Deciphering the impact of fructose metabolism on liver mitochondrial bioenergetics

Since its discovery in 1980, the interest in nonalcoholic fatty liver disease, the liver manifestation of metabolic syndrome, has expanded due to the growing impact on the world's health. Commonly characterized by [greater-than-or-equal-to]5% hepatic lipid accumulation in the absence of a secondary cause, NAFLD is a broad term used to encompass a variety of disease states within the liver. The largest predictor of NAFLD is said to metabolic syndrome itself, with type 2 diabetes being the most prevalent link. Currently, up to 75% of diabetic individuals also have a NAFLD diagnosis. It is also known that individuals diagnosed with NAFLD exhibit lower mitochondrial function overall. While the mechanism is largely unknown, fructose consumption has been specifically linked to the development and progression of NAFLD. Fructose metabolism differs from that of glucose metabolism. Unlike glucose, fructose has been shown to increase glucose, glycogen, lactate, uric acid, and pyruvate as end products, as well as a small increase in diet-induced thermogenesis. One major difference in metabolism is the production of uric acid following fructose metabolism. As fructose is brought into the liver, it is rapidly phosphorylated to fructose-1-phospate. This rapid use of ATP increases the generation of AMP and subsequent flux through the purine degradation pathway, ultimately leading to an increase in uric acid production in the liver. A second finding of note is that this phosphorylation process leads to an immediate drop in available ATP within the liver. Since it is unlikely that the cell's ATP usage outpaces its ability to resynthesize ATP using oxidative phosphorylation, there is a possibility that fructose metabolism leads to a decline in mitochondrial function, the primary site of ATP production. Overall, this project set out to determine if the metabolism of fructose, and subsequent production of uric acid, was the driving factor behind the decline of mitochondrial function in the liver. In Aim 1 of this project, the impact of fructose was directly determined. This was done by gavaging mice with both fructose and glucose to determine the mitochondrial changes associated with each sugar source. It was determined that an acute gavage of both glucose and fructose lead to increases in uric acid production in the liver, accompanied by increases in the oxygen consumption rates of the mitochondria isolated from these mice. It was also found that the measured differences between treatment groups was a transient measurement, in which there was only a difference 15-minutes post gavage, and by 60-minutes, there was no impact on mitochondrial function. Additionally, when treated over a 14-day period, there were no changes in mitochondrial function. This aim found that overall there were increase in oxygen consumption rates associated with increases in uric acid production. To better determine the potential direct impact of uric acid on mitochondrial function, Aim 2 was designed. It was found that the addition of uric acid to the system elicited a dose dependent increase in oxygen consumption rate. Using this dose dependent response, an optimal concentration was determined to carry out the remaining assays. From this aim, it was determined that there was an increase in oxygen consumption rate following the addition of uric acid. This increase was found to be present both in the presence and absence of adenylates. When each complex was examined individually, it was determined that there was no single mitochondrial source linked to the increase in oxygen consumption rate. Considering that rodent models retain urate oxidase, the known inhibitor, potassium oxonate, was used to determine if urate oxidase was in fact the source of residual oxygen consumption rate. Once added, it was determined that potassium oxonate was sufficient to inhibit the residual oxygen consumption. Additionally, the presence of potassium oxonate did not impact mitochondrial function alone. Overall, this project set out to determine the impact of fructose and uric acid on mitochondrial function. It was thought that both fructose and uric acid would lead to a decline in the mitochondrial function, resulting in an inability to produce, and therefore maintain, the ATP concentration. This was not what was found. It was determined that neither fructose nor uric acid was sufficient to lead to a decline in mitochondrial bioenergetic function. Instead, it was found that all oxygen consumption observations were due to the presence of urate oxidase and its consumption of oxygen during the metabolization of uric acid. This is a novel finding in that it does not agree with the current literature, which suggests both fructose and uric acid lead to mitochondrial decline