Processing of ¹²⁵I-insulin by polarized cultured kidney cells

Renal clearance of insulin is achieved by glomerular filtration and by passage from the postglomerular peritubular circulation into the renal interstitium. In the proximal tubule, filtered insulin binds to the apical membrane and is internalized and degraded, while insulin in the interstitium is taken up by receptor-mediated endocytosis and degraded. To study these processes we have utilized cultured opossum kidney cells. These cells have proximal-like features and process insulin in a manner consistent with that described in vivo. To study apical and basolateral uptake and metabolism of insulin independently, cells were grown on filters suspended in culture wells. Insulin was degraded to large insulin-size intermediates and low-molecular-weight products. This occurred whether the protein was internalized from the apical or basolateral pole of the cell. Analysis of the intermediate products by reverse-phase high-performance liquid chromatography revealed that products formed after apical or basolateral internalization were similar. Since products were preferentially released from the side of uptake, it is likely that apically and basolaterally internalized insulin is degraded in comparable organelles located in different regions of the cell. Most of the internalized insulin traversed the degradative pathway but some insulin followed a retroendocytic or minor transcytotic pathway. Degradation was inhibited by chloroquine, which also selectively increased the release of internalized insulin from the apical pole irrespective of the side of uptake. Thus while the polar degradative processes appear to be similar in nature, the polar exocytotic processes appear to be different.