Hypoxia-Activated Prodrug TH-302 Targets Hypoxic Bone Marrow Niches in Preclinical Leukemia Models.

PURPOSE: To characterize the prevalence of hypoxia in the leukemic bone marrow, its association with metabolic and transcriptional changes in the leukemic blasts and the utility of hypoxia-activated prodrug TH-302 in leukemia models. EXPERIMENTAL DESIGN: Hyperpolarized magnetic resonance spectroscopy was utilized to interrogate the pyruvate metabolism of the bone marrow in the murine acute myeloid leukemia (AML) model. Nanostring technology was used to evaluate a gene set defining a hypoxia signature in leukemic blasts and normal donors. The efficacy of the hypoxia-activated prodrug TH-302 was examined in the in vitro and in vivo leukemia models. RESULTS: Metabolic imaging has demonstrated increased glycolysis in the femur of leukemic mice compared with healthy control mice, suggesting metabolic reprogramming of hypoxic bone marrow niches. Primary leukemic blasts in samples from AML patients overexpressed genes defining a "hypoxia index" compared with samples from normal donors. TH-302 depleted hypoxic cells, prolonged survival of xenograft leukemia models, and reduced the leukemia stem cell pool in vivo In the aggressive FLT3/ITD MOLM-13 model, combination of TH-302 with tyrosine kinase inhibitor sorafenib had greater antileukemia effects than either drug alone. Importantly, residual leukemic bone marrow cells in a syngeneic AML model remain hypoxic after chemotherapy. In turn, administration of TH-302 following chemotherapy treatment to mice with residual disease prolonged survival, suggesting that this approach may be suitable for eliminating chemotherapy-resistant leukemia cells. CONCLUSIONS: These findings implicate a pathogenic role of hypoxia in leukemia maintenance and chemoresistance and demonstrate the feasibility of targeting hypoxic cells by hypoxia cytotoxins.

TGF-ß-Neutralizing Antibody 1D11 Enhances Cytarabine-Induced Apoptosis in AML Cells in the Bone Marrow Microenvironment.

Hypoxia and interactions with bone marrow (BM) stromal cells have emerged as essential components of the leukemic BM microenvironment in promoting leukemia cell survival and chemoresistance. High levels of transforming growth factor beta 1 (TGFß1) produced by BM stromal cells in the BM niche regulate cell proliferation, survival, and apoptosis, depending on the cellular context. Exogenous TGFß1 induced accumulation of acute myeloid leukemia (AML) cells in a quiescent G0 state, which was further facilitated by the co-culture with BM-derived mesenchymal stem cells (MSCs). In turn, TGFß-neutralizing antibody 1D11 abrogated rhTGFß1 induced cell cycle arrest. Blocking TGFß with 1D11 further enhanced cytarabine (Ara-C)-induced apoptosis of AML cells in hypoxic and in normoxic conditions. Additional constituents of BM niche, the stroma-secreted chemokine CXCL12 and its receptor CXCR4 play crucial roles in cell migration and stroma/leukemia cell interactions. Treatment with 1D11 combined with CXCR4 antagonist plerixafor and Ara-C decreased leukemia burden and prolonged survival in an in vivo leukemia model. These results indicate that blockade of TGFß by 1D11 and abrogation of CXCL12/CXCR4 signaling may enhance the efficacy of chemotherapy against AML cells in the hypoxic BM microenvironment.

Targeting of mTORC1/2 by the mTOR kinase inhibitor PP242 induces apoptosis in AML cells under conditions mimicking the bone marrow microenvironment.

The interactions between the bone marrow (BM) microenvironment and acute myeloid leukemia (AML) is known to promote survival of AML cells. In this study, we used reverse phase-protein array (RPPA) technology to measure changes in multiple proteins induced by stroma in leukemic cells. We then investigated the potential of an mTOR kinase inhibitor, PP242, to disrupt leukemia/stroma interactions, and examined the effects of PP242 in vivo using a mouse model. Using RPPA, we confirmed that multiple survival signaling pathways, including the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR), were up-regulated in primary AML cells cocultured with stroma. PP242 effectively induced apoptosis in primary samples cultured with or without stroma. Mechanistically, PP242 attenuated the activities of mTORC1 and mTORC2, sequentially inhibited phosphorylated AKT, S6K, and 4EBP1, and concurrently suppressed chemokine receptor CXCR4 expression in primary leukemic cells and in stromal cells cultured alone or cocultured with leukemic cells. In the in vivo leukemia mouse model, PP242 inhibited mTOR signaling in leukemic cells and demonstrated a greater antileukemia effect than rapamycin. Our findings indicate that disrupting mTOR/AKT signaling with a selective mTOR kinase inhibitor can effectively target leukemic cells within the BM microenvironment.

Human extramedullary bone marrow in mice: a novel in vivo model of genetically controlled hematopoietic microenvironment.

The interactions between hematopoietic cells and the bone marrow (BM) microenvironment play a critical role in normal and malignant hematopoiesis and drug resistance. These interactions within the BM niche are unique and could be important for developing new therapies. Here, we describe the development of extramedullary bone and bone marrow using human mesenchymal stromal cells and endothelial colony-forming cells implanted subcutaneously into immunodeficient mice. We demonstrate the engraftment of human normal and leukemic cells engraft into the human extramedullary bone marrow. When normal hematopoietic cells are engrafted into the model, only discrete areas of the BM are hypoxic, whereas leukemia engraftment results in widespread severe hypoxia, just as recently reported by us in human leukemias. Importantly, the hematopoietic cell engraftment could be altered by genetical manipulation of the bone marrow microenvironment: Extramedullary bone marrow in which hypoxia-inducible factor 1α was knocked down in mesenchymal stromal cells by lentiviral transfer of short hairpin RNA showed significant reduction (50% ± 6%; P = .0006) in human leukemic cell engraftment. These results highlight the potential of a novel in vivo model of human BM microenvironment that can be genetically modified. The model could be useful for the study of leukemia biology and for the development of novel therapeutic modalities aimed at modifying the hematopoietic microenvironment.

Targeting the leukemia microenvironment by CXCR4 inhibition overcomes resistance to kinase inhibitors and chemotherapy in AML.

SDF-1alpha/CXCR4 signaling plays a key role in leukemia/bone marrow microenvironment interactions. We previously reported that bone marrow-derived stromal cells inhibit chemotherapy-induced apoptosis in acute myeloid leukemia (AML). Here we demonstrate that the CXCR4 inhibitor AMD3465 antagonized stromal-derived factor 1alpha (SDF-1alpha)-induced and stroma-induced chemotaxis and inhibited SDF-1alpha-induced activation of prosurvival signaling pathways in leukemic cells. Further, CXCR4 inhibition partially abrogated the protective effects of stromal cells on chemotherapy-induced apoptosis in AML cells. Fetal liver tyrosine kinase-3 (FLT3) gene mutations activate CXCR4 signaling, and coculture with stromal cells significantly diminished antileukemia effects of FLT3 inhibitors in cells with mutated FLT3. Notably, CXCR4 inhibition increased the sensitivity of FLT3-mutated leukemic cells to the apoptogenic effects of the FLT3 inhibitor sorafenib. In vivo studies demonstrated that AMD3465, alone or in combination with granulocyte colony-stimulating factor, induced mobilization of AML cells and progenitor cells into circulation and enhanced antileukemic effects of chemotherapy and sorafenib, resulting in markedly reduced leukemia burden and prolonged survival of the animals. These findings indicate that SDF-1alpha/CXCR4 interactions contribute to the resistance of leukemic cells to signal transduction inhibitor- and chemotherapy-induced apoptosis in systems mimicking the physiologic microenvironment. Disruption of these interactions with CXCR4 inhibitors represents a novel strategy of sensitizing leukemic cells by targeting their protective bone marrow microenvironment.

Mutant FLT3: a direct target of sorafenib in acute myelogenous leukemia.

BACKGROUND: Internal tandem duplication (ITD) mutations in the juxtamembrane domain-coding sequence of the Fms-like tyrosine kinase 3 (FLT3) gene have been identified in 30% of acute myeloid leukemia (AML) patients and are associated with a poor prognosis. The kinase inhibitor sorafenib induces growth arrest and apoptosis at much lower concentrations in AML cell lines that harbor FLT3-ITD mutations than in AML cell lines with wild-type FLT3. METHODS: The antileukemic activity of sorafenib was investigated in isogenic murine Ba/F3 AML cell lines that expressed mutant (ITD, D835G, and D835Y) or wild-type human FLT3, in primary human AML cells, and in a mouse leukemia xenograft model. Effects of sorafenib on apoptosis and signaling in AML cell lines were investigated by flow cytometry and immunoblot analysis, respectively, and the in vivo effects were determined by monitoring the survival of leukemia xenograft-bearing mice treated with sorafenib (groups of 15 mice). In a phase 1 clinical trial, 16 patients with refractory or relapsed AML were treated with sorafenib on different dose schedules. We determined their FLT3 mutation status by a polymerase chain reaction assay and analyzed clinical responses by standard criteria. All statistical tests were two-sided. RESULTS: Sorafenib was 1000- to 3000-fold more effective in inducing growth arrest and apoptosis in Ba/F3 cells with FLT3-ITD or D835G mutations than in Ba/F3 cells with FLT3-D835Y mutant or wild-type FLT3 and inhibited the phosphorylation of tyrosine residues in ITD mutant but not wild-type FLT3 protein. In a mouse model, sorafenib decreased the leukemia burden and prolonged survival (median survival in the sorafenib-treated group vs the vehicle-treated group = 36.5 vs 16 days, difference = 20.5 days, 95% confidence interval = 20.3 to 21.3 days; P = .0018). Sorafenib reduced the percentage of leukemia blasts in the peripheral blood and the bone marrow of AML patients with FLT3-ITD (median percentages before and after sorafenib: 81% vs 7.5% [P = .016] and 75.5% vs 34% [P = .05], respectively) but not in patients without this mutation. CONCLUSION: Sorafenib may have therapeutic efficacy in AML patients whose cells harbor FLT3-ITD mutations.

Mechanisms of apoptosis sensitivity and resistance to the BH3 mimetic ABT-737 in acute myeloid leukemia.

BCL-2 proteins are critical for cell survival and are overexpressed in many tumors. ABT-737 is a small-molecule BH3 mimetic that exhibits single-agent activity against lymphoma and small-cell lung cancer in preclinical studies. We here report that ABT-737 effectively kills acute myeloid leukemia blast, progenitor, and stem cells without affecting normal hematopoietic cells. ABT-737 induced the disruption of the BCL-2/BAX complex and BAK-dependent but BIM-independent activation of the intrinsic apoptotic pathway. In cells with phosphorylated BCL-2 or increased MCL-1, ABT-737 was inactive. Inhibition of BCL-2 phosphorylation and reduction of MCL-1 expression restored sensitivity to ABT-737. These data suggest that ABT-737 could be a highly effective antileukemia agent when the mechanisms of resistance identified here are considered.

Synthetic triterpenoid 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid induces growth arrest in HER2-overexpressing breast cancer cells.

HER2 overexpression is one of the most recognizable molecular alterations in breast tumors known to be associated with a poor prognosis. In the study described here, we explored the effect of HER2 overexpression on the sensitivity of breast cancer cells to the growth-inhibitory effects of 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid (CDDO), a synthetic triterpenoid, both in vitro and in vivo in a xenograft model of breast cancer. Both cell growth and colony formation in the soft agar assay, a hallmark of the transformation phenotype, were preferentially suppressed in HER2-overexpressing cell lines at low concentrations of CDDO, whereas growth-inhibitory effects at high concentrations did not correlate with the expression level of HER2. CDDO dose-dependently inhibited phosphorylation of HER2 in HER2-overexpressing cells and diminished HER2 kinase activity in vitro. CDDO induced the transactivation of the nuclear receptor peroxisome proliferator-activated receptor-gamma in both vector control and HER2-transfected MCF7 cells. Dose-response studies showed that the growth inhibition seen at lower concentrations of CDDO correlated with induction of the tumor suppressor gene caveolin-1, which is known to inhibit breast cancer cell growth. CDDO also reduced cyclin D1 mRNA and protein expression. In vivo studies with liposomally encapsulated CDDO showed complete abrogation of the growth of the highly tumorigenic MCF7/HER2 cells in a xenograft model of breast cancer. These findings provide the first in vitro and in vivo evidence that CDDO effectively inhibits HER2 tyrosine kinase activity and potently suppresses the growth of HER2-overexpressing breast cancer cells and suggest that CDDO has a therapeutic potential in advanced breast cancer.

2-Cyano-3,12-dioxooleana-1,9-dien-28-imidazolide (CDDO-Im) directly targets mitochondrial glutathione to induce apoptosis in pancreatic cancer.

Surgical resection is the only curative strategy for pancreatic cancer (PC). Unfortunately, >80% of pancreatic cancer patients bear inoperable, locally advanced, chemoresistant tumors demonstrating the urgent need for development of novel therapeutic approaches to treat this disease. Here we report that the synthetic triterpenoid 2-cyano-3,12 dioxooleana-1,9 dien-28-imidazolide (CDDO-Im) antagonizes PC cell growth by inducing apoptosis at submicromolar concentrations. Notably, we demonstrate for the first time that the cytotoxicity of CDDO-Im is accompanied by the rapid and selective depletion of mitochondrial glutathione that results in accumulation of reactive oxygen species, oxidation of the cellular glutathione pool, loss of mitochondrial membrane potential, and phosphatidylserine externalization. The parent compound CDDO as well as the methyl ester of CDDO also depleted mitochondrial glutathione, demonstrating that this effect is mediated by the triterpenoid nucleus of these agents. Co-treatment with sulfhydryl nucleophiles completely prevented apoptosis and loss of viability induced by CDDO-Im, whereas alkylation of intracellular thiols by diethylmaleate or co-treatment with dithiothreitol decreased the accumulation of a biotinylated derivative of CDDO, TP-301, in PC cells, suggesting that intracellular reduced thiols are functional targets of the electrophilic triterpenoid nucleus of CDDO and its derivatives. In conclusion, our report is the first to identify mitochondrial glutathione as a target of CDDO and its derivatives and demonstrates that depletion of this antioxidant in the mitochondria is an effective strategy to induce cell death in PC cells. These results suggest that CDDO and its derivatives may offer a clinical benefit for the treatment of PC.

PAX6 suppresses growth of human glioblastoma cells.

PURPOSE: Glioblastomas (GBMs) are the most common primary malignant brain tumors. Majority of GBMs has loss of heterozygosity of chromosome 10. The PAX6 encodes a transcription factor that involves in development of the brain, where its expression persists. We have reported that the expression of PAX6 was significantly reduced in GBMs and that a low level of PAX6 expression is a harbinger of an unfavorable prognosis for patients with malignant astrocytic glioma. Interestingly, PAX6 expression was increased in suppressed somatic cell hybrids derived from introducing a normal human chromosome 10 into U251 GBM cells. Thus it is interesting to determine if repression of PAX6 expression is involved in anti-tumor suppression function in GBM. EXPERIMENTAL DESIGN: We overexpressed PAX6 in a GBM cell line U251HF via either stable transfection or infection with recombinant adenovirus, and examined cell growth in vitro and in vivo. RESULT: Although we did not observe changes in the cell doubling time for PAX6-stable transfectants, significantly fewer numbers of PAX6-positive colonies grew in soft agar. Transient overexpression of PAX6 via adenovirus, however, suppressed cell growth by increasing the number of cells in G1 and by decreasing the number of cells in S-phase, and later on caused a dramatic level of cell death. Repeated subcutaneous and intracranial implantation experiments in nude mice using PAX6-stable transfectants provided solid evidence that PAX6 suppressed tumor growth in vivo and significantly extended mouse survival. CONCLUSION: Our data demonstrate that PAX6exerts a tumor suppressor function that limits the growth of GBM cells.

VEGF(121), VEGF(165) overexpression enhances tumorigenicity in U251 MG but not in NG-1 glioma cells.

Vascular endothelial growth factor (VEGF) is a multifunctional cytokine with important roles in angiogenesis. VEGF is overexpressed in human cancers, including highly vascularized and infiltrative brain tumors. In our previous study of seven glioma cell lines, VEGF expression levels correlated with blood vessel density and tumorigenicity, and U251 MG and NG-1 cells were recognized as low-tumorigenic glioma cell lines. We hypothesized that low-tumorigenic cells can become highly tumorigenic when high levels of VEGF are expressed. To test this hypothesis, we constructed VEGF expression vectors containing 564 bp or 696 bp of VEGF(121) or VEGF(165) cDNA, respectively, and transfected them into U251 MG and NG-1 cells. In comparison to parental cells, the 20 VEGF-expressing clones examined had on average 8-10-fold more VEGF mRNA and 12-88-fold more secreted VEGF proteins. Four VEGF-overexpressing clones (U251 MG/V121-C2, U251 MG/V165-C3, NG-1/V121-C6, and NG-1/V165-C3) were selected for additional study. As VEGF production increased with population growth, U251 MG/V121-C2 and U251 MG/V165-C3 cells accumulated 47.9 and 22.0 ng of VEGF during a 5-day culture of 10(4) cells, a 313- and 144-fold overexpression when compared with that in parental U251 MG cells. NG-1/V121-C6 and NG-1/V165-C3 cells secreted 30.4 and 9.4 ng of VEGF, respectively, or 138- and 43-fold more than did the parental NG-1 cells. Subcutaneous implantation of the VEGF-overexpressing U251 MG cells into nude mice caused huge, soft hemorrhagic tumors to form, whereas controls maintained very small tumors. Intracranial implantation of the VEGF-overexpressing cell lines significantly shortened survival of the mice when compared with controls, and it caused formation of solid brain tumors with variable sized hemorrhages, whereas the controls had no apparent brain tumors. Tumorigenicity of U251 MG cells was synergized by co-overexpression of VEGF(121) and VEGF(165). In addition, VEGF(165) seemed to be more potent to the brain endothelium than was VEGF(121). More interestingly, except when an admixture of cells was implanted s.c., VEGF overexpression in NG-1 cells did not promote hemorrhagic tumor formation. These data suggested that a switch from a phenotype of low tumorigenicity to one of high tumorigenicity is possible when VEGF overexpression occurs, although other factors may also be required.