Molecular and Pharmacologic Suppression of MAPK Activity Rescues Differentiation of BCR/ABL⁺ Myeloid Progenitors by Releasing C/EBPa from the Inhibitory Effect of hnRNP E2.

Impaired differentiation is a common feature of many hematological malignancies including blast crisis chronic myelogenous leukemia (CML-BC). We previously reported that the inability of CML-BC myeloid progenitors to undergo terminal neutrophil differentiation depends on suppression of C/EBPα expression through the translation inhibitory activity of the BCR/ABL-regulated RNA binding protein hnRNP E2.

Here we show that p210-BCR/ABL oncoprotein post-translationally enhances hnRNP E2 expression and translation-regulatory function. In fact, hnRNP E2 protein but not mRNA expression directly correlates with BCR/ABL levels in myeloid progenitors expressing different BCR/ABL levels, in Ph1 K562 and EM3 and BCR/ABL-inducible TonB.210 cells. Likewise, graded BCR/ABL expression in primary Lin⁻ mouse bone marrow cells results in increasing hnRNP E2 levels. This, in turn, inhibits the expression of C/EBPα and that of the C/EBPα-regulated G-CSF (granulocytic-colony stimulating factor) receptor.

In these models of CML-BC, increased hnRNP E2 expression results from the BCR/ABL- and ERK1/2-dependent enhancement of hnRNP E2 protein stability, as hnRNP E2 levels are downregulated by treatment with imatinib or the MEK inhibitors PD098059 and U0126, and after expression of dominant negative ERK1/2 mutants. Accordingly, hnRNP E2 protein levels rapidly decrease and are barely detectable in parental 32Dcl3 cells treated for 30 min. with the protein synthesis inhibitor cycloheximide (CHX). By contrast, they remain unchanged in 32D-BCR/ABL cells CHX-treated for 4 hours. Accordingly, hnRNP E2 protein expression is markedly reduced in IL-3-deprived parental but not 32D-BCR/ABL cells through a proteasome-dependent mechanism. Mechanistically, we have evidence that the BCR/ABL-activated ERK1/2 kinases phosphorylate hnRNP E2 on four different sites (S173, S189, T213, and S272), and that serine to alanine substitution of these phosphorylation sites abolishes hnRNP E2 phosphorylation in living cells and in vitro kinase assay. Moreover, the non-phosphatable hnRNP E2^{S173A, S189A, T213A, S272A} mutant is less stable than wild-type and phosphomimetic hnRNP E2.

Functionally, suppression of hnRNP E2 phosphorylation/expression by chemical inhibition of ERK1/2 activity results in decreased hnRNP E2 binding to the C-rich element contained in the uORF-spacer region of C/EBPα mRNA. This, in turn, restores C/EBPα expression and allows G-CSF-driven neutrophilic maturation of differentiation-arrested 32D-BCR/ABL cells and Lin⁻ mouse bone marrow cells expressing high levels of p210-BCR/ABL oncoprotein. Likewise, treatment of G-CSF-cultured (48 hours) CML-BC^CD34+ bone marrow patient cells with the MEK1 inhibitor U0126 (25 mM, 32 h) destabilizes hnRNP E2, thus restoring C/EBPα expression.

Altogether our data not only indicate that the BCR/ABL-induced ERK1/2 activity is essential for the hnRNP E2 stability and suppression of C/EBPα-dependent granulocytic differentiation of CML-BC myeloid progenitors, but they also suggest the potential use of clinically-relevant MAPK inhibitors in the therapy of CML-BC.