Expression of p68 RNA helicase is closely related to the early stage of adipocyte differentiation of mouse 3T3-L1 cells

Identification of DDX5 as an indispensable activator of the glucocorticoid receptor in adipocyte differentiation

The expression of CCAAT/enhancer-binding protein (C/EBP) family members and peroxisome proliferator-activated receptor γ (PPAR γ) is essential for the differentiation of pre-adipocyte 3T3-L1 cells into mature adipocytes induced by a combined stimulation with dexamethasone, 3-isobutyl-1-methylxanthine and insulin (DMI). We herein demonstrated that the RNA helicase DDX5, the expression of which was induced by DMI, played an important role in the adipocyte differentiation of 3T3-L1 cells. The DMI-induced accumulation of lipid droplets and expression of adipocyte markers in 3T3-L1 cells were significantly inhibited by the knockdown of DDX5. The knockdown of DDX5 interfered with the expressional induction of C/EBPδ, which was the first to be induced in the transcription factor cascade, and inhibited the subsequent expression of the other transcription factors, C/EBPβ, PPARγ and C/EBPα. DDX5 interacted with the glucocorticoid receptor (GR), which induced the expression of C/EBPδ. The knockdown of DDX5 failed to induce the nuclear translocation of GR, suggesting the essential role of DDX5 in the early stage of adipocyte differentiation. Furthermore, the reconstitution of DDX5, but not the DDX5 mutant (K144N) lacking RNA helicase activity, restored DMI-induced GR activation and adipocyte differentiation in 3T3-L1 cells in which DDX5 was knocked down, confirming that the RNA helicase activity of DDX5 is essential for adipogenesis. Collectively, these results revealed for the first time that DDX5 is necessary for GR activation and plays an essential role in early adipocyte differentiation.

Induction of DEAD Box helicase 5 in early adipogenesis is regulated by Ten-eleven translocation 2

Dead box helicase 5 (DDX5) is an RNA helicase that is has cellular function on RNA splicing and transcriptional regulation. It has been reported to be involved in cell differentiation including adipogenesis. However, it is not clear how DDX5 is regulated during adipogenesis. Our previous report demonstrated that the Ten-eleven translocation methyl-cytosine dioxygenase 2 (TET2) is required for adipogenesis. This study was aimed to investigate DDX5 as a direct target of TET2 upon adipogenic induction of 3T3-L1 preadipocyte. Microarray-based screening of differentially expressed genes upon TET2 knockdown identified genes involved in cell cycle, DNA replication, and ribosome biology as major targets of TET2 in the initial step of adipogenic induction. The Ddx5 gene was identified and validated as the target. TET2-mediated epigenetic regulation of the Ddx5 gene was measured by two independent methods including immunoprecipitation against 5-hydroxymethylcytosine (5hmC) and 5-methylcytosine (5mC) as well as EpiMark 5hmC and 5mC analysis. Ddx5 expression was downregulated upon TET2 knockdown, coincided with a significant decrease of 5hmC at the Ddx5 locus. DDX5 knockdown significantly suppressed adipogenesis, while DDX5 overexpression promoted it. Importantly, DDX5 overexpression, when co-transfected, rescued the process of adipogenesis, which was hindered by TET2 siRNA treatment. The findings suggest TET2-mediated regulation of the Ddx5 gene is required for an initial step of adipogenesis.

The role of DEAD-box RNA helicase p68 (DDX5) in the development and treatment of breast cancer

RNA helicase p68 or DEAD (Asp-Glu-Ala-Asp) box polypeptide 5 (DDX5) is a unique member of the highly conserved protein family, which is involved in a broad spectrum of biological processes, including transcription, translation, precursor messenger RNA processing or alternative splicing, and microRNA (miRNA) processing. It has been shown that p68 is necessary for cell growth and participates in the early development and maturation of some organs. Interestingly, p68 is a transcriptional coactivator of numerous oncogenic transcription factors, including nuclear factor-κβ (NF-κβ), estrogen receptor α (ERα), β-catenin, androgen receptor, Notch transcriptional activation complex, p53 and signal transducer, and activator of transcription 3 (STAT3). Recent studies on the role of p68 (DDX5) in multiple dysregulated cellular processes in various cancers and its abnormal expression indicate the importance of this factor in tumor development. Discussion of the precise role of p68 in cancer is complex and depends on the cellular microenvironment and interacting factors. In terms of the deregulated expression of p68 in breast cancer and the high prevalence of this cancer among women, it can be informative to review the precise function of this factor in the breast cancer. Therefore, an attempt will be made in this review to clarify the tumorigenic function of p68 in association with its targeting potential for the treatment of breast cancer.

Characterization of the mammalian DEAD-box protein DDX5 reveals functional conservation with S. cerevisiae ortholog Dbp2 in transcriptional control and glucose metabolism

DEAD-box proteins are a class of nonprocessive RNA helicases that dynamically modulate the structure of RNA and ribonucleoprotein complexes (RNPs). However, the precise roles of individual members are not well understood. Work from our laboratory revealed that the DEAD-box protein Dbp2 in Saccharomyces cerevisiae is an active RNA helicase in vitro that functions in transcription by promoting mRNP assembly, repressing cryptic transcription initiation, and regulating long noncoding RNA activity. Interestingly, Dbp2 is also linked to glucose sensing and hexose transporter gene expression. DDX5 is the mammalian ortholog of Dbp2 that has been implicated in cancer and metabolic syndrome, suggesting that the role of Dbp2 and DDX5 in glucose metabolic regulation is conserved. Herein, we present a refined biochemical and biological comparison of yeast Dbp2 and human DDX5 enzymes. We find that human DDX5 possesses a 10-fold higher unwinding activity than Dbp2, which is partially due to the presence of a mammalian/avian specific C-terminal extension. Interestingly, ectopic expression of DDX5 rescues the cold sensitivity, cryptic initiation defects, and impaired glucose import in dbp2Δ cells, suggesting functional conservation. Consistently, we show that DDX5 promotes glucose uptake and glycolysis in mouse AML12 hepatocyte cells, suggesting that mammalian DDX5 and S. cerevisiae Dbp2 share conserved roles in cellular metabolism.

[Molecular Mechanisms of Early-stage Adipocyte Differentiation and Multi-functional Roles of Newly Isolated Adipogenic Factors]

Obesity is a major risk factor for diabetes, hypertension, hyperlipidemia, and arteriosclerosis. Although the middle and late stages of adipocyte differentiation are well characterized, the earliest step in the differentiation process has remained largely unknown. We isolated 102 genes expressed at the beginning of the differentiation of a mouse preadipocyte cell line, 3T3-L1 cells. Because approximately half of these genes were unknown, we named them factor for adipocyte differentiation (fad) genes. I first show how these genes regulate the early stage of adipocyte differentiation. We next generated fad104-deficient mice, and demonstrated that fad104-deficient mice died due to cyanosis-associated lung dysplasia with atelectasis. We also found that fad104 positively regulated adipocyte differentiation and negatively regulated osteoblast differentiation. We then demonstrated that fad24-knockdown inhibited mitotic clonal expansion (MCE) and that FAD24 contributed to the regulation of DNA replication by recruiting histone acetyltransferase binding to ORC1 (HBO1) to DNA replication origins. In vitro culture experiments revealed that fad24-null embryos developed normally to the morula stage, but acquired growth defects in subsequent stages. These results strongly suggest that fad24 is essential for pre-implantation in embryonic development, particularly for progression to the blastocyst stage. These findings together indicate that both fad104 and fad24 contribute not only to adipogenesis but also to other physiological events. The multi-functional roles of these genes are discussed.

DDX5/p68 RNA helicase expression is essential for initiating adipogenesis

Background

DDX5/p68 RNA helicase is a member of the DEAD (Asp-Glu-Ala-Asp) box proteins. Apart from RNA unwinding, DDX5 is an important transcriptional factor and co-activator in cell proliferation and differentiation.

Findings

Here, we have characterised the role of DDX5 in adipogenesis in 3T3-L1 cells using siRNA mediated suppression. Transient inhibition of Ddx5 mRNA expression at the start of adipogenesis impairs the differentiation programme even when DDX5 expression is restored later in adipogenesis. However transient suppression of Ddx5 at the later stages of adipogenesis do not impair adipogenesis or triglyceride accumulation suggesting Ddx5 expression is dispensable in a mature adipocyte.

Conclusion

These results implicate DDX5 as a crucial factor involved in the complex transcriptional cascade of events that regulate adipogenesis and essential to the initiation of adipogenesis.

P68 RNA helicase as a molecular target for cancer therapy

The DEAD-box family of RNA helicase is known to be required in virtually all cellular processes involving RNA, and p68 is a prototypic one of the family. Reports have indicated that in addition to ATPase and RNA helicase ability, p68 can also function as a co-activator for transcription factors such as estrogen receptor alpha, tumor suppressor p53 and beta-catenin. More than that, post-translational modification of p68 including phosphorylation, acetylation, sumoylation, and ubiquitylation can regulate the coactivation effect. Furthermore, aberrant expression of p68 in cancers highlights that p68 plays an important role for tumorgenesis and development. In this review, we briefly introduce the function and modulation of p68 in cancer cells, and put forward envisagement about future study about p68.

Detection of differential fetal and adult expression of chloride intracellular channel 4 (CLIC4) protein by analysis of a green fluorescent protein knock-in mouse line

Background

Chloride Intracellular Channel 4 (CLIC4) is one of seven members in the closely related CLIC protein family. CLIC4 is involved in multiple cellular processes including apoptosis, cellular differentiation, inflammation and endothelial tubulogenesis. Despite over a decade of research, no comprehensive in situ expression analysis of CLIC4 in a living organism has been reported. In order to fulfill this goal, we generated a knock-in mouse to express Green Fluorescent Protein (GFP) from the CLIC4 locus, thus substituting the GFP coding region for CLIC4. We used GFP protein expression to eliminate cross reaction with other CLIC family members.

Results

We analyzed CLIC4 expression during embryonic development and adult organs. During mid and late gestation, CLIC4 expression is modulated particularly in fetal brain, heart, thymus, liver and kidney as well as in developing brown adipose tissue and stratifying epidermis. In the adult mouse, CLIC4 is highly expressed globally in vascular endothelial cells as well as in liver, lung alveolar septae, pancreatic acini, spermatogonia, renal proximal tubules, cardiomyocytes and thymic epithelial cells. Neural expression included axonal tracks, olfactory bulb, Purkinje cell layer and dentate gyrus. Renal CLIC4 expression was most pronounced in proximal tubules, although altered renal function was not detected in the absence of CLIC4. Myeloid cells and B cells of the spleen are rich in CLIC4 expression as are CD4 and CD8 positive T cells.

Conclusions

In a comprehensive study detailing CLIC4 expression in situ in a mouse model that excludes cross reaction with other family members, we were able to document previously unreported expression for CLIC4 in developing fetus, particularly the brain. In addition, compartmentalized expression of CLIC4 in specific adult tissues and cells provides a focus to explore potential functions of this protein not addressed previously.

Proteomic analysis of mesenchymal stem cells from normal and deep carious dental pulp

Dental pulp stem cells (DPSCs), precursor cells of odontoblasts, are ideal seed cells for tooth tissue engineering and regeneration. Our previous study has demonstrated that stem cells exist in dental pulp with deep caries and are called carious dental pulp stem cells (CDPSCs). The results indicated that CDPSCs had a higher proliferative and stronger osteogenic differentiation potential than DPSCs. However, the molecular mechanisms responsible for the biological differences between DPSCs and CDPSCs are poorly understood. The aim of this study was to define the molecular features of DPSCs and CDPSCs by comparing the proteomic profiles using two-dimensional fluorescence difference gel electrophoresis (2-D DIGE) in combination with matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Our results revealed that there were 18 protein spots differentially expressed between DPSCs and CDPSCs in a narrow pH range of 4 to 7. These differently expressed proteins are mostly involved in the regulation of cell proliferation, differentiation, cell cytoskeleton and motility. In addition, our results suggested that CDPSCs had a higher expression of antioxidative proteins that might protect CDPSCs from oxidative stress. This study explores some potential proteins responsible for the biological differences between DPSCs and CDPSCs and expands our understanding on the molecular mechanisms of mineralization of DPSCs in the formation of the dentin-pulp complex.

The RNA helicase Ddx5/p68 binds to hUpf3 and enhances NMD of Ddx17/p72 and Smg5 mRNA

Non-sense-mediated mRNA decay (NMD) is a mechanism of translation-dependent mRNA surveillance in eukaryotes: it degrades mRNAs with premature termination codons (PTCs) and contributes to cellular homeostasis by downregulating a number of physiologically important mRNAs. In the NMD pathway, Upf proteins, a set of conserved factors of which Upf1 is the central regulator, recruit decay enzymes to promote RNA cleavage. In mammals, the degradation of PTC-containing mRNAs is triggered by the exon–junction complex (EJC) through binding of its constituents Upf2 and Upf3 to Upf1. The complex formed eventually induces translational repression and recruitment of decay enzymes. Mechanisms by which physiological mRNAs are targeted by the NMD machinery in the absence of an EJC have been described but still are discussed controversially. Here, we report that the DEAD box proteins Ddx5/p68 and its paralog Ddx17/p72 also bind the Upf complex by physical interaction with Upf3, thereby interfering with the binding of EJC. By activating the NMD machinery, Ddx5 is shown to regulate the expression of its own, Ddx17 and Smg5 mRNAs. For NMD triggering, the adenosine triphosphate-binding activity of Ddx5 and the 3′-untranslated region of substrate mRNAs are essential.

The DEAD box proteins DDX5 (p68) and DDX17 (p72): multi-tasking transcriptional regulators

Members of the DEAD box family of RNA helicases, which are characterised by the presence of twelve conserved motifs (including the signature D-E-A-D motif) within a structurally conserved 'helicase' core, are involved in all aspects of RNA metabolism. Apart from unwinding RNA duplexes, which established these proteins as RNA helicases, DEAD box proteins have been shown to also catalyse RNA annealing and to displace proteins from RNA. DEAD box proteins generally act as components of large multi-protein complexes and it is thought that interactions, via their divergent N- and C-terminal extensions, with other factors in the complexes may be responsible for the many different functions attributed to these proteins. In addition to their established crucial roles in the manipulation of RNA structure, it is becoming increasingly clear that several members of the DEAD box family act as regulators of transcription. In this review I shall focus on DDX5 (p68) and the highly related DDX17 (p72), two proteins for which there is a large body of evidence demonstrating that they function in transcriptional regulation. This article is part of a Special Issue entitled: The Biology of RNA helicases - Modulation for life.

CLIC4 is a tumor suppressor for cutaneous squamous cell cancer

Chloride intracellular channel (CLIC) 4 is a member of a redox-regulated, metamorphic multifunctional protein family, first characterized as intracellular chloride channels. Current knowledge indicates that CLICs participate in signaling, cytoskeleton integrity and differentiation functions of multiple tissues. In metabolically stressed skin keratinocytes, cytoplasmic CLIC4 is S-nitrosylated and translocates to the nucleus where it enhances transforming growth factor-β (TGF-β) signaling by protecting phospho-Smad 2 and 3 from dephosphorylation. CLIC4 expression is diminished in multiple human epithelial cancers, and the protein is excluded from the nucleus. We now show that CLIC4 expression is reduced in chemically induced mouse skin papillomas, mouse and human squamous carcinomas and squamous cancer cell lines, and the protein is excluded from the nucleus. The extent of reduction in CLIC4 coincides with progression of squamous tumors from benign to malignant. Inhibiting antioxidant defense in tumor cells increases S-nitrosylation and nuclear translocation of CLIC4. Adenoviral-mediated reconstitution of nuclear CLIC4 in squamous cancer cells enhances TGF-β-dependent transcriptional activity and inhibits growth. Adenoviral targeting of CLIC4 to the nucleus of tumor cells in orthografts inhibits tumor growth, whereas elevation of CLIC4 in transgenic epidermis reduces de novo chemically induced skin tumor formation. In parallel, overexpression of exogenous CLIC4 in squamous tumor orthografts suppresses tumor growth and enhances TGF-β signaling. These results indicate that CLIC4 suppresses the growth of squamous cancers, that reduced CLIC4 expression and nuclear residence detected in cancer cells is associated with the altered redox state of tumor cells and the absence of detectable nuclear CLIC4 in cancers contributes to TGF-β resistance and enhances tumor development.

An evolutionarily conserved, alternatively spliced, intron in the p68/DDX5 DEAD-box RNA helicase gene encodes a novel miRNA

The DEAD-box RNA helicase p68 (DDX5) plays important roles in several cellular processes, including transcription, pre-mRNA processing, and microRNA (miRNA) processing. p68 expression is growth and developmentally regulated, and alterations in p68 expression and/or function have been implicated in tumor development. The p68 gene encodes an evolutionarily conserved, alternatively spliced, intron the function of which has to date remained unclear. Although the intron-containing p68 RNA does not appear to yield an alternative p68 protein, it is differentially expressed in cell lines and tissues, indicating regulation of expression. Here we show that the p68 conserved intron encodes a novel putative miRNA, suggesting a previously unknown possible regulatory function for the p68 intron. We show that this miRNA (referred to as p68 miRNA) is processed from the intron via the canonical miRNA-processing pathway and that it associates with the Argonaute protein Ago2. Finally we show that the p68 miRNA suppresses an mRNA bearing complementary target sequences, suggesting that it is functional. These findings suggest a novel mechanism by which alterations in p68 expression may impact on the cell.

RNA helicases p68 and p72: multifunctional proteins with important implications for cancer development

The DEAD box RNA helicases p68 (DDX5) and p72 (DDX17) play important roles in multiple cellular processes that are commonly dysregulated in cancers, including transcription, pre-mRNA processing/alternative splicing and miRNA processing. Although p68 and p72 appear to have some overlapping functions, they clearly also have distinct, nonredundant functions. Furthermore, their ability to interact with a variety of different factors and act as multifunctional proteins has the potential to impact on several different processes, and alterations in expression or function of p68 and/or p72 could have profound implications for cancer development. However, their roles are likely to be context-dependent and both proteins have been reported to have pro-proliferation or even oncogenic functions as well as antiproliferative or tumor cosuppressor roles. Therefore, eludicating the precise role of these proteins in cancer is likely to be complex and to depend on the cellular environment and interacting factors. In this article, we review the many functions that have been attributed to p68 and p72 and discuss their potential roles in cancer development.

Direct discovery and validation of a peptide/MHC epitope expressed in primary human breast cancer cells using a TCRm monoclonal antibody with profound antitumor properties

The identification and validation of new cancer-specific T cell epitopes continues to be a major area of research interest. Nevertheless, challenges remain to develop strategies that can easily discover and validate epitopes expressed in primary cancer cells. Regarded as targets for T cells, peptides presented in the context of the major histocompatibility complex (MHC) are recognized by monoclonal antibodies (mAbs). These mAbs are of special importance as they lend themselves to the detection of epitopes expressed in primary tumor cells. Here, we use an approach that has been successfully utilized in two different infectious disease applications (WNV and influenza). A direct peptide-epitope discovery strategy involving mass spectrometric analysis led to the identification of peptide YLLPAIVHI in the context of MHC A*02 allele (YLL/A2) from human breast carcinoma cell lines. We then generated and characterized an anti-YLL/A2 mAb designated as RL6A TCRm. Subsequently, the TCRm mAb was used to directly validate YLL/A2 epitope expression in human breast cancer tissue, but not in normal control breast tissue. Moreover, mice implanted with human breast cancer cells grew tumors, yet when treated with RL6A TCRm showed a marked reduction in tumor size. These data demonstrate for the first time a coordinated direct discovery and validation strategy that identified a peptide/MHC complex on primary tumor cells for antibody targeting and provide a novel approach to cancer immunotherapy.

CLIC4 and Schnurri-2: a dynamic duo in TGF-beta signaling with broader implications in cellular homeostasis and disease

CLIC4 is a highly conserved, multifunctional member of the chloride intracellular channel family of proteins. The protein is largely cytoplasmic but translocates to the nucleus upon a variety of stimuli including TGF-beta, TNF-alpha and etoposide. Nuclear resident CLIC4 causes growth arrest, terminal differentiation and apoptosis. Recently, it was discovered that TGF-beta causes CLIC4 to associate with Schnurri-2 and together they translocate to the nucleus and dissociate thereafter. The nuclear function of CLIC4 was further illuminated by the discovery that CLIC4 enhances TGF-beta signaling by associating with phospho-Smad2 and 3 and preventing their dephosphorylation. Enhanced TGF-beta dependent gene expression and growth inhibition are downstream consequences of this activity of CLIC4. In this article, we speculate on other consequences of the CLIC4 relation to TGF-beta signaling and the potential for CLIC4 to participate in other cellular functions related to normal homeostasis and disease.

SRA and its binding partners: an expanding role for RNA-binding coregulators in nuclear receptor-mediated gene regulation

The discovery that SRA RNA can function as a nuclear receptor (NR) coactivator resulted in a fundamental change in the perception of how NRs and their coregulators may regulate hormone signaling pathways. The subsequent identification of molecules capable of binding SRA, including SHARP, p68, and more recently SLIRP, which also function as coregulators, has further broadened our understanding of NR-dependent gene regulation. The integral role that NRs play in directing developmental, metabolic and pathological programs of transcription has defined them as paramount targets for treating a broad range of human diseases. Thus with a greater understanding of SRA and its interactions with its binding partners, novel RNA-protein interactions may be identified and exploited for therapeutic gain. Here we discuss the isolation of SRA, its impact on NR activity and interactions with known binding partners.

Disruption of the novel gene fad104 causes rapid postnatal death and attenuation of cell proliferation, adhesion, spreading and migration

The molecular mechanisms at the beginning of adipogenesis remain unknown. Previously, we identified a novel gene, fad104 (factor for adipocyte differentiation 104), transiently expressed at the early stage of adipocyte differentiation. Since the knockdown of the expression of fad104 dramatically repressed adipogenesis, it is clear that fad104 plays important roles in adipocyte differentiation. However, the physiological roles of fad104 are still unknown. In this study, we generated fad104-deficient mice by gene targeting. Although the mice were born in the expected Mendelian ratios, all died within 1 day of birth, suggesting fad104 to be crucial for survival after birth. Furthermore, analyses of mouse embryonic fibroblasts (MEFs) prepared from fad104-deficient mice provided new insights into the functions of fad104. Disruption of fad104 inhibited adipocyte differentiation and cell proliferation. In addition, cell adhesion and wound healing assays using fad104-deficient MEFs revealed that loss of fad104 expression caused a reduction in stress fiber formation, and notably delayed cell adhesion, spreading and migration. These results indicate that fad104 is essential for the survival of newborns just after birth and important for cell proliferation, adhesion, spreading and migration.

The DEAD box RNA helicases p68 (Ddx5) and p72 (Ddx17): novel transcriptional co-regulators

DEAD box [a motif named after its amino acid sequence (Asp-Glu-Ala-Asp)] RNA helicases are known to play key roles in all cellular processes that require modulation of RNA structure. However, in recent years, several of these proteins have been found to function in transcriptional regulation. In the present paper, we shall review the literature demonstrating the action of p68 and, where data are available, p72 as transcriptional co-regulators for a range of transcription factors, namely ERalpha (oestrogen receptor alpha), the tumour suppressor p53, the myogenic regulator MyoD and Runx2, a transcription factor essential for osteoblast development. We shall also discuss evidence indicating that, in some cases at least, p68 and p72 have distinct, non-redundant, roles.

The RNA coregulator SRA, its binding proteins and nuclear receptor signaling activity

Nuclear receptor (NR) coregulators are key modulators of hormone signaling. Discovery of steroid receptor RNA activator (SRA), a coregulator that is active as a RNA, transformed thinking in the field of hormone action. The subsequent identification of SRA-binding coregulator proteins, including p68, SHARP and more recently SLIRP, has provided important insight into SRA's mechanism of action and potentially offers new opportunities to target NR signaling pathways for therapeutic gain. Here we outline advances in the field of NR coregulator biology, with a bias on recent progress in understanding SRA-protein interactions.

peg10, an imprinted gene, plays a crucial role in adipocyte differentiation

An imprinted gene, paternally expressed gene (peg) 10, was isolated as one of the genes expressed early in adipogenesis. The expression of peg10 was elevated after the addition of inducers, and was detected in adipocyte differentiable 3T3-L1 cells, but not observed in the non-adipogenic cell line NIH-3T3. Moreover, the knockdown of peg10 by RNA interference (RNAi) inhibited the differentiation of 3T3-L1 cells into lipid-laden adipocytes. Interestingly, peg10 RNAi-treatment reduced the expressions of C/EBPbeta and C/EBPdelta, and inhibited mitotic clonal expansion. These findings strongly indicate that peg10 plays a crucial role at the immediate early stage of adipocyte differentiation.

CLIC4 mediates and is required for Ca2+-induced keratinocyte differentiation

Keratinocyte differentiation requires integrating signaling among intracellular ionic changes, kinase cascades, sequential gene expression, cell cycle arrest, and programmed cell death. We now show that Cl(-) intracellular channel 4 (CLIC4) expression is increased in both mouse and human keratinocytes undergoing differentiation induced by Ca(2+), serum and the protein kinase C (PKC)-activator, 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Elevation of CLIC4 is associated with signaling by PKCdelta, and knockdown of CLIC4 protein by antisense or shRNA prevents Ca(2+)-induced keratin 1, keratin 10 and filaggrin expression and cell cycle arrest in differentiating keratinocytes. CLIC4 is cytoplasmic in actively proliferating keratinocytes in vitro, but the cytoplasmic CLIC4 translocates to the nucleus in keratinocytes undergoing growth arrest by differentiation, senescence or transforming growth factor beta (TGFbeta) treatment. Targeting CLIC4 to the nucleus of keratinocytes via adenoviral transduction increases nuclear Cl(-) content and enhances expression of differentiation markers in the absence of elevated Ca(2+). In vivo, CLIC4 is localized to the epidermis in mouse and human skin, where it is predominantly nuclear in quiescent cells. These results suggest that CLIC4 participates in epidermal homeostasis through both alterations in the level of expression and subcellular localization. Nuclear CLIC4, possibly by altering the Cl(-) and pH of the nucleus, contributes to cell cycle arrest and the specific gene expression program associated with keratinocyte terminal differentiation.

The transcriptional factor Osterix directly interacts with RNA helicase A

Osterix is an osteoblast-specific transcriptional factor, required for bone formation and osteoblast differentiation. Here, we identified new Osterix interacting factors by using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Among the candidates, RNA helicase A was identified to interact with Osterix. To determine the interaction of Osterix with RNA helicase A, immunoprecipitation assay was performed. Western analysis confirmed the association between Osterix and RNA helicase A. Immunocytochemical analysis also showed that Osterix and RNA helicase A were co-localized in HEK 293 cells. Our data suggest that RNA helicase A might be a component of Osterix regulation.

Characterization of Novel Genes Regulating Adipocyte Differentiation

The mid- and late stages of adipocyte differentiation are known to be regulated by transcription factors such as peroxisome proliferator-activated receptor (PPAR)gamma and CCAAT-box/enhancer binder protein (C/EBP) families. However, events in the early stage of adipocyte differentiation remain largely unknown. To gain insights into the molecular mechanisms underlying the beginning of adipocyte differentiation, we have isolated 102 genes, which are induced at the beginning of the differentiation of mouse 3T3-L1 preadipocytes, using the polymerase chain reaction (PCR)-subtraction method. Of these, 46 appear to be unknown genes. Since rapid amplification of cDNA end (RACE), cDNA library screening, and a genome database search have revealed that two of these genes are novel, we have named them factor for adipocyte differentiation (fad) 24 and fad158. The database research of amino acid sequences revealed that fad24 has a basic leucine zipper motif and an NOC domain, and fad158 has four transmembrane domains and eight leucine-rich repeats. The expression of fad24 and fad158 transiently increased after the addition of adipogenic inducers [insulin, dexamethasone, 3-isobutyl-1-methylxanthine, fetal bovine serum (FBS)]. RNAi-mediated knockdown of fad24 or antisense fad158 inhibited adipogenesis of 3T3-L1 preadipocytes and decreased expressions of PPARgamma and C/EBPalpha. Furthermore, the constitutive overexpression of fad24 or fad158 in the mouse fibroblast cell line NIH-3T3 resulted in adipocyte conversion when stimulated with adipogenic inducers and PPARgamma ligand BRL49653. Moreover, it was found that FAD24 localizes in the nucleus, especially within nuclear speckles and nucleolus, and FAD158 localizes to the endoplasmic reticulum (ER). Taken together, fad24 and fad158 appear to regulate adipocyte differentiation by activating the PPARgamma pathway.

Crystal structure of the soluble form of the redox-regulated chloride ion channel protein CLIC4

The structure of CLIC4, a member of the CLIC family of putative intracellular chloride ion channel proteins, has been determined at 1.8 Angstroms resolution by X-ray crystallography. The protein is monomeric and it is structurally similar to CLIC1, belonging to the GST fold class. Differences between the structures of CLIC1 and CLIC4 are localized to helix 2 in the glutaredoxin-like N-terminal domain, which has previously been shown to undergo a dramatic structural change in CLIC1 upon oxidation. The structural differences in this region correlate with the sequence differences, where the CLIC1 sequence appears to be atypical of the family. Purified, recombinant, wild-type CLIC4 is shown to bind to artificial lipid bilayers, induce a chloride efflux current when associated with artificial liposomes and produce an ion channel in artificial bilayers with a conductance of 30 pS. Membrane binding is enhanced by oxidation of CLIC4 while no channels were observed via tip-dip electrophysiology in the presence of a reducing agent. Thus, recombinant CLIC4 appears to be able to form a redox-regulated ion channel in the absence of any partner proteins.

RNA helicases: regulators of differentiation

RNA helicases are highly conserved enzymes that utilize the energy derived from NTP hydrolysis to modulate the structure of RNA. RNA helicases participate in all biological processes that involve RNA, including transcription, splicing and translation. Based on the sequence of the helicase domain, they are classified into families, such as DDX and DHX families of human RNA helicases. The specificity of RNA helicases to their targets is likely due to several factors, such as the sequence, interacting molecules, subcellular localization and the expression pattern of the helicases. There are several examples of the involvement of RNA helicases in differentiation. Human DDX3 has two closely related genes designated DDX3Y and DDX3X, which are localized to the Y and X chromosomes, respectively. DDX3Y protein is specifically expressed in germ cells and is essential for spermatogenesis. DDX25 is another RNA helicase which has been shown to be required for spermatogenesis. DDX4 shows specific expression in germ cells. The Drosophila ortholog of DDX4, known as vasa, is required for the formation of germ cells and oogenesis by a mechanism that involves regulating the translation of mRNAs essential for differentiation. Abstrakt is the Drosphila ortholog of DDX41, which has been shown to be involved in visual and CNS system development. DDX5 (p68) and its related DDX17 (p72) have also been implicated in organ/tissue differentiation. The ability of RNA helicases to modulate the structure and thus availability of critical RNA molecules for processing leading to protein expression is the likely mechanism by which RNA helicases contribute to differentiation.

SLC39A14, a LZT protein, is induced in adipogenesis and transports zinc

During adipocyte differentiation, there is an underlying complex series of gene expressions. We have previously isolated many genes whose expression levels are quickly elevated by the addition of inducers to mouse 3T3-L1 preadipocyte cells. Here we report the isolation and characterization of SLC39A14, a member of the LZT proteins, one of the subfamilies of ZIP transporters. The expression of the SLC39A14 gene was strongly and rapidly induced at the early stages of differentiation. Moreover, it was highly restricted to the potential differentiation state of 3T3-L1 cells and the expression level was quite low in the nonadipogenic NIH-3T3 cells, indicating a dominant expression in adipocyte differentiation. The zinc uptake assay revealed that SLC39A14 functions as a zinc transporter. Taken together, these results suggest that SLC39A14 plays a role as a zinc transporter during the early stages of adipogenesis.

The novel gene fad104, containing a fibronectin type III domain, has a significant role in adipogenesis

A novel gene named fad104 (factor for adipocyte differentiation-104), whose expression level quickly increased in the early stage of adipogenesis, was isolated and characterized. The deduced amino acid sequence of fad104 revealed the possible presence of a fibronectin type III domain and transmembrane domain. The expression of fad104 was detected in adipocyte differentiable 3T3-L1 cells but not observed in the non-adipogenic cell line NIH-3T3. Moreover, the ability of 3T3-L1 cells to differentiate declined with the knockdown of fad104 by RNA interference, strongly indicating that fad104 functions as a positive regulator of adipogenesis.

Fad24, a mammalian homolog of Noc3p, is a positive regulator in adipocyte differentiation

Adipocyte differentiation is controlled by complex actions involving gene expression and signal transduction. From metaphase to anaphase, peroxisome proliferator-activated receptor γ, the CCAAT/enhancer-binding protein family and sterol regulatory element-binding protein-1 are known to function as master regulators. However, the mechanism underlying the earliest step, which triggers the initiation of differentiation, remains unknown. In previous reports, we have isolated a number of genes, whose expression increases in the early stage of differentiation in the mouse 3T3-L1 preadipocyte cell line. Here we report the cloning of the full-length cDNA and characterization of an unknown gene isolated previously and named fad24 (factor for adipocyte differentiation 24). Fad24 encodes a protein consisting of 807 amino acids. The deduced amino acid sequence was shown to have a basic leucine zipper motif and a NOC domain. Expression of fad24 was rapidly induced after stimulation with inducers. Furthermore, overexpression of fad24 in NIH-3T3 cells promoted adipogenesis in the presence of a ligand for peroxisome proliferator-activated receptor γ. FAD24 localizes in the nucleus, especially within nuclear speckles. As the nuclear speckle functions as a nascent transcription and pre-mRNA splicing machinery, there is a possibility that FAD24 functions as one of the components for transcription and/or pre-mRNA splicing and positively regulates adipocyte differentiation.

The p68 and p72 DEAD box RNA helicases interact with HDAC1 and repress transcription in a promoter-specific manner

Background

p68 (Ddx5) and p72 (Ddx17) are highly related members of the DEAD box family and are established RNA helicases. They have been implicated in growth regulation and have been shown to be involved in both pre-mRNA and pre-rRNA processing. More recently, however, these proteins have been reported to act as transcriptional co-activators for estrogen-receptor alpha (ERα). Furthermore these proteins were shown to interact with co-activators p300/CBP and the RNA polymerase II holoenzyme. Taken together these reports suggest a role for p68 and p72 in transcriptional activation.

Results

In this report we show that p68 and p72 can, in some contexts, act as transcriptional repressors. Targeting of p68 or p72 to constitutive promoters leads to repression of transcription; this repression is promoter-specific. Moreover both p68 and p72 associate with histone deacetylase 1 (HDAC1), a well-established transcriptional repression protein.

Conclusions

It is therefore clear that p68 and p72 are important transcriptional regulators, functioning as co-activators and/or co-repressors depending on the context of the promoter and the transcriptional complex in which they exist.

Do human RNA helicases have a role in cancer?

Human RNA helicases (HRH) represent a large family of enzymes that play important roles in RNA processing. The biochemical characteristics and biological functions of the majority of HRH are still to be determined. However, there are examples of dysregulation of HRH expression in various types of cancer. In addition, some HRH have been shown to be involved in the regulation of, or the molecular interaction with, molecules implicated in cancer. Other helicases take part in fusion transcripts resulting from cancer-associated chromosomal translocation. These findings raise the question of whether HRH can contribute to cancer development/progression. In this review, I summarize the cancer-related features of HRH.

The Novel Gene fad158, Having a Transmembrane Domain and Leucine-rich Repeat, Stimulates Adipocyte Differentiation

Adipocyte differentiation is known to be regulated by a complex array of genes known as master regulators. Using a subtraction method, we previously isolated 102 genes that are expressed in the early stage of adipocyte differentiation. One of these genes named fad158 (factor for adipocyte differentiation 158) seems to be a novel gene, since there is no significantly similar gene listed in databases. Both mouse and human fad158 encode 803 amino acids and contain 4 transmembrane regions and 8 leucine-rich repeat motifs. Expression of fad158 was induced at an early stage in differentiating 3T3-L1 cells and was observed in the skeletal muscle. When the expression was knocked down with an antisense method in 3T3-L1 cells, the accumulation of oil droplets was reduced. Moreover, on overexpression of fad158 in NIH-3T3 cells, which are fibroblasts and do not usually differentiate into adipocytes, stable transformants accumulated oil droplets and showed an elevated expression of adipocyte marker genes, indicating that these cells had differentiated into mature adipocytes. fad158 has the ability to regulate adipocyte differentiation positively, especially at an early stage.

Interaction of the plant glycine-rich RNA-binding protein MA16 with a novel nucleolar DEAD box RNA helicase protein from Zea mays

The maize RNA-binding MA16 protein is a developmentally and environmentally regulated nucleolar protein that interacts with RNAs through complex association with several proteins. By using yeast two-hybrid screening, we identified a DEAD box RNA helicase protein from Zea mays that interacted with MA16, which we named Z. maysDEAD box RNA helicase 1 (ZmDRH1). The sequence of ZmDRH1 includes the eight RNA helicase motifs and two glycine-rich regions with arginine-glycine-rich (RGG) boxes at the amino (N)- and carboxy (C)-termini of the protein. Both MA16 and ZmDRH1 were located in the nucleus and nucleolus, and analysis of the sequence determinants for their cellular localization revealed that the region containing the RGG motifs in both proteins was necessary for nuclear/nucleolar localization The two domains of MA16, the RNA recognition motif (RRM) and the RGG, were tested for molecular interaction with ZmDRH1. MA16 specifically interacted with ZmDRH1 through the RRM domain. A number of plant proteins and vertebrate p68/p72 RNA helicases showed evolutionary proximity to ZmDRH1. In addition, like p68, ZmDRH1 was able to interact with fibrillarin. Our data suggest that MA16, fibrillarin, and ZmDRH1 may be part of a ribonucleoprotein complex involved in ribosomal RNA (rRNA) metabolism.

The Organellular Chloride Channel Protein CLIC4/mtCLIC Translocates to the Nucleus in Response to Cellular Stress and Accelerates Apoptosis

CLIC4/mtCLIC, a chloride intracellular channel protein, localizes to the mitochondria and cytoplasm of keratinocytes and participates in the apoptotic response to stress. We now show that multiple stress inducers cause the translocation of cytoplasmic CLIC4 to the nucleus. Immunogold electron microscopy and confocal analyses indicate that nuclear CLIC4 is detected prior to the apoptotic phenotype. CLIC4 associates with the Ran, NTF2, and Importin-alpha nuclear import complexes in immunoprecipitates of lysates from cells treated with apoptotic/stress-inducing agents. Deletion or mutation of the nuclear localization signal in the C terminus of CLIC4 eliminates nuclear translocation, whereas N terminus deletion enhances nuclear localization. Targeting CLIC4 to the nucleus via adenoviral transduction accelerates apoptosis when compared with cytoplasmic CLIC4, and only nuclear-targeted CLIC4 causes apoptosis in Apaf null mouse fibroblasts or in Bcl-2-overexpressing keratinocytes. These results indicate that CLIC4 nuclear translocation is an integral part of the cellular response to stress and may contribute to the initiation of nuclear alterations that are associated with apoptosis.

Cloning and Characterization of Inducible Genes at the Beginning of Adipocyte Differentiation

Adipocyte differentiation takes place via a complex series of steps. While PPAR gamma and C/EBP alpha are known to be master regulators, the events at the earliest stage of adipocyte differentiation are not yet known. In this study, we cloned the genes that are induced at the beginning of the differentiation of 3T3-L1 preadipocyte cells. Of 102 clones obtained, only several clones were already reported as genes that are expressed differentially during adipocyte development. The expression of TCL/TC10 beta L (TC10-like/TC10 beta Long) and RGS2 (regulators of G protein signaling 2) genes isolated here rapidly increased after the addition of inducers (insulin, dexemethasone, 3-isobutyl-1-methylxanthine, fetal bovine serum [FBS]). Further, the antisense TCL/TC10 beta L inhibited the adipogenesis of mouse 3T3-L1 preadipocyte cells, prevented cytoplasmic triglyceride accumulation, and decreased the expression of PPAR gamma and C/EBP alpha. Moreover, the constitutive overexpression of TCL/TC10 beta L or RGS2 in the mouse fibroblast cell line NIH-3T3 results in efficient adipocyte conversion when stimulated with 10% FBS, insulin, 3-isobutyl-1-methylxanthine, dexamethasone, and PPAR gamma ligand BRL49653. These results strongly suggest that TCL/TC10 beta L and RGS2 have crucial roles in the program of adipocyte differentiation, probably linked to the PPAR gamma pathway. Using a subtraction protocol, the genes specifically regulated by TCL/TC10 beta L were also isolated. The expression pattern of some was similar to TCL/TC10 beta L expression in adipogenesis, suggesting that these genes are regulated by TCL/TC10 beta L.

mtCLIC is up-regulated and maintains a mitochondrial membrane potential in mtDNA-depleted L929 cells

To explain why mitochondrial DNA (mtDNA)-depleted or rho0 cells still keep a mitochondrial membrane potential (Delta(psi)m) in the absence of respiration, several hypotheses have been proposed. The principal and well accepted one involves a reverse of action for ANT combined to F1-ATPase activity. However, the existence of other putative electrogenic channels has been speculated. Here, using mRNA differential display reverse transcriptase-polymerase chain reaction on L929 mtDNA-depleted cells, we identified mtCLIC as a differentially expressed gene in cells deprived from mitochondrial ATP production. Mitochondrial chloride intracellular channel (mtCLIC), a member of a recently discovered and expanding family of chloride intracellular channels, is up-regulated in mtDNA-depleted and rho0 cells. We showed that its expression is dependent on CREB and p53 and is sensitive to calcium and tumor necrosis factor alpha. Interestingly, up- or down-regulation of mtCLIC protein expression changes Delta(psi)m whereas the chloride channel inhibitor NPPB reduces the Delta(psi)m in mtDNA-depleted L929 cells, measured with the fluorescent probe rhodamine 123. Finally, we demonstrated that purified mitochondria from mtDNA-depleted cells incorporate, in a NPPB-sensitive manner, more 36chloride than parental mitochondria. These findings suggest that mtCLIC could be involved in mitochondrial membrane potential generation in mtDNA-depleted cells, a feature required to prevent apoptosis and to drive continuous protein import into mitochondria.

The highly related DEAD box RNA helicases p68 and p72 exist as heterodimers in cells

The RNA helicases p68 and p72 are highly related members of the DEAD box family of proteins, sharing 90% identity across the conserved core, and have been shown to be involved in both transcription and mRNA processing. We previously showed that these proteins co-localise in the nucleus of interphase cells. In this study we show that p68 and p72 can interact with each other and self-associate in the yeast two-hybrid system. Co-immunoprecipitation experiments confirmed that p68 and p72 can interact in the cell and indicated that these proteins preferentially exist as hetero-dimers. In addition, we show that p68 can interact with NFAR-2, a protein that is also thought to function in mRNA processing. Moreover, gel filtration analysis suggests that p68 and p72 can exist in a variety of complexes in the cell (ranging from approximately 150 to approximately 400 kDa in size), with a subset of p68 molecules being in very large complexes (>2 MDa). The potential to exist in different complexes that may contain p68 and/or p72, together with a range of other factors, would provide the potential for these proteins to interact with different RNA substrates and would be consistent with recent reports implying a wide range of functions for p68/p72.