Polymxin B inhibits insulin‐induced glucose transporter and IGF II receptor translocation in isolated adipocytes

Abstract

In isolated adipocytes, polymyxin B inhibited insulin‐induced glucose incorporation into lipids in a dose‐dependent manner, while polymyxin E, a structurally related antibiotic, was ineffective. To approach the mechanism of this effect, the subcellular distribution of the glucose transporter Glut 4 was investigated. Adipocytes were pretreated without or with polymyxin B before insulin stimulation, subcellular fractionation was performed and Glut 4 was detected by immunodetection. Incubation of adipocytes with polymyxin B prevented the insulin‐induced appearance of Glut 4 in the plasma membranes, but did not prevent their decrease from the low‐density microsomal fraction. A lower purity of the plasma membrane fractions, a detergent effect of polymyxin B on the membranes or an interference of the substance with the immunodetection of the Glut 4 molecules were excluded. These results suggest that polymyxin B was interfering with the Glut 4 translocation process stimulated by insulin in adipocytes. In a similar fashion, polymyxin B inhibited the insulin‐induced increase in IGF II binding to adipocytes. This resulted from a blockade of the appearance of IGF II receptors in the plasma membranes. Since low‐molecular‐mass GTP‐binding proteins have been implicated in the regulation of vesicular trafficking, we have used [α‐32P]GTP binding to analyze such proteins in adipocyte fractions, after SDS/PAGE and transfer to nitrocellulose. Specific and distinct subsets of GTP‐binding proteins were revealed in plasma membrane and low‐density microsomal fractions of control adipocytes, whether they were stimulated or not with insulin. Polymyxin B treatment of adipocytes markedly modified the profile of the low‐molecular‐mass GTP‐binding proteins in plasma membranes, but not in lowdensity microsomal fractions. Our results suggest that polymyxin B was interfering with the exocytotic process of the Glut 4 and IGF II receptor‐containing vesicles, perhaps at the fusion step between vesicles and plasma membranes. Copyright © 1992, Wiley Blackwell. All rights reserved