All trans retinoic acid induces apoptosis in acute promyelocytic NB4 cells when combined with isoquinolinediol, a poly(ADP-ribose) polymerase inhibitor

Abstract 

NB4 cells, a model of acute promyelocytic leukemia have been shown to undergo granulocytic differentiation in response to all trans retinoic acid (ATRA), or monocytic differentiation in response to 1α,25 dihydroxyvitamin D₃ (1,25 D₃) and phorbol ester. We have shown previously that the expression of poly(ADP-ribose) polymerase (PARP) is dramatically increased during monocytic differentiation and completely down-regulated during neutrophilic differentiation. Here we show that NB4 cells simultaneously treated with ATRA and isoquinolinediol (Iso-Q), a specific PARP inhibitor, fail to differentiate into neutrophils. Nitroblue tetrazolium (NBT) dye reduction was inhibited by 48% and neutrophil formation was reduced by 75%. NB4 cells treated with ATRA and Iso-Q instead showed features of apoptosis including nuclear condensation, DNA fragmentation, and PARP cleavage yielding a 85 kDa fragment. NB4 cells treated with ATRA in combination with 1,25 D₃, a monocytic differentiation inducer, while continuing to reduce NBT also failed to mature into neutrophils or monocytes and again showed features of apoptosis. Down-regulation of Bcl-2 protein expression was evident in NB4 cells treated with ATRA and ATRA in combination with Iso-Q or 1,25 D₃, but not in cells treated with a classic chemotherapeutic agent, arabinosycytosine, suggesting that Bcl-2 down-regulation is neither necessary, nor specific for apoptosis in this model.

Keywords: Apoptosis, All-trans retinoic acid, 1α,25 dihydroxyvitamin D₃, Differentiation, NB4, Poly(ADP-ribose) polymerase