ROLE OF INSULIN AND INSULIN-LIKE GROWTH-FACTOR RECEPTORS IN REGULATION OF T84 CELL MONOLAYER PERMEABILITY

We previously showed that insulin and insulin-like growth factor (IGF)-I and IGF-II caused a dose-dependent increase in permeability through the paracellular pathway of T84 cell monolayers over 3-4 days. Here we have determined which cell surface receptors were involved in this response. Using radioligand binding studies and receptor cross-linking studies, we found that T84 cells possess insulin and IGF-I receptors. There were similar to 20 x 10(3) insulin receptors/cell with a dissociation constant (K-D) of 0.5 nM and 29 x 10(3) IGF-I receptors with a K-D of 0.6 nM for IGF-I. Cross-linking studies identified the alpha-subunit of insulin and IGF-I receptors with deduced molecular weights of 126 x 10(3) and 128 x 10(3), respectively. IGF-II bound to T84 cells with an apparent K-D of similar to 2.0 nM. Radioreceptor crosslinking indicated that IGF-II interacted principally with the IGF-I receptor, although low levels of the IGF-II/mannose 6-phosphate receptor were also expressed on the cell surface. We then correlated the biological effect with the radioligand binding studies. It was first demonstrated that insulin and IGF-I were degraded in medium in the presence of cells. In addition, we showed that continuous exposure for 2-3 days to insulin or IGF-I was required to produce their biological effect on permeability. Taking into account the rate of degradation and the requirement for continuous exposure, we found a close correlation between radioligand binding and the half-maximal effective concentration for the hormonal effects on transepithelial permeability. These results establish that insulin acts through the insulin receptor, whereas IGF-I and IGF-II act through the IGF-I receptor to cause an increase in permeability across T84 cell monolayers.