RB loss sensitizes cells to replication-associated DNA damage after PARP inhibition by trapping.

The retinoblastoma tumor suppressor protein (RB) interacts physically and functionally with a number of epigenetic modifying enzymes to control transcriptional regulation, respond to replication stress, promote DNA damage response and repair, and regulate genome stability. To better understand how disruption of RB function impacts epigenetic regulation of genome stability and determine whether such changes represent exploitable weaknesses of RB-deficient cancer cells, we performed an imaging-based screen to identify epigenetic inhibitors that promote DNA damage and compromise the viability of RB-deficient cells. We found that loss of RB alone leads to high levels of replication-dependent poly-ADP ribosylation (PARylation) and that preventing PARylation by trapping PARP enzymes on chromatin enables RB-deficient cells to progress to mitosis with unresolved replication stress. These defects contribute to high levels of DNA damage and compromised cell viability. We demonstrate this sensitivity is conserved across a panel of drugs that target both PARP1 and PARP2 and can be suppressed by reexpression of the RB protein. Together, these data indicate that drugs that target PARP1 and PARP2 may be clinically relevant for RB-deficient cancers.

RB loss sensitizes cells to replication-associated DNA damage by PARP inhibition.

The retinoblastoma tumor suppressor protein (RB) interacts physically and functionally with a number of epigenetic modifying enzymes to control transcriptional regulation, respond to replication stress, promote DNA damage response and repair pathways, and regulate genome stability. To better understand how disruption of RB function impacts epigenetic regulation of genome stability and determine whether such changes may represent exploitable weaknesses of RB-deficient cancer cells, we performed an imaging-based screen to identify epigenetic inhibitors that promote DNA damage and compromise viability of RB-deficient cells. We found that loss of RB alone leads to high levels of replication-dependent poly-ADP ribosylation (PARylation) and that preventing PARylation through inhibition of PARP enzymes enables RB-deficient cells to progress to mitosis with unresolved replication stress and under-replicated DNA. These defects contribute to high levels of DNA damage, decreased proliferation, and compromised cell viability. We demonstrate this sensitivity is conserved across a panel of inhibitors that target both PARP1 and PARP2 and can be suppressed by re-expression of the RB protein. Together, these data indicate that inhibitors of PARP1 and PARP2 may be clinically relevant for RB-deficient cancers.

Suppression of Chromosome Instability Limits Acquired Drug Resistance.

Numerical chromosome instability, or nCIN, defined as the high frequency of whole chromosome gains and losses, is prevalent in many solid tumors. nCIN has been shown to promote intratumor heterogeneity and corresponds with tumor aggressiveness, drug resistance, and tumor relapse. Although increased nCIN has been shown to promote the acquisition of genomic changes responsible for drug resistance, the potential to modulate nCIN in a therapeutic manner has not been well explored. Here we assess the role of nCIN in the acquisition of drug resistance in non-small cell lung cancer. We show that the generation of whole chromosome segregation errors in non-small cell lung cancer cells is sensitive to manipulation of microtubule dynamics and that enhancement of chromosome cohesion strongly suppresses nCIN and reduces intratumor heterogeneity. We demonstrate that suppression of nCIN has no impact on non-small cell lung cancer cell proliferation in vitro nor in tumor initiation in mouse xenograft models. However, suppression of nCIN alters the timing and molecular mechanisms that drive acquired drug resistance. These findings suggest mechanisms to suppress nCIN may serve as effective cotherapies to limit tumor evolution and sustain drug response.

Suv420 enrichment at the centromere limits Aurora B localization and function.

Centromere structure and function are defined by the epigenetic modification of histones at centromeric and pericentromeric chromatin. The constitutive heterochromatin found at pericentromeric regions is highly enriched for H3K9me3 and H4K20me3. Although mis-expression of the methyltransferase enzymes that regulate these marks, Suv39 and Suv420, is common in disease, the consequences of such changes are not well understood. Our data show that increased centromere localization of Suv39 and Suv420 suppresses centromere transcription and compromises localization of the mitotic kinase Aurora B, decreasing microtubule dynamics and compromising chromosome alignment and segregation. We find that inhibition of Suv420 methyltransferase activity partially restores Aurora B localization to centromeres and that restoration of the Aurora B-containing chromosomal passenger complex to the centromere is sufficient to suppress mitotic errors that result when Suv420 and H4K20me3 is enriched at centromeres. Consistent with a role for Suv39 and Suv420 in negatively regulating Aurora B, high expression of these enzymes corresponds with increased sensitivity to Aurora kinase inhibition in human cancer cells, suggesting that increased H3K9 and H4K20 methylation may be an underappreciated source of chromosome mis-segregation in cancer. This article has an associated First Person interview with the first author of the paper.

Whole-genome doubling confers unique genetic vulnerabilities on tumour cells.

Whole-genome doubling (WGD) is common in human cancers, occurring early in tumorigenesis and generating genetically unstable tetraploid cells that fuel tumour development1,2. Cells that undergo WGD (WGD+ cells) must adapt to accommodate their abnormal tetraploid state; however, the nature of these adaptations, and whether they confer vulnerabilities that can be exploited therapeutically, is unclear. Here, using sequencing data from roughly 10,000 primary human cancer samples and essentiality data from approximately 600 cancer cell lines, we show that WGD gives rise to common genetic traits that are accompanied by unique vulnerabilities. We reveal that WGD+ cells are more dependent than WGD- cells on signalling from the spindle-assembly checkpoint, DNA-replication factors and proteasome function. We also identify KIF18A, which encodes a mitotic kinesin protein, as being specifically required for the viability of WGD+ cells. Although KIF18A is largely dispensable for accurate chromosome segregation during mitosis in WGD- cells, its loss induces notable mitotic errors in WGD+ cells, ultimately impairing cell viability. Collectively, our results suggest new strategies for specifically targeting WGD+ cancer cells while sparing the normal, non-transformed WGD- cells that comprise human tissue.

Alisertib plus induction chemotherapy in previously untreated patients with high-risk, acute myeloid leukaemia: a single-arm, phase 2 trial.

BACKGROUND: Increased aurora A kinase (AAK) expression occurs in acute myeloid leukaemia; AAK inhibition is a promising therapeutic target in this disease. We therefore aimed to assess the activity of alisertib combined with 7 + 3 induction chemotherapy in previously untreated patients with high-risk acute myeloid leukaemia. METHODS: We did a single-arm, phase 2 trial of patients recruited from the Dana-Farber/Harvard Cancer Center in the USA. Eligible patients had previously untreated acute myeloid leukaemia, an Eastern Cooperative Oncology Group performance status of 0-2, and were at high risk of disease as defined by the presence of an adverse-risk karyotype, the presence of secondary acute myeloid leukaemia arising from previous myelodysplastic syndrome or myeloproliferative neoplasm, the presence of therapy-related acute myeloid leukaemia, or being 65 years or older. Enrolled patients received 7 + 3 induction chemotherapy of continuous infusion of cytarabine (100 mg/m2 per day on days 1-7) and intravenous bolus of idarubicin (12 mg/m2 per day on days 1-3). Oral alisertib (30 mg) was given twice per day on days 8-15. Patients could receive up to four consolidation cycles with cytarabine and alisertib, and alisertib maintenance for 12 months. The primary endpoint was a composite including the proportion of patients achieving complete remission and those with a complete remission with incomplete neutrophil or platelet count recovery. Analyses were per-protocol. This study is registered with Clinicaltrials.gov, number NCT02560025, and has completed enrolment. FINDINGS: Between Dec 31, 2015, and Aug 1, 2017, we enrolled a total of 39 eligible patients. 19 (49%) of 39 patients had secondary acute myeloid leukaemia and three (8%) had therapy-related acute myeloid leukaemia. At mid-induction, 33 (85%) of 39 patients showed marrow aplasia, six (15%) received re-induction. The median follow-up was 13·7 months (IQR 12·7-14·4). Composite remission was 64% (two-stage 95% CI 48-79), with 20 (51%) of 39 patients achieving complete remission and five (13%) achieving complete remission with incomplete neutrophil or platelet count recovery. The most common grade 3 or 4 adverse events included febrile neutropenia (16 [41%] of 39), neutropenia (12 [31%]), thrombocytopenia (13 [33%]), anaemia (11 [28%]), anorexia (nine [23%]), and oral mucositis (four [10%]). No treatment-related deaths were observed. INTERPRETATION: These results suggest that alisertib combined with induction chemotherapy is active and safe in previously untreated patients with high-risk acute myeloid leukaemia. This study met criteria to move forward to a future randomised trial. FUNDING: Millennium Pharmaceuticals.

Identification of the kinase STK25 as an upstream activator of LATS signaling.

The Hippo pathway maintains tissue homeostasis by negatively regulating the oncogenic transcriptional co-activators YAP and TAZ. Though functional inactivation of the Hippo pathway is common in tumors, mutations in core pathway components are rare. Thus, understanding how tumor cells inactivate Hippo signaling remains a key unresolved question. Here, we identify the kinase STK25 as an activator of Hippo signaling. We demonstrate that loss of STK25 promotes YAP/TAZ activation and enhanced cellular proliferation, even under normally growth-suppressive conditions both in vitro and in vivo. Notably, STK25 activates LATS by promoting LATS activation loop phosphorylation independent of a preceding phosphorylation event at the hydrophobic motif, which represents a form of Hippo activation distinct from other kinase activators of LATS. STK25 is significantly focally deleted across a wide spectrum of human cancers, suggesting STK25 loss may represent a common mechanism by which tumor cells functionally impair the Hippo tumor suppressor pathway.

Aurora A inhibition limits centrosome clustering and promotes mitotic catastrophe in cells with supernumerary centrosomes.

The presence of supernumerary centrosomes is prevalent in cancer, where they promote the formation of transient multipolar mitotic spindles. Active clustering of supernumerary centrosomes enables the formation of a functional bipolar spindle that is competent to complete a bipolar division. Disruption of spindle pole clustering in cancer cells promotes multipolar division and generation of non-proliferative daughter cells with compromised viability. Hence molecular pathways required for spindle pole clustering in cells with supernumerary centrosomes, but dispensable in normal cells, are promising therapeutic targets. Here we demonstrate that Aurora A kinase activity is required for spindle pole clustering in cells with extra centrosomes. While cells with two centrosomes are ultimately able to build a bipolar spindle and proceed through a normal cell division in the presence of Aurora A inhibition, cells with supernumerary centrosomes form multipolar and disorganized spindles that are not competent for chromosome segregation. Instead, following a prolonged mitosis, these cells experience catastrophic divisions that result in grossly aneuploid, and non-proliferative daughter cells. Aurora A inhibition in a panel of Acute Myeloid Leukemia cancer cells has a similarly disparate impact on cells with supernumerary centrosomes, suggesting that centrosome number and spindle polarity may serve as predictive biomarkers for response to therapeutic approaches that target Aurora A kinase function.

Dendritic Cell RIPK1 Maintains Immune Homeostasis by Preventing Inflammation and Autoimmunity.

Necroptosis is a form of cell death associated with inflammation; however, the biological consequences of chronic necroptosis are unknown. Necroptosis is mediated by RIPK1, RIPK3, and MLKL kinases but in hematopoietic cells RIPK1 has anti-inflammatory roles and functions to prevent necroptosis. Here we interrogate the consequences of chronic necroptosis on immune homeostasis by deleting Ripk1 in mouse dendritic cells. We demonstrate that deregulated necroptosis results in systemic inflammation, tissue fibrosis, and autoimmunity. We show that inflammation and autoimmunity are prevented upon expression of kinase inactive RIPK1 or deletion of RIPK3 or MLKL. We provide evidence that the inflammation is not driven by microbial ligands, but depends on the release of danger-associated molecular patterns and MyD88-dependent signaling. Importantly, although the inflammation is independent of type I IFN and the nucleic acid sensing TLRs, blocking these pathways rescues the autoimmunity. These mouse genetic studies reveal that chronic necroptosis may underlie human fibrotic and autoimmune disorders.

RUNX1 is required for oncogenic Myb and Myc enhancer activity in T-cell acute lymphoblastic leukemia.

The gene encoding the RUNX1 transcription factor is mutated in a subset of T-cell acute lymphoblastic leukemia (T-ALL) patients, and RUNX1 mutations are associated with a poor prognosis. These mutations cluster in the DNA-binding Runt domain and are thought to represent loss-of-function mutations, indicating that RUNX1 suppresses T-cell transformation. RUNX1 has been proposed to have tumor suppressor roles in T-cell leukemia homeobox 1/3-transformed human T-ALL cell lines and NOTCH1 T-ALL mouse models. Yet, retroviral insertional mutagenesis screens identify RUNX genes as collaborating oncogenes in MYC-driven leukemia mouse models. To elucidate RUNX1 function(s) in leukemogenesis, we generated Tal1/Lmo2/Rosa26-CreERT2Runx1f/f mice and examined leukemia progression in the presence of vehicle or tamoxifen. We found that Runx1 deletion inhibits mouse leukemic growth in vivo and that RUNX silencing in human T-ALL cells triggers apoptosis. We demonstrate that a small molecule inhibitor, designed to interfere with CBFß binding to RUNX proteins, impairs the growth of human T-ALL cell lines and primary patient samples. We demonstrate that a RUNX1 deficiency alters the expression of a crucial subset of TAL1- and NOTCH1-regulated genes, including the MYB and MYC oncogenes, respectively. These studies provide genetic and pharmacologic evidence that RUNX1 has oncogenic roles and reveal RUNX1 as a novel therapeutic target in T-ALL.

Phase I study of the aurora A kinase inhibitor alisertib with induction chemotherapy in patients with acute myeloid leukemia.

Aberrant expression of aurora kinase A is implicated in the genesis of various neoplasms, including acute myeloid leukemia. Alisertib, an aurora A kinase inhibitor, has demonstrated efficacy as monotherapy in trials of myeloid malignancy, and this efficacy appears enhanced in combination with conventional chemotherapies. In this phase I, dose-escalation study, newly diagnosed patients received conventional induction with cytarabine and idarubicin, after which alisertib was administered for 7 days. Dose escalation occurred via cohorts. Patients could then receive up to four cycles of consolidation, incorporating alisertib, and thereafter alisertib maintenance for up to 12 months. Twenty-two patients were enrolled. One dose limiting toxicity occurred at dose level 2 (prolonged thrombocytopenia), and the recommended phase 2 dose was established at 30mg twice daily. Common therapy-related toxicities included cytopenias and mucositis. Only three (14%) patients had persistent disease at mid-cycle, requiring "5+2" reinduction. The composite remission rate (complete remission and complete remission with incomplete neutrophil recovery) was 86% (nineteen of twenty-two patients; 90% CI 68-96%). Among those over age 65 and those with high-risk disease (secondary acute leukemia or cytogenetically high-risk disease), the composite remission rate was 88% and 100%, respectively. The median follow up was 13.5 months. Of those treated at the recommended phase 2 dose, the 12-month overall survival and progression-free survival were 62% (90% CI 33-81%) and 42% (90% CI 17-65%), respectively. Alisertib is well tolerated when combined with induction chemotherapy in acute myeloid leukemia, with a promising suggestion of efficacy. (clinicaltrials.gov Identifier:01779843).

Deletion of the gene encoding G0/G 1 switch protein 2 (G0s2) alleviates high-fat-diet-induced weight gain and insulin resistance, and promotes browning of white adipose tissue in mice.

AIMS/HYPOTHESIS: Obesity is a global epidemic resulting from increased energy intake, which alters energy homeostasis and results in an imbalance in fat storage and breakdown. G0/G1 switch gene 2 (G0s2) has been recently characterised in vitro as an inhibitor of adipose triglyceride lipase (ATGL), the rate-limiting step in fat catabolism. In the current study we aim to functionally characterise G0s2 within the physiological context of a mouse model. METHODS: We generated a mouse model in which G0s2 was deleted. The homozygous G0s2 knockout (G0s2 (-/-)) mice were studied over a period of 22 weeks. Metabolic variables were measured including body weight and body composition, food intake, glucose and insulin tolerance tests, energy metabolism and thermogenesis. RESULTS: We report that G0s2 inhibits ATGL and regulates lipolysis and energy metabolism in vivo. G0s2 (-/-) mice are lean, resistant to weight gain induced by a high-fat diet and are glucose tolerant and insulin sensitive. The white adipose tissue of G0s2 (-/-) mice has enhanced lipase activity and adipocytes showed enhanced stimulated lipolysis. Energy metabolism in the G0s2 (-/-) mice is shifted towards enhanced lipid metabolism and increased thermogenesis. G0s2 (-/-) mice showed enhanced cold tolerance and increased expression of thermoregulatory and oxidation genes within white adipose tissue, suggesting enhanced 'browning' of the white adipose tissue. CONCLUSIONS/INTERPRETATION: Our data show that G0s2 is a physiological regulator of adiposity and energy metabolism and is a potential target in the treatment of obesity and insulin resistance.

Hematopoietic RIPK1 deficiency results in bone marrow failure caused by apoptosis and RIPK3-mediated necroptosis.

Receptor-interacting serine/threonine-protein kinase 1 (RIPK1) is recruited to the TNF receptor 1 to mediate proinflammatory signaling and to regulate TNF-induced cell death. RIPK1 deficiency results in postnatal lethality, but precisely why Ripk1(-/-) mice die remains unclear. To identify the lineages and cell types that depend on RIPK1 for survival, we generated conditional Ripk1 mice. Tamoxifen administration to adult RosaCreER(T2)Ripk1(fl/fl) mice results in lethality caused by cell death in the intestinal and hematopoietic lineages. Similarly, Ripk1 deletion in cells of the hematopoietic lineage stimulates proinflammatory cytokine and chemokine production and hematopoietic cell death, resulting in bone marrow failure. The cell death reflected cell-intrinsic survival roles for RIPK1 in hematopoietic stem and progenitor cells, because Vav-iCre Ripk1(fl/fl) fetal liver cells failed to reconstitute hematopoiesis in lethally irradiated recipients. We demonstrate that RIPK3 deficiency partially rescues hematopoiesis in Vav-iCre Ripk1(fl/fl) mice, showing that RIPK1-deficient hematopoietic cells undergo RIPK3-mediated necroptosis. However, the Vav-iCre Ripk1(fl/fl) Ripk3(-/-) progenitors remain TNF sensitive in vitro and fail to repopulate irradiated mice. These genetic studies reveal that hematopoietic RIPK1 deficiency triggers both apoptotic and necroptotic death that is partially prevented by RIPK3 deficiency. Therefore, RIPK1 regulates hematopoiesis and prevents inflammation by suppressing RIPK3 activation.

RIPK1 maintains epithelial homeostasis by inhibiting apoptosis and necroptosis.

Necroptosis has emerged as an important pathway of programmed cell death in embryonic development, tissue homeostasis, immunity and inflammation. RIPK1 is implicated in inflammatory and cell death signalling and its kinase activity is believed to drive RIPK3-mediated necroptosis. Here we show that kinase-independent scaffolding RIPK1 functions regulate homeostasis and prevent inflammation in barrier tissues by inhibiting epithelial cell apoptosis and necroptosis. Intestinal epithelial cell (IEC)-specific RIPK1 knockout caused IEC apoptosis, villus atrophy, loss of goblet and Paneth cells and premature death in mice. This pathology developed independently of the microbiota and of MyD88 signalling but was partly rescued by TNFR1 (also known as TNFRSF1A) deficiency. Epithelial FADD ablation inhibited IEC apoptosis and prevented the premature death of mice with IEC-specific RIPK1 knockout. However, mice lacking both RIPK1 and FADD in IECs displayed RIPK3-dependent IEC necroptosis, Paneth cell loss and focal erosive inflammatory lesions in the colon. Moreover, a RIPK1 kinase inactive knock-in delayed but did not prevent inflammation caused by FADD deficiency in IECs or keratinocytes, showing that RIPK3-dependent necroptosis of FADD-deficient epithelial cells only partly requires RIPK1 kinase activity. Epidermis-specific RIPK1 knockout triggered keratinocyte apoptosis and necroptosis and caused severe skin inflammation that was prevented by RIPK3 but not FADD deficiency. These findings revealed that RIPK1 inhibits RIPK3-mediated necroptosis in keratinocytes in vivo and identified necroptosis as a more potent trigger of inflammation compared with apoptosis. Therefore, RIPK1 is a master regulator of epithelial cell survival, homeostasis and inflammation in the intestine and the skin.

Cutting edge: RIPK1 Kinase inactive mice are viable and protected from TNF-induced necroptosis in vivo.

The serine/threonine kinase RIPK1 is recruited to TNFR1 to mediate proinflammatory signaling and to regulate TNF-induced cell death. A RIPK1 deficiency results in perinatal lethality, impaired NFκB and MAPK signaling, and sensitivity to TNF-induced apoptosis. Chemical inhibitor and in vitro-reconstitution studies suggested that RIPK1 displays distinct kinase activity-dependent and -independent functions. To determine the contribution of RIPK1 kinase to inflammation in vivo, we generated knock-in mice endogenously expressing catalytically inactive RIPK1 D138N. Unlike Ripk1(-/-) mice, which die shortly after birth, Ripk1(D138N/D138N) mice are viable. Cells expressing RIPK1 D138N are resistant to TNF- and polyinosinic-polycytidylic acid-induced necroptosis in vitro, and Ripk1(D138N/D138N) mice are protected from TNF-induced shock in vivo. Moreover, Ripk1(D138N/D138N) mice fail to control vaccinia virus replication in vivo. This study provides genetic evidence that the kinase activity of RIPK1 is not required for survival but is essential for TNF-, TRIF-, and viral-initiated necroptosis.

NOTCH1 inhibition in vivo results in mammary tumor regression and reduced mammary tumorsphere-forming activity in vitro.

INTRODUCTION: NOTCH activation has been recently implicated in human breast cancers, associated with a poor prognosis, and tumor-initiating cells are hypothesized to mediate resistance to treatment and disease relapse. To address the role of NOTCH1 in mammary gland development, transformation, and mammary tumor-initiating cell activity, we developed a doxycycline-regulated mouse model of NOTCH1-mediated mammary transformation. METHODS: Mammary gland development was analyzed by using whole-mount analysis and by flow cytometry in nulliparous transgenic mice maintained in the presence/absence of doxycycline (or intracellular NOTCH1). Mammary tumors were examined histologically and immunophenotyped by staining with antibodies followed by flow cytometry. Tumors were transplanted into mammary fat pads under limiting dilution conditions, and tumor-initiating cell frequency was calculated. Mammary tumor cells were also plated in vitro in a tumorsphere assay in the presence/absence of doxycycline. RNA was isolated from mammary tumor cell lines cultured in the presence/absence of doxycycline and used for gene-expression profiling with Affymetrix mouse arrays. NOTCH1-regulated genes were identified and validated by using quantitative real-time polymerase chain reaction (PCR). Mammary tumor-bearing mice were treated with doxycycline to suppress NOTCH1 expression, and disease recurrence was monitored. RESULTS: Similar to published studies, we show that constitutive expression of human intracellular NOTCH1 in the developing mouse mammary gland inhibits side branching and promotes luminal cell fate. These mice develop mammary adenocarcinomas that express cytokeratin (CK) 8/18. In vivo limiting-dilution analyses revealed that these mammary tumors exhibit functional heterogeneity and harbor a rare (1/2,978) mammary tumor-initiating cell population. With this dox-regulated NOTCH1 mammary tumor model, we demonstrate that NOTCH1 inhibition results in mammary tumor regression in vivo and prevents disease recurrence in four of six tumors tested. Consistent with the in vivo data, NOTCH1 inhibition reduces mammary tumorsphere activity in vitro. We also identify the embryonic stem cell transcription factor Nanog as a novel NOTCH1-regulated gene in tumorspheres and in mouse and human breast cancer cell lines. CONCLUSIONS: These data indicate that NOTCH1 inhibition results in mammary tumor regression in vivo and interferes with disease recurrence. We demonstrate that NOTCH1-transformed mouse mammary tumors harbor a rare mammary tumor-initiating population and that NOTCH1 contributes to mammary tumor-initiating activity. This work raises the possibility that NOTCH therapeutics may target mammary tumor-initiating cells in certain human breast cancer subtypes.

A cytosolic ATM/NEMO/RIP1 complex recruits TAK1 to mediate the NF-kappaB and p38 mitogen-activated protein kinase (MAPK)/MAPK-activated protein 2 responses to DNA damage.

In multiple tumor types, activation of the transcription factor NF-κB increases the resistance of tumor cells to anticancer therapies and contributes to tumor progression. Genotoxic stress induced by chemotherapy or radiation therapy triggers the ATM-dependent translocation of NF-κB essential modifier (NEMO), also designated IκB kinase γ (IKKγ), from the nucleus to the cytosol, resulting in IκB kinase activation by mechanisms not yet fully understood. RIP1 has been implicated in this response and found to be modified in cells with damaged DNA; however, the nature of the RIP1 modification and its precise role in the pathway remain unclear. Here, we show that DNA damage stimulates the formation of a cytosolic complex containing ATM, NEMO (IKKγ), RIP1, and TAK1. We find that RIP1 is modified by SUMO-1 and ubiquitin in response to DNA damage and demonstrate that modified RIP1 is required for NF-κB activation and tumor cell survival. We show that ATM activates TAK1 in a manner dependent on RIP1 and NEMO. We also reveal TAK1 as a central mediator of the alternative DNA damage response pathway mediated by the p38 mitogen-activated protein kinase (MAPK)/MAPK-activated protein 2 (MAPKAP-2) kinases. These findings have translational implications and reveal RIP1 and TAK1 as potential therapeutic targets in chemoresistance.

Targeting the Notch1 and mTOR pathways in a mouse T-ALL model.

Mutations in NOTCH1 are frequently detected in patients with T-cell acute lymphoblastic leukemia (T-ALL) and in mouse T-ALL models. Treatment of mouse or human T-ALL cell lines in vitro with gamma-secretase inhibitors (GSIs) results in growth arrest and/or apoptosis. These studies suggest GSIs as potential therapeutic agents in the treatment of T-ALL. To determine whether GSIs have antileukemic activity in vivo, we treated near-end-stage Tal1/Ink4a/Arf+/- leukemic mice with vehicle or with a GSI developed by Merck (MRK-003). We found that GSI treatment significantly extended the survival of leukemic mice compared with vehicle-treated mice. Notch1 target gene expression was repressed and increased numbers of apoptotic cells were observed in the GSI-treated mice, demonstrating that Notch1 inhibition in vivo induces apoptosis. T-ALL cell lines also exhibit PI3K/mTOR pathway activation, indicating that rapamycin may also have therapeutic benefit. When GSIs are administered in combination with rapamycin, mTOR kinase activity is ablated and apoptosis induced. Moreover, GSI and rapamycin treatment inhibits human T-ALL growth and extends survival in a mouse xenograft model. This work supports the idea of targeting NOTCH1 in T-ALL and suggests that inhibition of the mTOR and NOTCH1 pathways may have added efficacy.