Tissue hypoxia in inflammation induces the upregulation of a gene program associated with angiogenesis, glycolysis and adaptation to pH via the hypoxia-inducible transcription factors (HIFs). In this study we investigate the relationship between synovial tissue oxygen levels in inflammatory arthritis and tissue expression of HIF-1α and vascular endothelial growth factor (VEGF).
Novel silver microelectrode technology was used to measure synovial tissue oxygen levels under direct vision during arthroscopy in six patients with inflammatory arthritis of the knee. A sterile silver needle electrode comprising a teflon-insulated silver wire was embedded in a 19 G hypodermic needle of 10 cm length and introduced to the intra-articular space via a medial suprapatellar approach such that the electrode tip could be directly visualised by the arthro-scope introduced from a standard lateral infrapatellar approach. The voltage applied was adjusted to give a diffusion-limited current for the reduction of oxygen. Synovial biopsies were taken under direct vision from exactly the point at which tissue oxygen levels were measured and snap-frozen immediately. HIF-1α, HIF-2α and VEGF were detected by immunohistochemistry in fixed synovial biopsy sections and the immunopositive area fraction was determined by quantitative digital image analysis.
The mean and standard deviation for oxygen tension in the synovial tissue of the joints sampled was 15.4 ± 9.6 mmHg, range 24.4 mmHg. The mean and standard deviation for immunopositive area fraction for HIF-1α was 0.64 ± 0.39% and for HIF-2α was 4.9 ± 6.7%. HIF-1α and HIF-2α staining were detected predominantly in the synovial lining layer. There was a significant, inverse relationship between synovial tissue oxygen measurement in vivo and tissue expression of HIF-1 (Pearson r = -0.86, P = 0.03) and similarly between synovial tissue oxygen and tissue expression of VEGF (Pearson r = -0.89, P = 0.02). There was a positive, highly significant correlation between synovial expression of HIF-1α and VEGF (Pearson r = 0.97, P = 0.0005).
In this study we directly demonstrate that intra-articular is pO2 markedly reduced in inflammatory arthritis. Furthermore, in the same patients, synovial tissue oxygen tensions are inversely proportional to HIF-1α and VEGF expression, suggesting that this environment contributes to the perpetuation of inflammation and associated tissue destruction.