The role of insulin dissociation from its endosomal receptor in insulin degradation

Mechanisms that terminate signals from activated receptors have potential to influence the magnitude and nature of cellular responses to insulin. The aims of this study were to determine in rat liver endosomes (the subcellular site of insulin signal termination) whether dissociation of insulin from its receptor was a pre-requisite for ligand degradation and whether the state of receptor phosphorylation influenced the dissociation and hence endosomal degradation of insulin and/or receptor recycling. Following in vivo administration of 125I-[A14]-insulin or analogues (native, X10 or H2, relative binding affinities 1:7:67) livers were removed and endosomes prepared. In the endosomal preparations a significantly greater percentage of both analogues were receptor-bound than native insulin with concomitantly less ligand degradation. When rats were injected with protein-tyrosine phosphatase inhibitors (peroxovanadium compounds bpV(phen) or bpV(pic)) before insulin, endosomal insulin receptor phosphotyrosine content, assessed by Western blotting, was increased as was receptor-bound 125I-[A14]-insulin, whilst insulin degradation was decreased. Peroxovanadiums also completely inhibited recycling of insulin receptors from endosomes. However, treatment of freshly isolated endosomes with acid phosphatase which completely dephosphorylated the insulin receptor, did not return the rate of insulin dissociation and degradation to control values, suggesting that peroxovanadium compounds elicit their effect on binding and degradation via a mechanism other than as protein-tyrosine phosphatase inhibitors. We conclude that promotion of sustained receptor binding decreases endosomal insulin degradation and extends the half-life of the activated endosomal receptor, which in turn would be expected to potentiate insulin signalling from this intracellular compartment.