Inhibition by inflammatory signals

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Recent studies have suggested AMPK to play a crucial role in the inflammatory signaling pathways.  AMPK activity has been shown to be down-regulated upon pro-inflammatory stimulus (LPS) and up-regulated upon anti-inflammatory cytokine stimulation (IL-10 and TGF-b) (Sag et al., 2008). Also, inhibition of AMPK activity or expression increases the production of TNFa, IL-6 and IL-1 upon pro-inflammatory stimulus, whereas overexpression of AMPK results in the dampening of inflammatory response and increases the production of IL-10 (Jeong et al., 2009; Sag et al., 2008). The effects of AMPK deficiency on the regulation of inflammatory status, indicates that the presence of AMPK and its activation is important to counteract inflammation. Furthermore, increasing AMPK activity with AICAR, or by transfection of a constitutively active AMPK catalytic subunit, blunts the ability of free fatty acids (palmitate) or TNFa to activate NFkB (Cacicedo et al., 2004). Accordingly, in vivo AMPK activation decreases severity of LPS-induced lung injury (Zhao et al., 2008) and the expression of pro-inflammatory genes in adipose tissue of obese db/db mice (Bai et al., 2010). A defect in AMPK function has been found in various cells in animals with metabolic diseases. In diabetes and obesity, it is likely that AMPK activation is compromised in inflammation-related cells and leads to the development of inflammatory diseases. Thus, AMPK may be a promising pharmacologic target for the treatment of various chronic inflammatory diseases.

Obesity is a morbid condition characterized by an excess in fat mass and myriad co-morbidities. Among them, it is recognized that insulin resistance promotes the development of type 2 diabetes. Interestingly, insulin resistance varies greatly among obese people, some patients being severely insulin resistant while other remains insulin sensitive despite accumulation of body fat (Brochu et al., 2001; Guilherme et al., 2008). Different hypothesis have been discussed to explain this variability. One of them postulated that obesity related insulin resistance can be recognized as a state of chronic low-grade inflammation (Lumeng et al., 2007; Permana et al., 2006; Rasouli et al., 2005; Xu et al., 2003). Macrophages in obese patients are in an inflammatory state and display increased NFkB and TNFa expression. TNFa induces insulin resistance through the serine phosphorylation of IRS protein by JNK and Ik/NFkB and increases the expression of STAT3-suppressor of cytokine signaling 3 (SOCS3) (Kern et al., 2001; Shi et al., 2004). Obesity favors increased rates of fatty acid uptake and esterification leading to storage of bioactive lipids such as ceramides, diacylglycerol (DAG) and fatty acyl-CoA in tissues. These lipids contribute to the activation of inflammatory serine threonine kinases such as conventional PKCs, IKK-b and JNK (Schenk et al., 2008). Rates of fatty acid oxidation in skeletal muscle are also reduced in obese humans and rodents and this defect has been correlated with reduced AMPK activity. Nevertheless, the mechanism connecting excess of lipids and decreased AMPK activity in skeletal muscle has not been completely elucidated. To address this question, it has been shown in cultured L6 muscle cells that TNFa reduced AMPK activity without change in LKB1 activity. TNFa suppresses AMPK activity which leads to defective fatty-acid metabolism, an important contributing factor to the development of insulin resistance in obesity (Steinberg et al., 2006a). TNFa mediates its action through TNF receptor (TNFR) 1 to attenuate AMPK activity via transcriptional upregulation of PP2C which results in reduction of ACC phosphorylation, suppressing fatty-acid oxidation, increasing intramuscular diacylglycerol accumulation and causing insulin resistance in skeletal muscle. Using in vitro and in vivo approaches, Steinberg and coworkers provided for the first time conclusive evidence of AMPK as a link between inflammation and metabolic disease. According to these results, ob/ob mice have also reduced muscular AMPK activity, inhibited fatty acid oxidation, increased PP2C expression in their skeletal muscle and reduced muscular insulin sensitivity in vivo. In contrast, AMPK activity is not altered in ob/ob TNFR-/- mice indicating that disruption of TNF signaling prevents AMPK inhibition in this genetically obese mice model.

Circulating free fatty acids (FFA) are often increased in obesity and they activate TLR4 signaling-NFkB-inflammation cascade (TNFa production) in adipocytes and macrophages which contributes to insulin resistance in skeletal muscle (Shi et al., 2006). Interestingly, ablation of TLR4 signaling using TLR4 knockout mice protects against high fat diet-induced insulin resistance, due to reduced inflammation, linking innate immune system and metabolism. Activation of TLR4 by endotoxin also leads to loss of AMPK phosphorylation under similar conditions where NFkB pathway is activated in macrophage (Nath et al., 2009). These studies delineate a novel FFA/endotoxin-TLR4-NFkB-TNFa-loss of AMPK-insulin resistance pathway which could be implicated in metabolic disorders (Figure 3). In addition, it has been demonstrated that resistin, an adipocytokine elevated in obesity, inhibited skeletal muscle AMPK activity. Consequent accumulation of lipids and their mediators probably explains resistin-mediated insulin resistance during obesity. When insulin resistance occurs, reduced adiponectin levels can also contribute to continuous suppression of AMPK activity. Because AMPK is a critical factor for mitochondrial biogenesis, long-term reduction of its activity can lead to reduction of mitochondrial density/function in skeletal muscle, as observed in insulin resistance associated with obesity. Supporting this hypothesis, treatment of ob/ob mice by rosiglitazone or by adiponectin reduced TNFa synthesis and increased muscle mitochondrial biogenesis in parallel to metabolic improvement. In summary, it has been evidenced that excess of lipids can inhibit muscular AMPK activity through increased proinflammatory cytokines pathway. Similar conclusions have been obtained in hearts of mice during excess  lipids availability. Indeed, acute lipid excess (5 hours of lipids infusion) or diet-induced obesity was both associated with blunted myocardial glucose metabolism concomitantly with reduction of AMPK phosphorylation in heart (Ko et al., 2009). These deleterious effects of long-term or acute exposure to lipids in vivo are based on elevation of inflammatory cytokines (TNFa and IL-6) and increase in their myocardial signaling (Senn et al., 2002). Myocardial levels of STAT3, CD68 and SOCS3, reduction of AMPK activity and down-regulation of myocardial glucose metabolism are attenuated in IL-6 KO mice following high fat diet. This suggests that IL-6 is a key component of the diet-induced myocardial inflammation and subsequent metabolic changes in heart. Chronic exposure of IL-6 (as observed in obesity) promotes insulin resistance both in vitro and in vivo (Nieto-Vazquez et al., 2008). In contrast, during prolonged exercise, IL-6 is released acutely from the skeletal muscle (Febbraio and Pedersen, 2005; Kelly et al., 2004), AMPK is activated (Kelly et al., 2009) and leads to improved peripheral glucose uptake and insulin sensitivity at the whole body level (Glund et al., 2007; Ruderman et al., 2006). This dual effect of IL-6 on insulin sensitivity probably explains some conflicting results recently discussed in more details elsewhere (Nieto-Vazquez et al., 2008).

In general, AMPK functions solely to restore energy balance after depletion of energy stores. However, in T cells, Tamas and coworkers (Tamas et al., 2006) proposed that its unique ability to anticipate energy-consuming processes could be useful for immune cells that need a rapid response to an increased demand for ATP. Activation of AMPK by TCR engagement was shown to be abrogated by CaMKK inhibitor (STO-609) but not when it was activated by AMP/ATP ratio, suggesting two independent pathways for the regulation of AMPK in T cells. Recently, it was reported that the AMPKa1 protein is lost in spleen macrophages, total T cells and their subsets (CD4, CD8 and regulatory T cells) isolated from experimental autoimmune encephalomyelitis (EAE) afflicted animals, compared to control, without affecting its mRNA levels (Nath et al., 2009), suggesting a posttranscriptional modification. Genetic ablation of AMPKa1 in mice exhibited severe disease with profound infiltration of mononuclear cells in central nervous system (CNS) compare to wild type mice. Interestingly, AMPKa2 isoform does not participate in enhancing the severity of the disease.