title: Induction of apoptosis by retinoids and retinoic acid receptor gamma-selective compounds in mouse thymocytes through a novel apoptosis pathway.
abstract: Retinoic acids are morphogenic signaling molecules that are derived from vitamin A and involved in a variety of tissue functions. Two groups of their nuclear receptors have been identified: retinoic acid receptors (RARs) and retinoic acid X receptors (RXRs). All-trans retinoic acid 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 unknown mechanisms. It was previously shown that retinoic acids prevents activation-induced death of thymocytes. Here, we report that both all-trans and 9-cis retinoic acid induce apoptosis of mouse thymocytes and purified CD4+CD8+ cells in ex vivo cultures, with 9-cis retinoic acid being 50 times more effective. The induction of apoptosis by retinoic acids is mediated by RARgamma because (a) the phenomenon can be reproduced only by RARgamma-selective retinoic acid analogs, (b) the cell death induced by either retinoic acids or RARgamma analogs can be inhibited by RARgamma-specific antagonists, and (c) CD4+CD8+ thymocytes express RARgamma. In vivo administration of an RARgamma analog resulted in thymus involution with the concomitant activation of the apoptosis-related endonuclease and induction of tissue transglutaminase. The RARgamma pathway of apoptosis is RNA and protein synthesis dependent, affects the CD4+CD8+ double positive thymocytes, and can be inhibited by the addition of either Ca2+ chelators or protease inhibitors. Using various RAR- and RXR-specific analogs and antagonists, it was demonstrated that stimulation of RAR alpha inhibits the RARgamma-specific death pathway (which explains the lack of apoptosis stimulatory effects of all-trans retinoic acid at physiological concentrations) and that costimulation of the RXR receptors (in the case of 9-cis retinoic acid) can neutralize this inhibitory effect. It is suggested that formation of 9-cis retinoic acid may be a critical element in regulating both the positive selection and the "default cell death pathway" of thymocytes.
authors.affiliation: Department of Biochemistry, University Medical School of Debrecen, Hungary.
authors.forename: Z
authors.initials: Z
authors.lastname: Szondy
authors.forename: U
authors.initials: U
authors.lastname: Reichert
authors.forename: J M
authors.initials: JM
authors.lastname: Bernardon
authors.forename: S
authors.initials: S
authors.lastname: Michel
authors.forename: R
authors.initials: R
authors.lastname: Tóth
authors.forename: P
authors.initials: P
authors.lastname: Ancian
authors.forename: E
authors.initials: E
authors.lastname: Ajzner
authors.forename: L
authors.initials: L
authors.lastname: Fesus
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects* / Q000502:physiology*; D015496:CD4-Positive T-Lymphocytes / Q000166:cytology / Q000187:drug effects / Q000648:ultrastructure; D018414:CD8-Positive T-Lymphocytes / Q000166:cytology / Q000187:drug effects / Q000648:ultrastructure; D008297:Male; D051379:Mice; D008815:Mice, Inbred Strains; D018168:Receptors, Retinoic Acid / Q000187:drug effects* / Q000502:physiology*; D000072482:Retinoic Acid Receptor alpha; D047488:Retinoid X Receptors; D012176:Retinoids / Q000494:pharmacology*; D013950:Thymus Gland / Q000166:cytology* / Q000187:drug effects* / Q000648:ultrastructure; D014157:Transcription Factors / Q000187:drug effects / Q000502:physiology; D000097768:Retinoic Acid Receptor gamma
chemical_list: D018168:Receptors, Retinoic Acid; D000072482:Retinoic Acid Receptor alpha; D047488:Retinoid X Receptors; D012176:Retinoids; D014157:Transcription Factors; D000097768:Retinoic Acid Receptor gamma; C099555:CD 437; C000607072:Rara protein, mouse