Visceral obesity associates with insulin resistance and chronic inflammation, which are major risk factors for the metabolic syndrome, diabetes, and cardiovascular disease. Although the cellular hallmark of obesity is neutral lipid expansion in adipocytes, adipose tissue of obese mice and humans also accumulate macrophages and other leukocytes. It is well accepted that adipose tissue macrophages (ATMs) play a critical role in systemic insulin resistance, suggesting that inflammatory mediators produced by ATMs are important factors linking excess fat mass to insulin sensitivity, glucose intolerance, and increased atherosclerotic risk. Understanding the mechanistic basis of the pro-inflammatory ATM phenotype and ATM function is required to devise new strategies for attenuating inflammation in metabolic disease. Pro-inflammatory pathways in ATMs are commonly attributed to classical activation (exposure to bacteria, M1), establishing molecular links between innate immunity and metabolic dysfunction. Recent studies, including work from our lab, suggest that M1 activation fails to accurately represent the complex phenotype of ATMs in vivo. We have shown that ATMs in obese adipose tissue from humans and mice adopt a unique ?metabolically activated? (MMe) phenotype that is distinct from the M1 phenotype. Inhibiting MMe activation of macrophages in vivo, attenuates ATM inflammation and improved glucose tolerance in mice. Moreover, the abundance of MMe-like ATMs in visceral fat is positively correlated with insulin resistance in patients controlled for adiposity. These findings underscore the pathophysiological importance of MMe macrophages in mice and humans. Although MMe and M1 macrophages are both characterized by increased expression of NFkB-induced inflammatory cytokines (ie. Tnfa, Il1b, Il6), our preliminary studies demonstrate that the upstream signaling cascades driving NFkB activation are remarkably distinct. We provide evidence for a novel fatty acid-driven, ROS-dependent, tyrosine-kinase mediated, pro-inflammatory signaling cascade in MMe macrophages. Targeting this ?metabolically activated NFkB? (Me-NFkB) pathway at any point selectively attenuates inflammatory cytokine expression by MMe macrophages. Our work has positioned us to test the innovative hypothesis that this Me-NFkB pathway promotes ATM inflammation and insulin resistance during obesity but is dispensable for inflammation required for host defense during infection. Specifically, we plan to 1) Delineate the Me-NFkB pathway that drives inflammation in MMe macrophages, and 2) Determine if targeting the Me-NFkB pathway improves insulin sensitivity in mice. Overall, our proposed studies aim to demonstrate that macrophage inflammation in metabolic disease can be attenuated without blocking these same cytokines in bacterial infection, a conceptual milestone that may lead to an improved anti-inflammatory strategy in the clinic.
During obesity, adipose tissue macrophages (ATMs) adopt a metabolically activated phenotype that contributes to low-grade chronic inflammation in adipose tissue and insulin resistance. Although studies in humans and experimental animal models support targeting the inflammatory phenotype of ATMs to improve insulin sensitivity, the molecular mechanisms underlying this inflammation are incompletely understood. We have identified a novel proinflammatory signaling pathway in macrophages triggered by metabolic stimuli and distinct from infectious stimuli ? and, our overall goal is to delineate the contribution this pathway to obesity- associated insulin resistance.
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| Coats, Brittney R; Schoenfelt, Kelly Q; Barbosa-Lorenzi, Valéria C et al. (2017) Metabolically Activated Adipose Tissue Macrophages Perform Detrimental and Beneficial Functions during Diet-Induced Obesity. Cell Rep 20:3149-3161 |
