THE USE OF A RETINOID RECEPTOR ANTAGONIST IN A NEW MODEL TO STUDY VITAMIN-A-DEPENDENT DEVELOPMENTAL EVENTS

Multiple fetal anomalies occur in vitamin A deficient animals as well as in retinoic acid receptor gene 'knockout' mice, indicating that ret inoic acid tan active metabolite of vitamin A) performs some essential functions in normal development. Additional approaches are needed to probe directly the stages and sites in the embryo where a presence of endogenous retinoic acid is indispensable. We have employed a new stra tegy for this purpose which involved an intervention in retinoic acid receptor (RAR)-dependent functions at specific developmental stages by means of a highly effective RAR antagonist, AGN 193109. We report tha t in an in vitro cell differentiation bioassay, AGN 193109 completely reversed the inhibitory action of a potent RAR agonist, AGN 190121. In pregnant mice, a single oral 1mg/kg dose of the antagonist given on 8 day post coitum (dpc) produced a severe craniofacial anomaly (median cleft face or frontonasal dysplasia) and eye malformations in virtuall y all exposed fetuses. On the other hand, treatment on 11 dpc, a time in development when RARs are strategically expressed in the limb bud p rimordium, no limb anomalies could be induced by the antagonist. Even after a high dose of 100 mg/kg, limb development progressed normally i n spite of the fact that measurable concentrations of the antagonist w ere present. Because retinoids are long known to influence skin morpho logy, we next monitored the effects of the antagonist on skin developm ent. When given late in gestation, on 14 dpc, we found that the antago nist delayed differentiation and maturation of the fetal skin and hair follicles. We conclude that this model provides a convenient and pert inent system which enables us to seek and clarify true functions of re tinoic acid and its cognate receptors in embryogenesis and in adult an imals.