The mAb against adipocyte fatty acid-binding protein 2E4 attenuates the inflammation in the mouse model of high-fat diet-induced obesity via toll-like receptor 4 pathway

Islet-Resident Memory T Cells Orchestrate the Immunopathogenesis of Type 1 Diabetes through the FABP4-CXCL10 Axis

Type 1 diabetes (T1D) is a chronic disease characterized by self-destruction of insulin-producing pancreatic β cells by cytotoxic T cell activity. However, the pathogenic mechanism of T cell infiltration remains obscure. Recently, tissue-resident memory T (TRM) cells have been shown to contribute to cytotoxic T cell recruitment. TRM cells are found present in human pancreas and are suggested to modulate immune homeostasis. Here, the role of TRM cells in the development of T1D is investigated. The presence of TRM cells in pancreatic islets is observed in non-obese diabetic (NOD) mice before T1D onset. Mechanistically, elevated fatty acid-binding protein 4 (FABP4) potentiates the survival and alarming function of TRM cells by promoting fatty acid utilization and C-X-C motif chemokine 10 (CXCL10) secretion, respectively. In NOD mice, genetic deletion of FABP4 or depletion of TRM cells using CD69 neutralizing antibodies resulted in a similar reduction of pancreatic cytotoxic T cell recruitment, a delay in diabetic incidence, and a suppression of CXCL10 production. Thus, targeting FABP4 may represent a promising therapeutic strategy for T1D.

Diabetes and the fabkin complex: A dual-edged sword

The Fabkin complex, composed of FABP4, ADK, and NDPKs, emerges as a novel regulator of insulin-producing beta cells, offering promising prospects for diabetes treatment. Our approach, which combines literature review and database analysis, sets the stage for future research. These findings hold significant implications for both diabetes treatment and research, as they present potential therapeutic targets for personalized treatment, leading to enhanced patient outcomes and a deeper comprehension of the disease. The multifaceted role of the Fabkin complex in glucose metabolism, insulin resistance, anti-inflammation, beta cell proliferation, and vascular function underscores its therapeutic potential, reshaping diabetes management and propelling advancements in the field.

The role of fatty acid metabolism in acute lung injury: a special focus on immunometabolism

Reputable evidence from multiple studies suggests that excessive and uncontrolled inflammation plays an indispensable role in mediating, amplifying, and protracting acute lung injury (ALI). Traditionally, immunity and energy metabolism are regarded as separate functions regulated by distinct mechanisms, but recently, more and more evidence show that immunity and energy metabolism exhibit a strong interaction which has given rise to an emerging field of immunometabolism. Mammalian lungs are organs with active fatty acid metabolism, however, during ALI, inflammation and oxidative stress lead to a series metabolic reprogramming such as impaired fatty acid oxidation, increased expression of proteins involved in fatty acid uptake and transport, enhanced synthesis of fatty acids, and accumulation of lipid droplets. In addition, obesity represents a significant risk factor for ALI/ARDS. Thus, we have further elucidated the mechanisms of obesity exacerbating ALI from the perspective of fatty acid metabolism. To sum up, this paper presents a systematical review of the relationship between extensive fatty acid metabolic pathways and acute lung injury and summarizes recent advances in understanding the involvement of fatty acid metabolism-related pathways in ALI. We hold an optimistic believe that targeting fatty acid metabolism pathway is a promising lung protection strategy, but the specific regulatory mechanisms are way too complex, necessitating further extensive and in-depth investigations in future studies.

Endothelial-derived FABP4 constitutes the majority of basal circulating hormone and regulates lipolysis-driven insulin secretion

Fatty acid binding protein 4 (FABP4) is a lipid chaperone secreted from adipocytes upon stimulation of lipolysis. Circulating FABP4 levels strongly correlate with obesity and metabolic pathologies in experimental models and humans. While adipocytes have been presumed to be the major source of hormonal FABP4, this question has not been addressed definitively in vivo. We generated mice with Fabp4 deletion in cells known to express the gene - adipocytes (Adipo-KO), endothelial cells (Endo-KO), myeloid cells (Myeloid-KO), and the whole body (Total-KO) - to examine the contribution of these cell types to basal and stimulated plasma FABP4 levels. Unexpectedly, baseline plasma FABP4 was not significantly reduced in Adipo-KO mice, whereas Endo-KO mice showed ~87% reduction versus WT controls. In contrast, Adipo-KO mice exhibited ~62% decreased induction of FABP4 responses to lipolysis, while Endo-KO mice showed only mildly decreased induction, indicating that adipocytes are the main source of increases in FABP4 during lipolysis. We did not detect any myeloid contribution to circulating FABP4. Surprisingly, despite the nearly intact induction of FABP4, Endo-KO mice showed blunted lipolysis-induced insulin secretion, identical to Total-KO mice. We conclude that the endothelium is the major source of baseline hormonal FABP4 and is required for the insulin response to lipolysis.

Optimization of potent, selective and orally bioavailable biphenyl scaffold as FABP4 inhibitors for anti-inflammation

Fatty-acid binding protein 4 (FABP4) is an essential driver for the progression of metabolic-related inflammatory diseases including obesity, diabetes, atherosclerosis, and various lipid metabolism-related tumors. However, FABP4 inhibitors are not yet available for clinical use, which may be associated with their poor selectivity of FABP3, unsatisfactory efficacy and physicochemical properties. Herein, we reported a systematic optimization of a class of biphenyl scaffold molecules as potent FABP4 inhibitors. Further in vitro and in vivo pharmacokinetic studies identified a selective and orally bioavailable compound 10g, with K_i of 0.51 μM against FABP4, K_i of 33.01 μM against FABP3 and bioavailability F% value of 89.4%. In vivo anti-inflammatory efficacy and multi-organ protection study in LPS-induced inflammatory mice model highlighted the potential of compound 10g as a therapeutic candidate in inflammation-related diseases.

FABP4-mediated lipid droplet formation in Streptococcus uberis-infected macrophages supports host defence

Foamy macrophages containing prominent cytoplasmic lipid droplets (LDs) are found in a variety of infectious diseases. However, their role in Streptococcus uberis-induced mastitis is unknown. Herein, we report that S. uberis infection enhances the fatty acid synthesis pathway in macrophages, resulting in a sharp increase in LD levels, accompanied by a significantly enhanced inflammatory response. This process is mediated by the involvement of fatty acid binding protein 4 (FABP4), a subtype of the fatty acid-binding protein family that plays critical roles in metabolism and inflammation. In addition, FABP4 siRNA inhibitor cell models showed that the deposition of LDs decreased, and the mRNA expression of Tnf, Il1b and Il6 was significantly downregulated after gene silencing. As a result, the bacterial load in macrophages increased. Taken together, these data demonstrate that macrophage LD formation is a host-driven component of the immune response to S. uberis. FABP4 contributes to promoting inflammation via LDs, which should be considered a new target for drug development to treat infections.

Fatty Acid-Binding Protein 4 in Patients with and without Diabetic Retinopathy

BACKGROUND

Fatty acid-binding protein 4 (FABP4) has been demonstrated to be a predictor of early diabetic nephropathy. However, little is known about the relationship between FABP4 and diabetic retinopathy (DR). This study explored the value of FABP4 as a biomarker of DR in patients with type 2 diabetes mellitus (T2DM).

METHODS

A total of 238 subjects were enrolled, including 20 healthy controls and 218 T2DM patients. Serum FABP4 levels were measured using a sandwich enzyme-linked immunosorbent assay. The grade of DR was determined using fundus fluorescence angiography. Based on the international classification of DR, all T2DM patients were classified into the following three subgroups: non-DR group, non-proliferative diabetic retinopathy (NPDR) group, and proliferative diabetic retinopathy (PDR) group. Multivariate logistic regression analyses were employed to assess the correlation between FABP4 levels and DR severity.

RESULTS

FABP4 correlated positively with DR severity (r=0.225, P=0.001). Receiver operating characteristic curve analysis was used to assess the diagnostic potential of FABP4 in identifying DR, with an area under the curve of 0.624 (37% sensitivity, 83.6% specificity) and an optimum cut-off value of 76.4 μg/L. Multivariate logistic regression model including FABP4 as a categorized binary variable using the cut-off value of 76.4 μg/L showed that the concentration of FABP4 above the cut-off value increased the risk of NPDR (odds ratio [OR], 3.231; 95% confidence interval [CI], 1.574 to 6.632; P=0.001) and PDR (OR, 3.689; 95% CI, 1.306 to 10.424; P=0.014).

CONCLUSION

FABP4 may be used as a serum biomarker for the diagnosis of DR.

Pep19 Has a Positive Effect on Insulin Sensitivity and Ameliorates Both Hepatic and Adipose Tissue Phenotype of Diet-Induced Obese Mice

Peptide DIIADDEPLT (Pep19) has been previously suggested to improve metabolic parameters, without adverse central nervous system effects, in a murine model of diet-induced obesity. Here, we aimed to further evaluate whether Pep19 oral administration has anti-obesogenic effects, in a well-established high-fat diet-induced obesity model. Male Swiss mice, fed either a standard diet (SD) or high-fat diet (HFD), were orally administrated for 30 consecutive days, once a day, with saline vehicle or Pep19 (1 mg/kg). Next, several metabolic, morphological, and behavioral parameters were evaluated. Oral administration of Pep19 attenuated HFD body-weight gain, reduced in approximately 40% the absolute mass of the endocrine pancreas, and improved the relationship between circulating insulin and peripheral insulin sensitivity. Pep19 treatment of HFD-fed mice attenuated liver inflammation, hepatic fat distribution and accumulation, and lowered plasma alanine aminotransferase activity. The inguinal fat depot from the SD group treated with Pep19 showed multilocular brown-fat-like cells and increased mRNA expression of uncoupling protein 1 (UCP1), suggesting browning on inguinal white adipose cells. Morphological analysis of brown adipose tissue (BAT) from HFD mice showed the presence of larger white-like unilocular cells, compared to BAT from SD, Pep19-treated SD or HFD mice. Pep19 treatment produced no alterations in mice behavior. Oral administration of Pep19 ameliorates some metabolic traits altered by diet-induced obesity in a Swiss mice model.

Interplay between Fatty Acid Binding Protein 4, Fetuin-A, Retinol Binding Protein 4 and Thyroid Function in Metabolic Dysregulation

Signalling between the tissues integrating synthesis, transformation and utilization of energy substrates and their regulatory hormonal axes play a substantial role in the development of metabolic disorders. Interactions between cytokines, particularly liver derived hepatokines and adipokines, secreted from adipose tissue, constitute one of major areas of current research devoted to metabolic dysregulation. The thyroid exerts crucial influence on the maintenance of basal metabolic rate, thermogenesis, carbohydrate and lipid metabolism, while its dysfunction promotes the development of metabolic disorders. In this review, we discuss the interplay between three adipokines: fatty acid binding protein type 4, fetuin-A, retinol binding protein type 4 and thyroid hormones, that shed a new light onto mechanisms underlying atherosclerosis, cardiovascular complications, obesity, insulin resistance and diabetes accompanying thyroid dysfunction. Furthermore, we summarize clinical findings on those cytokines in the course of thyroid disorders.

TLR4 inhibition ameliorated glucolipotoxicity-induced differentiation suppression in osteoblasts via RIAM regulation of NF-κB nuclear translocation

TLR4 is a key innate immune signal that mediates glucolipid toxicity through yet unclear mechanisms. Here, TLR4 truncation ameliorated bone metabolism disorders in diabetic rats, and the underlying mechanisms were explored by proteomics. Our study showed that TLR4 truncation inhibited bone loss induced by diabetes in rats. In addition, a proteomic analysis screen exposed the differential proteins associated with immune reactivity and T cell activation (RIAM and Class II histocompatibility antigen, M β1 chain). Further cellular experiments showed that TLR4 mediated the inhibition of osteoblast differentiation induced by glucolipotoxicity and promoted an increase in the nuclear level of RIAM-NF-κB. Mechanistic studies showed that TLR4 mediated glucolipotoxicity induced damage in bone metabolism primarily by regulating RIAM-NF-κB interactions, which promoted RIAM-NF-κB nuclear translocation. In conclusion, we confirmed that TLR4 inhibition could delay bone metabolism disorders induced by glycolipid toxicity via RIAM regulation of NF-κB nuclear translocation.

Unveiling the Role of the Fatty Acid Binding Protein 4 in the Metabolic-Associated Fatty Liver Disease

Metabolic-associated fatty liver disease (MAFLD), the main cause of chronic liver disease worldwide, is a progressive disease ranging from fatty liver to steatohepatitis (metabolic-associated steatohepatitis; MASH). Nevertheless, it remains underdiagnosed due to the lack of effective non-invasive methods for its diagnosis and staging. Although MAFLD has been found in lean individuals, it is closely associated with obesity-related conditions. Adipose tissue is the main source of liver triglycerides and adipocytes act as endocrine organs releasing a large number of adipokines and pro-inflammatory mediators involved in MAFLD progression into bloodstream. Among the adipocyte-derived molecules, fatty acid binding protein 4 (FABP4) has been recently associated with fatty liver and additional features of advanced stages of MAFLD. Additionally, emerging data from preclinical studies propose FABP4 as a causal actor involved in the disease progression, rather than a mere biomarker for the disease. Therefore, the FABP4 regulation could be considered as a potential therapeutic strategy to MAFLD. Here, we review the current knowledge of FABP4 in MAFLD, as well as its potential role as a therapeutic target for this disease.

Adipose-Derived Lipid-Binding Proteins: The Good, the Bad and the Metabolic Diseases

Adipose tissue releases a large range of bioactive factors called adipokines, many of which are involved in inflammation, glucose homeostasis and lipid metabolism. Under pathological conditions such as obesity, most of the adipokines are upregulated and considered as deleterious, due to their pro-inflammatory, pro-atherosclerotic or pro-diabetic properties, while only a few are downregulated and would be designated as beneficial adipokines, thanks to their counteracting properties against the onset of comorbidities. This review focuses on six adipose-derived lipid-binding proteins that have emerged as key factors in the development of obesity and diabetes: Retinol binding protein 4 (RBP4), Fatty acid binding protein 4 (FABP4), Apolipoprotein D (APOD), Lipocalin-2 (LCN2), Lipocalin-14 (LCN14) and Apolipoprotein M (APOM). These proteins share structural homology and capacity to bind small hydrophobic molecules but display opposite effects on glucose and lipid metabolism. RBP4 and FABP4 are positively associated with metabolic syndrome, while APOD and LCN2 are ubiquitously expressed proteins with deleterious or beneficial effects, depending on their anatomical site of expression. LCN14 and APOM have been recently identified as adipokines associated with healthy metabolism. Recent findings on these lipid-binding proteins exhibiting detrimental or protective roles in human and murine metabolism and their involvement in metabolic diseases are also discussed.

A-FABP in Metabolic Diseases and the Therapeutic Implications: An Update

Adipocyte fatty acid-binding protein (A-FABP), which is also known as ap2 or FABP4, is a fatty acid chaperone that has been further defined as a fat-derived hormone. It regulates lipid homeostasis and is a key mediator of inflammation. Circulating levels of A-FABP are closely associated with metabolic syndrome and cardiometabolic diseases with imminent diagnostic and prognostic significance. Numerous animal studies have elucidated the potential underlying mechanisms involving A-FABP in these diseases. Recent studies demonstrated its physiological role in the regulation of adaptive thermogenesis and its pathological roles in ischemic stroke and liver fibrosis. Due to its implication in various diseases, A-FABP has become a promising target for the development of small molecule inhibitors and neutralizing antibodies for disease treatment. This review summarizes the clinical and animal findings of A-FABP in the pathogenesis of cardio-metabolic diseases in recent years. The underlying mechanism and its therapeutic implications are also highlighted.

Adipocyte Fatty Acid-Binding Protein, Cardiovascular Diseases and Mortality

It has been increasingly recognized that inflammation plays an important role in the pathogenesis of cardiovascular disease (CVD). In obesity, adipose tissue inflammation, especially in the visceral fat depots, contributes to systemic inflammation and promotes the development of atherosclerosis. Adipocyte fatty acid-binding protein (AFABP), a lipid chaperone abundantly secreted from the adipocytes and macrophages, is one of the key players mediating this adipose-vascular cross-talk, in part via its interaction with c-Jun NH2-terminal kinase (JNK) and activator protein-1 (AP-1) to form a positive feedback loop, and perpetuate inflammatory responses. In mice, selective JNK inactivation in the adipose tissue significantly reduced the expression of AFABP in their adipose tissue, as well as circulating AFABP levels. Importantly, fat transplant experiments showed that adipose-specific JNK inactivation in the visceral fat was sufficient to protect mice with apoE deficiency from atherosclerosis, with the beneficial effects attenuated by the continuous infusion of recombinant AFABP, supporting the role of AFABP as the link between visceral fat inflammation and atherosclerosis. In humans, raised circulating AFABP levels are associated with incident metabolic syndrome, type 2 diabetes and CVD, as well as non-alcoholic steatohepatitis, diabetic nephropathy and adverse renal outcomes, all being conditions closely related to inflammation and enhanced CV mortality. Collectively, these clinical data have provided support to AFABP as an important adipokine linking obesity, inflammation and CVD. This review will discuss recent findings on the role of AFABP in CVD and mortality, the possible underlying mechanisms, and pharmacological inhibition of AFABP as a potential strategy to combat CVD.

Fatty acid-binding protein 4 silencing protects against lipopolysaccharide-induced cardiomyocyte hypertrophy and apoptosis by inhibiting the Toll-like receptor 4-nuclear factor-κB pathway

Objective

To explore the effects and potential mechanisms of fatty acid-binding protein 4 (FABP4) in a lipopolysaccharide (LPS)-induced in vitro septic cardiomyopathy model.

Methods

Rat cardiomyocyte H9c2 cells were transfected with small interfering RNA (siRNA) against FABP4 (siFABP4), then induced with LPS. The following parameters were measured: cell viability, lactate dehydrogenase release, cardiac hypertrophy and related marker expression, apoptosis, inflammatory cytokine release and expression, and the activation of Toll-like receptor 4 (TLR4) and nuclear factor-κB (NF-κB) pathways.

Results

LPS increased the mRNA and protein expression of FABP4 in H9c2 cells. FABP4 silencing by siFABP4 significantly inhibited LPS-induced cardiac hypertrophy and reduced the mRNA expression of the myocardial hypertrophy markers atrial natriuretic peptide and brain natriuretic peptide. siFABP4 also attenuated LPS-induced increase in TUNEL-positive apoptotic cells, caspase-3 and caspase-9 activities, and the release and expression of proinflammatory cytokines. Mechanistically, we found that FABP4 silencing inhibited the mRNA and protein expression of TLR4 and suppressed the NF-kappa B signaling pathway, as evidenced by reduced nuclear NF-κB p65 and increased cytoplasmic I-κBα expression in LPS-stimulated H9c2 cells.

Conclusion

FABP4 silencing reduces LPS-induced cardiomyocyte hypertrophy and apoptosis by down-regulating the TLR4/NF-κB axis.

Lipid chaperones and associated diseases: a group of chaperonopathies defining a new nosological entity with implications for medical research and practice

Fatty acid-binding proteins (FABPs) are lipid chaperones assisting in the trafficking of long-chain fatty acids with functions in various cell compartments, including oxidation, signaling, gene-transcription regulation, and storage. The various known FABP isoforms display distinctive tissue distribution, but some are active in more than one tissue. Quantitative and/or qualitative changes of FABPs are associated with pathological conditions. Increased circulating levels of FABPs are biomarkers of disorders such as obesity, insulin resistance, cardiovascular disease, and cancer. Deregulated expression and malfunction of FABPs can result from genetic alterations or posttranslational modifications and can be pathogenic. We have assembled the disorders with abnormal FABPs as chaperonopathies in a distinct nosological entity. This entity is similar but separate from that encompassing the chaperonopathies pertaining to protein chaperones. In this review, we discuss the role of FABPs in the pathogenesis of metabolic syndrome, cancer, and neurological diseases. We highlight the opportunities for improving diagnosis and treatment that open by encompassing all these pathological conditions within of a coherent nosological group, focusing on abnormal lipid chaperones as biomarkers of disease and etiological-pathogenic factors.

TRB3 stimulates SIRT1 degradation and induces insulin resistance by lipotoxicity via COP1

Fatty acid-induced lipotoxicity plays an important role in the pathogenesis of diabetes mellitus. Our previous studies have documented that lipotoxicity contributes to the onset and development of diabetes via insulin resistance and/or compromised function of the pancreatic β-cells. However, the underlying molecular mechanisms associating lipotoxicity with insulin resistance remain to be fully elucidated. In this study, we explored the role of TRB3-COP1-SIRT1 in lipotoxicity leading to insulin resistance in hepatocytes. High fat diet (HFD)-fed mice and hepG2 cells stimulated with palmitate were utilized as models of lipid metabolism disorders. We analyzed the interactions of SIRT1 and COP1 with each other and with TRB3 using co-immunoprecipitation, western blotting. SIRT1 ubiquitination was also explored. Animal and cell experiments showed that lipotoxicity induced SIRT1 down-regulation at the protein level without altering the mRNA level, whereas, lipotoxicity led to up-regulation of TRB3 and COP1 at both the gene and protein levels. Mechanistic analysis indicated that COP1 functioned as an E3 Ub-ligase of SIRT1, responsible for its proteasomal degradation under lipotoxic conditions. TRB3 recruited COP1 to SIRT1 to promote its ubiquitination. Our data indicated for the first time that TRB3-COP1-SIRT1 pathway played an important role in lipotoxicity leading to insulin resistance in hepatocytes, and suggested that COP1 could be a potential therapeutic choice for the treatment of diabetes mellitus, with lipotoxicity being the important pathomechanism.

Macrophage FABP4 is required for neutrophil recruitment and bacterial clearance in Pseudomonas aeruginosa pneumonia

Fatty acid binding protein 4 (FABP4), an intracellular lipid chaperone and adipokine, is expressed by lung macrophages, but the function of macrophage-FABP4 remains elusive. We investigated the role of FABP4 in host defense in a murine model of Pseudomonas aeruginosa pneumonia. Compared with wild-type (WT) mice, FABP4-deficient (FABP4-/-) mice exhibited decreased bacterial clearance and increased mortality when challenged intranasally with P. aeruginosa. These findings in FABP4-/- mice were associated with a delayed neutrophil recruitment into the lungs and were followed by greater acute lung injury and inflammation. Among leukocytes, only macrophages expressed FABP4 in WT mice with P. aeruginosa pneumonia. Chimeric FABP4-/- mice with WT bone marrow were protected from increased mortality seen in chimeric WT mice with FABP4-/- bone marrow during P. aeruginosa pneumonia, thus confirming the role of macrophages as the main source of protective FABP4 against that infection. There was less production of C-X-C motif chemokine ligand 1 (CXCL1) in FABP4-/- alveolar macrophages and lower airway CXCL1 levels in FABP4-/- mice. Delivering recombinant CXCL1 to the airways protected FABP4-/- mice from increased susceptibility to P. aeruginosa pneumonia. Thus, macrophage-FABP4 has a novel role in pulmonary host defense against P. aeruginosa infection by facilitating crosstalk between macrophages and neutrophils via regulation of macrophage CXCL1 production.-Liang, X., Gupta, K., Rojas Quintero, J., Cernadas, M., Kobzik, L., Christou, H., Pier, G. B., Owen, C. A., Çataltepe, S. Macrophage FABP4 is required for neutrophil recruitment and bacterial clearance in Pseudomonas aeruginosa pneumonia.

Fatty Acid-Binding Protein 4 in Cardiovascular and Metabolic Diseases

Fatty acid-binding proteins (FABPs), a family of lipid chaperones, contribute to systemic metabolic regulation via several lipid signaling pathways. Fatty acid-binding protein 4 (FABP4), known as adipocyte FABP (A-FABP) or aP2, is mainly expressed in adipocytes and macrophages and plays important roles in the development of insulin resistance and atherosclerosis in relation to metabolically driven low-grade and chronic inflammation, referred to as ‘metaflammation’. FABP4 is secreted from adipocytes in a non-classical pathway associated with lipolysis and acts as an adipokine for the development of insulin resistance and atherosclerosis. Circulating FABP4 levels are associated with several aspects of metabolic syndrome and cardiovascular disease. Ectopic expression and function of FABP4 in cells and tissues are also related to the pathogenesis of several diseases. Pharmacological modification of FABP4 function by specific inhibitors, neutralizing antibodies or antagonists of unidentified receptors would be novel therapeutic strategies for several diseases, including obesity, diabetes mellitus, atherosclerosis and cardiovascular disease. Significant roles of FABP4 as a lipid chaperone in physiological and pathophysiological conditions and the possibility of FABP4 being a therapeutic target for metabolic and cardiovascular diseases are discussed in this review.

Effect of Low-Fat Diet in Obese Mice Lacking Toll-like Receptors

Background: This study aimed at assessing the effect of a low-fat diet (LFD) in obese mice lacking toll–like receptors (Tlr) and understanding the expression and regulation of microRNAs during weight reduction. Methods: C57BL/6, Tlr5^−/−, Tlr2^−/− and Tlr4^−/− mice were used in this study. A group of mice were fed with a high-fat diet (HFD) (58% kcal) for 12 weeks to induce obesity (diet-induced obesity, DIO). Another group that had been fed with HFD for eight weeks (obese mice) were switched to a low-fat diet (LFD) (10.5% kcal) for the next four weeks to reduce their body weight. The control mice were fed with a standard AIN-76A diet for the entire 12 weeks. The body weight of the mice was measured weekly. At the end of the experiment, epididymal fat weight and adipocyte size were measured. The differentially expressed miRNAs in the fat tissue was determined by next-generation sequencing with real-time quantitative reverse transcription polymerase chain reaction (RT–qPCR). Target prediction and functional annotation of miRNAs were performed using miRSystem database. Results: Switching to LFD significantly reduced the body weight and epididymal fat mass in the HFD-fed C57BL/6 and Tlr5^−/− mice but not in Tlr2^−/− and Tlr4^−/− mice. Weight reduction significantly decreased the size of adipocytes in C57BL/6 but not in the Tlr knockout mice. In Tlr2^−/− and Tlr4^−/− mice, feeding with HFD and the subsequent weight reduction resulted in an aberrant miRNA expression in the epididymal fat tissue unlike in C57BL/6 and Tlr5^−/−. However, target prediction and functional annotation by miRSystem database revealed that all the top 10 Kyoto Encyclopedia of Genes and Genomes (KEGG) database pathways of the dysregulated miRNAs during weight reduction in the C57BL/6 mice were also found in the regulated pathways of Tlr5^−/−, Tlr2^−/−, and Tlr4^−/− strains. However, among these pathways, gene sets involved in arginine and proline metabolism and glutathione metabolism were mainly involved in the Tlr knockout mice but not in the C57BL/6 mice. Conclusions: In this study, we demonstrated that feeding of LFD leads to significant body weight reduction in C57BL/6 and Tlr5^−/− mice, but not in Tlr2^−/− and Tlr4^−/− mice. Significant reduction in the size of adipocytes of epididymal fat was only found in C57BL/6, but not in Tlr5^−/−, Tlr2^−/−, and Tlr4^−/− mice. The dysregulated miRNAs in Tlr2^−/− and Tlr4^−/− mice were different from those in C57BL/6 and Tlr5^−/− strains. Among those miRNA-regulated pathways, arginine and proline metabolism as well as glutathione metabolism may have important roles in the Tlr knockout mice rather than in C57BL/6 mice.

FABP4 regulates eosinophil recruitment and activation in allergic airway inflammation

Fatty acid binding protein 4 (FABP4), a member of a family of lipid-binding proteins, is known to play a role in inflammation by virtue of its ability to regulate intracellular events such as lipid fluxes and signaling. Studies have indicated a proinflammatory role for FABP4 in allergic asthma although its expression and function in eosinophils, the predominant inflammatory cells recruited to allergic airways, were not investigated. We examined expression of FABP4 in murine eosinophils and its role in regulating cell recruitment in vitro as well as in cockroach antigen (CRA)-induced allergic airway inflammation. CRA exposure led to airway recruitment of FABP4-expressing inflammatory cells, specifically eosinophils, in wild-type (WT) mice. FABP4 expression in eosinophils was induced by TNF-α as well as IL-4 and IL-13. FABP4-deficient eosinophils exhibited markedly decreased cell spreading/formation of leading edges on vascular cell adhesion molecule-1 and significantly decreased adhesion to intercellular adhesion molecule-1 associated with reduced β2-integrin expression relative to WT cells. Furthermore, FABP4-deficient eosinophils exhibited decreased migration, F-actin polymerization, calcium flux, and ERK(1/2) phosphorylation in response to eotaxin-1. In vivo, CRA-challenged FABP4-deficient mice exhibited attenuated eosinophilia and significantly reduced airway inflammation (improved airway reactivity, lower IL-5, IL-13, TNF-α, and cysteinyl leukotriene C4 levels, decreased airway structural changes) compared with WT mice. In conclusion, expression of FABP4 in eosinophils is induced during conditions of inflammation and plays a proinflammatory role in the development of allergic asthma by promoting eosinophil adhesion and migration and contributing to the development of various aspects of airway inflammation.

Lychee (Litchi chinensis Sonn.) peel flour: effects on hepatoprotection and dyslipidemia induced by a hypercholesterolemic diet

Dyslipidemias are associated with the incidence of cardiovascular diseases, obesity, diabetes, hypertension and hepatic steatosis, being the cause of morbidity and mortality. This study investigated the effects of lychee peel flour (PF) on serum levels of total cholesterol (TC), low-density lipoprotein (LDL-c), triacylglycerols (TAG) and various parameters related to obesity, in rats fed a hypercholesterolemic diet. Therefore, 20 male rats were used. In the first 21 days, the animals were fed a hypercholesterolemic diet, except for control group. In the following 21 days, their diets were modified, and they received a standard diet (Control); hypercholesterolemic (Hyper); hypercholesterolemic + 5% PF (PF5) and hypercholesterolemic + 10% PF (PF10). The results revealed that PF intake attenuated weight gain, reduced body mass index, glucose and the levels of TAG, TC, LDL-c, hepatic enzymes and leptin, besides the percentage of hepatic lipids, liver lipid peroxidation and frequency of severe steatosis. Histological studies of the aorta did not show the formation of the atheromatous plaque. These results reinforce its potential to reduce the risk of diseases associated with obesity.

Role of the fatty acid-binding protein 4 in heart failure and cardiovascular disease

Obesity and ectopic fat accumulation in non-adipose tissues are major contributors to heart failure (HF) and cardiovascular disease (CVD). Adipocytes act as endocrine organs by releasing a large number of bioactive molecules into the bloodstream, which participate in a communication network between white adipose tissue and other organs, including the heart. Among these molecules, fatty acid-binding protein 4 (FABP4) has recently been shown to increase cardiometabolic risk. Both clinical and experimental evidence have identified FABP4 as a relevant player in atherosclerosis and coronary artery disease, and it has been directly related to cardiac alterations such as left ventricular hypertrophy (LVH) and both systolic and diastolic cardiac dysfunction. The available interventional studies preclude the establishment of a direct causal role of this molecule in CVD and HF and propose FABP4 as a biomarker rather than as an aetiological factor. However, several experimental reports have suggested that FABP4 may act as a direct contributor to cardiac metabolism and physiopathology, and the pharmacological targeting of FABP4 may restore some of the metabolic alterations that are conducive to CVD and HF. Here, we review the current knowledge regarding FABP4 in the context of HF and CVD as well as the molecular basis by which this protein participates in the regulation of cardiac function.

Metabolic functions of FABPs--mechanisms and therapeutic implications

Intracellular and extracellular interactions with proteins enables the functional and mechanistic diversity of lipids. Fatty acid-binding proteins (FABPs) were originally described as intracellular proteins that can affect lipid fluxes, metabolism and signalling within cells. As the functions of this protein family have been further elucidated, it has become evident that they are critical mediators of metabolism and inflammatory processes, both locally and systemically, and therefore are potential therapeutic targets for immunometabolic diseases. In particular, genetic deficiency and small molecule-mediated inhibition of FABP4 (also known as aP2) and FABP5 can potently improve glucose homeostasis and reduce atherosclerosis in mouse models. Further research has shown that in addition to their intracellular roles, some FABPs are found outside the cells, and FABP4 undergoes regulated, vesicular secretion. The circulating form of FABP4 has crucial hormonal functions in systemic metabolism. In this Review we discuss the roles and regulation of both intracellular and extracellular FABP actions, highlighting new insights that might direct drug discovery efforts and opportunities for management of chronic metabolic diseases.