The effect of short-term excessive dietary fat intake on subcutaneous white adipose tissue nuclear factor-κB inflammatory signalling

Metabolic-inflammation in tissues such as white adipose tissue (WAT) is an important contributing factor to obesity and obesity-associated disease progression. Therefore, lifestyle modifications aimed at manipulating WAT inflammatory responses may have important implications for improving public health and reducing the socioeconomic burden of obesity. The overconsumption of dietary fat has been suggested to modulate the WAT inflammatory environment. It has also been recognised that fats varying in degree of fatty acid saturation may elicit differential WAT inflammatory responses. However, much of this knowledge has originated from cell and animal experiments and translation into humans remains limited. Therefore, the aim of this thesis was to investigate the effect of short-term excessive dietary fat intake on subcutaneous WAT nuclear factor-κB (NF-κB) inflammatory signalling in human participants. Chapter 4 demonstrated that it may be possible to collect two abdominal subcutaneous WAT biopsies from a single incision in our laboratory without the technique, per se, significantly altering NF-κB inflammatory signalling. This method was employed in Chapter 5 which aimed to investigate the effect of high-fat meals (75 g test oil) rich in saturated vs monounsaturated fatty acid on abdominal subcutaneous WAT protein content and phosphorylation of proteins within the NF-κB pathway, systemic metabolic and inflammatory biomarkers and circulating concentration of monocyte subsets and monocyte inflammatory responses. The acute consumption of high-fat meals differing in fatty acid quality, did not alter postprandial WAT total content or phosphorylation of proteins analysed within the NF-κB pathway. The meals did however initiate a postprandial systemic glucose, insulin, triacylglycerol (TAG) and interleukin-6 response and there were differential responses between conditions for glucose, insulin, and TAG. There was also a tendency for both meals to increase the postprandial number of circulating CCR5+ classical monocytes and for the saturated fatty acid-rich meal to increase CX3CR1+ classical monocytes. In Chapter 6, a 7-d hyperenergetic (150% estimated energy requirement), high-fat (65% total fat) diet was employed to assess WAT NF-κB inflammatory, and systemic metabolic and inflammatory responses in the development of obesity. This intervention did not alter fasted WAT NF-κB inflammatory signalling despite modest weight gain (+1.2 kg) and negative metabolic alterations. Metabolic alterations included an increase in systemic glucose, total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol and lipopolysaccharide binding protein as well as a decrease in TAG concentration. Although short-term positive energy balance did not alter V WAT NF-κB signalling in Chapter 6, when individuals who are lean (LE) were compared to individuals with obesity (OB; i.e. chronic positive energy balance) in Chapter 7, alterations in several proteins within the WAT NF-κB inflammatory signalling pathway were present between groups. This cross-sectional comparison revealed that fasted WAT IκBα content was higher and p-IκBαser32/IκBα ratio tended to be lower in LE compared to OB which favoured increased inflammatory signalling in obesity. However, there was also a tendency for pP38Thr180/Tyr182 and MCP-1 to be higher in LE compared to OB which did not favour increased NF-κB signalling in obesity. This likely highlights the complex role of WAT NF-κB inflammatory signalling in the regulation of metabolic-inflammation in obesity. Collectively, these studies demonstrate that while short-term excessive dietary fat intake does lead to negative systemic metabolic alterations, it does not impact postprandial or fasted WAT NF-κB protein content or phosphorylation. However, individuals in chronic positive energy balance (obesity) do display an altered fasted WAT NF-κB inflammatory signalling profile compared to LE. Proteins within the WAT NF-κB inflammatory signalling pathway are therefore important to consider in the assessment of WAT inflammation in obesity. Nonetheless, these studies add weight to the suggestion that WAT inflammation may occur secondary to the development of obesity.