Inflammation and insulin resistance

Abstract

Over a hundred years ago, high doses of salicylates were shown to lower glucose levels in diabetic patients. This should have been an important clue to link inflammation to the pathogenesis of type 2 diabetes (T2D), but the antihyperglycemic and antiinflammatory effects of salicylates were not connected to the pathogenesis of insulin resistance until recently. Together with the discovery of an important role for tissue macrophages, these new findings are helping to reshape thinking about how obesity increases the risk for developing T2D and the metabolic syndrome. The evolving concept of insulin resistance and T2D as having immunological components and an improving picture of how inflammation modulates metabolism provide new opportunities for using antiinflammatory strategies to correct the metabolic consequences of excess adiposity.

Figure 1. Potential cellular mechanisms for activating inflammatory signaling.

Obesity and high-fat diet activate IKKβ/NF-κB and JNK pathways in adipocytes, hepatocytes, and associated macrophages. Stimuli that have been shown to activate these pathways during metabolic dysregulation include ligands for TNF-α, IL-1, Toll, or AGE receptors (TNFR, IL-1R, TLR, or RAGE, respectively), intracellular stresses including ROS and ER stress, ceramide, and various PKC isoforms. Obesity-induced IKKβ activation leads to NF-κB translocation and the increased expression of numerous markers and potential mediators of inflammation that can cause insulin resistance. Obesity-induced JNK activation promotes the phosphorylation of IRS-1 at serine sites that negatively regulate normal signaling through the insulin receptor/IRS-1 axis. Examples include serine-302 (pS302) and serine-307 (pS307). By contrast, evidence has not been reported for obesity-induced effects on transcription factors such as AP-1 that are regulated by JNK. IKKβ and/or NF-κB are inhibited or repressed by the actions of salicylates, TZDs, and statins.

Figure 2. Potential mechanisms for activation of inflammation in adipose tissue.

Dietary excess and obesity cause lipid accumulation in adipocytes, initiating a state of cellular stress and activation of JNK and NF-κB. These inflammatory signaling pathways regulate protein phosphorylation and cellular transcriptional events, thereby leading to increased adipocyte production of proinflammatory cytokines, including TNF-α, IL-6, leptin, and resistin, chemokines such as MCP-1, and other proatherogenic mediators, for example PAI-1. Endothelial adhesion molecules (e.g., ICAM-1 and VCAM-1) and chemoattractant molecules (designated CCX) bind integrins and chemokine receptors (CCR), respectively, on the monocyte surface to recruit them to the adipose tissues. The monocytes that differentiate into macrophages produce many of the same inflammatory cytokines and chemokines as those listed above, and additional ones, to further promote local inflammation and propagate the inflammatory diathesis systemically. pS, phosphoserine.

Figure 3. Potential mechanisms for adiposity-induced inflammation in the liver.

Healthy liver contains a broad repertoire of cells that participate in inflammatory and immune responses, including resident hepatic macrophages (Kupffer cells), B and T cells, NK and NKT cells, DCs, liver sinusoidal endothelial cells, hepatic stellate cells, and hepatocytes. Hepatic steatosis and obesity are accompanied by the activation of inflammatory signaling pathways in liver. Proinflammatory cytokines and FFAs, produced either by hepatocytes in response to steatosis or by abdominal fat tissue, may activate Kupffer cells. Numbers of regulatory NKT cells decrease in parallel with the Kupffer cell activation.

Figure 4. Local, portal, and systemic effects of inflammation in insulin resistance and atherogenesis.

Increasing adiposity activates inflammatory responses in fat and liver, with associated increases in the production of cytokines and chemokines. Immune cells including monocytes and macrophages are recruited and/or activated, and together these cause local insulin resistance. Portal delivery of abdominal fat–derived cytokines and lipids contributes to hepatic inflammation and insulin resistance. Proinflammatory and proatherogenic mediators are produced in the adipose tissue and liver and associated immune cells. This creates a systemic inflammatory diathesis that promotes insulin resistance in skeletal muscle and other tissues and atherogenesis in the vasculature.