C/EBPalpha or C/EBPalpha oncoproteins regulate the intrinsic and extrinsic apoptotic pathways by direct interaction with NF-kappaB p50 bound to the bcl-2 and FLIP gene promoters

Homodimer-mediated phosphorylation of C/EBPα-p42 S16 modulates acute myeloid leukaemia differentiation through liquid-liquid phase separation

CCAAT/enhancer binding protein α (C/EBPα) regulates myeloid differentiation, and its dysregulation contributes to acute myeloid leukaemia (AML) progress. Clarifying its functional implementation mechanism is of great significance for its further clinical application. Here, we show that C/EBPα regulates AML cell differentiation through liquid-liquid phase separation (LLPS), which can be disrupted by C/EBPα-p30. Considering that C/EBPα-p30 inhibits the functions of C/EBPα through the LZ region, a small peptide TAT-LZ that could instantaneously interfere with the homodimerization of C/EBPα-p42 was constructed, and dynamic inhibition of C/EBPα phase separation was observed, demonstrating the importance of C/EBPα-p42 homodimers for its LLPS. Mechanistically, homodimerization of C/EBPα-p42 mediated its phosphorylation at the novel phosphorylation site S16, which promoted LLPS and subsequent AML cell differentiation. Finally, decreasing the endogenous C/EBPα-p30/C/EBPα-p42 ratio rescued the phase separation of C/EBPα in AML cells, which provided a new insight for the treatment of the AML.

PHF6 maintains acute myeloid leukemia via regulating NF-κB signaling pathway

Acute myeloid leukemia (AML) is a major hematopoietic malignancy characterized by the accumulation of immature and abnormally differentiated myeloid cells in bone marrow. Here with in vivo and in vitro models, we demonstrate that the Plant homeodomain finger gene 6 (PHF6) plays an important role in apoptosis and proliferation in myeloid leukemia. Phf6 deficiency could delay the progression of RUNX1-ETO9a and MLL-AF9-induced AML in mice. PHF6 depletion inhibited the NF-κB signaling pathways by disrupting the PHF6-p50 complex and partially inhibiting the nuclear translocation of p50 to suppress the expression of BCL2. Treating PHF6 over-expressed myeloid leukemia cells with NF-κB inhibitor (BAY11-7082) significantly increased their apoptosis and decreased their proliferation. Taken together, in contrast to PHF6 as a tumor suppressor in T-ALL as reported, we found that PHF6 also plays a pro-oncogenic role in myeloid leukemia, and thus potentially to be a therapeutic target for treating myeloid leukemia patients.

NF-κB: A Druggable Target in Acute Myeloid Leukemia

Simple Summary

AML is a highly heterogeneous hematological disease and is the second most common form of leukemia. Around 40% of AML patients display elevated nuclear NF-κB activity, providing a compelling rationale for targeting the NF-κB pathway in AML. Here we summarize the main drivers of the NF-κB pathway in AML pathogenesis as well as the conventional and novel therapeutic strategies targeting NF-κB to improve the survival of AML patients.

Abstract

Acute Myeloid Leukemia (AML) is an aggressive hematological malignancy that relies on highly heterogeneous cytogenetic alterations. Although in the last few years new agents have been developed for AML treatment, the overall survival prospects for AML patients are still gloomy and new therapeutic options are still urgently needed. Constitutive NF-κB activation has been reported in around 40% of AML patients, where it sustains AML cell survival and chemoresistance. Given the central role of NF-κB in AML, targeting the NF-κB pathway represents an attractive strategy to treat AML. This review focuses on current knowledge of NF-κB’s roles in AML pathogenesis and summarizes the main therapeutic approaches used to treat NF-κB-driven AML.

Bclaf1 regulates c-FLIP expression and protects cells from TNF-induced apoptosis and tissue injury

TNF stimulation generates pro-survival signals through activation of NF-κB that restrict the build-in death signaling triggered by TNF. The competition between TNF-induced survival and death signals ultimately determines the fate of a cell. Here, we report the identification of Bclaf1 as a novel component of the anti-apoptotic program of TNF. Bclaf1 depletion in multiple cells sensitizes cells to TNF-induced apoptosis but not to necroptosis. Bclaf1 exerts its anti-apoptotic function by promoting the transcription of CFLAR, a caspase 8 antagonist, downstream of NF-κB activation. Bclaf1 binds to the p50 subunit of NF-κB, which is required for Bclaf1 to stimulate CFLAR transcription. Finally, in Bclaf1 siRNA administered mice, TNF-induced small intestine injury is much more severe than in control mice with aggravated signs of apoptosis and pyroptosis. These results suggest Bclaf1 is a key regulator in TNF-induced apoptosis, both in vitro and in vivo.

The AML-associated K313 mutation enhances C/EBPα activity by leading to C/EBPα overexpression

Mutations in the transcription factor C/EBPα are found in ~10% of all acute myeloid leukaemia (AML) cases but the contribution of these mutations to leukemogenesis is incompletely understood. We here use a mouse model of granulocyte progenitors expressing conditionally active HoxB8 to assess the cell biological and molecular activity of C/EBPα-mutations associated with human AML. Both N-terminal truncation and C-terminal AML-associated mutations of C/EBPα substantially altered differentiation of progenitors into mature neutrophils in cell culture. Closer analysis of the C/EBPα-K313-duplication showed expansion and prolonged survival of mutant C/EBPα-expressing granulocytes following adoptive transfer into mice. C/EBPα-protein containing the K313-mutation further showed strongly enhanced transcriptional activity compared with the wild-type protein at certain promoters. Analysis of differentially regulated genes in cells overexpressing C/EBPα-K313 indicates a strong correlation with genes regulated by C/EBPα. Analysis of transcription factor enrichment in the differentially regulated genes indicated a strong reliance of SPI1/PU.1, suggesting that despite reduced DNA binding, C/EBPα-K313 is active in regulating target gene expression and acts largely through a network of other transcription factors. Strikingly, the K313 mutation caused strongly elevated expression of C/EBPα-protein, which could also be seen in primary K313 mutated AML blasts, explaining the enhanced C/EBPα activity in K313-expressing cells.

C/EBPα induces Ebf1 gene expression in common lymphoid progenitors

C/EBPα is required for formation of granulocyte-monocyte progenitors (GMP) and also participates in B lymphopoiesis. The common lymphoid progenitor (CLP) and preproB populations but not proB cells express Cebpa, and pan-hematopoietic deletion of the +37 kb Cebpa enhancer using Mx1-Cre leads not only to reduced GMP but also to 2-fold reduced marrow preproB and >15-fold reduced proB and preB cells. We now show that IL7Rα-Cre-mediated deletion of the +37 kb Cebpa enhancer, which occurs in 89% of Ly6D⁺ and 65% of upstream Ly6D^- CLP, leads to a 2-fold reduction of both preproB and proB cells, and a 3-fold reduction in preB cells, with no impact on GMP numbers. These data support a direct role for C/EBPα during B lineage development, with reduced enhancer deletion in Ly6D^- CLP mediated by IL7Rα-Cre diminishing the effect on B lymphopoiesis compared to that seen with Mx1-Cre. Amongst mRNAs encoding key transcriptional regulators that initiate B lymphoid specification (PU.1, E2A, IKAROS, EBF1, FOXO1, and BACH2), only Ebf1 levels are altered in CLP upon Mx1-Cre-mediated Cebpa enhancer deletion, with Ebf1 reduced ~40-fold in Flt3⁺Sca-1^intc-kit^intIL7Rα⁺ CLP. In addition, Cebpa and Ebf1 RNAs were 4- and 14-fold higher in hCD4⁺ versus hCD4^- CLP from Cebpa-hCD4 transgenic mice. Histone modification ChIP-Seq data for CLP indicate the presence of active, intronic Ebf1 enhancers located 270 and 280 kb upstream of the transcription start sites. We identified a cis element in this region that strongly binds C/EBPα using the electrophoretic mobility shift assay. Mutation of this C/EBPα-binding site in an Ebf1 enhancer-TK-luciferase reporter leads to a 4-fold reduction in C/EBPα-mediated trans-activation. These findings support a model of B lymphopoiesis in which induction of Ebf1 by C/EBPα in a subset of CLP contributes to initiation of B lymphopoiesis.

Therapeutic Vulnerabilities of Transcription Factors in AML

Acute myeloid leukemia (AML) is characterized by impaired myeloid lineage differentiation, uncontrolled proliferation, and inhibition of proapoptotic pathways. In spite of a relatively homogeneous clinical disease presentation, risk of long-term survival in AML varies from 20% to 80% depending on molecular disease characteristics. In recognition of the molecular heterogeneity of AML, the European Leukemia Net (ELN) and WHO classification systems now incorporate cytogenetics and increasing numbers of gene mutations into AML prognostication. Several of the genomic AML subsets are characterized by unique transcription factor alterations that are highlighted in this review. There are many mechanisms of transcriptional deregulation in leukemia. We broadly classify transcription factors based on mechanisms of transcriptional deregulation including direct involvement of transcription factors in recurrent translocations, loss-of-function mutations, and intracellular relocalization. Transcription factors, due to their pleiotropic effects, have been attractive but elusive targets. Indirect targeting approaches include inhibition of upstream kinases such as TAK1 for suppression of NFκB signaling and downstream effectors such as FGF signaling in HOXA-upregulated leukemia. Other strategies include targeting scaffolding proteins like BrD4 in the case of MYC or coactivators such as menin to suppress HOX expression; disrupting critical protein interactions in the case of β-catenin:TCF/LEF, and preventing transcription factor binding to DNA as in the case of PU.1 or FOXM1. We comprehensively describe the mechanism of deregulation of transcription factors in genomic subsets of AML, consequent pathway addictions, and potential therapeutic strategies.

Loss of CCAAT-enhancer-binding protein alpha (CEBPA) is linked to poor prognosis in PTEN deleted and TMPRSS2:ERG fusion type prostate cancers

BACKGROUND

The transcription factor CCAAT-enhancer-binding protein alpha (CEBPA) is a crucial regulator of cell proliferation and differentiation. Expression levels of CEBPA have been suggested to be prognostic in various tumor types.

METHODS

Here, we analyzed the immunohistochemical expression of CEBPA in a tissue microarray containing more than 17 000 prostate cancer specimens with annotated clinical and molecular data including for example TMPRSS2:ERG fusion and PTEN deletion status.

RESULTS

Normal prostate glands showed moderate to strong CEBPA staining, while CEBPA expression was frequently reduced (40%) or lost (30%) in prostate cancers. Absence of detectable CEBPA expression was markedly more frequent in ERG negative (45%) as compared to ERG positive cancers (20%, P < 0.0001). Reduced CEBPA expression was linked to unfavorable phenotype (P < 0.0001) and poor prognosis (P = 0.0008). Subgroup analyses revealed, that the prognostic value of CEBPA loss was entirely driven by tumors carrying both TMPRSS2:ERG fusions and PTEN deletions. In this subgroup, CEBPA loss was tightly linked to advanced tumor stage (P < 0.0001), high Gleason grade (P < 0.0001), positive nodal stage (0.0003), and early biochemical recurrence (P = 0.0007), while these associations were absent or markedly diminished in tumors with normal PTEN copy numbers and/or absence of ERG fusion.

CONCLUSIONS

CEBPA is down regulated in about one third of prostate cancers, but the clinical impact of CEBPA loss is strictly limited to the subset of about 10% prostate cancers carrying both ERG fusion and deletions of the PTEN tumor suppressor. Our findings challenge the concept that prognostic molecular markers may be generally applicable to all prostate cancers.

Inhibition of Casein Kinase 2 Disrupts Differentiation of Myeloid Cells in Cancer and Enhances the Efficacy of Immunotherapy in Mice

The role of myeloid cells as regulators of tumor progression that significantly impact the efficacy of cancer immunotherapies makes them an attractive target for inhibition. Here we explore the effect of a novel, potent, and selective inhibitor of serine/threonine protein kinase casein kinase 2 (CK2) on modulating myeloid cells in the tumor microenvironment. Although inhibition of CK2 caused only a modest effect on dendritic cells in tumor-bearing mice, it substantially reduced the amount of polymorphonuclear myeloid-derived suppressor cells and tumor-associated macrophages. This effect was not caused by the induction of apoptosis, but rather by a block of differentiation. Our results implicated downregulation of CCAAT-enhancer binding protein-α in this effect. Although CK2 inhibition did not directly affect tumor cells, it dramatically enhanced the antitumor activity of immune checkpoint receptor blockade using anti-CTLA-4 antibody. These results suggest a potential role of CK2 inhibitors in combination therapies against cancer.Significance: These findings demonstrate the modulatory effects of casein kinase 2 inhibitors on myeloid cell differentiation in the tumor microenvironment, which subsequently synergize with the antitumor effects of checkpoint inhibitor CTLA4. Cancer Res; 78(19); 5644-55. ©2018 AACR.

The significance of c.690G>T polymorphism (rs34529039) and expression of the CEBPA gene in ovarian cancer outcome

The CEBPA gene is known to be mutated or abnormally expressed in several cancers. This is the first study assessing the clinical impact of CEBPA gene status and expression on the ovarian cancer outcome. The CEBPA gene sequence was analyzed in 118 ovarian cancer patients (44 platinum/cyclophosphamide (PC)-treated and 74 taxane/platinum (TP)-treated), both in tumors and blood samples, and in blood from 236 healthy women, using PCR-Sanger sequencing and Real-Time quantitative PCR (qPCR)-based genotyping methods, respectively. The CEBPA mRNA level was examined with Reverse Transcription quantitative PCR (RT-qPCR). The results were correlated to different clinicopathological parameters. Thirty of 118 (25.4%) tumors harbored the CEBPA synonymous c.690G>T polymorphism (rs34529039), that we showed to be related to up-regulation of CEBPA mRNA levels (p=0.0059). The presence of the polymorphism was significantly associated with poor prognosis (p=0.005) and poor response to the PC chemotherapy regimen (p=0.024). In accordance, elevated CEBPA mRNA levels negatively affected patient survival (p<0.001) and tumor response to the PC therapy (p=0.014). The rs34529039 SNP did not affect the risk of developing ovarian cancer. This is the first study providing evidence that the c.690G>T, p.(Thr230Thr) (rs34529039) polymorphism of the CEBPA gene, together with up-regulation of its mRNA expression, are negative factors worsening ovarian cancer outcome. Their adverse clinical effect depends on a therapeutic regimen used, which might make them potential prognostic and predictive biomarkers for response to DNA-damaging chemotherapy.

Long non-coding RNA CEBPA-AS1 correlates with poor prognosis and promotes tumorigenesis via CEBPA/Bcl2 in oral squamous cell carcinoma

Oral squamous cell carcinoma (OSCC) is one of the most aggressive and lethal malignancies affecting the head and neck region with a general 5-year survival rate about 50%. Long non-coding RNAs (lncRNAs) are believed to participate in diverse biological processes and are emerging as convenient and minimally invasive diagnostic/prognostic/therapeutic markers. The aim of this study was to explore CEBPA-AS1 role and mechanism in OSCC tumorigenesis. In this study, CEBPA-AS1 localized in the cytoplasm and the peri-nuclear cellular compartment functioning as a potential oncogene up-regulated in OSCC was correlated with poor differentiation, lymph node metastasis and high clinical stage, which made it considered to be a prognostic biomarker. Silence of CEBPA-AS1 inhibited OSCC cells proliferation and induced cells apoptosis, migration and invasion by targeting CEBPA and via a novel pathway CEBPA/Bcl2. Our findings provided the first evidence for the lncRNA CEBPA-AS1 regulatory network in OSCC tumorigenesis, which might be helpful to improve the effects of clinical treatment in OSCC.

NF-κB in Hematological Malignancies

NF-κB (Nuclear Factor Κ-light-chain-enhancer of activated B cells) transcription factors are critical regulators of immunity, stress response, apoptosis, and differentiation. Molecular defects promoting the constitutive activation of canonical and non-canonical NF-κB signaling pathways contribute to many diseases, including cancer, diabetes, chronic inflammation, and autoimmunity. In the present review, we focus our attention on the mechanisms of NF-κB deregulation in hematological malignancies. Key positive regulators of NF-κB signaling can act as oncogenes that are often prone to chromosomal translocation, amplifications, or activating mutations. Negative regulators of NF-κB have tumor suppressor functions, and are frequently inactivated either by genomic deletions or point mutations. NF-κB activation in tumoral cells is also driven by the microenvironment or chronic signaling that does not rely on genetic alterations.

In Vivo Deletion of the Cebpa +37 kb Enhancer Markedly Reduces Cebpa mRNA in Myeloid Progenitors but Not in Non-Hematopoietic Tissues to Impair Granulopoiesis

The murine Cebpa gene contains a +37 kb, evolutionarily conserved 440 bp enhancer that directs high-level expression to myeloid progenitors in transgenic mice. The enhancer is bound and activated by Runx1, Scl, GATA2, C/EBPα, c-Myb, Pu.1, and additional Ets factors in myeloid cells. CRISPR/Cas9-mediated replacement of the wild-type enhancer with a variant mutant in its seven Ets sites leads to 20-fold reduction of Cebpa mRNA in the 32Dcl3 myeloid cell line. To determine the effect of deleting the enhancer in vivo, we now characterize C57BL/6 mice in which loxP sites flank a 688 bp DNA segment containing the enhancer. CMV-Cre mediated germline deletion resulted in diminution of the expected number of viable Enh(f/f);CMV-Cre offspring, with 28-fold reduction in marrow Cebpa mRNA but normal levels in liver, lung, adipose, intestine, muscle, and kidney. Cre-transduction of lineage-negative marrow cells in vitro reduced Cebpa mRNA 12-fold, with impairment of granulocytic maturation, morphologic blast accumulation, and IL-3 dependent myeloid colony replating for >12 generations. Exposure of Enh(f/f);Mx1-Cre mice to pIpC led to 14-fold reduction of Cebpa mRNA in GMP or CMP, 30-fold reduction in LSK, and <2-fold reduction in the LSK/SLAM subset. FACS analysis of marrow from these mice revealed 10-fold reduced neutrophils, 3-fold decreased GMP, and 3-fold increased LSK cells. Progenitor cell cycle progression was mildly impaired. Granulocyte and B lymphoid colony forming units were reduced while monocytic and erythroid colonies were increased, with reduced Pu.1 and Gfi1 and increased Egr1 and Klf4 in GMP. Finally, competitive transplantation indicated preservation of functional long-term hematopoietic stem cells upon enhancer deletion and confirmed marrow-intrinsic impairment of granulopoiesis and B cell generation with LSK and monocyte lineage expansion. These findings demonstrate a critical role for the +37 kb Cebpa enhancer for hematopoietic-specific Cebpa expression, with enhancer deletion leading to impaired myelopoiesis and potentially preleukemic progenitor expansion.

Aberrant nuclear factor-kappa B activity in acute myeloid leukemia: from molecular pathogenesis to therapeutic target

The overall survival of patients with acute myeloid leukemia (AML) has not been improved significantly over the last decade. Molecularly targeted agents hold promise to change the therapeutic landscape in AML. The nuclear factor kappa B (NF-κB) controls a plethora of biological process through switching on and off its long list of target genes. In AML, constitutive NF-κB has been detected in 40% of cases and its aberrant activity enable leukemia cells to evade apoptosis and stimulate proliferation. These facts suggest that NF-κB signaling pathway plays a fundamental role in the development of AML and it represents an attractive target for the intervention of AML. This review summarizes our current knowledge of NF-κB signaling transduction including canonical and non-canonical NF-κB pathways. Then we specifically highlight what factors contribute to the aberrant activation of NF-κB activity in AML, followed by an overview of 8 important clinical trials of the first FDA approved proteasome inhibitor, Bortezomib (Velcade), which is a NF-κB inhibitor too, in combination with other therapeutic agents in patients with AML. Finally, this review discusses the future directions of NF-κB inhibitor in treatment of AML, especially in targeting leukemia stem cells (LSCs).

C/EBPα in normal and malignant myelopoiesis

CCAAT/enhancer binding protein α (C/EBPα) dimerizes via its leucine zipper (LZ) domain to bind DNA via its basic region and activate transcription via N-terminal trans-activation domains. The activity of C/EBPα is modulated by several serine/threonine kinases and via sumoylation, its gene is activated by RUNX1 and additional transcription factors, its mRNA stability is modified by miRNAs, and its mRNA is subject to translation control that affects AUG selection. In addition to inducing differentiation, C/EBPα inhibits cell cycle progression and apoptosis. Within hematopoiesis, C/EBPα levels increase as long-term stem cells progress to granulocyte-monocyte progenitors (GMP). Absence of C/EBPα prevents GMP formation, and higher levels are required for granulopoiesis compared to monopoiesis. C/EBPα interacts with AP-1 proteins to bind hybrid DNA elements during monopoiesis, and induction of Gfi-1, C/EBPε, KLF5, and miR-223 by C/EBPα enables granulopoiesis. The CEBPA ORF is mutated in approximately 10 % of acute myeloid leukemias (AML), leading to expression of N-terminally truncated C/EBPαp30 and C-terminal, in-frame C/EBPαLZ variants, which inhibit C/EBPα activities but also play additional roles during myeloid transformation. RUNX1 mutation, CEBPA promoter methylation, Trib1 or Trib2-mediated C/EBPαp42 degradation, and signaling pathways leading to C/EBPα serine 21 phosphorylation reduce C/EBPα expression or activity in additional AML cases.

CCAAT/enhancer binding protein α predicts poorer prognosis and prevents energy starvation-induced cell death in hepatocellular carcinoma

CCAAT enhancer binding protein α (C/EBPα) plays an essential role in cellular differentiation, growth, and energy metabolism. Here, we investigate the correlation between C/EBPα and hepatocellular carcinoma (HCC) patient outcomes and how C/EBPα protects cells against energy starvation. Expression of C/EBPα protein was increased in the majority of HCCs examined (191 pairs) compared with adjacent nontumor liver tissues in HCC tissue microarrays. Its upregulation was correlated significantly with poorer overall patient survival in both Kaplan-Meier survival (P=0.017) and multivariate Cox regression (P=0.028) analyses. Stable C/EBPα-silenced cells failed to establish xenograft tumors in nude mice due to extensive necrosis, consistent with increased necrosis in human C/EBPα-deficient HCC nodules. Expression of C/EBPα protected HCC cells in vitro from glucose and glutamine starvation-induced cell death through autophagy-involved lipid catabolism. Firstly, C/EBPα promoted lipid catabolism during starvation, while inhibition of fatty acid beta-oxidation significantly sensitized cell death. Secondly, autophagy was activated in C/EBPα-expressing cells, and the inhibition of autophagy by ATG7 knockdown or chloroquine treatment attenuated lipid catabolism and subsequently sensitized cell death. Finally, we identified TMEM166 as a key player in C/EBPα-mediated autophagy induction and protection against starvation.

CONCLUSION

The C/EBPα gene is important in that it links HCC carcinogenesis to autophagy-mediated lipid metabolism and resistance to energy starvation; its expression in HCC predicts poorer patient prognosis.

HTLV-1 bZIP factor supports proliferation of adult T cell leukemia cells through suppression of C/EBPα signaling

BACKGROUND

Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus etiologically associated with adult T-cell leukemia (ATL). The HTLV-1 bZIP factor (HBZ), which is encoded by minus strand of provirus, is expressed in all ATL cases and supports the proliferation of ATL cells. However, the precise mechanism of growth promoting activity of HBZ is poorly understood.

RESULTS

In this study, we showed that HBZ suppressed C/EBPα signaling activation induced by either Tax or C/EBPα. As mechanisms of HBZ-mediated C/EBPα inhibition, we found that HBZ physically interacted with C/EBPα and diminished its DNA binding capacity. Luciferase and immunoprecipitation assays revealed that HBZ repressed C/EBPα activation in a Smad3-dependent manner. In addition, C/EBPα was overexpressed in HTLV-1 infected cell lines and fresh ATL cases. HBZ was able to induce C/EBPα transcription by enhancing its promoter activity. Finally, HBZ selectively modulated the expression of C/EBPα target genes, leading to the impairment of C/EBPα-mediated cell growth suppression.

CONCLUSION

HBZ, by suppressing C/EBPα signaling, supports the proliferation of HTLV-1 infected cells, which is thought to be critical for oncogenesis.

Runx1 deletion or dominant inhibition reduces Cebpa transcription via conserved promoter and distal enhancer sites to favor monopoiesis over granulopoiesis

Deletion of Runx1 in adult mice produces a myeloproliferative phenotype. We now find that Runx1 gene deletion increases marrow monocyte while reducing granulocyte progenitors and that exogenous RUNX1 rescues granulopoiesis. Deletion of Runx1 reduces Cebpa mRNA in lineage-negative marrow cells and in granulocyte-monocyte progenitors or common myeloid progenitors. Pu.1 mRNA is also decreased, but to a lesser extent. We also transduced marrow with dominant-inhibitory RUNX1a. As with Runx1 gene deletion, RUNX1a expands lineage-Sca-1+c-kit+ and myeloid cells, increased monocyte CFUs relative to granulocyte CFUs, and reduced Cebpa mRNA. Runx1 binds a conserved site in the Cebpa promoter and binds 4 sites in a conserved 450-bp region located at +37 kb; mutation of the enhancer sites reduces activity 6-fold in 32Dcl3 myeloid cells. Endogenous Runx1 binds the promoter and putative +37 kb enhancer as assessed by ChIP, and RUNX1-ER rapidly induces Cebpa mRNA in these cells, even in cycloheximide, consistent with direct gene regulation. The +37 kb region contains strong H3K4me1 histone modification and p300-binding, as often seen with enhancers. Finally, exogenous C/EBPα increases granulocyte relative to monocyte progenitors in Runx1-deleted marrow cells. Diminished CEBPA transcription and consequent impairment of myeloid differentiation may contribute to leukemic transformation in acute myeloid leukemia cases associated with decreased RUNX1 activity.

C/EBPα dysregulation in AML and ALL

The transcription factor CCAAT/enhancer binding protein a (C/EBPα) is a critical regulator of myeloid development, directing granulocyte, and monocyte differentiation. As such, it is dysregulated in more than half of patients with acute myeloid leukemia (AML). C/EBPα expression is suppressed as result of common leukemia-associated genetic and epigenetic alterations such as AML1-ETO, BCR-ABL, FLT3-ITD, or CEBPA promoter methylation. In addition, 10-15% of patients with AML with intermediate risk cytogenetics are characterized by mutations of the CEBPA gene. Two classes of mutations are described. N-terminal changes result in expression of a truncated dominant negative C/EBPαp30 isoform. C-terminal mutations are in-frame insertions or deletions resulting in alteration of the leucine zipper preventing dimerization and DNA binding. Often, patients carry both N- and C-terminal mutations each affecting a different allele, and a mouse model recapitulates the human phenotype. Patients with mutated CEBPA AML comprise a clinically distinct group with favorable outcome consistently seen in patients with biallelic mutations. In addition, C/EBP family members are aberrantly expressing from the immunoglobulin heavy chain locus in 2% of pre-B ALLs. This review summarizes the normal hematopoietic developmental pathways regulated by C/EBPα and discusses the molecular pathways involved in mutated CEBPA AML and ALL.

Overexpression of wild-type or mutants forms of CEBPA alter normal human hematopoiesis

C/EBPα (CEBPA) is mutated in approximately 8 % of AML in both familial and sporadic AML and, with FLT3 and NPM1, has received most attention as a predictive marker of outcome in patients with normal karyotype disease. Mutations clustering to either the N- or C-terminal (N-and C-ter) portions of the protein have different consequences on the protein function. In familial cases the N-ter form is inherited with patients exhibiting long latency period before the onset of overt disease, typically with the acquisition of a C-ter mutation. Despite the essential insights murine models provide the functional consequences of wild-type C/EBPα in human hematopoiesis and how different mutations are involved in AML development have received less attention. Our data underline the critical role of C/EBPα in human hematopoiesis and demonstrate that C/EBPα mutations (alone or in combination) are insufficient to convert normal human hematopoietic stem/progenitors (HSC/HPCs) into leukemic initiating cells, although individually each altered normal hematopoiesis. It provides the first insight into the effects of N- and C-terminal mutations acting alone and to the combined effects of N/C double mutants. Our results mimicked closely what happens in CEBPA mutated patients.

C/EBPα, C/EBPα oncoproteins, or C/EBPβ preferentially bind NF-κB p50 compared with p65, focusing therapeutic targeting on the C/EBP:p50 interaction

Canonical nuclear factor kappaB (NF-κB) activation signals stimulate nuclear translocation of p50:p65, replacing inhibitory p50:p50 with activating complexes on chromatin. C/EBP interaction with p50 homodimers provides an alternative pathway for NF-κB target gene activation, and interaction with p50:p65 may enhance gene activation. We previously found that C/EBPα cooperates with p50, but not p65, to induce Bcl-2 transcription and that C/EBPα induces Nfkb1/p50, but not RelA/p65, transcription. Using p50 and p65 variants containing the FLAG epitope at their N- or C-termini, we now show that C/EBPα, C/EBPα myeloid oncoproteins, or the LAP1, LAP2, or LIP isoforms of C/EBPβ have markedly higher affinity for p50 than for p65. Deletion of the p65 transactivation domain did not increase p65 affinity for C/EBPs, suggesting that unique residues in p50 account for specificity, and clustered mutation of HSDL in the "p50 insert" lacking in p65 weakens interaction. Also, in contrast to Nfkb1 gene deletion, absence of the RelA gene does not reduce Bcl-2 or Cebpa RNA in unstimulated cells or prevent interaction of C/EBPα with the Bcl-2 promoter. Saturating mutagenesis of the C/EBPα basic region identifies R300 and nearby residues, identical in C/EBPβ, as critical for interaction with p50. These findings support the conclusion that C/EBPs activate NF-κB target genes via contact with p50 even in the absence of canonical NF-κB activation and indicate that targeting C/EBP:p50 rather than C/EBP:p65 interaction in the nucleus will prove effective for inflammatory or malignant conditions, alone or synergistically with agents acting in the cytoplasm to reduce canonical NF-κB activation.

SHP2 tyrosine phosphatase stimulates CEBPA gene expression to mediate cytokine-dependent granulopoiesis

G-CSF signals contribute to granulocyte lineage specification. We previously found that G-CSF induces SHP2 tyrosine phosphorylation and that chemical inhibition of SHP1/SHP2 reduces CFU-G and prevents G-CSF but not M-CSF activation of ERK. We now find that SHP2 shRNA knockdown in the 32Dcl3 granulocytic line reduces ERK activation, diminishes CEBPA protein and RNA expression and promoter histone acetylation, and inhibits granulopoiesis. Exogenous, shRNA-resistant SHP2 rescues these effects of SHP2 knockdown, exogenous C/EBPα rescues granulocytic markers, and exogenous RUNX1 rescues C/EBPα. 32Dcl3 lines with knockdown of ERK1 and ERK2 retain normal levels of C/EBPα and differentiate normally in G-CSF despite also having reduced proliferation. SHP2 knockdown reduces CEBPA levels in lineage-negative murine marrow cells cultured in TPO, Flt3 ligand, and SCF, without affecting the rate of cell expansion. On transfer to IL-3, IL-6, and SCF to induce myelopoiesis, levels of granulocytic RNAs are reduced and monocyte-specific RNAs are increased by SHP2 knockdown, and there is a reduction in the percentage of CFU-G that form in methylcellulose and of granulocytes that develop in liquid culture. In summary, SHP2 is required for induction of C/EBPα expression and granulopoiesis in response to G-CSF or other cytokines independent of SHP2-mediated ERK activation.

AP-1 protein induction during monopoiesis favors C/EBP: AP-1 heterodimers over C/EBP homodimerization and stimulates FosB transcription

AP-1 proteins heterodimerize via their LZ domains to bind TGACGTCA or TGACTCA, whereas C/EBPs dimerize to bind ATTGCGCAAT. We demonstrate that intact C/EBPα also heterodimerizes with c-Jun or c-Fos to bind a hybrid DNA element, TGACGCAA, or more weakly to TGATGCAA. A 2:1 ratio of c-Jun:C/EBPα or c-Fos:C/EBPα was sufficient for preferential binding. Semiquantitative Western blot analysis indicates that the summation of c-Jun, JunB, and c-Fos levels in differentiating myeloid cells is similar to or exceeds the entirety of C/EBPα and C/EBPβ, indicating the feasibility of heterodimer formation. Induction of AP-1 proteins during monocytic differentiation favored formation of C/EBP:AP-1 heterodimers, with C/EBPα homodimers more evident during granulopoiesis. Approximately 350 human and 300 murine genes contain the TGACGCAA motif between -2 kb and +1 kb of their transcription start sites. We focused on the murine Fosb promoter, which contains a C/EBP:AP-1 cis element at -56 and -253, with the hFOSB gene containing an identical site at -253 and a 1-bp mismatch at -56. C/EBPα:AP-1 heterodimers bound either site preferentially in a gel-shift assay, C/EBPα:c-Fos ER fusion proteins induced endogenous Fosb mRNA but not in the presence of CHX, C/EBP and AP-1 proteins bound the endogenous Fosb promoter, mutation of the -56 cis element reduced reporter activity fivefold, and endogenous FosB protein was expressed preferentially during monopoiesis versus granulopoiesis. Increased expression of Jun/Fos proteins elevates C/EBP:AP-1 heterodimer formation to potentially activate novel sets of genes during monopoiesis and potentially during other biologic processes.

C/EBPα and C/EBPα oncoproteins regulate nfkb1 and displace histone deacetylases from NF-κB p50 homodimers to induce NF-κB target genes

Mutated CEBPA defines a subgroup of acute myeloid leukemia (AML). We have previously shown that C/EBPα or its AML mutants synergize with NF-κB p50 to activate antiapoptotic genes, including BCL2 and FLIP. Furthermore, p50 binds and activates the CEBPA gene in myeloid cells. We now report that C/EBPα or C/EBPα leucine zipper AML mutants bind in vivo to the nfkb1 (p50) promoter and induce its expression even in the presence of cycloheximide. Induction of p50 by C/EBPα depends on 2 conserved κB sites in the nfkb1 promoter. C/EBPα did not induce p65 expression. Thus, C/EBPα and p50 reciprocally regulate each other's expression, establishing a positive feedback relationship. Although p50 homodimers inhibit transcription, C/EBPα and p50 synergistically activate antiapoptotic genes. ChIP analysis showed that C/EBPα diminishes the occupation of histone deacetylase 1 (HDAC1) or HDAC3 on the endogenous FLIP promoter but not in mice lacking p50. Coimmunoprecipitation confirmed that C/EBPα, its AML variants, or C/EBPβ disrupt interaction between p50 and HDACs dependent on the C/EBP basic region. These findings suggest that C/EBPs displace HDACs from p50 homodimers bound to antiapoptotic genes, contributing to NF-κB dysregulation in leukemia, and that the C/EBPα:p50 complex is a potential therapeutic target.

Bortezomib induces nuclear translocation of IκBα resulting in gene-specific suppression of NF-κB--dependent transcription and induction of apoptosis in CTCL

Cutaneous T-cell lymphoma (CTCL) is characterized by constitutive activation of nuclear factor κB (NF-κB), which plays a crucial role in the survival of CTCL cells and their resistance to apoptosis. NF-κB activity in CTCL is inhibited by the proteasome inhibitor bortezomib; however, the mechanisms remained unknown. In this study, we investigated mechanisms by which bortezomib suppresses NF-κB activity in CTCL Hut-78 cells. We demonstrate that bortezomib and MG132 suppress NF-κB activity in Hut-78 cells by a novel mechanism that consists of inducing nuclear translocation and accumulation of IκBα (nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha), which then associates with NF-κB p65 and p50 in the nucleus and inhibits NF-κB DNA binding activity. Surprisingly, however, while expression of NF-κB-dependent antiapoptotic genes cIAP1 and cIAP2 is inhibited by bortezomib, expression of Bcl-2 is not suppressed. Chromatin immunoprecipitation indicated that cIAP1 and cIAP2 promoters are occupied by NF-κB p65/50 heterodimers, whereas Bcl-2 promoter is occupied predominantly by p50/50 homodimers. Collectively, our data reveal a novel mechanism of bortezomib function in CTCL and suggest that the inhibition of NF-κB-dependent gene expression by bortezomib is gene specific and depends on the subunit composition of NF-κB dimers recruited to NF-κB-responsive promoters.

Bcl3 prevents acute inflammatory lung injury in mice by restraining emergency granulopoiesis

Granulocytes are pivotal regulators of tissue injury. However, the transcriptional mechanisms that regulate granulopoiesis under inflammatory conditions are poorly understood. Here we show that the transcriptional coregulator B cell leukemia/lymphoma 3 (Bcl3) limits granulopoiesis under emergency (i.e., inflammatory) conditions, but not homeostatic conditions. Treatment of mouse myeloid progenitors with G-CSF--serum concentrations of which rise under inflammatory conditions--rapidly increased Bcl3 transcript accumulation in a STAT3-dependent manner. Bcl3-deficient myeloid progenitors demonstrated an enhanced capacity to proliferate and differentiate into granulocytes following G-CSF stimulation, whereas the accumulation of Bcl3 protein attenuated granulopoiesis in an NF-κB p50-dependent manner. In a clinically relevant model of transplant-mediated lung ischemia reperfusion injury, expression of Bcl3 in recipients inhibited emergency granulopoiesis and limited acute graft damage. These data demonstrate a critical role for Bcl3 in regulating emergency granulopoiesis and suggest that targeting the differentiation of myeloid progenitors may be a therapeutic strategy for preventing inflammatory lung injury.

Involvement of NF-kappaB activation in the apoptosis induced by extracellular adenosine in human hepatocellular carcinoma HepG2 cells

Adenosine can exhibit cytotoxic activity in vivo and in vitro, though its mechanisms are still uncertain. In this study, we investigated the adenosine-mediated apoptotic signaling pathway and the role of NF-kappaB in human hepatocellular carcinoma HepG2 cells. HepG2 cells were treated with different concentrations of adenosine for 12-48 h, and the effect of adenosine on cell proliferation was evaluated by MTT assay. The cytotoxicity of adenosine alone or in combination with an NF-kappaB inhibitor, pyrrolidine dithiocarbamate (PDTC), was also evaluated by MTT assay and the mode of cell death was detected by Hoechst 33342 staining. Cell cycle progress was performed by flow cytometry with PI staining. The protein expressions of Bcl-2, p53, NF-kappaB subunit p65, and caspase-3 were assayed by Western blot. Caspase-3 activity was measured by spectrophotomteric assay. The results showed that adenosine significantly reduced the viability of HepG2 cells in a dose- and time-dependent manner, with IC 50 (24 and 48 h) of 2.52 and 1.89 mmol x L(-1), respectively. The apoptotic index (percentage of sub-G1 phase) of HepG2 cells in adenosine treatment alone for 12 and 24 h or in combination with PDTC were 8.30%, 22.32% and 20.18%, 30.89%, respectively. All of them were higher than that in the control group (0.81%, p < 0.01). The characteristic changes of cell apoptosis (chromatin condensation and sub-G1 peak) were observed under fluorescent microscopy and flow cytometry. We also found that the apoptotic process triggered by adenosine was involved in G0-G1 cell-cycle arrest, enhanced the activity of caspase-3, upregulated p53 and NF-kappaB p65 expression, and downregulated Bcl-2 expression. Inhibition of NF-kappaB by PDTC decreased NF-kappaB p65 expression, enhanced cell apoptosis ratio, and increased caspase-3 activity. NF-kappaB may play an anti-apoptosis role in adenosine-induced HepG2 cytotoxicity.

NF-kappa B p50 regulates C/EBP alpha expression and inflammatory cytokine-induced neutrophil production

NF-kappaB is a key transcriptional inducer of the inflammatory response in mature myeloid cells, and also stimulates cell survival, but its role in immature myeloid cell development has not been well characterized. C/EBPalpha is required for the development of monocytic and granulocytic myeloid cells from early progenitors, and NF-kappaB and C/EBPbeta cooperatively induce several inflammatory mediators. Having found that C/EBPalpha binds NF-kappaB p50 preferentially compared with NF-kappaB p65, we have now investigated myelopoiesis in nfkb1(-/-) mice lacking NF-kappaB p50. Absence of p50 leads to a significant reduction in the number of granulocytic progenitors, CFU-granulocyte, obtained with G-CSF or GM-CSF in vitro and reduces neutrophil production in vivo in response to G-CSF, with preservation of monopoiesis in vitro in response to cytokines or LPS. To gain insight into the mechanism underlying reduced granulopoiesis in the absence of NF-kappaB p50, we assessed the expression of several myeloid regulatory proteins in lineage-negative, immature myeloid cells. Although PU.1, C/EBPbeta, and STAT3 levels were unchanged, C/EBPalpha protein and RNA levels were reduced approximately 3-fold in the absence of NF-kappaB p50. In addition, NF-kappaB p50 and C/EBPalpha bound the endogenous C/EBPalpha promoter in a chromatin immunoprecipitation assay, and NF-kappaB p50 trans activated the C/EBPalpha promoter, alone or in cooperation with C/EBPalpha. Despite reduction of C/EBPalpha, G-CSFR and M-CSFR levels were maintained in total marrow and in lineage-negative cells. Together, these data indicate that acute inflammation not only activates mature myeloid cells, but also stimulates neutrophil production via NF-kappaB p50 induction of C/EBPalpha transcription.