Mechanisms of resistance against Fas-induced apoptosis in rheumatoid arthritis synovial fibroblasts

The activation of synovial fibroblasts (SF) is a hallmark of rheumatoid arthritis, and the resistance of rheumatoid arthritis (RA)-SF against Fas-induced cell death is one of their characteristic features. However, the mechanisms that are responsible for the low susceptibility of RA-SF to programmed cell death and, specifically, the pathways that link altered apoptosis to their aggressive behavior are only incompletely understood. In our present studies, we have compared the susceptibility of synovial fibroblasts from RA and osteoarthritis (OA) patients to Fas-induced apoptosis, and have studied mechanisms that contribute to both the invasiveness of RA-SF and their resistance against apoptosis.

As determined by different techniques (measurement of cytoplasmatic mononucleosomes and oligonucleosomes, FACS analysis), RA-SF were significantly less susceptible to Fas-induced cell death than OA-SF despite their abundant expression of Fas. Stimulation of RA-SF with tumor necrosis factor alpha (TNF-α) did not induce apoptosis, but in a dose-dependent manner reduced Fas-mediated cell death. This was accompanied by the activation of NF-κB and the upregulation of disease-relevant matrix metalloproteinases. Of interest, adenoviral gene transfer of TIMP-3 reduced the invasiveness of RA-SF in the SCID-mouse in vivo model of RA, and completely reversed the apoptosis-inhibiting effect of TNF-α, in part by reducing the activation of NF-κB. The effects of TNF-α on apoptosis were similar in OA-SF. However, RA-SF differed from OA-SF by their elevated expression of the small ubiquitin-like modifier sentrin-1/SUMO-1. Retroviral gene transfer of antisense and expression constructs of sentrin-1/SUMO-1 confirmed its involvement in the altered susceptibility of RA-SF to apoptosis. Analysis of the subcellular localization of sentrin-1/SUMO-1 and gene transfer of SUMO-1-specific proteases revealed that modifications of transcriptionally active nuclear proteins by sentrin-1/SUMO-1 contribute to the regulation of apoptosis and the production of matrix-degrading enzymes.

Collectively, our data suggest that in RA-SF there is a close functional association between pathways that confer the resistance against apoptosis and that mediate the progressive destruction of cartilage. Both cytokine-dependent and cytokine-independent mechanisms contribute to these processes. While TNF-α-mediated prevention of cell death is not specific for RA and is seen in a variety of fibroblast-like cells, activation of signaling pathways involving sentrin-1/SUMO-1 appears to be a characteristic feature of RA-SF.