THE EFFECT OF FRUCTOSE METABOLISM ON HEPATIC DE NOVO LIPOGENESIS AND ADIPOSE TISSUE INSULIN RESISTANCE

Abstract

There is a worldwide epidemic of obesity and metabolic dysfunction. The root cause of obesity is driven by a positive energy balance, which is a consequence of increased caloric intake and decreased energy expenditure. Energy intake is the more dominant force, as increasing energy expenditure is a time-consuming and labor-intensive process. The caloric surplus can be mainly accounted for by increased consumption of high-fat and high- sugar diets. While the risks associated with fat consumption are well-documented, the impact of sugar intake is less understood. Fructose is a simple sugar widely consumed in modern diets. Its intake has been associated with various metabolic disorders such as obesity, insulin resistance, chronic kidney disease, hypertension, metabolic dysfunction associated with steatotic liver disease (MASLD) and others. The most detrimental effects of fructose are observed when co- ingested with a high-fat diet (HFD). Fructose is mainly metabolized in the liver, kidney, and intestines. Its negative health effects have been linked to its strong propensity to induce de novo lipogenesis (DNL). This dissertation investigated whether fructose intake in the absence of HFD is sufficient to induce metabolic dysfunction and if its metabolism by other organs, such as adipose tissue (AT), plays a role in the development of metabolic dysfunction. To investigate whether fructose intake is sufficient to induce metabolic dysfunction, male and female mice were studied on three different normal chow diets: Boston Chow Diet (BCD), Lexington Chow Diet (LXD), and Low-Fat Diet (LFD). The diets had different fat-to-carbohydrate ratios, with BCD > LXD > LFD having the most fat. Additionally, the mice were provided regular water or 30% fructose solution in water. In males, fructose supplementation on BCD led to weight gain, glucose intolerance, and hepatic steatosis, whereas on LXD, fructose did not induce these adverse effects. Moreover, male mice on LFD did not gain weight, but once switched to BCD they did gain weight and developed metabolic dysfunction. Interestingly, female mice did not gain weight and remained insulin-sensitive even on BCD when supplemented with fructose. However, they did develop hepatic steatosis. These findings indicate that metabolic dysfunction associated with fructose intake is not a universal finding but, rather, is influenced by the dietary fat-to-carbohydrate ratio of the diet, duration of dietary exposure, and sex of the mice. AT is an important organ in orchestrating metabolic homeostasis. Fructose intake increases adiposity, but whether fructose can be directly metabolized in AT is not known. Male mice were fed a Chow diet or 60% high-fat diet (HFD) and supplemented with either regular water, 30% fructose-, or 30% glucose-solution for ten weeks. Fructose and glucose had similar effects on fat mass and adipocyte size, but only fructose and not glucose intake reduced beneficial adipokines, such as adiponectin and resistin, in HFD-fed mice. This is associated with increased crown-like structures, elevated inflammatory markers, and impaired insulin signaling (p-AKT) in visceral adipose tissue (VAT) of fructose-fed mice on a HFD. Moreover, when fructose metabolism in the liver was prevented via KHK siRNA, more fructose was available to be metabolized in AT, which worsened AT inflammation and metabolic dysfunction. In terms of its metabolism, fructose supplementation elevated HK1 and HK3 expression in VAT. The specific fructose transporter, Glut5, was absent in AT. However, Glut1 expression was increased with fructose supplementation in AT. In vitro, fructose was as sufficient as glucose to promote the differentiation of 3T3-L1 and stromal vascular fraction (SVF) into fully differentiated adipocytes. In human subjects, fructose supplementation was associated with elevated inflammatory markers and increased HK3 expression. HK3 expression was also increased in obese compared to the lean individuals. These findings indicate that adipose tissue may serve as an inducible site for fructose metabolism under conditions of high fructose intake or when fructose metabolism is prevented in the liver. In summary, fructose intake on a normal chow diet is sufficient to induce metabolic dysfunction only when consumed on diets with high fat-to-carbohydrate ratios. The effects of fructose are also sex-specific, as female mice are better at compensating for the detrimental effects of fructose largely due to estrogen, which enhances insulin sensitivity and promotes fat oxidation. While the liver, kidney, and intestine are the primary sites of fructose metabolism, AT may also participate in fructose metabolism under specific situations which raise serum fructose levels. It remains to be determined which specific cell types in AT mediate fructose metabolism.