INDUCTION OF APOPTOSIS BY RETINOIDS AND RETINOIC ACID RECEPTOR GAMMA-SELECTIVE COMPOUNDS IN MOUSE THYMOCYTES THROUGH A NOVEL APOPTOSIS PATHWAY

Retinoic acids are morphogenic signaling molecules that are derived fr om vitamin A and involved in a variety of tissue functions. Two groups of their nuclear receptors have been identified: retinoic acid recept ors (RARs) and retinoic acid X receptors (RXRs). All-trans retinoic ac id is the high affinity ligand for RARs, and 9-cis retinoic acid also binds to RXRs with high affinity. In cells at high concentrations, all -trans retinoic acid can be converted to 9-cis retinoic acid via unkno wn mechanisms. It was previously shown that retinoic acids prevents ac tivation-induced death of thymocytes. Here, we report that both all-tr ans and 9-cis retinoic acid induce apoptosis of mouse thymocytes and p urified CD4(+)CD8(+) cells in ex vivo cultures, with 9-cis retinoic ac id being 50 times more effective. The induction of apoptosis by retino ic acids is mediated by RAR gamma because (a) the phenomenon can be re produced only by RAR gamma-selective retinoic acid analogs, (b) the ce ll death induced by either retinoic acids or RAR gamma analogs can be inhibited by RAR gamma-specific antagonists, and (c) CD4(+)CD8(+) thym ocytes express RAR gamma. In vivo administration of an RAR gamma analo g resulted in thymus involution with the concomitant activation of the apoptosis-related endonuclease and induction of tissue transglutamina se. The RAR gamma pathway of apoptosis is RNA and protein synthesis de pendent, affects the CD4(+)CD8(+) double positive thymocytes, and can be inhibited by the addition of either Ca2+ chelators or protease inhi bitors, Using various RAR- and RXR-specific analogs and antagonists, i t was demonstrated that stimulation of RAR alpha inhibits the RAR gamm a-specific death pathway (which explains the lack of apoptosis stimula tory effects of all-trans retinoic acid at physiological concentration s) and that costimulation of the RXR receptors (in the case of 9-cis r etinoic acid) can neutralize this inhibitory effect, It is suggested t hat formation of g-cis retinoic acid may be a critical element in regu lating both the positive selection and the ''default cell death pathwa y'' of thymocytes.