IN-VITRO AND IN-VIVO INTERACTIONS BETWEEN NUCLEAR RECEPTORS AT ESTROGEN RESPONSE ELEMENTS

To study mechanisms involved in the antiestrogenic effect of retinoic acid (RA), previously described in mammalian cells, we used in vitro a nd in vivo approaches. One hypothesis was direct competition between n uclear receptors (ER, RAR and RXR) at the DNA level. We first showed i n vitro that the RAR/RXR heterodimer could weakly bind an ERE and that retinoid receptors reduced binding of ER to an ERE. We next checked w hether, in yeast, direct competition between receptors that recognize the same responsive element could be monitored in a reconstituted hete rologous estrogen-responsive system, by determining the expression of a reporter gene. We then co-transformed RAR and RXR in an estrogenic r esponsive strain. This model demonstrated that, even though RAR/RXR wa s able to bind an ERE, the addition of retinoic acid had no inhibitory effect on estrogen-induced responses in this yeast system, unlike in mammalian cells. Interference between these receptors should require o ther factors than interactions at the ERE level. This model could be u sed to identify mammalian factors interacting with estrogen and retino ic acid receptors which could play a role in crosstalk between these r eceptors. Copyright (C) 1996 Elesevier Science Ltd.