RAD 001 (everolimus) prevents mTOR and Akt late re‐activation in response to imatinib in chronic myeloid leukemia

Abstract

The mammalian target of rapamycin (mTOR) is one target of BCR-ABL fusion gene of chronic myeloid leukemia (CML). Moreover, it drives a compensatory route to Imatinib mesylate (IM) possibly involved in the progression of leukemic progenitors towards a drug-resistant phenotype. Accordingly, mTOR inhibitors are proposed for combined therapeutic strategies in CML. The major caveat in the use of mTOR inhibitors for cancer therapy comes from the induction of an mTOR-phosphatidylinositol 3 kinase (PI3k) feedback loop driving the retrograde activation of Akt. Here we show that the rapamycin derivative RAD 001 (everolimus, Novartis Institutes for Biomedical Research) inhibits mTOR and, more importantly, revokes mTOR late re-activation in response to IM. RAD 001 interferes with the assembly of both mTOR complexes: mTORC1 and mTORC2. The inhibition of mTORC2 results in the de-phosphorylation of Akt at Ser(473) in the hydrophobic motif of C-terminal tail required for Akt full activation and precludes Akt re-phosphorylation in response to IM. Moreover, RAD 001-induced inhibition of Akt causes the de-phosphorylation of tuberous sclerosis tumor suppressor protein TSC2 at 14-3-3 binding sites, TSC2 release from 14-3-3 sigma (restoring its inhibitory function on mTORC1) and nuclear import (promoting the nuclear translocation of cyclin-dependent kinase [CDK] inhibitor p27(Kip1), the stabilization of p27(Kip1) ligand with CDK2, and the G(0)/G(1) arrest). RAD 001 cytotoxicity on cells not expressing the BCR-ABL fusion gene or its p210 protein tyrosine kinase (TK) activity suggests that the inhibition of normal hematopoiesis may represent a drug side effect.

Extra-thymically induced regulatory T cells: Do they have potential in disease prevention?

“…The ability of rapamycin to promote Treg cell generation was therefore attributed to its ability to inhibit mTORC1. However, more recent studies confirmed that mTOR inhibitors as rapamycin or everolimus do inhibit mTORC2 as well, thereby causing a reduction of Akt activity in vitro and in vivo [85-88]. This fact appears to be in line with recent studies on the role of Foxo factors in regulating Foxp3 expression.…”

Utility of mTOR Inhibition in Hematologic Malignancies

“…When PP242 was combined with higher doses of dasatinib, the decrease in leukemic burden was significantly higher than with dasatinib monotherapy [17]. The combination of everolimus and imatinib enhanced the cytotoxicity induced by imatinib alone in Bcr-Abl-expressing cells via persistent inhibition of cell proliferation and apoptosis [32,33]. Greater nuclear translocation of the normal p145 c-Abl protein occurred in response to the combined presence of everolimus and imatinib, leading to apoptosis [33].…”

“…Everolimus also may have a role in the blockade of imatinib resistance in leukemic cells. One mechanism of imatinib resistance involves activation of the PI3K/Akt/mTOR pathway; however, treatment with everolimus antagonized the late reactivation of mTOR triggered by imatinib treatment in Bcr-Abl-expressing cells [32]. The T315I mutation in the Abl kinase domain coding region alters the imatinib binding site on tyrosine kinase and confers resistance to imatinib.…”

New Strategies in the Chemotherapy of Leukemia: Eradicating Cancer Stem Cells in Chronic Myeloid Leukemia

“…Currently, it is under phase I/II clinical trials (NCT00093639, www.clinicaltrials.gov) in combination with IM to treat CP CML patients who are not in complete cytogenetic remission after previous treatment with IM. Everolimus inhibits mTOR and revokes its late re-activation in response to IM [173] and enhances the effects of IM in CML by raising the nuclear expression of c-ABL proteins [174]. Ridaforolimus, previously called Deforolimus, is another mTOR inhibitor undergoing clinical trials for the treatment of aggressive, relapsed/refractory lymphoma [175] (NCT00086125, www.clinical trials.gov) and metastatic bone sarcomas [176].…”