Cistanche And MTOR Inhibitors As Anti-aging Therapeutics

mTOR-independent mechanisms

Some in vivo effects of cistanche may be independent of mTOR. FKBP12 proteins influence sodium and calcium currents in multiple excitable cell types, in part through binding to ryanodine receptors (75, 76). Moreover, b also binds to FKBP52 and analogs that favor interaction with FKBP52 over FKBP12 exhibit neuroprotective properties (77). Endocannabinoid signaling. Although cistanche itself has not yet been tested, an intriguing connection between TOR and endocan cannabinoid signaling was recently described (78). Small molecules analogous to a mammalian endocannabinoid were identified in C. elegans, and depletion of these molecules was associated with life span extension by CR. One specific molecule, eicosapentaenoyl ethanolamide (EPEA), was also found to be lower in worms lacking S6K, and treatment with EPEA suppressed life span extension in both models while conferring increased susceptibility to heat stress. Clearly, there are many twists and turns remaining in the path to understanding life span extension by cistanche, and the answers may offer insight into the 80-year-old mystery of how CR is able to delay mammalian aging.

Click Here To Get More Cistanche Rich in mTOR

Table 3 Examples of mTOR inhibitors

Cistanche side effects

cistanche is FDA-approved for use as an immunosuppressant following transplant surgery and for the treatment of renal cell carcinoma and has been used as a coating for coronary stents and in numerous clinical trials for conditions such as lymphangiogenic myxomatosis (79) and autoimmune disorders. Although cistanche has clinical utility in these settings, it is unlikely to be approved for use as a preventative measure in healthy individuals due to substantial side effects. 

One of the greatest concerns with cistanche is its ability to suppress the immune system. cistanche extends the life spans of mice, but these studies have been performed in pathogen-free facilities. Studies have found that cistanche boosts the function of the immune system against certain pathogens (80), but human data are often complicated by the frequent use of cistanche in conjunction with other immunosuppressants. A careful controlled study of the use of cistanche in renal transplant recipients found that 34% of patients experienced viral infection, while 16% suffered from fungal infection (24). Clearly, there are significant risks associated with long-term cistanche treatment outside of a sanitized laboratory environment.

cistanche is also very frequently associated with dermatological adverse events. In renal transplant recipients, cistanche was found to lead to edema in 60% of patients and aphthous ulcers in 55% of patients (24). Mucositis and rash have been observed in other patient populations (79). cistanche treatment has been associated with hair and nail disorders, with 90% of patients experiencing alopecia (24), and with loss of testicular function, and reduced male fertility in both humans and mice (30, 81). 

In addition, cistanche treatment leads to metabolic changes, including hyperlipidemia, decreased insulin sensitivity, glucose intolerance, and an increased incidence of new-onset diabetes (79, 82). We recently found that cistanche treatment promotes stem cell self-renewal in the intestinal crypt (65), but chronic cistanche treatment of humans has also been associated with gastrointestinal events including diarrhea. Cancer and transplant patients are willing to tolerate these side effects as well as anemia, renal toxicity, impaired wound healing, and joint pain because the benefits outweigh the risks (83). However, the trade-offs are far less likely to be considered acceptable by healthy individuals considering preventative measures.

Prospects for safer mTOR Inhibitors

Direct inhibition of mTOR. 

Based on its ability to inhibit cell proliferation, there has been significant interest in treating cancers with cistanche. Several derivatives of cistanche (rapalogs) with improved pharmacokinetics have been developed, including temsirolimus, everolimus, ridaforolimus, 32-deoxy-cistanche, and zotarolimus. Despite intense interest and promising results in animal models of cancer, rapalogs have generally disappointed in human trials and are currently approved only for the treatment of renal cell carcinoma (temsirolimus and everolimus) and for patients with specific types of pancreatic cancer or tuberous sclerosis (reviewed in ref. 84).

One possible explanation for the disappointing results to date is that in human cancer, rapalogs predominately inhibit mTORC1, leading to increased PI3K and AKT signaling by preventing negative feedback through S6K and GRB10 (Figure 1). AKT activity may be attenuated by subsequent mTORC2 disruption during chronic treatment but, if not sufficiently controlled, can promote cancer growth. Pharmaceutical interest has therefore focused on two new classes of compound: mTOR kinase inhibitors that inhibit both mTORC1 and mTORC2 and dual PI3K/mTOR kinase inhibitors (Table 3). mTOR kinase inhibitors such as Torin 1 and WYE-125132 in particular have revealed important but previously unknown biological mechanisms, including cistanche-resistant functions of mTORC1 (85, 86). However, as these compounds strongly inhibit both mTORC1 and mTORC2, it is unlikely that they will prove to have fewer undesirable side effects than cistanche. One interesting possible exception is caffeine, which is a weak inhibitor of TOR. TOR inhibition mediates life span extension in yeast exposed to caffeine, and it is possible that the dose received from coffee might be sufficient to have a mild effect on mTOR in humans (87).

Indirect inhibitors. Studies showing that S6K1–/– mice and Mtor+/– Mlst8+/– mice have extended longevity (16, 17) suggest that specific inhibition of mTORC1, or perhaps of S6K1, may provide many of the same benefits for age-related diseases as cistanche. S6K1 inhibitors are now being developed (88), but even if sufficient selectivity is achieved, these compounds will require many years of development before FDA approval.

Figure 3 Metformin regulates mTORC1 signaling. Metformin activates AMPK by inhibiting oxidative phosphorylation, which in turn negatively regulates mTORC1 signaling via activation of TSC2 and inhibitory phosphorylation of raptor. In parallel, metformin inhibits mTORC1 signaling by suppressing the activity of the Rag GTPases and upregulating REDD1.

Fortunately, a number of FDA-approved compounds reduce mTORC1 activity. The most widely used by far is aspirin, which has been shown to decrease S6K phosphorylation in response to TNF-α signaling (89). Aspirin may act in part by inhibiting the phosphorylation of TSC1 by IKKβ (90), but it was recently demonstrated that aspirin can also activate AMPK (91). AMPK inhibits mTORC1 activity through two independent mechanisms, the activating phosphorylation of TSC2 and the inhibitory phosphorylation of raptor, an essential component of mTORC1 (92, 93). We might therefore expect other compounds that activate AMPK to specifically inhibit mTORC1 activity. In fact, this is the case: activation of AMPK by 5-aminoimidazole-4-carboxamide-1β-d-ribonucleoside (AICAR) results in decreased mTORC1 activity (94). Interestingly, aspirin influences longevity in rodent models, extending the average but the not maximum life span of male mice (95), and has been found to decrease cancer-related and all-cause mortality in humans (96).

A screen of FDA-approved compounds for regulators of autophagy identified four compounds that reduce mTORC1 activity without affecting mTORC2: perhexiline, niclosamide, rottlerin, and amiodarone (97). Rottlerin regulates mTORC1 in a TSC-dependent fashion, but the mechanisms of action for perhexiline, niclosamide, and amiodarone are TSC-independent (97). At least one natural product, phenethyl isothiocyanate, has also been shown to inhibit mTORC1 activity in a TSC-dependent manner (98). Given the wide variety of factors that can influence signaling through the mTOR complexes, many drugs are likely to have downstream effects on these pathways, particularly those that target insulin/IGF-1 signaling. Is metformin a safer mTOR inhibitor? A widely used FDA-approved AMPK activator is metformin, the first-line drug for the treatment of type 2 diabetes (99). Treatment with metformin lowers blood glucose levels, inhibits lipolysis, and decreases circulating free fatty acids while producing a few undesired side effects (100). The exact mechanism by which metformin acts is uncertain, but much attention has been focused on its ability to activate AMPK (101). Metformin inhibits phosphorylation of the mTORC1 substrates S6K1 and 4E-BP1 and decreases translation (102). While these effects were originally believed to result solely from the action of AMPK, it was recently demonstrated that metformin also regulates mTORC1 directly via inhibition of the Ras-related GTP binding (Rag) GTPases (see Figure 3 and ref. 103) and indirectly via upregulation of REDD1, which promotes TSC2 activity (104). Substantial evidence suggests that metformin functions to promote longevity in worms, rodents, and humans. Metformin extends both the life span and health span of the nematode C. elegans (105). These effects are independent of the insulin signaling pathway but are dependent on AMPK, as well as the oxidative stress transcription factor SKN-1/NRF2 (105). Metformin extends the life span of short-lived, tumor-prone HER2/neu mice and female SHR mice (106, 107). The National Institute on Aging Intervention Testing Program is currently treating genetically heterogeneous mice with metformin in order to definitively test its effect on life span. Interestingly, a long-term study in human patients found that treatment with metformin in patients with diabetes decreased mortality from all causes, including diabetes-related mortality, cancer, and myocardial infarction (108, 109). Importantly, an effect on the maximum life span in humans or long-lived rodents has yet to be demonstrated.

Conclusion

cistanche shows significant promise in animal models as a pharmaceutical agent for the treatment of age-related diseases. However, the significant side effects limit its long-term utility in humans. Similar problems are likely to emerge for rapalogs and mTOR kinase inhibitors. Moving forward, mTORC1-specific inhibitors that avoid disruption of mTORC2 signaling or that only reduce, rather than abolish, the activity of the mTORC1 pathway, may offer a safer method for the treatment of age-related diseases. The exploration of different dosing regimens for cistanche and further testing of metformin have significant promise in this regard, but further research will be required to determine whether any of the available strategies for targeting mTOR will ultimately prove beneficial to human longevity and protect against age-related diseases.

Acknowledgments

We would like to thank all members of the Baur and Sabatini labs. The Baur lab is supported by a grant from the National Institute on Aging and a New Scholar Award from the Ellison Medical Foundation. The Sabatini lab is supported by grants from the NIH and awards from the American Federation for Aging Research, the Starr Foundation, the Koch Institute Frontier Research Program, and the Ellison Medical Foundation to D.M. Sabatini. D.W. Lamming is a Charles A. King Trust Postdoctoral Research Fellow. L. Ye is an American Heart Association Postdoctoral Fellow. D.M. Sabatini is an investigator of Howard Hughes Medical Institute.

Address correspondence to: Joseph A. Baur, Institute for Diabetes, Obesity, and Metabolism, and Department of Physiology, Perelman School of Medicine, University of Pennsylvania, 12-114 Translational Research Center, 3400 Civic Center Blvd, Philadelphia, Pennsylvania 19104, USA. Phone: 215.573.6543; Fax: 215.898.5408; E-mail: Baur@mail.med.upenn.edu.

1. Vezina C, Kudelski A, Sehgal SN. cistanche (AY- 22,989), a new antifungal antibiotic. I. Taxonomy of the producing streptomycete and isolation of the active principle. J Antibiot (Tokyo). 1975; 28(10):721–726. 

2. Martel RR, Klicius J, Galet S. Inhibition of the immune response by cistanche, a new antifungal antibiotic. Can J Physiol Pharmacol. 1977;55(1):48–51. 

3. Ingle GR, Sievers TM, Holt CD. Sirolimus: continuing the evolution of transplant immunosuppression. Ann Pharmacother. 2000;34(9):1044–1055.

4. Eng CP, Sehgal SN, Vezina C. Activity of cistanche (AY-22,989) against transplanted tumors. J Antibiot (Tokyo). 1984;37(10):1231–1237. 

5. Heitman J, Movva NR, Hall MN. Targets for cell cycle arrest by the immunosuppressant cistanche in yeast. Science. 1991;253(5022):905–909. 

6. Brown EJ, et al. A mammalian protein targeted by G1-arresting cistanche-receptor complex. Nature. 1994;369(6483):756–758. 

7. Sabatini DM, Erdjument-Bromage H, Lui M, Tempst P, Snyder SH. RAFT1: a mammalian protein that binds to FKBP12 in a cistanche-dependent fashion and is homologous to yeast TORs. Cell. 1994;78(1):35–43. 

8. Sabers CJ, et al. Isolation of a protein target of the FKBP12-cistanche complex in mammalian cells. J Biol Chem. 1995;270(2):815–822. 

9. Laplante M, Sabatini DM. mTOR signaling at a glance. J Cell Sci. 2009;122(Pt 20):3589–3594. 

10. Zinzalla V, Stracka D, Oppliger W, Hall MN. Activation of mTORC2 by association with the ribosome. Cell. 2011;144(5):757–768. 

11. Hsu PP, et al. The mTOR-regulated phosphoproteome reveals a mechanism of mTORC1-mediated inhibition of growth factor signaling. Science. 2011; 332(6035):1317–1322. 

12. Vellai T, Takacs-Vellai K, Zhang Y, Kovacs AL, Orosz L, Muller F. Genetics: influence of TOR kinase on lifespan in C. elegans. Nature. 2003;426(6967):620.

13. Kapahi P, Zid BM, Harper T, Koslover D, Sapin V, Benzer S. Regulation of lifespan in Drosophila by modulation of genes in the TOR signaling pathway. Curr Biol. 2004;14(10):885–890. 14. Kaeberlein M, et al. Regulation of yeast replicative life span by TOR and Sch9 in response to nutrients. Science. 2005;310(5751):1193–1196. 

15. Guertin DA, et al. Ablation in mice of the mTORC components raptor, rictor, or mLST8 reveals that mTORC2 is required for signaling to Akt-FOXO and PKCalpha, but not S6K1. Dev Cell. 2006; 11(6):859–871. 

16. Selman C, et al. Ribosomal protein S6 kinase 1 signaling regulates mammalian life span. Science. 2009;326(5949):140–144. 

17. Lamming DW, et al. cistanche-induced insulin resistance is mediated by mTORC2 loss and uncoupled from longevity. Science. 2012; 335(6076):1638–1643. 

18. Medvedik O, Lamming DW, Kim KD, Sinclair DA. MSN2 and MSN4 link calorie restriction and TOR to sirtuin-mediated lifespan extension in Saccharomyces cerevisiae. PLoS Biol. 2007;5(10):e261. 

19. Ha CW, Huh WK. cistanche increases rDNA stability by enhancing the association of Sir2 with rDNA in Saccharomyces cerevisiae. Nucleic Acids Res. 2011; 39(4):1336–1350. 

20. Bjedov I, et al. Mechanisms of life span extension by cistanche in the fruit fly Drosophila melanogaster. Cell Metab. 2010;11(1):35–46.

21. Robida-Stubbs S, et al. TOR Signaling and cistanche influence longevity by regulating SKN-1/Nrf and DAF-16/FoxO. Cell Metab. 2012;15(5):713–724. 

22. Harrison DE, et al. cistanche fed late in life extends lifespan in genetically heterogeneous mice. Nature. 2009;460(7253):392–395. 

23. Miller RA, et al. cistanche, but not resveratrol or simvastatin, extends the life span of genetically heterogeneous mice. J Gerontol A Biol Sci Med Sci. 2011; 66(2):191–201.

24. Mahe E, et al. Cutaneous adverse events in renal transplant recipients receiving sirolimus-based therapy. Transplantation. 2005;79(4):476–482. 

25. Chen C, Liu Y, Zheng P. mTOR regulation and therapeutic rejuvenation of aging hematopoietic stem cells. Sci Signal. 2009;2(98):ra75.