Polyamine depletion inhibits apoptosis following blocking of survival pathways in human chondrocytes stimulated by tumor necrosis factor-alpha

Chondrocyte apoptosis can bean important contributor to cartilage degeneration, thereby making it a potential therapeutic target in articular diseases. To search for new approaches to limit chondrocytic cell death, we investigated the requirement of polyamines for apoptosis favored by tumor necrosis factor-alpha(TNF), using specific polyamine biosynthesis inhibitors in human chondrocytes. The combined treatment of C-28/12 chondrocytes with TNF and cycloheximide (CHX) resulted in a prompt effector caspase activation and internucleosomal DNA fragmentation. Pre-treatment of chondrocytes with alpha-difluoromethylornithine (DFMO), an ornithine decarboxylase (C)DC) inhibitor, markedly reduced putrescine and spermidine content as well as the caspase-3 activation and DNA fragmentation induced by TNF and CHX. DFMO treatment also inhibited the increase in effector caspase activity provoked by TNF plus MG132, a proteasome inhibitor. DFMO decreased caspase-8 activity and procaspase-8 content, an apical caspase essential for TNF-induced apoptosis. Although DFMO increased the amount of active, phosphorylated Akt, inhibitors of the Akt pathway failed to restore the TNF-induced increase in caspase activity blunted by DFMO. DFMO also reduced the increase in caspase activity induced by staurosporine, but in this case Akt inhibition prevented the DFMO effect. Pre-treatment with CGP48664, an S-adenosylmethionine decarboxylase (SAMDC) inhibitor markedly reduced spermidine and spermine levels, and provoked effects similar to those caused by DFMO. Finally DFMO was effective even in primary osteoarthritis (OA) chondrocyte cultures. These results suggest that the intracellular depletion of polyamines in chondrocytes can inhibit both the death receptor pathway by reducing the level of procaspase-8, and the apoptotic mitochondrial pathway by activating Akt.