| The objective of these studies was to establish a potential role for IGF binding protein-2 (IGFBP-2) in the development and treatment of IGF-dependent hyperproliferative diseases, including diabetic glomerulosclerosis and breast cancer. Based on the known role of IGFBP-2 as an inhibitor of IGF action, I hypothesized that dysregulation of IGFBP-2 levels could contribute to the hypersensitivity to the IGFs observed in these disease states. I further reasoned that the binding site of IGFBP-2 could serve as a template for designing a therapeutic IGF antagonist. The major benefit of this non-classical antagonism would be to avoid altering insulin-insulin receptor interactions. To test this hypothesis, the effects of elevated glucose and IGF-1 receptor overexpression on IGFBP-2 secretion by glomerular mesangial cells (responsible for the development of diabetic glomerulosclerosis) were quantified. These conditions, as seen in diabetes, resulted in reduced IGFBP-2 secretion which was correlated with increased IGF-1-stimulated proliferation. It was subsequently demonstrated that purified rhIGFBP-2 could inhibit both basal and IGF-1-stimulated growth of MCF-7 breast cancer cells. Having demonstrated that IGFBP-2 exhibits an important IGF-inhibitory function in these situations, identification of the IGF-binding domain on IGFBP-2 was pursued. For these studies, an IGFBP-selective IGF-1 photoprobe was generated by synthetically adding a photoactivatable aryl azide to the a-amino group of Gly1, a residue within the IGFBP-binding domain on IGF-1. By using this photoprobe to photoaffinity label purified rhIGFBP-2, a site was identified within the carboxy-terminal region of IGFBP-2 which is in close three-dimensional proximity to the IGF-binding domain. These data suggest that the region of IGFBP-2 responsible for high affinity binding to IGF-1 is near the C-terminus, a finding that fits well with and extends current but limited data about IGFBP-2 structure. This new structural information will provide a starting point for the development of an IGFBP-based IGF antagonist. |
