We previously reported that constitutive activation of PI3K/AKT activity in primary blasts from patients with AML is related to the expression of the p110d isoform of class IA PI3K which is activated by an upstream, actually unknown kinase (

Sujobert P,
Blood
,
2005
,
106
,
1063
). Activation of PI3K activity might make AML cells susceptible to mTORC1 inhibition, which was recently shown to be activated in AML cells. mTORC1 inhibition by Rapamycin or RAD001 may control both proliferation and survival of leukemic cells. The mechanisms leading to mTORC1 activation in AML are actually unknown

In this report, we studied the interactions between PI3K and mTORC1 pathways in primary blast cells from the bone marrow of patients with primary AML at diagnosis. We observed that specific inhibition of mTORC1 activity with RAD001 upregulates PI3K activity, as evidenced by an increased phosphorylation of AKT on Ser 473. In 8 patients, the mean increase of AKT phosphorylation in the presence of RAD001 was 200% (130%–500%). Such results were observed in most patients with constitutive activation of PI3K, but very rarely in patients without constitutive PI3K activity.

We then studied the activation of the mTORC1 complex when a full inhibition of PI3K activity with IC87114, a specific inhibitor of p110d isoform, was performed. mTORC1 activity, as assessed by phosphorylation of downstream targets, p70S6K, S6 ribosomal protein and 4EBP1 was not inhibited by IC87114 (10μM) in PI3K+ patients, whereas RAD001 (10nM) completely or near completely inhibited all phosphorylation events. Similar results were detected in the MV4-11 cell line. Moreover, we detected constitutive activation of mTORC1 in several PI3K- patients. These results clearly indicate that mTORC1 activation is no related to PI3K activity. In accordance with that, we showed that simultaneous inhibition of PI3K and mTORC1, with IC87114 and RAD001 respectively, additively inhibited the proliferation of blast cells and the clonogenicity of leukemic progenitor cells. Our results provide evidence that RAD001 relieve mTORC1 mediated feedback inhibition on AKT activation, as recently described in multiple myeloma and some cancer cell lines. We also demonstrate that mTORC1 activation occurs independently of PI3K activity. We are currently studying the upstream mechanisms resulting in mTORC1 activation. Finally, functional studies support a clear strategy for combining an mTORC1 inhibitor with an inhibitor of PI3K.

Topics:
1-phosphatidylinositol 3-kinase, leukemia, myelocytic, acute, mechanistic target of rapamycin complex 1, phosphoinositide 3-kinase, proto-oncogene proteins c-akt, protein isoforms, leukemic cells, multiple myeloma, phosphotransferases, rapamycin

Disclosure: No relevant conflicts of interest to declare.

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2006, The American Society of Hematology
2006
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