Obesity and diabetes are highly prevalent. Both are fundamentally disorders of energy balance. In obesity, too much energy is stored and too little is utilized, while in diabetes available energy sources like glucose are not sufficiently used and/or are incompletely utilized. The role of mitochondria is to sense and control energy balance, thus mitochondrial metabolism is a natural area of focus for these conditions. The mitochondrial respiratory chain is the final energy-yielding pathway in the cell. The respiratory chain produces ATP from reducing equivalents derived from nutrients like fats, proteins, and carbohydrates. But when the mitochondrial respiratory chain is not functioning properly, whether from genetic mutations (in genetic disorders), nutrient excess or decreased physical activity (in obesity), less ATP is produced from metabolism derived reducing equivalents. The main reducing equivalent we are focusing on in the project is nicotinamide adenine dinucleotide (NADH). The impairment of the mitochondrial respiratory chain causes a build up of NADH relative to NAD+. This alteration in redox balance has myriad potential effects, one of which may include “reversal” of citrate acid cycle flux.
This flux reversal may result in glutamine being reduced to produce alpha- ketoglutarate, which is then converted to citrate, and ultimately fatty acids. Many individuals with mitochondrial respiratory chain disease patients have highly elevated triglyceride levels, which are the main constituents of fat. Hypertriglyceridemia can also occur in children and adults who have diet-induced obesity and insulin resistance. That is, the metabolic profile in both mitochondrial disease and obesity-related insulin resistance may be similar. This has the potential to guide development of efficient diagnostic tools for both obesity and mitochondrial disease.
To test this model, I focused on using in vitro human cell lines, HepG and RD cell systems to determine whether respiratory chain inhibition increases de-novo lipogeneis due to reversal of citric acid flux. This summer I was really given the opportunity, thanks to the College Alumni Society research grant, to work under the McCormack Lab7 able to dive deeper into wet lab and understand the analysis and process that goes into research. I gained a greater understanding of how mitochondrial disease and metabolic pathways work through actively working with muscle and liver like cells and treating them with inhibitors that mimic what occurs in diabetes and obesity, which was a very interesting and eye opening experience.