Polyamine depletion results in impairment of polyribosome formation and protein synthesis before onset of DNA synthesis in mitogen-activated human lymphocytes

Good cop, bad cop: Polyamines play both sides in host immunity and viral replication

Viruses rely on host cells for energy and synthesis machinery required for genome replication and particle assembly. Due to the dependence of viruses on host cells, viruses have evolved multiple mechanisms by which they can induce metabolic changes in the host cell to suit their specific requirements. The host immune response also involves metabolic changes to be able to react to viral insult. Polyamines are small ubiquitously expressed polycations, and their metabolism is critical for viral replication and an adequate host immune response. This is due to the variety of functions that polyamines have, ranging from condensing DNA to enhancing the translation of polyproline-containing proteins through the hypusination of eIF5A. Here, we review the diverse mechanisms by which viruses exploit polyamines, as well as the mechanisms by which immune cells utilize polyamines for their functions. Furthermore, we highlight potential avenues for further study of the host-virus interface.

Genome-wide RNAi screen identifies novel players in human 60S subunit biogenesis including key enzymes of polyamine metabolism

Ribosome assembly is an essential process that is linked to human congenital diseases and tumorigenesis. While great progress has been made in deciphering mechanisms governing ribosome biogenesis in eukaryotes, an inventory of factors that support ribosome synthesis in human cells is still missing, in particular regarding the maturation of the large 60S subunit. Here, we performed a genome-wide RNAi screen using an imaging-based, single cell assay to unravel the cellular machinery promoting 60S subunit assembly in human cells. Our screen identified a group of 310 high confidence factors. These highlight the conservation of the process across eukaryotes and reveal the intricate connectivity of 60S subunit maturation with other key cellular processes, including splicing, translation, protein degradation, chromatin organization and transcription. Intriguingly, we also identified a cluster of hits comprising metabolic enzymes of the polyamine synthesis pathway. We demonstrate that polyamines, which have long been used as buffer additives to support ribosome assembly in vitro, are required for 60S maturation in living cells. Perturbation of polyamine metabolism results in early defects in 60S but not 40S subunit maturation. Collectively, our data reveal a novel function for polyamines in living cells and provide a rich source for future studies on ribosome synthesis.

Exogenous putrescine affects polyamine and arginine metabolism in rat liver following bile ductus ligation

Rat bile duct ligation (BDL) represents a useful method that mimics obstructive extrahepatic cholestasis, which is known to be a frequent disorder in humans. Polyamines (putrescine, spermidine, and spermine) are one of the key molecules regulating cell proliferation and differentiation. This work aimed to evaluate the potential beneficial properties of putrescine in rat BDL model by studying several biochemical parameters reflecting liver function and polyamine metabolism. Rats that were subjected to BDL were injected with putrescine (150 mg/kg) for 9 days, while in parallel another group with BDL remained untreated. Two control groups were included as well, sham-opened and putrescine-treated group. The following plasma parameters: ALT, AST, γ-GT, ALP, bilirubin, bile acids, as well as liver malondialdehyde and polyamine concentration and the activity of enzymes involved in polyamine metabolism were studied. After BDL, significant alterations in plasma biochemical parameters occurred, where a 9-day putrescine treatment significantly alleviated liver function deterioration. Putrescine also increased liver polyamines’ concentrations and polyamine and diamine oxidase activities in rats submitted to BDL. Our results demonstrated, for the first time, that putrescine plays an important role in preserving liver tissue function in rats with experimentally induced cholestasis.

Metabolism of activated T lymphocytes

Activated T cells undergo metabolic reprogramming which promotes glycolytic flux and lactate production as well as elevated production of lipids, proteins, nucleic acids and other carbohydrates (i.e. induction of biomass) even in the presence of oxygen. Activated T cells show induced expression of, among other things, Glucose Transporter 1 and several glycolytic enzymes, including ADP-Dependent Glucokinase and the low affinity isoform Pyruvate Kinase-M2 (which promote glycolytic flux), as well Glutamine Transporters and Glycerol-3-phosphate Dehydrogenase 2 which make available glutamate and glycerol-3-phosphate as mitochondrial energy sources. Intracellular leucine concentrations critically regulate mammalian target of rapamycin (mTOR) signaling to promote Th1, Th2, and Th17 CD4(+) T effector cell differentiation. In contrast, T regulatory (Treg) cells are generated when AMP-Activating Protein Kinase signaling is activated and mTOR activation is suppressed. Unlike effector CD4(+) and CD8(+) T cells, Tregs and memory T cells oxidize fatty acids for fuel. Effector and memory T cells perform different functions and thus show distinct metabolic profiles which are exquisitely controlled by cellular signaling. Upon activation, T cells express the insulin and leptin receptors on their surface and become sensitive to insulin signaling and nutrient availability and show changes in differentiation. Thus, metabolism and nutrient availability influence T cell activation and function.

Polyamines and the intestinal tract

Owing to their high turnover, the intestinal mucosal cells have a particularly high requirement for polyamines. Therefore, they are an excellent charcol for the study of polyamine function in rapid physiological growth and differentiation. After a cursory introduction to the major aspects of polyamine metabolism, regulation, and mode of action, we discuss the contribution of the polyamines to the maintenance of normal gut function, the maturation of the intestinal mucosa, and its repair after injuries. Repletion of cellular polyamine pools with (D,L)-2-(difluoromethyl)ornithine has considerably improved our understanding of how the polyamines are involved in the regulation of normal and neoplastic growth. Unfortunately, the attempts to exploit polyamine metabolism as a cancer therapeutic target have not yet been successful. However, the selective inactivation of ornithine decarboxylase appears to be a promising chemopreventive method in familial adenomatous polyposis. Presumably, it relies on the fact that ornithine decarboxylase is a critical regulator of the proliferative response of the protooncogene c-myc, but not of its apoptotic response.

Spermine, a natural polyamine, suppresses LFA-1 expression on human lymphocyte

Natural polyamines, spermine, spermidine, and putrescine, play a pivotal role in the regulation of gene expression; therefore, the age-dependent decreases and the disease-dependent increases in polyamine synthesis suggest a possible contribution of polyamines to the age-related and disease-associated changes in cellular function. In this study, we examined the effects of polyamines on the cellular function and the expression of adhesion molecules on human PBMCs from healthy volunteers. Flow cytometry revealed that PBMCs cultured with spermine decreased mean fluorescent intensities (MFIs) of CD11a and CD18 in the lymphocyte light-scattered region, but not in the monocyte region. This suppression was observed in a dose- and time-dependent manner and found nonspecifically on all cell subsets we tested (CD3(+), CD4(+), CD8(+), CD19(+), CD45RA(+), CD45RO(+), CD4(+)CD45RA(+), CD4(+)CD45RO(+), CD8(+)CD45RA(+), CD8(+)CD45RO(+)). The decreases of CD11a and CD18 MFIs were accompanied by the decrease in adherent capacity of PBMCs to HUVECs. Spermine did not hinder cell activities or cell viability. Among 42 healthy volunteers (mean, 49.5 years old; from 26 to 69), blood spermine levels inversely correlated with the CD11a MFIs of cells in the lymphocyte region (r = -0.48; p = 0.001), but not with those in the monocyte region. The effects of spermidine seemed weaker than those of spermine, and blood spermidine levels had no correlation with CD11a MFIs of the lymphocyte region. Putrescine had no effect on the expressions of membrane molecules. Polyamines, especially spermine, decrease LFA-1 (CD11a/CD18) expression on human lymphocyte and adhesion capacity of PBMCs to HUVECs.

Differential effect on polyamine metabolism in mitogen- and superantigen-activated human T-cells

Polyamines are important for regulation of lymphocyte differentiation and proliferation. Mitogens induce synthesis of ornithine decarboxylase (ODC), the rate limiting enzyme in polyamine biosynthesis. Since mitogens stimulate T-cells by non-physiological routes, the role of polyamine metabolism in T-cell receptor (TCR)-mediated T-cell activation has not been adequately evaluated. The effect of phytohemagglutinin (PHA) and staphylococcal enterotoxin B (SEB) on T-cell ODC and polyamine synthesis was compared. ODC activity was 6-11-fold higher in PHA compared to SEB stimulated T-cells. These differences were not attributed to differences in the magnitude of T-cell proliferation. Kinetics of ODC and polyamine synthesis were also different in PHA- and SEB-stimulated T-cells. In PHA-stimulated cells ODC levels and the induction of putrescine and spermidine synthesis peaked 6 h prior to peak IL-2 production, while in SEB-stimulated cells, ODC levels and polyamine synthesis peaked 6-12 h after IL-2 production. Differences in the temporal relationship between IL-2 production and polyamine induction in mitogen- versus superantigen-stimulated cells may account for the significant inhibition of the proliferative response by alpha-difluoromethylornithine following PHA but not SEB stimulation. Polyamine metabolism is regulated differently in T-cells stimulated via TCR engagement than with polyclonal mitogens.

Polyamines may regulate S-phase progression but not the dynamic changes of chromatin during the cell cycle

Several studies suggest that polyamines may stabilize chromatin and play a role in its structural alterations. In line with this idea, we found here by chromatin precipitation and micrococcal nuclease (MNase) digestion analyses, that spermidine and spermine stabilize or condense the nucleosomal organization of chromatin in vitro. We then investigated the possible physiological role of polyamines in the nucleosomal organization of chromatin during the cell cycle in Chinese hamster ovary (CHO) cells deficient in ornithine decarboxylase (ODC) activity. An extended polyamine deprivation (for 4 days) was found to arrest 70% of the odc- cells in S phase. MNase digestion analyses revealed that these cells have a highly loosened and destabilized nucleosomal organization. However, no marked difference in the chromatin structure was detected between the control and polyamine-depleted cells following the synchronization of the cells at the S-phase. We also show in synchronized cells that polyamine deprivation retards the traverse of the cells through the S phase already in the first cell cycle. Depletion of polyamines had no significant effect on the nucleosomal organization of chromatin in G1-early S. The polyamine-deprived cells were also capable of condensing the nucleosomal organization of chromatin in the S/G2 phase of the cell cycle. These data indicate that polyamines do not regulate the chromatin condensation state during the cell cycle, although they might have some stabilizing effect on the chromatin structure. Polyamines may, however, play an important role in the control of S-phase progression.

Inhibition of methionine adenosyltransferase by the polyamines

The effect of the polyamines, putrescine, spermine, and spermidine, on the activity of extrahepatic methionine adenosyltransferase (MAT II) was studied. The polyamines inhibited MAT II activity at concentrations equal to or greater than 5 mm. Combinations of polyamines were more effective than individual polyamines in inhibiting MAT activity; maximum inhibition approached 80% with combinations of all three polyamines. S-Adenosylmethionine (AdoMet), Pi, and PPi, the products of the MAT reaction, are known to be synergistic inhibitors of the nonhepatic form of the enzyme. Combinations of polyamines plus Pi and/or PPi induced an additive inhibition of the enzyme. AdoMet plus polyamines also resulted in significant inhibition, but inhibition plateaued at about 80%, indicating the presence of a protective mechanism to maintain AdoMet synthesis. Extrahepatic MAT from human and rat tissues was inhibited by the polyamines, indicating that this phenomenon is not species specific. In addition, we examined the effect of polyamines on MAT activity in resting and activated human lymphocytes that were shown to differ in the relative expression of MAT II subunits. Although MAT from mitogen (phytohemagglutinin, PHA)- and superantigen (Staphylococcal enterotoxin B, SEB)-stimulated lymphocytes were similarly inhibited by 10 mM polyamines, at lower concentrations of polyamines (1-5 mM), MAT from SEB-stimulated cells appeared to be more susceptible to inhibition by the polyamines. Inasmuch as SEB is a more physiological stimulator of T cells than PHA, the data suggest a possible role of polyamines in regulating MAT activity.

Normal G1/S transition and prolonged S phase within one cell cycle after seeding cells in the presence of an ornithine decarboxylase inhibitor

We have previously found that DNA replication was affected within one cell cycle after seeding Chinese hamster ovary (CHO) cells in the presence of the polyamine biosynthesis inhibitor 2-difluoromethylornithine (DFMO). We could, however, not rule out if this was due to an effect on the G1/S transition and/or on DNA synthesis elongation. In the present paper, we use a bromodeoxyuridine- flow cytometric method to more specifically study the G1/S transition, the S phase length, and the progression of cells from S phase through G2+M and into G1, after seeding plateau phase CHO cells at low density in the absence or presence of 5 mM DFMO. We report here that DFMO-induced polyamine depletion increased the length of the S phase within one cell cycle after seeding of CHO cells in the presence of the inhibitor. No effect on the G1/S transition was observed until 2 days after seeding, suggesting that a DFMO-induced lengthening of the G1 phase occurred later than the effect on S phase progression. These results imply that the G2+M phase was not prolonged until 2 days after seeding CHO cells in the presence of DFMO.

Impairment of DNA replication within one cell cycle after seeding of cells in the presence of a polyamine-biosynthesis inhibitor

Chinese hamster ovary (CHO) cells in the plateau phase were seeded in the absence or presence of 5 mM 2-difluoromethylornithine (F2MeOrn), an enzyme-activated irreversible inhibitor of ornithine decarboxylase. The thymidine analogue bromodeoxyuridine (BrdUrd, 5 microM) was added to the culture medium 30 min before sampling of the cells, which occurred 1-17 h after seeding. Using flow cytometry, coupled with an indirect immunofluorescence technique, which utilized monoclonal BrdUrd and secondary fluorescein-isothiocyanate-conjugated antibodies, and the DNA stain propidium iodide, cellular BrdUrd and DNA contents were quantified. To determine if there was a perturbation in the progression of cells through the S phase, the distribution of BrdUrd-labelled cells in the S phase was evaluated in two ways: (a) by calculating the mean DNA content of BrdUrd-labelled cells in relation to the mean DNA contents of G1 and G2 cells (relative movementzero) and (b) by studying DNA histograms of BrdUrd-labelled cells. By using both evaluation methods, we show that DNA replication was impaired during the first cell cycle that was initiated after seeding CHO cells in the presence of F2MeOrn. The cells appeared to enter the S phase normally but were then delayed in their progression through this phase. The impairment of F2MeOrn treatment on DNA replication was apparent at 9 h after seeding, a time point at which the putrescine pool was depleted, the spermidine pool was approximately halved, and the spermine pool was unaffected, when compared to corresponding pools of control cells. When cells were seeded in the presence of F2MeOrn and putrescine, the effect on DNA replication was prevented. The rates of incorporation of [3H]uridine and [3H]leucine into RNA and protein, respectively, were the same in control and in F2MeOrn-treated cells for at least up to 11 h after seeding.

Regulation of the human spermidine synthase mRNA translation by its 5'-untranslated region

An increased mRNA content of spermidine synthase was found in phytohemagglutinin stimulated human peripheral lymphocytes and in cultured human myeloma (Sultan) cells stimulated to grow by change of the culture medium. The many-fold increase in the amount of the message was accompanied by stimulation of the enzyme activity in activated lymphocytes, but not in stimulated myeloma cells. In the present study the effect of the 5'-untranslated region of spermidine synthase mRNA on the post-transcriptional control of its expression was studied both in vitro in rabbit reticulocyte system and in cultured mammalian cells. The results show that the GC-rich 5'-untranslated region of spermidine synthase mRNA has an inhibitory effect on its translation.

Flow cytometric analysis of the cell cycle in polyamine-depleted cells

Polyamines are found in all cells but their function is not fully understood. We have studied the effect of polyamines on the passage of cells through the cell cycle using a polyamine auxotrophic mutant, CHO-P22, which has no detectable ornithine decarboxylase activity. The ability of these cells to grow without serum allows efficient polyamine depletion. A flow cytometric analysis of DNA content and bromodeoxyuridine labeling showed that without added polyamines the cells accumulated in S-phase, the rate of DNA synthesis was retarded, and the entry into mitosis was blocked. Addition of polyamines to cultures deprived of polyamines induced cells in all phases of the cell cycle to reinitiate cycling. Earlier studies have shown that cells with damaged DNA are blocked from entering into mitosis but caffeine can partly overcome this block and induce premature chromosome condensation. Polyamine-depleted CHO-P22 cells responded to caffeine in the same way as cells with damaged DNA. These results show that polyamine depletion in CHO-P22 cells primarily affects DNA synthesis. The finding that polyamine-starved cells continuously take up bromodeoxyuridine without a corresponding increase in the amount of DNA is compatible with extensive repair of erroneous and/or damaged DNA. Polyamine auxotrophic Chinese hamster ovary (CHO) cells might be useful in studies on the regulation of mitosis in mammalian cells.

Polyamines modulate streptomycin-induced mistranslation in Escherichia coli

The effects of intracellular levels of polyamines on both the in vivo inhibition of protein synthesis and the decrease of translation accuracy induced by streptomycin have been studied in polyamine-auxotrophic strains of Escherichia coli infected with the MS2 bacteriophage. The amount of viral coat protein formed was strongly reduced upon addition of increasing concentrations of streptomycin to polyamine-supplemented bacteria. In contrast, the antibiotic almost did not inhibit coat protein synthesis in polyamine-starved cells. The increase of mistranslation frequency elicited by streptomycin was only observed in bacteria grown with putrescine. In these cells several coat protein-satellites were detected after two-dimensional gel electrophoresis. These proteins, more basic than the normal MS2 coat protein, contain multiple substitutions of lysine for asparagine.

Polyamines are essential for cell transformation by pp60v-src: delineation of molecular events relevant for the transformed phenotype

Ornithine decarboxylase (ODC), the key enzyme of polyamine biosynthesis, becomes upregulated during cell proliferation and transformation. Here we show that intact ODC activity is needed for the acquisition of a transformed phenotype in rat 2R cells infected with a temperature-sensitive mutant of Rous sarcoma virus. Addition of the ODC inhibitor alpha-difluoromethyl ornithine (DFMO) to the cells (in polyamine-free medium) before shift to permissive temperature prevented the depolymerization of filamentous actin and morphological transformation. Polyamine supplementation restored the transforming potential of pp60v-src. DFMO did not interfere with the expression of pp60v-src or its in vitro tyrosine kinase activity. The tyrosine phosphorylation of most cellular proteins, including ras GAP, did not either display clear temperature- or DFMO-sensitive changes. A marked increase was, however, observed in the tyrosine phosphorylation of phosphatidylinositol 3-kinase and proteins of 33 and 36 kD upon the temperature shift, and these hyperphosphorylations were partially inhibited by DFMO. A DFMO-sensitive increase was also found in the total phosphorylation of calpactins I and II. The well-documented association of GAP with the phosphotyrosine-containing proteins p190 and p62 did not correlate with transformation, but a novel 42-kD tyrosine phosphorylated protein was complexed with GAP in a polyamine- and transformation-dependent manner. Further, tyrosine phosphorylated proteins of 130, 80/85, and 36 kD were found to coimmunoprecipitate with pp60v-src in a transformation-related manner. Altogether, this model offers a tool for sorting out the protein phosphorylations and associations critical for the transformed phenotype triggered by pp60v-src, and implicates a pivotal role for polyamines in cell transformation.

Ornithine decarboxylase gene of Neurospora crassa: isolation, sequence, and polyamine-mediated regulation of its mRNA

Ornithine decarboxylase (ODC), which initiates the biosynthesis of the polyamines putrescine, spermidine, and spermine, is encoded by the spe-1 gene of the fungus Neurospora crassa. This gene and its cDNA have been cloned and sequenced. The gene has a single 70-nucleotide intron in the coding sequence. The cDNA, comprising the entire coding region, recognizes a single 2.4-kb mRNA in Northern (RNA) blots. The mRNA transcript, defined by S1 mapping, has an extremely long, 535-base leader without strong secondary-structure features or an upstream reading frame. The translational start of the protein is ambiguous: a Met-Val-Met sequence precedes the Pro known to be the N terminus of the ODC polypeptide. The polypeptide encoded by the N. crassa spe-1 gene (484 amino acids) has 46% amino acid identity with that of Saccharomyces cerevisiae (466 amino acids) and 42% with that of mouse (461 amino acids). Alignment of the longer N. crassa sequence with S. cerevisiae and mouse sequences creates gaps in different sites in the S. cerevisiae and mouse sequences, suggesting that N. crassa ODC is closer to an ancestral form of the enzyme than that of either yeast or mouse ODC. N. crassa ODC, which turns over rapidly in vivo in the presence of polyamines, has two PEST sequences, found in most ODCs and other proteins with rapid turnover. In striking contrast to other eucaryotic organisms, the variation in the rate of ODC synthesis in response to polyamines in N. crassa is largely correlated with proportional changes in the abundance of ODC mRNA. Spermidine is the main effector of repression, while putrescine has a weaker effect. However, putrescine accumulation appears to increase the amount of active ODC that is made from a given amount of ODC mRNA, possibly by improving its translatability. Conversely, prolonged starvation for both putrescine and spermidine leads to the differentially impaired translation of ODC mRNA.

Ornithine decarboxylase gene of Neurospora crassa: isolation, sequence, and polyamine-mediated regulation of its mRNA

Ornithine decarboxylase (ODC), which initiates the biosynthesis of the polyamines putrescine, spermidine, and spermine, is encoded by the spe-1 gene of the fungus Neurospora crassa. This gene and its cDNA have been cloned and sequenced. The gene has a single 70-nucleotide intron in the coding sequence. The cDNA, comprising the entire coding region, recognizes a single 2.4-kb mRNA in Northern (RNA) blots. The mRNA transcript, defined by S1 mapping, has an extremely long, 535-base leader without strong secondary-structure features or an upstream reading frame. The translational start of the protein is ambiguous: a Met-Val-Met sequence precedes the Pro known to be the N terminus of the ODC polypeptide. The polypeptide encoded by the N. crassa spe-1 gene (484 amino acids) has 46% amino acid identity with that of Saccharomyces cerevisiae (466 amino acids) and 42% with that of mouse (461 amino acids). Alignment of the longer N. crassa sequence with S. cerevisiae and mouse sequences creates gaps in different sites in the S. cerevisiae and mouse sequences, suggesting that N. crassa ODC is closer to an ancestral form of the enzyme than that of either yeast or mouse ODC. N. crassa ODC, which turns over rapidly in vivo in the presence of polyamines, has two PEST sequences, found in most ODCs and other proteins with rapid turnover. In striking contrast to other eucaryotic organisms, the variation in the rate of ODC synthesis in response to polyamines in N. crassa is largely correlated with proportional changes in the abundance of ODC mRNA. Spermidine is the main effector of repression, while putrescine has a weaker effect. However, putrescine accumulation appears to increase the amount of active ODC that is made from a given amount of ODC mRNA, possibly by improving its translatability. Conversely, prolonged starvation for both putrescine and spermidine leads to the differentially impaired translation of ODC mRNA.

Modulation of ornithine decarboxylase mRNA following transient ischemia in the gerbil brain

Ornithine decarboxylase (ODC) is the rate-limiting enzyme that catalyzes the synthesis of polyamines from ornithine and is thought to be involved in the cellular response to growth, differentiation, and stress. Previous studies have demonstrated that transient cerebral ischemia results in an increase in ODC activity and polyamine synthesis. We have used the Mongolian gerbil as a model system to test the hypothesis that the cellular response to ischemia induces a distinct pattern of ODC gene expression. Our results indicate that transient ischemia, induced by bilateral carotid occlusion, elevates ODC mRNA within 1-4 h after reperfusion, which correlates with increased ODC activity and polyamine synthesis. Increased ODC mRNA can be detected in the forebrain, striatum, hippocampus, and midbrain but not the cerebellum, which is not subject to ischemic injury. In contrast, c-fos mRNA increased by 15 min after reperfusion and actin mRNA did not demonstrate alterations in level after ischemia. Pentobarbital prevented the increase in ODC mRNA, whereas the glutamate antagonist MK-801 had no effect on the elevation of ODC gene expression after ischemia. We conclude that the ischemia-induced increase in ODC enzyme activity may be attributed in part to transcriptional activation of the ODC gene.

Inhibition of polyamine synthesis in human B lymphocytes during primary infection with Epstein-Barr virus (EBV) blocks cellular DNA synthesis but not the expression of EBV-encoded nuclear antigens (EBNA)

Inhibition of polyamine synthesis by 2-difluoromethylornithine (DFMO) treatment had no apparent effect on the initial manifestation of Epstein-Barr virus (EBV) infection in human B lymphocytes, because the expression of EBV-encoded nuclear antigens (EBNA) occurred normally. However, many subsequent steps in the transformation process were inhibited by DFMO treatment. These include cellular DNA synthesis and immunoglobulin (IgM, IgG and IgA) synthesis and secretion. Consequently, DFMO treatment blocked the progression of the transformation process of EBV-infected B lymphocytes. EBV-carrying marmoset B lymphocytes (B95-8 cells) were also blocked in their DNA synthesis when treated with DFMO. At variance with other DNA synthesis inhibitors, which induce virus production very effectively in B95-8 cells, DFMO caused no increase in the number of cells expressing the early antigens associated with the lytic cycle.

Interleukin-2 regulates the activity of ornithine decarboxylase in a cloned murine T lymphocytic cell line: evidence for a protein kinase C-dependent pathway

The role of protein kinase C (PKC) in the regulation of ornithine decarboxylase (ODC) activity during interleukin-2 (IL-2)-dependent cell growth was investigated. A large biphasic increase in the activity of ODC was observed after treatment of IL-2-deprived CTLL-2 cells with recombinant human IL-2 (rec IL-2). The PKC activators phorbol 12-myristate 13-acetate (PMA) and 4 beta-phorbol 12,13-didecanoate (4 beta-PDD), but not the inactive analog 4 alpha-PDD, induced ODC activity in exponentially growing cultures. Unlike IL-2, however, phorbol esters were poor inducers of IL-2-depleted cultures. H-7, a potent inhibitor of PKC and cyclic nucleotide-dependent protein kinases (CN-PK), suppressed the IL-2-induced ODC activity, while HA1004, a more potent inhibitor of CN-PK than of PKC, had opposite effects depending on its concentration. The results suggest that activation of PKC is involved in but is not the sole mechanism for the induction of ODC by rec IL-2. At concentrations which suppressed the induction of ODC activity by IL-2, H-7 inhibited DNA synthesis and HA1004 did not.

Cell-cycle-related metabolic and enzymatic events in proliferating rat thymocytes

Cell-cycle progression of rat thymocytes stimulated with concanavalin A and interleukin 2 was monitored at 12-h intervals by pulse labeling aliquots of the cell culture with [3H]thymidine, by measuring cellular DNA and protein content and by counting the number of cells in the cultures. The cell cycle was completed after 96 h of culture with the S phase peaking at 48 h. Early events in thymocyte activation were enhanced phosphatidylinositol turnover and the induction of ornithine decarboxylase. Concomitant changes were observed in the rates of DNA synthesis and glycolysis accompanied by a 20-fold increase in glucose uptake 48 h after stimulation. However, the maximal increment in the glycolytic rate preceded that of DNA synthesis by 12 h. Apart from the quantitative changes which occurred during the cell-cycle progression, there was also a change from partial aerobic glucose degradation to CO2 (26%) to almost complete anaerobic conversion of glucose to lactate (85%) and less than 3% to CO2. Glycolytic enzyme levels increased fourfold to tenfold and reached their maxima 48 h after mitogenic stimulation. Maximal increments of glycolytic enzyme activities preceded or coincided with the maximal increments of the glycolytic rate. Actinomycin D (1.5 ng/ml) completely inhibited DNA and RNA synthesis but did not show any inhibitory effect either on glycolytic enzyme induction or on enhanced glycolysis. During mitosis and return of the cells to the non-proliferative state, all of the enhanced metabolic rates returned to their initial levels and the elevated enzyme activities were decreased also. The marked changes of metabolic rates and enzyme activities observed at the various phases of the cell cycle suggest that these biochemical events may also serve as suitable parameters for evaluating the response of lymphocytes towards mitogens and lymphokines.

Effects of polyamine depletion on serum stimulation of quiescent 3T3 murine fibroblast cells

Numerous reports have shown that polyamines are required for cell proliferation. A current model for regulating commitment to DNA replication in cultured fibroblasts stimulated from quiescence by serum addition postulates sequential action by specific growth factors. To temporally localize polyamine-dependent steps within this defined sequence, mouse Balb/c-3T3 fibroblasts were partially depleted of polyamines by treatment with DL-alpha-difluoromethylornithine (DFMO), next rendered quiescent by serum deprivation, then stimulated by 10% serum with or without exogenous putrescine (Pu). Depletion of polyamines was verified by HPLC, and entry of cells into S phase was monitored by autoradiography. After 24 h of incubation with [3H]-thymidine, polyamine-depleted cells had labeling indices similar to quiescent cells if they were serum-stimulated without Pu, but progressed to S phase to the same degree as control cultures if polyamines were restored by adding Pu at the time of serum stimulation. These observations suggested that commitment of quiescent cells to DNA replication may require polyamines. To determine if polyamine-dependent steps occur during the pre-commitment period (up to 12 h after serum stimulation) or only in traverse of G1 (12 h to 24 h, post-commitment), polyamine-depleted quiescent cells were serum-stimulated for 12 h without Pu, then returned to low serum with Pu. Labeling indices of these cultures remained nearly as low as those of unstimulated cells. Reducing serum concentration from 10% to 0.5% at 12 h after stimulation did not effect labeling indices of control cells not depleted of polyamines by DFMO. These results supported the postulated requirement for polyamines during pre-commitment events. However, polyamine-deficient quiescent cells serum-stimulated without Pu for periods longer than 24 h had labeling indices at 36 and 48 h significantly greater than at 24 h. This suggested that polyamine depletion may decrease the rate at which quiescent cells commit to DNA replication, rather than producing an absolute blockade during the pre-commitment period.

Growth signal transduction: rapid activation of covalently bound ornithine decarboxylase during phosphatidylinositol breakdown

We have previously shown that treatment of T lymphocytes with mitogenic ligands induces a rapid activation of ornithine decarboxylase (ODC) through a mechanism that is independent of protein synthesis but requires energy and an intact cytoskeleton. Here we show by immunoprecipitation experiments and by chemical analyses that ODC is covalently linked to the cell membrane by inositol. Treatment of sonicated cells with a phosphatidylinositol-specific phospholipase C from B. thuringiensis caused a rapid 3-fold increase in ODC activity. Similar treatment of intact cells had no effect, suggesting that the ODC is attached to the cytoplasmic surface of the membrane. We conclude that ODC release and activation occur by a novel mechanism involving phosphatidylinositol breakdown following ligand-receptor interaction.

Post-translational modification of the protein-synthesis initiation factor eIF-4D by spermidine in rat hepatoma cells

The rates of synthesis and turnover of the rare amino acid hypusine [N6-(4-amino-2-hydroxybutyl)-2,6-diaminohexanoic acid] in protein were studied in relationship to polyamine metabolism and growth rates in rat hepatoma tissue-culture (HTC) cells. Hypusine is selectively formed in the eukaryotic translation initiation factor eIF-4D, by a post-translational mechanism involving spermidine [Cooper, Park, Folk, Safer & Braverman (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 1854-1857]. The half-life of the hypusine-containing protein was longer than 24 h. In cells whose intracellular spermidine pools had been initially depleted, by using DL-alpha-difluoromethylornithine (DFMO), maximum synthesis rates of hypusine in protein were 5-10 times higher, on restoration of endogenous spermidine contents by exogenous addition, than those observed in untreated exponential-phase cultures. In cells pretreated with DFMO, the rate of hypusine synthesis was constant for up to 1 h after the addition of 5 microM-spermidine, whereas endogenous spermidine contents varied from less than 1 to more than 10 nmol/mg of protein. However, the overall amount of hypusine formed, during the first 1 h after the addition of various concentrations of spermidine (0.05-10 microM) to the culture medium, was markedly dependent on the final endogenous spermidine content achieved at the end of the 1 h measurement interval. Early in exponential-phase growth, protein-bound hypusine was synthesized at a rate of 1-2 pmol/h per mg of protein. This rate decreased to less than 0.5 pmol/h per mg of protein when cell growth rates decreased as cultures reached high cell densities. Analysis of the polyamine substrate specificity for hypusine formation showed that N1-acetylspermidine did not compete with spermidine in the reaction, nor did N1-(buta-2,3-dienyl)-N2-methylbutane-1,4-diamine, and irreversible inhibitor of polyamine oxidase, block the reaction. On the basis of comparative radiolabelling experiments, spermine was either a poor substrate, or not a substrate, for hypusine formation. These results confirm that spermidine is the likely precursor of the aminohydroxybutyl moiety of hypusine, and show that overall hypusine formation, but not necessarily the synthesis rate, is dependent on the endogenous spermidine concentration, especially under conditions where spermidine concentrations are initially low, as is the case after DFMO treatment, and then increase.

N1-monoacetylation abolishes the inhibitory effect of spermine and spermidine in the reticulocyte lysate translation system

At optimum magnesium, the translation of rat heart mRNA in the nuclease treated rabbit reticulocyte lysate system was inhibited by low concentrations of spermidine or spermine but not of putrescine. Spermidine and spermine cause a general reduction in the translation of all the heart mRNAs since no differential effects were observed when the translation products were examined by gel electrophoresis. Spermine was a five times more potent inhibitor than spermidine but no inhibition was obtained with N1-acetylspermidine or N1-acetylspermine. Since analyses of endogenous polyamines demonstrate that the inhibitory concentrations of spermine could be obtained by converting a small fraction of the endogenous spermidine to spermine, these results indicate that interconversions of the polyamines might be a sensitive regulatory mechanism for protein synthesis.