Role of MEK1 signaling in the regulation of hematopoiesis and leukemogenesis

The Raf/MEK/ERK pathway has long been implicated in leukemogenesis. Mostly, the proteins within the pathway are considered pro-oncogenic, and MEK inhibitors block K-RasG12D-driven myelomonocytic leukemia in preclinical mouse models. Intriguingly, however, silencing of the MEK1 gene has been reported to accelerate Myc- driven lymphomagenesis. We and others have shown that disruption of the mek1 gene results in a recessive lethal phenotype, with the mutant embryos dying at around 9.5 days of gestation due to abnormal placenta development. Embryonic lethality can be circumvented by epiblast-restricted deletion, which yields viable, fertile mek1 deficient mice. Using these mice, we have recently reported that MEK1 is essential for the regulation of the timing and strength of ERK signaling. By phosphorylating the T292 site in the proline-rich region of MEK1, ERK exerts negative feedback control on MEK1/MEK2 dimers. If MEK1 is absent, this control is disabled, leading to increased ERK signaling. More recently, we have discovered that the same ERK-dependent phosphosite of MEK1 is essential for proper membrane localization of the phosphatase and tensin homologue deleted on chromosome ten (PTEN), which functions as a tumor suppressor in leukemogenesis. PTEN converts PIP3 to PIP2, thereby inhibitingerminating AKT activation; therefore, both MEK1 ablation and MEK inhibition lead to AKT activation. In vivo, MEK1 ablation causes a complex immunopathology comprising inflammation, lupus-like autoimmune disease, and myeloproliferation, partially phenocopying PTEN heterozygous mice. We have not, however, found any evidence of leukemias or lymphoma in these animals. Based on this, we now intend to systematically investigate the role of MEK1 in myeloproliferation and leukemogenesis and the molecular mechanisms underlying it. Tissue-restricted MEK1 ablation and bone marrow transplantation will be employed to distinguish between cell-autonomous and non-cell autonomous effects of MEK1 ablation; mouse models of acute myeloid leukemia will be used to test whether MEK1 suppresses leukemogenesis by one or more of the oncogenes implicated in this disease. MEK1 deletion at different stages will elucidate whether MEK1 exerts its function during the establishment or maintenance of the disease. Finally, both candidate and unbiased approaches will be used to elucidate the molecular basis of MEK1`s function. The basic mechanistic information yielded by the proposed research will advance the understanding of the cross-talk between the MEK/ERK and PI3K pathway, provide a mechanistic basis for the observed upregulation of the AKT pathway in leukemic cells treated with MEK inhibitors, and therefore be useful for the design of novel therapies.

The Ras/Raf/MEK/ERK pathway has long been implicated in leukemogenesis. The proteins within the pathway are considered pro-oncogenic, and MEK inhibitors block Ras-driven myelomonocytic leukemia in preclinical mouse models. Intriguingly, however, silencing of the MEK1 gene has been reported to accelerate Myc-driven lymphomagenesis.The protein MEK has two sister forms, MEK1 and MEK2. We have previously shown that MEK1 and MEK2 form a complex that can only be inactivated when both partners are present. If MEK1 is absent, MEK2 can no longer be turned off and the ERK signaling pathway remains active. In order to switch off the MEK1/2 complex and thus the complete pathway, MEK1 must be labeled (phosphorylated) by active ERK at a specific residue. This phosphorylation of MEK1 by ERK is also necessary to terminate another growth and survival pathway, the PI3K/AKT pathway. Simultaneous deregulation of the ERK and AKT pathways is expected to promote uncontrolled cell growth. In vivo, the ablation of MEK1 causes lupus-like autoimmune disease and myeloproliferation. Leukemia or lymphomas were not observed in MEK1 knockout mice.We have now systematically investigated the role of MEK1 in hematopoiesis, myeloproliferation and leukaemogenesis. We have shown that MEK1 plays a role in hematopoiesis; also, the protein is necessary for the development of leukemias, but specifically of those caused by the oncogene Ras, which directly activates the MEK signaling pathway. The results support the use of MEK inhibitors specifically in RAS-mutated leukemias.