Abstract

In acute promyelocytic leukemia (APL), the translocation t(15;17) induces a block at the promyelocytic stage of differentiation in an all-trans-retinoic acid (ATRA)-responsive manner. Here we report that upon treatment with ATRA, t(15;17) cells (NB4) reveal a very rapid increase in protein level and binding activity of C/EBP, a C/EBP family member, which was not observed in an ATRA-resistant NB4 cell line. We further provide evidence that ATRA mediates a direct increase of C/EBP, only in PML–RARA (promyelocytic leukemia–retinoic acid receptor )-expressing cells. In addition, transactivation experiments indicate that the PML–RARA fusion protein, but not PML–RARA mutants defective in transactivation, strongly transactivates the C/EBP promoter. These results suggest that PML–RARA mediates ATRA-induced C/EBP expression. Finally, we demonstrate the importance of C/EBP in granulocytic differentiation. We show that not only does C/EBP induce granulocytic differentiation of non-APL myeloid cell lines independent of addition of ATRA or other cytokines, but also that C/EBP induction is required during ATRA-induced differentiation of APL cells. Taken together, C/EBP is an ATRA-dependent PML–RARA target gene involved in ATRA-induced differentiation of APL cells.