Multiple ways of C/EBPalpha inhibition in myeloid leukaemia

Dysregulated Expression of MiR-19b, MiR-25, MiR-17, WT1, and CEBPA in Patients with Acute Myeloid Leukemia and Association with Graft versus Host Disease after Hematopoietic Stem Cell Transplantation

Elham JamaliObjectives  Acute myeloid leukemia (AML) is a blood malignancy characterized by the proliferation of aberrant cells in the bone marrow and blood that interfere with normal blood cells. We have investigated whether changes in the level of micro-ribonucleic acid (miR)-19b, miR-17, and miR-25, Wilms' tumor (WT1), and CCAAT enhancer-binding protein α (CEBPA) genes expression affect disease prognosis and clinical outcome in AML patients. Materials and Methods  The expression level of miR-19-b, miR-17, and miR-25, as well as WT1 and CEBPA genes in a group of patients and controls as well as different risk groups (high, intermediate, and favorite risk), M3 versus non-M3, and graft-versus-host disease (GvHD) versus non-GvHD patients were assessed using a quantitative SYBR Green real-time polymerase chain reaction method. Results  When compared with the baseline level at the period of diagnosis before chemotherapy, the expression of miR-19b and miR-17 in AML patients increased significantly after chemotherapy. The level of miR-19b and miR-25 expression in AML patients with M3 and non-M3 French-American-British subgroups differ significantly. MiR-19b and miR-25 expression was elevated in GvHD patients, while miR-19b and miR-25 expression was somewhat decreased in GvHD patients compared with non-GvHD patients, albeit the difference was not statistically significant. Also, patients with different cytogenetic aberrations had similar levels of miR-19-b and miR-25 expression. Conclusion  MiR-19b, miR-17, and miR-25 are aberrantly expressed in AML patients' peripheral blood leukocytes, which may play a role in the development of acute GvHD following hematopoietic stem cell transplantation.

CDK2-instigates C/EBPα degradation through SKP2 in Acute myeloid leukemia

Transcription factor CCAAT/enhancer-binding protein-alpha (C/EBPα) regulates myelopoiesis by coupling growth arrest with differentiation of myeloid progenitors. Mutations in one or both alleles are observed in 10-14% AML cases that render C/EBPα functionally inactive. Besides, antagonistic protein-protein interactions also impair C/EBPα expression and function. In recent independent studies, we showed that CDK2 and SKP2 downregulated C/EBPα expression in an ubiquitin-dependent proteasome degradation manner leading to differentiation block in AML. Here, we demonstrate that CDK2-instigated C/EBPα downregulation is actually mediated by SKP2. Mechanistically, we show that CDK2 stabilizes SKP2 by phosphorylating it at Ser64 and thereby potentiates C/EBPα ubiquitination and subsequent degradation in AML cells. Immunoblot experiments showed that CDK2 inhibition downregulated SKP2 levels and concomitantly enhanced C/EBPα levels in myeloid cells. We further show that while CDK2 promoted C/EBPα ubiquitination and inhibited its protein levels, negatively affected its transactivation potential and DNA binding ability, simultaneous SKP2 depletion abrogated CDK2-promoted ubiquitination and restored C/EBPα expression and function. Taken together, these findings consolidate that CDK2 potentiates SKP2-mediated C/EBPα degradation in AML and targeting CDK2-SKP2 axis can be harnessed for therapeutic benefit in AML. Hypothetical model depicts that SKP2-mediated C/EBPα proteasomal degradation is reinforced by CDK2. CDK2 phopshorylates SKP2 leading to its enhanced stabilization which in turn exaggerates C/EBPα degradation leading to differentiation arrest in AML.

E3 ligase SCFSKP2 ubiquitinates and degrades tumor suppressor C/EBPα in acute myeloid leukemia

AIM

Transcription factor CCAAT/Enhancer binding protein alpha (C/EBPα) is a key regulator of myeloid differentiation, granulopoiesis in particular. Although CEBPA mutations are found in more than 10% in AML, functional inhibition of C/EBPα protein is also widely observed in AML. Here, we sought to examine if SKP2, an aberrantly enhanced E3 ubiquitin ligase in primary AMLs inhibits C/EBPα stability to induce differentiation block.

MAIN METHODS

Here we employed cell based assays such as transfections, immunoblotting, co-immunoprecipitation, luciferase and gel shift assays along with differentiation assays to investigate SKP2 regulated C/EBPα protein stability in acute myeloid leukemia.

KEY FINDINGS

Here we discovered that oncogenic E3 ubiquitin ligase SCF^skp2 ubiquitinates and destabilizes C/EBPα in a proteasome-dependent manner. Our data demonstrates that SKP2 physically interacts with C-terminal of C/EBPα and promotes its K48-linked ubiquitination-mediated degradation leading to its reduced transactivation potential, DNA binding ability and cellular functions. We further show that while overexpression of SKP2 inhibits both ectopic as well as endogenous C/EBPα in heterologous (HEK293T) as well as myeloid leukemia cells respectively, SKP2 depletion restores endogenous C/EBPα leading to reduced colony formation and enhanced myeloid differentiation of myeloid leukemia cells. Using Estradiol-inducible K562-C/EBPα-ER cells as yet another model of granulocytic differentiation, we further confirmed that SKP2 overexpression indeed inhibits granulocytic differentiation by mitigating C/EBPα stability.

SIGNIFICANCE

Our findings identify SKP2 as a potential negative regulator of C/EBPα stability and function in AML which suggests that SKP2 can be potentially targeted in AML to restore C/EBPα and overcome differentiation block.

Nano-LC based proteomic approach identifies that E6AP interacts with ENO1 and targets it for degradation in breast cancer cells

E6AP (E6 associated protein) is a HECT domain containing protein having dual E3 ligase and ERα coactivation activity in breast cancer cells. Although E6AP is known to possess antitumorigenic activity, the underlying molecular mechanism is poorly understood. In the present study, we applied nano-LC based proteomics approach to identify E6AP-interacting proteins where we performed GST-pull down using GST-E6AP from whole cell extracts of MCF7 cells, resolved the differentially interacting proteins on 1D-SDS-PAGE, excised the gel bands that were trypsin digested followed by fractionation and spotting on MALDI-TOF/TOF plate through Nano-LC MALDI spotter. Subsequently, fractionated and spotted peptides were identified using MALDI-TOF/TOF. We identified several E6AP interacting proteins including previously reported such as HSP70 and new ones such as Enolase-1. We further confirmed that E6AP and Enolase1 interacted and colocalized more in the cytoplasmic periphery in breast cancer cells and further demonstrated that E6AP also targeted ENO1 for ubiquitin-mediated degradation in these cells.

Prevalence and Prognostic Impact of CEBPA Gene Mutation (Simplified Assay Technique) in Egyptian Acute Myeloid Leukemia Patients with Normal Cytogenetics

Mutations of the CCAAT/enhancer binding protein alpha (CEBPA) gene have been associated with a favorable outcome in patients with acute myeloid leukemia (AML), especially in those with a normal cytogenetics. However, few studies were done on Egyptian AML patients and none of them look for easier and less expensive method for CEBPA mutation screening. This study is aimed to investigate the prevalence of CEBPA mutations and its clinical and prognostic impact in Egyptian patients with cytogenetically normal AML (CN-AML). This was done using fragment analysis to assess this method as a cheaper and less laborious screening method compared to sequencing. Fluorescent PCR was done to amplify CEBPA gene in DNA extracted from 40 CN-AML patients. This was followed by fragment analysis of post-PCR products using GeneMapper software for detection of CEBPA mutations. CEBPA gene mutations were found in 7/40 CN-AML patients (17.5 %) and it was significantly associated with lower LDH levels (p = 0.039). All patients with CEBPA mutations achieved clinical remission and none of them showed refractoriness, relapsed, or died by the end of the 2 years study period. Furthermore, those patients demonstrate significantly longer overall and disease free survival than those with wild type CEBPA gene (p = 0.001 and 0.004 respectively). CEBPA mutation has a favorable prognostic impact in CN-AML. Fragment analysis is a good, lees laborious and cheaper method that can be used for CEBPA mutation screening in patients with CN-AML.

E3 ubiquitin ligase E6AP negatively regulates adipogenesis by downregulating proadipogenic factor C/EBPalpha

CCAAT/Enhancer Binding Protein Alpha (C/EBPα) is a key transcription factor involved in the adipocyte differentiation. Here for the first time we demonstrate that E6AP, an E3 ubiquitin ligase inhibits adipocyte differentiation in 3T3-L1 cells as revealed by reduced lipid staining with oil red. Knock down of E6AP in mouse 3T3L1 preadipocytes is sufficient to convert them to adipocytes independent of external hormonal induction. C/EBPα protein level is drastically increased in E6AP deficient 3T3L1 preadipocytes while inverse is observed when wild type E6AP is over expressed. We show that transient transfection of wild type E6AP downregulates C/EBPα protein expression in a dose dependent manner while catalytically inactive E6AP-C843A rather stabilizes it. In addition, wild type E6AP inhibits expression of proadipogenic genes while E6AP-C843A enhances them. More importantly, overexpression of E6AP-C843A in mesenchymal progenitor cells promotes accumulation of lipid droplets while there is drastically reduced lipid droplet formation when E6AP is over expressed. Taken together, our finding suggests that E6AP may negatively control adipogenesis by inhibiting C/EBPα expression by targeting it to ubiquitin-proteasome pathway for degradation.

E6AP, an E3 ubiquitin ligase negatively regulates granulopoiesis by targeting transcription factor C/EBPα for ubiquitin-mediated proteasome degradation

CCAAT/enhancer-binding protein alpha (C/EBPα) is an important transcription factor involved in granulocytic differentiation. Here, for the first time we demonstrate that E6-associated protein (E6AP), an E3 ubiquitin ligase targets C/EBPα for ubiquitin-mediated proteasome degradation and thereby negatively modulates its functions. Wild-type E6AP promotes ubiquitin dependent proteasome degradation of C/EBPα, while catalytically inactive E6-associated protein having cysteine replaced with alanine at amino-acid position 843 (E6AP-C843A) rather stabilizes it. Further, these two proteins physically associate both in non-myeloid (overexpressed human embryonic kidney epithelium) and myeloid cells. We show that E6AP-mediated degradation of C/EBPα protein expression curtails its transactivation potential on its target genes. Noticeably, E6AP degrades both wild-type 42 kDa CCAAT-enhancer-binding protein alpha (p42C/EBPα) and mutant isoform 30 kDa CCAAT-enhancer-binding protein alpha (p30C/EBPα), this may explain perturbed p42C/EBPα/p30C/EBPα ratio often observed in acute myeloid leukemia (AML). We show that overexpression of catalytically inactive E6AP-C843A in C/EBPα inducible K562- p42C/EBPα-estrogen receptor (ER) cells inhibits β-estradiol (E2)-induced C/EBPα degradation leading to enhanced granulocytic differentiation. This enhanced granulocytic differentiation upon E2-induced activation of C/EBPα in C/EBPα stably transfected cells (β-estradiol inducible K562 cells stably expressing p42C/EBPα-ER (K562-C/EBPα-p42-ER)) was further substantiated by siE6AP-mediated knockdown of E6AP in both K562-C/EBPα-p42-ER and 32dcl3 (32D clone 3, a cell line widely used model for in vitro study of hematopoietic cell proliferation, differentiation, and apoptosis) cells. Taken together, our data suggest that E6AP targeted C/EBPα protein degradation may provide a possible explanation for both loss of expression and/or functional inactivation of C/EBPα often experienced in myeloid leukemia.

Repression of transcriptional activity of C/EBPalpha by E2F-dimerization partner complexes

The transcription factor CCAAT/enhancer-binding protein alpha (C/EBPalpha) coordinates proliferation arrest and the differentiation of myeloid progenitors, adipocytes, hepatocytes, keratinocytes, and cells of the lung and placenta. C/EBPalpha transactivates lineage-specific differentiation genes and inhibits proliferation by repressing E2F-regulated genes. The myeloproliferative C/EBPalpha BRM2 mutant serves as a paradigm for recurrent human C-terminal bZIP C/EBPalpha mutations that are involved in acute myeloid leukemogenesis. BRM2 fails to repress E2F and to induce adipogenesis and granulopoiesis. The data presented here show that, independently of pocket proteins, C/EBPalpha interacts with the dimerization partner (DP) of E2F and that C/EBPalpha-E2F/DP interaction prevents both binding of C/EBPalpha to its cognate sites on DNA and transactivation of C/EBP target genes. The BRM2 mutant, in addition, exhibits enhanced interaction with E2F-DP and reduced affinity toward DNA and yet retains transactivation potential and differentiation competence that becomes exposed when E2F/DP levels are low. Our data suggest a tripartite balance between C/EBPalpha, E2F/DP, and pocket proteins in the control of proliferation, differentiation, and tumorigenesis.

Ectopic expression of hC/EBPs in breast tumor cells induces apoptosis

CCAAT/enhancer binding proteins (C/EBPs) are a group of transcription factors which have been implicated in cellular proliferation, terminal differentiation, and apoptosis in a variety of tissues including mammary gland. Owing to its role in various cellular functions, inactivation of C/EBP proteins is central to the pathogenesis of many disorders. Recent reports suggest that expression as well as function of C/EBP proteins is deregulated in breast tumors. Although, role of C/EBPs in growth arrest in mammary tissues has been studied in much detail; their role in apoptosis is relatively less explored. In the present study, we have assessed if breast tumors evade apoptosis and grow faster by down regulating and inhibiting the C/EBP proteins, C/EBPalpha in particular. Our data shows that ectopic expression of human C/EBPs in breast tumor cells inhibits proliferation and induces apoptosis which is likely to be associated with caspase dependent pathway.

Differentiation therapy of leukemia: 3 decades of development

A characteristic feature of leukemia cells is a blockade of differentiation at a distinct stage in cellular maturation. In the 1970s and 1980s, studies demonstrating the capabilities of certain chemicals to induce differentiation of hematopoietic cell lines fostered the concept of treating leukemia by forcing malignant cells to undergo terminal differentiation instead of killing them through cytotoxicity. The first promising reports on this notion prompted a review article on this subject by us 25 years ago. In this review, we revisit this interesting field of study and report the progress achieved in the course of nearly 3 decades. The best proof of principle for differentiation therapy has been the treatment of acute promyelocytic leukemia with all-trans retinoic acid. Attempts to emulate this success with other nuclear hormone ligands such as vitamin D compounds and PPARgamma agonists or different classes of substances such as hematopoietic cytokines or compounds affecting the epigenetic landscape have not been successful on a broad scale. However, a multitude of studies demonstrating partial progress and improvements and, finally, the new powerful possibilities of forward and reverse engineering of differentiation pathways by manipulation of transcription factors support the continued enthusiasm for differentiation therapy of leukemia in the future.