AJP - Endocrinology and Metabolism, Vol 254, Issue 5 E601-E608, Copyright © 1988 by American Physiological Society
Insulin binding, internalization, and degradation by a cultured kidney cell line
C. Yagil, U. K. Ehmann, B. H. Frank and R. Rabkin
Department of Medicine, Stanford University 94305.
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 and 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
degrees C, 94% of cell-associated radioactivity was membrane bound, whereas
at 37 degrees 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.
