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

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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.