Research articleNetwork analysis of adipose tissue gene expression highlights altered metabolic and regulatory transcriptomic activity in high-fat-diet-fed IL-1RI knockout mice
Introduction
Obesity is the key etiological factor in the development of insulin resistance (IR) and type 2 diabetes (T2DM), the prevalence of which is increasing rapidly [1]. Adipose tissue remodelling is evident in high-fat-diet (HFD)-induced obesity with increased infiltration of T cells and macrophages [2], [3]. In the obese state, adipocytes and immune cells secrete proinflammatory cytokines, including interleukin (IL)-1β, tumour necrosis factor α (TNFα) and IL-6, which further promote macrophage recruitment and impede insulin signalling [4]. Furthermore, disruption of various components of the inflammatory signalling cascade including TNFα [5], I kappa B kinase β [6] and Toll-like receptor 4 [7] results in partial protection from obesity-induced IR.
IL-1 is a family of proteins that act as central mediators of innate immune responses. The most well-studied members of the IL-1 family, IL-1α and IL-1β, are produced by a variety of cells, including activated macrophages, and induce proinflammatory responses through binding to the type I IL-1 receptor (IL-1RI) [8]. High saturated fat intake induces IL-1 production and an ensuing subacute inflammatory phenotype [9]. Consequently, IL-1 signalling has received considerable attention in human and mouse studies of glycemic control [10], [11]. Treatment with IL-1 receptor antagonist (IL-1Ra) in type 2 diabetics improves pancreatic beta-cell function and insulin sensitivity [12]. IL-1Ra also protects from HFD-induced hyperglycemia in mice [13] and rats [14], and blockage of IL-1 signalling through an anti-IL-1β antibody also improves glucose control in mice fed an HFD [15], [16]. IL-1 modulates the function of adipose tissue through inhibition of adipocyte differentiation [17] and stimulation of lipolysis [18] and energy expenditure [19]. Impaired adipose tissue storage leads to lipid accumulation in liver, muscle and heart tissue, further exacerbating systemic IR [20].
Transcriptomic analysis has been critical in clarifying the role of adipose tissue remodelling in high-fat diet-induced IR [21]. Analysis and interpretation of transcriptomic profiles are challenging due to the high dimensionality of these data sets and inherent biological complexity of adipose tissue inflammation. To this end, pathway analysis (typically implemented using gene set enrichment analysis; GSEA) has become a standard in the field of transcriptomic data analysis [22]. However, from a systems biological perspective, global network-based approaches carry the advantage of highlighting inherent intersection and overlapping of functionally related pathways.
Despite growing evidence for a functional role of IL-1 in adipose tissue function, the complex downstream consequences of IL-1 signalling remain poorly clarified — particularly with respect to diet-induced obesity and adipose tissue remodelling. We therefore used a network-based approach to analyze adipose tissue gene expression profiles of IL-1RI−/− mice fed an HFD.
Section snippets
Mouse feeding trial
Six-week-old male C57BL/6 wild-type (WT) and IL-1RI−/− mice were fed an HFD for 16 weeks. HFD consisted of 60% kcal from fat (54% lard, 6% soybean oil), 20% kcal from carbohydrate (13% maltodextrin, 7% sucrose) and 20% kcal from protein (19.7% casein, 0.3% l-cystine) as described by de Roos et al. [23]. C57BL/6 mice were obtained from Harlan Ltd., and breeding pairs of IL-1RI−/− were purchased from Jackson Labs and bred at Trinity College Dublin under specific pathogen-free conditions; ethical
Analysis of gene expression and correlation with plasma markers
IL-1RI−/− and WT mice were fed an HFD for 16 weeks, and gene expression profiles were analyzed using Affymetrix microarrays. Comprehensive phenotype data have been presented previously [23]. Briefly, food intake did not differ between the intervention groups (data not shown), and both groups gained similar body weight over the course of the intervention (19.3±7.8 g in IL-1RI−/− mice and 23.5±4.9 g in control mice, P>.05).Single gene expression analysis of the microarrays highlighted 25 genes
Discussion
Adipose tissue inflammation is now recognized as a central player in the aetiology of HFD-induced IR [4]. Metabolic network analysis of transcriptomic effects identified up-regulation and co-expression of genes involved in lipolysis, lipogenesis and TCA cycle, coincident with an attenuated plasma inflammatory cytokines in HFD fed IL-1RI−/−, relative to weight-matched HFD-induced IR WT mice. Further assessment of gene expression in a network of protein interactions related to innate immunity
Acknowledgments
This work was supported by Science Foundation Ireland PI Programme (06/IM.1/B105; 11/PI/1119) (H.M.R.) and IRCSET postgraduate scholarship scheme (M.J.M.).
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