Therapeutic targeting Tudor domains in leukemia via CRISPR-Scan Assisted Drug Discovery

Epigenetic dysregulation has been reported in multiple cancers including leukemias. Nonetheless, the roles of the epigenetic reader Tudor domains in leukemia progression and therapy remain unexplored. Here, we conducted a Tudor domain-focused CRISPR screen and identified SGF29, a component of SAGA/ATAC acetyltransferase complexes, as a crucial factor for H3K9 acetylation, ribosomal gene expression, and leukemogenesis. To facilitate drug development, we integrated the CRISPR tiling scan with compound docking and molecular dynamics simulation, presenting a generally applicable strategy called CRISPR-Scan Assisted Drug Discovery (CRISPR-SADD). Using this approach, we identified a lead inhibitor that selectively targets SGF29's Tudor domain and demonstrates efficacy against leukemia. Furthermore, we propose that the structural genetics approach used in our study can be widely applied to diverse fields for de novo drug discovery.

Epigenetic Control of Translation Checkpoint and Tumor Progression via RUVBL1-EEF1A1 Axis

Epigenetic dysregulation is reported in multiple cancers including Ewing sarcoma (EwS). However, the epigenetic networks underlying the maintenance of oncogenic signaling and therapeutic response remain unclear. Using a series of epigenetics- and complex-focused CRISPR screens, RUVBL1, the ATPase component of NuA4 histone acetyltransferase complex, is identified to be essential for EwS tumor progression. Suppression of RUVBL1 leads to attenuated tumor growth, loss of histone H4 acetylation, and ablated MYC signaling. Mechanistically, RUVBL1 controls MYC chromatin binding and modulates the MYC-driven EEF1A1 expression and thus protein synthesis. High-density CRISPR gene body scan pinpoints the critical MYC interacting residue in RUVBL1. Finally, this study reveals the synergism between RUVBL1 suppression and pharmacological inhibition of MYC in EwS xenografts and patient-derived samples. These results indicate that the dynamic interplay between chromatin remodelers, oncogenic transcription factors, and protein translation machinery can provide novel opportunities for combination cancer therapy.

Dynamic phosphatase-recruitment controls B-cell selection and oncogenic signaling

Initiation of B-cell receptor (BCR) ¹ signaling, and subsequent antigen-encounter in germinal centers ^2,3 represent milestones of B-lymphocyte development that are both marked by sharp increases of CD25 surface-expression. Oncogenic signaling in B-cell leukemia (B-ALL) ⁴ and lymphoma ⁵ also induced CD25-surface expression. While CD25 is known as an IL2-receptor chain on T- and NK-cells ^6-9 , the significance of its expression on B-cells was unclear. Our experiments based on genetic mouse models and engineered patient-derived xenografts revealed that, rather than functioning as an IL2-receptor chain, CD25 expressed on B-cells assembled an inhibitory complex including PKCδ and SHIP1 and SHP1 phosphatases for feedback control of BCR-signaling or its oncogenic mimics. Recapitulating phenotypes of genetic ablation of PKCδ ¹⁰ - ¹² , SHIP1 ^13,14 and SHP1 ^{14, 15,16} , conditional CD25-deletion decimated early B-cell subsets but expanded mature B-cell populations and induced autoimmunity. In B-cell malignancies arising from early (B-ALL) and late (lymphoma) stages of B-cell development, CD25-loss induced cell death in the former and accelerated proliferation in the latter. Clinical outcome annotations mirrored opposite effects of CD25-deletion: high CD25 expression levels predicted poor clinical outcomes for patients with B-ALL, in contrast to favorable outcomes for lymphoma-patients. Biochemical and interactome studies revealed a critical role of CD25 in BCR-feedback regulation: BCR-signaling induced PKCδ-mediated phosphorylation of CD25 on its cytoplasmic tail (S ²⁶⁸ ). Genetic rescue experiments identified CD25-S ²⁶⁸ tail-phosphorylation as central structural requirement to recruit SHIP1 and SHP1 phosphatases to curb BCR-signaling. A single point mutation CD25 ^S268A abolished recruitment and activation of SHIP1 and SHP1 to limit duration and strength of BCR-signaling. Loss of phosphatase-function, autonomous BCR-signaling and Ca ²⁺ -oscillations induced anergy and negative selection during early B-cell development, as opposed to excessive proliferation and autoantibody production in mature B-cells. These findings highlight the previously unrecognized role of CD25 in assembling inhibitory phosphatases to control oncogenic signaling in B-cell malignancies and negative selection to prevent autoimmune disease.

Targeted engagement of β-catenin-Ikaros complexes in refractory B-cell malignancies

In most cell types, nuclear β-catenin functions as prominent oncogenic driver and pairs with TCF7-family factors for transcriptional activation of MYC. Surprisingly, B-lymphoid malignancies not only lacked expression and activating lesions of β-catenin but critically depended on GSK3β for effective β-catenin degradation. Our interactome studies in B-lymphoid tumors revealed that β-catenin formed repressive complexes with lymphoid-specific Ikaros factors at the expense of TCF7. Instead of MYC-activation, β-catenin was essential to enable Ikaros-mediated recruitment of nucleosome remodeling and deacetylation (NuRD) complexes for transcriptional repression of MYC. To leverage this previously unrecognized vulnerability of B-cell-specific repressive β-catenin-Ikaros-complexes in refractory B-cell malignancies, we examined GSK3β small molecule inhibitors to subvert β-catenin degradation. Clinically approved GSK3β-inhibitors that achieved favorable safety prof les at micromolar concentrations in clinical trials for neurological disorders and solid tumors were effective at low nanomolar concentrations in B-cell malignancies, induced massive accumulation of β-catenin, repression of MYC and acute cell death. Preclinical in vivo treatment experiments in patient-derived xenografts validated small molecule GSK3β-inhibitors for targeted engagement of lymphoid-specific β-catenin-Ikaros complexes as a novel strategy to overcome conventional mechanisms of drug-resistance in refractory malignancies.

HIGHLIGHTS

Unlike other cell lineages, B-cells express nuclear β-catenin protein at low baseline levels and depend on GSK3β for its degradation.In B-cells, β-catenin forms unique complexes with lymphoid-specific Ikaros factors and is required for Ikaros-mediated tumor suppression and assembly of repressive NuRD complexes. CRISPR-based knockin mutation of a single Ikaros-binding motif in a lymphoid MYC superenhancer region reversed β-catenin-dependent Myc repression and induction of cell death. The discovery of GSK3β-dependent degradation of β-catenin as unique B-lymphoid vulnerability provides a rationale to repurpose clinically approved GSK3β-inhibitors for the treatment of refractory B-cell malignancies.

GRAPHICAL ABSTRACT

Abundant nuclear β-cateninβ-catenin pairs with TCF7 factors for transcriptional activation of MYCB-cells rely on efficient degradation of β-catenin by GSK3βB-cell-specific expression of Ikaros factors Unique vulnerability in B-cell tumors: GSK3β-inhibitors induce nuclear accumulation of β-catenin.β-catenin pairs with B-cell-specific Ikaros factors for transcriptional repression of MYC.

SYK and ZAP70 kinases in autoimmunity and lymphoid malignancies

SYK and ZAP70 nonreceptor tyrosine kinases serve essential roles in initiating B-cell receptor (BCR) and T-cell receptor (TCR) signaling in B- and T-lymphocytes, respectively. Despite their structural and functional similarity, expression of SYK and ZAP70 is strictly separated during B- and T-lymphocyte development, the reason for which was not known. Aberrant co-expression of ZAP70 with SYK was first identified in B-cell chronic lymphocytic leukemia (CLL) and is considered a biomarker of aggressive disease and poor clinical outcomes. We recently found that aberrant ZAP70 co-expression not only functions as an oncogenic driver in CLL but also in various other B-cell malignancies, including acute lymphoblastic leukemia (B-ALL) and mantle cell lymphoma. Thereby, aberrantly expressed ZAP70 redirects SYK and BCR-downstream signaling from NFAT towards activation of the PI3K-pathway. In the sole presence of SYK, pathological BCR-signaling in autoreactive or premalignant cells induces NFAT-activation and NFAT-dependent anergy and negative selection. In contrast, negative selection of pathological B-cells is subverted when ZAP70 diverts SYK from activation of NFAT towards tonic PI3K-signaling, which promotes survival instead of cell death. We discuss here how both B-cell malignancies and autoimmune diseases frequently evolve to harness this mechanism, highlighting the importance of developmental separation of the two kinases as an essential safeguard.

Deciphering intratumoral heterogeneity using integrated clonal tracking and single-cell transcriptome analyses

Cellular heterogeneity is a major cause of treatment resistance in cancer. Despite recent advances in single-cell genomic and transcriptomic sequencing, it remains difficult to relate measured molecular profiles to the cellular activities underlying cancer. Here, we present an integrated experimental system that connects single cell gene expression to heterogeneous cancer cell growth, metastasis, and treatment response. Our system integrates single cell transcriptome profiling with DNA barcode based clonal tracking in patient-derived xenograft models. We show that leukemia cells exhibiting unique gene expression respond to different chemotherapies in distinct but consistent manners across multiple mice. In addition, we uncover a form of leukemia expansion that is spatially confined to the bone marrow of single anatomical sites and driven by cells with distinct gene expression. Our integrated experimental system can interrogate the molecular and cellular basis of the intratumoral heterogeneity underlying disease progression and treatment resistance.

DNA barcoding is a promising technology for the simultaneous analysis of genetic and phenotypic heterogeneity. Here, the authors combine DNA barcoding and single-cell RNA-seq to study heterogeneity, progression and response to therapy in B-cell acute lymphoblastic leukaemia patient-derived xenografts.

R-2-hydroxyglutarate attenuates aerobic glycolysis in leukemia by targeting the FTO/m6A/PFKP/LDHB axis

R-2-hydroxyglutarate (R-2HG), a metabolite produced by mutant isocitrate dehydrogenases (IDHs), was recently reported to exhibit anti-tumor activity. However, its effect on cancer metabolism remains largely elusive. Here we show that R-2HG effectively attenuates aerobic glycolysis, a hallmark of cancer metabolism, in (R-2HG-sensitive) leukemia cells. Mechanistically, R-2HG abrogates fat-mass- and obesity-associated protein (FTO)/N6-methyladenosine (m6A)/YTH N6-methyladenosine RNA binding protein 2 (YTHDF2)-mediated post-transcriptional upregulation of phosphofructokinase platelet (PFKP) and lactate dehydrogenase B (LDHB) (two critical glycolytic genes) expression and thereby suppresses aerobic glycolysis. Knockdown of FTO, PFKP, or LDHB recapitulates R-2HG-induced glycolytic inhibition in (R-2HG-sensitive) leukemia cells, but not in normal CD34+ hematopoietic stem/progenitor cells, and inhibits leukemogenesis in vivo; conversely, their overexpression reverses R-2HG-induced effects. R-2HG also suppresses glycolysis and downregulates FTO/PFKP/LDHB expression in human primary IDH-wild-type acute myeloid leukemia (AML) cells, demonstrating the clinical relevance. Collectively, our study reveals previously unrecognized effects of R-2HG and RNA modification on aerobic glycolysis in leukemia, highlighting the therapeutic potential of targeting cancer epitranscriptomics and metabolism.

Metabolic Gatekeepers of Pathological B Cell Activation

Unlike other cell types, B cells undergo multiple rounds of V(D)J recombination and hypermutation to evolve high-affinity antibodies. Reflecting high frequencies of DNA double-strand breaks, adaptive immune protection by B cells comes with an increased risk of malignant transformation. In addition, the vast majority of newly generated B cells express an autoreactive B cell receptor (BCR). Thus, B cells are under intense selective pressure to remove autoreactive and premalignant clones. Despite stringent negative selection, B cells frequently give rise to autoimmune disease and B cell malignancies. In this review, we discuss mechanisms that we term metabolic gatekeepers to eliminate pathogenic B cell clones on the basis of energy depletion. Chronic activation signals from autoreactive BCRs or transforming oncogenes increase energy demands in autoreactive and premalignant B cells. Thus, metabolic gatekeepers limit energy supply to levels that are insufficient to fuel either a transforming oncogene or hyperactive signaling from an autoreactive BCR.

IFITM3 functions as a PIP3 scaffold to amplify PI3K signalling in B cells

Interferon-induced transmembrane protein 3 (IFITM3) has previously been identified as an endosomal protein that blocks viral infection^1-3. Here we studied clinical cohorts of patients with B cell leukaemia and lymphoma, and identified IFITM3 as a strong predictor of poor outcome. In normal resting B cells, IFITM3 was minimally expressed and mainly localized in endosomes. However, engagement of the B cell receptor (BCR) induced both expression of IFITM3 and phosphorylation of this protein at Tyr20, which resulted in the accumulation of IFITM3 at the cell surface. In B cell leukaemia, oncogenic kinases phosphorylate IFITM3 at Tyr20, which causes constitutive localization of this protein at the plasma membrane. In a mouse model, Ifitm3^-/- naive B cells developed in normal numbers; however, the formation of germinal centres and the production of antigen-specific antibodies were compromised. Oncogenes that induce the development of leukaemia and lymphoma did not transform Ifitm3^-/- B cells. Conversely, the phosphomimetic IFITM3(Y20E) mutant induced oncogenic PI3K signalling and initiated the transformation of premalignant B cells. Mechanistic experiments revealed that IFITM3 functions as a PIP3 scaffold and central amplifier of PI3K signalling. The amplification of PI3K signals depends on IFITM3 using two lysine residues (Lys83 and Lys104) in its conserved intracellular loop as a scaffold for the accumulation of PIP3. In Ifitm3^-/- B cells, lipid rafts were depleted of PIP3, which resulted in the defective expression of over 60 lipid-raft-associated surface receptors, and impaired BCR signalling and cellular adhesion. We conclude that the phosphorylation of IFITM3 that occurs after B cells encounter antigen induces a dynamic switch from antiviral effector functions in endosomes to a PI3K amplification loop at the cell surface. IFITM3-dependent amplification of PI3K signalling, which in part acts downstream of the BCR, is critical for the rapid expansion of B cells with high affinity to antigen. In addition, multiple oncogenes depend on IFITM3 to assemble PIP3-dependent signalling complexes and amplify PI3K signalling for malignant transformation.

Rationale for targeting BCL6 in MLL-rearranged acute lymphoblastic leukemia

Chromosomal rearrangements of the mixed lineage leukemia (MLL) gene occur in ∼10% of B-cell acute lymphoblastic leukemia (B-ALL) and define a group of patients with dismal outcomes. Immunohistochemical staining of bone marrow biopsies from most of these patients revealed aberrant expression of BCL6, a transcription factor that promotes oncogenic B-cell transformation and drug resistance in B-ALL. Our genetic and ChIP-seq (chromatin immunoprecipitation [ChIP] combined with high-throughput sequencing) analyses showed that MLL-AF4 and MLL-ENL fusions directly bound to the BCL6 promoter and up-regulated BCL6 expression. While oncogenic MLL fusions strongly induced aberrant BCL6 expression in B-ALL cells, germline MLL was required to up-regulate Bcl6 in response to physiological stimuli during normal B-cell development. Inducible expression of Bcl6 increased MLL mRNA levels, which was reversed by genetic deletion and pharmacological inhibition of Bcl6, suggesting a positive feedback loop between MLL and BCL6. Highlighting the central role of BCL6 in MLL-rearranged B-ALL, conditional deletion and pharmacological inhibition of BCL6 compromised leukemogenesis in transplant recipient mice and restored sensitivity to vincristine chemotherapy in MLL-rearranged B-ALL patient samples. Oncogenic MLL fusions strongly induced transcriptional activation of the proapoptotic BH3-only molecule BIM, while BCL6 was required to curb MLL-induced expression of BIM. Notably, peptide (RI-BPI) and small molecule (FX1) BCL6 inhibitors derepressed BIM and synergized with the BH3-mimetic ABT-199 in eradicating MLL-rearranged B-ALL cells. These findings uncover MLL-dependent transcriptional activation of BCL6 as a previously unrecognized requirement of malignant transformation by oncogenic MLL fusions and identified BCL6 as a novel target for the treatment of MLL-rearranged B-ALL.

Abstract 5469: RAS and STAT5 pathway lesions are mutually exclusive in B-cell malignancies through mechanisms of biochemical cross-inhibition

Activation of STAT5- and RAS-signaling are segregated to early and later stages of normal B cell development, respectively. Studying B-lineage acute lymphoblastic leukemia (ALL; n=578), we found that STAT5 (e.g. BCR-ABL1, JAK2, cytokine receptors) and RAS (NRAS, KRAS, PTPN11, NF1) lesions were mutually exclusive, with only 9 cases (1.6%) carrying lesions in both pathways. Reverse phase protein array measurements revealed that phosphorylation of MEK and ERK1/2 were inversely correlated with STAT5-phosphorylation (MDACC, 1983-2007; P<0.001). These findings prompted us to study mechanisms of cross-inhibition between RAS and STAT5 pathways. Inducible NRASG12D activated ERK at the expense of STAT5 phosphorylation. This was due to stabilization and increased activation of the STAT5-phosphatase PTPN6 (SHP1). Inducible ablation of Ptpn6 elevated phospho-STAT5 levels, while genetic inactivation of Stat5 strongly increased ERK activity. Constitutively active STAT5 suppressed phosphorylation of ERK. Interestingly, STAT5 negatively regulated BCL6, which marks the transition from cytokine receptor-dependent pro-B cells (Stat5+) to pre-BCR dependent stages of development (ERK+). While oncogenic RAS suppressed STAT5, we also found that induction of RAS induced BCL6 expression at both the mRNA and protein levels. Increases in BCL6 expression in response to NRASG12D were abrogated upon treatment with a MEK kinase inhibitor or activation of STAT5. Studying a matched patient-derived pre-B ALL sample at diagnosis and at relapse (acquired KRASG12V mutation) revealed activation of ERK in association with increased BCL6 and decreased STAT5 levels in the KRASG12V relapsed ALL sample. With engagement of BCL6 and the STAT5-inhibitory phosphatase PTPN6 downstream of RAS and ERK signaling, these findings suggest that occupancy of either RAS or STAT5-pathway represents a commitment step that renders cells non-permissive to the respective alternative pathway. To test this hypothesis, we induced B cell transformation with either RAS or STAT5-pathway oncogenes and then subsequently transduced with either empty vectors (EV) or vectors carrying the alternative oncogene. While EVs were easily transduced, RAS- and STAT5-transformed B cells did not tolerate the alternative oncogene. Reflecting early (STAT5) and later (RAS) stages of B cell development, oncogenic activation of these pathways occurs in a mutually exclusive way, owing to biochemical cross-inhibition between STAT5 and RAS.

Citation Format: Lai N. Chan, Seyedmehdi Shojaee, Christian Hurtz, Rebecca Caeser, Gang Xiao, Huimin Geng, Steven Kornblau, Markus Muschen. RAS and STAT5 pathway lesions are mutually exclusive in B-cell malignancies through mechanisms of biochemical cross-inhibition [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 5469.

Abstract 368: B-lymphoid transcriptional repression of the pentose phosphate pathway reveals a unique therapeutic vulnerability of B cell malignancies

Introduction Activation during normal immune responses and oncogenic transformation impose increased metabolic demands on B-cells and their ability to retain redox homeostasis. While the serine/threonine-protein phosphatase 2A (PP2A) was identified as tumor suppressor in multiple types of cancer, our genetic studies revealed an unexpected lineage-specific dependency on PP2A in a broad range of B-cell malignancies.

Results PP2A regulates glycolysis rate and balances energy supply against anti-oxidant protection of B-cells through the pentose-phosphate pathway (PPP). In contrast to robust PPP-activity in other hematopoietic lineages, B-lymphoid transcription factors (PAX5, IKZF1) restrict PPP-activity by transcriptional repression of G6PD and other rate-limiting PPP enzymes. Constitutively low PPP-activity and limited capacity to balance redox fluctuations cause a unique dependency on PP2A in B-cell tumors. Pharmacological ablation of PP2A and PPP-activity identify this pathway as a novel lineage-specific therapeutic target in a broad range of B-cell malignancies.

Highlights • Conditional deletion of Ppp2r1a, the central scaffold assembling the PP2A holoenzyme, induces acute cell death of early and mature B-cells but does not affect other hematopoietic lineages. • PP2A redirects glucose carbon utilization from glycolysis to the PPP to mitigate oxidative stress. • B-cell malignancies exhibit constitutively low PPP activity, owing to B-cell-specific transcriptional repression of G6PD and other rate-limiting PPP enzymes. • Transcriptional repression of PPP activity in B-cells represents the mechanistic basis for the unique dependency of B-cell malignancies on PP2A. • Small molecule inhibitors of PP2A and G6PD act synergistically and overcome conventional drug-resistance in B-cell tumors.

Summary Genetic lesions of PP2A are frequent in solid tumors and myeloid leukemia, but not in B-cell malignancies. Unlike other types of cancer, our genetic studies revealed an essential role of PP2A in B-cell tumors. Thereby, PP2A redirects glucose carbon utilization from glycolysis to PPP to salvage oxidative stress. This unique vulnerability reflects constitutively low PPP activity in B-cells and transcriptional repression of G6PD and other key PPP enzymes by the B-cell transcription factors PAX5 and IKZF1. Reflecting B-cell-specific transcriptional repression of PPP activity, glucose carbon utilization in B-cells is heavily skewed in favor of glycolysis resulting in lack of PPP-dependent antioxidant protection. These findings reveal a novel gatekeeper function of the PPP in a broad range of B-cell malignancies that can be efficiently targeted by small molecule inhibition of PP2A and G6PD.

Citation Format: Gang Xiao, Zhengshan Chen, Lai N. Chan, Daniel Braas, Thomas G. Graeber, Huimin Geng, Hassan Jumaa, Xiaoyan Jiang, Markus Müschen. B-lymphoid transcriptional repression of the pentose phosphate pathway reveals a unique therapeutic vulnerability of B cell malignancies [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 368.

B-Cell-Specific Diversion of Glucose Carbon Utilization Reveals a Unique Vulnerability in B Cell Malignancies

B cell activation during normal immune responses and oncogenic transformation impose increased metabolic demands on B cells and their ability to retain redox homeostasis. While the serine/threonine-protein phosphatase 2A (PP2A) was identified as a tumor suppressor in multiple types of cancer, our genetic studies revealed an essential role of PP2A in B cell tumors. Thereby, PP2A redirects glucose carbon utilization from glycolysis to the pentose phosphate pathway (PPP) to salvage oxidative stress. This unique vulnerability reflects constitutively low PPP activity in B cells and transcriptional repression of G6PD and other key PPP enzymes by the B cell transcription factors PAX5 and IKZF1. Reflecting B-cell-specific transcriptional PPP-repression, glucose carbon utilization in B cells is heavily skewed in favor of glycolysis resulting in lack of PPP-dependent antioxidant protection. These findings reveal a gatekeeper function of the PPP in a broad range of B cell malignancies that can be efficiently targeted by small molecule inhibition of PP2A and G6PD.

Loss of Pax5 Exploits Sca1-BCR-ABLp190 Susceptibility to Confer the Metabolic Shift Essential for pB-ALL

Preleukemic clones carrying BCR-ABL^p190 oncogenic lesions are found in neonatal cord blood, where the majority of preleukemic carriers do not convert into precursor B-cell acute lymphoblastic leukemia (pB-ALL). However, the critical question of how these preleukemic cells transform into pB-ALL remains undefined. Here, we model a BCR-ABL^p190 preleukemic state and show that limiting BCR-ABL^p190 expression to hematopoietic stem/progenitor cells (HS/PC) in mice (Sca1-BCR-ABL^p190) causes pB-ALL at low penetrance, which resembles the human disease. pB-ALL blast cells were BCR-ABL-negative and transcriptionally similar to pro-B/pre-B cells, suggesting disease onset upon reduced Pax5 functionality. Consistent with this, double Sca1-BCR-ABL^p190+Pax5^+/- mice developed pB-ALL with shorter latencies, 90% incidence, and accumulation of genomic alterations in the remaining wild-type Pax5 allele. Mechanistically, the Pax5-deficient leukemic pro-B cells exhibited a metabolic switch toward increased glucose utilization and energy metabolism. Transcriptome analysis revealed that metabolic genes (IDH1, G6PC3, GAPDH, PGK1, MYC, ENO1, ACO1) were upregulated in Pax5-deficient leukemic cells, and a similar metabolic signature could be observed in human leukemia. Our studies unveil the first in vivo evidence that the combination between Sca1-BCR-ABL^p190 and metabolic reprogramming imposed by reduced Pax5 expression is sufficient for pB-ALL development. These findings might help to prevent conversion of BCR-ABL^p190 preleukemic cells.Significance: Loss of Pax5 drives metabolic reprogramming, which together with Sca1-restricted BCR-ABL expression enables leukemic transformation. Cancer Res; 78(10); 2669-79. ©2018 AACR.

Abstract 93: Transcriptional control of glucocorticoid responses in leukemia

Glucocorticoids (GCs) are central to all major therapy regimens for pre-B cell-derived acute lymphoblastic leukemia (ALL), but have no activity in myeloid leukemia. Such divergent responses represent an empirically established clinical standard; however, neither the mechanism by which GCs induce cell death nor the biological basis for the distinct responses in B-cell and myeloid leukemias is clear. Studying patient-derived samples revealed that NR3C1 (glucocorticoid receptor) levels were 6- to 20-fold higher in pre-B ALL compared to chronic myeloid leukemia (CML). High levels of Nr3c1 were reduced upon B- to myeloid-lineage conversion, suggesting that regulation of NR3C1 expression and GC responsiveness depend on a B-cell transcriptional program. B-cell transcription factors (e.g. PAX5, IKZF1) are critical for B-cell development, yet they are genetically lesioned in more than 80% of pre-B ALL cases. Despite such high frequency, the significance of these inactivating lesions remains elusive. Combining ChIP-seq and RNA-seq analyses, we identified a novel B-cell transcriptional program for activation of NR3C1 and its transcriptional target TXNIP (a negative regulator of glucose uptake). Reconstitution of PAX5 or IKZF1 expression in haploinsufficient patient-derived pre-B ALL cells increased NR3C1 and TXNIP levels. Conversely, expression of dominant negative mutant of PAX5 or IKZF1 abolished NR3C1 expression. Loss of Nr3c1 or Txnip in murine BCR-ABL1-driven pre-B ALL cells resulted in survival advantage in competitive growth assays. Importantly, loss of Nr3c1 or Txnip significantly elevated glucose uptake, lactate production and cellular ATP levels. These findings suggest that GCs induce cell death by exacerbating glucose and energy depletion. Notably, reconstitution of PAX5 or IKZF1 rendered haploinsufficient patient-derived pre-B ALL cells more sensitive to dexamethasone (dex) treatment. In contrast, dominant-negative PAX5 or IKZF1 largely de-sensitized pre-B ALL cells expressing wildtype PAX5 or IKZF1. These findings suggest that B-cell transcription factors set the threshold for GC responsiveness in pre-B ALL. Since relapsed ALL cells often acquire GC resistance, drug-combinations may be useful to prevent GC-resistance. As expected, loss of Nr3c1 abrogated responses to GCs. Interestingly, loss of Txnip also largely rescued GC-induced cell death in pre-B ALL cells. On this basis, we tested drug interactions between GCs and TXNIP agonists, 3-O-methylglucose (3-OMG) and D-allose. Treating patient-derived GC-refractory pre-B ALL cells with 3-OMG or D-allose strongly synergized with GC-treatment. Collectively, our findings provide a mechanistic explanation for the empiric finding that GCs are effective in the treatment of B-cell but not myeloid malignancies, and identify TXNIP as a novel therapeutic target in pre-B ALL.

Note: This abstract was not presented at the meeting.

Citation Format: Lai N. Chan, Zhengshan Chen, Gang Xiao, Jae Woong Lee, Kadriye Nehir Cosgun, Huimin Geng, Valeria Cazzaniga, Hilde Schjerven, Ross A. Dickins, Markus Muschen. Transcriptional control of glucocorticoid responses in leukemia [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 93. doi:10.1158/1538-7445.AM2017-93

B-cell identity as a metabolic barrier against malignant transformation

B-lineage and myeloid leukemia cells are often transformed by the same oncogenes, but have different biological and clinical characteristics. Although B-lineage acute lymphoblastic leukemia (B-ALL) cells are characterized by a state of chronic energy deficit, myeloid leukemia cells show abundant energy reserve. Interestingly, fasting has been demonstrated to inhibit selectively the development of B-ALL but not myeloid leukemia, further suggesting that lineage identity may be linked to divergent metabolic states in hematopoietic malignancies. The B-lymphoid transcription factors IKZF1, EBF1, and PAX5 are essential for early B-cell development and commitment to B-cell identity. However, in >80% of human pre-B-ALL cases, the leukemic clones harbor genetic lesions of these transcription factors. The significance of these defects has only recently been investigated. Here, we discuss the unexpected function of a B-lymphoid transcriptional program as a metabolic barrier against malignant transformation of B-cell precursor cells. The metabolic gatekeeper function of B-lymphoid transcription factors may force silent preleukemic clones carrying potentially oncogenic lesions to remain in a latent state. In addition, this program sets the threshold for responses to glucocorticoids in pre-B-ALL. Finally, the link between the tumor-suppressor and metabolic functions of B-lymphoid transcription factors is matched by observations in clinical trials: obesity and hyperglycemia are associated with poor clinical outcome in patients with pre-B-ALL.

Abstract 1124: Transcriptional control of B cell identity restricts metabolic fitness in human leukemia

Oncogenic lesions in multi-potent progenitor cells often give rise to either B-cell or myeloid lineage leukemia. While transformed by the same oncogenes (e.g. BCR-ABL1, RAS), B-lineage and myeloid leukemias are distinct diseases. Given that oncogenic tyrosine kinase signaling (e.g. BCR-ABL1) imposes significant metabolic requirements on energy supply, biogenesis and metabolic fitness, we studied whether the divergent characteristics of myeloid and B-lineage leukemias have a metabolic basis.

Metabolic analyses revealed that B-lineage acute lymphoblastic leukemia (Ph+ ALL) cells proliferate at maximum capacity of their glycolytic machinery. In contrast to myeloid leukemia (CML), B-lineage ALL cells lack metabolic adaptive fitness in response to metabolic fluctuations. C/EBPα-mediated reprogramming of B-lineage cells into the myeloid lineage induced glycolytic gene expression (Insr, Slc2a1, G6pdx, G6pd2, and Hk3). Frequent genetic lesions of transcription factors that determine B cell identity (IKZF1, PAX5, EBF1) partially mitigate B cell-instrinsic metabolic liability. Reconstitution of PAX5 expression in patient-derived B-lineage ALL cells reduced metabolic fitness by impacting glucose metabolism. Using genetic and metabolic experiments, we identified the metabolic liability observed in B-lineage ALL is in part dependent on the serine/threonine kinase LKB1.

In agreement with previous studies, Cre-mediated deletion of Lkb1 induced proliferation in myeloid leukemia. Surprisingly, Lkb1 deletion led to apoptosis and decreased leukemogenic capacity in B-lineage leukemia. Consistent with the above observations, Arf, p53 and p27 levels were reduced in Lkb1-deficient myeloid leukemia cells, while Lkb1 deletion in B-lineage ALL cells up-regulated Arf, p53 and p27 levels. Enhanced glucose consumption and lactate production were observed in Lkb1-deficient myeloid leukemia cells. In contrast, loss of Lkb1 led to defective glycolytic and mitochondrial activity in B-lineage ALL. Lkb1 deletion in B-lineage ALL caused global accumulation of metabolites, suggesting that LKB1 is required for maintaining metabolic homeostasis. Moreover, loss of Lkb1 decreased protein levels of mitochondrial, anti-apoptotic BCL-2 family proteins, BCL-xL and MCL1, in B-lineage ALL. Reverse Phase Protein Array analyses revealed that LKB1 levels positively correlated with BCL-xL and MCL1 in patient-derived Ph+ ALL samples (n = 51) as well as other subtypes of B-lineage ALL (n = 183; MDACC, 1983-2007). Importantly, C/EBPα-mediated reprogramming of B-lineage ALL cells to the myeloid linage relieved dependency on LKB1.

Taken together, we showed that transcriptional control of B cell identity causes unique metabolic liability. B-lineage ALL cells exhibit unique reliance on LKB1 for metabolic homeostasis and survival. Our findings revealed LKB1 as a potential therapeutic target in B-lineage ALL.

Note: This abstract was not presented at the meeting.

Citation Format: Lai N. Chan, Daniel Braas, Christian Hurtz, Seyedmehdi Shojaee, Huimin Geng, Valeria Cazzaniga, Carina Ng, Behzad Kharabi Masouleh, Yi Hua Qiu, Nianxiang Zhang, Kevin R. Coombes, Thomas Ernst, Giovanni Cazzaniga, Andreas Hochhaus, Steven Kornblau, Thomas Graeber. Transcriptional control of B cell identity restricts metabolic fitness in human leukemia. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1124. doi:10.1158/1538-7445.AM2015-1124

Erk Negative Feedback Control Enables Pre-B Cell Transformation and Represents a Therapeutic Target in Acute Lymphoblastic Leukemia

Studying mechanisms of malignant transformation of human pre-B cells, we found that acute activation of oncogenes induced immediate cell death in the vast majority of cells. Few surviving pre-B cell clones had acquired permissiveness to oncogenic signaling by strong activation of negative feedback regulation of Erk signaling. Studying negative feedback regulation of Erk in genetic experiments at three different levels, we found that Spry2, Dusp6, and Etv5 were essential for oncogenic transformation in mouse models for pre-B acute lymphoblastic leukemia (ALL). Interestingly, a small molecule inhibitor of DUSP6 selectively induced cell death in patient-derived pre-B ALL cells and overcame conventional mechanisms of drug-resistance.

Self-enforcing feedback activation between BCL6 and pre-B cell receptor signaling defines a distinct subtype of acute lymphoblastic leukemia

Studying 830 pre-B ALL cases from four clinical trials, we found that human ALL can be divided into two fundamentally distinct subtypes based on pre-BCR function. While absent in the majority of ALL cases, tonic pre-BCR signaling was found in 112 cases (13.5%). In these cases, tonic pre-BCR signaling induced activation of BCL6, which in turn increased pre-BCR signaling output at the transcriptional level. Interestingly, inhibition of pre-BCR-related tyrosine kinases reduced constitutive BCL6 expression and selectively killed patient-derived pre-BCR(+) ALL cells. These findings identify a genetically and phenotypically distinct subset of human ALL that critically depends on tonic pre-BCR signaling. In vivo treatment studies suggested that pre-BCR tyrosine kinase inhibitors are useful for the treatment of patients with pre-BCR(+) ALL.

Abstract 2447: Lineage-specific metabolic reprogramming in BCR-ABL1-driven leukemia

Background & Hypothesis: The serine-threonine liver kinase B1 (LKB1) activates AMP-activated protein kinase (AMPK) and negatively regulates aerobic glycoloysis (Warburg effect). LKB1 and AMPK have long been established as tumor suppressors, leading to clinical trials that test the efficacy of AMPK activators as cancer therapeutics. Paradoxically, we found that high expression levels of LKB1 and subunits of AMPK at diagnosis correlate with poor clinical outcome in patients with high risk B precursor acute lymphoblastic leukemia (ALL) (n = 207). These findings seem to contradict the historical notion of LKB1-AMPK as a tumor suppressor pathway, suggesting that the functions of LKB1-AMPK pathway may depend on cellular and genetic contexts.

Results: Here, we focus on the role of LKB1 in BCR-ABL1-driven leukemia - chronic myeloid leukemia (CML) and B cell lineage Ph+ ALL. To do so, genetic mouse models for 4-hydroxytamoxifen (4-OHT)-inducible deletion of Lkb1 in BCR-ABL1-transformed hematopoietic stem and progenitor cells (CML-like) and B cell progenitors (Ph+ ALL) were developed. In agreement with previous findings in solid tumors, Cre-mediated Lkb1 deletion in CML-like cells resulted in enhanced proliferation. Unexpectedly, deletion of Lkb1 in Ph+ ALL cells led to apoptosis and cell cycle arrest. Moreover, Lkb1deletion delayed the onset of Ph+ ALL development as well as prolonged overall survival of transplant recipient mice in vivo. Consistent with the above observations, Arf, p53 and p27 levels were reduced in Lkb1-deficient CML cells, while Lkb1 deletion in Ph+ ALL cells up-regulated Arf, p53 and p27 levels. Decreases in glucose consumption and lactate production were also observed in Lkb1-deificient Ph+ ALL cells; however, increases in the levels of glucose consumed and lactate produced were detected in CML cells following Lkb1 deletion.

Importantly, inhibition of AMPK using Compound C (an ATP-competitive inhibitor) resulted in apoptosis in patient-derived Ph+ ALL cells, while Compound C had no significant effects on the viability of a panel of lymphoma and multiple myeloma cell lines tested. Furthermore, patient-derived Ph+ ALL cells were resistant to treatment with various AMPK activators (metformin, phenformin and AICAR). Finally, Compound C showed synergistic responses in combination with Imatinib and different PI3K/AKT inhibitors (BKM120, AZD5363 and GSK690693) in Ph+ ALL. In vivo, Compound C in combination with BKM120, a PI3K inhibitor, exerted significantly more potent inhibitory effect on leukemia progression than each agent alone, prolonging the overall survival of recipient mice.

Conclusions: Taken together, our findings demonstrate that LKB1 plays divergent roles in myeloid lineage CML and B cell lineage Ph+ ALL. While AMPK activators were shown to be effective against CML cells in previous studies, inhibiting the LKB1-AMPK pathway may provide a better therapeutic avenue for treatment of Ph+ ALL.

Citation Format: Lai N. Chan, Seyedmehdi Shojaee, Christian Hurtz, Huimin Geng, Carina Ng, Behzad Kharabi, Markus Müschen. Lineage-specific metabolic reprogramming in BCR-ABL1-driven leukemia. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2447. doi:10.1158/1538-7445.AM2014-2447

Mechanistic rationale for targeting the unfolded protein response in pre-B acute lymphoblastic leukemia

The unfolded protein response (UPR) pathway, a stress-induced signaling cascade emanating from the endoplasmic reticulum (ER), regulates the expression and activity of molecules including BiP (HSPA5), IRE1 (ERN1), Blimp-1 (PRDM1), and X-box binding protein 1 (XBP1). These molecules are required for terminal differentiation of B cells into plasma cells and expressed at high levels in plasma cell-derived multiple myeloma. Although these molecules have no known role at early stages of B-cell development, here we show that their expression transiently peaks at the pre-B-cell receptor checkpoint. Inducible, Cre-mediated deletion of Hspa5, Prdm1, and Xbp1 consistently induces cellular stress and cell death in normal pre-B cells and in pre-B-cell acute lymphoblastic leukemia (ALL) driven by BCR-ABL1- and NRAS(G12D) oncogenes. Mechanistically, expression and activity of the UPR downstream effector XBP1 is regulated positively by STAT5 and negatively by the B-cell-specific transcriptional repressors BACH2 and BCL6. In two clinical trials for children and adults with ALL, high XBP1 mRNA levels at the time of diagnosis predicted poor outcome. A small molecule inhibitor of ERN1-mediated XBP1 activation induced selective cell death of patient-derived pre-B ALL cells in vitro and significantly prolonged survival of transplant recipient mice in vivo. Collectively, these studies reveal that pre-B ALL cells are uniquely vulnerable to ER stress and identify the UPR pathway and its downstream effector XBP1 as novel therapeutic targets to overcome drug resistance in pre-B ALL.

In vitro and in vivo effects of geranylgeranyltransferase I inhibitor P61A6 on non-small cell lung cancer cells

Background

Lung cancer is the leading cause of cancer-related mortality. Therapies against non-small cell lung cancer (NSCLC) are particularly needed, as this type of cancer is relatively insensitive to chemotherapy and radiation therapy. We recently identified GGTI compounds that are designed to block geranylgeranylation and membrane association of signaling proteins including the Rho family G-proteins. One of the GGTIs is P61A6 which inhibits proliferation of human cancer cells, causes cell cycle effects with G1 accumulation and exhibits tumor-suppressing effects with human pancreatic cancer xenografts. In this paper, we investigated effects of P61A6 on non-small cell lung cancer (NSCLC) cells in vitro and in vivo.

Methods

Three non-small cell lung cancer cell lines were used to test the ability of P61A6 to inhibit cell proliferation. Further characterization involved analyses of geranylgeranylation, membrane association and activation of RhoA, and anchorage-dependent and –independent growth, as well as cell cycle effects and examination of cell cycle regulators. We also generated stable cells expressing RhoA-F, which bypasses the geranylgeranylation requirement of wild type RhoA, and examined whether the proliferation inhibition by P61A6 is suppressed in these cells. Tumor xenografts of NSCLC cells growing in nude mice were also used to test P61A6’s tumor-suppressing ability.

Results

P61A6 was shown to inhibit proliferation of NSCLC lines H358, H23 and H1507. Detailed analysis of P61A6 effects on H358 cells showed that P61A6 inhibited geranylgeranylation, membrane association of RhoA and caused G1 accumulation associated with decreased cyclin D1/2. The effects of P61A6 to inhibit proliferation could mainly be ascribed to RhoA, as expression of the RhoA-F geranylgeranylation bypass mutant rendered the cells resistant to inhibition by P61A6. We also found that P61A6 treatment of H358 tumor xenografts growing in nude mice reduced their growth as well as the membrane association of RhoA in the tumors.

Conclusion

Thus, P61A6 inhibits proliferation of NSCLC cells and causes G1 accumulation associated with decreased cyclin D1/2. The result with the RhoA-F mutant suggests that the effect of P61A6 to inhibit proliferation is mainly through the inhibition of RhoA. P61A6 also shows efficacy to inhibit growth of xenograft tumor.

Abstract 2334: Targeting inhibitory phosphatase signaling in Ph+ ALL

Background: Current therapy approaches for tyrosine kinase-driven leukemias including Ph+ ALL and CML are almost entirely focused on the development of more potent tyrosine kinase inhibitors (TKI) with the goal to reduce oncogenic signaling below a minimum threshold that is required for the survival of leukemia cells. Studying regulators of BCR-ABL1 kinase signaling strength, we found that three key inhibitory phosphatases (INPP5D, PTEN and PTPN6) are expressed at high levels in Ph+ ALL cells. Both INPP5D and PTEN negatively regulate PI3K/AKT pathway. Like INPP5D, PTPN6 is recruited to ITIM motifs in the cytoplasmic tails of inhibitory surface recepotors and negatively regulates activation signals from tyrosine kinases and activating receptors.

Genetic models: We hypothesized that Cre-mediated deletion of INPP5D, PTEN and PTPN6 will result in increased oncogenic signaling downstream of BCR-ABL1 and a more aggressive form of leukemia. Surprisingly, however, genetic deletion of either INPP5D, PTEN or PTPN6 resulted in drastic upregulation of reactive oxygen species (ROS), accumulation of Arf, p53 and p21, cellular senescence and subsequent cell death of leukemia cells. Inducible deletion of any of these genes profoundly affected B cell lineage leukemia, deletion of these phosphatases and had no effects in myeloid leukemias. Studying BCR-ABL1-transformed Inpp5dfl/fl,Ptenfl/fl and Ptpn6fl/fl leukemia cells in vivo, we observed that induction of Cre-mediated deletion resulted in rapid leukemia regression and prolonged survival of leukemia transplant recipient mice.

Pharmacological targeting: Targeted blockade of inhibitory phosphatases for the treatment of Ph+ ALL seems counter intuitive because it represents effectively the opposite of current TKI-based therapies. Small molecule inhibition of Inpp5d using the compound 3AC effectively killed patient-derived Ph+ leukemia cells. 3AC inhibition of Inpp5d dramatically increased levels of ROS and increased phosphorylation of the stress-associated MAP kinases p38α and JNK and substantially prolonged overall survival of NOD/SCID transplant recipients of patient-derived Ph+ ALL cells. Based on these findings, we propose that pharmacological blockade of inhibitory phosphatases represents a powerful means to induce leukemia cell death owing to excessive oncogene signaling.

Conclusions: According to our concept, tyrosine kinase-driven leukemia cells can only thrive within a certain "comfort zone" of signal strength. Both attenuation below and exaggeration above this "comfort zone" of signal strength result in cell death. If validated, our approach of phosphatase-inhibition will lead to the discovery and development of multiple new targets for therapy and will significantly broaden currently available treatment options for B cell lineage Ph+ ALL and other acute leukemias carrying oncogenic tyrosine kinases.

Citation Format: Seyedmehdi Shojaee, Maike Buchner, Sigal Gery, Huimin Geng, Lai N. Chan, Ari Melnick, Phillip H. Koeffler, Markus Müschen. Targeting inhibitory phosphatase signaling in Ph+ ALL. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2334. doi:10.1158/1538-7445.AM2013-2334

Identification and characterization of mechanism of action of P61-E7, a novel phosphine catalysis-based inhibitor of geranylgeranyltransferase-I

Small molecule inhibitors of protein geranylgeranyltransferase-I (GGTase-I) provide a promising type of anticancer drugs. Here, we first report the identification of a novel tetrahydropyridine scaffold compound, P61-E7, and define effects of this compound on pancreatic cancer cells. P61-E7 was identified from a library of allenoate-derived compounds made through phosphine-catalyzed annulation reactions. P61-E7 inhibits protein geranylgeranylation and blocks membrane association of geranylgeranylated proteins. P61-E7 is effective at inhibiting both cell proliferation and cell cycle progression, and it induces high p21(CIP1/WAF1) level in human cancer cells. P61-E7 also increases p27(Kip1) protein level and inhibits phosphorylation of p27(Kip1) on Thr187. We also report that P61-E7 treatment of Panc-1 cells causes cell rounding, disrupts actin cytoskeleton organization, abolishes focal adhesion assembly and inhibits anchorage independent growth. Because the cellular effects observed pointed to the involvement of RhoA, a geranylgeranylated small GTPase protein shown to influence a number of cellular processes including actin stress fiber organization, cell adhesion and cell proliferation, we have evaluated the significance of the inhibition of RhoA geranylgeranylation on the cellular effects of inhibitors of GGTase-I (GGTIs). Stable expression of farnesylated RhoA mutant (RhoA-F) results in partial resistance to the anti-proliferative effect of P61-E7 and prevents induction of p21(CIP1/WAF1) and p27(Kip1) by P61-E7 in Panc-1 cells. Moreover, stable expression of RhoA-F rescues Panc-1 cells from cell rounding and inhibition of focal adhesion formation caused by P61-E7. Taken together, these findings suggest that P61-E7 is a promising GGTI compound and that RhoA is an important target of P61-E7 in Panc-1 pancreatic cancer cells.

A novel approach to tag and identify geranylgeranylated proteins

A recently developed proteomic strategy, the "GG-azide"-labeling approach, is described for the detection and proteomic analysis of geranylgeranylated proteins. This approach involves metabolic incorporation of a synthetic azido-geranylgeranyl analog and chemoselective derivatization of azido-geranylgeranyl-modified proteins by the "click" chemistry, using a tetramethylrhodamine-alkyne. The resulting conjugated proteins can be separated by 1-D or 2-D and pH fractionation, and detected by fluorescence imaging. This method is compatible with downstream LC-MS/MS analysis. Proteomic analysis of conjugated proteins by this approach identified several known geranylgeranylated proteins as well as Rap2c, a novel member of the Ras family. Furthermore, prenylation of progerin in mouse embryonic fibroblast cells was examined using this approach, demonstrating that this strategy can be used to study prenylation of specific proteins. The "GG-azide"-labeling approach provides a new tool for the detection and proteomic analysis of geranylgeranylated proteins, and it can readily be extended to other post-translational modifications.

Magnesium as NMDA receptor blocker in the traditional Chinese medicine Danshen

Aqueous extracts of the traditional Chinese medicine Danshen, the dried roots of Salvia miltiorrhiza Bunge (Labiatae), blocked N-methyl-D-aspartate (NMDA) evoked currents in cerebrocortical neurons in vitro. The block of the NMDA-evoked currents was voltage dependent and showed the negative slope conductance reminiscent of the effect of Mg2+ ions. Atomic absorption spectrophotometry (AAS) revealed that aqueous Danshen extracts contained approximately 9mM magnesium. Fractionation of the extracts by high performance liquid chromatography followed by patch clamp recording and AAS indicated that magnesium ions were present in two distinct fractions. One fraction contained approximately 5 mM magnesium and blocked NMDA-induced currents indicating that it contained mostly free Mg2+ ions, while a second fraction did not possess NMDA antagonist activity despite the presence of approximately 4 mM magnesium suggesting that Mg2+ in this fraction was mostly chelated. Following removal of the free Mg2+ by ion exchange chromatography, the previously observed block of the NMDA-induced currents was abolished. These data demonstrate that Danshen contains both free and chelated Mg2+. Free Mg2+ ions account for the NMDA antagonist activity of Danshen in vitro.

Inhibition of amiloride-sensitive Na(+) absorption by activation of CFTR in mouse endometrial epithelium

Previous studies have demonstrated amiloride-sensitive Na(+) absorption under basal conditions and cystic fibrosis transmembrane conductance regulator (CFTR)-mediated Cl(-) secretion following neurohormonal stimulation in the mouse endometrial epithelium. The present study investigated the inhibition of amiloride-sensitive Na(+) absorption accompanying activation of CFTR in the mouse endometrium using the short-circuit current ( I(sc)) technique. RT-PCR demonstrated the co-expression of CFTR and epithelial Na(+) channels (ENaC) in primary cultured mouse endometrial epithelia and cultured endometrial monolayers exhibited a basal amiloride-sensitive I(sc) of 5.4 +/- 0.6 microA/cm(2). The amiloride-sensitive current fell to 3.1 +/- 0.5 microA/cm(2) after stimulation with forskolin. When the possible contribution of Na(+) absorption to the I(sc) was eliminated by amiloride (1 microM) or Na(+) replacement, the forskolin-induced I(sc) was not reduced, but rather increased significantly compared with that in the absence of amiloride or in Na(+)-containing solutions ( P < 0.02), indicating that the forskolin-induced I(sc) was mediated by Cl(-) secretion, portion of which may be masked by concurrent inhibition of basal Na(+) absorption if the contribution of Na(+) is not eliminated. When the contribution of Cl(-) to the I(sc) was eliminated by diphenylamine 2,2'-dicarboxylic acid (DPC, 2 mM) or Cl(-) replacement, forskolin now decreased, rather than increased the I(sc), demonstrating the inhibition of Na(+) absorption upon stimulation. Our data suggest an interaction between CFTR and ENaC, which may be the underlying mechanism for balancing Na(+) absorption and Cl(-) secretion across the mouse endometrial epithelium.

Cellular signaling mechanisms underlying pharmacological action of Bak Foong Pills on gastrointestinal secretion

Bak Foong Pills (BFP, also known as Bai Feng Wan) is an over-the-counter traditional Chinese medicine that has long been used for treating gynecological disorders and improving overall body functions, including gastrointestinal (GI) function. However, the cellular signaling mechanism underlying BFP action, especially on the GI tract, has not been elucidated. In the present study, the human colonic epithelia cell line T(84) was used as a model to investigate the effect of BFP ethanol extract on ion transport in conjunction with the short-circuit current (I(SC)) technique. The results showed that the apical addition of BFP extract produced a concentration-dependent (10-1,000 microg/ml, EC(50) = 120 microg/ml) increase in I(SC). The maximal response was observed at 500 microg/ml with an increase in I(SC) of 24.4 +/- 2.3 microA/cm(2) and apical conductance. The BFP-induced I(SC) was not observed when extracellular Cl(-) was replaced or when treated with Bumetanide (100 microM), an inhibitor of the Na(+)-K(+)-2Cl(-) cotransporter. The BFP-induced I(SC) was insensitive to the Na(+) channel blocker, amiloride, but partially inhibited by the Cl(-) channel blocker, DIDS (100 microM), and completely blocked by DPC (2 mM) or glibenclamide (1 mM) with a significant reduction in the apical conductance. The BFP-induced I(SC) could be mimicked by forskolin (10 microM), but inhibited by a pretreatment of the cells with adenylate cyclase inhibitor, MDL-12330A (10 microM). Pretreatment with EGTA (5 mM) and thapsigargin (10 microM) decreased the BFP-induced I(SC) by 10%. These results demonstrated that BFP ethanol extract exerted a stimulatory effect on gastrointestinal Cl(-) secretion by predominantly activating adenylate cyclase and apical cAMP-dependent Cl(-) channels, with minor contributions from calcium-dependent Cl(-) channels. The effect of BFP may be explored to treat GI disorders such as constipation.

Distribution and regulation of ENaC subunit and CFTR mRNA expression in murine female reproductive tract

The present study investigated the regional distribution and cyclic changes in the mRNA expression of epithelial Na+ channel (ENaC) subunit and cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated Cl- channel, in adult female mouse reproductive tract. In situ hybridization revealed that in contrast to the abundant expression of CFTR, ENaC (alpha, beta, gamma) mRNA signal was not detected throughout the estrus cycle in the ovary and oviduct. Messenger RNA for all ENaC subunits was abundantly detected in the cervical and vaginal epithelia throughout the estrus cycle but for CFTR, mRNA was found only at proestrus. In the uterine epithelium, alphaENaC mRNA was detected at diestrus but not found at any other stage, while CFTR mRNA was only detected at early estrus but not other stages. Semi-quantitative RT-PCR detected mRNA for all ENaC subunits in the uterus throughout the cycle with maximal expression at diestrus and CFTR mRNA was only found in the early stages of the cycle. The involvement of ENaC and CFTR in Na+ absorption and Cl- secretion was demonstrated in cultured endometrial epithelia using the short-circuit current technique and found to be influenced by ovarian hormones. Taken together, these data indicate a main secretory role of the ovary and oviduct and a predominantly absorptive role of the cervix and vagina. The present results also suggest an ability of the uterus to secrete and absorb at different stages of the estrus cycle. Variations in the fluid profiles may be dictated by the regional and cyclic variations in expression of ENaC and CFTR and are likely to contribute to various reproductive events in different regions of the female reproductive tract.

Upregulation of cystic fibrosis transmembrane conductance regulator expression by oestrogen and Bak Foong Pill in mouse uteri

Although cystic fibrosis transmembrane conductance regulator (CFTR) has been shown to be expressed in the female reproductive tract, its functional role in the uterus is not fully understood. The present study investigated a possible physiological role of CFTR by comparing the effects of 17beta-oestradiol and Bak Foong Pill (BFP), an over-the-counter Chinese medicine used for centuries for the treatment of various gynaecological disorders, on uterus size and the expression of CFTR in the uterus of ovariectomised mice using RT-PCR. Treatment of ovariectomised mice with 17beta-oestradiol (0.2 mg/kg, p.o.) for 12 days caused a significant increase in uterine wet weight compared to vehicle. However, treatment with BFP (3 g/kg, p.o.) for the same period failed to increase uterine wet weight, indicating a lack of direct oestrogen-like activity of BFP. Analysis of CFTR mRNA expression in the harvested uteri using RT-PCR showed that both 17beta-oestradiol and BFP induced an increase in CFTR mRNA expression in mouse uteri compared to levels observed in vehicle-treated animals. These results suggest that CFTR can be upregulated by oestrogen and BFP, however, the effect exerted by BFP does not seem to be mediated by direct oestrogen-like activity. Regulation of CFTR expression by both oestrogen and gynaecological medication BFP indicates an important role of CFTR in reproductive functions.

Effect of phenol red and steroid hormones on cystic fibrosis transmembrane conductance regulator in mouse endometrial epithelial cells

Previous studies have demonstrated that cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-mediated Cl(-)channel found in most epithelia including reproductive tract, could be regulated by various culture conditions. The present study further investigated the effect of phenol red, a pH indicator widely used in growth medium, and steroid hormones, present in the supplement fetal bovine serum (FBS), on primary cultured endometrial epithelial cells by monitoring ion channel activities using the short-circuit current technique. When compared to the results obtained with normal medium supplemented with regular FBS, the forskolin-stimulated I(SC), presumably mediated by CFTR, obtained in phenol red-free medium was significantly reduced, from 16.95+/-1.53 microA/cm(2)(control) to 9.72+/-0.89 microA/cm(2)(medium without phenol red, P< 0.05). The forskolin-activated I(SC)was further attenuated to 5.29+/-0.46 microA/cm(2)in the phenol red-free medium when supplemented with charcoal/ dextran-treated FBS where steroid hormones were removed. Our data suggest that phenol red and steroid hormones present in culture medium and FBS supplement, respectively, may somehow upregulate CFTR expression in vitro. Our study demonstrates the need for carefully choosing the culture media and supplements due to the effect of steroid hormones.

Enhanced epithelial Na(+) channel (ENaC) activity in mouse endometrial epithelium by upregulation of gammaENaC subunit

The amiloride-sensitive epithelial Na(+) channel (ENaC), which is made of three different but homologous subunits, controls the rate of transepithelial Na(+) absorption in a variety of epithelia. The present study investigated the functional role of its subunits in regulating ENaC activity, measured as amiloride sensitive short-circuit current (I(SC)), in the mouse endometrial epithelium under different culture conditions. The treatment of the cultured epithelia with aldosterone (1 microM) or culturing cells on filters coated with concentrated Matrigel resulted in an increase in the amiloride-sensitive I(SC). Semiquantitative RT-PCR demonstrated that the expression of alpha and beta subunits was not significantly altered by these treatments, but an increase in the gamma subunit expression was observed. An 11-fold increase, induced by aldosterone, in the expression of the gamma subunit, but not in the alpha and beta subunits, was confirmed by capillary electrophoresis with laser-induced fluorescence (CE-LIF). The treatment of endometrial cells with antisense against the gammaENaC subunit abolished the aldosterone-enhanced amiloride-sensitive I(SC). The results indicated an important role of gammaENaC subunit in determining ENaC activity, and a possible role of the gammaENaC subunit in interacting with CFTR was also discussed.

Drug-nutrient interactions in transplant recipients

Drug-nutrient interaction refers to an alteration of kinetics or dynamics of a drug or a nutritional element, or a compromise in nutritional status as a result of the addition of a drug. The potentials for drug-nutrient interaction increase with the number of drugs taken by the patient. Organ transplant recipients are therefore at high risk for drug-nutrient interactions because multiple medications are used to manage graft rejection, opportunistic infections, and other associated complications. Unrecognized or unmanaged drug-nutrient interactions in this patient population can have an adverse impact on their outcomes. This paper reviews the importance of recognizing drug-nutrient interaction when using cyclosporine-based regimens.

Suppression of CFTR-mediated Cl(-) secretion by enhanced expression of epithelial Na(+) channels in mouse endometrial epithelium

The present study investigated the effect of enhanced expression of epithelial Na(+) channels (ENaC) on the cystic fibrosis transmembrane conductance regulator (CFTR)-mediated Cl(-) secretion in the mouse endometrium using the short-circuit current technique. The amiloride sensitivity of the basal current of the cultured endometrial epithelia was found to vary with the magnitude of the basal current, the higher the basal current the greater its sensitivity to amiloride, indicating possible elevation of ENaC expression. However, the magnitude of the forskolin-induced Isc, previously demonstrated to be mediated by CFTR, decreased as the amiloride sensitivity of the basal current increased, suggesting a possible inhibitory effect of elevated expression of ENaC on CFTR-mediated Cl(-) secretion. The Matrigel concentration for culturing the endometrial epithelia was found to affect the amiloride sensitivity of the basal current as well as the forskolin-induced Isc in opposite directions. However, competitive RT-PCR demonstrated that the expression of both ENaC and CFTR was enhanced in Matrigel-treated culture, suggesting that the reduced forskolin-induced Isc with enhanced amiloride sensitivity was not due to a reduction in CFTR expression, but rather suppression of CFTR function by enhanced ENaC expression. In addition to the previously demonstrated inhibition of ENaC by activation of CFTR, the present results reveal possible regulation of CFTR by ENaC. The interaction between the two may be one of the underlying mechanisms for balancing Na(+) absorption and Cl(-) secretion across epithelia.

Pyrimidinoceptors-mediated activation of Ca(2+)-dependent Cl(-) conductance in mouse endometrial epithelial cells

Previous studies have demonstrated the activation of endometrial Cl(-) secretion through P(2Y2) (P(2U)) purinoceptors by extracellular ATP. The present study further explored the presence of pyrimidine-sensitive receptors in the primary cultured mouse endometrial epithelial cells using the short-circuit current (I(SC)) and whole-cell patch-clamp techniques. UDP induced a transient increase in I(SC) in a concentration-dependent manner (EC(50) approximately 8.84 microM). The UDP-induced I(SC) was abolished after pretreating the epithelia with a calcium chelator, 1, 2-bis-(2-aminophenoxy)-ethane-N,N,N'N'tetraacetic acid-acetomethyl ester (BAPTA-AM), suggesting the dependence of the I(SC) on cytosolic free Ca(2+). The type of receptor involved was studied by cross-desensitization between ATP and UDP. ATP or UDP desensitized its subsequent I(SC) response. However, when ATP was added after UDP, or vice versa, a second I(SC) response was observed, indicating the activation of distinct receptors, possibly pyrimidine-sensitive receptors in addition to P(2Y2) (P(2U)) receptors. Similar results were observed in the patch-clamp experiments where UDP and ATP were shown to sequentially activate whole-cell current in the same cell. The UDP-activated whole-cell current exhibited outward rectification with delay activation and inactivation at depolarizing and hyperpolarizing voltages, respectively. In addition, the UDP-evoked whole-cell current reversed near the equilibrium potential of Cl(-) in the presence of a Cl(-) gradient across the membrane, and was sensitive to 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), indicating the activation of Ca(2+)-activated Cl(-) conductance. These characteristics were very similar to that of the ATP-activated whole-cell current. Taken together, our findings indicate the presence of distinct receptors, pyrimidinoceptors and P(2Y2) (P(2U)) receptors in mouse endometrial epithelial cells. These distinct receptors appear to converge on the same Ca(2+)-dependent Cl(-) channels.

Warfarin-acetaminophen drug interaction revisited

Physicians and pharmacists routinely advise patients receiving warfarin to take acetaminophen for pain or fever because of its relative safety; however, a recent study questioned the safety of such practice. A comprehensive search of MEDLINE and IPA for human studies and case reports from 1966-1999 revealed evidence that acetaminophen may potentiate the effect of warfarin by a mechanism that has yet to be elucidated. Due to lack of a safer alternative, acetaminophen still should be the analgesic and antipyretic of choice in patients taking warfarin, as long as excessive amounts and prolonged administration (> 1.3 g acetaminophen/day for > 2 wks) are avoided. With the high degree of interpatient variability and the unpredictability of various drug-drug interactions with warfarin, close and frequent monitoring of international normalized ratios is the key for safe oral anticoagulation therapy.

Activation of an adenosine 3',5'-cyclic monophosphate-dependent Cl- conductance in response to neurohormonal stimuli in mouse endometrial epithelial cells: the role of cystic fibrosis transmembrane conductance regulator

Previous studies have demonstrated that Cl- secretion by the mouse endometrial epithelium is under neurohormonal influence. The present study characterized the Cl- conductance activated by a number of agonists in the mouse endometrial epithelial cells using the whole-cell voltage-clamp technique. Adrenaline (1 microM), prostaglandin (PG) E2 (5-10 microM), and PGF2alpha (100 microM) activated a whole-cell current that exhibited a linear I-V relationship as well as time- and voltage-independent characteristics. However, the current magnitude varied with different agonists. The agonist-activated current could be mimicked by an adenylate cyclase activator, forskolin (10 microM), and suppressed by an adenylate cyclase inhibitor, MDL12330A, suggesting the involvement of cAMP. Current characteristics remained the same after cation replacement, leaving Cl- as the major permeant ion species in the solutions. The reversal potential of the agonist-induced current was close to the equilibrium potential of Cl- in the presence of a Cl- gradient, indicating the activation of Cl- conductance. The agonist-induced current was inhibited by the Cl- channel blocker diphenylamine 2,2'-dicarboxylic acid (DPC), but not by the Cl- channel blocker 4,4'-diisothiocyanatostibene-2, 2'-disulfonic acid (DIDS). The anion selectivity sequence of the current was NO3->Br->Cl->I-. The observed electrophysiological properties of the agonist-induced Cl- conductance were consistent with those reported for the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated Cl- channel expressed in many epithelia. The expression of CFTR in the mouse endometrial cells was also demonstrated by Western blot analysis. It appears that neurohormonal regulation of the uterine fluid in the mouse endometrium converges on the cAMP-activated Cl- channel, presumably CFTR.

Prolongation of the QT interval related to cisapride-diltiazem interaction

Cisapride, a cytochrome P450 3A4 (CYP3A4) substrate, is widely prescribed for the treatment of gastrointestinal motility disorders. Prolongation of QT interval, torsades de pointes, and sudden cardiac death have been reported after concomitant administration with erythromycin or azole antifungal agents, but not with other CYP3A4 inhibitors. A possible drug interaction occurred in a 45-year-old woman who was taking cisapride for gastroesophageal reflux disorder and diltiazem, an agent that has inhibitory effect on CYP3A4, for hypertension. The patient was in near syncope and had QT-interval prolongation. After discontinuing cisapride, the QT interval returned to normal and symptoms did not recur. We suggest that caution be taken when cisapride is prescribed with any potent inhibitor of CYP3A4, including diltiazem.

N-(4-hydroxyphenyl)retinamide induces apoptosis in T lymphoma and T lymphoblastoid leukemia cells

We demonstrate that N-(4-hydroxyphenyl)-all-trans-retinamide (4-HPR), a synthetic retinoic acid (RA) derivative, is a potent and selective inducer of apoptosis in malignant T lymphoid cells, but has little effect on normal lymphoid cells of the thymus or spleen. 4-HPR and its stereoisomer, 9-cis-4-HPR, are 50 to > 150 times more potent than 7 other retinoids in killing CEM-C7 human T lymphoblastoid leukemia cells and P1798-C7 murine T lymphoma cells. 4-HPR's apoptotic action requires the intact molecule bearing both the retinoid moiety and the hydroxyphenol ring; 4-HPR remains unmetabolized after uptake into CEM-C7 and P1798-C7 cells for up to 24 hours. We also show that glucocorticoid (GC)-resistant variants are equally susceptible to 4-HPR as are GC-sensitive cells. Thus, 4-HPR may be potentially important as a new chemotherapeutic drug for use as alternative to, or in combination with, conventional drugs for treating lymphoid malignancies.

Glucocorticoid/oxysterol-induced DNA lysis in human leukemic cells

Both glucocorticoids and oxysterols, steroids with quite different known transduction pathways, cause the death of lymphoid cells. Dual TUNEL/propidium iodide assays on sensitive human leukemic CEM-C7 clones treated with either steroid were clearly positive by 48 h, consistent with apoptosis. Both steroids evoked two distinctive types of DNA lysis: cleavage into large fragments of several different sizes and the classic "ladders", multiples of approximately 200 base pairs. Conventional gel electrophoresis showed that a small proportion of total DNA had undergone laddering 36-48 h after treatment with glucocorticoid or 24 h after oxysterol exposure. On field inversion gel electrophoresis of cellular DNA both steroids caused an increase in an array of large DNA fragments <50 kb in size. A 50 kb fragment appeared 36 h after treatment with either steroid, but only oxysterol treatment caused a significant increase in a 300 kb fragment. Oxysterol treatment did not result in DNA fragmentation in the resistant M10R5 subclone, which retained sensitivity to glucocorticoids. We conclude that glucocorticoids and oxysterols kill these cells with similar, but not identical, patterns of DNA lysis which occur just before or concomitant with the onset of cell death.

N-(4-hydroxyphenyl)retinamide prevents development of T-lymphomas in AKR/J mice

N-(4-Hydroxyphenyl)retinamide (4-HPR), a synthetic retinoic acid derivative, has chemopreventive effects on several types of cancer. We recently showed that 4-HPR is a potent inducer of apoptosis in malignant, but not normal, T-lymphoid cells in vitro. To test 4-HPR's effect in vivo, we used the virus-induced T-lymphoma in AKR/J mice as a model system. The AKR/J mice were fed 4-HPR at 0, 1 or 2 mmole/kg diet, and the animals were monitored as to tumor development, plasma level of 4-HPR, body weight, appetite, and general health. Our results show that in a 19-week period, 4-HPR prevented T-lymphoma development by 40% and 50% of animals fed 1 and 2 mmole 4-HPR/ kg diet, respectively. In the plasma, 4-HPR reached micromolar levels without causing any observable deleterious side-effects. Thus, 4-HPR is potentially useful in chemoprevention of lymphoid cancers.

Retinoic acid inhibition of serum-induced c-fos transcription in a fibrosarcoma cell line

We investigated the mechanism by which retinoic acid causes growth arrest and flat reversion of SSV-NRK, simian sarcoma virus-transformed normal rat kidney cells. Northern analysis revealed that both chronic (7 days) and acute (6 h) retinoic acid treatment of serum-stimulated SSV-NRK cells caused a 6-fold decrease in c-fos mRNA levels. In addition, nuclear run-on experiments showed that retinoic acid regulated c-fos expression in SSV-NRK cells at the transcriptional initiation level. Attenuation of c-fos transcription was equal in both retinoic acid-treated and control cells, and no increased c-fos mRNA turnover was detected in retinoic acid-treated cells. Furthermore, there was no observed change in the c-fos mRNA levels after only 30 min of retinoic acid treatment, suggesting that a mechanism involving the interruption of the signal transduction mechanism at the membrane level is unlikely. Because it has been shown that c-fos expression plays a pivotal role in mitogenesis of quiescent fibroblasts, we conclude that the retinoic acid-mediated down-regulation of c-fos expression is a mechanism for growth inhibition in SSV-NRK cells.

Transferrin receptor gene is hyperexpressed and transcriptionally regulated in differentiating erythroid cells

We have analyzed the developmental pattern of expression of the chicken transferrin receptor (CTR) gene in various chick embryonic tissues. Northern analyses of RNA from embryonic tissues at different stages of development and cultured chick embryonic fibroblasts (CEFs) show that CTR is hyperexpressed in differentiating erythroid cells such that the steady-state level of CTR mRNA in these cells could be 200 or more times higher than in nonerythroid cells. In vitro nuclear transcription assays using nuclei from embryonic erythroid and brain cells, as well as CEFs, demonstrate that the vast differences in CTR mRNA levels in these cells are reflected in their respective CTR gene transcriptional activities. During development, the steady-state level of CTR mRNA declines in all tissues and, in erythroid cells, this pattern is accompanied by a similar decline in beta-globin mRNA levels. These changes are concurrent with the decreases in CTR and beta-globin mRNA transcriptional activities during erythroid maturation. Taken together, our results indicate that the hyperexpression of the CTR gene in differentiating erythroid cells is regulated to a significant degree at the transcriptional level. We also demonstrate that, in erythroid cells, neither CTR gene transcription nor CTR mRNA stability is regulated by intracellular iron levels.

The cDNA sequence and primary structure of the chicken transferrin receptor

Recombinant cDNA clones encoding the chicken transferrin receptor (cTR) have been isolated and sequenced. Comparison of the deduced primary structure of cTR with those of the human transferrin receptor (hTR) and mouse transferrin receptor (mTR) shows that their size, hydropathy profile, location of sites for posttranslational modifications, and domain organization are highly similar. The cytoplasmic domain of cTR contains the motif Tyr-Xaa-Arg-Phe (YXRF) that is the recognition signal for high-efficiency endocytosis of hTR. The cTR has several highly conserved regions within its extracellular domain, including those flanking the putative N-glycosylation sites. Overall, however, the extracellular domain of cTR is only 53% identical to the extracellular domains of hTR and mTR. The cTR also lacks three of the six Cys residues found in the extracellular domains of the mammalian TRs. These differences can account for functional and structural properties that distinguish cTR and mammalian TRs.

Chicken transferrin receptor gene: conservation 3' noncoding sequences and expression in erythroid cells

Recombinant clones of the chicken transferrin receptor gene and cDNA have been isolated and sequenced. Two highly conserved regions have been identified in the 3' noncoding sequence of the human and chicken TR gene. The conserved regions include sequences that have been shown to be involved in the iron-dependent regulation of human TR mRNA stability. These sequences can be modeled as two different types of RNA secondary structures, one containing stem-loop structures that are similar to the iron-responsive elements found in ferritin mRNA and the other being a stable, duplex/stem-loop structure. Both forms show considerable similarity between chicken and human mRNA. The expression of TR is developmentally regulated during erythroid maturation, and immature erythroid cells express exceptionally high levels of TR mRNA.

A cytosolic protein binds to structural elements within the iron regulatory region of the transferrin receptor mRNA

The level of mRNA encoding the transferrin receptor (TfR) is regulated by iron, and this regulation is mediated by a portion of the 3' untranslated region (UTR) of the TfR transcript. This portion of 3' UTR of the human TfR mRNA contains five RNA elements that have structural similarity to the iron-responsive element (IRE) found as a single copy in the 5' UTR of the mRNA for ferritin, whose translation is regulated by iron. Moreover, five very similar elements are also contained in the 3' UTR of the chicken TfR mRNA. Cytosolic extracts of human cell lines are shown by a gel shift assay involving RNase T1 protection to contain an IRE-binding protein capable of specific interaction with the human TfR 3' UTR. When the protecting protein is removed, the protected RNA can be digested with RNase T1 to yield oligoribonucleotide fragments characteristic of two of the IREs contained in the TfR 3' UTR. As judged by cross-competition experiments, the same IRE-binding protein interacts with the ferritin IRE. The apparent affinity of RNA sequence elements for the IRE-binding protein is shown to depend upon the sequence of the RNA. A comprehensive secondary structure for the regulatory region of the TfR mRNA is proposed based on the experimentally demonstrated presence of at least two IRE-like structural elements.