mTOR signaling status was assessed using biochemical, immunostaining, and molecular regulation studies and functional assays performed to assess the impact of mTOR blockade on T-ALL proliferation, survival, and cell cycle.
We observed that mTOR signaling is highly activated in all T-ALL patients tested, with phosphorylation of its downstream substrates eIF4G and S6 ribosomal protein. mTOR activation was detected in vivo and was further increased in vitro by stimulation with interleukin-7, a potentially leukemogenic cytokine normally produced by the bone marrow microenvironment. In T-ALL cells, mTOR blockade was associated with accumulation of the cyclin-dependent kinase inhibitor p27kip1, which preferentially adopted a nuclear localization. Functional studies using rapamycin or CCI-779 showed a dominant inhibitory effect of mTOR blockade on interleukin-7−induced proliferation, survival, and cell-cycle progression of T-ALL cells. Furthermore, mTOR blockade markedly potentiated the antileukemia effects of dexamethasone and doxorubicin, and showed highly synergistic interactions in combination with specific inhibitors of phosphatidylinositol 3-kinase/Akt and Janus kinase 3 signaling.
This study shows activation of mTOR signaling in primary T-ALL cells evolving in the leukemic bone marrow, and supports the inclusion of mTOR antagonists in current therapeutic regimens for this cancer.