Justin I. Odegaard

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.

Eosinophils and Type 2 Cytokine Signaling in Macrophages Orchestrate Development of Functional Beige Fat

Beige fat, which expresses the thermogenic protein UCP1, provides a defense against cold and obesity. Although a cold environment is the physiologic stimulus for inducing beige fat in mice and humans, the events that lead from the sensing of cold to the development of beige fat remain poorly understood. Here, we identify the efferent beige fat thermogenic circuit, consisting of eosinophils, type 2 cytokines interleukin (IL)-4/13, and alternatively activated macrophages. Genetic loss of eosinophils or IL-4/13 signaling impairs cold-induced biogenesis of beige fat. Mechanistically, macrophages recruited to cold-stressed subcutaneous white adipose tissue (scWAT) undergo alternative activation to induce tyrosine hydroxylase expression and catecholamine production, factors required for browning of scWAT. Conversely, administration of IL-4 to thermoneutral mice increases beige fat mass and thermogenic capacity to ameliorate pre-established obesity. Together, our findings have uncovered the efferent circuit controlling biogenesis of beige fat and provide support for its targeting to treat obesity.

Pleiotropic actions of insulin resistance and inflammation in metabolic homeostasis

Metabolism and immunity are inextricably linked both to each other and to organism-wide function, allowing mammals to adapt to changes in their internal and external environments. In the modern context of obesogenic diets and lifestyles, however, these adaptive responses can have deleterious consequences. In this Review, we discuss the pleiotropic actions of inflammation and insulin resistance in metabolic homeostasis and disease. An appreciation of the adaptive context in which these responses arose is useful for understanding their pathogenic actions in disease.

Alternative Macrophage Activation and Metabolism

Obesity and its attendant metabolic disorders represent the great public health challenge of our time. Recent evidence suggests that onset of inflammation in metabolic tissues pathogenically links obesity to insulin resistance and type 2 diabetes. In this review, we briefly summarize the extant literature, paying special attention to the central role of the tissue-associated macrophage in the initiation of metabolic inflammation. We argue that rather than representing simple inflammatory disease, obesity and metabolic syndrome represent derangements in macrophage activation with concomitant loss of metabolic coordination. As such, the sequelae of obesity are as much products of the loss of positive macrophage influences as they are of the presence of deleterious inflammation. The therapeutic implications of this conclusion are profound because they suggest that pharmacologic targeting of macrophage activation, rather than simply inflammation, might be efficacious in treating this global epidemic.

Activated Type 2 Innate Lymphoid Cells Regulate Beige Fat Biogenesis

Type 2 innate lymphoid cells (ILC2s), an innate source of the type 2 cytokines interleukin (IL)-5 and -13, participate in the maintenance of tissue homeostasis. Although type 2 immunity is critically important for mediating metabolic adaptations to environmental cold, the functions of ILC2s in beige or brown fat development are poorly defined. We report here that activation of ILC2s by IL-33 is sufficient to promote the growth of functional beige fat in thermoneutral mice. Mechanistically, ILC2 activation results in the proliferation of bipotential adipocyte precursors (APs) and their subsequent commitment to the beige fat lineage. Loss- and gain-of-function studies reveal that ILC2- and eosinophil-derived type 2 cytokines stimulate signaling via the IL-4Rα in PDGFRα(+) APs to promote beige fat biogenesis. Together, our results highlight a critical role for ILC2s and type 2 cytokines in the regulation of adipocyte precursor numbers and fate, and as a consequence, adipose tissue homeostasis. PAPERCLIP:

PPAR-δ senses and orchestrates clearance of apoptotic cells to promote tolerance

Macrophages rapidly engulf apoptotic cells to limit the release of noxious cellular contents and to restrict autoimmune responses against self antigens. Although factors participating in recognition and engulfment of apoptotic cells have been identified, the transcriptional basis for the sensing and the silent disposal of apoptotic cells is unknown. Here we show that peroxisome proliferator–activated receptor-δ (PPAR-δ) is induced when macrophages engulf apoptotic cells and functions as a transcriptional sensor of dying cells. Genetic deletion of PPAR-δ decreases expression of opsonins such as complement component-1qb (C1qb), resulting in impairment of apoptotic cell clearance and reduction in anti-inflammatory cytokine production. This increases autoantibody production and predisposes global and macrophage-specific Ppard−/− mice to autoimmune kidney disease, a phenotype resembling the human disease systemic lupus erythematosus. Thus, PPAR-δ has a pivotal role in orchestrating the timely disposal of apoptotic cells by macrophages, ensuring that tolerance to self is maintained.

Mechanisms of macrophage activation in obesity-induced insulin resistance

Chronic inflammation is now recognized as a key step in the pathogenesis of obesity-induced insulin resistance and type 2 diabetes mellitus. This low-grade inflammation is mediated by the inflammatory (classical) activation of recruited and resident macrophages that populate metabolic tissues, including adipose tissue and liver. These findings have led to the concept that infiltration by and activation of macrophages in adipose tissue are causally linked to obesity-induced insulin resistance. Studies have shown, however, that alternatively activated macrophages taking residence in adipose tissue and liver perform beneficial functions in obesity-induced metabolic disease. Alternatively activated macrophages reduce insulin resistance in obese mice by attenuating tissue inflammation and increasing oxidative metabolism in liver and skeletal muscle. The discovery that distinct subsets of macrophages are involved in the promotion or attenuation of insulin resistance suggests that pathways controlling macrophage activation can potentially be targeted to treat these comorbidities of obesity. Thus, this Review focuses on the stimuli and mechanisms that control classical and alternative activation of tissue macrophages, and how these macrophage activation programs modulate insulin action in peripheral tissues. The functional importance of macrophage activation is further discussed in the context of host defense to highlight the crosstalk between innate immunity and metabolism.

IL-4/STAT6 immune axis regulates peripheral nutrient metabolism and insulin sensitivity

Immune cells take residence in metabolic tissues, providing a framework for direct regulation of nutrient metabolism. Despite conservation of this anatomic relationship through evolution, the signals and mechanisms by which the immune system regulates nutrient homeostasis and insulin action remain poorly understood. Here, we demonstrate that the IL-4/STAT6 immune axis, a key pathway in helminth immunity and allergies, controls peripheral nutrient metabolism and insulin sensitivity. Disruption of signal transducer and activator of transcription 6 (STAT6) decreases insulin action and enhances a peroxisome proliferator-activated receptor α (PPARα) driven program of oxidative metabolism. Conversely, activation of STAT6 by IL-4 improves insulin action by inhibiting the PPARα-regulated program of nutrient catabolism and attenuating adipose tissue inflammation. These findings have thus identified an unexpected molecular link between the immune system and macronutrient metabolism, suggesting perhaps the coevolution of these pathways occurred to ensure access to glucose during times of helminth infection.

Comparison of the Acute Inflammatory Response of Two Commercial Platelet-Rich Plasma Systems in Healthy Rabbit Tendons

Background: Numerous studies have shown platelet-rich plasma (PRP) preparations differ with respect to the inclusion of certain blood components, which may affect the host’s cellular response. Hypothesis: This study evaluated the inflammatory effect of Biomet GPS III leukocyte-rich PRP (LR-PRP) versus MTF Cascade leukocyte-poor PRP (LP-PRP) after intratendinous injection in an animal model. The authors anticipated that LR-PRP would incite a greater acute inflammatory response than LP-PRP. Study Design: Controlled laboratory study. Methods: A total of 17 skeletally mature New Zealand White rabbits were tested. In all cases, healthy patellar tendons were treated. In the control animals, one patellar tendon was injected with 2 mL autologous whole blood, and the other was injected with 2 mL sterile saline. Seven total tendons were injected with whole blood, and 7 tendons were injected with saline. In the experimental animals, one patellar tendon was injected with 2 mL LR-PRP, and the other was injected with 2 mL LP-PRP. Ten tendons were injected with LR-PRP, and 10 tendons were injected with LP-PRP. Animals were euthanized at 5 or 14 days after injection. Tendons were harvested and stained using hematoxylin and eosin and scored semi-quantitatively for total white blood cells (WBCs), mononuclear cells (macrophages and lymphocytes), polymorphonuclear cells (PMNs), vascularity, fiber structure, and fibrosis. Results: At 5 days after injection, tendons treated with LR-PRP had significantly greater overall tendon scores (6.3 ± 1.79 vs 1.8 ± 1.64, P = .012), as well as mean scores for fiber structure (1.4 ± 0.22 vs 0.50 ± 0.50, P = .012), denoting disrupted composition, total WBCs (1.1 ± 0.89 vs 0.10 ± 0.22, P = .014), mononuclear cells (macrophages and lymphocytes) (0.80 ± 0.45 vs 0.10 ± 0.22, P = .014), vascularity (1.7 ± 0.27 vs 0.80 ± 0.16, P = .008), and fibrosis (1.0 ± 0.35 vs 0.3 ± 0.45, P = .037) compared with tendons treated with LP-PRP. Otherwise, there were no significant differences in mononuclear cells (P = .590), PMN cells (P = 1.00), total WBCs (P = .811), vascularity (P = .650), or total tendon score (P = .596) in any of the treatment groups at 14 days. Conclusion: Compared with leukocyte-poor Cascade PRP, leukocyte-rich GPS III PRP causes a significantly greater acute inflammatory response at 5 days after injection. There is no significant difference in the inflammatory response or cellularity regardless of the injection type at 14 days after intratendinous injection. Clinical Relevance: Platelet-rich plasma injections are frequently prepared using commercial systems and are administered for clinical treatment of chronic tendinopathy. It is important to characterize the cellular responses elucidated by different injection preparations to further understand their effect on tissue healing and aid clinical decision making. Future investigations are necessary to apply these findings to the clinical setting.