Characterization of insulin-like growth factor-binding proteins in cultured rat pituitary cells

Binding proteins (BPs) for the insulin-like growth factors (IGFs) produced by cultured rat anterior pituitary (AP) and neurointermediate lobe (NI) cells were studied by competitive binding, affinity cross-linking, and Western ligand blot techniques. Conditioned medium from AP cultures contained specific high affinity IGF BPs with apparent mol wt of 35K, 27K, and 24K, while the 27K BP predominated in NI conditioned medium. Treatment of AP and NI conditioned media with endoglycosidase-F did not alter the 27K BP, but significantly reduced the apparent mol wt of the 35K BP into the 27-29K range, suggesting that the 35K BP may be a glycosylated form of the 27K BP. This 27K pituitary BP appeared similar to the BP produced by BRL-3A cells in both size and apparent lack of glycosylation. Although type 2 IGF receptors could be identified in conditioned medium from NI and GH₃ pituitary cells, binding of [¹²⁵I]IGF to pituitary BPs could not be inhibited, nor could the cross-linked BPs be immunoprecipitated, by antibody against the type 2 receptor. We conclude that cultured AP and NI cells produce a variety of related IGF BPs that are structurally distinct from the type 2 IGF receptor.