The underlying mechanisms of UCP3-dependent thermogenesis in skeletal muscle
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Mitochondrial uncoupling proteins (UCPs) are anion / solute transporters that dissipate the proton gradient used to drive ATP generation. By allowing protons to flow down their electrochemical gradient, UCP activation releases the energy generated from mitochondrial substrate oxidation as heat. This thermogenic process is important in normal thermoregulation (i.e. non-shivering thermogenesis), and also serves as an attractive target in the treatment of obesity by lowering metabolic efficiency. The skeletal muscle (SKM) enriched UCP homologue, UCP3, is associated with increased energy expenditure, fatty acid metabolism, and insulin sensitivity. Unlike the cold-induced prototypical pathway of UCP1-mediated non-shivering thermogenesis in brown adipose tissue (BAT), the mechanisms underlying the thermogenic actions of UCP3 in SKM are not well characterized. Although global UCP3 knockout mice exhibit normal thermoregulatory responses to cold under fed conditions, they exhibit an attenuated hyperthermic response when administered amphetamine-type drugs. In our initial investigation, we show that selective overexpression of UCP3 in SKM by the human α-skeletal actin promoter restored methamphetamine (Meth)-induced hyperthermia in the UCP3⁻/⁻ background (TgSKM UCP3⁻/⁻), but not in the UCP1/UCP3 double knockout background (TgSKM UCP1⁻/⁻+UCP3⁻/⁻). Taken together, these findings further bolster the role of UCP3 as a thermogenic mediator in SKM, and suggest a novel mechanism of crosstalk between BAT UCP1 and SKM UCP3 in Meth-induced hyperthermia. In the second aspect of my project, we characterized the underlying mechanisms of UCP3-dependent thermogenesis within SKM by utilizing an immunoprecipitation- based mass spectrometry approach to identify interacting partners of UCP3. These analyses corroborated previous work performed by our lab, and demonstrated that UCP3 interacts with a subset of fatty acid metabolizing enzymes. Interestingly, one such enzyme, enoyl-CoA hydratase-1 (ECH1), is involved in the metabolism of oleic acid, a known ligand activator of UCP3. This work reveals that ECH1:UCP3 complex formation enhances uncoupled-respiration and fatty acid metabolism, and that genetic mouse models in vivo show that UCP3 and ECH1 participate in a common pathway of thermogenesis. These findings support a new model by which UCP3-dependent thermogenesis in SKM is mediated in part through its cooperation with ECH1, and suggest new approaches for treatment of obesity and related metabolic diseases.