Ultraspiracle, a Drosophila retinoic X receptor alpha homologue, can mobilize the human thyroid hormone receptor to transactivate a human promoter

We have analyzed the functional domains of the Drosophila orphan receptor Ultraspiracle (usp), a homologue of the vertebrate retinoic X receptor alpha, as well as the ability of heterodimers between usp and the thyroid hormone receptor beta (T3R beta) to transactivate the human apolipoprotein A-II (apoA-II) promoter. DNA binding assays demonstrated that heterodimers of usp and the human T3R beta can bind to the hormone response element (HRE) of the regulatory element AIIJ (-734 to -716) of the human apoA-II promoter. Cotransfection experiments have shown that the combination of usp and T3R beta can transactivate the human apoA-II promoter in COS-1 cells 7-8-fold in the presence of thyroid hormone (T-3) The observed transactivation was not affected by the deletion of the amino-terminal residues 1-85 of usp, which represent a putative transactivation domain, suggesting that the function of usp is to recruit T3R beta. Furthermore, a mutant usp, with impaired DNA binding properties, can form heterodimers with T3R beta in vitro but has reduced ability to transactivate the human apoA-II promoter. A minimal thymidine kinase (tk) promoter driven by four AIIJ regulatory elements is repressed to 20% of its original activity by T;RB and the repression is relieved by usp/T3R beta heterodimers. Deletion analysis demonstrated that factors bound to the regulatory elements AIIJ, AIIAB, and AIM participate in the usp/T3R beta-mediated transactivation of the human apoA-II promoter. Similarly to element AIIJ, element AIIAB binds usp/T3R beta heterodimers, whereas element AIIH binds a COS-1 nuclear activity that is supershifted with anti-hepatic nuclear factor 1 antibodies. The findings suggest that optimal transactivation of the apoA-II promoter by usp/T3R beta heterodimers requires complex interactions between these heterodimers and factors bound to other regulatory elements. The observed transcriptional activation through heterodimer formation between nuclear receptors from species as divergent in the evolutionary scale as insects and mammals indicates that the functional domains of these proteins have been highly conserved.