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BRIEF REPORT |
Program in Nutritional Metabolism (D.K., C.H., S.M., S.G.), Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts 02114; and Division of Endocrinology, Diabetes, and Metabolism (M.L., M.A.L.), Department of Medicine and The Institute for Diabetes, Obesity, and Metabolism, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104
Address all correspondence and requests for reprints to: Colleen Hadigan, M.D., M.P.H., Program in Nutritional Metabolism, Massachusetts General Hospital, 55 Fruit Street, LON 207, Boston, Massachusetts 02114. E-mail: chadigan{at}partners.org.
| Abstract |
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agonist rosiglitazone (48 mg/d) on resistin in these subjects. Participants completed metabolic testing before and after rosiglitazone including fasting determination of resistin, adiponectin, and leptin levels, serum inflammatory markers, and hyperinsulinemic euglycemic clamp testing. Resistin concentration decreased significantly after rosiglitazone (12.17 ± 1.15 ng/ml to 10.23 ± 1.05 ng/ml; P = 0.02), in conjunction with significant increases in adiponectin- (P < 0.001) and insulin- stimulated glucose disposal (P = 0.004). Leptin levels, as well as TNF-
, did not change with rosiglitazone. In summary, among HIV-infected subjects with insulin resistance and lipoatrophy, resistin levels decreased significantly after rosiglitazone. Further investigation into the physiological role of this peroxisome proliferator-activated receptor-
-responsive adipocytokine in the metabolic abnormalities associated with HIV is warranted. | Introduction |
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Resistin is an adipocytokine highly expressed in murine adipose tissue (4). In animals, resistin is believed to function through effects on glucose flux into muscle (5) and hepatic glucose production (6). Currently, human physiology of resistin remains unclear (7). Diabetic and insulin-resistant subjects have elevated resistin in some studies (8, 9) but not others (10, 11). In vitro studies have not convincingly isolated resistin from cultured human adipocytes (12, 13), whereas macrophages may elaborate the bulk of circulating resistin in humans (14, 15).
Here, we characterize resistin concentrations in HIV- infected adults with lipoatrophy and hyperinsulinemia. In a recent study (16), we showed benefits of rosiglitazone on insulin sensitivity and sc fat in this population, and we now demonstrate that rosiglitazone reduced circulating resistin.
| Subjects and Methods |
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Subjects completed a comprehensive assessment of insulin sensitivity, as well as measurement of adipocytokines and inflammatory markers, before and at completion of both the 12-wk placebo-controlled and open-label portions of the study. Blood samples were obtained after a 12-h overnight fast and stored at 80 C until analyzed. Testing included determination of adiponectin, resistin, leptin, plasminogen activator inhibitor 1 (PAI-1), tissue plasminogen activator (tPA), C reactive protein (CRP), TNF-
, soluble TNF-
receptor 1 (sTNF-Rc1), and soluble TNF-
receptor 2 (sTNF-Rc2), as well as CD4 T-cell count and HIV viral load. Insulin sensitivity was determined by hyperinsulinemic euglycemic clamp testing, which consisted of a 2-h primed infusion of regular insulin at 40 mU/m2·min and variable-rate glucose to maintain blood sugar at 90 mg/dl. The effects of rosiglitazone on insulin sensitivity, lipid profile, and body composition during the 12-wk randomized controlled study were published previously (16). Written informed consent was obtained from each subject, and the protocol was approved by the Massachusetts General Hospital and the Massachusetts Institute of Technology institutional review boards.
We also compared mean fasting resistin with a reference group of nondiabetic adults from the Study of Inherited Risk of Coronary Atherosclerosis (SIRCA; n = 879) (17). Subjects in SIRCA were healthy men aged 3065 yr or women aged 3570 yr with a family history of premature coronary artery disease [male SIRCA participants: median age 46 yr (4152 interquartile range, IQR), body mass index 27.6 kg/m2 (25.330.5 IQR), n = 471; female SIRCA participants: median age 50 yr (4457 IQR), body mass index 25.7 kg/m2 (22.830.4 IQR), n = 408].
Plasma resistin was measured by enzyme immunoassay (LINCO Research, Inc., St. Charles, MO). Average correlation coefficient for standards was 0.99. The average intraassay coefficient of variation was 1.2% for low and 0.83% for high resistin standards and 16.67% for fresh aliquots of pooled human plasma, included in duplicate on all plates. Results for plasma samples across different assay plates were standardized by using the ratio of individual plate pooled plasma to the average established pooled plasma value, which was used as a reference across multiple studies. The sensitivity based on the lowest standard for the assay was 0.16 ng/ml. Resistin determinations for this study and the SIRCA study were completed in a single laboratory by using the same assay and control standards.
CRP was measured by ELISA (Diagnostic Systems Laboratories, Webster, TX). PAI-1 and tPA antigens were determined by ELISA (Biopool, Umea, Sweden, distributed by Biopool International, Ventura, CA). Adiponectin and leptin were measured by using a RIA (LINCO Research, Inc.). Insulin was measured by using a RIA (Diagnostic Products Corp., Los Angeles, CA). TNF-
, sTNF-R1, and sTNF-R2 were measured by quantitative sandwich enzyme immunoassay (R&D Systems, Minneapolis, MN).
CD4+ count was determined by flow cytometry (BD Biosciences, San Jose, CA) and HIV viral load by ultrasensitive assay (Amplicor HIV-1 Monitor Assay; Roche Molecular Systems, Branchburg, NJ).
Data analysis
We assessed the metabolic effects of rosiglitazone by comparing values obtained immediately before and after 12 wk of rosiglitazone therapy. We pooled data from subjects who initially received rosiglitazone (n = 16) during the randomized study with subjects who originally received placebo but subsequently received rosiglitazone (n = 8) for 12 wk to increase the power of the study. Paired t tests were performed to compare immediate prerosiglitazone baseline and posttreatment values for each variable. Linear correlation coefficients were generated to determine associations between adipocytokines and inflammatory markers. All data are presented as mean ± SE of the mean unless otherwise indicated. A two-level
of 0.05 was used to determine statistical significance. Statistical analyses were performed by using SAS JMP software, version 4.04 (SAS Institute, Inc., Cary, NC).
| Results |
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, CRP, and fibrinogen were not significantly different after rosiglitazone compared with baseline.
There was no significant difference in baseline resistin according to protease inhibitor use (current protease use 11.36 ± 1.26 ng/ml vs. no protease inhibitor 13.29 ± 2.16 ng/ml, P = 0.42). Baseline markers of inflammation/coagulation (TNF-
, sTNF-R1, sTNF-R2, CRP, fibrinogen, tPA, and PAI-1) did not correlate with baseline resistin (P > 0.05 for each). Similarly, change in inflammatory markers did not correlate with change in resistin. Neither baseline resistin, nor change in resistin with rosiglitazone, correlated with insulin sensitivity (r = 0.07, P = 0.8) or change in insulin sensitivity(r = 0.07, P = 0.7), respectively.
| Discussion |
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Some cross-sectional studies have found resistin is higher among diabetics (8, 9) and that resistin predicts glycemia (8) and homeostasis model assessment of insulin resistance (18). Other studies have come to nearly opposite conclusions, suggesting that resistin may correlate with adiposity but not with measures of glucose homeostasis (10). Here, we find no significant correlation between baseline fasting resistin and measures of glucose homeostasis. Heterogeneous findings may reflect population differences and/or relate to the sensitivity of individual assays (7).
In the present study of HIV-infected subjects, resistin was elevated compared with a reference population of nondiabetic adults of similar age and body weight (17); e.g. median resistin for HIV-infected women was 9.51 ng/ml (7.5316.26 IQR) compared with 5.88 ng/ml (4.427.84 IQR) for female SIRCA participants and 10.87 ng/ml (8.9813.00 IQR) for HIV-infected men compared with 5.20 ng/ml (3.876.90 IQR) for men in the SIRCA study (17). Here and in SIRCA, resistin was measured in the same laboratory by using the same assay and control standards, allowing direct comparability.
The physiological basis for elevated resistin in patients with insulin resistance is not yet known (7). Unlike in mice, in humans resistin is expressed in and elaborated by activated macrophages. In vitro experiments show resistin is regulated by inflammatory mediators (14), is elaborated in humans exposed to lipopolysaccharide (19), and is reduced when macrophages are exposed to peroxisome proliferator-activated receptor-
(PPAR-
) agonists (15). PPAR-
expression is reduced in response to nuclear HIV-1 Nef protein (20), and this may uniquely contribute to elevated resistin expression in HIV-infected patients compared with the general population.
Our findings are consistent with initial observations that PPAR-
agonists regulate resistin (4), but the physiological relevance of a reduction in resistin remains uncertain. In a recent study of obese type 2 diabetics, resistin, insulin resistance, and hepatic fat improved after 16 wk of pioglitazone (21). Similarly, we found resistin decreased and insulin sensitivity improved with rosiglitazone; however, in both studies resistin did not correlate with glucose disposal. The smaller effect size on resistin in our study, compared with the study of Bajaj et al. (21), may be related to differences in thiazolidinedione, severity of insulin resistance among groups, and potential ongoing effects of antiretroviral medication. Research suggests that resistin may be most influential with regard to hepatic insulin sensitivity (6, 22), which has been characterized as one component of insulin resistance in those with HIV lipodystrophy (23), and therefore resistin may be less closely related to peripheral insulin sensitivity.
Circulating markers of inflammation were no different after 3 months of rosiglitazone, nor were baseline markers of inflammation (e.g. TNF-
) related to circulating resistin. Similarly, TNF-
expression was not responsive to treatment with PPAR-
agonists among subjects with obesity and type 2 diabetes (24). Although the relatively small sample size of this study may have limited our ability to detect such relationships, it is possible that for HIV-infected individuals, inflammatory markers such as TNF-
are more closely linked to chronic viral or immune factors.
In summary, we show that HIV-positive subjects with lipodystrophy and insulin resistance have high circulating resistin and demonstrate that resistin levels decrease as a result of thiazolidinedione therapy in this population. We studied a relatively small group of HIV-infected individuals with significant abnormalities in fat distribution and hyperinsulinemia, and, therefore, our results regarding resistin levels may not be generalized to the overall population of HIV-infected individuals. Prospective studies in well- defined groups of both insulin-resistant and insulin-sensitive individuals are necessary to more clearly elucidate the mechanisms by which resistin may influence the development of insulin resistance in HIV patients.
| Acknowledgments |
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| Footnotes |
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First Published Online March 1, 2005
Abbreviations: CRP, C reactive protein; IQR, interquartile range; PAI-1, plasminogen activator inhibitor 1; PPAR-
, peroxisome proliferator-activated receptor
; SIRCA, Study of Inherited Risk of Coronary Atherosclerosis; sTNF-Rc, soluble TNF-
receptor; tPA, tissue plasminogen activator.
Received February 9, 2005.
Accepted February 23, 2005.
| References |
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action in humans. Diabetes 50:21992202
activators. Biochem Biophys Res Commun 300:472476[CrossRef][Medline]
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