Date of Award

8-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Nutrition Science

Committee Chair

Kolapo M. Ajuwon

Committee Member 1

Kimberly K. Buhman

Committee Member 2

Qing Jiang

Committee Member 3

Kee-Hong Kim

Abstract

Dietary fatty acids, in particular long-chain fatty acids (LCFA), are involved in the regulation of metabolic, oxidative, and inflammatory responses. This is a mechanism by which fatty acids participate in the regulation of energy homeostasis and impact development of obesity, type 2 diabetes, and cardiovascular diseases. White and brown adipose tissue play a significant role in energy storage and expenditure through fatty acid uptake and oxidation. LCFA, especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), have been reported to induce thermogenesis by activating brown and beige adipocytes to oxidize more fuel molecules. However, the effects of 18-carbon (18-C) LCFA with same length but with different number and position of double bonds on metabolism and expression of thermogenic genes in adipose tissue has not been well studied. It is important to study this group of fatty acids because they are some of the most abundant fatty acid class in human diets, and there are multiple isomers in terms of the number and the position of double bonds in their structure. Therefore, the research described in this dissertation were conducted to investigate effects of 18-C fatty acids on regulation of expression of metabolic and brown-specific genes.

To determine thermogenic responses of different adipocyte cell types with distinct stages of differentiation into mature white adipocytes to 18-C fatty acids, fully differentiated C3H10T1/2, 3T3-L1, murine, or porcine primary adipocytes were treated with 18-C fatty acids, including stearic acid (STA; C18:0), oleic acid (OLA; C18:1), linoleic acid (LNA; C18:2, n-6), α-linolenic acid (ALA; C18:3, n-3), γ-linolenic acid (GLA; C18:3, n-6), and pinolenic acid (PLA, C18:3, n-6), with or without norepinephrine (NE). In murine primary adipocytes, LNA, ALA, GLA, and PLA upregulated expression of thermogenic genes; however, other types of adipocytes did not respond to these PUFA. Although NE increased thermogenic gene expression in C3H10T1/2 adipocytes, there was no effect of NE in 3T3-L1 adipocytes. In porcine primary adipocytes, which lack of uncoupling protein 1 (UCP1), NE upregulated oxidative gene expression, suggesting that increasing fatty acid oxidation may be a compensatory mechanism to overcome absence of UCP1 in pigs. This study indicates that different adipocytes have different thermogenic responses to 18-carbon fatty acids and β-adrenergic agonist.

The second experiment was conducted to compare thermogenic effects of 18-C fatty acids in vivo. Mice (C57BL/6J) were fed high-fat diets (HFD) made of vegetable oils with different composition of 18-C fatty acids, including shea butter (SHB; STA-rich fat), olive oil (OO; OLA-rich oil), safflower oil (SFO; LNA-rich oil), and soybean oil (SBO; ALA-rich fat) for 12 weeks. This was followed with or without a terminal NE injection. Compared to SHB group that had the lowest thermogenic gene expression, OO, SFO, and SBO groups had lower weight gain and body fat accumulation. Among HFD-fed groups, OO group had the lowest white fat mass due to its highest heat production level, which led to improved glucose metabolism. Feeding of SFO, with the highest n-6:n-3 ratio of fatty acids, downregulated oxidative gene expression and upregulated lipogenic gene expression. Mice that were fed HFD had lower basal (without NE injection) brown-specific gene expression in both subcutaneous and epididymal white adipose tissue (WAT), and had diminished NE-induced upregulation of thermogenic genes in brown adipose tissue (BAT). These data indicate that quantity and quality of dietary fatty acids, especially the position of the double bonds of fatty acids, may be important in regulating their effects on thermogenic gene expression.

LCFA are also involved in regulation of inflammatory responses, which are known to cause dysregulation of lipid metabolism and thermogenesis, effects of HFD differing in 18-C fatty acid composition on inflammatory and thermogenic markers were also investigated. C57BL/6J mice were fed HFD made of SHB (saturated fatty acid-rich), OO (monounsaturated fatty acid-rich), and SBO (polyunsaturated fatty acid-rich) for 4 weeks with or without a terminal injection of lipopolysaccharides (LPS). Mice fed OO had the highest BAT mass and hypothalamic leptin receptor expression, indicating that they may have a higher thermogenesis than other groups because of the involvement of BAT in non-shivering thermogenesis. In contrast, SBO-fed mice with the highest weight gain had higher expression of pro-inflammatory cytokines and downregulation of oxidative genes in WAT compared to other groups. This is because upregulation of pro-inflammatory cytokines is associated with insulin resistance and lower thermogenic gene expression in WAT. In support of this, exposure to LPS has been shown to suppress thermogenic and oxidative gene expression in NE-treated murine primary adipocytes. However, there was no fatty acid-specific effect in the regulation of response to inflammatory effect of LPS. This suggests that the degree of saturation of dietary fatty acids with 18 carbons may not play a key role in regulation of metabolic and inflammatory responses to LPS treatment.

The results presented in this dissertation provide greater understanding of the effect of the number and position of double bonds in 18-C fatty acids on regulation of energy balance. A better understanding of the mechanisms of effects of 18-C fatty acids in the regulation of metabolism will increase our understanding of their roles in the development of obesity and associated metabolic diseases.

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