Natural Products in the Promotion of Healthspan and Longevity

Human longevity dramatically increased over the past century. According to World Health Organization (WHO), the number of people over 60-years-of-age will increase from 12% to 22% of the total global population between 2015 and 2050. The aging pace of the population is remarkable, and it has not been accompanied by an equivalent increase in healthy life expectancy [1]. Therefore, age has become the greatest risk factor for all major age related pathologies [2]. In recent years, it has been demonstrated that the molecular mechanisms underlying aging appear to be interconnected and affect the same pathways as those responsible for diseases such as cancer, cardiovascular disease and neurodegeneration. Therefore, there has been a great deal of attention toward identifying molecular mechanisms (markers) that will allow discoveries of therapeutic solutions (interventions) promoting healthy aging and increasing longevity.

During the last years, several health-promoting interventions have been reported as potentially being effective against age-related diseases. Caloric restriction (CR) is, to date, the most successful intervention for increasing health span and longevity in all model organisms including rodents [3, 4] and non-human primates [5], and it is suspected to favor healthy aging in humans as well [6–8]. CR has been defined as the reduction of energy intake in the absence of malnutrition. In addition to CR, a variety of eating patterns has been shown to have similar beneficial effects on health and longevity. These therapeutic fasting include intermittent fasting (IF) or periodic fasting (PF) or among others. CR and IF may have a number of beneficial effects on health including improved cardiovascular health, reduced inflammation, improved mitochondrial function, reduced insulin resistance and delayed the onset and progression of age-related metabolic diseases. One key mechanism responsible for many of these beneficial effects appears to be the induction of autophagy. In fact, CR or IF are effective autophagy inducers in human tissues, and this has multiple anti-aging effects as it promotes efficient quality control of organelles, supports optimal stem cell activity, improves immunological functions and inhibits malignant transformation [9]. Interestingly, one of the best correlations between anti-aging effects of CR and health span in humans has been reported in a centenarian population in Okinawa, Japan. Okinawans are a genetically distinct group with several specific gene variants, some of which may be related to longevity. However, it is also known that Okinawan elders have reduced caloric intake compared to the rest of the Japanese population and have been found to have low mortality from aging-related diseases [10]. Although the majority of the population is aware of the fact that reduction in caloric intake confers beneficial effects on health, adherence to continuous calorie restricted diets has been poor. Hence, search for effective natural or pharmacological bio-compounds with anti-aging properties has been intensified lately. Caloric restriction mimetics (CRM) are pharmacological compounds that mimic the main biochemical properties of CR, inducing autophagy by reduction of protein acetylation without provoke a sizeable weight loss [11]. Some compounds considered CRMs include rapamycin (an mTOR inhibitor), resveratrol (which stimulates SIRT1 (sirtuin1 expression) or metformin (an AMPK activator), among others. All of them are potent autophagy inducers [9].

Activation of autophagy is one of the convergent mechanism that links aging and nutrient availability [12, 13]. Autophagy is a highly conserved catabolic process that plays a key role in the maintenance of cellular and organismal homeostasis by facilitating the turnover of cytoplasmic structures, allowing cells to adapt to changing and stressful conditions [12,13]. Three different types of autophagy-microautophagy, macroautophagy and chaperone mediated autophagy (CMA)-contribute to degradation of intracellular components in lysosomes in mammalian cells. Macroautophagy (to which we refer to as “autophagy”) is the best-characterized type and consists of sequestration of portions of the cytoplasm (cytosol and organelles) within autophagosomes for their subsequent degradation by lysosomal enzymes [13]. Autophagy occurs at low basal levels in virtually all nucleated cells, permitting homeostatic functions such as protein and organelle turnover. It is rapidly upregulated when cells require to generate intracellular nutrients and energy, for example, during dietary restriction, growth factor withdrawal, or high bioenergetics demands. Autophagy is also upregulated when cells rid themselves of damaging cytoplasmic components, such as during oxidative stress, infection or protein aggregate accumulation. Genetic studies in yeast have identified more than 30 ATG genes that are required for autophagy, most of them are conserved from yeast to mammals. These ATG gene products are organized into several multiprotein complexes that regulate different stages of autophagic process [14].

CR triggers autophagic responses through nutrient sensors such as SIRT1, AMPK and mTORC1 [9]. One of the key regulators of autophagy is TOR kinase, which is the major signal inhibitor that shuts off autophagy in the presence of growth factors and nutrient availability [15]. mTOR is activated by signal transducers, including class I phosphatidylinositol-3-kinases (PI3K)/Akt, insulin-like growth factor (IGF) and other growth signals, and is inhibited by AMP-activated protein kinase (AMPK), among others. Indeed, CR and CRMs inhibit mTOR, and such inhibition by rapalogs, such as rapamycin, extends lifespan in worms, flies and mice [16, 17]. Of note, while inhibiting mTOR, CR and CRM activate AMPK and SIRT1. AMPK is activated as a result of changed ATP/ADP ratios, while the deacetylase SIRT1 activity is induced in response to changes in NADH/NAD+ ratios [18, 19]. Once autophagy is activated, class III Vps34/PI3-kinase is involved in the phagophore membrane nucleation via production of phosphatidylinositol 3-phosphate (PI(3)P). Beclin-1/Atg6 is an allosteric activator of Vps34, which is part of a dynamic multiprotein complex consisting of Vps34, Beclin-1 and Vps150 proteins. The molecular composition of this complex is altered in response to activators (e.g. UVRAG, Bif-1/endophilin B1, Ambra1) and/or inhibitors (e.g. Bcl2, Rubicon) [20, 21]. Phagophore elongation depends on two ubiquitin-like conjugation systems, the Atg5-Atg12 and the Atg8/LC3 complexes [22, 23]. Finally, mature autophagosomes fused with lysosomes to form autophagolysosomes, and its contents are degraded for recycling.

It has been presumed that all longevity extending interventions, including the nutritional, pharmacological or genetic manipulations, must induce autophagy to be efficient [11, 24, 25]. Indeed, impaired autophagy is directly involved in all major age-related human diseases including cancer [2], cardiovascular disease [26], neurodegeneration [27]. Thus, induction of autophagy has broad cytoprotective effects [13]. In fact, autophagy protects cells against otherwise lethal damage in vitro, favors the intracellular maintenance of high ATP levels, increases the capacity of cells to withstand metabolic stress (hypoxia and nutrient deprivation), facilitates genomic stability, reduces the abundance of potentially toxic proteins or damaged organelles responsible for neurodegeneration, and increases the lifespan of different animal species in vivo [11, 28–30]. This applies to nematodes (Caenorhabditis elegans), fruit flies (Drosophila melanogaster) or mice treated with CR or pharmacological autophagy inducers such as rapamycin, resveratrol or spermidine [11, 16]. When autophagy is inhibited by genetic manipulations, the life span-extending effect (antiaging effect) of these autophagy inducers is lost, indicating that autophagy is indeed one of the main positive longevity-determining factors.

Based on these premises, it is not surprising that aging research is searching for novel autophagy inducers to test their beneficial effects on health. Natural bee products such as honey, propolis and royal Jelly (RJ) have attracted increased attention due to their many biological and pharmacological properties. RJ has been reported to possess diverse health-beneficial properties as well as to extend the lifespan of nematodes, flies and mice. Most studies with RJ suggest that many of its components might be responsible for its beneficial effects. Royal jelly consists of water (50%−60%), proteins (18%), carbohydrates (15%), lipids (3%–6%), mineral salts (1.5%), and vitamins. Around the 40% of lipid fraction is composed by queen bee acid being the RJ the only know source of this compound. [31]. Several studies have reported the beneficial effects of queen bee acid in vitro, including antitumor activity, anti-inflammatory activity and antiangiogenic activity [32–34], although a clear mechanism of action has yet to be described. It has been shown that queen bee acid is an inhibitor of histone deacetylase, which is known to play a key role in epigenetic switch for the queen bee larvae phenotype, and stimulates neurogenesis from stem cells [35, 36]. Moreover, queen bee acid has been shown to upregulate the expression and activity of AMPK in muscle cells in an AMP/ATP radio-independent manner [37]. Studies exploring queen bee acid in vivo have focused on its benefits following chronic administration. Notably, queen bee acid was shown to extend lifespan of C. elegans, not via insulin-like signaling (ILS) but rather via dietary restriction and TOR signaling [38]. Another in vivo study has shown that long-term administration of queen bee acid to rodents reduces anxiety-like behavior, promotes neuronal health and improves body composition [39]. To date, there has been no in-depth investigation into the mechanism of action of queen bee acid following chronic administration.

In summary, although the health-beneficial properties of queen bee acid are well established, the precise molecular mechanisms by which this fatty acid provides these effects have yet to be elucidated. Based on previous results, we postulate that the beneficial effects of queen bee acid are derived from distinct cellular processes, in particular autophagy and this autophagic activation, in turn, promoting health span and longevity.