The RARa-PLZF Oncogenic Protein Inhibits C/EBPa Function in Myeloid Cells.

In acute promyelocytic leukemia (APL), the variant t(15;17) translocation is responsive to differentiation therapy with retinoic acid (RA) while the t(11;17) APL is a more aggressive disease with poor prognosis. The latter produces two fusion proteins, PLZF-RARa and RARa-PLZF, and both proteins are required for leukemogenesis. To define the role of RARa-PLZF, we ectopically expressed the fusion gene in 32D cells and in primary bone marrow cells. First, our results show that RARa-PLZF inhibits myeloid gene expression, specifically CEBPa targets, which fulfill important function in cell survival and differentiation along the granulocytic lineage. Second, we found that repression by RARa-PLZF is dependent on the binding of C/EBPa to its cognate sequence in the promoter of CEBPa target gene, GCSFR. Third, we confirmed by chromatin immuprecipitation that RARa-PLZF associate with C/EBPa on DNA. Fourth, we showed that as PLZF, RARa-PLZF interact directly with HDAC1 and that this interaction causes a deacetylation of histone H3 at the promoter. This inhibition is reversed by treatment with histone deacetylase inhibitor (TSA) both in vitro and in vivo. Thus, this repression is dependent on direct interaction of RP with C/EBPa and recruitment of HDAC1, causing histone deacetylation at C/EBPa target loci. Finally, our data indicate that C/EBPa activity is severely impaired in leukemic cells from patients with t(11;17) APL, as compared to the t(15;17) APL, which is more amenable to therapy. In summary, our study indicates that the oncogene RARa-PLZF inhibits C/EBPa function through direct protein-protein interaction, and thus contributes to leukemogenesis in t(11;17) APL.