An Inhibitor of a Cell Adhesion Receptor Stimulates Cell Migration**

The integrins are a family of heterodimeric receptors that play a universal role in mediating the adhesion of cells to the extracellular matrix. As such, the differential interactions of the receptors at the front and back ends of the cell are important in regulating the migration of cells. Approximately one-half of the integrins recognize the canonical RGD motif found in fibronectin, and a significant body of work has shown that model substrates presenting the RGD peptide support the adhesion and migration of cells. Inhibitors of the integrin receptor have been explored as reagents that may block cell migration and recent work has advanced a class of soluble RGD peptides as possible therapeutics that display anti-metastatic activity. But the molecular mechanisms by which these compounds block migration have not been elucidated and have recently been contradicted by work that demonstrates that the soluble antagonists can also promote the migration of tumor cells. Herein, we use a well-defined model system to assess the influence of soluble inhibitors on cell migration and we show that the integrin antagonists are able to promote the migration of cells. The RGD-mimetic compounds cilengitide and S36578 were developed as integrin antagonists and are now in early phase clinical trials for cancer therapy. 10] These compounds, at micromolar concentrations, were shown to block cell migration, but recent work reported that lower concentrations of these drugs can actually enhance the growth of tumors in vivo by promoting migration and VEGF-mediated angiogenesis. Other recent work has investigated the analogy between tumor angiogenesis and wound healing with the finding that the fibronectin-derived Pro-His-SerArg-Asn (PHSRN) peptide can stimulate migration of human kerotinocytes and fibroblasts and accelerate wound healing in obese diabetic mice. Our group 13] and others have suggested that PHSRN antagonizes RGD binding, and therefore may accelerate migration by inhibiting the integrinmediated adhesion. Additionally, it has been reported that addition of insulin-like growth factor binding protein (IGFBP-1) which contains the RGD sequence in the cell culture medium increases the migration of CHO cells twofold compared to cells treated with Trp-Gly-Asp (WGD). The literature contains several additional contradictory reports as to the effect of soluble RGD-containing proteins on adhesion and migration. Mechanistic studies of the roles of ligand–receptor interactions in cell migration are challenging because it is difficult to control which interactions operate between cell and matrix. This limitation is particularly relevant to in vivo experiments, where a broad range of signals and matrix constituents are present and not always defined. Even in the laboratory, it can be difficult to reproducibly present matrix ligands to a cell in order to obtain reproducible matrix formulations. To address these limitations, we use selfassembled monolayers that present RGD peptides against an otherwise inert background. We measure the migration profiles of large populations of individual cells and show that the addition of a soluble RGD peptide gives a dosedependent stimulation of cell migration and we rationalize this result in terms of the properties of polyvalent adhesion (Figure 1).