Basic Calcium Phosphate Crystals Induce Matrix Metalloproteinase-1 through the Ras/Mitogen-activated Protein Kinase/c-Fos/AP-1/Metalloproteinase 1 Pathway

Synovial fluid basic calcium phosphate (BCP) crystals are common in osteoarthritis and are associated with severe degenerative arthropathy. Besides stimulating synovial fibroblast-like cells to proliferate, BCP crystals are a potent inducer of human matrix metalloproteinases (hMMPs), which can speed up the articular joint tissue degeneration of osteoarthritis patients. Here, we report that transfections with hMMP1 luciferase reporter plasmids in fibroblast-like synoviocytes revealed that the induction of hMMP1 promoter by BCP crystals was mainly mediated through the −72AP-1 element. Elimination of the −72AP-1 element either by mutation or deletion abolished the induction of hMMP1 promoter activity by BCP crystals almost completely. Interestingly, a mutation at the −88PEA-3 site also abolished the induction of hMMP1 promoter. Further mutation at the −181AP-1 site resumed the induction, indicating that the −181AP-1 element had an effect opposite to the −72AP-1 element. The effect of −181AP-1 could be inactivated either by a mutation at this −181AP-1 site or by the −88PEA-3 element. In addition, dominant negative Ras, Raf, and MEK1/2 could block the induction of hMMP1, and a MEK1/2-specific inhibitor (UO126) could block the induction of hMMP1 and c-Fos by BCP crystals. Taken together, these data indicate that multiple elements, including at least AP-1 and PEA-3, are involved in the induction of hMMP1 gene expression by BCP crystals and that the induction follows the Ras/MAPK/c-Fos/AP-1/MMP1 signaling pathway.