SPARC/osteonectin induces matrix metalloproteinase 2 activation in human breast cancer cell lines

Activation of the matrix metalloproteinase 2 (MMP-2) has been shown to play a major role in the proteolysis of extracellular matrix (ECM) associated w ith tumor invasion. Although the precise mechanism of this activation remai ns elusive, levels of the membrane type 1-MMP (MT1-MMP) at the cell surface and of the tissue inhibitor of MMP-2 (TIMP-2) appear to be two important d eterminants. Induction of MMP-2 activation in cells cultivated on collagen type I gels indicated that the ECM is important in the regulation of this p rocess. In this study, we show that SPARC/osteonectin, a small ECM-associat ed matricellular glycoprotein, can induce MMP-2 activation in two invasive breast cancer cell lines (MDA-MB-231 and BT549) but not in a noninvasive co unterpart (MCF-7), which lacks MT1-MMP. Using a set of peptides from differ ent regions of SPARC, we found that peptide 1.1 (corresponding to the NH2-t erminal region of the protein) contained the activity that induced MMP-2 ac tivation. Despite the requirement for MT1-MMP, seen in MCF-7 cells transfec ted with MT1-MMP, the activation of MMP-2 by SPARC peptide 1.1 was not asso ciated with increased steady-state levels of MT1-MMP mRNA or protein in eit her MT1-MMP-transfected MCF-7 cells or constitutively expressing MDA-MB-231 and BT549 cells. We did, however, detect decreased levels of TIMP-2 protei n in the media of cells incubated with peptide 1.1 or recombinant SPARC; th us, the induction of MMP-2 activation by SPARC might be due in part to a di minution of TIMP-2 protein. We conclude that SPARC, and specifically its NH 2-terminal domain, regulates the activation of MMP-2 at the cell surface an d is therefore likely to contribute to the proteolytic pathways associated with tumor invasion.