Differential effects of paromomycin on ribosomes of Leishmania mexicana and mammalian cells

Paromomycin, an aminoglycoside antibiotic having low mammalian cell toxicity, is one of the drugs currently used in the chemotherapy of cutaneous and visceral leishmaniasis. In order to understand the mode of action of this antibiotic at the molecular level, we have investigated the effects of paromomycin on protein synthesis in Leishmania and its mammalian hosts. We were able to demonstrate that in vivo protein synthesis in the promastigote stage of the parasite and its proliferation rate are markedly inhibited by paromomycin while being only slightly affected by other aminoglycoside antibiotics, such as streptomycin and neomycin B. Furthermore, both in vitro polypeptide synthesis induced by poly(U) as mRNA and accuracy of translation are significantly decreased by paromomycin in cell-free systems containing ribosomal particles of Leishmania promastigotes. Conversely, when ribosomes from mammalian cells are used instead of the protozoan particles, polyphenylalanine synthesis is only barely reduced by the antibiotic and the translation misreading remains almost unaltered. Surface plasmon resonance analysis of the interaction between paromomycin and protozoan or mammalian cell ribosomal RNAs shows a strong binding of antibiotic to the parasite ribosomal decoding site and practically no interaction with the mammalian cell counterpart. Our results indicating differential effects of paromomycin on the translation processes of the Leishmania parasite and its mammalian hosts can explain the therapeutic efficiency of this antibiotic as an antileishmaniasis agent.

Polyamine biosynthesis in Phytomonas: biochemical characterisation of a very unstable ornithine decarboxylase

The metabolism of polyamines as well as their functions as growth regulators in plants have been extensively studied for many years. However, almost nothing is known about the biosynthesis and roles of these substances in Phytomonas spp., parasites of several plants. We have used HPLC and electrophoretic analyses to investigate the presence and metabolism of polyamines in Phytomonas Jma strain, detecting both putrescine and spermidine but not spermine. Experiments carried out by incubation of intact parasites with labelled ornithine or putrescine showed the formation of radioactive putrescine or spermidine, respectively. These results indicated that Phytomonas Jma can synthesise these polyamines through the action of ornithine decarboxylase (ODC) and spermidine synthase. On the other hand, we could not detect the conversion of arginine to agmatine, suggesting the absence of arginine decarboxylase (ADC) in Phytomonas. However, we cannot ensure the complete absence of this enzymatic activity in the parasite. Phytomonas ODC required pyridoxal 5'-phosphate for maximum activity and was specifically inhibited by α-difluoromethylornithine. The metabolic turnover of the enzyme was very high, with a half-life of 10-15 min, one of the shortest found among all ODC enzymes studied to date. The parasite proteasome seems to be involved in degradation of the enzyme, since Phytomonas ODC can be markedly stabilized by MG-132, a well known proteasome inhibitor. The addition of polyamines to Phytomonas cultures did not decrease ODC activity, strongly suggesting the possible absence of antizyme in this parasite.

Polyamine metabolism in Trypanosoma cruzi: studies on the expression and regulation of heterologous genes involved in polyamine biosynthesis

Biochemical studies have shown that Trypanosoma cruzi and Toxoplasma gondii are the only eukaryotic organisms so far described which are auxotrophic for polyamines. Both parasites are unable to carry out the de novo biosynthesis of putrescine, and therefore they need the addition of exogenous polyamines to the culture medium for their normal proliferation. Further investigations at the molecular level have demonstrated that the wild-type T. cruzi genome does not contain ornithine or arginine decarboxylase-like nucleic acid sequences, and that the corresponding genes have been presumably lost during evolution. Since T. cruzi behaves as a deletion mutant for ornithine decarboxylase (ODC) and arginine decarboxylase (ADC) genes, this parasite has been selected to study the regulation of the expression of heterologous genes involved in polyamine biosynthesis in other organisms. The resulting transgenic parasites have been useful tools to analyze the different stages of gene expression after transformation, as well as the mechanisms of drug resistance induction and the post-translational processing of enzyme precursors.

Trypanosoma cruzi as a model system to study the expression of exogenous genes coding for polyamine biosynthetic enzymes. Induction of DFMO resistance in transgenic parasites

Trypanosoma cruzi, the etiologic agent of Chagas' disease, is a polyamine auxotroph organism because its genome contains neither ornithine decarboxylase (ODC) nor arginine decarboxylase (ADC) genes, presumably lost during evolution. After transformation with a recombinant plasmid bearing the complete coding region of Crithidia fasciculata ODC gene, the transgenic parasites were able to synthesize putrescine and simultaneously became susceptible to alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ODC. We have studied the emergence of DFMO-resistant T. cruzi after one-step selection of ODC-transformed parasites cultivated in the presence of high levels of the drug (5 mM). Our results have indicated a duplication of the ODC gene copy number in the drug-resistant cell line. The ODC transcripts and the corresponding translation products showed very significant increases (about 7- and 25-fold, respectively) in DFMO-resistant parasites, while the ODC enzymatic activity was 5 times higher than in drug-sensitive T. cruzi. The unequal increases of ODC protein and enzymatic activity in DFMO-resistant protozoa strongly suggest that in addition to gene amplification and enhanced transcription and translation, the assembly of ODC polypeptide chains into dimeric active enzyme molecules might also contribute to regulate the development of DFMO resistance.

Molecular and functional characterization of a spermidine transporter (TcPAT12) from Trypanosoma cruzi

Trypanosoma cruzi, the etiological agent of Chagas' disease, is the only eukaryotic cell which lacks the ability to synthesize polyamines de novo. In this work, we describe for the first time the molecular and biochemical properties of a high-affinity spermidine transporter from T. cruzi. The transporter gene TcPAT12 was functionally expressed in Xenopus laevis oocytes, showing high levels of spermidine uptake. Similar apparent affinity constants for spermidine uptake were obtained when comparing T. cruzi epimastigotes and heterologous expressed TcPAT12 in X. laevis. In addition, TcPAT12 also transports putrescine and the amino acid l-arginine at lower rates than spermidine.

Modulation of oat arginine decarboxylase gene expression and genome organization in transgenic Trypanosoma cruzi epimastigotes

We have previously demonstrated that wild-type Trypanosoma cruzi epimastigotes lack arginine decarboxylase (ADC) enzymatic activity as well as its encoding gene. A foreign ADC has recently been expressed in T. cruzi after transformation with a recombinant plasmid containing the complete coding region of the oat ADC gene. In the present study, upon modulation of exogenous ADC expression, we found that ADC activity was detected early after transfection; subsequently it decreased to negligible levels between 2 and 3 weeks after electroporation and was again detected approximately 4 weeks after electroporation. After this period, the ADC activity increased markedly and became expressed permanently. These changes of enzymatic activity showed a close correlation with the corresponding levels of ADC transcripts. To investigate whether the genome organization of the transgenic T. cruzi underwent any modification related to the expression of the heterologous gene, we performed PCR amplification assays, restriction mapping and pulse-field gel electrophoresis with DNA samples or chromosomes obtained from parasites collected at different time-points after transfection. The results indicated that the transforming plasmid remained as free episomes during the transient expression of the foreign gene. Afterwards, the free plasmid disappeared almost completely for several weeks and, finally, when the expression of the ADC gene became stable, two or more copies of the transforming plasmid arranged in tandem were integrated into a parasite chromosome (1.4 Mbp) bearing a ribosomal RNA locus. The sensitivity of transcription to alpha-amanitin strongly suggests involvement of the protozoan RNA polymerase I in the transcription of the exogenous ADC gene.

Poly amine Requirement for Streptomycin Action on Protein Synthesis in Bacteria

The effect of streptomycin on polypeptide synthesis in vivo and in vitro has been investigated using polyamine auxotrophic mutants of Escherichia coli grown in the presence or in the absence of putrescine. We found that streptomycin caused a marked inhibition of protein synthesis in polyamine-supplemented cells whereas bacteria starved for polyamines were less sensitive to the action of the antibiotic. Neomycin, kanamycin and kasugamycin had a behaviour similar to streptomycin while spectinomycin, gentamicin and tetracycline brought about a strong inhibition of protein synthesis both in polyamine-starved and unstarved bacteria. The increase of misreading induced by the addition of streptomycin in vivo was higher in extracts derived from bacteria cultivated in the presence of polyamines. This effect was observed in cell-free systems of streptomycin-sensitive and resistant strains. In contrast, spermidine added in vitro caused an improvement in the accuracy of translocation. Analysis of sodium dodecyl sulphate/polyacrylamide gel electrophoresis of the labelled polypeptides synthesized in vivo seems to indicate that the starvation for polyamine or the presence of streptomycin may lead to premature termination with the appearance of unfinished peptide chains.

Heterologous expression of a plant arginine decarboxylase gene in Trypanosoma cruzi

Wild-type Trypanosoma cruzi epimastigotes lack arginine decarboxylase (ADC) enzymatic activity. However, the transformation of these parasites with a recombinant plasmid containing the oat ADC cDNA coding region gave rise to the transient heterologous expression of the enzyme, suggesting the absence of endogenous mechanisms that could inhibit the expression of a hypothetical own ADC gene or the assay used to measure its enzymatic activity. The foreign ADC enzyme expressed in the transgenic T. cruzi was characterized by identification of the products, the stoichiometry of the catalysed reaction, the specific inhibition by alpha-difluoromethylarginine (DFMA) and the study of its metabolic turnover. The half-life of the heterologous ADC activity in T. cruzi was about 150 min. Bioinformatics studies and polymerase chain reaction (PCR) analyses seem to indicate the absence of ADC-like DNA sequences in the wild-type T. cruzi genome.

Lack of Arginine Decarboxylase in Trypanosoma cruzi Epimastigotes

The presence of arginine decarboxylase (ADC) enzymatic activity in Trypanosoma cruzi epimastigotes is still a matter of controversy due to conflicting results published during the last few years. We have investigated whether arginine might indeed be a precursor of putrescine via agmatine in these parasites. We have shown that wild-type T. cruzi epimastigotes cultivated in a medium almost free of polyamines stopped their growth after several repeated passages of cultures in the same medium, and that neither arginine nor omithine were able to support or reinitiate parasite multiplication. In contrast, normal growth was quickly resumed after adding exogenous putrescine or spermidine. The in vivo labelling of parasites with radioactive arginine showed no conversion of this amino acid into agmatine, and attempts to detect ADC activity measured by the release of CO2 under different conditions in T. cruzi extracts gave negligible values for all strains assayed. The described data clearly indicate that wild-type T. cruzi epimastigotes lack ADC enzymatic activity.

Spermidine is essential for normal proliferation of trypanosomatid protozoa

Trypanosomatid parasites containing a metabolically unstable ornithine decarboxylase (ODC) are naturally resistant to high levels of alpha-difluoromethylornithine (DFMO) because this ODC inhibitor, though causing a drastic reduction of intracellular putrescine, elicits only a moderate decrease of the spermidine endogenous pool. In this study we have used a combination of DFMO with cyclohexylamine (CHA; bis-cyclohexylammonium sulfate), an inhibitor of spermidine synthase, to reach a more complete depletion of spermidine. Under these conditions we have observed the arrest of proliferation not only in trypanosomatids with stable ODC but also in parasites with an enzyme of high turnover rate. In all cases the reinitiation of proliferation occurred only after the addition of exogenous spermidine, and neither putrescine nor spermine were able to induce the same effect.

Sensitivity of trypanosomatid protozoa to DFMO and metabolic turnover of ornithine decarboxylase

alpha-Difluoromethylornithine (DFMO), the specific and irreversible inhibitor of ornithine decarboxylase (ODC), was able to induce the arrest of proliferation in Leishmania mexicana and ODC-transformed Trypanosoma cruzi cultures grown in a semi-defined medium essentially free of polyamines. Conversely, Crithidia fasciculata and Phytomonas 274 were not affected by the inhibitor. The drug-resistance of Crithidia and Phytomonas was neither caused by an impairment of DFMO uptake nor by a decrease of the enzyme affinity for the inhibitor. We were also able to rule out the possibility of ODC overexpression in the drug-tolerant parasites. The measurements of ODC metabolic turnover indicated that the enzymes from Crithidia and Phytomonas have a short half-life of 20-40 min, while ODC from Leishmania and transgenic Trypanosoma cruzi are rather stable with a half-life longer than 6 hours. Analyses of polyamine internal pools under different growth conditions have shown that DFMO was able to markedly decrease the levels of putrescine and spermidine in all parasites, but the depletion of spermidine was higher in trypanosomatids containing an ODC with slow turnover. Our results suggest that in these parasites cultivated in the presence of the drug, spermidine might decrease below critical levels needed to maintain trypanothione concentrations or other conditions essential for normal proliferation.

Trypanosoma cruzi epimastigotes lack ornithine decarboxylase but can express a foreign gene encoding this enzyme

Trypanosoma cruzi, a pathogenic protozoan causing Chagas disease, lacks ornithine decarboxylase (ODC), the enzyme catalyzing the first step of polyamine biosynthetic pathway in eukaryotic cells. Our results indicate that the auxotrophy for diamines of T. cruzi epimastigotes is due to the absence of an active ODC gene in these parasites and not to the inability for the expression of this gene. The introduction of an exogenous complete coding region from Crithidia fasciculata ODC gene inserted in an expression vector specific for trypanosomatids induces the normal expression of the foreign genetic information allowing the transformed T. cruzi to overcome the exogenous polyamine requirement for growth. The enzyme expressed in the transformed parasites has shown a considerably extended metabolic stability. The loss of ODC activity in T. cruzi might be related to the parasite adaptation to the intracellular stages of its life cycle.

Streptomycin bactericidal action is dependent on polyamine endogenous levels in E. coli

E. coli polyamine-supplemented and depleted cultures showed an important difference in survival to streptomycin; the bactericidal effect of the antibiotic was remarkably higher in cells with normal levels of polyamines. Similar results were observed with kanamycin. Analysis of the polyamine-containing cells pulse-labelled with 35S-methionine during streptomycin action indicated that the amounts of newly-synthesized peptides in various subcellular fractions was different from the amounts formed in the untreated controls; the most dramatic change was found in the residual particulate fraction where the antibiotic treatment caused a 3-fold increase of radioactive proteins. On the contrary, equivalent amounts of labelled peptides were detected in the different fractions prepared from polyamine-depleted bacteria incubated with or without antibiotic. In this case the corresponding residual fraction was only slightly increased. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the different fractions showed some changes elicited by streptomycin in the protein patterns of polyamine-containing bacteria, especially in the residual fractions. The electrophoretic profile corresponding to deprived cells was very similar in all cases. The role of polyamines in the conformation of the outer membrane and in the correct assembly of ribosomes is discussed on account of the enhancing effect of these polycations on the bactericidal action of streptomycin.

Characterization of a novel translational inhibitor from Leishmania mexicana promastigotes

An inhibitory activity blocking protein synthesis elongation in several eukaryotic systems has been detected in Leishmania mexicana extracts. This factor, which competes with aminoacylation of tRNA and also affects the subsequent polymerization step, is a strong inhibitor of polypeptide synthesis induced by poly U in wheat-germ extracts or by endogenous mRNAs in rat liver cell-free systems. The purified translational inhibitor has shown to be essentially free of proteins. Several chemical and biochemical properties of the inhibition factor have supported the conclusion that it behaves as a 200 bases RNA with a high content of secondary structure.

Alpha-difluoromethylornithine-resistant cell lines obtained after one-step selection of Leishmania mexicana promastigote cultures

Proliferation of Leishmania mexicana promastigotes in synthetic medium can be blocked by the depletion of intracellular polyamine pools induced by the presence of D,L-alpha-difluoromethylornithine (DFMO), a specific and irreversible inhibitor of ornithine decarboxylase (ODC). Here we report that DFMO-resistant cell lines growing normally at DFMO levels of 10 mM have been obtained from non-proliferating cultures after a single-step selection in the presence of high concentrations of the drug. The DFMO-resistant promastigotes underwent a morphological transformation into an 'amastigote-like' form after incubation for several hours at gradually increasing temperatures up to 35 degrees C. The uptake of DFMO was not significantly altered in the drug-resistant cell lines but in both cases (promastigote and 'amastigote-like' forms) the ODC specific activity was increased approx. 15-fold over the normal enzymic levels found in the wild-type Leishmania. The enzyme affinities for its substrate and for DFMO gave very similar values in the drug-resistant promastigotes and the wild-type parasites. In contrast, ODC from the 'amastigote-like' Leishmania showed a higher affinity for ornithine and a decreased capacity for the binding of DFMO. An 80-fold amplification of the ODC gene and a corresponding increase in its transcripts have been detected in both DFMO-resistant Leishmania cell lines. The drug-resistant phenotypes with their characteristic morphologies, the increased levels of ODC activity and the amplification of the ODC gene have been stable for at least 6 months in the absence of selective pressure.

Cloning of a trypanosomatid gene coding for an ornithine decarboxylase that is metabolically unstable even though it lacks the C-terminal degradation domain

Mammalian ornithine decarboxylase (ODC) is among the most labile of cellular proteins, with a half-life of usually less than an hour. Like other short-lived proteins ODC is degraded by the 26S proteasome. Its degradation is not triggered by ubiquitination, but is stimulated by the binding of an inducible protein, antizyme. Truncations and mutations in the C terminus of mammalian ODC have been shown to prevent the rapid turnover of the enzyme, demonstrating the presence of a degradation signal in this region. Moreover, ODCs from the trypanosomatid parasites Trypanosoma brucei and Leishmania donovani, which lack this C-terminal domain, are metabolically stable, and recombination of T. brucei ODC with the C terminus of mammalian ODC confers a short half-life to the fusion protein when expressed in mammalian cells. In the present study we have cloned and sequenced the ODC gene from the trypanosomatid Crithidia fasciculata. To our knowledge, this is the first protozoan shown to have an ODC with a rapid turnover. The sequence analysis revealed a high homology between C. fasciculata ODC and L. donovani ODC, despite the difference in stability. We demonstrate that C. fasciculata ODC has a very rapid turnover even when expressed in mammalian cells. Moreover, ODC from C. fasciculata is shown to lack the C-terminal degradation domain of mammalian ODC. Our findings indicate that C. fasciculata ODC contains unique signals, targeting the enzyme for rapid degradation not only in the parasite but also in mammalian cells.

Effect of polyamines on plasmid-mediated kanamycin resistance and kanamycin phosphotransferase gene expression in Escherichia coli

The emergence of kanamycin resistance in a polyamine-deficient mutant of E. coli transformed with a plasmid encoding the kanamycin phosphotransferase gene has been studied. The initial inhibition of growth and protein synthesis caused by the addition of the antibiotic could be reversed earlier in polyamine-supplemented bacteria than in those depleted of the organic bases. Concomitantly, we have observed that the increase of kanamycin phosphotransferase activity evoking the antibiotic resistance was higher in bacteria cultivated in the presence of putrescine. This result seems to depend exclusively on the enhanced capacity of the translation process in bacteria grown with polyamines since the transcription of phosphotransferase gene was higher in cells subjected to polyamine starvation.

Ornithine decarboxylase from Leishmania mexicana promastigotes: interaction with pyridoxal 5'-phosphate and alpha-difluoromethylornithine

The catalytic properties of ornithine decarboxylase (ODC) from Leishmania mexicana as well as the interaction with its cofactor pyridoxal 5'-phosphate (PLP) and the irreversible inhibitor alpha-difluoromethylornithine (DFMO) have been studied using partially purified preparations of the enzyme obtained from parasite promastigotes. Leishmania extracts prepared in the presence of saturating concentrations of PLP yielded an enzyme considerably more resistant to heat inactivation and with a three-fold higher activity than the ODC obtained without the addition of cofactor. The complete removal of PLP by treatment with hydroxylamine yielded the apoenzyme which shows an absolute requirement for PLP to recover its enzymatic activity. The Km values for L-ornithine and PLP were 0.7 mM and 25 microM, respectively, while Ki for DFMO was 0.2 mM. The restoration of ODC activity from apoenzyme and cofactor seems to involve time and temperature-dependent activation processes. L. mexicana ODC has an apparent molecular mass of 240 +/- 20 kDa.

Regulation of putrescine uptake in Leishmania mexicana promastigotes

Putrescine uptake of Leishmania mexicana promastigotes is tightly regulated by polyamine intracellular concentrations. This uptake, markedly stimulated after parasite treatment with alpha-difluoromethylornithine (DFMO) for 48 to 72 hrs., was strongly repressed by exposure of Leishmania cultures to exogenous putrescine or its derivative 1,4-dimethylputrescine. In contrast, spermidine, spermine, diaminopropane and cadaverine were unable to decrease putrescine transport. Both, the uptake induction as well as its specific feedback repression by increased levels of endogenous putrescine requires protein synthesis since they were abolished after addition of cycloheximide for several hours. Our results seem to indicate that putrescine transporter is a stable and specific protein which can be reversibly inactivated by a relatively unstable repressor.

Inhibition of in vivo transcription by actinomycin D treatment of ethylenediaminetetraacetic acid-permeabilized Escherichia coli: effects on bacteriophage proliferation

Rifampicin, which is able to block DNA-dependent RNA synthesis, has been widely used to selectively inhibit host protein synthesis in RNA bacteriophage-infected Escherichia coli without affecting the viral specific protein synthesis. However, in many cases it is necessary to increase rifampicin levels to 200 micrograms/ml in order to obtain an almost complete suppression of bacterial protein synthesis, and these high antibiotic concentrations cause at the same time a strong inhibition of phage proliferation resulting in a 50- to 100-fold reduction of phage yields. We have partially avoided this difficulty by using actinomycin D after permeabilization of bacteria by a brief incubation with EDTA. To optimize the method the effects of changing EDTA and actinomycin concentrations as well as the duration of the permeabilization period have been studied. With this procedure it has been possible to shut off bacterial RNA and protein synthesis with phage yields about 10 times higher than those observed in the presence of high levels of rifampicin. The usefulness of the described method is particularly evident when working with rifampicin-resistant strains of E. coli.

Polyamines prevent DFMO-mediated inhibition of angiogenesis

Tumor growth mainly depend on formation of new blood vessels. DFMO (alpha-difluoromethylornithine), an inhibitor of polyamine biosynthesis, inhibits tumor growth in many animal tumors. Our investigation was to evaluate the requirement of polyamines for induction of angiogenesis by tumor cells and spleen lymphocytes from tumor-bearing mice. In this regard, we have added DFMO to cell cultures. The neovascular response induced either by tumor cells or spleen lymphocytes was completely abrogated. This inhibition could be reversed by the addition of exogenous putrescine. These findings suggest that the effect of DFMO on angiogenesis is, in part, mediated by the inhibition of polyamine biosynthesis.

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.

Differential regulation of putrescine uptake in Trypanosoma cruzi and other trypanosomatids

Putrescine uptake in Trypanosoma cruzi epimastigotes is 10 to 50-fold higher than in Leishmania mexicana or Crithidia fasciculata. Polyamine transport in all these trypanosomatids is an energy-dependent process strongly inhibited by the presence of 2,4-dinitrophenol or KCN. Putrescine uptake in T. cruzi and L. mexicana was markedly decreased by the proton ionophore carbonylcyanide m-chlorophenylhydrazone but it was not affected by ouabain, a Na(+)-K+ pump inhibitor. The depletion of intracellular polyamines by treatment of parasite cultures with alpha-difluoromethylornithine elicited a marked induction of putrescine uptake in L. mexicana and C. fasciculata by increasing considerably the Vmax of this process. Conversely, the uptake of putrescine in T. cruzi was essentially unchanged by the same treatment. The differential regulation of putrescine transport in T. cruzi might be related to some distinctive features of polyamine metabolism in this parasite.

Ornithine decarboxylase from Crithidia fasciculata is metabolically unstable and resistant to polyamine down-regulation

Ornithine decarboxylase (ODC) of Crithidia fasciculata extracts shows maximal activity during exponential growth of the parasite and decreases markedly in the stationary phase. The inhibition of protein synthesis by cycloheximide evoked a rapid loss of enzyme activity with a half-life of about 30 min. Upon removal of DFMO from Crithidia cultures treated with the drug for 24 h, the ODC activity increased at the same rate as total protein synthesis. The addition of putrescine at high concentrations to parasites cultivated in a synthetic medium showed that Crithidia ODC levels were not reduced by polyamines.

Control of Leishmania mexicana proliferation by modulation of polyamine intracellular levels

Repeated treatments of Leishmania mexicana promastigote cultures with a-difluoromethylornithine could not block proliferation when the parasite was grown in a rich medium. Although the irreversible inhibitor of ornithine decarboxylase was able to abolish the enzymatic activity under these conditions, polyamine depletion was only partial probably due to the uptake of these substances from the external medium. Conversely, when Leishmania was cultivated in a defined medium essentially free of polyamines, a-difluoromethylornithine was able to decrease the growth rate and proliferation was arrested after several passages in the presence of the drug. Parasite multiplication could be resumed by addition of exogenous polyamines, and a strict correlation between Leishmania promastigote growth and intracellular levels of spermidine was observed.

Stable ornithine decarboxylase in promastigotes of Leishmania mexicana mexicana

Studies on the decarboxylation of ornithine in Leishmania mexicana have shown that this activity corresponds to a true ornithine decarboxylase rather than to an oxidative decarboxylation or aminotransferase reaction, both of which also give rise to the release of CO2. The stoichiometric relationship between substrate and products has indicated that extracts of L. mexicana were able to catalyse the formation of an unknown compound besides putrescine and CO2. The addition of cycloheximide to cultures of L. mexicana allowed us to demonstrate that ornithine decarboxylase degradation in vivo was extremely slow in this parasite. This remarkable stability of the enzyme is only comparable to that found in Trypanosoma brucei and contrasts with the high turnover rate of ornithine decarboxylases of different mammalian cells.

Polyamines and regulation of ornithine biosynthesis in Escherichia coli

The growth rate of several polyamine-deficient mutants of Escherichia coli was very low in minimal medium and increased markedly upon the addition of putrescine, spermidine, arginine, citrulline, or argininosuccinic acid. The endogenous content of polyamines was not significantly altered by the supplementation of polyamine-starved cultures with arginine or its precursors. In contrast, these compounds as well as putrescine or spermidine caused a 40-fold reduction in intracellular ornithine levels when added to polyamine-depleted bacteria. In vivo experiments with radioactive glutamic acid as a precursor and in vitro assays of the related enzymes showed that the decrease in ornithine levels was due to the inhibition of its biosynthesis rather than to an increase in its conversion to citrulline or delta 1-pyrroline-5-carboxylic acid and proline. High endogenous concentrations of ornithine were toxic for the E. coli strains tested. The described results indicate that the stimulatory effect of putrescine and spermidine on the growth of certain polyamine-starved bacteria may be partially due to the control of ornithine biosynthesis by polyamines.

Inhibition of protein synthesis by aminoglycoside antibiotics in polyamine-requiring bacteria

The effect of streptomycin and other aminoglycosides on protein synthesis has been studied using various streptomycin-sensitive strains unable to synthesize polyamines. We have confirmed and extended our previous results showing that the strong inhibition of translation caused by the antibiotic in polyamine-supplemented bacteria was markedly reduced in polyamine-starved cells. The analysis of polypeptides synthesized in the absence and presence of streptomycin in bacteria grown with and without putrescine has shown that the antibiotic provoked the accumulation of low molecular weight peptides partially bound to ribosomes in polyamine-unstarved cells. On the contrary, the drug did not induce major alterations in the patterns of proteins obtained from polyamine-depleted bacteria. The addition of the antibiotic did not evoke any change of proteolytic activity.

Protein synthesis in polyamine-deficient bacteria during amino-acid starvation

Amino-acid starvation in polyamine-auxotrophic bacteria grown in the presence of putrescine provokes a marked inhibition of protein synthesis. This inhibition is almost completely relieved in polyamine-depleted cells. The differential behaviour of bacterial protein synthesis depending on the endogenous levels of polyamines is not due to a change in the uptake of amino acids used to measure protein synthesis, nor to the decreased growth rate of polyamine-depleted cells. During leucine starvation, cells grown with putrescine synthesized a somewhat lower amount of high-molecular-weight proteins than polyamine-depleted bacteria. In addition, cells with normal endogenous levels of polyamines accumulated significant amounts of 62 and 41 kDa polypeptides as well as several low-molecular-weight peptides.

A novel mechanism of resistance to alpha-difluoromethylornithine induced by cycloheximide. Growth with abnormally low levels of putrescine and spermidine

Treatment of the chemically transformed fibroblasts BP-A31 and other cell lines with low concentrations of cycloheximide (CHM) for 72 h followed by the removal of the protein synthesis inhibitor leads to the proliferation of alpha-difluoromethylornithine (DFMO)-resistant phenotypes. These drug-resistant cells contain almost no ornithine decarboxylase (ODC) activity and concomitantly very low levels of putrescine and spermidine. Southern blot analysis and measurements of ODC activity and intracellular polyamine levels showed that the described mechanism of inducing resistance to DFMO triggered by CHM does not involve ODC gene amplification, altered transport of the drug or reduced affinity of the enzyme for DFMO.

A probable new pathway for the biosynthesis of putrescine in Escherichia coli

Some cultures of Escherichia coli BGA8, a mutant unable to synthesize putrescine, showed a change of behaviour and could grow almost equally well in either the absence or the presence of polyamines after repeated periods of polyamine starvation. Experiments in vivo with radioactive precursors showed that the bacteria which evaded the polyamine requirement had recovered their ability to synthesize putrescine from glucose or glutamic acid, but not from ornithine or arginine. These results are in agreement with the fact that the polyamine-independent cells were still deficient in the enzymes ornithine decarboxylase and agmatinase. Our findings seem to indicate the existence of a new pathway synthesize putrescine which does not involve ornithine or arginine as intermediates.

Effect of polyamines on translation fidelity in vivo

Polyamines, when added to cell-free protein-synthesizing systems, have been shown either to reduce mistranslation or to increase it depending upon the composition of the reaction mixture. To study this question under conditions as natural as possible we investigated the effects of polyamines on the fidelity of protein synthesis in intact Escherichia coli bacterial cells, using strains which were auxotrophic for polyamine synthesis. Error was measured in two ways: the incorporation of [3H]histidine into coat protein of bacteriophage MS2, the gene of which does not code for histidine, and the synthesis of a basic variant of MS2 coat protein in which a lysine erroneously replaces an asparagine, causing a change in isoelectric point. We found that when cell cultures were supplemented with polyamines there was no effect on the first type of error and the second type decreased twofold. The measured errors occurred at the level of translation because their frequency increased in the presence of streptomycin and decreased in cells bearing a streptomycin-resistance mutation known to lower the occurrence of translational misreading. The average erroneous incorporation per mol coat protein in the presence of polyamines was 1.43 +/- 0.59 mmol histidine and 25-34 mmol lysine/asparagine substitution. The reason for the different effect of polyamines on the two types of error is not known but could be related to the difference between their corresponding frequencies or to codon-specific effects.

Long-term treatment of tumor-operated mice with high doses of alpha-difluoromethylornithine

Mice inoculated subcutaneously with a mammary adenocarcinoma M3cell suspension and submitted 10 to 15 days later to the surgical removal of primary tumors have been chosen in our laboratory as the experimental model to study the effect of several drugs on metastasis spreading. A 90-day treatment of tumor-operated mice with high doses of alpha-difluoromethylornithine showed, in addition to a marked decrease of lung metastases, several side effects including a loss of body weight, a delay of surgical wound healing and a reduced number of megakaryocytes in bone marrow. The measurement of ceruloplasmin in serum has indicated that treated animals contained increased levels of this oxidase activity in spite of the polyamine depletion caused by alpha-difluoromethylornithine. These results rule out a direct correlation between ceruloplasmin activity and polyamine concentration.

Effect of alpha-difluoromethylornithine in lung metastases before and after surgery of primary adenocarcinoma tumors in mice

The depletion of polyamines by alpha-difluoromethylornithine (DFMO) treatment of mice after subcutaneous inoculation of adenocarcinoma M3 cells caused a remarkable inhibition in the growth rate of primary tumors as well as in the occurrence and number of lung metastases with a concomitant increase in survival time. Tumor-bearing mice submitted to the surgical removal of primary tumors and then treated with alpha-difluoromethylornithine also showed a significant reduction of lung metastases. In addition, a lower number of lung metastatic nodules correlated with decreased levels of polyamines in the same tissue. The described approach provides a useful experimental model for studies in human cancer therapy.

Putrescine distribution in Escherichia coli studied in vivo by 13C nuclear magnetic resonance

In order to study the intracellular polyamine distribution in Escherichia coli, 13C-NMR spectra of [1,4-13C]putrescine were obtained after addition of the latter to intact bacteria. The 13C-enriched methylene signal underwent line broadening. When the cells were centrifuged after 90 min the cell-bound putrescine peak had a linewidth of 23 Hz, while the supernatant liquid showed an unbound putrescine signal with a linewidth smaller than 1 Hz. By using 13C-enriched internal standards it could be shown that the linewidening was not due to the heterogeneity of the medium or to an in vivo paramagnetic effect. Cell-bound putrescine was liberated by addition of trichloroacetic acid and was therefore non-covalently linked to macromolecular cell structures. Cell-bound [13C]putrescine could be displaced by addition of an excess of [12C]putrescine. When samples of membranes, soluble protein, DNA, tRNA and ribosomes from E. coli were incubated with [1,4-13C]putrescine, strong binding was detected only in the ribosomal and membrane fractions. The ribosome-putrescine complex showed properties similar to those determined with the intact cells. By measuring the nuclear Overhauser enhancements eta, it was possible to estimate that only about 50% of the polyamine was linked to the macromolecules. Determination of the T1 values of free and ribosomal-bound putrescine allowed the calculation of a correlation time, tau c = 4 X 10(-7) s for the latter. T1 and tau c values found for the ribosome-putrescine complex were those expected for a motional regime of slowly tumbling molecules.

Surface proteins in different isolates of Trypanosoma cruzi epimastigotes

Epimastigotes from several Trypanosoma cruzi stocks were labeled by iodination with Chloramine T and their proteins detected by gel electrophoresis and autoradiography. The labeled proteins from the parasite surface were detected after immunoprecipitation with antisera against fixed trypanosomes or from infected rabbits. These antisera were able to recognize one or more proteins in all T. cruzi isolates analyzed, but the individual patterns differed from each other. Variations in the surface protein patterns were also observed in two Tulahuen stocks kept during several years under different conditions. Growth medium as well as the stage of growth at which the parasites were collected had also an effect upon the relative amount of the observed labeled proteins.

Differential effect of alpha-difluoromethyl-ornithine on the proliferation of Balb 3T3 and chemically transformed 3T3 cells

In order to study the effect of polyamine depletion on growth and proliferation of untransformed and chemically transformed cells, alpha-difluoromethyl-ornithine (DFMO) was added to cultures of 3T3 cells and their benzo[a]pyrene derivative BP-3T3. Both types of cells stopped their proliferation after 72 hr of treatment with the inhibitor. When DFMO was removed and cells were cultivated afterwards in fresh medium without the drug, untransformed cells resumed growth after a lag period, whereas transformed cells were unable to proliferate unless exogenous polyamine was added. These alterations showed a strict correlation with intracellular polyamine pools, since after removal of DFMO from the culture medium, polyamine concentrations increased to almost normal values in 3T3 cells, but remained at low levels or decreased even more in the transformed cells BP-3T3. The analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of labeled proteins of both cell extracts has indicated that the described control of cell proliferation by intracellular polyamine levels might be related to the synthesis of at least two proteins with molecular weights of about 36,000 and 55,000 daltons.

Polyamines and antibiotic effects on translation

Protein synthesis was studied in polyamine-auxotrophic mutants of Escherichia coli. The decreased protein synthesizing rate observed both in vivo and in vitro in polyamine-starved bacteria is due to defective 30S ribosomal subparticles which are impaired in the initiation step of translation. Analysis of peptides synthesized in vivo suggests a more extensive misreading by putrescine-depleted bacteria. Cells grown without polyamine show a markedly decreased response to the inhibitory effect and the misreading inducing action of streptomycin in vivo and in vitro. The polyamine-starved bacteria are also less sensitive to other aminoglycoside antibiotics such as neomycin, kanamycin and kasugamycin.

Studies on biosynthesis, assembly and expression of human major transplantation antigens

Biosynthesis and regulation of expression of transplantation as detected by a monoclonal antibody to HLA-A,B,C antigens (human leucocytic antigen) and a polyclonal antiserum to beta 2-microglobulin have been investigated using radioactive amino acids and sugars to label human lymphoid cells. We found unbalanced synthesis of HLA heavy chains and beta 2-microglobulin, the latter being in excess and secreted to the extracellular medium. In DAUDI cells, which are defective in beta 2-microglobulin, no HLA-A,B,C could be detected intracellularly even in the presence of added beta 2-microglobulin. Treatment of BRI-8 cells with tunicamycin, an antibiotic which inhibits glycosylation of polypeptides, almost had no effect on the levels of beta 2-microglobulin, while it markedly decreased that of HLA heavy chains, both on the cell surface and intracellularly. Glycosylation of the HLA heavy chains appeared to be an essential requirement for the normal expression of HLA-A,B,C antigens. The translation in vitro in a messenger-dependent reticulocyte system with total polysomes obtained from BRI-8 cells showed that beta 2-microglobulin was synthesized as a precursor. This larger polypeptide was converted into mature beta 2-microglobulin when protein synthesis was performed with microsomes instead of polysomes.

Improved method for the specific immunoprecipitation of albumin

The immunoprecipitates of many antigens are frequently contaminated by coprecipitation of unrelated substances. A method to overcome this type of contamination in the immunoprecipitation of albumin is described. The insoluble albumin-antibody complexes are solubilized by a brief treatment at high temperature in the presence of sodium dodecyl sulfate, and after dilution the mixture is submitted to a second immunoprecipitation.

Association and dissociation of Neurospora crassa cytoplasmic ribosomes

Dissociation and association factors of ribosomal particles were detected in extracts from Neurospora crassa at different stages of growth. The dissociation factor was easily released into the S100 supernatant fraction, whereas the association factor remained bound to the ribosomes.

Protein synthesis in resting and stimulated human lymphocytes

The ribosomal profiles in lysates from resting and phytohemagglutinin stimulated human lymphocytes have been analyzed by sucrose gradient centrifugation. The percentage of polyribosomes increased during lymphocyte transformation reaching a maximal value of 60 to 70% of the total ribosomes after 72 hours of mitogen addition. This time period coincides with maximal in vivo protein synthesis. On the other hand, in nonstimulated lymphocytes, about 25% of the ribosomal particles appeared as aggregates, independently of the incubation period. Experiments performed with homologous cell free systems containing ribosomes and supernatant fluids prepared from unstimulated or activated lymphocytes demonstrate that the mixtures containing both components from stimulated lymphocytes are several fold more active in polypeptide synthesis than the systems which contain ribosomal particles and cell sap from resting cells. Assays carried out with mixtures combining the components from both sources indicate that the increased activity depends on ribosomes as well as on the supernatant fractions.

Polyamines and protein synthesis: studies in various polyamine-requiring mutants of Escherichia coli

Different Escherichia coli mutants auxotrophic for polyamines were studied in order to investigate the relationships among polypeptide synthesis in cell-free systems, ribosomal distribution profiles and endogenous polyamine pools. The in vitro protein synthetic activity and the polyribosomal content were reduced in extracts from putrescine-starved cells of the double mutans MA 255 and MA 261, but not in the arginine-conditional auxotroph DK 6. Putrescine addition to the cultures of all these strains previously starved for polyamines, provoked a shift towards monomers in the equilibrium involving ribosomal particles. Concomitant changes in the intracellular levels of polyamines were observed: putrescine and spermidine increased markedly, and cadaverine disappeared.