Insulin activates the appearance of insulin-like growth factor II receptors on the adipocyte cell surface.

To evaluate the mechanism of insulin action to increase rat 125I-labeled insulin-like growth factor II (125I-IGF-II) binding to rat adipocytes, we raised a potent rabbit antiserum against purified IGF-II receptors from rat placental membranes. The antiserum elicited a positive reaction at a 1:5000 dilution against purified IGF-II receptor in an ELISA and markedly inhibited 125I-IGF-II binding to adipocyte plasma membranes when added prior to the growth factor. Immunoprecipitation lines formed between agar plate wells containing antiserum versus IGF-II receptor, both in the presence and absence of 1 microM IGF-II, indicating that binding of anti-receptor Ig to the IGF-II receptor is not affected by occupancy of the IGF-II binding site. Intact adipocytes treated with or without insulin were incubated with anti-IGF-II receptor Ig, washed, and further incubated with 125I-labeled goat anti-rabbit IgG to monitor the amount of anti-receptor Ig bound. Insulin induced parallel increases in anti-IGF-II receptor Ig binding (2.4-fold) and 125I-IGF-II binding (3-fold) to the isolated cells. The dose-response relationship of insulin action on 125I-IGF-II binding and anti-receptor Ig binding was essentially identical with a half-maximal effect at approximately 0.07 nM insulin. That insulin does not act to expose new types of antigenic sites on IGF-II receptors was indicated by the demonstration that control adipocytes could readily adsorb the anti-receptor Ig. These data demonstrate that increased numbers of IGF-II receptors are displayed in an exposed position on the adipocyte cell surface in response to insulin.