Differential induction of ‘metabolic genes’ after mitogen stimulation and during normal cell cycle progression

IL-10-producing B cells are characterized by a specific methylation signature

Among the family of regulatory B cells, the subset able to produce interleukin-10 (IL-10) is the most studied, yet its biology is still a matter of investigation. The DNA methylation profiling of the il-10 gene locus revealed a novel epigenetic signature characterizing murine B cells ready to respond through IL-10 synthesis: a demethylated region located 4.5 kb from the transcription starting site (TSS), that we named early IL10 regulatory region (eIL10rr). This feature allows to distinguish B cells that are immediately prone and developmentally committed to IL-10 production from those that require a persistent stimulation to exert an IL-10-mediated regulatory function. These late IL-10 producers are instead characterized by a delayed IL10 regulatory region (dIL10rr), a partially demethylated DNA portion located 9 kb upstream from the TSS. A demethylated region was also found in human IL-10-producing B cells and, very interestingly, in some B-cell malignancies, such as chronic lymphocytic leukemia and mantle cell lymphoma, characterized by an immunosuppressive microenvironment. Our findings define murine and human regulatory B cells as an epigenetically controlled functional state of mature B cell subsets and open a new perspective on IL-10 regulation in B cells in homeostasis and disease.

Inducible expression of immediate early genes is regulated through dynamic chromatin association by NF45/ILF2 and NF90/NF110/ILF3

Immediate early gene (IEG) transcription is rapidly activated by diverse stimuli. This transcriptional regulation is assumed to involve constitutively expressed nuclear factors that are targets of signaling cascades initiated at the cell membrane. NF45 (encoded by ILF2) and its heterodimeric partner NF90/NF110 (encoded by ILF3) are chromatin-interacting proteins that are constitutively expressed and localized predominantly in the nucleus. Previously, NF90/NF110 chromatin immunoprecipitation followed by deep sequencing (ChIP-seq) in K562 erythroleukemia cells revealed its enriched association with chromatin at active promoters and strong enhancers. NF90/NF110 specifically occupied the promoters of IEGs. Here, ChIP in serum-starved HEK293 cells demonstrated that NF45 and NF90/NF110 pre-exist and specifically occupy the promoters of IEG transcription factors EGR1, FOS and JUN. Cellular stimulation with phorbol myristyl acetate increased NF90/NF110 chromatin association, while decreasing NF45 chromatin association at promoters of EGR1, FOS and JUN. In HEK293 cells stably transfected with doxycycline-inducible shRNA vectors targeting NF90/NF110 or NF45, doxycycline-mediated knockdown of NF90/NF110 or NF45 attenuated the inducible expression of EGR1, FOS, and JUN at the levels of transcription, RNA and protein. Dynamic chromatin association of NF45 and NF90/NF110 at IEG promoters are observed upon stimulation, and NF45 and NF90/NF110 contribute to inducible transcription of IEGs. NF45 and NF90/NF110 operate as chromatin regulators of the immediate early response.

Combined positron emission tomography/computed tomography in sunitinib therapy assessment of patients with metastatic renal cell carcinoma

AIM

To assess the clinical benefit of combined functional imaging with [(18)F]2-fluoro-2-deoxy-d-glucose (18F-FDG) positron emission tomography/computed tomography (PET/CT) in patients with metastatic renal cell carcinoma (mRCC) treated with the tyrosine kinase inhibitor sunitinib.

MATERIALS AND METHODS

Fourteen patients with mRCC were prospectively enrolled in this study. All patients underwent PET/CT before receiving at least two cycles of sunitinib treatment. Three months after the onset of sunitinib treatment, a second PET/CT was carried out. The metabolic response evaluated from the PET (standard uptake value; SUV) was compared with the CT component of the PET/CT. The Response Evaluation Criteria in Solid Tumours criteria were used to assess the CT response and modified European Organization for Research and Treatment of Cancer criteria were used to assess the PET response.

RESULTS

Three main results were obtained: (1) Patients with relatively low 18F-FDG uptake before treatment (SUV<5) had a longer progression-free survival than those with a relatively high 18F-FDG uptake (P=0.006). (2) Patients with a partial metabolic response or stable metabolic disease after two courses of sunitinib had improved prognosis as compared with those with progressive metabolic disease (P=0.031). (3) There was a clear discrepancy between PET and CT as a tool for the evaluation of treatment response after two courses of sunitinib. PET indicated progressive disease in three patients, a partial response in six patients and stable disease in four patients. In contrast, CT concluded with progression in only one patient and stable disease in all other patients.

CONCLUSION

In patients with mRCC, a high baseline 18F-FDG uptake indicates aggressive disease, and the degree of reduction in 18F-FDG uptake after sunitinib treatment adds valuable prognostic information. Hence, the inclusion of PET results seems to improve the clinical counselling of patients with mRCC. Larger studies are needed to confirm these findings.

Immediate-early and delayed primary response genes are distinct in function and genomic architecture

The transcriptional program induced by growth factor stimulation is classically described in two stages as follows: the rapid protein synthesis-independent induction of immediate-early genes, followed by the subsequent protein synthesis-dependent induction of secondary response genes. In this study, we obtained a comprehensive view of this transcriptional program. As expected, we identified both rapid and delayed gene inductions. Surprisingly, however, a large fraction of genes induced with delayed kinetics did not require protein synthesis and therefore represented delayed primary rather than secondary response genes. Of 133 genes induced within 4 h of growth factor stimulation, 49 (37%) were immediate-early genes, 58 (44%) were delayed primary response genes, and 26 (19%) were secondary response genes. Comparison of immediate-early and delayed primary response genes revealed functional and regulatory differences. Whereas many immediate-early genes encoded transcription factors, transcriptional regulators were not prevalent among the delayed primary response genes. The lag in induction of delayed primary response compared with immediate-early mRNAs was because of delays in both transcription initiation and subsequent stages of elongation and processing. Consistent with increased abundance of RNA polymerase II at their promoters, immediate-early genes were characterized by over-representation of transcription factor binding sites and high affinity TATA boxes. Immediate-early genes also had short primary transcripts with few exons, whereas delayed primary response genes more closely resembled other genes in the genome. These findings suggest that genomic features of immediate-early genes, in contrast to the delayed primary response genes, are selected for rapid induction, consistent with their regulatory functions.

Fatty acid oxidation is a dominant bioenergetic pathway in prostate cancer

Most malignancies have increased glycolysis for energy requirement of rapid cell proliferation, which is the basis for tumor imaging through glucose analog FDG (2-deoxy-2-fluoro-D-glucose) with positron emission tomography. One of significant characteristics of prostate cancer is slow glycolysis and low FDG avidity. Recent studies showed that prostate cancer is associated with changes of fatty acid metabolism. Several enzymes involved in the metabolism of fatty acids have been determined to be altered in prostate cancer relative to normal prostate, which is indicative of an enhanced beta-oxidation pathway in prostate cancer. Increased fatty acid utilization in prostate cancer provides both ATP and acetyl-coenzyme A (CoA); subsequently, increased availability of acetyl-CoA makes acceleration of citrate oxidation possible, which is an important energy source as well. Dominant fatty acid metabolism rather than glycolysis has the potential to be the basis for imaging diagnosis and targeted treatment of prostate cancer.

ANT2 Isoform Required for Cancer Cell Glycolysis

The three adenine nucleotide translocator (ANT1 to ANT3) isoforms, differentially expressed in human cells, play a crucial role in cell bioenergetics by catalyzing ADP and ATP exchange across the mitochondrial inner membrane. In contrast to differentiated tissue cells, transformed cells, and their rho(0) derivatives, i.e. cells deprived of mitochondrial DNA, sustain a high rate of glycolysis. We compared the expression pattern of ANT isoforms in several transformed human cell lines at different stages of the cell cycle. The level of ANT2 expression and glycolytic ATP production in these cell lines were in keeping with their metabolic background and their state of differentiation. The sensitivity of the mitochondrial inner membrane potential (Deltapsi) to several inhibitors of glycolysis and oxidative phosphorylation confirmed this relationship. We propose a new model for ATP uptake in cancer cells implicating the ANT2 isoform, in conjunction with hexokinase II and the beta subunit of mitochondrial ATP synthase, in the Deltapsi maintenance and in the aggressiveness of cancer cells.

Activation/division of lymphocytes results in increased levels of cytoplasmic activation/proliferation-associated protein-1: prototype of a new family of proteins

We purified from activated T lymphocytes a novel, highly conserved, 116-kDa, intracellular protein that occurred at high levels in the large, dividing cells of the thymus, was up-regulated when resting T or B lymphocytes or hemopoietic progenitors were activated, and was down-regulated when a monocytic leukemia, M1, was induced to differentiate. Expression of the protein was highest in the thymus and spleen and lowest in tissues with a low proportion of dividing cells such as kidney or muscle, although expression was high in the brain. The protein was localized to the cytosol and was phosphorylated, which is consistent with a previous report that the Xenopus laevis ortholog was phosphorylated by a mitotically activated kinase (1 ). The cDNA was previously mischaracterized as encoding p137, a 137-kDa GPI-linked membrane protein (2 ). We propose that the authentic protein encoded by this cDNA be called cytoplasmic activation/proliferation-associated protein-1 (caprin-1), and show that it is the prototype of a novel family of proteins characterized by two novel protein domains, termed homology regions-1 and -2 (HR-1, HR-2). Although we have found evidence for caprins only in urochordates and vertebrates, two insect proteins exhibit well-conserved HR-1 domains. The HR-1 and HR-2 domains have no known function, although the HR-1 of caprin-1 appeared necessary for formation of multimeric complexes of caprin-1. Overexpression of a fusion protein of enhanced green fluorescent protein and caprin-1 induced a specific, dose-dependent suppression of the proliferation of NIH-3T3 cells, consistent with the notion that caprin-1 plays a role in cellular activation or proliferation.

Antisense targeting of c-fos transcripts inhibits serum- and TGF-beta 1-stimulated PAI-1 gene expression and directed motility in renal epithelial cells

Plasminogen activator inhibitor type-1 (PAI-1), the major regulator of pericellular plasmin generation, and the c-FOS transcription factor are expressed by migrating cells in response to monolayer wounding. Induced c-fos and PAI-1 transcripts were evident within 30 min and 2 h, respectively, of scrape injury to confluent, growth-arrested, cultures of NRK epithelial cells. Since c-FOS/AP-1 DNA-binding activity modulates both basal and inducible modes of PAI-1 gene control, and AP-1 motif binding factors were present in quiescent as well as stimulated NRK cells, a model of directionally regulated cell movement (migration into scrape-denuded "wounds") was used to assess the consequences of c-fos transcript targeting on PAI-1 expression and cell motility. This in vitro model of epithelial injury closely approximated in vivo wound repair with regard to the spatial and temporal emergence of cohorts of cells involved in migration, proliferation, and PAI-1 expression. Stable cell lines (NRKsof) were generated by transfection of parental NRK cells with a c-fos antisense expression vector. Serum-inducible c-fos transcripts and PAI-1 protein levels were significantly attenuated in NRKsof transfectants relative to parental controls or cells transfected with a neo(R) vector without the sof insert. NRKsof cells had a markedly impaired ability to repair scrape-generated monolayer wounds under basal, serum-stimulated, or TGF-beta 1-supplemented culture conditions. Since injury closure and PAI-1 induction were attenuated in c-fos antisense cells, it was important to clarify the role of specific AP-1 sites in serum-mediated PAI-1 transcription. PAI-1 "promoter"-driven CAT reporter expression was assessed within the real time of serum-stimulated PAI-1 induction. A segment of the PAI-1 promoter corresponding to nucleotides -533 to -764 upstream of the transcription start site functioned as a prominent serum-responsive region (SSR). The 9-fold increase in CAT mRNA levels attained with the -533 to -764 bp PAI-1 SRR ligated to a minimal PAI-1 promoter (i.e., 162 bp of 5' flanking sequence containing the basal transcription complex) closely approximated the serum-induced transcriptional activity of a fully responsive (1,230 bp) PAI-1 promoter construct as well as the endogenous PAI-1 gene. AP-1-like, CTF/NF-1-like, and AP-2 sites were identified in the SRR. The SRR AP-1 motif was homologous to the sequence TGACACA that mapped between nucleotides -740 and -703 in the human PAI-1 gene, a region essential for growth factor-inducible PAI-1 transcription. While the functionality of this AP-1 site in wound-regulated PAI-1 synthesis remains to be determined, antisense c-fos transcripts effectively attenuated PAI-1 induction and basal as well as growth factor-stimulated cell locomotion, indicating that expression of both the PAI-1 and c-fos genes is necessary for wound-initiated NRK cell migration.

Growth state-dependent binding of USF-1 to a proximal promoter E box element in the rat plasminogen activator inhibitor type 1 gene

Induced PAI-1 gene expression in renal epithelial (NRK-52E, clone EC-1) cells occurs as part of the immediate-early response to serum. PAI-1 transcripts are maximally expressed early in G(1) (within 4 h of serum addition to quiescent EC-1 cells) and then subsequently decline to basal levels prior to entry into DNA synthetic phase. Comparative analysis of PAI-1 mRNA abundance and de novo-synthesized thiolated RNA in quiescent cells, as well as at 4 h (early G(1)) and 20 h (late G(2)) postserum addition, in conjunction with RNA decay measurements indicated that PAI-1 gene regulation upon growth activation was predominantly transcriptional. An E box motif (CACGTG), important in the induced expression of some growth state-dependent genes, mapped to nucleotides -160 to -165 upstream of the transcription start site in the PAI-1 proximal promoter. Mobility-shift assessments, using a 18-bp deoxyoligonucleotide construct containing the E box within the context of PAI-1-specific flanking sequences, confirmed binding of EC-1 nuclear protein(s) to this probe and, specifically, to the E box hexanucleotide site. The specificity of this protein-probe interaction was verified by competition analyses with double-stranded DNA constructs that included E box deoxyoligonucleotides with non-PAI-1 flanking bases, mutant E box sequences incapable of binding NRK nuclear proteins, and unrelated (i.e., AP-1) target motifs. Extract immunodepletion and supershift/complex-blocking experiments identified one PAI-1 E box-binding protein to be upstream stimulatory factor-1 (USF-1), a member of the HLH family of transcription factors. Mutation of the CACGTG site to TCCGTG in an 18-bp PAI-1 probe inhibited the formation of USF-1-containing complexes confirming that an intact E box motif at -160 to -165 bp in the PAI-1 promoter and, in particular, the CA residues at -165 and -164 are essential for USF-1 binding. Incorporation of this 2 bp change into a reporter construct containing 764 bp of the proximal PAI-1 "promoter" ligated to a CAT gene effectively reduced (by 74%) CAT activity in cycling cells. An intact E box motif at nucleotides -160 to -165 in the PAI-1 promoter, thus, is an important functional element in the regulation of PAI-1 transcriptional activity in renal cells.

Regulation of sugar, amino acid and peptide plant membrane transporters

During the past few years, various cDNAs encoding the proton cotransporters which mediate the uptake of sucrose, hexoses, amino acids and peptides across the plant plasma membrane have been cloned. This has made possible some preliminary insight into the regulation of the activity of these transporters at various levels. The paper summarises the present status of knowledge and gaps relative to their transcriptional control (organ, tissue and cell specificity, response to the environment) and post-transcriptional control (targeting and turnover, kinetic and thermodynamic control, lipidic environment, phosphorylation). This outline and the description of a few cases (the sink/source transition of the leaf, the pollen grain, the legume seed) serve as a basis for suggesting some directions for future research.

Kinase inhibitors in cancer therapy: a look ahead

The most essential kinases involved in cell membrane receptor activation, signal transduction and cell cycle control or programmed cell death and their interconnections are reviewed. In tumours, the genes of many of those kinases are mutated or amplified or the proteins are overexpressed. The use of key kinases offers the possibility to screen in vitro for synthetic small molecule kinase inhibitors. In view of the many interconnections of cellular kinases, their role in preventing or inducing programmed cell death and the possibility that a considerable number of signal transducing proteins are still unknown, cellular test systems are recommended in which the respective key kinase or one of its main partner molecules are overexpressed.

Glyceraldehyde-3-phosphate dehydrogenase expression during apoptosis and proliferation of rat ventral prostate

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a multifunctional enzyme known to play a critical role in neuronal apoptosis. We undertook the current studies to determine whether GAPDH also plays a role in prostate epithelial cell apoptosis in response to androgen deprivation. To do so, we analyzed GAPDH staining by immunohistochemistry during castration-induced involution and androgen-induced regeneration of rat ventral prostate. We found that GAPDH was undetectable in secretory epithelial cells at baseline and that staining did not increase in the epithelium during the period of peak apoptosis from 1 to 3 days after castration. However, GAPDH levels did increase within nuclei of some basal epithelial cells 5 days after castration and within the cytoplasm of all secretory epithelial cells 7 days after castration. GAPDH was also abundant within the cytoplasm of secretory epithelial cells during the period of maximal cell proliferation from 2 to 3 days after androgen replacement and was clearly apparent within nuclei of some epithelial cells 4 days after androgen replacement. Our studies suggest that GAPDH plays multiple roles during prostate epithelial cell apoptosis and proliferation.

Regulation of prostate cancer cell division by glucose

Previous studies have shown that rapid cell proliferation is associated with elevated glucose consumption. However, those studies did not establish whether glucose is required for prostate cancer cell proliferation or define the molecular mechanisms by which glucose regulates cell division. We addressed these issues by studying two metastatic human prostate cancer cell lines: DU145, which is androgen independent and highly proliferative; and LNCaP, which is androgen dependent and relatively slow growing. We found that proliferation of DU145 cells was significantly inhibited by reduction of glucose in the medium to 0.5 g/L, which is half the physiologic concentration, whereas LNCaP cells grew at control rates even in the presence of only 0.05 g/L glucose. Glucose deprivation of DU145 cells caused a 90% reduction in DNA synthesis; a 10-20-fold reduction in cyclins D and E and CDK4 levels; and cell cycle arrest in G0-G1. However, glucose deprivation did not cause global inhibition of protein synthesis, since mutant p53 levels increased in glucose-deprived DU145 cells. This observed increase in mutant p53 levels was not associated with a rise in p21 levels. Glucose deprivation of DU145 cells also led to apparent dephosphorylation of mutant retinoblastoma (RB) protein. We conclude that: 1) high levels of glucose consumption are required for rapid proliferation of androgen-independent prostate cancer cells, 2) glucose may not be required for slow growth of androgen-dependent prostate cancer cells, and 3) glucose promotes passage of cells through early G1 by increasing the expression of several key cell cycle regulatory proteins that normally inhibit RB function.

Uptake of glucose analogues by colonic tumour cells during growth and after treatment with hydroxyurea

SW620 cells were grown in tissue culture flasks to various cell densities producing populations of cells with a range of proliferative indices. The uptake of the two glucose analogues, deoxy-D-glucose (DG) and 3-O-methylglucose (OMG) was determined and found to be associated with S-phase fraction. The strong correlation between DG and OMG uptakes suggested that proliferation-related changes in transmembrane transport accounted for the association with S-phase fraction. Treatment of SW620 cells with the cell cycle inhibitor hydroxyurea was found to increase the uptake of DG and OMG in a time-dependent manner.

Isolation and characterization of genomic sequences involved in the regulation of the human reduced folate carrier gene (RFC1)

Decreased reduced folate carrier (RFC) activity has been associated with MTX resistance in experimental models of transport-mediated MTX resistance, and has been attributed to changes in the expression of RFC1, the gene that encodes a protein with this activity. RNA transcripts of RFC1 may contain any one of four distinct 5' untranslated regions (UTRs). We cloned a human genomic DNA fragment upstream from the RFC1 translation start site and determined the nucleotide sequence of a 4.8kb region that contained the exons corresponding to each of the reported UTRs. To identify regulatory elements that may be involved in RFC1 transcription, we first developed a semi-quantitative RT-PCR assay using primers specific for each of the 5' UTRs to amplify RNA transcripts containing each of the RFC1 5' exons, and found evidence for differential transcription of RFC1 non-coding exons in tissues, during development, and in MTX-resistant, transport-deficient cells. We also found that RFC1 RNA levels are cell cycle regulated and peak at the G1 to S transition. Then, using a series of RFC1 promoter-reporter fusion constructs, we identified genomic sequences that may be involved in the regulation of expression of exons 1b and 1c of the RFC1 gene. These studies therefore identify regulatory regions of RFC1 promoters and potential models of regulation in which these regions may exert control.

Epidermal growth factor- and hepatocyte growth factor-receptor activity in serum-free cultures of human hepatocytes

BACKGROUND/AIMS

Serum-free primary cultures of hepatocytes are a useful tool to study factors triggering hepatocyte proliferation and regeneration. We have developed a chemically defined serum-free system that allows human hepatocyte proliferation in the presence of epidermal growth factor and hepatocyte growth factor.

METHODS

DNA synthesis and accumulation were determined by [3H]thymidine incorporation and fluorometry, respectively. Western blot analyses and co-immunoprecipitations were used to investigate the association of proteins involved in epidermal growth factor and hepatocyte growth factor activation and signaling: epidermal growth factor receptor, hepatocyte growth factor receptor (MET), urokinase-type plasminogen activator and its receptor, and a member of the signal transducer and activator of transcription family, STAT-3.

RESULTS

Primary human hepatocytes proliferated under serum-free conditions in a chemically defined medium for up to 12 days. Epidermal growth factor-receptor and MET were present and functional, decreasing over time. MET, urokinase-type plasminogen activator and urokinase-type plasminogen activator receptor co-precipitated to varying degrees during the culture period. STAT-3 co-precipitated with epidermal growth factor-receptor and MET to varying degrees.

CONCLUSIONS

Proliferation of human hepatocytes can improve by modification of a chemically defined medium originally used for rat hepatocyte cultures. In these long-term cultures of human hepatocytes, hepatocyte growth factor and epidermal growth factor can stimulate growth and differentiation by interacting with their receptors and initiating downstream signaling. This involves complex formation of the receptors with other plasma membrane components for MET (urokinase-type plasminogen activator in context of its receptor) and activation of STAT-3 for both receptors.

The plant cell cycle in context

Biological scientists are eagerly confronting the challenge of understanding the regulatory mechanisms that control the cell division cycle in eukaryotes. New information will have major implications for the treatment of growth-related diseases and cancer in animals. In plants, cell division has a key role in root and shoot growth as well as in the development of vegetative storage organs and reproductive tissues such as flowers and seeds. Many of the strategies for crop improvement, especially those aimed at increasing yield, involve the manipulation of cell division. This review describes, in some detail, the current status of our understanding of the regulation of cell division in eukaryotes and especially in plants. It also features an outline of some preliminary attempts to exploit transgenesis for manipulation of plant cell division.

Cell cycle-regulated transcription in mammalian cells

The periodic, phase-specific transcription of defined sets of genes is a hallmark of cell cycle progression in all organisms (1-3). In this article, we will summarise our current knowledge and views of the mechanisms governing the cross-coupling of cell cycle control and transcriptional regulation in mammalian cells, with particular emphasis on the transcription factor E2F and the retinoblastoma protein pRb (1-3). Excluded from this review will be the genomic response to mitogenic stimulation, which is part of the mitogen-triggered signal transduction cascades rather than a reflection of cell cycle regulation (4).

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

A nitrogen starvation-induced dormant G0 state in fission yeast: the establishment from uncommitted G1 state and its delay for return to proliferation

Fission yeast cells either remain in the mitotic cell cycle or exit to meiotic sporulation from an uncommitted G1 state dependent on the presence or absence of nitrogen source in the medium (Nurse and Bissett, 1981). We examined how heterothallic haploid cells, which cannot sporulate, behave under nitrogen-starvation for longer than 25 days at 26 degrees C. These cells were shown to enter a stable state (designated the dormant G0) with nearly full viability. Maintaining the dormant cells required glucose, suggesting that the cells remained metabolically active although cell division had ceased. They differed dramatically from mitotic and uncommitted G1 cells in heat resistance, and also in cytoplasmic and nuclear morphologies. After nitrogen replenishment, the initial responses of dormant G0 cells were investigated. The kinetics for reentry into the proliferative state were delayed considerably, and the changes in cell shape were enhanced particularly for those recovering from extended nitrogen starvation. A part of the delay could be accounted for by the duration of nuclear decondensation and cell elongation for the first cell division.

Cell-cycle-dependent gene expression studied by two-colour fluorescent detection of a mRNA and histone mRNA

We investigated whether a probe specific for histone H3 mRNA could be used as a marker to study cell-cycle dependency of gene expression by double-fluorescent RNA in situ hybridization (FISH). First, we showed that all S-phase cells in cell cultures having incorporated BrdU revealed histone H3 mRNA expression by RNA FISH, indicating that histone H3 expression is a reliable marker for S-phase cells. Second, we analysed whether the expression of human cytomegalovirus immediate early genes in rat 9G cells, which are known to be induced in an S-phase dependent way by cycloheximide, correlated with the expression of histone H3 mRNA. Double-hybridization experiments with a digoxigenin-labelled probe for IE mRNA and a fluoresceinated probe for histone H3 mRNA revealed that cells expressing IE mRNA also expressed histone H3 mRNA. Third, we examined the cell-cycle dependency of luciferase gene expression in X1 cells. Luciferase mRNA is heterogeneously expressed in X1 cell cultures, but cells expressing luciferase did not necessarily express histone H3 mRNA. This indicates that luciferase gene expression in X1 cells is not induced during S-phase. The results of our study show that histone H3 mRNA expression can be successfully used as a marker to establish cell-cycle dependency of gene expression by double-RNA FISH.

Analysis of growth factor stimulated nucleocytoplasmic protein transport using two-dimensional gel electrophoresis

The rapid activation of gene expression by growth factors frequently involves the nucleocytoplasmic redistribution of proteins which regulate the transcription of discrete sets of genes. In particular, it has become apparent that protein kinases and transcription factors including protein kinase A, mitogen activated protein kinase, protein kinase C, NF-kappa B and Stat91, can rapidly migrate from the cytosol to the nucleus in response to a wide variety of extracellular stimuli. Furthermore, it is assumed that these proteins represent important components of the signal transduction pathways which operate between the plasma membrane and the nucleus. Here two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) has been used to detect proteins which undergo rapid growth factor stimulated nucleocytoplasmic redistribution. Epidermal growth factor and insulin influenced the association of 15 individual protein spots with the nucleus. Estimates of the abundance of these proteins revealed that they represented approximately 1/1000 to 1/5000 of total nuclear protein. To permit identification by microsequencing, five of the protein spots have been recovered from multiple 2-D PAGE gels and concentrated by one-dimensional elution-concentration gel electrophoresis. The data obtained demonstrate that 2-D PAGE can be used to: (i) analyse putative regulators of growth factor activated gene transcription, and (ii) purify sufficient amounts of these proteins to facilitate identification by microsequence analysis.

Okadaic acid, cAMP, and selected nutrients inhibit hepatocyte proliferation at different stages in G1: modulation of the cAMP effect by phosphatase inhibitors and nutrients

The protein phosphatase inhibitor okadaic acid (> 100 nM) caused an abrupt and complete cessation of primary rat hepatocyte cell cycle progression at the restriction point in late G1. A decline in the G1/S transition rate was observed in response to elevated cAMP, excess selected nutrients, and okadaic acid (< 100 nM). Excess nutrients (40 mM glucose +/- 5 mM dihydroxyacetone) acted by imposing an incomplete block in early G1. The cAMP action was potentiated by the phosphatase inhibitor microcystin, which in itself did not affect DNA replication. This suggests that cAMP acted by phosphorylating substrate(s) that is dephosphorylated by a microcystin-sensitive phosphatase. The additive effects of submaximal concentrations of okadaic acid and cAMP analogs indicated that okadaic acid and cAMP acted via different pathways. In conclusion, okadaic acid, cAMP, and excess nutrients, acting through distinct pathways, inhibited hepatocytes in different parts of the G1 phase.

Calcium regulation of immediate-early response genes

A rise in intracellular Ca2+ concentration induces the transcription of a number of eukaryotic genes through transcription factors interacting with calcium response elements. Immediate-early response genes encode proteins that couple extracellular signals to phenotypic alterations by modulating the transcription rates of target genes. Since the activation of early response genes occurs within minutes, this class of genes has served as a paradigm for the understanding of the molecular mechanisms by which external signals are conveyed to the nucleus to induce changes in genetic programs. In this review, we outline the recent information which has been gained specifically on how the Ca2+ messenger system modulates early response gene expression. We also discuss some lines of research with the intent of linking closer Ca2+ homeostasis and gene expression studies which in the past have followed their own separate routes.

Identification of serum-inducible genes: different patterns of gene regulation during G0-->S and G1-->S progression

We have identified, by differential cDNA library screening, 15 serum inducible genes in the human diploid fibroblast cell line WI-38. The genes fall into two classes that are distinguished by their dependence on protein synthesis for the induction by serum, i.e., primary and secondary genes. While 11 of these genes encode known proteins, 4 other genes have not been described to date. The former genes encode proteins of diverse functions, including the monocyte-derived neutrophil chemotactic factor (MONAP), calmodulin, tropomyosin, tenascin, collagenase, plasminogen activator inhibitor-2a, the ‘sperm-specific’ cleavage signal-1 protein, metallothionein IIa and the mitochondrial chaperonin hsp-60. Interestingly, one of the unknown genes contains a large open reading frame for a polypeptide that is highly homologous to a previously unidentified long open reading frame in the opposite strand of the gene coding for the transcription factor HTF-4. We also studied the regulation of these serum-induced genes during cell cycle progression in normally cycling WI-38 and HL-60 cells separated by counterflow elutriation as well as in serum-stimulated HL-60 cells. Our results clearly show that, in contrast to the prevailing opinion, the expression of most genes induced after mitogen stimulation is not subject to a significant regulation in normally proliferating cells. This supports the hypothesis that the progression into S from either G0 or G1 are distinct processes with specific patterns of gene expression.