Objectives: To investigate the effect of poly(ADP-ribose) polymerase 1 (PARP-1) suppression on CD95/Apo-1 (Fas)-induced apoptosis in fibroblast-like synoviocytes (FLS) from RA patients.
Methods: Apoptosis, determined by Hoechst staining and quantification of nucleosomal release by ELISA, was induced by stimulation with anti-Fas antibody with or without pre-treatment with cycloheximide (CHX). PARP-1 and poly(ADP-ribose) glycohydrolase (PARG) were suppressed in RA FLS by small interfering RNA (siRNA) transfection. Fas-associated via death domain (FADD), pro-caspase-8, Fas, c-Fas-associated death domain-like IL-1b-converting enzyme-inhibitory protein (FLIP) expression, and AKT and GSK phosphorylation were analysed by western blot.
Results: PARP-1-deficient FLS showed significantly lower apoptosis than non-transfected and control siRNA-transfected FLS. The expression of death-inducing signaling complex (DISC) components such as Fas, FADD and pro-caspase-8 was not modified by PARP-1 suppression; however, FLS lacking PARP-1 showed high activation of the Akt-GSK survival pathway and up-regulation of the c-FLIP-S isoform after Fas triggering. Inhibition of PI3K/Akt pathway did not modify the difference between PARP-1-competent or -deficient FLS in Fas-mediated apoptosis or c-FLIP-S expression. Poly(ADP-ribose) accumulation induced by PARG supression did not modify the apoptotic response.
Conclusion: PARP-1 deficiency increases the resistance of RA FLS to Fas-induced apoptosis through activation of the Akt-GSK survival pathway and up-regulation of c-FLIP-S isoform.