DOI

10.17077/etd.msa1uou9

Document Type

Dissertation

Date of Degree

Fall 2016

Degree Name

PhD (Doctor of Philosophy)

Degree In

Human Toxicology

First Advisor

Larry W. Robertson

Second Advisor

Aloysius J. Klingelhutz

First Committee Member

Justin L Grobe

Second Committee Member

Gabriele Ludewig

Third Committee Member

Katherine Gibson-Corley

Abstract

Recently, persistent organic pollutants such as polychlorinated biphenyls (PCBs) were classified as “metabolic disruptors” for their suspected roles is altering metabolic and energy homeostasis through bioaccumulation in liver and adipose tissues. Among PCBs, a specific congener, 3,3',4,4',5-pentachlorobiphenyl (PCB126), is a potent arylhydrocarbon receptor (AhR) agonist and elicits toxicity similar to the classic dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). PCB126 levels found in human blood are particularly associated with diabetes and nonalcoholic fatty liver disease (NAFLD) in humans, however the mechanisms are unclear.

We hypothesized that the accumulation of PCB126 disrupts carbohydrate and lipid metabolism by altering the functions of liver and adipose tissues. Hence, our objective was to characterize PCB126 induced-metabolic disruption and the underlying molecular mechanisms that cause toxicity. Separate animal studies were performed using a rat model to understand the time- and dose-dependent effects after PCB126 administration. The chronology of PCB126 toxicity showed early decreases in serum glucose level at 9 h, worsened in a time-dependent way until the end of the study at 12 d. Lipid accumulation and the liver pathology also worsened over time between 3 d and 12 d post administration. These observed effects in the liver were also found to be dose-dependent. The decrease in serum glucose was a result of a decrease in the transcript levels of gluconeogenic and glycogenolytic enzymes, necessary for hepatic glucose production and hence the maintenance of steady glucose levels in the blood. Phosphoenolpyruvate carboxykinase (PEPCK-C), the rate limiting enzyme of gluconeogenesis, was found to be significantly decreased upon exposure to PCB126. The expression levels of peroxisome proliferator-activated receptor alpha (Pparα) and some of its targets involved in fatty acid oxidation were also found to be time and dose-dependently decreased upon exposure to PCB126. In an attempt to understand the molecular targets that may cause these dual effects on both gluconeogenic and fatty acid oxidation, we found that PCB126 significantly decreases phosphorylation of the cAMP response element-binding protein (CREB). CREB is a nuclear transcription factor that is activated in the liver through phosphorylation; to switch-on the transcription of enzymes that catalyze gluconeogenesis and fatty acid oxidation, in order to meet energy demands, especially during fasting.

Further, to understand the toxicity of PCB126 on adipose tissue, a human pre-adipocyte model that can be differentiated into mature adipocytes was used. In these studies, we found that exposure of preadipocytes to PCB126 resulted in a significant reduction in their ability to differentiate into adipocytes. This results in decreased lipid accumulation in the adipocyte. Reduction in the differentiation by PCB126 was associated with down regulation in transcript levels of a key adipocyte transcription factor, PPARγ and its transcriptional targets necessary for adipogenesis and adipocyte function. These inhibitory effects of PCB126 on the regulation of PPARγ and the initiation of adipogenesis were mediated through activation of AhR.

Overall, this work shows that PCB126 disrupts nutrient homeostasis through its effects on the function of target tissues; liver and adipose. PCB126 significantly alters the nutrient homeostasis through its effects on gluconeogenesis and fatty-acid oxidation necessary for glucose and energy regulation during fasting. In addition, PCB126 interrupts the storage functions of adipose tissue by inhibiting adipogenesis and thus disrupts lipid storage and distribution

Public Abstract

Contamination of food, air and water with certain “legacy chemicals” such as polychlorinated biphenyls (PCBs) released into the environment is of great concern for public health. PCBs were previously suggested to increase the risk of adverse health effects such as cancer and endocrine disruption in humans. Recently, PCBs are also emerging as “metabolic disruptors” for their possible roles in increasing the risk of silent metabolic epidemics such as obesity, diabetes and fatty liver.

Studies described in this work investigate the specific effects of PCBs on the function of liver and adipose tissues, which also happen to be their storage sites. PCB126 serves as representative chemical, whose mode of action is also similar to the chemicals like dioxin, present in the herbicide Agent Orange. Using PCB126, the primary effects on the functions of liver and adipose tissue to balance the normal sugar and fat metabolism, were assessed. PCB126 disrupts the ability of liver to produce glucose. Liver plays an important role in replenishing the blood glucose levels during fasting, by converting other stored nutrients into glucose. A disruption in this process by PCB126 leads to low blood glucose, and hence shrinks the primary source of energy for other organs. In addition to effects on glucose production, PCB126 also interferes with the capacity of liver to break-down fat for energy. This interruption leads to fat accumulation in the liver and hence results in fatty liver. PCB126 seems to cause these effects by interfering with the function of a certain protein called cAMP response element-binding protein (CREB), needed to relay a hormonal signal produced during fasting process. Thus, PCB126 causes a double-blow on the functions of liver in generating energy from both glucose and fat.

PCB126 also shows its effects on the functions of adipocytes. Adipose tissue plays a regulatory role in the storage of excess fat and releasing it into the blood during fasting. The released fat is then processed in the liver to generate energy. PCB126 inhibits the ability of the adipocyte to accumulate fat by interfering with their development. This diminished storage function of adipocyte leads to disorganization in the fat storage, necessary for proper fat distribution during need (eg. fasting) and may play a role is causing ectopic fat accumulation in the liver. The inability of the adipose tissue to store lipids may also further lead to inappropriate accumulation of fat in the other organs such as liver. In conclusion, this dissertation creates new insights into our understanding of the modes of action of pollutants such as PCB126 and their roles in causing metabolic disease.

Keywords

AhR, PPAR, CREB, gluconoegenesis, glucose metabolism, liver steatosis, metabolic disruption, PCBs, dioxin

Pages

xvii, 142 pages

Bibliography

Includes bibliographical references (pages 117-142).

Copyright

Copyright © 2016 Gopi Srinivas Gadupudi

Included in

Toxicology Commons

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