CUL-4A stimulates ubiquitylation and degradation of the HOXA9 homeodomain protein

The Implant-Induced Foreign Body Response Is Limited by CD13-Dependent Regulation of Ubiquitination of Fusogenic Proteins

Implanted medical devices, from artificial heart valves and arthroscopic joints to implantable sensors, often induce a foreign body response (FBR), a form of chronic inflammation resulting from the inflammatory reaction to a persistent foreign stimulus. The FBR is characterized by a subset of multinucleated giant cells (MGCs) formed by macrophage fusion, the foreign body giant cells (FBGCs), accompanied by inflammatory cytokines, matrix deposition, and eventually deleterious fibrotic implant encapsulation. Despite efforts to improve biocompatibility, implant-induced FBR persists, compromising the utility of devices and making efforts to control the FBR imperative for long-term function. Controlling macrophage fusion in FBGC formation presents a logical target to prevent implant failure, but the actual contribution of FBGCs to FBR-induced damage is controversial. CD13 is a molecular scaffold, and in vitro induction of CD13KO bone marrow progenitors generates many more MGCs than the wild type, suggesting that CD13 regulates macrophage fusion. In the mesh implant model of FBR, CD13KO mice produced significantly more peri-implant FBGCs with enhanced TGF-β expression and increased collagen deposition versus the wild type. Prior to fusion, increased protrusion and microprotrusion formation accompanies hyperfusion in the absence of CD13. Expression of fusogenic proteins driving cell-cell fusion was aberrantly sustained at high levels in CD13KO MGCs, which we show is due to a novel CD13 function, to our knowledge, regulating ubiquitin/proteasomal protein degradation. We propose CD13 as a physiologic brake limiting aberrant macrophage fusion and the FBR, and it may be a novel therapeutic target to improve the success of implanted medical devices. Furthermore, our data directly implicate FBGCs in the detrimental fibrosis that characterizes the FBR.

Molecular regulators of HOXA9 in acute myeloid leukemia

Dysregulation of the oncogenic transcription factor HOXA9 is a prominent feature for most aggressive acute myeloid leukemia cases and a strong indicator of poor prognosis in patients. Leukemia subtypes with hallmark overexpression of HOXA9 include those carrying MLL gene rearrangements, NPM1c mutations, and other genetic alternations. A growing body of evidence indicates that HOXA9 dysregulation is both sufficient and necessary for leukemic transformation. The HOXA9 mRNA and protein regulation includes multilayered controls by transcription factors (such as CDX2/4 and USF2/1), epigenetic factors (such as MLL-menin-LEDGF, DOT1L, ENL, HBO1, NPM1c-XPO1, and polycomb proteins), microRNAs (such as miR-126 and miR-196b), long noncoding RNAs (such as HOTTIP), three-dimensional chromatin interactions, and post-translational protein modifications. Recently, insights into the dynamic regulation of HOXA9 have led to an advanced understanding of the HOXA9 regulome and provided new cancer therapeutic opportunities, including developing inhibitors targeting DOT1L, menin, and ENL proteins. This review summarizes recent advances in understanding the molecular mechanisms controlling HOXA9 regulation and the pharmacological approaches that target HOXA9 regulators to treat HOXA9-driven acute myeloid leukemia.

CUL4high Lung Adenocarcinomas Are Dependent on the CUL4-p21 Ubiquitin Signaling for Proliferation and Survival

Cullin (CUL) 4A and 4B ubiquitin ligases are often highly accumulated in human malignant neoplasms and are believed to possess oncogenic properties. However, the underlying mechanisms by which CUL4A and CUL4B promote pulmonary tumorigenesis remain largely elusive. This study reports that CUL4A and CUL4B are highly expressed in patients with non-small cell lung cancer (NSCLC), and their high expression is associated with disease progression, chemotherapy resistance, and poor survival in adenocarcinomas. Depletion of CUL4A (CUL4Ak/d) or CUL4B (CUL4Bk/d) leads to cell cycle arrest at G1 and loss of proliferation and viability of NSCLC cells in culture and in a lung cancer xenograft model, suggesting that CUL4A and 4B are oncoproteins required for tumor maintenance of certain NSCLCs. Mechanistically, increased accumulation of the cell cycle-dependent kinase inhibitor p21/Cip1/WAF1 was observed in lung cancer cells on CUL4 silencing. Knockdown of p21 rescued the G1 arrest of CUL4Ak/d or CUL4Bk/d NSCLC cells, and allowed proliferation to resume. These findings reveal that p21 is the primary downstream effector of lung adenocarcinoma dependence on CUL4, highlight the notion that not all substrates respond equally to abrogation of the CUL4 ubiquitin ligase in NSCLCs, and imply that CUL4Ahigh/CUL4Bhigh may serve as a prognostic marker and therapeutic target for patients with NSCLC.

Tumor suppressor DCAF15 inhibits epithelial-mesenchymal transition by targeting ZEB1 for proteasomal degradation in hepatocellular carcinoma

Epithelial-mesenchymal transition (EMT) is an evolutionarily conserved developmental program that has been implicated in tumorigenesis and confers metastatic properties upon cancer cells. ZEB1 is a master transcription factor that activates the EMT process in various cancers. ZEB1 is reportedly degraded through the ubiquitin proteasome pathway, but the underlying molecular mechanism of this process remains largely unknown in hepatocellular carcinoma (HCC). Here, we identified ZEB1 as a substrate of the CRL4-DCAF15 (DDB1 and CUL4 associated factor 15) E3 ubiquitin ligase complex. DCAF15 acts as an adaptor that specifically recognizes the N-terminal zinc finger domain of ZEB1, then triggers its degradation via the ubiquitin-proteasome pathway. DCAF15 knockdown led to upregulation of ZEB1 and activation of EMT, whereas overexpression of DCAF15 suppressed ZEB1 and inhibited EMT. DCAF15 knockdown also promoted HCC cell proliferation and invasion in a ZEB1-dependent manner. In HCC patients, low DCAF15 expression was predictive of an unfavorable prognosis. These findings reveal the distinct molecular mechanism by which DCAF15 suppresses HCC malignancy and provides insight into the relationship between the CUL4-DCAF15 E3 ubiquitin ligase complex and ZEB1 in HCC.

Cullin 4-DCAF Proteins in Tumorigenesis

Cullin-RING ligase 4 (CRL4), a member of the cullin-RING ligase family, orchestrates a variety of critical cellular processes and pathophysiological events. Recent results from mouse genetics, clinical analyses, and biochemical studies have revealed the impact of CRL4 in development and cancer etiology and elucidated its in-depth mechanism on catalysis of ubiquitination as a ubiquitin E3 ligase. Here, we summarize the versatile roles of the CRL4 E3 ligase complexes in tumorigenesis dependent on the evidence obtained from knockout and transgenic mouse models as well as biochemical and pathological studies.

HOX family transcription factors: Related signaling pathways and post-translational modifications in cancer

HOX family transcription factors belong to a highly conserved subgroup of the homeobox superfamily that determines cellular fates in embryonic morphogenesis and the maintenance of adult tissue architecture. HOX family transcription factors play key roles in numerous cellular processes including cell growth, differentiation, apoptosis, motility, and angiogenesis. As tumor promoters or suppressors HOX family members have been reported to be closely related with a variety of cancers. They closely regulate tumor initiation and growth, invasion and metastasis, angiogenesis, anti-cancer drug resistance and stem cell origin. Here, we firstly described the pivotal roles of HOX transcription factors in tumorigenesis. Then, we summarized the main signaling pathways regulated by HOX transcription factors, including Wnt/β-catenin, transforming growth factor β, mitogen-activated protein kinase, phosphoinositide 3-kinase/Akt, and nuclear factor-κB signalings. Finally, we outlined the important post-translational modifications of HOX transcription factors and their regulation in cancers. Future research directions on the HOX transcription factors are also discussed.

Direct and Indirect Targeting of HOXA9 Transcription Factor in Acute Myeloid Leukemia

HOXA9 (Homeobox A9) is a homeotic transcription factor known for more than two decades to be associated with leukemia. The expression of HOXA9 homeoprotein is associated with anterior-posterior patterning during embryonic development, and its expression is then abolished in most adult cells, with the exception of hematopoietic progenitor cells. The oncogenic function of HOXA9 was first assessed in human acute myeloid leukemia (AML), particularly in the mixed-phenotype associated lineage leukemia (MPAL) subtype. HOXA9 expression in AML is associated with aggressiveness and a poor prognosis. Since then, HOXA9 has been involved in other hematopoietic malignancies and an increasing number of solid tumors. Despite this, HOXA9 was for a long time not targeted to treat cancer, mainly since, as a transcription factor, it belongs to a class of protein long considered to be an "undruggable" target; however, things have now evolved. The aim of the present review is to focus on the different aspects of HOXA9 targeting that could be achieved through multiple ways: (1) indirectly, through the inhibition of its expression, a strategy acting principally at the epigenetic level; or (2) directly, through the inhibition of its transcription factor function by acting at either the protein/protein interaction or the protein/DNA interaction interfaces.

A feedback regulation of CREB activation through the CUL4A and ERK signaling

CUL4A; an E3 ubiquitin ligase is involved in the degradation of negative regulators of cell cycle such as p21, p27, p53, etc., through polyubiquitination-mediated protein degradation. The functional role(s) of CUL4A proteins on their targets are well characterized; however, the transcriptional regulation of CUL4A, particularly at its promoter level is not yet studied. Therefore, in this study, using computational tools, we found cAMP responsive elements (CRE) at the locations of - 926 and - 764 with respect to transcription state site + 1 of CUL4A promoter. Hence, we investigated the role of CREB on the regulation of CUL4A transcription. Our chromatin immunoprecipitation (ChIP) data clearly showed increased levels of promoter occupancy of both CREB and pCREB on both CREs of CUL4A promoter. As expected, the expression of CUL4A increases and decreases upon the overexpression of and knocking down of CREB, respectively. Moreover, the inhibition of ERK pathway by U0126 not only reduces the CREB activation but also the CUL4A levels suggesting that CREB is the upstream activator of CUL4A transcription. The reduction of CUL4A levels upon the knocking down of CREB or by U0126 treatment increases the protein levels of CUL4A substrates such as p21 and p27. It is reported that CUL4A activates the ERK1/2 transcription and ERK1/2 pathway activates the CREB by phosphorylation. Based on our data and earlier findings, we report that CREB regulates the CUL4A levels positively which in turn activates the CREB through ERK1/2 pathway in the form of auto-regulatory looped mechanism.This suggests that CUL4A might be involved in proliferation of cancer cells by regulating the ERK1/2 and CREB signaling.

The emerging role for Cullin 4 family of E3 ligases in tumorigenesis

As a member of the Cullin-RING ligase family, Cullin-RING ligase 4 (CRL4) has drawn much attention due to its broad regulatory roles under physiological and pathological conditions, especially in neoplastic events. Based on evidence from knockout and transgenic mouse models, human clinical data, and biochemical interactions, we summarize the distinct roles of the CRL4 E3 ligase complexes in tumorigenesis, which appears to be tissue- and context-dependent. Notably, targeting CRL4 has recently emerged as a noval anti-cancer strategy, including thalidomide and its derivatives that bind to the substrate recognition receptor cereblon (CRBN), and anticancer sulfonamides that target DCAF15 to suppress the neoplastic proliferation of multiple myeloma and colorectal cancers, respectively. To this end, PROTACs have been developed as a group of engineered bi-functional chemical glues that induce the ubiquitination-mediated degradation of substrates via recruiting E3 ligases, such as CRL4 (CRBN) and CRL2 (pVHL). We summarize the recent major advances in the CRL4 research field towards understanding its involvement in tumorigenesis and further discuss its clinical implications. The anti-tumor effects using the PROTAC approach to target the degradation of undruggable targets are also highlighted.

Recombinant TAT-BMI-1 fusion protein induces ex vivo expansion of human umbilical cord blood-derived hematopoietic stem cells

Transplantation of hematopoietic stem cells (HSCs) is a well-established therapeutic approach for numerous disorders. HSCs are typically derived from bone marrow or peripheral blood after cytokine-induced mobilization. Umbilical cord blood (CB) represents an appealing alternative HSC source, but the small amounts of the individual CB units have limited its applications. The availability of strategies for safe ex vivo expansion of CB-derived HSCs (CB-HSCs) may allow to extend the use of these cells in adult patients and to avoid the risk of insufficient engraftment or delayed hematopoietic recovery.

Here we describe a system for the ex vivo expansion of CB-HSCs based on their transient exposure to a recombinant TAT-BMI-1 chimeric protein. BMI-1 belongs to the Polycomb family of epigenetic modifiers and is recognized as a central regulator of HSC self-renewal. Recombinant TAT-BMI-1 produced in bacteria was able to enter the target cells via the HIV TAT-derived protein transduction peptide covalently attached to BMI-1, and conserved its biological activity. Treatment of CB-CD34⁺ cells for 3 days with repeated addition of 10 nM purified TAT-BMI-1 significantly enhanced total cell expansion as well as that of primitive hematopoietic progenitors in culture. Importantly, TAT-BMI-1-treated CB-CD34⁺ cells displayed a consistently higher rate of multi-lineage long-term repopulating activity in primary and secondary xenotransplants in immunocompromised mice. Thus, recombinant TAT-BMI-1 may represent a novel, effective reagent for ex vivo expansion of CB-HSC for therapeutic purposes.

The O-GlcNAc transferase OGT interacts with and post-translationally modifies the transcription factor HOXA1

HOXA1 belongs to the HOX family of transcription factors which are key regulators of animal development. Little is known about the molecular pathways controlling HOXA1. Recent data from our group revealed distinct partner proteins interacting with HOXA1. Among them, OGT is an O-linked N-acetylglucosamine (O-GlcNAc) transferase modifying a variety of proteins involved in different cellular processes including transcription. Here, we confirm OGT as a HOXA1 interactor, we characterise which domains of HOXA1 and OGT are required for the interaction, and we provide evidence that OGT post-translationally modifies HOXA1. Mass spectrometry experiments indeed reveal that HOXA1 can be phosphorylated on the AGGTVGSPQYIHHSY peptide and that upon OGT expression, the phosphate adduct is replaced by an O-GlcNAc group.

PRDM14, a putative histone methyl-transferase, interacts with and decreases the stability and activity of the HOXA1 transcription factor

Understanding how the activity of transcription factors like HOX proteins is regulated remains a widely open question. In a recent screen for proteins interacting with HOXA1, we identified a PRDM protein family member, PRDM14, which is known to be transiently co-expressed with HOXA1 in epiblast cells before their specification towards somatic versus germ cell fate. Here, we confirm PRDM14 is an interactor of HOXA1 and we identify the homeodomain of HOXA1 as well as the PR domain and Zinc fingers of PRDM14 to be required for the interaction. An 11-His repeat of HOXA1 previously highlighted to contribute to HOXA1-mediated protein-protein interactions is also involved. At a functional level, we provide evidence that HOXA1 displays an unexpectedly long half-life and demonstrate that PRDM14 can reduce the stability and affect the transcriptional activity of HOXA1.

A novel lncRNA uc.134 represses hepatocellular carcinoma progression by inhibiting CUL4A-mediated ubiquitination of LATS1

BACKGROUND

Hepatocellular carcinoma (HCC) is one of the most common malignancies worldwide, and tumor recurrence and metastasis are major factors that contribute to the poor outcome of patients with HCC. Long noncoding RNAs (lncRNAs) are known to regulate different tumorigenic processes, and a growing body of evidence indicates that Hippo kinase signaling is inactivated in many cancers. However, the upstream lncRNA regulators of Hippo kinase signaling in HCC are poorly understood.

METHODS

Using a lncRNA microarray, we identified a novel lncRNA, uc.134, whose expression was significantly decreased in the highly aggressive HCC cell line HCCLM3 compared with MHCC97L cells. Furthermore, we evaluated uc.134 expression in clinical samples using in situ hybridization (ISH) and quantitative real-time polymerase chain reaction (qRT-PCR) analysis. The full-length transcript of uc.134 was confirmed using rapid amplification of cDNA ends (RACE) analyses. To investigate the biological function of uc.134, we performed gain-of-function and loss-of-function studies both in vitro and in vivo. The underlying mechanisms of uc.134 in HCC were investigated using RNA pulldown, RNA immunoprecipitation, ubiquitination assays, Western blotting, mRNA microarray analyses, and qRT-PCR analyses.

RESULTS

The ISH assay revealed that uc.134 expression was significantly decreased in 170 paraffin-embedded samples from patients with HCC compared with adjacent tissues and uc.134 expression directly correlated with patient prognosis. Furthermore, we defined a 1867-bp full-length transcript of uc.134 using 5'- and 3'-RACE analysis. The overexpression of uc.134 inhibited HCC cell proliferation, invasion, and metastasis in vitro and in vivo, whereas the knockdown of uc.134 produced the opposite results. Furthermore, we confirmed that uc.134 (1408-1867 nt) binds to CUL4A (592-759 aa region) and inhibits its nuclear export. Moreover, we demonstrated that uc.134 inhibits the CUL4A-mediated ubiquitination of LATS1 and increases YAP^S127 phosphorylation to silence the target genes of YAP. Finally, a positive correlation between uc.134, LATS1, and pYAP^S127 was confirmed in 90 paraffin-embedded samples by ISH and immunohistochemical staining.

CONCLUSIONS

Our study identifies that a novel lncRNA, uc.134, represses hepatocellular carcinoma progression by inhibiting the CUL4A-mediated ubiquitination of LATS1 and increasing YAP^S127 phosphorylation. The use of this lncRNA may offer a promising treatment approach by inhibiting YAP and activating Hippo kinase signaling.

Identification of a tumor suppressor network in T-cell leukemia

To identify novel cancer-related genes targeted by copy number alterations, we performed genomic profiling of T-cell acute lymphoblastic leukemia (T-ALL) cell lines. In 3/8, we identified a shared deletion at chromosomal position 2p16.3-p21. Within the minimally deleted region, we recognized several candidate tumor suppressor (TS) genes, including FBXO11 and FOXN2. An additional deletion at chromosome 14q23.2-q32.11 included FOXN3, highlighting this class of FOX genes as potential TS. Quantitative expression analyses of FBXO11, FOXN2, and FOXN3 confirmed reduced transcript levels in the identified cell lines. Moreover, reduced expression of these genes was also observed in about 7% of T-ALL patients, showing their clinical relevance in this malignancy. Bioinformatic analyses revealed concurrent reduction of FOXN2 and/or FOXN3 together with homeobox gene ZHX1. Consistently, experiments demonstrated that both FOXN2 and FOXN3 directly activated transcription of ZHX1. Taken together, we identified novel TS genes forming a regulatory network in T-cell development and leukemogenesis.

Molecular Analysis of the HOXA2-Dependent Degradation of RCHY1

The homeodomain transcription factor Hoxa2 interacts with the RING-finger type E3 ubiquitin ligase RCHY1 and induces its proteasomal degradation. In this work, we dissected this non-transcriptional activity of Hoxa2 at the molecular level. The Hoxa2-mediated decay of RCHY1 involves both the 19S and 20S proteasome complexes. It relies on both the Hoxa2 homeodomain and C-terminal moiety although no single deletion in the Hoxa2 sequence could disrupt the RCHY1 interaction. That the Hoxa2 homeodomain alone could mediate RCHY1 binding is consistent with the shared ability all the Hox proteins we tested to interact with RCHY1. Nonetheless, the ability to induce RCHY1 degradation although critically relying on the homeodomain is not common to all Hox proteins. This identifies the homeodomain as necessary but not sufficient for what appears to be an almost generic Hox protein activity. Finally we provide evidence that the Hoxa2-induced degradation of RCHY1 is evolutionarily conserved among vertebrates. These data therefore support the hypothesis that the molecular and functional interaction between Hox proteins and RCHY1 is an ancestral Hox property.

Distinct and overlapping functions of the cullin E3 ligase scaffolding proteins CUL4A and CUL4B

The cullin 4 subfamily of genes includes CUL4A and CUL4B, which share a mostly identical amino acid sequence aside from the elongated N-terminal region in CUL4B. Both act as scaffolding proteins for modular cullin RING ligase 4 (CRL4) complexes which promote the ubiquitination of a variety of substrates. CRL4 function is vital to cells as loss of both genes or their shared substrate adaptor protein DDB1 halts proliferation and eventually leads to cell death. Due to their high structural similarity, CUL4A and CUL4B share a substantial overlap in function. However, in some cases, differences in subcellular localization, spatiotemporal expression patterns and stress-inducibility preclude functional compensation. In this review, we highlight the most essential functions of the CUL4 genes in: DNA repair and replication, chromatin-remodeling, cell cycle regulation, embryogenesis, hematopoiesis and spermatogenesis. CUL4 genes are also clinically relevant as dysregulation can contribute to the onset of cancer and CRL4 complexes are often hijacked by certain viruses to promote viral replication and survival. Also, mutations in CUL4B have been implicated in a subset of patients suffering from syndromic X-linked intellectual disability (AKA mental retardation). Interestingly, the antitumor effects of immunomodulatory drugs are caused by their binding to the CRL4CRBN complex and re-directing the E3 ligase towards the Ikaros transcription factors IKZF1 and IKZF3. Because of their influence over key cellular functions and relevance to human disease, CRL4s are considered promising targets for therapeutic intervention.

KPC2 relocalizes HOXA2 to the cytoplasm and decreases its transcriptional activity

Regulation of transcription factor activity relies on molecular interactions or enzymatic modifications which influence their interaction with DNA cis-regulatory sequences, their transcriptional activation or repression, and stability or intracellular distribution of these proteins. Regarding the well-conserved Hox protein family, a restricted number of activity regulators have been highlighted thus far. In the framework of a proteome-wide screening aiming at identifying proteins interacting with Hoxa2, KPC2, an adapter protein constitutive of the KPC ubiquitin-ligase complex, was identified. In this work, KPC2 was confirmed as being a genuine interactor of Hoxa2 by co-precipitation and bimolecular fluorescence complementation assays. At functional level, KPC2 diminishes the transcriptional activity and induces the nuclear exit of Hoxa2. Gene expression analyses revealed that Kpc2 is active in restricted areas of the developing mouse embryo which overlap with the Hoxa2 expression domain. Together, our data support that KPC2 regulates Hoxa2 by promoting its relocation to the cytoplasm.

Akt Phosphorylates Wnt Coactivator and Chromatin Effector Pygo2 at Serine 48 to Antagonize Its Ubiquitin/Proteasome-mediated Degradation

Pygopus 2 (Pygo2/PYGO2) is an evolutionarily conserved coactivator and chromatin effector in the Wnt/β-catenin signaling pathway that regulates cell growth and differentiation in various normal and malignant tissues. Although PYGO2 is highly overexpressed in a number of human cancers, the molecular mechanism underlying its deregulation is largely unknown. Here we report that Pygo2 protein is degraded through the ubiquitin/proteasome pathway and is posttranslationally stabilized through phosphorylation by activated phosphatidylinositol 3-kinase/Akt signaling. Specifically, Pygo2 is stabilized upon inhibition of the proteasome, and its intracellular level is regulated by Cullin 4 (Cul4) and DNA damage-binding protein 1 (DDB1), components of the Cul4-DDB1 E3 ubiquitin ligase complex. Furthermore, Pygo2 is phosphorylated at multiple residues, and Akt-mediated phosphorylation at serine 48 leads to its decreased ubiquitylation and increased stability. Finally, we provide evidence that Akt and its upstream growth factors act in parallel with Wnt to stabilize Pygo2. Taken together, our findings highlight chromatin regulator Pygo2 as a common node downstream of oncogenic Wnt and Akt signaling pathways and underscore posttranslational modification, particularly phosphorylation and ubiquitylation, as a significant mode of regulation of Pygo2 protein expression.

Role of HOXA9 in leukemia: dysregulation, cofactors and essential targets

HOXA9 is a homeodomain-containing transcription factor that has an important role in hematopoietic stem cell expansion and is commonly deregulated in acute leukemias. A variety of upstream genetic alterations in acute myeloid leukemia lead to overexpression of HOXA9, which is a strong predictor of poor prognosis. In many cases, HOXA9 has been shown to be necessary for maintaining leukemic transformation; however, the molecular mechanisms through which it promotes leukemogenesis remain elusive. Recent work has established that HOXA9 regulates downstream gene expression through binding at promoter distal enhancers along with a subset of cell-specific cofactor and collaborator proteins. Increasing efforts are being made to identify both the critical cofactors and target genes required for maintaining transformation in HOXA9-overexpressing leukemias. With continued advances in understanding HOXA9-mediated transformation, there is a wealth of opportunity for developing novel therapeutics that would be applicable for greater than 50% of AML with overexpression of HOXA9.

HoxA10 Terminates Emergency Granulopoiesis by Increasing Expression of Triad1

Expression of the E3 ubiquitin ligase Triad1 is greater in mature granulocytes than in myeloid progenitor cells. HoxA10 actives transcription of the gene encoding Triad1 (ARIH2) during myeloid differentiation, but the contribution of increased Triad1 expression to granulocyte production or function is unknown. Mice with bone marrow-specific disruption of the ARIH2 gene exhibit constitutive inflammation with tissue infiltration by granulocytes and B cells. In contrast, disruption of the HOXA10 gene in mice neither constitutively activates the innate immune response nor significantly alters steady-state granulopoiesis. This study explores the impact of HoxA10-induced Triad1 expression on emergency (stress) granulopoiesis. We found that mice with HOXA10 gene disruption exhibited an overwhelming and fatal emergency granulopoiesis response that was characterized by tissue infiltration with granulocytes, but reversed by re-expression of Triad1 in the bone marrow. We determined that HoxA9 repressed ARIH2 transcription in myeloid progenitor cells, antagonizing the effect of HoxA10 on Triad1 expression. Also, we found that differentiation-stage-specific ARIH2 transcription was regulated by the tyrosine phosphorylation states of HoxA9 and HoxA10. Our studies demonstrate a previously undescribed role for HoxA10 in terminating emergency granulopoiesis, suggesting an important contribution by Hox proteins to the innate immune response.

CDK4/6 Inhibitor PD 0332991 Sensitizes Acute Myeloid Leukemia to Cytarabine-Mediated Cytotoxicity

Cyclin-dependent kinase (CDK)4 and CDK6 are frequently overexpressed or hyperactivated in human cancers. Targeting CDK4/CDK6 in combination with cytotoxic killing therefore represents a rational approach to cancer therapy. By selective inhibition of CDK4/CDK6 with PD 0332991, which leads to early G1 arrest and synchronous S-phase entry upon release of the G1 block, we have developed a novel strategy to prime acute myeloid leukemia (AML) cells for cytotoxic killing by cytarabine (Ara-C). This sensitization is achieved in part through enrichment of S-phase cells, which maximizes the AML populations for Ara-C incorporation into replicating DNA to elicit DNA damage. Moreover, PD 0332991 triggered apoptosis of AML cells through inhibition of the homeobox (HOX)A9 oncogene expression, reducing the transcription of its target PIM1. Reduced PIM1 synthesis attenuates PIM1-mediated phosphorylation of the proapoptotic BAD and activates BAD-dependent apoptosis. In vivo, timely inhibition of CDK4/CDK6 by PD 0332991 and release profoundly suppresses tumor growth in response to reduced doses of Ara-C in a xenograft AML model. Collectively, these data suggest selective and reversible inhibition of CDK4/CDK6 as an effective means to enhance Ara-C killing of AML cells at reduced doses, which has implications for the treatment of elderly AML patients who are unable to tolerate high-dose Ara-C therapy.

CUL4A ubiquitin ligase: a promising drug target for cancer and other human diseases

The ability of cullin 4A (CUL4A), a scaffold protein, to recruit a repertoire of substrate adaptors allows it to assemble into distinct E3 ligase complexes to mediate turnover of key regulatory proteins. In the past decade, a considerable wealth of information has been generated regarding its biology, regulation, assembly, molecular architecture and novel functions. Importantly, unravelling of its association with multiple tumours and modulation by viral proteins establishes it as one of the key proteins that may play an important role in cellular transformation. Considering the role of its substrate in regulating the cell cycle and maintenance of genomic stability, understanding the detailed aspects of these processes will have significant consequences for the treatment of cancer and related diseases. This review is an effort to provide a broad overview of this multifaceted ubiquitin ligase and addresses its critical role in regulation of important biological processes. More importantly, its tremendous potential to be exploited for therapeutic purposes has been discussed.

Role of HOXA9 and VEZF1 in endothelial biology

Proper development of the vascular system as one of the earliest and most critical steps during vertebrate embryogenesis is ensured by the exact spatial and temporal control of gene expression in cells forming the vessel network. Whereas the regulation of vascular system development is well elucidated on the level of ligand-receptor signaling, the processes on the transcriptional level are much less understood. As the signaling mechanisms in embryogenesis and pathological conditions are similar, the study of embryonic blood vessel development is of great interest for the treatment of cardiovascular diseases and cancer. This review discusses two transcription factors, HOXA9 and VEZF1, which are relevant for endothelial biology but are excluded in the bulk of transcription factor references discussing endothelial biology. To our knowledge, there is no comprehensive overview of these two transcription factors available to date. Here, we summarize the current knowledge of human HOXA9 and VEZF1 biology and function, we detail their target genes and roles in endothelial biology and propose that HOXA9 and VEZF1 also deserve consideration as relevant transcriptional regulators of endothelial biology. Due to their broad role in multiple aspects of endothelial biology, they might potentially become interesting targets for therapeutic manipulation of pathological blood vessel growth.

Improved ex vivo expansion of adult hematopoietic stem cells by overcoming CUL4-mediated degradation of HOXB4

Direct transduction of the homeobox (HOX) protein HOXB4 promotes the proliferation of hematopoietic stem cells (HSCs) without induction of leukemogenesis, but requires frequent administration to overcome its short protein half-life (∼1 hour). We demonstrate here that HOXB4 protein levels are post-translationally regulated by the CUL4 ubiquitin ligase, and define the degradation signal sequence (degron) of HOXB4 required for CUL4-mediated destruction. Additional HOX paralogs share the conserved degron in the homeodomain and are also subject to CUL4-mediated degradation, indicating that CUL4 likely controls the stability of all HOX proteins. Moreover, we engineered a degradation-resistant HOXB4 that conferred a growth advantage over wild-type HOXB4 in myeloid progenitor cells. Direct transduction of recombinant degradation-resistant HOXB4 protein to human adult HSCs significantly enhanced their maintenance in a more primitive state both in vitro and in transplanted NOD/SCID/IL2R-γ(null) mice compared with transduction with wild-type HOXB4 protein. Our studies demonstrate the feasibility of engineering a stable HOXB4 variant to overcome a major technical hurdle in the ex vivo expansion of adult HSCs and early progenitors for human therapeutic use.

Hoxa9 transduction induces hematopoietic stem and progenitor cell activity through direct down-regulation of geminin protein

Hoxb4, a 3′-located Hox gene, enhances hematopoietic stem cell (HSC) activity, while a subset of 5′-located Hox genes is involved in hematopoiesis and leukemogenesis, and some of them are common translocation partners for Nucleoporin 98 (Nup98) in patients with leukemia. Although these Hox gene derivatives are believed to act as transcription regulators, the molecular involvement of the Hox gene derivatives in hematopoiesis and leukemogenesis remains largely elusive. Since we previously showed that Hoxb4 forms a complex with a Roc1-Ddb1-Cul4a ubiquitin ligase core component and functions as an E3 ubiquitin ligase activator for Geminin, we here examined the E3 ubiquitin ligase activities of the 5′-located Hox genes, Hoxa9 and Hoxc13, and Nup98-Hoxa9. Hoxa9 formed a similar complex with the Roc1-Ddb1-Cul4a component to induce ubiquitination of Geminin, but the others did not. Retroviral transduction-mediated overexpression or siRNA-mediated knock-down of Hoxa9 respectively down-regulated or up-regulated Geminin in hematopoietic cells. And Hoxa9 transduction-induced repopulating and clonogenic activities were suppressed by Geminin supertransduction. These findings suggest that Hoxa9 and Hoxb4 differ from Hoxc13 and Nup98-Hoxa9 in their molecular role in hematopoiesis, and that Hoxa9 induces the activity of HSCs and hematopoietic progenitors at least in part through direct down-regulation of Geminin.

Two isoforms of HOXA9 function differently but work synergistically in human MLL-rearranged leukemia

HOXA9 plays a critical role in both normal hematopoiesis and leukemogenesis, particularly in the development and maintenance of mixed lineage leukemia (MLL)-rearranged leukemia. Through reverse transcription-polymerase chain reaction (RT-PCR) analysis of HOXA9 transcripts in human leukemia and normal bone marrow samples, we identified a truncated isoform of HOXA9, namely HOXA9T, and found that both HOXA9T and canonical HOXA9 were highly expressed in leukemia cell lines bearing MLL rearrangements, relative to human normal bone marrow cells or other subtypes of leukemia cells. A frameshift in HOXA9T in exon I causes a premature stop codon upstream of the PBX-binding domain and the homeodomain, which leads to the generation of a non-homeodomain-containing protein. Unlike the canonical HOXA9, HOXA9T alone cannot transform normal bone marrow progenitor cells. Moreover, HOXA9T cannot cooperate with MEIS1 to transform cells, despite the presence of a MEIS1-binding domain. Remarkably, although the truncated isoforms of many proteins function as dominant-negative competitors or inhibitors of their full-length counterparts, this is not the case for HOXA9T; instead, HOXA9T synergized with HOXA9 in transforming mouse normal bone marrow progenitor cells through promoting self-renewal and proliferation of the cells. Collectively, our data indicate that both truncated and full-length forms of HOXA9 are highly expressed in human MLL-rearranged leukemia, and the truncated isoform of HOXA9 might also play an oncogenic role by cooperating with canonical HOXA9 in cell transformation and leukemogenesis.

HOXA9 methylation by PRMT5 is essential for endothelial cell expression of leukocyte adhesion molecules

The induction of proinflammatory proteins in stimulated endothelial cells (EC) requires activation of multiple transcription programs. The homeobox transcription factor HOXA9 has an important regulatory role in cytokine induction of the EC-leukocyte adhesion molecules (ELAM) E-selectin and vascular cell adhesion molecule 1 (VCAM-1). However, the mechanism underlying stimulus-dependent activation of HOXA9 is completely unknown. Here, we elucidate the molecular mechanism of HOXA9 activation by tumor necrosis factor alpha (TNF-α) and show an unexpected requirement for arginine methylation by protein arginine methyltransferase 5 (PRMT5). PRMT5 was identified as a TNF-α-dependent binding partner of HOXA9 by mass spectrometry. Small interfering RNA (siRNA)-mediated depletion of PRMT5 abrogated stimulus-dependent HOXA9 methylation with concomitant loss in E-selectin or VCAM-1 induction. Chromatin immunoprecipitation analysis revealed that PRMT5 is recruited to the E-selectin promoter following transient HOXA9 binding to its cognate recognition sequence. PRMT5 induces symmetric dimethylation of Arg140 on HOXA9, an event essential for E-selectin induction. In summary, PRMT5 is a critical coactivator component in a newly defined, HOXA9-containing transcription complex. Moreover, stimulus-dependent methylation of HOXA9 is essential for ELAM expression during the EC inflammatory response.

Pathogenic Role of the CRL4 Ubiquitin Ligase in Human Disease

The cullin 4-RING ubiquitin ligase (CRL4) family employs multiple DDB1–CUL4 associated factors substrate receptors to direct the degradation of proteins involved in a wide spectrum of cellular functions. Aberrant expression of the cullin 4A (CUL4A) gene is found in many tumor types, while mutations of the cullin 4B (CUL4B) gene are causally associated with human X-linked mental retardation. This focused review will summarize our current knowledge of the two CUL4 family members in the pathogenesis of human malignancy and neuronal disease, and discuss their potential as new targets for cancer prevention and therapeutic intervention.

Cullins and cancer

The cullin family of ubiquitin ligases can potentially assemble hundreds of RING-type E3 complexes (CRLs) by utilizing different substrate receptors that share common interaction domains. Cullin receptors dictate substrate specificity, and cullin-mediated substrate degradation controls a wide range of cellular processes, including proliferation, differentiation, and apoptosis. Dysregulation of cullin activity has been shown to contribute to oncogenesis through the accumulation of oncoproteins or the excessive degradation of tumor suppressors. In this review, we will discuss cullin complexes and their substrates, the regulatory pathways that affect cullin activity, and the mechanisms by which cullins may facilitate or inhibit carcinogenesis.

HoxA10 influences protein ubiquitination by activating transcription of ARIH2, the gene encoding Triad1

HoxA10 is a homeodomain transcription factor that is maximally expressed in myeloid progenitor cells. An increase in HoxA10 expression correlates with poor prognosis in human acute myeloid leukemia (AML). Consistent with this scenario, HoxA10 overexpression in murine bone marrow induces a myeloproliferative neoplasm that advances AML over time. Despite the importance of HoxA10 for leukemogenesis, few genuine HoxA10 target genes have been identified. The current study identified ARIH2, the gene encoding Triad1, as a HoxA10 target gene. We identified two distinct HoxA10-binding cis elements in the ARIH2 promoter and determined that HoxA10 activates these cis elements in myeloid cells. Triad1 has E3 ubiquitin ligase activity, and we found that HoxA10-overexpressing myeloid cells exhibited a Triad1-dependent increase in protein ubiquitination. Therefore, these studies have identified the regulation of protein ubiquitination as a novel function of Hox transcription factors. Forced overexpression of Triad1 has been show previously to inhibit colony formation by myeloid progenitor cells. In contrast, HoxA10-overexpressing myeloid progenitor cells exhibited increased proliferation in response to low doses of various cytokines. We found that Triad1 knockdown further increased cytokine-induced proliferation in HoxA10-overexpressing cells. Therefore, these studies have identified a HoxA10 target gene that antagonizes the overall influence of overexpressed HoxA10 on myeloproliferation. This result suggests that the consequences of HoxA10 overexpression reflect a balance between the target genes that facilitate and antagonize proliferation. These results have implications for understanding the mechanisms of leukemogenesis in AML with Hox overexpression.

Hoxb4 transduction down-regulates Geminin protein, providing hematopoietic stem and progenitor cells with proliferation potential

Retrovirus-mediated transduction of Hoxb4 enhances hematopoietic stem cell (HSC) activity and enforced expression of Hoxb4 induces in vitro development of HSCs from differentiating mouse embryonic stem cells, but the underlying molecular mechanism remains unclear. We previously showed that the HSC activity was abrogated by accumulated Geminin, an inhibitor for the DNA replication licensing factor Cdt1 in mice deficient in Rae28 (also known as Phc1), which encodes a member of Polycomb-group complex 1. In this study we found that Hoxb4 transduction reduced accumulated Geminin in Rae28-deficient mice, despite increasing the mRNA, and restored the impaired HSC activity. Supertransduction of Geminin suppressed the HSC activity induced by Hoxb4 transduction, whereas knockdown of Geminin promoted the clonogenic and replating activities, indicating the importance of Geminin regulation in the molecular mechanism underlying Hoxb4 transduction-mediated enhancement of the HSC activity. This facilitated our investigation of how transduced Hoxb4 reduced Geminin. We showed in vitro and in vivo that Hoxb4 and the Roc1 (also known as Rbx1)-Ddb1-Cul4a ubiquitin ligase core component formed a complex designated as RDCOXB4, which acted as an E3 ubiquitin ligase for Geminin and down-regulated Geminin through the ubiquitin-proteasome system. Down-regulated Geminin and the resultant E2F activation may provide cells with proliferation potential by increasing a DNA prereplicative complex loaded onto chromatin. Here we suggest that transduced Hoxb4 down-regulates Geminin protein probably by constituting the E3 ubiquitin ligase for Geminin to provide hematopoietic stem and progenitor cells with proliferation potential.

CRL4s: the CUL4-RING E3 ubiquitin ligases

The evolutionarily conserved cullin family proteins can assemble as many as 400 distinct E3 ubiquitin ligase complexes that regulate diverse cellular pathways. CUL4, one of three founding cullins conserved from yeast to humans, uses a large beta-propeller protein, DDB1, as a linker to interact with a subset of WD40 proteins that serve as substrate receptors, forming as many as 90 E3 complexes in mammals. Many CRL4 complexes are involved in chromatin regulation and are frequently hijacked by different viruses.

CUL4A abrogation augments DNA damage response and protection against skin carcinogenesis

It is intuitively obvious that the ability of a cell to repair DNA damage is saturable, either by limitation of enzymatic activities, the time allotted to achieve their function, or both. However, very little is known regarding the mechanisms that establish such a threshold. Here we demonstrate that the CUL4A ubiquitin ligase restricts the cellular repair capacity by orchestrating the concerted actions of nucleotide excision repair (NER) and the DNA damage-responsive G1/S checkpoint through selective degradation of the DDB2 and XPC DNA damage sensors and the p21/CIP1/WAF1 checkpoint effector. We generated Cul4a conditional knockout mice and observed that skin-specific Cul4a ablation dramatically increased resistance to UV-induced skin carcinogenesis. Our findings reveal that wild-type cells do not operate at their full DNA repair potential, underscore the critical role of CUL4A in establishing the cellular DNA repair threshold, and highlight the potential augmentation of cellular repair proficiency by pharmacological CUL4A inhibition.

Proliferation defects and genome instability in cells lacking Cul4A

The Cul4A gene, which encodes a core component of a cullin-based E3 ubiquitin ligase complex, is over-expressed in breast and hepatocellular cancers. In breast cancers, over-expression of Cul4A strongly correlates with poor prognosis. Also, Cul4A is required for early embryonic development. Early lethality of mouse embryos prevented a detailed analysis of the functions of Cul4A. Here, we used a strain of mice carrying floxed alleles of Cul4A to study its role in cell division, in vitro and in vivo. Embryonic fibroblasts exhibit a severe deficiency in cell proliferation following deletion of Cul4A. We observed that the Cul4A protein is abundantly expressed in brain, liver and in the mammary tissue of pregnant mice. Deletion of Cul4A in liver impairs hepatocyte proliferation during regeneration following carbon tetrachloride induced injury. The Cul4A-deleted cells are slow in entering S phase, and are deficient in progressing through early M phase. Several cell cycle regulators, including p53 and p27Kip1, are de-regulated in the Cul4A-deleted cells. Expression of a dominant negative mutant of p53 causes significant reversal of the proliferation defects in Cul4A-deleted cells. The Cul4A-deleted cells exhibit aberrant number of centrosome, multipolar spindles and micronuclei formation. Furthermore, those cells are sensitive to UV irradiation and exhibit reduced levels of unscheduled DNA synthesis. Together, our observations indicate that Cul4A is required for efficient cell proliferation, control of the centrosome amplification and genome stability.

Morpholino-mediated knockdown in primary chondrocytes implicates Hoxc8 in regulation of cell cycle progression

Numerous experiments in mutant and transgenic mice have implicated Hox transcription factors in development of the skeletal system, postulating a role for these proteins in cell proliferation of precursor cells and regulation of cell differentiation. Our own data from Hoxc8 and Hoxd4 transgenic mice suggest that Hoxc8 is involved in cell proliferation during cartilage development. In order to directly assess its role in cell proliferation of a specific skeletal cell type, the cartilage-producing chondrocyte, we performed morpholino-mediated knockdown experiments in normal primary chondrocytes. Through analysis of PCNA expression and staining for phosphorylated Histone 3, two cell cycle markers, we show that interference with Hoxc8 expression in chondrocytes reduces cell proliferation, but in the absence of apoptosis. Instead, cells with a knockdown in Hoxc8 expression appear to be delayed in their progression through the cell cycle. Our results provide evidence for prolonged duration of and delayed exit from M-phase, thus implicating a role for Hoxc8 in controlling cell cycle progression at this critical check point.

Cul4A is required for hematopoietic cell viability and its deficiency leads to apoptosis

In vitro studies indicate that Cul4A ubiquitin ligases target for ubiquitin-mediated proteolysis regulators of cell-cycle progression, apoptosis, development, and DNA repair. In hematopoietic cell lines, studies by our group and others showed that Cul4A ligases regulate proliferation and differentiation in maturing myeloid and erythroid cells. In vivo, Cul4A-deficient embryos die in utero. Cul4A haploinsufficient mice are viable but have fewer erythroid and primitive myeloid progenitors. Yet, little more is known about Cul4A function in vivo. To examine Cul4A function in adults, we generated mice with interferon-inducible deletion of Cul4A. Cul4A deficiency resulted in DNA damage and apoptosis of rapidly dividing cells, and mutant mice died within 3 to 10 days after induction with dramatic atrophy of the intestinal villi, bone marrow, and spleen, and with hematopoietic failure. Cul4A deletion in vivo specifically increased cellular levels of the Cul4A ligase targets Cdt1 and p27(Kip1) but not other known targets. Bone marrow transplantation studies with Cul4A deletion in engrafted cells specifically isolated analysis of Cul4A function to hematopoietic cells and resulted in hematopoietic failure. These recipients died within 9 to 11 days, demonstrating that in hematopoietic cells, Cul4A is essential for survival.

Differential regulation of HOXA9 expression by nuclear factor kappa B (NF-kappaB) and HOXA9

HOXA9 is a homeobox transcription factor expressed in endothelial cells (EC) and its expression is rapidly downregulated during EC activation by inflammatory signals like tumor necrosis factor-alpha (TNF-alpha) and lipopolysaccharide (LPS). Recently, we have shown that HOXA9 overexpression prevents EC activation by inhibiting NF-kappaB activity, which suggests that HOXA9 downregulation is an essential event for EC activation. The present study is directed towards understanding the mechanism of HOXA9 regulation during EC activation. Here we show that nuclear factor-kappaB (NF-kappaB) activation is an essential step for HOXA9 downregulation. Deletion analyses of HOXA9 promoter in EC and NF-kappaB knockout cells have shown that NF-kappaB is a major transcription factor that is absolutely required for HOXA9 downregulation. Our 5' deletion analysis of HOXA9 promoter shows that NF-kappaB response element is localized within first 400 nucleotides, while minimal basal promoter is within 100 nucleotides upstream of its transcriptional start site. We demonstrate that HOXA9 regulates its own expression by positive feedback mechanism. To define mechanism by which HOXA9 autoregulates its expression, we show that HOXA9 DNA binding and transactivation domains are essential.

Cul4A is required for hematopoietic stem-cell engraftment and self-renewal

Several hematopoietic stem-cell (HSC) regulators are controlled by ubiquitin-mediated proteolysis, so the ubiquitin pathway might modulate HSC function. However, this hypothesis has not been formally tested. Cul4A encodes a core subunit of one ubiquitin ligase. Whereas Cul4A-deficient embryos die in utero, Cul4A-haploinsufficient mice are viable but exhibit abnormal hematopoiesis (fewer erythroid and primitive myeloid progenitors). Given these data, we examined whether Cul4A(+/-) HSCs might also be impaired. Using bone marrow transplantation assays, we determined that Cul4A(+/-) HSCs exhibit defects in engraftment and self-renewal capacity. These studies are the first to demonstrate that ubiquitin-mediated protein degradation is important for HSC function. Further, they indicate that a Cul4A ubiquitin ligase targets for degradation one or multiple HSC regulators.

HOXA9 participates in the transcriptional activation of E-selectin in endothelial cells

The homeobox gene HOXA9 has recently been shown to be an important regulator of endothelial cell (EC) differentiation and activation in addition to its role in embryonic development and hematopoiesis. In this report, we have determined that the EC-leukocyte adhesion molecule E-selectin is a key target for HOXA9. The depletion of HOXA9 protein in ECs resulted in a significant and specific decrease in tumor necrosis factor alpha (TNF-alpha)-induced E-selectin gene expression. In addition, HOXA9 specifically activated the E-selectin gene promoter in ECs. Progressive deletional analyses together with site-specific mutagenesis of the E-selectin promoter indicated that the Abd-B-like HOX DNA-binding motif, CAATTTTATTAA, located in the proximal region spanning bp -210 to -221 upstream of the transcription start site was crucial for the promoter induction by HOXA9. Both HOXA9 in EC nuclear extract and recombinant HOXA9 protein bound to this sequence in vitro. Moreover, we showed that HOXA9 binds temporally, in a TNF-alpha-dependent manner, to the region containing this Abd-B-like element in vivo. We have thus identified a novel and functionally critical cis-regulatory element for TNF-alpha-mediated transient expression of the E-selectin gene. Further, we provide evidence that HOXA9 acts as an obligate proinflammatory factor by mediating cytokine induction of E-selectin.

NUP98 Dysregulation in Myeloid Leukemogenesis

Nucleoporin 98 (NUP98) is a component of the nuclear pore complex that facilitates mRNA export from the nucleus. It is mapped to 11p15.5 and is fused to a number of distinct partners, including nine members of the homeobox family as a consequence of leukemia-associated chromosomal translocations. NUP98-HOXA9 is associated with the t(7;11)(p15;p15) translocation in acute myeloid leukemia (AML), myelodysplastic syndrome, and blastic crisis of chronic myeloid leukemia. Expression of NUP98-HOXA9 in murine bone marrow resulted in a myeloproliferative disease progressing to AML by 7-8 months. Transduction of NUP98 fusion genes into human CD34(+) cells confers a proliferative advantage in long-term cytokine-stimulated and stromal cocultures and in NOD-SCID engrafted mice, associated with a five- to eight-fold increase in hematopoietic stem cells. NUP98-HOXA9 expression inhibited erythroid and myeloid differentiation but enhanced serial progenitor replating. NUP98-HOXA9 upregulated a number of homeobox genes of the A and B cluster as well as MEIS1 and Pim-1, and downmodulated globin genes and C/EBPalpha. The HOXA9 component of the NUP98-HOXA9 fusion protein was protected from cullin-4A-mediated ubiquitination and subsequent proteasome-dependent degradation. In NUP98-HOX-transduced CD34(+) cells and cells from AML patients with t(7;11)(p15;p15) NUP98 was no longer associated with the nuclear pore complex but formed intranuclear aggregation bodies. Analysis of NUP98 allelic expression in AML and myelodysplastic syndrome showed loss of heterozygosity observed in 29% of the former and 8% of the latter. This was associated with poor prognosis.

Enforced expression of NUP98-HOXA9 in human CD34(+) cells enhances stem cell proliferation

The t(7;11)(p15;p15) translocation, observed in acute myelogenous leukemia and myelodysplastic syndrome, generates a chimeric gene where the 5' portion of the sequence encoding the human nucleoporin NUP98 protein is fused to the 3' region of HOXA9. Here, we show that retroviral-mediated enforced expression of the NUP98-HOXA9 fusion protein in cord blood-derived CD34(+) cells confers a proliferative advantage in both cytokine-stimulated suspension cultures and stromal coculture. This advantage is reflected in the selective expansion of hematopoietic stem cells as measured in vitro by cobblestone area-forming cell assays and in vivo by competitive repopulation of nonobese diabetic/severe combined immunodeficient mice. NUP98-HOXA9 expression inhibited erythroid progenitor differentiation and delayed neutrophil maturation in transduced progenitors but strongly enhanced their serial replating efficiency. Analysis of the transcriptosome of transduced cells revealed up-regulation of several homeobox genes of the A and B cluster as well as of Meis1 and Pim-1 and down-modulation of globin genes and of CAAT/enhancer binding protein alpha. The latter gene, when coexpressed with NUP98-HOXA9, reversed the enhanced proliferation of transduced CD34(+) cells. Unlike HOXA9, the NUP98-HOXA9 fusion was protected from ubiquitination mediated by Cullin-4A and subsequent proteasome-dependent degradation. The resulting protein stabilization may contribute to the leukemogenic activity of the fusion protein.

CUL4 associates with DDB1 and DET1 and its downregulation affects diverse aspects of development in Arabidopsis thaliana

Cullins are central scaffolding subunits in eukaryotic E3 ligases that facilitate the ubiquitination of target proteins. Arabidopsis contains at least 11 cullin proteins but only a few of them have been assigned biological roles. In this work Arabidopsis cullin 4 is shown to assemble with DDB1, RBX1, DET1 and DDB2 in vitro and in planta. In addition, by using T-DNA insertion and CUL4 antisense lines we demonstrate that corresponding mutants are severely affected in different aspects of development. Reduced CUL4 expression leads to a reduced number of lateral roots, and to abnormal vascular tissue and stomatal development. Furthermore, cul4 mutants display a weak constitutive photomorphogenic phenotype. These results therefore assign an important function to CUL4 during plant development and provide strong evidence that CUL4 assembles together with RBX1 and DDB1 proteins to form a functional E3 ligase in Arabidopsis.

Ubiquitylation in normal and malignant hematopoiesis: novel therapeutic targets

The modification of proteins with ubiquitin is involved in the regulation of various important biological pathways. A crucial step in this process is the modification of specific substrate proteins with ubiquitin by E3 ligases. The ubiquitylation of proteins can result in altered protein function or degradation by the 26S proteasome. Various proteins playing an important role during hematopoiesis are regulated via ubiquitin modification. Recently, alterations in ubiquitylation and proteasomal degradation have been implicated in hematological cancers. Based on these findings, novel therapies that specifically target ubiquitylation or the proteasome are currently being developed. In this review, we will highlight the role of ubiquitylation in normal and malignant hematopoiesis and discuss novel therapeutical approaches that are now tested in various hematological malignancies.

Cul4A and DDB1 associate with Skp2 to target p27Kip1 for proteolysis involving the COP9 signalosome

DDB1, a subunit of the damaged-DNA binding protein DDB, has been shown to function also as an adaptor for Cul4A, a member of the cullin family of E3 ubiquitin ligase. The Cul4A-DDB1 complex remains associated with the COP9 signalosome, and that interaction is conserved from fission yeast to human. Studies with fission yeast suggested a role of the Pcu4-Ddb1-signalosome complex in the proteolysis of the replication inhibitor Spd1. Here we provide evidence that the function of replication inhibitor proteolysis is conserved in the mammalian DDB1-Cul4A-signalosome complex. We show that small interfering RNA-mediated knockdown of DDB1, CSN1 (a subunit of the signalosome), and Cul4A in mammalian cells causes an accumulation of p27Kip1. Moreover, expression of DDB1 reduces the level of p27Kip1 by increasing its decay rate. The DDB1-induced proteolysis of p27Kip1 requires signalosome and Cul4A, because DDB1 failed to increase the decay rate of p27Kip1 in cells deficient in CSN1 or Cul4A. Surprisingly, the DDB1-induced proteolysis of p27Kip1 also involves Skp2, an F-box protein that allows targeting of p27Kip1 for ubiquitination by the Skp1-Cul1-F-box complex. Moreover, we provide evidence for a physical association between Cul4A, DDB1, and Skp2. We speculate that the F-box protein Skp2, in addition to utilizing Cul1-Skp1, utilizes Cul4A-DDB1 to induce proteolysis of p27Kip1.

TGFbeta/BMP inhibits the bone marrow transformation capability of Hoxa9 by repressing its DNA-binding ability

Homeobox (Hox) gene mutations and their altered expressions are frequently linked to human leukemia. Here, we report that transforming growth factor beta (TGFbeta)/bone morphogenetic protein (BMP) inhibits the bone marrow transformation capability of Hoxa9 and Nup98-Hoxa9, the chimeric fusion form of Hoxa9 identified in human acute myeloid leukemia (AML), through Smad4, the common Smad (Co-Smad) in the TGFbeta/BMP signaling pathway. Smad4 interacts directly with the homeodomain of Hoxa9 and blocks the ability of Nup98-Hoxa9 to bind DNA, thereby suppressing its ability to regulate downstream gene transcription. Mapping data revealed that the amino-terminus of Smad4 mediates this interaction and overexpression of the Hoxa9 interaction domain of Smad4 was sufficient to inhibit the enhanced serial replating ability of primary bone marrow cells induced by Nup98-Hoxa9. These studies establish a novel mechanism by which TGFbeta/BMP regulates hematopoiesis and suggest that modification of Hox DNA-binding activity may serve as a novel therapeutic intervention for those leukemias that involve deregulation of Hox.

Cul4A targets p27 for degradation and regulates proliferation, cell cycle exit, and differentiation during erythropoiesis

As erythroid progenitors differentiate into precursors and finally mature red blood cells, lineage-specific genes are induced, and proliferation declines until cell cycle exit. Cul4A encodes a core subunit of a ubiquitin ligase that targets proteins for ubiquitin-mediated degradation, and Cul4A-haploinsufficient mice display hematopoietic dysregulation with fewer multipotential and erythroid-committed progenitors. In this study, stress induced by 5-fluorouracil or phenylhydrazine revealed a delay in the recovery of erythroid progenitors, early precursors, and normal hematocrits in Cul4A(+/-) mice. Conversely, overexpression of Cul4A in a growth factor-dependent, proerythroblast cell line increased proliferation and the proportion of cells in S phase. When these proerythroblasts were induced to terminally differentiate, endogenous Cul4A protein expression declined 3.6-fold. Its enforced expression interfered with erythrocyte maturation and cell cycle exit and, instead, promoted proliferation. Furthermore, p27 normally accumulates during erythroid terminal differentiation, but Cul4A-enforced expression destabilized p27 and attenuated its accumulation. Cul4A and p27 proteins coimmunoprecipitate, indicating that a Cul4A ubiquitin ligase targets p27 for degradation. These findings indicate that a Cul4A ubiquitin ligase positively regulates proliferation by targeting p27 for degradation and that Cul4A down-regulation during terminal erythroid differentiation allows p27 to accumulate and signal cell cycle exit.

Control of Self-Renewal and Differentiation of Hematopoietic Stem Cells: HOXB4 on the Threshold

The homeodomain transcription factor HOXB4 is one of the most attractive tools to expand hematopoietic stem cells in vitro and in vivo and to promote the formation of hematopoietic cells from in vitro differentiated embryonic stem cells. However, the expression levels compatible with the favorable effect of enhanced self-renewal without perturbing differentiation, in vivo, remain to be determined. In this paper, we discuss the necessity to define the "therapeutic width" of HOXB4 expression, based on observations from our lab and others that demonstrate that ectopic HOXB4 expression leads to a concentration-dependent perturbation of lineage differentiation of mouse and human hematopoietic cells. In summary, the combined results argue in favor of the existence of certain threshold levels for HOXB4 activity that control the differentiation and self-renewal behavior of hematopoietic stem and progenitor cells. Indeed, existing evidence suggests that dosage effects of ectopically expressed transcription factors may be more the rule than an exception.

Simian virus 5 V protein acts as an adaptor, linking DDB1 to STAT2, to facilitate the ubiquitination of STAT1

The V protein of simian virus 5 (SV5) facilitates the ubiquitination and subsequent proteasome-mediated degradation of STAT1. Here we show, by visualizing direct protein-protein interactions and by using the yeast two-hybrid system, that while the SV5 V protein fails to bind to STAT1 directly, it binds directly and independently to both DDB1 and STAT2, two cellular proteins known to be essential for SV5-mediated degradation of STAT1. We also demonstrate that STAT1 and STAT2 interact independently of SV5 V and show that SV5 V protein acts as an adaptor molecule linking DDB1 to STAT2/STAT1 heterodimers, which in the presence of additional accessory cellular proteins, including Cullin 4a, can ubiquitinate STAT1. Additionally, we show that the avidity of STAT2 for V is relatively weak but is significantly enhanced by the presence of both STAT1 and DDB1, i.e., the complex of STAT1, STAT2, DDB1, and SV5 V is more stable than a complex of STAT2 and V. From these studies we propose a dynamic model in which SV5 V acts as a bridge, bringing together a DDB1/Cullin 4a-containing ubiquitin ligase complex and STAT1/STAT2 heterodimers, which leads to the degradation of STAT1. The loss of STAT1 results in a decrease in affinity of binding of STAT2 for V such that STAT2 either dissociates from V or is displaced from V by STAT1/STAT2 complexes, thereby ensuring the cycling of the DDB1 and SV5 V containing E3 complex for continued rounds of STAT1 ubiquitination and degradation.