Translational and pretranslational regulation of protein synthesis by amino acid availability in primary cultures of rat hepatocytes

Protein synthesis is inhibited in both rat liver and isolated rat hepatocytes following deprivation of single essential amino acids. The aim of the present study was to define the time course of changes in peptide-chain initiation, albumin synthesis, and albumin mRNA following histidine deprivation and the reversal of these changes in response to readdition of the deprived amino acid. A further aim was to ascertain whether there was an accommodation of the inhibition of initiation following long-term amino acid deprivation. Primary cultures of rat hepatocytes were maintained in serum-free medium containing either all amino acids (complete medium) or all except histidine. Synthesis of total protein was reduced to 34% of control values following 48 hr of histidine deprivation and was restored to control values within 1 hr of addition of complete medium to histidine-deprived cells. These changes in protein synthesis were due to translational regulation involving initiation. No accommodation of the inhibition was observed following long-term deprivation of histidine as has been observed under other conditions of cellular stress. The synthesis of albumin was reduced to a greater extent than that of total protein, and required 72 hr to recover to control values following return to complete medium. These changes in albumin synthesis were due to a combination of altered initiation and a mechanism involving pretranslational regulation as evidenced by corresponding alterations in albumin mRNA. The results show that amino acid availability controls protein synthesis in liver cells through both translational and pretranslational mechanisms.