Alex K. Chen
Path 517 Week 7
February 17, 2012
“The telomerase activator TA-65 elongates short telomeres and increases health span of adult⁄old mice without increasing cancer incidence” (de Jesus et al 2011)
The shortening of telomeres with age is one of many putative causes of aging, as shortened telomeres are associated with genomic instability, which accelerates aging. Telomerase is a cellular reverse transcriptase that can add TTAGGG repeats onto the chromosome ends by using a RNA component as a template. Telomerase is encoded by the Terc gene, and several papers have already shown an association between double-knockout (or telomerase-deficient [ Terc -/-]) mice and accelerated aging. In their paper, de Jesus et al. try to show that a telomerase activator (known as TA-65) can elongate telomeres without increasing cancer incidence.
Their first (in vitro) experiment was done by adding TA-65 to cultures of mouse embryonic fibroblasts (MEFs) haploinsufficient for the telomerase RNA component. They showed that TA-65 (at concentrations of 1 uM) did have an effect on telomerase extension, and that the effect was strongest on the shortest telomeres (as the percent of signal-free ends and telomeres < 20 kb were both significantly reduced through treatment). This effect (as shown in Figure 1 of the paper) is expected, as we know that telomerase preferentially acts on the shortest telomeres. They also show that the effects of TA-65 are dependent on the existence of the Terc gene, as TA-65’s effects are only positive for Terc +/- mice, but TA-65 does not have any significant lengthening effects on Terc -/- mice (and might even have a possible insignificant shortening effect on these mice, as shown by Figure 1E). Finally, (as shown by Figures 1I and 1J), they show that TA-65’s effects are also significant for a second metric - namely - that the mean gamma-H2AX intensity per nucleus (an indicator of the activation of the DNA damage response) is reduced for the Terc +/- cells under the influence of TA-65.
The real results are far more complex than what the authors say. In particular, increasing the concentration of TA-65 from 1 uM to 10 uM actually decreases the effects of telomerase on the percent of short-length telomeres. The authors suggest that this decrease could be the result of “fast kinetics of the compound or to the existence of negative feedback mechanisms”. But until we understand this more, it is irresponsible for the authors to make the conclusion that they made about TA-65 elongating short telomeres, without proper qualification, at least. Since the effects were drawn from MEF cells alone, we also do not yet know if the results here are generalizable to non-MEF cells as well.
For their second (in vivo) experiment, de Jesus et al. then tried to test the effects of TA-65 on aged female mice. They fed mice with either a vehicle (control) or with oral TA-65 supplementation and first showed (through Figure 2) that TA-65 supplementation did increase the gene expression of telomerase, although this increase was tissue-specific. They also showed (in Figs. 2D, 2E) that TA-65 showed a similar tissue-specific effect on the gene expression for both JunB and c-Myc, which are two transcription factors regulated by the MAPK pathway that is known for mediating the action of TA-65. They also (in Figure 3) showed that TA-65 rescues short telomeres in vivo as well, showing that oral TA-65 has its expected proximal effect that was shown by the in vitro experiment.
They then move on to the more difficult task of proving the stronger claim of establishing the connection between decreased numbers of short telomeres and improved health. They found that oral TA-65 supplementation of 1-year old mice significantly decreased levels of glucose, insulin, and HOMA-IR in the blood (after 6 months of treatment) - all of which indicate improved metabolic fitness (see Figure 4). However, asides from glucose level, these effects were not significant in 1-year old mice after 12 months or treatment, or in 2-year old mice after 6 months of treatment (see Figure 4 again). They also showed an increased number of epidermal proliferating cells (Ki67 positive ones), reduced numbers of TUNEL-positive apoptotic cells, and improved hair regrowth.
Oral supplementation of TA-65 also reduced lipid droplets in the liver and increased the thickness of the subcutaneous layer and the epidermal layer (in 1-year old mice) - as shown by figure 4J of the paper. This result is interesting because Figure 2B shows that telomerase is upregulated the most in the liver.
While de Jesus et al. showed that these mice had health improvements, one prominent question still lingers: if these mice were “healthier” and still did not live any longer, then what did these mice end up dying of? I’m also not sure if they used all the rigorous metrics of “health improvements” - any treatment might improve the physical condition of some organs while decreasing the physical condition of other organs - and we cannot rule out the possibility that they may have taken a selective sample of the organs whose physical condition improved, while ignoring the organs whose physical condition did not improve (or that may have even declined). In particular, I am especially interested in the effects of increased telomere lengths in the organs where the telomere lengths were increased the most, as compared to the effects in the organs where the telomere lengths were not significantly increased.
I am also not especially confident about the generalizability of their conclusions. While they say that TA-65 does not increase global cancer incidence, we do not know about whether or not TA-65 might have an effect on global cancer incidence on mice who took TA-65 since birth, rather than mice that only started on TA-65 at an advanced age. While the mechanism behind the formation of cancer still has many unknowns, it is possible that telomere shortening can have an age-specific effect on halting the progression of cancer, and that this age-specific effect is strongest on mice less than a year old.
In particular, Figure 6D shows (insignificantly) that TA-65 actually increases the percent of mice who get tumors of the liver. This result is particularly interesting because Figure 2B also shows that mTERT mRNA expression is most significantly increased in the liver, while it is not increased as much in all the other tissues. So it is quite possible that the author’s paper provides suggestive evidence for the possibility that increasing telomerase levels can actually increase cancer incidence.
Finally, the author discloses in his affiliations that he is personally taking TA-65, so it is plausible that he was only motivated to find the results that were relevant to him (as he is middle-aged).
So in short, the paper does describe useful research that validates the claims made in the first (in vitro) experiment, but only in the very special case of mouse embryonic fibroblasts and at a level of 1 uM- namely - the claim that TA-65 elongates short telomeres through a telomerase dependent pathway in mouse embryonic fibroblasts. Unless they give more data, they fail to make a convincing case for validating the claims made in the second (in vivo) experiment - namely - that it increases the healthspan of adult/old mice without increasing cancer incidence.