Paola Bergamo

The mitogen-activated protein kinase (MAPK) cascade controls phosphatase and tensin homolog (PTEN) expression through multiple mechanisms

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.

Co-targeting of Bcl-2 and mTOR pathway triggers synergistic apoptosis in BH3 mimetics resistant acute lymphoblastic leukemia

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.

Aberrant Proliferative and Apoptotic Pathways in Acute Lymphoblastic Leukemia (ALL): Molecular Therapies to Overcome Chemo-Resistance

Adult acute lymphoblastic leukemia (ALL) is characterized by a high relapse rate, with the majority of patients developing chemo-resistance and ultimately dying of the disease with a 5-year survival rate of 40% (Faderl et al., 2010). Significant advances, however, have been made in cases carrying the acquired genetic alteration BCR-ABL (ALL-Ph+) targeted by tyrosine-kinase inhibitors (Ottmann & Pfeifer, 2009). Therefore, several studies have recently been carried out to look for additional, therapeutically exploitable, genetic lesions. Aberrant activation of signal transduction pathways (STP) implicated in proliferation and survival mechanisms are generally involved in leukemogenesis and drug resistance (Zhao et al., 2010). Genes in the PI3K/PTEN/AKT/mTOR, RAS/RAF/MEK/ERK, and Jak/STAT pathways are frequently mutated and their expression is often altered in hematopoietic malignancies, including ALL (McCubrey et al., 2011; Steelman et al. 2008). In addition, deregulation of survival mechanisms may confer chemo-resistance to leukemic cells, particularly involving alterations of the Bcl-2 signaling cascade, which may represent one of the most important, potentially druggable, pathways for therapeutic intervention in ALL. Starting from our studies on chemo-resistance in ALL, particularly on multidrug resistance (MDR1) expression and prognostic significance, in this chapter we will illustrate the major pathways aberrantly activated in ALL PI3K/PTEN/AKT/mTOR, RAF/RAS/MEK/ERK, and the Bcl-2 family of proteins with the ultimate goal of summarizing novel targets for