INHIBITION OF IGFBP-5 BINDING TO EXTRACELLULAR-MATRIX AND IGF-1-STIMULATED DNA-SYNTHESIS BY A PEPTIDE FRAGMENT OF IGFBP-5

Insulin-like growth factor binding protein-5 (IGFBP-5) is synthesized and secreted by smooth muscle cells (SMC). IGFBP-5 synthesis is stimul ated five- to sixfold by IGF-I, and IGFBP-5 has been shown to augment IGF-I-stimulated DNA synthesis in this cell type. The ability of IGFBP -5 to augment the SMC response to IGF-I is dependent upon its binding to extracellular matrix. A highly charged region of IGFBP-5 that conta ins amino acids in positions 201-218 has been shown to mediate binding of IGFBP-5 to human fibroblast extracellular matrix (ECM), and a synt hetic peptide containing this sequence inhibits ICFBP-5 binding to fib roblast ECM. In this study we show that exposure of SMC cultures that are constituitively synthesizing IGFBP-5 to a synthetic peptide (terme d peptide A) containing this sequence has no effect on its synthesis b ut reduces its abundance within the ECM. The addition of increasing co ncentrations of the peptide to SMC cultures resulted in a concentratio n-dependent reduction in ECM-associated ICFBP-5. In contrast, a contro l peptide (peptide B), which contained the region of amino acids in po sitions 131-141 and had a similar charge-to-mass ratio, caused a minim al decrease in ECM binding. This effect was functionally significant s ince the addition of 10 mu g/ml of peptide A inhibited the cellular re plication response to 10 ng/ml IGF-I by 51%, and peptide B had no effe ct. The effects of peptide A were not due to nonspecific cytotoxicity since it had no inhibitory effect on the response of these cells to hu man serum and was associated with only minimal inhibition of the cellu lar response to platelet-derived growth factor. The findings suggest t hat inhibiting IGFBP-5 binding to porcine SMC ECM results in reduced c ellular responses to IGI-I. J. Cell. Biochem. 71 :375-381, 1998. (C) 1 998 Wiley-Liss, Inc.