Targeting DNA topoisomerases or checkpoint kinases results in an overload of chaperone systems, triggering aggregation of a metastable subproteome

A loss of the checkpoint kinase ataxia telangiectasia mutated (ATM) leads to impairments in the DNA damage response, and in humans causes cerebellar neurodegeneration, and an increased risk of cancer. A loss of ATM is also associated with increased protein aggregation. The relevance and characteristics of this aggregation are still incompletely understood. Moreover, it is unclear to what extent other genotoxic conditions can trigger protein aggregation as well. Here, we show that targeting ATM, but also ATR or DNA topoisomerases, results in the widespread aggregation of a metastable, disease-associated subfraction of the proteome. Aggregation-prone model substrates, including Huntingtin exon 1 containing an expanded polyglutamine repeat, aggregate faster under these conditions. This increased aggregation results from an overload of chaperone systems, which lowers the cell-intrinsic threshold for proteins to aggregate. In line with this, we find that inhibition of the HSP70 chaperone system further exacerbates the increased protein aggregation. Moreover, we identify the molecular chaperone HSPB5 as a cell-specific suppressor of it. Our findings reveal that various genotoxic conditions trigger widespread protein aggregation in a manner that is highly reminiscent of the aggregation occurring in situations of proteotoxic stress and in proteinopathies.

Cells are constantly perceiving and responding to changes in their surroundings, and challenging conditions such as extreme heat or toxic chemicals can put cells under stress. When this happens, protein production can be affected. Proteins are long chains of chemical building blocks called amino acids, and they can only perform their roles if they fold into the right shape. Some proteins fold easily and remain folded, but others can be unstable and often become misfolded. Unfolded proteins can become a problem because they stick to each other, forming large clumps called aggregates that can interfere with the normal activity of cells, causing damage.

The causes of stress that have a direct effect on protein folding are called proteotoxic stresses, and include, for example, high temperatures, which make proteins more flexible and unstable, increasing their chances of becoming unfolded. To prevent proteins becoming misfolded, cells can make ‘protein chaperones’, a type of proteins that help other proteins fold correctly and stay folded. The production of protein chaperones often increases in response to proteotoxic stress. However, there are other types of stress too, such as genotoxic stress, which damages DNA. It is unclear what effect genotoxic stress has on protein folding.

Huiting et al. studied protein folding during genotoxic stress in human cells grown in the lab. Stress was induced by either blocking the proteins that repair DNA or by ‘trapping’ the proteins that release DNA tension, both of which result in DNA damage. The analysis showed that, similar to the effects of proteotoxic stress, genotoxic stress increased the number of proteins that aggregate, although certain proteins formed aggregates even without stress, particularly if they were common and relatively unstable proteins.

Huiting et al.’s results suggest that aggregation increases in cells under genotoxic stress because the cells fail to produce enough chaperones to effectively fold all the proteins that need it. Indeed, Huiting et al. showed that aggregates contain many proteins that rely on chaperones, and that increasing the number of chaperones in stressed cells reduced protein aggregation.

This work shows that genotoxic stress can affect protein folding by limiting the availability of chaperones, which increases protein aggregation. Remarkably, there is a substantial overlap between proteins that aggregate in diseases that affect the brain – such as Alzheimer’s disease – and proteins that aggregate after genotoxic stress. Therefore, further research could focus on determining whether genotoxic stress is involved in the progression of these neurological diseases.

The Hippo signaling pathway effector YAP promotes salivary gland regeneration after injury

[Figure: see text].

Vps13 is required for timely removal of nurse cell corpses

Programmed cell death and consecutive removal of cellular remnants is essential for development. During late stages of Drosophila melanogaster oogenesis, the small somatic follicle cells that surround the large nurse cells promote non-apoptotic nurse cell death, subsequently engulf them, and contribute to the timely removal of nurse cell corpses. Here, we identify a role for Vps13 in the timely removal of nurse cell corpses downstream of developmental programmed cell death. Vps13 is an evolutionarily conserved peripheral membrane protein associated with membrane contact sites and lipid transfer. It is expressed in late nurse cells, and persistent nurse cell remnants are observed when Vps13 is depleted from nurse cells but not from follicle cells. Microscopic analysis revealed enrichment of Vps13 in close proximity to the plasma membrane and the endoplasmic reticulum in nurse cells undergoing degradation. Ultrastructural analysis uncovered the presence of an underlying Vps13-dependent membranous structure in close association with the plasma membrane. The newly identified structure and function suggests the presence of a Vps13-dependent process required for complete degradation of bulky remnants of dying cells.

CBX7 Induces Self-Renewal of Human Normal and Malignant Hematopoietic Stem and Progenitor Cells by Canonical and Non-canonical Interactions

In this study, we demonstrate that, among all five CBX Polycomb proteins, only CBX7 possesses the ability to control self-renewal of human hematopoietic stem and progenitor cells (HSPCs). Xenotransplantation of CBX7-overexpressing HSPCs resulted in increased multi-lineage long-term engraftment and myelopoiesis. Gene expression and chromatin analyses revealed perturbations in genes involved in differentiation, DNA and chromatin maintenance, and cell cycle control. CBX7 is upregulated in acute myeloid leukemia (AML), and its genetic or pharmacological repression in AML cells inhibited proliferation and induced differentiation. Mass spectrometry analysis revealed several non-histone protein interactions between CBX7 and the H3K9 methyltransferases SETDB1, EHMT1, and EHMT2. These CBX7-binding proteins possess a trimethylated lysine peptide motif highly similar to the canonical CBX7 target H3K27me3. Depletion of SETDB1 in AML cells phenocopied repression of CBX7. We identify CBX7 as an important regulator of self-renewal and uncover non-canonical crosstalk between distinct pathways, revealing therapeutic opportunities for leukemia.

CoA-dependent activation of mitochondrial acyl carrier protein links four neurodegenerative diseases

PKAN, CoPAN, MePAN, and PDH‐E2 deficiency share key phenotypic features but harbor defects in distinct metabolic processes. Selective damage to the globus pallidus occurs in these genetic neurodegenerative diseases, which arise from defects in CoA biosynthesis (PKAN, CoPAN), protein lipoylation (MePAN), and pyruvate dehydrogenase activity (PDH‐E2 deficiency). Overlap of their clinical features suggests a common molecular etiology, the identification of which is required to understand their pathophysiology and design treatment strategies. We provide evidence that CoA‐dependent activation of mitochondrial acyl carrier protein (mtACP) is a possible process linking these diseases through its effect on PDH activity. CoA is the source for the 4′‐phosphopantetheine moiety required for the posttranslational 4′‐phosphopantetheinylation needed to activate specific proteins. We show that impaired CoA homeostasis leads to decreased 4′‐phosphopantetheinylation of mtACP. This results in a decrease of the active form of mtACP, and in turn a decrease in lipoylation with reduced activity of lipoylated proteins, including PDH. Defects in the steps of a linked CoA‐mtACP‐PDH pathway cause similar phenotypic abnormalities. By chemically and genetically re‐activating PDH, these phenotypes can be rescued, suggesting possible treatment strategies for these diseases.

Ring sideroblasts in AML are associated with adverse risk characteristics and have a distinct gene expression pattern

Ring sideroblasts (RS) emerge as result of aberrant erythroid differentiation leading to excessive mitochondrial iron accumulation, a characteristic feature for myelodysplastic syndromes (MDS) with mutations in the spliceosome gene SF3B1. However, RS can also be observed in patients diagnosed with acute myeloid leukemia (AML). The objective of this study was to characterize RS in patients with AML. Clinically, RS-AML is enriched for ELN adverse risk (55%). In line with this finding, 35% of all cases had complex cytogenetic aberrancies, and TP53 was most recurrently mutated in this cohort (37%), followed by DNMT3A (26%), RUNX1 (25%), TET2 (20%), and ASXL1 (19%). In contrast to RS-MDS, the incidence of SF3B1 mutations was low (8%). Whole-exome sequencing and SNP array analysis on a subset of patients did not uncover a single genetic defect underlying the RS phenotype. Shared genetic defects between erythroblasts and total mononuclear cell fraction indicate common ancestry for the erythroid lineage and the myeloid blast cells in patients with RS-AML. RNA sequencing analysis on CD34+ AML cells revealed differential gene expression between RS-AML and non RS-AML cases, including genes involved in megakaryocyte and erythroid differentiation. Furthermore, several heme metabolism-related genes were found to be upregulated in RS- CD34+ AML cells, as was observed in SF3B1mut MDS. These results demonstrate that although the genetic background of RS-AML differs from that of RS-MDS, they have certain downstream effector pathways in common.

Differential redox-regulation and mitochondrial dynamics in normal and leukemic hematopoietic stem cells: A potential window for leukemia therapy

The prognosis for many patients with acute myeloid leukemia (AML) is poor, mainly due to disease relapse driven by leukemia stem cells (LSCs). Recent studies have highlighted the unique metabolic properties of LSCs, which might represent opportunities for LSC-selective targeting. LSCs characteristically have low levels of reactive oxygen species (ROS), which apparently result from a combination of low mitochondrial activity and high activity of ROS-removing pathways such as autophagy. Due to this low activity, LSCs are highly dependent on mitochondrial regulatory mechanisms. These include the anti-apoptotic protein BCL-2, which also has crucial roles in regulating the mitochondrial membrane potential, and proteins involved in mitophagy. Here we review the different pathways that impact mitochondrial activity and redox-regulation, and highlight their relevance for the functionality of both HSCs and LSCs. Additionally, novel AML therapy strategies that are based on interference with those pathways, including the promising BCL-2 inhibitor Venetoclax, are summarized.

CITED2 affects leukemic cell survival by interfering with p53 activation

CITED2 (CBP/p300-interacting-transactivator-with-an-ED-rich-tail 2) is a regulator of the acetyltransferase CBP/p300 and elevated CITED2 levels are shown in a number of acute myeloid leukemia (AML). To study the in vivo role of CITED2 in AML maintenance, AML cells were transduced with a lentiviral construct for RNAi-mediated knockdown of CITED2. Mice transplanted with CITED2-knockdown AML cells (n=4) had a significantly longer survival compared to mice transplanted with control AML cells (P<0.02). In vitro, the reduction of CITED2 resulted in increased p53-mediated apoptosis and CDKN1A expression, whereas BCL2 levels were reduced. The activation of p53 upon CITED2 knockdown is not a direct consequence of increased CBP/p300-activity towards p53, since no increased formation of CBP/p300/p53 complexes was demonstrated and inhibition of CBP/p300-activity could not rescue the phenotype of CITED2-deficient cells. Instead, loss of CITED2 had an inhibitory effect on the AKT-signaling pathway, which was indicated by decreased levels of phosphorylated AKT and altered expression of the AKT-pathway regulators PHLDA3 and SOX4. Notably, simultaneous upregulation of BCL2 or downregulation of the p53-target gene PHLDA3 rescued the apoptotic phenotype in CITED2-knockdown cells. Furthermore, knockdown of CITED2 led to a decreased interaction of p53 with its inhibitor MDM2, which results in increased amounts of total p53 protein. In summary, our data indicate that CITED2 functions in pathways regulating p53 activity and therefore represents an interesting target for AML therapy, since de novo AML cases are characterized by an inactivation of the p53 pathway or deregulation of apoptosis-related genes.

CITED2-mediated human hematopoietic stem cell maintenance is critical for acute myeloid leukemia

As the transcriptional coactivator CITED2 (CBP/p300-interacting-transactivator-with-an ED-rich-tail 2) can be overexpressed in acute myeloid leukemia (AML) cells, we analyzed the consequences of high CITED2 expression in normal and AML cells. CITED2 overexpression in normal CD34(+) cells resulted in enhanced hematopoietic stem and progenitor cell (HSPC) output in vitro, as well as in better hematopoietic stem cell (HSC) engraftability in NSG (NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ) mice. This was because of an enhanced quiescence and maintenance of CD34(+)CD38(-) HSCs, due in part to an increased expression of the cyclin-dependent kinase inhibitor CDKN1A. We demonstrated that PU.1 is a critical regulator of CITED2, as PU.1 repressed CITED2 expression in a DNA methyltransferase 3A/B (DNMT3A/B)-dependent manner in normal CD34(+) cells. CD34(+) cells from a subset of AML patients displayed higher expression levels of CITED2 as compared with normal CD34(+) HSPCs, and knockdown of CITED2 in AML CD34(+) cells led to a loss of long-term expansion, both in vitro and in vivo. The higher CITED2 expression resulted from reduced PU.1 activity and/or dysfunction of mutated DNMT3A/B. Collectively, our data demonstrate that increased CITED2 expression results in better HSC maintenance. In concert with low PU.1 levels, this could result in a perturbed myeloid differentiation program that contributes to leukemia maintenance.

Impact of genetic variation in the SMIM1 gene on Vel expression levels

BACKGROUND

Serologic determination of the Vel- phenotype is challenging due to variable Vel expression levels. In this study we investigated the genetic basis for weak Vel expression levels and developed a high-throughput genotyping assay to detect Vel- donors.

STUDY DESIGN AND METHODS

In 548 random Caucasian and 107 Vel+(w) donors genetic variation in the SMIM1 gene was studied and correlated to Vel expression levels. A total of 3366 Caucasian, 621 black, and 333 Chinese donors were screened with a high-throughput genotyping assay targeting the SMIM1*64_80del allele.

RESULTS

The Vel+(w) phenotype is in most cases caused by the presence of one SMIM1 allele carrying the major allele of the rs1175550 SNP in combination with a SMIM1*64_80del allele or in few cases caused by the presence of the SMIM1*152T>A or SMIM1*152T>G allele. In approximately 6% of Vel+(w) donors genetic factors in SMIM1 could not explain the weak expression. We excluded the possibility that lack of expression of another blood group system was correlated with weak Vel expression levels. Furthermore, using a high-throughput Vel genotyping assay we detected two Caucasian Vel- donors.

CONCLUSION

Weak Vel expression levels are caused by multiple genetic factors in SMIM1 and probably also by other genetic or environmental factors. Due to the variation in Vel expression levels, serologic determination of the Vel- phenotype is difficult and a genotyping assay targeting the c.64_80del deletion in SMIM1 should be used to screen donors for the Vel- phenotype.

STAT5-mediated self-renewal of normal hematopoietic and leukemic stem cells

The level of transcription factor activity critically regulates cell fate decisions such as hematopoietic stem cell self-renewal and differentiation. The balance between hematopoietic stem cell self-renewal and differentiation needs to be tightly controlled, as a shift toward differentiation might exhaust the stem cell pool, while a shift toward self-renewal might mark the onset of leukemic transformation. A number of transcription factors have been proposed to be critically involved in governing stem cell fate and lineage commitment, such as Hox transcription factors, c-Myc, Notch1, β-catenin, C/ebpα, Pu.1 and STAT5. It is therefore no surprise that dysregulation of these transcription factors can also contribute to the development of leukemias. This review will discuss the role of STAT5 in both normal and leukemic hematopoietic stem cells as well as mechanisms by which STAT5 might contribute to the development of human leukemias.

Ex vivo assays to study self-renewal, long-term expansion, and leukemic transformation of genetically modified human hematopoietic and patient-derived leukemic stem cells

With the emergence of the concept of the leukemic stem cell (LSC), assays to study them remain pivotal in understanding (leukemic) stem cell biology. Although the in vivo NOD-SCID or NSG xenotransplantation model is currently still the favored assay of choice in most cases, this system has some limitations as well such as its cost-effectiveness, duration, and lack of engraftability of cells from some acute myeloid leukemia (AML) patients. Here, we describe in vitro assays in which long-term expansion and self-renewal of LSCs isolated from AML patients can be evaluated. We have optimized lentiviral transduction procedures in order to stably express genes of interest or stably downmodulate genes using RNAi in primary AML cells, and these approaches are described in detail here. Also, we describe bone marrow stromal coculture systems in which cobblestone area-forming cell activity, self-renewal, long-term expansion, and in vitro myeloid or lymphoid transformation can be evaluated in human CD34(+) cells of fetal or adult origin that are engineered to express oncogenes. Together, these tools should allow a further molecular elucidation of derailed signal transduction in LSCs.

SMIM1 underlies the Vel blood group and influences red blood cell traits

The blood group Vel was discovered 60 years ago, but the underlying gene is unknown. Individuals negative for the Vel antigen are rare and are required for the safe transfusion of patients with antibodies to Vel. To identify the responsible gene, we sequenced the exomes of five individuals negative for the Vel antigen and found that four were homozygous and one was heterozygous for a low-frequency 17-nucleotide frameshift deletion in the gene encoding the 78-amino-acid transmembrane protein SMIM1. A follow-up study showing that 59 of 64 Vel-negative individuals were homozygous for the same deletion and expression of the Vel antigen on SMIM1-transfected cells confirm SMIM1 as the gene underlying the Vel blood group. An expression quantitative trait locus (eQTL), the common SNP rs1175550 contributes to variable expression of the Vel antigen (P = 0.003) and influences the mean hemoglobin concentration of red blood cells (RBCs; P = 8.6 × 10(-15)). In vivo, zebrafish with smim1 knockdown showed a mild reduction in the number of RBCs, identifying SMIM1 as a new regulator of RBC formation. Our findings are of immediate relevance, as the homozygous presence of the deletion allows the unequivocal identification of Vel-negative blood donors.

Seventy-five genetic loci influencing the human red blood cell

Anaemia is a chief determinant of global ill health, contributing to cognitive impairment, growth retardation and impaired physical capacity. To understand further the genetic factors influencing red blood cells, we carried out a genome-wide association study of haemoglobin concentration and related parameters in up to 135,367 individuals. Here we identify 75 independent genetic loci associated with one or more red blood cell phenotypes at P < 10(-8), which together explain 4-9% of the phenotypic variance per trait. Using expression quantitative trait loci and bioinformatic strategies, we identify 121 candidate genes enriched in functions relevant to red blood cell biology. The candidate genes are expressed preferentially in red blood cell precursors, and 43 have haematopoietic phenotypes in Mus musculus or Drosophila melanogaster. Through open-chromatin and coding-variant analyses we identify potential causal genetic variants at 41 loci. Our findings provide extensive new insights into genetic mechanisms and biological pathways controlling red blood cell formation and function.

Cited2 is an essential regulator of adult hematopoietic stem cells

The regulatory pathways necessary for the maintenance of adult hematopoietic stem cells (HSCs) remain poorly defined. By using loss-of-function approaches, we report a selective and cell-autonomous requirement for the p300/CBP-binding transcriptional coactivator Cited2 in adult HSC maintenance. Conditional deletion of Cited2 in the adult mouse results in loss of HSCs causing multilineage bone marrow failure and increased lethality. In contrast, conditional ablation of Cited2 after lineage specification in lymphoid and myeloid lineages has no impact on the maintenance of these lineages. Additional deletion of Ink4a/Arf (encoding p16(Ink4a) and p19(Arf)) or Trp53 (encoding p53, a downstream target of p19(Arf)) in a Cited2-deficient background restores HSC functionality and rescues mice from bone marrow failure. Furthermore, we show that the critical role of Cited2 in primitive hematopoietic cells is conserved in humans. Taken together, our studies provide genetic evidence that Cited2 selectively maintains adult HSC functions, at least in part, via Ink4a/Arf and Trp53.

Induced pluripotent stem cells: will they be safe?

Recent developments in stem cell research have enabled the reprogramming of somatic cells to a pluripotent state using exogenous factors. Induced pluripotent stem (iPS) cells have the potential to differentiate into any cell type, and are being used to elucidate the molecular events that permit the conversion of one cell type to another. iPS cells have potential uses in in vitro disease modeling and toxicology screening, and as cellular therapies and regenerative medicine; however, various safety concerns exist that must be resolved before iPS cell therapy becomes a reality. Potential risks are related to the delivery of the endogenous factors, alterations in target cells, the cellular effects of the expression and reactivation of the factors that induce pluripotency, and safety issues related to the incorrect characterization and incomplete differentiation of the reprogrammed cells. In this review, the technique used to generate iPS cells is described, followed by a discussion of the safety concerns and how these concerns are currently being addressed.

Mucin1 expression is enriched in the human stem cell fraction of cord blood and is upregulated in majority of the AML cases

OBJECTIVE

Mucin1 is a membrane glycoprotein that is overexpressed in a variety of human cancers. Here, we analyzed the role of Mucin1 in human hematopoietic stem/progenitor cells as well as in acute myeloid leukemia (AML) cells.

MATERIALS AND METHODS

Mucin1 expression was determined within the normal stem cell and progenitor compartment, as well as in the AML CD34+ and CD34- subfractions of patient samples. Stem cells were enumerated in long-term culture-initiating cell (LTC-IC) assays in limiting dilution and progenitor frequencies in colony-forming cell (CFC) assays in methylcellulose, and consequences of elevated Mucin1 expression were studied using retroviral overexpression systems in cord blood (CB) CD34+ cells.

RESULTS

Ten percent of CB and 5% of peripheral blood CD34+ cells expressed Mucin1. Retroviral overexpression of Mucin1 in CB CD34+ cells resulted in elevated stem cell and progenitor frequencies as determined in LTC-IC and CFC assays without affecting differentiation, which coincided with increased proliferation. Overexpression of intercellular adhesion molecule-1, a ligand for Mucin1, in MS5 stromal cells further increased LTC-IC frequencies. Mucin1 overexpression was associated with increased nuclear factor-kappaB p50 nuclear translocation, suggesting that Mucin1-induced phenotypes involve increased cell survival mechanisms. Finally, we observed increased Mucin1 expression in 70% of the AML cases (n=24), suggesting that elevated Mucin1 levels might be involved in regulating the proliferative potential of the immature leukemic compartment as well.

CONCLUSIONS

Our data indicate that hematopoietic stem cells as well as CD34+ AML subfractions are enriched for Mucin1 expression, and that overexpression of Mucin1 in CB cells is sufficient to increase both progenitor and LTC-IC frequencies.

Establishing long-term cultures with self-renewing acute myeloid leukemia stem/progenitor cells

OBJECTIVE

With the emergence of the concept of the leukemia stem cell, assays to study them remain pivotal in understanding (leukemic) stem cell biology.

METHODS

We have cultured acute myeloid leukemia CD34(+) cells on bone marrow stroma. Long-term expansion was monitored and self-renewal was addressed by replating of Leukemic-cobblestone area-forming cells (L-CAs). Also, lentiviral vectors were generated that could target L-CAs.

RESULTS

A strong expansion was observed in about 75% of the acute myeloid leukemia cases (n = 30) and long-term cultures could be maintained for up to 24 weeks on MS5 bone marrow stromal cells. Cells that were able to initiate leukemic cobblestone areas resided in the CD34(+) population and were absent from the CD34(-) population. Self-renewal within these L-CAs was determined by sequential passaging of these L-CAs onto new MS5 stromal layers, which resulted in the generation of second, third, and fourth L-CAs, which were able to sustain long-term expansion and generated high numbers of immature undifferentiated suspension cells. CD34(+) cells that were able to initiate long-term cultures all coexpressed MEIS1 and HOXA9, and expressed elevated BMI1 levels.

CONCLUSION

We present a novel long-term leukemic stem/progenitor assay in which new drugs can be tested and in which genes can be overexpressed or downmodulated using a lentiviral approach in order to obtain more insight into the process of leukemic transformation and self-renewal.

STAT5 is required for long-term maintenance of normal and leukemic human stem/progenitor cells

The transcription factor STAT5 fulfills a distinct role in the hematopoietic system, but its precise role in primitive human hematopoietic cells remains to be elucidated. Therefore, we performed STAT5 RNAi in sorted cord blood (CB) and acute myeloid leukemia (AML) CD34+ cells by lentiviral transduction and investigated effects of STAT5 downmodulation on the normal stem/progenitor cell compartment and the leukemic counterpart. STAT5 RNAi cells displayed growth impairment, without affecting their differentiation in CB and AML cultures on MS5 stroma. In CB, limiting-dilution assays demonstrated a 3.9-fold reduction in progenitor numbers. Stem cells were enumerated in long-term culture-initiating cell (LTC-IC) assays, and the average LTC-IC frequency was 3.25-fold reduced from 0.13% to 0.04% by STAT5 down-regulation. Single-cell sorting experiments of CB CD34+/CD38− cells demonstrated a 2-fold reduced cytokine-driven expansion, with a subsequent 2.3-fold reduction of progenitors. In sorted CD34+ AML cells with constitutive STAT5 phosphorylation (5/8), STAT5 RNAi demonstrated a reduction in cell number (72% ± 17%) and a decreased expansion (17 ± 15 vs 80 ± 58 in control cultures) at week 6 on MS5 stroma. Together, our data indicate that STAT5 expression is required for the maintenance and expansion of primitive hematopoietic stem and progenitor cells, both in normal as well as leukemic hematopoiesis.

Reintroduction of C/EBPalpha in leukemic CD34+ stem/progenitor cells impairs self-renewal and partially restores myelopoiesis

The CCAAT/enhancer binding protein (C/EBP) alpha transcription factor is indispensable for myeloid differentiation. In various myeloid leukemias, C/EBPalpha is mutated or functionally impaired due to decreased C/EBPalpha expression or phosphorylation. In order to investigate the functional consequences of decreased C/EBPalpha function in AML, we reintroduced C/EBPalpha in primary CD34(+) sorted acute myeloid leukemia (AML) cells using a lentiviral approach. Self-renewal and differentiation of primary AML stem cells were studied on long-term MS5 cocultures. Activation of C/EBPalpha immediately led to a growth arrest in all AML cultures (N = 7), resulting in severely reduced expansion compared with control cultures. This growth arrest corresponded with enhanced myeloid differentiation as assessed by fluorescence-activated cell sorter (FACS) analysis for CD14, CD15, and CD11b. Myeloid differentiation was further confirmed by the up-regulation of neutrophil elastase and granulocyte colony-stimulating factor (G-CSF) receptor in C/EBPalpha transduced cells. C/EBPalpha-expressing AML CD34(+) cells failed to generate second and third leukemic cobblestone areas (L-CAs) in serial replating experiments, while control cultures could be sequentially passaged for more than 4 times, indicating that reintroduction of C/EBPalpha impaired the self-renewal capacity of the leukemic CD34(+) compartment. Together, our data indicate that low C/EBPalpha levels are necessary to maintain self-renewal and the immature character of AML stem cells.

Constitutive activation of NF-kappa B is not sufficient to disturb normal steady-state hematopoiesis

Since nuclear factor-kappaB (NF-kappaB) is frequently activated in acute myeloid leukemia, we questioned whether active NF-kappaB can affect the cellular properties of cord blood CD34+ cells. The results demonstrated that NF-kappaB activation did not influence growth or differentiation properties of these cells. Furthermore, NF-kappaB activation was not sufficient to induce changes in stem- and progenitor cell numbers.

Cellular effects of imatinib on medullary thyroid cancer cells harboring multiple endocrine neoplasia Type 2A and 2B associated RET mutations

BACKGROUND

Activating mutations in the RET gene, which encodes a tyrosine kinase receptor, often cause medullary thyroid carcinoma (MTC). Surgical resection is the only curative treatment; no effective systemic treatment is available. We evaluated imatinib, a tyrosine kinase inhibitor currently used to treat chronic myelogenous leukemia and gastrointestinal stromal tumors, as a potential drug for systemic treatment of MTC, in 2 MTC-derived cell lines expressing multiple endocrine neoplasia-associated mutant RET receptors.

METHODS

We determined RET expression and Y1062 phosphorylation using Western blot analysis and quantitative polymerase chain reaction. We determined the effects on cell proliferation by a 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide assay, and we used fluorescence-activated cell sorter analysis with annexin V/propidium iodide staining to study imatinib-induced cell-cycle arrest, apoptosis, and cell death.

RESULTS

Imatinib inhibited RET Y1062 phosphorylation in a dose-dependent manner after 1.5 hours of exposure. After 16 hours both RET Y1062 phosphorylation and protein expression levels were affected. Dose-dependent decreases in cell proliferation of both cell lines after exposure to imatinib with inhibitory concentration of 50% levels of 23 +/- 2 micromol/L and 25 +/- 4 micromol/L were seen. These values are high, compared with those for chronic myelogenous leukemia and gastrointestinal stromal tumors. We further could show that imatinib induced cell-cycle arrest, and apoptotic and nonapoptotic cell death.

CONCLUSIONS

Imatinib inhibits RET-mediated MTC cell growth affecting RET protein levels in vitro in a dose-dependent manner. The concentration of imatinib necessary to inhibit RET in vitro, however, makes it impossible to conclude that imatinib monotherapy will be a good option for systemic therapy of MTC.

STAT5-induced self-renewal and impaired myelopoiesis of human hematopoietic stem/progenitor cells involves down-modulation of C/EBPα

Previously, we demonstrated that enforced activation of signal transducer and activator of transcription 5 (STAT5A) in human cord blood (CB)–derived stem/progenitor cells results in enhanced self-renewal and impaired myelopoiesis. The present study identifies C/EBPα as a critical component that is down-regulated by STAT5. Microarray and reverse transcriptase–polymerase chain reaction (RT-PCR) analysis on STAT5A1*6-transduced CD34+ cells identified C/EBPα as the most prominently down-regulated gene. To determine the cell-biological relevance of these observations, a 4-OHT-inducible C/EBPα-ER protein was co-expressed with the STAT5A1*6 mutant in CB CD34+ cells using a retroviral approach. Re-expression of C/EBPα in STAT5A1*6 cells resulted in a marked restoration of myelopoiesis. The proliferative advantage imposed on CD34+ cells by STAT5A1*6 depended on the down-modulation of C/EBPα, as reintroduction of C/EBPα induced a quick cell-cycle arrest and the onset of myeloid differentiation. Long-term culture–initiating cell (LTC-IC) frequencies were elevated from 0.8% ± 0.6% to 7.8% ± 1.9% by STAT5A1*6 as compared with controls, but these elevated LTC-IC frequencies were strongly reduced upon re-introduction of C/EBPα in STAT5A1*6 cells, and no second cobble-stone area–forming cells (CAFCs) could be generated from double-transduced cells. Enumeration of progenitors revealed that the number of colony-forming cells (CFCs) was reduced more than 20-fold when C/EBPα was co-expressed in STAT5A1*6 cells. Our data indicate that down-modulation of C/EBPα is a prerequisite for STAT5-induced effects on self-renewal and myelopoiesis.

Oncogenic Ras blocks transforming growth factor-beta-induced cell-cycle arrest by degradation of p27 through a MEK/Erk/SKP2-dependent pathway

OBJECTIVE

To examine whether oncogenic Ras affects transforming growth factor (TGF)-beta-mediated cell-cycle arrest in hematopoietic cells and the downstream signal transduction pathway involved in the interference with TGF-beta-induced cell-cycle arrest.

MATERIALS AND METHODS

Two leukemic cell lines bearing N-Ras(L61) mutations; HL-60 and TF-1, and the M1 cell line with wt Ras were investigated for their response to TGF-beta. Signal transduction inhibitors, overexpression and RNA interference studies were performed to investigate the involvement of the various proteins.

RESULTS

Although TGF-beta signal transduction was not affected, G0-G1 arrest was absent in HL-60 and TF-1 cells due to the absence of p27. Overexpression of p27 restored TGF-beta-induced cell-cycle arrest, as well as interfering in Ras-mediated signaling. The farnesyl transferase inhibitor L744832 and the MEK inhibitor U0126 both restored p27 levels and cell-cycle arrest in response to TGF-beta. The absence of p27 protein is due to elevated levels of the ubiquitin ligase SKP2, which complexes with and targets p27 for degradation. RNA interference for SKP2 and treatment of these cells with the proteasome inhibitor MG132 restored p27 levels, corresponding with decreasing SKP2 levels after interfering in N-Ras signal transduction. P27, phosphorylated at threonine 187, is nuclear localized in N-Ras-containing cells. Mutation of this residue to alanine rendered p27 insensitive to degradation.

CONCLUSION

N-Ras(L61) transformed cells lack a G0-G1 arrest upon TGF-beta treatment due to absence of p27. p27 is degraded through a MapK-, and SKP2-dependent pathway. Overexpression of p27 results in restoration of cell-cycle arrest upon TGF-beta treatment.

HSP27 protects AML cells against VP-16-induced apoptosis through modulation of p38 and c-Jun

OBJECTIVE

To investigate 1) the signal transduction pathways affected by heat shock protein 27 (HSP27) expression; and 2) the expression and regulation of HSP27 in acute myeloid leukemia (AML).

MATERIALS AND METHODS

RNA interference studies for HSP27 in leukemic TF-1 cells were used to investigate the effects on downstream signal transduction and apoptosis after VP-16 and CD95/Fas treatment. HSP27 expression and activation was investigated in AML blasts through Western blot analysis.

RESULTS

RNA interference for HSP27 resulted in a twofold increase in VP-16-induced apoptosis, which was preceded by enhanced p38 and c-Jun phosphorylation and a twofold increased cytochrome c release into the cytoplasm. DAXX co-immunoprecipitated with HSP27, suggesting an inhibitory role of HSP27 in VP-16-mediated activation of the ASK1/p38/JNK pathway. CD95/Fas-induced apoptosis, however, was unaffected by HSP27 siRNA, due to upregulation of HSP27. Although HSP27 was highly expressed and phosphorylated in primitive monocytic AML blasts (M4-M5, 91%, n=11) and undetectable in myeloid blasts (M1-M2, n=5), VP-16-mediated apoptosis correlated moderately with HSP27 expression. This is likely due to the co-expression of p21Waf1/Cip1, which is in the majority of the monocytic AML M4-M5 blasts constitutively localized in the cytoplasm. Overexpression of cytoplasmic p21 inhibited the enhanced p38 phosphorylation after HSP27 RNAi, suggesting a predominant anti-apoptotic role of p21 over HSP27.

CONCLUSION

1) HSP27 inhibits VP-16-mediated phosphorylation of p38 and c-Jun, cytochrome c release, and subsequent apoptosis; 2) HSP27 is expressed and activated in monocytic AML blasts; 3) cytoplasmic expression of p21 compensates for the lack of HSP27.

Constitutive NF-kappaB DNA-binding activity in AML is frequently mediated by a Ras/PI3-K/PKB-dependent pathway

In the present study, we aimed to elucidate the mechanism responsible for constitutive NF-kappaB DNA-binding activity in AML cells. Intervening in aberrant signaling pathway provides a rational approach for in vivo targeting of AML cells. Constitutive NF-kappaB DNA-binding activity was observed in 16 of 22 (73%) investigated AML cases and was, in general, associated with resistance to spontaneous apoptosis. Indeed, inhibition of NF-kappaB activity by the NF-kappaB inhibitor SN-50 peptide resulted in enhanced chemotherapy-induced apoptosis. In the majority of cases, constitutive NF-kappaB activity was mediated by a Ras/PI3 kinase (PI3-K)/protein kinase B (PKB)-mediated pathway. The PI3-K inhibitor Ly294002 and the Ras inhibitor L-744832 both inhibited PKB phosphorylation and NF-kappaB DNA-binding activity. The constitutive activation of Ras GTP-ase was caused by mutations in the gene encoding for N-Ras in 29% of the cases. The constitutive NF-kappaB activity could so far not be ascribed to the autocrine production of growth factors or to mutations in the Flt3 receptor, since anti-GM-CSF, -IL-1, -IL6, -TNFalpha or the tyrosine kinase inhibitor AG1296 did not affect the NF-kappaB DNA-binding activity. The present study demonstrates that Ras activation is an important pathway for triggering the NF-kappaB pathway in AML cells.

Constitutive cytoplasmic localization of p21(Waf1/Cip1) affects the apoptotic process in monocytic leukaemia

In the present study, we analysed the expression and localization of p21(Waf1/Cip1) in normal and malignant haematopoietic cells. We demonstrate that in normal monocytic cells, protein kinase C (PKC)-induced p21 gene activation, which is nuclear factor-kappaB (NF-kappaB) independent, results in predominantly cytoplasmic localized p21 protein. In acute monocytic leukaemia (M4, M5), monocytic blasts (N=12) show constitutive cytoplasmic p21 expression in 75% of the cases, while in myeloid leukaemic blasts (N=10), low nuclear and cytoplasmic localization of p21 could be detected, which is also PKC dependent. Constitutive p21 expression in monocytic leukaemia might have important antiapoptotic functions. This is supported by the finding that in U937 cells overexpressing p21, VP16-induced apoptosis is significantly reduced (20.0+/-0.9 vs 55.8+/-3.8%, P<0.01, N=5), reflected by a reduced phosphorylation of p38 and JNK. Similarly, AML blasts with high cytoplasmic p21 were less sensitive to VP16-induced apoptosis as compared to AML cases with low or undetectable p21 expression (42.25 vs 12.3%, P<0.01). Moreover, complex formation between p21 and ASK1 could be demonstrated in AML cells, by means of coimmunoprecipitation. In summary, these results indicate that p21 has an antiapoptotic role in monocytic leukaemia, and that p21 expression is regulated in a PKC-dependent and NF-kappaB-independent manner.

Ser727-dependent transcriptional activation by association of p300 with STAT3 upon IL-6 stimulation

Activation of the signal transducer and activator of transcription 3 (STAT3) in response to interleukin-6 (IL-6) type cytokines involves both phosphorylation of Tyr705, which enables dimerization, nuclear translocation and DNA binding, as well as ser727 phosphorylation. Here, we describe that the 65 C-terminal amino acids of STAT3 can function as an independent transcription activation domain (TAD), particularly when a negative charge is introduced at position 727 by mutation of the serine residue into aspartate. The strong transcriptional activity of the C-terminal STAT3 Ser727Asp TAD is coupled to a constitutive association with the co-activator p300. In HepG2 cells, p300 associates with STAT3 upon IL-6 stimulation, and overexpression of p300 enhances the transcriptional activity of STAT3alpha, but not of STAT3beta or STAT3 Ser727Ala. We conclude that Ser727 phosphorylation in the C-terminal region of STAT3 is required for transactivation by association with p300.

Mode of insertion of praziquantel and derivatives into lipid membranes

We report here about the relationship between the destabilization of the lipid organization induced by praziquantel and derivatives and their mode of insertion into the lipid matrix. Measurements of lipid transition temperature and efflux of 6-carboxyfluorescein encapsulated in liposomes establish the lipid destabilizing capacity of praziquantel as compared to praziquantel derivatives. IR spectroscopy (attenuated total reflection technique) applied to oriented lipid bilayers indicates that praziquantel or derivatives do not modify significantly the lipid structure. In order to give a molecular description of the position of the drug into the lipid bilayer, we applied a conformational analysis procedure making it possible to calculate the structure of amphiphilic molecules assembled in aggregates and the mode of insertion of amphiphilic drugs into a lipid layer. The praziquantel lipid destabilizing capacity is explained in terms of the high praziquantel-lipid interaction and the large area occupied per drug molecule in the lipid layer.

Eine Methode zur elektromagnetischen Impulsspeieherung bei cler szintillographischen Isotopendiagnostik

Es wird über die Entwicklung einer Apparatur zur elektromagnetischen Impulsspeicherung bei der szintillografischen Messung von Isotopenverteilungen berichtet. Die Vorteile des Gerätes bestehen im wesentlichen in relativ geringem apparativem Aufwand, sowie in der Möglichkeit, einfache quantitativ-statistische Analysen der aufgenommenen Szintillogramme durchzuführen.