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OBJECTIVE—Pioglitazone is a member of the thiazolidinediones (TZDs), insulin-sensitizing agents used to treat type 2 diabetes. The aim of this study was to define the effect of pioglitazone on the expression of genes related to carbohydrate and lipid metabolism in subcutaneous fat obtained from type 2 diabetic patients. RESEARCH DESIGN AND METHODS—Forty-eight volunteers with type 2 diabetes were divided into two groups treated for 12 weeks with placebo or pioglitazone (30 mg/day). The expression of several genes was quantified by real-time RT-PCR. RESULTS—Pioglitazone treatment increased the expression of genes involved in glycerol-3-phosphate synthesis. The mRNA expression of PEPCK-C and glycerol-3-phosphate dehydrogenase (GPDH) increased (P < 0.01) in patients treated with pioglitazone. There was no difference in glycerol kinase (GyK) mRNA levels. The expression of genes that regulate fatty acid availability in adipocytes, including lipoprotein lipase (LPL) and acetyl-CoA synthetase (ACS), was higher (P < 0.01) in pioglitazone-treated patients. Pioglitazone stimulated (P < 0.0001) expression of c-Cbl-associated protein (CAP), whereas tumor necrosis factor-α, leptin, resistin, angiopoietin like-4, and 11-β-hydroxysteroid dehydrogenase type 1 (11β HSD 1) were not affected by pioglitazone. The baseline peroxisome proliferator-activated receptor (PPAR)-γ1 mRNA was significantly correlated with mRNA for LPL, CAP, ACS, 11β HSD 1, GyK, fatty acid synthase, leptin, and GPDH, whereas PPAR-γ2 mRNA was correlated with CAP, PEPCK-C, leptin, and GPDH. CONCLUSIONS—Treatment with pioglitazone increased body weight, and this is associated with upregulation of some, but not all, genes previously demonstrated as “TZD responsive” in subcutaneous fat. The results suggest that TZDs might increase body weight through the upregulation of genes facilitating adipocyte lipid storage in vivo.

Insulin resistance is associated with metabolic inflexibility, impaired switching of substrate oxidation from fatty acids to glucose in response to insulin. Impaired switching to fat oxidation in response to a high-fat diet (HFD) is hypothesized to contribute to insulin resistance. The objective of this study was to test the hypothesis that defects in substrate switching in response to insulin and a HFD are linked to reduced mitochondrial biogenesis and occur before the development of diabetes. Metabolic flexibility was measured in young sedentary men with (n ‫؍‬ 16) or without (n ‫؍‬ 34) a family history of diabetes by euglycemichyperinsulinemic clamp. Flexibility correlated with fat oxidation measured in a respiratory chamber after a 3-day HFD. Muscle mitochondrial content was higher in flexible subjects with high fat oxidation after a HFD and contributed 49% of the variance. Subjects with a family history of diabetes were inflexible and had reduced HFD-induced fat oxidation and muscle mitochondrial content but did not differ in the amount of body or visceral fat. Metabolic inflexibility, lower adaptation to a HFD, and reduced muscle mitochondrial mass cluster together in subjects with a family history of diabetes, supporting the role of an intrinsic metabolic defect of skeletal muscle in the pathogenesis of insulin resistance. Diabetes 56:720 -727, 2007 A high-fat diet (HFD) is a risk factor for obesity and has been implicated in the development of insulin resistance (1). A short-term HFD, as well as a lipid infusion, causes insulin resistance (2-4), reduces oxidative metabolism in skeletal muscle in rats (5), and downregulates genes of oxidative phosphorylation and mitochondrial biogenesis, such as peroxisome proliferator-activated ␥ coactivator-1␣ (PGC-1␣) (6). Moreover, oxidative phosphorylation and PGC-1␣ gene expression are decreased in insulin resistance (7,8). Taken together, these findings point to HFD, reduced oxidative capacity, and lipotoxicity in the pathophysiology of insulin resistance (9,10).Substantial interindividual variability exists in the change of fat oxidation during adaptation to a HFD (11). Impaired fat oxidation during adaptation to a HFD is observed in restrained eaters (12), postobese (13) and obese (14) individuals, and in individuals with a family history of obesity (15), pointing toward a possible genetic basis for reduced fat oxidation (1). Impaired substrate switching in response to insulin (metabolic inflexibility) and dietary stimuli (attenuated adaptation to a HFD) are hypothesized to contribute to obesity and insulin resistance (1,16).Metabolic inflexibility, as defined by Kelley and Mandarino (17), represents impaired substrate switching in skeletal muscle in insulin resistance. Healthy lean individuals who are flexible rely on lipids as a main source of fuel under fasting conditions and readily switch to carbohydrate oxidation in response to insulin infusion (18). On the contrary, the inflexible muscle of an insulin-resistant individual is characterized by lower fasting ...

Objective: Alternate day fasting may extend lifespan in rodents and is feasible for short periods in nonobese humans. The aim of this study was to examine the effects of 3 weeks of alternate day fasting on glucose tolerance and skeletal muscle expression of genes involved in fatty acid transport/oxidation, mitochondrial biogenesis, and stress response. Research Methods and Procedures:Glucose and insulin responses to a standard meal were tested in nonobese subjects (eight men and eight women; BMI, 20 to 30 kg/m 2 ) at baseline and after 22 days of alternate day fasting (36 hour fast). Muscle biopsies were obtained from a subset of subjects (n ϭ 11) at baseline and on day 21 (12-hour fast). Results: Glucose response to a meal was slightly impaired in women after 3 weeks of treatment (p Ͻ 0.01), but insulin response was unchanged. However, men had no change in glucose response and a significant reduction in insulin response (p Ͻ 0.03). There were no significant changes in the expression of genes involved in mitochondrial biogenesis or fatty acid transport/oxidation, although a trend toward increased CPT1 expression was observed (p Ͻ 0.08). SIRT1 mRNA expression was increased after alternate day fasting (p ϭ 0.01).Discussion: Alternate day fasting may adversely affect glucose tolerance in nonobese women but not in nonobese men. The gene expression results indicate that fatty acid oxidation and mitochondrial biogenesis are unaffected by alternate day fasting. However, the increased expression in SIRT1 suggests that alternate day fasting may improve stress resistance, a commonly observed feature of calorierestricted rodents.

Pharmacological activation of the cAMP or PPARgamma pathway pushes the white adipocyte down the oxidative continuum. The direct energy-dissipating effects could be significant tools to treat obesity and to improve insulin resistance in type 2 diabetic patients by reduction of fat accumulation in adipocytes or by reprogramming fatty acid metabolism.