No abstract

SummaryCardiovascular disease affects approximately 60% of the adult population over the age of 65 and represents the number one cause of death in the United States. Coronary atherosclerosis is responsible for the vast majority of the cardiovascular events, and a number of cardiovascular risk factors have been identified. In recent years, it has become clear that insulin resistance and endothelial dysfunction play a central role in the pathogenesis of atherosclerosis. Much evidence supports the presence of insulin resistance as the fundamental pathophysiologic disturbance responsible for the cluster of metabolic and cardiovascular disorders, known collectively as the metabolic syndrome. Endothelial dysfunction is an important component of the metabolic or insulin resistance syndrome and this is demonstrated by inadequate vasodilation and/or paradoxical vasoconstriction in coronary and peripheral arteries in response to stimuli that release nitric oxide (NO). Deficiency of endothelial-derived NO is believed to be the primary defect that links insulin resistance and endothelial dysfunction. NO deficiency results from decreased synthesis and/or release, in combination with exaggerated consumption in tissues by high levels of reactive oxygen (ROS) and nitrogen (RNS) species, which are produced by cellular disturbances in glucose and lipid metabolism.Endothelial dysfunction contributes to impaired insulin action, by altering the transcapillary passage of insulin to target tissues. Reduced expansion of the capillary network, with attenuation of microcirculatory blood flow to metabolically active tissues, contributes to the impairment of insulinstimulated glucose and lipid metabolism. This establishes a reverberating negative feedback cycle in which progressive endothelial dysfunction and disturbances in glucose and lipid metabolism develop secondary to the insulin resistance. Vascular damage, which results from lipid deposition and oxidative stress to the vessel wall, triggers an inflammatory reaction, and the release of chemoattractants and cytokines worsens the insulin resistance and endothelial dysfunction.From the clinical standpoint, much experimental evidence supports the concept that therapies that improve insulin resistance and endothelial dysfunction reduce cardiovascular morbidity and mortality. Moreover, interventional strategies that reduce insulin resistance ameliorate endothelial dysfunction, while interventions that improve tissue sensitivity to insulin enhance vascular endothelial function. There is general agreement that aggressive therapy aimed simultaneously at improving insulin-mediated glucose/lipid metabolism and endothelial dysfunction represents an important strategy in preventing/delaying the appearance of atherosclerosis.

OBJECTIVE-Tall-like receptor (TLR)4 has been implicated in the pathogenesis of free fatty acid (FFA)-induced insulin resistance by activating inflammatory pathways, including inhibitor of B (IB)/nuclear factor B (NFB). However, it is not known whether insulin-resistant subjects have abnormal TLR4 signaling. We examined whether insulin-resistant subjects have abnormal TLR4 expression and TLR4-driven (IB/NFB) signaling in skeletal muscle. RESEARCH DESIGN AND METHODS-TLR4gene expression and protein content were measured in muscle biopsies in 7 lean, 8 obese, and 14 type 2 diabetic subjects. A primary human myotube culture system was used to examine whether FFAs stimulate IB/NFB via TLR4 and whether FFAs increase TLR4 expression/content in muscle.RESULTS-Obese and type 2 diabetic subjects had significantly elevated TLR4 gene expression and protein content in muscle. TLR4 muscle protein content correlated with the severity of insulin resistance. Obese and type 2 diabetic subjects also had lower IB␣ content, an indication of elevated IB/NFB signaling. The increase in TLR4 and NFB signaling was accompanied by elevated expression of the NFB-regulated genes interleukin (IL)-6 and superoxide dismutase (SOD)2. In primary human myotubes, acute palmitate treatment stimulated IB/NFB, and blockade of TLR4 prevented the ability of palmitate to stimulate the IB/NFB pathway. Increased TLR4 content and gene expression observed in muscle from insulin-resistant subjects were reproduced by treating myotubes from lean, normal-glucose-tolerant subjects with palmitate. Palmitate also increased IL-6 and SOD2 gene expression, and this effect was prevented by inhibiting NFB.CONCLUSIONS-Abnormal TLR4 expression and signaling, possibly caused by elevated plasma FFA levels, may contribute to the pathogenesis of insulin resistance in humans. Diabetes 57: [2595][2596][2597][2598][2599][2600][2601][2602] 2008 T he mechanism(s) by which free fatty acids (FFAs) cause insulin resistance is not fully understood. Considerable evidence suggests that the deleterious effect of FFAs on insulin action is caused by intramyocellular FFA metabolites that stimulate inflammatory pathways leading to impaired insulin signaling/action (1). However, recent reports demonstrate that FFAs directly can stimulate plasma membrane receptors (2,3), suggesting an alternate model in which FFAs cause insulin resistance by stimulating inflammatory pathways through the direct activation of plasma membrane receptors. Consistent with this hypothesis, FFAs have been shown to bind to toll-like receptor (TLR)4 (4), a transmembrane receptor, and TLR4-driven inflammatory cascades, such as the inhibitor of B (IB)/nuclear factor B (NFB) pathway, are implicated in the pathogenesis of insulin resistance (5-7).TLRs play an important role in the innate immune system by activating inflammatory pathways in response to microbial agents (8). TLR4 functions as the receptor for lipopolysaccharide (LPS) of gram-negative bacterial cell walls (8). Saturated FFAs acylated in the lipid A moiety of...

The effect of pioglitazone (PIO) on plasma adiponectin concentration, endogenous glucose production (EGP), and hepatic fat content (HFC) was studied in 11 type 2 diabetic patients (age, 52 ؎ 2 yr; body mass index, 29.6 ؎ 1.1 kg/m 2 ; HbA 1c , 7.8 ؎ 0.4%). HFC (magnetic resonance spectroscopy) and basal plasma adiponectin concentration were quantitated before and after PIO (45 mg/d) for 16 wk. Subjects received a 3-h euglycemic insulin (100 mU/m 2 ⅐min) clamp combined with 3-[ 3 H] glucose infusion to determine rates of EGP and tissue glucose disappearance (Rd) before and after PIO. PIO reduced fasting plasma glucose (10.0 ؎ 0.7 to 7.2 ؎ 0.6 mmol/liter, P < 0.01) and HbA 1c (7.8 ؎ 0.4 to 6.5 ؎ 0.3%, P < 0.01) despite increased body weight (83.0 ؎ 3.0 to 86.4 ؎ 3.0 kg, P < 0.01). PIO improved Rd (6.6 ؎ 0.6 vs. 5.2 ؎ 0.5 mg/kg⅐min, P < 0.005) and reduced EGP (0.23 ؎ 0.04 to 0.05 ؎ 0.02 mg/kg⅐min, P < 0.01) during the 3-h insulin clamp. After PIO treatment, HFC decreased from 21.3 ؎ 4.2 to 11.0 ؎ 2.4% (P < 0.01), and plasma adiponectin increased from 7 ؎ 1 to 21 ؎ 2 g/ml (P < 0.0001). Plasma adiponectin concentration correlated negatively with HFC (r ‫؍‬ ؊0.60, P < 0.05) and EGP (r ‫؍‬ ؊0.80, P < 0.004) and positively with Rd before (r ‫؍‬ 0.68, P < 0.02) pioglitazone treatment; similar correlations were observed between plasma adiponectin levels and HFC (r ‫؍‬ ؊0.65, P < 0.03) and Rd after (r ‫؍‬ 0.70, P ‫؍‬ 0.01) pioglitazone treatment. EGP was almost completely suppressed after pioglitazone treatment; taken collectively, plasma adiponectin concentration, before and after pioglitazone treatment, still correlated negatively with EGP during the insulin clamp (r ‫؍‬ ؊0.65, P < 0.001). In conclusion, PIO treatment in type 2 diabetes causes a 3-fold increase in plasma adiponectin concentration. The increase in plasma adiponectin is strongly associated with a decrease in hepatic fat content and improvements in hepatic and peripheral insulin sensitivity. The increase in plasma adiponectin concentration after thiazolidinedione therapy may play an important role in reversing the abnormality in hepatic fat mobilization and the hepatic/muscle insulin resistance in patients with type 2 diabetes. (J Clin Endocrinol Metab 89: 200 -206, 2004)

Activation of AMP-activated protein kinase (AMPK) by exercise induces several cellular processes in muscle. Exercise activation of AMPK is unaffected in lean (BMI ϳ25 kg/m 2 ) subjects with type 2 diabetes. However, most type 2 diabetic subjects are obese (BMI >30 kg/m 2 ), and exercise stimulation of AMPK is blunted in obese rodents. We examined whether obese type 2 diabetic subjects have impaired exercise stimulation of AMPK, at different signaling levels, spanning from the upstream kinase, LKB1, to the putative AMPK targets, AS160 and peroxisome proliferator-activated receptor coactivator (PGC)-1␣, involved in glucose transport regulation and mitochondrial biogenesis, respectively. Twelve type 2 diabetic, eight obese, and eight lean subjects exercised on a cycle ergometer for 40 min. Muscle biopsies were done before, during, and after exercise. Subjects underwent this protocol on two occasions, at low (50% VO 2max ) and moderate (70% VO 2max ) intensities, with a 4 -6 week interval. Exercise had no effect on LKB1 activity. Exercise had a time-and intensity-dependent effect to increase AMPK activity and AS160 phosphorylation. Obese and type 2 diabetic subjects had attenuated exercise-stimulated AMPK activity and AS160 phosphorylation. Type 2 diabetic subjects had reduced basal PGC-1 gene expression but normal exercise-induced increases in PGC-1 expression. Our findings suggest that obese type 2 diabetic subjects may need to exercise at higher intensity to stimulate the AMPK-AS160 axis to the same level as lean subjects. Diabetes 56:836 -848, 2007