RETINOIC ACID RECEPTOR BETA-2 MESSENGER-RNA IS ELEVATED BY RETINOIC ACID INVIVO IN SUSCEPTIBLE REGIONS OF MIDGESTATION MOUSE EMBRYOS

Many of the biological effects of retinoic acid are mediated by its nuclear receptors (RAR-alpha, RAR-beta, and RAR-gamma), and each of these three receptors exist in multiple isoforms. As a first step to identify if any of the receptor isoforms are involved in dysmorphogenesis which is induced in mouse embryos after treatment with retinoic acid (RA), we examined the levels of mRNA of several isoforms of each RAR in the limb buds and other embryonic regions of normal and RA-treated embryos. Within 3 to 6 hr after treatment of mice on day 11 of gestation with RA, RAR-beta2 mRNA levels in the whole embryo increased 7-fold while both RAR-alpha2 and RAR-gamma1 mRNA levels were elevated only 2-fold. Since RA treatment of day 11 embryos especially produces limb defects in virtually every embryo, we next examined individual embryonic regions separately. Limb buds showed the highest elevations in RAR-beta2 mRNA levels (12-fold) compared to a moderate elevation in the head/craniofacial region (8-fold) and a small elevation in the remainder of the body (4-fold). In contrast, RAR-alpha2 and RAR-gamma1 mRNA levels were elevated in all these tissues to a similar extent, which amounted to only about a 2-fold increase. Retinol, the precursor of RA in the embryo, was also capable of elevating RAR-beta2 mRNA levels in the limb bud, but the increase was delayed, apparently indicating that metabolic conversion of retinol to RA preceded the effect on mRNA levels. Finally, treatment of dams on day 14 of gestation, a time when embryos are relatively insensitive to RA, resulted in no elevation in RAR-alpha2 mRNA levels and a greatly reduced elevation (2- to 3-fold in all embryonic regions) in RAR-beta2 mRNA levels. Therefore, the elevation in RAR-beta2 mRNA correlates well with regions of the embryo, e.g., limb buds, which are specific targets for RA-induced teratogenesis. These results are consistent with the possibility that specific isoforms of the RARs, in particular RAR-132, may mediate the effects of RA during abnormal development.