Jo Ishizawa

ATF4 induction through an atypical integrated stress response to ONC201 triggers p53-independent apoptosis in hematological malignancies

ONC201 triggers an apoptotic cellular stress response in both solid and blood tumors. Stressing cancer cells to death The anticancer drug ONC201 triggers cell death in various tumor types. A pair of papers (see also the Focus by Greer and Lipkowitz) shows that ONC201 activated cell stress pathways that depended on the activation of the transcription factor ATF4. Kline et al. showed that this stress response to ONC201 occurred in cells derived from various types of solid tumors, in which ATF4 activation led to an increase in the abundance of the proapoptotic protein TRAIL and its receptor DR5. Ishizawa et al. demonstrated that in acute myeloid leukemias and mantle cell lymphoma, ONC201 triggered apoptosis and inhibited mTORC1 signaling, a pathway that promotes cell growth and proliferation. The findings reveal more details about ONC201’s mechanism of action, potentially enabling patient stratification and future development to improve its efficacy. The clinical challenge posed by p53 abnormalities in hematological malignancies requires therapeutic strategies other than standard genotoxic chemotherapies. ONC201 is a first-in-class small molecule that activates p53-independent apoptosis, has a benign safety profile, and is in early clinical trials. We found that ONC201 caused p53-independent apoptosis and cell cycle arrest in cell lines and in mantle cell lymphoma (MCL) and acute myeloid leukemia (AML) samples from patients; these included samples from patients with genetic abnormalities associated with poor prognosis or cells that had developed resistance to the nongenotoxic agents ibrutinib and bortezomib. Moreover, ONC201 caused apoptosis in stem and progenitor AML cells and abrogated the engraftment of leukemic stem cells in mice while sparing normal bone marrow cells. ONC201 caused changes in gene expression similar to those caused by the unfolded protein response (UPR) and integrated stress responses (ISRs), which increase the translation of the transcription factor ATF4 through an increase in the phosphorylation of the translation initiation factor eIF2α. However, unlike the UPR and ISR, the increase in ATF4 abundance in ONC201-treated hematopoietic cells promoted apoptosis and did not depend on increased phosphorylation of eIF2α. ONC201 also inhibited mammalian target of rapamycin complex 1 (mTORC1) signaling, likely through ATF4-mediated induction of the mTORC1 inhibitor DDIT4. Overexpression of BCL-2 protected against ONC201-induced apoptosis, and the combination of ONC201 and the BCL-2 antagonist ABT-199 synergistically increased apoptosis. Thus, our results suggest that by inducing an atypical ISR and p53-independent apoptosis, ONC201 has clinical potential in hematological malignancies.

Expression, function, and targeting of the nuclear exporter chromosome region maintenance 1 (CRM1) protein

Nucleocytoplasmic trafficking of proteins/RNAs is essential to normal cellular function. Indeed, accumulating evidence suggests that cancer cells escape anti-neoplastic mechanisms and benefit from pro-survival signals via the dysregulation of this system. The nuclear exporter chromosome region maintenance 1 (CRM1) protein is the only protein in the karyopherin-β protein family that contributes to the trafficking of numerous proteins and RNAs from the nucleus. It is considered to be an oncogenic, anti-apoptotic protein in transformed cells, since it reportedly functions as a gatekeeper for cell survival, including affecting p53 function, and ribosomal biogenesis. Furthermore, abnormally high expression of CRM1 is correlated with poor patient prognosis in various malignancies. Therapeutic targeting of CRM1 has emerged as a novel cancer treatment strategy, starting with a clinical trial with leptomycin B, the original specific inhibitor of CRM1, followed by development of several next-generation small molecules. KPT-330, a novel member of the CRM1-selective inhibitors of nuclear export (SINE) class of compounds, is currently undergoing clinical evaluation for the therapy of various malignancies. Results from these trials suggest that SINE compounds may be particularly useful against hematological malignancies, which often become refractory to standard chemotherapeutic agents.

Induction of p53-mediated transcription and apoptosis by exportin-1 (XPO1) inhibition in mantle cell lymphoma.

The nuclear transporter exportin‐1 (XPO1) is highly expressed in mantle cell lymphoma (MCL) cells, and is believed to be associated with the pathogenesis of this disease. XPO1‐selective inhibitors of nuclear export (SINE) compounds have been shown to induce apoptosis in MCL cells. Given that p53 is a cargo protein of XPO1, we sought to determine the significance of p53 activation through XPO1 inhibition in SINE‐induced apoptosis of MCL cells. We investigated the prognostic impact of XPO1 expression in MCL cells using Oncomine analysis. The significance of p53 mutational/functional status on sensitivity to XPO1 inhibition in cell models and primary MCL samples, and the functional role of p53‐mediated apoptosis signaling, were also examined. Increased XPO1 expression was associated with poor prognosis in MCL patients. The XPO1 inhibitor KPT‐185 induced apoptosis in MCL cells through p53‐dependent and ‐independent mechanisms, and p53 status was a critical determinant of its apoptosis induction. The KPT‐185‐induced, p53‐mediated apoptosis in the MCL cells occurred in a transcription‐dependent manner. Exportin‐1 appears to influence patient survival in MCL, and the SINE XPO1 antagonist KPT‐185 effectively activates p53‐mediated transcription and apoptosis, which would provide a novel strategy for the therapy of MCL.

Bone Marrow Adipocytes Facilitate Fatty Acid Oxidation Activating AMPK and a Transcriptional Network Supporting Survival of Acute Monocytic Leukemia Cells

Leukemia cells in the bone marrow must meet the biochemical demands of increased cell proliferation and also survive by continually adapting to fluctuations in nutrient and oxygen availability. Thus, targeting metabolic abnormalities in leukemia cells located in the bone marrow is a novel therapeutic approach. In this study, we investigated the metabolic role of bone marrow adipocytes in supporting the growth of leukemic blasts. Prevention of nutrient starvation-induced apoptosis of leukemic cells by bone marrow adipocytes, as well as the metabolic and molecular mechanisms involved in this process, was investigated using various analytic techniques. In acute monocytic leukemia (AMoL) cells, the prevention of spontaneous apoptosis by bone marrow adipocytes was associated with an increase in fatty acid β-oxidation (FAO) along with the upregulation of PPARγ, FABP4, CD36, and BCL2 genes. In AMoL cells, bone marrow adipocyte coculture increased adiponectin receptor gene expression and its downstream target stress response kinase AMPK, p38 MAPK with autophagy activation, and upregulated antiapoptotic chaperone HSPs. Inhibition of FAO disrupted metabolic homeostasis, increased reactive oxygen species production, and induced the integrated stress response mediator ATF4 and apoptosis in AMoL cells cocultured with bone marrow adipocytes. Our results suggest that bone marrow adipocytes support AMoL cell survival by regulating their metabolic energy balance and that the disruption of FAO in bone marrow adipocytes may be an alternative, novel therapeutic strategy for AMoL therapy. Cancer Res; 77(6); 1453-64. ©2017 AACR.

Mitochondrial Profiling of Acute Myeloid Leukemia in the Assessment of Response to Apoptosis Modulating Drugs.

BH3 profiling measures the propensity of transformed cells to undergo intrinsic apoptosis and is determined by exposing cells to BH3-mimicking peptides. We hypothesized that basal levels of prosurvival BCL-2 family proteins may modulate the predictive power of BH3 profiling and termed it mitochondrial profiling. We investigated the correlation between cell sensitivity to apoptogenic agents and mitochondrial profiling, using a panel of acute myeloid leukemias induced to undergo apoptosis by exposure to cytarabine, the BH3 mimetic ABT-199, the MDM2 inhibitor Nutlin-3a, or the CRM1 inhibitor KPT-330. We found that the apoptogenic efficacies of ABT-199 and cytarabine correlated well with BH3 profiling reflecting BCL2, but not BCL-XL or MCL-1 dependence. Baseline BCL-2 protein expression analysis increased the ability of BH3 profiling to predict resistance mediated by MCL-1. By utilizing engineered cells with overexpression or knockdown of BCL-2 family proteins, Ara-C was found to be independent, while ABT-199 was dependent on BCL-XL. BCL-2 and BCL-XL overexpression mediated resistance to KPT-330 which was not reflected in the BH3 profiling assay, or in baseline BCL-2 protein levels. In conclusion, mitochondrial profiling, the combination of BH3 profiling and prosurvival BCL-2 family protein analysis, represents an improved approach to predict efficacy of diverse agents in AML and may have utility in the design of more effective drug combinations.

Preclinical activity of the novel B-cell-specific Moloney murine leukemia virus integration site 1 inhibitor PTC-209 in acute myeloid leukemia: Implications for leukemia therapy

Curing patients with acute myeloid leukemia (AML) remains a therapeutic challenge. The polycomb complex protein B‐cell‐specific Moloney murine leukemia virus integration site 1 (BMI‐1) is required for the self‐renewal and maintenance of leukemia stem cells. We investigated the prognostic significance of BMI‐1 in AML and the effects of a novel small molecule selective inhibitor of BMI‐1, PTC‐209. BMI‐1 protein expression was determined in 511 newly diagnosed AML patients together with 207 other proteins using reverse‐phase protein array technology. Patients with unfavorable cytogenetics according to Southwest Oncology Group criteria had higher levels of BMI‐1 compared to those with favorable (P = 0.0006) or intermediate cytogenetics (P = 0.0061), and patients with higher levels of BMI‐1 had worse overall survival (55.3 weeks vs. 42.8 weeks, P = 0.046). Treatment with PTC‐209 reduced protein level of BMI‐1 and its downstream target mono‐ubiquitinated histone H2A and triggered several molecular events consistent with the induction of apoptosis, this is, loss of mitochondrial membrane potential, caspase‐3 cleavage, BAX activation, and phosphatidylserine externalization. PTC‐209 induced apoptosis in patient‐derived CD34+CD38low/− AML cells and, less prominently, in CD34− differentiated AML cells. BMI‐1 reduction by PTC‐209 directly correlated with apoptosis induction in CD34+ primary AML cells (r = 0.71, P = 0.022). However, basal BMI‐1 expression was not a determinant of AML sensitivity. BMI‐1 inhibition, which targets a primitive AML cell population, might offer a novel therapeutic strategy for AML.

Ribosomal Biogenesis and Translational Flux Inhibition by the Selective Inhibitor of Nuclear Export (SINE) XPO1 Antagonist KPT-185.

Mantle cell lymphoma (MCL) is an aggressive B-cell lymphoma characterized by the aberrant expression of several growth-regulating, oncogenic effectors. Exportin 1 (XPO1) mediates the nucleocytoplasmic transport of numerous molecules including oncogenic growth-regulating factors, RNAs, and ribosomal subunits. In MCL cells, the small molecule KPT-185 blocks XPO1 function and exerts anti-proliferative effects. In this study, we investigated the molecular mechanisms of this putative anti-tumor effect on MCL cells using cell growth/viability assays, immunoblotting, gene expression analysis, and absolute quantification proteomics. KPT-185 exhibited a p53-independent anti-lymphoma effect on MCL cells, by suppression of oncogenic mediators (e.g., XPO1, cyclin D1, c-Myc, PIM1, and Bcl-2 family members), repression of ribosomal biogenesis, and downregulation of translation/chaperone proteins (e.g., PIM2, EEF1A1, EEF2, and HSP70) that are part of the translational/transcriptional network regulated by heat shock factor 1. These results elucidate a novel mechanism in which ribosomal biogenesis appears to be a key component through which XPO1 contributes to tumor cell survival. Thus, we propose that the blockade of XPO1 could be a promising, novel strategy for the treatment of MCL and other malignancies overexpressing XPO1.

The novel BMI-1 inhibitor PTC596 downregulates MCL-1 and induces p53-independent mitochondrial apoptosis in acute myeloid leukemia progenitor cells

Disease recurrence is the major problem in the treatment of acute myeloid leukemia (AML). Relapse is driven by leukemia stem cells, a chemoresistant subpopulation capable of re-establishing disease. Patients with p53 mutant AML are at an extremely high risk of relapse. B-cell-specific Moloney murine leukemia virus integration site 1 (BMI-1) is required for the self-renewal and maintenance of AML stem cells. Here we studied the effects of a novel small molecule inhibitor of BMI-1, PTC596, in AML cells. Treatment with PTC596 reduced MCL-1 expression and triggered several molecular events consistent with induction of mitochondrial apoptosis: loss of mitochondrial membrane potential, BAX conformational change, caspase-3 cleavage and phosphatidylserine externalization. PTC596 induced apoptosis in a p53-independent manner. PTC596 induced apoptosis along with the reduction of MCL-1 and phosphorylated AKT in patient-derived CD34+CD38low/− stem/progenitor cells. Mouse xenograft models demonstrated in vivo anti-leukemia activity of PTC596, which inhibited leukemia cell growth in vivo while sparing normal hematopoietic cells. Our results indicate that PTC596 deserves further evaluation in clinical trials for refractory or relapsed AML patients, especially for those with unfavorable complex karyotype or therapy-related AML that are frequently associated with p53 mutations.

The Imipridone ONC201 Induces Apoptosis and Overcomes Chemotherapy Resistance by Up-Regulation of Bim in Multiple Myeloma

In multiple myeloma, despite recent improvements offered by new therapies, disease relapse and drug resistance still occur in the majority of patients. Therefore, there is an urgent need for new drugs that can overcome drug resistance and prolong patient survival after failure of standard therapies. The imipridone ONC201 causes downstream inactivation of ERK1/2 signaling and has tumoricidal activity against a variety of tumor types, while its efficacy in preclinical models of myeloma remains unclear. In this study, we treated human myeloma cell lines and patient-derived tumor cells with ONC201. Treatment decreased cellular viability and induced apoptosis in myeloma cell lines, with IC50 values of 1 to 1.5 μM, even in those with high risk features or TP53 loss. ONC201 increased levels of the pro-apoptotic protein Bim in myeloma cells, resulting from decreased phosphorylation of degradation-promoting Bim Ser69 by ERK1/2. In addition, myeloma cell lines made resistant to several standard-of-care agents (by chronic exposure) were equally sensitive to ONC201 as their drug-naïve counterparts, and combinations of ONC201 with proteasome inhibitors had synergistic anti-myeloma activity. Overall, these findings demonstrate that ONC201 kills myeloma cells regardless of resistance to standard-of-care therapies, making it promising for clinical testing in relapsed/refractory myeloma.

FZR1 loss increases sensitivity to DNA damage and consequently promotes murine and human B-cell acute leukemia

FZR1 (fizzy-related protein homolog; also known as CDH1 [cell division cycle 20 related 1]) functions in the cell cycle as a specific activator of anaphase-promoting complex or cyclosome ubiquitin ligase, regulating late mitosis, G1 phase, and activation of the G2-M checkpoint. FZR1 has been implicated as both a tumor suppressor and oncoprotein, and its precise contribution to carcinogenesis remains unclear. Here, we examined the role of FZR1 in tumorigenesis and cancer therapy by analyzing tumor models and patient specimens. In an Fzr1 gene-trap mouse model of B-cell acute lymphoblastic leukemia (B-ALL), mice with Fzr1-deficient B-ALL survived longer than those with Fzr1-intact disease, and sensitivity of Fzr1-deficient B-ALL cells to DNA damage appeared increased. Consistently, conditional knockdown of FZR1 sensitized human B-ALL cell lines to DNA damage-induced cell death. Moreover, multivariate analyses of reverse-phase protein array of B-ALL specimens from newly diagnosed B-ALL patients determined that a low FZR1 protein expression level was an independent predictor of a longer remission duration. The clinical benefit of a low FZR1 expression level at diagnosis was no longer apparent in patients with relapsed B-ALL. Consistent with this result, secondary and tertiary mouse recipients of Fzr1-deficient B-ALL cells developed more progressive and radiation-resistant disease than those receiving Fzr1-intact B-ALL cells, indicating that prolonged inactivation of Fzr1 promotes the development of resistant clones. Our results suggest that reduction of FZR1 increases therapeutic sensitivity of B-ALL and that transient rather than tonic inhibition of FZR1 may be a therapeutic strategy.