CD44 Is the Physiological Trigger of Fas Up-Regulation on Rheumatoid Synovial Cells

CD44 is a ubiquitous molecule known as a hyaluronan receptor. However, the relevance of CD44 to inflammatory processes, for example, rheumatoid synovitis, remains unclear. In this study, we propose a novel function for CD44 using synovial cells from rheumatoid arthritis (RA) patients and demonstrated that CD44 cross-linking augmented Fas expression and subsequent Fas-mediated apoptosis of the cells: 1) cross-linking of CD44 on RA synovial cells markedly augmented Fas expression and its mRNA transcription; 2) engagement of CD44 up-regulated Fas on the cells within 3 h, much more than IL-1beta and TNF-alpha did; 3) the Fas-mediated early apoptotic change of the cells was amplified by CD44 cross-linking; and 4) hyaluronan, especially when fragmented, also augmented Fas-mediated early apoptosis of the cells. Based on these findings, we postulate a new concept: that interaction of CD44 on RA synovial cells with hyaluronan fragments present in the surrounding extracellular matrix augments Fas expression as well as Fas-mediated apoptosis of synovial cells. This may lead to spontaneous growth arrest through Fas-Fas ligand pathway observed in synovial cells of RA synovitis in vivo.