Targeting of active mTOR inhibits primary leukemia T cells and synergizes with cytotoxic drugs and signaling inhibitors.

OBJECTIVE: Rationally designed therapies aim at the specific disruption of critical signaling pathways activated by malignant transformation or signals from the tumor microenvironment. Because mammalian target of rapamycin (mTOR) is an important signal integrator and a key translational regulator, we evaluated its potential involvement in T-cell acute lymphoblastic leukemia (T-ALL) and whether mTOR blockade synergizes with chemotherapeutic agents or other signaling antagonists to inhibit primary leukemia T cells. MATERIALS AND METHODS: 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. RESULTS: 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 p27(kip1), 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. CONCLUSIONS: 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.

IL-2 regulates FOXP3 expression in human CD4+CD25+ regulatory T cells through a STAT-dependent mechanism and induces the expansion of these cells in vivo.

IL-2 plays a critical role in the maintenance of CD4+CD25+ FOXP3(+) regulatory T cells (Tregs) in vivo. We examined the effects of IL-2 signaling in human Tregs. In vitro, IL-2 selectively up-regulated the expression of FOXP3 in purified CD4+CD25+ T cells but not in CD4+CD25- cells. This regulation involved the binding of STAT3 and STAT5 proteins to a highly conserved STAT-binding site located in the first intron of the FOXP3 gene. We also examined the effects of low-dose IL-2 treatment in 12 patients with metastatic cancer and 9 patients with chronic myelogenous leukemia after allogeneic hematopoietic stem cell transplantation. Overall, IL-2 treatment resulted in a 1.9 median fold increase in the frequency of CD4+CD25+ cells in peripheral blood as well as a 9.7 median fold increase in FOXP3 expression in CD3+ T cells. CD56+CD3- natural killer (NK) cells also expanded during IL-2 therapy but did not express FOXP3. In vitro treatment of NK cells with 5-aza-2'-deoxycytidine restored the IL-2 signaling pathway leading to FOXP3 expression, suggesting that this gene was constitutively repressed by DNA methylation in these cells. Our findings support the clinical evaluation of low-dose IL-2 to selectively modulate CD4+CD25+ Tregs and increase expression of FOXP3 in vivo.

Elevated levels of Rad51 recombination protein in tumor cells.

Rad51 is the key enzyme for homologous recombination, an evolutionarily conserved mechanism for the repair of DNA damage and the generation of genetic diversity. Given the observation that many tumors become resistant to radiation therapy and DNA-damaging chemotherapeutics and also that tumor cell populations can acquire a high number of genetic alterations and then expand clonally, dysfunction of the mammalian Rad51 recombinase could play a major role in the multistep process of tumorigenesis. The data we present provide further strong support for this hypothesis. Using anti-Rad51 immunofluorescence staining, widely different tumor cell lines displayed increased numbers of nuclei with focally concentrated Rad51 protein compared with nonmalignant control cell lines. These nuclear foci are thought to represent a repairosome-type assembly of Rad51 and other proteins required for recombinational DNA repair. By Western blot analyses, the net amount of Rad51 protein was increased 2-7-fold in all tested tumor cell lines. Inhibition of de novo protein synthesis by cycloheximide treatment showed a similar half-life of Rad51 protein in normal and tumor cells. Fluorescence in situ hybridization experiments did not detect Rad51 gene amplifications in tumors. Because Northern blot analysis demonstrated highly elevated Rad51 mRNA levels, we conclude that the increases in Rad51 protein and nuclear foci formation in tumor cells are the result of transcriptional up-regulation.

Formation of higher-order nuclear Rad51 structures is functionally linked to p21 expression and protection from DNA damage-induced apoptosis.

After exposure of mammalian cells to DNA damage, the endogenous Rad51 recombination protein is concentrated in multiple discrete foci, which are thought to represent nuclear domains for recombinational DNA repair. Overexpressed Rad51 protein forms foci and higher-order nuclear structures, even in the absence of DNA damage, in cells that do not undergo DNA replication synthesis. This correlates with increased expression of the cyclin-dependent kinase (Cdk) inhibitor p21. Following DNA damage, constitutively Rad51-overexpressing cells show reduced numbers of DNA breaks and chromatid-type chromosome aberrations and a greater resistance to apoptosis. In contrast, Rad51 antisense inhibition reduces p21 protein levels and sensitizes cells to etoposide treatment. Downregulation of p21 inhibits Rad51 foci formation in both normal and Rad51-overexpressing cells. Collectively, our results show that Rad51 expression, Rad51 foci formation and p21 expression are interrelated, suggesting a functional link between mammalian Rad51 protein and p21-mediated cell cycle regulation. This mechanism may contribute to a highly effective recombinational DNA repair in cell cycle-arrested cells and protection against DNA damage-induced apoptosis.