Abnormal accumulation and toxicity of polyamines in a difluoromethylornithine-resistant HTC cell variant

Knocking out Ornithine Decarboxylase Antizyme 1 (OAZ1) Improves Recombinant Protein Expression in the HEK293 Cell Line

Creating efficient cell lines is a priority for the biopharmaceutical industry, which produces biologicals for various uses. A recent approach to achieving this goal is the use of non-coding RNAs, microRNA (miRNA) and small interfering RNA (siRNA), to identify key genes that can potentially improve production or growth. The ornithine decarboxylase antizyme 1 (OAZ1) gene, a negative regulator of polyamine biosynthesis, was identified in a genome-wide siRNA screen as a potential engineering target, because its knock down by siRNA increased recombinant protein expression from human embryonic kidney 293 (HEK293) cells by two-fold. To investigate this further, the OAZ1 gene in HEK293 cells was knocked out using CRISPR genome editing. The OAZ1 knockout cell lines displayed up to four-fold higher expression of both stably and transiently expressed proteins, with comparable growth and metabolic activity to the parental cell line; and an approximately three-fold increase in intracellular polyamine content. The results indicate that genetic inactivation of OAZ1 in HEK293 cells is an effective strategy to improve recombinant protein expression in HEK293 cells.

Mechanism of polyamine toxicity in cultured cardiac myocytes

The goal of this study was to investigate the mechanism of polyamine-mediated injury to the cardiac myocytes isolated from neonatal rat hearts. The myocytes, cultured in Dulbecco's minimal essential medium-1% foetal calf serum (FBS), were exposed to spermidine or spermine. The toxicity to myocytes was determined by (a) increased release of creatine kinase (CPK) into the media and (b) decline in cell viability or functional activity. Spermidine, above 10 mum, increased the release of CPK into media, decreased cell viability and decreased the functional activity of the myocytes. The FBS exhibited polyamine oxidase activity and semicarbazide-sensitive amine oxidase activity. Aminoguanidine, MDL72,527 or semicarbazide, are the inhibitors of amine oxidases, polyamine oxidase (PAO) and semicarbazide-sensitive amine oxidase (SSAO), respectively. The addition of these inhibitors to the medium protected the myocytes from spermidine toxicity. To determine whether myocyte PAO is involved in polyamine toxicity, we used horse serum that contained high SSAO activity and negligible PAO activity. The myocyte extracts had negligible SSAO activity but high PAO activity. When myocytes were cultured in horse serum in lieu of FBS, spermine caused toxicity at above 100 mum. In horse serum, MDL72,527 and semicarbazide protected the myocytes from spermine toxicity. These observations show that extracellular amine oxidases and myocyte PAO are significant in mediation of polyamine toxicity.

Amine oxidases in apoptosis and cancer

Amine oxidases, the major enzymes of biogenic amines metabolism, are considered to be biological regulators, especially for cell growth and differentiation. A primary involvement of amine oxidases in cancer growth inhibition and progression, especially by means of aldehydes, H(2)O(2) and other reactive oxygen species, the amine oxidase-mediated products of biogenic amines oxidation, has been demonstrated. Amine oxidases are involved in cancer growth inhibition because of the higher content in tumour cells of biogenic amines in comparison to normal cells. The cytotoxic effect can be explained by a damage to cell membranes and/or nuclei or, indirectly, through modulation of membrane permeability transition and therefore apoptosis. The oxidation products of biogenic amines appears to be also carcinogenic, while acrolein, produced from the oxidation of spermine and spermidine, should be a key compound both carcinogenic and cytotoxic. The cancer inhibition/promotion effect of amine oxidases could be explained by taking into consideration the full pattern of the enzyme content of the cell. The balance of amine oxidases and antioxidant enzymes appear to be a crucial point for cancer inhibition or progression. A long lasting imbalance of these enzymes appears to be carcinogenic, while, for a short time, amine oxidases are cytotoxic for cancer cells.

Androgen responsiveness and intrarenal localization of transcripts coding for the enzymes of polyamine metabolism in the mouse

Polyamines, spermidine (SPD), and spermine (SPM) are intracellular polycations required for cell growth and differentiation. Their biosynthetic precursor, the diamine putrescine (PUT), is produced by regulatory ornithine decarboxylase (ODC). Spermidine/spermine N1-acetyltransferase (SSAT) is the ODC counterpart in the degradation pathway which retroconverts SPM and SPD into PUT. Castration of male mice for 7 days resulted in a 40% decrease of the renal levels of both SSAT and ODC transcripts. Administration of 5-alpha-dihydrotestosterone (DHT) to castrated mice for the last 3 days before sacrifice caused the levels of ODC and SSAT mRNAs to increase by 250% and 180%, respectively. Thus activation of the retroconversion pathway of polyamine metabolism appears to contribute towards the increase in PUT production known to be caused by androgens in the mouse kidney. In situ hybridization histochemistry experiments showed that the SSAT transcript is expressed only by the epithelial cells of the straight and convoluted distal tubules of the nephron, while the expression of the ODC transcript is confined to the epithelium of the convoluted and straight portion of the proximal tubules. The separation of the biosynthetic from the degradation pathway along the nephron suggests that PUT is mostly produced in the distal tubule, where it may play a physiological role, independent of androgen action, in protecting tubular cells from the very low osmolarity to which they are exposed in this nephron segment.

Effect of polyamine depletion on caspase activation: a study with spermine synthase-deficient cells

Activation of the caspase proteases represents a central point in apoptosis. The requirement for spermine for the processes leading to caspase activation has been studied in transformed embryonic fibroblasts obtained from gyro (Gy) mutant male mice. These cells lack spermine synthase activity and thus provide a valuable model to study the role of spermine in cell processes. Gy fibroblasts do not contain spermine and have a higher spermidine content. However, when compared with fibroblasts obtained from normal male littermates (N cells), Gy fibroblasts were observed to grow normally. The lack of spermine did not affect the expression of Bcl-2, and caspases 3 and 9 were activated by etoposide in both N and Gy cells, indicating that spermine is dispensable for caspase activation. Spermine deficiency did not significantly influence caspase activity in cells treated with etoposide, cycloheximide or staurosporine, but sensitized the cells to UV irradiation, which triggered significantly higher caspase activity in Gy cells compared with N cells. alpha-Difluoromethylornithine (DFMO), an inhibitor of polyamine synthesis that is able to deplete cells of putrescine and spermidine, but usually does not influence spermine content, was able to produce a more complete polyamine depletion in Gy cells. This depletion, which included spermine deficiency, dramatically increased caspase activation and cell death in Gy fibroblasts exposed to UV irradiation. On the other hand, in either N or Gy cells, DFMO treatment did not influence caspase activity triggered by staurosporine, but inhibited it when the inducers were cycloheximide or etoposide. In Gy cells depleted of polyamines by DFMO, polyamine replenishment with either spermidine or spermine was sufficient to restore caspase activity induced by etoposide, indicating that, in this model, polyamines have an interchangeable role in supporting caspase activation. Therefore, spermine is not required for such activation, and the effect and specificity of polyamine depletion on caspase activity may be very different, depending on the role of polyamines in the specific death pathways engaged by different stimuli. Some inducers of apoptosis, for example etoposide, absolutely require polyamines for caspase activation, yet the lack of polyamines, particularly spermine, strongly increases caspase activation when induced by UV irradiation.

Spermine oxidation leads to necrosis with plasma membrane phosphatidylserine redistribution in mouse leukemia cells

Oxidation by copper/quinone-containing serum amine oxidases (SAO) is a well-known cause of polyamine cytotoxicity. Spermine oxidation exerts potent immunosuppressive effects in animal cells, but the cell death mechanism involved remains unclear. We compared biochemical and morphological parameters of SAO-mediated cell death in L1210 mouse leukemia cells with normal or amplified ornithine decarboxylase gene expression with those observed during apoptosis induced by deregulated polyamine uptake or by okadaic acid. None of the characteristic features of apoptotic cell death (e.g., chromatin condensation, nuclear fragmentation, internucleosomal DNA cleavage, poly(ADP-ribose) polymerase cleavage) were observed during spermine oxidation-mediated cell death, which was clearly necrotic by morphological criteria. Inhibition of a wide spectrum of caspases did not prevent SAO-dependent cell death, whereas N-acetylcysteine completely abolished the cytotoxic effects of spermine oxidation. Catalase only delayed spermine oxidation-induced cell death without affecting its modality or preventing depletion of intracellular glutathione, suggesting that both H(2)O(2) and aminoaldehydes generated by SAO-mediated spermine oxidation contribute to SAO-induced necrosis. Interestingly, redistribution of phosphatidylserine to the outer leaflet of the plasma membrane, usually a diagnostic feature of apoptosis, preceded necrotic cytolysis triggered by spermine oxidation. Thus, L1210 cell death caused by SAO-mediated spermine oxidation has all the attributes of primary necrosis, but is also accompanied by loss of phospholipid asymmetry, indicating that the latter phenomenon may not be unique to apoptosis. Phosphatidylserine exposure, a potent engulfment signal for phagocytes, might contribute to the immunosuppressive effects of plasma polyamines through a controlled and rapid necrotic process involving SAO.

Polyamine depletion delays apoptosis of rat intestinal epithelial cells

The polyamines spermidine, spermine, and their precursor putrescine are essential for cell growth and the regulation of the cell cycle. Recent studies suggest that excessive accumulation of polyamines favors either malignant transformation or apoptosis, depending on the cell type and the stimulus. This study examines the involvement of polyamines in the induction of apoptosis by the DNA topoisomerase I inhibitor, camptothecin. In IEC-6 cells, camptothecin induced apoptosis within 6 h, accompanied by detachment of cells. Detached cells showed DNA laddering and caspase 3 induction, characteristic features of apoptosis. Depletion of putrescine, spermidine, and spermine by DL-alpha-difluoromethylornithine (DFMO), a specific inhibitor of ornithine decarboxylase (ODC) that is the first rate-limiting enzyme for polyamine biosynthesis, decreased the apoptotic index. Delayed apoptosis was accompanied by a decrease in caspase 3 activity in polyamine-depleted cells. Addition of putrescine restored the induction of apoptosis as indicated by an increase in the number of detached cells and caspase 3 activity. Polyamine depletion did not change the level of caspase 3 protein. Inhibition of S-adenosylmethionine decarboxylase by a specific inhibitor [diethylglyoxal bis-(guanylhydrazone); DEGBG] led to depletion of spermidine and spermine with a significant accumulation of putrescine and induction of ODC. The DEGBG-treated cells showed an increase in apoptosis, suggesting the importance of putrescine in the apoptotic process. Addition of putrescine to DFMO-treated cell extracts did not increase caspase 3 activity. The above results indicate that polyamine depletion delays the onset of apoptosis in IEC-6 cells and confers protection against DNA damaging agents, suggesting that polyamines might be involved in the caspase activating signal cascade.

Involvement of polyamines in apoptosis. Facts and controversies: effectors or protectors?

The natural polyamines (putrescine, spermidine and spermine) are ubiquitous low-molecular aliphatic amines that play multifunctional roles in cell growth and differentiation. Recently, evidence has merging that polyamines are actively involved in cell death. Changes in polyamine homeostasis have been reported during cell death of nerve cells, in programmed cell death of embryonic cells and in various in vitro models of apoptosis. Polyamines and many of their structural analogs exert cytotoxic effects in vitro as well in vivo. Furthermore, polyamine analogs and inhibitors of the polyamine anabolic/catabolic pathways modulate processes of cell death in a cell-type specific way. Much ambiguity exists in the working mechanisms by which polyamines mediate apoptosis since they have been shown to act as promoting, modulating or protective agents in apoptosis. Nevertheless, from the studies reviewed here it can be concluded that polyamines are critically involved in cellular survival which makes them suitable targets for therapeutic intervention that is specifically directed to cell death pathways.

Polyamine oxidase and tissue transglutaminase activation in rat small intestine by polyamines

Polyamine degradation was studied in the small intestine from rats fed on a polyamine-supplemented diet. Lactalbumin diet was given to Hooded-Lister rats, with or without 5 mg rat(-1) day(-1) of putrescine or spermidine for 5 days. Polyamine oxidase activity increased with putrescine and spermidine in the diet, whereas spermidine/spermine N(1)-acetyltransferase and diamine oxidase activities were unchanged. We also studied the calcium-dependent and -independent tissue transglutaminase activities, since they can modulate intestinal polyamine levels. Both types of enzymes increased in the cytosolic fraction after putrescine (about 65%) or spermidine (80-100%). Our results indicate that exogenous polyamines stimulate intestinal polyamine oxidase and tissue transglutaminase activities, probably to prevent polyamine accumulation, when other pathways of polyamine catabolism (acetylation and terminal catabolism) are not activated.

Coordinate changes of polyamine metabolism regulatory proteins during the cell cycle of normal human dermal fibroblasts

In human dermal fibroblasts, brought to quiescence (G0) by serum starvation, the S phase peaked 24 h and G2/M phases 36 h after serum re-addition. Under the same conditions, ornithine decarboxylase mRNA peaked at 12 h, decreased markedly in S phase and remained low until 48 h. Conversely, ornithine decarboxylase antizyme transcript dropped to its lowest level at 12 h, while reaching its highest values between 24 and 48 h. Ornithine decarboxylase activity followed essentially the pattern of its mRNA, but relative changes were much greater. S-Adenosylmethionine decarboxylase transcript and enzyme activity also peaked at around 12 h, decreasing thereafter. Spermidine/spermine N1-acetyltransferase mRNA and activity reached the highest values at 36-48 h. Putrescine concentration increased up to 18 h and fell dramatically in the S phase, remaining low thereafter. Both spermidine and spermine reached peaks at 18 h and decreased in the S phase, but not nearly as much as putrescine. We discuss how this comprehensive study may help to understand the involvement of polyamines in the control of cell proliferation.

Spermine causes caspase activation in leukaemia cells

Exposure of several leukaemia cell types to the polyamine spermine triggered caspase activation. In HL60 cells, the onset of caspase activity correlated with the accumulation of spermine, and was accompanied by the processing of the caspase-3 precursor and the digestion of the substrate proteins PARP and gelsolin. Spermine also induced the accumulation of cytochrome c in the cytosol. Caspase activation triggered by spermine was not blocked by antioxidants or inhibition of polyamine oxidase. The deregulation of polyamine uptake strongly sensitised the cells to spermine-induced caspase activation. These data show that an excessive intracellular level of spermine triggers caspase activation that is not mediated by oxidative mechanisms, and suggest a model where elevated free cytosolic polyamines may act as transducers of a death message.

Mammalian cell polyamine homeostasis is altered by the radioprotector WR1065

Mammalian cells become more susceptible to radiation-induced death and mutagenesis when restricted in their production of the natural polyamines putrescine, spermidine and spermine. The effects of polyamine deprivation are reversed by N-(2-mercaptoethyl)-1, 3-diaminopropane (WR1065), a simple aminothiol that has been extensively studied for its radioprotectant properties. Because this compound and its oxidized derivative WR33278 bear some resemblance to the polyamines, it was hypothesized that radioprotection by WR1065 or its metabolites is derived, at least in part, from their ability to supplement the natural polyamines. To evaluate the ability of these aminothiol compounds to emulate polyamine function in intact cells, rat liver hepatoma (HTC) cells were treated with radioprotective doses of WR1065; the ability of this compound to affect various aspects of normal polyamine metabolism was monitored. Although cellular WR1065 was maintained at levels exceeding those of the polyamines, this aminothiol did not have any polyamine-like effect on the initial polyamine biosynthetic enzyme, ornithine decarboxylase, or on polyamine degradative reactions. On the contrary, treatment with relatively low levels of WR1065 resulted in an unexpected increase in putrescine and spermidine synthesis. WR1065 treatment enhanced the stability, and consequently the activity, of ornithine decarboxylase. This stabilization seems to result from a WR1065-induced delay in the synthesis of antizyme, a critical regulatory protein required in the feedback modulation of polyamine synthesis and transport. The increase in cellular spermidine induced by WR1065 might explain its antimutagenic properties, but is probably not a factor in protection against cell killing by radiation. This is the first evidence that compounds can be designed to control polyamine levels by targeting the activity of the regulatory protein antizyme.

Excess putrescine accumulation inhibits the formation of modified eukaryotic initiation factor 5A (eIF-5A) and induces apoptosis

DH23A cells, an alpha-difluoromethylornithine-resistant variant of the parental hepatoma tissue culture cells, express high levels of stable ornithine decarboxylase. Aberrantly high expression of ornithine decarboxylase results in a large accumulation of endogenous putrescine and increased apoptosis in DH23A cells when alpha-difluoromethylornithine is removed from the culture. Treatment of DH23A cells with exogenous putrescine in the presence of alpha-difluoromethylornithine mimics the effect of drug removal, suggesting that putrescine is a causative agent or trigger of apoptosis. Accumulation of excess intracellular putrescine inhibits the formation of hypusine in vivo, a reaction that proceeds by the transfer of the butylamine moiety of spermidine to a lysine residue in eukaryotic initiation factor 5A (eIF-5A). Treatment of DH23A cells with diaminoheptane, a competitive inhibitor of the post-translational modification of eIF-5A, causes both the suppression of eIF-5A modification in vivo and induction of apoptosis. These data support the hypothesis that rapid degradation of ornithine decarboxylase is a protective mechanism to avoid cell toxicity from putrescine accumulation. Further, these data suggest that suppression of modified eIF-5A formation is one mechanism by which cells may be induced to undergo apoptosis.

Rapid induction of apoptosis by deregulated uptake of polyamine analogues

Treatment of Chinese hamster ovary cells with alpha-difluoromethylornithine for 3 days, followed by exposure to cycloheximide, led to an unregulated, rapid and massive accumulation of polyamine analogues. This accumulation led to cell death by apoptosis within a few hours. Clear evidence of DNA fragmentation was seen in response to both N-terminally ethylated polyamines and to polyamines containing methyl groups on the terminal carbon atoms. Programmed cell death was induced within 2-4 h of exposure to 1 microM or higher concentrations of N1,N11-bis(ethyl)norspermine. The presence of cycloheximide increased the uptake of the polyamine analogues and therefore led to cell death at lower analogue concentrations, but it was not essential for the induction of apoptosis, since similar effects were seen when the protein synthesis inhibitor was omitted and the concentration of N1, N11-bis(ethyl)norspermine was increased to 5 microM or more. The induction of apoptosis was blocked both by the addition of the caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone, or by the addition of the polyamine oxidase inhibitor N1-methyl-N2-(2,3-butadienyl)butane-1,4-diamine (MDL 72,527). These experiments provide evidence to support the concepts that: (1) polyamines or their oxidation products may be initiators of programmed cell death; (2) regulation of polyamine biosynthesis and uptake prevents the accumulation of toxic levels of polyamines; and (3) the anti-neoplastic effects of bis(ethyl) polyamine analogues may be due to the induction of apoptosis in sensitive tumour cells.

Identification of regulatory region of antizyme necessary for the negative regulation of polyamine transport

Antizyme is a negative regulator of ornithine decarboxylase (ODC) and of polyamine transport. Regions of antizyme necessary for the negative regulation of polyamine transport were determined by transfecting ODC-overproducing EXOD-1 cells with mutant antizyme genes containing different size deletions in the NH2- and COOH-terminal of antizyme (AZ69-227). When peptide 119-144 or peptide 211-216, which are responsible for the binding of ODC, were deleted from antizyme, the mutant antizyme could not reverse the inhibition of growth of EXOD-1 cells produced by spermine. In parallel with the decrease in antizyme effect on cell growth, spermine transport activity and the accumulation of spermine in EXOD-1 cells were not significantly altered by the mutant antizyme, whereas wild-type antizyme decreased spermine transport and accumulation. When the peptide 69-118, which is responsible for the degradation of ODC, was deleted from antizyme, the mutant antizyme showed a smaller effect compared with the normal antizyme in terms of the inhibition of spermine transport and the recovery from the spermine inhibition of cell growth. The results indicate that regions 119-144 and 211-216 in antizyme are necessary for the negative regulation of polyamine transport and that these regions overlap with ODC binding sites.

Loss of intracellular putrescine pool-size regulation induces apoptosis

Synthesis and uptake are two important regulated mechanisms by which eukaryotic cells maintain polyamine levels. The role that loss of synthesis and/or uptake regulation plays in mediating putrescine toxicity was investigated by comparing toxicity in an ornithine decarboxylase (ODC)-deficient Chinese hamster ovary cell line (C55.7) with a functional putrescine transport system and an ODC-overproducing rat hepatoma cell line (DH23b), which are transport regulation deficient. When C55.7 cells were transfected with either mouse ODC (M) or trypanosome ODC (Tb), intracellular putrescine content increased slightly in C55.7(Tb-ODC), compared to C55.7(M-ODC), due to the lack of response of Tb-ODC to polyamine regulation. The increase in putrescine content resulting from loss of ODC regulation had no impact on cell growth and viability. When the feedback repression of polyamine uptake was blocked with cycloheximide, C55.7 cells transfected with either ODC construct accumulated very high levels of putrescine from the medium, and underwent apoptosis in a putrescine dose-dependent manner. A similar correlation of deregulated putrescine uptake and increased apoptotic cells was observed in DH23b cells. These data demonstrate that loss of feedback regulation on the polyamine transport system, but not ODC activity, is sufficient to induce apoptosis. Thus, downregulation of the transport system is necessary to prevent accumulation of cytotoxic putrescine levels in rodent cells.

Feedback repression of ornithine decarboxylase synthesis mediated by antizyme

The induction of antizyme by spermidine and the resulting enhancement of ornithine decarboxylase (ODC) degradation have been well studied; however, little is known about the mechanism whereby elevated spermidine levels decrease synthesis of the polyamine biosynthetic enzyme. To evaluate the relative contribution of inhibited synthesis, as distinct from enhanced degradation of ODC, spermidine levels were manipulated in a variant cell line that overproduces a stable form of ODC. Spermidine did not selectively inhibit ODC synthesis in these variant cells, supporting the concept that spermidine diminishes ODC synthesis in normal cells owing to enhanced degradation of the protein in the presence of elevated antizyme levels. This model was further investigated in vitro by use of rabbit reticulocyte lysate, which catalyses simultaneous ODC mRNA translation and antizyme-stimulated degradation of ODC protein. Antizyme strongly repressed the incorporation of labelled amino acids into normal rat ODC. Unexpectedly it also diminished the apparent translation of ODC mRNA species coding for enzyme forms that are not destabilized by the post-translational addition of antizyme. The effect of antizyme on ODC translation was not observed in wheatgerm extract, in which there is no antizyme-induced degradation. Further, deletion of a short segment of antizyme necessary for the destabilization of ODC (amino acid residues 113-118) resulted in a form that bound ODC but did not diminish its apparent translation. These results suggest that the co-translational addition of antizyme to ODC results in a complex that is different from, and innately less stable than, that formed when antizyme is added post-translationally.

Ornithine decarboxylase stability in HMOA and DH23b cells is not due to post-translational truncation of a C-terminal recognition site

The normally labile ornithine decarboxylase (ODC) becomes unusually stable when Cys-441 is replaced with Trp in the variant cell lines HMOA and DH23b. This stable ODC is also observed to have higher mobility on SDS/PAGE. Because previous studies have shown that ODC stability can be achieved when as few as five amino acid residues are removed from its C-terminus, it was suggested that the amino acid substitution in the variant ODC might alter its conformation sufficiently to promote a similar proteolytic loss of a C-terminal degradation signal, resulting in a stable yet active ODC. To examine this mechanism, amino acids in the C-terminal regions of both wild-type and stable (Trp-441) ODC proteins were released, by means of carboxypeptidase-Y digestion, and identified by HPLC. The C-terminal ends were found to be the same, and are as predicted from the cDNA sequence. This study proves that stability of the Trp-441 form of ODC is not simply due to proteolytic removal of a C-terminal proteasome-targeting sequence, thereby implying that the stabilization of this mutant ODC form must result directly from a conformational change associated with the loss of Cys-441.

Polyamine transport in mammalian cells. An update

The uptake and release of the natural polyamines putrescine, spermidine and spermine by mammalian cells are integral parts of the systems that regulate the intracellular concentrations of these biogenic amines according to needs. Although a general feature of all tissues, polyamine uptake into intestinal mucosa cells is perhaps the most obvious polyamine transport pathway of physiological and pathophysiological importance. Mutant cell lines lacking the ability to take up polyamines from the environment are capable of releasing polyamines. This indicates that uptake and release are functions of two different transport systems. The isolation of a transporter gene from a mammalian cell line is still lacking. Overaccumulation of polyamines is controlled by release and by a feedback regulation system that involves de novo synthesis of antizyme, a well known protein that also regulates the activity of ornithine decarboxylase. Recent work has demonstrated that Ca(2+)-signalling pathways are also involved. Although there is consensus about the importance of polyamine uptake inhibitors in the treatment of neoplastic disorders, a practically useful uptake inhibitor is still missing. However, the attempts to target tumours, and to increase the selectivity of cytotoxic agents by combining them with the polyamine structure, are promising. New, less toxic and more selective anticancer drugs can be expected from this approach.

Overproduction of stable ornithine decarboxylase and antizyme in the difluoromethylornithine-resistant cell line DH23b

DH23b cells, a variant of the HTC line selected for their resistance to difluoromethylornithine, exhibit defective feedback regulation of ornithine decarboxylase (ODC) stability and polyamine transport, and accumulate ODC protein to > 1000 times normal concentrations. The components of the polyamine feedback regulation system have been examined in an attempt to understand these unusual responses. Southern-blot analysis revealed an amplification (approx. 10-fold) in ODC DNA sequence without any concomitant increase in antizyme. Moreover, the amplified ODC sequence contains a single base substitution that results in the conversion of Cys-441 into Trp. This modification has previously been shown to cause ODC stability in HMOA cells. Although antizyme activity has not been noted in DH23b cells, Western-blot analysis revealed the accumulation of antizyme protein to > 50 times that induced in parental HTC cells. This increase is consistent with a 6-9-fold increase in the half-life of antizyme in these cells, a consequence of the inability of the mutant ODC-antizyme complex to be degraded by 26 S proteasome. Associated with the stabilization of antizyme in both DH23b and HMOA cells is the appearance of two additional forms of antizyme protein with apparent molecular masses of 22 and 18.5 kDa. It is suggested that these result from proteolytic removal of discrete fragments from the N-terminal end of antizyme, perhaps an indication of an initial step in rapid antizyme turnover.

Regulation of apoptosis of interleukin 2-dependent mouse T-cell line by protein tyrosine phosphorylation and polyamines

We examined the effect of inhibitors of tyrosine kinase and tyrosine phosphatase on DNA fragmentation, protein tyrosine phosphorylation, and polyamine metabolism in the murine T-cell line CTLL-2. When cells were exposed to herbimycin A, a specific inhibitor of tyrosine kinase (Uehara et al., 1989, Biochem. Biophys. Res. Commun., 163:803-809), in the presence of interleukin 2 (IL-2), DNA was degraded into oligonucleosomal fragments in a dose-dependent fashion. Genistein, another inhibitor of tyrosine kinase (Akiyama et al., 1987, J. Biol. Chem., 262:5592-5596), had similar effects. Exposure of CTLL-2 cells to vanadate, a tyrosine phosphatase inhibitor, blocked with the DNA fragmentation induced by herbimycin A. Tyrosine phosphorylation of 55 Kd protein was inhibited by herbimycin A, and the inhibition was reduced by vanadate. Ornithine decarboxylase (ODC) activity decreased rapidly after herbimycin A was added to CTLL-2 cell cultures, while vanadate increased ODC activity. The exogenous addition of putrescine or spermine, but not that of spermidine, attenuated herbimycin A-induced DNA fragmentation. These findings suggest that phosphorylation of tyrosine residues of 55 Kd protein prevents DNA fragmentation and that polyamines are involved in regulation of apoptosis.

Induction of apoptosis by excessive polyamine accumulation in ornithine decarboxylase-overproducing L1210 cells

Deregulation of polyamine transport in L1210 cells overexpressing ornithine decarboxylase leads to a lethal accumulation of spermidine. We now provide evidence that over-accumulation of natural and synthetic polyamines, but not putrescine, rapidly induces apoptosis, as shown by hypercondensation of peripheral chromatin and internucleosomal cleavage, followed by nuclear fragmentation. Polyamine oxidation is not responsible for the apoptosis observed. Thus, abnormally high polyamine pools could be an important physiological trigger of apoptosis.

Use of aminopropyltransferase inhibitors and of non-metabolizable analogs to study polyamine regulation and function

The polyamines spermidine and spermine are essential for the growth of mammalian cells. This review describes the properties of the two aminopropyltransferases that are responsible for their biosynthesis, the synthesis and use of specific aminopropyltransferase inhibitors, and the use of analogs of the polyamines to investigate polyamine transport and function. Highly specific and potent multisubstrate adduct inhibitors of these enzymes have been synthesized while less potent inhibitors have been obtained by the synthesis of amines that bind at the active site. Studies with these inhibitors indicate that polyamines are needed for a normal rate of growth and that, although some of the functions of polyamines may be interchangeable, other functions may have a specific requirement for spermidine or spermine. Two groups of growth-promoting polyamine analogs can be distinguished: the many that are effective in short-term experiments compared to the few that can act over a prolonged period. The more stringent structural requirements for long-term growth are probably due to a need for spermidine, or a closely related analog, as a precursor of hypusine in the protein eIF-5A. Metabolically resistant polyamine analogs can be used as model substrates for studies of the polyamine transport system, which plays a critical role in maintaining normal cellular polyamine levels. The feedback regulation by high levels of polyamines that downregulates transport is essential to prevent the accumulation of polyamines at toxic levels. Such accumulation may be associated with apoptosis and, therefore, polyamine analogs are useful tools for investigating the mechanism(s) of polyamine-mediated toxicity.

Correlationships between enzymatic activity of lectins, putrescine content and chloroplast damage in Xanthoria parietina phycobionts

Lectins from the lichen Xanthoria parietina develop arginase activity. One of these lectins behaves as a secreted arginase whereas another is an endocellular enzyme. Both enzymes are glycosylated proteins differing in the occurrence of galactose instead of N-acetyl-D-glucosamine in secreted arginase. The affinity for the algal ligand (glycosylated cell wall urease) of secreted arginase is higher than that shown for the endocellular enzyme. When the lectin ligand is absent from the algal cell wall, both endocellular and secreted arginases seem to be able to enter algal cells. This uptake promotes the increase in the amount of algal putrescine, preferently as free polyamine, and the chloroplast is rapidly damaged. Induction of cell wall urease retains lectins outside the cells, on the cell wall, and chloroplast remains healthy.

Effects of the S-adenosylmethionine decarboxylase inhibitor, 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine, on cell growth and polyamine metabolism and transport in Chinese hamster ovary cell cultures

The regulation of polyamine transport and the roles of polyamine transport and synthesis in cell growth were investigated using cultured Chinese hamster ovary (CHO) cells and CHOMG cells which are mutants lacking polyamine-transport activity. Metabolically stable methylated polyamine analogues were used to measure polyamine accumulation, and the irreversible S-adenosyl-L-methionine decarboxylase inhibitor, 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (AbeAdo), was used to inhibit synthesis. Exposure to AbeAdo lead to a dose-dependent decrease in growth for both cell lines, although CHOMG cells were more sensitive. Intracellular putrescine levels were greatly increased in AbeAdo-treated CHO cells and to a lesser extent in CHOMG cells, whereas intracellular spermidine and spermine levels were substantially reduced in both. Treatment with AbeAdo increased putrescine content in the culture medium to a much greater extent in CHOMG cultures indicating that a portion of the excess putrescine synthesized in response to AbeAdo treatment is excreted, but that CHO cells salvage this putrescine whereas it is lost to CHOMG cells which cannot take up polyamines. AbeAdo treatment increased polyamine transport into CHO cells despite high intracellular putrescine, suggesting that spermidine and/or spermine, and not putrescine, are the major factors regulating transport activity. The accumulation of either 1-methylspermidine or 1,12-dimethylspermine was significantly increased by AbeAdo treatment. Accumulation was increased even further when protein synthesis was blocked by cycloheximide, indicating that a short-lived protein is involved in the regulation of polyamine uptake. In the presence of cycloheximide and AbeAdo or alpha-difluoromethylornithine, methylated polyamine derivatives accumulated to very high levels leading to cell death. These results show that the polyamine-transport system plays an important role in retaining intracellular polyamines and that down-regulation of the transport system in response to increased intracellular polyamine content is necessary to prevent accumulation of toxic levels of polyamines.

Antizyme protects against abnormal accumulation and toxicity of polyamines in ornithine decarboxylase-overproducing cells

Exposure of ornithine decarboxylase (ODC; L-ornithine carboxy-lyase, EC 4.1.1.17)-overproducing mouse FM3A cells to micromolar levels of spermine or spermidine caused abnormal accumulation and toxicity of polyamines. This was apparently due to the inefficiency of negative feedback control of polyamine transport by polyamines in ODC-overproducing cells. Since antizyme is the only protein thus far recognized that can interact with ODC, depletion of free antizyme was regarded as the reason for the abnormal accumulation of polyamines. Accordingly, ODC-overproducing cells were transfected with pMAMneoZ1 possessing rat antizyme cDNA under the control of a glucocorticoid-inducible promoter. In the transfected cells, the addition of dexamethasone caused an increase in the amount of antizyme with an apparent molecular mass of 27 kDa, a decrease in the amount of ODC, a decrease in the polyamine transport activity, and the recovery of growth inhibition or cell death. The results indicate that antizyme can regulate not only the amount of ODC but also the activity of polyamine transport.

Feedback repression of polyamine transport is mediated by antizyme in mammalian tissue-culture cells

Antizyme, a spermidine-induced protein that binds and stimulates ornithine decarboxylase degradation, is now shown also to mediate the rapid feedback inhibition of polyamine uptake into mammalian cells. Using a cell line (HZ7) transfected with truncated antizyme cDNA, and mutant ornithine decarboxylase cell lines, we demonstrate that this newly discovered action of antizyme is distinct from its role in modulating polyamine biosynthesis.

Consequences of aberrant ornithine decarboxylase regulation in rat hepatoma cells

DH23A cells, an alpha-difluoromethylornithine (DFMO)-resistant variant of rat hepatoma tissue culture cells (HTC), contain high levels of very stable ornithine decarboxylase (ODC). In the absence of DFMO, the high ODC activity results in a large accumulation of endogenous putrescine. Concomitant with the putrescine increase is a period of cytostasis and a subsequent loss of viable cells. In contrast, HTC cells with a moderate polyamine content can be maintained in exponential growth. This suggests that a moderate polyamine concentration is necessary for both optimal cell growth and survival. The cytotoxicity observed in the DH23A cells is apparently not due to byproducts of polyamine oxidation or alterations in steady state intracellular pH or free [Ca2+]. It is possible to mimic the effects of high levels of stable ODC by treatment of cells with exogenous putrescine in the presence of DFMO. This suggests that overaccumulation of putrescine is the causative agent in the observed cytotoxicity, although the mechanism is unclear. These data support the hypothesis that downregulation of ODC may be necessary to prevent accumulation of cytotoxic concentrations of the polyamines.

Correlation between the inhibition of cell growth by accumulated polyamines and the decrease of magnesium and ATP

The mechanism of the antiproliferation effect of spermidine and spermine was studied using a cell culture system of mouse FM3A cells. The addition of either 10 mM spermidine or 2 mM spermine to the growth medium containing 0.9 mM Mg2+ greatly inhibited cell growth (more than 90%). A decrease in the Mg2+ concentration to 50 microM in the growth medium, but without the polyamine addition, did not influence cell growth. However, the concentrations of spermidine and spermine necessary for the inhibition of cell growth when cells were cultured in the presence of 50 microM Mg2+ were much smaller (2 mM spermidine and 0.15 mM spermine). Nevertheless, the amount of polyamines accumulating in cells which could cause the inhibition of cell growth was almost the same, regardless of the large difference in the added polyamine concentrations. At the early stage of polyamine accumulation, the inhibition of cell growth correlated with the decrease of Mg2+ content, but not with a decrease of the ATP content. The decrease in Mg2+ content correlated well with the inhibition of macromolecular synthesis, especially protein synthesis. Thus, the inhibition of cell growth at the early stage of polyamine accumulation was thought to be due to the inactivation of ribosomes through the replacement of Mg2+ on magnesium-binding sites by polyamines. The decrease in Mg2+ content was mainly caused by the inhibition of Mg2+ transport by polyamines. At the later stage of polyamine accumulation, a decrease in ATP content was also observed. This was followed by swelling of the mitochondria, which may be a symptom of the subsequent cell death.

Regulation of polyamine transport by polyamines and polyamine analogs

Regulation of polyamine transport in murine L1210 leukemia cells was characterized in order to better understand its relationship to specific intracellular polyamines and their analogs and to quantitate the sensitivity by which it is controlled. Up-regulation of polyamine uptake was evaluated following a 48-hr treatment with a combination of biosynthetic enzyme inhibitors to deplete intracellular polyamine pools. The latter declined gradually over 48 hr and was accompanied by a steady increase in spermidine (SPD) and spermine (SPM) transport as indicated by rises in Vmax to levels approximately 4.5 times higher than control values. Restoration of individual polyamine pools during a 6-hr period following inhibitor treatment revealed that SPD and SPM uptake could not be selectively affected by specific pool changes. The effectiveness of individual polyamines in reversing inhibitor-induced stimulation of uptake was as follows: putrescine < SPD < SPM = the SPM analog, N1, N12-bis(ethyl)spermine (BESPM). In contrast to stimulation of transport, down-regulation by exogenous polyamines or analogs occurred rapidly and in response to subtle increases in intracellular pools. Following a 1-hr exposure to 10 microM BESPM, Vmax values for SPD and SPM fell by 70%, whereas the analog pool increased to only 400-500 pmol/10(6) cells--about 15-20% of the total polyamine pool (approximately 2.8 nmol/10(6) cells). SPM produced nearly identical regulatory effects on transport kinetics. Both BESPM and SPM were even more effective at down-regulating transport that had been previously stimulated four to fivefold by polyamine depletion achieved with enzyme inhibitors. A dose response with BESPM at 48 hr revealed a biphasic effect on uptake whereby concentrations of analog < 3 microM produced an increase in SPD and SPM Vmax values, whereas concentrations 3 microM and higher produced a marked suppression of these values. Cells treated with 3 microM BESPM for 2 hr and placed in analog-free medium recovered transport capability in only 3 hr. Thus, whereas stimulation of polyamine transport is a relatively insensitive and slowly responsive process that tends to parallel polyamine depletion, down-regulation of polyamine transport by exogenous polyamines and analogs and its reversal are rapidly responsive events that correlate with relatively small (i.e., 15-20%) changes in intracellular polyamine pools.

Regulation of spermidine/spermine N1-acetyltransferase by intracellular polyamine pools. Evidence for a functional role in polyamine homeostasis

Through its role in polyamine acetylation and the back-conversion pathway, spermidine/spermine N1-acetyltransferase (SSAT) has the potential to control intracellular polyamine pools by facilitating their catabolism and/or excretion. The possibility that the enzyme is subject to regulation by intracellular polyamine pools was investigated in MALME-3 human melanoma cells. Increases in intracellular polyamine pools by treatment with 3 microM exogenous spermidine or spermine for 48 h caused SSAT activity to increase 111% and 226%, respectively, and SSAT-specific mRNA to rise 19% and 66%, respectively. Decreases in polyamine pools by treatment with inhibitors of polyamine biosynthesis caused SSAT activity to decrease by 46% and mRNA to fall by 89%. Both SSAT activity and mRNA were more sensitive to changes in spermine than spermidine. The identification of a positive regulatory relationship between SSAT and intracellular polyamine pools further implicates this enzyme in a proposed model for polyamine pool homeostasis.