To investigate the expression of tumour necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) receptors in cultured synovial fibroblasts from rheumatoid arthritis (RA) and osteoarthritis (OA) patients, and to examine their susceptibility to TRAIL-induced apoptosis in the presence or absence of metabolic inhibitors.
The expression of TRAIL receptors in synovial fibroblasts was examined by Western blot and immunohistochemistry. Expression of TRAIL-receptor 1 (TRAIL-R1), FLICE-inhibitory protein (Fas-associating protein with death domain-like interleukin-1-converting enzyme), and Bcl-2 was assessed by Western blot. Synovial cell viability was measured by 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide assay (XTT), and apoptosis was determined both by DNA content analysis after propidium iodide staining and Annexin V stain.
TRAIL-R1 was constitutively expressed on cultured synovial fibroblasts from RA and OA, however, expression of TRAIL-R2 and TRAIL-R3 was not observed by immunohistochemistry and Western blot. Cultured synovial fibroblasts were resistant to apoptosis by TRAIL alone, but combined treatment of TRAIL with actinomycin D (ActD: 200 ng/mL), cycloheximide (CHX: 10 microg/mL), or proteasome inhibitor (MG132: 20 microM) induced apoptosis in a dose-dependent manner. The apoptosis was completely or partially inhibited by various caspase inhibitors, implicating an involvement of caspase pathway in TRAIL-induced apoptosis in the presence of these metabolic inhibitors. Expression of TRAIL-R1, FLIPL, and Bcl-2 did not account for the apoptosis by the combined treatment of TRAIL with ActD.
Although TRAIL-R1 was constitutively expressed; cultured synovial fibroblasts were resistant to apoptosis by TRAIL. ActD, CHX, and MG132 rendered cultured synovial fibroblasts susceptible to TRAIL-induced apoptosis by a caspase-dependent mechanism. However, the exact mechanism of sensitization by these metabolic inhibitors remains to be determined.