Cell type-specific variability of bacitracin's effects on insulin binding and intracellular accumulation

Bacitracin is known to inhibit proteolytic degradation of insulin and several other peptide hormones. Previous work with isolated rat adipocytes showed that bacitracin blocked insulin degradation by the plasma membrane and, even in the absence of detectable insulin degradation, bacitracin increased insulin binding by decreasing the rate of insulin dissociation. The present study examined the effects of bacitracin on insulin binding and degradation and on levels of intracellular insulin in a variety of cell types. Bacitracin inhibited insulin degradation in all cell types. Maximal inhibition varied from 70% (H4IIEC3 hepatoma cells) to 95% (rat adipocytes); concentrations giving half-maximal inhibition varied from 25 microM (3T3-A31 fibroblasts) to 250 microM (H4IIEC3). Dose-response curves showed three distinctive effects on insulin binding: dose-dependent stimulation (rat adipocytes), a biphasic curve with slight stimulation at low doses and inhibition at concentrations greater than 50 microM (human fibroblasts, H4IIEC3, and 3T3-L1 adipocytes), or dose-dependent inhibition of binding (3T3-L1 preadipocytes and 3T3-A31 fibroblasts). The intracellular accumulation of insulin rat adipocytes was not affected by bacitracin but was decreased in all other cell types. These data illustrated type-specific variability in the effects of bacitracin on insulin processing resulting from cellular heterogeneity either in processing insulin or in response to bacitracin, or both, and suggest that insulin binding studies performed in the presence of bacitracin can be biased.