Insulin binding and degradation in vascular endothelial cells: modulation by cell growth and culture organization.
The interaction of insulin with the vascular endothelium and its modulation by cell growth and culture organization was studied using bovine aortic endothelial cells in monolayer cultures. Three types of cultures were investigated: 1) confluent nondividing cultures, organized and differentiated as the in vivo tissue; 2) subconfluent, not yet organized cell cultures, representing proliferating endothelium; and 3) endothelial cell cultures modified to lose their property of contact inhibition, growing in multiple layers. All three types of cultures exhibited specific binding of 125I-insulin to high and low affinity cell surface receptor sites, and were capable of degrading 125I-insulin. Preexposure of the cultures to insulin resulted in a time dependent reduction in the availability of cell surface receptors (down-regulation). Insulin binding per cell was 2.4-fold and 10-fold higher in the subconfluent and modified cultures, respectively, as compared to the contact-inhibited confluent cultures. Similarly, the rate of insulin degradation was higher in the subconfluent and modified cultures (2.3-fold and 20-fold, respectively). Subconfluent cultures were more sensitive than confluent cultures to the down-regulatory effect of insulin. They exhibited a 60% decrease in insulin binding as compared to a 40% decrease in confluent cultures after preexposure to 50 ng/ml insulin. The increase in insulin binding and degradation in growing endothelial cells suggests a role for the hormone in the regulation of endothelial cell growth, e.g. in response to injury. This was further supported by the observation of a dose-dependent stimulation of [3H]thymidine incorporation into sparse, serum-starved endothelial cells by physiological concentrations of the hormone.