Encyclopedia
Please note this is a comparison between Version 1 by Kuniyasu Soda and Version 2 by Lindsay Dong.
The relationship between polyamines and healthy longevity has received much attention in recent years. The most fundamental consideration in conducting polyamine studies is that bovine serum used for cell culture contains bovine serum amine oxidase. Bovine serum amine oxidase, which is not inactivated by heat treatment, breaks down spermine and spermidine to produce the highly toxic aldehyde acrolein, which causes cell damage and activates autophagy. Polyamine catabolism does not produce toxic aldehydes under normal conditions, but inflammation and some pathogens provoke an inducible enzyme, spermine oxidase, which only breaks down spermine to produce acrolein, resulting in cytotoxicity and the activation of autophagy. Therefore, spermine oxidase activation reduces spermine concentration and the ratio of spermine to spermidine, a featurere recently reported in patients with age-related diseases. Spermine, which is increased by a long-term, continuous high polyamine diet, suppresses aberrant gene methylation and the pro-inflammatory status that progress with age and are strongly associated with the development of several age-related diseases and senescence. Changes in spermine concentration and the spermine/spermidine ratio should be considered as indicators of human health status.
- polyamine
- spermine
- spermidine
- lifespan extension
- polyamine-rich food
- age-related disease
1. Polyamines
Natural polyamines are low-molecular-weight aliphatic polycations found in the cells of all living organisms and their intracellular concentrations are very high, ranging from micromolar to low millimolar. They are known to be essential for cell growth and differentiation and are synthesized in cells on demand. In addition to de novo intracellular synthesis, cells can take up polyamines from the environment. Polyamines (spermine and spermidine) and their precursor, putrescine, contain multiple amino groups (-NH2). The molecular weights of spermine (SPM) with four amino groups and spermidine (SPD) with three amino groups are about 140 g/mol and 200 g/mol, respectively. Putrescine (PUT) with two amino groups is a precursor of polyamines and is called a diamine. SPD and SPM are then synthesized by the sequential addition of aminopropyl groups donated by decarboxylated S-adenosylmethionine (dcSAM), which is converted from S-adenosylmethionine (SAM) by the enzymatic activities of adenosylmethionine decarboxylase (AdoMetDC). The addition of aminopropyl groups to PUT and SPD is catalyzed by spermidine synthase and spermine synthase, respectively (Figure 1).
Figure 1. Polyamine synthesis pathway. Spermidine synthase and spermine synthase catalyze the addition of aminopropyl groups to putrescine and spermidine to synthesize spermidine and spermine, respectively. Aminopropyl groups are donated by decarboxylated S-adenosylmethionine (dcSAM), which is converted from S-adenosylmethionine (SAM) by the enzymatic activities of adenosylmethionine decarboxylase (AdoMetDC).
Polyamines and putrescine are universally required for cell growth and differentiation. However, their importance has been found to vary from organism to organism, as summarized in amy previous review [1][6]. For example, PUT is essential for cell growth in lower organisms such as bacteria and fungi, whereas spermine is not present in the cell. Or, in yeast and nematodes, spermine can be found in small amounts, but it is not essential for growth. All of this suggests that these are inconsequential roles for spermine in cell growth and differentiation, as well as cell function, in lower primitive organisms. Spermine is considered to be more important in highly developed animals. For example, a decrease in the spermine levels due to a deficiency in spermine synthase (Snyder–Robinson syndrome) has serious consequences in humans [2][7].
The degradation pathway of polyamines is also elucidated (Figure 2). Spermidine/spermine N-(1)-acetyltransferase (SSAT) and N1-acetylpolyamine oxidase (APAO) are enzymes that break down SPM and SPD in cells. SSAT is a highly inducible enzyme that catalyzes the transfer of acetyl groups from acetyl-coenzyme A to the terminal amines of SPM and SPD. APAO catalyzes the oxidation of N1-acetylspermine and N1-acetylspermidine produced by SSAT activity, releasing aldehyde and hydrogen peroxide to produce SPD and PUT, respectively. The resulting aldehyde, 3-acetamidopropanal, has been shown to have no cytotoxic activity [3][8]. This degradation pathway, along with a mechanism for transporting polyamines across the cell membrane, keeps the intracellular concentration of polyamines constant.
Figure 2. Polyamine degradation pathway under normal conditions. SPM and SPD are converted to N1-acetylspermine and N1-acetylspermidine, respectively, via the enzymatic activity of SSAT. N1-acetylpolyamine oxidase (APAO) preferentially catalyzes the oxidation of N1-acetylspermine and N1-acetylspermidine to SPD and putrescine, respectively. This degradation process produces non-toxic 3-acetamidopropanal. Abbreviations: SPM, spermine; SPD, spermidine; SSAT, spermidine/spermine N-(1)-acetyltransferase; APAO, N1-acetylpolyamine oxidase.
The alternative polyamine degrading enzyme, spermine oxidase (SMO), can directly convert SPM back to SPD (Figure 3). SMO, a highly inducible enzyme, specifically oxidizes spermine. The enzymatic activity of SMO degrades spermine, producing 3-aminopropanal (3-AP) as a by-product. Produced 3-AP spontaneously deaminates to form acrolein [4][9]. Unlike the metabolite (3-acetamidopropanal) produced by the enzymatic activities of SSAT and APAO, both 3-AP and acrolein are substances with potent cytotoxic activities. In fact, SMO activation in the presence of SPM has been shown to cause severe damage to cells, supporting the strong cytotoxic activity of these two substances produced by SMO [5][6][10,11].
Figure 3. Polyamine degradation via spermine oxidase (SMO). SMO, a highly inducible enzyme expressed in macrophages and epithelial cells, directly converts SPM back to SPD. SMO degrades SPM and produces 3-AP as a by-product. Generated 3-AP is spontaneously deaminated to form acrolein. Both 3-AP and acrolein are highly toxic and exhibit cytotoxic activity. Abbreviations: SPM, spermine; SPD, spermidine; SMO, spermine oxidase; 3-AP, 3-aminopropanal.
2. The Most Basic and Important Aspects of Conducting Polyamine Research
Recently, several researchers have postulated that spermidine activates autophagy function and that its bioactivity is the mechanism through which increased polyamine intake promotes longevity [7][8][9][10][2,3,4,5]. However, these studies used medium supplemented with fetal bovine serum (FBS) to perform polyamine experiments. Importantly, ruminant serum contains a copper-containing amine oxidase called bovine serum amine oxidase (BSAO). This enzymatic activity is not observed in humans or non-ruminants.
It has been known for about 70 years that polyamines are cytotoxic when added to cell cultures mixed with FBS [11][18]. Cytotoxic substances are produced during the conversion of spermine and spermidine catalyzed by copper-containing amine oxidase in FBS, i.e., BSAO. This enzymatic activity does not disappear even in serum deactivated by heat treatment. BSAO catalyzes the oxidation of SPM and SPD to produce SPD and PUT, respectively, while simultaneously producing acrolein as a by-product (Figure 4).
Figure 4. Polyamine degradation pathway via fetal bovine serum amine oxidase. BSAO in ruminant sera such as bovine serum catalyzes the oxidation of spermine and spermidine to spermidine and putrescine, respectively. The acrolein produced in this process is very toxic and can cause cell damage and death. Abbreviation: BSAO, bovine serum amine oxidase.
This means that SPD is degraded to produce acrolein in the culture medium, but the production of acrolein via SPD degradation has not been confirmed in vivo. In addition, while SMO breaks down one molecule of SPM to produce one molecule of acrolein, BSAO converts one molecule of SPM to one molecule of acrolein and one molecule of SPD, which in turn breaks down the SPD produced to produce one molecule of acrolein. Simply put, SPM should be at least twice as cytotoxic as SPD in FBS-supplemented medium. And it has been reported that treatment with SPM in culture supernatant supplemented with BSAO is more toxic than SPD treatment [12][19]. Therefore, when cultured with SPD, the autophagic function is activated by relatively mild cytotoxic activity and the cells do not die.
At this time, when the existence of enzymes that degrade SPD to produce acrolein is unknown, it can be said that at least an examination of the biological activity of SPD in culture medium mixed with FBS containing BSAO is not an examination of the biological activity that occurs in vivo in humans. Applying the reaction that occurs under these culture conditions to polyamine activity in vivo, the phenomenon is attributed to SPM degradation due to SMO activation induced by certain pathological conditions or the presence of inflammation (Figure 5). The finding of significant age-related increases in autophagy markers in the aged kidney suggests the compensatory activation of basal autophagy in response to the increased cytotoxic activities with aging [13][20] and supports an increase in various stimuli, such as chronic inflammation, with aging and the activation of autophagy responses induced by them. Spermine and spermidine themselves do not activate autophagy, indicating that the increase in autophagy markers upon the addition of SPD to cultured cells is only observed in the presence of BSAO [12][19].
Figure 5. Similarities between cells cultured in bovine serum medium in vitro (right) and cells under cytotoxic stress such as inflammation in vivo (left). (left): BSAO in bovine serum degrades both spermine and spermidine in the culture medium to produce acrolein. Heat treatment to deactivate the serum does not eliminate the enzymatic activity of BSAO. In response to the potent cytotoxic activity of acrolein, cells have been shown to activate autophagy. Spermine and spermidine themselves have no cytotoxic activity and therefore do not activate autophagy. (right): Inflammation and certain pathogens activate SMO, and SMO exclusively breaks down spermine and produces acrolein via the production of 3-AP. The background of autophagy activation observed in cell cultures mixed with bovine serum containing BSAO is similar to the background of autophagy activation observed in cells from the elderly and patients suffering from age-related diseases induced and promoted by chronic inflammation. Abbreviations: SMO, spermine oxidase; BSAO, bovine serum amine oxidase; 3-AP, 3-aminopropanal.
Polyamines are involved in various cellular functions such as transcription, RNA modification, protein synthesis, and the regulation of enzyme activity, and exist in association with DNA, RNA, and various protein molecules. As summarized in amy previous review article [1][6], a high percentage of all polyamines are bound by ionic interactions to nucleic acids, proteins, and other negatively charged molecules in the cell. Therefore, very few polyamines are free in the body. When measuring polyamine concentrations in serum or plasma using HPLC, polyamine peaks, especially SPM peaks, can be difficult to detect due to the very low levels [14][21]. If the concentration of polyamines, especially SPM, in serum or plasma is not high, HPLC may only detect a shimmer of the baseline or no peaks at all. Therefore, determining polyamine concentrations from an uncertain peak is difficult to measure accurately, especially with respect to SPM concentrations.
3. Age-Related Changes in Polyamine Concentrations
The relationship between aging and polyamine levels has already been summarized in amy previous review article [1][6]. During fetal and developmental stages, polyamine synthase is highly activated, but its activity gradually declines with age. From this, it can be inferred that polyamine levels decrease with age. In fact, when the relationship between age and polyamine levels is examined for all age groups, including the developing years, blood polyamine levels decrease with age, mainly as noted in the titles and abstracts of the papers [15][22]. However, when measured in healthy adults who have stopped growing, polyamine levels in tissue, blood, and urine have not been found to decrease with age [1][16][17][6,23,24].
Although there is no decrease in polyamine levels in the body with age, it is noteworthy that there are large individual differences in blood polyamine levels [16][17][23,24]. It is not clear what the biological basis for the large individual differences in blood polyamine levels is. However, this large individual variation in polyamine concentrations is an aspect that makes the clinical application of polyamines difficult. It is well known that in cancer patients, polyamines, which are synthesized in large amounts by cancer cells, are transferred into the blood, resulting in elevated blood polyamine levels. Therefore, attempts have been made to diagnose the presence of cancer based on differences in polyamine levels, but large individual differences have made clinical application difficult.
