The circulating level of the inflammatory cytokine interleukin (IL)-6 is elevated in various insulin-resistant states including type 2 diabetes, obesity, cancer, and HIV-associated lipodystrophy. To determine the role of IL-6 in the development of insulin resistance, we examined the effects of IL-6 treatment on whole-body insulin action and glucose metabolism in vivo during hyperinsulinemic-euglycemic clamps in awake mice. Pretreatment of IL-6 blunted insulin's ability to suppress hepatic glucose production and insulin-stimulated insulin receptor substrate (IRS)-2-associated phosphatidylinositol (PI) 3-kinase activity in liver. Acute IL-6 treatment also reduced insulin-stimulated glucose uptake in skeletal muscle, and this was associated with defects in insulin-stimulated IRS-1-associated PI 3-kinase activity and increases in fatty acyl-CoA levels in skeletal muscle. In contrast, we found that co-treatment of IL-10, a predominantly anti-inflammatory cytokine, prevented IL-6 -induced defects in hepatic insulin action and signaling activity. Additionally, IL-10 co-treatment protected skeletal muscle from IL-6 and lipidinduced defects in insulin action and signaling activity, and these effects were associated with decreases in intramuscular fatty acyl-CoA levels. This is the first study to demonstrate that inflammatory cytokines IL-6 and IL-10 alter hepatic and skeletal muscle insulin action in vivo, and the mechanism may involve cytokineinduced alteration in intracellular fat contents. These findings implicate an important role of inflammatory cytokines in the pathogenesis of insulin resistance.

SummaryRecent studies have demonstrated a strong relationship between aging-associated reductions in mitochondrial function, dysregulated intracellular lipid metabolism, and insulin resistance. Given the important role of the AMP-activated protein kinase (AMPK) in the regulation of fat oxidation and mitochondrial biogenesis, we examined AMPK activity in young and old rats and found that acute stimulation of AMPK-α2 activity by 5′-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) and exercise was blunted in skeletal muscle of old rats. Furthermore, mitochondrial biogenesis in response to chronic activation of AMPK with β-guanidinopropionic acid (β-GPA) feeding was also diminished in old rats. These results suggest that aging-associated reductions in AMPK activity may be an important contributing factor in the reduced mitochondrial function and dysregulated intracellular lipid metabolism associated with aging.

Alterations in mitochondrial function have been implicated in the pathogenesis of insulin resistance and type 2 diabetes. However, it is unclear whether the reduced mitochondrial function is a primary or acquired defect in this process. To determine whether primary defects in mitochondrial ␤-oxidation can cause insulin resistance, we studied mice with a deficiency of long-chain acylCoA dehydrogenase (LCAD), a key enzyme in mitochondrial fatty acid oxidation. Here, we show that LCAD knockout mice develop hepatic steatosis, which is associated with hepatic insulin resistance, as reflected by reduced insulin suppression of hepatic glucose production during a hyperinsulinemic-euglycemic clamp. The defects in insulin action were associated with an Ϸ40% reduction in insulin-stimulated insulin receptor substrate-2-associated phosphatidylinositol 3-kinase activity and an Ϸ50% decrease in Akt2 activation. These changes were associated with increased PKC activity and an aberrant 4-fold increase in diacylglycerol content after insulin stimulation. The increase in diacylglycerol concentration was found to be caused by de novo synthesis of diacylglycerol from medium-chain acyl-CoA after insulin stimulation. These data demonstrate that primary defects in mitochondrial fatty acid oxidation capacity can lead to diacylglycerol accumulation, PKC activation, and hepatic insulin resistance.diacylglycerol ͉ mitochondria ͉ nonalcoholic fatty liver disease ͉ PKC R ecent studies have implicated alterations in mitochondrial function in the pathogenesis of insulin resistance and type 2 diabetes mellitus (1-8). It has been proposed that decreased mitochondrial fatty acid oxidation can result in insulin resistance by promoting increased intracellular diacylglycerol content, which in turn leads to activation of novel PKCs in liver and skeletal muscle and decreased insulin signaling and action in these tissues (9). However, it remains to be determined whether reduced mitochondrial function plays a primary role in causing the insulin resistance or whether it is a result of the increase in intracellular lipid content or other acquired factors (6, 10). To address this question, we examined insulin action in liver and skeletal muscle, using the hyperinsulinemic-euglycemic clamp, in long-chain acyl-CoA dehydrogenase (LCAD)-deficient (LCAD Ϫ/Ϫ ) mice, a genetic model of defective fatty acid oxidation. LCAD is a mitochondrial matrix enzyme catalyzing the first step for the oxidation of long-chain fatty acyl-CoAs. LCAD Ϫ/Ϫ mice are known to have impaired fatty acid oxidation and develop a disease similar to other disorders of mitochondrial fatty acid oxidation (11-12). We also examined the impact of LCAD deficiency on whole-body glucose and fatty acid oxidation in these mice, using indirect calorimetry. Results Metabolic Profile of the LCAD ؊/؊ Mice. LCADϪ/Ϫ mice, fed a standard rodent diet, had similar body weights but a 60% increase in whole-body fat content compared with their WT littermates (Table 1). However, they ate 11% less of the standard r...

We generated VEGF‐null fibrosarcomas from VEGF‐loxP mouse embryonic fibroblasts to investigate the mechanisms of tumor escape after VEGF inactivation. These cells were found to be tumorigenic and angiogenic in vivo in spite of the absence of tumor‐derived VEGF. However, VEGF derived from host stroma was readily detected in the tumor mass and treatment with a newly developed anti‐VEGF monoclonal antibody substantially inhibited tumor growth. The functional significance of stroma‐derived VEGF indicates that the recruitment of stromal cells is critical for the angiogenic and tumorigenic properties of these cells. Here we identified PDGF AA as the major stromal fibroblast chemotactic factor produced by tumor cells, and demonstrated that disrupting the paracrine PDGFR α signaling between tumor cells and stromal fibroblasts by soluble PDGFR α‐IgG significantly reduced tumor growth. Thus, PDGFR α signaling is required for the recruitment of VEGF‐producing stromal fibroblasts for tumor angiogenesis and growth. Our findings highlight a novel aspect of PDGFR α signaling in tumorigenesis.

Recent studies have suggested that n-3 fatty acids, abundant in fish oil, protect against high-fat diet-induced insulin resistance through peroxisome proliferator-activated receptor (PPAR)-␣ activation and a subsequent decrease in intracellular lipid abundance. To directly test this hypothesis, we fed PPAR-␣ null and wild-type mice for 2 weeks with isocaloric high-fat diets containing 27% fat from either safflower oil or safflower oil with an 8% fish oil replacement (fish oil diet). In both genotypes the safflower oil diet blunted insulin-mediated suppression of hepatic glucose production (P < 0.02 vs. genotype control) and PEPCK gene expression. Feeding wild-type mice a fish oil diet restored hepatic insulin sensitivity (hepatic glucose production [HGP], P < 0.002 vs. wild-type mice fed safflower oil), whereas in contrast, in PPAR-␣ null mice failed to counteract hepatic insulin resistance (HGP, P ‫؍‬ NS vs. PPAR-␣ null safflower oil-fed mice). In PPAR-␣ null mice fed the fish oil diet, safflower oil plus fish oil, hepatic insulin resistance was dissociated from increases in hepatic triacylglycerol and acyl-CoA but accompanied by a more than threefold increase in hepatic diacylglycerol concentration (P < 0.0001 vs. genotype control). These data support the hypothesis that n-3 fatty acids protect from high-fat diet-induced hepatic insulin resistance in a PPAR-␣-and diacylglycerol-dependent manner. Diabetes