Ornithine decarboxylase as target of chemotherapy

An Undefined Interaction between Polyamines and Heat Shock Proteins Leads to Cellular Protection in Plasmodium falciparum and Proliferating Cells in Various Organisms

Environmental stimuli can distress the internal reaction of cells and their normal function. To react promptly to sudden environmental changes, a cascade of heat shock proteins (Hsps) functions to protect and act as housekeepers inside the cells. In parallel to the heat shock response, the metabolic polyamine (PA) status changes. Here, we discuss possible ways of putative interactions between Hsps and polyamines in a wide lineage of eukaryotic model organisms with a particular focus on parasitic protozoa such as Plasmodium falciparum (P. falciparum). The supposed interaction between polyamines and Hsps may protect the parasite from the sudden change in temperature during transmission from the female Anopheles mosquito to a human host. Recent experiments performed with the spermidine mimetic inhibitor 15-deoxyspergualine in Plasmodium in vitro cultures show that the drug binds to the C-terminal EEVD motif of Hsp70. This leads to inhibition of protein biosynthesis caused by prevention of eIF5A2 phosphorylation and eukaryotic initiation factor 5A (eIF5A) modification. These observations provide further evidence that PAs are involved in the regulation of protein biosynthesis of Hsps to achieve a protective effect for the parasite during transmission.

Local inhibition of ornithine decarboxylase reduces vascular stenosis in a murine model of carotid injury

OBJECTIVES

Polyamines are organic polycations playing an essential role in cell proliferation and differentiation, as well as in cell contractility, migration and apoptosis. These processes are known to contribute to restenosis, a pathophysiological process often occurring in patients submitted to revascularization procedures. We aimed to test the effect of α-difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase, on vascular cell pathophysiology in vitro and in a rat model of carotid arteriotomy-induced (re)stenosis.

METHODS

The effect of DFMO on primary rat smooth muscle cells (SMCs) and mouse microvascular bEnd.3 endothelial cells (ECs) was evaluated through the analysis of DNA synthesis, polyamine concentration, cell viability, cell cycle phase distribution and by RT-PCR targeting cyclins and genes belonging to the polyamine pathway. The effect of DFMO was then evaluated in arteriotomy-injured rat carotids through the analysis of cell proliferation and apoptosis, RT-PCR and immunohistochemical analysis of differential gene expression.

RESULTS

DFMO showed a differential effect on SMCs and on ECs, with a marked, sustained anti-proliferative effect of DFMO at 3 and 8 days of treatment on SMCs and a less pronounced, late effect on bEnd.3 ECs at 8 days of DFMO treatment. DFMO applied perivascularly in pluronic gel at arteriotomy site reduced subsequent cell proliferation and preserved smooth muscle differentiation without affecting the endothelial coverage. Lumen area in DFMO-treated carotids was 49% greater than in control arteries 4 weeks after injury.

CONCLUSIONS

Our data support the key role of polyamines in restenosis and suggest a novel therapeutic approach for this pathophysiological process.

Micronutrients and amino acids, main regulators of physiological processes

Human physiology is supposed to be a complex interaction of regulating processes, in which hormones, genes, their proteins and apoptosis are thought to play a dominant role. We hypothesize that regulation of physiological processes is mainly influenced by amino acids and micronutrients with hormones, proteins, apoptosis and gene modifications being their derivatives. Furthermore, we suppose that the cells power plant, the mitochondrion, is in fact an intracellular bacterium, living in absolute symbiosis. Because of its intracellular existence it depends on the host's micronutrients completely. Within the host these micronutrients regulate their own formation, degradation, uptake and excretion. Known deficiencies, such as iodine and vitamin D, affect billions of people. Many micronutrients neither have been investigated, nor have they been studied in relation to each other and solid data are not available. Optimal levels of many micronutrients and all amino acids are not known. Amino acids, vitamins and minerals are capable of altering gene expression, inducing apoptosis and regulating chemical processes. It makes them highly attractive for creating better health, against low cost, as we have already proven in the case of rickets, cretinism and scurvy in severe deficiencies. By creating optimal living conditions and study mitochondria from a symbiotic point of view we suppose that diseases not only can be prevented, but the course of diseases can be altered as well.

Effects of dietary polyamines and clofibrate on metabolism of polyamines in the rat

The activities of catalase, polyamine oxidase, diamine oxidase, ornithine decarboxylase, and peroxisomal beta-oxidation were assayed in homogenates from liver and small intestinal mucosa of rats which had been fed either a diet very low in polyamines or a diet containing five times the levels of dietary polyamines (putrescine, spermine, and spermidine) found in a standard rat diet. In rats fed the high polyamine diet, hepatic activities of catalase and polyamine oxidase were significantly decreased. Levels of the other activities were unchanged, except that intestinal ornithine decarboxylase was decreased. In rats treated simultaneously with clofibrate, the high polyamine diet restored activities of catalase, ornithine decarboxylase, and polyamine oxidase back to levels found in rats fed the low polyamine diet. The expected increase in activity of peroxisomal beta-oxidation was observed, although this was somewhat diminished in rats fed the high polyamine diet. Intestinal diamine oxidase activity was stimulated by clofibrate, particularly in rats fed the high polyamine diet. For the duration of the experiment (20 days), levels of putrescine, spermine, and spermidine in blood remained remarkably constant irrespective of treatment, suggesting that polyamine homeostasis is essentially independent of dietary supply of polyamines. It is suggested that intestinal absorption/metabolism of polyamines is of significance in this respect. Treatment with clofibrate appeared to alter polyamine homeostasis.

A perspective of polyamine metabolism

Polyamines are essential for the growth and function of normal cells. They interact with various macromolecules, both electrostatically and covalently and, as a consequence, have a variety of cellular effects. The complexity of polyamine metabolism and the multitude of compensatory mechanisms that are invoked to maintain polyamine homoeostasis argue that these amines are critical to cell survival. The regulation of polyamine content within cells occurs at several levels, including transcription and translation. In addition, novel features such as the +1 frameshift required for antizyme production and the rapid turnover of several of the enzymes involved in the pathway make the regulation of polyamine metabolism a fascinating subject. The link between polyamine content and human disease is unequivocal, and significant success has been obtained in the treatment of a number of parasitic infections. Targeting the polyamine pathway as a means of treating cancer has met with limited success, although the development of drugs such as DFMO (alpha-difluoromethylornithine), a rationally designed anticancer agent, has revolutionized our understanding of polyamine function in cell growth and provided 'proof of concept' that influencing polyamine metabolism and content within tumour cells will prevent tumour growth. The more recent development of the polyamine analogues has been pivotal in advancing our understanding of the necessity to deplete all three polyamines to induce apoptosis in tumour cells. The current thinking is that the polyamine inhibitors/analogues may also be useful agents in the chemoprevention of cancer and, in this area, we may yet see a revival of DFMO. The future will be in adopting a functional genomics approach to identifying polyamine-regulated genes linked to either carcinogenesis or apoptosis.

The Plasmodium falciparum bifunctional ornithine decarboxylase, S-adenosyl-L-methionine decarboxylase, enables a well balanced polyamine synthesis without domain-domain interaction

In the human malaria parasite Plasmodium falciparum (Pf), polyamines are synthesized by a bifunctional enzyme that possesses both ornithine decarboxylase (ODC) and S-adenosyl-l-methionine decarboxylase (AdoMetDC) activities. The mature enzyme consists of the heterotetrameric N-terminal AdoMetDC and the C-terminal dimeric ODC, which results in the formation of a heterotetrameric complex. For the native bifunctional protein a half-life longer than 2 h was determined, which is in contrast to the extreme short half-life of its mammalian monofunctional counterparts. The biological advantage of the plasmodial bifunctional ODC/AdoMetDC might be that the control of polyamine synthesis is achieved by only having to regulate the abundance and activity of one protein. An interesting feature in the regulation of the bifunctional protein is that putrescine inhibits PfODC activity approximately 10-fold more efficiently than the mammalian ODC activity, and in contrast to the mammalian AdoMetDC the activity of the PfAdoMetDC domain is not stimulated by the diamine. To analyze post-translational processing, polymerization, and domain-domain interactions, several mutant proteins were generated that have single mutations in either the PfODC or PfAdoMetDC domains. The exchange of amino acids essential for the activity of one domain had no effect on the enzyme activity of the other domain. Even prevention of the post-translational cleavage of the AdoMetDC domain or ODC dimerization and thus the interference with the folding of the protein hardly affected the activity of the partner domain. In addition, inhibition of the activity of the PfODC domain had no effect on the activity of the PfAdoMetDC domain and vice versa. These results demonstrate that no domain-domain interactions occur between the two enzymes of the bifunctional PfODC/AdoMetDC and that both enzymatic activities are operating as independent catalytic sites that do not affect each other.

Inhibitors of polyamine biosynthesis affect the expression of genes encoding cytoskeletal proteins

The polyamines are ubiquitous components of mammalian cells. Those compounds have been postulated to play an important role in different cellular functions including the reorganization of cytoskeleton associated with the cell cycle. In the studies reported here, it was found that inhibitors of polyamine biosynthesis, methylglyoxal-bis[quanylhydrazone] (MGBG) and difluoromethylornithine (DFMO), prevent mitogen-induced accumulation of mRNAs encoding major cytoskeletal components, beta-actin and alpha-tubulin, in mouse splenocytes. These findings suggest mechanisms through which polyamines may exert their effects on the cytoskeleton integrity.

The regulation of G0-S transition in mouse T lymphocytes by polyamines

While the role of polyamines in DNA synthesis during the S phase of the cell cycle has been repeatedly postulated, recent studies point also to polyamine involvement in the early phase of the G0-S transition. In order to determine polyamine-dependent steps in the cell cycle we have studied the effects of inhibitors of polyamine biosynthesis and exogenous polyamines on the proliferation of T lymphocytes as well as on the expression of some growth-regulated genes. The ability of Con A-stimulated mouse T lymphocytes to enter DNA synthesis was markedly inhibited by methylglyoxal bis(guanylhydrazone) in a dose-dependent manner. This inhibitory effect was stronger in the presence of fetal calf serum containing a high level of activities of polyamine oxidases than in the presence of horse serum. Putrescine and spermine added to T splenocyte culture instead of mitogen-Con A stimulated [3H]thymidine incorporation with kinetics similar to that observed with Con A. The growth-stimulating effects of polyamines were concentration-dependent. Polyamines at optimal growth-stimulating concentrations (10 microM spermine and 80 microM putrescine) induced the expression of genes encoding the cytoskeletal proteins beta-actin, vimentin, and alpha-tubulin to an extent and with kinetics similar to those of Con A. The results presented herein suggest that polyamines are capable of stimulating the transition of G0 cells to the S phase and that this effect may be mediated by their influence on the gene expression.

Molecular biology of cell activation

This article summarizes common features of activation of different types of cells along different physiological lines such as proliferation, differentiation, and execution of function of terminally differentiated cells. The common basis of many of these phenomena includes (i) first messengers (growth factors, cytokines, neurotransmitters, etc.) acting on membrane receptors, (ii) second messengers (cAMP, IP3, DAG, Ca2+) spreading an activating signal inside the cell, and (iii) elevated expression of some genes (c-fos, c-myc, ornithine decarboxylase). The role of the genetic correlate in cell activation is emphasized, and it is concluded that the aforementioned genes (their protein products) should be called third messengers, whose function is mediation of long-term phenotypic changes.

The effect of hyperthermia on ornithine decarboxylase activity in different rat tissues

Hyperthermia produced a decrease of ornithine decarboxylase activity in different tissues of adult rats. The fall in ornithine decarboxylase was dependent on time of exposure and temperature. The decay of ornithine decarboxylase activity in liver, brain, kidney, heart, spleen and testes was rather similar. The t1/2 for liver ornithine decarboxylase determined by the hyperthermic treatment (40 degrees ambient temperature) was 20 min. Ornithine decarboxylase activity was recovered in all tissues exposed to the hyperthermic shock after a period of 4 hours, although the degree of recovery was dependent on the type of tissue. The effect that hyperthermia produces on ornithine decarboxylase activity in rats could be related to an inhibition in the synthesis of active enzyme rather than to a specific degradation or inactivation of ornithine decarboxylase molecule.

Enzyme regulation as an approach to interference with polyamine biosynthesis--an alternative to enzyme inhibition

The progress reviewed here would seem to validate the regulatory approach to interference with polyamine biosynthesis as an antiproliferative strategy. To our knowledge, this is the first example, among anticancer drugs, of pharmacological intervention of a biochemical pathway based strictly on regulatory control. Several features of polyamine biology naturally favor this approach and may account for its relative success. These include (a) the nature of the regulatory mechanisms themselves, (b) the exquisite sensitivity of the pathway to regulatory control, (c) the rapid turnover of ODC and AdoMetDC, (d) the different structural specificity of ODC and AdoMetDC regulation versus growth-dependent functions, and (e) the direct dependence of growth on sustained polyamine biosynthesis. As such, the regulatory approach to interference with polyamine biosynthesis offers several advantages over the use of specific enzyme inhibitors (Table 10). Of these, perhaps, the more significant are the facts that more than one enzyme can be simultaneously and specifically suppressed and that compensatory mechanisms, which otherwise counter the effects of enzyme inhibitors (11), are not invoked. We are encouraged by the concurrence of in vitro mechanistic findings with the predictions of the hypothesis for the regulatory approach and by the in vitro and in vivo growth inhibitory effects of the analogs against murine leukemia. One disadvantage of the regulatory analogs, such as BESm, has been that, as with specific polyamine inhibitors such as DFMO, analog-induced polyamine depletion results in cytostatic growth inhibition. While this response may help to minimize host toxicities, it clearly compromises antitumor activity. An intriguing exception to this generality has recently been found among human lung carcinoma cell lines. Previously, Luk et al. (93, 94) and others (95) reported that, among a spectrum of human lung carcinoma lines, small cell carcinoma was exquisitely sensitive to the ODC inhibitor, DFMO. Not only did these cells display a cessation of growth but also an inability to survive during DFMO-induced polyamine depletion. Studies extending these findings to long term maintenance therapy in human small cell lung carcinoma implants in athymic mice revealed sustained growth inhibition of the tumor for longer than one year (96). Casero et al. (97) now find that human large cell carcinoma, which is otherwise refractory to chemotherapeutic intervention, displays a cytotoxic response in vitro to polyamine depletion induced by BES or BESm but not by DFMO.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetic study of the inhibition of rat liver ornithine decarboxylase by diamines; considerations on the mechanism of interaction between enzyme and inhibitor

1. Partially purified rat liver ornithine decarboxylase is inhibited by several diamines including putrescine, 1,3-diaminopropane, cadaverine and p-phenylenediamine. 2. The inhibition is dependent on pH, being strong at pH above 8 and negligible below pH 6.5. 3. The kinetic study of the inhibition showed that while the aromatic diamine behaved as a simple competitive inhibitor, the aliphatic diamines presented a more complex pattern of inhibition in which two molecules of inhibitor might bind to the enzyme active site. 4. The Ki values for the different inhibitors were calculated and the degree of affinity for the enzyme was p-phenylenediamine greater than putrescine greater than cadaverine greater than 1,3-diaminopropane. 5. A molecular mechanism explaining how one or two molecules of inhibitor can bind to the enzyme is proposed.