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Background  Cartilage is an avascular and aneural tissue. Chondrocytes thrive in this restricted environment of low oxygen tension and poor nutrient availability which has led to suggestions that hypoxia may be a protective mechanism against the development of osteoarthritis (OA). There is also a growing body of evidence to support the role of epigenetic factors in the pathogenesis of OA. However, few studies have investigated the epigenetic-OA process within a hypoxic environment. The current study has investigated the effects of hypoxia on gene expression and DNA methylation of anabolic and catabolic genes involved in the pathogenesis of OA.    Methods  Chondrocytes extracted from OA femoral heads were incubated in normoxia and hypoxia (20% and 2% oxygen concentrations respectively). Interleukin 1-beta (IL-1β) plus oncostatin M (OSM), 5-azadeoxycytidine (5-aza-dC) or media alone (control) were added twice weekly to the incubated samples. After 5 weeks, levels of Collagen type IX ( COL9A1 ),  IL1B , and matrix metalloproteinase-13 ( MMP13 ) gene expression were measured using SYBR Green-based qRT-PCR and were correlated with methylation status analysed by pyrosequencing methodology.    Results  Hypoxia resulted in a >50-fold and >10-fold increase in relative expression of  COL9A1  and  IL1B  respectively. This was inversely correlated to the DNA methylation status of these genes. Expression of  MMP13  was reduced at 2% oxygen tension in control cells. Relative expression of  MMP13  increased in cells stimulated with IL-1β and 5-aza-dC in normoxic conditions, and this effect was eliminated at low oxygen tension although no correlation with methylation status was observed.    Conclusions  These findings demonstrate a role for hypoxia in the regulation of anabolic and catabolic gene expression and the influence of changes in DNA methylation. These results further support the role of epigenetics in OA and, critically, highlight the complex relationship between the physiological environment of cartilaginous cells and the osteoarthritic process with implications for therapeutic intervention and our understanding of OA pathophysiology.    Electronic supplementary material  The online version of this article (doi:10.1186/1471-2474-15-431) contains supplementary material, which is available to authorized users.

Effects of hypoxia on anabolic and catabolic gene expression and DNA methylation in OA chondrocytes