The effects of cycloheximide and chloroquine on insulin receptor metabolism. Differential effects on receptor recycling and inactivation and insulin degradation.

The effects of protein synthesis inhibitors and the lysosomotropic agent chloroquine on the metabolism of the insulin receptor were examined. Through the use of the heavy-isotope density shift technique, cycloheximide was found to inhibit both the synthesis of new insulin receptor and the inactivation of old cellular insulin receptor. Upon investigation of the locus of this effect of protein synthesis inhibition, it was found that cycloheximide did not inhibit 1) the translocation of receptor from the cell surface to an intracellular site, 2) the recycling of receptor from the internal site back to the plasma membrane, nor 3) the degradation of insulin. Cycloheximide did, however, rapidly and completely inhibit the inactivation of the insulin receptor. In the presence of extracellular insulin, this effect of cycloheximide resulted in the long-term (6 h) accumulation of receptor in a trypsin-resistant intracellular compartment. Puromycin and pactamycin, protein synthesis inhibitors with mechanisms of action which differ from cycloheximide, produced the same effects on insulin receptor metabolism as cycloheximide, indicating that this effect on receptor metabolism is due to the inhibition of protein synthesis and not a secondary effect of cycloheximide. Actinomycin D also inhibited the inactivation of receptor. Chloroquine inhibited the receptor-mediated degradation of insulin, but had no effect on either the internalization or inactivation of the insulin receptor. The insulin-induced recycling of the internalized receptor was inhibited by chloroquine, possibly through the inhibition of the discharge of insulin from the insulin-receptor complex. From these observations, we suggest that 1) a protein factor is required to inactivate the insulin receptor, 2) this protein and the messenger RNA coding for the protein have short cellular half-lives, and 3) insulin degradation and insulin receptor inactivation are distinct, separable processes which not only occur at different rates, but possibly occur in distinct subcellular locations.