MADM, a novel adaptor protein that mediates phosphorylation of the 14-3-3 binding site of myeloid leukemia factor 1

NRBP1 and TSC22D proteins impact distal convoluted tubule physiology through modulation of the WNK pathway

The With No lysine (WNK) kinases regulate processes such as cell volume and epithelial ion transport through the modulation of Cation Chloride Cotransporters such as the NaCl cotransporter, NCC, present in the distal convoluted tubule (DCT) of the kidney. Recently, the interaction of WNKs with Nuclear Receptor Binding Protein 1 (NRBP1) and Transforming Growth Factor β-Stimulated Clone 22 Domain (TSC22D) proteins was reported. Here we explored the effect of NRBP1 and TSC22Ds on WNK signaling in vitro and in the DCT. TSC22D1.1, TSC22D2, and NRBP1 are localized in DCT WNK bodies, which are cytoplasmic biomolecular condensates associated with WNK activation. In HEK293 cells, long TSC22D isoforms and NRBP1 increase WNK4 activity. DCT-specific NRBP1 knockout mice have reduced NCC phosphorylation and activate a compensatory response. Thus, NRBP1 and long TSC22D proteins are positive modulators of WNK signaling and modulate Na+ reabsorption in the kidney. NRBP1 and TSC22Ds likely influence WNK signaling in other tissues, impacting various physiological processes.

The TSC22D, WNK, and NRBP gene families exhibit functional buffering and evolved with Metazoa for cell volume regulation

The ability to sense and respond to osmotic fluctuations is critical for the maintenance of cellular integrity. We used gene co-essentiality analysis to identify an unappreciated relationship between TSC22D2, WNK1, and NRBP1 in regulating cell volume homeostasis. All of these genes have paralogs and are functionally buffered for osmo-sensing and cell volume control. Within seconds of hyperosmotic stress, TSC22D, WNK, and NRBP family members physically associate into biomolecular condensates, a process that is dependent on intrinsically disordered regions (IDRs). A close examination of these protein families across metazoans revealed that TSC22D genes evolved alongside a domain in NRBPs that specifically binds to TSC22D proteins, which we have termed NbrT (NRBP binding region with TSC22D), and this co-evolution is accompanied by rapid IDR length expansion in WNK-family kinases. Our study reveals that TSC22D, WNK, and NRBP genes evolved in metazoans to co-regulate rapid cell volume changes in response to osmolarity.

The COG1-OsSERL2 complex senses cold to trigger signaling network for chilling tolerance in japonica rice

Improvement of chilling tolerance is a key strategy to face potential menace from abnormal temperature in rice production, which depends on the signaling network triggered by receptors. However, little is known about the QTL genes encoding membrane complexes for sensing cold. Here, Chilling-tolerance in Gengdao/japonica rice 1 (COG1) is isolated from a chromosome segment substitution line containing a QTL (qCS11-jap) for chilling sensitivity. The major gene COG1 is found to confer chilling tolerance in japonica rice. In natural rice populations, only the haplogroup1 encodes a functional COG1. Evolutionary analysis show that COG1 originates from Chinese O. Rufipogon and is fixed in japonica rice during domestication. COG1, a membrane-localized LRR-RLP, targets and activates the kinase OsSERL2 in a cold-induced manner, promoting chilling tolerance. Furthermore, the cold signal transmitted by COG1-OsSERL2 activates OsMAPK3 in the cytoplasm. Our findings reveal a cold-sensing complex, which mediates signaling network for the chilling defense in rice.

The pseudokinase NRBP1 activates Rac1/Cdc42 via P-Rex1 to drive oncogenic signalling in triple-negative breast cancer

We have determined that expression of the pseudokinase NRBP1 positively associates with poor prognosis in triple negative breast cancer (TNBC) and is required for efficient migration, invasion and proliferation of TNBC cells in culture as well as growth of TNBC orthotopic xenografts and experimental metastasis. Application of BioID/MS profiling identified P-Rex1, a known guanine nucleotide exchange factor for Rac1, as a NRBP1 binding partner. Importantly, NRBP1 overexpression enhanced levels of GTP-bound Rac1 and Cdc42 in a P-Rex1-dependent manner, while NRBP1 knockdown reduced their activation. In addition, NRBP1 associated with P-Rex1, Rac1 and Cdc42, suggesting a scaffolding function for this pseudokinase. NRBP1-mediated promotion of cell migration and invasion was P-Rex1-dependent, while constitutively-active Rac1 rescued the effect of NRBP1 knockdown on cell proliferation and invasion. Generation of reactive oxygen species via a NRBP1/P-Rex1 pathway was implicated in these oncogenic roles of NRBP1. Overall, these findings define a new function for NRBP1 and a novel oncogenic signalling pathway in TNBC that may be amenable to therapeutic intervention.

Myeloid leukemia factor 1: A "double-edged sword" in health and disease

The occurrence and development of malignancies are closely related to abnormal cell cycle regulation. Myeloid leukemia factor 1 (MLF1) is a small nucleocytoplasmic shuttling protein associated with cell cycle exit, apoptosis, and certain immune functions. Therefore, it is pertinent to explore the role of MLF1 in health and diseases. Studies to date have suggested that MLF1 could act as a double-edged sword, regulating biochemical activities directly or indirectly. In hematopoietic cells, it serves as a protective factor for the development of lineages, and in malignancies, it serves as an oncogenesis factor. The diversity of its functions depends on the binding partners, including tumor inhibitors, scaffolding molecules, mitochondrial membrane proteins, and transcription factors. Emerging evidence indicates that MLF1 influences immune responses as well. This paper reviews the structure, biological function, and research progress on MLF1 in health and diseases to provide new insights for future research.

Madm/NRBP1 mediates synaptic maintenance and neurodegeneration-induced presynaptic homeostatic potentiation

The precise regulation of synaptic connectivity and function is essential to maintain neuronal circuits. Here, we show that the Drosophila pseudo-kinase Madm/NRBP1 (Mlf-1-adapter-molecule/nuclear-receptor-binding protein 1) is required presynaptically to maintain synaptic stability and to coordinate synaptic growth and function. Presynaptic Madm mediates these functions by controlling cap-dependent translation via the target of rapamycin (TOR) effector 4E-BP/Thor (eukaryotic initiation factor 4E binding protein/Thor). Strikingly, at degenerating neuromuscular synapses, postsynaptic Madm induces a compensatory, transsynaptic signal that utilizes the presynaptic homeostatic potentiation (PHP) machinery to offset synaptic release deficits and to delay synaptic degeneration. Madm is not required for canonical PHP but induces a neurodegeneration-specific form of PHP and acts via the regulation of the cap-dependent translation regulators 4E-BP/Thor and S6-kinase. Consistently, postsynaptic induction of canonical PHP or TOR activation can compensate for postsynaptic Madm to alleviate functional and structural synaptic defects. Our results provide insights into the molecular mechanisms underlying neurodegeneration-induced PHP with potential neurotherapeutic applications.

NRBP1-Containing CRL2/CRL4A Regulates Amyloid β Production by Targeting BRI2 and BRI3 for Degradation

Alzheimer's disease (AD) is a progressive neurodegenerative disease caused by accumulations of Aβ peptides. Production and fibrillation of Aβ are downregulated by BRI2 and BRI3, which are physiological inhibitors of amyloid precursor protein (APP) processing and Aβ oligomerization. Here, we identify nuclear receptor binding protein 1 (NRBP1) as a substrate receptor of a Cullin-RING ubiquitin ligase (CRL) that targets BRI2 and BRI3 for degradation. Moreover, we demonstrate that (1) dimerized NRBP1 assembles into a functional Cul2- and Cul4A-containing heterodimeric CRL through its BC-box and an overlapping cryptic H-box, (2) both Cul2 and Cul4A contribute to NRBP1 CRL function, and (3) formation of the NRBP1 heterodimeric CRL is strongly enhanced by chaperone-like function of TSC22D3 and TSC22D4. NRBP1 knockdown in neuronal cells results in an increase in the abundance of BRI2 and BRI3 and significantly reduces Aβ production. Thus, disrupting interactions between NRBP1 and its substrates BRI2 and BRI3 may provide a useful therapeutic strategy for AD.

A myeloid leukemia factor homolog involved in encystation-induced protein metabolism in Giardia lamblia

BACKGROUND

Giardia lamblia differentiates into resistant cysts as an established model for dormancy. Myeloid leukemia factor (MLF) proteins are important regulators of cell differentiation. Giardia possesses a MLF homolog which was up-regulated during encystation and localized to unknown cytosolic vesicles named MLF vesicles (MLFVs).

METHODS

We used double staining for visualization of potential factors with role in protein metabolism pathway and a strategy that employed a deletion mutant, CDK2m3, to test the protein degradation pathway. We also explored whether autophagy or proteasomal degradation are regulators of Giardia encystation by treatment with MG132, rapamycin, or chloroquine.

RESULTS

Double staining of MLF and ISCU or CWP1 revealed no overlap between their vesicles. The aberrant CDK2m3 colocalized with MLFVs and formed complexes with MLF. MG132 increased the number of CDK2m3-localized vesicles and its protein level. We further found that MLF colocalized and interacted with a FYVE protein and an ATG8-like (ATG8L) protein, which were up-regulated during encystation and their expression induced Giardia encystation. The addition of MG132, rapamycin, or chloroquine, increased their levels and the number of their vesicles, and inhibited the cyst formation. MLF and FYVE were detected in exosomes released from culture.

CONCLUSIONS

The MLFVs are not mitosomes or encystation-specific vesicles, but are related with degradative pathway for CDK2m3. MLF, FYVE, and ATG8L play a positive role in encystation and function in protein clearance pathway, which is important for encystation and coordinated with Exosomes.

GENERAL SIGNIFICANCE

MLF, FYVE, and ATG8L may be involved an encystation-induced protein metabolism during Giardia differentiation.

Molecular Interaction Mechanism of a 14-3-3 Protein with a Phosphorylated Peptide Elucidated by Enhanced Conformational Sampling

Enhanced conformational sampling, a genetic-algorithm-guided multidimensional virtual-system coupled molecular dynamics, can provide equilibrated conformational distributions of a receptor protein and a flexible ligand at room temperature. The distributions provide not only the most stable but also semistable complex structures and propose a ligand-receptor binding process. This method was applied to a system consisting of a receptor protein, 14-3-3ε, and a flexible peptide, phosphorylated myeloid leukemia factor 1 (pMLF1). The results present comprehensive binding pathways of pMLF1 to 14-3-3ε. We identified four thermodynamically stable clusters of MLF1 on the 14-3-3ε surface and free-energy barriers among some clusters. The most stable cluster includes two high-density spots connected by a narrow corridor. When pMLF1 passes the corridor, a salt-bridge relay (switching) related to the phosphorylated residue of pMLF1 occurs. Conformations in one high-density spot are similar to the experimentally determined complex structure. Three-dimensional distributions of residues in the intermolecular interface rationally explain the binding constant changes resulting from the alanine mutation experiment for the residues. We also performed a simulation of nonphosphorylated peptide and 14-3-3ε, which demonstrated that the complex structure was unstable, suggesting that phosphorylation of the peptide is crucially important for binding to 14-3-3ε.

CRL4DCAF8 and USP11 oppositely regulate the stability of myeloid leukemia factors (MLFs)

Myeloid leukemia factors (MLF1 and MLF2) are proteins associated with leukemia and several other cancers. However, little is known about the regulatory mechanisms underlying the stability of these proteins. Here, we show that DDB1 and CUL4 associated factor 8 (DCAF8), which can form a functional E3 ligase complex (CRL4^DCAF8), has a strong interaction with the MLF2 protein. DCAF8 could promote MLF2 degradation through the ubiquitin-proteasome pathway. In contrast, ubiquitin specific peptidase 11 (USP11) associates with MLF2, thereby increasing its stability. Since MLF1 is highly related to MLF2, we demonstrated that MLF1 also interacts with DCAF8 and USP11, suggesting that CRL4^DCAF8 and USP11 may also regulate the expression of MLF1. TCGA analysis revealed that both the myeloid leukemia factors (MLF1 and MLF2) show significant differential expression in various tumors. The results of our study indicate that CRL4^DCAF8 and USP11 play opposite roles in the regulation of MLF1 and MLF2, which may, in turn, affect their biological functions in various cancers.

High NRBP1 expression promotes proliferation and correlates with poor prognosis in bladder cancer

Nuclear receptor binding protein 1 (NRBP1) is an evolutionarily highly conserved adaptor protein with multiple domains. Recently, its role in cancers has received increasing attention. To investigate whether NRBP1 is involved in the development of bladder cancer, we used tissue microarray and analyzed the association between the expression levels of NRBP1 and clinico-pathological features of 56 patients diagnosed with bladder cancer. Subsequently, NRBP1 was silenced using siRNA in bladder cancer cell lines T24 and 5637, and cell phenotype such as proliferation and apoptosis were observed. Further, in vivo tumor formation assay was performed. The expression of apoptosis markers was detected by Western blot. A significant positive correlation between increased NRBP1 expression and tumor stage, and lymph node metastasis was observed in 56 patients. High expression of NRBP1 was associated with poor prognosis and NRBP1 knockdown significantly inhibited cell proliferation and induced intrinsic apoptosis in vitro. Moreover, we also found that NRBP1 knockdown significantly suppress tumor growth in xenograft mouse model. Taken together, these data suggest that NRBP1 plays a critical role in the development of bladder cancer and may represent a potential target for bladder cancer treatment.

NPM-hMLF1 fusion protein suppresses defects of a Drosophila FTLD model expressing the human FUS gene

Fused in sarcoma (FUS) was identified as a component of typical inclusions in frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). In FTLD, both nuclear and cytoplasmic inclusions with wild-type FUS exist, while cytoplasmic inclusions with a mutant-form of FUS occur in many ALS cases. These observations imply that FUS plays a role across these two diseases. In this study, we examined the effect of several proteins including molecular chaperons on the aberrant eye morphology phenotype induced by overexpression of wild-type human FUS (hFUS) in Drosophila eye imaginal discs. By screening, we found that the co-expression of nucleophosmin–human myeloid leukemia factor 1 (NPM-hMLF1) fusion protein could suppress the aberrant eye morphology phenotype induced by hFUS. The driving of hFUS expression at 28 °C down-regulated levels of hFUS and endogenous cabeza, a Drosophila homolog of hFUS. The down-regulation was mediated by proteasome dependent degradation. Co-expression of NPM-hMLF1 suppressed this down-regulation. In addition, co-expression of NPM-hMLF1 partially rescued pharate adult lethal phenotype induced by hFUS in motor neurons. These findings with a Drosophila model that mimics FTLD provide clues for the development of novel FTLD therapies.

Nuclear receptor binding protein 1 correlates with better prognosis and induces caspase-dependent intrinsic apoptosis through the JNK signalling pathway in colorectal cancer

Nuclear receptor binding protein 1 (NRBP1) is a ubiquitously expressed and highly conserved pseudokinase that has important roles in cellular homoeostasis. Despite recent advances in understanding the biology of NRBP1, the role of NRBP1 and its underlying mechanism in colorectal cancer (CRC) have not been fully elucidated. In the present study, we observed that NRBP1 expression levels were significantly reduced in CRC tissues compared with corresponding adjacent normal tissues, and high NRBP1 expression correlated with better prognosis in CRC. Overexpression of NRBP1 inhibited CRC cell proliferation and promoted apoptosis in vitro and in vivo. In contrast, knockdown of NRBP1 expression increased cell proliferation and decreased the percentage of apoptotic cells. Moreover, overexpression of NRBP1 activated caspase-dependent intrinsic apoptosis. In addition, we further discovered that NRBP1 regulated the apoptotic pathway through interaction with JNK. Finally, NRBP1 overexpression led to attenuated CRC growth in a xenograft mouse model. Our study illustrates the suppressor role of NRBP1 in CRC and provides a potential therapeutic target.

Myeloid leukemia factor functions in anti-WSSV immune reaction of kuruma shrimp, Marsupenaeus japonicus

Myeloid leukemia factor (MLF) plays an important role in development, cell cycle, myeloid differentiation, and regulates the RUNX transcription factors. However, the function of MLF in immunity is still unclear. In this study, an MLF was identified and characterized in kuruma shrimp Marsupenaeus japonicus, and named as MjMLF. The full-length cDNA of MjMLF contained 1111 nucleotides, which had an opening reading frame of 816 bp encoding a protein of 272 amino acids with an MLF1-interacting protein domain. MjMLF could be ubiquitously detected in different tissues of shrimp at the transcriptional level. The expression pattern analysis showed that MjMLF could be upregulated in shrimp hemocytes and hepatopancreas after white spot syndrome virus challenge. The RNA interference and protein injection assay showed that MjMLF could inhibit WSSV replication in vivo. Flow cytometry assay showed that MjMLF could induce hemocytes apoptosis which functioned in the shrimp antiviral reaction. All the results suggested that MjMLF played an important role in the antiviral immune reaction of kuruma shrimp. The research indicated that MjMLF might function as a novel regulator to inhibit WSSV replication in shrimp.

Gromoff E, Wang Y, Maier W, Baumeister R. C. elegans DAF-16/FOXO interacts with TGF-ß/BMP signaling to induce germline tumor formation via mTORC1 activation

Activation of the FOXO transcription factor DAF-16 by reduced insulin/IGF signaling (IIS) is considered to be beneficial in C. elegans due to its ability to extend lifespan and to enhance stress resistance. In the germline, cell-autonomous DAF-16 activity prevents stem cell proliferation, thus acting tumor-suppressive. In contrast, hypodermal DAF-16 causes a tumorous germline phenotype characterized by hyperproliferation of the germline stem cells and rupture of the adjacent basement membrane. Here we show that cross-talk between DAF-16 and the transforming growth factor ß (TGFß)/bone morphogenic protein (BMP) signaling pathway causes germline hyperplasia and results in disruption of the basement membrane. In addition to activating MADM/NRBP/hpo-11 gene alone, DAF-16 also directly interacts with both R-SMAD proteins SMA-2 and SMA-3 in the nucleus to regulate the expression of mTORC1 pathway. Knocking-down of BMP genes or each of the four target genes in the hypodermis was sufficient to inhibit germline proliferation, indicating a cell-non-autonomously controlled regulation of stem cell proliferation by somatic tissues. We propose the existence of two antagonistic DAF-16/FOXO functions, a cell-proliferative somatic and an anti-proliferative germline activity. Whereas germline hyperplasia under reduced IIS is inhibited by DAF-16 cell-autonomously, activation of somatic DAF-16 in the presence of active IIS promotes germline proliferation and eventually induces tumor-like germline growth. In summary, our results suggest a novel pathway crosstalk of DAF-16 and TGF-ß/BMP that can modulate mTORC1 at the transcriptional level to cause stem-cell hyperproliferation. Such cell-type specific differences may help explaining why human FOXO activity is considered to be tumor-suppressive in most contexts, but may become oncogenic, e.g. in chronic and acute myeloid leukemia.

The transcription factor FOXO is a well-known tumor suppressor whose activity is controlled by nutrients and stress signaling. In the roundworm C. elegans, the activity of the FOXO protein DAF-16 is best known for its beneficial role in stress response and long lifespan. However, FOXO proteins may also promote tumor cell growth and maintenance in chronic and acute myeloid leukemia, suggesting that may have different roles in distinct contexts. Previously we have shown that selective activation of DAF-16 in the epidermis causes a tumorous growth in the stem cells of the C. elegans germline. Now we demonstrate that this oncogenic activity of DAF-16 is mediated by interactions with the transforming growth factor (TGFß)/Bone Morphogenic protein (BMP) signaling pathway. In the epidermis, direct binding of DAF-16 and R-SMAD proteins of the BMP pathway helps to activate genes involved in the mTORC1 signaling pathway that is frequently activated in tumors. We propose that the transcription factor DAF-16/FOXO may be controlled in different ways in the stem cells, in which its activity normally prevents tumor formation, and in other tissues, in which defects in controlling its activity may result in overwriting the beneficial stem cell activity to eventually promote tumor cell growth.

MLF1 is a proapoptotic antagonist of HOP complex-mediated survival

In the HAX1/HtrA2-OMI/PARL (HOP) mitochondrial protein complex, anti-apoptotic signals are generated by cleavage and activation of the serine protease HtrA2/OMI by the rhomboid protease PARL upon recruitment of both proteases to inner mitochondrial membrane protein HAX1 (HS1-associated protein X-1). Here we report the negative regulation of the HOP complex by human leukemia-associated myeloid leukemia factor 1 (MLF1). We demonstrate that MLF1 physically and functionally associates with HAX1 and HtrA2. Increased interaction of MLF1 with HAX1 and HtrA2 displaces HtrA2 from the HOP complex and inhibits HtrA2 cleavage and activation, resulting in the apoptotic cell death. Conversely, over-expressed HAX1 neutralizes MLF1's effect and inhibits MLF1-induced apoptosis. Importantly, Mlf1 deletion reverses B- and T-cell lymphopenia and significantly ameliorates the progressive striatal and cerebellar neurodegeneration observed in Hax1^-/- mice, with a doubling of the lifespan of Mlf1^-/-/Hax1^-/- animals compared to Hax1^-/- animals. Collectively, these data indicate that MLF1 serves as a proapoptotic antagonist that interacts with the HOP mitochondrial complex to modulate cell survival.

Modulation of mutant Huntingtin aggregates and toxicity by human myeloid leukemia factors

Increased poly glutamine (polyQ) stretch at N-terminal of Huntingtin (HTT) causes Huntington's disease. HTT interacts with large number of proteins, although the preference for such interactions with wild type or mutated HTT protein remains largely unknown. HYPK, an intrinsically unstructured protein chaperone and interactor of mutant HTT was found to interact with myeloid leukemia factor 1 (MLF1) and 2 (MLF2). To identify the role of these two proteins in mutant HTT mediated aggregate formation and toxicity in a cell model, both the proteins were found to preferentially interact with the mutated N-terminal HTT. They significantly reduced the number of cells containing mutant HTT aggregates and subsequent apoptosis in Neuro2A cells. Additionally, in FRAP assay, mobile fraction of mutant HTT aggregates was increased in the presence of MLF1 or MLF2. Further, MLF1 could release transcription factors like p53, CBP and CREB from mutant HTT aggregates. Moreover, in HeLa cell co-expressing mutant HTT exon1 and full length MLF1, p53 was released from the aggregates, leading to the recovery of the expression of the GADD45A transcript, a p53 regulated gene. Taking together, these results showed that MLF1 and MLF2 modulated the formation of aggregates and induction of apoptosis as well as the expressions of genes indirectly.

Stem-Cell-Based Tumorigenesis in Adult Drosophila

Recent studies suggest that a small subset of cells within a tumor, the so-called cancer stem cells (CSCs), are responsible for tumor propagation, relapse, and the eventual death of most cancer patients. CSCs may derive from a few tumor-initiating cells, which are either transformed normal stem cells or reprogrammed differentiated cells after acquiring initial cancer-causing mutations. CSCs and normal stem cells share some properties, but CSCs differ from normal stem cells in their tumorigenic ability. Notably, CSCs are usually resistant to chemo- and radiation therapies. Despite the apparent roles of CSCs in human cancers, the biology underlying their behaviors remains poorly understood. Over the past few years, studies in Drosophila have significantly contributed to this new frontier of cancer research. Here, we first review how stem-cell tumors are initiated and propagated in Drosophila, through niche appropriation in the posterior midgut and through stem-cell competition for niche occupancy in the testis. We then discuss the differences between normal and tumorigenic stem cells, revealed by studying Ras^V12-transformed stem-cell tumors in the Drosophila kidney. Finally, we review the biology behind therapy resistance, which has been elucidated through studies of stem-cell resistance and sensitivity to death inducers using female germline stem cells and intestinal stem cells of the posterior midgut. We expect that screens using adult Drosophila neoplastic stem-cell tumor models will be valuable for identifying novel and effective compounds for treating human cancers.

The novel tumour suppressor Madm regulates stem cell competition in the Drosophila testis

Stem cell competition has emerged as a mechanism for selecting fit stem cells/progenitors and controlling tumourigenesis. However, little is known about the underlying molecular mechanism. Here we identify Mlf1-adaptor molecule (Madm), a novel tumour suppressor that regulates the competition between germline stem cells (GSCs) and somatic cyst stem cells (CySCs) for niche occupancy. Madm knockdown results in overexpression of the EGF receptor ligand vein (vn), which further activates EGF receptor signalling and integrin expression non-cell autonomously in CySCs to promote their overproliferation and ability to outcompete GSCs for niche occupancy. Conversely, expressing a constitutively activated form of the Drosophila JAK kinase (hop^Tum−l) promotes Madm nuclear translocation, and suppresses vn and integrin expression in CySCs that allows GSCs to outcompete CySCs for niche occupancy and promotes GSC tumour formation. Tumour suppressor-mediated stem cell competition presented here could be a mechanism of tumour initiation in mammals.

Stem cell competition mediates the balance between tissue homeostasis and tumour formation, but how this occurs is unclear. Here, Singh et al. show that the tumour suppressor Mlfl-adaptor molecule regulates the balance between germline stem cell and somatic cyst stem cell growth in the Drosophila testis niche.

Drosophila myeloid leukemia factor acts with DREF to activate the JNK signaling pathway

Drosophila myelodysplasia/myeloid leukemia factor (dMLF), a homolog of human MLF1, oncogene was first identified by yeast two-hybrid screen using the DNA replication-related element-binding factor (DREF) as bait. DREF is a transcription factor that regulates proliferation-related genes in Drosophila. It is known that overexpression of dMLF in the wing imaginal discs through the engrailed-GAL4 driver causes an atrophied wing phenotype associated with the induction of apoptosis. However, the precise mechanisms involved have yet to be clarified. Here, we found the atrophied phenotype to be suppressed by loss-of-function mutation of Drosophila Jun N-terminal kinase (JNK), basket (bsk). Overexpression of dMLF induced ectopic JNK activation in the wing disc monitored with the puckered-lacZ reporter line, resulting in induction of apoptosis. The DREF-binding consensus DRE sequence could be shown to exist in the bsk promoter. Chromatin immunoprecipitation assays in S2 cells with anti-dMLF IgG and quantitative real-time PCR revealed that dMLF binds specifically to the bsk promoter region containing the DRE sequence. Furthermore, using a transient luciferase expression assay, we provide evidence that knockdown of dMLF reduced bsk gene promoter activity in S2 cells. Finally, we show that dMLF interacts with DREF in vivo. Altogether, these data indicate that dMLF acts with DREF to stimulate the bsk promoter and consequently activates the JNK pathway to promote apoptosis.

Nuclear receptor-binding protein 1: a novel tumour suppressor and pseudokinase

Pseudokinases are a class of kinases which are structurally designated as lacking kinase activity. Despite the lack of kinase domain sequence conservation, there is increasing evidence that a number of pseudokinases retain kinase activity and/or have critical cellular functions, casting aside previous notions that pseudokinases simply exist as redundant kinases. Moreover, a number of recent studies have implicated pseudokinases as critical components in cancer formation and progression. The present review discusses the interactions and potential functions that nuclear receptor-binding protein 1, a pseudokinase recently described to have a tumour-suppressive role in cancer, may play in cellular homoeostasis and protein regulation. The recent findings highlighted in the present review emphasize the requirement to fully determine the function of pseudokinases in vitro and in vivo, the understanding of which may ultimately uncover new directions for drug discovery.

Subcellular localization of full-length human myeloid leukemia factor 1 (MLF1) is independent of 14-3-3 proteins

Myeloid leukemia factor 1 (MLF1) is associated with the development of leukemic diseases such as acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS). However, information on the physiological function of MLF1 is limited and mostly derived from studies identifying MLF1 interaction partners like CSN3, MLF1IP, MADM, Manp and the 14-3-3 proteins. The 14-3-3-binding site surrounding S34 is one of the only known functional features of the MLF1 sequence, along with one nuclear export sequence (NES) and two nuclear localization sequences (NLS). It was recently shown that the subcellular localization of mouse MLF1 is dependent on 14-3-3 proteins. Based on these findings, we investigated whether the subcellular localization of human MLF1 was also directly 14-3-3-dependent. Live cell imaging with GFP-fused human MLF1 was used to study the effects of mutations and deletions on its subcellular localization. Surprisingly, we found that the subcellular localization of full-length human MLF1 is 14-3-3-independent, and is probably regulated by other as-yet-unknown proteins.

High NRBP1 expression in prostate cancer is linked with poor clinical outcomes and increased cancer cell growth

BACKGROUND

We recently established the rationale that NRBP1 (nuclear receptor binding protein 1) has a potential growth-promoting role in cell biology. NRBP1 interacts directly with TSC-22, a potential tumor suppressor gene that is differently expressed in prostate cancer. Consequently, we analyzed the role of NRBP1 expression in prostate cancer cell lines and its expression on prostate cancer tissue microarrays (TMA).

METHODS

The effect of NRBP1 expression on tumor cell growth was analyzed by using RNAi. NRBP1 protein expression was evaluated on two TMAs containing prostate samples from more than 1,000 patients. Associations with clinico-pathological features, the proliferation marker Ki67 and survival data were analyzed.

RESULTS

RNAi mediated silencing of NRBP1 expression in prostate cancer cell lines resulted in reduced cell growth (P < 0.05). TMA analysis revealed NRBP1 protein expression in benign prostate hyperplasia in 6% as compared to 60% in both, high-grade intraepithelial neoplasia and prostate cancer samples. Strong NRBP1 protein expression was restricted to prostate cancer and correlated with higher expression of the proliferation marker Ki67 (P < 0.05). Further, patients with strong NRBP1 protein expression showed poor clinical outcomes (P < 0.05). Analysis of matched localized cancer tissues before and after castration revealed that post-therapy-related repression of NRBP1 expression was significantly associated with better overall survival.

CONCLUSIONS

We demonstrate that expression of NRBP1 is up-regulated during the progression of prostate cancer and that high NRBP1 expression is linked with poor prognosis and enhanced tumor cell growth.

Myeloid leukemia factor: a return ticket from human leukemia to fly hematopoiesis

Even though deregulation of human MLF1, the founding member of the Myeloid Leukemia Factor family, has been associated with acute myeloid leukemia, the function and mode of action of this family of genes have remained rather mysterious. Yet, recent findings in Drosophila shed new light on their biological activity and suggest that they play an important role in hematopoiesis and leukemia, notably by regulating the stability of RUNX transcription factors, another family of conserved proteins with prominent roles in normal and malignant blood cell development.

The adaptor protein 14-3-3 binds to the calcium-sensing receptor and attenuates receptor-mediated Rho kinase signalling

A yeast two-hybrid screen performed to identify binding partners of the CaR (calcium-sensing receptor) intracellular tail identified the adaptor protein 14-3-3θ as a novel binding partner that bound to the proximal membrane region important for CaR expression and signalling. The 14-3-3θ protein directly interacted with the CaR tail in pull-down studies and FLAG-tagged CaR co-immunoprecipitated with EGFP (enhanced green fluorescent protein)-tagged 14-3-3θ when co-expressed in HEK (human embryonic kidney)-293 or COS-1 cells. The interaction between the CaR and 14-3-3θ did not require a putative binding site in the membrane-proximal region of the CaR tail and was independent of PKC (protein kinase C) phosphorylation. Confocal microscopy demonstrated co-localization of the CaR and EGFP–14-3-3θ in the ER (endoplasmic reticulum) of HEK-293 cells that stably expressed the CaR (HEK-293/CaR cells), but 14-3-3θ overexpression had no effect on membrane expression of the CaR. Overexpression of 14-3-3θ in HEK-293/CaR cells attenuated CaR-mediated Rho signalling, but had no effect on ERK (extracellular-signal-regulated kinase) 1/2 signalling. Another isoform identified from the library, 14-3-3ζ, exhibited similar behaviour to that of 14-3-3θ with respect to CaR tail binding, cellular co-localization and impact on receptor-mediated signalling. However, unlike 14-3-3θ, this isoform, when overexpressed, significantly reduced CaR plasma membrane expression. Results indicate that 14-3-3 proteins mediate CaR-dependent Rho signalling and may modulate the plasma membrane expression of the CaR.

Structural insights of the MLF1/14-3-3 interaction

Myeloid leukaemia factor 1 (MLF1) binds to 14-3-3 adapter proteins by a sequence surrounding Ser34 with the functional consequences of this interaction largely unknown. We present here the high-resolution crystal structure of this binding motif [MLF1(29-42)pSer34] in complex with 14-3-3ε and analyse the interaction with isothermal titration calorimetry. Fragment-based ligand discovery employing crystals of the binary 14-3-3ε/MLF1(29-42)pSer34 complex was used to identify a molecule that binds to the interface rim of the two proteins, potentially representing the starting point for the development of a small molecule that stabilizes the MLF1/14-3-3 protein-protein interaction. Such a compound might be used as a chemical biology tool to further analyse the 14-3-3/MLF1 interaction without the use of genetic methods. Database Structural data are available in the Protein Data Bank under the accession number(s) 3UAL [14-3-3ε/MLF1(29-42)pSer34 complex] and 3UBW [14-3-3ε/MLF1(29-42)pSer34/3-pyrrolidinol complex] Structured digital abstract •  14-3-3 epsilon and MLF1 bind by x-ray crystallography (View interaction) •  14-3-3 epsilon and MLF1 bind by isothermal titration calorimetry (View Interaction: 1, 2).

Distant homologs of anti-apoptotic factor HAX1 encode parvalbumin-like calcium binding proteins

Background

Apoptosis is a highly ordered and orchestrated multiphase process controlled by the numerous cellular and extra-cellular signals, which executes the programmed cell death via release of cytochrome c alterations in calcium signaling, caspase-dependent limited proteolysis and DNA fragmentation. Besides the general modifiers of apoptosis, several tissue-specific regulators of this process were identified including HAX1 (HS-1 associated protein X-1) - an anti-apoptotic factor active in myeloid cells. Although HAX1 was the subject of various experimental studies, the mechanisms of its action and a functional link connected with the regulation of apoptosis still remains highly speculative.

Findings

Here we provide the data which suggests that HAX1 may act as a regulator or as a sensor of calcium. On the basis of iterative similarity searches, we identified a set of distant homologs of HAX1 in insects. The applied fold recognition protocol gives us strong evidence that the distant insects' homologs of HAX1 are novel parvalbumin-like calcium binding proteins. Although the whole three EF-hands fold is not preserved in vertebrate our analysis suggests that there is an existence of a potential single EF-hand calcium binding site in HAX1. The molecular mechanism of its action remains to be identified, but the risen hypothesis easily translates into previously reported lines of various data on the HAX1 biology as well as, provides us a direct link to the regulation of apoptosis. Moreover, we also report that other family of myeloid specific apoptosis regulators - myeloid leukemia factors (MLF1, MLF2) share the homologous C-terminal domain and taxonomic distribution with HAX1.

Conclusions

Performed structural and active sites analyses gave new insights into mechanisms of HAX1 and MLF families in apoptosis process and suggested possible role of HAX1 in calcium-binding, still the analyses require further experimental verification.

Bunched and Madm: a novel growth-regulatory complex?

By combining Drosophila genetics and proteomics Gluderer et al. report in this issue of Journal of Biology the isolation of a novel growth-regulatory complex consisting of Bunched and Madm. Future study of this complex will address the precise mechanism of growth control, regulation of complex activity, the interface with other growth pathways and a potential role in human cancer.

See research article at http://jbiol.com/content/9/1/9.

Madm (Mlf1 adapter molecule) cooperates with Bunched A to promote growth in Drosophila

BACKGROUND

The TSC-22 domain family (TSC22DF) consists of putative transcription factors harboring a DNA-binding TSC-box and an adjacent leucine zipper at their carboxyl termini. Both short and long TSC22DF isoforms are conserved from flies to humans. Whereas the short isoforms include the tumor suppressor TSC-22 (Transforming growth factor-beta1 stimulated clone-22), the long isoforms are largely uncharacterized. In Drosophila, the long isoform Bunched A (BunA) acts as a growth promoter, but how BunA controls growth has remained obscure.

RESULTS

In order to test for functional conservation among TSC22DF members, we expressed the human TSC22DF proteins in the fly and found that all long isoforms can replace BunA function. Furthermore, we combined a proteomics-based approach with a genetic screen to identify proteins that interact with BunA. Madm (Mlf1 adapter molecule) physically associates with BunA via a conserved motif that is only contained in long TSC22DF proteins. Moreover, Drosophila Madm acts as a growth-promoting gene that displays growth phenotypes strikingly similar to bunA phenotypes. When overexpressed, Madm and BunA synergize to increase organ growth.

CONCLUSIONS

The growth-promoting potential of long TSC22DF proteins is evolutionarily conserved. Furthermore, we provide biochemical and genetic evidence for a growth-regulating complex involving the long TSC22DF protein BunA and the adapter molecule Madm.

Gene expression profiles classify human osteosarcoma xenografts according to sensitivity to doxorubicin, cisplatin, and ifosfamide

PURPOSE

In osteosarcoma, aggressive preoperative and postoperative multidrug chemotherapy given to all patients has improved patient survival rate to the present level of approximately 60%. However, no tumor marker is available that reliably can identify those patients who will or will not respond to chemotherapy.

EXPERIMENTAL DESIGN

In an attempt to find leads to such markers, we have obtained microarray gene expression profiles from a panel of 10 different human osteosarcoma xenografts and related the results to their sensitivity to ifosfamide, doxorubicin, and cisplatin.

RESULTS

The expression data identified genes with highly significant differential expression between poor and good responder xenografts to the three different drugs: 85 genes for doxorubicin, 74 genes for cisplatin, and 118 genes for ifosfamide. Technical validation with quantitative reverse transcription-PCR showed good correlation with the microarray expression data. Gene Ontology-guided analysis suggested that properties of the poorly responsive xenografts were resistance to undergo programmed cell death and, particularly for ifosfamide, a drive toward dedifferentiation and increased tumor aggressiveness. Leads toward metabolic alterations and involvement of mitochondrial pathways for apoptosis and stress response were more prominent for doxorubicin and cisplatin. Finally, small interfering RNA-mediated gene silencing of IER3 and S100A2 sensitized the human osteosarcoma cell line OHS to treatment with 4-hydroperoxyifosfamide.

CONCLUSIONS

The expression profiles contained several novel biomarker candidates that may help predict the responsiveness of osteosarcoma to doxorubicin, cisplatin, and ifosfamide. The potential of selected candidates will be further validated on clinical specimens from osteosarcoma patients.

The myeloid leukemia factor interacts with COP9 signalosome subunit 3 in Drosophila melanogaster

The human myeloid leukemia factor 1 (hMLF1) gene was first identified as an NPM-hMLF1 fusion gene produced by chromosomal translocation. In Drosophila, dMLF has been identified as a protein homologous to hMLF1 and hMLF2, which interacts with various factors involved in transcriptional regulation. However, the precise cellular function of dMLF remains unclear. To generate further insights, we first examined the behavior of dMLF protein using an antibody specific to dMLF. Immunostaining analyses showed that dMLF localizes in the nucleus in early embryos and cultured cells. Ectopic expression of dMLF in the developing eye imaginal disc using eyeless-GAL4 driver resulted in a small-eye phenotype and co-expression of cyclin E rescued the small-eye phenotype, suggesting the involvement of dMLF in cell-cycle regulation. We therefore analyzed the molecular mechanism of interactions between dMLF and a dMLF-interacting protein, dCSN3, a subunit of the COP9 signalosome, which regulates multiple signaling and cell-cycle pathways. Biochemical and genetic analyses revealed that dMLF interacts with dCSN3 in vivo and glutathione S-transferase pull-down assays revealed that the PCI domain of the dCSN3 protein is sufficient for this to occur, possibly functioning as a structural scaffold for assembly of the COP9 signalosome complex. From these data we propose the possibility that dMLF plays a negative role in assembly of the COP9 signalosome complex.

Shuttling imbalance of MLF1 results in p53 instability and increases susceptibility to oncogenic transformation

Myeloid leukemia factor 1 (MLF1) stabilizes the activity of the tumor suppressor p53 by suppressing its E3 ubiquitin ligase, COP1, through a third component of the COP9 signalosome (CSN3). However, little is known about how MLF1 functions upstream of the CSN3-COP1-p53 pathway and how its deregulation by the formation of the fusion protein nucleophosmin (NPM)-MLF1, generated by t(3;5)(q25.1;q34) chromosomal translocation, leads to leukemogenesis. Here we show that MLF1 is a cytoplasmic-nuclear-shuttling protein and that its nucleolar localization on fusing with NPM prevents the full induction of p53 by both genotoxic and oncogenic cellular stress. The majority of MLF1 was located in the cytoplasm, but the treatment of cells with leptomycin B rapidly induced a nuclear accumulation of MLF1. A mutation of the nuclear export signal (NES) motif identified in the MLF1 sequence enhanced the antiproliferative activity of MLF1. The fusion of MLF1 with NPM translocated MLF1 to the nucleolus and abolished the growth-suppressing activity. The introduction of NPM-MLF1 into early-passage murine embryonic fibroblasts allowed the cells to escape from cellular senescence at a markedly earlier stage and induced neoplastic transformation in collaboration with the oncogenic form of Ras. Interestingly, disruption of the MLF1-derived NES sequence completely abolished the growth-promoting activity of NPM-MLF1 in murine fibroblasts and hematopoietic cells. Thus, our results provide important evidence that the shuttling of MLF1 is critical for the regulation of cell proliferation and a disturbance in the shuttling balance increases the cell's susceptibility to oncogenic transformation.

Non-pathogenic protein aggregates in skeletal muscle in MLF1 transgenic mice

Protein aggregate formation in muscle is thought to be pathogenic and associated with clinical weakness. Over-expression of either wild type or a mutant form of myeloid leukemia factor 1 (MLF1) in transgenic mouse skeletal muscle and in cultured cells resulted in aggregate formation. Aggregates were detected in MLF1 transgenic mice at 6 weeks of age, and increased in size with age. However, histological examination of skeletal muscles of MLF1 transgenic mice revealed no pathological changes other than the aggregates, and RotaRod testing did not detect functional deficits. MLF1 has recently been identified as a protein that could neutralize the toxicity of intracellular protein aggregates in a Drosophila model of Huntington's disease (HD). We also demonstrate that MLF1 interacts with MRJ, a heat shock protein, which can independently neutralize the toxicity of intracellular protein aggregates in the Drosophila HD model. Our data suggest that over-expression of MLF1 has no significant impact on skeletal muscle function in mice; that progressive formation of protein aggregates in muscle are not necessarily pathogenic; and that MLF1 and MRJ may function together to ameliorate the toxic effects of polyglutamine or mutant proteins in myodegenerative diseases such as inclusion body myositis and oculopharyngeal muscular dystrophy, as well as neurodegenerative disease.

Characterization of the Drosophila myeloid leukemia factor

In human, the myeloid leukemia factor 1 (hMLF1) has been shown to be involved in acute leukemia, and mlf related genes are present in many animals. Despite their extensive representation and their good conservation, very little is understood about their function. In Drosophila, dMLF physically interacts with both the transcription regulatory factor DREF and an antagonist of the Hedgehog pathway, Suppressor of Fused, whose over-expression in the fly suppresses the toxicity induced by polyglutamine. No connection between these data has, however, been established. Here, we show that dmlf is widely and dynamically expressed during fly development. We isolated and analyzed the first dmlf mutants: embryos lacking maternal dmlf product have a low viability with no specific defect, and dmlf(-)- adults display weak phenotypes. We monitored dMLF subcellular localization in the fly and cultured cells. We were able to show that, although generally nuclear, dMLF can also be cytoplasmic, depending on the developmental context. Furthermore, two differently spliced variants of dMLF display differential subcellular localization, allowing the identification of regions of dMLF potentially important for its localization. Finally, we demonstrate that dMLF can act developmentally and postdevelopmentally to suppress neurodegeneration and premature aging in a cerebellar ataxia model.

Myeloid Leukemia Factor 1 Associates with a Novel Heterogeneous Nuclear Ribonucleoprotein U-like Molecule

Myeloid leukemia factor 1 (MLF1) is an oncoprotein associated with hemopoietic lineage commitment and acute myeloid leukemia. Here we show that Mlf1 associated with a novel binding partner, Mlf1-associated nuclear protein (Manp), a new heterogeneous nuclear ribonucleoprotein (hnRNP) family member, related to hnRNP-U. Manp localized exclusively in the nucleus and could redirect Mlf1 from the cytoplasm into the nucleus. The nuclear content of Mlf1 was also regulated by 14-3-3 binding to a canonical 14-3-3 binding motif within the N terminus of Mlf1. Significantly Mlf1 contains a functional nuclear export signal and localized primarily to the nuclei of hemopoietic cells. Mlf1 was capable of binding DNA, and microarray analysis revealed that it affected the expression of several genes, including transcription factors. In summary, this study reveals that Mlf1 translocates between nucleus and cytoplasm, associates with a novel hnRNP, and influences gene expression.

Isolation and characterization of a novel gene, xMADML, involved in Xenopus laevis eye development

We have identified Xenopus MADM-like (xMADML), a Xenopus laevis gene related to the murine MADM and the human NRBP genes. xMADML is expressed throughout early development and is expressed most strongly in the developing lens and more weakly in the retina and other anterior tissues. We demonstrate that disruption of xMADML translation by means of morpholino injection results in impaired retina and lens development. Reciprocal transplantation of the presumptive lens ectoderm between morpholino-injected embryos and those injected solely with a dextran lineage tracer demonstrates that xMADML is necessary in both the lens and the retina for correct development of these eye tissues. Analysis of gene expression after knockdown of xMADML revealed significant alterations in the expression of some genes, including Pax6, xSix3, Sox2, and Sox3, suggesting that xMADML plays a role in regulating gene expression during development of the eye. This investigation is the first in vivo study examining the developmental role of this novel gene and reveals an important role of xMADML in eye tissue development and differentiation.

Adapter protein NRBP associates with Jab1 and negatively regulates AP-1 activity

Jun activation domain-binding protein 1 (Jab1) is a coactivator of activating protein-1 (AP-1) and is the fifth component of the COP9 signalosome complex. It interacts with a variety of proteins and plays important roles in diverse signaling pathways and cellular function including oncogenesis. We show here that Jab1 interacts in vivo with nuclear receptor binding protein (NRBP), an evolutionarily conserved adapter protein with a kinase-like domain. We further show that NRBP inhibits Jab1-induced phosphorylation of c-Jun and AP-1 activation. Finally, overexpression of NRBP in mammalian cells specifically inhibits AP-1 activation by various stimuli. Taken together, our data suggest that NRBP may be an important negative regulator of Jab1-mediated functions such as gene transcription and tumor progression.

The non-structural 3 (NS3) protein of dengue virus type 2 interacts with human nuclear receptor binding protein and is associated with alterations in membrane structure

Flaviviral infections produce a distinct array of virus-induced intracellular membrane alterations that are associated with the flaviviral replication machinery. Currently, it is still unknown which flaviviral protein(s) is/are responsible for this induction. Using yeast two-hybrid and co-immunoprecipitation analyses, we demonstrated that the NS3 protein of dengue virus type 2 interacted specifically with nuclear receptor binding protein (NRBP), a host cellular protein that influences trafficking between the endoplasmic reticulum (ER) and Golgi, and that interacts with Rac3, a member of the Rho-GTPase family. Co-expression of NS3 and NRBP in baby hamster kidney cells exhibited significant subcellular co-localization, and revealed the redistribution of NRBP from the cytoplasm to the perinuclear region. Furthermore, a set of membrane structures affiliated with the rough ER at the perinuclear region was induced in cells transfected with NS3. These structures are reminiscent of the virus-induced convoluted membranes previously observed in flavivirus-infected cells. This interaction between dengue viral and host cell proteins as well as the formation of the NS3-induced membrane structures suggest that NS3 may subvert the role of NRBP in ER-Golgi trafficking.

Myeloid Leukemia Factor 1 inhibits erythropoietin-induced differentiation, cell cycle exit and p27Kip1 accumulation

Myeloid leukemia factor 1 (MLF1) is a novel oncoprotein involved in translocations associated with acute myeloid leukemia (AML), especially erythroleukemias. In this study, we demonstrate that ectopic expression of Mlf1 prevented J2E erythroleukemic cells from undergoing biological and morphological maturation in response to erythropoietin (Epo). We show that Mlf1 inhibited Epo-induced cell cycle exit and suppressed a rise in the cell cycle inhibitor p27(Kip1). Unlike differentiating J2E cells, Mlf1-expressing cells did not downregulate Cul1 and Skp2, components of the ubiquitin E3 ligase complex SCF(Skp2) involved in the proteasomal degradation of p27(Kip1). In contrast, Mlf1 did not interfere with increases in p27(Kip1) and terminal differentiation initiated by thyroid hormone withdrawal from erythroid cells, or cytokine-stimulated maturation of myeloid cells. These data demonstrate that Mlf1 interferes with an Epo-responsive pathway involving p27(Kip1) accumulation, which inhibits cell cycle arrest essential for erythroid terminal differentiation.

cDNA cloning and characterization of a novel gene encoding the MLF1-interacting protein MLF1IP

Myelodysplasia/acute myeloid leukemia (MDS/AML) is characterized by a t(3;5)(q25.1;q34) chromosomal translocation that forms a fusion gene between nucleophosmin (NPM) and MDS/myeloid leukemia factor 1 (MLF1). We identified a novel protein, MLF1-interacting protein (MLF1IP), that specifically associates with MLF1 by yeast two-hybrid analysis and in pulldown assays, and colocalizes with it in both the nuclei and cytoplasm of cells. The MLF1IP gene locus is at chromosome 4q35.1 and is composed of 14 exons spanning 75.8 kb of genomic DNA. The MLF1IP cDNA encodes a 46-kDa protein that contains two bipartite and two classical nuclear localization signals, two nuclear receptor-binding motifs (LXXLL), two leucine zippers, two PEST residues and several potential phosphorylation sites. MLF1IP transcripts are expressed in a variety of tissues (e.g. fetal liver, bone marrow, thymus and testis). MLF1IP appears to be a lineage-specific gene whose expression is confined exclusively to the CFU-E erythroid precursor cells, but not in mature erythrocytes. These observations, together with previous data demonstrating a role for MLF1 in suppressing red cell maturation, suggest a possible role for MLF1IP and MLF1 deregulation in the genesis of erythroleukemias.

HLS5, a Novel RBCC (Ring Finger, B Box, Coiled-coil) Family Member Isolated from a Hemopoietic Lineage Switch, Is a Candidate Tumor Suppressor

Hemopoietic cells, apparently committed to one lineage, can be reprogrammed to display the phenotype of another lineage. The J2E erythroleukemic cell line has on rare occasions developed the features of monocytic cells. Subtractive hybridization was used in an attempt to identify genes that were up-regulated during this erythroid to myeloid transition. We report here on the isolation of hemopoietic lineage switch 5 (Hls5), a gene expressed by the monocytoid variant cells, but not the parental J2E cells. Hls5 is a novel member of the RBCC (Ring finger, B box, coiled-coil) family of genes, which includes Pml, Herf1, Tif-1alpha, and Rfp. Hls5 was expressed in a wide range of adult tissues; however, at different stages during embryogenesis, Hls5 was detected in the branchial arches, spinal cord, dorsal root ganglia, limb buds, and brain. The protein was present in cytoplasmic granules and punctate nuclear bodies. Isolation of the human cDNA and genomic DNA revealed that the gene was located on chromosome 8p21, a region implicated in numerous leukemias and solid tumors. Enforced expression of Hls5 in HeLa cells inhibited cell growth, clonogenicity, and tumorigenicity. It is conceivable that HLS5 is one of the tumor suppressor genes thought to reside at the 8p21 locus.