MMP9 and breast neoplasm: We suggest that IL-8 and/or PTHrP produced by inflammatory cells or osteoblasts stimulate secretion of MMP-13 by breast tumour cells; MMP-13 then indirectly induces OC differentiation by activating pro-MMP-9 that, together with MMP-13 itself, could contribute to cleave the osteoclastogenesis inhibitor galectin-3, and cooperates with MMP-9 to directly degrade bone matrix (Figure 8).