A synthetic glyoxylate shunt for increased fatty acid degradation in hepatocytes

The obesity epidemic is widespread in developed countries and is associated with the constellation of clinical problems known as the metabolic syndrome: insulin resistance, diabetes, cardiovascular disease, dyslipidemia, and fatty liver. Elevated levels of plasma free fatty acids are a risk factor for the development of type-2 diabetes and are a biochemical hallmark of obesity. Whereas adipose tissue is specialized to store lipids, excess triglyceride storage in the liver is a pathologic condition. With this in mind, we investigated whether increasing hepatic fatty acid oxidation has a favorable effect on co-morbidities associated with obesity.;Increasing fatty acid oxidation in the liver necessitates consideration of the hepatic regulatory and metabolic networks to simultaneously divert inhibitory molecules and provide additional channels for oxidation. To achieve this, we investigated the effect of expressing the non-native metabolic glyoxylate shunt pathway from Escherichia coli in hepatocytes both in vitro and in vivo. Pathway analysis demonstrated that the resulting phenotype from glyoxylate shunt expression can not be predicted: it may increase, decrease, or have no effect on fatty acid oxidation based on how the pathway integrates into the native hepatic regulatory and metabolic networks.;We found that a human hepatocyte HepG2 cell line expressing the glyoxylate shunt had increased fatty acid degradation and decreased glucose uptake compared to wild type cells and cytosolic malic enzyme expression was necessary for this phenotype. Based on these results, we tested the effect of the glyoxylate shunt on whole body metabolism in a C57BL/6 mouse model. Mice expressing the glyoxylate shunt in the liver resisted diet-induced obesity on a high fat diet compared to mice injected with beta-galactosidase. This decrease in weight gain was accompanied by a decreased respiratory exchange ratio, indicative of increased whole body fatty acid oxidation, and lower malonyl-CoA levels.;Given the success of engineering synthetic phenotypes in microbes and mammalian cells via non-native pathways, constructing non-native pathways in mammals has become increasingly attractive. This work demonstrates the potential of using synthetic biology and non-native pathways for understanding and identifying potential targets for treating metabolic disorders.