Ordered cell cycle phase perturbations in Chinese hamster ovary cells treated with an S-adenosylmethionine decarboxylase inhibitor

Altered gene expression in CHO cells following polyamine starvation

AIM

To investigate the impact of polyamine deprivation on the transcriptome of CHO cells RESULTS: Polyamines play a central but poorly-understood role in cell proliferation. Most studies to date have utilised chemical inhibitors to probe polyamine function. Here we exploit the fact that CHO cells grown in serum-free medium have an absolute requirement for putrescine supplementation due to their deficiency in activity of the enzyme arginase. A gene expression microarray (Affymetrix) analysis of CHO-K1 cells starved of polyamines for 3 days showed that cessation of growth, associated with increased G1/S transition and inhibition of M/G1 transition was accompanied by increased mRNA levels of mitotic complex checkpoint genes (Mad2l1, Tkk, Bub1b) and in the transition of G1- to S-phase (such as Skp2 and Tfdp1). mRNAs associated with DNA homologous recombination and repair (including Fanconi's anaemia-related genes) and with RNA splicing were consistently increased. Alterations in mRNA levels for genes related to protein processing in the ER, to ER stress, and to p53-related and apoptosis pathways were also observed. mRNAs showing highest levels of fold-change included several which code for membrane-localised proteins and receptors (Thbs1, Tfrc1, Ackr3, Extl1).

CONCLUSIONS

Growth-arrest induced by polyamine deprivation was associated with significant alterations in levels of mRNAs associated with cell cycle progression, DNA repair, RNA splicing, ER trafficking and membrane signalling as well as p53 and apoptosis-related pathways.

Norspermidine and novel Pd(II) and Pt(II) polynuclear complexes of norspermidine as potential antineoplastic agents against breast cancer

Background

New strategies are needed for breast cancer treatment and one initial step is to test new chemotherapeutic drugs in breast cancer cell lines, to choose candidates for further studies towards clinical use.

Methodology and Findings

The cytotoxic effects of a biogenic polyamine analogue – norspermidine – and its trinuclear Pd(II) and Pt(II) complexes – Pd₃NSpd₂ and Pt₃NSpd₂, respectively – were investigated in one immortalized normal-like and three breast cancer cell lines. The normal-like MCF-10A cells were least sensitive to the compounds, while growth inhibition and cell death was observed in the cancer cell lines. Norspermidine and its Pd(II) complex were generally shown to have stronger antiproliferative effects than the corresponding Pt(II) complex. Moreover, both norspermidine and the Pd(II) complex reduced the cellular activity of the growth-related enzyme, ornithine decarboxylase (ODC) to a lower level than the Pt(II) complex in most of the cell lines examined. Treatment with norspermidine or the Pd(II) complex reduced the number of colonies formed in a soft agar assay performed with the breast cancer cell lines, indicating that these compounds reduced the malignancy of the breast cancer cells. The effect of norspermidine or the Pd(II) complex on colony formation was much stronger than that observed for the Pt(II) complex. The results from a new mammalian genotoxicity screen together with those of a single cell gel electrophoresis assay indicated that none of the drugs were genotoxic at a 25 µM concentration.

Main Conclusions

Overall, norspermidine and its Pd(II) complex were shown to have strong antiproliferative effects. In comparison, the effects obtained with the Pd(II) complex were much stronger than that of the Pt(II) complex. The results obtained in the present study demonstrate that the trinuclear Pd(II) complex of norspermidine (Pd₃NSpd₂) may be regarded as a potential new metal-based drug against breast cancer, coupling a significant efficiency to a low toxicity.

Cells and polyamines do it cyclically

Cell-cycle progression is a one-way journey where the cell grows in size to be able to divide into two equally sized daughter cells. The cell cycle is divided into distinct consecutive phases defined as G(1) (first gap), S (synthesis), G(2) (second gap) and M (mitosis). A non-proliferating cell, which has retained the ability to enter the cell cycle when it receives appropriate signals, is in G(0) phase, and cycling cells that do not receive proper signals leave the cell cycle from G(1) into G(0). One of the major events of the cell cycle is the duplication of DNA during S-phase. A group of molecules that are important for proper cell-cycle progression is the polyamines. Polyamine biosynthesis occurs cyclically during the cell cycle with peaks in activity in conjunction with the G(1)/S transition and at the end of S-phase and during G(2)-phase. The negative regulator of polyamine biosynthesis, antizyme, shows an inverse activity compared with the polyamine biosynthetic activity. The levels of the polyamines, putrescine, spermidine and spermine, double during the cell cycle and show a certain degree of cyclic variation in accordance with the biosynthetic activity. When cells in G(0)/G(1) -phase are seeded in the presence of compounds that prevent the cell-cycle-related increases in the polyamine pools, the S-phase of the first cell cycle is prolonged, whereas the other phases are initially unaffected. The results point to an important role for polyamines with regard to the ability of the cell to attain optimal rates of DNA replication.

Effect of polyamine deficiency on proteins involved in Okazaki fragment maturation

Polyamine depletion causes S phase prolongation, and earlier studies indicate that the elongation step of DNA replication is affected. This led us to investigate the effects of polyamine depletion on enzymes crucial for Okazaki fragment maturation in the two breast cancer cell lines MCF-7 and L56Br-C1. In MCF-7 cells, treatment with N(1),N(11)-diethylnorspermine (DENSPM) causes S phase prolongation. In L56Br-C1 cells the prolongation is followed by massive apoptosis. In the present study we show that L56Br-C1 cells have substantially lower basal expressions of two Okazaki fragment maturation key proteins, DNA ligase I and FEN1, than MCF-7 cells. Thus, these two proteins might be promising markers for prediction of polyamine depletion sensitivity, something that can be useful for cancer treatment with polyamine analogues. DENSPM treatment affects the cellular distribution of FEN1 in L56Br-C1 cells, but not in MCF-7 cells, implying that FEN1 is affected by or involved in DENSPM-induced apoptosis.

Importance of polyamines in cell cycle kinetics as studied in a transgenic system

Polyamines are organic cations, which are considered essential for normal cell cycle progression. This view is based on results from numerous studies using a variety of enzyme inhibitors or polyamine analogues interfering with either the metabolism or the physiological functions of the polyamines. However, the presence of non-specific effects may be hard to rule out in such studies. In the present study, we have for the first time used a transgenic cell system to analyze the importance of polyamines in cell growth. We have earlier shown that expression of trypanosomal ODC in an ODC-deficient variant of CHO cells (C55.7) supported growth of these otherwise polyamine auxotrophic cells. However, one of the transgenic cell lines grew much slower than the others. As shown in the present study, the level of ODC activity was much lower in these cells, and that was reflected in a reduction of cellular polyamine levels. Analysis of cell cycle kinetics revealed that reduction of growth was correlated to prolongation of the G1, S, and G2+M phases in the cells. Providing exogenous putrescine to the cells resulted in a normalization of polyamine levels as well as cell cycle kinetics indicating a causal relationship.

Differential effects of ferulic acid and p-coumaric acid on S phase distribution and length of S phase in the human colonic cell line Caco-2

Ferulic acid (FA) and para-coumaric acid (p-CA) may mediate the protective effects of whole-grain cereals against colon cancer. Therefore, the effects of FA and p-CA on the metabolic activity, proliferation, cell cycle phase distribution, and kinetics of the colonic endothelial tumor cell line Caco-2 was studied. Both compounds at 1500 microM decreased the number of cells to 43-75% of control after 2-3 days of treatment. Cell cycle phase distribution and cell cycle kinetics were determined by flow cytometric analysis after bromodeoxyuridine labeling. Each compound at 1500 microM decreased the proportion of cells in the G(1) phase and increased the proportion of cells in the S and G(2) phases. Treatment with 1500 microM FA significantly increased the length of the S phase, while p-CA did not. It was concluded that FA and p-CA inhibited cell proliferation by presumably affecting different cell cycle phases, and this warrants further investigations because this inhibition may be one explanation for the diet-related protection against cancer.

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.

Analysis of polyamines as markers of (patho)physiological conditions

The aliphatic polyamines, putrescine, spermidine and spermine, are normal cell constituents that play important roles in cell proliferation and differentiation. The equilibrium between cellular uptake and release and the balanced activities of biosynthetic and catabolic enzymes of polyamines are essential for normal homeostasis in the proliferation and functions of cells and tissues. However, the intracellular polyamine content increases in hyperplastic or neoplastic growth. Although the involvement of polyamines in physiological and pathological cell proliferation and differentiation has been well established, the role they play is quite different in relation to cell systems and animal models and is dependent on inducer agents and stimuli. Also, the experimental procedures used to deplete polyamines have been shown to influence the cell responses. In this paper, the assay methods currently in use for polyamines are reviewed and compared with respect to sensitivity, reproducibility and applicability to routine analysis. The relevance of polyamine metabolism and the uptake/release process in many physiological and pathological processes is highlighted, and the cellular polyamine pathways are discussed in relation to the possible diagnostic and therapeutic significance of these mediators.

Absolute requirement of spermidine for growth and cell cycle progression of fission yeast (Schizosaccharomyces pombe)

Schizosaccharomyces pombe cells that cannot synthesize spermidine or spermine because of a deletion-insertion in the gene coding for S-adenosylmethionine decarboxylase (Deltaspe2) have an absolute requirement for spermidine for growth. Flow cytometry studies show that in the absence of spermidine an overall delay of the cell cycle progression occurs with some accumulation of cells in the G(1) phase; as little as 10(-6) M spermidine is sufficient to maintain normal cell cycle distribution and normal growth. Morphologically some of the spermidine-deprived cells become spherical at an early stage with little evidence of cell division. On further incubation in the spermidine-deprived medium, growth occurs in most of the cells, not by cell division but rather by cell elongation, with an abnormal distribution of the actin cytoskeleton, DNA (4', 6-diamidino-2-phenylindole staining), and calcofluor-staining moieties. More prolonged incubation in the spermidine-deficient medium leads to profound morphological changes including nuclear degeneration.

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

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

The organization of replicon clusters is not affected by polyamine depletion

Earlier investigations have shown that polyamine depletion affects DNA replication negatively. DNA is synthesized in replicons which are gathered in replicon clusters. DNA replication is initiated simultaneously in every replicon of a replicon cluster. By pulse labeling cells with the thymidine analog bromodeoxyuridine and then detecting bromodeoxyuridine in situ with immunofluorescence, replicon clusters can be studied. We have used this method to investigate the effects of 2-difluoromethylornithine (DFMO)- and 4-amidinoindan-1-one 2'-amidinohydrazone (CGP 48664)-mediated polyamine depletion on the organization of replicon clusters. The cells were studied by fluorescence microscopy and confocal laser scanning microscopy. Our studies give at hand that neither the number nor the distribution of replicon clusters were affected even after 4 days of treatment with 5 mM DFMO or 20 microM CGP 48664, indicating that polyamine depletion did not affect the organization of replicon clusters. However, the fluorescence intensity of the replicon clusters was much lower in inhibitor-treated cells. The results indicate that the impaired DNA replication observed in polyamine-depleted cells is not due to an effect on the initiation step of DNA replication, but rather on the elongation process. To confirm that it is possible to observe changes in the organization of replicon clusters using bromodeoxyuridine, we treated the cells with various drugs that affect DNA replication. Aphidicolin, which inhibits DNA elongation, gave results similar to those of DFMO and CGP 48664.

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.