Cell-Type-Specific Regulation of the Retinoic Acid Receptor Mediated by the Orphan Nuclear Receptor TLX

Abstract

Malformations in the eye can be caused by either an excess or deficiency of retinoids. An early target gene of the retinoid metabolite, retinoic acid (RA), is that encoding one of its own receptors, the retinoic acid receptor β (RARβ). To better understand the mechanisms underlying this autologous regulation, we characterized the chick RARβ2 promoter. The region surrounding the transcription start site of the avian RARβ2 promoter is over 90% conserved with the corresponding region in mammals and confers strong RA-dependent transactivation in primary cultured embryonic retina cells. This response is selective for RAR but not retinoid X receptor-specific agonists, demonstrating a principal role for RAR(s) in retina cells. Retina cells exhibit a far higher sensitivity to RA than do fibroblasts or osteoblasts, a property we found likely due to expression of the orphan nuclear receptor TLX. Ectopic expression of TLX in fibroblasts resulted in increased sensitivity to RA induction, an effect that is conserved between chick and mammals. We have identified a cis element, the silencing element relieved by TLX (SET), within the RARβ2 promoter region which confers TLX- and RA-dependent transactivation. These results indicate an important role for TLX in autologous regulation of the RARβ gene in the eye.

ACKNOWLEDGMENTS

We thank H. Ohizumi, K. Nozaki, H. Otani, and H. Ono for technical assistance and members of the Yasuda Lab and Umesono Lab for valuable discussions during the study. We also thank Jochen Buck, Lonny R. Levin, Leonard P. Freedman, and David J. Mangelsdorf for critical reading of the manuscript and Richard A. Heyman and Yoshiko Ishimi for kindly providing synthetic retinoids and the MC3T3-E1 cell line, respectively.

This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture of Japan; the Research for the Future Program of the Japan Society for the Promotion of Science; and the Human Frontier Science Program.