INDUCTION OF APOPTOSIS WITHOUT DIFFERENTIATION BY RETINOIC ACID IN PLB-985 CELLS REQUIRES THE ACTIVATION OF BOTH RAR AND RXR

Retinoic acid (RA) induces differentiation, followed by apoptosis in a cute promyelocytic leukemia (APL) cells, both in vitro and in patients . One problem in understanding these mechanisms is to distinguish mole cular events leading to differentiation from those leading to apoptosi s. We have identified a leukemic cell line, PLB-985, where RA directly induces apoptosis with no morphologic, genetic, or cell-surface marke r evidence of differentiation. These cells differentiate following dim ethyl sulfoxide (DMSO), but not RA, treatment. Two-color flow cytometr y showed no alteration of the cell cycle after RA treatment, and cell- surface marker analysis of CD11a, CD11b, and CD13 showed no modulation typical of differentiating cells. RNA expression of myeloblastin and transglutaminase, genes regulated by RA-induced differentiation in NB4 cells, was unchanged by RA treatment. Instead, RA induced apoptosis, as shown by typical apoptotic morphological features, genomic DNA ladd ering, and positive labeling in the TUNEL assay. We found that inducti on of apoptosis in this model requires a different pattern of retinoid receptor binding and transcriptional activation than is seen in APL c ells. As previously described, treatment with retinoid receptor-select ive ligands showed that stimulation of RAR alone is sufficient to indu ce differentiation and apoptosis in NB4 cells, and that stimulation of RXR has no effect on the parameters analyzed. In PLB-985 cells, on th e other hand, apoptosis was induced only upon costimulation of both RA R and RXR. Stimulation of either receptor alone had no effect on the c ells. Consistent with these findings, bcl-2 RNA and protein levels wer e downregulated after stimulation of both RAR and RXR, but not with an RAR-specific ligand alone, as in NB4 cells. The expression of several other bcl-2 family members (bcl-X, ich-1, bax, bag, and bak) and reti noid receptors (RAR alpha, RXR alpha, and RXR beta) was not affected b y treatment with RAR- and/or RXR-activating retinoids; RAR beta RNA wa s undetectable before and after retinoid treatment. Thus, our cell mod el provides a useful tool in determining the genetic events mediating apoptosis as a response to RA, unobscured by events implicated in diff erentiation. (C) 1997 by The American Society of Hematology.