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.

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.

Oxidative metabolism and PGC-1beta attenuate macrophage-mediated inflammation.

Complex interplay between T helper (Th) cells and macrophages contributes to the formation and progression of atherosclerotic plaques. While Th1 cytokines promote inflammatory activation of lesion macrophages, Th2 cytokines attenuate macrophage-mediated inflammation and enhance their repair functions. In spite of its biologic importance, the biochemical and molecular basis of how Th2 cytokines promote maturation of anti-inflammatory macrophages is not understood. We show here that in response to interleukin-4 (IL-4), signal transducer and activator of transcription 6 (STAT6) and PPARgamma-coactivator-1beta (PGC-1beta) induce macrophage programs for fatty acid oxidation and mitochondrial biogenesis. Transgenic expression of PGC-1beta primes macrophages for alternative activation and strongly inhibits proinflammatory cytokine production, whereas inhibition of oxidative metabolism or RNAi-mediated knockdown of PGC-1beta attenuates this immune response. These data elucidate a molecular pathway that directly links mitochondrial oxidative metabolism to the anti-inflammatory program of macrophage activation, suggesting a potential role for metabolic therapies in treating atherogenic inflammation.

PPAR-delta 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-delta (PPAR-delta) is induced when macrophages engulf apoptotic cells and functions as a transcriptional sensor of dying cells. Genetic deletion of PPAR-delta 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-delta has a pivotal role in orchestrating the timely disposal of apoptotic cells by macrophages, ensuring that tolerance to self is maintained.

History of the biomedical studies PhD program: a joint graduate program of the Baylor Health Care system and Baylor University.

On a sweltering summer morning, throngs of people filed into Jones Theatre at Baylor University in Waco for the graduate student orientation. One could look around and notice the diversity of not only the student population, but also the disciplines being represented. Many students had stepped off planes only hours prior, but even those who had been traveling for days could not contain their excitement. As for me, I was nowhere near any of this. I was still 40 miles north of Waco in Waxahachie, having been pulled over for speeding. After 4 days of traveling with my life in my Volkswagon Jetta, all the way from San Francisco, on one of the most important days of my life, I was late. When I finally arrived at the Hooper Schafer Fine Arts Auditorium, out of breath from running all the way from the parking structure, all of the graduate students were quietly listening to the first introductory speech. I snuck into the back and sat down. My mind was racing, as I knew very little about Waco and Baylor University except for the growing accomplishments of the biomedical studies program. What little I did know about Baylor seemed so different from my very liberal upbringing in California. What would this experience be like for me? But, as I listened to the talks, met with other students, and finally met the entire biomedical studies entering class of 2007, I knew that I had made the right decision in coming to Baylor. This would be an experience unlike any other, and I was wholeheartedly open to embracing it. -Christine Morel, PhD candidate, Institute of Biomedical Studies.

Alternative M2 activation of Kupffer cells by PPARdelta 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.