Final Report: TCDD-Induced Insulin ResistanceEPA Grant Number: R827685E03
Title: TCDD-Induced Insulin Resistance
Investigators: Kern, Philip A. , Kennedy, Richard H.
Institution: University of Arkansas for Medical Sciences
EPA Project Officer: Hunt, Sherri
Project Period: August 1, 1999 through July 31, 2001
RFA: EPSCoR (Experimental Program to Stimulate Competitive Research) (1999) RFA Text | Recipients Lists
Research Category: EPSCoR (The Experimental Program to Stimulate Competitive Research)
The objective of this research project was to examine the role of environmental dioxin exposure in the development of insulin resistance, a prediabetic state in which normal blood glucose levels are maintained by high circulating levels of insulin. The most toxic of the dioxin compounds is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). TCDD was the primary dioxin component of "Agent Orange" and has been found at numerous toxic waste sites, including the Vertac Superfund site in Jacksonville, AR. Thus, the problem is not limited to Vietnam veterans; many Americans have been exposed to TCDD. The objectives of this project were to document further this pathology in humans, and to use an animal model to determine if the toxic mechanism includes effects on tumor necrosis factor (TNF)-, glucose transporters, lipase activities, or peroxisome proliferator-activated receptor-.
Effect of TCDD on Oxidative Enzymes in Adipocytes and Liver
Reactive oxygen species are produced in response to environmental toxins, and previous studies have suggested that TCDD damages a number of target organs through the generation of oxygen free radicals and oxidative stress. Upon exposure, TCDD becomes concentrated in adipose tissue and adversely affects many organs, including liver. This study examined whether oxidative stress was induced in adipocytes and liver that were exposed to TCDD.
3T3-F442A adipocyte cultures were treated with TCDD (5 to 200 nM) for up to 72 hours, and the activity and mRNA levels of superoxide dismutase (SOD), catalase, and glutathione peroxidase (GSH-Px) in adipocyte cell lysates were measured. The addition of 50 nM TCDD induced a twofold increase in SOD activity after 48 hours (p < 0.05). In contrast, TCDD had no significant effect on the activity of catalase or GSH-Px in the adipocytes, and the increase in SOD activity was not accompanied by a change in SOD mRNA levels. To assess the effects of TCDD on oxidative stress enzymes in vivo, male Sprague-Dawley rats were injected weekly for 8 weeks with 30 ng/kg TCDD. In addition, the rats were fed either a low fat complex carbohydrate (LFCC) diet, or a high fat sucrose (HFS) diet. The HFS diet previously has been shown to induce mild obesity and insulin resistance, without inducing diabetes. SOD, catalase, and GSH-Px activities were measured in the liver and adipose tissue of these rats. TCDD injection resulted in a 52 percent decrease in catalase activity in the adipose tissue of HFS rats (p < 0.05). In contrast, SOD and GSH-Px activities were not altered in adipose tissue of TCDD-injected rats. In liver, however, there were significant decreases in GSH-Px activity in response to TCDD. This effect of TCDD was observed in both the LFCC and HFS dietary groups. In addition, GSH-Px activity in the HFS rats was decreased significantly when compared to GSH-Px activity in LFCC rats in both TCDD-treated and control groups, suggesting that TCDD and a high fat diet may combine to exacerbate oxidative stress.
Thus, TCDD induces complex changes in enzymes of oxidative stress in both adipocytes and liver. In adipocytes, these changes occurred post-transcriptionally, as there were no changes in mRNA levels. In addition, a high fat diet also resulted in a decrease in GSH-Px activity in liver.
Stimulation of TNF and Inhibition of Glucose Transport and Lipoprotein Lipase in Adipose Cells by TCDD
TCDD is found throughout the environment in industrialized countries, and most people have had some exposure. TCDD has very high lipid solubility, and it is concentrated in adipose tissue. Because an epidemiological association between TCDD exposure and diabetes has been described, we examined the effects of TCDD in adipocytes.
The addition of TCDD to 3T3-F442a cells, both at the initiation of differentiation and after cells were fully differentiated, resulted in a twofold increase in the secretion of TNF. When added during differentiation, there also was a 25 percent decrease in lipid accumulation. In addition to the stimulation of TNF, TCDD affected glucose transport and lipoprotein lipase (LPL) activity. When added to cultures of cells that were undergoing differentiation, TCDD inhibited total 2-deoxyglucose transport in a dose-dependent fashion, with 50 percent inhibition of glucose transport when added to cultures for 48 hours at 5 nM TCDD. In addition, when cells were exposed to 50 nM TCDD for 48 hours, there was a 40 percent reduction in LPL activity.
Thus, the addition of TCDD to adipocyte cultures resulted in an increase in TNF secretion and a decrease in glucose transport and LPL activity. Because TCDD is concentrated in adipose tissue, these studies provide a possible physiologic mechanism for epidemiologic studies that link dioxin to diabetes.
TCDD Exposure and Hyperinsulinemia and Insulin Resistance
High exposures of Vietnam veterans to TCDD, commonly referred to as "dioxin" and contained in the herbicide mixture Agent Orange, have been associated with an increased prevalence of diabetes and hyperinsulinemia in nondiabetic subjects. Sixty-nine persons were identified who were in good health and had normal glucose levels during glucose tolerance testing. These subjects lived within 25 miles of the Vertac/Hercules Superfund site located in Jacksonville, AR. The blood sera lipid concentrations of TCDD for the 69 subjects ranged between 2 and 94 ppt. When subjects with blood sera lipid TCDD levels in the top 10 percent (TCDD > 15 ppt, n = 7) were compared to subjects with lower levels (2-15 ppt, n = 62), there were no group differences in age, obesity, gender distribution, total lipids, or glucose levels. However, plasma insulin concentrations, at fasting and 30, 60, and 120 minutes following a 75 g glucose load, were significantly higher in the group with the "high TCDD." These findings could not be explained by other known risk factors for hyperinsulinemia. The finding of the TCDD-hyperinsulinemia relationship is consistent with studies in Vietnam veterans and suggests that high blood TCDD levels may cause insulin resistance.
The addition of TCDD to cells in culture yields findings consistent with increased oxidative stress, along with a proinflammatory condition. Humans with high blood levels of TCDD also demonstrate evidence of insulin resistance, and these findings cannot be explained by traditional risk factors.
This project was continued and expanded by funding from the U.S. Air Force (contract with the Air Force Health Study of Brooks Air Force Base).
Journal Articles on this Report : 3 Displayed | Download in RIS Format
|Other project views:||All 3 publications||3 publications in selected types||All 3 journal articles|
||Cranmer M, Louie S, Kennedy RH, Kern PA, Fonseca VA. Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is associated with hyperinsulinemia and insulin resistance. Toxicological Sciences 2000;56(2):431-436.||
||Kern PA, Fishman RB, Song W, Dicker-Brown A, Fonseca V. The effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on oxidative enzymes in adipocytes and liver. Toxicology 2002;171(2-3):117-125.||
||Kern PA, Dicker-Brown A, Said S, Kennedy R, Fonseca V. The stimulation of tumor necrosis factor and inhibition of glucose transport and lipoprotein lipase in adipose cells by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Metabolism 2002;51(1):65-68.||