Insulin-like growth factor (IGF)-I binding to a cell membrane associated IGF binding protein-3 acid-labile subunit complex in human anterior pituitary gland

The binding characteristics of [125I]insulin-like growth factor (IGF)-I were studied in human brain and pituitary gland. Competition binding studies with DES(1-3)IGF-I and R3-IGF-I, which display high affinity for the IGF-I receptor and low affinity for IGF binding proteins (IGFBPs), were performed to distinguish [125I]IGF-I binding to IGF-I receptors and IGFBPs. Specific [125I]IGF-I binding in brain regions and the posterior pituitary was completely displaced by DES(1-3)IGF-I and R3-IGF-I, indicating binding to IGF-I receptors. In contrast, [125I]IGF-I binding in the anterior pituitary was not displaced by DES(1-3)IGF-I and R3-IGF-I, suggesting binding to an IGF-binding site that is different from the IGF-I receptor. Binding affinity of IGF-I to this site was about 10-fold lower than for the IGF-I receptor. Using western immunoblotting we were also unable to detect IGF-I receptors in human anterior pituitary. Instead, western immunoblotting and immunoprecipitation experiments showed a 150-kDa IGFBP-3-acid labile subunit (ALS) complex in the anterior pituitary and not in the posterior pituitary and other brain regions. RT-PCR experiments showed the expression of ALS mRNA in human anterior pituitary indicating that the anterior pituitary synthesizes ALS. In the brain regions and posterior pituitary, IGFBP-3 was easily washed away during pre-incubation procedures as used in the [125I]IGF-I binding experiments. In contrast, the IGFBP-3 complex in the anterior pituitary could not be removed by these washing procedures. Our results indicate that the human anterior pituitary contains a not previously described tightly cell membrane-bound 150-kDa IGFBP-3–ALS complex that is absent in brain and posterior pituitary.