Obesity is a serious health problem characterized by an excessive expansion of the white adipose tissue coupled with a state of chronic, low-grade inflammation.1 Obesity-associated inflammation occurs as a result of immune cell infiltration of the adipose tissue and increased production of pro-inflammatory cytokines.2 These changes negatively affect normal adipocyte functions, such as triglyceride storage and lipolysis, leading to high circulating levels of free fatty acids and ectopic lipid accumulation.3 In addition, elevated levels of pro-inflammatory cytokines, including IL-1 beta, IL-6, and TNF-alpha, reduce insulin sensitivity, which can lead to the development of type II diabetes if insulin production by the pancreatic beta cells is not sufficiently increased.4, 5, 6 Although it has been recognized for some time that pro-inflammatory cytokines inhibit insulin signaling, the molecular mechanisms that trigger inflammation and cytokine production during obesity are not well understood.
The first evidence of an obesity-related signal that could initiate the inflammatory response came from reports that saturated fatty acids activate Toll-like receptors (TLRs).7, 8, 9 TLRs are a family of membrane-associated pattern recognition receptors that detect invading pathogens and activate the innate immune response. These studies demonstrated that fatty acids, like microbial pathogens, promote TLR-induced inflammation. It was subsequently shown that disruption or a loss of TLR4 function protects against high fat diet-induced insulin resistance in mice, suggesting that fatty acid-induced TLR4 signaling provides a link between obesity, inflammation, and insulin resistance.10, 11
||Obesity-Induced Inflammasome Activation in Key Metabolic Tissues Promotes Chronic Inflammation and Insulin Resistance. Obesity-related danger signals, such as palmitate, ceramide, high glucose concentrations, islet amyloid polypeptide deposition, defective autophagy, or mitochondrial dysfunction may lead to the generation of reactive oxygen species (ROS) and subsequent activation of the Nlrp3 inflammasome in adipocytes, pancreatic islet cells, or infiltrating macrophages present in these tissues. Nlrp3 inflammasome activation promotes the cleavage and activation of Caspase-1, resulting in the secretion of IL-1 beta, a pro-inflammatory cytokine that negatively affects adipocyte differentiation, inhibits insulin signaling, and has a cytotoxic effect on insulin-producing pancreatic beta cells. Identification of a connection between obesity, the inflammasome, and insulin resistance links inflammasome activation with metabolic tissue dysfunction, which may contribute to the pathogenesis of type II diabetes, and other diseases associated with obesity-induced inflammation.
Several recent studies suggest that another subset of pattern recognition receptors, the Nod-like receptors (NLRs), may also play a critical role in detecting obesity-associated signals and initiating the inflammatory response. Unlike TLRs, NLRs are cytoplasmic receptors that detect microbial components or danger signals. Upon activation, some NLR proteins, including Nlrp3, form multiprotein inflammasome complexes that cleave and activate Caspase-1, leading to the secretion of IL-1 beta and IL-18.12 Inflammasome oligomerization requires two signals, a priming signal that results in the transcription of IL-1 beta and IL-18, and a second signal that promotes indirect activation of the inflammasome such as reactive oxygen species (ROS), ion or membrane perturbations, or extracellular ATP.12 A connection between the Nlrp3 inflammasome and the development of insulin resistance was established by the observation that Nlrp3-/- mice fed a high fat diet were more glucose tolerant and insulin sensitive than similarly fed wild-type mice.13 The Nlrp3 inflammasome was also suggested to be involved in the pathogenesis of type II diabetes since Nlrp3 was found to be activated in pancreatic islet cells by both high glucose concentrations and deposition of amyloid polypeptide, two conditions associated with type II diabetes.13, 14 The authors speculated that Nlrp3-dependent IL-1 beta secretion leads to impaired pancreatic beta cell function and destruction, which likely plays a causative role in promoting disease progression.13, 14, 15
Stienstra et al. further investigated the effects of inflammasome signaling on adipocyte function and insulin sensitivity.16 In vitro studies demonstrated that adipocyte differentiation was enhanced in the presence of Caspase-1 or IL-1 beta inhibitors. This was indicated by Oil Red O staining of differentiating adipocytes and increased expression of the adipogenic markers, GLUT4, Adiponectin, and PPAR gamma. A similar increase in adipogenesis was observed in preadipocytes isolated from Caspase-1- or Nlrp3-deficient mice. In addition, total white adipose tissue explants from these mice displayed enhanced insulin signaling, suggesting that activation of the inflammasome not only alters adipocyte differentiation but also negatively affects normal adipocyte functions. Subsequent analysis of Caspase-1-deficient mice revealed a reduction in total fat mass, lower levels of free fatty acids, enhanced insulin sensitivity, and increased diurnal fat oxidation rates compared to wild-type mice, despite similar food intake and fecal output.16 Significantly, treatment of leptin-deficient (ob/ob) obese mice with an oral Caspase-1 inhibitor improved insulin sensitivity, suggesting that inhibition of Caspase-1 may be a valid strategy for restoring metabolic functions in obesity and type II diabetes.16
Two additional studies identified ceramide and palmitate respectively, as high fat diet-associated signals that induce Nlrp3 activation in lipopolysaccharide (LPS)-primed bone marrow-derived macrophages.17, 18 Palmitate-induced Nlrp3 inflammasome activation was dependent on the generation of ROS, which occurred as a result of defective autophagic degradation caused by a decrease in the activity of AMP-activated protein kinase (AMPK), a positive regulator of the autophagy initiator protein ULK1.18 Both studies also confirmed that diet-induced obese Nlrp3–/– mice displayed enhanced insulin signaling in the liver, muscle, and adipose tissue.17, 18 Notably, the adipose tissue of these mice was found to have a reduced pro-inflammatory profile compared to wild-type mice on the same diet.17 This was indicated by a significant decrease in the expression of pro-inflammatory M1 macrophage markers, combined with an increase in the expression of anti-inflammatory M2 macrophage markers and a reduction in the number of adipose tissue associated effector T cells in Nlrp3-deficient mice. These observations suggested that inhibition of Nlrp3-dependent cytokine secretion may prevent the chronic inflammatory state associated with obesity and the development of insulin resistance. Supporting this hypothesis, IL-1 beta was shown to both directly and indirectly inhibit insulin signaling.18 Treatment of mouse liver cells with IL-1 beta inhibited insulin-induced Akt phosphorylation and significantly enhanced serine phosphorylation of Insulin Receptor Substrate 1 (IRS-1) to directly inhibit insulin signaling. At the same time, IL-1 beta induced the expression of TNF-alpha, which independently impaired insulin signaling both in vitro and in vivo.18
Collectively, these observations demonstrate that the Nlrp3 inflammasome serves as a molecular sensor of obesity-associated danger signals. Nlrp3 activation leads to Caspase-1-dependent IL-1 beta secretion and inflammation in key metabolic tissues, including the pancreas and adipose tissue. Chronic inflammation in these tissues negatively affects their functions and promotes insulin resistance. As a result, the Nlrp3 inflammasome and/or Caspase-1 may represent novel therapeutic targets for restoring insulin sensitivity and inhibiting the pathogenesis of type II diabetes, and other diseases linked to obesity-induced inflammation.
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