Effects of tunicamycin on the expression of beta-adrenergic receptors in human astrocytoma cells during growth and recovery from agonist-induced down-regulation

Tunicamycin, which inhibits formation of asparagine-linked glycoproteins, caused a concentration-dependent blockade of beta-adrenergic receptor (beta-AR) accumulation in 1321N1 human astrocytoma cells during growth in culture. A concentration of tunicamycin (0.1 microgram/ml) that inhibited receptor accumulation and [3H]mannose or [3H]glucosamine incorporation into glycoproteins by 90% had only a small effect (10%) on [3H]leucine incorporation into protein, and reduced the rate of cell growth. Incubation in drug-free medium subsequent to treatment of 1321N1 cells with tunicamycin for 48 hr resulted in recovery of beta-AR to control levels within an additional 48 hr. Exposure of cultures to isoproterenol (0.1 microM, 12 hr) caused an 80-90% loss of beta-AR in both pre- and postconfluent cultures; beta-AR recovered to control levels upon removal of isoproterenol. Although both tunicamycin and the protein synthesis inhibitor cycloheximide blocked beta-AR accumulation during growth of 1321N1 cells, neither agent inhibited the appearance of beta-AR during recovery from the down-regulated state in preconfluent cultures. However, cycloheximide, but not tunicamycin, blocked recovery of beta-AR after isoproterenol-induced loss of receptors in postconfluent cultures. In a previous report (Mol. Pharmacol. 26:424-429, 1984), we provided direct evidence that recovery of beta-AR from down-regulation in postconfluent cultures requires de novo synthesis of receptor protein. Thus, the results with tunicamycin are consistent with the idea that recovery of beta-AR in postconfluent cultures requires the synthesis of new beta-AR molecules, but as aglycoproteins that exhibit radioligand-binding characteristics similar to those of native glycoprotein beta-AR.