Mediation of interleukin‐1β–induced transforming growth factor β1 expression by activator protein 4 transcription factor in primary cultures of bovine articular chondrocytes: Possible cooperation with activator protein 1

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Abstract
Objective Interleukin‐1 (IL‐1) and transforming growth factor β1 (TGFβ1) play major roles in osteoarticular diseases, exerting opposite effects on both the catabolism and anabolism of cartilage matrix. Previous findings suggest that IL‐1 and TGFβ1 could function in a feedback interaction. However, the effect exerted by IL‐1 on expression of TGFβ by articular chondrocytes is, so far, poorly understood. The present study was carried out to determine the influence of IL‐1β on the expression of TGFβ1 by bovine articular chondrocytes (BACs) in primary culture. Methods BAC primary cultures were treated with IL‐1β, and TGFβ1 messenger RNA (mRNA) steady‐state levels and protein expression were measured by real‐time reverse transcription–polymerase chain reaction and enzyme‐linked immunosorbent assay, respectively. Transient transfection of TGFβ1 gene promoter constructs was performed to delineate the DNA sequences that mediate the IL‐1β effect. Electrophoretic mobility shift assays (EMSAs) and supershift analysis were used to characterize the transcription factors binding to these sequences. Results Cultured BACs responded to IL‐1β exposure by exhibiting an increase of TGFβ1 expression at both the mRNA and protein levels. The effect was found to be mediated by a major 80‐bp sequence located between −732 and −652 upstream of the transcription initiation site. EMSA and supershift analysis revealed that the transcription factors activator protein 4 (AP‐4) and AP‐1 specifically bound to the −720/−696 part of this sequence under IL‐1β treatment. Overexpression of AP‐4 in the BAC cultures resulted in stimulation of the transcriptional activity of the −732/+11 TGFβ1 promoter construct through the same IL‐1β–responsive element. Conclusion IL‐1β induces an increase of TGFβ1 in articular chondrocytes through activation of AP‐4 and AP‐1 binding to the TGFβ1 gene promoter. These findings may help us understand the role of IL‐1β in the disease process. Notwithstanding its deleterious effect on cartilage, IL‐1 could initiate the repair response displayed by injured cartilage in the early stages of osteoarthritis through its ability to enhance TGFβ1 expression by local chondrocytes.