FLS and MLS at the cartilage-pannus junction of RA patients contribute most to the joint destruction and they produce a great amount of MMPs. These enzymes act in a synergistic manner to destroy connective tissue components [24
]. The combination of MMPs is capable of degrading all essential extracellular matrix components of the synovial membrane, articular cartilage and subchondral bone. Imbalanced activity between MMPs and tissue inhibitors of metalloproteinases caused by enhanced expression of CD147 eventually leads to joint destruction in RA [26
CD147 is more highly expressed on human carcinoma cells than on normal cells and its expression correlates with the MMP expression level and the cancer invasive potential [29
]. The aggressive characteristics of RA synovium are similar to those of neoplastic tissues [33
]. The proliferating mass of synoviocytes locally invades the cartilage and bone, destroying the joint. MMPs, mainly produced in FLS, play a central role in arthritic joint destruction. Beyond the proof that the expressions of MMP-2 and MMP-9 as well as the invasive potential of RA FLS are higher than those of OA FLS, we have also found in this study, by cell isolation and culture, that the expression of CD147 on RA FLS is higher than that on OA FLS.
The monocyte/macrophage lineage, especially MLS, is also known to play an important role in RA pathogenesis. In this study, we found that expressions of CD147, MMP-2 and MMP-9 of differentiated THP-1 cells or monocytes/macrophages from synovial fluid of RA patients are higher than those of undifferentiated THP-1 cells or monocytes from peripheral blood of healthy humans. PMA might be a main upregulative factor that enhances expression of CD147 on the differentiated THP-1 cells. Some cytokines in the synovial fluid of RA patients, such as interferon gamma and granulocyte–macrophage colony-stimulating factor, may also enhance the expression of CD147 on the monocytes/macrophages from synovial fluid of RA patients. The overexpression of CD147 on differentiated monocytes/macrophages suggests that CD147 may be important in the early phases of direct cell migration and of both autocrine and paracrine stimulation of MMP expression. However, further work is needed to test this supposition.
In our previous study, we found that CD147 was expressed predominantly on the MLS and FLS in the lining and sublining layers of RA synovium, and macrophages acted as the amplifier of the pathogenetic cascade in RA via the increase of MMP production by interacting macrophages with fibroblasts [10
]. To explore the effect of cell–cell interaction on the production of MMPs and the invasiveness of RA synoviocytes, RA FLS were co-cultured with THP-1 cells in the present study. We have found that the expression of CD147 on RA FLS is enhanced by cell–cell interaction, and that the secretion and activation of MMPs and the invasive potential of RA FLS are also enhanced. These results suggest that the overexpression of CD147 on macrophages may accelerate the production of MMPs and the invasive ability of RA FLS by cell–cell interaction. The overexpression of CD147 on FLS and the monocyte/macrophage lineage, especially on MLS, suggests that CD147 may be important in both autocrine and paracrine stimulation of MMPs. From the findings that homophilic CD147 binding has occurred in the context of both heterotypic and homotypic cell–cell interactions and that CD147 can be a receptor in itself to induce MMP production, not only in primary fibroblast cells but also in tumor cells themselves [34
], we presume that the increased expression of CD147 on FLS and the monocyte/macrophage lineage, especially on MLS, in RA synovium could possibly induce MMP production through interaction.
To further confirm the effect of cell–cell interactions in rheumatic joints, monocytes/macrophages from synovial fluid of RA patients and from peripheral blood of healthy humans were co-cultured with RA FLS in this study. The environments of co-cultured RA FLS and monocytes/macrophages from synovial fluid of RA patients or from peripheral blood of healthy humans are more like the real environment in vivo than that of co-cultured RA FLS and THP-1 cells. The results from this co-culture were consistent with those of RA FLS co-cultured with THP-1 cells, indicating that the overexpression of CD147 induces elevated levels of MMPs and their activated forms in RA FLS – and the elevated levels of MMPs in turn enhance the invasive ability of RA FLS.
In this study, CD147 antagonistic peptide was added in the condition medium of co-cultured cells. We have found that the addition of CD147 antagonistic peptide has some inhibitory effect not only on the MMP production, but also on the invasive potential of the co-cultured FLS. The inhibition of MMP-2 and MMP-9 production by CD147 antagonistic peptide indicates that CD147 may induce MMP-2 and MMP-9 production. The mechanism by which CD147 regulates MMP-2 and MMP-9 production is largely unclear, although a recent study has suggested that the mitogen-activated protein kinase p38 pathway may be involved during MMP induction in dermal fibroblasts [35
]. Our previous studies have indicated that overexpression of HAb18G/CD147 enhances metastatic potentials in human hepatoma cells by disrupting the regulation of store-operated Ca2+
entry by nitric oxide/cGMP. CD147 is required to mediate the effect of HAb18G/CD147 on the secretion and activation of MMPs and metastasis-related processes in human hepatoma cells by disrupting the regulation of nitric oxide/cGMP-sensitive intracellular Ca2+
]. These findings also indicate that CD147 is involved in promoting the secretion and activation of MMPs, which in turn increases the invasive potential of FLS. These results also suggest that CD147 expression may be correlated to MMP (MMP-2, MMP-9) secretion, activation and invasive potential in RA FLS.
Very low levels of CD147 have been reported in most normal adult tissues, including the epidermis, retinal pigment epithelium, and breast lobules and ductules. CD147 may therefore play a physiologic role in tissue remodeling by inducing MMPs [36
]. A correlation between CD147 and progressive joint destruction in RA is suggested in this study based on the findings that the overexpression of CD147 on monocytes can facilitate enhancement of the production of MMPs and the invasion ability of FLS, and that CD147 antagonistic peptide can block the enhancement.