Anthony W. Ferrante

Macrophage-specific PPARγ controls alternative activation and improves insulin resistance

Obesity and insulin resistance, the cardinal features of metabolic syndrome, are closely associated with a state of low-grade inflammation. In adipose tissue chronic overnutrition leads to macrophage infiltration, resulting in local inflammation that potentiates insulin resistance. For instance, transgenic expression of Mcp1 (also known as chemokine ligand 2, Ccl2) in adipose tissue increases macrophage infiltration, inflammation and insulin resistance. Conversely, disruption of Mcp1 or its receptor Ccr2 impairs migration of macrophages into adipose tissue, thereby lowering adipose tissue inflammation and improving insulin sensitivity. These findings together suggest a correlation between macrophage content in adipose tissue and insulin resistance. However, resident macrophages in tissues display tremendous heterogeneity in their activities and functions, primarily reflecting their local metabolic and immune microenvironment. While Mcp1 directs recruitment of pro-inflammatory classically activated macrophages to sites of tissue damage, resident macrophages, such as those present in the adipose tissue of lean mice, display the alternatively activated phenotype. Despite their higher capacity to repair tissue, the precise role of alternatively activated macrophages in obesity-induced insulin resistance remains unknown. Using mice with macrophage-specific deletion of the peroxisome proliferator activated receptor-γ (PPARγ), we show here that PPARγ is required for maturation of alternatively activated macrophages. Disruption of PPARγ in myeloid cells impairs alternative macrophage activation, and predisposes these animals to development of diet-induced obesity, insulin resistance, and glucose intolerance. Furthermore, gene expression profiling revealed that downregulation of oxidative phosphorylation gene expression in skeletal muscle and liver leads to decreased insulin sensitivity in these tissues. Together, our findings suggest that resident alternatively activated macrophages have a beneficial role in regulating nutrient homeostasis and suggest that macrophage polarization towards the alternative state might be a useful strategy for treating type 2 diabetes.

Alternative M2 Activation of Kupffer Cells by PPARδ Ameliorates Obesity-Induced Insulin Resistance

Macrophage infiltration and activation in metabolic tissues underlie obesity-induced insulin resistance and type 2 diabetes. While inflammatory activation of resident hepatic macrophages potentiates insulin resistance, the functions of alternatively activated Kupffer cells in metabolic disease remain unknown. Here we show that in response to the Th2 cytokine interleukin-4 (IL-4), peroxisome proliferator-activated receptor delta (PPARdelta) directs expression of the alternative phenotype in Kupffer cells and adipose tissue macrophages of lean mice. However, adoptive transfer of PPARdelta(-/-) (Ppard(-/-)) bone marrow into wild-type mice diminishes alternative activation of hepatic macrophages, causing hepatic dysfunction and systemic insulin resistance. Suppression of hepatic oxidative metabolism is recapitulated by treatment of primary hepatocytes with conditioned medium from PPARdelta(-/-) macrophages, indicating direct involvement of Kupffer cells in liver lipid metabolism. Taken together, these data suggest an unexpected beneficial role for alternatively activated Kupffer cells in metabolic syndrome and type 2 diabetes.

C-C Chemokine Receptor 2 (CCR2) Regulates the Hepatic Recruitment of Myeloid Cells That Promote Obesity-Induced Hepatic Steatosis

OBJECTIVE Obesity induces a program of systemic inflammation that is implicated in the development of many of its clinical sequelae. Hepatic inflammation is a feature of obesity-induced liver disease, and our previous studies demonstrated reduced hepatic steatosis in obese mice deficient in the C-C chemokine receptor 2 (CCR2) that regulates myeloid cell recruitment. This suggests that a myeloid cell population is recruited to the liver in obesity and contributes to nonalcoholic fatty liver disease. RESEARCH DESIGN AND METHODS We used fluorescence-activated cell sorting to measure hepatic leukocyte populations in genetic and diet forms of murine obesity. We characterized in vivo models that increase and decrease an obesity-regulated CCR2-expressing population of hepatic leukocytes. Finally, using an in vitro co-culture system, we measured the ability of these cells to modulate a hepatocyte program of lipid metabolism. RESULTS We demonstrate that obesity activates hepatocyte expression of C-C chemokine ligand 2 (CCL2/MCP-1) leading to hepatic recruitment of CCR2+ myeloid cells that promote hepatosteatosis. The quantity of these cells correlates with body mass and in obese mice represents the second largest immune cell population in the liver. Hepatic expression of CCL2 increases their recruitment and in the presence of dietary fat induces hepatosteatosis. These cells activate hepatic transcription of genes responsible for fatty acid esterification and steatosis. CONCLUSIONS Obesity induces hepatic recruitment of a myeloid cell population that promotes hepatocyte lipid storage. These findings demonstrate that recruitment of myeloid cells to metabolic tissues is a common feature of obesity, not limited to adipose tissue.

The Immune Cells in Adipose Tissue

Although the pathological role of the immune system in several metabolic disorders, including type 1 diabetes mellitus (T1DM) and Addisons disease, has long been recognized and studied, only in the last decade has it become apparent that the immune system plays a broad and more subtle role in local and systemic metabolism. It is now apparent that the immune system monitors and responds to specific metabolic cues in both pathologic and non‐pathologic settings through a set of processes dubbed immunometabolism. Expansion of adipose tissue mass, activation of lipolysis, eating a high fat diet and even non‐shivering thermogenesis all lead to the recruitment and activation of immune cells in key metabolic tissues. The responses are complex and not completely defined, and indeed, as is typical of rapidly evolving research areas, there are some conflicting reports, especially related to the metabolic consequences of manipulation of immune function. However, what is clear is the consensus that metabolic processes, especially obesity and obesity‐related complications, activate both the innate and adaptive arms of the immune system. Canonical immune processes consist of discrete steps: surveillance, recognition, effector action and resolution. Over the last decade evidence for each part of the immune response has been found at the intersection of the immune system with metabolism. Although evidence for immune surveillance and modulation of metabolism has been found in the liver, muscle, hypothalamus and pancreas, immune cell function has been most intensively studied and best understood in adipose tissue where studies continue to provide insights into the intersection of the metabolic and immune systems. Here we review the modulation of immune cell populations in adipose tissue and discuss regulatory processes implicated in controlling the interface between metabolism and immunologic function.

Macrophage content in subcutaneous adipose tissue: associations with adiposity, age, inflammatory markers, and whole-body insulin action in healthy Pima Indians.

OBJECTIVE— In severely obese individuals and patients with diabetes, accumulation and activation of macrophages in adipose tissue has been implicated in the development of obesity-associated complications, including insulin resistance. We sought to determine whether in a healthy population, adiposity, sex, age, or insulin action is associated with adipose tissue macrophage content (ATMc) and/or markers of macrophage activation. RESEARCH DESIGN AND METHODS— Subcutaneous ATMc from young adult Pima Indians with a wide range of adiposity (13–46% body fat, by whole-body dual-energy X-ray absorptiometry) and insulin action (glucose disposal rate 1.6–9 mg/kg estimated metabolic body size/min, by glucose clamp) were measured. We also measured expression in adipose tissue of factors implicated in macrophage recruitment and activation to determine any association with ATMc and insulin action. RESULTS— ATMc, as assessed by immunohistochemistry (Mphi) and by macrophage-specific gene expression (CD68, CD11b, and CSF1R), were correlated with percent body fat, age, and female sex. Gene expression of CD68, CD11b, and CSF1R but not Mphi was correlated negatively with glucose disposal rate but not after adjustment for percent body fat, age, and sex. However, adipose tissue expression of plasminogen activator inhibitor type-1 (PAI-1) and CD11 antigen-like family member C (CD11c), markers produced by macrophages, were negatively correlated with adjusted glucose disposal rate (r = −0.28, P = 0.05 and r = −0.31, P = 0.03). CONCLUSIONS— ATMc is correlated with age and adiposity but not with insulin action independent of adiposity in healthy human subjects. However, PAI-1 and CD11c expression are independent predictors of insulin action, indicating a possible role for adipose tissue macrophage activation.

RAGE Regulates the Metabolic and Inflammatory Response to High Fat Feeding in Mice

In mammals, changes in the metabolic state, including obesity, fasting, cold challenge, and high-fat diets (HFDs), activate complex immune responses. In many strains of rodents, HFDs induce a rapid systemic inflammatory response and lead to obesity. Little is known about the molecular signals required for HFD-induced phenotypes. We studied the function of the receptor for advanced glycation end products (RAGE) in the development of phenotypes associated with high-fat feeding in mice. RAGE is highly expressed on immune cells, including macrophages. We found that high-fat feeding induced expression of RAGE ligand HMGB1 and carboxymethyllysine-advanced glycation end product epitopes in liver and adipose tissue. Genetic deficiency of RAGE prevented the effects of HFD on energy expenditure, weight gain, adipose tissue inflammation, and insulin resistance. RAGE deficiency had no effect on genetic forms of obesity caused by impaired melanocortin signaling. Hematopoietic deficiency of RAGE or treatment with soluble RAGE partially protected against peripheral HFD-induced inflammation and weight gain. These findings demonstrate that high-fat feeding induces peripheral inflammation and weight gain in a RAGE-dependent manner, providing a foothold in the pathways that regulate diet-induced obesity and offering the potential for therapeutic intervention.

Obesity, Inflammation, and Macrophages

The World Health Organization estimates that since 1980 the prevalence of obesity has increased more than threefold throughout much of the world, and this increase is not limited to developed nations. Indeed, the incidence of obesity is increasing most rapidly among rapidly industrializing countries raising the spectre of a burgeoning epidemic in obesity-associated diseases, including diabetes, dyslipidemia, nonalcoholic fatty liver disease and atherosclerosis. Reducing the rates of obesity and its attendant complications will require both coordinated public health policy and a better understanding of the pathophysiology of obesity. Obesity is associated with low grade chronic inflammation, a common feature of many complications of obesity that appears to emanate in part from adipose tissue. In obese individuals and rodents adipose tissue macrophage accumulation is a critical component in the development of obesity-induced inflammation. The macrophages in adipose tissue are bone marrow-derived and their number is strongly correlated with bodyweight, body mass index and total body fat. The recruited macrophages in adipose tissue express high levels of inflammatory factors that contribute to systemic inflammation and insulin resistance. Interventions aimed at either reducing macrophage numbers or decreasing their inflammatory characteristics improves insulin sensitivity and decreases inflammation. Macrophage accumulation and adipose tissue inflammation are dynamic processes under the control of multiple mechanisms. Investigating the role of macrophages in adipose tissue biology and the mechanisms involved in their recruitment and activation in obesity will provide useful insights for developing therapeutic approaches to treating obesity-induced complications.

Genomic Profiling of Left and Right Ventricular Hypertrophy in Congenital Heart Disease

BACKGROUND The right ventricle (RV) has a lower ability than the left ventricle (LV) to adapt to systemic load. The molecular basis of these differences is not known. We compared hypertrophy-signaling pathways between the RV and the LV in patients with congenital heart disease (CHD). METHODS Gene expression was measured using DNA microarrays in myocardium from children with CHD with LV or RV obstructive lesions undergoing surgery. The expression of 175 hypertrophy-signaling genes was compared between the LV (n=7) and the RV (n=11). Hierarchic clustering was performed. RESULTS Seventeen genes (10%) were differentially expressed between the LV and the RV. Expression of genes for angiotensin, adrenergic, G-proteins, cytoskeletal, and contractile components was lower (P < .05) and expression of maladaptive factors (fibroblast growth factors, transforming growth factor-beta, caspases, ubiquitin) was higher in the RV compared with the LV (P < .05). Five of 7 LV samples clustered together. Only 4 of 11 RV samples clustered with the LV. Genes critical to adaptive remodeling correlated with the degree of LV hypertrophy but not RV hypertrophy. CONCLUSION The transcription of pathways of adaptive remodeling was lower in the RV compared with the LV. This may explain the lower ability of the RV to adapt to hemodynamic load in CHD.

A Missing Link in Body Weight Homeostasis: The Catabolic Signal of the Overfed State

Mammals regulate fat mass so that increases or reductions in adipose tissue mass activate responses that favor return to ones previous weight. A reduction in fat mass activates a system that increases food intake and reduces energy expenditure; conversely, overfeeding and rapid adipose tissue expansion reduces food intake and increases energy expenditure. With the identification of leptin nearly two decades ago, the central circuit that defends against reductions in body fat was revealed. However, the systems that defend against rapid expansion of fat mass remain largely unknown. Here we review the physiology of the overfed state and evidence for a distinct regulatory system, which unlike the leptin-mediated system, we propose primarily measures a functional aspect of adipose tissue and not total mass per se.

Expanded Granulocyte/Monocyte Compartment in Myeloid-Specific Triple Foxo Knockout Increases Oxidative Stress and Accelerates Atherosclerosis in Mice

Rationale: Increased neutrophil and monocyte counts are often associated with an increased risk of atherosclerosis, but their relationship to insulin sensitivity is unknown. Objective: To investigate the contribution of forkhead transcription factors (FoxO) in myeloid cells to neutrophil and monocyte counts, atherosclerosis, and systemic insulin sensitivity. Methods and Results: Genetic ablation of the 3 genes encoding FoxO isoforms 1, 3a, and 4, in myeloid cells resulted in an expansion of the granulocyte/monocyte progenitor compartment and was associated with increased atherosclerotic lesion formation in low-density lipoprotein receptor knockout mice. In vivo and ex vivo studies indicate that FoxO ablation in myeloid cells increased generation of reactive oxygen species. Accordingly, treatment with the antioxidant N-acetyl-L-cysteine reversed the phenotype, normalizing atherosclerosis. Conclusions: Our data indicate that myeloid cell proliferation and oxidative stress can be modulated via the FoxO branch of insulin receptor signaling, highlighting a heretofore-unknown link between insulin sensitivity and leukocytosis that can affect the predisposition to atherosclerosis.