Disease-Associated Matrix Fragment Suppresses Osteoblastic Differentiation by Inducing MMP-1

Objectives: Fibronectin (FN) fragments found in chronic inflammatory diseases, including periodontal disease and arthritis, may contribute to tissue destruction in part via induction of matrix metalloproteases (MMPs). We previously showed that the 120-kDa FN fragment containing the cell binding domain, dose-dependently induces MMP-1 in human periodontal ligament (PDL) cells, whereas intact FN did not elicit this response. Recently, we found that increased MMP-1 expression correlates with a decreased osteoblastic phenotype in PDL cells. We therefore hypothesized that the 120-kDa FN fragment inhibits osteoblastic differentiation of PDL cells by inducing MMPs.

Methods: The effects of increasing concentrations of the 120-kDa FN fragment on expression of collagenase and osteoblastic markers in the absence or presence of MMP-1 siRNA was assessed in cultured PDL cells using Western blots, qRT-PCR, collagen degradative and alkaline phosphatase (AP) activity assays.

Results: The 120-kDa fragment produced a dose-dependent increase in collagenase activity concomitant with a decrease in AP activity. The increase in collagenase activity was largely attributed to increased MMP-1 expression. Concurrent with the decrease in AP activity, the 120-kDa fragment reduced expression of specific osteoblastic markers, including Runx2 and osteonectin mRNAs. Treatment of cells with dexamethasone, which is a known inhibitor of MMP-1 expression, rescued the 120-kDa FN fragment-mediated decrease in AP activity and of osteoblastic marker levels. Finally, cells treated with FN fragment in which MMP-1 expression was inhibited by siRNA transfection showed reduced collagenase expression levels that were accompanied by a rescue in AP activity to baseline levels.

Conclusion: These findings suggest that in addition to their direct effects on tissue destruction by upregulation of MMPs, disease-associated FN fragments may contribute to periodontal disease progression by impeding osteoblast differentiation, thereby decreasing the available pool of bone reparative cells.

Supported by NIH/NIDCR ROI DE16671 to SK