Deficiencies in DNA replication and cell-cycle progression in polyamine-depleted HeLa cells

Role of polyamines at the G1/S boundary and G2/M phase of the cell cycle

The role of polyamines at the G1/S boundary and in the G2/M phase of the cell cycle was studied using synchronized HeLa cells treated with thymidine or with thymidine and aphidicolin. Synchronized cells were cultured in the absence or presence of α-difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase, plus ethylglyoxal bis(guanylhydrazone) (EGBG), an inhibitor of S-adenosylmethionine decarboxylase. When polyamine content was reduced by treatment with DFMO and EGBG, the transition from G1 to S phase was delayed. In parallel, the level of p27(Kip1) was greatly increased, so its mechanism was studied in detail. Synthesis of p27(Kip1) was stimulated at the level of translation by a decrease in polyamine levels, because of the existence of long 5'-untranslated region (5'-UTR) in p27(Kip1) mRNA. Similarly, the transition from the G2/M to the G1 phase was delayed by a reduction in polyamine levels. In parallel, the number of multinucleate cells increased by 3-fold. This was parallel with the inhibition of cytokinesis due to an unusual distribution of actin and α-tubulin at the M phase. Since an association of polyamines with chromosomes was not observed by immunofluorescence microscopy at the M phase, polyamines may have only a minor role in structural changes of chromosomes at the M phase. In general, the involvement of polyamines at the G2/M phase was smaller than that at the G1/S boundary.

Metabolomics integrated with transcriptomics: assessing systems response to sulfur-deficiency stress

Sulfur-containing amino acids, cysteine and methionine synthesized in plants are essential for human and animal nutrition. That is why understanding of how inorganic sulfur is taken up by plants and built into the organic molecules in the process of sulfur assimilation is important. As complex biological systems, plants subsist as integrated molecular, organelle, cell, tissue and organ entities, being in permanent synergistic coordination. The process of sulfur uptake and assimilation is an integral part of this dense network of influences, its reconstruction may help in manipulating the bioproduction of organic sulfur-containing compounds. New high-throughput technologies allow the systems' view on the coordination of complex processes in living organisms. Among them, transcriptomics and metabolomics studies were applied to Arabidopsis plants subjected to sulfur-deficiency stress. From the integrated analysis of the obtained data, the mosaic picture of distinct sulfur stress response events and processes are starting to be assembled into the whole systems' network of sulfur assimilation. At the time trajectory of sulfur stress response, two system states can be distinguished. The first state of short-term responses is characterized by the development of enhanced lateral roots exploring the space in search for the lacking nutrient. When this physiological reaction cannot be accomplished by bringing the system back to the initial state of sulfur sufficiency, a new program is toggled aiming at saving the organismal resources for vital seed production. Here, we describe the biological reasoning in these two system states and the process of state transition between them.

On the processing of metabolic information through metabolite-gene communication networks: an approach for modelling causality

Gene-metabolite correlation networks of three independent biological systems were interrogated using an approach to define, and subsequently model, causality. The major goal of this work was to analyse how information from those metabolites, that displayed a rapid response to perturbation of the biological system, is processed through the response network to provide signal-specific adaptation of metabolism. For this purpose, comparison of network topologies was carried out on three different groups of system elements: transcription factors, other genes and metabolites, with special emphasis placed on those features which are possible sites of metabolic regulation or response propagation. The degree of connectivity in all three analysed gene-metabolite networks followed power-law and exponential functions, whilst a comparison of connectivities of the various cellular entities suggested, that metabolites are less involved in the regulation of the sulfur stress response than in the ripening of tomatoes (in which metabolites seem to have an even greater regulatory role than transcription factors). These findings reflect different degree of metabolic regulation for distinct biological processes. Implementing causality into the network allowed classification of metabolite-gene associations into those with causal directionality from gene to metabolite and from metabolite to gene. Several metabolites were positioned relatively early in the causal hierarchy and possessed many connections to the downstream elements. Such metabolites were considered to have higher regulatory potential. For the biological example of hypo-sulfur stress response in Arabidopsis, the highest regulatory potential scores were established for fructose and sucrose, isoleucine, methionine and sinapic acid. Further developments in profiling techniques will allow greater cross-systems comparisons, necessary for reliability and universality checks of inferred regulatory capacities of the particular metabolites.

The incorporation of nucleoside analogs by human immunodeficiency virus type 1 reverse transcriptase decreases in the presence of polyamines

Nucleoside analogs (NAs) are an important class of anti-retroviral compounds used against human immunodeficiency virus (HIV). We have analyzed the potential effect of polyamines on the incorporation of NAs during DNA synthesis by HIV type-1 (HIV-1) reverse transcriptase (RT). The polyamines exert the ability to decrease the incorporation of various dideoxynucleoside triphosphates (ddATP, ddTTP or ddCTP) with both RNA/DNA and DNA/DNA substrates in the following order: spermine>spermidine>putrescine. The reduction is a sequence-independent effect, taking place at different sequence context. The results suggest that polyamines might affect the inhibition of reverse transcription by nucleoside analogs HIV-1 RT directed.

An endosymbiont positively modulates ornithine decarboxylase in host trypanosomatids

Some trypanosomatids, such as Crithidia deanei, are endosymbiont-containing species. Aposymbiotic strains are obtained after antibiotic treatment, revealing interesting aspects of this symbiotic association. Ornithine decarboxylase (ODC) promotes polyamine biosynthesis and contributes to cell proliferation. Here, we show that ODC activity is higher in endosymbiont-bearing trypanosomatids than in aposymbiotic cells, but isolated endosymbionts did not display this enzyme activity. Intriguingly, expressed levels of ODC were similar in both strains, suggesting that ODC is positively modulated in endosymbiont-bearing cells. When the aposymbiotic strain was grown in conditioned medium, obtained after cultivation of the endosymbiont-bearing strain, cellular proliferation as well as ODC activity and localization were similar to that observed in the endosymbiont-containing trypanosomatids. Furthermore, dialyzed-heated medium and trypsin treatment reduced ODC activity of the aposymbiont strain. Taken together, these data indicate that the endosymbiont can enhance the protozoan ODC activity by providing factors of protein nature, which increase the host polyamine metabolism.

The fidelity of DNA synthesis by human immunodeficiency virus type 1 reverse transcriptase increases in the presence of polyamines

The high error rates characteristic of human immunodeficiency virus type-1 reverse transcriptase (HIV-1 RT) are a presumptive source of the viral hypervariability that substantially affect viral pathogenesis and resistance to drug therapy. We have analyzed the potential role of polyamines in the fidelity of DNA synthesis by HIV-1 RT. The current study suggest that polyamines tested has the potential to be "antimutator". The polyamines exert the ability to reduce the misincorporation and mispair extension with both RNA/DNA and DNA/DNA template-primers in the following order: spermine > spermidine > putrescine. In view of the significance of mutations of HIV, the possible roles of polyamines in the accuracy of DNA synthesis could be of particular importance; polyamines may affect the mutation rate of the virus.

Uptake of polyamines by the unicellular green alga Chlamydomonas reinhardtii and their effect on ornithine decarboxylase activity

Uptake of exogenous polyamines by the unicellular green alga Chlamydomonas reinhardtii and their effects on polyamine metabolism were investigated. Our data show that, in contrast to mammalian cells, Chlamydomonas reinhardtii does not contain short-living, high-affinity polyamine transporters whose cellular level is dependent on the polyamine concentration. However, exogenous polyamines affect polyamine metabolism in Chlamydomonas cells. Exogenous putrescine caused a slow increase of both putrescine and spermidine and, vice versa, exogenous spermidine also led to an increase of the intracellular levels of both spermidine and putrescine. No intracellular spermine was detected under any conditions. Exogenous spermine was taken up by the cells and caused a decrease in their putrescine and spermidine levels. As in other organisms, exogenous polyamines led to a decrease in the activity of ornithine decarboxylase, a key enzyme of polyamine synthesis. In contrast to mammalian cells, this polyamine-induced decrease in ornithine decarboxylase activity is not mediated by a polyamine-dependent degradation or inactivation, but exclusively due to a decreased synthesis of ornithine decarboxylase. Translation of ornithine decarboxylase mRNA, but not overall protein biosynthesis is slowed by increased polyamine levels.

Gut-enriched Kruppel-like factor represses ornithine decarboxylase gene expression and functions as checkpoint regulator in colonic cancer cells

Gut-enriched Krüppel-like factor (GKLF, KLF4) is an epithelial-specific transcription factor that expresses in the gastrointestinal tract and mediates growth arrest of colonic epithelium. The molecular mechanisms governing its growth inhibitory effect have not been fully elucidated. In the present study, we showed that induction of GKLF mRNA and protein expression by interferon-gamma treatment was associated with reduction of ornithine decarboxylase (ODC) gene expression and enzyme activity in colon cancer HT-29 cells. Overexpression of GKLF in HT-29 cells significantly reduced ODC mRNA and protein levels as well as enzyme activity and resulted in growth arrest, indicating that ODC might be a downstream target of GKLF. This conclusion was further supported by data showing that GKLF mRNA and protein concentrations were the highest at the G(1)/S boundary of the cell cycle, where ODC mRNA and protein levels were the lowest and that overexpression of GKLF resulted in cell arrested at the G(1) phase. Reporter gene transfection studies and electrophoretic mobility gel shift assays demonstrated that GKLF repressed ODC promoter activity and that these effects appeared to be mediated through interaction with a GC box in the proximal portion of the promoter. Transfection studies using reporter constructs and chromatin immunoprecipitation assays also demonstrated that GKLF inhibited transactivation of the ODC gene by interfering with the binding of Sp1 to the ODC promoter. These results indicate that GKLF may function as a G(1)/S checkpoint regulator and exert its growth arrest effect through down-regulation of ODC gene expression. Furthermore, GKLF is a transcriptional repressor of the ODC gene, and these effects are mediated by interaction with the GC-rich region on the promoter.

Regulation by polyamines of ornithine decarboxylase activity and cell division in the unicellular green alga Chlamydomonas reinhardtii

Polyamines are required for cell growth and cell division in eukaryotic and prokaryotic organisms. In the unicellular green alga Chlamydomonas reinhardtii, biosynthesis of the commonly occurring polyamines (putrescine, spermidine, and spermine) is dependent on the activity of ornithine decarboxylase (ODC, EC 4.1.1.17) catalyzing the formation of putrescine, which is the precursor of the other two polyamines. In synchronized C. reinhardtii cultures, transition to the cell division phase was preceded by a 4-fold increase in ODC activity and a 10- and a 20-fold increase, respectively, in the putrescine and spermidine levels. Spermine, however, could not be detected in C. reinhardtii cells. Exogenous polyamines caused a decrease in ODC activity. Addition of spermine, but not of spermidine or putrescine, abolished the transition to the cell division phase when applied 7 to 8 h after beginning of the light (growth) phase. Most of the cells had already doubled their cell mass after this growth period. The spermine-induced cell cycle arrest could be overcome by subsequent addition of spermidine or putrescine. The conclusion that spermine affects cell division via a decreased spermidine level was corroborated by the findings that spermine caused a decrease in the putrescine and spermidine levels and that cell divisions also could be prevented by inhibitors of S-adenosyl-methionine decarboxylase and spermidine synthase, respectively, added 8 h after beginning of the growth period. Because protein synthesis was not decreased by addition of spermine under our experimental conditions, we conclude that spermidine affects the transition to the cell division phase directly rather than via protein biosynthesis.

Polyamine depletion arrests growth of IEC-6 and Caco-2 cells by different mechanisms

The polyamines spermidine and spermine and their precursor, putrescine, are required for the growth and proliferation of eukaryotic cells. This study compares and contrasts growth arrest caused by polyamine depletion in the untransformed IEC-6 cell line with that in the p53-mutated colon cancer Caco-2 cell line. Cells were grown in the presence or absence of alpha-difluoromethylornithine (DFMO), a specific inhibitor of ornithine decarboxylase, the first rate-limiting enzyme in the synthesis of polyamines. Depletion of polyamines inhibited the growth of both cell lines equally and over the same time frame. However, whereas IEC-6 cells were arrested in the G(1) phase of the cell cycle, there was no accumulation of Caco-2 cells in any particular phase. In IEC-6 cells, growth arrest was accompanied by elevated levels of p53 and p21(Waf1/Cip1) (p21). There were no changes in p53 levels in Caco-2 cells. Levels of p21 increased in Caco-2 cells on day 2 without any effect on cell cycle progression. The amount of cyclin-dependent kinase (Cdk)2 protein was unchanged by polyamine depletion in both cell lines. However, the activity of Cdk2 was significantly inhibited by DFMO in IEC-6 cells. These data suggest that in the untransformed IEC-6 cells the regulation of Cdk2 activity and progression through the cell cycle are p53- and p21 dependent. Growth arrest in the p53-mutated Caco-2 line after polyamine depletion occurs by a different, yet unknown, mechanism.

Manipulation of the expression of regulatory genes of polyamine metabolism results in specific alterations of the cell-cycle progression

We have previously reported that cyclical phases of accumulation and depletion of polyamines occur during cell-cycle progression. Regulatory ornithine decarboxylase (ODC) catalyses the first step of polyamine biosynthesis. Ornithine decarboxylase antizyme (OAZ), induced by high polyamine levels, inhibits ODC activity and prevents extracellular polyamine uptake. Spermidine/spermine N1-acetyltransferase (SSAT) regulates the polyamine degradation/excretion pathway. Here we show that 24 h transient transfection of immortalized human prostatic epithelial cells (PNT1A and PNT2) with antisense ODC RNA or OAZ cDNA, or both, while effectively causing marked decreases of ODC activity and polyamine (especially putrescine) concentrations, resulted in accumulation of cells in the S phase of the cell cycle. Transfection with SSAT cDNA led to more pronounced decreases in spermidine and spermine levels and resulted in accumulation of cells in the G2/M phases. Transfection with all three constructs together produced maximal depletion of all polyamines, accompanied by accumulation of PNT1A cells in the S phase and PNT2 cells in the G0/G1 and G2/M phases. Accumulation of PNT1A cells in the S phase progressively increased at 15, 18 and 24 h of transfection with antisense ODC and/or OAZ cDNA. At 24 h, the DNA content was always reduced, as a possible outcome of altered chromosome condensation. A direct link between polyamine metabolism, cell proliferation and chromatin structure is thus proposed.

Treatment of cells with the polyamine analog N, N11-diethylnorspermine retards S phase progression within one cell cycle

When Chinese hamster ovary cells were seeded in the presence of the spermine analog N1,N11-diethylnorspermine (DENSPM), cell proliferation ceased; this was clearly apparent by cell counting 2 days after seeding the cells. However, 1 day after seeding there was a slight difference in cell number between control and DENSPM-treated cultures. To investigate the reason for this easily surpassed slight difference, we used a sensitive bromodeoxyuridine/flow cytometry method. Cell cycle kinetics were studied during the first cell cycle after seeding cells in the absence or presence of DENSPM. Our results show that DENSPM treatment did not affect the progression of the cells through G1 or the first G1/S transition that took place after seeding the cells. The first cell cycle effect was a delay in S phase as shown by an increase in the DNA synthesis time. The following G2/M transition was not affected by DENSPM treatment. DENSPM treatment inhibited the transient increases in putrescine, spermidine, and spermine pools that took place within 24 h after seeding. Thus, in conclusion, the first cell cycle phase affected by the inhibition of polyamine biosynthesis caused by DENSPM was the S phase. Prolongation of the other cell cycle phases occurred at later time points, and the G1 phase was affected before the G2/M phase.

Spermine inhibition of the 2,5-diaziridinyl-1,4-benzoquinone (DZQ) crosslinking reaction with DNA duplexes containing poly(purine). poly(pyrimidine) tracts

Upon reduction, 2,5-diaziridinyl-1,4-benzoquinone (DZQ) can form an interstrand guanine to guanine crosslink with DNA duplexes containing a d(GC).d(GC) dinucleotide step. The reaction is enhanced by a thymine positioned 5[prime] to each guanine [i.e. in a d(TGCA). d(TGCA) duplex fragment]. Here we show that spermine can inhibit DZQ crosslink formation in duplexes of sequence d[C(N6)TGCA(M6)C]. d[G(M[prime]6)TG-CA(N[prime]6)G]. For N6= M6= GGGGGG, N6= M6= a 'random' sequence and N6= GGGGGG and M6= a 'random' sequence, spermine concentrations of 20, 1 and 3 microM, respectively, were required for 50% inhibition of the DZQ crosslink. This suggests that spermine is more strongly bound to the polyguanosine tract than the random sequence, making it less available for crosslink inhibition. When the polyguanosine tract is interrupted by N 7-deazaguanine (D) located three bases, d(CGGGDGGTGCAGGDGGGC), and four bases, d(CG-GDGGGTGCAGGGDGGC), from the d(TGCA).d(TGCA) site, 30 and 3 microM spermine, respectively, were required for 50% crosslink inhibition. We suggest that this difference is due to the relative proximity of the three-guanosine tract to the d(TGCA).d(TGCA) site. We were able to confirm these conclusions with further experiments using duplexes containing three-guanosine and two-guanosine tracts and from computer simulations of the spermine-DNA complexes.

N8-acetyl spermidine protects rat cerebellar granule cells from low K+-induced apoptosis

The endogenous polyamines have been extensively studied with respect to their role in cellular death mechanisms, although the results are contradictory. In contrast, their primary metabolites, the N-acetyl polyamines, have not been much studied. It has been hypothesized that the N-acetyl metabolites may play a role in cellular death mechanisms, and some of the variability between different reports may be due to altered polyamine metabolic capacities. Using primary cultures of rat cerebellar granule cells, the effects of N-acetyl metabolites have been examined on basal, cytosine beta-D-arabinofuranoside (Ara-C)-induced and low K+-induced apoptosis. None of the compounds affected either basal or Ara-C-induced apoptosis at low doses. At higher doses, all compounds were toxic. Two compounds, N8-acetyl spermidine and N1-acetyl spermine, were found to protect cells from low K+-induced apoptosis, which has been shown to be p53-independent. In contrast, the parent polyamines were devoid of protective activity at subtoxic doses. This represents the first time that an antiapoptotic effect of N-acetyl polyamines has been demonstrated. These results raise the possibility that these compounds may act as endogenous neuroprotectants. The lack of effect on basal apoptosis provides evidence of at least two forms of p53-independent apoptosis that can be regulated independently.

Involvement of polyamines in retinoblastoma protein phosphorylation

Hepatocyte growth factor (HGF) increased both levels of phosphorylated and non-phosphorylated forms of retinoblastoma protein (RB) in primary cultured rat hepatocytes. Combined treatment of HGF and a specific inhibitor of ornithine decarboxylase (ODC), alpha-difluoromethylornithine (DFMO), reduced the levels of hyper-phosphorylated and hypo-phosphorylated forms of RB and increased the levels of the non-phosphorylated form, compared to HGF alone, but did not affect the total level of RB. Polyamines added exogenously overcame the effects of DFMO; they increased hyper- and hypo-phosphorylated forms and decreased non-phosphorylated RB. TGF-beta1 inhibited the increases in ODC activity, RB phosphorylation, and DNA synthesis induced by HGF. However, polyamines added exogenously could not overcome the inhibition by RB phosphorylation and DNA synthesis by TGF-beta1. These results suggest that polyamines are involved in the phosphorylation of RB, but the inhibition of polyamine biosynthesis by TGF-beta1 did not result in the inhibition of RB phosphorylation and DNA synthesis.

N1,N12-bis(ethyl)spermine effect on growth of cis-diamminedichloroplatinum(II)-sensitive and -resistant human ovarian-carcinoma cell lines

The results presented here demonstrate that a cis-diamminedichloroplatinum(II) (DDP)-resistant human ovarian-carcinoma cell line is also cross-resistant to the spermine analogue N1,N12-bis(ethyl)spermine (BESPM). We report that C13* cells, which are approximately 20-fold resistant to DDP, similarly showed 7-fold resistance to BESPM by colony-forming assay with an IC50 value of 24.6 +/- 2 microM vs. 3.4 +/- 0.8 microM of 2008 cells. Resistance appears to be the result of many effects, such as different morphological and functional modifications of mitochondria. Furthermore, although BESPM accumulation was almost identical in sensitive and resistant cells, the intracellular polyamine pool of the 2 cell lines was differentially affected by this polyamine analogue. In fact, when spermidine (SPD) was still detectable in C13* cells, in 2008 cells it was not, and the spermine (SPM) content was always more markedly reduced in sensitive cells than in the resistant variant. The lower polyamine content of 2008 cells could be related to a higher degree of induction of spermidine/ spermine N1-acetyltransferase (SSAT) activity by BESPM in sensitive cells than in their resistant counterpart. Despite the observed cross-resistance, the combination of the 2 drugs resulted in supra-additive and synergistic effects in both cell lines, depending on concentration, as assessed by median-effect analysis of the survival data. The effectiveness of this combination was also confirmed by the increased accumulation of cells in the G2/M phase of the cell cycle in both cell lines. Taken together, these data suggest that BESPM effect on cell growth of DDP-sensitive and DDP-resistant cells involves multiple mechanisms that are differently modulated by the DDP-resistant phenotype.

Effects of polyamine levels on the degradation of short-lived and long-lived proteins in cultured L-132 human lung cells

Biogenic polyamines have important regulatory functions in various biological processes and it has also been suggested that they could modulate intracellular protein degradation. For an overall assessment of the role of polyamines in this process, we have investigated the effect that the decrease in intracellular polyamine levels caused by inhibitors of polyamine biosynthesis brings about on the degradation of the pools of short- and long-lived proteins in cultured L-132 human lung cells. Treatment of cells with 100 microM (2R,5R)-delta-methyl acetylenic putrescine (MAP), a potent enzyme-activated irreversible inhibitor of ornithine decarboxylase, or with 100 microM MAP plus 50 microM N-butyl 1,3-diaminopropane, a specific inhibitor of spermine synthase, caused a similar decrease (65-70% of control) in the total intracellular levels of polyamines, although they affected the concentrations of spermidine and spermine differently. The effect of the two treatments on protein degradation was essentially the same. In polyamine-depleted cells we observed an inhibition of degradation in long-lived proteins of 16% (P<0.05), with a significant increase in the half-life (t12) of this pool from 100.5 to 120.1 h. This was concomitant with an increase of 26% (P<0. 05) in degradation in short-lived proteins, with a significant decrease in the t12 of this pool from 0.85 to 0.67 h. Recovery of polyamine levels by the addition of 50 microM spermidine to polyamine-depleted cells resulted in a restoration of the degradation rates in both pools of proteins. The way(s) by which polyamines could modulate proteolysis are discussed.

The early region 1B 55-kilodalton oncoprotein of adenovirus relieves growth restrictions imposed on viral replication by the cell cycle

The E1B 55-kDa oncoprotein of adenovirus enables the virus to overcome restrictions imposed on viral replication by the cell cycle. Approximately 20% of HeLa cells infected with an E1B 55-kDa mutant adenovirus produced virus when evaluated by electron microscopy or by assays for infectious centers. By contrast, all HeLa cells infected with a wild-type adenovirus produced virus. The yield of E1B mutant virus from randomly cycling HeLa cells correlated with the fraction of cells in S phase at the time of infection. In synchronously growing HeLa cells, approximately 75% of the cells infected during S phase with the E1B mutant virus produced virus, whereas only 10% of the cells infected during G1 produced virus. The yield of E1B mutant virus from HeLa cells infected during S phase was sevenfold greater than that of cells infected during G1 and threefold greater than that of cells infected during asynchronous growth. Cells infected during S phase with the E1B mutant virus exhibited severe cytopathic effects, whereas cells infected with the E1B mutant virus during G1 exhibited a mild cytopathic effect. Viral DNA synthesis appeared independent of the cell cycle because equivalent amounts of viral DNA were synthesized in cells infected with either wild-type or E1B mutant virus. The inability of the E1B mutant virus to replicate was not mediated by the status of p53. These results define a novel property of the large tumor antigen of adenovirus in relieving growth restrictions imposed on viral replication by the cell cycle.

Stimulation in vitro of vitamin B12-dependent methionine synthase by polyamines

Vitamin B12-dependent methionine synthase is an important enzyme for sulphur amino acid, folate polyamine metabolism, S-adenosylmethionine metabolism and also in the methylation pathway of DNA, RNA, proteins and lipids. Consequently, studies aiming at exploring the control and regulation of methionine synthase are of particular interest. Here we report the modulation of enzyme activity in vitro by polyamines. Although putrescine, cadaverine, spermine and spermidine all stimulated enzyme activity, the last two were the most potent, causing increases in enzyme activity up to 400%. The EC50 for spermine was determined as 8 microM and for spermidine 40 microM. The physiological concentration for spermine has been reported to be 15-19 microM. Spermine was found to increase both the Km and the V(max) with respect to methyltetrahydrofolate for the enzyme. These data support the hypothesis that spermine and spermidine are feedback regulators of methionine synthase both in vivo and in vitro and are consistent with the polyamines' regulating cell signalling pathways.

Procyclic Trypanosoma brucei cell lines deficient in ornithine decarboxylase activity

Ornithine decarboxylase (ODC) is a rate limiting enzyme in the biosynthesis of polyamines. We report here the construction of ODC gene deficient Trypanosoma brucei brucei cell lines by homologous recombination and disruption of the two alleles of the ODC gene. With our first stable transfection vector, we replaced the 2.8 kb SacII ODC gene-containing fragment with a hygromycin-B-phosphotransferase gene (hph) cassette transcribed under the control of the endogenous promoter. For the second ODC allele knock-out, we stably transfected similar constructs that contained either the phleomycin or G418 resistance gene cassette, and included 1 mM putrescine in the media. These experiments resulted in two separate ODC- lines: one hygromycin and phleomycin resistant, the other hygromycin and G418 resistant. The two ODC gene knockout lines were verified by Southern and Northern hybridization, and confirmed by Western blot and enzymatic activity assay. There is no ODC expression in the two ODC- lines and the ODC messages in the single ODC gene knockouts were only half of that of the wild type. When grown in the presence of putrescine, the ODC- lines showed little difference, morphologically, from wild type trypanosomes. The growth rate of these lines varied greatly, depending on the concentration of the putrescine. Interestingly, when putrescine was completely withdrawn from the media, the ODC- trypanosomes soon reached a plateau phase and some cells remained viable for 7-8 weeks. The starved cells could be rescued by the addition of putrescine or introducing back the ODC gene. Cell cycle analysis suggested that putrescine is required for G1-S transition in the procyclic form T. brucei.

Endogenous reverse transcription of human immunodeficiency virus type 1 in physiological microenviroments: an important stage for viral infection of nondividing cells

Endogenous reverse transcription (ERT) of retroviruses has long been considered a somewhat artificial process which only mimics reverse transcription occurring in target cells, as detergents or amphipathic peptides have classically been used to make the envelopes of retroviruses in these reaction systems permeable. Recently, several studies suggested that ERT of human immunodeficiency virus type 1 (HIV-1) might occur without detergent treatment. However, this phenomenon could be due to damage of the retroviral envelope during the process of virion purification or freezing and thawing. In this report, intravirion HIV-1 ERT, without detergent-induced permeabilization, is demonstrated to occur in the natural microenvironments of HIV-1 virions and is not caused by artificial processes. Therefore, this stage of the viral life cycle was termed natural ERT (NERT). The efficiency of NERT in HIV-1 virions was markedly augmented by several physiological substances in the extracellular milieu, such as polyamines and deoxyribonucleoside triphosphates. In addition, HIV-1 virions in seminal plasma samples harbored dramatically higher levels of full-length or nearly full-length reverse transcripts than virions isolated from peripheral blood plasma samples of HIV-1-seropositive men. When HIV-1 virions were incubated with seminal plasma samples, infectivity in initially nondividing cells was also significantly enhanced. Thus, we suggest that HIV-1 virions are actively altered by the extracellular microenvironment and that NERT may play an important role in viral infection of nondividing cells.

A new model for disruption of the ornithine decarboxylase gene, SPE1, in Saccharomyces cerevisiae exhibits growth arrest and genetic instability at the MAT locus

Ornithine decarboxylase (ODC) is a rate-determining enzyme of the polyamine-biosynthetic pathway. We sought to produce cells with impaired ODC function in order to study the biological functions of polyamines. Saccharomyces cerevisiae strains were obtained by one-step gene replacement of a 900 bp fragment of the yeast ODC gene (SPE1) with the yeast URA3 gene. Spores derived from SPE1/spe1 cells germinated at reduced efficiency relative to SPE1/SPE1. Sustained growth of spe1 haploid mutants in polyamine-free medium led to intracellular polyamine depletion, reduction in budding index, G1 arrest and cessation of growth, and cells that were large and misshapen. All of these effects were completely reversed by adding polyamines to the medium, even after 5 days of polyamine starvation. A diploid yeast strain bearing two copies of disrupted spe1 lost heterozygosity at the mating-type locus more often when grown in the absence of polyamines than when grown in their presence, indicating that polyamine deficiency leads to either chromosome loss or to mitotic recombination.

Multiple DNA binding modes of anthracene-9-carbonyl-N1-spermine

The poly(dAdT)2 complex of anthracene-9-carbonyl-N1-spermine, a spermine derivative terminally substituted with an anthracene moiety, has been studied using fluorescence, linear dichroism, circular dichroism, normal absorption spectroscopy (as a function of temperature) and computer modelling. For comparison, some data are also provided for the same ligand with poly(dGdC)2 and calf thymus DNA. Following detailed fluorescence and CD spectroscopic studies, we propose that anthracene-9-carbonyl-N1-spermine intercalates in at least two different binding orientations with poly(dAdT)2. Based on computer simulation data, we deduce that the ligand can intercalate from both the minor groove and the major groove. In contrast, intercalation with poly(dGdC)2 probably occurs only from the major groove. At high ligand concentrations, the CD spectra suggest anthracene-anthracene interactions, whilst the LD data point towards a groove-bound anthracene. Again from computer simulations, we propose binding modes consistent with these observations. Other data from the LD spectra suggest a sequential nature to the binding of the ligand to calf thymus DNA, with GC-rich sites being occupied first. At low ligand concentrations, anthracene-9-carbonyl-N1-spermine is able to stabilize poly(dAdT)2 against thermal decomposition, but not as effectively as spermine. The reverse is found to be true with calf thymus DNA. Both the anthracene-9-carbonyl-N1-spermine and spermine complexes of poly(dAdT)2 show pre-melt transitions in their melting curves. The anthracene-9-carbonyl-N1-spermine complex with poly(dAdT)2 also shows a post-melt transition.

Two polyamine analogs (BE-4-4-4 and BE-4-4-4-4) directly affect growth, survival, and cell cycle progression in two human brain tumor cell lines

1,14-Bis-(ethyl)-amino-5,10-diazatetradecane N1,N11-bis(ethyl)norspermine (BE-4-4-4) and 1,19-bis-(ethylamino)-5,10,15 triazanonadecane (BE-4-4-4-4) are two relatively new polyamine analogs synthesized for use as antineoplastic agents. In human brain tumor cell lines U-251 MG and SF-767, both agents inhibited cell growth, were cytotoxic, induced a variable G1/S block, and depleted intracellular polyamines. Since intracellular polyamine depletion did not always correlate with growth inhibition, cell survival, or cell cycle progression, it cannot completely explain the effects of these agents on growth, survival, and cell cycle progression in U-251 MG and SF-767 cells.

Inhibition of immediate-early-gene induction in renal mesangial cells by depletion of intracellular polyamines

Mitogens have been shown to stimulate the activity of the rate-limiting enzyme for polyamine synthesis, ornithine decarboxylase (ODC), and ODC mRNA expression in cultured rat mesangial cells (MCs). In addition, inhibition of ODC by alpha-difluoromethylornithine (DFMO) results in growth arrest of MCs. To elucidate the mechanisms involved in the inhibition of MC proliferation due to polyamine depletion, we studied the effects of DFMO on the activation of phospholipase C and induction of the immediate early genes (IEGs), c-fos, c-jun and Egr-1, which are thought to regulate cell growth. Mitogenic 10% fetal-calf serum (FCS) and 1 unit/ml thrombin activated phospholipase C in MCs within 30 s, as assessed by generation of [3H]inositol phosphates. This activation was not affected by DFMO. mRNAs of the IEGs c-fos, c-jun and Egr-1 were induced by FCS within 15 min. Expression of these genes reached a peak at 60 min and disappeared at 3 h. Treatment of MCs with a growth-suppressing dose of DFMO (5 mM) inhibited mRNAs of all three IEGs by 52-87% at 1 h. Total expression of Egr-1 over 20-120 min was diminished by 41%, and the time point of maximal expression was delayed by 40 min. This inhibitory effect was abolished in a time-dependent manner (1-3 days) by prior addition of 200 microM putrescine, the reaction product of ODC. Egr-1 mRNA expression was super-induced by the inhibitor of protein synthesis, cycloheximide. This effect was also blocked by DFMO. The results indicate that the DFMO-induced process of MC growth inhibition involves steps necessary for IEG activation. The signal-transduction step sensitive to polyamines occurs distal to the activation of phospholipase C. Since reconstitution of normal induction of IEGs requires 3 days, it seems likely that polyamine depletion affects the regulation of IEG expression in an indirect fashion. We conclude that activation of IEGs requires the presence of polyamines and plays a significant role in the induction of MC replication.