Pyrimidine depletion enhances targeted and immune therapy combinations in acute myeloid leukemia

Acute myeloid leukemia (AML) is a fatal disease characterized by the accumulation of undifferentiated myeloblasts, and agents that promote differentiation have been effective in this disease but are not curative. Dihydroorotate dehydrogenase inhibitors (DHODHi) have the ability to promote AML differentiation and target aberrant malignant myelopoiesis. We introduce HOSU-53, a DHODHi with significant monotherapy activity, which is further enhanced when combined with other standard-of-care therapeutics. We further discovered that DHODHi modulated surface expression of CD38 and CD47, prompting the evaluation of HOSU-53 combined with anti-CD38 and anti-CD47 therapies, where we identified a compelling curative potential in an aggressive AML model with CD47 targeting. Finally, we explored using plasma dihydroorotate (DHO) levels to monitor HOSU-53 safety and found that the level of DHO accumulation could predict HOSU-53 intolerability, suggesting the clinical use of plasma DHO to determine safe DHODHi doses. Collectively, our data support the clinical translation of HOSU-53 in AML, particularly to augment immune therapies. Potent DHODHi to date have been limited by their therapeutic index; however, we introduce pharmacodynamic monitoring to predict tolerability while preserving antitumor activity. We additionally suggest that DHODHi is effective at lower doses with select immune therapies, widening the therapeutic index.

Inhibition of Enhancer of Zeste Homolog 2 Induces Blast Differentiation, Impairs Engraftment and Prolongs Survival in Murine Models of Acute Myeloid Leukemia

BACKGROUND

Acute myeloid leukemia (AML) is the malignant proliferation of immature myeloid cells characterized by a block in differentiation. As such, novel therapeutic strategies to promote the differentiation of immature myeloid cells have been successful in AML, although these agents are targeted to a specific mutation that is only present in a subset of AML patients. In the current study, we show that targeting the epigenetic modifier enhancer of zeste homolog 2 (EZH2) can induce the differentiation of immature blast cells into a more mature myeloid phenotype and promote survival in AML murine models.

METHODS

The EZH2 inhibitor EPZ011989 (EPZ) was studied in AML cell lines, primary in AML cells and normal CD34+ stem cells. A pharmacodynamic assessment of H3K27me3; studies of differentiation, cell growth, and colony formation; and in vivo therapeutic studies including the influence on primary AML cell engraftment were also conducted.

RESULTS

EPZ inhibited H3K27me3 in AML cell lines and primary AML samples in vitro. EZH2 inhibition reduced colony formation in multiple AML cell lines and primary AML samples, while exhibiting no effect on colony formation in normal CD34+ stem cells. In AML cells, EPZ promoted phenotypic evidence of differentiation. Finally, the pretreatment of primary AML cells with EPZ significantly delayed engraftment and prolonged the overall survival when engrafted into immunodeficient mice.

CONCLUSIONS

Despite evidence that EZH2 silencing in MDS/MPN can promote AML pathogenesis, our data demonstrate that the therapeutic inhibition of EZH2 in established AML has the potential to improve survival.

Mass Cytometry as a Tool for Investigating Senescence in Multiple Model Systems

Cellular senescence is a durable cell cycle arrest as a result of the finite proliferative capacity of cells. Senescence responds to both intrinsic and extrinsic cellular stresses, such as aging, mitochondrial dysfunction, irradiation, and chemotherapy. Here, we report on the use of mass cytometry (MC) to analyze multiple model systems and demonstrate MC as a platform for senescence analysis at the single-cell level. We demonstrate changes to p16 expression, cell cycling fraction, and histone tail modifications in several established senescent model systems and using isolated human T cells. In bone marrow mesenchymal stromal cells (BMSCs), we show increased p16 expression with subsequent passage as well as a reduction in cycling cells and open chromatin marks. In WI-38 cells, we demonstrate increased p16 expression with both culture-induced senescence and oxidative stress-induced senescence (OSIS). We also use Wanderlust, a trajectory analysis tool, to demonstrate how p16 expression changes with histone tail modifications and cell cycle proteins. Finally, we demonstrate that repetitive stimulation of human T cells with CD3/CD28 beads induces an exhausted phenotype with increased p16 expression. This p16-expressing population exhibited higher expression of exhaustion markers such as EOMES and TOX. This work demonstrates that MC is a useful platform for studying senescence at a single-cell protein level, and is capable of measuring multiple markers of senescence at once with high confidence, thereby improving our understanding of senescent pathways.

TP-0903 Is Active in Preclinical Models of Acute Myeloid Leukemia with TP53 Mutation/Deletion

Acute myeloid leukemia (AML) with mutations in the tumor suppressor gene TP53 confers a dismal prognosis with 3-year overall survival of <5%. While inhibition of kinases involved in cell cycle regulation induces synthetic lethality in a variety of TP53 mutant cancers, this strategy has not been evaluated in mutant TP53 AML. Previously, we demonstrated that TP-0903 is a novel multikinase inhibitor with low nM activity against AURKA/B, Chk1/2, and other cell cycle regulators. Here, we evaluated the preclinical activity of TP-0903 in TP53 mutant AML cell lines, including a single-cell clone of MV4-11 containing a TP53 mutation (R248W), Kasumi-1 (R248Q), and HL-60 (TP 53 null). TP-0903 inhibited cell viability (IC50, 12−32 nM) and induced apoptosis at 50 nM. By immunoblot, 50 nM TP-0903 upregulated pChk1/2 and pH2AX, suggesting induction of DNA damage. The combination of TP-0903 and decitabine was additive in vitro, and in vivo significantly prolonged median survival compared to single-agent treatments in mice xenografted with HL-60 (vehicle, 46 days; decitabine, 55 days; TP-0903, 63 days; combination, 75 days) or MV4-11 (R248W) (51 days; 62 days; 81 days; 89 days) (p < 0.001). Together, these results provide scientific premise for the clinical evaluation of TP-0903 in combination with decitabine in TP53 mutant AML.

Abstract 1060: Introducing a novel DHODH inhibitor with superior in vivo activity as monotherapy or in novel combination regimen with immunotherapy for hematological malignancies

Acute myeloid leukemia (AML) is characterized by the uncontrolled expansion of un-differentiated hematopoietic progenitor myeloblasts. AML treatment is very challenging owing to its complex heterogeneity resulting in a dismal 5-year overall survival rate particularly in elderly patients unfit for standard induction chemotherapy. The expansion of AML requires the availability of sufficient nucleotides supporting the anabolic processes required for AML growth thus, targeting nucleotide biosynthesis can halt AML progression. Indeed, targeting dihydroorotate dehydrogenase (DHODH), a critical rate-limiting step in the de novo pyrimidine synthesis pathway not only induced cytotoxicity but has been shown to promote blast differentiation in a HOXA9/MEIS1 over-expressing model. We sought to develop a DHODH inhibitor that had superior properties to those reported for AML therapy. Compound 41 (cmpd 41) demonstrates sub-nanomolar 50% inhibitory concentration for DHODH biochemical activity and potent in vitro activity across several AML cell lines and primary AML cells independent of mutational subtype, including mutated TP53. Cmpd 41 also demonstrated superior in vivo anti-leukemic activity in multiple AML xenograft models as monotherapy and demonstrated synergy with a hypomethylating agent, decitabine in TP53 mutated AML. Given the heterogeneity of AML and frequent emergence of resistant clones, we aimed to investigate ways to enhance response to DHODH inhibitors through combination. After in vitro treatment of AML cell lines and primary patient samples with DHODH inhibitors, we observed an increase in CD38 surface expression suggesting synergy with CD38 targeting monoclonal antibody (mAb) immunotherapies. Indeed, we are the first to report synergy between DHODH inhibitors and anti-CD38 mAb in AML which emphasizes the synergy between this promising novel class of agents with immunotherapies via recruiting innate immunity. Consequently, given the relevance of CD38 mAb therapy to multiple myeloma (MM), we extended these studies to MM and remarkably found that cmpd 41 was highly efficacious as a monotherapy and in combination with CD38 mAb, resulted in complete tumor regression in a subcutaneous MM xenograft model. In summary, we introduce a best in class DHODH inhibitor with a data-driven combination strategy for both AML and MM. Our studies suggest a highly synergistic combination strategy involving immunotherapy for AML and other hematologic malignancies.

Citation Format: Ola A. Elgamal, Sandip Vibhute, Sydney Fobare, Abeera Mehmood, Mariah L. Johnson, Jean Truxall, Emily Stahl, Bridget Carmichael, Shelley J. Orwick, Ramasamy Santhanam, Kasey Hill, Susheela Tridandapani, Christopher C. Coss, Alice S. Mims, Karilyn T. Larkin, Mitch A. Phelps, Sharyn D. Baker, Alex Sparreboom, Thomas E. Goodwin, Gerard Hilinski, Chad E. Bennett, Erin Hertlein, John C. Byrd. Introducing a novel DHODH inhibitor with superior in vivo activity as monotherapy or in novel combination regimen with immunotherapy for hematological malignancies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1060.

Abstract 1824: EZH2 inhibition induces blast differentiation in acute myeloid leukemia

Enhancer of zeste homolog 2 (EZH2) is a part of the polycomb repressive complex and catalyzes the trimethylation of lysine 27 on histone H3 (H3K27me3). EZH2 inhibition has a complex role in the pathogenesis of acute myeloid leukemia (AML), in that it has been shown to be either a tumor suppressor or an oncogene depending on the stage of AML development and the genes that EZH2 is regulating during each stage. Unlike follicular and diffuse large B-cell lymphoma where EZH2 mutations result in gain of function, EZH2 mutations are typically loss of function in myeloid diseases. However, we hypothesized that in AML patients without EZH2 mutations, loss of EZH2 function may produce a phenotype that would allow for therapeutic targeting without influencing normal hematopoiesis.

We used EPZ011989 (EPZ), an EZH2 inhibitor tool compound, to inhibit H3K27me3 in our studies. We started by treating the MOLM-13 AML cell line with EPZ and confirmed a decrease in H3K27me3. This reduction in H3K27me3 resulted in a slight decrease in metabolic activity via MTS assays as well as decreased colony formation in methocult. These studies were followed up with EPZ inhibition in primary AML samples in vitro. We found that EZH2 inhibition resulted in decreased self-renewal of primary AML samples but not of CD34+ bone marrow cells from normal donors. Furthermore, we found that after 7-day treatment with EPZ, primary AML samples undergo moderate differentiation as suggested by an increase in CD11b surface expression via flow cytometry. These results are further supported by the morphological changes seen after 14-days of EPZ treatment in vitro. Based on these results, we hypothesize that EZH2 inhibition in primary AML samples promotes the differentiation of AML blasts. Furthermore, our preliminary data suggests that daily treatment with 150 mg/kg of EPZ results in a survival advantage and reduced disease burden in the MOLM-13-luciferase murine xenograft model.

Despite loss of function EZH2 mutations portending poor outcomes in myeloid malignancies, we demonstrate that pharmacologic EZH2 inhibition reduces AML blast stemness and promotes differentiation into mature myeloid cells. In contrast, no change in normal CD34+ stem cells occurs with EZH2 inhibition, offering the opportunity to selectively target myeloid leukemia.

Citation Format: Sydney Fobare, Ola A. Elgamal, Emily H. Stahl, Abeera Mehmood, Jean Truxall, Mariah L. Johnson, Amina Abdul-Aziz, John C. Byrd, Erin Hertlein. EZH2 inhibition induces blast differentiation in acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1824.

PP2A is a therapeutically targetable driver of cell fate decisions via a c-Myc/p21 axis in human and murine acute myeloid leukemia

Dysregulated cellular differentiation is a hallmark of acute leukemogenesis. Phosphatases are widely suppressed in cancers but have not been traditionally associated with differentiation. In this study, we found that the silencing of protein phosphatase 2A (PP2A) directly blocks differentiation in acute myeloid leukemia (AML). Gene expression and mass cytometric profiling revealed that PP2A activation modulates cell cycle and transcriptional regulators that program terminal myeloid differentiation. Using a novel pharmacological agent, OSU-2S, in parallel with genetic approaches, we discovered that PP2A enforced c-Myc and p21 dependent terminal differentiation, proliferation arrest, and apoptosis in AML. Finally, we demonstrated that PP2A activation decreased leukemia-initiating stem cells, increased leukemic blast maturation, and improved overall survival in murine Tet2-/-Flt3ITD/WT and human cell-line derived xenograft AML models in vivo. Our findings identify the PP2A/c-Myc/p21 axis as a critical regulator of the differentiation/proliferation switch in AML that can be therapeutically targeted in malignancies with dysregulated maturation fate.

Preclinical evaluation of the Hsp90 inhibitor SNX-5422 in ibrutinib resistant CLL

B-cell receptor (BCR) antagonists such as the BTK inhibitor ibrutinib have proven to effectively target chronic lymphocytic leukemia (CLL) tumor cells, leading to impressive response rates in these patients. However patients do still relapse on ibrutinib, and the progressive disease is often quite aggressive requiring immediate treatment. Several strategies are being pursued to treat patients who relapse on ibrutinib therapy. As the most common form of relapse is the development of a mutant form of BTK which limits ibrutinib binding, agents which lead to degradation of the BTK protein are a promising strategy. Our study explores the efficacy of the Hsp90 inhibitor, SNX-5422, in CLL. The SNX Hsp90 inhibitor was effective in primary CLL cells, as well as B-cell lines expressing either BTK wild type or C481 mutant BTK, which has been identified as the primary resistance mechanism to ibrutinib in CLL patients. Furthermore the combination of SNX-5422 and ibrutinib provided a remarkable in vivo survival benefit in the Eμ-TCL1 mouse model of CLL compared to the vehicle or single agent groups (51 day median survival in the vehicle and ibrutinib groups versus 100 day median survival in the combination). We report here preclinical data suggesting that the Hsp90 inhibitor SNX-5422, which has been pursued in clinical trials in both solid tumor and hematological malignancies, is a potential therapy for ibrutinib resistant CLL.

TP-0903 is active in models of drug-resistant acute myeloid leukemia

Effective treatment for AML is challenging due to the presence of clonal heterogeneity and the evolution of polyclonal drug resistance. Here, we report that TP-0903 has potent activity against protein kinases related to STAT, AKT, and ERK signaling, as well as cell cycle regulators in biochemical and cellular assays. In vitro and in vivo, TP-0903 was active in multiple models of drug-resistant FLT3 mutant AML, including those involving the F691L gatekeeper mutation and bone marrow microenvironment–mediated factors. Furthermore, TP-0903 demonstrated preclinical activity in AML models with FLT3-ITD and common co-occurring mutations in IDH2 and NRAS genes. We also showed that TP-0903 had ex vivo activity in primary AML cells with recurrent mutations including MLL-PTD, ASXL1, SRSF2, and WT1, which are associated with poor prognosis or promote clinical resistance to AML-directed therapies. Our preclinical studies demonstrate that TP-0903 is a multikinase inhibitor with potent activity against multiple drug-resistant models of AML that will have an immediate clinical impact in a heterogeneous disease like AML.

TP-0903, a multikinase inhibitor, demonstrates preclinical activity in models of drug-resistant AML, including those involving FLT3 mutations, bone marrow microenvironment-mediated factors and recurrent mutations.

Abstract 6118: A primary mouse model of AML as a new way to screen novel therapeutics and treatment combinations

Introduction

Acute myeloid leukemia (AML) is a disease characterized by the proliferation of undifferentiated myeloid blasts, promoting bone marrow failure, infectious complications, and death. Progress in developing treatments for AML has been slow and clinical outcomes outside of bone marrow transplant are poor. A limitation to progress is the availability of reproducible murine models of AML. As such, there is a critical need to develop new models to quickly screen new therapeutics and treatment combinations. Here we have optimized the engraftment of a spontaneous Idh2R140Q/Flt3ITD murine AML (developed by Shih et al 2017) for rapid in vivo screening of compounds and combination strategies.

Methods

Spleen derived AML cells from a spontaneous Idh2R140Q/Flt3ITD murine AML were engrafted i.v. into NCG mice. Disease progression was monitored by flow cytometry and LC-MS/MS measurement of plasma D-2-hydroxyglutarate (D2HG), the product of mutant IDH2. At 1 week post-engraftment animals were enrolled onto treatment arms, which included enasidenib (IDH2), gilteritinib (Flt3), a dihydroorotate dehydrogenase (DHODH) inhibitor, and combination arms. In follow-up studies, non-tumor bearing C57Bl/6 mice were treated with various doses of treatment combinations and blood counts, serum liver chemistry, and weight loss were used to evaluate toxicity.

Results

The time from engraftment to terminal disease can be precisely adjusted to experimental needs, enabling an aggressive 21-day disease course for rapid screening or a more protracted 1.5 month course that limits the initial number of cells required and allows for the testing of slower-acting differentiating agents. We show that enasidenib, gilteritinib, and a DHODH inhibitor all slow disease progression and prolong survival, and that DHODH inhibition may have a greater effect on survival than IDH2 inhibition. We also show that DHODH inhibition lowers plasma D2HG. Importantly, we identify that combining DHODH and IDH2 inhibition is untenable and results in lethal GI toxicity. This was validated in non-tumor bearing mice, where even low doses of DHODH/IDH2 inhibitors in combination resulted in cytopenias and rapid weight loss.

Discussion

Developing combination treatment regimens is likely to be key to achieving long-term remission for many AML patients; however, efforts to develop these combinations are often stymied by in vitro results that fail to translate to the in vivo setting, particularly unexpected toxicities that are not recognized in vitro. We show that our AML engraftment model can be used to identify and develop pre-clinical compounds and allow us to quickly identify and abandon treatment combinations that are toxic in vivo. Our hope is that this work enables the rapid identification and development of the best novel therapeutics and treatment combinations, such that valuable time and resources can be utilized to their greatest translational effect.

Citation Format: Mariah L. Johnson, Ola A. Elgamal, Bonnie Harrington, Allison Mustonen, Amy Lehman, Chad Bennett, Kyle R. Dzwigalski, Kasey L. Hill, Mitch A. Phelps, Jean Truxall, Thomas Goodwin, Alan H. Shih, Ross L. Levine, Erin Hertlein, John C. Byrd. A primary mouse model of AML as a new way to screen novel therapeutics and treatment combinations [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6118.

Preclinical efficacy for a novel tyrosine kinase inhibitor, ArQule 531 against acute myeloid leukemia

BACKGROUND

Acute myeloid leukemia (AML) is the most common type of adult leukemia. Several studies have demonstrated that oncogenesis in AML is enhanced by kinase signaling pathways such as Src family kinases (SFK) including Src and Lyn, spleen tyrosine kinase (SYK), and bruton's tyrosine kinase (BTK). Recently, the multi-kinase inhibitor ArQule 531 (ARQ 531) has demonstrated potent inhibition of SFK and BTK that translated to improved pre-clinical in vivo activity as compared with the irreversible BTK inhibitor ibrutinib in chronic lymphocytic leukemia (CLL) models. Given the superior activity of ARQ 531 in CLL, and recognition that this molecule has a broad kinase inhibition profile, we pursued its application in pre-clinical models of AML.

METHODS

The potency of ARQ 531 was examined in vitro using FLT3 wild type and mutated (ITD) AML cell lines and primary samples. The modulation of pro-survival kinases following ARQ 531 treatment was determined using AML cell lines. The effect of SYK expression on ARQ 531 potency was evaluated using a SYK overexpressing cell line (Ba/F3 murine cells) constitutively expressing FLT3-ITD. Finally, the in vivo activity of ARQ 531 was evaluated using MOLM-13 disseminated xenograft model.

RESULTS

Our data demonstrate that ARQ 531 treatment has anti-proliferative activity in vitro and impairs colony formation in AML cell lines and primary AML cells independent of the presence of a FLT3 ITD mutation. We demonstrate decreased phosphorylation of oncogenic kinases targeted by ARQ 531, including SFK (Tyr416), BTK, and fms-related tyrosine kinase 3 (FLT3), ultimately leading to changes in down-stream targets including SYK, STAT5a, and ERK1/2. Based upon in vitro drug synergy data, we examined ARQ 531 in the MOLM-13 AML xenograft model alone and in combination with venetoclax. Despite ARQ 531 having a less favorable pharmacokinetics profile in rodents, we demonstrate modest single agent in vivo activity and synergy with venetoclax.

CONCLUSIONS

Our data support consideration of the application of ARQ 531 in combination trials for AML targeting higher drug concentrations in vivo.

Leukemic B Cell CTLA-4 Suppresses Costimulation of T Cells

The clinical benefit of CTLA-4 blockade on T cells is known, yet the impact of its expression on cancer cells remains unaddressed. We define an immunosuppressive role for tumor-expressed CTLA-4 using chronic lymphocytic leukemia (CLL) as a disease model. CLL cells, among other cancer cells, are CTLA-4⁺ Coculture with activated human T cells induced surface CTLA-4 on primary human CLL B cells. CTLA-4 on CLL-derived human cell lines decreased CD80 expression on cocultured CD80⁺ cells, with restoration upon CTLA-4 blockade. Coculture of CTLA-4⁺ CLL cells with CD80-GFP⁺ cell lines revealed transfer of CD80-GFP into CLL tumor cells, similar to CTLA-4⁺ T cells able to trans-endocytose CD80. Coculture of T cells with CTLA-4⁺ CLL cells decreased IL-2 production. Using a human CTLA-4 knock-in mouse lacking FcγR function, antitumor efficacy was observed by blocking murine CTLA-4 on tumor cells in isolation of the T cell effect and Fc-mediated depletion. These data implicate tumor CTLA-4 in cancer cell-mediated immunosuppression in vitro and as having a functional role in tumor cells in vivo.

Abstract 1964: Preclinical efficacy of the HSP90 inhibitor SNX-5422 in targeting C481S mutant BTK and ibrutinib resistant CLL

While chronic lymphocytic leukemia (CLL) is effectively treated by the Bruton's tyrosine kinase (BTK) inhibitor ibrutinib, patients that do relapse on ibrutinib (approximately 19%) have an extremely poor clinical outcome without further intervention. Before clinical diagnosis of relapse, signs of molecular relapse can be identified through deep sequencing for mutations in BTK (C481S) and the immediate downstream BTK target, PLCγ2. The BTK C481S mutation (which reduces ibrutinib binding) can be identified at frequencies as low as 5%, and expansion of this mutant close inevitably leads to clinical relapse. Therefore, identifying methods to target this mutant clone before clinical relapse represents a novel and potentially effective therapeutic strategy.

HSP90 is a chaperone protein responsible for maintaining the stability of a large number of client proteins involved in cancer development. Cancer cells, particularly those reliant on a mutated oncogene, rely heavily on HSP90 for survival, making this an attractive clinical target. Here we have explored the preclinical efficacy of SNX-5422, a chemically unique, orally bioavailable HSP90 inhibitor, in ibrutinib resistant CLL. We determined that SNX-5422 can degrade C481S mutated BTK by utilizing BTK-null B-cell lines over-expressing wildtype or C481S BTK. Whereas ibrutinib reduces BTK activity in only the wildtype BTK transfected cell line, SNX-5422 degrades both wildtype BTK and C481S mutant BTK and impairs downstream signaling. In fact, C481S mutant BTK shows increased degradation compared to wildtype BTK suggesting that the C481S mutant BTK may rely on HSP90 more for stabilization.

We next examined the in vivo efficacy of SNX-5422 in a mouse model of ibrutinib-resistant CLL. Leukemic splenocytes isolated at removal criteria from TCL1 mice continuously treated with ibrutinib were adoptively transferred into syngeneic WT B6 mice. Upon reaching 10% leukemia burden in the peripheral blood, the mice were randomized into 1 of 4 treatment groups: vehicle, SNX-5422 (50 mg/kg MWF), ibrutinib (continuous in drinking water), or SNX-5422 plus ibrutinib (concurrent). Mice treated with the combination of SNX-5422 and ibrutinib exhibit prolonged survival and decreased disease burden in the peripheral blood compared to the vehicle or single agent groups.

In summary, we show that SNX-5422 degrades C481S BTK in vitro and prolongs survival when combined with ibrutinib in vivo. We therefore propose that upon molecular relapse, the use of SNX-5422 may prevent clinical relapse and deepen the response to ibrutinib. A phase 1 study is currently open at Ohio State to investigate whether the addition of SNX-5422 to an established dose of ibrutinib can eliminate the C481S mutant clone and prevent or delay disease progression in subjects with CLL (NCT02914327).

EH and JAW contributed equally as senior authors.

Citation Format: Timothy L. Chen, Bonnie Harrington, Jean Truxall, Ronni Wasmuth, Amy Lehman, Eric Orlemans, John C. Byrd, Jennifer A. Woyach, Erin Hertlein. Preclinical efficacy of the HSP90 inhibitor SNX-5422 in targeting C481S mutant BTK and ibrutinib resistant CLL [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1964.

Abstract 1882: Preclinical evaluation of the tyrosine kinase inhibitor ARQ 531 in AML

Acute Myeloid Leukemia (AML) is a rapidly progressing hematopoietic malignancy arising from bone marrow myeloid progenitor cells. Treatment with cytotoxic chemotherapy has not changed for over four decades resulting in poor survival. The dismal prognosis could be attributed to the heterogeneity of this disease, where multiple genetically aberrant clones exist within the same patient. Mutations in the FMS-like tyrosine kinase (FLT3) occurs in 30% of AML patients, typically as an internal tandem duplication (ITD) resulting in a constitutively active FLT3 survival pathway. This has prompted the generation of selective FLT3 inhibitors such as Quizartinib and Gilteritinib which are currently being pursued in clinical trials. Still, acquired resistance to these selective FLT3 inhibitors due to the acquisition of tyrosine kinase domain mutations (TKD) can occur. This suggests that the use of an agent with a broader kinome inhibition profile (such as the recently granted FDA approved Midostaurin) could achieve more durable clinical benefit.

ARQ 531 is a novel potent BTK inhibitor currently being investigated in a Phase 1 trial in patients with relapsed/refractory hematological malignancies (ClinicalTrials.gov Identifier: NCT03162536). We have found that ARQ 531 also has inhibitory activity against members of the Src family of kinases (SFK; including downstream target SYK) as well as FLT3. SYK directly binds to and trans-activates FLT3 which is essential for FLT3-ITD tumorigenicity, suggesting that ARQ 531 has therapeutic potential in AML. Therefore we have investigated the in vitro and in vivo efficacy of ARQ 531 in AML.

Our preliminary studies demonstrate cytotoxicity for ARQ 531 in patient-derived primary AML cells harboring FLT3 wild type and FLT3-ITD, as well as multiple AML cell lines. Importantly, ARQ 531 is effective in a MOLM-13 tyrosine kinase inhibitor (TKI) resistant cell line harboring a FLT3-ITD-TKD-D835Y mutation. Furthermore, we show that ARQ 531 can reduce the level of phosphorylated FLT3, but unlike selective FLT3 inhibitors, it can also inhibit Src family phosphorylation and SYK phosphorylation. Additionally, ARQ 531 exhibited an anti-clonogenic effect on primary patient blasts using Methocult colony forming unit assay. Finally, to investigate the in vivo effect of ARQ 531, we used an aggressive AML MOLM-13 disseminated xenograft mouse model. NSG mice were randomized one-week post engraftment to either vehicle or daily oral gavage of 50 mg/kg ARQ 531. The estimated median survival for the ARQ 531 group was 23 days compared to 21 days for the vehicle group (p = 0.002) suggesting in vivo efficacy for ARQ 531 in AML.

Collectively, we provide for the first time promising preclinical efficacy for ARQ 531 in AML supporting further mechanistic investigation of this agent, and potentially, expansion of the ongoing clinical studies to include AML patients.

E. H. and J.C. B. contributed equally as co-senior authors to this work

Citation Format: Ola A. Elgamal, Bridget Carmichael, Amy Lehman, Shelley J. Orwick, Minh Tran, Virginia M. Goettl, Shaneice Mitchell, Rosa Lapalombella, Jae Yoon Jeon, Sharyn D. Baker, Sudharshan Eathiraj, Brian Schwartz, Erin Hertlein, John C. Byrd. Preclinical evaluation of the tyrosine kinase inhibitor ARQ 531 in AML [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1882.

Hsp90 inhibition increases SOCS3 transcript and regulates migration and cell death in chronic lymphocytic leukemia

Epigenetic or transcriptional silencing of important tumor suppressors has been described to contribute to cell survival and tumorigenesis in chronic lymphocytic leukemia (CLL). Using gene expression microarray analysis, we found that thousands of genes are repressed more than 2-fold in CLL compared to normal B cells; however therapeutic approaches to reverse this have been limited in CLL. Following treatment with the Hsp90 inhibitor 17-DMAG, a significant number of these repressed genes were significantly re-expressed. One of the genes significantly repressed in CLL and up-regulated by 17-DMAG was suppressor of cytokine signaling 3, (SOCS3). SOCS3 has been shown to be silenced in solid tumors as well as myeloid leukemia; however little is known about the regulation in CLL. We found that 17-DMAG induces expression of SOCS3 by via the activation of p38 signaling, and subsequently inhibits AKT and STAT3 phosphorylation resulting in downstream effects on cell migration and survival. We therefore suggest that SOCS3 is an important signaling protein in CLL, and Hsp90 inhibitors represent a novel approach to target transcriptional repression in B cell lymphoproliferative disorders which exhibit a substantial degree of gene repression.

Abstract 3014: BCL3 over-expression contributes an in vivo growth advantage in a B-cell lymphoma xenograft model and is a risk factor for ibrutinib relapse in CLL

Chronic lymphocytic leukemia (CLL) relies on chronic B-cell receptor (BCR) signaling, and as such is effectively treated with Bruton's tyrosine kinase (BTK) inhibitors. However patients who relapse on BTK inhibitors such as ibrutinib have an extremely poor prognosis without additional intervention. Therefore, identifying and characterizing risk factors that predict relapse to BTK inhibitors is important. A series of 308 CLL patients have been enrolled on various ibrutinib clinical trials at the Ohio State University. Seven of these patients were identified to carry the translocation t(14;19), which leads to over-expression of B-cell leukemia 3 (BCL3), Of these 7, 6 (85%) have relapsed on ibrutinib while only 27% of patients (83/308) overall have progressed. BCL3 is known to regulate NF-κB transcription and influence B-cell function, and we found that overall BCL3 expression is increased in CLL compared to normal B-cells, therefore hypothesized that BCL3 may provide a competitive advantage to promote ibrutinib resistance. In order to test the role of BCL3 in ibrutinib resistance using an in vitro system, we overexpressed BCL3 or an empty vector (EV) control in ibrutinib responsive B-cell lines (BCL3 was at least 2-fold over-expressed relative to endogenous BCL3). Cells were treated with vehicle or 1uM ibrutinib for 1 hour followed by a washout, and proliferation and viability were evaluated at various time points. Ibrutinib inhibited cell proliferation and induced apoptosis, however BCL3 expression did not abrogate these effects. However, while examining BCR signaling proteins we found that BCL3 expression enhanced phosphorylation of BTK, suggesting that BCL3 stabilizes activated BTK to circumvent ibrutinib treatment. We next engrafted these EV or BCL3 over-expressing cells into the left or right flank, respectively, of immune-compromised (NSG) mice. Mice were sacrificed when either tumor reached a volume of 2000cm3, and the tumors were isolated and weighed. Tumors which developed from the BCL3 over-expressing cells at sacrifice were larger than those which developed from the EV cells by an average of 1332g. We also evaluated a disseminated tumor model where HBL-1 EV or BCL3 over-expressing cells were injected via the tail vein. Disease was monitored by weekly peripheral blood flow cytometry for human CD19. We saw that 6 of 11 mice intravenously engrafted with HBL-1 BCL3 present with peripheral disease when the mice meet removal criteria, whereas only 1 of 11 mice engrafted with HBL-1 EV cells developed peripheral disease. We conclude that the presence of t(14;19) indicates a higher risk of relapse in patients undergoing ibrutinib therapy, and these patients should be closely monitored for evidence of progression. Our studies also suggest that BCL3 promotes more aggressive disease in a lymphoma xenograft model, and may be involved in tumor migration in vivo.

Citation Format: Timothy L. Chen, Bonnie K. Harrington, Larry Beaver, Amy S. Ruppert, Nyla A. Heerema, Xiaoli Zhang, Hatice Gulcin Ozer, Amy J. Johnson, Jennifer A. Woyach, Erin Hertlein, John C. Byrd. BCL3 over-expression contributes an in vivo growth advantage in a B-cell lymphoma xenograft model and is a risk factor for ibrutinib relapse in CLL [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3014. doi:10.1158/1538-7445.AM2017-3014

Leukemic cell expressed CTLA-4 suppresses T cells via down-modulation of CD80 by trans-endocytosis

Despite clinical utility in targeting the immune checkpoint, Cytotoxic T Lymphocyte Antigen 4 (CTLA-4), on T cells, its function on non-T cells remains unaddressed, especially in the context of therapy. We define an immunosuppressive role for tumor expressed CTLA-4 in Chronic Lymphocytic Leukemia (CLL). Most primary CLL samples were intracellularly CTLA-4+ (12/14) but surface CTLA-4− (14/14). Co-culture with activated T cells induced surface expression of CTLA-4 on CLL B cells. To mechanistically study CLL B cell expressed CTLA-4, we generated CLL-derived Mec1 and OSU-CLL cell lines to inducibly express CTLA-4. These cell lines express the ligands for CTLA-4, CD80 and CD86. Expression of CTLA-4 on Mec1 and OSU-CLL resulted in decreased CD80 expression on CD80+ cells with rescue upon CTLA-4 blockade (N=3). Co-culture of CTLA-4+ Mec1 and CTLA-4+ primary CLL cells with CD80-GFP+ Hek293 cells revealed transfer of CD80-GFP into primary CLL cells (N=2) and the Mec1 cell line. This finding was consistent with the ability of CTLA-4+ T cells to trans-endocytose CD80. Co-culture of T cells with Mec1 CTLA-4+ cells resulted in decreased IL2 production (N=3, p=0.0172). The loss of IL2 signified decreased co-stimulation as a result of tumor expressed CTLA-4. We generated an in vivo model to study tumor CTLA-4 using the TCL1 murine model of CLL. Murine CTLA-4+ (mCTLA-4) TCL1 tumor cells were engrafted into human CTLA-4+/+mFcγR−/− immune competent mice. MCTLA-4 on the tumor cells but not human CTLA-4 on the host cells was blocked with an anti-mCTLA-4 antibody. Effect on disease progression and overall survival are currently being assessed. We present a paradigm shift whereby CTLA-4 is an immunosuppressive protein irrespective of expression on T cells.

Abstract 4520: The effect of EPZ011989, an enhancer of Zeste homolog 2 inhibitor, in acute myeloid leukemia

Enhancer of Zeste Homolog 2 (EZH2) is a catalytic core subunit of the Polycomb repressive complex 2, PRC2. EZH2 catalyzes the tri-methylation of lysine 27 on histone H3 (H3K27me3) which often results in the silencing of associated gene promoters. The over-expression of EZH2 is associated with cancer progression and poor prognosis in solid tumors and hematological malignances. EZH2 mutations in lymphoma generally are activating whereas those in myelodysplasia and acute myeloid leukemia are typically loss of function. Additionally, EZH2 mutations are not isolated to a hotspot and therefore discerning phenotype is challenging. The purpose of this study was to determine if the inhibition of EZH2 induces cell differentiation or apoptosis in AML, and to determine the significance of EZH2 mutations in AML patients. As H3K27me3 deposition is catalyzed by EZH2, we hypothesized that analysis of H3K27me3 levels could differentiate an inactivating EZH2 mutation. We analyzed H3K27me3 levels in AML patient samples with wild type (N = 5) and mutant (N = 3) EZH2, and found that overall H3K27me3 is reduced in the mutant patient samples. Furthermore, the sample with the highest variant allele frequency (VAF) of the EZH2 mutation had noticeably lower levels of H3K27me3 compared to those that had a lower VAF. This finding demonstrates that diminished H3K27me3 levels can predict the presence of an inactivating EZH2 mutation in AML cells. We next assessed if the EZH2 tool molecule EPZ011989 produced a similar phenotype in wild-type EZH2 AML cell lines (Kasumi-1, MOLM-13, and MV4-11). EPZ011989 inhibited H3K27me3 in all of the cell lines at a concentration of 0.625 μM after only four days of treatment. At these concentrations and this time point we demonstrated no change in viability among these three cell lines and only minimal proliferation inhibition in MV4-11 and MOLM-13 cells. Longer time points are currently being investigated as EZH2 inhibitors often induce anti-proliferative effects only after longer incubation periods.

Therefore, our results support that inactivation of EZH2 with EZP011989 in AML cell lines induces the same cell-biochemical consequence (i.e. H3K27me3 loss) associated with EZH2 mutations in AML patients. This potentially will allow diverse pharmacologic studies in cell lines relative to gene targets and also synergistic lethality studies utilizing genetic and pharmacologic screening techniques to potentially develop therapeutic agents to treat this subtype of AML.

Citation Format: Sydney Fobare, Cecelia Miller, Shelley J. Orwick, Heike Keilhack, Erin Hertlein, John C. Byrd. The effect of EPZ011989, an enhancer of Zeste homolog 2 inhibitor, in acute myeloid leukemia. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4520.

Lenalidomide Induces Interleukin-21 Production by T Cells and Enhances IL21-Mediated Cytotoxicity in Chronic Lymphocytic Leukemia B Cells

The immunomodulatory drug lenalidomide has demonstrated efficacy in patients with chronic lymphocytic leukemia (CLL), despite a lack of direct cytotoxic effects in vitro The mechanism of lenalidomide efficacy in vivo is thought to occur via a combination of enhanced immune activity and an alteration of tumor cell-microenvironment interactions. We demonstrate in whole blood from patients with CLL that lenalidomide significantly depletes malignant B cells. Lenalidomide also induced production of interleukin-21 (IL21) and its mRNA in T cells from patients with CLL. In addition, lenalidomide enhanced upregulation of functional IL21 receptor (IL21R) on the cell surface and increased receptor mRNA in vitro The in vitro combination of IL21 and lenalidomide enhanced IL21-mediated cytotoxicity toward CLL cells through a variety of mechanisms. We show association of cell death with upregulation of Bid by IL21, enhanced upregulation of Bid by the combination therapy, and diminished Lck and downstream BCR signaling activation of Syk and PLCG2. Collectively, we demonstrated an immune cell-tumor cell interaction through lenalidomide-mediated induction of IL21 and IL21R, with enhanced IL21-mediated cytotoxicity, which provides justification for this combination in clinical trials for patients with CLL. Cancer Immunol Res; 4(8); 698-707. ©2016 AACR.

BRAFV600E induces ABCB1/P-glycoprotein expression and drug resistance in B-cells via AP-1 activation

A subset of patients with chronic lymphocytic leukemia (CLL) and nearly all patients with classic hairy cell leukemia (HCL) harbor somatic BRAF activating mutations. However, the pathological role of activated BRAF in B-cell leukemia development and progression remains unclear. In addition, although HCL patients respond well to the BRAFV600E inhibitor vemurafenib, relapses are being observed, suggesting the development of drug resistance in patients with this mutation. To investigate the biological role of BRAFV600E in B-cell leukemia, we generated a CLL-like B-cell line, OSUCLL, with doxycycline-inducible BRAFV600E expression. Microarray and real-time PCR analysis showed that ABCB1 mRNA is upregulated in these cells, and P-glycoprotein (P-gp) expression as well as function were confirmed by immunoblot and rhodamine exclusion assays. Additionally, pharmacological inhibition of BRAFV600E and MEK alleviated the BRAFV600E-induced ABCB1/P-gp expression. ABCB1 reporter assays and gel shift assays demonstrated that AP-1 activity is crucial in this mechanism. This study, uncovers a pathological role for BRAFV600E in B-cell leukemia, and provides further evidence that combination strategies with inhibitors of BRAFV600E and MEK can be used to delay disease progression and occurrence of resistance.

Characterization of a new chronic lymphocytic leukemia cell line for mechanistic in vitro and in vivo studies relevant to disease

Studies of chronic lymphocytic leukemia (CLL) have yielded substantial progress, however a lack of immortalized cell lines representative of the primary disease has hampered a full understanding of disease pathogenesis and development of new treatments. Here we describe a novel CLL cell line (OSU-CLL) generated by EBV transformation, which displays a similar cytogenetic and immunophenotype observed in the patient’s CLL (CD5 positive with trisomy 12 and 19). A companion cell line was also generated from the same patient (OSU-NB). This cell line lacked typical CLL characteristics, and is likely derived from the patient’s normal B cells. In vitro migration assays demonstrated that OSU-CLL exhibits migratory properties similar to primary CLL cells whereas OSU-NB has significantly reduced ability to migrate spontaneously or towards chemokine. Microarray analysis demonstrated distinct gene expression patterns in the two cell lines, including genes on chromosomes 12 and 19, which is consistent with the cytogenetic profile in this cell line. Finally, OSU-CLL was readily transplantable into NOG mice, producing uniform engraftment by three weeks with leukemic cells detectable in the peripheral blood spleen and bone marrow. These studies describe a new CLL cell line that extends currently available models to study gene function in this disease.

Fcγ receptor-induced soluble vascular endothelial growth factor receptor-1 (VEGFR-1) production inhibits angiogenesis and enhances efficacy of anti-tumor antibodies

Monocytes/macrophages are potent mediators of antitumor antibody therapy, where they engage target cells via Fcγ receptors (FcγR). Binding of these cells to opsonized tumor targets elicits cytokine production, phagocytosis, and antibody-mediated cellular cytotoxicity. Here we show for the first time that activation of monocyte FcγR results in the secretion of soluble vascular endothelial growth factor receptor-1 (VEGFR-1/sFlt-1), which serves to antagonize VEGF-mediated angiogenesis and tumor growth. Consistent with this, using a murine solid tumor model of antibody therapy, we show that sFlt-1 is involved in restricting tumor growth. Analyzing the mechanism of induction of sFlt-1, we found that the Erk and PI3K pathways were required for transcription, and NF-κB was required for translation. Upon closer examination of the role of NF-κB, we found that a microRNA, miR181a, negatively regulates FcγR-mediated sFlt-1 production and that NF-κB serves to antagonize this microRNA. Taken together, these results demonstrate a novel and biologically important function of monocytes and macrophages during antibody therapy.

The proteasome inhibitor carfilzomib functions independently of p53 to induce cytotoxicity and an atypical NF-κB response in chronic lymphocytic leukemia cells

PURPOSE

The proteasome consists of chymotrypsin-like (CT-L), trypsin-like, and caspase-like subunits that cleave substrates preferentially by amino acid sequence. Proteasomes mediate degradation of regulatory proteins of the p53, Bcl-2, and nuclear factor-κB (NF-κB) families that are aberrantly active in chronic lymphocytic leukemia (CLL). CLL remains an incurable disease, and new treatments are especially needed in the relapsed/refractory setting. We therefore investigated the effects of the proteasome inhibitor carfilzomib (CFZ) in CLL cells.

EXPERIMENTAL DESIGN

Tumor cells from CLL patients were assayed in vitro using immunoblotting, real-time polymerase chain reaction, and electrophoretic mobility shift assays. In addition, a p53 dominant-negative construct was generated in a human B-cell line.

RESULTS

Unlike bortezomib, CFZ potently induces apoptosis in CLL patient cells in the presence of human serum. CLL cells have significantly lower basal CT-L activity compared to normal B and T cells, although activity is inhibited similarly in T cells versus CLL. Co-culture of CLL cells on stroma protected from CFZ-mediated cytotoxicity; however, PI3K inhibition significantly diminished this stromal protection. CFZ-mediated cytotoxicity in leukemic B cells is caspase-dependent and occurs irrespective of p53 status. In CLL cells, CFZ promotes atypical activation of NF-κB evidenced by loss of cytoplasmic IκBα, phosphorylation of IκBα, and increased p50/p65 DNA binding, without subsequent increases in canonical NF-κB target gene transcription.

CONCLUSIONS

Together, these data provide new mechanistic insights into the activity of CFZ in CLL and support phase I investigation of CFZ in this disease.

Milatuzumab-conjugated liposomes as targeted dexamethasone carriers for therapeutic delivery in CD74+ B-cell malignancies

PURPOSE

Corticosteroids are widely used for the treatment of B-cell malignancies, including non-Hodgkin lymphoma, chronic lymphocytic leukemia (CLL), and acute lymphoblastic leukemia; however, this class of drug is associated with undesirable off-target effects. Herein, we developed novel milatuzumab-conjugated liposomes as a targeted dexamethasone carrier for therapeutic delivery in CD74(+) B-cell malignancies and explored its effect against the disease.

EXPERIMENTAL DESIGN

The targeting efficiency of milatuzumab-targeted liposomes to CD74(+) cells was evaluated in vitro. The effect of CD74-targeted liposomal dexamethasone was compared with free dexamethasone in primary CLL cells and cell lines in vitro. The therapeutic efficacy of CD74-targeted liposomal dexamethasone was evaluated in a Raji-severe combined immunodeficient (SCID) xenograft model in vivo.

RESULTS

Milatuzumab-targeted liposomes promoted selective incorporation of carrier molecules into transformed CD74-positive B cells as compared with CD74-negative T-cells. The CD74-dexamethasone-targeted liposomes (CD74-IL-DEX) promoted and increased killing in CD74-positive tumor cells and primary CLL cells. Furthermore, the targeted drug liposomes showed enhanced therapeutic efficacy against a CD74-positive B-cell model as compared with free, or non-targeted, liposomal dexamethasone in SCID mice engrafted with Raji cells in vivo.

CONCLUSIONS

These studies provide evidence and support for a potential use of CD74-targeted liposomal dexamethasone as a new therapy for B-cell malignancies.

Combination anti-CD74 (milatuzumab) and anti-CD20 (rituximab) monoclonal antibody therapy has in vitro and in vivo activity in mantle cell lymphoma

Mantle cell lymphoma (MCL) is an aggressive B-cell malignancy with a median survival of 3 years despite chemoimmunotherapy. Rituximab, a chimeric anti-CD20 monoclonal antibody (mAb), has shown only modest activity as single agent in MCL. The humanized mAb milatuzumab targets CD74, an integral membrane protein linked with promotion of B-cell growth and survival, and has shown preclinical activity against B-cell malignancies. Because rituximab and milatuzumab target distinct antigens and potentially signal through different pathways, we explored a preclinical combination strategy in MCL. Treatment of MCL cell lines and primary tumor cells with immobilized milatuzumab and rituximab resulted in rapid cell death, radical oxygen species generation, and loss of mitochondrial membrane potential. Cytoskeletal distrupting agents significantly reduced formation of CD20/CD74 aggregates, cell adhesion, and cell death, highlighting the importance of actin microfilaments in rituximab/milatuzumab-mediated cell death. Cell death was independent of caspase activation, Bcl-2 family proteins or modulation of autophagy. Maximal inhibition of p65 nuclear translocation was observed with combination treatment, indicating disruption of the NF-κB pathway. Significant in vivo therapeutic activity of combination rituximab and milatuzumab was demonstrated in a preclinical model of MCL. These data support clinical evaluation of combination milatuzumab and rituximab therapy in MCL.

17-DMAG targets the nuclear factor-kappaB family of proteins to induce apoptosis in chronic lymphocytic leukemia: clinical implications of HSP90 inhibition

The HSP90 client chaperone interaction stabilizes several important enzymes and antiapoptotic proteins, and pharmacologic inhibition of HSP90 results in rapid client protein degradation. Therefore, HSP90 inhibition is an attractive therapeutic approach when this protein is active, a phenotype commonly observed in transformed but not normal cells. However, preclinical studies with HSP90 inhibitors such as 17-AAG demonstrated depletion of only a subset of client proteins and very modest tumor cytotoxicity in chronic lymphocytic leukemia (CLL) cells. Herein, we describe another HSP90 inhibitor, 17-DMAG, which is cytotoxic to CLL but not normal lymphocytes. Treatment with 17-DMAG leads to depletion of the HSP90 client protein IKK, resulting in diminished NF-kappaB p50/p65 DNA binding, decreased NF-kappaB target gene transcription, and caspase-dependent apoptosis. Furthermore, treatment with 17-DMAG significantly decreased the white blood cell count and prolonged the survival in a TCL1-SCID transplant mouse model. The ability of 17-DMAG to function as an NF-kappaB inhibitor is of great interest clinically, as few currently available CLL drugs target this transcription factor. Therefore, the effect of 17-DMAG on NF-kappaB signaling pathways represents a novel therapy warranting further clinical pursuit in this and other B-cell lymphoproliferative disorders.

Epigenetic alterations in a murine model for chronic lymphocytic leukemia

Early stages in the development of chronic lymphocytic leukemia (CLL) have not been explored mainly due to the inability to study normal B-cells en route to transformation. In order to determine such early events of leukemogenesis, we have used a well established mouse model for CLL. Over-expression of human TCL1, a known CLL oncogene in murine B-cells leads to the development of mature CD19+/CD5+/IgM+ clonal leukemia with a disease phenotype similar to that seen in human CLL. Herein, we review our recent study using this TCL1-driven mouse model for CLL and corresponding human CLL samples in a cross-species epigenomics approach to address the timing and relevance of epigenetic events occurring during leukemogenesis. We demonstrated that the mouse model recapitulates the epigenetic events that have been reported for human CLL, affirming the power and validity of this mouse model to study early epigenetic events in cancer progression. Epigenetic alterations are detected as early as three months after birth, far before disease manifests at about 11 months of age. These mice undergo NFkappaB repressor complex mediated inactivation of the transcription factor Foxd3, whose targets become aberrantly methylated and silenced in mouse and human CLL. Overall, our data suggest the accumulated epigenetic alterations during CLL pathogenesis as a consequence of gene silencing through TCL1 and NFkappaB repressor complex, suggesting the relevance for NFkappaB as a therapeutic target in CLL.

NF-kappaB regulation of YY1 inhibits skeletal myogenesis through transcriptional silencing of myofibrillar genes

NF-kappaB signaling is implicated as an important regulator of skeletal muscle homeostasis, but the mechanisms by which this transcription factor contributes to muscle maturation and turnover remain unclear. To gain insight into these mechanisms, gene expression profiling was examined in C2C12 myoblasts devoid of NF-kappaB activity. Interestingly, even in proliferating myoblasts, the absence of NF-kappaB caused the pronounced induction of several myofibrillar genes, suggesting that NF-kappaB functions as a negative regulator of late-stage muscle differentiation. Although several myofibrillar promoters contain predicted NF-kappaB binding sites, functional analysis using the troponin-I2 gene as a model revealed that NF-kappaB-mediated repression does not occur through direct DNA binding. In the search for an indirect mediator, the transcriptional repressor YinYang1 (YY1) was identified. While inducers of NF-kappaB stimulated YY1 expression in multiple cell types, genetic ablation of the RelA/p65 subunit of NF-kappaB in both cultured cells and adult skeletal muscle correlated with reduced YY1 transcripts and protein. NF-kappaB regulation of YY1 occurred at the transcriptional level, mediated by direct binding of the p50/p65 heterodimer complex to the YY1 promoter. Furthermore, YY1 was found associated with multiple myofibrillar promoters in C2C12 myoblasts containing NF-kappaB activity. Based on these results, we propose that NF-kappaB regulation of YY1 and transcriptional silencing of myofibrillar genes represent a new mechanism by which NF-kappaB functions in myoblasts to modulate skeletal muscle differentiation.

ASC directs NF-kappaB activation by regulating receptor interacting protein-2 (RIP2) caspase-1 interactions

Receptor interacting protein-2 (RIP2) is a caspase recruitment domain (CARD)-containing kinase that interacts with caspase-1 and plays an important role in NF-kappaB activation. Apoptosis-associated speck-like protein containing a CARD (ASC) is a PYRIN and CARD-containing molecule, important in the induction of apoptosis and caspase-1 activation. Although RIP2 has also been linked to caspase-1 activation, RIP2 knockout animals fail to show a defect in caspase-1-mediated processing of proIL-1beta to its active form. Therefore, RIP2 function in binding to caspase-1 remains poorly understood. We hypothesized that caspase-1 may serve as a scaffolding molecule that promotes RIP2 interaction with IkappaB kinase-gamma thus inducing NF-kappaB activation. We further hypothesized that ASC, which also interacts with caspase-1 via its CARD, may interfere with the caspase-1 RIP2 interaction. In HEK293 cells, ASC induced prominent activation of caspase-1 and proIL-1beta processing. RIP2 transient transfection induced transcription of an NF-kappaB reporter gene. This RIP2-induced NF-kappaB activity and caspase-1 binding was inhibited in a dose-dependent fashion by ASC. Consistent with a role for caspase-1 as a scaffold for RIP2, caspase-1 knockout macrophages were suppressed in their ability to activate NF-kappaB, and septic caspase-1 knockout animals produced less IL-6, a functional marker of NF-kappaB activity. Lastly, THP-1 cells treated with small interfering RNA for ASC decreased their caspase-1 activity while enhancing their NF-kappaB signal. These data suggest that ASC may direct caspase-1 away from RIP2-mediated NF-kappaB activation, toward caspase-1-mediated processing of proIL-1beta by interfering with the RIP2 caspase-1 interaction.

RelA/p65 regulation of IkappaBbeta

IkappaB inhibitor proteins are the primary regulators of NF-kappaB. In contrast to the defined regulatory interplay between NF-kappaB and IkappaBalpha, much less is known regarding the regulation of IkappaBbeta by NF-kappaB. Here, we describe in detail the regulation of IkappaBbeta by RelA/p65. Using p65(-/-) fibroblasts, we show that IkappaBbeta is profoundly reduced in these cells, but not in other NF-kappaB subunit knockouts. This regulation prevails during embryonic and postnatal development in a tissue-specific manner. Significantly, in both p65(-/-) cells and tissues, IkappaBalpha is also reduced, but not nearly to the same extent as IkappaBbeta, thus highlighting the degree to which IkappaBbeta is dependent on p65. This dependence is based on the ability of p65 to stabilize IkappaBbeta protein from the 26S proteasome, a process mediated in large part through the p65 carboxyl terminus. Furthermore, IkappaBbeta was found to exist in both a basally phosphorylated and a hyperphosphorylated form. While the hyperphosphorylated form is less abundant, it is also more stable and less dependent on p65 and its carboxyl domain. Finally, we show that in p65(-/-) fibroblasts, expression of a proteolysis-resistant form of IkappaBbeta, but not IkappaBalpha, causes a severe growth defect associated with apoptosis. Based on these findings, we propose that tight control of IkappaBbeta protein by p65 is necessary for the maintenance of cellular homeostasis.