Melatonin Exerts Positive Effects on Sepsis Through Various Beneficial Mechanisms

Introduction

Sepsis is defined as a life-threatening organ dysfunction caused by a dysregulated host response to infection.¹ A recent global burden of disease report highlights that sepsis is highly prevalent, with nearly 50 million cases worldwide each year.² Despite over three decades of research, including more than 200 randomized controlled trials and numerous animal experiments, we do not have a treatment method that can consistently save the lives of sepsis patients.^3–5

In recent years, the use of certain drugs has been shown to positively impact the prognosis of sepsis. Ulinastatin is a broad-spectrum protease inhibitor that modulates the inflammatory response by inhibiting NF-κB and NLRP3 inflammasome activation, thereby reducing cytokine levels such as TNF-α and IL-1β. It has been demonstrated to improve the APACHE II score and the short-term prognosis of the patients with severe sepsis.^6,7 However, its efficacy varies among patients, and it may not significantly reduce overall mortality. Xuebijing injection, a traditional Chinese medicine, possesses anti-inflammatory and immunomodulatory properties. It effectively reduces pro-inflammatory cytokines and enhances organ function in septic patients.^8,9 However, the optimal dosing and duration of treatment are still unclear. Continuing to explore effective pharmacological agents and their mechanisms of action for the treatment of sepsis is a crucial area of focus in sepsis research.

The development of sepsis is multifaceted, extending beyond the specific infection type and the initial response of the host. It encompasses diverse aspects, including inflammation, coagulation activation, vascular endothelial dysfunction, complement system activation, immune suppression, and changes in the microbiome.^10,11 Melatonin (N-acetyl-5-methoxytryptamine) is an endogenous indoleamine widely distributed in plants, unicellular organisms, algae, bacteria, invertebrates, and vertebrates.¹² Its widespread distribution forms an important basis for its protective effects against multiorgan damage.^13,14 Numerous studies have documented the wide-ranging properties of melatonin, including antioxidant, anti-inflammatory, anti-tumor, and anti-aging effects.¹⁵

This article provides an overview of the protective and therapeutic effects of melatonin in sepsis and emphasizes its potential clinical applications. Firstly, we reviewed the relevant studies on melatonin and the prognosis of sepsis patients in clinical research. Next, we discuss the mechanisms through which melatonin-mediated protection affects inflammatory responses, oxidative-reductive homeostasis (As shown in Figure 1), and the regulation of epithelial cell ion channels and endothelial cell permeability, etc. (As shown in Figure 2). Additionally, we explore the synergistic effects of combining melatonin with other biological agents in the treatment of sepsis in vivo. Overall, we offer insights into the anti-septic effects of melatonin and propose prospective strategies for guiding future melatonin-based sepsis treatments.

Figure 1 Melatonin can effectively inhibit inflammatory and oxidative stress responses in sepsis. Abbreviations: MT, Melatonin; VEGF, Vascular Endothelial Growth Factor; MMP-2, Matrix Metallopeptidase-2; IL-1β, Interleukin-1 Beta; IL-6, Interleukin-6; TNF-α, Tumor Necrosis Factor Alpha; PI3K, Phosphoinositol-3 Kinase; AKT, Protein Kinase B; SIRT1, Sirtuin 1; SIRT3, Sirtuin 3; FOXO1, Forkhead Box O1; NFKB, Nuclear Factor Kappa B; SOD2, Superoxide Dismutase 2; Oxidative Stress; Ac, Acetylation; K68, Lysine 68; K122, Lysine 122.

Figure 2 Melatonin can effectively restore damaged cellular homeostasis in sepsis, such as mitochondrial autophagy, impaired glucose metabolism, regulation of epithelial ion channels, and restoration of endothelial cell homeostasis. Abbreviations: SIRT1, Silent Information Regulator 1; SIRT3, Silent Information Regulator 3; Ac, Acetylation; TFAM, Transcription Factor A, Mitochondrial; STAT3, Signal Transducer and Activator of Transcription 3; Tyr705, Tyrosine 705; P, Phosphorylation; Lysine685, Lysine 685; Sck1, Sterol-Cis-Delta-Isomerase 1; P-Nedd4-2, Pro-Neural Cell Adhesion Molecule 2; Usp8, Ubiquitin Specific Peptidase 8; NICD, Notch Intrinsically Disordered Region; Nlrp3, Nucleotide-Binding Domain, Leucine-Rich Repeat, Pyrin Domain Containing 3; Nrf2, Nuclear Factor Erythroid 2-Related Factor 2; HO-1, Heme Oxygenase-1; GPX4, Glutathione Peroxidase 4.

Melatonin Has a Protective Effect Against Sepsis Caused by Various Pathogens

Sepsis can be caused by various pathogens, with bacterial infections being the primary cause. However, up to 42% of sepsis patients have negative cultures, indicating that non-bacterial infections are also significant contributors to sepsis development.¹⁶ Melatonin has shown positive therapeutic effects not only in bacterial sepsis but also holds promising potential in the management of viral sepsis. For example, in sepsis caused by the ongoing Covid-19 viral infection, significant differences in the development of sepsis have been observed between patients in the melatonin group and the control group. On the 17th day of symptom onset, 35.5% of patients in the control group (n=76 average age 55.7) developed sepsis, while only 8.5% of patients in the melatonin group (n=82 average age 56.8) did so (P<0.001).¹⁷ In a recent clinical trial, it was found that daily intravenous administration of a 60 mg melatonin formulation improved sepsis patients, reducing their mortality rate to zero and decreasing their hospitalization time by 40%.¹⁸

Another trial involving daily use of 9 mg melatonin in COVID-19 patients showed significant reductions in hospitalization time in the melatonin group (n=24 average age 50.75) (P<0.05) compared to the control group (n=20 average age 52.95). Clinical symptoms, CRP levels, and lung involvement were also improved in the melatonin group (P<0.01).¹⁹ These studies indicate that melatonin has a broad regulatory effect on sepsis caused by different pathogens, demonstrating promising prospects for its application in the treatment of sepsis.

The Positive Role of Melatonin in Septic Patients in Clinical Practice

Early identification of sepsis is of great significance in providing appropriate and timely treatment to improve prognosis. Clinical studies on late-onset sepsis have shown that compared to the control group, the sepsis group has increased concentrations of melatonin (27.2±3.3 vs 11.4±3.2 pg/mL, p=0.001), which is positively correlated with HsCRP (r=0.952, p=0.001) and the Immature/Total Neutrophil ratio (I/T ratio) (r=0.326, p=0.015). When melatonin is combined with HsCRP in the detection of neonatal sepsis, the sensitivity and specificity are improved to 97.3% and 93.3%, respectively.²⁰ This indicates that endogenous melatonin levels are significantly increased in late-onset neonatal sepsis and can serve as a biomarker for sepsis, especially when used in conjunction with CRP. In a clinical study, it was found that septic AKI patients had higher levels of plasma melatonin. Additionally, plasma melatonin levels were higher in AKI recovery patients compared to AKI non-recovery patients, and patients with higher plasma melatonin levels recovered faster after AKI.²¹ This suggests that melatonin can serve as a potential biomarker for the diagnosis of sepsis and is closely associated with the prognosis of septic patients.

In a controlled clinical trial, vitamin C (group 1: 1000 mg every 6 hours), vitamin E (group 2: 400 IU every 8 hours), and N-acetylcysteine (NAC) (group 3: 600 mg every 12 hours), along with melatonin (group 4: 50 mg once daily), were administered as adjuvant therapies to standard care in adult patients with septic shock and multiple organ dysfunction. This treatment was compared to standard care alone (group 5) over a period of 5 consecutive days. The results indicate that each of these antioxidants, particularly vitamin C and melatonin, significantly reduced the severity of organ dysfunction, as assessed by SOFA scores. Furthermore, notable reductions in plasma levels of oxidative stress and pro-inflammatory biomarkers, such as C-reactive protein (CRP), procalcitonin, and lipid peroxidation, were observed in patients receiving melatonin treatment.²²

The Mechanisms Underlying the Beneficial Effects of Melatonin on Sepsis

Anti-Inflammatory Effects

The excessive expression of pro-inflammatory cytokines is necessary for the clearance of invading pathogens, but an excessive inflammatory response can also result in tissue inflammation and MODS.^23,24 Therefore, early suppression of the inflammatory response and maintaining a balance between pro-inflammatory and anti-inflammatory cytokines are considered key aspects of sepsis treatment.²⁵ Current research is focused on finding more effective treatment methods to simultaneously reduce inflammatory responses and enhance antimicrobial activity in order to achieve better treatment outcomes.

Many biomarkers used to monitor the clinical condition of patients. One of these biomarkers is vascular endothelial growth factor (VEGF). Elevated levels of VEGF are associated with the severity of sepsis and mortality, suggesting a poorer prognosis.²⁶ Matrix metalloproteinase-2 (MMP-2) can be considered as a poor prognostic factor because it causes extracellular matrix (ECM) damage during the process of sepsis.^27,28 TGF-β is another effective cytokine during the inflammatory process, influencing the differentiation and function of T cells. Research suggested that TGF-β has anti-inflammatory effects in inflammation and can inhibit MMP activation, thereby limiting ECM damage. In addition to its intervention on the aforementioned pro-inflammatory factors,²⁹ melatonin intervention can effectively reduce the levels of pro-inflammatory cytokines such as IL-1β, IL-6, TNF-α in sepsis.

One of the important pathways through which melatonin exerts its effects is by activating two types of membrane-specific receptors: MT1 and MT2. By binding to these receptors, melatonin regulates the expression of cytokines and the activation of signaling pathways.³⁰ Li et al found that in immature mice, macrophages express both MT1 and MT2 receptors, while neutrophils only express MT2 receptors. Other immune cells, including T cells, B cells, natural killer cells, and dendritic cells, do not express MT1 or MT2 receptors. In an in vitro experiment using a mixture of Escherichia coli and Staphylococcus aureus to infect isolated macrophages and neutrophils, the results showed an upregulation of MT2 mRNA levels in neutrophils, while the levels of MT1 and MT2 mRNA in macrophages remained unchanged. These data suggested that melatonin may act on neutrophils after bacterial infection.³¹

There were studies have shown that melatonin can effectively increase the levels of p-Akt in septic myocardial injury, indicating activation of the Akt pathway. Melatonin induced an increase in Bcl-2 expression and a decrease in Bax expression, suggesting it’s inhibition of cell apoptosis. However, the PI3K inhibitor LY294002 can abolish all protective effects of melatonin.³² The melatonin-mediated upregulation of SIRT1 is an important anti-inflammatory mechanism. The upregulation of SIRT1 can lead to the deacetylation of FoxO1 (Forkhead box O1), p53, and NF-κB, thereby inhibiting oxidative stress, cell apoptosis, and inflammatory responses, and reducing the expression levels of inflammatory factors.^33–36 Furthermore, melatonin can inhibit inflammatory responses in sepsis through various mechanisms, such as inhibiting the release of extracellular histones and directly blocking the NLRP3 inflammasome activation induced by histones.³⁷

Antioxidant Effects

The enhanced generation of reactive oxygen species (ROS) is associated with the pathogenesis of MODS in sepsis.^38,39 Under normal physiological conditions, the production of ROS is balanced to maintain cellular activities.⁴⁰ However, excessive production of ROS can deplete many endogenous antioxidant systems, including superoxide dismutase (SOD) and catalase (CAT), and may lead to cell damage.⁴¹ In sepsis-induced injury models, melatonin has been shown to effectively reduce the levels of malondialdehyde (MDA), myeloperoxidase (MPO), total oxidant status (TOS), and oxidative stress index (OSI). Additionally, melatonin increased the expression of SOD, glutathione (GSH), CAT, and glutathione peroxidase (GPx). These effects indicated that melatonin can attenuate oxidative stress and enhance antioxidant defense mechanisms in sepsis.⁴²

Previously, it has been demonstrated that NO (nitric oxide) produced by iNOS is associated with the pathogenesis of organ dysfunction induced by endotoxins.^43–45 NO is primarily involved in maintaining physiological homeostasis. However, due to its free radical nature, NO can rapidly react with ROS, leading to the formation of high-energy oxidant peroxynitrite. This peroxynitrite is considered to be the cellular toxic potential of NO, causing oxidative stress and tissue damage.⁴⁶ Melatonin can reverse gastrointestinal dysfunction in septic mice. This beneficial effect of melatonin is associated with reduced lipid peroxidation, decreased transcription and expression of iNOS, and reduced production of nitrites in the intestinal tissue of septic mice. It also inhibits the activation of MAPK and NF-kappaB.⁴⁷

Another potential mechanisms underlying the antioxidant action of melatonin is its upregulating effect on Sirtuins (SIRTs).⁴⁸ SIRTs regulate various physiological functions, from energy metabolism to stress responses, and they also exhibit significant antioxidant activity, primarily through their deacetylase and severe antioxidant enzyme activation. Melatonin can trigger the deacetylation of SOD2, increasing its activity and thereby reducing oxidative stress. However, this improvement can be blocked by the selective inhibitor of SIRT3, 3-TYP.³⁶

As the powerhouse of the cell,⁴⁹ mitochondria play a crucial role in various aspects of cell signaling, intracellular calcium homeostasis, gene expression, as well as processes involving cell apoptosis, necrosis, ferroptosis, and autophagy.⁵⁰ Mitochondrial dynamics, mitophagy, and mitochondrial redox regulation are three key mechanisms that promote mitochondrial quality control by maintaining a balance between fission and fusion and by determining cellular fate and overall homeostasis to eliminate unhealthy mitochondria.⁵¹ Excessive oxidative stress can lead to impaired mitochondrial quality control. The mitochondrial antioxidant enzyme SOD2 plays a crucial role in clearing free radicals by converting them into hydrogen peroxide and water.⁵² Activation of SIRT3 can alleviate sepsis-induced acute lung injury (ALI) by improving mitochondrial bioenergetics and redox homeostasis.⁵³ However, reduced expression and activity of SIRT3 may lead to SOD2 inactivation through enhanced acetylation at K68 and K122 sites of SOD2.⁵⁴ In sepsis-induced lung injury, RNA sequencing was performed to analyze the differentially expressed genes (DEGs) in the lung tissues of septic mice and septic mice with melatonin intervention. The enrichment analysis of these DEGs indicated their association with mitochondrial fission, mitochondrial fusion, and oxidative stress. Melatonin can shift the dynamic processes of mitochondria from fission to fusion and inhibit mitochondrial autophagy and fatty acid oxidation in lung epithelial cells treated with LPS, both in vitro and in vivo. However, the protective effect of melatonin in acute lung injury was abolished by SIRT3 inhibition. Mechanistically, melatonin increased the activity and expression of SIRT3, further promoting the deacetylation of SOD2 at K122 and K68 sites.⁵⁵

Regulating Metabolic Disorders

As a highly lethal disease, sepsis can induce numerous severe complications, including metabolic disturbances characterized by reduced food intake, increased energy expenditure, and impaired glucose homeostasis.⁵⁶ Sepsis commonly causes significant hyperglycemia and insulin resistance, primarily due to enhanced hepatic gluconeogenesis.⁵⁷

In the liver of septic rats, the expression of glucose 6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK), which are two crucial rate-limiting enzymes involved in gluconeogenesis, was significantly increased. Additional, the septic rats are characterized by significant increases in blood glucose and serum insulin levels, accompanied with enhanced gluconeogenesis and glucose intolerance, and the reduced insulin sensitivity.³⁵ Previous studies have confirmed the intricate and complex relationship between melatonin and glucose metabolism.^56,58 The signal transducer and activator of transcription 3 ((STAT3) are widely involved in various key cellular processes, especially in liver glycolipid metabolism.^59,60 The phosphorylation and deacetylation of STAT3 mediated by SIRT1 play a crucial metabolic regulatory role under septic conditions.^61–63 The upregulation of Sirt1 by melatonin has been widely confirmed.^64–66 Melatonin-induced SIRT1 expression partially restored phosphorylation of STAT3 at Tyr705, accompanied by the downregulation of lysine 685 acetylation. Furthermore, inhibition experiments using the specific inhibitor EX527 suggested that SIRT1 activity is necessary for the anti-inflammatory and anti-hyperglycemic effects of melatonin.³⁵

Regulating Hemodynamics

Disseminated intravascular coagulation (DIC) is a common pathological phenomenon in the systemic inflammatory response of the body to infection, and it is a strong predictor of mortality in severe sepsis patients.^67,68 The characteristics of DIC include systemic coagulation activation, simultaneous loss of compensatory mechanisms such as fibrinolysis and anticoagulant proteins, leading to increased microvascular thrombosis formation and a tendency for bleeding. In septic patients and animal models of endotoxemia, fibrin deposition in the blood vessels caused by DIC may also result in red blood cell membrane damage, leading to the release of hemoglobin.⁶⁹

A clinical trial targeting sepsis in newborns has provided initial evidence of the impact of melatonin on platelet count during sepsis. Compared to baseline values and untreated septic newborns, septic infants treated with melatonin showed a significant increase in platelet count at 24 and 48 hours after melatonin administration.⁷⁰

The Regulation on Epithelial Sodium Channels

In most patients with ARDS, the ability to clear excessive alveolar fluid (AFC) is impaired, and the reduction in AFC is associated with higher mortality rates.⁷¹ Epithelial sodium channels (ENaC) are heteromeric trimeric proteins that play a crucial role in reducing pulmonary edema and promoting AFC.⁷²

The clearance rate of alveolar fluid is associated with active translocation of Na+ across the alveolar epithelium via apical epithelial sodium channels (ENaC) and basolateral Na+-K+-ATPase. ENaC is composed of three subunits and is located in the apical membrane of alveolar epithelial cells. The α subunit is the essential subunit for the formation of functional ENaC, while the β and γ subunits enhance the activity of the channel. The transport of Na+ creates an osmotic gradient that drives water reabsorption.⁷³ Therefore, ENaC is considered a rate-limiting factor for AFC in acute lung injury.

All subtypes of glucocorticoid regulated kinase (SGK) have been shown to promote ENaC activity, with SGK1 being the most potent stimulator of ENaC activity. Nedd4-2 is an E3 ubiquitin ligase that interacts with ENaC and promotes its ubiquitination, leading to the internalization and degradation of ENaC. However, SGK1 can phosphorylate neural precursor cell expressed developmentally downregulated 4-like (Nedd4-2) and inhibit its activity, thereby reducing the ubiquitination of ENaC. Wang et al found that melatonin increased the protein expression of α-ENaC and SIRT1 in lung tissue and in vitro. Additionally, melatonin improved the expression of SGK1 and inhibited the decreased expression of p-Nedd4-2 induced by sepsis.⁷⁴ The effect of melatonin was abolished by the SIRT1 inhibitor EX527. These results suggest that melatonin may upregulate ENaC expression and attenuate sepsis-induced reduction in alveolar fluid clearance (AFC) by activating the SIRT1/SGK1/Nedd4-2 signaling pathway.

Regulate Endothelial Cell Function

The abnormal expression and activity of endothelial nitric oxide synthase (eNOS) contribute to the pathogenesis of vascular dysfunction in sepsis.^75,76 The NLRP3 inflammasome is involved in vascular dilation and endothelial dysfunction during the early stages of sepsis by inhibiting the activity of eNOS.⁷⁶ In the mesenteric arteries of mice with sepsis induced by cecal ligation and puncture (CLP), the expression of eNOS was found to be downregulated. However, when NLRP3 was inhibited using a selective inhibitor called MCC950, the expression of eNOS was significantly upregulated. This suggests that NLRP3 may be responsible for the decreased expression of eNOS observed in late-stage sepsis. Furthermore, the NLRP3 inflammasome is involved in the maturation and release of proinflammatory cytokine IL-1β.⁷⁷ Interestingly, IL-1β was found to downregulate the expression of eNOS in human aortic endothelial cells (HAECs) in a time- and concentration-dependent manner. These findings indicate that the NLRP3/IL-1β axis may contribute to the downregulation of eNOS expression through proteasomal degradation pathways. Furthermore, melatonin was found to suppress the NLRP3/IL-1β axis and reduce eNOS proteolysis, resulting in improved endothelium-dependent vasorelaxation.⁷⁸

The LPS can directly cause endothelial cells (ECs) injury, disrupting the cytoskeletal proteins, intercellular connections, and adhesion of ECs, which leads to the increased vascular permeability and fluid extravasation into the extravascular tissue, resulting in edema. Moreover, damaged ECs produce a significant amount of nitric oxide, and sustained elevation of nitric oxide levels affects the function of vascular smooth muscle, further impairing vascular reactivity. This renders fluid resuscitation and vasopressor therapy ineffective.⁷⁹ The Notch1 signaling pathway is a conserved pathway that regulates vascular EC function.⁸⁰ When the Notch1 receptor is bound by its ligand, the Notch1 protein is cleaved by specific enzymes into extracellular and intracellular fragments. The intracellular segment region (Notch intracellular domain, NICD) is transported to the nucleus, where it forms a complex with DNA-binding proteins, thereby relieving the inhibition of target genes, particularly Hes1, and allowing them to be transcriptionally activated to fulfill their biological roles. There were studies have shown that LPS, IL-6, and sera from septic patients can inhibit the expression of NICD and its downstream regulator Hes1, thereby impairing endothelial barrier function and inducing endothelial cell (EC) apoptosis through the AKT pathway.⁸¹ NICD is regulated by deubiquitinating enzymes (DUBs), and LPS increases the ubiquitination of NICD.^82,83 Melatonin can significantly increase the expression of ubiquitin-specific proteases 8 (USP8), thereby enhancing the continuous expression of NICD and its downstream signaling molecules by inhibiting NICD ubiquitination.

Regulation of Autophagy

Increasing evidence suggests that autophagy is associated with organ dysfunction induced by sepsis.⁸⁴ In sepsis, the activation of autophagy can significantly improve organ damage.^85–87 Recently, the antioxidant properties of melatonin have been acknowledged for their ability to safeguard mitochondria.⁸⁸ One potential mechanism underlying positive role of melatonin involves the upregulation of SIRTs.⁴⁸ SIRT1 is a conserved nicotinamide adenine dinucleotide (NAD+)-dependent protein deacetylase that is associated with various intracellular signals, such as aging, inflammation, cell apoptosis, and autophagy.^89,90 Melatonin is an effective modulator of SIRT1 in many diseases.³⁴ In sepsis-induced MODS, the beneficial effects of melatonin also demonstrate a close association with SIRT1.

Increased protein acetylation has been demonstrated to induce autophagy inhibition⁹¹ and mitochondrial damage, while SIRT3, a major mitochondrial deacetylase, is involved in mitochondrial protein deacetylation.⁹² Upon translocation into mitochondria, the full-length 44-kD SIRT3 undergoes activation through cleavage, resulting in the formation of a smaller 28-kD SIRT3.⁹³ In sepsis-induced kidney injury, Melatonin has been shown to significantly enhance the activation of SIRT3 and mitotic flux, thereby exerting a positive protective effect. Specifically, TFAM (Transcription Factor A, Mitochondrial) serves as the primary regulatory factor for mitochondrial function, with its expression being responsible for mtDNA transcription initiation.⁹⁴ In the case of impaired lysosomal function, defects in TFAM lead to compromised mitotic flux.⁹⁵ Acetylation at the K154 site of TFAM reduces its activity, while SIRT3-induced deacetylation at the K154 site promotes its activity.⁹⁶ Overall, Melatonin promotes mitochondrial autophagy by activating SIRT3 and enhancing the activity of TFAM.

Beclin-1 is an important autophagy effector that plays a central role in regulating the initiation and maturation of autophagosomes.⁹⁷ The genetic deficiency of Beclin-1 could lead to the inhibition of autophagy, thereby exacerbating the sepsis-induced organ damage.⁸⁵ The acetylation of Beclin-1 enhances its interaction with Rubicon (a negative regulator of autophagy), which is an important mechanism for autophagy inhibition.⁹⁸ The acetylation status of Beclin-1 is controlled by Sirt1.⁹⁹ In septic myocardial tissue, intervention with melatonin can increase the expression level of Sirt1 and inhibit the acetylation of Beclin-1. The inhibition of Sirt1 activity significantly attenuates the effect of melatonin on the deacetylation of Beclin-1. These results suggested that the deacetylation of Beclin-1 mediated by Sirt1 is also an important mechanism by which melatonin regulates autophagy.

Ferroptosis

Ferroptosis is a new form of cell death regulation caused by Fe2+-dependent lipid peroxidation, which was identified in 2012.¹⁰⁰ In recent years, ferroptosis has been extensively confirmed to be involved in the pathological processes of various diseases,¹⁰¹ ferroptosis is also believed to exacerbate the development of sepsis.^102,103 The levels of the end products of lipid peroxidation, 4-hydroxynonenal (4-HNE) and malondialdehyde (MDA), are considered as biomarkers of ferroptosis. Among the key enzymes regulating the occurrence of ferroptosis, the most well-known is glutathione peroxidase 4 (GPX4). GPX4 expression is negatively correlated with iron-mediated cell death, as GPX4 functions by degrading lipid peroxides.¹⁰⁴ Melatonin can attenuate organ damage through the GPX4-related pathway.¹⁰⁵ Heme oxygenase-1 (HO-1) is a cellular protective enzyme that plays a crucial role in maintaining redox balance and responds to cellular stress and iron levels.¹⁰⁶ As an upstream molecule of HO-1, nuclear factor erythroid 2-related factor 2 (Nrf2) is also a transcription factor that plays a key role in antioxidant defense.¹⁰⁷ In a sepsis-induced kidney injury model, intervention with melatonin significantly upregulates the expression of Nrf2/HO-1/GPX4, inhibits iron death, and thereby alleviates kidney damage. However, the HO-1 inhibitor Znpp and the Nrf2 inhibitor ML385 partially attenuate the upregulation of GPX4 by melatonin, reducing the inhibitory effect of melatonin on iron death. This suggests that the Nrf2/HO-1/GPX4 pathway may be an important mechanism through which melatonin inhibits iron death.

Synergistic Effects of Other Formulations

The synergistic effects of melatonin with other agents have been extensively studied. Melatonin can interact with various drugs or compounds such as antibiotics, antioxidants, anti-inflammatory drugs, enhancing their effects or producing synergistic effects.

Ascorbic Acid

Ascorbic acid, also known as vitamin C, is an essential micronutrient with potent antioxidant properties.¹⁰⁸ Its administration has a positive effect on improving the prognosis of patients with sepsis.^109–113 The combined application of melatonin and ascorbic acid significantly alleviates sepsis-induced organ injury in male rats. The combination therapy significantly improves cardiac and renal function, as evidenced by decreased levels of CK-MB, cardiac troponin I, creatinine, and urea. Furthermore, this combination therapy effectively modulates key markers of inflammation, cellular injury, oxidative stress, and vascular function, including NO, VEGF, and SIRT1 expression. The combined application demonstrates a more effective protective effect than when these two agents are applied individually.¹¹⁴ In a sepsis-induced lung injury model, the combined treatment of melatonin and ascorbic acid significantly reduced the levels of TNF-α and IL-1β, and improved the levels of peroxisome proliferator-activated receptor (PPAR), aromatic esterase (ARE), and paraoxonase (PON) in lung tissue. Histopathological examination showed reduced edema and lymphocyte infiltration. This suggests that melatonin and ascorbic acid have a beneficial synergistic effect in the treatment of sepsis.⁴²

Traditional Antibiotics

Both azithromycin and vancomycin are traditional antibiotic used in the treatment of sepsis.^115,116 In experimental animal models of sepsis, both azithromycin and vancomycin were found to effectively reduce sepsis score, increase survival rates, and improve blood routine parameters, blood biochemical parameters, and cardiac function parameters. The combination of melatonin and azithromycin significantly reduces the sepsis score. However, compared to the use of azithromycin alone, it does not have a significant impact on blood biochemical parameters, blood routine parameters, and cardiac function parameters. Additionally, there is no significant difference in the effects of melatonin combined with vancomycin and the use of vancomycin alone on septic mice.¹¹⁷

Discussion

In recent years, melatonin has been the subject of extensive research due to its diverse positive regulatory effects and potential therapeutic applications. However, its effectiveness, safety, and potential adverse reactions are critical factors that must be considered in clinical practice, particularly for patients with sepsis. Clinical research on melatonin has made significant progress. Administered 20 mg of melatonin, which significantly improved the clinical prognosis of newborns with purulent infections.⁶⁹ Similarly, Abbas et al reported that in patients experiencing early adult purulent toxic shocks, a daily dose of 50 mg of melatonin for five consecutive nights significantly reduced SOFA scores and the need for vasopressor medications.¹¹⁷ This treatment also decreased the duration of mechanical ventilation and the time on vasopressors, as well as reducing the lengths of ICU and hospital stays; however, no significant differences were observed in 28-day mortality rates. Furthermore, a single-center study suggested that the daily use of 10 mg of melatonin may help reduce the incidence of purulent infections and the mortality rate among patients with COVID-19.¹⁷

Melatonin is generally considered safe for short-term use in adults, presenting minimal risk of dependency or significant side effects. Long-term use has been investigated in certain populations, revealing no major adverse effects. Nevertheless, its safety profile in specific groups, including pregnant women, breastfeeding mothers, and children, remains uncertain due to a lack of comprehensive research. Currently, there is no evidence of serious adverse reactions, such as hepatotoxicity and nephrotoxicity; however, potential risks including drowsiness, headache, and exacerbation of depression may arise in certain patients with specific conditions, such as mental and psychological disorders, or autoimmune diseases. Consequently, the use of melatonin in patients with sepsis must take into account the individual circumstances of each patient, carefully consider the timing and dosage of administration, and remain vigilant regarding potential interactions with other medications. Moreover, the research on the extensive regulatory mechanisms of melatonin in sepsis-induced organ damage primarily derives from animal studies, revealing a significant gap in investigations involving human subjects. Further clinical trials are necessary to elucidate the specific mechanisms through which melatonin exerts its beneficial effects in sepsis.

In summary, melatonin has shown promising clinical significance in sepsis. Recent clinical trials and basic research on the topic have demonstrated encouraging effects of melatonin in the treatment of sepsis. In our work, we have integrated and summarized the relevant studies on the application of melatonin in sepsis in recent years. Our findings are of great significance for advancing the understanding of melatonin’s mechanisms of action in sepsis and promoting further clinical applications of melatonin. Melatonin is characterized by minimal side effects and a high safety profile, and it can exert protective effects through various molecular mechanisms. It may represent a promising therapeutic option for treating sepsis; however, its clinical application remains in the exploratory stage, necessitating a series of large-scale clinical studies to verify its effectiveness and safety.