[Show abstract][Hide abstract] ABSTRACT: Several chemo-resistance mechanisms including the Bcl-2 protein family overexpression and constitutive activation of the PI3K/Akt/mTOR signaling have been documented in acute lymphoblastic leukemia (ALL), encouraging targeted approaches to circumvent this clinical problem. Here we analyzed the activity of the BH3 mimetic ABT-737 in ALL, exploring the synergistic effects with the mTOR inhibitor CCI-779 on ABT-737 resistant cells. We showed that a low Mcl-1/Bcl-2 plus Bcl-xL protein ratio determined ABT-737 responsiveness. ABT-737 exposure further decreased Mcl-1, inducing apoptosis on sensitive models and primary samples, while not affecting resistant cells. Co-inhibition of Bcl-2 and the mTOR pathway resulted cytotoxic on ABT-737 resistant models, by downregulating mTORC1 activity and Mcl-1 in a proteasome-independent manner. Although Mcl-1 seemed to be critical, ectopic modulation did not correlate with apoptosis changes. Importantly, dual targeting proved effective on ABT-737 resistant samples, showing additive/synergistic effects. Together, our results show the efficacy of BH3 mimetics as single agent in the majority of the ALL samples and demonstrate that resistance to ABT-737 mostly correlated with Mcl-1 overexpression. Co-targeting of the Bcl-2 protein family and mTOR pathway enhanced drug-induced cytotoxicity by suppressing Mcl-1, providing a novel therapeutic approach to overcome BH3 mimetics resistance in ALL.
[Show abstract][Hide abstract] ABSTRACT: 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 G1-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
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.
Journal of Molecular Medicine 03/2012; 90(10):1133-44. DOI:10.1007/s00109-012-0886-z · 5.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The mitogen-activated protein kinase (MAPK) and PI3K pathways are regulated by extensive crosstalk, occurring at different levels. In tumors, transactivation of the alternate pathway is a frequent "escape" mechanism, suggesting that combined inhibition of both pathways may achieve synergistic antitumor activity. Here we show that, in the M14 melanoma model, simultaneous inhibition of both MEK and mammalian target of rapamycin (mTOR) achieves synergistic effects at suboptimal concentrations, but becomes frankly antagonistic in the presence of relatively high concentrations of MEK inhibitors. This observation led to the identification of a novel crosstalk mechanism, by which either pharmacologic or genetic inhibition of constitutive MEK signaling restores phosphatase and tensin homolog (PTEN) expression, both in vitro and in vivo, and inhibits downstream signaling through AKT and mTOR, thus bypassing the need for double pathway blockade. This appears to be a general regulatory mechanism and is mediated by multiple mechanisms, such as MAPK-dependent c-Jun and miR-25 regulation. Finally, PTEN upregulation appears to be a major effector of MEK inhibitors' antitumor activity, as cancer cells in which PTEN is inactivated are consistently more resistant to the growth inhibitory and anti-angiogenic effects of MEK blockade.
Journal of Molecular Medicine 01/2012; 90(6):667-79. DOI:10.1007/s00109-011-0844-1 · 5.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recently, strategies for acute myeloid leukemia (AML) therapy have been developed that target anti-apoptotic BCL2 family members using BH3-mimetic drugs such as ABT-737. Though effective against BCL2 and BCL-X(L), ABT-737 poorly inhibits MCL-1. Here we report that, unexpectedly, ABT-737 induces activation of the extracellular receptor activated kinase and induction of MCL-1 in AML cells. MEK inhibitors such as PD0325901 and CI-1040 have been used successfully to suppress MCL-1. We report that PD0325901 blocked ABT-737-induced MCL-1 expression, and when combined with ABT-737 resulted in potent synergistic killing of AML-derived cell lines, primary AML blast and CD34+38-123+ progenitor/stem cells. Finally, we tested the combination of ABT-737 and CI-1040 in a murine xenograft model using MOLM-13 human leukemia cells.Whereas control mice and CI-1040-treated mice exhibited progressive leukemia growth, ABT-737, and to a significantly greater extent, ABT-737+CI-1040 exerted major anti-leukemia activity. Collectively, results demonstrated unexpected anti-apoptotic interaction between the BCL2 family-targeted BH3-mimetic ABT-737 and mitogen-activated protein kinase signaling in AML cells: the BH3 mimetic is not only restrained in its activity by MCL-1, but also induces its expression. However, concomitant inhibition by BH3 mimetics and MEK inhibitors could abrogate this effect and may be developed into a novel and effective therapeutic strategy for patients with AML.
Leukemia: official journal of the Leukemia Society of America, Leukemia Research Fund, U.K 11/2011; 26(4):778-87. DOI:10.1038/leu.2011.287 · 10.43 Impact Factor