Sequential processing of insulin by cultured kidney cells.

It is now well established that in kidney, as in liver, endosomes participate in the degradation of insulin. Degradation in this compartment involves the action of the insulin-degrading enzyme or a similar enzyme with the formation of large intermediate products. The role of lysosomes is less clear, for although earlier studies suggested that they are the major or sole site of degradation, this has been increasing questioned. More recently, it has been concluded that endosomes, at least in liver, are the major site of insulin degradation and that classic lysosomes are only involved in the latter stages of degradation if at all. As intracellular insulin processing varies among cell types, we set out to examine directly the processing of insulin within cultured proximal-like opossum kidney cells. By means of analytical subcellular fractionation and reverse phase HPLC analysis of products, we established the following sequence of events. After internalization, [125I]A14-insulin is partially degraded in endosomes. The formed products together with intact insulin, which accounts for most of the radioactive material in the endosomes, are then directed to the lysosomal compartment, where degradation proceeds rapidly to completion. Bacitracin inhibited degradation in both compartments and, although not eliminating insulin trafficking, may impair the transfer of insulin from endosomes to lysosomes. This study establishes a major role for lysosomes in kidney cell insulin degradation.