Figure 3.
RXR-specific ligand increases effects of natural and synthetic RAR ligands, but not of 9-cis-RA (9cRA) or 9-cis-4-oxo-RA (9c4oRA) on DR5-TATAluc activation (A) or on axis formation (B) in Xenopus. (A) Xenopus zygotes were injected with DR-5-TATAluc reporter. At stage 10, embryos were treated at 10 embryos/1 ml 3 × 10−7 M retinoid in the absence or presence of 2.5 × 10−7 M RXR-specific ligand SR11246. 9-Cis-4-oxo-RA also proved to be an efficient activator of DR-5 transcription via RAR–RXR heterodimers in COS-1 cells (using cotransfection of RARβ, RXRα, and DR5-TATA-luc), its activity being in between the activities of RA and 9-cis-RA (not shown). Data are means ± SEM. (B) Stage 10 Xenopus embryos treated with retinoids as in A. Larvae were scored for anteroposterior defects using a dorsoanterior index (DAI) as described (21). Index 5 represents normal larvae, and index 0 the most ventro-posteriorized larvae. At 3 × 10−7 M, Am80 induced the maximal response, DAI value 0, in the presence of SR11246. At lower Am80 concentrations, synergy with SR11246 was comparable to that of RA or 4-oxo-RA (not shown). Data are means of 10 embryos ± SEM. (C) 9-Cis-4-oxo-RA binds both subunits of RARβ–RXRα heterodimers. Indicated retinoids (at 10−5 M) were tested for competition with 10 nM 3H-9-cis-RA for binding to RARβ–RXRα heterodimers. Data are means of two experiments.