Macrophages block insulin action in adipocytes by altering expression of signaling and glucose transport proteins

Key reports from the 9th International Workshop on Adverse Drug Reactions and Lipodystrophy in HIV 2007

The 9th International Workshop on Adverse Drug Reactions and Lipodystrophy in HIV provided a forum for the presentation of basic and clinical research focused on the pathogenesis and management of lipodystrophy and other adverse events associated with antiretroviral therapy. New data were reported on the contribution of both antiretroviral therapy and HIV infection itself on the development of metabolic abnormalities in patients with lipodystrophy, including insulin resistance and dyslipidaemia, which are associated with an increased risk of diabetes and cardiovascular disease. In addition, an emerging role of HIV and antiretroviral therapy in bone, liver and kidney disease were highlighted. A major focus of the data presented in these areas concerned the identification and evaluation of risk factors and appropriate surrogate markers for defining cardiovascular disease risk as well as other outcomes of long-term treatment. The complexity of defining such risk factors was underscored by data describing the impact of race, age and gender in the progression of metabolic disease and related complications among different HIV-infected populations. Finally, advances in the development of pharmacovigilance reporting systems in resource-limited settings and their impact upon healthcare policies and the provision of patient care were also described.

Adipose tissue macrophages

It is now broadly accepted that low-grade chronic inflammation associated with obesity leads to the onset of insulin resistance and type 2 diabetes mellitus. Obesity-associated inflammation is characterized by an increased abundance of macrophages in adipose tissue along with production of inflammatory cytokines. Adipose tissue macrophages (ATMs) are suspected to be the major source of inflammatory mediators such as TNF-alpha and IL-6 that interfere with adipocyte function by inhibiting insulin action. However, ATMs phenotypically resemble alternatively activated (M2) macrophages and are capable of anti-inflammatory mediator production challenging the concept that ATMs are simply the "bad guys" in obese adipose tissue. Triggers promoting ATM recruitment, ATM functions and dysfunctions, and stimuli and molecular mechanisms that drive them into becoming detrimental to their environment are subject to current research. Strategies to interfere with ATM recruitment and adverse activation could give rise to novel options for treatment and prevention of insulin resistance and type 2 diabetes mellitus.

Macrophage-specific PPARgamma 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-gamma (PPARgamma), we show here that PPARgamma is required for maturation of alternatively activated macrophages. Disruption of PPARgamma 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.

Palmitic acid induces IP-10 expression in human macrophages via NF-kappaB activation

It is now recognized that cross-talk between adipocytes and adipose tissue stromal cells such as macrophages contributes to local and systemic inflammation. One factor from adipocytes that may participate in this interaction and that is frequently elevated in inflammatory conditions such as obesity, insulin resistance, and type 2 diabetes is free fatty acids (FFA). To investigate the potential for FFA to enhance macrophage inflammation, we exposed U937 macrophages to physiological levels (150 microM) of FFA. Palmitic acid (PA), the predominant saturated FFA released from adipose tissue, but not unsaturated FFA, induced an approximately 6-fold (p<0.05) increase in IP-10 gene expression (and 2- to 4-fold increases in IL-8, MCP-1, COX-2, and MIG). PA also induced an approximately 2-fold increase (p<0.05) in active NF-kappaB, and two structurally distinct NF-kappaB inhibitors effectively blocked PA-induced IP-10 gene expression. Conditioned medium from PA-treated cells increased lymphocyte migration 41% (p<0.05) which was significantly reduced by IP-10-neutralizing antibody. These results suggest that elevated concentrations of PA commonly present in obese and insulin resistant individuals can increase NF-kappaB-mediated expression of IP-10 in macrophages. These events in turn may lead to an increasing feed-forward loop of chronic inflammation.

Obesity induces a phenotypic switch in adipose tissue macrophage polarization

Adipose tissue macrophages (ATMs) infiltrate adipose tissue during obesity and contribute to insulin resistance. We hypothesized that macrophages migrating to adipose tissue upon high-fat feeding may differ from those that reside there under normal diet conditions. To this end, we found a novel F4/80(+)CD11c(+) population of ATMs in adipose tissue of obese mice that was not seen in lean mice. ATMs from lean mice expressed many genes characteristic of M2 or "alternatively activated" macrophages, including Ym1, arginase 1, and Il10. Diet-induced obesity decreased expression of these genes in ATMs while increasing expression of genes such as those encoding TNF-alpha and iNOS that are characteristic of M1 or "classically activated" macrophages. Interestingly, ATMs from obese C-C motif chemokine receptor 2-KO (Ccr2-KO) mice express M2 markers at levels similar to those from lean mice. The antiinflammatory cytokine IL-10, which was overexpressed in ATMs from lean mice, protected adipocytes from TNF-alpha-induced insulin resistance. Thus, diet-induced obesity leads to a shift in the activation state of ATMs from an M2-polarized state in lean animals that may protect adipocytes from inflammation to an M1 proinflammatory state that contributes to insulin resistance.