Connecting Amino Acid Metabolism to Inflammatory Modulation in Liver Ischemia-Reperfusion Injury

Liver ischemia-reperfusion injury (IRI) remains a major cause of early allograft dysfunction and graft failure in liver transplantation, yet clinically effective strategies to mitigate this injury are lacking. While much of the field has focused on immune and hepatocellular signaling and responses to injury, emerging data reveal that these pathways are influenced by nutrient intake and liver metabolism. Metabolic shifts triggered by fasting or feeding can recalibrate hepatocellular signaling, modulate the immune response to damage-associated molecular patterns, and ultimately shape the intensity of inflammatory injury. In particular, the availability of essential amino acids through nutrient intake can reshape metabolic pathways, and their metabolites can modulate the hepatocellular stress responses and the downstream inflammatory cascade in liver IRI.

Previous work on deciphering metabolomic signatures in human donor livers associated the disruption of histidine pathways and increased levels of histidine-derived metabolites with early allograft dysfunction.¹ Building on this, work by Kawamoto et al² further dissects the histidine-histamine axis and its mechanistic role in liver IRI, using a warm murine hepatic IRI model and complementary transgenic approaches. Through extensive metabolomic profiling of mouse liver tissues, they demonstrate that hepatic histamine level rises sharply following hepatic IRI but is suppressed by preoperative fasting. Histamine, derived from histidine, functions as a potent inflammatory mediator that engages the histamine H₁ receptor (H1R) to promote vascular permeability, immune cell recruitment, and release of damage-associated molecular patterns. In the absence of histamine-H1R signaling, hepatocellular injury, cell death, inflammatory cytokine and chemokine expression, and circulating High Mobility Group Box 1 (HMGB1) are reduced, implicating this signaling pathway as a critical driver of inflammatory injury in liver IRI.

Further evidence from mast cell degranulation studies confirmed that histamine-H1R signaling potentiates early HMGB1 secretion. These findings prompted the exploration of a clinically available H1R antagonist, chlorpheniramine, as an anti-inflammatory agent. Chlorpheniramine pretreatment, administered 1 h before injury, significantly attenuated liver damage in a mouse model, reducing immune cell infiltration and suppressing HMGB1 release. Given chlorpheniramine’s long-standing clinical use, these results support the possibility that perioperative H₁ blockade could offer a practical, low-cost intervention, one that can be quickly evaluated in a clinical trial. In addition to modulating the inflammatory response, H₁ antagonists may mimic the protective metabolic effects of fasting without the risks associated with caloric deprivation or metabolic dysfunction-associated liver disease.

However, the expanded application of H₁ antagonists will require careful optimization of dosing, timing, and route of administration in large animal or human studies. Whether H1R blockade can protect steatotic or otherwise marginal grafts presents another important topic for future investigation. Moreover, while Kawamoto et al² focus specifically on histamine-H1R engagement, which is linked to mast cells and supported by prior evidence in cholangiocytes and endothelial cells, histamine also signals through 3 additional receptors: H2R, H3R, and H4R.² These receptors are differentially expressed across immune, neuronal, and epithelial cell populations, and their combined activity introduces functional complexity in vivo. The cell-type-specific co-expression of these receptors can generate distinct context-dependent responses. This broad but intricate network of histamine receptors poses additional challenges to a comprehensive understanding and therapeutic targeting of histamine in liver injury.

Histamine, although central to this study, is not the only essential amino acid-derived metabolite implicated in liver injury. Kynurenine and its downstream metabolite, kynurenic acid, are products of tryptophan catabolism via indoleamine 2,3-dioxygenase and have also been closely associated with immune modulation and metabolic stress signaling. Specifically, kynurenine has been implicated in multiple liver injury contexts, including viral hepatitis, cholangiocyte ischemia, and hepatic IRI.² Elevated kynurenine levels in hepatic IRI were also found to be accompanied by a metabolic skewing toward kynurenic acid at the expense of Nicotinamide Adenine Dinucleotide-generating intermediates.² As an endogenous ligand of the aryl hydrocarbon receptor, kynurenine can also promote mast cell activation and propagate inflammatory signaling.³ However, similar to the histamine receptor family, aryl hydrocarbon receptor is widely expressed across diverse immune, epithelial, and stromal cell types, contributing to context-dependent effects of metabolic conditioning in IRI pathogenesis.

Preoperative fasting has been recognized as a beneficial intervention in experimental liver IRI. However, its clinical adoption remains limited because of concerns regarding nutritional fragility, particularly in patients with cirrhosis. Alternatively, branched-chain amino acid (ie, leucine, isoleucine, and valine) supplementation has been associated with improved nutritional status, reduced cirrhotic complications, and enhanced survival, particularly with high branched-chain amino acid-to-aromatic amino acid ratios.⁴ These observations highlight the modulation of nutrient metabolism, particularly involving essential amino acids, as a promising therapeutic strategy for inflammatory liver injury.

Beyond amino acid metabolism, the dataset by Kawamoto et al² also suggests broader metabolic involvement. Lipid-derived metabolites such as cholic acid and prostaglandin F₂α, along with vitamin B-complex derivatives including riboflavin, pyridoxine, and 4-pyridoxic acid, exhibit a shared pattern: upregulation during IRI and dampening by fasting.² Hepatic lipidomic remodeling in IRI has been studied in the context of oxidative stress and metabolic dysfunction.¹ Meanwhile, pilot clinical trials of vitamins have shown only modest reductions in inflammatory biomarkers with limited mechanistic insights, and the role of the vitamin B complex in modulating hepatic inflammation remains largely unexplored.⁵

Collectively, these findings expand the range of liver IRI studies beyond immune and cellular signaling, positioning nutrient metabolism as a modifiable determinant of inflammatory liver injury. More than a background physiological process, this approach offers mechanistic pathways for noninvasive nutritional conditioning strategies and a foundation for developing nutrient-informed interventions in transplantation and hepatic surgery.