Metabolic engineering by acetate : monitoring effects at a molecular level

Finding effective life style interventions is paramount in the control of obesity and its comorbidities such as NAFLD. In this thesis I investigate the effects of a short chain fatty acid (SCFA) on obesity and liver fat, using firstly two different carbohydrates, inulin (an indigestible but fermentable carbohydrate) and isomaltulose (a digestible carbohydrate). I go on to investigate the effects of acetate, through the development and implementation of a novel nanoparticle carrier to study the effects of this SCFA on hepatic metabolism. To study the effects of isomaltulose and inulin on adiposity, magnetic resonance imaging (MRI) and spectroscopy (MRS) techniques were used on mice fed on a high fat diet supplemented with these two carbohydrates. Manganese enhanced MRI (MEMRI), was used to monitor hypothalamic brain activation. A significant decrease in hypothalamic activation was detected in both inulin and isomaltulose supplemented animals in the absence of detectable phenotypic changes, including body adiposity and liver fat. As some of the effects of fermentable carbohydrates are thought to occur through the increased production of SCFAs, the potential physiological effects of one of the SCFA, acetate, were further explored. PET imaging was employed to study whole body 11C-acetate biodistribution on a murine model. The highest uptake of 11C-acetate was observed in the heart followed by liver, colon, brown adipose tissue (BAT), brain, fat and muscle (20, 10, 4, 3.6, 3, 2.9 and 2 peak %ID/g respectively). Colonic administration caused significant difference in uptake pattern of heart, liver, brain and BAT (p=0.001), muscle (p=0.0001) and colon (p=0.004) compared with i.v. No difference was observed in fed vs fasted animals. Pre-administration of “cold” acetate prior to systemic administration of 11C-acetate increased uptake of the latter in the liver, heart, brain and BAT suggesting that priming with CA saturates either the GPR43/41 receptors or the transport system of acetate In order to assess the potential beneficial effects of acetate, it was necessary to administer this SCFA in a chronic and consistent manner. Administration of large concentrations of pure SCFA to mice or human is known to have significant detrimental effects, so an alternative nanoparticle based strategy was developed for this purpose. Acetate was encapsulated in liposomal nanoparticles, capable of carrying millimolar concentrations of SCFA. In a murine model fed on normal fat diet, liposomal-encapsulated acetate significantly decreased liver adiposity, but not total body fat, while serum markers of obesity were reduced although they did not reach significance. In a murine model fed on high fat diet, liposomal-encapsulated acetate decreased whole body adiposity, liver fat content and serum free fatty acid (FFA) concentrations and serum markers of liver disease were significantly reduced whereas ketone concentrations in serum were significantly increased. This thesis shows that alterations in dietary carbohydrate composition can lead to significant effects on appetite, probably through the increase production of SCFAs. Furthermore, the use of liposomal-nanoparticles for direct SCFA delivery appears a potentially fast and effective way to treat some of the physiological and metabolic abnormalities associated with obesity