Dietary Amino Acids and Amelioration of Liver Ischemia-reperfusion Injury

The demand for liver transplants significantly outstrips the available supply of donor livers. Consequently, ensuring the survival of every transplanted liver is critical. Organ Procurement and Transplantation Network/Scientific Registry of Transplant Recipients reports indicate that 7.9% and 12.5% of liver transplants experience graft failure within 1 and 5 y, respectively.1 Despite advancements in transplant technologies and therapeutics, donor livers inevitably undergo periods of ischemia followed by reperfusion, leading to injury (IRI). Ischemia results in hypoxia, oxidative stress and subsequent reperfusion-induced inflammation. At the cellular level, this leads to metabolic disruption, including acidosis, ATP depletion, elevated intracellular free calcium, cellular edema, and cell death (Figure 1). Resident innate immune cells in the liver, such as Kupffer cells, detect this initial necrotic cellular damage and release inflammatory cytokines and chemokines (Figure 1). Responding neutrophils infiltrate the liver and release reactive oxygen species, proteases, and neutrophil extracellular traps. These cellular responses contribute to the early graft loss, as well as alloimmune activation and rejection. Current immunosuppressive therapy in liver transplant patients primarily aims to prevent rejection by the recipient’s immune system, by targeting T and B cells. No specific treatment has been designed to alleviate the innate immune dominated IRI. Therefore, understanding the cellular mechanisms of IRI and finding ways to minimize it would provide additional therapeutic benefit to the patients.FIGURE 1.: The pathophysiology of liver ischemia-reperfusion injury and potential therapeutic targets of SG. Ischemia-induced hypoxia alters intracellular metabolism in both hepatocytes and immune cells resulting in cell death and inflammatory responses. Nonessential amino acids (NEAAs) have been shown to activate expression of anti-inflammatory and cytoprotective genes (green) blocking the vicious cycle of inflammation and cell death. CYP450, cytochrome P450; DAMPs, damage-associated molecular patterns; GSH, glutathione; IR, ischemia reperfusion; SG, serine and glycine.In this issue of Transplantation, Yang et al2 studied the immune regulatory effect of amino acids (AAs) in liver IRI in a mouse model. Inspired by the findings in literature on the effect of dietary manipulation of nonessential amino acid (NEAA) in tissue inflammation and injury and tumor growth in vivo, they set up experiments to test whether and how dietary manipulation of serine and glycine (SG) impacted liver IRI. Indeed, mice fed with SG-deficient diet suffered significantly higher levels of liver IRI while those fed with serine supplemented diet were protected. RNA sequencing analysis revealed that the CYP gene family was altered most significantly by the SG in IR livers (Figure 1). Functional relevance of CYP genes in the cytoprotective mechanism was tested by overexpression of cytochrome P51 and cytochrome P26B1, 2 of the most downregulated genes by SG deficiency, in vitro and in vivo, which protected cells against hypoxia and SG-deficient livers from IRI, respectively. Several critical questions remain to be addressed, such as how to gauge the effect of SG manipulation in vivo, whether and how SG would regulate innate immune cells, and whether there are potential differential effects of dietary manipulation of AAs in terms of its duration and doses. Furthermore, mechanistic studies are warranted to dissect molecular and cellular targets of these NEAAs. Despite these issues, the results suggest that manipulating dietary AAs could represent a readily applicable therapeutic approach to minimizing IRI in liver transplantation. This concept of using nutritional regulation of specific AAs to control or influence inflammatory responses and tissue injury in liver IRI is intriguing. AAs and their metabolites have well-documented roles in physiology and pathology beyond protein synthesis, via various signaling pathways.3 Both hepatocytes and immune cells, 2 key players in liver IRI, are influenced by AAs (Figure 1). SG are 2 NEAAs, which can be synthesized de novo inside our body. Specific receptors and transporters, for example, sodium-coupled neutral amino acid transporter, solute carrier 38, and glycine receptors, are required to sense exogenous SG and carry them into cells.3 NEAAs, in general, have been shown to promote hepatocyte proliferation and maturation via Cyclin D1, induction of CYP450 family of proteins,4 protect hepatocytes against toxin or oxidative stress (glutamine, asparagine), delay progression of metabolic dysfunction-associated steatotic liver disease via solute carrier 7A11, and control cellular growth by regulating the expression of the microRNA processing protein Drosha. L-serine has a multifaceted role in macrophage inflammatory response, acting as both pro- and anti-inflammatory agent. As a substrate for glutathione synthesis in cells, serine is required for optimal lipopolysaccharide induction of interleukin (IL)-1 beta messenger RNA expression.5 Extracellular serine regulates macrophage cytokine production upon lipopolysaccharide stimulation. Serine deprivation significantly reduces IL-10, but not IL-6, expressions.6 In transplantation-related models, infusion of glycine into donors before rat partial-liver transplantation diminished Kupffer cell responses and enhanced graft survival and function.7 Glycine infusion also protected biliary and liver function in a rabbit model of warm IRI.8 In clinical liver transplantation, a double-blinded, placebo-controlled multicenter trial (hepatoprotective effects of glycine in the postoperative phase of liver transplantation trial) investigated intravenous glycine administration during the anhepatic phase in liver transplant recipients. The study found that the very high plasma concentrations of glycine achieved after intravenous administration were safe and showed hepatoprotective and nephroprotective effects.9 A meta-analysis suggests that perioperative “immunonutrition” might improve outcomes by reducing infectious complications and shortening hospital stays, although a direct impact on rejection rates or long-term graft survival have not always demonstrated statistical significance.10 While promising preclinical and preliminary clinical data exist, much more work is warranted to delineate the cellular and mechanistic pathways mediating the beneficial effect of NEAAs. To translate the innovative finding into clinical practice, well-controlled clinical trials with defined cellular/molecular measurements and robust endpoints are needed to definitively establish the efficacy of AA interventions. IRI research is a rapidly evolving field. In additional to dietary AAs, several novel mechanisms related to cell death (ferroptosis) and neutrophil recruitment/activation/function have been revealed recently in IRI models, which provide us new targets of drug intervention.11,12 Interestingly, exercise has been shown in mouse models as an effective preconditioning and therapeutic modality to increase liver graft resilience against IRI.13 Thus, an integrated approach combining nutrition, lifestyle, and drugs may ultimately help transplant patients in protecting organs from IRI, improving long-term liver graft survival and reducing rejection. ACKNOWLEDGMENTS The authors acknowledge Tori Huey for her help in creating Figure 1.