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¹ Reproduction, Perinatal and Child Health, CHUQ Research Centre, CHUL Room T1-49, 2705 Laurier Boulevard, Quebec City, Quebec, Canada G1V 4G2² Department of Obstetrics and Gynecology, Faculty of Medicine, Centre for Research in Biology of Reproduction, Université Laval, Quebec City, Quebec, Canada G1V 0A6

(Correspondence should be addressed to J J Tremblay; Email: jacques-j.tremblay{at}crchul.ulaval.ca)

Expression of steroidogenic enzyme-encoding genes in testicular Leydig cells is complex and involves several transcription factors including the orphan nuclear receptor NUR77 (NR4A1) and the bZIP factor CCAAT/enhancer binding protein β (EBPβ). How these transcription factors are integrated into a functional network, however, remains to be fully understood. Here, we report that the transcription factor C/EBPβ can activate the Nur77 promoter as revealed by transient transfections in MA-10 Leydig cells. Through 5' progressive deletions and site-directed mutagenesis, the C/EBPβ-mediated activation of the Nur77 promoter was found to be dependent on a novel species-conserved C/EBP element located at –110 bp. We also demonstrate using electromobility shift assay that C/EBPβ specifically binds to this element. Furthermore, we report a functional cooperation between C/EBPβ and the p50 subunit of NF-B that involves a previously uncharacterized B element located at –18 bp. Promoter analysis revealed that either the C/EBP or the B element was sufficient to sustain the C/EBPβ-p50 cooperation thus suggesting that both factors physically interact. Altogether, our results provide new data regarding Nur77 transcription in testicular Leydig cells in addition to providing new insights into the interplay between transcription factors involved in Leydig cell gene expression and function.

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