High levels of the BCR/ABL oncoprotein are required for the MAPK-hnRNP-E2–dependent suppression of C/EBPα-driven myeloid differentiation

Abstract

The inability of myeloid chronic myelogenous leukemia blast crisis (CML-BC) progenitors to undergo neutrophil differentiation depends on suppression of C/EBPα expression through the translation inhibitory activity of the RNA-binding protein hnRNP-E2. Here we show that “oncogene dosage” is a determinant factor for suppression of differentiation in CML-BC. In fact, high levels of p210-BCR/ABL are required for enhanced hnRNP-E2 expression, which depends on phosphorylation of hnRNP-E2 serines 173, 189, and 272 and threonine 213 by the BCR/ABL-activated MAPKERK1/2. Serine/threonine to alanine substitution abolishes hnRNP-E2 phosphorylation and markedly decreases its stability in BCR/ABL-expressing myeloid precursors. Similarly, pharmacologic inhibition of MAPKERK1/2 activity decreases hnRNP-E2 binding to the 5′UTR of C/EBPα mRNA by impairing hnRNP-E2 phosphorylation and stability. This, in turn, restores in vitro and/or in vivo C/EBPα expression and G-CSF–driven neutrophilic maturation of differentiation-arrested BCR/ABL+ cell lines, primary CML-BCCD34+ patient cells and lineage-negative mouse bone marrow cells expressing high levels of p210-BCR/ABL. Thus, increased BCR/ABL oncogenic tyrosine kinase activity is essential for suppression of myeloid differentiation of CML-BC progenitors as it is required for sustained activation of the MAPKERK1/2-hnRNP-E2-C/EBPα differentiation-inhibitory pathway. Furthermore, these findings suggest the inclusion of clinically relevant MAPK inhibitors in the therapy of CML-BC.

BCR and BCR-ABL regulation during myeloid differentiation in healthy donors and in chronic phase/blast crisis CML patients

“…This hypothesis is further supported by the ability of BCR-ABL to block myeloid differentiation. 5,6 Overall, these data suggest that the 'oncogene dosage' is a determinant factor for the differentiation block in BC-CML and that the progression to BC could be linked to an abnormal control of BCR-ABL expression during myeloid differentiation with ensuing maturation block. Unfortunately, the mechanisms leading to the deregulated expression of BCR-ABL in BC are not known.…”

“…Previous works showed that the BCR-ABL expression level is critical for this differentiation block. 5 CEBPa represents the principal inducer of granulocytic differentiation; in BC-CML primary cells, CEBPa protein is almost undetectable, although its mRNA is expressed at high levels. 6 Perrotti et al 6 showed that BCR-ABL upregulates the expression of hnRNPE2, an RNA-binding protein that, by its interaction with CEBPa mRNA, inhibits CEBPa translation and causes its suppression at the protein level: thus hnRNPE2 and CEBPa protein expression levels inversely correlate.…”

CDDO induces granulocytic differentiation of myeloid leukemic blasts through translational up-regulation of p42 CCAAT enhancer–binding protein alpha

“…6.15 (32D-BCR/ABL) cells have been previously described. 26 Ficoll-separated bone marrow cells from patients with AML were used freshly or after freezing in liquid nitrogen and cultured in IMDM (Gibco) with 20% FBS, 1% P/S, and 1% glutamine. CDDO was synthesized by Dr Sporn, Dartmouth, NH, and was diluted in dimethyl sulfoxide (DMSO) to obtain working concentrations, and identical volumes of DMSO and CDDO (in DMSO) were added to the cultures.…”

“…24,25 In these AML subtypes, re-expression of functional CEBPA restores granulocytic differentiation, suggesting that suppression of CEBPA is essential for the phenotype of AML blasts. Likewise, differentiation of myeloid progenitors in chronic myelogenous leukemia (CML) blast crisis is disturbed by BCR/ABL-induced suppression of CEBPA mRNA translation through the activation of the MAPKhnRNP E2 pathway 23,26 . Here, we provide evidence that CDDO potently induces granulocytic differentiation of leukemic cell lines and patient-derived primary AML blasts by translationally enhancing the expression and function of CEBPA through a mechanism that involves increase of p42 and the p42/p30 ratio.…”

“…For personal use only. on August 28, 2018. by guest www.bloodjournal.org From (CML) blast crisis is disturbed by BCR/ABL-induced suppression of CEBPA mRNA translation through the activation of the MAPKhnRNP E2 pathway 23,26 . Here, we provide evidence that CDDO potently induces granulocytic differentiation of leukemic cell lines and patient-derived primary AML blasts by translationally enhancing the expression and function of CEBPA through a mechanism that involves increase of p42 and the p42/p30 ratio.…”