We have recently reported that osteopontin (OPN) stimulates tumor growth and activation of promatrix metalloproteinase-2 (pro-MMP-2) through nuclear factor kappaB (NFkappaB)-mediated induction of membrane type 1 matrix metalloproteinase (MT1-MMP) in murine melanoma cells (Philip, S., Bulbule, A., and Kundu, G. C. (2001) J. BioL Chem. 276, 44926-44935). However, the molecular mechanism by which OPN activates NFkappaB and regulates pro-MMP-2 activation in murine melanoma (B16F10) cells is not well defined. We also investigated the mechanism of action of curcumin (diferulolylmethane) on OPN-induced NFkappaB-mediated activation of pro-MMP-2 in B16F10 cells. Here we report that OPN induces phosphorylation and degradation of the inhibitor of nuclear factor kappaB (IkappaBalpha) by inducing the activity of IkappaB kinase (IKK) in these cells. OPN also induces the nuclear accumulation of NFkappaB p65, NFkappaB-DNA binding, and transactivation. However, curcumin a known antiinflammatory and anticarcinogenic agent suppressed OPN-induced IkappaBalpha phosphorylation and degradation by inhibiting the IKK activity. Moreover, our data revealed that curcumin inhibited the OPN-induced translocation of p65, NFkappaB-DNA binding, and NFkappaB transcriptional activity. The OPN-induced pro-MMP-2 activation and MT1-MMP expression were also drastically reduced by curcumin. Curcumin also inhibited OPN-induced cell proliferation, cell migration, extracellular matrix invasion, and synergistically induced apoptotic morphology with OPN in these cells. Most importantly, curcumin suppressed the OPN-induced tumor growth in nude mice, and the levels of pro-MMP-2 expression and activation in OPN-induced tumor were inhibited by curcumin. To our knowledge, this is the first report that OPN induces NFkappaB activity through phosphorylation and degradation of lkappaBalpha by activating IKK that ultimately triggers the activation of pro-MMP-2 and further demonstrates that curcumin potently suppresses OPN-induced cell migration, tumor growth, and NFkappaB-mediated pro-MMP-2 activation by blocking the IKK/IkappaBalpha signaling pathways.