Elucidating the role of rhodoquinone in adipocyte differentiation and obesity

Project Summary/Abstract During obesity, white adipose tissue (WAT) expands to store excess calories from the diet. WAT expands either by increasing the size of preexisting adipocytes (hypertrophy) or by increasing the number of adipocytes through the differentiation (hyperplasia). Hypertrophic growth has been associated with hypoxia (low oxygen (O2)) in WAT due to ineffective vascularization during its expansion. Hypoxia in WAT has been linked to insulin resistance, ectopic lipid deposition, and mitochondrial dysfunction, although the mechanisms connecting hypoxia to these effects are not well understood. A major function of O2 is that it serves as the terminal electron acceptor (TEA) in the electron transport chain (ETC), which sustains mitochondrial functions including de novo pyrimidine synthesis, reactive oxygen species production, and ATP generation. However, it is unknown how obesity- induced hypoxia impacts the ETC, and if these changes mechanistically explain adipocyte dysfunction upon obesity. In preliminary work, we discovered rhodoquinone (RQ), a novel mammalian metabolite that functions as an electron carrier in the ETC. The RQ-directed ETC circuit employs fumarate, instead of O2, as the TEA, enabling the RQ circuit to support certain mitochondrial functions in hypoxia. Through lipidomic analysis of tissues from two distinct obese mouse models, we found that RQ levels profoundly and specifically rise in the WAT of obese mice. These data inspired the hypothesis that obese adipose tissue reprogram their ETC to the RQ/fumarate circuit to support mitochondrial functions in WAT during hypertrophic/hypoxic expansion. To address this hypothesis, Aim 1 will leverage primary human adipocytes to explore how the RQ- directed ETC circuit impacts differentiation and lipid droplet formation. This will be achieved using stable isotope tracing to measure lipogenesis and western blotting to monitor signaling cascades associated with lipid droplet formation and turnover. In Aim 2 we will test which ETC circuit (UQ/O2 or RQ/fumarate) is preferentially used in the WAT of lean versus obese mice. To this end, we will perform stable isotope tracing assays and respirometry experiments that distinguish these two ETC pathways in vivo. Moreover, this aim will test the therapeutic potential of reprogramming the ETC to the RQ/fumarate circuit during diet-induced obesity using small molecule analogs of RQ. We will determine the impact of RQ on insulin sensitivity, lipid storage, and metabolic parameters via metabolic cages in lean and obese conditions to reveal if RQ can mitigate obesity-induced metabolic dysfunction. Taken together, this work will explore the role of a novel mammalian metabolite in adipocytes during differentiation and upon obesity induction. Beyond defining the fundamental metabolic changes induced by the RQ circuit in differentiating adipocytes, the proposed research is translational, as it will investigate, for the first time, the therapeutic potential of reprogramming the ETC in diet-induced obesity.