© 2002 by the American Diabetes Association, Inc.
Synthetic Peroxisome Proliferator-Activated Receptor-
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| ABSTRACT |
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(PPAR-
) agonist in diabetic patients.
RESEARCH DESIGN AND METHODSType 2 diabetic patients (30 in the treatment group and 34 in the placebo group) were recruited for a randomized double-blind placebo-controlled trial for 6 months with the PPAR-
agonist rosiglitazone. Blood samples were collected and metabolic variables and adiponectin levels were determined in all patients before initiation of the study.
RESULTSIn the rosiglitazone group, mean plasma adiponectin level was increased by more than twofold (P < 0.0005), whereas no change was observed in the placebo group. Multivariate linear regression analysis showed that whether rosiglitazone was used was the single variable significantly related to the changes of plasma adiponectin. The amount of variance in changes of plasma adiponectin level explained by the treatment was
24% (r2 = 0.24) after adjusting for age, sex, and changes in fasting plasma glucose, HbA1c, insulin resistance index, and BMI.
CONCLUSIONSRosiglitazone increases plasma adiponectin levels in type 2 diabetic subjects. Whether this may contribute to the antihyperglycemic and putative antiatherogenic benefits of PPAR-
agonists in type 2 diabetic patients warrants further investigation.
Abbreviations: HOMA, homeostasis model assessment TNF, tumor necrosis factor PPAR-
, peroxisome proliferator-activated receptor-
| INTRODUCTION |
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-induced monocyte adhesion, nuclear factor-
B signaling, and expression of intracellular adhesion molecule-1, endothelial cell adhesion molecule-1 and E-selectin in endothelial cells in vitro (5,6). It also attenuates production of TNF-
in lipopolysaccharide-treated macrophages (7). Recently, it was also demonstrated to reduce cholesterol ester accumulation and class A scavenger receptor gene expression in cultured human monocyte-derived macrophages (8). These studies indicate that adiponectin is an adipocyte and adipose tissue-specific anti-inflammatory and antiatherogenic molecule and may be a bridge between obesity and atherosclerosis. Furthermore, adiponectin was detectable only in the walls of catheter-injured vessels but not in the intact vascular walls in an animal model and in humans (8,9). Lower plasma levels of adiponectin have been documented in human subjects with obesity, type 2 diabetes, or coronary artery disease (5,10,11). Plasma levels of adiponectin were shown to correlate negatively with plasma glucose, insulin, triglyceride levels, and BMI but positively with plasma levels of HDL cholesterol (11). Plasma adiponectin levels were also demonstrated to correlate positively with insulin-stimulated glucose disposal measured by hyperinsulinemic-euglycemic clamp (12). Taken together, these suggest that low plasma adiponectin according to levels may be a novel biomarker of insulin resistance syndrome.
Recent animal studies indicate that adiponectin has a wide array of metabolic effects. Injection of recombinant adiponectin in experimental animals was shown to reduce plasma fatty acid and glucose levels, increase fatty acid ß-oxidation and decrease triglyceride content in skeletal muscles, improve insulin sensitivity, and reduce body weight (1315). These studies further support that a low plasma adiponectin level may play a role in the pathophysiology of type 2 diabetes.
The expression of adiponectin is developmentally regulated and is activated by day 4 during adipocyte differentiation in cultured 3T3-L1 cells (1). Furthermore, we have tested whether proliferator-activated receptor-
(PPAR-
), the master regulator of adipocyte differentiation, might upregulate its expression in fully differentiated 3T3-L1 adipocytes. We showed that the steady-state mRNA of adiponectin in differentiated 3T3-L1 adipocytes was increased by administration of rosiglitazone for 24 h (16). Rosiglitazone is a synthetic PPAR-
agonist and is now widely used to treat patients with type 2 diabetes through enhancing insulin sensitivity (17). Therefore, we asked in this study whether plasma adiponectin levels might increase in type 2 diabetes patients treated with rosiglitazone.
| RESEARCH DESIGN AND METHODS |
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50 years. The baseline characteristics of these subjects are shown in Table 1. This protocol has been approved by the Institutional Review Board and Department of Health of Taiwan. Written informed consent was obtained from each participant.
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Statistical analyses
Data were presented as means and SDs. Statistical analyses, including Students t test and multivariate linear regression analysis, were performed using Statistical Analysis System software (Edition 6.12; SAS Institute, Cary, NC). Differences in clinical characteristics between those treated with rosiglitazone and those treated with placebo were tested by Students t test. Several multivariate linear regression models were performed that included age, sex, changes in fasting plasma glucose level, HbA1c, insulin resistance index by HOMA, BMI, and treatment status as independent variables and changes in plasma adiponectin levels as dependent variables. The amount of variance explained by treatment status was indicated by the r2 or partial r2 in the models.
| RESULTS |
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38% of variance in the changes of plasma adiponectin (r2 = 0.38). After adjusting the other variables in model 5, treatment status was attributed to
24% of variance (r2 = 0.24), whereas the r2 of the model was 0.44. These results suggest that the increase in plasma adiponectin level may primarily be the result of rosiglitazone treatment.
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| CONCLUSIONS |
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Recently, adiponectin also has been shown to have various metabolic effects. It was reported that the decrease in plasma adiponectin paralleled the development of insulin resistance and diabetes among monkeys in a longitudinal observation study (19). Plasma level of adiponectin was also related to the direct measurement of insulin sensitivity by clamp studies in humans (12). Furthermore, injection of recombinant adiponectin in experimental animals was shown to increase fatty acid oxidation in muscle, improve insulin sensitivity, and decrease hepatic glucose output (1315). This indicates that low plasma adiponectin may contribute to the pathophysiology of insulin resistance among patients with type 2 diabetes.
The mechanism of adiponectin in improving insulin sensitivity has been elucidated, at least in part. Recombinant adiponectin injection in experimental animal models showed a decrease in plasma fatty acid levels (13,14). Fatty acid ß-oxidation in skeletal muscle increased in these animals, probably secondary to enhanced expression of genes involved in ß-oxidation and energy dissipation, such as acyl-CoA oxidase and uncoupling protein-2 (14). As a result, adiponectin also decreased triglyceride content in muscle. Both the increased fatty acid levels in plasma and increased triglyceride content in muscle have been shown to cause insulin resistance (20). In addition, insulin-stimulated tyrosine phosphorylation of signaling molecules, including insulin receptor, insulin receptor substrate-1, and Akt in skeletal muscle, was also enhanced by adiponectin (14).
In view of its potential beneficial effects, any measures that could increase plasma adiponectin levels would likely have some clinical significance. We recently also showed that weight reduction with gastric partition surgery significantly increased plasma adiponectin levels and insulin sensitivity among severely obese individuals (21). Whether certain beneficial metabolic effects, such as improvement in insulin sensitivity by weight reduction or by treatment with PPAR-
agonists, were at least partially mediated by adiponectin remains to be investigated.
Although we did not investigate the expression of adiponectin in adipose tissue in these subjects, it should be plausible to speculate that the mechanism of increased plasma adiponectin by rosiglitazone treatment was secondary to increased transcription of adiponectin gene by activating the transcription factor, PPAR-
. We have also shown, in cultured mouse adipocytes, that rosiglitazone increased the steady-state mRNA of adiponectin (16). The proximal promoter region (1 kb) of the human adiponectin gene contains only a half-site of PPAR-
response element (AGGTCA between -610 and -605 relative to ATG start codon) (22,23). Its proximal promoter also contains several regulatory elements commonly observed in the promoters of genes expressed in adipose tissue. These cis-elements include C/EBPs, SREBP, E-box, and GATA-1 (22,23). However, it cannot be excluded that rosiglitazone may enhance the stability of adiponectin mRNA, as well as the synthesis, stability, and secretion of adiponectin protein.
In this study, we observed an increase in plasma adiponectin along with weight gain after rosiglitazone treatment. This may seem contradictory to the previously reported negative correlation between plasma adiponectin level and body weight. It is plausible that activation of PPAR-
by rosiglitazone may, on one hand, promote body weight gain by increasing adipocyte differentiation and the number of small adipocytes as previously shown (24) and, on the other hand, enhance adiponectin gene transcription.
In conclusion, this study clearly demonstrates that treatment with rosiglitazone in type 2 diabetic patients increased plasma adiponectin levels. This effect may potentially protect diabetic patients from macrovascular complications and may improve their insulin sensitivity and glycemic control. Therefore, further studies on its clinical and biological relevance are warranted.
| Acknowledgments |
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We thank Chia-Ling Chao for technical assistance.
| Footnotes |
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Received for publication 20 June 2001 and accepted in revised form 24 October 2001.
L.M.C. has received funding from Smith-Kline Beecham.
A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances.
| References |
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