Protein Synthesis Escapes mTORC1 Control and Constitutes a Promising Therapeutic Target in Acute Myeloid Leukemia.

The deregulation of translation, leading to a preferential translation of oncogene-related mRNAs, markedly contributes to the malignant phenotype in human cancers. The assembly of the eukaryotic translation Initiation Factor 4F (eIF4F) complex is limiting for malignancy-related translation events and is hence an attractive anticancer therapeutic target. The Mammalian Target Of Rapamycin Complex 1 (mTORC1) is thought to positively regulate eIF4F assembly and subsequent oncogenic protein synthesis through eIF4E Binding Protein 1 (4E-BP1) phosphorylation. The mTORC1 inhibitor rapamycin has been developed as an anti-cancer agent but is of limited efficacy in the treatment of Acute Myeloid Leukemia (AML). We show in our study that in primary AML cells, 4E-BP1 phosphorylation is mTORC1-independent and is in fact controlled by the Pim- 2 serine/threonine kinase: neither the rapamycin derivate RAD001 nor Raptor siRNA inhibited 4E-BP1 phosphorylation contrary to siRNA-mediated Pim-2 knock-down. We utilized the 4E-BP1 mimetic 4EGI-1, as a direct inhibitor of translation in AML. In contrast to RAD001, 4EGI-1

• totally inhibited eIF4F assembly;

• markedly decreased the proportion of polysome-bound c-Myc mRNA; and

• dramatically decreased the accumulation of oncogenic proteins that are regulated at the translational level.

Moreover, 4EGI-1 demonstrated potent anti-leukemic activity ex vivo in primary AML blast cells through a selective suppression of proteins implicated in proliferation and in anti-apoptotic mechanisms. Significantly, these effects occurred with a minimal degree of toxicity against normal hematopoietic processes. Our present results therefore strongly implicate an mTORC1-independent mechanism of deregulated synthesis of oncogenic proteins in human myeloid leukemogenesis. The direct inhibition of the eIF4F translation initiating complex, by 4EGI-1, by the recently developed eIF4E antisense oligonucleotides or by specific Pim-2 inhibitors when available, thus represents an attractive option for the development of new therapies for AML.