Longevity

Almost all life forms constantly sit on a balance between production and maintenance, and under low nutrient conditions when reproduction is more challenging, in order to ensure reproductive success, increasing somatic maintenance is necessary to prolong the reproductively competent period and consequently, lifespan.5 Hence, calorie restriction (CR) without malnutrition is one of the most reliable approaches in extending both lifespan and healthspan in various vertebrate and non-vertebrate species. However, CR is difficult to sustain and implement since individuals must remain in a state of hunger and endure feelings of starvation, fatigue, and irritations. Besides, individuals who practiced CR are more susceptible to viral infections6 and resistant to wound-healing7, both of which impede its widespread use. Alternatively, metformin and rapamycin can act as calorie-restriction mimetics (CRM) by triggering the nutrient sensing pathways that sense and respond to the changing intracellular and extracellular energy and nutrient levels without actually restricting calorie intake.8
Rapamycin and metformin target respectively the mechanistic target of rapamycin (mTOR) and 5’-AMP-activated protein kinase (AMPK) (Fig.1). The mTOR comprises of two complexes, mTORC1 and mTORC2, and they coordinate a wide range of cellular metabolic processes concerning production, growth, and somatic maintenance, such as protein synthesis, mitochondrial function, and cell proliferation. Activated mTORC1 enhances mRNA translation and protein synthesis in the cell by phosphorylating the p70 ribosomal protein S6 kinase (S6K) and eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1);9 it also suppresses autophagy by phosphorylating ULK1, ULK2, and ATG13 of the ULK complex and the transcription factor EB, both of which essential for the autophagy process.10 Not surprisingly, mounting studies have shown that deregulated mTOR signaling is implicated in the aging process and the progression of age-related disease such as cancer and diabetes.10,11 Rapamycin suppresses mTOR signaling by first binding to its immunophilin FK binding protein (FKBP12) and then acting upon mTORC1 and mTORC2.12 While inhibiting mTORC1 extends life expectancy and confers protection for age-related diseases, inhibiting mTORC2 is associated with unwanted effects such as glucose intolerance and abnormal lipid profiles. Nevertheless, mTORC2 is less sensitive to rapamycin and its inhibition can only be achieved through long-term treatment.13
AMPK is the upstream controller of the mTOR signaling pathway (Fig. 1). Numerous studies have indicated that the activating capacity of the AMPK signaling pathway declines with aging, and its decline disturbs autophagy, increases cellular stress, and promotes inflammation, which further provoke many age-associated diseases, such as cardiovascular disease, diabetes, and cancer.9,14 Correspondingly, increased activation of the AMPK pathway has been shown to extend lifespan in lower organisms in response to CR and pharmaceutical agents, such as metformin.15 Activated AMPK phosphorylates and activates ULK1 of the ULK complex to promote autophagy as well as activates the FOXO transcription factors that transactivate the genes involved in detoxification, autophagy, tumorigenesis suppression, and energy homeostasis.5 Furthermore, AMPK activation attenuates the aging process by inhibiting NF-κB, the major regulator of innate and adaptive immunity, and relieves ER stress and oxidative stress by promoting the expression of mitochondrial uncoupling protein (UCP-2).16
Metformin and rapamycin are also implicated in DNA methylation, an omnipresent regulatory mechanism for gene expression in our genome. DNA methylation is facilitated by DNA methyltransferases DNMT3A, DNMT3B, and DNMT1 and adds a methyl group to the 5th carbon on cytosine. DNA methylation usually leads to gene silencing by interacting with transcription mechanisms, and global hypomethylation and promoter hypermethylation are often observed in aged people.17Since DNA methylation is reversible, it is a promising target for therapeutic interventions.
Rapamycin regulates DNA methylation by inhibiting the mTOR signaling pathway to reduce serine production and glycolytic metabolism, which tune down the serine and 1C metabolism that uses serine to produce SAM, the methyl donor in the methylation reaction.18Metformin alters methylomes globally via the H19/SAHH axis. H19, a long noncoding RNA that should be downregulated in adults, causes an aberrant methylation profile by binding to and inhibiting s-adenosylhomocysteine hydrolase (SAHH), which normally hydrolyzes s-adenosylhomocysteine (SAH) and removes its inhibition of DNMT3B. Metformin activates AMPK and upregulates let-7, a family of microRNAs that bind to and degrade H19.19
Quite a few studies have corroborated the proposed life-extending effects of metformin and rapamycin. Metformin has been shown to extend healthspan and lifespan in the roundworm C. elegans (Table 1).20 Furthermore, a low dose of metformin supplemented for middle-aged male mice’s diet lead to a 5.83% extension of mean lifespan.21 A longitudinal study that compares the methylation profiles of the white blood cells from 12 healthy individuals at the beginning, 10 hours, and 7 days after metformin treatment revealed 11 consistently differentially methylated sites. By looking at the associated genes, regions, and networks, the study found several related genes including CAMKK1, a regulator of AMPK and glucose uptake, BACE2, involved in neurodegenerative disorders and insulin production, and ADAM8, which is related to monocyte adhesion and migration and contributes to disorders caused by excessive inflammation such as neurodegenerative disorders, allergy, asthma, and acute lung inflammation. As the cells were from healthy individuals, having ADAM8 in the result shows that metformin’s anti-inflammatory effect is independent of diabetic status.22
Treating yeast with rapamycin resulted in extended lifespan in a process that has been postulated to mimic CR.23 Heterogeneous male mice that received a daily dosage of 2.24 mg of rapamycin per kg of body weight beginning at the age of 20 months had 9% extended lifespan while female mice had 14%.24 In another mice study, the Intervention Testing Program (ITP), genetically outbred mice were used to test the potential of multiple anti-aging manipulations including drugs, diets, and other interventions, and rapamycin was one of the only two drugs that had robust anti-aging effects.25 Mice treated with rapamycin at 42mg/kg from 4-month to 22-month old had a 6-month decrease on average in epigenetic aging compared with control mice.26 Moreover, healthy participants in a double blind randomized study aged 65 and older who had taken Everolimus, also a mTOR inhibitor, for 6 weeks then stopped for 2 weeks before a flu shot was given to them had 20% stronger immune responses compared with the control, suggesting that a low dose of rapamycin may delay immunesenescence in the elderly instead of suppressing their immune system.27