Upregulated PDK4 expression is a sensitive marker of increased fatty acid oxidation

Ina Katrine Nitschke Pettersen,Deusdedit Tusubira,Hanan Ashrafi,Sissel Elisabeth Dyrstad,Lena Hansen,Xiao-Zheng Liu,Linn Iren Hodneland Nilsson,Nils Gunnar Løvsletten,Kjetil Berge,Hege Wergedahl,Bodil Bjørndal,Øystein Fluge,Ove Bruland,Arild Christian Rustan,Nils Halberg,Gro Vatne Røsland,Rolf Kristian Berge,Karl Johan Tronstad

doi:10.1016/j.mito.2019.07.009

Abstract

Fatty acid oxidation is a central fueling pathway for mitochondrial ATP production. Regulation occurs through multiple nutrient- and energy-sensitive molecular mechanisms. We explored if upregulated mRNA expression of the mitochondrial enzyme pyruvate dehydrogenase kinase 4 (PDK4) may be used as a surrogate marker of increased mitochondrial fatty acid oxidation, by indicating an overall shift from glucose to fatty acids as the preferred oxidation fuel. The association between fatty acid oxidation and PDK4 expression was studied in different contexts of metabolic adaption. In rats treated with the modified fatty acid tetradecylthioacetic acid (TTA), Pdk4 was upregulated simultaneously with fatty acid oxidation genes in liver and heart, whereas muscle and white adipose tissue remained unaffected. In MDA-MB-231 cells, fatty acid oxidation increased nearly three-fold upon peroxisome proliferator-activated receptor α (PPARα, PPARA) overexpression, and four-fold upon TTA-treatment. PDK4 expression was highly increased under these conditions. Further, overexpression of PDK4 caused increased fatty acid oxidation in these cells. Pharmacological activators of PPARα and AMPK had minor effects, while the mTOR inhibitor rapamycin potentiated the effect of TTA. There were minor changes in mitochondrial respiration, glycolytic function, and mitochondrial biogenesis under conditions of increased fatty acid oxidation. TTA was found to act as a mild uncoupler, which is likely to contribute to the metabolic effects. Repeated experiments with HeLa cells supported these findings. In summary, PDK4 upregulation implies an overarching metabolic shift towards increased utilization of fatty acids as energy fuel, and thus constitutes a sensitive marker of enhanced fatty acid oxidation.

Highlights

Mitochondrial fatty acid oxidation is a crucial and highly regulated fueling pathway for aerobic ATP production in mammalian organisms
In order to investigate the relationship between fatty acid oxidation and pyruvate dehydrogenase kinase 4 (PDK4) expression in different contexts of metabolic adaptation, we studied the effects of long-term (6 days) treatment with tetradecylthioacetic acid (TTA) and selective activators of AMP-dependent protein kinase (AMPK) (AICAR) and PPARα (WY 14,643) in MDAMB-231 cells
This study shows that upregulated PDK4 mRNA expression is strongly associated with increased activity of mitochondrial fatty acid oxidation in metabolically adapted cells and rat tissues

Summary

Introduction

Mitochondrial fatty acid oxidation is a crucial and highly regulated fueling pathway for aerobic ATP production in mammalian organisms. Increased rates of fatty acid oxidation, e.g. due to starvation or insulin-deficient diabetes, is associated with suppression of PDH activity (reviewed in (Holness and Sugden, 2003)) This is important for conserving 3‐carbon precursors for glucose synthesis when glucose is scarce. Increased level of PDK4, e.g. due to starvation (Wu et al, 2000), favors inactivation of PDH in oxidative tissues, and thereby implements an influential metabolic shift from glucose to fatty acid oxidation Under such conditions, reduced production of malonyl-CoA, an important inhibitor of mitochondrial fatty acid uptake and oxidation (the CPT system), will support increased rates of fatty acid oxidation

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