Inhibition of activation-induced apoptosis of thymocytes by all-trans- and 9-cis-retinoic acid is mediated via retinoic acid receptor alpha.

Thymocytes can be induced to undergo apoptotic cell death by activation through the T-cell receptor (TCR). This process requires macromolecular synthesis and has been shown to be inhibited by retinoic acids (RAs). Two groups of nuclear receptors for RAs have been identified: retinoic acid receptors (RARs) and retinoid X receptors (RXRs). All-trans-RA is the high-affinity ligand for RARs, and 9-cis-RA additionally binds to RXRs with high affinity. Because 9-cis-RA is much more potent in inhibiting TCR-mediated death than all-trans-RA, it was suggested that RXRs participate in the process. In the present study various synthetic retinoid analogues were used to address this question further. The results presented suggest that the inhibitory effect of RAs on activation-induced death of thymocytes is mediated via RARalpha, because (1) it can be reproduced by various RARalpha analogues both in vitro and in vivo, (2) the effect of RAs can be inhibited by the addition of an RARalpha antagonist, (3) CD4+CD8+thymocytes, which die on TCR stimulation, express RARalpha. Stimulation of RARgamma, in contrast, enhances the activation-induced death of thymocytes and inhibits its prevention by RARalpha stimulation. RXR co-stimulation suspends this inhibitory effect of RARgamma and permits the preventive function of RARalpha on activation-induced death. Our results suggest a complex interaction between the various isoforms of retinoid receptors and demonstrate that low (physiological) concentrations of all-trans-RA do not affect the activation-induced death of thymocytes because the RARalpha-mediated inhibitory and the RARgamma-mediated enhancing pathways are in balance, whereas if 9-cis-RA is formed, additional stimulation of RXRs permits the inhibitory action of RARalpha.