The molecular diversity of receptors in human blood vessels remains largely unexplored. We developed a selection method in which peptides that home to specific vascular beds are identified after administration of a peptide library. Here we report the first in vivo screening of a peptide library in a patient. We surveyed 47,160 motifs that localized to different organs. This large-scale screening indicates that the tissue distribution of circulating peptides is nonrandom. High-throughput analysis of the motifs revealed similarities to ligands for differentially expressed cell-surface proteins, and a candidate ligand-receptor pair was validated. These data represent a step toward the construction of a molecular map of human vasculature and may have broad implications for the development of targeted therapies.

Several lines of evidence suggest that tumor growth, angiogenesis, and metastasis are dependent on matrix metalloproteinase (MMP) activity. However, the lack of inhibitors specific for the type IV collagenase/gelatinase family of MMPs has thus far prevented the selective targeting of MMP-2 (gelatinase A) and MMP-9 (gelatinase B) for therapeutic intervention in cancer. Here, we describe the isolation of specific gelatinase inhibitors from phage display peptide libraries. We show that cyclic peptides containing the sequence HWGF are potent and selective inhibitors of MMP-2 and MMP-9 but not of several other MMP family members. Our prototype synthetic peptide, CTTHWGFTLC, inhibits the migration of human endothelial cells and tumor cells. Moreover, it prevents tumor growth and invasion in animal models and improves survival of mice bearing human tumors. Finally, we show that CTTHWGFTLC-displaying phage specifically target angiogenic blood vessels in vivo. Selective gelatinase inhibitors may prove useful in tumor targeting and anticancer therapies.

Human fibroblasts and HT-1080 fibrosarcoma cells express membrane-type-1 matrix metalloproteinase (MT1-MMP), the cell surface activator of gelatinase A, in separate forms of 63 kDa, 60 kDa and in some cases 43 kDa. In the present work the interrelationships between MT1-MMP processing and gelatinase A activation were analysed using HT-1080 fibrosarcoma cells as a model. It was found that MT1-MMP was synthesized as a 63 kDa protein, which was constitutively processed to a 60 kDa active enzyme with N-terminal Tyr112, as shown by immunoprecipitation, immunoblotting and sequence analyses. Co-immunoprecipitation results indicated that only the active 60 kDa form of MT1-MMP bound gelatinase A at the cell surface. Both the activation of pro-MT1-MMP and the membrane binding of the tissue inhibitor of metalloproteinases type 2 (TIMP-2) and gelatinase A, and subsequent activation of gelatinase A, were inhibited by calcium ionophores. Although the active MT1-MMP was required for cell surface binding and activation of gelatinase A, it was inefficient in activating gelatinase A in fibroblasts or in control HT-1080 cells alone. Low expression levels of TIMP-2 and rapid synthesis of MT1-MMP were found to be critical for gelatinase A activation. In HT-1080 cells, MT1-MMP was further processed to an inactive, 43 kDa cell surface form when overexpressed, or when the cells were treated with PMA. Under these conditions, the activated gelatinase A was detected in the culture medium, in cell membrane extracts and in MT1-MMP-containing complexes. These results indicate that proteolytic processing (activation and degradation/inactivation) of MT1-MMP and MT1-MMP/TIMP-2 relationships at the cell surface are important regulatory levels in the control of gelatinolytic activity.

Membrane-type-1 matrix metalloproteinase (MT1-MMP) has transmembrane and cytoplasmic domains, which target it to invasive fronts. We analyzed the role of the cytoplasmic tail by expressing wild type MT1-MMP and three mutants with progressively truncated C termini in human Bowes melanoma cells. We examined gelatinase A activation and the localization and processing of recombinant proteins in stable cell clones using gelatin zymography, immunoblotting, and immunofluorescence. Cell invasion was analyzed in vitro by Matrigel invasion assays. Gelatinase A was activated in all cell clones. However, the localization of MT1-MMP to the leading edge of migrating cells and cell invasion through Matrigel were strongly enhanced only in cells expressing either wild type or truncated MT1-MMP lacking 6 C-terminal amino acid residues (⌬577). Truncations of 10 or 16 amino acid residues in the cytoplasmic domain (⌬567 and ⌬573, respectively) disturbed MT1-MMP localization. The expression of wild type and ⌬577 MT1-MMPs induced also their cleavage to 43-kDa cell surface forms and the release of soluble, ϳ20-kDa N-terminal fragments containing the catalytic center. A synthetic MMP inhibitor but not a gelatinase inhibitor prevented the processing, suggesting that autocatalytic cleavage occurs. Purified soluble MT1-MMP was also autoproteolytically processed to 43-and 20-kDa forms in vitro. Our results indicate that the cytoplasmic domain has an important role in cell invasion by controlling both the targeting and degradation/turnover of MT1-MMP.

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