Polyamines are polycations that interact with negatively charged molecules such as DNA, RNA and proteins. They play multiple roles in cell growth, survival and proliferation. Changes in polyamine levels have been associated with aging and diseases. Their levels decline continuously with age and polyamine (spermidine or high-polyamine diet) supplementation increases life span in model organisms. Polyamines have also been involved in stress resistance. On the other hand, polyamines are increased in cancer cells and are a target for potential chemotherapeutic agents. In this review, we bring together these various results and draw a picture of the state of our knowledge on the roles of polyamines in aging, stress and diseases.

There are also two exporters (PotE and CadB), uptaking polyamines at neutral pH and excreting them at acidic pH. Finally, a spermidine excretion protein, MdtII was recently identified [9]. S. cerevisiae polyamine transport is energy-dependent and regulated mainly by phosphorylation and dephosphorylation. The proteins DUR3 and SAM3, and to a lesser extent GAP1 and AGP2, are responsible for polyamine uptake across the plasma membrane. Putrescine can be taken into the vacuole by the 4- aminobutyric acid transporter UGA4. TPO1 to 4 excrete polyamines at acidic pH (at which yeast cells usually grow) but uptake them into the yeast cell at pH 8. A polyamine preferential excretion protein, TPO5, has been identified on Golgi and post-Golgi vesicles. Recently, Teixeira et al. [10] showed that the gene QDR3, coding for a plasma membrane drug: H+ antiporter, is involved in resistance to spermine and spermidine, but not putrescine in yeast. Δqdr3 yeast cells grew less when plated on food containing spermidine or spermine at high concentrations. They accumulated more spermidine, suggesting it is likely involved in polyamine excretion.In mammals, the TATA-binding associated factor 7 (TAF7) rescues the lack of polyamine transport in methylglyoxal bis (guanylhydrazone) Resistant Chinese hamster ovary cells [11]. Antizyme, the protein responsible for ODC inhibition and degradation also enhanced polyamines and acetylpolyamines excretion. Finally, a diamine transporter has been identified in colon epithelial cells, which could be responsible for putrescine as well as acetylated polyamines excretion [12]. 

The same authors reported that the total polyamine level increased but no difference in free polyamines was observed in Prunus persica. Despite these correlative changes in polyamines with age, it is only recently that the causative involvement of polyamines in the aging process has been investigated. The study of the effects of polyamines on aging seems to be more advanced in plant biology, where they have been identified as “juvenility” factors for some time. For instance, Serafini-Fracassini et al. [19] showed that spermine treatment delayed senescence in excised flowers of Nicotiana tabacum. After 48 hours, 70% of the flowers were still at the stage at which they had been cut (anthesis or peak bloom) whereas in the controls, 50% were at an early senescence stage and the other 50% senescent. Spermine delayed DNA degradation and preserved chlorophyll content. Spermidine and putrescine also delayed senescence of excised flowers, but to a lesser extent than spermine. More recently, the same group [20] studied senescence in Lactuca sativa. In cut leaves, spermine postponed chlorophyll loss, as observed in N. tabacum, probably via chlorophyll stabilization by a higher transglutaminase activity. In intact plants, the same preservation of chlorophyll and higher transglutaminase activity were observed after spermine spraying, especially in senescent plants. In contrast, the treatment did not affect the protein content decrease observed with age.

Transgenic animal models, particularly rodents, have been designed to modulate the activity of the polyamine pathway. However, these models have rarely been used in the context of studying aging. Suppola et al. [21] reported the creation of a mouse model overexpressing both ODC and SSAT under the methallothionein I promoter. These mice accumulated high levels of putrescine and exhibited a depletion of spermine and spermidine. They showed no overt organ-specific histopathological changes, but permanently lost their hair at 8 to 9 weeks of age. This hair loss was already observed in single transgenics for SSAT overexpression, which also exhibited extensive wrinkling upon aging. Finally, the double transgenic mice were very short-lived. Cerrada-Gimenez et al. [22] also reported a decreased life span in mice overexpressing SSAT. They noticed that in these mice, p53 expression in the liver was increased and that the SSAT overexpressing mice exhibited similar aging phenotypes to mice with activated p53 expression. However, it is difficult to really know if such phenotypes reflect an acceleration of the aging process or whether they reflect a general disturbance of the organism physiology leading to general weakness. Another strategy to study the effects of polyamines in aging is an exogenous administration to organisms. When polyamines are provided with food or water, their endogenous levels increase. 

For instance, Soda et al. [23] observed an increase in spermine after 26 weeks in mice and after two months in humans under a highpolyamine diet. We also reported [4] that providing spermidine in food or water increased its endogenous levels in yeast, flies, and mouse liver. This is thus a promising strategy, particularly valuable in the context of extending polyamine use to humans. Using such an experimental approach to modulate polyamine levels, Soda et al. [24] fed male mice a low, normal or highpolyamine chow. They showed that mortality in mice fed a high-polyamine chow was lower in the first 88 weeks. Unfortunately, the mice were sacrificed at 88 weeks of age, precluding the gathering of mortality data after that age. The authors also reported a lower incidence of age-related kidney glomerular atrophy kept on high-polyamine diet. Finally, they observed that old mice on high-polyamine chow kept a thicker coat with age and appeared more active. However, these last two observations were only qualitative as coat thickness and activity were not measured. The lower mortality in mice fed a high-polyamine chow was not due to a dietary restriction effect as these mice ate more than the other two groups and exhibited similar body mass measured between 11 and 55 weeks of age. These results are promising and further studies in rodents are urgently required. We have recently published a study following the consequences of external spermidine administration in various model organisms, including yeast, worms, flies, mice and human cells [4]. 

Polyamines are particularly important for adaptation and resistance to cold stress [reviewed in 25] and polyamine levels increase in plants during abiotic stress such as salinity, extreme temperature, paraquat or heavy metals. This regulation of polyamines under stress is achieved by differential expression of polyamine biosynthesis enzymes, such as arginine decarboxylase, spermidine and spermine synthase or AdoMetDC. Exogenous application of polyamines led to, in varying degrees, preserved membrane integrity and lower growth inhibition during stress, reduced accumulation of ROS and increased activity of antioxidant enzymes such as catalase. In contrast, polyamine synthesis inhibitors triggered decreased stress resistance, a phenotype counteracted by the simultaneous treatment with polyamines. Mutants in arginine decarboxylase or spermine synthase are sensitive to stress. Using another approach to study the role of polyamines in stress resistance, many groups have engineered plants to overexpress polyamine biosynthesis genes, especially arginine and ornithine decarboxylases and AdoMetDC. Many studies are described in [2] and only a few, published later are reported here. Mohapatra et al. [26] studied aluminum stress in interaction with calcium in poplar cell cultures with normal or high levels of putrescine. Reducing or increasing the amount of calcium had no effect on growth in the control cells except when drastically reduced, but respectively decreased and increased growth in cells with high levels of putrescine. 

The addition of aluminum with normal or low calcium again had no effect in the control cells. In contrast, the cells with high putrescine content exhibited a better growth. These latter had a lower mitochondrial activity than control cells, but a higher mitochondrial activity after 48 hours of exposure to aluminum. Taken together, these results suggest that in these cells, a high putrescine content is a disadvantage in a low-calcium environment but an advantage in the presence of aluminum. Wang et al. [27] cloned the arginine decarboxylase gene from trifoliate orange and carried out its functional study in transgenic Arabidopsis. The transgenic plants had higher levels of putrescine but not of spermidine and spermine. They displayed a lower stomatal density, longer roots, larger cell size and higher relative water content than the controls both in normal conditions and under stress. They had similar germination to the controls in normal conditions and germinated better under osmotic stress. During dehydration, the transgenic plants lost less water, had a better electrolyte leakage and leaf turgor. Upon sustained dehydration, they exhibited better growth, survival and electrolyte leakage. The transgenic plants were also more resistant to cold. This higher stress resistance might be due to lower accumulation of H2O2, O2 - and lipid peroxidation. Finally, A. thaliana overexpressing spermine synthase or wild-type exogenously supplied with spermine were shown to be more resistant to infection with Pseudomonas viridiflava.

 In contrast, mutants in spermine synthase with low levels of spermine were less resistant to the same infection [28]. In model organisms, the effect of polyamines on stress resistance has not been so widely studied, but as in plants, has shown various results. SSAT overexpressing mice exhibited an increased production of H2O2 from polyamine degradation, a higher carbonyl content, and a decreased expression of superoxide dismutase (SOD), catalase and CYP450 2E1, suggesting these mice may be more prone to stress [22]. However, these mice were more resistant to thioacetamide or carbon tetrachloride, because these compounds need to be metabolized before becoming toxic, and were not metabolized to the same degree in the transgenic mice. Kaasinen et al. [29] had earlier shown that SSAT overexpressing mice exhibited decreased spontaneous activity, climbing and wheel running behavior. The transgenic mice were more phlegmatic and less aggressive. They had increased levels of ACTH and corticosterone, and decreased levels of TSH and T4, again suggesting they may be more prone to the effects of some stress. Putrescine, spermidine and spermine decreased survival under hypoxia in D. melanogaster [30]. In contrast, we have shown [4] that spermidine treatment increased resistance to heat and H2O2 in yeast. It also decreased age-related oxidative stress in mice as measured by higher levels of free thiol groups. Markers of oxidative stress were also reduced in yeast. From these studies, we can conclude that polyamines are involved in stress resistance but in a much more complex way than just allowing a general higher resistance to stress when present at higher levels.