Sustained expression of lipocalin-2 during polymicrobial sepsis

Dynamic changes and clinical value of lipocalin 2 in liver diseases caused by microbial infections

Lipocalin 2 (LCN2) plays a pivotal role in iron metabolism, particularly in the context of microbial infection resistance (e.g., viruses, bacteria, parasites, etc.). LCN2 combats microbial infection by directly assisting the body in competing with microorganisms for iron, inducing immune cells to secrete various cytokines to enhance systemic immune responses, or recruiting neutrophils to infectious sites. The liver serves as the primary organ for LCN2 secretion during microbial infections. This review encapsulates recent advances in dynamic changes, clinical values, and the effects of LCN2 in infectious liver diseases caused by various microbial microorganisms.

Lipocalin-2 participates in sepsis-induced myocardial injury by mediating lipid accumulation and mitochondrial dysfunction

BACKGROUND

Sepsis-induced cardiomyopathy (SIC) is one major cause of death for sepsis but lacks timely diagnosis and specific treatment due to unclear mechanisms. Lipocalin-2 (LCN-2) is a key regulator of lipid metabolism which has been recently proved closely related to sepsis, however, the relationship between LCN-2 and septic myocardial injury remains unknown. We aim to explore the role of LCN-2 in the pathological progress of SIC based on clinical and laboratory evidence.

METHODS

Consecutive patients admitted to the intensive care unit (ICU) from August 2021 to April 2022 fulfilling the criteria of severe sepsis were included. The level of LCN-2 in plasma was assayed and analyzed with clinical characteristics. Biostatistical analysis was performed for further identification and pathway enrichment. Mouse model for SIC was thereafter established, in which plasma and tissue LCN-2 levels were tested. RNA sequencing was used for verification and to reveal the possible mechanism. Mitochondrial function and intracellular lipid levels were assayed to further assess the biological effects of targeting LCN-2 in cardiomyocytes with small interference RNAs (siRNAs).

RESULTS

The level of LCN-2 in plasma was markedly higher in patients with severe sepsis and was associated with higher cardiac biomarkers and lower LVEF. In the in vivo experiment, circulating LCN-2 from plasma was found to increase in SIC mice. A higher level of LCN-2 transcription in myocardial tissue was also found in SIC and showed a clear time relationship. RNA sequencing analysis showed the level of LCN-2 was associated with several gene-sets relevant to mitochondrial function and lipid metabolism-associated pathways. The suppression of LCN-2 protected mitochondrial morphology and limited the production of ROS, as well as restored the mitochondrial membrane potential damaged by LPS. Neutral lipid staining showed prominent lipid accumulation in LPS group, which was alleviated by the treatment of siLCN2.

CONCLUSION

The level of LCN-2 is significantly increased in SIC at both circulating and tissue levels, which is correlated with the severity of myocardial injury indicators, and may work as an early and great predictor of SIC. LCN-2 probably participates in the process of septic myocardial injury through mediating lipid accumulation and affecting mitochondrial function.

Lipocalin-2 in neutrophils induces ferroptosis in septic cardiac dysfunction via increasing labile iron pool of cardiomyocytes

Cardiac dysfunction is a common complication of sepsis with high mortality. The present study was designed to identify the effect of neutrophil-derived lipocalin-2 (LCN2) in septic cardiac dysfunction (SCD) and its potential mechanism. Wild-type (WT) and LCN2-knockout (LCN2 KO) mice were peritoneally injected with lipopolysaccharide (LPS) to induce SCD. The cardiac function was assessed 12 h after LPS injection by echocardiography. Cardiac tissue was harvested for the evaluation of malonaldehyde (MDA) and prostaglandin E synthase 2 (PTGS2) mRNA levels. LPS induced ferroptosis and SCD in mice. LCN2 deficiency attenuated cardiac injury post-LPS administration. In vitro, LCN2 expression in neutrophils increased in response to LPS. Ferroptosis of cardiomyocytes induced by conditioned medium (CM) from LPS-induced neutrophils of WT mice could be attenuated in CM from LPS-induced neutrophils of LCN2 KO mice. Exogenous LCN2 induced H9C2 cell ferroptosis via increasing labile iron pool (LIP). In conclusion, our results showed that LCN2 deficiency prevented heart dysfunction and ferroptosis in SCD mice and suggested that neutrophil-derived LCN2 might be a promising therapeutic target for SCD.

[Inhibiting ferroptosis attenuates myocardial injury in septic mice: the role of lipocalin-2]

OBJECTIVE

To explore the contribution of ferroptosis to myocardial injury in mouse models of sepsis and the role lipocalin-2 (Lcn2) in ferroptosis.

METHODS

Adult male C57BL/6 mice were randomized equally into sham-operated group, cecal ligation and puncture (CLP)-induced sepsis group, and CLP + Fer-1 group where the mice received intraperitoneal injection of 5 mg/mL Fer-1 (5 mg/kg) 1 h before CLP. The left ventricular functions (including LVEF%, LVFS%, LVIDd and LVIDs) of the mice were assessed by echocardiography at 24 h after CLP. Myocardial injury in the mice was observed with HE staining, and the changes of myocardial ultrastructure and mitochondria were observed using transmission electron microscopy (TEM). Serum TNF-α level was measured with ELISA, and the changes of myocardial iron content were detected using tissue iron kit. The protein expressions of myocardial Lcn2, glutathione peroxidase 4 (GPX4) and ferroptosis suppressor protein 1 (FSP1) were determined with Western blotting.

RESULTS

The septic mice showed significantly decreased LVEF%, LVFS% and LVIDd and increased LVIDs at 24 h after CLP (P < 0.05), and these changes were significantly improved by Fer-1 treatment. Sepsis caused obvious myocardial pathologies and changes in myocardial ultrastructure and mitochondria, which were significantly improved by Fer-1 treatment. Fer-1 treatment also significantly ameliorated sepsis-induced elevations of serum TNF-α level, myocardial tissue iron content, and Lcn2 protein expression and the reduction of GPX4 and FSP1 protein expression levels (P < 0.05).

CONCLUSION

GPX4- and FSP1-mediated ferroptosis are involved in myocardial injury in mice with CLP-induced sepsis, and inhibition of ferroptosis can attenuate septic myocardial injury, in which Lcn2 may play a role.

The Transcription Factor FRA-1/AP-1 Controls Lipocalin-2 Expression and Inflammation in Sepsis Model

Sepsis is a life-threatening condition characterized by excessive inflammation in its early phase. This is followed by an aberrant resolution phase associated to a prolonged period of immune suppression that can ultimately lead to multiple organ dysfunctions. This immunosuppression can be mediated by the functional reprogramming of gene transcription in monocytes/macrophages in response to prolonged lipopolysaccharide (LPS) exposure. Surprisingly, there is no report on the role of AP-1 transcription factors in this reprogramming process. Herein, we used the endotoxin tolerance model on murine bone marrow-derived macrophages in which tolerant cells stimulated twice with LPS were compared to naïve cells stimulated once. Out of all AP-1 transcription factors tested, Fosl1 gene stood out because of its unique regulation in tolerized cells. Moreover, we could correlate FRA-1 expression to the expression of an essential anti-inflammatory molecule involved in sepsis response, Lipocalin 2 aka NGAL. Identical results were obtained in human PBMC following the endotoxin tolerance model. When using FRA-1 deficient macrophages, we could confirm that FRA-1 regulates NGAL expression during the tolerant state. Interestingly, ChIP-seq and ChIP-qPCR revealed the binding of FRA-1 on Lcn2 promoter after LPS stimulation in these cells. Finally, we used an in vivo septic model of consecutive injection of LPS, in which the second stimulation is performed before the resolution of inflammation, in wild type and FRA-1 deficient mice. NGAL secretion was elevated in lung, spleen and serum of wild type tolerant mice, whereas it was significantly lower in tolerant FRA-1 deficient mice. Moreover, an increased inflammatory state likely dependent of the low level of NGAL was observed in these FRA-1 deficient mice. This was characterized by an increase of neutrophil infiltration in lung and an increase of apoptotic follicular cells in spleen. This suggests that FRA-1 expression supports resolution of inflammation in this model. Collectively, our data indicate that FRA-1 is involved in myeloid cell tolerance responses by mediating the functional reprogramming of Lcn2 transcription in response to prolonged LPS exposure. In conclusion, FRA-1 may have a protective role in the tolerance response of sepsis through the regulation of NGAL, leading to resolution of inflammation.

Multi-Organ Transcriptome Dynamics in a Mouse Model of Cecal Ligation and Puncture-Induced Polymicrobial Sepsis

PURPOSE

During sepsis, an excessive inflammatory immune reaction contributes to multi-organ dysfunction syndrome (MODS), a critical condition associated with high morbidity and mortality; however, the molecular mechanisms driving MODS remain elusive.

METHODS

We used RNA sequencing to characterize transcriptional changes in the early phase of sepsis, at 6, 12, 24 hour time points in lung, kidney, liver, and heart tissues, in a cecal ligation and puncture (CLP)-induced polymicrobial sepsis murine model.

RESULTS

The CLP surgery induced significant changes (adj. p-value<0.05) in expression of hundreds of transcripts in the four organs tested, with the highest number exceeding 2,000 differentially expressed genes (DEGs) in all organs at 12 hours post-CLP. Over-representation analysis by functional annotations of DEGs to the Reactome database revealed the immune system, hemostasis, lipid metabolism, signal transduction, and extracellular matrix remodeling biological processes as significantly altered in at least two organs, while metabolism of proteins and RNA were revelaed as being liver tissue specific in the early phase of sepsis.

CONCLUSION

RNA sequencing across organs and time-points in the CLP murine model allowed us to study the trajectories of transcriptome changes demonstrating alterations common across multiple organs as well as biological pathways altered in an organ-specific manner. These findings could pave new directions in the research of sepsis-induced MODS and indicate new sepsis treatment strategies.

Regulation of the Nfkbiz Gene and Its Protein Product IkBζ in Animal Models of Sepsis and Endotoxic Shock

Sepsis is a life-threatening condition that arises from a poorly regulated inflammatory response to pathogenic organisms. Current treatments are limited to antibiotics, fluid resuscitation, and other supportive therapies. New targets for monitoring disease progression and therapeutic interventions are therefore critically needed. We previously reported that lipocalin-2 (Lcn2), a bacteriostatic mediator with potent proapoptotic activities, was robustly induced in sepsis. Other studies showed that Lcn2 was a predictor of mortality in septic patients. However, how Lcn2 is regulated during sepsis is poorly understood. We evaluated how IkBζ, an inducer of Lcn2, was regulated in sepsis using both the cecal ligation and puncture (CLP) and endotoxemia (lipopolysaccharide [LPS]) animal models. We show that Nfkbiz, the gene encoding IkBζ, was rapidly stimulated but, unlike Lcn2, whose expression persists during sepsis, mRNA levels of Nfkbiz decline to near basal levels several hours after its induction. In contrast, we observed that IkBζ expression remained highly elevated in septic animals following CLP but not LPS, indicating the occurrence of a CLP-specific mechanism that extends IkBζ half-life. By using an inhibitor of IkBζ, we determined that the expression of Lcn2 was largely controlled by IkBζ. Altogether, these data indicate that the high IkBζ expression in tissues likely contributes to the elevated expression of Lcn2 in sepsis. Since IkBζ is also capable of promoting or repressing other inflammatory genes, it might exert a central role in sepsis.

Immunosuppression is Inappropriately Qualifying the Immune Status of Septic and SIRS Patients

Immunosuppression is the most commonly used concept to qualify the immune status of patients with either sterile systemic inflammatory response syndrome (SIRS) or sepsis. In this review we attempt to demonstrate that the concept of immunosuppression is an oversimplification of the complex anti-inflammatory response that occurs in patients dealing with a severe sterile or infectious insult. Particularly, the immune status of leukocytes varies greatly depending on the compartment from where they are derived from. Furthermore, although certain functions of immune cells present in the blood stream or in the hematopoietic organs can be significantly diminished, other functions are either unchanged or even enhanced. This juxtaposition illustrates that there is no global defect. The mechanisms called reprogramming or trained innate immunity are probably aimed at preventing a generalized deleterious inflammatory reaction, and work to maintain the defense mechanisms at their due levels.

The role of the innate immune system on pulmonary infections

Inhalation is required for respiration and life in all vertebrates. This process is not without risk, as it potentially exposes the host to environmental pathogens with every breath. This makes the upper respiratory tract one of the most common routes of infection and one of the leading causes of morbidity and mortality in the world. To combat this, the lung relies on the innate immune defenses. In contrast to the adaptive immune system, the innate immune system does not require sensitization, previous exposure or priming to attack foreign particles. In the lung, the innate immune response starts with the epithelial barrier and mucus production and is reinforced by phagocytic cells and T cells. These cells are vital for the production of cytokines, chemokines and anti-microbial peptides that are critical for clearance of infectious agents. In this review, we discuss all aspects of the innate immune response, with a special emphasis on ways to target aspects of the immune response to combat antibiotic resistant bacteria.

Functions and regulation of lipocalin-2 in gut-origin sepsis: a narrative review

Lipocalin-2 (Lcn2), an innate immune protein, has come to be recognized for its roles in iron homeostasis, infection, and inflammation. In this narrative review, we provide a comprehensive description based on currently available evidence of the clinical implications of Lcn2 and its therapeutic potency in gut-origin sepsis. Lcn2 appears to mitigate gut barrier injury via maintaining homeostasis of the microbiota and exerting antioxidant strategy, as well as by deactivating macrophages and inducing immune cell apoptosis to terminate systemic hyper-inflammation. We propose that development of a therapeutic strategy targeting lipocalin-2 could be highly promising in the management of gut-origin sepsis.

Lipocalin 2 Plays an Important Role in Regulating Inflammation in Retinal Degeneration

It has become increasingly important to understand how retinal inflammation is regulated because inflammation plays a role in retinal degenerative diseases. Lipocalin 2 (LCN2), an acute stress response protein with multiple innate immune functions, is increased in ATP-binding cassette subfamily A member 4 (Abca4) ^-/- retinol dehydrogenase 8 (Rdh8) ^-/- double-knockout mice, an animal model for Stargardt disease and age-related macular degeneration (AMD). To examine roles of LCN2 in retinal inflammation and degeneration, Lcn2^-/-Abca4^-/-Rdh8^-/- triple-knockout mice were generated. Exacerbated inflammation following light exposure was observed in Lcn2^-/-Abca4^-/-Rdh8^-/- mice as compared with Abca4^-/-Rdh8^-/- mice, with upregulation of proinflammatory genes and microglial activation. RNA array analyses revealed an increase in immune response molecules such as Ccl8, Ccl2, and Cxcl10 To further probe a possible regulatory role for LCN2 in retinal inflammation, we examined the in vitro effects of LCN2 on NF-κB signaling in human retinal pigmented epithelial (RPE) cells differentiated from induced pluripotent stem cells derived from healthy donors. We found that LCN2 induced expression of antioxidant enzymes heme oxygenase 1 and superoxide dismutase 2 in these RPE cells and could inhibit the cytotoxic effects of H₂O₂ and LPS. ELISA revealed increased LCN2 levels in plasma of patients with Stargardt disease, retinitis pigmentosa, and age-related macular degeneration as compared with healthy controls. Finally, overexpression of LCN2 in RPE cells displayed protection from cell death. Overall these results suggest that LCN2 is involved in prosurvival responses during cell stress and plays an important role in regulating inflammation during retinal degeneration.

Lipocalin 2 (LCN2) Expression in Hepatic Malfunction and Therapy

Lipocalin 2 (LCN2) is a secreted protein that belongs to the Lipocalins, a group of transporters of small lipophilic molecules such as steroids, lipopolysaccharides, iron, and fatty acids in circulation. Two decades after its discovery and after a high variety of published findings, LCN2's altered expression has been assigned to critical roles in several pathological organ conditions, including liver injury and steatosis, renal damage, brain injury, cardiomyopathies, muscle-skeletal disorders, lung infection, and cancer in several organs. The significance of this 25-kDa lipocalin molecule has been impressively increased during the last years. Data from several studies indicate the role of LCN2 in physiological conditions as well as in response to cellular stress and injury. LCN2 in the liver shows a protective role in acute and chronic injury models where its expression is highly elevated. Moreover, LCN2 expression is being considered as a potential strong biomarker for pathological conditions, including rheumatic diseases, cancer in human organs, hepatic steatosis, hepatic damage, and inflammation. In this review, we summarize experimental and clinical findings linking LCN2 to the pathogenesis of liver disease.

Macrophagic control of the response to uropathogenic E. coli infection by regulation of iron retention in an IL-6-dependent manner

Introduction

Uropathogenic Escherichia coli (UPEC), the causative agent of over 85% of urinary tract infections (UTIs), elaborate a number of siderophores to chelate iron from the host. On the other hand, the host immune imperative is to limit the availability of iron to the bacteria. Little is known regarding the mechanisms underlying this host‐iron‐UPEC interaction. Our objective was to determine whether macrophages, in response to UPEC infection, retain extracellular siderophore‐bound and free iron, thus limiting the ability of UPEC to access iron.

Methods

Quantitative PCR, immunoblotting analysis, and gene expression analysis of wild type and IL‐6‐deficient macrophages was performed.

Results

We found that (1) macrophages upon UPEC infection increased expression of lipocalin 2, a siderophore‐binding molecule, of Dmt1, a molecule that facilitates macrophage uptake of free iron, and of the intracellular iron cargo molecule ferritin, and decreased expression of the iron exporter ferroportin; (2) bladder macrophages regulate expression of genes involved in iron retention upon UPEC infection; (3) IL‐6, a cytokine known to play an important role in regulating host iron homeostasis as well as host defense to UPEC, regulates this process, in part by promoting production of lipocalin 2; and finally, (4) inhibition of IL‐6 signaling genetically and by neutralizing antibodies against the IL‐6 receptor, promoted intra‐macrophagic UPEC growth in the presence of excess iron.

Conclusions

Together, our study suggests that macrophages retain siderophore‐bound and free iron in response to UPEC and IL‐6 signaling is necessary for macrophages to limit the growth of UPEC in the presence of excess iron. IL‐6 signaling and iron regulation is one mechanism by which macrophages may mediate UPEC clearance.