The homeotic protein Six3 is a coactivator of the nuclear receptor NOR-1 and a corepressor of the fusion protein EWS/NOR-1 in human extraskeletal myxoid chondrosarcomas.
Nuclear receptors represent a large family of transcription factors involved in development, differentiation, homeostasis, and cancer. In recent years, a growing number of cofactors has been discovered that participate in the regulation of the transcriptional activity of these proteins. We present in this study the identification of a cofactor, the homeotic protein Six3, which differentially regulates the transcriptional activity of the orphan nuclear receptor NOR-1 (NR4A3). NOR-1 is normally involved in the balance between cell proliferation and cell death, and is implicated in oncogenesis as part of the EWS/NOR-1 fusion protein found in human extraskeletal myxoid chondrosarcoma (EMC) tumors. Reverse transcription-PCR analyses indicate that EMC tumors expressing the EWS/NOR-1 mRNA also express mRNAs encoding NOR-1 and Six3. Glutathione S-transferase fusion protein assays show that Six3 binds in vitro the DNA-binding domain of NOR-1 and the EWS domain of EWS/NOR-1 and that the homeodomain of Six3 is required for these interactions. Mammalian two-hybrid experiments, using immortalized human chondrocytes as a model, indicate that Six3 also interacts with NOR-1 and EWS/NOR-1 in vivo. Cotransfection experiments show that Six3 stimulates the transcriptional activity of NOR-1, whereas it represses that of EWS/NOR-1. Considering the highly specific expression pattern of Six3, our finding that it is expressed in EMC suggests that it plays a pivotal role in the development of these tumors. We propose that Six3 maintains a transcriptional balance between the activities of NOR-1 and EWS/NOR-1, the net effect being to deregulate the expression of specific target genes and push the equilibrium toward uncontrolled cell proliferation.
Laflamme
Cynthia
C
Human and Molecular Genetic Research Unit, Pavillon Saint-François d'Assise, CHUQ, Quebec, Qc, G1L 3L5 Canada.
Filion
Christine
C
Bridge
Julia A
JA
Ladanyi
Marc
M
Goldring
Mary B
MB
Labelle
Yves
Y
D000818:Animals; D002813:Chondrosarcoma / Q000235:genetics / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D005136:Eye Proteins; D018398:Homeodomain Proteins / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D051379:Mice; D008819:Mice, Nude; D009419:Nerve Tissue Proteins / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D015514:Oncogene Proteins, Fusion / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D034802:RNA-Binding Protein EWS / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D011987:Receptors, Steroid; D011988:Receptors, Thyroid Hormone; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012983:Soft Tissue Neoplasms / Q000235:genetics / Q000378:metabolism*; D015533:Transcriptional Activation; D014162:Transfection; D014407:Tumor Cells, Cultured; D000097607:Homeobox Protein SIX3
D004268:DNA-Binding Proteins; D005136:Eye Proteins; D018398:Homeodomain Proteins; D009419:Nerve Tissue Proteins; D015514:Oncogene Proteins, Fusion; D012333:RNA, Messenger; D034802:RNA-Binding Protein EWS; D011987:Receptors, Steroid; D011988:Receptors, Thyroid Hormone; D000097607:Homeobox Protein SIX3; C490412:NR4A3 protein, human; C490411:Nr4a3 protein, mouse