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.

Public Health Relevance

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.

Agency
National Institute of Health (NIH)
Institute
National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
Type
Research Project (R01)
Project #
5R01DK089312-04
Application #
8538370
Study Section
Integrative Physiology of Obesity and Diabetes Study Section (IPOD)
Program Officer
Laughlin, Maren R
Project Start
2010-07-01
Project End
2014-06-30
Budget Start
2013-07-01
Budget End
2014-06-30
Support Year
4
Fiscal Year
2013
Total Cost
$340,791
Indirect Cost
$121,259
Name
Duke University
Department
Pharmacology
Type
Schools of Medicine
DUNS #
044387793
City
Durham
State
NC
Country
United States
Zip Code
27705
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
Kien, C Lawrence; Matthews, Dwight E; Poynter, Matthew E et al. (2015) Increased palmitate intake: higher acylcarnitine concentrations without impaired progression of β-oxidation. J Lipid Res 56:1795-807
Baker 2nd, Peter R; Boyle, Kristen E; Koves, Timothy R et al. (2015) Metabolomic analysis reveals altered skeletal muscle amino acid and fatty acid handling in obese humans. Obesity (Silver Spring) 23:981-8
Fisher-Wellman, Kelsey H; Lin, Chien-Te; Ryan, Terence E et al. (2015) Pyruvate dehydrogenase complex and nicotinamide nucleotide transhydrogenase constitute an energy-consuming redox circuit. Biochem J 467:271-80
Seiler, Sarah E; Koves, Timothy R; Gooding, Jessica R et al. (2015) Carnitine Acetyltransferase Mitigates Metabolic Inertia and Muscle Fatigue during Exercise. Cell Metab 22:65-76
Keenan, Melissa M; Liu, Beiyu; Tang, Xiaohu et al. (2015) ACLY and ACC1 Regulate Hypoxia-Induced Apoptosis by Modulating ETV4 via α-ketoglutarate. PLoS Genet 11:e1005599
Wong, Kari E; Mikus, Catherine R; Slentz, Dorothy H et al. (2015) Muscle-Specific Overexpression of PGC-1α Does Not Augment Metabolic Improvements in Response to Exercise and Caloric Restriction. Diabetes 64:1532-43
Huynh, Frank K; Muoio, Deborah M; Hirschey, Matthew D (2015) SIRT3 Directs Carbon Traffic in Muscle to Promote Glucose Control. Diabetes 64:3058-60
Huffman, Kim M; Koves, Timothy R; Hubal, Monica J et al. (2014) Metabolite signatures of exercise training in human skeletal muscle relate to mitochondrial remodelling and cardiometabolic fitness. Diabetologia 57:2282-95
Muoio, Deborah M (2014) Metabolic inflexibility: when mitochondrial indecision leads to metabolic gridlock. Cell 159:1253-62

Showing the most recent 10 out of 20 publications