In
hematological malignancies, constitutive activation of the RAF/MEK/ERK pathway is frequently observed, conveys a poor prognosis, and constitutes a promising target for therapeutic intervention. Here, we investigated the molecular and functional effects of pharmacological
MEK inhibition in cell line models of
acute myeloid leukemia (AML) and freshly isolated primary AML samples. The small-molecule,
ATP-non-competitive,
MEK inhibitor
PD0325901 markedly inhibited ERK phosphorylation and growth of several AML cell lines and approximately 70 % of primary AML samples. Growth inhibition was due to G(1)-phase arrest and induction of apoptosis. Transformation by constitutively active upstream pathway elements (HRAS, RAF-1, and
MEK) rendered FDC-P1 cells exquisitely prone to PD0325901-induced apoptosis. Gene and
protein expression profiling revealed a selective effect of
PD0325901 on ERK phosphorylation and compensatory upregulation of the RAF/
MEK and AKT/p70( S6K )
kinase modules, potentially mediating resistance to drug-induced growth inhibition. Consequently, in appropriate cellular contexts, both "vertical" (i.e., inhibition of RAF and
MEK along the MAPK pathway) and "lateral" (i.e., simultaneous inhibition of the
MEK/ERK and mTOR pathways) combination strategies may result in synergistic anti-leukemic effects. Overall,
MEK inhibition exerts potent growth inhibitory and proapoptotic activity in preclinical models of AML, particularly in combination with other pathway inhibitors. Deeper understanding of the molecular mechanisms of action of
MEK inhibitors will likely translate into more effective targeted strategies for the treatment of AML.