ADAM28, a member of a disintegrin and metalloproteinase (ADAM) family, has two isoforms, membrane-type form (ADAM28m) and secreted form (ADAM28s). Although ADAM28 is expressed and synthesized in a precursor form (proADAM28) by lymphocytes and some cancer cells, its activation mechanism and substrates remain unclear. Here, we report that proADAM28s of 65 kDa is processed with active matrix metalloproteinase-7 (MMP-7) to 42- and 40-kDa forms which corresponds to active ADAM28s without propeptide. Processed ADAM28s digested insulin-like growth factor binding protein-3 (IGFBP-3) in both free and complex forms with IGF-I or IGF-II, and the digestion was prevented with EDTA, 1,10-phenanthroline, KB-R7785, tissue inhibitor of metalloproteinases-3 (TIMP-3), and TIMP-4. These data provide the first evidence that proADAM28s is activated by MMP-7 and ADAM28 digests IGFBP-3. (C) 2004 Elsevier Inc. All rights reserved.