Introduction: Acute promyelocytic leukemia (APL) patients with t(15;17)(PML/RARalpha positive) achieve remission upon t-RA treatment, whereas patients with t(11;17)(PLZF/RARalpha positive) do not. Both APL translocation products bind to the histone deacetylase (HD)-recruiting nuclear co-repressor complex (HD-NCR) in a ligand-dependent manner through their RARalpha portion. Differently to PML/RARalpha, PLZF/RARalpha also binds the HD-NCR in a ligand-independent manner through the PLZF portion of the fusion protein (PLZF#), which seems to be crucial for the t-RA resistance of t(11;17) APL patients.

Materials and methods: The t-RA sensitivity of U937 cells was tested by the nitro-blue tetrazolium reduction (NBT) assay and by analysis of t-RA-induced type II transglutaminase activity. The interaction between HD-NCR and PLZF/RARalpha was investigated by in vitro binding assays.

Results: (i) Deletions in PLZF# convert PLZF/RARalpha from a repressor to an activator of t-RA response in U937 cells; (ii) the effect of PLZF/RARalpha on t-RA-signaling is regulated by the POZ-domain and its down-stream regions of PLZF#; (iii) there are additional binding sites for HD-NCR in PLZF# and (iv) PLZF# not only directly binds but also regulates the binding of PLZF/RARalpha to the HD-NCR.

Conclusions: At least two different mechanisms responsible for the aberrant recruitment of HD-NCR complexes by PLZF# are regulating the different t-RA-sensitivity of the PLZF/RARalpha and PML/RARalpha positive APL blasts: one is related to the direct binding of the different members of the HD-NCR complex to PLZF#; the other is an enforcing effect of PLZF# on the affinity of the PLZF/RARalpha fusion protein to the HD-NCR complex.