DIFFERENTIATION INDUCTION OF HUMAN LEUKEMIA-CELLS (HL-60) BY A COMBINATION OF 1,25-DIHYDROXYVITAMIN D-3 AND RETINOIC ACID (ALL-TRANS OR 9-CIS)

1,25(OH)(2)D-3 and two stereoisomers of retinoic acid, all trans and 9 -cis retinoic acid, are regulators of cell proliferation and different iation. The aim of this study was to evaluate the effects of a combina tion of 1,25(OH)(2)D-3 and retinoic acid (all trans or 9-cis) on proli feration and cell differentiation of the human promyelocytic leukemia cell line HL60, and to test the reversibility of the induced different iation. Cell proliferation was inhibited as expected by 1,25(OH)(2)D-3 and all trans retinoic acid alone (IC50 of cell survival was 4 x 10(- 7) M, 9 x 10(-6) M and 9 x 10(-7) M for 1,25(OH)(2)D-3, all trans and 9-cis retinoic acid, respectively). Combination of 1,25(OH)(2)D-3 and either form of retinoic acid resulted in a partially additive decrease in cell proliferation. 1,25(OH),D, induced a monocytic differentiatio n (100% CD14+ cells with 10(-7) M 1,25(OH)(2)D-3), while retinoic acid led to a predominantly granulocytic differentiation (36 and 42% CD67 cells with 10(-6) M all trans and 9-cis retinoic acid, respectively). Additive effects on differentiation were observed upon combination of subtherapeutical doses of the drugs, achieving a mainly monocytic pop ulation, demonstrating the dominant role of 1,25(OH)(2)D-3 in determin ing the direction of differentiation. The effects on proliferation and differentiation of the solitary drugs were reversible, while the prol iferation arrest and differentiation induced by the combination persis ted and even progressed after withdrawal of the drugs. We conclude tha t 1,25(OH)(2)D-3 and retinoic acid (all trans or 9-cis) exert additive effects on inhibition of proliferation and induction of cell differen tiation of HL60 cells, leading to a persistent differentiation, even a fter drug withdrawal.