Drugs that inhibit mTORC1 are logical candidates to mimic DR as prolongevity agents. First we consider metformin. Although proposed as an activator of adenosine monophosphate-activated protein kinase (AMPK), metformin has no direct effect on it or its upstream kinase, LKB1 [62]. Through inhibition of mitochondrial function that increases AMP and/or ADP levels, metformin indirectly activates AMPK. Metformin also indirectly inhibits mTORC1 via two pathways: first by inhibiting the RagGTPase system [63], which functions in the amino acid sensing system associated with lysosomes [52, 64], and second inhibiting mTORC1 through REDD1 and p53 [65].

For 30 years beginning with phenformin, metformin and other biguanide antidiabetic drugs extend survival in models of carcinogen-induced, genetically prone, and spontaneously arising tumors, suggesting that they could possibly function as prolongevity drugs. Interestingly, chronic treatment with metformin in the drinking water extended mean and maximum life-span of outbred SHR female mice (prone to mammary carcinoma and leukemia) without any effect on the incidence of spontaneous malignant tumors [66]. Metformin alone and in combination with rapamycin is currently under study by the ITP for prolongevity effects in UM-HET3 mice. This is an important test as metformin is one of the most prescribed drugs in the world. A systematic review and meta-analysis revealed that metformin “was the only antidiabetic agent not associated with harm in patients with heart failure and diabetes” [67].

Next, we consider resveratrol, an activator of SIRT1 and one of seven mammalian sirtuins, which has been extensively investigated for its anticancer and antiaging effects (reviewed by Baur et al. [68]). Numerous studies demonstrated that resveratrol reduces mTORC1 [69–73], suggesting a possible mechanism underlying its aging and cancer effects. Importantly, resveratrol extended the life-span of mice fed with a high fat diet [68]. However, two doses of resveratrol (300 and 1200 ppm in standard diet) did not extend the life-span of UM-HET3 mice fed with a normal diet [5].

Finally, we discuss rapamycin, an obvious candidate for a direct mTORC1 inhibitor that could mimic DR and/or growth factor restriction to extend life-span. Numerous studies have now shown rapamycin prolongevity efficacy in a variety of experimental settings. In budding yeast (Saccharomyces cerevisiae) cultures, adding rapamycin produces a state resembling DR [74], resulting in a longer chronological life-span [38]. Separately or combined, rapamycin and caffeine extended chronological life-span in Schizosaccharomyces pombe (fission yeast) [75]. Rapamycin and DR extended the life-span of Drosophila melanogaster, and rapamycin also further extended the life-span of DR flies [76]. These and other data convinced Bjedov et al. [76] that mTORC1 (not mTORC2) specifically regulates aging in fruit flies. These data strongly suggest that the link between mTOR and aging has deep evolutionary roots [77].

As discussed at the outset, eRapa is the first drug formulation that extends both median and maximum life-span in both sexes in a mammal, a feat previously achieved with DR and growth factor restriction models. In addition, rapamycin also extends life-span when given as a 6-week treatment to old C57BL/6 mice [78], as subcutaneous injections to female mice carrying the tumorigenic HER-2/neu transgene [79], or to female inbred 129/Sv mice [80]. Interestingly, Neff et al. [81] found that eRapa extended the life-span of male C57BL/6 mice and performed a comprehensive examination of the antiaging effect and toxicities associated with chronic treatment. Nephrotoxicity and testicular degeneration were noted in their study. However, Zhang et al. [9] did not find nephrotoxicity in C57BL/6 mice, and Wilkinson et al. [8] also did not report nephrotoxicities in UM-HET3 mice. Thus, testicular degeneration represents the only common toxicity associated with chronic rapamycin treatment [82]. Hemizygous deletion of mTOR and mLST8 extends the life-span of female (but not male) mice in a C57BL/6 and 129S5 background [83], indicating that mTORC1 could be key to the control of aging and age-related diseases, similar to fruit flies. Finally, small mice carrying two hypomorphic alleles of mTOR [84] lived 20 % longer than wild-type controls and had reductions in several aging tissue biomarkers and preservation of some, but not all, organ system function [85]. Overall, these data strengthen the case for mTORC1 and a central regulator of aging and its associated diseases.

We posit that one effect of chronic treatment with eRapa in mice is a delay in cancer development and progression and/or an improved tolerance of their cancers. How does chronic treatment with rapamycin do this? A detailed elucidation of how rapamycin works in vivo to extend life-span and repress cancer will be as complicated and difficult to understand as DR’s mechanism, which has been intensely studied for 30 years, with many hypotheses tested and debated [86]. Our recent study of chronic rapamycin effects in a preclinical model of cancer driven by loss of the tumor suppressor, pRb1, illustrates the difficulties in understanding how DR and chronic rapamycin work in cancer prevention. We found that eRapa treatment of male and female Rb1 ^+/− mice extended their life-span by preventing or delaying growth of Rb1 ^−/− neuroendocrine tumors [87]. This result is in stark contrast to 50 % DR, which had minimal effect on life-span, tumor incidence, or multiplicity in this model [88]. These results suggest that rapamycin and DR are not epistatic and that pRb function is critical for DR but not rapamycin-induced tumor suppression. A more detailed explanation for these results awaits further study.

Our group recently reported that chronic eRapa prevented small intestinal polyps and restored a normal life and health span in Apc ^Min/+ mice [89]. Since intestinal crypt stem cells (ICSC) originate polyps in Apc ^Min/+ mice [90], and rapamycin promotes stem cell renewal [3], we postulate that direct effects of intestinally delivered rapamycin on ICSC result in the prevention of polyps in this model of familial adenomatous polyposis. We also posit that the remarkable life-span extension in Apc ^Min/+ mice by chronic rapamycin results from polyp prevention in combination with a general delay in the other mortal diseases associated with aging.

The pro-growth state (biomass accumulation for proliferation) of most cancer cells is to a large degree addicted to active mTORC1 [91], which should make most, if not all, vulnerable to growth inhibitors. mTORC1 nutrient sensing provides a key decision point between anabolic and catabolic metabolisms regardless of the situation [64], but especially in cancer cells. Laplante and Sabatini [52] provided an excellent review of the processes (e.g., ribosome biogenesis, translation of cell cycle regulators important in proliferation, antiapoptotic factors, angiogenic regulators, metastatic factors, and energy-promoting factors) that cancer cells exploit and in which mTORC1 has a regulatory role.

Translation, especially translation initiation, is an overlooked opportunity for the development of new drugs that target cancer [92] and aging [93]. Transcription on the other hand has been studied exhaustively in both fields. Until recently, little was known about how transcription and translation regulation are coordinated. Addressing this question, Santagata et al. [94] performed a detailed study to determine how malignant cells coordinate translation and transcription to maintain an anabolic state. In response to inhibition of translation, they identified heat shock transcription factor 1 (HSF1) as a key coordinator. A chemical screen for HSF1 inhibitors identified the natural product rocaglamide, which was previously known to have potent anticancer activity [95–97] and, interestingly in common with rapamycin, anti-inflammatory activity [98] and antifungal properties [99]. Importantly, rohinitib, a more potent derivative of rocaglamide, is a strong translation initiation inhibitor [94]. This study also emphasizes the crucial role that translation initiation plays in maintenance of oncogenic anabolism and the opportunities for the development of new drugs that target this event.

In addition to initiation, translation elongation is also an opportunity for the development of anticancer and, perhaps, antiaging drugs. Ribosome profiling [100–102], a higher resolution variation on an older technique called polysome profiling, compares the translational footprint of cells and was used effectively for the development of a unified “model for mTORC1-mediated regulation of mRNA translation” [103]. Liu et al. used ribosome profiling to study translation elongation in response to proteotoxic stress, which revealed an association with ribosome stalling due to reductions of the Hsc70/Hsp70 chaperones needed for exit of nascent polypeptide chains from ribosomes [104]. Small molecule inhibitors of Hsc/Hsp70 are under investigation as anticancer agents [105] and might serve to promote increased longevity and improve health span.

The anabolic program is coordinated with supporting processes regulated by mTORC1 [106]. One of these upregulated programs to support cancer cell growth and proliferation is de novo fatty acid and lipid synthesis [52, 107, 108]. mTORC1 relays oncogenic and growth factor signaling to pro-lipogenic transcription factor SREBP1 [109]. In addition to increased uptake of glucose, activated mTORC1 also promotes gene expression supporting the pentose phosphate pathway (PPP) for its oxidative, NADPH-producing branch, which is coordinated through SREBP (reviewed in [64]). Ribose production by PPP is also important for nucleic acid biosynthesis, which is also acutely regulated in parallel with the metabolic flux through the de novo pyrimidine synthetic pathway regulated by S6K1-mediated phosphorylation of enzyme CAD (carbamoyl phosphate synthetase 2, aspartate transcarbamoylase, dihydroorotase) [110, 111]. Another branch of regulation is mTORC1-promoted translation of hypoxia-inducible factor-1α (HIF-1α), which upregulates glucose transporters and enzymes for glycolysis and promotes a change to aerobic glycolysis (Warburg effect) seen in most growing cancer cells [112]. Notch signaling, which is important in tumorigenesis [113], appears to regulate both glucose and lipid biosyntheses in the liver via mTORC1. All of these points of regulation represent opportunities for new drug targets to prevent cancer and positively impact aging. How chronic inhibition by rapamycin affects these processes is currently unknown. Short-term inhibition by rapamycin or active site inhibitors has been studied in some detail.

Ribosome profiling studies [114] revealed that prostate cancer cells treated with rapamycin or active-site mTOR inhibitors, PP242 and clinical grade INK128, have interesting transcript-specific control mediated by oncogenic mTORC1 signaling that included a specific set of pro-invasion and metastasis genes. The question of tumor cell specificity of this response is unknown, but it is known that tumors driven by oncogenic signaling have increased ribosome biogenesis linked to mTOR activation. These studies also revealed that active site inhibitors of the mTOR kinase are more efficient in generating this response than rapamycin, an allosteric inhibitor. The new generation of ATP-competitive inhibitors, which target the mTOR catalytic site directly, shows promise as more effective cancer therapeutic agents [115]. Their effectiveness as both cancer prevention and antiaging agents remains to be tested.

In sum, there are numerous critical points of control in the PI3K–mTORC1 pathways that would be targets of opportunity for the development of safe and effective drugs to intervene in both the cancer and aging processes. The question remains whether these drugs will be any safer or more effective than the founding drug rapamycin.

Intestinally delivered rapamycin for prophylaxis against tumors would require it to have little toxicity in healthy adults. Rapamycin, marketed to prevent organ allograft rejection, carries a US Food and Drug Administration (FDA) black box warning for immunosuppression. As an immunosuppressive, clinicians often use rapamycin in combination with other more potent calcineurin inhibitor-based immunosuppressants, meaning that its individual effects in humans are not well understood. We know of no published studies that show rapamycin is immunosuppressive in healthy subjects. The fact that rapamycin has been rigorously documented to increase maximum life-span of genetically heterogeneous mice in nine studies conducted in three geographically separate laboratories is not consistent with any clinically relevant immunosuppression. In fact, there is preclinical evidence to the contrary. Araki et al. [116] specifically examined effects of rapamycin on immunity and found it boosts immunity to infections. To address this paradox, Ferrer et al. [117] investigated the effects of rapamycin in an experimental setting in which CD8+ T cell responses to a pathogen or to a skin transplant could be compared. To achieve this, they used a transgenic model in which an identical monoclonal cell population would respond to the same epitope in either an infection or transplant setting. Remarkably, they found that rapamycin had disparate effects depending on the setting, whereas rapamycin boosted antigen-specific T cell responses to a bacterium, it did not to a transplant. This prompted the authors to state in their discussion “many facets to the mTOR signaling pathway in immune cells that are still poorly understood” [117]. Jagannath et al. [118] showed that rapamycin pretreatment enhances immune function in tuberculosis. Pretreatment also enhances immune function in antitumor vaccine responses in mice [119], influenza [78], and vaccinia vaccine responses in non-human primates [116]. Pretreatment with eRapa also enhanced resistance of old mice to pneumococcal pneumonia through reduced cell senescence [33]. Our studies in C57BL/6 mice showed no detrimental effects of chronic rapamycin on immune function [89]. Another paradox is that rapamycin and rapalogs are being tested in a variety of clinical trials (reviewed in [120]) and are FDA approved for the treatment of certain cancers. It is not likely that rapamycin is immunosuppressive in these populations; in fact, reports suggest otherwise [121].

The age-related decline in the immune system has been well recognized and appreciated for some time [122]. Naive T cells exhibit age-associated reduction in function by acquiring functional defects including reduced ability to proliferate, alterations in cytokine secretion, and deficits in the ability to undergo effector T cell differentiation [123–125]. Immune surveillance of cancer [126] could be negatively impacted by this decline. However, abrogation of age-associated decline in immunity is reversible by specific interventions [127, 128] and can improve efficacy of immunotherapy [129]. Since mTOR regulates aging and modulates the immune system including effects on immune mediators important for anticancer immune defenses [116, 130–134], could the longevity and cancer prevention effects of chronic eRapa treatment be, in part at least, through immune system modulation? Most explanations for how mTOR inhibition inhibits cancer focus on its growth, nutrient, and metabolic functions [52]. Remarkably, little is known about the role of immune effects by mTOR inhibition in longevity extension and cancer prevention.

Available data do not support the prevailing notion that single-agent rapamycin in healthy, normal subjects suppresses immunity, while preclinical data do support the concept that it can be an immune enhancer and/or modulator and a health span extender, including preclinical studies showing improvements in a broad range of diseases including those affecting cognition [37].

The fact that rapamycin and its analogs are used therapeutically for cancer treatment (e.g., renal cell carcinoma and breast cancer) suggests that chronic rapamycin treatment could be beneficial in a cancer prevention, antiaging setting. On the basis of all these and the above considerations, we believe it is time to give serious thought to the use of mTORC1 inhibitors as cancer prevention agents, especially for at-risk individuals, and perhaps at the same time address other age-associated diseases so that we can start to get a small handle on the huge economic burden, not to mention human suffering, facing the world.