Induction of amino acid transport system A in rat hepatocytes is blocked by tunicamycin.

Primary cultures of rat hepatocytes respond to hormones or amino acid deprivation by increasing System A-mediated neutral amino acid transport. Previous reports have shown this stimulation to be dependent on RNA and protein synthesis, whereas the present report describes the inhibition of System A by tunicamycin (TM), an inhibitor of asparagine-linked glycoprotein biosynthesis. The basal System A activity, as monitored by Na+-dependent 2-aminoisobutyric acid uptake, was decreased by TM when hepatocytes were cultured for 24 h in the presence of the antibiotic. System Gly activity was also sensitive to TM, whereas the activities of Systems L1, L2, and N were relatively resistant and that of System ASC was only moderately affected. The increase in System A-mediated uptake after incubation of hepatocytes in the absence of amino acids (i.e. adaptive control) was almost completely abolished by including TM. Likewise, stimulation of hepatic 2-aminoisobutyric acid transport by glucagon, dexamethasone, insulin, or vasopressin was also blocked by the inhibitor. When glucagon alone or glucagon plus dexamethasone was added, the inhibition by TM was transient such that the degree of inhibition decreased with incubation time after the initial 2 h. Addition of TM to cells which had been treated previously for 2 h to 4 h with glucagon and dexamethasone blocked any further increase in transport indicating that the glycoprotein component of System A must be continually synthesized to sustain the increase in activity. Treatment of hepatocytes with various lectins did not inhibit 2-aminoisobutyric acid transport.