Essential Amino Acids Regulate Fatty Acid Synthase Expression through an Uncharged Transfer RNA-dependent Mechanism

Steven M Dudek,Clay F Semenkovich

doi:10.1074/jbc.270.49.29323

Steven M Dudek, Clay F Semenkovich

Open Access

https://doi.org/10.1074/jbc.270.49.29323

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Abstract

To better understand the regulation of gene expression by amino acids, we studied the effects of these macronutrients on fatty acid synthase (FAS), an enzyme crucial for energy storage. When HepG2 cells were fed serum-free media selectively deficient in each amino acid, the omission of any single classic essential amino acid as well as Arg or His (essential in some rapidly growing cells) resulted in FAS mRNA levels that were about half of those in complete medium. Control message levels were unaffected and omission of nonessential amino acids did not alter FAS expression. FAS mRNA levels peaked 12-16 h after feeding complete and Ser (nonessential)-deficient media but did not increase in cells fed Lys (essential)-deficient medium. With Lys, FAS mRNA increased over the physiologic concentration range of 15-150 microM, and low concentrations of lysine decreased FAS but not apoB protein mass. Transcription inhibitors mimicked treatment with Lys-deficient media, and nuclear run-off assays showed that Lys-deficient media abolished FAS but not apoB transcription. After treatment with Lys-deficient media, the intracellular Lys pool was rapidly depleted in association with an increase of uncharged (deacylated) tRNA Lys from < 1 to 64% of available tRNA Lys. Even in the presence of the essential amino acid His, increasing the level of uncharged tRNA His with histidinol, a competitive inhibitor of the histidinyl-tRNA synthetase, blocked FAS expression. Tyrosinol treatment did not alter FAS mRNA levels. These results suggest that essential amino acids regulate FAS expression by altering uncharged tRNA levels, a novel mechanism for nutrient control of gene expression in mammalian cells.

Highlights

Fatty acid synthase (FAS)1 is a large multifunctional protein that synthesizes the fatty acid palmitate from acetyl-CoA, malonyl-CoA, and NADPH [1]
Several enzymes are critical for the synthesis of fatty acids, it appears that FAS is rate-limiting in the long term control of lipogenesis [2]
A combination of essential and nonessential amino acids is necessary for maximal induction of FAS protein levels [13], but nothing else is known about the effects of individual amino acids on FAS gene expression

Summary

Introduction

Fatty acid synthase (FAS) is a large multifunctional protein that synthesizes the fatty acid palmitate from acetyl-CoA, malonyl-CoA, and NADPH [1] This process is essential for the conversion of dietary calories into a storage form suitable for use during periods of fasting. MRNA levels, and enzyme activity in fasted rats requires feeding of both carbohydrate and protein [5]. A combination of essential and nonessential amino acids is necessary for maximal induction of FAS protein levels [13], but nothing else is known about the effects of individual amino acids on FAS gene expression. We used human HepG2 cells, known to functionally resemble hepatocytes [17], to test the hypothesis that amino acids regulate FAS expression. We present data supporting this hypothesis and show that the effect of amino acids is likely mediated by changes in the intracellular concentrations of uncharged tRNA

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