Modulation of Ten-Eleven Translocation 1 (TET1), Isocitrate Dehydrogenase (IDH) Expression, α-Ketoglutarate (α-KG), and DNA Hydroxymethylation Levels by Interleukin-1β in Primary Human Chondrocytes

Background: Cytosine hydroxymethylation in the genomic DNA controls gene expression. Results: Treatment of primary chondrocytes with IL-1 alone or in conjunction with TNF- inhibited TET1 and IDH expression and suppressed cytosine hydroxymethylation. Conclusion: IL-1 and TNF- decrease DNA hydroxymethylation by suppressing the expression and activity of TET1 and IDH. Significance: Pro-inflammatory cytokines likely control gene expression by modulating the 5-hmC levels. 5-Hydroxymethylcytosine (5-hmC) generated by ten-eleven translocation 1-3 (TET1-3) enzymes is an epigenetic mark present in many tissues with different degrees of abundance. IL-1 and TNF- are the two major cytokines present in arthritic joints that modulate the expression of many genes associated with cartilage degradation in osteoarthritis. In the present study, we investigated the global 5-hmC content, the effects of IL-1 and TNF- on 5-hmC content, and the expression and activity of TETs and isocitrate dehydrogenases in primary human chondrocytes. The global 5-hmC content was found to be approximate to 0.1% of the total genome. There was a significant decrease in the levels of 5-hmC and the TET enzyme activity upon treatment of chondrocytes with IL-1 alone or in combination with TNF-. We observed a dramatic (10-20-fold) decrease in the levels of TET1 mRNA expression and a small increase (2-3-fold) in TET3 expression in chondrocytes stimulated with IL-1 and TNF-. IL-1 and TNF- significantly suppressed the activity and expression of IDHs, which correlated with the reduced -ketoglutarate levels. Whole genome profiling showed an erasure effect of IL-1 and TNF-, resulting in a significant decrease in hydroxymethylation in a myriad of genes including many genes that are important in chondrocyte physiology. Our data demonstrate that DNA hydroxymethylation is modulated by pro-inflammatory cytokines via suppression of the cytosine hydroxymethylation machinery. These data point to new mechanisms of epigenetic control of gene expression by pro-inflammatory cytokines in human chondrocytes.