mTORC1 Activity Is Independent of PI3K/Akt but Controlled by AMPK in Acute Myeloid Leukemia.

In most cancer models, the PI3K/Akt pathway controls the activity of the mammalian Target of Rapamycin Complex 1 (mTORC1) that regulates cell growth through the phosphorylation of P70S6K and 4E-BP1. In Acute Myeloid Leukemia (AML), both constitutive activation of PI3K (PI3K+) and of mTORC1 are detected. We previously reported in AML that PI3K activity is due to the expression of the p110d isoform of class Ia PI3K and is constitutive in 50% of samples at diagnosis. We show here in primary AML samples that the constitutive activity of mTORC1, detected by the phosphorylation of P70S6K on Thr389, is not due to PI3K/Akt activation. In PI3K+ samples, the specific inhibition of p110d with IC87114 does not inhibit mTORC1. Furthermore, a constitutive activity of mTORC1 is detected in almost all AML samples regardless of their PI3K/ Akt activation status. Similarly, the inhibition of ERK/MAPK activity with UO126 does not result in mTORC1 inhibition. Interestingly, in primary AML cells, we shows that mTORC1 is fully inhibited following leucine starvation as observed in non-transformed cells, suggesting that the kinase activity of mTOR is not deregulated and remains accessible to target inhibition. We thus focused on the links between the AMP kinase (AMPK) and mTORC1. AMPK is a physiological cellular energy sensor and becomes phosphorylated at Thr172 downstream of LKB1 in response to a decrease in cellular ATP levels. AMPK then controls multiple metabolic processes such as fatty acid oxidation, glucose production and protein synthesis. Interestingly, AMPK is usually unphosphorylated in cancer cells including AML and thereby represents a potential therapeutic target. We thus investigated the effects of metformin, a potent AMPK activator developed as an oral anti-diabetic drug, in primary AML cells. This compound induces a dose-dependent activation of AMPK that correlates with a decrease of mTORC1 activity. Primary AML blast cells cultured with metformin show a significant inhibition of proliferation, of clonogenic growth and apoptosis suggesting that AMPK may be an attractive anticancer therapeutic target in AML. However, as metformin may exerts AMPK-independent effects, we are producing a lentiviral construct expressing a constitutively activated AMPK mutant to more specifically determine the consequences of AMPK activation in mTORC1 signaling and blast cells survival.