Cell membrane association of insulin-like growth factor binding protein-2 (IGFBP-2) in the rat brain olfactory bulb

Abstract

Identification of sites of expression of IGF, IGF receptors and IGFBPs in the olfactory bulb of the rat brain suggested the presence of a paracrine IGF system. Since cell association of IGFBPs has been suggested as an important factor in their modulation of IGF action, we investigated whether IGFBPs are cell associated in olfactory bulb (OB). This was supported by des(1–3)IGF-I only partially competing for [¹²⁵I]IGF-I binding to rat OB membrane, suggesting the presence of a cell associated IGFBP. Affinity cross-linking of [¹²⁵I]IGF-I to rat OB membrane demonstrated a 39-kDa complex which was reduced by IGF-I and IGF-II, but not by des(1–3)IGF-I or insulin. Western ligand blotting of solubilised membrane showed a 38-kDa IGFBP which was immunoprecipitated by anti-IGFBP-2 antiserum but not by anti-IGFBP-5 antiserum. We conclude that in the rat IGFBP-2 is associated with membranes from OB. Whether the cell membrane association is due to integrin binding via its RGD sequence or glycosaminoglycan binding is currently under investigation. Cell associated IGFBP-2 may modulate IGF action in the neonatal rat OB.