Glucocorticoid receptor knock down reveals a similar apoptotic threshold but differing gene regulation patterns in T-cell and pre-B-cell acute lymphoblastic leukemia

PRC2 Inhibitors Overcome Glucocorticoid Resistance Driven by NSD2 Mutation in Pediatric Acute Lymphoblastic Leukemia

Mutations in epigenetic regulators are common in relapsed pediatric acute lymphoblastic leukemia (ALL). Here, we uncovered the mechanism underlying the relapse of ALL driven by an activating mutation of the NSD2 histone methyltransferase (p.E1099K). Using high-throughput drug screening, we found that NSD2-mutant cells were specifically resistant to glucocorticoids. Correction of this mutation restored glucocorticoid sensitivity. The transcriptional response to glucocorticoids was blocked in NSD2-mutant cells due to depressed glucocorticoid receptor (GR) levels and the failure of glucocorticoids to autoactivate GR expression. Although H3K27me3 was globally decreased by NSD2 p.E1099K, H3K27me3 accumulated at the NR3C1 (GR) promoter. Pretreatment of NSD2 p.E1099K cell lines and patient-derived xenograft samples with PRC2 inhibitors reversed glucocorticoid resistance in vitro and in vivo. PRC2 inhibitors restored NR3C1 autoactivation by glucocorticoids, increasing GR levels and allowing GR binding and activation of proapoptotic genes. These findings suggest a new therapeutic approach to relapsed ALL associated with NSD2 mutation. SIGNIFICANCE: NSD2 histone methyltransferase mutations observed in relapsed pediatric ALL drove glucocorticoid resistance by repression of the GR and abrogation of GR gene autoactivation due to accumulation of K3K27me3 at its promoter. Pretreatment with PRC2 inhibitors reversed resistance, suggesting a new therapeutic approach to these patients with ALL.This article is highlighted in the In This Issue feature, p. 1.

Crosstalk between Hedgehog pathway and the glucocorticoid receptor pathway as a basis for combination therapy in T-cell acute lymphoblastic leukemia

Notwithstanding intensified therapy, a considerable fraction of T-cell acute lymphoblastic leukemia (T-ALL) patients face a dismal prognosis due to primary resistance to treatment and relapse, raising the need for more efficient and targeted therapies. Hedgehog (HH) signaling is a major developmental pathway frequently deregulated in cancer, for which a role in T-ALL is emerging. Mounting evidence suggests that ligand-independent activation of HH pathway occurs in cancer including T-ALL, emphasizing the necessity of dissecting the complex interplay between HH and other signaling pathways regulating activation. In this work, we present a therapeutically relevant crosstalk between HH signaling and the glucocorticoid receptor (NR3C1) pathway acting at the level of GLI1 transcription factor. GLI inhibitor GANT61 and dexamethasone were shown to exert a synergistic anti-leukemic effect in vitro in T-ALL cell lines and patient-derived xenografts. Mechanistically, dexamethasone-activated NR3C1 impaired GLI1 function by dynamically modulating the recruitment of PCAF acetyltransferase and HDAC1 deacetylase. Increased GLI1 acetylation was associated with compromised transcriptional activity and reduced protein stability. In summary, our study identifies a novel crosstalk between GLI1 and NR3C1 signaling pathway which could be exploited in HH-dependent malignancies to increase therapeutic efficacy.

Huai Qi Huang Potentiates Dexamethasone-Mediated Lethality in Acute Lymphoblastic Leukemia Cells by Upregulating Glucocorticoid Receptor α

Background

Glucocorticoids are important components of a number of chemotherapeutic regimens used to treat pediatric acute lymphoblastic leukemia (ALL). A primary cause of treatment failure of ALL is acquired resistance to glucocorticoids. Recently, traditional Chinese medicines were effectively used to treat solid tumors. Thus, the aim of this study was to investigate whether Huai Qi Huang (HQH), a traditional Chinese medicine, increased the efficacy of glucocorticoids in the treatment of ALL, and if so, to determine the underlying mechanism.

Material/Methods

Various concentrations of HQH were used to treat Jurkat and Nalm-6 cells for 24 to 72 hours. Subsequently, cells were co-treated with HQH and the glucocorticoid receptor agonist, dexamethasone (DEX), or a MEK inhibitor (PD98059) to verify the synergistic effects on apoptosis in Jurkat and Nalm-6 cells for 24 hours. Cell Counting Kit-8 assay and flow cytometry were used to measure cell viability and apoptosis, respectively. Protein and mRNA expression levels were assessed using western blotting and quantitative polymerase chain reaction.

Results

The results revealed that cell survival was reduced and apoptosis was increased as the HQH concentration was increased, and this was accompanied with increases in the levels of BAX, cleaved-caspase-3 and glucocorticoid receptor α (GRα) and decreases in the levels of Bcl-2 and phospho-ERK (pERK). Glucocorticoid receptor β (GRβ) and total ERK (t-ERK) had no significant changes. Combined treatment with HQH and DEX or PD98059 increased apoptosis in Jurkat and Nalm-6 cells, and concurrently increased BAX, cleaved-caspase-3, GILZ, NFKBIA, and GRα and decreased Bcl-2 and pERK.

Conclusions

HQH enhanced the sensitivity of ALL cells to glucocorticoids by increasing the expression of GRα and inhibiting the MEK/ERK pathway, thus providing a rational foundation for the treatment of ALL with HQH.

MicroRNAs and Glucocorticoid-Induced Apoptosis in Lymphoid Malignancies

The initial response of lymphoid malignancies to glucocorticoids (GCs) is a critical parameter predicting successful treatment. Although being known as a strong inducer of apoptosis in lymphoid cells for almost a century, the signaling pathways regulating the susceptibility of the cells to GCs are only partly revealed. There is still a need to develop clinical tests that can predict the outcome of GC therapy. In this paper, I discuss important parameters modulating the pro-apoptotic effects of GCs, with a specific emphasis on the microRNA world comprised of small players with big impacts. The journey through the multifaceted complexity of GC-induced apoptosis brings forth explanations for the differential treatment response and raises potential strategies for overcoming drug resistance.

Role of c-Myb in the survival of pre B-cell acute lymphoblastic leukemia and leukemogenesis

Acute lymphoblastic leukemia (ALL) is the most common cancer in children. The current treatment protocol for ALL involves an intense chemotherapy regimen yielding cure rates of nearly 80%. However, new therapies need to be designed not only to increase the survival rate but also to combat the risk of severe therapy associated toxicities including secondary malignancies, growth problems, organ damage, and infertility. The c-Myb proto-oncogene is highly expressed in immature hematopoietic cells. In this study, we demonstrate that loss of c-Myb itself decreased the viability of these leukemic cells. Additionally, the inhibition of c-Myb caused a decrease in cell proliferation, significantly increased the number of cells in G(0) /G(1) phase of the cell cycle, increased the sensitivity of pre-B-ALL cells to cytotoxic agents in vitro, and significantly delayed disease onset in a mouse model of leukemia. Furthermore, we demonstrate that Bcl-2 is a target of c-Myb in pre-B-ALL cells. Our results identify c-Myb as a potential therapeutic target in pre-B-ALL and suggest that suppression of c-Myb levels or activity, in combination with currently used therapies and/or dose reduction, may lead to a decrease in toxicity and an increase in patient survival rates. Because c-Myb is aberrantly expressed in several other malignancies, targeting c-Myb will have broad clinical applications.

c-Myb interacts with the glucocorticoid receptor and regulates its level in pre-B-acute lymphoblastic leukemia cells

Glucocorticoid (GC) hormones are used in the treatment of hematopoietic malignancies. When the GC binds to the glucocorticoid receptor (GR) protein, c-Myb and GR are recruited at the Glucocorticoid Response Unit in the DNA. Here we demonstrate that c-Myb interacts with the GR and that decreasing c-Myb amounts reduces the levels of GR transcripts and protein in 697 pre-B-acute lymphoblastic leukemia (ALL) cells. Furthermore, the auto-upregulation of GR promoter 1C and promoter 1D is blunted at reduced c-Myb levels. Taken together, these data show that c-Myb is a direct, key regulator of the GR. Unexpectedly, the reduction in c-Myb levels increased the sensitivity of the cells to steroid-mediated apoptosis. This was because the reduction in c-Myb itself decreases cell viability, and the residual GR remained above the threshold needed to trigger apoptosis. These studies show the mutual importance of c-Myb and the GR in controlling survival of pre-B ALL cells.

A facile, branched DNA assay to quantitatively measure glucocorticoid receptor auto-regulation in T-cell acute lymphoblastic leukemia

Glucocorticoid (GC) steroid hormones are used to treat acute lymphoblastic leukemia (ALL) because of their pro-apoptotic effects in hematopoietic cells. However, not all leukemia cells are sensitive to GC, and no assay to stratify patients is available. In the GC-sensitive T-cell ALL cell line CEM-C7, auto-up-regulation of RNA transcripts for the glucocorticoid receptor (GR) correlates with increased apoptotic response. This study aimed to determine if a facile assay of GR transcript levels might be promising for stratifying ALL patients into hormone-sensitive and hormone-resistant populations. The GR transcript profiles of various lymphoid cell lines and 4 bone marrow samples from patients with T-cell ALL were analyzed using both an optimized branched DNA (bDNA) assay and a real-time quantitative reverse transcription-polymerase chain reaction assay. There were significant correlations between both assay platforms when measuring total GR (exon 5/6) transcripts in various cell lines and patient samples, but not for a probe set that detects a specific, low abundance GR transcript (exon 1A3). Our results suggest that the bDNA platform is reproducible and precise when measuring total GR transcripts and, with further development, may ultimately offer a simple clinical assay to aid in the prediction of GC-sensitivity in ALL patients.

Thymocyte responsiveness to endogenous glucocorticoids is required for immunological fitness

Generation of a self-tolerant but antigen-responsive T cell repertoire occurs in the thymus. Although glucocorticoids are usually considered immunosuppressive, there is also evidence that they play a positive role in thymocyte selection. To address the question of how endogenous glucocorticoids might influence the adaptive immune response, we generated GRlck-Cre mice, in which the glucocorticoid receptor gene (GR) is deleted in thymocytes prior to selection. These mice were immunocompromised, with reduced polyclonal T cell proliferative responses to alloantigen, defined peptide antigens, and viral infection. This was not due to an intrinsic proliferation defect, because GR-deficient T cells responded normally when the TCR was cross-linked with antibodies or when the T cell repertoire was "fixed" with αβ TCR transgenes. Varying the affinity of self ligands in αβ TCR transgenic mice showed that affinities that would normally lead to thymocyte-positive selection caused negative selection, and alterations in the TCR repertoire of polyclonal T cells were confirmed by analysis of TCR Vβ CDR3 regions. Thus, endogenous glucocorticoids are required for a robust adaptive immune response because of their promotion of the selection of T cells that have sufficient affinity for self, and the absence of thymocyte glucocorticoid signaling results in an immunocompromised state.

Research resource: transcriptional response to glucocorticoids in childhood acute lymphoblastic leukemia

Glucocorticoids (GC) induce apoptosis in lymphoblasts and are thus essential in the treatment of acute lymphoblastic leukemia (ALL). Their effects result from gene regulations via the GC receptor (NR3C1/GR), but it is unknown how these changes evolve, what the primary GR targets are, and to what extent responses differ between ALL subtypes and nonlymphoid malignancies. We delineated the transcriptional response to GC on the exon level in a time-resolved manner in a precursor B- and a T childhood ALL model employing Exon microarrays and combined this with genome-wide NR3C1-binding site detection using chromatin immunoprecipitation-on-chip technology. This integrative approach showed that the response was strongly influenced by kinetics and extent of GR autoinduction in both models. Although remarkable differences between the ALL systems were apparent, we defined a set of common response genes enriched in apoptosis-related processes. Globally, GR binding was higher for GC-induced vs. -repressed genes, suggesting that GR mediates gene repression by interaction with distant enhancers or by cross talk with other transcription factors. Exon level analysis defined several new GC-regulated transcript variants of genes, including ATP4B, GPR98, TBCD, and ZBTB16. Our study provides unprecedented insight into the transcriptional response to GC in ALL cells, essential to understand this biologically and clinically important phenomenon. We found evidence of cell type-specific as well as common responses, possibly related to apoptosis induction, and detected induction of novel transcript variants by GC in the investigated systems. Finally, we implemented a bioinformatic framework that might be useful for high-density microarray analyses to identify alternative transcript variant expression.

A new, lineage specific, autoup-regulation mechanism for human glucocorticoid receptor gene expression in 697 pre-B-acute lymphoblastic leukemia cells

Glucocorticoid (GC) steroid hormones induce apoptosis in acute lymphoblastic leukemia (ALL). Autoup-regulation of human GC receptor (hGR) levels is associated with sensitivity to GC-mediated apoptosis. Among the major hGR promoters expressed in 697 pre-B-ALL cells (1A, 1B, 1C, and 1D), only promoters 1C and 1D are selectively activated by the hormone. Promoter 1B is unresponsive, and promoter 1A is down-regulated by dexamethasone (Dex) in 697 cells, whereas they are both up-regulated in CEM-C7 T-ALL cells. Autoup-regulation of promoter 1C and 1D in 697 cells requires sequences containing GC response units (GRUs) (1C GRU, -2915/-2956; 1D GRU, -4525/-4559) that were identified previously in CEM-C7 cells. These GRUs potentially bind GR, c-myeloblastosis (c-Myb), and E-twenty six (Ets) proteins; 697 cells express high levels of c-Myb protein, as well as the E-twenty six family protein members, PU.1 and Spi-B. Dex treatment in 697 cells elevates the expression of c-Myb and decreases levels of both Spi-B and PU.1. Chromatin immunoprecipitation assays revealed the specific recruitment of GR, c-Myb, and cAMP response element-binding protein binding protein to the 1C and 1D GRUs upon Dex treatment, correlating to observed autoup-regulated activity in these two promoters. These data suggest a hormone activated, lineage-specific mechanism to control the autoup-regulation of hGR gene expression in 697 pre-B-ALL cells via steroid-mediated changes in GR coregulator expression. These findings may be helpful in understanding the mechanism that determines the sensitivity of B-ALL leukemia cells to hormone-induced apoptosis.

Protein kinase networks regulating glucocorticoid-induced apoptosis of hematopoietic cancer cells: fundamental aspects and practical considerations

Glucocorticoids (GCs) are integral components in the treatment protocols of acute lymphoblastic leukemia, multiple myeloma, and non-Hodgkin lymphoma owing to their ability to induce apoptosis of these malignant cells. Resistance to GC therapy is associated with poor prognosis. Although they have been used in clinics for decades, the signal transduction pathways involved in GC-induced apoptosis have only partly been resolved. Accumulating evidence shows that this cell death process is mediated by a communication between nuclear GR affecting gene transcription of pro-apoptotic genes such as Bim, mitochondrial GR affecting the physiology of the mitochondria, and the protein kinase glycogen synthase kinase-3 (GSK3), which interacts with Bim following exposure to GCs. Prevention of Bim up-regulation, mitochondrial GR translocation, and/or GSK3 activation are common causes leading to GC therapy failure. Various protein kinases positively regulating the pro-survival Src-PI3K-Akt-mTOR and Raf-Ras-MEK-ERK signal cascades have been shown to be activated in malignant leukemic cells and antagonize GC-induced apoptosis by inhibiting GSK3 activation and Bim expression. Targeting these protein kinases has proven effective in sensitizing GR-positive malignant lymphoid cells to GC-induced apoptosis. Thus, intervening with the pro-survival kinase network in GC-resistant cells should be a good means of improving GC therapy of hematopoietic malignancies.