Abstract
The activation of the NF-kappaB pathway by pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNFalpha), can be an important contributor for the re-programming of chondrocyte gene expression, thereby making it a therapeutic target in articular diseases. To search for new approaches to limit cartilage damage, we investigated the requirement of polyamines for NF-kappaB activation by TNFalpha in human C-28/I2 chondrocytes, using alpha-difluoromethylornithine (DFMO), a specific polyamine biosynthesis inhibitor. The NF-kappaB pathway was dissected by using pharmacological inhibitors or by expressing a transdominant IkappaBalpha super repressor. Treatment of C-28/I2 chondrocytes with TNFalpha resulted in a rapid enhancement of nuclear localization and DNA binding activity of the p65 NF-kappaB subunit. TNFalpha also increased the level and extracellular release of interleukin-8 (IL-8), a CXC chemokine that can have a role in arthritis, in an NF-kappaB-dependent manner. Pre-treatment of chondrocytes with DFMO, while causing polyamine depletion, significantly reduced NF-kappaB DNA binding activity. Moreover, DFMO also decreased IL-8 production without affecting cellular viability. Restoration of polyamine levels by the co-addition of putrescine circumvented the inhibitory effects of DFMO. Our results show that the intracellular depletion of polyamines inhibits the response of chondrocytes to TNFalpha by interfering with the DNA binding activity of NF-kappaB. This suggests that a pharmacological and/or genetic approach to deplete the polyamine pool in chondrocytes may represent a useful way to reduce NF-kappaB activation by inflammatory cytokines in arthritis without provoking chondrocyte apoptosis.