Hypoxia, RONS and energy metabolism in articular cartilage

Objective: Increased pro-inflammatory cytokines and reactive oxygen and nitrogen species (RUNS) occur in osteoarthritis (OA). Oxygen tension can alter the levels of RUNS induced by interleukin-1 (IL-1). RUNS such as nitric oxide (NO) can alter energy metabolism. The aim of this study was to determine if oxygen tension alters energy metabolism, in articular cartilage, in response to IL-1 or NO and to determine if cell death occurred. Design: Porcine articular chondrocytes were incubated with IL-1 or the NO donor NOC-18 for 48 h in either 1, 5 or 20% O-2. Adenosine triphosphate (ATP) levels were measured and immunoblots for adenosine monophosphate-activated protein kinase (AMPK) were done. Protein translation was measured by S6 activation. Senescence and autophagy were determined by increased caveolin or conversion of LC3-I to LC3-II respectively. Results: One percent O-2 significantly reduced ATP levels compared with 20% O-2. Five percent O-2 significantly increased ATP levels compared with 20% O-2. One percent O-2 significantly increased phospho-AMPK (pAMPK) protein expression compared with 5 or 20% O-2. Oxygen tension had no effects on pS6, caveolin or LC3-II levels. IL-1-induced NO production was significantly reduced with decreased oxygen tension, and significantly reduced ATP levels at all oxygen tensions, but pAMPK was only significantly increased at 5% O-2. IL-1 significantly reduced p56 at all oxygen tensions. IL-1 had no effects on caveolin and significantly increased LC3-II at 20% O-2 only. NOC-18 significantly reduced ATP levels at all oxygen tensions, and significantly increased pAMPK at 5% O-2 only, and significantly decreased pAMPK at 1% O-2. NOC-18 significantly reduced pS6 at 1% O-2 and significantly increased caveolin at 5% O-2, and LO-II at 1% O-2. Conclusion: Our data suggest 5% O-2 is optimal for energy metabolism and protective to some effects of IL-1 and NO. NO has the greatest effects on ATP levels and the induction of autophagy at 1% O-2. (C) 2010 Osteoarthritis Research Society International. Published by Elsevier Ltd. All rights reserved.