We have previously shown that interleukin‐1β (IL‐1β) impairs transforming growth factor β (TGFβ) signaling through TGFβ receptor type II (TGFβRII) down‐regulation and Smad7 up‐regulation. This mechanism could account for the reduced responsiveness of osteoarthritic chondrocytes to TGFβ and the cartilage breakdown linked to this disease. The aim of this study was to investigate the molecular mechanism underlying the IL‐1β–induced stimulation of Smad7 in human articular chondrocytes.

Human articular chondrocytes were treated with IL‐1β in the presence of TGFβ1, pyrrolidine dithiocarbamate (a repressor of the NF‐κB pathway), or cycloheximide. Then, steady‐state messenger RNA and protein levels were estimated by real‐time reverse transcription–polymerase chain reaction and immunocytology. In addition, transient transfections of p65 expression vector or p65‐targeted short hairpin RNA were performed to define the effect of NF‐κB on Smad7 expression.

TGFβRII overexpression restored the TGFβ response of human articular chondrocytes. However, this effect was transient, implying that a secondary mechanism was responsible for the alteration of the TGFβ response with long‐term exposure to IL‐1β. Moreover, IL‐1β caused a late induction of the inhibitory Smad7. This effect was direct, since it did not require de novo synthesis. In addition, we established, by experiments with gain/loss of function, that the up‐regulation of Smad7 by IL‐1β is mediated through the NF‐κB pathway, especially the p65 subunit.

These findings clarify the regulatory process of IL‐1β on Smad7 expression. Understanding the molecular basis of IL‐1β induction of Smad7 and the reduction of chondrocyte responsiveness to TGFβ provides new insights into the molecular mechanisms of osteoarthritis and may facilitate the identification of novel approaches for its treatment.