Miguel A. Iglesias

Diabetes and obesity are two metabolic diseases characterized by insulin resistance and a low-grade inflammation. Seeking an inflammatory factor causative of the onset of insulin resistance, obesity, and diabetes, we have identified bacterial lipopolysaccharide (LPS) as a triggering factor. We found that normal endotoxemia increased or decreased during the fed or fasted state, respectively, on a nutritional basis and that a 4-week high-fat diet chronically increased plasma LPS concentration two to three times, a threshold that we have defined as metabolic endotoxemia. Importantly, a high-fat diet increased the proportion of an LPS-containing microbiota in the gut. When metabolic endotoxemia was induced for 4 weeks in mice through continuous subcutaneous infusion of LPS, fasted glycemia and insulinemia and whole-body, liver, and adipose tissue weight gain were increased to a similar extent as in high-fat-fed mice. In addition, adipose tissue F4/80-positive cells and markers of inflammation, and liver triglyceride content, were increased. Furthermore, liver, but not whole-body, insulin resistance was detected in LPS-infused mice. CD14 mutant mice resisted most of the LPS and high-fat diet-induced features of metabolic diseases. This new finding demonstrates that metabolic endotoxemia dysregulates the inflammatory tone and triggers body weight gain and diabetes. We conclude that the LPS/CD14 system sets the tone of insulin sensitivity and the onset of diabetes and obesity. Lowering plasma LPS concentration could be a potent strategy for the control of metabolic diseases.

Nondigestible fermentable dietary fibers such as oligofructose (OFS) exert an antidiabetic effect and increase the secretion of glucagon-like peptide 1 (GLP-1). To determine the importance of GLP-1 receptor-dependent mechanisms for the actions of OFS, we studied high-fat-fed diabetic mice treated with OFS for 4 weeks in the presence or absence of the GLP-1 receptor antagonist exendin 9-39 (Ex-9). OFS improved glucose tolerance, fasting blood glucose, glucose-stimulated insulin secretion, and insulin-sensitive hepatic glucose production and reduced body weight gain. Ex-9 totally prevented the beneficial effects of OFS. Furthermore, GLP-1 receptor knockout mice (GLP-1R(-/-)) were completely insensitive to the antidiabetic actions of OFS. At the molecular level, the effects of OFS on endogenous glucose production correlated with changes of hepatic IRS (insulin receptor substrate)-2 and Akt phosphorylation in an Ex-9-dependent manner. As inflammation is associated with diabetes and obesity, we quantified nuclear factor-kappaB and inhibitor of kappaB kinase beta in the liver. The activity of both intracellular inflammatory effectors was reduced by OFS but, importantly, this effect could not be reverted by Ex-9. In summary, our data show that the antidiabetic actions of OFS require a functional GLP-1 receptor. These findings highlight the therapeutic potential of enhancing endogenous GLP-1 secretion for the treatment of type 2 diabetes.

Peroxisome proliferator-activated receptor (PPAR)-alpha agonists lower circulating lipids, but the consequences for muscle lipid metabolism and insulin sensitivity are not clear. We investigated whether PPAR-alpha activation improves insulin sensitivity in insulin-resistant rats and compared the effects with PPAR-gamma activation. Three-week high fat-fed male Wistar rats were untreated or treated with the specific PPAR-alpha agonist WY14643 or the PPAR-gamma agonist pioglitazone (both 3 mg x kg(-1) x day(-1)) for the last 2 weeks of high-fat feeding. Like pioglitazone, WY14643 lowered basal plasma levels of glucose, triglycerides (-16% vs. untreated), and leptin (-52%), and also muscle triglyceride (-34%) and total long-chain acyl-CoAs (LCACoAs) (-41%) (P < 0.05). In contrast to pioglitazone, WY14643 substantially reduced visceral fat weight and total liver triglyceride content (P < 0.01) without increasing body weight gain. WY14643 and pioglitazone similarly enhanced whole-body insulin sensitivity (clamp glucose infusion rate increased 35 and 37% and glucose disposal 22 and 15%, respectively, vs. untreated). Both agents enhanced insulin-mediated muscle glucose metabolic index (Rg') and reduced muscle triglyceride and LCACoA accumulation (P < 0.05). Although pioglitazone had more potent effects than WY14643 on muscle insulin sensitization, this was associated with its greater effect to reduce muscle LCACoA accumulation. Overall insulin-mediated muscle Rg' was inversely correlated with the content of LCACoAs (r = -0.74, P = 0.001) and with plasma triglyceride levels (r = -0.77, P < 0.001). We conclude that even though WY14643 and pioglitazone, representing PPAR-alpha and PPAR-gamma activation, respectively, may alter muscle lipid supply by different mechanisms, both significantly improve muscle insulin action in the high fat-fed rat model of insulin resistance, and this effect is proportional to the degree to which they reduce muscle lipid accumulation.

Exercise improves insulin sensitivity. As AMP-activated protein kinase (AMPK) plays an important role in muscle metabolism during exercise, we investigated the effects of the AMPK activator 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) on insulin action in insulin-resistant high-fat-fed (HF) rats. Rats received a subcutaneous injection of 250 mg/kg AICAR (HF-AIC) or saline (HF-Con). The next day, euglycemic-hyperinsulinemic clamp studies were performed. Glucose infusion rate during the clamp was enhanced (50%) in HF-AIC compared with HF-Con rats. Insulin-stimulated glucose uptake was improved in white but not in red quadriceps, whereas glycogen synthesis was improved in both red and white quadriceps of HF-AIC rats. HF-AIC rats also showed increased insulin suppressibility of hepatic glucose output (HGO). AICAR-induced responses in both liver and muscle were accompanied by reduced malonyl-CoA content. Clamp HGO correlated closely with hepatic triglyceride content (r = 0.67, P < 0.01). Thus, a single dose of AICAR leads to an apparent enhancement in whole-body, muscle, and liver insulin action in HF rats that extends beyond the expected time of AMPK activation. Whether altered tissue lipid metabolism mediates AICAR effects on insulin action remains to be determined. Follow-up studies suggest that at least some of the post-AICAR insulin-enhancing effects also occur in normal rats. Independent of this, the results suggest that pharmacological activation of AMPK may have potential in treating insulin-resistant states and type 2 diabetes.