AJP - Cell Physiology, Vol 254, Issue 6 C822-C828, Copyright © 1988 by American Physiological Society
Internalization and catabolism of insulin by an established renal cell line
C. Yagil, B. H. Frank and R. Rabkin
Department of Medicine, Stanford University, California 94305.
Proximal renal tubules are a key site of insulin metabolism. To explore the
kinetics and metabolic requirements of insulin internalization and
catabolism, we used the opossum kidney cell line, which has proximal
tubular-like features and possesses insulin-specific receptors.
Internalization was determined by separating membrane-bound insulin from
intracellular insulin by exposure to an acidified medium. Internalization
of membrane-bound insulin was rapid, and half-maximal internalization
occurred within 2.5 min. Degradation products did not accumulate in the
cell but appeared in the medium after a delay of 5 min from the onset of
internalization. In other experiments, addition of KCN (2 mM) or omission
of glucose did not alter degradation, but KCN, combined with the omission
of glucose, inhibited degradation by 64%. This was associated with a 240%
increase in membrane-bound insulin and an 81% decrease in intracellular
insulin. Accordingly, it appears that under these circumstances impaired
degradation was a consequence of impaired internalization. In contrast,
although 0.1 mM chloroquine, an endosomal-lysosomal inhibitor, also
depressed degradation (by 57%), intracellular insulin increased fourfold,
indicating failure of intracellular processing. We conclude that these
cultured kidney cells rapidly internalize and degrade insulin and that
internalization, a prerequisite for degradation, is dependent on energy
that can be derived from anaerobic glycolysis or oxidative metabolism.
Furthermore, the intracellular degradative processing of insulin by these
cells involves a chloroquine-sensitive pathway.
