ICER expression inhibits leukemia phenotype and controls tumor progression

The role and application of transcriptional repressors in cancer treatment

Gene expression is modulated through the integration of many regulatory elements and their associated transcription factors (TFs). TFs bind to specific DNA sequences and either activate or repress transcriptional activity. Through decades of research, it has been established that aberrant expression or functional abnormalities of TFs can lead to uncontrolled cell division and the development of cancer. Initial studies on transcriptional regulation in cancer have focused on TFs as transcriptional activators. However, recent studies have demonstrated several different mechanisms of transcriptional repression in cancer, which could be potential therapeutic targets for the development of specific anti-cancer agents. In the first section of this review, "Emerging roles of transcriptional repressors in cancer development," we summarize the current understanding of transcriptional repressors and their involvement in the molecular processes of cancer progression. In the subsequent section, "Therapeutic applications," we provide an updated overview of the available therapeutic targets for drug discovery and discuss the new frontier of such applications.

Impact of Histone Modifications and Their Therapeutic Targeting in Hematological Malignancies

Hematologic malignancies are a large and heterogeneous group of neoplasms characterized by complex pathogenetic mechanisms. The abnormal regulation of epigenetic mechanisms and specifically, histone modifications, has been demonstrated to play a central role in hematological cancer pathogenesis and progression. A variety of epigenetic enzymes that affect the state of histones have been detected as deregulated, being either over- or underexpressed, which induces changes in chromatin compaction and, subsequently, affects gene expression. Recent advances in the field of epigenetics have revealed novel therapeutic targets, with many epigenetic drugs being investigated in clinical trials. The present review focuses on the biological impact of histone modifications in the pathogenesis of hematologic malignancies, describing a wide range of therapeutic agents that have been discovered to target these alterations and are currently under investigation in clinical trials.

Role of proton-activated G protein-coupled receptors in pathophysiology

Local acidification is a common feature of many disease processes such as inflammation, infarction, or solid tumor growth. Acidic pH is not merely a sequelae of disease but contributes to recruitment and regulation of immune cells, modifies metabolism of parenchymal, immune and tumor cells, modulates fibrosis, vascular permeability, oxygen availability and consumption, invasiveness of tumor cells, and impacts on cell survival. Thus, multiple pH-sensing mechanisms must exist in cells involved in these processes. These pH-sensors play important roles in normal physiology and pathophysiology, and hence might be attractive targets for pharmacological interventions. Among the pH-sensing mechanisms, OGR1 (GPR68), GPR4 (GPR4), and TDAG8 (GPR65) have emerged as important molecules. These G protein-coupled receptors are widely expressed, are upregulated in inflammation and tumors, sense changes in extracellular pH in the range between pH 8 and 6, and are involved in modulating key processes in inflammation, tumor biology, and fibrosis. This review discusses key features of these receptors and highlights important disease states and pathways affected by their activity.

The long non-coding RNA CDK6-AS1 overexpression impacts on acute myeloid leukemia differentiation and mitochondrial dynamics

Patients with acute myeloid leukemia (AML) carrying high-risk genetic lesions or high residual disease levels after therapy are particularly exposed to the risk of relapse. Here, we identified the long non-coding RNA CDK6-AS1 able to cluster an AML subgroup with peculiar gene signatures linked to hematopoietic cell differentiation and mitochondrial dynamics. CDK6-AS1 silencing triggered hematopoietic commitment in healthy CD34+ cells, whereas in AML cells the pathological undifferentiated state was rescued. This latter phenomenon derived from RUNX1 transcriptional control, responsible for the stemness of hematopoietic precursors and for the block of differentiation in AML. By CDK6-AS1 silencing in vitro, AML mitochondrial mass decreased with augmented pharmacological sensitivity to mitochondria-targeting drugs. In vivo, the combination of tigecycline and cytarabine reduced leukemia progression in the AML-PDX model with high CDK6-AS1 levels, supporting the concept of a mitochondrial vulnerability. Together, these findings uncover CDK6-AS1 as crucial in myeloid differentiation and mitochondrial mass regulation.

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CDK6-AS1 acts in concert with CDK6

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High CDK6-AS1 levels trigger RUNX1 early differentiation arrest in myeloid cells

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CDK6-AS1 controls mitochondrial mass of AML blasts

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CDK6-AS1 levels impact on mitochondrial-targeted agents sensitivity

Drug repurposing for targeting cyclic nucleotide transporters in acute leukemias - A missed opportunity

While current treatment regimens for acute leukemia can dramatically improve patient survival, there remains room for improvement. Due to its roles in cell differentiation, cell survival, and apoptotic signaling, modulation of the cyclic AMP (cAMP) pathway has provided a meaningful target in hematological malignancies. Several studies have demonstrated that gene expression profiles associated with increased pro-survival cAMP activity or downregulation of various pro-apoptotic factors associated with the cAMP pathway are apparent in acute leukemia patients. Previous work to increase leukemia cell intracellular cAMP focused on the use of cAMP analogs, stimulating cAMP production via transmembrane-associated adenylyl cyclases, or decreasing cAMP degradation by inhibiting phosphodiesterase activity. However, targeting cyclic nucleotide efflux by ATP-binding cassette (ABC) transporters represents an unexplored approach for modulation of intracellular cyclic nucleotide levels. Preliminary studies have shown that inhibition of cAMP efflux can stimulate leukemia cell differentiation, cell growth arrest, and apoptosis, indicating that targeting cAMP efflux may show promise for future therapeutic development. Furthermore, inhibition of cyclic nucleotide transporter activity may also contribute multiple anticancer benefits by reducing extracellular pro-survival signaling in malignant cells. Hence, several opportunities for drug repurposing may exist for targeting cyclic nucleotide transporters.

Control of CREB expression in tumors: from molecular mechanisms and signal transduction pathways to therapeutic target

The cyclic AMP response element binding (CREB) protein has pleiotropic activities in physiologic processes. Due to its central position downstream of many growth signaling pathways CREB has the ability to influence cell survival, growth and differentiation of normal, but also of tumor cells suggesting an oncogenic potential of CREB. Indeed, increased CREB expression and activation is associated with tumor progression, chemotherapy resistance and reduced patients' survival. We summarize here the different cellular functions of CREB in tumors of distinct histology as well as its use as potential prognostic marker. In addition, the underlying molecular mechanisms to achieve constitutive activation of CREB including structural alterations, such as gene amplification and chromosomal translocation, and deregulation, which could occur at the transcriptional, post-transcriptional and post-translational level, will be described. Since downregulation of CREB by different strategies resulted in inhibition of cell proliferation, invasion and induction of apoptosis, the role of CREB as a promising target for cancer therapy will be also discussed.

CREB engages C/EBPδ to initiate leukemogenesis

cAMP response element binding protein (CREB) is frequently overexpressed in acute myeloid leukemia (AML) and acts as a proto-oncogene; however, it is still debated whether such overactivation alone is able to induce leukemia as its pathogenetic downstream signaling is still unclear. We generated a zebrafish model overexpressing CREB in the myeloid lineage, which showed an aberrant regulation of primitive hematopoiesis, and in 79% of adult CREB-zebrafish a block of myeloid differentiation, triggering to a monocytic leukemia akin the human counterpart. Gene expression analysis of CREB-zebrafish revealed a signature of 20 differentially expressed human homologous CREB targets in common with pediatric AML. Among them, we demonstrated that CREB overexpression increased CCAAT-enhancer-binding protein-δ (C/EBPδ) levels to cause myeloid differentiation arrest, and the silencing of CREB-C/EBPδ axis restored myeloid terminal differentiation. Then, C/EBPδ overexpression was found to identify a subset of pediatric AML affected by a block of myeloid differentiation at monocytic stage who presented a significant higher relapse risk and the enrichment of aggressive signatures. Finally, this study unveils the aberrant activation of CREB-C/EBPδ axis concurring to AML onset by disrupting the myeloid cell differentiation process. We provide a novel in vivo model to perform high-throughput drug screening for AML cure improvement.

Histone acetylation: novel target for the treatment of acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) has been generally considered a genetic disease (disorder) with an aggressive tumor entity of highly proliferative malignant lymphoid cells. However, in recent years, significant advances have been made in the elucidation of the ALL-associated processes. Thus, we understand that histone acetylation is involved in the permanent changes of gene expression controlling ALL developmental outcomes. In this article, we will focus on histone acetylation associated with ALL, their implications as biomarkers for prognostic, and their preclinical and clinical applications.

Specific genomic and transcriptomic aberrations in tumors induced by partial hepatectomy of a chronically inflamed murine liver

Resection of hepatocellular carcinoma (HCC) tumors by partial hepatectomy (PHx) is associated with promoting hepatocarcinogenesis. We have previously reported that PHx promotes hepatocarcinogenesis in the Mdr2-knockout (Mdr2-KO) mouse, a model for inflammation-mediated HCC. Now, to explore the molecular mechanisms underlying the tumor-promoting effect of PHx, we compared genomic and transcriptomic profiles of HCC tumors developing in the Mdr2-KO mice either spontaneously or following PHx. PHx accelerated HCC development in these mice by four months. PHx-induced tumors had major chromosomal aberrations: all were amplifications affecting multiple chromosomes. Most of these amplifications were located near the acrocentric centromeres of murine chromosomes. Four different chromosomal regions were amplified each in at least three tumors. The human orthologs of these common amplified regions are known to be amplified in HCC. All tumors of untreated mice had chromosomal aberrations, including both deletions and amplifications. Amplifications in spontaneous tumors affected fewer chromosomes and were not located preferentially at the chromosomal edges. Comparison of gene expression profiles revealed a significantly enriched expression of oncogenes, chromosomal instability markers and E2F1 targets in the post-PHx compared to spontaneous tumors. Both tumor groups shared the same frequent amplification at chromosome 18. Here, we revealed that one of the regulatory genes encoded by this amplified region, Crem, was over-expressed in the nuclei of murine and human HCC cells in vivo, and that it stimulated proliferation of human HCC cells in vitro. Our results demonstrate that PHx of a chronically inflamed liver directed tumor development to a discrete pathway characterized by amplification of specific chromosomal regions and expression of specific tumor-promoting genes. Crem is a new candidate HCC oncogene frequently amplified in this model and frequently over-expressed in human HCC.

Cyclic AMP responsive element binding proteins are involved in 'emergency' granulopoiesis through the upregulation of CCAAT/enhancer binding protein β

In contrast to the definitive role of the transcription factor, CCAAT/Enhancer binding protein α (C/EBPα), in steady-state granulopoiesis, previous findings have suggested that granulopoiesis during emergency situations, such as infection, is dependent on C/EBPβ. In this study, a novel lentivirus-based reporter system was developed to elucidate the molecular switch required for C/EBPβ-dependency. The results demonstrated that two cyclic AMP responsive elements (CREs) in the proximal promoter region of C/EBPβ were involved in the positive regulation of C/EBPβ transcription during granulocyte-macrophage colony-stimulating factor (GM-CSF)–induced differentiation of bone marrow cells. In addition, the transcripts of CRE binding (CREB) family proteins were readily detected in hematopoietic stem/progenitor cells. CREB was upregulated, phosphorylated and bound to the CREs in response to GM-CSF stimulation. Retroviral transduction of a dominant negative CREB mutant reduced C/EBPβ mRNA levels and significantly impaired the proliferation/differentiation of granulocyte precursors, while a constitutively active form of CREB facilitated C/EBPβ transcription. These data suggest that CREB proteins are involved in the regulation of granulopoiesis via C/EBPβ upregulation.

MicroRNA-34b promoter hypermethylation induces CREB overexpression and contributes to myeloid transformation

MicroRNA-34b down-regulation in acute myeloid leukemia was previously shown to induce CREB overexpression, thereby causing leukemia proliferation in vitro and in vivo. The role of microRNA-34b and CREB in patients with myeloid malignancies has never been evaluated. We examined microRNA-34b expression and the methylation status of its promoter in cells from patients diagnosed with myeloid malignancies. We used gene expression profiling to identify signatures of myeloid transformation. We established that microRNA-34b has suppressor ability and that CREB has oncogenic potential in primary bone marrow cell cultures and in vivo. MicroRNA-34b was found to be up-regulated in pediatric patients with juvenile myelomonocytic leukemia (n=17) and myelodysplastic syndromes (n=28), but was down-regulated in acute myeloid leukemia patients at diagnosis (n=112). Our results showed that hypermethylation of the microRNA-34b promoter occurred in 66% of cases of acute myeloid leukemia explaining the low microRNA-34b levels and CREB overexpression, whereas preleukemic myelodysplastic syndromes and juvenile myelomonocytic leukemia were not associated with hypermethylation or CREB overexpression. In paired samples taken from the same patients when they had myelodysplastic syndrome and again during the subsequent acute myeloid leukemia, we confirmed microRNA-34b promoter hypermethylation at leukemia onset, with 103 CREB target genes differentially expressed between the two disease stages. This subset of CREB targets was confirmed to associate with high-risk myelodysplastic syndromes in a separate cohort of patients (n=20). Seventy-eight of these 103 CREB targets were also differentially expressed between healthy samples (n=11) and de novo acute myeloid leukemia (n=72). Further, low microRNA-34b and high CREB expression levels induced aberrant myelopoiesis through CREB-dependent pathways in vitro and in vivo. In conclusion, we suggest that microRNA-34b controls CREB expression and contributes to myeloid transformation from both healthy bone marrow and myelodysplastic syndromes. We identified a subset of CREB target genes that represents a novel transcriptional network that may control myeloid transformation.

CREB signalling in neural stem/progenitor cells: recent developments and the implications for brain tumour biology

This paper discusses the evidence for the role of CREB in neural stem/progenitor cell (NSPC) function and oncogenesis and how these functions may be important for the development and growth of brain tumours. The cyclic-AMP response element binding (CREB) protein has many roles in neurons, ranging from neuronal survival to higher order brain functions such as memory and drug addiction behaviours. Recent studies have revealed that CREB also has a role in NSPC survival, differentiation and proliferation. Recent work has shown that over-expression of CREB in transgenic animals can impart oncogenic properties on cells in various tissues and that aberrant CREB expression is associated with tumours in patients. It is the central position of CREB, downstream of key developmental and growth signalling pathways, which give CREB the ability to influence a spectrum of cell activities, such as cell survival, growth and differentiation in both normal and cancer cells.

Cyclic AMP induces IPC leukemia cell apoptosis via CRE-and CDK-dependent Bim transcription

The IPC-81 cell line is derived from the transplantable BNML model of acute myelogenic leukemia (AML), known to be a reliable predictor of the clinical efficiency of antileukemic agents, like the first-line AML anthracycline drug daunorubicin (DNR). We show here that cAMP acted synergistically with DNR to induce IPC cell death. The DNR-induced death differed from that induced by cAMP by (1) not involving Bim induction, (2) being abrogated by GSK3β inhibitors, (3) by being promoted by the HSP90/p23 antagonist geldanamycin and truncated p23 and (4) by being insensitive to the CRE binding protein (CREB) antagonist ICER and to cyclin-dependent protein kinase (CDK) inhibitors. In contrast, the apoptosis induced by cAMP correlated tightly with Bim protein expression. It was abrogated by Bim (BCL2L11) downregulation, whether achieved by the CREB antagonist ICER, by CDK inhibitors, by Bim-directed RNAi, or by protein synthesis inhibitor. The forced expression of BimL killed IPC-81_WT cells rapidly, Bcl2-overexpressing cells being partially resistant. The pivotal role of CREB and CDK activity for Bim transcription is unprecedented. It is also noteworthy that newly developed cAMP analogs specifically activating PKA isozyme I (PKA-I) were able to induce IPC cell apoptosis. Our findings support the notion that AML cells may possess targetable death pathways not exploited by common anti-cancer agents.

ICER evokes Dusp1-p38 pathway enhancing chemotherapy sensitivity in myeloid leukemia

PURPOSE

The inducible cyclic adenosine monophosphate (cAMP) early repressor (ICER) is found downregulated in acute myeloid leukemia (AML), failing to control cAMP response element binding protein (CREB) transcriptional activity, recently demonstrated to mediate AML progression. We aimed to characterize ICER's role in drug sensitivity by treating myeloid cell lines and primary AML with chemotherapics.

EXPERIMENTAL DESIGN

The effects on CREB target genes induced by ICER restoration and drug treatment were studied by quantitative real-time PCR (qRT-PCR) and western blot. Cell cycle and apoptosis analysis were performed. Possible ICER-evoked pathways were investigated in vitro. The mechanism involved in enhanced drug sensitivity was described in primary AML cultures by silencing ICER main target genes.

RESULTS

AML cell lines reduced cell growth and enhanced apoptotic behavior after chemotherapy treatment if ICER was expressed. A significantly lowered expression of CREB target genes involved in cell cycle control (CyA1, B1, D1), and in the mitogen-activated protein kinase signaling pathway (ERK, AKT, DUSP1/4), was found after Etoposide treatment. The dual-specificity phosphatases DUSP1 and DUSP4, directly repressed by ICER, activated the p38 pathway, which triggered enhanced caspase-dependent apoptosis. The silencing of DUSP1/4 in HL60 confirmed the same enhanced drug sensitivity induced by ICER. Primary AML cultures, silenced for DUSP1 as well as restored of ICER expression, showed DUSP1 downregulation and p38 activation.

CONCLUSION

ICER mediates chemotherapy anticancer activity through DUSP1-p38 pathway activation and drives the cell program from survival to apoptosis. ICER restoration or DUSP1 inhibition might be possible strategies to sensitize AML cancer cells to conventional chemotherapy and to inhibit tumor growth.

Activation of the cAMP/CREB/inducible cAMP early repressor pathway suppresses andrographolide-induced gene expression of the pi class of glutathione S-transferase in rat primary hepatocytes

Andrographolide (Ap) is a bioactive compound in Andrographis paniculata that is a Chinese herb. The pi class of glutathione S-transferase (GSTP) is one kind of phase II detoxification enzyme. Here we show that induction of GSTP protein and mRNA expression in rat primary hepatocytes by Ap was inhibited by forskolin and a variety of cAMP analogues. The inhibitory effect of the cAMP analogues was partially blocked by pretreatment with H89. In the presence of Ap, forskolin, or both, the expression of phospho-cAMP response element-binding protein (CREB) was increased. Ap alone had no effect on inducible cAMP early repressor (ICER) mRNA expression; however, Ap played a potentiating role in forskolin-induced ICER mRNA expression. An EMSA and immunoprecipitation assay showed that ICER binding to cAMP-response element (CRE) was increased in cells cotreated with Ap and forskolin for 3 and 8 h. Taken together, these results suggest that ICER is likely to be involved in the suppression of Ap-induced GSTP expression caused by the increase of cAMP in rat primary hepatocytes.

Sequence and expression analysis of the gene encoding inducible cAMP early repressor in tilapia

Suppression subtractive hybridization library was generated by comparison of cDNA populations isolated from peripheral leukocytes of pre- and post-immunized tilapia. One cDNA sequence encoding complete inducible cAMP early repressor was obtained from the library. The sequence was characterized by the presence of the basic structure of ICER IIgamma. Expression of ICER was in the tissues of four types of tilapia was decreased after infection with Streptococcus. After immunization, expression of ICER was initially decreased and then increased after 7 days. In addition, the order for the overall expression of ICER gene after infection and the increases of ICER expression later after immunization in these four types of tilapia was positively correlated to the disease resistance and productivity of these four species of tilapia. Our results provided molecular mechanisms for the different disease resistance capability in different species of tilapia. In addition, our results also provided reference molecular marker for breeding disease resistant tilapia, cAMP responsive element modulator.

CREB: A Key Regulator of Normal and Neoplastic Hematopoiesis

The cAMP response element-binding protein (CREB) is a nuclear transcription factor downstream of cell surface receptors and mitogens that is critical for normal and neoplastic hematopoiesis. Previous work from our laboratory demonstrated that a majority of patients with acute myeloid leukemia (AML) and acute lymphoid leukemia (ALL) overexpress CREB in the bone marrow. To understand the role of CREB in leukemogenesis, we examined the biological effect of CREB overexpression on primary leukemia cells, leukemia cell lines, and CREB overexpressing transgenic mice. Our results demonstrated that CREB overexpression leads to an increase in cellular proliferation and survival. Furthermore, CREB transgenic mice develop a myeloproliferative disorder with aberrant myelopoiesis in both the bone marrow and spleen. Additional research from other groups has shown that the expression of the cAMP early inducible repressor (ICER), a CREB repressor, is also deregulated in leukemias. And, miR-34b, a microRNA that negative regulates CREB expression, is expressed at lower levels in myeloid leukemia cell lines compared to that of healthy bone marrow. Taken together, these data suggest that CREB plays a role in cellular transformation. The data also suggest that CREB-specific signaling pathways could possibly serve as potential targets for therapeutic intervention.