FABP4 is secreted from adipocytes by adenyl cyclase-PKA- and guanylyl cyclase-PKG-dependent lipolytic mechanisms

Postprandial fatty acid-binding protein 4 is associated with muscle insulin resistance

AIMS/HYPOTHESIS

Fatty acid-binding protein 4 (FABP4) has been reported to act as a hepatic insulin resistance factor. We previously reported that fasting FABP4 was correlated with insulin resistance measurements derived from the glucose clamp, and another study reported that postprandial FABP4 levels were decreased in healthy volunteers but were not reported (or known) in participants with type 2 diabetes. We have limited knowledge about the direct effect of FABP4 on muscle cells. We investigated the postprandial FABP4 levels in participants with type 2 diabetes, and the basic mechanism of muscle insulin resistance and FABP4.

METHODS

We performed a meal tolerance test and hyperinsulinaemic-euglycaemic clamp in 22 participants with type 2 diabetes and 26 participants without diabetes. We measured fasting and postprandial serum FABP4. We cultured mouse C2C12 muscle cells, and investigated the effect of FABP4 on glucose uptake. We analysed insulin signalling by western blot and insulin binding assay.

RESULTS

The postprandial FABP4 level in participants with type 2 diabetes was higher than that in participants without diabetes. Participants without diabetes had lower postprandial FABP4 than fasting except for one participant, whereas one-third of participants with type 2 diabetes had higher postprandial FABP4 than fasting. Postprandial FABP4 was correlated with the muscle insulin resistance M/I value from a glucose clamp in participants without diabetes (r=-0.42, p<0.05). The increase in FABP4 after a meal correlated with the muscle insulin resistance M/I value (r=-0.44, p<0.05) and the difference between fasting and postprandial glucagon in participants with type 2 diabetes (r=0.36, p<0.05). FABP4 alone appears to increase glucose uptake, and the combination of FABP4 and insulin decreases glucose uptake when compared with insulin alone. FABP4 inhibits insulin signalling of muscle cells through decreases in phosphorylation of insulin receptor substrate 1 and Akt. The physiological concentration of FABP4 did not inhibit insulin binding to muscle cells.

CONCLUSIONS/INTERPRETATION

These results suggested that the postprandial FABP4 level is associated with insulin resistance, and FABP4 may suppress insulin signals.

Self-Assembled Oligopeptoplex-Loaded Dissolving Microneedles for Adipocyte-Targeted Anti-Obesity Gene Therapy

Advancements in gene delivery systems are pivotal for gene-based therapeutics in oncological, inflammatory, and infectious diseases. This study delineates the design of a self-assembled oligopeptoplex (SA-OP) optimized for shRNA delivery to adipocytes, targeting obesity and associated metabolic syndromes. Conventional systems face challenges, including instability due to electrostatic interactions between genetic materials and cationic oligopeptides. Additionally, repeated injections induce discomfort and compromise patient well-being. To circumvent these issues, a dissolvable hyaluronic acid-based, self-locking microneedle (LMN) patch is developed, with improved micro-dose efficiency, for precise SA-OP delivery. This platform offers pain-free administration and improved SA-OP storage stability. In vitro studies in 3T3-L1 cells demonstrated improvements in SA-OP preservation and gene silencing efficacy. In vivo evaluation in a mice model of diet-induced type 2 diabetes yielded significant gene silencing in adipose tissue and a 21.92 ± 2.51% reduction in body weight with minimum relapse risk at 6-weeks post-treatment, representing a superior therapeutic efficacy in a truncated timeframe relative to the GLP-1 analogues currently available on the market. Additionally, SA-OP (LMN) mitigated insulin resistance, inflammation, and hepatic steatosis. These findings establish SA-OP (LMN) as a robust, minimally invasive transdermal gene delivery platform with prolonged storage stability for treating obesity and its metabolic comorbidities.

Carbohydrate ingestion does not suppress increases in fatty acid-binding protein 4 concentrations post-acute aerobic exercise in healthy men: a randomized crossover study

BACKGROUND

Fatty acid-binding protein 4 (FABP4) has been associated with cardiovascular disease and diabetes. Acute aerobic exercise increases circulating FABP4 concentrations, but the underlying mechanisms remain unclear. The purpose of this study was to investigate the effects of inhibition of lipolysis by carbohydrate ingestion on circulating FABP4 concentrations during and after acute aerobic exercise in healthy men.

METHODS

Men aged between 20 and 40, with no exercise habits and no metabolic diseases, were recruited. In a randomized crossover design, the participants underwent a carbohydrate-ingestion exercise (CE) and a fasted exercise (FE) trial. The CE trial consisted of 40-min acute aerobic exercise with ingestion of carbohydrates and 60-min bed rest. The FE trial followed the same protocol as the CE trial but without carbohydrate ingestion. Venous blood samples were collected to measure hormones (adrenaline, noradrenaline, and insulin) metabolites (glycerol, free fatty acids, and glucose), and FABP4 concentrations. Ventilation and gas exchange were also collected to measure substrate oxidation.

RESULTS

Thirteen healthy men participated in and completed both the CE and FE trials. The insulin concentration was more than 4 times higher in the CE trial than in the FE trial (p < 0.004, effect size [ES] > 2.00). Free fatty acid concentrations were more than 4 times lower in the CE trial than in the FE trial (p < 0.02, ES > 2.04). However, there was no significant difference in the changes in circulating FABP4 concentrations between the CE and FE trials (p = 0.108), which did not change during aerobic exercise and significantly increased post-aerobic exercise in both trials (p < 0.002, ES > 1.212). Changes in FABP4 concentrations following aerobic exercise were not significantly correlated with changes in glycerol or free fatty acid concentrations during aerobic exercise.

CONCLUSIONS

The results suggest that suppression of lipolysis and elevation of insulin are not strongly involved in increases in FABP4 secretion following acute aerobic exercise.

p21-activated kinase 4 counteracts PKA-dependent lipolysis by phosphorylating FABP4 and HSL

Adipose tissue lipolysis is mediated by cAMP-protein kinase A (PKA)-dependent intracellular signalling. Here, we show that PKA targets p21-activated kinase 4 (PAK4), leading to its protein degradation. Adipose tissue-specific overexpression of PAK4 in mice attenuates lipolysis and exacerbates diet-induced obesity. Conversely, adipose tissue-specific knockout of Pak4 or the administration of a PAK4 inhibitor in mice ameliorates diet-induced obesity and insulin resistance while enhancing lipolysis. Pak4 knockout also increases energy expenditure and adipose tissue browning activity. Mechanistically, PAK4 directly phosphorylates fatty acid-binding protein 4 (FABP4) at T126 and hormone-sensitive lipase (HSL) at S565, impairing their interaction and thereby inhibiting lipolysis. Levels of PAK4 and the phosphorylation of FABP4-T126 and HSL-S565 are enhanced in the visceral fat of individuals with obesity compared to their lean counterparts. In summary, we have uncovered an important role for FABP4 phosphorylation in regulating adipose tissue lipolysis, and PAK4 inhibition may offer a therapeutic strategy for the treatment of obesity.

Fatty Acid-Binding Proteins Identification during the Evolution of Metabolic Syndrome: A Raman Spectroscopy-Based Approach

Excess fat in abdominal deposits is a risk factor for multiple conditions, including metabolic syndrome (MetS); lipid metabolism plays an essential role in these pathologies; fatty acid-binding proteins (FABPs) are dedicated to the cytosolic transport of fat. FABP4, whose primary source is adipose tissue, is released into the circulation, acting as an adipokine, while FABP5 also accompanies the adverse effects of MetS. FABP4 and 5 are potential biomarkers of MetS, but their behavior during syndrome evolution has not been determined. Raman spectroscopy has been applied as an alternative method to disease biomarker detection. In this work, we detected spectral changes related to FABP4 and 5 in the serum at different points of time, using an animal model of a high-fat diet-induced MetS. FABP4 and 5 spectral changes show a contribution during the evolution of MetS, which indicates alteration to a molecular level that predisposes to established MetS. These findings place FABPs as potential biomarkers of MetS and Raman spectroscopy as an alternative method for MetS assessment.

The Multifunctional Family of Mammalian Fatty Acid-Binding Proteins

Fatty acid-binding proteins (FABPs) are small lipid-binding proteins abundantly expressed in tissues that are highly active in fatty acid (FA) metabolism. Ten mammalian FABPs have been identified, with tissue-specific expression patterns and highly conserved tertiary structures. FABPs were initially studied as intracellular FA transport proteins. Further investigation has demonstrated their participation in lipid metabolism, both directly and via regulation of gene expression, and in signaling within their cells of expression. There is also evidence that they may be secreted and have functional impact via the circulation. It has also been shown that the FABP ligand binding repertoire extends beyond long-chain FAs and that their functional properties also involve participation in systemic metabolism. This article reviews the present understanding of FABP functions and their apparent roles in disease, particularly metabolic and inflammation-related disorders and cancers.

Circulating Concentrations of Cathelicidin Anti-Microbial Peptide (CAMP) Are Increased during Oral Glucose Tolerance Test

Recent investigation has revealed the significant role of Cathelicidin antimicrobial peptide (CAMP) in infection defense and innate immunity processes in adipose tissue. Meanwhile, knowledge of its regulation and functions in metabolic contexts as an adipokine remains sparce. The present study investigated the postprandial regulation of circulating CAMP levels during oral glucose tolerance tests (OGTTs). Eighty-six metabolically healthy volunteers participated in a standardized 75 g-2 h-OGTT setting. The effects of exogenous glucose, insulin, and incretins on CAMP expression in human adipocyte culture (cell-line SGBS) were studied in vitro. CAMP concentrations in blood serum samples were measured by ELISA techniques and adipocyte gene expression levels were quantified by real-time PCR. Of note, base-line CAMP serum quantities were negatively correlated with HDL cholesterol levels as well as with the anti-inflammatory adipokine adiponectin. During the 2 h following glucose ingestion, a significant rise in circulating CAMP concentrations was observed in considerable contrast to reduced quantities of fatty acid binding proteins (FABP) 2 and 4 and dipeptidyl peptidase 4 (DPP4). In SGBS adipocytes, neither differing glucose levels nor insulin or incretin treatment significantly induced CAMP mRNA levels. According to our data, glucose represents a positive postprandial regulator of systemic CAMP. This effect apparently is not mediated by the regulatory impact of glucose metabolism on adipocyte CAMP expression.

Sympathetic tone dictates the impact of lipolysis on FABP4 secretion

Levels of circulating fatty acid binding protein 4 (FABP4) protein are strongly associated with obesity and metabolic disease in both mice and humans, and secretion is stimulated by β-adrenergic stimulation both in vivo and in vitro. Previously, lipolysis-induced FABP4 secretion was found to be significantly reduced upon pharmacological inhibition of adipose triglyceride lipase (ATGL) and was absent from adipose tissue explants from mice specifically lacking ATGL in their adipocytes (ATGLAdpKO). Here, we find that upon activation of β-adrenergic receptors in vivo, ATGLAdpKO mice unexpectedly exhibited significantly higher levels of circulating FABP4 as compared with ATGLfl/fl controls, despite no corresponding induction of lipolysis. We generated an additional model with adipocyte-specific deletion of both FABP4 and ATGL (ATGL/FABP4AdpKO) to evaluate the cellular source of this circulating FABP4. In these animals, there was no evidence of lipolysis-induced FABP4 secretion, indicating that the source of elevated FABP4 levels in ATGLAdpKO mice was indeed from the adipocytes. ATGLAdpKO mice exhibited significantly elevated corticosterone levels, which positively correlated with plasma FABP4 levels. Pharmacological inhibition of sympathetic signaling during lipolysis using hexamethonium or housing mice at thermoneutrality to chronically reduce sympathetic tone significantly reduced FABP4 secretion in ATGLAdpKO mice compared with controls. Therefore, activity of a key enzymatic step of lipolysis mediated by ATGL, per se, is not required for in vivo stimulation of FABP4 secretion from adipocytes, which can be induced through sympathetic signaling.

Tisochrysis lutea F&M-M36 Mitigates Risk Factors of Metabolic Syndrome and Promotes Visceral Fat Browning through β3-Adrenergic Receptor/UCP1 Signaling

Pre-metabolic syndrome (pre-MetS) may represent the best transition phase to start treatments aimed at reducing cardiometabolic risk factors of MetS. In this study, we investigated the effects of the marine microalga Tisochrysis lutea F&M-M36 (T. lutea) on cardiometabolic components of pre-MetS and its underlying mechanisms. Rats were fed a standard (5% fat) or a high-fat diet (20% fat) supplemented or not with 5% of T. lutea or fenofibrate (100 mg/Kg) for 3 months. Like fenofibrate, T. lutea decreased blood triglycerides (p < 0.01) and glucose levels (p < 0.01), increased fecal lipid excretion (p < 0.05) and adiponectin (p < 0.001) without affecting weight gain. Unlike fenofibrate, T. lutea did not increase liver weight and steatosis, reduced renal fat (p < 0.05), diastolic (p < 0.05) and mean arterial pressure (p < 0.05). In visceral adipose tissue (VAT), T. lutea, but not fenofibrate, increased the β3-adrenergic receptor (β3ADR) (p < 0.05) and Uncoupling protein 1 (UCP-1) (p < 0.001) while both induced glucagon-like peptide-1 receptor (GLP1R) protein expression (p < 0.001) and decreased interleukin (IL)-6 and IL-1β gene expression (p < 0.05). Pathway analysis on VAT whole-gene expression profiles showed that T. lutea up-regulated energy-metabolism-related genes and down-regulated inflammatory and autophagy pathways. The multitarget activity of T. lutea suggests that this microalga could be useful in mitigating risk factors of MetS.

Glomerular expression and urinary excretion of fatty acid-binding protein 4 in IgA nephropathy

BACKGROUND

Fatty acid-binding protein 4 (FABP4) is secreted from adipocytes and macrophages in adipose tissue and acts as an adipokine. It has recently been reported that FABP4, but not liver-type FABP (L-FABP/FABP1), is also expressed in injured glomerular endothelial cells and infiltrating macrophages in the glomerulus and that urinary FABP4 (U-FABP4) is associated with proteinuria and kidney function impairment in nephrotic patients. However, the link between glomerular FABP4 and U-FABP4 has not been fully addressed in IgA nephropathy (IgAN).

METHODS

We investigated the involvement of FABP4 in human and mouse IgAN.

RESULTS

In patients with IgAN (n = 23), the ratio of FABP4-positive area to total area within glomeruli (G-FABP4-Area) and U-FABP4 were positively correlated with proteinuria and were negatively correlated with eGFR. In 4-28-week-old male grouped ddY mice, a spontaneous IgAN-prone mouse model, FABP4 was detected in glomerular endothelial cells and macrophages, and G-FABP4-Area was positively correlated with urinary albumin-to-creatinine ratio (r = 0.957, P < 0.001). Endoplasmic reticulum stress markers were detected in glomeruli of human and mouse IgAN. In human renal glomerular endothelial cells, FABP4 was induced by treatment with vascular endothelial growth factor and was secreted from the cells. Treatment of human renal glomerular endothelial cells or mouse podocytes with palmitate-bound recombinant FABP4 significantly increased gene expression of inflammatory cytokines and endoplasmic reticulum stress markers, and the effects of FABP4 in podocytes were attenuated in the presence of an anti-FABP4 antibody.

CONCLUSION

FABP4 in the glomerulus contributes to proteinuria in IgAN, and U-FABP4 level is a useful surrogate biomarker for glomerular damage in IgAN.

FABP4 Expression in Subcutaneous Adipose Tissue Is Independently Associated with Circulating Triglycerides in Obesity

Hypertriglyceridemia (HTG) has been associated with an increased risk of pancreatitis and cardiovascular disease. Adipose tissue plays a major role in lipid metabolism, mobilization and distribution. We have compared the histological and transcriptomic profiles of the subcutaneous (SAT) and visceral (VAT) adipose tissues from subjects with severe obesity undergoing bariatric surgery with (Ob-HTG, n = 37) and without HTG (Ob-NTG, n = 67). Mean age and BMI were 51.87 ± 11.21 years, 45.78 ± 6.96 kg/m² and 50.03 ± 10.17 years, 44.04 ± 4.69 kg/m², respectively. The Ob-HTG group showed higher levels of glycosylated hemoglobin, fasting plasma glucose, high-sensitivity C-reactive protein and prevalence of hypertension. The degree of fibrosis was increased by 14% in SAT from the Ob-HTG group (p = 0.028), while adipocyte size distribution was comparable. Twenty genes were found differentially expressed in SAT and VAT between study groups. Among them, only SAT expression of FABP4 resulted significantly associated with circulating triglyceride levels after adjusting for other covariates and independently explained 5% of the variance in triglyceride levels in the combined model. This relationship was not found in the cohort of lean or overweight patients with normotriglyceridemia (non-Ob, n = 21). These results emphasize the contribution of SAT to triglyceride concentrations in obesity and indicate that FABP4 may be a potential drug target for the treatment of HTG.

Approaches to Measuring the Activity of Major Lipolytic and Lipogenic Enzymes In Vitro and Ex Vivo

Since the 1950s, one of the goals of adipose tissue research has been to determine lipolytic and lipogenic activity as the primary metabolic pathways affecting adipocyte health and size and thus representing potential therapeutic targets for the treatment of obesity and associated diseases. Nowadays, there is a relatively large number of methods to measure the activity of these pathways and involved enzymes, but their applicability to different biological samples is variable. Here, we review the characteristics of mean lipogenic and lipolytic enzymes, their inhibitors, and available methodologies for assessing their activity, and comment on the advantages and disadvantages of these methodologies and their applicability in vivo, ex vivo, and in vitro, i.e., in cells, organs and their respective extracts, with the emphasis on adipocytes and adipose tissue.

Insulin prevents fatty acid induced increase of adipocyte size

Metabolic disorders related to obesity are largely dependent on adipose tissue hypertrophy, which involves adipocyte hypertrophy and increased adipogenesis. Adiposize is regulated by lipid accumulation as a result of increased lipogenesis (mainly lipid uptake in mature adipocytes) and reduced lipolysis. Using realtime 2D cell culture analyses of lipid uptake, we show (1) that high glucose concentration (4.5 g/L) was required to accumulate oleic acid increasing lipid droplet size until unilocularization similar to mature adipocytes in few days, (2) oleic acid reduced Peroxisome-Proliferator Activated Receptor Gamma (PPARG) gene transcription and (3) insulin counteracted oleic acid-induced increase of lipid droplet size. Although the lipolytic activity observed in high versus low glucose (1 g/L) conditions was not altered, insulin was found to inhibit oleic acid induced gene transcription required for lipid storage such as Cell Death Inducing DFFA Like Effectors (CIDEC) and G0S2 (G0 switch gene S2), possibly through PPARA activity. Although this signalling pathway requires more detailed investigation, the results point out the differential mechanisms involved in the pro-adipogenic effect of insulin in absence versus its protective effect on adiposity in presence of oleic acid uptake.

Abbreviations: AICAR, 5-Aminoimidazole-4-carboxamide-1-D-ribofuranoside; AMPK, AMP-Activated protein kinase, ASCs, adipose stem cell; ATGL, adipose triglyceride lipase; BSA, Bovine serum albumin; CEBPA, CCAAT enhancer binding protein alpha; CIDEs, Cell Death Inducing DFFA Like Effectors; dA, differentiated adipocyte; DMEM, Dulbecco’s Modified Eagle’s Medium; FABPs, Fatty Acid Binding Proteins; FAT/CD36, Fatty acid translocase; FCS, Foetal calf serum; FN1, fibronectin 1; FFA, free fatty acid; G0S2, G0 switch gene S2; GLUTs, Glucose transporters; GPR120, G protein-coupled receptor 120; HG, high glucose; HSL, hormone sensitive lipase; INSR, insulin receptor; LG, low glucose; OA, oleic acid; PBS, Phosphate buffer saline; PPARs, Peroxisome-Proliferator Activated Receptors; PKA, Protein kinase cyclic AMP-dependent; PKG, Protein kinase cyclic GMP dependent; PTGS2, cytochrome oxidase 2; RTCA, realtime cell analysis; TG, triglyceride.

The Molecular Brakes of Adipose Tissue Lipolysis

Adaptation to changes in energy availability is pivotal for the survival of animals. Adipose tissue, the body’s largest reservoir of energy and a major source of metabolic fuel, exerts a buffering function for fluctuations in nutrient availability. This functional plasticity ranges from energy storage in the form of triglycerides during periods of excess energy intake to energy mobilization via lipolysis in the form of free fatty acids for other organs during states of energy demands. The subtle balance between energy storage and mobilization is important for whole-body energy homeostasis; its disruption has been implicated as contributing to the development of insulin resistance, type 2 diabetes and cancer cachexia. As a result, adipocyte lipolysis is tightly regulated by complex regulatory mechanisms involving lipases and hormonal and biochemical signals that have opposing effects. In thermogenic brown and brite adipocytes, lipolysis stimulation is the canonical way for the activation of non-shivering thermogenesis. Lipolysis proceeds in an orderly and delicately regulated manner, with stimulation through cell-surface receptors via neurotransmitters, hormones, and autocrine/paracrine factors that activate various intracellular signal transduction pathways and increase kinase activity. The subsequent phosphorylation of perilipins, lipases, and cofactors initiates the translocation of key lipases from the cytoplasm to lipid droplets and enables protein-protein interactions to assemble the lipolytic machinery on the scaffolding perilipins at the surface of lipid droplets. Although activation of lipolysis has been well studied, the feedback fine-tuning is less well appreciated. This review focuses on the molecular brakes of lipolysis and discusses some of the divergent fine-tuning strategies in the negative feedback regulation of lipolysis, including delicate negative feedback loops, intermediary lipid metabolites-mediated allosteric regulation and dynamic protein–protein interactions. As aberrant adipocyte lipolysis is involved in various metabolic diseases and releasing the brakes on lipolysis in thermogenic adipocytes may activate thermogenesis, targeting adipocyte lipolysis is thus of therapeutic interest.

Fatty acid-binding protein 4: a key regulator of ketoacidosis in new-onset type 1 diabetes

AIMS/HYPOTHESIS

Fatty acid-binding protein 4 (FABP4) is an adipokine with a key regulatory role in glucose and lipid metabolism. We prospectively evaluated the role of FABP4 in the pathophysiology of diabetic ketoacidosis (DKA) in new-onset type 1 diabetes.

METHODS

Clinical and laboratory data were prospectively collected from consecutive children presenting with new-onset type 1 diabetes. In addition to blood chemistry and gases, insulin, C-peptide, serum FABP4 and NEFA were collected upon presentation and 48 h after initiation of insulin treatment. In a mouse model of type 1 diabetes, glucose, insulin, β-hydroxybutyrate and weight were compared between FABP4 knockout (Fabp4^-/-) and wild-type (WT) mice.

RESULTS

Included were 33 children (mean age 9.3 ± 3.5 years, 52% male), of whom 14 (42%) presented with DKA. FABP4 levels were higher in the DKA group compared with the non-DKA group (median [IQR] 10.1 [7.9-14.2] ng/ml vs 6.3 [3.9-7] ng/ml, respectively; p = 0.005). The FABP4 level was positively correlated with HbA_1c at presentation and inversely correlated with venous blood pH and bicarbonate levels (p < 0.05 for all). Following initiation of insulin therapy, a marked reduction in FABP4 was observed in all children. An FABP4 level of 7.22 ng/ml had a sensitivity of 86% and a specificity of 78% for the diagnosis of DKA, with an area under the receiver operating characteristic curve of 0.78 (95% CI 0.6, 0.95; p = 0.008). In a streptozotocin-induced diabetes mouse model, Fabp4^-/- mice exhibited marked hypoinsulinaemia and hyperglycaemia similar to WT mice but displayed no significant increase in β-hydroxybutyrate and were protected from ketoacidosis.

CONCLUSIONS/INTERPRETATION

FABP4 is suggested to be a necessary regulator of ketogenesis in insulin-deficient states.

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.

Adipokines, Hepatokines and Myokines: Focus on Their Role and Molecular Mechanisms in Adipose Tissue Inflammation

Chronic low-grade inflammation in adipose tissue (AT) is a hallmark of obesity and contributes to various metabolic disorders, such as type 2 diabetes and cardiovascular diseases. Inflammation in ATs is characterized by macrophage infiltration and the activation of inflammatory pathways mediated by NF-κB, JNK, and NLRP3 inflammasomes. Adipokines, hepatokines and myokines - proteins secreted from AT, the liver and skeletal muscle play regulatory roles in AT inflammation via endocrine, paracrine, and autocrine pathways. For example, obesity is associated with elevated levels of pro-inflammatory adipokines (e.g., leptin, resistin, chemerin, progranulin, RBP4, WISP1, FABP4, PAI-1, Follistatin-like1, MCP-1, SPARC, SPARCL1, and SAA) and reduced levels of anti-inflammatory adipokines such as adiponectin, omentin, ZAG, SFRP5, CTRP3, vaspin, and IL-10. Moreover, some hepatokines (Fetuin A, DPP4, FGF21, GDF15, and MANF) and myokines (irisin, IL-6, and DEL-1) also play pro- or anti-inflammatory roles in AT inflammation. This review aims to provide an updated understanding of these organokines and their role in AT inflammation and related metabolic abnormalities. It serves to highlight the molecular mechanisms underlying the effects of these organokines and their clinical significance. Insights into the roles and mechanisms of these organokines could provide novel and potential therapeutic targets for obesity-induced inflammation.

Circulating level of fatty acid-binding protein 4 is an independent predictor of metabolic dysfunction-associated fatty liver disease in middle-aged and elderly individuals

Aims/Introduction

Metabolic dysfunction‐associated fatty liver disease (MAFLD), defined as hepatosteatosis with type 2 diabetes mellitus, overweight/obesity or metabolic dysregulation, has been proposed as a new feature of chronic liver disease. Fatty acid‐binding protein 4 (FABP4) is expressed in adipose tissue, and secreted FABP4 is associated with the development of insulin resistance and atherosclerosis. However, the relationship between MAFLD and FABP4 has not been fully addressed.

Materials and Methods

Associations of MAFLD with metabolic markers, including FABP4, fibroblast growth factor 21 and adiponectin, were investigated in 627 individuals (men/women 292/335) in the Tanno‐Sobetsu Study, a population‐based cohort.

Results

The mean age was 65 years (range 19–98 years, median [interquartile range] 68 [56–76] years). Hepatosteatosis was determined by the fatty liver index (FLI), and FLI ≥35 for men and FLI ≥16 for women were used for detection of fatty liver, as previously reported using 14,471 Japanese individuals. FLI was positively correlated with systolic blood pressure and levels of FABP4 (r = 0.331, P < 0.001), fibroblast growth factor 21, homeostasis model assessment of insulin resistance as an insulin resistance index and uric acid, and was negatively correlated with levels of high‐density lipoprotein cholesterol and adiponectin. FABP4 concentration was independently associated with FLI after adjustment of age, sex, systolic blood pressure and levels of uric acid, high‐density lipoprotein cholesterol, homeostasis model assessment of insulin resistance, adiponectin and fibroblast growth factor 21 in multivariable regression analysis. Logistic regression analysis showed that FABP4 was an independent predictor of MAFLD after adjustment of age, sex, presence of diabetes mellitus, hypertension and dyslipidemia, and levels of uric acid, homeostasis model assessment of insulin resistance, adiponectin and fibroblast growth factor 21.

Conclusions

FABP4 concentration is independently associated with FLI and is an independent predictor of MAFLD in middle‐aged and elderly individuals.

Fatty Acid-Binding Protein 4 (FABP4) Is Associated with Cartilage Thickness in End-Stage Knee Osteoarthritis

BACKGROUND

There is no single blood biomarker for the staging of knee osteoarthritis (KOA). The purpose of this study was to assess the relationship of obesity, serum biomarkers, the hip-knee-ankle angle (HKAA) with sonographic cartilage thickness.

METHODS

We conducted a cross-sectional study of n = 33 patients undergoing knee arthroplasty. Body mass index (BMI) was recorded, and patients were grouped based on BMI. Serum blood samples were collected, and the following biomarkers were measured using the ELISA technique (subgroup of n = 23): oxidized low-density lipoprotein (oxLDL), soluble receptor for advanced glycation end-products (sRAGE), fatty acid-binding protein 4 (FABP4), membrane-bound phospholipase A2 (PLA2G2A). The HKAA was analyzed on full-length limb standing x-ray images. Cartilage thickness was assessed on ultrasound images. Multivariable regression analysis was performed to account for confounding.

RESULTS

After adjusting for age, gender, and HKAA, obese patients had thicker medial femoral cartilage (β = 0.165, P = 0.041). Furthermore, lateral cartilage thickness was negatively correlated with FABP4 level after adjusting for of age, gender, BMI, and HKAA (β = -0.006, P = 0.001). Confirming previous studies, after adjustment, FABP4 level was associated with a higher BMI group (β = 42.99, P < 0.001). None of the other markers (oxLDL, PLA2G2A, and sRAGE) was associated with BMI or cartilage thickness.

DISCUSSION

Our results indicate that BMI has a weak, positive association with cartilage thickness in end-stage KOA patients. FABP4 levels were negatively associated with cartilage thickness. While our study is limited by a small sample size, these results further highlight the role of FABP4 as promising biomarkers of burden of disease in KOA.

MicroRNA-29b/29c targeting CTRP6 influences porcine adipogenesis via the AKT/PKA/MAPK Signalling pathway

Porcine fat deposition is an important economic trait of pig breeds, and understanding the gene regulatory network in adipocytes is essential for modern pig breeding. In a previous study, we demonstrated that miR-29a negatively regulates the differentiation of porcine adipocytes. In this study, we investigated the roles of miR-29b/c in porcine adipocytes and the underlying mechanisms. Using EdU staining and the CCK-8 assay, we observed that transfection with the miR-29b/c agomir promoted the proliferation of porcine intramuscular (IM) and subcutaneous (SC) adipocytes by altering the expression of cell-cycle-related genes. According to the results of oil red O staining and western blot analysis, transfection with the miR-29b/c agomir suppressed the differentiation of porcine SC and IM adipocytes via the AKT/PKA/MAPK signalling pathway. Furthermore, we proved that miR-29b/c regulates porcine adipocytes by directly targeting the 3ʹ-untranslated region (3ʹUTR) of CTRP6 using a dual-luciferase reporter assay. Finally, co-transfection with miR-29b/c and CTRP6 partially restored the changes of phenotype and gene expression induced by miR-29b/c overexpression in 3T3-L1 adipocytes. Taken together, our data demonstrate that both miR-29b and miR-29 c negatively regulate porcine adipogenesis by targeting CTRP6, which furthers our understanding of the gene network that regulates fat deposition in pigs.

The adipokine FABP4 is a key regulator of neonatal glucose homeostasis

During pregnancy, fetal glucose production is suppressed, with rapid activation immediately postpartum. Fatty acid–binding protein 4 (FABP4) was recently demonstrated as a regulator of hepatic glucose production and systemic metabolism in animal models. Here, we studied the role of FABP4 in regulating neonatal glucose hemostasis. Serum samples were collected from pregnant women with normoglycemia or gestational diabetes at term, from the umbilical circulation, and from the newborns within 6 hours of life. The level of FABP4 was higher in the fetal versus maternal circulation, with a further rise in neonates after birth of approximately 3-fold. Neonatal FABP4 inversely correlated with blood glucose, with an approximately 10-fold increase of FABP4 in hypoglycemic neonates. When studied in mice, blood glucose of 12-hour-old WT, Fabp4^–/+, and Fabp4^–/– littermate mice was 59 ± 13 mg/dL, 50 ± 11 mg/dL, and 43 ± 11 mg/dL, respectively. Similar to our observations in humans, FABP4 levels in WT mouse neonates were approximately 8-fold higher compared with those in adult mice. RNA sequencing of the neonatal liver suggested altered expression of multiple glucagon-regulated pathways in Fabp4^–/– mice. Indeed, Fabp4^–/– liver glycogen was inappropriately intact, despite a marked hypoglycemia, with rapid restoration of normoglycemia upon injection of recombinant FABP4. Our data suggest an important biological role for the adipokine FABP4 in the orchestrated regulation of postnatal glucose metabolism.

Elevated S-adenosylhomocysteine induces adipocyte dysfunction to promote alcohol-associated liver steatosis

It has been previously shown that chronic ethanol administration-induced increase in adipose tissue lipolysis and reduction in the secretion of protective adipokines collectively contribute to alcohol-associated liver disease (ALD) pathogenesis. Further studies have revealed that increased adipose S-adenosylhomocysteine (SAH) levels generate methylation defects that promote lipolysis. Here, we hypothesized that increased intracellular SAH alone causes additional related pathological changes in adipose tissue as seen with alcohol administration. To test this, we used 3-deazaadenosine (DZA), which selectively elevates intracellular SAH levels by blocking its hydrolysis. Fully differentiated 3T3-L1 adipocytes were treated in vitro for 48 h with DZA and analysed for lipolysis, adipokine release and differentiation status. DZA treatment enhanced adipocyte lipolysis, as judged by lower levels of intracellular triglycerides, reduced lipid droplet sizes and higher levels of glycerol and free fatty acids released into the culture medium. These findings coincided with activation of both adipose triglyceride lipase and hormone sensitive lipase. DZA treatment also significantly reduced adipocyte differentiation factors, impaired adiponectin and leptin secretion but increased release of pro-inflammatory cytokines, IL-6, TNF and MCP-1. Together, our results demonstrate that elevation of intracellular SAH alone by DZA treatment of 3T3-L1 adipocytes induces lipolysis and dysregulates adipokine secretion. Selective elevation of intracellular SAH by DZA treatment mimics ethanol's effects and induces adipose dysfunction. We conclude that alcohol-induced elevations in adipose SAH levels contribute to the pathogenesis and progression of ALD.

FABP4 deactivates NF-κB-IL1α pathway by ubiquitinating ATPB in tumor-associated macrophages and promotes neuroblastoma progression

Neuroblastoma (NB) is the most common and deadliest pediatric solid tumor. Targeting and reactivating tumor-associated macrophages (TAMs) is necessary for reversing immune suppressive state and stimulating immune defense to exert tumoricidal function. However, studies on the function and regulation of TAMs in NB progression are still limited. Fatty acid binding protein 4 (FABP4) in TAMs was correlated with advanced clinical stages and unfavorable histology of NB. FABP4-mediated macrophages increased migration, invasion, and tumor growth of NB cells. Mechanically, FABP4 could directly bind to ATPB to accelerate ATPB ubiquitination in macrophages. The consequently decreased ATP levels could deactivate NF-κB/RelA-IL1α pathway, which subsequently results in macrophages reprogrammed to an anti-inflammatory phenotype. We also demonstrated that FABP4-enhanced migration and invasion were significantly suppressed by IL1α blocking antibody. Furthermore, circulating FABP4 was also associated with the clinical stages of NB. Our findings suggest that FABP4-mediated macrophages may promote proliferation and migration phenotypes in NB cells through deactivating NF-κB-IL1α pathway by ubiquitinating ATPB. This study reveals the pathologic and biologic role of FABP4-mediated macrophages in NB development and exhibits a novel application of targeting FABP4 in macrophages for NB treatment.

Elevated circulating FABP4 concentration predicts cardiovascular death in a general population: a 12-year prospective study

Fatty acid-binding protein 4 (FABP4) is secreted from adipose tissue and acts as an adipokine, and an elevated circulating FABP4 level is associated with metabolic disorders and atherosclerosis. However, little is known about the causal link between circulating FABP4 level and mortality in a general population. We investigated the relationship between FABP4 concentration and mortality including cardiovascular death during a 12-year period in subjects of the Tanno-Sobetsu Study, a population-based cohort (n = 721, male/female: 302/419). FABP4 concentration at baseline was significantly higher in female subjects than in male subjects. All-cause death occurred in 123 (male/female: 74/49) subjects, and 34 (male/female: 20/14) and 42 (male/female: 26/16) subjects died of cardiovascular events and cancer, respectively. When divided into 3 groups according to tertiles of FABP4 level at baseline by sex (T1–T3), Kaplan–Meier survival curves showed that there were significant differences in rates of all-cause death and cardiovascular death, but not cancer death, among the groups. Multivariable Cox proportional hazard model analysis with a restricted cubic spline showed that hazard ratio (HR) for cardiovascular death, but not that for all-cause death, significantly increased with a higher FABP4 level at baseline after adjustment of age and sex. The risk of cardiovascular death after adjustment of age, sex, body mass index and levels of brain natriuretic peptide and high-sensitivity C-reactive protein in the 3rd tertile (T3) group (HR: 4.96, 95% confidence interval: 1.20–22.3) was significantly higher than that in the 1st tertile (T1) group as the reference. In conclusion, elevated circulating FABP4 concentration predicts cardiovascular death in a general population.

Differences in circulating fatty acid-binding protein 4 concentration in the venous and capillary blood immediately after acute exercise

Background

Circulating fatty acid-binding protein 4 (FABP4) is a marker for various diseases. It would be highly useful to have simple and less invasive techniques for the assessment of FABP4 concentrations in the clinical research setting. The purpose of the present study was to assess the concordance of circulating FABP4 concentrations in venous and capillary blood both at rest and immediately after acute exercise in healthy young males.

Results

Thirty-eight healthy young male adults aged from 19 to 25 years (mean age, 20.8 ± 1.2 years) were recruited. Paired blood samples were taken from the cubital vein (venous) and fingertip (capillary) blood at rest (resting state) and immediately after incremental exercise (exercising state). Blood samples were analyzed to determine the circulating FABP4 concentration using an enzyme-linked immunosorbent assay. Pearson’s correlation coefficients for circulating FABP4 concentrations between venous and capillary blood samples indicated a strong positive correlation in both the resting and exercising state (resting state: r = 0.982, exercising state: r = 0.989, both p < 0.001). The mean FABP4 concentration was similar between venous and capillary blood in the resting state (p = 0.178), whereas it was significantly higher in capillary blood than in venous blood in the exercising state (p < 0.001). Furthermore, Bland–Altman plots showed a non-significant bias (− 0.07 ± 0.61 ng/mL, p = 0.453) in the resting state, whereas a significant bias (− 0.45 ± 0.61 ng/mL, p < 0.001) was observed in the exercising state.

Conclusions

These results indicate that capillary blood sampling can slightly overestimate circulating FABP4 concentrations under a physiologically dynamic state. However, the association between the venous and capillary blood in terms of FABP4 concentration was very strong, suggesting that capillary blood sampling can detect changes in FABP4 concentration in both physiologically steady and dynamic states.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40101-021-00255-z.

Through the back door: Unconventional protein secretion

Proteins are secreted from eukaryotic cells by several mechanisms besides the well-characterized classical secretory system. Proteins destined to enter the classical secretory system contain a signal peptide for translocation into the endoplasmic reticulum. However, many proteins lacking a signal peptide are secreted nonetheless. Contrary to conventional belief, these proteins are not just released as a result of membrane damage leading to cell leakage, but are actively packaged for secretion in alternative pathways. They are called unconventionally secreted proteins, and the best-characterized are from fungi and mammals. These proteins have extracellular functions including cell signaling, immune modulation, as well as moonlighting activities different from their well-described intracellular functions. Among the pathways for unconventional secretion are direct transfer across the plasma membrane, release within plasma membrane-derived microvesicles, use of elements of autophagy, or secretion from endosomal/multivesicular body-related components. We review the fungal and metazoan unconventional secretory pathways and their regulation, and propose experimental criteria to identify their mode of secretion.

Significance of urinary fatty acid-binding protein 4 level as a possible biomarker for the identification of minimal change disease in patents with nephrotic-range proteinuria

Background

Fatty acid-binding protein 4 (FABP4), but not FABP1 (liver-type FABP), is ectopically induced in injured glomerular endothelial cells, and urinary FABP4 (U-FABP4) level is associated with proteinuria and renal dysfunction in a general population.

Methods

The clinical significance of U-FABP4 was investigated in 81 patients (male/female: 43/38, age: 57 ± 17 years) who underwent kidney biopsy.

Results

U-FABP4 was negatively correlated with estimated glomerular filtration rate (eGFR) (r = − 0.56, P < 0.01) and was positively correlated with age, blood pressure, triglycerides, proteinuria (r = 0.58, P < 0.01), plasma FABP4 and urinary FABP1 (U-FABP1) (r = 0.52, P < 0.01). Multivariable regression analysis showed that eGFR, proteinuria and U-FABP1 were independent predictors of U-FABP4. The level of U-FABP4, but not that of proteinuria, eGFR or U-FABP1, in minimal change nephrotic syndrome (MCNS) was significantly lower than the level in membranous nephropathy (MN) and that in diabetic nephropathy. Receiver operating characteristic curve analysis indicated that U-FABP4 level ≤ 0.78 μg/gCr predicted MCNS in patients who had nephrotic-range proteinuria with a high level of accuracy. When divided by the median value of U-FABP4 at baseline in 33 of the 81 patients who could be followed up, the yearly change (post–pre) in eGFR in the low U-FABP4 group was significantly greater than that in the high U-FABP4 group (median: 11.0 vs. -5.0 mL/min/1.73m²/year).

Conclusions

U-FABP4 level is independently associated with proteinuria and renal dysfunction in patients with glomerular kidney disease. A low U-FABP4 level may predict MCNS in patients with nephrotic syndrome and would be a useful biomarker for differential diagnosis of MCNS and MN, which are common causes of nephrotic syndrome.

Supplementary Information

Supplementary information accompanies this paper at 10.1186/s12882-020-02122-y.

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.

Adipocyte lipolysis: from molecular mechanisms of regulation to disease and therapeutics

Fatty acids (FAs) are stored safely in the form of triacylglycerol (TAG) in lipid droplet (LD) organelles by professional storage cells called adipocytes. These lipids are mobilized during adipocyte lipolysis, the fundamental process of hydrolyzing TAG to FAs for internal or systemic energy use. Our understanding of adipocyte lipolysis has greatly increased over the past 50 years from a basic enzymatic process to a dynamic regulatory one, involving the assembly and disassembly of protein complexes on the surface of LDs. These dynamic interactions are regulated by hormonal signals such as catecholamines and insulin which have opposing effects on lipolysis. Upon stimulation, patatin-like phospholipase domain containing 2 (PNPLA2)/adipocyte triglyceride lipase (ATGL), the rate limiting enzyme for TAG hydrolysis, is activated by the interaction with its co-activator, alpha/beta hydrolase domain-containing protein 5 (ABHD5), which is normally bound to perilipin 1 (PLIN1). Recently identified negative regulators of lipolysis include G0/G1 switch gene 2 (G0S2) and PNPLA3 which interact with PNPLA2 and ABHD5, respectively. This review focuses on the dynamic protein-protein interactions involved in lipolysis and discusses some of the emerging concepts in the control of lipolysis that include allosteric regulation and protein turnover. Furthermore, recent research demonstrates that many of the proteins involved in adipocyte lipolysis are multifunctional enzymes and that lipolysis can mediate homeostatic metabolic signals at both the cellular and whole-body level to promote inter-organ communication. Finally, adipocyte lipolysis is involved in various diseases such as cancer, type 2 diabetes and fatty liver disease, and targeting adipocyte lipolysis is of therapeutic interest.

Hepatic Lipidomics and Molecular Imaging in a Murine Non-Alcoholic Fatty Liver Disease Model: Insights into Molecular Mechanisms

An imbalance between hepatic fatty acid uptake and removal results in ectopic fat accumulation, which leads to non-alcoholic fatty liver disease (NAFLD). The amount and type of accumulated triglycerides seem to play roles in NAFLD progression; however, a complete understanding of how triglycerides contribute to NAFLD evolution is lacking. Our aim was to evaluate triglyceride accumulation in NAFLD in a murine model and its associations with molecular mechanisms involved in liver damage and adipose tissue-liver cross talk by employing lipidomic and molecular imaging techniques. C57BL/6J mice fed a high-fat diet (HFD) for 12 weeks were used as a NAFLD model. Standard-diet (STD)-fed animals were used as controls. Standard liver pathology was assessed using conventional techniques. The liver lipidome was analyzed by liquid chromatography–mass spectrometry (LC–MS) and laser desorption/ionization–mass spectrometry (LDI–MS) tissue imaging. Liver triglycerides were identified by MS/MS. The transcriptome of genes involved in intracellular lipid metabolism and inflammation was assessed by RT-PCR. Plasma leptin, resistin, adiponectin, and FABP4 levels were determined using commercial kits. HFD-fed mice displayed increased liver lipid content. LC–MS analyses identified 14 triglyceride types that were upregulated in livers from HFD-fed animals. Among these 14 types, 10 were identified in liver cross sections by LDI–MS tissue imaging. The accumulation of these triglycerides was associated with the upregulation of lipogenesis and inflammatory genes and the downregulation of β-oxidation genes. Interestingly, the levels of plasma FABP4, but not of other adipokines, were positively associated with 8 of these triglycerides in HFD-fed mice but not in STD-fed mice. Our findings suggest a putative role of FABP4 in the liver-adipose tissue cross talk in NAFLD.

Mechanisms of Artemisia scoparia's Anti-Inflammatory Activity in Cultured Adipocytes, Macrophages, and Pancreatic β-Cells

OBJECTIVE

An ethanolic extract of Artemisia scoparia (SCO) improves adipose tissue function and reduces negative metabolic consequences of high-fat feeding. A. scoparia has a long history of medicinal use across Asia and has anti-inflammatory effects in various cell types and disease models. The objective of the current study was to investigate SCO's effects on inflammation in cells relevant to metabolic health.

METHODS

Inflammatory responses were assayed in cultured adipocytes, macrophages, and insulinoma cells by quantitative polymerase chain reaction, immunoblotting, and NF-κB reporter assays.

RESULTS

In tumor necrosis factor α-treated adipocytes, SCO mitigated ERK and NF-κB signaling as well as transcriptional responses but had no effect on fatty acid-binding protein 4 secretion. SCO also reduced levels of deleted in breast cancer 1 protein in adipocytes and inhibited inflammatory gene expression in stimulated macrophages. Finally, in pancreatic β-cells, SCO decreased NF-κB-responsive promoter activity induced by IL-1β treatment.

CONCLUSIONS

SCO's ability to promote adipocyte development and function is thought to mediate its insulin-sensitizing actions in vivo. Our findings that SCO inhibits inflammatory responses through at least two distinct signaling pathways (ERK and NF-κB) in three cell types known to contribute to metabolic disease reveal that SCO may act more broadly than previously thought to improve metabolic health.

Treatment with anagliptin, a DPP-4 inhibitor, decreases FABP4 concentration in patients with type 2 diabetes mellitus at a high risk for cardiovascular disease who are receiving statin therapy

Background

Fatty acid-binding protein 4 (FABP4) acts as a novel adipokine, and elevated FABP4 concentration is associated with obesity, insulin resistance and atherosclerosis. Dipeptidyl peptidase-4 (DPP-4) inhibitors, a class of antidiabetic drugs, have distinct structures among the drugs, possibly leading to a drug class effect and each drug effect. Sitagliptin, a DPP-4 inhibitor, has been reported to decrease FABP4 concentration in drug-naïve and sulfonylurea-treated patients with type 2 diabetes mellitus. Anagliptin, another DPP-4 inhibitor, was shown to decrease low-density lipoprotein cholesterol (LDL-C) level to a greater extent than that by sitagliptin in the Randomized Evaluation of Anagliptin vs. Sitagliptin On low-density lipoproteiN cholesterol in diabetes (REASON) trial.

Aim and methods

As a sub-analysis study using data obtained from the REASON trial, we investigated the effects of treatment with anagliptin (n = 148, male/female: 89/59) and treatment with sitagliptin (n = 159, male/female: 93/66) for 52 weeks on FABP4 concentration in patients with type 2 diabetes mellitus at a high risk for cardiovascular events who were receiving statin therapy.

Results

The DPP-4 inhibitor had been administered in 82% of the patients in the anagliptin group and 81% of the patients in sitagliptin group prior to randomization. Serum FABP4 level was significantly decreased by 7.9% by treatment with anagliptin (P = 0.049) and was not significantly decreased by treatment with sitagliptin (P = 0.660). Change in FABP4 level was independently associated with basal FABP4 level and changes in waist circumference and creatinine after adjustment of age, sex and the treatment group.

Conclusion

Anagliptin decreases serum FABP4 concentration independent of change in hemoglobin A1c or LDL-C in patients with type 2 diabetes mellitus and dyslipidemia who are on statin therapy.

Trial registration ClinicalTrials.gov number NCT02330406. Registered January 5, 2015, https://clinicaltrials.gov/ct2/show/NCT02330406

Exogenous FABP4 interferes with differentiation, promotes lipolysis and inflammation in adipocytes

PURPOSE

Fatty acid binding protein 4 (FABP4) has been demonstrated to be secreted from adipocytes in an unconventional pathway associated with lipolysis. Circulating FABP4 is elevated in metabolic disorders and has been shown to affect various peripheral cells such as pancreatic β-cells, hepatocytes and macrophages, but its effects on adipocytes remains unclear. The aim of this study was to investigate the effects of exogenous FABP4 (eFABP4) on adipocyte differentiation and function.

METHODS

3T3-L1 pre-adipocytes or mature adipocytes were treated with recombinant FABP4 in the absence or presence of FABP4 inhibitor I-9/p38 MAPK inhibitor SB203580; Meanwhile male C57BL/6J mice were subcutaneously injected twice a day with recombinant FABP4 (0.35 mg/kg) with or without I-9 (50 mg/kg) for 2 weeks. The effects of eFABP4 on differentiation, lipolysis and inflammation were determined by triglyceride measurement or lipolysis assay, western blotting, or RT-qPCR analysis.

RESULTS

eFABP4 treatment significantly reduced intracellular triglyceride content and decreased expression of adipogenic markers peroxisome proliferator-activated receptor gamma (PPARγ), CCAAT/enhancer binding protein alpha (C/EBPα), intracellular FABP4, and adiponectin in 3T3-L1 cells. Besides, eFABP4 promoted lipolysis and inflammation in differentiated 3T3-L1 adipocytes as well as in adipose tissue of eFABP4-treated C57BL/6J mice, with elevated gene expression of monocyte chemoattractant protein (MCP)-1, tumor necrosis factor (TNF)-α, and elevated protein expression of adipose triglyceride lipase (ATGL), phosphorylation of hormone-sensitive lipase (HSL) (Ser-660), p38, and nuclear factor-kappa B (NF-κB). The pro-inflammatory and pro-lipolytic effects of eFABP4 could be reversed by SB203580/I-9.

CONCLUSIONS

These findings indicate that eFABP4 interferes with adipocyte differentiation, induces p38/HSL mediated lipolysis and p38/NF-κB mediated inflammation in adipocytes in vitro and in vivo.

Independent and Distinct Associations of FABP4 and FABP5 With Metabolic Parameters in Type 2 Diabetes Mellitus

Objective: Among fatty acid-binding proteins (FABPs), secreted forms of FABP4 and FABP5, which are expressed in adipocytes and macrophages, act as bioactive molecules. We investigated concentrations of FABP4 and FABP5 in patients with type 2 diabetes mellitus. Methods: As a sub-analysis study of the Randomized Evaluation of Anagliptin vs. Sitagliptin On low-density lipoproteiN cholesterol in diabetes (REASON) trial, 256 patients (male/female: 146/110, age: 68 ± 10 years) with type 2 diabetes mellitus and dyslipidemia who were receiving statin therapy were recruited. Patients who had been treated with a thiazolidinedione were excluded. Results: Several drugs which may modulate FABP4 levels including statins, dipeptidyl peptidase-4 inhibitors and angiotensin II receptor blockers had been administered in 100, 81, and 51% of the recruited patients, respectively. The level of FABP4, but not that of FABP5, was significantly higher in females than in males. Multivariable linear regression analysis demonstrated that waist circumference (β = 0.21), estimated glomerular filtration rate (β = -0.31), triglycerides (β = 0.16), and FABP5 (β = 0.39) were independent predictors of FABP4 level after adjusting age and sex. On the other hand, FABP5 level was independently associated with levels of FABP4 (β = 0.57) and high-density lipoprotein (HDL) cholesterol (β = -0.12). Conclusions: Concentrations of FABP4 and FABP5 are independent predictors of each other in patients with type 2 diabetes mellitus. There are distinct independent associations of FABP4 with renal dysfunction, adiposity and hypertriglyceridemia and there is a distinct independent association of FABP5 with a low HDL cholesterol level in type 2 diabetic patients with dyslipidemia at high risks for cardiovascular disease who are receiving statin therapy.

Comparison of plasma fatty acid binding protein 4 concentration in venous and capillary blood

Circulating fatty acid binding protein 4 (FABP4) is associated with various diseases and simple and less invasive techniques for assessment of FABP4 concentration are required in clinical research setting. The purpose of the present study was to assess the correlation of plasma FABP4 concentration between venous and capillary blood in healthy young adults. Twenty-eight healthy young adults aged from 20 to 26 years (mean age, 22.2 ± 1.4 years, 14 males and 14 females) were included. Paired resting blood samples were taken from the cubital vein (venous) and fingertip (capillary) blood. Plasma FABP4 concentration in both blood was analyzed by enzyme-linked Immunosorbent assay. Plasma FABP4 concentration did not differ significantly between venous and capillary blood (−0.11± 0.75 ng/mL, p = 0.447, 95%CI: -0.402–0.182). Pearson’s correlation coefficient for plasma FABP4 concentration between venous and capillary blood samples suggests strong correlation (r = 0.961, p < 0.001). The Bland & Altman plot showed a non-significant bias (−0.11 ± 0.75 ng/mL, p = 0.684) and the 95% limits of agreement ranged from −1.59 to 1.37 ng/mL. FABP4 concentration in both venous and capillary blood was significantly higher in females than in males (venous blood: p = 0.041; capillary blood: p = 0.049). These results suggest that capillary blood sampling can detect gender difference and is useful for the assessment of FABP4 concentration.

IL-17A promotes fatty acid uptake through the IL-17A/IL-17RA/p-STAT3/FABP4 axis to fuel ovarian cancer growth in an adipocyte-rich microenvironment

Pro-inflammatory cytokines are crucial mediators of cancer development, representing potential targets for cancer therapy. The molecular mechanism of a vital pro-inflammatory cytokine, IL-17A, in cancer progression and its potential use in therapy through influencing fatty acid (FA) metabolism, especially FA uptake of cancer cells, remains unknown. In the present study, we used IL-17A and ovarian cancer (OvCa), a representative of both obesity-related and inflammation-related cancers, to explore the interactions among IL-17A, cancer cells and adipocytes (which can provide FAs). We found that in the presence of palmitic acid (PA), IL-17A could directly increase the cellular uptake of PA, leading to the proliferation of OvCa cells via the IL-17A/IL-17RA/p-STAT3/FABP4 axis rather than via CD36. Moreover, in vivo experiments using an orthotopic implantation model in IL-17A-deficient mice demonstrated that endogenous IL-17A could fuel OvCa growth and metastasis with increased expression of FABP4 and p-STAT3. Furthermore, analysis of clinical specimens supported the above findings. Our data not only provide useful insights into the clinical intervention of the growth and metastasis of the tumors (such as OvCa) that are prone to growth and metastasis in an adipocyte-rich microenvironment (ARM) but also provides new insights into the roles of IL-17A in tumor progression and immunomodulatory therapy of OvCa.

The Transcription Factor FOXN1 Regulates Skin Adipogenesis and Affects Susceptibility to Diet-Induced Obesity

FOXN1, a transcription factor expressed in the epidermis, regulates keratinocyte differentiation and participates in skin wound healing. In this study, we explored the impact of FOXN1 insufficiency on diet-stimulated weight gain and dermal white adipose tissue regulation in the intact and wounded skin of FOXN1^eGFP/+ (heterozygotes, FOXN1-insufficient) mice in the context of age and diet. The results showed that on a high-fat diet, FOXN1^eGFP/+ mice gained significantly less body weight than their FOXN1^+/+ counterparts (FOXN1-sufficient mice). The intact and wounded skin of FOXN1^eGFP/+ mice displayed abrogated expression of the master regulators of adipogenesis, PPARγ, FABP4, and leptin, which decreased with age in FOXN1^+/+ mice. FOXN1 insufficiency also resulted in a decreased percentage of adipocyte-committed precursor cells (CD24⁺) in the skin. The proadipogenic pathway genes Bmp2, Igf2, and Mest showed a gradual decrease in expression that accompanied the gradual inactivation of FOXN1 in the skin of FOXN1^+/+, FOXN1^eGFP/+, and FOXN1^eGFP/eGFP (lack of FOXN1) mice. Bone morphogenetic protein 2 and insulin-like growth factor 2 signals colocalized with FOXN1-eGFP in the epidermis and in hair follicles. These data demonstrated that FOXN1 initiates the cascade of adipogenic signaling that regulates skin homeostasis and wound healing and affects susceptibility to diet-induced obesity.

Unconventional Secretion of Adipocyte Fatty Acid Binding Protein 4 Is Mediated By Autophagic Proteins in a Sirtuin-1-Dependent Manner

Fatty acid binding protein 4 (FABP4) is a leaderless lipid carrier protein primarily expressed by adipocytes and macrophages that not only functions intracellularly but is also secreted. The secretion is mediated via unconventional mechanism(s), and in a variety of species, metabolic dysfunction is correlated with elevated circulating FABP4 levels. In diabetic animals, neutralizing antibodies targeting serum FABP4 increase insulin sensitivity and attenuate hepatic glucose output, suggesting the functional importance of circulating FABP4. Using animal and cell-based models, we show that FABP4 is secreted from white, but not brown, adipose tissue in response to lipolytic stimulation in a sirtuin-1 (SIRT1)-dependent manner via a mechanism that requires some, but not all, autophagic components. Silencing of early autophagic genes such as Ulk1/2, Fip200, or Beclin-1 or chemical inhibition of ULK1/2 or VPS34 attenuated secretion, while Atg5 knockdown potentiated FABP4 release. Genetic knockout of Sirt1 diminished secretion, and serum FABP4 levels were undetectable in Sirt1 knockout mice. In addition, blocking SIRT1 by EX527 attenuated secretion while activating SIRT1 by resveratrol-potentiated secretion. These studies suggest that FABP4 secretion from adipocytes is regulated by SIRT1 and requires early autophagic components.

NTN1 Affects Porcine Intramuscular Fat Content by Affecting the Expression of Myogenic Regulatory Factors

Simple Summary

Intramuscular fat (IMF) is a key meat quality trait in the pork industry. In this study, we validated the effect of the copy number of Netrin-1 (NTN1-CNV) on Netrin-1 (NTN1) protein expression and explored the possible affective mechanism of NTN1 on IMF. The results indicated that NTN1-CNV may affect the expression of NTN1 protein by its gene dose, and the expression of NTN1 may affect the proliferation and differentiation of muscle cells by the AMP-activated protein kinase (AMPK) pathway and finally influence the IMF content.

Abstract

Intramuscular fat (IMF) content is an important economic trait for pork quality. Our previous results regarding the genome-wide association between IMF content and copy number variations (CNVs) indicated that the CNV within Netrin-1(NTN1-CNV) was significantly associated with IMF. In order to validate the effect of NTN1-CNV, we detected the Netrin-1 (NTN1) gene dose and protein expression content in the longissimus dorsi of different IMF content pigs using Western blotting and investigated the expression of NTN1 RNA in different tissues using real-time quantitative polymerase chain reaction (qPCR). The knock-down of the NTN1 gene in C2C12 and 3T3-L1 cells and over-expression in C2C12 cells during the proliferation and differentiation stage were also investigated to explore the possible pathway of action of NTN1. The results showed that in individuals with IMF content differences, the gene dose of NTN1 and the expression of NTN1 protein were also significantly different, which indicated that NTN1-CNV may directly affect IMF by its coding protein. NTN1 had the highest expression in pig longissimus dorsi and backfat tissues, which indicates that NTN1 may play an important role in muscle and fat tissues. The in vitro validation assay indicated that NTN1 silencing could promote the proliferation and inhibit the differentiation of C2C12 cells, with no effect on 3T3-L1 cells. Additionally, NTN1 over-expression could inhibit the proliferation and promote the differentiation of C2C12 cells. Combined with previous research, we conclude that NTN1-CNV may affect IMF by its gene dose, and the expression of NTN1 may affect the proliferation and differentiation of muscle cells by the AMP-activated protein kinase (AMPK) pathway and finally influence the IMF.

Independent links between plasma xanthine oxidoreductase activity and levels of adipokines

AIMS/INTRODUCTION

Xanthine oxidoreductase (XOR) is a rate-limiting enzyme that catalyzes uric acid formation in the purine metabolism, is involved in an increase in reactive oxygen species. Plasma XOR activity has been shown to be associated with obesity, smoking, liver dysfunction, hyperuricemia, dyslipidemia and insulin resistance.

MATERIALS AND METHODS

The association between plasma XOR activity, measured by using liquid chromatography and mass spectrometry, and levels of adipokines, including adiponectin, fatty acid-binding protein 4 (FABP4) and fibroblast growth factor 21 (FGF21), was investigated in 282 participants (male/female: 126/156) of the Tanno-Sobetsu Study who were not taking medication.

RESULTS

Women had lower plasma XOR activity than did men. Smoking habit was associated with increased activity. Plasma XOR activity was positively correlated with concentrations of FABP4 (r = 0.192, P < 0.001) and FGF21 (r = 0.208, P < 0.001), homeostasis model assessment of insulin resistance as an index of insulin resistance and uric acid, and was negatively correlated with adiponectin level (r = -0.243, P = 0.001). Multivariate regression analyses showed that levels of adiponectin, FABP4 and FGF21 were independent determinants of plasma XOR activity after adjusting age, sex, uric acid and homeostasis model assessment of insulin resistance. With additional adjustment of smoking habit, the level of FABP4, but not that of adiponectin or FGF21, remained as an independent predictor of plasma XOR activity.

CONCLUSIONS

Plasma XOR activity was independently associated with levels of adipokines in a general population of individuals not taking medication.

Adipose Tissue-Derived Signatures for Obesity and Type 2 Diabetes: Adipokines, Batokines and MicroRNAs

: Obesity is one of the main risk factors for type 2 diabetes mellitus (T2DM). It is closely related to metabolic disturbances in the adipose tissue that primarily functions as a fat reservoir. For this reason, adipose tissue is considered as the primary site for initiation and aggravation of obesity and T2DM. As a key endocrine organ, the adipose tissue communicates with other organs, such as the brain, liver, muscle, and pancreas, for the maintenance of energy homeostasis. Two different types of adipose tissues-the white adipose tissue (WAT) and brown adipose tissue (BAT)-secrete bioactive peptides and proteins, known as "adipokines" and "batokines," respectively. Some of them have beneficial anti-inflammatory effects, while others have harmful inflammatory effects. Recently, "exosomal microRNAs (miRNAs)" were identified as novel adipokines, as adipose tissue-derived exosomal miRNAs can affect other organs. In the present review, we discuss the role of adipose-derived secretory factors-adipokines, batokines, and exosomal miRNA-in obesity and T2DM. It will provide new insights into the pathophysiological mechanisms involved in disturbances of adipose-derived factors and will support the development of adipose-derived factors as potential therapeutic targets for obesity and T2DM.

Serum fatty acid binding protein 4 is positively associated with early stroke recurrence in nondiabetic ischemic stroke

Adipocyte fatty acid–binding protein (FABP4) played critical roles in metabolic syndrome, inflammatory responses and cardiovascular diseases. It aimed to investigate the associations of serum FABP4 levels with early stroke recurrence. This study included the 206 acute ischemic stroke patients hospitalized in our institution. Stroke recurrence events were assessed at the 3-month follow-up. The median of FABP level was 22.6 (IQR, 17.9-31.6) ng/mL in patients with stroke recurrence (N=36), which was higher than in patients without stroke recurrence [16.9 (IQR, 11.8-21.4) ng/mL] (P<0.001). As a continuous variable, the unadjusted and adjusted risk of stroke recurrence would be increased by 12% (OR=1.12 [95% CI 1.06–1.17], P<0.001) and 8% (1.08 [1.02–1.14], P=0.006) for every 1 ng/ml increment of FABP4. The Area under the curve (AUC) of serum FABP4 and NIH Stroke Scale (NIHSS) score for predicting stroke recurrence was 0.73 (95% CI: 0.64–0.82) and 0.72 (95% CI: 0.64–0.81), presenting no discriminating capacity (P=0.45). In the combining model, the AUC of NIHSS score was further improved to 0.77 by FABP4 (0.77; 95% CI: 0.69–0.85), which was significant (P=0.01). The risk of stroke recurrence can be predicted by elevated FABP4 levels in serum of nondiabetic patients with first-ever ischemic stroke.

Adipokine FABP4 integrates energy stores and counterregulatory metabolic responses

Although counterregulatory hormones and mediators of the fight-or-flight responses are well defined at many levels, how energy stores per se are integrated into this system remains an enigmatic question. Recent years have seen the adipose tissue become a central focus for mediating intracellular signaling and communication through the release of a variety of bioactive lipids and substrates, as well as various adipokines. A critical integration node among these mediators and responses is controlled by FA binding protein 4 (FABP4), also known as adipocyte protein 2 (aP2), which is highly expressed in adipose tissue and functions as a lipid chaperone protein. Recently, it was demonstrated that FABP4 is a secreted hormone that has roles in maintaining glucose homeostasis, representing a key juncture facilitating communication between energy-storage systems and distant organs to respond to life-threatening situations. However, chronic engagement of FABP4 under conditions of immunometabolic stress, such as obesity, exacerbates a number of immunometabolic diseases, including diabetes, asthma, cancer, and atherosclerosis. In both preclinical mouse models and humans, levels of circulating FABP4 have been correlated with metabolic disease incidence, and reducing FABP4 levels or activity is associated with improved metabolic health. In this review, we will discuss the intriguing emerging biology of this protein, including potential therapeutic options for targeting circulating FABP4.

Impact of arsenic exposure on clinical biomarkers indicative of cardiovascular disease risk in Mexican women

An appropriate and precise identification of high-risk individuals to develop cardiovascular diseases (CVD) is of high importance to reduce these kinds of diseases, a major health concern worldwide. Therefore, the aim of this research was to evaluate prognostic CVD biomarkers in Mexican women exposed to inorganic arsenic via drinking water. Then, a cross-sectional study including 190 women was achieved. Urinary arsenic (UAs) levels were analyzed as exposure biomarker to that metalloid. While, plasma asymmetric dimethylarginine (ADMA), adipocyte fatty acid-binding protein (FABP4), adiponectin, and chemerin levels, hypertriglyceridemic waist (HW) phenotype, atherogenic index of plasma (AIP), and Framingham risk score (FRS) were assessed as prognostic CVD biomarkers. Mean UAs level detected in the evaluated urinary samples was 45.0 ± 40.0 μg/g creatinine. In addition, mean plasma ADMA, FABP4, chemerin and adiponectin levels were 0.68 µmol/L, 20.3 ng/mL, 12.5 μg/mL, and 255 ng/mL, correspondingly. Approximately, 54% of women participants displayed an HW phenotype. Regarding AIP and FRS values, 0.12 ± 0.15 and 7.50 ± 8.00 were found, respectively. Besides, strong and significant associations (p < 0.05) between UAs and AIP, ADMA, and FABP4 were distinguished. Also, after a multivariate analysis, the association between those variables persisted after adjustment for traditional risk factors of CVD. In conclusion, according to the results found in this research, the most sensible CVD biomarkers distinguished in this study were AIP, ADMA, and FABP4. Nevertheless, more studies are necessary to confirm the results found in this investigation.

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.

Circulating FABP4 is eliminated by the kidney via glomerular filtration followed by megalin-mediated reabsorption

Circulating fatty acid binding protein 4 (FABP4), secreted from adipocytes, is a potential biomarker for metabolic and cardiovascular diseases. Circulating FABP4 levels are positively associated with adiposity and adrenergic stimulation, but negatively with renal function. In this study, we addressed the issue of how the kidney regulates clearance of circulating FABP4. Tracing study revealed remarkable accumulation of ¹²⁵I-labeled FABP4 in the kidney. Exogenous FABP4 was exclusively detected in the apical membrane of proximal tubule epithelial cells (PTECs). Bilateral nephrectomy resulted in marked elevation of circulating FABP4 levels. Accelerated lipolysis by β-3 adrenergic stimulation led to a marked elevation in circulating FABP4 in mice with severe renal dysfunction. Megalin, an endocytic receptor expressed in PTECs, plays a major role in reabsorption of proteins filtered through glomeruli. Quartz-crystal microbalance study revealed that FABP4 binds to megalin. In kidney-specific megalin knockout mice, a large amount of FABP4 was excreted in urine while circulating FABP4 levels were significantly reduced. Our data suggest that circulating FABP4 is processed by the kidney via the glomerular filtration followed by megalin-mediated reabsorption. Thus, it is likely that circulating FABP4 levels are determined mainly by balance between secretion rate of FABP4 from adipocytes and clearance rate of the kidney.

KMUP-1, a GPCR Modulator, Attenuates Triglyceride Accumulation Involved MAPKs/Akt/PPARγ and PKA/PKG/HSL Signaling in 3T3-L1 Preadipocytes

Xanthine-based KMUP-1 was shown to inhibit phosphodiesterases (PDEs) and modulate G-protein coupled receptors (GPCRs) to lower hyperlipidemia and body weight. This study further investigated whether KMUP-1 affects adipogenesis and lipolysis in 3T3-L1 preadipocytes. KMUP-1 (1–40 µM) concentration-dependently attenuated Oil Red O (ORO) staining and decreased triglyceride (TG) accumulation, indicating adipogenesis inhibition in 3T3-L1 cells. In contrast, the β-agonist ractopamine increased ORO staining and TG accumulation and adipogenesis. KMUP-1 (1–40 µM) also reduced MAPKs/Akt/PPARγ expression, PPARγ1/PPARγ2 mRNA, and p-ERK immunoreactivity at the adipogenesis stage, but enhanced hormone sensitive lipase (HSL) immunoreactivity at the lipolysis stage. Addition of protein kinase A (PKA) or protein kinase G (PKG) antagonist (KT5720 or KT5728) to adipocytes did not affect HSL immunoreactivity. However, KMUP-1 did increase HSL immunoreactivity and the effect was reduced by PKA or PKG antagonist. Simvastatin, theophylline, caffeine, and sildenafil, like KMUP-1, also enhanced HSL immunoreactivity. Phosphorylated HSL (p-HSL) was enhanced by KMUP-1, indicating increased lipolysis in mature 3T3-L1 adipocytes. Decreases of MAPKs/Akt/PPARγ during adipogenesis contributed to inhibition of adipocyte differentiation, and increases of PKA/PKG at lipolysis contributed to HSL activation and TG hydrolysis. Taken together, the data suggest that KMUP-1 can inhibit hyperadiposity in 3T3-L1 adipocytes.

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.