ESSENTIAL AMINO-ACIDS REGULATE FATTY-ACID SYNTHASE EXPRESSION THROUGHAN UNCHARGED TRANSFER RNA-DEPENDENT MECHANISM

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 fe d serum-free media selectively deficient in each amino acid, the omiss ion of any single classic essential amino acid as well as Arg or His ( essential in some rapidly growing cells) resulted in FAS mRNA levels t hat were about half of those in complete medium. Control message level s were unaffected and omission of nonessential amino acids did not alt er FAS expression. FAS mRNA levels peaked 12-16 h after feeding comple te and Ser (nonessential)-deficient media but did not increase in cell s fed Lys (essential)-deficient medium. With Lys, FAS mRNA increased o ver the physiologic concentration range of 15-150 mu M, and low concen trations of lysine decreased FAS but not apoB protein mass. Transcript ion inhibitors mimicked treatment with Lys-deficient media, and nuclea r run-off assays showed that Lys-deficient media abolished FAS but not apoB transcription. After treatment with Lys-deficient media, the int racellular Lys pool was rapidly depleted in association with an increa se of uncharged (deacylated) tRNA(Lys) from <1 to 64% of available tRN A(Lys). Even in the presence of the essential amino acid His, increasi ng the levels of uncharged tRNA(His) with histidinol, a competitive in hibitor of the histidinyl-tRNA synthetase, blocked FAS expression. Tyr osinol treatment did not alter FAS mRNA levels. These results suggest that essential amino acids regulate FAS expression by altering uncharg ed tRNA levels, a novel mechanism for nutrient control of gene express ion in mammalian cells.