Expression of two types of receptor for insulinlike growth factors in human colonic epithelium

The presence of receptors for insulinlike growth factor I and II in human colonic epithelium is demonstrated. Scatchard analysis of binding data obtained with 125I-insulinlike growth factor I showed an insulinlike growth factor I receptor with a dissociation constant of 8.6 nM and a binding capacity of 0.8 pmol/mg membrane protein. Distinct insulinlike growth factor II receptors labeled with 125I-insulinlike growth factor II were also found with a dissociation constant of 6.9 nM and a binding capacity of 4.7 pmol/mg membrane protein. Two sets of observations make it possible to discriminate between the two types of insulinlike growth factor receptors. (a) Unlabeled insulinlike growth factor I was 3 times more potent than insulinlike growth factor II in inhibiting [125I] insulinlike growth factor I binding. Conversely, unlabeled insulinlike growth factor II was 10 times more potent than insulinlike growth factor I in competing with 125I-insulinlike growth factor II for binding to membranes. Insulin and proinsulin did not compete with either of the tracers. (b) Affinity labeling of membranes followed by sodium dodecylsulfate polyacrylamide gel electrophoresis under reducing conditions, revealed a radio-ligand-receptor complex of molecular weight 130,000 and 250,000 using 125I-insulinlike growth factor I and 125I-insulinlike growth factor II, respectively. These observations indicate that adult human colonic epithelium is abundantly equipped with two sets of receptors that recognize preferentially either insulinlike growth factor I or insulinlike growth factor II.