Clare M. Reynolds

Fats, inflammation and insulin resistance: insights to the role of macrophage and T-cell accumulation in adipose tissue.

High-fat diet-induced obesity is associated with a chronic state of low-grade inflammation, which pre-disposes to insulin resistance (IR), which can subsequently lead to type 2 diabetes mellitus. Macrophages represent a heterogeneous population of cells that are instrumental in initiating the innate immune response. Recent studies have shown that macrophages are key mediators of obesity-induced IR, with a progressive infiltration of macrophages into obese adipose tissue. These adipose tissue macrophages are referred to as classically activated (M1) macrophages. They release cytokines such as IL-1β, IL-6 and TNFα creating a pro-inflammatory environment that blocks adipocyte insulin action, contributing to the development of IR and type 2 diabetes mellitus. In lean individuals macrophages are in an alternatively activated (M2) state. M2 macrophages are involved in wound healing and immunoregulation. Wound-healing macrophages play a major role in tissue repair and homoeostasis, while immunoregulatory macrophages produce IL-10, an anti-inflammatory cytokine, which may protect against inflammation. The functional role of T-cell accumulation has recently been characterised in adipose tissue. Cytotoxic T-cells are effector T-cells and have been implicated in macrophage differentiation, activation and migration. Infiltration of cytotoxic T-cells into obese adipose tissue is thought to precede macrophage accumulation. T-cell-derived cytokines such as interferon γ promote the recruitment and activation of M1 macrophages augmenting adipose tissue inflammation and IR. Manipulating adipose tissue macrophages/T-cell activity and accumulation in vivo through dietary fat modification may attenuate adipose tissue inflammation, representing a therapeutic target for ameliorating obesity-induced IR.

Lack of Interleukin-1 Receptor I (IL-1RI) Protects Mice From High-Fat Diet–Induced Adipose Tissue Inflammation Coincident With Improved Glucose Homeostasis

OBJECTIVE High-fat diet (HFD)-induced adipose tissue inflammation is a critical feature of diet-induced insulin resistance (IR); however, the contribution of interleukin-1 receptor I (IL-1RI)-mediated signals to this phenotype has not been defined. We hypothesized that lack of IL-1RI may ameliorate HFD-induced IR by attenuating adipose tissue inflammation. RESEARCH DESIGN AND METHODS Glucose homeostasis was monitored in chow- and HFD-fed wild-type (WT) and IL-1RI−/− mice by glucose tolerance and insulin tolerance tests. Macrophage recruitment and cytokine signature of adipose tissue macrophages was evaluated. Insulin sensitivity and cytokine secretion from adipose explants was quantified. Cytokine secretion and adipocyte insulin sensitivity was measured in cocultures of WT or IL-1RI−/− macrophages with 3T3L1 adipocytes. Synergistic effects of IL-1β with tumor necrosis factor (TNF)-α on inflammation was monitored in WT and IL-1RI−/− bone-marrow macrophages and adipose explants. RESULTS Lean and obese IL-1RI−/− animals exhibited enhanced glucose homeostasis by glucose tolerance test and insulin tolerance test. M1/M2 macrophage number in adipose tissue was comparable between genotypes; however, TNF-α and IL-6 secretion was lower from IL-1RI−/− adipose tissue macrophages. IL-1RI−/− adipose exhibited enhanced insulin sensitivity, elevated pAKT, lower cytokine secretion, and attenuated induction of phosphorylated signal transducer and activator of transcription 3 and suppressor of cytokine signaling molecule 3 after HFD. Coculture of WT, but not IL-1RI−/− macrophages, with 3T3L1 adipocytes enhanced IL-6 and TNF-α secretion, reduced adiponectin secretion, and impaired adipocyte insulin sensitivity. TNF-α and IL-1β potently synergized to enhance inflammation in WT macrophages and adipose, an effect lost in the absence of IL-1RI. CONCLUSIONS Lack of IL-1RI protects against HFD-induced IR coincident with reduced local adipose tissue inflammation, despite equivalent immune cell recruitment.

Docosahexaenoic acid attenuates macrophage-induced inflammation and improves insulin sensitivity in adipocytes-specific differential effects between LC n-3 PUFA ☆

OBJECTIVE Adipose tissue inflammation with immune cell recruitment plays a key role in obesity-induced insulin resistance (IR). Long-chain (LC) n-3 polyunsaturated fatty acids (PUFA) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) have anti-inflammatory potential; however, their individual effects on adipose IR are ill defined. We hypothesized that EPA and DHA may differentially affect macrophage-induced IR in adipocytes. METHODS J774.2 macrophages pretreated with EPA or DHA (50 μM for 5 days) were stimulated with lipopolysaccharide (LPS, 100 ng/ml for 30 min-48 h). Cytokine secretion profiles and activation status of macrophages were assessed by enzyme-linked immunosorbent assay and flow cytometry. Pretreated macrophages were seeded onto transwell inserts and placed over 3T3-L1 adipocytes for 24-72 h; effects on adipocyte-macrophage cytokine cross-talk and insulin-stimulated ³H-glucose transport into adipocytes were monitored. RESULTS DHA had more potent anti-inflammatory effects relative to EPA, with marked attenuation of LPS-induced nuclear factor (NF)κB activation and tumor necrosis factor (TNF)α secretion in macrophages. DHA specifically enhanced anti-inflammatory interleukin (IL)-10 secretion and reduced the expression of proinflammatory M1 (F4/80⁺/CD11⁺) macrophages. Co-culture of DHA-enriched macrophages with adipocytes attenuated IL-6 and TNFα secretion while enhancing IL-10 secretion. Conditioned media (CM) from DHA-enriched macrophages attenuated adipocyte NFκB activation. Adipocytes co-cultured with DHA-enriched macrophages maintained insulin sensitivity with enhanced insulin-stimulated ³H-glucose transport, GLUT4 translocation and preservation of insulin-receptor substrate-1 expression compared to co-culture with untreated macrophages. We confirmed that IL-10 expressed by DHA-enriched macrophages attenuates the CM-induced proinflammatory IR phenotype in adipocytes. CONCLUSIONS We demonstrate an attenuated proinflammatory phenotype of DHA-pretreated macrophages, which when co-cultured with adipocytes partially preserved insulin sensitivity.

Monounsaturated fatty acid enriched high fat-diets impede adipose NLRP3 inflammasome mediated IL-1β secretion and insulin resistance despite obesity

Saturated fatty acid (SFA) high-fat diets (HFDs) enhance interleukin (IL)-1β–mediated adipose inflammation and insulin resistance. However, the mechanisms by which different fatty acids regulate IL-1β and the subsequent effects on adipose tissue biology and insulin sensitivity in vivo remain elusive. We hypothesized that the replacement of SFA for monounsaturated fatty acid (MUFA) in HFDs would reduce pro-IL-1β priming in adipose tissue and attenuate insulin resistance via MUFA-driven AMPK activation. MUFA-HFD–fed mice displayed improved insulin sensitivity coincident with reduced pro-IL-1β priming, attenuated adipose IL-1β secretion, and sustained adipose AMPK activation compared with SFA-HFD–fed mice. Furthermore, MUFA-HFD–fed mice displayed hyperplastic adipose tissue, with enhanced adipogenic potential of the stromal vascular fraction and improved insulin sensitivity. In vitro, we demonstrated that the MUFA oleic acid can impede ATP-induced IL-1β secretion from lipopolysaccharide- and SFA-primed cells in an AMPK-dependent manner. Conversely, in a regression study, switching from SFA- to MUFA-HFD failed to reverse insulin resistance but improved fasting plasma insulin levels. In humans, high-SFA consumers, but not high-MUFA consumers, displayed reduced insulin sensitivity with elevated pycard-1 and caspase-1 expression in adipose tissue. These novel findings suggest that dietary MUFA can attenuate IL-1β–mediated insulin resistance and adipose dysfunction despite obesity via the preservation of AMPK activity.

Dietary saturated fatty acids prime the NLRP3 inflammasome via TLR4 in dendritic cells—implications for diet-induced insulin resistance

SCOPE Inflammasome-mediated inflammation is a critical regulator of obesity-induced insulin resistance (IR). We hypothesized that saturated fatty acids (SFA) directly prime the NLRP3 inflammasome via TLR4 concurrent with IR. We focused on dendritic cells (DCs) (CD11c(+) CD11b(+) F4/80(-) ), which are recruited into obese adipose tissue following high-fat diet (HFD) challenge and are a key cell in inflammasome biology. METHODS AND RESULTS C57BL/6 mice were fed HFD for 16 weeks (45% kcal palm oil), glucose homeostasis was monitored by glucose and insulin tolerance tests. Stromal vascular fraction (SVF) cells were isolated from adipose and analyzed for CD11c(+) CD11b(+) F480(-) DC. Following coculture with bone marrow derived DC (BMDC) insulin-stimulated (3) H-glucose transport into adipocytes, IL-1β secretion and caspase-1 activation was monitored. BMDCs primed with LPS (100 ng/mL), linoleic acid (LA; 200 μM), or palmitic acid (PA; 200 μM) were used to monitor inflammasome activation. We demonstrated significant infiltration of DCs into adipose after HFD. HFD-derived DCs reduce adipocyte insulin sensitivity upon coculture co-incident with enhanced adipocyte caspase-1 activation/IL-1β secretion. HFD-derived DCs are skewed toward a pro-inflammatory phenotype with increased IL-1β secretion, IL-1R1, TLR4, and caspase-1 expression. Complementary in vitro experiments demonstrate that TLR4 is critical in propagating SFA-mediated inflammasome activation. CONCLUSION SFA represent metabolic triggers priming the inflammasome, promoting adipocyte inflammation/IR, suggesting direct effects of SFA on inflammasome activation via TLR4.

Maternal Obesity, Inflammation, and Developmental Programming

The prevalence of obesity, especially in women of child-bearing age, is a global health concern. In addition to increasing the immediate risk of gestational complications, there is accumulating evidence that maternal obesity also has long-term consequences for the offspring. The concept of developmental programming describes the process in which an environmental stimulus, including altered nutrition, during critical periods of development can program alterations in organogenesis, tissue development, and metabolism, predisposing offspring to obesity and metabolic and cardiovascular disorders in later life. Although the mechanisms underpinning programming of metabolic disorders remain poorly defined, it has become increasingly clear that low-grade inflammation is associated with obesity and its comorbidities. This review will discuss maternal metainflammation as a mediator of programming in insulin sensitive tissues in offspring. Use of nutritional anti-inflammatories in pregnancy including omega 3 fatty acids, resveratrol, curcumin, and taurine may provide beneficial intervention strategies to ameliorate maternal obesity-induced programming.

Conjugated linoleic acid and inflammatory cell signalling

Conjugated linoleic acids (CLA) are a family of polyunsaturated fatty acids (PUFA), some isomers occurring naturally in beef and dairy products and others being formed as a result of bihydrogenation of vegetable oils to form margarine. Synthetic and natural sources of CLA may have beneficial effects in a range of inflammatory conditions including colitis, atherosclerosis, metabolic syndrome and rheumatoid arthritis. Most of the biological effects have been attributed to the cis9, trans11- (c9, t11-) and the trans10, cis12- (t10, c12-) isomers. Evidence suggests that c9, t11-CLA is responsible for the anti-inflammatory effect attributed to CLA while t10, t12-CLA appears to be responsible for anti-adipogenic effects. This review will focus on the effects of CLA on the inflammatory components associated with insulin resistance, atherosclerosis and Th1 mediated inflammatory disease, at a cellular, systemic and clinical level. Whist CLA may ameliorate certain aspects of the inflammatory response, particularly within cellular and animal models, the relevance of this has yet to be clarified within the context of human health.

Omega-3 fatty acids attenuate dendritic cell function via NF-κB independent of PPARγ☆

Long-chain n-3 polyunsaturated fatty acids (n-3 PUFA) have been shown to modulate the immune response and have therapeutic effects in inflammatory disorders. PUFA are also peroxisome proliferators-activator receptor-gamma (PPARγ) ligands; a family of ligand-activated transcription factors, which when activated antagonise the pro-inflammatory capability of nuclear factor κB (NF-κB). PPARγ plays a role in dendritic cell (DC) maturation and n-3 PUFA have been shown to affect DC maturation by decreasing activation of NF-κB. While n-3 PUFA can function as PPAR ligands, it is not known whether the NF-κB-mediated immunomodulatory properties of n-3 PUFA are PPARγ-dependent. In this study we examined whether the immunomodulatory effects of n-3 PUFA on DC activation were mediated through activation of PPARγ. Treatment of murine bone marrow derived DCs with docosahexaenoic acid (DHA; 25 μM) and eicosapentaenoic acid (EPA; 25 μM) attenuated LPS-induced DC maturation. This was characterised by suppression of IL-12 production and expression of CD40, CD80, CD86 and MHC II and enhanced production of IL-10 and expression of IL-10R. This was coincident with enhanced PPARγ expression, suppressed NF-κB activity and increased the physical interaction and cellular colocalization between NF-κB with PPARγ. To understand the functional implication of the physical association of PPARγ with NF-κB, we determined whether the specific PPARγ inhibitor, GW9662 could abolish the anti-inflammatory effect of n-3 PUFA Inhibiting PPARγ did not impede the NF-κB-mediated anti-inflammatory cytokine profile induced by EPA and DHA alone. Thus n-3 PUFA activate PPARγ and interact with NF-κB in DC. However, the anti-inflammatory effects of EPA and DHA on DCs are independent of PPARγ.

Insights into the role of macrophage migration inhibitory factor in obesity and insulin resistance

High-fat diet (HFD)-induced obesity has emerged as a state of chronic low-grade inflammation characterised by a progressive infiltration of immune cells, particularly macrophages, into obese adipose tissue. Adipose tissue macrophages (ATM) present immense plasticity. In early obesity, M2 anti-inflammatory macrophages acquire an M1 pro-inflammatory phenotype. Pro-inflammatory cytokines including TNF-α, IL-6 and IL-1β produced by M1 ATM exacerbate local inflammation promoting insulin resistance (IR), which consequently, can lead to type-2 diabetes mellitus (T2DM). However, the triggers responsible for ATM recruitment and activation are not fully understood. Adipose tissue-derived chemokines are significant players in driving ATM recruitment during obesity. Macrophage migration inhibitory factor (MIF), a chemokine-like inflammatory regulator, is enhanced during obesity and is directly associated with the degree of peripheral IR. This review focuses on the functional role of macrophages in obesity-induced IR and highlights the importance of the unique inflammatory cytokine MIF in propagating obesity-induced inflammation and IR. Given MIF chemotactic properties, MIF may be a primary candidate promoting ATM recruitment during obesity. Manipulating MIF inflammatory activities in obesity, using pharmacological agents or functional foods, may be therapeutically beneficial for the treatment and prevention of obesity-related metabolic diseases.

Effects of taurine supplementation on hepatic markers of inflammation and lipid metabolism in mothers and offspring in the setting of maternal obesity.

Maternal obesity is associated with obesity and metabolic disorders in offspring. However, intervention strategies to reverse or ameliorate the effects of maternal obesity on offspring health are limited. Following maternal undernutrition, taurine supplementation can improve outcomes in offspring, possibly via effects on glucose homeostasis and insulin secretion. The effects of taurine in mediating inflammatory processes as a protective mechanism has not been investigated. Further, the efficacy of taurine supplementation in the setting of maternal obesity is not known. Using a model of maternal obesity, we examined the effects of maternal taurine supplementation on outcomes related to inflammation and lipid metabolism in mothers and neonates. Time-mated Wistar rats were randomised to either: 1) control : control diet during pregnancy and lactation (CON); 2) CON supplemented with 1.5% taurine in drinking water (CT); 3) maternal obesogenic diet (high fat, high fructose) during pregnancy and lactation (MO); or 4) MO supplemented with taurine (MOT). Maternal and neonatal weights, plasma cytokines and hepatic gene expression were analysed. A MO diet resulted in maternal hyperinsulinemia and hyperleptinemia and increased plasma glucose, glutamate and TNF-α concentrations. Taurine normalised maternal plasma TNF-α and glutamate concentrations in MOT animals. Both MO and MOT mothers displayed evidence of fatty liver accompanied by alterations in key markers of hepatic lipid metabolism. MO neonates displayed a pro-inflammatory hepatic profile which was partially rescued in MOT offspring. Conversely, a pro-inflammatory phenotype was observed in MOT mothers suggesting a possible maternal trade-off to protect the neonate. Despite protective effects of taurine in MOT offspring, neonatal mortality was increased in CT neonates, indicating possible adverse effects of taurine in the setting of normal pregnancy. These data suggest that maternal taurine supplementation may ameliorate the adverse effects observed in offspring following a maternal obesogenic diet but these effects are dependent upon prior maternal nutritional background.