Effects of monensin on insulin processing in adipocytes. Evidence that the internalized insulin-receptor complex has some physiological activities.

In the presence of 10-100 microM monensin (a monovalent cation ionophore), a considerable amount of 125I activity of iodoinsulin accumulated in isolated rat epididymal adipocytes during a 30-min incubation. The accumulation was secondary to the action of monensin to inhibit dissociation of a certain fraction of the cellbound 125I activity. This monensin effect was reversible. The accumulation of 125I activity was ATP dependent and so was the discharge of the accumulated radioactivity. Approximately 91% of the accumulated radio-activity was precipitable with trichloroacetic acid, and at least 84% was reactive to anti-insulin antibody. Monensin at 100 microM appeared to have only mild effects on the cellular activities of glucose transport and cAMP phosphodiesterase. Nevertheless, when cells were first exposed to 10 nM insulin in the presence of 100 microM monensin and then transferred into a hormone-free buffer that contained monensin, the phosphodiesterase activity in cells remained partially activated as if cells were kept exposed to approximately 0.5 nM insulin. Under similar conditions, glucose transport activity remained partially activated as if cells were incubated with approximately 70 pM insulin. Monensin did not inhibit the reversal of the insulin effect per se. Like monensin, 20-100 microM chloroquine (a lysosomotropic inhibitor) induced a considerable accumulation of [125I] iodoinsulin. However, cells that had been exposed to insulin in the presence of chloroquine retained little hormonal effect after washing. Based on these observations and on the reported biological effects of monensin, it is suggested (a) that monensin may induce intracellular accumulation of the insulin-receptor complex by blocking the acidification of endocytic vesicles and (b) that the accumulated insulin-receptor complex may retain a weak, but significant, capacity to stimulate both glucose transport and phosphodiesterase activities.