Induction of SNAT2 by amino acid starvation requires eIF2α phosphorylation

Amino acid starvation lowers global protein synthesis via phosphorylation at serine 51 of the α subunit of the translation initiation factor eIF2 (eIF2α) and stimulates neutral amino acid transport through the ubiquitous transporter SNAT2 (system A). We show here that phosphorylation of eIF2α is required for the induction of SNAT2‐mediated amino acid transport. Upon total amino acid starvation, mouse embryonic fibroblasts, homozygous for a mutant eIF2α (Ser51→Ala), exhibited lower system A transport activity, decreased SNAT2 mRNA levels, and impaired cell viability compared with wild type cells. The presence of an internal ribosome entry site in the 5′‐UTR of the SNAT2 mRNA was also investigated. Transient transfections with dicistronic or hairpin‐containing monocistronic vectors, both including the SNAT2 5′‐untranslated region (UTR), indicated that the SNAT2 5′‐UTR contained an IRES. Consistently, using monocistronic vectors with a reporter gene downstream of the SNAT2 5′‐UTR, translation proceeded even when cap‐dependent translation was inhibited by amino acid starvation. These results demonstrate that eIF2α phosphorylation is required for SNAT2 mRNA induction by amino acid starvation. Moreover, the IRES in the SNAT2 5′‐UTR may allow an efficient cap‐independent synthesis of the transporter proteins under conditions of limiting substrate availability. This work was supported by University of Parma, FIL 2004‐2005 (RFG, OB) and RO1 DK60596 (MH).