Longevity Extension by Phytochemicals

Phytochemicals are structurally diverse secondary metabolites synthesized by plants and also by non-pathogenic endophytic microorganisms living within plants. Phytochemicals help plants to survive environmental stresses, protect plants from microbial infections and environmental pollutants, provide them with a defense from herbivorous organisms and attract natural predators of such organisms, as well as lure pollinators and other symbiotes of these plants. In addition, many phytochemicals can extend longevity in heterotrophic organisms across phyla via evolutionarily conserved mechanisms. In this review, we discuss such mechanisms. We outline how structurally diverse phytochemicals modulate a complex network of signaling pathways that orchestrate a distinct set of longevity-defining cellular processes. This review also reflects on how the release of phytochemicals by plants into a natural ecosystem may create selective forces that drive the evolution of longevity regulation mechanisms in heterotrophic organisms inhabiting this ecosystem. We outline the most important unanswered questions and directions for future research in this vibrant and rapidly evolving field.


Introduction
Plants use a diverse set of secondary biochemical pathways not to fulfill their primary metabolic needs in energy and biosynthetic products, but to generate a number of secondary metabolites called phytochemicals [1][2][3][4][5]. Some phytochemicals are produced not by plants, but by non-pathogenic endophytic bacteria and fungi that live within the plants [6][7][8][9][10][11].
A body of evidence supports the notion that, in addition to being beneficial to survival and reproduction of the plants producing them, phytochemicals can extend longevity and/or improve health in various heterotrophic organisms [5,[13][14][15][16][17][18]21,22,27,39,[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60]. In this review, we discuss recent progress in understanding mechanisms underlying such longevity-extending and health-improving effects of phytochemicals on heterotrophic organisms across phyla. We also propose a hypothesis in which phytochemicals that have been released by plants into an ecosystem create xenohormetic, hormetic and cytostatic selective forces that may drive the evolution of longevity regulation mechanisms in heterotrophic organisms inhabiting this ecosystem. Table 1 recapitulates numerous findings on how different phytochemicals prolong longevity in various heterotrophic organisms by modulating certain cellular processes . The mechanisms by which these phytochemicals extend lifespan in organisms across phyla have begun to emerge. In this section, we discuss such mechanisms.

Phytochemicals Extend Lifespan in Evolutionarily Distant Heterotrophic Organisms by Targeting an Evolutionarily Conserved Set of Longevity-Defining Cellular Processes
C. elegans defines resistance to osmotic stress, arsenic, and pathogen infection [67,74,101]; (8) the nicotinic acetylcholine receptor EAT-2, which is essential for longevity regulation in the nematode C. elegans because it defines the rate of pharyngeal pumping in this organism [70,80,81,114]; (9) NHR-8, a non-canonical nuclear hormone receptor which is essential for longevity regulation-likely because it defines resistance to xenobiotic stress and plays essential roles in the metabolism of cholesterol, bile acids, and neutral lipids in the nematode C. elegans [72]; (10) the mitogen-activated protein kinase (MAPK) kinase MEK-1, which in the nematode C. elegans is involved in protein synthesis and stress-induced apoptosis and defines resistance to pathogen infection and heavy metals [74]; (11) the MEV-1 subunit of succinate-coenzyme Q oxidoreductase, a component of the mitochondrial electron transport chain that defines longevity of the nematode C. elegans [70,72,81,114]; (12) the histone acetyl transferase CBP-1, a transcriptional activator which is involved in mRNA processing and neurogenesis in the nematode C. elegans [70,71]; (13) TPH-1, a tryptophan hydroxlase enzyme involved in serotonin synthesis in the nematode C. elegans [104]; (14) the sirtuins Sir1 in the yeast S. cerevisiae [66], SIR-2.1 in the nematode C. elegans [67,74,106], Sir2 in the fruit fly D. melanogaster [106], and SIRT1 in mice on a high-calorie diet [42,54,60,109]-all of which define longevity by modulating numerous cellular processes; (15) the target of rapamycin complex 1 (TORC1), which in the yeasts S. cerevisiae and Sch. pombe controls cell metabolism, protein synthesis, resistance to many stresses, and autophagy [17,68,69]; (16) the nutrient-sensing protein kinases Sch9 and Gcn2, which define longevity by modulating cell cycle progression, transcription, protein synthesis, responses to various stresses, amino acid synthesis and sphingolipid synthesis in the yeast S. cerevisiae [17]; (17) cytosolic and mitochondrial superoxide dismutases Sod1 and Sod2 (respectively), both playing essential roles in longevity regulation by detoxifying the superoxide radical, modulating cellular respiration, and controlling cell response to various stresses in the yeast S. cerevisiae [17,97]; and (18) the non-selective autophagy pathway for degradation of various cellular organelles and macromolecules in the yeast S. cerevisiae, nematode C. elegans, and fruit fly D. melanogaster [111][112][113].

Yeasts
In the yeast S. cerevisiae, the changes elicited by longevity-extending phytochemicals include the following: (1) caffeine enhances transcription of genes encoding heat-shock proteins and molecular chaperones [68]; (2) cryptotanshinone reduces cellular levels of reactive oxygen species (ROS) [17]; (3) phloridzin decreases cellular levels of ROS, increases resistance to oxidative stress and superoxide dismutase activity, and activates transcription of the SOD1 (cytosolic superoxide dismutase), SOD2 (mitochondrial superoxide dismutase) and SIR2 (sirtuin) genes [97]; (4) quercetin reduces cellular levels of ROS, the efficiencies of glutathione oxidation and lipid peroxidation, the extent of protein carbonylation, and cell susceptibility to oxidative stress [98]; (5) resveratrol decreases the frequency of rDNA recombination [66]; and (6) spermidine reduces activities of histone acetyltransferases, increases the extent of histone H3 deacetylation, activates transcription of many autophagy-related genes, induces autophagy and delays onset of age-related necrotic cell death [111] (Table 1). In the yeast Sch. pombe, caffeine decelerates growth, causes cell-cycle arrest in G2, alters transcription of many nuclear genes, attenuates protein synthesis and inhibits phosphorylation of ribosomal S6 proteins [69] (Table 1).

The Nematode C. elegans
In the nematode C. elegans, longevity-extending phytochemicals cause the following changes: (1) caffeic and rosmarinic acids decrease susceptibility to thermal stress, reduce oxidative damage to macromolecules, lower body size, alter lipid metabolism and delay reproductive timing [67]; (2) caffeine delays the onset of paralysis and reduces protein aggregation in nematode models of Alzheimer's and Huntington's diseases [70,71]; (3) catechin lowers body length, reduces susceptibility to thermal stress, and elevates pumping rate [72]; (4) curcumin and tetrahydrocurcumin decrease cellular levels of ROS, the extent of oxidative damage to macromolecules, susceptibility to oxidative and thermal stresses, body length, and pumping rate [74]; (5) diallyl trisulfide alters expression of many nuclear genes involved in metabolism and stress response [80]; (6) ellagic acid delays the beginning of egg deposition and lowers the extent of oxidative damage to water-soluble metabolites [81]; (7) epigallocatechin gallate lowers cellular levels of ROS, reduces susceptibility to oxidative stress, decreases the extent of oxidative damage to lipids, attenuates expression of nuclear genes encoding HSP-16, enhances nuclear import of the transcription factor DAF-16/FOXO, and mitigates the formation of Aβ deposits [82,83]; (8) ferulsinaic acid reduces susceptibility to oxidative and thermal stresses, lowers the extent of oxidative damage to lipids, and slows down the formation of advanced glycation end products [85]; (9) fisetin decreases cellular levels of ROS, lowers susceptibility to oxidative stress, reduces the extent of oxidative damage to macromolecules and stimulates nuclear import of the transcription factor DAF-16/FOXO [86]; (10) gallic acid increases body length, delays the beginning of egg deposition, and reduces the extent of oxidative damage to water-soluble metabolites [81]; (11) glaucarubinone increases the rate of oxygen consumption and reduces cellular levels of neutral lipids [87]; (12) icariin and icariside II lower susceptibility to oxidative and thermal stresses, decelerate age-related decline in locomotion, delay the onset of paralysis elicited by the proteotoxicity of polyQ and Aβ , and stimulate transcription of the SOD-3 and HSP-12.3 genes [89]; (13) kaempferol lowers cellular levels of ROS, reduces susceptibility to oxidative stress, decreases the extent of oxidative damage to macromolecules, and accelerates nuclear import of the transcription factor DAF-16/FOXO [86]; (14) myricetin decreases cellular levels of ROS, lowers the extent of oxidative damage to proteins, stimulates nuclear import of the transcription factor DAF-16/FOXO and enhances transcription of the SOD-3 gene [90]; (15) quercetin lowers cellular levels of ROS, decreases the extent of oxidative damage to macromolecules, elevates anti-oxidative activities, reduces susceptibility to thermal and oxidative stresses, reduces cellular levels of neutral lipids, and stimulates nuclear import of the transcription factor DAF-16/FOXO [67,[99][100][101]; (16) reserpine decreases susceptibility to thermal stress, decelerates the age-related declines in locomotion and pharyngeal pumping, and delays postembryonic development [104,105]; in the nematode model of Alzheimer's disease it also postpones the onset of paralysis caused by the proteotoxicity of Aβ [105]; (17) resveratrol and spermidine induce autophagy [110,111]; (18) tannic acid decreases body length, lowers susceptibility to thermal and oxidative stresses, reduces cellular levels of triglycerides, and enhances anti-oxidant capacity [70,81,114]; and (19) tyrosol lowers susceptibility to thermal and oxidative stresses, decelerates the onset of an age-related decline in pharyngeal pumping, and stimulates transcription of nuclear genes encoding several heat-shock proteins [115] (Table 1).

The Fruit Fly D. melanogaster
In the fruit fly D. melanogaster, the alterations caused by longevity-extending phytochemicals include the following: (1) curcumin and tetrahydrocurcumin lower the extent of macromolecular oxidative damage, reduce susceptibility to oxidative stress, and improve locomotor performance [75][76][77]; (2) nordihydroguaiaretic acid decreases the rate of oxygen consumption [94]; and (3) spermidine lowers susceptibility to oxidative stress, induces autophagy, decelerates age-related decline of locomotor activity, increases cellular levels of triglycerides, and alters relative levels of fatty acid species and phospholipid classes [112,113] (Table 1).

The Fish Nothobranchius Furzeri
In the naturally short-lived fish N. furzeri, resveratrol delays age-related decay of locomotor activity and cognitive performances [107]. This phenolic phytochemical is also known to reduce neurofibrillary degeneration in the brain of N. furzeri [107] (Table 1).

Laboratory Mouse
In laboratory mice (including transgenic mice models of several age-related diseases and mice on a high-calorie diet), longevity-extending phytochemicals elicit the following changes: (1) allicin improves memory retention and acquisition in senescence-accelerated mice models [62][63][64][65]; (2) celastrol decelerates weight loss, improves motor performance, increases the number of neurons and delays the onset of amyotrophic lateral sclerosis (ALS) in a transgenic mouse model of ALS [73]; (3) crocin increases hemoglobin and lymphocytes, and decreases white blood cell count and neutrophils in Dalton's lymphoma ascites-bearing mice [78]; (4) epicatechin reduces degeneration of aortic vessels and fat deposition, decreases hydropic degeneration in the liver and markers of systematic inflammation, lowers levels of serum LDL cholesterol and circulating insulin-like growth factor 1, improves skeletal muscle stress output, increases concentration of hepatic glutathione and total superoxide dismutase activity, and elevates AMP-activated protein kinase activity in diabetic mice [84]; (5) nordihydroguaiaretic acid reduces motor dysfunction in a transgenic mouse model of ALS [91]; and (6) resveratrol increases insulin sensitivity, stimulates activities of AMP-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor-gamma coactivator 1α (PGC-1α), lowers levels of insulin-like growth factor-1 (IGF-I), elevates the number of mitochondria, and alters transcription of many nuclear genes in mice on a high-calorie diet [109] (Table 1).

Phytochemicals: Interspecies Chemical Signals That May Contribute to the Evolution of Longevity Regulation Mechanisms within Natural Ecosystems
Findings summarized in the previous section provided the comprehensive evidence that many phytochemicals can extend lifespans of heterotrophic organisms across phyla via evolutionarily conserved mechanisms. These findings gave rise to a hypothesis on how such lifespan-extending capabilities of phytochemicals may contribute to the evolution of longevity regulation mechanisms in various organisms inhabiting a natural ecosystem. In this section, we discuss and extend this hypothesis.

The "Xenohormesis" Hypothesis
The term "hormesis" has been introduced to define a special kind of response of cells and organisms to different doses of a stress agent. In this kind of stress response: (1) an exposure of a cell or an organism to low ("hormetic") doses of a stress agent stimulates its growth, proliferation and/or survival; whereas (2) high doses of the same stress agent exhibit adverse effects on growth, proliferation and/or survival of this cell or organism [134][135][136][137]. Graphically, hormetic stress response is defined by a nonlinear and biphasic dose-response curve, which could be U-shaped, inverted U-shaped or J-shaped [44,57,138,139]. It is commonly accepted that an exposure of a cell or an organism to low doses of a hormetic stress agent elicits certain adaptive changes; by preconditioning the cell or the organism to a moderate stress, such changes can help to protect it against higher doses of the same (or related) stress agent [44,57,[138][139][140].
The xenohormesis hypothesis posits that plants synthesize phytochemicals, in part, as a response to such hormetic environmental stresses as UV light, heat and cold stresses, osmotic stress and high salinity, water deficit and dehydration, nutrient deprivation, and infection [66,141,142]. Within the plant that synthesizes such phytochemicals, these secondary metabolites present at the concentrations that are not toxic but create a mild stress. Thus, within the host plant, such phytochemicals function as hormetic stress agents capable of inducing certain defense systems; these systems protect the plant against higher doses of the environmental stress that caused the synthesis of the phytochemicals and, possibly, against related environmental stresses [66,141,142]. According to the xenohormesis hypothesis, after the plant has released the phytochemicals into the ecosystem, these secondary metabolites (1) do not have any hormetic effect on the heterotrophic organisms inhabiting the ecosystem (and, thus, act as hormetic stress agents only within the host plant)-likely because the concentration of the phytochemicals outside the plant is below a hormetic threshold; (2) provide the heterotrophic organisms within the ecosystem with chemically encoded information on various changes in the environmental conditions taking place in the ecosystem; and (3) operate as interspecies chemical signals that can extend lifespans of various heterotrophic organisms within the ecosystem via evolutionarily conserved mechanisms, as described in the previous section [66,141,142]. The xenohormesis hypothesis postulates that the ability of heterotrophic organisms to remodel their metabolism and physiology in response to phytochemicals as messages on environmental changes within the ecosystem will increase their chances of survival, thus creating selective forces aimed at maintaining such ability [66,141,142]. Because many of these phytochemicals are also known to extend longevity in heterotrophic organisms across phyla by targeting evolutionarily conserved mechanisms, these selective forces may provide a mode of selection for the most efficient longevity regulation mechanisms [66,141,142]. Thus, phytochemicals may function as xenohormetic stress signals that drive the ecosystemic evolution of such mechanisms in heterotrophic organisms.

Many Observations Contradict the Xenohormesis Hypothesis
One of the predictions of the xenohormesis hypothesis is that phytochemicals do not act as hormetic stress agents in heterotrophic organisms inhabiting the ecosystem [66,141,142]. However, recent findings imply that the dose-response effect of some phytochemicals on the longevity of heterotrophic organisms can be graphically described as an inverted U-shaped curve; these phytochemicals include caffeic acid, caffeine, ellagic acid, epigallocatechin gallate, quercetin, rosmarinic acid, and tannic acid [43,44,67,71,81,141]. Thus, at least caffeic acid, caffeine, ellagic acid, epigallocatechin gallate, quercetin, rosmarinic acid, and tannic acid can elicit a hormetic stress response in heterotrophic organisms; these phytochemicals are likely to operate as hormetic stress agents within both the host plants synthesizing them and heterotrophic organisms exposed to them.
Furthermore, some phytochemicals exhibit moderate cytostatic effects in heterotrophic organisms across phyla. These phytochemicals include resveratrol and caffeine; they both act as cytostatic agents that retard cellular and organismal growth because of their abilities to attenuate the pro-aging TOR signaling pathway, a key driver of proliferative growth in heterotrophic organisms [68,126,[143][144][145][146][147][148][149].
Moreover, some phytochemicals are not secondary metabolites that exist at low concentrations and are synthesized in secondary biochemical pathways, but primary metabolites that are relatively abundant and synthesized in primary biochemical pathways-one example of such phytochemicals is unsaturated fatty acids synthesized by plants in response to cold stress [13,150,151]. After being consumed by mammals, these unsaturated fatty acids are incorporated into cellular membranes in substantial quantities, thereby increasing membrane fluidity; this, in turn, elicits a longevity-extending process of heat shock response aimed at enhancing such age-delaying process as proteostasis maintenance [13,152,153].

An Extended Hypothesis on the Role of Phytochemicals in the Ecosystemic Evolution of Longevity Regulation Mechanisms
Because of the above contradictions to some projections of the xenohormesis hypothesis, we extend it by proposing a hypothesis in which phytochemicals that have been released by plants into a natural ecosystem may create xenohormetic, hormetic and cytostatic selective forces driving the evolution of longevity regulation mechanisms in heterotrophic organisms within this ecosystem. Our extended hypothesis posits that phytochemicals creating such selective forces after being released by plants into the ecosystem: (1) are either low-abundance chemical compounds formed by plants in secondary biochemical pathways or high-abundance products of primary biochemical pathways; (2) do not act as hormetic stress agents in heterotrophic organisms inhabiting the ecosystem or, alternatively, can trigger hormetic stress response in these organisms; (3) do not slow down growth of heterotrophic organisms within the ecosystem or, alternatively, cause a moderate delay of such growth by attenuating the pro-aging TOR signaling pathway; and (4) can increase lifespans of heterotrophic organisms inhabiting the ecosystem by modulating several evolutionarily conserved signaling pathways regulating longevity in these organisms; these signaling pathways and mechanisms of their modulation by phytochemicals have been described in the previous section.

Conclusions and Future Perspectives
Growing evidence supports the view that phytochemicals prolong lifespan in heterotrophic organisms across phyla by modulating a complex network of evolutionarily conserved signaling pathways; these pathways orchestrate a compendium of longevity-defining cellular processes that have been conserved in the course of evolution. The molecular and cellular mechanisms by which structurally diverse phytochemicals extend longevity of various heterotrophic organisms have emerged. Based on these findings, a hypothesis has been proposed on how the release of phytochemicals by plants into a natural ecosystem may create selective forces that guide the evolution of longevity regulation mechanisms in heterotrophic organisms inhabiting this ecosystem. The major challenge now is to empirically validate this hypothesis, perhaps by conducting experimental evolution of longevity regulation mechanisms in naturally short-lived heterotrophic organisms (such as the yeast S. cerevisiae, nematode C. elegans, fruit fly D. melanogaster, and fish N. furzeri) exposed to phytochemicals under laboratory conditions that mimic the natural stressful environment of cyclical starvation. It will be interesting to see if such long-term exposure of these organisms to phytochemicals can lead to selection of species that live longer than their predecessors. If such laboratory-evolved long-lived species can be selected, it will be intriguing to measure the relative fitness of each of them in a direct competition assay with their relatively short-lived ancestors. These experiments will provide an empirical validation test of the numerous evolutionary theories of aging trying to explain how the evolutionary force actively limits organismal lifespan at an age unique to each species [154][155][156][157][158][159][160][161][162][163].