Proteomic identification of C/EBP-DBD multiprotein complex: JNK1 activates stem cell regulator C/EBPalpha by inhibiting its ubiquitination

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.

FBW7 Inhibits Myeloid Differentiation in Acute Myeloid Leukemia via GSK3-Dependent Ubiquitination of PU.1

Glycogen synthase kinase 3β (GSK3β), an ubiquitously expressed serine/threonine kinase is reported to be overexpressed and hyperactivated in cancers including acute myeloid leukemia (AML) where it promotes self-renewal, growth, and survival of AML cells. Therefore, GSK3β inhibition results in AML cell growth inhibition and myeloid differentiation. Here we identified master transcription factor PU.1 of monocyte-macrophage differentiation pathway as potential GSK3β target. We demonstrate that GSK3β phosphorylates PU.1 at Ser41 and Ser140 leading to its recognition and subsequent ubiquitin-mediated degradation by E3 ubiquitin ligase FBW7. This GSK3-dependent degradation of PU.1 by FBW7 inhibited monocyte-macrophage differentiation. We further showed that a phospho-deficient PU.1 mutant (PU.1-S41, S140A) neither bound to FBW7 nor was degraded by it. Consequently, PU.1-S41, S140A retained its transactivation, DNA-binding ability and promoted monocyte-macrophage differentiation of U937 cells even without phorbol 12-myristate 13-acetate (PMA) treatment. We further showed that FBW7 overexpression inhibited both PMA as well as M-CSF-induced macrophage differentiation of myeloid cell lines and peripheral blood mononuclear cells (PBMC) from healthy volunteers, respectively. Contrarily, FBW7 depletion promoted differentiation of these cells even without any inducer suggesting for a robust role of GSK3β-FBW7 axis in negatively regulating myeloid differentiation. Furthermore, we also recapitulated these findings in PBMCs isolated from patients with leukemia where FBW7 overexpression markedly inhibited endogenous PU.1 protein levels. In addition, PBMCs also showed enhanced differentiation when treated with M-CSF and GSK3 inhibitor (SB216763) together compared with M-CSF treatment alone. IMPLICATIONS: Our data demonstrate a plausible mechanism behind PU.1 restoration and induction of myeloid differentiation upon GSK3β inhibition and further substantiates potential of GSK3β as a therapeutic target 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.

Up-regulation of interferon-stimulated gene 15 and its conjugation machinery, UbE1L and UbcH8 expression by tumor necrosis factor-α through p38 MAPK and JNK signaling pathways in human lung carcinoma

Interferon-stimulated gene 15 (ISG15) is a member of the family of ubiquitin-like proteins. Similar to ubiquitin, conjugation of ISG15 to cellular proteins requires cascade reactions catalyzed by at least 2 enzymes, UbE1L and UbcH8. Expression of ISG15 and its conjugates is up-regulated in many cancer cells, yet the underlying mechanism of up-regulation is still unclear. In this study, we showed that TNF-α, similar to the response by IFN-β, could directly induce expression of ISG15 and its conjugation machinery, UbE1L and UbcH8, in human lung carcinoma, A549. The early response of their expression was effectively blocked by specific inhibitors of p38 MAPK (SB202190) and JNK (SP600125), but not by B18R, a soluble type-I IFN receptor. In addition, luciferase reporter assay together with serial deletions and site-directed mutagenesis identified a putative C/EBPβ binding element in the ISG15 promoter, which is necessary to the response by TNF-α. Taken together, expression of ISG15 and ISG15 conjugation machinery in cancer cells is directly up-regulated by TNF-α via p38 MAPK and JNK pathways through the activation of C/EBPβ binding element in the ISG15 promoter. This study provides a new insight toward understanding the molecular mechanism of ISG15 system and inflammatory response in cancer progression.

Further dissection of QTLs for salt-induced stroke and identification of candidate genes in the stroke-prone spontaneously hypertensive rat

We previously revealed that two major quantitative trait loci (QTLs) for stroke latency of the stroke-prone spontaneously hypertensive rat (SHRSP) under salt-loading were located on chromosome (Chr) 1 and 18. Here, we attempted further dissection of the stroke-QTLs using multiple congenic strains between SHRSP and a stroke-resistant hypertensive rat (SHR). Cox hazard model among subcongenic strains harboring a chromosomal fragment of Chr-1 QTL region showed that the most promising region was a 2.1 Mbp fragment between D1Rat177 and D1Rat97. The QTL region on Chr 18 could not be narrowed down by the analysis, which may be due to multiple QTLs in this region. Nonsynonymous sequence variations were found in four genes (Cblc, Cxcl17, Cic, and Ceacam 19) on the 2.1 Mbp fragment of Chr-1 QTL by whole-genome sequence analysis of SHRSP/Izm and SHR/Izm. Significant changes in protein structure were predicted in CBL-C and CXCL17 using I-TASSER. Comprehensive gene expression analysis in the kidney with a cDNA microarray identified three candidate genes (LOC102548695 (Zinc finger protein 45-like, Zfp45L), Ethe1, and Cxcl17). In conclusion, we successfully narrowed down the QTL region on Chr 1, and identified six candidate genes in this region.

Development of MTL-CEBPA: Small Activating RNA Drug for Hepatocellular Carcinoma

BACKGROUND

Oligonucleotide drug development has revolutionised the drug discovery field. Within this field, 'small' or 'short' activating RNAs (saRNA) are a more recently discovered category of short double-stranded RNA with clinical potential. saRNAs promote transcription from target loci, a phenomenon widely observed in mammals known as RNA activation (RNAa).

OBJECTIVE

The ability to target a particular gene is dependent on the sequence of the saRNA. Hence, the potential clinical application of saRNAs is to increase target gene expression in a sequence-specific manner. saRNA-based therapeutics present opportunities for expanding the "druggable genome" with particular areas of interest including transcription factor activation and cases of haploinsufficiency.

RESULTS AND CONCLUSION

In this mini-review, we describe the pre-clinical development of the first saRNA drug to enter the clinic. This saRNA, referred to as MTL-CEBPA, induces increased expression of the transcription factor CCAAT/enhancer-binding protein alpha (CEBPα), a tumour suppressor and critical regulator of hepatocyte function. MTL-CEBPA is presently in Phase I clinical trials for hepatocellular carcinoma (HCC). The clinical development of MTL-CEBPA will demonstrate "proof of concept" that saRNAs can provide the basis for drugs which enhance target gene expression and consequently improve treatment outcome in patients.

E6AP inhibits G-CSFR turnover and functions by promoting its ubiquitin-dependent proteasome degradation

Granulocyte colony-stimulating factor receptor (G-CSFR) plays a crucial role in regulating myeloid cell survival, proliferation, and neutrophilic granulocyte precursor cells maturation. Previously, we demonstrated that Fbw7α negatively regulates G-CSFR and its downstream signaling through ubiquitin-proteasome mediated degradation. However, whether additional ubiquitin ligases for G-CSFR exist is not known. Identifying multiple E3 ubiquitin ligases for G-CSFR shall improve our understanding of activation and subsequent attenuation of G-CSFR signaling required for differentiation and proliferation. Here, for the first time we demonstrate that E6 associated protein (E6AP), an E3 ubiquitin ligase physically associates with G-CSFR and targets it for ubiquitin-mediated proteasome degradation and thereby attenuates its functions. We further show that E6AP promoted G-CSFR degradation leads to reduced phosphorylation of signal transducer and activator of transcription 3 (STAT3) which is required for G-CSF dependent granulocytic differentiation. More importantly, our finding shows that E6AP also targets mutant form of G-SCFR (G-CSFR-T718), frequently observed in severe congenital neutropenia (SCN) patients that very often culminate to AML, however, at a quite slower rate than wild type G-CSFR. In addition, our data showed that knockdown of E6AP restores G-CSFR and its signaling thereby promoting granulocytic differentiation. Collectively, our data demonstrates that E6AP facilitates ubiquitination and subsequent degradation of G-CSFR leading to attenuation of its downstream signaling and inhibition of granulocytic differentiation.

Proteomic discovery of MNT as a novel interacting partner of E3 ubiquitin ligase E6AP and a key mediator of myeloid differentiation

Perturbed stability of regulatory proteins is a major cause of transformations leading to cancer, including several leukemia subtypes. Here, for the first time we demonstrate that E6-associated protein (E6AP), an E3 ubiquitin ligase negatively targets MAX binding protein MNT for ubiquitin-mediated proteasome degradation and impedes ATRA mediated myeloid cell differentiation. MNT is a member of the Myc/Max/Mad network of transcription factor that regulates cell proliferation, differentiation, cellular transformation and tumorigenesis. Wild-type E6AP promoted proteasome dependent degradation of MNT, while catalytically inactive E6AP having cysteine replaced with alanine at amino-acid 843 position (E6APC843A) rather stabilized it. Further, these proteins physically associated with each other both in non-myeloid (HEK293T) and myeloid cells. MNT overexpression induced G0-G1 growth arrest and promoted myeloid differentiation while its knockdown mitigated even ATRA induced differentiation suggesting MNT to be crucial for myeloid differentiation. We further showed that ATRA inhibited E6AP and stabilized MNT expression by protecting it from E6AP mediated ubiquitin-proteasome degradation. Notably, E6AP knockdown in HL60 cells restored MNT expression and promoted myeloid differentiation. Taken together, our data demonstrated that E6AP negatively regulates granulocytic differentiation by targeting MNT for degradation which is required for growth arrest and subsequent myeloid differentiation by various differentiation inducing agents.

Ubiquitin Ligase, Fbw7, Targets CDX2 for Degradation via Two Phosphodegron Motifs in a GSK3β-Dependent Manner

Drosophila caudal-related homeobox transcription factor 2 (CDX2) drives differentiation of the intestinal epithelium. Loss of CDX2 expression has been reported in several colorectal cancers and cancer cell lines with a potential inverse correlation between CDX2 levels and tumor stage. Ubiquitination of CDX2 leading to its downregulation has been implicated in several studies; however, the E3 ubiquitin ligases involved in CDX2 ubiquitination have largely remained unknown. Here, it is mechanistically determined that the E3 ubiquitin ligase Fbw7 promotes CDX2 ubiquitination and degradation through two phosphodegron motifs present within CDX2 in a GSK3β-dependent manner leading to its reduced expression and function in colon cancer cells. Fbw7, through its WD domain, interacted with CDX2 both in a heterologous HEK293T cell system and in colon cancer cells. GSK3β was also present in the same complex as determined by coimmunoprecipitation. Furthermore, overexpression of both Fbw7 or GSK3β down regulated endogenous CDX2 expression and function; however, both failed to inhibit endogenous CDX2 when either of them were depleted in colon cancer cells. Fbw7-mediated inhibition of CDX2 expression also led to reduced CDX2 transactivation and growth arrest of colon cancer cells. Both GSK3β and Fbw7 degraded mutant-CDX2 having either of the Cdc4-phosphodegron (CPD) motifs disrupted (CDX2-S60A or CDX-S281A), but were unable to degrade mutant-CDX2 having both CPDs disrupted (CDX2-S60,64,281A).

IMPLICATIONS

Taken together, these findings demonstrate that Fbw7 negatively regulates CDX2 expression in a GSK3β-dependent manner through two CPDs present in CDX2. Mol Cancer Res; 14(11); 1097-109. ©2016 AACR.

Acetylation of C/EBPα inhibits its granulopoietic function

CCAAT/enhancer-binding protein alpha (C/EBPα) is an essential transcription factor for myeloid lineage commitment. Here we demonstrate that acetylation of C/EBPα at lysine residues K298 and K302, mediated at least in part by general control non-derepressible 5 (GCN5), impairs C/EBPα DNA-binding ability and modulates C/EBPα transcriptional activity. Acetylated C/EBPα is enriched in human myeloid leukaemia cell lines and acute myeloid leukaemia (AML) samples, and downregulated upon granulocyte-colony stimulating factor (G-CSF)- mediated granulocytic differentiation of 32Dcl3 cells. C/EBPα mutants that mimic acetylation failed to induce granulocytic differentiation in C/EBPα-dependent assays, in both cell lines and in primary hematopoietic cells. Our data uncover GCN5 as a negative regulator of C/EBPα and demonstrate the importance of C/EBPα acetylation in myeloid differentiation.

C/EBPα is an essential transcription factor for myeloid lineage commitment. Here, the authors show that acetylation of C/EBPα at K298 and K302, mediated at least in part by GCN5, impairs C/EBPα DNA binding ability and modulates C/EBPα transcriptional activity.

E3 Ubiquitin Ligase Fbw7 Negatively Regulates Osteoblast Differentiation by Targeting Runx2 for Degradation

Runx2, a master regulator of osteoblast differentiation, is tightly regulated at both transcriptional and post-translational levels. Post-translational modifications such as phosphorylation and ubiquitination have differential effects on Runx2 functions. Here, we show that the reduced expression and functions of Runx2 upon its phosphorylation by GSK3β are mediated by its ubiquitin-mediated degradation through E3 ubiquitin ligase Fbw7α. Fbw7α through its WD domain interacts with Runx2 both in a heterologous (HEK293T cells) system as well as in osteoblasts. GSK3β was also present in the same complex as determined by co-immunoprecipitation. Furthermore, overexpression of either Fbw7α or GSK3β was sufficient to down-regulate endogenous Runx2 expression and function; however, both failed to inhibit endogenous Runx2 when either of them was depleted in osteoblasts. Fbw7α-mediated inhibition of Runx2 expression also led to reduced Runx2 transactivation and osteoblast differentiation. In contrast, inhibition of Fbw7α restored Runx2 levels and promoted osteoblast differentiation. We also observed reciprocal expression levels of Runx2 and Fbw7α in models of bone loss such as lactating (physiological bone loss condition) and ovariectomized (induction of surgical menopause) animals that show reduced Runx2 and enhanced Fbw7α, whereas this was reversed in the estrogen-treated ovariectomized animals. In addition, methylprednisolone (a synthetic glucocorticoid) treatment to neonatal rats showed a temporal decrease in Runx2 with a reciprocal increase in Fbw7 in their calvarium. Taken together, these data demonstrate that Fbw7α negatively regulates osteogenesis by targeting Runx2 for ubiquitin-mediated degradation in a GSK3β-dependent manner and thus provides a plausible explanation for GSK3β-mediated bone loss as described before.

Tumor Suppressive Function of p21-activated Kinase 6 in Hepatocellular Carcinoma

Our previous studies identified the oncogenic role of p21-activated kinase 1 (PAK1) in hepatocellular carcinoma (HCC) and renal cell carcinoma (RCC). Contrarily, PAK6 was found to predict a favorable prognosis in RCC patients. Nevertheless, the ambiguous tumor suppressive function of PAK6 in hepatocarcinogenesis remains obscure. Herein, decreased PAK6 expression was found to be associated with tumor node metastasis stage progression and unfavorable overall survival in HCC patients. Additionally, overexpression and silence of PAK6 experiments showed that PAK6 inhibited xenografted tumor growth in vivo, and restricted cell proliferation, colony formation, migration, and invasion and promoted cell apoptosis and anoikis in vitro. Moreover, overexpression of kinase dead and nuclear localization signal deletion mutants of PAK6 experiments indicated the tumor suppressive function of PAK6 was partially dependent on its kinase activity and nuclear translocation. Furthermore, gain or loss of function in polycomb repressive complex 2 (PRC2) components, including EZH2, SUZ12, and EED, elucidated epigenetic control of H3K27me3-arbitrated PAK6 down-regulation in hepatoma cells. More importantly, negative correlation between PAK6 and EZH2 expression was observed in hepatoma tissues from HCC patients. These data identified the tumor suppressive role and potential underlying mechanism of PAK6 in hepatocarcinogenesis.

Pharmacological targeting of the Wdr5-MLL interaction in C/EBPα N-terminal leukemia

The CEBPA gene is mutated in 9% of patients with acute myeloid leukemia (AML). Selective expression of a short (30-kDa) CCAAT-enhancer binding protein-α (C/EBPα) translational isoform, termed p30, represents the most common type of CEBPA mutation in AML. The molecular mechanisms underlying p30-mediated transformation remain incompletely understood. We show that C/EBPα p30, but not the normal p42 isoform, preferentially interacts with Wdr5, a key component of SET/MLL (SET-domain/mixed-lineage leukemia) histone-methyltransferase complexes. Accordingly, p30-bound genomic regions were enriched for MLL-dependent H3K4me3 marks. The p30-dependent increase in self-renewal and inhibition of myeloid differentiation required Wdr5, as downregulation of the latter inhibited proliferation and restored differentiation in p30-dependent AML models. OICR-9429 is a new small-molecule antagonist of the Wdr5-MLL interaction. This compound selectively inhibited proliferation and induced differentiation in p30-expressing human AML cells. Our data reveal the mechanism of p30-dependent transformation and establish the essential p30 cofactor Wdr5 as a therapeutic target in CEBPA-mutant AML.

Signal, transduction, and the hematopoietic stem cell

The hematopoietic stem cell (HSC) is a unique cell positioned highest in the hematopoietic hierarchical system. The HSC has the ability to stay in quiescence, to self-renew, or to differentiate and generate all lineages of blood cells. The path to be actualized is influenced by signals that derive from the cell's microenvironment, which activate molecular pathways inside the cell. Signaling pathways are commonly organized through inducible protein-protein interactions, mediated by adaptor proteins that link activated receptors to cytoplasmic effectors. This review will focus on the signaling molecules and how they work in concert to determine the HSC's fate.

Positive feedback between PU.1 and the cell cycle controls myeloid differentiation

Regulatory gene circuits with positive-feedback loops control stem cell differentiation, but several mechanisms can contribute to positive feedback. Here, we dissect feedback mechanisms through which the transcription factor PU.1 controls lymphoid and myeloid differentiation. Quantitative live-cell imaging revealed that developing B cells decrease PU.1 levels by reducing PU.1 transcription, whereas developing macrophages increase PU.1 levels by lengthening their cell cycles, which causes stable PU.1 accumulation. Exogenous PU.1 expression in progenitors increases endogenous PU.1 levels by inducing cell cycle lengthening, implying positive feedback between a regulatory factor and the cell cycle. Mathematical modeling showed that this cell cycle-coupled feedback architecture effectively stabilizes a slow-dividing differentiated state. These results show that cell cycle duration functions as an integral part of a positive autoregulatory circuit to control cell fate.

HDAC3 interacts with sumoylated C/EBPα to negatively regulate the LXRα expression in rat hepatocytes

The expression changes of liver X receptor alpha (LXRα), histone deacetylase 3 (HDAC3) and CCAAT/enhancer binding protein alpha (C/EBPα) were detected in liver tissues of our high-fat-diet E3 rat model. The aim of this study is to pinpoint the molecular mechanism of HDAC3 and C/EBPα to orchestrate LXRα expression in hepatocytes. We confirmed that LXRα and its target genes were negatively regulated by HDAC3 in stable expressed clones with pEGFP-Hdac3 or shRNA-Hdac3 vector. However, transient pEGFP-C/EBPα plasmid transfection showed an upregulation of LXRα expression and C/EBPα enhanced LXRα promoter activity in a dose-dependent manner in CBRH-7919 cells. By using 5'-serial deletion reporter analysis, we identified that fragment from -2881 to -1181bp of LXRα promoter was responsible for C/EBPα binding to the promoter, especially CBS1 and CBS4 were identified essentially by using ChIP and luciferase reporter assay. Co-IP, qRT-PCR and ChIP revealed that HDAC3 interacted with C/EBPα co-regulated LXRα expression. Sumoylation of C/EBPα at lysine 159 was detected in CBRH-7919 cells with transient overexpressed C/EBPα, and Co-IP assay detected that sumoylated C/EBPα interacted with more HDAC3 than C/EBPα K159L mutant. Luciferase reporter assay demonstrated that C/EBPα participated in HDAC3-repressed LXRα transcription, and HDAC3 was involved in sumoylated C/EBPα-inactivated LXRα activity. Luciferase reporter assay demonstrated that sumoylation of C/EBPα by SUMO-1 directly reversed the activation of C/EBPα on LXRα promoter. The results suggested that HDAC3 interacts with sumoylated C/EBPα to negatively regulate the LXRα expression.

E3 ubiquitin ligase Fbw7 negatively regulates granulocytic differentiation by targeting G-CSFR for degradation

Tight control between activation and attenuation of granulocyte colony stimulating factor receptor (G-CSFR) signaling is essential to regulate survival, proliferation and differentiation of myeloid progenitor cells. Previous studies demonstrated negative regulation of G-CSFR through endosomal-lysosomal routing and ubiquitin-proteasome mediated degradation. However, very few E3 ubiquitin ligases are known to target G-CSFR for ubiquitin-proteasome pathway. Here we identified F-box and WD repeat domain-containing 7 (Fbw7), a substrate recognizing component of Skp-Cullin-F box (SCF) E3 ubiquitin Ligase physically associates with G-CSFR and promotes its ubiquitin-mediated proteasomal degradation. Our data shows that Fbw7 also interacts with and degrades G-CSFR-T718 (a truncated mutant of G-CSFR found in severe congenital neutropenia/acute myeloid leukemia (SCN/AML patients)) though at a quite slower rate compared to G-CSFR. We further show that glycogen synthase kinase 3 beta (GSK3β), like Fbw7 also targets G-CSFR and G-CSFR-T718 for degradation; however, Fbw7 and GSK3β are interdependent in targeting G-CSFR/G-CSFR-T718 for degradation because they are unable to degrade G-CSFR individually when either of them is knocked down. We further show that Fbw7 mediated downregulation of G-CSFR inhibits signal transducer and activator of transcription 3 (STAT3) phosphorylation which is required for G-CSF dependent granulocytic differentiation. In addition, our data also shows that inhibition of Fbw7 restores G-CSFR signaling leading to enhanced STAT3 activity resulting in massive granulocytic differentiation. These data indicate that Fbw7 together with GSK3β negatively regulates G-CSFR expression and its downstream signaling.

Interaction and cooperation of the CCAAT-box enhancer-binding protein β (C/EBPβ) with the homeodomain-interacting protein kinase 2 (Hipk2)

CCAAT box/enhancer-binding protein β (C/EBPβ) is a bZip transcription factor that plays crucial roles in important cellular processes such as differentiation and proliferation of specific cell types. Previously, we showed that C/EBPβ cooperates with the coactivator p300 through a novel mechanism that involves the C/EBPβ-induced phosphorylation of multiple sites in the carboxyl-terminal domain of p300 by protein kinase Hipk2. We have now examined the interaction and cooperation of C/EBPβ, p300, and Hipk2 in more detail. We show that Hipk2 and C/EBPβ are direct physical binding partners whose interaction is mediated by sequences located in the amino-terminal and central domains of Hipk2 and the amino-terminal part of C/EBPβ. In addition to phosphorylating p300 recruited to C/EBPβ, Hipk2 also phosphorylates C/EBPβ at sites that have previously been shown to plays key roles in the regulation of C/EBPβ activity. Silencing of Hipk2 expression disrupts adipocyte differentiation of 3T3-L1 cells, a physiological C/EBPβ-dependent differentiation process indicating that the cooperation of C/EBPβ and Hipk2 is functionally relevant. Finally, we demonstrate that C/EBPα, a related C/EBP family member whose amino-terminal sequences differ significantly from that of C/EBPβ, is unable to interact and cooperate with Hipk2. Instead, our data suggest that C/EBPα cooperates with the protein kinase Jnk to induce phosphorylation of p300. Overall, our data identify Hipk2 as a novel regulator of C/EBPβ and implicate different protein kinases in the cooperation of p300 with C/EBPβ and C/EBPα.

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.

Proteomic analysis of rosiglitazone and guggulsterone treated 3T3-L1 preadipocytes

Adipogenesis is the differentiation of preadipocytes to adipocytes which is marked by the accumulation of lipid droplets. Adipogenic differentiation of 3T3-L1 cells is achieved by exposing the cells to Insulin, Dexamethasone and IBMX for 5-7 days. Thiazolidinedione drugs, like rosiglitazone are potent insulin sensitizing agents and have been shown to enhance lipid droplet formation in 3T3-L1 cells, a model cell line for preadipocyte differentiation. Guggulsterone is a natural drug extracted from the gum resin of tree Commiphora mukul. Guggulsterone has been shown to inhibit adipogenesis and induce apoptosis in 3T3-L1 cells. In this study we treated the 3T3-L1 preadipocytes with rosiglitazone and guggulsterone and assessed the protein expression profile using 2D gel electrophoresis-based proteomics to find out differential target proteins of these drugs. The proteins that were identified upon rosiglitazone treatment generally regulate cell proliferation and/or exhibit anti-inflammatory effect which strengthens its differentiation-inducing property. Guggulsterone treatment resulted in the identification of the apoptosis-inducing proteins to be up regulated which rightly is in agreement with the apoptosis-inducing property of guggulsterone in 3T3-L1 cells. Some of the proteins identified in our proteomic screen such as Galectin1, AnnexinA2 & TCTP were further confirmed by Real Time qPCR. Thus, the present study provides a better outlook of proteins being differentially regulated/expressed upon treatment with rosiglitazone and guggulsterone. The detailed study of the differentially expressed proteins identified in this proteomic screen may further provide the better molecular insight into the mode of action of these anti-diabetic drugs rosiglitazone and guggulsterone.

Proteomic identification of E6AP as a molecular target of tamoxifen in MCF7 cells

Tamoxifen (Tam) is most widely used selective estrogen receptor modulator (SERM) for treatment of hormone-responsive breast cancer. Despite being regularly used in clinical therapy for breast cancer since 1971, the mechanism of Tam action remains largely unclear. In order to gain insights into Tam-mediated antibreast cancer actions, we applied 2DE and MS based proteomics approach to identify target proteins of Tam. We identified E6-associated protein, i.e. E6AP (UBE3A) among others to be regulated by Tam that otherwise is upregulated in breast tumors. We confirmed our 2DE finding by immunoblotting and further show that Tam leads to inhibition of E6AP expression presumably by promoting its autoubiquitination, which is coupled with nuclear export and subsequent proteasome-mediated degradation. Furthermore, we show that Tam- and siE6AP-mediated inhibition of E6AP leads to enhanced G0-G1 growth arrest and apoptosis, which is also evident from significant upregulation of cytochrome-c, Bax, p21, and PARP cleavage. Taken together, our data suggest that, Tam-targeted E6AP inhibition is in fact required for Tam-mediated antibreast cancer actions. Thus, E6AP may be a therapeutic target in breast cancer.

C/EBPα and DEK coordinately regulate myeloid differentiation

The transcription factor C/EBPα is a critical mediator of myeloid differentiation and is often functionally impaired in acute myeloid leukemia. Recent studies have suggested that oncogenic FLT3 activity disrupts wild-type C/EBPα function via phosphorylation on serine 21 (S21). Despite the apparent role of pS21 as a negative regulator of C/EBPα transcription activity, the mechanism by which phosphorylation tips the balance between transcriptionally competent and inhibited forms remains unresolved. In the present study, we used immuno-affinity purification combined with quantitative mass spectrometry to delineate the proteins associated with C/EBPα on chromatin. We identified DEK, a protein with genetic links to leukemia, as a member of the C/EBPα complexes, and demonstrate that this association is disrupted by S21 phosphorylation. We confirmed that DEK is recruited specifically to chromatin with C/EBPα to enhance GCSFR3 promoter activation. In addition, we demonstrated that genetic depletion of DEK reduces the ability of C/EBPα to drive the expression of granulocytic target genes in vitro and disrupts G-CSF-mediated granulocytic differentiation of fresh human BM-derived CD34(+) cells. Our data suggest that C/EBPα and DEK coordinately activate myeloid gene expression and that S21 phosphorylation on wild-type C/EBPα mediates protein interactions that regulate the differentiation capacity of hematopoietic progenitors.

Protein kinase R as mediator of the effects of interferon (IFN) gamma and tumor necrosis factor (TNF) alpha on normal and dysplastic hematopoiesis

IFNγ and TNFα are potent inhibitors of hematopoiesis and have been implicated in the pathophysiology of bone marrow failure and myelodysplastic syndromes (MDS). We examined the role of protein kinase R (PKR) in the generation of the inhibitory effects of these myelosuppressive cytokines on hematopoiesis. Our data demonstrate that PKR is rapidly phosphorylated/activated in response to engagement of IFNγ or TNFα receptors in normal human hematopoietic progenitors. Such engagement of PKR is important for the suppressive effects of these cytokines on normal hematopoiesis. Pharmacological targeting of PKR using a specific inhibitor or siRNA-mediated PKR knockdown results in partial reversal of the suppressive effects of IFNγ and TNFα on normal human CD34+-derived myeloid (colony-forming unit-granulocyte-monocytic) and erythroid (burst-forming unit-erythroid) progenitors. Importantly, inhibition of PKR activity or expression increases hematopoietic colony formation from human MDS progenitors, suggesting that drugs that target PKR may provide a novel approach for the treatment of MDS and marrow failure syndromes. Altogether, our data establish that beyond its key role in the induction of IFN-antiviral responses, PKR plays important roles in signaling for IFNγ and other myelosuppressive cytokine receptors as a common mediator of signals for hematopoietic suppression.

2-D gel electrophoresis-based proteomic analysis reveals that ormeloxifen induces G0-G1 growth arrest and ERK-mediated apoptosis in chronic myeloid leukemia cells K562

Ormeloxifen is a nonsteroidal selective estrogen receptor modulator (SERM) and has been shown to possess anticancer activities in breast and uterine cancer. Here, we show that ormeloxifen induces apoptosis in dose-dependent manner in a variety of leukemia cells, more strikingly in K562. 2-DE-gel electrophoresis of K562 cells induced with ormeloxifen showed that 57 and 30% of proteins belong to apoptosis and cell-cycle pathways, respectively. Our data demonstrate that ormeloxifen-induced apoptosis in K562 cells involves activation of extracellular signal-regulated kinases (ERKs) and subsequent cytochrome c release, leading to mitochondria-mediated caspase-3 activation. Ormeloxifen-induced apoptosis via ERK activation was drastically inhibited by prior treatment of K562 cells with ERK inhibitor PD98059. Ormeloxifen also inhibits proliferation of K562 cells by blocking them in G0-G1 phase by inhibiting c-myc promoter via ormeloxifen-induced MBP-1 (c-myc promoter-binding protein) and upregulation of p21 expression. We further show that ormeloxifen-induced apoptosis in K562 is translatable to mononuclear cells isolated from chronic myeloid leukemia (CML) patients. Thus, ormeloxifen induces apoptosis in K562 cells via phosphorylation of ERK and arrests them in G0-G1 phase by reciprocal regulation of p21 and c-myc. Therefore, inclusion of ormeloxifen in the therapy of chronic myeloid leukemia can be of potential utility.

Proteomics of AML1/ETO Target Proteins: AML1–ETO Targets a C/EBP–NM23 Pathway

Introduction

The rational design of targeted therapies for acute myeloid leukemia (AML) requires the discovery of novel protein pathways in the systems biology of a specific AML subtype. We have shown that in the AML subtype with translocation t(8;21), the leukemic fusion protein AML1–ETO inhibits the function of transcription factors PU.1 and C/EBPα via direct protein–protein interaction. In addition, recently using proteomics, we have also shown that the AML subtypes differ in their proteome, interactome, and post-translational modifications.

Methods

We, therefore, hypothesized that the systematic identification of target proteins of AML1–ETO on a global proteome-wide level will lead to novel insights into the systems biology of t(8;21) AML on a post-genomic functional level. Thus, 6 h after inducible expression of AML1–ETO, protein expression changes were identified by two-dimensional gel electrophoresis and subsequent mass spectrometry analysis.

Results

Twenty-eight target proteins of AML1–ETO including prohibitin, NM23, HSP27, and Annexin1 were identified by MALDI-TOF mass spectrometry. AML1–ETO upregulated the differentiation inhibitory factor NM23 protein expression after 6 h, and the NM23 mRNA expression was also elevated in t(8;21) AML patient samples in comparison with normal bone marrow. AML1–ETO inhibited the ability of C/EBP transcription factors to downregulate the NM23 promoter. These data suggest a model in which AML1–ETO inhibits the C/EBP-induced downregulation of the NM23 promoter and thereby increases the protein level of differentiation inhibitory factor NM23.

Conclusions

Proteomic pathway discovery can identify novel functional pathways in AML, such as the AML1–ETO–C/EBP–NM23 pathway, as the main step towards a systems biology and therapy of AML.

Molecular mechanisms underlying deregulation of C/EBPalpha in acute myeloid leukemia

The CEBPA gene encodes a transcription factor protein that is crucial for granulocytic differentiation, regulation of myeloid gene expression and growth arrest. Mutations in one or both alleles of CEBPA are observed in about 10% of patients with acute myeloid leukemia (AML). Moreover, other genetic events associated with AML have been identified to deregulate C/EBPalpha expression and function at various levels. Recently developed mouse models that accurately mimic the genetic C/EBPalpha alterations in human AML demonstrate C/EBPalpha's gatekeeper function in the control of self-renewal and lineage commitment of hematopoietic stem cells (HSCs). Moreover, these studies indicate that CEBPA mutations affect HSCs in early leukemia development by inducing proliferation and limiting their lineage potential. However, the exact relationship between 'pre-leukemic' HCSs and those cells that finally initiate leukemia (leukemia-initiating cells) with disturbed differentiation and aberrant proliferation remains elusive. More research is needed to identify and characterize these functionally distinct populations and the exact role of the different genetic alterations in the process of leukemia initiation and maintenance.

The emerging importance of group II PAKs

The Rho-family GTPases Rho Rac and Cdc42 regulate many intracellular processes through their interaction with downstream effector proteins. The PAKs (p21-activated kinases) are a family of effector proteins for Rac and Cdc42. PAKs are important regulators of actin cytoskeletal dynamics, neurite outgrowth, cell survival, hormone signalling and gene transcription. There are six mammalian PAKs that can be divided into two groups: group I PAKs (PAK1-3) and group II PAKs (PAK4-6). Although the two PAK groups are architecturally similar, there are differences in their mode of regulation, suggesting that their cellular functions are likely to be different. Whereas much is known about group I PAKs, less is known about the more recently discovered PAK4, PAK5 and PAK6. This review will focus on the latest structural and functional results relating to the group II PAKs and discuss the emerging importance of group II PAKs in disease progression.

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.

MAPK signaling pathways in the regulation of hematopoiesis

The MAPKs are a family of serine/threonine kinases that play an essential role in connecting cell-surface receptors to changes in transcriptional programs. MAPKs are part of a three-component kinase module consisting of a MAPK, an upstream MEK, and a MEKK that couples the signals from cell-surface receptors to trigger downstream pathways. Three major groups of MAPKs have been characterized in mammals, including ERKs, JNKs, and p38MAPKs. Over the last decade, extensive work has established that these proteins play critical roles in the regulation of a wide variety of cellular processes including cell growth, migration, proliferation, differentiation, and survival. It has been demonstrated that ERK, JNK, and p38MAPK activity can be regulated in response to a plethora of hematopoietic cytokines and growth factors that play critical roles in hematopoiesis. In this review, we summarize the current understanding of MAPK function in the regulation of hematopoiesis in general and myelopoiesis in particular. In addition, the consequences of aberrant MAPK activation in the pathogenesis of various myeloid malignancies will be discussed.

Dysregulation of the C/EBPalpha differentiation pathway in human cancer

While much is known about aberrant pathways affecting cell growth and apoptosis, our understanding of another critical step of neoplastic transformation, differentiation arrest, remains poor. The differentiation-inducing transcription factor CCAAT enhancer binding protein alpha (C/EBPalpha) is required for proper control of adipogenesis, glucose metabolism, granulocytic differentiation, and lung development. Studies investigating the function of this protein in hematopoietic malignancies as well as in lung and skin cancer have revealed numerous ways how tumor cells abrogate C/EBPalpha function. Genetic and global expression analysis of acute myeloid leukemia (AML) cases identifies C/EBPalpha-deficient AML as a separate entity yielding novel classification schemes. In patients with a dysfunctional C/EBPalpha pathway, targeted therapies may overcome the block in differentiation, and in combination with conventional chemotherapy, may lead to complete eradication of the malignant clone. Overall, a better understanding of the mechanisms of how C/EBPalpha dysregulation participates in the neoplastic process has opened new gateways for differentiation biology research.

Multiple ways of C/EBPalpha inhibition in myeloid leukaemia

Transcription factors play a crucial role in myeloid differentiation and lineage determination. Tumour suppressor protein C/EBPalpha is a key regulator of granulocytic differentiation whose functional inactivation has become a pathophysiological signature of myeloid leukaemia. In this review we describe various mechanisms such as antagonistic protein-protein interaction, mutation and posttranslational modifications of C/EBPalpha which lead to its transcriptional inhibition and render C/EBPalpha inactive in its functions.

Proteomic identification of the MYST domain histone acetyltransferase TIP60 (HTATIP) as a co-activator of the myeloid transcription factor C/EBPalpha

The transcription factor C/EBPalpha (CEBPA) is a key player in granulopoiesis and leukemogenesis. We have previously reported the interaction of C/EBPalpha with other proteins (utilizing mass spectrometry) in transcriptional regulation. In the present study, we characterized the association of the MYST domain histone acetyltransferase Tat-interactive protein (TIP) 60 (HTATIP) with C/EBPalpha. We show in pull-down and co-precipitation experiments that C/EBPalpha and HTATIP interact. A chromatin immunoprecipitation (ChIP) and a confirmatory Re-ChIP assay revealed in vivo occupancy of the C/EBPalpha and GCSF-R promoter by HTATIP. Reporter gene assays showed that HTATIP is a co-activator of C/EBPalpha. The co-activator function of HTATIP is dependent on its intact histone acetyltransferase (HAT) domain and on the C/EBPalpha DNA-binding domain. The resulting balance between histone acetylation and deacetylation at the C/EBPalpha promoter might represent an important mechanism of C/EBPalpha action. We observed a lower expression of HTATIP mRNA in undifferentiated U937 cells compared to retinoic acid-induced differentiated U937 cells, and correlated expression of CEBPA and HTATIP mRNA levels were observed in leukemia samples. These findings point to a functional synergism between C/EBPalpha and HTATIP in myeloid differentiation and suggest that HTATIP might be an important player in leukemogenesis.