All trans retinoic acid (ATRA) induces remission in patients with acute promyelocytic leukemia (APL) by induction of granulocytic differentiation. Since CCAAT/enhancer binding protein (C/EBP) α, β and ε play important roles in normal granulocytic differentiation we compared their expression and regulation in ATRA differentiation inducible NB4 and HL-60 cells to their ATRA differentiation resistant subclones R4 and HL-60/Res cells. All four cell lines robustly express C/EBPα but have low or absent C/EBPβ and ε expression. ATRA treatment increases the levels of C/EBPβ and ε protein in NB4 and HL-60 cells but not in the R4 and HL-60/Res cells which is correlated with the degree of differentiation induction. Knockdown of C/EBPβ or ε using shRNA decreases ATRA differentiation induction of HL-60 cells. HL-60 cells with BCR-ABL stable transfection lose expression of C/EBPα, and ATRA-induced differentiation and expression of C/EBPβ and ε are no longer seen. K562 cells which express BCR-ABL do not have detectable or inducible C/EBPα, β and ε after ATRA treatment and are resistant to ATRA differentiation induction. Ectopic expression of C/EBPα-estrogen receptor (ER) or C/EBPβ-ER, but not C/EBPε-ER, induces granulocytic differentiation in K562 cells after addition of estradiol or tamoxifen which activates these fusion factors. C/EBPα-ER or C/EBPβ-ER infected K562 cells is followed by induction of C/EBPε expression and knockdown of C/EBPε in C/EBPα-ER or C/EBPβ-ER infected K562 cells decreases induction of differentiation. The induction of C/EBPα, β, and ε expression by ATRA is investigated in ten additional myeloid leukemia cell lines and it is found that the expression of C/EBPβ and ε, but not C/EBPα, are induced in THP-1 and ML-1 cells which are responsive to ATRA differentiation induction. These results indicate that induction of C/EBPβ is required and C/EBPε plays a collaborative role with C/EBPβ in ATRA-induced differentiation of myeloid leukemia cells.

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