Immunoblotting revealed that although STAT6 is expressed in liver and white adipose tissue (WAT) (Fig

Immunoblotting revealed that although STAT6 is expressed in liver and white adipose tissue (WAT) (Fig. 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. Keywords:insulin resistance, obesity, cytokines, liver, Th2 immunity Defense against pathogens, the primordial function of the immune system, is bioenergetically intensive and exquisitely sensitive to nutrient availability. For example, the sensing of bacteria by the Toll-like receptor 4 complex initiates signaling cascades that culminate in the activation of NF-B, which, in turn, induces the expression of inflammatory and microbicidal molecules (1,2). Interestingly, the ability of macrophages and neutrophils to support antimicrobial programs is completely dependent on the maintenance of high flux through the glycolytic pathways (3,4). Because immune cells lack significant glycogen stores, the metabolic demands of these activated cells are primarily met by rapid uptake of extracellular glucose (5,6). Thus, in the context of bacterial infection, the inhibition of insulin signaling by inflammatory molecules might be an adaptive response that redirects glucose from storage to host defense (7,8). Although this nutrient reallocation strategy is protective during bacterial infection, it becomes maladaptive in the setting of obesity, resulting in insulin resistance and type 2 diabetes (7,911). In addition to bacteria, parasitic worms, the helminths, comprise another major group of vertebrate pathogens (12). Unlike bacterial infections that tend to be acute and limited, infections with extracellular helminths are chronic and widely prevalent. For example, although certain helminth species are known to reside in human hosts for decades (12), others Valproic acid exhibit near ubiquitous infestation of animals in the wild (13). Moreover, the immune response invoked by helminths is also distinct, one dominated by Th2 cytokines, such as IL-4 and IL-13 (12), whose functions in nutrient metabolism remain unexplored. Because infective helminths continually parasitize nutrients from the host, we hypothesized that signaling pathways Valproic acid mediating immunity against helminths might also regulate nutrient homeostasis via modulation of insulin action. Th2 cytokines IL-4 and IL-13, which are locally produced in tissues, exert their biologically effects via activation of the transcription factor STAT6 (14). Confirming the importance of this pathway in antihelminth immunity and Valproic acid allergies, mice lacking STAT6 are severely compromised in their ability to mount allergic inflammatory responses (15,16) or defend against various gastronintestinal helminths (12,17). Thus, using STAT6-null mice and IL-4 as surrogates, we explored the role of Th2-type immune responses in regulation of nutrient metabolism and insulin action. == Results == == IL-4 and STAT6 Regulate Hepatic Metabolism. == We first analyzed the expression and activity of STAT6 in major metabolic tissues: liver, fat, and skeletal Valproic acid muscle. Immunoblotting revealed that although STAT6 is expressed in liver and white adipose tissue (WAT) (Fig. 1A), treatment with IL-4 increased tyrosine phosphorylation of STAT6 in the liver with more variable effects in adipose tissue (Fig. 1A). These findings are consistent with presence of IL-4R, which mediates IL-4dependent activation of STAT6, in hepatocytes but not adipocytes (Fig. S1) (18,19). Because liver contains many hematopoietic cells, which express IL-4R and STAT6, we evaluated the patency of IL-4/STAT6 signaling in primary hepatocytes. Treatment of mouse hepatocytes with IL-4 increased tyrosine phosphorylation of STAT6 (Fig. 1B), suggesting its potential Valproic acid involvement in the regulation of hepatic metabolism. == Fig. 1. == IL-4 and STAT6 regulate liver nutrient metabolism. (A) IL-4 enhances phosphorylation of STAT6 in the liver, but not skeletal muscle or WAT. NS, nonspecific. (B) STAT6 is expressed in primary hepatocytes and becomes tyrosine-phosphorylated in response to IL-4. (CandD) IL-4 regulates Ntn1 macronutrient metabolism in isolated primary hepatocytes. Changes in rates of fatty acid (C) and glucose oxidation (D) in hepatocytes treated with IL-4 (10 ng/mL). (EandF) STAT6 regulates fuel oxidation in primary hepatocytes. Rates of fatty acid (E) and glucose oxidation (F) in wild-type and STAT6-null hepatocytes. (GandH) Induction of PPAR and its transcriptional program in livers of STAT6/mice. (G) Immunoblotting for PPAR and STAT6 in wild-type and STAT6-null livers. -actin is used as a loading control. (H) Quantitative RT-PCR analyses for PPAR and its target genes, acyl-CoA thioesterase 1 (Acot1), fatty-acid binding protein 1 (Fabp1), and fibroblast growth element 21 (Fgf21), in livers of wild-type and STAT6/mice. All results are displayed as means.