YAN, B; LI, S; DUAN, C: Molecular interaction between fibronectin and insulin-like growth factor binding protein-5 (IGFBP-5)

Previous studies have shown that IGFBP-5 is not only capable of regulating IGF activity by modulating the IGF ligand and receptor interaction, it also has intrinsic biological activity in vascular smooth muscle cells (SMC). The intrinsic action of IGFBP-5 may involve its cell surface association, internalization, and nuclear translocation. To identify proteins that interact with IGFBP-5 in the extracellular and intracellular compartments, we performed a yeast-2 hybrid screening of the human aorta cDNA library using full-length human IGFBP-5 as the bait. 63 positive yeast clones were obtained as a result of three rounds of screening under high stringency. Plasmids isolated from these yeast clones were transformed into E. coli. Bacterial plasmid DNA was isolated and subjected to DNA sequencing and/or restriction endonuclease mapping. Of the 48 clones that have been identified to date, 26 are fibronectin, an extracellular protein that is known to be important for SMC growth and migration. To determine the functional importance of this interaction, SMCs were grown on different extracellular matrix proteins and their responsiveness to IGF-I were examined. When SMCs were plated on fibronectin, they had a 5-fold greater DNA synthesis response to IGF-I compared with those maintained on laminin/type IV collagen. Addition of echistatin, a disintegrin that blocks fibronectin binding to alpha5beta3 intergrin, significantly inhibited the DNA synthesis response of the fibronectin-maintained cells to IGF-I. The anti alpha5beta 3 intergrin antibody partially reversed the inhibitory effect of laminin and type IV collagen on IGF-I-stimulated DNA synthesis. These results indicate that fibronectin and IGFBP-5 bind to each other and this interaction may alter the cellular responses to IGF-I (Supported by NIHRO1HL60679).