A novel antihypoglycemic role of inducible nitric oxide synthase in liver inflammatory response induced by dietary cholesterol and endotoxemia

Apolipoprotein A-I Binding Protein Inhibits the Formation of Infantile Hemangioma through Cholesterol-Regulated Hypoxia-Inducible Factor 1α Activation

Infantile hemangioma (IH) is the most frequent vascular tumor of infancy with unclear pathogenesis; disordered angiogenesis is considered to be involved in its formation. Apolipoprotein A-I binding protein (AIBP)-also known as NAXE (NAD [P]HX epimerase)-a regulator of cholesterol metabolism, plays a critical role in the pathological angiogenesis of mammals. In this study, we found that AIBP had much lower expression levels in both tissues from patients with IH and hemangioma endothelial cells (HemECs) than in adjacent normal tissues and human dermal vascular endothelial cells, respectively. Knockout of NAXE by CRISPR-Cas9 in HemECs enhanced tube formation and migration, and NAXE overexpression impaired tube formation and migration of HemECs. Interestingly, AIBP suppressed the proliferation of HemECs in hypoxia. We then found that reduced expression of AIBP correlated with increased hypoxia-inducible factor 1α levels in tissues from patients with IH and HemECs. Further mechanistic investigation demonstrated that AIBP disrupted hypoxia-inducible factor 1α signaling through cholesterol metabolism under hypoxia. Notably, AIBP significantly inhibited the development of IH in immunodeficient mice. Furthermore, using the validated mouse endothelial cell (ie, EOMA cells) and Naxe-/- mouse models, we demonstrated that both endogenous AIBP from tumors and AIBP in the tumor microenvironment limit the formation of hemangioma. These findings suggested that AIBP was a player in the pathogenesis of IH and could be a potential pharmacological target for treating IH.

Cholesterol suppresses human iTreg differentiation and nTreg function through mitochondria-related mechanisms

BACKGROUND

Both the crystalline and soluble forms of cholesterol increase macrophage secretion of interleukin 1β (IL-1β), aggravating the inflammatory response in atherosclerosis (AS). However, the link between cholesterol and regulatory T cells (Tregs) remains unclear. This study aimed to investigate the effect of cholesterol treatment on Tregs.

METHODS

Differentiation of induced Tregs (iTregs) was analyzed using flow cytometry. The expression of hypoxia-inducible factor-1a (HIF-1a) and its target genes was measured by western blotting and/or RT-qPCR. Two reporter jurkat cell lines were constructed by lentiviral transfection. Mitochondrial function and the structure of natural Tregs (nTregs) were determined by tetramethylrhodamine (TMRM) and mitoSOX staining, Seahorse assay, and electron microscopy. The immunoregulatory function of nTregs was determined by nTreg-macrophage co-culture assay and ELISA.

RESULTS

Cholesterol treatment suppressed iTreg differentiation and impaired nTreg function. Mechanistically, cholesterol induced the production of mitochondrial reactive oxygen species (mtROS) in naïve T cells, inhibiting the degradation of HIF-1α and unleashing its inhibitory effects on iTreg differentiation. Furthermore, cholesterol-induced mitochondrial oxidative damage impaired the immunosuppressive function of nTregs. Mixed lymphocyte reaction and nTreg-macrophage co-culture assays revealed that cholesterol treatment compromised the ability of nTregs to inhibit pro-inflammatory conventional T cell proliferation and promote the anti-inflammatory functions of macrophages. Finally, mitoTEMPO (MT), a specific mtROS scavenger, restored iTreg differentiation and protected nTreg from further deterioration.

CONCLUSION

Our findings suggest that cholesterol may aggravate inflammation within AS plaques by acting on both iTregs and nTregs, and that MT may be a promising anti-atherogenic drug.

iNOS as a metabolic enzyme under stress conditions

Nitric oxide (NO) is a free radical acting as a cellular signaling molecule in many different biochemical processes. NO is synthesized from l-arginine through the action of the nitric oxide synthase (NOS) family of enzymes, which includes three isoforms: endothelial NOS (eNOS), neuronal NOS (nNOS) and inducible NOS (iNOS). iNOS-derived NO has been associated with the pathogenesis and progression of several diseases, including liver diseases, insulin resistance, obesity and diseases of the cardiovascular system. However, transient NO production can modulate metabolism to survive and cope with stress conditions. Accumulating evidence strongly imply that iNOS-derived NO plays a central role in the regulation of several biochemical pathways and energy metabolism including glucose and lipid metabolism during inflammatory conditions. This review summarizes current evidence for the regulation of glucose and lipid metabolism by iNOS during inflammation, and argues for the role of iNOS as a metabolic enzyme in immune and non-immune cells.

Tylophorine-based compounds are therapeutic in rheumatoid arthritis by targeting the caprin-1 ribonucleoprotein complex and inhibiting expression of associated c-Myc and HIF-1α

Interruption of the Warburg effect - the observation that un-stimulated macrophages reprogram their core metabolism from oxidative phosphorylation toward aerobic glycolysis to become pro-inflammatory M1 macrophages upon stimulation - is an emerging strategy for the treatment of cancer and anti-inflammatory diseases such as rheumatoid arthritis. We studied this process with view to the discovery of novel therapeutics, and found that tylophorine-based compounds targeted a ribonucleoprotein complex containing caprin-1 and mRNAs of c-Myc and HIF-1α in LPS/IFN-γ stimulated Raw264.7 cells, diminished the protein levels of c-Myc and HIF-1α, and consequently downregulated their targeted genes that are associated with the Warburg effect, as well as the pro-inflammatory iNOS and COX2. The tylophorine-based compound DBQ 33b significantly meliorated the severity and incidence of type II collagen-monoclonal antibody-induced rheumatoid arthritis and diminished gene expressions of c-Myc, HIF-1α, iNOS, COX2, TNFα, and IL-17A in vivo. Moreover, pharmacological inhibition of either c-Myc or HIF-1α exhibited similar effects as the tylophorine-based compound DBQ 33b, even though inhibition of c-Myc reversed the induction of iNOS and COX2 in LPS/IFN-γ stimulated Raw264.7 cells to a lesser degree. Therefore, simultaneous inhibition of both c-Myc and HIF-1α is efficacious for anti-inflammation in vitro and in vivo and merits further study.

Lipidomic biomarkers and mechanisms of lipotoxicity in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) represents the most common form of chronic liver disease worldwide (about 25% of the general population) and 3-5% of patients develop non-alcoholic steatohepatitis (NASH), characterized by hepatocytes damage, inflammation and fibrosis, which increase the risk of developing liver failure, cirrhosis and hepatocellular carcinoma. The pathogenesis of NAFLD, particularly the mechanisms whereby a minority of patients develop a more severe phenotype, is still incompletely understood. In this review we examine the available literature on initial mechanisms of hepatocellular damage and inflammation, deriving from toxic effects of excess lipids. Accumulating data indicate that the total amount of triglycerides stored in the liver cells is not the main determinant of lipotoxicity and that specific lipid classes act as damaging agents. These lipotoxic species affect the cell behavior via multiple mechanisms, including activation of death receptors, endoplasmic reticulum stress, modification of mitochondrial function and oxidative stress. The gut microbiota, which provides signals through the intestine to the liver, is also reported to play a key role in lipotoxicity. Finally, we summarize the most recent lipidomic strategies utilized to explore the liver lipidome and its modifications in the course of NALFD. These include measures of lipid profiles in blood plasma and erythrocyte membranes that can surrogate to some extent lipid investigation in the liver.

Cholesterol Prevents Hypoxia-Induced Hypoglycemia by Regulation of a Metabolic Ketogenic Shift

Blood cholesterol levels have been connected to high-altitude adaptation. In the present study, we treated mice with high-cholesterol diets following exposure to acute hypoxic stress and evaluated the effects of the diets on whole-body, liver glucose, and liver fat metabolism. For rapid cholesterol liver uptake, 6-week-old male C57BL/J6 mice were fed with high-cholesterol/cholic acid (CH) diet for 6 weeks and then were exposed to gradual oxygen level reduction for 1 h and hypoxia at 7% oxygen for additional 1 hour using a hypoxic chamber. Animals were than sacrificed, and metabolic markers were evaluated. Hypoxic treatment had a strong hypoglycemic effect that was completely blunted by CH treatment. Decreases in gluconeogenesis and glycogenolysis as well as an increase in ketone body formation were observed. Such changes indicate a metabolic shift from glucose to fat utilization due to activation of the inducible nitric oxide synthase/AMPK axis in the CH-treated animals. Increased ketogenesis was also observed in vitro in hepatocytes after cholesterol treatment. In conclusion, our results show for the first time that cholesterol contributes to metabolic shift and adaptation to hypoxia in vivo and in vitro through induction of HIF-1α and iNOS expression.

Intestinally derived bacterial products stimulate development of nonalcoholic steatohepatitis

Fatty livers are susceptible to factors that cause inflammation and fibrosis, but fat deposition and the inflammatory response can be dissociated. While nonalcoholic fatty liver disease (NAFLD), caused by pathologic fat accumulation inside the liver, can remain stable for several years, in other cases NAFLD progresses to nonalcoholic steatohepatitis (NASH), which is characterized by fat accumulation and inflammation and is not a benign condition. In this review, we discuss the NASH host cells and microbial mechanisms that stimulate inflammation and predispose the liver to hepatocyte injury and fibrotic stages via increased lipid deposition. We highlight the interactions between intestine-derived bacterial products, such as lipopolysaccharide, and nutritional models of NAFLD and/or obese individuals. The results of modulating enteric microbiota suggest that gut-derived endotoxins may be essential determinants of fibrotic progression and regression in NASH.

High Fat Diets Composed of Palm Stearin and Olive Oil Equally Exacerbate Liver Inflammatory Damage and Metabolic Stress in Mice

SCOPE

People with fatty liver could be subject to acute infections such as sepsis. The aim of the study is to evaluate the effect of high fat diets (HFD) of olive oil and palm stearin on liver inflammation induced by lipopolysaccharides (LPS).

METHODS AND RESULTS

C57BL/6J male mice were treated with high fat diets with different sources of oils: palm stearin and olive oil for 8 weeks followed by LPS injection. The proinflammatory effect of olive oil was also studied using gavage treatment and IP injection of LPS. Animals fed with HFDs showed an increase in body weight, elevated blood glucose levels, and fatty liver phenotype. HFDs aggravated the effect of LPS treatment to induce inflammatory response compared to low fat diet (LFD) effect. Following HFD supplementation, LPS induced hyperinsulinemia, more liver damage than in animals that consumed LFD. In addition, both gavage and long-term feeding with high lipids in the presence of LPS resulted in inhibition of gluconeogenic genes expression.

CONCLUSION

HFDs of both monounsaturated and saturated fat potentiated liver inflammation induced by LPS treatment indicate that the total amount of fat consumed is the main proinflammatory factor rather than the type of fat.

Hypoxic Signaling and Cholesterol Lipotoxicity in Fatty Liver Disease Progression

Cholesterol is the only lipid whose absorption in the gastrointestinal tract is limited by gate-keeping transporters and efflux mechanisms, preventing its rapid absorption and accumulation in the liver and blood vessels. In this review, I explored the current data regarding cholesterol accumulation in liver cells and key mechanisms in cholesterol-induced fatty liver disease associated with the activation of deleterious hypoxic and nitric oxide signal transduction pathways. Although nonalcoholic fatty liver disease (NAFLD) affects both obese and nonobese individuals, the mechanism of NAFLD progression in lean individuals with healthy metabolism is puzzling. Lean NAFLD individuals exhibit normal metabolic responses, implying that liver damage is not associated with impaired metabolism per se and that direct lipotoxic effects are crucial for disease progression. Several redox and oxidant signaling pathways involving cholesterol are at play in fatty liver disease development. These include impairment of the mitochondrial and lysosomal function by cholesterol loading of the inner-cell membranes; formation of cholesterol crystals and hepatocyte degradation; and crown-like structures surrounding degrading hepatocytes, activating Kupffer cells, and evoking inflammation. The current review focuses on the induction of liver inflammation, fibrosis, and steatosis by free cholesterol via the hypoxia-inducible factor 1α (HIF-1α), a main oxygen-sensing transcription factor involved in all stages of NAFLD. Cholesterol loading in hepatocytes can result in chronic HIF-1α activity because of the decreased oxygen availability and excessive production of nitric oxide and mitochondrial reactive oxygen species.

Translational Advances of Hydrofection by Hydrodynamic Injection

Hydrodynamic gene delivery has proven to be a safe and efficient procedure for gene transfer, able to mediate, in murine model, therapeutic levels of proteins encoded by the transfected gene. In different disease models and targeting distinct organs, it has been demonstrated to revert the pathologic symptoms and signs. The therapeutic potential of hydrofection led different groups to work on the clinical translation of the procedure. In order to prevent the hemodynamic side effects derived from the rapid injection of a large volume, the conditions had to be moderated to make them compatible with its use in mid-size animal models such as rat, hamster and rabbit and large animals as dog, pig and primates. Despite the different approaches performed to adapt the conditions of gene delivery, the results obtained in any of these mid-size and large animals have been poorer than those obtained in murine model. Among these different strategies to reduce the volume employed, the most effective one has been to exclude the vasculature of the target organ and inject the solution directly. This procedure has permitted, by catheterization and surgical procedures in large animals, achieving protein expression levels in tissue close to those achieved in gold standard models. These promising results and the possibility of employing these strategies to transfer gene constructs able to edit genes, such as CRISPR, have renewed the clinical interest of this procedure of gene transfer. In order to translate the hydrodynamic gene delivery to human use, it is demanding the standardization of the procedure conditions and the molecular parameters of evaluation in order to be able to compare the results and establish a homogeneous manner of expressing the data obtained, as ‘classic’ drugs.

Technical Improvement and Application of Hydrodynamic Gene Delivery in Study of Liver Diseases

Development of an safe and efficient in vivo gene delivery method is indispensable for molecular biology research and the progress in the following gene therapy. Over the past few years, hydrodynamic gene delivery (HGD) with naked DNA has drawn increasing interest in both research and potential clinic applications due to its high efficiency and low risk in triggering immune responses and carcinogenesis in comparison to viral vectors. This method, involving intravenous injection (i.v.) of massive DNA in a short duration, gives a transient but high in vivo gene expression especially in the liver of small animals. In addition to DNA, it has also been shown to deliver other substance such as RNA, proteins, synthetic small compounds and even viruses in vivo. Given its ability to robustly mimic in vivo hepatitis B virus (HBV) production in liver, HGD has become a fundamental and important technology on HBV studies in our group and many other groups. Recently, there have been interesting reports about the applications and further improvement of this technology in other liver research. Here, we review the principle, safety, current application and development of hydrodynamic delivery in liver disease studies, and discuss its future prospects, clinical potential and challenges.

Liver-targeted hydrodynamic gene therapy: Recent advances in the technique

One of the major research focuses in the field of gene therapy is the development of clinically applicable, safe, and effective gene-delivery methods. Since the first case of human gene therapy was performed in 1990, a number of gene-delivery methods have been developed, evaluated for efficacy and safety, and modified for human application. To date, viral-vector-mediated deliveries have shown effective therapeutic results. However, the risk of lethal immune response and carcinogenesis have been reported, and it is still controversial to be applied as a standard therapeutic option. On the other hand, delivery methods for nonviral vector systems have been developed, extensively studied, and utilized in in vivo gene-transfer studies. Compared to viral-vector mediated gene transfer, nonviral systems have less risk of biological reactions. However, the lower gene-transfer efficiency was a critical hurdle for applying them to human gene therapy. Among a number of nonviral vector systems, our studies focus on hydrodynamic gene delivery to utilize physical force to deliver naked DNA into the cells in the living animals. This method achieves a high gene-transfer level by DNA solution injections into the tail vein of rodents, especially in the liver. With the development of genome editing methods, in vivo gene-transfer therapy using this method is currently the focus in this research field. This review explains the method principle, efficiency, safety, and procedural modifications to achieve a high level of reproducibility in large-animal models.

Does Citrulline Have Protective Effects on Liver Injury in Septic Rats?

Citrulline (Cit) supplementation was proposed to serve as a therapeutic intervention to restore arginine (Arg) concentrations and improve related functions in sepsis. This study explored whether citrulline had positive effects on liver injury and cytokine release in the early stages of sepsis. The cecal ligation and puncture (CLP) model was utilized in our study. Rats were divided into four groups: normal, Cit, CLP, and CLP+Cit. The CLP group and CLP+Cit group were separated into 6-, 12-, and 24-hour groups, according to the time points of sacrifice after surgery. Intragastric administration of L-citrulline was applied to rats in Cit and CLP+Cit groups before surgery. Serum AST and ALT levels and levels of MDA, SOD, NO, and iNOS in the liver tissues were evaluated. Plasma concentrations of Cit and Arg were assessed using HPLC-MS/MS. Serum concentrations of cytokines and chemokines were calculated by Luminex. Results showed SOD activities of CLP+Cit groups were significantly higher than that of CLP groups, contrasting with the MDA and NO levels which were significantly lower in CLP+Cit groups than in CLP groups. In addition, plasma concentrations of TNF-α, IL-6, and IL-1β were significantly lower in the CLP+Cit 6-hour group than in the CLP 6-hour group.

Image-Guided Hydrodynamic Gene Delivery: Current Status and Future Directions

Hydrodynamics-based delivery has been used as an experimental tool to express transgene in small animals. This in vivo gene transfer method is useful for functional analysis of genetic elements, therapeutic effect of oligonucleotides, and cancer cells to establish the metastatic cancer animal model for experimental research. Recent progress in the development of image-guided procedure for hydrodynamics-based gene delivery in large animals directly supports the clinical applicability of this technique. This review summarizes the current status and recent progress in the development of hydrodynamics-based gene delivery and discusses the future directions for its clinical application.

The role of iNOS in cholesterol-induced liver fibrosis

Accumulation of cholesterol in the liver is associated with the development of non-alcoholic steatohepatitis-related fibrosis. However, underlying mechanisms are not well understood. The present study investigated the role of inducible nitric oxide synthase (iNOS) in cholesterol-induced liver fibrosis by feeding wild-type (WT) and iNOS-deficient mice with control or high-cholesterol diet (HCD) for 6 weeks. WT mice fed with HCD developed greater liver fibrosis, compared with iNOS-deficient mice, as evident by Sirius red staining and higher expression levels of profibrotic genes. Enhanced liver fibrosis in the presence of iNOS was associated with hypoxia-inducible factor-1α stabilization, matrix metalloproteinase-9 expression, and enhanced hepatic DNA damage. The profibrotic role of iNOS was also demonstrated in vivo using a selective inhibitor of iNOS as well as in vitro in a rat liver stellate cell line (HSC-T6). In conclusion, these findings suggest that iNOS is an important mediator in HCD-induced liver fibrosis.

Hydrodynamic delivery

Hydrodynamic delivery (HD) is a broadly used procedure for DNA and RNA delivery in rodents, serving as a powerful tool for gene/protein drug discovery, gene function analysis, target validation, and identification of elements in regulating gene expression in vivo. HD involves a pressurized injection of a large volume of solution into a vasculature. New procedures are being developed to satisfy the need for a safe and efficient gene delivery in clinic. Here, we summarize the fundamentals of HD, its applications, and future perspectives for clinical use.

Mechanism for HIF-1 activation by cholesterol under normoxia: a redox signaling pathway for liver damage

Cholesterol and chronic activation of hypoxia-inducible factor-1 (HIF-1) have been separately implicated in the pathogenesis and progression of liver diseases. In AML12 hepatocytes increased HIF-1α protein accumulation was evident after 2 h of incubation with cholesterol, whereas enhanced HIF-1 transcriptional activity was observed after 6 h. Investigations into the molecular mechanism have shown that cholesterol inhibited HIF-1α degradation. Mitochondrial dysfunction and enhanced mitochondrial reactive oxygen species (ROS) generation were observed in 2-h cholesterol-treated cells along with augmented nitric oxide (NO) levels. Further analysis indicated that HIF-1α stabilization at later time (6h), but not after 2h, of incubation with cholesterol was dependent on NO production. To elucidate the role of mitochondrial dysfunction in HIF-1α stabilization, mitochondrial DNA-depleted hepatocytes were prepared. In these cells the ability of cholesterol to activate the HIF-1 pathway was abolished. Similarly, catalase overexpression also attenuated cholesterol-induced HIF-1α accumulation. These results demonstrate that cholesterol promotes HIF-1 activation in a ROS- and NO-dependent manner. Chronic liver activation of HIF-1 by cholesterol may mediate its deleterious effects in the liver.