Revisiting the trafficking of insulin and its receptor in rat liver:in vivoand cell-free studies

Abstract

Endosomes are the main locus where, upon endocytosis in liver cells, insulin and the activated insulin receptor accumulate and insulin is degraded. In this study, ligand and receptor pathways in rat liver have been revisited using analytical subcellular fractionation. Density gradient analysis of microsomal and light mitochondrial fractions confirmed that, upon in vivo uptake into liver, radiolabeled insulin was rapidly translocated from plasma membranes to endosomes. Internalized radiolabeled proinsulin and biotinylated insulin, which are less degraded than insulin, were in part transferred from endosomes to lysosomes. Following injection of native insulin, receptor and ligand were also both translocated from the plasma membrane to endosomes. Receptor translocation was progressively reversed and neither its endocytosis nor its recycling were affected by cycloheximide treatment. Fractionation of endosomes on density gradients showed that the receptor was recovered at low density, whereas insulin progressively migrated towards a higher density at the position of acid phosphatase. On sodium dodecyl sulfate polyacrylamide gels, insulin receptor alpha and beta subunits were identified in plasma membrane, endosomal and lysosomal fractions by affinity crosslinking and Western immunoblotting, respectively. Following insulin treatment, receptor expression rapidly decreased in plasma membranes while increasing in endosomes. In a cell free system, labeled insulin was in part transferred from endosomes to lysosomes, as was, with organelle content mixing, labeled biotinylated insulin. It is concluded that, upon endocytosis, the insulin receptor progressively segregates from its ligand, and that neither endocytosis no recycling of the receptor requires protein synthesis.