cChronic inflammation in obese adipose tissue is linked to endoplasmic reticulum (ER) stress and systemic insulin resistance. Targeted deletion of the murine fatty acid binding protein (FABP4/aP2) uncouples obesity from inflammation although the mechanism underlying this finding has remained enigmatic. Here, we show that inhibition or deletion of FABP4/aP2 in macrophages results in increased intracellular free fatty acids (FFAs) and elevated expression of uncoupling protein 2 (UCP2) without concomitant increases in UCP1 or UCP3. Silencing of UCP2 mRNA in FABP4/aP2-deficient macrophages negated the protective effect of FABP loss and increased ER stress in response to palmitate or lipopolysaccharide (LPS). Pharmacologic inhibition of FABP4/aP2 with the FABP inhibitor HTS01037 also upregulated UCP2 and reduced expression of BiP, CHOP, and XBP-1s. Expression of native FABP4/aP2 (but not the non-fatty acid binding mutant R126Q) into FABP4/aP2 null cells reduced UCP2 expression, suggesting that the FABP-FFA equilibrium controls UCP2 expression. FABP4/aP2-deficient macrophages are resistant to LPS-induced mitochondrial dysfunction and exhibit decreased mitochondrial protein carbonylation and UCP2-dependent reduction in intracellular reactive oxygen species. These data demonstrate that FABP4/aP2 directly regulates intracellular FFA levels and indirectly controls macrophage inflammation and ER stress by regulating the expression of UCP2.O besity-linked metabolic disorders, including insulin resistance, fatty liver disease, and coronary arterial disease, share the common signature of chronic inflammation and endoplasmic reticulum (ER) stress (1, 2). Macrophage and T cell infiltration and activation in adipose tissue play a key role in affecting adipokine synthesis and secretion, thereby regulating systemic insulin resistance (3). Inflammatory cytokines increase oxidative stress and decrease the protein-folding efficiency of the ER, initiating a counterregulatory unfolded protein response (UPR) (4) involving pancreatic ER kinase (PERK), activating transcription factor-6 (ATF6), and inositol-requiring enzyme 1 (IRE1). Such concomitant activation leads to the downstream activation of response pathways and the induction of inflammatory signaling networks via c-Jun N-terminal kinase (JNK) and/or NF-B (nuclear factor kappa B) (1).Lipid metabolism in macrophages has been shown to play an important role in triggering inflammation and ER stress (5, 6) and has led to the identification of critical proteins that regulate the obesity-metabolic disease axis. For example, genetic ablation of the fatty acid binding protein (FABP4, also known as aP2) in macrophages alone is sufficient to protect mice from development of atherosclerosis and dyslipidemia (7). FABP4/aP2 is a cytoplasmic fatty acid (FA) carrier protein that mediates intracellular fatty acid trafficking, and a number of hypotheses have been proposed to explain why the loss of FABP4/aP2 results in metabolic improvement (6). Moreover, small molecules that target FABP4/aP2 have ...

Activation of proinflammatory macrophages plays an important role in the pathogenesis of insulin resistance, type 2 diabetes, and atherosclerosis. Previous work using high fat-fed mice has shown that ablation of the adipocyte fatty acid binding protein (FABP4/aP2) in macrophages leads to an antiinflammatory state both in situ and in vivo, and the mechanism is linked, in part, to increased intracellular monounsaturated fatty acids and the up-regulation of uncoupling protein 2. Here, we show that loss of FABP4/aP2 in macrophages additionally induces sirtuin 3 (SIRT3) expression and that monounsaturated fatty acids (C16:1, C18:1) lead to increased SIRT3 protein expression. Increased expression of SirT3 in FABP4/aP2 null macrophages occurs at the protein level with no change in SirT3 mRNA. When compared with controls, silencing of SIRT3 in Raw246.7 macrophages leads to increased expression of inflammatory cytokines, inducible nitric oxide synthase and cyclooxygenase 2. In contrast, loss of SIRT3 in FABP4/aP2-deficient macrophages attenuates the suppressed inflammatory signaling, reduced reactive oxygen species production, lipopolysaccharide-induced mitochondrial dysfunction, and increased fatty acid oxidation. These results suggest that the antiinflammatory phenotype of FABP4/aP2 null mice is mediated by increased intracellular monounsaturated fatty acids leading to the increased expression of both uncoupling protein 2 and SirT3.

Obesity-linked metabolic disease is mechanistically associated with the accumulation of proinflammatory macrophages in adipose tissue, leading to increased reactive oxygen species (ROS) production and chronic low-grade inflammation. Previous work has demonstrated that deletion of the adipocyte fatty acidbinding protein (FABP4/aP2) uncouples obesity from inflammation via upregulation of the uncoupling protein 2 (UCP2). Here, we demonstrate that ablation of FABP4/ aP2 regulates systemic redox capacity and reduces cellular protein sulfhydryl oxidation and, in particular, oxidation of mitochondrial protein cysteine residues. Coincident with the loss of FABP4/aP2 is the upregulation of the antioxidants superoxide dismutase (SOD2), catalase, methionine sulfoxide reductase A, and the 20S proteasome subunits PSMB5 and ␣␤. Reduced mitochondrial protein oxidation in FABP4/ aP2 Ϫ/Ϫ macrophages attenuates the mitochondrial unfolded-protein response (mtUPR) as measured by expression of heat shock protein 60, Clp protease, and Lon peptidase 1. Consistent with a diminished mtUPR, FABP4/aP2 Ϫ/Ϫ macrophages exhibit reduced expression of cleaved caspase-1 and NLRP3. Secretion of interleukin 1␤ (IL-1␤), in response to inflammasome activation, is ablated in FABP4/aP2 Ϫ/Ϫ macrophages, as well as in FABP4/aP2 inhibitor-treated cells, but partially rescued in FABP4/aP2-null macrophages when UCP2 is silenced. Collectively, these data offer a novel pathway whereby FABP4/aP2 regulates macrophage redox signaling and inflammasome activation via control of UCP2 expression.KEYWORDS FABP, inflammation, UCP2, inflammasome, mitochondrial metabolism, obesity O besity is linked to a variety of metabolic diseases, including type II diabetes, dyslipidemia, and cardiovascular disease. Accumulation of visceral adipose tissue (VAT) is critical for disease development, as it contains inflammatory macrophages that secrete cytokines, chemokines, and other signaling molecules, leading to both local and systemic effects (1, 2). Increased abundance of reactive oxygen species (ROS) is one critical consequence of adipose tissue inflammation, and hydrogen peroxide (H 2 O 2 ) serves as a secondary messenger in various immunometabolic signaling processes (3, 4). H 2 O 2 leads to reversible cysteine and methionine oxidation that alters protein activity and/or interactions (5, 6). Moreover, chronically elevated oxidative species diminish the cellular pool of chemical antioxidants (glutathione [GSH] and NADPH), leading to a variety of pathophysiologies (7-10). Sustained protein oxidation leads to not only dysregulated signaling cascades but also protein unfolding and potentially aggregation (11,12). Multiple quality control systems exist in the endoplasmic reticulum (ER) and mitochondria to prevent oxidative damage and protein aggregation (4,(13)(14)(15). Despite this, the mitochondria in particular are vulnerable to sustained oxidative stress due to the high capacity for reactive oxygen species synthesis in the electron transport chain (16-18). Mitochond...

BackgroundBariatric surgery remains the most effective treatment for reducing adiposity and eliminating type 2 diabetes, however the mechanism(s) responsible have remained elusive. Peroxisome proliferator activated receptors (PPAR) encompass a family of nuclear hormone receptors that upon activation exert control of lipid metabolism, glucose regulation, and inflammation. Their role in adipose tissue following bariatric surgery remains undefined.Materials and MethodsSubcutaneous adipose tissue biopsies and serum were obtained and evaluated from at time of surgery and on postoperative day 7 in patients randomized to Roux-en-Y gastric bypass (n=13) or matched caloric restriction (n=14), as well as patients undergoing vertical sleeve gastrectomy (n=33). Fat samples were evaluated for changes in gene expression, protein levels, β-oxidation, lipolysis, and cysteine oxidation.ResultsWithin 7 days, bariatric surgery acutely drives a change in the activity and expression of PPARγ and PPARδ in subcutaneous adipose tissue thereby attenuating lipid storage, increasing lipolysis and potentiating lipid oxidation. This unique metabolic alteration leads to changes in downstream PPARγ/δ targets including decreased expression of FABP4 and SCD1 with increased expression of carnitine palmitoyl transferase 1 (CPT1) and uncoupling protein 2 (UCP2). Increased expression of UCP2 not only facilitated fatty acid oxidation (increased 15-fold following surgery) but also regulated the subcutaneous adipose tissue redoxome by attenuating protein cysteine oxidation and reducing oxidative stress. The expression of UCP1, a mitochondrial protein responsible for the regulation of fatty acid oxidation and thermogenesis in beige and brown fat, was unaltered following surgery.ConclusionsThese results suggest that bariatric surgery initiates a novel metabolic shift in subcutaneous adipose tissue to oxidize fatty acids independently from the beiging process through regulation of PPAR isoforms. Further studies are required to understand the contribution of this shift in expression of PPAR isoforms as a contributor to weight loss following bariatric surgery.

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.