Although the involvement of Wnt/beta-catenin signaling pathway in the regulation of COX-2 expression has been suggested, the underlying molecular mechanisms are still unclear. Here, we demonstrate that lymphoid enhancer-binding factor 1 (Left) in concert with P-catenin is associated with the transcriptional regulation of the gene encoding COX-2 in mouse chondrocytes. Our results show that co-overexpression of Left and P-catenin synergistically upregulates COX-2 expression. Furthermore, decrease of Left expression using Left siRNA results in the downregulation of COX-2 expression. Using bioinformatic analysis, we identified a conserved Lef1-binding site that is mapped at the 3' region of the genomic COX-2 locus. In vivo and in vitro binding of Left at the predicted binding site was proved using chromatin immunoprecipitation assays and electrophoretic mobility shift assays, respectively. Moreover, binding of Left and P-catenin to the Lef1-binding site transactivates COX-2 promoter activity. Our results indicate a pivotal role of Left in the regulation of COX-2 transcription in arthritic chondrocytes. (C) 2007 Elsevier Inc. All rights reserved.