Defect in the regulation of 4E-BP1 and 2, two repressors of translation initiation, in the retinoid acid resistant cell lines, NB4-R1 and NB4-R2.

We recently reported evidence for differential regulation of the translation machinery during human myeloid differentiation, specific to the monocytic/macrophage pathway or to the granulocytic pathway. A decrease in translation rates and concomitant regulation of two repressors of translation initiation, 4E-BP1 and 4E-BP2 (eIF4E-binding proteins 1 and 2), occur in cells induced to differentiate along the monocytic/macrophage pathway or along the granulocytic pathway. Induction of HL-60 and U-937 cell differentiation into monocytes/macrophages results in a dephosphorylation and consequent activation of 4E-BP1. In contrast, following treatment of HL-60 cells with retinoic acid (RA) which results in a granulocytic differentiation of these cells, 4E-BP1 protein expression is decreased whereas 4E-BP2 protein expression is strongly increased. In this study, we further investigated the regulation of 4E-BP1 and 4E-BP2 in the RA-induced differentiation process using the NB4 promyelocytic cell line and the RA maturation-resistant NB4 subclones, NB4-R1 and NB4-R2. RA treatment resulted in a decrease in 4E-BP1 protein and mRNA expression and concomitant increase in 4E-BP2 protein expression, in NB4 cells, but not in NB4-R1 and NB4-R2 cells. The increase in 4E-BP2 protein expression was not correlated to an increase in 4E-BP2 mRNA level suggesting a post-transcriptional regulation of 4E-BP2 expression. In RA-primed cells, cAMP induce maturation of NB4-R1, but not NB4-R2 cells. cAMP treatment resulted in a down-regulation of 4E-BP1 protein and mRNA expression in RA-primed NB4-R1, but not NB4-R2 cells. However, 4E-BP2 expression was not modified in both cell types following cAMP treatment. This indicates that 4E-BP1 down-regulation is associated with granulocytic maturation, whereas post-transcriptional regulation of 4E-BP2 expression is associated with the early action of RA.