Differential regulation of 4E-BP1 and 4E-BP2, two repressors of translation initiation, during human myeloid cell differentiation. 
Human myeloid differentiation is accompanied by a decrease in cell proliferation. Because the translation rate is an important determinant of cell proliferation, we have investigated translation initiation during human myeloid cell differentiation using the HL-60 promyelocytic leukemia cell line and the U-937 monoblastic cell line. A decrease in the translation rate is observed when the cells are induced to differentiate along the monocytic/macrophage pathway or along the granulocytic pathway. The inhibition in protein synthesis correlates with specific regulation of two repressors of translation initiation, 4E-BP1 and 4E-BP2. Induction of HL-60 and U-937 cell differentiation into monocytes/macrophages by IFN-gamma or PMA results in a dephosphorylation and consequent activation of 4E-BP1. Dephosphorylation of 4E-BP1 was also observed when U-937 cells were induced to differentiate into monocytes/macrophages following treatment with retinoic acid or DMSO. In contrast, treatment of HL-60 cells with retinoic acid or DMSO, which results in a granulocytic differentiation of these cells, decreases 4E-BP1 amount without affecting its phosphorylation and strongly increases 4E-BP2 amount. Taken together, these data provide evidence for differential regulation of the translational machinery during human myeloid differentiation, specific to the monocytic/macrophage pathway or to the granulocytic pathway.