Rheumatoid arthritis (RA) is characterized by hyperplasia of synovial lining tissue, which is involved directly in the damage of cartilage and bone. One of the factors thought to contribute to this synovial lining hyperplasia is dysregulation of, or functional abnormality in, the tumor suppressor protein p53. The protein known as murine double-minute protein 2 (MDM2) is the major negative regulator of p53, and in tumors contributes to increased cell proliferation. The detection of MDM2 in rheumatoid synovium has not previously been described. We investigated whether this protein is detectable in cells and tissues derived from patients with RA.
Expression of MDM2 protein was examined in fibroblast-like synoviocytes (FLS) by methods including permeabilization flow cytometry, immunofluorescence, and Western blotting, and in synovial tissues using immunohistochemistry. The proliferative capacity of these cells was also examined using 3H/thymidine incorporation. Cell cycle analysis was performed by propidium iodide incorporation.
MDM2 was detected in RA FLS and synovial tissues. MDM2 protein was identified in CD14-positive and CD14-negative synovial lining cells and CD14-positive sublining cells. RA FLS exhibited faster proliferative rates and higher levels of MDM2 expression than FLS derived from patients with osteoarthritis (OA). Both OA and RA FLS were found to be in similar phases of the cell cycle at the time of MDM2 protein analysis.
The abundant expression of MDM2 in RA may be a contributing factor to the hypoapoptotic phenotype of lining tissue through its capacity to downregulate p53 levels and effects. Further studies are required to determine the relationship between this cell-cycle protein profile, tissue hyperplasia, and the functional abnormality of p53 in RA.