At sites of chronic
inflammation, such as in the inflamed rheumatoid joint, activated neutrophils release
hydrogen peroxide (H(2)O(2)) and the
enzyme myeloperoxidase to catalyse the formation of
hypochlorous acid (HOCl).
3-chlorotyrosine, a marker of HOCl in vivo, has been observed in synovial fluid
proteins from
rheumatoid arthritis patients. However the mechanisms of HOCl-induced cytotxicity are unknown. We determined the molecular mechanisms by which HOCl induced cell death in human mesenchymal progenitor cells (MPCs) differentiated into a chondrocytic phenotype as a model of human cartilage cells and show that HOCl induced rapid Bax conformational change, mitochondrial permeability and release of intra-mitochondrial
pro-apoptotic proteins which resulted in nuclear translocation of AIF and EndoG.
siRNA-mediated knockdown of Bax substantially prevented mitochondrial permeability, release of intra-mitochondrial
pro-apoptotic proteins. Cell death was inhibited by
siRNA-mediated knockdown of Bax, AIF or EndoG. Although we observed several
biochemical markers of apoptosis,
caspase activation was not detected either by western blotting, fluorescence activity assays or by using
caspase inhibitors to inhibit cell death. This was further supported by findings that (1) in vitro exposure of recombinant human
caspases to HOCl caused significant inhibition of
caspase activity and (2) the addition of HOCl to
staurosporine-treated MPCs inhibited the activity of cellular
caspases. Our results show for the first time that HOCl induced Bax-dependent mitochondrial permeability which led to cell death without
caspase activity by processes involving AIF/EndoG-dependent pathways. Our study provides a novel insight into the potential mechanisms of cell death in the inflamed human joint.