ARTHRITIS & RHEUMATISM Vol. 46, No. 4, April 2002, pp 968–975 DOI 10.1002/art.10213 © 2002, American College of Rheumatology Influence of Hypoxia and Reoxygenation on Cytokine-Induced Production of Proinflammatory Mediators in Articular Cartilage Julie Cernanec, 1 Farshid Guilak, 1 J. Brice Weinberg, 2 David S. Pisetsky, 2 and Beverley Fermor 1 Objective. Articular cartilage is an avascular tis- sue that functions at a lower oxygen tension than do most tissues. With mobilization, arthritic joints may undergo cycles of hypoxia and reoxygenation. The goal of this study was to determine the effects of hypoxia and reoxygenation on cytokine-induced nitric oxide (NO) and prostaglandin E 2 (PGE 2 ) production in articular cartilage. Methods. Porcine cartilage explants were incu- bated at 37°C for 72 hours in either 1% O 2 (hypoxia) or 20% O 2 (normoxia) in media supplemented with interleukin-1 (IL-1) or tumor necrosis factor  (TNF), with or without the NO synthase 2 (NOS2) selective inhibitor 1400W. Culture media were then removed and replaced with freshly prepared media and incubated for a further 24 hours in normoxia. Results. NO levels were significantly higher in explants supplemented with IL-1 and TNF compared with controls, in both hypoxia and normoxia. Compared with normoxia, hypoxia decreased IL-1– and TNF- induced NO production significantly. Reoxygenation of hypoxic explants resulted in sustained significant NO production in response to either cytokine. However, comparably high levels of NO production were not sustained in explants cultured continuously in nor- moxia. Although IL-1 alone did not significantly in- crease PGE 2 production, significant PGE 2 superinduc- tion occurred in cartilage stimulated with IL-1 and the NOS2 inhibitor 1400W compared with stimulation with IL-1 alone in hypoxia, but not in normoxia. Conclusion. Oxygen tension significantly affects cytokine-induced proinflammatory mediator production in articular cartilage. Furthermore, hypoxia alters NO mediation of PGE 2 production. Hypoxia and reoxygen- ation can affect cytokine-induced proinflammatory me- diator production, suggesting that oxygen tension may influence inflammation associated with cartilage injury and disease. Articular cartilage is an avascular tissue that functions at an oxygen tension that is lower than that of most tissues (1). Because oxygen and other nutrients must diffuse into the tissue from the synovial fluid surrounding the joint, an oxygen gradient is created in cartilage. In the normal joint, the superficial zones of the tissue exist at 6% O 2 (45.6 mm Hg) and the deep zones exist at 1% O 2 (1). Furthermore, in the setting of diseases such as rheumatoid arthritis (RA) and osteoar- thritis (OA), a further decrease in synovial fluid oxygen tension may occur (2) because of decreased capillary density and deep placement of capillaries within the synovium (3). Hypoxia may therefore represent an im- portant feature of the joint microenvironment. With mobilization, arthritic joints may be ex- posed to hypoxia–reoxygenation events (4). Short-term exercise results in increased lactate production and increased levels of pCO 2 , suggesting that oxygen levels in the synovial fluid decrease as pressure is applied to the arthritic joint (5). At the start of motion, capillaries flatten as intraarticular pressure increases, decreasing blood flow and limiting the delivery of oxygen to the joint. Capillaries return to their open configuration as intraarticular pressure returns to normal levels. The synovium is then reperfused with blood, allowing the Supported by NIH grants AR-39162, AR-43876, and AG- 15768 and by the Veterans Affairs Medical Research Service, Duke University Undergraduate Research Support, and the Howard Hughes Foundation. 1 Julie Cernanec, BS, Farshid Guilak, PhD, Beverley Fermor, PhD: Duke University Medical Center, Durham, North Carolina; 2 J. Brice Weinberg, MD, David S. Pisetsky, MD, PhD: VAMC and Duke University Medical Center, Durham, North Carolina. Address correspondence and reprint requests to Farshid Guilak, PhD, Orthopaedic Research Laboratories, 375 MSRB, Box 3093, Duke University Medical Center, Durham, NC 27710. E-mail: guilak@duke.edu. Submitted for publication June 15, 2001; accepted in revised form December 5, 2001. 968