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

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 M, 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 levels 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. Fatty acid synthase (FAS)1 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. Although 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).Feeding increases FAS expression (3). The intake of high carbohydrate diets following periods of fasting increases FAS mRNA levels and enzyme concentration (4). Hormones, many of which are affected by feeding and fasting, play a role in this regulation. In diabetic rats, insulin increases hepatic FAS transcription, mRNA levels, and enzyme levels 5-15-fold (5). Dexamethasone enhances FAS mRNA induction by insulin in primary rat hepatocyte cultures (6). Triiodothyronine stimulates FAS gene transcription in cultured chick embryo hepatocytes (7) and mouse 3T3-L1 adipocytes (8). Progesterone stabilizes FAS mRNA levels (9).Hormonal regulation of FAS expression has appropriately been the focus of many previous studies. However, individual dietary components, especially macronutrients, can also affect FAS expression. The carbohydrate glucose stabilizes FAS mRNA levels in HepG2 cells cultured in serum-free media (10, 11). Polyunsatu...

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