This project seeks to elucidate the role of carnitine acetyltransferase (CrAT) as an important regulator of mitochondrial function and glucose tolerance, and the principal mediator of the antidiabetic actions of L-carnitine therapy. Carnitine acetyltransferase (CrAT) is a mitochondrial matrix enzyme that plays a key role in the synthesis and efflux of short chain carnitine conjugates, such as acetyl-carnitine. This enzyme is most abundant in carnitine-rich tissues such as skeletal muscle and heart, but its precise metabolic function remains largely unexplored. CrAT activity is mainly regulated by availability of L-carnitine, a conditionally essential nutrient that is best known for its obligatory role in permitting mitochondrial uptake and oxidation of long chain fatty acids. In addition to its requisite role in fat oxidation, carnitine also permits the intramitochondrial formation of acylcarnitine conjugates, thereby facilitating mitochondrial efflux of excess carbon fuels. Recent studies by our laboratory suggest that carnitine insufficiency caused by aging and/or overnutrition impairs fuel metabolism and insulin action by compromising CrAT activity. Remarkably, dietary carnitine supplementation improved metabolic outcomes in these models in association with robust increases in plasma and urinary acetyl-carnitine levels. The physiological relevance of acetyl-carnitine production and efflux is poorly understood and surprisingly understudied. We seek to understand the specific role of CrAT as a carnitine effector that defends metabolic homeostasis. We will address two central hypotheses: 1) CrAT plays a key role in regulating mitochondrial substrate switching between glucose and fatty acid fuels, and 2) mitochondrial efflux of CrAT-derived acylcarnitines affords protection against muscle insulin resistance and oxidative stress caused by chronic overnutrition. These hypotheses will be tested using gain- and loss-of-function genetic engineering approaches in primary human skeletal myocytes as well as knockout mouse models. Primary outcome measures will include indirect calorimetry, multiple measures of insulin action and metabolic flux, along with state-of-the-art mass spectrometry-based metabolic profiling.
This project examines the role of carnitine insufficiency a contributing factor to the development and progression of metabolic disorders such as insulin resistance and type 2 diabetes. L- carnitine is a conditionally essential amino acid derivative that is often advertised as an energy enhancing nutrient. Recent animal studies by our laboratory have shown that systemic carnitine homeostasis is compromised by aging and obesity, whereas carnitine supplementation improved metabolic regulation and glucose tolerance. The salutary effects of carnitine therapy were linked to increased activity of a mitochondrial enzyme known as carnitine acetyltransferase (CrAT). The overarching goal of this project is to elucidate mechanisms through which supplemental L-carnitine improves glucose tolerance and to determine the precise role of the CrAT enzyme in regulating energy metabolism. Results from the proposed studies are likely to yield new insights regarding the therapeutic properties of L-carnitine while also advancing understanding of the interplay between mitochondrial function and insulin action. These are clinically relevant topics of intense scientific interest and controversy.
|Fisher-Wellman, Kelsey H; Davidson, Michael T; Narowski, Tara M et al. (2018) Mitochondrial Diagnostics: A Multiplexed Assay Platform for Comprehensive Assessment of Mitochondrial Energy Fluxes. Cell Rep 24:3593-3606.e10|
|Muoio, Deborah M (2017) HDAC3 sets the timer on muscle fuel switching. Nat Med 23:148-150|
|Anderson, Kristin A; Huynh, Frank K; Fisher-Wellman, Kelsey et al. (2017) SIRT4 Is a Lysine Deacylase that Controls Leucine Metabolism and Insulin Secretion. Cell Metab 25:838-855.e15|
|Stöckli, Jacqueline; Fisher-Wellman, Kelsey H; Chaudhuri, Rima et al. (2017) Metabolomic analysis of insulin resistance across different mouse strains and diets. J Biol Chem 292:19135-19145|
|Krycer, James R; Fisher-Wellman, Kelsey H; Fazakerley, Daniel J et al. (2017) Bicarbonate alters cellular responses in respiration assays. Biochem Biophys Res Commun 489:399-403|
|Huffman, Kim M; Jessee, Ryan; Andonian, Brian et al. (2017) Molecular alterations in skeletal muscle in rheumatoid arthritis are related to disease activity, physical inactivity, and disability. Arthritis Res Ther 19:12|
|Davies, Michael N; Kjalarsdottir, Lilja; Thompson, J Will et al. (2016) The Acetyl Group Buffering Action of Carnitine Acetyltransferase Offsets Macronutrient-Induced Lysine Acetylation of Mitochondrial Proteins. Cell Rep 14:243-54|
|Crown, Scott B; Kelleher, Joanne K; Rouf, Rosanne et al. (2016) Comprehensive metabolic modeling of multiple 13C-isotopomer data sets to study metabolism in perfused working hearts. Am J Physiol Heart Circ Physiol 311:H881-H891|
|Consitt, Leslie A; Koves, Timothy R; Muoio, Deborah M et al. (2016) Plasma acylcarnitines during insulin stimulation in humans are reflective of age-related metabolic dysfunction. Biochem Biophys Res Commun 479:868-874|
|Kien, C Lawrence; Matthews, Dwight E; Poynter, Matthew E et al. (2015) Increased palmitate intake: higher acylcarnitine concentrations without impaired progression of ?-oxidation. J Lipid Res 56:1795-807|
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