Fibroblast growth factor 3, a protein with dual subcellular localization, is targeted to the nucleus and nucleolus by the concerted action of two nuclear localization signals and a nucleolar retention signal

Primer on FGF3

Though initially discovered as a proto-oncogene in virally induced mouse mammary tumors, FGF3 is primarily active in prenatal stages, where it is found at various sites at specific times. FGF3 is crucial during development, as its roles include tail formation, inner ear development and hindbrain induction and patterning. FGF3 expression and function are highly conserved in vertebrates, while it also interacts with other FGFs in various developmental processes. Intriguingly, while it is classified as a classical paracrine signaling factor, murine FGF3 was uniquely found to also act in an intracrine manner, depending on alternative translation initiation sites. Corresponding with its conserved role in inner ear morphogenesis, mutations in FGF3 in humans are associated with LAMM syndrome, a disorder that include hearing loss and inner ear malformations. While recent studies indicate of some FGF3 presence in post-natal stages, emerging evidences of its upregulation in various human tumors and cariogenic processes in mouse models, highlights the importance of its close regulation in adult tissues. Altogether, the broad and dynamic expression pattern and regulation of FGF3 in embryonic and adult tissues together with its link to congenital malformations and cancer, calls for further discoveries of its diverse roles in health and disease.

SERPINA1 long transcripts produce non-secretory alpha1-antitrypsin isoform: In vitro translation in living cells

SERPINA1 is a well-studied serpin gene due to its dramatic impact on human health. Translation initiation at the main SERPINA1 start codon produces the only known alpha1-antitrypsin (AAT) isoform intended for secretion. AAT performs essential functions by inhibiting proteases and modulating immunity. However, SERPINA1 expression at the level of translation is not sufficiently studied. Here we hypothesize that the main SERPINA1 ORF can be alternatively translated, producing a non-secretory AAT isoform by either masking or excluding a signal peptide. We defined SERPINA1 long mRNA isoforms specific for prostate (DU145) and liver (HepG2) cell lines and studied their individual expression by in vitro assay. We found that all long transcripts produce both glycosylated secretory AAT-eGFP fusion protein and non-glycosylated intracellular AAT-eGFP (initiated from an alternative AUG-2 start codon), with the proportion regulated by the SERPINA1 5'-UTR. Both fusion proteins localize to distinct cellular compartments: in contrast to a fusion with the secretory AAT accumulating in the ER, the intracellular one exhibits nuclear-cytoplasmic shuttling. We detected putative endogenous AAT isoform enriching the nuclear speckles. CONCLUSION: Alternative translation initiation might be a mechanism through which SERPINA1 expands the biological diversity of its protein products. Our findings open up new prospects for the study of SERPINA1 gene expression.

Identification of a homozygous frameshift mutation in the FGF3 gene in a consanguineous Iranian family: First report of labyrinthine aplasia, microtia, and microdontia syndrome in Iran and literature review

BACKGROUND

To date, over 400 syndromes with hearing impairment have been identified which altogether constitute almost 30% of hereditary hearing loss (HL) cases around the globe. Manifested as complete or partial labyrinthine aplasia (severe malformations of the inner ear structure), type I microtia (smaller outer ear with shortened auricles), and microdontia (small and widely spaced teeth), labyrinthine aplasia, microtia, and microdontia (LAMM) syndrome (OMIM 610706) is an extremely rare autosomal recessive condition caused by bi-allelic mutations in the FGF3 gene.

METHODS

Using the whole-exome sequencing (WES) data of the proband, we analyzed a consanguineous Iranian family with three affected members presenting with congenital bilateral HL, type I microtia, and microdontia.

RESULTS

We discovered the homozygous deletion c.45delC in the first exon of the FGF3 gene, overlapping a 38.72 Mb homozygosity region in chromosome 11. Further investigations using Sanger sequencing revealed that this variant co-segregated with the phenotype observed in the family.

CONCLUSION

Here, we report the first identified case of LAMM syndrome in Iran, and by identifying a frameshift variant in the first exon of the FGF3 gene, our result will help better clarify the phenotype-genotype relation of LAMM syndrome.

Nuclear action of FGF members in endocrine-related tissues and cancer: Interplay with steroid receptor pathways

Dysregulation of the fibroblast growth factors/fibroblast growth factor receptor (FGF/FGFR) pathway has been implicated in a wide range of human disorders and several members have been localized in the nuclear compartment. Hormone-activated steroid receptors or ligand independent activated receptors form nuclear complexes that activate gene transcription. This review aims to highlight the interplay between the steroid receptor and the FGF/FGFR pathways and focuses on the current knowledge on nuclear action of FGF members in endocrine-related tissues and cancer. The nuclear trafficking and targets of FGF/FGFR members and the available evidence on the interplay with steroid hormones and receptors is described. Finally, the data on aberrant FGF/FGFR signaling is summarized and the nuclear action of FGF members on endocrine resistant breast cancer is highlighted. Identifying the mechanisms underlying FGF-induced endocrine resistance will be important to understand how to efficiently target endocrine-related diseases and even enhance or restore endocrine sensitivity in hormone receptor positive tumors.

Blocked O-GlcNAc cycling disrupts mouse hematopoeitic stem cell maintenance and early T cell development

Small numbers of hematopoietic stem cells (HSCs) balance self-renewal and differentiation to produce the diversity and abundance of cell types that make up the blood system. How nutrients are recruited to support this massive differentiation and proliferation process remains largely unknown. The unique metabolism of adult HSCs, which rely on glycolysis and glutaminolysis, suggests a potential role for the post-translational modification O-GlcNAc as a critical nutrient signal in these cells. Glutamine, glucose, and other metabolites drive the hexosamine biosynthetic pathway (HBP) ultimately leading to the O-GlcNAc modification of critical intracellular targets. Here, we used a conditional targeted genetic deletion of the enzyme that removes O-GlcNAc, O-GlcNAcase (OGA), to determine the consequences of blocked O-GlcNAc cycling on HSCs. Oga deletion in mouse HSCs resulted in greatly diminished progenitor pools, impaired stem cell self-renewal and nearly complete loss of competitive repopulation capacity. Further, early T cell specification was particularly sensitive to Oga deletion. Loss of Oga resulted in a doubling of apoptotic cells within the bone marrow and transcriptional deregulation of key genes involved in adult stem cell maintenance and lineage specification. These findings suggest that O-GlcNAc cycling plays a critical role in supporting HSC homeostasis and early thymocyte development.

Lamin B2 Modulates Nucleolar Morphology, Dynamics, and Function

The nucleolus is required for ribosome biogenesis. Human cells have 2 or 3 nucleoli associated with nucleolar organizer region (NOR)-bearing chromosomes. An increase in number and altered nucleolar morphology define cancer cells. However, the mechanisms that modulate nucleolar morphology and function are unclear. Here we show that in addition to localizing at the nuclear envelope, lamin B2 localizes proximal to nucleolin at the granular component (GC) of the nucleolus and associates with the nucleolar proteins nucleolin and nucleophosmin. Lamin B2 knockdown severely disrupted the nucleolar morphology, which was rescued to intact and discrete nucleoli upon lamin B2 overexpression. Furthermore, two mutually exclusive lamin B2 deletion mutants, ΔHead and ΔSLS, rescued nuclear and nucleolar morphology defects, respectively, induced upon lamin B2 depletion, suggesting independent roles for lamin B2 at the nucleolus and nuclear envelope. Lamin B2 depletion increased nucleolin aggregation in the nucleoplasm, implicating lamin B2 in stabilizing nucleolin within the nucleolus. Lamin B2 knockdown upregulated nucleolus-specific 45S rRNA and upstream intergenic sequence (IGS) transcripts. The IGS transcripts colocalized with aggregates of nucleolin speckles, which were sustained in the nucleoplasm upon lamin B2 depletion. Taken together, these studies uncover a novel role for lamin B2 in modulating the morphology, dynamics, and function of the nucleolus.

MUC1-ARF-A Novel MUC1 Protein That Resides in the Nucleus and Is Expressed by Alternate Reading Frame Translation of MUC1 mRNA

Translation of mRNA in alternate reading frames (ARF) is a naturally occurring process heretofore underappreciated as a generator of protein diversity. The MUC1 gene encodes MUC1-TM, a signal-transducing trans-membrane protein highly expressed in human malignancies. Here we show that an AUG codon downstream to the MUC1-TM initiation codon initiates an alternate reading frame thereby generating a novel protein, MUC1-ARF. MUC1-ARF, like its MUC1-TM 'parent’ protein, contains a tandem repeat (VNTR) domain. However, the amino acid sequence of the MUC1-ARF tandem repeat as well as N- and C- sequences flanking it differ entirely from those of MUC1-TM. In vitro protein synthesis assays and extensive immunohistochemical as well as western blot analyses with MUC1-ARF specific monoclonal antibodies confirmed MUC1-ARF expression. Rather than being expressed at the cell membrane like MUC1-TM, immunostaining showed that MUC1-ARF protein localizes mainly in the nucleus: Immunohistochemical analyses of MUC1-expressing tissues demonstrated MUC1-ARF expression in the nuclei of secretory luminal epithelial cells. MUC1-ARF expression varies in different malignancies. While the malignant epithelial cells of pancreatic cancer show limited expression, in breast cancer tissue MUC1-ARF demonstrates strong nuclear expression. Proinflammatory cytokines upregulate expression of MUC1-ARF protein and co-immunoprecipitation analyses demonstrate association of MUC1-ARF with SH3 domain-containing proteins. Mass spectrometry performed on proteins coprecipitating with MUC1-ARF demonstrated Glucose-6-phosphate 1-dehydrogenase (G6PD) and Dynamin 2 (DNM2). These studies not only reveal that the MUC1 gene generates a previously unidentified MUC1-ARF protein, they also show that just like its ‘parent’ MUC1-TM protein, MUC1-ARF is apparently linked to signaling and malignancy, yet a definitive link to these processes and the roles it plays awaits a precise identification of its molecular functions. Comprising at least 524 amino acids, MUC1-ARF is, furthermore, the longest ARF protein heretofore described.

FGF1 C-terminal domain and phosphorylation regulate intracrine FGF1 signaling for its neurotrophic and anti-apoptotic activities

Fibroblast growth factor 1 (FGF1) is a prototypic member of the FGFs family overexpressed in various tumors. Contrarily to most FGFs, FGF1 lacks a secretion peptide signal and acts mainly in an intracellular and nuclear manner. Intracellular FGF1 induces cell proliferation, differentiation and survival. We previously showed that intracellular FGF1 induces neuronal differentiation and inhibits both p53- and serum-free-medium-induced apoptosis in PC12 cells. FGF1 nuclear localization is required for these intracellular activities, suggesting that FGF1 regulates p53-dependent apoptosis and neuronal differentiation by new nuclear pathways. To better characterize intracellular FGF1 pathways, we studied the effect of three mutations localized in the C-terminal domain of FGF1 (i.e., FGF1^K132E, FGF1^S130A and FGF1^S130D) on FGF1 neurotrophic and anti-apoptotic activities in PC12 cells. The change of the serine 130 to alanine precludes FGF1 phosphorylation, while its mutation to aspartic acid mimics phosphorylation. These FGF1 mutants kept both a nuclear and cytosolic localization in PC12 cells. Our study highlights for the first time the role of FGF1 phosphorylation and the implication of FGF1 C-terminal domain on its intracellular activities. Indeed, we show that the K132E mutation inhibits both the neurotrophic and anti-apoptotic activities of FGF1, suggesting a regulatory activity for FGF1 C terminus. Furthermore, we observed that both FGF1^S130A and FGF1^S130D mutant forms induced PC12 cells neuronal differentiation. Therefore, FGF1 phosphorylation does not regulate FGF1-induced differentiation of PC12 cells. Then, we showed that only FGF1^S130A protects PC12 cells against p53-dependent apoptosis, thus phosphorylation appears to inhibit FGF1 anti-apoptotic activity in PC12 cells. Altogether, our results show that phosphorylation does not regulate FGF1 neurotrophic activity but inhibits its anti-apoptotic activity after p53-dependent apoptosis induction, giving new insight into the poorly described FGF1 intracrine/nuclear pathway. The study of nuclear pathways could be crucial to identify key regulators involved in neuronal differentiation, tumor progression and resistances to radio- and chemo-therapy.

Dual protein trafficking to secretory and non-secretory cell compartments: clear or double vision?

Approximately 18% of Arabidopsis thaliana proteins encode a signal peptide for translocation to the endoplasmic reticulum (ER), the gateway of the eukaryotic secretory pathway. However, it was recently discovered that some ER proteins can undergo both co-translational import into the ER/secretory pathway and trafficking to compartments outside of the secretory pathway. This phenomenon is observed among members of the protein disulfide isomerase (PDI) family, which are traditionally regarded as ER enzymes involved in protein folding. Although classical PDIs possess an N-terminal signal peptide and a C-terminal ER retention signal, some also dual localize to secretory and non-secretory compartments, including mammalian PDI ERp57, Chlamydomonas reinhardtii PDI RB60, and A. thaliana AtPDI2. ERp57 is present in both the ER and nucleus where it influences gene transcription. RB60 localizes to the ER and chloroplast where it modulates the redox state of polyadenylate-binding protein RB47. AtPDI2, which interacts with transcription factor MEE8, localizes to the ER-secretory pathway and the nucleus. A model proposing secretory trafficking of AtPDI2 and nuclear co-translocation of an AtPDI2-MEE8 complex illustrates the diversity of dual targeting mechanisms, the multifunctional roles of some PDIs, and the potential co-translocation of other proteins to multiple subcellular compartments.

A model to explain specific cellular communications and cellular harmony:- a hypothesis of coupled cells and interactive coupling molecules

BACKGROUND

The various cell types and their relative numbers in multicellular organisms are controlled by growth factors and related extracellular molecules which affect genetic expression pathways. However, these substances may have both/either inhibitory and/or stimulatory effects on cell division and cell differentiation depending on the cellular environment. It is not known how cells respond to these substances in such an ambiguous way. Many cellular effects have been investigated and reported using cell culture from cancer cell lines in an effort to define normal cellular behaviour using these abnormal cells.A model is offered to explain the harmony of cellular life in multicellular organisms involving interacting extracellular substances.

METHODS

A basic model was proposed based on asymmetric cell division and evidence to support the hypothetical model was accumulated from the literature. In particular, relevant evidence was selected for the Insulin-Like Growth Factor system from the published data, especially from certain cell lines, to support the model. The evidence has been selective in an attempt to provide a picture of normal cellular responses, derived from the cell lines.

RESULTS

The formation of a pair of coupled cells by asymmetric cell division is an integral part of the model as is the interaction of couplet molecules derived from these cells. Each couplet cell will have a receptor to measure the amount of the couplet molecule produced by the other cell; each cell will be receptor-positive or receptor-negative for the respective receptors. The couplet molecules will form a binary complex whose level is also measured by the cell. The hypothesis is heavily supported by selective collection of circumstantial evidence and by some direct evidence. The basic model can be expanded to other cellular interactions.

CONCLUSIONS

These couplet cells and interacting couplet molecules can be viewed as a mechanism that provides a controlled and balanced division-of-labour between the two progeny cells, and, in turn, their progeny. The presence or absence of a particular receptor for a couplet molecule will define a cell type and the presence or absence of many such receptors will define the cell types of the progeny within cell lineages.

Fibroblast growth factors: from molecular evolution to roles in development, metabolism and disease

Fibroblast growth factors (FGFs) are a family of structurally related polypeptides that are essential for embryonic development and that function postnatally as homoeostatic factors, in the response to injury, in the regulation of electrical excitability of cells and as hormones that regulate metabolism. In humans, FGF signalling is involved in developmental, neoplastic, metabolic and neurological diseases. Fgfs have been identified in metazoans but not in unicellular organisms. In vertebrates, FGFs can be classified as having intracrine, paracrine and endocrine functions. Paracrine and endocrine FGFs act via cell-surface FGF receptors (FGFRs); while, intracrine FGFs act independent of FGFRs. The evolutionary history of the Fgf family indicates that an intracrine Fgf is the likely ancestor of the Fgf family. During metazoan evolution, the Fgf family expanded in two phases, after the separation of protostomes and deuterostomes and in the evolution of early vertebrates. These expansions enabled FGFs to acquire diverse actions and functions.

Characterization and prediction of protein nucleolar localization sequences

Although the nucleolar localization of proteins is often believed to be mediated primarily by non-specific retention to core nucleolar components, many examples of short nucleolar targeting sequences have been reported in recent years. In this article, 46 human nucleolar localization sequences (NoLSs) were collated from the literature and subjected to statistical analysis. Of the residues in these NoLSs 48% are basic, whereas 99% of the residues are predicted to be solvent-accessible with 42% in α-helix and 57% in coil. The sequence and predicted protein secondary structure of the 46 NoLSs were used to train an artificial neural network to identify NoLSs. At a true positive rate of 54%, the predictor’s overall false positive rate (FPR) is estimated to be 1.52%, which can be broken down to FPRs of 0.26% for randomly chosen cytoplasmic sequences, 0.80% for randomly chosen nucleoplasmic sequences and 12% for nuclear localization signals. The predictor was used to predict NoLSs in the complete human proteome and 10 of the highest scoring previously unknown NoLSs were experimentally confirmed. NoLSs are a prevalent type of targeting motif that is distinct from nuclear localization signals and that can be computationally predicted.

Nuclear variants of bone morphogenetic proteins

BACKGROUND

Bone morphogenetic proteins (BMPs) contribute to many different aspects of development including mesoderm formation, heart development, neurogenesis, skeletal development, and axis formation. They have previously been recognized only as secreted growth factors, but the present study detected Bmp2, Bmp4, and Gdf5/CDMP1 in the nuclei of cultured cells using immunocytochemistry and immunoblotting of nuclear extracts.

RESULTS

In all three proteins, a bipartite nuclear localization signal (NLS) was found to overlap the site at which the proproteins are cleaved to release the mature growth factors from the propeptides. Mutational analyses indicated that the nuclear variants of these three proteins are produced by initiating translation from downstream alternative start codons. The resulting proteins lack N-terminal signal peptides and are therefore translated in the cytoplasm rather than the endoplasmic reticulum, thus avoiding proteolytic processing in the secretory pathway. Instead, the uncleaved proteins (designated nBmp2, nBmp4, and nGdf5) containing the intact NLSs are translocated to the nucleus. Immunostaining of endogenous nBmp2 in cultured cells demonstrated that the amount of nBmp2 as well as its nuclear/cytoplasmic distribution differs between cells that are in M-phase versus other phases of the cell cycle.

CONCLUSIONS

The observation that nBmp2 localization varies throughout the cell cycle, as well as the conservation of a nuclear localization mechanism among three different BMP family members, suggests that these novel nuclear variants of BMP family proteins play an important functional role in the cell.

Molecular pathology of the fibroblast growth factor family

The human fibroblast growth factor (FGF) family contains 22 proteins that regulate a plethora of physiological processes in both developing and adult organism. The mutations in the FGF genes were not known to play role in human disease until the year 2000, when mutations in FGF23 were found to cause hypophosphatemic rickets. Nine years later, seven FGFs have been associated with human disorders. These include FGF3 in Michel aplasia; FGF8 in cleft lip/palate and in hypogonadotropic hypogonadism; FGF9 in carcinoma; FGF10 in the lacrimal/salivary glands aplasia, and lacrimo-auriculo-dento-digital syndrome; FGF14 in spinocerebellar ataxia; FGF20 in Parkinson disease; and FGF23 in tumoral calcinosis and hypophosphatemic rickets. The heterogeneity in the functional consequences of FGF mutations, the modes of inheritance, pattern of involved tissues/organs, and effects in different developmental stages provide fascinating insights into the physiology of the FGF signaling system. We review the current knowledge about the molecular pathology of the FGF family.

Syndromic congenital sensorineural deafness, microtia and microdontia resulting from a novel homoallelic mutation in fibroblast growth factor 3 (FGF3)

We identified a homozygous missense mutation (c.196G-->T) in fibroblast growth factor 3 (FGF3) in 21 affected individuals from a large extended consanguineous Saudi family, phenotypically characterized by autosomal recessive syndromic congenital sensorineural deafness, microtia and microdontia. All affected family members are descendents of a common ancestor who had lived six generations ago in a geographically isolated small village. This is the second report of FGF3 involvement in syndromic deafness in humans, and independently confirms the gene's positive role in inner ear development. The c.196G-->T mutation results in substitution of glycine by cysteine at amino acid 66 (p.G66C). This residue is conserved in several species and across 18 FGF family members. Conserved glycine/proline residues are central to the 'beta-trefoil fold' characteristic of the secondary structure of FGF family proteins and substitution of these residues is likely to disrupt structure and consequently function.

Homozygous FGF3 mutations result in congenital deafness with inner ear agenesis, microtia, and microdontia

Homozygous mutations in the fibroblast growth factor 3 (FGF3) gene have recently been discovered in an autosomal recessive form of syndromic deafness characterized by complete labyrinthine aplasia (Michel aplasia), microtia, and microdontia (OMIM 610706 - LAMM). In order to better characterize the phenotypic spectrum associated with FGF3 mutations, we sequenced the FGF3 gene in 10 unrelated families in which probands had congenital deafness associated with various inner ear anomalies, including Michel aplasia, with or without tooth or external ear anomalies. FGF3 sequence changes were not found in eight unrelated probands with isolated inner ear anomalies or with a cochlear malformation along with auricle and tooth anomalies. We identified two new homozygous FGF3 mutations, p.Leu6Pro (c.17T>C) and p. Ile85MetfsX15 (c.254delT), in four subjects from two unrelated families with LAMM. The p.Leu6Pro mutation occurred within the signal site of FGF3 and is predicted to impair its secretion. The c.254delT mutation results in truncation of FGF3. Both mutations completely co-segregated with the phenotype, and heterozygotes did not have any of the phenotypic findings of LAMM. Some affected children had large skin tags on the upper side of the auricles, which is a distinctive clinical component of the syndrome. Enlarged collateral emissary veins associated with stenosis of the jugular foramen were noted on computerized tomographies of most affected subjects with FGF3 mutations. However, similar venous anomalies were also detected in persons with non-syndromic Michel aplasia, suggesting that a direct causative role of impaired FGF3 signaling is unlikely.

Hepatic stellate cells display a functional vascular smooth muscle cell phenotype in a three-dimensional co-culture model with endothelial cells

Hepatic stellate cells (HSCs) are pericytes of liver sinusoidal endothelial cells (LSECs) and activation of HSC into a myofibroblast-like phenotype (called transdifferentiation) is involved in several hepatic disease processes including neovascularization during liver metastasis, chronic and acute liver injury. While early smooth muscle cell (SMC) differentiation markers including SM alpha-actin and SM22alpha are expressed in a variety of non-SMC, expression of late-stage markers is far more restricted. Here, we found that in addition to early SMC markers, activated rat HSC express a large panel of characteristic late vascular SMC markers including SM myosin heavy chain, h1-calponin and h-caldesmon. Furthermore, myocardin, which is present exclusively in SMCs and cardiomyocytes and controls the transcription of a subset of early and late SMC markers, is highly expressed in activated HSC. We further studied activated HSC in a functional three-dimensional spheroidal co-culture system together with endothelial cells (EC). Co-culture spheroids of EC and SMC differentiate spontaneously and organize into a core of SMC and a surface layer of EC representing an inside-outside model of the physiological assembly of blood vessels. Replacing SMC by in vitro activated HSC resulted in a similar organized spheroid with differentiated, von-Willebrand factor producing, surface lining quiescent human umbilical vein endothelial cell and a core of HSC. In an in vitro angiogenesis assay, activated HSC induced quiescence in vascular EC-the hallmark of vascular SMC function. Co-spheroids of LSEC and activated HSC formed capillary-like sprouts in gel angiogenesis assays expressing the vascular EC marker VE-cadherin. Our findings indicate that activated HSC are capable to adapt a functional SMC phenotype and to induce formation of tubular sprouts by LSEC and vascular endothelial cells. Since tumors and tumor metastasis induce HSC activation, HSC may take part in tumor-induced neoangiogenesis by adapting SMC-like functions.

A nuclear export sequence located on a beta-strand in fibroblast growth factor-1

Receptor-bound and endocytosed fibroblast growth factor-1 (FGF-1) is able to cross the vesicle membrane and translocate to cytosol and nucleus. This suggests an intracellular role of FGF-1, which also signals by activating transmembrane FGF receptors. Phosphorylation of internalized FGF-1 by nuclear protein kinase C delta induces rapid export from the nuclei by a leptomycin B-sensitive pathway. In the present work, we have searched for and identified a Leu-rich nuclear export sequence (NES) at the C terminus of FGF-1 required for its nuclear export and able to confer nuclear export activity to a reporter protein in an in vivo system. Mutants where hydrophobic amino acids within the NES were exchanged for alanine exhibited reduced or abolished nuclear export. As demonstrated in co-immunoprecipitation experiments, a complex containing FGF-1, exportin-1, and its co-factor Ran-GTP, was formed in vitro. Formation of this complex in vivo was demonstrated by a peroxisomal targeting assay. Formation of the FGF-1-exportin-1-Ran-GTP complex in vitro as well as nuclear export of FGF-1 in vivo was dependent on phosphorylation of FGF-1, and it was abolished by leptomycin B. The FGF-1 NES was found to be situated along a beta-strand, which has not been reported before, since NESs usually are alpha-helical.

Role of estrogen receptor beta in uterine stroma and epithelium: Insights from estrogen receptor beta-/- mice

In this study, we compared the uterine tissue of estrogen receptor (ER)beta(-/-) mice and their WT littermates for differences in morphology, proliferation [the percentage of labeled cells 2 h after BrdUrd injection and EGF receptor (EGFR) expression], and differentiation (expression of progesterone receptor, E-cadherin, and cytokeratins). In ovariectomized mice, progesterone receptor expression in the uterine epithelium was similar in WT and ERbeta(-/-) mice, but E-cadherin and cytokeratin 18 expression was lower in ERbeta(-/-) mice. The percentage of cells in S phase was 1.5% in WT mice and 8% in ERbeta(-/-) mice. Sixteen hours after injection of 17beta-estradiol (E(2)), the number of BrdUrd-labeled cells increased 20-fold in WT mice and 80-fold in ERbeta(-/-) mice. Although ERalpha was abundant in intact mice, after ovariectomy, ERalpha could not be detected in the luminal epithelium of either WT or ERbeta(-/-) mice. In both untreated and E(2)-treated mice, ERalpha and ERbeta were colocalized in the nuclei of many stromal and glandular epithelial cells. However, upon E(2) + progesterone treatment, ERalpha and ERbeta were not coexpressed in any cells. In WT mice, EGFR was located on the membranes and in the cytoplasm of luminal epithelium, but not in the stroma. In ERbeta(-/-) mice, there was a marked expression of EGFR in the nuclei of epithelial and stromal cells. Upon E(2) treatment, EGFR on cell membranes was down-regulated in WT but not in ERbeta(-/-) mice. These findings reveal an important role for ERbeta in response to E(2) and in the organization, growth, and differentiation of the uterine epithelium.

Nuclear trafficking of secreted factors and cell-surface receptors: new pathways to regulate cell proliferation and differentiation, and involvement in cancers

Secreted factors and cell surface receptors can be internalized by endocytosis and translocated to the cytoplasm. Instead of being recycled or proteolysed, they sometimes translocate to the nucleus. Nuclear import generally involves a nuclear localization signal contained either in the secreted factor or its transmembrane receptor, that is recognized by the importins machinery. In the nucleus, these molecules regulate transcription of specific target genes by direct binding to transcription factors or general coregulators. In addition to the transcription regulation, nuclear secreted proteins and receptors seem to be involved in other important processes for cell life and cellular integrity such as DNA replication, DNA repair and RNA metabolism.

Nuclear secreted proteins and transmembrane receptors now appear to induce new signaling pathways to regulate cell proliferation and differentiation. Their nuclear localization is often transient, appearing only during certain phases of the cell cycle. Nuclear secreted and transmembrane molecules regulate the proliferation and differentiation of a large panel of cell types during embryogenesis and adulthood and are also potentially involved in wound healing. Secreted factors such as CCN proteins, EGF, FGFs and their receptors are often detected in the nucleus of cancer cells. Nuclear localization of these molecules has been correlated with tumor progression and poor prognosis for patient survival. Nuclear growth factors and receptors may be responsible for resistance to radiotherapy.

Subcellular localization of CIAPIN1

Cytokine-induced apoptosis inhibitor 1 (CIAPIN1) is a newly identified anti-apoptotic molecule. Our previous studies have demonstrated that CIAPIN1 is ubiquitously expressed in normal fetal and adult human tissues and confers multidrug resistance in gastric cancer cells, possibly by upregulating the expression of multidrug resistance gene 1 and multidrug resistance-related protein 1. However, fundamental biological functions of CIAPIN1 have not been elucidated. In this study, we first predicted the subcellular localization of CIAPIN1 with bioinformatic approaches and then characterized the intracellular localization of CIAPIN1 in both human and mouse cells by a combination of techniques including (a)immunohistochemistry and immunofluorescence, (b) His-tagged CIAPIN1 expression, and (c)subcellular fractionation and analysis of CIAPIN1 in the fractions by Western blotting. All methods produced consistent results; CIAPIN1 was localized in both the cytoplasm and the nucleus and was accumulated in the nucleolus. Bioinformatic prediction disclosed a putative nuclear localization signal and a putative nuclear export signal within both human and mouse CIAPIN1. These findings suggest that CIAPIN1 may undergo a cytoplasm-nucleus-nucleolus translocation.

Nuclear localisation of endogenous SUMO-1-modified PDGF-C in human thyroid tissue and cell lines

We investigated post-translational modification and subcellular localisation of endogenous platelet-derived growth factor-C (PDGF-C) in human thyroid papillary carcinomas (PTC), non-neoplastic thyroid tissues, and a selection of cultured cell lines. PDGF-C expressed nuclear localisation in 95% of all tested cell types in culture and in 10% of the thyrocytes from both PTC and non-neoplastic tissue. The cell lines expressed two forms of full-length PDGF-C, approximately 39 and approximately 55 kDa, in cell membrane and cytosol, while the approximately 55 kDa form dominated in the nucleus where it was partly chromatin-associated. The approximately 55 kDa form was post-translationally modified by SUMO-1. The putative PDGF-C SUMOylation site is the surface exposed (314)lysine part of a positively charged loop ((312)RPKTGVRGLHK(322)) with characteristics of a nuclear localisation signal. The tissue thyrocytes expressed a non-SUMOylated approximately 43 kDa and the 55 kDa PDGF-C. The SUMO-1 modified approximately 55 kDa PDGF-C expression was low in PTC where the approximately 43 kDa PDGF-C dominated. This is in contrast to non-neoplastic tissue and cultured cells where the SUMOylated approximately 55 kDa PDGF-C was strongly expressed. Our data provide novel evidence for nuclear localisation of PDGF-C, post-translational modification by SUMOylation and the expression of a novel form of PDGF-C in human papillary thyroid carcinomas.

Nucleolar localization of cirhin, the protein mutated in North American Indian childhood cirrhosis

Cirhin (NP_116219), the product of the CIRH1A gene is mutated in North American Indian childhood cirrhosis (NAIC/CIRH1A, OMIM 604901), a severe autosomal recessive intrahepatic cholestasis. It is a 686-amino-acid WD40-repeat containing protein of unknown function that is predicted to contain multiple targeting signals, including an N-terminal mitochondrial targeting signal, a C-terminal monopartite nuclear localization signal (NLS) and a bipartite nuclear localization signal (BNLS). We performed the direct determination of subcellular localization of cirhin as a crucial first step in unraveling its biological function. Using EGFP and His-tagged cirhin fusion proteins expressed in HeLa and HepG2, cells we show that cirhin is a nucleolar protein and that the R565W mutation, for which all NAIC patients are homozygous, has no effect on subcellular localization. Cirhin has an active C-terminal monopartite nuclear localization signal (NLS) and a unique nucleolar localization signal (NrLS) between residues 315 and 432. The nucleolus is not known to be important specifically for intrahepatic cholestasis. These observations provide a new dimension in the study of hereditary cholestasis.

Fibroblast growth factor 3, a protein with a dual subcellular fate, is interacting with human ribosomal protein S2

The secreted isoform of fibroblast growth factor 3 (FGF3) induces a mitogenic cell response, while the nuclear form inhibits cell proliferation. Recently, we identified a nucleolar FGF3-binding protein which is implicated in processing of pre-rRNA as a possible target of nuclear FGF3 signalling. Here, we report a second candidate protein identified by a yeast two-hybrid screen for nuclear FGF3 action, ribosomal protein S2, rpS2. Recombinant rpS2 binds to in vitro translated FGF3 and to nuclear FGF3 extracted from transfected COS-1 cells. Characterization of the FGF3 binding domain of rpS2 showed that both the Arg-Gly-rich N-terminal region and a short carboxyl-terminal sequence of rpS2 are necessary for FGF3 binding. Mapping the S2 binding domains of FGF3 revealed that these domains are important for both NoBP and rpS2 interaction. Transient co-expression of rpS2 and nuclear FGF3 resulted in a reduced nucleolar localization of the FGF. These findings suggest that the nuclear form of FGF3 inhibits cell proliferation by interfering with ribosomal biogenesis.

Identification of an unconventional nuclear localization signal in human ribosomal protein S2

Ribosomal proteins must be imported into the nucleus after being synthesized in the cytoplasm. Since the rpS2 amino acid sequence does not contain a typical nuclear localization signal, we used deletion mutant analysis and rpS2-beta-galactosidase chimeric proteins to identify the nuclear targeting domains in rpS2. Nuclear rpS2 is strictly localized in the nucleoplasm and is not targeted to the nucleoli. Subcellular localization analysis of deletion mutants of rpS2-beta-galactosidase chimeras identified a central domain comprising 72 amino acids which is necessary and sufficient to target the chimeric beta-galactosidase to the nucleus. The nuclear targeting domain shares no significant similarity to already characterized nuclear localization signals in ribosomal proteins or other nuclear proteins. Although a Nup153 fragment containing the importinbeta binding site fused to VP22 blocks nuclear import of rpS2-beta-galactosidase fusion proteins, nuclear uptake of rpS2 could be mediated by several import receptors since it binds to importinalpha/beta and transportin.

Identification of a novel nucleolar localization signal and a degradation signal in Survivin-deltaEx3: a potential link between nucleolus and protein degradation

For a long time, as the most prominent subnuclear structure, nucleolus has been recognized as a main site where rRNA processing and ribosomal subunit assemblies take place. It has not been until recently that additional functions of nucleolus have begun to be proposed. In this study, we for the first time demonstrate that Survivin-deltaEx3, a novel functionally splice variant of Survivin localizes in the nucleoli where it degrades rapidly through ubiquitin-proteosome pathway. Several lines of evidences provided in this report support this finding (i) a novel nucleolar localization sequence (NoLS, MQRKPTIRRKNLRLRRK) and a novel degradation signal (aa92-aa137) within Survivin-deltaEx3 were identified (ii) proteasome inhibitors MG132 or ALLN greatly inhibits degradation of Survivin-deltaEx3 and polyubiquitination of Survivin-deltaEx3 was detected (iii) heterologous proteins such as TAT-PTD or p14ARF, when fused to this putative degradation signal, result in a significant degradation within the nucleolus. In addition, the nucleolar localization and degradation of Survivin-deltaEx3 appear to be required for its antiapoptotic function, since neither NoLS-deleted nor degradation signal-deleted Survivin-deltaEx3 retains protective effect against Doxorubicin-induced apoptosis. Thus, our results have provided evidences to suggest that besides cytosol, nucleus, endoplsmic reticulum (ER) or lysosomes, nucleolus may also operate important protein degradation pathway, which has been overlooked previously.

Nuclear and Nucleolar Localization of 18-kDa Fibroblast Growth Factor-2 Is Controlled by C-terminal Signals

Members of high (22-, 22.5-, 24-, and 34-kDa) and low (18-kDa) molecular mass forms of fibroblast growth factor-2 (FGF-2) regulate cell proliferation, differentiation, and migration. FGF-2s have been previously shown to accumulate in the nucleus and nucleolus. Although high molecular weight forms of FGF-2 contain at least one nuclear localization signal (NLS) in their N-terminal extension, the 18-kDa FGF-2 does not contain a standard NLS. To determine signals controlling the nuclear and subnuclear localization of the 18-kDa FGF-2, its full-length cDNA was fused to that of green fluorescent protein (GFP). The fusion protein was primarily localized to the nucleus of COS-7 and HeLa cells and accumulated in the nucleolus. The subcellular distribution was confirmed using wild type FGF-2 and FGF-2 tagged with a FLAG epitope. A 17-amino acid sequence containing two groups of basic amino acid residues separated by eight amino acid residues directed GFP and a GFP dimer into the nucleus. We systematically mutated the basic amino acid residues in this nonclassical NLS and determined the effect on nuclear and nucleolar accumulation of 18-kDa FGF-2. Lys(119) and Arg(129) are the key amino acid residues in both nuclear and nucleolar localization, whereas Lys(128) regulates only nucleolar localization of 18-kDa FGF-2. Together, these results demonstrate that the 18-kDa FGF-2 harbors a C-terminal nonclassical bipartite NLS, a portion of which also regulates its nucleolar localization.

Signaling, internalization, and intracellular activity of fibroblast growth factor

The fibroblast growth factor (FGF) family contains 23 members in mammals including its prototype members FGF-1 and FGF-2. FGFs have been implicated in regulation of many key cellular responses involved in developmental and physiological processes. These includes proliferation, differentiation, migration, apoptosis, angiogenesis, and wound healing. FGFs bind to five related, specific cell surface receptors (FGFRs). Four of these have intrinsic tyrosine kinase activity. Dimerization of the receptor is a prerequisite for receptor transphosphorylation and activation of downstream signaling molecules. All members of the FGF family have a high affinity for heparin and for cell surface heparan sulfate proteoglycans, which participate in formation of stable and active FGF-FGFR complexes. FGF-mediated signaling is an evolutionarily conserved signaling module operative in invertebrates and vertebrates. It seems that some members of the family have a dual mode of action. FGF-1, FGF-2, FGF-3, and FGF-11-14 have been found intranuclearly as endogenous proteins. Exogenous FGF-1 and FGF-2 are internalized by receptor-mediated endocytosis, in a clathrin-dependent and -independent way. Internalized FGF-1 and FGF-2 are able to cross cellular membranes to reach the cytosol and the nuclear compartment. The role of FGF internalization and the intracellular activity of some FGFs are discussed in the context of the known signaling induced by FGF.

Contact call-driven zenk mRNA expression in the brain of the budgerigar (Melopsittacus undulatus)

Contact call-driven zenk (zif268, egr1, NGF1A, Krox 24) mRNA expression was mapped with in situ hybridization histochemistry in a vocal learning parrot, the budgerigar (M. undulatus). Relative to controls, call stimulation induced high zenk mRNA expression in all auditory areas including those closely associated with the vocal system within the anterior forebrain (Brauth et al. (2001) J. Comp. Neurol. 432, 481; (2002) Learn. Memory 9, 76). Thus there is a high correspondence between the distributions of neurons exhibiting contact call-driven zenk protein and mRNA expression in budgerigars. Field L2a, an area reported previously to express only perinucleolar zenk protein localization (Brauth et al. (2002) Learn. Memory 9, 76) also showed zenk mRNA expression.

Cloning and molecular characterization of a novel gene strongly induced by the adenovirus E1A gene in rat thyroid cells

Expression of the adenovirus E1A gene in the rat thyroid differentiated cell line PC Cl 3 induces thyrotropin-independent cell growth and impairs differentiation. However, the malignant phenotype is achieved only when the PC E1A cells are infected with other murine retroviruses carrying the v-abl, v-raf or polyoma middle-T genes. To determine through which genes E1A affects thyroid cells, we differentially screened PC Cl 3 and PC E1A cells. Here we report a new gene, named CL2, that is upregulated in PC E1A cells. The CL2 transcript is 4.4 kb long and encodes a 949 amino-acid protein. Conceptual translation of the open reading frame showed one product with a signal peptide, multiple nuclear localization signals and three newly described domains. Furthermore, in vivo, this protein was located juxtanuclear, which is suggestive of Golgian localization, and also in cytoplasm and nucleus/nucleolus. Finally, CL2 gene expression was drastically downregulated in human thyroid neoplastic cell lines and tissues.

Nucleolar localization of an isoform of the IGF-I precursor

BACKGROUND Alternative exons encode different isoforms of the human insulin-like growth factor-I (IGF-I) precursor without altering mature IGF-I. We hypothesized that the various IGF-I precursors may traffic IGF-I differently. Chimeric IGF-I precursors were made with green fluorescent protein (GFP) cloned between the signal and mature IGF-I domains. RESULTS Chimeras containing exons 1 or 2 were located in the cytoplasm, consistent with a secretory pathway, and suggesting that both exons encoded functional signal peptides. Exon 5-containing chimeras localized to the nucleus and strongly to the nucleolus, while chimeras containing exon 6 or the upstream portion of exon 5 did not. Nuclear and nucleolar localization also occurred when the mature IGF-I domain was deleted from the chimeras, or when signal peptides were deleted. CONCLUSIONS We have identified a nucleolar localization for an isoform of the human IGF-I precursor. The findings are consistent with the presence of a nuclear and nucleolar localization signal situated in the C-terminal part of the exon 5-encoded domain with similarities to signals in several other growth factors.

c-erbB-3: a nuclear protein in mammary epithelial cells

c-erbB receptors are usually located in cell membranes and are activated by extracellular binding of EGF-like growth factors. Unexpectedly, using immunofluorescence we found high levels of c-erbB-3 within the nuclei of MTSV1-7 immortalized nonmalignant human mammary epithelial cells. Nuclear localization was mediated by the COOH terminus of c-erbB-3, and a nuclear localization signal was identified by site-directed mutagenesis and by transfer of the signal to chicken pyruvate kinase. A nuclear export inhibitor caused accumulation of c-erbB-3 in the nuclei of other mammary epithelial cell lines as demonstrated by immunofluorescence and biochemical cell fractionation, suggesting that c-erbB-3 shuttles between nuclear and nonnuclear compartments in these cells. Growth of MTSV1-7 on permeable filters induced epithelial polarity and concentration of c-erbB-3 within the nucleoli. However, the c-erbB-3 ligand heregulin beta1 shifted c-erbB-3 from the nucleolus into the nucleoplasm and then into the cytoplasm. The subcellular localization of c-erbB-3 obviously depends on exogenous stimuli and on the stage of epithelial polarity and challenges the specific function of c-erbB-3 as a transmembrane receptor protein arguing for additional, as yet unidentified, roles of c-erbB-3 within the nucle(ol)us of mammary epithelial cells.

Fibroblast growth factor 8 is expressed at higher levels in lactating human breast and in breast cancer

Fibroblast growth factor 8 can transform NIH3T3 cells and its expression has been found to be associated with breast and prostate cancer. Following our finding that fibroblast growth factor 8 mRNA expression is increased in breast cancer, we have undertaken an immunohistochemistry study of fibroblast growth factor 8 expression in a series of human breast tissues and other normal tissues. Our findings confirm increased expression of fibroblast growth factor 8 in malignant breast tissue but also show significant fibroblast growth factor 8 expression in non-malignant breast epithelial cells. No significant difference in fibroblast growth factor 8 expression was found between different grades of ductal carcinoma, lobular carcinoma and ductal carcinoma in-situ or cancer of different oestrogen receptor, progesterone receptor or nodal status. The highest levels of fibroblast growth factor 8 expression were found in lactating breast tissues and fibroblast growth factor 8 was also detected in human milk. A survey of other normal tissues showed that fibroblast growth factor 8 is expressed in the proliferative cells of the dermis and epithelial cells in colon, ovary fallopian tube and uterus. Fibroblast growth factor 8 appears to be expressed in several organs in man and appears to have an importance in lactation.

Hepatoma-derived growth factor stimulates cell growth after translocation to the nucleus by nuclear localization signals

Hepatoma-derived growth factor (HDGF) is the original member of the HDGF family of proteins, which contains a well-conserved N-terminal amino acid sequence (homologous to the amino terminus of HDGF; hath) and nuclear localization signals (NLSs) in gene-specific regions other than the hath region. In addition to a bipartite NLS in a gene-specific region, an NLS-like sequence is also found in the hath region. In cells expressing green fluorescence protein (GFP)-HDGF, green fluorescence was observed in the nucleus, whereas it was detected in the cytoplasm of cells expressing GFP-HDGF with both NLSs mutated or deleted. GFP-hath protein (GFP-HATH) was distributed mainly in the nucleus, although some was present in the cytoplasm, whereas GFP-HDGF with a deleted hath region (HDGFnonHATH) was found only in the nucleus. Exogenously supplied GFP-HDGF was internalized and translocated to the nucleus. GFP-HATH was internalized, whereas GFP-HDGFnonHATH was not. Overexpression of HDGF stimulated DNA synthesis and cellular proliferation, although HDGF with both NLSs deleted did not. Overexpression of HDGFnonHATH caused a significant stimulation of DNA synthesis, whereas that of hath protein did not. HDGF containing the NLS sequence of p53 instead of the bipartite NLS did not stimulate DNA synthesis, and truncated forms without the C- or N-terminal side of NLS2 did not. These findings suggest that the gene-specific region, at least the bipartite NLS sequence and the N- and C-terminal neighboring portions, is essential for the mitogenic activity of HDGF after nuclear translocation.

Ectopic expression of FGF-3 results in abnormal prostate and Wolffian duct development

To evaluate the effects of FGF-3 expression in the prostate and male reproductive tract, we employed a bitransgenic system to target FGF-3 to these organs. We present a first study that ectopic FGF-3 expression resulted in exuberant hyperplasia of all bigenic prostatic lobes typified by epithelial stratification, cribiform structures and papillary tufts. These cells displayed increased nuclear-to-cytoplasmic ratios and bromodeoxyuridine (BrdU) proliferative index but retained relatively uniform nuclear androgen receptor (AR) and the tumor suppressor C-CAM1 staining. Furthermore, the dysmorphogenic prostatic cells also resembled PIN (prostatic intraepithelial neoplasia)-like lesions but did not appear to have invaded the basal lamina. In addition to these phenotypes, profound disorders of the bigenic Wolffian duct derivatives were observed. The bigenic ampullary glands and vas deferens were extremely cystic, hypertrophic and hyperplastic; the enlarged epididymi showed a reduction of spermatozoa and the seminal vesicles exhibited a dramatic reduction of seminal secretions. Because of these severe abnormalities, these infertile males presented with diaphragmatic hernias, hemoperitoneum and many secondary abnormalities at sacrifice. Taken together, we show that ectopic FGF-3 expression severely perturbs normal prostate development and our system should be useful for the analyses of early changes in prostatic hyperplasia.

Mechanisms of nuclear import and export that control the subcellular localization of class II transactivator

The presence of the class II transactivator (CIITA) activates the transcription of all MHC class II genes. Previously, we reported that deletion of a carboxyl-terminal nuclear localization signal (NLS) results in the cytoplasmic localization of CIITA and one form of the type II bare lymphocyte syndrome. However, further sequential carboxyl-terminal deletions of CIITA resulted in mutant forms of the protein that localized predominantly to the nucleus, suggesting the presence of one or more additional NLS in the remaining sequence. We identified a 10-aa motif at residues 405-414 of CIITA that contains strong residue similarity to the classical SV40 NLS. Deletion of this region results in cytoplasmic localization of CIITA and loss of transactivation activity, both of which can be rescued by replacement with the SV40 NLS. Fusion of this sequence to a heterologous protein results in its nuclear translocation, confirming the identification of a NLS. In addition to nuclear localization sequences, CIITA is also controlled by nuclear export. Leptomycin B, an inhibitor of export, blocked the nuclear to cytoplasmic translocation of CIITA; however, leptomycin did not alter the localization of the NLS mutant, indicating that this region mediates only the rate of import and does not affect CIITA export. Several candidate nuclear export sequences were also found in CIITA and one affected the export of a heterologous protein. In summary, we have demonstrated that CIITA localization is balanced between the cytoplasm and nucleus due to the presence of NLS and nuclear export signal sequences in the CIITA protein.

NoBP, a nuclear fibroblast growth factor 3 binding protein, is cell cycle regulated and promotes cell growth

Secreted and nuclear forms of fibroblast growth factor 3 (FGF3) have opposing effects on cells. The secreted form stimulates cell growth and transformation, while the nuclear form inhibits DNA synthesis and cell proliferation. By using the yeast two-hybrid system we have identified a nucleolar FGF3 binding protein (NoBP) which coimmunoprecipitated and colocalized with FGF3 in transfected COS-1 cells. Characterization of the NoBP binding domain of FGF3 exactly matched the sequence requirements of FGF3 for its translocation into the nucleoli, suggesting that NoBP might be the nucleolar binding partner of FGF3 essential for its nucleolus localization. Carboxyl-terminal domains of NoBP contain linear nuclear and nucleolar targeting motifs which are capable of directing a heterologous protein beta-galactosidase to the nucleus and the nucleoli. While NoBP expression was detected in all analyzed proliferating established cell lines, NoBP transcription was rapidly downregulated in the promyelocytic leukemia cell line HL60 when induced to differentiate. Analysis on the expression pattern of NoBP mRNA throughout the cell cycle in HeLa cells synchronized by lovastatin demonstrated a substantial upregulation during the late G(1)/early S phase. NoBP overexpression conferred a proliferating effect onto NIH 3T3 cells and can counteract the inhibitory effect of nuclear FGF3, suggesting a role of NoBP in controlling proliferation in cells. We propose that NoBP is the functional target of nuclear FGF3 action.

UV induces nucleolar translocation of ING1 through two distinct nucleolar targeting sequences

The ING1 candidate tumor suppressor is downregulated in a variety of primary tumors and established cancer cell lines. Blocking its expression experimentally promotes unregulated growth in vitro and in vivo, using cell and animal models. Alternative splicing products encode proteins that localize to the nucleus, inhibit cell cycle progression and affect apoptosis in different model systems. Here we show that ING1 proteins translocate to the nucleolus 12-48 h after UV-induced DNA damage. When a small 50 amino acid portion of ING1 was fused to green fluorescent protein, the fusion protein was efficiently targeted to the nucleolus, indicating that ING1 possesses an intrinsic nucleolar targeting sequence (NTS). We mapped this activity to two distinct 4 amino acid regions, which individually direct fused heterologous proteins to the nucleolus. Overexpression of ING1 induced apoptosis of primary fibroblasts in the presence and absence of UV exposure. In contrast, NTS mutants of ING1 that were not targeted to the nucleolus did not efficiently induce apoptosis when overexpressed and instead protected cells from UV-induced apoptosis. Taken together, these results indicate that UV induces ING1 to translocate to the nucleolus and that this translocation may facilitate apoptosis.

Altered intracellular localization of fibroblast growth factor receptor 3 in human breast cancer

Immunohistochemical staining of human breast tissues, using an antibody against fibroblast growth factor receptor 3 [FGFR-3], showed differences in cellular distribution. Both malignant and non-malignant epithelial cells contained FGFR-3 immunoreactivity, but myoepithelial cells and stroma were negative. The staining pattern in malignant epithelial cells was predominantly nuclear, whereas epithelial cells in normal breast tissue showed both cytoplasmic and nuclear elements. Reverse transcription-polymerase chain reaction (RT-PCR) revealed two isoforms of FGFR-3 corresponding to the FGFR-3-IIIb variant and a previously described exon-deleted nuclear form of FGFR-3, which were present in both malignant and non-malignant epithelial cells. The higher level of nuclear staining and loss of cytoplasmic staining seen in malignant epithelial cells did not correspond to an increase in expression of the exon-deleted form of FGFR-3, nor to any detectable activating point mutations. Since receptor activation can result in its movement to a perinuclear localization, an alternative explanation for the redistribution of FGFR-3-IIIb could be different degrees of activation by a ligand (FGF1 or FGF9). No FGF9 was detected by immunohistochemistry in breast tissues. FGF1, however, is present in the majority of breast cancers and a different tissue distribution of FGF1 was found in breast tissues, showing predominantly nuclear, or a mix of nuclear and cytoplasmic FGFR-3. The difference in FGFR-3 staining patterns may implicate this ligand-receptor pair in breast cancer.

Cell-cycle dependent tyrosine phosphorylation on mortalin regulates its interaction with fibroblast growth factor-1

We previously reported that endogenously expressed, intracellularly localized fibroblast growth factor (FGF)-1 interacts with mortalin. Here we report that FGF-1 added to the culture medium of quiescent BALB/c3T3 cells is taken up by the cells and interacts with mortalin in the cells in a regulated manner. Although both the internalized FGF-1 and mortalin were present at high levels throughout the FGF-1-initiated cell cycle, their interaction became apparent only in late G1 phase. Interestingly, mortalin was preferentially tyrosine phosphorylated at the same time, and when its normally weak phosphorylation in early G1 phase was augmented by treating the cells with vanadate, a strong interaction between mortalin and FGF-1 was established. Conversely, when phosphorylated mortalin was treated with tyrosine phosphatase, its interaction with FGF-1 was abrogated. These results indicate that FGF-1 taken up by cells preferentially interacts with mortalin in late G1 phase of the cell cycle, and that tyrosine phosphorylation of mortalin regulates this interaction.

Nuclear localization of epidermal growth factor and epidermal growth factor receptors in human thyroid tissues

Epidermal growth factor (EGF) has widespread growth effects, and in some tissues proliferation is associated with the nuclear localization of EGF and epidermal growth factor receptor (EGFR). In the thyroid, EGF promotes growth but differs from thyrotropin (TSH) in inhibiting rather than stimulating functional parameters. We have therefore studied the occurrence and cellular distribution of EGF and EGFR in normal thyroid, in Graves' disease, where growth is mediated through the thyrotropin receptor (TSHR), and in a variety of human thyroid tumors. In the normal gland the staining was variable, but largely cytoplasmic, for both EGF and EGFR. In Graves' disease there was strong cytoplasmic staining for both EGF and EGFR, with frequent positive nuclei. Nuclear positivity for EGF and particularly for EGFR was also a feature of both follicular adenomas and follicular carcinomas. Interestingly, nuclear staining was almost absent in papillary carcinomas. These findings document for the first time the presence of nuclear EGF and EGFR in thyroid. Their predominant occurrence in tissues with increased growth (Graves' disease, follicular adenoma, and carcinoma) may indicate that nuclear EGF and EGFR play a role in growth regulation in these conditions. The absence of nuclear EGF and EGFR in papillary carcinomas would suggest that the role played by EGF in growth control differs between papillary carcinoma and follicular adenomas/carcinomas of the thyroid.

Fibroblast growth factors

SUMMARY

Fibroblast growth factors (FGFs) make up a large family of polypeptide growth factors that are found in organisms ranging from nematodes to humans. In vertebrates, the 22 members of the FGF family range in molecular mass from 17 to 34 kDa and share 13-71% amino acid identity. Between vertebrate species, FGFs are highly conserved in both gene structure and amino-acid sequence. FGFs have a high affinity for heparan sulfate proteoglycans and require heparan sulfate to activate one of four cell-surface FGF receptors. During embryonic development, FGFs have diverse roles in regulating cell proliferation, migration and differentiation. In the adult organism, FGFs are homeostatic factors and function in tissue repair and response to injury. When inappropriately expressed, some FGFs can contribute to the pathogenesis of cancer. A subset of the FGF family, expressed in adult tissue, is important for neuronal signal transduction in the central and peripheral nervous systems.

The tetrabasic KKKK(147-150) motif determines intracrine regulatory effects of PthrP 1-173 on chondrocyte PPi metabolism and matrix synthesis

Expression of PTHrP is a major regulator of growth cartilage development and also becomes robust in osteoarthritic cartilage. We further defined how PTHrP 1-173, which we observed to be the preferentially expressed PTHrP isoform in normal and osteoarthritic cartilage, functions in chondrocytes. We transfected both immortalized human juvenile costal chondrocytes (TC28 cells) and rabbit articular chondrocytes with wild-type PTHrP 1-173 and mutants of putative PTHrP 1-173 endoproteolytic processing sites. Wild-type PTHrP 1-173 inhibited collagen synthesis and decreased extracellular PPi (which critically regulates hydroxyapatite deposition) by 50-80% in both chondrocytic cell types. In contrast, PTHrP 1-173 mutated at the PTHrP 147-150 motif KKKK (but not the other site-directed mutants) and increased both extracellular PPi and collagen synthesis by >50%. Synthetic PTHrP 140-173 mutated at amino acids 147-150 and also increased extracellular PPi, and wild-type 140-173 decreased extracellular PPi in permeabilized cells. The 147-nuclear localization of PTHrP. We conclude that the tetrabasic 147-150 motif functions to determine how PTHrP 1-173 regulates collagen synthesis and levels of extracellular PPi by an intracrine mechanism in chondrocytes, and it may prove useful as a therapeutic target for regulation of mineralization.

An acidic amino acid cluster regulates the nucleolar localization and ribosome assembly of human ribosomal protein L22

The control of human ribosomal protein L22 (rpL22) to enter into the nucleolus and its ability to be assembled into the ribosome is regulated by its sequence. The nuclear import of rpL22 depends on a classical nuclear localization signal of four lysines at positions 13-16. RpL22 normally enters the nucleolus via a compulsory sequence of KKYLKK (I-domain, positions 88-93). An acidic residue cluster at the C-terminal end (C-domain) plays a nuclear retention role. The retention is concealed by the N-domain (positions 1-9) which weakly interacts with the C-domain as demonstrated in the yeast two-hybrid system. Once it reaches the nucleolus, the question of whether rpL22 is assembled into the ribosome depends upon the presence of the N-domain. This suggests that the N-domain, on dissociation from its interaction with the C-domain, binds to a specific region of the 28S rRNA for ribosome assembly.

Fibroblast growth factor-10: a stromal mediator of epithelial function in the ovine uterus

Fibroblast growth factor-10 (FGF-10) is a stromal-derived paracrine growth factor considered to be important during embryogenesis; however, its expression by cells in the female reproductive tract has not been investigated. Therefore, an ovine FGF-10 cDNA was cloned from an ovine endometrial cDNA library to investigate expression and potential paracrine characteristics of FGF-10 in the ovine uterus. The ovine FGF-10 cDNA encodes a protein of 213 amino acids and possesses an unusually long 5' untranslated region (UTR). In situ hybridization demonstrated that ovine FGF-10 mRNA was expressed by endometrial stromal cells and by mesenchymal cells of the chorioallantoic placenta. The mRNA for FGF-7, a homologue of FGF-10, was localized in the tunica muscularis of blood vessels in endometrium and myometrium. In contrast, FGF receptor 2IIIb, the high-affinity receptor for both FGF-10 and FGF-7, was expressed exclusively in luminal epithelium, glandular epithelium, and placental trophectoderm. The in vivo spatial expression pattern suggests that FGF-10 is a novel endometrial stromal cell-derived mediator of uterine epithelial and conceptus trophectodermal functions. The nonoverlapping spatial patterns of expression for FGF-10 and FGF-7 in ovine uterus and conceptus suggest independent roles in uterine function and conceptus development.

The homeodomain coordinates nuclear entry of the Lhx3 neuroendocrine transcription factor and association with the nuclear matrix

LIM homeodomain transcription factors regulate development in complex organisms. To characterize the molecular signals required for the nuclear localization of these proteins, we examined the Lhx3 factor. Lhx3 is essential for pituitary organogenesis and motor neuron specification. By using functional fluorescent derivatives, we demonstrate that Lhx3 is found in both the nucleoplasm and nuclear matrix. Three nuclear localization signals were mapped within the homeodomain, and one was located in the carboxyl terminus. The homeodomain also serves as the nuclear matrix targeting sequence. No individual signal is alone required for nuclear localization of Lhx3; the signals work in combinatorial fashion. Specific combinations of these signals transferred nuclear localization to cytoplasmic proteins. Mutation of nuclear localization signals within the homeodomain inhibited Lhx3 transcriptional function. By contrast, mutation of the carboxyl-terminal signal activated Lhx3, indicating that this region is critical to transcriptional activity and may be a target of regulatory pathways. The pattern of conservation of the nuclear localization and nuclear matrix targeting signals suggests that the LIM homeodomain factors use similar mechanisms for subcellular localization. Furthermore, upon nuclear entry, association of Lhx3 with the nuclear matrix may contribute to LIM homeodomain factor interaction with other classes of transcription factors.

Translation of p15.5INK4B, an N-terminally extended and fully active form of p15INK4B, is initiated from an upstream GUG codon

The expression of the cyclin-dependent kinase inhibitor p15INK4B in normal cells after induction with TGF-beta1, or following overexpression from an adenovirus-encoded cDNA, appears on an SDS-polyacrylamide gel as a doublet. Here, the underlying mechanism behind the synthesis of the two species has been studied. By expressing cDNAs truncated at their 5' end, we found that the synthesis of the more slowly migrating form, called p15.5INK4B, is dependent on a sequence upstream of the first AUG codon thought to initiate translation of p15INK4B. Two potential, in frame, alternative upstream initiation codons, ACG and GUG, were individually changed to GCA encoding alanine. Analysis by in vitro translation, or immunoblotting of lysates from transfected 293 cells, showed that translation of p15.5INK4B is initiated at the GUG located 13 codons upstream of the first AUG. When this AUG was mutated, p15INK4B was no longer made. Instead, a shorter form, initiated at an in frame AUG located seven codons downstream, was synthesized. Finally, when both these AUGs were mutated, only p15.5INK4B was generated. Both p15INK4B and p15.5INK4B bound to CDK4 and CDK6, inhibited DNA synthesis, and caused replicative senescence of a human glioma cell line. We thus conclude that p15INK4B and p15.5INK4B, encoded by the CDKN2B gene, are functionally indistinguishable as based on these assays.