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

The inability of myeloid chronic myelogenous leukemia blast crisis (CIVIL-BC) progenitors to undergo neutrophil differentiation depends on suppression of C/EBP alpha 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 MAPK(ERK1/2). 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/EBP alpha mRNA by impairing hnRNP-E2 phosphorylation and stability. This, in turn, restores in vitro and/or in vivo C/EBP alpha 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 CIVIL-BC progenitors as it is required for sustained activation of the MAPK(ERK1/2-) hnRNP-E2-C/EBP alpha differentiation-inhibitory pathway. Furthermore, these findings suggest the inclusion of clinically relevant MAPK inhibitors in the therapy of CML-BC.