Mice Lacking Fatty Acid-Binding Protein 5 Are Resistant to Listeria monocytogenes

Proteomic Analysis of Signaling Pathways Modulated by Fatty Acid Binding Protein 5 (FABP5) in Macrophages

Although acute inflammation serves essential functions in maintaining tissue homeostasis, chronic inflammation is causally linked to many diseases. Macrophages are a major cell type that orchestrates inflammatory processes. During inflammation, macrophages undergo polarization and activation, thereby mobilizing pro-inflammatory and anti-inflammatory transcriptional programs that regulate ensuing macrophage functions. Fatty acid binding protein 5 (FABP5) is a lipid chaperone highly expressed in macrophages. FABP5 deletion is implicated in driving macrophages toward an anti-inflammatory phenotype, yet signaling pathways regulated by macrophage-FABP5 have not been systematically profiled. We leveraged proteomic and phosphoproteomic approaches to characterize pathways modulated by FABP5 in M1 and M2 polarized bone marrow-derived macrophages (BMDMs). Stable isotope labeling by amino acids-based analysis of M1 and M2 polarized wild-type and FABP5 knockout BMDMs revealed numerous differentially regulated proteins and phosphoproteins. FABP5 deletion impacted downstream pathways associated with inflammation, cytokine production, oxidative stress, and kinase activity. Toll-like receptor 2 (TLR2) emerged as a novel target of FABP5 and pharmacological FABP5 inhibition blunted TLR2-mediated activation of downstream pathways, ascribing a novel role for FABP5 in TLR2 signaling. This study represents a comprehensive characterization of the impact of FABP5 deletion on the proteomic and phosphoproteomic landscape of M1 and M2 polarized BMDMs. Loss of FABP5 altered pathways implicated in inflammatory responses, macrophage function, and TLR2 signaling. This work provides a foundation for future studies seeking to investigate the therapeutic potential of FABP5 inhibition in pathophysiological states resulting from dysregulated inflammatory signaling. SIGNIFICANCE STATEMENT: This research offers a comprehensive analysis of fatty acid binding protein 5 (FABP5) in macrophages during inflammatory response. The authors employed quantitative proteomic and phosphoproteomic approaches to investigate this utilizing bone marrow-derived macrophages that were M1 and M2 polarized using lipopolysaccharide with interferon γ and interleukin-4, respectively. This revealed multiple pathways related to inflammation that were differentially regulated due to the absence of FABP5. These findings underscore the potential therapeutic significance of macrophage-FABP5 as a candidate for addressing inflammatory-related diseases.

Potential safety implications of fatty acid-binding protein inhibition

Fatty acid-binding proteins (FABPs) are small intracellular proteins that regulate fatty acid metabolism, transport, and signalling. There are ten known human isoforms, many of which are upregulated and involved in clinical pathologies. As such, FABP inhibition may be beneficial in disease states such as cancer, and those involving the cardiovascular system, metabolism, immunity, and cognition. Recently, a potent, selective FABP5 inhibitor (ART26.12), with 90-fold selectivity to FABP3 and 20-fold selectivity to FABP7, was found to be remarkably benign, with a no-observed-adverse-effect level of 1000 mg/kg in rats and dogs, showing no genotoxicity, cardiovascular, central, or respiratory toxicity. To understand the potential implication of FABP inhibition more fully, this review systematically assessed literature investigating genetic knockout, knockdown, and pharmacological inhibition of FABP3, FABP4, FABP5, or FABP7. Analysis of the literature revealed that animals bred not to express FABPs showed the most biological effects, suggesting key roles of these proteins during development. FABP ablation sometimes exacerbated symptoms of disease models, particularly those linked to metabolism, inflammatory and immune responses, cardiac contractility, neurogenesis, and cognition. However, FABP inhibition (genetic silencing or pharmacological) had a positive effect in many more disease conditions. Several polymorphisms of each FABP gene have also been linked to pathological conditions, but it was unclear how several polymorphisms affected protein function. Overall, analysis of the literature to date suggests that pharmacological inhibition of FABPs in adults is of low risk.

Proteomic analysis of signaling pathways modulated by FABP5 in macrophages

Background

While acute inflammation serves essential functions in maintaining tissue homeostasis, chronic inflammation is causally linked to many diseases. Macrophages are a major cell-type that orchestrates inflammatory processes. During inflammation, macrophages undergo polarization and activation, thereby mobilizing pro-inflammatory and anti-inflammatory transcriptional programs that regulate ensuing macrophage functions. Fatty acid binding protein 5 (FABP5) is a lipid chaperone that is highly expressed in macrophages. FABP5 deletion is implicated in driving macrophages towards an anti-inflammatory phenotype, yet the signaling pathways regulated by macrophage FABP5 have not been systematically profiled. Herein, we leveraged proteomic and phosphoproteomic approaches to characterize pathways modulated by FABP5 in M1 and M2 polarized bone marrow derived macrophages (BMDMs).

Results

Stable isotope labeling by amino acids (SILAC) based analysis of M1 and M2 polarized wild-type (WT) and FABP5 knockout (KO) BMDMs revealed numerous differentially regulated proteins and phosphoproteins. FABP5 deletion impacted several downstream pathways associated with inflammation, cytokine production, oxidative stress, and kinase activity. Kinase enrichment analysis based on phosphorylated sites revealed key kinases, including members of the GRK family, that were altered in FABP5 KO BMDMs. Reactive oxygen species (ROS) levels were elevated in M1 polarized KO macrophages, consistent with the differential protein expression profiles.

Conclusions

This study represents a comprehensive characterization of the impact of FABP5 deletion upon the proteomic and phosphoproteomic landscape of M1 and M2 polarized BMDMs. Loss of FABP5 altered multiple pathways implicated in inflammatory responses and macrophage function. This work provides a foundation for future studies seeking to investigate the therapeutic potential of FABP5 inhibition in pathophysiological states resulting from dysregulated inflammatory signaling.

Macrophage programming is regulated by a cooperative interaction between fatty acid binding protein 5 and peroxisome proliferator-activated receptor γ

Resolution of inflammation is an active process that is tightly regulated to achieve repair and tissue homeostasis. In the absence of resolution, persistent inflammation underlies the pathogenesis of chronic lung disease such as chronic obstructive pulmonary disease (COPD) with recurrent exacerbations. Over the course of inflammation, macrophage programming transitions from pro-inflammatory to pro-resolving, which is in part regulated by the nuclear receptor Peroxisome Proliferator-Activated Receptor γ (PPARγ). Our previous work demonstrated an association between Fatty Acid Binding Protein 5 (FABP5) expression and PPARγ activity in peripheral blood mononuclear cells of healthy and COPD patients. However, a role for FABP5 in macrophage programming has not been examined. Here, using a combination of in vitro and in vivo approaches, we demonstrate that FABP5 is necessary for PPARγ activation. In turn, PPARγ acts directly to increase FABP5 expression in primary human alveolar macrophages. We further illustrate that lack of FABP5 expression promotes a pro-inflammatory macrophage programming with increased secretion of pro-inflammatory cytokines and increased chromatin accessibility for pro-inflammatory transcription factors (e.g., NF-κB and MAPK). And finally, real-time cell metabolic analysis using the Seahorse technology shows an inhibition of oxidative phosphorylation in FABP5-deficient macrophages. Taken together, our data indicate that FABP5 and PPARγ reciprocally regulate each other's expression and function, consistent with a novel positive feedback loop between the two factors that mediates macrophage pro-resolving programming. Our studies highlight the importance of defining targets and regulatory mechanisms that control the resolution of inflammation and may serve to inform novel interventional strategies directed towards COPD.

FABP5 Deficiency Impairs Mitochondrial Function and Aggravates Pathological Cardiac Remodeling and Dysfunction

Fatty acid-binding protein 5 (FABP5) is an important member of the FABP family and plays a vital role in the metabolism of fatty acids. However, few studies have examined the role of FABP5 in pathological cardiac remodeling and heart failure. The aim of this study was to explore the role of FABP5 in transverse aortic constriction (TAC)-induced pathological cardiac remodeling and dysfunction in mice. Quantitative RT-PCR (qRT-PCR) and western blotting (WB) analysis showed that the levels of FABP5 mRNA and protein, respectively, were upregulated in hearts of the TAC model. Ten weeks after TAC in FABP5 knockout and wild type control mice, echocardiography, histopathology, qRT-PCR, and WB demonstrated that FABP5 deficiency aggravated cardiac injury (both cardiac hypertrophy and fibrosis) and dysfunction. In addition, transmission electron microscopy, ATP detection, and WB revealed that TAC caused severe impairment to mitochondria in the hearts of FABP5-deficient mice compared with that in control mice. When FABP5 was downregulated by siRNA in primary mouse cardiac fibroblasts, FABP5 silencing increased oxidative stress, reduced mitochondrial respiration, and increased the expression of myofibroblast activation marker genes in response to treatment with transforming growth factor-β. Our findings demonstrate that FABP5 deficiency aggravates cardiac pathological remodeling and dysfunction by damaging cardiac mitochondrial function.

New Classification of Macrophages in Plaques: a Revolution

PURPOSE

Macrophages play vital roles in the development of atherosclerosis in responding to lipid accumulation and inflammation. Macrophages were classified as inflammatory (M1) and alternatively activated (M2) macrophage types based on results of in vitro experiments. On the other hand, the composition of macrophages in vivo is more complex and remains unresolved. This review summarizes the transcriptional variations of macrophages in atherosclerosis plaques that were discovered by single-cell RNA sequencing (scRNA-seq) to better understand their contribution to atherosclerosis.

RECENT FINDINGS

ScRNA-seq provides a more detailed transcriptional landscape of macrophages in atherosclerosis, which challenges the traditional view. By mining the data of GSE97310, we discovered the transcriptional variations of macrophages in LDLR^-/- mice that were fed with high-fat diet (HFD) for 11 and 20 weeks. Cells were represented in a two-dimensional tSNE plane and clusters were identified and annotated via Seurat and SingleR respectively, which were R toolkits for single-cell genomics. The results showed that in healthy conditions, Trem2^hi (high expression of triggering receptors expressed on myeloid cells 2)-positive, inflammatory, and resident-like macrophages make up 68%, 18%, and 6% of total macrophages respectively. When mice were fed with HFD for 11 weeks, Trem2^hi, monocytes, and monocyte-derived dendritic cells take possession of 40%, 18%, and 17% of total macrophages respectively. After 20 weeks of HFD feeding, Trem2^hi, inflammatory, and resident-like macrophages occupied 12%, 37%, and 35% of total macrophages respectively. The phenotypes of macrophages are very different from the previous studies. In general, Trem2^hi macrophages are the most abundant population in healthy mice, while the proportion of monocytes increases after 11 weeks of HFD. Most importantly, inflammatory and resident-like macrophages make up 70% of the macrophage populations after 20 weeks of HFD. These strongly indicate that inflammatory and resident-like macrophages promote the progression of atherosclerosis plaques.

Single-cell RNA-sequencing reveals profound changes in circulating immune cells in patients with heart failure

AIMS

Identification of signatures of immune cells at single-cell level may provide novel insights into changes of immune-related disorders. Therefore, we used single-cell RNA-sequencing to determine the impact of heart failure on circulating immune cells.

METHODS AND RESULTS

We demonstrate a significant change in monocyte to T-cell ratio in patients with heart failure, compared to healthy subjects, which were validated by flow cytometry analysis. Subclustering of monocytes and stratification of the clusters according to relative CD14 and FCGR3A (CD16) expression allowed annotation of classical, intermediate, and non-classical monocytes. Heart failure had a specific impact on the gene expression patterns in these subpopulations. Metabolically active genes such as FABP5 were highly enriched in classical monocytes of heart failure patients, whereas β-catenin expression was significantly higher in intermediate monocytes. The selective regulation of signatures in the monocyte subpopulations was validated by classical and multifactor dimensionality reduction flow cytometry analyses.

CONCLUSION

Together this study shows that circulating cells derived from patients with heart failure have altered phenotypes. These data provide a rich source for identification of signatures of immune cells in heart failure compared to healthy subjects. The observed increase in FABP5 and signatures of Wnt signalling may contribute to enhanced monocyte activation.