Insulin binding, internalization, and degradation by a cultured kidney cell line

Proximal tubules are a key site of insulin metabolism, which is in part a receptor-mediated process. To explore the interaction between insulin and the kidney to evaluate the role of receptors in insulin uptake and processing, a study was carried out with a cultured proximal-like opossum kidney (OK) cell line, ¹²⁵I-insulin associated with confluent monolayers in a specific manner, and this interaction was competitively inhibited by insulin; unrelated peptides were relatively ineffective. Insulin degradation exhibited time and temperature dependency and up to a concentration of 5 x 10^-8 M was not saturable. Degradation exhibited partial hormone specificity. Separation of plasma membrane bound from internalized insulin was achieved by lowering extracellular pH. At 4°C, 94% of cell-associated radioactivity was membrane bound, whereas at 37°C, in the steady state, 33% was membrane bound and 67% was internalized. There was a significant correlation between membrane-bound insulin and the rate of degradation. These findings reveal that the binding and processing of insulin by the kidney cell line are compatible with the description of the uptake of filtered insulin by the proximal tubule in the intact kidney. Accordingly we conclude that this cell line provides a good model for studying renal epithelial uptake and metabolism of insulin.