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CHEMOTHERAPY, ANTIBIOTICS, AND GENE THERAPY
Department of Medicine III, Division of Endocrinology and Metabolism, Medical University of Vienna, Vienna, Austria (Z.S., B.B., K.S., P.N., A.L., C.F.); and 55pharma Drug Discovery & Development AG, Vienna, Austria (L.B.)
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Abstract |
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), their metabolic deviations are known to depend on gender and diet. Females slowly develop hyperglycemia only if fed a highly diabetogenic diet, but males are much more hyperglycemia-prone and, in combination with a defined diet (Purina LabDiet Formulab 5008, Richmond, IN), emerged as a standard model for human type 2 diabetes. The diabetes-like syndrome of ZDF rats is caused by a mutation in the gene encoding the leptin receptor, which results in severe dysregulation of appetite and body weight (Phillips et al., 1996). The predominant factor driving metabolic deterioration is severe adiposity-associated insulin resistance, which explains that ZDF rats are particularly favored for the study of the pharmacology of insulin-sensitizing drugs like thiazolidinediones (TZDs). In males on Purina 5008, regular administration of the TZDs rosiglitazone or pioglitazone fully prevents hyperglycemia or restores normoglycemia, if the treatment is initiated at a young age of less than approximately 9 weeks. More severe hyperglycemia prevailing at ages 10 to 12 weeks can only be ameliorated by TZDs, and intervention at 21 weeks of age has been reported to lack any glucose-lowering effect (Sturis et al., 1995; Smith et al., 2000; Finegood et al., 2001; Brand et al., 2003; Yang et al., 2004; Pickavance et al., 2005).
The aim of the present study was to thoroughly investigate and characterize the age-dependent development of the ZDF syndrome as well as the associated changes in responses to treatment with pioglitazone under standard conditions (males on Purina 5008). The spectrum of parameters measured included several readouts of glucose and lipid metabolism as well as plasma adiponectin and p70S6 kinase activity in skeletal muscles. Although p70S6 kinase has been associated with the regulation of cellular insulin sensitivity (Um et al., 2004; Krebs et al., 2007), its possible role in TZD-induced insulin sensitization has not yet been studied. The goal of the study was to provide a detailed understanding of parallels and differences between the etiologies and pathophysiologies of the ZDF syndrome and human type 2 diabetes, which is an essential prerequisite for the appropriate design and interpretation of preclinical studies using ZDF rats.
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Materials and Methods |
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Treatment. Oral gavage treatment with pioglitazone (12 mg/kg/day, in 2 ml/kg 5% gum arabic), a dose with distinct glucose-lowering activity in 8-week-old ZDF rats (Yang et al., 2004), was initiated in three groups of ZDF rats (fa/fa), which were 7, 10.5, and 15.5 weeks old, and was continued for 5.5, 4.5, and 2.5 weeks, respectively (n = 6–12 per group). Controls were treated with the vehicle, and an additional group of vehicle-treated lean littermates that were not affected by the ZDF syndrome (Fa/–) was studied for comparison starting at 10.5 weeks of age.
On the first and third day of treatment, as well as every 7 days thereafter, rats were fasted for 4 h and weighed. Before the rats received their daily dose of pioglitazone (i.e., approximately 24 h after the last dose), capillary blood for the determination of blood glucose was collected without physical restriction by gently pricking the tip of the tail. On the last day, this procedure was not followed by gavage, but rats were deeply anesthetized with sevoflurane (Abbott Laboratories, Queenborough, Kent, UK) and subjected to cervical dislocation. A large amount of blood was sampled immediately by heart puncture, and specimens of liver, extensor digitorum longus (EDL) muscle, and gastrocnemius muscle (red part), as well as the left epididymal fat pad were collected. Plasma and tissue samples were stored at –20° and –80°C, respectively, until further analyzed. Food intake per rat was calculated from consumption per cage.
Analytics. Blood glucose, plasma triglycerides, and plasma cholesterol were measured with an enzymatic analyzer (Falcor 350; Menarini, Florence, Italy) according to the manufacturer's standard procedures. Plasma free fatty acids were determined with a kit from Wako (Neuss, Germany), and radioimmunoassays from Linco (St. Charles, MO) were used to measure rat insulin and adiponectin in plasma.
p70S6 kinase activity in specimens of EDL muscle is given as percentage of the protein in the active (phosphorylated) state as determined by Western blotting. Procedures for quantification of the phosphorylation status exploited the different electrophoretic mobilities of the active (phosphorylated) and inactive (dephosphorylated) forms of the kinase and have been previously described in detail (Krebs et al., 2007). The activity of Akt (also referred to as protein kinase B) was determined in EDL muscle by Western blot measurements of total Akt protein and its activated form, phospho-Akt (Ser473), according to a previously described procedure (Anderwald et al., 2007).
For the determination of glycogen content, frozen specimens of liver and gastrocnemius muscle were lysed in 1 mol/l KOH at 70°C. Glycogen in the lysate was degraded to glucose with amyloglucosidase (Roche Diagnostics, Mannheim, Germany) followed by the measurement of glucose with an enzymatic kit (Human, Taunusstein, Germany).
Statistics. Results are given as mean± S.E.M. Differences were analyzed by two-tailed paired Student's t test (for comparison of results obtained from the same individuals at different ages) or by two-tailed unpaired Student's t test (for comparison of results obtained from different individuals), and p < 0.05 was considered as significant. Intraindividual associations between two variables are indicated by Pearson's correlation coefficient (r), and a two-tailed p < 0.05 was considered as significant.
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Results |
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25 mM and plasma insulin levels of 22 mU/l, the latter was not different from those prevailing in healthy 15-week-old littermates (Fig. 2). All other measured parameters likewise remained more or less stable between 15 and 18 weeks of age (Figs. 2 and 3). One old vehicle-treated rat, which was still normoglycemic at this age, was excluded from the study (Fig. 1).
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Effects of Pioglitazone Treatment. When treatment with pioglitazone was initiated at 7 weeks of age, the development of hyperglycemia as seen in vehicle-treated ZDF rats was completely suppressed over the entire treatment period of 5.5 weeks (Fig. 1). The same intervention at an age of 10.5 weeks caused only a transient amelioration of hyperglycemia (22.2 ± 0.6 versus 17.7 ± 1.1 mM after 10 days of treatment, p = 0.004), but the glucose-lowering effect of pioglitazone faded progressively thereafter. After 4.5 weeks of treatment, mean blood glucose was similar in rats treated with pioglitazone or vehicle (25.2 ± 0.9 versus 23.5 ± 0.7 mM, N.S.; Fig. 1). When started at an age of 15.5 weeks, treatment with pioglitazone remained without any effect on blood glucose (Fig. 1).
At the end of the treatment period, distinctly lower blood glucose seen in pioglitazone versus vehicle-treated rats belonging to the youngest group was accompanied by a very marked increase in plasma adiponectin (17.8 ± 0.4 versus 4.6 ± 0.4 mg/l, p < 10–10; Fig. 3). Pioglitazone treatment also resulted in markedly lower plasma triglycerides (–82%) and free fatty acids (–69%, p < 0.001 each; Figs. 2 and 3) as well as in elevated weight gain and epididymal fat pad weight (p < 0.001 each; Figs. 1 and 2). The activity of p70S6 kinase in EDL muscle was reduced (19.0 ± 3.0 versus 31.7 ± 2.7% in phosphorylated state, p = 0.01; Fig. 3), but this was obviously not mediated by inhibition of Akt, which showed unchanged activity in muscle from pioglitazone-treated rats (97.9 ± 15.3% of control muscles). Furthermore, increased appetite and trends toward lower plasma insulin (p = 0.053) and muscle glycogen content (p = 0.054) were seen in the pioglitazone-treated animals of young age (Fig. 2).
The lack of significant differences between blood glucose concentrations at the end of the treatment periods in the two older groups of ZDF rats was paralleled by the absence of significant differences in plasma insulin, triglycerides, free fatty acids, and muscle p70S6 kinase activity (Figs. 2 and 3; with the exception of a modest effect on fatty acids in the oldest group). In spite of losing any influence on glycemia and these other parameters, pioglitazone maintained its effects on muscle glycogen content, weight gain, fat pad weight, and appetite in older rats (Figs. 1 and 2). Likewise, pioglitazone elevated plasma adiponectin in the older rats, but the increase was clearly blunted in comparison to the distinct effect seen in association with pioglitazone-induced glucose lowering in young rats (approximately 1.5 versus 4-fold; Fig. 3). Plasma cholesterol and liver glycogen content were not affected by pioglitazone treatment in any age group examined.
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; Finegood et al., 2001; Brand et al., 2003; Pickavance et al., 2005). Because higher blood glucose was age-dependently as well as intraindividually associated with lower plasma insulin, deterioration of glycemia can be attributed to an insufficient capacity to produce insulin, which in turn was associated with lower body weight and adiposity (i.e., a catabolic state). So far, the ZDF syndrome resembles type 2 diabetes, but other results point to several important differences, which are to be considered when using these animals as a model of human disease.
Type 2 diabetes is a progressive disease characterized by gradual glycemic deterioration and loss of β-cell function over time, which can ultimately lead to life-threatening insulin deficiency (DeFronzo, 1988; UK Prospective Diabetes Study Group, 1995). At variance to the continuous worsening seen in human type 2 diabetes, ZDF rats develop severe hyperglycemia within a short dynamic phase of disease progression, but at a relatively young age of approximately 13 to 14 weeks they seem to reach a new metabolic equilibrium characterized by severe but stable hyperglycemia, hyperlipidemia, and relative insulin deficiency (Smith et al., 2000; Finegood et al., 2001; Erdely et al., 2004; and this study). At this stage, we found glycogen content of the skeletal muscle and liver to be approximately 70% higher in hyperglycemic ZDF rats than in their lean littermates, which clearly contrasts with reduced tissue glycogen stores in diabetic humans (Hwang et al., 1995; Carey et al., 2003; He and Kelley, 2004; Krssak et al., 2004). Different hyperglycemia-associated glycogen stores hint at a much higher potential of glucose to drive its own disposal in rats than humans. As soon as a certain level of hyperglycemia is reached in ZDF rats, glucose seems to promote its own disposal via a mass effect, which compensates for deficient insulin stimulation and allows for long-lasting glucose homeostasis on a severely hyperglycemic but stable level. Such species difference in the potential for insulin-independent cellular glucose uptake and glycogen storage could also explain why only rat muscle shows glucose 6-phosphate accumulation and stimulation of glycogen storage in response to high ambient fatty acids (Randle, 1998; Roden, 2004). In this context, note that in the ZDF rats, a more deranged metabolic state was age-dependently and intrain-dividually associated with relatively lower circulating free fatty acids and triglycerides. Hence, the role of high plasma free fatty acids as a driving force in the etiology of insulin resistance and diabetes could be different in rats versus humans (Smith, 2003; Bays et al., 2004).
Effects of Intervention with Pioglitazone. In agreement with previous reports (Sturis et al., 1995; Smith et al., 2000; Finegood et al., 2001; Brand et al., 2003; Yang et al., 2004; Pickavance et al., 2005), early initiation of pioglitazone treatment fully prevented the deterioration of blood glucose, whereas the same intervention failed to restore normoglycemia and even lacked any glucose-lowering effect in older ZDF rats. The most straightforward explanation for fading responsiveness to the insulin sensitizer pioglitazone is loss of sufficient circulating insulin, which for obvious reasons is a prerequisite for glucose lowering via insulin sensitization. Particular efficacy of TZDs at the earlier stages of disease development has also been observed in humans (The DREAM Trial Investigators, 2006), but clinical exploitation of this potential remains limited by concerns about side effects (Nesto et al., 2003; Nissen and Wolski, 2007). Beyond confirmatory findings about age-dependent responsiveness to TZDs, our study is the first to document that secondary failure, as seen with TZD monotherapy in type 2 diabetes (Kahn et al., 2006), can also occur in ZDF rats. Taken together, our results confirm certain similarities between TZD effects on glucose homeostasis in diabetic humans and ZDF rats and suggest that the often claimed superior efficacy of TZDs in rodents versus humans could, at least mainly, reflect initiation of treatment at different stages of disease development rather than a species difference.
Mechanisms of Pioglitazone Action. TZDs are believed to exert their insulin-sensitizing action mainly via adipogenic action and remodeling of adipose tissue, which improves insulin sensitivity of skeletal muscle and liver via adipocyte-derived signaling molecules like adiponectin (Smith, 2003; Whitehead et al., 2006). Even in old ZDF rats, and hence without concomitant glucose lowering, pioglitazone had adipogenic action (reflected by larger fat pads and accelerated weight gain) and increased plasma adiponectin. This is in agreement with the assumption that such events are upstream of TZD-induced changes in glucose homeostasis. Although TZD treatment elevated plasma adiponectin in all age groups, this increase was by far more pronounced in the youngest group of ZDF rats (approximately 4-fold in the youngest versus only 1.5-fold in the older groups). At first glimpse, this seems to confirm that a robust increase in plasma adiponectin is a prerequisite of glucose-lowering action, but our results actually support another interpretation. Intraindividual and age-dependent associations of higher blood glucose with lower plasma adiponectin in vehicle-treated rats imply that the degree of glucose deterioration predicts plasma adiponectin independently of TZD treatment. In 12.5-week-old ZDF rats, the association of low adiponectin with high glycemia even outdid the established association of low adiponectin with high body weight (Arita et al., 1999; Bays et al., 2004). Thus, the impact of metabolic deterioration (hyperglycemia) on circulating adiponectin is by far superior to established direct influences of body weight and pioglitazone, which suggests that most of the TZD-induced increases in plasma adiponectin seen in ZDF rats and type 2 diabetic humans (Miyazaki et al., 2004; Yang et al., 2004; and this study) could be the consequence rather than cause of metabolic improvement.
The next step in the proposed causal chain of TZD action would be that adiponectin acts on insulin target tissues like muscle and liver. In these tissues, adiponectin is known to immediately reduce the cellular energy charge (Yamauchi et al., 2002), which, in concert with a proposed direct effect of TZDs on energy availability (Brunmair et al., 2004; Saha et al., 2004; LeBrasseur et al., 2006), should stimulate fuel recruitment from glycogen for compensatory ATP synthesis. This could explain why pioglitazone reduced muscle glycogen content independently of the ambient glucose and insulin concentrations. Reduced glycogen in turn increases insulin-stimulated glucose disposal (Laurent et al., 2000; Nielsen et al., 2001) and, hence, could contribute to the beneficial actions of this drug in ZDF rats.
In addition to causing glycogen depletion, cellular energy shortage is known to inhibit p70S6 kinase (Dennis et al., 2001). Although impaired activity of p70S6 kinase has been associated with insulin sensitization via dephosphorylation of serine residues on insulin receptor substrate-1 (Dennis et al., 2001; Um et al., 2004; Khamzina et al., 2005), accordant TZD effects have been reported only in studies dealing with possible antitumor activities (Cho et al., 2006; Han and Roman, 2006; He et al., 2006). Our study is the first to document reduced activity of p70S6 kinase in the context of TZD-induced glucose lowering and in the skeletal muscle, which is the predominant tissue of insulin-induced glucose disposal. Nevertheless, it should be noted that TZD-induced inhibition of p70S6 kinase was observed only in association with glucose lowering and not in the older groups of ZDF rats, which does not allow us to differentiate whether this inhibition is upstream or downstream of insulin sensitization and glucose lowering in the causal chain of TZD action.
Conclusions. Although our study basically corroborates the suitability of ZDF rats for preclinical studies on TZD pharmacology, it also pinpoints differences between the ZDF syndrome and human type 2 diabetes that are to be considered in this context. Beyond this, our study raises the questions of how far changes in tissue glycogen content and p70S6 kinase activity could contribute to the insulin-sensitizing action of TZDs, and to what extent TZD-induced increases in plasma adiponectin are the result rather than the cause of metabolic improvement.
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Acknowledgements |
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Footnotes |
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
ABBREVIATIONS: ZDF, Zucker diabetic fatty; TZD, thiazolidinedione; EDL, extensor digitorum longus.
Address correspondence to: Dr. Clemens Fürnsinn, Department of Medicine III, Division of Endocrinology and Metabolism, Währinger Gürtel 18-20, A-1090 Vienna, Austria. E-mail: clemens.fuernsinn{at}meduniwien.ac.at
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References |
|---|
|
TopAbstractMaterials and MethodsResultsDiscussionReferences |
|---|
Anderwald C, Brunmair B, Stadlbauer K, Krebs M, Fürnsinn C, and Roden M (2007) Effects of free fatty acids on carbohydrate metabolism and insulin signalling in perfused rat liver. Eur J Clin Invest 37: 774–782.[CrossRef][Medline]
Arita Y, Kihara S, Ouchi N, Takahashi M, Maeda K, Miyagawa J, Hotta K, Shimomura I, Nakamura T, Miyaoka K, et al. (1999) Paradoxical decrease of an adipose-specific protein, adiponectin, in obesity. Biochem Biophys Res Commun 257: 79–83.[CrossRef][Medline]
Bays H, Mandarino L, and DeFronzo RA (2004) Role of the adipocyte, free fatty acids, and ectopic fat in pathogenesis of type 2 diabetes mellitus: peroxisomal proliferators-activated receptor agonists provide a rational therapeutic approach. J Clin Endocrinol Metab 89: 463–478.
Brand CL, Sturis J, Gotfredsen CF, Fleckner J, Fledelius C, Hansen BF, Andersen B, Ye J-M, Sauerberg P, and Wassermann K (2003) Dual PPAR
/
activation provides enhanced improvement of insulin sensitivity and glycemic control in ZDF rats. Am J Physiol Endocrinol Metab 284: E841–E854.
Brunmair B, Staniek K, Gras F, Scharf N, Althaym A, Clara R, Roden M, Gnaiger E, Nohl H, Waldhäusl W, et al. (2004) Thiazolidinediones, like metformin, inhibit respiratory complex I: a common mechanism contributing to their antidiabetic actions? Diabetes 53: 1052–1059.
Carey PE, Halliday J, Snaar JEM, Morris PG, and Taylor R (2003) Direct assessment of muscle glycogen storage after mixed meals in normal and type 2 diabetic subjects. Am J Physiol Endocrinol Metab 284: E688–E694.
Cho DH, Choi YJ, Jo SA, Ryou J, Kim JY, Chung J, and Jo I (2006) Troglitazone acutely inhibits protein synthesis in endothelial cells via a novel mechanism involving protein phosphatase 2A-dependent p70 S6 kinase inhibition. Am J Physiol Cell Physiol 291: C317–C326.
Clark JB, Palmer CJ, and Shaw WN (1983) The Zucker diabetic fatty rat. Proc Soc Exp Biol Med 173: 68–75.[CrossRef][Medline]
DeFronzo RA (1988) The triumvirate: β-cell, muscle, liver. A collusion responsible for NIDDM. Diabetes 37: 667–687.[Medline]
Dennis PB, Jaeschke A, Saitoh M, Fowler B, Kozma SC, and Thomas G (2001) Mammalian TOR: a homeostatic ATP sensor. Science 294: 1102–1105.
Erdely A, Freshour G, Maddox DA, Olson JL, Samsell L, and Baylis C (2004) Renal disease in rats with type 2 diabetes is associated with decreased renal nitric oxide production. Diabetologia 47: 1672–1676.[CrossRef][Medline]
Finegood DT, McArthur MD, Kojwang D, Thomas MJ, Topp BG, Leonard T, and Buckingham RE (2001) β-Cell mass dynamics in Zucker diabetic fatty rats. Rosiglitazone prevents the rise in net cell death. Diabetes 50: 1021–1029.
Han SW and Roman J (2006) Rosiglitazone suppresses human lung carcinoma cell growth through PPAR-dependent and PPAR-independent signal pathways. Mol Cancer Ther 5: 430–437.
He G, Sung YM, DiGiovanni J, and Fischer SM (2006) Thiazolidinediones inhibit insulin-like growth factor-I-induced activation of p70S6 kinase and suppress insulin-like growth factor-I tumor-promoting activity. Cancer Res 66: 1873–1879.
He J and Kelley DE (2004) Muscle glycogen content in type 2 diabetes mellitus. Am J Physiol Endocrinol Metab 287: E1002–E1007.
Hwang J-H, Perseghin G, Rothman DL, Cline GW, Magnusson I, Petersen KF, and Shulman GI (1995) Impaired net hepatic glycogen synthesis in insulin-dependent diabetic subjects during mixed meal ingestion. A 13C nuclear magnetic resonance spectroscopy study. J Clin Invest 95: 783–787.[Medline]
Kahn SE, Haffner SM, Heise MA, Herman WH, Holman RR, Jones NP, Kravitz BG, Lachin JM, O'Neill C, Zinman B, et al. (2006) Glycemic durability of rosiglitazone, metformin, or glyburide monotherapy. N Engl J Med 355: 2427–2444.
Khamzina L, Veilleux A, Bergeron S, and Marette A (2005) Increased activation of the mammalian target of rapamycin pathway in liver and skeletal muscle of obese rats: possible involvement in obesity-linked insulin resistance. Endocrinology 146: 1473–1481.
Krebs M, Brunmair B, Brehm A, Artwohl M, Szendroedi J, Nowotny P, Roth E, Fürnsinn C, Promintzer M, Anderwald C, et al. (2007) The mammalian target of Rapamycin pathway regulates nutrient-sensitive glucose uptake in man. Diabetes 56: 1600–1607.
Krssak M, Brehm A, Bernroider E, Anderwald C, Nowotny P, Dalla Man C, Cobelli C, Cline GW, Shulman GI, Waldhäusl W, et al. (2004) Alterations in postprandial hepatic glycogen metabolism in type 2 diabetes. Diabetes 53: 3048–3056.
Laurent D, Hundal RS, Dresner A, Price TB, Vogel SM, Petersen KF, and Shulman GI (2000) Mechanism of muscle glycogen autoregulation in humans. Am J Physiol Endocrinol Metab 278: E663–E668.
LeBrasseur NK, Kelley M, Tsao TS, Farmer SR, Saha AK, Ruderman NB, and Thomas E (2006) Thiazolidinediones can rapidly activate AMP-activated protein kinase in mammalian tissues. Am J Physiol Endocrinol Metab 291: E175–E181.
Miyazaki Y, Mahankali A, Wajcberg E, Bajaj M, Mandarino LJ, and DeFronzo RA (2004) Effect of pioglitazone on circulating adipocytokine levels and insulin sensitivity in type 2 diabetic patients. J Clin Endocrinol Metab 89: 4312–4319.
Nesto RW, Bell D, and Bonow RO, Fonseca V, Grundy SM, Horton ES, Le Winter M, Porte D, Semenkovich CF, Smith S, et al. (2003) Thiazolidinedione use, fluid retention, and congestive heart failure: a consensus statement from the American Heart Association and American Diabetes Association. October 7, 2003. Circulation 108: 2941–2948.
Nielsen JN, Derave W, Kristiansen S, Ralston E, Plough T, and Richter EA (2001) Glycogen synthase localization and activity in rat skeletal muscle is strongly dependent on glycogen content. J Physiol 531: 757–769.
Nissen SE and Wolski K (2007) Effect of rosiglitazone on the risk of myocardial infarction and death from cardiovascular causes. N Engl J Med 356: 2457–2471.
Phillips MS, Liu Q, Hammond HA, Dugan V, Hey PJ, Caskey CT, and Hess JF (1996) Leptin receptor missense mutation in the fatty Zucker rat. Nature Genet 13: 18–19.[Medline]
Pickavance LC, Brand CL, Wassermann K, and Wilding JPH (2005) The dual PPAR/ agonist, ragaglitazar, improves insulin sensitivity and metabolic profile equally with pioglitazone in diabetic and dietary obese ZDF rats. Br J Pharmacol 144: 308–316.[CrossRef][Medline]
Randle PJ (1998) Regulatory interactions between lipids and carbohydrates: the glucose fatty acid cycle after 35 years. Diabetes Metab Rev 14: 263–283.[CrossRef][Medline]
Roden M (2004) How free fatty acids inhibit glucose utilization in human skeletal muscle. News Physiol Sci 19: 92–96
Saha AK, Avilucea P, Ye JM, Assifi MM, Kraegen EW, and Ruderman NB (2004) Pioglitazone treatment activates AMP-activated protein kinase in rat liver and adipose tissue in vivo. Biochem Biophys Res Commun 314: 580–585.[CrossRef][Medline]
Smith SA (2003) Central role of the adipocyte in the insulin-sensitising and cardiovascular risk modifying actions of thiazolidinediones. Biochimie 85: 1219–1230.[Medline]
Smith SA, Lister CA, Toseland CDN, and Buckingham RE (2000) Rosiglitazone prevents the onset of hyperglycaemia and proteinuria in the Zucker diabetic fatty rat. Diabetes Obes Metab 2: 363–372.[CrossRef][Medline]
Sturis J, Pugh WL, Tang J, and Polonsky KS (1995) Prevention of diabetes does not completely prevent insulin secretory defects in the ZDF rat. Am J Physiol Endocrinol Metab 269: E786–E792.
The DREAM (Diabetes REduction Assessment with ramipril and rosiglitazone Medication) Trial Investigators (2006) Effect of rosiglitazone on the frequency of diabetes in patients with impaired glucose tolerance or impaired fasting glucose: a randomised controlled trial. Lancet 368: 1096–1105.[CrossRef][Medline]
UK Prospective Diabetes Study Group (1995) U.K. prospective diabetes study 16. Overview of 6 years' therapy of type II diabetes: a progressive disease. Diabetes 44: 1249–1258.[Abstract]
Um SH, Frigerio F, Watanabe M, Picard F, Joaquin M, Sticker M, Fumagalli S, Allegrini PR, Kozma SC, Auwerx J, et al. (2004) Absence of S6K1 protects against age- and diet-induced obesity while enhancing insulin sensitivity. Nature 431: 200–205.[CrossRef][Medline]
Whitehead JP, Richards AA, Hickman IJ, Macdonald GA, and Prins JB (2006) Adiponectin: a key adipokine in the metabolic syndrome. Diabetes Obes Metab 8: 264–280.[CrossRef][Medline]
Yamauchi T, Kamon J, Minokoshi Y, Ito Y, Waki H, Uchida S, Yamashita S, Noda M, Kita S, Ueki K, et al. (2002) Adiponectin stimulates glucose utilization and fattyacid oxidation by activating AMP-activated protein kinase. Nat Med 8: 1288–1294.[CrossRef][Medline]
Yang B, Brown KK, Chen L, Carrick KM, Clifton LG, McNulty JA, Winegar DA, Strum JC, Stimpson SA, and Pahel GL (2004) Serum adiponectin as a biomarker for in vivo PPARgamma activation and PPARgamma agonist-induced efficacy on insulin sensitization/lipid lowering in rats. BMC Pharmacol 4: 23.[CrossRef][Medline]
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, p < 0.05, 
