The cleavage product δPML-RARα contributes to all-trans retinoic acid-mediated differentiation in acute promyelocytic leukemia cells

PML-RARα protein, the leukemogenic product of t(15,17) in acute promyelocytic leukemia, is cleaved into a truncated form termed ΔPML-RARα during all-trans retinoic acid (ATRA)-induced differentiation of NB4 cells. ΔPML-RARα is not formed in ATRA differentiation resistant NB4 subclones. As₂O₃ inhibits ΔPML-RARα formation and differentiation-induction when given in combination with ATRA. Treatment with hexamethylene bisacetamide (HMBA) combined with ATRA enhances ATRA-induced differentiation in ATRA-insensitive NB4-CI and arsenic-resistant NB4/As cells, and is associated with stabilization of PML-RARα protein and increased ΔPML-RARα formation. Unlike forced expression of PML-RARα, forced ΔPML-RARα expression based on an estimated deletion of the N-terminal PML portion does not repress RARE-tk-luc reporter activity mediated by endogenous retinoic acid receptors. The cleavage of PML-RARα is blocked by RARα antagonist Ro-41-5253 and cycloheximide and therefore requires a RARα transactivation-dependent pathway. Proteasome inhibitor MG-132 and caspase inhibitor Z-VAD-FMK do not block ATRA-induced PML-RARα cleavage and differentiation. These data suggest that (a) ATRA treatment induces PML-RARα cleavage by induction of unknown enzymes independent of proteasome- and caspase-mediated pathways; (b) ΔPML-RARα might function differently from both PML-RARα and RARα; (c) failure to cleave PML-RARα and form ΔPML-RARα after ATRA treatment may contribute to ATRA resistance in APL cells.