INSULIN-LIKE-GROWTH-FACTOR BINDING PROTEIN-2 BINDS TO CELL-SURFACE PROTEOGLYCANS IN THE RAT-BRAIN OLFACTORY-BULB

A family of six insulin-like growth factor binding proteins (IGFBPs) b ind IGF-I and modulate its biological activity. IGFBPs may bind to mac romolecules on the cell surface or pericellular extracellular matrix, and this interaction may modulate their effect on IGF activity. To dat e, Little is known about the specificity of IGFBPs in the regulation o f IGF action in the brain. We therefore explored whether IGFBPs were a ssociated with cell membrane or extracellular matrix components in the rat brain. IGF-I binding sites with the characteristics of an IGFBP w ere found in the olfactory bulb mitral cell layer. This IGFBP was iden tified as IGFBP-2 by immunoprecipitation of both solubilized membrane preparations and cross-linked I-125-IGF: IGFBP complexes. While bindin g of IGFBP-2 to cell membranes was unaffected by RGD-containing peptid e, it was inhibited by high salt concentration, suggesting interaction with proteoglycans. IGFBP-2 bound in vitro to the glycosaminoglycans chondroitin-4 and -6-sulfate, keratan sulfate, and heparin. IGFBP-2 al so hound the proteoglycan aggrecan, an effect reduced by digestion of its glycosaminoglycans. Binding of IGFBP-2 to chondroitin-6-sulfate de creased the binding affinity of IGFBP-2 for IGF-I approximately 3-fold . Finally, an IGFBF-2 antibody coimmunoprecipitated IGFBP-2 and an app roximately 200 kDa proteoglycan containing chondroitin-sulfate side ch ains from the rat olfactory bulb, providing definitive evidence for IG FBP-2 binding to olfactory bulb proteoglycans. These findings indicate that IGFBP-2 binds to proteoglycans in cell. membranes of the rat olf actory bulb. Because we have previously shown that IGFs are highly exp ressed in the rat olfactory bulb, cell associated IGFBP-2 may have an important role in directing IGFs to specific sites in this brain regio n.