A composite element binding the vitamin D receptor and the retinoic X receptor alpha mediates the transforming growth factor-beta inhibition of decorin gene expression in articular chondrocytes

Decorin, a small leucine-rich proteoglycan may play an important role in th e attempt of cartilage repair initiated by chondrocytes in early stages of osteoarthritis, through its ability to bind collagen fibrils and growth fac tors such as transforming growth factor-beta (TGF-beta). We previously demo nstrated that TGF-beta decreased decorin mRNA steady state levels in articu lar chondrocytes (Demoor, M., Redini, F., Boittin, M., and Pujol, J.-P. (19 98) Biochim. Biophys. Acta 1398, 179-191). Here, we investigated the effect of TGF-beta on decorin gene expression in both primary cultures of articul ar chondrocytes and chondrocytes dedifferentiated by serial passages. Trans ient transfection of cells with plasmid constructs of the decorin promoter linked to the luciferase reporter gene revealed transcriptional repression by TGF-beta, in fully differentiated as well as dedifferentiated chondrocyt es. Experiments with 5'-deleted constructs allowed characterization of a TG F-beta -responsive element in the shortest construct (base pairs (bp) -155/ +269). DNase I footprinting analysis delineated a negative TGF-beta -respon sive region between -140 and -111 bp in the decorin proximal promoter. Get retardation assays demonstrated that TGF-beta modulates decorin gene expres sion through transcription factors, the nature and mode of action of which depend on the differentiation state of the chondrocytes; two DNA-protein co mplexes were formed in the region -1441-127 bp with nuclear extracts from p rimary chondrocytes, whereas a higher mobility complex was observed in the -127/-111 bp region for dedifferentiated cells. Antibodies against vitamin D and retinoic acid receptors used in supershift experiments showed that th ese nuclear receptors are involved in the regulation of decorin gene expres sion in articular chondrocytes.