Immunophenotypic aberrant hematopoietic stem cells in myelodysplastic syndromes: a biomarker for leukemic progression.

Myelodysplastic syndromes (MDS) comprise hematological disorders that originate from the neoplastic transformation of hematopoietic stem cells (HSCs). However, discrimination between HSCs and their neoplastic counterparts in MDS-derived bone marrows (MDS-BMs) remains challenging. We hypothesized that in MDS patients immature CD34+CD38- cells with aberrant expression of immunophenotypic markers reflect neoplastic stem cells and that their frequency predicts leukemic progression. We analyzed samples from 68 MDS patients and 53 controls and discriminated HSCs from immunophenotypic aberrant HSCs (IA-HSCs) expressing membrane aberrancies (CD7, CD11b, CD22, CD33, CD44, CD45RA, CD56, CD123, CD366 or CD371). One-third of the MDS-BMs (23/68) contained IA-HSCs. The presence of IA-HSCs correlated with perturbed hematopoiesis (disproportionally expanded CD34+ subsets beside cytopenias) and an increased hazard of leukemic progression (HR = 25, 95% CI: 2.9-218) that was independent of conventional risk factors. At 2 years follow-up, the sensitivity and specificity of presence of IA-HSCs for predicting leukemic progression was 83% (95% CI: 36-99%) and 71% (95% CI: 58-81%), respectively. In a selected cohort (n = 10), most MDS-BMs with IA-HSCs showed genomic complexity and high human blast counts following xenotransplantation into immunodeficient mice, contrasting MDS-BMs without IA-HSCs. This study demonstrates that the presence of IA-HSCs within MDS-BMs predicts leukemic progression, indicating the clinical potential of IA-HSCs as a prognostic biomarker.

Cellular Assay to Study ß-Arrestin Recruitment by the Cannabinoid Receptors 1 and 2.

Cannabinoid receptor 1 (CB1R) and cannabinoid receptor 2 (CB2R) are G protein-coupled receptors (GPCRs) that activate a variety of pathways upon activation by (partial) agonists including the G protein pathway and the recruitment of ß-arrestins. Differences in the activation level of these pathways lead to biased signaling. Here, we describe a detailed protocol to characterize the potency and efficacy of ligands to induce or inhibit ß-arrestin recruitment to the human CB1R and CB2R using the PathHunter® assay. This is a cellular assay that uses a ß-galactosidase complementation system which has a chemiluminescent read-out and can be performed in 384-well plates. We have successfully used this assay to characterize a set of reference ligands (both agonists, antagonists, and an inverse agonist) on human CB1R and CB2R, of which some examples will be presented here.

Targeting histone methylation to reprogram the transcriptional state that drives survival of drug-tolerant myeloid leukemia persisters.

Although chemotherapy induces complete remission in the majority of acute myeloid leukemia (AML) patients, many face a relapse. This relapse is caused by survival of chemotherapy-resistant leukemia (stem) cells (measurable residual disease; MRD). Here, we demonstrate that the anthracycline doxorubicin epigenetically reprograms leukemia cells by inducing histone 3 lysine 27 (H3K27) and H3K4 tri-methylation. Within a doxorubicin-sensitive leukemia cell population, we identified a subpopulation of reversible anthracycline-tolerant cells (ATCs) with leukemic stem cell (LSC) features lacking doxorubicin-induced H3K27me3 or H3K4me3 upregulation. These ATCs have a distinct transcriptional landscape than the leukemia bulk and could be eradicated by KDM6 inhibition. In primary AML, reprogramming the transcriptional state by targeting KDM6 reduced MRD load and survival of LSCs residing within MRD, and enhanced chemotherapy response in vivo. Our results reveal plasticity of anthracycline resistance in AML cells and highlight the potential of transcriptional reprogramming by epigenetic-based therapeutics to target chemotherapy-resistant AML cells.

The Novel Oral BET-CBP/p300 Dual Inhibitor NEO2734 Is Highly Effective in Eradicating Acute Myeloid Leukemia Blasts and Stem/Progenitor Cells.

Acute myeloid leukemia (AML) is a disease characterized by transcriptional dysregulation that results in a block in differentiation and aberrant self-renewal. Inhibitors directed to epigenetic modifiers, aiming at transcriptional reprogramming of AML cells, are currently in clinical trials for AML patients. Several of these inhibitors target bromodomain and extraterminal domain (BET) proteins, cyclic AMP response binding protein-binding protein (CBP), and the E1A-interacting protein of 300 kDa (p300), affecting histone acetylation. Unfortunately, single epigenetic inhibitors showed limited efficacy due to appearance of resistance and lack of effective eradication of leukemic stem cells. Here, we describe the efficacy of 2 novel, orally available inhibitors targeting both the BET and CBP/p300 proteins, NEO1132 and NEO2734, in primary AML. NEO2734 and NEO1132 efficiently reduced the viability of AML cell lines and primary AML cells by inducing apoptosis. Importantly, both NEO drugs eliminated leukemic stem/progenitor cells from AML patient samples, and NEO2734 increased the effectiveness of combination chemotherapy treatment in an in vivo AML patient-derived mouse model. Thus, dual inhibition of BET and CBP/p300 using NEO2734 is a promising therapeutic strategy for AML patients, making it a focus for clinical translation.

Escape From Treatment; the Different Faces of Leukemic Stem Cells and Therapy Resistance in Acute Myeloid Leukemia.

Standard induction chemotherapy, consisting of an anthracycline and cytarabine, has been the first-line therapy for many years to treat acute myeloid leukemia (AML). Although this treatment induces complete remissions in the majority of patients, many face a relapse (adaptive resistance) or have refractory disease (primary resistance). Moreover, older patients are often unfit for cytotoxic-based treatment. AML relapse is due to the survival of therapy-resistant leukemia cells (minimal residual disease, MRD). Leukemia cells with stem cell features, named leukemic stem cells (LSCs), residing within MRD are thought to be at the origin of relapse initiation. It is increasingly recognized that leukemia "persisters" are caused by intra-leukemic heterogeneity and non-genetic factors leading to plasticity in therapy response. The BCL2 inhibitor venetoclax, combined with hypomethylating agents or low dose cytarabine, represents an important new therapy especially for older AML patients. However, often there is also a small population of AML cells refractory to venetoclax treatment. As AML MRD reflects the sum of therapy resistance mechanisms, the different faces of treatment "persisters" and LSCs might be exploited to reach an optimal therapy response and prevent the initiation of relapse. Here, we describe the different epigenetic, transcriptional, and metabolic states of therapy sensitive and resistant AML (stem) cell populations and LSCs, how these cell states are influenced by the microenvironment and affect treatment outcome of AML. Moreover, we discuss potential strategies to target dynamic treatment resistance and LSCs.

IGFBP7 activates retinoid acid-induced responses in acute myeloid leukemia stem and progenitor cells.

Treatment of acute promyelocytic leukemia (APL) with all-trans retinoic acid (ATRA) in combination with low doses of arsenic trioxide or chemotherapy leads to exceptionally high cure rates (>90%). ATRA forces APL cells into differentiation and cell death. Unfortunately, ATRA-based therapy has not been effective among any other acute myeloid leukemia (AML) subtype, and long-term survival rates remain unacceptably low; only 30% of AML patients survive 5 years after diagnosis. Here, we identified insulin-like growth factor binding protein 7 (IGFBP7) as part of ATRA-induced responses in APL cells. Most importantly, we observed that addition of recombinant human IGFBP7 (rhIGFBP7) increased ATRA-driven responses in a subset of non-APL AML samples: those with high RARA expression. In nonpromyelocytic AML, rhIGFBP7 treatment induced a transcriptional program that sensitized AML cells for ATRA-induced differentiation, cell death, and inhibition of leukemic stem/progenitor cell survival. Furthermore, the engraftment of primary AML in mice was significantly reduced following treatment with the combination of rhIGFBP7 and ATRA. Mechanistically, we showed that the synergism of ATRA and rhIGFBP7 is due, at least in part, to reduction of the transcription factor GFI1. Together, these results suggest a potential clinical utility of IGFBP7 and ATRA combination treatment to eliminate primary AML (leukemic stem/progenitor) cells and reduce relapse in AML patients.

Uncoupling DNA damage from chromatin damage to detoxify doxorubicin.

The anthracycline doxorubicin (Doxo) and its analogs daunorubicin (Daun), epirubicin (Epi), and idarubicin (Ida) have been cornerstones of anticancer therapy for nearly five decades. However, their clinical application is limited by severe side effects, especially dose-dependent irreversible cardiotoxicity. Other detrimental side effects of anthracyclines include therapy-related malignancies and infertility. It is unclear whether these side effects are coupled to the chemotherapeutic efficacy. Doxo, Daun, Epi, and Ida execute two cellular activities: DNA damage, causing double-strand breaks (DSBs) following poisoning of topoisomerase II (Topo II), and chromatin damage, mediated through histone eviction at selected sites in the genome. Here we report that anthracycline-induced cardiotoxicity requires the combination of both cellular activities. Topo II poisons with either one of the activities fail to induce cardiotoxicity in mice and human cardiac microtissues, as observed for aclarubicin (Acla) and etoposide (Etop). Further, we show that Doxo can be detoxified by chemically separating these two activities. Anthracycline variants that induce chromatin damage without causing DSBs maintain similar anticancer potency in cell lines, mice, and human acute myeloid leukemia patients, implying that chromatin damage constitutes a major cytotoxic mechanism of anthracyclines. With these anthracyclines abstained from cardiotoxicity and therapy-related tumors, we thus uncoupled the side effects from anticancer efficacy. These results suggest that anthracycline variants acting primarily via chromatin damage may allow prolonged treatment of cancer patients and will improve the quality of life of cancer survivors.

IGFBP7 Induces Differentiation and Loss of Survival of Human Acute Myeloid Leukemia Stem Cells without Affecting Normal Hematopoiesis.

Leukemic stem cells (LSCs) are thought to be the major cause of the recurrence of acute myeloid leukemia (AML) due to their potential for self-renewal. To identify therapeutic strategies targeting LSCs, while sparing healthy hematopoietic stem cells (HSCs), we performed gene expression profiling of LSCs, HSCs, and leukemic progenitors all residing within the same AML bone marrow and identified insulin-like growth factor-binding protein 7 (IGFBP7) as differentially expressed. Low IGFBP7 is a feature of LSCs and is associated with reduced chemotherapy sensitivity. Enhancing IGFBP7 by overexpression or addition of recombinant human IGFBP7 (rhIGFBP7) resulted in differentiation, inhibition of cell survival, and increased chemotherapy sensitivity of primary AML cells. Adding rhIGFBP7 reduced leukemic stem and/or progenitor survival and reversed a stem-like gene signature, but it had no influence on normal hematopoietic stem cell survival. Our data suggest a potential clinical utility of the addition of rhIGFBP7 to current chemotherapy regimens to decrease AML relapse rates.

Reprogramming acute myeloid leukemia into sensitivity for retinoic-acid-driven differentiation.

The success of all-trans retinoic acid (ATRA) therapy for acute promyelocytic leukemia (APL) provides a rationale for using retinoic acid (RA)-based therapy for other subtypes of acute myeloid leukemia (AML). Recently, several studies showed that ATRA may drive leukemic cells efficiently into differentiation and/or apoptosis in a subset of AML patients with an NPM1 mutation, a FLT3-ITD, an IDH1 mutation, and patients overexpressing EVI-1. Because not all patients within these molecular subgroups respond to ATRA and clinical trials that tested ATRA response in non-APL AML patients have had disappointing results, the identification of additional biomarkers may help to identify patients who strongly respond to ATRA-based therapy. Searching for response biomarkers might also reveal novel RA-based combination therapies with an efficient differentiation/apoptosis-inducing effect in non-APL AML patients. Preliminary studies suggest that the epigenetic or transcriptional state of leukemia cells determines their susceptibility to ATRA. We hypothesize that reprogramming by inhibitors of epigenetic-modifying enzymes or by modulation of microRNA expression might sensitize non-APL AML cells for RA-based therapy. AML relapse is caused by a subpopulation of leukemia cells, named leukemic stem cells (LSCs), which are in a different epigenetic state than the total bulk of the AML. The survival of LSCs after therapy is the main cause of the poor prognosis of AML patients, and novel differentiation therapies should drive these LSCs into maturity. In this review, we summarize the current knowledge on the epigenetic aspects of susceptibility to RA-induced differentiation in APL and non-APL AML.

Structural basis of ligand interaction with atypical chemokine receptor 3.

Chemokines drive cell migration through their interactions with seven-transmembrane (7TM) chemokine receptors on cell surfaces. The atypical chemokine receptor 3 (ACKR3) binds chemokines CXCL11 and CXCL12 and signals exclusively through ß-arrestin-mediated pathways, without activating canonical G-protein signalling. This receptor is upregulated in numerous cancers making it a potential drug target. Here we collected over 100 distinct structural probes from radiolytic footprinting, disulfide trapping, and mutagenesis to map the structures of ACKR3:CXCL12 and ACKR3:small-molecule complexes, including dynamic regions that proved unresolvable by X-ray crystallography in homologous receptors. The data are integrated with molecular modelling to produce complete and cohesive experimentally driven models that confirm and expand on the existing knowledge of the architecture of receptor:chemokine and receptor:small-molecule complexes. Additionally, we detected and characterized ligand-induced conformational changes in the transmembrane and intracellular regions of ACKR3 that elucidate fundamental structural elements of agonism in this atypical receptor.

Cannabinoid CB2 receptor ligand profiling reveals biased signalling and off-target activity.

The cannabinoid CB2 receptor (CB2R) represents a promising therapeutic target for various forms of tissue injury and inflammatory diseases. Although numerous compounds have been developed and widely used to target CB2R, their selectivity, molecular mode of action and pharmacokinetic properties have been poorly characterized. Here we report the most extensive characterization of the molecular pharmacology of the most widely used CB2R ligands to date. In a collaborative effort between multiple academic and industry laboratories, we identify marked differences in the ability of certain agonists to activate distinct signalling pathways and to cause off-target effects. We reach a consensus that HU910, HU308 and JWH133 are the recommended selective CB2R agonists to study the role of CB2R in biological and disease processes. We believe that our unique approach would be highly suitable for the characterization of other therapeutic targets in drug discovery research.

Protocol to Study ß-Arrestin Recruitment by CB1 and CB2 Cannabinoid Receptors.

Cannabinoid CB1 and CB2 receptors are G-protein-coupled receptors (GPCRs) that recruit ß-arrestins upon activation by (partial) agonists. ß-Arrestin recruitment is induced by phosphorylation of their C-terminal tails, and is associated with the termination of GPCR signaling; yet, it may also activate cellular signaling pathways independent of G-proteins. Here, we describe a detailed protocol to characterize the potency and efficacy of ligands to induce or inhibit ß-arrestin recruitment to the human CB1 and CB2 receptors, by using the PathHunter(®) assay. The latter is a cellular assay that can be performed in plates with 384-wells. The PathHunter(®) assay makes use of ß-galactosidase complementation, and has a chemiluminescent readout. We used this assay to characterize a set of reference ligands (both agonists and antagonists) on human CB1 and CB2 receptors.

The novel, orally available and peripherally restricted selective cannabinoid CB2 receptor agonist LEI-101 prevents cisplatin-induced nephrotoxicity.

BACKGROUND AND PURPOSE: Here, we have characterized 3-cyclopropyl-1-(4-(6-((1,1-dioxidothiomorpholino)methyl)-5-fluoropyridin-2-yl)benzyl)imidazolidine-2,4-dione hydrochloride (LEI-101) as a novel, peripherally restricted cannabinoid CB2 receptor agonist, using both in vitro and in vivo models. EXPERIMENTAL APPROACH: We investigated the effects of LEI-101 on binding and functional activity. We assessed its in vitro and in vivo selectivity. Efficacy of LEI-101 was determined in a mouse model of cisplatin-induced nephrotoxicity. KEY RESULTS: LEI-101 behaved as a partial agonist at CB2 receptors using ß-arrestin and GTPγS assays and was ~100-fold selective in CB2 /CB1 receptor-binding assays. It did not display any activity on endocannabinoid hydrolases and nor did it react with serine hydrolases in an activity-based protein profiling assay. In mice, LEI-101 had excellent oral bioavailability reaching high concentrations in the kidney and liver with minimal penetration into the brain. LEI-101 up to a dose of 60 mg·kg(-1) (p.o.) did not exert any CNS-mediated effects in the tetrad assay, in mice. LEI-101 (p.o. or i.p.) at 3 or 10 mg·kg(-1) dose-dependently prevented kidney dysfunction and/or morphological damage induced by cisplatin in mice. These protective effects were associated with improved renal histopathology, attenuated oxidative stress and inflammation in the kidney. These effects were absent in CB2 receptor knockout mice. CONCLUSION AND IMPLICATIONS: These results indicate that LEI-101 is a selective, largely peripherally restricted, orally available CB2 receptor agonist with therapeutic potential in diseases that are associated with inflammation and/or oxidative stress, including kidney disease.