in linking hepatic metabolic dysfunction to central nervous system pathology through intermediary metabolites in the exacerbation of neuroinflammation and oxidative stress. We also explore therapeutic strategies targeting NOX-NOS interactions, including selective NOX inhibitors, NOS modulators, and redox homeostasis regulators, providing new insights into redox regulation and the management of metabolic neurodegenerative diseases.

Keywords: Liver-brain axis; NADPH oxidase; NOX-NOS crosstalk; Neurodegenerative diseases; Nitric oxide synthase; Oxidation-reduction reaction.

Graphical abstract

Fig. 1

Synergistic interplay between NOX- and NOS-derived reactive species along the liver–brain axis.

Fig. 2

Metabolic “trigger” stage of the NOX-NOS crosstalk in liver–brain axis: NOX4-centred crosstalk initiates NOX–iNOS crosstalk and pro-inflammatory signalling. Dyslipidaemia and the influx of free fatty acids (FFAs) activate membrane NOX4 and suppress the LKB1 AMPK checkpoint, while reactive carbonyl precursors (4-HNE/4-ONE, acrolein, MDA, MGO/GO, 3-DG) provide a secondary hit that upregulates iNOS. NOX4-derived H₂O₂ and iNOS-derived •NO combine to form ONOO⁻, creating a NOX4-iNOS feed-forward loop (red dashed box) that depletes NADPH, oxidises BH₄ and drives eNOS uncoupling. Mitochondrial dysfunction, ER stress, lysosomal permeabilisation and NLRP1/3/10 inflammasome activation amplify ROS/RNS release, while Kupffer cell cytokines (IL 1β, TNF α, MIP 1α) further stimulate NOX1/2/4/5 and iNOS. The cumulative effect creates a chronic metabolic inflammation milieu that drives the transition from MASLD to NASH (progress bar, bottom). The red arrow indicates the starting point of the fibrosclerotic stage shown in Fig. 3.

Fig. 3

Intermediate “fibro-sclerotic” stage of the liver–brain axis: circulating R PAMPs prime NOX-driven eNOS uncoupling and multi-organelle stress. Reactive protein-adduct-making precursors (R PAMPs) in the bloodstream are subclassified into AOPP-P (AOPP precursors including HOCl, HOBr, Fe-Fenton reagents, and ONOO⁻), HL-A-P (high-lipid aldehyde precursors: 4-HNE/4-ONE, acrolein, MDA-LDL, oxLDL, IsoLG) and HG-C-P (high-glucose carbonyl precursors: MGO, GO, 3-DG, glycol-aldehyde). These ligands bind NOX1/2, NOX4, and NOX5 in the vascular endothelial cells and organ epithelial cells, and consume cytosolic NADPH, culminating in eNOS uncoupling. Consequent mitochondrial dysfunction, ER stress, lysosomal membrane permeabilisation and peroxisomal burnout amplify O₂•^-/H₂O₂ and ONOO⁻ release and activate profibrotic/prosclerotic pathways. The aggregate effect steers the axis toward tissue fibrosis and sclerosis, setting the stage for later neurodegeneration. These changes by the NOX-NOS crosstalk are important in regulating several related pathways to produce endothelial and organ fibrosis and sclerosis, and mediate the first steps leading to neurodegenerative disease development by compromising BBB integrity. Among the RCS, acrolein (56 Da), 4-HNE (156 Da), and others are able to pass through regardless of BBB integrity impairment and may mediate M1-like microglial activation in the CNS even before progression to chronic fibrosis and sclerosis through these pathways. Abbreviations: NOX, NADPH oxidase; NOS, nitric oxide synthase; eNOS, endothelial NOS; iNOS, inducible NOS; NADPH, nicotinamide adenine dinucleotide phosphate (reduced form); NADP⁺, oxidized NADPH; O₂•^-, superoxide anion; H₂O₂, hydrogen peroxide; ONOO⁻, peroxynitrite; AOPP, advanced oxidation protein product; ALE, advanced lipid peroxidation end-product; AGE, advanced glycation end-product; AOPP-P, AOPP precursor; HL-A-P, high-lipid aldehyde precursor; HG-A-P, high-glucose aldehyde precursor; HOCl, hypochlorous acid; HOBr, hypobromous acid; ONOO⁻, peroxynitrite; Fe²⁺/Fe³⁺, ferrous/ferric iron; 4-HNE, 4-hydroxy-2-nonenal; 4-ONE, 4-oxo-2-nonenal; MDA-LDL, malondialdehyde-modified low-density lipoprotein; oxLDL, oxidized low-density lipoprotein; IsoLG, isolevuglandin; MGO, methylglyoxal; GO, glyoxal; 3-DG, 3-deoxyglucosone; ER, endoplasmic reticulum; BBB, blood-brain barrier; RAGE, receptor for advanced glycation end-products; SMAD, mothers against decapentaplegic homolog; ROCK, Rho-associated coiled-coil-containing protein kinase; NLR, nucleotide-binding oligomerization domain-like receptor; TAK1, transforming growth factor beta-activated kinase 1; MAPK, mitogen-activated protein kinase; IKK, IκB kinase; NF-κB, nuclear factor-kappa B; PI3K, phosphoinositide 3-kinase; AKT, protein kinase B; mTORC1, mechanistic target of rapamycin complex 1; NICD, Notch intracellular domain; YAP/TAZ, Yes-associated protein/transcriptional co-activator with PDZ-binding motif.

Fig. 4

Phased escalation of NOX–NOS crosstalk in the CNS leading to neurodegeneration. (A) Timeline of redox escalation. (1) NOX4–iNOS crosstalk accompanied by p65-NF-κB and NLRP1/2/3 activation (green segment). (2) NOX–iNOS synergy + nNOS hyper-activation triggered by glutamate excitotoxicity (yellow). (3) NOX–nNOS uncoupling with ONOO⁻ amplification culminating in neuronal demise (red). (B) Astrocyte–neuron dyad at stage (3). NOX-rich astrocytic end-feet accumulate R PAMPs, secrete cytokines, and fail to recycle glutamine, disrupting the glutamate–GABA–glutamine (GGG) cycle. Excess synaptic glutamate opens NMDA/AMPA receptors, causing Ca²⁺ influx and nNOS activation/uncoupling in neurons. Intracellular O₂•^- reacts with •NO to form ONOO⁻ (red dashed box), while mitochondrial, lysosomal, and ER stress drive apoptosis, necroptosis, and pyroptosis. Collagen-rich fibrotic and glial scars further isolate synapses and perpetuate redox imbalance. Abbreviations: NOX, NADPH oxidase; NOS, nitric oxide synthase; iNOS, inducible NOS; nNOS, neuronal NOS; eNOS, endothelial NOS; O₂•^-, superoxide anion; H₂O₂, hydrogen peroxide; ONOO⁻, peroxynitrite; •NO, nitric oxide; RCSs, reactive carbonyl species; R PAMPs, reactive protein-adduct-making precursors; ALEs, advanced lipid peroxidation end-products; AGEs, advanced glycation end-products; AOPPs, advanced oxidation protein products; MDA-LDL, malondialdehyde-modified low-density lipoprotein; oxLDL, oxidized low-density lipoprotein; IsoLG, isolevuglandin; MGO, methylglyoxal; GO, glyoxal; 3-DG, 3-deoxyglucosone; 4-HNE, 4-hydroxy-2-nonenal; 4-ONE, 4-oxo-2-nonenal; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; NLRP, NOD-like receptor family pyrin domain containing; NMDA, N-methyl-d-aspartate receptor; AMPA, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor; GABA, γ-aminobutyric acid; GAT1, GABA transporter 1; EAAT, excitatory amino acid transporter; GGG cycle, glutamate–GABA–glutamine cycle; Ca²⁺, calcium ion; Na⁺, sodium ion; ER, endoplasmic reticulum; Apoptosis, programmed cell death; Necroptosis, programmed necrosis; Pyroptosis, inflammatory cell death.

Fig. 5

The Potential of NOX- and NOS-Targeted Therapeutic Strategy from the Perspective of the Liver-Brain Axis. By leveraging biomarker-based stratification and highly specific, efficient diagnostic platforms, three therapeutic strategies targeting NOX–NOS crosstalk can be concretely implemented. Restoring redox, metabolic, and immune homeostasis embodies healthy living within the liver–brain axis and broader multi-organ network paradigms.