Pioglitazone improves lipid and insulin levels in overweight rats on a high cholesterol and fructose diet by decreasing hepatic inflammation

Background and purpose: Nutrient overload leads to obesity and insulin resistance. Pioglitazone, a selective peroxisome proliferator-activated receptor (PPAR)gamma agonist, is currently used to manage insulin resistance, but the specific molecular mechanisms activated by PPAR gamma are not yet fully understood. Recent studies suggest the involvement of suppressor of cytokine signalling (SOCS)-3 in the pathogenesis of insulin resistance. This study aimed to investigate the hepatic signalling pathway activated by PPAR gamma activation in a non-genetic insulin-resistant animal model. Experimental approach: Male Wistar rats were maintained on a high-cholesterol fructose (HCF) diet for 15 weeks. Pioglitazone (3 mg center dot kg-1) was administered orally for the last 4 weeks of this diet. At the end of the treatment, serum was collected for biochemical analysis. Levels of PPAR gamma, SOCS-3, pro-inflammatory markers, insulin receptor substrate-2 and Akt/glycogen synthase kinase-3 beta phosphorylation were assesed in rat liver. Key results: Rats fed the HCF diet exhibited hyperlipidemia, hyperinsulinemia, impaired glucose tolerance and insulin resistance. Pioglitazone administration evoked a significant improvement in lipid metabolism and insulin responsiveness. This was accompanied by reduced hepatic expression of SOCS-3, interleukin-6, tumour necrosis factor-alpha and markers of neutrophil infiltration. Diet-induced PPAR gamma expression was unaffected by the pioglitazone treatment. Conclusion and implications: Chronic pioglitazone administration reduced hepatic inflammatory responses in rats fed a HCF diet. These effects were associated with changes in hepatic expression of SOCS-3, which may be a crucial link between the reduced local inflammation and the improved insulin signalling. This article is commented on by Chatterjee, pp. 1889-1891 of this issue. To view this commentary visit http://dx.doi.org/10.1111/j.1476-5381.2010.00739.x.