Role of hepatocyte plasma membrane in insulin degradation

Abstract

An important role of the cell membrane in insulin degradation by cultured rat hepatocytes is supported by studies using the surface-active antibiotic bacitracin. Bacitracin inhibited degradation of cell-associated insulin (both randomly and A14 labeled) by 80–90% at 15 degrees C and by 60% at 37 degrees C. At 37 degrees C, inhibition of degradation was observed only with bacitracin present during dissociation and was accompanied by a compensatory increase in release of trichloroacetic acid (TCA)-precipitable insulin. This profile suggests inhibition of insulin degradation on the membrane after either primary binding or diacytosis (endocytosis-reverse endocytosis). In contrast, at 15 degrees C, bacitracin's inhibitory effect was greater with the antibiotic present during association and was not accompanied by a compensatory increase in TCA-precipitable insulin. This profile was compatible with inhibition of partial cleavage of insulin on the membrane. Internalization and degradation through chloroquine-sensitive pathways may be required to complete degradation at this temperature because chloroquine exhibited an inhibitory effect on insulin degradation equally potent to that of bacitracin at 15 degrees C (no effect at 37 degrees C).