Different effect of amino acid deprivation on syntheses of intra- and extracellular proteins in rat hepatocytes in primary culture.

Primary cultured rat hepatocytes synthesize various proteins and secrete half of them into the medium. When the cells were cultured in medium deficient in amino acids for 30 h, they maintained their normal level of intracellular protein synthesis, but their synthesis of extracellular proteins decreased by half. This reduced production of extracellular proteins could be restored by addition of amino acid mixture to the medium. Since secretion of protein was not inhibited and no albumin accumulated in the cells on amino acid deprivation, the reduced secretion of extracellular protein was due to inhibition of its synthesis rather than inhibition of its secretion. Addition of the protease inhibitor leupeptin to cells cultured in medium supplemented with amino acids inhibited protein degradation 60%, but it did not change the rate of protein synthesis. Addition of this inhibitor to cells in amino acid deficient medium strongly inhibited the syntheses of both intra- and extracellular proteins. Electromicroscopic examination of the cells showed that amino acid deprivation markedly decreased the amount of rough endoplasmic reticulum and caused the appearance of autophagic vacuoles. These results show that extracellular amino acids control the syntheses of both intra- and extracellular proteins, but have a more direct effect on the latter, the intracellular proteins being synthesized by reutilization of amino acids liberated on protein degradation. Moreover, protein degradation seems to be accelerated by formation of phagosomes, when the medium is deficient in amino acids. When this protein degradation is inhibited by a protease inhibitor, the synthesis of intracellular proteins decreases. Therefore, there seem to be two compartments of amino acids in hepatocytes, one supplied by lysosomal degradation of proteins and used mainly for synthesis of intracellular protein, and the other supplied from exogenous amino acids and used for syntheses of both intra- and extracellular proteins.