Multiple growth factor induction of a murine early response gene that complements a lethal defect in yeast ribosome biogenesis

Nucleolar Sik-similar protein (Sik-SP) is required for the maintenance of uterine estrogen signaling mechanism via ERα

Sik-similar protein (Sik-SP), a small nucleolar ribonucleoprotein, has been shown to be primarily involved in ribosome biogenesis. However, its role in the hormone-directed nuclear receptor signaling is largely unknown. Here, we provide novel evidence that Sik-SP is required for appropriate regulation of estrogen receptor (ER)α-mediated estradiol-17β (E2)-dependent uterine physiologic responses in mice. Studies by Western blotting using the newly developed antibodies for Sik-SP showed that this protein is up-regulated in both the ovariectomized wild-type and ERα null uteri by E2. Immunohistochemical analyses in uterine sections showed that this protein is induced in the epithelial and stromal cells. Coimmunoprecipitation studies revealed that E2 directs molecular interaction between Sik-SP and ERα. Furthermore, gel-mobility shift and chromatin immunoprecipitation analyses provided evidence that Sik-SP is recruited with ERα to estrogen-responsive uterine gene promoters. Overexpression of Sik-SP in vitro demonstrated a role for Sik-SP in cellular growth and viability. In a primary uterine epithelial-stromal coculture system, E2 exhibited early induction of Sik-SP in both the epithelial and stromal cells. Interestingly, suppression of Sik-SP in this coculture model, for the stromal but not epithelial cells, caused perturbation of E2-dependent proliferation in the epithelial cell layer. Similarly, in vivo uterine suppression of Sik-SP also caused inhibition of epithelial cell proliferation and aberrant prolongation of water imbibition in the late phase by E2. Finally, studies showed that Sik-SP is physiologically important during the onset of implantation by E2. In conclusion, Sik-SP, an early E2-responsive nucleolar protein, is necessary to induce E2-dependent ERα-mediated appropriate physiologic responses in the uterus.

Proteomic snapshot analyses of preribosomal ribonucleoprotein complexes formed at various stages of ribosome biogenesis in yeast and mammalian cells

Proteomic technologies powered by advancements in mass spectrometry and bioinformatics and coupled with accumulated genome sequence data allow a comprehensive study of cell function through large-scale and systematic protein identifications of protein constituents of the cell and tissues, as well as of multi-protein complexes that carry out many cellular function in a higher-order network in the cell. One of the most extensively analyzed cellular functions by proteomics is the production of ribosome, the protein-synthesis machinery, in the nucle(ol)us--the main site of ribosome biogenesis. The use of tagged proteins as affinity bait, coupled with mass spectrometric identification, enabled us to isolate synthetic intermediates of ribosomes that might represent snapshots of nascent ribosomes at particular stages of ribosome biogenesis and to identify their constituents--some of which showed dynamic changes for association with the intermediates at various stages of ribosome biogenesis. In this review, in conjunction with the results from yeast cells, our proteomic approach to analyze ribosome biogenesis in mammalian cells is described.

B cell activation leads to upregulated expression of the murine Sik-similar protein gene

B cells stimulated by the combination of CD40L plus anti-Ig were screened for upregulated gene expression in an unbiased fashion through differential display. An inducible transcript was obtained that corresponds to a sequence previously isolated from pre-B cells and termed murine Sik-similar protein (mSSP). The mSSP gene is predominantly expressed in lymphoid tissues, including spleen and thymus, as well as testis, with lesser amounts in kidney and ovary. Among lymphocyte cell lines, mSSP expression varies widely. The mSSP protein is localized to the nucleus in NIH3T3 cells, and its expression in BAL-17 B cells varies with cell cycle progression. Expression of mSSP increased significantly within the first hours of B cell treatment with either CD40L, anti-IgM, or phorbol myristate acetate (PMA) with or without ionomycin. The predicted amino acid sequence of mSSP bears some homology to proteins involved in ribosomal RNA synthesis and processing. mSSP is a previously identified pre-B cell gene now shown for the first time to be an activation-responsive transcript in mature, primary B cells.

Fibroblast growth factor 2 is necessary for the growth of glutamate projection neurons in the anterior neocortex

Basic fibroblast growth factor (Fgf2) is required for the generation of founder cells within the dorsal pseudostratified ventricular epithelium, which will generate the cerebral cortex, but the ganglionic eminences are not affected. We report here that the Fgf2 null mutant mice show an approximately 40% decrease in cortical glutamatergic pyramidal neurons. In contrast, no change in pyramidal or granule cell number is detected in the hippocampus of Fgf2 -/- mice. In addition, the soma of the pyramidal cells in the frontal and parietal cortices are smaller in Fgf2 knock-out mice. The decrease in the number and size of glutamatergic neuronal population affects all cortical layers but is restricted to the frontal and parietal cortices without any change in the occipital cortex, indicating that Fgf2 is necessary to regulate cell number and size in the anterior cerebral cortex. In contrast to pyramidal neurons, cortical GABA interneurons are unaffected by the lack of Fgf2. The resulting imbalance between the excitatory and inhibitory neurotransmission in the cerebral cortex is reflected by an increased duration of sleep when the animals receive a GABA receptor agonist. Thus, Fgf2 signaling may contribute to the regional specification of the cerebral cortex and may play a role in increasing the size of anterior cortical regions during vertebrate evolution.

Increased expression of a nucleolar Nop5/Sik family member in metastatic melanoma cells: evidence for its role in nucleolar sizing and function

F10 and BL6 cells of B16 mouse melanoma cells are metastatic after intravenous injection, but only BL6 cells can metastasize to lungs after subcutaneous injection. Differences in gene expression between the two cell lines were examined, and a greater expression of the Sik-similar protein (Sik-SP) gene was found in BL6 cells. Structurally, Sik-SP belongs to the nucleolar Nop5/Sik family whose members play central roles in ribosome biogenesis; however, the function of Sik-SP has not been examined. Cytology with green fluorescent protein-fused proteins showed that Sik-SP was localized to the nucleolus. To examine whether Sik-SP is involved in ribosome biogenesis, two parameters were measured: magnitude of ribosomal RNA synthesis per nucleus and magnitude of protein production from the same amount of mRNA of an exogenous luciferase gene. Both values and, in addition, nucleolar size were larger in COS-7 monkey kidney cells overexpressing Sik-SP and BL6 cells than in mock-transfected COS-7 and F10 cells, respectively. Sik-SP seemed to promote ribosome biogenesis in the nucleolus. Furthermore, the expression of Sik-SP seemed to confer a greater cell growth response to serum, because such a response was greater in BL6 cells and F10 cells overexpressing Sik-SP than in untreated and mock-transfected F10 cells. Sik-SP may render melanoma cells more competent to survive through augmenting the activity of nucleolus.

Isolation and characterization of a novel PDGF-induced human gene

Using a differential display RT-PCR strategy to identify novel growth-factor-induced transcripts, we cloned and characterized the human homolog of yeast NOP5/NOP58, whose gene product has been implicated in the execution of early pre-rRNA processing steps. Human NOP5 cDNA was isolated from an M426 fibroblast cDNA library. Determination of the cDNA nucleotide sequence revealed an open reading frame of 1587 nucleotides encoding a predicted gene product of 529 amino acids and mass of 59554Da. The yeast and human NOP5 gene products were found to share 63% homology and 46% identity. NOP5 mRNA was induced within 2h of platelet-derived growth factor (PDGF) treatment of human M426 fibroblasts. Pretreatment with cycloheximide enhanced, while actinomycin blocked induction of the NOP5 transcript. In vitro translational analysis of the cDNA revealed a 60kDa species, consistent with the predicted molecular weight of the gene product. Ubiquitous, but differential NOP5 mRNA expression was revealed after Northern blot analysis of total RNA from several human tissues. Moreover, NOP5 mRNA expression was also demonstrated in cell lines of fibroblast, epithelial, and myeloid origin. A highly charged carboxy terminal domain and consensus phosphorylation sites were identified. The presence of potential regulatory elements, together with growth factor induction and widespread expression is consistent with the hypothesis that the NOP5 gene product may play a role in fundamental cellular growth processes.