When cells become senescent, they no longer proliferate, but that doesn’t mean they don’t grow. In fact, a very interesting recent review1 explains how cellular senescence involves both blocked cell cycling (discontinuation of replication) as well as excessive growth-promoting pathways.
The review’s author explains, “[w]hen the cell cycle is arrested, a continuation of cellular-mass growth results in senescent morphology.” In fact, in another paper by the same author (and his co-author)2 it is noted that older cells are indeed larger. “An increase in cell size is a hallmark of senescent fibroblasts. Their cell volume is several fold greater compared with proliferating cells. Cell size is progressively increased in cell culture as cells progress toward senescence.” “In other words, when the cell cycle is blocked in the presence of growth-promoting signaling, then cells increase in size.”
A major growth-promoting pathway includes TOR (target of rapamycin) along with its upstream regulators and downstream effectors. The TOR gene is structurally and functionally conserved from yeast to humans (including worms, flies, plants, and mice), acting as a cell growth regulator. “… [E]xcessive growth is a driving force for aging.” Indeed, it has been reported that inhibition of TOR signaling increases lifespan in worms, flies, and possibly mammals. In mice, decreased signaling through the insulin/insulin-like growth factor (IGF-1) pathway in adipose tissue results in less mTOR (mammalian TOR) signaling and increases lifespan. Reduced caloric intake (as in dietary restriction) also reduces signaling through mTOR and is a well-known method of increasing lifespan in many species, possibly including monkeys.
TOR (target of rapamycin) is inhibited by rapamycin, a natural secondary metabolite produced by soil bacteria to inhibit growth of fungal competitors. Interestingly, rapamycin is a prescription drug in clinical practice; it is administered to renal (kidney) transplant patients everyday for several years to prevent organ rejection. Results in these patients have included the unexpected prevention of cancer and even cures of some pre-existing cancers. Moreover, two years after renal transplantation, the body-mass index of patients treated with rapamycin was significantly lower than the patients treated with cyclosporine, another immunosuppressant. A study was described2 in which 11 healthy men were treated with 6 mg of rapamycin, with the prevention of insulin resistance that accompanies the large increase of nutrients (that ordinarily induces mTOR signaling) during feeding. At present, rapamycin is being investigated in clinical trials as a treatment for cancer.
Another paper describes the activity of pharmaceutical companies seeking to develop rapamycin derivatives to inhibit mTOR. The drug companies are interested in mTOR inhibitors for possible treatment, in addition to cancer, of autoimmune disorders, type 1 and type 2 diabetes, and obesity. For example, in relation to type 2 diabetes, chronic hyperglycemia can lead to chronic activation of mTOR in pancreatic beta cells. Rapamycin (which reduces the activity of mTOR) has been shown to induce autophagy, a process of programmed self-digestion which, for example, helps to clear aggregated proteins in neurodegenerative diseases such as Alzheimer’s disease.
The mTOR Pathway is Sensitive to Redox State
A further paper explains how a complex containing mTOR and the protein raptor “is a key component of a nutrient-sensitive signaling pathway that regulates cell size by controlling the accumulation of cellular mass.” In this very interesting study, the authors found that HEK293T cells treated with potent oxidants activated the raptor-mTOR pathway even under nutrient-deprived conditions, when this pathway is ordinarily suppressed. The authors suggest that “[i]f the oxidizing compounds are mimicking an endogenous oxidant that normally activates the raptor-mTOR pathway, the reducing reagent should inhibit pathway activation caused by nutrients.” Indeed the authors found this to be the case; incubating cells with a reducing agent called BAL (2,3-dimercapto-1-propanol) significantly reduced the phosphorylation of S6K1 (an effector of the raptor-mTOR pathway) “and was associated with an increase in the amount of raptor recovered with mTOR as is seen in cells in nutrient-deprived conditions.”
This is an exciting finding as it suggests that it might be possible to suppress mTOR activation even under conditions of full feeding by using appropriate (safe and effective) doses of certain powerful reducing agents. We haven’t seen any further work on this (though it may be that such research is being done but is being kept proprietary).
Natural Products That Inhibit mTOR
There are two natural products that have been reported to be possible inhibitors of mTOR: curcumin and resveratrol. A recent paper describes how curcumin disrupts the mTOR-raptor complex (mTORC1) that results from the activation of the mTOR pathway, thus leading the authors to propose that “curcumin may represent a new class of mTOR inhibitor.” Curcumin, along with possibly active (in inhibiting mTOR) molecularly related curcuminoids, can be gotten by supplementing with turmeric root powder.
A very interesting paper reports that resveratrol inhibits the mitogenic signaling (growth promoting) by mTOR that causes smooth muscle cells to proliferate in response to oxidized LDL.5a This could be a very important protective effect of resveratrol since the proliferation of smooth muscle cells is a major part of atherosclerotic development. Rapamycin dose dependently inhibited the DNA synthesis (marker of cellular proliferation) and cell proliferation of smooth muscle cells in culture, with complete inhibition taking place at 10-100 nM, indicating that the smooth muscle cell proliferation was under the control of mTOR. This effect was not due to cytotoxicity of rapamycin because in cells treated with oxidized LDL (50µg apoB/ml), rapamycin was not toxic up to 100 nM. Since resveratrol has been reported to have inhibitory effects on smooth muscle cell (SMC) proliferation, the authors tested it for its effects on mTOR and SMC proliferation. They report: “Dose-response experiments showed that DNA synthesis and cell proliferation were significantly inhibited by 25 µM resveratrol without any significant apoptotic effects [indicative of toxicity] at this concentration. It may be noted that 50 µM resveratrol exhibited a slight toxic effect in the presence of oxLDL [oxidized LDL].” They conclude that “[t]his strongly suggests that resveratrol acts on an upstream target in the PI3K/Akt/mTOR signaling pathway.”
Resveratrol Dose Limited by Toxicity
Another paper reports that resveratrol can inhibit mTOR and thus suppress cellular senescence but that the concentration required is close to the high dose at which resveratrol is toxic to cells. At lower doses, 8-25 µM, resveratrol was reported to “slightly but detectably” prevent the loss of proliferative activity (e.g., senescence) of the cells in which it was tested. Still, 6.25 – 12.5 µM resverarol was shown to block the cell cycle and 25 µM caused apoptosis in vascular smooth cells in another study.
Another recent paper reports that a “low dose” of dietary resveratrol (4.9 mg/kg) partially mimics caloric restriction and retards aging parameters in mice on a non-calorically restricted diet (though effects on mTOR were not reported in this study).
A further recent paper reported that standard diet fed rats receiving 6 mg of resveratrol/liter of drinking water had a reduced ratio of GSH/GSSG (reduced glutathione/oxidized glutathione) and enhanced GSSG, indicative of increased oxidative stress, in liver cells; in the same study, rats on a high fat diet receiving the same amount of resveratrol in their drinking water had reduced GSSG with GSH/GSSG not significantly different from controls on a standard diet, indicative of less oxidative stress. Though this dose of resveratrol (6 mg/liter of water) is, the authors say, below the maximal tolerated dose, the study suggests that the dose ingested by the standard diet fed rats (an average of a total of 48.2 mg/kg of body weight of resveratrol over 45 days or about 1 mg of resveratrol/kg body weight/day) had toxic effects, particularly (as noted above) increased oxidative stress in the liver. Meanwhile, the total amount of resveratrol, 14.8 mg/kg, ingested over 15 days (about 1 mg/kg body weight per day) by the high fat diet fed rats had protective effects.
Further research is needed to understand the varying effects of different doses of resveratrol in rats (and, indeed, in humans) fed different diets to determine optimal doses. It has already been found that dietary composition may affect the degree of life extension resulting from caloric restriction in fruit flies.8a
The amount of resveratrol in red wine is reportedly about 90 µg of resveratrol/fluid ounce of red wine).
The authors of paper speculate that “even transient inhibition of mTOR is already sufficient to slightly suppress senescence.” They also suggest that “a combination of non-toxic doses of resveratrol with rapamycin would also extend life span in animals on a standard diet.” Resveratrol has already been shown to extend the lifespan of mice on a high-fat diet. We, of course, would like to see a test of non-toxic doses of resveratrol along with curcumin for its effects on mTOR and on life extension in animals on a standard diet. We would also be interested in the effects on mTOR of the curcumin-related curcuminoids found in turmeric root powder.
We ourselves take our turmeric root powder (2 capsules four times a day) rather than taking only curcumin due to the possible additional benefits of the curcuminoids. We do not know what the optimal amount of resveratrol is for the purpose of decreasing cellular senescence and inhibiting mTOR, though we do drink moderate amounts of red wine and also take resveratrol supplements.
The Durk Pearson & Sandy Shaw®
Life Extension News, Volume 13 No. 1 • February 2010
