Rhein Inhibits Interleukin-1β-Induced Activation of MEK/ERK Pathway and DNA Binding of NF-κB and AP-1 in Chondrocytes Cultured in Hypoxia: A Potential Mechanism for Its Disease-Modifying Effect in Osteoarthritis

In the present report, we show that bovine articular chondrocytes cultured in low oxygen tension, i.e. in conditions mimicking their hypoxic in vivo environment, respond to IL-1β (10 ng/mL) by an increased DNA binding activity of NF-κB and AP-1 transcription factors. Incubation of the cells with 10−5 M rhein for 24 h was found to reduce this activity, particularly in the case of AP-1. Mitogen activated kinases (ERK-1 and ERK-2) were activated by exposure of the chondrocytes to 1-h treatment with IL-1ß. This effect was greater in hypoxia (3% O2) than in normoxia (21% O2). Rhein was capable of reducing the IL-1β-stimulated ERK1/ERK2 pathway whatever the tension of oxygen present in the environment. The level of c-jun protein, an element of AP-1 complex, was increased by exposure of the chondrocytes to IL-1β after 2, 6, and 24 h. Addition of rhein at 10−5 M for 24 h did not reduce the c-jun protein amount. The mRNA steady-state levels of collagen type II (COL2A1) and aggrecan core protein were found to be significantly increased by a 24-h treatment with 10−5 M rhein. This stimulating effect was also observed in the presence of IL-1β, suggesting that the drug could prevent or reduce the IL-1β-induced inhibition of extracellular matrix synthesis. IL-1-induced collagenase (MMP1) expression was significantly decreased by rhein in the same conditions. In conclusion, rhein can effectively inhibit the IL-1-activated MAPK pathway and the binding of NF-κB and AP-1 transcription factors, two key factors involved in the expression of several proinflammatory genes by chondrocytes. In addition, the drug can reduce the procatabolic effect of the cytokine, by reducing the MMP1 synthesis, and enhance the synthesis of matrix components, such as type II collagen and aggrecan. These results may explain the antiosteoarthritic properties of rhein and its disease-modifying effects on OA cartilage, in spite of absence of inhibition at prostaglandin level.