EVIDENCE FOR THE INVOLVEMENT OF BOTH RETINOIC-ACID-RECEPTOR-X AND RETINOIC-X-RECEPTOR-DEPENDENT SIGNALING PATHWAYS IN THE INDUCTION OF TISSUE TRANSGLUTAMINASE AND APOPTOSIS IN THE HUMAN MYELOMA CELL-LINE RPMI-8226

In this study, we show that both all-trans-retinoic acid (atRA) and 9- cis-retinoic acid (9-cis-RA) are potent inducers of tissue transglutam inase (TGase II), an enzyme involved in apoptosis, at the level of bot h enzyme activity and mRNA in the human myeloma cell line RPMI 8226. R PMI 8226 cells were shown to express mRNAs for all the retinoid recept ors subtypes, ie, RAR alpha, RAR beta, RAR gamma, RXR alpha, RXR beta, and RXR gamma. To identify which of these receptors are involved in r egulating TGase II expression, several receptor-selective synthetic re tinoids were used. Neither CD367, a very potent retinoid that selectiv ely binds and activates receptors of the RAR family nor CD2425, an RXR -selective agonist, induced TGase II when used alone. However, combina tion of CD367 and CD2425 resulted in nearly full induction of the enzy me. Moreover, when used in combination with atRA, CD367 partially inhi bited the atRA-dependent induction of TGase II, whereas CD2425 enhance d it. The effects of Am 580, CD417, and CD437, three synthetic retinoi ds selective for the RARs subtypes RAR alpha, RAR beta, and RAR gamma, respectively, were also investigated. None of these compounds was abl e to induce TGase II when used alone; however, the combination of each of them with CD2425 resulted in strong induction of the enzyme activi ty, reaching 30% to 50% of the values obtained in the presence of reti noic acid and suggesting functional redundancy between the RAR subtype s. Finally, treatment with atRA or the combination of CD367 and CD2425 , but not with CD367 or CD2425 alone, was also shown to trigger apopto sis in RPMI 8226 cells, with prominent accumulation of TGase II immuno reactivity in apoptotic cells. Taken together these data suggest that the induction of TGase II expression and apoptosis in the RPMI 8226 my eloma cell line required ligand-dependent activation of both the RAR a nd RXR receptors. (C) 1998 by The American Society of Hematology.