Granulocytic differentiation of human NB4 promyelocytic leukemia cells induced by all-trans retinoic acid metabolites

The metabolism of all-trans retinoic acid (ATRA) has been reported to be pa rtly responsible for the ill vivo resistance to ATRA seen in the treatment of human acute promyelocytic leukemia (APL). However, ATRA metabolism appea rs to be involved in the growth inhibition of several cancer cell lines in vitro. The purpose of this study was to evaluate the ill vitro activity of the principal metabolites of ATRA [4-hydroxy-retinoic acid (4-OH-RA), 18-hy droxy-retinoic acid (18-OH-RA), 4-oxo-retinoic acid (4-oxo-RA), and 5,6-epo xy-retinoic acid (5,6-epoxy-RA)] in NB4, a human promyelocytic leukemia cel l line that exhibits the APL diagnostic t(15;17) chromosomal translocation and expresses the PML-RAR alpha fusion protein. We established that the fou r ATRA metabolites were indeed formed by the NB4 cells in vitro. NB4 cell g rowth was inhibited (69-78% at 120 h) and cell cycle progression in the G(1 ) phase (82-85% at 120 h) was blocked by ATRA and all of the metabolites at 1 muM concentration. ATRA and its metabolites could induce NB4 cells diffe rentiation with similar activity, as evaluated by cell morphology, by the n itroblue tetrazolium reduction test (82-88% at 120 h) or by the expression of the maturation specific cell surface marker CD11c. In addition, nuclear body reorganization to macropunctated structures, as well as the degradatio n of PML-RAR alpha, was found to be similar for ATRA and all of its metabol ites. Comparison of the relative potency of the retinoids using the nitrobl ue tetrazolium reduction test showed effective concentrations required to d ifferentiate 50% of cells in 72 h as follows: ATRA, 15.8 +/- 1.7 nM; 4-oxo- RA, 38.3 +/- 1.3 nM; 18-OH-RA, 55.5 +/- 1.8 nhl; 4-OH-RA, 79.8 +/- 1.8 nar; and 5,6-epoxy-RA, 99.5 +/- 1.5 nhl, The ATRA metabolites were found to exe rt their differentiation effects via the RAR alpha nuclear receptors, becau se the RAR alpha -specific antagonist BMS614 blocked metabolite-induced CD1 1c expression in NB4 cells. These data demonstrate that the principal ATRA Phase 1 metabolites can elicit leukemia cell growth inhibition and differen tiation in vitro through the RAR alpha signaling pathway, and they suggest that these metabolites may play a role in ATRA antileukemic activity in viv o.