The biological activity of metal ion-gastrin complexes

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DESCRIPTION (provided by applicant): The long-term aim of this project is to understand the biological significance of the interactions between metal ions and peptides derived from the prohormone progastrin. Recent data from this laboratory has shown that progastrin-derived peptides (PDPs) selectively bind 2 ferric ions, and that recombinant human progastrin also binds a calcium ion with high affinity, at a site distinct from the ferric ion binding site. The specific aims of the project are: (1) to define the properties and structure of the complexes between metal ions and PDPs, (2) to define the role of metal ions in the biological activities of PDPs including receptor binding, cell proliferation and cell migration, and (3) to determine the role of the complexes in modulation of progastrin processing and in metal ion uptake by the gastrointestinal tract. The health significance of the project lies in the facts that PDPs act as growth factors for the normal gastric and colonic mucosa, accelerate the development of both gastric and colorectal cancer, and may be involved in disorders of iron homeostasis. The research design mirrors the specific aims. Firstly, the structures of the complexes between metal ions and PDPs will be determined by a combination of fluorescence, EPR and NMR spectroscopy. The structures will be used as the basis for the design of recombinant and synthetic PDPs with single amino acid substitutions which prevent the binding of either ferric or calcium ions. Secondly, the ability of the parent and mutant PDPs to bind to, stimulate proliferation in, and reduce adhesion of, a panel of gastrointestinal cell lines will be compared. Proliferation will be assessed by measurement of DNA synthesis in cell lines, isolated crypts, and the defunctioned colon in vivo. Adhesion will be assessed by immunochemical analysis of adhesion protein complexes, and migration and wound healing assays in tissue culture. Thirdly, the effect of changes in ferric ion concentration on progastrin processing will be measured in cell lines and in mice with altered iron status. The role of PDPs in the cellular uptake of ferric ions will also be determined. The demonstration that ferric and calcium ions are essential components of the biologically active forms of PDPs would alter completely our understanding of the role of metal ions in hormone function. The studies may also reveal an unexpected role for peptide hormones in ferric ion homeostasis. In the longer term definition of high affinity receptors for PDP-metal ion complexes may permit the development of novel therapies for treatment of disorders of iron metabolism and of gastrointestinal cancers.