Matrix metalloproteinases degrade insulin-like growth factor-binding protein-3 in dermal fibroblast culture

Insulin-like growth factor binding protein-3 (IGFBP-3) is degraded by a Zn²⁺-dependent protease(s) produced by human dermal fibroblasts in vitro (Fowlkes, J. (1994) Endocrine J. 2, 63-68). Initial studies using IGFBP-3-substrate zymography identified several IGFBP-3-degrading proteases with M_r 52,000-72,000, which were inhibitable by EDTA and were shifted to lower M_r species after treatment of conditioned medium with an organomercurial, suggesting that they might represent one or more of the matrix metalloproteinases (MMPs). Immunoblotting of conditioned medium demonstrated the presence of proMMP-1 (52 and 55 kDa), proMMP-3 (58 and 60 kDa), and proMMP-2 (72 kDa) whose molecular masses corresponded identically to those of the IGFBP-3-degrading proteases. Degradation of recombinant human (rh) IGFBP-3 by conditioned media was blocked (>80% inhibition) by tissue inhibitor of metalloproteinases-1, a specific inhibitor of all MMPs, while removal of MMPs -1, -2, and -3 from conditioned medium by sequential immunoaffinity and gelatin-Sepharose chromatography resulted in the complete loss of IGFBP-3-degrading proteinase activity. Furthermore, human MMP-1, MMP-3, and to a lesser extent MMP-2 degraded rhIGFBP-3 ire vitro. Sequence analysis of rhIGFBP-3 cleavage sites produced by MMP-1, -2, or -3 demonstrated that each cleaved within the mid-region of the binding protein, a domain with little or no homology with the other five cloned IGFBPs. These studies suggest that MMPs, beyond their previously described functions as extracellular degrading enzymes, may also exert effects on cellular growth and proliferation via degradation of IGFBP-3, thus enhancing IGF bioavailability.