Differential expression of elongation factor-2, alpha4 phosphoprotein and Cdc5-like protein in prolactin-dependent/independent rat lymphoid cells

Low resolution structure of the human alpha4 protein (IgBP1) and studies on the stability of alpha4 and of its yeast ortholog Tap42

The yeast Tap42 and mammalian alpha4 proteins belong to a highly conserved family of regulators of the type 2A phosphatases, which participate in the rapamycin-sensitive signaling pathway, connecting nutrient availability to cell growth. The mechanism of regulation involves binding of Tap42 to Sit4 and PPH21/22 in yeast and binding of alpha4 to the catalytic subunits of type 2A-related phosphatases PP2A, PP4 and PP6 in mammals. Both recombinant proteins undergo partial proteolysis, generating stable N-terminal fragments. The full-length proteins and alpha4 C-terminal deletion mutants at amino acids 222 (alpha4Delta222), 236 (alpha4Delta236) and 254 (alpha4Delta254) were expressed in E. coli. alpha4Delta254 undergoes proteolysis, producing a fragment similar to the one generated by full-length alpha4, whereas alpha4Delta222 and alpha4Delta236 are highly stable proteins. alpha4 and Tap42 show alpha-helical circular dichroism spectra, as do their respective N-terminal proteolysis resistant products. The cloned truncated proteins alpha4Delta222 and alpha4Delta236, however, possess a higher content of alpha-helix, indicating that the C-terminal region is less structured, which is consistent with its higher sensitivity to proteolysis. In spite of their higher secondary structure content, alpha4Delta222 and alpha4Delta236 showed thermal unfolding kinetics similar to the full-length alpha4. Based on small angle X-ray scattering (SAXS), the calculated radius of gyration for alpha4 and Tap42 were 41.2 +/- 0.8 A and 42.8 +/- 0.7 A and their maximum dimension approximately 142 A and approximately 147 A, respectively. The radii of gyration for alpha4Delta222 and alpha4Delta236 were 21.6 +/- 0.3 A and 25.7 +/- 0.2 A, respectively. Kratky plots show that all studied proteins show variable degree of compactness. Calculation of model structures based on SAXS data showed that alpha4Delta222 and alpha4Delta236 proteins have globular conformation, whereas alpha4 and Tap42 exhibit elongated shapes.

Role of O-linked beta-N-acetylglucosamine modification in the subcellular distribution of alpha4 phosphoprotein and Sp1 in rat lymphoma cells

The mTOR alpha4 phosphoprotein is a prolactin (PRL)-downregulated gene product that is found in the nucleus of PRL-dependent rat Nb2 lymphoma cells. Alpha4 lacks a nuclear localization signal (NLS) and the mechanism of its nuclear targeting is unknown. Post-translational modification by O-linked beta-N-acetylglucosamine (O-GlcNAc) moieties has been implicated in the nuclear transport of some proteins, including transcription factor Sp1. The nucleocytoplasmic enzymes O-beta-N-acetylglucosaminyltransferase (OGT) and O-beta-N-acetylglucosaminidase (O-GlcNAcase) adds or remove O-GlcNAc moieties, respectively. If O-GlcNac moieties contribute to the nuclear targeting of alpha4, a decrease in O-GlcNAcylation (e.g., by inhibition of OGT) may redistribute alpha4 to the cytosol. The present study showed that alpha4 and Sp1 were both O-GlcNAcylated in quiescent and PRL-treated Nb2 cells. PRL alone or PRL + streptozotocin (STZ; an O-GlcNAcase inhibitor) significantly (P <or=.05) increased the O-GlcNAc/alpha4 ratio above that in control quiescent cells. However, PRL + alloxan (ALX; an OGT inhibitor) or ALX alone did not decrease O-GlcNAcylation of alpha4 below that of controls and alpha4 remained nuclear. In comparison, PRL (+/-ALX/STZ) greatly increased Sp1 protein levels, caused a significant decrease in the GlcNAc/Sp1 ratio (P <or=0.05, n = 3) as compared to controls and partially redistributed Sp1 to the cytosol. Finally, a 50% downregulation of OGT gene expression by small interfering RNA (i.e., siOGT) partially redistributed both alpha4 and Sp1 to the cytosol. The alpha4 protein partner PP2Ac had no detectable O-GlcNAc moieties and its nuclear distribution was not affected by siOGT. In summary, alpha4 and Sp1 contained O-GlcNAc moieties, which contributed to their nuclear targeting in Nb2 cells.

Transcriptional profiling of mefloquine-induced disruption of calcium homeostasis in neurons in vitro

Mefloquine is associated with adverse neurological effects that are mediated via unknown mechanisms. Recent in vitro studies have shown that mefloquine disrupts neuronal calcium homeostasis via liberation of the endoplasmic reticulum (ER) store and induction of calcium influx across the plasma membrane. In the present study, global changes in gene expression induced in neurons in response to mefloquine-induced disruption of calcium homeostasis and appropriate control agents were investigated in vitro using Affymetrix arrays. The mefloquine transcriptome was found to be enriched for important regulatory sequences of the unfolded protein response and the drug was also found to induce key ER stress proteins, albeit in a manner dissimilar to, and at higher equivalent concentrations than, known ER-tropic agents like thapsigargin. Mefloquine also down-regulated several important functional categories of genes, including transcripts encoding G proteins and ion channels. These effects may be related to intrusion of extracellular calcium since they were also observed after glutamate, but not thapsigargin, hydrogen peroxide, or low-dose mefloquine treatment. Mefloquine could be successfully differentiated from other treatments on the basis of principle component analysis of its "calcium-relevant" transcriptome. These data may aid interpretation of expression of results from future in vivo studies.

DAP-like kinase interacts with the rat homolog of Schizosaccharomyces pombe CDC5 protein, a factor involved in pre-mRNA splicing and required for G2/M phase transition

DAP-like kinase (Dlk, also termed ZIP kinase) is a leucine zipper-containing serine/threonine-specific protein kinase with as yet unknown biological function(s). Interaction partners so far identified are either transcription factors or proteins that can support or counteract apoptosis. Thus, Dlk might be involved in regulating transcription or, more generally, survival or apoptosis. Here we report on a new interaction partner, the rat homolog of Schizosaccharomyces pombe CDC5 protein, a presumptive transcription and splicing factor involved in the G(2)/M transition. In vitro, rat CDC5 forms complexes with, but is not phosphorylated by, Dlk. Rather, it was phosphorylated by an associated kinase which was identified as CK2. The interaction domain of Dlk was mapped to the leucine zipper, while that of CDC5 was mapped to the C-terminal region between residues 500 and 802. In vivo, both proteins co-localize perfectly in distinct speckle-like structures in the nucleus, some of which overlap with promyelocytic leukemia protein. Interestingly, splicing factor SC35, which also resides in speckles, was partially displaced upon overexpression of either CDC5 or Dlk, perhaps due to phosphorylation by Dlk. Together with previous data, these results suggest that Dlk might play a role in coordinating specific transcription and splicing events.

Prolactin, interleukin-2 and FGF-2 stimulate expression, nuclear distribution and DNA-binding of rat homolog of pombe Cdc5 in Nb2 T lymphoma cells

Pombe and human Cdc5 have been implicated in G2/M progression, but recently Cdc5 was identified as a component of a multiprotein complex essential for pre-mRNA splicing. We have previously isolated a prolactin (PRL)-inducible partial cDNA (1907 bp) encoding rat Cdc5. In the present study, the full length rCdc5 sequence (2847 bp) was obtained by 5'-RACE and cytokine regulation of Cdc5 expression was examined. PRL and interleukin-2 (IL2) act as mitogens in Nb2 T-lymphoma cells. Fibroblast growth factor (FGF-2) is not mitogenic in Nb2 cells but inhibits apoptosis of PRL-deprived cells. This study showed that PRL, IL-2 and FGF-2 rapidly increased Nb2 Cdc5 expression (3.4 kb mRNA) to reach 2-3-fold above controls at 4 h, and Cdc5 mRNA levels remained elevated at 24 h. There was a corresponding 2-3-fold increase in Cdc5 protein (105 kDa) levels at 24 h. Immunoblotting and fluorescent confocal microscopy showed predominant nuclear/perinuclear Cdc5 in quiescent Nb2 cells. PRL or FGF-2 treatment transiently increased nuclear Cdc5-specific immunofluorescence at 4 h but IL-2 gave maximal nuclear accumulation of Cdc5 at 24 h. The deduced rCdc5 protein has approximately 98% amino acid identity with human Cdc5. Like other Cdc5 family members, the N-terminus of rCdc5 contains two repeats of a DNA-binding domain found in a-, b- and c-Myb. Gel shift assays using (32)P-labeled Myb consensus oligonucleotides revealed two Myb-specific DNA-protein complexes in Nb2 nuclear extracts. Formation of both complexes was increased by PRL or FGF-2 at 1-5 and at 20 h and was partially inhibited by anti-Myb or anti-Cdc5 antibodies. In summary, rapid activation of Cdc5 in response to mitogenic and non-mitogenic stimuli suggests a complex role for Cdc5 in cellular regulation and this may not be restricted to mitotic entry or G2/M progression as previously supposed.

Analysis of prolactin-modulated gene expression profiles during the Nb2 cell cycle using differential screening techniques

BACKGROUND

Rat Nb2-11C lymphoma cells are dependent on prolactin for proliferation and are widely used to study prolactin signaling pathways. To investigate the role of this hormone in the transcriptional mechanisms that underlie prolactin-stimulated mitogenesis, five different techniques were used to isolate differentially expressed transcripts: mRNA differential display, representational difference analysis (RDA), subtractive suppressive hybridization (SSH), analysis of weakly expressed candidate genes, and differential screening of an organized library.

RESULTS

About 70 transcripts were found to be modulated in Nb2 cells following prolactin treatment. Of these, approximately 20 represent unknown genes. All cDNAs were characterized by northern blot analysis and categorized on the basis of their expression profiles and the functions of the known genes. We compared our data with other cell-cycle-regulated transcripts and found several new potential signaling molecules that may be involved in Nb2 cell growth. In addition, abnormalities in the expression patterns of several transcripts were detected in Nb2 cells, including the constitutive expression of the immediate-early gene EGR-1. Finally, we compared the differential screening techniques in terms of sensitivity, efficiency and occurrence of false positives.

CONCLUSIONS

Using these techniques to determine which genes are differentially expressed in Nb2 lymphoma cells, we have obtained valuable insight into the potential functions of some of these genes in the cell cycle. Although this information is preliminary, comparison with other eukaryotic models of cell-cycle progression enables identification of expression abnormalities and proteins potentially involved in signal transduction, which could indicate new directions for research.

A new member of the alpha4-related molecule (alpha4-b) that binds to the protein phosphatase 2A is expressed selectively in the brain and testis

A murine alpha4, identified in lymphocytes, binds to protein phosphatase 2A (PP2A). We found another murine alpha4-related gene (named alpha4-b) expressed selectively in the brain and testis. The alpha4-b transcript is expressed in the brain and testis, but is not detected in the spleen, thymus, bone marrow, liver, kidney, lung, heart or muscle. In-situ RNA hybridization analysis suggested that alpha4-b is expressed in most neuronal cells in the brain, but it is not expressed in the glial cells. The alpha4-b cDNA encodes a putative protein that is highly homologous (66% identity in amino-acid sequence) to the alpha4 molecule. The alpha4-b protein associates with the catalytic subunit of PP2A (PP2Ac), suggesting that the alpha4-b protein is involved in the regulation of phosphatase activity in neuronal cells.

Estrogen stimulates expression of adenine nucleotide translocator ANT1 messenger RNA in female rat hearts

The identification of estrogen-responsive genes in the heart, is necessary to understand estrogen-induced changes in cardiac function. Using Delta RNA fingerprinting, we demonstrate that a single injection of estradiol benzoate (50 microg, s.c.) revealed mRNA species that were elevated, down-regulated, or were unaffected in the heart tissue of ovariectomized female rats. One of the upregulated genes was identified, by cloning and sequencing, to have 95.8% (230/240) identity with the 3' end of the rat ant1 gene encoding the mitochondrial adenine nucleotide translocator, ANT1. Using the isolated ANT1 cDNA (280 bp) as a probe in Northern analysis, estrogen was shown to upregulate the expression of cardiac ANT1, by at least 3-fold in female rats, from as early as 1 h to as long as 24 h. In contrast, estrogen treatment had no effect on ANT1 expression in heart tissue from male rats. RNA yields were low in rat atria and no transcript was detectable by Northern analysis. Using primers specific to the known rat ANT1 gene, the estrogen upregulation of the cardiac ANT1 transcript in female rat was confirmed by reverse transcriptase-polymerase chain reaction (RT-PCR); a predicted product of 249 bp was obtained and this was stimulated by at least 3-fold upon estrogen treatment for 24 h.

Malignant progression of rat Nb2 lymphoma cells: chromosomal alterations and metastatic properties

Cultured rat pre-T Nb2 lymphoma cell lines have provided a useful model for tumor progression of T-cell cancers. Comparative analysis of the non-metastatic, prolactin (PRL)-dependent parental Nb2-U17 line and its PRL-independent and/or metastatic sublines, can be used in a search for progression-related genomic alterations. In the present study, the PRL-dependent, cloned Nb2-11C and PRL-independent Nb2-Sp sublines were used to examine development of metastatic ability and PRL independence relative to chromosomal alterations. Metastatic ability was determined using Noble rats carrying subcutaneous tumor transplants; PRL dependence/autonomy was checked in culture. Nb2-11C tumor transplants quickly gave rise to morbidity, associated with metastases in kidney and liver. Transplants of the slower growing Nb2-Sp cells showed variable tumorigenicity as metastases developed in only 40% of the rats (in lungs, kidney, stomach). G-banded chromosome analysis showed the Nb2-11C culture had the karyotype of the parental Nb2-U17 line plus an extra chromosome 19, thus, indicating an association between the development of metastatic ability in Nb2-11C cells and trisomy 19. The Nb2-Sp subline was not clonal. Its stemline showed two alterations in the parental karyotype: acquisition of an add(7)(q10) and loss of the extra chromosome add(15)(p12). Additional abnormalities, add(6)(q11) and trisomy 19, occurred in 15% and 5% of the Nb2-Sp population, respectively. Passaging of the Nb2-Sp subline in vivo resulted in generation and/or outgrowth of new sublines, a major one of which showed an apparent transient growth requirement for lactogens. Possible mechanisms underlying the PRL independence and in vivo properties of the Nb2-Sp cells are discussed.

The gene structure and promoter analysis of mouse lymphocyte signal transduction molecule alpha 4 that is related to the yeast TAP42 involved in a rapamycin-sensitive pathway

The mouse alpha 4 phosphoprotein encoding a component associated with the B cell antigen receptor (BCR)-mediated signal transduction is suggested to be involved in a unique rapamycin-sensitive pathway. We studied the structure and the molecular mechanism of the expression of alpha 4 gene by isolating two phage clones, named #10 and #23, covering entire exons of the mouse alpha 4 gene. The alpha 4 gene is located within about 25 kb and composed of six exons. To analyze the regulation of alpha 4 gene expression, we determined the nucleotide sequence toward 2 kb upstream of the translation start site of the alpha 4 gene. The 5'-flanking region does not contain a typical TATA box or the initiation consensus sequence, but it contains a CCAAT box, E-boxes, and several DNA binding motifs such as c-Myc, c-Myb, and c-Ets. Transcription of the alpha 4 gene starts at four different sites, determined by primer extension analysis, that were surrounded by Y-rich sequences. We further characterized the functional promoter of the alpha 4 gene at the region between -263 and the transcription start site of alpha 4 gene by luciferase assay system and suggested that the 5' upstream region of alpha 4 gene contains the silencer element of MT repetitive sequence.

Prolactin induces expression of FGF-2 and a novel FGF-responsive NonO/p54nrb-related mRNA in rat lymphoma cells

The rat Nb2-11C lymphoma cell line expresses high affinity prolactin (PRL) receptors, and requires lactogenic hormones for survival and proliferation. We have applied differential display to identify genes which are differentially induced in Nb2-11C cells following PRL stimulation, or which are constitutively expressed in the PRL-independent Nb2-Sp cells. In the present study we characterized a clone (22c.2) which was expressed in Nb2-Sp cells, and in Nb2-11C cells given PRL for 3 h but not in untreated cells. The 279 bp cDNA had 95% homology with the 3' end of the murine 2.6 kb FGF-inducible gene 14 (FIN14). When clone 22c.2 was used to screen a Nb2-Sp cDNA library to obtain a longer cDNA, a unique 1039 bp clone PNR (Prolactin-responsive/ NonO-Related) was isolated, subcloned and sequenced. The deduced amino acid sequence encoded by the PNR open reading frame had significant homology with a family of RNA- and DNA-binding proteins which include the human polypyrimidine tract-binding protein (PTB)-associated splicing factor (PSF), the murine non-POU-domain-containing octamer-binding protein (NonO) and the human NonO homologue p54nrb. Nb2-11C cells expressed three PNR-related mRNA transcripts of 2.5, 3.0 and > 10 kb. Expression of the 2.5 and 3.0 kb transcripts were increased at least 4-fold within 3 h of PRL treatment. PNR expression was also significantly stimulated within 3 h by addition of FGF-2 to either Nb2-11C or Nb2-Sp cells, although alone FGF-2 was not mitogenic for either cell line. Reverse transcription-polymerase chain reaction (RT-PCR) confirmed the expression of both FGF-2 and FGF receptor mRNA in Nb2 cells. raising the possibility of an autocrine or paracrine function for FGF-2 in lymphoma cells. Furthermore, PRL rapidly stimulated the expression of FGF-2 mRNA in a time- and dose-dependent manner in both Nb2-11C and Nb2-Sp cells. FGF-2 expression was increased within 1 h and was maintained at a high level for at least 10 h following treatment with 2 ng/ml PRL. Western blotting with anti-FGF2 antisera demonstrated PRL stimulation of intracellular accumulation, but not secretion of immunoreactive FGF-2. The observation of PRL-responsive expression of FGF-2 in Nb2 cells suggests a previously unrecognized pathway for PRL action in lymphoid cells.