Inhibition of Inducible Nitric-oxide Synthase by Activators of AMP-activated Protein Kinase

Abstract

AMP-activated protein kinase (AMPK), an energysensing enzyme that is activated in response to cellular stress, is a critical signaling molecule for the regulation of multiple metabolic processes. AMPK has recently emerged as an attractive novel target for the treatment of obesity and type 2 diabetes because its activation increases fatty acid oxidation and improves glucose homeostasis. Here we show that pharmacological activation of AMPK by insulin-sensitizing drugs markedly inhibits inducible nitric-oxide synthase (iNOS), a proinflammatory mediator in endotoxic shock and in chronic inflammatory states including obesity-linked diabetes. AMPK-mediated iNOS inhibition was observed in several cell types (myocytes, adipocytes, macrophages) and primarily resulted from post-transcriptional regulation of the iNOS protein. AMPK activation in vivo also blunted iNOS induction in muscle and adipose tissues of endotoxin-challenged rats. Reduction of AMPK expression by small interfering RNA reversed the inhibitory effects of AMPK activators on iNOS expression and nitric oxide production in myocytes. These results indicate that AMPK is a novel anti-inflammatory signaling pathway and thus represents a promising therapeutic target for immune-inflammatory disorders. AMP-activated protein kinase (AMPK)1 is emerging as an important energy-sensing/signaling system in mammalian tissues. It is a member of a metabolite-sensing protein kinase family that acts as a fuel gauge by monitoring cellular energy levels (1). When AMPK "senses" decreased energy storage, it acts to switch off ATP-consuming pathways and switch on alternative pathways for ATP regeneration. AMPK is a heterotrimer consisting of a catalytic ␣-subunit and two regulatory subunits, ␤ and ␥. In response to cellular energy depletion, as reflected by an increase in the AMP/ATP ratio, AMPK is phosphorylated and activated by a still uncharacterized upstream AMPK kinase (2). It also can be activated allosterically by increases in the AMP/ATP and creatine/creatine-P ratios. The metabolic function of AMPK perhaps has been documented best in exercising skeletal muscle, where its activation seems to contribute to increased glucose transport and fatty acid oxidation (3, 4). AMPK can be activated chemically with 5-aminoimidazole-4-carboxamide riboside (AICAR), which is taken up by cells and phosphorylated by adenosine kinase to form 5-aminoimidazole-4-carboxamide ribonucleoside, a nucleotide that mimics the effect of AMP (5). More recent studies also have identified AMPK as the mediator of the metabolic effects of the adipose-derived peptidic hormones leptin and adiponectin in skeletal muscle and liver (6 -8).Chronic treatment of animal models of type 2 diabetes with the AMPK activator AICAR improves glucose homeostasis and insulin sensitivity (9 -11). These beneficial effects are thought to be explained mainly by the well known actions of AMPK on glucose metabolism and lipid oxidation in muscle and liver. Accordingly, the anti-diabetic drugs metformin and rosiglitazone also activat...

Wnt10b Inhibits Obesity in ob/ob and Agouti Mice

“…Our assessment of mRNA levels of MCP-1 revealed that expression of MCP-1 is decreased in Wnt10b-A y mice. While only a trend toward reduced expression of F4/80 was also noted (not shown), iNOS expression, one possible mediator of inflammation-associated insulin resistance (43,45), is reduced by 73% in white adipose tissue of Wnt10b-A y mice (Fig. 3E).…”

5-Aminoimidazole-4-Carboxamide Ribonucleoside: A Novel Immunomodulator with Therapeutic Efficacy in Experimental Autoimmune Encephalomyelitis

“…AMPK can also inhibit cholesterol and fatty acid biosynthesis via phosphorylation and inhibition of acetyl CoA-carboxylase and HMG CoA-reductase (9,11). Previously, it was reported by us and others that AICAR inhibits the production of proinflammatory mediators TNF-␣, IL-1␤, IL-6, and NO in primary astrocytes, microglia, and M (23,37,38). Therefore, the observed antiinflammatory effects of AICAR on EAE are likely mediated through both immune-independent and -dependent mechanisms.…”

How Metabolism Generates Signals during Innate Immunity and Inflammation

“…Yuan et al (41) demonstrated that the treatment of obese mice with salicylate results in a significant decrease in blood glucose levels, whereas clinical trials with salsalate, a non-acetylated prodrug of salicylate, showed a reduction in blood glucose levels in patients with T2D (42). In obesity and T2D, the accumulation of saturated fatty acids such as palmitate results in deactivation of AMPK, and this decreases autophagy and NO production and increases mitochondrial dysfunction and mitochondrial ROS production (43). This leads, in turn, to activation of the NLRP3 inflammasome.…”